GB2460724A

GB2460724A - Torque-sensing control system for a brushless doubly fed machine (BDFM)

Info

Publication number: GB2460724A
Application number: GB0810867A
Authority: GB
Inventors: Ehsan Abdi Jalebi; Richard Anthony Mcmahon
Original assignee: WIND TECHNOLOGIES Ltd
Current assignee: WIND TECHNOLOGIES Ltd
Priority date: 2008-06-13
Filing date: 2008-06-13
Publication date: 2009-12-16
Anticipated expiration: 2028-06-13
Also published as: GB2460724B; GB0810867D0

Abstract

The machine comprises a brushless doubly fed motor or generator (BDFG), a controller coupled to an ac mains power supply line connection, and a rotor speed sensor coupled to said controller. The controller has inputs for a current signal and a voltage signal from the power and control stator windings, and from the rotation sensor. A first control module generates a torque demand signal from angular rotation speed data and information defining a target angular rotation speed of said rotor. A torque estimation module generates a torque estimate from current and voltage from said power and control stator windings, said estimate being dependent on a resistance torque being applied by said generator. A second control module receives said torque demand signal and said torque estimation signal and having a quadrature voltage control output. A voltage reference frame conversion module receives said quadrature voltage and l transforms a reference frame of said quadrature and direct voltage signals to a reference frame of said control stator winding. A control stator winding driver for providing a three phase drive to said control stator winding accordingly. This control system is said to improve open-loop stability. A power factor correction system is also discussed.

Description

POWER GENERATORS

FIELD OF THE INVENTION

This invention relates to power generators, more particularly to brushless doublyfed generators. The techniques we describe are particularly useful for generators in wind turbines, although applications are not limited to this field.

BACKGROU1\D 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.

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. Voltage and frequency conversion are possible using power electronics single phase inverter; the use of a controlled rectifier will giving better performance.

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 output then has to be electronically converted to a fixed voltage and frequency, which requires an expensive converter.

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 rotor 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. Nevertheless, the rating of the converter may be a modest fraction of the total power generated by the turbine.

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 normally 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 maj or 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, brushless doubly fed machines (BDFM5) 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 single frame BDFM has two stator windings of different pole numbers; alternatively a twin stator configuration can be used. 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 culTents 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. The techniques we describe later are applicable to all three types of BDFM, although they can be especially useful 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. lEE 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. TEE 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 Tnt. 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 technology for a brushless doubly-fed machine (bdfrn). In Tnt. Conf. Power Electronics, Machines and Drives, volume 1, pages 120- 125. TEE, 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 (Inducjp 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. mt. Conf. Power Electronics, Machines and Drives (PEMD), vol. 1, PP. 606-610, Clontarf Castle, Dublin, Ireland, 4th-Ã�th April 2006; R. A. McMahon, X. Wang, E. Abdi-Jalebi, P.J. Tavner, P. C. Roberts and M. Jagiela The BDFM as a Generator in Wind Turbines Tnt. 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-Sth 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, "Performance 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 W003/026 121. 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.

We have described in another UK Patent application, filed on the same day as this application by the same Applicants (hereby incorporated by reference) a technique for deliberately dissipating power in the control stator winding of a BDFM by over-exciting this winding above the natural speed of the machine, which provides unexpected advantages, particularly in small scale or micro-wind applications.

There is, however, a further problem to address, which is that a BDFM is typically not open-loop stable under all conditions. In practice this can be observed as increasingly noisy operation followed by sudden loss of synchronisation. There is a need for improved control schemes for brushless doubly fed machines.

SUMMARY OF THE INVENTION

According to the first aspect of the 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 motor or 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 ac mains power supply comprises a three phase power supply, and wherein said controller comprises: a first input to receive a current signal and a voltage signal from said power stator winding; a second input to receive a current signal and a voltage signal from said control stator winding; a third input to receive a signal from said rotation sensor to provide angular position data and angular rotation speed data from said rotor; a first control module coupled to said third input and configured to generate a torque demand signal from said angular rotation speed data and information defining a target angular rotation speed of said rotor, and having a torque demand signal output; a torque estimation module coupled to said first and second inputs and configured to generate a torque estimation signal from said current and voltage signals from said power stator winding and said sensed current and voltage signals from said control stator winding, said torque estimation signal being dependent on a resistance torque being applied by said motor or generator, said torque estimator module having a torque estimation signal output; a second control module coupled to said torque demand signal output of said first control module and to said torque estimation signal output of said torque estimation module, and having a quadrature voltage control output to provide a quadrature voltage control signal; a direct voltage control signal source having a direct voltage control output; a voltage reference frame conversion module coupled to said quadrature voltage control output, to said direct voltage control output, and to said third input, to convert a reference frame of said quadrature voltage control signal and said direct voltage control signal to output quadrature and direct voltage signals in a reference frame of said control stator winding, and having an output; and a control stator winding driver coupled to said control stator winding and to said output of said voltage reference frame conversion module to convert said quadrature and direct voltage signals in said reference frame of said control stator winding to a three phase voltage control signal for providing a three phase drive to said control stator winding in accordance with said three phase voltage signal.

