DK178199B1

DK178199B1 - Method for controlling the shape of the rising and falling edges of an output voltage pulse of a PWM converter

Info

Publication number: DK178199B1
Application number: DKPA201300602A
Authority: DK
Inventors: Paul Bach Tøgersen
Original assignee: Wind Solutions As Kk
Priority date: 2013-10-23
Filing date: 2013-10-23
Publication date: 2015-08-03
Also published as: DK201300602A1; WO2015058773A1

Abstract

Method for controlling the shape of the rising and falling edges of an output voltage pulse of a PWM converter for a renewable power generating device, the PWM converter comprising a plurality of parallel inverter phase legs forming a phase group, the plurality of parallel inverter phase legs are adapted to be controlled individually, wherein the individual control includes independent switching of each of the inverter phase legs and wherein the switching of the inverter phase legs is controlled according to a micro shifting pattern.

Description

Method for controlling the shape of the rising and falling edges of an output voltage pulse of a PWM converter

Field of invention

The invention relates to an apparatus and method for controlling the shape of the rising and falling edges of an output voltage pulse of a PWM converter.

Background of the Invention

Power converters are known to be used for controlling the output voltage of a generator, for example in wind turbines, in order to connect the generator to the utility grid.

Conventionally, a generator, for example a wind turbine generator, is connected to the utility grid via power converters in a back-to-back configuration. Such back-to-back power converter configuration comprises a machine side inverter/converter which via a DC link (DC; Direct Current) is connected to a grid/line side inverter/converter.

US5852554 discloses a power inverter which can drive in parallel three or more PWM-type inverting units with each phase current balanced. This is obtained by providing to a PWM generator voltage and frequency instructions, the PWM generator then in response hereto generates a PWM pulse to each phase line with a period and width being dependent on the voltage and frequency instructions.

The voltage from a generator is rectified by the first PWM converter (PWM; Pulse Width Modulation) into a constant magnitude DC voltage. This DC voltage is then converted, by use of the second PWM converter, into an AC voltage (AC; Alternating Current) for the utility grid. The second PWM converter is adapted to deliver AC voltage that complies with the grid codes for the specific utility grid that it is connected to. In addition it is known to supply a motor (or the generator) with voltage from the utility grid via the power converters as it is also known to at least partly control the output of a DFIG generator (DFIG; Doubly Feed Induction Generator) by means of the power converter.

Between the first PWM converter and the electrical machine e.g. a generator a dU/dt filter is situated in order to protect especially the windings of the generator from voltage peaks and/or high dU/dt values. The dU/dt filter normally comprises an inductor, a capacitor and a damping resistance and/ or a clamping circuit.

The voltage peaks and/or the high dU/dt value at the generator windings are caused by the switching of the semiconductor switches of the inverter phase leg(s) of the first PWM converter,

In the absence of a filter these voltage peaks and/or the high dU/dt value may damages the generator windings.

Summary of the invention

Considering the prior art described above, it is an object of the present invention, to provide a method for controlling rising and falling edges of an output voltage pulse of a PWM converter, which makes it possible to omit the dU/dt filter, resulting in a system with less components and therefore cheaper, and minimising the electrical loss without generating damaging voltage peaks.

The object can be achieved by means of a method for controlling the shape of the rising and falling edges of an output voltage pulse of a PWM converter for a renewable power generating device, the PWM converter comprising a plurality of parallel inverter phase legs forming a phase group, the plurality of parallel inverter phase legs are adapted to be controlled individually by a controller, wherein the individual control includes independent switching of each of the inverter phase legs and wherein the switching of the inverter phase legs is controlled according to a micro shifting pattern.

Throughout this application when referred to “switching inverter phase leg” in practice a reference is made to switching of the switches of the inverter phase leg.

Thus, it is possible to get the same performance as prior art systems without producing the above mentioned voltage peaks (also referred to as overvoltage peaks) and/or high dU/dt values. Therefore, the method makes it possible to omit (or at least reduce the size of) the conventionally used dU/dt filter, which will reduce the electrical loss in the electrical system and make it cheaper, as fewer / smaller components are used.

