9.11.2 Application SEMOD151283-4 v5The task of a generator in a power plant is to convert mechanical energy availableas a torque on a rotating shaft to electric energy.Sometimes, the mechanical power from a prime mover may decrease so much thatit does not cover bearing losses and ventilation losses. Then, the synchronousgenerator becomes a synchronous motor and starts to take electric power from therest of the power system. This operating state, where individual synchronousmachines operate as motors, implies no risk for the machine itself. If the generatorunder consideration is very large and if it consumes lots of electric power, it maybe desirable to disconnect it to ease the task for the rest of the power system.Often, the motoring condition may imply that the turbine is in a very dangerousstate. The task of the reverse power protection is to protect the turbine and not toprotect the generator itself.Steam turbines easily become overheated if the steam flow becomes too low or ifthe steam ceases to flow through the turbine. Therefore, turbo-generators shouldhave reverse power protection. There are several contingencies that may causereverse power: break of a main steam pipe, damage to one or more blades in thesteam turbine or inadvertent closing of the main stop valves. In the last case, it ishighly desirable to have a reliable reverse power protection. It may prevent damageto an otherwise undamaged plant.During the routine shutdown of many thermal power units, the reverse powerprotection gives the tripping impulse to the generator breaker (the unit breaker). Bydoing so, one prevents the disconnection of the unit before the mechanical powerhas become zero. Earlier disconnection would cause an acceleration of the turbinegenerator at all routine shutdowns. This should have caused overspeed and highcentrifugal stresses.When the steam ceases to flow through a turbine, the cooling of the turbine bladeswill disappear. Now, it is not possible to remove all heat generated by the windagelosses. Instead, the heat will increase the temperature in the steam turbine andespecially of the blades. When a steam turbine rotates without steam supply, theelectric power consumption will be about 2% of rated power. Even if the turbinerotates in vacuum, it will soon become overheated and damaged. The turbineoverheats within minutes if the turbine loses the vacuum.The critical time to overheating a steam turbine varies from about 0.5 to 30 minutesdepending on the type of turbine. A high-pressure turbine with small and thinblades will become overheated more easily than a low-pressure turbine with longand heavy blades. The conditions vary from turbine to turbine and it is necessary toask the turbine manufacturer in each case.Power to the power plant auxiliaries may come from a station service transformerconnected to the secondary side of the step-up transformer. Power may also comefrom a start-up service transformer connected to the external network. One has to1MRK 506 369-UEN B Section 9Current protectionLine distance protection REL670 2.2 IEC 515Application manual
