| With the increase of the load density and capacity of the power grid in China, short circuit current has become evident large, compared to the breaking capacity of high voltage (HV) circuit breakers, and this has also become critical problem to need solving urgently. Hence, the investigation on the new type of fault current limiter with preferable performance in the technology and in the economy is of significance for the construction and development of the power grid. The economical-type fault current limiter (FCL) based on conventional electrical devices and components presents the following advantages, i.e., strong ability of the limitation to short-circuit current, better reliability in operation, and the auto-switching realization without additional control. Due to these, it is expected to be widely applied in the high-voltage system.In order to advance application of the economical-type FCL mentioned above in the high voltage power grid, in this dissertation the systematic studies on the key technical problems for the application of the economical-type FCL have been made. The studies cover several aspects as follows:investigation on a series resonant type of FCL based on Metal Oxide Varistor(MOV) and fast transfer switch, including the analysis of dynamic current limiting characteristic of new type of FCL, improvement of practical topology for FCL, and its optimization of component parameters; impact of FCL on the transient recovery voltage property of circuit breakers; calculation analysis and design for the ventilation structure of MOV with large capacity, including the optimization of the ventilation structure and the temperature characteristic analysis; numerical direct coupling calculation of both the flow field and temperature field in MOV; simulation analysis of high-voltage and fast transfer switch based on electromagnetic repulsion mechanism, including finite elements simulation of electromagnetic repulsion mechanism and the test for fast transfer switch;the engineering application calculation and its parameters design of FCL. These aspects are described below.Based on the analysis on the topologies of different types of the series resonant FCL, a new fault current limiter topology is proposed. This topology consists mainly of MOV and fast transfer switch. By comparing the calculated short circuit current before installing FCL with that after installing FCL, it is shown that the limitation of this topology to fault current is remarkable. For a10kV simulation system of installing the new fault current limiter, the dynamic performance of limitation of the new topology to the short circuit current is simulated by using EMTP code, and all the factors of influencing the performance of FCL are systematically analyzed. On the basis of the above, both the improvement for the practical topology and the optimization of the component parameters have been carried out. These provide fundamental theory for the engineering application of FCL.The topological combination for the new type of FCL based on MOV and fast transfer switch are carried out. Following this, a comprehensive fault current limiter is proposed and its equivalent model is established. For different running mode of the comprehensive FCL and for its different location in transmission line, an analytical expression has been presented for describing the relationship between the rate of rise of recovery voltage (RRRbr) of circuit breakers and current limit factor, series compensation degree, and short-line fault distance. Using the presented expression, the systematical calculations and analyses have been performed to reveal the influence of the several factors on RRRV. These several factors include the running mode, the location of the comprehensive FCL in transmission line, and fault type. The results obtained above provide the basic and theoretical reference for the parameter optimization of comprehensive FCL and for reliable selection of the breaking characteristics of IIV circuit breakers.As the key component of FCL, MOV will absorb short-circuit energy during the short-circuit fault in power system, leading to the instantaneous temperature increase of itself. The heat transfer ability of air-cooling system is of critical importance for the MOV reliability. Therefore, it is very important to reasonably optimize the ventilation structure of MOV used in power system. In this dissertation, based on the structure design of large capacity MOV performed early and its corresponding experimental researches, two optimization designs about MOV structure (marked by MOV-1and MOV-2, respectively) have been proposed. With Computational Fluid Dynamics (CFD) principle and finite volume method, the fluid field velocity and temperature distribution of both basic MOV structure and the two optimized ones have been systematically calculated and analyzed. It is shown that when compared to those due to basic MOV structure, more heat can be taken away from the optimized MOVs and the decrease of the maximum temperature of the two optimized MOVs is about35℃and40℃, respectively, indicating evident improvement for the heat dissipation by MOV-1and MOV-2. In addition, the MOV-2is obtained by adding the radial air-cooling hole on the basis of the MOV-1, and its heat dissipation is obviously improved, compared to the MOV-1. According to this, a practical MOV-2has been constructed and its temperature increase has been determined experimentally. Good agreement between the determined temperature increase and the corresponding theoretical calculations by use of3D finite volume analysis is obtained, showing that the MOV-2can be implemented in power system and indicating the reliability of the calculations performed in this dissertation.With the finite volume method, the three dimensional analysis model used for the direct coupling calculation of the flow field and the temperature field of MOV is given. A simplified experiment model of MOV is established and the more accurate calculation results for the flow field and the temperature field of MOV are obtained. These examine the reliability of the simulation model and overcome the difficulty that the accurate results of the two fields of MOV can not be derived from other calculation methods because of being not able to exactly determine the coupling parameters, such as heat dissipation coefficient. Further, the influences of the major factors on MOVs temperature characteristics have been simulated and analyzed in detail. These major factors are the inlet wind velocity of the radial wind channel, the radial cooling channel radius, the materials used for the conductive ring and thickness of the conductive ring.It is shown that during the period of t<300s,the heat conduction of metal conductive ring is a dominative factor of influencing cooling of MOV, the effects of the increase of only inlet wind velocity on the cooling of MOV are not obvious, and the heat conduction of metal conductive ring is weakened by increasing the cooling channel radius. When the constraint conditions for the height and weight of MOV are satisfied, better cooling effect could be obtained by increasing thickness of the conductive ring and by constructing the conductive ring using metal material with larger thermal conductivity. During the period of t>300s, the heat convection play leading roles in the ventilation structure of MOV for the cooling of MOV, the cooling effect of MOV could be strengthened obviously through the increase of the inlet wind velocity and the cooling channel radius. Due to the above results, the general design rules for obtaining a preferable MOV have been given, which is the theoretical basis for optimizing cooling structure of MOV.The high voltage and fast transfer switch is one of the key facilities governing the current limiting performance and reliability of the economical-type fault current limiter. The parameters, such as the opening time, closing time, and so on, influence directly the current limiting effect and safety operation of the FCL. In this dissertation, a two dimensional finite element model and coupling circuit model for the analysis of the transient characteristics of the electromagnetic repulsion mechanism is presented. According to the results simulated using the different parameters, i.e., coil turns number, thickness and radius of copper dish, and capacity and voltage of energy storage capacitor, the influences of these parameters on the electromagnetic repulsion force are obtained. As a result, the general principles of designing the electromagnetic repulsion mechanism are given. Based on the simulation results, a10kV high voltage and fast transfer switch with an electromagnetic repulsion mechanism was developed.lt can open within4.8ms from the trigger signal, and the closing time is16.2ms, which satisfied the demands of economical-type FCL for open and close speed of the fast transfer switch.In the end, the research on engineering application calculation and its design parameters of economical-type FCL at convenience of installing the FCL in site has been made. Based on the research, the specific design parameters were obtained and the verification calculation of design parameters has been carried out.The achievements obtained in this dissertation provide the fundamental theory and new analysis methodologies for the development of core component of economical-type FCL and are of great significance for promoting the engineering application of economical-type Fault Current Limiter in high voltage power grid. |
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