NO320903B1 - Procedure for limiting the grid connection current of a wind turbine generator - Google Patents

Description

Method for limiting the grid connection current for a wind turbine generator

The invention includes a procedure for limiting the grid connection current and the surplus effect of a wind turbine or the like. electricity generating system for the use of renewable energy, as stated in the introduction to patent claim 1.

Background

When an induction motor is operated above synchronous speed, it acts as an electrical generator that converts the supplied mechanical power on the shaft into electrical power. If the motor - now an induction generator - is connected to an alternating current network, the current generated will be absorbed by the network. This is used e.g. in modern wind turbines etc. energy systems to utilize renewable energy sources. The induction motors used as generators are developed for this purpose.

The electrical power developed by a wind turbine varies due to of the variable and unstable nature of the wind. The connection to the grid creates major technical difficulties, the connection takes place at a point where the turbine runs almost load-free, often while the wind is rising. Ideally, it would be possible to control the wind speed as another controllable operating parameter: the wind speed could then be made to rise slowly until the exact point where the turbine rotor rotates at synchronous speed. As soon as the generator was in phase with the grid, the generator could be connected to the grid through a relay or the like. simple electrical switch mechanism, without causing power surges in the grid or torque jumps in the wind system. In practice, however, this is not possible, because the wind speed is not a controllable factor.

In practice, one is forced to take into account that the wind speed, and thus the energy it contains, can vary very quickly. Strong gusts of wind can thus cause the turbine rotor to accelerate above synchronous speed very quickly, provided that the turbine's starting point is idle speed, with no load on the generator and that there is therefore no load on the rotor either. The connection of the generator to the grid must take place at the latest when the engine reaches synchronous speed, to avoid the turbine accelerating and rushing. In order to prevent unacceptable power surges on the grid and torque surges, it is necessary to make the connection to the grid "soft", i.e. that the power transfer from the generator to the grid during the switching process must be increased evenly and gradually from a minimum to a full connection of power. For this purpose, an electronically controllable electrical connector has been developed which satisfies the need for a fully controllable connection process. Reference can be made to Danish patent application 0758/97.

However, the desire to make a soft connection to the grid is in direct conflict with the need for a hard and fixed load on the rotor when the dynamic connections that surround the wind turbine

requires it, i.e. when the turbine, during idling, is exposed to a strong gust of wind as described above. The reason for this is, of course, that in order to avoid the turbine rushing, you have to keep a tight control, so that it does not accelerate in an uncontrolled way during the connection process. It is precisely during the connection process that, in order to achieve a soft connection to the grid, one wants to gradually increase the generator load over a time interval, during which the braking effect from the generator on the rotor is reduced as a result. In practice, it is necessary to choose a compromise, where the generator is connected to the grid by a controlled connection process that takes into account the possible effects on the wind turbine structure (gear and rotor shaft torque, bending stresses on the blades, etc.). It is also necessary to take into account the maximum power level for grid connection, which is acceptable in relation to the grid's adaptability.

As soon as an induction-type wind turbine is connected, the grid frequency and voltage will control the generator and thus also the turbine. In practice, this means that the speed of the turbine engine is determined by the grid frequency. Because the power developed by the turbine depends on the speed of the wind, it is clear that the electrical power delivered to the grid is constantly varying. Wind power systems are designed to develop a certain nominal yield (normed capacity) at a certain wind speed, which will hereafter be called "nominal wind speed". For economic reasons, the grid is usually designed for the same electrical output (ie rated capacity). Wind speeds above the nominal - more specifically in the interval between the nominal wind speed and the wind turbine's cut-out speed - will result in a higher output power from the generator than the network is designed to receive. Two different methods are used in the attempt to control the power generated by wind turbines. One method is called "saturation control" and it exploits the tendency of the blades to go into saturation (lose their lift), when the angle of attack of the blade in relation to the air flow exceeds a certain threshold value determined by the shape of the blade and fixed pitch rotor blades are used, with particularly developed aerodynamic blade shapes. This control method is not very accurate, and it requires oversized gears and generators, so that the braking torque from the generator against the turbine rotor is sufficient to prevent the saturation condition of the blades at the set wind speed. Just before the blades go into saturation ("stalls"), there is often an unwanted and uncontrollable power surge. In cases where the requirements for power quality are high, this control method is insufficient.

