We have a 25 mw extraction cum condensing turbines and we have a low load limiter which comes in picture when machine is on speed control (droop control) otherwise same is deactivated when we are on load control.

This low load limiter is interfering with speed control and is brining down the mw generation to it's limit, i.e. 3 mw and this is happening whenever we change from load control to speed control.

The output of speed/load control & load limiter are connected to a MIN selector block and whichever control is having the lower output controls the machine.

 

A couple of questions (or rather food for thought):

1. Was this set-up functioning and lately it stopped functioning properly, or is it a new set-up which never functioned properly?

2. Are the signals reaching the MIN selector directly or inversly proportional to main parameters being measured?

3. I would say that if the signals are directly proportional, you need a MAX selector rather than a MIN selector block. Inversly proportional signals need a MIN selector block. What does your setup really need?

4. Are the controls of the electronic or pneumatic type, and especially if of the latter, are things set-up properly?

5. In your last sentence you stated that whichever control has the lower output takes control. In view of points 2 and 3 above, is your reasoning correct?

6. One final question, why isn't the load limiter in action also when you are in load control? What limits the steam turbine from being set at a too low load setpoint?

 

Many steam turbines in combined cycle power plants are not operated in droop speed control, or, if they are, it is the rare condition and usually with some restrictions imposed by the extraction system which could be the reason for the 3 MW limit, which could be some kind of minimum limit instead of a maximum limit.

Are you trying to control frequency with the steam turbine?

If you have Droop Speed Control, there should be some method for changing the turbine speed reference, buttons or switches or targets on some operator interface to raise or lower the turbine speed reference. Have you tried changing the turbine speed reference after enabling droop speed control? What happens?

I would agree with jojo's queries: Was this working previously and now has ceased to work? Or, is this some mode of operation which is being used for the first time or for the first time in a very long time?

You say "load limiter"; is this value a constant value or some variable value? If it's variable, what are the conditions for changing the value? You also talk about speed/load control (which is sometimes how droop speed control is described), but in your case are the inputs to the MIN select block separate? Is there an input for speed control and an input for load control and an input for the load limiter?

 

I will try to make you understand my system

a> There is a MIN selector block to which are connected three PID control outputs.

b> These PID's are speed controller ,Load controller(MW) & Low load limit controller.

c> STG is started on speed control and after synchronisation the control is transferred to load control with operator given MW setpoint.

d> When STG is on load control and Generator breaker is closed then low limit controller output is forced to 100% by the logic and hence remains inactive.

e> The speed controller output tracks the load controller output and changes it's setpoint accordingly.

f> When operator transfers the control from load to speed (which is very rare)the STG comes on speed control and this is a bumpless transfer due to tracking feature.Now operator can change the speed setpoint to change the load accordingly.

g> But as soon as m/c comes on speed control and generator breaker is closed the third PID load limit controller becomes active and starts decreasing it's output and when it's output is less then speed controller it takes the control and starts reducing the load on machine to it's operator variable setpoint i.e. 3 mw.

h> operation is keeping this load limit setpoint as 3 mw since commissioning and nobody knows from where this value has come.

i> We are very rarely operating on speed control so this problem was never noticed ,recently when one of the operator took it on speed control this 3mw low load limit was noticed .

j> so I wanted to know for what purpose a low load limiter can be defined in STG.

k> another thought I am having is that can it be a high load limiter because if we keep it's setpoint on higher side then same would not interfere with speed control.

can anyone discuss on load limit in STG

 

In view of our comments,

1. There could be the possibility that what you are thinking is a low load limiter is actually a high load limiter. I don't know exactly what is the mechical setup of your STG apart from the fact that it is an extraction turbine.

The problem with extraction turbines, is that you canget the same shaft power output to generator from different flows in the various stages of the steam turbine. The extremes are you get X MW with the turbine running in fully condensing mode, and thus taking mechanical work from all the turbine blades, or else you go practically to full extraction, get the same X MW, but in such a case, only the blades that lie between the inlet to steam turbine and the extraction point are doing the work. Thus as you can imagine in the latter case, the steam turbine blades which are producing the work, are really being driven hard, with respect to the case when the steam turbine is running in fully condensing mode. In such a situation I would expect some sort of instruction which limits STG power output with respect to extraction (STG manufacturer normally give limiting curves for such operation) and these have to be used in conjunction with your load limiter. In such a case the load limiter would act as a maximum limiter.

2. On the other hand, steam turbines normally also have minimum limiters, as by the nature of the steam turbine, when this is run at very low loads, with respect to its rating, exhaust temperatures start to rise (even though condensor is running properly) resulting in extreme cases of trip out due to excessive exhaust hood temperatures. So in such a case your limiter would be a minimum limiter.

3. Is your STG connected to a conventional boiler or is it running in combined cycle mode? In addition what is the primary function of your STG, provide constant power to the grid, or maintain extraction pressure/flow constant to support a process? I would see some difficulty in trying to achieve both simultaneously.

 

Hi,

Apart from driving the turbine, steam flow also doing the cooling function to the steam exhaust path.

Let imaging that the Turbine is in speed control mode, under certain situation the inlet valve might be closing in order to ensure that speed of the turbine is not above the setpoint.

The load limiter 3MW is the minimum value that ensure the minimum steam flow is going to the exhaust path for cooling.

Regards,

PRAN Sammsanti

 

In response to tclmarkv, it would seem that the 3 MW "limit" is just the low value of a PID control, which has some kind of operator adjustable value (at least that's what it seems like from your discussion). Has anyone tried increasing the setpoint to the "Load Limiter" PID controller?

I would agree with PRAN Sammsanti that the 3 MW is probably a minimum limit to prevent heating up the exhaust. Most people don't realize it, but as he says, steam flow actually "cools" the turbine by preventing overheating due to windage (spinning with low or no steam flows). There are some large steam turbine-generators (units rated at 600 MW) which can't be operated at less than 200 MW for more than 30 minutes because there is insufficient steam flow to prevent overheating. The units can only be operated for extended periods from 200 MW to 600 MW, and will trip if load goes below about 160 MW. They will also be tripped at loads between 160 MW and 200 MW if operated there for anything longer than 30 minutes. Start-ups have to be "perfect" because the boiler trips when the steam turbine trips, and it takes a crew of people and about 8 hours to get it started again and ready for a very quick roll-up of the steam turbine to get above 200 MW.

 

To original Poster:
I think the 3 inputs (Are really two one is defeate) to MIN selection are:

1. speed control/load control

2. your extraction control (this got 0 when you put load control on active, otherwise this controls the load)

3. output of load limiter, actually as soon as TG are synchronise to grid it goes out of service and setpoint is not given in MW but % it could be 3% of valve lift.

some manufacturer call it valve lift reference and is used to put speed ref setter to manual mode during rolling (valve feedback <2% and man PB pressed....)

speed ref setter goes out of manual mode as soon as GB is closed. simultaneously its output tp your first mentioned MIN block is also blocked.

what is happening in your case is this output is reaching MIN block.

-You should check GBi/p contact to to s-R flipflop (reset point)

hope it should help

regards
Adi