I>: Current level for the Stub protection, set in % ofIBase. This parameter should be set so thatall faults on the stub can be detected. The setting should thus be based on fault calculations.t: Time delay of the operation. Normally the function shall be instantaneous.6.11 Directional underpower protection GUPPDUP SEMOD156693-1 v46.11.1 IdentificationSEMOD158941-2 v4Function description IEC 61850identificationIEC 60617identificationANSI/IEEE C37.2device numberDirectional underpower protection GUPPDUPP <2SYMBOL-LL V2 EN-US376.11.2 Application SEMOD151283-4 v5The task of a generator in a power plant is to convert mechanical energy available as a torqueon a rotating shaft to electric energy.Sometimes, the mechanical power from a prime mover may decrease so much that it does notcover bearing losses and ventilation losses. Then, the synchronous generator becomes asynchronous motor and starts to take electric power from the rest of the power system. Thisoperating state, where individual synchronous machines operate as motors, implies no risk forthe machine itself. If the generator under consideration is very large and if it consumes lots ofelectric power, it may be desirable to disconnect it to ease the task for the rest of the powersystem.Often, the motoring condition may imply that the turbine is in a very dangerous state. The taskof the reverse power protection is to protect the turbine and not to protect the generatoritself.Steam turbines easily become overheated if the steam flow becomes too low or if the steamceases to flow through the turbine. Therefore, turbo-generators should have reverse powerprotection. There are several contingencies that may cause reverse power: break of a mainsteam pipe, damage to one or more blades in the steam turbine or inadvertent closing of themain stop valves. In the last case, it is highly desirable to have a reliable reverse powerprotection. It may prevent damage to an otherwise undamaged plant.During the routine shutdown of many thermal power units, the reverse power protection givesthe tripping impulse to the generator breaker (the unit breaker). By doing so, one prevents thedisconnection of the unit before the mechanical power has become zero. Earlier disconnectionwould cause an acceleration of the turbine generator at all routine shutdowns. This shouldhave caused overspeed and high centrifugal stresses.When the steam ceases to flow through a turbine, the cooling of the turbine blades willdisappear. Now, it is not possible to remove all heat generated by the windage losses. Instead,the heat will increase the temperature in the steam turbine and especially of the blades. Whena steam turbine rotates without steam supply, the electric power consumption will be about2% of rated power. Even if the turbine rotates in vacuum, it will soon become overheated anddamaged. The turbine overheats within minutes if the turbine loses the vacuum.1MRK 505 355-UEN A Section 6Current protection105Application manual
