Adaptability Analysis And Optimization Control For An LCL-type Grid-connected Inverter Under Non-ideal Grid Conditions

The grid-connected inverters are used as the interface unit of the distributed power generation systems, such as wind, photovoltaic, etc, of which the control performance partly determines the output power quality of the systems. Due to the characteristics of the geographical distribution of renewable energy in China, many grid-connected inverters are installed at the area which away from the electric power backbone network. Thus, the power grid in the interface of the distributed power generation systems is generally weak, such as unbalance, grid harmonics and grid impedance. These non-ideal grid conditions will directly affect the injected power quality of grid-connected inverters, even threatening the safe operation of equipment when a serious situation occurs. Taking an LCL-type grid-connected inverter as the research object, this paper investigates its adaptability and optimization method under the above three kinds of non-ideal grid conditions.Eliminating the second-order harmonics or fluctuations in the dc-side voltage and output instantaneous power, and achieving three-phase balanced output current are the main control objectives of grid-connected inverters under unbalanced grid conditions. In order to operate the grid-connected inverters at an optimal state and achieve multiple control objectives, this paper analyzes the amplitude and phase relation between the positive- and negative-sequence output current under different control objectives, and then proposes two unbalanced control strategies with multi-objective optimization based on grid-side and converter-side instantaneous output model, respectively. The proposed strategies introduce a negative-sequence current compensation without changing the structure of the traditional control strategy under balanced conditions. The control structure is simple, and each of the control objectives can be realized and transformed into another by regulating the factor k. Particularly, the output filter effect has been considered in the development of the control strategy, and the difference between the two unbalanced control strategies is that the strategy based on grid-side power model can realize constant instantaneous active power, while the strategy based on converter-side power model can eliminate second-order harmonics in the dc-side voltage.In this paper, the influence of grid harmonics on the output current of grid-connected inverters is investigated by means of the output admittance. After three kinds of current harmonics control strategies, including the proportional-resonant regulator plus harmonics compensator(PR+HC), the full-feedforward scheme of grid voltage and the proportional feedforward of grid voltage, are discussed, a feedforward compensation scheme based on the band-pass filter(BPF) is proposed. And on basis of reshaping the output admittance at the grid harmonics frequencies, the parameters of the BPF are derived. Furthermore, the mathematical model of the commonly-used software phase-locked loop(SPLL) is built with the harmonic linearization method, and the additional admittance introduced by SPLL is derived. In order to suppress the influence of grid harmonics which propagate to the output current through the SPLL, a modified SPLL is proposed, which can achieve suppression of the output current harmonics when a high-bandwidth SPLL is adopted.The stability issue of a single LCL-type grid-connected inverter is investigated in the discrete-time domain. First, the July’s criterion is applied to analyze the stability of the current loop when no damping method and the proportional capacitor-current-feedback active damping(AD) are adopted, and the stability region and stability conditions are given respectively. Considering the poor robustness of the above methods against the grid impedance, a capacitor-current-feedback AD method with a wide resonance damping region is proposed. The upper limit of the damping region is extended to fs/4 from fs/6, which improves the robustness of the control system against the grid impedance variation. Moreover, the influence of damping coefficient on damping performance is analyzed. The optimal damping position is derived, and an approximate calculation for the optimal damping coefficient is given.Finally, the stability issue of multi-parallel grid-connected inverters caused by the coupling of grid impedance is studied from the perspective of system. According to the closed-loop Norton equivalent circuit of multi-parallel inverters, the Nyquist stability criterion is adopted to analyze the minor-loop gain. Through the comparison of the system stability between with and without the feedforward of grid voltage, an active revision method of the output impedance/admittance based on the hybrid-filter feedforward of grid voltage is proposed. Then the stability margin of the control system is improved by raising the phase-frequency curve of the closed-loop output impedance. Furthermore, the negative damping of the output impedance/admittance introduced by SPLL is investigated, and the influence of the power level and the bandwidth of SPLL on the negative damping of the output impedance/admittance are analyzed. In order to eliminate the negative damping, the feedforward compensation of grid voltage is introduced to the current command value, and the minimum theoretical value of compensation coefficient is derived on the basis of minimum effects on other frequencies characteristic of the output impedance/admittance, which can effectively eliminate the negative damping of the output impedance/ admittance.