FOR A WIND TURBINE POWER CONVERTER

Technical Field

Aspects of the invention relate to the configuration and control of a power dissipation system for a wind turbine power converter particularly, though not exclusively, during grid fault ride through events. Background to the Invention

Megawatt-scale wind turbine generators are usually required to comply with so-called grid code requirements in order to provide voltage and frequency support to the electrical grid. To this end, such wind turbine generators typically include a power converter system to convert the variable frequency alternating (AC) voltage/current from the generator to a substantially constant frequency AC voltage/current suitable for onward transmission to the electricity distribution networks or grids by a suitable step-up transformer.

One known architecture of a wind turbine generator includes a so-called full-scale power converter system which typically includes a generator side converter and a grid side converter coupled by a DC bus or link. Such an architecture is also referred to as a 'back-to- back' voltage source frequency converter. Other types of known systems also incorporate back-to-back converters coupled by a DC-link, one example of which is the 'DFIG' or doubly- fed induction generator system.

In such power converter systems the generator-side converter regulates the power of the generator that is passed to the DC-link. In turn, the grid-side converter feeds power from the DC-link into the grid. The DC-link is thus an intermediary stage in such a power converter and in effects decouples the generator-side converter from the grid-side converter. The voltage on the DC-link is maintained at a substantially constant level during operation of the power converter by way of a reference voltage parameter. The reference voltage parameter may be around 1 100V, whereas the DC-link would usually be permitted to fluctuate around this point, for example between 1050V and 1 150V. In some circumstances it may be necessary to shut down or curtail operation of a wind turbine generator. This may be due to a component failure of the wind turbine generator, due l to a fault in the power distribution grid, or even in response to a direct shutdown request from a power plant controller, for example, in order to curtail the power production of an associated power plant. During a grid fault, for example, a known approach involves disabling the control signals to the generator-side and the grid-side converters and opening at least the circuit breaker between the power converter and the grid. In this situation, the grid-side converter is no longer able to deliver power to the grid. This can result in a voltage increase on the DC-link that must be managed. For this reason it is known to include a power dissipation system on the DC-link in the form of a DC chopper circuit. A DC chopper circuit may include one or more dissipating modules that are connected to the DC-link and which are controllable in order to drain current from the DC-link through a resistor, thereby dissipating the energy on the DC-link as heat. The dissipating modules may include a semiconductor switch coupled to a resistor.

Summary of the Invention

According to an aspect of the present invention there is provided a power dissipation system for a power converter of a wind turbine generator, the power dissipation system comprising at least a first dissipation module and a second dissipation module, each of the dissipation modules including a switch unit coupled to a respective dissipating element, a controller configured to provide a respective switch control signal to each of the switch units, and wherein the switch control signal of the first switch unit is different to the switch control signal of the second switch unit.

In another aspect, the invention resides in a method of controlling a power dissipation system for a power converter of a wind turbine generator, wherein the power dissipation system comprises at least a first switch unit and a second switch unit, each of the switch units coupled to a respective dissipating element, wherein the method includes; regulating the power dissipation of the power dissipation system by controlling the first switch unit and the second switch unit by way of respective first and second switch control signals; and configuring the first switch control signal to be different to the second switch control signal. Beneficially, by configuring the switch control signals to be different, it is possible to reduce the voltage ripple on the DC-link. Where the switch control signal are periodic waveforms, it becomes possible to reduce the switching frequency of the switch control signals thereby reducing operational losses.

Each of the switch control signals may be a PWM waveform having a controllable duty cycle to control the power dissipation of the respective dissipation modules. In some embodiments, each of the switch control signals may have a predetermined frequency, wherein the on-time of the switch control signals can be controlled to control the power dissipation of the respective dissipation modules. Alternatively, each of the switch control signals may have a predetermined on-time, wherein the frequency of the switch control signals may be controlled to control the power dissipation of the respective dissipation modules. In some embodiments, at least one of the switch control signals is configured to define a phase difference with at least one other of the switch control signals. The phase difference may be at least 10 degrees, more preferably at least 50 degrees but less than 310 degrees, and more preferably at least 100 degrees but less than 260 degrees. In one embodiment, however, the phase difference between the switch control signals may be calculated by dividing the waveform period by the number of switch units. So, in the case where there are three switch units, the phase difference between each of the switch control signals may be 120 degrees, by way of example.

