Research On Non-PLL Control Strategy Of Three-phase Bidirectional Converter

With the shortage of fossil fuels and increasingly serious environmental pollution, the application of the new energy has become a pressing matter of the moment, especially solar and wind power. Three-phase bidirectional converters play an important role in the interface between renewable energy and the grid, the phase-lock loop(PLL) technique has been used as a traditional way of synthesizing the phase and frequency information. However, the grid voltage unbalance and distortion may exist, so the calculation of phase angle may have errors. Therefore control strategies without PLL of three-phase AC/DC converter become one of the hotspots.Firstly, the current research status and trends of PLL and non-PLL control strategies of three-phase AC/DC converter are introduced. The paper presents research on the influence of the conventional PLL performance by considering different grid conditions and calculates the errors of phase angle. 18 kVA three-phase dual-buck converter prototype based on DSP digital control is built. The paper proposes two SVPWM control strategies without PLL for three-phase grid-connected inverter: Direct Power Control Strategy and Direct Current Control Strategy and builds the mathematical models to verify the feasibility. The simulation and experimental results show that the non-PLL strategies can accurately and rapidly extract the positive sequences from unbalanced and distorted grid voltage compared with PLL strategy, the operation capability of the system under non-ideal grid voltage is enhanced. These non-PLL strategies are also summarized and compared by detectable quantity, applicability, complexity and dynamical response. Analyzing the effect of frequency deviation, the paper proposes a frequency locked loop(FLL) technology based on second-order generalized integrator(SOGI), build the mathematical models in the frequency-domain, and analyzes the speed of the dynamical response. The simulation and experimental results show that SOGI-FLL just need one integrator with negative gain to lock the frequency under unbalanced and distorted grid voltage and frequency deviation. Finally,the paper compares SOGI-FLL strategy with traditional PLL strategy under real grid voltage to verify the superiority of SOGI-FLL.