Function Calculation Principles - ABB RELION 670 Series Technical Manual

1MRK 502 066-UUS B
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 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.
6.1.7.1

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.
Fundamental frequency differential currents
The fundamental frequency differential current is a vectorial sum (sum of fundamental frequency
phasors) of the individual phase currents from the different sides of the protected power
transformer.
Before any differential current can be calculated, the power transformer phase shift, and its
transformation ratio, must be accounted for. Conversion of all currents to a common reference is
performed in two steps:
• all current phasors are phase-shifted to (referred to) the phase-reference side, (whenever
possible the first winding with wye connection)
• all currents magnitudes are always referred to the first winding of the power transformer
(typically transformer high-voltage side)
The two steps of conversion are made simultaneously on-line by the pre-programmed coefficient
matrices, as shown in equation
three-winding power transformer.
Technical manual
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.
1 for a two-winding power transformer, and in equation
These are the internal compensation within the differential function. The protected
power transformer data is always entered per its nameplate. The Differential
function will correlate nameplate data and select proper reference windings.
Section 6
Differential protection
M13039-12 v4
M13039-15 v9
2 for a
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