When the load varies in the system, the consumption of reactive power changes, and consequently the voltage fluctuates on the line. In order to control and correct the voltage fluctuations, there are several voltage regulating devices that are used in the power system; which are briefly described as follows:
1. Synchronous Generators:
Synchronous generators are not only used for generating active power; they can also be used for both generation and consumption of reactive power. When they are in over-excitation mode, they deliver reactive power to the power system; while they absorb reactive power from the grid when working in under excitation mode. Thus a synchronous generator could be considered as a device to regulate the voltage.
2. Regulating Power Transformers:
It is possible to change a power transformer’s voltage ratio by using tap-changer which consequently affects the voltage level and also the reactive power balance.
3. Shunt Capacitors:
Capacitors generate reactive power. By connecting capacitors to a transmission line, they deliver reactive power and increase the voltage level; so they have typically been used during heavy load conditions. Depending on the voltage of the line, the capacitors can be connected directly or via a transformer to the line.
4. Shunt Reactors:
In contrast with capacitors, reactors absorb (consume) reactive power by connecting them to a transmission line. They are typically used during light load conditions since they decrease the voltage level. The shunt reactor can be directly connected to the line or through the tertiary winding of a three winding transformer.
5. Static VAR Compensators:
In case of applications that need very fast compensation (generation or consumption) of reactive power, Static VAR Compensators (SVC) have been used. For example, when a fault happens on the network, an immediate compensation of reactive power is needed in order to keep the voltage stability in the electrical power system. To control the reactive power continuously and dynamically, SVCs are equipped with electrical components like capacitors and reactors which are switched in and out very fast with the help of semi conductive electronic devices. Since these electronic devices cannot usually be connected directly to the high voltage power line, it is necessary to also install an intermediate power transformer.