Development of an Improved Control System for Active Rectifiers with Additional Negative Sequence Currents Control Loops

Modern main electric drives of rolling mills are performed using asynchronous (synchronous) AC electric motors and three-level frequency converters with active rectifiers and autonomous voltage inverters. Semiconductor switches are controlled by pulse width modulation method with orientation by the supply voltage vector. AR control systems are sensitive to the quality of the mains voltage. Occurrence of voltage sags in the mains leads to FC-AR emergency shutdowns. AR emergency shutdown occurs due to the increase in the voltage swing in the DC link and the increase in the currents consumed by FC, which is caused by the flow of reverse sequence currents. In this connection, the actual task is the development of measures to improve the stability of the main electric drives of the mill at voltage perturbations on the part of the electric power system. The paper develops an improved AR control system with an additional control loop of orthogonal components of the reverse sequence currents to improve the AR operation reliability under asymmetrical voltage dips by maintaining the reverse sequence currents on active and reactive components equal to zero for the entire duration of the voltage dips.