WO2018068843A1

WO2018068843A1 - Adaptive delay of a third harmonic component

Info

Publication number: WO2018068843A1
Application number: PCT/EP2016/074435
Authority: WO
Inventors: Axel Andersson
Original assignee: Abb Schweiz Ag
Priority date: 2016-10-12
Filing date: 2016-10-12
Publication date: 2018-04-19

Abstract

A controller controlling the phase of a third harmonic component of a control signal (Uv_ref_tot) used for forming a waveform on an AC terminal of a phase leg comprises a third harmonic delay determining module (18) operative to obtain a modulation reference (R_P) of the control signal used for an upper phase arm of the phase leg, obtain a modulation reference (R_N) of the control signal used for a lower phase arm of the phase leg,determine a first extreme value based on the two modulation references (R_P, R_N) in a first half period of the waveform, determine a second extreme value based on the two modulation references (R_P, R_N) in a second half period of the waveform,determine a difference between the first and second extreme values, process the difference in order to obtain a delay (φ) and adjust the phase of the third harmonic component using the delay.

Description

ADAPTIVE DELAY OF A THIRD HARMONIC COMPONENT

FIELD OF INVENTION The present invention generally relates to voltage source converters. More particularly the present invention relates to a method, controller, voltage source converter and computer program product for controlling the phase of a third harmonic component added to a fundamental frequency component of a control signal used for controlling a phase leg of the voltage source converter.

BACKGROUND

There exist different types of voltage source converters, where one type that has become of interest lately is the cell based or modular multilevel voltage source converter. This converter is for instance described in DE 10103031.

It is in many voltage source converter applications of interest to add an odd harmonic component, like a zero sequence third harmonic

component, to a basic reference waveform used for controlling the converter, where the basic reference waveform then forms a fundamental component of the resulting waveform. This has the effect of reducing the peak value of the modulation reference, which in turn allows an increase of the modulation range, i.e. of the converter output voltage.

However, for the cell based voltage source converters such as modular multilevel converters, there are some problems with adding such third harmonics.

It may as an example be necessary to shift the phase of the third harmonics in relation to the fundamental component. A phase shift of this added third harmonic is not trivial. It depends on several factors, such as an operating point as defined by the power being delivered, circulating current, cell voltage ripple etc. If the third harmonic is not optimally shifted in phase, there may be negative effects since more cells than necessary need to be inserted or bypassed in order to fulfill the output voltage reference, leading to lower control margins and lower active and reactive power delivery capability of the converter.

One way in which the third harmonics may be shifted is described in WO 2011/032581, where an adjustment factor that is determined based on an analytical expression determined for an operating point of the converter is used to shift the third harmonic.

This type of adjustment generally functions well. However it may be fairly static and not adapt to control system variations around the operating point. There is therefore a need for a more flexible phase adjustment of the added third harmonics.

The present invention is directed towards this type of flexible third harmonic phase adjustment.

SUMMARY OF THE INVENTION

The present invention is directed towards the adaptive adjustment of the third harmonic component of a waveform used to control a voltage source converter.

This object is according to a first aspect achieved through a method for controlling the phase of a third harmonic component added to a fundamental frequency component of a control signal used for forming a waveform on an AC terminal of a phase leg of a voltage source converter, the method comprising the steps of:

- obtaining a modulation reference of the control signal used for an upper phase arm of the phase leg,

- obtaining a modulation reference of the control signal used for a lower phase arm of the phase leg,

- determining a first extreme value based on the two modulation

references in a first half period of the waveform,

- determining a second extreme value based on the two modulation references in a second half period of the waveform,

- determining a difference between the first and second extreme values,

- processing the difference in order to obtain a delay, and

- adjusting the phase of the third harmonic component using the delay.

