2. Basics of SFC Operation
February 7, 2023 Footer text 2
SFC outputs power to the generator stator to create a
magnetic field in the stator.
The exciter magnetizes the generator rotor.
The SFC creates a rotating magnetic
field in the stator to pull the rotor.
Stator magnetic field is adjustable and
determines the speed of the rotor.
3. What is a Static Frequency Converter
February 7, 2023 Footer text 3
• A frequency converter (frequency changer) is an electronic or
electromechanical device that converts alternating current (AC) of
one frequency to alternating current of another frequency.
• The device may also change the voltage, but if it does, that is
incidental to its principal purpose.
• Traditionally, these devices were electromechanical machines called
a motor-generator set (rotary frequency converter).
• With the advent of solid state electronics, it has become possible to
build completely electronic frequency converter.
4. How does a Frequency Converter work
• A frequency converter usually consists of a rectifier stage
(producing direct current) which is then inverted to produce AC of the
desired frequency.
• The inverter may use thyristors, IGCTs or IGBTs.
• If voltage conversion is desired, a transformer will usually be included in
either the ac input or output circuitry and this transformer may also
provide galvanic isolation between the input and output ac circuits.
February 7, 2023 Footer text 4
5. What is the purpose of SFC in a Gas Turbine
• A gas turbine compressor does not supply enough air for the machine to
be self-sustaining until it reaches approximately 60 per cent of its rated
speed.
• Therefore the gas turbine must be accelerated to the speed at which it
can produce enough power itself to continue acceleration.
• Gas Turbine need starting equipment to accelerate from standstill and
follow a preset speed-time profile as required by the ignition and start-up
procedure.
• Once the Gas Turbine can deliver enough torque, further acceleration up
to the synchronous speed is accomplished by the turbine itself and the
starting equipment is no more necessary.
• When it comes to larger machines, it is common to use SFC, using the
synchronous machine as a motor.
February 7, 2023 Footer text 5
6. Single Line Diagram for SFC starting of GT Generator
February 7, 2023 Footer text 6
7. Operation of an SFC Inverter
February 7, 2023 Footer text 7
The output of the SFC uses SCR’s
to switch AC current flow to the
Generator Stator windings.
An analogy of a SCR is a switch.
The switch is either open or closed,
same as the SCR being ON, switch
closed or OFF, switch open. The
switches close in the sequence
shown. At anytime there are 2
switches closed.
SFC converter with switches
8. Operation of an SFC Inverter
February 7, 2023 Footer text 8
Switches 3 & 4 closed
1) In this view switches 4 and 3 are closed.
AC current flow is into phase A winding
and out of phase C winding. The current
creates a magnet in the machine stator
iron. The poles of the magnet are shown
with the N and S indicating North and
South pole
2) Magnet created in the Stator. As the
switches closing sequence continues the
magnet will begin to rotate
9. Operation of an SFC Inverter
February 7, 2023 Footer text 9
Switches 4 & 5 closeds Switches 5 & 6 closed Switches 6 & 1 closed
10. SFC Starting Sequence
February 7, 2023 Footer text 10
1. Start Command given from MARK VIe.
2. After approximately 25 s delay, Ventilators of reactor and
thyristors starts. (1s difference between these operations). There
are 3 nos of ventilators in all, third one is for transformer cabinet.
3. 6.6KV MV breaker gets closed and SFC transformer (3.2MVA
6.6KV/1.5KV) is charged.
4. Internal disconnector switch 89SFC1 (GT-1) / 89SFC2 (GT-2) is
closed .
5. External disconnector switch 89SS1 (GT-1) / 89SS2 (GT-2) is
closed .
6. Internal 1SKV (Q570) breaker is closed.
7. Rotor brush lift off device position is detected and thyristor
pulses given. Excitation starts simultaneously.
8. An excitation current is given, the feedback of which is received
at MARK VIe (250 A DC initial set point).
9. Excitation transformer (rating 415V/ 80V) supplies the excitation
voltage which is converted to 60V DC in Excitation panel and fed
to generator rotor through brushes.
10. Simultaneously, current is injected in stator of the synchronous
machine and it starts to rotate as a synchronous motor.
