What are the key factors to consider when designing distribution system protection schemes?

1 Load flow and short circuit analysis

Before selecting and setting the protection devices, you need to perform a load flow and short circuit analysis of the distribution system. This will help you determine the normal and abnormal operating conditions, the voltage and current levels, the fault types and locations, and the available fault current. You can use software tools or analytical methods to perform these analyses and obtain the necessary data for protection design.

2 Protection device selection

The next step is to choose the appropriate protection devices for the distribution system. These may include fuses, circuit breakers, reclosers, sectionalizers, relays, and switches. You need to consider the characteristics, ratings, functions, and costs of each device and how they fit the system requirements. You also need to ensure that the devices are compatible and coordinated with each other and with the upstream and downstream protection systems.

3 Protection device coordination

Protection device coordination is the process of setting the operating times and characteristics of the protection devices to achieve selective isolation of faults. This means that only the faulty section of the system is disconnected, while the rest remains in service. You need to consider the time-current curves, the coordination intervals, the fault clearing times, and the sensitivity and reliability of the devices. You can use graphical or numerical methods to coordinate the protection devices and verify their performance.

4 Protection device communication

Some protection devices may require communication with other devices or with a central control system to enhance their functionality and coordination. For example, some relays may use communication channels to exchange information, such as current and voltage measurements, trip signals, or logic commands. You need to consider the communication methods, protocols, media, and equipment that are suitable for the distribution system and the protection devices. You also need to ensure the security and reliability of the communication system.

6 Protection system evaluation and improvement

Finally, you need to evaluate and improve the protection system based on the actual performance and feedback from the operation and maintenance activities. You need to monitor and analyze the protection system events, such as faults, trips, recloses, or alarms. You also need to review and adjust the protection system settings, design, and coordination based on the changes in the distribution system conditions, such as load growth, network expansion, or generation integration. You can use simulation tools or field measurements to evaluate and improve the protection system.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?