Tech Tip 18
Power Factor Correction and Capacitor Issues
This Tech Tip briefly describes issues related to the installation and operation of power factor correction capacitors. If you have questions regarding the issues presented here or for further assistance, please contact Power Quality.
Power factor is the ratio of the real power (kW) to the apparent power (kVA) that a system draws. Power factor is an indication of the inefficiency of a load or loads. Many utilities charge an additional amount for low power factor to compensate for the additional system losses and generating capacity. By adding a source of reactive power (kVAR), such as capacitors, near the load(s), power factor can be improved and additional charges reduced or eliminated. Other benefits include voltage improvement and more available system capacity. When it is decided for financial or other reasons that power factor correction capacitors may be beneficial, the following issues should be addressed.
Overvoltage is the predominant cause of capacitor failure. Capacitors should not be operated at more than 110% of their rated terminal voltage. Lack of ventilation or placement with exposure to heat or sun will shorten capacitor life. The NEC requires certain discharge characteristics.
Harmonics are a measure of the distortion in a current or voltage waveshape. Non-linear loads such as adjustable speed drives and electronics require current that is heavy in harmonics. This current causes distortion in the voltage waveshape. Capacitors form parallel resonant circuits with system impedance. If the resonant frequency is at the same frequency as the harmonic frequency, increased voltage distortion, damaged capacitors and blown capacitor fuses can result. Solutions to harmonics problems include installing the capacitors as a filter, sizing the capacitors to avoid harmonic frequencies or relocating the capacitors.
When capacitors are switched on, they will tend to raise the voltage level at the bus they are on. When there is little load on, such as overnight or on a weekend, this can lead to voltages outside the acceptable tolerance of connected equipment. Calculate the voltage rise due to capacitors on a bus on the low side of a transformer as follows:
If this calculated voltage rise added to the highest voltage is unacceptable, consideration should be given to either switching the capacitors off manually when the load goes down or installing the capacitors with automatic switching control.
Capacitors can be located in one place such as at the main bus or distributed on distribution buses or at loads such as large motors. Placing capacitors at large motors can provide two benefits: they will come on when they are required and losses in the feed to the motor will be reduced. Care must be taken when applying capacitors at motors, as harmonic issues become much more difficult to address with distributed capacitors. There are several types of motor applications where capacitors should definitely not be installed.
Capacitor switching transients
A capacitor creates a momentary short circuit at the instant it is energized. Current surges into the capacitor and line voltage collapses for a few millionths of a second. As the current surge declines, it causes a ringing transient with overshoot on the peak voltage. This can create several problems with connected equipment. The overshoot can cause adjustable speed drives to trip out. Capacitors on the low side of a transformer can amplify the transient caused by a capacitor switching on the high side causing more frequent trips or equipment damage. The switching transient can also cause a surge in neutral current level. This may cause ground fault circuit interrupters to trip. Care must be taken in location and sizing of capacitors to head off these potential problems.
The issues presented here are easier and less expensive to deal with if addressed prior to installation of power factor correction capacitors. The following references contain more detailed information about the topics discussed here.
- IEEE Red Book Chapter 8
- IEEE Standard 519
- Power Clinic Tech Tips 2, 5, 7, 10 & 11
The information and diagrams presented herein are for general educational purposes only, and should not be relied upon as instructions for customer self-wiring. Customers should at all times seek the assistance of qualified electricians or utility personnel for all wiring projects.