Transient overvoltage fault analysis in thyristors controlled rectifiers in medium voltage and very high power applications

EasyChair Preprint 5937

Abstract

This paper introduces the analysis of the transient phenomena that occur when energizing or de-energizing a three-phase power transformer whose load on the secondary side is a three-phase rectifier controlled by thyristors which in turn feeds, through a large inductance, an electric arc furnace from a steel melting facility. The arc power is regulated by the firing control of six large thyristors blocks. To understand the transient phenomena in the process of turning on and off large transformers, the present study first presents some theoretical concepts about the origin of common-mode transient overvoltage that occur when energizing power transformers and that are dangerous when feeding power electronics circuits. Then a series of circuits are presented that have the function of damping the sudden changes in voltage that could erratically trigger the thyristors by dv/dt and other circuits to dampen the common mode surge spikes. Finally, a circuit is presented that safely channels the energy stored in the filter inductance when, due to operator actions, like sudden decreases of current in the load, or sudden failures in the electrical feeding grid. The present study was originated to identify and solve a series of erratic and catastrophic failures, of high cost in electronic components and factory production, which began to occur when one of the three power transformers of an 18-pulse rectifier was changed by a new one. Thanks to having a good understanding of the concepts, presented in this document and mostly not covered in classic power electronics texts, a measurement campaign was carried out to capture the moments in which these erratic faults occurred and solve the problem in a satisfactory way.

Keyphrases: Capacidad parasita, Transitorios en rectificadores de 18 pulsos Inductancia de dispersion, sobretensiones

@booklet{EasyChair:5937,
  author    = {Armando Llamas Terres and Osvaldo Miguel Micheloud},
  title     = {Transient overvoltage fault analysis in thyristors controlled rectifiers in medium voltage and very high power applications},
  howpublished = {EasyChair Preprint 5937},
  year      = {EasyChair, 2021}}