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This course covers detailed solutions for questions for all the questions given in the last pages of CL WADHWA textbook.

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CL Wadhwa Backside Bits Solutions (Part - 2)

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This lesson is the overview of the CL wadhwa backside bits solution course ( questions from 125- ....)

Overview

125. The phase comparators in case of static relays and electro-mechanical relays normally
are:

(a) Sine and cosine comparators respectively
(b) Cosine and sine comparators respectively
(c) Both are cosine comparators
(d) Both are sine comparators.


126. The order of the lightning discharge current is:

(a) 10,000 amp (b) 100 amp
(c) 1 amp (d) 1 microampere.


127. The magnetising-inrush-current in a transformer is rich in:

(a) 3rd harmonics (b) 5th harmonics
(c) 7th harmonics (d) 2nd harmonics.


128. For effective use of a counterpoise wire:

(a) Its leakage resistance should be greater than the surge impedance
(b) Its leakage resistance should be less than the surge impedance
(c) Its leakage resistance should be equal to the surge impedance
(d) The two resistances may have any relation.


129. The inertia constants of two groups of machines which do not swing together are M1 and M2. The equivalent inertia constant of the system is:

(a) M1 + M2 
(b) M1 – M2 if M1 > M2
(c)M M
M M
1 2
1 + 2
(d) M1-M2

CL Wadhwa Bits Solutions (125-129)

130. If the inductance and capacitance of a system are 1 H and 0.01 μF respectively and the
instantaneous value of current interrupted is 10 amps, the value of shunt resistance
across the breaker for critical damping is:
(a) 100 kΩ (b) 10 kΩ
(c) 5 kΩ (d) 1 kΩ
.
131. For load flow solution the quantities specified at a load bus are:
(a) P and Q (b) P and |V|
(c) Q and |V| (d) P and δ.

132. The solution of coordination equations takes into account:
(a) All the system constraints
(b) All the operational constraints
(c) All the system and operation constraints
(d) None of the above.

133. For a two-bus system if the change in load at bus 2 is 5 MW and the corresponding
change in generation at bus 1 is 8 MW, the penalty factor of bus 1 is:
(a) 0.6 (b) 1.67
(c) 0.625 (d) None of the above.

134. If the penalty factor for bus 1 in a two-bus system is 1.25 and if the incremental cost of
production at bus 1 is Rs. 200 per MWhr, the cost of received power at bus 2 is:
(a) Rs. 250/MWhr (b) Rs. 62.5/MWhr
(c) Rs. 160/MWhr (d) None of the above.

CL wadhwa bits solutions (130-134)

135. The cost of generation is theoretically minimum if:
(a) The system constraints are considered
(b) The operational constraints are considered
(c) (a) and (b)
(d) The constraints are not considered.

136. The incremental transmission loss of a plant is:
(a) Positive always (b) Negative always
(c) Can be positive or negative.

137. If Pm is the maximum power transferred, the loss on the system is:
(a)
Pm
4
(b)
Pm
2
(c)
3
4
Pm 
(d) None of the above.

138. If X is the system reactance and R its resistance, the power transferred is maximum
when:
(a) X = R (b) X = 2R
(c) X = 3R (d) X = 2R.
 	
	
139. If the inertia constant H of a machine of 200 MVA is 2 p.u. its value corresponding to
400 MVA will be:
(a) 4 p.u (b) 2 p.u.
(c) 1.0 p.u. (d) 0.5 p.u.

CL wadhwa bits solutions (135-139)

140. The inertia constant of two groups of machines which do not swing together are M1 and
M2 such that M1 > M2. It is proposed to add some inertia to one of the two groups of
machines for improving the transient stability of the system. It should be added to:
(a) M1 (b) M2
(c) It does not matter whether to add to M1 or M2.

141. Tick out the correct one:
(a) Higher the SCR of a machine the heavier is its rotor
(b) Higher the SCR of a machine the lighter is its rotor
(c) The SCR and size of the rotor are not at all related.

142. A 100 V/10 V, 50 VA transformer is converted to 100 V/110 V auto transformer, the
rating of the auto transformer is:
(a) 550 VA (b) 500 VA
(c) 110 VA (d) 100 VA.

143. A 100 KVA transformer has maximum efficiency of 98% when operating at half full
load. Its full load losses are:
(a) Cu = 2.04 kW and Iron loss = 0.51 kW
(b) Cu = 4.08 kW and Iron loss = 1.02 kW
(c) Cu = 3.0 kW and Iron loss = 0.75 kW
(d) None of the above.

