WO2019032083A1

WO2019032083A1 - Distributed power supply system including inductive power transfer for a medium voltage variable frequency drive

Info

Publication number: WO2019032083A1
Application number: PCT/US2017/045666
Authority: WO
Inventors: Marc F. Aiello; Peter Willard Hammond
Original assignee: Siemens Aktiengesellschaft
Priority date: 2017-08-07
Filing date: 2017-08-07
Publication date: 2019-02-14

Abstract

A distributed power supply system (400, 800) for a modular multilevel converter (100) includes an inverter (170) comprising multiple power cells (402, 802), and a voltage source power supply (410, 810), wherein control power is transferred from the voltage source power supply (410, 810) directly in each of the multiple power cells (402, 802) by inductive power transfer.

Description

DISTRIBUTED POWER SUPPLY SYSTEM INCLUDING INDUCTIVE POWER TRANSFER FOR A MEDIUM VOLTAGE VARIABLE FREQUENCY DRIVE

BACKGROUND

1. Field [0001] Aspects of the present invention generally relate to a distributed power supply system including inductive power transfer for a medium voltage variable frequency drive.

2. Description of the Related Art

[0002] Medium voltage variable frequency drives, such as for example multilevel power converters are used in the applications of medium voltage alternating current (AC) drives, flexible AC transmission systems (FACTS), and High Voltage DC (HVDC) transmission systems, because single power semiconductor devices cannot handle high voltage. Multilevel converters typically include a plurality of power cells for each phase, each power cell including an inverter circuit having semiconductor switches that are capable of altering the voltage output of the individual cells. [0003] One example of a multilevel power converter is a cascaded H-bridge converter system having a plurality of H-bridge cells as described for example in U.S. Patent No. 5,625,545 to Hammond, the content of which is herein incorporated by reference in its entirety. Another example of a multilevel power converter is a modular multilevel converter system having a plurality of M2C or M2LC subsystems. The M2C or M2LC subsystems are herein also referred to as M2C or M2LC cells or simply as power cells. The M2LC topology has recently become popular in medium to high voltage applications since it provides a number of advantages over other topologies, for example simple process of scaling the number of output voltage levels by a linear addition of identical cells, capacitor free direct current (DC)-link, continuous link currents, reduced voltage rating of the switches and redundant switching operations. However, M2C or M2LC cells are not independently supplied from isolated voltage sources or secondary windings. The cells are typically supplied from a common DC link via for example AC/DC rectifier systems or batteries, wherein for a given cell, the amount of energy processed at the two terminals depends on the amount of energy supplied to the cell by the link it is connected to and to some extent the ability of the cell to store and release energy. This may cause a problem in controlling the DC link voltages in these cells during pre-charge of the power circuit or during abnormal operation when one or more of the cells needs to be bypassed or made inactive. Thus, there is a need for an autonomous high isolation voltage power supply which allows the subsystem or cell to communicate with and to be controllable by a higher level controller (also known as "hub") it is connected to under any normal or abnormal operating conditions.

SUMMARY

[0004] Briefly described, aspects of the present invention relate to a distributed voltage source power supply system including inductive power transfer for a modular medium voltage variable frequency drive.

[0005] A first aspect of the present invention provides a distributed power supply system for a modular multilevel converter comprising an inverter comprising multiple power cells, and a voltage source power supply, wherein control power is transferred from the voltage source power supply directly in each of the multiple power cells by inductive power transfer. A second aspect of the present invention provides a variable frequency drive comprising a distributed power supply system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] FIG. 1 illustrates a schematic of a basic configuration of a modular multilevel converter system in accordance with an exemplary embodiment described herein.

[0007] FIG. 2 illustrates a known two-level configuration of an M2LC subsystem having two terminals in accordance with an exemplary embodiment described herein.

[0008] FIG. 3 illustrates a known three-level configuration of an M2LC subsystem having two terminals in accordance with an exemplary embodiment described herein. [0009] FIG. 4 illustrates another known three-level configuration of an M2LC subsystem having two terminals in accordance with an exemplary embodiment described herein.

[0010] FIG. 5 illustrates a schematic of a first embodiment of a distributed power supply system for a modular multilevel converter system including inductive power transfer in accordance with an exemplary embodiment of the present invention.

[0011] FIG. 6 illustrates a three dimensional view of a first embodiment of a magnetic circuit for inductive power transfer in accordance with an exemplary embodiment of the present invention. [0012] FIG. 7 illustrates a schematic of an equivalent circuit topology of a magnetic circuit in accordance with an exemplary embodiment of the present invention.

[0013] FIG. 8 illustrates a schematic of a simplified circuit of the circuit as illustrated in FIG. 7 of a magnetic circuit in accordance with an exemplary embodiment of the present invention. [0014] FIG. 9 illustrates a first schematic of a second embodiment of a distributed power supply system for a modular multilevel converter system including inductive power transfer in accordance with an exemplary embodiment of the present invention.

[0015] FIG. 10 illustrates a second schematic of the first embodiment of the distributed power supply system of FIG. 5 in accordance with an exemplary embodiment of the present invention.

