Studies On Power Balance Control Of A DC Grid With DC-DC Converters

The integration of large-scale renewable energy brings many technical challenges to traditional AC grid.As an effective method to solve the problems,the multi-terminal HVDC(MTDC)and DC grid technology have already attracted the focus of industry and academe in recent years.However,there is no exactly definiation of DC grid and the research of DC grid topology and control is still at the start stage.This thesis attempts to promote the application of DC grid and research the control method of DC grid based on the proposed benchmark topology.Firstly,this thesis studies the characteristics and technical requirements of a DC grid and proposes a benchmark topology of DC grid incorporating DC-DC converters.As for the four-terminal DC ring system,this thesis proposes a dual-limiting power balance controller after analyzing the advantages and disadvantages of master-slave control and droop control.Simulations verify the efficacy of the proposed dual-limiting controller.The dual-limiting controller combines the advantage of master-slave control that active power of each power terminal is precisely controlled in normal operation and the advantage of droop control that system stability is retained in case of un-balanced power or faults.In the meantime,drawbacks of these two conventional controllers have been discarded.The controller is suitable for the control of VSC-MTDC.As for the LCC-HVDC system interconnecting with the DC ring through DC-DC converter,this thesis analyzed the impact of DC-DC converter on the control of LCC-HVDC and proposes a local power balance controller and a centralized power balance controller,respectively.Simulations verify the efficacy of proposed controllers.Lastly,the control methods proposed in the study on each part of the topology are applied to the control of the DC grid.Power balance control of the DC grid incorporating DC-DC converters is developed.Control methods for starting-up,DC fault isolating and tripping of a converter station of the DC grid are proposed.Simulation results validate the effectiveness of the proposed control strategies for flexible and reliable operation of the presented DC grid.