Schweitzer Engineering Laboratories SEL-311L, Instruction Manual, Page: 59 / 734

Date Code 20011112 Line Current Differential, Distance, Out-of-Step, Overcurrent, 3-5
Voltage, Synchronism Check, and Frequency Elements
SEL-311L Instruction Manual
(an extreme case). The SEL-311L Relay estimates the one-way channel delay as half the round
trip delay. In this situation, the round trip delay is about 2 ms (100 µs one way, 2 ms the other
way, for a total round trip of about 2 ms).
In this extreme case both SEL-311L Relays estimate a one-way channel delay of 1 ms, and each
relay uses local currents measured 1 ms earlier to align the local data with the received remote
data. Thus, both relays have a 1 ms data alignment error (one relay leading, the other lagging).
This causes the angle of the Alpha plane ratio to be in error by about 22 degrees on a 60 Hz
system. In one relay, the error is positive (counter clockwise on the Alpha plane); in the other
relay the error is negative (clockwise on the Alpha plane).
Depending on the angular shift at a particular relay, this error could add to or subtract from the
angles caused by the system non-homogeneity and load angle discussed above. Assume the
angles add, as a worst case. For an internal fault, the Alpha plane angle could be as much as
+/-(20 + 22) = +/-42 degrees.
Next consider CT saturation. As shown in Figure 3.5, a severely saturated CT might temporarily
cause the fundamental component of the secondary current to lead the primary current by as
much as 40 degrees. Considering CT saturation, system non-homogeneity, load angle, and
asymmetrical channel delay, the Alpha plane angle for phase currents could be as much as
+/-(40 + 22 + 20) = +/-82 degrees for an internal three-phase fault.