[0016] FIG. 11 illustrates a second schematic of the second embodiment of the distributed power supply system of FIG. 9 in accordance with an exemplary embodiment of the present invention.

[0017] FIG. 12 illustrates a two dimensional view of a second embodiment of a magnetic circuit for inductive power transfer in accordance with an exemplary embodiment of the present invention. [0018] FIG. 13 illustrates a three dimensional view of the second embodiment of a magnetic circuit for inductive power transfer in accordance with an exemplary embodiment of the present invention.

[0019] FIG. 14 illustrates a three dimensional view of a third embodiment of a magnetic circuit for inductive power transfer in accordance with an exemplary embodiment of the present invention.

[0020] FIG. 15 illustrates a three dimensional view of a section of a modular multilevel converter system including inductive power transfer in accordance with an exemplary embodiment of the present invention. DETAILED DESCRIPTION

[0021] To facilitate an understanding of embodiments, principles, and features of the present invention, they are explained hereinafter with reference to implementation in illustrative embodiments. In particular, they are described in the context of being a distributed power supply system including inductive power transfer for a medium voltage variable frequency drive, in particular a modular multilevel converter comprising M2C or M2LC topology or any other modular power electronic system requiring high isolation voltage. Embodiments of the present invention, however, are not limited to use in the described devices or methods.

[0022] The components and materials described hereinafter as making up the various embodiments are intended to be illustrative and not restrictive. Many suitable components and materials that would perform the same or a similar function as the materials described herein are intended to be embraced within the scope of embodiments of the present invention.

[0023] FIG. 1 illustrates a schematic of a basic configuration of a modular multilevel converter system 100 in accordance with an exemplary embodiment described herein. In an example, the converter system 100 comprises a basic input module 130 and an output module 160 deploying M2C or M2LC technology. The basic input module 130 generates a DC voltage and provides energy for the output module 160 connected to the basic input module 130. In an example, the basic input module 130 can comprise series-connected six-pulse rectifiers 140. The output module 160 provides power for a connected motor 190, which can 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, which represents a DC link. The output module 160 comprises an inverter unit 170 with M2C or M2LC technology comprising multiple semiconductors, in particular Insulated Gate Bipolar Transistors (IGBTs). The output module 160 including M2C or M2LC subsystems, herein also referred to as power cells, provides the motor 190 with almost sinusoidal voltages. In an example, the inverter 170 can comprises 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.

[0024] FIG. 1 further illustrates a circuit-breaker 110 and transformer 120 as an example for a power supply for the converter system 100. Furthermore, the converter system 100 can comprise one or more measuring units 150, 180 used to measure voltages and currents. For example, measuring unit 150 measures voltages and currents of the basic line module 130, and measuring unit 180 measures voltages and currents on the motor side. Voltages can be measured using AVT (actual value transmission) combination modules, currents can be measured using electronic current transformers and AVT combination modules. The AVT combination modules convert analog signals into digital signals and transfer the signals to a control unit for example via fiber-optic cables. It should be noted that the converter system 100 of FIG. 1 may comprise more components, such as for example control module(s), cooling module(s), braking module(s) and/or bypass module(s). Control module(s) are typically used for open-loop and closed- loop control of the drive as well as operating control and diagnostics of the drive.

[0025] FIG. 2 illustrates a known two-level configuration of an M2LC subsystem 200 having two terminals, and FIG. 3 and FIG. 4 illustrate known three-level configurations of an M2LC subsystem 300 an M2LC subsystem 350 having two terminals. As shown in FIG. 2, the M2LC subsystem 200 includes two switching devices, two diodes, a capacitor and two terminals. The two switching devices can be controlled such that one of two different potentials (e.g., zero volts or Vcap) may be present across the two terminals. As shown in FIG. 3 and FIG. 4, the M2LC subsystems 300, 350 include four switching devices, four diodes, two capacitors and two terminals, wherein the four switching devices can be controlled such that one of three different potentials (e.g., zero volts, Neap or 2Ycap) may be present across the two terminals. Note that three levels can also be produced with the same number of switching devices and capacitors by parallel arrangement as shown in FIG. 4, where the output voltage is zero volts, +Ycap and -Vcap. Arrangements such as shown in FIG. 4 are traditionally known as Cascaded H-Bridge. Although other topologies of the M2LC subsystems 200, 300, 350 are possible, all of the topologies may be defined as two-terminal subsystems or power cells with internal capacitor energy storage(s) which are capable of producing various levels of voltages between the two terminals depending on the state of the switching devices.

[0026] FIG. 5 illustrates a schematic of a first embodiment of a distributed power supply system 400 for a modular multilevel converter system including inductive power transfer in accordance with an exemplary embodiment of the present invention.

[0027] The distributed voltage source power supply system 400 is provided for multiple subsystems or power cells 402 connected in series. FIG. 4 illustrates three power cells 402 as an example, labelled as 'Power Cell # , 'Power Cell #2' and 'Power Cell #3'. Typically, a converter system can comprise 24, 30, 36 or 42 cells according to a required output voltage. For example, a converter system, such as for example converter system 100 as illustrated in FIG. 1, with 24 cells provides an output voltage of 4.16 kV (3 phase alternating current (AC)). Each cell 402 comprises a control unit, for example a cell control board 418, for controlling and operating the cell 402 as well as for communicating with other controllers, in particular with a higher level controller, also known as hub of the converter system 100.

