Troubleshooting power factor correction capacitors

Power quality

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 on a regular basis to make sure they are 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 a fire. This article describes how to inspect power factor correction capacitors and avoid these problems.

Safety first!

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 will exist for an extended period of time. When testing is required with the voltage applied, you must take extreme care. Capacitor bank maintenance requires training specific to the equipment, its application, and the task you are expected to perform. In addition, the proper personal protective equipment (PPE) per 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 is a danger of electrical shock from the CT wiring. If the CT circuit is opened when there is a load on the switchboard, the CT can develop a lethal voltage across its terminals.

What is power factor?

Power factor is defined as the percentage ratio between the true power, measured in kilowatts (kW), and 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 or not 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 / 52KVA = .96 (a good power factor of 96%)
50 KW / 63 KVA = .79 (a poor power factor of 79%)

Motor inductance is the most typical cause of poor power factor, and the problem only increases when motors are not loaded to their full capacity. Harmonic currents reflected back into the systems also reduce power factor.

Measuring power factor requires a meter that can simultaneously measure voltage, current, power and demand over at least a one-second period. A digital multimeter cannot perform these measurements, but a power quality analyzer such as the Fluke 43B used with a current clamp will 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 parameters, over time.

Low power factor can be corrected by adding power factor correction capacitors to the facility's 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 expenses, and in extreme cases create the potential for fires or explosion. So, it's important to inspect power factor correction capacitors on a regular basis to ensure they are working properly. Most manufacturers post the service bulletins on their web sites. Their typical recommended preventative maintenance interval is twice annually.

Inspection with infrared imager

The most valuable tool for evaluating capacitor banks is a thermal imager. The system should be energized for at least an hour prior to 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 prior to opening the doors. And, based on your arc-flash assessment, wear the required personal protective equipment.

Damage to circuit breaker feeding a capacitor bank. A thermal examination would have detected abnormal heating.

Examine power and control wiring with the thermal imager, looking for loose connections. A thermal evaluation will identify a bad connection by showing a temperature increase due to the additional resistance at the point of connection. A good connection should measure no more than 20 degrees above the ambient temperature. There should be little or no difference in temperature phase-to-phase or bank-to-bank at points of connection.

The difference in temperature indicates that the fuse on the left is blown.
This infrared image indicates that a capacitor has failed.

An infrared evaluation will detect a blown fuse by highlighting temperature differences between blown and intact fuses. A blown fuse in a capacitor bank stage reduces the amount of correction available. Some units are equipped with blown fuse indicators but others are not. If you find a blown fuse, shut down the entire bank and determine what caused the fuse to blow. Some common causes are bad capacitors, reactor problems; and bad connections at line fuse connections, load fuse connections, or fuse clips.

Look for differences in the temperatures of individual capacitors. If a capacitor is not called for or connected at the time of examination then it should be cooler. Also, keep in mind that the temperatures of components might be higher in the upper sections due to convection. But if, according to the controller, all stages are connected, then temperature differences usually indicate a problem. For example, high pressure may cause the capacitor's internal pressure interrupter to operate before the external fuse, thus removing the capacitor from the circuit without warning.

Current measurements

As part of preventative maintenance, a current measurement on all three phases of each stage should be taken and recorded using a multimeter and a current clamp. Also use the multimeter to measure the current input to the controller from the current transformer in the switchboard, using a current clamp around the CT secondary conductor. A calculation is required to convert the measured current value to the actual current flowing through the switchboard. If the current transformer is rated 3000 A to 5 A, and you measure 2 A, the actual current is . In addition, measure the current through the breaker feeding the capacitor bank for phase imbalance, with all stages connected. Maintain a log of all readings, to provide a benchmark for readings taken at a later date.

Capacitance measurements

Before measuring capacitance, de-energize the capacitor bank and wait for the period specified in the manufacturer's service bulletin. While wearing the proper personal protective equipment, confirm with a properly rated meter there is no ac present. Follow your facility's lockout/tagout procedure. Using a dc meter rated for the voltage to be tested and set to 1000 V dc, test each stage phase-to-phase and phase-to-ground. There should be no voltage. The presence of voltage indicates the capacitor may not be discharged. If no voltage is detected, measure capacitance with the meter and compare the reading to the manufacturer's specifications for each stage.

Visual inspection and cleaning

Also perform a complete visual inspection. Look for discolored components, bulging and/or leaking capacitors, and signs of heating and/or moisture. Clean and/or replace filters for cooling fans. Clean the units using a vacuum - never use compressed air. Prior to re-energizing the capacitors, perform an insulation integrity test from the bus phase-to-phase and phase-to-ground. The control power transformer line side breaker or fuses must be removed to prevent erroneous readings phase-to-phase. Power factor correction capacitors are designed to provide years of service when properly maintained in accordance with the manufacturer's instructions. Inspecting capacitor banks on a regular basis provides assurance that they are operating safely while delivering the anticipated energy cost savings.