Working of brushless and static excitation

Both are modern excitation systems for synchronous generators. They differ in where the rectifier is and how DC reaches the field.

Brushless excitation system

• A PMG (pilot exciter) on the shaft supplies the AVR.
• The AVR feeds DC to the stationary field of the main AC exciter.
• The AC exciter armature rotates with the shaft and produces 3-phase AC.
• A rotating diode bridge on the shaft rectifies it and feeds the main field directly. No brushes are needed.
• The AVR controls the exciter field, which in turn controls the main field.

 PMG ---> AVR ---> Exciter field (stationary)
                         :  (air gap)
 [ROTOR] Exciter armature -> Diode bridge -> Main field
                                                :
 Main stator ---> Generator terminals ---> PT/CT to AVR

Static excitation system

• An excitation transformer fed from the generator terminals supplies a thyristor bridge.
• The AVR compares the terminal voltage with the set value and changes the firing angle of the thyristors.
• The DC output goes to the rotor through a field breaker and slip rings/brushes.
• At start, field flashing from the battery gives the initial field until the voltage builds up.

 Gen terminals -> Exc. transformer -> Thyristor bridge
                                          |  (alpha)
                       AVR <- PT, CT      |
                                          v
 Battery -> Field flashing -> Field CB -> Slip rings
                                              |
                                          Main field