By converter is meant electrical power converter; a device that change electrical energy from one form to another, for example, converting the electrical energy from AC to DC, DC to AC and/ or changing the voltage or frequency. Other words can also be used instead of converter; among them are inverter and rectifier. As an example in a power converter for use e.g. in a renewable power generating device such as a solar or a wind turbine application the output voltage from the generator of the wind turbine are controlled by one or more power stacks (also referred to as inverter sections comprising inverter phase legs) each comprising several semiconductors switches, such as IGBTs (IGBT; Insulated Gate Bipolar Transistor), diodes, Thyristors, GTOs (GTO; Gate Turn Off) etc. typically by PWM. It should be mentioned that the converter or inverter may be of a single-phase or multi-phase type.

Control according to an advantageous embodiment of the invention is understood as a controller, such as a higher level controller (e.g. Wind Turbine Controller), a converter controller, power stack controller etc., which is determining or receiving information on how to control or shape the output voltage from the PWM converter. Such information could include height (voltage) and/or duration (time) of an output voltage pulse i.e. the area of the output voltage pulse. When this information is obtained e.g. preferably the converter controller or power stack controller is applying a micro shifting pattern to control the switching of the individual inverter phase legs including the ordered time shifted switching of the inverter phase legs of a phase group. In this way the shape of the rising and falling edges of the output voltage pulsed is controlled in a desired manner to achieve the advantages of the present invention. The control of the shape of the rising and falling edges of the output voltage pulse may be referred to both as shape and control.

The machine side inverter/converter is also referred to as first PWM converter or simply PWM converter and the grid side inverter/converter is also referred to as second PWM converter i.e. the converters are being controlled based on PWM principles.

It should be mentioned that throughout this document when referring to control of inverter phase legs reference is made to control of parallel inverter phase legs in a phase group. In case of a multi-phase converter multiple phase groups are present each having a plurality of inverter phase legs controlled according to the present invention.

In an embodiment of the invention, the micro shifting pattern is at least partly implemented as time delays separating the switching of the inverter phase legs of a phase group in time. This is advantageous in that by introducing a time delay and/or adjusting the duration of each time delay the rising and falling edges of the output voltage pulse is shaped accordingly.

The micro shifting pattern could be predefined hence e.g. the order in which the individual inverter phase legs are switched, the time delay between switching the inverter phase legs etc. are manually configured. Alternatively the micro shifting pattern may be created automatically e.g. in real time based on measurements in relation to the PWM converter or the electrical system surrounding it.

In an embodiment of the invention the time delay between the switching of a first and a second inverter phase leg of the plurality of parallel inverter phase legs are equal to the time delay between switching of subsequent inverter phase leg of the plurality of parallel inverter phase legs. This is advantageous in that loads may be distributed more equally between the inverter phase legs than compared to control where the time delay are not equal. The latter may be possible and allows flexible control of the inverter phase legs in that it allows to e.g. shaping the rising and falling edges of the output voltage in any desired way or load one inverter phase leg more than another inverter phase leg.

Further it is advantageous that the time delay between switching of inverter phase legs are the same in that the inverter phase legs are contributing equally to the output voltage pulse both in relation to time and voltage. It should be mentioned that the output voltage pulse is also sometimes simply referred to as output voltage.

In an embodiment of the invention the switching of the plurality of inverter phase legs are symmetric in time around the reference voltage. In this way, the pulse shaping will not affect (increase or decrease) the (average) output voltage, The reference voltage is understood as the ideal output voltage pulse created in a situation where e.g. only one inverter phase leg is present or two inverter phase legs are switched at the same time forming a (ideally) vertical rising and falling edges of the output voltage pulse. Symmetric in time should be understood as, in the case of two phase legs in a phase group, the switching of a first inverter leg is done at half the time delay before the reference voltage switching time and the second inverter phase leg is switched the half the time delay after the reference voltage switching time This may also be part of the micro shifting pattern.

