GB2460723A - Operating a brushless doubly fed machine (BDFM) - Google Patents

Abstract

A BDFM comprises a brushless doubly fed generator (BDFG) having a rotor, a power stator winding and a control stator winding, said rotor having a natural speed of rotation. The operating method comprising reducing an efficiency of the BDFM above its natural speed of rotation by driving power into said control stator winding above said natural speed of rotation to increase losses in said machine. Preferably driving power into the control stator winding comprises driving the control stator winding with a voltage greater than a voltage defined by a speed of rotation of said rotor, the natural speed of rotation of said rotor and a characteristic curve of the machine. The method may also comprise compensating for increased reactive power from said power stator winding due to said driving of said control stator winding with an increased voltage. This operating method finds particular application with wind turbines that are subject to gusts of wind.

Description

Power Generators

FIELD OF THE INVENTION

This invention relates to power generators, more particularly to doubly-fed generators.

The techniques we describe are particularly useful for generators in wind turbines, although applications are not limited to this field.

This application is related to another, filed on the same day as the present application, by the same applicants/inventors, hereby incorporated by reference.

BACKGROUND TO THE INVENTION

The generation of electricity from the wind is a proven means of obtaining energy without the emission of carbon dioxide. Wind turbines range from large machines with peak outputs of several Megawatts to small machines designed for domestic use. For home use the maximum output power, at least in part limited by the acceptable size of blades, is 1 to 2 kW maximum, this maximum occurring at a wind speed of around 30 mph in most designs. Average output is rather less as wind speeds vary.

Some generators are intended for charging a battery and such systems find application, for example, in boats. There is no intention of feeding the power into the mains and the approach is one oriented to an isolated electrical system. However, there is growing interest in the use of small-scale turbines which supply power into the mains, to reduce the power drawn from the mains or even return power to the electricity supply company. An example of this is the Windsave' (RTM) product designed to supply power into the single-phase domestic mains. Tariff regimes are evolving which reward consumers for power supplied to the grid e.g. green energy.

A generator is an essential part of the wind turbine. For wind turbines of modest rating, up to say 2 kW, the normal choice is an ac machine with a permanent magnet rotor which has the advantage of robustness and compactness but several drawbacks, including cost. The output of the generator is variable voltage and variable frequency, both rising in proportion to the rotational speed of the blades and most generators will produce a three-phase output as these machines are more compact.

Whilst it is possible to construct wind turbines which run at constant speed, the harvesting of the available power is poor and in practice variable speed is used to achieve acceptable performance. However, the desired output is fixed frequency and voltage for injection into the mains and in many countries is single-phase output is required. Voltage and frequency conversion is possible using power electronics and two of many possibilities are an uncontrolled three-phase rectifier, or a controlled three-phase rectifier, followed by a single phase inverter; the use of a controlled rectifier will giving better performance. In addition, the electronic circuitry is to be designed with the peak output power in mind to avoid failures but this results in a poor utilization of the capacity of the electronics for most of the time. The result is a relatively expensive system with a long or even no payback time. Example of such systems are power conversion systems include (Windy Boy RTM).

There are additional issues associated with the supply of power to the mains. Current codes have a threshold of 2.5 kW -above this level more complex regulations apply.

Nevertheless, even below 2.5 kW, there are strict regulatory requirements. One form of these, as found in the UK, is the G83 requirement which requires that in the event of mains failure the wind turbine must cease supplying power, the so-called anti-islanding requirement. In addition, most jurisdictions insist that any accessible parts should be galvanically isolated.

A somewhat different strategy in terms of generation is adopted in most large wind turbines. Variable speed operation is used to achieve satisfactory performance over the whole range of wind speeds. One possibility is to use large polyphase permanent magnet machines but these generate an output of variable frequency and voltage. The same issues then apply as found with small turbines, namely that the output has to be electronically converted to a fixed voltage and frequency. In particular, the need to convert large quantities of power is expensive.

