ELECTRICAL POWER CONVERTER

Abstract

An electrical three-phase AC-DC converter includes first and second converter stages and a controller. The first converter stage converts between three phase AC terminals and first and second intermediate nodes. The second converter stage has a boost circuit to convert between fourth and fifth intermediate nodes and first and second DC terminals. A link connects the first and second intermediate nodes to the fourth and fifth intermediate nodes. A phase selector selectively connects the three phase terminals to a third intermediate node and a current injection circuit connects the third intermediate node to the first and second DC terminals. In a mode, a current path through the third intermediate node is obtained acting parallel to a current path through the first intermediate node, through the second intermediate node, or through the first and the second intermediate nodes in alternation.

Claims

1. An electrical converter for converting between an AC signal having three phase voltages and a DC signal, the electrical converter comprising: three phase terminals (A, B, C), a first DC terminal (P), and a second DC terminal (N); a first converter stage operably coupled to the three phase terminals and comprising a first intermediate node (x) and a second intermediate node (y), wherein the first converter stage is configured to convert between the AC signal at the three phase terminals and a first DC signal at the first intermediate node (x) and the second intermediate node (y), wherein the first converter stage further comprises a phase selector comprising first active switches (S.sub.aza, S.sub.bzb, S.sub.czc) configured for selectively connecting the three phase terminals to a third intermediate node (z); a second converter stage operably coupled to the first and second DC terminals (P, N) and comprising a fourth intermediate node (r) and a fifth intermediate node (s), wherein the second converter stage comprises a boost circuit operable to convert between a second DC signal at the fourth and fifth intermediate nodes (r, s) and a third DC signal at the first and second DC terminals (P, N) through at least one second active switch (S.sub.xm, S.sub.my, S.sub.xy), wherein the second converter stage further comprises a current injection circuit comprising third active switches (S.sub.Pz, S.sub.zN) operable to connect the third intermediate node (z) to the first DC terminal (P) and to the second DC terminal (N); a link connecting the first intermediate node (x) to the fourth intermediate node (r) and the second intermediate node (y) to the fifth intermediate node (s); and a controller implemented with a first mode of operation configured to convert between the AC signal and the third DC signal; wherein the controller is implemented with a second mode of operation configured to convert between a single phase AC signal applied between at least two of the three phase terminals and a fourth DC signal at the first and second DC terminals (P, N); wherein in the second mode of operation the controller is configured to operate the first active switches (S.sub.aza, S.sub.bzb, S.sub.czc) and the third active switches (S.sub.Pz, S.sub.zN), such that a third current path through the third intermediate node (z) is obtained acting parallel to a first current path through the first intermediate node (x), or acting parallel to a second current path through the second intermediate node (y), or acting in alternation parallel to the first and second current paths.

2. The electrical converter of claim 1, wherein in the first mode of operation the at least one second active switch (S.sub.xm, S.sub.my, S.sub.xy) is configured to operate through pulse width modulation such that the second converter stage operates as a boost converter and the first active switches (S.sub.aza, S.sub.bzb, S.sub.czc) are operated according to a switching pattern in which the phase terminal having a smallest absolute instantaneous voltage value of the three phase voltages is continuously connected to the third intermediate node (z).

3. The electrical converter of claim 1, wherein in the second mode of operation, the at least one second active switch (S.sub.xm, S.sub.my, S.sub.xy) is configured to operate through pulse width modulation such that the electrical converter operates as a single phase boost converter.

4. The electrical converter of claim 1, wherein in the second mode of operation the controller (40) is configured to operate the at least one second active switch (S.sub.xm, S.sub.my, S.sub.xy) and the third active switches (S.sub.Pz, S.sub.zN) via pulse width modulation.

5. The electrical converter of claim 4, wherein in the second mode of operation, the controller (40) is configured to operate the first active switches (S.sub.aza, S.sub.bzb, S.sub.czc) according to one or more of: a first selection mode configured to connect the third intermediate node to a phase terminal of the three phase terminals having a highest instantaneous voltage to obtain the third current path through the third intermediate node (z) acting parallel to the first current path through the first intermediate node (x), and a second selection mode configured to connect the third intermediate node to a phase terminal of the three phase terminals having a lowest instantaneous voltage to obtain the third current path through the third intermediate node (z) acting parallel to the second current path through the second intermediate node (y).

6. The electrical converter of claim 4, wherein in the second mode of operation, the controller (40) is configured to operate the at least one second active switch (S.sub.xm, S.sub.my, S.sub.xy) and the third active switches (S.sub.Pz, S.sub.zN) in an interleaved mode.

7. The electrical converter of claim 4, wherein in the second mode of operation, the controller (40) is configured to operate the at least one second active switch (S.sub.xm, S.sub.my, S.sub.xy) and the third active switches (S.sub.Pz, S.sub.zN) synchronously.

8. The electrical converter of claim 1, wherein the boost circuit is a single boost circuit, and wherein the link does not comprise inductive storage elements between the second intermediate node (y) and the fifth intermediate node (s), or between the first intermediate node (x) and the fourth intermediate node (r).

9. The electrical converter of claim 1, wherein the boost circuit comprises a first boost circuit and a second boost circuit stacked between the first DC terminal (P) and the second DC terminal (N), wherein the first and second boost circuits comprise a common node (m).

10. The electrical converter of claim 9, wherein each of the first boost circuit and the second boost circuit comprises one of the at least one second active switch (S.sub.xm, S.sub.my), wherein in the second mode of operation, the controller is configured to operate the at least one second active switches of the first boost circuit and of the second boost circuit synchronously.

11. The electrical converter of claim 9, wherein either one or both the first boost circuit and the second boost circuit is a multi-level boost circuit.

12. The electrical converter of claim 9, wherein the common node (m) is connected to a middle voltage node (q) between the first DC terminal (P) and the second DC terminal (N).

13. The electrical converter of claim 1, wherein the first converter stage comprises a bridge converter comprising three bridge legs.

14. The electrical converter of claim 1, comprising a fourth switch connected between one or more of: the first intermediate node (x) and the fourth intermediate node (r), and the second intermediate node (y) and the fifth intermediate node (s); wherein the controller is operable to open the fourth switch during startup for pre-charging a voltage between the first and second DC-terminals.

15. A battery charging system comprising a power supply, the power supply comprising the electrical converter of claim 1.

16. The battery charging system of claim 15, further comprising a battery, wherein the battery is configured to drive an electric vehicle.

17. An electric motor drive system, comprising a power supply, the power supply comprising the electrical converter of claim 1.

18. The electrical converter of claim 1, wherein in the second mode of operation the controller is configured to operate the first active switches (S.sub.aza, S.sub.bzb, S.sub.czc) and the third active switches (S.sub.Pz, S.sub.zN), such that a third current path through the third intermediate node (z) is obtained acting parallel to a first current path through the first intermediate node (x), and wherein a return current being a sum of currents of the first current path and the third current path is configured to flow through the second intermediate node (y).

