Publication Date: 2016-Nov-25
The IP.com Prior Art Database
The disclosure relates to the control of a generator with unsymmetrical power supplies. A solution for realizing a flux reduction by the exciter is disclosed. Generally, the idea is an active control of magnetic flux in the exciter machine of a generator, e.g. a synchronous generator, using unsymmetrical voltage sources. With unsymmetrical power supplies the output current/voltage can be set to both directions at a different level. The control accuracy is to scale on both sides.
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The disclosure relates to the control of a generator with unsymmetrical power supplies.
In a typical case in a testing use, for example, the generator terminal voltage is controlled by the generator excitation current. In a testing use a nominal generator terminal voltage Un = 400 … 690 V depending on the manufacturer or the user requirements, for example. Excitation can be used to control the generator terminal voltage Uout = 0 … ~Un. Limiting factors in the voltage control can be U/f ratio and an open-circuit voltage caused by self- excitation, for example.
Figure 1. The frequency is controlled by the speed of the traction motor and the terminal voltage by the excitation current.
The excitation current produced by the exciter can be controlled by controllable semiconductor switch, such as an IGBT (Insulated-Gate Bipolar Transistor). The current control may be implemented by a PWM control and in high-current devices by a thyristor converter, for example. The IGBT switch can be controlled by the PWM control. Already the lowest control step starts to amplify the magnetic flux of the exciter machine.
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Figure 2. An exemplary circuit diagram of excitation.
With the control of the excitation current the machine's own excitation can be amplified and the generator's output voltage can be controlled to a desired level within limits. Often generators equipped with an internal exciter are made self-exciting. In other words, when the axes are running, a magnetic flux in the stator of the exciter machine starts to create a source voltage to the rotor of the exciter machine (self-excitation). The voltage is rectified and conveyed to the excitation circuit of the main generator. As a result of this, the generator starts to generate a terminal voltage even though the control has not been switched on. This kind of behaviour is undesirable in view of the test field application. The generator output voltage is desired to be controlled steplessly 0 … Un. It is a safety risk if Uolo >> Uref. Self-excitation can be prevented by a separate de-magnetizing coil or a large artificial load. De-magnetization weakens the magnetic flux of the stator of the exciter machine. De-magnetizing coils are singular solutions as per application and they are usually not readily available in low voltage generators.
The disclosed solution generally proposes an operational change in the exciter machine. The electric flux in the exciter machine can be reduced by flux reversal. The reduction of the magnetic flux obtained by the flux reversal should be very small such that the magnetic polarity of the exciter machine is not reversed. Generally, the exciter should be able to supply 2 to 8 times terminal voltage to the exciter (forward direction). In this case the supply voltage of this magnitude cannot be used for the flux reduction because