C37.118.1 Extension
The IEEE C37.118.1 standard for Synchrophasor Measurements for Power Systems was released in 2011, superseding the previous standard C37.118 (2005) and the IEEE 1344 (1995) standards. Each of these standards were improved to keep up with the need for real-time monitoring of parameters such as current, frequency, load, and voltage to avoid blackouts. With the introduction of Phasor Measurement Units (PMUs), the need for high accuracy, reliable time stamping became a strict requirement when recording and comparing samples. Timing errors between two locations due to an unsynchronised clock could result in false tripping, causing operators to make incorrect and potentially costly decisions.
With IRIG-B being a one-way signal, i.e. no feedback to the clock from the slave, additional fields needed to be added to the IRIG-B signal to allow the slave devices to decide if the timing source meets their accuracy requirements, and to stop operating if the reported accuracy was too low. This brought about the use of the control fields by the IEEE standards, with the fields in table 6 being added to the signal.
|
Bit# |
Value |
Definition |
|
60 |
0 |
Leap Second Pending (LSP) – This field becomes a 1 up to 59 seconds BEFORE a leap is insert/deleted. Returns to 0 after the event. |
|
61 |
0 |
Leap Second (LS) -0 = Add a second (most common) and 1 subtract a second |
|
62 |
0 |
Daylight Saving Pending (DSP) - This field becomes a 1 up to 59 seconds before a DST event. Returns to 0 after the event |
|
63 |
0 |
Daylight savings Time (DST) – Becomes 1 during DST |
|
64 |
0 |
Time Offset Sign- 0 = +and 1 = - |
|
65 |
1 |
Time Offset – This is the offset from the IRIG-B time to UTC time ie. The local time offset. Taking this offset and the IRIG time you can get the UTC time |
|
66 |
2 |
|
|
67 |
4 |
|
|
68 |
8 |
|
|
69 |
P7 – Position ID |
|
|
70 |
0 |
Time offset 0.5 Hours -0= No offset and 1- 0.5 hour offst |
|
71 |
1 |
Time quality bit- this is a 4-bit code representation of the approximate clock time error from UTC |
|
72 |
2 |
|
|
73 |
4 |
|
|
74 |
8 |
|
|
75 |
0 |
Parity – this is the for all of the proceeding bits. Acts as a check t ensure the data (SBS and CF) make sense. This will toggle between ODD and EVEN every second |
|
76 |
1 |
Continuous Time Quality – This is a 3-bit code representation of the estimated time error in the transmitted message |
|
77 |
2 |
|
|
78 |
4 |
|
|
79 |
P8 – Position ID |
|
Time Quality Bit
The TQ field gives an indication of the time accuracy of the IRIG-B signal at the “on time” point relative to UTC. When in a locked state this value remains at 0, and will only change when the clock loses its fix with the visible satellite constellations, entering holdover.
|
Value |
Definition |
|
0 |
Clock is locked to a UTC traceable source |
|
1 |
Time is within <1ns of UTC |
|
2 |
Time is within <10ns of UTC |
|
3 |
Time is within <100ns of UTC |
|
4 |
Time is within <1us of UTC |
|
5 |
Time is within <10us of UTC |
|
6 |
Time is within <100us of UTC |
|
7 |
Time is within <1ms of UTC |
|
8 |
Time is within <10ms of UTC |
|
9 |
Time is within <100ms of UTC |
|
10 |
Time is within <1s of UTC |
|
11 |
Time is within <10s of UTC |
|
15 |
Fault- Clock Failure, time is not reliable |
Continuous Time Quality
The CTQ field gives an indication of the time accuracy of the IRIG-B signal at the “on time” point with respect to UTC for each IRIG-B message. CTQ was added to the IRIG-B signal to give an indication of accuracy when in sync as the Time Quality Indicator always shows 0 when in sync. This field was not available in the IEE1344 standard, being added to the later C37.118 standard. A full table of the available values is shown in table 8.
|
0 |
Note Used (indicates code from previous version of standard) |
|
1 |
Estimated maximum time error <100ns |
|
2 |
Estimated maximum time error <1us |
|
3 |
Estimated maximum time error <10us |
|
4 |
Estimated maximum time error <100us |
|
5 |
Estimated maximum time error <1ms |
|
6 |
Estimated maximum time error <10ms |
|
7 |
Estimated maximum time error <10ms – time error unknown |
The C37.118.1 extension should always be used in IRIG-B signals received by Tekron clocks. We cannot guarantee accurate synchronization to IRIG-B without this extension, and daylight savings and leap second shift events will also be unaccounted for.
AFNOR S87-500 Extension
The AFNOR standard is a French standard that is very similar to the IRIG-B code. The extensions can include additional information about the day of week, month, day of month, and SBS. Even though not widely adopted in the power industry, this standard is still supported by most clock vendors.