Fuzzy logic control of a unified power flow controller for wind farm applications

Due to the stochastic nature of the wind, the power fed into the grid from a wind farm is variable. This leads to a highly nonlinear variation in the terminal voltage. This variation depends largely upon the energy penetration of the wind farm, the load density at the wind farm node as well as the transmission line characteristics. A unified power flow controller (UPFC) is considered as a means of providing wind farm reactive power and maintaining the voltage of the local grid. To simplify the discussion only the control of the series branch of the UPFC is discussed in some detail. A classical control approach might be to use simple P+I controllers for both the series and shunt branches of the UPFC. However, the linearisation of power electronic devices plus the characterisation and the nonlinear nature of a wind farm makes it difficult to model accurately. Fuzzy logic provides an attractive alternative. It has a fairly straightforward theory and implementation. It does not necessarily require mathematical modelling of the system under control, but, the system dynamics need to be appreciated before the controller is designed. In the study the series branch of the UPFC is under fuzzy control whilst the shunt branch remains under P+I control. This paper is provided as a tutorial for fuzzy control as much more sophisticated forms of fuzzy (e.g. adaptive) and classical control (robust, optimal) methods are available.