Starting of Synchronous Gas Turbine Generator with Load Couumtated Inverter 2

[rockman7892]

I recently came across an application where a 230MVA Gas Turbine generator used a Load Commutated Inverter (LCI)to bring the generator and turbine up to speed during startup. From what I have found the use of an LCI for generator startup seems to be typical for a gas turbine application.

Seeing this, I was hoping to learn more about the role of the LCI in the startup process. I am not that familiar with an LCI but from what I have read it appears to be a preferred method of speed control with a synchronous motor. Can someone provide an explanation or good resource as to why an LCI is preferred over a PWM drive from a synchronous motor? I am familiar with how a PWM drive works but and not quite sure about how an LCI works in comparison to a PWM drive? Can anyone provide a good explanation or reference on LCI operation?

Is this LCI method a typical method for starting of gas turbine generators? I've never really given much thought to the startup of generators so it got me thinking about what other starting methods are used in industry. Are there other preferable starting methods for synchronous generators? Do steam turbine generators use this LCI application?

I'm also curious to hear any protection considerations that are crucial to examine for protection during startup with an LCI.

Thanks for the help

 

[davidbeach]

I can't really talk to the LCI itself, but it is only the combustion turbine that would need such a beast. A steam turbine or a hydro machine can be brought up from zero rpm to full speed simply with the throttle valve or wicket gate control. A combustion turbine can't produce self-sustaining power until it is running at some large fraction of rated speed. The LCI is the means of using the generator as a motor and spinning the turbine up to a speed sufficient to produce its own running power.

 

[jraef]

2 minute lesson:
LCI technology is used for extremely large Synchronous motors for two reasons; they are good at a wide speed range and the amount of power is difficult to deal with.

Voltage Source Inverter (VSI) drives that use transistors are fine for low voltage and some ranges of medium voltage, but for extremely large power applications the transistors must be run in large parallel arrays to get the amount of current you need, and series arrays to get the voltage. The end result is a ridiculous number of devices, resulting in a very low MTBF.

Current Source Inverter (CSI) and Load Commutated Inverter (LCI) drives both use much simpler thyristor based power switching technologies that are much more robust and have a much much lower component count. Basic CSI drives have issues running motors at low speeds however.

LCI drives use similar power switching technology and so remain robust, but also take advantage of the characteristics of synchronous motors to overcome the limitations in CSI drives; the fact that they can be made with high pole counts so as to have lower speeds and the fact that power can go both ways in them. But LCI technology ONLY works with synchronous motors, not the more common induction motors.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[rockman7892]

Thank you both for your replies

It sounds like I need to do some further reading to understand the differences between VSI and CSI drives. From my very limited knowledge I understand that VSI's essentially control the voltage output to the motors and thus the current output requirement changes depending on motor torque demand. With a CSI drive the drive is able to control the current output to the motor therefore it is not totally a function of motor torque demand whereas voltage output to the motor with a CSI drives is dependent on motor torque requirements. Do I have this right?

Can you give a simple explanation of the role that the DC reactor plays in a CSI or LSI? With a VSI drive I am sued to seeing a capacitor on the DC link to smooth voltage ripple, and it appears that the DC reactor on the DC link of a CSI is doing something similar with the rectified current but I'm not exactly sure what role it plays and how it affects the speed control and current regulation on a synchronous machine. For this particular application that I am looking at a generation station actually has some sort of existing converter and inverter (appear to be separate units from drawing) with an external DC Link Reactor located outside of the building where the converter/inverter are located. This existing arrangement is being replaced with an LCI drive which I'm assuming as an integral DC reactor inside of the unit. I'm assuming that the DC link in the current setup is doing the same thing as it would in an LCI?

How does the fact that power can go both ways with a synchronous machine make an LCI an attractive feature?

I'm curious if there are any generator protection settings that may need special settings or may need to be considered during this startup process?

 

[davidbeach]

Other than differential, the generator protections are turned off during the LCI start. The LCI system should have its own "motor" protection for the time generator is being used as a motor.

 

[rockman7892]

In reading I notice the terms excitation and Automatic Voltage Regulator (AVR) but don't exactly understand the difference between the two. I understand that the exciter supplies the filed current to the generator rotor and controls voltage, excitation, V/Hz etc... and that the AVR monitors and controls the generators terminal voltage but I don't understand the difference physically between the two. Is the AVR just a control system/algorithm that is part of the exciter equipment or is the AVR a separate unit? I always though the AVR was a control loop within the exciter that was used as an input to control field excitation.

