Study On Current Source Converter-long Cable Transmission System

At present, there are a great many of variable frequency drive industrial products in the market and each of them using different power switching device, topology, control strategy, has their own characteristics, wherein the current source converter(CSC) received wide Applications in high-power applications due to its excellent control characteristics. For the harsh environments as well as the complexity of the explosion-proof converter design, the coal mine often adopts long-distance transmission system. At the same time, due to the increase in power and switching frequency, the negative effects such as over-voltage and conduction disturbances become more and more serious. Based on the R & D projects of surface coal mine centralized inverter system, combining with the current source inverter and the long cable process problems, the work mainly carried out as follows.Firstly, the structure characteristics of the system are analyzed. Based on current source inverter, this thesis conducts in-depth study of the modulation strategy, and mainly focuses on two modulation strategies: trapezoidal pulse width modulation(TPWM) and the current space vector pulse width modulation(SVPWM). Simulation models based on the two modulation strategies are established respectively in MATLAB. The simulations achieve the control of frequency and amplitude of the inverter output current by changing the modulation wave frequency and DC current. Then the two control algorithms are compared.Secondly, based on the principle of the transmission line the pulse wave reflection phenomenon and the mechanism of over-voltage are analyzed, and the analysis shows that the impedance the mismatch is the main reason of motor terminal overvoltage. The expression of CSI common-mode voltage is defined firstly. Then the mechanism and expressions of the common-mode voltage is analyzed according to formula derivation. A common-mode equivalent circuit is established. On this basis, the nine switch states of the two-level inverters and their common-mode voltage value are discussed. Simulation model is created in the simulation software and the common-mode voltage simulations are analyzed. The high-frequency modeling for cable and inducted motor suitable for long cable system is established. The parameters of the models are calculated according to the experimental data, verifying the correctness of the models.Thirdly, system simulation model is built by the combination of MATLAB and PSPice two kinds of simulation software. The reflection rule of current pulse is analyzed comparing with the voltage pulse. Then two kinds of CSI modulation strategy are used to verify the impact of cable length to motor terminal voltage, which confirms that the output of current source inverter is not limited by the length of the cable. A current source converter experiment platform is built and the experimental waveforms are consistent with the simulation results.Finally, this thesis makes further study on the common-mode voltage suppression strategies from hardware and software two aspects. In hardware method, a kind of integrated reactor structure and a neutral point connection structure which can reduce the influence of system common-mode voltage are given. In software method, three kinds of nonzero vector suppression strategy and one suppression strategy with three-phase four-leg power inverter are given respectively and the common-mode voltage is suppressed by using optimizing the modulation strategy. And the simulation results show the effectiveness of the methods.