SE1200606A1 - Power synchronization control of a single-phase voltage-rigid inverter - Google Patents

Abstract

EFFEKTSYN KRONISERINGSSTYRNING AV EN ENFASIG SPÄNNINGSSTYV STRÖMRIKTARE. Beskrivningen hänför sig till en enfasig spänningsstyv strömriktare for anslutning till ett kraftledningsnät liksom även till en datorprogramprodukt och en metod för att styra den enfasiga spänningsstyva strömriktaren. Enligt metoden erhålls en effektsynkroniseringsvinkel (9p) från en effektsynkroniseringsslinga (4) som använder en referens (P) för den spänningsstyva strömriktarens aktiva effekt och en uppmätt aktiv uteffekt (P) hos den enfasiga spänningsstyva strömriktaren som insignal. Dessutom härleds en växelspänningsreferens (u) baserad på effektsynkroniseringsvinkeln (0p) från effektsynkroniseringsslingan (4) och den enfasiga spänningsstyva strömriktarens funktion styrs enligt en pulsbreddsmoduleringsmetod (3) baserat på växelspänningsreferensen (u).(Fig. 1)POWER VIEW CHRONICATION CONTROL OF A SINGLE-PHASE VOLTAGE RIGID CURRENT CONVERTER. The description relates to a single-phase voltage-rigid rectifier for connection to a power line network, as well as to a computer program product and a method for controlling the single-phase voltage-rigid rectifier. According to the method, a power synchronization angle (9p) is obtained from a power synchronization loop (4) which uses a reference (P) for the active power of the solid-state converter and a measured active output power (P) of the single-phase solid-state converter as input. In addition, an AC voltage reference (u) based on the power synchronization angle (0p) is derived from the power synchronization loop (4) and the operation of the single-phase voltage rigid rectifier is controlled according to a pulse width modulation method (3) based on the AC voltage reference (u).(Fig. 1)

Description

15 20 25 30 A second embodiment provides a single-phase voltage source converter configured for connection to a power grid. The voltage source converter comprises a controller configured to control the single-phase voltage source converter according to the method of the first embodiment. 15 20 25 30 A second embodiment provides a single-phase voltage source converter configured for connection to a power grid. The voltage source converter comprises a controller configured to control the single-phase voltage source converter according to the method of the first embodiment.

A third embodiment provides a computer program product, stored on a computer-readable medium. The computer program product comprises computer-readable code portions, which when loaded into and executed by a controller of a single-phase voltage source converter causes the controller to carry out the method of the first embodiment.A third embodiment provides a computer program product, stored on a computer-readable medium. The computer program product comprises computer-readable code portions, which when loaded into and executed by a controller of a single-phase voltage source converter causes the controller to carry out the method of the first embodiment.

An advantage of some of the embodiments of this disclosure is that the low- frequency instability problems which often appear for weak grids may be solved.An advantage of some of the embodiments of this disclosure is that the low-frequency instability problems which often appear for weak grids may be solved.

Another advantage of some of the embodiments of this disclosure is that phased-locked loop (PLL) for maintaining synchronism with the grid voltage may be omitted.Another advantage of some of the embodiments of this disclosure is that phased-locked loop (PLL) for maintaining synchronism with the grid voltage may be omitted.

Further advantages and features of embodiments of the present invention will become apparent when reading the following detailed description in conjunction with the drawings.Further advantages and features of embodiments of the present invention will become apparent when reading the following detailed description in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic block diagram illustrating a VSC for a single-phase traction application along with the control block diagram according to an example embodiment.BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic block diagram illustrating a VSC for a single-phase traction application along with the control block diagram according to an example embodiment.

DETAILED DESCRIPTION The embodiments of this disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which different example embodiments are shown. These example embodiments are provided so that this disclosure will be thorough and complete and not for purposes of limitation. ln the drawings, like reference signs refer to like elements. 10 15 20 25 30 The inventors have realized that the phenomenon of low-frequency instability in electric traction power systems is very similar to instability of VSC High Voltage Direct Current (HVDC) terminals connected to weak ac networks. Even though the latter uses three-phase ac transmission, the converter control systems are usually very similar for the two cases. ln such converter control systems a control loop for the grid current is the innermost, and fastest, control loop. This is typically a propoitional-integral (P|)-type controller with grid-voltage feedforvvard.DETAILED DESCRIPTION The embodiments of this disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which different example embodiments are shown. These example embodiments are provided so that this disclosure will be thorough and complete and not for purposes of limitation. ln the drawings, like reference signs refer to like elements. 10 15 20 25 30 The inventors have realized that the phenomenon of low-frequency instability in electric traction power systems is very similar to instability of VSC High Voltage Direct Current (HVDC) terminals connected to weak ac networks. Even though the latter uses three-phase ac transmission, the converter control systems are usually very similar for the two cases. ln such converter control systems a control loop for the grid current is the innermost, and fastest, control loop. This is typically a propositional-integral (P |) -type controller with grid-voltage feedforvvard.

