P.3.336
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
Protection Application Examples
EBFL2 := 1 Breaker 2 Failure Logic (N, 1,2)
E25BK1 := Y Synchronism Check for Breaker 1 (Y, N)
E25BK2 := Y Synchronism Check for Breaker 2 (Y, N)
E79 := Y Reclosing (Y, Y1, N)
EMANCL := Y Manual Closing (Y, N)
ELOP := Y1 Loss-of-Potential (Y, Y1, N)
We would have the following at Relay T:
E50G := 1 Res. Ground Inst./Def.-Time O/C Elements (N, 1–4)
87 Current Differential Protection
We would have the following at all relays:
87CTWL := 1 Terminal W included in 87L Element (SEL
OGIC control
equation)
87CTPWL := P CT Polarity of Terminal W (P = Pos., N = Neg.)
Set the unrestrained pickup greater than the largest inrush you could expect
during transformer energization.
87LPU := 8 Unrest. Diff. Element Pickup (OFF, 3.0–15 pu)
CT Performance for External Faults
The steps for determination of CT performance are as follows:
➤ Calculate the current (IF) that each CT sees for an external
fault.
➤ Calculate the system X/R ratio.
➤ Use Equation 3.115 to determine if CT saturation is possible.
➤ In a case where saturation is possible and the CTs comprising
the zone have different characteristics, ratios, or burdens, use
the simulation tools to evaluate CT performance.
Two-Terminal 500 kV Line Differential Example provides details of these
calculations. For this example, an external three-phase fault at Terminal T
yields 9.3 A secondary.
The corresponding X/R ratio for this fault is 43. In applying Equation 3.115,
we obtain the following:
Equation 3.126
CT saturation can therefore occur. Application of the saturation tools show
that the differential is secure for this fault when we apply typical settings (see
Figure 3.221).
400 V
9.3 A 43 1+
--------------------------------------- 0.98=
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