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					Excitation Systems
The equipment providing Field current
For a synchronous machine , including
all power, regulating, control, and
protective elements is called as
excitation system.




          Excitation System
      Types of Excitation Systems:

   Most popular excitation systems from the older
days to the present modern synchronous machines
are:

 Conventional DC
 AC ( High Frequency) Excitation system
 Brushless Excitation system
 Static Excitation system
         Conventional DC :
• Station DC supply shall be connected to the
  Generator rotor winding through the
  Rheostat. Control of the excitation shall be
  done by varying the rheostat. This scheme
  is applicable for lower rating machines as
  the size of Rheostat and source of external
  power are the limitations.
 AC ( High Frequency) Excitation
             system:
• Commutator and Brush gear assembly can be
  avoided in this scheme. Generator main shaft
  driven AC pilot exciter having rotating permanent
  Magnetic field and a stationery armature feeds the
  DC field current of the main high frequency AC
  exciter through controlled rectifiers. The High
  frequency output of the stationery armature is
  rectified by stationery diodes and fed via the slip-
  rings to the field of main turbo generator.
               S.E.E. IN OFF CONDITION


                                   GEN. TRANS. CB          220 KV
   GEN                                                        BUS
               GEN. PT   GEN. CT


ROTOR                              EXC. TRANS.
                                   2.5 MVA 15.75KV / 575 V
          FB        AVR

                                           THYRISTOR BRIDGES
                                           3 x 1820 Amps


         FF                                415/25 V
                                                             415 V AC
        SEE WITH INITIAL EXCITATION
              ie.. F.F. CIRCUIT


                                   GEN. TRANS. CB          220 KV
   GEN                                                        BUS
               GEN. PT   GEN. CT


ROTOR                              EXC. TRANS.
                                   2.5 MVA 15.75KV / 575 V
          FB        AVR

                                           THYRISTOR BRIDGES
                                           3 x 1820 Amps


         FF                                415/25 V
                                                             415 V AC
 AT 10% OF Gen. KV, THYRISTORS STARTS
             CONDUCTION


                                   GEN. TRANS. CB          220 KV
   GEN                                                        BUS
               GEN. PT   GEN. CT


ROTOR                              EXC. TRANS.
                                   2.5 MVA 15.75KV / 575 V
          FB        AVR

                                           THYRISTOR BRIDGES
                                           3 x 1820 Amps


         FF                                415/25 V
                                                             415 V AC
   SEE WITH SELF EXCITATION- FF OFF
          READY FOR SYNCHRONISATION


                                   GEN. TRANS. CB          220 KV
   GEN                                                        BUS
               GEN. PT   GEN. CT


ROTOR                              EXC. TRANS.
                                   2.5 MVA 15.75KV / 575 V
          FB        AVR

                                           THYRISTOR BRIDGES
                                           3 x 1820 Amps


         FF                                415/25 V
                                                             415 V AC
    SEE WITH SELF EXCITATION AFTER
                     SYNCHRONISATION


                                   GEN. TRANS. CB          220 KV
   GEN                                                        BUS
               GEN. PT   GEN. CT


ROTOR                              EXC. TRANS.
                                   2.5 MVA 15.75KV / 575 V
          FB        AVR

                                           THYRISTOR BRIDGES
                                           3 x 1820 Amps


         FF                                415/25 V
                                                             415 V AC
         IF UNIT TRIPS..
• Whenever Unit Trips, Field Breaker
  Trips on Interlock. The Generator Rotor
  will connect to Field Discharge Resistor
  through Field Breaker Contact, so that
  the Residual energy is dissipated through
  the Field discharge Resistor.
              FIELD BRKR.




                             FIELD DISCHARGE
GEN   ROTOR                       RESISTOR
                            2480 A, 900V, 20 kW
       Brushless Excitation:
• In addition to the above AC Excitation
  system, main exciter is also available along
  with rotating diodes in this system.
  Controlled output from the Pilot exciter is
  given to Main exciter armature and the
  generated AC in the rotor of the main
  exciter is given to the rotor winding through
  rotating diodes.
       Brushless Excitation System
  Stator    Field Wndg.   Rotating Diodes   Main exciter        Pilot Exciter




GENERATOR



                                                  Rectified              3-Ph AC
                                                  DC


                                                                     AVR
                                                           FB




                                                                 PT and CT
                                                                 of Generator
    Static Excitation system :
• No rotating parts are in this system and
  hence the name Static excitation system.
  Power required for the excitation is drawn
  from the machine terminals and controlled
  through the rectifiers and given to the rotor
  winding through the Brush gear and slip-
  rings assembly.
Block Diagram of Excitation System
                                                         Excitation
                                                         Transformer




