Calculator Inputs
Example Data Table
| Generator Type | Rating | PF | Pickup | CT | PT | Primary Pickup | Relay Watts |
|---|---|---|---|---|---|---|---|
| Diesel standby | 1000 kVA | 0.80 | 8% | 1200/5 | 11000/110 | 64 kW | 2.667 W |
| Steam turbine | 50000 kVA | 0.85 | 2% | 3000/1 | 11000/110 | 850 kW | 2.833 W |
| Gas unit | 20000 kVA | 0.80 | 4% | 1500/1 | 6600/110 | 640 kW | 7.111 W |
Formula Used
Rated kW: Rated kW = Generator kVA × Power Factor
Full Load Current: I = kVA × 1000 ÷ (√3 × Line Voltage)
Reverse Pickup kW: Pickup kW = Rated kW × Pickup % ÷ 100
CT Ratio: CT Ratio = CT Primary ÷ CT Secondary
PT Ratio: PT Ratio = PT Primary ÷ PT Secondary
Relay Secondary Watts: Secondary W = Primary Pickup W ÷ (CT Ratio × PT Ratio)
Pickup Current: I = Pickup W ÷ (√3 × Line Voltage × Power Factor)
Motoring Margin: Margin = Estimated Motoring kW − Pickup kW
How to Use This Calculator
- Enter the generator rated kVA and rated power factor.
- Enter the generator line voltage in volts.
- Add CT primary, CT secondary, PT primary, and PT secondary values.
- Enter the desired reverse power pickup percentage.
- Add the estimated motoring power percentage from plant data.
- Enter the required safety margin and relay time delay.
- Use a negative present kW value when checking reverse flow.
- Press calculate and review pickup, relay watts, margin, and status.
Generator Reverse Power Relay Setting Guide
A reverse power relay protects a generator from motoring. Motoring happens when the prime mover stops giving torque. The generator then starts taking power from the bus. It behaves like a motor and drives the engine or turbine. This can overheat parts, damage couplings, waste fuel, and disturb the plant.
Why the setting matters
The pickup should be low enough to detect real reverse power. It should also be high enough to ignore small power swings. A very low value may trip during synchronization, load rejection, or governor hunting. A very high value may miss dangerous motoring. The best value depends on generator rating, prime mover type, CT ratio, PT ratio, and relay design.
Practical protection approach
This calculator starts with the generator rated apparent power and power factor. It converts them into rated kilowatts. Then it applies the selected reverse power percentage. The result is the primary pickup in kilowatts. That value is also converted into secondary relay watts by using the CT and PT ratios. This helps compare the setting with relay test kit values.
Relay timing and margin
Time delay is important. A delay prevents unwanted trips during short transients. Diesel engines often need a higher pickup than steam turbines. Steam turbines can be damaged by motoring with a lower reverse power level. Gas turbines and hydro units may use different plant rules. Always compare the calculated pickup with manufacturer data and site standards.
Field checking
During commissioning, inject voltage and current at the correct phase angle. Confirm that the relay sees reverse watts, not forward watts. Check wiring polarity before relying on the setting. CT polarity, PT polarity, and phase sequence can change the measured direction. Record pickup, dropout, timing, and final values. Keep the record with the protection study.
Using the result
Use the output as an engineering worksheet. The relay setting is not a replacement for a protection coordination study. Review reverse power limits from the prime mover vendor. Check grid code needs where they apply. Confirm that the breaker trip path is healthy. Test alarm and trip contacts separately. Use conservative judgment when the plant history shows unstable loading. Document every change before returning the unit online.
FAQs
What is reverse power in a generator?
Reverse power occurs when a generator absorbs active power from the system instead of supplying it. The generator then acts like a motor and can drive the prime mover.
Why is reverse power protection needed?
It protects engines, turbines, couplings, and plant equipment from motoring damage. It also prevents unnecessary mechanical stress and unsafe operating conditions.
How is pickup percentage selected?
The pickup percentage is usually based on prime mover motoring power, generator rating, plant standards, and manufacturer recommendations. It should stay below the damaging motoring level.
Why do CT and PT ratios matter?
The relay measures secondary current and voltage. CT and PT ratios convert primary reverse power into secondary relay watts for setting and testing.
What does relay secondary watts mean?
Relay secondary watts show the measured reverse power at the relay terminals. This value is useful during relay injection testing and commissioning.
Should the relay trip instantly?
No. A time delay is commonly used to ride through brief power swings, synchronization errors, and short governor transients.
What does negative present kW mean?
A negative present kW value means power is flowing into the generator. The calculator compares that reverse power with the selected pickup value.
Can this replace a protection study?
No. It is a calculation aid. Final settings should be reviewed with relay manuals, generator data, prime mover limits, and site protection standards.