20.6.3.2
Real-time clock (RTC) operation
The IED has a built-in real-time clock (RTC) with a resolution of one second. The
clock has a built-in calendar that handles leap years through 2038.
Real-time clock at power off
During power off, the system time in the IED is kept by a capacitor-backed real-time
clock that will provide 35 ppm accuracy for 5 days. This means that if the power is off,
the time in the IED may drift with 3 seconds per day, during 5 days, and after this time
the time will be lost completely.
Real-time clock at startup
Time synchronization startup procedure
The first message that contains the full time (as for instance LON, SNTP and GPS)
gives an accurate time to the IED. After the initial setting of the clock, one of the three
following conditions happens with each of the coming synchronization messages
configured as
FineSynchSource
:
•
If the synchronization message, which is similar to the other messages, has an
offset compared to the internal time in the IED, the message is used directly for
synchronization, which means, for adjusting the internal clock to obtain zero
offset at the next coming time message.
•
If the synchronization message has a large offset compared to the other messages,
a spike-filter in the IED removes this time-message.
•
If the synchronization message has a large offset and the following message also
has a large offset, the spike filter does not act and the offset in the synchronization
message is compared to a threshold that defaults to 500 milliseconds. If the offset
is more than the threshold, the clock jumps a whole number of seconds so the
remaining offset is less than 500ms. The remaining offset is then slowly adjusted
with 1000 ppm until the offset is removed. With an adjustment of 1000ppm it
takes 500 seconds to remove an offset of 500 milliseconds.
Synchronization messages configured as coarse are only used for initial setting of the
time. After this has been done, the messages are checked against the internal time and
only an offset of more than 10 seconds resets the time.
Rate accuracy
The Rate accuracy is the accuracy of the internal clock while the external time signal
is lost (e.g. GPS lost). In the IED, the rate accuracy at cold start is 100 ppm but if the
IED is synchronized for a while, the rate accuracy is approximately 1 ppm if the
surrounding temperature is constant. It can take up to one hour after a cold start of the
IED to reach to a stable rate due to the normal warming-up procedure of the IED.
Time-out on synchronization sources
All synchronization interfaces have a time-out, and a configured interface must
receive time synchronization messages regularly in order not to give an error signal
(TSYNCERR) that is reported from the INTERRSIG function block. The time-out is
1MRK502052-UEN B
Section 20
Basic IED functions
1111
Technical manual
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