Research On The Optimal Operation Of High Proportion Renewable Energy DC Transmission System Considering The Short Circuit Ratio Constraint Of The Sending End

In order to address climate change and reduce greenhouse gas emissions caused by the power industry,new energy power generation represented by wind energy and solar energy has been developed and utilized on a large scale in China.High-voltage direct current transmission has become the main way to send new energy across regions.The large-scale new energy DC transmission also brings potential hidden dangers to the safe and stable operation of the system,and the transient overvoltage problem at the sending end after the DC fault can easily cause the DC near-zone new energy unit to go off the network,which poses a threat to the safe and stable operation of the system.Therefore,this paper takes the short-circuit ratio as the breakthrough point to reduce the overvoltage level from the perspective of changing the system operation mode and improving the system strength,and deeply considers the optimal operation of the new energy DC transmission system constrained by the short-circuit ratio of the sending end.Firstly,the short-circuit ratio requirement algorithm of the sending system expressed by the new energy off-grid voltage rise threshold is studied.By analyzing the causes of transient overvoltage in the sending system.an equivalent calculation model of the transient overvoltage of the bus bus of the sending converter station is constructed.Based on the calculation method of transient overvoltage of converter station busbar and the analysis of the influencing factors of transient overvoltage.the transient voltage rise at the grid connection point of new energy is derived by using the equivalent value of the system network.Using the relationship between transient overvoltage and short circuit ratio,and through the transient voltage rise off-grid threshold of new energy units,the short-circuit ratio demand algorithm of the sending system to prevent new energy units from going off the grid is derived.Secondly,the construction method of short circuit ratio constraint of new energy delivery system under different operation modes is studied.The characteristics of the traditional power system short-circuit ratio calculation method and its application to the high proportion of new energy power system are analyzed.On the basis of this calculation method,the contribution of synchronous generator sets and new energy units to the system strength is analyzed,and then the relationship between the operation mode of the two and the short-circuit current provided by the converter station node is derived through Thevenin’s equivalence theorem and superposition principle.and then the calculation method of the short-circuit ratio of the sending end under different operation modes is obtained,and then the short-circuit ratio constraint corresponding to the off-grid threshold of the grid-connected point of the new energy unit is determined,and the rationality of the proposed constraint is verified by example.The results show that the proposed short-circuit ratio constraint has higher rationality and accuracy in systems with a relatively high proportion of new energy and many nodes.Finally,the optimal operation model of new energy DC transmission system considering the short-circuit ratio constraint of the sending end is studied.For the new energy transmission system with energy storage,with the goal of minimizing the system operating cost and curtailment cost,considering the voltage support capacity of the sending power grid,a collaborative scheduling optimization operation model of power generation and DC transmission power considering the short-circuit ratio constraint of the sending end is established,and the internal point method is used to solve it,and the IEEE14 node and IEEE118 node system are verified by the sending and receiving end to improve the IEEE14 node and the IEEE118 node system.The calculation results show that the built model can comprehensively consider the system economy and safety to give the generator set output plan and the DC transmission plan curve,improve the new energy absorption rate,and prevent the wind turbine off-grid accident caused by transient overvoltage to a certain extent,which provides a reference basis for the power grid dispatching department.