EP1097499A1

EP1097499A1 - Method and device for limiting making current and excess power from an alternating-current induction generator

Info

Publication number: EP1097499A1
Application number: EP99911642A
Authority: EP
Inventors: Carsten Henriksen
Original assignee: Mita-Teknik AS; Mita Teknik AS
Current assignee: Mita-Teknik AS; Mita Teknik AS
Priority date: 1998-03-30
Filing date: 1999-03-30
Publication date: 2001-05-09
Also published as: NO320903B1; JP2002510951A; DK174466B1; NO20004889L; WO1999050945A1; NO20004889D0; DK43899A; AU3025799A

Abstract

The invention comprises a technique for limiting the grid-connection current and surplus output of a wind turbine or a similar electricity-generating system for the utilisation of renewable energy where the electrical generator of the system is an alternating-current induction generator. The generator is coupled to the grid by means of a variable, electronically controlled electrical connector of the type based on thyristors whose acceptance angle plus the phase angle of the generator determine the actual degree of connection. The technique in the invention is special in that the generator is loaded with a variable, grid-independent dump-load during the connection process and otherwise during all other operating situations in which the generator yield is higher than desired with respect to grid compatibility. This dump-load is stepwise or continuously controlled within a relatively wide output range.

Description

METHOD AND DEVICE FOR LIMITING MAKING CURRENT AND EXCESS POWER FROM AN ALTERNATING-CURRENT INDUCTION GENERATOR

The invention comprises a technique for limiting the grid-connection current and surplus output of a wind turbine or a similar electricity-generating system for the utilization of renewable energy, and this technique is more specifically of a nature as described in the preface to Claim 1. The invention also comprises an electronically controllable power diverter (electrical brake load) for use in this technique.

10

When an induction motor is driven above synchronous speed, it functions as an electrical generator which converts the added mechanical power of the axle into electric power. If the motor - now an induction generator - is coupled with an alternating-current grid, the current generated will be accepted by the grid. This effect is employed, for example, in modern wind 15 turbines and similar energy systems used to utilize renewable energy sources. It should be noted, however, that the induction motors used in generators are specially developed for this purpose.

Naturally, the electric power generated by a wind turbine varies because of the varying and 20 unstable nature of wind. Connection with the grid presents major technical difficulties: connection takes place at a point when the turbine is running almost load-free, often while the wind is on the rise. Ideally, it would be possible to control the windspeed just like any other controllable operating parameter: the windspeed could then be made to rise slowly until the exact point where the turbine rotor rotates at synchronous speed. Once the generator was in 25 phase with the grid, then the generator could be coupled to the grid through a relay or some similar, simple electric switch mechanism without causing power surges in the grid or torque discontinuities in the wind power system. In practice, of course, this method is impossible, since windspeed is not a controllable factor.

30 In practice, we are forced to take into consideration the fact that the speed of the wind and thus the energy it contains can change very quickly indeed. 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 at an "idle", at which there is no load on the generator - and thus no load on the rotor, either. The coupling of the generator with the grid must occur no later than at the moment

35 the motor reaches synchronous speed in order to prevent the turbine from accelerating and overspeeding. To prevent unacceptable power surges on the grid and torque surges, it is necessary to make the connection with the grid "softly," i.e. the power transmission from the generator to the grid during the coupling process must be increased smoothly and evenly from a 2

minimum to a full connection of power. For this purpose, MITA TEKNIK A/S has developed an electronically controllable electrical connector which satisfies the need for a fully controllable coupling process. Please see the description in DK Patent Application No.0758/97.

The desire to make a "soft" connection with the grid, however, is in direct conflict with the need for a hard and fast loading of the rotor when the dynamic conditions surrounding the wind turbine require it, i.e. when the turbine, while "idling", is subjected to a strong gust of wind as described above. The reason for this is, naturally, that in order to keep the turbine from overspeeding, it necessary to "keep a tight reign" on it so that it does not accelerate in an uncontrolled fashion during the connecting process. It is exactly during this connecting process that, in order to obtain a "soft" connection with the grid, we want to increase the generator load gradually over a period of time, during which the braking effect of the generator on the rotor is reduced as a result. In practice, it is necessary to choose a compromise in which the generator is connected to the grid in a controlled coupling process which takes into consideration possible effects on the windmill structure (gear and rotor axle torque, bending stress on the blades, etc.); it is also necessary to take into account the maximum grid-connection current level which is acceptable with respect to grid compatibility.

