MCQ For Electrical Eng.-4

From the economic point of view , a power system should have a load factor

1. About 0.12
2. About 0.5
3. About1

Ans. c

From the economic point of view , a power system should have a diversity factor

1. Much less than 1
2. Nearly equal to but less than one
3. Greater than one

Ans. c

Peak load hydro power plants compared to base load thermal plants have plant capacity factor

1. Much lower
2. Much higher
3. Almost equal

Ans. a

Run-of-river plants are used for

1. Base load operation
2. Peak load operation
3. Near base load

Ans. a
The two-part tariff is based on

1. MW and MVar
2. MW and MVA
3. MWH and MVA

Ans. c

 CIRCUIT -BREAKERS - OBJECTIVE TYPE QUESTIONS

1.      Which of the following results when a resistance is placed across the contacts of a circuit breaker

i successful and effective interruption of current

ii undue power loss in the resistor

iii effective fault power dissipation

iv control of rate of rise of restriking voltage.

a.       all four above

(b) i and iii

(c) i and iv.

Ans: (c)

1.      The circuit breaker rating relevant to transient stability is

a.       rated voltage

b.      rated operating time

c.       rated breaking current

Ans: c

ECONOMIC OPERATION -OBJECTIVE TYPE QUESTIONS

1.      When generating units in a power plant are loaded with equal incremental costs, the cost of generation is

a.       maximum

b.      minimum

c.       not affected

Ans.: (b)

1.      Incremental heat - rate curves for thermal generating units are used to determine the

a.       fuel cost in $/hour

b.      values to which the units should be loaded to decrease the fuel costs to a minimum

c.       none of the above.

Ans.: (b)

1.      In economic operation of a power system , the effect of increased penalty factor between a generating plant and system load centre is

a.       to decrease the load on the generating plant

b.      to increase the load on the plant

c.       to hold the plant load constant.

Ans.: (a)

1. Reactive power in a single-phase a.c circuit is a product of

1. Voltage times the current in a circuit
2. Voltage times the current times the cosine of the angle between the voltage & current
3. Voltage times current times the sine of the angle between the voltage & current

Ans.c

1. In a lightly loaded transmission line of such a length that the capacitive reactance is appreciable, the receiving end voltage

1. Is always less than the sending end voltage
2. May exceed the sending end voltage
3. Is always equal to the sending end voltage

Ans.b

3. On a long high voltage transmission line under heavy load conditions, Var compensation can be provided by installing

1. Series inductive reactors
2. Series capacitors
3. Shunt inductive reactors

Ans.b

4. If a synchronous motor(properly synchronized to the supply) is running on no load with negligible loss then

1. The stator current will be very high
2. the stator current will be zero
3. the stator current will be very small
4. the back emf will be more than the supply voltage

Ans.c

5. The minimum armature current of he synchronous condenser corresponds to

1. 0.8 lagging p.f
2. zero pf lagging
3. 0.8 pf leading
4. unity pf

Ans.d

5. When synchronous condensers are used to provide vars, leading Vars are provided by

1. Increasing the field current
2. Decreasing the field current
3. Increasing the speed of the machine

Ans.a

5. Vars are characterized by the fact that they always flow

1. From points of low voltage to points of high voltage
2. From points of high voltage to points of low voltage
3. Between two points having the same voltage

Ans.b
SHORT-CIRCUIT ANALYSIS -OBJECTIVE TYPE QUESTIONS

1. If a positive sequence current passes through a transformer and its phase shift is 30 degrees, the negative sequence current flowing through the transformer will have a phase shift of

1. 30 deg.
2. -30deg
3. 120 deg.

Ans: (b)

2. The zero sequence impedances of an ideal star-delta connected transformer (star-grounded)

1. looking from star side is zero and looking from delta side is infinite
2. looking from star side is zero and looking from delta side is also zero
3. looking from star side is infinite and looking from delta side is zero

Ans: (a)

2. The positive, negative and zero sequence impedances of a transmission line are 0.5,0.5 and 1.1 pu respectively. The self (Zs) and mutual (Zm) impedances of the line will be given by

(a) Zs = 0.7 pu, Zm =0.2 pu

(b) Zs = 0.5 pu, Zm =0.6 pu

(c)Zs = 1 pu, Zm =0.6 pu

Ans: (a)

3. Symmetrical component method of analysis is more useful when

1. system has unsymmetrical fault and the network is otherwise balanced
2. system has symmetrical fault and the network is otherwise unbalanced
3. system has unsymmetrical fault and the network is unbalanced.

Ans: (a)

2. For a line to line fault analysis using symmetrical components,

(a) the positive and negative sequence networks at the fault point are

connected in series

(b) the positive and negative sequence networks at the fault point are

connected in parallel

(c) the positive, negative, and zero sequence networks at the fault point are

connected in parallel

Ans: (b)

3. The machine reactances used for computation of short circuit current ratings of a circuit breaker are

1. synchronous reactance
2. transient reactance
3. sub-transient reactance

Ans: (c)

2. The load currents in short-circuit calculations are neglected because

1. short-circuit currents are much lager than load currents
2. short -circuit currents are greatly out of phase with load currents

The correct alternative is

1. both (i) and (ii) are wrong
2. (i) is wrong and (ii) is correct
3. both (i) and (ii) are correct.

