WO2013137749A1

WO2013137749A1 - Electrical systems with inductive power transfer-based energy balancing

Info

Publication number: WO2013137749A1
Application number: PCT/NZ2013/000037
Authority: WO
Inventors: Udaya Kumara Madawala
Original assignee: Auckland Uniservices Limited
Priority date: 2012-03-16
Filing date: 2013-03-13
Publication date: 2013-09-19

Abstract

The present invention provides an electrical system comprising a plurality of electrically coupleable energy storage elements operable to supply power to an output of the electrical system, and an inductive power transfer (IPT) sub-system independently operable to balance energy in the plurality of energy storage elements. The IPT sub-system balances energy across the energy storage elements without any direct electrical connections but through magnetic coupling and with galvanic isolation. The electrical system may comprise a modular multi-level converter (M2LC), a multi-level converter, or a multi-cell battery, for example.

Description

ELECTRICAL SYSTEMS WITH INDUCTIVE POWER TRANSFER-BASED ENERGY

BALANCING

Field of the Invention

This invention relates to electrical systems using inductive power transfer (IPT) to balance energy between a plurality of energy storage elements. More particularly, though not exclusively, the invention relates to a modular multi-level converter (often referred to as an "M2LC") system which uses IPT technology to at least balance capacitor voltages in a plurality of M2LC system modules.

Background

In a number of electrical systems, it is necessary or desirable to "balance" or maintain a substantially consistent voltage or energy between a plurality of energy storage elements during operation. Examples of such systems include, but are not limited to, the capacitors of multi-level converters or modular multi-level converters, and each of the cells of a multi-cell battery during charging and/or discharging.

Lesnicar et al. (A. Lesnicar and R. Marquardt. An innovative modular multilevel converter topology suitable for a wide power range. In Proc. IEEE Power Tech. Conf., Bologna, Italy, Jun. 2003), the contents of which are incorporated herein by reference, discloses an example of a multi-level converter topology particularly suitable for high-voltage applications, termed a "modular multi-level converter" (M2LC).

The M2LC topology is becoming increasingly popular in both medium- and high-voltage applications. The modular structure of the M2LC offers a number of advantages over other available multi-level converter topologies, such as the Neutral Point Clamped Voltage Source Converter (NPC VSC), Flying Capacitor Voltage Source Converter (FC VSC) and Series Connected H-Bridge Voltage Source Converter (SCHB VSC).

Some of the advantageous features of M2LCs are simple scaling of the number of output voltage levels (by a linear addition of identical modules), a capacitor-free DC-link, continuous arm currents and redundant switching operations. These features of the M2LC topology make it suitable for various applications such as high power motor drives, high-voltage direct current (HVDC) transmission, traction motors, static synchronous compensator (STATCOM), battery energy storage systems, and as a general grid connected converter. A general layout of the M2LC topology of the prior art with three output voltage levels is shown in Fig. 1. The converter comprises a DC link made up of a DC voltage source V_dc, a DC link inductance L_dc, and a DC link resistance R _c; and a number of sub-converter modules. The DC link voltage is usually derived from rectifying the utility AC supply voltage from which the M2LC can be powered with or without DC link capacitors, represented here by a DC link voltage source. For simplicity, these features are not shown in Fig. 1. Each phase or leg of the converter is divided into two halves, called arms. In this example, the M2LC comprises three legs and each arm consists of two sub-converter modules, SM_qn (where q≡ {a, b, c}, and n≡ {1, 2, 3, 4}), a resistor R that models conduction losses, and an arm inductor L. Each module SM_qn (shown in detail in the lower left corner of the diagram) consists of two switches and two diodes forming a half-bridge, and a capacitor C_qn. Each module is operated in a similar manner to a chopper and, hence, referred to as a 'chopper module'. Each individual chopper module has two switching states u_qn {0, 1}, where 1 means that the capacitor is connected in the circuit, i.e. switch S_qri:u is turned on. The two switches in each module are operated complementary. The resistor f?_cap is connected in parallel to the capacitor to model the leakage current of the capacitor.

It is also possible to operate the M2LC as an AC-AC converter, where input to the converter can either be a single or three phase AC supply. In this case, the half bridge configuration of the chopper module is replaced with a full bridge configuration with 4 switches. Consequently, the individual chopper module will have three switching states u_qn e {-1 , 0, 1}, where -1 means that the capacitor can be connected in an opposite polarity and 0 and 1 are as same as for the half-bridge situation as described previously. During standard operation, the module capacitors C_qn are charged or discharged depending on the module's switching state u_qn and the polarity of the arm current i_qm (where q e {a, b, c}, and m e {P,N}).

The example M2LC converter of Fig. 1 provides three voltages, (V_do 2, 0, and -V_d< 2), at its output terminal V_q (where q≡ {a, b, c}) with respect to the supply ground (node N). The terminal is connected to the load, which consists of an inductor /., in series with a resistor R_: and grid voltage V_g,q.

Inherent to the M2LC converter topology are circulating currents or balancing currents, generated by the variation in the capacitor voltages, which are connected in parallel to the supply bus. Uncontrolled or non-minimized circulating currents increase the required rating of the switches, and conduction losses. In addition, the capacitor voltage ripple is inversely proportional to the frequency of the load current and directly proportional to the magnitude of the load current, which restrict the converter's operation at low output frequencies unless complex procedures to add harmonic currents in the arms or use of common mode voltages are considered. This demands a control algorithm which can balance the capacitor voltages, meet the converter objectives such as load current, maintain a low switching frequency and provide the best possible performance during various operating conditions. The control complexity and computational burden of such a controller increase tremendously with the number of modules, compromising the controller's ability to maintain the expected performance of the system.

Several control schemes have been proposed to balance the capacitor voltages, each employing a modulator to drive the M2LC switches and utilize the redundancy of the converter. However, these schemes have the following drawbacks:

i) a requirement for complex controls for low output frequency operations as described by Korn et al. (A. J. Korn, M. Winkelnkemper, and P. Steimer. Low output frequency operation of the modular multi-level converter. In Proc. IEEE Energy Conversion Congress and Exposition (ECCE), pages 3993-3997, 2010);

ii) limited control over peak capacitor voltages or the energy stored in the converter, and the total stored energy in the converter can be higher than conventional converter topologies; iii) limited control over reducing circulating currents or balancing currents, which dictate the rating of switches and conduction losses; and

iv) at start-up, there is a need for an auxiliary circuit to control the charging process of capacitors, which can be bypassed during normal operations.

Accordingly, there is a need for an improved or alternative means for controlling capacitor voltages in at least an M2LC converter.

