4.22
SEL-787 Relay Instruction Manual Date Code 20150130
Protection and Logic Functions
Basic Protection
set of compensation settings, you can set them accordingly. In this review, it is
assumed that wye-connected CTs are used on either side of the transformer.
To understand how the preferred settings are more secure for external faults,
refer to Figure 4.10. Consider the application with a DY1 transformer
connection and an external phase-to-phase fault on the wye side with
10 percent CT error resulting from saturation on Phase A of the LV side.
Figure 4.10 presents the secondary fault currents the relay measures before
compensation, after compensation, and the resulting operate and restraint
currents for Case 1 (preferred compensation settings) and Case 2 (a different
pair of valid compensation settings). Observe the compensated currents and
the operate/restraint currents in both the cases. It can be seen that in Case 2,
the 10 percent ratio error results in an operate current greater than the O87P
setting and an undesired operation. Upon analysis of the different external
fault types, the preferred compensation settings in this application are
W1CTC = 0 (delta side of transformer) and W2CTC = 1 (wye side of the
transformer); this combination, Case 1, provides adequate restraint current
and prevents the undesired operation.
Table 4.5 Preferred Compensation Settings (W1CTC and W2CTC)
XFMR
Connection
Settings
W1CTC/W2CTC
Applications
DY1 0/1 Preferred compensation for XFMR step down
to sub-transmission or distribution systems;
also for generator step-up XFMRs. (Always
compensate delta winding with WnCTC = 0, if
possible.)
DY7 0/7
DY11 0/11
DY5 0/5
YDY 1/1 Use WnCTC = Odd/Odd (e.g., 1/1, 3/3) if
possible.
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