Siemens SIPROTEC 7SJ62 Manual page 206

Functions
2.11 Monitoring Functions
Single- and Two-pole 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 a single- or two-pole voltage failure, 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-pole Fail-
ure:
If a fault occurs in the voltage transformer secondary system, the following rules apply to the Two-pole Failure:
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 < 0.1 V
1
Three-pole Faults in Voltage Transformer Circuits
A three-pole failure in the voltage transformer secondary system cannot be detected via the positive- and neg-
ative 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-pole 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.
206
) and current (I < 0.1 I
Nom 1 Nom
).
C53000-G1140-C207-2, Release date 01.2008
SIPROTEC, 7SJ62/64, Manual