Furthermore, voltage and current ratings of currently available semiconductor devices are limited and hence cannot be used on their own to meet high power demands. As a solution, both multilevel converters and 2LCs, which facilitate the use of existing and freely available low power semiconductor devices to meet the demand of high power applications, have been proposed. However, the complete system, comprising a multitude of levels, legs or sub-modules, is still powered by a single source, which is rated for the full power capability of the complete system. If the high input power can easily be provided by using a number of low power modules, the flexibility for control and implementation, reliability, safety, ride through capability, cost effectiveness and simplicity of the complete system can be improved. Therefore there is also a need for improved or alternative means of supplying power to a whole converter system.

Object of the Invention

It is therefore an object of the invention to provide an electrical system and/or control method which overcomes or at least ameliorates one or more disadvantages of the prior art, or alternatively to at least provide the public with a useful choice.

Further objects of the invention will become apparent from the following description. Summary of Invention

In a first aspect the invention may broadly be said to consist in a modular multi-level converter (M2LC) system comprising:

a converter circuit for converting a DC or AC input to an AC output, the converter circuit comprising a plurality of chopper modules each comprising a capacitor, wherein said chopper modules are arranged in two arms of at least one leg; and

an inductive power transfer (IPT) sub-system for selectively charging and discharging said chopper module capacitors to substantially balance capacitor voltages, the IPT sub-system comprising a primary IPT converter, an IPT track supplied with an alternating current by said primary IPT converter, and a plurality of IPT pick-up circuits each electrically coupled with one of said chopper modules and inductively coupled with said IPT track.

The DC or AC input to the converter circuit is preferably be provided by a DC or AC power supply connected in parallel to the one or more legs of the converter. Alternatively, or additionally, a DC input may be supplied directly to each chopper module by the associated IPT sub-system, wherein the system comprises a plurality of IPT sub-systems.

Preferably said IPT pick-up sub-systems are bi-directional.

Preferably each chopper module further comprises a pair of switches forming a chopper circuit with the capacitor.

Preferably the M2LC system further comprises a main controller adapted to selectively operate said chopper module switches to meet load objectives of the M2LC system.

Preferably the M2LC system further comprises a balancing controller adapted to operate the pick-up circuits to substantially balance the capacitor voltages. Preferably the balancing controller comprises a secondary controller associated with each pick-up circuit, each secondary controller being adapted to control a direction and magnitude of power flow from/to the IPT track dependent upon the chopper module capacitor voltage. Preferably the balancing controller is adapted to operate the pick-up circuits to maintain the capacitor voltages within a predefined range. In particular, each secondary controller is preferably adapted to charge the associated chopper module capacitor when the capacitor voltage is below a predefined lower threshold, and to discharge the capacitor when the capacitor voltage is greater than a predefined upper threshold, wherein said charging and discharging occurs via the inductive coupling between the pick-up circuit and the IPT track.

Preferably each pick-up circuit comprises a pick-up coil and a reversible rectifier (a converter operating in both controlled inverting and rectifying modes) coupled to the pick-up coil and said chopper module. The reversible rectifier preferably comprises four switches in an H-bridge configuration, or two switches in a half-bridge.

Preferably the balancing controller further comprises a primary controller associated with the or each primary IPT converter, wherein the primary controller acts to provide a constant AC current in the associated IPT track. More preferably, the primary IPT converter comprises a bi-directional converter.

Preferably the or each IPT sub-system is inductively coupled with a plurality of said IPT pick-up circuits, whereby a discharging capacitor of one chopper module may at least in part supply a charging capacitor of another chopper module inductively coupled with the same IPT track.

Preferably the system comprises an IPT sub-system for each leg or each arm of the converter circuit.

In a second aspect, the invention may broadly be said to consist in a modular multi-level converter (M2LC) chopper module comprising:

a resonant tank comprising an inductive pick-up coil;

a secondary converter coupled to the resonant tank;

a capacitor coupled to the secondary converter; and

a chopper circuit coupled to the capacitor and a pair of terminals.

Preferably the resonant tank further comprises an inductor and a tuning capacitor forming an LCL circuit with the inductive pick-up coil. A DC blocking capacitor may also be provided in series with the inductor. Alternatively, the resonant tank may comprise any other suitable combination of inductors and capacitors.

Preferably the secondary converter comprises an inverter or a controlled rectifier.

Preferably the chopper module further comprises a secondary controller associated with the secondary converter and adapted to control the same dependent upon the capacitor voltage. In particular, the secondary controller is preferably adapted to control the secondary converter to maintain a voltage across the capacitor within a predefined range.

Preferably the controller is adapted to charge the capacitor when the capacitor voltage is below a lower threshold, and to discharge the capacitor when the capacitor voltage exceeds an upper threshold. Preferably the controller is adapted to charge and discharge the capacitor from/to an inductive power transfer (IPT) track with which the inductive pick-up coil is inductively coupled in use.

Preferably the secondary controller is further adapted to supply a DC input directly to the chopper module.

In a third aspect, the invention may broadly be said to consist in a method for controlling a modular multi-level converter (M2LC) system comprising a plurality of chopper modules each comprising a capacitor, wherein said chopper modules are arranged in two arms of at least one leg, the method comprising the steps of:

providing an inductive power transfer (IPT) sub-system comprising at least one primary IPT converter, at least one IPT track supplied with an alternating current by a primary IPT converter, and at least one IPT pick-up circuit inductively coupled with the or each IPT track and electrically coupled with a chopper module; and

operating said plurality of pick-up circuits to substantially balance the capacitor voltages of said plurality of chopper modules.

Preferably the step of operating said plurality of pick-up circuits comprises operating each pick-up circuit to maintain a voltage across the chopper module capacitor within a predefined range. In particular, this preferably comprises selectively charging and discharging the capacitor via the inductive coupling. Preferably the step of operating said plurality of pick-up circuits comprises independently operating each pick-up circuit to charge the capacitor of the chopper module electrically coupled therewith when the capacitor voltage is below a predefined lower threshold, and to discharge the capacitor when the capacitor voltage exceeds a predefined upper threshold.

Preferably the step of operating each pick-up circuit comprises selectively operating a plurality of switches of a secondary converter to control the direction and/or magnitude of power flow between the primary IPT converter and the associated chopper module. Preferably the method further comprises the step of operating said plurality of chopper modules to meet load objectives of the M2LC system.

Preferably the method further comprises operating said plurality of pick-up circuits to directly supply a DC input to each of said plurality of chopper modules.

In a fourth aspect, the invention may broadly be said to consist in an electrical system comprising a plurality of electrically coupleable energy storage elements operable to supply power to an output of the electrical system, and an inductive power transfer (IPT) sub-system independently operable to balance energy in the plurality of energy storage elements.

Preferably the IPT sub-system comprises a conductive path and a plurality of pick-up circuits each electrically coupled to an energy storage element and capable of being inductively coupled with the conductive path, wherein each pick-up circuit comprises a pick-up controller operable to maintain a voltage across the associated energy storage element within a predefined range by selectively charging and/or discharging the energy storage element via the conductive path.