There is generally also a phase-locked loop (PLL), which maintains synchronism with the grid voltage, and a dc-bus-voltage controller (typically also a Pl controller), which feeds a current reference to a current controller. By aligning the grid current in phase with the grid voltage (using the PLL phase angle), the active power is controlled so as to maintain a constant dc-bus voltage, while the reactive power is controlled to zero (unity input power factor). The latter is often a requirement by the rail infrastructure providers. ln the international patent application WO 2010/022766 and a number of papers, e.g., “Power-synchronization control of grid-connected voltage-source convertersf' by L. Zhang, L. Harnefors, and H.-P. Nee, published in IEEE Trans.There is generally also a phase-locked loop (PLL), which maintains synchronism with the grid voltage, and a dc-bus-voltage controller (typically also a Pl controller), which feeds a current reference to a current controller. By aligning the grid current in phase with the grid voltage (using the PLL phase angle), the active power is controlled so as to maintain a constant dc-bus voltage, while the reactive power is controlled to zero (unity input power factor). The latter is often a requirement by the rail infrastructure providers. ln the international patent application WO 2010/022766 and a number of papers, e.g., “Power-synchronization control of grid-connected voltage-source convertersf 'by L. Zhang, L. Harnefors, and H.-P. No, published in IEEE Trans.

Power Systems, May 2010, pp. 809-820 and “lnterconnection of two very weak ac systems by VSC-HVDC links using power-synchronization contro|,” by L.Power Systems, May 2010, pp. 809-820 and “lnterconnection of two very weak ac systems by VSC-HVDC links using power-synchronization contro |,” by L.

Zhang, L. Harnefors, and H.-P. Nee, published in IEEE Trans. Power Systems, Feb. 2011, pp. 344-355 the inventors proposed an alternative, power- synchronization control (in WO 2010/022766 called synchronous-machine- emulating control), to the common current-control-based method. ln power- synchrcnization control, active power is controlled via adjustment of the phase angle of the converter voltage, in the same fashion as for a synchronous machine. The amplitude of the converter voltage can be held constant, or be adjusted via an outer control loop. ln the above mentioned paper entitled “lnterconnection of two very weak ac systems by VSC-HVDC links using power- synchronization control" power-synchronization control was analyzed and shown to have excellent stability properties for weak networks. 10 15 20 25 30 ln the following embodiments which use power-synchronization control to overcome the aforementioned stability problems in single-phase traction applications are described.Zhang, L. Harnefors, and H.-P. No, published in IEEE Trans. Power Systems, Feb. 2011, pp. 344-355 the inventors proposed an alternative, power-synchronization control (in WO 2010/022766 called synchronous-machine- emulating control), to the common current-control-based method. ln power- synchrcnization control, active power is controlled via adjustment of the phase angle of the converter voltage, in the same fashion as for a synchronous machine. The amplitude of the converter voltage can be held constant, or be adjusted via an outer control loop. ln the above mentioned paper entitled “lnterconnection of two very weak ac systems by VSC-HVDC links using power- synchronization control" power-synchronization control was analyzed and shown to have excellent stability properties for weak networks. 10 15 20 25 30 ln the following embodiments which use power-synchronization control to overcome the aforementioned stability problems in single-phase traction applications are described.

