Dual power paths and single-cord equipment

Protecting your data center against power failure is crucial to providing maximum availability. Power loss or poor power quality is a major contributor to data center server down time. This task takes more than simply having UPS and a backup power source, such as a generator. Dual Power Paths: A Crucial Element in a High-Availability Data Center article tells that in in high availability environments, a common way to provide redundancy is to supply two independent power paths to each piece of computing equipment. All Dual Feeds Are Not Created Equal white paper explains the classification system for data center power feeds and provides the pros/cons of each configuration.

The use of dual power path architecture in combination with IT equipment with dual power supplies and power cords is an industry best-practice. Data centers designed and built utilizing Tier IV requirements are by definition “concurrently maintainable,” which means any system or component in the data center may be shut down for maintenance or may fail without affecting the delivery of services to the end user. In the case of a dual-powered data center, this typically is achieved by delivering at least two power circuits to each cabinet, one from the A power source and one from the B power source. The equipment accepts the two power feeds via independent, parallel power supplies that are sized such that the equipment will continue to operate with only one power path. To make this work the equipment to be powered need to have Redundant Power Supplies.

Redundant Power Supplies are one advanced feature available on high-end server machines. In essence, this is a power supply that actually includes two (or more) units within it, each of which is capable of powering the entire system by itself. If for some reason there is a failure in one of the units, the other one will seamlessly take over to prevent the loss of power to the PC. You can usually even replace the damaged unit without taking the machine down.

Design Tips for the Dual-Powered Data Center and Four A-B Design Failures to Avoid in the Dual-Powered Data Centerarticles gives some more tips how to design dual power paths. You need to be careful in designing dual-powered data center. Failure to properly design, size and implement dual power infrastructure at the cabinet may lead to breaker trip during restart (starting current of many computing devices and storage systems could easily exceed 200% of the running load for some time). You need to have enough spare capacity, but on the other hand failing to fully load power circuits to their rated capacity may not result in downtime but may drive power subscription costs higher than necessary. Proper power planning and budgeting involves loading every circuit to the proper rated capacity while respecting safety margins.

For dual power paths approach to be effective, you’ve got to meet two requirements:

  • The protected equipment must support dual power feeds and operate with one feed faulted.
  • The loading of breakers within each power path must always be less than 50% of trip rating during normal conditions, so the breaker doesn’t trip if the alternate path has to take on the full load.

Meeting these two requirements can be a challenge. Especially because some computing and networking equipment is only available with a single power cord. It’s good design principle to disallow the use of single power cord computer devices in a high-availability data center environment, but there are case where those can’t be avoided. For example some network products or legacy servers may only have single power supplies.

One good way to over-come single power input equipment problem is to use an Automatic Transfer Switch (ATS), which generates a single feed from two inputs. These single power supply devices can still be used with reliability by utilizing automatic transfer switches. Redundant Power Supply article has the following picture to show the use if automatic switch.

Powering single-cord equipment in dual path data center environments article tells about a new white paper from APC-Schneider Electric addresses the concern of powering single-corded equipment in dual path data center environments. However, equipment with a single power supply introduces a weakness into an otherwise highly available data center. Transfer switches are often used to enhance single-corded equipment availability by providing the benefits of redundant utility paths. You need to understand the use of power transfer switches well because there are several possible configurations how to use them, with their pros and cons.

Powering Single-corded Equipment in a Dual Path Environment white paper goes on to describe three fundamental approaches to powering single-corded equipment in a dual path data center environment. There are a number of options for integrating single-corded devices into a high availability dual path data center. Powering Single-corded Equipment in a Dual Path Environment paper explains the differences between the various options and provides a guide to selecting the appropriate approach. The conclusion is that Power availability to the single-corded equipment below 10 kVA is optimized by bringing utility redundancy directly to the rack. This can be done by using a rack mount static transfer switch or a rack mount ATS, and the optimal solution is a rack mount ATS.

A well designed adaptable rack enclosure power system would be able to support a single or dual path environment or a hybrid of both single and dual equipment. Automatic Transfer Switch (ATS) used in data center is typically rack mountable and occupy 1U or rack unit of space. They feature dual input cords and are able to switch from one power circuit to the other in a few micro seconds when power failure is detected on one of the input leads.

The idea to write about this topic to this blog came to me after reading Powering Single-corded Equipment in a Dual Path Environment white paper.


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  2. Tomi Engdahl says:

    Super Bowl XLVII blackout: Power redundancy, paralleling and LED lighting

    No, Beyoncé’s revival of Destiny’s Child did not cause the blackout during this year’s Super Bowl game on February 3, 2013. It was combination of technical and communication failures that contributed to the partial power outage according to an independent analysis.

    Dr. John Palmer of Palmer Engineering & Forensics did an investigation and found that a recently installed relay had a malfunction or “misoperation” that caused it to trip in an unpredictable way.

    The report also notes the relay had a design defect, and under testing it did not perform entirely as its instruction manual said it was supposed to. It says the factory default setting of the relay was inappropriate.

    Kornblit says that Field-of-play (FOP) lighting is typically a very large imperative load and every aspect of its design, including the power supply must receive special attention. Most FOP lighting relies on metal-halide technology even a minimal voltage disturbance can cause an extended outage.

    The power supply must be fully redundant, backed up by generation, and the lighting divided into two equal groupings. During competition each group must be isolated from the other and fed from a separate source, each source capable of feeding the full load if necessary.

    The IBC electrical power distribution system consisted of numerous 400V switchgear lineups distributed throughout the facility

    Under normal conditions, the system is powered by main source S1.

    If the main source is lost, then S1’s main breaker opens and the tie breaker automatically closes, connecting backup power S2 to the bus. At the same time, because an abnormal condition exists, the backup emergency generators start-up and serve as backup to S2.

    All power sources are intended to run independently, only one source provides power to the system at any given time under the automatic switching conditions described and all critical control is through PLC (Programmable Logic Controller).

    Power loss, unfortunately, does occur on the bus during these emergency-operating conditions. The outage time per switching cycle, including sensing, signal transmissions, and internal trip times, is roughly ten to one hundred milliseconds.

    It was agreed to minimize such outages wherever possible by incorporating a transitional parallel operation mode between sources. That is, paralleling would be permitted, for this application only, and for periods not to exceed a maximum defined length of time.

    Parallel operation of supply sources within a power system has been shown by many studies to provide added reliability to the overall operation of the power system and may be necessary for certain critical applications. Such operation adds system complexity and requires suitable planning, appropriate system protection and control, and critical evaluation of the system’s needs and capabilities.

    When power sources are paralleled, additional protection must be added at various key positions in the system. This protection includes a means of confirming that the voltage magnitude, phase angles, and frequencies between the two systems are within a safe tolerance level. Directional sensing capabilities must also be included in order to limit the flow of power in undesirable directions, and to assure proper coordination in both directions, since the protective settings are usually different in each direction.

    For this application normal power is supplied by main source S1, the backup power (S2) is in hot standby, and the gensets are under cool-standby. When the main source power is lost, the PLC commands the S1 main breaker to open. Once confirmed and following verification of a sound bus and availability of the alternate source, a close command is sent to close the S2 tie breaker. Power is automatically transferred to the backup source with minimal delay, and the gensets are commanded to start via communications. Within about 15 seconds the genset are available as the hot-standby.

    Separate sequences must also be considered and included for the recovery cycle to permit a return to normal power conditions

    A major concern for parallel transfer schemes is that the fault current capacity of the system, which dramatically increases during the period of parallel operation, may exceed the short-circuit ratings of the equipment being paralleled.

    The power outage at the Super Bowl causing the lights to go through a 34-minute recycle to warm up

    LED lighting has reached the stage of technology that enables it to be implemented even in field and stadium lighting venues. This technology is another possible help in reducing down time in failures like that in New Orleans at the Super Bowl this year.

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  4. Tomi Engdahl says:

    The Perfect Power Microgrid at IIT

    Microgrids – self-contained electrical generation and distribution grids that may or may not be connected to the national electric grid – offer increased reliability for those connected to them. When the main grid loses power, a microgrid can disconnect from it and continue to generate and distribute power to its local customers. Microgrids are popping up in applications where power failures would be catastrophic: military bases, medical facilities, and now … college campuses

    While you might not think of a college campus as a “mission critical” place, Illinois Institute of Technology (IIT) experienced three to four power outages per year, resulting in numerous research experiments being delayed or ruined. University officials estimated that those power failures cost IIT about $1 million per year. Also, IIT wanted to incorporate renewable energy sources – partly for educational purposes and partly to save money. Combining those two factors, they decided to build a microgrid.

    When the main grid goes down, the campus grid needs to generate its own power. Although they want to move to renewable sources long term, their current on-campus generation is mostly natural gas powered turbines with 9 MW of capacity. They have 140 kW of rooftop solar on one building and a small wind turbine that’s mostly used for educational purposes, and they also incorporated 500 kWh of flow batteries for energy storage. The batteries provide energy for short durations, making it less likely that the generators need to power up at all.

    One of the first design decisions was to change the distribution from a radial layout to a multiple loop configuration with each building being fed from two different locations. A fault in one loop wouldn’t necessarily bring the entire system down, as buildings that weren’t part of the fault could get their power from another loop. Intelligent substations are capable of isolating and bypassing a fault, allowing unaffected buildings to remain powered and providing fault-location data for technicians.

    What does the future hold for the IIT microgrid? The addition of smart metering and communication equipment will help to make real-time decisions about when it’s better to buy energy from the grid and when it’s more economical to generate it locally.

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  7. Tomi Engdahl says:

    Protecting standby generators for mission critical facilities
    The generator and standby power systems for mission critical facilities require a higher level of reliability and availability

    How important are generators in a standby power system for a mission critical facility? When the lights go out and you find yourself counting the seconds until they come back on, that generator is the most important piece of equipment in the facility. During utility power outages, mission critical facilities rely on generators to keep the facility operating

    If the generator fails to start or if there is a fault in the standby power distribution system, that facility will eventually stop operating.

    This is not an option for mission critical facilities. Whether for public safety, national security, or business continuity reasons, mission critical facilities must remain operational. The reliability of the generator and the standby power system is crucial to the continued operation of the facility.

