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2.7.5 Nature of CT saturation in case of external fault
Figure 2.7-12 shows that an external fault occurs out of the protected line GH. In the DIFL
function the magnitude of an entering current (I
G
) is approximately equal to the one of a
leaving current (I
H
). Thereby, the error made by the CT saturation can be offset in the DIFL
calculation, if both CTs have the same characteristic.
Figure 2.7-12 CT saturation induced by an external fault beyond the protected line
On the other hand, as shown in Figure 2.7-13, if an external fault occurs beyond the
terminal-H, the DIFL function applied for the three-terminal line cannot offset the error. This
is because the same magnitude of the fault current does not flow at respective terminals; the
terminal-H, which is in close proximately the external fault, may experience the outflowing
fault current larger than the terminal-G and the terminal-K. In other words, the DIFL function
cannot offset the error yielded by the CT saturation, unlike in the above case. Thus, the user
has to check a case that CT saturation is particularly produced in when a terminal has an
outflow; the CT saturation must be accommodated utilizing the DIFI2 setting in the DIFL
function.
Figure 2.7-13 CT saturation induced by an external fault on a three-terminal line
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