Fig. 1 is a schematic block diagram illustrating a VSC for a single-phase traction application along with the control block diagram according to an example embodiment. Fig. 1 illustrates a motor 1 which is connected to a grid (not shown) via a VSC. Fig. 1 illustrates a VSC phase reactor 2 at a VSC-grid connection as an inductor LC and a resistor RC. ln Fig. 1 current and power directions are shown as generating to the grid, although the actual power flow is mostly the opposite for a rail vehicle (except when braking). The motor 1 is shown to be a dc motor, but consists in most cases of one or several three- phase inverters feeding induction or permanent-synchronous traction motors. A logical Pulse-Width Modulation (PWM) block 3 is schematically illustrated in Fig.1 to show that the VSC is controlled by means of PWM according to a control scheme. ln the following, theory for the control scheme is presented.Fig. 1 is a schematic block diagram illustrating a VSC for a single-phase traction application along with the control block diagram according to an example embodiment. Fig. 1 illustrates a motor 1 which is connected to a grid (not shown) via a VSC. Fig. 1 illustrates a VSC phase reactor 2 at a VSC-grid connection as an inductor LC and a resistor RC. ln Fig. 1 current and power directions are shown as generating to the grid, although the actual power flow is mostly the opposite for a rail vehicle (except when braking). The motor 1 is shown to be a dc motor, but consists in most cases of one or several three-phase inverters feeding induction or permanent-synchronous traction motors. A logical Pulse-Width Modulation (PWM) block 3 is schematically illustrated in Fig.1 to show that the VSC is controlled by means of PWM according to a control scheme. In the following, theory for the control scheme is presented.

A control signal is provided to the PWM block 3 in the form of a reference voltage ucæf. According to the embodiment illustrated in Fig. 1 the reference voltage uJef is derived based on a synchronization angle GPSL provided from a Power-Synchronization Loop (PSL) 4 and the output of current controller 6 which converts reference currents given from a current reference control 7 into d and q components of the voltage reference uJef . The d and q components refer to dq decomposition from the converter dq frame, i.e. a rotating reference frame with the d axis aligned with the vector VSC voltage.A control signal is provided to the PWM block 3 in the form of a reference voltage ucæf. According to the embodiment illustrated in Fig. 1 the reference voltage uJef is derived based on a synchronization angle GPSL provided from a Power-Synchronization Loop (PSL) 4 and the output of current controller 6 which converts reference currents given from a current reference control 7 into d and q components of the voltage reference uJef. The d and q components refer to dq decomposition from the converter dq frame, i.e. a rotating reference frame with the d axis aligned with the vector VSC voltage.

Somewhat surprisingly, single-phase VSCs are more difficult to control than three-phase VSCs, because in the latter case, abc/dß and dß/dq transforrnations (i.e. transformations between the three phase quantities and a stationary orß-reference frame and transformations between the stationary oiß- reference frame and a rotating dq-reference frame) can be made with ease, giving quantities that are constant in the steady state. ln a single-phase system, the or signal - or a bandpass filtered variant, see below - may represent the actual phase quantity, but unlike a three-phase system, the orthogonal ß signal 10 15 20 25 cannot immediately be obtained. lt has to be artificialiy generated. The simplest method for generating such a signal may be to shift the existing cx signal 90 degrees, corresponding to a delay of a quarter of a fundamental period Ts/4. For the orthogonal signal generation in this single-phase model, a second-order generalized integrator (SOGI) is used, as Hm = ya ___ KSÛGIwls ync (s) _ sz + KSOG/æls + ælz yß (s) = Ksocrwiz ya: (s) S2 + Ksocrmis + 5012 HN) = (Eq. 2) ln Fig. 1 it can be seen that Iogical blocks 8 and 9 are SOGI blocks which decomposes a low-pass filtered feedfon/vard term uf of the Point of Common Coupling (PCC) voltage and the converter current ic into oi- and ß-components of the respective quantities.Somewhat surprisingly, single-phase VSCs are more difficult to control than three-phase VSCs, because in the latter case, abc / dß and dß / dq transforrnations (ie transformations between the three phase quantities and a stationary orß-reference frame and transformations between the stationary oiß- reference frame and a rotating dq-reference frame) can be made with ease, giving quantities that are constant in the steady state. ln a single-phase system, the or signal - or a bandpass filtered variant, see below - may represent the actual phase quantity, but unlike a three-phase system, the orthogonal ß signal 10 15 20 25 cannot be immediately obtained. lt has to be artificially generated. The simplest method for generating such a signal may be to shift the existing cx signal 90 degrees, corresponding to a delay of a quarter of a fundamental period Ts / 4. For the orthogonal signal generation in this single-phase model, a second-order generalized integrator (SOGI) is used, as Hm = ya ___ KSÛGIwls ync (s) _ sz + KSOG / æls + ælz yß (s) = Ksocrwiz ya: (s) S2 + Ksocrmis + 5012 HN) = (Eq. 2) ln Fig. 1 it can be seen that Iogical blocks 8 and 9 are SOGI blocks which decomposes a low-pass filtered feedfon / vard term uf of the Point of Common Coupling (PCC) voltage and the converter current ic into oi- and ß-components of the respective quantities.