This object is according to a second aspect achieved through a controller for controlling the phase of a third harmonic component added to a fundamental frequency component of a control signal used for forming a waveform on an AC terminal of a phase leg of a voltage source converter, the controller comprising a third harmonic delay determining module operative to :

- obtain a modulation reference of the control signal used for an upper phase arm of the phase leg,

- obtain a modulation reference of the control signal used for a lower phase arm of the phase leg,

- determine a first extreme value based on the two modulation

references in a first half period of the waveform,

- determine a second extreme value based on the two modulation

references in a second half period of the waveform,

- determine a difference between the first and second extreme values,

- process the difference in order to obtain a delay, and

adjust the phase of the third harmonic component using the delay. The object is according to a third aspect achieved through a voltage source converter comprising a controller according to the second aspect.

The object is according to a fourth aspect achieved through a computer program product for controlling the phase of a third harmonic component added to a fundamental frequency component of a control signal used for forming a waveform on an AC terminal of a phase leg of a voltage source converter, the computer program product comprising a data carrier with computer program code configured to cause a controller of the converter to:

obtain a modulation reference of the control signal used for an upper phase arm of the phase leg,

obtain a modulation reference of the control signal used for a lower phase arm of the phase leg,

determine a first extreme value based on the two modulation references in a first half period of the waveform,

determine a second extreme value based on the two modulation references in a second half period of the waveform,

determine a difference between the first and second extreme values, process the difference in order to obtain a delay (φ), and

adjust the phase of the third harmonic component using the delay.

The present invention has a number of advantages. It allows an optimal phase shift of the third harmonic to be obtained for all operating points. This is furthermore done adaptively as the controller is used. The controller is thereby able to adapt to instantaneous variations through using feedback of the two modulation references. This is also achieved with limited additions to the controller. It only requires limited additional software in order to be implemented. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will in the following be described with reference being made to the accompanying drawings, where fig. l schematically shows a voltage source converter that may be controlled using a voltage reference comprising a third harmonic frequency component combined and a fundamental frequency component, fig. 2 shows a block schematic of a controller for controlling the voltage source converter,

fig. 3 schematically shows a control loop for controlling a phase leg and comprising modules comprising a delay determining module and various blocks of a third harmonic waveforming control module,

fig. 4 shows a modulation reference used for an upper phase arm and a modulation reference used for a corresponding lower phase arm of a phase leg of the converter,

fig. 5 shows the two half periods of the voltage reference,

fig. 6 schematically shows a first and second extreme value formed for the two half periods in relation to the modulation references,

fig. 7 shows the modulation references after delaying the phase of the third harmonic component using a delay determined by the delay determining module,

fig. 8 shows a flow chart of a number of method steps performed by the delay determining module.

fig. 9 shows a block schematic of a combination of the delay determining module together with a combined fundamental and third harmonic waveforming control block, and

fig. io schematically shows a computer program product comprising computer program code for forming the controller. DETAILED DESCRIPTION OF THE INVENTION

In the following, a detailed description of preferred embodiments of the invention will be given.

The present invention is directed towards the adding of third harmonics to a basic reference waveform used to control a voltage source converter. This basic reference waveform may with advantage be sinusoidal and provide a fundamental frequency. Thereby a a control signal that comprises a fundamental frequency component and a third harmonic frequency components is obtained and used.

A voltage source converter where such control is used may as an example be provided in a converter station that provides an interface between a direct current (DC) network and an Alternating Current (AC) network, where the DC network for instance may be a High Voltage Direct Current (HVDC) network. Thereby the converter station may also be an HVDC converter station. Such a converter station may also comprise an AC line connected between a secondary side of a transformer and an AC side of the converter, where the primary side of the transformer is connected to the

AC network and a DC side of the converter is connected to the DC network.

The DC network may in turn be a transmission network covering long distances, for instance in order to transfer power over these long distances.

