| Under the background of the national strategy of "carbon peak and carbon neutrality",the power industry bears the expectation of being the first to achieve carbon neutrality.Compared with AC power distribution,DC power distribution has become an important direction in the research and development of distribution network due to its advantages of large power supply capacity,long power supply radius,high operation efficiency,good controllability and adaptability to new energy access.Compared with unipolar network,bipolar DC distribution network can provide two voltage levels,which is convenient for distributed power supply and DC load access,high power supply reliability,and large transmission capacity.Therefore,this paper chooses this topology to carry out research.However,in bipolar DC distribution networks,due to the differences in cable resistance,the traditional sag control has inherent contradictions in achieving small voltage deviation and high shunt accuracy.When the structure of the distribution network is complex and all kinds of loads are switched frequently,the load distribution will be unreasonable,and the voltage at the outlet of the positive and negative converters will be seriously unbalanced,affecting the stability of the system and the economy of operation.At the same time,in terms of power flow optimization,a high proportion of renewable energy access and a high proportion of power electronic equipment application improve the controllability of the distribution network,and provide more possibilities for DC distribution network to operate more efficiently and economically.Most of the existing studies focus on unipolar DC distribution networks,but ignore the influence of the unique positive and negative voltage imbalance in bipolar networks.However,the few researches on bipolar networks do not involve the action scenarios of DC/DC converter(DC/DC transformer,DCT)between DC distribution networks.Therefore,this paper studies the control strategy and power flow optimization model for bipolar DC distribution network.The main work and achievements are as follows:(1)The topology of bipolar DC distribution network with optical storage system is proposed,the quantitative evaluation indexes and limits of the operation of DC distribution network are given,and the models of DC/DC converter,photovoltaic unit,energy storage unit and their respective grid-connected control strategies are analyzed,which lays a foundation for the subsequent research on control strategy and power flow optimization of DC distribution network.(2)The causes of unreasonable load distribution between the same electrode and unbalance positive and negative voltage in the bipolar DC distribution network connected by DCT are analyzed.Based on the bottom control,a voltage-current secondary control strategy suitable for bipolar topology based on DCT is proposed.The strategy mainly uses the consistency algorithm to control the shunt and voltage regulation of each converter to achieve the dual objectives of improving the shunt accuracy and suppressing the voltage imbalance of positive and negative terminals.The proposed secondary control strategy can effectively reduce the unbalance of positive and negative voltage and make the load distribution more reasonable.For different network topologies such as two ends and rings,the dual control goal can be achieved well.When a DCT exits due to fault,it can still achieve the established control objective.It has a good adaptability to communication delay,and only has a small impact on the dynamic performance of the system,but has no impact on the steady-state performance.(3)The controllable quantity,objective function and constraint conditions of bipolar DC distribution network are analyzed and determined,and a multi-objective power flow optimization model suitable for bipolar DC distribution network is proposed.The validity and applicability of the proposed optimization model are verified by numerical examples of bipolar DC distribution network based on 5 nodes and modified IEEE 14 nodes and IEEE 69 nodes respectively.By adjusting the sag coefficient of each DCT,the output of energy storage and the power of controllable load,the proposed model can fully absorb distributed photovoltaic,adjust the voltage of each node within the allowable range,reduce the voltage imbalance of positive and negative terminals,and reduce the line loss of the whole DC distribution network.This model can be applied to the adaptive ultra-short term optimal dispatching of DC distribution network,which is conducive to energy saving. |
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