Motoring Generator by Breaker Failure 1

[ahung]

Does anyone know about the consequences of Motoring of Electric Generator by Breaker failure (Trip Coil was failed and the Generator breaker didn´t open to disconect to the grid).

When the operator unload the machine to shutdown it the control system send the signal to open the generator breaker the trip coil fail and the unit without load and with excitation on the rotor (the field breaker stay still close)the unit get power from the grid and motoring the generator.

The motoring of generator was around five (5) minutes until the operator call to Central Dispatch for clean all bus bar tripped all circuits on the electric substation 69 kV.The operator can´t pull the manual trip on SF6 SIEMENS breaker because he didn´t know how is the procedure to do that.

Our generators are made by BRUSH and the nominal power is 133 MVA and gas turbines are model W501D5 Siemens Westinghouse the nominal capacity is 100 MW ISO conditions

What we do to inspect the generator? What kind of failures are typical when motoring generator? We consulted to Siemens for install a redundant trip coil on SF6 breaker to prevent the same problem on the future.





Regards

Alberto J. Hung C
Caracas Venezuela

 

[buzzp]

From a hydro generator standoint, there would be no damage. Not sure about a gas system.
Breaker failure is typically handled by monitoring the current through the breaker after a trip signal is issued. If x time passes, and current is still flowing, then a breaker failure alarm is annunciated and you take the appropriate action to clear the problem. Whether you have two trip coils or not, I would implement something like I mentioned or you can buy dedicated breaker failure relays if your protective relaying you have now does not have an option for this.

 

[dpc]

In general, motoring and reverse power concerns relate to the prime mover not the generator. The generator is happy as a motor as long as you are operating within its excitation limits.

The gas turbine might be an issue since it did not its normal gas flow while rotating, which might cause some high temperatures. In a steam turbine, the main concern is overheating in the low pressure section due to low flow. I'm not sure about a combustion turbine.

My guess is that the generator is fine.

 

[rmw]

If you did not slip a pole while motoring the generator, you are probably OK. That is the real danger with motoring. A CT doesn't have the same issues with last stage overheating that a steam turbine has. At that point, it was just a large air compressor.

rmw

 

[ScottyUK]

Did the generator remain synchronous: i.e. did the field breaker open? If the machine ran as a synchronous motor the generator will be ok; if it ran as an induction motor then the rotor will experience very high losses which can damage the core and / or windings. A gas turbine is perhaps the worst load from this respect because the compressor presents a significant motoring load compared with the steam turbine which presents a very light load. Can you confirm what mode the machine operated in? Five minutes is probably not long enough to get too concerned about either way.


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Your body may be a temple. Mine is an amusement park...

 

[ahung]

Thanks for all responses. For clear some doubts about the motoring generator I explain the event with more details:

Definitely the generator (syncronous machine) ran as syncronous motor because the rotor still has excitation current. The speed maintain around 3600 rpm.

The CT normally when receive the shutdown order first open the generator breker. When the control system receive the open status of generator breaker the machine disconnect from the grid and start the cooldown process (The CT ran at 3600 rpm (sync speed)for three (3) minutes for cooling the turbine stages with air from axial compressor using natural gas just for ran on 3600 rpm. When the cooldown process finish the machine close the main natural gas and the speed decrease from 3600 rpm to 0 rpm (When the speed reach 3550 rpm the field breaker open).

According with I understand about the problem the real danger is when the generator ran as a induction motor without excitation current because the induction motor should move all axial compressor and reaction turbine. In this case according with ScottyUK the generator rotor perhaps suffered some damage by high temperature.

I have an additional question about generator ran as induction motor without excitation current:
Is possible that the machine change the rotation way (clockwise or anticlockwise) in this condition as induction motor.



Regards

Alberto J. Hung C
Caracas Venezuela

 

[byrdj]

I pulled a field than motored with out excitation many years back for others to inspect. The field forging was damaged from circulating current on its surface. This was arc spots from coil wedges to forging.

A small operator's instruction manual i looked through did not even mention motoring as abnormal, but running without excitation was listed as severe

 

[SlideRuleEra]

Eleven year ago, I was involved in the repair of a 270 MW coal-fired unit where the generator was accidentally motorized. Within a few seconds, this created a cascading failure that went something like this:

1. The unit was on turning gear. Acceleration of the (motorized) generator/steam turbine stripped the turning gear and caused other mechanical damage, throwing the rotating mass out of balance.