In an embodiment of the invention the order of the switching of the inverter phase legs are alternating. By alternating is understood that the order, in which the individual inverter phase legs in a phase group is switched, is changed. Hence in a first period inverter phase leg 1 is switched first and subsequently inverter phase leg 2. In the second period the order is reversed and thereby inverter phase leg 2 is switched first followed by inverter phase leg 1 (in configurations where a phase group comprises two parallel inverter phase legs). This balances the loading of the inverter phase legs if e.g. the load is greatest when the first inverter phase leg of a phase group is switched.

It should be mentioned that the micro shifting pattern could also include e.g. the order in which the individual inverter phase legs are switched.

Preferably, the output voltage of the plurality of parallel inverter phase legs of the PWM converter are connected to an electrical machine, preferably a generator. The method according to the invention is suitable for reducing voltage peaks (also referred to as overvoltage peaks) and /or high dU/dt values at the machine/generator windings/terminals when converting the electrical energy generated by the machine/generator by, performing the above mentioned method, on the generator side inverter (also referred to as PWM converter or first PWM converter). In this way the method according to the invention is suitable for shaping the resulting output voltage in a suitable manner for the connected machine i.e. generator, motor, etc. with a more simple filter with lower power losses compared to prior art.

In an embodiment, the plurality of parallel inverter phase legs, are connected to the electrical machine via at least one inductor, preferably one inductor per inverter phase leg. The electrical machine can be any kind of generator e.g. DFIG generator of a wind turbine or motor. Even though the present invention makes it possible to omit / reduce the size of the dU/dt filter, it is preferred to use an inductor to smoothen the current.

The inductor can for example be the natural or physical inductance in a conductor such as one or more cables, rails, etc. connecting the plurality of parallel inverter phase legs with the electrical machine e.g. a generator.

As mentioned the inductance (the physical inductance of e.g. a cable, inductor, etc.) may act as a filter (e.g. a small dU/dt filter) e.g. together with one or more control algorithms for shaping the edges and displace these edges mutually within a phase group of the parallel inverter legs.

It should be mentioned that the inductor mentioned above in a three phase configuration could be implemented as three single inductors or as one three phase inductor.

In an embodiment, the time delay between the switching of two inverter phase legs are less than 0.5% of the maximum width of the output voltage pulse. Preferably, the time delay is less than 0.2% of the maximum width of the output voltage pulse. These relatively short time delays compared to the maximum width of the output voltage pulse are advantageous as it enables generation of output voltage pulses with relative steep voltage edges, and at the same time avoiding the (over)voltage peaks and/or high dU/dt value generated because of the simultaneously switching of inverter phase legs when prior art methods are used. It should be mentioned that the output voltage pulses are preferably measured on the machine side of the PWM converter.

Advantageously, the time delay between the shifting of two parallel inverter phase legs are less than 1.0ps, preferably less than 0.5ps. The width of the output voltage pulse can for example be 500ps and the time delays can then be equal to or less than 0.5ps, which leads to an output voltage pulse similar to prior art methods, but without the (over)voltage peak produced when using prior art methods. As mentioned this is possible because of the control of the rising and falling edges of the output voltage of the individual inverter phase legs. The parallel inverter phase legs within a phase group is individually controlled to enable the mutual delay in switching of parallel inverter phase legs leading to the small displacements shaping the rising and falling edges of the final output voltage of that phase.

Advantageously, the PWM converter comprises three, four or five parallel inverter phase legs. The number of inverter phase leg determines the number of steps in the rising and falling edges of the final output voltage pulse. Thus, it is possible to shape the output voltage in a suitable manner.

It should be noted that in case of more than two inverter phase legs the time delay between the switching of leg 1 and 2 does not necessarily have to be exact the same as the time delay between switching of leg 2 and 3. The time delay depends on the desired shape of the rising and/or falling edge of the output voltage pulse. Typically a phase group comprises between 2 and 8 inverter phase legs but higher numbers of inverter phase legs could also be controlled according to the present invention.

