Capacitor failure analysis: a troubleshooting case study

Adding power factor correction capacitors provides well-known benefits to industrial plants. These benefits include power factor correction, voltage support, and release of system capacity. Engineering judgment must be used, however, when applying capacitors in power systems with excessive harmonics and/or transients. Capacitors might not survive long in such environments if they are improperly applied. Capacitors are also known to cause power quality problems, such as oscillatory switching transients, and can possibly amplify other problems, such as harmonics. The same piece of equipment that solves some problems can cause other problems. Adjustable speed drives (ASDs) are another example of such equipment. This paper investigates capacitor failures and fuse operations in an automatically switched capacitor bank in an industrial facility. The fuses that cleared were protecting individual capacitor steps in the bank. It was initially believed that harmonics were the source of the problem. The investigation determined that transients from an unlikely source were to blame.