Fig. l shows one way of realizing the converter 10. The converter 10 may be a three-phase voltage source converter for converting between AC and DC. The converter 10 may therefore comprise three phase legs PLi, PL2 and PL3, for instance connected in parallel between a first and a second DC terminal DCi and DC2, where the first DC terminal DCi may be connected to a first pole of the DC network and the second DC terminal DC2 may be connected to a second pole of the DC network or to ground. Each phase leg furthermore comprises a set of converter valves, which in this example is a pair of converter valves. The first phase leg PLi therefore comprises a first and a second converter valve CVAi and CVA2, the second phase leg comprises a first and a second converter valve CVBi and CVB2 and the third phase leg PL3 comprises a first and a second converter valve CVCi and CVC2. The mid points of the phase legs are connected to

corresponding AC terminals ACi, AC2, AC3, where each AC terminal is connected to a corresponding phase of the previously mentioned AC line. A phase leg is in this example divided into two halves, a first upper half and a second lower half, where such a half is also termed a phase arm.

The first DC pole furthermore has a first potential that may be positive. The first pole may therefore also be termed a positive pole. A phase arm between the first DC terminal DCi and a first, second or third AC terminal ACi, AC2 and AC3 may be termed a first phase arm or an upper phase arm, while a phase arm between the first, second or third AC terminal ACi, AC2 and AC3 and the second DC terminal DC2 may be termed a second phase arm or a lower phase arm. The phase arm mid points may

furthermore be connected to the AC terminals via phase reactors LACi, LAC 2 and LAC3.

Moreover, the upper phase arms may be joined to the first DC terminal DCi via a corresponding first or upper arm reactor LAi, LBi and LCi, while the lower phase arms may be joined to the second DC terminal DC2 via a second or lower arm reactor LA2, LB2 and LC2.

The voltage source converter 10 may be a two-level converter, where each converter valve is made up of a number of series connected switching units. Alternatively the converter may be a modular multilevel converter (MMC) where each converter valve is formed through a series-connection of a number of cells, where a cell may be a half-bridge cell or a full-bridge cell. A cell then comprises one or two strings of series connected switching units in parallel with an energy storage element like a capacitor. A switching unit may be realized in the form a transistor with anti-parallel diode. However, it is also known to be realized using other types of semiconducting units.

It should here also be realized that there exist countless variations of voltage source converters, where a converter may for instance be an n-level converter, such as a neutral point clamped three-level converter. Also a modular multilevel converter may be made up of a number of different types of cells. There may also exist hybrid converters that use cells in an n- level environment.

There is finally a controller 12, which controls the operation of the converter 10 and more particularly controls each converter valve. The controller 12 is provided for controlling all the phase arms of the converter. However, in order to simplify the figure only the control of the first converter valve CVAi of the upper phase arm and the second converter valve CVA2 of the lower phase arm of the first phase leg PL is indicated. The operation according to aspects of the invention will later be described in relation to the two converter valves CVAi and CVA2. It should be realized that all converter valves are controlled by the controller 12.

Therefore the control to be described will be performed also in relation to the other converter valves of the other phase legs. The controller 12 may be implemented through a computer or a processor with associated program memory or dedicated circuit such Field-Programmable Gate Arrays (FPGAs).

Fig. 2 shows a block schematic of one way of realizing controller 12. The controller 12 comprises a fundamental waveforming control module FWF 14, a third harmonics waveforming control module 3HWF 16 and a third harmonics delay determining module 3HD 18. The fundamental

waveforming control module 14 is used for forming a fundamental component of a waveform such as a sine wave, while the third harmonics waveforming control module 16 is used for forming a third harmonics component of the waveform, which component may likewise be a sine wave. The modules 14 and 16 are here shown as being separate from each other. However, as will be shown later, their functionality may as an alternative be combined into one module. The fundamental waveforming control module 14 is thus used for forming a fundamental component of a waveform to be output via an AC terminal of the converter, which in this example is the first AC terminal ACi. For this reason the fundamental waveforming control module 14 determines a modulation or voltage reference representing the fundamental voltage of the waveform that is to appear on the AC terminal and which fundamental component is also to be conveyed to the AC network. A typical frequency of such a fundamental wave is 50 Hz. However, also other frequencies may be used, such as 60 Hz. To this component the third harmonics

waveforming control module 16 then adds zero sequence third harmonics. These harmonics are thereafter removed before entering the AC system, for instance using the previously mentioned transformer.

