Resonance Damping for an LCL filter Type Grid-Connected Parallel Inverters Using Active Disturbance Rejection Control

Abstract

In this dissertation, the resonance damping for an LCL filter type single and multi-parallel grid-connected inverters is proposed. For reducing the switching harmonics, an LCL filter is more commonly integrated into the grid-connected inverter. However, the inherent resonance in an LCL filter may cause instability of the system if it is not handled properly. Generally, it is mandatory to damp the resonance when the ratio of resonance to sampling frequency is above 1/6 or else the whole system is unstable. In the literature, there are several methods to damp the resonance with proportional-integral control classified as an active and passive damping method. The active damping is preferred generally due to no compromise over filter effectiveness and efficiency of the system. However, active damping may increase the cost of the system when sensing based active damping is employed and complexity, when the filter based active damping, is employed. In addition, the filter based active damping remain very sensitive to the parameter variation and the grid condition at the point of common coupling (PCC). It is also common to integrate many inverters in parallel at the PCC to enhance the capabilities of the power grid. In such a system, several resonances are excited and a mutual current circulating among inverters instead of injecting into the grid is activated when different current reference to each inverter is applied. Generally, it is difficult to guarantee a similar current reference to each inverter due to uncertain or inconsistent nature of renewable energy sources. Therefore, in this dissertation, the active disturbance rejection control (ADRC) with reduced-order extended state observer (RESO) is proposed for damping the single resonance in a single grid-connected inverter. Later, the same method is extended to the multi-parallel grid-connected inverter to damp the multiple resonances. The resonance damping is achieved by pole-zero crenelation with the minimum information about the system required. In this way, the proposed method achieves simplicity and robustness in handling the inductive and inductive-capacitive grid impedance at PCC or filter parameter variation. Being a disturbance observer-based control structure, the ADRC provides an effective way to handle the complex/uncertain system by considering them as a disturbance, estimating with RESO and rejecting in the control law of ADRC. The proposed method also greatly reduces the mutual current or harmonic in parallel GCIs irrespective of current reference condition. The stability of the system with the proposed method is analyzed using frequency response analysis via Bode plot, root locus and pole-zero map in the discrete time domain. The inherent delay in the control loop equal to 1.5 times of sampling time is also considered. To verify the dynamics of the system with the proposed method, the several experimental results are conducted for the LCL filter type single and two parallel grid-connected inverters under inductive and inductive-capacitive grid impedance separately. The harmonic analysis is also presented in this dissertation to explain the lower total harmonic distortion achieved with the proposed method compared to the conventional method.