In one aspect, the invention may reside in a power converter for a wind turbine generator, comprising a generator-side converter, a grid-side converter, a DC-link coupled between the generator-side converter and the grid-side converter, and a power dissipation system as described above.

Aspects of the invention may also be expressed as a controller for a wind turbine power dissipation system, wherein the controller comprises a processor, a memory module, and an input/output system, and wherein the memory includes a set of program code instructions which when executed by the processor, implement a method as described above.

Aspects of the invention may also be expressed as a computer program product downloadable from a communication network and/or stored on a machine readable medium, comprising program code instructions for implementing a method as described above, and also to a machine readable medium having stored thereon such a computer program product.

For the purposes of this disclosure, it is to be understood that the control system described herein can comprise a control unit or computational device having one or more electronic processors. Such a system may comprise a single control unit or electronic controller or alternatively different functions of the controller(s) may be embodied in, or hosted in, different control units or controllers. As used herein, the term "control system" will be understood to include both a single control unit or controller and a plurality of control units or controllers collectively operating to provide the required control functionality. A set of instructions could be provided which, when executed, cause said controller(s) or control unit(s) to implement the control techniques described herein (including the method(s) described below). The set of instructions may be embedded in one or more electronic processors, or alternatively, the set of instructions could be provided as software to be executed by one or more electronic processor(s). For example, a first controller may be implemented in software run on one or more electronic processors, and one or more other controllers may also be implemented in software run on or more electronic processors, optionally the same one or more processors as the first controller. It will be appreciated, however, that other arrangements are also useful, and therefore, the aspects of the invention are not intended to be limited to any particular arrangement.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

Brief Description of the Drawings

So that it may be more fully understood, the invention will now be described by way of example only with reference to the following drawings, in which: Figure 1 is a front view of a wind turbine generator in which the embodiments of the invention may be incorporated;

Figure 2 is a schematic view of power generation components of a wind turbine generator, including a power dissipation system in accordance with an embodiment of the invention;

Figure 3 is an enlarged schematic view of the power dissipation system in Figure 2; Figures 4a and 4b show two sets of control signals in respect of the power dissipation system for comparison; and

Figure 5 is a flow chart of an embodiment of the invention. Detailed Description of Embodiments

Figure 1 provides an overview of a typical horizontal-axis wind turbine generator 2 (HAWT) within which the embodiments of the invention may be implemented. Note, however, that this is given by way of example and that the aspects of the invention apply equally to other configurations of wind turbine generators and to electrical generation machines more generally.

The illustrated wind turbine generator 2 comprises a nacelle 4 mounted to a tower 6, which is itself securely mounted on the ground 8 or other type of solid foundation. The nacelle 4 supports a rotor 10 which is rotatable by the flow of air past it. The rotor 10 comprises a hub 12 and a plurality of blades 14, although in this common configuration of wind turbine generator the hub 12 includes three blades.

As is known, the blades 14 have an aerodynamic profile to cause them to generate lift, thus driving the hub 12 to rotate in the wind. The hub 12 is coupled to a suitable power generation system comprising many components housed in the nacelle 4.

The power generation system will now be described with reference to Figure 2. Represented schematically as a system diagram, an illustrated power generation system 20 includes many features that are significant for this discussion, but it should be appreciated that other conventional features are not shown here for brevity, for example yaw control equipment, control networks, local power distribution networks and so on. However, the skilled person would understand that these features would be present in a practical implementation. Also it should be noted that the specific architecture discussed here is used as an example to illustrate the technical functionality of the embodiments of the invention, and so the embodiments may be implemented by a system having a different specific architecture.

