Published by David Horning, November 2014. Power Monitors, Inc., White Paper: Steps to Find the Source of Harmonics
Abstract. Harmonics are “Non-Linear” current or voltage in an electrical system. Any waveform that deviates from a perfect sine wave has harmonics. Any nonlinear load draws harmonic currents and therefore produces harmonic voltage distortion, by producing non-sinusoidal voltage drops across system wiring and transformers. Harmonics are voltages or currents that are multiples of the fundamental frequency in a circuit. These are specified by their harmonic number or multiple of the fundamental frequency, as shown in Figure 1.
For example, with 60Hz fundamental frequency of the third harmonic is 180Hz. In this example, for every cycle of the fundamental frequency, there are three cycles of the harmonic frequency. Any complex, periodic waveform can be uniquely broken down in terms of harmonics, making a harmonic analysis a useful way of analyzing nonlinear distortion.
Finding the Source
Finding the source of significant harmonic distortion observed on a power system is an important part of being able to mitigate the distortion. When harmonic distortion occurs in a distribution system, sometimes it is not apparent which customer is responsible for the power line distortion. At this point it is important to formulate a strategy of determining the origin of the power distortion in order to put in place a barrier to keep the distortion from propagating into the rest of the distribution system and on to other customers. The following are several methods that can be used to help find the source of harmonics.
Method 1. Harmonic Current Flow – “Follow the Current”
Without capacitors, normal harmonic current flow is back to substation (lowest impedance). The nonlinear load is the source of the harmonics and the harmonic current flows from it (see Figure 2).
Power factor capacitors can alter flow for at least one harmonic (see Figure 3). Current flows into a capacitor in series resonance, and not back to the substation. It’s often necessary to disconnect capacitors to reliably locate source of harmonics. With power factor correction caps out, monitor the flow of harmonic currents on the feeder – follow harmonic current “downstream” until the offending load is found.
Method 2. Harmonic Power Flow Direction
Both harmonics and the fundamental frequency cause energy to flow at their characteristic frequencies in a distribution system. The power at any harmonic is equal to the harmonic voltage times the harmonic current, times the cosine of the harmonic phase angle difference. If the capacitive reactance happens to become equal to the inductive reactance at one of the harmonic frequencies, resonances will occur. Resonances will exaggerate the effect and may give misleading results. Therefore it is important to consider the harmonics as a group, and place more emphasis on the odd harmonics, typically between the 3rd and 11th to reduce the effects of resonance at one given harmonic. It is natural to assume that the direction of the power flow is from the customer whose load is causing the harmonics back into the distribution system. However, this isn’t always the case.
The relative phase angle between the voltage and current for each harmonic, as measured at an intermediate point in the circuit, is affected by the line impedance, the impedance of all other loads in between, and also the specific nonlinear nature of the load itself.
The frequency response of the distribution network and the nonlinear nature of the loads themselves vary with time and position on the network, making it very difficult to draw any conclusions from the power flow of any specific harmonic. In addition, unless the voltage distortion is large, the magnitude of harmonic power flow is often very small, making reliable direction measurements difficult (Figure 4).
Negative power at a harmonic can indicate that the load is the source of harmonic current injection. There may be very little harmonic active power, with most of the harmonic flow producing reactive power flow. This could result in insufficient real power at a harmonic to get an accurate power flow direction. Proper CT polarity is important for this technique. Negative power can indicate source of harmonics IF…
• Background harmonic levels (utility %VTHD) are not significant.
• Capacitor banks are not producing resonance near harmonic component being evaluated – altered harmonic current flow.
• Level of harmonic Watts sufficient (vs. 60Hz power) for valid measurements – low harmonic Watts can produce inaccurate, meaningless power flow direction (sign).
