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.
