Design of Double Decoupled Phase-Locked Loop of Microgrid and Smooth Switching Control Strategy for On-grid and Off-grid Connection

摘要

The control methods of microgrid are generally divided into micro-source level control, system level control and scheduling level control. Based on the equivalent structure of the AC microgrid, the transient mode of the AC microgrid switched from off-grid to grid-connected is proposed, the dual-decoupled phase-locked loop of the microgrid and the smooth control strategy switching from grid-connected to off-grid are proposed. The phase-locked loop is a key link for extracting the voltage amplitude, frequency and phase of the grid. In the case of grid voltage asymmetry, the phase-locking accuracy of the traditional phase-locked loop is insufficient. Under the grid voltage distortion state, the accuracy of the extracted grid voltage phase, amplitude and frequency is poor. A new phase-locked loop structure based on double decoupling is adopted so as to eliminate the double frequency disturbance of the grid voltage and improve the tracking accuracy of the phase-locked loop. The voltage amplitude, frequency and phase are the three key parameters switching from grid-connected to off-grid. So a new grid-connected pre-synchronization controller consisting of voltage regulator, frequency regulator and phase regulator is designed. The voltage and frequency operating values at the AC bus-bar divide the microgrid operation into four intervals, which can be better used to judge the switching condition between off-grid and grid-connected of the microgrid. The simulation experiments show that the proposed dual-decoupled phase-locked loop and the off-grid and grid-connected smooth switching control strategy of the microgrid can effectively reduce voltage impact of microgrid during off-grid and grid-connected switching process.