    By definition, a mission critical facility is essential to the survival of a business or organization. Mission critical facility operations are significantly affected when the power system fails or is interrupted. Important aspects of a mission critical facility power system are availability, reliability, and security.

    Mission critical facilities can be divided into two categories: private and public safety. The private mission critical facility contains systems that must remain operational for business continuity reasons. The public safety facility contains systems that must remain operational to protect the safety of the public.

  8. Tomi Engdahl says:

    Be prepared with hydrogen fuel cell back-up power

    For small and mid-sized businesses, however, power outages are not so amusing. According to Price Waterhouse research, once an outage has occurred:

    • 33 percent of companies take more than a day to recover.
    • 10 percent of companies take more than a week.
    • It can take up to 48 hours to reconfigure a network.
    • It can take days or weeks to re-enter lost data.
    • 90 percent of companies that experience computer downtime and don’t have a contingency plan go out of business within 18 months.

    Recently, the emergence of advanced hydrogen fuel cell technology has more business owners and managers re-thinking their back-up power strategy. Electrical current hydrogen fuel cells (HFC) are a low-maintenance solution that is gaining momentum in the market as a reliable alternative source of back-up power. HFCs are electro-chemical devices that use hydrogen fuel and atmospheric oxygen to generate clean and quiet direct current electricity.

  9. Tomi Engdahl says:

    Data center tier classifications
    Tier classifications have been established to address several issues within data centers.

  10. Tomi Engdahl says:

    Be prepared with hydrogen fuel cell back-up power

    For small and mid-sized businesses, however, power outages are not so amusing. According to Price Waterhouse research, once an outage has occurred:

    • 33 percent of companies take more than a day to recover.
    • 10 percent of companies take more than a week.
    • It can take up to 48 hours to reconfigure a network.
    • It can take days or weeks to re-enter lost data.
    • 90 percent of companies that experience computer downtime and don’t have a contingency plan go out of business within 18 months.

    Traditionally, businesses rely on a combination of generators powered by diesel fuel and lead-acid batteries for back-up power. These solutions, known as gen-sets, are permanently installed on an exterior concrete slab in a climate-controlled enclosure and hardwired into the building’s electrical system.

    Still, commercial users are plagued by persistent issues with generators, including maintenance and space requirements, unreliable operation, inefficient delivery of power and the impact of noise and air pollution.

    Over the years, a variety of HFC technologies were developed to optimize performance for specific applications.

  11. Tomi Engdahl says:

    NFPA 110: Standard for Emergency and Standby Power Systems extended Q&A

    Recent Consulting-Specifying Engineer webcast presenters Tom Divine, PE, Project Manager, Smith Seckman Reid Inc., and Kenneth Kutsmeda, PE, LEED AP, Jacobs Engineering, answer reader questions about what new code requirements will mean for consulting engineers.

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  13. Tomi Engdahl says:

    Power plant reliability

    The ASME committee for reliability, availability, and maintainability helps power plant owners and operators achieve their goals.

  14. Tomi Engdahl says:

    Using eco mode in UPS systems
    Saving energy is the ultimate goal when implementing UPS eco mode.

    The choice of using UPS eco mode operation for critical systems is a widely debated topic in the industry today, as energy costs rise and corporations seek to enhance their green image by showing improved power usage effectiveness (PUE) in their annual reports.

    There are many different interpretations of what “eco mode” operation means in a UPS system. The reality is that eco mode is a broad term used to describe any UPS mode of operation that improves the efficiency of the system. This efficiency increase comes with a particular trade-off in performance, which varies by vendor. This means that, depending on the manufacturer, UPS systems that use an eco mode may have entirely different modes of operation, affecting reliability and energy savings—again, stressing the importance of understanding the operating characteristics of the specific equipment within your facility.

    UPS systems with an eco mode use the same configuration as double conversion units, but with different operational characteristics that provide an increase in efficiency. When placed in eco mode, the UPS system typically allows utility power to bypass the rectifier and inverter and directly feed the critical load. In the event of a power disturbance, the UPS can provide conditioned power to the load by returning to normal mode. This process can significantly reduce the losses in the UPS system, depending on the manufacturer, and generally improves UPS efficiencies by 2% to 4%.

    A common misconception of UPS systems is the difference between offline and line interactive, both of which can be considered eco mode.

  15. Tomi Engdahl says:

    Kinetic energy storage fulfills a vital role for diverse applications
    Data centers, hospitals, and other applications can benefit from the reliability, scalability, low cost of ownership, and other advantages kinetic energy storage systems offer.

    Batteries have long been the mainstay energy storage partner for UPS and for good reason. They are ubiquitous, have a low initial purchase cost, and are readily available. However, their reliability is always in question and they have a limited life.

    The service life of valve-regulated sealed lead-acid (VRLA) batteries depends on many factors, including ambient temperature, proper maintenance, and usage frequency (cycling), as well as the quality of connections and terminals.

    Rapid advances in virtual, cloud, and mobile computing have driven computer networks to be operational 24×7, no matter what. And as we move closer to an Internet of Things (IoT) infrastructure—linking physical and virtual objects—downtime is just not tolerable. According to a recent study on the Cost of Data Center Outages by the Ponemon Institue, the cost of downtime is $627,418 per incident. If a data center is unable to process e-commerce transactions, serve up important data, or provide fast networked communications instantly, customers are unhappy, which results in a less-than-optimal bottom line.

    In a typical data center, health care facility, or manufacturing facility, flywheels free up 50% to 75% of space that would be taken up by an equivalent power-rated battery bank.

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  17. Tomi Engdahl says:

    Power plant reliability

    The ASME committee for reliability, availability, and maintainability helps power plant owners and operators achieve their goals.

  18. Tomi Engdahl says:

    Casino selects backup power system

    A 4-MW standby power system backs up life safety equipment, gaming, and security loads for an entertainment venue in the Arizona desert.

    “Reliable backup power is critical for us as a gaming facility,” said Jim Wanamaker, facilities manager for NNGE. “For us, life safety is always of the utmost importance. Beyond that, the critical requirement for us as a gaming operation is that we don’t lose power to the gaming floor or the gaming slot systems to maintain play on the floor. Security systems are also crucial: if security were to go down, we’d have to shut down the gaming systems as well.”

    The resort was designed by the Friedmutter Group architects of Las Vegas in collaboration with the Navajo tribal members and JBA Consulting Engineers. To ensure power is available at all times, Twin Arrows has several redundant systems that include dual utility feeds, multiple uninterruptible power supply (UPS) systems, and a pair of standby diesel generators in a 2N configuration.

    The two standby generators operate at 480 V and produce 2 MW each. With a current total facility load of about 1.25 MW, one generator is sufficient to supply the total load in the event of an outage. The second or redundant generator provides several advantages: it is a backup to the first generator, it allows maintenance to be performed without jeopardizing system reliability, and, in an extended outage, it can take over if the first generator consumes its 24-hour supply of fuel.

    The generators also operate in parallel and share load using the Cummins PowerCommand digital generator controls and parallel with the utility grid using the PowerCommand digital master control (DMC 300). When an outage occurs, both generators start and assume load. However, the digital master control will shut down the redundant generator when the facility load drops below 80% of a single unit, saving fuel and engine wear. Each generator is housed in a sound-attenuated enclosure, located outdoors.

    The casino’s first line of defense is the facility’s three UPS backup units. The battery-powered units maintain power to key casino operations that include emergency lighting, security and surveillance, gaming floor and player management systems, HVAC, and other systems. The UPS systems bridge the approximate 10-second gap between when normal power goes out and the load is assumed by the diesel generators.

  19. Tomi Engdahl says:

    Choosing between 3-pole and 4-pole transfer switches

    The choice between a 3-pole and 4-pole transfer switch depends on whether the emergency power system will be a separately-derived source.

    There are many design choices to make when planning a backup power system. But perhaps one that isn’t understood as well as it should be is whether to specify a 3-pole or 4-pole automatic transfer switch (ATS). At the heart of this choice is one simple consideration: whether or not your emergency power system will be separately derived.

    On systems using a 3-pole ATS, the neutral is continuous through the entire system. This is known as a solid neutral, and it’s bonded to ground at only one location: the facility’s utility service entrance.

    On systems using a 4-pole ATS, each source’s neutral is bonded to ground at its source, so each source is considered to be “separately derived.” Regardless of which source the customer load is switched to, if a ground fault occurs, the fault current will travel through ground directly back to the source that is presently supplying the loads. This is known as a switched-neutral system, and the neutral switching can be open or overlapping (closed).

    The NEC (and many other local codes and standards) provides the system designer with guidance to determine whether the emergency system must be a separately-derived source. It’s important to consult the appropriate code when planning your system to ensure there is clarity regarding the need to have the emergency system as a separately derived source.

    While there are many factors that determine whether to use a 3-pole or 4-pole transfer switch, it should be emphasized that in systems with multiple ATSs, it is important to stick with one or the other neutral switching schemes. In other words, all the transfer switches serving 3-phase, 4-wire loads should be of the same type-either all 3-pole or all 4-pole. This is essential for maintaining the integrity of the ground-fault scheme.

  20. Tomi Engdahl says:

    Specifying a multi-mode UPS in data centers
    Engineers should consider lifecycle cost and energy efficiency models for evaluating data center uninterruptable power supply (UPS) systems.

    When consulting-specifying engineers look at the hundreds of technology factors that go into a data center’s design, they know that even small variables, when multiplied by big numbers, add up quickly. That’s the case with seemingly small incremental increases in energy efficiency for uninterruptible power supply (UPS) systems used in data centers around the world.

    Unfortunately, in this scenario power efficiency is the price paid for protection. Transformer-based double conversion UPS systems have a typical power efficiency rating in the range of 88% to 92%. As a result, double conversion UPS systems place a steep toll on annual data center energy operating budgets.

    Newer three-level insulated gate bipolar transistor (IGBT) UPS technologies, which reduce switching and filtering power conversion losses, offer efficiency levels approaching 97% in double conversion mode, and up to 99% efficiency when operating in energy-saving multi-mode. These new, three-level UPS topologies create new OpEx rationales when designing data center power systems and specifying UPS technologies.

