DE29621449U1 - Device for improving the grid compatibility of wind turbines with asynchronous generators - Google Patents

Description

DEVICE FOR IMPROVING THE GRID COMPATIBILITY OF WIND TURBINES WITH ASYNCHRONOUS GENERATORS

According to the preamble of claim 1, the present invention relates to a device for improving the grid compatibility of wind turbines with asynchronous generators that are connected to a power supply network.

Wind turbines that are connected to a power grid are well known in the art. The problem with these grid-connected wind turbines is the fact that the electrical fluctuations of such a wind turbine are relatively large, since fluctuations in wind power have a direct effect on the power output of the asynchronous generators.

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^; A problem is the interaction of wind turbines ^{that are} 'coupled' with each other. A major cause of such disturbances is the so-called tower jam effect, ie a power fluctuation occurs when the rotor blade of a wind turbine crosses a zone of reduced wind speed in the tower area. During synchronous operation, the fluctuations of the individual turbines overlap at the common grid feed-in point to create a maximum voltage fluctuation, which leads to considerable flicker. The interaction between the wind turbines occurs via the voltage fluctuations resulting from the power fluctuations.

The operation of a wind turbine on the grid is permitted if certain limit values for voltage fluctuations are not exceeded. The voltage changes Δδ must be < 3% and the long-term flicker Δt must be kept < 0.1.

Switching on and off also leads to voltage changes. Variable speed wind turbines have grid-controlled frequency converters, which can be responsible for harmonics. These have a disruptive effect on other consumers and generate additional losses and heating in the grid.

The switch-on process of a wind turbine with a directly grid-connected asynchronous generator usually takes place as follows: After the switch-on signal, the turbine initially remains in the idle position for a certain time, for example 1 minute. The turbine is then started up by the wind without being connected to the grid until the synchronous speed is reached. At this moment, a thyristor controller switches the generator to the grid, whereby the thyristor controller limits the generator's switch-on current. After a few seconds, the thyristor controller is bridged by a grid contactor, so that the generator is directly connected to the grid. When switching on a single wind turbine, a relatively large reactive power requirement is necessary. This reactive power also puts a load on the grid. Similar conditions prevail when switching on wind turbines in a so-called wind farm,

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A thyristor controller generates higher order harmonics through phase control. The long-wave power fluctuations that also occur are generated by a variety of parameters that will not be explained in detail here.

The overall complex of the problems outlined above involves a number of individual tasks that need to be solved, which appear particularly relevant due to the large number of wind turbines and the increase in output in the megawatt range. In order to make the most of the existing capacity of the utility grids, an improvement in the grid compatibility of the wind turbines is therefore desirable. Many wind turbines that are already directly connected to the utility grid have the advantage of producing robust generators, simple control cabinets and harmonic-free grid currents, and are also simple and inexpensive, but due to the rigid grid connection they have considerable grid flicker effects, depending on the rotor speed, between 2 and 4 Hz. This effect occurs particularly in large wind turbines and means that only a few wind turbines can be connected to an existing utility grid.

It is therefore an object of the present invention to provide a device which is able to improve the grid compatibility of at least one wind turbine using simple, cost-effective means.

This object is achieved according to the invention by the characterizing features of the main claim.

According to this, the wind power plant according to the invention with a wind-driven rotor that is mechanically coupled to an asynchronous generator that is connected to a public power grid is characterized by a computer-controlled

controlled converter with a DC intermediate circuit and a buffer that absorbs short-term positive power fluctuations that deviate from the average power value and releases them again with a time delay.

The method by which this wind turbine works to improve grid compatibility with asynchronous generators when connected to the public power grid is characterized by the fact that an electronically controlled device absorbs the short-term positive power fluctuations that deviate from the average value and releases them again with a time delay.

A basic idea of this invention is to improve the grid compatibility of an asynchronous generator by having an intermediate storage unit absorb excess power from the average value and release it again at a suitable time later. This smooths out the power output and reduces the flicker value. If only the energy from the power fluctuations is to be absorbed, a storage unit that has, for example, 10% of the nominal power with a storage time of a maximum of a few seconds is sufficient. In order to prevent power peaks when switching operations occur when the intermediate storage unit is charged, the excess energy is then converted into heat.

