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2.3.2 Check relay characteristic (EFD)
To remove unnecessary operations in the DIFGL function, a check relay are accommodated;
consequently, the operation of the DIFGL function is possible during the check relay being in
operation. Figure 2.3-4 shows the check relay and its logic.
Figure 2.3-4 Check relay logic
(i) Earth-fault-current-change detection element (EFD-DIFGL)
Figure 2.3-5 shows a current vector (I
M
) measured at the present and a pre-current vector (I
N
)
measured at one sampling cycle† ago. When the magnitude of difference between (I
s
) is larger
than a setting, the operation the EFD-DIFGL element is introduced. The EFD-DIFGL element
is expressed by the following equation:
where I
s
is referred in order to determine the operation of the EFD-DIFGL element; the
user can set the value using setting [EFD-DIFGL], which is applied to adjust the decision point
for the operation. The user can set the value among 0.05 to 0.20A when 1A rating‡ is set for
the VCT module. When 5A rating‡ is set for the VCT module, the setting value is chosen among
0.25 to 1.00A.
Figure 2.3-5 Current change I
s
detected by I
M
and I
N
†Note: One sampling cycle is discussed later.
‡Note: For the rating and the VCT module, see Chapter
Technical description:
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