Real-Time Clock (Rtc) Operation - ABB REG650 Technical Manual

1MRK 502 034-UEN -
16.3.8.2
Technical Manual

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 SNTP and IRIG-B)
gives an accurate time to the IED. The IED is brought into a safe state and the time
is set to the correct value. After the initial setting of the clock, one of three things
happens with each of the coming synchronization messages, configured as "fine":
• If the synchronization message, which is similar to the other messages, from
its origin has an offset compared to the internal time in the IED, the message is
used directly for synchronization, that is, for adjusting the internal clock to
obtain zero offset at the next coming time message.
• If the synchronization message has an offset that is large compared to the other
messages, a spike-filter in the IED removes this time-message.
• If the synchronization message has an offset that is large, 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 IED is brought into a
safe state and the clock is set to the correct time. If the offset is lower than the
threshold, the clock is adjusted with 10 000 ppm until the offset is removed.
With an adjustment of 10 000 ppm, it takes 50 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
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. Normally, it takes 20 minutes to reach full accuracy.
Time-out on synchronization sources
All synchronization interfaces has a time-out and a configured interface must
receive time-messages regularly in order not to give an error signal (TSYNCERR).
Normally, the time-out is set so that one message can be lost without getting a
TSYNCERR, but if more than one message is lost, a TSYNCERR is given.
Section 16
Basic IED functions
493