| Utral HVDC technology will be the main solution in power transport of large quantity across a country or a continent,and strong AC with strong DC power grid interconnected will become the main form of global energy internet.Because of the increasing capacity of DC transmission power and the intensive landing points of which are very rare in the world,the existing DC connection mode to AC grid will not be conducive to power flow evacuation and would bring a series of voltage support problems to receiving end power grid.State Power Grid Corp will apply hierarchical connection technology in the planning UHVDC transmission project.Based on this background,this paper studies the new dc access mode from the following four aspects.Firstly,the modeling of UHVDC with the connection mode and the realization of dc control system are studied.In view of the characteristics of the high side converter and the low side converter at the inverter side needing to be controlled independently,the level of the control system is redefined and designed.Taking a practical project,Ximeng-Taizhou UHVDC transmission project which will be put into operation,as an example,a model is built up in electromagnetic transient simulation software and it can provide guidance and basis for the design of the actual system.Furthermore,by using the proposed model and practical grid data,the advantage in power flow evacuation and improving the voltage support ability are analyzed.Next,the voltage stability of converter buses of UHVDC with hierarchical connection mode is studied in this paper.power output characteristic of inverters of different layers is analyzed firstly according to mathematical model of the new connection mode and numerical calculation method of comprehensive voltage stability factor are derived by using reduced order jacobian matrix to evaluate voltage stability of converter buses.It is shown that being granter than and closer to zero of the factor indicates the better stability of converter buses.Furthermore voltage stability of converter buses under various DC control modes is studied by using the factor and the sort of voltage stability is obtained.The effect of the transmission power on voltage stability is also investigated.Finally,simulation based on Ximeng-Taizhou UHVDC project shows the analytical method of this paper is reasonable and effective.Then,the voltage stability margin index of each buses of the UHVDC is studied.Based on the node impedance matrix of the power network and the equivalent method of multi-port network,the hierarchical DC is equivalent to two independent single port networks.By using impedance matching theorem,the expression of the DC voltage stability margin for each layer is derived.Furthermore,difference of the voltage stability margin index and the reasons for the difference of the virtual voltage source,the virtual impedance and the virtual power equivalent scheme are studied.Finally,the feasibility of the voltage stability margin index of converter bus in the real power network is verified by the actual grid data.Finally,influence of the AC voltage on the DC transmission power of UHVDC with the connection mode is studied.According to the characteristics of the connection mode itself structure and the control system,three kinds of switching schemes of DC current are designed in this paper.The influence of the DC power on the variation of the voltage at the receiving end is compared under the different current switching schemes,and it is found that the DC power transmission can be guaranteed by the combined control scheme of the high and low side converter in inverter side. |
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