SOFT START ON GENERATOR USED AS TURNING GEAR

[xxjohnh]

Some vendors now offer a variable speed drive to use for acceleration and turning gear for Gas Turbines instead of starter motors.. Since synchronous motors are not inherently self starting, use an embedded squirrel cage, does any one have an explanation how this is applied to use the generator as a synchronous motor.

 

[TurbineGen]

The embeeded squirrel cage windings are known as amortissuer windings. They are often reduced conductivity material for higher torque such as silicon doped copper or similar metal.

As you may know, the gas turbine requires a rolling start. Often it is run at 20% speed for extended time before it is lit off to purge the unit. The variable speed drive allows you to maintain this speed without needing the starter. Since you are not trying to get the machine up to synchronous speed, you need the variable speed drive.

So basically, the amortissuer windings make the unit a large induction motor driven by the variable speed drive until the unit is lit off. Then the varibale speed drive is taken off line and the unit is acceleratedd by the gas turbine up to synchronous speed where it is synchronized to the grid and driven as a generator.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[TurbineGen]

This could be used in fairly low speed as the turning gear for the cool down period. After that you are probably better off going to the ratcheting turning gear.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[xxjohnh]

That is how I thought it worked, but Alstom literature states it starts and operates aa a soft start synchronous motor, so the field is applied.

 

[xxjohnh]

Alstom uses the ABB megatrol package.

 

[TurbineGen]

I've never heard of that being done with the field applied. Usually that is done just before the breaker closes. I'll have to ask a friend of mine who has a few Alstom units to see if he starts them up with the field applied.

Does your generator have amortissuer windings? If it doesn't, then you would apply the field. The variable speed drive would then start this up as a brushless DC motor rather than an inductive motor. If your varibale controller has a feedback path for sensing rotor position (it could even use feedback voltage) then you would need the field to be applied and the amortissuer windngs are unneccessary.

What make is the generator?

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[GTstartup]

ABB Megatrol is a combined LCI SFC/AVR. The AVR part is used to appy the field for starting the generator as a synchronous motor. Then after 90% speed is reached the AVR part is used again for the generator excitation

 

[GTstartup]

Turning gear speed on an ALSTOM/ABB GT is too slow for practical SFC operation. It could be used for fast cooling however.

 

[rcw retired EE]

The GE and Siemens CTG's use a similar system. The starting system operates the excitation to run the generator as a variable speed synchronous motor. The excitation is controlled to obtain the needed torque without exceeding any of the generator capabilities.

A small turning gear motor rolls the generator and turbine at slow speed (12 rpm?) when it is off line. During a start the VFD runs the unit at 30% speed for a few minutes to purge the system of any unspent fuel. The speed is then reduced to keep from blowing out the ignition pilot flame and fuel is injected. After successful ignition, the VFD goes to full power (5-11 MW) and accelerates up to 90% speed where the turbine takes over and the VFD and the excitation shuts down.

The VFD operates at about 4.16 kV on an 18 kV generator winding and is connected to the generator terminals via a motor-operated switch. It is isolated before excitation is applied. Also, the generator neutral ground is automatically disconnected during the starting sequence to prevent circulating current problems. The generator excitation is interlocked to prevent normal energization unless the starting and neutral disconnect switches are in the correct position.

 

[ScottyUK]

Something has gone astray with terminology here: turning gear (or barring gear) keeps the shaft moving at a couple of rpm to ensure that the shaft does not bow or develop a thermal bend. Starting the gas turbine isn't a function of the turning gear. Gas turbines are used to purge the HRSG for perhaps 20 minutes at roughly 20% rated speed, and then the LCI provides accelerating power to the turbine until it reaches about 60% of rated speed after which it is self-sustaining.

All the static starts I'm aware of use an LCI connected to the generator IPB via a disconnector and require the field to be energised. The machines which do not use an LCI usually have a torque converter or something similar.


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If we learn from our mistakes I'm getting a great education!

 

[rcw retired EE]

Good explanation, Scotty.

I only mentioned turning gear because the LCI doesn't have to start the machine from a dead stop. That said, there's not much difference between dead stop and turning gear speed!

 

[ScottyUK]

Hi rcwilson,

It was the use of 'turning gear' in the title of the thread I'd picked up on rather than your mention. In answer to the original post, the LCI does start the machine from a couple of rpm and it can be a bit 'lumpy' at very low speed. As the unit accelerates it becomes very smooth.


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If we learn from our mistakes I'm getting a great education!

 

[xxjohnh]

To operate as a synchronous motor it needs to start as an induction motor or be brought up to near synch speed by some other means. The barring gear is hydraulic and could be used to bring it up to a very low speed, perhaps the VFD is able to bring the frequency down to that speed to allow the generator to operate as a synchronous motor.
The normal use for the drive is to bring the unit up to 90% speed to start.

 

[ScottyUK]

GE's LS2100 LCI is the workhorse for pretty much all their heavy turbines from 120MW upward, and it runs as a synchronous drive from barring up to the point where the turbine becomes self-sustaining, at which point it disconnects from the IPB.


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If we learn from our mistakes I'm getting a great education!

 

[xxjohnh]

Thanks Scotty. The GE Literature is much better than for the ABB megatrol.It seems then the barring low revs are matched by the VFD to produce synchronous motor operation. I note the GE is water cooled and ABB aircooled.

 

[GTstartup]

XXjohn, I think you need to read a little closer, the barring gear is not related to the SFC in any way at all. The synchronous motor operation is simply a LCI with field applied via the AVR part to the slip rings.

 

[xxjohnh]

Read page 16. A sychronous motor can not start from standstill, so the turning or barring gear gets it rolling
and the VFD matches the speed frequency and applies the field. Then it becomes a synchronous motor.

 

[GTstartup]

Quote

"Read page 16. A sychronous motor can not start from standstill"

It actually says that in a GE Brochure? Point it out to me please. What line?

 

[ScottyUK]

Turning gear is required for other reasons, like preventing the rotor developing a thermal bow or, in the case of big turbo machines, a sag. Both conditions can result in rotor imbalance and severe vibration, rendering the set unfit for service until a prolonged rollout period has elapsed. Siemens Westinghouse were requiring a standard 12 hour rollout prior to firing, and in the case of a severe thermal bow it could be a couple of days until the rotor cooled enough to be able to rotate.

I'm pretty sure the LCI would move the rotor from standstill, it just happens to be rotating at a couple of RPM for other reasons when the LCI is energised. The same situation applies to the torque converter systems: the rotor is already on barring before the torque converter kicks in.

Why do you think the rotor needs to be rotating prior to the LCI being able to operate?


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If we learn from our mistakes I'm getting a great education!

 

[xxjohnh]

Basic theory about synchronous motors, they are not inherently self starting and usually start as induction motors using an embedded squirell cage in the rotor.