Large Motor starts on Generators

[Umtully1]

Hi I was hoping someone could point me in the direction of some literature or advice regarding starting large motors on generators only?

We have a very weak grid, only 1 18MVA generator and we want to start a 2000hp synchronous motor. Our plan is use a couple smallish diesel gensets as synchronous condensors to supply the VARs needed to get it up and running, while also having another genset to provide MW if needed. Has anyone heard of this strategy? could it work? Or should we just have all gensets pumping out MW and disconnect from the grid while doing the motor start?

 

[cranky108]

Are you intending on starting the motor under load?
If not, you could use a pony motor to spin it up so as to reduce the spin up time.
Or you could use reduced voltage starting.

 

[waross]

Consider bringing the motor up with a gen set.
I hope that your 1.8 MVA is engine, water or steam turbine powered.
Use an external power supply for the Automatic Voltage Regulator power.
Direct connect the motor to the gen set. Use an external power supply to energize the motor contactor.
Start the gen set and let it accelerate the motor as it builds up speed.
The Under Frequency Roll Off provision of the AVR should maintain the Volts per Hertz ratio and the motor should start with similar currents to a VFD start. 1.8 MVA should start the motor nicely. The generator may be a 5% or 10% overloaded if starting under full load. If this is a prime rated set, you are allowed 10% overload, no problem. If this is a standby rated set, the prime mover capability may act as a limit. I hope that the gen set will be cold when the motor is started. That will buy you enough thermal capacity that a motor starting overload should not damage the set.
Once up to speed synchronize and transfer to the grid.
Using condensers to supply MVARs is a technique that many of us have read about but few have seen.
Starting current may consist of more KVARs than KWs. You will probably need more than 1.8 MVARs for a DOL start. Then you must be cutting back the KVARs as the motor accelerates to avoid over voltage issues. Supplying starting KVARs with capacitors is a theoretically valid starting method but it is not a recommended starting method.
If it is possible to start the motor unloaded there are some other schemes to bring it up to speed.

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

 

[waross]

cranky108 posted as I was composing.
The pony motor is one good method cranky.
Another option is a combination of the pony motor and reduced voltage starting. Use the method I have described to bring up the motor with a small generator. The UFRO will maintain the V/Hz ratio so the current stays low. The large motor will act as its own pony motor. You may even tweak the fuel stop so that the gen set is incapable of exceeding your desired starting load limit.

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

 

[Umtully1]

Thanks for the help, I should've posted some more details.
Being relativily new to motor starts, I've been told that synchronous motors start uncoupled from the load, and once they're up to speed they close a clutch, so that answers one question. We're also planning on using a reduced voltage auto transformer to limit the inrush. The utility gave us a 300kVAR limit, which is tiny, considering the size of our motors.

From what I gather, the simpliest starting method is starting the motor using reduced voltage on only generator power.
But I'm not sure how mobile the generators are, so I don't know if we could direct connect it to the motor and then woudln't we need some sort of transfer switch then?

Oh, and the grid is an 18MVA hydro generator, about 300km away, not a 1.8MVA generator. There's about 4 small towns inbetween us and the generator so voltages must not dip, (hence the diesel gensets on our site).

 

[waross]

The cheapest solution may be a diesel generator large enough to accelerate the motor.
A VFD works by varying the effective voltage and the frequency so that the Volts per Hertz ratio is maintained.
If the voltage and frequency are ramped up together a motor may be started at a low frequency and low voltage and there will be none of the starting surge associated with DOL starting.
If the motor is unloaded it may be started by a VFD that is much smaller and cheaper than would be required to start under full load.
The motor may be spun up by a small pony motor and energized when it is up to speed. The pony motor may have to be a high slip motor to handle the inertia of the large motor.
A small pony motor may be VFD driven.
The motor may be connected to a generator and the generator started. The generator need only be large enough to bring the motor up to speed. The UFRO function of the AVR will keep the Volts per Hertz in the proper ratio to avoid large currents.
The fuel limit adjustment may be used to limit the energy to the motor and the motor and generator will accelerate as fast as the diesel engine is able.
I would be comparing the price of VFDs and generators.
By the way, if you use anything but a direct grid start you will need synchronizing gear to close on-line at the correct instant. The motor starter may be used to put the motor on-line with a sync check relay. A shunt trip may be added to the generator breaker to clear the generator. A contactor would be safer.
A reversing starter may be modified to provide a fast open transition.

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

 

[LionelHutz]

We need more info. This doesn't really make sense. Are you planning on running a genset whenever this motor is running? Is the 300kVAR limit inductive and capacitive? What is the load?

The "good" news is that a synchronous motor with a clutch is usually very easy to start. I've spun up unloaded synchronous motors on soft-starters using a current which was about equal to the motor FLA.

The "bad" news is that a synchronous motor can be both inductive and capacitive, inductive when starting and either inductive or capacitive when running depending on the field current applied. So, supplying some inductive VAR's during the start may not be enough to satify the utility.

If the utility limited you to 300kVAR always, then you may not even be able to engage the clutch or run that motor on utility. That motor will be well over 300kVAR capacitive with any reasonable field current applied. And you will likely need to apply a field current close to the rated value to be able to successfully energize the clutch and get the load spinning. Then, any time you unload the motor it will likely go over the 300kVAR limit, clutch engaged or not.

FYI, synchronous motors do not always start with a clutch. You'd better verify that a clutch is planned if you have just been told that in general.