19. The electrical converter of claim 1, wherein in the second mode of operation the controller is configured to operate the first active switches (S.sub.aza, S.sub.bzb, S.sub.czc) and the third active switches (S.sub.Pz, S.sub.zN), such that a third current path through the third intermediate node (z) is obtained acting parallel to a second current path through the second intermediate node (y), and wherein a return current being a sum of currents of the second current path and the third current path is configured to flow through the first intermediate node (x).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Aspects of the present disclosure will now be described in more detail with reference to the appended drawings, wherein same reference numerals illustrate same features and wherein:

[0019] FIG. 1 schematically shows an electrical converter that is unidirectional according to an embodiment of the present disclosure.

[0020] FIG. 2A is a diagram showing three-phase mains voltages v.sub.a, v.sub.b and v.sub.c during a 360° period of the AC mains voltage.

[0021] FIG. 2B is a diagram showing voltages between the intermediate nodes of the electrical converter during a 360° period of the AC mains voltage, and illustrates the overall operating principle of the electrical converter according to an embodiment of the present disclosure.

[0022] FIG. 2C is a diagram showing voltages across the DC link capacitors C.sub.x, C.sub.y, C.sub.z of the electrical converter according to an embodiment of the present disclosure during a 360° period of the AC mains voltage.

[0023] FIG. 2D and 2E are diagrams showing currents of the electrical converter during a 360° period of the AC mains voltage, and illustrate the overall operating principle of the electrical converter according to an embodiment of the present disclosure.

[0024] FIG. 2F is a diagram showing switching states of the phase-selector switches during a 360° period of the AC mains voltage, and illustrates the overall operating principle of the electrical converter according to an embodiment of the present disclosure.

[0025] FIG. 2G is a diagram showing switching states of the switches of the boost (upper and lower) and buck-boost circuits during a 360° period of the AC mains voltage, and illustrates the overall operating principle of the electrical converter according to an embodiment of the present disclosure.

[0026] FIG. 3 shows a block diagram of an advantageous implementation of a central control unit and control method according to an embodiment of the present disclosure.

[0027] FIG. 4A, 4B, 4C show diagrams with voltages, currents and switching states within five consecutive switching cycles of the boost (upper and lower) and buck-boost bridge legs of the electrical converter, and illustrates the PWM modulation of these bridge legs according to an embodiment of the present disclosure.

[0028] FIG. 5 schematically shows an electrical converter that is bidirectional according to an embodiment of the present disclosure.

[0029] FIG. 6 schematically shows an electrical converter that is unidirectional, and that has an input filter that is placed before instead of after the first converter stage according to an embodiment of the present disclosure.

[0030] FIG. 7A, FIG. 7B show different variants of the first converter stage that can be used in electrical converters of the present disclosure.

[0031] FIG. 8A and FIG. 8B show other variants of a first converter stage that can be used in electrical converters of the present disclosure.

[0032] FIG. 9 represents an electrical converter according to aspects of the present disclosure that is unidirectional and comprises a connection terminal for connecting to the neutral conductor of the grid (fourth phase).

[0033] FIG. 10 represents the electrical converter of FIG. 1 connected to a single phase gird.

[0034] FIG. 11 represents the single phase grid voltage and current over one period of the grid signal for the converter of FIG. 10.

[0035] FIG. 12 represents the rectified single-phase grid voltage and current at the upper and lower intermediate nodes of the first converter stage over the period of FIG. 11.

[0036] FIG. 13 represents the current flows through the electrical converter of FIG. 10 in single phase mode of operation and a positive grid voltage.

[0037] FIG. 14 represents the current flows through the electrical converter of FIG. 10 in single phase mode of operation and a negative grid voltage.

[0038] FIG. 15 represents an alternative electrical converter according to the present disclosure, having a two-level boost circuit.

[0039] FIG. 16 represents the single phase grid voltage and current over one period of the grid signal for the converter of FIG. 15.

[0040] FIG. 17 represents the rectified single-phase grid voltages and currents at the upper, lower and middle intermediate nodes of the first converter stage over the period of FIG. 16 in a second type of single phase operation applied to the electrical converter of FIG. 15.

[0041] FIG. 18 represents the current flows through the electrical converter of FIG. 15 in the second type of single phase mode of operation and a positive grid voltage.

[0042] FIG. 19 represents the current flows through the electrical converter of FIG. 15 in the second type of single phase mode of operation and a negative grid voltage.

[0043] FIG. 20 represents another embodiment of electrical converter according to the present disclosure comprising a switch for partially disabling the bridge rectifier during startup.

[0044] FIG. 21 represents a diagram of a battery charging system comprising an electrical converter according to the present disclosure.

DETAILED DESCRIPTION

[0045] FIG. 1 shows an electrical converter 100, referred to as the DUTCH

[0046] RECTIFIER, comprising two converter stages 11, 12 in the form of a three-phase active phase selector 11 and a DC/DC stage 12. Electrical converter 100 further comprises an input filter 13, and an output filter 15.

[0047] The electrical converter 100 is an AC-to-DC converter that has three phase inputs A, B, C which are connected to a three-phase voltage of a three-phase AC grid 21, and two DC outputs P, N which for example may be connected to a DC load 22 such as, for example, a high voltage (e.g. 800 V) battery of an electric car.

[0048] The first converter stage 11 comprises three phase connections a, b, c that are connected to the three phase inputs A, B, C, and three outputs x, y, z. These outputs may be seen as an upper intermediate voltage node x, a lower intermediate voltage node y, and a middle intermediate voltage node z.

[0049] The first converter stage 11 comprises a three-phase bridge rectifier 24 consisting of three bridge legs 16, 17, 18 which each comprise two passive semiconductor devices (diodes D.sub.ax and D.sub.ya for leg 16, D.sub.bx and D.sub.yb for leg 17, D.sub.cx and D.sub.yc for leg 18) connected in the form of a half bridge configuration, and a phase selector 25 comprising three selector switches (S.sub.aza, S.sub.bzb, and S.sub.czc) which each comprise two anti-series connected actively switchable semiconductor devices. Each such switchable semiconductor device advantageously has an anti-parallel diode. In this example, Metal Oxide Field Effect Transistors (MOSFETs) are used for the actively switchable semiconductor devices, and each includes an internal anti-parallel body diode that may replace an external anti-parallel diode.

[0050] The DC/DC stage 12 comprises, or consists of, two stacked boost bridge legs 19, 20 and one buck-boost bridge leg 14. Each boost bridge leg (19, 20) comprises a boost switch (S.sub.xm for the upper boost bridge leg 19 and S.sub.my for the lower boost bridge leg 20) and boost diode (D.sub.xP for the upper boost bridge leg 19 and D.sub.Ny for the lower boost bridge leg 20) connected in a half-bridge configuration. The buck-boost bridge leg 14 comprises two buck-boost switches (S.sub.Pz and S.sub.zN) connected in a half-bridge configuration. The middle node r of the upper boost bridge leg 19 is connected to intermediate voltage node x via an upper boost inductor L.sub.x, the middle node s of the lower boost bridge leg 20 is connected to intermediate voltage node y via a lower boost inductor L.sub.y, and the middle node t of the buck-boost bridge leg 14 is connected to intermediate voltage node z via a middle buck-boost inductor L.sub.z.

