Reverse power

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Thread Starter

Shaheed ul Islam

Dear Phil Corso
Sir

I would like to know if during exporting power to the utility steam throttle valve of my turbine get closed but the outgoing breaker not opened for any reason. The generator will become a motor running on reverse power supply from utility,

1- will the direction of rotation of turbine be reversed with reverse power supply.

2- What damage can lead to this reversal.

Regards
Shaheed
 
Shaheed...

The primary purpose of the Rev-Pwr relay is to protect the Steam Turbine, not the Generator!

The generator will continue to supply the same kVAr component of line-current to the utility! However, its magnitude will be insufficient to operate any Over-Current relays!

But, the kW component of line-current will substantially decrease and its direction reverse, causing the Rev-Pwr relay to operate!

Answer to Q1):
No! The turbine's rotation does not change!

Answer to Q2):
Failure of the generator's CB to clear the condition is a serious threat to the turbine. That is, the low-steam flow condition and subsequent turbulence can cause overheating of the blading! Continued operation in this condition can lead to catastrophic damage!

Answer to Q3):
Although it wasn't asked; typically the event is cleared by "back-up" protective relays!

Regards,
Phil Corso
 
If I may,

I would like to add to Phil's reply and concern that the primary PROTECTION is for the turbine.

the reverse power LOGIC is recommended in verifying that a turbine trip has indeed reduced steam flow to the turbine BEFORE the generator breaker is open. thus a several second time delay is used following a turbine trip before the generator breaker is opened. This logic was recommended to reduce the probability of an overspeed event due to stuck open turbine valves.
 
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Shaheed ul Islam

Sir

more clarification is requested about your answer to Q-2. If the turbine throtal valve get closed during say 10MW power export to utility but the out going breaker not opened for any reason.
You have told that:

1- it is low steam flow condition.If the valve 100% closed how this condition would be considered as low steam flow?
The direction of turbine rotation does not change in reverse power condition. It means during motoring condition through utility electric supply the turbine blades will rotate in the same direction freely as they were rotating by steam before under torqued condition.

2- I need some more information that what thing will create turbulence that can cause over heating of free rotating blades? Your more guidance is requested please.

In practical I faced this condition when the throttle valve open sensor got faulty the turbine was turning in motoring condition and the reverse power relay failed to issue breaker open common. Load on meter was displaying -ve 400KW for about 20 seconds.

Regards
Shaheed
 
Shaheed...

the generator was operating as a Synchronous Motor turning the turbine grid-frequency. Actually the generator operates like a Synchronous Condenser, that is, Low kW, but probably pre-disturbance kVAr!

During the disturbance, that is, until the Rev-Pwr relay operated, the inlet-steam flow was negligible thus eliminating the cooling effect of the turbine’s blades, thereby exposing them to over-temperature!

And now three questions:

1) What protective-action eventually separated the generator from the grid? If it was the Rev-Pwr relay after 20 seconds, then the delay is intentional!

2) What is the rated capacity of the prime-mover?

3) What was the pre-disturbance load on the generator

Phil Corso
 
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Shaheed ul Islam

Sir

The steam temperature is 500 deg C. I think may be wrong thinking that when steam is not entering to blades by closing throttle valve the temperature of blade would be drop down as steam is not a cooling media of blades so temperature of blades should not rise.

Ans to Q1- The operator closed the grid VCB manually during motoring period when he saw that KW is not increasing in +ve direction.

Ans to Q-2 rated capacity of prime mover is 12MW.

Ans to Q-3 pre disturbance load, During starting export to utility the gen set start hunting from 4MW to 9MW +ve and sudden mechanical trip lever of hydraulic relay tripped while sensor of trip lever was already faulty.

Regards
Shaheed
 
again, if I may add to Mr Corso comments.

to discuss motoring, I use the illustration that once the generator becomes a motor due to reduction in steam flow, the turbine then becomes a compressor (a poorly designed compressor).

the turbine blading heating is mainly for the last stages of a condensing unit. thus the better the vacuum the less amount of windage heating will occur for the last stage blading.

as long as the generator stays synchronized, its design does not mind being motored, however increase the temperature of last stage turbine blades will cause a catastrophic failure in a short time.

the time it takes the last stage blades to over heat depends greatly on the vacuum they are spinning in. If the vacuum is near absolute, then very little windage heating and the unit may be motored for several minutes with not damage. with the vacuum in the normal to alarm range, the motoring should be keep less than a couple minutes.

I do not know what the motor recommendations would be on a non condensing turbine.
 
JFB...

the typical setting for a steam-turbine is about 3-5% of the rated kW of the turbine! For a non-condensing machine it's somewhat higher when operating against atmospheric or higher exhaust pressure!

However, in my experience development of the Trip-setting and Time-delay is usually a collaborative effort by the facility owner's rep, the facility design engineer, and the turbine supplier!

Shaheed… I also believe that "because Lightning is more likely to strike twice in the same place" you should develop a tripping scheme to cover the incident you experienced!

Regards,
Phil
 
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