We have two gas turbines; control system of one is Mark-II (TG-701) and other is Mark-V (TG-702). Currently Mark-II is on “ISO” while TG-702 is on “DROOP”. We are facing load variation problem. TG-702 picks load of 1.5 MW and TG-701 drops load of 1.5 MW simultaneously. Pressure transmitter is installed on inlet fuel gas of both turbines. Inlet Fuel Pressure of TG-702 shows increases up to 400 NMC and TG-701 shows decreases up to 400 NMC simultaneously. We conformed that no heavy machine / motor start or stop at that time.

We took real time plot of fuel gas from TG-702 (Mark-V), detail of observation is as below.

• FAG (GCV Servo current) shows almost constant reading while FAGR (SRV Servo Current) varies up to 4.0 units approximately.
• GCV_FDBACK and GCV_OUTPUT show almost constant value while minor changes observe in FSGR, GRV_OUTPUT and P2_Pressure.

From above mentioned detail, we come to conclude that SRV of TG-702 might be suspected. Please share your knowledge about that problem. Also suggest what other things we should look in such load variation problem.

 

Is this a normal operating condition for your site: one unit operating in Isochronous Mode and the other operating in Droop mode?

If so, has this condition just started?

Are both gas turbines supplied from the same gas fuel source? (This author isn't familiar with a unit of measure NMC--newton meter carrots??? Whatever happened to kPA or barg or atm???)

You say there's a gas fuel supply pressure drop on the unit which picks up load, and a pressure increase on the unit which drops load; do the pressures equalize again after some period of time?

When a GE-design heavy-duty gas turbine is operating at rated speed (and rated frequency if it's a generator drive), the Stop/Ratio Valve (SRV) has a constant reference--NOT a constant position, but a constant reference. The reference is the pressure to be maintained between the SRV
and the GCV (Gas Control Valve), usually called "P2 pressure." If the GCV opens to increase load, the tendency is for the P2 pressure to decrease, but the Speedtronic tells the SRV to open to maintain P2 pressure constant. As the unit is loaded, and the GCV opens, so does the SRV. (The PREsumption here is that the gas fuel supply pressure remains within limits and doesn't vary, in other words it's fairly stable, say, +/- 20 psig (pounds-per-square inch, gage).

Conversely, as the GCV is closed to reduce load the tendency would be for the P2 pressure to increase, but the SRV is closed to maintain the P2 pressure reference.

Now, it makes sense that there would be very slight pressure changes with changes in gas fuel flow, especially if the gas supply to the two units wasn't coming from a common supply, or there was some kind of orifice between them, or a great distance.

There is usually a slight deadband on frequency/speed control for units operating in Isochronous mode. For most Speedtronic units, it's approximately +/- 0.13%, in other words, the speed frequency can vary from 99.87% of rated to 100.13 % of rated. Mk II controls were analog control systems, and used pots (potentiometers) in analog circuits to set these limits. Drift and misadustment have been known to occur in analog circuits... It wouldn't have to be a large change in load to occur to cause the isoch unit to respond unusually intermittently--especially if it occurred when the unit was near one of the limits of the deadband.

How much is frequency varying at the site--maximum, peak-to-peak?

The servo-valve output current of a Speedtronic turbine control panel doesn't behave like most people expect it to--in other words, it is not proportional to the reference. In fact, it only changes when there is an error between the reference to the device and the feedback. When the reference and the feedback are equal, the servo-valve output is steady and unchanging. In reality, the output of the regulator is actually zero milliamps when the error between the reference and the feedback is zero--but there is a null bias current added to the output to overcome the force of the null bias spring in the servo-valve.

When you say the FAGR changes by "4 units" do you mean 4% or 0.4% or 0.04% (since the value is usually expressed in percent with two decimal places?) (What's the problem with units of measure here???)

When you witness a slight change in FAGR does it return to normal or does it stay at the new level?

If it doesn't return to normal and remains at the new level, it may mean that the SRV is sticking, or requiring more force to move to or remain at the required level. If the GCV position isn't changing when the SRV position is changing, then a sticking SRV could be the culprit. If the SRV closes slightly and the GCV remained at the same position, the flow through the GCV would be reduced--and also the load of the unit would decrease. Which would cause the Isoch unit to pick up the load.

What's happening to the P2 pressure of TG-702 when the unit load is reduced and TG-701 picks up the load?

Sometimes,there could also be a problem with the SRV LVDT (Linear Variable Differential Transformer) feedback which could cause the problem. The SRV regulator uses LVDT feedback in the control loop (see a recent thread for more information).

