4.23
Date Code 20081022 Instruction Manual SEL-787 Relay
Protection and Logic Functions
Basic Protection
Fourth-Harmonic Blocking
Transformer magnetizing inrush currents are generated during transformer
energization when the current contains a dc offset due to point-on-wave
switching. Inrush conditions typically are detected using even harmonics and
are used to prevent misoperations due to inrush. The largest even-harmonic
current component is usually second harmonic followed by fourth harmonic.
Use fourth-harmonic blocking to provide additional security against inrush
conditions; set PCT4 less than PCT2.
Fifth-Harmonic Blocking
Fourier analysis of transformer currents during overexcitation indicates that a
35 percent fifth harmonic setting is adequate to block the percentage
differential element. To disable fifth harmonic blocking, set PCT5 to OFF.
You may use the presence of fifth-harmonic differential current to assert an
alarm output during startup. This alarm indicates that the rated transformer
excitation current is exceeded. At full load, a TH5P setting of 0.1 corresponds
to 10 percent of the fundamental current. A delay, TH5D, that can be set by
the user prevents the relay from indicating transient presence of fifth-
harmonic currents.
You may consider triggering an event report if transformer excitation current
exceeds the fifth harmonic threshold.
There are two criteria for setting TH5P:
TH5P • TAPn ≥ 0.05 • I
nom
where:
n = 1, 2 and I
nom
is nominal current of corresponding CT
Example of Setting the SEL-787 Relay
The example represents a typical transformer application and demonstrates
the use of CT compensation settings and tap calculations.
Figure 2.19 illustrates the application. The transformer is a 138 kV to 13.8 kV.
The transformer has a maximum rating of 30 MVA. Both windings have wye-
connected current transformers, with ratios of 200/5 A at 138 kV, and
2000/5 A at 13.8 kV. We have connected the transformer per IEEE standards,
with the low voltage delta lagging the high-voltage wye by 30 degrees.
1. Enable the differential settings as follows:
E87 := Y
2. Select settings for the current transformer connection and ratio
for each winding. All CTs connect in wye. The ratios are equal
to primary current divided by secondary current. The settings
are as follows:
3. Set the transformer maximum rating. We use this rating for all
windings in the later tap calculation:
MVA = 30
138 kV 13.8 kV
W1CT = WYE W2CT = WYE
CTR1 = 40 CTR2 = 400
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