GB2489411A

GB2489411A - Control of a single phase brushless doubly fed generator

Info

Publication number: GB2489411A
Application number: GB1104988.9A
Authority: GB
Inventors: Thomas George Logan; Teng Long
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-03-25
Filing date: 2011-03-25
Publication date: 2012-10-03
Also published as: GB201104988D0

Abstract

A brushless doubly fed machine (BDFM) comprising a brushless doubly fed generator (BDFG) 21 and a controller 28 and a single phase ac mains power supply line connection 24. The brushless doubly fed generator comprises a power winding 232 and a control winding 23. A frequency converter 26 is provided having an input connected to the single phase mains supply and a three phase output 27 connected to the control winding of the generator. The controller controls the waveform of the three phase output voltage of the frequency converter. Sensors 21, 29 and 220 are provided to provide the controller with information about the rotor position and/or speed, and the voltages and currents flowing in the power and control windings. The voltage waveform of the output of the frequency converter contains components at two distinct frequencies, a forward control frequency component, consistent with a poly-phase BDFM excited with forwards sequence voltage, and a backwards control frequency component consistent with the same BDFM excited with backwards sequence voltages. The generator may be used for a wind turbine or hydro-electric plant. The BDFM may be used as a motor.

Description

I

FIELD OF THE INVENTION

This invention relates to power generators, more particularly to brushless double fed generators.

The techniques we describe are particularly useful for generators in wind turbines, although

applications are not limited to this field.

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. For the agricultural and light industrial markets the size may be larger, up to 10 to 15 kW maximum. 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, "feed in tariffs".

A generator is an essential part of the wind turbine. For wind turbines of modest rating, up to say 2kW, 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 payback time or which never generates enough energy to offset its initial cost. 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, altemative 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.

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 1000 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.

Tn 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 (2p 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).

A number of papers have been published 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. 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 Tnt. Conf. Power Electronics, Machines and Drives, pages 341-346. TEE, 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. (P5C2003), Iran, volume 5, pages 1-9, October 2003; P. C. Roberts, E. Abdi-Jalebi, R. A. McMahon, and I J. Flack.

Real-time rotor bar current measurements using bluetooth technology for a brushless doubly-fed machine (bdfm).

In mt. Conf. Power Electronics, Machines and Drives, volume 1, pages 120-125. lEE, March 2004; V 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; V 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, V 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. 606-6 10, Clontarf Castle, Dublin, Ireland, 4th-6th 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 mt. Conf. Electrical Machines (ICEM), 2nd-Sth September 2006, Chania, Crete, Greece paper no. 439; and D. Feng, R 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 I 980s and the inventors are aware of five patents which resulted from this work: US4994684, U55028804, U55083077, U5523925 1 and U5579863 1. However despite this work there remained problems in producing a design suitable for commercial applications.

The patents GB2460723 and GB2460724 describe more recent inventions related to the BDFM by Wind Technologies Limited, a UK company. GB2460723 describes a method of operation by which the control winding of the BDFM is overexcited by applying a higher voltage to the control winding of the machine than that defined by the speed of rotation of the rotor, which reduces the efficiency of the machine but allows the converter connected to the control winding to have a lower rating, and preferably be unidirectional, resulting in a lower system cost. GB2460724 describes a method by which the BDFM may be controlled so as to achieve stable operation, in which the control scheme uses the terminal voltages and currents of the BDFM to estimate its torque, and this torque estimate is used to control the voltage applied to the control winding so as to achieve a demanded rotor speed.

This document describes improvements to BDFMs which are operated with their power windings connected to supplies which are either single-phase or unbalanced. To aid understanding of the descriptions of these improvements it is useful to explain some background details of the workings of this machine. The structure and operation of such BDFMs is most easily understood by considering the steps of the evolution of such a machine. First consider a single-phase induction motor, with a wound rotor and slip rings. The rotor winding may be fed from a VVVF (Variable Voltage Variable Frequency) converter and, much like an ordinary DFIG, with appropriate voltages and frequencies applied synchronous-machine-like performance can be achieved over a range of speeds. Next it is desirable to eliminate the brushes. For the synchronous generator this may be achieved using a brushless exciter and rotating rectifiers to give the dc rotor current. For this machine an ac control voltage is required, which may be obtained from the rotor windings of another control' induction machine mounted on the same shaft as the main (power') machine, providing we apply the right voltages to its stator windings. (Such a machine would be termed a Single-Phase Cascaded Doubly Fed Machine.) However, having two machines in the same case results in a physically large machine. Some significant improvement can be made by arranging the two machines not adjacent to each other on the shaft but concentrically, because among other things the bulk of the extra set of end-windings is removed. A machine of this structure is a single-phase BDFG.

