Beatrice Lang
VAR, or volt-ampere reactive, is a unit of measurement for reactive power in electricity. Reactive power is a type of power that exists in AC circuits when the current and voltage are not in phase, and it is represented by the unit var, which is allowed by the International System of Units (SI). Reactive power is important in power transmission and distribution, as it can affect the efficiency of the system and the amount of power that is consumed by loads.
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What You'll Learn
- VAR is a unit of measurement for reactive power
- Reactive power exists in AC circuits when voltage and current are out of phase
- VARs are used to indicate the total of real and non-real power
- Capacitors can be used to compensate for high-VAR inductive loads
- Utilities prefer customers to have power factors close to 1.0
VAR is a unit of measurement for reactive power
VAR, or volt-ampere reactive, is a unit of measurement for reactive power. Reactive power is a form of power that exists in an AC circuit when the current and voltage are not in phase. This occurs when there is a phase difference between voltage and current, resulting in a power factor of less than 1. In such cases, the notation is changed from watts (volts x amps) to volt-amps or VARs to indicate the total of real and non-real power. VARs are allowed by the International System of Units (SI) and are used in electric power transmission and distribution.
The term VAR was introduced in 1930 by the IEC in Stockholm, proposed by Romanian electrical engineer Constantin Budeanu. VARs are particularly relevant in power systems, where they are used to describe the portion of current that is delivered by generators to create magnetic fields. This magnetic field allows for the flow of electrical MW through the system to perform real work. However, if the system is turned off, the portion of the current that created the magnetic field "disappears," indicating that it did no work.
VARs are important in understanding power quality and efficiency in electrical systems. A high VAR inductive load can be compensated for by installing capacitor banks, which draw reactive power and store it in an electric field instead of a magnetic field. This stored energy is then released at the opposite times of an inductive load, ensuring that the extra reactive power cycles between the capacitor and inductive loads without being seen by the power company.
It is important to distinguish between real power and reactive power. While real power is the power that is consumed by the load, reactive power is the extra power "borrowed" from the power company and later returned when the magnetic field collapses. This reactive power is not typically billed to customers, but it does impact the power factor, which utilities prefer to be as close to 1.0 as possible.
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Reactive power exists in AC circuits when voltage and current are out of phase
Reactive power is a term used to describe the power generated by a capacitor or inductor in an AC circuit. Inductors and capacitors are energy storage elements that can cause periodic reversals in the direction of energy flow in an AC circuit. This is distinct from a direct current (DC) circuit, where electrical power is equal to the voltage multiplied by the current, and the current and voltage are in phase with each other.
In an AC circuit, the voltage and current waveforms do not align perfectly due to the presence of inductance or capacitance. This misalignment is known as the phase angle. When the voltage and current are out of phase, the power factor is less than 1, and the total power includes both real and non-real power. The unit "var" or volt-amp reactive, represents this total power, accounting for the phase difference between voltage and current.
In a purely resistive circuit, the current and voltage are in phase, and all the power is consumed by the resistance, typically in the form of heat. No power is returned to the source or circuit. However, in a purely inductive or capacitive circuit, the current leads or lags the voltage by 90 degrees (the phase angle), resulting in power being stored and returned to the source. Over one full cycle, the average power in such a circuit is zero, indicating that no real work is performed.
In a circuit with both resistance and reactance, there will be a combination of active and reactive power. While some power is dissipated by the load, a portion of it is absorbed and returned. This reactive power does not perform any real work and can lead to higher losses and reduced transmission efficiency.
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VARs are used to indicate the total of real and non-real power
Volt-ampere reactive (VAR) is a unit of measurement of reactive power in an AC circuit. Reactive power is a type of non-real power that exists when there is a phase difference between the voltage and current, resulting in a power factor less than 1. This occurs when the current and voltage are not in phase with each other. The term "var" was first proposed by Romanian electrical engineer Constantin Budeanu and officially introduced in 1930 by the IEC in Stockholm.
While real power, or true power, refers to the actual amount of power being consumed and dissipated in a circuit, reactive power does not directly electrify items. Instead, it plays a crucial role in maintaining a balanced power grid by facilitating the smooth flow of electricity between generators and capacitors. Reactive power is measured in VARs, while real power is measured in watts.
In a practical system, higher currents can lead to higher losses and reduced transmission efficiency. This is where VARs come into play. By indicating the total of real and non-real power, VARs help in managing the reactive power flow. Reactive power is generated by capacitors and consumed by inductors, and it is this dynamic that forms the basis for controlling the power factor in electric power transmission.
The power triangle, consisting of real power, reactive power, and apparent power, further illustrates the relationship between these concepts. Apparent power is the combination of reactive power and true power and represents the power sent out to power up devices. It is calculated as the product of a circuit's voltage and current without considering the phase angle. Apparent power is measured in Volt-Amps (VA) and is useful for sizing equipment and wiring.
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Capacitors can be used to compensate for high-VAR inductive loads
Volt-amp reactive (VAR) is a unit of measurement of reactive power, which exists in an AC circuit when the current and voltage are not in phase. Capacitors are used to compensate for high-VAR inductive loads because they can absorb the high voltages generated by inductive loads, blocking them from the contacts of the relay. This improves the lifespan of the relay and the reliability of the board.
Capacitors are often used for VAR compensation because most large industrial users have to worry about correcting the power factor, and most of their loads are motors, which are inductive. Reactive power is consumed by inductive loads, which is why capacitor banks or other reactive power compensators are used.
In addition, capacitors are used to control power flow, as they can improve the power factor when it is brought down by motors. They are also used to reduce electromagnetic interference that could damage the board or interfere with the board's performance.
Capacitors are easy to install and are connected in parallel with the load being controlled. They are not polarized, so they can be used in both AC and DC applications. The correct capacitor is chosen based on the maximum voltage requirement of the device.
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Utilities prefer customers to have power factors close to 1.0
Volt-amp reactive (VAR) is a unit of measurement of reactive power in electric power transmission and distribution. Reactive power exists in an AC circuit when there is a phase difference between voltage and current, resulting in a power factor of less than 1. A power factor of 1, also known as a unity power factor, indicates that all the energy supplied by the source is consumed by the load.
In an electric power system, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. This results in increased energy losses and requires larger wires and equipment, leading to higher costs for utilities. Utilities typically charge additional costs to commercial customers with a low power factor, usually defined as below 0.9 to 0.95.
To improve efficiency and reduce costs, utilities prefer customers to have power factors close to 1.0. A high power factor reduces losses and improves voltage regulation at the load. Power factor correction can be achieved by supplying or absorbing reactive power and adding capacitors or inductors to cancel the inductive or capacitive effects of the load. This brings the power factor closer to 1.0, reducing the apparent power demand on the supply system.
Dynamic power factor correction (DPFC) is a technique used for electrical stabilisation in cases of rapid load changes, such as at large manufacturing sites. DPFC uses semiconductor switches to quickly connect and disconnect capacitors or inductors, improving the power factor. By ensuring that their customers have power factors close to 1.0, utilities can minimise operational risks and improve the stability and efficiency of their networks.
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Frequently asked questions
VAR stands for Volt-Amperes Reactive.
VAR is a unit of measurement of reactive power. Reactive power exists in an AC circuit when the current and voltage are not in phase.
Reactive power occurs when the magnetic field built up by alternating current power systems collapses, and the energy stored in the magnetic field is converted back into power. This power is then returned to the power company.
A "capacitor" can be used to compensate for high VAR inductive loads. A capacitor stores energy in an electric field instead of a magnetic field, and releases its reactive energy at the opposite times of an inductive load.
