Function Calculation Principles; Fundamental Frequency Differential Currents - ABB REG650 Technical Manual

1MRK 502 043-UUS B
6.1.3.1
6.1.3.2
Technical Manual
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
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.

Fundamental frequency differential currents

The fundamental frequency differential current is a vectorial sum (sum of fundamental
frequency phasors) of the individual phase currents from different side of the protected
power transformer.
Before any differential current can be calculated, the power transformer phase shift,
and its transformation ratio, must be allowed 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 a 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
in equation 2 for a three-winding power transformer.
Differential protection
1 for a two-winding power transformer, and
Section 6
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