By using a combination of fixed (left) and switched (right) capacitor banks, reactive power compensation will better track load and minimize losses.

feeder voltage profile.

A number of utilities have adopted the “two-thirds rule,” or some variant, for distribution capacitor placement. It calls for installing a quantity of capacitive volt-amperes reactive (VARs) equal to two-thirds of the total feeder peak inductive VARs at a distance of two-thirds of the overall feeder length from the substation. The rule works best for a feeder of constant load that is uniformly distributed along the feeder’s length. These two conditions are more theoretical than realistic and are mostly found in textbooks and technical papers.

Feeder reactive power varies with load throughout the day and throughout the year. If reactive power compensation were only supplied with fixed capacitor banks, it would likely result in over-com-pensation (too much capacitive current) during light feeder loading and under-compensation (not enough capacitive current) during peak load. This increases total current—leading to increased I²R loss—and possible over-voltage ( steady-state capacitive current raises voltage over conductor inductance) during light

feeder loading.

Placing fixed capacitors according to the two-thirds rule (or a similar variation) can be performed relatively quickly with minimal engineering time and will likely produce immediate power and energy savings. However, it does not meet full reactive power compensation (i.e. unity power factor where real power current equals total current) to produce the least feeder I²R losses at peak. By using a combination of fixed and switched capacitor banks, reactive power compensation will better track the load and minimize losses. This has proven to be economical and well worth the extra effort.

To save the most energy annually through capacitive reactive power compensation, the amount of fixed capacitance (VARs) should approximately equal the feeder’s reactive power requirements at minimum annual load. Switched capacitor banks should then be added to the fixed capacitor banks on the feeder until the total peak feeder reactive power requirements are met. Modeling distribution feeders in a power flow computer program will yield the

most economical fixed and switched capacitor placement. Without the luxury of power flow modeling, capacitor bank placement can be determined by placing capacitors near concentrated or lumped feeder loads using an 85 percent load power factor approximation. It is worth noting that capacitor bank current can flow both downstream and upstream of the bank itself—ideally half going each way. Hence, capacitors should rarely be placed close to the feeder breaker unless a fair amount of load is concentrated there. Likewise, it is best not to place capacitors at the end of distribution feeders unless load or voltage needs dictate it. Low-voltage concerns during contingency backfeeding may also call for capacitors near the feeder breaker.

Capacitors may be switched according to a variety of factors, so selecting a capacitor switching method warrants a more detailed discussion. Switching determination methods (i.e. trip or close decisions) include: VAR requirements, voltage, ambient temperature and time-of-day. Switching on reactive power requirements with voltage override is best for reducing power losses while maintaining proper operating voltage. The simplest way to switch by VAR requirements is installing a local switch controller that uses a single-phase voltage transformer and a single-phase current transformer (CT) to determine the power factor and current magnitude immediately downstream of the capacitor bank and closes it on adjustable settings. The same local capacitor controller can house voltage override controls to close the capacitor bank for low voltage and trip it for high voltage.

Unfortunately, it can be difficult and time-consuming to evaluate whether local capacitor bank switch controllers are working properly after several years of service. Receiving a graph of a feeder’s VARs (reactive power) over time should show capacitors switching on and off as