100 Most Important MCQ Of Switchgear and Protection with explanation

Ques.61. Bus coupler is very essential in the arrangement

  1. Single bus
  2. Double bus, double breaker
  3. Main and transfer bus
  4. All of the above

Answer.3. Main and transfer bus

Explanation:-

Bus coupler is a device that is used to couple one bus to the other without any interruption in the power supply and without creating hazardous arcs. A bus coupler is a breaker used to couple two busbars in order to perform maintenance on other circuit breakers associated with that busbar. It is achieved with the help of a circuit breaker and isolators.

Main and Transfer Bus Arrangement

The main and transfer bus arrangement has two buses: one is the main bus, and the other is the transfer bus. With the help of isolator switches, it is connected to the transfer bus, which is called bypass isolators and with the help of circuit breakers and isolator switches, it is connected to the main bus. A bus coupler is also required in this arrangement, as shown in Figure.

In normal conditions, the feeders are fed through the main bus, but during fault conditions, the load is transferred to the transfer bus. To transfer load from the main bus to the transfer bus, we need to close the bus coupler first and then close bypass isolators of the feeder to be connected to the transfer bus. Then we must open the isolator switch off the feeder coupled to the main bus and then open the bus coupler breaker.

When the maintenance on any breaker is required then there is a changeover frown main bus to transfer bus and controlled through bus coupler breaker. The system uses an additional isolator on each circuit which increases the cost of the system As the changeover from the main bus to the transfer bus in this system is through isolators, careful interlocking is necessary with the bus coupler breaker.

Advantages:

  • It ensures supply in case of a bus fault. In case of any fault in a bus, the circuit can be transferred to the transformer bus.
  • It is easy to connect the circuit from any bus.
  • The maintenance cost of the substation decreases.
  • The bus potential can be used for relays.

Disadvantages:

  • Requires one extra breaker for the bus tie
  • Switching is somewhat complicated while maintaining a breaker.
  • Failure of the bus or any circuit breaker results in the shutdown of the entire substation.

 

Ques.62. For cost and safety, the outdoor substations are installed for voltages above

  1. 11 kV
  2. 33 kV
  3. 60 kV
  4. 110 kV

Answer.2. 33 kV

Explanation:-

Indoor substation: Generally, this substation is used for voltage up to 11 kV. However, when the atmosphere is polluted with impurities, this substation is also erected up to 33 kV. All items of the equipment are installed indoors for economic reasons. The indoor substation consists of either an enclosed or open compartments or chambers in which the main equipment of the substation is installed.

Outdoor substation: It is used for voltages above 33 kV. Due to high voltages, the clearance between conductors and the space required for switches, circuit breakers, and other types of equipment is very large and the indoor substations are not suitable economically.

 

Ques.63. The short circuit in any winding of the transformer is the result of

  1. Mechanical vibration
  2. Insulation failure
  3. Loose connection
  4. Impulse voltage

Answer.4. Impulse voltage

Explanation:-

Failure of the Transformer

Faults may occur in different parts and components of the transformer due to mechanical, electrical or thermal stress caused due to different conditions. Some of the most commonly occurring failures of the transformer and the] causes are listed below.

 Winding failure

Windings are an important part of a transformer. In distribution side transformers there are commonly two windings. One of the primary side and the second on the secondary side.

High voltage/low current flows in the primary side winding and through electromagnetic induction voltage is stepped down and current stepped up in the secondary side winding These windings withstand dielectric, thermal, and mechanical stress during this process. The faults that occur in the winding are due to these stresses. This causes the breaking of the windings or the burn-out. 

Dielectric faults occur in the winding due to turn-to-turn insulation breakdown. These are the insulation between the turns of the winding. Insulation breakdown commonly occurs due to high current and voltage which are high above the rated values. The breakdown of the insulation results in the flashover of the winding turns and causes a short circuit. Two reasons for the high rating are

i. Lightning impulse attack with no lightning arresters

ii. Fault voltages

⇒The windings are usually of copper. Due to the copper line resistance, thermal losses occur. These thermal losses make hotspots in the winding due to bad or lack of maintenance. This overtime cause wear and tear and the decrease of the physical strength up to the point of breaking of the winding.

