(SEM VII) THEORY EXAMINATION 2022-23 HVDC & AC TRANSMISSION

SECTION A – Short Answers (2 Marks Each)

(a) Effect of radio interference on EHV AC lines

Radio interference is caused mainly due to corona discharge and results in noise in radio and communication systems, reducing signal quality and causing electromagnetic disturbances.

(b) Preference of bundled conductors in EHV transmission

Bundled conductors reduce corona loss, radio interference, surface voltage gradient, and increase power handling capacity of EHV lines.

(c) Relation of corona power loss with frequency

Corona power loss increases with increase in frequency of the supply due to higher rate of ionization.

(d) Principle of half-wave transmission

In half-wave transmission, the line length is equal to half the wavelength of the operating frequency, resulting in minimum voltage regulation and reactive power flow.

(e) Effect of pollution on EHV AC lines

Pollution increases surface conductivity of insulators, leading to higher leakage current, flashover, and reduced insulation strength.

(f) Need for testing of EHV AC lines

Testing ensures reliability, safety, insulation strength, and proper performance of EHV AC transmission systems under operating conditions.

(g) Controlled vs uncontrolled rectifier

Controlled rectifiers use thyristors and allow control of output voltage, whereas uncontrolled rectifiers use diodes with fixed output voltage.

(h) Effect of source inductance on average output voltage

Source inductance causes overlap during commutation, reducing the average DC output voltage.

(i) Types of faults in HVDC converters

Converter faults include valve short-circuit, valve open-circuit, commutation failure, and control system faults.

(j) Need of smoothing reactor

Smoothing reactors reduce ripple in DC current, limit fault current, and improve system stability.

SECTION B – Long Answers (10 Marks Each)

(a) Technical and economic reasons for adopting EHV transmission

EHV transmission reduces line losses, improves voltage regulation, allows bulk power transfer over long distances, reduces conductor material, and lowers overall transmission cost despite higher initial investment.

(b) Terms used in EHVAC transmission

Sub-transient reactance represents initial reactance immediately after fault.
Transient reactance exists during transient period.
Synchronous reactance is steady-state reactance.
Interrupting current capacity is the maximum fault current a circuit breaker can safely interrupt.

(c) Design factors of EHV line under steady-state limits

Factors include conductor size, spacing, bundled conductors, corona effects, insulation coordination, thermal limits, voltage regulation, and stability constraints.

(d) Use of higher pulse converters in HVDC

Higher pulse converters reduce harmonic content, ripple, and filter size, improve efficiency, and enhance power quality of HVDC transmission.

(e) HVDC projects in India

Examples include Rihand-Dadri HVDC link, Talcher-Kolar HVDC link, and Champa-Kurukshetra HVDC link. These projects enable long-distance bulk power transfer with reduced losses.

SECTION C – Long Answers (10 Marks Each)

3(a) Surface voltage gradient on conductors

Surface voltage gradient is the electric field intensity at conductor surface. For bundled conductors, gradient reduces due to increased effective radius, lowering corona effects. Expression is derived using electrostatic field theory.

3(b) Power handling capacity of EHVAC lines

As transmission voltage increases, power handling capacity increases and line losses decrease. Higher voltage levels enable efficient bulk power transfer over long distances.

4(a) Formation of corona and affecting factors

Corona is formed when electric field exceeds critical value, ionizing surrounding air. Factors include conductor size, spacing, surface condition, atmospheric pressure, humidity, and supply voltage.

4(b) Short notes

Ferro-resonance: Non-linear resonance causing overvoltages in transformer-line systems.
Radio interference: Caused by corona discharge affecting communication signals.

5(a) Impulse generator numerical (Method)

Wave front and wave tail resistances are calculated using standard impulse equations.
Maximum output voltage equals number of stages × charging voltage × efficiency.

5(b) Impulse generator circuit and voltage efficiency

Impulse generator consists of charging resistors, capacitors, wave-shaping resistors, and spark gaps. Voltage efficiency is the ratio of output voltage to ideal voltage.

6(a) Six-pulse inverter operation

Extinction angle, overlap angle, and DC voltage are calculated using inverter equations considering leakage reactance and DC current.

6(b) Types of HVDC links

Types include monopolar, bipolar, and homopolar HVDC links. Applications include long-distance transmission, submarine cables, and interconnection of asynchronous grids.

7(a) Reduction of THD and ripple

THD and ripple are reduced using higher pulse converters, filters, smoothing reactors, and PWM control techniques.

7(b) Importance of multi-terminal DC transmission

MTDC systems allow power exchange among multiple terminals, improve reliability, and enhance flexibility of power flow control.