[0051] The common node m of the upper and lower boost bridge legs 19, 20 is advantageously connected to the middle voltage node q of the output filter 15 to form two stacked 2-level boost circuits. The output filter 15 comprises two output filter capacitors C.sub.Pm, C.sub.mN that are connected in series between the upper output node P and the lower output node N and middle voltage node q forming the middle node between capacitors C.sub.Pm and C.sub.mN. It will be convenient to note that the middle node t of the buck-boost bridge leg 14 acts as a switch node between middle intermediate node z, and the DC output terminals P and N. Switch node t is not connected to middle voltage node q of the output filter 15.

[0052] The upper boost bridge leg 19 is connected between the upper output node P and the common node m (i.e. in parallel with the upper output filter capacitor C.sub.Pm), and is arranged in a way that current can flow from the intermediate voltage node x to the upper output node P via the diode D.sub.xP when the switch S.sub.xm is open (not conducting, off state), and current can flow from the intermediate voltage node x to the common node m (or vice versa) via the switch S.sub.xm when the switch S.sub.xm is closed (conducting, on state). The boost switch (S.sub.xm) of the boost bridge leg 19 is an actively switchable semiconductor device, for example a MOSFET.

[0053] The lower boost bridge leg 20 is connected between the common node m and the lower output node N (i.e. in parallel with the lower output filter capacitor C.sub.mN), and is arranged in a way that current can flow from the lower output node N to the intermediate voltage node y via the diode D.sub.Ny when the switch S.sub.my is open (not conducting, off state), and current can flow from the common node m to the intermediate voltage node y (or vice versa) via the switch S.sub.my when the switch S.sub.my is closed (conducting, on state). The boost switch (S.sub.my) of the boost bridge leg 20 is an actively switchable semiconductor device, for example a MOSFET.

[0054] The buck-boost bridge leg 14 is connected between the upper output node P and the lower output node N (i.e. in parallel with the DC load 22) and acts as a current injection circuit arranged such that current flows from the intermediate voltage node z to the upper output node P (or vice versa) when the switch S.sub.Pz is closed (conducting, on state) while the switch S.sub.zN is open (not conducting, off state), and current flows from the intermediate voltage node z to the lower output node N (or vice versa) when the switch S.sub.zN is closed (conducting, on state) while the switch S.sub.Pz is open (not conducting, off state). The buck-boost switches (S.sub.Pz, S.sub.zN) of the buck-boost bridge leg 14 are actively switchable semiconductor devices, e.g. MOSFETs, which are controlled in a complementary way (i.e. the one is closed while the other is open and vice versa).

[0055] Advantageously, three high-frequency (HF) filter capacitors C.sub.x, C.sub.y, C.sub.z, which are part of the input filter 13, are interconnecting the intermediate voltage nodes x, y, z in the form of a star-connection. Generally, it is advantageous that the three capacitors C.sub.x, C.sub.y, C.sub.z have substantially equal value in order to symmetrically load the AC grid.

[0056] According to an aspect of the present disclosure, the controller is configured to operate according to a first mode of operation, referred to as three-phase operation, and to a second mode of operation, referred to as single-phase operation as will be further described herein.

[0057] The central control unit 40 advantageously controls all the controllable semiconductor devices (switches) of the electrical converter 100, sending control signals to each switch via a communication interface 50. In particular, semiconductor devices S.sub.aza, S.sub.bzb, S.sub.czc, S.sub.xm, S.sub.my, S.sub.Pz, S.sub.zN are controlled by controller 40. Furthermore, the control unit has measurement input ports (42, 43, 44, 45), for receiving measurements of: [0058] 42: the AC-grid phase voltages v.sub.a, v.sub.b, v.sub.c; [0059] 43: the inductor currents i.sub.Lx, i.sub.Ly, i.sub.Lz; [0060] 44: the DC bus voltage V.sub.DC; [0061] 45: the DC bus mid-point voltage V.sub.mN=−V.sub.Nm,
and an input port 41 to receive a set-value, which may be a requested DC output voltage V*.sub.DC. Controller operation allows particularly to accomplish the piece-wise sinusoidal shapes of inductor currents i.sub.Lx, i.sub.Ly, i.sub.Lz during normal operation.

[0062] The electrical converter 100 shown in FIG. 1 is unidirectional since the input stage 11 and the output power stage 12 contain diodes, only allowing power to be drawn from the electrical AC grid 21 and provide this power at the output to a load 22. FIG. 5, on the other hand, shows an electrical converter 200 according to the present disclosure that is bidirectional. Electrical converter 200 differs from converter 100 in that the diodes (D.sub.ax, D.sub.bx, D.sub.cx, D.sub.ya, D.sub.yb, D.sub.yc) of the input stage 11 and the diodes (D.sub.xP, D.sub.Ny) of the output power stage 12 of the converter shown in FIG. 1 have been replaced with controllable semiconductor switches (S.sub.xa, S.sub.xb, S.sub.xc, S.sub.ay, S.sub.by, S.sub.cy) in the input stage 211 and (S.sub.yN, S.sub.Px) in the output power stage 212 respectively. The switching device 23 is provided as a semiconductor switch, e.g. MOSFET.

[0063] In FIG. 6, an electrical converter 300 is shown which differs from converter 100 in that the input filter 13 is placed before (instead of after) first converter stage 11, i.e. the input filter 13 is connected between the phase input terminals A, B, C and the first converter stage 11. The first converter stage 11 connects the phase input terminals A, B, C to the intermediate nodes x, y, z via the corresponding inductor L.sub.a, L.sub.b, L.sub.c of the input filter 13. Capacitors C.sub.a, C.sub.b, C.sub.c are arranged between the phase input terminals and the inductors. The capacitors are connected in a star configuration, advantageously with the star point connected to a midpoint of the output filter 15, just like in the previous examples. Alternatively, the capacitors C.sub.a, C.sub.b, C.sub.c can be arranged in a delta configuration across the three phase input lines. It will be convenient to note that in the example of FIG. 6, the voltage signal at the three intermediate nodes x, y, z is somewhat different as compared to the previous examples (FIG. 1, FIG. 5), since the voltages at switch nodes r, s and t are identical to the voltages at the intermediate nodes x, y, z. As a result, high frequency currents will be flowing through the first converter stage 11, whereas in the previous examples (FIG. 1 and FIG. 5) the high frequency currents only occur in the output power stage downstream of the input filter 13.

[0064] In either electrical converters 100, 200, and 300, diodes may be replaced by actively switchable semiconductor devices to allow for bidirectional power flow of the electrical converter.

