UNIT 1: POWER SYSTEM PROTECTION - EXAM-FOCUSED SHORT NOTES

1.0 FUNDAMENTALS OF FAULT ANALYSIS

1.1 Types of Power System Faults

Symmetrical (Balanced) Fault: Three-phase short circuit (L-L-L). All three phases affected equally. Rare (5-10%) but most severe. Analysis uses only positive sequence network.
Unsymmetrical (Unbalanced) Faults:
- Single Line-to-Ground (LG): Phase 'a' to ground. Most common (70-80%).
- Line-to-Line (LL): Phases 'a' and 'b' shorted. No ground involvement.
- Double Line-to-Ground (DLG): Phases 'a' and 'b' to ground.
Causes: Insulation failure, lightning, wind, fire, mechanical damage, human error.
Effects: High fault current → equipment damage, fire; voltage dip → instability; system separation.
Fault Level / Fault MVA: The maximum short-circuit MVA that would flow at a point if the system voltage were maintained at its nominal value. It is a measure of the strength of the system at that bus. Higher fault level means higher fault current for a given fault.

Fault MVA = $$\displaystyle \frac{\sqrt{3} \times V_{base}}{Z_{base}} $$ or $$\displaystyle \frac{(V_{base})^2}{Z_{base}} $$ (3-phase)

1.2 Symmetrical Components (Fortescue's Theorem)

Significance: Any set of three unbalanced phasors ($$\displaystyle V_a, V_b, V_c $$) can be resolved into three balanced sets:
1. Positive Sequence ($$\displaystyle V_1 $$): $$\displaystyle V_{a1}, V_{b1}=V_{a1}\angle-120^\circ, V_{c1}=V_{a1}\angle120^\circ $$. Rotates clockwise (same as original system).
2. Negative Sequence ($$\displaystyle V_2 $$): $$\displaystyle V_{a2}, V_{b2}=V_{a2}\angle120^\circ, V_{c2}=V_{a2}\angle-120^\circ $$. Rotates counter-clockwise.
3. Zero Sequence ($$\displaystyle V_0 $$): $$\displaystyle V_{a0}=V_{b0}=V_{c0} $$. All in-phase.
Transformation:

$$ \begin{bmatrix} V_a \\ V_b \\ V_c \end{bmatrix} = \begin{bmatrix} 1 & 1 & 1 \\ 1 & a^2 & a \\ 1 & a & a^2 \end{bmatrix} \begin{bmatrix} V_0 \\ V_1 \\ V_2 \end{bmatrix} \quad \text{where } a = 1\angle120^\circ $$

Inverse: $$\displaystyle V_0 = \frac{1}{3}(V_a + V_b + V_c) $$, etc.

Sequence Networks: Separate single-phase networks representing the path for each sequence component.
- Generator: $$\displaystyle V_1 = E_f - I_1 Z_1 $$; $$\displaystyle V_2 = -I_2 Z_2 $$; $$\displaystyle V_0 = -I_0 Z_0 $$.
- Transformer: Positive/Negative: impedance referred to appropriate side. Zero-sequence path depends on connection (e.g., blocked by delta, passes through star with grounded neutral).
- Line: All sequences have impedance ($$\displaystyle Z_0 > Z_1 \approx Z_2 $$).
Interconnection for Faults at Generator Terminals (Unloaded):

| Fault Type | Sequence Network Connection | Key Point | | :--- | :--- | :--- | | 3-Phase | Only Positive sequence in series. $$\displaystyle I_f = \frac{E_f}{Z_1} $$ | Balanced, no zero/neg seq. | | LG (a-g) | All three sequences in series. $$\displaystyle I_{f(a)} = 3I_0 = \frac{3E_f}{Z_1+Z_2+Z_0} $$ | Most important formula. | | LL (a-b) | Positive & Negative sequences in parallel. $$\displaystyle I_{f(a)} = \sqrt{3} I_1 \angle30^\circ = \frac{\sqrt{3}E_f}{Z_1+Z_2} $$ | No zero sequence. | | DLG (a-b-g) | Positive in series with (Zero // Negative). $$\displaystyle I_{f(a)} = \frac{\sqrt{3}E_f}{Z_1 + \frac{Z_0 Z_2}{Z_0+Z_2}} $$ | Complex parallel combo. |

