Multi-level power converter and a method for controlling a multi-level power converter

Abstract

A multi-level power converter for one or more phases includes one or more converter arms including a plurality of serial connected switching cells. Each switching cell includes a plurality of switching devices, a primary energy storage, a secondary energy storage and a first inductor. The switching devices are arranged to selectively provide a connection to the primary energy storage, wherein each switching cell includes a bridge circuit including the switching devices and the primary energy storage, a battery circuit connected to the bridge circuit and including the secondary energy storage, and an arm circuit providing a connection between two adjacent switching cells. The first inductor of each switching cell is arranged in the arm circuit.

Claims

1. A multi-level power converter for one or more phases, the converter comprises one or more converter arms comprising a plurality of serial connected switching cells, each switching cell comprises a plurality of switching devices, a primary energy storage, a secondary energy storage and a first inductor, the switching devices being arranged to selectively provide a connection to the primary energy storage, wherein each switching cell comprises a bridge circuit comprising the switching devices and the primary energy storage, a battery circuit connected to the bridge circuit and comprising the secondary energy storage, and an arm circuit providing a connection between two adjacent switching cells, wherein the bridge circuit comprises a first switching device and second switching device connected in series between a first and second terminal of the primary energy storage and a third switching device and fourth switching device connected in series between the first and second terminal of the primary energy storage, wherein the first inductor of each switching cell is arranged in the arm circuit, said arm circuit being connected between the second terminal of the primary energy storage and a point of connection to an adjacent switching cell, said battery circuit being connected between the point of connection of the adjacent cell and the bridge circuit, wherein the battery circuit is connected to the bridge circuit between the third switching device and the fourth switching device of the primary energy storage.

2. The multi-level power converter according to claim 1, wherein the switching devices of the bridge circuit comprise one of an integrated gate-commutated thyristor, a gate turn-off thyristor and an insulated-gate bipolar transistor.

3. The multi-level power converter according to claim 1, wherein each of the one or more converter arms comprises at least one second inductor arranged connected in series with the switching cells.

4. The multi-level power converter according to claim 1, wherein the converter comprises a first arm part and a second arm part per phase.

5. The multi-level power converter according to claim 1, wherein the primary energy storage is a capacitor.

6. The multi-level power converter according to claim 1, wherein the secondary energy storage comprises one or more battery or one or more supercapacitors.

7. The multi-level power converter according to claim 1, wherein a first terminal of said battery circuit is directly connected to a point of connection of the third switching device and the fourth switching device, and a second terminal of said battery circuit is directly connected to the first inductor.

8. The multi-level power converter according to claim 2, wherein each of the one or more converter arms comprises at least one second inductor arranged connected in series with the switching cells.

9. The multi-level power converter according to claim 2, wherein the converter comprises a first part and a second arm part per phase.

10. The multi-level power converter according to claim 2, wherein the primary energy storage is a capacitor.

11. The multi-level power converter according to claim 3, wherein the converter comprises a first arm part and a second arm part per phase.

12. The multi-level power converter according to claim 3, wherein the primary energy storage is a capacitor.

13. The multi-level power converter according to claim 4, wherein each of the first aim part and the second arm part comprises a second inductor.

14. A method for operating a multi-level power converter for one or more phases, the converter comprises one or more converter arms comprising a plurality of serial connected switching cells, each switching cell comprises a plurality of switching devices, a primary energy storage, a secondary energy storage and a first inductor, the switching devices being arranged to selectively provide a connection to the primary energy storage, wherein each switching cell comprises a bridge circuit comprising the switching devices and the primary energy storage, a battery circuit connected to the bridge circuit and comprising the secondary energy storage, and an arm circuit providing a connection between two adjacent switching cells, wherein the bridge circuit comprises a first switching device and second switching device connected in series between a first and second terminal of the primary energy storage and a third switching device and fourth switching device connected in series between the first and second terminal of the primary energy storage, wherein the first inductor of each switching cell is arranged in the arm circuit, said arm circuit being connected between the second terminal of the primary energy storage and a point of connection to an adjacent switching cell, said battery circuit being connected between the point of connection of the adjacent cell and the bridge circuit, wherein the battery circuit is connected to the bridge circuit between the third switching device and the fourth switching device of the primary energy storage, and wherein the method comprises connecting the primary energy storage with the arm circuit by closing the first switching device and opening the second switching device.

15. The method according to claim 14, wherein the method comprises disconnecting the primary energy storage from the arm circuit by means of opening the first switching device and closing the second switching device.

16. The method according to claim 14, wherein the method comprises connecting the primary energy storage with the battery circuit by means of closing the third switching device and opening the fourth switching device.

17. The method according to claim 14, wherein the method comprises disconnecting the primary energy storage from the battery circuit by means of opening the third switching device and closing the fourth switching device.

