Unit bypass for variable frequency drive

Abstract

A driving system includes: a power circuit (600, 700, 800) having a plurality of power cells supplying power to output phases (A, B, C), each output phase including a phase group of power cells connected in series; a voltage source power supply; and a bypass mechanism (650, 750, 850) comprising a backup power unit and a bypass contactor, where the phase groups of the power circuit are connected at the common coupling point, and where the backup power unit is mounted at the common coupling point.

Description

Technical Field

[0001] Aspects of the present disclosure relate to a variable frequency drive (also known as a VFD) that powers an electric motor to drive a load such as a pump, compressor, fan, or reciprocating compressor system. Throughout this specification, the terms "drive," "drive system," "multilevel power converter," "converter," "power supply," and "variable frequency drive (VFD)" are used interchangeably. Background Art

[0002] Medium voltage variable frequency drives, such as multilevel power converters, are used in applications such as medium voltage alternating current (AC) drives, flexible AC transmission systems (FACTS), and high voltage DC (HVDC) transmission systems because a single power semiconductor device cannot handle the high voltage. A multilevel converter typically includes multiple power cells for each phase, each power cell including an inverter circuit having semiconductor switches that are capable of varying the voltage output of the individual cells. An example of a multilevel power converter is a cascaded H-bridge converter system having multiple H-bridge cells, such as described in U.S. Patent No. 5,625,545 to Hammond, the contents of which are incorporated herein by reference in their entirety.

[0003] Another example of a multilevel power converter is a modular multilevel converter system with multiple M2C or M2LC subsystems. The M2C or M2LC subsystems are also referred to herein as M2C or M2LC units or simply power units. The M2LC topology is popular in medium and high voltage applications because it offers several advantages over other topologies, such as a simple process for expanding the number of output voltage levels by linearly adding identical units, a capacitor-free DC link, continuous link current, reduced switch voltage ratings, and redundant switching operations. However, the M2C or M2LC units are not independently powered by isolated voltage sources or secondary windings. The units are typically powered from a common DC link via, for example, an AC / DC rectifier system or a battery, where the amount of energy handled at the two terminals of a given unit depends on the amount of energy supplied to the unit by the link to which it is connected and, to a certain extent, on the unit's ability to store and release energy.

[0004] In addition, various methods of implementing cell bypass have been adopted, where redundant cells are added to the M2CL subsystem. These methods provide N+1 redundancy, N+2 redundancy, etc. by adding one extra row (bank) of cells, two extra rows of cells, etc., on both the positive and negative legs of the power cell. Summary of the Invention

[0005] Briefly described, aspects of the present disclosure relate to a power circuit of a drive system including a cell bypass, the drive system being configured as, for example, a medium voltage variable frequency drive.

[0006] More specifically, a drive system includes: a power circuit comprising a plurality of power cells supplying power to output phases, each output phase comprising a phase group of power cells connected in series; a voltage source power supply; and a bypass mechanism comprising a backup power cell and a bypass contactor, wherein the phase groups of the power circuit are connected at a common coupling point, and wherein the backup power cell is mounted at the common coupling point. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 A schematic diagram shows a basic configuration of a modular multilevel converter system according to exemplary embodiments described herein.

[0008] Figure 2 A known two-level configuration of an M2LC subsystem with two terminals according to exemplary embodiments described herein is shown.

[0009] Figure 3 A known three-level configuration of an M2LC subsystem with two terminals according to exemplary embodiments described herein is shown.

[0010] Figure 4 Another known three-level configuration of an M2LC subsystem with two terminals according to exemplary embodiments described herein is shown.

[0011] Figure 5 A schematic diagram shows a known basic configuration of a cascaded H-bridge converter system according to exemplary embodiments described herein.

[0012] Figure 6 A schematic diagram of a first embodiment of a modular multi-level converter system having multiple M2C or M2LC subsystems and including cell bypass according to an exemplary embodiment of the present disclosure is shown.

