Automatic Grid Control Functions

I

Thread Starter

ITT

How do advanced/automatic grid control functions in combined and simple cycle power plant? it seems easy in simple cycle that demand is divided into multiple running gts, but want to understand controls. however, not control guy so this is my assumption.

Thanks in advance
 
ITT,

What do you mean by "advanced/automatic grid control functions"?

The big difference between simple- and combined cycle power plants is that the steam turbine load is a function of the gas turbine load. For example, if the steam turbine is rated for 100 MW, and each of the two gas turbines (in this example there are two GTs and one ST), one can't start the steam turbine before one of the Gas turbines has been started. And if only one of the gas turbines is running the steam turbine load can't be increased much above 50 MW--and if only one gas turbine is running at 100 MW the steam turbine can only produce about 50 MW maximum.

The basic "rule" is the steam turbine in a combined cycle power plant can produce about half as much power as a gas turbine. So in our example the two gas turbines can produce 200 MW and the steam turbine can produce half as much--100 MW.

In a combined cycle power plant steam production is related to gas turbine exhaust flow, which is a function of gas turbine load. The steam turbine is considered to be a "follower." Most combined cycle steam turbine control systems just open the steam turbine control valve full open once above a minimum load which means the steam turbine load is "controlled" by how much steam is produced by the gas turbines. In a combined cycle power plant the steam turbine is just there to make use of the exhaust heat from the gas turbines. More exhaust heat, more steam; less exhaust heat, less steam. So, while one has more direct control over the loads of the gas turbines, the load of the steam turbine can't be so easily controlled since steam production is related to gas turbine exhaust flow/load.

There are some slight variations on this depending on plant design, but if the heat used to make steam has to come from the gas turbine exhaust the load on the steam turbine is a function of gas turbine load and exhaust heat.

As for grid frequency, well, that's basically the same for all power plants synchronized to a grid. When multiple power plants are synchronized together they are effectively a single power plant, in the same way that all motors and lights and TVs and computers and computer monitors are a single load to the power plant(s). To provide power at some frequency all the power plants have to be producing power at the same frequency--that's called 'synchronism.' And that happens when the total amount of generation equals the total amount of load. If one power plant increases the load it is producing but the total number of motors and lights and TVs and computers and computer monitors doesn't change and no other power plant reduces the amount of power it's producing then the grid frequency will increase until either some power plant reduces its load OR the total number of motors and lights and TVs and computers and computer monitors decreases. It's a balancing act to keep the amount of generation equal to the amount of load to keep the grid frequency at rated. Some grid operators are better than others.

But that is all done outside of the individual power plants--regardless if it's a simple- or combined cycle power plant.

Hope this helps!
 
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