Power Factor No Longer a Factor for Pipe Manufacturer
An East Texas pipe manufacturer recently experienced some unexpected consequences from its modernization efforts. Managers at the company’s fabrication plant, which flattens steel ingots to pipe-wall thickness, had replaced its old single-speed motor-generator sets with variable-speed drive models. The move meant greater control over the massive rolling mills, but it also resulted in current surges when ingots entered one of the large machines. The surges, in turn, caused voltage sags of up to 8% on the 13.8-kV bus of the plant’s motor room.
In addition to forcing nuisance tripping of other motors connected to the affected bus, these conditions also resulted in poor power-factor conditions for the entire facility. With a peak reactive power demand of almost 11 MVAR, VAR compensation was required.
Several options were considered, including fixed capacitors, an inverter-based compensator and a capacitor-based compensator. Fixed capacitors were seen as a less costly but also less complete solution, because of ongoing losses associated with capacitors always being in the circuit. In addition, these units weren’t seen as capable of handling large reactive-power swings associated with the range of mill motors operating. Managers realized inverter-based compensators could handle these swings, but thought them too expensive for their needs.
A 5,000-kVAR capacitor-based compensator was the chosen option. The unit offered a 2-1/2-year payback, compared to the assumed 6-1/2-year payback of the inverter-based solution. In addition, the selected unit was more compact, allowing for closer installation to the affected mill.
The compensator was installed in January 2000 and has maintained voltage variation within +/-3.2% of nominal. Power factor is now typically above 92%, and total voltage and current harmonic distortion are now less than 10% and 5%, respectively.