In short. Voltage and frequency relays trip or alarm when supply leaves a safe band. They protect equipment from damage, detect loss of mains and stop a generator from running into abnormal conditions. Settings must allow for normal transient dips and recoveries.
Protection elements
| Element | Protects against |
|---|---|
| Undervoltage (27) | Motor stalling, contactor dropout, utility loss |
| Overvoltage (59) | Insulation stress, AVR faults, regulation loss |
| Underfrequency (81U) | Overload, governor problems, utility instability |
| Overfrequency (81O) | Loss of load, governor faults, overspeed |
| Volts per hertz (24) | Overexcitation at low speed or high voltage |
Setting logic
- Set bands wide enough to ride through starting and load-step transients, and tight enough to protect.
- Use time delays that grade with other protection and motor ride-through capability.
- Use more than one stage: alarm, then delayed trip, then faster trip for severe deviations.
- Different settings may apply in grid-parallel and island modes.
Loss-of-mains and interconnection
For a generator that parallels with the utility, voltage and frequency relays, with rate-of-change-of-frequency or vector-shift elements, detect loss of mains and disconnect to prevent unplanned islanding. Utility requirements specify settings. See utility plus DG synchronisation.
Interaction with generator controls
The generator controller has its own voltage and speed limits. The protection relay should be independent and set so that the controller can act first for ordinary deviations, with the relay as backup.
Steps
- List equipment limits for voltage and frequency.
- Study expected transients. See voltage dip and frequency dip.
- Set stages and delays.
- Test with injection and verify trip logic.
Illustrative example
A generator feeding motor loads has an undervoltage alarm at a modest dip with a delay, and a trip at a deeper dip with a longer delay, so that a motor start does not cause a trip. An overfrequency trip is set above the governor’s overshoot after a load rejection, and an underfrequency stage sheds load before tripping the generator.
Why voltage and frequency protection exist
Abnormal voltage or frequency damages equipment, upsets loads and can signal a failing source. Under-voltage causes motors to draw excess current and stall. Over-voltage stresses insulation. Frequency departures stress turbines and engines, change motor speed and show that supply and demand are out of balance.
Typical functions and uses
| Function | Use |
|---|---|
| Undervoltage (27) | Mains failure detection for AMF; motor protection; generator alarm |
| Overvoltage (59) | Protection against AVR failure and surges |
| Underfrequency (81U) | Detect overloaded or slowing engine, load shedding trigger |
| Overfrequency (81O) | Detect overspeed or governor failure |
Setting principles
- Pick thresholds based on equipment tolerance, not nominal values only.
- Add time delays to ride through disturbances that clear on their own.
- Use different settings for utility and generator sources if their normal ranges differ.
- Coordinate with downstream devices that have their own undervoltage release.
Interaction with ATS logic
The mains-failure detection settings decide when the generator starts. Too sensitive: nuisance starts on brief dips. Too slow: long outages for sensitive loads. Return-to-mains settings should confirm stable supply before retransfer. See ATS logic.
Frequency and load shedding
An underfrequency signal can trigger staged load shedding to rescue the generator from overload. See load shedding and frequency dip.
Testing
Inject voltage or frequency steps from a test set and check pickup, time delay and output action. Record results.
Common mistakes
- Delays shorter than the motor starting dip duration.
- Same settings in all operating modes.
- No protection independent of the controller.
What to send us
Send the equipment limits and operating modes. Send your application details.
Questions engineers ask
Why does the relay trip during motor start?
The voltage dip may exceed its pickup for longer than its delay. Adjust the delay or reduce the dip.
What is volts-per-hertz protection?
Protection against overexcitation when voltage is high relative to frequency.
Is loss-of-mains protection mandatory?
For parallel operation with the utility, usually yes, as set by the utility.
Can one relay cover all of these?
Modern multifunction relays include most elements, with independent settings.
This guide gives general principles. Settings must follow equipment and utility requirements.