Research On Power Balance Strategy Of Cascaded Inverter Based On Carrier Reconstruction

Compared with the two-level inverter,the cascaded H-bridge multilevel inverter has the characteristics of good output voltage waveform quality,low electromagnetic interference,easy modular expansion,and strong fault tolerance.For the traditional modulation strategy of the CHB inverter,only a single control goal of optimal harmonic characteristics of the inverter output voltage or power balance between cascaded cells can be achieved,and coordinated control between the two cannot be achieved.Poor harmonic characteristics of the output voltage will affect the output waveform quality of the inverter.Unbalanced output power will affect the harmonic current elimination on the input side of the phase-shifting transformer,or affect the battery life.Aiming at the shortcomings of the existing modulation strategies in the application of cascaded H-bridge inverters,this paper combines the advantages of carrier phase dispositon and carrier phase-shift modulation strategies,and proposes a new random carrier full-cycle power balance strategy and a new power balance modulation method which is based on carrier reconstruction.Among them,the new random carrier full-cycle power balance strategy can realize the uniform distribution of active power among cascaded cells in 1/2 new carrier cycles through the random and uniform distribution of carriers in each layer domain,but the number of carriers required for modulation is large.By reconstructing the triangular carrier in the optimized and improved CPS modulation strategy,a carrier arrangement with the advantages of carrier phase shifting is obtained.The inverter output voltage waveform in this carrier arrangement is exactly the same as that under the PD modulation strategy.Therefore,this modulation strategy can not only achieve the natural balance of inverter output power,but also ensure that the inverter output line voltage harmonic characteristics are optimal,and the number of carriers required for modulation is small,and the implementation is simple.In this paper,a three-cell cascaded H-bridge inverter is used as an example to study the carrier reconstruction method.The principle of the new power balance modulation method and the time required for power balance are analyzed.The proposed modulation strategy is extended to make it suitable for n-cell topology.Finally,a simulation and experimental platform for a cascaded H-bridge inverter is built,and the power balance performance and scalability of the proposed power balance control method are simulated and experimentally verified.The final results show that the proposed control method is feasible and the theoretical analysis is correct.