Combining Flux Conditioning and Controlled Switching Techniques to Eliminate Transformer Energization Voltage Dips When Connecting DERs to the Grid: A Field Demonstration

The connection of renewable energy resources to a distribution system whose short-circuit capacity may be limited is a notorious source of significant voltage disturbances. The causes of such disturbances can include the high inrush currents occurring when the step-up transformers of distributed energy resources are energized. The mitigation of such transients through controlled switching is impeded by the presence of inverters on their low voltage side. Indeed, most circuit breakers in such installations are three-pole gang-operated. The control strategy for such breakers leverages the presence of residual magnetic flux in the iron core of the de-energized transformer, but DER inverters tend to erase that residual flux. A novel method combining flux conditioning and controlled switching was developed to compensate for this drawback and eliminate, virtually in full, the transformer switching transients. This paper reports on a full demonstration of the method on a transformer tied to a 25-kV distribution line first attached to the main grid, and then to a microgrid's battery energy storage system.