ABB Relion 670 series, Technical Reference Manual

Page: 93 / 1232

to choose the source with the best quality, and to adjust its internal clock after this
source. The maximum error of a clock can be defined as:
• The maximum error of the last used synchronization message
• The time since the last used synchronization message
• The rate accuracy of the internal clock in the function.
4.3.2.2 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 three
things happens with each of the coming synchronization messages configured as “fine”:
• 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, 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 100
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 1000 ppm until the offset is removed. With an
adjustment of 1000 ppm, it takes 100 seconds or 1.7 minutes to remove an offset
of 100 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.
1MRK505222-UUS C
Section 4
Basic IED functions
87
Technical reference manual