144. A voltmeter gives 120 oscillations per minute when connected to the rotor. The stator
frequency is 50 Hz. The slip of the motor is:
(a) 2% (b) 4%
(c) 5% (d) 2.5%.

CL wadhwa bits solutions (140-144)

145. If the excitation of the synchronous generator fails, it acts as a:
(a) Synchronous motor (b) Synchronous generator
(c) Induction motor (d) Induction generator.

146. I and T are the line current and the torque respectively when DOL starter is used, these
quantities when Y/Δ starter is used, are:
(a)
I T
3 3
, (b) I T
3 3
,
(c) I T
2 2
, (d)
I T
2 4
, .

147. A 100 KVA transformer has 4% impedance and 50 KVA transformer has 3% impedance.
When they are operated in parallel, which transformer will reach full load first.
	
	
 
(a) 3% (b) 4%
(c) The data is insufficient to judge.

148. In EHV transmission lines, efficiency of transmission can be increased by decreasing
the corona loss. This is achieved by
(a) Increasing the distance between the line conductors
(b) Using bundled conductors
(c) Using thick conductors
(d) Using thin conductors.

149. An alternator having induced emf. of 1.6 p.u. is connected to an infinite bus of 1.0 p.u. If
the busbar has reactance of 0.6 p.u. and alternator has reactance of 0.2 p.u., the maximum
power that can be transferred is given by
(a) 8 p.u. (b) 2 p.u.
(c) 2.67 p.u. (d) 5.0 p.u.

CL wadhwa bits solutions (145 - 149)

150. If one phase of supply goes off in the case of 3-phase induction motor, the motor
(a) Comes to a stop
(b) Draws double the initial current and continues to run
(c) Draws 1/ 3 times the initial current and continues to run
(d) None of the above.

151. Induction generator works between the slip
(a) 1 < s < 2 (b) 0.1 < s < 1.0
(c) s < 0.0 (d) None of the above.

152. The motor which can be used on both a.c. and d.c. is
(a) Reluctance motor (b) Induction motor
(c) d.c. series motor (d) None of the above.

153. If transformer frequency is changed from 50 Hz to 60 Hz, the ratio of eddy current loss
50 Hz to 60 Hz at constant voltage is
(a) 5/6 (b) 25/36
(c) 6/5 (d) 1.0.

154. An alternator of 300 kW is driven by a prime mover of speed regulation 4% and another
alternator of 200 kW driven by a prime mover of speed regulations 3%, the total load
they can take is
(a) 500 kW (b) 567 kW
(c) 425 kW (d) 257 kW

CL wadhwa bits solutions (150-154)

155. The minimum oil circuit breaker has less volume of oil because:
(a) There is insulation between contacts
(b) The oil between the breaker contacts has greater strength
(c) Solid insulation is provided for insulating the contacts from earth
(d) None of the above is true.
 	


156. Load flow study is carried out for
(a) Fault calculations (b) Stability studies
(c) System planning (d) Load frequency control.

157. Arcing on transmission lines is prevented by connecting a suitable
(a) Circuit breaker (b) Protective relay
(c) Inductor in the neutral (d) Capacitor in the neutral.

158. The in-rush current of a transformer at no load is maximum if the supply voltage is
switched on
(a) At zero voltage value (b) At peak voltage value
(c) At V/2 value (d) At 3 /2 V value.

159. If the loading of the line corresponds to the surge impedance loading the voltage at the
receiving end is
(a) Greater than sending end (b) Less than sending end
(c) Equal to the sending end (d) None of the above is necessary.

CL wadhwa bits solutions (155-159)

160. The voltages of a generator and an infinite bus are given as 0.92∠10° and 1.0∠0°
respectively. The generator acts as a
(a) Shunt coil (b) Shunt capacitor
(c) The data is insufficient to judge.

161. For a synchronous phase modifier the load angle is
(a) 0° (b) 25°
(c) 30° (d) None of the above.

162. The voltages of a generator and an infinite bus are given as 0.92∠10° and 1.0∠0°
respectively. The active power will flow from
(a) Generator to infinite bus (b) Infinite bus to generator
(c) The data is insufficient to judge.

163. If the penalty factor of a plant is unity, its incremental transmission loss is
(a) 1.0 (b) – 1.0
(c) Zero (d) None of the above.

