GAS TURBINE - STARTING SYSTEM

1-STARTING SYSTEM

•  GT starting system used here is Static Frequency Converter (SFC).
•   When all permissive of GTG are met and auxiliary checks are satisfied, GTG TCS sends to DCS a signal of “READY TO START”. On obtaining the signal, DCS sends GT start signal to GT TCS. The gas turbine is started up by turning the generator via the starting frequency converter (SFC).
• The starting frequency converter also turns the turbine via the generator for compressor cleaning and for heat-recovery steam-generator purging in combined-cycle operation.
• The power section of the SFC consists of the following components:

                   −Supply via the SFC transformer

                   −Two fully controlled three-phase thyristor bridges, line-side converter and the generator side converter

                   −Current DC link reactor decouples line- and generator side converters 

                   −Radial fan for heat dissipation

2-STARTING FREQUENCY CONVERTER (SFC)

•    Power Supply                                                                                                                                           −The SFC is fed via the SFC transformer from the medium-voltage supply of the plant auxiliary power supply system i.e. BSDG (Black Start Diesel Generator).

•   Perating Aspects                                                                                                                                         −A SFC is used to drive the generator in synchronous motor operation to accelerate the gas turbine train. Synchronous motor operation requires static excitation equipment (SEE) for field control over the complete speed range. During SFC operation the SEE controls the excitation current according to the SFC requirements.

            −The SEE, which operates only one generator, is also used for signal interchange between SFC and control system.

  −SFC operation is controlled fully automatic by the control system sub group control, except of the compressor cleaning mode.

          −The SFC is designed for four gas turbine starts in sequence. After this, a cooling down time of at least two hours must be observed.

      

3-SFC OPERATING MODES

•   SFC Standard Operating Modes are:

         −Unit start: SFC provides rated output power to start up the gas turbine train.   Gas turbine  will be fired in low speed range and SFC supports the   acceleration above the self sustaining speed of gas turbine and   switches off at 70% of nominal speed.

     −Compressor Cleaning: The turbine-generator is turned to clean the gas turbine compressor   blades. Depending on turbine type constant or various speeds can   be provided in low speed range. The gas turbine remains unfired   during this operation. This mode is operated manually from the   control system. Turning in this mode is at a various speed between   approximately 23 to 27% of rated speed.

      −HRSG Purging (Option): On boiler request the purging mode will be executed prior gas turbine start. Ignitable gases, which might be injected during the last   gas turbine operation, are flushed out of the heat-recovery steam   generator (HRSG) of combined-cycle units. Depending on turbine   type, constant or various speeds can be provided in low speed   range. The gas turbine remains un-fired during this operation.

CHANGE OVER FUNCTION

•   A change over function can be supplied as an add-on option that allows the start-up multiple gas turbines with one SFC. Start up availability can be improved when more than one SFC is used to start all gas turbines in a SFC starting bus.
•   The changeover function requires beside the starting bus power connection among the units a data link.