Stability Analysis of Grid-Connected VSC Dominated by PLL Using Electrical Torque Method

The instability issue of the grid-connected Voltage Source Converter (VSC) dominated by phase-lock loop (PLL) is investigated using the electrical torque method in this paper. The system dynamic model and steady-state relationship of variables are first established. Then, a basic two-order model representing the PLL natural oscillation mode is extracted. The impacts of the current control loop and altering current grid (ACG) on the PLL oscillation mode are derived using the electrical torque method. As a result, analytical expressions of the system synchronous and damping coefficients are obtained. Based on the derived synchronous and damping coefficient, qualitative evaluations of the impacts of the system parameters on the oscillation frequency and system stability can be achieved. It is drawn that a small PLL time constant, low infinite source voltage, heavy load, reactive power absorption, and long electrical distance will deteriorate system stability. Moreover, a sufficient analytical stability criterion for the grid-connected VSC is derived from the damping coefficient. The system stability can be directly evaluated through the operation points and system parameters, and the quantitative evaluation of the system stability is achieved. The eigenvalue analysis and electromagnetic transient simulation have verified the reliability and effectiveness of the theoretical analysis.