Research On Cascaded Multilevel Converter With Open Neutral Point

With the increasing exhaustion of fossil fuels,vigorous development of clean energy has become a hot issue in today's society.Renewable energy generation such as solar energy and wind energy can promote the construction of low-carbon energy systems,and has been vigorously promoted by countries around the world.Multilevel converters are the first choice for high voltage and high power applications.Due to their high power quality,low switching losses,high voltage rating,and easy expansion,they have been widely used in large-scale photovoltaic systems and energy storage systems in recent years.This article will introduce a new type of hybrid multilevel converter---cascaded multilevel converter with open neutral point.The converter is a combination of a cascaded H-bridge and a half-bridge circuit.The effect of almost doubling the level can be achieved by adding a simple half-bridge circuit.At the same time,the half-bridge circuit does not require a DC voltage source,only uses capacitors for power supply,and the capacitor voltage can achieve self-balancing without additional control.This article will make in-depth research on its topology,working principle and control strategy.First,this article takes the cascaded multilevel converter with open neutral point as the research object,introduces its working principle in detail,analyzes the advantages of this topology,and establishes its mathematical model under gridconnected conditions.Based on this,a simplified algorithm is introduced as its modulation strategy.At the same time,the high and low frequency combined duty cycle direct distribution method is used to allocate the duty cycle of each power unit,so as to reduce the equivalent switching frequency of the system and reduce the switching loss.In addition,a modulation extension algorithm is introduced to extend its linear modulation range and improve the utilization rate of the DC side voltage.Secondly,this paper proposes an inter-phase power active control strategy to achieve accurate allocation of inter-phase power.In view of the small control range of this method,this paper proposes a power control range extension algorithm to improve the system's power control capability.At the same time,this paper makes further improvements on this basis,and proposes an optimized power control range extension algorithm to achieve the optimal control of system power.In addition,this paper also analyzes and compares the power control limits of the three methods.Finally,a simulation model is built in Matlab / Simulink,and an experimental platform is set up under laboratory conditions to verify the feasibility and effectiveness of the topology and control strategy proposed in this paper.