Therefore, alternative approaches have evolved. The most commonly used employs a wound rotor induction machine with double feed. The stator of the machine is connected directly to the three-phase mains, and the stator winding is standard. The rotor of the machine is wound also with three-phase windings and connections are made to them by slip rings. An electronic power conversion circuit is used to link the rotor to the grid -the converter applies variable voltage and frequency to the rotor and can either supply power to the rotor or return power from the rotor to the grid. The machine operates in a synchronous mode with a fixed relationship between the grid frequency (i.e. the stator frequency), the rotor frequency and the shaft speed of the machine. The relationship is well documented in the literature. A further consideration is that the power flow in or out of the rotor, the power being inwards below the synchronous speed of the machine and out above the synchronous speed, increases in proportion to the deviation from synchronous speed. For example, if the induction generator is a 4-pole machine the synchronous speed is 1500 rpm. If the speed is increased to 1650 rpm, a 10% rise, the power output from the stator is 10% of the power being supplied directly from the stator. In reality, this simple relationship is complicated by losses and the effect of the flow of reactive power (VArs) has to be taken in to account when sizing the converter and the machine windings.

The use of doubly fed wound rotor induction generators is attractive enough to make them the system of choice in most wind turbines. In large wind turbines, a gearbox is used to increase the shaft speed at the blades, say 50 rpm, typically to 100 to 1500 rpm to allow a 4 pole or 6 pole generator to be used. These are relative compact machines.

However, the presence of brush gear is a major drawback as there is a maintenance issue, particularly acute offshore, and the brush gear is an expensive part of the machine, also increasing its size. Recently, brush less doubly fed machines (BDFMs) have been increasingly considered, as, as their name implies, they do not have brushes.

In these machines, there are stator windings, one of which is connected directly to the fixed frequency mains and the other is supplied with a variable frequency and voltage from a power converter which is bi-directional, as in the case for the doubly fed induction generator. Also these machines are run in a synchronous mode with a fixed relationship between the two stator frequencies and the shaft speed. Speed variation is achieved by changing the frequency applied to the second stator. The power rating of the converter supplying the variable frequency stator winding need only be a fraction of the desired power output of the machine, leading to substantial economic benefits.

A BDFM has two stator windings of different pole numbers, in general on a single frame. The pole numbers are chosen so that there is no direct coupling between them, the rotor coupling with both stator fields. One of the stator windings, the power winding (2p1 poles) is connected to the power grid with a fixed voltage and frequency and the other, the control winding (2p2 poles) is supplied by a frequency converter with variable voltage and variable frequency. The frequency driving the control winding depends upon the rotor speed and is adjusted so that the frequency of the power winding output matches that of the grid, so achieving synchronous operation.

There are three principal types of brushless doubly-fed generators, namely the Brushless Doubly-Fed induction Generator (BDFG), the Brushless Doubly-Fed Reluctance Generator (BDFRG) (which has a reluctance type rotor), and the Brushless Doubly-Fed Twin Stator Induction Generator (BDFTSIG). Typically in a BDFG operation is via currents flowing in the rotor bars, which is not the case for a BDFRG (where the rotor has salient poles). In both the BDFG and BDFRG in general both stator windings are in a single frame, in general in the same slots, whereas a BDFTSIG has twin frames and two rotors on a common shaft. For a detailed classification and comparison of doubly- fed machines, reference can be made to: B.Hopfensperger and D.J.Atkinson, "Doubly-fed a.c. machines: classification and comparison," in Proc. 9th. European Conf Power Electronics and Applications, August 2001 -and the specific definitions therein are hereby incorporated by reference. Here we are particularly concerned with the brushless doubly-fed induction generator (BDFG).

One of the inventors has published a number of papers relating to BDFM design, to which reference may be made for background information. These include: P. C. Roberts, R. A. McMahon, P. J. Tavner, J. M. Maciejowski and T. J. Flack.

Equivalent circuit for the Brushless Doubly-Fed Machine (BDFM) including parameter estimation. In Proc. TEE B -Elec. Power App., vol. 152, Issue 4, pp932-942, July 2005; R. A. McMahon, P. C. Roberts, P. J. Tavner, and X. Wang. Performance of BDFM as a generator and motor Proc. lEE B -Elec. Power App., vol. 153, Issue 2, pp289-299, March 2006; P. C. Roberts, T. J. Flack, J. M. Maciejowski, and R. A. McMahon. Two stabilising control strategies for the brushless doubly-fed machine (BDFM). In Int. Conf. Power Electronics, Machines and Drives, pages 34 1-346. lEE, April 2002; E. Abdi-Jalebi, P. C. Roberts, and R. A. McMahon. Real-time rotor bar current measurement using a rogowski coil transmitted using wireless technology. In 18th Intl.

