DK201600797A1 - Control system for pulse-width modulation in a wind turbine - Google Patents

Abstract

A method of controlling a wind turbine generator comprising an electrical generator and a power converter, the power converter comprising an electrical switch that is configured to process electrical power produced by the electrical generator. The method comprises: controlling an output from the electrical switch using a discontinuous pulse-width modulated control signal comprising a clamped region during which the control signal has a constant value, thereby to control characteristics of output power from the power converter; acquiring sample data relating to an electrical current received by the electrical switch from the generator; and dynamically adjusting the clamped region of the discontinuous pulse-width modulated control signal to centre the clamped region around a peak or a trough of the electrical current received by the electrical switch. The embodiments of the invention provide an approach by which a converter control system can manage the PWM drive signal to reduce switching losses compared with known approaches, by ensuring that switching of both electrical switches occur when the load current is as low as possible. This is achieved by aligning the centre of the clamped region or "flat top region" with the peak load current, and not with the peak load voltage as in the prior art approach.

Description

CONTROL SYSTEM FOR PULSE-WIDTH MODULATION IN A WIND TURBINE

TECHNICAL FIELD

The embodiments of the invention relate to a method, a controller and system for controlling a power converter of a wind turbine generator.

BACKGROUND

Power converters are used to provide frequency conversion in wind power plants to convert variable AC input signals output by a wind turbine generator, whose characteristics vary in dependence on local wind conditions around the power plant, into AC output signals of suitable frequency and form for supplying a power network. Such power converters typically incorporate a rectifier stage in series with an inverter stage.

Modern rectifiers and inverters typically use semi-conductor switching devices to maximise the efficiency of the frequency conversion. The switching devices are controlled using respective pulse-width modulation (PWM) drive signals, to alter the output of the rectifier or inverter. The drive signal is generated according to a number of parameters, such as the PWM method used, the switching method and switching frequency as well as the number of switching devices.

As the skilled reader will appreciate, the specific PWM method adopted depends on the situation, the expected input, and the intended output. For example, the PWM method may be carrier-based or space-vector PWM, and the method used may comprise continuous or discontinuous PWM. Each method is effective in reducing inefficiencies or losses in specific circumstances, but typically introduces other losses or inefficiencies relative to other methods outside of those circumstances.

It is desirable to optimise the PWM method used to maximise the efficiency of frequency conversion with minimal losses.

It is against this background that the present invention has been devised.

SUMMARY OF THE INVENTION

According to an aspect of the present invention there is provided a method of controlling a wind turbine generator comprising an electrical generator and a power converter, the power converter comprising an electrical switch that is configured to process electrical power produced by the electrical generator. The method comprises controlling an output from the electrical switch using a discontinuous pulse-width modulated control signal comprising a clamped region during which the control signal has a constant value, thereby to control characteristics of output power from the power converter; acquiring sample data relating to an electrical current received by the electrical switch from the generator; and dynamically adjusting the clamped region of the discontinuous pulse-width modulated control signal to centre the clamped region around a peak or a trough of the electrical current received by the electrical switch.

The embodiments of the invention can also be considered to encompass a controller for controlling a wind turbine generator comprising an electrical generator and a power converter, the power converter comprising an electrical switch that is configured to process electrical power produced by the electrical generator. The controller comprises a current controller and a pulse-width modulator, wherein the current controller is configured to receive sample data relating to an electrical current received by the electrical switch from the generator; and the pulse-width modulator is configured to i) generate a discontinuous pulse-width modulated drive signal comprising a clamped region during which the control signal has a constant value; ii) dynamically adjust the clamped region of the discontinuous pulse-width modulated control signal to centre the clamped region around a peak or a trough of the electrical current received by the electrical switch; and iii) communicate the control signal to the electrical switch to control the output of the electrical switch.

The embodiments of the invention provide an approach by which a converter control system can manage the PWM drive signal to reduce switching losses compared with known approaches, by ensuring that switching of both electrical switches occur when the load current is as low as possible. This is achieved by aligning the centre of the clamped region or “flat top region” with the peak load current, and not with the peak load voltage as in the prior art approach.

The method may further include determining a shift angle that determines the position of the clamped region of the control signal, thereby enabling the drive signal to be shifted to align the centre of the flat top region with the peak load current in each phase. The shift angle may correspond to a phase angle between the electrical current received by the electrical switch and an electrical voltage output from the electrical generator.

