Research On The Frequency Control Of Asynchronous System After HVDC Bipole Block

The power flow transferring on the AC system after HVDC bipole block is a key problem that may influence the stability of AC/DC hybrid system. Asynchronous operation can eliminate the influence of power flow transferring, enhance the system’s stability. However, under asynchronous operation, the receiving end system will suffer from severe power gaps, while the sending end system will suffer from severe power surplus after HVDC bipole block. The frequency of both ends will face a serious impact, even could not meet the control requirements.Frequency is an important parameter in power system operation, and it is also an important indicator of power quality. The frequency control after HVDC bipole block has became a key problem of asynchronous system. In this paper, combined with simulation, the dynamic response of frequency after HVDC bipole block under two different mode of asynchronous operation — islanded operation and regional power system asynchronous interconnect are analyzed and improvement approaches are proposed. The major works of this paper include:(1) Analyzes the process of frequency dynamic response under large capacity power gaps in detail. The process of frequency dynamic response is divided into three stages according to the main factors. Combined with practical case, the theory is verified, and the severity of the HVDC bipole block’s impact on frequency is revealed.(2)Analyzes and points out that compared with AC-DC parallel operation, under sending end islanded operation, the receiving end system will suffer from larger power gaps after HVDC bipole block and the impact on frequency is more severe. The simulation of Yun-Guang UHVDC bipole block is given, the reserve capacity of primary frequency regulation needed to meet the frequency control requirements is provided. The paper also proposes approach to improve frequency control from the perspective of the spatial distribution of reserve capacity, in order to reduce the risk of AC tie line overload.(3) The simulation of Yun-Guang UHVDC bipole block is given. The impact of regional power system asynchronous interconnect on frequency dynamic response of sending and receiving end system is analyzed. The effect of chain generator shedding and emergency DC power support is analyzed based on simulation. Approaches are proposed to improve the problem of intercoordination between different DCs and between sending and receiving end during the process of DC power support, in order to reduce the risk of AC tie line overload.