Power Systems Conf. (PSC2003), Iran, volume 5, pages 1-9, October 2003; P. C. Roberts, E. Abdi-Jalebi, R. A. McMahon, and T. J. Flack. Real-time rotor bar current measurements using bluetooth teclmology for a brushless doubly-fed machine (bdfm). In Int. Conf. Power Electronics, Machines and Drives, volume 1, pages 120- 125. lEE, March 2004; P. C. Roberts, R. A. McMahon, P. J. Tavner, J. M. Maciejowski, T. J. Flack, and X. Wang. Performance of Rotors for the Brushless Doubly-fed (induction) Machine (BDFM). In Proc. 16th Tnt. Conf. Electrical Machines (ICEM), Sth-8th September 2004, Cracow, Poland; P. C. Roberts, J. M. Maciejowski, R. A. McMahon, T. J. Flack. A simple rotor current observer with an arbitrary rate of convergence for the Brushless Doubly-Fed (Induction) Machine (BDFM). In Proc. Proc. IEEE Joint CCA, ISIC, CACSD, September 2-4 2004, Taipai; X. Wang, P. C. Roberts and R. A. McMahon. Studies of inverter ratings of BDFM adjustable drive or generator systems Proc. IEEE Conf. Power Electronics and Drive Systems (PEDS) 2005, Kuala Lumpur, Malaysia 28th Nov -1st Dec 2005; X. Wang, P. C. Roberts and R. A. McMahon. Optimisation of BDFM Stator Design Using an Equivalent Circuit Model and a Search Method Proc. tnt. Conf. Power Electronics, Machines and Drives (PEMD), vol. 1, PP. 606-610, Clontarf Castle, Dublin, Ireland, 4th-6th April 2006; R. A. McMahon, X. Wang, E. Abdi-Jalebi, P.S. Tavner, P. C. Roberts and M. Jagiela The BDFM as a Generator in Wind Turbines hit Conf. Power Electronics and Motion Control Conference (EPE-PEMC), Portoroz, Solvenia, 30th August -1st September 2006; P. J. Tavner, R. A. McMahon, P. C. Roberts, E. Abdi-Jalebi, X. Wang, M. Jagiela, T. Chick Rotor & Design Performance for a BDFM. In Proc. 17th Tnt. Conf. Electrical Machines (ICEM), 2nd-5th September 2006, Chania, Crete, Greece paper no. 439; and D. Feng, P. Roberts, R. McMahon Control Study on Starting of BDFM. In Proc. 41st International Universities Power Engineering Conference (UPEC) 2006, 6th-8th September 2006, Northumbria University, Newcastle upon Tyne, UK.

Further background information relating to brushless doubly-fed generators other than the BDFG can be found in: WO 2005/046044; WO 01/91279; WO 00/48295; Seman S et al, "PerfonTnance Study of a Doubly Fed Wind-Power Induction Generator under Network Disturbances", published 2005, IEEE; Basic D et al, "Transient Performance Study of a Brushless Doubly Fed Twin Stator Induction Generator", published 2003, IEEE; and Duro Basic et al, "Modelling and Steady-State Performance Analysis of a Brushless Doubly Fed Twin Stator Induction Generator". Further background information can be found in, "A short review of models for grid-connected doubly-fed variable speed wind turbines, M. Hokkanen, H. J. Salminen, T. Vekara.

A field orientated control technique for a doubly-fed induction machine is described in W003/026121. The Oregon State University worked on aspects of BDFG design and operation in the 1 980s and the inventors are aware of five patents which resulted from this work: US4994684, US5028804, US5083077, US5239251 and US5798631.

However despite this work there remained problems in producing a design suitable for commercial applications.

There therefore exists a need for further development of brushless doubly-fed machines.

SUMMARY OF THE INVENTION

According to the present invention there is therefore provided a brushless doubly fed machine (BDFM) for coupling to an ac mains power supply line to deliver power to said ac mains power supply line, the machine comprising an ac mains power supply line connection, a brushless doubly fed generator, a controller coupled to said ac mains power supply line connection, and a sensor coupled to said controller, said brushless doubly fed generator having a rotor, said sensor being configured to sense rotation of said rotor, a power stator winding to provide an ac power supply from the machine to said ac mains power supply line connection and a control stator winding driven by said controller, wherein said controller is configured to provide a variable frequency drive to said control stator winding, wherein said brushless doubly fed generator has a natural speed of rotation of said rotor at which said frequency of said drive is zero, and wherein said controller is configured to draw power from said ac mains power supply line to drive said control stator winding when a speed of said rotor is below said natural speed of rotation and to dissipate power via said control stator winding when said speed of said rotor is above said natural speed of rotation such that when said speed of said rotor is above said natural speed of rotation said machine dissipates excess energy to thereby enable a substantially unidirectional power flow from said ac mains power supply line towards said control stator winding by reducing the efficiency of said machine above said natural speed of rotation.

