Modular multilevel converter

Abstract

An electrical power converter consists of a plurality of power converter modules 300. A power converter module consists of a DC energy source 303, a first bidirectional switch 302 electrically connect

Description

TECHNICAL FIELD The subject matter described herein relates generally to module-based energy systems and modules for use therein, and systems, devices, and methods that facilitate the interconnection and control of modules in module-based energy systems. BACKGROUND The subject matter of this disclosure relates generally to power converters, and more particularly to power converter topologies based multiple DC energy sources that are coupled to one or more multi-phase electromagnetic energy conversion devices including without limitation, multiphase transformers with open windings and / or electric machines with open windings, to provide a regenerative or partial regenerative power converter. Energy systems having multiple energy sources or sinks are commonplace in many industries. One example is the automobile industry where high voltage (HV) battery packs are typically organized in a serial chain of lower voltage battery modules. Each such module is further comprised of a serially connected set of individual cells and a simple embedded battery management system to regulate basic cell related characteristics, such as state of charge and voltage. An advanced approach to converting the HV DC to the AC required to control an electric motor is to utilise the individual DC energy sources in a multilevel converter. Multilevel converters offer reduced motor and converter losses which in turn extend the mileage range of the vehicle. Power converter topologies associated with variable frequency drive (VFD) applications continue to receive attention. New power converter topologies are continually being introduced. Some known power converter cell topologies have employed H-bridge converters. DEFINITIONS DC: Direct current. AC: Alternating current. HV: High voltage. Generally, an AC or DC voltage above 60V. Switch: A device that can be controlled to change state, either conducting or isolating. Electrical switches such as IGBTs (Insulated Gate Bipolar Transistors). Other useful switches include GTOs (Gate Turn Off Thyristors), IGCTs (Integrated Gate Commutated Thyristors), lEGTs (Injection Enhanced Gate Transistors) and MOSFETs (Metal Oxide Semiconductor Field Effect Transistors). The standard switches would be paired with an anti-parallel freewheeling diode to accommodate, for example, inductive motor load currents. A switch may be implemented using multiple semiconductor die. Switch rating: The voltage, current and power dissipation limits of the switch, in terms of continues operation, peak operation (short term duration) and overload operation (during failure). Bidirectional switch: Can control conduction in either direction of current. This is different from a standard switch which only controls condition in one direction (of current) due to the inclusion of an anti-parallel diode which always provides a condition bath in the other current direction. Devices: Power semiconductor devices, sometimes called die. Converter: A power electronics topology that includes switches that provides a function such as converting DC to AC (or vice-versa). 2-level inverter: A power electronics topology that provides AC from 2 levels of DC, for example from the OV and positive DC from a battery. Multi-level inverter: A power electronics topology that provides AC from more than 2 levels of DC. The levels may be fed directly from a DC battery through multiple voltage terminals, or from multiple batteries. Bipolar operation: An AC wave produced via the PWM of switches where the resultant voltage spends time at +supply and zero. Unipolar operation: An AC wave produced via the PWM of switches where the resultant voltage spends time at +supply, zero and -supply. Smoother generated AC wave: A converter generated AC where the steps in voltage quantisation are smaller so that the harmonic spectrum of the generated AC has less power in the switching frequency harmonics. PWM: Pulse width modulation. Complementary PWM switching: This occurs when one switch is conducting while its complimentary switch is isolating and vice versa. An example would be the upper and the lower switches of one phase of a 2-level inverter. Note thata period of “deadtime” (when neither switch is conducting) is provided when the state of the switches needs to change. This is to avoid the possibility of both switches conducting at the same time which would effectively short the DC supply terminal together. Bidirectional power flow: Occurs when power can be both transferred from the DC source and transferred to the DC source. Note that this does not occur at the same time. Multiphase AC output: A number of AC phases that combine to form a balanced set. For example, three phase to power a three-phase motor. Controller: A controller is generally used for controlling each of the switches. The controller may comprise, for example, a computer and / or a digital signal processor. Variable frequency drive (VFD): Provides speed (or torque) control of a motor (or other type of AC load) by varying the AC frequency. Energy storage: The amount of energy being stored by an energy storage device or supply. Stress: The amount of stress experienced by a power circuit or device. This may be the result of a combination of items or a single item. Items include, but are not limited to, current, power, heat, vibration etc Lifetime remaining: The initial estimate of the functioning time available (or the full