[0013] FIG7 shows a schematic diagram of a second embodiment of a modular multilevel converter system having multiple M2C or M2LC subsystems and including cell bypass according to an exemplary embodiment of the present disclosure.

[0014] Figure 8 A schematic diagram illustrating an embodiment of a cascaded H-bridge converter system including cell bypassing according to an exemplary embodiment of the present disclosure is shown. DETAILED DESCRIPTION

[0015] To facilitate understanding of the embodiments, principles, and features of the present disclosure, the following description is provided in conjunction with implementation in illustrative embodiments. Specifically, the embodiments, principles, and features of the present disclosure are described in the context of a drive system, such as a medium voltage (MV) variable frequency drive including a multi-unit power supply comprising a modular multilevel converter system and a cascaded H-bridge converter system. Like reference numerals denote like elements throughout.

[0016] As used herein, "medium voltage" is a voltage greater than about 690 V and less than about 69 kV, and "low voltage" is a voltage less than about 690 V. Those skilled in the art will appreciate that other voltage levels may also be designated as "medium voltage" and "low voltage." For example, in some embodiments, "medium voltage" may be a voltage between about 3 kV and about 69 kV, and "low voltage" may be less than about 3 kV.

[0017] The components and materials described below as making up various embodiments are intended to be illustrative and not limiting. Many suitable components and materials that perform the same or similar functions as the materials described herein are intended to be included within the scope of embodiments of the present invention.

[0018] Figure 1 A schematic diagram illustrates the basic configuration of a modular multilevel converter system 100 according to exemplary embodiments described herein. In this example, converter system 100 includes a basic input module 130 employing M2C or M2LC technology and an output module 160. The basic input module 130 generates a DC voltage and provides energy to an output module 160 connected to the basic input module 130. In this example, the basic input module 130 may include a six-pulse rectifier 140 connected in series. The output module 160 provides power to a connected motor 190, which may be, for example, a high-voltage AC motor. The output module 160 is supplied with power for the motor 190 via the basic input module 130, representing a DC link. The output module 160 includes an inverter unit 170 employing M2C or M2LC technology, comprising multiple semiconductors, specifically insulated-gate bipolar transistors (IGBTs). The output module 160 includes an M2C or M2LC subsystem (also referred to herein as a power unit) that provides a substantially sinusoidal voltage to the motor 190. In this example, the inverter 170 may include three phases. Each phase comprises two so-called M2C or M2LC branches.The six branches of the inverter 170 each consist of identical subsystems (power cells) connected in series.

[0019] Figure 1Also shown are a circuit breaker 110 and a transformer 120 as examples of power supplies for the converter system 100. Furthermore, the converter system 100 may include one or more measuring units 150, 180 for measuring voltage and current. For example, the measuring unit 150 measures the voltage and current of the basic line module 130, and the measuring unit 180 measures the voltage and current on the motor side. The voltage can be measured using an AVT (actual value transmission) combination module, and the current can be measured using an electronic current transformer and an AVT combination module. The AVT combination module converts the analog signal into a digital signal and transmits the signal to the control unit, for example via a fiber optic cable. It should be noted that Figure 1 The converter system 100 may include further components, such as, for example, a control module, a cooling module, a braking module and / or a bypass module. The control module is typically used for open-loop and closed-loop control of the drive as well as operational control and diagnosis of the drive.

[0020] Figure 2 shows a known two-level configuration of an M2LC subsystem 200 having two terminals, and Figure 3 and Figure 4 A known three-level configuration of an M2LC subsystem 300 , 350 having two terminals is shown.

