As the load in the distribution system changes, a VARiable voltage drop occurs in the system impedance, which is mainly reactive. Assuming the generator voltage remains constant, the voltage on the bus will change. Voltage drop is a function of the reactive power component of the load current and the reactance of the system and transformer. When loads change very quickly or fluctuate frequently, they can cause "voltage flickering" in customers' loads. Voltage flickering can be annoying to customers because it is glittering. Some loads may also be sensitive to these fast voltage fluctuations.
Static VAR compensator is a very mature FACTS device that emerged in the 1970s and has now been developed sophisticatedly. And at the current stage, it is widely used in load compensation and transmission line compensation including voltage and reactive power compensation, while in high-power grids, a static VAR compensator is a device used for voltage control or for obtaining other benefits, such as improving the damping and stability of the system.
A static VAR compensator is a device that can compensate for the voltage drop of a load change and maintain a constant voltage by controlling the duration of the current passing through the reactor in each cycle. The current in the reactor can be controlled by controlling the strobe of the thyristor, which controls the on-cycle of the thyristor in each cycle, from zero conduction (gate signal off) to full-cycle conduction.
For example, assuming the MVA of a fixed capacitor bank is equal to the MVA of the reactor when the reactor branch is fully cycled, and when the reactor branch undergoes a full cycle, the static VAR compensator, a combination of capacitor bank and thyristor control reactor, is drawing zero net reactive power. When the load reactive power, usually inductive, changes, a static VAR compensator is a device that can change the reactive power to match the load reactive power by controlling the duration of the current in the reactive power branch controlled by the thyristor.
With a fixed capacitor bank, the net reactive power of the static VAR compensator is changed from 0 to a full-capacity VAR only. This is sufficient for most voltage regulation applications, as in most cases, only a capacitive VAR is required to compensate for the inductive VAR of the load.
If the capacitor can be turned on and off, the MVAR can change from full inductance to full capacitance up to the rating of the inductive and capacitive branches. If the time delay is not taken into account (typically 5 to 10 cycles), the capacitor bank can be switched by mechanical circuit breaker, or quickly, less than one cycle at most, by thyristor switch.
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