    Newer multi-mode UPS technologies, such as GE’s eBoost (which operates at 97% to 99% efficiency),

    So what’s that optimum switching or transfer time? Some earlier white papers and blogs suggest that anything over 8 to 10 milliseconds (ms) is problematic, given that not all data center sensitive equipment (e.g., servers) have tolerances at or above these levels. According to a Green Grid white paper on multi-mode (or eco-mode), “if, for example, a UPS has a transfer time of greater than 10 ms and is paired with information technology (IT) equipment that has ride-through capabilities of only 10 ms, the UPS may not be able to support the IT equipment.”

  21. Tomi Engdahl says:

    Reconditioning backup batteries—the right way
    Proper backup battery reconditioning methods lead to increased reliability, performance, and savings.

    With an aging infrastructure, power outages are a concern across the U.S. It is estimated that U.S. businesses lose on average $15,709 every 30 min during a power outage, totaling $104 billion annually.

    As such, the data center’s stored energy system is designed to provide:

    Instant availability of supply power to the critical load via the UPS in the event of sags, spikes, complete utility failure, or any other power disturbance that requires a switchover to a backup power source
    Proper sizing to supply the critical load that is normally supported by the utility via the UPS
    Sufficient operating time for backup power to come online (typically the time required for a generator to start and be able to accept load).

    While there are several types of rechargeable batteries available for backup applications, the data center designer turned to EnerSys for NiCd batteries to power its switchgear because of their good cycle life, capacity, and performance at low temperatures. The selected NiCd batteries—with a nominal cell voltage of 1.2 Vdc—were specifically designed for mixed loads that include both high and low discharge rates.

    Diagnosis: Floating effect

    After reviewing the test data, EnerSys engineers realized that the NiCd batteries were experiencing a phenomenon known as “floating effect.”

    This is a common occurrence. The conditions that can cause this effect include:

    Maintaining the NiCd cell at a fixed floating voltage for a period of time
    Storing a battery for long periods of time without proper activation
    Applying an improper commissioning charge.

    When a NiCd cell is maintained at a fixed floating voltage over a period of time as required in switchgear applications, there is a decrease in the voltage level of the discharge curve. Known as floating effect, this process begins after one week at a fixed floating voltage and reaches its maximum in about 3 months. It can yield very poor capacity, with results as low as 10%. It also can significantly decrease available battery run time.

    Following the battery manufacturer’s recommended directions, the data center completed a reconditioning cycle and load test by:

    Discharging the cells as a string to 1.1 V per cell at 211 A for 5 hr
    Recharging the cells as a string at 1.62 V per cell for 30 hr
    Allowing the cells to remain on open circuit for 4 hr before discharge
    Discharging the cells as a string at 456 A to 1.15 V per cell.

    During the charge and discharge steps, the data center monitored the cell temperatures, cell voltages, and string voltage.

    The reconditioning charge worked, and the floating effect was completely reversed.

  22. Tomi Engdahl says:

    Designing medium-voltage genset systems

    Good working knowledge of medium-voltage applications is essential in making a smooth transition from low-voltage system experience to successful medium-voltage system design.

    A common electric machine voltage tends to be 4,160 V. This voltage becomes very prevalent in industrial environments as motor horsepower exceeds 500. It is common to find medium-voltage motors ranging from 2,400 to 6,900 V. Some of the classical definitions of medium voltage extend to 35 kV, or even 69 kV. This operational range makes sense from a utility/transformer perspective, but not necessarily from an on-site generator perspective. It is common for alternator manufacturers to reference alternators in the 5 kV class (2,400, 4,160, and 6,900 V) as medium voltage and alternators in the 15 kV (12.47, 13.2, and 13.8 kV) class as high voltage-though from the broader sense, both classes are in the medium-voltage category.

    The transition from low-voltage to medium-voltage on-site generation is influenced by a mix of economic and system design considerations: cabling distance, genset, and switchgear costs; and bus capacity, fault-current capacity, and utility interconnection/integration configurations.

    Paralleling generators in prime power applications
    Paralleling generators can increase efficiency, improve reliability, and save money.

    Power: it’s a necessity that keeps the lights on and equipment running in every building and on every job site. But in many cases, it’s the true lifeblood of an operation, and an outage can be downright costly in critical operations, such as asphalt plants, concrete plants, or mines. In scenarios like these, it’s especially important to have predictable and reliable power. And while specifying the right size generator is a good starting point, one might find there’s greater dependability, cost savings, and efficiency in numbers.

    Since the advent of portable generators, it has been common practice in many prime power applications to choose a single generator with enough power output to operate all of the tools and equipment needed. While this certainly serves the immediate need, operating several smaller generators in parallel might be a more efficient and versatile option

    The synchronizing process-which at one time involved a vast network of wires, manually adjusting the rpm, and frequently monitoring the loads-is now fairly simple with most portable generators. In fact, some generators feature plug-and-play technology that allows the control to be connected with just one cable. After the connection is made, the user connects the power cables to a common bus, sets a few parameters in the controller, then hits the start button, and the process takes care of itself.

  23. Tomi Engdahl says:

    Specifying a multi-mode UPS in data centers
    Engineers should consider lifecycle cost and energy efficiency models for evaluating data center uninterruptable power supply (UPS) systems.

    When consulting-specifying engineers look at the hundreds of technology factors that go into a data center’s design, they know that even small variables, when multiplied by big numbers, add up quickly. That’s the case with seemingly small incremental increases in energy efficiency for uninterruptible power supply (UPS) systems used in data centers around the world.

    According to the Uptime Institute, traditional transformer-based UPS devices represent only 12% of a typical data center’s energy consumption, given power use and energy conversion inefficiencies and heat loss. Although they account for only a fraction of the total energy consumption in a data center, even small improvements in UPS energy conversion efficiencies can add up to significant lifecycle operational cost savings. A study by Frost and Sullivan found that the U.S. could reduce its yearly consumption of electricity by up to $3 billion by increasing the energy efficiency of UPS units in data centers from 90% to 98%.

    In evaluating efficiency and lifecycle costs for multi-mode UPS systems, some might ask: If our UPS running in double conversion already gets us to 93% efficiency, why take a “risk” for a few percentage points in efficiency? Can that extra energy efficiency provide a significant return?

    If we look at a UPS deployment at a typical 10 MW data center realizing just a 1% gain in efficiency, we can see a significant impact over 10 years.

  24. Tomi Engdahl says:

    When utility power is interrupted, standby power system failure is not an option for mission critical facilities. Mission critical facilities such as hospitals, data centers, and other highly critical buildings must remain operational—period. Vital mission critical power system characteristics include availability, reliability, security—and now efficiency.

    Designing reliable and efficient standby power for mission critical facilities poses unique challenges, such as determining the size and quantity of standby generators; installation concerns such as elevation and heat de-rating; analyzing the types of loads and those load characteristics that may affect de-rating of generators; calculating the amount of on-site fuel and different types of fuels (i.e. natural gas or diesel); determining the required “ride-through” time to allow engines to start and assume the load; consideration of testing and ongoing maintenance; and anticipating a multitude of scenarios that can affect system performance.

    Source: https://event.webcasts.com/starthere.jsp?ei=1052723

  25. Tomi Engdahl says:

    Standby vs. emergency power in mission critical facilities
    System designers must interpret the requirements of NFPA 110, ensure their designs follow them, and educate their clients about how the standard affects their operations.

    NFPA 101 defines the minimum requirements for equipment needed to support life safety systems, such as elevators, exit signs, and emergency lighting. This code relies on other codes, such as the NEC and NFPA 110, to provide information on how those systems should be installed and how they should perform.

    The NEC is primarily concerned with installation requirements and safety. While it defines some operational requirements, it does not list specific requirements for performance, testing, or maintenance

  26. Tomi Engdahl says:

    Rack transfer switch provides data center power monitoring at the outlet level

    Raritan recently announced new power management capabilities for its Intelligent Rack Transfer Switch, a product the company says “helps data centers keep equipment operating through power failures and provides insights on energy usage and rack-equipment health.”

    The Intelligent Rack Transfer Switch uses a patent-pending hybrid design, Raritan explains, “for fast and durable load transfers from one power source to another.” With recently added capabilities, the switch “now offers outlet-level metering and power switching. Ideal for cloud computing infrastructure and data center racks filled with one-power-supply devices, the new switch is one of the fastest in the industry—twice as fast as standard automatic transfer switches—and one of the most reliable,” the company proclaims.

    Similar to the company’s intelligent rack power distribution units, the new transfer switch can monitor data center power at the outlet level as well as the inlet level, Raritan says, “providing more granular energy information useful for capacity planning and managing energy costs. The transfer switch’s new secure switching capabilities enable power to a device to be turned on and off remotely, eliminating visits to the data center.”

    “Before Raritan released its first intelligent rack transfer switch model, the choices were limited.” He added that automatic transfer switches (ATS) were inexpensive but suffered from longer transfer times of 8 to 16 ms and frequently failed as a result of electrical arcing that welded contacts together. Plus, he said, static transfer switches offered very fast transfers of 4 to 6 ms, but were nearly six times the cost of an ATS, drew more energy, and produced excess heat that consumed cooling resources.

    “Raritan overcame these limitations with a new hybrid system that uses the best of both electromechanical relay and silicon-controlled rectifier technologies to deliver fast performance and better energy efficiency and reliability,”

  27. Tomi Engdahl says:

    Energy performance in mission critical facilities
    Mission critical facilities, such as data centers, are judged carefully on their energy use. Engineers should focus on the codes and standards that dictate energy performance and how building energy performance can be enhanced.

    Mission critical facilities support a wide variety of vital operations where facility failure will result in complications that range from serious disruptions to business operations, to circumstances that can jeopardize life safety of the general public. To minimize or eliminate the chance of facility system failure,mission critical facilities have three hallmarks that make them different from other type of commercial buildings

    Standby vs. emergency power in mission critical facilities
    System designers must interpret the requirements of NFPA 110, ensure their designs follow them, and educate their clients about how the standard affects their operations.

