Restrained And Unrestrained Limits Of The Differential Protection - ABB REG650 Technical Manual

Section 6
Differential protection
6.1.3.6
90
• the protected power transformer cannot transform the zero sequence currents to the
other side, for any reason.
• the zero sequence currents can only flow on one side of the protected power
transformer.
In most cases, power transformers do not properly transform the zero sequence current
to the other side. A typical example is a power transformer of the wye-delta type, for
example YNd1. Transformers of this type do not transform the zero sequence
quantities, but zero sequence currents can flow in the grounded wye connected
winding. In such cases, an external ground-fault on the wye-side causes the zero
sequence currents to flow on the wye-side of the power transformer, but not on the
delta side. This results in false differential currents - consisting exclusively of the zero
sequence currents. If high enough, these false differential currents can cause an
unwanted disconnection of the healthy power transformer. They must therefore be
subtracted from the fundamental frequency differential currents if an unwanted trip is
to be avoided.
For delta windings this feature shall be enabled only if an grounding transformer exist
within differential zone on the delta side of the protected power transformer.
Removing the zero sequence current from the differential currents decreases to some
extent sensitivity of the differential protection for internal ground-faults. In order to
counteract this effect to some degree, the zero sequence currents are subtracted not
only from the three fundamental frequency differential currents, but automatically from
the bias current as well.

Restrained and unrestrained limits of the differential protection

Power transformer differential protection function uses two limits, to which actual
magnitudes of the three fundamental frequency differential currents are compared at
each execution of the function.
The unrestrained (that is, non-stabilized, "instantaneous") part of the differential
protection is used for very high differential currents, where it should be beyond any
doubt, that the fault is internal. This settable limit is constant (that is, not proportional
to the bias current). Neither harmonic, nor any other restrain is applied to this limit,
which is therefore capable to trip power transformer instantaneously.
The restrained (that is, stabilized) part of the differential protection compares the
calculated fundamental differential (that is, operating) currents, and the bias (that is,
restrain) current, by applying them to the operate - restrain characteristic. The operate -
restrain characteristic is represented by a double-slope, double-breakpoint diagram,
where the operating current is set against the bias current, as shown in figure
characteristic is determined by the following 5 settings:
1MRK 502 043-UUS B
32 The
Technical Manual