Ans: (c)

2. The positive sequence network of a sample power system is shown in Fig.2 and the primitive reactance of each element is marked in ohms. The element Zcc of the bus impedance matrix Zbus will be

1. 1.0
2. 0.1
3. 0.21

Fig.2
Ans: (c)

2. Which one of the following statements is not true?

1. Fault levels in an all a. c system are less than in an a. c system with a D.C. link operating
2. Large systems may be interconnected with d. c link of small capacity
3. Limitation on the critical length of underground cables for use in A. C no longer exists if D.C transmission by cables is used.
4. Corona loss and radio and TV interference with D.C. transmission is less

Ans. a

2. Advantage of power system interconnection is

1. Large size circuit breakers are required because of large short-circuit currents
2. Machines of one system remain in step with machines of another system
3. Fewer machines are required as reserve for operation at peak loads

Ans. c

2. The interenal voltages of a 3-phase synchronous generator correspond to

1. Positive sequence
2. Negative sequence
3. Zero sequence

PER UNIT SYSTEM- OBJECTIVE TYPE QUESTIONS

1. The synchronous reactance of a modern large size turbogenerator in pu is nearest to

1. 1
2. 2
3. 3

Ans: (b)

2. The base value of a power system is chosen based upon

1. the rating of one machine in the system
2. the largest machine in the system
3. some average (rounded) values such that the pu machine ratings are not too large or too small.

Ans: (c)

3. A bank of three single-phase transformers is star-star connected and has a % reactance of 10%. The same bank is then converted to satisfy rated star-delta connections. The per cent reactance of the transformer will now be

1. 10/  3 %
2. 10 3 %
3. 10 %

Ans: (c)

4. A transmission line has a pu reactance of 30 %. If the working voltage is now increased to 110% of its original voltage ( the MVA rating of the line remaining the same), the pu reactance of the line will now be

1. 33%
2. 36.3%
3. 24.8%

Ans: (c)

4. The percentage reactance can be converted into ohmic value with the following formula

1. X(ohms)=X(%)*100*kV² /kVA
2. X(ohms)=X(%)*1000*kV² /kVA
3. X(ohms)=X(%)*kV² /(1000*kVA)

Ans. None of the above

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 c.   The speed-torque characteristics of a DC series motor are approximately similar to those of the _________motor.

                  

(A)    Universal                                       (B) synchronous

(C)  DC shunt                                      (D) two-phase

             d.   The rotor frequency for a 3 phase 1000 RPM 6 pole induction motor with a slip of 0.04 is________Hz

(A)    8                                                      (B) 4

(C)  6                                                 (D)  2       

             e.   The torque-speed characteristics of an a.c. operated universal motor has a ______characteristic and it______ be started under no-load condition.

(A) inverse, can                                  (B) nearly inverse, can

(C)  inverse, cannot                             (D) nearly inverse, cannot  

             f.    In the heating process of the ________type a simple method of temperature control is possible by means of a special alloy which loses its magnetic properties at a particular high temperature and regains them when cooled to a temperature below this value.

(A)     Indirect induction over                  (B) core type induction furnace

(C)  Coreless induction furnace             (D) high frequency eddy current

             g.   In order to reduce the harmful effects of harmonics on the a.c. side of a high voltage D.C. transmission system ______are provided.

(A)     Synchronous condensers               (B) shunt capacitors

(C)  Shunt filters                                   (D) static compensators

             h.   An a.c. tachometer is just a ________with one phase excited from the carrier frequency.

(A)    Two-phase a.c. servomotor           (B) two-phase induction motor

(C) a.c. operated universal motor         (D) hybrid stepper motor.

             i.    The torque, in a _____________is proportional to the square of the armature current

(A)   DC shunt motor                            (B) stepper motor

(C) 2-phase servomotor                      (D) DC series motor

             j.    The synchronous speed for a 3 phase 6-pole induction motor is 1200 rpm. If the number of poles is now reduced to 4 with the frequency remaining constant, the rotor speed with a slip of 5% will be _________.

(A) 1690 rpm                                      (B) 1750 rpm

(C) 1500 rpm                                      (D) 1710 rpm

       

a.       Thevenin’s equivalent circuit consists of _________.

                   (A)  Series combination of R_Th, E_Th and R_L.

                   (B)  Series combination of R_Th, E_Th.

(C)    Parallel combination of R_Th, E_Th. 

(D)   Parallel combination of R_Th, E_Th and R_L.

       

b.      In an R – L –C circuit, the phase of the current with respect to the circuit voltage will be_________.

(A)    Leading.                                       (B)  Same.

(C)  Lagging.                                       (D)  Depends upon the value of Land C.