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Today in Energy

November 21, 2011

Electric systems respond quickly to the sudden loss of supply or demand

Source: University of Tennessee, Power Information Technology Laboratory, FNET Web Display.
Note: Normal system frequency of 60 cycles per second appear as blue. Green and red colors indicate higher than normal frequency.

A large region of electric systems, called the Western Interconnection, responded to the San Diego blackout on September 8 in less than a minute. A YouTube video vividly illustrates how changes in frequency across interconnected alternating current (AC) electric systems ripple across such a large area in a very short time.

The video shows the variation of frequency (the number of times per second that the electric charge reverses direction) in the Western Interconnection associated with the San Diego blackout on September 8. An event, which has still not been fully characterized, drove system frequency below 60 cycles per second for about 25 seconds. Interconnected electric systems in North America are designed to operate within a very narrow frequency band around 60 cycles per second (hertz). A frequency below 60 cycles per second indicates that instantaneous demand exceeds supply of electricity. After an extended period of insufficient supply (25 seconds), the system automatically shed load in San Diego. The blackout suddenly reduced system demand and frequency shot up well above 60 cycles per second.

The video uses data from a network of small, low-cost devices that plug into wall outlets. They reveal—for the first time—electric system dynamics during disturbances. Frequency, phase angle, and voltage of the power system are measured every few seconds and transmitted via the internet to computers at the University of Tennessee and Virginia Tech. The resulting GPS-synchronized wide-area network makes possible the geographic presentation of the data and system disturbance animations.

Frequency is shown as the vertical scale on the graph on the left and the color-coded map legend on the right. The red dots on the map are the locations of the monitors. Each line on the graph represents the frequency recorded over time by a monitor.

There are monitors in the Eastern Interconnection as well. The video below shows changes in electric system frequency in the 12 seconds after the loss of the two large North Anna nuclear units near Richmond Virginia due to the east coast earthquake on August 23.

Source: University of Tennessee, Power Information Technology Laboratory, FNET Web Display.
Note: Normal system frequency of 60 cycles per second appear as red. Green and blue colors indicate lower than normal frequency.