[0021] like Figure 2 As shown, the M2LC subsystem 200 includes two switching devices, two diodes, a capacitor, and two terminals. The two switching devices can be controlled so that one of two different potentials (e.g., zero volts or Vcap) can exist across the two terminals. Figure 3 and Figure 4 As shown, the M2LC subsystem 300, 350 includes four switching devices, four diodes, two capacitors, and two terminals, wherein the four switching devices can be controlled so that one of three different potentials (e.g., zero volts, Vcap, or 2Vcap) can exist at the two terminals. Figure 4 The parallel arrangement shown can also generate three levels using the same number of switching devices and capacitors, where the output voltage is zero volts, +Vcap and -Vcap. Figure 4 The arrangement shown is conventionally referred to as a cascaded H-bridge. Although other topologies of the M2LC subsystems 200, 300, 350 are possible, all topologies can be defined as a two-terminal subsystem or power cell with an internal capacitor energy storage device capable of generating various levels of voltage between the two terminals depending on the state of the switching device.

[0022] Figure 5 A schematic diagram showing a known basic configuration of a cascaded H-bridge converter system according to exemplary embodiments described herein is shown.

[0023] Multi-cell power supply 10 receives three-phase power from an alternating current (AC) power source and delivers the power to a load 12, such as a three-phase AC motor. Load 12 may include AC motors such as synchronous motors, asynchronous motors, and permanent magnet motors, and may be rated for low voltage, medium voltage, or high voltage. For example, medium voltage AC motors (such as those used in industrial process control) may operate in the range of 4.16 kV to 13.8 kV. Higher or lower voltages may be used. More than one motor may be connected. Other loads may be used in place of or in addition to the motors. The motor responds to the voltage applied by multi-cell power supply 10 on the three phases to, for example, increase, decrease, or maintain speed or position.

[0024] Multi-cell power supply 10 includes a transformer 14, a power circuit 16, and a central control system 18 (also referred to herein as a controller). Transformer 14 includes a primary winding that excites nine secondary windings, and power circuit 16 includes a plurality of printed circuit board (PCB) power cells 26 (referred to herein as power cells 26 or power modules), each of which is operably coupled to the secondary windings of transformer 14. Since power supply 10 includes nine secondary windings, each operably coupled to a power cell 26, power supply 10 includes nine power cells 26. Of course, depending on the type of power supply 10 and / or the type of load 12 coupled to power supply 10, power supply 10 may include more or fewer than nine power cells 26 and / or more or fewer than nine secondary windings. Power cells 26 are configured to provide a medium voltage output to load 12. Each output phase A, B, and C of power circuit 16 is fed by a set of series-connected power cells 26. The outputs of the power cells 26 are coupled in series in the first phase group 30, in the second phase group 32, and in the third phase group 34. Each phase output voltage is the sum of the output voltages of the power cells 26 in the corresponding phase groups 30, 32, and 34. For example, the first phase group 30 includes power cells 26 labeled A1, A2, and A3, where the phase output voltage of output phase A is the sum of the output voltages of power cells A1, A2, and A3. The same applies to output phase B with power cells B1, B2, and B3, and output phase C with power cells C1, C2, and C3. In this regard, the power circuit 16 provides a medium voltage output to the output load 12 using lower voltage rated power cells 26 that include components rated for the lower voltage standard.

[0025] Each power cell 26 is coupled to the central control system 18 (eg, via a fiber optic communication link), which can utilize current feedback and voltage feedback to control the operation of the power cells 26 .

[0026] It should be noted that in Figure 1In the embodiment, the number of power cells 26 in each phase group 30, 32, 34 can be between 2 and 12 to provide different (medium voltage) outputs required by the load 12. Figure 1 In the embodiment shown, the number of secondary windings of transformer 14 matches the number of power cells 26. Those skilled in the art will appreciate that other numbers of cells and diode bridges may be used depending on the application, and that the configurations shown and described herein are intended to be exemplary in nature.

[0027] Figure 6 1 shows a schematic diagram of a first embodiment of a modular multilevel converter system having multiple M2C or M2LC subsystems and including cell bypass according to an exemplary embodiment of the present disclosure. More specifically, Figure 6 1 shows a schematic diagram of a power circuit 600, which is also referred to herein as an inverter 600 (see also Figure 1 The inverter unit 170 in FIG. 1 includes an improved unit bypass.