As mentioned earlier, the third harmonic component is added to the fundamental component in order for the converter to produce an output signal on the AC side resembling the basic reference waveform with added harmonic component, where the basic reference waveform is with advantage sinusoidal. One reason for this addition is that it reduces the peak value of the modulation reference, thereby allowing an increase of the modulation range, i.e. an increase of the converter output voltage. The most common harmonic component added is a zero-sequence third harmonic having the opposite polarity or opposite sign in relation to the polarity or sign of the fundamental component. It is also possible to add higher order harmonics that are multiples of three (and too are zero sequence). However the third harmonic has the highest influence. For a standard two-level voltage-source converter, the converter basic reference waveform u^T , which is with advantage the output voltage reference, is for one of the three phases ideally given by

where ω_ι is the fundamental angular frequency and U_v is the fundamental reference amplitude or the output AC voltage amplitude. Then the optimal third-harmonic addition is - in the sense that the peak value of the basic reference waveform is reduced as much as possible -

In the controller 12, the fundamental waveforming control module 14 is set to provide the voltage reference of equation (1) and the combination of fundamental waveforming control module 14 and third harmonics waveforming control module 16 together provide the voltage reference in equation (2). Here it may be added that the fundamental waveforming control module also determines a phase Θ of the fundamental component.

There is a problem with the addition of third harmonics in this way in that a phase shift may be needed on the third harmonic contribution in order to obtain the desired benefits. How an optimal phase shift of this added third harmonic is obtained is not trivial. It depends on several factors, including the used operating point of the control, circulating currents, cell voltage ripple etc.

If the third harmonic is not optimally shifted in phase in a modular multilevel converter, there may be negative effects because more cells than necessary may need to be inserted or bypassed in order to fulfil the output voltage reference, leading to lower control margins and lower active and reactive power delivery capability of the converter.

Aspects of the invention are directed towards adaptively adjusting the phase of the added third harmonics.

In order to understand how this may be implemented reference is now also being made to fig. 3, which shows a block schematic of a control loop for controlling a phase leg comprising the delay determining module 18 and various blocks of the third harmonic waveforming control module 16 as well as some further blocks for combining the waves and forming modulation references.

In the control loop the voltage reference Uv_ref representing the fundamental component is provided by the fundamental waveforming control module 14 to an amplitude calculating block 20 of the third harmonic waveform control module 16 as well as to a first summing block 26. The amplitude calculating block 20 determines the amplitude of the fundamental voltage reference and supplies it to a multiplying block 22 of the third harmonic waveform control module 16, which multiplies the amplitude with a set value k, which as an example is the value of 1/6, in order to obtain the amplitude of the third harmonic component in relation to the amplitude of the fundamental component. The amplitude k is then supplied to a third harmonic waveforming control block 24 of the third harmonic waveform control module 16. The delay determining module 18 in turn determines a delay φ of the third harmonic waveform contribution and also supplies this delay φ to the third harmonic waveforming control block 24. The way the delay φ is being determined will be described shortly.

The third harmonic waveforming control block 24 generates a voltage reference representing the third harmonic component and having the amplitude k and a phase that is delayed with the delay φ. The voltage reference representing the third harmonic component is then supplied to the first summing block 26. The summing block 26 then sums the fundamental voltage reference Uvref and the third harmonic voltage reference in order to obtain the signal Uv_ref_tot, which is a reference signal comprising both the fundamental and third harmonic component.

