Study of inverter-interfaced wind power generation system under balanced & unbalanced grid voltage conditions

Abstract

This thesis presents the control of grid-side converter (GSC) for wind turbine based power generation system under balanced and unbalanced grid voltage conditions. For that Doubly-Fed Induction Generator (DFIG) with two back-to-back bi-directional voltage source converters are considered and are examined in the laboratory experimental set-up through DSP TMS320F2812 digital processor platform. The major role of GSC is to interact with the utility grid. Moreover, the control of GSC is necessary to meet the following bjectives 1): control of active power delivered to the grid, control of reactive power exchanged between the DFIG and the utility grid, control of the DC-link voltage, power quality improvement and grid synchronization etc. Grid-Side converter (GSC) control is an obvious choice to optimize the power quality problems, harmonic compensation and full reactive power support to the grid by reactive current injection during grid abnormalities. Under balanced condition, voltage oriented control (VOC) algorithm is studied in the grid-side converter (GSC) in order to control the active and reactive power of the grid independently through control of id and iq components of the grid currents. However, the converter performance largely depends on the applied current control strategy in the inner control loop. In this study, there are three current controllers are projected namely; SVPWM with Proportional Integral (PI) current controller, hysteresis current controller (HCC), and adaptive hysteresis current controller (AHCC) and their performance is analyzed during steady state and transient conditions. SVPWM with portional Integral (PI) current controller is implemented in the synchronous rotating reference frame (dq reference frame) control algorithm. HCC and AHCC are implemented in the natural reference frame (abc reference frame) control algorithm. Further, a comparative study is made between the three current controllers, in order to ensure the sinusoidal current injected into the grid. The performance indices include THD of the grid current, DC-link utilization, dynamic response etc. In addition to that, control of grid-side converter control is also studied during unbalanced grid voltage conditions. During unbalanced condition of the grid voltages, operation of grid side converter is decisive due to failure of detecting exact grid flux position for voltage oriented control (VOC) algorithm; unless an independent control for the positive and the negative sequence is implemented. In this study, detection of positive fundamental component and negativesequence component are achieved by phase locked loop (PLL) using double rotating reference frames to transform the input signal. The control strategy applied to grid-side converter is implemented in stationary (αβ) reference frame using fast acting proportional-resonant (PR) current controller. The effectiveness of the techniques studied in this dissertation is demonstrated by both simulation and experimental results based on TMS320F2812 digital signal processor platform.