Power Quality Symptoms & Solutions

Power Quality Symptoms & Solutions e-book is is written from an electronics point of view, rather than a power engineering one. And in so doing, provides the bridge between theory and real life. According to the book introduction more and more lecturers are using this material as a reference in their courses. You can find lots of interesting reading here for many industry fields and links to other resources.

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11 Comments

  1. Tomi Engdahl says:

    Mitigating harmonics in electrical systems
    http://www.csemag.com/single-article/mitigating-harmonics-in-electrical-systems/0fdc552157dd758226bc8f757fe2b252.html

    Although devices using power electronics can produce distortion in electrical distribution systems, it’s up to the engineer to apply effective solutions to mitigate them.

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  2. Tomi Engdahl says:

    Southern Fried Transformer
    http://www.designnews.com/author.asp?section_id=1368&doc_id=273075&cid=nl.dn11

    It is common practice to connect a three-phase generators and transformers in this configuration, wye-wye-delta. The delta connected transformer’s secondary absorbs the harmonic distortion from the generator, which is typically just a few percent. And that’s good because it cleans up the waveform. This particular generator had about 5% distortion, not bad at all, but the generator and transformer are usually the same size. Ours weren’t.

    But 5% of our generator amounts to several hundred percent of our poor little transformer, and the better transformer, with its lower impedance, were even less able to resist the harmonic current. And that’s why it burned up so much quicker.

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  3. Tomi Engdahl says:

    Welcome to Open Power Quality
    http://openpowerquality.org/

    Open source hardware, software, and data for low cost, crowd-sourced power quality monitoring, storage, and analysis

    OPQBox: Low cost, open source hardware

    Our first generation OPQBox costs less than US$75, and the schematics are published under an open source hardware license if you want to build it yourself.

    OPQHub: Cloud-based, open source software service

    Each OPQBox sends power quality events and data to OPQHub, an open source cloud-based service.

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  4. Tomi Engdahl says:

    Home made power quality analyser
    https://www.youtube.com/watch?v=x5BWcP8UZnQ

    Built from a scrap step-down transformer and a few resistors, and a lot of software finesse.

    Demonstrates some truly terrible power quality, including harmonic distortion, commutation notches and other problems.

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  5. Tomi Engdahl says:

    Understanding Power Quality
    https://www.youtube.com/watch?v=qUGgOm6s0aM

    ONEAC AC Power, a business of Emerson Network Power, demonstrates the dramatic difference an ONEAC power conditioner can make in your power quality and protecting your system from power disturbances.

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  6. Tomi Engdahl says:

    Monitor Ground Fault Leakage Currents
    http://ecmweb.com/content/monitor-ground-fault-leakage-currents

    Most power quality problems are due to incorrect connections of an electrical system. Using CTs and leakage current monitors, you can check an electrical system during acceptance testing of the installation and also during maintenance and renovation of the system. Watch out for partial or complete short circuits between neutrals and your grounding network. They’ll often create power quality disturbances

    Most power quality problems are due to incorrect connections of an electrical system. Using CTs and leakage current monitors, you can check an electrical system during acceptance testing of the installation and also during maintenance and renovation of the system.

    Yes, there’s a certain amount of normal leakage current going from the neutral and the phase conductors to ground in all electrical systems. Usually, the level of this leakage current is from about 10mA to some 100mA, depending on the size of electrical system.

    Voltage differences are between different grounding points.

    Because of contact between the neutral and ground at Point A, the return current flowing from the load also flows through the grounding network and the grounding circuits in Devices D1 and D2.

    This current may disrupt their operation because the voltage difference and resulting current are often quite high.

    So, how does ground noise enter a sensitive electronic device? Current flowing in a neutral conductor consists of many kinds of disturbances such as harmonic waves and distortion, high frequency disturbances, transients, etc.

