Question 1

Q.1. Why all overhead lines use ACSR conductors?

Accepted Answer

ACSR (Aluminium Conductor Steel Reinforced) conductors combine a lightweight, cheap, conductive aluminium outer layer with a strong steel core, giving the best strength-to-weight and cost-to-conductivity ratio for the long spans of overhead lines.

Question 2

Q.2. Why we use Vacuum Circuit Breaker in High Transmission System and not Ai…

Accepted Answer

Vacuum circuit breakers are preferred over air circuit breakers in high-voltage systems because vacuum offers far superior dielectric recovery and arc-quenching capability with minimal contact erosion and maintenance, allowing compact, reliable interruption at high voltage/current levels that air breakers cannot match economically.

Question 3

Q.3. Why HVDC line do not require any reactive power compensation?

Accepted Answer

HVDC lines do not require reactive power compensation because DC current flows without any oscillating electric/magnetic field exchange along the line, so there is no line reactance-based reactive power consumption or capacitive charging current at DC, unlike AC lines.

Question 4

Q.4. What are the advantages of adopting EHV/UHV for transmission of AC elect…

Accepted Answer

EHV/UHV transmission reduces line current for a given power (since P=√3VIcosφ), thereby reducing I²R losses, voltage drop, and conductor size requirements, while increasing the power transfer capacity of a given right-of-way corridor.

Question 5

Q.5. What will happen when power factor is leading in distribution of power?

Accepted Answer

A leading power factor in power distribution causes the receiving-end voltage to rise above the sending-end voltage (voltage boost, opposite of the usual voltage drop with lagging pf), due to the leading reactive current interacting with the line reactance in a way that adds to, rather than subtracts from, the voltage.

Question 6

Q.6. What is infinite bus in power system?

Accepted Answer

An infinite bus is an idealized reference bus in power system analysis whose voltage magnitude and frequency remain constant regardless of the amount of active or reactive power drawn from or injected into it, representing a source of effectively infinite generation capacity.

Question 7

Q.7. What is the difference between Isolator and Circuit Breaker?

Accepted Answer

An isolator is a manually operated, off-load switch used only to provide a visible, physical isolation gap for safe maintenance access after a circuit has already been de-energized, whereas a circuit breaker is an automatic device capable of safely making and breaking both normal load current and fault current under energized conditions.

Question 8

Q.8. How can we reduce the effect of corona?

Accepted Answer

Corona effects can be reduced by increasing conductor size/diameter (reducing surface voltage gradient), increasing spacing between conductors, using bundled conductors (multiple sub-conductors per phase), and ensuring smooth conductor surfaces free of sharp points, scratches or dirt.

Question 9

Q.9. What is the difference between grid station and substation?

Accepted Answer

A grid station is a major high-voltage transmission substation that interconnects bulk transmission lines and steps voltage down for onward regional distribution, whereas a substation (in general) is any facility that transforms/switches voltage levels, ranging from small distribution substations to large grid stations.

Question 10

Q.10. For a fault at a given location, rank the various faults in the order o…

Accepted Answer

In decreasing order of severity, faults are typically ranked: three-phase fault (most severe, symmetrical, highest fault current) > double line-to-ground fault > line-to-line fault > single line-to-ground fault (least severe, though most frequent in occurrence).

Question 11

Q.1. Two generators rated 10MVA, 13.2kV and 15MVA, 13.2kV are connected in pa…

Accepted Answer

Choosing a 50 MVA, 13.8 kV base, the per-unit reactances of G1 (0.15×50/10=0.75 p.u.), G2 (0.15×50/15=0.50 p.u.), M1 (0.20×50/8=1.25 p.u.) and M2 (0.20×50/12=0.833 p.u.) are computed by base-conversion and assembled into a per-unit reactance diagram with both generators feeding a common bus connected to both motors.

Question 12

Q.2. Derive the expression for the capacitance of three phase unsymmetrical s…

Accepted Answer

For an unsymmetrically spaced, fully transposed 3-phase line, the capacitance per phase to neutral is C = 2πε₀ / ln(GMD/r), where GMD = (Dab·Dbc·Dca)^(1/3) is the geometric mean distance between conductors and r is the conductor radius — identical in form to the symmetrical-spacing formula but with GMD replacing the fixed conductor spacing.