Preferably the IPT sub-system comprises a plurality of conductive paths and each IPT pick-up circuit is capable of being inductively coupled with one of said plurality of conductive paths.

Preferably the IPT sub-system further comprises an IPT converter electrically coupled to the or each conductive path and operable to maintain a substantially constant alternating current in the associated conductive path. Preferably the IPT sub-system is bi-directional and operable to selectively charge and discharge each of the plurality of energy storage elements. Preferably energy discharged from one energy storage element is at least partially used to charge another energy storage element.

Preferably the electrical system comprises a modular multi-level converter (M2LC) system for converting a DC or AC input to an AC output, the M2LC system comprising a plurality of chopper modules each comprising an energy storage element in the form of a capacitor.

Preferably the electrical system further comprises a main controller operable to selectively couple the capacitors to the output to meet a load objective of the M2LC system.

Preferably the chopper modules are arranged in two arms of at least one leg, and the DC or AC input to the M2LC system is provided by a DC or AC power supply connected in parallel to the one or more legs of the converter. Alternatively, a DC input to the M2LC system may be provided by the IPT sub-system.

Preferably each chopper module further comprises a pair of switches forming a chopper circuit with the capacitor. Rather than an M2LC system, the electrical system may alternatively comprise a multi-level converter wherein the plurality of energy storage elements each comprise a capacitor; or a multi-cell battery wherein the plurality of energy storage elements each comprise a battery cell.

In a fifth aspect, the invention may broadly be said to consist in a method for balancing energy in an electrical system comprising a plurality of coupleable energy storage elements operable to supply power to an output of the electrical system, comprising the steps of:

sensing the energy stored by each energy storage element; and

selectively charging and/or discharging each energy storage element via an inductive coupling to maintain the sensed energy of each energy storage element within a predefined range.

Preferably the steps of sensing energy and selectively charging and/or discharging each energy storage element are performed independently from a method for controlling the output of the electrical system to meet a load objective.

Further aspects of the invention, which should be considered in all its novel aspects, will become apparent from the following description. Drawing Description

A number of embodiments of the invention will be described below by way of example with reference to the drawings in which:

Fig. 1 is a circuit diagram of an example M2LC system according to the prior art;

Fig. 2 is a circuit diagram of a first embodiment of an 2LC system according to the present invention;

Fig. 3 is a circuit diagram of an example primary IPT converter of the prior art suitable for use with the present invention;

Fig. 4 is a diagram illustrating a control scheme of the prior art suitable for use in the primary IPT converter of Fig. 3 to maintain a constant current in the IPT track;

Fig. 5 shows circuit diagrams of two possible M2LC chopper modules according to the present invention, with: (a) an H-bridge converter, and (b) a half-bridge converter;

Fig. 6 is a circuit diagram of a single phase M2LC system according to another embodiment of the present invention;

Fig. 7 shows simulated waveforms of the chopper module capacitor voltages V_Ciqn of both (a) the conventional M2LC converter of the prior art; and (b) the M2LC system of Fig. 6;

Fig. 8 shows simulated waveforms of the arm currents at steady-state for both (a) the conventional M2LC converter of the prior art; and (b) the M2LC system of Fig. 6;

Fig. 9 is a circuit diagram of an alternative embodiment of a single phase M2LC system according to the present invention;

Fig. 10 shows simulated waveforms of the chopper module capacitor voltages V_C:qn of both (a) the conventional M2LC converter of the prior art; and (b) the M2LC system of Fig. 9;

Fig. 11 shows simulated waveforms of the arm currents at steady-state for both (a) the conventional M2LC converter of the prior art; and (b) the M2LC system of Fig. 9;

Fig. 12 shows simulated waveforms for the supply current according to (a) the prior art; and (b) the M2LC system of Fig. 9;

Fig. 13 is a circuit diagram of a three phase embodiment of an M2LC system according to the present invention;

Fig. 14 shows simulated waveforms of the chopper module capacitor voltages V_Ciqn of both (a) the conventional M2LC converter of the prior art; and (b) the M2LC system of Fig. 13;

Fig. 15 shows simulated waveforms of the arm currents at steady-state for both (a) the conventional M2LC converter of the prior art; and (b) the M2LC system of Fig. 13;

Fig. 16 is a circuit diagram of another alternative embodiment of a single phase M2LC system according to the present invention;

Fig. 17 is a circuit diagram of a further alternative embodiment of a single phase M2LC system according to the present invention; Fig. 18 is a circuit diagram of yet another alternative embodiment of a single phase M2LC system according to the present invention;

Fig. 19 is a circuit diagram of an alternative embodiment of a three phase M2LC system according to the present invention;

Fig. 20 is a circuit diagram of another alternative embodiment of a three phase M2LC system according to the present invention;

Fig. 21 is a block diagram of a non-modular multi-level converter according to the present invention (with the primary converters and conductive paths omitted for clarity); and

Fig. 22 is a circuit diagram of a multi-cell battery system according to the present invention (with the primary converters and conductive paths again omitted for clarity).

Detailed Description of the Drawings

Throughout the description like reference numerals will be used to refer to like features in different embodiments. The present invention provides a system and method for controlling or balancing capacitor voltages or energy in an electrical system by way of inductive power transfer (IPT). Although the invention is described below with reference to a preferred application of the invention in a modular multi-level converter ( 2LC), it is to be appreciated that the invention may alternatively be applied to any other multi-level converter topology requiring accurate balancing of capacitor voltages to ensure proper functioning, or indeed any other electrical system requiring voltage or energy balancing between a plurality of energy storage elements during operation. The energy storage elements may comprise capacitors or battery cells, for example. IPT technology is now widely accepted as an efficient method for contactless power transfer, commonly used in industrial applications (e.g. for supplying power wirelessly to automated guided vehicles), and in consumer electronics (e.g. for wireless battery charging).

An IPT system is commonly said to comprise a primary side and a secondary side. The IPT primary side power converter is most commonly supplied by an external power source, such as a utility network or electricity "grid", and supplies power 'inductively' (magnetically) to the secondary side of the IPT system, which is connected to load. The primary side thus typically comprises a primary IPT converter (commonly an inverter) which energises a primary conductive path (such as a track or inductor coil) with an alternating current (AC) at a suitable frequency. The secondary side typically comprises one or more pick-up circuits, each comprising a secondary pick-up coil for inductive coupling with the primary track. The pick-up circuit generally also includes a secondary converter, commonly including at least a rectifier to supply DC power to a load electrically coupled therewith. The primary and/or secondary converter may also each comprise a controller to regulate power transfer within the IPT system.

