Increasing stability of electric drives of rolling mills with active front ends at voltage sag

Abstract

Modern adjustable electric drives of rolling mills are commonly manufactured on AC with a two-link frequency converter. Stable operation of electric drives with active front ends can be provided at fulfilling certain requirements to supply voltage quality. In the systems of plant power supply at single grid-to-ground faults in the grid 110-220 kV one can observe unbalanced voltage sags in the grid 10 kV. At occurrence of unbalanced voltage sags the conditions for negative sequence over-current flow on the part of active front ends are generated. Negative sequence over-current occurrence leads to the active front end disconnection and violation of an operating cycle. Such situations are especially inadmissible at electric drive power supply of continuous production to which continuous rolling mills also relate. The problem of increasing the stability of active front ends can be provided due to circuit designs as well as by adapting the systems of active front end control. The article considers methods for improving the stability of active front ends to unbalanced voltage sags due to control system upgrade. The authors analyze control systems with feedforward control on negative phase-sequence voltage and grid voltage correction. The simulation results demonstrated that critical values limits capable of causing converter disconnection - grid current and voltage fluctuation in the DC link - don't exceed permissible values in case one applies the control system with correction on the grid voltage and a filter of sliding average with a finite impulse response.