1.3 Fault Current Calculation & Analysis

General Bus Fault (with system Thevenin impedance $$\displaystyle Z_{th} $$):
- 3-φ: $$\displaystyle I_f = \frac{E_{bus}}{Z_1^{th}} $$
- LG: $$\displaystyle I_f = \frac{3E_{bus}}{Z_1^{th}+Z_2^{th}+Z_0^{th}} $$
Effect of Fault Impedance ($$\displaystyle Z_f $$):
- Adds in series with the fault path.
- LG Fault with $$\displaystyle Z_f $$: $$\displaystyle I_f = \frac{3E_f}{Z_1+Z_2+Z_0+3Z_f} $$. Fault current decreases as $$\displaystyle Z_f $$ increases.
Per Unit System:
- Step 1: Choose common Base MVA ($$\displaystyle S_{base} $$) and Base kV ($$\displaystyle V_{base} $$) for the system.
- Step 2: Calculate base impedance: $$\displaystyle Z_{base} = \frac{(V_{base})^2}{S_{base}} $$.
- Step 3: Convert each element: $$\displaystyle Z_{pu(new)} = Z_{pu(old)} \times \frac{S_{base(old)}}{S_{base(new)}} \times \left(\frac{V_{base(new)}}{V_{base(old)}}\right)^2 $$.
TIP: Always convert all reactances to a common base before connecting sequence networks.

1.4 Current Limiting Reactors

Purpose: Limit maximum short-circuit current to a safe value for equipment (CBs, CTs, conductors) during faults, especially when system expansion increases fault level.
Types:
- Air-cored: No iron, linear inductance, no saturation. Used for high currents.
- Iron-cored: Gassy, prone to saturation under high currents.
- Ring-type: Installed around busbars (busbar reactors).
Location:
- Feeder reactors: In series with each incoming feeder.
- Busbar reactors: Connected between bus sections (sectionalizing).
- Generator reactors: In series with generator (now less common due to voltage drop).
Selection: Rated for continuous load current, impedance % chosen to limit fault current below CB breaking capacity.

2.0 PROTECTION RELAYS: PRINCIPLES & CHARACTERISTICS

2.1 Basic Relay Concepts

Pick-up Value: Minimum input (current/voltage) to close relay contacts.
Reset Value: Maximum input to open relay contacts. Reset Ratio = $$\displaystyle \frac{\text{Reset Value}}{\text{Pick-up Value}} $$ (ideally close to 1).
Drop-out Ratio: Same as reset ratio.
Time Delay: Intentional lag between pick-up and operation.
Reach: For distance relays, the maximum fault distance up to which it operates.
Plug Setting Multiplier (PSM): $$\displaystyle \text{PSM} = \frac{\text{Fault Current}}{\text{Relay Rated Current} \times \text{Plug Setting}} $$.
Time Multiplier Setting (TMS): Multiplier for time dial setting in IDMT relays.
Security: No operation for non-faults (no unwanted trips).
Dependability (Reliability): Operation for all faults within its zone.
Selectivity: Only the relay nearest to the fault operates.
Speed: Fast operation to minimize damage.

