This Fluke article originally appeared in the March edition of Plant Engineering.
Several options exist to mitigate harmonic problems in plants. Unfortunately, there is no one solution that fits all applications. Finding and choosing the correct option for your needs is the key. Otherwise, failure to reduce these unwanted electrical currents can result in problems ranging from overheating neutral conductors and transformers to inefficient operation of electronic equipment. Spurious alarms on VFD's, PLC problems and computer lock-ups are common issues resulting from excessive harmonics in a distribution system.
To determine the best solution for mitigating unwanted harmonics, start by investigating the equipment suspected of producing the bulk of the harmonic currents. Most often these are Variable Frequency Drives (VFD's). Use measuring equipment to measure and analyze the frequencies and amplitudes of the harmonics. This is much simpler than it sounds. Power quality analyzers and recorders measure and record these values, calculate the distortion of the original 60 Hz sine wave created by the harmonics, and then allow for a data download to a PC for further analysis. The key parameter calculated by the analysis equipment is Total Harmonic Distortion (THD). THD is calculated for both voltage and current. Once the measurement process is complete, the VFD manufacturer will help bring about a solution.
What is the best way, then, to go about measuring the needed values? Simply measuring the amount of THD at one point in time and then purchasing a filter to mitigate these harmonic currents is probably not the best solution. The reason: the frequency and magnitudes of the harmonic currents being created by the VFD will typically vary as the load changes on the drive. For example, a certain VFD may create many harmonics of relatively low magnitude when lightly loaded, but produce more prominent harmonic frequencies under heavy load. If harmonic data is recorded only once for a relatively short duration of time, chances are you will miss the data you need.
Filters, chokes and other devices used to mitigate harmonics are designed for certain harmonic and operational conditions. Selecting a filter to mitigate harmonics generated while the VFD is lightly loaded would most likely result in purchasing the wrong harmonic filter. You may find your most severe problems are generated at other times during plant operation.
If you want to increase mitigate harmonic issues, follow these steps: