Performing a coordination study for electrical protective devices is an important process in ensuring the reliability and safety of an electrical power distribution system. The coordination study aims to determine the optimal settings for protective devices such as fuses, circuit breakers, and relays to ensure selective coordination. Selective coordination means that only the protective device closest to the fault should operate to isolate the fault while allowing other devices further away to remain unaffected, thereby minimizing downtime and disruption to the system.
Here are the general steps to perform a coordination study for electrical protective devices:
Gather Data: Collect all relevant data about the electrical power distribution system, including single-line diagrams, system specifications, equipment ratings, load information, cable sizes, and manufacturer data for protective devices.
Define Objectives: Determine the objectives of the coordination study, such as achieving selective coordination, minimizing downtime, ensuring compliance with codes and standards, and optimizing equipment settings.
Identify Protective Devices: Identify all the protective devices in the system, including circuit breakers, fuses, relays, motor starters, etc.
Select Time-Current Curves: Obtain time-current curves for all protective devices from manufacturers. These curves show the operating time of the devices at various current levels.
Coordinate Devices: Plot the time-current curves on a coordination graph. This graph typically has a logarithmic scale on the X-axis (current) and a logarithmic or linear scale on the Y-axis (time). The curves for different protective devices are plotted on the same graph.
Determine Settings: Analyze the coordination graph to identify any areas of overlap or close proximity between the curves of adjacent protective devices. Determine the settings (trip times, current settings, etc.) for each device to achieve selective coordination and eliminate nuisance tripping.
Model the System: Use specialized electrical system analysis software, such as ETAP or SKM PowerTools, to model the entire power distribution system. Input all relevant data, including protective device settings.
Run Study: Perform a simulation using the software to evaluate the performance of protective devices under various fault scenarios. The software will calculate the time taken for each device to clear the fault.
Evaluate Results: Review the simulation results to ensure that the protective devices operate selectively, i.e., only the nearest protective device to the fault operates while others remain unaffected.
Adjust Settings: If the coordination is not achieved or if there are undesirable operating characteristics, adjust the settings of the protective devices and rerun the simulation until the desired coordination is achieved.
Document the Study: Document all the data, settings, and results of the coordination study. This documentation is essential for future reference and for compliance with safety standards.
Implement Settings: Apply the final approved settings to the protective devices in the field.
Regular Maintenance: Periodically review and update the coordination study as changes are made to the electrical system to ensure ongoing coordination and safety.
Performing a coordination study may require expertise in electrical engineering and the use of specialized software. It is often carried out by electrical engineers or consultants with experience in power system analysis.