2.2 Electromechanical Relays

Electromagnetic Attraction Type:
- Construction: Moving iron piece, coil, spring, contacts.
- Principle: Magnetic force $$\displaystyle F \propto I^2 $$ attracts iron, closes contacts.
- Torque: $$\displaystyle T \propto I^2 $$. No time delay inherently (instantaneous).
- Applications: Instantaneous overcurrent, undercurrent, auxiliary relays.
Induction Disc Relay:
- Construction: Disc (aluminum), two electromagnets (operating & braking), spring, contact.
- Operating Torque: $$\displaystyle T_o \propto I^2 \sin \theta $$ (where $\theta$ is angle between fluxes).
- Braking Torque: Constant from permanent magnet.
- Net Torque: $$\displaystyle T_{net} = T_o - T_b $$. Disc rotates when $$\displaystyle T_{net} > 0 $$.
- Applications: Overcurrent (definite/IDMT), directional (with voltage polarizing).
Induction Cup Relay:
- Construction: Hollow cylindrical cup (conducting), field system, contacts.
- Advantages over Disc: Faster (less inertia), less overrun, more sensitive.
- Torque: Similar to disc but higher.
- Reset to Pick-up Ratio: Typically 0.85 to 0.95 (much better than disc ~0.7). High reset ratio is desirable.
Minimizing Overrun/Backswing:
1. Strong braking magnet.
2. High reset ratio (cup relay inherently better).
3. Shorted turn on moving part (to damp oscillations).

2.3 Static Relays (Analog/Digital)

Advantages: No moving parts → fast, accurate, long life; low burden; multiple characteristics; self-test; no mechanical wear.
Functional Building Blocks:
- Level Detector: Compares input (I or V) with a reference. Hysteresis (dead-band) prevents chatter.
  DiagramCANVAS: Comparator with positive feedback for hysteresis
- Comparator:
  - Amplitude Comparator: Outputs 1 if $$\displaystyle |S_1| > K|S_2| $$. Used in impedance relays.
  - Phase Comparator: Outputs 1 if angle between $$\displaystyle S_1 $$ and $$\displaystyle S_2 $$ is within $\pm \beta$. Used in directional relays.
  - Conversion: Phase comparator can be made from two amplitude comparators using $$\displaystyle 90^\circ $$ phase shift.
- Logic Circuits: AND, OR, NOT gates to implement trip logic (e.g., Trip = (Overcurrent AND NOT Directional Block)).
- Timing Circuits: RC networks or digital counters for definite/inverse time delays.
Static Directional Relay: Compares operating quantity (e.g., line current $I$) with polarizing/reference quantity (e.g., voltage $V$). Operates when phase angle between them is within $$\displaystyle 90^\circ $$ of the fault direction.
Static Distance Relay: Measures impedance $$\displaystyle Z = V/I $$. Operates if $$\displaystyle Z < \text{Setting} $$ (within reach).

2.4 Microprocessor-Based (Numerical) Relays

Schematic & Components:

DiagramSEARCH: "numerical relay block diagram CT VT anti-aliasing filter ADC microprocessor"
1. Analog Input: CT/VT → Anti-aliasing Filter (low-pass).
2. A/D Converter: Samples filtered analog signals.
3. Microprocessor/DSP: Executes protection algorithms (FFT for phasors, impedance calculation).
4. Memory: Stores settings, firmware, fault records.
5. Output: Trip/Close contacts, alarm.
6. Communication Port: IEC 61850, Modbus, etc.
7. Front Panel: LCD, keys for settings.
Functions:
- Protection Algorithms: Implement all characteristics (OC, Distance, Diff, etc.).
- Fault Recording: COMTRADE files (.cfg, .dat).
- Communication: Phasor Measurement Unit (PMU) data, GOOSE messages.
- Self-Monitoring: Watchdog timer, hardware checks.
- Settings Management: Via software.
Software Development Importance: Robustness (no crashes), cybersecurity (firewalls, authentication), validation (simulation testing), standards compliance (IEC 61850).

2.5 Relay Characteristics & Plots

Time-Current Characteristics:

| Type | Equation (Standard Inverse) | Plot | | :--- | :--- | :--- | | Definite Time (DT) | $$\displaystyle t = TMS \times k $$ (constant) | Horizontal line. | | IDMT | $$\displaystyle t = \frac{0.14 \times TMS}{PSM^{0.02} - 1} $$ | Inverse curve, flattens at high PSM. | | Very Inverse | $$\displaystyle t = \frac{13.5 \times TMS}{PSM - 1} $$ | Steeper than IDMT. | | Extremely Inverse | $$\displaystyle t = \frac{80 \times TMS}{PSM^2 - 1} $$ | Very steep at low PSM. |

