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

Chapter Ten: Power Stations and Grids 135heating, and can even be run for up to four hours at their “long-time emergencyrating” without damage. But before time and temperature limits on thelines are reached, operators must reroute power or shed (cut off) loads tobring the lines within safe limits.Similar readjustments may also be needed after the initial rapid redistributionof power flows that accompanies the sudden trip of a loaded generator.Further, the generator itself must rapidly bypass steam from its turbine inorder to avoid serious damage from spinning too fast without load. Thereafterthe turbogenerator cannot be rapidly reconnected to the grid, but must bebrought up gradually from almost zero load. 72In practice, the detailed electrical phenomena occurring when normal bulkpower flows are interrupted are very complex and demand elaborate mathematicalanalysis. It is not simply a matter of electricity’s flowing or not flowing;rather, the current tends to rush out of some parts of the grid and intoothers, producing “transients”—abnormally high voltages or currents whichcan severely damage equipment. The effect is somewhat analogous to whathappens when a complex mechanical structure held together by various stiffstruts is sharply struck at a single point. A wave of stress propagates throughthe structure. Depending on the structural details, shocks arriving from differentdirections may concentrate at single points, doing disproportionatedamage to components far from the site of the original blow. How the shockspropagate and focus depends on the relative stiffness and strength of the variousmembers and on how they have been assembled.Electrical networks have analogous elements and properties. Transientsurges of high voltage can break down insulation in a cable or transformer,thereby causing a secondary fault which can itself propagate new transientsthrough the network. A surge of current can likewise trip a protective breakerand needlessly disconnect a circuit. The electrical properties of long transmissionlines and (especially) of long underground cable tend to increase transients.Alternating-current power grids can also become unstable by losing theirsynchronization:In normal operation, all of the [generator] rotors...are rotating in precise synchronism.Further, the power output and other electrical quantities associated with eachgenerator are absolutely dependent on this synchronous operation. If a generator issubjected to a sufficiently large disturbance,...as...from a nearby fault, it may... “pullout” of synchronism, even though the original disturbance is momentary. Oncesynchronism is lost, the power output of the unit drops rapidly.... 73and it must be instantly taken off-line until ready for exact resynchronization.Steam-driven turbines, if run without load, will ordinarily gain too much