The PSL 4 provides the synchronization angle GPSL of the voltage-source converter with the ac system. This is in contrast to many prior art solutions where such a synchronization signal is usually given by a phase locked loop (PLL) for a vector-current control scheme. The control law is given by k ÛPSL = “Lugi S el" - P) (Eq. s) where P is the measured active power from the VSC, Pæf is the reference for the active powe, kp is the controller gain and BPSL supplies the transform angle of dq/oiß and dß/dq blocks 5, 10 and 11.The PSL 4 provides the synchronization angle GPSL of the voltage-source converter with the ac system. This is in contrast to many prior art solutions where such a synchronization signal is usually given by a phase locked loop (PLL) for a vector-current control scheme. The control law is given by k ÛPSL = "Lugi S el" - P) (Eq. S) where P is the measured active power from the VSC, Pæf is the reference for the active powe, kp is the controller gain and BPSL supplies the transform angle of dq / oiß and dß / dq blocks 5, 10 and 11.

The aß signals are transformed to the dq signals by blocks 10 and 11 as sin 0 n _ wsßm m n yq -sinâm cosâm yß where GPSL is given by the PSL block 4.The aß signals are transformed to the dq signals by blocks 10 and 11 as sin 0 n _ wsßm m n yq -sinâm cosâm yß where GPSL is given by the PSL block 4.

(Eq- 4) The dq signals are transformed to the oß signals by block 5 as ya _ cosólm -SinÛPSL ya, yß sinßm cosólpsL yq where GPSL is also given by the PSL block 4.(Eq- 4) The dq signals are transformed to the oß signals by block 5 as ya _ cosólm -SinÛPSL ya, yß sinßm cosólpsL yq where GPSL is also given by the PSL block 4.

(Eq. 5) 10 15 20 25 30 The PSL is based on the knowledge that synchronous machines (SMs) in an ac system maintain synchronism by means of power synchronization, i.e., a transient power transfer. This power transfer involves a current which is network. synchronization is used to control a VSC, it cannot be combined with a vector- determined by the interconnecting Therefore when power current controller, which requires a known current reference. The alternating current at the fundamental frequency is not controlled by means of power- synchronization control since the current control conflicts with the power synchronization mechanism. The active power output from the VSC is instead controlled directly by the PSL and the alternating voltage or reactive power is controlled by adjusting the magnitude of the voltage. Consequently, an inner current loop is not necessary. The only exception is during severe ac system faults. However, for VSC applications, it is important to limit the current flowing into the converter valve to prevent the valve from over-current blocking. The current controller 6 and current reference control 7 may be used for this purpose.(Eq. 5) 10 15 20 25 30 The PSL is based on the knowledge that synchronous machines (SMs) in an ac system maintain synchronism by means of power synchronization, i.e., a transient power transfer. This power transfer involves a current which is network. synchronization is used to control a VSC, it cannot be combined with a vector- determined by the interconnecting Therefore when power current controller, which requires a known current reference. The alternating current at the fundamental frequency is not controlled by means of power-synchronization control since the current control conflicts with the power synchronization mechanism. The active power output from the VSC is instead controlled directly by the PSL and the alternating voltage or reactive power is controlled by adjusting the magnitude of the voltage. Consequently, an inner current loop is not necessary. The only exception is during severe ac system faults. However, for VSC applications, it is important to limit the current wing owing into the converter valve to prevent the valve from over-current blocking. The current controller 6 and current reference control 7 may be used for this purpose.

