IEEE C37.117-2007 pdf free download.IEEE Guide for the Application of Protective Relays Used for Abnormal Frequency Load Shedding and Restoration.
Each region within the NERC applies rules that mainly define the amount of connected load to be shed and the threshold and operating time. As an example, the Northeast Power Coordinating Council (NPCC) region has defined the objective [B52] as follows: “The intent of the Automatic Underfrequency Load Shedding program is to stabilize the system frequency in an area during an event leading to declining frequency while recognizing the generation characteristics in each area. The goal of the program is to arrest the system frequency decline and to return the frequency to at least 58.5 Hertz in ten seconds or less and to at least 59.5 Hertz in thirty seconds or less, for a generation deficiency of up to 25% of the load.” To ensure satisfactory voltage and loading conditions after automatic load shedding each area normally performs studies. The shedding is accomplished system-wide by applying underfrequency relays at distribution or transmission stations where major load feeders can be controlled by tripping breakers automatically when frequency relays reach the setting threshold. All generators require underfrequency tripping. This may be to protect the production facility from the effects of underfrequency conditions, but if the generator is located in an area of the system that could easily become islanded then the generation is tripped to prevent significant impact (over or underfrequency and over or undervoltage) on the customers in the local area. When generators are disconnected due to an inability to support recovery from underfrequency conditions, some regions have defined certain criteria to maintain coordination between underfrequency tripping of generators and the automatic underfrequency load shedding program. For example, it may be required to assign additional load shedding that is equivalent to the amount of generation to be tripped.
Automatic load shedding programs on the transmission power system provide the initial underfrequency protection for the system turbine-generators. Load shedding programs should be designed for the maximum possible overload conditions and should ensure that sufficient load is shed to quickly restore system frequency to normal or to return the frequency to acceptable continuous operating range as soon as possible. This will prevent additional loss of generation due to generator frequency relay operation. Generator underfrequency protection is implemented based on the turbine-generator design and is a critical input to the load shedding design studies. It is critical for the generation interconnection to recognize and follow the regional council requirements for off-nominal frequency performance when specifying the generator, and to make certain the generator underfrequency set points fall well within acceptable regional council requirements. System inertia constant, percentage overload, islanding patterns, and reactive support by generation during system disturbances are also very critical inputs in studying and designing load shedding schemes. The result of these studies determines the number of frequency steps and the percentage of required load to be shed at each step. Load shedding programs with fewer frequency steps, shedding higher percentage of load, work well for some overload levels, but may result in shedding too much load under light load conditions resulting in system overfrequency. In such cases, more steps with smaller percentage of load shed at each step may be better suited. Time delays at each step are also determined by the studies. The coordination of the transmission system load shedding scheme with individual generators is critical in maintaining the integrity of the system and should not intrude on the reliability of the electrical systems.