Power harmonic problems at a plastics extrusion plant

A plastics extrusion plant in Massachusetts, USA, operated a 250 hp thyristor DC adjustable speed drive from a 300 kVA 13.8 kV to 480 V utility service transformer on two miles of 13.8 kV line. The operation was marginal. Tripping of the drive occurred on undervoltage dips. To raise the power factor and reduce the voltage drop, the plant personnel installed 300 kVAr of capacitors on the 480 V system. The installation was made without a power harmonic study. The result was frequent tripping of the drives and noisy operation. It was apparent that resonance occurred between the capacitors and the source impedance. To gain insight into the system operation, a PSpice computer simulation was performed on the plant electrical system. It was determined that the modified power system, with the capacitors installed, had a resonance frequency at the 5th harmonic. This resonance condition caused severe problems of harmonic voltage distortion, aggravated voltage drop, and caused the DC drives to trip. The PSpice computer simulation was also performed using power harmonic filters, consisting of tuning reactors in series with the capacitors.