Embodiments of the above-described controller are able to provide improved stability and/or greater operating efficiency. The first and second inputs may be separate or shared; a skilled person will understand there are many ways of determining or sensing voltages and currents on the power and control stator windings. In embodiments the rotation sensor may comprise a shaft encoder providing multiple pulses per revolution, for example an optical encoder or a Hall sensor. The skilled person will understand that from such a sensor it is possible to derive both angular position and angular rotation speed data.

In some preferred embodiments the voltage reference frame conversion module is configured to use the angular position data to convert the direct and quadrature voltage control signals from a reference frame of the power stator winding to a reference frame of the rotor and then from the reference frame of the rotor to the reference frame of the control stator winding.

In some preferred embodiments a further, power factor control ioop is provided, the torque control loop controlling the quadrature axis voltage and the power factor control loop controlling the direct axis voltage component. The power factor control loop may either employ a fixed reference power factor or may have a power factor input; the latter is particularly useful in medium and large scale machines where controlling reactive power delivered to the grid is important. In embodiments the power factor/reactive power control loop responds to the sensed current and voltage on the power stator winding and controls the direct voltage axis component responsive to a comparison of the reactive power component on the power stator winding and a reference reactive power, which may be of fixed value or derived from a referenced power factor.

In some particularly preferred embodiments the reactive power comparison takes into account reactive power supplied by the line converter, that is by a converter supplying real power to and/or delivering real power from the control stator winding (depending upon the implementation and whether the rotor is low or above its natural speed of rotation). In still further embodiments one or more sets of capacitors, for example in a delta or star configuration, may be coupled across the power stator winding to provide further reactive power, and the power factor/reactive control loop may also take into account the effect of these capacitors (in embodiments the reactive due to the effect of the capacitors and/or from the line converter may be combined with the determined reactive power from the power stator winding and the combination used in the control loop to control to a reference reactive power/power factor).

Thus in a 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 line, the machine comprising an ac mains power supply line connection, a brushless doubly fed motor or generator, and a controller coupled to said ac mains power supply line connection, said brushless doubly fed generator having a 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, said controller having a converter coupled to said ac mains power supply line, and wherein said controller is configured to control a power factor of said machine when coupled to said ac mains power supply line by controlling a component of reactive power supplied to said ac mains power supply line by said converter.

Preferably the controller includes a reactive power control module to implement a control ioop as described above, responsive to a combination of reactive power contributions from the power stator winding and from the converter coupled to the control stator winding. (Optionally any reactive power contribution due to the capacitors which may be connected to parallel to the machine).

In embodiments the converter includes an inverter coupled between said ac mains power supply line and a dc link within the converter, and the reactive power supplied to the AC mains power supply line may be controlled by controlling one or both of the effective amplitude and phase of a set of three-phase pulse width modulation signals driving said inverter (effectively the amplitude and/or phase of signals defined by the PWJVI signals). The skilled person will appreciate that reactive power may be supplied to the AC mains inespective of whether real power is being input from or output to the AC mains.

Particularly for medium to large scale machines it is desirable to be able to meet a desired target power factor for the grid, for example, say, 0.9. The need for this is greater as more power is delivered to the grid mains (and thus the control technique could even be power dependent, employing greater reactive power control at higher overall output powers). With embodiments of the technique described reactive power may be generated either using the power stator winding or using the (line) converter coupled to the control stator winding, or both. This relieves the burden from the power stator winding of delivering of all the reactive power. Further the ability to supply reactive power from the line converter provides a further degree of freedom which can be employed to reduce losses/increase efficiency of the machine, either at the design stage or during operation.

