Siemens 7UT51 Instruction Manual page 40

4
Transformer Differential Protection (87T and 87HS)
The ground current of one or two grounded wye
windings can be input to a 7UT513 relay. As an
example, the current-matching matrix equation for the
"reference" wye winding would be:
I'
A
1 0 0
×
= k
I'
0 1 0
B
0 0 1
I'
C
See Section 1.2 on page 11, Definition of Terms, for an
explanation of "reference" winding and Section 4.3.5
on page 49 for further discussion of options for
handling the zero sequence current.
Once the vector group (see Table 4.1 on page 46 and
Table 4.2 on page 47) and rated transformer data,
such as rated power, rated voltage and rated primary
current of the current transformers, has been entered,
the relay is capable of performing the current
comparison according to fixed formulae.
Conversion of the currents is performed by the
programmed coefficient matrices that simulate the
difference currents in the transformer windings (see
Figure 4.3). All conceivable vector groups are
possible.
The following factors are available:
k CT 1 = current processed by 7UT51
current through the relay terminals
for Side 1 of the protected object and similar for the
other sides: k CT2 =
k CT3=(only 7UT513)
ksCT 1=(only 7UT513)
ksCT 2=(only 7UT513)
ksCT 3=(only 7UT513)
40
I
I
G
A
× ×
1
-- -
+ I
I
3 G
B
I I
G
C
where ksCT are the factors for inputs from the CT in
the star-point connection of a grounded transformer
winding (if used).
Note: The "k" matching factors are calculated by the
relay from the entered rated data and can be
read out in the operational annunciation in
(4.5)
Address Block 5100.
Electromechanical Relays require
Delta-Wye CT's to eliminate
the zero-sequence current and to
achieve correct phase angle
IA
A
IB
B
IC
C
Figure 4.1 Classic Phase Shift Correction
No Delta-Wye CT connection is necessary.
Phase-shift correction is done internally.
Result: Waveform Capture records original
current waveforms !
I
A
A
I
B
B
I
C
C
Figure 4.2 7UT51 Phase Shift Correction
I1
I2
I3
Transformer Protection
I
1
I
2
I
3
Transformer Protection 7UT51X
7UT51 v3
a
b
c
a
I
a
b
I
b
c
I
c
PRIM-2330C