Voltage Support

A requirement for electric grid operators is to maintain voltage within specified limits. In most cases, this requires management of reactance, which is caused by grid-connected equipment that generates, transmits, or uses electricity and often has or exhibits characteristics like those of inductors and capacitors in an electric circuit. To manage reactance at the grid level, system operators need voltage support resources to offset reactive effects so that the transmission system can be operated in a stable manner.

Normally, designated power plants are used to generate reactive power (VAR) to offset reactance in the grid. These power plants could be displaced by strategically placed energy storage within the grid at central locations or taking the distributed approach and placing multiple VAR-support storage systems near large loads.

Technical Considerations

Storage System Size Range: 1 – 10 mega volt-ampere reactive (MVAR)
Target Discharge Duration Range: Not Applicable
Minimum Cycles/Year: Not Applicable

The PCS of the storage systems used for voltage support must be capable of operating at a non- unity power factor, to source and sink reactive power or volt-ampere reactive (VARs). This capability is available in all PCSs used in today’s storage systems. Real power is not needed from the battery in this mode of operation and thus discharge duration and minimum cycles per year are not relevant in this case.

The nominal time needed for voltage support is assumed to be 30 minutes — time for the grid system to stabilize and, if necessary, to begin orderly load shedding to match available generation. Figure 7 shows three discharges of storage: with active injection of real power and VARs, with absorbing power to balance voltage while providing VARs, and providing VARs only without real power injection or absorption as needed by the grid.