Applying Time-Graded Overcurrent Protection to High-Voltage Overhead Transmission Lines

The application of time-graded overcurrent protection to high-voltage overhead transmission lines (HV circuits) is a practical and cost-effective solution for enhancing system reliability and safety. While the term time-graded overcurrent protection (IDMT) may seem crude when compared to modern alternatives, it remains a reliable method for protecting critical transmission infrastructure.

Terminology and Context

It is important to distinguish between transmission and distribution. In the UK, distribution generally involves voltages up to 11,000V, while transmission lines operate at much higher voltages, typically starting from 132,000V. Time-graded overcurrent protection is most commonly utilized on circuits up to 11,000V, where it serves as the primary protection measure. However, for overhead transmission lines, it is often enhanced with additional measures such as instantaneous overcurrent and sensitive earth fault protection in conjunction with an auto-reclosing system. This setup helps to minimize fault damage and quickly restore power following transient faults, such as debris coming into contact with a line and then falling away.

Protection Systems for Primary Distribution

For primary distribution, particularly in Extra High Voltage (EHV) systems, protection schemes can be more complex. These systems cover voltages between 33,000V and 66,000V and typically employ one of two main protection methods:

1. Unit Protection

Unit protection involves comparing the current flowing into and out of the feeder circuit. This method is considered the best but requires a signaling channel between the circuit ends. This signaling channel can either be rented from a telecommunications provider or privately owned and maintained by the electricity company, which incurs ongoing costs. However, if the power circuit is underground and requires excavation, it can be cost-effective to install a private communication cable in the same trench as the power cable.

2. Distance Protection

Distance protection is another method commonly used for high voltage circuits. It offers high-speed impedance protection, making it a reliable choice for overheat conditions. Distance protection can be implemented using either high-speed impedance or balanced earth fault plus high-set overcurrent, with time-graded overcurrent as backup.

Transmission Lines and Advanced Protection

At higher transmission voltages, such as 275kV and 400kV, the choice of main protection becomes even more critical. At these voltages, minimizing fault duration is essential due to the significant energy involved and the potential for voltage depression. If a fault lasts more than a couple of seconds, it could cause generators to disconnect or lose synchronism, leading to system instability.

Time-graded IDMT overcurrent protection, though somewhat crude, remains a simple, relatively cheap, and reliable method for higher voltage circuits. The type of protection chosen often depends on the circuit configuration (whether it is a radial feeder or interconnected with parallel paths) and the cost considerations. The cost of more advanced protection systems should be balanced against the costs of the equipment being protected and the potential consequences of failure, including the risk of fire or other catastrophic events.

Conclusion

In conclusion, while modern protection systems offer advanced capabilities, time-graded overcurrent protection remains a practical and effective solution for high-voltage overhead transmission lines. Its simple implementation, reliability, and cost-effectiveness make it a valuable tool in the arsenal of any electrical engineer responsible for managing high-voltage circuits.