Voltage Regulation: Tapping Distributed Energy Resources
to deliver the services provided by the rapid control of static var compensators (SVC).5 The SVC behaves like a shunt-connected variable reactance and generally is used for transmission voltage regulation. It can either produce or absorb reactive power to regulate voltage. It is basically a capacitor bank in parallel with a thyristor controlled reactor. The strength of the SVC is that it is very fast acting and can dampen power system oscillations. Thus, the transmission voltage is directly regulated at high speed. The droop (slope) setting of a SVC is usually small compared to generators regulating terminal voltage. This means the SVC will respond much more quickly, including responding to transients while the generator responds more slowly, usually based on a voltage schedule.
We suggest that the DER's voltage regulator (or switching converter) be controlled to provide services such as control of the customer's voltage, assistance in regulation of distribution voltage, and providing unity power factor both in the distribution system and transmission system. The most important service may be the increase in margin of reactive power.
To meet the rising demand for reactive power, American Superconductor has developed a superconducting synchronous condensor with an 8 MVAR continuous rating and a 64 MVAR short-term rating. It is designed to provide steady state VAR support while maintaining this large reserve for transient problems. One is being installed near a steel mill in Gallatin, Tenn., and if it works well, Tennessee Valley Authority (TVA) has plans to install five more soon. American Superconductor also has plans to develop much larger devices.
DER generally will increase voltage along a feeder, but the impact depends on its active and reactive power and the feeder loading. The DER easily can be controlled to help regulate voltage. If voltage increases when DER active power increases, decreasing reactive power will typically cause voltages to drop. In some cases, reactive power control alone may not be adequate to control voltage. In these cases, the voltage would need to be controlled by a voltage regulator. Present-day voltage regulators sometimes make decisions as to the optimum tap setting based on the sensed voltage, the secondary current, the secondary voltage and the line resistance and inductance.
When there is a mix of voltage control devices such as capacitor banks, voltage regulators and DER control, the local control decision making will become more complicated, but not excessive. A local intelligent agent would be ideal for sensing local conditions and making decisions regarding tap settings, DER reactive power output, and capacitor bank connections. Central dispatch would still control the voltage and power factor of the central generators, and would provide the local intelligent agent with instructions such as the voltage schedule to be maintained.
Devices using power electronic interfaces will simplify coordination greatly among the conventional distribution equipment and DER. Response to some types of transients, such as voltage sags, will have to be quite fast. Power electronics devices are fast enough to distinguish between a large non-linear load or motor start and an actual sag caused by short circuit current flowing into a fault.