[0028] Inverter 600 includes M2C or M2LC subsystems 610, also referred to herein as power cells 610, which include semiconductors, particularly insulated gate bipolar transistors (IGBTs). In the example, inverter 600 includes three phases A, B, and C. Each phase A, B, and C includes two so-called M2C or M2LC branches or arms A1, A2, B1, B2, C1, and C2. The six branches / arms A1, A2, B1, B2, C1, and C2 are each composed of identical subsystems (power cells) connected in series. Branches A1, B1, and C1 are referred to as the upper arm phase group 620, and branches A2, B2, and C2 are referred to as the lower arm phase group 630. Figure 6 The example includes 24 power cells 610, with 12 power cells 610 in the upper arm phase group 620 and 12 power cells in the lower arm phase group 630. However, it should be noted that the power circuit 600 may include more or fewer power cells than the illustrated power cells 610. For example, the power circuit 600 may include 18 power cells (9 cells per arm) or 30 power cells (15 cells per arm).

[0029] As described above, known cell bypass systems require installation of spare (redundant) power cells in each output phase leg (upper and lower), for example due to a fixed output phase (voltage) reference.

[0030] According to an exemplary embodiment of the present disclosure, a converter system, specifically a power circuit (inverter) 600, includes a bypass mechanism 650, which includes a backup power unit and a bypass contactor. By using the bypass contactor (switch), a single backup power unit can be installed at a common coupling point and switched to any output phase as needed, thereby providing redundancy for the power unit 610.

[0031] Figure 6 The embodiment shows N+1 redundant cell bypass, which means that a single backup power cell 652 is provided for the upper arm phase group 620, and a single backup power cell 654 is provided for the lower arm phase group.

[0032] The branches A1, B1, and C1 of the upper arm phase group 620 are connected at a first common coupling point, and a first backup power unit 652 is installed at the first common coupling point. The branches A2, B2, and C2 of the lower arm phase group 630 are connected at a second common coupling point, and a second backup power unit 654 is installed at the second common coupling point.

[0033] Bypass mechanism 650 also includes a bypass contactor, wherein a bypass contactor is arranged for each upper arm branch A1, B1, C1 and each lower arm branch A2, B2, C2. Specifically, bypass contactor 660 is coupled between backup power cell 652 and branch A1, bypass contactor 662 is coupled between backup power cell 652 and branch B1, and bypass contactor 664 is coupled between backup power cell 652 and branch C1. On the other hand, bypass contactor 666 is coupled between backup power cell 654 and branch A2, bypass contactor 668 is coupled between backup power cell 654 and branch B2, and bypass contactor 670 is coupled between backup power cell 654 and branch C2.

[0034] In an exemplary embodiment, the bypass contactor comprises a semiconductor switch or a mechanical switch or a pyrotechnic control switch or a combination thereof.

[0035] In an embodiment, the upper arm legs A1, B1, and C1 are connected via a WYE connection, wherein a first backup power unit 652 is installed at the "Y" (neutral point) of the WYE connection. The lower arm legs A2, B2, and C2 are connected via a WYE connection, wherein a second backup power unit 654 is installed at the "Y" (neutral point) of the connection. The backup power units 652 and 654 can be installed at either the positive (+) busbar or the negative (-) busbar of the common coupling.

[0036] In an embodiment, the drive system includes a control system 680, which is configured to: control the bypass contactors 660, 662, 664 so that in response to a faulty power unit in any one of the upper arm branches A1, B1, C1, the first backup power unit 652 is switched to any one of the upper arm branches A1, B1, C1; and control the bypass contactors 666, 668, 670 so that in response to a faulty power unit in any one of the lower arm branches A2, B2, C2, the second backup power unit 654 is switched to any one of the lower arm branches A2, B2, C2.

[0037] The voltage source power supply includes a common DC link 690 , wherein the power unit 610 including the backup power units 652 , 654 and the bypass contactors 660 , 662 , 664 , 666 , 668 , 670 is supplied with voltage from the common DC link 690 .

