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

SECTION – A

Attempt all parts (Short Answer Type)

Audible noise is the sound produced in high voltage transmission lines due to corona discharge around conductors. It is commonly heard as a buzzing or humming noise.

Corona is a phenomenon in which air around a conductor becomes ionized due to high electric field intensity, resulting in power loss and noise.

Standard transmission voltages are commonly used voltage levels such as 132 kV, 220 kV, 400 kV, and 765 kV.

HVDC systems are preferred for long-distance transmission because they have lower losses, better voltage control, and no reactive power issues compared to EHV AC systems.

Filters used in HVDC systems include harmonic filters and DC filters, which remove unwanted harmonics from the system.

Surface voltage gradient refers to the electric field intensity at the surface of a conductor. High gradients can lead to corona discharge.

An impulse generator is a device used to produce high voltage impulses for testing insulation strength.

Impulse tests are important for checking the ability of electrical equipment to withstand sudden voltage surges such as lightning.

Flashover occurs when an insulating material fails and allows current to pass across its surface. The 50% flashover voltage is the voltage at which flashover occurs in half of the test cases.

Overcurrents are caused by faults, short circuits, overloads, and sudden load changes.

Questions (Section A)

Define audible noise.

What is corona?

Why is HVDC preferred for long-distance transmission?

What is impulse generator?

What are causes of overcurrent?

SECTION – B

Attempt any five (Long Answer Type)

Overvoltages in converter stations occur due to lightning, switching operations, and system faults. Protective devices such as surge arresters and filters are used to protect equipment.

The electric intensity on conductor surfaces depends on conductor spacing, radius, and system voltage. Mathematical formulas are used to calculate maximum intensity.

Mechanical considerations in transmission lines include conductor strength, sag, wind loading, and tower design.

Switching surges in EHV lines can be reduced using surge arresters, controlled switching, and proper insulation design.

Dampers and spacers are used in transmission lines to reduce vibrations and maintain conductor spacing.

Design aspects of EHV lines include insulation coordination, conductor configuration, and voltage regulation.

Corona loss depends on voltage, conductor radius, and atmospheric conditions and can be calculated using Peek’s formula.

Corona pulses are high-frequency disturbances generated due to corona discharge and affect communication systems.

Questions (Section B)

What causes overvoltage in converter stations?

Explain mechanical considerations in transmission lines.

How are switching surges reduced?

What are dampers and spacers?

What is corona loss?

SECTION – C

Attempt any two (Detailed Explanation)

MTDC (Multi-Terminal DC) systems are HVDC systems with more than two terminals. They allow power transmission between multiple locations.

Voltage multiplier circuits are used to generate very high DC voltages by multiplying input voltage using capacitors and diodes.

High impulse currents are measured using special instruments such as current transformers and oscilloscopes. Sphere gap measurement is a method for measuring high voltages accurately.

Questions (Section C)

What is MTDC system?

Explain voltage multiplier circuits.

What is sphere gap measurement?