  28. Tomi Engdahl says:

    Static transfer switches are designed for critical infrastructure applications
    GE’s Critical Power’s Zenith STS-3 Series three-phase static transfer switches and Zenith STS-1 Series single-phase static transfer switches are designed for critical infrastructure applications.

    GE Critical Power (NYSE: GE) released the GE Zenith STS-3 Series three-phase static transfer switches and GE Zenith STS-1 Series single-phase static transfer switches for critical infrastructure applications.

    The GE Zenith STS-3 static transfer switches provide power source redundancy in UPS system architectures to increase product mean time between failure between the power source and the critical load, using features such as internal triple redundancy of control power and logic.

    The GE Zenith STS-1 Series static transfer switches provide a single-phase solution for customers who want to deploy the switches in a rack-mount form factor to provide dual input source capabilities to rack-mounted single-cord information technology (IT) servers or other non-IT critical process equipment.

    GE Zenith STS-3 Series three-phase units are ideal for data center, telecommunications, financial and industrial critical infrastructure customers seeking centralized power source redundancy

  29. Tomi Engdahl says:

    4 Things Your Business Should Know Before Renting a Temporary Generator

    Temporary generators can be ideal for all kinds of business applications, whether it’s powering equipment at a construction site or serving as a backup plan to help keep an industrial process on track.

    1. Temporary generators are more portable than ever before – generators are often surprisingly easy to move
    2. Generators without safety features can be a hazard – ground fault protection system and circuit breakers
    3. The right fuel makes a difference – diesel, propane or natural gas, gasoline
    4. Not all rental companies provide the same level of support.

  30. Tomi Engdahl says:

    Energy performance in mission critical facilities

    Mission critical facilities, such as data centers, are judged carefully on their energy use. Engineers should focus on the codes and standards that dictate energy performance and how building energy performance can be enhanced.

    Mission critical facilities support a wide variety of vital operations where facility failure will result in complications that range from serious disruptions to business operations, to circumstances that can jeopardize life safety of the general public. To minimize or eliminate the chance of facility system failure,mission critical facilities have three hallmarks that make them different from other type of commercial buildings:

    1. The facility must support operations that run continuously without shutdowns due to equipment failure or maintenance. Seasonal or population changes within the facility have a small impact on the energy use profile; generally, the facility is internally loaded with heavy electrical consumption.

    2. Redundant power and cooling systems are required to support the 24/7/365 operation. Depending on the level of redundancy, there will be additional efficiency losses in the power and cooling systems brought on by running the equipment at small percentages of the capacity.

    3. The technical equipment used in the facility, such as computers; medical and laboratory equipment;and monitoring , communications, and surveillance systems, will have high power requirements that translate into heat gain and energy use.

    Putting these hallmarks together, mission critical facilities need to run continuously, providing less efficient power and cooling to technical equipment that has very high electrical requirements, all without failure or impacts from standard maintenance procedures. This is why energy use (and ways to reduce it) in mission critical facilities has been, and will continue to be, of great concern.

  31. Tomi Engdahl says:

    Powering Converged Infrastructure

    Converged infrastructures utilize virtualization and automation to achieve high levels of availability in a cost-effective manner. In fact, converged infrastructures are so resilient that some IT managers believe they can be safely and reliably operated without the assistance of uninterruptible power systems (UPSs), power distribution units (PDUs) and other power protection technologies. In truth, however, such beliefs are dangerously mistaken.

    What is converged infrastructure?
    Simply put, converged infrastructures are pre-integrated hardware and software bundles designed to reduce the cost and complexity of deploying and maintaining virtualized solutions. Most converged infrastructure products include these four elements:
    1. Server hardware
    2. Storage hardware
    3. Networking hardware
    4. Software (including a hypervisor, operating system, automated management tools and sometimes email systems, collaboration tools or other applications)

    Why use converged infrastructure?
    According to analyst firm IDC, the worldwide market for converged infrastructure solutions will expand at a compound annual growth rate of 40 percent between 2012 and 2016, rising from $4.6 billion to $17.8 billion. Sales of non-converged server, storage and networking hardware, by contrast, will increase at a CAGR of just a little over two percent over the same period. Benefits like the following help explain why adoption of converged infrastructures is rising so sharply:
    Faster, simpler deployment. Converged infrastructures are pre-integrated and tested, so they take far less time to install and configure. According to a study from analyst firm IDC, in fact, Hewlett-Packard converged infrastructures typically enable businesses to cut application provisioning time by 75 percent.
    Lower costs. Converged infrastructure products usually sell for less than the combined cost of their individual components, enabling businesses to conserve capital when rolling out new solutions. Furthermore, the automated management software included with most converged infrastructure offerings decreases operating expenses by simplifying system administration. Indeed, the HP converged infrastructure users studied by IDC shifted over 50 percent of their IT resources from maintenance to innovation on average.
    Enhanced agility. Thanks to their ease of deployment, affordability and scalability, converged infrastructures enable companies to add new IT capabilities or augment existing ones more quickly and cost-effectively.

    Power protection equipment plays a key role in automatically triggering virtual machine migration processes during utility outages. Converged infrastructures execute automated failover routines only when informed that there’s a reason to do so. During utility failures, network connected UPSs can provide that information by notifying downstream devices that power is no longer available. At companies without UPSs, technicians must initiate the virtual machine transfer processes manually, which is far slower and less reliable.

    A converged infrastructure’s failover features can’t function without electrical power.

    Converged infrastructures are vulnerable to power spikes and other electrical disturbances.

    The fifth element of converged infrastructures: Intelligent power protection
    Power distribution units suitable for use with converged infrastructures do more than simply distribute power.

    Management software
    Most converged infrastructure solutions come with built-in system management software that helps make them highly resilient. Adding VM-centric power management software increases resilience even further by enabling technicians to do the following:
    Manage all of their converged IT and power protection assets through a single console.

  32. Tomi Engdahl says:

    Reduce diode losses in redundant systems with integrated power MUXes

    Many power management applications use Schottky diodes for the parallel operation of multiple power sources. This type of power redundancy is often found in systems with solid-state drives (SSDs), hard disk drives (HDDs), programmable logic controllers (PLCs), peripheral component interconnect express (PCIe) cards, network and graphic cards, and some others used in automotive, industrial, personal electronics and telecommunications infrastructure applications. The diodes do a great job of isolating redundant power sources to keep the system operational in the event that any one power source fails, while also preventing current flow from one supply to the other.

    The diode power-muxing configuration gives a seamless transition from one voltage rail to the other

  33. Tomi Engdahl says:

    Is a series rated panel right for you?

    It is not always necessary to have every branch circuit breaker rated for the full available fault current in an electrical panel or switchboard.

    Designers, owners, and contractors are motivated by budget constraints to save costs wherever possible. Circuit breakers found in electrical panels and switchboards become increasingly expensive as the rated current interrupting capacity (AIC) increases. In facilities where the available fault current is high, this will lead to increasingly expensive circuit breakers. All of the available options to reduce cost should be considered.

    Series-rating a panel or switchboard and using lower AIC-rated circuit breakers is one option that could reduce the electrical installation costs of a project.

    Because of the great risk to the facility and personnel in the event of an electrical fault, properly rating a panel or switchboard’s current interrupting capacity must be a priority. An underrated circuit breaker could fail to interrupt a fault event, and poses a serious fire and personal safety hazard to the facility. The resulting cost of an improperly rated circuit breaker in the event of a fault could be substantial.

    Finally, if all the required conditions cannot be met to series-rate a panel, there are other options to reduce the necessary rated current interrupting capacity of a panel. Although a panel may be fully rated, if the calculated available fault current at a panel or switchboard is reduced, the cost to install and maintain that equipment may be reduced initially and well into the future.

    Circuit breakers are rated by their interrupting rating or ampere interrupting capacity (AIC). This is often expressed in kilo-amperes as the KAIC rating or simply “Interrupting Rating ”

    In the event the fault current exceeds the circuit breaker’s capacity to clear the fault, the results could be disastrous. The circuit breaker could fail to open during a fault and its contacts may fuse together causing catastrophic failure. This scenario poses significant risk to the facility and its personnel.

    For this reason, accurate maximum fault current calculations are imperative. While erring on the side of conservatism is safe and prudent, beware of the tendency to be overly conservative in these calculations. If the maximum available fault current is calculated to be higher than reality, it may force the affected equipment to be needlessly overrated.

    What is a series rated circuit breaker?

    It is not always necessary to have every branch circuit breaker rated for the full available fault current in an electrical panel or switchboard. After substantial lab testing, many circuit breaker manufacturers have the ability to install circuit breakers rated for a lower fault current, but maintain the panel or switchboard’s overall AIC rating. A panel or switchboard with this setup would then rely on the main circuit breaker to clear the fault. A panel with this arrangement is called a series-rated panel.

    Currently, there is no way to accurately calculate the outcome when two circuit breakers are paired together in a series rating. The only way to determine if the two circuit breakers will operate properly together is through extensive lab testing. Two specific circuit breakers are then paired together and must be from the same manufacturer. The pair can then be listed by UL as being safe for use in this application.

    A fully rated panel would not use this method of pairing tested circuit breakers together. Instead, all circuit breakers in the panel, including the branch circuit breakers, would be rated for the maximum available fault current

    When considering a series-rated panel, the first step is to verify that the installation will adhere to the requirements of the NEC.

    Effects on future modifications to the panel
    One drawback to consider if series-rating is performed by the manufacturer, is that the panel is now limited to that manufacturer when selecting replacement or additional circuit breakers in the future. Currently, no manufacturer goes through the process of having their circuit breakers UL-listed for use with a competitor’s circuit breakers, and this will probably not happen anytime in the near future.

    It is possible that over the lifecycle of the equipment, a manufacturer may no longer produce replacement circuit breakers, making maintenance and future expansion difficult or impossible.

    Consider reducing the calculated available fault current
    An alternative to series rating a panel is to reduce the facility’s calculated available fault current. This would allow less expensive circuit breakers for the facility’s electrical panels and switchboards. If the calculated available fault current of the facility is substantial, it could also significantly reduce the initial construction cost as well as the cost of maintenance well into the future.
    Reducing the calculated available fault current has the added benefit of reducing the potential arc flash hazard.