In one embodiment, the shift angle may be determined by calculating a voltage reference, and comparing the voltage reference with the acquired sample data relating to the electrical current received at the electrical switch. The voltage reference may be calculated by applying a transformation, for example an alpha-beta transformation, to the acquired sample data relating to the electrical current received at the electrical switch.

In one embodiment, the acquired sample data relating to the electrical current received at the electrical switch comprises a magnitude of the electrical current. Furthermore, the method may further comprise determining a frequency of the electrical current received at the electrical switch based on the acquired sample data.

In the illustrated embodiment, the pulse-width modulated signals have carrier frequencies of at least 1000 Hz, although this is intended only to be representative of a suitable frequency.

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

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a front view of a wind turbine generator;

Figure 2 is a schematic representation of a wind power plant including the wind turbine generator in Figure 1, within which embodiments of the invention may be incorporated;

Figure 3 is a schematic representation of a conventional two-level power converter;

Figure 4 is a chart illustrating a discontinuous pulse-width modulated drive signal of a known approach;

Figure 5 is a chart illustrating a discontinuous pulse-width modulated drive signal according to an embodiment of the present invention;

Figure 6 is a schematic representation of a control system according to an embodiment of the present invention for use in the wind power plant of Figure 2; and

Figure 7 is a flow chart illustrating a process governing the generation of a pulse-width modulated drive signal according to an embodiment of the present invention.

DETAILED DESCRIPTION

In order to place the embodiments of the invention in a suitable context, reference will firstly be made to Figure 1, which shows a wind turbine, or wind turbine generator, 10 comprising a rotor 12 including a hub 14 to which three blades 16 are attached. The rotor 12 is rotatably supported by a nacelle 18 that is mounted to the top of a tower 20 in the usual way.

The nacelle 18 houses and supports various power generating components of the wind turbine 10, as will be described with reference to Figure 2. As is known, a flow of wind acting on the blades 16 spins the rotor 12, which drives the power generation equipment housed in the nacelle 18.

The wind turbine generator 10 illustrated in Figure 1 is an onshore wind turbine, although the invention may equally be applied to an offshore wind turbine. Here, the wind turbine 10 is a horizontal axis wind turbine (HAWT) having three blades 16, which is a common type of system, although other types having different numbers of blades exist to which the invention is also applicable.

Figure 2 illustrates an example of a wind power plant 30 that incorporates the wind turbine generator 10 of Figure 1. In the wind power plant 30 of Figure 2, the rotor 12 drives a transmission 32 by way of an input drive shaft 34. Although the transmission 32 is shown here in the form of a gearbox, it is also known for wind turbines to have direct-drive architectures which do not include a gearbox. The transmission 32 has an output shaft 36 which drives an electrical generator 38 for generating three-phase electrical power. In this way, the rotor 12 drives the electrical generator 38 through the transmission 32.

The generator 38 is connected to a power converter 40 by a suitable three-phase electrical connector 42 such as a cable or bus. The power converter 40 converts the output frequency of the generator 38 to a frequency that is suitable for supplying to an electrical grid 43. The output of the power converter 40 is transmitted to the grid 43 through a transformer 44.

The power converter 40 includes a generator-side (or ‘machine-side’) converter 46, and a line-side (or ‘grid-side’) converter 48, which are coupled by a DC link 50 that incorporates a smoothing capacitor 52.

The wind power plant 30 also comprises a control system 54, which controls the operation of both the generator-side converter 46 and the grid-side converter 48 for efficient power conversion. In Figure 2, the control system 54 is housed within the nacelle 18 of the wind turbine 10, and connects to the generator-side converter 46, the grid-side converter 48, to the generator 38, the connector 42 that connects the generator-side converter 46 and the generator 38, and the DC link 50.

While the control system 54 is depicted here as being housed in the nacelle 18, it will be appreciated that the control system 54 may be located anywhere within the wind power plant 30.

The control system 54 is operable to detect the angle of the load current, namely the current of the electrical power output from the generator 38, and to alter control commands issued to the power converter 40 depending upon the detected angle. The control system 54 modulates a drive signal sent to the power converter 40 to reduce the losses that result from a desired control method.

