Virtual Lab — Reverse Power Relay for Generator Protection

1. Experiment objective

START HERE

What are we trying to understand?

A generator normally converts mechanical energy into electrical energy and supplies real power to the grid. If the mechanical input to the generator is lost while the generator remains connected to the grid, the direction of real power can reverse.

GENERATOR → GRID
Normal generating condition

→

GRID → GENERATOR
Reverse power / motoring

The purpose of the reverse-power relay is to detect this condition and disconnect the generator before prolonged motoring causes undesirable operation of the generating unit.

Learning idea: Do not think of relay 32 as simply a current relay. It is a directional real-power protection function.

2. Principle of reverse-power protection

THEORY

For a balanced three-phase system, real power can be represented by:

P_{3φ} = \sqrt{3} V_{L} I_{L} \cos φ

The important quantity for this experiment is not only the magnitude of power, but also its direction.

Power	Direction	Generator condition	Relay interpretation
+P	Generator → Grid	Generating	Normal
0	No real-power transfer	Transition	No reverse-power operation
−P	Grid → Generator	Motoring	Reverse-power region

P_{actual} \leq P_{pickup} \Rightarrow Relay timer starts

t_{operating} \geq t_{set} \Rightarrow Trip

3. What happens physically?

CONCEPT MAP

Generator
supplies

Mechanical
input falls

P approaches
zero

Power
reverses

Relay
times

Breaker
trips

Normal: The prime mover provides enough mechanical input for the generator to export real power.

Transition: As mechanical input is reduced, electrical output approaches zero.

Motoring: If the grid begins supplying real power to the machine, the generator is absorbing power. Relay 32 detects the reverse direction.

Why use a time delay?
Temporary disturbances and transient power reversals should not necessarily cause an immediate generator trip. The relay can therefore be coordinated with an intentional operating delay.

4. Generator protection single-line diagram

PUBLICATION-STYLE SVG

5. Live relay indication

LIVE

NORMAL — GENERATOR EXPORTING

Real power

+2.50 MW

Pickup

−0.25 MW

Timer

0.00 s

Power direction: Generator → Grid

The generator is supplying real power to the grid. Reverse-power protection is not required to operate.

6. Guided demonstration — from generating to motoring

WATCH THE SEQUENCE

The demonstration automatically reduces the simulated mechanical input. Watch the power flow change from export to import and observe when relay 32 starts timing.

7. Virtual relay test set

EXPERIMENT

Line voltage 11.0 kV

Line current 131.2 A

Power factor 0.99

Power direction EXPORT

Reverse pickup 5.0 %

Time delay 1.0 s

8. Live engineering calculation

MATHML

P_{3φ} = \sqrt{3} V_{L} I_{L} \cos φ

P = \sqrt{3} \times 11.0 \times 131.2 \times 0.99 \approx 2.47 MW

P_{pickup} = \frac{R_{pickup}}{100} \times P_{rated}

P_{pickup} = 0.05 \times 5.0 = 0.25 MW

9. Reverse-power operating characteristic

GRAPH

Stable region Reverse-power region Present operating point

10. Power versus time

OSCILLOGRAPH

The red dashed line represents the reverse-power pickup threshold. A trip occurs only when the reverse-power condition remains beyond pickup for the selected time delay.

11. Directional power visualization

SVG

12. Observation table

RECORD

Parameter	Observed value	Meaning
Line voltage	11.0 kV	Generator terminal voltage
Line current	131.2 A	Measured current
Power factor	0.99	Phase relationship
Real power	+2.47 MW	Direction of real-power flow
Pickup setting	−0.25 MW	Reverse-power threshold
Relay state	STABLE	Present protection condition
Operating time	0.00 s	Relay timing

13. Think while performing the experiment

GUIDED LEARNING

Observe 1 — Direction Watch the arrow when power is positive and negative. Ask yourself: Who is supplying whom?

Observe 2 — Zero power Notice what happens as real power approaches zero. This is the transition between generating and motoring.

Observe 3 — Pickup Compare the actual negative power with the pickup value. The relay does not operate merely because power is negative.

Observe 4 — Time delay Once pickup is crossed, watch the timer. The trip requires the condition to persist for the set delay.

Important distinction:
Negative power means power is flowing toward the generator.
Negative power beyond the pickup setting for the required time causes the simulated relay to trip.

14. What should you understand after this experiment?

A reverse-power relay protects a generator against sustained reverse real-power flow. The relay determines the direction of real power, compares the measured reverse power with its pickup setting, applies the specified time delay, and then issues a trip command when the operating condition is satisfied.

Generator \to Grid : P > 0

Grid \to Generator : P < 0

P \leq P_{pickup} + time \geq t_{set} \Rightarrow TRIP