    A comprehensive list of options for reducing the calculated available fault current is beyond the scope of this article, however below we discuss some common and cost effective options. One of the most common options when reducing the calculated available fault current is to employ current limiting fuses. Current limiting fuses are designed to interrupt a fault condition before the fault can reach its peak current. They have been tested extensively by the manufacturer and are UL labeled for this application. When used in the design of a facility, this could ensure that the calculated available fault current downstream from the fuses will never reach the maximum calculated level.

    Another option is the careful selection of an upstream transformer. If the upstream transformer serving a facility, panel or switchboard is being selected as a part of the project, it might be more cost effective to deliberately use a transformer with an increased internal impedance.

    A third option, and often times the most cost effective option, is to increase the linear length of the feeder serving the panel or switchboard. If the fault current calculations approach one of the standard AIC ratings, even a small amount of additional feeder length could reduce the available fault current to a level below the next interval.

    The final option for reducing the calculated available fault current is to use more accurate fault current calculations. It is common for the engineer on record to perform calculations that are not only excessively conservative, but are calculated before the conduit and feeders can be routed. As a result, the calculated available fault current often times represents a “worst case scenario” instead of actual conditions of the installation. If possible, the calculations should be done with the aid of accurate system modeling.

  34. Tomi Engdahl says:

    Broadcast Transmitter

    When the transmitter was put in operation, it was said to be powered from a 2-pole power generator that was spinning at 3600 RPM. Unfortunately, when transmitter operation was attempted, the generator’s output line voltage became unstable. Other equipment in the building that was taking power from this generator were affected too.

    The transmitter was of very modern design. It was proudly touted as being power efficient which suggested to me that it must be using switchmode power supplies

    It looked like the source impedance of the power generator was too high for the purpose at hand and that it must have exceeded the absolute value of the negative dynamic input impedance presented by the transmitter. As a result, the system would have a net negative dynamic impedance value and because of that, it would be unstable, just as we were seeing.

    The thing to bear in mind is to make certain be in a situation like this that the power generator is more than merely capable of delivering the needed number of kilowatts. Its source impedance as reflected in its load regulation properties must be scaled appropriately to the dynamic input impedance of the load it will be feeding.

    If the system’s net impedance looks dynamically negative, there will be the instability problem that was seen here.

    Switchmode Dynamic Input Impedance – John Dunn, Consultant, Ambertec, P.E., P.C.

  35. Tomi Engdahl says:

    Ensuring power quality in mission critical facilities

    Many industrial, commercial, and service businesses are sensitive to power quality problems because they affect a company’s ability to compete in a global economy.

    The generally accepted definition of clean power is “current and voltage waveforms that are purely sinusoidal.” However, this clean, or high-quality power does not have to be absolutely sinusoidal. So, what is the definition of high-quality power? Does the mere presence of harmonics on a power system indicate poor power quality? What about intermittent transients? Is that poor power quality?

    Technically, there is no single accepted definition of “quality power.” Standards exist that help define criteria that can be measured, such as voltage. However, the real measure of power quality is determined by the performance and productivity of end-user equipment. If the equipment is not performing correctly, verification of proper mechanical and electrical installation and maintenance is necessary. A faulty piece of equipment, bad bearings, or poor internal connections can affect performance. If this doesn’t resolve the problem then power quality is most likely inadequate.

    Because there is a close relationship between voltage and current, we must address the current to understand many of the power problems that exist. For example:

    A short circuit can cause a voltage sag—or cause voltage to even disappear completely—due to extremely high current passing through the system impedance.
    Lightning generates high impulse voltages that can travel on the power distribution system.
    Distorted currents from harmonic loads also cause the voltage to distort as the current passes through the system impedance.

    Since the advent of electricity, reliable, high-quality power has been desirable. In the late 1980s, computers became commonplace in our offices and homes. In the 1990s, we were able to network this equipment together to increase equipment performance. Today, we face new problems, such as faster processing speeds, increased computer chip densities, and equipment that is more sensitive to the quality of power it receives. Factories, offices, hotels, shopping centers, hospitals, and homes depend heavily on microprocessor-based loads, such as lighting controls, computers, copiers, appliances, scanners, control systems, monitoring devices, etc. It’s difficult to find equipment that lacks a microprocessor. While this electronic equipment is relatively small in size and power consumption, it is large in quantity and is in close proximity to one another.

    We are interested in power quality because of its economic impact. An increasing majority of industrial, commercial, and service businesses are sensitive to power quality problems because they affect a company’s ability to compete in a global economy.

    The costs related to a power quality disturbance can be categorized as direct costs, indirect costs, and inconveniences.

    Direct costs: include reduced equipment efficiency, loss of raw material and production, equipment/product damage, corrupt data communications/storage, and nonproductive employee wages.

    Indirect costs: more difficult to quantify and may include missed delivery deadlines, which may cause future orders to be lost.

    Inconvenience: Items in this category are not expressed in lost revenue dollars but rather in how much someone is willing to pay to avoid having to deal with the inconvenience.

    Ultimately, the end user is responsible for preparing appropriate performance criteria for the equipment as well as for the proper installation and correction of inadequacies in the power and grounding system. Unfortunately, many end users are unaware of the installation pitfalls and need assistance.

    Many times, the local utility company can provide guidance on how to properly install sensitive electronic equipment as well as modifications to the power and grounding system. The utility is motivated to provide customer service in regard to power quality to help build and maintain confidence in its distribution system. Utility engineers can provide troubleshooting analysis of harmonic issues

    Grounding, bonding, and wiring

    Around 80% of all power quality problems are related to grounding, bonding, and wiring problems within a facility. Is this percentage exaggerated? Possibly, but many power problems are resolved simply by fixing a few grounding connections or replacing a couple of grounding cables.

    Grounding and bonding are not the same. However, they are closely related.

    Electrical systems do not need to be grounded to function. In fact, not all electrical systems are grounded. But when discussing electrical systems, usually the voltages are with respect to ground.

    Another common grounding problem is using an isolated ground with the idea of obtaining a clean ground. Isolated grounds are typically misunderstood and misapplied because they are not actually isolated, but rather insulated, thereby eliminating parallel return paths.

    Sometimes isolated or dedicated grounds are recommended by equipment manufacturers. These recommendations can compromise the safety and performance of the equipment, are dangerous, violate the NEC, and are unlikely to solve power quality problems.

  36. Tomi Engdahl says:

    Complying with NFPA 110 in mission critical facilities

    Design engineers must consider the implications of combining emergency, legally required, and optional standby systems to ensure code compliance, maintainability, and economics.

    Design engineers have many factors to consider when designing a backup system for a facility. Safety, maintainability, code compliance, and economics play crucial roles in determining the topology of a backup system for a critical facility. In large facilities where electrical system downtime results in significant economic loss, a backup power system usually is employed. Owners frequently desire to use their backup systems to support their emergency and legally required standby loads. Due to the requirements of NFPA 110-2013: Standard for Emergency and Standby Power Systems, and NFPA 70-2014: National Electrical Code (NEC), the design engineer must carefully consider the implications of combining emergency, legally required, and optional standby systems to ensure code compliance with maintainability and economics in mind.

    The first major challenge to meeting the requirements of NFPA 110 is properly defining system levels. This requires careful evaluation of the loads you are serving and coordination with your authority having jurisdiction (AHJ). According to Annex A.4.4.1, “Level 1 systems are intended to automatically supply illumination or power, or both, to critical areas and equipment … Essential electrical systems can provide power for the following essential functions: life safety illumination, fire detection and alarm systems, elevators, fire pumps, public safety communications systems, industrial processes where current interruption would produce serious life safety or health hazards, and essential ventilating and smoke removal systems.”

    The next significant challenge to meeting NFPA 110 is fuel storage requirements.

    Another challenge to NFPA 110 compliance is serving the relatively small code-required loads in a mission critical facility such as a data center. A data center is certainly a major example of mission critical facilities that have spawned publications and organizations to support them, but there are other types of mission critical facilities. Other examples of mission critical systems are those that support research where the failure can result in millions of dollars of loss, or response centers where power failure could hinder the response of a company to a crisis. Based on NFPA definitions, mission critical loads are generally classified as optional standby loads. Despite the fact that these types of loads are not life safety loads, in the owner’s perception, they are no less critical to maintain. As such, the electrical distribution that supports them can be as robust, and many times are more robust than the Level 1 EPSS that supports life safety loads.

    Finally, it can be challenging to economically scale NFPA 110 on a large system for a large system load.

    The examples given in Annex B of NFPA 110 are well-suited for applications lower than 600 Vac (see Figure 1). Large power systems are typically designed at system voltages of 12 kV and higher.

  37. Tomi Engdahl says:

    Selective Coordination

    Smart Mission Critical Solutions and Expert Guidance for Meeting Codes and Standards

    Certain electrical power system designs need to ensure the selection and setting of protective devices cause the smallest possible portion of the system to be de-energized due to an abnormal condition. In a selectively coordinated system, only the circuit breaker that directly supplies the overloaded/faulted part of the system will trip, limiting the service interruption to only the circuit experiencing the problem and not shutting down the larger portion of the power system.
    Achieving Selective Coordination

    Designing a coordinated electrical distribution system requires knowledge about the application and circuit breakers being specified. Considering selective coordination early in the design phase makes the process easier and more cost-effective. Conducting a comprehensive short-circuit current study and an overcurrent coordination study of the electrical power system is the only true method of achieving selective coordination and equipment protection.

  38. Tomi Engdahl says:

    Selective Coordination Using Circuit Breakers

    How to design an electrical system in which the upstream protective device (fuse or circuit breaker) nearest to the system fault clears the fault without affecting the protective devices that are upstream from it.

    Selective coordination generally describes the design of an electrical system in which the upstream protective device (fuse or circuit breaker) nearest to the system fault clears the fault without affecting the protective devices that are upstream from it.