functionality) of a power electronic circuit or device minus the accumulated indication of wear / usage of that power electronic circuit or device. The wear indication is often based on the stress. Load: The sink for the energy transferred by the converter. Its requirements may be defined as power, current, voltage etc. Switching frequency: The frequency of the PWM switching period. Switching frequency harmonic: A multiple of the switching frequency. Switching transition: The transition between the conducting (ON) state and the isolating (OFF) state, or vice versa, of a switch. SUMMARY A first aspect provides an electrical power converter consisting of a plurality of power converter modules. Wherein a power converter module consisting of a DC energy source, a bidirectional switch electrically connected to the positive node of the DC energy source and an output node, a bidirectional switch electrically connected to the negative node of the DC energy source and a reference node, a positive voltage node electrically connected to the DC energy source and a negative voltage node electrically connected to the DC energy source. And further that the output nodes of all power converter modules are electrically connected together, the reference nodes of all power converter modules are electrically connected together, and the negative voltage node of a first power converter module is electrically connected to the positive voltage node of a second power converter. And further consist of a switch electrically connected between the reference node and the positive voltage node of the first power converter module, and a switch electrically connected between the output node and the negative voltage node of a third power converter module. A second aspect where the negative voltage of the second power converter module is electrically connected to the positive voltage node of the third power converter module. A third aspect where the positive voltage node of the third power converter module is electrically connected to a DC energy source, which is not part of the power converter module, and is also connected to the reference node via a switch. A fourth aspect where the negative voltage node of the second power converter module is electrically connected to a DC energy source, which is not part of the power converter module. A fifth aspect where a plurality of the electrical power converters are electrically connected via their reference nodes. A sixth aspect where the individual output nodes of the electrical power converters configured are used as a multiphase AC output. A seventh aspect where the switches within the electrical power converter are controlled to balance the multiple DC energy sources within in terms of storage level and / or temperature and / or load requirements. An eighth aspect where the switches within the electrical power converters are controlled to balance the multiple DC energy sources within in terms of storage level and / or temperature and / or load requirements. A nineth aspect where the method of control selects the power converter module whose reference node will be in the conductive state, and the required set of output nodes that switch to provide the required output voltage with respect to this selected reference node, in order to balance the power converter modules, within the electrical power converter, in respect of a metric including one or more of: energy storage and / or temperature and / or power and / or current and / or voltage and / or stress, and / or lifetime remaining. A tenth aspect where the method of control is optimised to reduce the power of the switching frequency harmonics through the selection of the power converter module whose reference node will be in the conductive state and the selection of output nodes that switch to provide the required output voltage with respect to this selected reference node. An eleventh aspect where the method of control guarantees a minimum distance in time between the switching transition of the electrical power converter and any other connected electrical power converters. A twelfth aspect provides a method for the design of the electrical power converter that optimises the number of power converter modules in terms of lowest number of parts, highest converter efficiency and lowest harmonic loss in the load. A thirteenth aspect of two of the electrical power converters where the most positive nodes of the DC energy sources are electrically connected, and the most negative nodes of the DC energy sources are electrically connected. A fourteenth aspect where the DC energy source within a first power converter module of the electrical power converter has a different DC voltage value to that of the DC energy source within a second power converter module within the same electrical power converter. A fifteenth aspect of provides a computer comprising a processor and a memory configured to implement a method, a computer program comprising instructions which, when executed by a computer comprising a processor and a memory, cause the computer to perform a method or a non-transient computer-readable storage medium comprising instructions which, when executed by a computer comprising a processor and a memory, cause the computer to perform a method. OBJECTS OF THE INVENTION Accordingly, a primary object of the present invention provides an electrical power converter consisting of a plurality of power converter modules. An additional object of the present invention provides a plurality of the electrical power converters which are electrically connected via their reference nodes and where the individual output nodes of the electrical power converters are used as a multiphase AC output. An additional object of the present invention provides two of the electrical power converters where the most positive nodes of the DC energy sources are electrically connected, and the most negative nodes of the DC energy sources are electrically connected. BRIEF DESCRIPTION OF THE DRAWINGS The details of the subject matter set forth herein, both as to its structure and operation, may be apparent by study of the accompanying figures, in which like reference numerals refer to like parts. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the subject matter. Moreover, all illustrations are intended to convey concepts, where relative sizes, shapes and other detailed attributes may be illustrated schematically rather than literally or precisely. Figure 1 schematically depicts prior art H bridge topology. Figure 2 schematically depicts prior art H bridge topology combined into a multilevel converter. Figure 3 schematically depicts the basic module of the modular converter. Figure 4 schematically depicts the basic module used in a modular converter. Figure 5 schematically depicts how output voltages may be created using the modular converter. Figure 6 schematically depicts how negative output voltages with respect to the reference may be created using the modular converter. Figure 7 schematically depicts how an AC voltage with respect to the reference can be produced. Figure 8 schematically depicts how an additional DC energy source can be added to the modular converter with the additional minimum number of switches while the system provides the additional multi-level in the voltage output. Figure 9 schematically depicts how an AC voltage with respect to the reference can be produced with an additional DC energy source. Figure 10 schematically depicts how the selection of activated switches may be used to balance the multiple DC energy sources. Figure 11 schematically depicts how the modular converters can be combined to produce a 3-phase voltage output. Figure 12 schematically depicts optimisation of the converter system design in terms of lower number of parts, higher converter efficiency and lower harmonic loss in the load. Figure 13 schematically depicts modular converters where the most positive and most negative nodes are connected. DETAILED DESCRIPTION PRIOR ART METHODS Turing to figure 1 with schematically depicts prior art H bridge topology. The H bridge is a well-known topology that enables a bipolarity output (102) with respect to the reference (103) to be created from a single DC energy source (100) using four switches (on shown as 101). Image 110 shows when two diagonally opposite switches conduct and the positive voltage with respect to the reference that results. Image 111 shows when two switches (both connected to the positive terminal of the DC energy source) conduct and the zero voltage with respect to the reference. Note that zero voltage also results if the two switches both connected to the negative terminal of the DC energy source conduct instead of those connected to the positive terminal of the DC energy source. Image 112 shows when the other two diagonally opposite switches conduct and the negative voltage with respect to the reference that results. In this way, the H bridge can produce three levels when controlled in unipolar operation. Turing to figure 2 with schematically depicts prior art H bridge topology combined into a multilevel converter. The H bridge topology of figure 1 is shown as 202 in figure 2 and then shown as simplified blocks (203 and alike). Three H bridges are shown in series per phase (Phase A, Phase B and Phase C). This topology can provide a multi-level converter with the associated benefit of a smoother generated AC wave when compared to a 2-level converter. Smoother relating to smaller steps in voltage resulting in lower harmonic content A disadvantage is that this topology requires four switches per H bridge module and thus 12 per phase in the example shown in figure 2. Another disadvantage is that the switches when in series are required to carry the full phase current Another disadvantage is that there is no access to the combined voltage of the three cells per phase. INVENTION Turning to figure 3 which schematically depicts the basic module of the modular converter. The module consists of two bidirectional switches (301 and 302) and a DC energy source (303). The bidirectional switches (301 and 302) could be a single device, or a set of single parallel devices. Alternatively, the bidirectional switches (301 and 302) could be made of two devices (304 and 305) with opposing anti-parallel diodes. A plurality of the devices 304 and 305 maybe connected in parallel. The DC energy source (303) could be an external power supply or an internal energy store, for example, a battery. The bidirectional switch (301) is connected to the lowest voltage level (negative polarity) node (309) of the DC energy source (303) of the DC energy source (303). The bidirectional switch (302) is connected to the highest voltage level (positive polarity) node (308) of the DC energy source (303) of the DC energy source (303). The bidirectional switch (301) is also connected to the reference node (307). The bidirectional switch (302) is also connected to the output node (306). Note that the output may have bidirectional power flow. Turning to figure 4 which schematically depicts the basic module used in a modular converter. The basic module (400) of figure 3 has its lowest voltage level (negative polarity) node connected to the highestvoltage level (positive polarity) node of an additional basic module (405). The