Accuracy of Px class CT 2

[HiSet]

Is there a way to find/derive the accuracy class of protection class CT with following specs ?
Ratio- 1000/1A, 0.1 Px 500 R 3.0 ohm
Is it fair to say that such a CT will equivalent to metering CT of class 1 accuracy?

 

[RRaghunath]

The kind of guidance you are seeking is available readily for P-class CTs - A 5P class CT is equivalent to a Class 1 metering CT and a 10P class CT to Class 3P metering CT.
The subject PX class CT has 10% error (0.1A Imag) at Vk. Thus, this CT will definitely be equivalent to Class 3 CT or better. Whether it can be equivalent to Class 1 metering CT needs to be established by looking at the CT excitation characteristic curve.

 

[dfdt]

Thanks Raghu. 100 mA is at the rated Ek of 500 V but in practice, depending on connected burden, the CT will only develop fraction of rated Ek at rated current of 1000 A and hence the excitation current will be very low.

 

[scottf]

RRaghunath-

I don't think an automatic assumption of 5P = class 1 and 10P = class 3 metering is valid all of the time. It very much can depend on the burden rating, ratio, and type of core material being used.

As an example, if you have a 5P - 100VA rated core and a relative low ratio (like 400:1A), the core will have a relatively large cross-sectional area. Class 1 requires 3% ratio error and 180min phase error at 5% rated current. At 5% rated current with such a large cross-section core, the flux density could be low enough where the B-H or excitation curve is in the "boot" region, which is where the performance relationship becomes non-linear again. I ran a quick calculation using our design tool on something similar and it would not meet a class 1 metering rating.

The types of core material used for such protection cores can "drop off" rather steeply when the flux density gets too low.

A PX rated CT with a Vkp >= 500V is also going to be a rather large cross-section core.

 

[RRaghunath]

scottf,
Thanks for your post. Just to clarify,
My understanding comes from IEC 61869-2 Table-205.

 

[scottf]

RRaghunath-

Right...that table lists the allowable ratio and phase error at rated current only .

It is typical for protection CTs to have relatively high ratios as compared to expected maximum continuous current and/or the current level trying to be metered. The original question asked about metering class 1, which is spelled out in table 201. That class requires, for example, ratio error of 1.5% at 20%Inom and 3% ratio error at 5%Inom. What I'm saying above is that it is not a given that a CT with a large protection core will meet the error requirements for the metering class at the lower primary current ranges...and it is conceivable and expected that the current level trying to be metered will be in those lower ranges.