4
Transformer Differential Protection (87T and 87HS)
7UT51 v3
56 PRIM-2330C
• If there is a short circuit within the protected
transformer with current entering from only one
side, , so:
(4.15)
(4.16)
Note:
For an internal short-circuit fault,
I
diff
= I
rest
.
The differential protection function uses the
instantaneous values of
I
diff
and
I
rest
to plot the
location of the “operating point” on the trip
characteristic shown in Figure 4.9. The shape of the
trip characteristic is configured using the addresses
described in the following subsections.
The 87T or 87HS function will pickup if either (1) the
operating point of any phase moves above 85% of the
height of the trip characteristic for the simultaneous
restraining current, or (2) the restraining current of any
phase exceeds four times the rated nominal current of
the reference winding of the protected transformer
(
I
rest
> 4
I
n
). This makes it possible for the
saturation-detection algorithms to become active
prior
to actual trip conditions
, as well as for the relay to
record waveform data (see Section 14.7 on page 147)
for the entire fault event, including during external
faults indicated by a high restraining current. The
operating point must be on or above the trip
characteristic for a trip event to occur. Individual
pickup and trip events are stored and annunciated that
report which phase(s) have picked up or tripped (see
Section 4.10 on page 62).
After pickup or trip occurs, the 87T or 87HS function
will dropout (or reset) once the operating point stays
below 70% of the trip setting for two full cycles.
4.4.1 Minimum 87T Trip
Threshold
Constant error currents, such as magnetizing currents,
can result in a small, continuous differential current. To
prevent this current from causing a pickup or trip, the
differential current,
I
diff
, must be at or above the
threshold set in Address 1603 before an 87T trip can
occur.
If the transformer has a tap changer, then the setting
should be increased by 10% for every 10% of tap
changing range. For example, if the pickup value
would otherwise be 0.20, but there is 10% tap
changing, the setting should be 0.22.
4.4.2 High Set (87HS) Trip
Threshold
If the level of the differential current becomes so high
that it cannot be due to an external fault, the
differential protection should trip regardless of the
level of restraining current. A differential current above
the level set in Address 1604 will cause an 87HS
pickup regardless of any restraint measures in effect
(unless the entire 87/87HS function is being blocked
by a binary-signal input).
The 87HS function features an algorithm that
responds to the instantaneous current value (sampled
12 times per cycle), so the 87HS function can trip even
if the fundamental wave is being clipped due to CT
saturation.
′
=
I
2
0
I
diff
=
′
+
′
=
′
+=
′
II I I
12 1 1
0
I
rest
=
′
+
′
=
′
+=
′
II I I
12 1 1
0
1603 87 PICKUP
Minimum trip level of the 87T differential
protection function specified as a fraction (or
multiple) of the rated nominal current of the
protected transformer’s reference winding, I
n
.
Range: 0.15
–
2.00 I
n
Default: 0.20 I
n
1604 87HS RMS
High
-
set pickup level of the differential
protection (87HS) specified as a fraction (or
multiple) of the rated nominal current of the
protected transformer’s reference winding, I
n
.
Range: 0.5
–
20.0 I
n
Default: 7.5 I
n
Setting 0.2
tap setting range (%)()
0.10
-------------------------------------------------------
0.10 0.20××+=
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