DiagramSEARCH: "IDMT very inverse extremely inverse time current characteristic curve"
R-X Diagram (Impedance Plane):

DiagramSEARCH: "R-X diagram impedance relay mho offset mho directional quadrilateral"
- Impedance Relay (MHO): Circular characteristic passing through origin. Operates for $$\displaystyle |Z| < Z_{set} $$. Circle diameter = $$\displaystyle Z_{set} $$. Directional (operates in first and third quadrants if voltage reference is correct).
- OFF-SET MHO Relay: Circle offset from origin (center not at 0). Used for long lines where reach must be adjusted for line impedance.
- Directional Relay: Straight line (semi-circle) at $$\displaystyle 90^\circ $$ to reference voltage axis.
- Distance Relay Zones:
  - Zone 1: 80-90% of line length, no time delay.
  - Zone 2: Covers full line + backup for next line (e.g., 120-150%), time delay ($$\displaystyle t_2 $$).
  - Zone 3: Backup for next two lines, longest time delay ($$\displaystyle t_3 $$).
  - Often implemented as MHO circles or quadrilaterals (better for resistive faults).

3.0 EQUIPMENT-SPECIFIC PROTECTION SCHEMES

3.1 Generator / Alternator Protection

Stator Winding - Percentage Differential:
- Principle: $$\displaystyle \sum I_{CT} = 0 $$ under normal/external faults. Internal fault → differential current $$\displaystyle I_{diff} = I_1 - I_2 $$.
- Slope Characteristic: I_{diff} > \text{Slope} \times I_{avg} + \text{Minimum}. Slope (e.g., 15-30%) accounts for CT mismatch during high through-fault currents.
- Stability Check: For a fault, $$\displaystyle I_{diff} = 400A $$, $$\displaystyle I_{avg} = \frac{400+320}{2}=360A $$, Slope=15% → Threshold = $$\displaystyle 0.15 \times 360 = 54A $$. Since $$\displaystyle 400A > 54A $$, relay trips.
Rotor Winding: Loss of excitation (impedance relay), field failure (undervoltage), negative sequence current (rotor heating).
Other: Overcurrent (backup), earth fault (95% stator earth fault), overvoltage, underfrequency, reverse power (motoring).
Generator-Transformer Unit: Single differential protection covering both.

3.2 Power Transformer Protection

Buchholz Relay:
- Construction: Gas-filled chamber, float, flap.
- Operation:
  - Incipient Fault (slow heating): Gas bubbles → float drops → alarm contact.
  - Severe Fault (arc, oil surge): Oil rushes → flap deflects → trip contact.
- Application: Oil-filled transformers (≥ 500 kVA). Not for dry-type.
Percentage Differential:
- CT Ratio Selection: Must account for vector group (star-delta shift) and tap changer.
- Star-Delta Example:
  - HT side (star): CT ratio = $$\displaystyle \frac{I_{HV}}{\text{CT sec}} $$.
  - LT side (delta): CT ratio = $$\displaystyle \frac{I_{LV}}{\text{CT sec}} \times \frac{1}{\sqrt{3}} $$ (due to delta-star phase shift).
- Problem: 66/11 kV star-delta, LT CT ratio 420:5. Find HT CT ratio.
  - LT side line current: $$\displaystyle I_{LT} = \frac{S}{\sqrt{3} \times 11} $$.
  - HT side line current: $$\displaystyle I_{HT} = \frac{S}{\sqrt{3} \times 66} = \frac{I_{LT}}{6} $$.
  - But LT CTs are on delta side → CT secondary current must match HT CT secondary after accounting for $\sqrt{3}$ shift.
  - Solution: HT CT ratio = $$\displaystyle \frac{I_{HT}}{I_{CT-sec}} = \frac{I_{LT}/6}{I_{LT}/(420\sqrt{3})} = \frac{420}{6\sqrt{3}} \approx 40.4 $$. Choose standard ratio like 400:5 or 420:5.
Restricted Earth Fault (REF): Sensitive earth fault protection for faults near neutral (where normal differential is insensitive). Uses CTs in neutral and phase.