    Reviewing a typical protective device trip curve (click here to see Fig. 2), it can be divided into three regions. First is the “instantaneous region,” which is the region intended to interrupt high-level faults. On a UL breaker (typical molded-case unit), this area operates with no intentional delay. For ANSI-rated breakers, this region may operate with a delay, or may be defeated entirely, within the withstand capability of the breaker.

    Next is the “short-time region,” which operates in a general range of between three and 30 cycles. The short-time region will cause the breaker to trip at a lower current than the instantaneous region, but acts after a short delay (by definition). This is a feature of many electronic-trip circuit breakers, and is not intentionally provided on standard thermal-magnetic, molded-case units. The final region is the “long-time region,” operating in the range of 30 cycles to beyond 8 min. (Fig. 2). This region causes a trip at the trip setting of the circuit breaker (i.e., a 1,600A circuit breaker will trip when the nominal current exceeds 1,600A for significantly more than 8 sec — approximately 60 sec for the breaker shown).

    Molded-case breakers

    By their nature, breakers are more difficult to selectively coordinate than fuses. Their time current curves are not as smooth, and the tolerances of thermal-magnetic molded-case breakers (as indicated by their trip curves) are not as narrow as those of fuses.

    Consider a typical electrical system, again referencing Fig. 1. A 75kVA step-down transformer (F) is serving a 208V emergency receptacle panel (H). Based upon a typical impedance of 2.6%, the available fault current at the secondary terminals of the transformer is more than 7,900A. The typical 250A molded-case secondary circuit breaker (G) will have an instantaneous trip of between five and 10 times the trip rating (depending on the manufacturer, model, and frame), or 1,250A to 2,500A.

    However, selective coordination of circuit breakers is a possibility. In fact, in many cases, breakers that do not appear to selectively coordinate (based solely upon their individual curves) do selectively coordinate in specific pairings. This is possible using the same principle that allows series rating of breakers — dynamic impedance.

    Circuit breakers, by their nature, do have current-limiting properties, which are inherently tied to the action of the breaker. When interrupting a fault, the breaker will “blow open,” separating its contacts and opening the circuit. This action causes a variable (or dynamic) impedance, which changes the short circuit current of the system, changing the action of the other circuit breakers.

    As the requirements for selective coordination have grown, breaker manufacturers are making information more readily available. This information is being provided in “coordination tables,” which are somewhat similar to (although more cumbersome than) the fuse manufacturer coordination ratios.

    Unfortunately, these tables do not “reflect” through a transformer. Therefore, although you can see that the 200A mission-critical main circuit breaker coordinates with the downstream QO branch breaker, the QO breaker must also coordinate with the breaker protection on the primary side of the transformer.

    Electronic-trip and ANSI breakers

    Where coordination of molded-case circuit breakers proves to be unwieldy or impossible, there are other options available for selective coordination. These include the use of electronic trip breakers, which can be “trimmed” to provide greater coordination. In certain instances, ANSI-rated breakers will allow the elimination of the instantaneous region of the electronic breakers, which requires the use of 30-cycle-rated (ANSI-rated) switchgear.

    These methods are more versatile than coordination of the fixed trip curves available for molded-case circuit breakers. Nevertheless, this flexibility comes at a price. Electronic trip circuit breakers are more costly than the molded-case variety, and must be properly set and field maintained to assure the curves in the field match the intent of the drawings.

    Selective coordination of a system

    In addition to the methods described above, there are a few other ways to enhance the ability to selectively coordinate the system. The most important is to minimize the number of “levels” in the path of coordination. Naturally, a coordination path with three protective devices will be easier to design, maintain, and coordinate than a path with six such devices.

    One of the easiest ways to reduce these levels is to eliminate main circuit breakers in panel risers.
    This increases the ability to coordinate the system, but has the drawback of requiring the entire riser to be shut down for service, as the individual panels do not have main protective devices

    Ground fault protection

    One last concern in coordination of systems is ground fault protection. The NEC requires ground fault protection for systems between 150V and 600V to ground, for all electrical services rated 1,000A and above (per 230.95).

    Normally, the phase curves are coordinated, and the ground fault curves are coordinated separately. However, in cases where a protective device does not have a ground fault trip sensor, a ground fault on its load side will be treated the same as a phase fault. Therefore, it’s imperative the phase trip curve be coordinated with the upstream ground fault protection.

    Often, the ground-fault short circuit current available on a 20A feeder circuit is sufficient to trip the ground-fault protection of a building main circuit breaker, which makes it imperative that the ground-fault sensors be properly set up to assure selective coordination.

    In summary

    Selective coordination is an important part of designing emergency electrical systems. Full compliance with the selective coordination requirements of the NEC is mandatory in most jurisdictions.

    Finally, it’s imperative that ground fault coordination be considered in all cases where selective coordination is a requirement, especially in cases where an upstream breaker has ground fault protection and the downstream breaker does not. Without this key piece, a seemingly minor downstream event could cause the entire electrical system to fail.

  39. Tomi Engdahl says:

    Article that touches potential dangers of dual power paths when not correctly handled:

    $600k Fine Over Data Center Death

    UK contractors Balfour Beatty and Norland have been fined £380,000 ($580k) after an electrician was electrocuted while working on a data center owned by finance firm Morgan Stanley. The fine follows mounting concern that safety is being compromised because of the need for data centers to remain online non-stop.

    Norland, Balfour Beatty fined after data center engineer dies at work

    Norland Managed Services and the Balfour Beatty group have been fined more than £380,000 ($583,000) by a British court after a contractor employed by Norland died while installing cables at a Morgan Stanley data center.

    Martin Walton, a 27-year old cable joiner, accidentally touched a 415V live terminal and passed away on the spot in 2010.

    The court ruled that the equipment was kept switched on due to management mistakes and employees were forced to work on live systems – a situation which was entirely preventable.

    Balfour Beatty blamed client pressure to complete the project as the cause.

    Part of the work involved installing new PDUs to enable two independent power supplies for the servers. The existing power supply was under the control of Norland while Balfour Beatty had control of the connections to a new substation.

    As a result of last minute changes, PDUs were tested on a live network to make sure they worked properly. The first test was successful, but the second resulted in an engineer’s death.

    “Not one person involved in the work at the time of the accident had an accurate overall understanding of the work being carried out and, as a consequence, Martin Walton and others were unknowingly working in the vicinity of exposed live electrical terminals.”

  40. Tomi Engdahl says:

    Emergency and standby power in hospitals

    Consulting engineers who specify emergency power equipment understand that installations for hospitals are required to comply with NFPA 110 in conjunction with NFPA 70. System designers must interpret the requirements of these standards, ensure their designs follow them, and educate their clients about how the standard affects their operations.

    Hospitals have been evolving for the past few decades in both size and complexity, and in many cases have grown from a single structure to multiple buildings in a campus setting. A primary challenge for many health care facilities is to provide a high-quality source of electrical power that is backed up with highly reliable emergency and standby power systems to ensure uninterrupted flow of electricity to the entire facility, particularly during crisis and natural disasters.

    The terminology used for backup (emergency and standby) power systems in health care facilities is different than other facilities. It is designated as the “essential electrical system”

    Most buildings experience power interruptions that are caused by utility outages, equipment failures, testing, and maintenance, and generally are easier to manage due to the cause, timing, and duration. In many cases, these power outages are planned and easily handled. However, power outages due to natural disasters and unexpected events are much more difficult to deal with, and in some cases the entire facility has to rely solely on the emergency and standby power system to continue operating for several days.

    Unlike most standard commercial buildings, delivering emergency and standby power to health care facilities is a major undertaking due to its complexity and size. It involves many different systems consisting of alternate sources of power, switching equipment, controls, and distribution equipment.

    Generators, paralleling, and distribution equipment

    For larger hospitals or campus environments, it is highly recommended to use multiple generators controlled by paralleling switchgear with intelligence (supervisory control and data acquisition and programmable logic controller). This will not only improve the reliability of the alternate source of power, but also provide flexibility to add or shed loads as required, among many other benefits.

    A single generator may not be able to sufficiently handle the total capacity of life safety and critical loads in larger facilities. Consideration should be given to design a multiple-bus arrangement to isolate the load breakers from generators; or simply use a split-bus paralleling switchgear arrangement to allow simultaneous start of two generators that could be up and running within the 10-sec time constraint while connected to the separate buses. Once the generators are synchronized, they can then be connected via a tie breaker

    Separate transfer switches are required for each of the branches required by the EES Type 1 or 2 systems. In larger hospitals, there may be multiples of each, depending on the distribution methods used. In small facilities where maximum demand on EES is 150 kVA or less, a single transfer switch can be used to serve one or more branches together.

    Open (or delayed)-transition ATSs are generally used to transfer loads from one source to another and back for applications where short interruptions can be tolerated.

    Nonemergency and optional transfer switches may also be manual as allowed by the codes.

    In recent years, many facilities have elected to include SCADA as part of the emergency power control system to provide interactive monitoring and testing—not only to comply with all the regulations, but, more importantly, to improve the overall reliability of the system and save in operational costs.

    As stated earlier, the hospital electrical system is very complex and must comply with several codes and standards that have undergone extensive changes in recent years.

  41. Tomi Engdahl says:

    Data center power can be software defined too

    More and more data is being collected, stored and transacted today thanks to the internet, social networking, smartphones and credit cards. All this activity takes place in real time, so application availability is more important than ever and reliability requirements are increasingly stringent. Much application downtime today is caused by power problems, either in a data center’s power delivery network or the utility distribution grid. This is likely to become even more so as reliability of the electrical grid continues to deteriorate.

    Part of the reason power is such a frequent cause of application downtime is the effort to abstract IT hardware from applications through virtualization and “software defined data center” technologies. While abstracting servers, storage and networking, the concept of software defined infrastructure has ignored power.

    It is a purely IT-centric view of the data center. Standing separately are facilities staff who operate building management systems and other infrastructure components. If you want an integrated management environment for this infrastructure you get what is called data center infrastruture management (DCIM) software.

    Software defined data center technologies and DCIM software are valuable tools for their respective purposes, neither addresses power-related downtime. This problem is generally addressed by setting up multiple geographically dispersed, often fully redundant data centers, configured for either hot or cold backup and failover. But automated failover and recovery is still very often plagued by problems.