[0028] As noted before, there is a need for an autonomous high isolation voltage power supply which allows the power cells 402 to communicate with and be controlled by a higher level controller, for example the hub, which they are connected to under any normal or abnormal operating conditions. This is important specifically for M2LC systems, because an "on-board" power supply fed from the local DC link becomes inoperable for example under fault or unstable operation, thus rendering the system of cells inoperable until a stable state can be achieved. This may take considerable time which cannot be tolerated and also requires significant redundancy, leading to cost and in-efficiency in the system. In addition, it may be a very desirable feature in all variations of modular power converters to know and control state(s) of the cells before pre-charge or main power is applied so as to both diagnose and simplify the pre-charge sequence. Also allowing complete control of the cells down to zero voltage on their DC links allows maximum extraction of energy during brown-out or disruptions of the power source feeding the system of cells.

[0029] In an exemplary embodiment, the system 400 comprises technology to transfer control power from a central power supply 410 near ground potential directly into the power cells 402. The technology includes inductive (magnetic) power transfer which is illustrated using arrows 430. In the embodiment according to FIG. 5, the central power supply 410 is configured as a dual-redundant AC power supply.

[0030] In order to transfer power into the cells 402, magnetic coupling is used employing for example high or low frequency AC, provided by the power supply 410, and magnetic circuits comprising magnetic transmitters 412 and magnetic receivers 414. Frequency of the AC power supply as used herein refers typically to a frequency between 1 and 100 kHz. The inductive (magnetic) power transfer utilized is also known as near field resonant coupled power transfer (wireless power transmission) which uses a parallel resonant primary (transmitter 412) and series resonant secondary transformer (receiver 414) to allow high gap power to be transferred over significant isolation distance wireless ly. [0031] For each cell 402, a magnetic circuit comprising a magnetic transmitter 412 and a magnetic receiver 414 is provided. When employing high or low frequency AC, the magnetic transmitter 412 and receiver 414 are configured as high or low frequency magnetic transmitter and receiver. The magnetic transmitter 412 and receiver 414 are used to pass control power from the central power supply 410 to the cells 402. The shared power supply 410 of high or low frequency AC powers all of the magnetic transmitters 412

[0032] The magnetic transmitter 412 and receiver 414 are configured such that the control power is passed across dielectric isolation gap(s) 420 between transmitter 412 and receiver 414. For example, for each power cell 402, control power up to 50W over dielectric isolation gap(s) 420 is transferred between transmitter 412 and receiver 414. The gap(s) 420 provide isolation between the cells 402 and the power supply 410, since the transmitters 412 and receivers 414 operate at very different voltage levels. The magnetic transmitters 412 operate near ground potential (since the power supply 410 operates near ground potential), whereas the magnetic receivers 414 operate at medium voltage corresponding to an output voltage of the converter system provided to for example an electrodynamic machine, such as an electric AC motor.

[0033] Since the power supply 410 represents a single point of failure, the power supply 410 comprises dual-redundancy. A fuse 416 is assigned to each transmitter 412 to disconnect a failed transmitter so that the remaining operating transmitters 412 can continue to operate. In an example, the fuses 416 are located at the transmitters 412, rather than at the shared power supply 410, so that a single wire 440 feeds multiple transmitters 412.

[0034] Each cell 402 comprises a magnetic receiver 414 which can be located at a side or back of the cell 402. The magnetic receiver 414 is aligned with the magnetic transmitter 412, located for example in a cell compartment of a converter enclosure. Further details of location and position of magnetic transmitter 412 and receiver 414 are described for example with reference to FIG. 15.

[0035] FIG. 6 illustrates a three dimensional view of a first embodiment of a magnetic circuit 500 for inductive power transfer in accordance with an exemplary embodiment of the present invention.

[0036] In an exemplary embodiment, the magnetic circuit 500 comprises high or low frequency magnetic transmitter 512 and high or low frequency magnetic receiver 514 comprising Ferrite, powdered iron or even electrical steel components. Magnetic circuit(s) 500 as shown in FIG. 6 may be used in the distributed voltage source power supply system 400 as described and shown in FIG. 5.

[0037] Table 9 of IEC Standard 61800-5-1 (2007) requires a minimum gap 560 (at 2000 MASL or less) of 60 mm (2.36 inches) between circuits 500 operating at a peak voltage difference of 16kV or an impulse test of 40 kV (see also gap 420 in FIG. 5). Such a large gap 560 may degrade magnetic coupling between the transmitter 512 and receiver 514. In order to improve the magnetic coupling between transmitter 512 and receiver 514, considering that the spacing of the gap 560 is fixed, a gap area between transmitter 512 and receiver 514 is increased or in some case may be filled with higher strength dielectric material.

