(SEM VI) THEORY EXAMINATION 2023-24 POWER SYSTEM-II

POWER SYSTEM-II – KEE601

Section-wise Important Questions & Ready Answers

SECTION A

(Attempt all – 2 marks each)

(a) Need of Sequence Components

Sequence components are used to simplify the analysis of unbalanced three-phase power systems. By resolving unbalanced quantities into positive, negative, and zero-sequence components, complex fault 
and stability problems can be analyzed using simple single-phase equivalent circuits.

(b) Advantages of Per Unit System

The per unit system simplifies calculations by normalizing electrical quantities with respect to chosen base values. It avoids confusion due to different voltage levels, simplifies transformer calculations, and allows easy comparison of system parameters.

(c) Load Flow Analysis

Load flow analysis determines bus voltages, power flows, and losses in a power system under steady-state conditions. It is essential for system planning, operation, and economic dispatch.

(d) Need for Slack Bus

The slack bus compensates for real and reactive power mismatches in the power system. It maintains system balance by supplying or absorbing power required to cover losses.

(e) Voltage Surge

A voltage surge is a sudden rise in voltage caused by lightning, switching operations, or faults. These surges can damage insulation and equipment if not properly controlled.

(f) Characteristic Impedance Loading (SIL)

SIL is the power transmitted over a transmission line when the reactive power generated equals the reactive power absorbed. At SIL, the line operates at natural loading without voltage rise or drop.

(g) Steady-State and Transient Stability

Steady-state stability refers to the system’s ability to maintain synchronism under small disturbances, while transient stability deals with large disturbances like faults and switching operations.

(h) Stability Limit

The stability limit is the maximum power that can be transmitted over a system without losing synchronism. Beyond this limit, the system becomes unstable.

(i) Types of Faults

Faults in power systems include symmetrical faults (three-phase faults) and unsymmetrical faults such as single line-to-ground, line-to-line, and double line-to-ground faults.

(j) Need for Power System Protection

Power system protection ensures safety of equipment and personnel by detecting faults and isolating 
faulty sections quickly to maintain system stability and continuity of supply.

SECTION B

(Attempt any three – 10 marks each)

1. Per Unit Impedance and Reactance Diagram

To draw the per unit reactance diagram, all reactances of generators, transformers, motors, and transmission lines are converted to a common base. Resistance is neglected, and the resulting diagram represents the power system in a simplified and standardized form.

2. Load Flow Equation by Gauss-Seidel Method

The Gauss-Seidel method is an iterative technique used for solving nonlinear load flow equations. It updates bus voltages sequentially using previously calculated values until convergence is achieved. It is simple but slow compared to Newton-Raphson.

3. Bewley’s Lattice Diagram and Surge Phenomenon

Bewley’s lattice diagram graphically represents traveling waves on transmission lines. Surge phenomenon occurs due to lightning or switching, producing high-frequency overvoltages. Protection methods include surge arresters and shielding wires.

4. Equal Area Criterion

The equal area criterion determines transient stability of a power system by comparing accelerating and decelerating areas on the power-angle curve. Stability exists if both areas are equal.

5. Differential Relay

A differential relay operates by comparing currents entering and leaving a protected zone. It trips when the difference exceeds a preset value. A restraining coil is used to prevent false tripping during external faults.

SECTION C

Q3(a) Reactance Diagram for Parallel Generators and Motors

All generators and motors are converted to a common base using given ratings. Their per unit reactances are calculated and represented on a reactance diagram connected to a common bus.

Q3(b) Single Line-to-Ground Fault Analysis

For an LG fault, positive, negative, and zero-sequence networks are connected in series. The fault current is obtained by summing the sequence impedances and applying symmetrical component theory.

Q4(a) Newton-Raphson Method for Load Flow

The Newton-Raphson method uses Jacobian matrices to solve load flow equations efficiently. It converges faster than Gauss-Seidel and is suitable for large power systems.

Q4(b) Types of Buses and Bus Admittance Matrix

Buses are classified as slack, PV, and PQ buses. The bus admittance matrix (Y-bus) represents network connections and is essential for load flow analysis.

Q5(a) Surge on Transmission Line (Numerical)

When a surge reaches a capacitive termination, part of the wave is reflected. Using surge impedance and capacitance, the voltage across the capacitor and reflected wave are calculated.

Q5(b) Wave Equation of Lossless Transmission Line

The voltage and current wave equations are derived using distributed inductance and capacitance. These equations describe the propagation of traveling waves along the line.

Q6(a) Swing Equation and Angular Momentum

The swing equation represents rotor dynamics of a synchronous machine. Angular momentum is calculated using inertia constant and machine rating, indicating stored kinetic energy.

Q6(b) Step-by-Step Stability Method

This method numerically solves the swing equation to analyze rotor angle variation during disturbances, providing detailed insight into transient stability.

Q7(a) Overcurrent Relay and Distance Protection

Overcurrent relays operate when current exceeds a preset value, while distance protection operates based on impedance measurement. Both ensure fast fault clearance.

Q7(b) Definitions

Recovery voltage is the voltage across circuit breaker contacts after arc extinction. Restriking voltage is the transient voltage that appears immediately after current zero. RRRV indicates how fast restriking voltage rises.