Question 13

Q.3. Discuss series and shunt compensation of a transmission line.

Accepted Answer

Series compensation adds capacitors in series with the transmission line to cancel part of its inductive reactance, reducing voltage drop and increasing power transfer capability, while shunt compensation adds reactors or capacitors in parallel with the line to control reactive power balance and voltage profile under varying load conditions.

Question 14

Q.4. Write short notes on the following: (a) Insulation coordination (b) Radi…

Accepted Answer

Insulation coordination is the systematic grading of equipment insulation levels relative to protective devices to ensure controlled, safe failure locations under overvoltage; radial distribution networks supply loads via a single unbranched path from the substation (simple, cheap, but less reliable), while mesh networks interconnect multiple paths for higher reliability at greater cost and complexity.

Question 15

Q.5. Draw a simple scheme of HVDC converter station and describe briefly comp…

Accepted Answer

An HVDC converter station consists of converter transformers, thyristor/IGBT valve groups (rectifier/inverter), AC and DC harmonic filters, smoothing reactors, and reactive power compensation equipment, all working together to convert AC to DC (or vice versa) for transmission.

Question 16

Q.6. A synchronous motor, having a power consumption of 50kW, is connected in…

Accepted Answer

With a 200 kW, 0.8 pf lagging load combined with a 50 kW synchronous motor to give an overall 0.9 lagging power factor, the synchronous motor is found to operate at a leading power factor of approximately 0.866, drawing a kVA input of about 57.76 kVA, since its leading reactive power partially cancels the load's lagging reactive power.

Question 17

Q.7. Differentiate between synchronous and asynchronous grid.

Accepted Answer

A synchronous grid maintains all interconnected generators in exact frequency and phase lock through direct AC ties, allowing unrestricted power exchange and mutual inertial support, while an asynchronous grid consists of separately operating AC systems interconnected only via a non-synchronizing link (typically HVDC), decoupling their frequencies/phases.

Question 18

Q.1. (a) Define the term feeder, distributor and service mains.

Accepted Answer

A feeder carries bulk power without tapped loads, a distributor has loads tapped along its length, and service mains connect an individual consumer's premises to the nearest distributor; raising a 2-wire DC system's voltage from 250V to 500V for the same power/loss/distance gives a 75% saving in feeder copper, since conductor volume varies inversely as the square of the voltage.

Question 19

Q.2. (a) Explain different types of AC transmission line. Derive the expressi…

Accepted Answer

AC transmission lines are classified as short (<80km, series impedance only), medium (80-250km, nominal-T or nominal-π lumped models) and long (>250km, distributed-parameter rigorous solution); for the nominal-T circuit, A=D=1+ZY/2, B=Z(1+ZY/4), C=Y; applying the given ABCD constants (A=D=0.94∠1°, B=130∠73°, C=0.001∠90°) with Vs=240kV and nominal Vr=220kV gives a voltage regulation of approximately 16.05%.

Question 20

Q.3. (a) Derive an expression for fault current for double line-to-ground (LL…

Accepted Answer

For a double line-to-ground (LLG) fault, the sequence networks are connected in parallel with the boundary conditions Ia=0 and Vb=Vc=0, giving Ia1 = E/[Z1+Z2∥(Z0+3Zf)]; for the given alternator with X0=0.05, X1=X2=0.15 p.u., the LG-fault current is found to be about 1.286 times the 3-phase fault current.

Question 21

Q.4. (a) What is the difference between On-grid and Off-grid solar power?

Accepted Answer

On-grid solar power feeds directly into and relies on the utility grid (no battery, cheaper), while off-grid solar power operates independently with battery storage for isolated locations; DC link control regulates the DC-side voltage/power balance between the rectifier and inverter stages of an HVDC or back-to-back converter system by adjusting firing/gating angles to maintain the desired power transfer and voltage set point.

Question 22

Q.5. (a) Explain the working of SF6 gas circuit breaker.

Accepted Answer

SF6 gas circuit breakers extinguish the arc using the superior dielectric strength and cooling capability of sulphur hexafluoride gas, either via a puffer mechanism or self-blast/rotating-arc designs; for the given 11kV line (R=1.5Ω, X=4Ω/phase) delivering 5000kVA at 0.8 lagging pf, the calculated percentage regulation is approximately 5.85% and transmission efficiency approximately 97.87%.

Power System - I