[0065] In either electrical converters 10, 200 and 300, the HF capacitors C.sub.x, C.sub.y, C.sub.z (or C.sub.a, C.sub.b, C.sub.c in case of FIG. 6) are connected in a star configuration. The voltage in the star point connection can be controlled by controlling the voltage at the common node m.

[0066] FIG. 7A, 7B show different variants of the first converter stage 11, which may be used in the electrical converters of either FIG. 1, FIG. 5, FIG. 6.

[0067] In FIGS. 8A-B yet other variants of the first converter circuit 11 are shown. In these variants, the three bridge legs 16, 17 and 18 of the phase selector are arranged as half-controlled thyristor legs (FIG. 8A), i.e. comprising thyristors Thy.sub.ax, Thy.sub.bx, Thy.sub.cx in the bridge leg portions connected to the upper intermediate node and diodes in the other bridge leg portion connected to the lower intermediate node (or vice versa), or as full-controlled thyristor legs (FIG. 8B), i.e. comprising a thyristor Thy.sub.ax, Thy.sub.bx, Thy.sub.cx, Thy.sub.ya, Thy.sub.yb, Thy.sub.yc, in each bridge half leg, instead of diodes. Such a phase selector allows for controllably pre-charging the output filter capacitors C.sub.Pm, C.sub.mN, or C.sub.PN without requiring an additional pre-charge circuit.

[0068] Referring to FIG. 9, the electrical converter 100 (and which may alternatively be the electrical converter 200 or 300) can comprise a connection terminal n for connecting the neutral conductor of the three-phase AC grid. In some applications, such as for example the charging of electric vehicles, it is often required that the amplitude of the sinusoidal current drawn from each phase of the three-phase grid can be independently controlled in order to be able to decrease the loading of a certain phase such that other consumer devices are still able to draw power from that particular phase during the charging of the vehicle's battery while not overloading the phase. In this case, the connection terminal n is advantageously connected to the neutral conductor of the three-phase grid, allowing a return current substantially equal to the sum of the three phase currents to flow back to the neutral conductor of the grid. In an advantageous aspect, the three phase currents can be fully independently controlled by providing a common node connected to the neutral conductor of the input.

[0069] The neutral connection terminal n is advantageously connected to the star-point of the AC capacitors C.sub.x, C.sub.y, C.sub.z and to the common node m of the stacked boost bridges 19, 20 (and thus also to the midpoint of the output filter 15). This results in a fully symmetrical converter structure. In this case, the voltage at the star-point and at the common node is equal to the voltage of the neutral conductor of the grid.

Three-Phase Operation of the Electrical Converter

[0070] Referring again to FIG. 1, the bridge leg of the bridge rectifier 24 that is connected with the phase input A, B, or C that has the highest voltage of the three-phase AC input voltage is switched in a way that the corresponding phase input A, B, or C is connected to the upper intermediate voltage node x. To achieve this, the bridge leg connects the corresponding phase connection a, b, or c with the node x via the upper diode (D.sub.ax, D.sub.bx, D.sub.cx) of the bridge leg, while the corresponding selector switch (S.sub.aza, S.sub.bzb, S.sub.czc) of the bridge leg is open (not conducting, off state). The bridge leg of the rectifier 24 that is connected with the phase input A, B, or C that has the lowest voltage of the three-phase AC input voltage is switched in a way that the corresponding phase input A, B, or C is connected to the lower intermediate voltage node y. To achieve this, the bridge leg connects the corresponding phase connection a, b, or c with the node y via the lower diode (D.sub.ya, D.sub.yb, D.sub.yc) of the bridge leg, while the corresponding selector switch (S.sub.aza, S.sub.bzb, S.sub.czc) of the bridge leg is open (not conducting, off state). The phase input A, B, or C that has a voltage between the highest voltage and the lowest voltage of the three-phase AC input voltage is connected by phase selector 25 to the middle intermediate voltage node z. To achieve this, the by phase selector 25 connects the corresponding phase connection a, b, or c with the node z via the selector switch (S.sub.aza, S.sub.bzb, S.sub.czc) which is closed (conducting, on state).

[0071] In a three-phase AC grid with substantially balanced phase voltages, for example as shown in FIG. 2A, the three-phase AC input voltage (shown in FIG. 2A) is converted into three intermediate DC voltages (v.sub.xz, v.sub.zy, v.sub.xy; shown in FIG. 2B) provided between the upper intermediate voltage node x, the lower intermediate voltage node y and the middle intermediate voltage node z. These DC voltages thus show piece-wise sinusoidal shapes. The conversion of the three-phase AC input voltage into three intermediate DC voltages is the result of the operation of the first converter stage 11, as explained above. The switching states (switch on.fwdarw.S=1, switch off.fwdarw.S=0) of the selector switches (S.sub.aza, S.sub.bzb, S.sub.czc) are shown in FIG. 2F. It can be seen that the switches are ‘on’ or ‘off’ continuously during whole particular 60° sectors within the period (360°) of the AC mains voltage. Also the diodes of the bridge rectifier 24 are ‘conducting’ or ‘not conducting’ during whole particular sectors, e.g. of 60°, within the period (360°) of the AC mains voltage. The combination of states of the switches and diodes is unique for every 60° sector of the three-phase AC input voltage and depends on the voltage value of the phase inputs (A, B, C). The sequence of the 6 unique states of the switches and diodes repeats itself every period (360°) of the AC mains voltage.

[0072] Seen from the viewpoint of the intermediate voltage nodes x, y, z towards the output terminals P, N, a conventional DC-DC boost circuit (upper boost circuit) is formed, comprising the HF filter capacitor C.sub.x, the upper boost inductor L.sub.x, the upper boost bridge leg 19, and the upper output capacitor C.sub.Pm. The input voltage of this upper boost circuit is the voltage v.sub.Cx (shown in FIG. 2C) across capacitor C.sub.x and the output voltage of this upper boost circuit is the voltage V.sub.Pm across the upper output capacitor C.sub.Pm, having a voltage value that is substantially equal to half the total DC bus voltage (V.sub.Pm≈V.sub.DC/2). The formed upper boost circuit may be operated by PWM modulation of the switch S.sub.xm at a specified, possibly variable, switching frequency f.sub.s in order to control the current in the upper boost inductor L.sub.x.

[0073] Seen from the viewpoint of the intermediate voltage nodes x, y, z towards the output terminals P, N, a conventional ‘inversed’ (negative input voltage and negative output voltage) DC-DC boost circuit (lower boost circuit) is formed, comprising the HF filter capacitor C.sub.y, the lower boost inductor L.sub.y, the lower boost bridge leg 20, and the lower output capacitor C.sub.mN. The input voltage of this lower boost circuit is the voltage v.sub.Cy (shown in FIG. 2C) across capacitor C.sub.y and the output voltage of this lower boost circuit is the voltage V.sub.Nm across the lower output capacitor C.sub.mN, having a voltage value that is substantially equal to minus half the total DC bus voltage (V.sub.Nm≈−V.sub.DC/2). The formed lower boost circuit may be operated by

[0074] PWM modulation of the switch S.sub.my at a specified, possibly variable, switching frequency f.sub.s in order to control the current in the lower boost inductor L.sub.y.

