Hi everybody,

Could someone possibly explain to me how does the system which uses a SFC instead of using a gearbox to start up a turbine work? Thanks in advance.

 

Also referred to as a "static starter" or and LCI (Load Commutated Inverter), this type of starting means uses the generator as a motor during starting. Current, at a variable frequency, is applied to the generator stator terminals (excitation must also be applied to the generator rotor) to accelerate the unit from zero speed through purging, firing, and up to near rated speed, before the generator then reverts to its normal role as a converter of torque to electricity.

Units which have a gearbox do not use the gearbox as an accelerating means; it's simply a speed converter. The gearbox requires some sort of torque input. And in the case of GE-design heavy duty gas turbines, the shaft which transmits torque from the starting means (electric motor, diesel motor, expander turbine, etc.) to the turbine-generator shaft is direct--meaning the speed of the starting motor is not changed as it's applied to the turbine-generator shaft.

 

Hai. we have load commutated inverter called LCI.In this the synchronous generator works as the starting motor upto 91% speed and then cuts off. At this time we need excitation also.

I think the principle is the same. The LCI gives power to the stator,where as the exciter gives the power to the rotor, then the generator works as a synchronous motor. By varying the frequency of the power supplied the speed is gradually increased.

The main components of the LCI are
1)LCI input transformer. Its input voltage is 6.9kv and outlet is 2080 v. the transformer is Delta connected on primary and star-delta on secondary side.

2)There are two SCR bridges which will covert the LCI transformer output to DC volatge.These two The two SCR bridge output the same magnitude of DC voltage. The DC voltages are added together, similar to adding 2 batteries. This is the voltage that will be applied to the generator(motor) stator. The top row is the Source A bridge and the bottom row is Source B bridge. It may be difficult to picture but there are 3 SCR’s in series where as the inverter explained earlier had 4. Source A and Source B bridge SCR’s must be able to handle the input AC voltage of 2080. If one SCR is rated about 1500 Volts than 2 are need in series too obtain a 3000 Volt rating which can handle the 2080 Volts.

3) this Dc volatge goes to the DC link reactor.The purpose of the DC link reactor is to store a huge magnetic field. The huge magnetic field will provide a big current source so that there will be a continuous current flow into the generator windings. You wont run out of current. If current were to be interrupted and then restarted the stator magnet would not move smoothly and the stator magnet would jump around and may pull the rotor backwards, worst case.

4) Then this output will go to the Liverter which will convert it to AC. This will be fed to the stator through a disconnector switch 89SS. Generally this connection will be connected before the generator breaker. This disconnector switch is normally open and only closes during the start-up.

5) the generator grounding switch 89ND-1 should be open during the LCI operation. Then only the LCI can be started.

6)The main part of LCI is the LCI controller.the LCI controller. The control cabinet controls the firing of the SCR’s in inverter and rectifier.

Fiber optic connections are used between the control and the circuit boards that initiate the SCR firing.

7) at the same time when the LCI is started,the Exciater should also start to make the generator to become the synchronous generator.

8) The LCi control will make torque command in the start up and upto purge speed,then reduces the turbine speed to ignition speed. After the firing and warm-up step it will accelarate the turbine to 91% which is self sustainable speed for our machine. At this speed the LCI is turned off and the disconnector will open.The exciator will also get switched off and again starts after 1 min at 95%. Now it will act for generating the voltage.

So the summary how the LCI works is the following.

The LCI changes the generator/motor speed by?

A.) Regulating the generator field
B.) Changing the voltage magnitude applied to the stator
C.) Changing the current magnitude applied to the stator
D.) Switching the rate and sequence of DC power applied to the stator windings
E.) Applying a pulse width modulated waveform to the stator windings

Hope this will clear how the static frequency starter works?In this there is no another rotary starting means and the starting aid is the static starter.So it is called the static frequecny converter.

Ravi kishore. p

 

Thanks to CSA and Ravi Kisori alike for this explanation. It is really helpful.

 

>5) the generator grounding switch 89ND-1 should be open during the LCI operation. Then only the LCI can be started.

What is the exact purpose for the 89ND breaker during normal operation??? Our 89ND breaker is currently closed while the unit is online putting out 175MW.

 

89ND Breaker need to be open during normal operation. It is normally used when unit is stand still to ground the circulating current that present during the operation.

 

I will disagree with this.

> It is normally used when unit is stand still to ground the circulating current that present during the operation. <

I don't believe circulating currents which were present when the unit was running will still be there when the unit is at standstill, so this just doesn't compute.

I believe the neutral disconnect switch (89ND) is to be closed at all times, except during starting when the LCI is providing power to the generator.

Generator neutrals are normally grounded, and per some other threads on control.com, it's basically to limit fault currents. (Search control.com for other threads and "more information".)

Motor neutrals are not generally grounded, so when using the generator as a motor to start the turbine, the neutral disconnect switch is opened.

I would really like to have had a chance to look at the sequencing or application code for a unit with an LCI to be sure, but that's my recollection and my understanding.