The power generation system 20 includes the bladed rotor 10, which drives a transmission 24. Although the transmission 24 is shown here in the form of a gearbox, direct-drive architectures are known which do not include a gearbox. The transmission 24 drives an electrical generator 26 for generating electrical power. Three-phase electrical power generation is usual in utility scale wind turbine systems, but this is not essential for the purpose of this discussion.

The electrical generator 26 is connected to a power converter system 28 by a suitable three- phase electrical connector such as a cable or bus 30. Three-phase systems are usual, but not necessarily universal in such systems. The power converter system 28 may be of a conventional architecture and, as is known, converts the output frequency of the electrical generator 26 (an AC signal) to a suitable output voltage level and frequency (also an AC signal) that is suitable for supplying to an internal electrical grid 31 via a transformer 32. A first (grid-side) breaker arrangement 34 is located between the power converter system 28 and the grid 6 in order to control connection of the converter 28 to the grid 6, and a second (generator-side) breaker arrangement 36 is located between the electrical generator 26 and the power converter system 28 to control connection of the electrical generator 26 to the converter 28.

A grid choke 38 may be located between the power converter system 28 and the grid-side breaker arrangement 34 to remove high frequency switching characteristics from the voltage waveform output by the transformer 32.

It will be appreciated that the specific architecture described here is a two-level back-to-back voltage source full scale power converter system (FSC) system 28, which includes a generator-side converter 40 and a grid-side converter 42 which are coupled via a DC-link 44. A similar architecture is also known to be used in a doubly-fed induction generation (DFIG) system in which a portion of the generated power is fed through the power converter and a portion of the generated power is fed directly to the grid. The DC-link 44 comprises capacitor 46 which acts to smooth out the voltage ripple in the output of the generator-side converter 40. Although not shown in Figure 2, the power converter system 28 may also include a DC- link pre-charge unit coupled to the DC-link 44 and being operable to charge the DC-link 44 to a voltage level that enables the converter 28 to operate correctly. Such a pre-charge unit is conventional and will not be described in further detail here.

As is conventional, the generator-side converter 40 and the grid-side converter 42 comprise a plurality of semiconductor switches 50, which in this case are insulated gate bipolar transistors (IGBTs), but other switching devices may be used. In this embodiment, the converters 40, 42 comprise six semiconductor switches 50 but other full-scale converter configurations would be understood by the skilled person. Together, the generator-side converter 40 and the grid-side converter 42 are responsible for converting one or more AC voltage signals to a DC voltage signal and then to convert the DC voltage signal back again to one or more AC voltage signals. More specifically, the AC voltage signal(s) from the electrical generator 26 are converted by the generator-side converter to a DC voltage signal on the DC link, which is then converted by the grid-side converter back to an AC voltage signal(s). However, it is normal, under certain conditions, for wind turbine generators to be supplied with power from the electrical grid, and then the grid-side converter would convert AC signals to DC signals, and the generator-side converter would convert DC signals to AC signals.

In order to achieve power control objectives, the wind turbine system 20 is equipped with a control system, or more simply 'controller' 52 that is operable to control the power converter system 28 appropriately. The controller 52 may be implemented in any combination of hardware, software and/or firmware to provide a suitable processor 80, memory module 82 and input/output system 84.

As part of this, the controller 52 includes a generator-side converter drive module 54 and a grid-side converter drive module 56. Here the drive modules 54, 56 are shown as being integrated into the controller 52, but it should be noted that the drive modules 54, 56 may also be separate hardware units. The controller 52 is also responsible for issuing control commands to other parts of the system, for example the grid-side and generator-side breaker arrangements 34, 36, and also a power dissipation system 60 by command signal S, as will become apparent.

The generator-side converter drive module 54 provides suitable drive signals 58 to the control gates of the semiconductor switches 50 of the generator-side converter 40 in order to convert the alternating voltage generated by the electrical generator 26 to a substantially constant voltage on the DC-link 44 at a predetermined voltage level. Similarly, the grid-side converter drive module 56 provides suitable drive signals 60 to the control gates of the semiconductor switches 50 of the grid-side converter 42 in order to convert the substantially constant voltage on the DC-link 44 to an alternating voltage at a predetermined voltage level and frequency.

