(SEM VII) THEORY EXAMINATION 2024-25 POWER SYSTEM PROTECTION

SECTION A

(2 × 10 = 20 Marks)

a) Discuss frequency of occurrence of different types of faults on overhead lines

In overhead transmission lines, faults occur mainly due to lightning, insulation failure, wind, and external objects. Among all types, single line-to-ground faults occur most frequently, accounting for nearly seventy percent of total faults. Line-to-line faults occur less frequently, while double line-to-ground faults are comparatively rare. Three-phase faults occur very infrequently but are the most severe, causing maximum damage to the system.

b) Define time setting multiplier

The time setting multiplier is defined as the factor by which the operating time of an inverse definite minimum time relay is multiplied. It adjusts the relay operating time without changing the current setting, allowing coordination between relays in a protection scheme.

c) Define IDMT relay

An IDMT relay is an inverse definite minimum time relay whose operating time decreases as the fault current increases, but beyond a certain value of current, the operating time becomes constant. These relays are widely used in overcurrent protection due to their flexibility and coordination capability.

d) Discuss the advantages of differential relay

Differential relays provide fast and selective protection by comparing currents at two ends of the protected zone. They offer high sensitivity, excellent discrimination, and are unaffected by external faults. Due to their inherent selectivity, they are widely used for generator, transformer, and busbar protection.

e) Define Plug Setting Multiplier (PSM)

Plug Setting Multiplier is defined as the ratio of actual fault current to the relay pick-up current. It indicates the severity of the fault and determines the operating time of an inverse-time overcurrent relay.

f) Explain the need for Pilot wire protection

Pilot wire protection is required to protect feeders and transmission lines where high-speed and selective protection is needed. It provides instantaneous fault detection within the protected zone and ensures stability for external faults by comparing currents through pilot wires connecting relays at both ends.

g) Illustrate applications of Minimum oil and Bulk oil circuit breakers

Minimum oil circuit breakers are used in medium voltage systems where reduced oil volume and compact design are required. Bulk oil circuit breakers are mainly used in older high-voltage installations where oil acts as both arc quenching and insulation medium.

h) Define the insulation level of a circuit breaker

The insulation level of a circuit breaker refers to its ability to withstand normal operating voltages and abnormal overvoltages such as lightning and switching surges without insulation failure. It is specified in terms of power frequency and impulse withstand voltages.

i) Discuss differences between Static and Electromagnetic relays

Static relays use electronic components and have no moving parts, resulting in faster operation and higher sensitivity. Electromagnetic relays operate using mechanical movement and are slower, bulkier, and require more maintenance compared to static relays.

j) Explain reliability and security of static relays

Reliability of static relays refers to their ability to operate correctly when required, while security ensures that the relay does not operate unnecessarily. Static relays offer high reliability due to fewer mechanical components and better security through precise electronic control.

SECTION B

(10 × 3 = 30 Marks)

2(a) Explain in detail the zones of protection

Zones of protection divide a power system into discrete sections, each protected by its own relay and circuit breaker arrangement. The objective is to isolate only the faulty section while keeping the rest of the system in service. Overlapping zones ensure that no part of the system remains unprotected, thereby increasing system reliability.

2(b) State and classify distance relays

Distance relays operate by measuring the impedance between the relay location and the fault point. Based on their operating characteristics, they are classified into impedance relays, reactance relays, and mho relays. These relays are widely used for transmission line protection.

2(c) Protection of transformer using Merz–Price scheme

The Merz–Price protection scheme is a differential protection method used for transformers. Current transformers are installed on both primary and secondary sides, and the difference between the currents is monitored. Under normal conditions, the differential current is zero, while an internal fault produces a differential current that operates the relay.

2(d) Methods of testing circuit breakers

Circuit breakers are tested through routine tests, type tests, and special tests. These include mechanical operation tests, dielectric tests, short-circuit tests, and temperature rise tests to ensure proper operation under all conditions.

2(e) Short note on Schmitt trigger with transistor level detector

A Schmitt trigger with a transistor level detector is used in static relays to convert analog signals into digital signals. It improves noise immunity and ensures precise switching by introducing hysteresis in the input-output characteristics.

SECTION C

3(a) Define potential transformer and explain its construction

A potential transformer is an instrument transformer used to step down high voltage to a lower value suitable for measurement and protection. It consists of a laminated magnetic core with primary and secondary windings. The primary is connected across the high-voltage line, while the secondary supplies voltage to meters and relays.

3(b) Classification of protective schemes

Protective schemes are classified based on the principle of operation, zone of protection, and type of equipment protected. These schemes include overcurrent protection, differential protection, distance protection, and pilot protection schemes.

4(a) Explain Mho relay and its operating characteristics

A mho relay is a voltage-controlled distance relay whose operating characteristic is a circle passing through the origin on the impedance plane. It is inherently directional and provides stable operation under power swings, making it suitable for long transmission line protection.

4(b) Directional induction type overcurrent relay

This relay operates based on the relative phase angle between current and voltage. It allows operation only in a specified direction, preventing unwanted tripping during reverse power flow or external faults.

5(a) Restricted earth fault protection of generators

Restricted earth fault protection provides sensitive protection for stator winding earth faults within a limited zone. The earth resistance affects the percentage of winding protected, with higher resistance increasing the unprotected portion of the winding.

5(b) Protection of alternator field winding against ground faults

Ground faults in the field winding are detected using insulation monitoring or resistance measurement methods. Early detection prevents severe damage and ensures continuous operation of the alternator.

6(a) Construction and working of vacuum circuit breaker

A vacuum circuit breaker uses a vacuum as the arc quenching medium. The contacts are enclosed in a vacuum interrupter, and when separation occurs, the arc is rapidly extinguished due to the absence of ionizable medium, resulting in fast and reliable operation.

6(b) Sequence of operation of circuit breaker, isolator, and earthing switch while closing

While closing, the earthing switch is opened first, followed by the isolator, and finally the circuit breaker is closed to energize the system safely.

7(a) Advantages of static relays

Static relays offer fast operation, high sensitivity, improved accuracy, reduced maintenance, and better reliability compared to conventional relays.

7(b) Functional circuits in a static relay

A static relay consists of sensing circuits, measuring units, comparators, logic circuits, and output units. These circuits collectively ensure accurate detection and isolation of faults.