| ABSTRACT:Ice coating on power transmission line in winter brought a series of problems has gottern more and more attention. investigation on the ice melting method has been a hot topic in power transmission and distribution system field in recent years. By comprehensive consideration of the power supply capacity,efficiency of ice melting and the time of ice melting, the joule heat ice melting especially the method based on direct current were the preferred selected method. DC ice melting require an additional DC power to send direct current to ice coating lines, and the DC power can be achieved by some kinds of circuit topology and technology routes. when the power is operating, the power quality of the network also need to be taken more attention, in order to improve the ability of ice melting power, the functions such as power governance and energy transportation should be acquired. The flexible HVDC technology, especially the application in small and medium-side field are becoming a new research direction and becoming very important in the occasions of the in below20MW and emergency power restoration. Meanwhile, STATCOM was always applied to reactive power compensation and require operating all-weather. Study on the converter with all these functions is a novel and meaningful topic.In this dissertation, after the research and analysis of DC ice melting, flexible HVDC transmission, reactive power compensation, a new multifunctional converter technology, which was based on IGBT, was proposed. it had many functions, including DC ice melting, static VAR generator and flexible HVDC transmission technology. Two kinds of new circuit topologies of converter and control modes were proposed, and the device had been developed and applied too. The main contents are as below:1. Accord to the different line ice melting requirement and the problem of multifarious ice melting lines connected, the ice melting mechanism of power transmission line was studied, the equivalent model of ice coating and ice melting process were proposed and the calculation principle of ice melting current was determined, the load line connected method and automatic switching strategy was studied. By equivalent calculation of the load of power line, the maximum ice melting current and preserving line current were acquired. Because the output of DC power only has positive and negative poles and ice coating lines are always three-phase, in addition to railway traction network with single-phase line. A new method of ice melting was proposed, which was based on "one intake and two returns", in order to melt ice automatically, a new ice melting control strategy of automatic switching for three-phase transmission lines was proposed and achieved.2. The multifunctional and integrated problem of flexible HVDC transmission, DC ice melting, reactive power compensation in35kV and above application field were researched, and the technology route of flexible HVDC transmission was studied too, what’s more, a new IPOC circuit topology which had many functions, including DC ice melting, static VAR generator and flexible HVDC transmission technology was proposed, and it was combined with cascading pulses rectifier and hybrid DC/DC chopper circuit topology. The mathematical model and state equation of power unit and main circuit cascading topology was deduced, the equivalent circuits of converter unit was gottern, the controllability and observability present in working process was analyzed, and the unified technical framework and control system was also obtained.3. The problem of How to integrate the DC ice melting and power compensation in10kV and below was analyzed, a hybrid integrated converter topology based on H-Bridge Cascaded which including static VAR generator function was proposed. Based on the analysis of the operating mechanism and operating switch process of topology, the equivalent circuit and mathematical model of power unit and main circuit system were derived, based on the studied of the control strategies in including static VAR generation operating mode and ice melting operating mode, the operating mode of power unit and control method of power unit,the unified framework and hardware control system were obtained.4. According to multiple operating conditions,problem of coordination of the same integrated system control and how to achieve soft-switching was researched, control strategy for multiple operating conditionswas designed, the stability of boundary conditions of cascaded circuit unit voltage sharing was analyzed, the power module power balance control method was proposed too. The continuously and adjustable voltage output in the ice melting state and unity power factor control was achieved, and the harmonic content can be controlled within the range of GB. In the flexible HVDC state, the voltage difference of DC-side between two converter stations was used to control Transmission power by chopper control, and the double-ended converter stations were completely decoupled and achieved, in the static VAR generator state, the dynamic compensation of the reactive power was achieved.5. Through the simulation and experimental verification, two kinds of integrated converter technology were realized on the equipment with excellent result. The integrated equipment based on IPOC achieved many functions, including large capacity DC ice melting based on IGBT, point-to-point flexible HVDC transmission and static VAR generation, by switch of the main circuit and control of software. The integrated equipment based on H-Bridge can be switch between DC ice melting and static VAR generation by the switch in the main circuit. The results showed that compensation effect in inductive or capacitive state was well and the ability of DC ice melting was steady. |
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