Obviously the windings of the two machines would ordinarily couple together by transformer action if they were both of the same pole-number, but if the two machines are chosen to have different pole-numbers this coupling is avoided. It can be shown that by containing the fields of both machines in the same core, less iron is needed than for the case of the two separate machine on the same shaft. Finally, rather than having two separate concentric rotor windings, other windings have been found for the BDFG which can couple both fields simultaneously and so allow the resistance seen by currents flowing through the rotor to be reduced and the machine rating increased. Most notable is the nested-loop' structure described extensively in the literature.

Despite this work the BDFMs produced to date have an inherently low efficiency when operated from a single phase supply because, when controlled using any of the methods previously described in the literature, an additional uncontrolled component of current flows in the control and rotor windings of the machine at a frequency different to that which arises in BDFMs which are operated with a balanced voltage applied to the power winding. The techniques described in this document improve the efficiency and/or power rating and/or dynamic performance of the machine by controlling this additional current.

SUMMARY OF THE INVENTION

According to the present invention there is a brushless doubly fed machine (BDFM) connected to a single-phase ac mains power supply line to deliver power into said single-phase ac mains power supply line, wherein the brushless doubly fed machine comprises of a brushless doubly fed generator, a controller and an ac mains power supply line connection, said brushless doubly fed generator having a stator and a rotor, said stator having a single-phase or poly-phase power winding and a poly-phase control winding, and said rotor having a sensor, said sensor being configured to sense rotation of said rotor, and said power winding of said brushless doubly fed generator being connected to said single-phase ac mains power supply line, said control winding of said brushless doubly fed generator being connected to the output of a power electronic frequency converter drive, the input of said drive being connected to said single-phase ac mains power supply line, and the output of said drive controlled by said controller, wherein said controller is configured to command said drive to apply a control voltage to said control winding, the waveform of said control voltage containing substantial components at two distinct frequencies, the former of said frequencies (henceforth referred to as the "forwards control frequency") being substantially given by f=p+q)N-w (1) (where p. q, N, w and fare half the number of poles of said power winding, half the number of poles of said control winding, the speed of said rotor (in Hertz), said ac mains power power supply line frequency (in Hertz), and said forwards control frequency (in Hertz), respectively, all frequency terms being measured in the same angular direction), and the later of said frequencies (henceforth referred to as the "backwards control frequency") being substantially given by b = (p+q)r -w (2) (where b is said backwards control frequency (in Hertz)).

In certain implementations the rotor position input to the controller may come not from a physical sensor but from an estimate of the rotor position from the voltages and currents at the terminals of the machine.

A person skilled in the art will be aware that all of the controllers used in previous attempts to produce brushless doubly fed machines for wind turbines control the machine by providing (via several strategies, for example "vector control") a control voltage containing substantially a single frequency at the afore mentioned forward control frequency, as this is the only frequency appropriate for controlling brushless doubly fed machines that have their power windings connected to balanced poly-phase ac mains power supply lines (in practice the control voltage will normally contain additional components at high frequency (for example at 10 to 20 kHz) due to the switching pattem (such as PWM) used to control the output of the converter, however in this discussion concems the power frequency voltage components of the output of the converter, at frequencies typically less that 250Hz). However the inventors have recognised that additional control over brushless doubly fed machines which have their power winding connected to single-phase ac mains power supply lines may be obtained by including a component at the backwards control frequency in the control voltage output of the drive.

It is well known to persons skilled in the art that the oscillating flux produced by a single phase winding may be considered as two sequence components, one rotating in a forwards direction and one in a backwards direction. In a brushless doubly fed generator with its power winding connected to a single-phase ac mains power supply line the flux produced by the power winding has significant forwards and backwards sequence components, and in the machine the forward control frequency component of the control voltage applied to the control winding controls the forward sequence component of current in the power winding and the backward control frequency component of the control voltage applied to the control winding controls the backward sequence component of current in the power winding. Thus in previous attempts to control the brushless doubly fed machine using drives with outputs containing only a component at the forward control frequency only the forwards sequence component of the power winding current (and the current at the forwards control frequency in the control winding) could be controlled, the backwards sequence component being uncontrolled, where as by employing a drive outputting components at both frequencies to the control winding both components of the power winding current can be controlled.

