Grid converter synchronization techniques for distributed generation systems

Abstract

The exponential growth of renewable energy resources based distributed generation (DG) systems is due to the ever increasing power demand and environmental concerns. The DGs are integrated to the grid through power electronic converters and they contribute to the enhancement of reliability and efficiency of power system. However, due to the increased penetration, DGs cause concern about stability due to their interaction with the grid. Hence, grid codes stipulate that DGs must have fault ridethrough capability and specify the range of values of voltage and frequency for which they should remain connected to the grid. Therefore, fast and accurate estimation of grid variables such as amplitude, phase and fre quency is essential. Further, these information are vital in the reference current generation for converters. In this regard, this thesis focuses on developing grid variables monitoring scheme suitable for normal and abnormal grid conditions.

This thesis presents sliding DFT prefiltered synchronous reference frame phase locked loop (PLL) for tracking the grid voltage attributes. With this prefilter, dc offset and harmonics present in the input are blocked from entering PLL. Further, the input voltage is amplitude normalized. The proposed scheme is suitable for singlephase as well as threephase applications. In the singlephase scheme, slid ing DFT acts as prefilter and orthogonal signal generator. In case of threephase scheme, with the help of sliding DFT, the instantaneous symmetrical components (ISC) method succeeds in identifying the fundamental positivesequence (FPS) component. With this capability, even during severe disturbance and unbalance, rapid and precise tracking of utility variables is achieved. Synchronous sampling is essential for the deployment of sliding DFT. However, when the grid frequency deviates from the nominal value, sampling becomes asynchronous. To address this problem, two variants of this scheme are proposed.