[0038] Installing a single backup power unit 652, 654 at each phase group (upper arm phase group 620 and lower arm phase group 630) provides significant cost savings by:

[0039] - Reduce the number of backup power units from six to two (N+1 redundancy),

[0040] - Reduce the overall footprint of the drive system,

[0041] - Reduced drive system cooling requirements, and

[0042] - Reduce the complexity of the drive system (wiring, fiber optics, etc.).

[0043] As described with reference to Figure 7, installing two backup power cells per phase group (N+2 redundancy) adds the ability to bypass two cells in one or more output phases, which enhances the functionality of the system. Further, more backup power cells can be installed (N+3 redundancy, N+4 redundancy, etc.).

[0044] Figure 7 shows a schematic diagram of a second embodiment of a modular multilevel converter system having multiple M2C or M2LC subsystems and including cell bypass according to an exemplary embodiment of the present disclosure. More specifically, Figure 7 shows a schematic diagram of a power circuit or inverter 700 including an improved cell bypass.

[0045] Figure 6 The embodiment of FIG shows N+1 redundant cell bypass, while FIG7 shows N+2 redundant cell bypass. For N+2 cell bypass, two spare power cells are provided for the upper arm phase group and two spare power cells are provided for the lower arm phase group.

[0046] More specifically, power circuit 700 includes M2C or M2LC subsystems 710, also referred to herein as power cells 710, which include semiconductors, particularly insulated gate bipolar transistors (IGBTs). Power circuit 700 includes three phases A, B, and C. Each phase A, B, and C includes two so-called M2C or M2LC branches or arms A1, A2, B1, B2, C1, and C2. The six branches / arms A1, A2, B1, B2, C1, and C2 are each composed of identical subsystems (power cells) connected in series. Branches A1, B1, and C1 are referred to as an upper arm phase group 720, and branches A2, B2, and C2 are referred to as a lower arm phase group 730.

[0047] The bypass mechanism 750 includes a backup power unit and a bypass contactor, wherein by using the bypass contactor (switch), a backup (redundant) power unit can be installed at the common coupling point and switched into any output phase as needed, thereby providing redundancy for the power unit.

[0048] As noted, the embodiment of FIG. 7 shows N+2 redundant cell bypassing, meaning that two spare (redundant) power cells 752 and 754 are provided for the upper arm phase group 720 , and two power cells 756 and 758 are provided for the lower arm phase group 730 .

[0049] The bypass mechanism 750 further includes bypass contactors, wherein two bypass contactors are arranged for each branch A1 , B1 , C1 of the upper arm phase group 720 , and two bypass contactors are arranged for each branch A2 , B2 , C2 of the lower arm phase group 730 due to N+2 redundancy.

[0050] Specifically, bypass contactors 760 and 762 are coupled between backup power cells 752 and 754 and branch A1, bypass contactors 764 and 766 are coupled between backup power cells 752 and 754 and branch B1, and bypass contactors 768 and 770 are coupled between backup power cells 752 and 754 and branch C1. Furthermore, since two backup power cells 752 and 754 are provided in each arm, an additional bypass contactor 772 is also provided. The same concept applies to the lower arm phase group 730, which has two backup power cells 756 and 758 and seven bypass contactors. In an exemplary embodiment, the bypass contactors include semiconductor switches, mechanical switches, pyrotechnic switches, or a combination thereof.

[0051] Further, the control system 780 is configured to: control the bypass contactors 760-772 of the upper arm phase group 720 and the bypass contactors of the lower arm phase group 730, so that in response to a faulty power unit in any one of the upper arm branches A1, B1, C1, the first backup power unit 752 and, if necessary, the second power unit 754 are switched to any one of the upper arm branches A1, B1, C1; and control the bypass contactors so that in response to a faulty power unit in any one of the lower arm branches A2, B2, C2, the third backup power unit 756 and, if necessary, the fourth backup power unit 758 are switched to any one of the lower arm branches A2, B2, C2.

