- 2069 Chaitra · 8 marks
Types of excitation systems
Short answer
An excitation system provides regulated DC field current. Types: DC (shaft-driven DC generator, slow, brush maintenance); AC (brushless with rotating diodes, medium response, low maintenance); static (thyristor bridge from generator terminals, very fast, high ceiling voltage but needs slip rings).
What is excitation system? Explain the various types of excitation system employed in power plant on the basis of their performance with suitable connection diagram.
An excitation system supplies the DC field current of a synchronous generator and controls it to hold the terminal voltage and the reactive power output at the desired values.
On the basis of their power source and performance, excitation systems are of three types.
1. DC excitation system
A shaft-driven DC generator (main exciter), often with a pilot exciter, supplies the field through slip rings.
Pilot exc --> Main DC exc ==slip rings==> Gen field
^ |
+------ AVR <---- PT <-----+
- Slow response (exciter time constant 0.5 to 1 s); commutator and brushes need maintenance.
- Used in old and small units.
2. AC excitation system
An AC exciter (alternator) is on the same shaft and its output is rectified.
(a) Stationary rectifier: rectifiers are static, output reaches the field through slip rings.
(b) Rotating rectifier (brushless): the exciter has stationary field and rotating armature; diodes mounted on the shaft feed the field directly, so no brushes or slip rings.
Stationary | Rotating parts on shaft |
AVR -> exc | exciter rotating gen field |
field ======>| armature -> diodes --> winding |
(PMG supply) | (3-ph AC) (DC) |
- Medium response; brushless type is almost maintenance-free and suits large turbo-generators and hazardous areas.
- Field current cannot be measured or de-excited directly.
3. Static excitation system
Power is taken from the generator terminals through an excitation transformer and rectified by a thyristor bridge; DC goes to the field through slip rings. The AVR controls the firing angle.
Gen bus -> Exc. transformer -> Thyristor bridge
| (AVR firing)
v
slip rings -> Gen field
- Very fast response and high ceiling voltage; best for transient stability.
- Needs field flashing at start; supply falls during close-in faults.
Comparison of performance
| Feature | DC | AC (brushless) | Static |
|---|---|---|---|
| Response speed | Slow | Medium | Very fast |
| Brushes/slip rings | Yes, plus commutator | None | Slip rings |
| Maintenance | High | Low | Low |
| De-excitation | Slow | Slow | Fast (inversion) |
| Typical use | Old, small units | Large turbo units | Modern hydro and thermal |
✓ Answer checked by Claude Opus 5.5 (high effort)
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