3.3 Busbar Protection

Importance: High fault current → system instability if not cleared fast (< 100 ms).
Differential Protection:
- CTs on all incoming/outgoing feeders.
- Internal Fault: $$\displaystyle \sum I_{CT} \neq 0 $$ → trip all feeders.
- External Fault: $$\displaystyle \sum I_{CT} = 0 $$ → no trip.
- Must be high-speed.
Frame Leakage Protection:
- CTs on main connections to bus structure.
- Fault current flows from bus → frame → ground → returns via frame CT → operates.
Other: High-impedance differential (with series stabilizing resistor), biased differential, overcurrent with time grading.

3.4 Transmission Line Protection

Protection Zones:
- Zone 1 (Primary): 80-90% line length, instantaneous.
- Zone 2 (Backup): Covers full line + 30-50% of next line, time delay ($$\displaystyle t_2 $$).
- Zone 3 (Backup): Covers next two lines, longest delay ($$\displaystyle t_3 $$).
Distance Relaying: Primary for EHV lines. Measures impedance. Uses MHO/quadrilateral characteristics on R-X.
Pilot Relaying Schemes:
- Pilot: Communication channel between line ends for fast tripping over entire line (Zone 1 speed).
- Types:
  
  | Pilot Type | Medium | Merits | Demerits | Application | | :--- | :--- | :--- | :--- | :--- | | Power Line Carrier (PLC) | HV line itself | Low cost, uses existing line | Affected by line conditions, frequency limits | Up to 200 km, < 230 kV | | Microwave | Dedicated radio link | Fast, reliable, independent | High cost, terrain sensitive | EHV, important lines | | Fiber Optic | Optical fiber | Very high speed, immune to EMI, large bandwidth | Cable laying cost, fragility | Modern substations, GIS |
- Phase Comparison (PLC):
  - Principle: Compare phase angles of currents at both ends. Internal fault: currents in-phase (or $$\displaystyle 180^\circ $$ out depending on CT polarity). External fault: currents opposite.
  - Permissive Scheme: Trip = (Local Zone 1) AND (Permissive signal from remote). Fast.
  - Blocking Scheme: Trip = (Local Zone 2) AND (NOT Blocking signal). More secure.
  DiagramSEARCH: "phase comparison carrier protection block diagram permissive"
Overcurrent Protection: Used for distribution lines and as backup for transmission lines. Requires time grading.

4.0 CIRCUIT BREAKERS: THEORY, TYPES & APPLICATIONS

4.1 Arc Phenomenon & Interruption Theory

Arc Formation: When contacts separate, ionization of medium creates conductive path.
Arc Characteristics: Voltage across arc ($$\displaystyle V_a $$) vs current ($I$). Negative resistance region (as $I$ increases, $$\displaystyle V_a $$ decreases slightly).
Theory of Current Interruption - Energy Balance:
- Condition for extinction: Rate of rise of dielectric strength ($du/dt$) > Rate of rise of recovery voltage ($dv/dt$).
- At current zero, arc extinguishes if arc resistance increases rapidly (quenching) so that $$\displaystyle V_a > V_{rec} $$.
Key Terms:
- Restriking Voltage ($$\displaystyle v_r $$): Transient voltage across contacts immediately after current zero. $$\displaystyle v_r = L \frac{di}{dt} $$. Can cause re-ignition.
- Recovery Voltage ($$\displaystyle V_{rec} $$): Voltage that appears across breaker after arc extinction. RMS value of first few cycles. Must be withstood by breaker.
DiagramSEARCH: "restriking voltage recovery voltage transient RRRV circuit breaker"

4.2 Circuit Breaker Ratings & Selection

Breaking Capacity: RMS value of symmetrical current it can break at rated voltage. Often given in kA or MVA. Must be ≥ system fault level.
- Symmetrical Breaking Current: $$\displaystyle I_{b,sym} = \frac{I_{sym}}{\sqrt{3}} $$.
- Asymmetrical Breaking Current: Includes DC component. $$\displaystyle I_{b,asym} = I_{sym} \times \text{Asymmetry factor} $$.
Making Capacity: Peak value of first current loop when closing onto a fault. Must be ≥ peak fault current. Making current ≈ 1.8 × Sym. breaking current.
Other Ratings: Rated voltage, rated continuous current, duty cycle (O-0.3s-CO-3min-CO).
Selection Factors: System voltage, fault level, location (indoor/outdoor), duty, cost, maintenance, environmental (SF6 handling).

