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The neutral of the three phase 20 MVA, 11kV alternator is earthed through a resistance of 5 ohms, the relay is set to operate when there is an out of balance current of 1.5 A. The CTs have a ratio of 100/5. What percentage of the winding is protected against L-G faults?
- a)76.4
- b)77.8
- c)73
- d)None of the mentioned
Correct answer is option 'D'. Can you explain this answer?
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The neutral of the three phase 20 MVA, 11kV alternator is earthed thro...
The given system will not protect for the LG faults, instead it will protect for earth faults.
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The neutral of the three phase 20 MVA, 11kV alternator is earthed thro...
To determine the percentage of the winding protected against L-G faults, we need to calculate the current flowing through the earthed resistor when there is an out of balance current of 1.5 A.
Given data:
- Three-phase alternator: 20 MVA, 11 kV
- Neutral resistance: 5 ohms
- Relay set to operate at an out of balance current of 1.5 A
- CT ratio: 100/5
First, let's calculate the out of balance current in the alternator winding:
Since the relay operates at an out of balance current of 1.5 A, the total out of balance current in the alternator winding can be calculated using the CT ratio.
Out of balance current in the winding = CT ratio × Relay operating current
= (100/5) × 1.5 A
= 30 A
Now, let's calculate the voltage across the neutral resistor:
The voltage across the neutral resistor can be calculated using Ohm's Law.
Voltage across the neutral resistor = Neutral resistance × Out of balance current
= 5 ohms × 30 A
= 150 V
To find the percentage of the winding protected against L-G faults, we need to determine the voltage across the winding during a L-G fault.
During a L-G fault, the voltage across the winding will be equal to the voltage across the neutral resistor, which is 150 V.
Now, let's calculate the percentage of the winding protected against L-G faults:
Percentage of winding protected = (Voltage across the winding during L-G fault / Line-line voltage) × 100
= (150 V / 11 kV) × 100
= 1.36%
Therefore, the correct answer is option 'D' (None of the mentioned) as none of the given options match the calculated percentage.
Given data:
- Three-phase alternator: 20 MVA, 11 kV
- Neutral resistance: 5 ohms
- Relay set to operate at an out of balance current of 1.5 A
- CT ratio: 100/5
First, let's calculate the out of balance current in the alternator winding:
Since the relay operates at an out of balance current of 1.5 A, the total out of balance current in the alternator winding can be calculated using the CT ratio.
Out of balance current in the winding = CT ratio × Relay operating current
= (100/5) × 1.5 A
= 30 A
Now, let's calculate the voltage across the neutral resistor:
The voltage across the neutral resistor can be calculated using Ohm's Law.
Voltage across the neutral resistor = Neutral resistance × Out of balance current
= 5 ohms × 30 A
= 150 V
To find the percentage of the winding protected against L-G faults, we need to determine the voltage across the winding during a L-G fault.
During a L-G fault, the voltage across the winding will be equal to the voltage across the neutral resistor, which is 150 V.
Now, let's calculate the percentage of the winding protected against L-G faults:
Percentage of winding protected = (Voltage across the winding during L-G fault / Line-line voltage) × 100
= (150 V / 11 kV) × 100
= 1.36%
Therefore, the correct answer is option 'D' (None of the mentioned) as none of the given options match the calculated percentage.
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The neutral of the three phase 20 MVA, 11kV alternator is earthed through a resistance of 5 ohms, the relay is set to operate when there is an out of balance current of 1.5 A. The CTs have a ratio of 100/5. What percentage of the winding is protected against L-G faults?a)76.4b)77.8c)73d)None of the mentionedCorrect answer is option 'D'. Can you explain this answer?
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