The first summing block 26 in turn supplies the voltage reference signal Uv_ref_tot to a modulation reference calculating block 27, which determines a modulation reference R_P for the upper phase arm and a modulation reference R_N for the lower phase arm using the voltage reference signal Uvref_tot. How such modulation references may be determined is known and will therefore not be described in any further detail. The references are then used for controlling the upper and lower phase arm valves CVAl and CVA2 in a known fashion for instance using Pulse Width Modulation (PWM). These modulation references R_P and R_N are also provided to the third harmonic delay determining module 18. The modulation references may each vary between +1 and -1. Moreover, the fundamental waveforming control module also provides the phase Θ of the fundamental component to the delay determining module 18.

It can here be seen that the signal Uv_ref_tot corresponds to the signal of equation (2) above. There is a difference though and that is that there is a delay in the phase of the third harmonic waveform contribution being output by the first summing block 26. Aspects of the invention are directed towards the determining of this delay.

How the delay may be formed by the delay determining module 18 will now be described in some more detail with reference also being made to fig- 4, 5, 6, 7 and 8, where fig. 4 shows the modulation reference R_P used for the upper phase arm and the modulation reference R_N used for the corresponding lower phase arm, fig 5 shows the two half periods of the generated waveform WF, fig. 6 schematically shows the first and second extreme value formed for the two half periods based on the modulation references, fig. 7 shows the modulation references after delaying the phase of the third harmonic component and fig. 8 shows a flow chart of a number of method steps being performed by the delay determining module 18.

The delay determining module 18 uses the modulation references R_P and R_N of the positive and negative arms in order to determine the delay φ. In order to do this it obtains the modulation reference R_P of the positive phase arm, step 30, and it obtains the modulation reference R_N of the negative phase arm, step 32, which in this embodiment is done through receiving them from the modulation reference calculating block 28. It thus obtains the modulation reference R_P of the control signal Uvref_tot used for the upper phase arm of the phase leg and the modulation reference R_N of the control signal Uvref_tot used for the lower phase arm of the phase leg PLIT

Figure 4 shows the modulation references R_P and R_N for a certain operating point, such as a certain level of active power P and/or a certain level of reactive power being delivered by the converter 10. From these signals, the delay determining module 18 determines which of the upper and lower phase arm reference has the highest value.

The determination of highest value is more particularly performed for different half periods, where the division into half-period may be made based on an extreme value of the fundamental component. In the example given here one border between two half-periods is at the positive peak or maximum of the fundamental component and the other at the negative peak or minimum of the fundamental component as shown in Figure 5. Each half period may therefore stretch between the maximum and minimum of the fundamental component. The wave shown in fig. 5 is a wave comprising both fundamental and harmonic components and therefore the peak of the fundamental component is not the peak of the waveform but a local minimum between two waveform peaks. The phase angle Θ of the fundamental wave may in this case be used to set the point where the border between a first and a second half period HPi and HP2 is to occur. In order to indicate the different half periods, the first half period HPi, which is here a half period with rising voltage, is enclosed in a dashed box and the second half period HP2, , which is here a half period with falling voltage, is enclosed in a solid box in the figure.

The delay determining module 18 thereafter determines a first extreme value EVi in the first half period HPi based on the modulation references R_P and R_N for the upper and lower phase arms in this half period HPi, step 34, and determines a second extreme value EV2 in the second half period HP2 based on the modulation indexes R_P and R_N for the upper and lower phase arms in this half period HP2, step 36. In this embodiment the delay determining module 18 determines the first extreme value EVi as the highest value of the two modulation references in the first half period and the second extreme value EV2 as the highest of the two modulation references in the second half period. The determining of the first and second extreme values may thus comprise a determining of which of the upper and lower phase arm modulation references R_P, R_N has the most extreme value in the first half period HPi and which of the upper and lower phase arm modulation references R_P, R_N has the most extreme value in the second half period HP2. The determining of the extreme value may thus be a determining of the most extreme of the extreme values of the two modulation references in the half period to be the extreme value of this half period, where in this embodiment the highest of the two maximum values, i.e. positive peaks, in the half period may be determined to be the extreme value of the half period. This is shown in Figure 6, where the first extreme value EVi of the first half period is the peak of the first modulation reference R_P and is shown as a dashed line and the peak EV2 of the second half period is the peak of the second modulation reference

