Studies Of MMC Control Scheme Under Unbalanced Voltage Conditions

With the development of power electronics technology and control technology,a new full-controlded of semiconductor devices IGBT is applied in HVDC field.In 1997,the world's first HVDC engineering-Hulse Yang experimental project is put into operation.After that,there are a number of flexible HVDC engineering application in the fields of renewable energy grid,large power motor drive,grid connected distributed generation,the city power grid.The modular multilevel converter(Modular Multilevel Converter,MMC)is a kind of voltage source converter topology.Compared with other topologies,the MMC has low harmonic content,high quality of output waveform,good scalability,advantages of simple redundant backup.But due to their own deficiencies inherent in the MMC topology,compared with other traditional two level,three level topology,MMC has problem of circulating current;and when the system occurs a fault,due to the presence of anti parallel diode.MMC can't block the fault current through their own structure.So the study of DC fault current blocking and MMC circulating current problems is of important both theoretically and practically.Studies the MMC circulating current are mainly concentrated in the steady state,and control for unbalanced grid voltage is very complex,need to introduce more than one controller.This paper makes a deep research on the above two points,the main research contents include the following:(1)Modular multilevel converter(MMC)is considered as a promising topology for voltage-source converter(VSC)highvoltage,direct current(HVDC)applications.This paper presents a new control strategy for MMC-HVDC under unbalanced gridconditions.First,a new inner loop current control strategy based on nonideal proportional resonant(PR)controllers in stationary ?? frame is designed,which is more concise compared to the existing dual sequence current control scheme.Second,an analytical expression for circulating current is obtained which shows that the circulating currents will be asymmetric under unbalanced grid conditions and can be decomposed into positive-,negative-,and zero-sequence component.In order to suppress all these components,a new circulating current suppressing strategy is analyzed and designed also based on nonideal PR controllers.Application of nonideal PR controllers makes the control system well adapt to the fluctuation of grid frequency.(2)This paper proposes an enhanced control method for modular multilevel converter based high-voltage direct-current(MMC-HVDC).To control an MMC-HVDC system properly,the ac current,circulating current,and submodule(SM)capacitor voltage are considered.The ac-side current is a fundamental frequency component,and the circulating current is a double-line frequency component.Existing control methods control the ac current and circulating current by separating each component.However,the existing methods have a disadvantage in that the ac-side current must be separated into the positive and negative sequences for control under an unbalanced voltage condition.The circulating current consists of not only negative-sequence components but also positive-and zero-sequence components under an unbalanced voltage condition.Therefore,an additional controlmethod is necessary to consider the positive-and zero-sequence components of the circulating current.The proposed control method has the advantage of controlling not only the ac-side current of the MMC but also the circulating current without separating each of the current components to control each arm current of the MMC.In addition,it can stably control the positive and zero-sequence components of the circulating current under the unbalanced voltage condition.