Load shedding (power plant)

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As load shedding refers to a control situation in a power plant in which the generator runs abruptly without load (load rejection to zero) or has its own consumer maintains (load rejection to own use ). Such a condition is caused by, among other things

Immediately after the load shedding increases in thermal power plants and used there turbine sets the rotational speed of turbine and turbine generator strongly, since the standing available torque of the turbine, only the mass moment of inertia to overcome both machines must (the turbine will "pass"). With a large turbo generator, the speed increase within three seconds is about 10% from 3000 to 3300 revolutions per minute. The machine must be intercepted during this period in order to avoid destruction by the centrifugal forces that occur .

Process for different types of power plants

Load shedding in steam power plants

Load shedding on personal use

If the disturbance that leads to load shedding is exclusively on the grid side , thermal power plants provide for load shedding on own demand. This is to ensure that the power plant remains on standby and can be reconnected to the grid at any time. This is solved by immediately throttling or interrupting the energy supply in the event of overspeed. In steam power plants, which are control valves fed in. The output of the steam generator or nuclear reactor is quickly reduced to the minimum continuous load (approx. 35% of the nominal output), excess steam is discharged via the diversion station into the condenser or via the safety valves directly into the atmosphere.

Load shedding to zero

In the event of malfunctions on the power plant side that require load shedding to zero, a turbine shutdown (TUSA, in a nuclear power plant) or a turbine shutdown (TSS, in a fossil power plant) is carried out. The quick- closing valves shut off the steam supply to the turbine immediately and completely. The generator is also disconnected from the grid. The output of the steam generator or nuclear reactor is quickly reduced to approx. 35% of the nominal output, the steam generated is discharged by the safety devices directly into the atmosphere and into the condenser . If a long-term turbine failure is foreseeable, the steam generator / nuclear reactor will also be shut down. A quick shutdown of the steam generator is usually not necessary.

Load shedding in wind turbines

Load shedding can also occur in wind turbines , for example due to network disturbances, and results in a sudden increase in speed. This is associated with much greater loads on all parts of the system compared to normal operation. The system is then regulated down to idle / spin mode within a few seconds by adjusting the rotor blades . The complete execution of a load shedding is part of the commissioning procedure after a system has been set up.

In the event of brief grid disruptions (e.g. voltage drops), however, wind turbines may not disconnect from the grid, but must instead contribute to dynamic grid support. If wind turbines were to disconnect themselves completely from the grid in such situations, there could otherwise be power outages. The exact criteria are regulated in the medium voltage directive .

Load shedding in hydropower plants

In hydropower plants , the water supply cannot usually be reduced fast enough, but the turbines there rotate at significantly lower speeds and the salient pole machines used in these types of power plant are sufficiently speed-stable, so they can withstand an overspeed after load shedding without damage. After the water supply has been closed, the water is either dammed or passed past the turbine via a bypass.

See also

literature

Individual evidence

  1. ↑ Volker Quaschning , Regenerative Energy Systems. Technology - calculation - simulation . 8th updated edition. Munich 2013, p. 308 f.