Previously attempts to produce brushless doubly-fed machines for wind turbines have focused on maximising efficiency. By contrast the inventors have recognised that substantial improvements may be made by deliberately making the machine less efficient for gusts of wind -which are experienced relatively infrequently -instead optimising the design for closer to average conditions. As is well known to those skilled in the art (and is outlined later) a brushless doubly-fed generator has a natural speed of rotation of the rotor and the inventors have recognised that improvements can be made by deliberately wasting or dissipating energy when the rotor speed is greater than this natural speed of rotation, more particularly by increasing a drive to the controls data winding. Therefore, instead of this control of winding generating power, as would otherwise be the case, a voltage supplied to inhibit this winding from generating power and, more particularly to force power into the control winding when the rotor speed is greater than its natural speed of rotation. In this way energy is drawn from the ac mains supply and is dissipated in the machine by the control and power windings. By increasing the voltage into the control winding to guarantee the current flows into this winding the drive circuitry for the control winding may be made unidirectional because substantially no power flows back from the control winding into the ac mains power supply. This in turn enables the drive circuitry to be simpler, cheaper to manufacture and more reliable and hence facilitates use of the machine in a commercially viable consumer product.

As the skilled person will understand, a brushless doubly-fed machine has a characteristic curve which relates speed of rotation of the rotor to voltage and frequency of the (three-phase) drive to the control winding. Thus in embodiments when the speed of rotation of the rotor is above its natural speed a voltage greater than defined by this characteristic curve is applied to the control winding, to thereby force current into the winding. The voltage applied may be defined by a lookup table relating speed of rotation (or equivalently, frequency) to the voltage to be applied. This information may be stored in memory or, for example, hard wired into an FPGA (field programmable gate array). In embodiments the applied voltage may be varied by varying a dc power supply voltage to an inverter driving the three-phase control winding. In some preferred embodiments the voltage applied to the control winding is limited to a maximum of fifty volts peak value which facilitates the use of inexpensive components, for example automotive grade MOSFETs.

The drive to the three-phase control winding may be a positive sequence drive or zero sequence currents may be employed, for example by adding a third, ninth or other triple harmonic component to the drive waveform for each of the windings.

To facilitate control of the machine preferably the sensor provides multiple pulses per revolution, for example at least 10 or 15 pulses per revolution; suitable sensors include an optical coder and a Hall sensor. In some preferred embodiments the frequency of the drive wavefonri for the control winding is adjusted by adjusting the phase of the waveform (stretching or compressing the sine wave data) using measured changes in the instantaneous speed of the rotor. This facilitates stability.

Increasing the volts per Hertz drive to the control winding at greater than the natural speed of rotation tends to increase the reactive power delivered by the generator. Thus preferred embodiments include a system to control or reduce this reactive power. This may comprise a circuit to sense the output phase angle and then to adjust the slope of the volts per Hertz controller, in particular to decrease the slope to export (reduce) reactive power. When the generator is delivering power at a lagging power factor (the grid looks inductive/resistive to the generator) -and when the control stator winding is over-excited -the generator looks like a capacitor. However in embodiments a capacitor may also be coupled across the output from the power stator winding, for example in the range 1-lOOpF, say 1O-2OpF, to help provide VARs when the machine is operating below natural speed.

In a related aspect the invention provides a method of operating a brushless doubly fed machine (BDFM), said brushless doubly fed machine comprising a brushless doubly fed generator having a rotor, a power stator winding and a control stator winding, said rotor having a natural speed of rotation, the method comprising reducing an efficiency of said BDFM above said natural speed of rotation of said rotor by driving power into said control stator winding above said natural speed of rotation of said rotor to increase losses in said machine.

Preferably the control winding is driven with a voltage greater than that defined by the characteristic of the machine, to force current into the control winding when the machine is operating above its natural speed of rotation.

In a further related aspect the invention provides a brushless doubly fed machine (BDFM) for coupling to an ac mains power supply line to deliver power to said ac mains power supply live, the brushless doubly fed machine comprising an ac mains power supply line connection, a brushless doubly fed generator, a controller coupled to said ac mains power supply line connection, and a sensor coupled to said controller, said brushless doubly fed generator having a rotor, said sensor being configured to sense notation of said rotor, a power stator winding to provide an ac power supply from the machine to said ac mains power supply line connection and a control stator winding driven by said controller, wherein said controller is configured to provide a variable frequency drive to said control stator winding, wherein said sensor is configured to sense rotation of said rotor by a fraction of a revolution to provide multiple indications of said speed of rotation of said rotor per revolution of said rotor, and wherein said controller is configured to vary said frequency of said drive to said control stator winding by varying a phase of a waveform of said drive in response to a signal from said sensor indicating fractional rotation of said rotor.

