(SEM VII) THEORY EXAMINATION 2022-23 POWER SYSTEM PROTECTION

SECTION A – Short Answers (2 Marks Each)

(a) Types of protection relays

Protection relays are classified as electromechanical relays, static relays, digital relays, and numerical relays based on their operating principle.

(b) Summation transformer

A summation transformer is used to add or subtract currents from different CTs to obtain a resultant current for protection purposes, commonly used in differential protection.

(c) Why Buchholz relay is called gas actuated relay

Buchholz relay operates due to gas produced by internal faults in oil-filled transformers, hence it is called a gas-actuated relay.

(d) Advantages of differential relay

Differential relay offers high sensitivity, fast operation, selective protection, and is suitable for protecting generators, transformers, and bus bars.

(e) Types of faults and their effects

Faults are classified as symmetrical (3-phase) and unsymmetrical (L-G, L-L, L-L-G). Effects include high fault current, voltage drop, equipment damage, and system instability.

(f) Pilot protection schemes

Pilot protection schemes use communication channels between line ends to compare currents or phase angles for fast and selective protection of transmission lines.

(g) Steps in test procedure

Test procedure includes visual inspection, secondary injection testing, primary injection testing, performance verification, and calibration.

(h) Properties of arc

Arc has high temperature, low resistance, ionized path, and emits light and heat, making interruption difficult in circuit breakers.

(i) Use of logic circuit in protection system

Logic circuits process relay signals, implement decision making, coordinate tripping actions, and avoid false operations.

(j) Advantages of computer-based protection system

They provide high accuracy, fast response, self-diagnostics, event recording, and easy modification of protection settings.

SECTION B – Long Answers (10 Marks Each)

2(a) Need of power system protection & attributes

Power system protection is required to detect faults, isolate faulty sections, minimize damage, ensure safety, and maintain system stability.

Attributes of a protection system:                        Reliability

Selectivity                                                                  Sensitivity

Speed                                                                        Stability

Economy

An effective protection system ensures uninterrupted and safe power supply.

2(b) Protection relays

(i) Shaded pole induction disc relay

It works on electromagnetic induction principle. Shading rings create phase displacement, producing torque on aluminum disc. Used mainly in overcurrent protection.

(ii) Induction cup relay

Uses a rotating cup instead of disc, providing high speed operation, better sensitivity, and directional characteristics.

2(c) Merz-Price circulating current protection of alternator

This scheme compares currents at both ends of stator windings using CTs. Under normal conditions, currents balance. During internal faults, differential current flows, operating the relay and isolating the alternator.

2(d) Vacuum circuit breaker

Vacuum circuit breaker interrupts current by separating contacts inside a vacuum chamber.
 

Advantages:                                                          Fast arc extinction

Long life                                                                  Low maintenance

Compact size                                                          Used in medium voltage systems.

2(e) Electronic relay

Electronic relays use solid-state components instead of mechanical parts.
 

Advantages:                                                          No moving parts

High accuracy                                                         Fast response

Low power consumption

SECTION C – Long Answers (10 Marks Each)

3(a) Primary protection and back-up protection

Primary protection is the first line of defense that isolates the faulty element directly.

Back-up protection is required in case primary protection fails.

Types of back-up protection:                             Local back-up

Remote back-up                                                    Back-up protection ensures system reliability and safety.

3(b) Essential qualities of protective relays & classification

Essential qualities:                                               Reliability

Selectivity                                                               Sensitivity

Speed                                                                     Simplicity

Classification of protection schemes:                Unit protection

Non-unit protection                                              Pilot protection

Differential protection

4(a) Distance relay / Directional relay

Distance relay operates based on impedance measured between relay and fault point. It is widely used in transmission line protection.

Directional relay operates when fault current flows in a specified direction, preventing unnecessary tripping.

4(b) Three-zone distance relay characteristics

Zone-1: Instantaneous, protects 80–90% of line

Zone-2: Time-delayed, covers remaining line + part of next line

Zone-3: Long delay, provides remote back-up

5(a) Overvoltage protection of overhead transmission line

Methods include:                                                     Lightning arresters

Shield wires                                                              Surge absorbers

Proper grounding                                                    Insulation coordination

5(b) Reverse power protection of alternator

Reverse power relay protects alternator from motoring action when prime mover fails. It detects power flow reversal and trips the breaker to prevent mechanical damage.

6(a) Re-striking voltage transient & arc interruption

Re-striking voltage is transient voltage across breaker contacts after current zero.
Arc interruption methods include:                           Oil interruption

Vacuum interruption                                                SF₆ gas interruption

6(b) Short notes

(i) SF₆ Circuit Breaker

Uses sulfur hexafluoride gas with excellent arc quenching properties, high dielectric strength, and compact design.

(ii) Bulk oil circuit breaker

Uses large quantity of oil for arc extinction and insulation. Oil decomposes during arcing and absorbs energy.

7(a) Microprocessor-based digital static relay

It consists of:                                                             Input transducers

Signal conditioning                                                   A/D converter

Microprocessor                                                         Memory

Output relays                                                            Provides adaptive, reliable, and fast protection.

7(b) Level detector & PNP/NPN transistor

A level detector senses voltage crossing a reference level.

PNP transistor: Conducts when base is negative relative to emitter

NPN transistor: Conducts when base is positive relative to emitter

Used in protection and control circuits.