⌂ Home

Virtual Laboratory: Differential Protection of Generator

Static Differential Relay — Interactive Demonstration

1. Aim of the Experiment

To perform the differential protection of a three-phase generator using a static differential relay, and to study the behavior of the relay under normal load, external fault, internal fault and CT saturation conditions.

Core idea: The relay continuously compares the compensated currents at the two ends of the protected generator. Under healthy conditions and external faults the currents balance. For an internal fault, a substantial differential current appears and the relay issues a trip command.
Id = | I1 − I2 |

2. Virtual Laboratory Control Panel

RELAY STABLE — PROTECTED ZONE HEALTHY

3. Live Protection Quantities

CT Ratio 200 : 1
I₁ secondary 4.000 A
I₂ secondary 4.000 A
Differential Iᵈ 0.000 A
Percentage differential 0.00 %
Relay decision STABLE

4. Animated Generator Differential Protection System

PROTECTED GENERATOR ZONE Differential protection boundary Generator terminal side A B C CT-A CT-B CT-C G 3-Phase Generator Grid / Load side CT-A′ CT-B′ CT-C′ Circuit Breaker STATIC DIFFERENTIAL RELAY Comparator + Rectifier + Trip Circuit STABLE INTERNAL FAULT Differential current rises EXTERNAL FAULT Through-current; relay remains restrained CT SATURATION Secondary current becomes distorted
Primary current CT secondary circuit Relay stable Relay trip / fault CT saturation

5. How Differential Protection Works

Two sets of current transformers are installed at the boundaries of the generator. The secondary currents are connected so that they oppose each other under normal operating conditions.

Id = | I1 − I2 |
Isec = Ipri × ICT,sec ICT,pri

When the differential current exceeds the pickup threshold, the static relay energizes its trip circuit and opens the generator circuit breaker.

6. Live Calculation

CT ratio = ICT,pri ICT,sec = 200
I1 = 4.000 A
I2 = 4.000 A
Id = | 4.000 − 4.000 | = 0.000 A
% differential = Id I1 + I2 2 × 100 = 0.00 %

7. Phasor Representation

Current Phasor Diagram

In the healthy condition, the CT secondary currents are equal and oppositely directed, giving a small resultant differential current. During an internal fault, the balance is destroyed.

8. Relay Operating Characteristic

The dashed vertical line represents the relay pickup current. The moving point represents the present operating condition.

9. Three-Phase / Differential Current Waveforms

CT-1 current CT-2 current Differential current

10. Protection Sequence Demonstration

STEP 1 — NORMAL
Generator supplies load. CT currents are balanced. Differential current remains below pickup.
STEP 2 — FAULT
An internal fault creates an imbalance between currents entering and leaving the protected zone.
STEP 3 — TRIP
The static differential relay operates and sends a trip command to the circuit breaker.

11. Observation Table

Condition I₁ secondary I₂ secondary Iᵈ Pickup % Differential Relay Breaker
Normal Load 4.000 A 4.000 A 0.000 A 0.500 A 0.00 % STABLE CLOSED

12. Experimental Procedure

  1. Set the generator rated current and CT ratio.
  2. Set the static relay pickup current.
  3. Run the simulation under normal load.
  4. Observe the equality of the two CT secondary currents.
  5. Observe that the differential current remains below pickup.
  6. Select External Fault and observe the high through-current and relay restraint.
  7. Select Internal Fault and observe the current imbalance.
  8. Observe the increase in differential current and movement of the relay operating point.
  9. Verify that the relay operates when the differential current exceeds pickup.
  10. Observe the circuit breaker opening after relay operation.
  11. Select CT Saturation to study a non-ideal external-fault condition.

13. Result

The generator differential protection is stable for the present normal-load condition. The differential current is below the relay pickup setting.