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A star connected 3 phase, 12 MVA, 8 KV alternator is protected by Merz-Price circulating current principle using 1000/5 amperes current transformers. The start point of the alternator is earthed through a resistance of 10 Ω. If the minimum operating current for the relay is 1 A. Calculate the percentage of each phase of the stator winding which is unprotected against earth-faults when the machine is operating at normal voltage.
Correct answer is between '40,46'. Can you explain this answer?
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Verified Answer
A star connected 3 phase, 12 MVA, 8 KV alternator is protected by Merz...
Let x% of the winding be unprotected.
Earth resistance, r = 10 Ω
Voltage per phase, 
Minimum fault current which will operate the relay =
EMF induced in x% winding =
= 46.19 x volts
Minimum fault current which will operate the relay =
EMF induced in x% winding =
= 46.19 x volts
Earth fault current which x% winding will cause
⇒ 4.62x = 200
⇒ 4.62x = 200
⇒ x = 43.29%
Hence, 43.29% of alternator winding is left unprotected.
Most Upvoted Answer
A star connected 3 phase, 12 MVA, 8 KV alternator is protected by Merz...
Ohms.
Assuming a balanced three-phase fault occurs on the alternator terminals, the fault current will flow through the 10 ohm resistance to earth, and a circulating current will be induced in the Merz-Price system. The circulating current will be proportional to the fault current and will flow through the CTs, causing the relay to operate.
Let's calculate the circulating current:
The fault current can be calculated using the formula:
I_fault = MVA / (√3 x kV)
where MVA is the rated power of the alternator and kV is the rated voltage.
I_fault = 12 x 10^6 / (√3 x 8 x 10^3) = 866.03 A
The circulating current can be calculated using the formula:
I_circulating = k x I_fault
where k is the ratio of the CTs.
k = 1000 / 5 = 200
I_circulating = 200 x 866.03 = 173,206 A
Therefore, if a balanced three-phase fault occurs on the alternator terminals, a circulating current of 173,206 A will flow through the CTs, causing the relay to operate and trip the circuit breaker.
Assuming a balanced three-phase fault occurs on the alternator terminals, the fault current will flow through the 10 ohm resistance to earth, and a circulating current will be induced in the Merz-Price system. The circulating current will be proportional to the fault current and will flow through the CTs, causing the relay to operate.
Let's calculate the circulating current:
The fault current can be calculated using the formula:
I_fault = MVA / (√3 x kV)
where MVA is the rated power of the alternator and kV is the rated voltage.
I_fault = 12 x 10^6 / (√3 x 8 x 10^3) = 866.03 A
The circulating current can be calculated using the formula:
I_circulating = k x I_fault
where k is the ratio of the CTs.
k = 1000 / 5 = 200
I_circulating = 200 x 866.03 = 173,206 A
Therefore, if a balanced three-phase fault occurs on the alternator terminals, a circulating current of 173,206 A will flow through the CTs, causing the relay to operate and trip the circuit breaker.
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A star connected 3 phase, 12 MVA, 8 KV alternator is protected by Merz-Price circulating current principle using 1000/5 amperes current transformers. The start point of the alternator is earthed through a resistance of 10 Ω. If the minimum operating current for the relay is 1 A. Calculate the percentage of each phase of the stator winding which is unprotected against earth-faults when the machine is operating at normal voltage.Correct answer is between '40,46'. Can you explain this answer?
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A star connected 3 phase, 12 MVA, 8 KV alternator is protected by Merz-Price circulating current principle using 1000/5 amperes current transformers. The start point of the alternator is earthed through a resistance of 10 Ω. If the minimum operating current for the relay is 1 A. Calculate the percentage of each phase of the stator winding which is unprotected against earth-faults when the machine is operating at normal voltage.Correct answer is between '40,46'. Can you explain this answer? for Electrical Engineering (EE) 2024 is part of Electrical Engineering (EE) preparation. The Question and answers have been prepared according to the Electrical Engineering (EE) exam syllabus. Information about A star connected 3 phase, 12 MVA, 8 KV alternator is protected by Merz-Price circulating current principle using 1000/5 amperes current transformers. The start point of the alternator is earthed through a resistance of 10 Ω. If the minimum operating current for the relay is 1 A. Calculate the percentage of each phase of the stator winding which is unprotected against earth-faults when the machine is operating at normal voltage.Correct answer is between '40,46'. Can you explain this answer? covers all topics & solutions for Electrical Engineering (EE) 2024 Exam. Find important definitions, questions, meanings, examples, exercises and tests below for A star connected 3 phase, 12 MVA, 8 KV alternator is protected by Merz-Price circulating current principle using 1000/5 amperes current transformers. The start point of the alternator is earthed through a resistance of 10 Ω. If the minimum operating current for the relay is 1 A. Calculate the percentage of each phase of the stator winding which is unprotected against earth-faults when the machine is operating at normal voltage.Correct answer is between '40,46'. Can you explain this answer?.
A star connected 3 phase, 12 MVA, 8 KV alternator is protected by Merz-Price circulating current principle using 1000/5 amperes current transformers. The start point of the alternator is earthed through a resistance of 10 Ω. If the minimum operating current for the relay is 1 A. Calculate the percentage of each phase of the stator winding which is unprotected against earth-faults when the machine is operating at normal voltage.Correct answer is between '40,46'. Can you explain this answer? for Electrical Engineering (EE) 2024 is part of Electrical Engineering (EE) preparation. The Question and answers have been prepared according to the Electrical Engineering (EE) exam syllabus. Information about A star connected 3 phase, 12 MVA, 8 KV alternator is protected by Merz-Price circulating current principle using 1000/5 amperes current transformers. The start point of the alternator is earthed through a resistance of 10 Ω. If the minimum operating current for the relay is 1 A. Calculate the percentage of each phase of the stator winding which is unprotected against earth-faults when the machine is operating at normal voltage.Correct answer is between '40,46'. Can you explain this answer? covers all topics & solutions for Electrical Engineering (EE) 2024 Exam. Find important definitions, questions, meanings, examples, exercises and tests below for A star connected 3 phase, 12 MVA, 8 KV alternator is protected by Merz-Price circulating current principle using 1000/5 amperes current transformers. The start point of the alternator is earthed through a resistance of 10 Ω. If the minimum operating current for the relay is 1 A. Calculate the percentage of each phase of the stator winding which is unprotected against earth-faults when the machine is operating at normal voltage.Correct answer is between '40,46'. Can you explain this answer?.
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