There are two functions of AutoCalibrate which can be used to check for sticking valves/actuators or bad LVDT feedback. 'Verify Current' and 'Verify Position' are the two functions. One function outputs a constant current to make the valve move and plots valve position vs. time; the other outputs a variable current in order to make the feedback change at a constant rate. Of course, the unit has to be shut down to perform these tests, and the gas fuel must be isolated and bled off before the SRV is stroked by AutoCalibrate to perform these tests. Also, an AutoCalibrate of the SRV LVDT feedback must have been recently performed (i.e., since the last time the <Q> processors were re-booted).

But, it does indeed seem that there may some problem with the SRV--either the actuator or the valve stem (sometimes they get scored and cause the rod or stem to stick where they pass through the packing--which can usually be seen by examining the actuator rod or the valve stem), one of the LVDTs having some kind of "jump" or "dead spot" in its feedback at some position, or even the servo-valve.

Write back and let us know what you find.

markvguy

 

Sorry for delayed reply as I was away from my office. The problem is still persisting. In response to your queries here are the answers:

• It is our normal operating condition that we run one turbine on ISOH and other on DROOP. Periodically we change the mode of both turbines.
• The condition started from few days back. Otherwise both were running normal with no load variation.
• Both turbines are supplied with same fuel source. NMC is the unit used for fuel gas flow. NMC stand for Normal Meter Cube. Approximately flow is 9000 NMC for 12.4 MW.
• Flow of TG-702 shows increases up to 400 NMC and TG-701 shows decreases up to 400 NMC simultaneously.
• There is minor frequency variation, which is negligible. It is varying between 50.01 to 50.04 Hz.
• FAGR (SRV Servo Current) varies up to 4.0 % approximately.
• The plot of FAGR shows that it continuously varying, i.e. going up to 4.0% and comes back to normal line.
• In normal condition P2 remain almost constant.

The frequency of problem occurrence is reduced from last few days. How can we find out which turbine is basically initiating this load shedding problem? Is there any technique to pinpoint the exact turbine to isolate the problem?

 

If you switch the DROOP/ISOCH units, does the problem go away?

You seem to be pretty convinced that the problem is the unit with the Mk V, but that may be because data is more readily available than on the Mk II-equipped unit, right? Do you have some knowledge/experience that would cause you to believe the SRV of the Mk V-equipped unit would be suspect? Prior history of problems with the valve?

One needs to be aware that servo-valve currents (such as FAGR - Fuel, Current - Gas Ratio Valve) are only reported to the CDB (Control Signal Database) at the rate of 4 Hz (or, 4 times-per-second), yet the regulator on the TCQA card changes the current 128 times per second to make the actual P2 Pressure equal to the P2 Pressure reference.

You refer to a single value of FAGR. If you're only looking at a plot of FAGR, you're only looking at the median value of FAGR, as determined from a value that's only reported to the CDB (for display/logging/trending) four times a second. Note that 1% servo-valve current is equal to 0.1 mA (10 mA equals 100% servo current). So, 4% is equal to 0.4 mA. Remember: There are three processors supplying current to each of the three coils of every servo-valve, and it's not unusual for the amounts of current to vary amongst the three processors even when the unit is operating at a steady-state condition.

As was mentioned previously, there are two features of AutoCalibrate which might be helpful in determining if the problem is really the Mk V-equipped unit or the Mk II-equipped unit.

While it's unusual for this load swapping to occur, at least the frequency is being maintained and the units are continuing to run.

What happens if you try to increase the load on the Mk V-equipped unit? Does it load at a normal rate? Does the load increase suddenly, with a corresponding load decrease on the Mk II-quipeed unit? Have you tried increasing the load on the Mk V-equipped unit above the load it was running at before it lost the 1.5 MW? Or below what it was running at when it lost the 1.5 MW? The thought process here that either there's some problem with sticking valve stem/actuator at that particular load or with the LVDTs at that particular load (and SRV position).

It makes perfect sense that the fuel flow decreases on the unit which loses load and increases by the same amount on the unit which picks up the load.

You say that P2 pressure on the Mk V-equipped unit stays pretty constant under normal conditions; exactly what does it do during this condition? How much does it change? How long does the pressure stay lower (it is presumed it's decreasing since fuel flow is decreasing and load is decreasing)? Does an operator reload the DROOP unit back to it's original, pre-variation value?

Much thought has been given to your scenario, and it's difficult to say how to prove it's one unit or the other. If you can say for certain that the Mk V-equipped unit experiences a drop in P2 pressure and fuel flow and load BEFORE the ISOCH unit experiences an increase in load and fuel flow, then you can isolate it to a particular unit. But, again, while it's unusual to see load swings like this, they are not affecting frequency or stability--though if left unchecked they might.

Sometimes it's very difficult to pinpoint a problem when it's minor, and much easier to do so when it grows in magnitude. You may just have to wait to see if the problem gets worse (you say it's getting better) or just decide that you have determined the problem is the valve actuator or the valve stem or the servo-valve or the LVDTs the SRV of the Mk V-equipped unit and start analyzing these things further.

markvguy