From minor control trips to compressor and transformer failures, harmonics can impact every type of equipment, especially HVAC equipment. Harmonics can wreak havoc on systems, often being overlooked because of the additional symptoms they cause.
Finding harmonic currents in an electrical distribution system
Harmonic currents flow in a circuit at multiples of the fundamental 60 hertz (Hz) frequency. For example, current flowing in a circuit at 180 Hz is the third harmonic (60 Hz multiplied by 3). These kinds of currents are not usually directly shown with a multimeter measurement, and usually aren’t even found until equipment down the line begins to show problems.
The production and reflection of these harmonic currents back into the electrical distribution system can cause problems.
- Improper operation of control circuits
- Faulty shutdowns of electronically controlled chillers and air handler units (AHUs)
- Overheating of solenoid coils, requiring replacement
- Overheating of 480-volt transformers supplying 208Y/120-volt HVAC systems
- Overheating of fan and chilled water pump motors
One way to start looking for these issues is by comparing meter readings. Comparing the current readings from an average-responding meter to that of a good quality true-rms meter on the same circuit will help indicate harmonic issues. The average-responding meter will indicate only the 60 Hz current and the true-rms meter will indicate a combination of 60 Hz and harmonic currents.
Understanding harmonic currents
Knowing what to look for and how to spot harmonics in your electrical distribution system can help save a lot of time and headaches, and letting you know to start mitigating their effects with replacement items or installing harmonic filters.
Modern electronic circuits must convert the supplied 60 Hz alternating current (AC) into direct current (DC) since the electronics operate using DC voltage and current. These electronic loads are referred to as “nonlinear” loads because the waveform of the current drawn does not correspond to the voltage waveform applied. These nonlinear loads produce harmonic currents reflected into the system. Harmonic currents appear across a wide spectrum, but generally diminish as the frequencies get higher and higher.
While different harmonic frequencies produce their own unique effect in a circuit, when multiples are combined, they distort the original 60 Hz sine wave. That distorted power at the input can cause erroneous trips and alarms in control circuits. Some harmonic currents produce excessive heat. Others produce a reverse torque in motors—reducing efficiency and overheating motors in the process.
Measuring and interpreting harmonic numbers
Harmonic numbers are assigned in their relationship to the fundamental (or 60 Hz) frequency. To find the frequency of the harmonic current, multiply the harmonic number by the first, or fundamental, frequency of 60 Hz.
All harmonics tend to distort the original fundamental 60 Hz sine wave. The total harmonic distortion (THD) for the voltage sine wave should not exceed 5% when measured on a power quality analyzer.
Many VFDs in use today are 6-pulse drives (six diodes in their converter circuits). These drives produce harmonics at the 5th, 11th, 13th, 17th, 19th, and so on, harmonic. If high levels of harmonics exist at these frequencies, consider placing a filter on the input power to the drive.
|Frequency of Harmonic Current
|60 Hz (fundamental)
Troubleshooting in any circuit means properly identifying the root cause of the problem and isolating the source. If routine preventative maintenance checks of your nonlinear loads do not reveal the problem, consider looking for harmonics.
To look for the presence of harmonics, start by measuring with a clamp meter that can indicate power quality issues, like the Fluke 378 FC Non-Contact Voltage True-rms Clamp Meter. This PQ issue can help indicate total harmonic distortion (THD) and show the total of the harmonics present.
- THD for voltage should not exceed 5-6% and can be easily read on a clamp meter.
- THD for current will run considerably higher.
Excessive THD for voltage means any number of problems could be occurring, and corrective action should be taken. To get more detailed information, you need to use a power quality analyzer to further investigate the magnitude and effects of the individual harmonics.
A power quality analyzer measures the level of each harmonic frequency as well as many other issues related to power quality. To measure for harmonics, the Fluke 1770 Series Power Quality Analyzer would be placed on the circuit for a period to record disturbances in the line power.
In addition to measuring harmonics, power quality analyzers record other disturbances that can cause the malfunction of control circuits. From swells and dips, to wasted energy and electrical disturbances, power quality analyzers are powerful tools that can help you solve many issues within your facility.
THD and harmonic levels should be measured at the point of common coupling (PCC). When troubleshooting, the PCC is the point at which the nonlinear loads suspected of causing the problem connect to the remainder of the distribution system.
For example, a quick check at the motor control center (MCC) cubicle supplying a VFD will indicate whether the VFD is creating a potential harmonic problem. Look for THD for voltage approaching 5% and check for the presence and the levels of different harmonic frequencies. Harmonic output from the VFD will vary as the VFD output varies, so it may be necessary to set up the power quality analyzer to record values for a period, because the system requirements vary.
What to do when you find excessive harmonics?
If you find excessive harmonics in your system, it’s best to look at each case individually before making decisions. In some cases, it may be best to purchase harmonic filters and place them as close as possible to the equipment producing the harmonic currents. It is best to consult the manufacturer of the equipment, or an outside engineering consultant to find the best harmonic filter for the problem. There is no “one size fits all” when it comes to harmonic filters. You must consider the size of the load and the harmonics being generated.
In other situations, it may be best to isolate the problem equipment using an isolation transformer. Relocating either the nonlinear load causing the problem, or the affected circuit to another distribution panel, may help clean up any harmonics. For example, if the affected controls are supplied from the same panelboard as the nonlinear load causing the problem, moving the control circuit to another panelboard may help alleviate the problem. Harmonic problems tend to diminish moving farther away from the nonlinear load.
Steps to isolate and troubleshoot harmonics
CAUTION: Troubleshooting requires work on live circuits; be sure to follow the requirements of NFPA 70E: Standard for Electrical Safety in the Workplace®.
- Check possible causes unrelated to harmonics
- Inputs and outputs to electronic controls
- Faulty relays, sensors, etc.
- Determine the possibility of harmonics in the distribution system by taking readings with an instrument that measures total harmonic distortion.
- Use a power quality analyzer to identify the harmonic frequencies present and their magnitudes. Record values over time at the feeder supplying the equipment.
- Verify that total harmonic distortion (THD) for voltage does not exceed 5%. This is the generally accepted maximum value and would indicate potential problems.
- Take corrective action.
- May include adding a specially purchased harmonic filter on the input power to minimize reflection of harmonic currents back into the system.
- May require relocating or isolating the problem equipment or load.