Method 3. Relative Magnitudes Approach
Utility voltage is generated as a pure 60Hz sine wave with no harmonic distortion. When nonlinear loads are attached, harmonic currents flow. These harmonic currents result in corresponding voltage drops along the distribution wiring and across transformers, due to their non-zero impedances. In a typical distribution system, the voltage source impedance is very low (ideally zero) compared to the load impedance. Stated another way, the available short circuit current is much higher than the typical (or even maximum) load current. At 60Hz, this difference insures that voltage sags due to high load current are a small percentage of the line voltage. Similarly, harmonic voltages developed from harmonic currents are correspondingly smaller, and the resulting voltage THD is much smaller than the current THD causing the distortion.
If the voltage has a non-zero THD, even a perfectly benign linear load (eg. electric heater or incandescent lighting) will draw harmonic currents in proportion to the harmonic voltage. In this case, the current THD will be similar in magnitude and the to voltage THD, rather than much higher. In general, if the current THD is roughly similar in size to the THD, it’s likely that the monitored load in not responsible for the voltage THD.
Unfortunately, the current THD is often much higher than the voltage THD. In these cases, examining the voltage and current THDs along with the load current can provide some clues as to the source of harmonics. In Figure 5, the RMS current is graphed with the voltage THD. There’s a clear correlation between the voltage THD and current – the voltage THD jumps from a mildly elevated 1.5% to a very high 4.5-5% when the large 2500A load turns on. The high load current is a significant faction of the short circuit current, and thus has a large influence on the voltage THD.
The opposite case is shown in Figure 6. Here, the voltage THD is over 6%, but shows little correlation to changes in RMS current. This is a strong indication that the monitored current is not the cause of the voltage THD. There are large step changes in current with no change in voltage THD, and the voltage THD varies over a wide range with no change in load current.
The voltage THD and current relationship is not always so clear-cut. If the current is a mix of linear and nonlinear loads, RMS current shifts can produce unexpected voltage THD changes.
Compare time variation of VTHD with specific customer load characteristics:
• Correlate VTHD patterns with customer load characteristics – equipment types, usage patterns
• Does VTHD vary with customer shift changes, breaks, etc. – commercial, industrial customers
• Interval graphs – compare VTHD vs RMS and harmonic load current time trends
• Compare VTHD to Customer Load
Figure 7 shows some correlation of VTHD to load current.
Figure 8 shows a strong correlation of VTHD to load current.
Method 4. Common Sense Approach – Evaluate Likely Sources, Customers
Evaluate likely sources – larger industrial, commercial customers On the customer side of the transformer:
• Look for significant harmonic currents
• Elevated VTHD (greater than 5%) usually indicates resonance condition
• Measure capacitor currents
• Correlation of VTHD with customer RMS and harmonic currents
• Look for dissimilar Load and Supply Harmonics (Figure 9).
Dissimilar load (current) and supply (voltage) harmonics, as seen in Figure 9, indicate that the monitored load is not a dominant cause of voltage harmonics. Here, the largest current harmonic is the 5th, but the largest voltage harmonic is the 3rd. The voltage THD must be mostly from another load (or aggregation of distribution loads), resulting in background distribution voltage THD.
Method 5. Resonance Clues
Resonance induced problems typically have one dominant harmonic. Where harmonic problems exist, measure current in capacitors – single, large harmonic current nearly always indicates that a power factor correction capacitor is in resonance with the inductive system impedance. High voltage distortion is often a combination of excessive harmonic current injection and system response that magnifies harmonic currents due to a resonance. Temporarily disconnecting power factor capacitors can help identify resonance problems.
Conclusion
Several methods have been given to help identify the source of harmonic distortion. The best method depends on the details of the problem, especially if power factor correction capacitors are involved. With a systematic approach, and recording of voltage and current total harmonic distortion and individual harmonics, nonlinear loads can be identified. This is the first step towards mitigation or harmonic filtering.
Author: David Horning, Software Developer, Email: dhorning@powermonitors.com, Website: http://www.powermonitors.com, Phone no. (800) 296-4120
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