Function Calculation Principles - ABB REG670 Technical Manual

Section 6
Differential protection
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IEC05000186 V1 EN
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 it. This is 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.
First, compensation for the protected transformer transformation ratio and connection
group is made, and only then are the currents 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 off-line matrix coefficients required in order to perform the on-
line current comparison by means of a fixed equation.
The negative-sequence-current-based internal-external fault discriminator, is used
with advantage in order to determine whether a fault is internal or external. It not only
positively discriminates between internal and external faults, but can also
independently detect minor faults which may not be sensed by the "usual" differential
protection based on operate-restrain characteristic.
For all differential functions it is the common trip that is used to
initiate a trip of a breaker. The separate trip signals from the different
parts lacks the safety against maloperation. This will in some cases
result in a 6 ms time difference between, for example restrained trip is
issued and common trip is issued. The separate trip signals are only
used for information purpose of which part that has caused the trip.

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
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Technical manual