 

[DHambley]

Rockman, I can't tell from your first post if this LCI topology which you are looking at is a separate circuit for start up than the normal generator circuit. As you most likely know, you can use the inverter for both a generator and motor starter. Starter/generators I have designed utilized a constant torque for start up which also implies constant AC current. This allows the quickest starting while keeping the switches in their safe operating current range. At about 40% or so, light-off occurs and the starting torque can decrease while the turbine drives itself to the operating rpm.

Darrell Hambley P.E.
SENTEK Engineering, LLC

 

[rockman7892]

DHambley

LCI is part of a separate startup circuit. Once generator is up to speed a switch is opened which removes the LCI from the generator circuit.

 

[jraef]

Think of it this way perhaps. In this application, the LCI drive is being used as a very big, very expensive soft starter. It is only used to ramp the motor up to speed. After that the prime mover takes over to make it a generator. So in that capacity, the drive is unconcerned with power factor itself, or the AVR. They will not come into play until AFTER it has done its job.

But the reason that you don't use a simpler soft starter, aside from the fact that nobody makes one that big, is that a soft starter can only control the voltage, not the frequency. So in starting, the torque is reduced by the square of the voltage change, and at the same time the starting current percentage will equal the voltage percentage. So if I want to keep the line current at 100% of FLA, but the normal starting current is 500% of FLA, I would have to reduce my voltage to 20% on startup, which would not leave me with enough starting torque to accelerate even the motor with no load on it at all. If I adjust the soft starter current to give me enough torque, the line current will end up, even on an unloaded turbine, somewhere in the 250-300% of FLA range. For something that large, that amount of current is usually untenable.

The LCI however, because it can control both the voltage AND frequency together, can in theory accelerate that motor with 100% FLA. It may take it a long time to do so, but because it is a drive, you can take all the time you need without harming the motor. You could do this with a VSI, CSI or LCI drive, but it comes down to the things I mentioned earlier as to why LCI is chosen for these large synchronous motor applications.

The "power can go both directions" is by the way POWER, not power FACTOR. This is important because in this application the motor is expected to become a generator, so at the end of the acceleration cycle when it does, the power can flow back through the LCI. You cannot do that with a VSI unless you make it an Active Front End design, which is essentially another drive on that front, doubling the size and cost. You could do an Active Front End on a CSI drive as easily as on the LCI, but since the motor is synchronous already, the LCI makes more sense because it can give you better torque control at the lowest speeds, which helps with acceleration. Technically CSI drive technology has improved to make that less of an issue, but again, your motor is going to be synchronous ANYWAY, so no point in reinventing the wheel for no gain. An LCI drive is a variant of a CSI anyway, so if anyone made a CSI drive that large, it would cost the same. So there would be no point in doing it when we already KNOW the LCI works fine.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Parchie]

On older single-shaft GT's that I have known in my other life, the starter motor/exciter only needs to bring the shaft speed near 1,200 rpm. At that compressor speed, the igniter fires up the combustor and GT revs up shaft to 3,600 rpm!

I guess you'll just have to take note at what speed your GT fires its combustor--> the maximum speed your LCI mode will have to power the GT shaft/s. Just my two cents.

 

[ScottyUK]

Light-off speed is not the speed at which the engine becomes self-sustaining. Without the LCI or torque converter to assist it will fail to accelerate from light-off becasue the compressor is so inefficient at low speeds.

For a 50Hz machine (MW-701DA) light-off was about 650 rpm, with the machine becoming self-sustaining at about 1950rpm when the starter gear shut down. Compressor bleed valves didn't close until about 2750 rpm bringing the machine up to full power.

 

[waross]

The exciter is basically a small generator that powers the field in a generator.
The AVR provides exciting current to the exciter.
If the generator voltage is low, the AVR increases the voltage on the field of the exciter and the exciter output voltage rises to increase the field strength until the terminal voltage is correct.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

The following

http://www.mhi-global.com/company/technology/review/pdf/e342/e342049.pdf

may be of help

NB: I'm not employed by Mitsubishi Heavy Industries.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[ScottyUK]

Interesting. Not sure I'm sold on the 'benefits' over the LCI but haven't seen that system applied to the 701 before. The earlier design of that engine used a 2000HP electric motor and torque converter to accelerate the machine, or occasionally to test the fire suppression system.