Other Related Uses

Energy storage used for transmission support improves the transmission system performance by compensating for electrical anomalies and disturbances such as voltage sag, unstable voltage, and sub-synchronous resonance. The result is a more stable system. It is similar to the network stability ancillary service that is not addressed in this course. Benefits from transmission support are highly situation-specific and site-specific. Two cases are briefly described:

Transmission Stability Damping: Increase load-carrying capacity by improving dynamic stability.

Sub-synchronous Resonance Damping: Increase line capacity by allowing higher levels of series compensation by providing active real and/or reactive power modulation at sub- synchronous resonance modal frequencies.

Technical Considerations

Storage System Size Range: 10 – 100 MW
Target Discharge Duration Range: 5 seconds – 2 hours
Minimum Cycles/Year: 20 – 100

Energy storage must be capable of sub-second response, partial state-of-charge operation, and many charge-discharge cycles. For storage to be most beneficial as a transmission support resource, it should provide both real and reactive power. Typical discharge durations for transmission support are between one and 20 seconds.

Figure 13 shows two plots that illustrate the storage response to momentary voltage sag and a deviation in the phase angle that persists for a few seconds, as shown in the upper plot. The storage response is a quick discharge and recharge to damp the oscillation caused by the voltage sag and phase angle deviation. As shown in the lower plot, the storage response needs to be very fast and requires high power but lower energy capacity.