Research On Virtual Harmonic Impedance Control Of Grid-connected-inverter In Distributed Generation

With the large-scale application of the distributed power generation (DG) and growing severe grid power issues, the grid connected inverter in DG is required to provide the necessary harmonic suppression services, in addition to transfer power from DG to grid. The multi-functional grid-connected inverter(MFGCI) in DG with harmonic compensation control, which means a grid-connected inverter in DG is combined with active power filter(APF) services and can compensate harmonics in grid, has been garnered a lot of attention. But, it is very difficult to obtain the harmonic currents information of distant loads to compensate them and it can’t compensate the voltage distortion at Point of Common Coupling (PCC) caused by background harmonic voltage in power systems. Aiming at this issue, the virtual harmonic impedance control for MFGCI is researched in this thesis. Compared with the harmonic compensation control of grid connected inverter, the virtual harmonic impedance control can not only compensate harmonic currents without the limitation of harmonic current detection, but also suppress PCC voltage harmonic caused by background harmonic voltage. Face to the increasingly desired ancillary services of grid-connected inverter in DG, some detailed and comprehensive studies on the virtual harmonic impedance control for MFGCI have highlighted in this dissertation thesis. And some novel and important conclusions are drawn as follows.1. Aiming at quantitatively evaluating harmonic suppression effect of the harmonic impedance, A harmonic suppression coefficient based visual method to analyze the suppressing ability of virtual harmonic impedances is proposed. Literatures have mainly focused on the qualitative analysis of resistive harmonic impedance, and the lack of a quantitative study of its harmonic suppression effect. This thesis proposes a concept of harmonic suppression coefficient. The coefficient can show suppressing ability of the virtual harmonic impedance to PCC voltage and grid current distortion which is caused by grid background harmonic voltage or harmonic loads. It makes the coefficient is very suitable as a criteria to evaluate the effect of virtual harmonic impedances. On this basis, a visual method to analyze the suppressing ability of virtual harmonic impedances is proposed. The proposed method can visually show variation trends of virtual harmonic impedances suppressing ability by a three-dimensional image.2. A D-partition based virtual harmonic impedance stability analysis method is researched in order to avoid the influence of current controller parameters in harmonic impedance stability analysis. In the traditional stability analysis methods, such as bode plots, zero pole map or root locus, all the parameters of system are necessary. And the different current controller parameters will inevitably affect the result of stability analysis. So, a D-decomposition based virtual harmonic impedance stability analysis method is proposed in the thesis to avoid the influence of current controller parameters. The method can show the stability of virtual harmonic impedance by comparing the stability region area of current controller parameters and does not need specific controller parameter value.3. Aiming at the conflict of harmonic suppression effect and stability in Resistive harmonic impedance control, the Resistive and Inductive (RL) harmonic impedance control strategy for MFGCI is proposed in this thesis. The harmonic impedance in the most of literature is resistive harmonic impedance. But, is the resistive the best form of harmonic impedance? For this problem, the system is modeled and the mathematical expression of optimal harmonic impedance is derived. From this, a resistive and inductive harmonic impedance control strategy is proposed which has better harmonic suppression ability at low frequency and system stability.4. In order to avoid the high frequency harmonic amplification in the RL harmonic impedance control, a composite harmonic impedance control strategy for MFGCI is proposed in this thesis. Aiming at the problem of high frequency harmonic amplification due to resistive and inductive harmonic impedance control strategy, composite harmonic impedance is proposed. The composite harmonic impedance is composed by resistive harmonic impedance and resistive and inductive harmonic impedance in parallel. The composite harmonic impedance has a better harmonic suppression effect at low frequency and no harmonic amplification at high frequency. In addition, the composite harmonic impedance is more stable then resistive harmonic impedance.5. As to the bandwidth requirement of out put filter in MFGCI, an LCL-LC filter topology and its parameter design are presented in the thesis. In order to improve the bandwidth of output filter, the evolution of filter is studied, and an LCL-LC filter topology is proposed, which is a combination of a LCL filter and a LC series resonant circuit. The filter has the advantages of high bandwidth and low cost compared with the traditional LCL filter. Two resonant frequency characteristics of the filter are analyzed and a parameter design method on the base of the characteristics is also proposed in the paper.On the other hand, the number of parameters in presented LCL-LC filter is too big to analyze its parameter robustness by the traditional way. And then a parameter robustness analysis method based on four-dimensional graphics is proposed to analyze parameter robustness of the presented filter. Compared with the traditional ones, the proposed analysis method can analyze the filter performance under variations of several parameters at a time without any iteration and show the trend of filter performance due to filter parameters variation.6. According to the requirement of bandwidth in MFGCI, A high bandwidth PI controller parameters design on the base of D-partition method is researched. Firstly, the continuous domain D-partition method is extended to the discrete domain, and on this basis, the author proposes a PI controller parameters design method with high bandwidth. The method calculates the stability region under the setting magnitude margin and phase margin by discrete D-partition method, and then choose the select the PI parameters with the highest bandwidth.