Time Overcurrent Protection - Siemens siprotec 7SA6 User Manual

Functions
Definite Time
Overcurrent
Stage 3I >
0
Inverse Time
Overcurrent
Stage 3I
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Forward
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Non-Directional
forwards
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reverse
determination
Figure 6-62 Logic diagram of the 3I
6-104
The logic of the overcurrent stage 3I
3I0>>> is therefore replaced with ,!, then, Figure 6-61 also applies for 3I
This stage operates with a specially optimized digital filter that completely suppresses
all harmonic components beginning with the 2nd harmonic. Therefore it is particularly
suited for a highly-sensitive earth fault detection. A fourth, definite time stage can be
implemented by setting the "inverse" stage (refer to the next paragraph) to a definite
time stage.
The logic of the inverse time stage in principle functions the same as the other stages.
This stage operates with a specially optimized digital filter that completely suppresses
all harmonic components beginning with the 2nd harmonic. Therefore it is particularly
suited for a very sensitive earth fault detection. The delay times in this case are
however determined by the set characteristic (Parameter ,(& &XUYH), the magnitude
of the earth current and the time multiplier ,S 7LPH 'LDO (Figure 6-62). A pre-
selection of the optional characteristics was already done during the configuration of
the protection functions. Furthermore, an additional fixed delay $GG7'(/$< may
be selected. The optional characteristics are illustrated in the technical data of Section
10.5.
Figure 6-62 shows the logic diagram. As an example, the setting addresses for the IEC
curves are shown in the diagram. The different setting addresses are referred to in
more detail in the setting information (Sub-section 6.5.2)
It is also possible to implement this stage as a further definite time stage. In this case
,S 3,&.83 is the pick up threshold and $GG7'(/$< the definite time delay. The
inverse time characteristic is then effectively bypassed.
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7SA6 Manual