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Voltage Regulation: Tapping Distributed Energy Resources

Voltage Regulation
Fortnightly Magazine - September 2004

reactive reserves available from generators, synchronous condensers, static var compensators, and other inverter-based devices are the most effective and reliable means to prevent voltage collapse due to unanticipated system contingencies (line loss, generator loss, etc.).

Today, most spinning reserve is provided from a few high-operating-cost machines. Spin is not evenly distributed across a control area. Reactive reserves are available from these same machines because they are operating in a "backed off" mode and have high reactive power levels available. Thus the reactive reserves are not distributed evenly. With an unequal pick up of reserves around an area, there is an impact on power flow. When planning reactive reserves for contingency cases, these "lumpy" reactive reserves can cause problems. Flow paths have to be held open to provide enough flow capacity, and some paths actually have to be de-rated. This, in turn, has caused the curtailment of operating units. One solution is to build more transmission. This will take 5 to 10 years in many cases. A better solution is to provide reactive power from DER units that are evenly spread across the control area.

Who Could Provide Voltage Regulation?

Utility-owned DER would be possible in areas where there is no market for local generation. Ancillary service or regulated resource contracts could reimburse small generators for reactive power produced during heavy load periods or absorbed during light load conditions. In fact, some utility contracts already provide financial inducements for power factor correction. This concept would simply tighten the band because the capability to provide real-time sensing and control is available, and the incentive signal would increase the availability of power factor correction.

Instead of switching capacitors in and out of the grid, there is a way to generate controllable reactive power directly by switching power converters. These converters are operated as voltage or current sources and produce reactive power without energy storage components by switching alternating current among the phases of the AC system. Their operation is similar to that of an ideal synchronous machine whose reactive power output is varied by excitation control.4 If they are supplied with an energy source, they can also supply active power to the AC system. These converters are often called static synchronous generators (SSGs) when supplied with an energy source and static synchronous compensators (SSCs) when operated without an energy source.

These compensators are capable of providing both direct voltage support and transient and dynamic stability improvements to increase stability margin and provide power oscillation damping. Regulation of the voltage at intermediate points and selected loads can limit voltage variation significantly, increase the capability to transport active power, prevent voltage collapse, increase transient stability limits, and even provide power system oscillation damping as well as reducing energy losses.

As the cost of the power electronics switches in converters decreases with technology improvements and the devices become mass-produced, it is likely that their use in distribution systems and to interface small power generators will become ubiquitous.

Fast-Acting Voltage Regulation: An Answer

There is a possibility that the voltage regulators on generators could be controlled much more quickly