base load on a frame v machine

K

Thread Starter

karen

It's a hypothetical query. Request to kindly help me understand it.

A single frame v machine is put on base load. Not connected to grid. Assuming there's enough load on the process side, it's supplying, at given ambient conditions, 20MW load under a steady condition.

Now, suddenly an additional load of 10MW is put on it. Will it continue to run on temp control mode still? How will be the machine respond? Will the speed fall? If yes, then by how much? What will happen to PCD, Gas flow etc because the fall in speed will lower the air intake to the turbine!

Thanks in advance
Regards
 
Karen,

What do you think will happen, and why?

(By the way, all gas turbines will respond the same way under the conditions you describe.)
 
karen,

The reason for my question was to try to understand your knowledge of the overall "system" as opposed to just the turbine and generator.

As CuriousOne has said, the unit should "trip" on under-frequency--which is not directly related to the Frame 5 gas turbine, but to the electrical system protection.

First some definitions and clarifications. The under-frequency relay we are referring to in this discussion is monitoring the generator (and this case the grid since the unit is operating in Island mode). It's not part of the turbine control system. When an under-frequency relay operates because it detects an under-frequency condition it does one of two things: it either trips the generator breaker without tripping the turbine, or it trips the turbine which trips the generator breaker. ("Tripping the turbine" refers to an emergency shutdown of the turbine by very quickly shutting off the flow of fuel. "Tripping the generator breaker" refers to opening the generator breaker. If the generator breaker is not open when the turbine is tripped, the generator breaker will be tripped (opened) by a reverse power relay/function.)

Also, the speed of a generator, and speed of a GE-design Frame <b>5</b> heavy-duty gas turbine since it's directly coupled to the generator, is directly proportional to frequency of the alternating current by the formula F=(P*N)/120 (where F=Frequency in Hertz; P=Number of poles of the generator rotor; and N=Speed of the generator rotor, in RPM).

When a gas turbine is running at Base Load, that means it's producing as much power as it can for the current conditions (ambient temperature and humidity and pressure; inlet filter condition; axial compressor cleanliness; hot gas path component conditions; exhaust back-pressure; etc.). In the situation you describe the turbine control system should be operating in Isochronous Speed Control mode to maintain a stable frequency (speed).

If any additional load were added to the system, the speed would begin to decrease--because the turbine control system can't increase the fuel flow because the unit is already producing as much power as it can.

Unfortunately, when the speed decreases the air flow through the axial compressor decreases. If the fuel flow were held constant this would cause the exhaust temperature to increase. BUT, the governor would see the exhaust temperature increase and limit the fuel to limit the exhaust temperature increase. And as the fuel decreases the power output of the turbine decreases which causes the speed to decrease further which causes the fuel to be further reduced which causes the speed to decrease further which causes the fuel to be further reduced which causes the speed to decrease further which causes the fuel to be further reduced which causes the speed to decrease even further which causes ....

You get the idea. It's a downward spiral that gets worse and eventually ends badly if the load isn't reduced or the frequency continues to decrease.

If the load increase is very fast, I would think it would be a race between whether the unit tripped on exhaust overtemperature or under-frequency. Generally, an under-frequency condition only results in an opening ("tripping") of the generator breaker, and not necessarily in a trip of the turbine (immediate, emergency shutdown of the fuel to the turbine). But, in a condition where the unit is being driven to under-frequency very quickly while at base load if the generator breaker tripped before the turbine tripped on exhaust overtemperature the amount of fuel flow and the inertia of the turbine might not be enough to prevent a trip, either on exhaust overtemperature or a shutdown due to low speed. In any case, I don't think the turbine would continue to run if only the generator breaker opened.

Now, if you'd said the unit was operating in Island mode in Isochronous Speed Control governor mode selected and at 50% of rated load and 50% of rated load was suddenly "thrown on" the unit, then the unit should increase it's output to rated with very little change in speed/frequency. But, when the unit is already at rated load and speed/frequency and the load increases the turbine control system can't increase the power output because it's already at rated, and so the speed starts to decrease and then the downward spiral begins, and continues.

This is a dirty little secret of gas turbines: When they are at rated load (Base Load), or reach rated load in an under-frequency condition, and the load increases they can't maintain that output because as the speed decreases the power output decreases, which contributes to a decrease in the ability of the system to contribute/maintain to the load on the grid (be that a single unit operating in Island mode or a larger, even infinite, grid). Which contributes to a worsening of the situation, which makes for even more problems.

In some parts of the world, the grid regulators require special sequencing in the turbine control systems of gas turbines to temporarily increase power output when a grid frequency decrease occurs <b>and</b> the turbine is at or near rated power output. This is limited to a few seconds, in the hopes that the grid can shed some load or otherwise increase generation sufficiently to prevent further frequency decrease, but that has never really been tested with a LOT of gas turbines providing power to a large grid.

