Topic wise previous Gate question answers Power System

Topic wise previous Gate question answers Power System

1.A negative sequence relay is commonly used to protect

(A) an alternator 

(B) an transformer 

(C) a transmission line 

(D) a bus bar 

Ans: Option (A) is correct.

Negative phase sequence relay is used for the protection of alternators against unbalanced loading that may arise due to phase-to-phase faults.

2. For enhancing the power transmission in along EHV transmission line, the most preferred method is to connect a 

(A) Series inductive compensator in the line 

(B) Shunt inductive compensator at the receiving end 

(C) Series capacitive compensator in the line 

(D) Shunt capacitive compensator at the sending end

Ans: Option (C) is correct. Steady state stability or power transfer capability Pmax X E V = To improve steady state limit, reactance X should be reduced. The stability may be increased by using two parallel lines. Series capacitor can also be used to get a better regulation and to increase the stability limit by decreasing reactan

3. The total reactance and total suspectance of a lossless overhead EHV line, operating at 50 Hz, are given by 0.045 pu and 1.2 pu respectively. If the velocity of wave propagation is 3 105 # km/s, then the approximate length of the line is 

(A) 122 km 

(B) 172 km 

(C) 222 km 

(D) 272 km

Ans: Option (B) is correct.

4.In a biased differential relay the bias is defined as a ratio of (A) number of turns of restraining and operating coil
(B) operating coil current and restraining coil current
 (C) fault current and operating coil current
D) fault current and restraining coil current

Ans: Option (B) is correct. Ratio of operating coil current to restraining coil current is known as bias in biased differential relay.

5.The Gauss Seidel load flow method has following disadvantages. Tick the incorrect statement.
(A) Unreliable convergence
(B) Slow convergence
(C) Choice of slack bus affects convergence
 (D) A good initial guess for voltages is essential for convergence

Ans: Option (A) is correct. Unreliable convergence is the main disadvantage of gauss seidel load flow method.

6.The insulation strength of an EHV transmission line is mainly governed by
(A) load power factor
 (B) switching over-voltages
 (C) harmonics
 (D) corona

Ans: Option (B) is correct. In EHV lines the insulation strength of line is governed by the switching over voltages. 

7.High Voltage DC (HVDC) transmission is mainly used for
(A) bulk power transmission over very long distances
(B) inter-connecting two systems with same nominal frequency
C) eliminating reactive power requirement in the operation
 (D) minimizing harmonics at the converter stations

Ans: Option (A) is correct. For bulk power transmission over very long distance HVDC transmission preferably used.

8.Bundled conductors are mainly used in high voltage overhead transmission lines to
 (A) reduces transmission line losses (B) increase mechanical strength of the line
(C) reduce corona
(D) reduce sag

Ans: Option (C) is correct. In case of bundled conductors, We know that self GMD of conductor is increased and in a conductor critical disruptive voltage of line depends upon GMD of conductor. Since GMD of conductor is increased this causes critical disruptive voltage is being reduced and if critical disruptive voltage is reduced, the corona loss will also be reduced.

9. A lossless radial transmission line with surge impedance loading
 (A) takes negative VAR at sending end and zero VAR at receiving end
(B) takes positive VAR at sending end and zero VAR at receiving end
(C) has flat voltage profile and unity power factor at all points along it
(D) has sending end voltage higher than receiving end voltage and unity power factor at sending end

Ans: Option (C) is correct. A lossless radial transmission line with surge impedance loading has flat voltage profile and unity power factor at all points along it. 

10. Consider a power system with three identical generators. The transmission losses are negligible. One generator(G1) has a speed governor which maintains its speed constant at the rated value, while the other generators(G2 and G3) have governors with a droop of 5%. If the load of the system is increased, then in steady state.

(A) generation of G2 and G3 is increased equally while generation of G1 is unchanged. 

(B) generation of G1 alone is increased while generation of G2 and G3 is unchanged. 

(C) generation of G1, G2 and G3 is increased equally

(D) generally of G1, G2 and G3 is increased in the ratio 0.5 : 0.25 : 0.25.

Ans: Option (B) is correct. Given power system with these identical generators G1 has Speed governor G2 and G3 has dr0op of 5% When load increased, in steady state generation of G1 is only increased while generation of G2 and G3 are unchanged.