Question 1

Q.1. What is slack bus? What is its significance?

Accepted Answer

The slack (swing) bus is a reference bus in load flow analysis with fixed voltage magnitude and angle (usually 0 degrees), whose significance is to supply the unknown system losses and balance the overall power mismatch.

Question 2

Q.2. What is market equilibrium?

Accepted Answer

Market equilibrium in an electricity market is the operating point at which the quantity of electricity supplied (via generator bids) equals the quantity demanded (via load/buyer bids) at a mutually agreed market clearing price.

Question 3

Q.3. Define transient stability.

Accepted Answer

Transient stability is the ability of a power system to maintain synchronism among its generators following a large, sudden disturbance such as a fault, sudden loss of load, or line switching.

Question 4

Q.4. What is the difference between state estimation and load flow problem?

Accepted Answer

Load flow determines bus voltages and power flows assuming perfectly known, error-free network and injection data, whereas state estimation uses redundant, noisy real-time measurements (with measurement errors) to statistically estimate the most likely true system state.

Question 5

Q.5. How power angle affects stability of power system?

Accepted Answer

Power angle (the phase angle difference between internal generator EMF and the receiving-end/bus voltage) directly determines the electrical power transferred, and stability requires the system to operate on the ascending (stable) portion of the power-angle curve, generally for angles less than 90 degrees.

Question 6

Q.6. Define per unit inertia constant known as H constant.

Accepted Answer

The per-unit inertia constant H is defined as the ratio of the kinetic energy stored in the rotating mass at synchronous speed to the machine's rated MVA, expressed in units of MJ/MVA or seconds, quantifying how quickly a machine accelerates for a given power imbalance.

Question 7

Q.7. What are the advantages of GS method over NR method?

Accepted Answer

The Gauss-Seidel method requires less computer memory (no Jacobian matrix storage) and is simpler to program, making it advantageous for smaller systems, though it converges more slowly than Newton-Raphson for large systems.

Question 8

Q.8. What is automatic voltage control in power system?

Accepted Answer

Automatic voltage control (via the Automatic Voltage Regulator, AVR) is a closed-loop control system that continuously adjusts a synchronous generator's field excitation to maintain its terminal voltage at a desired setpoint despite load variations.

Question 9

Q.9. What is synchronizing power coefficient?

Accepted Answer

The synchronizing power coefficient is the rate of change of electrical power output with respect to power angle (dP/d-delta), representing the restoring torque per unit angular deviation that keeps a synchronous machine in step with the system.

Question 10

Q.10. Write down the benefits of deregulation of power industry.

Accepted Answer

Deregulation of the power industry brings benefits including increased competition and efficiency, lower electricity prices for consumers, greater consumer choice of supplier, encouragement of new investment and innovation, and improved transparency in electricity pricing.

Question 11

Q.1. A loss-free generator supplied 50 MW to an infinite bus, the steady stat…

Accepted Answer

Applying the equal area criterion, the generator remains in synchronism after the sudden 30MW increase in mechanical input, since the available decelerating area (approximately 17.04 pu-radians) exceeds the required accelerating area (approximately 5.69 pu-radians).

Question 12

Q.2. What does system security means? Draw a diagram showing power system sta…

Accepted Answer

Power system security refers to the system's ability to withstand credible contingencies (equipment outages, faults) without violating operating limits or cascading into wider failure, typically classified into normal, alert, emergency, in-extremis, and restorative security states/levels.

Question 13

Q.3. How speed governing system of a steam turbine works? What are the differ…

Accepted Answer

The speed governing system of a steam turbine senses shaft speed deviation and adjusts steam valve position (via a speed governor, hydraulic amplifier/servomotor, linkage, and steam control valve) to regulate turbine power output and maintain speed/frequency close to its scheduled value, incorporating a deliberate droop characteristic for stable load sharing among parallel generators.

Question 14

Q.4. Illustrate the comparison between Gauss-Seidel and Newton-Raphson method…

Accepted Answer

Newton-Raphson converges quadratically and in far fewer iterations regardless of system size (making it the standard choice for large power systems), whereas Gauss-Seidel converges linearly with a rapidly increasing iteration count as system size grows, but requires much less memory per iteration and is simpler to program, making GS more suitable mainly for small systems or generating a starting solution.

