Reactive Power

Major power system disturbances have, in one way or another, involved unstable oscillations of electrical quanti ties. Dynamic changes in power flows, or in system frequency (departures from 60 hertz), or in voltage reduction are all signs of system instability. Frequency excursions take place when the balance between supply and demand for power is upset. Too much demand produces a lower frequency, and too much supply results in a higher frequency. As the power system came apart in August 2003, there were islands with excess generation and islands with too little generation.

There is another kind of power in alternating current systems, associated with the magnetic fields produced by currents flowing in transmission lines, generators, and motors. This power is called reactive power and is measured in vars (for volt-ampere reactive).3 Reactive power represents energy stored in the magnetic field and later released. Motors such as those in air conditioners and refrigerators also require reactive power to function correctly.

Reactive power also is essential for the smooth operation of the transmission grid. It helps hold the voltage to desired levels. Inadequate reactive power leads to a decrease in the voltage of the system in which the shortage exists. For an interconnected system where active power is exactly in balance, the frequency is constant and the same everywhere, and the system is said to be in synchronous operation. Voltage, however, varies from location to location, depending largely on the reactive power balance. If a given load has a large reactive demand, the voltage will be lower at that point than at others. Low voltage can damage equipment and, if low enough, can cause system instability and a voltage collapse. There have been a few voltage collapses solely because of a shortage of reactive power. It is more common that reactive power problems aggravate active power problems in large power system disturbances, as was the case in the August 2003 event (U.S.-Canada Power System Outage Task Force, 2004).

Active power can be transmitted over great distances, while reactive power problems must be solved locally. Generators themselves are an excellent source of reactive power but at some cost. Increasing the reactive output of a generator results in a decrease in the possible active power output and, if not specifically compensated, a loss of income received for real power output. Capacitors can be a second source of reactive power by storing energy in electrostatic fields rather than electromagnetic fields. Capacitors can be fixed or variable in size. Distributed generators—for example, microturbines and synchronous motors—can also supply reactive power, but these units are outside the control of the system operator and cannot necessarily be counted on when needed.

Indian Point is a large supplier of reactive power to the grid in southeastern New York State, capable of providing about 1,000 megavars of reactive power. If it is shut down, that reactive power must be replaced. Insofar as replacement generation is located upstate or even farther away, it becomes even more important to ensure adequate supplies of reactive power. That could be done by installing capacitors at the Indian Point site or in the general area. Generating vars is not expensive, but it is a critical necessity that must be planned for if Indian Point is to be closed.

NYISO projects that, even with Indian Point operating, voltage constraints due to reactive power deficiencies in the Lower Hudson Valley will lower system reliability to unacceptable levels. Consequently, NYISO has solicited market-based and regulated backstop solutions to correct the reliability deficiency.4


DOE/EIA (Department of Energy/Energy Information Administration). 2005. “Natural Gas Weekly Update.” December 22. Available at http:/ / Accessed December 22, 2005.

EPRI (Electric Power Research Institute). 2005. “Making Billion Dollar Advanced Generation Investments in an Emissions-Limited World.” Background paper for the EPRI Summer Seminar, August 8-9, 2005, San Diego, Calif.

Hinkle, G., G. Jordan, and M. Sanford. 2005. “An Assessment of Alternatives to Indian Point for Meeting Energy Needs.” Unpublished report for the National Research Council, GE-Energy, Schenectady, N.Y., December 19.

NYISO (New York Independent System Operator). 2005a. Comprehensive Reliability Planning Process. October 25.

—. 2005b. Comprehensive Reliability Planning Process (CRPP), Reliability Needs Assessment, and NYISO Comprehensive Reliability Planning Process, Supporting Document and Appendices for the Draft Reliability Needs Assessment. December 21.

U.S.-Canada Power System Outage Task Force. 2004. Final Report on the August 14, 2003 Blackout in the United States and Canada. April. Available at Accessed March 2006.


Active power, the familiar type of power that keeps lightbulbs burning, is measured in watts. Consumers pay for active power (1,000 watts used for an hour is a kilowatt-hour) but usually not for reactive power.


See M. Calimano, NYISO solicitation letter to S.V. Lant, R.M. Kessel, E.R. McGrath, and J. McMahon, December 22, 2005.

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