Considering the generator-side converter 46 in more detail, three-phase AC power received by the generator-side converter 46 from the generator 38 is converted into a direct current (DC) signal. As already noted, this is necessary as the power produced in the electrical generator 38 is not in a form suitable for delivery to the grid 43. This is typically because the power is not at the correct frequency or phase angle as these values are determined, at least in part, by the speed of rotation of the rotor 12, which in turn is dependent on wind conditions.

The conversion is performed to supply a DC voltage to the grid-side converter 48 for re-conversion to an AC voltage having a form suitable for supply to the grid 43. In general terms, therefore, the power converter 40 provides AC to AC conversion, which it achieves by feeding electrical current through an AC-DC converter 46 followed by a DC-AC converter 48 in series.

Semiconductor switching devices 56 (shown in Figure 3) are used in the generator-side and grid-side converters 46, 48 to convert the AC or DC signais as required. Typically, this is achieved by switching the devices 56 between ‘on’ and ‘off’ states at high frequency and at a particular duty cycle to produce an intended output. For example, using a duty cycle of 50% produces an output voltage from the switching device that is half of its maximum output voltage when in the ‘on’ state.

Suitable switching devices for this purpose include integrated gate bipolar transistors (IGBTs) or metal-oxide-semiconductor field-effect transistors (MOSFETs).

Each converter 46, 48 may be a modular, multilevel converter (MMC), a two-level or three-level back-to-back converter or another converter configuration/topology that contains switching devices. Figure 3 illustrates an example of a two-level converter 58 that is suitable for this purpose. This arrangement 58 wiil be familiar to the skilled reader, and so is only described in overview here to establish context for embodiments of the invention.

In the arrangement shown, each phase of the three-phase power supplied by the generator 38 connects to a respective leg 60 of the generator-side converter 46. Each leg 60 includes two switching devices 56 in series, each of which is paired with a respective parallel diode 62. The three legs 60 together form the generator-side converter 46, and are connected to the DC link output 50 In parallel with one another. Three further legs 64 are disposed beyond the DC link 50, each of which connects to a respective phase of the converter 58 output to form the grid-side converter 48. The legs 64 of the grid-side converter 48 are connected in parallel with one another and in parallel with the DC link 50 and the legs of the generator-side converter 46.

Each switching device 56 can be switched between on and off states as described above, and the switching devices 56 of each leg 60, 64 are controlled in tandem, in that a common PWM drive signal is sent to each device 56 of the pair, it is noted that different PWM drive signals are sent to each leg 60, 64 to account for the difference between the phases. The PWM drive signais each have a carrier frequency of at least 1 kHz, and typically 50 kHz.

In each leg 60, 64 of the converter 58, either both switching devices 56 may be switched off so that no current flow is possible through that leg 60. 64 in either direction, or one of the switching devices 56 of the ieg 60, 64 is on so that electrical power is channelled through the leg 60, 64 in one direction to the relevant output.

Therefore, a PWM drive signal for a converter comprises either commands corresponding to the ‘on’ state of each switching device or commands corresponding to the ‘off’ state. To re-iterate, when one switching device of a ieg is switching, the other device of that ieg is switched off.

Figure 4 shows the relationship between load voltage 70, load current 72 and a conventional discontinuous PWM drive signa! 74 in a ieg 60, 64 of the power converter 40 over the course of a single cycle. The load current 72 and load voitage 70 are represented by respective sinusoidal waveforms, the waveform of larger amplitude corresponding to the load voltage 70. The drive signal is represented by a discontinuous square wave of high frequency relative to the sinusoidal waveforms. In the drive signal 74, portions of high frequency square wave are interrupted by damped, or ‘fiat top’, regions during which the drive signal 74 has a constant value. When used to operate a switching device 56, the clamped regions are implemented as periods in which the switching devices 56 are on and are not switched. Time is shown on the X-axis of Figure 4, while the Y-axis shows a per-unit measurement for the load voltage and current.

It is noted that the load voitage 70 and load current 72 shown in Figure 4 correspond, respectively, to the voitage and current of one of the phases of the electrical power output from the generator 38.

Per-unit voltage or current would be understood by the skilled person as an expression of the voltage or current with respect to a base value which is used as a reference. Using a per-unit system allows for normalization of values across transformers and other components that may change the value by an order of magnitude.

