Design of a Priority-Based Load Shed Scheme and Operation Tests

Many large industrials operate internal cogeneration systems to provide a substantial portion of their own power and to use the resulting output heat as part of their industrial process. Depending on the amount of cogeneration, 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 cogeneration or rather shedding 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 undervoltage 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.