basic module (405) has its lowest voltage level (negative polarity) node connected to the highest voltage level (positive polarity) node of an additional basic module (406). The connection of these nodes creates a highest total voltage node (403) and a lowest total voltage node (404). All of the basic modules (400,405 and 406) have their reference nodes connected together (401). All of the basic modules (400, 405 and 406) have their output nodes connected together (402). Turing to figure 5 which schematically depicts how output voltages may be created using the modular converter. Image 500 shows how voltage up to the sum of all three energy stores may be provided between the reference and the output. The arrows show the voltage potential difference between the reference and the output In image 500 the lowest (as drawn) basic module has its bidirectional switch connecting to the reference in the conducting state. The highest basic module has its bidirectional switch 7 connecting to the output in the conducting state. All other bidirectional switches are in the nonconducting state. Image 501 shows how voltage up to the sum of all two energy stores may be provided between the reference and the output. The arrows show the voltage potential difference between the reference and the output In image 501 the lowest (as drawn) basic module has its bidirectional switch connecting to the reference in the conducting state. The middle basic module has its bidirectional switch connecting to the output in the conducting state. All other bidirectional switches are in the nonconducting state. Image 502 is an alternative means of how voltage up to the sum of all two energy stores may be provided between the reference and the output The selection of where the voltage is to be provided as image 501 or 502 can be made based on state of charge of the associated basic modules. Alternately on the temperature or stress of the associated basic modules. Those skilled in the art will realise that there are many reasons to select one mode of operation over another when the operations may result in the same output. Image 503 shows how voltage up to the sum of all one energy stores may be provided between the reference and the output. The arrows show the voltage potential difference between the reference and the output In image 503 the lowest (as drawn) basic module has its bidirectional switch connecting to the reference in the conducting state. The lowest basic module has its bidirectional switch connecting to the output in the conducting state. All other bidirectional switches are in the non-conducting state. Images 504 and 505 are alternative means of how voltage up to the sum of all one energy stores may be provided between the reference and the output The selection of where the voltage is to be provided as image 503, 504 or 505 can be made based on the examples given before. Those skilled in the art will appreciate that intermediate voltages can be provided by complementary PWM switching of the bidirectional switches connecting to the output on two or more of the basic modules. Turning to figure 6 which schematically depicts how negative output voltages with respect to the reference may be created using the modular converter. Image 500 shows how negative voltage up to the sum of all two energy stores may be provided between the reference and the output The arrows show the voltage potential difference between the reference and the output In image 500 the basic module (602) has its bidirectional switch connecting to the reference in the conducting state. The basic module (604) has its bidirectional switch connecting to the output in the conducting state. All other bidirectional switches are in the non-conducting state. The voltage potential is shown by the arrows. The original structure of the stacked basic modules cannot provide the full negative voltage of the three energy stores without the addition of two further bidirectional switches. Image 601 shows the addition of two further bidirectional switches (605 and 606). Additional bidirectional switch 605 connects the highest voltage level (positive polarity) node (403 in figure 4) of the DC energy sources to the reference node. Additional bidirectional switch 605 connects the lowest voltage level (positive polarity) node (404 in figure 4) of the DC energy sources to the reference node. To create the largest negative voltage at the output with respect to the reference node, both the additional bidirectional switches (605 and 606) conduct. All other bidirectional switches are in the non-conducting state. The voltage potential is shown by the arrows. Those skilled in the art will appreciate that negative outputs with respect to the reference can be provided using PWM as described for the positive voltage case in connection with earlier figures. Turning to figure 7 which schematically depicts how an AC voltage with respect to the reference can be produced. The AC voltage wave is created using a multilevel approach and is separated into voltage ranges denoted RI through R6. The modular converter of figure 6 image 601 is used to create the entire AC wave when the AC wave has a peak of three energy sources. The six configurations used, in this example, are shown with the associated range denoted RI through R6. In each configuration, the bidirectional switch connected to the reference is shown by an arrow. The two bidirectional switches connected to the output that run with complementary PWM switching are those near the black box. Those skilled in the art will appreciate from the earlier figures that the ranges could be produced with alternative configurations