[0038] High or low frequency magnetic transmitter 512 and receiver 514 are each built from multiple components including electromagnetic coil(s) and suitable magnetic components for the value of frequency utilized. In an embodiment, transmitter 512 is built from multiple components and receiver 514 is built from multiple components. In an example and as FIG. 5 illustrates, transmitter 512 and receiver 514 are identical in size and shape. For example, transmitter 512 may be built from transmitter coil 520 (also referred to as primary coil) and multiple magnetic components 522, 524, 526, 528 and 530. Magnetic core 522 is positioned within and surrounded by the primary coil 520. Magnetic components 524, 526, 528 and 530 are configured as elongated components having rectangular surfaces and are positioned adjacent to the coil 520 in Z-direction as illustrated in FIG. 5. Receiver 514 may be built from receiver coil 540 (also referred to as secondary coil) and multiple magnetic components 542, 544, 546, 548 and 550. Magnetic core 542 is positioned within the coil 540. Magnetic components 544, 546, 548, 550 are configured as elongated components having rectangular surfaces and are positioned adjacent to the coil 540 in Z-direction.

[0039] Magnetic transmitter 512 and 514 are arranged opposite each other so that transmitter coil 520 and receiver coil 540 as well as components 528, 530 and 548, 550 face each other and form the gap 560. Components 528, 530 and 548, 550 are configured as elongated components in order to increase the gap area between transmitter 512 and receiver 514 and thus to improve the magnetic coupling between transmitter 512 and receiver 514.

[0040] In the embodiment as illustrated in FIG. 6, the gap 560 is configured as an air gap, and the high or low frequency magnetic circuit 500 is designed to operate with a large air core gap. This means that the transmitter 512 and receiver 514 may be physically independent components. The gap 560 is specifically configured as an air gap when using either high or low frequency sources, such as the shared voltage source power supply 410 (see FIG. 5), configured as high or low frequency AC power supply including typically 1-100 kHz. The frequency for the shared voltage source power supply 410 is chosen for AC based loss data of the magnetic components of the transmitters 512 and receivers 514.

[0041] In alternative embodiments, the gap 560 can be configured as an electrically insulated gap, and the magnetic circuit 500 can be operated with the electrically insulated gap, which means that transmitter 512 and receiver 514 are constructed as a single physical unit.

[0042] It should be noted that the implementation of FIG. 6 is only one example for a transmitter-receiver pair of magnetic circuit 500 and that one of ordinary skill in the art can conceive many other embodiments, shapes and forms for transmitter 512 and receiver 514. For example, transmitter 512 and receiver 514 may comprise different numbers of components and/or different shapes and sizes of components.

[0043] FIG. 7 illustrates a schematic of an equivalent circuit topology 600 of a magnetic circuit in accordance with an exemplary embodiment of the present invention, for example for magnetic circuit 500 as illustrated in FIG. 6. [0044] Circuit topology 600 comprises voltage source V0. Voltage source V0 represents a high or low frequency AC voltage source configured for example as typically 1-100 kHz voltage source (see also voltage source 410 in FIG. 5). Resistor R represents load at a high frequency magnetic receiver, see for example high frequency magnetic receiver 514 in FIG. 6. Inductances LI, L2, and L3 are an equivalent circuit 610 for structure of the magnetic circuit 500 as described before with reference to FIG. 6. Inductances LI and L2 form a voltage divider, so that open-circuit voltage at point V2 of circuit 610 is V0*L2/(L1+L2). Source impedance at point V2 is equal to LI in parallel with L2. Source impedance at point V3 is equal to LI in parallel with L2, plus L3.

[0045] In an exemplary embodiment, topology 600 comprises a resonant capacitor C which is connected in series with circuit 610, wherein resonant capacitor C is chosen along with the value of the inductance Leq to have the same resonant frequency of the voltage source V0, so that source impedance at point V4 will be nearly zero. [0046] FIG. 8 illustrates a schematic of a simplified circuit 700 of the circuit 600 as illustrated in FIG. 7 of a magnetic circuit in accordance with an exemplary embodiment of the present invention. Circuit 700 is equivalent to the circuit 600 of FIG. 7, from a perspective of the load R.

[0047] The circuit 700 has a same open-circuit voltage and short-circuit current at point V2 as the circuit 600 of FIG. 6. Inductance LI is now in parallel with L2, with L3 in series with both LI and L2. The combination of LI, L2 and L3 can be replaced by an equivalent inductance Leq = K * LI + L3, wherein K = L2/(L1+L2).

[0048] As noted before, the resonant capacitor C can be chosen to resonate with Leq at the source frequency, which means that the source impedance at point V4 is nearly zero. By adding a series capacitor, i.e. the resonant capacitor C, which resonates with V0 at the source frequency, the impedance of Leq is cancelled thereby improving regulation at the load R. The resonant capacitor C can be represented as C = l/(Leq*co^A2), where ω is the source frequency in radians per second.

