ERL 8700/BUS, User Manual, Page: 53 / 166

4 Protection Functions
D01908R02.12 B-PRO User Manual 8700 4-9
Transformer
Protection
87T Transformer Differential
Device 87T is the differential protection for the transformer zone. It can be
used when the differential zone is selected as Bus1 & Transformer or Trans-
former Only. The device operates for internal faults and provides restraint for
external faults, transformer energization or transformer over excitation condi-
tions. CT input 5 and CT input 6 are dedicated for the differential device when
2 differential zones (bus and transformer) are selected, or if only 1 differential
zone (transformer) is selected. If external control is enabled, and asserted, the
ac analog current will be excluded from all relay, metering and recording func-
tions.
Restraint quantities are summed; the magnitude sum is then divided by two.
Operating quantities are summed vectors.
Device 87T has 2nd and 5th harmonic blocking, each element is set indepen-
dent of each other. If an input phase current is less than 5% of I nominal, this
current will not be used for the 2nd and 5th harmonic blocking calculation. For
a 5A relay this equals 0.25A.
As shown below, the 2nd harmonics restraint signal is stretched for 5ms in the
first cycle on transformer energization to prevent the 2nd harmonics restraint
signal from any possible momentary reset due to the current signal transition
in the first cycle. Note that this logic only becomes active when the transformer
has been de-energized or very lightly loaded (the restraint current is less than
30% of IOmin setting)
IOmin Minimum level that device 87 operates
IRs Point of intersection between slope 1 and slope 2 of the characteristic
S1 Slope of first part of characteristic meeting IO min and slope 2
S2 Slope of second part of characteristic meeting slope 1 and high current
unrestrained setting
I2 Ratio of 2nd harmonic current to fundamental, used to provide energiz-
ing harmonic restraint
I5 Ratio of 5th harmonic current to fundamental, used to provide restraint
on over excitation
IOH High Set Unrestrained high set overcurrent, operates if a heavy fault occurs on
the transformer irrespective of restraint
37 IRA (30% of IOmin)
37 IRB (30% of IOmin)
37 IRC (30% of IOmin)
10 s
17 ms
0
5 ms
Transformer has
Device 37: under-current
been de-energized
2nd Harmonics Restraint Signal
Block 87
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