    Application failover to another site requires manual intervention nearly 80% of the time. A study by Symantec found that even before you get to manual intervention, 25% of disaster recovery failover tests fail completely even before getting to the manual part.

    Today, software defined data center and DCIM solutions do not address the relationship between applications and power. Power should be the next resource to become software defined. While you can use software to allocate IT resources, it is not possible with power. You cannot dynamically adjust the amount of power going to a rack or an outlet, but you can, however, dynamically change the amount of power consumed by IT gear plugged into an outlet by shifting the workload. Software defined power involves adjusting server capacity to accommodate workloads and indirectly manage the power consumed.

    The approach could combine power capacity management with disaster recovery procedures and other functions, such as participation in utility demand response programs.

    Because load shifting does not occur until availability of the destination has been verified, the process is risk free, and when disaster does strike, the chances of smooth transition are dramatically improved.

    implementation of software defined power brings together application monitoring, IT management, DCIM, power monitoring, enterprise-scale automation, analytics and energy market intelligence

  42. Tomi Engdahl says:

    World’s first locking rewireable C13 IEC Connector

    MEGA Electronics Inc introduces the new IEC C13 Lock Rewireable – the world’s first locking rewireable C13 IEC connector.

    Designed to guard against accidental disconnection of computers, PDU’s servers and most network devices, the new IEC Lock adds additional flexibility and ease of use, as well as LSZH (low smoke zero halogen) compatibility, to its list of features.

    The beautifully straightforward high-visibility locking mechanism is combined with a simple release mechanism which allows for disconnection from all sides. Ideal as a retrofit solution, it requires no other equipment or special inlets to secure it, simply plug it into an appliance as you would a standard IEC lead. The red cage release mechanism also marks the connector out from standard IEC leads to allow maintenance and other data centre staff to identify critical power sources.

  43. Tomi Engdahl says:

    IL13 Series IEC Lock Power Cords
    Schaffner announces the addition of the IEC Lock power cords

    Schaffner’s IEC Lock power cords lock into any IEC C14 connector or power entry module. This simple and attractive solution does not require the exchange or modification of the power inlet, making it an easy retrofit for all electronic equipment and devices where safe/reliable power connections are a must.

    The Schaffner power cords with IEC Lock reduce installation time and material by eliminating the need for bale or clamp accessories, and are easily installed and released from standard IEC 60320 style C14 inlets with the press of a button.

    Data centers
    Industrial equipment
    Medical devices
    In-vitro diagnostic devices
    Broadcasting stations
    Mobile applications

  44. Tomi Engdahl says:

    Unique Lockable Female C13 & C19 IEC Connectors and Outlets

    he IEC Lock range can be used with any standard IEC inlets & Guards against accidental disconnection of Computers, PDU’s, Servers & most Network Devices

    IEC-lock’s patented locking mechanism is beautifully simple…

    • Unique ‘Patented’ Female connectors, suitable for use with any standard IEC inlet

    • Protects computer equipment from
    accidental disconnection

    • Ideal for protecting appliances that are vulnerable to vibration

    • Suitable for various Data Communication appliances that require a secure
    power source

  45. Tomi Engdahl says:

    Understanding transfer switch operation

    Consulting engineers should understand transfer switch construction, performance requirements, selection criteria, and desired operation to ensure that critical systems and equipment are supplied with reliable backup power when needed.

    When utility power is interrupted, power system failure is not an option for many facilities. Standby power systems have many components, including transfer switches that must be designed correctly. During power transitions, transfer switch timing and sequence is critical to ensure proper system operation. Consulting engineers must understand transfer switch types, timing requirements, ratings, and the types of standby systems where transfer switches are used to transfer to backup power. The basis of this article is NFPA 70-2014: National Electrical Code (NEC) unless otherwise noted.

    Transfer switches are responsible for transitioning electrical power from the primary source to a secondary source in the event of primary source interruption, maintenance, or failure. The primary source most commonly consists of the utility service. The secondary source typically consists of the backup or emergency power source. The sequence of operation typically occurs as follows:

    1. The primary source is interrupted or fails.
    2. When the secondary source is stable and within voltage and frequency tolerances, the transfer switch transitions to the secondary power source. This transition can occur automatically or manually.
    3. When the primary source is restored and stabilized, the transfer switch transitions back to the primary source and resumes under normal operation. This transition back to the primary source can occur automatically or manually.

    Standby system types include emergency systems, legally required standby systems, optional standby systems, critical operations power systems (COPS), and systems that support health care facilities

    Emergency systems (NEC Article 700):
    Transfer equipment, including transfer switches, are required to be automatic, identified for emergency use, and approved by the authority having jurisdiction (AHJ). Transfer equipment shall be designed and installed to prevent inadvertent, simultaneous connection of primary and secondary supplies of power. Transfer equipment shall supply only emergency system loads. Power must be transferred to the secondary source in 10 sec or less.

    Legally required standby systems (NEC Article 701):
    Transfer equipment, including transfer switches, are required to be automatic, identified for standby use, and approved by the AHJ. Transfer equipment shall be designed and installed to prevent inadvertent, simultaneous connection of primary and secondary supplies of power. Power must be transferred to the secondary source in 60 sec or less.

    Optional standby systems (NEC Article 702): Optional standby systems are defined by the NFPA as “intended to supply power to public or private facilities or property where life safety does not depend on the performance of the system.” These systems may include data processing and communication systems, and mission critical systems that are not legally required by the AHJ.
    Transfer equipment, including transfer switches, for optional standby systems are not restricted to the same requirements as emergency and legally required system transfer equipment. However, transfer equipment shall be designed and installed to prevent inadvertent, simultaneous connection of primary and secondary supplies of power. There are no code requirements for power to be transferred to the secondary source within a certain time frame.

    Critical operations power systems (COPS) (NEC Article 708): Interruptions or outages to designated critical operations areas may negatively impact national security, economy, public health, or safety. The requirement to comply with NEC Article 708 is provided by any governmental agency having jurisdiction or by a facility providing documentation establishing the necessity for such a system.

    Health care facilities (NEC Article 517): Essential electrical systems for hospitals consist of the emergency system and equipment system to supply a limited amount of lighting and power essential for life safety and effective hospital operation when the normal service is disconnected. The number of transfer switches used “shall be based on reliability, design, and load considerations”

    Transfer switch types

    Transfer switch types include open-, closed-, fast closed-, soft closed-transition, and bypass/isolation.

    Open-transition transfer switches: Open-transition transfer is commonly described as “break-before-make.” This means that the transfer switch disconnects from the primary source before establishing the connection to the secondary source

    Closed-transition transfer switches: Closed-transition transfer is commonly described as “make-before-break.” This means that the transfer switch creates a connection to the secondary source while connected to the primary source (see Figure 3). When the connection to the secondary source is established, the primary source will disconnect. This enables a continuous source of supply to the electrical system as the two sources are paralleled together.
    Closed-transition switches transfer when both sources are synchronized in phase, voltage, and frequency.

    Fast closed-transition transfer switches: Fast closed-transition transfer switches use a momentary paralleling of sources (typically less than 100 msec) using a control system similar to the open-transition transfer switch system.
    Although the intent is to not parallel the sources for an extended amount of time, utility service providers commonly require reverse-power relay protection to protect their systems from sustained paralleled operation.

    Soft closed-transition transfer switches: Soft closed-transition transfer switches use an automatic synchronizer to enable the generator to synchronize with the utility service and transfer the loads. The transfer time may vary from seconds to several minutes, usually depending on requirements of the utility provider. During this process, there is a sustained duration of paralleled operation between the two sources.

    Bypass/isolation transfer switches: As the name implies, bypass or isolation capabilities may be provided to the transfer systems listed above to bypass the primary transfer switch components without interrupting power to the facility.

    Paralleling switchgear

    Paralleling switchgear is typically used to combine multiple power sources (commonly two or more generators) and connect to a common bus to use the aggregate capacity of the sources (see Figure 4). The power sources must be synchronized where the frequency, voltage, phase angle, and phase rotation are within prescribed limits and the sources can be paralleled together.

    Transfer switch operation

    Transfer switch operation occurs based on the initiation and transfer processes. The initiation process is what identifies that the transfer needs to occur. This event may consist of a loss of, or inconsistent voltage from, the primary source. The transfer is the process of shifting load from the secondary or alternate source, and vice versa.

    Automatic: In automatic mode, the transfer switch controller manages the entire process, and initiation begins when the controller senses a loss of the primary source.

    Nonautomatic: In nonautomatic mode, the transfer switch is manually initiated by an operator

    Manual: In manual mode, the entire process is completed manually by an operator.

    Transfer switch construction, performance requirements

    Codes and standards, such as UL 1008-8: Transfer Switch Equipment, UL 1008A-1: Standard for Medium-Voltage Transfer Switches, and NFPA 110-2016: Standard for Emergency and Standby Power Systems provide transfer switch construction and performance requirements.

    Transfer switch construction, performance requirements

    Codes and standards, such as UL 1008-8: Transfer Switch Equipment, UL 1008A-1: Standard for Medium-Voltage Transfer Switches, and NFPA 110-2016: Standard for Emergency and Standby Power Systems provide transfer switch construction and performance requirements.

  46. Tomi Engdahl says:

    Coordinating protective devices in mission critical facilities

    A coordination study ensures that the most reliable electrical system has been installed. Applicable codes and standards help engineers get it right.

    A sudden power failure will have a dramatic effect on business, especially in a critical environment. Isolating a fault condition to the smallest area possible is essential in providing the most reliable electrical system with maximum uptime for your facility. Expensive electronic distribution protection equipment is not worth the extra cost unless a proper protective-device coordination study is provided by an experienced engineer.

    NFPA 70-2014: National Electrical Code (NEC) defines selective coordination as: “Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the selection and installation of overcurrent protective devices and their ratings or settings for the full range of available overcurrents, from overload to the maximum available fault current, and for the full range of overcurrent protective device opening times associated with those overcurrents.”

    In other words, a properly coordinated system will limit disconnection to the nearest upstream protective device.