Recently, developments in IPT technology have enabled efficient bi-directional power transfer, which may be applied to achieve bi-directional power transfer to/from electric vehicles (EVs) in a vehicle-to-grid (V2G) system for example. International Patent Publication No. WO 2010/062198 (the contents of which are incorporated herein by reference), for example, discloses a phase control technique and circuits for controlling both the direction and magnitude of power transfer.

For the sake of clarity, in the case of a bi-directional IPT system in which power may flow in either direction, the term "primary" as used herein refers to the conductive path and/or the converter electrically coupled to the conductive path, and the term "secondary" refers to the one or more pick-up circuits inductively coupled with the conductive path, irrespective of the direction of power flow. Power may therefore flow in a first direction from the primary to a pick-up on the secondary side of the system, or in a second direction from a pick-up circuit on the secondary side to the primary side of the system.

The present invention makes use of an IPT sub-system, preferably a bi-directional IPT sub-system, to substantially balance the voltage or energy between a plurality of energy storage elements, and is largely described below with reference to particular example embodiments in which the IPT sub-system balances the voltage across a plurality of chopper module capacitors in an M2LC system. Other example applications of the invention include, but are not limited to, balancing the voltage across capacitors in a (non-modular) multi-level converter, or across each of the plurality of battery cells in a multi-cell battery.

The terms "balance" and "balancing" are intended as meaning that a substantially consistent, though not necessarily identical, energy level (as represented by the voltage across each energy storage element, for example) is maintained among the plurality of energy storage elements during operation of the electrical system. That is, the objective of balancing energy is met where the voltage of a plurality of capacitors is maintained within a predefined range. Although energy discharged from one energy storage element may, in at least some embodiments, be used to charge another energy storage element at least in part, it is to be understood that this is not an essential feature of the invention. In most applications of IPT systems of the prior art, the loads supplied by the plurality of pick-up circuits, such as automated guided vehicles (AGVs) for example, are typically galvanically isolated from one another. The present invention is distinguished from such prior art in that the plurality of energy storage elements are in fact electrically coupled, or coupleable, to each other and therefore not galvanically isolated. That is, two or more of the energy storage elements balanced by independent pick-up circuits may, at various times, be both selectively coupled to the output of the electrical system.

In an example M2LC system according to the present invention, the M2LC topology of the prior art, as shown by way of example in Fig. 1 , is modified to include a bi-directional IPT charge/discharge sub-system, as shown by way of example in Fig. 2. No other changes need necessarily be made to the physical layout of the prior art M2LC circuit except to add an IPT pick-up circuit to each of the chopper modules. The IPT charge/discharge sub-system is configured to discharge each sub-converter or chopper module capacitor C_qn when its voltage is greater than an upper threshold, and charge it when its voltage is below a lower threshold, thereby maintaining the capacitor voltage within a predefined range. The values of the upper and lower threshold are, preferably, symmetrical about the optimal capacitor voltage and based on the power transfer capability of the IPT sub-system. The upper and lower thresholds may be ±10%, more preferably within ±5%, or yet more preferably within ±3% of the optimal capacitor voltage, for example. Ideally, the difference between the upper and lower thresholds is as small as possible provided that the IPT sub-system is capable of either injecting or absorbing energy at the relevant thresholds. In addition, the threshold value is related to the circulating currents of the M2LC, and smaller threshold values will result in smaller circulating currents, leading to lower losses in the converter.

The capacitor voltages are not controlled by the IPT sub-system when the voltage is between the upper and lower threshold levels (bounds), i.e. their voltage trajectories are determined by the arm currents. However, in other embodiments the IPT sub-system may be configured to continuously regulate the capacitor to a particular predetermined voltage rather than maintaining it within a predetermined voltage range.

In the present invention, each sub-converter or chopper module SM_qn may be inductively (loosely) coupled with the IPT supply track L_T. For each leg, a primary side converter Conv_q maintains the track current /> at a fixed value. In alternative embodiments of the invention, the three IPT tracks and primary IPT converters may be replaced by a single primary converter and track inductively coupled with all the chopper modules of the three-phase M2LC, or various other combinations of primary converters and tracks.

An example of a suitable primary IPT converter or power supply for use in supplying the primary IPT track of the present invention is shown in Fig. 3. The primary IPT converter comprises a DC voltage source V_in and a controlled rectifier (comprising switches S_p1 to S_p4 and associated diodes) to control the track current /> at a desired frequency f_T. The track frequency would typically be in the range of 10-40 kHz. The reactive power (var) requirements are minimized by using L_prC-rL_T arrangement as a resonant circuit, with a DC blocking capacitor connected in series with L_pi. The primary track forms the inductor L_T, and may comprise either a single long wire or a series connection of small inductors, each intended for inductive coupling with one of the plurality of chopper modules of the M2LC circuit.

The primary IPT converter (which can alternatively be considered a bi-directional converter, or an active or reversible rectifier) preferably comprises four switches S_p1 to S_p4 in an H-bridge configuration, as shown. A primary controller (not shown) coupled to the primary inverter switches S_p1 to S_p4 determines the switching instants of the inverter such that a constant AC current is maintained in the primary track inductor L_T. A phase modulated square wave voltage V_Pi is preferably generated by the converter to maintain the constant track current and the implemented control scheme is shown in Fig. 4.

A triangle wave generator at 20 kHz, or any other desired frequency of track current, determines the gating signals of the switch pairs S_p1/S_p2 and S_p3/S_P4, such that the switches in each pair operate complementarily (i.e. one conducts while the other does not). In the left leg of the H-bridge, the top switch S_p1 is turned on for half the track current period and then turned off. The switches in the right leg are switched relative to the left leg such that the error between the track current /> and the desired track current (represented by reference current ) is minimized. A proportional-integral (PI) controller is implemented to determine the phase delay e_dei_ay between the left and the right legs of the inverter.

Although not shown in Fig. 2, each of the three primary converters (or more, in other embodiments) may be electrically coupled to a common voltage source V_in and, since they are preferably bi-directional, may share power. That is, one primary converter receiving power from one or more pick-up circuits discharging a chopper capacitor may convert the alternating current to a direct current which may be supplied to another primary converter to at least partially supply power to a pick-up circuit inductively coupled therewith and charging another chopper capacitor simultaneously. The secondary side of the IPT sub-system comprises a plurality of pick-up circuits, each electrically coupled to a chopper module of the M2LC circuit as shown in Fig. 5(a). Each pick-up circuit comprises at least a secondary/pick-up coil or inductor L_siiqn which, in use, is magnetically coupled through mutual inductance M_qn to the primary track inductor L_T. The secondary pick-up preferably forms an LCL resonant tank circuit with capacitor C_s/j(?n and inductor L_so,qn to which a suitable DC blocking capacitor is connected in series, however any other secondary pick-up topology may alternatively be used without departing from the scope of the invention. The pick-up circuit need not necessarily be designed to resonate at the frequency of the current supplied to the primary conductor, although this will generally be preferred for efficiency reasons.

