Tips for troubleshooting capacitor banks
When it comes to the most typical cause of poor power factor in a facility, motor inductance is a likely culprit. The problem worsens when motors are not loaded to their full capacity. Harmonic currents reflected back into the system also reduce power factor (What is Power Factor).
The good news is you can correct low power factor 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.
Power factor correction capacitors can reduce your energy costs by avoiding the premium rates electric utilities charge when your power factor falls below specified values. Facilities typically install these capacitors when their inductive loads cause power factor problems for their neighbors or the electric utility.
Under normal conditions, capacitors should operate trouble-free for many years. However, conditions such as harmonic currents, high ambient temperatures, and poor ventilation can cause premature failures in power factor correction capacitors and related circuitry. These failures can lead to substantial increases in energy expenses and — in extreme cases — create the potential for fires or explosion (Photo 1). Therefore, it’s critical to inspect power factor correction capacitors on a regular basis to ensure they’re working properly. In fact, most manufacturers recommend that preventive maintenance be performed twice a year.
These energy storage devices can deliver a lethal shock long after the power serving them has been disconnected. Although most capacitors are equipped with a discharge circuit, when the circuit fails, a shock hazard exists for an extended period of time. When testing is required with the voltage applied, you must use extreme caution. Capacitor bank maintenance requires training specific to the equipment, its application, and the task you are expected to perform. The proper personal protective equipment (PPE) per NFPA 70E is also 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.
Start by performing a complete visual inspection of the system. 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 points. Note: The control power transformer line-side breaker or fuses must be removed to prevent erroneous readings phase-to-phase.
The most valuable tool for evaluating capacitor banks is a thermal imager. The system should be energized for at least 1 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. Based on your arc flash assessment, wear the required PPE when performing tasks near energized equipment.
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°F above the ambient temperature. There should be little or no difference in temperature phase-to-phase or bank-to-bank at points of connection.
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; 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, load-fuse, or fuse clip points.
You should also look for differences in temperature of individual capacitors (Photo 2). 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 temperature of components might be higher in the upper sections due to convection. However, 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.
As part of your preventive maintenance procedures, take a current measurement on all three phases of each stage and record it using a multimeter and current clamp. 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 3,000A to 5A — and you measure 2A — the actual current is: (3,000A ÷ 5A) × 2A = 1,200A.
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.
Measuring power factor requires a meter that can simultaneously measure voltage, current, power, and demand over at least a 1-second period. A digital multimeter (DMM) cannot perform these measurements, but a power quality analyzer with a current clamp will measure all of these elements over time, helping you to build an accurate picture of the facility’s 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.
Before measuring capacitance, de-energize the capacitor bank and wait for the period specified in the manufacturer’s service bulletin. While wearing the proper PPE, confirm with a properly rated meter that no AC is present. Follow your facility’s lockout/tagout procedure. Using a DC meter rated for the voltage to be tested and set to 1,000VDC, test each stage — phase-to-phase and phase-to-ground. There should be no voltage present. 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. Although power factor correction capacitors are designed to provide years of service, the key is performing proper maintenance as recommended by the individual manufacturer.
Kennedy is a licensed electrical contractor and technical author for Fluke Corp., Everett, Wash. He also serves as an OSHA-authorized general industry outreach safety trainer and instructor/curriculum developer for Prairie State College, Lakeland College, Joliet Junior College, and the Indiana Safety Council.