Research on the stability caused by phase-locked loop for LCL-type grid-connected inverter in weak grid condition

For grid-connected inverter, the current injected into utility network has to be synchronized with the grid voltage, and Phase-Locked Loop (PLL) is widely used as an efficient approach to detect grid phase angle. The inverter uses detected grid phase angle to generate synchronized current reference, thus PLL itself acts as a part of the control loop of the inverter and it will certainly affect system stability. This article focused on the single-phase LCL-type grid-connected inverter, and presented the small signal model of PLL. Meanwhile, the impedance-based stability criterion was developed. Based on the criterion, it was pointed out that in stiff grid, where the grid impedance can be neglected, the stability of the inverter will not be affected by PLL. But if the grid is weak, PLL has strong effect on the stability of the inverter, and the effect mainly depends on the bandwidth of PLL, load condition and power factor of the inverter. In order to ensure the stability of the system, a method which design the control parameters of PLL based on phase margin requirements was also proposed in this paper. The experimental results were given to verify the validity of theoretical analysis presented above. © 2014 Chinese Society for Electrical Engineering.