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Examoogle • User • info@examoogle.com • EE-2024-8821
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Merz prize protection system is used for
Transformer
Alternator
Both A and B
None of above
Both A and B
Quick Summary: The Merz-Price protection system is a form of unit protection (differential protection) based on the principle of circulating currents. It is widely used to provide high-speed, selective protection for both power transformers and AC generators (alternators) against internal faults.
The Merz-Price protection system is a form of unit protection (differential protection) based on the principle of circulating currents. It is widely used to provide high-speed, selective protection for both power transformers and AC generators (alternators) against internal faults.
Idiff=∣I1−I2∣=0 (Normal Condition)
Idiff=∣I1−I2∣=0 (Fault Condition)
The system compares the current entering and leaving the protected zone using Current Transformers (CTs). Under healthy conditions, the secondary currents circulate between CTs such that the net current through the relay is zero. During an internal fault, the balance is disrupted, causing a differential current to flow through the relay, which triggers the circuit breaker.
It is also known as the Biased Differential Protection system when used with a restraining coil.
The CTs at both ends must have identical ratios and phase characteristics.
It provides instantaneous clearing of faults within the protected zone.
Used extensively for large machines where high-speed protection is mandatory.
High-speed operation
Provides inherent selectivity (only clears faults within the protected zone)
No time-delay grading required
Requires pilot wires for communication between distant CTs
Susceptible to mal-operation due to CT saturation during external through-faults
Large Alternator winding protection
Power Transformer differential protection
The system is often referred to as 'Merz-Price' (or Mertz-Price).
The protection is limited to the zone between the two sets of CTs.
C is correct — Merz-Price protection is a differential protection scheme applicable to both transformer and alternator windings.
Always remember that 'Unit Protection' schemes like Merz-Price do not require coordination with other relays, unlike 'Non-unit' schemes like Overcurrent protection.