3-24
Differential, Restricted Earth Fault,
Date Code 20020129
Overcurrent, Voltage, and Frequency Elements
SEL-387E Instruction Manual
Common Harmonic Blocking (IHBL = N) blocks all of the percentage differential
elements if any one phase has a harmonic magnitude above the blocking threshold.
Common Harmonic Blocking is a more secure scheme but may slightly delay percentage
differential element operation since harmonics in all three phases must drop below their
thresholds.
Example of Setting the SEL-387E Relay for a Three-Winding Transformer
In this section we use an example that forms the basis of the default differential settings we
entered at the factory before shipping the relay. The example represents a typical three-winding
transformer application and demonstrates the use of CT compensation settings and tap
calculations.
Figure 2.8 in
Section 2: Installation
illustrates the application. The transformer is a 230 kV to
138 kV autotransformer with a 13.8 kV delta tertiary whose terminals we have brought out and
included in the differential zone of protection. The transformer primary and secondary have a
maximum rating of 100 MVA, while the tertiary has a rating of 30 MVA. All three windings
have wye-connected current transformers, with ratios of 600/5 A at 230 kV, 1200/5 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. Set the appropriate enables. Because we need only three terminals for the differential
zone, make the first settings as follows:
E87W1 = Y
E87W2
= Y
E87W3 = Y
These settings enable Windings 1, 2, and 3.
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:
230
kV
138
kV
13.8
kV
W1CT =
Y
W2CT
= Y
W3CT = Y
CTR1 = 120
CTR2
= 240 CTR3 = 400
3. Set the transformer maximum rating. We use this rating for all windings in the later tap
calculation:
MVA = 100
4. Decide whether to use internal CT compensation and determine compensation settings.
Because there are both wye and delta transformer windings but only wye CTs, we must
adjust for the phase angle shift. In the “traditional” differential relay connection the wye
transformer windings would have their CTs connected in delta to produce a shift in the
same direction as that produced in the transformer. In this case a “DAC” or “30-degree
lagging” connection would have been used. This would not only shift the currents, but it
would remove the zero-sequence current component by physically subtracting the
appropriate phase currents via the delta connection. We achieve the same effect within
the relay by using the selected compensation. The settings are:
