Speed Hunting

V

Thread Starter

VKGUPTA

Dear CSA,

Thanks for your all valuable replies asked by me related to Gas turbine. You are doing best job on global forum which is helping to many peoples. One problem i am facing in our machine.

Gas turbine Frame-V with control system Mark-Vie is running on 4 MW load. Normal speed is 5120 RPM.Inter valve FPG2 pressure: 15 Kg/cm2.

Problem: machine speed is hunting 5100-5120 RPM intermittently (2-3 Times in two days). subsequently inter-valve pressure also hunting 14.1 to 15 Kg/cm2 and inter-valve pressure low alarm L30FPG2L_ALM appeared. Speed/load raise/lower command went as per speed signal.
Load remains constant.

63FG calibrated and found ok.

Kindly guide me.
 
VKGUPTA,

You have a GE-design Frame <b>5</b> heavy duty gas turbine with a Mark VIe control system.

Is the unit a generator drive?

Is it connected to a large grid, or does it supply a small load independently of a large grid?

Because, when a generator is connected to--synchronized with--a grid with other generators the speed of ALL generators connected to the grid--synchronized together on the grid--is a function of the grid frequency. So, if the unit is connected to--synchronized with--a grid and the unit speed is fluctuating then the grid frequency is fluctuating.

The Stop-Ratio Valve is also called the Speed-Ratio Valve and even the Stop/Speed-Ratio Valve. That's because when the unit is running it controls the pressure between the SRV and the GCV (Gas Control Valve), which is called P2 pressure, and it does so in proportion to the unit speed. So, if the unit speed is unstable, then the P2 pressure will be unstable. If the unit speed is stable, then P2 pressure will be stable.

The magnitude and frequency of the speed variations aren't known, but, if the gas fuel supply pressure isn't sufficient then the pressure could drop when the valve opened suddenly, as well as rising quickly when the valve closed suddenly.

So, the thing to know about speed variations when the unit is synchronized to a grid with other generators is this: Your unit is a slave to grid frequency, and if the grid frequency isn't stable then your unit's speed isn't going to be stable. And that's just a fact that many people don't--and fail to--understand. They think their machine speed (and load) should ALWAYS be stable regardless of whether or not the grid frequency is stable.

Now just think about that for a few minutes. What they're proposing is that generators can run at different speeds (frequencies) when they are connected to a grid. So, if the grid frequency is 50.47 Hz their machine should run at 50.0 Hz, and if the grid frequency is 50.47 Hz that all the other machines should also be at 50.0 Hz. That just can't--and doesn't--happen. If that could happen, then it wouldn't be necessary to have synchronization circuits and relays, and synchronization wouldn't be as critical as it is. But synchronization is VERY critical, and ALL machines synchronized to a grid operate at speeds that are directly related to the frequency of the grid.

Hope this helps! It would seem there was some kind of grid frequency disturbance during the time the unit's speed was unstable. Which in turn led to an unstable P2 pressure (because P2 pressure is related to speed).

But without more information, nothing more can be said.

Thank you for the kind words.
 
Thanks you sir for your reply.

Our machine is connected with generator drive and using as a captive power for internal power of plant.

It is not connected with grid.
SRV inlet pressure is also remains constant during speed variation.
 
Kind of an important detail, that bit about being an island power system....

Is the unit operated in Droop speed control or Isochronous speed control? If the unit is operated in Droop speed control where does the speed/load reference signal come from? Does it come from some kind of PMS (Power Management System) that sends signals to the Mark VI to change load (speed reference) to maintain frequency? If so, what was happening to that signal during the frequency/speed disturbance?

What was the load in the plant doing at the time of the frequency/speed disturbance?

What was the GCV doing during the frequency/speed disturbance?

Were their any alarms (Process or Diagnostic) annunciated before or during the disturbance, other than the one mentioned?
 
V
Dear CSA,

Machine is operating in Droop mode only.

During speed hunting, load will be normal 3.7 MW and at that time no activity is being carried out.

L70L alarm also appeared during speed hunting.

Speed is reducing from this L70L, same time operator increased the speed manually by pressing the L74R4R_CPB (speed/load manual raise).
Other than this no alarm is generated.

During speed hunting time, below parameters are vary

TNH Speed in RPM
5100 to 5119

FSR Fuel stroke reference in %
20.5 to 22.2

FPG2 Intervalve stage pressure in bar
14.3 to 16.3

FSG GCV position in %
20.7 to 22.5

FSGR SRV position in %
15.5 to 17.2

MW Load in MW
3.62 to 3.67

TTXM Exhaust temp in deg
348 to 351

We suspected from SRV inlet side. We have flushed all pipe lines and replaced the filters in Skid network during machine shut down condition. After that also speed hunting observed.

Walkies talkie is not using inside the Access compartment.
 
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