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Introduction to Protective Device Coordination Analysis

Velimir Lackovic, MScEE.

The objective of a protection scheme in a power system is to minimize hazards to personnel and equipment while allowing the least disruption of power service. Coordination studies are required to select or verify the clearing characteristics of devices such as fuses, circuit breakers, and relays used in the protection scheme. These studies are also needed to determine the protective device settings that will provide selective fault isolation. In a properly coordinated system, a fault results in interruption of only the minimum amount of equipment necessary to isolate the faulted portion of the system. The power supply to loads in the remainder of the system is maintained. The goal is to achieve an optimum balance between equipment protection and selective fault isolation that is consistent with the operating requirements of the overall power system. Short-circuit calculations are a prerequisite for a coordination study. Short-circuit results establish minimum and maximum current levels at which coordination must be achieved and which aid in setting or selecting the devices for adequate protection. Traditionally, the coordination study has been performed graphically by manually plotting time-current operating characteristics of fuses, circuit breaker trip devices, and relays, along with conductor and transformer damage curves—all in series from the fault location to the source. This course reviews the subject of protective device coordination in a very structured fashion. First, modelling technique is shown for a generic power system, protective relays are modelled in details and relay settings are determined based on short circuit current calculations. A detailed explanation of modelling and calculation procedure is offered. Explained techniques can be implemented in any commercial power system software package.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

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