Concerns about global warming, rising fossil-fuel prices, and summer blackouts have put the ways that America generates and uses electricity on the front page. This year, for the first time, President Bush mentioned climate change in his State of the Union address and proposed building another generation of nuclear power plants to mitigate it.
California's and Australia's proposals for eliminating incandescent bulbs have been newsworthy as well. Business magazines have endorsed Wal-Mart's and Whole Foods' wind power ventures. Computer firms advertise the energy efficiency of their new chips, not just their clock speed. Automotive magazines say hybrid vehicles that can be "refueled" by plugging them into an AC socket are on the near horizon.
All of these policy shifts and products may reduce emissions of heat-trapping gases, but they will do nothing to relieve the strain on already-congested U.S. transmission grids. Businesses already use fluorescent bulbs, and households are lighted during non-peak hours. Plug-in hybrids will reduce consumption of gasoline but increase electrical traffic on grids. Wind farms and geothermal plants are usually located far from loads, and they need new transmission lines to bring their output to market.
Many of the anticipated changes in usage patterns will increase the need for reactive power. Reactive power is needed to ensure grid stability—it maintains the voltage levels required to drive "real" power, or megawatt-hours, over transmission lines. A shortage of reactive power can compromise the safe operation of a grid; indeed, it was a key factor in the great Northeast blackout of August 2003. Power lines operating at unity power factor also operate more efficiently; they can carry more current without overheating than identically rated lines operating out of phase.
A grid's overall need for reactive power can't be satisfied by one generator. Volt-amperes reactive (VARs) must be supplied and absorbed near each load center. Let's now take a closer look at some of the technologies and methods available for supplying and absorbing reactive power.
Higher currents, more VARs
The U.S. Department of Energy (DOE) and private companies alike are working on new technologies—such as high-temperature superconducting (HTS) cables and ultrahigh-voltage lines—that will increase the carrying capacity and reduce the losses of transmission lines. Inevitably, both will increase demand for reactive power.
By how much? The Western Governors' Association's Draft Report of the Transmission Task Force issued in March 2006 predicted the size of the increase caused by use of HTS cables (and its price tag) by explaining that "Reactive power consumption is proportional to the square of the current (or power). Thus, doubling the current in a device will quadruple the reactive power consumption. This reactive power consumption must be managed by adding reactive compensating devices—an additional cost."
The Task Force found similar problems with ultrahigh-voltage lines. "Ultrahigh-voltage lines would enable more power to be transmitted over paths that currently [use] conventional transmission lines, [at ratings] such as 230 kV, 345 kV, and 500 kV. The highest transmission line voltage in North America is 765 kV. Ultra high-voltage lines are technologically possible, but would require larger rights-of-way, generate stronger electromagnetic fields, and produce much more reactive power (which must be managed by adding reactive compensating devices)."
The traditional sources of reactive power generation on grids and at central-station power plants won't provide enough VARs in the future. But—according to an April 2006 report prepared for the DOE by Oak Ridge National Laboratory and Energetics Inc. and titled "A Preliminary Analysis of the Economics of Using Distributed Energy as a Source of Reactive Power"—distributed generation (DG) plants could provide up to 10,000 MVAR.
"Distributed energy-based reactive supply can provide dynamic support capabilities that static devices like capacitors cannot match," said the report's authors. "Furthermore, industry experts believe that supplying reactive power from synchronized distributed energy sources can be 2 to 3 times more effective than providing reactive support in bulk from longer distances at the transmission or generation levels."
Email
Comments
Print
Reuse