The control preferably consists of a computer-controlled converter with a direct current intermediate circuit in which coils or capacitors or other storage devices can be used as intermediate storage. The energy dissipation of short-term power peaks of a higher order is dissipated by means of a passive component, whereby the component can be designed as a heating element.

Advantageously, the converter is an inverter with a direct current intermediate circuit, which means that the alternating current is converted into direct current and then back into alternating current.

are commercially state of the art

Further features essential to the invention can be found in the dependent claims.

In the following, the present invention is explained in more detail using drawings. It shows:

Fig. 1 a schematic circuit diagram of the entire wind turbine (1) ;

Fig. 2 is a time-power diagram in which a switching peak (8) occurs;'

Fig. 3 is a time-power diagram in which a power peak (8 ¹ ) occurs above the maximum permissible power (Pmax).

Fig. 1 shows a basic circuit of the entire wind turbine 1. The wind turbine 1 has a rotor 2, the rotor blades of which (three here) can be adjusted depending on the wind direction and wind strength. The rotor plane can be vertical or horizontal. An asynchronous generator 3 is connected to the rotor 2 in a force-locking manner, which generates electrical power to be output. This electrical power is fed into the public network 5 as required, which generally has the phases R, S, T. The device 4 according to the invention, which contains a plurality of switches 11, 12, 13, is connected in parallel to this public network 5. This electronically controlled device 4 also contains a commercially available converter that converts alternating current into alternating current via a direct current circuit. Since this converter is not the subject of the present invention, it will not be described in detail here. The electronically controlled device 4 has as its core a buffer 7, which absorbs short-term positive power fluctuations that deviate from the average value and releases them again with a time delay using electronically controlled switches 11, 12, 13. The time interval is determined by the

electronic control. In the event that the maximum permissible power output of the wind turbine 1 is reached, the energy stored in the storage unit 7 is diverted via a switch 12 and a passive component 10. This passive component 10 can be, for example, a heating element, i.e. an ohmic resistor.

Various storage means can be used as the intermediate storage device 7, for example a coil with sufficient inductance (L) or a capacitor with sufficient capacity (C). Mechanical energy storage by means of a flywheel or the like is also conceivable.

Fig. 2 shows a time-power diagram P(t) in which fluctuations from the average power value are entered. In the front part of the diagram, regular power fluctuations are shown, which are primarily generated when the blade passes through the tower shadow. The frequency of such regular fluctuations is between 1 and 3 Hz. Irrespective of this, uncontrolled switching peaks 8 occur during various switching processes, which have a disruptive effect on the grid compatibility of the wind turbine 1, as already mentioned above. These voltage peaks 8 must not generally exceed a limit value of approx. 3% specified in the DIN regulations. These switching peaks 8 can be absorbed by means of an intermediate storage device 7 according to the invention in order to achieve a relative smoothing of the fed-in power, whereby grid compatibility is significantly improved.

Fig. 3 shows a time-power diagram in which the maximum power to be fed in Pmax runs parallel to the time axis (t) as a limit value. In the event that the maximum permissible feed-in power (Pmax) is exceeded, it is the task of the electronic control device 4 to divert the excess power 8' via a passive component 10 by closing the switch 12.

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is therefore used to reduce power or voltage peaks that occur during switching operations in the network, in order to improve the network compatibility of wind turbines or other power generation systems such as photovoltaic generators.

Claims (4)

1. Wind power plant with a wind-driven rotor {2) which is mechanically coupled to an asynchronous generator (3), wherein the asynchronous generator (3) is connected to a public power supply network {5), characterized by a computer-controlled converter {6) with a direct current intermediate circuit with buffer (7) which absorbs short-term positive power fluctuations (8) deviating from the line mean value (9) and releases them again with a time delay. 2. Wind power plant according to claim 1, characterized in that the at least one intermediate storage device (7) absorbs approximately 10% of the nominal power of the asynchronous generator (3) for at least one second. 3. Wind power plant according to claim 1 and 2, characterized in that the computer-controlled converter (6) has at least one electronically controlled switch (11, 12, 13). »4 — 2 4. Wind power plant according to claim 1 and 2, characterized in that the intermediate storage device (7) is discharged by means of a passive component (10) via a switch (12).