In a further related aspect the invention therefore provides a method of controlling a power factor of 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 mmotor or generator, and a controller coupled to said ac mains power supply line connection, said brushless doubly fed generator having a 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 a converter in said controller coupled to said ac mains power supply line, the method comprising controlling a component of reactive power supplied to said ac mains power supply line by controlling said converter coupled to said ac mains power supply line.

The invention still further provides a controller for a brushless doubly fed machine, the controller being as described above in any of the aspects and/or embodiments of the invention.

A brushless doubly fed machine as described above may be combined with a source of mechanical renewable energy such as a wind turbine, and in such an arrangement a target speed of rotation of the rotor may be derived from the renewable energy source, for example to maximise efficiency of the source. Preferred applications of the machine, include but are not limited to, combination with a wind turbine. In some preferred embodiments the generator is a brushless doubly-fed induction generator (BDFG).

BRIEF DESCRIPTION OF THE DRAWThGS

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: Figures 1 a to 1 c show, respectively, a brushless doubly-fed machine according to an embodiment of the invention, a graph of control winding power input/output against rotor speed of rotation for the BDFM, and an overall general arrangement of a BDFM; Figure 2 shows a more detailed schematic diagram of parts of the brushless doubly-fed machine of figure 1 a with a unidirectional converter; Figure 3 shows a second embodiment of a brushless doubly-fed machine according to the invention, employing a bi-directional converter.

Figure 4 shows an example of a controller for use with the BDFM of Figure 3; Figure 5 shows a block diagram of a control system of the controller of Figure 4; Figure 6 shows a block diagram of a reference frame conversion module for the controller of Figure 4; and Figure 7 shows a schematic block diagram of a system for reactive power control in a BDFM in which reactive power supplied by the line converter is employed to supplement the overall reactive power supplied to the grid, according to an embodiment of a second aspect of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Figure 1 a shows an outline block diagram of an embodiment of a brushless doubly fed machine (BDFM) 100 according to the invention. The BDFM has two stator windings, a power stator winding (Si) which in embodiments is intended for direct connection to three-phase grid mains 102, and a control stator winding (S2) which is connected to mains 102 via a (power) converter 104. We will described embodiments with a 3-phase power stator winding and a 3-phase control stator winding; the converter may be unidirectional (from the mains to the control winding) or bi-directional. An example converter is shown inset, comprising a pair of series coupled AC-DC converters operating at a variable frequency on the machine side (to enable frequency control on this side) and at a fixed frequency on the line side (locked to the grid). The converter may, for example, comprise a 3-phase inverter on the machine side, such as a six switch inverter comprising three half-bridges, linked by a DC connection to a full bridge converter on the mains side; alternatively a switch mode configuration may be employed; a transformer may be employed for isolation, if required.

The BDFM has a rotor 106 which may be driven via a gearbox (not shown) by a wind turbine. The BDFM may be, for example, a 4-pole/8-pole machine with a natural speed of 500 rpm or a 2-pole/6-pole 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 & 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) 1 + 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 out of the control winding above natural speed.

This is illustrated in figure lb 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.

Referring now to figure 2, this shows a more detailed schematic diagram of a brushless doubly-fed machine 100 with a unidirectional converter. 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 p1 pole pairs) for connection to a three-phase mains supply.

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 Hall effect sensor, preferably providing at least 16 pulses per revolution to facilitate small phase angle adjustments. This provides an input to controller 214. Further inputs to the controller are provided by a sensed current and voltage (Ii, Vi) of one of the three phases of the power stator winding and a sensed current and voltage (2, V2) of one of the three phases of the control stator winding (in each case only one phased need to be sensed, assuming the three phases are balanced). The skilled person will be aware of many techniques which may be employed for the sensing ofT1, 2, Vi, and V2. The controller 214, responsive to the determined instantaneous position & speed of rotation of the rotor, and I, 2, V1 and V2, 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 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 PWIV1 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 amptitude of the sinusoidal waveform may be varied (shorter pulses giving a lower on "average" PWIVI voltage). Thus both voltage and frequency can be controlled.

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. 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.

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 (described in more detail below) may be implemented using a field progranmiable gate array (FPGA) or microcontroller.

Figure 3 shows a second embodiment of a brushless doubly-fed machine according to the invention, employing a bi-directional converter.

The converter 104 comprises a pair of AC-DC converters, 1 04a,b, in the illustrated example each comprising a three-phase 6-switch inverter with three half-bridges; these are coupled by a DC link. The switches may be thyristors, as illustrated, or preferably IGBTs. A second control unit 300 provides PWM signals to control inverter 104a which provides power to the grid mains; this has an input (Qre) used to control the reactive power supplied to the grid, for example by controlling the phase shift of the PWM signals relative to the three phase mains voltage.

