- Loss of Excitation Definition: Loss of excitation in a generator occurs when the excitation system fails, causing the generator to run above synchronous speed.
- Induction Generator Mode: Without excitation, the generator becomes an induction generator, which can lead to overheating and overloading issues.
- Undercurrent Relay Protection: An undercurrent relay can protect against loss of field by operating when the excitation current falls below a certain value.
- Timing Relays for Stability: Using timing relays helps stabilise the protection scheme against slip frequency effects and prevent false operations.
- Advanced Protection for Large Generators: For larger generators, advanced schemes with offset mho relays and under voltage relays are used to maintain system stability through load shedding and master tripping relays.
Loss of field or excitation means the field supply has failed. On a large machine that supply is often a separate auxiliary source or a separately driven DC generator. If that auxiliary supply or its driving motor fails, the machine loses field. After loss of excitation it runs above synchronous speed.
The generator or alternator then behaves as an induction generator and draws magnetizing current from the system. The system does not collapse at once. If the machine stays connected, the stator overloads and the rotor heats from slip-frequency current. Restore the field or isolate the machine before that heating lasts.
Two older schemes are used against loss of field. The first is an undercurrent relay in the main field circuit. It operates if excitation current falls below a set value. Older manuals often quote about 8 percent of rated field current. That figure is not a universal IEEE setpoint. If the field circuit stays closed but the exciter fails, slip-frequency current can pick up and drop the relay. Timing and pickup must be set for that chatter.
Some of those same manuals then recommend about 5 percent of normal field current. Use the maker’s figure. The undercurrent relay has a normally-closed contact that stays open while its coil is energised by the shunted excitation current. When the field fails the coil drops out, the contact closes and timing relay T1 is energised.
When T1 picks up, its normally-open contact energises a second timing relay T2. Older schemes give that relay an adjustable pickup of about 2 to 10 seconds. T1 is delayed on drop-off so slip-frequency current does not chatter the scheme. T2 then trips the set or raises an alarm. Its pickup delay rides through an external fault.
Larger machines use an offset-mho (device 40) looking into the generator, with an instantaneous undervoltage element to accelerate the trip if system voltage collapses. A time delay covers a stable swing. Immediate isolation is not always required if voltage holds.
An under voltage relay, often set near 70 percent of rated voltage on older schemes, marks that collapse. If voltage holds, the offset-mho may start a plant load reduction and then a delayed master trip. Those times and the 70 percent figure follow the maker, not a single IEEE number.
