ABB Relion 670 series, Техническое руководство

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PID-3713-MONITOREDDATA v6
Table 56: T3WPDIF (87T) Monitored data
Name Type Values (Range) Unit Description
IDMAG_A REAL - A Magnitude of fundamental frequency
differential current, phase A
IDMAG_B REAL - A Magnitude of fundamental frequency
differential current, phase B
IDMAG_C REAL - A Magnitude of fundamental frequency
differential current, phase C
IBIAS REAL - A Magnitude of the bias current, which is
common to all phases
IDMAG_NS REAL - A Magnitude of the negative sequence
differential current
6.1.7 Operation principle
M13039-3 v8
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 35 ), 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 Wye connected. The main CTs can be grounded in
anyway (that is, either "ToObject" or "FromObject"). However internally the differential function
will always use reference directions towards the protected transformer as shown in Figure 35 .
Thus the IED will always internally measure the currents on all sides of the power transformer with
the same reference direction towards the power transformer windings as shown in Figure 35 . For
more information see the Application manual.
en05000186_ansi.vsd
IED
I
W1
I
W2
I
W1
I
W2
E1
S1
Z1
S1
E1
S2
Z1
S2
ANSI05000186 V1 EN-US
Figure 35: Typical CT location and definition of positive current direction
Even in a healthy power transformer, the currents are generally not equal when they flow through
it. This is due to the ratio of the number of turns of the windings and the connection group of the
protected transformer. Therefore the differential protection must first correlate all currents to
each other before any calculation can be performed.
First, compensation for the protected transformer transformation ratio and connection group is
made, and only then are the currents compared phase-wise. This makes external auxiliary
(interposing) current transformers unnecessary. Conversion of all currents to the common
Section 6 1MRK 502 066-UUS B
Differential protection
118
Technical manual