Referring now to Figure 4, this shows details of the controller (Control Unit I) 214 of Figure 3. The controller has two main modules, a first, control module 402 (Block A) and a second, reference frame conversion module 404 (Block B). In embodiments to the control module 402 receives current and voltage sense inputs from each of the power and control stator windings as well as data indicating the angular rotation speed of the rotor (w1). Control module 402 also receives a target angular speed input (Wref), and optionally a target power factor input (pfef), and further optionally a signal (Q*lef) indicating the reactive power supplied by the control unit 300 to the grid mains, optionally taking into account any further reactive power supplied by capacitors connected across the machine. The Control Module 402 provides direct and quadrature voltage outputs VS12q and Vsl 2d (where S1 denotes the Stator 1 flux reference frame), which together define a set of three phase voltages V2a, V2b, V2 to be applied to the control stator winding. The reference frame conversion 404 receives the direct quadrature voltage signals from the control module 402, and also an angular position measurement signal for the rotor, which it uses to convert the quadrature and direct voltage signals to three phase voltages.

It is helpful at this point to explain a little further about direct and quadrature voltages: because of redundancy in a three phase representation, two sets of voltages can be employed to represent the desired three phases. A conversion from the three phases to direct and quadrature voltages is known as a Park-Clark transformation; a conversion from direct and quadrature to three phase voltages performed by an inverse of that transformation, and this requires angular position information. In a d-q, model the quadrature component is related to the magnetic flux and hence is proportional to the torque, in effect the resisting or breaking torque being applied by the generator. (This can be understood by considering the effect on the torque of increasing or decreasing the magnetic field). The direct voltage component controls the power factor.

In the controller at Figure 4 the direct and quadrature voltages are in the reference frame of the power stator winding (Si) flux and thus a transformation is applied to convert to the reference frame of the control stator winding. This is explained in more detail below.

To illustrate this, consider the example of a 4-pole, 8-pole machine with a 25 Hz current in to the rotor bars. The 4-pole field produced by the rotor rotates at 750 rpm relative to the rotor and the 8-pole field performed by the rota rotates at 375 rpm relative to the rotor (in the opposite direction). When the machine is in synchronisation the 4-pole field synclironises with one stator winding, for example the power winding and the 8-pole synchronises with the other winding, for example the control stator winding. Thus if, say, the rotor is rotating at 750 rpm then the 4-pole field rotates at 1500 rpm and synchronises with the power stator winding and the 8-pole field rotates at 375 rpm and synchronises with the control stator winding. Therefore the 4-pole stator winding frequency is 50Hz and the 8-pole stator winding frequency is 25Hz.

Referring now to Figure 5, this shows details of the Control Module 402 on Figure 4.

The Control Module 402 has first and second inputs, 502, 504 to receive voltage and culTent sense signals from the power and control stator windings respectively, and these provide the inputs to an equivalent circuit model 506 for the generator, which has an output 508 comprising a signal Test estimating the torque in the generator. For an example and a detailed description of a suitable equivalent circuit model reference may be made to P.C. Roberts, R.A. McMahon, P.J. Tavner, J.M. Maciejjowski, and T. J. flack; "Equivalent circuit for the brushless doubly fed machine (BDFM) including parameter estimation and experimental verification", Electrical Power Applications, lEE Proceedings, 152(40):933-942, July 2005. Alternatively but less preferably, a torque transducer may be employed to sense the torque at the shaft of the generator.

The Control Module 402 also has a third input 510 from an angular position/speed sensor to provide measured angular rotation date (w1). The module may also receive a reference or target angular rotation input ((.ref) and a differencer 512 determines the difference between the measured and target rotation speeds. This information is provided to a first PD (proportional integral derivative) Control Unit 514, which provides an output 516 bearing a signal which defines a target or a reference torque (Tref) for the generator. The estimated or measured torque and the target or reference torque are differenced in differencer 518 and provided to a further PD Control Unit 520 which provides a quadrature voltage output 522 for the control stator winding, to apply a torque to bring the rotor speed towards the target angular rotation speed.

The skilled person will understand that the PID units described above (and below) may be replaced by other control mechanisms, of which many types are available. For example the derivative term may be omitted; in other embodiments fuzzy logic control may be employed.