164. In a two plant system, the load is connected at plant no. 2. The loss coefficients
(a) B11, B12, B22 are nonzero
(b) B11 and B22 are nonzero but B12 is zero
(c) B11 and B12 are nonzero but B22 is zero
(d) B11 is nonzero but B12 and B22 are zero.

CL wadhwa bits solutions (160-164)

165. The diagonal elements of a nodal admittance matrix are strengthened by adding
(a) Shunt inductances (b) Shunt capacitors
(c) Loads (d) Generators.

 
166. The charging reactance of 50 km length of line is 1500 Ω. The charging reactance for 100
km length of line will be
(a) 1500 Ω (b) 3000 Ω
(c) 750 Ω (d) 600 Ω.

167. The eddy current loss in the core of a transformer is
(a) Inversely proportional to resistivity of the core material
(b) Directly proportional to resistivity of the core material
(c) Directly proportional to square of resistivity of the core material
(d) None of the above is true.

168. The normal practice to specify the making current of a circuit breaker is in terms of
(a) r.m.s value (b) Peak value
(c) Average value (d) Both r.m.s. and peak value.

CL wadhwa bits solutions (165-168)

169. A transmission line of 80 km length is operating at 400 Hz, it can be classified as
(a) Short length line (b) Medium length line
(c) Long length line (d) all of the above.

170. A transmission line of 210 km length has certain values of parameters A, B, C, D. If the
length is made 100 km thus the parameter
(a) A increase B decreases (b) A decreases B also decreases
(c) A and B both increase (d) A decreases B increases

171. In the above problem, the parameters
(a) C increases D decreases (b) C and D both decrease
(c) C and D both increase (d) C decreases D increases

172. When two transformers of different KVA ratings are connected in parallel they share
the load in proportion to their respective KVA rating only when their
(a) KVA ratings are identical (b) Efficiencies are equal
(c) pu impedances are equal (d) equivalent impedances are equal.

CL wadhwa bits solutions (169-172)

173. A salient pole synchronous motor is running at no load, its field current is switched off.
The motor will
(a) come to a stop
(b) continue to run at synchronous speed
(c) continue to run at a speed slightly less than synchronous speed
(d) none of these.

174. In a network the sum of currents entering a node is 5∠60°. The sum of the currents
leaving the node is
(a) 5 ∠60° (b) 5 ∠ – 60°
(c) 5 ∠240° (d) 15 A.
 	


175. In a synchronous machine, in case the axis of field flux is in line with the armature flux,
the machine is working
(a) as synchronous motor (b) as synchronous generator
(c) as floating machine (d) the m/c will not work
.
176. An ideal voltage source is connected across a variable resistance. The variation of current
as a function of resistance is given by
(a) A straight line passing through the origin
(b) a rectangular hyperbola
(c) a parabola
(d) it could be anyone of the above.

CL wadhwa bits solutions (173-176)

177. Pure inductive circuit takes power (reactive) from the a.c. line when
(a) both applied voltage and current rise
(b) both applied voltage and current decrease
(c) applied voltage decreases but current increases
(d) (a) and (b).

178. The current in the primary of the given transformer is
200
100 V
10 
(a) 10 A (b) 5 A
(c) 10/ 3 A (d) none of the above.

179. A synchronous machine has its field winding on the stator and armature winding on the
rotor. Under steady running condition, the air gap field
(a) rotates at synchronous speed w.r.t. stator
(b) rotates at synchronous speed in the direction of rotation of the rotor
(c) remains stationary w.r.t. stator
(d) remains stationary w.r.t. rotor.

180. For a given applied voltage to a transformer, if its magnetic core is replaced by a nonmagnetic
core
The magnitude of magnetic flux produced by the primary winding
(a) remains same (b) decreases
(c) increases (d) all the above alternatives are possible.

CL wadhwa bits solutions (177-180)

181. If a transformer is switched on to a voltage more than the rated voltage, for a certain
loading condition (lag p.f.)
(a) its p.f. will improve (b) its p.f. will deteriorate
(c) no efect on p.f. (d) it may or may not improve
.
182. Three phase transformers which can not be connected in parallel are
(a) YY with ΔΔ (b) YΔ with ΔY
(c) YY with YΔ (d) (b) and (c).

