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7UT51 v3
The ratio of the interposing transformers is chosen
such that the winding with the lower rating (usually the
tertiary winding of the transformer) is roughly matched
to the winding with the highest rating, i.e. the current
ratio of the interposing transformers should be:
S
of the high
N
TR
=
1
S
of the low
N
Thus, the winding ratio of the interposing CTs is
S
of the low
N
N
1
=
N
S
of the high
2
N
where:
N
-
Number of winding turns on the side facing the
1
main CTs
N
-
Number of winding turns on the side facing the
2
7UT51 relay
The exact current matching is performed when setting
the parameters for this winding.
Example: The following data should apply for the
winding with the highest power rating:
rated power
rated voltage
current transformers
The following data should apply for the winding
with the lowest power rating:
rated power
rated voltage
current transformers
This results in a winding ratio of the interposing
transformers of:
S
of the low
N
N
1
=
N
S
of the high
2
N
12.5 MVA
=
57 MVA
≈ 11/50 turns
PRIM-2330C
-
power winding
-
power winding
-
power winding
-
power winding
S
=57 MVA
N
U
=110 kV
N
300 A/ 1A
S
=12.5 MVA
N
U
=10 kV
N
800 A/ 1A
-
power winding
-
power winding
=
0.22
Transformer Differential Protection (87T and 87HS)
which results in a current transformer ratio of:
Thus, a primary rated current of 3636A for the current
transformers is to be set to the protection unit in this
example case.
4.3.4
Orientation of CTs
The orientation of the CTs monitoring each winding of
the protected transformer must be specified in order to
ensure that the correct polarity is used when
calculating the differential currents.
1105 CT1 STARPT (for Winding 1)
1125 CT2 STARPT (for winding 2)
1145 CT3 STARPT (for winding 3, 7UT513 only)
Orientation of which end of each winding's CTs are
connected to the corresponding non
the relay's phase current inputs. For wye connected CTs,
-
this is the non
connected to a common point (or star
Option
TOWARDS TRANSF
TWDS LINE/BUSBAR
For example, if terminal 3A2 (non-polarity, phase A
input of Winding 1) is connected to the end of the CT
closest to the transformer (as shown in Figure 4.4),
Address 1105 should be set to "Towards
Transformer."
4.3.5
Processing the Zero
Sequence Current
If a winding of the protected transformer has any
connection to ground (as is usually the case for a
wye-winding), there is likely to exist a zero sequence
current in the winding. Any zero sequence current will
by itself cause a differential current for one or more of
the phases (A, B, and/or C). Using the measured
phase currents, the relay can calculate the expected
zero sequence current for each winding and
mathematically eliminate it when calculating the
matched currents for the differential protection;
however, if the measured ground current(s) are
available (see Section 4.3.6 on page 50) the relay can
correct the matched currents, resulting in more
sensitive differential protection.
800 A/1 A
= 3636 A/1 A
11/50
-
polarity terminal of
polarity ends of the CTs, which are usually
-
point).
Description
Towards the transformer
(default).
Away from the transformer.
4
49
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