Operation And Control Of MMC-UPFC Under Unbalanced Grid Conditions

China is a country with a vast territory,its production and consumption of energy is in the state of obvious reverse distribution.Therefore,it is necessary to develop long-distance transmission line.However,long-distance transmission is not only limited by the environment and cost,but also brings the adverse influence on the power system.Furthermore,the West-to-East power transmission project will face the serious bottleneck of capacity.The flexible AC transmission system can improve the transmission capacity of the grid and the controllability of the grid operation.Therefore,it has become one of the most important supplements to long-distance transmission.As one kind of the flexible AC transmission device with superior performance,the unified power flow controller has received extensive attention.On the other hand,with the further development of the global energy interconnection,the domestic and foreign transmission systems have been developed rapidly.With the increase of loop networks in power systems,the large-scale application of power electronic equipment and the diversity of user loads,the three-phase unbalance in transmission lines has become increasingly obvious.Thus,the power quality problem urgently needs us to solve.In this paper,a general control strategy based on modular multilevel converter-unified power flow controller is proposed to control the three-phase unbalance of transmission line by series negative sequence and zero sequence voltage in the line.The dual-loop controller of MMC-UPFC in positive,negative and zero sequence synchronous rotating reference frame is established and the effect of different transformer connection modes on zero sequence components is analyzed.The zero sequence current can be suppressed by the selection of the connection mode.As a result,the zero-sequence controller can be omitted and the design of the overall controller is simplified.In addition,the inner circulating current of MMC-UPFC is analyzed and the corresponding circulating current suppressing controller is designed.The operation range of MMC-UPFC is analyzed quantitatively,and the mathematic analysis formula of the operation range of MMC-UPFC is given.The accuracy of the formula is verified against the point-by-point scanning method.Based on the analysis of operating regions,the voltage limit control is proposed.And the final cascaded control strategy is composed of voltage limit control strategy and symmetric component control strategy.When the UPFC is over-modulation,the set point value of the power flow is reduced correspondingly,thereby ensuring the governance effect of the three-phase unbalanced line.The cascaded control strategy enables the actual point to operate at the boundary of the maximum controllable region,ensuring that the power flow can be as close to the set point as possible.Thus,the MMC-UPFC can operate continuously and steadily under unbalanced grid conditions.Finally,MMC-UPFC and related control system model are built in PSCAD/EMTDC,and simulation of the cascaded control strategy is carried out in different situations.The simulation results are provided to validate the effectiveness and robust performance of the proposed control strategies.