Siemens siprotec 7SA6 User Manual page 440

Distance protection

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FFM I< (max)

Figure 6-133

Logic diagram of the fuse failure monitor with zero and negative sequence system

Fuse Failure

(Three-Phase)

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

differential (current differential criterion). A three-pole voltage failure is detected if

• all three phase–earth voltages are smaller than the threshold ))0 8PD[ 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 8PD[ 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

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, undervoltage

protection) are blocked until the voltage failure is removed; thereafter the blocking is

automatically removed. Definite time overcurrent emergency operation is possible

FFM Pickup Delaid

0170 VT FuseFail

0169 VT FuseFail > 10s

C53000-G1176-C133-1

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