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3.3 Automatic Supervision
3.3.1 Basic Concept of Supervision
Though the protection system is in a non-operating state under normal conditions, it is waiting for
a power system fault to occur at any time and must operate for the fault without fail. Therefore, the
automatic supervision function, which checks the health of the protection system during normal
operation by itself, plays an important role. A numerical relay based on microprocessor
technology is able to implement such as automatic supervision function. GRT100 implements an
automatic supervision function based on the following concept:
• The supervising function should not affect protection performance.
• Perform supervision with no omissions wherever possible.
• When a failure occurs, it should be possible to easily identify the failure location.
Note: Automatic supervision function includes automatic monitor function and automatic test
function. For the terminology, refer to IEC IEV 60448.
3.3.2 Relay Monitoring and Testing
The relay is supervised with the following items.
AC input imbalance monitoring
The AC current input is monitored such that the following equation is satisfied and the health of
the AC input circuit is checked.
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
= 20% of rated current
A/D accuracy checking
An analogue reference voltage is transmitted to a prescribed channel in the analogue-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.
Watchdog Timer
A hardware timer which is cleared periodically by software is provided and it is checked that the
software is running normally.