Gt generator neutral grounding 2

[stratford]

Dear all. On site we ahve a GT generator that is started by means of a static starter (LCI load commutated inverter). At this moment the neutral (star point of generator) is isolated from ground by a disconnector. When GT reaches full speed no load (FSNL) then the neutral disconnector closes and the generator synchronizes to the grid. During the start up the generator operates as a synchronous motor. Can you give me your opinion on this? I suppose that it could not operate as a motor with the star point grounded.
Generator 15.75 kV, 325MVA, Windings Y, grounded Y through transformer and resistor.

 

[RRaghunath]

Is the generator neutral high impedance grounded?

 

[stratford]

rraghunath
The generator is grounded with 25kVA 12kV/120V transformer whose secondary is in parallel with 0,525 Ohm/350A resistance.

 

[Kiribanda]

Startford,

What do you mean by "The sync. generator is started as a sync. motor and then sync to the grid?". How do you run as a motor initially? A generator is normally run by a prime mover.

It is a new thing for me. Could you please elaborate on the starting method?

Kiri

 

[stratford]

The gas turbine is a GE 9FA DLN II with a generator coupled and the GT is started not by a prime mover (another generator) but as a motor supplied with variable frequency and voltage through a frequency converter (GE LCI).

 

[TurbineGen]

Stratford, since you asked for an opinion, I'll wager a hypothesis that it is to keep the unit isolated from the grid until the unit is ready to come online to avoid false ground fault indication. The ground connection may interfere with your inverter and trip a breaker. Again this is just my guess. I have seen a similar system in to what you described in Texas, where they also used a ground-neutral disconnect.

We have a similar generator starting style in one of our generating stations, but we start the unit by inserting a reactor in series (and running as a sync motor) and then switching the reactor out once the unit is up to synchronous speed. This system seems to work well for us.

Maybe if I try, I can get both feet in my mouth.

 

[ScottyUK]

kiri,

The turbine only becomes self-sustaining as it approaches rated speed. below that speed it requires an auxiliary drive to provide cranking power to the shaft in order to reach the speed where it becomes self-sustaining. This can be done using a electric or hydraulic starter motor and a clutch arrangement, or in more modern designs the generator itself is used as a synchronous motor driven by a variable speed drive to accelerate the turbine. The power requirements are large for a big turbine, so the choice of drives is limited.


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[Kiribanda]

ScottyUK,

Thanks for the explanation.

Kiri

 

[subtech]

Scotty,
In this instance, roughly how much energy would be drawn from the bus to get the gt turning and making drive power?
From what I've read in my protection literature, a motoring gen that is normally driven by a gas turbine will draw 30-40% of its MVA rating because it must "drag" the compressor at synchronous speed. Can I assume approx. the same figures in this case?
Thanks

 

[ScottyUK]

Hi subtech,

Our turbines are an older generation design which uses an electric motor and fluid torque converter to provide cranking power rather than a direct drive through the generator. To give some idea of relative size, the shaft power delivered to the generator is about 165MW. The starter motor is 2000HP.

The normal cranking speed for the turbine is about 650 rpm at which point the turbine is ignited after a HRSG purge. At this point the turbine is drawing little power and the compressor is behaving like an inefficient fan. Once ignited the turbine accelerates gently up to synchronous speed on a pre-defined curve. At around 1800 rpm the starter motor and torque converter disengage, bleed valves close allowing compressor discharge pressure to rise, and the turbine becomes self-sustaining, although it still has little power available to drive any load or to provide acceleration torque because the engine output is only slightly greater than compressor load. As the machine continues to accelerate the compressor efficiency improves dramatically and the compressor discharge pressure rises steeply. The engine output power is rising more quickly than the compressor load at this stage.

There are a lot of things missing from that description, but it should help a little. The main thing to understand is that the bleed valves, when open, effectively spoil the compressor operation and this keeps the shaft load down allowing a smaller starter motor to be used.


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[RRaghunath]

Stratford,

The NGT/NGR for the GTG limits the earth fault current to 350A as mentioned. In your case, the generator is not high resistance grounded rather it can be said to be medium resistance grounded. Had the value been 100A or lower, then there would have been no need for isolating the neutral - earth circuit during run as motor.
This is because the high resistance required to limit the current to low value itself would control the magnitude of third / triplen harmonics in the circuit (to below the acceptable levels). These triplen harmonics have potential to cause additional heating in the motor (generator turned motor, especially the rotor part) windings and may be, in the LCI too.

I have seen the generators with high resistance neutral grounding that limits the E/f currents to ~25A and there is no disconnecting switch in the neutral circuit.

 

[stratford]

subtech The generator nominal apparent power is 324MVA and the power used to start the generator is maximum 11MVA.

rraghunath Thanks once again for the detailed justification.

 

[subtech]

Thanks to all!