by Alex Apostolov, Editor-in-Chief

It is time for a change

Protection, automation and control specialists have been using for a century current and voltage transformers as the interface of protection relays and measuring instruments with the primary components of the electric power system. Now we are at a point when we need to change.

I am sure that many people will read this and say "Why do we need to change? Everything is working fine. CTs and VTs are proven technology. We have been using them for so long, so we can continue using them in the future as well."
Is that true? Is everything working fine? Not really.
We know about the many challenges that we have to deal with when we use conventional instrument transformers. Let me just briefly mention a few things that are not fine:

  • The CTs can saturate and lead to an undesired operation of the protection relay. The same can happen because of a CCVT transient
  • If we are to change the zone of the bus differential protection, we need to be very careful with the switching of the CT circuits in order to avoid an open circuit that could kill someone
  • CTs and VTs sometimes explode, spreading sharp pieces of porcelain all over the substation yard - I have seen them myself in a 400 kV substation
  • You need different instrument transformers for protection and metering
  • They take a lot of space in the substation yard

That is why we decided that it is time to look at the non-conventional instrument transformers. To try to show that they represent a much better alternative to the conventional CTs and VTs that can help us in our transition towards the Smart Grid.
For many people NCITs are an exotic technology that is not mature enough. But that is not the case. When I was working at New York State Electric & Gas in the early 1990s we had a demonstration project with an optical CT interfacing with microprocessor relays through a low voltage level interface. That was not the best option due to the multiple conversions ? optical to low level voltage and then to current in order to interface with the IED?s current input, where it is converted to digital in order to be used for protection and measurement functions.

Then came IEC 60044 - 8 to provide as one of the possible interfaces a digital interface of the optical sensor. Based on it, IEC TC 57 working group 12 developed IEC 61850 9-1 and 9-2. In order to ensure interoperability between the merging units and the IEDs a task force under the UCA International Users Group developed a document - actually a profile of the standard that became known as IEC 61850 9-2 LE. But the people that were looking for excuses not to use the technology were saying that this is not a standard, but just an ?implementation agreement.?
Now this is not a valid argument anymore, because IEC TC 38 recently published a new standard IEC 61869-9: Instrument Transformers - Part 9: Digital interface for instrument transformers, based on IEC 61850 9-2. So now all NCITs, as well as the stand alone merging units, will be expected to become compliant with it.

The NCITs have many benefits that address the issues with conventional transformers listed above. For example:

  • They do not saturate and have linear characteristics which allows them to be used for both protection and metering
  • They are much safer, because they do not explode and there is no danger of an open CT circuit
  • There is no need for changing the analog circuits when there is a change in the substation topology - just change the subscription to different merging units
  • They can be integrated on the same structure as the breakers, thus resulting in significant space savings
  • They weigh very little which makes them much easier to transport and install

These and other benefits, together with the fact that NCITs today are available from the global suppliers shows that it is time for a change, that it is time to move our industry to the 21st century and take advantage of the digital technology that is changing everything around us.  

"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

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