Variable Gain Controller for Generator Governor

Thread Starter

stevesummers

Hi All,

I am a bit of a newbie on control theory and could do with some advice on an application I am looking at.

The application is for a generator governor controller that runs in speed control in an island network (I.e not droop) and will experience frequent load steps. The problem is that it’s ability to pick up load is nonlinear and depends heavily on its existing load. For example at light load (<25%) it can pickup around 25% of its rating in one step whilst maintaining its frequency limits, but at medium load (25% to 40%) its ability to manage the same step load drops off linearly, and above 40% it drops off very quickly.

Most standard governor models just use a basic PID control with the gain (and other constants) that are tuned to the specific capability of the generator. In this it would work and would end up trying to overstep the capability, soI am wondering what would be the best strategy.

My first thought would be to replace the normal gain factor with a variable gain factor set by a feedback loop monitoring the generator loading. Then I could reduce the gain factor down as the loading increases. I could use either a number of different gain values or even a group of piecewise functions.

Is this approach common? I see it referenced as a gain scheduling controller, in a few textbooks, but I have never used it before. Is it practical? I can see lots of potential problems with tuning and instability. Would this be a form of cascade control? It seems to be , but I am not sure on doesn’t seem like it as, like it meets the criteria.

CSA

stevesummers,

Speed control can take a couple of forms: Isochronous or Droop. In AC (Alternating Current) power systems the frequency of the system is directly related to the speed of the generator—which is a function of the speed of the prime mover driving the generator. The prime mover might be a steam turbine or a hydro turbine or a combustion turbine or a reciprocating engine or even a wind turbine. One controls the frequency of the generator output by controlling the speed of the prime mover. And the term for a generator prime mover energy control system is governor—because it’s primary function is to control speed—to control frequency but also to prevent over speed.

Isochronous speed control is the form of speed control that monitors prime mover speed and when it senses a change in speed it changes the energy flow-rate into the prime mover to return the prime mover speed to nominal (rated). As the load on the generator and prime mover changes the first effect of the change is to change the speed of the generator and prime mover. Isochronous speed control is usually very fast acting because frequency is important on an AC power system.

Droop speed control, on the other hand, does not really try to control the speed of the generator and prime mover. In fact, Droop speed control relies on the error between a speed reference and the actual speed to control the energy flow-rate into the generator prime mover. In the PID vernacular, Droop speed control is proportional control. Generator prime mover governors operating in Droop speed control rely on other methods to maintain system frequency and speed. In simpler power generation and distribution systems this is quite often a generator prime mover governor operating in Isochronous speed control. And, most governors presume energy flow-rate is proportional to electrical power output

With the necessary information it should be possible to implement a variable gain to obtain the tight speed (frequency) control the power island requires. Tuning could be problematic if the variable gain gets quite complicated.

I see no reason why you can’t implement a variable gain for your generator prime mover governor to characterize the ability of the control system to respond to changes in load (frequency).

Best of luck!