Signal reforming algorithm based three-phase pll under unbalanced and distorted conditions in the grid

Abstract

Fast and accurate estimation of the grid voltage phase angle is crucial for grid synchronization of power converters, flexible AC transmission systems (FACTS) devices, power line conditioners, regenerative drives, uninterrupted power supplies, distributed generations and energy storage systems. Several phase-angle estimation methods, such as- Fourier analysis, frequency locked loop, notch filter, least-error squares, and demodulation, have been developed for phase-angle estimation. Although these methods are accurate at nominal frequency, they require modification of design when the frequency is altered. Synchronous reference frame phase-locked loop (SRF-PLL) is the simplest and the fastest in phase-angle estimation of purely sinusoidal balanced three-phase voltage system. But its accuracy decreases when the system is not balanced or is affected by the presence of harmonics. In this thesis, a signal reforming algorithm based three-phase phase-locked loop (PLL) has been developed to estimate the phase angles of an unbalanced and harmonics affected three-phase voltage system by combining a harmonics attenuation algorithm, a signal reforming algorithm and the conventional SRF-PLL algorithm into a single PLL system. This technique attenuates the harmonic components from the three-phase signal and then reforms the unbalanced three-phase signal into a balanced one. This makes the conventional synchronous reference frame PLL (SRF-PLL) suited for accurate estimation of the phase angle of one phase. The phase angles of the other two unbalanced phases are also calculated accurately without using additional PLLs. The developed PLL works perfectly even when the frequency of the system is varied. The developed PLL has been tested in MATLAB/Simulink environment for different types of imbalances and distortions. The harmonics attenuation technique is simpler and faster. The signal reforming algorithm is effective in all unbalanced conditions. The developed PLL algorithm not only solves the phase estimation difficulties of the conventional SRF-PLL working under harmonics affected unbalanced grid conditions, but also avoids the use of three single-phase PLLs to estimate the phase angles of a three-phase voltage system.