As would be known to the skilled person, the drive signals 58, 60 sent to the generator-side converter 40 and the grid-side converter 42 may be any suitable drive signal, one example of which is a pulse-width modulated (PWM) drive signal, but other drive signal types could also be used. The drive signals 58, 60 may be enabled by the respective converter drive modules 54, 56 in order to transfer energy across the associated converter 40, 42. Likewise, the drive signals 58, 60 may be disabled by the respective converter drive modules 54, 56 in order to prevent the transfer of energy across the associated converter 40, 42.

The wind turbine system 20 described above may be controlled in an operating state in order to provide power to the grid 31 at a predetermined voltage and frequency. It is common for a wind turbine system 20 to spend the majority of its lifetime in the operating state in order to generate as much power as possible from a renewable energy resource. As mentioned above, during operation of the power convert system 28, the DC-link 44 is controlled so that its voltage level is maintained at a predetermined operational level. This may be in the region of 1100V, as an example.

However, during a grid fault the wind turbine may be required to operate in a grid fault ride through (GFRT) state to prevent the voltage on the DC-link 44 increasing to an unacceptable level. One technique to reduce the voltage on the DC-link 44 is through the use of the power dissipation system 60, which is shown coupled to the DC-link 44 in Figure 2.

The power dissipation system 60 is shown in more detail in Figure 3 and, in overview, includes first, second and third power dissipating modules 62a, 62b, 62c and a dissipation control system comprising a control module 64 or 'controller'. Although three power dissipating modules 62a-c are shown here, it should be appreciated that any number may be provided, provided that there are at least two. It should be noted at this point that although the dissipation control module 64 is shown as separate to the power dissipation modules 62a-c, this is merely for convenience and in practice the various components may be incorporated on the same electronics platform. Also, it should be noted that although the control module 64 is shown as a separate component in Figure 2, in practice the functionality provided by that module may instead be incorporated into the main control system 52 of the power converter system 20. In any event, it should be appreciated the control module 64 may be implemented in a suitable combination of hardware, software or firmware and as such includes a processor 64a, memory module 64b and input/output system 64c to carry out its control tasks. Accordingly the processor 64a may be configured to carry out a set of program code instructions stored on the memory module 64b which may implement a method as described herein, and as will become apparent in the foregoing description.

Each of the power dissipation modules 62a-c is identical in this embodiment and includes a respective switch unit 66a, b,c and a dissipation element 68a, b,c. Hereinafter, references made in general to the dissipation modules, switch units or the dissipation elements will exclude a suffix, whereas references to a specific one of the switch units or the dissipation elements will include the suffix.

Each of the switch units 66 may be a semiconductor switch such as an IGBT or MOSFET. The dissipation element 68 is shown as being a resistor in this embodiment, although embodiments are envisaged in which resistors may be combined with capacitors and/or inductors. As would be known to the skilled person, such a dissipation module 62 is sometimes referred to as a 'DC chopper' in the art.

The switch units 66 are controlled by respective switch control signals. The switch control signals are generated by the control module 64 in response to the command signal S sent from the wind turbine controller 52 (as shown in Figure 2). The switch control signals may be configured to control and regulate the amount of power that is dissipated by the respective dissipating elements 68. The dissipation control module 64 outputs three switch control signals each of which is associated with a respective one of the switch units 64. The switch control signals are labelled S1 , S2 and S3 on Figure 3 as output from the dissipation control module 64 and input into a respective one of the switch units 64a, b,c, as is illustrated

A suitable type of switching signal may be a periodic waveform having a frequency and phase such as a pulse-width modulated (PWM) signal. As is known, a PWM signal uses a rectangular pulse wave having a period whose pulse width is modulated with respect to the waveform period in order to vary the average value of the signal. An example of such a waveform is shown in the inset panel in Figure 3 in which the signal S1 has a period T, a minimum value 'XV, a maximum value 'X2' and a duty cycle 'd'. As would be understood by the skilled person, the term 'duty cycle' describes the proportion of 'on' time to the signal period T, such that a low duty cycle corresponds to low power, because the power is off for most of the time. Duty cycle is expressed in percent; 0% being fully off, in effect there being no signal, whereas 100% is fully on. As is shown in Figure 3, the signal S1 has about a 50% duty cycle which represents that the dissipation module 62a is dissipating 50% of its maximum dissipation capacity.

