Functions
2.4 Single-Phase Overcurrent Protection
SIPROTEC, 7SJ61, Manual
C53000-G1140-C210-1, Release date 02.2008
83
2.4 Single-Phase Overcurrent Protection
The single-phase overcurrent protection evaluates the current that is measured by the sensitive I
NS
- or the
normal I
N
input. Which input is used depends on the device version according to the order number.
Applications
• Plain ground fault protection at a power transformer;
• Sensitive tank leakage protection.
2.4.1 Functional Description
The single-phase definite time overcurrent ground protection is illustrated by the tripping characteristic as
shown in Figure 2-19. The current to be measured is filtered by numerical algorithms. Because of the high sen-
sitivity a particularly narrow band filter is used. The current pickup thresholds and tripping times can be set. The
detected current is compared to the pickup value 50 1Ph-1 PICKUP or 50 1Ph-2 PICKUP and reported if
this is violated. After expiry of the respective delay time 50 1Ph-1 DELAY or 50 1Ph-2 DELAY, the trip
command is issued. The two elements together form a two-stage protection. The dropout value is approximate-
ly 95% of the pickup value for currents greater than I > 0.3 · I
Nom
.
The current filter is bypassed if currents are extremely high in order to achieve a short tripping time. This occurs
automatically as soon as the instantaneous value of the current exceeds the set value of the 50 1Ph-2
PICKUP element by at least factor 2 · √2
.
Figure 2-19 Two-stage characteristic of the single-phase time-overcurrent protection
The following figure shows the logic diagram of the single-phase overcurrent protection function.
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