of the modular converter of figure 6 image 601. The present invention provides a means that a multi-level voltage can be provided where the bidirectional switches connected to the reference are switched at a much lower rate than the bidirectional switches connected to the output while still providing a smooth output voltage. In the example of the production of an AC output wave, the bidirectional switches connected to the reference are switched at a multiple of the AC output wave frequency, for example 500Hz, while the bidirectional switches connected to the output are switched at a much higher frequency, for example 20kHz. Thus, the bidirectional switches connected to the reference can be of lower rating and cost to those connected to the output. Turning to figure 8 which schematically depicts how an additional DC energy source can be added to the modular converter with the additional minimum number of switches while the system provides the additional multi-level in the voltage output. The lowest voltage level (negative polarity) node (803) of basic module (603) has been disconnected from the highest voltage level (positive polarity) node (801) of basic module (604). The additional DC energy source (802) has been connected between nodes 803 and 801. An additional bidirectional switch (800) is connected between lowest voltage level (negative polarity) node (801) of the additional DC energy source (802) to the reference node. Those skilled in the art will appreciate that further levels can be added by simply adding additional switches like 800 and additional DC energy sources like 802. Adding additional DC energy stores reduces the size in voltage of the voltage quantisation from the PWM used to generate an AC output waveform. This reduction in quantisation greatly reduces the switching harmonic content of the output AC. The present invention permits the addition of an DC energy store with only the addition of one more bidirectional switch. This enables the designer to reduce switching harmonic content of the output AC for less cost Turning to figure 9 which schematically depicts how an AC voltage with respect to the reference can be produced with an additional DC energy source. The AC voltage wave is created using a multilevel approach and is separated into voltage ranges denoted RI through R8. The modular converter of figure 8 is used to create the entire AC wave when the AC wave has a peak of four energy sources. The eight configurations used, in this example, are shown with the associated range denoted RI through R8. In each configuration, the bidirectional switch connected to the reference is shown by an arrow. The two bidirectional switches connected to the output that run with complementary PWM switching are those near the black box. When the AC voltage is positive with respect to the reference the control strategy is to run complementary PWM switching on the bidirectional switches connected to the output associated with the topmost (in respect of DC source total positive protentional) basic modules and change the bidirectional switches connected to the reference on the basic modules to move between ranges 1 to 4. When the AC voltage is negative with respect to the reference the control strategy is to run complementary PWM switching on the bidirectional switches connected to the output associated with the lowest (in respect of DC source total positive protentional) basic module and the additional switch (606 in figure 8) and change the bidirectional switches connected to the reference on the basic modules and the additional switch (605 on figure 8) to move between ranges 5 to 8. The invention disclosed has provided an extra level using an additional DC energy source and only one additional bidirectional switch connected to the reference. As described referenced to earlier figures, this additional bidirectional switch connected to the reference runs at a lower switching frequency and so can be of lower rating and cost to those connected to the output. The difference multilevel ranges shown in figures 7 and 9 have different lengths in terms of time within a sinusoidal output voltage wave. The ranges with the greatest lengths (R3 and R6 in figure 7, and R4 and R8 in figure 9) utilise all the DC energy sources resulting in a good balance of loading of the DC energy sources. Those skilled in the art will appreciate that further balancing of the DC energy sources at lower peak output AC voltage waves is possible utilising the different switching configurations shown in figure 5. Alternative switching configurations exist or will be discovered that can be considered covered by the presently disclosed invention. Compared to the H bridge module-based converter of figure 2 which require 12 switches for a phase containing three DC energy sources, the invention only requires 6 switches. 