METHOD FOR OPERATING A DOUBLE-FED ASYNCHRONOUS MACHINE

Abstract

In a method for operating a double-fed asynchronous machine, an exciter winding of the rotor is excited by adjusting an amplitude or a frequency of a voltage or current independently of armature values of the stator to attain a predetermined phase position and amplitude in the stator. While the stator is disconnected from an electrical grid, the rotation speed of the rotor is increased during startup and the amplitude of the voltage and/or of the current flow is adjusted to less than a start-up limit value, while the frequency of the voltage and/or of the current flow is adjusted to a grid frequency. The winding arrangement is then connected to the electrical grid, and the amplitude of the voltage and/or of the current flow is adjusted to an operating value which is greater than the start-up limit value by a predetermined amount.

Claims

1. (canceled)

2. A method for operating a double-fed asynchronous machine, comprising: exciting an exciter winding of a rotor of the asynchronous machine by adjusting at least one electrical variable independently of armature values of a winding arrangement of a stator of the asynchronous machine so as to attain a predetermined phase position and a predetermined amplitude in the stator, wherein the at least one electrical variable comprises an amplitude and/or a frequency of a voltage or of a current flow of the rotor; during a start-up process of the asynchronous machine and while the exciter winding is adjusted and while a winding arrangement of the stator is disconnected from an electrical grid; increasing a speed of rotation of the rotor, adjusting the amplitude of the voltage and/or of the current flow to less than a predetermined start-up limit value, and adjusting the frequency of the voltage and/or of the current flow to a grid frequency of the electrical grid; thereafter connecting the winding arrangement to the electrical grid; and adjusting the amplitude of the voltage and/or of the current flow to a predetermined operating value which is greater than the predetermined start-up limit value by at least a predetermined amount.

Description

[0042] In the figures:

[0043] FIG. 1 shows a block diagram of a double-fed asynchronous machine, which is connected to an electrical grid;

[0044] FIG. 2 shows the course of electrical variables related to the rotor and to the stator during a start-up process of the asynchronous machine;

[0045] FIG. 3 shows the course of further electrical variables related to the rotor and to the stator during a start-up process of the asynchronous machine;

[0046] FIG. 4 shows a block diagram of a possible arrangement of the asynchronous machine; and

[0047] FIG. 5 shows a further block diagram of a possible arrangement of the asynchronous machine.

[0048] FIG. 1 shows a block diagram of a double-fed asynchronous machine 1, which comprises a stator 2, a rotor 3, an exciter unit 4 and also a control unit 5. An exciter winding 30 of the rotor 3 is able to be connected to an electrical grid 6 via the exciter unit 4. A winding arrangement 20 (only shown extremely schematically in the figure) is able to be connected to the electrical grid 6 via a switching unit 21. In this case the winding arrangement 20 is designed in particular as a three-phase arrangement, for which reason a connection has three phase legs. The switching unit 21 can be a part of the asynchronous machine 1. The electrical connection 22 between the stator 2 and the electrical grid 6 is able to be switched by means of the switching unit 21. In particular the electrical connection 22 is able to be disconnected by the switching unit 21.

[0049] The rotor 3 or the asynchronous machine 1 is able to be excited via the exciter winding 30 of the rotor 3. In this case the excitation occurs via the exciter unit 4. The excitation is controlled by the control unit 5, which predetermines at least one electrical variable for the excitation. The control unit 5 can comprise a memory unit 50, in which pre-specified criteria for adjusting the at least one electrical variable are able to be stored. For example a characteristic field for the at least one electrical variable is stored in the memory unit 50.

[0050] The at least one electrical variable, which is adjusted for the excitation, relates in particular to a frequency and an amplitude of a current flow (rotor current) or of a voltage (rotor voltage) in the exciter winding 30. In other words preferably at least two electrical variables are adjusted for the excitation. By adjusting the at least one electrical variable the excitation of the asynchronous machine 1 or of the rotor 3 can be controlled.

