A water district pump station was supplied with 480 vac, 3 phase power. The system included a variable frequency drive and a back up generator in the event of a grid down situation, or a system trip due to “an imbalance in the phase to phase current that exceeded +/- 3%.” (reference: IEEE STD 519). Frequent trips of the system requiring the back up generator to take over, and a visit by maintenance personnel to reconnect the grid, caused an investigation to be initiated by the owner.
In 2007 GES Tech Group conducted an on site investigation using sensors designed to detect wave forms and a Fast Fourier Transform (FFT) spectrum analyzer to evaluate the harmonics in the supplied power.
A perfect sine waveform and typical power spectrum, with no harmonics, was first generated by a signal generator in order to calibrate our equipment.
A known clean power source was then tested to establish a base line.
The three phases of the grid 480 vac system were then tested for wave forms and power spectrum with and without the VFD on line in order to isolate the root cause of the power quality trips. Finally, the genset was tested to confirm the root cause of the trip behavior was the extremely poor quality of grid power. The utility power provider was contacted and given a copy of the GES Tech Group findings.
