After the CT ratio correction, the measured currents and corresponding setting
values of TR2PTDF are expressed in multiples of the rated power transformer
current I
r
(xI
r
) or percentage value of I
r
(%I
r
).
The rated input current (1A or 5A) of the relay does not have to be same for the HV
and the LV side. For example, the rated secondary current of 5 A can be used on the
HV side, while 1A is used on the LV side or vice versa.
Example
The rated power of the transformer is 25 MVA, the ratio of the CTs on the
110 kV side is 300/1 and that on the 21 kV side is 1000/1
Figure 279: Example of two-winding power transformer differential
protection
The rated load of the transformer is calculated:
HV side: I
nT_Wnd1
= 25 MVA / (1.732 x 110 kV) = 131.2 A
LV side: I
nT_Wnd2
= 25 MVA / (1.732 x 21 kV) = 687.3 A
Settings:
CT ratio Cor Wnd 1= 300 A / 131.2 A = “2.29”
CT ratio Cor Wnd 2= 1000 A / 687.3 A = “1.45”
Vector group matching and elimination of the zero-sequence component
The vector group of the power transformer is numerically matched on the high
voltage and low voltage sides by means of the
Winding 1 type, Winding 2 type
and
Clock number settings. Thus no interposing CTs are needed if there is only
a power transformer inside the protected zone. The matching is based on phase
shifting and a numerical delta connection in the protection relay. If the neutral of
a star-connected power transformer is earthed, any earth fault in the network is
perceived by the protection relay as a differential current. The elimination of the
zero-sequence component can be selected for that winding by setting the
Zro A
elimination parameter.
1MRS757644 H
Protection functions
620 series
Technical Manual
537
