| After the synchronous power grid of Ultra High Voltage (UHV) is completely constructed, the interconnection of regional power grids and a wider range of resource optimization will be achieved. Due to the remarkable increase of the active and reactive power, new challenges to the safe and stable operation of UHV power system now are confronted with, which mainly are manifested in:(1) Due to the restriction of system impedance characteristics and the stability limit, the transmission capacity of transmission lines is impacted, which may not satisfy the need of the increase of transmition power. (2) Because of the increase of power exchange between regions and the aggravation of power flow variety, the voltage amplitude and reactive power in UHV system is acceleratedly changed. The series-parallel hybrid reactive power compensation (HRPC), which is consisted of series compensation and stepped controllable shunt reactor (SCSR), is capable of adjusting reactive power frequently at the case of the sharp growth of power transmission, increasing the capacity of power transmission system and changing the output capacity by adjusting SCSR, suppressing the overvoltage emerging the on-off operation, overcoming the disadvantages of the traditional simple reactive compensation mode. The application of HRPC will make the UHV transmission system much more complex. In order to ensure the safe operation and the stability of UHV system, it is essential to study this subject in depth. The research on the switching characteristics of UHV transmission system installed with HRPC is based on the background above. The detail of the research includes:The electric field intensity and magnetic induction intensity are regarded as fundamental physical variables, based on the general form of Maxwell equations, separating the bound sources which include conduction current, magnetization current, polarization current from field source, the partial element equivalent circuit (PEEC) equivalent circuit model of component which takes surface charge into consideration is obtained. With the method of geometric mean distance and complex component segmentation, the inductance parameter which is composed of multiple integration can also be calculated quickly and precisely. And equivalent structure and its circuit equation of SCSR inner winding are presented. The results reveal that the transient voltage on the high voltage winding will be several times when the frequency of VFTO is close to the fixed oscillation frequency of SCSR.The very fast transient(VFT) is generated by the switching on of disconnect switches(DS) at different positions, which will be influenced by HRPC installed in UHV substation. Based on the equivalent model of HRPC, the theory of transmission line and the simplified model of substation, the expressions of VFTO and VFTC are deduced by mathematical analysis method. The influence of the length of bus-bar, the length of line and the equivalent capacitance of SCSR to the amplitude and frequency of VFTO and VFTC within substation is analyzed. Besides, with regard to switching order of DS, the installation position of parallel capacitor, closing resistor and ferrite, the impact to VFTO on the port of SCSR is provided.The influence of HRPC on transient earth voltage (TEV) is also put forward. The results show that the installation of HRPC will increase the difficulty of insulation design of GIS. It is better to apply closing resistor and ferrite for suppressing VFTO on the port of SCSR.The amplitude of TEV main frequency increases when HRPC is installed in UHV substation.Based on the simplified model of UHV transmission system with HRPC, amplitude-frequency characteristic of overvoltage on the end of unloaded UHV transmission line is analyzed before and after installing the HRPC. Under different compensation degrees of SCSR, the overvoltage of unloaded UHV transmission line is calculated. In view of the condition that the series compensation capacitor banks have not been short-circuited, the relationships between the transient recovery voltage (TRV) characteristics of cicut breaker and the length of lines, the location of fault, the compensation degree of HRPC are obtained. The operating characteristics of circuit breaker are analyzed by the short-circuited time of series compensation capacitor and the compensation degree of HRPC. The results show that the HRPC is beneficial to decrease the closing overvoltage of unloaded line. When gound fault happen and the series compensation capacitor banks have not been short circuited, the circuit breaker of transmission line with HRPC might fail to open. If the power frequency voltage amplitude at the time when the series compensation capacitor banks are short circuited is bigger, the ciruit breaker is harder to interrupt. It is beneficial for the circuit breakers to increase the compensation degree of HRPC.The dynamic arc resistance model of secondary arc is built on the basis of the Mayer modified equation. With regard to the single-phase transient ground fault in the UHV transmission system with HRPC, the change rules of secondary arc and the recovery voltage of arc path are studied. Based on transient simulation and Laplace transform, the low-frequency components of secondary arc current are obtained. With different compensation degrees of series compensation and SCSR, the influence of small reactance at neutral point and the arc resistance on the changing principle of secondary arc current’s frequency component and arcing time is simulated. On the basis of existing suppression measures of secondary arc, the sequence cooperating strategy of bypass breaker in series compensation with single phase autoreclose is put forward. Based on the submerged arc data derived from single variable, the submerged arc under multi-variable function can be predicted by support vector regression (SVR), which is optimized by fast non dominated sorting genetic algorithm. The result shows that the suppression effect of secondary arc is the best when the compensation degree of SCSR is 80%, the neutral reactance is about 700mH, the resistor of arc path is big, and the transient ground fault occurs in the middle of the transmission line.The research achievements in the dissertation have further developed the fundamental theory and analyzing methodologies of switching characteristics of UHV lines with the hybrid reactive power compensation, and present great significance for both the scientific research and engineering application of hybrid reactive power compensation. |
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