Abstract
To address the instability issues of LCL-type wind power inverter's operation under weak grid conditions, stability control methods are investigated in this paper. Based on an inverter impedance model that considers an inner current control loop, a precise impedance model that considers dynamic characteristics of dc bus voltage and a reactive power outer loop is created in this paper. The phasor diagram method and an analytical method are used to interpret frequency characteristics of the inverter output impedance. The impedance analysis results show that the instability of the inverter in a weak grid has a weak correlation with the LCL filter parameter and a strong correlation with the inner and outer control loops. Phase-locked loop bandwidth reduction improves inverter's operation stability to some extent. However, it also reduces the inverter's tracking and anti-interference performance. Therefore, a capacitor current-feedback-based stability control method is proposed in this paper. The feasibility of the proposed control method is proven via general Nyquist-criteria-based impedance analysis. Finally, an RT-LAB hardware loop test platform is created to verify the validity of impedance modeling proposed in this paper and the effectiveness of the stability control strategy.
Highlights
Renewable energy such as wind power and photovoltaic power has grown rapidly in recent years, and its proportion in power systems grows year over year.1 As a power transformation device, safe and stable operation of a grid-side converter has a significant impact on the connection of the wind power generation system to the grid.2 A voltage source inverter normally adopts the vector decoupling control based on the phase-locked loop (PLL’s) voltage-oriented effect.3In a stiff grid, the inverter provides fast and stable current to the grid and is characterized as an ideal current source
As the proportion of renewable energy connected to the grid increases continuously and distributed grid-connection scenarios increase, the electrical support capacity of the grid declines and the grid gradually becomes weak
Because the entire control algorithm is executed in a two-phase rotational dq coordinate system, PLL in a synchronous reference coordinate system is used to observe the grid voltage phase h for rotational coordinate transformation between the three-phase sinusoidal quantity and the two-phase dc value
Summary
Renewable energy such as wind power and photovoltaic power has grown rapidly in recent years, and its proportion in power systems grows year over year. As a power transformation device, safe and stable operation of a grid-side converter has a significant impact on the connection of the wind power generation system to the grid. A voltage source inverter normally adopts the vector decoupling control based on the phase-locked loop (PLL’s) voltage-oriented effect.. Operation characteristics of an inverter in a weak grid should be analyzed further, and the corresponding stability control methods should be applied. The PLL frequency stability of a wind power inverter and the suggested positive feedback effect of an inverter’s output current on the PLL structure were analyzed.. An inverter’s output impedance model in the DQ frequency domain that considered high-order characteristics of the LCL filter was created, and a power-voltage control method to improve the inverter’s operation stability in a weak grid was proposed. An inverter’s output impedance model that considers the inner current control loop, outer dc bus voltage-reactive power control loop, and PLL’s dynamic characteristics is established. A capacitor current-feedback-based stabilization control method is proposed, which effectively improves LCL-type wind power inverter’s operation stability in a weak grid.
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