R N and is shown as a solid line. The delay determining module 18 thereafter determines the difference between the first and second extreme values EVi and EV2, step 38, which difference is then processed in order to obtain a delay φ, step 40. In the presently described embodiment the processing involves applying integrating activity on the difference, i.e. integrating the difference EVi - EV2. The processed difference is then applied as the delay φ to be used by the third harmonic waveforming control block 24 in relation to the fundamental wave. The delay φ is thus used in the adjusting of the phase of the third harmonic component in relation to the fundamental frequency component using the adjustment factor φ, step 42. This will shift the phase of the added third harmonic so that the peaks of the half periods become equal, increasing control margin, thus increasing the capability of the converter, see figure 7. This type of adaptive phase shift adjusting is then continuously performed, which improves the efficiency of the conversion.

In the above given example shown in fig. 4 and 7, the max value of the modulation references was decreased from 0.957 to 0.947, increasing the control margin by 1%.

Thereby it is possible to achieve optimal phase shift of the third harmonic for all operating points. This is furthermore done adaptively as the controller is used. The controller is thereby able to adapt to instantaneous variations through using feedback of the two modulation references. This is also achieved with limited additions to the controller. It only requires limited additional software in order to be implemented.

It is possible to combine the above determined delay with an adjustment factor determined according to WO 2011/032581, which document is herein incorporated by reference. Such a combination has the advantage of limiting the initial differences between the upper and lower phase arm modulation indexes, thereby speeding up the time at which modulation index balance is achieved at the operating point.

As described in WO 2011/032581, it is possible to derive an expression of a an adjustment factor δ that is related to the operating point of the converter through

NP

δ (4)

As can be seen from equation (3) this adjustment factor δ has a

dependence on the number of cells N in a phase arm, the cell capacitance Cc, the fundamental reference waveform angular frequency coi and amplitude U_v , modulation index m and above all the operating point or power and here active output power P, where it can be seen in equation (4) that it is possible to omit the modulation index m .

This means that the adjustment factor δ may be is proportional to the output power P with a constant set by the other voltage source converter operational data. The constant is here based on cell specifying data that defines a relationship between the number of cells in the converter, i.e. in the arms, and the cell capacitance as well as data specifying the basic reference waveform in the form of basic reference waveform angular frequency and amplitude.

A controller that combines the adjustment factor with a determined delay is schematically shown in fig. 9. The controller includes an adjustment factor determining block 44 that receives measured signals in the form of output power which in one embodiment of the invention is output effective power P, i.e. effective power output by the voltage source converter, fundamental system constants in the form of basic reference waveform angular frequency coi, i.e. AC system angular frequency, number N of cells in each arm of the phase legs and cell capacitance Cc, i.e. the capacitance of the capacitors in the cells of the phase legs. This block 44 also receives internally generated signals including a modulation index m and basic reference waveform amplitude U_v . The adjustment factor determining block 44 thus obtains operational data of the voltage source converter 10, i.e. the measured value of the active output power P, the fundamental system constants, basic reference waveform angular frequency coi, phase arm cell number N and cell capacitance Cc as well as the internally generated signals modulation index m and fundamental reference waveform amplitude U_v . The fundamental reference waveform amplitude U_v can be measured as the amplitude of the output AC voltage. It may therefore alternatively be considered as a measured value. Based on this data, the adjustment factor determining block 44 determines an adjustment factor δ that is delivered to a low pass filter 46. As can be seen above, the adjustment factor is set in relation to the operating point of the converter.

The low pass filter 46 in turn low pass filters the adjustment factor in order to obtain a low pass filtered adjustment factor 5F that is supplied to a first terminal of a second summing block 48.

