Question 1

Q.1. What is the need of grounding the neutral in power system?

Accepted Answer

The neutral is grounded to limit overvoltages during earth faults, provide a path for earth-fault current so protective relays can detect and clear the fault, and stabilize the system voltage with respect to ground.

Question 2

Q.2. What are the issues in connecting the renewable energy system to the grid?

Accepted Answer

Key grid-integration issues include voltage/frequency fluctuations due to intermittency, power quality problems (harmonics, flicker), reverse power flow, synchronization requirements, protection coordination challenges, and grid stability/inertia concerns from inverter-based renewable sources.

Question 3

Q.3. Define critical disruptive voltage and critical visual voltage.

Accepted Answer

Critical disruptive voltage is the minimum line voltage at which corona discharge begins on a conductor, while critical visual voltage is the minimum voltage at which the corona glow becomes visible to the naked eye, and is always slightly higher than the disruptive voltage.

Question 4

Q.4. What is meant by insulation co-ordination?

Accepted Answer

Insulation co-ordination is the process of selecting the dielectric strength (insulation levels) of various equipment in a power system in a graded manner, relative to protective devices, so that insulation failure occurs only in a controlled, least-costly, and safely protected part of the system.

Question 5

Q.5. What is the necessity of maximum power point tracking in PV system?

Accepted Answer

Maximum Power Point Tracking (MPPT) is necessary in PV systems because a solar panel's power output varies non-linearly with irradiance and temperature, and MPPT continuously adjusts the operating voltage/current to always extract the maximum available power from the panel.

Question 6

Q.6. What is grid interactive inverter? State its significance.

Accepted Answer

A grid interactive (grid-tie) inverter converts DC power from a renewable source into AC power synchronized in voltage, frequency and phase with the utility grid, allowing power to be fed directly into the grid without battery storage.

Question 7

Q.7. What are the factors which affect corona?

Accepted Answer

Factors affecting corona include conductor surface condition and radius, spacing between conductors, supply voltage and frequency, atmospheric conditions (air density, humidity, rain, dust), and the presence of surface irregularities or dirt.

Question 8

Q.8. What are the two basic requirements for the firing pulse generation of H…

Accepted Answer

The two basic requirements for HVDC valve firing pulse generation are: precise synchronization of the firing instant with the AC system voltage (phase reference), and sufficient pulse power/amplitude with adequate duration to reliably trigger the thyristor valve into conduction.

Question 9

Q.9. What do you mean by symmetrical and unsymmetrical spacing?

Accepted Answer

Symmetrical spacing means the three conductors of a transmission line are placed at the vertices of an equilateral triangle (equal spacing between all pairs), giving equal and balanced inductance/capacitance per phase, while unsymmetrical spacing has unequal conductor spacing, causing unbalanced inductance/capacitance unless transposed.

Question 10

Q.10. What role do micro grids play during a power outage?

Accepted Answer

Microgrids can automatically island themselves from the main grid during an outage and continue supplying local loads using their own distributed generation and storage, maintaining critical power supply and improving overall resilience.

Question 11

Q.1. A balanced star connected load takes 150 A from a balanced 3-phase, 4-wi…

Accepted Answer

In a balanced 3-phase, 4-wire system, before the fuses are removed all currents are purely positive-sequence (150 A) with zero negative and zero-sequence components; after two line fuses (say B and C) blow, only line A carries 150 A, giving equal positive, negative and zero-sequence components of 50 A each.

Question 12

Q.2. Explain the switching operation in an R-L circuit. Also explain sub-tran…

Accepted Answer

Switching an R-L circuit onto an AC supply produces a total current with a steady-state (forced) sinusoidal component and a decaying DC offset (transient) component whose magnitude depends on the switching instant; in a synchronous machine short-circuit, the fault current similarly has sub-transient, transient and steady-state periods corresponding to progressively decreasing effective reactance (and hence decreasing current) as damper/rotor flux decays.

Question 13

Q.3. Discuss in detail the grid system characteristics and explain with a nea…

Accepted Answer

Grid system characteristics include voltage/frequency regulation, load-following capability, fault level and stability; a stand-alone solar system operates independently with battery storage for isolated loads, while a grid-integrated (grid-tied) solar system synchronizes with and exports/imports power to the utility grid without necessarily requiring battery storage.

