Under normal operation of a power system the current is essentially determined by the connected ohmic and inductive loads. High voltage transmission lines and cables however have an inherent capacitance, causing a capacitive charging current. Thus capacitive VArs are generated.
Increased Power Efficiency
In lightly loaded lines or cables this capacitive current will increase the voltage at the end of the line. By the use of shunt reactors the capacitive VArs will be compensated and the voltage increase at the end of the line will be limited. The efficiency of the power system will be increased by allowing the transmission of more active energy.
Air-core dry-type shunt reactors are normally connected to the tertiary winding (e.g. at 20kV) of the high voltage transformer (e.g. 400 kV/110 KV transformers) (a). For system voltages up to 115kV, air-core dry-type shunt reactors can also be directly connected to the system (b).
Current limiting reactors (a) are series connected to the transmission/distribution line or to the feeder in order to limit the short-circuit power on the load side of the reactor.
The Classical Application
The reactor limits the short-circuit current to a level which can be handled by the components installed in the electrical system, such as breakers, switches or fuses.This represents the classical application of air-core reactors. Due to the linear inductance-characteristics over the current range the full reactorimpedance is also maintained during system fault conditions.
Other special applications of current limiting reactors
Load balancing reactors (b) for load sharing in parallel circuits
Bus tie reactors (c) installed between two different bus systems
Capacitor inrush current limiting or damping reactors (d)