Functions
2.14 Differential Protection and Its Protected Objects
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
119
Quantitative matching of Measured values
The input currents are converted in relation to the power transformer rated current. The nominal values of the
transformer, i.e. rated apparent power, rated voltages and primary rated CT currents, are entered in the protec-
tive device, and a correction factor k
CT
is calculated according to the following formula:
with
I
N CTprim
Primary CT rated current
I
N,Obj.
Nominal primary current of the protected object
S
N
Nominal apparent power of protected object
U
N
Rated voltage
k
CT
Correction factor
This correction is performed for each side of the protected object.
Once the vector group has been entered, the protective device is capable of performing the current comparison
according to fixed formulae.
Matching of Vector Group
Unit transformers often have a wye-delta connection, with the delta connection being on the generator side. To
allow a maximum of versatility in the use of the 7UM62, all imaginable vector group combinations have been
provided for in the software. The basic principle of numerical vector group correction is now explained by way
of example for a Y(N)d5 transformer.
The higher voltage side has a wye connection and the lower voltage side a delta connection. The phase rotation
is n · 30° (i.e. 5 · 30° = 150°). Side 1 (higher voltage side) is the reference system. The vector group correction
feature transforms the currents flowing from side 2 to side 1.
Isolated Starpoint
The following figure shows the vector group, the vector diagram for symmetrically flowing currents and the
transformation rules for a system with an isolated starpoint.
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