[0051] The exciter unit 4 in the present case is designed as a current converter, in particular as an inverter. Via the exciter unit 4, the rotor current and/or the rotor voltage of the exciter winding 30 are controlled and regulated in such a way that the rotor current or the rotor voltage correspond to the at least one electrical variable that is adjusted by the control unit 5. For example the exciter unit 4 has one or more switching elements that are controlled by the control unit 5. The switching elements in particular involve transistors, preferably field effect transistors, or IGBTs (bipolar transistor with insulated gate electrode).

[0052] Depending on the operating mode of the asynchronous machine 1, electrical power can be fed by the exciter winding 30 into the electrical grid 6 or by the electrical grid 6 into the exciter winding 30 via the exciter unit 4.

[0053] The excitation of the asynchronous machine 1 or of the rotor 3 is controlled by the control unit 5 completely independently of electrical stator variables of the stator 2. Examples of stator variables are stator currents or stator voltages for example. Stator currents and stator voltages are for example individual phase currents of the individual phases of the winding arrangement 20 as well as an overall current or an overall voltage resulting therefrom. Further examples of stator variables are the speed of rotation of the rotor 3 in relation to the stator 2 and also an angle of the rotor 3 in relation to the stator 2, This involves variables that are measured by a rotary position transducer of the stator in the prior art for example.

[0054] The excitation of the rotor 3 or the at least one electrical variable is controlled here in the sense of an open regulation circuit. Therefore interaction between the stator 2 and the exciter unit 4 as well as the control unit 5 can be dispensed with. Different values for the at least one physical variable can be predetermined in the characteristic field for a number of operating points or operating states of the asynchronous machine 1. The rotor voltage or the rotor current is thus controlled independently of the stator-related variables.

[0055] Because of the absence of interaction with the stator 2 the exciter unit 4 can be designed as a universally applicable current converter. In particular no special adaptation of the exciter unit 4 to the asynchronous machine 1 is necessary. The excitation of the asynchronous machine 1 is adapted exclusively here by the control unit 5.

[0056] In particular the stator 2 or the winding arrangement 20, in normal operation of the asynchronous machine 1, is connected directly, meaning in an unregulated way, to the electrical grid 6. This means that the switching unit 21 establishes a direct electrical connection 22 of the phase legs to the electrical grid 6. If the electrical grid involves the general 50 Hz interconnected grid, then the grid voltage of the electrical grid 6 is fixed and cannot be changed by the operation of the asynchronous machine 1 as electric motor or as generator. Therefore in this case the electrical grid 6 involves a voltage source. For this case it has proved advantageous to control the excitation of the rotor 3 with guided current. In other words the at least one electrical variable for the rotor current is predetermined. The exciter unit 4 represents a current source in this case.

[0057] In general it is possible for both the winding arrangement 20 and also the exciter winding 30 to each be connected to a current source or to a voltage source. It has proved advantageous however for the winding arrangement 20 to be connected to a current source and the exciter winding 30 to a voltage source or conversely for the winding arrangement 20 to be connected to a voltage source and the exciter winding 30 to a current source, Since the stator 2 is preferably connected statically to the electrical grid 6, which often represents a voltage source, it has proved advantageous to designed the exciter unit 4 as a current source.

[0058] Advantageously the rotor 3 or a shaft, which is part of the rotor 3 or is connected to the rotor 3, has a torsional vibration damper. The torsional vibration damper has a moment of inertia, which for example amounts to 10% of the rotor 3. The moment of inertia of the torsional vibration damper can be supported sprung on the shaft. In particular the moment of inertia of the torsional vibration damper is linked to the shaft in an elastic and damped manner. As an alternative or in addition a fan wheel can be arranged on the shaft.

[0059] FIG. 2 shows a number of stator-related and rotor-related electrical variables represented on a time scale t (in seconds). FIG. 2 shows the following: The rotor current 10, the rotor voltage 11, the stator current 12 and the stator voltage 13. In this case the four variables are each shown in volts (v). In this figure the asynchronous machine is at a standstill at a time t of 0 s. Thus FIG. 2 shows a start-up process of the asynchronous machine 1. The start-up process of the asynchronous machine 1 is especially important in the present method for controlling the excitation, since the fields of stator 2 and rotor 3 initially usually do not coincide. This means that the phase position between stator 2 and rotor 3 is initially undefined. It is first necessary to align the fields of stator 2 and rotor 3 in relation to one another.

