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Which of the following is a criterion used for selecting the ratio of the minimum fault current to the maximum load current in over-current protection of a transmission line?
To increase the possibility of maloperation under normal operating conditions
To reduce the maximum load current on the transmission line
To prevent the possibility of maloperation under normal operating conditions
To decrease the sensitivity of the protection system to faults
To prevent the possibility of maloperation under normal operating conditions
In over-current protection, the relay setting must be chosen such that the pickup current is higher than the maximum possible load current but lower than the minimum expected fault current. This ensures the relay remains stable during heavy load conditions while reliably detecting fault conditions.
IEC 60255 / IS 3231
In over-current protection, the relay setting must be chosen such that the pickup current is higher than the maximum possible load current but lower than the minimum expected fault current. This ensures the relay remains stable during heavy load conditions while reliably detecting fault conditions.
Sensitivity┬аRatio(S)=IpickupтАЛIfault,minтАЛтАЛ
Iload,maxтАЛ<IpickupтАЛ<Ifault,minтАЛ
The relay must be able to distinguish between an overload condition (where the current is high but within limits) and a fault condition. If the pickup current is set lower than the maximum load current, the relay will trigger a trip during normal, heavy load operation, leading to a maloperation (nuisance tripping). By maintaining a margin, we ensure that IpickupтАЛ>Iload,maxтАЛ and IpickupтАЛ<Ifault,minтАЛ.
The protection system should not trip for transient or steady-state peak load currents.
The ratio of minimum fault current to maximum load current is a measure of the sensitivity and security of the relay setting.
A higher ratio implies a larger margin between operating and blocking conditions.
Modern digital relays use definite time or inverse definite minimum time (IDMT) characteristics to further prevent maloperation.
Ensures high system reliability by avoiding unwanted outages.
Maintains discrimination between fault and normal load.
High margins might lead to slower detection of high-resistance faults.
Cannot detect faults where current is lower than the relay pickup setting.
Radial distribution feeder protection.
Transmission line backup over-current relays.
Industrial motor protection against sustained overloads.
Standard values for pickup current settings are often selected to be 1.25 to 1.5 times the maximum load current.
Option A is incorrect because we want to minimize maloperation, not increase it.
Option B is incorrect because relay settings do not control the actual load current flowing through the line.
Option D is incorrect because increasing sensitivity is usually desired, provided it does not lead to maloperation.
C is correct тАФ The ratio is selected to ensure that the relay pickup current remains above the maximum load current, thereby preventing unnecessary tripping during normal high-load conditions.
Always remember that in protection relaying, 'Security' (not tripping when there is no fault) is just as critical as 'Dependability' (tripping when there is a fault).
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