| The reverse distribution of energy centers and load centers makes the construction of power grids inevitably move towards Ultra high-voltage direct current(UHVDC)transmission with large capacity and long-distance transmission capacity.Some DC access methods place high requirements on the reactive power support capacity of the receiving power grid,and are not conducive to the power evacuation of the receiving power system.Therefore,the State Grid started with the power grid structure and proposed a way for UHVDC to be hierarchically connected to AC grids with different voltage levels.The rheology is connected to the 500KV/1000KV receiving end power grid,and there are many DC transmission engineering applications in actual projects.The AC/DC interconnected power grids with hierarchical connection(HC)mode are very different in topology and control from the conventional DC access.In order to ensure the safe and stable operation of the AC/DC system and in-depth understanding of its topology,this article aims at Based on the multi-layer hierarchical DC access,the equivalent model and power flow calculation method of the AC-DC interconnected power grid and the simulation analysis of the system voltage stability under the interaction of multi-circuit hierarchical DC are made.The main research contents of the paper are as follows:First,this paper establishes an HVDC equivalent model that considers multi-layer hierarchical DC access.Starting from the equivalent structure of the LCC converter,the basic DC constraints considering the layered structure are obtained,and a new equivalent structure is proposed to obtain the admittance matrix of the DC bus,and then a high-voltage DC transmission system with a layered structure is obtained The general equivalent model of the model is obtained,and its corresponding converter relationship formula is obtained,and the model is improved by considering the unbalanced operation of the system.Then,this paper derives an equivalent method to calculate AC/DC power flows.First,based on the equivalent model,the DC constraint equation and the converter equation,a unified power flow model with a hierarchical structure is proposed.Then,considering the increase in the calculation burden due to the increase in the number of Jacobian matrix,the equivalent formula of the AC state variable Represent the DC system to further improve the model and get the equivalent model.The two power flow calculation methods of unified method and equivalent method are compared and analyzed by different power flow calculation examples.Finally,the VQ sensitivity analysis of HVDC with hierarchical structure based on Jacobian matrix was conducted,and the influence of LCC converter station on VQ sensitivity was also analyzed.Finally,based on data from Shandong Power Grid,PS ASP and PSCAD as simulation tools,a series of simulation analysis of system voltage stability under multi-layer hierarchical DC interaction is carried out.Firstly,it analyzes the influence of DC layered structure on the reactive power support of the receiving power grid,and compares it with the difference between the conventional access mode.Secondly,based on the data of Shandong Power Grid,simulation analysis is carried out through PSASP.Obtained the influence of the operation mode and the change of DC injection quantity on the stability of the receiving end connected to the bus.At the same time,the correlation relationship between the voltage of the same layer of multiple layered DC and the different layers of different layers was analyzed.A comprehensive voltage instability simulation was carried out,and the difference between the multi-circuit hierarchical DC access system and the conventional DC access system during voltage instability was compared through the above simulations.The evolution rule of voltage instability of the end system is obtained by PSCAD/EMTDC simulation,and the voltage correlation relationship between multi-layer layered DC layers is obtained.Finally,the transient voltage instability simulation was carried out through PSASP,and the difference between the conventional DC access and the multi-layer hierarchical DC access in transient voltage instability simulation was obtained. |
PDF Full Text Request