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Where can I find more information and details on Optical CTs and PTs, as well as architecture used in digital substations?

I'm keen to know more about different technologies that are adopted in Optical CT's & Optical PT's. 

I'm also curious to know of any installation of Optical CT & Optical PT without the use of conventional CT/PT in that substation meeting IEC 61850-9-2 standard.

What was the architecture adopted for such a digital substation which used only Optical CT & Optical PT? How was a dynamic response & harmonic response of such a Optical CT & Optical PT? Where can I find more information on these topics?


Dear Anand,


That's a very good question. First, let me say a few words about terminology.

There is no such thing as a "digital substation". It's just a marketing term coined to indicate that the secondary systems, the protection and control devices in the substation are using digital rather than analog current and voltage measurements.


There are two major architectures that can be used to communicate digital measurements in a substation: switched network based and point to point.


Switched network based architecture is based on Ethernet network.


It's advantage is that once the measurement is available on the network, any device can "subscribe" to it.


The disadvantage of this architecture are:

  • Non-deterministic nature of the Ethernet.
  • Need for careful network engineering (protection system may not work in poorly designed networks).
  • Need for external, high accuracy time synchronisation. Typically in a form of a GPS clock. Note that this clock is required for the protection system to work.
  • Cybersecurity posture needs to be carefully evaluated.
  • Need for new commissioning and troubleshooting tools.


The second architecture is based on point-to-point, fiber-optic connections between CT/PT and P&C devices.


The main advantage of this architecture are:

  • It is not dependent on Ethernet network, meaning no network engineering.
  • Does not require external time synchronisation.
  • Better Cybersecurity posture, since there is no network to connect to.
  • Easier troubleshooting of communication problems.


Disadvantages include:

  • Requires dedicated fiber-optic connection to each device "subscribing" to measurements.
  • Is less flexible in cases where subscribers are often added/removed to/from the system.


Hope this helps,


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