THEORY EXAMINATION (SEM–VI) 2016-17 SWITCH GEAR AND PROTECTION

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SWITCH GEAR AND PROTECTION (NEE602)

Section-wise Solved Answers & Notes

SECTION – A (10 × 2 = 20 Marks)

Very short & precise answers

(a) Switchgear

Switchgear is an assembly of electrical devices such as switches, circuit breakers, fuses, relays and isolators used to control, protect and isolate electrical equipment.

(b) Purpose of circuit breakers

Circuit breakers are used to:
• Interrupt fault current • Protect equipment from damage
• Isolate faulty sections • Ensure safety of power system

(c) Primary and backup protection

• Primary protection: First line of defense for a protected element
• Backup protection: Operates when primary protection fails

(d) Block diagram of static relay

Main blocks: • Input transducer (CT / PT)
• Comparator • Amplifier
• Decision logic • Output unit (trip circuit)

(Neat block diagram must be drawn in exam)

(e) Advantages of static relay • High sensitivity
• Fast operation • No moving parts
• Low maintenance • High reliability

(f) Arc extinction

Arc extinction is the process of quenching or extinguishing the electric arc formed between contacts of a circuit breaker during current interruption.

(g) Working of air-break circuit breaker

In air-break CB, arc is drawn in air and extinguished by lengthening, cooling and splitting the arc using arc chutes.

(h) Types of electromagnetic attraction relays

• Plunger type relay • Hinged armature type relay
• Balanced beam type relay

(i) Classification of generator faults • Stator faults: Phase-to-earth, phase-to-phase
• Rotor faults: Field winding faults • Abnormal conditions: Overload, over-speed

(j) Abnormal conditions in induction motor & protection

Abnormal conditions: • Overload
• Single phasing • Over-voltage / under-voltage
• Overheating

Protection used: • Thermal relay
• Over-current relay • Phase failure relay

SECTION – B (Attempt Any Five) (5 × 10 = 50 Marks)

(a) Basic principle of operation of circuit breaker

A circuit breaker operates by separating current-carrying contacts under fault conditions.
An arc is formed which is extinguished using a suitable arc quenching medium (air, oil, SF₆, vacuum).

Main steps: Fault detection

Trip signal from relay Contact separation

Arc extinction

(b) Phenomenon of arc

Arc: A luminous discharge between contacts due to ionization of medium.

Properties of arc: • High temperature
• Low resistance • Emits light and heat

Initiation: Occurs when contacts separate carrying current.

Maintenance: Maintained by continuous ionization and supply voltage.

(c) Numerical – IDMT overcurrent relay

Given: CT ratio = 400/5
Relay current setting = 150% Fault current = 5000 A
TMS = 0.6

Step-1: Relay secondary current

Is=5000×5400=62.5 AI_s = \frac{5000 \times 5}{400} = 62.5 \text{ A}Is=4005000×5=62.5 A

Step-2: Plug setting current (PSC)

PSC=1.5×5=7.5 APSC = 1.5 \times 5 = 7.5 \text{ A}PSC=1.5×5=7.5 A

Step-3: Plug setting multiplier (PSM)

PSM=62.57.5=8.33PSM = \frac{62.5}{7.5} = 8.33PSM=7.562.5=8.33

Step-4: Time of operation
From relay characteristics curve, time corresponding to PSM ≈ 3.2 s

T=0.6×3.2=1.92 sT = 0.6 \times 3.2 = \boxed{1.92 \text{ s}}T=0.6×3.2=1.92 s

(d) Current chopping

Current chopping is the sudden interruption of current before natural current zero, mainly in air-blast and vacuum CBs.

Effect: • Produces high over-voltage • Can damage insulation

(e) Protection circuit for induction motor

Protection against: • Overload → Thermal relay
• Single phasing → Phase failure relay • Short circuit → HRC fuse / CB
• Over-temperature → Temperature sensors

Purpose is to prevent winding damage and rotor heating.

(f) Problems in differential protection

• CT mismatch • CT saturation
• Spill current during external faults • Harmonic currents

Solutions:
• Percentage differential protection • Harmonic restraint

(g) Torque equation of induction type relay

Operating torque:

T∝Φ1Φ2sin⁡αT \propto \Phi_1 \Phi_2 \sin \alphaT∝Φ1Φ2sinα

Where: • Φ1,Φ2\Phi_1, \Phi_2Φ1,Φ2 = fluxes • α\alphaα = phase angle

Used in overcurrent and directional relays.

(h) Phase comparison method of carrier current protection

Used for transmission line protection.

Principle: • Compares phase of currents at both ends of line
• If currents are in phase → internal fault
• If out of phase → external fault

Provides fast and selective protection.

(i) Numerical – RRRV of circuit breaker

Given: Voltage = 132 kV
Restriking frequency = 20 kHz
Power factor = 0.15

RRRV:

RRRV=Vm×2πf1−cos⁡2ϕRRRV = \frac{V_m \times 2\pi f}{\sqrt{1-\cos^2\phi}}RRRV=1−cos2ϕVm×2πf RRRV≈6.5 kV/μs (approx.)RRRV \approx \boxed{6.5 \text{ kV/μs (approx.)}}RRRV≈6.5 kV/μs (approx.)

(Exact value depends on waveform assumption; show steps in exam)