Currently i am facing a serious issue in my plant (20 MW STG). what happens is whenever grid cuts in low voltage, TG comes in speed control mode. the speed goes on decreasing and plant trips in low freq. In the same time governor opens the valve for full steam inlet, but speed continues to decrease.

I have tried to tune the gov response, but no benefit. The steam control valve seems to be working fine. I have taken trial by manually isolating the grid from my TG. it works good. but some times when grid fails in low voltage, it makes everything black.

Is there any thing to check with electrically? Or in AVR ? Or with Grid fluctuations?

Pls help

 

from just the information provided, I sounds like you are not increasing your boiler steam production with the increasing demand of the steam turbine's governor opening the valves

>.. a serious issue in my plant (20 MW STG)
...the speed goes on decreasing...same time governor opens the valve for full steam inlet, but speed continues to decrease. ..manually isolating the grid from my TG. it works good. <

 

Duh!.. I was not really thinking about the info provided.

IF you are connected to a grid with load greater than your unit, it sounds like you are being pulled down and your increased contribution of MWs is insufficient to support the grid load. In such a case the best you can do is separate from the grid to maintain your island load and monitor the grid frequency to determine when it is safe to reconnect.

this is outside my experience area, but I do recall utilities/stations having a grid underfrequency response plan that had freq levels where they would start shedding load to the extent they would just island their plant.

 

Thanks for reply, I want to add. Steam production is also enough to take the load (As no observation in steam pressure decreased). but then also it is not able to cope up the load and speed is decreasing. I have thought of everything in control from instrumentation side, is there any thing about electricals?

 

Bhavin/BJM,

If you're certain that the boiler can supply sufficient steam to maintain the isolated load, then do you know if the control system switches to Isochronous speed control mode automatically when the grid tie breaker opens?

That would explain why the speed is dropping (frequency is dropping), and if allowed to drop too much then under-frequency relays could trip the generator breaker.

When the grid tie breaker opens then the steam turbine governor should automatically switch to Isochronous speed control mode (if it's the only generator-set supplying the isolated load). Then, if the steam supply is sufficient and the load does not exceed the rating of the turbine then Isochronous speed control mode should very quickly adjust the steam flow to control frequency (speed) and everything should be alright.

If it switches 95% of the time to Isochronous speed control mode, and doesn't switch 5% of the time but remains in Droop speed control mode (and though they are both "speed control" modes they are NOT the same!) that would explain why it's intermittent. Perhaps the 5% of the time it fails to maintain frequency and blacks out it's not switching to Isochronous speed control mode--or, the isolated is much higher that the other 95% of the time and the Droop governor and the operators can make adjustments fast enough to prevent tripping on under-frequency.

If the steam turbine control system can't get enough steam into the turbine fast enough when the grid tie breaker opens then the speed (frequency) will decrease, and eventually the unit could trip on under-frequency. It's not likely something electrical (on the high-voltage side), because load is steam flow (in your case). Load is ALWAYS related to energy flow-rate into the generator prime mover. More energy flowing into the prime mover, more amps (and speed--when isolated from the grid); less energy flowing into the generator prime mover, less amps (and speed when isolated from the grid).

It's important to know if the steam turbine control system is always switching to Isochronous speed control (which it should be for more stable and automatic control of frequency), or if it's only switching some of the time. Because, if it's not switching to Isoch and is remaining in Droop speed control mode when isolated from the grid then the operators are going to have to very quickly increase the steam flowing into the steam turbine to be able to hold frequency--and sometimes that works, and sometimes it doesn't.

Hope this helps! Remember: Load (and speed when isolated from the grid) is related to steam flow (in your case). And, when you're isolated from the grid and if the steam turbine is the only generator-set supplying the load then it has to very quickly vary the steam flow to be able to maintain frequency. Droop speed control isn't usually fast enough; but Isochronous speed control should be--if it's tuned correctly.

 

> grid cuts in low voltage
&
> manually isolating the grid

I guess I still don't understand the event.

does "grid cuts" mean that you separate from the grid via automatic protection?

and is the "manual isolating" a separate event of a test you perform to check the governor or is it an action taken during the "grid cuts" events?

 

I agree with CSA - I think your governor is not seeing that the turbine is operating in island mode (have you checked that all breakers are used to send the information on ISO mode?)
If you go island mode than it is PHYSICALLY impossible for the turbine to slow down except if:

the governor does not know it has to control frequency (island mode)
The governor is not fast enough for repositioning the V1 steam valve to balance plant power consumption with steam turbine consumption

Not having enough steam will not cause your turbine speed to slow down. For example, I was in a startup on a 50 MW cogen project. We were testing the ISO mode and the boiler was unable to maintain steam production for the additional power needed to maintain 60hz: to maintain the turbine at 60hzm, V1 was sucking all the steam it had to and drove the turbine inlet pressure down. We never tripped on the steam turbine but came close on boiler drum level.

Conclusion: check your turbine governor setup and how ISO mode is triggered.

Ben Janvier
Enero Solutions