No damage to 2 pole turbo-generator rotor after extended motoring (40mins) without excitation? 3

[jt2001]

Hello,

We were contacted to advise on our recommended inspection after a generator had been motored at synchronous speed for an extended period of time (40mins)

The entire event was approximately 80 minutes.

1. The field exitation was lost - and generator protection failed to trip - the generator was motoring but still driven by the turbine for approx 40mins.
2. The Turbine power was removed once motoring was noticed - and the generator motored at approx 2998 - 3003 RPM for 40mins still without excitation.
3. The stator was finally isolated and the unit spun down.

Given this sequence of events we advised a caps-off inspection in which we anticipated to see some arcing damage between the wedges and the end caps.

We have since recieved and partially completed the caps off inspection. Identifying almost no damage to either the retaining rings or the slot wedges.

Is this typical of a loss of excitation event when it occurs at synchronous speed (50hz)?
What detail could we have missed that would have protected the rotor from damage and avoided the arguably uneccessary forced shutdown?
Are these events only destructive when the rotor speed is significantly different to the synchronous speed of the stator?

Kind regards,

JT

 

[waross]

The generator will have been running as an induction motor on the damper, or squirrel cage winding.
First damage will be to the squirrel cage winding.
Depending on the Horse Power needed to spin the turbine, there may not be over heating damage to the squirrel cage.
Has there been any damage to the turbine?
Steam turbine or gas turbine?
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jt2001]

Hi waross,

Gas turbine - no damage that I am aware of.

The stator had approximately 60% of its operating current on its windings during the event.

Is there a way to estimate the maximum horsepower the damper winding can sustain without overheating?

 

[edison123]

15 MW, 2 pole generator motoring effects.

Rotor winding got cooked with overheating of pole faces & retaining rings, wedges meltdown near the RR's. etc.

Rotor rewind with new RR's and wedges done by us in 2017.

Muthu

http://www.edison.co.in

 

[wcaseyharman]

What type of gas turbine? How is it started?

Some of the newer gas turbines use a “static start” where they spin the shaft with the generator acting as an induction motor. My understanding is that these machines are designed to handle significant currents in the ammortisseur windings without damage. Perhaps the gas turbine generator in question is designed for static start and thus could handle the extended loss of excitation?

 

[davidbeach]

at approx 2998 - 3003 RPM for 40mins still without excitation

Is it really possible that the generator, acting as an induction motor, could have driven the gas turbine in that speed range? 2998 RMP is a mere 49.967Hz; to say that it was acting as an induction motor implies a slip of, what, about 0.07%. Bah, humbug. I think that the system frequency simply varied between 49.967 and 50.05Hz and that the machine had sufficient field strength to maintain synchronism throughout.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[jt2001]

Edison, That is the condition I expected the rotor to be in given the explanation of the event I recieved. It is actually a similiar design to that unit.

davidbeach, What speed range/difference from synchronous speed would you expect/(or have observed) during events like this?

wcaseyharman, It is a considerably older unit - I do not have details of the turbine design/operating characteristics.

 

[jt2001]

The attached data may assist.

 

[davidbeach]

Even a 1% slip would have taken the speed down to 2970 RPM. A 3% slip is in the probable ball park range. So, if it truly tracked normal system frequency it wasn't acting as an induction motor.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[Hoxton123]

Did the turbine go into idle, just ran near synchronous speed expecting a re-synchronisation?

is there a clutch between turbine and generator, so the turbine was not connected to generator?

 

[jt2001]

Allegedly it was driven by the generator for 40mins.

How much resistance does inactive gas turbine require to keep that kind of speed?

 

[edison123]

david

that the machine had sufficient field strength to maintain synchronism throughout.

Since the excitation off, it can't run in synchronous mode. It can run either as induction motor below 3000 RPM or as induction generator above 3000 RPM. Both these modes in sustained operation would damage the rotor and possibly stator also.


OP
At a minimum, I would check rotor IR, DC resistance and AC impedance and compare the later two with OEM values.

Muthu

http://www.edison.co.in

 

[wcaseyharman]

The resistance depends on the type of gas turbine - a “heavy frame turbine,” where the compressor, turbine and generator are all on the same shaft, takes a lot to spin because it’s still turning the compressor. It’s at least 1/3 the total power output.
If the gas turbine is an “aero derivative,” meaning its built using a jet engine designed for aircraft, the turbine driving the generator is not coupled to the compressor at all, and thus requires a lot less to turn.

Definitely an interesting case you have here.

 

[davidbeach]

Were those RPM values measured or were they calculated from the electrical frequency?

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[jt2001]

davidbeach, I believe they were measured.

wcaseyharman, I believe it is an aeroderivative.

edison, We have completed all tests mentioned. There is no trace of damage to the windings. We do not have OEM values - But previous inspections to compare values with . Negligible difference in test values from these.

We have concluded the inspection finding no evidence of heating/arcing in the damper winding.

Given the condition of the rotor we assume there must be a misunderstanding or assumption from the end user concerning the actual running conditions during the event.

And given the lack of slip frequency mentioned in davidbeach's response - There must have been some field energy during the suspected motoring period.

If anyone could share some generator telemetry data from an actual motoring event/loss of excitation at synchronous speed. I would be very curious to study this.

Thankyou all for your responses.

 

[waross]

The slip frequency of a standard four pole induction motor rune from 2% to 3% of line frequency at full load.
The slip is proportional to the total load, that is driven load plus windage and bearing friction.
At 10% load, you may see as little as 0.25% slip.
I would expect a higher slip percentage from a generator but I may be in error on that assumption.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jt2001]

Hi Waross,

Do you know of any documentation that adresses this?

Klempner addresses it in "Handbook of Large Turbo-Generator Operation and Maintenance". But based on these equations the stator current (approx 520A) would be high enough to cause damage.

Noting the damper winding in the slot section consists of 1 x 2mm thick aluminium strip and the retaining ring acts as the shorting ring for the winding (there is not "under-cap" amortisseur).


Is there any ways of determining damper winding load based on slip % + load?