This gives better control over the machine than in previous systems.

The afore mentioned additional control may be used chiefly in two ways. Firstly skilled persons will see that as there is a backwards sequence component of the current in the power winding and some current of the backwards control frequency in the control winding, and that by using the backwards control frequency component of the output of the drive these currents may be controlled, and the controller may be configured so as to make these current components smaller in magnitude than in the case where the drive output contains no component at the backwards control frequency.

Reducing the magnitude of said currents reduces the power dissipated in the windings and improves the machine efficiency and/or power handling capacity and/or torque (depending on the control strategy chosen).

Secondly, skilled persons will see that the torque of the machine and the power flow in both the power and control windings are controlled in part by the forward control frequency component of the output of the drive and in part by the backward control frequency component of the output of the drive. Depending on the rotor speed and the desired torque of the machine it is possible for the forward control frequency component of the output of the drive to be either emitting or absorbing power. Tn many situations it is also possible to control the backwards control frequency component of the drive to be either emitting or absorbing power as desired, with power being exchanged with the power winding and/or the mechanical system connected to the rotor. Using this aspect of the invention it is then possible to control the machine such that all or part of the power requirement from the drive for the forward control frequency component of its output may be taken from the backwards control frequency component of its output (or vice-versa). By using such a strategy the amount of power that the drive has to exchange with the ac mains power supply line may be reduced, allowing a drive of lower power rating to be employed, potentially reducing the cost of the system and increasing its commercial viability. It is also possible for the machine to be controlled so that power is exchanged with the ac mains power supply line only in one direction, which allows a considerably cheaper drive to be used (for example, in some embodiments the machine can be controlled such that the drive only draws power from the line, and as such a unidirectional drive with a simple passive rectifier as the line-side part of the drive may be used, which is considerably cheaper than the active circuits required for bi-directional power flow).

In some embodiments the drive may not exchange power directly with the ac mains power supply line at all, only transferring power between the forwards frequency component and backwards frequency component of the power flowing in the control winding. In such a system several components may be emitted from the drive, potentially reducing its cost and increasing its commercial viability.

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 section through a conventional brushless doubly fed generator, taken normal to the direction of the shaft, showing the positions of stator windings in the machine.

Figure 2a shows a diagram of a brushless doubly fed machine with its power winding connected to the single phase ac grid mains, its control winding connected to the three-phase output of a frequency converter drive, with said drive being controlled by a controller which monitors the rotor position of the brushless doubly fed machine and the voltages and currents flowing in its windings in order to make its control decisions.

Figure 2b shows a typical frequency spectrum of the voltage applied to the control winding of the bntshless doubly fed generator by the frequency converter drive shown in figure 2a, in which it can be seen that there are significant components at two distinct frequencies.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

We will describe the implementation of the improvements to the brushless double fed generator (BDFG) detailed in the previous sections in this case to a machine to be used for the application of a generator in a small wind turbine.

The overall structure of a standard BDFG is shown in figure 1, which shows a section though the machine normal to the shaft, and it is improvements to this machine that the invention relates. The standard BDFG is made up of a slotted laminate iron stator part 11, of the type commonly used in induction motors, a frame or supporting structure 13 and a rotor part 12. No modification is made to the rotor part by this invention, it may be of one of the standard types described in several of the publications cited, typically the "nested-loop" design. Two layers of windings (14 and 15) are fitted into the stator slots, one layer will form the control winding of the machine and the other the power winding. Whether the inner or outer layer is the power or control winding does not significantly alter the performance of the machine.

To prevent direct coupling between the windings they must be chosen to be of different pole-numbers. There are a large number of acceptable combinations, but, for example, suitable choices for the above described application would be 2-pole/6-pole and 4-pole/S-pole which give maximum shaft speeds round of 750-900rpm and 500-750rpm respectively. These speed ranges are well matched to the blade speeds of small wind turbines, but for larger wind turbines as gear-box may be required between the BDFG shaft and the wind turbine blades.

