Viewed externally in the substation yard, a large high voltage Reactor does not differ much from a transformer. The easiest way to distinguish a Reactor from a transformer is to observe its terminals and bushings on top of the device. To explain more, contrary to a 3-phase transformer which has three primary and three secondary voltage terminals, a 3-phase Shunt Reactor has only three voltage connections. Figure 6-1 illustrates a 3-phase Shunt Reactor.
Figure 6-1: Schematic of a 3-phase Shunt Reactor
The shunt reactor is the most cost efficient equipment for maintaining voltage stability on the transmission lines. It does this by compensating for the capacitive charging of the high voltage AC-lines and cables, which are the primary generators of reactive power. The reactor can be seen as the voltage control device which is often connected directly to the high voltage lines. Figure 6-2 shows how the generated capacitive reactive power of the line is consumed by the reactors.
Figure 6-2: Generated reactive power by the line is consumed by the reactor’s inductance
There are two main applications for Shunt Reactors. First, Shunt Reactors can be used for stability reasons especially on long transmission lines and cables (EHV and HV lines/cables); which in this case, Shunt Reactors are required to be permanently in service. Second, for the purpose of voltage control, where they can only switched in during light loaded conditions and are used in the underlying system and near to load centers.
Although Reactors reduce over voltages during light load conditions, they can also reduce the line load ability if they are not removed under full-load condition.
As seen in Figure 6-3, the Switched Reactor is connected to the bus bar for voltage regulation while the Non-switched Reactor is connected to the line for stability reason. However, the Switched Reactor could be also connected directly to the line.
Figure 6-3: General application of Switched and Non-switched Fixed Reactors
Shunt Reactors are commonly installed at both ends of EHV lines, and sized to prevent the line voltage from exceeding a designed value when energized from one end. The reason that they are installed at both ends of the line is that there is usually some uncertainty regarding which end of a line may be energized (or de-energized) first. Figure 6-4 shows the equivalent П-Model of a transmission line with Switched Shunt Reactors connected at both ends.
Figure 6-4: Single line diagram of the Switched Shunt Reactors connected at both ends of the line
The shunt reactor could be permanently connected or switched via a circuit breaker (Fixed Shunt Reactor). To improve the adjustment of reactive power consumption, the reactor can also be variable (Variable Shunt Reactor).