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Series capacitors on transmission lines are of little use when the load VAR requirement is
Large
Small
Fluctuating
Any of the above
Small
Series capacitors are installed in transmission lines to compensate for inductive reactance, thereby improving power transfer capability and voltage regulation. When the load VAR (reactive power) requirement is small, the inherent voltage drop across the line inductance is already negligible, rendering the capacitive compensation redundant or ineffective.
Series capacitors are installed in transmission lines to compensate for inductive reactance, thereby improving power transfer capability and voltage regulation. When the load VAR (reactive power) requirement is small, the inherent voltage drop across the line inductance is already negligible, rendering the capacitive compensation redundant or ineffective.
XeffтАЛ=XLтАЛтИТXcтАЛ тАФ Effective reactance of a compensated line
╬ФVтЙИI(Rcos╧Х+(XLтАЛтИТXcтАЛ)sin╧Х) тАФ Voltage drop expression
The series capacitor provides a capacitive reactance XcтАЛ that opposes the line's inductive reactance XLтАЛ. The effective impedance is Z=R+j(XLтАЛтИТXcтАЛ). When the load is small, the current I is low, making the total voltage drop I(XLтАЛтИТXcтАЛ) insignificant. Since the primary objective of series capacitors is to mitigate large voltage drops and stabilize power flow under heavy loading, they provide little functional value under light load conditions.
Series capacitors reduce the total inductive impedance of the line.
They are primarily used to enhance transient stability in long EHV transmission lines.
They do not significantly improve efficiency when the reactive power flow (which determines the voltage drop) is inherently low.
Potential risk of Sub-Synchronous Resonance (SSR) exists with excessive compensation.
Increased power transfer capability
Improved steady-state and transient stability
Better voltage regulation along the line
Risk of sub-synchronous resonance (SSR)
Protection complexity due to high fault currents
Risk of voltage instability under certain conditions
Long-distance EHV and UHV transmission lines
Inter-area tie lines to improve synchronizing power
Series capacitors provide 'Self-Regulation': the compensation magnitude increases as the load current increases, making them ideal for heavy loading.
Option B is the correct choice because when the load VAR requirement is small, the line impedance is not a limiting factor for voltage regulation, making the fixed or controlled compensation unnecessary.
B is correct тАФ Series capacitors are designed to mitigate large voltage drops and stability issues; when the load VAR requirement is small, these issues are absent, rendering the capacitor ineffective.
Always remember that series compensation targets the inductive reactance of the line (XLтАЛ), while shunt compensation targets the voltage profile directly at a specific node.