A secondary converter, preferably comprising a controlled reversible rectifier, is used to maintain the chopper module capacitor voltage within bounds, and is controlled accordingly by a secondary controller (not shown) coupled to switches S_{qnt S}i to S_{qni S}4. Power is thus exchanged between the primary side (i.e. the power supply and track, carrying sinusoidal current />), and the secondary side (i.e. the pick-up circuits electrically coupled to a chopper module) of the IPT sub-system. The total number of switches required in each chopper module according to the illustrated example of Fig. 5(a) is six, compared to two in the prior art. However, the 4 switches of the preferred H-bridge secondary converter in each pick-up circuit can be replaced by two switches in a half-bridge configuration as shown in Fig. 5(b), for example. There is a trade off between the benefits of the four switch operation, such as a higher power transfer capability, and those of two-switch operation, such as the reduced component count and associated losses.

The secondary controller may be operated to control the direction and magnitude of power transfer by controlling the relative phase angle of the reversible rectifier with respect to the primary inverter and track current i_T as described in WO 2010/062198, for example. Each secondary controller is operated independently relative to the primary controller and each other, such that the chopper modules each receive, deliver, or do not exchange power with the IPT track. The "output" (i.e. the output when power flows from the primary to the secondary side) voltage V_so,_qn of the pick-up circuit preferably leads or lags the primary inverter voltage V_pi by 90°. A leading phase angle of +90°will result in the pick-up circuit receiving power from the primary, and the chopper module capacitor C_qn will be charged. Conversely, a lagging phase angle of -90° will result in the pick-up circuit de livering power, discharged from the chopper module capacitor C_qn, to the primary. The secondary side controller is preferably only operational when the chopper module capacitor voltage violates its bounds. Within the upper and lower threshold voltages, no power is transferred to the primary. The design of a suitable secondary controller for controlling operation of the pick-up circuit to maintain the capacitor voltage between the predefined upper and lower thresholds is within the capabilities of a person skilled in the art. The secondary controllers may be similar to the secondary controllers disclosed in WO 2010/062198, for example. Each controller may comprise analogue and/or digital components, and in particular may comprise a logic or computing device such as a microcontroller programmed to perform the required control method. More particularly, each secondary controller is preferably adapted or programmed to sense the energy in the associated energy storage element, by measuring the voltage across the chopper capacitor for example, and control operation of the switches S_qn,_Si to S_qn,s4 accordingly to selectively charge and/or discharge the capacitor via the inductive coupling to maintain the capacitor voltage within the predefined range. Each secondary controller thus acts independently with respect to the main M2LC controller, the other secondary controllers, and the primary controllers of the M2LC system.

As explained above, control of chopper module capacitor voltage ripple in M2LC converters of the prior art becomes difficult at low output frequency operation. This inhibits the converter's applicability in variable speed drives, or variable frequency applications in general. Korn et al. discloses generation of common mode voltage to overcome this issue, but the common mode voltage could have negative effects on the motor bearings. Alternatively, the ripple on the capacitor voltage can be reduced by increasing the capacitor size, but this increases the energy requirements of the converter. Voltage ripple can also be minimized by injecting even order harmonics in the arm currents or in the circulating currents. However, this method increases the root mean square (RMS) magnitude of the arm currents or increases the losses in the converter and does not decrease the capacitor size at low output frequency operations. According to the present invention, however, the charging capacitors of one or more M2LC chopper modules can exchange their power with discharging capacitors of one or more other M2LC chopper modules through the IPT track or tracks. With this topology, even at low output frequency, the capacitor voltages can be easily controlled within predefined bounds. Even with small chopper module capacitors, the capacitor voltage ripple can be controlled by exchanging power between arms and/or legs of the M2LC system, via the conductive track or tracks. At start-up, the low power IPT sub-system charges the chopper module capacitors C_qn in a controllable manner using the inherent characteristics of the IPT system. As soon as the capacitors are charged to a desired voltage level (i.e. at least the lower threshold voltage), the IPT sub-system stops providing power. An M2LC controller (not shown) associated with the chopper module switches S_q„_iU and S_q„_tL monitors the status of the chopper module capacitor and if the capacitors are charged to an adequate level then the M2LC converter is made operational. The same procedure can also be applied to start M2LC when it is connected to a grid experiencing a power outage. A battery backup may therefore be provided for the control logic and switch drivers in case of an emergency.

The IPT sub-system may also be used to increase the reliability of the converter system. For example, if the DC link is unavailable it will not be possible for a standard M2LC converter of the prior art to supply power to the load without clearing the fault in the DC link and restarting the complete system. Each chopper module of a high-voltage and high-power M2LC converter provides a fraction of the converter's rated power, provided that there are sufficient number of modules. It is therefore possible to design a system where relatively low-powered IPT sub-systems provide the required power to the individual M2LC chopper modules without a DC link connection, or to provide a redundant backup to the DC link. The IPT sub-systems can thus continue feeding the chopper module capacitors for supplying power to the load, even without the DC link connection. A low-voltage low-power input can be connected to the primary IPT converters to achieve high-voltage high-power output at the M2LC side. Once the fault in the DC link is cleared, the IPT controllers can return to simply balancing the capacitor voltages. It is also possible to design an M2LC which is powered purely by the IPT sub-systems. In this case, the input power to the M2LC is supplied entirely by a plurality of primary IPT converters, each of which supplies power contactlessly to individual legs, arms, or other groups of chopper modules; any combination of legs, arms, or chopper modules; or any individual chopper modules of the M2LC topology, as shown in Figs. 17-20 for example. The number of IPT sub-systems to be employed depends largely on the ratings, availability and power handling capability of the switching devices. The low power primary IPT converters can be supplied with power from the utility grid or any other energy sources, and are preferably bi-directional so that power can also be supplied to the utility grid or other energy source. An M2LC system according to the present invention has a reduced capacitor size requirement compared to an M2LC system of the prior art with the same capacitor voltage ripple. Smaller voltage variations of the chopper module capacitors C_qn, which are connected in parallel to the DC link, means that the circulating or arm currents are reduced. This improves the current de-rating as needed in the converter of the prior art, and reduces the conduction losses.

Referring again to Fig. 5(a), the converter switches S_qn,_Si, S_qn,s2, S_qr,_{i S}3 and S_qn,s4 are rated to the same voltage level as the main chopper module switches (S_{qn U} and S_qn,L) due to the common module capacitor C_qn. However, the current requirements on the pick-up circuit to balance the capacitor voltage are smaller, and the converter switches S_qP:Si to S_qn:S4 may therefore have lower current ratings. On the other hand, improper choice of the operational quality factor Q of the pickup circuit will generate circulating currents in the L_SOtqn-C_3iiqn-L_siiqn resonant circuit and will demand switches with higher current requirements. However, this is a design issue which can be easily addressed with appropriate selection of the operational Q.

