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3.3.2 Relay Monitoring
The relay is supervised by the following functions.
AC input imbalance monitoring
The AC current input is monitored to check that the following equation is satisfied and the health of
the AC input circuit is verified. This monitoring is NOT for models 820 and 821.
• CT circuit current monitoring
Max(|I a |, |I b |, |I c |) − 4 × Min(|I a |, |I b |, |I c |) ≥ k 0
where,
Max(|I a |, |I b |, |I c |) = Maximum amplitude among I a , I b and I c
Min(|I a |, |I b |, |I c |) = Minimum amplitude among I a , I b and I c
k 0 = 0.2A
The CT circuit current monitoring allows high sensitivity detection of failures that have occurred in
the AC input circuit. If the imbalance detected, the relay issues an alarm shown in Table 3.3.1.
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
Memory is monitored as follows, depending on the type of memory, and checks are done to verify
that memory circuits are healthy:
• Random access memory monitoring:
• Program memory monitoring: Checks the checksum value of the written data.
• Setting value monitoring:
Watchdog Timer
A hardware timer that is cleared periodically by the software is provided, which checks that the
software is running normally.
Power supply Monitoring
The secondary voltage level is monitored, and is checked to see that the DC voltage is within a
prescribed range.
Issuing of Alarms
The alarms are issued when the failure continues for a predetermined time. The times for each
monitoring item are as follows;
• A/D accuracy checking, memory monitoring, Watch Dog Timer, DC supply monitoring:
less than 1s
• AC input imbalance monitoring, sampling synchronization monitoring : 15s
Writes/reads prescribed data and checks the storage function.
Checks discrepancies between the setting values stored in
duplicate.
70
6 F 2 T 0 1 7 2
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