Feeder Protection in Distribution Systems
Feeder protection plays a crucial role in safeguarding distribution systems, ensuring their reliable and uninterrupted operation. Distribution systems are the final stage of power delivery, responsible for supplying electricity to end-users, such as residential, commercial, and industrial consumers. While transmission systems transmit bulk power over long distances, distribution systems step down the voltage and distribute electricity to consumers at lower voltages.
Feeder protection involves the application of protective devices and schemes to detect and clear faults that may occur in distribution feeders. A distribution feeder is a circuit that connects a substation to the distribution transformers or directly to consumers. It consists of medium-voltage cables or overhead lines and protective devices such as circuit breakers and relays.
Specifics of Feeder Protection:
Fault Detection and Clearing: The primary objective of feeder protection is to detect and quickly clear faults to minimize disruptions and reduce the impact on the power system. Faults in distribution feeders can occur due to various reasons, including insulation failure, equipment malfunctions, or external factors like lightning strikes and tree-branch contact.
Coordination and Selectivity: Feeder protection should be coordinated with other protective devices in the system to ensure selective operation. Selectivity ensures that only the device nearest to the fault operates to isolate the faulty section while keeping the rest of the system operational. Coordination is achieved by setting the time-current characteristics of protective devices in a coordinated manner, ensuring faster operation of the nearest device to the fault.
Overcurrent Protection: Overcurrent protection is a fundamental component of feeder protection. Overcurrent relays are used to detect abnormal current levels caused by faults or overloads in the feeder. These relays can be set to operate based on different characteristics such as inverse, very inverse, extremely inverse, or definite time characteristics. The choice of the characteristic depends on equipment and system characteristics, along with regulatory standards like IEC 60255 and IEEE C37.112.
Directional Protection: Directional protection is useful in limiting the operation of protective devices to the desired direction of current flow. This is particularly important in meshed distribution networks, where multiple sources and feeders are interconnected. Directional relays detect the direction of current flow and allow operation only when a fault occurs in the intended direction.
Communication and Automation: Feeder protection can be enhanced by incorporating communication and automation technologies. Communication systems allow for faster fault detection, localization, and restoration. Automation enables the automatic reconfiguration of the network to isolate the faulty section and restore power to unaffected areas.
Numerical Example:
Consider a radial distribution feeder supplying power at 11 kV to various consumers. The feeder is protected by overcurrent relays set to operate based on the following characteristics:
- Relay A: Inverse-time characteristic with a time dial setting (TDS) of 0.8 and a relay pickup current (Ip) of 150 A.
- Relay B: Extremely inverse-time characteristic with TDS of 0.9 and Ip of 200 A.
- Relay C: Definite-time characteristic with a time setting (TS) of 0.2 s and Ip of 100 A.
During a fault, the fault current reaches 2500 A. Using the relay characteristic equations, we can determine the operation times for each relay.
- Relay A: Operation time (TA) = 0.8 * (150/2500)^2 = 0.092 s
- Relay B: Operation time (TB) = 0.9 * (200/2500)^2 = 0.104 s
- Relay C: Operation time (TC) = 0.2 s (fixed setting)
Based on the operation times, we can observe that Relay A would operate the fastest, followed by Relay B, and Relay C only if the fault persists beyond 0.2 s. This coordination ensures that the nearest relay to the fault operates first, minimizing the fault’s impact on the distribution system.
In conclusion, feeder protection in distribution systems plays a crucial role in ensuring the reliable and uninterrupted supply of electricity to consumers. By using appropriate protective devices and schemes, such as overcurrent relays, directional protection, and coordination, faults can be detected and cleared swiftly, minimizing disruptions and maintaining the integrity of the distribution network.