| With the development of modern power electronic technology and the wide application of new energy technologies,the DC distribution network can flexibly connect to the existing distribution network and be compatible with multiple types of DC power generation and consumption devices,which is the development trend of the future distribution network.However,the access of large-scale renewable energy generation devices,the complementation and local consumption of multiple types of energy,and the complex interaction between devices pose severe challenges to the efficient,reliable,and stable operation of the DC distribution network.In this context,it is necessary to study the coordinated control of large-scale power electronic devices connected to the DC distribution network and the stability of the interaction of multiple types of power electronic devices.In terms of coordinated operation and control,the increase in the number and types of power electronic equipment has led to a sharp increase in the amount of communication data and information calculations for DC distribution networks to achieve predetermined control goals.Especially in the context of low-bandwidth communication lines,the lack of operation and control information may cause the device to lose control or the distribution network to collapse.In addition,the dynamic interaction between power electronic devices with different dynamic characteristics is complex,and the increase in the number of devices leads to a higher order of the overall mathematical model,leading to order disasters and failure to analyze and evaluate the stability of the DC power distribution system.On the one hand,the development of the DC distribution network can achieve large-scale access to new energy sources and improve energy efficiency.However,if it cannot solve its coordinated control and stability analysis problems,it will restrict the expansion of the DC distribution network.Therefore,it is of practical significance to carry out the coordinated control of multiple power electronic devices and the stability analysis of the multi-level busbar DC distribution network for the DC distribution network connected by multiple converters.Therefore,this paper focuses on the outstanding problems of coordinated operation control and stability analysis faced by DC distribution networks under the background of multiple power electronic devices access,and the following results have been achieved:(1)When the DC converters are running in parallel,the traditional DC droop control method causes large deviation of the DC bus voltage on the load side due to the influence of virtual resistance and line impedance.Aiming at the problem that direct bus voltage compensation may reduce the effect of current sharing,a Integra droop control method that can simultaneously realize bus voltage deviation compensation and improve the current sharing effect of the converter is proposed.By using the load terminal voltage as the input variables,the line impedance is converted into the equivalent output impedance of the converter itself,combined with the Integra droop controller;on the basis of maintaining the original power-sharing control effect,the bus voltage drop is compensated to reduce The adverse effect of the line impedance of the DC bus voltage.The proposed Integra droop control method can reduce the DC bus voltage control deviation and improve the current sharing control effect between converters under the influence of line impedance and improve system reliability.(2)The increase in the number of connected converters in the DC distribution network has led to a sharp increase in the complexity of coordinated control between devices and the amount of communication data,limiting the further development of the DC distribution network.This paper proposes a secondary power coordinated control method based on a differential delay control algorithm.The differential delay algorithm is introduced through the multiplexing of the converter’s own power coefficients,and the delay competition mechanism is combined to realize the automatic selection and conversion of the main control unit.The realization of the secondary power sharing control of the converter.In realizing the secondary control,the communication line only needs to transmit the power coefficient signal of the maximum output unit.Therefore,the advantage of the proposed secondary power sharing control method is that the complexity of the control algorithm and the amount of communication data do not increase with the increase in the number of converters.It is suitable for the coordination of DC distribution networks with large-scale converter access.Operational control.(3)The Nyquist stability criterion is used to determine the impedance ratio of the source-load cascade system,which is difficult to reflect the instability mechanism of impedance mismatch intuitively.Based on the Nyquist stability criterion and the Cauchy argument theorem,a simplified stability determination method based on the real and imaginary parts of the impedance in the frequency domain is proposed.In the analysis process,the envelope of the amplitude-phase frequency characteristic curve of the impedance sum to the origin is mapped to crossing the real and imaginary parts of the impedance sum in the frequency domain to the real axis.Considering the physical meaning of impedance and real and imaginary parts,the impedance mismatched instability is explained from the perspective of resonant oscillation and negative damping.The stability judgment condition of this criterion is that when the converters in the source-load cascade system are stable,there is no zero-crossing point of the impedance and the imaginary part in the negative damping frequency band of the impedance sum.The simplified criterion can be used to accurately determine the frequency of oscillation instability,analyze the causes of instability and oscillation intuitively,and provide directional guidance for impedance remodeling.(4)Aiming at the problem that the global stability analysis of the multi-level DC distribution system is challenging to obtain the information of the weak links of the system,and thus can not provide guidance for the stability measures,a classification judgment method for the stability analysis of the DC distribution system is proposed.This method conducts stability analysis in the order of device level,bus level and system level.First,determine the stability of each converter itself,then analyze the DC bus’ s stability when the upper-level bus is equivalent to an ideal source,and finally complete the overall stability judgment of the conversion of the bus impedance through the interface converter.This method can obtain the overall stability analysis conclusion while retaining the impedance stability information of each flow bus,and provide support for the stability analysis of the complex DC distribution network.This paper researches the difficulties of coordinated control and stability analysis faced by the DC distribution network after large-scale power electronic devices are connected and improve the complex DC distribution network’s control and stability analysis system.The results obtained can be used for the DC distribution network.Provide technical support for coordinated control and stability analysis. |
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