IN>
alt
Three phase CT
summated
Single CT
YN/D or YN/Y
transformer
Single phase-to-
earth fault
3I
0
IEC05000493-en-2.vsd
IEC05000493 V2 EN-US
Figure 57: Step 1 fault calculation 1
The fault is located at the borderline between instantaneous and delayed operation
of the line protection, such as Distance protection or line residual overcurrent
protection. This calculation gives the current fed to the protection: 3I
0fault2
The setting of step 1 can be chosen within the interval shown in equation
32.
0fault 2 step1 0fault1
3I lowmar I 3I highmar× < < ×
EQUATION1455 V2 EN-US
(Equation 32)
Where:
lowmar is a margin to assure selectivity (typical 1.2) and
highmar is a margin to assure fast fault clearance of busbar fault (typical 1.2).
Setting of step 2
SEMOD55591-57 v5
The setting of the sensitive step 2 is dependent of the chosen time delay. Often a
relatively long definite time delay or inverse time delay is chosen. The current
setting can be chosen very low. As it is required to detect earth faults in the
transformer winding, close to the neutral point, values down to the minimum
setting possibilities can be chosen. However, one must consider zero-sequence
currents that can occur during normal operation of the power system. Such currents
can be due to un-transposed lines.
In case to protection of transformer windings not feeding residual current at
external earth faults, a fast low current step can be acceptable.
Section 7 1MRK 504 169-UEN A
Current protection
132 Transformer protection RET650 2.2 IEC
Application manual
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