Research On Control Strategy For Power Electronic Traction Transformer

Traction transformer is an important part in the electric locomotive traction drive system.As a power frequency transformer,its volume and weight are large,and thus it is not conducive to the lightweight development of high speed railway.Therefore,it's of great significance to develop a new type of traction transformer,which is small,light and efficient.Power electronic transformer,an entirely new class of transformer based on power electronic conversion technology,can be used to replace conventional transformer and achieve more intelligent control.In this dissertation,a three-stage AC-DC-AC type,input-series-output-parallel cascaded power electronic transformer is used as the research subject.Specific problems such as voltage balance control and power balance control are studied.The causes of DC voltage imbalance in the rectifier are studied in the dissertation.Through analysis and calculation of the loss equivalent model,it is found that the main factors affecting DC bus voltage are the load,the parallel loss,the hybrid loss,and the delay of the pulse.Through analysis of unit transmission power,the principle of voltage balance control strategy is explained,that is,to make the power absorbed from gird match with the unit transmission power.A new proportional pulse compensation voltage balance strategy is proposed in the dissertation.Traditional proportional pulse compensation strategy has poor adaptability for different load conditions.In order to solve this problem,the variation of the average DC current is introduced into the regulator.Thus,the rectifier can achieve voltage balance with a good performance under different load conditions.A controllable operation area analysis method for voltage balance strategy is proposed in the dissertation.Through analysis of the constraint condition needed to be met in the operation process,the expression of two-level CHBR voltage balance control strategy is deduced.In the same way,the expression can be further extended to a three-level or multilevel CHBR.The boundary constraint condition can be expressed as the upper limit of CHBR transmission power,and it can be used to guide the design and application.The control principle of DC-DC converter in the isolation stage is studied in this dissertation.Through analysis of the converter equivalent model,the expressions of fundamental transmission power and total transmission power are obtained and comparied,from the two aspects of fundamental phasor relationship and converter operating waveforms.The basic control method of the converter is thus deduced.The feedback power balance control strategy is also studied in the dissertation.In this strategy,the actual transmission power of each unit is collected and took as feedback value.And the difference of transmission power can be eliminated rapidly by the power loop regulator,which can realize the power balance with a good performance.A power reference compensation voltage balance control strategy is proposed in the dissertation.According to the principle of voltage balance strategy from the transmission power point of view and the power balance strategy in the isolation stage,a new idea of voltage balance control is presented,that is the DC voltages can be balanced by adjusting the transmission power distribution of DC-DC converters.A compensation value based on DC voltage deviation is added to the power reference in the feedback power balance strategy,and thus voltage balance is realized.As this power reference compensation strategy is realized in the isolation stage,the AC terminal voltage will not be affected,which makes a better performance in the aspect of grid harmonic characteristics.