183. For certain geometry and operating voltage of the uncompensated transmission line the
ratio of power transfer capability to the surge impedance loading with increase in length
(a) increases (b) remains unchanged
(c) decreases (d) no fixed criterion.

184. If the normal system frequency is 50 Hz and if it is operating at 53 Hz, the equipment on
the system most adversely affected is
(a) Power transformer (b) Alternator
(c) Turbine (d) all the above are equally affected.

CL wadhwa bits solutions (181-184)

185. The operation of the relay which is most affected due to arc resistance is
(a) Mho relay (b) reactance relay
(c) Impedance relay (d) all are equally affected
.
186. The relay which is most sensitive to power swings (maloperation) is
(a) Mho relay (b) reactance relay
(c) Impedance relay (d) all are equally affected.

187. A 100/5 A bar primary current transformer supplies an O/C relay set at 25% pick up and
it has a burden of 5 VA. The secondary voltage is
(a) 1 volt (b) 1.25 volt
(c) 2.5 volt (d) 4 volt.

188. A 100 : 5 current transformer has secondary winding and lead resistance of 0.2Ω and
the secondary burden is 5 VA. The VA output is
(a) 5 VA (b) 10 VA
(c) 7.5 VA (d) 2.5 VA.

189. The phase angle error of a current transformer can be reduced by connecting an element
across its secondary which is predominantly
(a) resistive (b) inductive
(c) capacitive (d) independent of the nature of element.

CL wadhwa bits solutions ( 185 - 189)

190. Auto reclosing is used in case of
(a) Lightning arrester (b) Air blast C.B.
(c) Bulk oil C.B. (d) Minimum oil C.B.
191. If the normal frequency is 50 Hz, the setting of the under-frequency relay for the most
important load could be
 	


(a) 49 Hz
(b) 48.5 Hz
(c) 47.5 Hz
(d) there is no grading for under frequency relays.

192. A distance relay is said to be inherently directional if its characteristic on R-X diagram
(a) is a straight line off-set from the origin
(b) is a circle that passes through the origin
(c) is a circle that encloses the origin
(d) always a separate directional relay is required.

193. The standard current ratings of an electromagnetic relay are
(a) 5 A and 15 A (b) 15 A and 20 A
(c) 1 A and 5 A (d) any one of the above.

CL wadhwa bits solutions (190-193)

194. Standard voltage ratings of the electromagnetic relay are
(a) 110 V, 63.5 (L-N) (b) 220 V 110 V
(c) 230 V 154 V (d) 400 V 231 V.

195. In order to quench the arc quickly and also optimise the dimensions generally
(a) oil C.B. is preferred (b) air blast C.B. is preferred
(c) SF6 C.B. is preferred (d) minimum oil C.B. is preferred.

196. The frequency of the carrier in the case of carrier current-pilot scheme is in the range of
(a) 1 KHz to 10 KHz (b) 15 KHz to 25 KHz
(c) 25 KHz to 50 KHz (d) 50 KHz to 500 KHz.

197. The unit protection scheme provides
(a) primary protection (b) Back up protection
(c) simultaneous protection (d) remote protection.
.

CL wadhwa bits solutions (194-197)

198. The X : R ratio for 220 KV line as compared to 400 KV line is
(a) greater (b) smaller
(c) equal (d) it could be anything
.
199. If a line is 100% series compensated it may result into series resonance at
(a) power frequency (50 Hz) (b) 100 Hz
(c) 150 Hz (d) none of the above
.
200. SSR phenomenon is
(a) purely electrical (b) purely mechanical
(c) purely hydraulic (d) (a) and (b).

201. The order of the subharmonic during SSR for 50 Hz normal frequency is
(a) 25 Hz (b) 16
2
3
Hz
(c) 10 Hz (d) none of the above.

CL wadhwa bits solutions (198-201)

202. A transformer rated for 500 KVA 11 KV/0.4 KV has an impedance of 10% and is connected
to an infinite bus. The fault level of the transformer is
(a) 500 KVA (b) 5000 KVA
(c) 500 3 KVA (d) none of the above.

203. With the help of a reactive compensator it is possible to have
(a) constant voltage operation only
(b) unity p.f. operation only
(c) both constant voltage and unity p.f.
(d) either constant voltage or unity p.f.

204. Efficiency of a power transformer under no load condition is approximately
(a) 75% (b) 50%
(c) 25% (d) none of the above.