The current controller 6 converts reference currents iæfcd and iæfcq given from a current reference control 7 into output signals uæfcd and uæfcq, which after transformation by block 5 forms a voltage reference ucæf for the PWM block. A proportional-type control law can be applied “ff z aflfiüfef _i:)+jwilæi: 'TH/.Pßsöuy (EQ. 6) where LC is the inductance of the phase reactor; dc is the bandwidth of the current controller. m1 is the grid frequency. The function HLp(S) is a low-pass filter to improve the disturbance rejection capability of the current controller 6.The current controller 6 converts reference currents iæfcd and iæfcq given from a current reference control 7 into output signals uæfcd and uæfcq, which after transformation by block 5 forms a voltage reference ucæf for the PWM block. A proportional-type control law can be applied “ff z afl fi üfef _i:) + jwilæi: 'TH / .Pßsöuy (EQ. 6) where LC is the inductance of the phase reactor; dc is the bandwidth of the current controller. m1 is the grid frequency. The function HLp (S) is a low-pass filter to improve the disturbance rejection capability of the current controller 6.

The reference of the vector-current control is in block 7 given by im = a IL [(110 + Au)- HW (m: - HL, (Sw, - jwlLcijlJf i: ° “ (Eq- 7) The control law is so designed as vf' = (m, + Au) - HH., (Sh: (Eq. s) 10 15 20 25 30 where HHP(s) is a high-pass filter for damping purpose, uo is the voltage magnitude of the VSC according to the selected operating point and Au is the output of a an alternating-voltage control (UAC) block 12.The reference of the vector-current control is in block 7 given by im = a IL [(110 + Au) - HW (m: - HL, (Sw, - jwlLcijlJf i: ° “(Eq- 7) The control law is so designed as vf '= (m, + Au) - HH., (Sh: (Eq. s) 10 15 20 25 30 where HHP (s) is a high-pass fi lter for damping purpose, uo is the voltage magnitude of the VSC according to the selected operating point and Au is the output of an alternating-voltage control (UAC) block 12.

The VSC control system should preferably maintain the alternating-voltage level at the PCC. The UAC 12 controls the alternating voltage at the filter bus. The control law is given by Au=å s (Uf -Ufl (Eq. 9) where k., is the controller gain, uf is the low-pass filtered feedforward term of the PCC voltage as mentioned above and ufæf is a voltage reference for uf. As an alternative, this controller 12 may be replaced by a reactive-power controller, which acts so as to keep unity input power factor in the steady state.The VSC control system should preferably maintain the alternating-voltage level at the PCC. The UAC 12 controls the alternating voltage at the filter bus. The control law is given by Au = å s (Uf -Ufl (Eq. 9) where k., Is the controller gain, uf is the low-pass filtered feedforward term of the PCC voltage as mentioned above and ufæf is a voltage reference for uf. As an alternative, this controller 12 may be replaced by a reactive-power controller, which acts so as to keep unity input power factor in the steady state.

For direct-voltage control (UDC), a Pl controller 13 with the control law Pm = _(Kpø + Iíd )ku:f y _ ušc] (Eq. 10) is proposed. udc is the measured VSC dc voltage, udcæf is the reference of udc, Kpd is the proportional gain and Kid is the integral gain of the controller 13. By controlling the square of the direct voltage the controller 13 is independent of the operating point which is convenient. However, it is also possible to use a direct voltage controller which operates directly on the error udJef - udc, but then the closed-loop dynamics would be dependent on the operating point The output of the UDC controller 13 is the power reference Pref to the PSL 4.For direct-voltage control (UDC), a Pl controller 13 with the control law Pm = _ (Kpø + Iíd) ku: f y _ ušc] (Eq. 10) is proposed. udc is the measured VSC dc voltage, udcæf is the reference of udc, Kpd is the proportional gain and Kid is the integral gain of the controller 13. By controlling the square of the direct voltage the controller 13 is independent of the operating point which is convenient. However, it is also possible to use a direct voltage controller which operates directly on the error udJef - udc, but then the closed-loop dynamics would be dependent on the operating point The output of the UDC controller 13 is the power reference Pref to the PSL 4.

Fig. 2 is a flow diagram illustrating an embodiment of a basic method for controlling a single-phase voltage source converter 21 configured for connection to a power grid. The method comprises a step 22 of deriving a power synchronization angle from a power synchronization loop using a reference for active power of the single-phase voltage source converter 21 and a measured active power output of the single-phase voltage source 21 converter as input. ln a further step 23 an ac voltage reference is derived based on the power synchronization angle obtained from the power synchronization loop. The method also comprises a step 24 of controlling operations of the single-phase 10 15 20 25 voltage source converter 21 according to a pulse width modulation method based on the derived ac voltage reference.Fig. 2 is a diagram illustrating an embodiment of a basic method for controlling a single-phase voltage source converter 21 configured for connection to a power grid. The method comprises a step 22 of deriving a power synchronization angle from a power synchronization loop using a reference for active power of the single-phase voltage source converter 21 and a measured active power output of the single-phase voltage source 21 converter as input. ln a further step 23 an ac voltage reference is derived based on the power synchronization angle obtained from the power synchronization loop. The method also comprises a step 24 of controlling operations of the single-phase 10 15 20 25 voltage source converter 21 according to a pulse width modulation method based on the derived ac voltage reference.