10 Compared to the H bridge module-based converter of figure 2 where the switches in series are required to carry the full phase current, the invention only requires 2 switches in series to carry the full phase current This has the advantage of reducing the losses of the converter and reduces the rating and cost of the switches. Compared to the H bridge module-based converter of figure 2 where there was no access to the combined voltage of the three cells per phase, the invention always has access to the combined voltage of the three cells per phase. Turning to figure 10 which schematically depicts how the selection of activated switches may be used to balance the multiple DC energy sources. Images 1000,1001 and 1002 show how the same range of voltages, between the reference (1006) and the output (1007), can be provided using different DC energy sources. In image 1000, the refence is connected via module 1003 and the output is via the PWM switching of 1003 and 1004. In image 1001, the refence is connected via module 1004 and the output is via the PWM switching of 1004 and 1005. In image 1002, the refence is connected via module 1004 and the output is via the PWM switching of 1005 and the lower bidirectional output switch. The selection of which image would be employed to provide the voltage range could depend on the storage levels and / or temperatures and / or load requirements. Additionally, the selection could be based on switch or DC energy storage stress, and / or power and / or current and / or voltage and / or lifetime remaining. Those skilled in the art will be able to identify additional metrics by which the selection can be made while still aligning to the invention. Turning to figure 11 which schematically depicts how the modular converters can be combined to produce a 3-phase voltage output. Three modular converters (1101, 1102 and 1103) have their reference nodes electrically connected to form a common neutral node. Their individual output nodes become the three phase outputs (Phase one, Phase two and Phase three). Turning to figure 12 which schematically depicts optimisation of the converter system design in terms of lower number of parts, higher converter efficiency and lower harmonic loss in the load. Image 1200 shows three oftheprior art H bridge topology converters explained in figure 1. Image 1201 shows three of the modular converters of this invention. Both the converter (Images 1200 and 1201) can provide the same voltage output level and control, when the DC sources are comparable. The light line arrow in image 1200 shows a typical path for the full output current. It is required to pass through six standard switches. The light line arrow in image 1201 also shows a typical path for the full output current. It is required to pass through three bidirectional switches. Bidirectional switches can be created with the same loss as the standard switches, so the converter of 1200 has more greater losses. The control of switches requires independent interface circuits including signal isolation, power supplies and protection circuits. The converter of image 1200 requires the independent control of 12 switches whereas the converter of image 1201 requires the independent control of 6 switches. Figure 8 has already shown that the number of switches can be further reduced when there are four or more DC energy stores. This is not the case for H-bridge based converters. Turning to figure 13 which schematically depicts modular converters where the most positive and most negative nodes are connected. Three modular converters (1305,1306 and 1307) have been connected in a delta configuration and provide three phase nodes (1304, 1302 and 1303). Athree-phase motor (1301) has been connected to the three phase nodes (1304,1302 and 1303). The most positive DC node of modular converter 1306 have been connected to the most positive DC node of modular converter 1307 to create a Primary +DC node. Likewise, most negative DC node of modular converter 1306 have been connected to the most negative DC node of modular converter 1307 to create a Primary-DC node. These +DC and -DC nodes can be used to power HV DC items. Modular converter 1305 does notshare DC nodes and so creates a Secondary -DC and a Secondary +DC node. During charging, the Primary and Secondary -DC nodes can be connected while the Primary and Secondary +DC nodes are connected. This permits one DC charging supply to be used to charge all the modular converters. The use of two sets of floating DC supplies (Primary and Secondary) through this delta configuration has reduced the number of supplies by one. Those skilled in the art will appreciate that there are many other ways of connecting the most positive nodes and the most negative nodes that still align with this invention. The invention disclosed enables the level of voltage quantisation of the PWM wave seen on the output AC to be controlled to the lowestvalue which those skilled in the art will appreciate in turn reduces the switching harmonic losses within the load. The control method used to command the switching of the output switches may contain a minimum limit (in time) which is imposed between the switching of one AC output phase and the other AC output phases when the modular converters are combined to produce a 3-phase voltage output as schematically depicted in figure 11. This is in order to reduce the possibly of switching superposition which is known to contribute to higher electro-magnetic emissions. This disclosure is provided to reveal a preferred embodiment of the invention and a best mode for practicing the invention. Having thus described the invention in this way, it should be apparent that various different modifications can be made to the preferred embodiment without departing from the scope and spirit of this invention disclosure. When structures are identified as a means to perform a function, the identification is intended to include all structures which can perform the function specified. When structures of this invention are identified as being coupled together, such language should be interpreted broadly to include the structures being coupled directly together or coupled together through intervening structures. Such coupling could be permanent or temporary and either in a rigid fashion or in a fashion which allows pivoting, sliding 12 or other relative motion while still providing some form of attachment, unless specifically restricted.

