Improvement of Stability of Frequency Converters with Active Rectifiers during Voltage Sags and Parallel Operation with an Electric Arc Furnace

The paper gives an efficiency estimation of stability of the rolling mill main electric drives based on three-level frequency converters with active rectifiers during compensation of voltage sags due to reactive current generation. The research group presented the simulation results of the parallel operation of frequency converters with active rectifiers and the electric steel-making complex during surge voltage caused by switching of filter compensating circuits of the SVC. A new method was proposed to improve the stability of operation of frequency converters with active rectifiers during parallel operation with an electric arc furnace (EAF) at additional distortions caused by transient processes during switching of the SVC and the filter compensating circuits. Modern electric drives of rolling mills are designed on the basis of synchronous motors and frequency converters consisting of an active rectifier and a self-commutated voltage inverter. Asymmetrical voltage sags of the supply mains cause shutdown of the main electric drives thus resulting in economic losses and product waste. Stable operation of the rolling mill in such conditions is still an important issue. The object under investigation is the hot rolling mill 1750 of the CJSC MMK Metalurji. The software package Matlab Simulink was used to develop a mathematical model of a frequency converter for pulse-width modulation algorithms with elimination of extracted harmonics and the diagnostic system of voltage sags. The research group used the mathematical model to investigate the operation of the active rectifier in the nominal operation conditions and during voltage sags when various control systems were applied. The mathematical model was also used to carry out the analysis of the line current at the input of the active rectifier and the voltage in the DC link. In accordance with the research, the frequency converters with active rectifiers shut down due to triggering of the overcurrent protection and due to triggering of the undervoltage protection in the DC link. The results were used to carry out the comparative analysis of the frequency converter operation during voltage sags when the automatic diagnostic system of voltage sags was applied and when the enhanced control system of the active rectifier with the comparing unit of the mains voltage with the voltage at the active rectifier input was applied. A conclusion was made about the influence of parameters of current and voltage regulators of the active rectifier on the stability of operation of frequency converters with active rectifiers during asymmetrical voltage sags.