Real-Time Clock (Rtc) Operation; Synchronization Options - Hitachi Relion 650 Series Technical Manual

Section 14
Basic IED functions
14.2.8.2

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
At IED startup, the internal time is free running. If the RTC is still alive since the last up time, the time
in the IED will be accurate (may drift 35 ppm), but if the RTC power has been lost during power off
(will happen after 5 days), the IED time will start at 1970-01-01.
Time synchronization startup procedure
Coarse time synchronization is used to set the time on the very first message and if any message
has an offset of more than ten seconds. If no FineSyncSource is given, the CoarseSyncSource is
used to synchronize the time.
Fine time synchronization is used to set the time on the first message after a time reset or if the
source may always set the fine time, and the source gives a large offset towards the IED time. After
this, the time is used to synchronize the time after a spike filter, that is, if the source glitches
momentarily or there is a momentary error, this is neglected. FineSyncSource that may always set
the time is only IRIG-B.
It is not recommended to use SNTP as both fine and coarse synchronization source, as some clocks
sometimes send out a bad message. For example, Arbiter clocks sometimes send out a "zero-time
message", which if SNTP is set as coarse synchronization source (with or without SNTP as fine
synchronization source) leads to a jump to "2036-02-07 06:28" and back. In all cases, except for
demonstration, it is recommended to use SNTP as FineSynchSource only.
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.
14.2.8.3

Synchronization options

Two main options of external time synchronization are available. The synchronization message is
applied either via any of the communication ports of the IED as a telegram message including date
and time or via IRIG-B.
Synchronization via SNTP
SNTP provides a ping-pong method of synchronization. A message is sent from an IED to an SNTP
server, and the SNTP server returns the message after filling in a reception time and a transmission
time. SNTP operates via the normal Ethernet network that connects IEDs via IEC 61850 station bus.
For SNTP to operate properly, there must be an SNTP server present, preferably in the same station.
The SNTP synchronization provides an accuracy that gives +/- 1 ms accuracy for binary inputs. The
IED itself can be set as an SNTP-time server.
SNTP provides complete time-information and can be used as both fine and coarse time synch
source. However shall SNTP normally be used as fine synch only. The only reason to use SNTP as
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