[0075] Seen from the viewpoint of the intermediate voltage nodes x, y, z towards the output terminals P, N, a conventional DC-DC buck-boost circuit (middle buck-boost circuit) is formed, comprising the HF filter capacitor C.sub.z, the middle buck-boost inductor L.sub.z, the buck-boost bridge leg 14, and the series connection of the output capacitors C.sub.Pm, C.sub.mN. This DC-DC buck-boost circuit may be seen as to be similar to a single-phase half-bridge voltage-source converter (VSC). The input voltage of this middle buck-boost circuit is the voltage v.sub.Cz (shown in FIG. 2C) across capacitor C.sub.z and the output voltage of this middle buck-boost circuit is the output voltage V.sub.PN across the series connection of the output capacitors C.sub.Pm, C.sub.mN. The formed middle buck-boost circuit may be operated by PWM modulation of the switches S.sub.Pz, S.sub.zN at a specified, possibly variable, switching frequency f.sub.s in order to control the current in the middle buck-boost inductor L.sub.z.

[0076] FIG. 2G shows the state of the switch S.sub.xm of the upper boost bridge leg 19, the state of the switch S.sub.my of the lower boost bridge leg 20, and the state of the switch S.sub.Pz (note that the state of the switch S.sub.zN is the complement of the state of the switch S.sub.Pz) of the middle buck-boost bridge leg 14. The switches S.sub.xm,S.sub.my,S.sub.Pz,S.sub.zN are all PWM modulated as can be seen from the black-colored bars, indicating PWM modulation of the corresponding switch.

[0077] An example of the currents i.sub.Lx, i.sub.Ly, i.sub.Lz in the inductors L.sub.x, L.sub.y, L.sub.z is shown in FIG. 2D. As can be seen, these currents are controlled to have piece-wise sinusoidal shapes and are transformed, i.e., as a result of the operation of the first converter stage 11, into three sinusoidal AC phase currents i.sub.a, i.sub.b, i.sub.c which are shown in FIG. 2E.

[0078] FIG. 3 shows a block diagram of an advantageous implementation of the central control unit 40 of FIG. 1 during the first mode of operation referred to as normal operation. The electrical converter 100 is represented in FIG. 3 as a ‘single-wire’ equivalent circuit, wherein the annotations of the elements correspond with those given in FIG. 1. Three slashes in a signal line indicate the bundling of three phase signals, and may represent the transition to a vector representation.

[0079] The goal of the control unit 40 is to control the output voltage V.sub.DC to a requested set-value V*.sub.DC that is received from an external unit via input port 41, and to balance the voltage across the two output capacitors C.sub.Pm and C.sub.mN, for example by controlling the voltage across the lower output capacitor C.sub.mN to be substantially equal to half the DC bus voltage. Additionally, the current drawn from the phase inputs (a,b,c) needs to be shaped substantially sinusoidal and controlled substantially in phase with the corresponding phase voltage. As explained previously, this can also be achieved by controlling the inductor currents i.sub.Lx, i.sub.Ly, i.sub.Lz, i.e., instead of directly controlling the phase currents i.sub.a, i.sub.b, i.sub.c, to have piece-wise sinusoidal shapes. In particular, the low-pass filtered values of the inductor currents are controlled while the high-frequency ripple of the inductor currents is filtered by the HF filter capacitors (C.sub.x, C.sub.y, C.sub.z).

[0080] The control of the output voltage V.sub.DC is advantageously done using a cascaded control structure, comprising an outer voltage control loop 60 and inner current control loop 70. The set-value of the output voltage is input to a comparator 61 via input port 41, and is compared with the measured output voltage obtained from a measurement processing unit 95 (for example comprising a low-pass filter). The output of comparator 61 is the control-error signal of the output voltage, which is further input to a control element 62 (for example comprising a proportional-integral control block) that outputs the instantaneous set-values of the amplitudes of the phase currents. These amplitudes are input to multiplier 63, and multiplied with signals that are obtained from calculation element 64 that outputs normalized instantaneous values of the phase voltages. The input of calculation element 64 are the measured phase voltages obtained from a measurement processing unit 93 (for example comprising a low-pass filter). The output of the multiplier 63 are set-values i*.sub.a, i*.sub.b, i*.sub.c for the instantaneous, for example low-pass filtered, phase currents i.sub.a, i.sub.b, i.sub.c, and are shaped substantially sinusoidal and positioned substantially in phase with the corresponding phase voltages. The set-values i*.sub.a, i*.sub.b, i*.sub.c are input to the current controller 70 after passing an addition element 67 and a selection element 81 whose functions are further detailed in the following text.

[0081] The current controller 70 is split into three individual current controllers 71, 74, 77, wherein: [0082] individual current controller 71 is used for controlling the current in the middle buck-boost inductor L.sub.z. This control is done by PWM modulation of the switches S.sub.Pz, S.sub.zN of the middle buck-boost circuit containing middle buck-boost bridge leg 14. As a result of the operation of the first converter stage 11, therewith, controller 71 controls the current of the phase input A,B,C, that has a voltage between the highest voltage and the lowest voltage of the three-phase AC voltage; [0083] individual current controller 74 is used for controlling the current in the upper boost inductor L.sub.x. This control is done by PWM modulation of the switch S.sub.xm of the upper boost circuit containing upper boost bridge leg 19. As a result of the operation of the first converter stage 11, therewith, controller 74 controls the current of the phase input A,B,C, that has the highest voltage of the three-phase AC voltage; [0084] individual current controller 77 is used for controlling the current in the lower boost inductor L.sub.y. This control is done by PWM modulation of the switch S.sub.my of the lower boost circuit containing lower boost bridge leg 20. As a result of the operation of the first converter stage 11, therewith, controller 77 controls the current of the phase input A,B,C, that has the lowest voltage of the three-phase AC voltage.

[0085] Selector element 81 is used to send the set-values i*.sub.a, i*.sub.b, i*.sub.c (shown in FIG. 2D) for the instantaneous phase currents to the correct individual current controller (71, 74, 77) depending on the voltage value of the phase inputs (A, B, C), resulting in inductor current set-values i*.sub.Lx, i*.sub.Ly, i*.sub.Lz (shown in FIG. 2E) for each inductor current controller, wherein: [0086] the set-value of the phase current of the phase input A,B,C, that has the highest voltage of the three-phase AC voltage is sent to individual current controller 74, resulting in set-value i*.sub.Lx; [0087] the set-value of the phase current of the phase input A,B,C, that has the lowest voltage of the three-phase AC voltage is sent to individual current controller 77, resulting in set-value i*.sub.Ly; [0088] the set-value of the phase current of the phase input A,B,C, that a voltage between the highest voltage and the lowest voltage of the three-phase AC voltage is sent to individual current controller 71, resulting in set-value i*.sub.Lz.

