Power System Protection

• Essential for: Equipment safety, personnel safety, system reliability

• Protection zones: Generators, transformers, buses, transmission lines

• Basic requirements: Selectivity, speed, sensitivity, reliability

• Protection system components: Relays, CTs, VTs, circuit breakers

• Coordination: Primary and backup protection

• Stability: Ability to remain inoperative during external faults

Over-current Protection - Principles

• Operates when current exceeds preset value

• Types:

  • Instantaneous (no intentional delay)

  • Time-delayed (IDMT characteristic)

• Applications: Feeders, motors, transformers

• Relay types: Electromechanical, digital, numerical

• Pickup current: 1.25 to 1.5 times full load current

Over-current Protection - Characteristics

• IDMT (Inverse Definite Minimum Time) curves:

  • Standard inverse (SI): \(t = \frac{0.14 \times TMS}{(I/I_s)^{0.02} - 1}\)

  • Very inverse (VI): \(t = \frac{13.5 \times TMS}{I/I_s - 1}\)

  • Extremely inverse (EI): \(t = \frac{80 \times TMS}{(I/I_s)^2 - 1}\)

  • Long time inverse

• Settings: Pickup current, time multiplier setting (TMS)

• Coordination: Grading margin (0.2-0.5 sec)

• Plug setting multiplier (PSM) = Fault current / Pickup current

Differential Protection - Principles

• Principle: Compare current at both ends of protected zone

• Operates when \(|I_1 - I_2| >\) threshold

• Kirchhoff’s Current Law: \(\sum I_{in} = \sum I_{out}\)

• Applications: Transformers, generators, busbars, transmission lines

• Merits: High sensitivity, complete protection zone

• Unit protection: Internal faults only

Differential Protection - Practical Issues

• CT mismatches and saturation

• Magnetizing inrush currents in transformers

• Percentage differential relay: \(\frac{|I_1 - I_2|}{|I_1 + I_2|} \times 100\%\)

• Biased differential protection

• Restricted earth fault protection

• Harmonic restraint for transformer protection (2nd harmonic)

• Slope settings: Typically 10-40%

Directional Protection - Principles

• Responds to power/current flow direction

• Requires reference quantity (voltage)

• Power direction: \(P = VI\cos\phi\)

• Torque equation: \(T = K_1 V I \cos(\theta - \phi)\)

• Maximum torque angle (MTA): Usually 30°-45° lagging

• Relay connection: 90° connection most common

Directional Protection - Applications

• Applications: Parallel feeders, ring mains, interconnected systems

• Directional over-current relay

• Power directional relay

• Reverse power protection for generators

• Coordination with non-directional relays

• Dead zone problem and solutions

• Voltage polarization: Cross-polarization, self-polarization

Distance Protection - Principles

• Measures impedance (\(Z = V/I\))

• Independent of current magnitude

• Fault location estimation

• Types: Impedance, reactance, mho relays

• Reach setting: Based on line impedance

• Suitable for long transmission lines

Distance Protection - Zones

• Zone 1: 80-85% of line (instantaneous)

• Zone 2: 120-150% of line (time delayed, 0.3-0.5 sec)

• Zone 3: Backup for adjacent lines (1-3 sec)

• Zone 4: Reverse direction (optional)

• Coordination between zones

• Load encroachment considerations

• Under-reach and over-reach concepts

Distance Protection - Relay Characteristics

• Impedance relay: Circle passing through origin

• Reactance relay: Straight line parallel to R-axis

• Mho relay: Circle passing through origin and relay point

• Quadrilateral characteristic: Combination of multiple elements

• Blinders: Prevent operation on load or out-of-zone faults

• Offset mho: For ground fault protection

Circuit Breakers - Principles

• Functions: Interrupt fault current, isolate faulty section

• Arc interruption: Key principle

• Recovery voltage: RRRV (Rate of Rise of Recovery Voltage)

• Breaking capacity: Maximum current it can interrupt

• Making capacity: Peak current it can close onto

• DC time constant: \(\tau = L/R\) (affects asymmetrical current)

Circuit Breakers - Types

• Oil Circuit Breakers:

  • Bulk oil (BOB): Up to 132 kV

  • Minimum oil (MOCB): Up to 36 kV

• Air blast circuit breakers: Up to 800 kV

• Vacuum circuit breakers: Up to 38 kV

• SF6 circuit breakers: Most common for high voltage (>145 kV)

• Comparison: Speed, maintenance, environmental impact

Circuit Breakers - Ratings and Selection

• Rated voltage and current

• Breaking capacity (symmetrical and asymmetrical)

• Making capacity = \(2.55 \times\) breaking capacity

• Operating sequence: O-0.3s-CO-15s-CO

• TRV (Transient Recovery Voltage) withstand

• X/R ratio consideration: Higher X/R = more severe duty

• Selection criteria based on system requirements

• Capability curve: Current vs time characteristic

Protection Coordination

• Primary and backup protection philosophy

• Coordination between different protection schemes

• Grading: Time grading, current grading, both

• Discrimination: Achieved through proper coordination

• Fault study requirements for protection design

• Communication-based protection (pilot protection)

• Selectivity index: Measure of coordination effectiveness

Earth Fault Protection

• Sensitive earth fault (SEF): Uses residual current

• Standby earth fault (SBEF): Backup protection

• Residual current: \(I_R = I_A + I_B + I_C\)

• Core balance CT: Encircles all three phases

• Neutral displacement relay: Voltage-based

• Earth fault factor: Voltage rise in healthy phases

• Restricted earth fault: For transformer protection

Generator Protection

• Stator protection: Differential, earth fault, inter-turn fault

• Rotor protection: Earth fault, inter-turn fault

• System-related protection: Reverse power, loss of excitation

• Over/under frequency protection

• Negative sequence protection: Unbalanced loading

• Loss of synchronism protection

• Capability curve: P-Q diagram limits

Transformer Protection

• Gas actuated relay (Buchholz): Mechanical protection

• Thermal protection: Winding temperature indicator

• Electrical protection: Differential, overcurrent, earth fault

• Magnetizing inrush current: 2nd harmonic content

• Vector group compensation in differential protection

• Tap changer protection

• Through fault withstand capability

Motor Protection

• Starting protection: Thermal overload, locked rotor

• Running protection: Phase unbalance, earth fault

• Stalling protection: Thermal and electrical

• Bearing protection: RTD-based temperature monitoring

• Surge protection: For variable frequency drives

• Single phasing protection: Negative sequence relay

• Starting time limitation: Thermal capacity curve

Busbar Protection

• High impedance scheme: Simple, economical

• Low impedance scheme: More sensitive, complex

• Differential protection principle

• Zone overlap: Avoiding blind spots

• CT saturation effects

• Check synchronism relay

• Bus transfer schemes

Current Transformers (CTs)

• Accuracy classes: 0.1, 0.2, 0.5, 1.0, 3.0, 5.0

• Protection class: 5P, 10P (P = Protection)

• Burden: Connected load in VA

• Saturation: Knee point voltage

• CT ratio: Primary/Secondary (e.g., 400/1A)

• Polarity: Subtractive or additive

• Core balance CT: Zero sequence current measurement

Voltage Transformers (VTs)

• Accuracy classes: 0.1, 0.2, 0.5, 1.0, 3.0

• Burden: Connected load in VA

• Ferranti effect: Capacitive loading

• Ratio: Primary/Secondary (e.g., 11000/110V)

• Electromagnetic VT: Up to 35 kV

• Capacitive VT: Above 35 kV

• Neutral grounding: Broken delta connection

GATE Focus Areas

• Numerical problems on relay settings and coordination

• Zone of protection diagrams and overlap

• CT ratios and burden calculations

• Comparison of different protection schemes

• Circuit breaker ratings and selection criteria

• Time-current coordination curves

• Fault calculation for protection applications

• Distance relay characteristics and settings

• IDMT relay calculations and TMS settings

Sample GATE Questions - 1

Answer: B) 25 kA

Sample GATE Questions - 2

Answer: A) 16 \(\Omega\)

Sample GATE Questions - 3

Answer: B) Prevent maloperation due to CT errors

Sample GATE Questions - 4

Answer: B) Prevent operation during inrush current

Sample GATE Questions - 5

Answer: A) 0.19 sec (Using SI formula: \(t = \frac{0.14 \times 0.5}{4^{0.02} - 1} \approx 0.19\) sec)