The DC link of the M2LC system can be connected to the utility network or electricity "grid" through a six or twelve pulse diode rectifier, and the split chopper module capacitors thus assume the DC voltage across the DC link. The M2LC controller of the prior art, which drives the M2LC switches, should therefore also act to maintain a constant DC link voltage. According to the present invention, the main controller is dedicated to modulating or driving the M2LC switches, and the plurality of secondary IPT controllers in effect comprise a distributed balancing controller which ensures that capacitor voltages remain within the specified bounds, independent of the M2LC controller. This simplifies the control process so that the main M2LC controller is dedicated to meeting the load objectives. The DC source of the primary IPT converter(s) can also be connected to the grid using a low power rectifier, which is preferably reversible to enable bi-directional power flow. Similarly, DC link voltage control of two back-to-back connected M2LCs (HVDC transmission) can be easily controlled by the IPT sub-system. In such a case, control of the split (chopper module) capacitor voltages implicitly controls the DC link voltage within a tight voltage band.

The IPT sub-system is galvanically isolated from the M2LC chopper modules and does not affect the modularity of the M2LC. Additional modules can be easily added to the existing setup and the faulty modules can be bypassed without affecting either the converter or the IPT sub-system.

To further illustrate operation of the M2LC system of the present invention, simulation results are presented below and in the accompanying drawings. The system is simulated with both a single-phase (using two different configurations) and a three phase load setup. In both cases, input to the converter is connected to a six pulse diode rectifier, where L_s is the supply line inductance. A six pulse rectifier is used instead of a 12 pulse rectifier (which requires a phase shifting transformer) because of its lower weight and footprint requirements. A pulse width modulation (PWM) based control scheme is used to drive the M2LC switches. For a three level converter, a sinusoidal modulating signal is compared against two carrier waveforms (at 1050 Hz) in phase disposition, to generate pulse patterns. In case of the IPT powered M2LC concept, the switching pattern of the chopper module does not depend on the voltage status of the capacitor voltages, which in fact is one of the key advantages as it significantly reduces the computation burden and control complexity. The IPT-based sub-system operates independently of the M2LC and keeps the capacitor voltages within a 6 V band or range about the optimal voltage. The IPT sub-system only acts to control a capacitor voltage when it violates its bounds.

For the purpose of comparison, simulation results for an M2LC topology of the prior art are also presented below and in the drawings, in which the capacitor voltages are balanced by the selection process as disclosed by Lesnicar et al. This process is based on the polarity of the arm current, such that for a charging current the capacitors with lowest voltage are selected first and conversely, capacitors with highest voltages are selected for a discharging arm current. Since arm current is a measure of both switching and conduction losses in the converter, it is used as a performance indicator for comparing the two topologies.

Figure 6 is a circuit diagram of a first configuration of a single phase M2LC system according to the present invention, with a single leg of two arms each comprising a pair of chopper modules, as used in the simulations. The four chopper modules are each inductively coupled with a primary IPT converter supplying a single primary conductive path or track. A proportional-resonant controller is implemented to control the load current. The circuit parameters for the M2LC and IPT sub-systems are shown in Tables 1 and 2, below.

M2LC system

Output frequency 50 Hz IPT syst em

Supply voltage ¾ ¾oy ^' acks freajueney h 20 kHz ^'

Load current 12,5 A Primary voltage Vin ^: 00 V

Capacitance ^'Cqn 3 mF Track current IT 42 k

Load resistance Ri 5 Ω ' Track capacitance CT

Arm resistance R 100 mil Secondary capacitance 4.3 /iF

Load inductance Li 5.2 mH Track inductance ί"χ

Arm inductance ■L- l mH Secondary inductance ¾si,_¾n Μ.74./ΪΗ

Supply inductance 0.5 rnH Mutual inductance

de-link capacitance 20 mF

Table 1 Table 2

Simulated waveforms of the chopper module capacitor voltages V_C:qn of both the conventional M2LC converter of the prior art and the M2LC system of the present invention are shown in Fig. 7 (a) and (b), respectively. The capacitor voltages in the M2LC system of the present invention are controlled within a 6 V band around the optimal voltage of 100 V, while it can be seen that the capacitor voltage of the prior art M2LC system ranges between approximately 90 and 110 V - a 20 V band. Achieving the same capacitor voltage ripple using the standard topology of the prior art would require double the capacitance per module. In other words, the capacitor size can be reduced by 50% in the present invention for a given capacitor voltage ripple.

Simulated waveforms of the arm currents at steady-state, when the chopper module capacitor voltages V_C:qn are controlled within a 6 V peak-peak range in the present invention, are shown in Fig. 8 for both the M2LC system of (a) the prior art, and (b) the present invention, respectively. The second harmonic in the arm currents is significantly reduced in the topology of the present invention. The RMS value of arm currents is decreased by 36% relative to the prior art.

Figure 9 is a circuit diagram of a second configuration of a single phase M2LC system according to the present invention, simulated to illustrate the operation and advantages of the present invention. In this embodiment, the system comprises two legs for a total of four arms, each arm comprising two chopper modules inductively coupled with a primary IPT converter supplying a single primary conductive path. The load current is controlled with a proportional-resonant controller, which generate two out of phase modulating waveforms to be compared against carrier waveforms. The outcome of this out of phase modulating waveforms is the generation of a five-level voltage waveform between nodes A and B. The circuit parameters of both the M2LC and IPT sub-systems are shown in Tables 3 and 4, below.