The second summing block 48 also has a second positive terminal on which it receives the delay φ determined by the delay determining module 18 and adds the delay φ to the adjustment factor δ. The second summing block thus sums the adjustment factor and the delay and provides the sum δ + φ to a waveforming block 50, which waveforming block 50 is responsible for forming the waveform comprising both the fundamental and third harmonic components. In order to be able to do this, the waveforming block 50 also receives the basic reference waveform amplitude U_v and angular frequency coi and the amplitude k of the third harmonic component amplitude, which is again typically 1/6. The waveforming block 50 then provides the fundamental reference waveform based on the basic reference waveform angular frequency coi and amplitude. Here it also determines the phase Θ of the fundamental component, which is supplied to the delay determining module 18. The waveforming block 50 also determines the frequency of the third harmonic component based on the basic reference waveform angular frequency coi and the relative harmonic amplitude k based on the amplitude of the basic reference waveform. The harmonic component may also receive the opposite polarity in relation to the polarity of the fundamental reference waveform, i.e. it may receive the opposite sign in relation to the

fundamental component. Also the phase of the third harmonic component is shifted with a delay or phase shift 5F + φ. The phase shifted third harmonic component is then added to the fundamental component to form an actual reference waveform Uv_ref_tot, which is then supplied to the modulation reference calculator 28 that calculates the modulation references R_P and R_N for the upper and lower arm phase arm, which modulation references are also supplied to the delay determining module 18. The modulation references are then again used to cause the cells of the voltage source converter 10 to reproduce the actual reference waveform on the first AC terminal ACi, i.e. the waveform of the fundamental reference with phase adjusted added third harmonics. The cells are here each controlled to provide a voltage contribution, where the sum of the voltage contributions over time provides the waveform.

This type of control is then continuously performed by the controller. There are several variations that are possible to make of the invention. In the example given above the first and second extreme values were selected as the highest of the modulation references in the first and second half periods. It is just as well possible to select the first and second extreme values as the lowest of the modulation references in two half periods, i.e. at the negative peaks. Moreover, the above described processing involved integrating control of the difference signal. As an alternative or in addition it is likewise possible to use proportional control.

The controller may be realized in the form of discrete components, such as FPGAs. However, it may also be implemented in the form of a processor with accompanying program memory comprising computer program code that performs the desired control functionality when being run on the processor. A computer program product carrying this code can be provided as a data carrier such as one or more CD ROM discs or one or more memory sticks carrying the computer program code, which performs the above-described control functionality when being loaded into a controller of a voltage source converter. One such data carrier in the form of a CD Rom disk 52 carrying computer program code 54 is shown in fig. 10.

From the foregoing discussion it is evident that the present invention can be varied in a multitude of ways. It shall consequently be realized that the present invention is only to be limited by the following claims.

Claims

1. A method for controlling the phase of a third harmonic component added to a fundamental frequency component (Uv_ref) of a control signal (Uv_ref_tot) used for forming a waveform on an AC terminal (ACi) of a phase leg of a voltage source converter (10), the method comprising the steps of:

- obtaining (30) a modulation reference (R_P) of the control signal used for an upper phase arm of the phase leg (PLi),

- obtaining (32) a modulation reference (R_N) of the control signal used for a lower phase arm of the phase leg (PLi),

- determining (34) a first extreme value (EVi) based on the two

modulation references (R_P, R_N) in a first half period (HPi) of the waveform,

- determining (36) a second extreme value (EV2) based on the two

modulation references (R_P, R_N) in a second half period (HP2) of the waveform,

- determining (38) a difference between the first and second extreme values,

- processing (40) the difference in order to obtain a delay (φ), and

- adjusting (42) the phase of the third harmonic component using the delay.

2. The method according to claim 1, wherein the determining of the first and second extreme values comprises determining which of the upper and lower phase arm modulation references (R_P, R_N) has the most extreme value in the first half period (HPi), selecting said most extreme value to be the first extreme value (EVi), determining which of the upper and lower phase arm modulation references (R_P, R_N) has the most extreme value in the second half period (HP2) and selecting said most extreme value to be the second extreme value (EV2).

