Multiratio CT disadvantages? 2

[StevenR]

Are there any disadvantages of using multiratio CT's for substation protection? The possibility of selecting different ratios adds ofcourse flexibiliy, but is there a negative side? Any protections that do not work well with multiratio (high impedance?)?

 

[dpc]

One major disadvantage is that the performance of the CT decreases as the effective ratio is reduced. A C200 relaying CT, 1200/5, will be only C100 at 600/5.

The CT's accuracy will always be worse at the lower tapped values.

 

[DanDel]

They also make CT burden calculations more difficult. As dpc touched on, the rated output is at the full tap; lower taps have lower output and may cause a problem with higher impedance relays.

 

[jbartos]

Suggestion: Typically, the multiratio CT is wired to some close-by terminal box. There will be more conductors wired from the CT to the terminal box. This increases the risk of potential CT malfuctions.

 

[scottf]

To further complicate things, IEEEC57.13 (CT and VT standard) does not require CT secondary windings to be equally distributed. Which means that many times the accuracy/saturation performance is not linear. For example, if you buy a 1200:5A MR C800 rated CT, if the performance was linear, you could expect a C400 rating at 600:5A. However, if the windings are not equally distributed, the performance at 600:5A tap may be worse/lower than C400.

Most of the time, the lower ratios are used to match up to other CTs for differential protection. Of course, most of the 'C' ratings for modern substation applications are over-kill for newer electronic relays, which bails many a protection engineer out of mis-applications.

Advice to users from a vendor of IT's, always specify that your multi-ratio CT windings are equally distributed.

You would be surprised at how many people think you can apply the full-winding performance to any tap arrangement and get the same performance. Some even try to use them for revenue meterings.

 

[dpc]

Scott makes a great point about distributed windings. I agree that there are a lot of misconceptions regarding multi-ratio CTs.

Many times they are specified because it seems to provide much greater flexibility and adapatbility. This is often done so the local analog ammeter will read up in a reasonable part of the scale for each feeder.

But there's definitely a price to pay for this flexibility.

 

[StevenR]

Thank you all for the response. Requesting equally distributed windings was a good point. Thanks scottf.