Research On Control Of Modular Multilevel Converter Under Unbalanced Grid

High-voltage direct current transmission has the advantages of large transmission capacity,long distance,high efficiency and low loss.The development of high-voltage direct current transmission is conducive to the intensive development of hydropower resources,coal-fired resources,nuclear power resources and renewable energy bases.As a bridge between HVDC and AC power grids,converters are the key to achieving safe,efficient and stable operation of the system.When MMC is used in a DC transmission system,the AC side is generally connected to the grid.The “asymmetry” of the grid is ubiquitous,which seriously affects the reliable operation of the system.MMC is a kind of multi-level converter topology.Since its introduction,it has many advantages.Due to its modular design,low output voltage harmonic content and low switching frequency,it has been widely concerned by scholars at home and abroad,but due to its own structural characteristics,MMC also has many problems such as capacitor voltage equalization and circulation suppression.Under unbalanced grid voltage conditions,MMC control is more complicated.Several aspects of MMC have been studied.Firstly,in view of the large computational complexity and high switching frequency of traditional capacitor voltage equalization control,a capacitor voltage equalization control strategy based on linear time selection algorithm and capacitor voltage equalization based on variable reference value comparison are proposed.And based on this,the dispersion of the capacitor voltage is introduced to reduce the switching frequency of the device while maintaining the voltage equalization of the capacitor.The simulation results show that the proposed strategy can significantly reduce the calculation of the capacitor voltage equalization and reduce the switching frequency of the device.Secondly,in the modular multilevel converter,due to the difference of the capacitor voltage,there is a phase-to-phase circulation between the three-phase bridge arms.The existence of the bridge arm circulation not only increases the current capacity of the device,but also increases the switching loss of the device.It is necessary to adopt a circulation suppression strategy to suppress the circulation.The generation mechanism of phase-to-phase circulation is analyzed.The main components of the circulation are DC component and double fundamental frequency component.The circulation suppression strategy based on quasi-degradation quasi-vector controller is proposed.The simulation results show that the bridge circulation is obviously supressed after the circulation suppression strategy is started.Finally,the operating conditions of the modular multilevel converter under unbalanced grid voltage are analyzed.Under the unbalanced grid voltage,the negative sequence current will appear in the output current of the modular multilevel converter.The active power andreactive power generates two-frequency fluctuation,and the unbalanced control strategy based on quasi-degraded vector resonator is proposed.The simulation results show that the proposed strategy can achieve three control targets separately,and compared with the unbalanced control strategy based on double dq-PI,the proposed imbalance strategy has faster dynamic response speed.