18. The method according to claim 14, wherein the method comprises isolating the secondary energy storage by means of opening the third switching device and the fourth switching device.

19. The method according to claim 14, wherein the method comprises switching the third switching device and the fourth switching device at a higher frequency than the switching frequency of the first switching device and the second switching device.

20. The method according to claim 14, wherein a first terminal of said battery circuit is directly connected to a point of connection of the third switching device and the fourth switching device, and a second terminal of said battery circuit is directly connected to the first inductor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be explained more closely by the description of different embodiments of the invention and with reference to the appended figures.

(2) FIG. 1 discloses an example of a prior art multi-level power converter for three phases.

(3) FIG. 2 discloses an example of a switching cell of the converter in FIG. 1.

(4) FIG. 3 discloses an example of a schematic view of one phase of the converter comprising N switching cells.

(5) FIG. 4 disclosed a method for operating a multi-level power converter according to an embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

(6) FIG. 1 shows an example of a prior art multi-level power converter 1 for converting DC electric power to AC electric power for three phases. The converter 1 comprises an arm 3 for each phase. Each arm 3 comprises a first arm part 5 connected to an input terminal 7 with first potential of the DC power and a second arm part 10 connected to an input terminal 12 with a second potential of the DC power. The first arm part 5 and the second arm part 10 are connected to an output terminal 15 of the AC power for the respective phase.

(7) Each arm 3 comprises plurality of switching cells 20 connected in serial. In FIG. 1, the first arm part 5 and the second arm part 10 each includes six switching cells 20. Each of the first arm part 5 and the second arm part 10 also comprises a second inductor 17 for reducing flow of transient current between the arms 3. In the disclosed example, a capacitor 24 is connected in parallel to the three arms 3.

(8) An example of a switching cell 20 is shown in FIG. 2. The switching cell 20 comprises four switching devices 40a, 40b, 40c, 40d, a primary energy storage 50, a secondary energy storage 52 and a first inductor 54.

(9) The switching cells 20 are arranged so that it comprises a bridge circuit 60, a battery circuit 62 and an arm circuit 64.

(10) The bridge circuit 60 comprises the four switching devices 40a, 40b, 40c, 40d and the primary energy storage 50 arranged as a full H-bridge arrangement. The bridge circuit 60 is arranged comprising a first switching device 40a and a second switching device 40b connected in series between a first and second terminal of the primary energy storage 50, and a third switching device 40c and a fourth switching device 40d connected in series between the first and second terminal of the primary energy storage 50.

(11) The switching operation of the first switching device 40a and the second switching device 40b is adapted to be controlled by a controller 34 so that the primary energy storage 50 of the switching cells 20 is discharged or charged, wherein the desired AC power is formed.

(12) The battery circuit 62 comprises the secondary energy storage 52 that is adapted to be charged and discharged during operation of the converter 1 in order to even out irregularities in supplied power that is to be converted by the converter 1, such as supplied power from renewable energy.

(13) The primary energy storage 50 is preferably a capacitor. However other types of energy storage devices such as a battery may be used. The secondary energy storage 52 is preferably a battery, such as an electrochemical battery. However, other types of energy storage devices such as a plurality of supercapacitors may be used.

(14) The switching operation of the third switching device 40c and the fourth switching device 40d is adapted to be controlled by the controller 34 so that the secondary energy storage 52 of the switching cell 20 is discharged or charged in order to boost the conversion of power in case of low supply from the energy source.

(15) The first inductor 54 that has the function to provide a smooth current to and from the second energy storage 52 that is arranged connected in parallel to the bridge circuit 60 and the arm circuit 64. The arm circuit 64 is arranged to provide a connection between two adjacent switching cells 20.

(16) An advantage of arranging the first inductor 54 in the arm circuit 64 instead of in the battery circuit 62 is that it allows the self-inductance of the first inductor 54 to be significantly reduced. The self-inductance is reduced but can however not be completely eliminate without implication to the function of converter 1.

(17) The function of the converter will be discussed in further details with reference to FIGS. 2 and 3. FIG. 3 discloses an example of the converter arm 3 where the first arm part 5 comprises N switching cells 20 and where the second arm part 10 comprises N switching cells 20.

(18) The circuit in FIG. 2 shows the circuit diagram of the i-th switching cell 20. The letter v indicates the electric potential at the point marked by an adjacent dot. All switching cells 20 are assumed to have equal inductances L.sub.c and capacitances C.sub.c. Resistances are neglected.