[0060] To do this, in accordance with FIG. 2, the rotor 3 is first operated with a rotor current 10 of which the amplitude is less than a pre-specified start-up limit value. The start-up limit value is less by a pre-specified amount than a pre-specified operating value to which the amplitude of the rotor current 10 is adjusted in a normal mode of operation, for example at rated power, of the asynchronous machine 1. By injecting the rotor current 10 into the exciter winding 30, a rotor voltage 11 is produced at the exciter winding 30. The frequency of the rotor current 10 is fixed at the grid frequency of the electrical grid 6. The stator 2 or the winding arrangement 20 is initially still disconnected from the electrical grid 6.

[0061] The stator 2 or the winding arrangement 20 is connected to the electrical grid 6. This takes place for example by closing respective switches of the switch unit 21.

[0062] This produces a compensation process, in which flow in the winding arrangement currents 20 and generate a torque at the rotor 3, so that the rotor 3 aligns in the field of the stator 2. Through this the phase position between the stator 2 and the rotor 3 is adjusted appropriately. In particular the chosen pre-specified limit value is sufficiently small for the compensation currents not to exceed a pre-specified amount.

[0063] With the asynchronous machine 1 it is possible to change the speed of rotation. In other words the asynchronous machine 1 offers the option of regulating the speed. This is done in particular through a continuous change of the frequencies of stator 2 or of rotor 3. In particular the frequencies of the stator current and/or of the stator voltage or the frequencies of the rotor current and/or of the rotor voltage are changed continuously in order to change the speed of rotation. By continuously changing the frequencies, the synchronism between stator 2 and rotor 3 is not lost. As well as the frequency of the rotor current and/or rotor voltage (rotational frequency), the direction of rotation of the rotor rotary field can also be changed, so that any given speed of rotation of the rotor can be reached. As an alternative or in addition the amplitude of the rotor current or of the rotor voltage can be changed, in order to control the active power and also the reactive power of the asynchronous machine 1. For example, by predetermining a pre-specified value for the amplitude of the rotor current or of the rotor voltage, the active power and/or the reactive power of the asynchronous machine can be adjusted to a further pre-specified value.

[0064] FIG. 3 shows the same start-up process of the asynchronous machine 1, wherein other values are plotted on the same time axis t (in seconds: Electrical reactive power of the rotor 40 (in kVA), electrical reactive power of the stator 41 (in kVA), electrical active power of the rotor 42 (in kW), electrical active power of the stator 43 (in kW), mechanical power of the asynchronous machine (in kW) and also speed of rotation of the rotor 46 (in revolutions per minute, rpm).

[0065] FIG. 4 and FIG. 5 each show an arrangement with the asynchronous machine 1, These figures can involve test rigs for the asynchronous machine 1. For example the characteristic field for the asynchronous machine can be created on the test rig.

[0066] In FIG. 4 the asynchronous machine 1 is shown in a test environment or on a test rig. Here the asynchronous machine 1 is operating in generator mode, in the present example the asynchronous machine 1 is driven mechanically via two load machines 64. Electrical energy is fed from the rotor 3 of the asynchronous machine 1 into the electrical grid 6, Moreover electrical energy is fed from the stator 2 of the asynchronous machine 1 into a synchronous machine 60, The two load machines 64 are controlled by respective inverters 63. The inverters 63 in their turn are supplied with an electrical voltage via voltage-regulated inverters 61, 62. The exciter unit 4 is embodied in this example as a current-regulated inverter, Electrical and mechanical energy flows are to be taken from FIG. 4.

[0067] FIG. 5 shows the asynchronous machine 1 in another test setup. Here too the exciter unit 4 is embodied as a current-regulated inverter. The electrical power from the stator 2 of the asynchronous machine 1 is fed directly to the current-regulated inverter 61 here. In the example of FIG. 5, the test setup has only one load machine 64, which is controlled via the inverter 63,