| In order to implement the national policy of "dual carbon" policy and achieve sustainable development,the distributed renewable clean energy has received a lot of attention.Therefore,the high efficiency and stability of distribution system are required.The medium voltage DC distribution system is a potential unit for the future power grid.Because it can realize the collection and consumption of the renewable energy power,and realize the interconnection of the different AC distribution areas.It also provides an effective solution for increasing the power supply capacity of high-density and narrow channel load areas.The stability of the DC bus voltage plays an important role in the medium voltage DC distribution system.However,the power electronic converter in the system always behaves like a constant power load,which has the negative impedance characteristic and would lead to the bus voltage instability.That will threat the normal operation of the load,and even lead to the maloperation of the protection system.Meanwhile,that will affect the stability of the entire grid.Based on this,the research work is as follows.First,the working principle,the mathematical model and the control strategy for the modular multilevel converter（MMC）are studied,and as well as the input series output parallel DC transformer（ISOP DCT）.They are the key equipment of the medium voltage DC distribution system.The Middlebrook stability criterion,gain margin and phase margin criterion and passivity-based stability criterion of the medium voltage DC power distribution system are analyzed.The output impedance scan model of the MMC and the generalized average model of the input impedance for the ISOP DCT are established,which are used as the source impedance and the load impedance of the medium voltage dc distribution system,respectively.Second,the cases of the bus voltage oscillation of the medium voltage DC distribution system are analyzed.According to the stability criterion and the established impedance model,the impedance stability of the system is analyzed.The influence of the control parameter optimization on system stability is discussed,and the impedance phase reshaping（IPR）method is proposed.By weakening the negative impedance characteristic of the ISOP DCT,the proposed method can quickly suppress the voltage oscillation of the medium voltage DC bus voltage.The controller parameter optimization method includes the optimization of control parameters of the MMC and the ISOP DCT.The IPR control strategy includes the establishment of the impedance model,the convergence of the impedance phase determination interval,the design of the impedance phase controller and the selection of parameters.Third,based on the RT-LAB platform,the real-time simulation model of the voltage oscillation case for the medium voltage dc distribution system is built and verified.The accuracy of the theoretical model is verified by comparing the established generalized average model with the actual scanning results.The main resonant frequency is obtained by the fast Fourier transform of the medium voltage bus voltage,which verifies the correctness of the resonance frequency analysis in the oscillation case.The stability and the transient characteristics of the system are compared between the proposed IPR method and the traditional controller parameter optimization method.It is verified that the proposed IPR method can achieve rapid suppression of the bus voltage oscillation.Besides,the dynamic response is good,and the medium voltage DC distribution system is stable. |
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