Research On Modulation And Electrolytic Capacitor-less Control Method Of Three-phase Three-level T-type Grid-connected Inverter

Compared with d the I-type topology,the three-level T-type topology has the advantages of fewer power devices and evenly distributed switching losses due to the lack of two power diodes per phase,so it has been widely used in high-voltage and high-power applications.At present,different modulation methods are used for grid-connected inverters in the three-phase three-wire and three-phase four-wire distribution power network system.Meanwhile,in order to suppress the power fluctuation,the electrolytic capacitor with larger capacity value is adopted for the DC bus of the traditional two-stage grid-connected inverter,but the electrolytic capacitor has the drawback of short use time and low withstand voltage,which is difficult to meet the practical requirements of high stability and long use time of the grid-connected inverter.Therefore,in this paper,the modulation and electrolytic capacitor-less control methods of the three-level T-type grid-connected inverters are studied in three-phase threewire and three-phase four-wire system,and the research contents are summarized as follows:Firstly,the current research status of the subject is introduced.The working principle of the three-level T-type grid-connected inverter in three-phase three-wire system and threephase four-wire system is analyzed,and its mathematical model is also established.Secondly,in view of the disadvantages of the traditional three-phase three-wire threelevel space vector pulse width modulation(SVPWM)method is not suitable for the application of three-phase four-wire system,an improved SVPWM method based on abc coordinates is proposed for the three-phase four-wire system,which is convenient for digital implementation.Meanwhile,in order to meet the practical requirement of three-phase three-wire and threephase four-wire system,a unified carrier modulation method is proposed.In this method,the modulation wave is decomposed and compared with the triangular carrier wave,and the balance of DC capacitors voltage is realized by injecting the offset voltage which is composed of the direction of the grid current and the value of the DC capacitors voltage,and a lot of sector judgement and triangulation calculation are avoided.The control structure of the proposed carrier modulation method adopts voltage and current double closed-loop control,and the current inner loop adopts the quasi-proportional resonance control method under the abc coordinate system to realize the non-static tracking of the grid-connected current.Then,based on the power feedforward strategy,a control method of DC-link electrolytic capacitor-less is proposed for the two-stage grid inverter,and the voltage fluctuation of DC bus capacitor caused by the power variation is reduced.Therefore,film capacitors with long service life and small capacitance value can be used to replace electrolytic capacitors with short service life and large capacitance value,the service life of the grid inverter system is improved.Meanwhile,the stability of the system is analyzed in detail.Finally,according to the research content and simulation parameters,an experimental prototype platform with a rated power of 20 k W was built to evaluate the practicability of the space vector modulation,unified carrier modulation,closed-loop control,and electrolytic capacitor control methods proposed in this article.