205. A 3-phase induction motor is drawing 5A when a single phasing takes place. The current
now drawn by the motor is
(a) 5 2 (b) 5 3
(c) 10 3 (d) none of the above.

CL wadhwa bits solutions (202-205)

206. When a d.c. source is switched in a purely inductive circuit the current response is
(a) an exponentially rising curve
(b) an exponentially decaying curve
(c) a straight line passing through the origin
(d) a straight line off-set from the origin.

207. The voltage and current in a circuit are given by
v = 10 sin (ωt – π/6)
i = 10 sin (ωt + π/6)
The power consumed is given as
(a) 100 watts (b) 50 watts
(c) 86.6 watts (d) 25 watts

208. In a large interconnected power system, consider three buses having short-circuit
capacities 1500 MVA (1), 1200 MVA (2) and 1000 MVA (3) respectively. The voltages of
all the buses are 1.0 p.u. If a 3-phase fault takes place on bus 2, the change in bus
voltage is described as
(a) ΔV1 > ΔV2 > ΔV3 (b) ΔV1 < ΔV3 < ΔV2
(c) ΔV1 > ΔV3 > ΔV2 (d) None of the above.

209. For a long uncompensated line the limit to the line loading is governed by
(a) Thermal limit (b) Voltage drop
(c) Stability limit (d) Corona loss.

CL wadhwa bits solutions (206-209)

210. For a lossless line terminated by its surge impedance, the natural reactive power loading
is
(a) V 2/X (b) V 2/Z
(c) V 2/Zc (d) Zero.

211. A 60 Hz 320 km lossless line has sending end voltage 1.0 p.u. The receiving end voltage
on no load is
(a) 1.1 p.u. (b) 1.088 p.u.
(c) 1.116 p.u. (d) none of the above.

212. The impedance of a circuit is given by z = 3 + j4, its conductance is given by
(a)
1
3
(b) 3/5
(c) 3/25 (d) 4/5.

213. With 100% series compensation of lines
(a) the circuit is series resonant at power frequency
(b) low transient voltage
(c) high transient current
(d) (a) and (c).

CL wadhwa bits solutions (210-213)

214. The effect of increasing gating angle in a thyristorised controlled reactor is
(a) to increase the effective inductance of the reactor
(b) to reduce the effective reactive power
(c) (a) and (b)
(d) none of the above.

215. The effect of increasing gating angle in a thyristorised controlled reactor is
(a) to decrease power loss in thyristor controller
(b) to decrease power loss in the reactor
(c) to make current wave form less sinusoidal
(d) all of the above.

216. The inertia constant H of a synchronous condenser is
(a) greater than that of hydro generator
(b) greater than that of turbo alternator
(c) equal to that of turbo alternator
(d) none of the above.

217. The effect of series capacitance compensation is
(a) to decrease the virtual surge impedance
(b) to decrease the effective length of the line
(c) to increase virtual surge impedance loading
(d) all of the above

CL wadhwa bits solutions - (214-217)

218. For any fixed degree of series compensation additional capacitive shunt compensation
(a) increases the effective length of line
(b) increases virtual surge impedance of line
(c) decreases virtual surge impedance loading of the line
(d) (b) and (c).

219. For any fixed degree of inductive shunt compensation, additional series capacitive
compensation
(a) increases the effective length of line
(b) increases virtual surge impedance of line
(c) decreases virtual surge impedance loading of the line
(d) none of the above.

220. With 100% inductive shunt compensation the voltage profile is flat for
(a) 100% loading of line (b) 50% loading of line
(c) Zero loading of line (d) None of the above.

221. A loss less line terminated with its surge impedance has
(a) Flat voltage profile
(b) transmission line angle is greater that actual length of the line
(c) transmission line angle is less than the actual length
(d) (a) and (b).

CL wadhwa bits solutions (218-221)

CL wadhwa bits solutions (222-225)

CL wadhwa bits solutions (226-229)

CL wadhwa bits solutions (230-233)

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Lessons

26 lessons • 4h 58m

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Overview

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CL Wadhwa Bits Solutions (125-129)

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CL wadhwa bits solutions (130-134)

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CL wadhwa bits solutions (135-139)

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CL wadhwa bits solutions (140-144)

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CL wadhwa bits solutions (145 - 149)

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CL wadhwa bits solutions (150-154)

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CL wadhwa bits solutions (155-159)

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CL wadhwa bits solutions (160-164)

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CL wadhwa bits solutions (165-168)

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