Describing The Ct Scheme; Nominal Secondary Rating Of Cts; Nominal Primary Rating Of Cts; Three-Winding Transformers With Considerably Different Rated Powers - Siemens 7UT51 Instruction Manual

4
Transformer Differential Protection (87T and 87HS)
4.3 Describing the CT
Scheme
Unlike electromechanical relays, numerical relays do
not require a complicated current transformer (CT)
scheme to match the measured currents. Instead, the
relay uses entered information about the protected
object and the current transformers to calculate the
matched currents from the measured currents. The
relay must know the following information about the
current transformers and how they are wired:
• Nominal secondary current rating of the CTs
• Nominal primary current of each CT set
• Orientation of the CTs relative to the protected
transformer
• Choice of how to process the zero-sequence
current
• How the ground-current inputs are used (7UT513
only, and, if used, information about the ground
current CT(s)
4.3.1 Nominal Secondary Rating
of CTs
The relay is manufactured with either 1A or 5A current
inputs. The choice should match the rated nominal
secondary current of the current transformers (CTs)
monitoring the protected transformer. The relay is
shipped with the same rating for all current inputs
(except for the tank leakage current input, if ordered).
A hardware modification can be made to have the
rated currents (1A or 5A) for different current inputs
(see Section 17.9 on page 189). If any current input
will be rated 5A, order the relay with 5A current inputs.
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4.3.2 Nominal Primary Rating
of CTs
When designing the protection application, choose
CTs whose nominal primary current rating results in a
secondary current close to 5A (or 1A, if the relay is
ordered with 1A current inputs) when the transformer
winding's rated nominal current is flowing through the
winding. Enter the CT primary current rating of each
winding in addresses 1104, 1124, and 1144.
1104 In CT WIND1 (for Winding 1)
1124 In CT WIND2 (for winding 2)
1144 In CT WIND3 (for winding 3, 7UT513 only)
Nominal primary current rating of each winding's CTs.
–
Range: 1 100,000 A
Default: 200 A for Winding 1, 2000 A for windings 2 and 3
If a winding has an interposing current transformer set
(see Sections 4.3.3), divide the actual CT primary
rated current by the winding ratio of the
interposing CT.
When using delta connected CTs, divide the rated CT
primary current by .
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4.3.3 Three Winding
-
Transformers with
Considerably Different
Rated Powers
In most cases, all current matching is performed
mathematically by the relay. The ratings of the
reference winding (Winding 1, and usually the winding
with the highest MVA power), is used to calculate a
current, In.
For three-winding transformers with different MVA
ratings of the windings, the unit automatically refers all
currents to the power of the largest winding. When the
power ratings of the windings differ significantly from
each other (more than the factor 4), interposing
transformers are recommended.
Since the current inputs in the 7UT51 are galvanically
isolated from each other, the interposing transformers
may be designed as auto-transformers. This better
utilizes the power rate of the interposing transformers
and the smaller transformer type 4AM5170-7AA is
sufficient.
7UT51 v3
PRIM-2330C