Function Calculation Principles - ABB Relion Technical Manual

1MRK 502 048-UUS A
6.1.3.1
Technical manual
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ANSI05000186 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 phase shift, 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.

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.
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en05000186_ansi.vsd
Section 6
Differential protection
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