31. The torque produced in induction type relay (shaded pole structure) is
A. inversely proportional to the current
B. inversely proportional to the square of the current
C. proportional to the current
D. proportional to square of the current
Answer
B. inversely proportional to the square of the current
Explanation:
- A shading ring or coil is made of copper (Cu) and encircles the part of the pole face of each pole at air-gap.
- Thus, two alternating fluxes are produced and are displaced in time and space.
- The air-gap flux of the shaded poles lags behind that of the non-shaded poles.
- The torque produced in induction type relay (shaded pole structure) is inversely proportional to the square of the current.
32. What will be the reflection co-efficient of the wave of load connected to transmission line if surge impedance of the line is equal to load ?
A. Zero
B. Unity
C. Infinity
D. None of the above
Answer
A. Zero
Explanation:
Transmission line impedance is surge impedance Zs.
Load impedance is ZL.
V = incident voltage
V’= reflected voltage
V’’= refracted or transmitted voltage.
Here is given that Zs = ZL.
=Zs - Zs /Zs + Zs =0
So Answer is Zero.
33. Burden of a protective relay is the power
A. required to operate the circuit breaker
B. absorbed by the circuit of relay
C. developed by the relay circuit
D. none of the above
Answer
B. absorbed by the circuit of relay
Explanation:
- In a Current Transformer (CT), there is lesser turns on primary side than secondary.
- Bar primary is the term given to a CT having a single (1) primary turn.
- This is done to lower down the current in secondary for ease in current measurement by CT.
- There should always be a resistive element in secondary so that secondary is never open circuited with current becoming almost zero and voltage becoming very high which can damage insulation.
- This resistor is called burden as it is always present and draws power.
- Since burden is resistive element so it will always absorb power.
- This power is absorbed by relay circuit and is used to trip the circuit breaker.
- It is measured in V A (Volt - Ampere).
34. The line insulation ...... is the insulation level of the station equipment.
A. less than
B. same as
C. more than
D. proportional to
E. not directly related with
Answer
E. not directly related with
Explanation:
- The line insulation is not directly related with the insulation level of the station equipment.
- When lightning impulse over voltage appears in the system, it is discharged through surge protecting devices before the equipments of the system gets damaged.
- Hence, the insulation of such equipment must be designed to withstand a certain minimum voltage before the lightning impulse over voltage gets discharged through surge protecting devices.
- Therefore, operating voltage level of surge protecting devices must be lower than the said minimum voltage withstanding level of the equipment.
- This minimum voltage rating is defined as BIL or basic insulation level of electrical equipment.
35. A balanced 3-phase system consists of
A. zero sequence currents only
B. positive sequence currents only
C. negative and zero sequence currents
D. zero, negative and positive sequence currents
Answer
B. positive sequence currents only
Explanation:
- A balanced three-phase network is one in which the impedances in the three phases are identical.
- A balanced three-phase voltage or current is one in which the size of each phase is the same, and the phase angles of the three phases differ from each other by 120 degrees.
- A balanced 3-phase system consists of positive sequence currents only.
36. The delay fuses are used for the protection of ________ .
A. motors
B. power outlet circuits
C. fluorescent lamps
D. light circuits
Answer
A. motors
Explanation:
- The time-delay, or "slow blow", fuses were developed for situations where acceptable momentary overloads are encountered, such as starting a motor.
- As the name implies, this type of fuse will carry an overload several times the normal load for a short period of time without blowing.
37. Ungrounded neutral transmission system is not recommended because of system
A. insulation being overstressed due to over voltages
B. insulation overstress may lead to failure and subsequent phase to phase faults
C. being inadequately protected against ground fault
D. all of the above
Answer
D. all of the above
Explanation:
In an ungrounded neutral system, the neutral is not connected to the ground i.e. the neutral is isolated from the ground. Therefore, this system is also called isolated neutral system or free neutral system.
- The potential of the faulty phase becomes equal to ground potential. However, the voltages of the two remaining healthy phases rise from their normal phase voltages to full line value. This may result in insulation breakdown.
- The capacitive current in the two healthy phases increase to √3 times the normal value.
- The capacitive fault current (IC) becomes 3 times the normal per phase capacitive current.
- This system cannot provide adequate protection against earth faults. It is because the capacitive fault current is small in magnitude and cannot operate protective devices.
- The capacitive fault current IC flows into earth. Experience shows that IC in excess of 4A is sufficient to maintain an arc in the ionized path of the fault. If this current is once maintained, it may exist even after the earth fault is cleared. This phenomenon of persistent arc is called arcing ground. Due to arcing ground, the system capacity is charged and discharged in a cyclic order. This sets up high-frequency oscillations on the whole system and the phase voltage of healthy conductors may rise to 5 to 6 times its normal value. The overvoltages in healthy conductors may damage the insulation in the line.
38. Induction cup relay is operated due to changes in
A. current
B. voltage
C. impedance
D. all of the above
Answer
D. all of the above
Explanation:
- The relay which works on the principle of electromagnetic induction is known as the induction cup relay.
- The relay has two or more electromagnet which is energized by the relay coil.
- Induction cup relay is operated due to changes in current, voltage and impedance.
39. Which of the following parameter can be neglected for a short line?
A. Inductance
B. Capacitance
C. Resistance
D. Reactance
Answer
B. Capacitance
Explanation:
- A transmission line having its length less than 80 km is considered as a short transmission line.
- In short transmission line capacitance is neglected because of small leakage current and other parameters (resistance and inductance) are lumped in the transmission line.
40. Shunt capacitance is neglected while considering
A. short transmission line
B. medium transmission line
C. long transmission line
D. medium and long transmission lines
Answer
A. short transmission line
Explanation:
- A short transmission line is defined as a transmission line with an effective length less than 80 km (50 miles), or with a voltage less than 69 kV.
- Unlike medium transmission lines and long transmission lines, the line charging current is negligible, and hence the shunt capacitance can be ignored.
- For short length, the shunt capacitance of this type of line is neglected and other parameters like electrical resistance and inductor of these short lines are lumped.