In short. A generator needs electrical protection that is independent of its engine controller where size or criticality demands it. The architecture covers which functions are included, which relay hosts them, where CTs and VTs sit, how trip signals reach the breaker and the engine, and how redundancy is provided.
Functions by duty
| Function | Small standby set | Parallel or larger set |
|---|---|---|
| Overcurrent / short circuit | Yes | Yes, voltage-restrained |
| Earth fault | Yes | Yes, plus REF |
| Over/under voltage, frequency | Yes | Yes |
| Reverse power | Not needed | Yes |
| Loss of excitation | Optional | Yes |
| Negative sequence | Optional | Yes |
| Differential | Rare | Yes, for larger sets |
| Synchro-check | Not needed | Yes |
Relay arrangement
- Controller-integrated protection: economical for standby and small prime sets.
- Dedicated multifunction relay: independent of the controller, with disturbance records.
- Redundant relays: for critical or large units, with separate CT cores and DC supplies.
Instrument transformers
CTs at the neutral and terminal ends for differential and REF; VTs on the generator terminals for voltage, frequency and power. Choose classes and burdens for the functions. See CTs and VTs.
Trip paths
An electrical protection trip should open the generator breaker and stop the engine (or run it down on cooling for some trips). A lockout relay or latching logic prevents restart until acknowledged. Trip circuits need monitoring. See DC and trip circuits.
Design steps
- Choose functions from generator size, duty and grid requirements.
- Decide on relay hosting and redundancy.
- Specify CT and VT arrangement.
- Define trip logic and lockout.
- Prepare settings and test plan.
Illustrative example
A 2 MVA set in parallel with the utility gets a dedicated multifunction relay with overcurrent, earth fault, REF, differential, reverse power, loss of excitation, negative sequence, voltage and frequency, and synchro-check. The controller keeps engine protections. Each protection trips the breaker and stops the engine as defined in a trip matrix.
Layers of protection for a generator
| Layer | Typical functions |
|---|---|
| Electrical faults inside the machine | Differential (87G), stator earth fault, restricted earth fault |
| External faults | Overcurrent with voltage restraint or control, backup earth fault |
| Abnormal operating conditions | Reverse power, loss of excitation, negative sequence, over and under voltage, over and under frequency, overload |
| Mechanical and engine | Oil pressure, coolant temperature, overspeed, vibration, fuel and start failure |
Scale the scheme to the machine
- Small sets (tens of kVA): basic engine protections plus overcurrent and voltage.
- Medium sets (hundreds of kVA): add reverse power, earth fault, frequency and unbalance.
- Large or critical sets: full numerical relay package with differential and excitation protection.
Numerical relay advantages
- Many functions in one device.
- Event and disturbance records for analysis.
- Multiple setting groups for utility and generator operation.
- Self-supervision and communications.
Trip logic design
Different faults need different responses: trip the breaker only; trip breaker and shut down with cool-down; trip breaker, shutdown and lock out. Define these explicitly. See interlocking.
Independence
Critical protections should not depend on the same control power, CTs or logic as the function they monitor. Consider duplication for the most important functions.
Documentation
Keep a table showing each function, its setting, time delay, action and the reason for the setting. This table makes reviews and audits efficient.
Common mistakes
- Relying only on the engine controller for electrical protection.
- No trip circuit supervision.
- No trip matrix documented.
What to send us
Send the generator rating, duty and single-line diagram. Send your application details.
Questions engineers ask
Can the controller replace a protection relay?
For small standby sets, often yes. For larger or parallel sets, a dedicated relay is recommended.
What is a trip matrix?
A table showing which protection trips which breaker or engine action.
Do I need redundancy?
For critical applications, yes, to avoid a single relay failure leaving the set unprotected.
What is a lockout?
A latch that prevents closing or restart until the fault is investigated.
This guide gives general principles. Scheme design needs project data.