P.3.85
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Differential Elements
87TTAPW and 87TTAPX
These read-only settings are the tap values used by the relay to scale its local
currents connected to the W and X current terminals, respectively, to per unit
values for use in the 87L function.
The relay allows CT ratio mismatching of 10:1; for 5 A nominal relays the
TAP range is 1.75 to 17.55, and the range is 0.35 to 3.5 if the relay is ordered
as a 1 A nominal relay.
You can apply 1 A and 5 A nominal CTs in the same 87L scheme, but not in
the same relay. For example, both CTs connected to Relay 1 in Figure 3.44
must be 5 A nominal, but the CT at Relay 3 can be 1 A nominal.
In applications with in-line transformers the tap values are calculated as
follows.
Equation 3.36
Equation 3.37
The 1 pu base value for the 87L function in applications with in-line
transformers equals the transformer rated current.
Consider the example shown in Figure 3.44. Relay 1 applies
200 • 10
3
/(sqrt(3) • 240 • 200) = 2.4056 as the tap for its W and X current
input terminals. Relay 2 applies 200 • 10
3
/(sqrt(3) • 240 • 240) = 2.0047 as the
tap for its W and X current input terminals. Relay 3 applies
200 • 10
3
/(sqrt(3) • 138 • 200) = 4.1837 as the tap for its W current input
terminal.
The 1 pu of the 87L zone amounts to 481.1 A primary on the 240 kV side of
the transformer, and amounts to 836.7 A primary on the 138 kV side of the
transformer. In other words, 481.1 A/200 = 2.4056 A secondary in the W or X
current input terminals of Relay 1 is 1 pu of the 87L zone;
481.1 A/240 = 2.0046 A secondary in the W or X current input terminals of
Relay 2 is 1 pu of the 87L zone; and 836.7 A/200 = 4.1835 A secondary in the
W current input terminal of Relay 3 is 1 pu of the 87L zone.
When selecting sensitive pickup threshold settings for the 87LP and 87LQ
functions (the 87LG function is not available with an in-line transformer) to
detect high-resistive faults on the transmission line, be sure to use the
appropriate 1 pu value for the line voltage level. For applications without in-
line transformers, there is only one base value, calculated with reference to the
CT with the highest primary value. In the example of Figure 3.44, sensitivity
of 120 A primary for short circuits on the transmission line, requires a pickup
setting of 120/481.1 = 0.25 pu.
Harmonic Blocking and Restraining Logic
You can use harmonic blocking, harmonic restraining or both in the relay to
secure the 87LP element for magnetizing inrush when protecting in-line
transformers. Harmonic blocking is used to secure both the 87LP and 87LQ
elements for overexcitation conditions and for transformer inrush conditions.
Table 3.32 lists settings applicable to harmonic blocking or restraining in
applications with in-line transformers. Table 3.33 lists analog quantities used
in the harmonic sensing logic. Table 3.34 lists Relay Word bits associated with
harmonic blocking or restraining for the 87L elements.
87TTAPW
87MVA 10
3
387VTWL CTRW
-------------------------------------------------------------=
87TTAPX
87MVA 10
3
387VTXL CTRX
-----------------------------------------------------------=
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