Utility Capacitor Bank Studies

By: Tom Grebe, Principal Consultant



Recent and upcoming power plant retirements are resulting in a need to reinforce our transmission systems. One measure being adopted is to install additional reactive power in the form of capacitor banks that often require detailed harmonic or transient simulation studies to assure a successful design.

The analysis of utility transmission capacitor banks often includes measurements and computer simulation for studying application considerations that include capacitor bank configurations, insulation withstand levels, switchgear capabilities, grounding, overcurrent protection, overvoltage protection, energy duties of protective devices, and unbalance detection. In addition, harmonic and transient concerns for the utility and their customers should be evaluated.

The capacitor bank energizing transient is important because it is one of the most frequent utility switching operations and it can produce high phase-to-phase transformer overvoltages, excite circuit resonances, or cause problems with sensitive customer equipment. The frequent switching of utility capacitor banks, coupled with the increasing sensitivity of customer equipment has led to a heightened awareness of several important power quality concerns which include magnification of capacitor bank switching transients and nuisance tripping of power electronic-based customer loads.

The most common methods for controlling utility capacitor switching transients include switching control (e.g., synchronous closing, pre-insertion inductors/resistors), fixed series inductances, and MOV surge arresters. The application of transmission and distribution system capacitor banks has long been accepted as a necessary step in the design and operation of electric power systems. Design considerations often include traditional factors such as voltage support, power factor, and released capacity. However, as customer systems evolve through the use of power electronics, future system designs should also consider power quality.

EnerNex’s utility capacitor bank study includes a transient analysis of overvoltages and overcurrents during capacitor bank energizing and capacitor bank switch restrike events. The primary issues evaluated during the study include:

  • Evaluation of transient overvoltage magnitudes for normal capacitor bank energizing operations, including the effects of other capacitor banks and system loads.
  • Evaluation of the effectiveness (control of energizing transients) of various transient control methods (e.g., pre-insertion inductors/resistors, synchronous closing control, etc.).
  • Evaluation of arrester duties during capacitor bank restrike conditions.
  • Evaluation of inrush currents for normal and back-to-back switching operations.
  • Evaluation of outrush currents for nearby fault conditions.
  • Evaluation of capacitor switching transients on lower voltage systems:

-Magnified transients at lower voltage distribution buses and within customer facilities.

-Nuisance tripping of adjustable-speed drives.

  • Evaluation of system frequency response characteristics (resonances).

The study includes recommended equipment ratings and requirements for protecting against excessive transients for the power system and also presents guidelines for applying transmission capacitor banks with respect to overvoltage mitigation. The study includes the following equipment recommendations:

  • Options for current limiting reactors for controlling inrush and outrush currents.
  • Requirements for capacitor switching devices, including the effectiveness of transient overvoltage mitigation techniques (i.e., synchronous closing control, pre-insertion resistors/inductors).
  • Arrester requirements associated with a capacitor bank restrike event.
  • Requirements to protect against excessive transients at lower voltage capacitor banks, including capacitor switching controls, surge arresters, reactors, or capacitor size limitations.
  • Requirements to protect against excessive transients at low voltage adjustable-speed drives, including capacitor switching controls, and reactors.
  • Guidelines for developing a standard capacitor bank design with respect to overvoltage mitigation, reactor requirements, and surge arrester applications.