[0052] The voltage source power supply includes a common DC link 790 , wherein the power unit 710 , including the backup power units 752 , 754 , 756 , 758 and the bypass contactors, is supplied with voltage from the common DC link 790 .

[0053] about Figure 6 7 (bypass mechanism for modular multilevel converter), the control system 680, 780 can be used to operate the power circuit (inverter) 600, 700 according to different control principles. Figure 6 According to the first control principle, backup power cells 652 and 654 are normally outside the circuit. In the event of a cell failure, backup cells 652 and 654 are inserted into the faulty output phase via corresponding bypass contactors. According to the second control principle, backup cells 652 and 654 are normally part of the inverter circuit and can be used, for example, to charge capacitors. In the event of a failure, backup cells are inserted into the faulty output phase via corresponding bypass contactors. These control principles can also be applied to the embodiment of Figure 7 (N+2 redundancy).

[0054] Figure 8 A schematic diagram illustrating an embodiment of a cascaded H-bridge converter system including cell bypassing according to an exemplary embodiment of the present disclosure is shown.

[0055] More specifically, Figure 8 A power circuit 800, also referred to herein as an inverter 800, is shown, comprising a plurality of power cells, such as PCB power cells. Each output phase A, B, and C of the power circuit 800 is fed by a set of power cells connected in series. For example, output phase A comprises power cells A1, A2, A3, and A4, wherein the phase output voltage of output phase A is the sum of the output voltages of power cells A1, A2, A3, and A4. The same applies to output phase B and power cells B1, B2, B3, and B4, and output phase C and power cells C1, C2, C3, and C4.

[0056] Each power cell is coupled to a central control system 880 (eg, via a fiber optic communication link), which can use current feedback and voltage feedback to control the operation of the power cells. The control system 880 is also configured to control the bypass mechanism 850 .

[0057] According to an exemplary embodiment of the present disclosure, the power circuit includes an N+2 redundant bypass mechanism 850. Two backup power units 852, 854 are installed so that each backup power unit 852, 854 can be switched / inserted into the output phase A, B, or C where a fault occurs. Bypass contactors 860, 862, 864 are coupled between the first backup power unit 852 and phases A, B, and C, respectively, and bypass contactors 866, 868, 870 are coupled between the second backup power unit 854 and phases A, B, and C, respectively. Further, a bypass contactor 872 is arranged between the two backup power units 852, 854.

[0058] Because power cells A1-A4, B1-B4, and C1-C4 of the cascaded H-bridge converter are independently powered by isolated voltage sources or secondary windings, an additional bypass contactor is assigned to each power cell. For example, bypass contactor 874 is assigned to and coupled to power cell A1. When power cell A1 fails, bypass contactor 874 and bypass contactor 860 close to insert backup power cell 852.

[0059] The described power circuits 600, 700, and 800 including cell bypassing provide the following advantages:

[0060] - Achieve N+1 redundancy with a minimum number of spare units.

[0061] - Add a spare unit to the output phase where the unit failure occurred.

[0062] - The control system does not have to adapt to an unbalanced set of units (as in conventional approaches), i.e. no neutral point displacement is required.

[0063] - Due to the modular multilevel converter ( Figure 6 ) requires only one spare unit for each half of the topology, and only one unit is needed for the cascaded H-bridge converter, thus achieving cost savings.

[0064] - The cell bypass mechanism can be easily expanded to provide N+2 redundancy.

[0065] - Further, the proposed system and method can be applied to modular multilevel converters (MMC) and cascaded H-bridge converters (CHB) using other cell types such as dual cells and H-bridge (for MMC) and NPC (for CHB).

Claims

1. A drive system comprising: A power circuit (600, 700, 800) comprising a plurality of power cells supplying power to output phases (A, B, C), each output phase (A, B, C) comprising a phase group of power cells connected in series; Voltage source power supply; as well as Bypass mechanism (650, 750, 850), including a backup power unit and a bypass contactor; wherein the phase groups of the power circuit are connected at a common coupling point; and The backup power unit is installed at the common coupling point.

