Brittle Power- PARTS 1-3 (+Notes) - Natural Capitalism Solutions

136Disasters Waiting to Happenspeed, so they are normally shut down altogether. They then take twelvehours or more to restart 74 (sometimes days in certain nuclear plants whereneutron-absorbing fission products accumulate after shutdown). Restartingtime can be reduced to minutes or less by “tripping to house load” 75 —that is,letting the plant continue to meet auxiliary loads in the power station itselfwhile bypassing surplus steam around the turbine. Thus the turbine is notcompletely shut down and remains ready for rapid reconnection with thegrid. This is common practice in Europe and mandatory in Japan, but not universalin American fossil-fueled power stations. The technique could haveeliminated the 1977 New York blackout. 76If a power grid is more than momentarily subjected to a load larger than itcan sustainably supply, and if “spinning reserve” capacity already synchronizedwith the grid cannot be brought into full production to make good thedeficit, the extra energy must come from somewhere. It comes out of thestored rotational energy of the operating generators. They will therefore slowdown, 77 and the frequency of the whole interconnected system will be pulleddown below the normal sixty cycles per second. This can cause more powerto flow toward the deficit area, perhaps further overloading transmissionlines 78 and probably tripping protective breakers. (If protective devices did notwork properly, different elements of a grid could try to operate at significantlydifferent frequencies, “bucking” each other. This would cause enormousinternal stresses and, probably, serious damage.) Some modern turbogeneratorsof very large capacity (well over a thousand megawatts of electrical outputin a single unit) work so close to the yield limits of their materials that theyhave little safety margin for the stresses generated by loss of synchronization.Some will reportedly suffer gross mechanical failure (e.g., by the shaft’s flyingapart) if the frequency deviates by one or two percent while they are underfull load. Similar cost-cutting savings in generator materials have greatlydecreased the rotors’ stored energy “and thus increased the probability thatsynchronism will be lost in the event of a fault.” 79Instabilities caused by the gridThe stability of a grid depends not only on how its generators can performrelative to their loads and to each other, but also on how well the transmissionlines (and their associated switchgear, transformers, and controls) can knitthese ingredients together. Transmission lines, because of their electrical properties,are subject to two kinds of limits on how much power they can safelyhandle: thermal limits, set by how much heat they can dissipate to their surroundingswithout sagging, and “system stability limits.” These arise from the