The above described components of the Control Module 402 provide a rotor speed control system 500. Optionally a power factor or reactive power control system 501 may also be included in Control Module 402. In embodiments the power factor control system is coupled to the first input 502 to receive current and voltage sense signals from the power stator winding and these are input to a module 524 which calculates real power P1 and reactive power Qi of the power stator winding. These may be determined as shown by the equations below (where is the phase angle between the voltage and current, assuming balanced three phase sets): = lvii I' I cos Q 1V11 Iii isin pf1 = cos Ã�b Optionally power factor control system 501 also has a target power factor input (pfi.ef), and this together with P1 is used by module 526 to determine a target reactive power Qref. For example using the following equation: Qref = Pi tan (cos' Pef) A differencer is used to compare the reactive power from the power stator winding with the target reactive power Qief and this provides a signal to a further PID unit 530, which in turn provides a direct axis voltage output V2dsl, this defining the control stator winding voltage for the desired reactive power.

Optionally as illustrated in Figure 5, differencer 528 may have a further input 532 (Q*ref) which defines a reactive power contribution supplied by a converter 104 to the grid, optionally also including any reactive power supplied by capacitors connected across the output of the machine. This reactive power component may be subtracted from the target reactive power, together with the reactive power supplied from the power stator winding in a machine in which the converter is employed to supplement the overall reactive power supplied to the grid mains. This is described in more detail later.

Refening now to Figure 6, this contains details of the reference frame conversion module 404 of the controller at Figure 4. The converter module 404 receives quadrature 522 and direct 534 voltage inputs from Control Module 402, and also sensed angular position data (Or) for the rotor. As illustrated, when used in the reference frame transformation the rotor position is multiplied by the number of pole pairs of the power and control stator windings. The reference frame conversion is implemented by two reference frame transformation units 600, 602, a first to transform from the reference frame of the power stator winding flux to the reference frame of the rotor (still using the d-q representation), and a second to transfonn from the reference frame of the rotor to the reference frame of the control stator winding flux. A Park-Clark transformation unit 604 is then employed to convert the d-q representation to a three -phase voltage representation V2a, V2b, V2 which is used to control PWM signals applied to converter 104 to drive the control stator winding.

Refening now to Figure 7, this shows a simplified block diagram of an embodiment of a brushless doubly-fed machine 700 in which the converter is used to supply reactive power to the grid mains. In Figure 7 like elements to those previously described are indicated by like reference numerals. Using the converter 104 to supply reactive power eases the requirement on reactive power on the power stator winding and helps efficiency.

The converter 104 at Figure 7 is a bi-directional converter with a real power, P3 which varies with speed of rotation as illustrated in Figure lb. We use Q3 to represent the reactive power component of the converter, and cos to represent the power factor of the converter. We can then define a net reactive power and net real power according to the equations below: Qnet = Qi + Q + optional Qcaps Pnet = Pl + P3 where Qcaps comprises reactive power supplied by capacitors connected across the machine, as schematically illustrated. A controller 702 may then be employed to control the reactive power supplied by the converter using the net reactive power Qnet, Qi and a target reactive power, Qtaiget. The reactive power controller 702 provides a reactive power control signal to converter 104 to control the reactive power delivered to the grid mains.

Refening again to Figure 3, if the grid voltage is Vi and the (three phase) voltage defined by the PWT\4 signal from Control Unit II 300 is V2, the effective reactance between inverter 1 04a and the grid is X (optionally this may be increased with the addition of series inductance), and the phase angle between V1 and V2 is, then the real power supplied from or delivered to the grid is given by, per phase: 17 V sin 8 x The reactive power, per phase, is: Q = V (V1 -V cos 6) =Q. (target) No doubt many other effective alternatives will occur to the skilled person. For example corresponding techniques could be used to control speed and/or torque and/or reactive power supply of a BDFM configured as a motor. It will be understood that the invention is not limited to the described embodiments but encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.

