Research On The AC Drive System With The Back-to-Back Converter

The system of the AC drives with the Back-to-Back converter have such the features asfour-quadrant operation, sinusoidal line current, controllable power factor and etc. All ofthese features make the Back-to-Back converter be a hot issue studied by researchers. Thispaper mainly focuses the researches on the problems of the Back-to-Back converter, and thecomputer simulation and the experimental prototype are made to investigate and solve theexited problems of the converter.This paper first develops the mathematical model of the Back-to-Back converter andanalyzes the transient and steady performance of the system by simulation. Then, theparameter designs of the line inductors and the dc-link capacitor are analyzed in detailed, andthe drawback of the traditional dc-link capacitor design method which doesn't take one-stepdelay of the PWM signal into account is pointed out. Thus, it leads to the possibility that boththe converter and the inverter charge to the dc-link capacitor at the same time, and causes thefluctuation of the dc-link voltage beyond the safe range. Therefore, the maximum energy ofthe dc-link capacitor in this case is studied and presents a design method called the minimumof the dc-link capacitance in this paper, which will assure the stability of the system.In order to improve the transient response and restrain the fluctuation of the dc-link voltage,this paper analyzes the limitation of the conventional power feed-forward strategy, and usingthe technology of the small-signal model, an improved feed-forward strategy is presentedwhich can reduce the effect on the dc-link voltage from not only the variation of the load butalso the variation of the grid voltage. In addition, a state observer of load current is proposedto replace the current sensor. The proposed strategies are analysed and testified by both thesimulated results and the experimental results.Due to the effect of the dead time and the minimum pulse width of the switching devices, themodulation index of the space vector PWM can't reach the ideal maximum value, which mayreduce the modulation range of the system. To solve this problem, a method of the minimumdead time and a method of redistributing the dwelling time of the zero vectors are presented.The two methods are carried out at one time, which can reduce the effect of the dead timeand the minimum pulse width on the modulation index greatly and the system modulationrange is increased from the simulation and experimential results.The high-order harmonics in the input current of the three phases PWM converter have agreat influence of EMI on the other sensitive electronic equipments. In order to eliminatethose high-order harmonics, this paper analyzes three kinds of filters which can be used inthe input stage firstly, and then for the problems produced by LC-type filter such as theresonance and the power factor degrading, this paper presents a novel active damping controlstrategy, from the simulation results it is found the proposed strategy can restrain the filterresonance and improve the power factor effectively.