Voltage Transformer Ratio

[bee79]

Hi Fellow Professionals,

Usually the VT ratio is given as for e.g 11kv/110V. But I've seen a different representation like 11kV/√3 / 110V/√3. Schneider's instrument transformer catalogue explains the first ratio representation is when primary voltage is connected between phases and the second representation is when the primary voltage is connected between phase and earth. How do we choose to connect the VT between phases or between phase and earth? Is there any specific requirement for that?

This is my first post. Please let me know if the question is not clear or additional information required.

 

[davidbeach]

That seems to be an IEC thing. I don't think I've seen such on our system here in the ANSI domain. So both that you list have a ratio of 100:1; one designed for 11kV across the primary winding and one designed for 6350 across the primary winding. I'd be more likely to spec a 6400:64 or some such, if I wanted a 11kV primary. Our distribution is at 13.2kV and we use 7200:120 VTs at slightly over their nominal rating.

 

[bee79]

Thanks for your reply guys. I've found out that VT connection depends on earthing conditions of the network. Also the VT has a rated voltage factor and rated withstand time which determine the maximum voltage for which the transformer must attain the require heating and accuracy as specified. Usually VT manufacturers comply with VT phase/earth 1.9(voltage factor) during 8 hour and VT phase/phase 1.2(voltage factor) continuous. Thus the connection will be based on this design as well.

 

[scottf]

davidbeach-

Something in your post caught my attention.

You said you use VTs rated for 12470V systems (7200:120V) on your 13200V system. Given that the typical overvoltage factor is 1.1 continuous (could be 1.05 depending on the class per IEEE C57.13), that doesn't give you much head-room.

12470 * 1.1 = 13717V

13717/13200 = 1.039

Does your 13200V system have an overvoltage limitation of less than 4%?

 

[davidbeach]

We call it 13.2kV at the substations, but the primary customers all know it as 12.47kV, so the 13.2 is pretty close to the upper limit.