Brushless excitation pros and cons

A brushless excitation system supplies the synchronous generator field from a shaft-mounted AC exciter whose output is rectified by a rotating diode bridge, so no brushes or slip rings are needed between the exciter and the main field.

Working

1. A small permanent magnet generator (PMG) or station supply feeds the AVR.
2. The AVR controls a thyristor bridge that feeds the stationary field of the AC exciter.
3. The AC exciter has a rotating armature on the main shaft. Its 3-phase output goes to a rotating diode rectifier mounted on the same shaft.
4. The rectified DC goes directly to the main generator field winding.

 PMG --> AVR --> Exciter field (stator)
                       |
   ===== shaft ========|=====================
   Exciter armature -> Rotating diodes -> Main field
   (rotating)          (rotating)        (rotor)
                                            |
                         Main stator --> Gen terminals

Best alternative

The best alternative is the static (thyristor) excitation system, where the field is fed from the generator terminals through an excitation transformer and thyristor bridge, using slip rings.

Pros of brushless excitation (compared with static)

• No brushes or slip rings: low maintenance, no carbon dust, no brush sparking; good for remote, unmanned plants and hazardous areas.
• Excitation power is taken from the shaft, so it is independent of generator terminal voltage; it can supply field current during close-in faults (good for fault current support and relay operation).
• Self-contained with a PMG: no need for an external supply or field flashing at start.
• Fewer external power components (no large excitation transformer or high-current thyristor cubicle).

Cons of brushless excitation (compared with static)

• Slow response: the exciter field time constant (about 0.5–1 s) adds delay, so transient stability support is poorer than static excitation (response in tens of ms).
• No fast field de-excitation: the main field cannot be forced negative or shorted quickly; field suppression after a fault is slow because the decay depends on the rotor time constant.
• Rotating diodes and fuses cannot be inspected while running; diode failure needs special monitoring.
• Longer shaft and extra rotating mass; higher cost for very large machines.
• Rotor field current and voltage cannot be measured directly.