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Reports until 18:05, Friday 19 August 2011
H2 General
robert.schofield@LIGO.ORG - posted 18:05, Friday 19 August 2011 (1262)
Elimination of power grid 60 Hz Time Error Correction should not hurt Crab detection
Summary. Field trials are about to begin for elimination of Time Error Correction (TEC), the practice of running the power grids at 59.98 or 60.02 Hz for short periods to keep 60 Hz clocks accurate. This change in 60 Hz regulation should be neutral or help with detection of the Crab pulsar at 59.4 Hz because it is expected to reduce low frequency excursions and the new average grid frequencies are expected to be higher, about 60.002 (LLO) and 60.0009 (LHO).

The North American Electric Reliability Corporation (NERC), is planning on changing the regulation of the 60 Hz frequency for the eastern (LLO) and western (LHO) power grids, raising questions about how this may effect detection of the Crab by changing the tails of the 60 Hz peak.  I talked with Andy Rodriquez (andy.rodriquez@nerc.net), Director of Standards Development at NERC, and reviewed NERC documents to asses the proposed elimination of Time Error Correction, which would mean that, over the long term, the average line frequency would not be precisely 60 Hz.

The power grid frequency varies from 60 Hz when supply and demand become unbalanced. When power use increases with morning activities, or a generator shuts down, the frequency decreases. Figure 1 shows a low frequency excursion from loss of generation. As demand drops off at night or as more generators come on line, the frequency increases. Figure 2 shows that morning and night are particularly susceptible to frequency excursions. Figure 3 shows the rms deviation from 60 Hz for the different grids in 1996.

Several control mechanisms affect the line frequency and are used either to minimize frequency departures from 60 Hz or to adjust financial balances between the generating authorities. Organized by time scales, the control mechanisms are:  1) primary control, on a second to minute scale, consisting of independent generator governors that increase or decrease, e.g., the flow of dam water to turbines when a deadband of, typically, 36 mHz is exceeded. Primary control also includes Automatic Generation Control (AGC), automatic control of clusters of generators,  2) secondary control, on a scale of 1 – 10 minutes, includes AGC and manual phone calls to power plants and other load management actions, 3) tertiary control, on a scale of 10 minutes to hours, includes adjustments of purchases from generating facilities and bringing generators on or off-line to meet demand, and 4) time and inadvertent interchange control, on a scale of hours. Inadvertent interchange control involves frequency changes when financial balances between generating authorities are manipulated. For example, if a regional authority inadvertently provided less energy than it sold, in order to compensate it may run its generators slow while other authorities on the grid run fast. Finally, on a multi-hour scale, the difference between the average frequency and 60 Hz is corrected by purposefully running the grid for a short time at 60.02 or 59.98 Hz. These Time Error Corrections are instigated manually (e.g. by phone to power plants) in the eastern grid and automatically (WATEC) or manually in the western grid. They are initiated when a clock using the 60 Hz line frequency would be inaccurate by +/- 10 s (Eastern Interchange) or  +/- 2 s (Western Interchange). 

NERC proposes to eliminate Time Error Correction, claiming that the only benefit lies in improving the accuracy of 60 Hz clocks, which are becoming outdated by commonly available and more accurate time pieces. In fact, Time Error Correction tends to decrease frequency stability: a study showed that 40% of the excursions below 59.95 Hz (an alarm threshold) in the Eastern Interconnection occurred during periods when the grid was running at 59.98 Hz to correct clocks. 

So, from a LIGO perspective, elimination of Time Error Correction will be beneficial because it will reduce the extent of low frequency excursions that further reduce frequency when the grid is running at 59.98 Hz. During frequency emergencies, e.g. for a large unscheduled shutdown of generation facilities, the frequency is raised by the primary through tertiary controls mentioned above. Time Error Correction plays no role in these emergencies and is only used after the fact to make up for frequency excursions. 

Furthermore, there is a tendency for power authorities to err on the side of over-generation rather than under-generation  because negative consequences begin at smaller deviations on the low frequency side than on the high frequency side. So without time error correction, the average frequency is expected to be slightly higher than 60 Hz. Based on past time error corrections, the new average grid frequency for the Eastern Interconnection (LLO) is expected to be about 60.002 Hz and, for the Western Interconnection (LHO) about 60.0009 Hz. The Crab gravitational wave frequency is expected to be about 59.4 at the beginning of aLIGO, so elimination of Time Error Correction should be neutral or slightly helpful. 
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