Study On Fault Interaction Analysis And Calculation Model Of AC/DC System And Its Application

With the rapid advance of HVDC projects construction, the complex multi-infeed directcurrent (MIDC) system has been formed in China Southern Power Grid and East China PowerGrid. For MIDC system, especially in the case of ac faults, the interactions between ac systemand dc system and the interactions among each dc system are extremely complicatedprocesses. The mechanism of those processes and the study of related issue can provideimportant theoretical and practical value for the safe and stable operation of large scale ac/dchybrid system. Therefore, fault steady state interaction analysis and calculation method ofac/dc system and mechanism analysis of fault transient processes influence on ac powernetwork relay protection under ac network faults are studied in this dissertation. For faultsteady state, a dc system equivalent model under no commutation failure condition isestablished. And base on it, a MIDC system ac power network fault analysis and harmoniccalculation method is proposed. Besides, the mechanism of harmonic instability and the newcriterion of dc harmonic protections in HVDC transmission system are studied. For faulttransient processes, the dynamic phasor model of dc system equivalent power-frequencycurrent and equivalent power-frequency variation current under commutation failurecondition is deduced. And based on it, the mechanism of HVDC commutation failureinfluence on ac power network relay protection is analyzed.The main work is summarized as following:1. By consider the dynamic switching characteristics of inverter and the steady-stateresponse characteristics of dc control system under various fault-types comprehensively, a dcsystem equivalent model which considering the dc control characteristic is established. Themodel reflects the relationship between each order current which injected by dc system andeach order converter bus voltage, which determined by dc system. It provides the interfacemodel for MIDC system ac power network fault analysis and harmonic calculation.2. Base on the dc system equivalent model which considering the dc controlcharacteristic, a MIDC system ac power network fault analysis and harmonic calculationmethod is proposed with the topology structure of receiving-end ac system and the boundaryconditions of ac asymmetric fault. It can improve the computational accuracy of each ordercurrent which injected by dc system and each order converter bus voltage effectively. Basedon CIGRE model, a three-infeed HVDC power transmission model is built and the methodhas been tested widely for fault analysis and harmonic calculation. The results are comparedwith those obtained by simulation using PSCAD/EMTDC software. It is shown that the proposed method is accurate and effective, and provides the analysis basis of harmonicmitigation and protection analysis in MIDC system.3. The combined action of HVDC converter harmonic modulation characteristics andconverter transformer saturation characteristic can lead to converter transformer coresaturation instability. The author analyses harmonic modulation characteristics of converterand the frequency transformation feature of saturation converter transformer. Based on theclosed-loop variable characteristics of harmonic transfer back and forth through saturationconverter transformer and converters in the ac side and dc side, a new criterion to determinewhether the core saturation instability occurs is proposed. Based on the criterion, the effect ofthe inverter operation parameters, the parameters of the converter transformer and ac/dcsystem impedance on the core saturation instability is theoretically analyzed. The criterion isverified by dynamic simulation of several systems using PSCAD/EMTDC software. Thestudy results indicate that the proposed criterion simple, physical meaning clear, parametersaccessible, which can quantitative evaluation of harmonic instability degree of convertertransformer core saturation instability, and provides the theory basis for risk evaluation andsuppression measures of HVDC system harmonic instability.4. Discrimination between converter valve fault and ac asymmetric fault has been achallenging issue in dc100Hz protections. Based on the different mechanism in generatingpower frequency negative sequence component of converter current switching functionbetween converter valve fault and ac asymmetric fault, a new method based on powerfrequency negative sequence component amplitude ratio of converter station six-pulse bridgecurrent switching function is proposed to discriminate the converter valve fault and the acasymmetric fault. The proposed new method is not only can discriminate the converter valvefault and the ac asymmetric fault, but also can achieve the fault converter bridge locationunder converter valve fault condition. The method has been tested widely based on GuiguangII HVDC system. Result show that the proposed method is simple, reliable and has well usevalue.5. In the ac/dc interconnection system, the dc commutation failure caused by ac powernetwork fault can lead to maloperation of ac power network protection. In this paper, underthe commutation failure condition, a switching function model of inverters and a transientchange model of dc current are established. According to modulation theory and convolutiontheorem, the dynamic phasor model of dc system equivalent power-frequency current andequivalent power-frequency variation current are deduced. The analysis shows that thevariation characteristics of the equivalent power-frequency current and equivalent power-frequency variation current are different from those of pure ac system, which may leadto maloperation of ac power network protection under the dc commutation failure caused byac power network fault. At last, combining the two specific ac power network protection, themechanism for maloperation of ac power network protection caused by dc commutationfailure is analyzed, which provides a theoretical basis for research on protection strategy ofac/dc interconnection system.