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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,


For basic info you can look up "Non conventional instrument transformers" you'll find some great information on ABB, Schneider and Pffeifer websites. For details your community member can contact me. There was a pilot project performed with NCITs here a few years ago... the NCITs here however didn't used optical CT/PT, but the merging unit was on the transformer and SVs were used to transfer the data from field to the station bus.

Prospects for Implementation of Non-conventional Instrument Transformers

The Brainstorm block is a collection of industry experts’ opinions on actual problems. It allows specialists to look at different problems from different perspectives.

Energy professionals from around the world answered several questions about non-conventional instrument transformers:

  1. How would you evaluate the prospects for implementation of non-conventional current and voltage transformers? What implementation strategy of non-conventional instrument transformers should be chosen? What types of instrument transformers should be implemented in different types of bays and equipment?
  2. What factors prevent non-conventional current and voltage transformers from being widespread? If some of the factors are technical, what are they?
  3. How can you evaluate the prospects of the implementation of merging units? Will they be implemented in the future in anticipation of mass adoption of non-conventional current and voltage transformers, and if yes, for what purposes?
  4. What types of current and voltage transformers for what voltage levels can be used in the most reasonable way — and why?
  5. In what ways will mass adoption of non-conventional current and voltage transformers influence secondary systems of substations?
  6. How long will it take non-conventional current and voltage transformers to be massively adopted? What is their mass adoption determined by (e.g. in terms of volume or market share)?

1. It is inappropriate to talk about the prospects of all types of non-conventional instrument transformers (NCITs). In power electronics, such devices have been used for decades and have shown only their best side. At our stations, dozens of excitation systems are equipped with so-called LEM sensors, and these are just electronic instrument transformers. The application of such measuring devices in high-voltage circuits of power plants and networks is more of a confirmation of the competitive advantages of technologies used in NCIT design, compared with conventional instrument transformers, which becomes more apparent with the advent of optical instrument transformers.

However, we must not forget about the ’infantile sicknesses’ of new devices, which do not yet allow one to state that the optical electrical technologies (OET), according to their performance characteristics, have reached the level of conventional transformers.

Nevertheless, currently, I believe that optical transformers are almost ready to be used in industrial facilities of an average responsibility level. As for power plants, in the next three years, RusHydro plans to install such devices in one of the facilities currently under construction.

2. Mass introduction of optical instrument transformers is primarily hampered by the following factors:

  • A lack of a single-industry, standard documentation, establishing the requirements for NCITs. During the design, testing and certification of NCITs, manufacturers currently apply regulatory documents that establish requirements for traditional current transformers/voltage transformers (CT/VT), which eliminates several potential advantages of NCITs.
  • Designers lack an understanding of the qualities, characteristics and properties of new equipment; inertness in the development of new, specific knowledge, computer networks, for example, a shortage of computer-aided design (CAD) diagrams for designing digital substations.
  • An absence of reference materials and methods of technical maintenance of new equipment that would define the requirements of personnel qualifications, tooling and work deadlines.
  • A lack of accumulated statistics characterizing the operational indicators, in particular, life-time metrics. It should be noted that these obstacles are relevant for virtually any new solution and technology. The mechanism for overcoming these issues is traditionally based on a gradual transition from research and development (R&D) to single implementations at low-priority facilities and then extend to wider applications in the industry. Meanwhile, each next step should be followed by a period sufficient for the collection and analysis of the obtained results, namely design modification, service, personnel training and subsequent organization of the production environment at equipment suppliers. I believe that at present we are in the stage of transition to single implementations of such equipment.

3. It should be noted, that even now, without mass implementation of NCITs, it cannot be said that MUs are widely used at power facilities.

In the current understanding, an MU is a device designed to convert analogue measurements into digital form with subsequent transmission of the results via the SV IEC 61850 protocol. In this form, obviously, the need for such devices will be further reduced with the mass introduction of NCITs.

There are good prospects of such devices as bay controllers (connection controllers).

However, in addition to measuring circuits, there are also signalling and monitoring circuits, the transfer of which into digital lines of communication will remain a vital task. To solve this problem, perhaps, there are good prospects of such devices as bay controllers (connection controllers). At the same time, I believe that development of such devices is directed towards their combination with circuit breaker automatic monitoring and soon we will see the mass application of devices such as a circuit breaker automatic-monitoring control processor, which will play the role of grass-roots devices in automation systems of both stations and substations. Furthermore, the wide introduction of NCITs will only contribute to this. Specifically, in our company, there are already three realized projects of gas insulated switch-gear (GIS) automation with voltage from 110 to 330 kV, complete with application of a circuit breaker automatic-monitoring control processor.

4. Optical current transformers — for all voltage classes from 110 kV and higher, as well as for applications at generator voltages. Regarding the voltage instrument transformers, the answer is not so obvious yet, and I believe that it will take another one or two years to test the devices already proposed, to draw some conclusions.

There are many technical, organizational and regulatory issues to be resolved, but there are no fundamental obstacles here.

5. Obviously, the impact will be quite strong. As a result, we will get one device (NCIT), which will be able to provide any required number of secondary devices with measuring information, and the concept of instrument transformers (IT) system separation on current transformer (CT) cores will be history, and will remain only as a measure for ensuring reliability due to hardware redundancy. There are many technical, organizational and regulatory issues to be resolved, but there are no fundamental obstacles here.

6. It will take 5–7 years before the full-edged competition of NCITs with traditional instrument CTs becomes apparent.

Hi Anand,

As others have mentioned, there are several vendors of Non-Conventional Instrument Transformers (NCIT), Low-Power Instrument Transformers (LPIT), and Digital Instrument Transformers (DIT).  All of these terms are used to describe Optical Instrument Transformers.

The IEC standards refer to them as LPITs and the relevant standards for Optical Instrument Transformers are IEC 60044-7 and 60044-8 (which will soon be replaced by IEC 61869-7 and 61869-8).

GE has been manufacturing Optical CTs and PTs for almost 20 years now.  They are used extensively in industrial applications (for high current DC measurement), in the utility space (primarily for revenue metering), and for protection and control on HVDC systems.

The thing that has limited adoption of LPITs in the High Voltage AC market has been the interface with conventional IEDs.  Up until recently, most IEDs required 1A and 5A secondary CT inputs .  Because Optical CTs are not true "transformers" they cannot practically reproduce 1A/5A secondaries (or more specifically the secondary currents during faults, i.e. 20..100A).

In the last few years, most of the major IED manufacturers have released relays that support Process Bus based on IEC 61850-9-2.  This makes it possible to use Optical CTs and PTs (and standalone Merging Units) to exchange current and voltage measurements digitally over an Ethernet network.  This in turn has enabled utilities to move forward with "digital" substation projects based on Process Bus and LPITs (including several in India).  The result is a safer substation (galvanic isolation between the switchyard and the control room), a smaller environmental footprint (reduction of oil and SF6) and lower total cost of ownership (reduced cabling, reduced Engineering costs, etc.).

If you would like more information, please don't hesitate to contact me.

Dear Anand, 

Here is a power point that will offer some finer details of Optical CTs, and give a good direction for Optical PTs. 


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