Siemens siprotec 7SA522 User Manual page 369

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L1
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2
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Figure 6-117 Logic diagram of the fuse failure monitor with zero and negative sequence system
Fuse Failure
Monitor
(Three-Phase)
7SA522 Manual
C53000-G1176-C119-2
≥1
≥1
&
≥1
&
≥1
A three-phase failure of the secondary measured voltage can be distinguished from
an actual system fault by the fact that the currents have no significant change in the
event of a failure in the secondary measured voltage. For this reason, the sampled
current values are routed to a buffer, so that the difference between the present and
stored current values can be analysed to recognize the magnitude of the current dif-
ferential (current differential criterion). A three-pole voltage failure is detected if
• all three phase–earth voltages are smaller than the threshold ))0 8 PD[ SK ,
• the current differential in all three phases is smaller than the threshold
))0 ,GHOWD S , and
• all three phase current amplitudes are greater than the minimum current ,SK! for
impedance measurement by the distance protection.
If no stored current values are present (yet), the current magnitude criterion is resorted
to. A three-pole system voltage failure is detected in this case if
• all three phase–earth voltages are smaller than the threshold ))0 8 PD[ SK ,
• all three phase current amplitudes are smaller than the minimum current ,SK! for
impedance measurement by the distance protection, and
• all three phase current amplitudes are greater than a fixed set noise threshold
(40 mA).
If such a voltage failure is recognized, the distance protection and all other functions
that operate on the basis of undervoltage (e.g. also weak infeed tripping) are blocked
until the voltage failure is removed; thereafter the blocking is automatically removed.
Definite time overcurrent emergency operation is possible during the voltage failure if
the overcurrent protection was configured accordingly (refer to Section 6.9).
&
≥1
≥1
Functions
0170 VT FuseFail
10 s 0
VT FuseFail>10s
0169
6-221