In the above described aspects and embodiments of the invention it is particularly preferable that the generator is a brushless doubly-fed induction generator (BDFG).

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which: Figure 1 shows a diagram of a brushless doubly-fed machine with a power stator winding directly connected to single-phase ac grid mains, a control stator winding driven from the ac mains supply via a frequency converter and with a brushless rotor; Figure 2 shows a schematic block diagram of a brushless doubly-fed machine (BDFM) according to an embodiment of the invention; Figure 3 shows a flow diagram of a procedure which, in embodiments, is implemented by the controller of figure 2; and Figures 4a and 4b show, respectively, a graph of control winding power input/output against rotor speed of rotation for a BDFM, and an example of a voltage-frequency curve stored in the lookup table of the controller of figure 2 illustrating excess drive of the control winding to dissipate power which would otherwise be output from the control winding.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

We will describe the application of a BDFM, in this case comprising brush less doubly fed induction generator (BDFG), to small-scale wind generation. The overall arrangement of BDFM 100 is shown in Figure 1: one of the two windings of the BDFG 102, the power winding, as it is intended for direct coimection to the mains 106, is reconfigured as a single phase winding as the majority of domestic installations are single phase. The control winding is fed from a small three-phase inverter 104. The rotor 108 of the BDFG maybe driven, via a gearbox (not shown) by a wind turbine.

In more detail, the BDFM has a single phase power winding connected directly to the mains, and this winding will be designed for the prevailing mains supply, for example 230 V, 50 Hz. The three-phase control winding is supplied by an inverter which can be a conventional six switch inverter comprising three half-bridges. The control winding can be wound for any convenient voltage and it is advantageous to use a voltage that can be derived from system using no more than a maximum of 50 V to simplify compliance with regulations. Note that there is electrical isolation between the power winding and the control winding and this insulation can be made good enough to ensure compliance with regulations. One possible means of achieving this is to apply a layer of insulation in the slots between windings.

The BDFM may be for example a 4-pole/8 pole machine with a natural speed of 500 rpm. For small turbines the maximum shaft speeds lie in the range 500 to 750 rpm so the BDFM is well matched to this range. Another alternative BDFM could be a 2pole/6pole machine with a natural speed of 750 rpm.

In general (using the nomenclature above) the relationship between the speed of rotation of the rotor shaft in rpm (revolutions per minute), N and the inverter frequency x (that is, the frequency of the drive to the control winding) is given by: 60.(50 + x) N= (1) p1 +p2 Where p1 and p2 are the numbers of pole pairs on the power and control windings respectively, the factor of 60 converts from revolutions per second to revolutions per minute and the constant 50 refers to the frequency of the mains supply (and can therefore be adjusted according to the particular country in which the machine is operating). At the natural speed of rotation, Nnat, x = 0 and thus 60.(50+ 0) Nnat (2) + The BDFM has the characteristic that the flow of power in the control winding is related to the deviation from the natural speed, and in an ideal BDFM the power in the control winding is proportional to the power in the power winding times the change in speed from the natural speed divided by the natural speed, i.e. the fractional deviation in speed. In generating mode, power flows into the control winding below natural speed and into the control winding above natural speed.

This is illustrated in figure 4a which shows an ideal power input/output of the control winding on the vertical axis against rotor speed (in rpm) on the horizontal axis. It can be seen that above the natural speed of rotation the control winding generates power.

In reality there are resistive losses and other losses in a BDFM and the effect is to make the machine more efficient above its natural speed. In addition, there are reactive powers in the system. Both magnetic fields require magnetizing current which is associated with reactive power, and there are VArs associated with the leakage reactances in the machine. Furthermore, changing the excitation on the control winding will change the flow of VArs through the machine in that increasing the control winding excitation will tend to lead to the generation of VArs on the power winding and the reduction of excitation will lead to the absorption of VArs.

Experience shows that there is a balanced' excitation which gives minimum current for a given amount of load torque and hence electrical power output. This is a preferable condition for achieving minimum resistive losses but the power factor of the power winding may not be acceptable. In this case there will be trade-off between losses and power factor.