Claims

1. An electrical power converter consisting of:a plurality of power converter modules wherein a power converter module consisting of: a DC energy source;a first bidirectional switch electrically connected to the positive connection of the DC energy source and an output node;a second bidirectional switch electrically connected to the negative connection of the DC energy source and a reference node;the output nodes of all power converter modules are electrically connected together; the reference nodes of all power converter modules are electrically connected together; the negative connection of the DC energy source of a number n power converter module is electrically connected to the positive connection of the DC energy source of a number n+1 power converter module.

2. The electrical power converter of claim 1 includes a bidirectional switch electrically connected between the reference node and the positive connection of the DC energy source of the number n power converter module and a bidirectional switch electrically connected between the output node and the negative connection of the DC energy source of the highest number converter module.

3. The electrical power converter of claim 1 is electrically connected to a DC energy source, which is not part of the power converter module, and is also connected to the reference node of claim 1 via a bidirectional switch.

4. The electrical power converter of claim 1 is electrically connected to a DC energy source, which is not part of the power converter module.

5. A plurality of the electrical power converters of claim 1 are electrically connected via their reference nodes.

6. The plurality of electrical power converter of claim 5 are used as a multiphase AC output7. The switches within the electrical power converter of claim 1 are controlled to balance the multiple DC energy sources within in terms of storage level and / or temperature and / or load requirements.

8. The plurality of electrical power converter of claim 5 are controlled to balance the multiple DC energy sources within in terms of storage level and / or temperature and / or load requirements.

9. A method of controlling the electrical convert of claim 1 where the method of control selects the power converter module whose second bidirectional switch will be in the conductive state, and the required set of first bidirectional switches that switch to provide the required output voltage with respect to this selected reference node, in order to balance the power converter modules, within the electrical power converter of claim 1, in25 06 25respect of a metric including one or more of: energy storage and / or temperature and / or power and / or current and / or voltage and / or stress, and / or lifetime remaining.

10. A method of controlling the electrical converter of claim 1 where the method of control is optimised to reduce the power of the switching frequency harmonics through the selection of the power converter module whose second bidirectional switch will be in the conductive state and the selection of first bidirectional switches that switch to provide the required output voltage with respect to this selected second bidirectional switch.

11. The electrical power converters of claim 1 where the method of control imposes a minimum limit in time between the switching transition of the electrical power converter and any other connected electrical power converter.

12. Two of the electrical power converters of claim 1 where the most positive connections of the DC energy sources of their respective number n power converters are electrically connected, and the most negative connections of the DC energy sources of their respective highest number converters are electrically connected.

13. The DC energy source within a first power converter module of the electrical power converter of claim 1 has a different DC voltage value to that of the DC energy source within another power converter module within the same electrical power converter of claim 1.

14. A computer comprising a processor and a memory configured to implement a method according to any of claims 7 to 12, a computer program comprising instructions which, when executed by a computer comprising a processor and a memory, cause the computer to perform a method according to any of claims 7 to 12 or a non-transient computer-readable storage medium comprising instructions which, when executed by a computer comprising a processor and a memory, cause the computer to perform a method according to any of claims 7 to 12.

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