01․ In case of failure of prime mover of the generator, which type of relay is used?
In case of failure of prime mover of the generator and the excitation is present then the generator draws the power from the other parallel generators and start working as a motor and for this motor, turbine is working as a load. Because of huge mass of the turbine the motor draws huge amount of current which damages the winding. For protecting the generator from this abnormalities reverse power relay is used.
02․ Which of the following relays is/are unit protection relays?
If the relay is used for the protection of faults within a specified device like transformer, generator, motor etc., are called as unit protection relays.
Examples:
1. Differential relays
2. Buccholz relays
03․ Differential relays does not used for which of the following faults?
If the relay is used for the protection of faults within a specified device like transformer, generator, motor etc., are called as unit protection relays.
Examples:
1. Differential relays
2. Buccholz relays
If the fault outside the unit protection area or zone are called through faults.
Differential relay does not operate for the through faults, but if the one of the current transformer core is saturated, relay mall operates for the through faults.
04․ Which type of connection is employed for current transformers for the protection of delta- star 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 delta-star 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.
05․ The neutral of 10 MVA, 11 kV alternator is earthed through a resistance of 5 ohms. The fault ready is set to operate at 0.75 A. The current transformers have a ratio of 1000/5. What is the percentage of the alternator winding is protected?
% Winding unprotected = I0*Rn/Vph*100
Where,
I0 = Minimum pick up current of relay * CT ratio
Rn = Neutral resistance
Vph = Phase voltage of the generator
% Winding protected = 100 - % Winding unprotected
I0 = 0.75*1000/5 = 150 A
Vph = 11/√3 = 6.35 kV
Rn = 5 Ω
% Winding unprotected = 150*5/(6.35*10³)*100 = 11.8%
% Winding protected = 100 - 11.8 = 88.2%
06․ Which of the following faults is/are treated as external faults?
Some of external faults are
1. Failure of prime mover of the generator
2. Failure of excitation
3. Failure of cooling medium
4. unbalanced loading
5. Over load
6. Over voltage
7. Over speed
07․ Which of the following is/are the stator faults?
Stator faults:
1. Phase - phase faults
2. Inter phase faults
3. Phase earth faults
08․ Mho relays are used for the protection of which of the following lines?
Relays and their applications:
1. Reactance relays ----- Short transmission lines
2. Impedance relays ------ Medium transmission lines
3. Mho relays ------------- Long transmission lines
In long transmission lines power swings are more frequent. It is not a fault but impedance relay and admittance relay mall operates during power swings. To overcome this problem mho relay is used.
09․ A thermal protection relay provides protection against what?
Thermal relays are used for protection of the equipment from variation of temperatures. To protect from the variations of temperature thermal or temperature sensitive relay is used, which consisting of bimetallic strips made of nickel and chromium . These provides higher bending movement if the temperature is increased. SO that initially operates alarm and the trip signal.
10․ Characteristics of arc quenching medium is/are
Characteristics of arc quenching medium:
1. High dielectric strength
2. High thermal conductivity
3. Thermally and chemically stable
4. Non flammable
5. Arc extinguishing ability
6. Commercially available at moderate cost
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