Load sharing between STG and Utility Grid


Thread Starter


Hi All,

This post is about understanding load and speed set point for STG while working in island operation and working in parallel with grid.

I have 3X16.75MW (2W +1S) STG running in parallel with grid or sometimes in islanding operation. I want to understand the set points to be sent to governors and AVR in both conditions. I have ECS (Electrical Control System) for performing the load sharing system.
Hi buddy

Can you share more details about your system and explain a bit more about what you need... would be great ;)

Droop Speed Control, used for operating prime movers and generators synchronized to a grid with many other prime movers and generators, is how units are typically controlled.

From the information you provided, I suspect the Power Management System is over-riding the governors of the prime movers in both grid and islanded operation, sending either a variable speed reference signal to each governor or sending discrete increase-and decrease signals to each governor to change the turbine speed reference.

When a governor is operating in Droop Speed Control mode when synchronized to a grid with many other prime movers and generators, as one increases the speed reference the load of the generator will vary (presuming the frequency of the grid or the island is stable and not changing)--but the speed and frequency will remain relatively unchanged. When a prime mover is operating a generator that is NOT producing any power (Watts; KW; MW) at rated frequency both the prime mover and generator are said to be at 100% speed. When the generator is synchronized to a grid or island that is operating at rated frequency and the speed reference is increased above 100% then the load being produced by the prime mover and generator will increase. So, speed reference and load are related. I presume the PMS is programmed to load or unload specific units in a particular order during grid operation in response to some load setpoint.

When your units are separated from the grid and operating as an island, the PMS is likely programmed to send speed reference changes to the prime mover governors, which are operating in Droop Speed Control mode, in a particular order in order to keep the frequency of the island at or near rated as load changes. This is in contrast to using one prime mover the control island frequency as load changes (its governor would likely be in Isochronous Speed Control mode) and the other units operating with their governors in Droop Speed Control mode.

There are MANY threads about Droop Speed Control mode on control.com which can be accessed using the 'Search' feature of the site. It's important to understand that when AC power is being generated that prime mover and generator speed are directly related. When a prime mover and generator are synchronized to a grid with many other prime movers and generators the speed of all prime movers and generators is directly related to grid frequency and as long as the grid frequency is stable then the frequencies--and speeds--of all the synchronized units will also be stable. Watch the speeds of the units at your site as the loads of the units at your site are changing when synchronized to the grid--they will be very constant, because their frequency--and speed--are controlled by the gird frequency. Changing their speed reference doesn't actually change their speed--bit it does change their load(s).

When operating in island mode, some control system or governor has to be controlling speed--and frequency--of the island as load changes. If the load of all the prime movers and generators remained constant as the island load increased the frequency of the island would decrease, and the speeds of the units would also decrease. So, the PMS does something to at least one of the units governors to maintain island frequency. On a well-maintained grid with many prime movers and generators the grid frequency is stable and load changes don't have to much of an effect on grid frequency, not like on an islanded system.

Hope this helps! Research past threads on control.com, and if you have questions--we'll try to help clarify and answer them.