Editorial Opinions

Developing Transmission Line Protection for a Changing Grid – Issue 063 March 2023

by Alex Apostolov, Editor-in-Chief

For more than a century we have lived in synchronous machines-based electric power systems that we have studied extensively and understand quite well. This has helped us develop protection devices and systems that operate efficiently in an environment characterized with high levels of fault current, DC offset, CT saturation and other phenomena.

Transmission lines are one of the crucial components of the electrical power system. Their large numbers and in many cases long length exposes them to weather related conditions resulting in short circuit fault, broken conductors, arcing insulators, etc.

If we ask, “What are we doing?” in this issue of the magazine the answer is we are concentrating on the developments in transmission line protection.

The answer to the second typical question “Why are we doing it?” is because of the changes in the electric power systems driven by the efforts for greener energy production, as well as availability of different new sensing devices, protection methods and communication technologies.

The answers to the third typical question “How are we doing it?” are provided by the different articles included in the magazine.

Protection is designed to ensure that the power system is operated safely, efficiently, and reliably. Over the past few decades, there have been significant developments in transmission line protection, which have steadily improved their functionality and performance. One of the major developments in transmission line protection at the end of the last century is the transition from the use of electromechanical and solid-state relays to microprocessor based multifunctional devices which has become increasingly common in transmission line protection.

Digital relays are in many cases faster and more accurate than their analog counterparts, they can adapt their behavior to changing electric power system conditions and they also provide more information about the status of the system before, during and after a fault. They can be programmed to perform a wide range of functions that in the past were performed by individual protection relays.

Today a multi-functional transmission line protection intelligent electronic device (IED) includes phase and ground over-current protection, distance and differential protection, breaker failure protection, auto reclosing, fault recording, measurements and many other functions. In addition, digital relays can be connected to a central control system, allowing for real-time monitoring and control of the power system.

With the availability of advanced communication technologies, it is now possible to use in many more cases accelerated transmission line protection schemes which ensures the stability and reliability of the electric power grid. Even that the IEC 61850 standard was developed predominantly for substation protection, automation and control applications it immediately had also an impact on transmission line protection. Using GOOSE messages between substations for communications-based schemes improves the interoperability between devices from different manufacturers. The later development of routable GOOSE allows the use of accelerated transmission line protection schemes without the need of significant capital investments. This is especially important in areas with large penetration of inverter based distributed energy resources that may not provide enough short circuit fault current for the operation of conventional overcurrent and distance protection.

This, combined with the improved implementation of protection schemes such as weak infeed logic can meet the requirements for reduced fault clearing time which will help keeping wind or solar generators in service during short circuit faults in the system.

Another improvement in transmission line protection schemes can be achieved by taking advantage of the much higher precision of the timing elements in digital protection devices. As a result, the time delays of backup protection elements can be significantly reduced which will also help with shortening the fault clearing times and keeping the DERs in service.

Travelling wave protection is another important tool available to us today that can help in operating for faults that are difficult to detect using traditional protection methods.

All of the above shows that our industry is taking very seriously the challenges that today’s changing electric power grid presents to us and is changing itself by developing and implementing advanced transmission line protection solutions based on the latest technology and innovative algorithms. 

“Change is the law of life. And those who look only to the past or present are certain to miss the future.”

John F. Kennedy