# Finding Impendence if problem has a current transformer



## sbsklewis (Jun 30, 2015)

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?


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## cupojoe PE PMP (Jul 1, 2015)

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.


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## Ryan P.E. (Oct 11, 2015)

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.


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