        AVR
                         PROTECTION


                                                                          Thyristor
                                                                          Bridges

                         MONITORING




                                   PULSE       PULSE
          AVR                   GENERATION   AMPLIFIER




                LOGIC CONTROL




                                                          FIELD SUPPRESSION
  GEN                                                       ( FB, FF & DISCH.
                                                               RESISTOR)




                                                                       FB, FF
                 Block Diagram of Static Excitation System
                    Excitation Transformer
                  2.5 MVA, 15.75 kv / 575 V

                                                                                                                         415 V AC




                                                                    Synch
                                                                    Transformer



                               Usyn


                                                                                                             Thyristor                Diode
                                                                                                              Bridges                 Stack
                                                                                                               3 Nos
                       If
                                      AUTO-1
                       Ig      A/D
        PT-1
                                                                                                                                      Field
                               CON             AVR              GATE              INTERMEDIATE
                                                              CO NTRO L               PULSE                                         Flashing
                       Uf      VER
                                                                                                                                    Breaker
                               TER                                                                 PULSE
                       Usyn
                                                                                        Block    FINAL STG


                       If                               Follow Up
                                                                                        Block
                       Ig             AUTO-1
        PT-2                   A/D                                                                 PULSE
                               CON                              GATE              INTERMEDIATE   FINAL STG
                                               AVR
                               VER                            CO NTRO L               PULSE
                       Uf
                               TER
                       Usyn
                                                                                                   PULSE
                                                                                                 FINAL STG

                       If             MANUAL
                               A/D
                               CON             If Reg           GATE              INTERMEDIATE                               Field
                               VER                            CO NTRO L               PULSE                                 Breaker
                               TER
                       Usyn




GENER
ATOR
               ROTOR
Auto Channel Functions..
• Voltage Regulation
• Follow Up
• Limiter Action
     Stator Current Limiter`
     Rotor Current Limiter
     Load Angle Limiter
     V / Hz Limiter
• Power System Stabilizer
Stator Current Limiter - Inductive

• In the Case of Over Excited Generators,
  this Limiter holds the Stator Current
  within permissible Limits by reducing the
  Field Current accordingly.
Stator Current Limiter - Capacitive


• In the Case of Under Excited Generators,
  this Limiter holds the Stator Current
  within permissible Limits by reducing the
  Field Current accordingly.
Field Current Limiter
• This Limiter protects Generator Rotor from
  Over Currents occurring in Steady State and
  Transient operation.
   High Field Currents are due to :
Ø Sharp drop in Network Voltage
Ø Improper raising the Stator Voltage Set
     point.
Load Angle Limiter
• This Limiter prevents the Generator from
  Slipping phase due to slipping of the Rotor.
• The Load Angle is the Difference between
  in Phase between the Rotor and Stator
  Rotating Field, results mainly from the
  Driving Torque (Active Power – P) acting
  on the Generator and level of the Rotor
  Current.
   If the Driving torque remains constant, an
  Increase in Field Current Reduces Load
  Angle.
V / Hz Limiter
• As V/Hz represents the Flux produced in
  the Transformer, the V/Hz limiter monitors
  the Frequency and changes the Voltage, if
  the Ratio V/Hz exceeding the Limits.
Power System Stabiliser
• The purpose of PSS is to use the Generator
  excitation to damp electromechanical
  oscillations between the Network and the
  Generator.
       Brushless Excitation System
  Stator    Field Wndg.   Rotating Diodes   Main exciter        Pilot Exciter




GENERATOR



                                                  Rectified              3-Ph AC
                                                  DC


                                                                     AVR
                                                           FB




                                                                 PT and CT
                                                                 of Generator
   Comparisons of SEE & B’less
        SEE                          B’less
Very Fast response           Fast response
Slip rings and brushes for   Slip rings and brushes for
rotor are required           rotor are eliminated
Gen. Field Parameters and    Gen. Field Parameters and
Rotor Temp can be            Rotor Temp can not be
monitored                    monitored directly
Limitation upto 600 MVA      No Limitation in
only                         Generator MVA
Higher rating FB             Lower rating FB
Higher Rating Thyristors     Lower Rating Thyristors
No Rotating Parts            Rotating Parts
Four Quadrant Diagram
          X
Quadrant 2:   Quadrant 1:
P : - Ve      P : + Ve
Q : + Ve      Q : + Ve


                            R

Quadrant 3:   Quadrant 4:
P : - Ve      P : + Ve
Q : - Ve      Q : - Ve

				
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posted:9/1/2012
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