Research On Control Strategy Of Back-to-back Parallel Three-level Converter System

In recent years,with the rapid growth of the installed capacity of wind power generation,higher and higher requirements have been put forward for the single unit capacity,efficiency and reliability of wind power generation units.As the energy conversion interface,three-level back-to-back parallel converters have been widely used due to their large capacity,high reliability,low loss,and good output harmonic performance.However,the zero-sequence circulating current(ZSCC)of the parallel system and the unbalanced midpoint potential of the three-level converter will adversely affect the stable operation of the parallel converter.In this paper,a detailed study on the control strategy of the parallel system of back-to-back three-level converters is done.This paper first analyzes the working principle and different modulation strategies of a single three-level converter.For the back-to-back parallel three-level system,the circulation path of the parallel system is analyzed first,and the mathematical model of the ZSCC of the parallel system is established.It is pointed out that the control of the parallel system circulating current is essentially to eliminate the zero-sequence voltage between the parallel converters.This paper proposes a PIR ZSCC controller based on carrier modulation and a hysteresis ZSCC controller based on a DPWM modulation strategy.It is verified through simulation that the circulating current suppression strategies under different modulation strategies can achieve good circulating current suppression effects under the same or different current setting values of the parallel converters.Regarding the DC side neutral point potential control of the parallel system,this article first analyzes the midpoint unbalance mechanism of a single three-level converter,and obtains the conclusion that the redundant small vector has an opposite effect on the midpoint potential.Based on this,the neutral point potential balance method of zero-sequence component injection is proposed.In the parallel system,the ZSCC and the neutral-point potential unbalance are mutually excited,and the control of both is realized by adjusting the zero-sequence component.In view of the mutual coupling between the neutral point potential and the ZSCC,this paper proposes a coordinated control scheme that takes into account the zero sequence circulating current of the parallel system and the neutral point potential control of the DC side,which can effectively simultaneously consider the control of the neutral point potential balance and ZSCC of the parallel system.The back-to-back parallel three-level system has different performance requirements on the machine side and the network side.Therefore,the generator-side converter adopts the control based on the DPWM modulation strategy to reduce the system switching loss,and the grid-side converter adopts the control scheme based on the carrier-based continuous PWM modulation strategy to improve the waveform quality of the grid-connected current.The overall control strategy of the back-to-back parallel system is proposed,which can take into account the circulation and midpoint control on the machine-grid side of the system,which reduce system loss of the parallel system while ensuring the grid-connected current quality and improve the system performance.Finally,a three-level back-to-back parallel experiment platform was built with three-level power modules in the laboratory,and related experiments including the circulation suppression method under different modulation strategies and the control of the midpoint potential of the DC side of the three-level converter were completed.And the back-to-back parallel system’s circulating midpoint cooperative control and overall control are verified by experiments.