Effects of harmonics on power systems - Part 2

Once you've recognized that harmonics are in a circuit or in an electrical system, the next step is to carry out tests to determine the magnitude and type of harmonics.As more and more VFDs, electronic ballasts, battery chargers, and static var compensators are installed in facilities, the problems related to harmonics are expected to get worse. As such, it's important that you' re able to determine

Once you've recognized that harmonics are in a circuit or in an electrical system, the next step is to carry out tests to determine the magnitude and type of harmonics.

As more and more VFDs, electronic ballasts, battery chargers, and static var compensators are installed in facilities, the problems related to harmonics are expected to get worse. As such, it's important that you' re able to determine harmonic levels and analyze system data so that you can implement corrective measures and avoid serious problems.

Extent of harmonics

While harmonic voltages and currents are, by themselves, imperceptible, the physical phenomena that accompany them are perceivable. The adverse effects of harmonics in electrical power systems are very real, and failures related to voltage and current harmonics very often occur without warnings. When you have an indication that harmonics are present, your next step is to carry out testing to measure their level in the power system; you'll need this information to determine what mitigation system to use.

The degree to which harmonics affect electrical power system components depends on several factors: physical location, installation practices, electrical loading, and ambient temperatures. This means the same magnitude of harmonics might affect two separate installations differently. Some of the symptoms associated with large magnitudes of harmonics include large neutral currents, excessive temperature rise, vibration, audible noise, and protective device malfunction.

If you think large harmonic components are present, you first should learn what the electrical system is actually carrying on its lines. To do this, you must use harmonic measurement meters. The following instrumentation are commonly employed.

True-rms meters

The root mean square (rms) value, also known as the effective value, is the true measure of electrical parameters. For example, rms current represents the net heating effect of current on electrical equipment, thereby determining the thermal rating of the equipment. Operation of fuses and thermal magnetic circuit breakers is based on rms current. In transformers, the rms voltages determine the magnetic flux density levels in the transformer core. The rms voltage ratings determine the operating limits of electrical equipment. The relationship between the rms, average, and peak values of a pure sinusoidal current waveform follows. Form factor (FF) and peak factor (PF) are two elements that further define electrical waveforms. For a pure sinusoidal wave, the following relationships are true:

[I.sub.AVE] = (2/[Pi]([I.sub.M]) = (0.636)([I.sub.M])

[I.sub.RMS] = (1/[square root of 2])([I.sub.M]) = (0.707)([I.sub.M])

FF = [I.sub.RMS] [divided by] [I.sub.AVE] = 1.11

PF = [I.sub.M] [divided by] [I.sub.RMS] = 1.414

where [I.sub.M] = peak current

[I.sub.RMS] = rms current

[I.sub.AVE] = average current

Harmonics in electrical systems distort the waveforms and alter the rms and average values. Under such conditions, the relationship between the rms, peak, and average values are not represented by the above equations.

Conventional analog style meters do not accurately measure the rms values of nonsinusoidal voltages and currents due to deficiencies in their response to higher frequency components. Some earlier forms of digital meters measure the average or peak values and use multiplication factors to derive rms values. In a harmonic-rich environment, this is not valid. True-rms meters, by a process that involves high rate of signal sampling, recreate the waveform, and use frequency transformation techniques to obtain the true-rms values. True-rms meters may indicate that harmonics are present in an electrical system but may not provide a breakdown of the significant harmonics.

Harmonic analyzers

Harmonic analyzers are effective instruments for determining the waveshapes of voltage and current and measuring the respective frequency spectrum. Several types of harmonic measuring instruments are available, with each type having a different capability.

The simplest ones measure single-phase harmonic voltage and current, and provide information on the harmonic spectrums. These handheld instruments are easy to carry around. Fig. 1 shows voltage and current waveshapes and their harmonic frequency distribution recorded using a handheld harmonic analyzer. In addition, power factor and phase angle information are also measured by the harmonic analyzer used. The data shown in developing Fig. 1 were measured at the supply terminals of a power distribution panel feeding main frame computer-type loads.

Three-phase harmonic analyzers measure the harmonic characteristics of the three phases and the neutral simultaneously. Furthermore, some of the 3-phase analyzers provide graphs of the current and voltage distortion variations with time. These graphs are useful for determining if adverse harmonic loading conditions exist within the facility during plant operation. Fig. 2, on page 62, shows voltage and current harmonic distortion measured by a 3-phase analyzer, with the instrument leads connected at a main switchboard supplying an office building.

In addition to harmonic measurement, some analyzers are capable of measuring power, power factor, and transient disturbance data to help assess power quality within the power system. As expected, these instruments are less portable and considerably more expensive than the simple handheld-type units.

