172
6
F
2
S
0
8
3
4
the presence of the zero sequence current on the power system by introduction of the residual
circuit current.
Only zero sequence monitoring is carried out for the current input circuit, because zero sequence
monitoring with the introduction of the residual circuit current can be performed with higher
sensitivity than negative sequence monitoring.
A/D accuracy checking
An analog reference voltage is input to a prescribed channel in the analog-to-digital (A/D)
converter, and it is checked that the data after A/D conversion is within a prescribed range and that
the A/D conversion characteristics are correct.
Memory monitoring
The memories are monitored as follows depending on the type of the memory and checked that the
memory circuits are healthy:
•
Random access memory monitoring: Writes/reads prescribed data and checks the storage
function.
•
Program memory monitoring:
Checks the checksum value of the written data.
•
Setting value monitoring:
Checks discrepancy between the setting values stored
in duplicate.
Watch Dog Timer
A hardware timer which is cleared periodically by software is provided and it is checked that the
software is running normally.
DC Supply monitoring
The secondary voltage level of the built-in DC/DC converter is monitored and checked that the
DC voltage is within a prescribed range.
3.3.3 CT Circuit Current Monitoring
The CT circuit is monitored to check that the following equation is satisfied and the health of the
CT circuit is checked.
Max(|Ia|, |Ib|, |Ic|)
−
4
×
Min(|Ia|, |Ib|, |Ic|)
≥
k0
where,
Max(|Ia|, |Ib|, |Ic|) = Maximum amplitude among Ia, Ib and Ic
Min(|Ia|, |Ib|, |Ic|) = Minimum amplitude among Ia, Ib and Ic
k0 = 20% of rated current
The CT circuit current monitoring allows high sensitivity detection of failures that have occurred
in the AC input circuit. This monitoring can be disabled by the scheme switch [CTSV].
3.3.4 Signal Channel Monitoring for Integral Digital Communication
Signal channel monitoring
If a failure occurs or noise causes a disturbance in the communication channel, it may interrupt the
data transmission or generate erroneous data, thus causing the relay to operate incorrectly.
The GRZ100 detects data failures by performing a cyclic redundancy check and a fixed bit check
on the data. The checks are carried out for every sampling.
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Summary of Contents for GRZ100-211B
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