Preferably, the PWM converter is a multiphase PWM converter. Preferably, a three phase converter. The multiphase PWM converters comprise a plurality of inverter sections (also referred to as Power stacks), wherein each inverter section has multiple inverter phase legs, preferably three inverter phase legs. The plurality of inverter sections are arranged in parallel in such a way that the first inverter phase legs of one inverter section is arranged in parallel to the first inverter phase legs of the other inverter sections.

The paralleled first inverter phase legs are together forming the final or total output voltage of the first phase according to the overall converter control. Likewise the paralleled second inverter phase legs are together forming the final or total output voltage of the second phase and finally the paralleled third inverter phase legs are together forming the final or total output voltage of the third phase.

In a three phased configuration, the paralleled first inverter phase legs, the paralleled second inverter phase legs and the paralleled third inverter phase legs are sometimes referred to as first, second and third phase group respectfully.

The invention further regards an apparatus for shaping the rising and falling edges of an output voltage pulse comprising, a PWM converter for a renewable power generating device having a plurality of parallel inverter phase legs adapted to be controlled individually, and, a controller adapted to control switching of each of the plurality of parallel inverter phase legs according to a micro shifting pattern. This apparatus ensures that harmful voltage peaks are reduced at least at the machines side of the PWM power converter as described above whereby a dU/dt filter positioned between the power converter and the machine can be omitted or at least significantly reduced resulting in a cheaper product.

Preferably, the micro shifting pattern includes a time delay between the switching of two parallel inverter phase legs, wherein the time delay is less than 0.5% of the width of the output voltage pulse. Preferably, the time delay is less than 0.2% of the width of the output voltage pulse. Thus, the rising edge of the output voltage pulse can be steep in order to be as close to a square pulse as possible.

Preferably, the time delay between the switching of two parallel inverter phase legs are less than 1 .Ops, preferably less than 0.5ps. Thus, an output voltage pulse similar to prior art voltage pulses can be generated, but without the (over)voltage peak produced by prior art devices.

In an embodiment, the PWM converter comprises at least three parallel inverter phase legs, preferably the time delays between the switching of the parallel inverter phase legs are the same. With this said situations could occur where difference in the time delays are not the same. Thus, it is possible to shape the output voltage in a desired manner.

Advantageously, the PWM converter is an AC to DC PWM converter. The PWM converter can then, for example, be used to convert a voltage generated by a generator, e.g. of a wind turbine, and be in a back-to-back configuration with a second PWM converter which delivers power to the supply grid.

As mentioned the electrical machine may at least partly be controlled or supplied from the utility grid through the PWM converter.

It is to be understood that the apparatus can be modified to perform any of the embodiments of the methods mentioned above.

The invention also regards the use of an apparatus according to the invention, to shape the rising and falling edges of an output voltage pulse of a PWM converter. The invention reduces the size of the (over)voltage peaks generated, in a cheap and efficient way, and therefore it reduces the risk of the generator winding insulation is damaged in comparison with prior art systems.

Description of the drawings

The invention will in the following be described in greater detail with reference to the accompanying drawings:

Fig. 1 a schematic view of a power converter in a back-to-back configuration.

Fig. 2a a schematic view of a first embodiment of the invention.

Fig. 2b a graph of an output voltage pulse created by the embodiment of fig. 2a.

Fig. 3 a schematic view of a second embodiment of the invention.

Fig. 4 a graph of an output voltage pulse of a third embodiment of the invention.

Detailed description of the invention

Fig. 1 shows a schematic view of a power converter 8 in a back-to-back configuration. The power converter 8 comprises a first PWM converter 1 connected to a second PWM converter 9 via a DC link 10 and controlled by a controller 14. The second PWM converter is connected to the utility grid 11 and the first PWM converter is connected to an electrical machine 12 such as a generator 2 suitable for a renewable power generating device preferably a wind turbine. In some wind turbine applications the power converter 8 is connected to the rotor of such generator 2 and the stator of the generator 2 is connected directly to the utility grid 11.