2. The drive system according to claim 1, wherein: The power circuit (600, 700) having the plurality of power cells is configured as an M2C or M2CL subsystem.

3. The drive system according to claim 1 or 2, in, Each output phase includes an upper arm phase group and a lower arm phase group, wherein the upper arm phase group is connected at a first common coupling point, wherein the lower arm phase group is connected at a second common coupling point; and The first backup power unit is installed at the first common coupling point, and the second backup power unit is installed at the second common coupling point (N+1 redundancy).

4. The drive system according to claim 1, 2 or 3, wherein: The bypass contactors include a bypass contactor for each upper arm phase group and a bypass contactor for each lower arm phase group.

5. The drive system according to any one of claims 1 to 4, further comprising: The control system (680, 780) is configured to: controlling the bypass contactor so that, in response to a faulty power unit in any one of the upper arm phase groups, the first backup power unit is switched to any one of the upper arm phase groups; as well as The bypass contactor is controlled so that, in response to a faulty power unit in any one of the lower arm phase groups, the second backup power unit is switched to any one of the lower arm phase groups.

6. The drive system according to any one of claims 1 to 5, wherein: The upper arm phase groups are connected via a WYE connection, and wherein the first standby power unit is installed at a "Y" (neutral point) of the WYE connection.

7. The drive system according to any one of claims 1 to 6, wherein: The lower arm phase groups are connected via a WYE connection, and wherein the second standby power unit is installed at the "Y" (neutral point) of the connection.

8. The drive system according to claims 1 to 7, in, The voltage source power supply (690, 790) includes a common DC link; and The plurality of power units, the backup power unit, and the bypass contactor are powered from the common DC link.

9. The drive system according to any one of claims 1 to 8, wherein: The backup power unit is installed at the commonly coupled positive bus or negative bus.

10. The drive system according to any one of claims 1 to 9, wherein: The bypass contactor comprises a semiconductor switch, a mechanical switch, a pyrotechnic control switch or a combination thereof.

11. The drive system according to claims 3 to 10, wherein: Two backup power units are installed for the upper arm phase group and the lower arm phase group respectively (N+2 redundancy).

12. The drive system according to claim 1, wherein: The power circuit (800) having the plurality of power cells is configured as a cascaded H-bridge subsystem.

13. The drive system according to claim 12, wherein: A first backup power unit and a second backup power unit are installed at the common coupling point.

14. The drive system according to claim 12 or 13, in, Install at least one bypass contactor for each output phase (A, B, C); and In this case, an additional bypass contactor is assigned and installed for each individual power unit.

15. The drive system according to claim 13 or 14, further comprising: The control system (880) is configured to: controlling the bypass contactor so that, in response to a faulty power unit in any one of the phase groups, the first backup power unit is switched to any one of the phase groups; as well as The bypass contactor is controlled so that in response to a faulty power unit in any one of the phase groups, a second backup power unit is switched into any one of the phase groups.

16. The drive system according to any one of claims 12 to 15, wherein: The common coupling point comprises a WYE connection, and wherein the first power unit and / or the second power unit is mounted at a “Y” (neutral point) of the WYE connection.

17. The drive system according to any one of claims 12 to 16, wherein: The bypass contactor comprises a semiconductor switch, a mechanical switch, a pyrotechnic control switch or a combination thereof.

18. A medium voltage variable frequency drive comprising a power circuit (600, 700, 800) according to claim 1.

19. The medium voltage variable frequency drive according to claim 18, wherein: The power circuit (600, 700) having the plurality of power cells is configured as an M2C or M2CL subsystem.

20. The medium voltage variable frequency drive according to claim 18, wherein: The power circuit (800) having the plurality of power cells is configured as a cascaded H-bridge subsystem.

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