Potential Transformers Metering Accuracy Used for Protection 1

[ppinuelas]

Hello all,

I have a 138kV potential transformers with two metering secondary windings 0.3WXYZ. I am wondering if there would be a problem saturating the PT's if a connect the signals to the relays as well for overvoltage and undervoltage protection.

Thank you,

-Pablo

 

[scottf]

No problem at all.

In fact, IEEE MV and HV VTs essentially only come in metering accuracy. In the IEEE world, the PT accuracy classes are 0.15,0.3,0.6, and 1.2. 1.2 is normally referred to as protection accuracy and 0.15 and 0.3 as metering accuracy. 0.3 class limits are essentially 4 times more accurate versus 1.2 accuracy class limits.

I think you'll find 138kV VTs really only come in 0.15 and 0.3 accuracy as a standard offering.

You asked about saturating the PT. I believe you're confusing CTs and PTs.

 

[ppinuelas]

Hi Scott,

Thank you so much for your quick response. I was not sure if the VTs would saturate just like the CTs do because I have seen that in the IEC world they specify a lot of protection windings class 3P, which means 3% error.

 

[stevenal]

Per IEEE C57.13.2, the rated accuracy applies from 90 to 110% of rated voltage. I don't believe you need metering accuracy for over and under-voltage, though.

 

[scottf]

Stevenal-

The point is, in the IEEE world, if you specify a MV or HV PT with 0.6 or 1.2 accuracy, you're going to get a 0.3 accuracy unit proposed/supplied. All of the standard designs are 0.3 as a general statement.

Now...for CCVTs, that's a different story and unique designs exist for each accuracy class and multiple burden levels.

 

[stevenal]

scottf,

So I have 0.3 accuracy from 90 to 110% of rated voltage. What accuracy will I get outside that range?

 

[scottf]

The standard requires that the accuracy class be maintained from 90% to 110% rated voltage.

Below that range, the accuracy is pretty stable down to about 30% rated voltage, but it varies with design. Certainly it's good for indicating/protection down to that range and lower.

Above 110%, it depends on the rated over-voltage factor as to how high you can go, but in general, the accuracy is stable as well until the VT approaches saturation. A 138kV VT, but the standard would have an OVF of 1.73 - 1 minute / 1.1 - continuous.