Schweitzer Engineering SEL-387-0, Instruction Manual

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3.20
SEL-387-0, -5, -6 Relay Instruction Manual Date Code 20170601
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Differential Element
Example of Setting the SEL-387 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 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.
Step 1. Enable the differential settings as follows:
E87W1 = Y
E87W2 = Y
E87W3 =
Y
These settings enable Windings 1, 2, and 3, leaving Winding 4
available for stand-alone overcurrent duty.
Step 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:
Step 3. Set the transformer maximum rating. We use this rating for all
windings in the later tap calculation:
MVA =
100
Step 4. Determine compensation settings.
Because the CTs on either side of the transformer (wye and
delta sides) are wye-connected, we must adjust for the phase
angle shift. In the “traditional” differential relay connection, the
wye transformer windings have their CTs connected in delta to
produce a shift in the same direction as that produced in the
transformer. Using the guidelines in
Steps to Determine the
Compensation Settings (WnCTC) on page H.5 , it can be
determined that the phase-to-transformer bushing connections
are standard. The primary delta current Ia lags the system
primary wye current IA by 30 degrees. The CT connections are
standard, which results in IAW3 leading IAW1 and IAW2 by
150 degrees as seen by the relay. Primary currents and
secondary currents as measured by the relay are shown in
Figure 3.10 . For simplicity, only Terminal 1 and Terminal 3
currents are shown. Terminal 2 currents are the same as
Terminal 1.
230 kV 138 kV 13.8 kV
W1CT = Y W2CT = Y W3CT = Y
CTR1 = 120 CTR2 = 240 CTR3 = 400