| In recent years,a large number of power electronic devices have been invested,along with non-ideal conditions such as sudden rise and fall of grid voltage,flicker,and fluctuations,and power quality has declined.Traditional passive power filters and active power filters are difficult to comprehensively solve the problem of poor voltage and current quality.The unified power quality conditioner(UPQC)has two parts on the series side and the parallel side,which can realize voltage and current compensation and power quality adjustment.However,in the medium and high voltage power grid,due to the large voltage amplitude,it is difficult to adapt the two-level and three-level unified power quality regulators.Modular multi-level converters(MMC)is widely used in DC transmission due to their high withstand voltage,low switching frequency and smooth output waveform.Combining MMC and UPQC can realize its application in medium and high voltage power grids.This paper takes the UPQC based on MMC as the research object,and studies the compensation control strategy of MMC-UPQC in the case of grid voltage imbalance in medium and high voltage systems It aims to realize the recovery of voltage and current on the MMC-UPQC series side and parallel side,improve the quality of the power grid under the condition of unbalanced power grid voltage,and reduce the harmonic content.First of all,in order to ensure the normal operation of the MMC sub-module,this paper analyzes the operating principle of the MMC,based on this,chooses the carrier phase-shift modulation method and builds the MMC circulating current controller and voltage equalization controller.On the basis of the internal stability of MMC,under the unbalanced grid voltage,the PI controller of MMC-UPQC is designed according to the idea of separation of positive and negative sequence,which can restore voltage and current.Secondly,due to the large number of traditional PI control parameters and the general control effect,this paper proposes a passivity-based control(PBC)strategy in the unbalanced state of the grid voltage.According to the mathematical model and PBC control theory of MMC-UPQC,the Euler-Lagrange(E-L)of MMC-UPQC is derived model,and proved the passivity of this model.Finally,the PBC controller of MMC-UPQC inner loop is designed through E-L mathematical model.The simulation results show that the PBC controller requires fewer parameters and can achieve good control effects.However,because passive controllers are more sensitive to system parameter changes,larger disturbances will make the passive controller’s compensation ability worse.Sliding mode control is a kind of non-linear control which is not easily disturbed,and it can overcome the influence of system parameter change.Therefore,this paper proposes a passive sliding mode control strategy.The sliding mode surface is introduced on the basis of the previous passive controller.By selecting appropriate coefficients,the robustness and adaptability of the system are further improved.Finally,this paper proposes a differential flat control(DFC)strategy.According to the principle of DFC,the flatness and stability of the MMC-UPQC system are analyzed.On this basis,a DFC controller with inner and outer loops on the series side and parallel side is designed.Compared with the traditional PI control,it is proved that the DFC has a short response time,good compensation effect,and has certain advantages in the unbalanced state of the grid voltage. |
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