SWITCHGEAR AND PROTECTION (NEE602)
Section-Wise Solved Answers
SECTION A – Short Answer Questions (2 × 10 = 20 Marks)
(a) Pick-up value of actuating quantity
The pick-up value is the minimum value of the actuating quantity (current, voltage, power, etc.) at which a protective relay just starts operating. Below this value, the relay does not respond.
(b) Stability of a protective relay
Stability means the ability of a relay to remain inoperative for faults outside its protected zone. A stable relay does not operate during external faults or normal operating conditions.
(c) Time setting of over-current relay
Time setting determines the operating time delay of the relay after current exceeds the pick-up value. It is adjusted so that proper coordination is maintained between primary and backup relays.
(d) Comparison between very inverse relay and IDMT relay
IDMT relay: Operating time decreases as fault current increases.
Very inverse relay: Operating time decreases more sharply with increase in current and is suitable for feeder protection where fault current reduces with distance.
(e) Reactance relay characteristic on R-X diagram
The reactance relay operates when the reactance (X) seen by the relay is less than a preset value. On the R-X diagram, its characteristic is a horizontal straight line, independent of resistance.
(f) Under-reach
Under-reach occurs when a distance relay fails to operate for faults within its intended zone, usually due to arc resistance or system conditions reducing measured impedance.
(g) Protection against rotor overheating of alternator
Temperature-sensitive devices such as resistance temperature detectors (RTDs) or thermal relays are used to protect the alternator rotor against overheating.
(h) Magnetizing inrush current
Magnetizing inrush current is a high transient current drawn by a transformer when it is energized, due to saturation of the core. It may be several times the rated current.
(i) Energy balance theory of arc interruption
According to this theory, an arc is extinguished when the rate of heat loss exceeds the rate of heat generation in the arc. Circuit breakers aim to increase cooling to interrupt the arc.
(j) Breaking capacity of a circuit breaker
Breaking capacity is the maximum current that a circuit breaker can safely interrupt at rated voltage without damage.
SECTION B – Long Answer Questions (Attempt Any Three)
(a) Thermal relays – working, types, and applications
Thermal relays operate on the principle of heating effect of current. When current exceeds safe limits, heat produced causes mechanical movement, leading to tripping.
Types:
Bimetallic relay Hot-wire relay Thermal overload relay
Applications:
Motor overload protection Transformer protection Industrial motor drives
(b) Stepped time-distance characteristics of impedance relays
Distance protection uses three zones:
Zone-I: Instantaneous protection (80–90% of line length)
Zone-II: Time-delayed backup for remaining line
Zone-III: Remote backup protection
Time delay increases with zone number to ensure selectivity.
(c) Carrier current protection
Carrier current protection uses high-frequency signals transmitted over transmission lines to detect faults.
Merits:
Fast operation Suitable for long lines High selectivity
Demerits:
High cost Complex equipment Sensitive to noise
(d) Protection of alternator field winding against ground faults
Field winding ground faults are detected using:
DC ground detection relay Voltage monitoring methods
Early detection prevents severe damage to rotor and excitation system.
(e) Vacuum circuit breaker – principle and advantages
In a vacuum circuit breaker, arc interruption occurs in a vacuum chamber. Vacuum has high dielectric strength, causing rapid arc extinction.
Advantages:
Long life Low maintenance
No fire hazard Fast arc extinction
SECTION C – Very Long Answer Questions
Q3(a) Types of over-current relays and applications
Over-current relays operate when current exceeds a preset value.
Types:
Instantaneous over-current relay Definite time relay
Inverse definite minimum time (IDMT) relay
Applications:
Feeder protection Motor protection
Transformer backup protection
Q3(b) Primary and backup protection
Primary protection clears faults within its zone first.
Backup protection operates if primary protection fails.
Methods of backup protection:
Relay backup Circuit breaker backup
Time-graded backup
Q4(a) Attracted armature relays and noise
Types include: Hinged armature relay
Plunger relay
They are noisy due to vibration and magnetic hum caused by alternating magnetic flux.
Q4(b) Amplitude comparators and rectifier bridge comparator
Amplitude comparators compare magnitudes of two signals.
A rectifier bridge comparator uses diodes to rectify signals and compare DC outputs for relay operation.
Q5(a) MHO relay characteristic and power surge effect
MHO relay has a circular characteristic on R-X diagram passing through origin.
Power surges may cause incorrect operation due to voltage distortion.
Q5(b) Unit protection and phase comparison scheme
Unit protection protects a specific zone only.
Phase comparison compares current phase angles at both ends of the line using carrier communication.
Q6(a) Testing of circuit breakers
Methods:
Direct testing Synthetic testing Routine testing
Each method differs in cost, accuracy, and feasibility.
Q6(b) Re-striking voltage, recovery voltage, and RRRV
Re-striking voltage: Appears immediately after arc extinction
Recovery voltage: Voltage across breaker contacts after arc extinction
RRRV: Rate of rise of restriking voltage
These depend on system inductance and capacitance.
Q7(a) Protection against loss of excitation
Loss of excitation causes rotor heating and instability.
Protection methods: Impedance relay Offset MHO relay
Q7(b) Differential protection of bus-zone
Bus-zone protection uses current transformers on each feeder.
If fault occurs within bus, differential current flows and relay trips.