[0049] In reality, the load R as illustrated in FIG. 7 and FIG. 8 is more complex, comprising for example a bridge rectifier to convert the AC voltage of V4 to DC voltage, followed by a filter capacitor and then a DC load. If the AC source voltage V0 in FIG. 7 were to be turned on suddenly, there may be a large inrush current 13 while the filter capacitor was being charged. This inrush current flowing through Leq and resonant capacitor C would cause large voltages across each of them, even though the total voltage across both would be small. Since Leq is dominated by the receiver windings, this brief over-voltage might cause a turn-to-turn failure. Also the size and cost of capacitor C might need to increase to tolerate the over-voltage. Thus, some form of soft-starting may be provided for the source voltage V0.

[0050] FIG. 9 illustrates a schematic of a second embodiment of a distributed power supply system 800 for a modular multilevel converter system including inductive power transfer in accordance with an exemplary embodiment of the present invention. The distributed power supply system 800 is similar to the system 400 as illustrated in FIG. 5. However, significant modifications of the system 400 are incorporated into the distributed power supply system 800.

[0051] The distributed voltage source power supply system 800 is provided for multiple subsystems or cells 802 connected in series, for example of inverter 170 of modular multilevel converter 100 as shown in FIG. 1. FIG. 9 illustrates three power cells 802 as an example, labelled as 'Power Cell # , 'Power Cell #2' and 'Power Cell #3'. Each cell 802 comprises a control unit, for example a cell control board 818, for communicating with other controllers, in particular with higher level controller, also known as hub of the converter system 100.

[0052] The system 800 comprises technology to transfer control power from a central power supply 810 near ground potential directly into the cells 802. The technology includes inductive (magnetic) power transfer which is illustrated using arrows 830 by magnetic coupling. For each cell 802, a magnetic circuit comprising a magnetic transmitter 812 and a magnetic receiver 814 are provided. The magnetic transmitter 812 and receiver 814 are used to pass control power from the central power supply 810 to the cells 802 (for example up to 50W of power transfer for each cell 802 over a dielectric isolation gap 820). The magnetic transmitter 812 and receiver 814 are configured such that the control power is passed across gap(s) 820 between transmitter 812 and receiver 814. A fuse 816 is assigned to each transmitter 812 to disconnect a failed transmitter so that the remaining operating transmitters 812 can continue to operate. In an example, the fuses 816 are located at the transmitters 812, rather than at the shared power supply 810, so that a single wire 840 feeds multiple transmitters 812. Components and elements of the system 800 that are not described here and correspond to components and elements of the system 400, are described with reference to the system 400 and to FIG. 5. [0053] In the embodiment according to FIG. 9, the central power supply 810 is configured as dual-redundant direct current (DC) power supply (instead of an AC power supply as shown in FIG. 5). Driven by the central power supply 810 configured as dual- redundant DC power supply, a dedicated inverter 850 is located near each transmitter 812, and is connected to drive the transmitter coil 520, see FIG. 6, of the respective transmitter 812. The inverter 850 converts DC voltage of the supply 810 into AC at a desired frequency. Thus, the magnetic transmitter 812 and receiver 814 are configured as low or high frequency transmitter and receiver.

[0054] In addition to providing current limiting, the provided system 800 including DC power supply 810 and inverter(s) 850 offers several other benefits. Current through each transmitter 812, in particular through each transmitter coil 520 (see FIG. 6) has a large reactive component. According to the example of FIG. 5, this reactive component flows through the fuses 416, the wiring 440, and the power supply 410. In FIG. 9, the reactive component flows only through the local inverter(s) 850. The fuses 816, the wiring 840, and the power supply 810 of FIG. 9 carry only enough DC to produce the power being transferred. This allows the current rating of each fuse 816 to be reduced significantly from about 5 amps to 0.5 amps. Further, the dual-redundant power supply 810 provides enough fault current to clear fuse(s) 816 quickly, which is much easier to achieve with smaller fuses 816 (lower fuse rating), and with a simple DC power supply 810. [0055] FIG. 10 illustrates a second schematic of the first embodiment of the distributed voltage source power supply system 400 of FIG. 5 in accordance with an exemplary embodiment of the present invention.

[0056] As described with reference to FIG. 5, the power supply system 400 comprises technology to transfer control power from the central power supply 410 near ground potential directly into the cells 402, wherein the central power supply 410 is configured as dual-redundant high or low frequency AC power supply. In an embodiment, the central power supply 410 is configured as a central sine wave excitation source, also known as sine wave generator. A sine wave generator is a much more efficient excitation source operating at a designed resonance of the system 400. In comparison to FIG. 5, capacitors CI have been added in parallel with the transmitter coils, to cancel most of the reactive current drawn by these coils. This allows the current rating of the fuses 416 to be reduced significantly, in the same manner as the local inverters 850 of FIG. 9.

[0057] The power cells 402 are configured as M2CL subsystems. M2CL topology 417 is shown for one of the cells 402 as an example. For each of the cells 402, a magnetic circuit is provided comprising transmitter 412 and receiver 414, wherein power supply 410 supplies control power for the subsystems 402 and powers the transmitters 412 via wiring 440. Control power is transferred via magnetic coupling to the receivers 414 and power cell 402. Cell control board 418 is operably coupled to receiver 414 and receives the control power for controlling the cell 402. Cell control board 418 is further configured to communicate with a higher level controller, for example the hub of the converter system.

