Abstract
The stability of grid-connected inverters is very sensitive to varying grid impedance, especially in the weak grid condition. In this paper, based on the Lyapunov method and with the micro-grid impedance identification, an active damping adaptive control strategy is proposed to guarantee local asymptotic stability for an LCL inverter system. First, using the backstepping design and taking the state variable errors as new variables, an appropriate Lyapunov function is constructed, so that the asymptotic stability control law can be obtained. Then, with this asymptotic stability controller, taking the varying impedance into account, an adaptive controller is developed using the Lyapunov direct method. Hence, the controller presented here can assure asymptotic stability, the system performance can also be improved instantly according to the adaptive control, and the expected stability and performance can also be achieved. Finally, the feasibility of the proposed control strategy was verified via a simulation and an experiment.
Highlights
A grid-connected inverter (GCI) with an LCL filter is widely used in the renewable energy generation (REG) systems [1,2]
To widen the stability region of a GCI under a wide-range grid impedance, an auto-tuning high-performance multiloop control scheme was employed in [24], but for sinusoidal inputs, the zero steady-state error could not be obtained because of the PI controller used in the outer loop
2) Second, as the impedance parameters can be identified online, a local asymptotic stability adaptive controller is developed carefully through a combination of backstepping design and the Lyapunov direct method, so that asymptotic tracking of the reference signal can be obtained and the plant state can tend to a desired value
Summary
A grid-connected inverter (GCI) with an LCL filter is widely used in the renewable energy generation (REG) systems [1,2]. For a GCI system under a weak grid, to ensure robustness against impedance variation, many control methods [5]-[19] have been proposed to achieve large stability regions, such as feedforward of the grid voltage at the point of common coupling (PCC) [8][9], a phase-locked loop(PLL) [10]-[12], damping of LCL resonance [13]-[15], and a current controller [16][17] among others[18]-[21]. A solution for the instability problem caused by changing grid impedance is presented in this paper, where additional active damping is not needed, which can greatly simplify the controller design. The state space equation of the inverter can be written as follows: dx
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