(19) All variables except the arm current i.sub.a are unique for each switching cells 20 and are denoted with the index i. By subtracting the potentials at the points on each side of the inductor, the following relation is established:

(20) $\begin{array}{cc}{L}_c\frac{d\left(i_a-i_{\mathrm{bi}}\right)}{\mathrm{dt}}=E_i-s_i{v}_{\mathrm{ci}}.& \left(1\right)\end{array}$

(21) It is desired that i.sub.bi should be smooth, i.e., di.sub.bi/dt=0. Hence, the insertion index for the right-side switches shall ideally be selected as

(22) $\begin{array}{cc}{s}_i=\frac{E_i-L_c\frac{{\mathrm{di}}_a}{\mathrm{dt}}}{v_{\mathrm{ci}}}.& \left(2\right)\end{array}$

(23) However, it shall be noted that this is only the average insertion index and therefore does not take into account switching harmonics. The voltage drop v.sub.i across the cell is given by
v.sub.i=E.sub.i+(n.sub.is.sub.i)v.sub.ci(3)

(24) Substituting (2) in (3) yields

(25) $\begin{array}{cc}\begin{array}{c}{v}_i=E_i+\left(n_i-\frac{E_i-L_c\frac{{\mathrm{di}}_a}{\mathrm{dt}}}{v_{\mathrm{ci}}}\right)v_{\mathrm{ci}}\\ =n_i{v}_{\mathrm{ci}}-L_c\frac{{\mathrm{di}}_a}{\mathrm{dt}}\end{array}& \left(4\right)\end{array}$
and adding up the voltages of all N cells in the arm yields, for the first arm part 5

(26) $\begin{array}{cc}\frac{v_d}{2}-\underset{i=1}{\overset{N}{.Math.}}\left(n_i{v}_{\mathrm{ci}}-L_c\frac{{\mathrm{di}}_a}{\mathrm{dt}}\right)-L_a\frac{{\mathrm{di}}_a}{\mathrm{dt}}=v_g& \left(5\right)\\ .Math.\left(L_a+{\mathrm{NL}}_c\right)\frac{{\mathrm{di}}_a}{\mathrm{dt}}=\frac{v_d}{2}-\underset{i=1}{\overset{N}{.Math.}}{n}_i{v}_{\mathrm{ci}}-v_g.& \end{array}$

(27) This shows that, as long as switching harmonics are disregarded, when cascading the switching cells 20 in a converter arm 5, 10, the invention gives a total equivalent arm inductance of L.sub.a+NL.sub.c. However, this does not account for harmonics. Inductance L.sub.a and the switching frequencies of the two pairs of switching devices 40a, 40b, 40c, 40d must be properly selected to give an acceptably low harmonic content of i.sub.a.

(28) FIG. 4 disclosed a method for operating a multi-level power converter 1 according to any of claim 1-9. The method comprises controlling the switching devices 40a, 40b, 40c, 40d of the bridge circuit 60.

(29) The method comprises a step 110 comprising connecting the primary energy storage 50 with the arm circuit 64 by means of closing the first switching device 40a and opening the second switching device 40b. The method step results in a positive output voltage from the switching cell 20.

(30) The method comprises a step 120 comprising disconnecting the primary energy storage 50 from the arm circuit 64 by means of opening the first switching device 40a and closing the second switching device 40b. The method step results in zero output voltage from the switching cell 20. The primary energy storage 50 is being charged until it is fully charged

(31) It shall be understood that the method steps 110 and 120 relate to independent states of the switching cells and accordingly the steps of the method are not executed in any particular order. Instead the controller 34 controls the plurality of switching cells 20 by applying step 110 and 120 according to a certain switching algorithm over time so that the desired current is formed.

(32) The method comprises a step 130 comprising connecting the primary energy storage 50 with the battery circuit 62 by means of closing the third switching device 40c and opening the fourth switching device 40d. The method step results in boosted output voltage from the switching cell 20.

(33) The method comprises a step 140 comprising disconnecting the primary energy storage 50 from the battery circuit 62 by means of opening the third switching device 40c and closing the fourth switching device 40d. The method step results in that the secondary energy storage 52 is not involved in boosting the output voltage of the converted power. The secondary energy storage 52 is instead being charged until it is fully charged

(34) The method comprises a step 150 comprising isolating the secondary energy storage 52 by means of opening the third switching device 40c and the fourth switching device 40d. The method step can be used in fault situation to disconnect the second energy storage 52 from the switching cell 20.

(35) It shall further be understood that the method steps 110 and 120 are alternated during operation of the converter 1. Likewise, the method steps 130 and 140 are alternated. Accordingly, the process of alternating between the control of the primary energy storage 50 and between the control of the secondary energy storage 52 take place simultaneously. Under normal condition the switching frequency of the switching operation of the third switching device 40c and the fourth switching device 40s is executed at a higher frequency than the switching frequency of the switching operation of the first switching device 40a and the second switching device 40b.

(36) The present invention is not limited to the disclosed embodiments but may be modified within the framework of the claims.