Variable Slip Generators for Wind Turbines
Manufacturers of electric motors have for many years been faced with the problem that their motors can only run at certain almost fixed speeds determined by the number of poles in the motor.
As we learned on the previous page, the motor (or generator) slip in an asynchronous (induction) machine is usually very small for reasons of efficiency, so the rotational speed will vary with around 1 per cent between idle and full load.
The slip, however is a function of the (DC) resistance (measured in ohms) in the rotor windings of the generator. The higher resistance, the higher the slip. so one way of varying the slip is to vary the resistance in the rotor. In this way one may increase generator slip to e.g. 10 per cent.
On motors, this is usually done by having a wound rotor, i.e. a rotor with copper wire windings which are connected in a star , and connected with external variable resistors, plus an electronic control system to operate the resistors. The connection has usually been done with brushes and slip rings, which is a clear drawback over the elegantly simple technical design of an cage wound rotor machine. It also introduces parts which wear down in the generator, and thus the generator requires extra maintenance.
Opti Slip®
An interesting variation of the variable slip induction generator avoids the problem of introducing slip rings, brushes, external resistors, and maintenance altogether.
By mounting the external resistors on the rotor itself, and mounting the electronic control system on the rotor as well, you still have the problem of how to communicate the amount of slip you need to the rotor. This communication can be done very elegantly, however, using optical fibre communications, and sending the signal across to the rotor electronics each time it passes a stationary optical fibre.
Running a Pitch Controlled Turbine at Variable Speed
As mentioned on the next page, there are a number of advantages of being able to run a wind turbine at variable speed.
One good reason for wanting to be able to run a turbine partially at variable speed is the fact that pitch control (controlling the torque in order not to overload the gearbox and generator by pitching the wind turbine blades) is a mechanical process. This means that the reaction time for the pitch mechanism becomes a critical factor in turbine design.
If you have a variable slip generator, however, you may start increasing its slip once you are close to the rated power of the turbine. The control strategy applied in a widely used Danish turbine design is to run the generator at half of its maximum slip when the turbine is operating near the rated power. When a wind gust occurs, the control mechanism signals to increase generator slip to allow the rotor to run a bit faster while the pitch mechanism begins to cope with the situation by pitching the blades more out of the wind. Once the pitch mechanism has done its work, the slip is decreased again. In case the wind suddenly drops, the process is applied in reverse.
Although these concepts may sound simple, it is quite a technical challenge to ensure that the two power control mechanisms co-operate efficiently.
Improving Power Quality
You may protest that running a generator at high slip releases more heat from the generator, which runs less efficiently. That is not a problem in itself, however, since the only alternative is to waste the excess wind energy by pitching the rotor blades out of the wind.
One of the real benefits of using the control strategy mentioned here is that you get a better power quality, since the fluctuations in power output are "eaten up" or "topped up" by varying the generator slip and storing or releasing part of the energy as rotational energy in the wind turbine rotor.
© Copyright 1997-2003 Danish Wind Industry Association
Updated 8 October 2003
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