DK174466B1 - Method for limiting switch-on current and surplus power from a wind turbine or similar electricity-generating plant for the utilization of renewable energy, and an adjustable electric power dissipator (brake load) for use in this method - Google Patents

Description

The invention relates to a method for limiting switch-on current and the surplus power of a wind turbine or similar electricity-generating plant for the utilization of renewable energy, which method is more specifically defined in the preamble of claim 1.

5 When an asynchronous motor is operated over the synchronous orbital number, the machine will act as an electrical generator which converts the applied mechanical shaft power into electrical power. Provided the motor - now an asynchronous generator - is connected to an AC power grid, the generated current will be sent to this. This property is utilized, among other things, in modern 10 wind turbines and similar energy plants for the utilization of renewable energy. However, it should be noted that the asynchronous motors used for generator operation are specially wound for the purpose.

The electrical power produced by a wind turbine is inherently variable due to the variable, unstable nature of the wind. The connection on the grid is associated with great technical difficulties, the connection being made from a state where the mill is running almost unloaded, and often at the same time that the wind speed is increasing. Ideally, the wind speed should be regulated as another controllable operating parameter, and one would then be able to slowly adjust the wind speed until the wind turbine rotor just rotated with synchronous orbital speed. When the generator phases had subsequently aligned themselves synchronously with the grid phases, the connection to the grid could be made through a relay or a similar simple electrical contact mechanism, without this causing electric shock on the grid or torque discontinuities in the wind turbine. In practice, of course, this approach is not possible since the wind speed is not controllable.

25 In practice, it is forced to take into account that the speed of the wind and thus the energy content can change even very quickly. Thus, in strong gusts, the wind turbine rotor can accelerate upwards over the synchronous orbital speed in a very short time, provided it is started from an "idle state" where the turbine rotates with sub-synchronous rpm and where the generator - and thus also the rotor - is unloaded. The generator must be switched on at the latest when the synchronous rpm is passed, unless the wind turbine is to run. Especially prevent uaceepia: '· ..;'. For large power surges on the grid and torque shocks on the rotor, it is necessary to make the connection "soft", ie the power transmission between the generator and the grid during the switch-on process must be increased evenly from a minimum to fully switched power. For this purpose, Μ1ΤΛ TEKNIK A / S has developed an electronically controllable, electrical coupling that meets the need for a fully controllable switch-on process. Reference is made to the description of DK patent application no. 0758/97.

However, the desire for a soft connection on the grid is in direct contradiction to the need for a fast, cash load on the rotor when the dynamic conditions around the mill require this, ie when the wind turbine is subjected to heavy gusts during idle travel as described. The reason for this is, of course, that in order to avoid '' running '', one is forced to keep the mill (loaded), so that the speed does not increase uncontrolled during the switch-on process. And it is precisely during the switch-on process that, for reasons of 45 for the soft switch-on on the grid, it is desired to increase the generator load successively over a period, during which period the generator's braking effect on the rotor is consequently reduced. In practice, it is necessary to choose a compromise in which the generator shutdown is done during a controlled shut-down process, taking into account 2 DK 174466 B1 to the effects that the wind turbine design as such can be subjected to (torque load on the gear and rotor shaft, bending effects on the blades, etc), as well account must be taken of the maximum switch-on current acceptable j taking into account the network quality.

5

When connecting an asynchronous generator to the grid, a short-term increase in current is caused by the magnetization current. In addition to this magnetization current, it has been found in practice that immediately after the passage of the synchronous point, a current (current shock) occurs, which far exceeds what might be expected as magnetization current alone. The phenomenon can be explained by the fact that the generator is in practice connected to the grid immediately before the turbine reaches the synchronous point, so that the generator acts briefly as motor until the synchronous point is reached. As the synchronous point passes during the continued acceleration of the rotor, the generator shifts from motor to generator operation.

15 This shift involves not only a change in the electrical conditions around the generator (phase angle, saturation current, etc.), but also a shift in the load on the physical structure and machine components of the wind turbine. This is because the torque between the rotor and the generator changes direction as the generator changes function from motor to generator. This shift in load direction (torque direction) means the release of 20 clamped elastic energy in the system. For example, the resilient bias in the rotor shaft, rotor blades, gear suspension and resilient shaft couplings will be released as motion energy at the torque change. The triggered motion energy is subsequently converted into a short-lived increase in the generator's rpm and will therefore lead to a short-term power gain. It is precisely this increase in power that can be measured as a strong switch-on current that far exceeds the expected magnetization current. The release of the elastic energy at the same time as the rotor during the switch-on process works quickly in revolutions, further contributes to the above-mentioned problems in connection with the connection of the generator to the grid.

Thus, the generator must be able at one and the same time to slow down the kinetic energy which the rotor has been able to collect during the '' sliding '' in: switch-off process. that is, the generator must provide a suitable deceleration of the rotor for operating speed figures, and this must be done at the same time as the generator takes up the elastic energy released from the turbine assembly as mentioned. The result is the measured, relatively high switch-on current which occurs for a short period of time after passing the synchronous point.

