Steady-state Analysis of Voltage Source Converter (VSC) Connected to Weak AC System and Optimal Control Strategies

The power transfer limitation of voltage source converter (VSC) connected to weak AC system is restricted by three main elements: steady-state characteristic, small signal stability, and transient stability. In this paper, according to steady-state analysis, constraint condition and mathematical model of VSC system are established to figure out the appropriate operating point. A novel outer-loop control diagram is proposed. Meanwhile, a small signal model of VSC system is established. Based on participation factor analysis, the dynamic performance of phase-locked loop (PLL) have significant impacts on the system stability assessment. Therefore, an advanced PLL control appending damping factor is proposed. Simulations validated the following conclusions: 1) The optimal AC voltage of point of common coupling is associated with the equivalent AC system voltage, impedance, and the current limitation. The novel outer-loop controller can enhance the power transfer limitations. 2) The small signal stability of VSC can be enhanced by appending an alternative damping factor control on PLL. The advanced PLL has robustness which can endure a great active power disturbance in AC system. Simulations and calculations have been performed to validate the findings.