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By Bennie Kennedy
Power factor correction capacitors reduce energy costs by avoiding the premium rates that utilities charge when power factor falls below specified values. Facilities typically install these capacitors when inductive loads cause power factor problems. Capacitor banks normally provide years of service, but they need to be inspected regularly to ensure they’re working properly. Problems such as loose connections, blown fuses or failing capacitors can reduce the amount of power correction available and, in extreme cases, even cause a total system failure or fire. Here is how to inspect power factor correction capacitors and avoid these problems.
“Even after the capacitor bank has been de-energized, there’s a danger of electrical shock from the CT wiring.”
Capacitors are energy storage devices that can deliver a lethal shock long after the power to them is disconnected. Most capacitors are equipped with a discharge circuit but, when the circuit fails, a shock hazard still exists for an extended period of time. When testing is required with the voltage applied, you must take extreme care. Capacitor bank maintenance requires training that’s specific to the equipment, its application and the task you’re expected to perform. In addition, the proper personal protective equipment (PPE) in accordance with NFPA 70E is required.
Additional hazards are involved in working with current transformer (CT) circuits, including the wiring and shorting block. The CT itself is normally located in the switchboard, not in the capacitor bank enclosure. Even after the capacitor bank has been de-energized, there’s a danger of electrical shock from the CT wiring. If the CT circuit is opened when there’s a load on the switchboard, the CT can develop a lethal voltage across its terminals.
Power factor is the ratio of true power, measured in kiloWatts (kW), to apparent power, measured in kiloVolt Amperes (kVA). The apparent power is the total requirement that a facility places upon the utility to deliver voltage and current, without regard to whether it does actual work. Utilities usually charge a higher rate when power factor falls below a certain level, often 90%.
True power (kW) / apparent power (kVA) = power factor
50 kW / 52 kVA = 0.96 (a good power factor of 96 %)
50 kW / 63 kVA = 0.79 (a poor power factor of 79 %)
Motor inductance is the most typical cause of poor power factor, and the problem increases if motors aren’t loaded to full capacity. Harmonic currents reflected back into the systems also reduce power factor.
Measuring power factor requires a meter that can measure voltage, current, power and demand simultaneously over at least a one-second period. A digital multimeter (DMM) can’t perform these measurements, but a power quality analyzer used with a current clamp can measure all of these elements over time and build an accurate picture of power consumption. A power logger, another type of power quality tool, can perform a 30-day load study to provide an even better understanding of power factor and other variables.
Low power factor can be corrected by adding power factor correction capacitors to the power distribution system. This is best accomplished via an automatic controller that switches capacitors, and sometimes reactors, on and off. The most basic applications use a fixed capacitor bank.
Under normal conditions, capacitors should operate trouble-free for many years. But conditions such as harmonic currents, high ambient temperatures and poor ventilation can cause premature failures in power correction capacitors and related circuitry. Failures can cause substantial increases in energy cost and, in extreme cases, produce the potential for fires or explosion, so it’s important to inspect power factor correction capacitors regularly to ensure they are working properly. Most manufacturers post the service bulletins on their websites. Their typical recommended preventive maintenance interval is twice annually.
The most valuable tool for evaluating capacitor banks is a thermal imager (Figure 1). The electrical distribution system should be operating for at least an hour before testing. To begin, check the controller display to determine if all the stages are connected. Next, verify that the cooling fans are operating properly. Conduct an infrared examination of the enclosure before opening the doors. And, based on your arc-flash assessment, wear the required personal protective equipment.
Figure 1. A thermal examination would have detected abnormal heating, and damage to a circuit breaker feeding a capacitor bank might have been avoided.
Examine the power and control wiring with the thermal imager, looking for loose connections. A thermal evaluation can identify a bad connection by showing a temperature increase caused by the additional resistance at the point of connection. A good connection should measure no more than 20 °F above the ambient temperature. There should be little or no difference in phase-to-phase or bank-to-bank temperatures at points of connection.
Figure 2. The difference in temperature indicates that the fuse on the left is blown.