    As you can see, a filter or surge suppressor circuit does not prevent disturbances from entering the device. Often sensitive electronic devices are connected to a reference ground. The problem above results from the devices’ grounding conductor connected to the conduit (which is also serving as a ground) rather than to a dedicated reference grounding system tied to the main grounding point.

    Sometimes the shield of the communication or data cable connects to conductive parts of the building.

    Watch out for magnetic field interference. In a normal electrical system, ground current produces a low frequency (60 Hz) magnetic field. At this frequency, a clean electrical system has a magnetic field between 0.1 mGaus to 45 mGaus. In an industrial working environment the magnetic flux density can be much higher, anywherefrom 20 mGaus to 15 Gaus. Normal sources of stray magnetic fields are transformers, large motors, and various industrial production machines. However, the major significant cause for magnetic stray fields is often a faulty grounding connection or an equipment failure in an electrical system.

    Relatively small magnetic fields cause disturbances to sensitive electronic devices. For example, fields higher than 13 mGaus can disturb a computer monitor.

    In the U.S., definitive standards on electric magnetic fields aren’t yet established. However, in Europe, the Cenelec (European Committee for Electrotechnical Standardization) EMC Standard EN-50082-1 gives limit values for residential, commercial, and light industry environments. The maximum value of magnetic flux density is 38 mGaus for industrial situations and 13 mGaus for computer monitoring locations. In a facility’s electrical system, you may see very high peaks of magnetic flux density during starting current impulse. These peaks can directly effect the circuits of sensitive devices.

    How do you monitor ground leakage current to avoid problems? To prevent electronic noise from disturbing electronic devices, you can use different kinds of noise attenuation circuits, such as filters, isolation transformers, photo-couplers, etc. But one of the most important aspects in preventing electronic noise from disturbing electronic devices is to evaluate the wiring and grounding systems first. This can often be expensive and time consuming. The simplest and most economical method is to continuously monitor leakage current of the whole electrical system or the most critical parts of electrical system.

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  7. Tomi Engdahl says:

    Save Energy Through Smart Feeder Design
    http://ecmweb.com/electrical-testing/save-energy-through-smart-feeder-design

    Energy savings is often thought of something extra you do, after the fact, to reduce thermal losses.

    Let’s look at a very common area of error: nominal voltage. Commercial and industrial facilities typically use some combination of 480/277V and 208/120V. Often, the electric utility provides 480V at the service. A center tap on that service transformer provides 277V.

    For offices and general receptacles, you need 120V, so stepdown transformers supply 208/120V. Sometimes, this is just one big transformer and one big panel. While that arrangement saves the design engineer time, it typically increases the cost of construction and definitely increases the cost of operation.

    Ideally, you will distribute 480V as far inside the building as you can — so far that all feeders are 480V and only branch circuits are at a lower voltage. First of all, it costs less in labor and materials to carry that same power at 480V than at 120V (smaller wire due to lower current, smaller raceway due to smaller wire, etc.). More important, the distribution itself is more efficient at 480V. The closer the 120V transformer is to its 120V loads, the more energy-efficient your system will be. Thus, it’s smart to:

    Use several small, strategically situated 480V-120/208V transformers instead of one large, centrally situated one. The idea here is to get the shortest 208/120V branch circuits that are practical.
    Use a similar approach for 277V. Rather than a center tap off the service transformer, use several smaller 480V-480/277V transformers. Better yet, see if you can replace 277V with 480V, since the typical uses (lighting and reheat boxes) are available in 480V versions.

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  8. Tomi Engdahl says:

    Branch circuit power meter delivers data center power-quality data
    http://www.cablinginstall.com/articles/2015/04/bcpm-data-center-power-monitor.html

    TrendPoint Systems Inc. recently launched Branch Circuit Power Meter 2.0, the newest version of its branch circuit power meter (BCPM) product that the company says “delivers utility-grade power data.” TrendPoint provides a monitoring platform for high-density power consumers including data centers.

