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A static VAR compensator (SVC) is an electrical device that forms part of a flexible alternating current (AC) power transmission system. The term "static" is used to indicate that there are no moving parts to an SVC. To understand the rest of the term, what it is and how it works, it is first necessary to understand reactive power, VARs, and high-voltage electricity transmission networks.
In an electric circuit, power is measured as the flow of energy as it moves past a certain point on the circuit. With AC circuits, the energy can reverse the direction in which it is flowing from time to time. When calculated over the complete wave of the AC cycle, real power is the average net transfer of energy in one direction or the other. The net power that returns to the source over each complete cycle is known as reactive power. A static VAR compensator is designed to return reactive power to the source quickly.
VAR is an acronym that stands for "volt-ampere reactive." It is a unit of measure for reactive power in an AC electrical system. VARs describe the level of power that is flowing into a reactive load.
A high-voltage electric power transmission system moves energy from power plants to substations. One system is comprised of a series of transmission lines that are interconnected. These are usually called "power grids" in the United States. The Eastern Interconnection, the Western Interconnection, and the Electric Reliability Council of Texas (ERCOT) grid are the three systems that form the backbone of US energy transmission. In the UK, the transmission system is simply referred to as the "national grid."
In an electric power transmission system, a static VAR compensator helps to regulate voltage and stabilize the grid. The SVC responds to the type of reactive load that the system is carrying. If the load is capacitive or leading, the SVC will use a Thyristor-Controlled Reactor or other similar type of reactor to absorb VARs and lower the total voltage of the system. When the reactive load is inductive or lagging, the SVC will switch on the capacitor banks to increase the system's voltage. These changes happen automatically and help the system to run at optimum levels.
There are several advantages to a static VAR compensator. First and foremost, a SVC is fast. It can provide almost instantaneous adjustments to the power system's voltage. SVCs are also usually cheaper, more reliable and able to handle a higher power capacity than other options that could be used to manage system voltage.