Operation Principle; Function Calculation Principles - ABB RET650 Technical Manual

1MRK 504 135-UEN A
6.1.4
6.1.4.1
Technical manual

Operation principle

The task of the power transformer differential protection is to determine whether a
fault is within the protected zone, or outside of the protected zone. The protected zone
is limited by the position of current transformers (see figure 32), and in principle can
include more objects than just a transformer. If the fault is found to be internal, the
faulty power transformer must be quickly disconnected from the system.
The main CTs are normally supposed to be star connected and can be earthed in any
direction (that is, either "ToObject" or "FromObject"). Internally the IED will always
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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IEC05000186 V1 EN
Figure 32: 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 vector group, 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
Differential protection
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Section 6
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