Research On The Partition Operation Mode Of The System With Multiple LCC-HVDC Infeed

In order to realize the carbon neutral before 2060,a large number of clean energy power generation bases will be built in the western area of China,and transmitted the electricity to the eastern China.However,considering the advantages of line commutated converter based high voltage DC(LCC-HVDC)transmission in transmitting a large power capacity through long distance in a near future,more LCC-HVDCs will infeed into the load center in the eastern China,which will cause a severer multi-infeed LCC-HVDC system.It is possible for the multi-infeed LCC-HVDC system to have simultaneous commutation failures.And the simultaneous commutation failure has a great influence on the stable operation of the multi-infeed system.At present,the methods to solve this problem are:(1)expand the scale of the AC system to form a stronger receiving end;(2)make LCC-HVDCs hierarchical and partitionable infeed into the receiving end.However,the scale of the AC system is restricted by many factors,and cannot be expanded blindly.Therefore,making LCC-HVDCs hierarchical and partitionable infeed into the receiving end has become an important research direction.In this paper,the partition of the multi-infeed LCC-HVDC system is adopted to solve the simultaneous commutation failure.The main ideas are that making each partition has fewer LCC-HVDC infeed by partitioning the multi-infeed sy stem,cutting off the AC interconnection among partitions,and using VSC-HVDC to interconnect.Also,VSC-HVDCs can be integrated to DC grid to realize coordinated control.Therefore,the main contents of this paper are:(1)How to realize the partition;(2)How to determine the interconnection location of VSC-HVDC among partitions;(3)How to realize the coordinated control of the DC grid.As for how to realize the partition,this paper proposes the partition method for the multi-infeed system based on the local fitness measure theory.Firstly,the reasons and developing process of the simultaneous commutation failure is analyzed.By setting the N-1 scan,lines whose three-phase permanent fault will cause simultaneous commutation failure are selected,among which with high load rate are removed.After that,the calculation method of voltage/reactive power sensitivity matrix considering PV node and LCC-HVDC is presented,and the sensitivity matrix is corrected considering the interconnection relations between nodes.Then,the node aggregation function and partition fitness function are applied to calculate the node aggregation and partition fitness respectively,and the initial partition is obtained.Finally,considering the line collection and short-circuit ratio,the initial partition is modified,and the overall partition process is established.The effectiveness of the whole segmentation process is verified on IEEE 39 bus system,and a partition scheme is proposed on an actual power grid.PSD-BPA was used to verify the result.As for how to determine the interconnection location of VSC-HVDC among partitions,an evaluation system with 9 indexes considering both the normal operation state and the N-1 operation state is established.Firstly,the topology of using VSC-HVDC to interconnect the partitions is given.Secondly,based on Jacobian transfer matrix,a small signal model of the AC system considering LCC-HVDC and VSC-HVDC is established.Therefore,the operation impedance of VSC-HVDC can be obtained,which leads to calculate the impedance based effective short circuit ratio(IESCR).Then,the relationships among the interconnection location,power transmission and IESCR are analyzed.Finding that it is not enough to determine the interconnection location of VSC-HVDC only according to IESCR and the capacity of VSC.Therefore,considering the normal operation and N-1 operation state,9 evaluation indexes including IESCR are proposed,and the evaluation process of VSC-HVDC interconnection locations is given.Then,an example is given based on the IEEE 39 nodes model.Three VSC-HVDC interconnection locations are chosen to calculate their evaluation index values,and their scores are obtained to determine the optimal interconnection location.Finally.according to the demand of an actual power grid,a VSC-HVDC reform plan is given.To deal with the problem of weak AC systems interconnected by VSC-HVDC after partition,the paper proposes the DC voltage synchronization control(DCSC).Based on the power balance of DC side,power synchronization control,and the feedback loop of the virtual synchronous generator control,the DCSC is given.Thereafter,a small-signal model of DCSC is established based on Jacobian transfer matrix,and the system stability influenced by the main control parameters in DCSC is analyzed.Finally,a simulation model is built in PSCAD/EMTDC,the operations of DCSC and phase lock loop(PLL)under different AC system strength are compared.It can be concluded that DCSC is better than PLL.As for how to realize the coordinated control if the DC grid is adopted to interconnect the partitions,the autonomous decentralized control is proposed.Firstly,the system control layer and the station control layer are designed.In the system control layer,the communication reliability and the time delay for each converter to obtain global information are analyzed,and an optimal power flow model is designed considering the minimum loss of the system and the minimum deviation of DC voltage.By using convex optimization,the reference values of DC voltage and active power are calculated.In the station control layer,a mathematical method for simplifying a complex DC grid into a radial DC grid is proposed,and an example is given.Then,a method of P-V droop control based on virtual resistance is given.The control dead zone of its limited operation state is analyzed,and its control strategy is improved.After that,three types of control strategies are simulated on a multi-terminal radial VSC system,and their advantages and disadvantages are compared.Finally,considering the autonomous decentralized control,the simplification method of the DC grid,P-V droop control based on virtual resistance,and the DC voltage synchronization control,a simulation model is established.The influence on the system economy and stability is compared whether the DC voltage control is adopted by a converter in the system.Simulation results show that whether this kind of converter is set or not has little effect on the operation of the system.