In this application, a single phase output from the BDFG is required. For this purpose, the power winding should be wound with a standard single phase winding of the appropriate pole-number.

Alternatively the power winding may be a three-phase winding with each phase of the three-phase winding connected in series, and this series network be treated as a single-phase winding. A machine with the former winding is likely to be more efficient than one with the latter, so the former would normally be preferred. However, a machine with the latter winding could be used for either single phase or three-phase operation, so in certain circumstances may be more economic to produce. The control winding would in all cases be poly-phase, preferably three-phase.

The BDFG just described may be connected to the grid as shown in figure 2a. In figure 2a the BDFG 21 has its power winding 22 connected directly to the single phase ac mains grid 24. The control winding 23 of the BDFG is is connected to the three-phase output 27 of a frequency converter 25. The input of the frequency converter 26 is connected to the single phase ac mains grid 24. For stable operation at variable speeds the BDFG requires a controller 28, which controls the waveform of the output voltage of the frequency converter which is fed to the control winding of the BDFG. In order to control this waveform the controller requires information about the state of the BDFG, which may include the rotor position and/or speed gathered from a suitable sensor 221, and the voltages and currents flowing in the power and control windings of the BDFG captured with suitable sensors 29 and 220.

The present invention is a method of operating the machine where the voltage waveform of the output of the frequency converter (27) contains components at two distinct frequencies. To explain why this is offers improved machine performance we must explain some aspects of the operation of the machine. The machine we will analyse will have a single-phase power winding of p pole-pairs and a control winding of p2 pole-pairs. We can understand the operation of the BDFG with a single phase power winding by considering the interaction of the fields in the machine. If the pi-pole-pair single-phase winding is connected to the grid with a frequency of w1, it will produce an oscillating field in the airgap also at o1 when viewed from a frame attached to the stator. The oscillating field may also be considered as the superposition of two rotating fields, a forward component rotating at +Wi electrical rad/s and a backward component at Wi. When viewed from a frame attached to the rotor (which rotates at w1 rad/s), the forwards and backwards components of this flux appear to be rotating at the frequencies Wf and Web respectively, where these are given by Wef = WI -P1Wr (5) Wb W1PIWr (6) Thus components of current at two different frequencies will appear in the rotor windings. The P2-pole-pair component of the rotor winding can be configured to have the same or opposite phase sequence as the pi-pole-pair component, however it can be shown that the torque rating of the machine is better in the opposite phase sequence configuration, so this would almost always be used. Taking this configuration, the p7-pole-pair components of flux will rotate at WCf and Web when viewed from the rotor frame. Viewed from the stator frame, the two components of flux rotate at frequencies Wzf and W*2h respectively, where these are given by W7f = Wef + P2Wr = U)i + (pi + P2)Wr (7) W'b = Web + P2Wr = Wi + (pi + P2)Wr (8) Thus two components of emf and current will appear in the control winding of the BDFM, one is consistent with a poly-phase BDFM excited with forwards sequence voltage and one consistent with the same BDFM excited with backwards sequence voltages. Thus the BDFM with a single phase power winding appears like the superposition of two BDFMs, one forward and one backwards, and it is control over this backwards effective BDFM which is key to this invention.

The frequencies 0zf and t02h are important and are referred to as the forwards control frequency and the backwards control frequency respectively.

In all previous attempts at controlling the BDFM the output of the converter has been controlled to produce an output voltage waveform which is substantially sinusoidal at the forwards control frequency By including a component at the backwards control frequency in the output voltage waveform of the frequency converter, control over the backwards sequence aspect of the BDFM may be obtained. Figure 2b shows the typical voltage spectrum of the output of the frequency converter under this control, where 230 is the forwards control frequency component and 231 is the backwards control frequency component (in practice there will be some additional noise at other frequencies due to distortions in the waveform from the output of the frequency converter).

Control over the backwards sequence aspect of the machine may be used in several ways: Firstly some settings of the voltage and phase of the backwards control frequency component cause a reduction in the current flowing in the control winding at the backwards control frequency the reduction of which reduces the T2R resistive heating losses in the control winding improving the efficiency of the BDFM.

