In electric power transmission and distribution, volt-ampere reactive (var) is a unit by which reactive power is expressed in an AC electric power system. Reactive power exists in an AC circuit when the current and voltage are not in phase. The correct symbol is var and not Var, VAr, or VAR, but all three terms are widely used, and VAR is widely used throughout the power industry. The term var was proposed by the Romanian electrical engineer Constantin Budeanu and introduced in 1930 by the IEC in Stockholm, which has adopted it as the unit for reactive power.
Special instruments called varmeters are available to measure the reactive power in a circuit.
Vars may be considered as either the imaginary part of apparent power or the power flowing into a reactive load, where voltage and current are specified in volts and amperes. The two definitions are equivalent.
The unit "var" does not follow the recommended practice of the International System of Units (SI) because the unit var is representative of a form of power, and SI practice is not to include information about the type of power being measured in the unit name. The SI unit of power is the watt, which is numerically equivalent to the var.
A sinusoidally alternating voltage applied to a purely resistive load results in an alternating current that is fully in phase with the voltage. However, in many applications it is common for there to be a reactive component to the system, that is, the system possesses capacitance, inductance, or both. These electrical properties cause the current to change phase with respect to the voltage: capacitance tending the current to lead the voltage in phase, and inductance to lag it.
For sinusoid currents and voltages at the same frequency, reactive power in vars is the product of the RMS voltage and current, or the apparent power, multiplied by the sine of (phase angle between the voltage and the current). The reactive power (measured in units of volt-amperes reactive or var) is given by:
where is the phase angle between the current and voltage. refers to the maximum value of the instantaneous power absorbed by the reactive component of the load.
Only effective power, the actual power delivered to or consumed by the load, is expressed in watts. The imaginary part is properly expressed in volt-amperes reactive.
Physical significance of reactive power
In power transmission, since loads such as motors are inductive, reactive power is present in the system. Since reactive power does not do any real work, the extra current supplied to provide the reactive power means greater line losses and higher thermal limits for equipment which translates to higher cost to operators which is why industrial users are charged extra if they have a low power factor, the ratio between real power and apparent power in the circuit.
Managing the reactive power flow in addition to real power flow becomes a very important task for operators to ensure voltage stability throughout the system. In general terms, decreasing a supply of reactive power to the system causes voltage to fall while increasing it causes voltage to rise. A voltage collapse occurs when the system serves a transient load that has a higher reactive power demand than the system can supply.
Practical significance of reactive power
Reactive power is required for electrical components that make use of an alternating magnetic field, primarily motors and transformers. In addition, many switching power supplies in computers and TVs draw current only during a part of the cycle, thus creating a net reactive load. Similar remarks apply for mercury-vapor lamps (but not, of course, incandescent lamps). There are no common household or industrial devices that present a capacitive load.
Reactive power is required to maintain voltage on motors and transformers, and hence, on the power system. A modern utility grid issue with reactive power is that many solar generators (with DC-AC inverters) and wind turbines (with induction generators) may generate no reactive power. Conventional central station power plants can generate reactive power, but fuel is not required to generate it. In fact, reactive power can be generated with passive capacitors, common on distribution systems. Advanced DC-AC inverters on solar plants can be set to generate or absorb reactive power as needed for voltage control. In some cases, old generators are used only to provide reactive power; these units, which no longer burn any fuel, are called synchronous condensers. Other sources of reactive power at the bulk level include the static var compensator, static synchronous compensator and the dynamic var compensator: the first being essentially a thyristor-controlled bank of capacitors, the last a more complex high-power electronic device.
Notes and references
- Council Directive on units of measurements 80/181/EEC: "Special names for the unit of power: the name volt–ampere (symbol ‘VA’) when it is used to express the apparent power of alternating electric current, and var (symbol ‘var’) when it is used to express reactive electric power." Chapter 1.2.3., p. 6
- Wildi, Theodore (2002). Electrical Machines, Drives and Power Systems. Pearson. p. 137. ISBN 978-0130930835.
- SI Brochure, 8th ed.: "Just as the quantity symbol should not imply any particular choice of unit, the unit symbol should not be used to provide specific information about the quantity [...]" Section 5.3.2.
- Jeff Palermo, Reactive power: what it is; why it is important (2012)