Control strategy without phase-locked loop based on coordinate transformation for three-phase AC/DC converter

A novel control strategy without phase-locked loop (PLL) for the three-phase AC/DC converter is proposed in this study. There is no need to detect the phase angle of the grid and the defect of inaccurate phase-locking can be remedied. The frequency of the grid can be detected by a frequency tracking method. A linear changed angle from -π to π with the detected grid frequency is generated by the DSP controller to take part in the coordinate transformation. The grid frequency tracking method is realised by negative feedback of reactive power. It can ensure that the frequency of the linear changed angle is the same as the grid frequency. The principle of the proposed control strategy is analysed in details. Compared with the control strategy with PLL, the proposed control strategy does not need complex PLL and it can realise high power factor and low total harmonic distortion in the non-ideal grid. The structure of the proposed grid frequency tracking method is established and the parameters of it are designed. An 18 kW DSP-based bidirectional three-phase AC/DC converter is established. Simulation and experimental results verify theoretical analysis.