The need to handle reactive power also affects the ratings of the control windings. Put simply, the greater the reactive power to be processed the lower the real power output of a given sized machine will be.

The fact that power flow in the control winding is hi-directional implies that the electronic converter associated with the control winding is bi-directional. This can be envisaged as a conventional three-phase inverter on the machine side linked by a dc connection to a full bridge converter on the mains side. However, this arrangement is complex and there also problems associated with returning this power to the mains. In addition, this arrangement does not offer galvanic isolation which may be required for safety reasons. A transformer can be used for isolation but a mains frequency transformer is a large and expensive component. Alternatively a switch mode configuration can be used, but this leads to further circuit complexity.

We therefore use an arrangement which avoids the need for a hi-directional converter.

The converter is unidirectional in that power is drawn from the mains and no power is returned to the mains. When the BDFM goes below its natural speed, i.e. when the turbine is turning slowly, the driving torque and hence power falls (turbine torque is proportional to wind speed cubed). Therefore the power flow from the power winding falls and even if the fractional speed deviation reaches say 50% the actual power required by the control winding will only be one eight of the nominal rating of the power winding. This power can easily be supplied by a low voltage inverter with a dc supply from an inexpensive consumer type computer power supply.

When the turbine and hence the BDFM goes above natural speed, the power output from the control winding becomes positive but by increasing the excitation increased losses will occur which will absorb this extra power. As a side effect the power winding power factor will move from lagging towards leading.

Although this strategy may seem wasteful, it provides an economical means of dealing with relatively infrequent strong gusts, allowing the machine to optimized for the more commonly occurring lower wind speeds. This approach leads overall to good exploitation of the available wind resource.

Referring now to figure 2, this shows a schematic diagTam of a brushless doubly-fed machine 200 according to an embodiment of the invention. The machine comprises a brushless doubly-fed induction generator 202 which has a three-phase control stator winding 204, and a three-phase power stator winding 206 coupled by a rotor 208. The control winding 204 has three terminals, u, v, w and P2 pole pairs; the power winding 206 has three terminals A, B, C and p' pole pairs. As illustrated the three terminals of the power winding are connected to provide a single phase output 210 for direct connection to ac grid mains (the arrangement shown is equivalent to the illustrated single phase winding 206'). In some countries, for example Germany, a three-phase mains supply is employed in which case an alternative configuration 206a of the power winding may be used.

A sensor 212 is coupled to rotor 208 to sense rotation of the rotor and hence its speed of rotation. Sensor 212 may comprise, for example, an optical or Haul effect sensor, preferably providing at least 16 pulses per revolution to facilitate small phase angle adjustments. This provides an input to controller 214 which, responsive to the determined instantaneous speed of rotation of the rotor, provides a control signal output for controlling the voltage and frequency of three-phase sinusoidal waveforms applied to terminals u, v and w of control windings 204. In embodiments the controller 214 is coupled to a lookup table 218 which stores values of voltage, and optionally frequency, for use by controller 214 as described in more detail below.

In the illustrated embodiment the control signals 216 are provided to a pulse width modulation module 220 which provides a set of three-phase sinusoidal waveforms for driving inverter 222 (via level shift circuitry, not shown in figure 2) and thence control winding terminals u, v, w. The PWM sinusoidal waveforms may, for example, represent the instantaneous amplitude of a sinusoidal waveform by an "on" pulse width. By varying the widths of the PWM pulses whilst maintaining their "sinusoidal distribution" the overall amplitude of the sinusoidal waveform may be varied (shorter pulses giving a lower "average" PWM voltage). Thus both voltage and frequency can be controlled.

Preferably the switching frequency is chosen to be above human audible range, for example greater than 18KHz.

The inverter module 222 receives dc power from a dc power supply 224, in turn powered by the ac mains supply to which the power winding is connected. The dc output voltage from this power supply on the dc Link line is preferably less than SO volts, for example 48 volts. It will be appreciated that the voltage of the three-phase waveforms driving the control winding may be varied either by varying the amplitude of the sinusoidal waveforms using the PWM controller 220 and/or by controlling the voltage on dc link line 226. Thus in embodiments there may be a voltage control connection from controller 214 to dc power supply 224 to enable the controller to control the dc link voltage and therefore the voltage applied to the control winding. In embodiments the PWM controller 220 may be configured to apply zero sequence currents to control winding 204, for example by adding some third harmonic to the waveforms driving the control winding.

