What the methods of voltage control in power system?
There are several methods of voltage control in power system, which are used to maintain the voltage within acceptable limits. Some of the common methods are:
Using excitation control or voltage regulators at generating stations: This method adjusts the field current of the synchronous generator (alternator) to control the terminal voltage. It uses an automatic voltage regulator (AVR) system that detects the terminal voltage and compares it with a reference voltage. The AVR then controls the excitation voltage of the alternator to cancel out the error voltage¹.
Using tap changing transformers: This method changes the turn ratio of the transformer by varying the number of secondary turns. This alters the secondary voltage of the transformer and hence the line voltage. There are two types of tap changing transformers: off-load tap changing transformer (OLTC) and on-load tap changing transformer (OLTC). The OLTC requires disconnecting the transformer from the supply before changing the tap, while the OLTC can change the tap without interrupting the load current².
Using shunt reactors: This method compensates the reactive power generated by long transmission lines or underground cables by connecting shunt reactors in parallel with the line. The shunt reactors are inductive elements that absorb reactive power and reduce the line voltage. They are usually installed at the sending end, receiving end and intermediate substations of long EHV and UHV lines¹.
Using shunt capacitors: This method injects reactive power into the line by connecting shunt capacitors in parallel with the line. The shunt capacitors are capacitive elements that generate reactive power and increase the line voltage. They are usually installed at the receiving end, distribution substations and switching substations².
Using synchronous condensers: This method uses synchronous motors running without a mechanical load to control the reactive power in the system. The synchronous condensers can absorb or generate reactive power by varying their field excitation. They can also improve the power factor and keep the voltage constant at any load condition. They are usually connected with the load at the receiving end of the line².
Using static VAR systems (SVS): This method uses static devices such as thyristors, inverters, converters, etc. to inject or absorb reactive power to the system when the voltage becomes higher or lower than a reference value. The SVS can provide fast and flexible voltage control by switching on or off different combinations of shunt reactors and shunt capacitors².