Event Timing

Abstract:

The use of a master clock to provide an accurate event-time stamp can eliminate ambiguity and maximize analysis efficiency when reconstructing major events such as cascading faults and blackouts.

Event Tagging

When complex interactions across the grid, usually in reaction to external stimuli, lead to major events such as cascading faults and large blackouts, recording devices installed at various points in the grid generate large numbers of reports and data files. Making sense of these files on a systemwide basis requires establishment of a common frame of reference. This consists of a spatial frame of reference; i.e. what happened where, as well as a temporal frame of reference, i.e. what happened when.

The spatial frame of reference comes directly from the network topology, which is generally well known to the engineers involved. Without some sort of accurate time mark in the data files, however, establishing the temporal frame of reference is challenging at best, and may be impossible. And, where possible, it often requires a great deal of effort. Indeed, in the analysis of the large blackout of August 14, 2003 affecting much of eastern North America, more time was spent ordering and manually time-tagging unsynchronized event recordings than on any other task.

This is done by looking for common, recognizable characteristics in the data files, usually by plotting them and visually inspecting the printouts. If one of the files has an accurate time tag, then the other may be ‘tagged’ by inference. As the events get more complex, with more and more ‘noise’ from other things happening at the same time, this becomes more difficult and eventually, impossible. And, as the ‘degrees of separation’ from each record to one with an accurate time-tag increases, the expected accuracy of the time frame of that record gets worse.

Establishing a precision timing system that shares a common, master time source reduces ambiguity and provides the foundation by which events can be accurately and efficiently analyzed.

Accuracy Requirements

Event reconstruction generally requires that records can be re-aligned within a fraction of a power line cycle. At 60 Hz, a power line cycle is 16.7 ms; at 50 Hz, 20 ms. Successful reconstruction is possible with time tags delivering an accuracy of about a quarter of a cycle (4 ms to 5 ms) but better accuracy usually makes the job easier. NERC standard PRC-018-0 requires that the clock in a substation Disturbance Measuring Equipment (DME) be set to an accuracy of 1 ms, which (allowing for other errors in the DME) will generally allow overall time accuracy in the recorded data file of less than 2 ms. Commonly, 1 ms has been considered the goal for synchronizing DME devices such as sequence of event recorders, digital fault recorders, and relays with event recording functions.

All Arbiter clock-based products support Event Timing.

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