The controller 14 is a converter controller and could receive reference values such as output voltage reference from a higher level controller such as a wind turbine controller (not illustrated). In addition the converter controller may control lower level controllers such as power stack controllers (not illustrated) implementing the control (e.g. the micro shifting pattern) of the inverter phase legs of the inverter section (also referred to as power stacks).

Fig. 2a shows in more details a single-phase PWM converter 1 having two inverter phase legs 3, 4 in parallel forming one phase group. The output voltage from the PWM converter 1 is measured in point A and between the point A and the inverter phase legs 3, 4 inductors 7 are located. The PWM converter 1 is able to convert an AC voltage from a machine (e.g. a generator or motor) 2 to a DC voltage (indicated as VDC+ and VDc") and vice versa. The first inverter phase leg 3 and a second inverter phase leg 4 of the PWM converter is also referred to as first and second inverter section or power stack.

The first inverter phase leg 3 has two switches 5A and 5B (e.g. IGBTs or the like) which are switched by use of the control signal SA and SB respectively. The second inverter phase leg 4 also has two switches 5C and 5D which are switched by use of the control signal Sc and SD respectively. The basic principles of the switching strategy for the switches in a PWM controlled converter is known to the skilled person and thus not described further.

The control signals SA, SB, Sc and SD is preferably generated by a controller 14 such as a dedicated converter controller or a power stack controller so that the individual inverter phase legs 3, 4 contributes to the output voltage as required according to the micro shifting pattern.

In order to shape the rising edge of the output voltage, the first inverter phase leg 3 is switched so that a constant output voltage is generated of maybe half the maximum value of the output voltage of point A (see reference 17 on figure 2b). After a time delay of e.g. 0.5ps preferably less the second inverter phase leg 4 is switched (see reference 18 on figure 2b), whereby the resulting output voltage rises to the maximum value in point A i.e. Vdc+. This is explained in further details in figure 2b and in a configuration with four inverter phase legs in relation to figure 4 below.

Shaping the falling edge of the output voltage is done in similar fashion, firstly the first inverter phase leg 3 is switched whereby the output voltage is lowered to an intermediate value e.g. to half the maximum value (see reference 19 of figure 2b). After a time delay of 0.5ps or less the second (preferably equal to the time delay between the rising edges of inverter phase legs 3, 4) inverter phase leg 4 is switched (see reference 20 of figure 2b) reducing the output voltage in point A to the minimum voltage value i.e. Vdc-.

By using the switching strategy explained above for fig. 2a the shaping of the output voltage in point A is performed by fast sequencing or micro shifting of the two parallel inverter phase legs 3, 4. Hereby, the (over)voltage pulses generated by prior art systems are avoided. Hence the sequencing or micro shifting is understood as timing control of the switching of the switches of the inverter phase legs in a phase group.

The switching strategy is preferably a part of the micro shifting pattern which may be implemented as control algorithms processed by the controller 14.

Fig. 2b discloses a graph of part of a voltage waveform for a PWM converter 1 with 2 parallel inverter phase legs 3, 4 controlled according to an embodiment of the present invention. The graph has; time (t) on the x-axis and output voltage (U) of the PWM converter on the y-axis measured at point A of figure 2a.

The dotted line 15 illustrates a reference voltage pulse as explained above. The solid line 16 illustrates that the two inverter phase legs 3, 4 are micro shifted symmetric around the rising and falling edges of the reference voltage pulse 15. The time delay constituting the micro shifting in this embodiment is illustrated as the arrow denoted td

One way of controlling the inverter phase legs 3, 4 is to make sure that the area of the voltage reference pulse 15 is equal to the area of the curve 16 created by the switching of the inverter phase legs 3, 4.

With this said an error could be introduced or occur leading to the above describe areas are not equal. In this case the error may be corrected at either the next pulse and/or in the same control period. A control period may be equal to a voltage pulse or e.g. be twice the frequency of the output voltage. In the latter case the error may be corrected halfway through a pulse i.e. at the next either rising or falling edge.

