Respected sir,

In our CPP there are 6 GTG, out of the four are running in preselect mode & remaining are not running. All GTGS are connected to power grid. I observed that our plant load varies with the grid frequency variation. So my question is what is the relation between grid frequency variation & plant load?

My second question is "in preselect mode how the Mark-6 set TNR (reference speed) value." What is the 96GW term used in generator control panel.

 

Regarding your first question, please clarify:

1) Presumably you have recorded the variations experienced by your local generation regarding their output "load", that is, MVA, MW, or MVAr. If so can you provide values?

2) Similarly, can you provide some idea as to the variation in bus Volts, bus Amps, and PF?

3) Are you observing a variation in just the local plant "load", that is, a variation of plant MVA, MW, and MVAr as well as plant Amps and PF?

4) What is frequency excursion in Hertz or percent?

5) Have you observed MW increase when operating at the higher frequency?

Regards, Phil Corso ([email protected])

 

I'm going to answer your questions in reverse order.

There are quite a number of very good descriptions of Droop Speed Control here on control.com. The 'Search' feature of control.com is very fast, but does take some getting used to. To search for droop speed control, you need to enter the term '+droop +speed +control' (without the quotation marks). You could also try searching for TNR ('+TNR').

I'm surprised to hear that the unit loads are fluctuating when operating at Part Load with Pre-selected Load Control enabled and active. We don't know how much or how fast the grid frequency is varying, but *in general* when Pre-selected Load Control is enabled and active the Speedtronic will automatically change TNR to try to keep the generator output (signal name DWATT, usually) equal to the setpoint (usually signal name LK90PSEL_CMD -> LK90SPEL). So when the frequency decreases, the error between actual turbine speed (TNH) and turbine speed reference increases and the power output of the generator will increase. *BUT* since the setpoint is MW, the Pre-selected Load Control function will decrease TNR to decrease load to maintain the Pre-selected Load setpoint.

Usually, when a unit is being operated at Part Load and the frequency decreases, one wants the power output to increase in proportion to the frequency decrease to try to support the load. So, Pre-selected Load Control is not allowing the unit to help support the grid frequency.

Now for the *not* in general part. GE has begun selling and providing something they call Primary Frequency Response, which will allow a unit being operated in Part Load with Pre-selected Load Control enabled to appropriately respond to frequency excursions to help maintain a stable grid frequency.

Without being able to know how the grid frequency is changing, it's impossible to say if your unit has Primary Frequency Response or just "plain" Pre-selected Load Control.

Without going into any more detail in this thread (on my part), the relationship between turbine power output and grid frequency is expressed by the Droop Speed Control setpoint (and, no, it's no a number you can find easily in the Speedtronic). If the unit has 4% droop, then every 1% change in frequency is equal to approximately 25% change in power output. If the unit has 5% droop, then every 1% change in frequency is equal to approximately 20% change in power output.

To answer the question, "How do you know what the droop setpoint is?" the quickest way is to look at the turbine speed reference (TNR) when the unit is at Base Load. Remembering the power output of a combustion turbine is a function of *many* things, if TNR is approximately 104% at Base Load, then the unit has 4% droop. If the TNR is approximately 105%, then the unit has a 5% droop.

For any more than that, see:

http://control.com/thread/1026248312

 

If your power generation fluctuates with grid frequency it means that your turbine governors are operating in frequency control mode (or droop control - which is normal).Frequency control is required by grid operators to ensure a fast response to load demand or unit generation trip on the grid. This is how it works:

Your governor system is in fact a speed controller (Proportional-only)where the PV (controller value) is the turbine speed. Since the turbine speed is proportional to the grid frequency, if a change in frequency occurs, the turbine governor will open/close valves. The interesting thing with this feature is that this control action is an open-loop action.

Let's take an example of why grid operators require large power generating units to operate in grid control:

1- A power plant shuts down (let's say the drum level tripped). This will lead to a net decline in power into the grid.
2- The frequency will start dropping.
3- As the frequency is dropping, all the governors on the grid that are in frequency control will increase their load to an amount proportional to the frequency change.
4- If you are in frequency control BUT you are at MAX load condition then you cannot participate in increasing the load.

Hope this helps,

Ben Janvier, Senior Control Consultant
Enero Solutions
514-207-5678

 

I agree with u,but in our CPP we are have 5 DG sets operated in parallel with grid,during this time our governor will be in MW control mode(and our droop setting on governor is 4%) ,so during frequency increase as I observe DG load is constant,but total plant load is increasing and ,can u clear me.........