Question 14

Q.4. Explain the significance of "α" operator. Also show that the power in a …

Accepted Answer

The operator α = 1∠120° rotates a phasor by 120° without changing its magnitude, and is used to relate the three phase quantities to sequence components; total 3-phase power can be shown to equal 3(Va1·Ia1* + Va2·Ia2* + Va0·Ia0*) when expressed in terms of symmetrical components.

Question 15

Q.5. Draw the single phase model of negative and zero sequence impedance of s…

Accepted Answer

The negative-sequence network of a synchronous machine is modeled as an EMF-free reactance (negative-sequence reactance X2, close to the sub-transient reactance) in series with the sequence network reference, while the zero-sequence network uses an EMF-free zero-sequence reactance X0, generally smaller than X2 and Xd; transformer zero-sequence networks depend critically on winding connection (star/delta) and grounding.

Question 16

Q.6. Why grading is required in cables? Also discuss different methods of gra…

Accepted Answer

Grading in cables is required to equalize the non-uniform electric stress distribution across the dielectric (which is naturally highest at the conductor surface), thereby making better use of the insulation and permitting a more compact, economical cable design; the two main methods are capacitance grading and intersheath grading.

Question 17

Q.7. Discuss line commutated converters with the help of schematic diagram.

Accepted Answer

Line-commutated converters (LCC) use naturally-commutated thyristor valves, relying on the AC system voltage itself to force commutation (turn-off) of the conducting thyristor as the next phase voltage becomes more positive, and are configured as 6-pulse (or 12-pulse) bridge circuits, forming the traditional basis of HVDC converter stations.

Question 18

Q.1. (a) Define positive, negative and zero sequence components in 3 phase sy…

Accepted Answer

The three sets of symmetrical components — positive (balanced, same phase sequence as original), negative (balanced, opposite phase sequence) and zero (three equal in-phase phasors) — together represent any unbalanced 3-phase phasor set; applying them to an unloaded generator under a line-to-ground fault shows how the sequence networks are connected in series to compute the fault current.

Question 19

Q.2. (a) Explain the per unit system for analyzing power system problem. Disc…

Accepted Answer

The per-unit system expresses all quantities as fractions of chosen base values, eliminating transformer turns ratios and simplifying multi-voltage-level analysis; it can be shown that a two-winding transformer's per-unit impedance is identical whether referred to the HV or LV side, provided consistent base values (related by the turns ratio) are used on each side; surge impedance Zc=√(L/C) determines a line's natural voltage/current relationship for traveling waves and is evaluated differently for overhead lines (higher, ~400Ω) versus underground cables (much lower, ~40-60Ω) due to their different L and C per unit length.

Question 20

Q.3. (a) Draw Bewley lattice diagram for an open circuited transmission line …

Accepted Answer

A Bewley lattice diagram graphically tracks a traveling voltage/current wave as it reflects repeatedly back and forth between the sending and receiving ends of a transmission line, with each reflection scaled by the reflection coefficient at that end; for the given open-circuited line (R=0.5Ω/km, G=10×10⁻⁷S/km, l=400km, initial Vs=2p.u.), the receiving end is fully reflecting (ρr=+1) causing the voltage to build up, while for a bifurcated (branched) line, the lattice diagram must additionally account for the refraction coefficient at the junction where the wave splits into two lines of different surge impedance.

Question 21

Q.4. (a) A three phase line delivers 3600 kW at a power factor 0.8 lagging to…

Accepted Answer

For a 3-phase line delivering 3600 kW at 0.8 lagging pf with 33 kV sending-end voltage, R=5.31Ω and X=5.54Ω per conductor, solving iteratively gives receiving-end line voltage ≈ 31.93 kV, line current ≈ 81.4 A, and transmission efficiency ≈ 97.15%; ACSR (Aluminium Conductor Steel Reinforced) conductors use a steel core for tensile strength and aluminium strands for conductivity, offering a high strength-to-weight ratio ideal for overhead lines.

Question 22

Q.5. (a) What is a circuit breaker? Also discuss the various types of circuit…

Accepted Answer

A circuit breaker is a mechanical switching device capable of making, carrying and safely interrupting normal and abnormal (fault) currents; common types are classified by arc-quenching medium as oil, air-blast, SF6 and vacuum circuit breakers; current chopping is the premature interruption of a small inductive current before its natural zero, causing dangerous transient overvoltages, mitigated by resistance switching, surge arresters or specially designed contact materials.

Power System - I