    The main types of overcurrent protection used in mission critical environments are circuit breakers, fuses, and relays.

    Depending on the circuit breaker type, there may be several parameters that can be selected for each protective device. A thermal magnetic breaker may have no adjustment at all, or only minimal adjustment to the instantaneous region, whereas a fully adjustable electronic trip breaker may have many.

    Adjustment of these parameters allows for what is referred to as “curve shaping.” Curve shaping enables better coordination between upstream and downstream overcurrent-protection devices.

    When performing electrical engineering studies for mission critical environments, the required documentation includes:

    Description, rating, make, and catalog numbers of protective devices
    Full-load current at the protective device (3-phase and line-to-ground)
    Transformer kVA, impedance, and inrush current data and connection type (delta-wye, etc.)
    Available fault current at the protective device
    Cable and conductor sizes
    Protective-device design requirements from the serving utility
    Voltage at each bus.

    After the aforementioned critical information is typed into the software database, the function of protective devices can be graphically presented. The resulting graphic representation is called a time-current curve (TCC). When more than one electrical device is overlaid on a single graph, the relationship of the characteristics among the devices is presented. Any potential issue, such as overlapping of curves or long time intervals between devices, are illustrated. Fault-current conditions can be illustrated by indicating on the current scale the maximum and minimum value of short-circuit currents (3-phase and line-to-ground) that can occur at various points in the circuit

    It is common today to perform complicated electrical protection coordination studies with computer software.

  47. Tomi Engdahl says:

    A guideline to success in the challenging MIL/Aero Power arena

    In this article we will discuss the military/aerospace grade power supply and what the key differences are as compared to commercial off-the-shelf or COTS designs. We will also discuss some design techniques used to help ensure high reliability which is so critical in these types of designs.


    Military supplies are typically used in aircraft, sea-going vessels and transportation equipment. These obviously need to have a more robust construction for the extreme environments in which they will operate than a typical commercial power supply. Power supplies need a far better mean time between failure (MTBF) rating in military usage because reliable operation without failure can save lives.

    The defense market, in many cases, may build systems using COTS power supplies, but there may be risks involved. Things like obsolescence, process changes, environmental vulnerability, faulty electrical performance or EMI issues need to be addressed to ensure reliable performance in combat or critical situations. There is a balance that must be struck in the design between the high cost of a full military class power supply and the manpower spent on designing in modifications in a COTS supply


    Obsolescence is out of the question on programs that are mission critical and need to be in field operation for 20-30 years. Ultimately form, fit, and function replacement for field supplies need to ensure no or acceptable minimal documentation and specifications differences from the original power supply.


    Redundancy in design ensures that no single failure will disable the supply or hinder its proper operation. Over and above redundancy the system must not be compromised via any effect on other equipment with fire, smoke, noise or any other hindrance to other equipment.

    Design Margins

    The use of commercial components may be considered as long as a comfortable and generous design margin is chosen between the component worst case operating point and its rated data sheet specifications. It is a fact that as temperature goes up, reliability will begin to degrade.

    FET ORing2

    Redundancy can be achieved in Fault-tolerant Power supplies by using the diode ORing technique of the outputs on multiple power supplies. Since diodes can be inefficient due to their large forward voltage drop, especially in low voltage designs, more efficient FETs are typically used. Since a FET is able to conduct in both directions, unlike a uni-directional diode, the added controller must be used to turn off the reverse path.

    More Electric Aircraft Power Systems (MEAPS)3

    In the last ten years the aircraft industry, military and commercial, has been making a concerted effort to move away from traditional power presently used to a more electric approach in non-propulsive power systems as a first order of business. This secondary power has been in the form of hydraulic, pneumatic, electrical and mechanical power extracted from the engines of the aircraft.

    Energy efficiency is a big driver of this effort

    The industry is looking into 270VDC systems as a high voltage DC power distribution main. This will mean lower currents at higher voltages, but lower currents give way to less copper and thus lower weight on the aircraft. This translates into better fuel economy.

    A 3 Phase, 4-Leg Inversion Power Supply for MEA

    Powering many legacy 115VAC/400Hz loads on an aircraft is facilitated by an inversion power supply. The most recent effort in this area has been with Space Vector Pulse Width Modulation (SVPWM) controlled 3-Phase, 3-Leg inverter architectures. Unfortunately, this architecture cannot operate in an unbalanced load condition, especially in the event of a short-circuit. This will not be acceptable in military or commercial power in an aircraft.

    Designers have tried Sinusoidal pulse width modulation (SPWM) controlled 3-Leg, 4-Phase inverter architectures, but its DC voltage utility is too low to meet the higher voltage bus levels of 270VDC

    Next-gen MIL vehicle power6

    Next generation MIL vehicles will use higher voltage energy storage plants such as Nickel-Metal Hydride or Lithium-Ion Batteries, or Super Capacitors. These will typically operate at around 300VDC to optimize hybrid motor operation. These higher voltage power units have the ability to provide significant amounts of power with drastically reduced audible and thermal signatures, the availability of this higher voltage power presents some significant advantages to the export power conversion system. Distribution currents are reduced by 90% (for example from 500A to 50A) from the traditional 28V case

    Most times export power converters will be located outside the crew compartment and will be exposed to a very harsh and wet environment. Operating temperatures typically will range from –46 to +54o C. Mechanical vibration and shock requirements are characterized in MIL-STD-810F, Methods 514.5 and 516.5.

    Power conversion densities in a vehicle can range from 3.4W/in3 to as high as 10.3W/in3 . Most cooling requirements will necessitate either forced air cooling or liquid cooling

    An indirect system which has no direct air blowing onto internal components is known as a “wind tunnel” design. This system uses fans to cool heat sinks and allows the electronic components to be protected from the environment by locating them on the other side of the aluminum heat exchanger. A drawback with this technique is a larger package

    In the case where a fully environmentally sealed package is needed, a liquid cooling architecture is often used. In this type of a system the heat exchanger may be located in the front of the vehicle. The normal cooling liquid employed is either water or a mixture of water and ethylene glycol (a.k.a., antifreeze).

    The cold plate approach has its drawbacks though. This architecture necessitates a flat shapes for heat transfer. This will spread the layout over a larger area, which can make electromagnetic noise difficult to deal with.


    The Military/Aerospace sector is not all that different than any other design using COTS products. There are more stringent temperature, shock, humidity and vibration expectations due to the harsh environment these designs might encounter as well as the increased reliability that is expected and needed.

  48. Tomi Engdahl says:

    Paralleling power supplies: Many viable options, but know the tradeoffs

    There are a variety of reasons why a system designer may want to consider paralleling of DC power supplies. Some of these are related to the bill of materials and logistics issues, others are focused on satisfying system current, performance or reliability objectives.

    On the non-design side, the ability to parallel supplies may allow a single supply model to be used singly or in combinations across a broad product line. This can simplify sourcing, increase per-unit volume, and streamline inventory management.

    The technical reasons to consider paralleling supplies are more complex, of course. First, using parallel supplies can be a form of “insurance” in case the product actually needs more current than budgeted, perhaps due to unavailability of lower-power components or new features and capabilities added by marketing. Second, parallel supplies may support N+1 and even N+2 redundancy to safeguard against single-point failures, or to enable hot-swapping of a failed supply without system impact. Third, it permits the use of a known, proven supply with well-understood features, characteristics and form factor, thus reducing design-in risk and uncertainty. Finally, it allows for “heat spreading” by adding flexibility in physical placement of the power converters, if a single higher-capacity unit would dissipate too much heat in a highly localized area.

    The flexibility and potential benefits offered by the paralleling of supplies brings an obvious question: can any supply be used, as-is, in parallel configuration? The answer is “no.” It depends on the design of the supply, the technique used to connect the supplies, and the reason the supplies are being used in parallel.

    The most obvious and simplest way to hope to put supplies in parallel is to simply tie their outputs together. In general, this won’t work

    One way this direct-connect topology can work well is if one supply is set to constant-voltage (CV) mode and the others are set to constant-current (CC) mode, but at slightly higher output voltage; note that not all supplies allow choice of output mode.

    The direct-connect approach is viable if the supply is specifically designed to support that topology

    Another approach adds small ballast resistors in series with each supply’s output, to equalize the distribution of the load current among the supplies
    However, these ballast resistors also dissipate heat, which degrades system efficiency.

    A seemingly “simple” solution to this direct-connect dilemma is to just use a diode between each supply and the common tie point of all supplies, a technique commonly referred to as diode-ORing, Figure 2. ORing diodes are very effective at preventing a supply from sinking current away from the shared output, but are generally insufficient to address sharing errors among supplies with independent error amplifiers, because the conduction knee is abrupt enough that parametric differences in the supplies’ setpoints will still lead to significant sharing imbalances.

    Diode ORing is generally required for supplies acting independently, whose outputs can both sink and source current (two quadrant-operation). The effect of directly paralleling such supplies without ORing diodes is far worse than it is for single-quadrant supplies.

    Also, if the diodes have a negative temperature coefficient for their conduction threshold, they will actually promote current hogging in the array. One way to minimize the problem is to use a method of rectification with a positive tempco – Schottky diodes, or via a diode-like function but built using FETs and a rectifier in an active-ORing implementation

    Under some circumstances, diode ORing can still offer reliability improvements at the system level.

    Who’s in charge here?

    Supplies generally must be designed specifically for parallel operation in order to operate reliably and predictably in an array. Startup synchronization, fault-protection coordination, and control-loop stability must all be considered.

    For a parallel array of supplies to deliver increased levels of usable current to a load, some type of control-loop strategy that factors in array use is needed. A popular control strategy is to run the supplies with no internal voltage regulation amplifier, but instead group them together with a common control-signal input which is controlled by a single error amplifier. This error amplifier regulates the output of the system, and then its single feedback signal is distributed to all of the power supplies in the system.

    A major benefit of this popular control strategy is the regulation of the output voltage is excellent, and sharing errors are dominated by part-to-part variations in modulator gain. On the downside, use of a single error amplifier and single wired control bus represents a single point of failure, which may present a problem for some types of high-reliability systems.

    Paralleling supplies may have negative consequences on transient response and load regulation.


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