Schweitzer Engineering SEL-387-0, Instruction Manual

Page: 530 / 556

H.12
SEL-387-0, -5, -6 Relay Instruction Manual Date Code 20170601
Protection Application Examples
Transformer Winding and CT Connection Compensation Settings Examples
Step 2. Select the Reference Winding and Associated Relay Terminal
If there is a delta winding on the power transformer, the delta winding should
be selected as the reference winding regardless of whether it is the high- or
low-voltage winding. The reference winding can be associated with any
analog current measurement terminal on the relay. For example, if the delta
winding current is measured by the W1-terminal inputs on the relay, W1CTC
is the setting that corresponds to the reference winding.
The compensation for the delta winding should be set to matrix CTC(0)
(W
n CTC = 0) unless there is a grounding bank on the delta winding within the
differential zone. Grounding banks are a source of zero-sequence current and
this current needs to be filtered to avoid operation of the differential element
for external ground faults. If there is a grounding bank within the differential
zone, use W
n CTC = 12. Both W n CTC = 0 and W n CTC = 12 result in no
phase shift, but W n CTC = 12 additionally removes zero-sequence current
from the differential calculation.
If there is no delta winding, select one of the wye windings as the reference
and set the compensation setting to 11 (W
n CTC = 11) for the reference
winding.
Step 3. Determine the Required Compensation Settings for All Other
Windings
Use the following guidelines for choosing the remaining CT compensation
settings.
1. Compensate delta windings with matrix CTC(0)
2. Compensate wye windings with odd matrices
3. Avoid the use of even matrices
There may be applications that require one or more of the guidelines to be
violated, but they should be followed when possible. The following examples
illustrate the steps required to determine the compensation setting(s) for the
remaining winding(s).
Application Examples
EXAMPLE H.1 Delta-Wye Transformer With Standard
Phase-to-Bushing Connections, Standard CT Connections, and an
ABC System Phase Rotation
Consider the system shown in Figure H.16. The primary current phase
shift for these connections was determined in Figure H.6. Figure H.17
shows the phase relationship of both the primary system phase
currents and the secondary phase currents as seen by the relay. The
system primary current Ia (X-side) lags the system primary current IA
(H-side) by 30 degrees. The CT connections are standard, which
results in IAW2 leading IAW1 by 150 degrees.