Operating a static transfer switch to eliminate transformer inrush current in the presence of residual flux

摘要

A method is provided for operating a static transfer switch to eliminate transformer inrush current in the presence of residual flux. The method includes: detecting an interrupt of electric power at an output terminal of the static transfer switch; controlling amplitude of voltage at the output terminal using phase fired control of one or more silicon controlled rectifiers interposed between a voltage source and the output terminal, where the amplitude of voltage is set at a value less than a peak value of the sinusoidal waveform; and increasing the amplitude of voltage at the output terminal incrementally until the amplitude reaches a peak value.

主权项

1. A static transfer switch, comprising:
a preferred input terminal configured to receive electric power from a preferred voltage source; an alternate input terminal configured to receive electric power from an alternate voltage source, where the electric power from the preferred voltage source and the alternate voltage source has a sinusoidal waveform; a load terminal electrically coupled to a transformer; a first switch arrangement electrically connected between the preferred input terminal and the load terminal, wherein the first switch arrangement includes one or more semiconductor switches; a second switch arrangement electrically connected between the alternate input terminal and the load terminal, wherein the second switch arrangement includes one or more semiconductor switches; and a controller configured to monitor operating parameters of at least one of the preferred voltage source and the alternate voltage source, and operates to selectively couple one of the preferred voltage source or the alternate voltage source to the load terminal in accordance with the monitored operating parameters, wherein the controller is interfaced with the semiconductor switches and, upon detecting an interruption of electric power at the load terminal, controls conductive state of the semiconductor switches in a startup mode such that amplitude of the voltage at the load terminal is set to a value substantially less than a peak value of the sinusoidal waveform and increases incrementally to the peak value to thereby remove any residual flux in the transformer and thus maintain magnetic flux in the transformer within an acceptable range, wherein the sinusoidal waveform alternatively applies positive and negative polarity voltages.