In general, with regard to grid connected wind power plants (and similar renewable energy plants), it can be said that the requirements for the quality of the electrical power produced by the plants increase as the plants grow steadily and the number of 40 plants grows. The wind turbine industry has, among other things, in Germany met a number of stricter quality requirements with regard to grid quality, which means that the above-mentioned problems in connection with the wind turbines' grid connection and electricity generation must necessarily find a solution. Grid quality means a number of requirements specifications that the grid connected wind turbines must comply with before they can be approved, including requirements for a 45 maximum switch-on current, narrow limit values for power surplus, etc. Known methods for controlling and regulating grid connected wind turbines during the switch-on cycle are inadequate solving the problems associated with the 3 DK 174466 B1; greatly increased currents that occur during switching on, cf. explanation above.

The object of the present invention is to provide a solution to how the requirements of the network quality 5 can be met. More specifically, the object is to provide a method for limiting the generator's current flow on the grid, while at the same time taking into account the necessary bridging (load) of the wind turbine during the switching process.

The method according to the invention is characterized in that the generator is decelerated / loaded by '' dumping '' power via a control unit in a resistance network during the switch-on process, ie while the thyristors in the main circuit are not yet fully open. In this way, the grid is effectively spared from power surges that exceed a specified maximum current. That is, the method enables at the same time to comply with the requirements for grid quality, while at the same time loading the mill generator sufficiently for the mill to be effectively defrosted during the switch-on process.

Conveniently, the dump load (resistance network) used for power dumping is stepwise or infinitely adjustable over a relatively wide power range, the size of which is selected 20, taking into account that any overload of the network with power surges or periodic overruns of the maximum current set for the network must in any case be possible. is prevented by appropriate dumping of surplus power during the switch-on process. According to the invention, the dumpload switch-on is controlled by a computer, which continuously calculates the required dumpload load on the basis of the phase angle and current of the generator, for which a fixed algorithm is included in the computer. The dumpload technique causes the generator power to stabilize around the maximum allowable output without being exceeded, and without unnecessarily '' burning '' power off during periods when the power could be transmitted to the grid. That is, the technique partly ensures optimum performance and partly helps to meet the requirements for network quality.

30

In the process vm; j>: message, the dumpload may advantageously be assembled! all 'resistive, capacitive or inductive resistance or any combination thereof. The switching speed of the dumpload is controlled depending on the generator power and / or other relevant operating parameters which indicate the generator's performance. The control 35 can follow a predetermined program, for example, determined as a calculation algorithm in a control computer belonging to the system. The program ensures that the dump load is switched off during the periods when the generating capacity is less than or equal to the nominal output (ie grid dimensioning). This is achieved, in part, by taking into account the grid quality (power peaks that exceed the nominal power limit value, 40 '' burned 'in the dump load) and partly that the utilized power from the generator is optimized.

The invention will be explained in more detail in connection with the drawing, where

FIG. 1 is a graph showing the operating conditions around the time of switching on a wind turbine;

FIG. 2 is a graph showing the effect of the dumpload technique under operating conditions where the generating capacity exceeds the permissible maximum power on the grid, and 4 DK 174466 B1

FIG. 3 is a cross-section of a mill tower showing the location of dump load resistance elements. '5 The diagrams, which are immediately understandable, illustrate how the dumpload technique is used to bridle a wind turbine during connection to the grid without causing a shock (Fig. I), and how the generating performance is'' regulated '' by dump loading under more common operating conditions (Fig. 2). The regulated grid power is plotted in waveform in the figure. The curve shows how both the consideration for the grid quality and the consideration 10 for the greatest possible performance from the mill are met.

1 FIG. 3, it is shown how the dumpload resistance elements of the system can be placed inside the turbine 1 using the tower itself as a cooling surface. The toe consists of bent or rolled steel plate 2. The dumpload resistance elements - or at least their resistive components - are constituted by electric heaters 3 embedded in holders 4 with a U-profile. The holders 4 are bolted to the inside of the tower plate 2 in close contact with it. The heat generated by the power dissipation is delivered to the toe plate and distributed along it. That is, the toe plate acts as a cooling element. For heating of the tower, the heaters are symmetrically distributed along the perimeter of the tower, so that the temperature variation along the perimeter is as small as possible.

Claims (3)

A method for limiting switch-on current and surplus power from a wind turbine or similar renewable energy generating plant, wherein the plant's electricity generating generator is an asynchronous alternator 5 (usually 3-phase generator) coupled to an alternating voltage grid (3-phase AC) and where the connection to the network is made by a variable, electronically controllable electrical coupling of the type based on thyristors whose opening angle together with the phase of the generator determines the current degree of coupling (implied: percentage power on ) and where the switching on the 10 grid is made successively by adjusting the thyristor opening angle depending on, inter alia, the instantaneous phase angle of the generator and other relevant operating parameters, so that both generator current and torque load during the switching process can be controlled and gradually increased according to a predetermined curve (a called '' soft '' switching), characterized in that the generator is decelerated / loaded by '' dumping '' power through a control unit in a resistance network during the switch-on process, ie while the thyristors in the main circuit are not yet fully open. Method according to claim 1, characterized in that the dump load (resistance network) used for power dumping is stepwise or infinitely controllable over a relatively wide power range, the size of which is chosen taking into account any overload of the network with power surges or periodic overruns of the power dump. the maximum current set for the grid must in any case be averted by appropriate dumping of surplus power during the switch-on process, 25 and - that the dumpload switch-on is controlled by a computer which continuously calculates the required dumpload load on the basis of the phase angle and current of the generator, for which calculation a fixed algorithm is embedded in the computer. 30 Method according to claim 1, characterized in! by. that an electrical load composed of re.Msiiv is used for the dumpload load. capacitive resistive inductive resistance or any combination thereof.