A synchronous generator can fail in the stator, the rotor, the excitation, or through system and prime mover conditions. Each has its protection function: differential for phase faults, stator and rotor ground protection for ground faults, and loss of field, reverse power, negative sequence and overexcitation for abnormal conditions. A trip is analyzed with the record and with the trip mode each function activates.
Short answer
Synchronous generator faults fall into four families. Those of the stator (phase-to-phase, to ground, turn-to-turn), those of the rotor and excitation (field ground, loss of field, overexcitation), those of the prime mover (motoring) and the external ones, which the generator suffers from the grid (unbalance, nearby fault, loss of synchronism). Each family has its protection. This guide relates them and explains how to analyze a trip; the complete scheme is in generator protection: zones, backup and trips. See also generator reverse power (32) trip.
What fails and what detects it
| Fault or condition | What happens | Function that detects it |
|---|---|---|
| Phase-to-phase fault in the stator | Very high current; damage to the iron and windings | 87G differential, generator differential protection |
| Ground fault in the stator | Depends on how the neutral is grounded; with high impedance the current is small | Neutral overvoltage 59N and 100 % stator ground (64S) |
| Turn-to-turn fault in the same phase | Phase input and output currents remain equal; the ordinary differential does not see it | Split-phase differential or other functions, depending on the design |
| Rotor ground | A first ground does no damage; the second does | Rotor ground fault (64F) |
| Loss of field | The machine draws reactive power from the grid and may lose synchronism | Loss of excitation (40) |
| Motoring | The machine takes active power and spins the turbine | Reverse power (32) |
| Unbalanced currents | Rotor heating from double-frequency currents | Negative sequence (46) |
| Excess flux (high V/Hz) | Iron saturation and heating | Overexcitation (24) |
| Loss of synchronism | The angle between the machine and the grid rotates uncontrolled | Out-of-step (78) |
| Thermal overload | Heating from sustained load | Thermal overload (49) |
This table does not include the prime mover protections (turbine, combustion engine, boiler), which have their own systems, nor those of the unit as a whole when the generator shares a step-up transformer.
Which function covers which fault is summarized in Generator protection: which function covers which fault.
What changes between an internal fault and an abnormal condition
The difference matters because it defines how urgent things are and what must be reviewed:
- Internal stator fault: trips immediately and completely, with the unit breaker, the field and the prime mover. Damage to the iron may require dismantling the machine. Before re-energizing, the insulation must be tested and the core inspected.
- Abnormal condition (reverse power, negative sequence, V/Hz, loss of field): generally there is no immediate damage, but an effect that accumulates over time. The trip is delayed to allow the condition to be corrected, and the analysis looks for why it happened.
- External condition (nearby fault, loss of synchronism): the generator is the victim. The protection that tripped may have acted correctly, and the question is whether the grid should have cleared the fault sooner.
- Was there fault current in the stator?
- Yes, phase-to-phase: 87G differential; test insulation and inspect the core before re-energizing
- Yes, to ground: Which function tripped?
- 59N or 64S: Stator ground fault; test insulation
- None of the stator functions: Review the measurement and the trip logic
- No: Which abnormal condition did the relay record?
- Reverse power: Check the prime mover and the speed governor
- Loss of field: Check the excitation and the voltage regulator
- Negative sequence or overexcitation: Check the grid and the excitation
How a trip is analyzed
- Which function tripped and which trip mode it activated. The unit’s trip matrix says whether it opened the breaker, the field and the prime mover, or only the breaker.
- What the records say. Terminal and neutral currents and voltages, and the quantities specific to each function: impedance seen by the 40, power for the 32, V/Hz for the 24. See how to read an oscillography record.
- What the prime mover was doing. If the turbine or combustion engine lost power before the electrical trip, the cause is mechanical.
- What the excitation was doing. The regulator and the field explain loss of field and overexcitation; see excitation and voltage regulator.
- What the grid was doing. An external fault or an out-of-synchronism closing shows in the voltage and the angle.
- If needed, tests. Insulation resistance of the stator and rotor, and review of the measurement connections.
To read the event record, see Fault analysis from oscillography records; the case of a generator set is in Generator set protection. To open the file you can use the free COMTRADE viewer.
Common mistakes
- Trips caused by settings: a delay that is too short on reverse power or negative sequence trips the generator on grid transients that do not damage it.
- Trips caused by measurement: a CT with reversed polarity or a voltage transformer with an open fuse makes the 40 or the 32 see false quantities; see VT and CT supervision.
- Wrong trip mode: a system fault that causes a full trip leaves the unit out longer than necessary.
- No synchronism check: closing out of synchronism produces a torque shock that does not always trip a protection but damages the shaft; see synchronism check 25.
What not to do
- Do not re-energize the generator after a differential or stator ground trip without testing the insulation.
- Do not lengthen the delays of abnormal-condition functions to avoid trips; first find what causes them.
- Do not erase the records of the relays or the excitation before copying them.
When to call us
If your generator tripped and the cause is not clear, send us the records of the unit relays, the trip matrix and the settings. We quote the trip analysis and deliver the sequence and the cause. We also review and correct the settings with the remote relay configuration and verify the scheme with secondary injection.
Related guides
- Generator protection: zones, backup and trips
- Synchronous generator and its protections
- Fault analysis from oscillography records
- Generator protection: which function covers which fault
- Generator set protection
- Generator Reverse Power (32) Trip: Causes and Fixes
References
- IEEE C37.102, Guide for AC Generator Protection.
- IEC/IEEE 60255-24:2013 (IEEE C37.111-2013), COMTRADE.
Frequently asked questions
Why does a generator trip on reverse power if there was no electrical fault?
Because reverse power does not detect an electrical fault but the loss of mechanical power: the machine stops delivering and starts taking power from the grid, spinning the turbine as a motor. It happens through closing of the turbine admission, loss of fuel or a prime mover trip. The speed governor and the fuel or water supply are checked first, not the generator.
Does a first rotor ground fault require stopping the generator?
The field circuit is ungrounded, so a first ground produces no current and no immediate damage. The danger is the second ground, which shorts part of the winding and unbalances the flux. That is why the protection usually alarms on the first and trips, or trips after a delay, on the second; the exact policy is set by the owner.
Which records must be kept after a generator trip?
The events and oscillography records of all the generator relays, the position of the unit and field breakers, the excitation voltages and currents, and the speed and mechanical power of the prime mover. Without them an electrical fault cannot be separated from a prime mover or grid condition.