[QuadList] Mains frequencies, control, similarities to genlocking

[Ted Langdell]

On Apr 10, 2010, at 10:53 AM, Chuck Reti wrote:

> Not documented anywhere I've looked, but wonder if 60Hz might have  
> been chosen since it correlates with 60sec/min - 60min/hr timekeping.
> Did make it easier to do electric clocks!
> -- 
> Chuck Reti
> Detroit MI


Keeping time with motor-based clocks is an interesting situation. See  
the link and text below.

The process has some elements of pre-framesync Genlock... (Think  
rubidium) and since this is Saturday, would have been something  
networks were doing with sports events from remote locations.

Googling "Genlock Remotes" brought up a 2009 patent application by  
CISCO, the data router people in regard to Genlocking remote video  
sources:
http://www.wipo.int/pctdb/en/wo.jsp?WO=2009105593&IA=US2009034598&DISPLAY=STATUS

Genlock and framesyncs turn up in the April "CBS Retirees Ramblings"  
here:
http://www.cbsretirees.com/blog/blogger.html

You'll see a couple of items from Harold Deppe (Sr.,) who was in  
Maint. at CBS in NYC and is on the QuadList.  We may have the only  
Father/Son pair of engineers on a video-related discussion list, for  
all I know.

Back to power for a tad:

The section below on Frequency and Load has me wondering whether it  
would be practical to monitor the powerline freq., with limit alarms  
to indicate that a power "event" like a brownout, load shed or  
blackout was about to happen.

Ted

http://en.wikipedia.org/wiki/Utility_frequency#Long-term_stability_and_clock_synchronization
Long-term stability and clock synchronization
Regulation of power system frequency for timekeeping accuracy was not  
commonplace until after 1926 and the invention of the electric clock  
driven by a synchronous motor. Network operators will regulate the  
daily average frequency so that clocks stay within a few seconds of  
correct time. In practice the nominal frequency is raised or lowered  
by a specific percentage to maintain synchronization. Over the course  
of a day, the average frequency is maintained at the nominal value  
within a few hundred parts per million.[17] In the synchronous grid of  
Continental Europe, the deviation between network phase time and UTC  
is calculated at 08:00 each day in a control center in Switzerland,  
and the target frequency is then adjusted by up to ±0.02% from 50 Hz  
as needed, to ensure a long-term frequency average of exactly  
24×3600×50 cycles per day is maintained.[18] In North America,  
whenever the error exceeds 10 seconds for the east, 3 seconds for  
Texas, or 2 seconds for the west, a correction of ±0.02 Hz (0.033%) is  
applied. Time error corrections start and end either on the hour or on  
the half hour.[19][20] A dynamicdemand.co.uk/grid - Real-time  
frequency meter for power generation in the United Kingdom is  
available online. Smaller power systems may not maintain frequency  
with the same degree of accuracy.


Frequency and load
The primary reason for accurate frequency control is to allow the flow  
of alternating current power from multiple generators through the  
network to be controlled. The trend in system frequency is a measure  
of mismatch between demand and generation, and so is a necessary  
parameter for load control in interconnected systems.
Frequency of the system will vary as load and generation change.  
Increasing the mechanical input power to a synchronous generator will  
not greatly affect the system frequency but will produce more electric  
power from that unit. During a severe overload caused by tripping or  
failure of generators or transmission lines the power system frequency  
will decline, due to an imbalance of load versus generation. Loss of  
an interconnection, while exporting power (relative to system total  
generation) will cause system frequency to rise. AGC (automatic  
generation control) is used to maintain scheduled frequency and  
interchange power flows. Control systems in power plants detect  
changes in the network-wide frequency and adjust mechanical power  
input to generators back to their target frequency. This counteracting  
usually takes a few tens of seconds due to the large rotating masses  
involved. Temporary frequency changes are an unavoidable consequence  
of changing demand. Exceptional or rapidly changing mains frequency is  
often a sign that an electricity distribution network is operating  
near its capacity limits, dramatic examples of which can sometimes be  
observed shortly before major outages.
Frequency protection relays on the power system network sense the  
decline of frequency and automatically initiate load shedding or  
tripping of interconnection lines, to preserve the operation of at  
least part of the network. Small frequency deviations (i.e.- 0.5 Hz on  
a 50 Hz or 60 Hz network) will result in automatic load shedding or  
other control actions to restore system frequency.
Smaller power systems, not extensively interconnected with many  
generators and loads, will not maintain frequency with the same degree  
of accuracy. Where system frequency is not tightly regulated during  
heavy load periods, the system operators may allow system frequency to  
rise during periods of light load, to maintain a daily average  
frequency of acceptable accuracy.[21][22]


Ted Langdell
Secretary

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