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∝Φ1Φ2sinαT \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−cos2ϕRRRV = \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)
SECTION – C (Attempt Any Two) (2 × 15 = 30 Marks)
Q3. Electromagnetic relay
Working principle
Based on electromagnetic attraction. When current flows through coil, magnetic field attracts armature causing contacts to close.
Advantages • Simple construction
• Reliable • Low cost
Disadvantages
• Slow operation • High maintenance
• Mechanical wear
Applications • Overcurrent protection
• Distance protection • Motor protection
Q4. Distance relays
Types of distance relays
(i) Impedance relay
• Operates when impedance < set value • Circular characteristic
(ii) Reactance relay
• Operates on reactance • Immune to arc resistance
(iii) Mho relay
• Directional relay • Circular characteristic passing through origin
Used mainly in transmission line protection.
Q5. Air blast circuit breaker
Construction
• Fixed & moving contacts • Air blast nozzle
• Compressor • Arc chute
Working
High-pressure air blast:
• Cools arc • Lengthens arc
• Removes ionized gases
Advantages
• Fast arc extinction • Suitable for high voltage
Disadvantages
• High noise • Requires air compressor
