once gas turbine started LCI sends speed reference of 23.6% and 14.6 during ignition. Once ignited it goes to 100%. Does it mean that LCI will try to reach 100% speed? when reaches 3250approx motor stops, how?

Do not know LCI relationship completely Please help.

 

cris,

The LCI is connected to--and disconnected from--the generator stator terminals via a very large "swtich." When the turbine is being started the generator is being used as an AC motor. And the speed of an AC motor is directly proportional to the frequency of the applied voltage and current. The LCI is the means of converting a voltage and current source (from the grid) which is at a constant frequency (or relatively constant frequency in most parts of the world) to a variable frequency voltage and current source. As the frequency of the voltage and current applied to the generator stator terminals is increased or decreased the speed of an AC motor will increase or decrease. (Most of the AC motors we work with every day run at a constant speed--because the frequency of voltage and current from the grid is constant.)

When the turbine is being started, the generator breaker (used to connect the turbine-generator to the grid) is open until it's time to synchronize the generator to the grid. The switch connecting the LCI is closed and the Speedtronic sends a signal to the LCI to control the frequency of the LCI output voltage and current being applied to the generator terminals--effectively making the generator a motor to turn the turbine shaft.

Most large GE-design heavy duty gas turbines purge at a higher speed (for a faster purge time) and are then decelerated to a lower speed for firing (this reduces the amount of fuel required to establish flame which reduces the thermal stresses on the hot gas path parts in the turbine).

When fuel is first burned in a gas turbine there is not enough torque developed from the expansion of hot gases in the turbine section to keep the turbine shaft spinning without an assist from a starting means--in the case of the 7FA that's the generator being used as an electric motor that's being powered by the LCI.

The Speedtronic tells the LCI how to control the frequency of its output to control the speed of the turbine during starting and acceleration.

At about 95% speed the 7FA turbine is said to be "self-sustaining" and the LCI is disconnected from the generator terminals by opening the switch and the fuel being burned in the turbine is now sufficient produce enough torque to continue increasing the turbine speed to approximately 100%, when synchronization occurs.

A motor is an electric device for converting amperes into torque. A generator is a device for converting torque into amperes. And electricity is just a method for transmitting and distributing torque over long distances using wires. In an AC system, the frequency of the voltage and current is VERY important, and the speed of generators must be kept very steady (constant) in order to produce a steady (constant) frequency of voltage and current.

An LCI could also be thought of a very large variable frequency AC drive--which is a device for converting constant frequency voltage and current into variable frequency voltage and current to power a motor so that the motor speed can be easily varied. Again, AC motors directly connected "to the grid" run at a constant speed because the AC grid frequency is constant. Vary the frequency applied to the motor, however, and the speed of the motor will vary.

Hope this helps!

 

Don't think so. If your gas turbine works the same way as our 6FAs, the load commutated inverter speeds up/down to the different speeds (purge speed, firing speed, etc.) following the set points (in my case CSFSTP) the speedtronic sends to the LCI control, until the set has reached a sufficient speed for the gas turbine to operate autonomously. When this speed has been reached, the gas turbine can speed up by its own means, so the LCI is switched off.

Hope this helps

 

You can take a look at this post:

http://www.control.com/thread/1362173773#1362350146

 

> You can take a look at this post:

> http://www.control.com/thread/1362173773#1362350146

Thanks a lot everyone !!!