Finally, the under-frequency relay does as much to protect the turbine as it does to protect the generator (from excessive volts/hertz) and grid (from low frequency power). When the axial compressor speed is below rated, air flow decreases almost exponentially and that significantly reduces the ability of the turbine to produce power. If fuel were not reduced when air flow was reduced the turbine and hot gas path components could be severely damaged. If turbine speed ever drops below approximately 94.5% (or 14HS drop-out for GE-design heavy duty gas turbines) the Speedtronic will automatically initiate a normal fired shutdown to protect the turbine.

That's it; that's all the variables there are. Speed, air flow, fuel flow. Speed is proportional to frequency. Air flow is proportional to speed. Fuel flow is generally proportional to load, except when it is limited by exhaust temperature which is what Base Load means: exhaust temperature control (limit).

Hope this helps!

Write back if you have more questions, or you need further clarification.
 
CSA, thanks for the nice explaination.

Yes, I could intutively guess that when the turbine is on base load AND the load were increased further, its speed has to reduce. I thought, it's like driving a car on an incline with the accelerator at its maximum limit and then suddenly encountering a steeper incline. The result would be a fall in speed.

But this raised following specific queries regarding the fuel flow condition and air flow vis-a-vis the temp control curve and also the proportion of speed drop:

1) Will the speed drop be random OR proportional to the droop line? Or How much will the speed drop be? For a machine configured for 3%, 4% and 5% droop, when subjected to similar condition, will the speed drop be proportional to the droop setting? (I have this query because in my understanding, at base load, the turbine isnt bothered about load - it just tries to maintain the combustion temp. So will the turbine follow any 'logic' while decreasing the speed?)

2) As you explained, with the fall in speed, the air flow will reduce. With the fall in air flow, the temp increases. With the increase in temp, the governer will begin to close and reduce the fuel flow further.

But when the whole thing is seen from the angle of temp control curve, the sequence seemingly reverses. With the fall speed, the air flow reduces. With the reduction in air flow, PCD will fall. With the fall of comp discharge pressure, at base load, the temp control set point will go up, following the standard exhaust temp vs comp discharge pressure line. Hence, more fuel is likely to be pumped in (Because Temp set point is higher at lower PCD).

So what will the turbine control block look at? Temp set point based on PCD OR just the temp alone?

Thanks in advance
Regards
 
Karen,

The exhaust temperature control curve can be misleading. Remember: The exhaust temperature control curve represents a constant firing temperature. And, it has a negative slope, which means when the unit is operating at Base Load that as CPD increases the exhaust temperature will decrease for the same firing temperature.

Yes, this is counter-intuitive and opposite of what happens when the unit is not at Base Load; as CPD increases so does exhaust temperature. But, then firing temperature is also increasing as CPD increases and exhaust temperature is increasing when fuel in increasing at less than Base Load.

Now, to the issue of how much speed will decrease when the unit is operating at Base Load and load increases. No; I don't believe it will be proportional to the increase in load--mostly because when the unit is operating in Island mode the speed and air flow will be decreasing. The IGVs will remain fully open, but air flow through an axial compressor is not proportional to speed; not at all.

I don't believe the speed decrease can be predicted under your conditions. It's not like a unit operating on Droop Speed Control in parallel with many other units on a large or infinite grid. Under this condition, the speed drop is predictable--for units operating at less than Base Load (i.e., Part Load). But under your stated conditions, with the unit already at Base Load when the load suddenly increased by 50%, I don't think the speed decrease can be predicted without a lot of knowledge about the machine (inertia; axial compressor characteristics; turbine characteristics; inlet air temperature; fuel flow; etc.) and the rate of load increase.

When a unit is operating at Base Load the turbine control system is always trying to control firing temperature by looking at CPD and exhaust temperature. If speed is at rated and constant, the turbine control system will dump as much fuel as it can make the actual exhaust temperature equal to the exhaust temperature reference. When the speed is decreasing (or increasing for that matter) the turbine control system will still be trying to make the actual exhaust temperature equal to the exhaust temperature reference. The power produced by the turbine will be whatever it will be while the turbine control system is adjusting fuel to make the actual exhaust temperature equal to the exhaust temperature reference--regardless of speed/frequency.

You might find it easier to plot exhaust temperature vs. CPD while the unit is loading. As CPD increases while loading, the exhaust temperature does indeed increase. So, as CPD increases, load increases. When the unit is operating at Base Load, as CPD increases load increases. And as CPD decreases, load decreases. The difference is hat happens to exhaust temperature. When the unit is loading exhaust temperature increases as CPD increase--but then so is firing temperature. However, when the unit is at Base Load, the firing temperature is not changing as CPD changes, but the exhaust temperature is--decreasing as CPD (and load) increases.

It seems strange, but that's the way it is. Don't stress about it. You'll drive yourself crazy.
 
Top