With the release of the G5/4-1 SYSTEM PLANNING LEVELS FOR HARMONIC DISTORTION which supersedes document G5/4 we thought it would be a good idea to put this information together as a general reference and put to bed some of the misconceptions regarding this subject. As you will appreciate it is quite a far reaching subject with many twists and turns eg; we found that as you answer one question, another question manifests itself but, we have tried to keep the subject as brief and simple as possible.
In years past, most electrical equipment operated on an ideal voltage and current waveform. However, in the past 25 years (particularly since the late 1980's) there has been an explosion in the use of solid-state electronic technology. This new, highly efficient, electronic technology provides improved product quality with increased productivity by the use of smaller and lighter electrical components. Today we are able to produce products that costs less than in years past, but this new technology requires clean electric power and is highly sensitive to power distortions.
Electronic devices convert 50 Hz alternating current to direct current by the use of switching power supplies that contain rectifiers and often capacitors. In addition to converting alternating current to direct current, sometimes the current is converted back to alternating current but into a different frequency.
Electronic equipment (switching power supplies) draws current differently than non-electronic equipment. Instead of a load having a constant impedance drawing current in proportion to the sinusoidal voltage, electronic devices change their impedance by switching on and off near the peak of the voltage waveform.
Switching loads on and off during part of the waveform results in short, abrupt, nonsinusoidal current pulses during a controlled portion of the incoming peak voltage waveform. These abrupt pulsating current pulses introduce unanticipated reflective currents (harmonics) back into the power distribution system. The currents operate at frequencies other than the fundamental 50 Hz. Harmonic currents can be likened to the vibration of water in a water line when a valve is open and closed suddenly.
Harmonics affect us all; from the secretary operating a computer, to the engineer trouble shooting equipment failure, the electrical contractor having to absorb the cost of equipment replacement, the inspector who must investigate the cause of electric fires, to the facilities management interested in effective and efficient equipment operation and the avoidance of down time. The scope of harmonics impacts consultants, engineers, designers, suppliers, equipment manufactures, and, of course your plant operation.
What types of equipment loads can cause the problem?
For example the largest contributor of reflective harmonic currents for commercial buildings is the personal computer. There are, however, as we know in our industry other large contributors too, such as:
- Variable Speed Drives (VSD)
- Arc Equipment
- Battery Chargers
- Computer Power Units (CPU)
- Discharge Lighting (fluorescent, mercury, sodium, etc.)
- Electronic Ballasts
- Personal Computers (PC)
- Rectifiers
- Uninterrupted Power Supplies (UPS)
Clean power is required for today's equipment
Electronic microprocessor PLC equipment requires clean power. This type of equipment needs undistorted voltage to function properly, and it is particularly sensitive to voltage transients (notches or spikes) and flat topping of the voltage waveform caused by the large pulsating currents. High frequency harmonic currents can introduce voltage (noise) in electronic cables or components.
Electronic equipment installation manuals often require the total voltage distortion to be no more than 10%. Voltage distortion can cause poor product performance, but in general, it is not a safety hazard. Strangely, electronic equipment requires clean power, but its power supplies generate the reflective harmonic currents that cause the voltage distortions!!!!
Exactly what is the problem?
The actual problems of any Project will vary, depending on the types and number of installed harmonic producing loads. Most Projects can withstand nonlinear loads of up to 15% of the total electrical system capacity without concern, but, when the nonlinear loads exceed 15% some non-apparent negative consequences can be expected. For Projects that have nonlinear loading of more than 25%, particular problems can be become apparent. The following is a short summary of most, but not all of the problems caused by harmonics:
- Capacitor Failure - Harmonic Resonance
- Circuit Breakers Tripping - Inductive Heating and Overload
- Computer Malfunction or Lockup - Voltage Distortion
- Conductor Failure - Inductive Heating
- Electronic Equipment Shutting down - Voltage Distortion
- Flickering of Fluorescent Lights - Transformer Ballast Saturation
- Fuses Blowing for No Apparent Reason - Inductive Heating and Overload
- Motor Failures (overheating) - Voltage Drop
- Neutral Conductor and Terminal Failures - Additive Currents
- Overheating of Metal Enclosures - Inductive Heating
- Power Interference on Voice Communication - Harmonic Noise
- Transformer Failures - Inductive Heating
The heating effects of harmonic currents can cause destruction of equipment, conductors, and fires. The results can be unpredictable legal and financial ramifications. Voltage distortions can lead to overheating of equipment, electronic equipment failure, expensive downtime, and maintenance difficulties. Harmonic currents and voltage distortion are becoming the most severe and complex electrical challenge for the electrical industry. The problems associated with nonlinear loads were once limited to isolated devices and computer rooms, but now the problem can appear throughout the power and utility system.
