Multivariable control analysis of a hydraulic turbine

Dinorwig is a pumped storage hydroelectric power generation station which is used for frequency regulation on the national grid. Its control system consists of separate PID governors for each of the six 300 MW turbine-generators. Accuracy and speed of response in counteracting supply frequency disturbances is improved by increasing the PID gains but recent experience has shown that this can sometimes lead to oscillatory behaviour. At Dinorwig, water is supplied to the turbines by means of a common tunnel from the upper lake which is then split by a manifold into six separate penstocks for individual machines. This paper uses a linearised multivariable model of two machines, coupled through a non-elastic water column, to show that this reduces the system's stability margin and that PID parameters tuned for optimum single machine operation are not suitable when two (or more) machines are in use. The Direct Nyquist Array method is then used to design a control system, consisting of a decoupling compensator and conventional PID controllers, which reduces the interaction between the two machines. The results show that some reduction in the speed of response must be accepted during multi-machine operation but that, whilst the multivariate controller is more complex to implement than the current individual governors, it yields a better stability margin and reduces interaction between machines.