Claims

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 motor or generator, a controller coupled to said ac mains power supply line connection, and a sensor coupled to said controller, said brushless doubly fed motor or 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 ac mains power supply comprises a three phase power supply, and wherein said controller comprises: a first input to receive a current signal and a voltage signal from said power stator winding; a second input to receive a current signal and a voltage signal from said control stator winding; a third input to receive a signal from said rotation sensor to provide angular position data and angular rotation speed data from said rotor; a first control module coupled to said third input and configured to generate a torque demand signal from said angular rotation speed data and information defining a target angular rotation speed of said rotor, and having a torque demand signal output; a torque estimation module coupled to said first and second inputs and configured to generate a torque estimation signal from said current and voltage signals from said power stator winding and said sensed current and voltage signals from said control stator winding, said torque estimation signal being dependent on a resistance torque being applied by said motor or generator, said torque estimator module having a torque estimation signal output; a second control module coupled to said torque demand signal output of said first control module and to said torque estimation signal output of said torque estimation module, and having a quadrature voltage control output to provide a quadrature voltage control signal; a direct voltage control signal source having a direct voltage control output; a voltage reference frame conversion module coupled to said quadrature voltage control output, to said direct voltage control output, and to said third input, to convert a reference frame of said quadrature voltage control signal and said direct voltage control signal to output quadrature and direct voltage signals in a reference frame of said control stator winding, and having an output; and a control stator winding driver coupled to said control stator winding and to said output of said voltage reference frame conversion module to convert said quadrature and direct voltage signals in said reference frame of said control stator winding to a three phase voltage control signal for providing a three phase drive to said control stator winding in accordance with said three phase voltage signal.
2. A brushless doubly fed machine (BDFM) as claimed in claim 1 wherein said voltage reference frame conversion module is configured to use said angular position data to convert said quadrature voltage control signal and said direct voltage control signal to a reference frame of said rotor and thence to said reference frame of said control stator winding.
3. A brushless doubly fed machine (BDFM) as claimed in claim 1 or 2 wherein said direct voltage control signal source comprises a third control module coupled to said first input to control said direct voltage control signal to control a component of reactive power supplied to said ac mains power supply by said power stator winding.
4. A brushless doubly fed machine (BDFM) as claimed in claim 3 wherein said direct voltage control signal source includes a power factor input to receive a target power factor signal, and wherein said control module is configured to control said direct voltage control signal responsive to a difference between a reactive power component defined by said target power factor signal and a reactive power component determined from said current signal and said voltage signal from said power stator winding.
5. A brushless doubly fed machine (BDFM) as claimed in claim 3 or 4 wherein said control stator winding driver includes a converter to drive or receive power to or from said control stator winding, wherein said third control module is coupled to said converter to receive converter reactive power data defining a component of reactive power supplied to said ac mains power supply by said converter, and wherein said third control module is configured to control said reactive power supplied to said ac mains power supply responsive to said component of reactive power supplied to said ac mains power supply by said converter.
6. A brushless doubly fed machine (BDFM) as claimed in claim 5 further comprising one or more capacitors or sets of capacitors connected across said machine, and wherein said third control module is further configured to control said reactive power supplied to said ac mains power supply responsive to a reactive component of power provided by said one or more capacitors or sets of capacitors.
7. 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 motor or generator, and a controller coupled to said ac mains power supply line connection, said brushless doubly fed generator having a 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, said controller having a converter coupled to said ac mains power supply line, and wherein said controller is configured to control a power factor of said machine when coupled to said ac mains power supply line by controlling a component of reactive power supplied to said ac mains power supply line by said converter.
8. A brushless doubly fed machine (BDFM) as claimed in claim 7 wherein said controller includes a reactive power control module having a first input to receive reactive power data from said converter and a second input to receive reactive power data from said power stator winding and an output coupled to said converter to control said component of reactive power supplied to said ac mains power supply line in response to a combination of reactive power contributions from said converter and from said power stator winding.
9. A brushless doubly fed machine (BDFM) as claimed in claim 8 further comprising one or more capacitors or sets of capacitors connected across said machine, and wherein said combination of reactive power contributions from said converter and from said power stator winding further comprises a reactive power contribution from said one or more capacitors or sets of capacitors.
10. A brushless doubly fed machine (BDFM) as claimed in claim 7, 8 or 9 wherein said converter includes an inverter coupled between said ac mains power supply line and a dc link within said converter, and wherein said controller is configured to control said component of reactive power supplied to said ac mains power supply line by said converter by controlling one or both of an amplitude and phase defined by a set of pulse width modulation signals driving said inverter.
11. A method of controlling a power factor of 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, and a controller coupled to said ac mains power supply line connection, said brushless doubly fed motor or generator having a 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 a converter in said controller coupled to said ac mains power supply line, the method comprising controlling a component of reactive power supplied to said ac mains power supply line by controlling said converter coupled to said ac mains power supply line.
12. A controller for a brushless doubly fed machine (BDFM) the controller being as defined in any one of claims 1 to 10.