4.3 Types of Circuit Breakers & Operation

Oil Circuit Breakers (OCB):
- Bulk Oil: Oil acts as both dielectric and quenching medium. Large oil volume.
- Minimum Oil: Less oil, only for quenching. Explosion Pot:
  - Function: Constrains arc, gas pressure blows arc.
  - Types:
    - Plain/Open End: Simple, low pressure.
    - Corrugated: Corrugations increase gas pressure.
    - Self-Generated Pressure: Piston/ring creates high pressure → fast quenching.
Air Blast Circuit Breakers (ABCB):
- Principle: High-pressure air blast through nozzle extinguishes arc.
- Types:
  - Axial Blast: Air along arc axis. Less affected by current chopping (high pressure maintains).
  - Cross Blast: Air perpendicular to arc.
  - Radial Blast: Air radially inward.
- Application: EHV systems (≥ 132 kV). Fast, no fire risk.
SF6 Circuit Breaker:
- Principle: SF6 gas has high dielectric strength and excellent arc-quenching (negative $dI/dt$). Arc decomposes SF6 → recombines quickly.
- Advantages: Compact, silent, low maintenance, no fire/explosion risk, excellent for EHV.
- Voltage Range: ≥ 72.5 kV (standard for EHV: 145 kV, 245 kV, 420 kV, 800 kV).
Vacuum Circuit Breaker (VCB):
- Construction: Vacuum interrupter (glass/ceramic), Cu-Cr contacts, mechanism.
- Principle: Arc in high vacuum ($$\displaystyle 10^{-6} $$ torr). Contacts separate → metal vapor arc → current zero → rapid dielectric recovery.
- Advantages: Long life (no erosion), low maintenance, no gas handling, fast operation (< 3 cycles), environmentally friendly.
- Voltage Range: Medium Voltage (typically up to 38 kV, commonly 11 kV & 33 kV in India).

5.0 MODERN & INTEGRATED PROTECTION CONCEPTS

5.1 Security and Reliability

Security: No trip for non-faults. Ensured by: proper coordination, high-quality CTs/VTs (no saturation), secure communication, fail-safe design (loss of power → trip? depends), cybersecurity.
Reliability (Dependability): Trip for all faults in zone. Ensured by: redundancy (dual relays), regular testing, robust hardware, diverse protection principles (e.g., differential + overcurrent).
Balance: Often trade-off. More security may reduce dependability.

5.2 Role of Microprocessor/Numerical Relays

Consolidation: One unit can provide OC, Earth Fault, Distance, Differential, Auto-reclose.
Advantages:
- Flexibility: Settings change via software.
- Self-Supervision: Continuous health monitoring.
- Communication: IEC 61850 → substation automation.
- Fault Recording: Detailed COMTRADE files.
- Adaptive Protection: Settings change with system conditions.
- Advanced Functions: Synchrocheck, fault location, sequence of events.

5.3 Software Development for Protection

Importance: The brain of numerical relay. Bugs can cause misoperation.
Aspects:
- Protection Algorithms: Accurate phasor estimation (FFT, DFT), impedance calculation, logic implementation.
- Settings Tools: User-friendly GUI for configuration.
- HMI: Human-machine interface.
- Communication Stacks: IEC 61850 (MMS, GOOSE, SV).
- Testing Simulators: Offline/online testing.
- Cybersecurity: Encryption, authentication, secure boot, firewalls.
- Standards: IEC 60870, IEC 61850, IEEE C37.90.