Past, present, and future trends
In the past, most electric power was consumed by "linear loads." Reflective harmonic currents from nonlinear loads (fluorescent lighting) were a relatively minor component of the total Project power usage. In 1992, 15 to 20% of the total load was nonlinear, and by this year (2007) it is expected that 50 to 70% of all loads will be nonlinear. As we can see from the projection, the problems (or opportunities) of harmonics will be growing with the expanded used or electronics. With this information we hope to inform the engineers as we have found many people in the electrical industry as yet do not fully understand the basics of harmonics; much less have a working knowledge of the problems.
Is there anything we can do?
Be sure that the Project Team has been made aware of the causes, the effects, and the solutions of harmonic currents. Because harmonics are here to stay, we must adjust our thinking on electrical system design, installation, inspection, and maintenance. We must anticipate the non-apparent overload of the electrical system and the associated distortions to the voltage waveforms. Think of harmonic currents as the symptoms of the common cold; there is no cure, but we can treat the symptoms. Before we apply any treatments or preventive measures, we must understand the symptoms and their cause.
What types of loads cause harmonic currents?
Let's understand the difference between linear and nonlinear loads. A linear load is a load that opposes the applied voltage with constant impedance resulting in a current waveform that changes in direct proportion to the change in the applied voltage. Examples of these loads are resistance heating, incandescent lighting, motors, etc. If the impedance is constant, then the applied voltage is sinusoidal, and the current waveform will also be sinusoidal.
A nonlinear load, on the other hand, is a load that does not oppose the applied voltage with constant impedance. The result is a nonsinusoidal current waveform that does not conform to the waveform of the applied voltage. Nonlinear loads have high impedance during part of the voltage waveform, and when the voltage is at or near the peak the impedance is suddenly reduced.
The reduced impedance at the peak voltage results in a large, sudden, rise in current flow until the impedance is suddenly increased resulting in a sudden drop in current. Because the voltage and current waveforms are no longer related, they are said to be "nonlinear." Nonlinear loads are loads that have diode-capacitor power supplies such as: computers; laser printers; welders; Variable Speed Drives; UPS systems; fluorescent lighting; etc., which draw current in short pulses during the peak of the line voltage. These nonsinusoidal current pulses introduce unanticipated reflective currents back into the power distribution system, and the currents operate at frequencies other than the fundamental 50 Hz. Harmonic is a term that describes sinusoidal waveforms that operate at a frequency that is a multiple of the fundamental 50 Hz frequency.
When a current, or voltage, operates at other than the fundamental 50 Hz frequency it is said to operate at a specific harmonic order (3rd harmonics operate at 150 Hz; 5th harmonics operate at 250 Hz). Because reflective harmonic currents operate at frequencies higher than the fundamental, we must be concerned with their effect in the electrical distribution system.
The most significant effects of high frequency harmonic currents are as follows: Inductive heating of transformers, generators, and other electromagnetic devices such as motors, relays, and coils (due to the inductive heating effects of eddy currents, skin effect, and hysteresis). Inductive heating of conductors, breakers, fuses, and all other devices that carry current (because of eddy currents, skin effect, and hysteresis).
Inductive heating of metal parts such as cable ways, metal enclosures, and other ferrous (iron or steel) metal parts (because of eddy currents and hysteresis). Voltage distortion resulting in unpredictable equipment operation because of harmonics. Excessive neutral current resulting in equipment overheating or failure because of additive harmonic currents, excessive voltage drop, and distortion.
Finally, how serious Is this problem?
The effects of harmonic currents on electrical distribution systems are not understood by most in the electrical industry. The number one hazard with harmonic currents is equipment failure because of current overload that result in fires. In addition to the electrical safety aspects, harmonics cause voltage waveform distortions that affect many different types of loads in different ways.
Engineering research on the problems and solutions is still in its infancy; solutions recommended today may not be viewed as correct further down the line.
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