Abstract
A less-intrusive solution to stabilize a voltage source converter (VSC) over an unknown grid strength is presented in this article. The existence of equilibrium point is investigated as a prerequisite to stabilization. By partially imposing grid forming control, a simple auxiliary outer loop is proposed to exhaust the physical limit of power delivery in steady state and provide support to fault-ride-through (FRT) operations over a wide range of grid strength. The proposed control can be used to upgrade a commissioned VSC with inner current loop intact and overcome the impact of negative voltage sensitivity; it also offers a robust nonintrusive solution to stabilize VSCs externally. The effectiveness of the proposed schemes is verified by analysis in frequency domain and simulations in electromagnetic transient (EMT) time domain considering change of grid strength and FRT.
Highlights
Indexed by Short Circuit Ratio (SCR), the grid strength of a power network can be variable, which can impact the integration of a Voltage Source Converter (VSC) [1]
In order to analyze the performance of a vector control based VSC in weak grids, a benchmark system is established in Fig. 1 as shown, where the classical average converter model is used in this paper [20,21,22,23,24,25,26,27,28]
Due to a power loop, the active power recovers 0.9 p.u. after the transient, but the voltage Vc drops from 1.015 p.u. to 0.998 p.u., which implies that an increase of reactive power to the grid is decreasing local voltage so the system is operating in an dQ/dV
Summary
Indexed by Short Circuit Ratio (SCR), the grid strength of a power network can be variable , which can impact the integration of a Voltage Source Converter (VSC) [1]. It is subject to dedicated design of gain scheduling scheme, which is of high complexity when grid strength is highly variable [25] For both PLL and outer loop approaches reported, the access to the internal control of the main VSC is essential [17,18,19,20,21,22,23,24,25,26]. The proposed scheme can provide an improved performance with the following benefits altogether: (1) robustness against unknown grid strength over a wide range (from SCR = 0.9~∞) including the region of dQ/dV
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