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Why are utilities hesitant to replace CTs/PTs?

Shaun Peden's picture
Marketing Specialist Micatu

Marketing Specialist at Micatu Inc., where we manufacture innovative optical sensing solutions that measure current, voltage, harmonics, non-shifted phase angle, power factor & more, for the...

  • Member since 2021
  • 6 items added with 1,157 views
  • Jun 28, 2022

With the increase in REs and mandates, I am curious why some utilities are hesitant to move forward with modern technologies, such as optical sensors, to replace CTs/PTs, and Rogowski coil designed sensors? Is it because there is a lack of knowledge around the newer technologies? 

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The main difference between Rogowski Coils and CTs is that Rogowski Coil windings are wound over an air core (non-magnetic), instead of over an iron core. Therefore, Rogowski Coils are linear since the air core cannot saturate.


The mutual coupling between the primary conductor and the secondary winding in Rogowski Coils is much smaller than in CTs. Therefore, Rogowski Coil output power is small, so it cannot drive current through low-resistance burden like CTs are able to drive. Rogowski Coils can provide input signals for numerical relays that have a high input resistance; therefore, these devices must measure the voltage across the Rogowski Coil secondary output terminals. Relay manufacturers tend not to support technology since the signal processing required to extract the power frequency signal for applications in phasor-based protective relays and must be designed to accept these types of signals. There are other disadvantages to the Rogowski Coil as well, which include:

  • The output of the coil must be pass through an integrator circuit to obtain the current waveform
  • The integrator circuit requires a power supply of 3 – 24 Vdc
  • It cannot measure dc current
  • Sensitivity – compared with a CT, the Rogowski Coil has lower sensitivity due to the of a high permeability curve

Been out of the design and equipment selection game for a while now, but there is a few comments that I would have relating this technology from when I was there.  First, I think it comes back to application - this equipment is generally going to be used for metering, relaying, and instrumentation...each of which may have a different reason for trying new technology out.  I can also say the entry of optical equipment into the utility market was not the smoothest - there were many issues related to equipment design and accuracy of signal, so there is some early experiences that would lead one to question the reliability compared to traditional units.  We pull traditional wound units out of the field that test with the same accuracy that they were put in the filed with after 50-60 years - its a set and forget it technology, and although not perfect, the tiny amount of O&M related to those units over their lifetime is also a benefit.  Overall, I think if there is a use case and track record, most utilities are willing to try and pilot any newer technology - but it needs to have similar performance characteristics or greater benefits than those that have served us for 100 years.  Hopefully this helps...and for full disclosure, my knowledge base in this area is aged a bit.   

Shaun Peden's picture
Shaun Peden on Jun 30, 2022

Thanks for your input Daniel and that is something I have noticed as well. Manufacturers like Micatu &GE have been proving out the technologies through plenty of pilots. I am sure there were kinks here and there in the beginning, but the amount of data pulled down from optical sensors, shows how advantageous it can be to the grid operator. Especially since PTs/CTs cannot handle bidirectional flow of energy, on top of the increase in REs, EV's, etc. Rapid change in the industry has definitely caused some concern, so grid visibility and insight through data analytics seems to be pressing at the moment. I appreciate your thoughts here, thank you! 

John Pollard's picture
John Pollard on Jul 26, 2022

I don't understand your statement of PTs and CTs cannot handle bidirectional energy flow. Please explain.

It could be as simple as the utility's culture. Many utilities hesitate to be the first one to try something new. There is no rush to become more competitive in a regulated market. 

Shaun Peden's picture
Shaun Peden on Jun 30, 2022

That is definitely something I have noticed. Utilities are hard to change, but with the integration of REs and mandates, legacy tech cannot handle bidirectional flow. Optical technology is still new and I am sure once bigger players in the industry continue to roll out projects revolving around data insight collected from optical technology, more players will want to join in on the fun. 

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