As the skilled person will be aware, a range of different circuits may be employed for the PWM module 220, inverter 222, and dc power supply 224. The controller 214 and lookup table 218 may be implemented using a field programmable gate array (FPGA) or microcontroller.

Referring now to figure 3, this shows a flow diagram of a procedure which may be implemented by controller 214 either in software or in hardware or in a combination of the two to control the frequency of the drive waveforms applied to control winding 204.

In a simple embodiment the frequency of the control winding drive may be determined using equation (one) above where N is the sensed speed of rotation and x is the calculated frequency of the PWM drive. However, because a difference between the mechanical phase angle and magnetic phase angle can vary depending upon the load it is preferable to use a control loop which controls the phase of the applied waveform (and therefore, indirectly, its frequency) based upon the measured phase of the rotor.

Thus in the procedure of figure 3, at step S300 an initial drive frequency is set based upon the sensed speed of rotation but there after a phase control loop (steps S302-S308) is employed.

Thus at step S302 the procedure receives a signal from the rotation sensor indicating that a pulse has been detected and that the rotor shaft has therefore turned through a fraction, for example 1 over 16, of a full revolution. The instantaneous speed may be calculated based upon the time interval between successive pulses or in some more complex manner for example employing a moving average. At step S304 the procedure determines a change in the instantaneous speed of the rotor and then changes the phase of the drive waveform to compensate. Thus at step S306 the procedure reads fine wave data from a table and either time extends of time compresses this data to achieve a desired phase change, for example a phase change which over a complete revolution will provide a frequency change satisfying equation (one). If there is less than a threshold change in instantaneous speed there may be no time stretchlcompress applied to the drive waveform. Then at step S308 the sign wave data is provided to the PWM module which generates three waveforms 120° out of phase with one another to provide the three-phase drive or the control winding 204. The procedure then loops back to step S 302.

Referring now to figure 4b, this shows a graph of control winding drive voltage against control winding drive frequency implemented by controller 214 using lookup table 218.

The zero frequency point on the x-axis corresponds to the natural speed of rotation of rotor 208. Frequencies to the left of this on the graph ("negative") relate to a situation where the speed of rotation of rotor 208 is below its natural speed of rotation; frequencies to the right of this point ("positive") relate to a speed of rotation greater than the natural speed of rotation.

As can be seen, below the natural speed of rotation of the rotor there is a generally linear relationship between the frequency of the drive and the voltage applied to the drive; the slope of this (in volts per Hertz) is characteristic of a particular machine although there is a limit to the maximum volts per Hertz set by saturation of iron within the machine and the like. There is a minimum voltage Vmjn which is applied to the control winding at zero drive frequency (and therefore the curved parts from familiarity at this point). This is the voltage required for rated current flow through the control winding at dc; an example value might be 2.5 volts for a 0.5 winding with a 5 amp desired current. The slope of the curve is typically of order of magnitude 1 volt per Hertz.

Dashed line 400 shows a linear voltage -frequency relationship above the natural speed of rotation of the rotor, corresponding to power generation by the control winding as shown in figure 4a. However, in embodiments of the machine we describe the voltage -frequency curve 402 is arranged so that at drive frequencies greater than zero, that is rotor speeds above the natural speed of rotation, the voltage applied to the control winding is increased above that which would be defined by a linear voltage -frequency relationship, to thereby enable current to be forced into the control winding even at speeds above the natural speed of rotation of the rotor. This draws energy from the ac mains supply and dissipates it in the windings of the machine, effectively wasting energy. However the machine only briefly operates in this regime, during gusts of wind and therefore this waste of energy can be tolerated since it enables the inverter 222 (and power supply 224) to be unidirectional.

Thus an arrangement of this type is able to employ simpler, cheaper and more reliable electronic circuitry, which in turn has a substantial impact on the economic viability of wind powered electricity generation.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.

