ABB RED650 Technical Manual page 104

Section 6
Differential protection
98
An internal fault can be suspected. In the opposite case, when one negative
sequence current is entering and the other is leaving the protected object, the phase
difference will ideally be 180 degrees. An external fault can be suspected. If either
the local or the sum of the remote negative sequence currents or both is below the
set level, the fault discriminator does not make any fault classification and the
value 120 degrees is set. This value is an indication that negative sequence
directional comparison has not been possible. In this case, neither internal nor
external fault is signalized. When an external fault is detected, the 2
harmonic analysis is activated for 200 ms. This gives better stability against
unwanted trips. Under an external fault condition, the cross block logic algorithm is
active as well.
When a fault is classified as internal by the negative sequence fault discriminator, a
trip command is issued under the condition that at least one start signal has been
issued, while all eventual block signals (issued by the harmonic analysis of the
instantaneous differential currents) are ignored.
For all differential functions it is the common trip that shall be used
to initiate a trip of a breaker. The separate trip signals from the
different parts lacks the safety against maloperation. This does 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 shall only be used for information purpose of which part
that has caused the trip.
With reference to Figure 33, the outputs from the three analysis blocks are fed to
the output logic. Figure
where only trip commands and no alarm signals are shown for simplicity.
36 shows a simplified block diagram of this output logic
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Line differential protection RED650 2.2 IEC
Technical manual
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