Superimposed on the voltage and current signals is a PWM drive signal 74 for the switching devices 56 of the leg 60, 64, the PWM drive signal 74 switching between either 0 and 1 or -1 and 0, to operate one or the other of the switching devices 56 of the leg 60, 64, For both switching devices 56 of the leg 60, 64, a value of 0 in the drive signal 74 corresponds to the ‘off’ state, whereas a drive signal value of 1 defines the ‘on’ state for one switching device 56 and a value of -1 defines the ‘on’ state for the other switching device 56.

In Figure 4, the PWM drive signal 74 shown is generated by a method known as ‘discontinuous pulse-width modulation’ (DPWM) for a two-level converter. The drive signal 74 for the cycle can be spilt into two sections 76, 78, one section for each switching device 56. The first section 76 is defined by the first half of the cycle shown in Figure 4, which can be characterised as positive in polarity, and the second section 78 therefore is defined by the second half of the cycle, which can be characterised as negative. As the sections 76, 78 are substantially identical except for their polarities, which indicates that they relate to different switching devices 56, only the first section 76 wiil be considered here.

The first section 76 can be separated out into a first switching period 80, a flat top region 82 and a second switching period 84. The first and second switching periods 80, 84 include a plurality of puises to switch the switching device 56 between on and off states. The width of the pulses - aiso referred to as the ‘duty cycle’ of the PWM drive signal 74 - may be varied to change the voltage output of the switching device 56. For example, if the duty cycle is 50%, the switching device 56 will output half of the maximum possible voltage.

Between the first and second switching periods 80, 84 is positioned the flat top region 82, during which the switching device 56 is ‘clamped’ in the ‘on’ state and so not switched, in this conventionai approach, it can be seen that the centre of the fiat top region 82 is aligned with the peak load voltage 70. The two switching periods 80, 84 and the flat top region 82 all last for a period corresponding to a phase angle of 60 degrees each, in this example. inaccuracy in the output signa! manifests as rippie in the output DC signai. To minimise this rippie, appropriate compensation can be applied; many known compensation techniques are suitable for this purpose and are not described in detail here. it is noted that in Figure 4 the ioad current lags the ioad voltage, in that the peak load current 72 occurs approximately 10 degrees later than the peak load voltage 70.

As the flat top region 82 is centred around the load voltage 70, which is out-of-phase with the ioad current 72, it follows that the fiat top region 82 is not centred around the load current 72. Indeed, the peak load current 72 occurs dose to the end of the flat top region 82 In Figure 4. Recognising that switching losses are proportional to the switched current level, allowing the second switching period 84 to begin at or ciose to the peak load current value will cause greater losses than if the switching was resumed when load current 72 was lower.

In view of this, the invention provides an approach by which a converter control system 54 can manage the PWM drive signal 74 to reduce switching losses compared with the known approach, by ensuring that switching of both devices 56 occurs when the load current 72 is as low as possible. This is achieved by aligning the centre of the flat top region 82 with the peak load current 72, and not with the peak load voltage 70 as in the prior art approach.

Figure 5 illustrates how a discontinuous PWM drive signal 90 can be shifted to align the centre of its flat top region 92 with the peak load current 72 according to an embodiment of the invention. Figure 6 illustrates a converter control system 54 to implement the shift or modulation shown in Figure 5, and is described later.

As can be seen in Figure 5, each switching device 56 is switched in the same manner as in Figure 4, with each drive signal 90 comprising a first switching period 94, a flat top region 92 and a second switching period 96, with each period 92, 94, 96 constituting 1/6th of a complete cycle. The centre of the flat top region 92 is aligned with the peak load current 72, and so is not aligned with the peak load voltage 70.

Figure 5 is purely used as an example of a shift that may be implemented using the invention. In Figure 5, the load current 72 lags the load voltage 70 by a phase angle of approximately 10 degrees, although it will be understood that the load current 72 may lead or lag the load voltage 70 by any amount so that a phase angle can be calculated between them. It is noted that there is no requirement to shift the PWM drive signal 90 relative to the known approach if the current and voltage are in-phase.