In the illustrated embodiment, each of the dissipation modules 62 are operated in parallel by the respective switch control signals S1-S3. However, the dissipation modules 62 are controlled so as to reduce the voltage fluctuation or ripple on the DC-link 44. The benefit of this is that by reducing the voltage ripple on the DC-link 44, it is then possible to reduce the switching frequency of the dissipation modules 62 which reduces the operational losses of these components, as will be described.

More specifically, the switch units 66 of the dissipation modules 62 are controlled with switch control signals that are different for each one of the switch units 66. The switch control signals S1-S3 may be configured such that the respective switch control signal for a first one of the switch units 66 has a frequency that is the same or similar to that of a second one of the switch control signals, but has a different phase. In other words, at least two of the switch control signals may have essentially identical waveforms, but are phase shifted with respect to one another.

The amount of phase shift may be optimised in order to maximise the reduction of voltage ripple on the DC-link. In one embodiment, the phase shift between switch control signals is a function of the number of the dissipation modules 62 in the power dissipation system 60. For example, the required phase shift between the switch control signals S1-S3 may be calculated by dividing the waveform period (360 degrees) by the number of dissipation modules 62. So, in the illustrated embodiment having three dissipation modules, the phase shift between switch control signals S1-S3 may be 120 degrees. It follows therefore, that for embodiments having two dissipation modules 62, the phase shift may be 180 degrees, and for embodiments having four dissipation modules, the phase shift may be 90 degrees.

It should be noted that in Figure 3 the control module 64 embodies one implementation which could be used in order to configure the switch control signals S1-S3 to achieve the advantages discussed above. In the illustrated implementation, the control module 64 comprises a first signal processing module 70 and a second signal processing module 72. In the following explanation, reference will also be made to Figure 5 that depicts the specific implementation as a process.

As has been mentioned above, the wind turbine controller 52 is responsible for activating the power dissipation system 60 and does this by sending an activation command signal S to the control module 64. The activation signal S may indicate that the power dissipation system 60 must be activated and may also indicate a required power dissipation rate.

The first signal processing module 70 receives the activation command S, as shown at step 100, and interprets this to produce a primary activation signal 'A' The primary activation signal A may consist of a reference PWM signal having a duty cycle that is calculated to produce the desired power dissipation rate from each of the dissipation modules 62 in order to provide the total required power dissipation rate. This is illustrated at step 102. The first signal processing module 72 receives the primary activation signal A and generates the three switch control signals S1-S3 that can also be considered to be secondary activation signals. This is illustrated at step 104.

The second processing module 72 generates the specific control signals S1-S3 for as long as it is instructed to do so by the first processing module 70. So, the first processing module 70 may monitor for required deactivation of the power dissipation function by virtue of the removal of the command signal S. This is illustrated by step 106.

The switch control signals S1 to S3 are configured and controlled to reduce the voltage ripple on the DC-link 44. Instead of switching all of the dissipation modules 62 in synchronisation, in which the voltage across the dissipation modules 62 (and therefore also current flow), steps from zero to a maximum value in a discontinuous manner, the switch control signals S1 to S3 may be configured so that the current steps with a smaller amplitude.

To this end, the switch control signals S1-S3 may be configured and controlled so that selected ones of the switch control signals S1-S3 are different to the other switch control signals. One option for this, as shown in the illustrated embodiment, is to phase-shift at least one of the switch control signals S1-S3 with respect to the other switch control signals S1- S3.

