Research On Active-clamp Multilevel Converter With Common DC-link Capacitors

Multilevel converters have important research value and broad application prospect in the field of medium/high voltage high power conversion due to their advantages,such as low voltage stress of switches,low harmonic distortion of output waveform and low system EMI.Among them,the multilevel converter with common DC-link capacitors are widely used in modern industry because of its simple structure,and the compact DC-link capacitors are easy to control in industrial applications.This paper proposes a novel derivation method of multilevel converters,which is based on the common DC-link capacitors multilevel converters.On this basis,this paper mainly investigates V-clamp Multilevel Converter(VMC)topology family,and carries out related research on its operation principle,switching control method and snubber design.Firstly,according to the principle of equal voltage stress of power devices,this paper proposes a topology derivation method of common DC-link capacitors multilevel converters based on topology network graph,which makes the deduction of multilevel converters’ topologies complete.This paper constructs the matrix expression model and the matrix constraint conditions,and describes the main steps of the topology derivation method.According to the proposed topology derivation method,various common DClink capacitors multilevel converters can be obtained.On this basis,this paper mainly investigates the derivated novel family of topologies,named V-clamp multilevel converters,which have a smaller number of devices.Besides,VMC has capacitors for only common DC link without flying capacitors for phase legs,which makes the control of VMC much easier and simpler.This paper reveals the composition of the VMC topology family,including the expansion method,general form,and evolution type.On this basis,this paper investigates the operation principle and the commutation process,analyzes the direct clamping mechanism and blocking voltage distribution,which provide theoretical guidance for the application of VMC.Since the actual circuit is not ideal,the stray parameters of the circuit will cause an overvoltage when the switch is turned off,which will affect the safe operation of the converter.Aiming at the turn-off overvoltage problem in multilevel converters,this paper analyzes the generation mechanism of voltage spikes and the influence of circuit parasitic parameters.According to the characteristics of VMC,this paper divides the VMC structure into six basic conversion units,proposes a simple and effective snubber circuit,and analyzes the mechanism of the snubber circuit.The simulation results show that the snubber circuit can effectively suppress the turn-off overvoltage spike.In addition,the snubber circuit is simple and practical.Finally,this paper introduces the design of the seven-level VMC experimental platform,and conducts experiments on the platform to verify the feasibility of the VMC circuit and its snubber circuit scheme.