The main CTs are normally supposed to be star connected and can be earthed in any
direction (that is, either "ToObject" or "FromObject"). Internally the IED will always
measure the currents on all sides of the power transformer with the same reference
direction towards the power transformer windings as shown in figure
en05000186.vsd
IED
I
W1
I
W2
I
W1
I
W2
E1
S1
Z1
S1
E1
S2
Z1
S2
IEC05000186 V1 EN
Figure 31:
Typical CT location and definition of positive current direction
Due to the ratio of the number of turns of the windings and the connection group of the
protected transformer, the current between two windings can not be directly compared
to each other. Therefore the differential protection must first correlate all currents to
each other before any calculation can be performed.
In numerical differential protections this correlation and comparison is performed
mathematically. First, compensation for the protected transformer transformation
ratio and connection group is made, and only then the currents are compared phase-
wise. This makes external auxiliary (interposing) current transformers unnecessary.
Conversion of all currents to the common reference side of the power transformer is
performed by pre-programmed coefficient matrices, which depends on the protected
power transformer transformation ratio and connection group. Once the power
transformer vector group, rated currents and voltages have been entered by the user,
the differential protection is capable to calculate the matrix coefficients required in
order to perform the on-line current comparison by means of a fixed equation.
6.1.3.1
Function calculation principles
To make a differential IED as sensitive and stable as possible, restrained differential
characteristics have been developed and is now adopted as the general practice in the
protection of power transformers. The protection should be provided with a
proportional bias, which makes the protection operate for a certain percentage
differential current related to the current through the transformer. This stabilizes the
protection under through fault conditions while still permitting the system to have
good basic sensitivity. The following chapters explain how these quantities are
derived.
Section 6
1MRK 502 048-UEN A
Differential protection
86
Technical manual
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