As described in relation to Figure 2, the control system 54is provided to communicate control commands to the generator-side converter 46 of the power converter 40, to control the conversion of the AC power output from the generator 38 into a DC signal.

Now considering both Figure 5 and Figure 6, the operation of the control system 54 will be explained.

The control system 54 shown in Figure 6 operates to calculate all PWM drive signals for each phase simultaneously. Alternatively, a separate control system may be specifically assigned to each phase, each separate control system being arranged as in Figure 6.

Additionally, it should be noted that although the control system 54 in Figure 6 is shown to be communicating its output drive signal 90 to an AC to DC, generator-side converter 46, the control system 54 may also, or alternatively, be adapted to communicate with a DC to AC, grid-side converter 48.

In this embodiment, the control system 54 comprises a current controller 102, a current angle detection (CAD) module 104 and a pulse-width modulator 106.

The control system 54 probes the connector 42 between the generator 38 and the generator-side converter 46 to sample electrical signals that are indicative of the load current 72 of each phase of the electrical power output from the generator 38. The control system 54 then outputs respective PWM drive signals 90 for each phase to the generator-side converter 46 based on the gathered signals.

Load currents 72 for each phase that are measured from the connector 42 between the generator 38 and the converter 46 form inputs to the current controller 102.

Current references 108 are also input to the current controller 102. The current references 108 are calculated according to the generator power required by the system to supply to the grid 43. To arrive at a current reference 108, the required power is converted to a converter compensation reference, which is used in a separate process by a converter controller (not shown) to compensate for losses in the wind turbine generator 10. The converter compensation reference is modified by a power converter (not shown), forming the current references 108 that are input to the current controller 102.

In this respect, it is noted that as the legs 60 of the generator-side converter 46 are connected in parallel, the electrical currents flowing on those legs 60 combine when output to the DC link 50. Therefore, the current controller 102 operates the generator-side converter 46 to modify the individual load currents of each phase of the generator output so that when combined they create a steady, constant current in the DC link 50. Thus, the current references 108 are determined to provide the required steady DC current at a desired magnitude.

Based on these inputs, the current controller 102 generates two outputs for each phase: first voltage references 110 and second voltage references 112. The first voltage references 110 are three-phase voltage readings calculated from the measured load currents using a suitable transformation such as the alpha-beta transformation. The second voltage references 112 are voltage outputs of the calculated first voltage references 110 at their fundamental frequency and with no harmonics present. In a permanent magnet synchronous generator or other variable speed constant frequency generator, this frequency is calculated based upon a measured rotational speed and pole number of the generator. For an induction generator, a slip controller calculates the desired fundamental frequency.

The current controller 102 communicates the second voltage references 112 to the CAD module 104. The CAD module 104 also receives the measured load currents as further inputs. The current controller 102 communicates the first voltage references 110 to the pulse-width modulator 106.

The CAD module 104 compares the second voltage references 112 and the measured load currents 72 to generate a shift angle 114 that is applied for each phase. The calculation of the shift angle 114 by the CAD module 104 is achieved by a known method such as a direct calculation of the angle or using a dedicated phase lock loop. The shift angle 114 corresponds to the phase angle between the load voltage 70 and load current 72, and is the angle by which the PWM drive signal 90 must be shifted to align the centre of the flat top region 92 with the peak load current in each phase.

Still considering Figure 6, the shift angle 114 output from the CAD module 104 is received by the pulse-width modulator 106, which also receives the first voltage references 110 from the current controller 102. Using these input values, the pulse-width modulator 106 generates a PWM drive signal 90 for each phase that is shifted relative to the respective load current 72 using the shift angle 114 generated by the CAD module 104. As discussed above, this modulation is in the form of shifting the flat top region 92 of the drive signal 90 so that the peak load current and centre of the flat top region 92 are aligned.

The generation of a PWM drive signal 90 by a modulator 106 is performed by comparison of a reference signal with a carrier signal and subsequent calculations to implement the flat top region 92 correctly. Once the drive signal 90 has been generated, it can be shifted relative to the load current 72 using the shift angle 114. As the skilled person is assumed to be familiar with the process for generating a PWM drive signal, this process is not described in any further detail here to avoid obscuring the invention.

