The corporate office sent a consultant to your plant to analyze energy waste and make recommendations. The plant engineer called you into his office to view and discuss the consultant’s report.
The crux of the report was that the plant is wasting energy due to harmonics. The report links to a Dropbox account with video after video of harmonics readings and they all look pretty scary. The report explains that the harmonics are all wasted energy and they actually reduce the life of transformers, conductors, and utilization equipment, with motors in particular being affected.
The report says the plant has circulating ground currents full of harmonics, and the solution is to install an isolated ground. The plant engineer asks for your thoughts on this.
The presence of harmonics is not in itself a problem. Keep in mind that with each order of harmonic, you have a corresponding decrease in magnitude. Thus, the third harmonic is one-third the magnitude of the fundamental and so on.
The phrase “circulating ground current” is often misused, but the remedy is not what the consultant recommends. You want to bond all non-current-carrying metallic objects per Art. 250, Part V and ensure your equipment grounding conductor (EGC) fully complies with Part VI of Art. 250.
Start with motor drives (VFDs). Look at any that lack harmonics correction and consider replacing them with ones that have it. The major sources of harmonics will typically be switching power supplies. If your plant has LED lighting, guess what? Switching power supplies! Filters (typically capacitor-based) can direct the harmonic currents to a dedicated “ground” conductor that connects at the grounding connection at the source.