5.4 Logic in Protection Schemes

Basic Gates: AND, OR, NOT implemented in software/firmware.
Applications:
- Interlocking: Trip = (OC) AND (NOT Switchgear Status).
- Permissive Schemes: Trip = (Zone 2) AND (Permissive from remote).
- Blocking Schemes: Trip = (OC) AND (NOT Blocking from pilot).
- Breaker Failure Protection: Trip = (BF Timer) AND (Current still flowing) → trip upstream breakers.
- Auto-reclose: Logic for single/triple pole, dead time, reclaim time.

6.0 AUXILIARY & SUPPORTING ELEMENTS

6.1 Current Transformers (CTs) & Voltage Transformers (VTs)

Role: Isolate protection/relay from high voltage, provide low-level signals proportional to primary.
CT Requirements for Protection:
- Accuracy Class: 5P or 10P (protection class). P = "protection", number = % composite error at rated burden & accuracy limit factor (ALF).
- ALF: e.g., 5P10 → 10 times rated current, composite error ≤ 5%.
- CT Ratio Selection for Differential:
  - Star-Delta Transformer: CT ratios must compensate for 30° phase shift and magnitude change ($1/\sqrt{3}$).
  - Tap Changers: CT ratios must match maximum/minimum tap to maintain balance.
Problems:
- Saturation: During high faults, flux exceeds core limit → secondary current distorted → differential relay may maloperate (see slope characteristic).
- Ratio Mismatch: Due to tap changer or vector group not fully compensated → residual current.

6.2 Fuses

HRC (High Rupturing Capacity) Fuses:
- Construction: Cercelain body, sand filling (arc quenching), fusible element (silver/copper).
- Working: Fault current → element melts ($$\displaystyle I^2t $$) → arc in sand → sand quenches arc.
- Characteristics: $$\displaystyle I^2t $$ is constant for given fuse → inverse time.
- Applications: LT switchgear (≤ 1000V), motor protection, transformer protection (backup), capacitor protection.

6.3 Buchholz Relay

(Covered in 3.2). Reiterated here as a key auxiliary device.
Key Point: Only for oil-immersed transformers. Detects incipient faults (gas) and severe faults (oil surge).

EXAM TIPS & COMMON PITFALLS:

Symmetrical Components: Always draw sequence networks separately first, then interconnect according to fault type. Remember LG has all three in series.

CT Ratio for Star-Delta: The $$\displaystyle \frac{1}{\sqrt{3}} $$ factor is often forgotten. HT side CT ratio = $$\displaystyle \frac{\text{LT side CT ratio} \times \text{LT voltage}}{\text{HT voltage} \times \sqrt{3}} $$ for star-delta.

Differential Slope: Calculate $$\displaystyle I_{diff} $$ and $$\displaystyle I_{avg} $$ correctly. $$\displaystyle I_{avg} = \frac{|I_1|+|I_2|}{2} $$. Compare $$\displaystyle I_{diff} $$ with $$\displaystyle \text{Slope} \times I_{avg} + \text{Min} $$.

IDMT Calculation: Use correct formula. $$\displaystyle t = \frac{0.14 \times TMS}{PSM^{0.02} - 1} $$. PSM = $$\displaystyle \frac{\text{Fault Current}}{\text{Relay Rated Current} \times \text{Plug Setting}} $$.

MHO vs OFF-SET MHO: MHO circle passes through origin. OFF-SET circle is shifted (center not at 0).

VCB vs SF6: VCB for MV (≤ 38 kV), SF6 for EHV (≥ 72.5 kV). Don't mix up.

Breaking vs Making Capacity: Breaking = RMS symmetrical current. Making = Peak current (≈ 2.7 × symmetrical RMS for first loop).

Restriking vs Recovery Voltage: Restriking is transient immediately after zero (high $du/dt$). Recovery is steady-state RMS after transient dies.

Buchholz: Gas → alarm. Oil surge → trip. Not for dry transformers.

Pilot Protection: Phase comparison compares phase angles at both ends. Permissive is faster, blocking is more secure.

\boxed{\text{End of Unit 1 Notes - Focus on derivations (fault currents, sequence networks), CT ratio calculations, relay characteristic plots, and CB theory.}}

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Power System Protection (EX-701) - Unit 1 Short Notes