Following this, the PWM drive signals 90 are communicated to the generator-side converter 46 and implemented at the switching devices 56, thereby to adjust the load currents 72 so that they sum to a constant DC current over time, while shifting the flat top region 92 to align with the peak load current in each phase.

Figure 7 is a flow chart illustrating a method 200 of controlling the wind turbine generator 10 using the control system 54. Referring to both Figures 6 and 7, initially the three-phase current levels of the generator 38 are measured at step 202 by acquiring sample data relating to the generator load currents 72.

The current controller 102 calculates three-phase voltage reference signals at the next step 204, which are transmitted to the CAD module 104. At the following step 206, the CAD module 104 then uses the voltage references 112 and the acquired current data 72 to calculate the shift angle 114, which is transmitted to the pulse-width modulator 106.

The pulse-width modulator 106 then generates a set of discontinuous PWM drive signals 90 at step 208, a respective signal for each phase, and at next step 210 dynamically adjusts them based upon the calculated shift angle 114, so that the flat top or clamped region 92 is centred around the peak or trough load current 72 for each phase. The PWM drive signals 90 are then communicated to the converter 46 at step 212 and implemented on the switching devices 56 to effect the conversion.

Many modifications may be made to the above examples without departing from the scope of the present invention as defined in the accompanying claims. For example, while the control system 54 is shown here as controlling a generator-side converter 46, it will be appreciated that the techniques and control methods discussed are equally applicable to controlling the grid-side converter 48.

Claims (11)

1. A method (200) of controlling a wind turbine generator (10) comprising an electrical generator (38) and a power converter (40), the power converter (40) comprising an electrical switch (56) that is configured to process electrical power produced by the electrical generator (38), the method (200) comprising: controlling an output from the electrical switch (56) using a discontinuous pulse-width modulated control signal (90) comprising a clamped region (92) during which the control signal (90) has a constant value, thereby to control characteristics of output power from the power converter (40); acquiring sample data relating to an electrical current (72) received by the electrical switch (56) from the generator (38); and dynamically adjusting the clamped region (92) of the discontinuous pulse-width modulated control signal (90) to centre the clamped region (92) around a peak or a trough of the electrical current (72) received by the electrical switch (56).

2. The method (200) of claim 1, comprising determining a shift angle (114) that determines the position of the clamped region (92) of the control signal (90).

3. The method (200) of claim 2, wherein the shift angle (114) corresponds to a phase angle between the electrical current (72) received by the electrical switch (56) and an electrical voltage output (70) from the electrical generator (38).

4. The method (200) of claim 2 or claim 3, wherein the shift angle (114) is determined by calculating a voltage reference (112), and comparing the voltage reference (112) with the acquired sample data relating to the electrical current (72) received at the electrical switch (56).

5. The method (200) of claim 4, wherein the voltage reference (112) is calculated by applying a transformation to the acquired sample data relating to the electrical current (72) received at the electrical switch (56).

6. The method (200) of claim 5, wherein the transformation comprises an alpha-beta transformation.

7. The method (200) of any preceding claim, wherein the acquired sample data relating to the electrical current (72) received at the electrical switch (56) comprises a magnitude of the electrical current (72).

8. The method (200) of any preceding claim, comprising determining a frequency of the electrical current (72) received at the electrical switch (56) based on the acquired sample data.

9. The method (200) of any preceding claim, wherein the pulse-width modulated signals (90) have carrier frequencies of at least 1000 Hz.

10. A controller (54) for controlling a wind turbine generator (10) comprising an electrical generator (38) and a power converter (40), the power converter (40) comprising an electrical switch (56) that is configured to process electrical power produced by the electrical generator (38), the controller (54) comprising a current controller (102) and a pulse-width modulator (106), wherein: the current controller (102) is configured to receive sample data relating to an electrical current (72) received by the electrical switch (56) from the generator (38); and the pulse-width modulator (106) is configured to: i) generate a discontinuous pulse-width modulated drive signal (90) comprising a clamped region (92) during which the control signal (90) has a constant value; ii) dynamically adjust the clamped region (92) of the discontinuous pulse-width modulated control signal (90) to centre the clamped region (92) around a peak or a trough of the electrical current (72) received by the electrical switch (56); and iii) communicate the control signal (90) to the electrical switch (56) to control the output of the electrical switch (56).

11. 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 in accordance with any of Claims 1 to 9.