Research on Digital Phase Locked Method in PWM Rectifier

Abstract

Underground coal mine power grid load fluctuates violently, and problems such as reactive power, harmonic and load asymmetry are prominent. Active Power Filter (APF) and Static Var Generator (SVG) with Pulse Width Modulation (PWM) as the core are the effective ways to solve this problem, PWM rectifier is required to obtain phase and frequency information of grid voltage accurately and quickly. For traditional Synchronous Reference Frame Phase-Locked Loop(SRF-PLL), voltage amplitude and phase are seriously distorted in three phase unbalance, the performance of the PWM rectifier is degraded. the method of Decoupled Double Synchronous Reference Frame Phase-Locked Loop(DDSRF-PLL) is studied. The positive and negative reference transformation of grid voltage is carried out, and the decoupling network is constructed to separate fundamental positive and negative sequence components, and the influence of cross-coupled 2ω harmonic components is effectively suppressed, A Low Pass Filter(LPF) is used to eliminate the harmonic effect, the positive and negative sequence components of grid voltage base wave are accurately locked. The simulations and experiments are performed for DDSRF-PLL and SRF-PLL, the results show that amplitude and phase of voltage positive and negative sequence components are accurately detected under unbalanced three-phase conditions by DDSRF-PLL, which shows better phase locking characteristics.