Turbine govenor failure, intertie reverse power trip

[obut4]

Hello,

I would like your thoughts on the following situtation.

A steam turbine of 750kW (480V) had a governor failure and the steam valve was fully.
The load on the bus to which the turbine is connected was 1200 kW.
The intertie relay tripped on reverse power because active power was exported to the grid.

Questions:
How is it possible to export power to the grid if the internal load of the plant is higher than the rated power of the steam turbine power?
In case the steam valve is fully open, can the steam turbine produce much more than its rated power?

Did somebody already met this situation?

Thanks in advance,

Obut4

 

[waross]

Is the 750 kW rating the steam turbine or the generator connected to the steam turbine?
The turbine and the generator are two separate machines each with its own power rating.
If the steam turbine is capable of 1200 kW mechanical power output then the generator will probably be capable of outputting 1200 kW for long enough to trip a reverse power relay.
Does the plant load vary at all? The steam turbine generator may have been putting out less than 1200 kW and the plant load may have dipped below the output of the generator.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[collies99]

Are you able to check under normal fully loaded condition that you valve position and steam temp / pressure is the same as during the last event ? Perhaps at full load it is normally running at lower gate openning or pressure ?

Could you have an external event where the remote utility bus torque angle and bus voltage changed ?

The capability curve is only for mormal opersting condition for boiler limit and ststor heating, but as noted already the generator can produce more though not without consequence if operated pronlongly outside its capability curve.

 

[waross]

Reminds me of some issues we had with a lumber kiln. Our salesman contacted the boiler operator to determine how much steam was available to heat the proposed kilns.
Our boiler is rated at 26.000 lbs per hr, but we can get 32,000 lbs per hr.
The design of the steam coils for the three kilns proceeded on the basis of 32,000 lbs per hr.
As the construction was in progress we heard again the boast of 32,000 lbs per hr.
Then came the start-up. Gross underachievement and disappointment.
It turns out that the existing kilns had a primitive on-off control that cycled in about four minutes. The steam flow would peak at 32,000 lbs/hr and then drop to 20,000 lbs/hr.
As the flow meter was going up, the pressure was going down. Both flow and pressure were painting the charts.
And we had not been informed about the 20,000 lb/hr base load.
With a decent proportional control smoothing out the flow, (and a second controller to maintain the pressure at set point) The flow straight lined at 26,000 lbs/hr.
What does this have to do with a reverse power trip?
Unexpected transients can and do happen when normal operating parameters are changed abruptly. Boilers put out more for a short time than nameplate value.
The turbine may well have been oversized and throttled down by the valve that the governor threw wide open.
Check the specs on the turbine. HP, pressure, and back pressure. The boiler pressure may have been higher than the pressure at which the turbine was rated. If this was a back pressure turbine, a below spec back pressure would allow more than nameplate power to be developed.
Short answer to the question YES, but more information is required to say if that happened in your case.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[obut4]

Thank you for your answers.

Find attached the steam turbine datasheet.
Its potential max power is 1000 kW.
The steam conditions are also given.

According to your mails, i would say that the steam pressure is higher than rated.
During normal operation, the steam throttle valve is slighty open.
When the meachnical linkage between the turbine control and the steam valve broke, the valve went fully open.
As the steam pressure was higher than rated, the steam turbine was able to produce more power than rated, thus reverse power on the intertie.

I will check the real steam pressure/temperature and valve opening on site.

Thanks again,

Obut4

 

[waross]

As I read the turbine data I think that it s probable that your generator did develop more than 1200 kW. If the reverse power trip had not shut it down, then the generator protection would have soon shut it down on overload.
Note that it is common to rate generators at 80% power factor so a 750 kW generator operating at unity power factor will safely generate over 900 kW if the prime mover has the power available.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[collies99]

Thanks for the data sheet.

You can load the unit at max power within the limits of its rated PF as long as the stator winding temperature is keep within operating limits, that is the main criteria for long winding life. All about heat generation and dissipation.