Function Calculation Principles - ABB REG650 Technical Manual

Section 5
Differential protection
5.1.3.1
70
The main CTs are normally supposed to be star connected. The main CTs can be
earthed in anyway (that is, either "ToObject" or "FromObject"). However
internally the differential function will always use reference directions towards the
protected transformer as shown in figure 32. Thus the IED will always internally
measure the currents on all sides of the power transformer with the same reference
direction towards the power transformer windings as shown in figure 32.
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Figure 32: Typical CT location and definition of positive current direction
Even in a healthy power transformer, the currents are generally not equal when
they flow through the power transformer, due to the ratio of the number of turns of
the windings and the connection group of the protected transformer. 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 the external auxiliary (that is, 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 has been
entered by the user, the differential protection is capable to calculate off-line matrix
coefficients required in order to perform the on-line current comparison by means
of a fixed equation.

Function calculation principles

To make a differential IED as sensitive and stable as possible, restrained
differential characteristic have been developed and are 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 calculated.
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