Research On Fault Analysis And Its Protection Of AC-DC Hybrid Systems

With the development and utilization of large DC power transmission systems, the DC transmission systems become indispensable for power systems. The interaction between the sending end grid and the receiving end grid becomes significant. A more deeply characteristics analysis of the network of AC-DC hybrid system is requested. Some problems are resolved, but others are still been short of effective analysis, precise calculation and reasonable avoidance. Now, the theoretical analysis and innovations about the AC-DC hybrid system are important, and become a challenge. This thesis makes researches on several typical issues in fault analysis and its protection of the network connected by the AC-DC hybrid system, and the main works are organized as follows:(1) The possibility of the inverter commutation failure during open-conductor fault is established. The sensitivities of the commutation failure to the AC voltage disturbances caused by the changes of the AC superposition voltage are analyzed. It is concluded that the sensitivities of the commutation failure with different transformer wiring schemes and fault types are diverse. Before the commutation failure happens in the inverter station during the open-conductor fault, the commutation failure may occur in other inverter stations of the muti-terminal DC systems.(2) The analysis results based on switching function show that the main factors influencing the backup protection of HVDC transmission system are negative-sequence component in HVAC system and the third harmonic component, and the negative-sequence component is the principal factor. The main factors influencing the backup protection of the HVDC transmission system are analyzed by the symmetrical component. It is found that the influences of main fault parameters in HVAC system, such as fault types, fault locations, reactive power compensation and short-circuit ratio, on backup protection of HVDC transmission system are different. Some are monotonic and the others vary with fault types and operation modes. The analysis results can be used to the protection checking analysis in engineering, and the operational range of inverse time-lag protection is also determined.(3) By analyzing the characteristics of the valve commutating voltage, it can be concluded that the ground fault at the converter station will cause short circuit across the bridges and multiple complex AC faults. Moreover, the line-to-line fault will result in unbalanced commutation. The fault current is characterized as AC short circuit among multiple phases and this can be illustrated by the time function of the commutating valve current after ground fault. It analyzes the AC and DC protections by the equivalent model of the AC fault circuit. Additionally, the distance components of AC relay have overreach phenomenon when a AC ground fault at the high-voltage side of the valves occurs. A method using zero-sequence compensation component, which can improve the performance of the startup component, was proposed.(4) It is analyzed the traditional protection cannot locate the fault because there is no smoothing reactor in the transmission lines when a fault occurs on the DC transmission line of a MTDC system. In the circumstance of AC/DC conductor contact fault, the varieties of the transient voltage and current components are given based on the symmetrical fault analysis. According to Bergeron Equation, the traveling wave characteristic is studied. A new criterion of fault locating using the change rate of the current traveling wave is presented which can reduce the impacts of the related fault on the AC system.(5) Based on the characteristics of DC filter, the impedance characteristics of the combinational circuits are analyzed, and a method to identify the fault location by the characteristic harmonic is proposed. Two protection criteria of the fundamental wave comparison and the non-integer harmonics comparison are composed with the phase-frequency characteristics curve derived from complex wavelet transform. Blind zone exists at the terminal of transmission lines just like distance protection in AC systems. Meanwhile, it indicates that the protection performance using the non-integer harmonics comparison is better than that using the fundamental wave comparison.(6) It is concluded that the traditional grounding electrode protection is not feasible to use in the configuration of common grounding electrode lines. The electric parameters under every kind of fault according to the circuit theory are calculated. It is concluded that the unbalanced current is fixed when single line is broken, and all the single-line-faults have very little affect on the over-voltage protection. The affect of the unbalanced currents in bipolar operation on the electrode current unbalanced protection depends on its direction and magnitude. It is suggested that the current unbalanced protection should adopt the fault location and the inverse-time function considering the characteristics of the fault circuit.