It should be mentioned that the longer time delay td of the micro shifting of inverter phases in a phase group the larger inductors 7 is required. The summation of all time delays of the micro shifting of inverter phase legs is preferred to be around 0,5ps (microsecond).

The embodiment of figure 2b illustrates one phase group comprises two inverter phase legs 3, 4. When the inverter phase leg 3 is switched illustrated by the raising edge 17 the output voltage in point A is half its maximum i.e. half Vdc+. Next is the inverter phase leg 4 switched illustrated by raising edge 18 increasing the output voltage in point A to its maximum i.e. Vdc+. In the same way the inverter phase leg 3 is switched illustrated by the falling edge 19 before the second inverter phase leg 4 illustrated by the falling edge 20. The result of this is half the output voltages minimum value i.e. half Vdc-, and the minimum value Vdc-respectively.

The time delay td may be 0,5ps which when compared to the time of e.g. 500ps the output voltage is on its maximum Vdc+ makes the edges steep. The reason why this is not seen from the illustration on figure 2b is that this illustration does not give the accurate dimensions.

As illustrated the switching of the first and second inverter phase legs 3, 4 is micro shifted according to a micro shifting pattern consisting of at least the time delay td symmetric around the edges of the reference voltage 15 leaving the areas below the curves 15 and 16 equal. As mentioned the symmetric shifting is advantageous in that it will not affect (increase or decrease) the (average) output voltage.

Fig. 3 discloses a 3-phase PWM converter 13 (i.e. a multi-phase converter) comprising two inverter sections 6A, 6B (also referred to as power stacks) arranged in parallel. The inverter phase legs 6A-| and 6B1: 6A2 and 6B2i 6A3 and 6B3 respectively are forming first, second and third phase groups. The PWM converter 13 is connected to a three-phase electrical machine 12 such as a generator. The three-phase PWM converter 13 is similar in function as the single-phase PWM converter 1 of fig. 2 i.e. PWM controlled and the total output voltage of the inverter sections 6A and 6B is measured in point A.

Between the generator 2 and the inverter sections 6A and 6B, inductors LA and LB are arranged so LA is between the generator 2 and the inverter section 6A and LB is between the generator 2 and the inverter section 6B. As an alternative to the three phased inductors LA, LB six single phased inductors could also have been used. Each inverter section 6A and 6B has three inverter phase legs ΘΑ^ 6A2, 6A3, ΘΒ^ 6B2, 6B3, respectively. The inverter phase leg 6A-i is arranged in parallel with the inverter phase leg 6B-|, the inverter phase leg 6A2 is arranged in parallel with the inverter phase leg 6B2 and inverter phase leg 6A3 is arranged in parallel with the inverter phase leg 6B3.

As mentioned between the inverter phase legs 6A-i, 6A2, 6A3, 6B-i, 6B2, 6B3, and the generator 2 the inductors 7A1; 7A2, 7A3, 7B1; 7B2, 7B3 are arranged. The PWM converter 13 connects the generator 2 to a DC voltage denoted VDc+ and VDc"·

The generator 2 delivers AC current in three phases. Each phase is delivered to two parallel inverter phase legs, such as 6A-1 and 6B-1, the current passes an inductor, such as 7A-i and 7B-|. The inductor functions as a simple filter such as a simple dU/dt filter. The parallel inverter phase legs ΘΑ^ 6A2, 6A3, ΘΒ^ 6B2, 6B3of the inverter sections 6A, 6A are controlled in similar fashion as described above by power stack controllers 12 connected to converter controller 14 and/or higher level controller such as a wind turbine controller (not illustrated). Each inverter phase leg βΑ^ 6A2, 6A3, βΒ^ 6B2, 6B3 can operate simultaneously and independent of the other inverter phase leg(s) in the inverter section / power stack 6A, 6B.

The PWM converter 13 (of figure 3) and 1 (of figure 1 and 2a) could be controlled by means of the same type / level controller and same principles resulting in an output voltage in line with what is illustrate on figure 2a and figure 4.

