parallel operation of generator

S

Thread Starter

Sagar

We are having 18MW captive power plant to fulfill requirement of power and steam in company. This TG set will run in LOAD mode parallel with GRID.

Lets assume that we are running with 14MW on our TG set when there is sudden tripping of load e.g. 07MW then our TG set will continuous run on 14MW and additional load of 07MW will pump into the GRID.

Now in the GRID there are so many TG set running parallel with LOAD mode so my question is which TG set will reduce this load of 07MW??
 
Sagar,

Hmmmm....My understanding of the definition of a captive power plant is one that is isolated from any grid and supplies only the power required by "the plant"--the power plant is "captive" to "the plant" and does not supply any power to a larger grid.

It seems you are saying that a captive power plant is planning on synchronizing to a grid with many other generators and their prime mover.

Another place where I'm having difficulty understanding is the rated capacity of the captive power plant, and the load of the power plant.

Anyway, let's try this for an explanation. A generator that is synchronized to a larger grid with other machines is usually operated in Droop Speed Control mode--even if a load setpoint is used to control the amount of power being produced by the machine. If the load of the grid (which at this point includes the load of the plant to which the power plant was captive before being synchronized to the grid) decreases the machines operating in Droop Speed Control will, for all intents and purposes, maintain their load. It is the job of the grid operators to sense the drop in load (even if it comes from the plant that was formerly supplied exclusively by the captive power plant) and adjust the output of one or more generators over which they have control so as to maintain the frequency of the grid (which is the frequency of all the generators connected to the grid).

On any grid, if load drops and no generator reduces it output by an amount equal to the change in load then the grid frequency will increase by an amount that is proportional to the percentage of decrease relative to the total load on the grid. If the grid is VERY large, 07 MW could be a very tiny amount, so the frequency increase could be infinitesimally small. This, in fact, does happen. In some parts of the world, the grid regulator agencies have websites that have real-time graphs of actual grid frequency. It's very interesting to watch these graphs when there are large load changes on the grid--such as in the morning when people wake and go to work, and then in the afternoon when lots of businesses close, and then later in the evening when people turn off their TVs and lights and go to bed. Sometimes many generators must be started and stopped in order to keep the grid frequency at or near rated, or the loads of many more generators must be increased or decreased to help keep grid frequency relatively constant. (In some parts of the world, grid frequency control isn't very good at all.)

Now, if you're unit is operating in LOAD mode, we don't quite understand exactly what LOAD mode is or how exactly it operates on the generator's prime mover at the soon-to-be-formerly captive power plant. If the grid to which you are connected is fairly large, and you are operating your unit in LOAD mode, meaning it is trying to control power output to maintain a load setpoint (18 MW???) and the load on the grid (of which the plant that was exclusively supplied by the formerly captive power plant) drops then it's likely that the formerly captive power plant will remain at 18 MW (if that is the load setpoint). If the breaker connecting the plant (including the formerly captive power plant) opens and the generator's prime mover governor does not switch quickly to Isochronous speed control mode and the plant load is less than 18 MW then the frequency of the plant (and the once-again captive power plant) will increase until some operator changes the load setpoint to match the actual plant load, or until some operator switches the generator's prime mover governor to Isochronous speed control mode.

So, if I understand the post and the information provided, if the formerly captive power plant its plant are synchronized/connected to a larger grid, if the plant load drops the output of the formerly captive power plant will not drop--whether it's operating in LOAD mode or Droop speed control mode. That is, unless the formerly captive power plant can be controlled by the grid regulators via some remote load setpoint scheme.

Does this answer the question?
 
Because of the vibration in the turbine we have to restrict the load on the TG set.

The total load demand in our company is 17 MW. We draw 3MW from the GRID. My question is that we haven't change the set point of turbine load at the time of tripping of 7MW. At this time instead of drawing 3MW we will supply 07MW to GRID. so we can say that there will be reduction in the load demand of the generators connected in the GRID.
But all the generators must be running in the fix load set point. I am confused which TG will reduced the load.
 
Sagar,

All the generator prime movers do NOT need to be running in the fixed load setpoint. And, only those generator prime movers whose output is above zero and below rated AND are operating in Droop Speed Control mode will respond.

Think about a very large grid, with hundreds of generator-sets. (A generator is really a very stoopid device; it doesn't have a governor or control system to limit power (only voltage). It's really the generator's prime mover governor (control system) that's at the heart of all this. Generators just convert torque to amps; and the amount of amps is a function of the amount of torque.) If the load changes by 07 MW and there are even 50 or 100 generator prime movers operating in Droop Speed Control mode, that load change is going to be split amongst those units, so the change in grid speed/frequency is going to be very, very little--especially if the capacity of those 50 to 100 generator prime movers is MUCH more than 07 MW.

If you haven't checked out any of the other speed control threads on control.com, look for the ones with the bicycle analogy. Bicycles convert torque to speed--keep the torque constant and the speed stays constant (when the road being ridden in flat and relatively smooth--and for the purposes of our understanding the road is flat and smooth).

Hope this helps!
 
Top