Leap Seconds

The rotational period of the earth is not exactly 24 hours, or more specifically 23 hours and 56 minutes, which is the nominal length of the 'sidereal' or astronomical day. As a result, the time when the celestial meridian appears overhead at any given point on the earth's surface changes slightly from day to day. This change is small, but given the high magnification of astronomical telescopes, it is important to astronomers.

UTC progresses forward at a fixed rate, based on the natural frequency of certain atomic vibrations (which define the atomic clock). As the earth's rotation diverges from this, the difference is accumulated as a correction that can be added to UTC to give sidereal time. Astronomers use this correction when directing their telescopes.

When this correction grows to approximately one second, the international timing community adds (or subtracts) one second to the UTC time scale, to keep UTC nearly aligned with sidereal time (figure 9). These added or subtracted seconds are called 'leap seconds.' By convention, they happen at midnight UTC, June 30 or December 31.

Fig. 9

When a leap second is added, it results in two seconds having the same time in many digital time representations. The time of the leap second is reported as 23:59:60, which will when converted to binary time often have the same representation as 00:00:00 the next day. This ambiguity can cause confusion when an event occurs during or overlapping a leap second. Consequently, some people think that NERC did not go far enough in recommending UTC, but should have recommended utilities use TAI for event recording (remember that TAI could also be easily converted in a file viewer to UTC or local time).

TAI, the International Atomic Time, does not have leap seconds. As leap seconds are added to UTC, there is a difference (TAI - UTC) which is an increasing integer, presently 33 seconds. GPS time also does not have leap seconds, but it 'started' more recently than TAI, so the difference is less. Today, (GPS - UTC) is 14 seconds. And, the difference (TAI - UTC) is fixed at 19 seconds. Using TAI (or GPS) time would eliminate the potential ambiguity caused by leap seconds, but would make the 'native' time tag format less familiar to a reader.

To add a little extra complexity, there is now a discussion recommending that 'civil time,' that is, UTC, not have any more leap seconds. The difference between UTC and sidereal time would be allowed to increase past one second. If the difference ever got large enough to be a concern, then a 'leap minute' could be added to 'fix' it (this would likely happen approximately once every 100 years). If leap seconds are eliminated from UTC, the advantage of TAI would become moot, since the only difference between TAI and UTC is lack of leap seconds.

Lifts
Thanks to modern satellite navigation systems, accurate time is now available to the power industry at minimal cost.

Numerous applications of accurate time exist in the substation, including time tagging IED event files, synchrophasors, measuring system time deviation, and multi-rate billing.

Various approaches to distributing accurate time in the substation are possible. Today, the most common method is IRIG-B time code, hard-wired to each IED. In the future, the new IEEE-1588 Precision Time Protocol will allow accurate synchronization over the same network connections used for data.

Careful consideration should be given to the choice of time format used for event recording. While local time is the obvious choice, UTC is actually better, because it reduces confusion when comparing data from different utilities in different locations.