Mechanical faults are the distortion, loosening, or displacement of the windings. This results in the decrease of the performance of the transformer and the tearing of the turn-to-turn ratio. The main reasons that cause this fault are improper repair, bad maintenance, corrosion, manufacturing deficiencies, vibration, and mechanical movement within the transformer.

 

Ques.64. A mho relay is used for protection of:

  1. Protection of a transformer against external fault
  2. Long Transmission Line
  3. Protection of a transformer against all the internal faults and external fault
  4. Medium Length lines

Answer.2. Long Transmission Line

Explanation:-

MHO relay is a high-speed relay and is also known as an admittance relay. lt is well known that a long line is less stable than a short line; that is, a long line has a larger swing angle δmax compared to a short line. The short line has a higher Pmax than that of a long line.

Why is Mho’s Relay used for Long Transmission Line?

  • Mho relay comes in the category of the distance relay protection scheme.
  • The mho type relay is most suited for long lines because there are more chances of severe synchronizing power surges on the system.
  • MHO Relay is less affected by the power swing.
  • It does not need any additional equipment to prevent tripping during these surges.
  • The mho relay occupies the least space on an R-X diagram for a given line section and is, therefore, least affected by abnormal system conditions except for the line faults.
  • Since the mho relay is most affected by arc resistance, it is used for long lines.

 

Ques.65. For which of the following protection from negative sequence currents is provided?

  1. Generators
  2. Motors
  3. Transmission line
  4. Transformers

Answer.1. Generators

Explanation:-

Negative Sequence Overcurrent Relay

Negative sequence overcurrent relays are used to detect the unbalanced load on a generator which may cause excessive rotor heating. The relay is also used to detect unbalanced load currents in motors.

The stator windings may have to carry unbalanced currents due to (i) unbalanced load current, and (ii) due to the unsymmetrical fault. Whatever may be the reason, these unbalanced stator currents will have a negative sequence component, the magnitude of which depends on the degree of unbalance. The negative sequence currents in the stator windings will produce a magnetic field that rotates at synchronous speed but in an opposite sense to the dc field system. The relative speed between the negative sequence flux and the rotor, therefore, is twice the synchronous speed. As a result, double frequency voltage and current will be induced in the rotor body and the field winding. These currents produce severe rotor heating. Modern synchronous machines have a limited negative phase sequence current capability. In other words, there is a limit to the degree of unbalanced current that an alternator can withstand without suffering serious rotor overheating.

A negative phase sequence (NPS) relay is provided to an alternator to disconnect the machine before a dangerous temperature is reached. The negative sequence current heating is proportional to the square of the current.

The negative sequence current capability of a generator is specified in terms of:

(a) The maximum continuous current that can flow without damage;

(b) The maximum value of I2t can be withstood without damage when t is small (less than 10 see). I2 is the current in p.u. and t is in seconds.

To protect against negative sequence current, the negative phase sequence relay should have:

(i) A negative sequence segregating network which will separate the negative sequence component from the other sequence components of line current;

(ii) An IDMT over-current relay fed with the negative sequence current, thus segregated.

The relay characteristic is matched with the requirement, I2t = constant. The protection system is schematically shown in Figure.

negative sequence relay

 

Ques.66. A relay that measures impedance or a component of the impedance at the relay location is known as

  1. Induction Relay
  2. Moving Coil Relay
  3. IDMT Relay
  4. Distance Relay

Answer.4. Distance Relay

Explanation:-

Distance relay: A relay that measures the impedance or a component of the impedance at the relay location is known as a distance relay. It is used for the protection of a transmission line. As the impedance of a line is proportional to the length of the line, a relay that measures the impedance or its component is called a distance relay.