Secondly some settings of the voltage and phase of the backwards control frequency component cause an adjustment to the torque of the machine, allowing an increase in the torque rating of the BDFG.

Thirdly, some settings of the voltage and phase of the backwards control frequency component cause an adjustment in the power flow to/from the control winding. By utilizing this control it is possible to reduce the amount of power that the frequency converter has to exchange with the grid.

The amount power flowing through the control winding at the forwards control frequency, and the direction in which it flows, is largely determined by the torque and speed of the machine. Typically, when generating at speeds lower than that which the forwards control frequency is zero in equation (7) power flows from the frequency converter into the control winding, where as at higher speeds power typically flows from the control winding into the frequency converter (in practice resistive losses mean that the speed at which the power flow changes direction is typically slightly higher than the speed at which the forwards control frequency is zero). In some preferred embodiments, the power flow in the control winding at the backwards control frequency may be controlled so as the make the power flow in the frequency converter substantially unidirectional, for example if the machine speed is high such that power is being transferred to the converter at the forwards control frequency, the backwards control frequency component of voltage in the control winding may be set such that an equal amount of power is transferred away from the converter at the backwards control frequency. A frequency converter which only has to draw power from the mains grid line can operate with simpler circuitry than one which must also export power into the mains grid line, for example such a frequency converter may use a simple passive rectifier for its line side part, and thus may be substantially less costly than a bi-directional converter.

In some embodiments, the power flow in the control winding at the backwards control frequency may be controlled such that it always balances the power flow at the forwards control frequency, allowing a frequency converter to the used which does not exchange power with the grid at all, allowing a further reduction in required circuity and reduction in cost, though at some expense in terms of machine efficiency Although it is the inclusion of the component of voltage at the backwards control frequency in the output of the frequency converter and the performance improvements which may be obtained by using this additional control in the three ways just outlined that make up this aspect of the invention, it may aid understanding to outline a simple active controller that may be used to to control this component. The forward control frequency component of the output of the voltage may be controlled in the normal way using any of the published control schemes (for example the "phase angle control" scheme or the "dq vector control" scheme). Tn a simple control scheme for the backwards control voltage, by prior experiment on the generator a lookup table may be built up of the required power flow in the control winding at the backwards control frequency to give the desired performance characteristic (one of the three outlined above) for each operating speed and torque (or power winding power) of the machine, along with suitable magnitudes for the backwards control frequency voltage component in the output of the converter. In operation the controller may measure the speed and the power winding power using suitable sensors (as shown in figure 2a), from the lookup table find the required backwards control frequency components of power and vohage. The magnitude of the control frequency voltage component may be set immediately, however the phase of the voltage must be controlled in order the achieve the desired power flow. A simple iterative tracking control scheme may be used, for example: -1) The power in the control winding at the backwards control frequency may be measured, for example by capturing the power product of voltage and current) waveform for a fraction of a second, for example 5Oms, and computing the FFT (Fast Fourier Transform) of the waveform -2) The phase of the backwards control winding voltage may be adjusted, for example by degrees.

-3) The power is be measured again -4) If the measured power is closer to the required power than before the phase shift, another phase shift in the same direction is made, otherwise the direction of shifting is changed and a shift is made in the opposite sense.

-Step 3 and 4 are repeated endlessly such that the phase settles to being adjusted back and forth about the value required to give the required power flow. At appropriate intervals (say every second) step 1 will be repeated to update the target for the current machine operating conditions.

Such a controller may be implemented for example on a DSP (Digital Signal Processor) or FPGA (Field Programmable Gate Array). The skilled person will see that this is merely the simplest scheme which could be used to control the backwards aspect of the machine, and that other schemes which actively control the backwards control frequency voltage component so as the achieve the one of the three outlined aims directly are likely to offer better performance with better dynamic response. The structure of such a controller is may for example be similar to that used to control forwards aspect of the machine, in particularly preferred embodiments vector control would be used to control the backwards aspect of the machine.

