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

A method for operating a static transfer switch electrically coupled to a transformer, comprising:

• detecting, by a controller, an interrupt in a voltage applied to at a load terminal of the static transfer switch, where the voltage is selectively supplied from one of two voltage sources having a sinusoidal waveform;
• controlling, by the controller, amplitude of the voltage at the load terminal using phase fired control of one or more silicon-controlled rectifiers interposed between each of the voltage sources and the load terminal, where controlling amplitude of the voltage is in response to detecting the interrupt in voltage at the load terminal and the amplitude of voltage, which alternates between positive and negative polarity voltages, is set at a value less than a peak value of the sinusoidal waveform;
• and
• increasing the amplitude of voltage at the load terminal incrementally over a given time period until the amplitude reaches a peak value to thereby remove any residual flux in the transformer and thus maintain magnetic flux in the transformer within an acceptable range.

A static transfer switch, comprising:

• a preferred input terminal configured to receive an AC voltage signal from a preferred voltage source;
• an alternate input terminal configured to receive an AC voltage signal from an alternate voltage source;
• a load terminal;
• a transformer coupled directly to the load terminal;
• 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 that monitors voltage at the load terminal and controls conductive state of the semiconductor switches to selectively output voltage from either the preferred input terminal or the alternate input terminal to the load terminal, wherein the controller, 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.