Distance protection is a widely used protective scheme for the protection of high and extra-high voltage (EHV) transmission and sub-transmission lines. This scheme employs a number of distance relays that measure the impedance or some components of the line impedance at the relay location. The measured quantity is proportional to the line length between the location of the relay and the point where the fault has occurred. As the measured quantity is proportional to the distance along the line, the measuring relay is called a distance relay. Overcurrent relays have been found unsuitable for the protection of transmission lines because of their inherent drawbacks of variable reach and variable operating time due to changes in source impedance and fault type. Distance relays have been developed to overcome the problems associated with the use of overcurrent relays for the protection of transmission lines.

 

Ques.67. Distance relays are generally

  1. Mho relays
  2. Reactance relays
  3. Impedance relays
  4. All of the above

Answer.1. All of the above

Explanation:-

A distance protection scheme is a non-unit system of protection. A single scheme provides both primary and backup protection.

The most important and versatile family of relays is the distance-relay group. It includes the following types:

  1. Impedance relays
  2. Reactance relays
  3. MHO relays
  4. Angle impedance relays
  5. Quadrilateral relays
  6. Elliptical and other conic section relays

 

Ques.68. For which of the following ratings of the transformer differential protection is recommended?

  1. Above 30 kVA
  2. Equal to and above 5 MVA
  3. Equal to and above 25 MVA
  4. None of the above

Answer.2. Equal to and above 5 MVA

Explanation:-

The inherent characteristics of power transformers introduce a number of unique problems that are not present in the protection of transmission lines, generators, motors or other power system apparatus. Transformer faults – i.e. short circuits – are the result of internal electrical faults, the most common one being the phase-to-ground fault. Somewhat less common are the turn-to-turn faults. Unlike a transmission line, the physical extent of a transformer is limited to within a substation, and consequently differential relaying, the most desirable form of protection available, can be used to protect transformers. In general, a transformer may be protected by fuses, overcurrent relays, differential relays, and pressure relays.

Transformer size:- The high cost of transformers justifies fast, reliable protection for all sizes, although additional protection is justified above about 5 MVA. The simplest form of protection for transformers below about 2 MVA and at 69 kV and below is the fuse. Although fuses are available for 115 kV, their low fusing current can make them unreliable. For transformers up to this size that is equipped with circuit breakers, phase-instantaneous and phase-ground inverse-time overcurrent relays (50/51) can provide good protection if the fault level is large enough to get satisfactory settings. For transformers rated at 5 MVA and above that are important to system operation, or where high sensitivity is necessary (generator transformers), differential protection is recommended in addition to high- and low-side overcurrent relays. Buchholz protection should be standard for all conservator-type transformers together with the winding-temperature trip for transformers of more than 5 MVA or just a temperature trip for the smaller unit.

differential relay

Location and function:- In addition to the size of the transformer, the decision regarding the specific protection application is significantly affected by consideration of the importance of the transformer within the power network. If the transformer is an integral part of the bulk power system, it will probably require more sophisticated relays in terms of design and redundancy. If it is a distribution station stepdown transformer, a single differential relay and overcurrent backup will usually suffice. If the transformer is near a generation source, the high XtR ratio of the fault path will require harmonic restraint relays to accommodate the higher magnetic inrush currents.

Voltage:- Generally, higher voltages demand more sophisticated and costly protective devices, due to the deleterious effect of a delayed fault clearing on the system performance, and the high cost of transformer repair.

Connection and design:- The protection schemes will vary considerably between autotransformers, and two- or three-winding transformers. The winding connection of a three-phase

 

Ques.69. A _______ is used to measure the stator % winding temperature of the generator.

