Research on AC Asymmetrical Fault Control Strategy AC/AC System Based on Hexverter
Hexagonal modular multilevel converter,as a new type of AC/AC power converter,has broad application prospects in grid connection of offshore wind farms in the future and low-frequency transmission technology.For achieving the stable operation of the Hexverter in asymmetrical condition of the power system,the topological structure and working principle of the Hexverter are simply introduced at first in this paper,and its branch current and branch power are analyzed.Then,the control strategy of the AC side of AC/AC system under the asymmetrical condition is studied,the power distribution among six branches of Hexverter is obtained on the basis of theoretical derivation(i.e.,feature of equal power of three branches that are not adjacent),the method of sub-module voltage balancing control,consisting of capacitive voltage balancing control among six sub-modules and the balancing control among capacitive voltage of n pieces of sub-modules,is designed.Considering that the branch currents contain different frequency components of AC systems on both sides,a branch current control method based on proportional resonance controller is proposed.Finally,its working principle and the effectiveness of the control strategy proposed in this paper are verified at the steady and faulty condition of the example.The results show that in case of short circuit fault of phase B and C at the AC system of the secondary side,the whole system maintains stable operation,the voltage of each sub-module always remains balanced and the effectiveness of the control strategy proposed in this paper is verified by the simulation result.
AC asymmetrical faultproportional resonancemodular multilevel converterloop currentsub-module voltage equalization
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