M2I system

IPT system

Output frequency 50 Hz

Track frequency

Supply voltage /T 20 kHz

¾ 200 V

Primary voltage v_m 800 V

Load current

Track current h 42 A

Capacitance 3mE

Track capacitance Or 537.02 nF

L oad resistance R_t Π

Secoiidaiy capacitance 4.3//F

Arm resistance % 100 mil

Track inductance Lf 117.92 /^11 toad inductance Li 5.2 rnH

Secoiidaiy ductarice 14.7-1 μΗ

Arm inductance 1 mH

Mutual inductance .42//H

Supply inductance 0,5 mil

Table 3 Table 4 This configuration of the single-phase M2LC does not require DC link capacitors C_DC, which reduces neutral point stability issues and the likelihood of developing large surge currents in case of a DC link short circuit. Moreover, with this configuration the M2LC can supply twice the output voltage than the configuration of Fig. 6, with M2LC chopper modules having the same power ratings. Alternatively, or additionally, at higher power levels the same output power requires switches with merely half the current ratings in this configuration. In addition, the five-level output voltage results in a significant reduction in total harmonic distortion (THD) of the output voltage. These advantages come at the expense of a higher component count. Also, the energy storage demand of the converter is higher due to the additional modules. The simulated waveforms of the chopper module capacitor voltages V_c,qn are shown in Fig. 10, where it can again be seen in Fig. 10(b) that the capacitor voltages are controlled to remain within a 6 V peak-peak band in the present invention, while the capacitor voltages in the equivalent prior art M2LC system vary between approximately 92 and 108 V - a 16 V band. The simulated waveforms of the arm currents at steady-state are shown in Fig. 1 for M2LC systems of (a) the prior art, and (b) the present invention with IPT voltage balancing. With the prior art topology, the second harmonic in the arm currents is significant. However, it can be seen from Fig. 11 (b) that the second harmonic is substantially reduced in the current waveforms of the present invention. The RMS value of the arm current decreases from 13.33A in the prior art M2LC system to 12.55A in the present invention, i.e. 5.64 % decrease, by limiting the peak-peak voltage ripple to 6V. Figure 12 shows simulated waveforms for the supply current i_s according to (a) the prior art, and (b) the present invention as shown in Fig. 9. It can be seen that the source current i_s has less harmonic distortion in the present invention, which improves the voltage quality at the point of common coupling (at utility supply).

As described above, the ripple on the capacitor voltages is inversely proportional to the frequency of the load current. This limits the use of the prior art M2LC systems for variable speed drives. However, it is possible to limit the peak of the capacitor voltages by using the IPT based technology of the present invention.

Figure 13 is a circuit diagram of a three phase embodiment of an M2LC system also simulated to further illustrate the present invention. In this embodiment, the system comprises three legs of two arms each, with two chopper modules in each arm for a total of twelve modules. A vector control (VC) scheme with a pulse width modulator (PWM) is used to drive the M2LC switches. In this scheme, two orthogonal current control loops are used; a current reference i_d,ref of one for the d axis, and a current reference i_vef of zero for the q axis. These d and q axis signals are then transformed to the abc domain and compared against carrier-based PWM with phase disposition (PD) to generate the gating signals for the chopper modules. In case of the IPT based M2LC of the present invention, the M2LC switching pattern does not consider the voltage status of the capacitor voltages. The IPT sub-system operates independently of the M2LC and keeps the capacitor voltages within a 6 V band (in this example) or range about the optimal voltage. The IPT sub-system only operates when a capacitor voltage violate its bounds. A single primary IPT converter and track is used for capacitor balancing in this embodiment, instead of an individual converter and track for each leg as shown in Fig. 2. The system is run to provide the rated power, 3.06 kVA, to a load again represented by series inductor L_t and resistor R,. The circuit parameters are shown in Tables 5 and 6, below. The M2LC chopper module capacitor voltage V_c,_qn waveforms are substantially similar to those of the previously described embodiments, but with different peak voltages as shown in Fig. 14. That is, the capacitor voltages vary within the band of 97-103V as opposed to 92-110V in the prior art M2LC system. M2LC system

IPT system

Output fre¾iiency 50 Hz

Track frequency Λ 20 kHz

Supply voltage 20by

Primary voltage Vin. 1200 V

Load curren 13.4 A

Track ciirrent ;i_T 42A

Capacitance 3rnF

Track capacitance 358 MF

Load resistance Ri S O

Secondary capacitance &si,i)n 4.3/iF

Ann resistance 11 100 ηιΩ

Track inductance 176.88 Η

Load inductance Li 5.2inH Secondar inductance 14.74 μΗ

Ami inductance L l mH Mutual inductance Mijn 4.42 μ.Η

Supply inductance L_s 0.5inH

Table 5 Table 6

The simulated waveforms of the arm currents at steady-state, when capacitor voltages are controlled within a 6V peak-peak in the present invention, are shown in Fig. 15 for both (a) the prior art, and (b) the present invention. The arm current is a measure of both switching and conduction losses in the converter and its RMS value is decreased by 18.08% in the present invention, relative to the RMS arm current of 10.62A according to the prior art.

It is to be appreciated that many variations to the circuits described above are possible without departing from the present invention. Several examples of such variations are briefly described below.

Figure 16 shows a variation of a single phase M2LC system according to the invention. Here, the IPT sub-system is employed to merely balance the capacitor voltages of chopper modules, as above. However, unlike the single-phase embodiment of Fig. 9, it can be seen that the system in this embodiment is provided with an independent primary IPT converter and IPT track for each leg of the M2LC circuit.

Figures 17 and 18 show two further example embodiments of single phase M2LC systems according to the invention, in which the systems are entirely powered by the IPT sub-systems. In Fig. 17, separate IPT sub-systems are provided to supply power to all four chopper modules in each leg. A different approach is presented in Fig. 18, where each arm has its own dedicated IPT sub-system to supply power to all the chopper modules of that arm.

Similarly, two further example embodiments of three phase M2LC systems according to the present invention are shown in Figs. 19 and 20. In each of these examples, the number of chopper modules that can be inductively coupled to each primary converter depends on the power transfer capability of the IPT sub-systems. Although the M2LC embodiments of the invention illustrated in the drawings and described herein preferably comprise bi-directional IPT sub-systems whereby each secondary IPT controller can selectively charge or discharge the associated chopper capacitor to balance the capacitor voltages, in other embodiments the IPT sub-systems may alternatively comprise uni-directional IPT sub-systems provided to solely charge or discharge the chopper capacitors. That is, the or each IPT sub-system would act only to maintain the capacitor voltage either above the lower threshold or below the upper threshold. In that context the "predefined range" which the IPT system acts to maintain the capacitor voltage within may be any voltage greater than 97V (for an optimal capacitor voltage of 100V), or any voltage less than 103V, for example. In such embodiments, additional circuitry may be provided to perform the other function of discharging or charging the capacitors, respectively, or the M2LC controller may be adapted to do so using the methods described by Lesnicar et a/., for example. Accordingly, at least some of the advantages of the present invention, such as simplification of the M2LC controller, may be obtained with uni-directional rather than bi-directional IPT sub-systems. Suitable uni-directional primary and secondary IPT converters are known to those skilled in the art.

From the foregoing it will be seen that an M2LC system and method is provided which permits direct control of ripple in the chopper module capacitors and alleviates at least some of the disadvantages of M2LC systems of the prior art. More specifically, the present invention enables:

i) control of the capacitor voltages with a minimum voltage ripple around its optimal voltage, and control of energy stored in the converter during both transient and steady-state operating conditions;

ii) operation at a low output frequency;

iii) reduced circulating currents and losses in the converter;

iv) reduced energy storage requirements in the M2LC; and

v) reduced control complexity by dividing switching control and capacitor voltage balancing.