[0089] In each individual current controller the received set-value (i*.sub.Lx, i*.sub.Ly, i*.sub.Lz) for the instantaneous inductor current is input to a comparator, for example comparator 72 of individual current controller 71, and compared with the measured inductor current obtained from a measurement processing unit 94 (for example comprising a low-pass filter). The output of the comparator is the control-error signal of the current, which is further input to a control element, for example control element 73 of individual current controller 71, whose output is input to a PWM generation element, for example PWM generation element 54 of individual current controller 71. The PWM generation element of the individual current controllers generate the PWM-modulated control signals for the controllable semiconductor switches of the PWM-controlled bridge legs, i.e. the upper boost bridge leg 19 of the upper boost circuit, the lower boost bridge leg 20 of the lower boost circuit, and the middle buck-boost bridge leg 14 of the middle buck-boost circuit. These PWM-modulated control signals are sent to the appropriate bridge legs via communication interface 50.

[0090] The selector switches of the first converter stage 11 are either ‘on’ or ‘off’ during each 60° sector of the three-phase AC input voltage, depending on the voltage value of the phase inputs (A, B, C). The control signals for the selector switches are generated by switch-signal generators 51, 52, 53.

[0091] DC bus mid-point balancing can be done by adding an offset value to the set-values i*.sub.a, i*.sub.b, i*.sub.c for the instantaneous, for example low-pass filtered, phase currents i.sub.a, i.sub.b, i.sub.c, which are output by multiplier 63. The offset value is obtained by comparing the measured DC bus midpoint voltage obtained from a measurement processing unit 96 (for example comprising a low-pass filter) with a set-value (for example V.sub.DC/2) using comparator 65 and feeding the error signal output by the comparator 65 into a control element 66.

[0092] The phase currents i.sub.a, i.sub.b, i.sub.c shown in FIG. 2E are obtained by controlling the electrical converter 100 using such control unit 40 and control method detailed in the foregoing text. Also shown in FIG. 2E are the set-values i*.sub.a, i*.sub.b, i*.sub.c for the instantaneous, for example low-pass filtered, phase currents i.sub.a, i.sub.b, i.sub.c, as input to selector element 81 shown in FIG. 3. As explained above, the phase currents i.sub.a, i.sub.b, i.sub.c are indirectly controlled, i.e., they are the result of the controlling of the inductor currents i.sub.Lx, i.sub.Ly, i.sub.Lz (shown in FIG. 2D) and the operation of the first converter stage 11. The set-points for the inductor currents (i*.sub.Lx, i*.sub.Ly, i*.sub.Lz) are derived from set-values i*.sub.a, i*.sub.b, i*.sub.c by selector element 81 based on the measured phase voltages.

[0093] FIGS. 4A-4C show diagrams within five consecutive switching cycles (i.e., each having a switching period T.sub.s equal to 1/f.sub.s, with f.sub.s the switching frequency) of the bridge legs of the electrical converter 100, for a time interval around ωt=45° which lies within the sector of the three-phase AC input voltage where 0≤ωt<60° (see FIG. 2). Within this sector, the selector switches and diodes of the first converter stage 11 are in the following switching states: [0094] Switch S.sub.aza=0 (off), diode D.sub.ax=1 (conducting), diode D.sub.ya=0 (blocking); phase connection a is connected with node x; [0095] Switch S.sub.bzb=0 (off), diode D.sub.bx=0 (blocking), diode D.sub.yb=1 (conducting); phase connection b is connected with node y; [0096] Switch S.sub.czc=1 (on), diode D.sub.cx=0 (blocking), diode D.sub.yc=0 (blocking); phase connection c is connected with node z;

[0097] The diagrams of FIGS. 4A-4C show voltages, currents, and switching signals on a milliseconds time axis. FIG. 4A corresponds with the operation of the upper boost circuit, showing the corresponding inductor current i.sub.Lx (and the set-value i*.sub.Lx of this current), the inductor voltage v.sub.Lx, and the control signal S.sub.xm of the switch of the PWM-modulated upper boost bridge leg 19. FIG. 4B corresponds with the operation of the lower boost circuit, showing the corresponding inductor current i.sub.Ly (and the set-value i*.sub.Ly of this current), the inductor voltage v.sub.Ly, and the control signal S.sub.my of the switch of the PWM-modulated lower boost bridge leg 20. FIG. 4C corresponds with the operation of the middle buck-boost circuit, showing the corresponding inductor current i.sub.Lz (and the set-value i*.sub.Lz of this current), the inductor voltage v.sub.Lz, and the control signal S.sub.Pz of the upper switch of the PWM-modulated bridge leg 14. Note that the control signal S.sub.zN of the lower switch of the PWM-modulated bridge leg 14 is the complement of the control signal S.sub.Pz.

[0098] In order to minimize the Total Harmonic Distortion (THD) of the AC input current of the electrical converter, the high-frequency ripple of phase currents i.sub.a, i.sub.b, i.sub.c is advantageously minimized.

[0099] An advantage of the electrical converter 100 is that the half-switching-period volt-seconds product/area of the upper boost inductor and of the lower boost inductor are smaller than the volt-seconds products/areas of the boost inductors of a conventional six-switch boost-type PFC rectifier. This is because the voltages applied to these inductors are smaller than in the case of a conventional six-switch boost-type PFC rectifier. For the middle buck-boost inductor, the applied voltages are not necessarily smaller but the value of the current flowing in the inductor is smaller than the value of the currents flowing in inductors of a conventional six-switch boost-type PFC rectifier. As a result, smaller inductors with less magnetic energy storage are feasible, resulting in a higher power-to-volume ratio of the electrical three-phase AC-to-DC converter 100 that is provided by the present disclosure.

[0100] Three-phase operation of the electrical converters 200-400 as represented in FIGS. 5, 6 and 9 is analogous to three-phase operation of converter 100 described above.

Single-Phase Operation of the Electrical Converter

[0101] According to the present disclosure, the controller 40 is implemented with a second mode of operation, referred to as single-phase operation, which is chosen when at the AC side, the converter is connected to a single-phase grid v.sub.gr. Referring to FIG. 10, showing the electrical converter 100, in single-phase operation, one of the AC phase terminals, e.g. A is connected to the forward conductor of the single-phase grid v.sub.gr and another AC phase terminal, e.g. C is connected to the return conductor of v.sub.gr. The third phase terminal, e.g. B, is not connected. Different single-phase operation modes can be contemplated.

