First let’s de ne the area we are working in is the area in between the CTs. This is called the “differential
zone” which means that the currents going inside from another side must come out from the other
side. It doesn’t matter if the signal is scaled either higher or lower or its phase angle is shifted, both side
currents have to match. Otherwise there is problem within the protected zone which either blocks or
keeps the current inside the zone.
Following example shows a typical minimum information from the transformer name plate data and
what to do with it.
Figure. 5.4.10. - 97. Transformer name-plate data.
According to name plate data this transformer is designed for three phase usage and it has two
windings. Nominal design power of the transformer is 2 MVA and its vector group is Yd1 which means
that the HV side is connected to star and LV side to delta so that the LV side has 30 degree lag to HV
side. Also the HV side nominal voltage is 10 kV and LV side nominal voltage is 1kV. Transformer short
circuit impedance is 4.95% which comes from the transformer nal test and basically it presents how
much the transformer shall be able to feed short circuit current. This information is normally available in
the transformer name plate and documentation. If the transformer has a tap changer its information is
normally also available in name plate data.
Nominal current matching is rst thing to consider in the differential protection. Normally modern
numerical protection relay can calculate these factors by itself when the transformer nominal power
and voltage levels are known. However below are the formulas to calculate the amplitude matching
coef cients.
Let’s say that in this example HV side CTs are 150/5A and LV side CTs are 1200/5A
Primary side per unit factor and current calculation
Secondary side per unit factor and current calculation
AQ-T216
Instruction manual
Version: 2.00
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