October 16, 2012
In my previous blogs, I outlined the challenges the consulting engineer may encounter in designing a time synchronization source that produces the appropriate degree of resolution to properly analyze events. I also made recommendations on how the consulting specifying engineer can assess vendor claims on time resolution. But these are complex topics for relatively brief blogs, so I’d like to complete the discussion with more insight on near-term strategies with a few words on current standards work in this area.
How can the consulting specifying engineer finesse the issues of system design and the selection of devices in the market?
One way is to request from the vendor a statement on absolute time accuracy. There are many ways to specify it, but one way to visualize it would be to ask a vendor to state whether, for example, if he had 10 of his intelligent electronic devices (IEDs) connected to the same time source and all of those IEDs connected to a single contact and the contact was made, to what degree of accuracy would all of those 10 IEDs report? Would they be within +/-1 millisecond? Would they be within +/-10 milliseconds? This is one way the consulting specifying engineer, if he’s going to rely on this accuracy, can specify it. Then he can begin drilling down into what we refer to as “absolute accuracy” or accuracy of the sequence-of-events records with reference to a common time reference of the substation.
So if the IED is reporting that an event took place on this day, hour, minute, second, millisecond point, to what degree of accuracy is the vendor willing to guarantee that, in fact, the event took place at that exact time and date in conjunction with the station reference time?
This particular strategy puts the onus on the vendor to draw conclusions about any unintended consequences of their proprietary approach to, say, putting a filter on a waveform, and whether they properly compensate for that filter. Maybe the vendor doesn’t compensate for it. Perhaps the vendor or its manufacturer hasn’t considered absolute accuracy in making a claim that a device has a “1 millisecond resolution.” As noted, in many instances, relative accuracy has served just fine. The technology of time-stamping events records to one millisecond has been around for 20 or 30 years, but the relevance of absolute accuracy between multiple IEDs is only something that is now being revealed with the greater resolution of the clocks themselves. Also, time synchronization now has to occur across wireless communications channels rather than just hardwired channels.
A joint working group between the IEEE Power System Relay Committee and the Power and Energy Society’s Substations Committee is charged with writing a standard on time-tagging in protection and disturbance IEDs. And that group is charged with developing the specifications so that vendors will begin to do this in a consistent manner. Also, the PES Substations Committee, Working Group C-16, is beginning a standard on the design and testing of time signal distribution.
These IEEE efforts are important because they will aid in testing the time distribution between devices, which is typically beyond the capabilities even of large electric utilities, let alone smaller utilities or private substation owners. Thus, creating a standard that can generate reproducible results—and to the degree of accuracy that’s needed—is critical.
Once the standard is written and approved, the consulting specifying engineer will be able to use this as a reference. Until that time, the consulting specifying engineer will need to carefully determine what level of time accuracy is actually needed by the various applications in the substation, or the applications that will be used in the substation, and look at available documentation, papers, vendor recommendations, etc., on how to build a time distribution system. That will avoid the challenges outlined in the preceding two blogs: taking a high-quality time signal from a clock and losing its usefulness by degrading it as it’s distributed to various devices.
The standards effort may require a two-year effort. In the interim, the consulting specifying engineer can assess the progress and gain insights on the direction of the two working groups involved, by contacting the chair of the IEEE PES Substations Committee.
Sam Sciacca is an active senior member in the IEEE and the International Electrotechnical Commission (IEC) in the area of utility automation. He has more than 25 years of experience in the domestic and international electrical utility industries. Sciacca serves as the chair of two IEEE working groups that focus on cyber security for electric utilities: the Substations Working Group C1 (P1686) and the Power System Relay Committee Working Group H13 (PC37.240). Sciacca also is president of SCS Consulting.
Sunday, 25-08-13 13:44
Kindly throw some light as to whether a standard for this variation between the IEDs been formulated?
As per the schedule put up at : "http://pestechnical.org/Schedule.asp?CID=3&quo[..] meetings should have taken place.