Functions
2.10 Monitoring Functions
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A4, Release date 08.2010
165
Single- and Two-phase Faults in Voltage Transformer Circuits
The measuring voltage failure detection is based on the fact that a significant negative sequence system is
formed in the voltage during single- or two-phase voltage failure, however without influencing the current. This
enables a clear distinction from asymmetries impressed by the power system. If the negative sequence system
is related to the current positive sequence system, the following rules apply to the Fault-free Case:
If a fault occurs in the voltage transformer secondary system, the following rules apply to the Single-phase
Failure:
If a fault occurs in the voltage transformer secondary system, the following rules apply to the Two-phase Fail-
ure:
In case of a failure of one or two phases of the primary system, the current also shows a negative sequence
system of 0.5 or 1. Consequently, the voltage monitoring does not respond since no voltage transformer fault
can be present. In order to avoid occurrence of an overfunctioning of the measuring voltage failure detection
due to inaccuracy, the function is blocked below a minimum threshold of the positive sequence systems of
voltage (V
1
< 0.1 V
Nom
) and current (I
1
< 0.1 I
Nom
).
Three-phase Faults in Voltage Transformer Circuits
A three-phase failure in the voltage transformer secondary system cannot be detected via the positive- and
negative sequence system as described above. The monitoring of the progress of current and voltage in
respect of time is required here. If a voltage dip to almost zero occurs (or if the voltage is zero), and the current
remains unchanged, a three-phase failure in the voltage transformer secondary system can be concluded. The
exceeding of an overcurrent threshold (parameter 5307 I> BLOCK) is used here. This threshold value should
be identical to the definite time overcurrent protection. If the threshold value is exceeded the measuring-circuit
voltage failure monitoring is blocked. This function is also blocked if a pickup by an (overcurrent) protection
function has already occurred.
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