[0102] In a first, conventional single-phase operation mode, the bridge rectifier 24 rectifies/folds the grid voltage v.sub.gr into v.sub.xy between the intermediate nodes x and y, as shown in FIG. 11 and FIG. 12. The boost circuit legs 19 and 20 can be operated to generate currents i.sub.x and i.sub.y, respectively, which are in phase with v.sub.xy as shown in FIG. 12, with i.sub.x=−i.sub.y. In particular, active switches S.sub.xm and S.sub.my are operated with PWM and diodes D.sub.xp and D.sub.ny are conducting when the respective S.sub.xm and S.sub.my is open. The bridge 24 unfolds i.sub.x and −i.sub.y into the grid current i.sub.gr. In this embodiment, the phase selector switches S.sub.aza, S.sub.bzb, S.sub.czc and the third harmonic current injection leg 14 with switches S.sub.pz, S.sub.zn are not operational (i.e. they are open and non-conducting) and no current flows through intermediate node z (i.sub.z=0). FIG. 13 shows the current paths during the interval where the grid voltage at A is positive. FIG. 14 shows the current paths during the interval where the grid voltage at A is negative.

[0103] The above single-phase operation allows to convert at least one third of the power as compared to three-phase operation. Assuming three-phase operation allows for converting 22 kW, i.e. 3×32 Arms in the phases for 400 Vrms line-to-line voltage. In three-phase operation, the peak current at node x=√{square root over (2)}*32=45.2 Apk (i.e. equal to the positive amplitude value of the phase currents). The peak current at node y=−√{square root over (2)}*32=−45.22 Apk (i.e. equal to the negative amplitude value of the phase currents). The peak current at node z=±√{square root over (2)}*32/2=±22.6 Apk (i.e. equal to the current value at the crossings of the phase currents). These respective currents are generated by the respective HF bridge legs 19-20 and 14. In single phase operation, only the upper and lower boost bridge legs 19, 20 are active and carry the same current (i.e. they do not act in parallel). This means that the peak phase current is equal to 45.2 Apk, meaning 32 Arms is obtained as well. The converted power is then 230 Vrms×32 Vrms=7.36 kW or about one third of 22 kW, assuming 230 Vrms phase voltage.

[0104] It is possible to obtain an even higher power rating in single-phase operation by allowing the inductors of the input filter stage (differential mode (DM) inductors) to go into controlled saturation, without the need to overdimension the inductive components as compared to three-phase operation.

[0105] In another circuit topology, referring to FIG. 15, electrical converter 500 shows a single (two-level) boost circuit 29 connected between nodes P and N, and inductor L.sub.y is missing in the DC-link between nodes y and s. In single-phase operation, the converter 500 can be operated analogously to converter 100 as described above, i.e. the boost circuit 29 is operated through PWM while the current injection leg 14 and phase selector 25 are not operated. A same current path as shown in FIGS. 13 and 14 is obtained.

[0106] Still referring to FIG. 15, in a second single-phase operation mode, according to aspects of the present disclosure, single-phase operation can be performed by operating the injection leg 14 as well as the boost circuit 29 through PWM. The single-phase grid is still connected to two phase terminals A, C. The bridge rectifier 24 rectifies/folds the grid voltage v.sub.gr as shown in FIG. 16 into the rectified voltage v.sub.xy between nodes x and y as shown in FIG. 17. The phase selector 25 is operated (by controller 40) to connect the middle intermediate node z to phase terminal A during the positive half-cycle of v.sub.gr and to connect the middle intermediate node z to phase terminal C during the negative half-cycle of v.sub.gr. By so doing, the phase selector 25 rectifies/folds the grid voltage v.sub.gr as shown in FIG. 16 into the rectified voltage v.sub.zy between nodes z and y as shown in FIG. 17, and obtains at middle intermediate node z a parallel current path i.sub.z to the current path i.sub.x at upper intermediate node x, i.e. the single phase conductor that is connected to node x by bridge 24 is also connected to node z by phase selector 25. It will be convenient to note that the respective switches of the phase selector are operated at a low frequency, e.g. grid frequency.

[0107] The switches S.sub.xy and S.sub.Pz, S.sub.zN of the boost leg 29 and injection leg 14 are operated through PWM in order to generate DC-link currents i.sub.x and i.sub.z, respectively, which are in phase with v.sub.xy and v.sub.zy as shown in FIG. 17. DC-link currents i.sub.x and i.sub.z are combined into i.sub.y according to Kirchhoff's law: i.sub.z+i.sub.x=−i.sub.y. The bridge rectifier 24 unfolds i.sub.z+i.sub.x and −i.sub.y as shown in FIG. 17 into i.sub.gr as shown in FIG. 16. FIG. 18 shows the current paths during the interval where the grid voltage at A is positive. FIG. 19 shows the current paths during the interval where the grid voltage at A is negative.

[0108] In this case it will be clear that i.sub.gr can be higher than in the example of FIGS. 11-14, and this type of single phase operation allows for converting at least half of the power as compared to three-phase operation. Assuming three-phase operation allows for converting 22 kW, i.e. 3×32 Arms in the phases for 400 Vrms line-to-line voltage. In three-phase operation, the peak current at node x=√{square root over (2)}*32=45.2 Apk (i.e. equal to the positive amplitude value of the phase currents). The peak current at node y=−√{square root over (2)}*32=−45.22 Apk (i.e. equal to the negative amplitude value of the phase currents). The peak current at node z=±√{square root over (2)}*32/2=±22.6 Apk (i.e. equal to the current value at the crossings of the phase currents). These respective currents are generated by the respective HF current legs (i.e., switches S.sub.xy and S.sub.Pz, S.sub.zN) of the boost circuit 29 and buck-boost circuit 14. In the second single-phase operation mode, these two HF current legs are active and act in parallel. Particularly, the HF current legs can be operated in phase (non-interleaved mode). This means that the peak phase current is equal to 45.2 Apk+22.6=67.8 Apk meaning 48 Arms as well. The power is then 230 Vrms×48=11 kW˜=½×22 kW, assuming 230 Vrms phase voltage.

[0109] However, assuming the mains-side (input) filter is designed for 32 Arms for three-phase operation, it now must carry 48 Arms in single phase operation (carried through node y), potentially driving the DM inductors into saturation, which is allowed by appropriate selection of core materials. The resulting reduction of the attenuation of the filter can be counteracted or largely reduced by interleaving the generation of currents i.sub.x and i.sub.z. In the latter case, the HF current legs of circuits 29 and 14 are operated out of phase (interleaved mode).

[0110] One advantage of operating the HF current legs, both in interleaved mode and in non-interleaved mode as described hereinabove is that it allows to control distribution of the grid current i.sub.a between DC link currents i.sub.x and i.sub.z. By so doing, the current ripple of i.sub.a can be reduced.

[0111] It will be convenient to note that the second single-phase operation mode can be equally applied to the electrical converter 100 (FIG. 10), with minimal or even no overdimensioning of the inductor L.sub.y needed in case of interleaving the generation of currents i.sub.x and i.sub.z. The two switches S.sub.xm and S.sub.my of the boost circuits 19, 20 between nodes r and s can be operated synchronously, i.e. simultaneously open or closed. Alternatively, it is possible to generate a multi-level voltage when operating the two switches S.sub.xm and S.sub.my in an interleaved fashion. This can be performed in particular regions of the grid voltage period, reducing the HF current ripple.

