(SEM VI) THEORY EXAMINATION 2017-18 SWITCHGEAR AND PROTECTION

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.