The second method is to use variable pitch blades. The system required is technically demanding and in practice it is impossible to vary the pitch of the blades sufficiently quickly to match changes in the wind speed. The result is an unwanted periodic overload of the network and intermediate periods of power loss, because the blades are not able to follow the changes in wind speed satisfactorily, and that they therefore periodically adapt to a pitch that develops less power than what they are able to produce at the given the wind speed.

Other methods of power management are based on mechanical auxiliary brakes that can be activated for short periods of time during the grid connection process, in order to limit the problematic shocks in the developed power. So-called "stand-alone" wind power systems are also used, where the frequency and power are controlled with a system with variable load. Dumping loads (load resistors) are connected to the grid or disconnected, depending on the user-induced load on the grid and the current wind speed, so that the turbine encounters a constant electrical impedance that allows it to rotate at a certain frequency and no more. These last two procedures are not relevant for the type of wind power systems that the invention is about.

Regardless of the chosen control method, there is a phenomenon called flickering which often occurs in connection with wind power development, i.e. voltage variations which typically have their cause in periodic load variations. One of the characteristics of this phenomenon is a flickering of the electric light, which is unacceptable. Flicker created by wind turbines is usually in a frequency range of 0-8 Hz. The causes of the flickering can vary: the blades run through the lee area of the wind turbine tower, periodic turbulence phenomena in the wind caused by special types of terrain, trees growing near the windmill etc. There is currently no known procedure to effectively investigate or remove flickering from wind turbines.

In the case of grid-connected wind power systems and similar systems for utilizing renewable energy, the requirements for the quality of the electrical current generated by the wind turbines generally increase as larger wind power systems are designed and their number increases. In Germany and other countries, the wind turbine manufacturers have faced strict power quality standards, which has made it necessary to find a solution to the problems of wind power quality and electrical production as described above. "Grid quality" refers to a set of mandated specifications, - including a maximum grid connection current, a narrow range of acceptable grid loads, a maximum permitted level of flicker etc, which grid-connected wind power systems must meet before they can be approved.

Purpose

The main purpose of the invention is to create a solution to the problem of satisfying the requirements for power quality. More specifically, the purpose is to create a method to limit the grid connection current of wind generators and divert the excess power generated in those situations where the generator output exceeds normal performance, i.e. when there is flicker and when the wind speed is generally higher than the normal operating conditions. The purpose of the invention is also to provide a controllable electric brake load for use in accordance with the invention.

The invention

The invention is stated in patent claim 1. It is special in that the generator during the grid connection - i.e. during the operating periods when the generator performance is higher than desired in relation to the grid's specifications - is loaded with a variable power arrester that is independent of the grid. This load resistor has a stepwise or continuously variable control system that spans a relatively wide power range. The output interval chosen should be large enough to allow a limitation of the power transferred to the grid, to prevent the generator output power from exceeding the nominal level of the grid. This "dumping" technique stabilizes the generator's output power to the grid at the maximum allowable output without allowing it to exceed this level and without unnecessarily wasting power that would otherwise be supplied to the grid. This ensures optimal performance, as well as the generator's compliance with the requirements from the grid.

In the invention, this dumping technique is thus used primarily in operating situations, where the wind speed varies in the interval between the turbine's nominal wind speed and the maximum permitted wind speed, plus in situations where there are voltage variations (flickering) that cause periodic shocks. This technique is also used, as mentioned above, during the process of connecting the generator to the grid, when the external dumping load acts as an additional load, so that the turbine rotor can be controlled efficiently during the connection process. This means that the turbine can be connected to the grid without power surges and without the risk of it accelerating out of control.

The dumping load - which in accordance with the invention can consist of a resistor, a capacitor or inductor or a combination of these - is stepwise or continuously variable, that is to say it can be connected in a stepwise or continuously variable way via the generator by means of a thyristor switch or similar electrical or electromechanical coupling of a known type. In this way, the dumping load can dump all types of electrical power, including capacitive and reactive power from the generator. In principle, the dumping load function is a resistor placed directly above the generator and independent of the grid.

