## Instrument trafo

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Instrument
Transformers

**
Voltage
Transformer
For a voltage transformer of voltampere rating S, rated primary voltage V _{P}
and rated secondary voltage V_{S}, the maximum secondary current I_{Smax},
maximum secondary burden conductance G_{Bmax} and maximum primary
current I_{Pmax} are:
I_{Smax} = S / V_{S}
G_{Bmax} = I_{Smax} / V_{S} = S / V_{S}^{2}
I_{Pmax} = S / V_{P} = I_{Smax}V_{S} / V_{P}
**

**
Current
Transformer
For a measurement current transformer of voltampere rating S, rated primary
current I _{P} and rated secondary current I_{S}, the maximum
secondary voltage V_{Smax}, maximum secondary burden resistance R_{Bmax}
and maximum primary voltage V_{Pmax} are:
V_{Smax} = S / I_{S}
R_{Bmax} = V_{Smax} / I_{S} = S / I_{S}^{2}
V_{Pmax} = S / I_{P} = V_{Smax}I_{S} / I_{P}
**

**
For
a protection current transformer of voltampere rating S, rated primary
current I _{P}, rated secondary current I_{S} and rated
accuracy limit factor F, the rated secondary reference voltage V_{SF},
maximum secondary burden resistance R_{Bmax} and equivalent primary
reference voltage V_{PF} are:**

V_{SF} = SF / I_{S}

R_{Bmax} = V_{SF} / I_{S}F = S / I_{S}^{2}

V_{PF} = SF / I_{P} = V_{SF}I_{S} / I_{P}

**
Impedance
Measurement
If the primary voltage V _{pri} and the primary current I_{pri}
are measured at a point in a system, then the primary impedance Z_{pri}
at that point is:
Z_{pri} = V_{pri} / I_{pri} **

**
If
the measured voltage is the secondary voltage V _{sec} of a voltage
transformer of primary/secondary ratio N_{V} and the measured
current is the secondary current I_{sec} of a current transformer of
primary/secondary ratio N_{I}, then the primary impedance Z_{pri}
is related to the secondary impedance Z_{sec} by:**

Z_{pri} = V_{pri} / I_{pri} = V_{sec}N_{V} / I_{sec}N_{I} = Z_{sec}N_{V} / N_{I} = Z_{sec}N_{Z}

where N_{Z} = N_{V} / N_{I}

**
If
the no-load (source) voltage E _{pri} is also measured at the point,
then the source impedance Z_{Tpri} to the point is:**

Z_{Tpri} = (E_{pri} - V_{pri}) / I_{pri} = (E_{sec} - V_{sec})N_{V} / I_{sec}N_{I} = Z_{Tsec}N_{V} / N_{I} = Z_{Tsec}N_{Z}