Protection of medium voltage SCR driven soft-starter from high frequency switching transients

Abstract

Starting of an induction motor is a process that creates many challenging problems for the motor and operations of the power system. The induction motor can be damaged, characteristics can be changed, and performances of the motor can be worsened. Having a significantly higher starting current than the rated current can produce mechanical and thermal stress on the motor and the loads. High-voltage fluctuations, dips, and sags can occur in electrical power systems associated with the motor. Many different methods have been developed to address particular induction motor starting problems associated with motor size and the stability of the connected network. A soft starter is a relatively new device and is used for smooth start of induction motors, particularly medium voltage motors, and adds flexibility in the operation and the interoperability. It is an integration of complex power control apparatus for reducing starting current in a medium voltage solid state soft starter. Electrical transients during switching ON and OFF processes are a major concern for the silicon controlled rectifiers (SCRs), current rise rate during switching ON—di/dt, and over voltages during switching OFF—dv/dt. During the transient process, the currents rise and di/dt passing through the SCRs can damage them. This paper is focused on identification of parameters of the transient process, protection, and limitation of di/dt by inserting an inductor in the soft starter circuit to limit di/dt to a safe level. Mathematical analyses for the purposes of the SCR protection and measurements are presented.