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Which of the following equipment is used to limit short circuit current level in a substation?
Isolator
Lightning arrester
Coupling capacitor
Series reactor
Series reactor
A series reactor is an inductor connected in series with a power line to limit the magnitude of fault currents during a short circuit condition. By adding inductive reactance to the system, it increases the total impedance of the circuit, thereby reducing the fault current IscтАЛ to a level that equipment like circuit breakers can safely interrupt.
A series reactor is an inductor connected in series with a power line to limit the magnitude of fault currents during a short circuit condition. By adding inductive reactance to the system, it increases the total impedance of the circuit, thereby reducing the fault current IscтАЛ to a level that equipment like circuit breakers can safely interrupt.
IscтАЛ=ZeqтАЛVтАЛ тАФ General fault current calculation
XLтАЛ=2╧АfL тАФ Inductive reactance of the reactor
When a short circuit occurs, the fault current is primarily limited by the source impedance. By inserting a series reactor with reactance XLтАЛ=2╧АfL, the total impedance of the fault loop is increased to ZtotalтАЛ=(R+RLтАЛ)2+(X+XLтАЛ)2тАЛ. This significantly reduces the peak short-circuit current, protecting downstream switchgear from excessive thermal and mechanical stresses.
Series reactors are essentially large air-core or iron-core inductors.
They act as current-limiting devices without significantly affecting normal operating conditions.
They help in reducing the kVA rating requirements for circuit breakers.
The primary disadvantage is the continuous voltage drop across the reactor during normal operation.
Protects circuit breakers from exceeding their MVA breaking capacity.
Reduces mechanical stress on busbars and insulators during a fault.
Simple construction and low maintenance.
Causes a constant voltage drop in the system.
Creates an additional power loss (I2R).
Degrades voltage regulation under heavy load conditions.
Substation busbar protection.
Feeder protection in high-voltage networks.
Limiting fault levels in interconnected power grids.
Lightning arresters (Option B) are used for overvoltage protection from surges, not for limiting short-circuit current.
Isolators are mechanical switches meant to disconnect circuits during no-load conditions.
Coupling capacitors are primarily used for power line carrier communication (PLCC).
D is correct тАФ A series reactor increases the total circuit impedance during a fault, thereby limiting the maximum prospective short-circuit current.
Always remember: Series reactors for current limiting, Shunt reactors for voltage control (compensation of capacitive effect in long lines).