Over-current, Differential, Directional, and Distance Protection

POWER SYSTEM PROTECTION – BASIC PRINCIPLE

Protection system detects faults and isolates faulty part using:

• Relays → detect fault
• Circuit breaker → isolate fault

Relay operates when electrical quantity exceeds preset value.

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1. OVER-CURRENT PROTECTION

Principle

Relay operates when current exceeds preset value.

$$I_{fault} > I_{pickup}$$

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Operating Condition

$$\text{Plug Setting Multiplier (PSM)} = \frac{I_{fault}}{I_{pickup}}$$

Relay operates if:

$$PSM > 1$$

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Types of Overcurrent Relays

1. Instantaneous Relay

Operates immediately (no delay)

Condition:

$$I > I_{pickup}$$

Used for close faults.

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2. Definite Time Relay

Operates after fixed time delay.

Independent of current magnitude.

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3. Inverse Time Relay (MOST IMPORTANT FOR GATE)

Operating time inversely proportional to current.

Higher current → lower operating time

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IDMT Relay (Important Formula)

IEC standard inverse equation:

$$t = \frac{0.14 \times TMS}{PSM^{0.02} - 1}$$

Where:

• TMS = Time multiplier setting
• PSM = Plug setting multiplier

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Applications

Feeders
Distribution lines

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Advantages

Simple
Cheap

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Disadvantages

Not selective for complex networks

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2. DIFFERENTIAL PROTECTION

Principle

Based on comparison of currents entering and leaving protected zone.

Kirchhoff's Current Law:

$$I_{in} = I_{out}$$

If not equal → fault exists.

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Differential Current

$$I_{diff} = I_1 - I_2$$

Relay operates if:

$$I_{diff} > I_{pickup}$$

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Percentage Differential Protection (VERY IMPORTANT)

Operating condition:

$$I_{diff} > \text{Slope} \times I_{restraint}$$

Where:

Restraint current:

$$I_{restraint} = \frac{I_1 + I_2}{2}$$

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Operating Characteristic

Two slopes:

• Slope 1 → normal faults
• Slope 2 → heavy faults

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Applications

Transformer protection
Generator protection
Busbar protection

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Advantages

Highly selective
Fast operation

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Disadvantages

Expensive

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3. DIRECTIONAL PROTECTION

Principle

Operates only for faults in specific direction.

Uses:

• Current magnitude
• Phase angle between voltage and current

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Torque Equation (VERY IMPORTANT FOR GATE)

$$T = K_1 VI \cos(\theta - \phi) - K_2 V^2$$

Where:

• θ = angle between V and I
• φ = relay characteristic angle

Relay operates if torque > 0

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Working Principle

Forward fault → relay operates
Reverse fault → relay does not operate

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Applications

Parallel feeders
Ring systems

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Advantages

Provides direction discrimination

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4. DISTANCE PROTECTION (MOST IMPORTANT FOR GATE)

Principle

Measures impedance between relay location and fault.

$$Z = \frac{V}{I}$$

Since impedance ∝ distance,

Relay operates when:

$$Z_{measured} < Z_{set}$$

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Operating Condition

$$\frac{V}{I} < Z_{reach}$$

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Types of Distance Relays

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1. Impedance Relay

Characteristic:

Circle centered at origin

Equation:

$$Z < Z_{set}$$

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2. Reactance Relay

Measures reactance only.

Equation:

$$X < X_{set}$$

Unaffected by fault resistance.

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3. Mho Relay (MOST IMPORTANT)

Directional relay.

Characteristic:

Circle passing through origin.

Equation:

$$\frac{V}{I} \cos(\theta - \phi) < constant$$

Best for long transmission lines.

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DISTANCE PROTECTION ZONES (VERY IMPORTANT)

Three zones used:

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Zone 1

Protects:

80–90% of line

No time delay

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Zone 2

Protects:

Remaining line + next line part

Time delay:

0.3–0.5 sec

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Zone 3

Backup protection

Time delay:

1 sec

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COMPARISON TABLE (VERY IMPORTANT FOR GATE)

Protection	Principle	Measures
Overcurrent	Current magnitude	Current
Differential	Current difference	Current difference
Directional	Direction of fault	Angle between V and I
Distance	Impedance measurement	V/I

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ADVANTAGES OF DISTANCE PROTECTION

Fast
Selective
Reliable

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GATE IMPORTANT TORQUE EQUATIONS SUMMARY

Overcurrent relay:

$$T \propto I^2$$

Directional relay:

$$T \propto VI \cos(\theta - \phi)$$

Distance relay:

$$Z = \frac{V}{I}$$

Differential relay:

$$I_{diff} = I_1 - I_2$$

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GATE IMPORTANT CHARACTERISTICS SUMMARY

Overcurrent → current based

Differential → current difference based

Directional → angle based

Distance → impedance based

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ONE-PAGE REVISION SHEET

Overcurrent:

Operates when current exceeds limit

Differential:

Operates when current difference exists

Directional:

Operates only in one direction

Distance:

Operates when impedance less than set value