Anyone with experience in converting existing synchronous generator in synchronous condenser? My company is considering the opportunity to convert our old generators which are going to be dismissed.
Which kind of issues have to be considered?

We operate in both mode for vertical hydro machine up to 250 mva rating.

Not an issue

Francesco,

Assuming that you have decoupled the prime mover to reduce the windage and bearing losses, your biggest issue will be getting it started. Depending upon the size of the generator relative to the system, you need to determine if it will require a starter or a small (relatively speaking) motor to bring it up to near synchronous speed. You also have to look at synchronization, metering/protective relaying, lube oil pumps, excitation settings, and other auxiliary/control systems.

Alan

Thank you everybody. Alan could you please explain me how do you determine wether a starter or a motor is required?

Francesco, The calculations are no different than those for determining how to start a large synchronous motor, it all depends upon the type of generator and how big it is relative to the system that you are connecting it to. Take a look at the SLD (Single Line Diagram) and get the GSU (Generator Step Up) transformer rating and impedance, transmission line impedance to the grid, as well as any information that you have on the generator. Then you can determine how much the voltage will dip if the generator is started DOL (Direct On Line). If it is unacceptable you will need some form of reduced voltage starter.

But it also depends on the construction of the machine, if this an old machine with damper windings then it may be possible to start as an induction motor, and have the field energized near synchronous speed.

The small motor (also known as a pony motor) eliminates the above concerns, you just need an induction motor large enough to accelerate the rotor to near synchronous speed and then engage the field. Usually a few hp will do since you don't care how long it takes (within reason) to get to speed. The only problem will be connecting and aligning it with the generator shaft.

As you can see there's a fair amount of engineering involved, and we haven't even discussed the excitation, control and protective system modifications involved.

I'd like to be more specific but there's just so much one can do without much more in-depth knowledge of your overall installation. The more information that you provide the better the guesstimate will be.

Alan

Thanks Alan, your comment is very usefull. The generator is a relatively small one (12.5MVA) and the prime mover (that we are going to decouple) is a steam turbine. Step up is a 6/33 kV, 14MVA transformer. Excitation and protection system have recently been substituted, I guess we can easily modify excitation and protection settings, since they are PLC based systems. Lube oil and generator cooling will have to modified, since they are strictly connected with steam turbine auxiliary systems.

My main concern was about starting means, but we prefer not to dismantle the steam turbine so may be a static start system is the best solution (there is not enough room to place an induction motor with clutch system)

Twenty-five years ago I played with the same idea for some diesel-generators that were supplying a load with a very low power factor.

Today I would definitely try to use a separate LV motor to run up your generator. Even if there is a bit of the turbine in the way physically.

That way you would overcome the expense of installing a MV Variable Frequency Drive. Otherwise you would have to ensure excitation before accelerating, and that would be difficult with most modern brush-less excitation systems, and even with a brush-ed exciter you would have to adjust excitation to the VFD output.

Would you not have sufficient space near the non-drive-end of the generator to install this small motor? - Or maybe even building a small permanent magnet rotor around the coupling part from the turbine, and a surrounding specially built stator, supplied from a VFD? Some wind turbines use a technique similar to this.

By "the non-drive-end" you mean the collector end of the rotor? Yes there we would have enough space to place a LV motor. But I have some concern regarding the mechanical torque applied by the starting motor during a fault: normally the collector end of the rotor shaft is not designed to withstand such a torque transient. On the other end, the space between generator and the steam turbine is very low..

It should be fairly simple matter to install some kind of coupling. It would not have to be particularly "stiff". It is only there to bring your generator up to speed, and the driving motor only need to be a fraction of the rating of the turbine, so even during a fault there would only be minimal mechanical torque transmitted to that motor.

The inertia of the generator should be able to minimise the torque impact considerably.

Typically the starting motor is de-energized once synchronous speed is reached so no clutching is required. If you're worried about a fault during startup simply trip the motor starter with the protective relaying.