Protection of Microgrids During Utility Voltage Sags

Microgrids are systems with clusters of microgenerators, which are installed for distributed power generation. When interfaced to the utility grid, microgrids are exposed to common utility power-quality disturbances. In particular, during utility-voltage sags, large line currents can flow along distribution feeders connecting the micro- and utility grids. To limit this flow of large line currents and, hence, protect the microgrids, this paper proposes two current-limiting algorithms, namely, the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$RL$</tex> feedforward and flux-charge-model feedback algorithms, for controlling a series inverter connected between the micro- and utility grids during utility voltage sags. Both methods function by inserting a large virtual <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$RL$</tex> or <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</tex> impedance in series with the distribution feeder to limit the line-current flow. Detailed descriptions, controller designs, and comparisons of both algorithms are presented. Phasor analyses of both methods are also presented to show how the inserted <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$RL$</tex> or <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</tex> values can be optimally tuned to improve the inverter damping performance and minimize its injected voltages and circulating power under all operation conditions. Lastly, both methods have been tested in simulation and in experiments using an emulated laboratory microgrid system.