In order to configure the switch control signals S1-S3 to be different to the reference signal A, the second processing unit 72 may be coupled to a suitable memory module 74 that may store the necessary information that instructs the second signal processing unit 72 to configure the switch control signals S1-S3 in a specific manner. In one specific implementation, the memory module 74 may contain data, information or instructions that cause the second processing module 74 to generate switch control signals S1-S3 that are phase-shifted with respect to the reference signal A. For example the first signal processing module 70 may sets the phase-shift of the three switch control signals S1-S3 with respect to the reference signal A as follows: first switch signal, S1 0 degrees phase shift

second switch signal, S2 120 degrees phase shift

third switch signal, S3 240 degrees phase shift

These specific values are calculated by dividing the waveform period (360 degrees) with the number of dissipation modules 62 in the power dissipation system 60. However, this should not be considered to be limiting and benefits are still achievable with a different phase shift. Preferably, the switching signals S1-S3 are phase shifted by at least 10 degrees, but less than 350 degrees, more preferably at least 50 degrees but less than 310 degrees, and still more preferably greater than 100 degrees but less than 260 degrees.

Figure 4b illustrates the three switch control signals S1-S3 generated in accordance with the embodiments of the invention discussed above and the resultant current flow on the DC link

44.

As is shown, taking switch control signal S1 as the reference signal, which as discussed above may correspond to the activation signal A, switch control signal S2 has a phase difference θι of approximately 120 degrees relative to switch control signal S1. Similarly, switch control signal S3 has a phase difference θ ₂ of approximately 120 degrees relative to switch signal S2, and, accordingly, has a phase difference of approximately 240 degrees relative to switch signal S1. The significance of configuring the three switch control signals S1-S3 in this way can be appreciated by referring to Figure 4a. Here, alternative switch control signals S4-S6 are shown in which each of the signals has the same frequency and phase. Since all of the dissipation modules 62 are coupled to the DC-link 44 in parallel, the synchronised switching signals S4-S6 shown in Figure 4a result in a large voltage step profile applied across the dissipation modules 62, which results in a discontinuous current flow through them, as indicated by the signal trace o er on Figure 4a It will be appreciated, therefore, that the current flow into the dissipation modules 62 is discontinuous as the chopper current steps between l ₀ and l _max . This discontinuous current characteristic manifests on the DC-link as a voltage ripple. In order to avoid a large voltage build up on the DC-link 44 during the period in which the switch control signals S4-S6 are off (V), the switching frequency is preferably very high, for example in the order of 2kHz. However, this results in relatively high switching losses. Turning to Figure 4b, it can be appreciated that the switch control signals S1-S3 are staggered so that at least one of the dissipation modules 62 is conducting at any instant. This means that the current flowing through the dissipation modules 62 fluctuates with a much smaller amplitude. This can be appreciated by referring to the lowermost trace in which the current o er fluctuates at an intermediate level between l ₀ and l _max with a greatly reduced amplitude l _am p and a higher frequency of fluctuation. This means that current is always flowing out of the DC-link 44 through the power dissipation system 60 and so the DC- link 44 voltage is not able to spike during Off periods' like the situation in Figure 4a. A further benefit of this approach is that since the voltage ripple on the DC-link is reduced, it enables the switching frequency to be reduced, which therefore reduces switching losses in the switch units 66 of the dissipation modules 62.

The skilled person will appreciate that the specific embodiments discussed above may be adapted or modified in various ways without departing from the scope of the invention as defined by the claims.

For example, in the above embodiments, the switch control signals S1-S3 have been described as having a certain frequency and a controllable duty cycle in order to control how much power is dissipated by the dissipation modules 62. However, embodiments are envisaged in which the absolute value of On time' of the switch control signals could be fixed at a predetermined value, for example 100με, and the frequency of the switch control signals could be controlled so as to vary the level of power dissipation. For example, in the example given of an on time of 100με, a 50% duty cycle could be achieved with a switching frequency of 5kHz. However, with the same 100με on time, a duty cycle of 10% could be achieved with a 1 kHz switching frequency.