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Electrical braking... Braking... Stop... Why electrical braking? Why braking? Why stop?

So many questions! Because we want to stop the unit, or we need to!
We want to stop it in order to periodically maintain the unit, or we need to stop it for a series of reasons such as:

  • the power system requires less power,
  • the down stream level is too high, or
  • the up stream level is too low and the levels have to come back to their own limits,
  • there is an electrical or a mechanical problem at the unit,
  • the energy program of the unit or of the hydro power plant is over for a pre established period of time (one day)...

How it happens? Considering an automatic stop sequence, we just press the STOP button.
The unit unloads the active (and reactive) power.
The opening command is given at the CB. The generator is off the grid. Now the generator's torque is zero, and there is no access of the water to the turbine by completely closing the wicket gates.
After we close the water access to the turbine, the unit keeps revolving inertially, lowering its speed.
At speeds lower than the rated one:

  • the oil film at the thrust bearing become thinner and thinner until it breaks. It breaks, but the unit is far from stop, so the metalic elements of the thrust bearing, the moving parts and the static parts will enter in direct contact, causing heating. This heating damages their surfaces and the oil quality of the thrust bearing;
  • cavitation act on the turbine, usually causing material loss at the runner blades and the runner chamber.

In order to minimize the consequences of the heating and the cavitation it is desirable to reduce the time during the stop process of the unit by using the electrical braking. The electromagnetic torque of a hydro generator is a braking one, in opposition with the one of the hydraulic turbine (the prime mover) which is an active torque.

How do we obtain a braking torque after the deexcitation and the disconection from the grid? We'll shortcircuit the stator's terminals and we'll excite the rotor using an ancillary supply (the main supply is the one from the terminals). We'll obtain the rated current in the stator, though the voltage is much much lower (the terminals are short circuited). Now the torque is back! This torque act like a braking one, so we obtained the electrical braking! The hydro generator will not be pushed over its limits because the electrical parameters/ values does not exceed the nominal/ rated ones.

The result of the electrical braking is a reduced time during the stop process of the unit, with two main advantages:

  • a longer life for the thrust bearing, and
  • less time spent by the turbine in a zone with cavitation.


HPG Home
Hydro Generators
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About MBM
About HG
Armature Reaction
Excitation Systems
Paralleling
Loading
Design
Electrical Braking
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