THEORY EXAMINATION (SEM–VIII) 2016-17 EHV AC & DC TRANSMISSION

SECTION – A

Attempt all parts (Short Answer Type)

EHV AC transmission is needed to transmit large amounts of electrical power over long distances with minimum losses and improved efficiency. Higher voltage reduces current, which decreases power loss.

Overvoltage in power systems is caused by lightning strikes, switching operations, sudden load changes, and faults.

Corona is affected by factors such as conductor size, surface condition, atmospheric pressure, and operating voltage.

Switching operations cause overvoltage because sudden changes in current and voltage create transient waves in transmission lines.

Streamer theory explains electrical breakdown in gases when ionization spreads rapidly due to high electric fields.

Surge impedance loading refers to the natural loading level of a transmission line at which reactive power generated equals reactive power consumed.

Insulation coordination ensures proper selection of insulation levels to protect equipment from overvoltages.

Pollution affects EHV lines by reducing insulation strength and increasing leakage currents, which can lead to flashover.

Paschen’s law states that breakdown voltage in gases depends on pressure and gap distance.

MTDC systems are used to connect multiple power stations or grids through HVDC transmission.

Questions (Section A)

Why is EHV AC transmission needed?

What causes overvoltage?

List factors affecting corona.

What is surge impedance loading?

What is insulation coordination?

SECTION – B

Attempt any five (Long Answer Type)

High impulse currents are measured using current transformers, oscilloscopes, and specialized sensors.

Sub-synchronous resonance occurs when electrical system frequency interacts with mechanical system frequency, causing instability. Countermeasures include damping systems and protective devices.

Corona loss current depends on voltage, conductor spacing, atmospheric conditions, and conductor configuration.

Extinction angle control is used in HVDC systems to control converter operation and ensure proper commutation.

Power reversal in HVDC links allows power flow to be reversed by changing current direction without altering voltage polarity.

HVDC links include monopolar, bipolar, and homopolar types based on configuration.

DC harmonics occur due to converter operation and are reduced using harmonic filters.

Audible noise is produced by corona discharge around conductors and depends on weather conditions and voltage levels.

Questions (Section B)

How are high impulse currents measured?

What is sub-synchronous resonance?

Explain extinction angle control.

What are types of HVDC links?

What is audible noise?

SECTION – C

Attempt any two (Detailed Explanation)

High voltage measurement methods include sphere gap and potential divider techniques. Sphere gaps provide accurate measurement but have limitations in dynamic conditions.

Voltage multiplier circuits use capacitors and diodes to generate very high DC voltages. Cascade transformers are used to produce very high AC voltages for testing.

A 12-pulse converter in HVDC systems reduces harmonic distortion by combining two six-pulse converters. It improves efficiency and reduces ripple.

Questions (Section C)

Explain sphere gap measurement.

What are voltage multiplier circuits?

Explain 12-pulse converter operation.