Claims (12)

1. CLAIMS: 1. A brushless doubly fed machine (BDFM) for coupling to an ac mains power supply line to deliver power to said ac mains power supply line, the machine comprising an ac mains power supply line connection, a brushless doubly fed generator, a controller coupled to said ac mains power supply line connection, and a sensor coupled to said controller, said brushless doubly fed generator having a rotor, said sensor being configured to sense rotation of said rotor, a power stator winding to provide an ac power supply from the machine to said ac mains power supply line connection and a control stator winding driven by said controller, wherein said controller is configured to provide a variable frequency drive to said control stator winding, wherein said brushless doubly fed generator has a natural speed of rotation of said rotor at which said frequency of said drive is zero, and wherein said controller is configured to draw power from said ac mains power supply line to drive said control stator winding when a speed of said rotor is below said natural speed of rotation and to dissipate power via said control stator winding when said speed of said rotor is above said natural speed of rotation such that when said speed of said rotor is above said natural speed of rotation said machine dissipates excess energy to thereby enable a substantially unidirectional power flow from said ac mains power supply line towards said control stator winding by reducing the efficiency of said machine above said natural speed of rotation.
2. 2. A brushless doubly fed machine as claimed in claim 1 wherein said machine has a characteristic curve relating speed of rotation of said rotor or said frequency of said drive to said control stator winding, to a voltage of said drive to said control stator winding, and wherein said controller is configured to drive said control stator winding with a voltage greater than that defined by said characteristic curve when said speed of rotation of said rotor is greater than said natural speed of rotation such that above said natural speed of rotation of said rotor power flows from said ac mains supply line into said control stator winding.
3. 3. A brushless doubly fed machine as claimed in claim 2 wherein said controller comprises a lookup table storing data defining a said voltage of said drive to apply to said control stator winding in response to sensed speed of rotation of said rotor or in response to said frequency of said drive to said control stator winding.
4. 4. A brushless doubly fed machine as claimed in claim 2 or 3 wherein said controller comprises a power supply unit coupled to said ac mains power supply line connection to provide an intermediate dc power supply, and an inverter coupled to said intermediate dc power supply to provide said drive to said control stator winding, and wherein said controller is configured to control said voltage of said drive to said control stator winding by controlling a voltage of said intermediate dc power supply subject to a maximum value of said voltage of substantially fifty volts.
5. 5. A brushless doubly fed machine as claimed in any preceding claim wherein said control stator winding is a three phase winding, and wherein said controller is configured to apply zero sequence currents to said three phase winding.
6. 6. A brushless doubly fed machine as claimed in any preceding claim wherein said sensor is configured to sense rotation of said rotor by a fraction of a revolution to provide multiple indications of said speed of rotation of said rotor per revolution of said rotor, and wherein said controller is configured to vary said frequency of said drive to said control stator winding by varying a phase of a waveform of said drive in response to a signal from said sensor indicating fractional rotation of said rotor.
7. 7. A brushless doubly fed machine as claimed in any preceding claim wherein said ac mains power supply line connection comprises a single phase connection, wherein said power stator winding comprises a three phase winding having the three phases connected to provide a single phase output to said ac mains power supply line connection, and wherein said machine further comprises a system to control or reduce reactive power at said ac mains power supply connection.
8. 8. A method of operating a brushless doubly fed machine (BDFM), said brushless doubly fed machine comprising a brushless doubly fed generator having a rotor, a power stator winding and a control stator winding, said rotor having a natural speed of rotation, the method comprising reducing an efficiency of said BDFM above said natural speed of rotation of said rotor by driving power into said control stator winding above said natural speed of rotation of said rotor to increase losses in said machine.
9. 9. A method as claimed in claim 8 wherein said driving of said power into said control stator winding comprises driving said control stator winding with a voltage greater than a voltage defined by a characteristic curve of said machine, a speed of rotation of said rotor and said natural speed of rotation of said rotor.
10. 10. A method as claimed in claim 9 further comprising compensating for increased reactive power from said power stator winding due to said driving of said control stator winding with an increased said voltage.
11. 11. A brushless doubly fed machine (BDFM) for coupling to an ac mains power supply line to deliver power to said ac mains power supply live, the brushless doubly fed machine comprising an ac mains power supply line connection, a brushless doubly fed generator, a controller coupled to said ac mains power supply line connection, and a sensor coupled to said controller, said brushless doubly fed generator having a rotor, said sensor being configured to sense notation of said rotor, a power stator winding to provide an ac power supply from the machine to said ac mains power supply line connection and a control stator winding driven by said controller, wherein said controller is configured to provide a variable frequency drive to said control stator winding, wherein said sensor is configured to sense rotation of said rotor by a fraction of a revolution to provide multiple indications of said speed of rotation of said rotor per revolution of said rotor, and wherein said controller is configured to vary said frequency of said drive to said control stator winding by varying a phase of a waveform of said drive in response to a signal from said sensor indicating fractional rotation of said rotor.
12. 12. A machine or method as recited in any preceding claim wherein said brushless doubly fed generator is a brushless doubly-fed induction generator (BDFG).