[0058] The magnetic circuit further comprises resonant capacitor C, coupled in series with the magnetic receiver 414 and configured to have the same but opposite impedance as L3 in series with both LI and L2 (see FIG. 7 and FIG. 8) at the operating frequency of the power supply 410 in order to improve the voltage regulation for the load, i.e. the cell control board 418. Thus, the distributed power supply system 400 can also be referred to as distributed resonant coupled power supply system. Capacitor CI coupled in parallel to the transmitter 412 is optional, if it is not necessary to minimize the current rating of fuses 416.

[0059] FIG. 11 illustrates a second schematic of the second embodiment of the distributed voltage source power supply system 800 of FIG. 9 in accordance with an exemplary embodiment of the present invention.

[0060] The power cells 802 are configured as M2CL subsystems. M2CL topology is shown for one of the cells 802 as an example. For each of the cells 802 a high frequency magnetic circuit is provided comprising transmitter 812 and receiver 814, wherein power supply 810 supplies control power for the subsystems 802 and powers the transmitters 812 via wiring 840. [0061] The power supply system 800 comprises redundant centralized DC power supply 810 feeding multiple subsystems (power cells) 802, wherein each power cell 802 is fused using fuses 816 to eliminate a single point of failure. Further to the central power supply 810 configured as dual-redundant DC power supply, a dedicated inverter 850 is located near each transmitter 812, and is connected to drive the transmitter coil of the respective transmitter 812. The inverter 850 converts DC voltage of the supply 810 into AC at the desired frequency, for example 100 kHz. Control power is transferred via magnetic coupling to the receivers 814 and power cell 802. Cell control board 818 is operably coupled to receiver 814 and receives the control power for controlling the cell 802. Cell control board 818 is further configured to communicate with a higher level controller, for example the hub of the converter system. The magnetic circuit further comprises resonant capacitor C, coupled in series with the magnetic receiver 814 and configured to have the same but opposite impedance as Leq (see FIG. 7 and FIG. 8) at the operating frequency of the power supply 810, for example 100 kHz, in order to improve the voltage regulation for the load, i.e. the cell control board 802. Thus, the distributed power supply system 800 can be referred to as distributed resonant coupled power supply system for power cells (subsystems) 802, in particular comprising M2CL subsystems.

[0062] FIG. 12 and FIG. 13 illustrate two dimensional and three dimensional views of a second embodiment of a magnetic circuit 900 for inductive power transfer in accordance with an exemplary embodiment of the present invention. Magnetic circuit 900 comprises components as described before with reference to magnetic circuit 500 and FIG. 6. However, insulating sheets 970 and 972 have been added in FIG. 12 and FIG. 13. The magnetic circuit 900 comprises magnetic transmitter 912 and magnetic receiver 914.

[0063] As mentioned before with reference to FIG. 6, IEC Standard 61800-5-1 requires a minimum gap 960 of 60 mm between the transmitter 912 and the receiver 914, for a peak working voltage between them of 16 kV, or an impulse voltage of 40 kV. Section 5.2.3.3 of the same standard also requires that partial discharge (PD) shall be under 10 pico-Coulombs for 15 seconds at 150% of the peak working voltage, or 24 kV. PD can still occur even when the gap 960 is compliant, if there are local concentrations in the electric field (for example, due to sharp edges or points).

[0064] Depending on an implementation of the transmitter 912 and receiver 914, magnetic components 928, 930 and 948, 950 may comprise sharp edges leading to local concentrations in the electric field. In case that the magnetic circuit 900 comprises local concentrations in the electric field, magnetic circuit 900 may comprise corresponding insulating protection. For example, transmitter 912 and receiver 914 each can comprise a protective cover 970, 972 in order to avoid PD of the magnetic circuit 900. Transmitter 912 comprises protective cover 970 and receiver 914 comprises protective cover 972. Protective covers 970 and 972 are configured as sheets comprising electrical insulating properties. In an exemplary embodiment, covers 970 and 972 are configured as sheets of fiberglass laminate, also known as G10. Covers 970, 972 are placed directly over pole faces of transmitter 912 and receiver 914 and face the gap 960.

[0065] FIG. 14 illustrates a three dimensional view of a third embodiment of a magnetic circuit 1000 for inductive power transfer in accordance with an exemplary embodiment of the present invention. [0066] Previously disclosed embodiments of magnetic circuits, for example high frequency circuits 500 and 900, see FIG. 6 and FIG. 12 and FIG. 13, comprise magnetic, for example Ferrite, components and operate for example at a frequency of about 100 kHz. But other mechanizations of a large gapped resonate structure (designed for maximum coupling) work at any frequency with proper choice of materials and resonant capacitor C. For the series resonant capacitor C in the magnetic receiver, which is crucial for operation of the magnetic circuit, a lower frequency, i.e. lower than 100 kHz, has the advantage that more suitable values are available at lower cost.

