WHY WE DO OVERCURRENT PROTECTION COORDINATION?
An overcurrent coordination study is the comparison and selection of operating times of the protective devices that achieve the objectives of the protection system under abnormal system conditions. This study should include all devices from the utilization equipment to the source.
A coordination study also provides data useful for the selection of
—Instrument transformer ratios
—Protective relay characteristics and settings
—Fuse characteristics and ratings
—Low-voltage circuit breaker ratings, characteristics, and settings
It also provides other information pertinent to the provision of optimum protection and selectivity in the coordination of these devices. In new installations, electrical equipment ratings often change prior to plant startup, but after protective devices have been ordered. These changes should be anticipated when selecting protective devices so that the device characteristics are sufficiently flexible to protect the individual load or branch circuit.
A preliminary coordination study should be made during the early stages of a new system design to verify that the protective device ratings can be selective and that the source utility’s protection practices have been considered. The protective device settings should be determined after the design has been completed and all load and fault currents have been calculated.
Protective devices are applied to a power system as primary and backup protection. Primary protection is the first line of defense against further damage caused by a fault or other abnormal operating condition. These devices are generally set to operate faster and remove less of the power system from service than backup protection.
Backup protection takes over when the primary protection fails to clear the abnormal condition. Backup protective devices and settings are selected to operate at some predetermined time interval after the primary device operates.
Thus, a backup device should be able to withstand the fault conditions for a greater time period than the primary protective device. For most applications, the operation of the backup device isolates circuits in addition to the faulted or overloaded circuit. Therefore, a greater portion of the power system is interrupted with backup protection.
In applying protective devices, it is occasionally necessary to compromise between protection and selectivity. While experience may suggest one alternative over another, the preferred approach is to favor protection over selectivity. Which choice is made, however, is dependent on the equipment damage and the affect on the process.
In existing facilities, system configurations and operating conditions often change. A new coordination study should be performed when the available short-circuit current to a plant changes or when significant changes in plant loading occur. This study determines the ratings or settings necessary to ensure that selectivity and protection are maintained after system changes occur.
A coordination study should definitely be performed when a fault on the periphery of an existing plant unexpectedly shuts down a major portion of the system. Such an event may indicate a need to change or reset devices.