Once a wind turbine of the induction type is online, the grid frequency and voltage will control the generator and thus the turbine itself. In practice, this means that the speed of the turbine motor is determined by the grid frequency. Since the power generated by the turbine depends on the speed of the wind, it is clear that the electric power delivered to the grid varies constantly. Wind power systems are designed to develop a certain nominal yield (rated capacity) at a certain windspeed which from now on will be referred to as the nominal windspeed. For reasons of economy, the grid is usually designed for the same electric power output (i.e. rated capacity). Windspeeds above the nominal - more specifically, in the interval between the nominal windspeed and the windmill's cut-out windspeed - will result in a higher output of electricity from the generator than the grid is designed to accept. Two different methods are used in the attempt to control the power generated by wind turbines. One method is called "stall controlling", and it utilizes the tendency of the blades to stall (lose their lift) when the angle of attack of the blade relative to the airflow exceeds a certain threshold value which is determined by the shape of the blade, and fixed-pitch rotor blades with specially developed aerodynamic blade shapes are used. This method of control is not very precise, and it requires oversized gears and generators so the braking torque of the generator on the turbine rotor is sufficient to provoke blade stall at the set windspeed. Just before the blades stall, there is often an undesirable (and uncontrollable) surge in power. In cases where requirements to power quality are high, this control method is inadequate. 3

The second method is using variable-pitch blades. The system required is technically demanding, and in practice it is impossible to vary the pitch of the blades fast enough to match changes in windspeed. The result is an undesirable periodical overload of the grid and intermittent periods of power loss because the blades are unable to follow the changes in windspeed adequately, and they thus periodically adopt a pitch which generates less power than they are potentially capable of producing at the given windspeed.

Other power control methods are based on mechanical auxiliary brakes which can be activated for short periods of time during the grid connection process in order to limit the problematic surges in the power generated. Also used are so-called "stand-alone" wind power systems in which the frequency and power are controlled through a variable-load system. Dump-loads (load resistors) are connected to the grid or disconnected again, depending on the user-induced load on the grid and the windspeed at the time, so that the turbine meets a constant electrical impedance that allows it to rotate at the set frequency and no more. These two last methods are not relevant to the type of wind power system for which this invention is intended.

Irrespective of the control method selected, there is a phenomenon called "flicker" which often occurs in connection with wind power generation, i.e. voltage fluctuations which are typically due to periodical load variations. One of the manifestations of this phenomenon is a visual perception of electric lights flickering, which is, of course, unacceptable. Flicker caused by wind turbines usually lies within a frequency range of 0 - 8 Hz. The causes of flicker may vary: the blades passing through the lee of the windmill tower, periodic turbulence phenomena in the wind caused by particular types of terrain, trees growing near the windmill, etc. There is no currently known method of effectively suppressing or removing flicker from wind turbines.

As regards grid-connected wind power systems (and similar systems to exploit renewable energy), the requirements to the quality of the electric power generated by wind turbines generally grow more rigorous as larger wind power systems are designed and their numbers increase. In Germany and other countries, wind turbine manufacturers have run up against a number of stricter power quality standards which have made it necessary to find a solution to the problems with wind power quality and electricity generation discussed above. "Grid quality" refers to a set of required specifications - including a maximum grid-connection current, a narrow range of acceptable grid loads, a maximum permissible level of flicker, etc. - with which grid-connected wind power systems must comply before they can be approved.

The purpose of this invention is to provide a solution to the problem of how to comply with power quality requirements. More specifically, the purpose is to provide a method to limit the grid-connection current of wind generators and to divert the surplus power generated in the 4

situations where generator production exceeds the nominal yield, i.e. when there is flicker and when the windspeed is generally higher than nominal operating conditions. The purpose of the invention is also to indicate a controllable electric brake load for the technique in the invention.

The technique in the invention is distinctive in that the generator during the grid connection process - also at all times of operation when the generator yield is higher than desired with respect to grid compatibility - is loaded by a variable power diverter which is independent of the grid. This power diverter has a stepwise or continuously variable control system spanning a relatively wide output spectrum. The output interval chosen should be wide enough to enable a limitation of the power loaded onto the grid in order to keep the generator output from exceeding nominal grid design levels. This "dump-load" technique stabilizes the generator output to the grid at the maximum permissible yield without permitting it to exceed this level and without unnecessary waste of power that could otherwise be supplied to the grid. Thus this technique ensures optimal yield as well as generator compliance with grid quality requirements

In the invention, the dump-load technique is thus used primarily in operational situations in which the windspeed varies in the interval between the turbine's nominal windspeed and the cut-out windspeed, plus in situations in which there are voltage fluctuations (flicker) which cause periodical surges in excess of the nominal voltage. The dump-load technique is also used, as mentioned above, during the process of connecting the generator to the grid, when the external dump-load serves as a supplemental load so that the turbine rotor can be effectively "reined in" during the connection process. This means that the turbine can be connected to the grid without power surges and without risk of it accelerating out of control.

The dump-loader - which according to the inventions may consist of a resistor, capacitator or inductor or any combination thereof - is stepwise or continuously variable, i.e. can be connected in a stepwise or continuously variable manner via the generator by means of a thyristor switch or a similar electrical or electromechanical connection device of a known type (Claim 4). In this way, the dump-loader can dump all kinds of electric output, including capacitive and reactive power from the generator. In principle, the dump-loader functions as a resistor which is located directly above the generator and independent of the grid.