2. Vibration from the out of balance rotor ripped the turbine/generator pedestal anchor bolts loose (these are typically about 4" diameter, 6 feet long).

3. Simultaneouly, both the hydrogen cooling and lubricating oil systems ruptured.

4. Sparking from the motorized generator caused a hydrogen explosion, which started a lube oil fire.

Windings in both the stator & rotor were destroyed. Both had to be removed, and shipped by rail to GE for total rebuild & rewinding. The steam turbine had to be totally overhauled, on-site. The pedestal needed extensive repairs.

Repairs went well, the unit was out of service for about 18 months.

http://www.SlideRuleEra.net

 

[bradh01]

ahung,

We also have W501D5 with Brush machines. We have a FUJI generator breaker with redundant trip coils, and Beckwith generator protection.

I would be inclined to at least borescope the surface of your rotor and look for evidence of heat or arcing as byrdj describes above. Also look around the retaining ring/forging contact area.

Our breaker failure current setpoint is .5 amps (secondary). This setpoint would probably have been too high to detect your motoring current and provide a clearing trip.

We also have a directional power (reverse current) setpoint of .08 amps (secondary). However, this trips the generator breaker, which in your case was inoperative.

Your installation of a redundant trip coil is definitely justified.

 

[ahung]

Hi bradh01 you are correct about breaker failure protection We have 50BF (Breaker Failure) relay but when the motoring generator occur the current setpoint is too high for detect it and not operate "cleaning" the bus bar on 69 kV. The Central Dispatch open all circuit of bus bar and clear the fail cut the power from the grid to feed generator as syncronous motor.

The generator breakers were made by Siemens and use SF6. We contact the provider to install a redundant trip coil on our generator breakers.

Note: What are the name of your power plant and where is locate? How many W501D5 machines were installed in your plant?
Here in our power plant (AES Electricidad de Caracas Planta Osacr A. Machado)we have five W501D5 machines running at base load condition and in some cases following the demand. We use natural gas as main fuel.
May be we can interchange some information about operation & maintenance of W501D5 units.



Regards

Alberto J. Hung C
Caracas Venezuela

 

[waross]

Is it possible to install de-energize to trip coils instead of energize to trip coils. In this case a failed coil would trip the breaker. My experience is with much much smaller equippment, but I have used 1000 amp, 600 Volt breakers with de-energize to trip coils. If the trip coil fails, you cannot even close the breaker until the coil has been repaired.

 

[Chad12]

waross, are you referring to an undervoltage trip coil?

Chad Snyder
Director of Operations
American Power Technology

http://www.ampowertech.com

 

[waross]

Chad12;
Yes I am, actually. I'm not trying to nit-pick, just curious.
I know there must be a good reason.
Small plant I worked in (350 KW an 600KW diesels) used undervoltage trip coils on the breakers to trip on reverse power feed. Of course we had to pick our voltage off of the generator rather than the buss, or we would never have been able to restart after a complete outage.

 

[ScottyUK]

waross,

The tripping coils on large switchgear typically consume significant amounts of power for the fraction of a second for which they are energised. 10A at 110V DC per pole is entirely reasonable. Coils are short-term rated because they can not dissipate over a kilowatt continuously. Substation supplies are usually battery maintained, so the UV release method quickly becomes a very power-hungry solution in a reasonable sized substation. It is more practical to use redundant tripping coils and tripping circuits of the energise-to-trip type than to provide massive battery systems to support the large continuous load of numerous UVTs. There would also be the problem of having to remove a significant heat input from the substation.


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Your body might be a temple. Mine is an amusement park...

 

[swgrmfg]

Mr. Hung:

One suggestion to help avoid this problem in the future is to add trip coil monitors on both trip coils. These are inexpensive, yet very useful. They maintain a wetting current on the coils, give visual indication at the control panel when coils are healthy, and issue an alarm form-C contact on coil failure.

Good luck.

 

[GTstartup]

Didn't see this question answered.


Quote

"Is possible that the machine change the rotation way (clockwise or anticlockwise) in this condition as induction motor."


Answer is no, not during a simple motoring occurance. Also the main concern on gas turbines during motoring is forced cooling.

 

[ahung]

Thanks a lot for all advices and suggestions

Regards

Alberto J. Hung C
Caracas Venezuela