3. The method according to claim 2, wherein one border between two half periods is at the maximum of the fundamental frequency component.

4. The method according to claim 2 or 3, wherein the most extreme value in both half periods is the highest value.

5. The method according to claim 2 or 3, wherein the most extreme value in both half periods is the lowest value.

6. The method according to any previous claim, wherein the processing comprises performing integrating control of the difference.

7. The method according to any previous claim, wherein the processing comprises performing proportional control of the difference.

8. The method according to any previous claim, wherein the third harmonic component of the control signal has an initial delay set by an adjustment factor (δ), where the adjustment factor is set in relation to the operating point of the converter and the adjusting of the phase of the third harmonic component comprises adjusting the adjustment factor with the determined delay (φ).

9. The method according to claim 8, wherein the adjustment factor is proportional to the output power.

10. The method according to claim 8 or 9, wherein the adjustment factor is based on the output power, number of cells and cell voltages of the voltage source converter.

11. A controller (12) for controlling the phase of a third harmonic component added to a fundamental frequency component (Uv_ref) of a control signal (Uv_ref_tot) used for forming a waveform on an AC terminal (ACi) of a phase leg of the voltage source converter, the controller (12) comprising a third harmonic delay determining module (18) operative to :

- obtain a modulation reference (R_P) of the control signal used for an upper phase arm of the phase leg (PLi),

- obtain a modulation reference (R_N) of the control signal used for a lower phase arm of the phase leg (PLi),

- determine a first extreme value (EVi) based on the two modulation references (R_P, R_N) in a first half period (HPi) of the waveform, - determine a second extreme value (EV2) based on the two modulation references (R_P, R_N) in a second half period (HP2) of the waveform,

- determine a difference between the first and second extreme values,

- process the difference in order to obtain a delay (φ), and

- adjust (42) the phase of the third harmonic component using the delay.

12. The controller according to claim 11, wherein the third harmonic delay determining module (18) when determining the first and second extreme values is operative to determine which of the upper and lower phase arm modulation references (R_P, R_N) has the most extreme value in the first half period (HPi), select said most extreme value to be the first extreme value (EVi), determine which of the upper and lower phase arm modulation references (R_P, R_N) has the most extreme value in the second half period (HP2) and select said most extreme value to be the second extreme value (EV2).

13. The controller according to claim 12, wherein one border between two half periods is at the maximum of the fundamental frequency component.

14. The controller according to claim 12 or 13, wherein the most extreme value in both half periods is the highest value.

15. The controller according to claim 12 or 13, wherein most extreme value in both half periods is the lowest value.

16. The controller according to any of claims 11 - 15, wherein the third harmonic delay determining module (18) when processing the difference is further operative to perform integrating control of the difference.

17. The controller according to any of claims 11 - 16, wherein the third harmonic delay determining module (18) when processing the difference is further operative to perform proportional control of the difference signal.

18. The controller according to any of claims 11 - 17, further comprising an adjustment factor determining block (44) operative to generate an adjustment factor (δ) set in relation to the operating point of the converter and providing an initial delay, where the third harmonic delay determining module (18) when adjusting the phase of the third harmonic component comprises is operative to adjust the adjustment factor with the determined delay (φ).

19. A voltage source converter (10) comprising a controller according to any of claims 11 - 18.

20. A computer program product for controlling the phase of a third harmonic component added to a fundamental frequency component (Uv_ref) of a control signal (Uv_ref_tot) used for forming a waveform on an AC terminal (ACi) of a phase leg of a voltage source converter (10), the computer program product comprising a data carrier (52) with computer program code (52) configured to cause a controller (12) of the converter

(14) to:

- obtain a modulation reference (R_P) of the control signal used for an upper phase arm of the phase leg (PLi), - obtain a modulation reference (R_N) of the control signal used for a lower phase arm of the phase leg (PLi),

- determine a difference between the first and second extreme values,

- process the difference in order to obtain a delay (φ), and

- adjust (42) the phase of the third harmonic component using the delay.