In the invention, control of the degree to which the dump-loader is connected may be made dependent on generator output and/or other relevant operating parameters which provide an indication of the generator yield.

The control system can follow a preset program, e.g. in the form of an algorithm in one of the system's control computers. The program ensures that the dump-loader is disconnected when 5

the generator yield is less than or equal to the nominal yield (i.e. grid design capacity). In this way, power quality is taken into account (output spikes which exceed the nominal power threshold are "burned off' in the dump-loader) and utilization of the power generated by the turbine is optimized. In summary, the dump-load technique in the invention has the effect of stabilizing the generator output to the grid at the maximum permissible yield without exceeding it and without unnecessarily diverting power during times when the power could be delivered to the grid. In other words, the technique ensures an optimal yield while ensuring compliance with power quality requirements.

The invention is explained in greater detail in connection with the drawing, where

Fig.1 is a curve which illustrates the operating conditions before, during and after the time a wind generator is connected to the grid; Fig. 2 is a curve that shows the effect of the dump-load technique under operating conditions in which the generator yield exceeds the maximum permissible grid voltage; and

Fig. 3 is a cross-section of windmill tower showing the location of the dump-load resistor elements.

These diagrams, which are readily understandable, illustrate how the dump-load technique is used to control a wind turbine while it is being connected to the grid to prevent it from causing power surges (Figure 1 ) and how the generator yield is "adjusted" by dump-loading during more ordinary operating conditions (Figure 2). The controlled output to the grid is plotted in a curve in Figure 2. The curve shows how power quality requirements are met and the greatest possible utilization of the turbine yield is achieved.

Figure 3 shows how the system's dump-load-resistor elements can be installed in the inside of the windmill tower (1 ), while the tower itself is utilized as a cooling surface. The tower consists of bent or rolled steel plate (2). The dump-load resistors - or, at least, their resistive components - consist of electric heat sinks (3) which sit in U-profile holders (4). These holders (4) are bolted tightly onto the interior of the tower plate. The heat from the power diverter is released into the tower plate and distributed along it, i.e. the tower plate functions as a cooling element. The heat sinks are symmetrically distributed along the outside of the tower, equidistant from each other, so that the temperature variation around the circumference of the tower is as low as possible.

Claims

1. A technique for the limitation of the grid-connection current and surplus output from a wind turbine or similar electricity-generating system for the utilization of renewable energy, in which the electric generator is an alternating-current induction generator (usually a three-phase generator) which is connected to an alternating-current grid (a three-phase alternating-current grid) and where connection to the grid is gradual by means of a variable, electronically controlled electrical connector of the type based on thyristors whose acceptance angle plus the phase angle of the generator determine the actual degree of connection (implied: gradual introduction of power to the grid) and where connection with the grid takes place by adjusting the thyristor acceptance angle depending on, among other things, the instantaneous phase angle of the generator and other relevant operating parameters, so that both the generator voltage and the torque load during the connection process can be controlled and increased gradually in accordance with a preset curve (a so-called "soft start"), characterized in that the generator is loaded with a variable dump-load during the connection process and otherwise during periods of operation when the generator yield is higher than desired with respect to grid compatibility and that this dump-load is stepwise or continuously controlled within a relatively wide output range which is sufficient to rein in the system power output to the grid under all conceivable operating conditions, i.e. during the connection process and in other operating situations in which, for example, a wind turbine continually or periodically generates a higher electrical output than the nominal design capacity of the grid, operating situations which typically occur when the windspeed varies in the interval between the turbine's nominal windspeed and its cut-out speed, and in situations in which voltage fluctuations (so-called "flicker") occur and periodical cause output in excess of the nominal value.

2. The technique specified in Claim 1 , characterized in that the variable dump-load is regulated in a manner electronically dependent on the generator output and/or other relevant operating parameters which indicate the yield of the generator, and at the same time according to a preset program which ensures disconnection of the dump-loader when the generator yield is less than or equal to the nominal yield (i.e. grid requirements).

3. The technique specified in Claim 1 or 2, characterized in that the dump-load is controlled via a computer which collects relevant operating parameters and on the basis of these parameters and on a preset algorithm continually calculates the optimal load for each individual operating situation.

4. Electric dump-loader for use in the technique specified in Claim 1 , 2 or 3, characterized in that it consists of a resistor, capacitator or inductor, or any combination thereof, whose resistance is stepwise or continuously variable in its connection via the generator (i.e. in principle independent of the grid) by means of a thyristor switch or a similar electric or electromechanical switch device of a known type.

5. Electric dump-loader as specified in Claim 4 and in which the technique is used in connection with a wind turbine of the type with a steel-plate tower with a closed conical or cylindrical shape, characterized in that the resistive components in the dump-loader are made up of heat sinks located on the inside of the tower tightly fastened to the inside of the tower wall, the tower itself being used to dissipate the heat.

6. Electric dump-loader as specified in Claim 5, characterized in that the heat sinks are distributed symmetrically (and equidistant to each other) along the inside circumference of the tower wall.