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

CLAIMS1. A brushless doubly fed machine (BDFM) connected to a single-phase ac mains power supply line to deliver power into said single-phase ac mains power supply line, wherein said brushless doubly fed machine comprises of a brushless doubly fed generator, a controller and an ac mains power supply line connection, said brushless doubly fed generator having a stator and a rotor, said stator having a single-phase or poly-phase power winding and a poly-phase control winding, and said rotor having a sensor, said sensor being configured to sense rotation of said rotor, and said power winding of said brushless doubly fed generator being connected to said single-phase ac mains power supply line, said control winding of said brushless doubly fed generator being connected to the output of a power electronic frequency converter drive, the input of said drive being connected to said single-phase ac mains power supply line, and the output of said drive controlled by said controller, wherein said controller is configured to command said drive to apply a control voltage to said control winding, the waveform of said control voltage containing substantial components at two distinct frequencies, the former of said frequencies (henceforth referred to as the "forwards control frequency") being substantially given by the sum of the product of the rotational speed of said rotor and half the number of poles of said power winding and the product of the rotational speed of said rotor and half the number poles of said control winding, less the frequency of said single-phase ac mains power supply line, and the later of said frequencies (henceforth referred to as the "backwards control frequency") being substantially given by the sum of the product of the rotational speed of said rotor and half the number of poles of said power winding, the product of the rotational speed of said rotor and half the number poles of said control winding, and the frequency of said single-phase ac mains power supply line, wherein the units of all said frequencies and speeds are Hertz and all said frequencies and speeds are measured in the same angular direction, and wherein said controller controls the forwards control frequency component of the output voltage of said drive so as to control the speed of said rotor or the torque developed by said rotor, and controls the backwards control frequency component of the output voltage of said drive so as to lower the losses in the machine by reducing the component of the current in the control windings at the backwards control frequency.
2. A brushless doubly fed machine (BDFM) connected to a single-phase ac mains power supply line to deliver power into said single-phase ac mains power supply line, wherein said brushless doubly fed machine comprises of a brushless doubly fed generator, a controller and an ae mains power supply line connection, said brushless doubly fed generator having a stator and a rotor, said stator having a single-phase or poly-phase power winding and a poly-phase control winding, and said rotor having a sensor, said sensor being configured to sense rotation of said rotor, and said power winding of said brushless doubly fed generator being connected to said single-phase ac mains power supply line, said control winding of said brushless doubly fed generator being connected to the output of a power electronic frequency converter drive, the input of said drive being connected to said single-phase ac mains power supply line, and the output of said drive controlled by said controller, wherein said controller is configured to command said drive to apply a control voltage to said control winding, the waveform of said control voltage containing substantial components at two distinct frequencies, the former of said frequencies (henceforth referred to as the "forwards control frequency") being substantially given by the sum of the product of the rotational speed of said rotor and half the number of poles of said power winding and the product of the rotational speed of said rotor and half the number poles of said control winding, less the frequency of said single-phase ac mains power supply line, and the later of said frequencies (henceforth referred to as the "backwards control frequency") being substantially given by the sum of the product of the rotational speed of said rotor and half the number of poles of said power winding, the product of the rotational speed of said rotor and half the number poles of said control winding, and the frequency of said single-phase ac mains power supply line, wherein the units of all said frequencies and speeds are Hertz and all said frequencies and speeds are measured in the same angular direction, and wherein said controller controls the forwards control frequency component of the output voltage of said drive so as to control the speed of said rotor or the torque developed by said rotor, and controls the backwards control frequency component of the output voltage of said drive so develop additional torque.
3. A brushless doubly fed machine (BDFM) connected to a single-phase ac mains power supply line to deliver power into said single-phase ac mains power supply line, wherein said brushless doubly fed machine comprises of a brushless doubly fed generator, a controller and an ac mains power supply line connection, said brushless doubly fed generator having a stator and a rotor, said stator having a single-phase or poly-phase power winding and a poly-phase control winding, and said rotor having a sensor, said sensor being configured to sense rotation of said rotor, and said power winding of said brushless doubly fed generator being connected to said single-phase ac mains power supply line, said control winding of said brushless doubly fed generator being connected to the output of a power electronic frequency converter drive, the input of said drive being connected to said single-phase ac mains power supply line, and the output of said drive controlled by said controller, wherein said controller is configured to command said drive to apply a control voltage to said control winding, the waveform of said control voltage containing substantial components at two distinct frequencies, the former of said frequencies (henceforth referred to as the "forwards control frequency") being substantially given by the sum of the product of the rotational speed of said rotor and half the number of poles of said power winding and the product of the rotational speed of said rotor and half the number poles of said control winding, less the frequency of said single-phase ac mains power supply line, and the later of said frequencies (henceforth referred to as the "backwards control frequency") being substantially given by the sum of the product of the rotational speed of said rotor and half the number of poles of said power winding, the product of the rotational speed of said rotor and half the number poles of said control winding, and the frequency of said single-phase ac mains power supply line, wherein the units of all said frequencies and speeds are Hertz and all said frequencies and speeds arc measured in the same angular direction, and wherein said controller controls the forwards control frequency component of the output voltage of said drive so as to control the speed of said rotor or the torque developed by said rotor, and controls the backwards control frequency component of the output voltage of said drive so as to transfer power from said drive to said brushlcss doubly fed machine and thenceforth to said ac mains power supply line via said power winding or to a mechanical system connected to said rotor or to both entities.
4. A brushless doubly fed machine (BDFM) connected to a single-phase ac mains power supply line to deliver power into said single-phase ac mains power supply line, wherein said brushless doubly fed machine comprises of a brushless doubly fed generator, a controller and an ac mains power supply line connection, said brushless doubly fed generator having a stator and a rotor, said stator having a single-phase or poly-phase power winding and a poly-phase control winding, and said rotor having a sensor, said sensor being configured to sense rotation of said rotor, and said power winding of said brushless doubly fed generator being connected to said single-phase ac mains power supply line, said control winding of said brushless doubly fed generator being connected to the output of a power electronic frequency converter drive, the input of said drive being connected to said single-phase ac mains power supply line, and the output of said drive controlled by said controller, wherein said controller is configured to command said drive to apply a control voltage to said control winding, the waveform of said control voltage containing substantial components at two distinct frequencies, the former of said frequencies (henceforth referred to as the "forwards control frequency") being substantially given by the sum of the product of the rotational speed of said rotor and half the number of poles of said power winding and the product of the rotational speed of said rotor and half the number poles of said control winding, less the frequency of said single-phase ac mains power supply line, and the later of said frequencies (henceforth referred to as the "backwards control frequency") being substantially given by the sum of the product of the rotational speed of said rotor and half the number of poles of said power winding, the product of the rotational speed of said rotor and half the number poles of said control winding, and the frequency of said single-phase ac mains power supply line, wherein the units of all said frequencies and speeds are Hertz and all said frequencies and speeds are measured in the same angular direction, wherein said controller controls the forwards control frequency component of the output voltage of said drive so as to control the speed of said rotor or the torque developed by said rotor, and controls the backwards control frequency component of the output voltage of said drive so as to transfer power to said drive from said brushless doubly fed machine, said power coming ultimately from said ac mains power supply line via said power winding or from a mechanical system connected to said rotor or from both sources.
5. A brushless doubly fed machine according to claim 3 wherein said controller controls the power flow in said control winding using the backwards control frequency component of the output of said drive such that under normal operating conditions there is no net power flow out of said control winding of said brushless doubly fed generator such that said drive may be unidirectional.
6. A brushless doubly fed machine according to claim 4 wherein said controller controls the power flow in said control winding using the backwards control frequency component of the output of said drive such that under normal operating conditions there is no net power flow in to said control winding of said brushless doubly fed generator such that said drive may be unidirectional.
7. A brushless doubly fed machine according at times to claims 3 and at other times to claim 4, wherein said controller controls the power flow in said control winding using the backwards control frequency component of the output of said drive such that under normal operating conditions the component of power transferred to said drive from said control winding at the forwards control frequency substantially equals the component of power transferred from said drive to said control winding at the backwards control frequency, such that said drive exchanges power only with the control winding and may be single-ended.
8. A brushless double fed machine according to any of the preceding claims in which the roles performed by the forwards control frequency output of said drive and the backwards control frequency output of said drive are exchanged.
9. A brushless doubly fed machine according to any of the preceding claims in which said rotation of rotor input into said controller comes not from a physical position sensor but from an estimate of the position of said rotor deduced from the voltages and currents measured at the terminals of said windings.
10. The use of a brushless doubly fed machine as described in any of the preceding claims as a generator for a wind turbine.
11. The use of a brushless doubly fed machine as described in any one of claims 1 to 9 as a generator for a hydro-electric plant.
12. The use of a brushless doubly fed machine as described in any one of claims 1 to 9 as a motor.