Harmonic analyzers calculate the total harmonic distortion (THD) of the waveform, so that overall distortion limits, as established under the guidelines of professional organizations, such as the Institute of Electrical and Electronic Engineers (IEEE), or the International Electrotechnical Commission (IEC), are not exceeded. IEEE defines THD by the following equation:

THD = [square root of [[([I.sub.2]).sup.2] + [([I.sub.3]).sup.2] + [([I.sub.4]).sup.2] + [([I.sub.5]).sup.2] + ...[([I.sub.n]).sup.2]] [divided by] [I.sub.1]

where, [I.sub.1] is the fundamental component of the current (60 Hz in the U.S.) and [I.sub.2], [I.sub.3], [I.sub.4]...[I.sub.n] are the harmonic frequency components of the current (multiples of the fundamental frequency).

While using a harmonic analyzer, it's important that you verify that voltage and current transformers (PTs and CTs) used with the analyzer have satisfactory higher frequency response characteristics. Normally, these instrument transformers are designed for optimum performance up to a cutoff frequency, beyond which their accuracies drop off considerably, introducing errors in the measurement. For example, to measure waveform distortion data up to the 50th harmonic, the PTs and CTs must have a frequency response of at least 3 kHz. (50th harmonic x 60 Hz = 3000 Hz). The manufacturers of harmonic analyzers are usually able to provide CTs with excellent harmonic frequency response.

Using an oscilloscope

Oscilloscopes have traditionally been used to troubleshoot electronic and electrical circuits. The older analog scopes had low high-frequency response characteristics and were very limited in their ability to perform waveform analyses. The digital storage-style scopes, which are now available, in addition to performing their traditional functions, have the capability to acquire and store signals and perform mathematical operations for determining frequency characteristics. These units collect and store the waveform data using voltage and current probes. This data then is downloaded into computers and synthesized to determine the waveform frequency characteristics.

Using harmonic analyzers

Harmonic analyzers are provided with voltage probes and current sensors. Some analyzers have seven to nine channels for simultaneous measurement of 3-phase voltages and currents and the neutral (each channel reading one parameter). There are other instruments available that offer less channels. If you have just a limited number of electrical parameters to be read simultaneously, the latter devices will work fine. Fig. 3 shows a typical installation of a harmonic analyzer to measure harmonics in a 3-phase, 4-wire lighting panel. If you want to make power and power factor measurements, make sure the polarities of the voltage probes and the current sensors are properly maintained; failing to do this will produce inaccurate results.

For high voltage (greater than 600V) and high current installations, PTs and CTs should be used. Again, make sure the high frequency response of these instrument transformers is adequate.

The location where an analyzer is to be installed depends upon the type of data required. If you suspect that a certain piece of equipment is generating harmonics, then the analyzer must be located in the lines feeding the equipment at a location close to the equipment. You must understand that as you move the analyzer upstream toward the power source, the harmonic currents become a decreasing percentage of the total load. This is partly due to the combination of harmonic non-linear loads and linear loads at upstream locations. Also, harmonic component currents generated by the different sources have varying phase angles between them. The net effect is cancellation of some of the harmonic currents as the measurement location is moved upstream.

When taking measurements, remember that while certain combinations of operating conditions could subject an electrical system to dangerous levels of harmonic distortions, operating conditions may be normal during the rest of the time. The best way you can address this is to take readings for 24-hr or longer periods.

Analyses of test results

By examining harmonic voltage and current data, you can get important information about the operating characteristics of a power system. Abnormal voltage as well as abnormal current conditions can cause problems. For example, most kinds of electrical equipment have maximum voltage distortion ratings for satisfactory operation. When the voltage distortion exceeds the established tolerance, certain types of equipment can malfunction or fail.

For current distortion, the magnitude of the fundamental current and/or the frequency distribution and magnitude of the harmonic current can cause equipment failure. A by-product of current distortion is excess thermal stress, which is a leading cause of equipment failure.

From harmonic spectrum data, k factors can be calculated to see if transformers can safely handle the harmonic load currents.

Emergency engine-generator sets, installed to provide power during utility outages, usually aren't very large. Thus, they're very limited in their capacity to handle harmonic loads and may fail during an emergency.

Motors can experience mechanical failures due to shaft torsional oscillations produced by the flow of harmonic currents in the motor windings. Therefore, it is vital that you carefully analyze harmonic data so that measures can be taken to prevent serious damage to equipment. Of course, you have to obtain information on equipment ratings to make such judgments.

Waveform distortion signatures

Harmonic distortions are characterized by the nature of the source responsible for the distortion. By examining the waveform, it's possible that you can determine the nature of the load producing the distortion. For example, variable frequency drives (VFDs), which use bridge rectifier circuits, produce a unique current waveform with two humps. Computer loads produce sharp peaks due to capacitive charging currents drawn by the power supply. Fluorescent lighting currents exhibit a flat current waveform due to striking of the arc in the light bulbs; at this point, the voltage and the current across the arc become flat. Current drawn by large arc furnaces produces extreme waveform distortions, with unequal positive and negative half cycles of currents.

Certain types of equipment produce even-order harmonics. These harmonics (2nd, 4th, 6th, 8th, etc.) are insignificant if the current's positive and negative half cycles are equal, as in a symmetrical power system. Even-order harmonic frequency current is a product of dissimilar current draw during two half cycles. Metering equipment will not read even-order harmonics because these harmonics cancel [TABULAR DATA OMITTED] themselves out. However, when the current's positive and negative halves are not equal, and even-order harmonics are present, then metering equipment will measure what's going on.

