Finding Impendence if problem has a current transformer

Professional Engineer & PE Exam Forum

Help Support Professional Engineer & PE Exam Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

sbsklewis

Member
Joined
Oct 12, 2012
Messages
9
Reaction score
0
I know how to find the impendence/current on secondary side is using a voltage tx, but practice exam 101 solves for current with a current tx by not inverting turns ratio, so would the same logic apply if finding impendence?

 
I'm not sure I understand what you are asking...but S is constant on both sides of transformer, so if voltage changes by a certain ratio, the current will change to keep S the same on both sides.

 
Are you talking about secondary impedance calculations like for distance relays like: Z2= Z1 * CTR/PTR?

I never knew how this formula was derived, but I made sense looking at it like so:

CTR=I1/I2 (primary divided by secondary)

PTR=V1/V2 (primary divided by secondary)

V1/I1 = Z1 (primary values)

V2/I2= Z2 (secondary values)

Z1 * I1/V1 = 1

Z2 * I2/V2 = 1

Z1 * I1/V1 = Z2 * I2/V2

Z2 = Z1 * (I1/V1) / (I2/V2)

Z2 = Z1 * (I1/I2) * (V2/V1)

Z2= Z1 * (I1/I2) / (V1/V2)

Z2 = Z1 * CTR/PTR

since the values of impedance (like transmission lines) are usually calculated in primary values from tables and configuration, using the CTR and PTR values you can get it in secondary impedances to set the microprocessor relays.

 
Back
Top