Design of a priority-based load shed scheme and operation tests

Many large industrials operate internal co-generation systems to provide a substantial portion of their own power as well as to use the resulting output heat as part of their industrial process. Depending on the amount of co-generation, on loss of connection to the main grid or instability of the grid, these industrials may have the option of supplying all internal electrical loads from the co-generation or rather, shed a portion of the load in order to maintain plant stability. In order to maintain and maximize plant operations during either an islanded condition, a severe (and dropping) underfrequency condition, or a prolonged under voltage condition, a load shed scheme needs to dynamically monitor the load-generation unbalance in the plant and, upon detecting or determining any of the above conditions, needs to quickly initiate shedding of the optimal amount of load - based on a user-determined priority schedule. Examples of an actual load shed test are shown and analyzed.