Even-order harmonics, where the current's positive and negative halves are not equal, are produced by arc furnaces, single-phase bridge, and half-wave rectifier circuits (as used in battery charges and power supplies for plating operations), and by transformer magnetizing currents. When metering equipment measure conditions of even-order harmonics, there usually should be no cause for alarm because the equipment operates that way.

If silicon controlled rectifiers (SCRs) are on the line (as used in some VFDs), however, then a reading of even-order harmonics is an indication of malfunctioning, such as the SCRs being unmatched due to manufacturing imperfections. In this condition, the SCRs may not turn on or off precisely. Therefore, conduction timing is not equal, or the SCRs are firing incorrectly. In such instances, the current flow during the positive and negative half cycles occurs during different durations, resulting in current mismatch during the two half cycles.

A small amount of mismatch can be tolerated by the equipment, and the signals produced may not be significant enough to stand out when taking instrument readings. If the mismatch is extreme, instrumentation will readily show the even-order harmonics. Here, you should take prompt corrective action.

As you can see, it's important that you have an understanding of the types of equipment connected to the electrical system when taking measurements. To help you in identifying problem sources, some harmonic analyzer manufacturers have published books containing samples of signature waveforms. You can compare your waveform with those published to determine the problem source.

In some instances, taking harmonic measurement at one location is all you'll need to define the problem. In other cases, you'll have to do a complete harmonic survey and analysis to assess the harmonic problem. The harmonic survey might involve data collection at several locations using harmonic analyzers and meters as required. Once the data is collected, harmonic analysis must be performed to identify potential problems, such as series and parallel resonance, harmonic heating, and motor torsional oscillations.

IEEE 519-1992 compliance

The above analysis will also reveal if the facility's power system complies with requirements noted in IEEE 519-1992, Recommended Practice And Requirements For Harmonic Control In Electric Power Systems, as indicated in the table above, for harmonic current injection into the utility lines. These requirements have been established to ensure that excessive harmonic currents are not so injected, which would affect the quality of power to other users sharing the same power lines and further overstress utility equipment. Presently, a number of utilities are considering placing contractual limitations in their rate structure regarding harmonic injection by their customers. Noncompliance could lead to penalty charges, higher rate schedules, or even electric service cutoff.

The harmonic current limitations established by IEEE 519-1992 are also applicable to equipment within a facility, as implementation of the standard will help enhance good operation. The point of harmonic current measurement in this case is the common junction between the offending loads and other equipment. For an example of using the IEEE standard, and making reference to the table, assume [I.sub.SC]/[I.sub.L] is 16. Then the net harmonic current distortion of all the harmonics up to and including the 10th is not to exceed 4.0%; the net harmonic current distortion of all the currents 11th to 16th is not to exceed 2.0%; and so on. The THD due to all harmonics must not exceed 5.0%. The reasoning behind this form of graded limits is to ensure that the larger users supplied by the utility are not allowed to inject larger quantity of harmonic frequency currents than the smaller users. It's expected that the use of IEEE 519-1992 will result in satisfactory power system operation within a facility, without placing undue burden on other loads or other utility customers sharing the same power source.

Practicing safely is important

Personnel safety is of primary concern when installing harmonic measurement equipment in electrical circuits. Awareness of the dangers that exist in a situation is the first step toward personnel safety. This awareness must be augmented by education about proper safety procedures and about equipment needed to protect against each hazard. Obviously, one safe practice action is to deenergize the electrical equipment prior to the installation of any harmonic measuring instrumentation. A procedure for safely deenergizing an electrical circuit can involve two parts. The first step is to evaluate the circuit for switching points and possible back feeds. This is done by comparing the single-line diagram and other available information associated with the circuit being deenergized and then to prepare a plan for switching off the live connections. The second step is to perform the switching in the order established by the plan.

Once the circuit is deenergized, you must install locks and tags on all applicable disconnecting devices and handles to ensure that the circuit can't be energized. Lockout devices are now available for all sizes of switches, fuse clips, breakers, and other devices. If you can't use locks, a tag should be supplemented by at least one additional safety measure, such as racking out a drawout circuit breaker, or disconnecting load conductors.

After lockout is completed, you should verify that the circuit is, in fact, deenergized. This can be done by using test equipment rated for the system line-to-line voltage. Any test meter used to verify the circuit must be checked for proper operation before and after the measurements.

Sometimes, it's not practical to deenergize a circuit for installation of harmonic measuring instrumentation. In such cases, you should wear proper protective equipment when installing the instruments. This equipment includes fire-resistant clothing, safety glasses, safety hats, rubber mats, electrical gloves, and electrical sleeves. Also, a second person trained in CPR and other first aid should be present during the installation of test leads. Never attempt to install instrumentation test leads on energized high voltage circuits (above 480V). The photo above shows the proper method of installing probes in electrical equipment.

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