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Abstract
The gains of phase-locked-loop (PLL) have significant impacts on the power transfer limits for the voltage source converter (VSC) connected to weak AC system. Therefore, in this paper, an improved PLL control, respectively, with alternative damping factors for rectifier and inverter is proposed. First, it is proved that the impedance angle of AC system has a great impact on the small-signal stability of the VSC system. With the same variation tendency of Thévenin equivalent resistance, the limits of power transmission are changing in opposite trends for rectifier and inverter. Second, the improved PLL with alternative damping factors is proposed based on the participation factor analysis. Third, the optimal damping factors of the improved PLL control for rectifier and inverter are calculated. Simulations and calculations validated the following three conclusions: (1) in rectifying operation, the equivalent system resistance has a negative impact on the stability of the system and this is not the case for inverting operation; (2) adding the alternative damping factors to PLL control shows similar results compared with changing the impedance angle of AC system; (3) the proposed optimal damping factors of PLL can effectively extend the power transfer limits under both rectifier and inverter modes.
Highlights
Renewable energy resources are emerging as a future energy vector, and the voltage source converters (VSCs) are widely used to integrate such energies into power system [1,2,3,4]
This paper aims to propose an improved PLL control to extend the power transfer limitations
Simulation results show that, with the rising of impedance angle, the VSC system operating in rectifier mode gets larger small-signal stable margin
Summary
Renewable energy resources are emerging as a future energy vector, and the voltage source converters (VSCs) are widely used to integrate such energies into power system [1,2,3,4]. The conventional vector-current control in d-q frame exhibits poor dynamic performance when applied to VSC connected to it This brings a problem that the transmitted power cannot reach the ideal limitation for the unstable of small-signal model [8,9,10,11,12,13,14,15,16,17]. An important observation is that for VSCs under different operation modes the equivalent resistance (related to the impedance angle directly) has opposite effects on the stability of VSCs. An advanced PLL with damping factor is proposed in this paper to enhance the power transfer limitations of grid-connected VSCs. The rest of this paper is organized as follows.
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