The primary objective of the Integrated Volt/VAR Control (IVVC) function is to reduce electric feeder losses while minimizing distribution voltage within acceptable operating limits.
The controls used to achieve these objectives are transformer Load Tap Changers (LTCs), substation and feeder capacitor bank controls plus substation and feeder voltage regulators.
Inherently the IVVC function improves energy conservation by reducing load demand in both peak and non-peak periods of operation of the distribution system. The load demand reduction is achieved through minimizing the power loss while maintaining voltage as low as possible without violating distribution voltage constraints. IVVC attains power loss reduction by setting transformer taps and by controlling capacitor banks while feeder voltages are kept above the low limit through a coordinated adjustment of voltage regulators. In order to determine an optimum control strategy without adjusting and readjusting the controls, which interact with each other, a real time three-phase unbalanced distribution Load Flow is used. The Load Flow is used to determine reactive power requirements at various capacitor bank locations as well as for the entire feeder under different iterations of the scenario.
In the process of capacitor bank control, individually operable capacitors on the feeder are identified by topology tracing from a feeder breaker downstream. Feeder loads are estimated to calculate voltage, branch flows, and power factors. The branch flows at capacitor locations are analyzed so that the capacitor banks are sorted in descending order based on their branch reactive power flows. The capacitor with the largest branch reactive power is selected as a control candidate. Its impact on feeder voltages is calculated and checked against the limits. If any constraint is violated, this capacitor bank will be passed over and the next capacitor is processed. Otherwise, a control command is issued to operate this capacitor bank. For example a capacitor needs to be operated if there exists a significant amount of lagging reactive power flow on the feeder breaker. Likewise, it needs to be turned off if there is a large amount of leading reactive power flow on the feeder breaker. The decision can be made through a series of load flow calculations. Finally, to verify if a given capacitor operation violates any voltage constraints, the changes in voltage and power factor are calculated considering the effect of the capacitor operation.
To prevent unbalance switching of a capacitor bank, a verification procedure is followed to check if any switch operation has failed. If so, this capacitor bank will be disabled for future control and an alarm is issued to notify the dispatcher. The above process is repeated until distribution loss is minimized or no capacitor is available for control. The disabled capacitor bank can be re-enabled by the dispatcher once it has been repaired and is ready for use. Defined rules are applied to determine the control action for each capacitor bank, considering maximum number of control operations, minimum on, or minimum off times and an adjustable dead band to prevent unnecessary controls.