Study On AC Side Support To DC Side With Voltage Source Converter Based AC/DC Hybrid Power System

As the development of global energy internet technology,power system will be the most important network for energy transportation and distribution in the world,whose significance is beyond words.Compared with traditional line communication technology,the direct-current transmission technology which is based on VSC(voltage source converter),has more advantages,like higher flexibility and better applicability.Therefor,there is more and more attention paid by scholars at home and abroad in optimizing operation and research of VSC-based AC/DC hybrid network.The VSC-based AC/DC has a various way for access device,and a flexible control mode,which brings a new challenge for the operation and analysis of power system.First of all,this paper discusses several ways of VSC-based AC/DC hybrid network,including AC/DC double-terminal interconnection,AC/DC back-to-back interconnection and AC/DC multi-terminal interconnection,and describes specific circumstance of every connection way.At the same time,make an analysis of different control modes beside the AC/DC of VSC converter station,application condition of all composite control modes and equivalent-node type in the load flow calculation.Due to the diversity of VSC access system,the traditional way which is based on node-modeling solves the boundedness existed in optimal power flow problem of VSC AC/DC hybrid network.This paper proposes optimal power flow model of VSC-HVDC AC/DC system which is based on branch extension,conducts an unified modeling of VSC-based AC/DC hybrid.Through the simulating calculation of examples on the different access ways of two groups,the result shows that,the proposed model in the paper can apply into the optimal power flow calculation of VSC-based AC/DC hybrid network,back-to-back direct current transmission system,multi-terminal direct current transmission system etc.For the multi-terminal direct current interconnection,the result shows that the interconnection of several AC networks through VSC can enhance the economic benefit of generator in AC network when comparing with individual operation of several AC networks.In AC/DC hybrid power system,there are mutual influences between AC side and DC side.This paper analyzes the AC and DC voltage system interaction relations,applies system load margin index to describe voltage stability margin of AC/DC hybrid power system.The strategies and implementation methods of the AC side support to DC side problem in AD/DC hybrid system have been analyzed.And presenting load margin and AC support to DC side voltage computation models to solve these problems.Finally,load margin calculations are performed on 24-hour load fluctuations,activating AC support to DC side voltage computation models when the load margin is too low.Optimize calculations of the testing case were performed which shows that the AC support to DC side voltage computation models can solve the problem of the DC voltage and the VSC transmission power crossing a line when the DC side is overloaded.