01․ Which device is used for protection of equipment from from damage caused by overload or short circuit?
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and interrupt current flow.
02․ Isolator in sub station is open under ------------- condition?
Isolator does not contain any arc quenching medium, it must be open under no load condition only. On the both side of the every equipment isolators are provided. For attending maintenance working on equipments these isolators are opened.
03․ Earthing switch is used for
Isolator does not contain any arc quenching medium, it must be open under no load condition only. On the both side of the every equipment isolators are provided. For attending maintenance working on equipments these isolators are opened.
At the time of attending maintenance work, the earth switch is closed for discharging residual energies available in the equipment for the safety purpose.
04․ To protect the power transformer ( Y-Y with neutral grounded) against fault, which type of connection do the current transformers have?
A current transformer is an instrument transformer in which the secondary current is substantially proportional to primary current and differs in phase from it by ideally zero degree. In case of Y-Y with neutral grounded, current transformers on both sides should be connected in delta otherwise the relay will also operate for the external faults due to zero sequence currents.
05․ Which type of connection is employed for current transformers for the protection of star- delta connected 3-phase transformer?
A current transformer is an instrument transformer in which the secondary current is substantially proportional to primary current and differs in phase from it by ideally zero degree.
In case of star-delta transformer, the currents on the two sides differ in phase by 30°. This is compensated by connecting current transformers in delta on the star side and in star on the delta side of the transformer.
06․ A 50 Hz, 17.32 kV generator is connected to a power system. The system reactance and capacitance per phase are 10 mH and 0.2 mF respectively. What is the maximum voltage across the contacts of the circuit breaker at an instant when it passes through zero?
Maximum value of restriking voltage = 2*peak value of the system voltage
= 2*13.32*10³/√3*√2
= 28.28 kV
07․ A buchholz relay is used for
Buchholz relay is the gas detector relay used to protect transformers from internal or incipient faults, which operates when the oil level in the conservator pipe of a transformer is lowered by the accumulation of gas caused by a poor connection or by incipient breakdown of insulation.
08․ What is the preferred type of circuit breaker(CB) to be installed in extra high voltage AC system?
Circuit breakers and their voltage ranges:
Vacuum CB --- up to 33 kV
Air blast or air break or minimum oil CB ----- up to 132 kV
Sulphur hexafluoride CB --- 132 kV to 765 kV
09․ Which of the following relays are used in protection of medium transmission lines?
Relays and their applications:
1. Admittance relays ----- Short transmission lines
2. Impedance relays ------ Medium transmission lines
3. Mho relays ------------- Long transmission lines
Impedance relays are used for protecting medium transmission lines in which arcing ground faults are less frequent.
10․ Offset mho relay is used for the protection of equipment when
Under normal condition, synchronous generators delivers both active and reactive power. If excitation is failed and the turbine is in service then it delivers active power and absorbs reactive power from other parallel generators. So impedance seen by the relay will change. For identifying this change and protect the generator from loss of excitation DC offset is added to the normal mho relay for identifying the fault. This relay is called offset mho relay.
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