Research On Grid-fault Ride-through Control Strategy Of Variable Frequency Transformer

Building global energy internet,and realizing the power energy substitution and clean energy substitution of fossil energy are the main trends in energy development in the future.The construction of the global energy internet must solve the problem of asynchronous interconnection between power grids with different frequencies,which can be achieved by high voltage direct current and variable frequency transformer.In order to further promote the application of variable frequency transformer,it is necessary to realize the uninterrupted operation of the variable frequency transformer during the grid faults,i.e.,to improve its gridfault ride-through capability.The current researches of fault ride-through operation are only focus on symmetrical faults,while the operation and control under asymmetrical faults have not been studied.Therefore,this paper will focus on the operation of variable frequency transformer under asymmetrical faults,and propose a new topology and control strategies to improve the grid-fault ride-through capability of variable frequency transformer,and lay the foundation for its further promotion and application.Firstly,the status and problems of building global energy internet are discussed.The principle and characteristics of high voltage direct current and variable frequency transformer are introduced.The main control technologies of variable frequency transformer proposed by General Electric Company of the United States are discussed in detail.Secondly,the basic structure and working principle of variable frequency transformer are introduced.The complete mathematical model of variable frequency transformer under dualside asymmetrical faults is derived by using the symmetrical component method and the circuit superposition theorem.A hardware-in-loop experimental platform based on OPAL-RT OP5600 and dSPACE DS1103 was built.The platform was used to evaluate the influence of dual-side asymmetrical faults on variable frequency transformer system and verify the correctness of complete mathematical model of variable frequency transformer proposed in this paper.Thirdly,considering that the variable frequency transformer is a system with small controllable degrees of freedom,in order to seek the optimal principle of grid-fault ride-through,this paper adds a series compensation converter on the stator side of the variable frequency transformer to make the stator voltage and the stator current controllable and improve the grid-fault ride-through capability of the variable frequency transformer: A control strategy considering the stator negative sequence voltage and the stator negative sequence current is proposed,which can maintain independent control of reactive power,and eliminate the torque,active power and reactive power fluctuations caused by dual-side asymmetrical faults.The hardware-in-loop experiment verifies that the control strategy considering the stator negative sequence voltage and the stator negative sequence current can completely eliminate the torque,active power and reactive power fluctuations at twice the rotor rotating angular speed and twice the stator synchronous speed,and effectively eliminate the torque,active power and reactive power fluctuations at twice the rotor synchronous speed.The gridfault ride-through control under dual-side asymmetrical faults is achieved.Finally,considering a series compensation converter is hard to solve dual-side asymmetrical faults,it is decided to add another series compensation converter on the rotor side of the variable frequency transformer to make the rotor voltage controllable and further improve the grid-fault ride-through capability of variable frequency transformer: A control strategy considering stator negative sequence voltage and rotor negative sequence voltage is proposed,which can maintain independent control of reactive power,and eliminate the torque,active power and reactive power fluctuations caused by dual-side asymmetrical faults.The hardware-in-loop experimental results show that the proposed topology of the variable frequency transformer system based on two series compensation converter is reasonable,and the new grid-fault ride-through control strategy can not only completely eliminate the torque,active power and reactive power fluctuations at twice the rotor rotating angular speed and twice the stator synchronous speed,but also completely eliminate the torque,active power and reactive power fluctuations at twice the rotor synchronous speed.It is better to realize the gridfault ride-through control under dual-side asymmetrical faults.The research work of this paper has been funded by the National Natural Science Foundation of China,“Control research of variable speed variable frequency offshore wind energy conversion system based on variable frequency transformer”.The proposed topology and control strategy have been verified by hardware-in-loop experiments and are based on variable frequency transformer.The grid asynchronous interconnection method provides theoretical basis and experience.