by Alex Apostolov, Editor-in-Chief

It is a Changing World

The electric power grid is changing in front of our eyes and we must change along with it in order to meet the requirements for reliability and stability that protection systems should support. We can no longer afford to use the excuse: "That is the way we always did it!";

If we consider a three phase transmission line connecting two substations as a line with simple configuration, we can say that everything else is a line with a complex configuration.

They all have some characteristic that presents challenges to the typical distance protection based non-communications schemes.

The good news is that technology is changing as well, which allows us to find solutions to all of our problems. That is why we decided to focus this issue of the magazine on the protection of transmission lines with complex configuration - to plant the seed of change and to show with some examples how state-of-the-art multifunctional IEDs and IEC 61850 based communications schemes can be used to improve the protection and control functionality.

A huge advantage that we have today compared to our colleagues in the past is advanced communications. Instead of having to use a dedicated communications channel like we did, let’s say fifty years ago, we can now use wide area communications based on optical, radio or cellular technology.

If we are using wide area communications we need to think about cyber security. We can follow the guidelines for conventional cyber security and implement authentication and encryption. But we can also implement what I call “functional security.”

For example if we are implementing a Permissive Overreaching Transfer Trip scheme on a double circuit transmission line using GOOSE message over cellular communications, even if a hacker is able to send permissive messages, they are not going to result in tripping of a breaker because the Zone 2 distance element has not started. Even if a Zone 2 element may not be available to provide the permissive signal supervision, we can always use an Undervoltage fault detector to do the job.

Another technology available to us is non-conventional instrument transformers.

For example a flexible wrap-around sensing cable can be installed on the bushing of a transformer connected directly to a transmission line and used to provide sampled values to both the transformer and line differential protection.

The benefit is that this will allow the protection scheme to distinguish between a line or transformer fault and based on this enable or block the autoreclosing.

Communications are also essential for the protection of multi-terminal lines used to connect DERs to the electric power grid. But communications are not the only tool available to us.

For example in traditional protection schemes we use three zones of distance protection – an underreaching Zone 1 and an overreaching Zone 2, plus a Zone 3 that provides remote backup. However, modern transmission line protection IEDs have more than three zones which allows us to use some of them to distinguish between a fault on the overhead section and the underground section of a hybrid circuit in order to make a decision to allow or block reclosing.

All of the above demonstrates that today we have available to us a wide range of tools that can help us change the way we think about the protection of transmission circuits with complex configuration and solve the problems that they present to conventional protection schemes.  

"The world as we have created it is a process of our thinking. It cannot be changed without changing our thinking."


Albert Einstein

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