[0112] In the second single phase operation mode, the phase selector 25 can alternatively be operated such that i.sub.z and i.sub.y act in parallel, instead of i.sub.x and i.sub.z. In this case, the phase selector 25 is operated (by controller 40) to connect the middle intermediate node z to phase terminal C (instead of A) during the positive half-cycle of v.sub.gr and to connect the middle intermediate node z to phase terminal A (instead of C) during the negative half-cycle of v.sub.gr. It may also be possible to alternate between the two options.

[0113] The third phase terminal B, which is left disconnected in the previous examples, can alternatively be connected to either the forward conductor (i.e. shorted with A), or the return conductor (i.e., shorted with C). It is possible to connect the third phase terminal B in parallel with the current path through phase terminal A or through phase terminal C by operating the corresponding switch of phase selector 25. Referring e.g. to FIGS. 18 and 19, one would then operate switch S.sub.bzb of phase selector 25 instead of S.sub.aza or S.sub.czc in conjunction with S.sub.Pz or S.sub.zN depending on whether phase terminal B acts in parallel with phase terminal A or C.

[0114] Referring to FIG. 20, an electrical converter 600 is shown which differs from electrical converter 200 of FIG. 5 in the presence of a switch 23. Switch 23 advantageously allows for pre-charging the converter at startup. In three-phase mode of operation, at start-up, switching device 23 is opened to interrupt conduction between the upper nodes of the bridge rectifier 24 and the upper intermediate node x. No current flows through inductor L.sub.x. The phase selector 25 is now operated to apply at the middle intermediate node z a phase input voltage which is slightly higher than the (instantaneous) output voltage V.sub.DC across the output terminals P, N for a limited amount of time (e.g. 1 μs). By so doing, during the limited amount of time, the positive voltage difference between the voltage at the middle intermediate node z and the output voltage V.sub.DC is applied across the inductor L.sub.z causing a phase current to flow through inductor L.sub.z and further to the upper output terminal P due to the conduction of the (internal) anti-parallel diode connected to switch S.sub.Pz between switch node t and terminal P. The current path hence goes from middle intermediate node z through switch node t through the anti-parallel diode of switch S.sub.Pz and through the capacitors C.sub.Pm, C.sub.mN of the output filter 15 and back to lower intermediate node y and back to a phase of the grid through one of the lower corresponding diodes/switches of the bridge rectifier 24. By so doing, the output voltage V.sub.DC can be stepped up gradually.

[0115] A same pre-charge operation can advantageously be performed when in single-phase mode of operation, i.e. opening switch 23 and operating the phase selector 25 as described above. In this case either switch S.sub.aza or S.sub.czc, or both S.sub.aza and S.sub.czc of phase selector 25 are operated.

[0116] During normal operation, switch 23 is closed continuously, both in three-phase and single-phase mode of operation. Switch 23 can be provided as a relay switch instead of a semiconductor switch and is advantageously operably coupled to controller 40.

[0117] In one particular aspect according to the present disclosure, there is therefore provided an electrical converter (600) for converting between an AC signal having three phase voltages and a DC signal, comprising: [0118] three phase terminals (A, B, C), a first DC terminal (P) and a second DC terminal (N), [0119] a first converter stage (11) operably coupled to the three phase terminals and comprising a first intermediate node (x) and a second intermediate node (y), wherein the first converter stage is configured for converting between the AC signal at the three phase terminals and a first DC signal at the first intermediate node (x) and the second intermediate node (y), wherein the first converter stage further comprises a phase selector (25) comprising first active switches (S.sub.aza, S.sub.bzb, S.sub.czc) configured for selectively connecting the three phase terminals to a third intermediate node (z), [0120] a second converter stage (12) operably coupled to the first and second DC terminals (P, N) and comprising a fourth intermediate node (r) and a fifth intermediate node (s), wherein the second converter stage comprises a boost circuit (19, 20, 29) operable to convert between a second DC signal at the fourth and fifth intermediate nodes (r, s) and a third DC signal at the first and second DC terminals (P, N) through at least one second active switch (S.sub.xm, S.sub.my), wherein the second converter stage further comprises a current injection circuit (14) comprising third active switches (S.sub.Pz, S.sub.zN) operable to connect the third intermediate node (z) to the first DC terminal (P) and to the second DC terminal (N), [0121] a link connecting the first intermediate node (x) to the fourth intermediate node (r) and the second intermediate node (y) to the fifth intermediate node (s), [0122] a controller (40) implemented with a first mode of operation configured to converting between the AC signal and the third DC signal,
wherein the controller (40) is implemented with a second mode of operation configured to convert between a single phase AC signal applied between at least two of the three phase terminals and a fourth DC signal at the first and second DC terminals (P, N), and
wherein the converter comprises a fourth switch (23) between the first intermediate node (x) and the fourth intermediate node (r) and/or between the second intermediate node (y) and the fifth intermediate node (s), wherein the controller (40) is operable to open the fourth switch (23) during startup for pre-charging a voltage between the first and second DC-terminals. The present aspect can be provided in combination with any one of the other aspects described in the present disclosure, e.g. as recited in the appended claims.

[0123] Electrical converters according to the present disclosure can, for example, be used for converting a three-phase AC voltage or a single phase AC voltage from an electrical grid, which may be a low voltage (e.g. 380-400 or 240 Vrms at 50 Hz frequency) grid, into a high DC output voltage (e.g. 700-1000 V for three-phase AC and typically 350-500 V for single phase AC).

[0124] Referring to FIG. 21, a battery charging system 700 comprises a power supply unit 704. The power supply unit 704 is coupled to an interface 702, e.g. comprising a switch device, which allows to connect the power supply unit 704 to a battery 703. The power supply unit 704 comprises any one of the electrical converters as described hereinabove, e.g. converter 500, coupled to a DC-DC converter stage 701. The DC-DC converter stage 701 can comprise or consist of one or more isolated DC-DC converters. The DC-DC converter stage can comprise a transformer effecting galvanic isolation, particularly in case of wired power transfer between power supply unit 704 and the battery 703. The DC-DC converter stage can comprise a pair of coils which are inductively coupled through air, such as in case of wireless power transfer. In some cases, the interface 702 can comprise a plug and socket, e.g. in wired power transfer. Alternatively, the plug and socket can be provided at the input (e.g., at terminals A, B, C). Particularly, the DC-DC converter stage 701 can comprise a plurality of DC-DC converters which are selectively connectable in parallel and in series. When operating the electrical converter in three-phase AC mode, the DC-DC converters are typically connecter in series. When operating in single phase AC mode as described above, the DC-DC converters are typically connected in parallel. Switching between parallel and series mode of connection of the DC-DC converters can be effected using relays, as known in the art.