Fault ride-through circuit with variable frequency transformer and control method thereof

A control method of a fault ride-through circuit with a variable frequency transformer (VFT), wherein the fault ride-through circuit with the VFT comprises:

• a first grid, a second grid, the VFT, a first compensating transformer and a first pulse width modulation (PWM) convertor;
• the first grid is connected to a stator interface of the VFT through secondary coils of the first compensating transformer, primary coils of the first compensating transformer are inserted in series between a first output end and a second output end of the first PWM convertor, and a rotor interface of the VFT is connected to the second grid;
• and the control method comprises the following steps:
• S101:
• pretreating a voltage of the first grid Ug1abc, a voltage of the second grid Ug2abc, a voltage of the stator interface of the VFT Usabc and a voltage of the rotor interface Urabc to obtain a stator positive sequence voltage DC component Usdq+, a stator negative sequence voltage DC component Usdq−, a rotor positive sequence voltage DC component Urdq+ and a rotor negative sequence voltage DC component Urdq−;
• S102:
• testing and treating the voltage of the first grid Ug1abc and the voltage of the second grid Ug2abc with a digital phase-locked loop to obtain a voltage frequency of the first grid fg1 and a voltage frequency of the second grid fg2;
• S103:
• carrying out a reference voltage calculation to the stator positive sequence voltage DC component Usdq+, the stator negative sequence voltage DC component Usdq−, the rotor positive sequence voltage DC component Urdq+, the rotor negative sequence voltage DC component Urdq−, the voltage frequency of the first grid fg1 and the voltage frequency of the second grid fg2 to obtain a positive voltage reference value Ut1dq+ and a negative voltage reference value Ut1dq− of the first PWM convertor;
• and
• S104:
• processing the voltage reference values Ut1dq+ and Ut1dq− to obtain switch signals for controlling the first PWM convertor Sa, Sb, Sc.