High dynamic control of a three-level voltage-source-converter drive for a main strip mill

A high dynamic control system for the Alspa VDM 7000 medium-voltage drive was implemented, which provides fast torque response times of a few milliseconds despite the typically low switching frequency of gate-turn-off thyristors which is necessary to achieve high efficiency. The drive system consists of a three-level voltage-source converter with active front end and a synchronous motor. The drive has most recently been applied to a main strip mill. It provides a maximum of 8.3-MW mechanical power with a rated motor voltage of 3 kV. Besides motor torque as the main control objective, the control system has to comply with a number of additional objectives and constraints like DC-link voltage regulation and balancing, current and torque harmonics, motor flux, and excitation.