Research On The Application Of Three-Phase SPLL In SAPF

Along with the widely used of power electronics and non-linear loads, harmonic pollution is increasingly serious. Active power filter has become an important device for harmonic control on account of its excellent performance. Since harmonic current detection module has an essential influence on the performance of APF, it should be able to timely and accurately extract harmonic current in various grid voltage conditions. Harmonic current extraction based on ip-iq algorithm is less affected by grid voltage and widely used in applications. But the above-mentioned algorithm depends on the phase angle of grid voltage, which requires phase-locked loop to detect the real-time phase angle of grid voltage. However, due to influence of various factors, the waveform of the actual grid voltage is usually in non-ideal state, or unbalanced and distorted conditions in other words. So, it's necessary for PLL to detect the real-time phase angle of fundamental positive sequence voltage accurately under unbalanced and distorted utility, which ensures the accuracy of harmonic current detection.This paper introduces the structure and working principle of PLL firstly, and analyzes the working principle of three-phase synchronous reference frame software PLL in ideal grid voltage, then gets the equivalent mathematical model. The performance of SRF-SPLL is analyzed based on the above-mentioned model. Secondly, the representation of three-phase unbalanced voltage in the positive and negative dual synchronous reference frame is introduced, and the decoupling method of unbalanced voltage in DSRF is analyzed. The q-axis component of unbalanced voltage fundamental positive sequence in positive SRF is obtained by decoupling network, and the block diagram and equivalent structure of decoupled double synchronous reference frame software PLL are further analyzed. Then, the unbalanced grid voltage phase is accurately detected. Thirdly, the unbalanced and distorted three-phase voltage is analyzed. The repetitive controller is added to SRF-SPLL which forms RC-SPLL. The structures of RC-SPLL and repetitive controller are designed and analyzed. The simulation model of RC-SPLL is built using MATLAB/Simulink and its performance under various input voltage conditions is simulated and analyzed.In order to compare and analyze the actual performance of PLL, a SAPF experimental platform is designed and established. It includes start-stop control unit, power conversion module and the main control unit. The data transmission between DSP, dual-port RAM and AD7606 is designed, and the software of both DSP is completed. Using SAPF experimental platform, the performance of each PLL under different input voltage is tested, and the experimental results are compared and analyzed. The harmonic current compensation effect of SAPF using different SPLL is measured, and the impact of SPLL on the performance of SAPF is given according to experimental results.