Correctly measuring and interpreting voltage and current waveforms are critical when you're trying to determine what's behind any power quality problem.
Since so many types of loads can cause harmonic currents, finding the culprit behind your power quality problem is anything but predictable. Adjustable speed drives, electronic ballasts, data processing equipment, and uninterruptible power systems are just a few of the devices that generate harmonic currents. However, their presence does not necessarily mean the phenomenon of harmonics exists. One of the most difficult things to determine in power quality analysis is whether a site is a prime candidate for harmonic problems. To help you in your troubleshooting efforts, let's look at the proper methods of measuring harmonic voltages and currents as well as review some rules for deciphering the results.
Voltage and current measurements. Troubleshooting a power system for a suspected harmonics problem must include voltage and current measurements made with true RMS (root-means-square) digital meters. By using such a meter, you can accurately determine the voltage and current amplitudes in the presence of non-sinusoidal loads. Most meters used today are average-responding RMS, meaning they measure the amplitude of the waveform and multiply it by 1.414.
Although the readings would be the same on circuits feeding linear loads, the results from average-responding meters can be misleading on those with nonlinear loads. Voltage readings taken from the average-responding meter are greater than those made with a true RMS meter. However, current readings from average-responding ammeters are less than measurements made by true RMS ammeters. Because of this discrepancy, you can see how using the wrong type of meter can affect everything from kVA calculations to conductor sizing.
Other meters provide graphic representation of the harmonics waveform. Spectrum analyzers, oscilloscopes, power disturbance monitors, and harmonic analyzers are just a few. These can display the waveform distortion and calculate the exact level of harmonic distortion at various frequencies. Infrared meters find overheated connections in electrical panelboards or transformer windings, especially along the neutral conductor path.
Voltage and current distortion. To simplify your investigation, it helps to divide harmonic problems into two categories: voltage waveform distortion and current waveform distortion. Voltage distortion concerns electric utilities more than current distortion because it affects electrical delivery to the facility.
IEEE standard 519-1992, Recommended Practice and Requirements for Harmonic Control in Electric Power Systems, states the total harmonic distortion (THD) of the voltage waveform provided by the utility cannot exceed 3% of the ideal sine wave. Make this measurement at the point of common coupling (PCC). This is where the utility and facility wiring meet (usually at the meter box). If the voltage distortion exceeds 3%, the utility should provide some form of mitigation to correct the problem.
Why does voltage and current waveform distortion concern you? Distorted voltage waveforms affect the RMS input voltage for equipment. In some cases, they occur too far downstream from the PCC for the utility to monitor it accurately. It's best to measure the voltage waveform distortion at the effected equipment location. Taking measurements at the suspected source of distortion will also help you find a solution.
High levels of harmonic current can overload the facility's wiring and electrical equipment. The overriding concern for current waveform distortion is THD versus the actual loading of the equipment power source. For example, let's say you determine the current waveform from a personal computer has a harmonic distortion of 75%. Although this appears high, the amplitude of the current is relatively low (maybe 2A).
This may have little impact to the overall harmonic distortion of the voltage sine wave if the power source is lightly loaded. As with voltage waveform distortion, the best location to measure current waveform distortion is at the power source that feeds the equipment location (i.e., isolation transformer or main service entrance). This gives you the best representation of the overall current waveform distortion. Remember: This distortion should not exceed 4% at any point of the facility's electrical distribution.
Although nearly all of the loads used today in the commercial and industrial sector generate harmonics, some believe complications from harmonics make up less than 3% of all power quality problems. What's the lesson here? Harmonics is not necessarily a problem just because of a customer's nonlinear loads.