With the invention, the degree of connection can be controlled depending on the generator power and/or other relevant operating parameters that can provide information about the generator power.

The control system can follow a predetermined program, e.g. in the form of an algorithm in one of the system's computers for management. The program ensures that the dumping load is disconnected when the output from the generator is less than or equal to the nominal output, i.e. the capacity for the network design. In this way, the power quality is taken into account. Output peaks that exceed the nominal power threshold are "burned off" in the load resistor. The utilization of the power generated by the turbine is made optimal. In summary, the method described in accordance with the invention will have the effect that it stabilizes the generator power output to the grid at the maximum permitted output, without exceeding it and without unnecessary diversion of power during the time when the power could have been delivered to the grid. In other words, the invention ensures an optimal yield while ensuring compliance with the requirements for power quality.

Example

The invention is described below in more detail with reference to the drawing, where

Fig. 1 shows a curve that illustrates the operating conditions, before, during and after the time when a wind generator is connected to the grid. Fig. 2 shows a curve that illustrates the effect of the dumping technique under operating conditions in which the generator output exceeds the maximum permitted grid voltage, and Fig. 3 shows a cross section through a wind turbine tower showing the location of the resistance elements in the dump load resistor.

These diagrams illustrate how the dumping technique is used to control a wind turbine while connected to the grid, to prevent it from creating a power surge (Fig. 1) and how the generator output is "adjusted" with the dumping load under more ordinary operating conditions (Fig. 2) . The controlled output power to the grid is drawn in a curve in Fig. 2. The curve shows how the power quality requirements are satisfied and the greatest possible utilization of the turbine performance is achieved.

Fig. 3 shows how the system's resistance elements can be mounted inside the wind turbine tower 1, while the tower is used as a cooling surface. The tower consists of bent or rolled steel plate 2. The load resistors, or at least their respective components, consist of electric heat sinks 3 which sit in U-profile holders 4. These holders 4 are bolted to the inside of the. the plate in the tower. The heating from the power dissipater is released into the tower plate and distributed along it, i.e. the lower surface acts as a cooling element. The cooling elements are symmetrical and distributed along the outside of the tower, at mutually equal distances, so that the temperature variation around the circumference of the tower is as low as possible.

Claims (3)

1. Procedure for limiting the grid connection current and excess power from a wind turbine etc. electricity generating system for use with renewable energy, where the electrical generator is an alternating current induction generator, in particular a three-phase generator, which is connected to an alternating current grid and where the connection to the grid is made gradually by means of a variable, electronically controlled electrical connector of the type which is based on thyristors, whose acceptance angle plus the phase angle of the generator determines the real degree of connection, with a gradual supply of power to the grid, and where the connection to the grid is made by adjusting the thyristor acceptance angle depending at least on the instantaneous phase angle of the generator, so that both the generator voltage and the torque load during the connection process can be controlled and increased gradually in accordance with a predetermined curve, characterized by the generator being loaded with a variable dumping load during the connection process and otherwise during operating periods where the generator performance is higher than desired in relation to grid compatibility, and that this dumping load is stepwise or continuously controlled with a relatively wide output range, which is sufficient to control the output to the grid under all relevant operating conditions, i.e. during the connection process and in other operating situations, in particular when a wind turbine continuously or periodically generates a higher electrical output than the nominal absorption capacity of the grid, operating situations that typically occur when the wind speed varies in the interval between the turbine's nominal wind speed and its cut-out speed, and in situations where the voltage fluctuations occur and periodically cause an effect above the nominal value, as the dumping load consists of a resistance, capacity and or inductor , or a combination of these, whose resistance is stepwise or continuously variable in its connection via the generator, by means of an electrical or electromechanical switching device, in particular a thyristor, and that the variable dumping load is regulated in a way e which is electronically dependent on parameters linked to the generator's output power. 2. Method in accordance with patent claim 1, characterized in that the variable dumping load is regulated in accordance with a predetermined program which ensures disconnection of the dumping load when the generator output is less than or equal to the nominal output given by the grid requirements. 3. Method in accordance with patent claim 1 or 2, characterized in that the dumping load is controlled with a computer which collects relevant operating parameters and on the basis of these parameters and according to a predetermined algorithm continuously calculates the optimal load for each individual operating situation.