    “BCPM 2.0 is the latest evolution of TrendPoint’s exceptional technology, delivering power quality data down to the branch circuit level,”

    “The BCPM 2.0 meter is the first of its kind to offer waveform capture, along with harmonics. [It] improves upon the industry’s method of using three-phase PQM meters for switchgear, switchboards, and distribution panels.”

    http://www.trendpoint.com/

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  9. Tomi Engdahl says:

    From the Ground Up
    Answering five of the most frequently asked questions on grounding and ground-fault current
    http://ecmweb.com/power-quality/ground

    Although it’s one of the most important aspects of electrical design and installation, grounding continues to be one of the least understood and most misinterpreted concepts in the industry. It’s also one of the most expensive when errors are made. The dollar value of equipment in-operation and/or loss — not to mention the potential liability — associated with ground-fault arcing can be staggering. So if grounding implementation problems leave you dazed and confused, don’t worry. You’re not alone.

    Electrical contractors, plant/facility electrical maintenance personnel, and electrical engineers continue to demand more complete and concise information on grounding-related issues. That’s why interest in grounding and ground faults has not diminished throughout the various Code cycles.

    Question No. 1: What are the advantages and disadvantages of the various grounding methods for medium-voltage systems in power plants? Also, what practices are adopted by electric utilities both nationally and internationally?

    Answer: You can broadly classify medium-voltage (MV) grounding systems into four categories: solidly grounded, low-resistance grounded (LRG), high-resistance grounded (HRG), and insulated neutral (ungrounded) systems. A good reference is ANSI/IEEE Std. 242 (Buff Book), “Protection and Coordination of Industrial and Commercial Power Systems.”

    With the solidly grounded system, as shown in Fig. 1, there is no intentional impedance in the neutral-to-earth path. Instead, the neutral is solidly connected to earth.
    The protective device closest to the fault must trip and isolate the circuit as fast as possible.

    With LRG systems, as shown in Fig. 2, the ground-fault current is controlled and normally limited to between 25A and 1,000A. The voltage to ground on the un-faulted phases can increase up to the phase-to-phase voltage level, so you must use adequately rated insulation systems and surge suppression devices. You also must detect and isolate the ground fault. Since the ground-fault current is smaller and controlled, ground-fault relaying still has the requirement of fast tripping.

    With an HRG system, as shown in Fig. 3, the ground-fault current is in the 10A range. The intention here is to allow the system to operate without tripping, even with a phase-to-ground fault on one phase. When a ground fault does occur, only an alarm is raised. This permits time to locate the fault while power continuity is maintained.
    If the fault is in a rotating machine, there usually is no iron damage in the stator.

    With the insulated neutral (ungrounded) system, as shown in Fig. 4 on page 36, there is no intentional connection of the system to ground. In effect, the three phases of the system float. When a ground fault occurs, the fault current is contributed by the system capacitance to earth on the un-faulted phases. This is usually small, and the system can be operated without tripping. Because the system is floating, if the ground fault is of the arcing or intermittent type, then there is the possibility of substantial transient overvoltages, which can be six to eight times the phase voltage.

    The standards and best practices in various countries generally follow ANSI or IEC standards. The technical literature supports these practices. In power plant applications, MV systems occur in two places: generation and station service. In practice, both station service and generators are low- or high-resistance grounded.

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  10. Tomi Engdahl says:

    System
    Grounding Impact on reliability and Safety
    http://southern-alberta.ieee.ca/files/2011/12/mar-11_presentation1.pdf

    May 11, 2009
    Transient Overvoltages on Ungrounded
    Systems from Intermittent Ground Faults
    http://www.eaton.com/ecm/groups/public/@pub/@electrical/documents/content/ia08700001e.pdf

    Reply
  11. Tomi Engdahl says:

    GROUNDING CONSIDERATIONS
    FOR INDUSTRIAL POWER SYSTEMS
    http://www.hv-eng.com/2010IASGrounding.pdf

    Reply

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