[0067] According to an exemplary embodiment, the magnetic circuit 1000 is designed for operation at a low or medium frequency, in particular a frequency lower than 100 kHz as used before. Medium frequency as used herein refers to a frequency less than 100 kHz, and low frequency refers to a frequency less than 1 kHz, for example a frequency of 50 Hz or 60 Hz. For employing low or medium frequency, for example 60 Hz, magnetic transmitter 1012 and magnetic receiver 1014 can comprise amorphous powdered iron. Specifically, the transmitter 1012 and receiver 1014 comprise multiple powdered iron slugs. Transmitter 1012 comprises powdered iron slugs 1022, 1024. Powdered iron slug 1022 is configured as core and positioned within transmitter coil 1020. Powdered iron slugs 1024 are identical in shape and dimensions, designed for example as cuboids, wherein three slugs 1024 are arranged in series and adjacent to the coil 1020 on one side (see Y direction), and three slugs 1024 are arranged in series and adjacent to the coil 1020 on the other, opposite, side. Magnetic receiver 1014 is constructed in the same manner as transmitter 1012, and comprises receiver coil 1040 comprising powdered iron slug core 1022 and powdered iron slugs 1044 arranged on both sides of the coil 1040.

[0068] It should be noted that the magnetic circuit 1000 comprising powdered iron slugs is only one example for a design of the transmitter 1012 and receiver 1014. Many other designs are conceivable and can comprise for example powdered iron slugs of different forms and sizes and/or different arrangements. Powdered iron slugs are simple to assemble and can be made modular so that for higher voltages requiring greater strike distance, more surface area can be added to the transmitter 1012 and receiver 1014 (pole structures) by adding blocks of powdered iron.

[0069] In another exemplary embodiment, a magnetic circuit 500, 900, 1000 of magnetic transmitter and receiver can comprise silicon steel lamination for low frequency applications. For example, magnetic transmitter and receiver can comprise silicon steel lamination. In particular, transmitter and receiver can be entirely constructed from silicon steel lamination including electromagnetic coils.

[0070] In another exemplary embodiment, the distributed power supply system 400, 800, in particular the magnetic circuit comprising transmitter 412, 812 and receiver 414, 814, can comprise a circuit for pre-charge of the capacitors of the power cells 402, 802. As noted before, the power cells 402, 802 comprise semiconductors, such as IGBTs, providing the inverter circuitry. The power cells 402, 802 further comprise capacitors for storing and supplying the energy for the semiconductors. In particular, the cell control board 418, 818 can comprise a circuit for a smooth pre-charge from zero charge state of the capacitors, using energy provided by the inductive power transfer. [0071] In another exemplary embodiment, the distributed power supply system 400, 800, in particular the magnetic circuit comprising transmitter 412, 812 and receiver 414, 814, can comprise a control for initiating latched bypass of a failed power cell 402, 802.

[0072] FIG. 15 illustrates a three dimensional view of a section 1100 of a modular multilevel converter system including inductive power transfer in accordance with an exemplary embodiment of the present invention. Section 1100 includes multiple subsystems or power cells 1200 of a modular multilevel converter system, for example converter system 100 as shown for example in FIG. 1. Section 1100 includes exemplary three power cells 1200, wherein the converter system typically comprises 24, 30, 36 or 42 cells 1200 according to a required output voltage. For example, a converter system 100 with 24 cells 1200 provides an output voltage of 4.16 kV (3 phase AC).

[0073] FIG. 15 illustrates possible locations and arrangements of a magnetic circuit, such as circuits 500, 900 or 1000, for power cells 1200. Magnetic transmitters 1212 and magnetic receivers 1214 are positioned so that inductive power transfer for transferring control power directly in each of the power cells 1200 is provided. [0074] Each power cell 1200 comprises a magnetic receiver 1214 which can be located at a side or back of the power cell 1200. In an example, the receiver 1214 is captive to an inside of the power cell 1200, for example to an inside of a molded insulated power cell 1200. The magnetic transmitters 1212 are aligned with the magnetic receivers 1214 and can be captive to a system containing the power cells 1200. For example, the transmitters 1212 can be located in a cell compartment of a converter enclosure 1300. As FIG. 15 further illustrates, power cells 1200 are easily removable for service or replacement. It should be noted that many other positions of transmitters 1212 and receivers 1214, which allow inductive power transfer transferring control power directly in the power cells 1200, are conceivable. [0075] Distributed power supply systems 400, 800 as described herein provide autonomous high isolation voltage power supplies which allow the subsystem or power cells 402, 802 to be visible and controllable via the hub (higher level controller or master control) it is connected to under any normal or abnormal operating condition. Such an autonomous voltage power supply is specifically important for converters with M2LC subsystems, because an "on-board" power supply fed from a local DC bus becomes inoperable under fault or unstable operation thus rendering the inverter comprising the subsystems inoperable until a stable state is achieved. This may take considerable time which cannot be tolerated and also requires significant redundancy, cost and in-efficiency in the system. Further, the hub or the master control is able to communicate with the power cells, to perform diagnostics, and to pre-charge the power cells before main medium-voltage supply was energized.

