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Examoogle • User • info@examoogle.com • EE-2024-8821
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Voltage balance type protection of lines used in
Parallel feeder
Ring mains
Both A and B
None of above
Both A and B
Quick Summary: Voltage balance protection, also known as the Merz-Price circulating current system for feeders, works on the principle that the current entering the line must be equal to the current leaving it under normal or through-fault conditions. It is used for the unit protection of both parallel feeders and ring mains where individual section isolation is required.
Voltage balance protection, also known as the Merz-Price circulating current system for feeders, works on the principle that the current entering the line must be equal to the current leaving it under normal or through-fault conditions. It is used for the unit protection of both parallel feeders and ring mains where individual section isolation is required.
Idiff=Iin−Iout — Differential current principle
Vpilot=I2×Zpilot — Voltage drop across pilot wires
Identical current transformers (CTs) are placed at both ends of the protected feeder section. Their secondary windings are connected in series with pilot wires such that the currents induced circulate within the loop, resulting in zero current through the relay. If a fault occurs within the zone, the balance is disturbed, causing the fault current to flow through the relay, which then trips the associated circuit breakers.
Operates on the Kirchhoff's Current Law (KCL) principle for balanced systems.
Requires pilot wires to connect CT secondaries at remote ends.
Specifically designed for feeders where the protection zone must be clearly defined.
Effectiveness depends on the stability and resistance of pilot wires.
Provides high-speed unit protection.
Does not require time-grading, allowing instantaneous operation for faults within the zone.
Requires long pilot wires which can be expensive.
Sensitivity is limited by pilot wire impedance and capacitance effects.
Protection of parallel transmission lines.
Protection of ring main distribution systems.
Voltage balance protection is generally restricted to shorter feeders due to pilot wire charging current issues.
Option A and B are both standard applications for this type of differential protection.
C is correct — Voltage balance type protection is suitable for both parallel feeders and ring mains as it ensures selective protection of the specific feeder section.
Always remember that unit protection schemes (like differential protection) provide instantaneous tripping for faults within the protected zone without needing time delay coordination.