Question 1

Q.1. How transmission voltage affects power losses in power system?

Accepted Answer

Higher transmission voltage reduces line current for the same power (P=√3VIcosφ), and since I²R losses scale with the square of current, raising transmission voltage sharply reduces power losses in a power system.

Question 2

Q.2. What does Geometric Mean Distance (GMD) and Geometric Mean Radius (GMR) …

Accepted Answer

GMD (Geometric Mean Distance) is the equivalent geometric mean of the spacings between conductors of different phases, used in inductance/capacitance formulas for unsymmetrically spaced lines, while GMR (Geometric Mean Radius) is the equivalent self-distance of a single conductor (or bundle) accounting for internal flux linkage, replacing the physical radius in the same formulas.

Question 3

Q.3. What is Skin effect? How skin effect affects resistance of conductor?

Accepted Answer

Skin effect is the tendency of alternating current to concentrate near the surface of a conductor rather than distributing uniformly across its cross-section, which effectively reduces the usable conductor area and increases the conductor's effective AC resistance compared to its DC resistance.

Question 4

Q.4. What do you mean by subtransient reactance in synchronous machines?

Accepted Answer

Sub-transient reactance (Xd″) is the effective reactance of a synchronous machine immediately following a sudden disturbance (such as a short circuit), the smallest of the machine's reactances, representing the combined demagnetizing-opposition effect of both the damper winding and field winding in the first few cycles before their currents decay away.

Question 5

Q.5. What is the importance of volt-time curves?

Accepted Answer

Volt-time curves show the relationship between the peak voltage a piece of insulation can withstand and the time duration for which that voltage is applied before breakdown occurs, and are essential for correctly coordinating the insulation strength of equipment with the characteristics of protective surge arresters.

Question 6

Q.6. Define positive, negative and zero sequence components.

Accepted Answer

Positive-sequence components are three equal-magnitude phasors 120° apart with the same phase sequence as the original system; negative-sequence components are similarly balanced but with reversed phase sequence; zero-sequence components are three equal-magnitude, in-phase (0° apart) phasors.

Question 7

Q.7. What is the difference between primary and back-up protection?

Accepted Answer

Primary protection is the first line of defense, designed to clear a fault within its own designated zone as quickly as possible, while back-up protection operates with an intentional time delay to clear the fault only if the primary protection fails to operate, providing a safety net against relay/breaker failure.

Question 8

Q.8. Why VSC based converters are widely used in comparison to LCC based conv…

Accepted Answer

VSC (Voltage Source Converter) based HVDC converters are increasingly preferred over LCC because they can independently control active and reactive power, do not require an external commutation voltage (allowing connection to weak or passive AC networks), have no risk of commutation failure, and produce lower harmonic content requiring smaller filters.

Question 9

Q.9. What is the working principle of a tap changing transformer?

Accepted Answer

A tap-changing transformer varies its output voltage by changing the effective number of turns on one winding (usually the higher-voltage winding) via a tap-selector switch, allowing voltage ratio adjustment either off-load (de-energized) or on-load (under full load current, using a make-before-break switching mechanism).

Question 10

Q.10. What is the difference between synchronous and asynchronous grid?

Accepted Answer

A synchronous grid keeps all interconnected generators locked at the same frequency and phase through direct AC ties, while an asynchronous grid links two or more independently operating AC systems only through a non-synchronizing interface (typically HVDC), allowing each side to maintain its own frequency/phase reference.

Question 11

Q.1. A single phase load of 5 MW is transmitted by a pair of conductors. If a…

Accepted Answer

Adding a third identical conductor and converting a single-phase 5 MW load to 3-phase, keeping voltage between conductors and percentage loss unchanged, allows a 3-phase load of 5√2 ≈ 7.07 MW to be transmitted — a 41.4% increase in capacity using just 50% more conductor material.

Question 12

Q.2. Find the geometric mean radius of a conductor in terms of the radius r o…

Accepted Answer

For a bundle of touching identical strands each of radius r, the geometric mean radius (GMR) works out to Ds = 1.46r for three strands arranged in an equilateral triangle, and Ds = 1.72r for four strands arranged in a square, both derived from the general formula Ds = [(r')^n · (product of all pairwise distances)^2]^(1/n²), where r' = 0.7788r is the self-GMR of each individual solid round strand.

Question 13

Q.3. Explain the flow of zero sequence current in different three-phase trans…

Accepted Answer

Zero-sequence current flow through a 3-phase transformer depends critically on winding connection: Y-Y grounded transformers allow zero-sequence current to pass through only if both neutrals are grounded; delta windings block zero-sequence current from flowing externally but allow it to circulate internally within the closed delta loop; and delta-delta transformers block zero-sequence current entirely on both sides.