Fig. 4 discloses a graph of part of a voltage waveform for a PWM converter with a phase group having 4 parallel inverter phase legs, controlled according to an embodiment of the method of the present invention. The graph has; time (t) on the x-axis and output voltage (U) of the PWM converter on the y-axis measured at point A of figure 2a and 3. UA, UB, Uc, UD is the contributing voltage from each parallel inverter phase leg A, B, C, D. In the shown embodiment, the voltages UA, UB, Uc and UD are equal i.e. one quarter of the maximum output voltage Vdc+. In this embodiment the rising edge of the output voltage Vdc+ (Ua+b+c+d) is shaped as follows. Inverter phase leg A is switched at time t1+ resulting in an output voltage UA. At time t2+ inverter phase leg B is switched resulting in an output voltage UA+B (=UA + UB). At time t3+ inverter phase leg C is switched, resulting in the output voltage UA+B+C (=UA + UB + Uc) and at the time t4+ inverter phase leg D also adds to the output voltage resulting in Ua+b+c+d (=Ua + UB + Uc + UD) also referred to as Vdc+.

The falling edge of the output voltage pulse is shaped as follows. At time t-i_ the voltage is then lowered by, firstly, switching leg A so the resulting output voltage becomes Ub+c+d (=UB + Uc + UD) and at time t2_ the output voltage is lowered to Uc+d (=Uc + UD) by switching leg B, at t3_ to UB by switching leg C and finally at time t4_ the final leg, D, is switched and the resulting voltage from the PWM converter is Vdc-

In this embodiment the time difference between t1+, t2+, t3+ and t4+ and between ti., t2_, t3. and t4. is equal to 0.125ps. The output voltage is then kept at Vdc+ for e.g. 200ps. Hence it is noted, that the micro shifting pattern duration of the rising and falling edges of the output voltage pulse is substantially smaller (0.125ps per. step) compared to the duration of the total width of the output voltage pulse (200ps). Therefore, the output voltage pulse can be considered to have a rectangular form. Further it is noted that the sum of the four steps are 0,5ps i.e. 0,25% of the duration of the output voltage at Vdc+.

As mentioned neither the time nor the contribution of voltage needs to be equally shared between the inverter phase legs in a phase group even though this is preferred.

The switching of the inverter phase legs could as mentioned be done in a reversed order so to speak i.e. according to specific predetermined micro shifting patterns.

Hence the four inverter phase legs forming the output voltage of figure 4 could be switched in the following order: first period leg 1,2, 3, 4; second period leg 2, 3, 4, 1; third period leg 3, 4, 1,2; etc.

In wind turbine applications the generator needs to be magnetised during start-up. This is done by supplying the generator with power from the grid or another energy source. Here, the method according to the invention, can ensure that no voltage peaks are generated, which can harm the windings of the generator during the magnetising of the generator.

As shown on fig. 4 the invention can relate to a PWM converter with a plurality (e.g. four) of parallel inverter phase legs, where the legs are controlled individually in order to shape the output voltage in a suitable manner for the connected device (e.g. a generator). The shaping of the output voltage is performed by control (e.g. fast sequencing) of the plurality of parallel inverter phase legs in each phase group by use of software (the micro shifting described above) and/or hardware based control such as programmable logic circuitry. Thereby, the complicated prior art hardware based on dU/dt filter can be omitted or at least significantly reduced, and in some embodiments be replaced by a simple inductor. This is advantageous as it minimises the costs, electrical loss, lowers the volume and weight and improves the reliability of the power converter 8 configuration.

It should be mentioned that the above described figures are all describing embodiments of the same invention and that features from the figures can be used mutually in relation to all figures.

