Hello every body,

i have been working on the GE MS9001E and i have a cyclic alarm which is appeared during the unit is at BASELOAD:
DWATT is too low to support TNR-TNR lower

it's worth mentioning that it generally appears on all the units at the same time, but rarely it may appear on one or two units out of four
can any one help me?

thank you very much

 

sherif elzoghby

That's the ONLY alarm I've never completely understood. I think it's primarily used on units with DLN combustors (do the units at your site have DLN combustors?). When I was working for GE I could never find a person who could explain exactly what the alarm was trying to indicate and how an operator or technician should respond to the alarm.

I believe what it's trying to do is inform the operator and technician that the Turbine Speed Reference (TNR) is "excessively" high for the load (DWATT). For example if the load is, say, 109 MW, and the corresponding TNR is, say, 104.9%, that is not typical of a machine with 4% Droop Regulation.

And, is this alarm annunciated when there are also frequency disturbances occurring?

And, Or, do you get fuel from different sources that has varying BTU content?

 

sherif elzoghby

> For example if the load is, say, 109 MW, and the corresponding TNR is, say,
> 104.9%, that is not typical of a machine with 4% Droop Regulation.

The above statement was referring to a typical Frame 9E with a typical Base Load rating of 120 MW, and a droop regulation setpoint of 4%.

Sorry for any confusion!

 

Thank you CSA for your precious help

you are right NO One can explain the reason even the GE technical assistant member.

about your question - NO my combustors are standard mode NOT DLN
and the fuel supplier is the same in all cases (National Grid)

 

sherif elzoghby,

What is happening to the generator frequency when the alarm is occurring?

 

Hello CSA

i didn't notice any unusual variable during this alarm and the speed within (100% to 99%)

 

sherif elzoghby,

100.what to 99.what?

 

100% of the turbine speed (3000 rpm)
to about 99% of the turbine speed

 

0.5% high or low might cause this alarm. Fractions of a percent can make a lot of difference in Speedtronic turbine control panels when it comes to alarms (especially nuisance alarms).

Precision is important in technical discussions; 100% to 99% (a 1% change) is not very precise when that means a 0.5 Hz change in generator frequency. I'm trying to understand if the grid frequency is stable and by how much it might be changing when this alarm occurs, because you say it sometimes comes up on more than one unit at a time....

 

I have seen this alarm come in many times in the past on our site, But we “took care” of it a few years ago. It is caused by sudden drops in frequency on units with constant settable droop and kicks the unit out of preselect or base load mode and issue a lower signal for as long as the condition is true.

The condition is caused by TNRL-TNH > 0.7% for more than 5 seconds. Actually it is designed to prevent FSR windup on dropping gas fuel pressure where the load starts to drop off and the constant settable droop will cause TNRL to increase and the FSR to wind up, possibly ending up with exhaust temperature trip is the gas pressure(and therefore load) suddenly recovers.

The side effect is that if frequency suddenly drops it has the same result, and GE put in block on that, L30AFL, but it is set too low at 49.25 Hz to be of any use (The alarm usually comes in way before the frequency drops to that level)

We changed the constants as follows to make the event much more rare.

TNKERRLIM increase from 0.7 to 1.2 %
K3TNRERRX increase from 5 to 10 seconds

We also added an additional sequence to monitor the stop speed ratio valve position and if it is over 90% then cancel Base or preselect load and also issue a load lower signal for as long as it remains at or over 90%. This provides the same protection in a more direct manner in my opinion.