DESIGN OF LINEAR AND NONLINEAR CONTROLLERS FOR GRID INTEGRATED PHOTOVOLTAIC SYSTEM

Abstract

The grid integration of photovoltaic system has gained popularity among researchers in recent past. In this thesis report a detail discussion on two stage grid integrated photovoltaic system has been provided. The working of Photo-Voltaic (PV) cell, its mathematical modelling has been discussed briefly. The stable operation of PV array and maximum extraction of available power under varying environmental condition is ensured by the first stage in which DC/DC boost converter has been used. A neural network based Maximum Power Point Tracking (MPPT) algorithm has been designed for instantaneous determination of Maximum power Point (MPP). A dual loop Proportional-Integral (P1) controller for the boost converter has been designed to fulfil the objective of fast dynamic response. The power generated by the PV array is transferred to the grid by the second stage, which is a single phase grid connected Voltage Source Inverter (VSI). In order to control the active and reactive power supplied by this VS[ a synchronous frame proportional-integral (P1) controller has been designed. Due to the frame/coordinate transformation, the design procedure of this controller is complex and hence increases the cost and is difficult to implement. Therefore a stationary frame proportional-resonant controller has been designed to replace the previous controller. These controllers are designed for the small signal linear model of VSI, which doesn't consider the inherent nonlinearities of VSI. Hence stability and performance of these controllers an't be guaranteed over wide range of operating points. Therefore a nonlinear controller based on inputoutput linearization has been designed, which linearizes all kind of nonlinearities converting the grid connected VSI into a reduced order linear system. In order to ensure the stable internal dynamic, a linear controller for the internal state has been designed and included in the network VA for calculating reference signal for the new linear system. A proportional resonant (PR) has been designed for the new linear system. The reference signal has been calculated by using synchronous reference frame components of inverter output current so that active and reactive power flow can be controlled as per standards. The complete control scheme has been implemented in Matlab Simulink environment in order to examine its performance.