Reference list: A. output voltage 1. PWM converter / first PWM converter / machine side converter 2. electrical machine such as a generator 3. first inverter phase leg 4. second inverter phase leg 5A, 5B, 5C, 5D. switch (e.g. IGBT) 6A, 6B. inverter section ΘΑ^ 6A2, 6A3, ΘΒ^ 6B2, 6B3. inverter phase leg 7. 7Ai, 7A2, 7A3, 7B15 7B2, 7B3. inductor 8. power converter 9. second PWM converter / grid side converter 10. DC link 11. utility grid 12. power stack controller 13. three phase PWM converter 14. controller

Claims

A method of controlling the shape of the rising and falling flanks of an output voltage pulse from a PWM power converter (1) to a sustained power generation device, the PWM power converter (1) comprising a plurality of parallel inverter phase legs (3, 4, 6), forming a phase group, the plurality of parallel inverter phase legs (3, 4, 6) being arranged to be controlled individually, the individual control unit including independent switching of each of the inverter phase legs (3, 4, 6), thereby providing the output voltage pulse to a generator (2) characterized in that the switching of the inverter phase legs (3, 4, 6) is controlled according to a micro-offset pattern which forms the rising and falling flanks of the output voltage pulse in a desired manner.

The method of claim 1, wherein the micro-offset pattern is implemented as time delays (td) which temporarily separate the switching of a phase group's inverter phase legs (3, 4, 6).

A method according to any one of the preceding claims, wherein the time delay (td) between the switching of a first and a second inverter phase leg by the plurality of parallel inverter phase legs (3, 4, 6) is the same as the time delay between switching the subsequent inverter phase leg the plurality of parallel inverter phase legs (3, 4, 6).

A method according to any one of the preceding claims, wherein the switching of the plurality of inverter phase legs (3, 4, 6) is symmetrical about a reference voltage.

A method according to any one of the preceding claims, wherein the order of switching of the inverter phase legs (3, 4, 6) is alternating.

A method according to any one of the preceding claims, wherein the output voltage of the plurality of the parallel inverter phase legs (3, 4, 6) of the PWM power converter is connected to an electric machine (2), preferably a generator.

A method according to any one of the preceding claims, wherein the plurality of parallel inverter phase legs (3, 4, 6) are connected to the electric machine (2) via at least one inductor, preferably one inductor per second. inverter phase legs (3, 4, 6).

A method according to any one of the preceding claims, wherein the time delay (td) between the switching of two inverter phase legs (3, 4, 6) is less than 0.5% of the output voltage pulse width.

A method according to any one of the preceding claims, wherein the time delay (td) between the switching of two parallel inverter phase legs (3, 4, 6) is less than 0.5 ps, preferably less than 0.1 ps.

A method according to any one of the preceding claims, wherein the PWM power converter (1) comprises three, four or five parallel inverter phase legs (3, 4, 6).

A method according to any one of the preceding claims, wherein the PWM power converter (1) is a multiphase PWM power converter (1).

An apparatus for forming the rising and falling flanks of an output voltage pulse, comprising: - a PWM power converter (1) for a sustained power generating device having a plurality of parallel inverter phase legs (3, 4, 6) arranged to individually controlled, and - a control unit adapted to control the switching of each of the plurality of parallel inverter phase legs (3, 4, 6) according to a micro-offset pattern wherein the output voltage pulse is configured to be provided to a generator (2), characterized in that the switching of the inverter phase legs (3, 4, 6) is configured to be controlled according to the micro-offset pattern so as to shape the rising and falling flanks of the output voltage pulse in a desired manner.

The apparatus of claim 12, wherein the micro-offset pattern includes a time delay (td) between the switching of two parallel inverter phase legs (3, 4, 6), wherein the time delay (td) is less than 0.5% of the output voltage pulse width.

Apparatus according to any one of claims 12 to 13, wherein the time delay (td) between the switching of two parallel inverter phase legs (3, 4, 6) is less than 1.0 ps, preferably less than 0.5 ps.

Apparatus according to any one of claims 12 to 14, wherein the PWM power converter (1) comprises at least three parallel inverter phase legs (3, 4, 6).

Apparatus according to any one of claims 12 to 17, wherein the PWM power converter (1) is an AC-to-DC PWM power converter.

Use of an apparatus according to any one of claims 12 to 16 for forming an output voltage pulse of a PWM power converter (1).