Study On Control Strategy Of MMC Type Power Electronic Transformer

With the improvement of quality requiremnent of users,traditional power transformers are unable to meet people's demands due to their large volume and heavy environmental pollution.Power electronic transformers combined with high-frequency transformer technology,can connect energy storage equipment and improve power supply reliability.It has become more suitable for modern power grid.The power electronic transformer with the new modular multi-level converter structure has the advantages of high modularity and low environmental pollution,which can provide more reliable power for the power gird.The power electronic treinsformer with modular multilevel converter structure is composed of three stages:rectifier stage,intermediate bidirectional DC/DC double active bridge isolation stage,four-arm three-phase inverter stage.This paper studies the new topological structures at different levels were studied,their working principles were analyzed,and corresponding control strategies were put forward.Targeting at solving the problem of poor waveform quality at low input level of recitifier,the level approximation method has been improved.The rectifier stage control strategy of modularized multilevel converter was proposed,which consists of DC voltage outer loop,positive and negative sequence current inner loop control and capacitor voltage equalization.Single-phase-shift(SPS)control has been adopted in the intermediate stage of DC/DC dual active bridge.Aiming at the problem that the parameters of each intermediate converter are not identical in actual production,the balance control of power feedback was added on the basis of the single-phase-shifl control,which effectively improved the stability of the intermediate stage.Four-leg three-phase inverters can drive unbalanced loads.Three-phase three-leg uses positive and negative sequence voltage control to achieve the control purpose.Fourth-leg uses neutral current to track the negative sum of the load current to achieve the control effect.Under the simulation environment of MATLAB,the simulation models of power electronic transformer with modular multi-level converter structure were built and simulated.The expected control effect has been achieved at all levels.The three-level connection was made in turn,and the overall simulation model of power electronic transformer was built to verify the feasibility of the control methods at all levels.