As previously described, in addition to M2LC systems the invention may alternatively be applied in any other system requiring the balancing of energy or voltages between a plurality of energy storage elements. For example, Fig. 21 illustrates a multi-level converter and Fig. 22 illustrates a battery according to the present invention. As in the M2LC embodiments of the invention described above, in the operation of both multi-level converters and batteries it is important to balance the voltage across a plurality of energy storage elements (capacitors or battery cells, respectively). Operation of these systems is largely similar to the M2LC system described above, in that secondary controllers independently act to maintain the voltage across each energy storage element within a predefined range. The IPT sub-systems are preferably bi-directional so that energy may be shared between energy storage elements.

More specifically, Fig. 21 shows an example of a three-phase, three-level converter according to the present invention. Each of the capacitors C C₉ is electrically coupled to an IPT pick-up circuit (represented by the nine blocks labelled ΊΡΤ') which acts to maintain the voltage across the associated capacitor within a predefined range. The IPT pick-up circuit coupled to capacitor C₃ is shown in further detail in the inset of Fig. 21 , in which it can be seen that the chopper module of Fig. 5 is simply replaced by the capacitor of the multi-level converter.

As in the M2LC systems described above, each of the IPT pick-up circuits are inductively coupled with a primary converter and conductive path which are omitted from the drawing for clarity. The number of primary converters and conductive paths, and thus the number of IPT sub-systems, may be selected based upon on the power transfer capability of the IPT sub-systems. The primary converter may be identical to that shown in Fig. 3, for example.

Figure 22 shows a multi-cell battery system in which the IPT pick-up circuits are also represented by the blocks labelled ΊΡΤ', and the primary converters and conductive paths are omitted for clarity. The battery may comprise any number n of battery cells, each electrically coupled with a controllable pick-up circuit (represented by the blocks labelled ΊΡΤ') operable to selectively charge and/or discharge the associated battery cell via an inductive coupling (not shown, for clarity). The IPT sub-system may be used to independently balance the voltage of each battery cell during charging and/or discharging. The battery charging system controlling power supplied to the battery cells during charging may therefore comprise any known battery charging system, and the IPT sub-system will ensure that the voltages are balanced.

Although the invention is described substantially herein with respect to the topologies or circuit layouts of specific example M2LC, multi-level converter, and battery systems, the invention is not limited to those particular topologies and it will be further appreciated that the invention may alternatively be said to consist in a method for controlling such systems to balance energy across a plurality of energy storage elements without any direct electrical connections but through magnetic coupling and with galvanic isolation, the steps of which will be apparent to a person skilled in the art in view of the foregoing description of the system, and as set out at least in part in the Summary of Invention. In particular, the invention provides a method for controlling a modular multi-level converter (M2LC) system broadly comprising the steps of providing and/or operating at least one IPT sub-system to substantially balance the capacitor voltages of a plurality of chopper modules.

Unless the context clearly requires otherwise, throughout the description, the words "comprise", "comprising", and the like, are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense, that is to say, in the sense of "including, but not limited to".

Although this invention has been described by way of example and with reference to possible embodiments thereof, it is to be understood that modifications or improvements may be made thereto without departing from the scope of the invention. The invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, in any or all combinations of two or more of said parts, elements or features. Furthermore, where reference has been made to specific components or integers of the invention having known equivalents, then such equivalents are herein incorporated as if individually set forth.

Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.

Claims

1. An electrical system comprising a plurality of electrically coupleable energy storage elements operable to supply power to an output of the electrical system, and an inductive power transfer (IPT) sub-system independently operable to balance energy in the plurality of energy storage elements.

2. The electrical system of claim 1 , wherein the IPT sub-system comprises a conductive path and a plurality of pick-up circuits each electrically coupled to an energy storage element and capable of being inductively coupled with the conductive path, wherein each pick-up circuit comprises a pick-up controller operable to maintain a voltage across the associated energy storage element within a predefined range by selectively charging and/or discharging the energy storage element via the conductive path.

3. The electrical system of claim 2, wherein the IPT sub-system comprises a plurality of conductive paths and each IPT pick-up circuit is capable of being inductively coupled with one of said plurality of conductive paths.

4. The electrical system of claim 2 or claim 3 wherein the IPT sub-system further comprising an IPT converter electrically coupled to the or each conductive path and operable to maintain a substantially constant alternating current in the associated conductive path.

5. The electrical system of any one of claims 1 to 4, wherein the IPT sub-system is bi-directional and operable to selectively charge and discharge each of the plurality of energy storage elements.

6. The electrical system of any one of claims 1 to 5, wherein energy discharged from one energy storage element is at least partially used to charge another energy storage element.

7. The electrical system of any one of claims 1 to 6, wherein the electrical system comprises a modular multi-level converter (M2LC) system for converting a DC or AC input to an AC output, the M2LC system comprising a plurality of chopper modules each comprising an energy storage element in the form of a capacitor.

8. The electrical system of claim 7, further comprising a main controller operable to selectively couple the capacitors to the output to meet a load objective of the M2LC system.

9. The electrical system of claim 7 or claim 8, wherein the chopper modules are arranged in two arms of at least one leg, and the DC or AC input to the M2LC system is provided by a DC or AC power supply connected in parallel to the one or more legs of the converter.

10. The electrical system of claim 7 or claim 8, wherein a DC input to the M2LC system is provided by the IPT sub-system.

11. The electrical system of any one of claims 7 to 10, wherein each chopper module further comprises a pair of switches forming a chopper circuit with the capacitor.

12. The electrical system of any one of claims 1 to 6, wherein the electrical system comprises a multi-level converter and the plurality of energy storage elements each comprise a capacitor.

13. The electrical system of any one of claims 1 to 6, wherein the electrical system comprises a multi-cell battery and the plurality of energy storage elements each comprise a battery cell.

14. A method for balancing energy in an electrical system comprising a plurality of coupleable energy storage elements operable to supply power to an output of the electrical system, comprising the steps of:

sensing the energy stored by each energy storage element; and

15. The method of claim 14, wherein the steps of sensing energy and selectively charging and/or discharging each energy storage element are performed independently from a method for controlling the output of the electrical system by selectively coupling the energy storage elements to the output to meet a load objective.

16. A modular multi-level converter (M2LC) system comprising: a converter circuit for converting a DC or AC input to an AC output, the converter circuit comprising a plurality of chopper modules each comprising a capacitor, wherein said chopper modules are arranged in two arms of at least one leg; and

17. A modular multi-level converter (M2LC) chopper module comprising:

a resonant tank comprising an inductive pick-up coil;

a secondary converter coupled to the resonant tank;

a capacitor coupled to the secondary converter; and

a chopper circuit coupled to the capacitor and a pair of terminals.

18. A method for controlling a modular multi-level converter (M2LC) system comprising a plurality of chopper modules each comprising a capacitor, wherein said chopper modules are arranged in two arms of at least one leg, the method comprising the steps of: