Research On Reactive Power Control And Operating Characteristics Of HVDC Receiving-end Connected To Weak AC System

Ultra-high voltage direct current(UHVDC)projects must be sustainedly and rapidly constructed in China to realize the clean energy efficient configuration in large scale and long distance,which is decided by the converse distribution characteristics of primary energy bases and load centers.During the transition period,problems of high proportion of imported electrity and lack of internal power supply come out in local power system.Strong DC and weak AC characteristics of AC/DC hybrid grid endanger the safe and stable operation.Thus dynamic var compensation devices will be installed in HVDC converter station connected to weak AC system,according to the plan of State Grid.Based on the above background,the paper takes the ±800kV DC power transmission project from Jiuquan to Hunan as experiment object,and studies operating characteristics of HVDC system connected to weak AC system from three parts.At first,on the basis of CIGRE-HVDC foundation model and real parameters of the ±800kV DC power transmission project from Jiuquan to Hunan,an electromagnetic transient simulation model is established.Then different working conditions are carried out.While,by analyzing the simulation results,this article illustrates the following problems exist in "Strong DC and weak AC" system:more prone to commutation failure on the DC side,and lack of transient voltage stability on the DC side.Then,impacts of DC power transmission on transient voltage stability of Hunan power grid is analyzed,combining present characteristics of Hunan power system.And the necessity of installation of reactive power compensation devices in Xiangtan converter station is indicated.Next,according to real parameters,electromagnetic transient simulation models are established for the large capacity cascade STATCOM system and synchronous condenser system.Accordingly,two HVDC power transmission systems containing STATCOM or synchronous condensers are built.Through comparative analysis of reactive power control features and operating characteristics of the two systems,this article summarizes strengths and weaknesses of STATCOM and synchronous condensers in HVDC application.Finally,this paper analyzes steady characteristics of bus voltage while HVDC is respectively connected to AC systems with different strength.,and points out many problems existing in HVDC connected to weak AC system.For instance,DC power fluctuations are inclined to induce bus voltage instability,as well as switching fixed capacitors is prone to fluctuation range overproof of bus voltage.Thus,mutiple control objectives against the above problems are proposed.Based on the coordination of static synchronous compensator and fixed capacitors,this paper proposes a multi-mode control strategy:steady state voltage regulation mode,the AVC control mode and transient control mode.In steady voltage regulation mode,the converter bus voltage can be controlled at its rated value with zero steady-state error by the coordinated control of the STATCOM and capacitor bank,while the Reactive component devices of the HVDC converter will participate voltage regulation of nearby power grid in the AVC control mode,and transient control mode is constructed for mitigating voltage fluctuation and reducing the probability of commutation failure during grid fault period.Then different working conditions are carried out by simulation model of HVDC transmission sysytem from Jiuquan to Hunan.The correctness ad effectiveness of the proposed control strategy is verified by the simulation results.The revelent research work about improving operating characteristics of HVDC system connected to weak AC system in this thesis will provide technical references for the safe and stable operation of practical projects.