The basic method illustrated in Fig. 2 may be implemented using some of the control blocks described in connection with Fig. 1 above. A more sophisticated control method can be obtained if all of the control blocks illustrated in Fig. 1 are used, e.g. including the current controller 6 and the current reference controller 7.The basic method illustrated in Fig. 2 may be implemented using some of the control blocks described in connection with Fig. 1 above. A more sophisticated control method can be obtained if all of the control blocks illustrated in Fig. 1 are used, e.g. including the current controller 6 and the current reference controller 7.

The method illustrated in Fig. 2 may be carried out by a VSC controller, which generally would be implemented by means of a processor. The processor may e.g. be configured to execute computer-readable code portions of a computer program product, stored on a computer-readable medium, such as a volatile or non-volatile storage medium. The code portions may comprise instructions which cause the controller to carry out the method illustrated in Fig. 2 when executed.The method illustrated in Fig. 2 may be carried out by a VSC controller, which generally would be implemented by means of a processor. The processor may e.g. be configured to execute computer-readable code portions of a computer program product, stored on a computer-readable medium, such as a volatile or non-volatile storage medium. The code portions may comprise instructions which cause the controller to carry out the method illustrated in Fig. 2 when executed.

The embodiments described herein are adapted for power-synchronization control to single-phase VSCs, and may be particularly useful for rail traction applications. Power-synchronization control according to an embodiment of this disclosure solves the low-frequency instability problems which often appear for weak grids. ln the drawings and specification, there have been disclosed typical embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.The embodiments described herein are adapted for power-synchronization control to single-phase VSCs, and may be particularly useful for rail traction applications. Power-synchronization control according to an embodiment of this disclosure solves the low-frequency instability problems which often appear for weak grids. In the drawings and specification, there have been disclosed typical embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.

Claims (3)

10 15 20 CLAIMS10 15 20 CLAIMS 1. A method for controlling a single-phase voltage source converter (21) configured for connection to a power grid, the method comprising deriving (22) a power synohronization angle (GPSL) from a power synohronization loop (4) using a reference (Pfef) for active power of the single- phase voltage source converter (3) and a measured active power output (P) of the single-phase voltage source converter (21) as input; and deriving (23) an ac voltage reference (uJef) based on the power synohronization angle (GPSL) obtained from the power synohronization loop (4); controlling (24) operations of the single-phase voltage source converter (21) according to a pulse width modulation method (3) based on said ac voltage reference (ucæf).1. A method for controlling a single-phase voltage source converter (21) con fi gured for connection to a power grid, the method comprising deriving (22) a power synohronization angle (GPSL) from a power synohronization loop (4) using a reference ( Pfef) for active power of the single-phase voltage source converter (3) and a measured active power output (P) of the single-phase voltage source converter (21) as input; and deriving (23) an ac voltage reference (uJef) based on the power synohronization angle (GPSL) obtained from the power synohronization loop (4); controlling (24) operations of the single-phase voltage source converter (21) according to a pulse width modulation method (3) based on said ac voltage reference (ucæf). 2. A single-phase voltage source converter (21) configured for connection to a power grid, the single-phase voltage source converter (21) comprising a controller configured to control the single-phase voltage source converter according to the method of claim 1.A single-phase voltage source converter (21) configured for connection to a power grid, the single-phase voltage source converter (21) comprising a controller configured to control the single-phase voltage source converter according to the method of claim 1 . 3. A computer program product, stored on a computer-readable medium, comprising computer-readable code portions which when loaded into and executed by a controller of a single-phase voltage source converter (21) causes the controller to carry out the method of claim 1.3. A computer program product, stored on a computer-readable medium, comprising computer-readable code portions which when loaded into and executed by a controller of a single-phase voltage source converter (21) causes the controller to carry out the method of claim 1.