Research On The Strategy Of SHEPWM And Beat-less Control For Traction Inverter

AC-DC-AC topology is widely used in locomotive converters, whose power level is up to MW and switching frequency is generally less than 500 Hz. This thesis mainly aims to solve the problem caused by AC-DC-AC topology and low switching frequency.On the basis of SHEPWM theory, solution of acquiring switching angles is converted to one single objective optimization problem without constraints. The problem is solved using improved artificial fish swarm algorithm. This can accelerate the convergence speed and enhance the calculation accuracy. Then the harmonic characteristic of different modes of SHEPWM is analyzed quantitatively. According to the traction inverter features of low switching frequency and wide operating frequency range, a strategy of multi-mode modulation and smooth transition is studied. Most attention is paid on the transition between different modes of SHEPWM. And the surge current by transition has been avoided. A simulation system on matlab is designed to verify the strategy.A scheme of repetitive-control feed-forward compensation is studied to solve the dc link voltage pulsation problem. It can predict the average value of the dc link voltage to calculate the required pulse width. Base on the otherness of different modulation modes, a three-segment beat-suppression strategy is presented, which is firstly using repetitivecontrol feed-forward compensation method in the SPWM mode, secondly utilizing traditional feed-forward compensation method in the SHEPWM mode, and thirdly turning into frequency compensation method when the one-pulse mode starts. Also a realization means based on integral is used for improving the traditional frequency compensation method. Above strategies are verified through matlab/simulation, coding by S-Function.