Question 15

Q.5. Derive the swing equation of single machine infinite bus system and expl…

Accepted Answer

The swing equation, derived from Newton's second law applied to the rotating turbine-generator mass, relates the machine's rotor angle acceleration to the difference between mechanical input and electrical output power, expressed in per-unit form as (2H/omega_s)(d^2-delta/dt^2)=Pm-Pe, and forms the fundamental basis for analyzing rotor dynamics and stability of a single machine connected to an infinite bus.

Question 16

Q.6. Explain the working of Static Compensator (STATCOM) with the help of dia…

Accepted Answer

A Static Compensator (STATCOM) is a shunt-connected Flexible AC Transmission System (FACTS) device based on a voltage-source converter, which rapidly and continuously injects or absorbs reactive power at its point of connection by controlling the magnitude of its internally-generated AC voltage relative to the system bus voltage, providing fast, smooth voltage support and reactive power compensation without relying on large banks of switched capacitors/reactors.

Question 17

Q.7. Draw generator cost curves. Why these curves are drawn? Explain the sign…

Accepted Answer

Generator (input-output and incremental/heat-rate) cost curves plot a thermal plant's fuel cost or heat input against its power output; they are drawn to characterize each plant's cost behavior across its operating range, and are of central significance in economic dispatch, since the equal-incremental-cost criterion used to allocate load among multiple plants directly depends on each plant's incremental cost curve.

Question 18

Q.1. Two generators rated 200 MW and 400 MW are operating in parallel. The dr…

Accepted Answer

Using governor droop characteristics, the no-load frequencies are found as 52 Hz (Gen A, 200MW, 4% droop) and 52.5 Hz (Gen B, 400MW, 5% droop); solving for the common operating frequency when total load reduces to 400MW gives f=50.77Hz, with Gen A supplying approximately 123.08 MW and Gen B supplying approximately 276.92 MW.

Question 19

Q.2. Apply equal area criterion method for transient stability analysis:

Accepted Answer

The equal area criterion, applied to a sudden mechanical power increase, requires the accelerating area (between the new higher mechanical power line and the power-angle curve, from the old to the new equilibrium angle) to be matched by an equal decelerating area beyond the new equilibrium, up to the maximum swing angle; for a three-phase fault at the sending end (reducing transmitted electrical power to zero during the fault), the accelerating area builds up during the fault-on period at constant mechanical power against zero electrical output, and stability requires sufficient post-fault decelerating area to be available before the rotor angle reaches the critical clearing angle.

Question 20

Q.3. Figure shows the single-line diagram of a simple three-bus power system …

Accepted Answer

The Y-bus for the three-bus system (with line reactances j0.025 pu bus1-bus2, j0.025 pu bus2-bus3, j0.05 pu bus1-bus3) is formed directly from the line admittances; applying the Gauss-Seidel method for two iterations (with V1=1.025<0 fixed slack, V3 magnitude fixed at 1.03 pu as a PV bus, and bus 2 as a PQ load bus of -4-j2 pu) gives V2 approximately 1.00008-j0.0406 pu after iteration 2, and V3 approximately 1.0298+j0.0221 pu (magnitude renormalized to 1.03) after iteration 2, with computed Q3 approximately -1.37 pu at that stage.

Question 21

Q.4. The fuel-cost function in $/h of two thermal plants are:

Accepted Answer

Solving the coordination equations for economic dispatch with transmission losses, the optimal generation schedule is P1 approximately 161.18 MW and P2 approximately 258.60 MW, with total transmission loss approximately 7.43 MW, at a system incremental cost (lambda) of approximately 7.804 $/MWh, with both units within their specified generation limits.

Question 22

Q.5. Explain the different types of electricity market models in detail with …

Accepted Answer

Electricity market models are broadly classified into the monopoly (regulated, vertically-integrated) model, the single-buyer model, the wholesale competition model, and the retail (full) competition model, each representing a progressively greater degree of unbundling and competitive market participation across generation, wholesale trading, and retail supply of electricity.

Power System 2