Research On V-clamp Multilevel Converter With Control Strategy

Due to the advantages of high output voltage and superior output harmonic characteristics,multilevel converters have been widely studied and applied in the field of high-power energy conversion.Among them,the type of directly-clamped multilevel converters does not need multiple dc power supplies and flying capacitors.All phases share the common dc-link cascade capacitors.Hence,this type of converter has the advantages of compact structure and simple switch control.The work of this dissertation is based on a new V-clamp multilevel converter(VMC),focusing on the operating principle,the snubber circuit scheme and the capacitor balancing control methods of single-ended and back-to-back VMC systems.The corresponding research work is carried out by throughing theoretical analysis and experimental verification.The traditional directly-clamped topologies contain a large number of clamping devices,which makes its level inconvenient to expand.To address this problem,a new multilevel converter in which the clamping branches resemble a “V” shape,named as V-vlamp multilevel converter,is proposed.The basic and deformation topologies of VMC are introduced,and its level expansion method is expounded.The operating principle and characteristics of VMC are analyzed.Each switch branch in VMC is directly clamped by the dc-side capacitor,and only sustains a single-level voltage of dc-link capacitor before or after commutation process.Hence the static over-voltage problem of switches and dynamic voltage balancing problem of series-connected switches are avoided.By comparing VMC with the common multilevel topologies,it is found that VMC reduces the number of clamping devices and simplifies the topology,which provides a preferred solution for the high-power energy conversation in the medium-voltage field.In the practical multilevel converter circuit,if the electromagnetic energy of the long stray inductance is not handled properly,it is easy to form a large voltage sipke on the junction capacitor of the switches which causes the break down fault.Aiming at this problem,a directional migration and absorption scheme for VMC line stray inductance energy is proposed.According to the working characteristics of the VMC series-connected switch branch,its switches are divided into main switches and auxiliary switches,and the corresponding switching modulation method is designed.This scheme transfers the energy of the line stray inductance to the conversion unit composed of the main switches,thereby eliminating the risk of turn-off overv-oltage of the auxiliary switches;by adding absorption capacitors at both ends of the conversion unit,the concentrated absorption of the line stray inductance energy is realized.The level expansion method of the absorption circuit scheme is further introduced,and its parameter design method is analyzed.Through comparative experiments,it can be seen that the proposed scheme only needs a small amount of absorbing capacitors to realize the turn-off over-voltage protection of all switches in the VMC.It has the advantages of low cost,simple structure and easy expansion,which provides effective support for the practical application of VMC.Maintaining the balance of the DC side capacitor voltage is the basic premise for the normal operation of the VMC system.Aiming at the issue of unbalanced capacitor voltage in VMC system,a capacitor voltage balance strategy based on equally-spaced carrier and partition modulation is proposed.Taking the three-phase seven-level VMC system as an example,its mathematical model is established,and the voltage-drift phenomenon of capacitor is analyzed.Based on the principle of equal output duty ratio of each intermediate voltage level,the expression of output duty ratio of each voltage level is deduced,and the equally-spaced carrier of VMC is constructed to realize the capacitor-voltage balance in the full range of operating conditions.On this basis,the constraint condition of the zero-sequence voltage component are analyzed,and a partition modulation method of VMC is designed with the optimization goal of reducing the number of additional switching actions.A capacitor-voltage active compensation control with dynamically adjusted duty ratio is proposed,which improves the dynamic performance of the balancing strategy.By comparing the performance with the commonly used virtual space vector modulation strategy,it can be seen that the proposed balancing strategy reduces the number of additional switching actions and effectively solves the problem of unbalanced dc-side capacitor voltage.Aiming at the problem of capacitor voltage unbalance in back-to-back VMC system,a balance strategy combining zero-sequence voltage component injection and reactive power injection is proposed,which can achieve a wide balanced range of dc-side capacitors without increasing additional switching actions.The voltage-drift phenomenon of capacitors in back-to-back three-phase seven-level VMC system is studied,the balancing control based on zero-sequence voltage component injection is designed,and the corresponding capacitor voltage balanced range under this method is discussed in detail.The influence of the power factor of the converter on the capacitor voltage balance is theoretically analyzed.According to the balance situation of the capacitor voltage,the reactive power of the rectifier is adjusted,so that it can provide sufficient itermediate current regulation capability to maintain the balance of capacitor voltage under different load conditions.The proposed balance stragegy makes full use of the active adjustment capability of the back-to-back system,and extends the balanced range of the dc-side capacitor voltage without increasing the switching frequency.91 figures,10 tables,147 references.