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1.
Segregated-phase Current Differential Element DIF
(1) Small current region DIFI1
The characteristic of the DIF for small current region is expressed by the following equation.
I d ≥ (1/6)I r + (5/6)DIFI1
Where, DIFI1 defines the minimum operating current. Therefore, DIFI1 is determined to detect
minimum fault current with margin of 1.5.
Examples: Minimum fault current = 3000A, CT ratio = 2000
(2) Large current region DIFI2
The characteristic of the DIF for large current region is expressed by the following equation.
I d ≥ I r - 2 × DIFI2
Where, DIFI2 defines the maximum out-flow current during an internal fault.
The characteristic has stronger restraint and prevents the element from operating falsely in
response to the erroneous current caused by the CT saturation. The CT saturation occurs in smaller
current than the current determined CT over current constant when the fault current includes
transient DC. Therefore, DIFI2 is preferable as small as possible, but it must be larger than the
maximum out-flowing current during an internal fault.
In case of two terminal network, the maximum out-flowing current is the maximum load current.
In this case, DIFI2 is determined to the maximum load current with margin of 3 or 4.
Examples: Maximum load current = 2000A, CT ratio = 2000
(3) Zero Sequence Current Differential Element DIFG
The minimum operating sensitivity of DIFGI is determined to detect high impedance earth fault.
DIFGI is set to 30 to 50% of the minimum fault current DIFI1 setting.
DIFG must not operate in response to the erroneous current caused by transient errors of the CT
during an external fault. Therefore, the time delay TDIFG setting is preferable larger than 0.1s.
DIFI1 = 3000A/1.5/2000 = 1 A
DIFI2 = 2000A × 3/2000 = 3 A
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