Question 14

Q.4. What is the working principle of directional relays? Explain its working…

Accepted Answer

Directional relays operate only when both the fault current magnitude exceeds a threshold and its phase angle relative to a reference (polarizing) voltage indicates the fault is in the intended tripping direction, achieved using a directional element that compares current and voltage phasors via a phase-angle-sensitive comparator.

Question 15

Q.5. Explain the phenomena of lightning and switching surges in power system.

Accepted Answer

Lightning surges are steep-fronted, high-magnitude transient overvoltages caused by direct or induced lightning strikes on transmission lines, while switching surges are transient overvoltages of somewhat lower magnitude but longer duration, caused by sudden switching operations (line energization, fault clearing, capacitor/reactor switching) that excite the system's natural oscillatory response.

Question 16

Q.6. Discuss merits and demerits of HVDC transmission system.

Accepted Answer

HVDC transmission offers merits of lower losses, no stability/reactive-power limits over long distances, asynchronous interconnection capability and reduced right-of-way, but suffers demerits of costly converter stations, harmonic generation, difficult DC circuit breaking, and complex multi-terminal tapping compared to AC transmission.

Question 17

Q.7. Derive an expression for fault current for single line to ground fault.

Accepted Answer

For a single line-to-ground (LG) fault at phase a, applying the boundary conditions Ib=Ic=0 and Va=0 in the symmetrical component method gives the fault current as If = 3E/(Z1+Z2+Z0+3Zf), obtained by connecting the positive-, negative- and zero-sequence networks in series.

Question 18

Q.1. (a) Derive an expression of inductance of three-phase transmission line.…

Accepted Answer

The inductance of a 3-phase transmission line is derived from the total flux linkage of a conductor (self flux + mutual flux from other phases), giving L = 2×10⁻⁷ ln(GMD/GMR) H/m per phase; for the given 345kV line (A=D=0.98182+j0.0012447, B=4.035+j58.947, C=j0.00061137) delivering 400MVA at 0.8 lagging pf, the sending-end line voltage works out to about 387.0 kV, sending current about 592.4 A, voltage regulation about 14.26%, and transmission efficiency about 98.48%.

Question 19

Q.2. A single line diagram of a simple power system is shown in Fig. The neut…

Accepted Answer

Using the Zbus reduction method on the given 3-bus, 2-generator system (100 MVA base), the positive- and negative-sequence Thevenin reactances at bus 3 both work out to X1=X2=0.22 p.u., and (assuming Y-Y grounded transformer connections with the given 0.25/3 p.u. neutral reactors) the zero-sequence reactance works out to X0=0.44 p.u.; applying these with Zf=j0.1 pu gives fault currents of 3.125 p.u. (3-phase), 2.542 p.u. total / 0.848 p.u. per sequence (LG), 3.208 p.u. in the faulted phases (LL), and Ia1=1.840 p.u. with a ground-return current of 1.667 p.u. (LLG).

Question 20

Q.3. (a) Examine the causes of over-voltages in a power system. [5]

Accepted Answer

Over-voltages in a power system arise mainly from lightning strikes, switching operations, and system faults/resonance conditions; common over-voltage protection devices include surge (lightning) arresters, ground/earth wires, spark gaps, and neutral grounding, each working by providing a controlled low-impedance path to limit the transient voltage before it damages equipment insulation.

Question 21

Q.4. (a) Illustrate voltage source converters (VSC) used in HVDC transmission…

Accepted Answer

A Voltage Source Converter (VSC) in HVDC uses self-commutating IGBT valves with pulse-width modulation to independently synthesize AC voltage of controllable magnitude, frequency and phase from a constant-polarity DC bus, enabling independent active/reactive power control; compared to AC transmission, DC transmission (whether LCC or VSC) offers lower long-distance losses and no stability limit but at higher converter station cost and complexity.

Question 22

Q.5. (a) Draw and explain the structure of electrical power system indicating…

Accepted Answer

The electrical power system structure consists of generation (11-25 kV), step-up transmission (up to 765 kV EHV), sub-transmission (33-132 kV) and distribution (11 kV primary, 415V/230V secondary) levels in a hierarchical chain; comparing a 3-phase 4-wire lighting system (same lamp voltage, same total power and loss, neutral csa = phase csa) against an equivalent 2-wire DC system shows the 3-phase 4-wire system requires only one-third (33.3%) of the copper volume of the 2-wire DC system, a 66.7% saving.

Power System - I