  1. Thermocouple
  2. Pyrometer
  3. Resistance thermometer
  4. Thermometer

Answer.4. Thermometer

Explanation:-

Heat Run test on induction motor

This test is carried out to determine the temperature rise of different parts of the motor while it is running at rated speed. For motors with the continuous rating and for motors for periodic duty and for continuous duty with intermittent load, the test of heat run should be continued till thermal equilibrium is reached. In the case of intermittent duty cycle motors, the duration of one cycle should be 10 minutes.

The temperature should be measured both while the motor is running and after it is shut down. Whenever thermal equilibrium is reached, the motor should be stopped as quickly as possible. When temperatures are measured after shutdown, the cooling curve should be plotted.

The temperature rise of a part of the motor is the difference between the temperature of the part and the temperature of the cooling medium.

Methods of measuring the temperature of windings

(a) Embedded temperature detector method: These detectors are either resistance thermometers or thermocouples built in the machine during manufacture at inaccessible points such as the slot portion of stator windings.

(b) Resistance method: Where embedded temperature detectors are not used, this method is used. The increase in resistance of the stator windings is measured and then the temperature rise of the windings is determined.

The temperature rise of the winding is found out by the expression

heat un

where

ta = temperature °C of cooling air or gas at the end of the test.

t2 = temperature °C of the winding at the end of the test.

t1 = temperature °C of the winding (cold) initially.

R1 = initial resistance of the winding.

R2 = resistance of the winding at the end of the test.

Thermometer method: Wherever the last two methods are not applicable, this method is used for measurement of the temperature. This method uses thermometers applied to the accessible surface of the motor. Where bulb thermometers are used in any moving or varying magnetic field, alcohol thermometers should be used in preference to mercury thermometers.

Measurement of cooling air or gas temperature:

Measure the temperature of the cooling medium at the entrance of the motor when using forced ventilation.

In other cases, the cooling air temperature should be measured by several thermometers at various points around the motor at a distance of one or two metros.

Generally, the heat run test on induction motors is carried out by a pump test on two identical coupled machines so that the net energy used during the test is corresponding to the losses in the two machines only and not as per full load on the machines as in that case output is wasted in load.

 

Ques.70. The under voltage relay can be used for

  1. Generators
  2. Busbars
  3. Motors
  4. All of the above

Answer.4. All of the above

Explanation:-

An under-voltage relay operates when the voltage that flows through a relay’s coil falls below a predetermined value. Under-voltage relays protect loads against voltage drops that can lead to power shortages and brown-outs. When the voltage drops due to some reason, the current drawn by the machine increases to cater to the constant load torque. Current beyond the rated value may damage the windings, melt the insulation or cause the thermal failure of the motor.

The typical uses for this relay function include

a) Bus Undervoltage protection:- The Undervoltage relay may either alarm or trip voltage-sensitive loads, such as induction motors, whenever the line voltage drops below the calibrated setting. A time-delay relay is normally used to enable it to ride through momentary sags and thus prevent nuisance operation. For electromechanical relays, to prevent the inertia (or overtravel) of the time-delay relay from tripping the circuit, an instantaneous Undervoltage relay with its contacts connected in series with the time Undervoltage relay contacts may also be used to provide a fast reset time.

b) Source transfer scheme:- The Undervoltage relay is used to initiate the transfer and, when desired, retransfer of a load from its normal source to standby or emergency power source. Due to the possibility of a motor load, this relay has a time delay in order to preclude out-of-synchronism closures.

c) Permissive functions:- An instantaneous Undervoltage relay is used as a permissive device to initiate or block certain actions when the voltage falls below the dropout setting.

d) Backup functions:– A time-Undervoltage relay may be used as a backup device following the failure of other devices to operate properly. For example, a long-time-delay relay may be used to trip an isolated generator and its auxiliaries if the primary protective devices fail to do so.

e) Timing applications:- A time-Undervoltage relay can be used to insert a precise amount of time delay in an operating sequence. Certain protective functions, such as a negative-sequence overvoltage relay, may require a time delay to prevent nuisance tripping

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