Performance Analysis of DSOGI PLL under Balanced and Unbalanced Conditions

Abstract

DPGS (distributed power generation system) gives an efficient and economic way of generating electricity by using renewable energy sources near to the load requirement. And it is the better way for reducing the transmission and distribution losses. But the process of synchronizing the DPGS system with grid is becoming one new challenge. As a consequence, the control of grid-connected power converters, grid synchronization method are very important because in order to implement stable control strategies under generic grid conditions a accurate and fast detecting method of the grid voltage characteristics is required .In actual practice the grid code requirements such as grid stability fault ride through, power quality improvement, power control and grid synchronization etc. should be satisfied by power plant operators. There are so many methods for grid synchronization , has been explained in the literature review to control the parameters like active and reactive power by tracking the phase angle of the supply grid voltage or grid current for proper synchronization of grid and DG system. In this thesis different types of grid synchronization system with DPGS system has been explained and later different types of PLL has been introduced for single phase and 3 phase system. For balanced 3 phase supply for getting better result SRF PLL is advisable, it uses the advanced method of conversion to 2 constant voltages from 2 orthogonal signals known as Park's transform and the Clarke's transform, it will take 3 phase supply as input and gives the 2 orthogonal signals as the output. But whenever the supply voltage is unbalanced the SRF PLL gives the output with the oscillating error means it is going to fail, so in case of the unbalanced supply voltage Decoupled Double Synchronous Reference Frame (DDSRF) PLL is suggested .the DDSRF PLL can detect the sequence components and positive sequence phase angle under unbalancing conditions. For generating the orthogonal signals Second Order Generalized Integrator (SOGI) is the better replacement of Clarke’s transform and it has high capacity of harmonic rejection because it can perform current controller duty and also sequence components also can be detected easily. For grid synchronization another advanced method is Dual Second Order Generalized Integrator - Phase Locked Loop (PLL) has been implemented by using Matlab simulink and LABVIEW under unbalanced conditions like sag and swell.