[0076] While embodiments of the present invention have been disclosed in exemplary forms, it will be apparent to those skilled in the art that many modifications, additions, and deletions can be made therein without departing from the spirit and scope of the invention and its equivalents, as set forth in the following claims.

Claims

1. A distributed power supply system (400, 800) for a modular multilevel converter (100) comprising:

an inverter (170) comprising multiple power cells (402, 802), and

a voltage source power supply (410, 810),

wherein control power is transferred from the voltage source power supply (410, 810) directly in each of the multiple power cells (402, 802) by inductive power transfer.

2. The distributed power supply system (400, 800) of Claim 1, wherein the multiple power cells (402, 802) are configured as M2C subsystems or M2CL subsystems or Cascaded H-B ridge subsystems.

3. The distributed power supply system (400, 800) of Claim 1 or 2, wherein the voltage source power supply (410) is configured as dual-redundant alternating current

(AC) power supply.

4. The distributed power supply system (400, 800) of Claim 1 or 2, wherein the voltage source power supply (810) is configured as dual -redundant direct current (DC) power supply.

5. The distributed power supply system (400, 800) of any of the preceding Claims, wherein the inductive power transfer is configured as near field resonant coupled power transfer.

6. The distributed power supply system (400, 800) of any of the preceding Claims, further comprising:

a plurality of magnetic circuits (500, 900, 1000) performing the inductive power transfer, each magnetic circuit (500, 900, 1000) comprising a magnetic transmitter (412, 512, 812, 912, 1012) and a magnetic receiver (414, 514, 814, 914, 1014), wherein each power cell (402, 802) is assigned a magnetic circuit (500, 900, 100).

7. The distributed power supply system (400, 800) of any of the preceding Claims, further comprising:

a resonant capacitor (C) configured to resonate at a source frequency of the voltage source power supply (410, 810) and to cancel a reactive impedance of the magnetic receiver (414, 814).

8. The distributed power supply system (400, 800) of Claim 6 and 7, wherein the resonant capacitor (C) is coupled in series with the magnetic receiver (412, 512, 812,

912, 1012) of the magnetic circuit (500, 900, 1000).

9. The distributed power supply system (400, 800) of any of the preceding Claims, wherein each power cell (402, 802) comprises a control unit (418, 818), and wherein each magnetic receiver (414, 514, 814, 914, 1014) is operably coupled to a control unit (418, 818) of the power cell (402, 802).

10. The distributed power supply system (400, 800) of any of the preceding Claims, wherein the voltage source power supply (410, 810) powers all of the magnetic transmitters (412, 512, 812, 912, 1012).

11. The distributed power supply system (400, 800) of Claim 6, wherein the magnetic circuit (500, 900, 1000) is configured to operate with an air gap (560, 960) between the magnetic transmitter (412, 512, 812, 912, 1012) and the magnetic receiver (414, 514, 814, 914, 1014), the magnetic transmitter (412, 512, 812, 912, 1012) and magnetic receiver (414, 514, 814, 914, 1014) being independent physical components.

12. The distributed power supply system (400, 800) of Claim 6, wherein the magnetic circuit (500, 900, 1000) is configured to operate with an electrically insulated gap (560, 960) between the magnetic transmitter (412, 512, 812, 912, 1012) and the magnetic receiver (414, 514, 814, 914, 1014), wherein the magnetic circuit (500, 900, 1000) is constructed as a single physical unit.

13. The distributed power supply system (400, 800) of Claim 6, wherein the magnetic circuit (500, 900) comprises silicon steel laminations.

14. The distributed power supply system (400, 800) of Claim 6, wherein the magnetic circuit (1000) comprises amorphous powdered iron.

15. The distributed power supply system (400, 800) of Claim 14, wherein the magnetic transmitter (1012) and the magnetic receiver (1014) are constructed from amorphous powdered iron slugs and electromagnetic coils.

16. The distributed power supply system (400, 800) of Claim 6, wherein the magnetic circuit (500, 900) comprises Ferrite.

17. The distributed power supply system (400, 800) of Claim 16, wherein the magnetic transmitter (512, 912) and the magnetic receiver (514, 914) are constructed from Ferrite components and electromagnetic coils.

18. The distributed power supply system (400, 800) of Claim 6, wherein energy is stored in capacitors of the power cell (402, 802), and wherein the magnetic circuit (500, 900, 1000) comprises a circuit for pre-charge of the capacitors of the power cell (402, 802).

19. The distributed power supply system (400, 800) of Claim 6, wherein the magnetic circuit (500, 900, 1000) comprises a control for initiating latched bypass of a failed power cell (402, 802).

20. The distributed power supply system (400, 800) of Claim 6, wherein a fuse (416, 816) is assigned to each transmitter (412, 512, 812, 912, 1012) for disconnecting a transmitter (412, 512, 812, 912, 1012) during a fault.

21. The distributed power supply system (400, 800) of Claim 20, further comprising means for reducing reactive current in the fuses (416, 816) so that the fuses (416, 816) comprise reduced current ratings to facilitate clearing during the fault.

22. A variable frequency drive comprising a distributed power supply system (400, 800) as claimed in any of the preceding Claims.