| With the development of large-scale technical transformation in coal mine, greater demands are being placed on mine power quality by more and more sensitive loads in coal mine. At the same time, various power quality problems, such as harmonic pollution, reactive power impact and voltage fluctuation, are caused by a lot of nonlinear, resistant, impact loads in mine grid. The contradiction between them is increased acutely. Decline in power quality of coal mine not only endangers the security of power grid, but also affects the safe and stable operation of power equipment in coal mine. Improving the power quality of mine grid and meeting needs of production development of the coal mine are increasingly becoming the common desire of supply and demand sides in mine.In order to improve the power quality of mine grid and provide the user with pure power, it is necessary to compensate the power quality of mine grid. While the current compensating devices, such as active power filter, static reactive power compensator, static reactive power generator, have certain limitations. Only one aspect of the power quality problems is solved by the devices. If different power quality compensation devices are installed together, other problems, such as maintaining, coordination resource wasting, will occur. Therefore, it is urgent to study the comprehensive compensation device of the power quality for mine grid.As a kind of comprehensive control device of power quality Unified Power Quality Controller (UPQC) has changed the traditional control method of coal power quality which respectively compensates voltage or current. It can control coal power and improve the power quality of mine grid according to the requirements of the user. It not only can rapidly compensate fluctuations, mutation, interrupt and flicker of the supply voltage, but also can suppress the harmonic and compensate reactive power. It is a multi machine which can save cost and reduce the complexity of equipment maintenance. It is considered as the most promising power quality conditioner. Based on the medium voltage distribution network in coal mine, the UPQC of coal mine was studied deeply and systematically.The power quality of power grids in coal mine was analyzed in order to provide theoretical basis for the research of UPQC in coal mine. Fristly, the major power quality problems of power grid in coal mine, including voltage deviation, voltage fluctuation, power factor, harmonic and voltage drop, were analyzed. Then the high power three-phase bridge rectifier circuit and mine hoist which were the main harmonic sources in coal mine were analysized from the aspects of power quality, respectively. And the corresponding simulations were performed. The results showed that many kinds of problems about power quality of power grid were in coal mine. Therefore, in order to improve the power quality, comprehensive compensation was necessary for mine grid. It is necessary for coal mine to study the UPQC which can comprehensively regulate the power quality.The topology of UPQC which was applicated in the medium voltage distribution network in coal mine was researched. Small capacity of UPQC experimental prototype has been reported at home and abroad, which is unable to meet the requirements of comprehensive compensation of power quality in the medium voltage distribution network in coal mine due to the limitation of compression capacity of the individual power device, but the large capacity of UPQC is still blank. Therefore, it is necessary to study a new topology of UPQC with the application of multilevel technology to solve the power quality problem of medium voltage grid in coal mine.Compared with other traditional multilevel converter, the modular multilevel converter (MMC) has obvious application advantages in the field of high voltage and large capacity. A new kind of MMC-UPQC topology was put forward with the application of MMC in the field of UPQC.According te the working mechanism of modular multilevel converter, the mathematical model of MMC was established. Based on the physical circuit model, the switching model of MMC in static coordinate was obtained. The mean value model of MMC in static coordinate and in dq rotating coordinate were developed through the continuous switch model and the coordinate transformation. And small-signal linear model of MMC was obtained by adopting the methods of small-signal modeling. Then it could provide guidance for the research on the system characteristics, the design of the main circuit parameters and the design of the control system for MMC-UPQC.A novel compensation detecting method for MMC-UPQC was proposed. The fundamental voltage component was obtained by the online adaptive adjustable learning rate of linear neuron; positive sequence voltage components of fundamental was extracted based on symmetrical component method and voltage compensation was acquired too. The current compensation was calculated by using the obtained results and the energy balance principle. The power quality could be compensated effectively by the compensating method of UPQC. The compensating method had the characteristics of fast dynamic response, simple structure and wide adjustment range of learning rate. Many hardwares, such as digital signal processor chip, could be easily implemented through it.Modulation strategy of MMC-UPQC was studied further and a new strategy of the space vector pulse-width modulation (SVPWM) was proposed,which was suitable for MMC. The sector judgment was required only by the values of the intermediate variables and their symbols without the arctan function and irrational number operation process and at the same time the calculation amount was greatly reduced. Then the criterion of the small triangle for the reference vector was given and multilevel vector synthesis and the calculation of the conduction time were transformed into the problem of two-level. The judgement of the small triangle and the calculation of the vector conduction time were acquired only by computing the line voltage of reference vector with simple calculation process.Finally, the optimal output switching sequence was given by using the redundant switching state of the vector, with the result that the loss of the switching devices was reduced, look-up table procedure was saved and implementation was convenient. The modulation strategy could be extended to any level converter.The sub-modules capacitor voltage balance problem of MMC was analyzed. In view of the fact that the traditional control method of capacitor voltage balance could waste a lot of system resources due to the sorting operation, switching frequency could be higher, and the initial investment and the cutting states of the sub-modules were not considered,an optimization method of sub-modules capacitor voltage balance was presented, which reduced the switching frequency and did not need sorting. In the premise of ensuring the quality of output voltages and the relative consistency of sub-modules voltages,the optimization method could make sub-modules keep original states and avoid frequent switching actions caused by the minor change of capacitance voltages by introducing the upper and lower coefficients of capacitor voltage. At the same time it could ensure that the capacitor voltage difference between the sub-modules was not too large by setting the threshold of the voltage difference between the capacitor voltage of sub-modules. It not only eliminated the traditional sorting process,but also improved the dynamic response speed of the system.Circulation exists in the modular multilevel converter. To solve the problem, the generation mechanism of inner circulating current of MMC was analyzed in detail. The circulating current suppressor was designed based on quasi-proportional resonant controller. Double frequency circulation was fully restrained by the controller without two frequency negative sequence of coordinate transformation and phase current decoupling.The control strategy for the parallel side and the series side of the MMC-UPQC was studyed. Since the inductance of bridge arm was directly connected in series with bridge arm of each phase in MMC,the current coupling problem between d and q axis was still existed in the small-signal current model of MMC in dq coordinate system coordinate system, which was difficult to control. Aiming at this problem, in order to implement the good compensation effect for MMC-UPQC, based on the mathematical model of the MMC the double-loop control system to parallel side and series side of UPQC was designed, respectively. It included outer-loop voltage control and the inner-loop current decoupling control. It realized the decouple of the d axis component from the q axis component and fast track of reference current and voltage at the side of inverter AC side. The simulation results proved the correctness of control strategy to parallel and serial side of MMC-UPQC.The system design of MMC-UPQC prototype was studied and the proposed methods were verified by taking the prototype as the experimental platform. The overall structure of the MMC-UPQC prototype was given and the main circuit parameters of MMC-UPQC were designed. An Architecture of three level control system was presented for MMC-UPQC and the software and hardware in the control system of prototype was designed. The experimental results verified the feasibility of the proposed MMC-UPQC strategy. |
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