Electronic – Capacitor exploding in a paper plant


Two years ago I was in an internship in a paper plant in the Superintendence of Electrocontrol, in the area of maintenance of electrical equipment. Well, they had a problem regarding some capacitor banks that were installed to mantain the power factor at a certain level. The system was simple, it had multiple capacitor banks that were switched to change the reactive power and control the power factor, and they did have many switchings over a day. The problem they had is that, capacitors would not last long, they kind of exploded on a month basis, and this was expensive due to the maintenance and the need of more capacitors to replace the useless ones. I am kind of puzzled, because it was a 3-phase system, and the capacitors nominal voltage was higher than the voltage of the source they were connected (380 line voltage, and capacitors were approximately 600 V)

My question is, why did capacitors "explode" so quickly? They should have lasted at least 3 – 6 months (1 year expected). I was thinking of over-voltage due to the transients when switching and producing like a "chain-reaction", less capacitors => higher voltage transient. However there is long time since I have not solved a circuit problem, could someome explain it in more mathematical terms? Am I correct in my statement?

Best Answer

I would suspect harmonic distortion of the system voltage due to nonlinear loads. The capacitors are sized to carry current due to the complex impedance of the capacitors at the line frequency. If the actual voltage waveform has significant content of voltages at harmonic frequencies, that increases the capacitor current because the impedance is lower at those frequencies. In addition, harmonic distortion can precipitate a harmonic resonance situation further increasing the capacitor current.

The nonlinear loads can be electronic motor speed controls or other power electronic equipment on the three-phase distribution system or even electronic equipment on single-phase distribution systems that are fed from the three-phase systems.

Many large motors in paper plants are powered through electronic power conversion speed controls.