In an incident involving several relays, breakers and meters, the exact order of events is the most important clue for root-cause analysis. If device clocks differ by even a few seconds, sequence-of-events (SOE) reports and disturbance files lose their value. In Sampled Values networks, a time difference of microseconds directly causes measurement error and protection mal-operation.
Required time accuracy
| Application | Required accuracy |
|---|---|
| Incident analysis and SOE | about 1 millisecond |
| Disturbance recorders (DFR) and relays | 1 millisecond or better |
| Synchrophasors (PMU, IEEE C37.118) | below 1 microsecond |
| Sampled Values (IEC 61850-9-2) | about 1 microsecond |
Common methods
IRIG-B
The time signal reaches all devices over a separate cable (modulated or unmodulated TTL). Accuracy is in the microsecond range and is independent of network traffic. Drawbacks: separate cabling, limits on length and number of outputs, and the need for an IRIG-B input on each device.
NTP and SNTP
Over Ethernet using UDP packets (port 123). Typical accuracy is a few to a few tens of milliseconds, adequate for control-centre event logging but not for accurate SOE or SV. SNTP is a simplified version with less filtering and network delay calculation.
PTP (IEEE 1588 v2)
Synchronisation over the same Ethernet network with sub-microsecond accuracy. Substations use the power profile (IEEE C37.238 and IEC/IEEE 61850-9-3). Switches must support transparent or boundary clock functions to compensate transit delay. Suited to process bus and PMUs.
1PPS
A pulse once per second, usually a complement for high accuracy.
Suggested architecture
- GNSS/GPS reference clock with a proper antenna, lightning protection and an open-sky location.
- A second (standby) clock or holdover with an OCXO oscillator for satellite outages.
- Distribution with IRIG-B for sensitive devices, PTP for the SV network and NTP for HMIs and servers.
- Monitoring of clock health with alarms sent to SCADA.
Time zone and leap seconds
It is best for all IEDs to keep UTC, with the time zone applied only in the HMI/SCADA. Daylight-saving changes and leap seconds should be defined before the project.
Testing synchronisation
- Check the time-quality bits (Clock Not Synchronised and Clock Failure) in every IED.
- Apply the same event (for example a common input signal) to several relays and read the time-stamp differences in the SOE.
- Remove the time source and observe holdover and alarms, then restore it and observe any time jump.
- Measure offset and path delay for PTP with a network analysis tool.
- Check that IED time agrees with SCADA time.
Common problems
- IEDs without a time input or with a poor internal clock
- Wrong year and time zone in IRIG-B (no year extension)
- A PTP path crossing a switch that does not support it
- Several uncoordinated time sources in one substation
- A GPS antenna installed in a covered location
ENP designs, configures and tests time synchronisation systems in substation automation projects.