Variable DC based AC generation

Abstract

An electrical power conversion system dynamically sets the DC voltage supplied to a multiphase inverter according to the difference between the two AC output demands that are most different from each

Description

TECHNICAL FIELD The present invention relates to an improved efficiency DC / AC electrical power conversion system. BACKGROUND High efficiency is needed for electric vehicles (EVs) to improve their range, reduce operating costs, conserve resources, and lessen the strain on the electric grid. More efficient vehicles use less energy per mile, which translates to a longer driving range from a smaller battery, lower charging costs, and a reduced need for critical minerals required for battery production. High efficiency also reduces the load on the electricity grid, especially during peak charging times. The primary benefit of high efficiency is extended range, which helps to alleviate "range anxiety". More Miles Per Charge: A more efficient EV can travel a longer distance on the same amount of stored energy (kWh). Smaller Batteries for the Same Range: Higher efficiency means manufacturers can achieve a target range with a smaller, lighter battery pack. This reduces the vehicle's overall weight, further enhancing efficiency. Better Performance: Effective energy use allows the powertrain components (motor, inverter, etc.) to deliver power more effectively. Efficiency translates into direct financial savings for both consumers and manufacturers. Lower Operating Costs: More efficient EVs use less electricity per mile, leading to lower charging costs over the vehicle's lifetime. Reduced Purchase Price: Since the battery is one of the most expensive components of an EV, using a smaller battery pack to achieve a competitive range can help lower the initial purchase price, making EVs more accessible to a wider market. Faster Charging: High efficiency also means getting more miles of range per minute of charging, as less energy is wasted as heat during the charging process. High efficiency is vital for maximizing the environmental advantages of electric mobility. Resource Conservation: By allowing the use of smaller batteries, higher efficiency reduces the demand for critical minerals like lithium, cobalt, and nickel that must be mined from the earth. Reduced Strain on the Grid: Higher efficiency lessens the overall demand on the electrical infrastructure, making it easier for the grid to handle a large influx of EVs and supporting a smoother transition to mass electrification. In short, maximum efficiency ensures that every watt-hour of electricity is used as effectively as possible, making EVs more practical, affordable, and sustainable. Improved efficiency DC / AC electrical power conversion systems apply the principle of combining two or more individual converter stages to achieve higher power output, improved efficiency, and better performance than a single, larger converter. This approach is used for both DC-DC (Direct Current to Direct Current) and DC-AC (Direct Current to Alternating Current) conversion. Unfortunately, most prior art designs that combine two or more individual converter stages are much higher cost and complexity than simple single stage 2-level inverters. Power semiconductor devices that can switch faster (have lower switching losses) are used in DC-DC converters to significantly increase efficiency and power density, allowing for smaller and lighter designs due to its faster switching speeds and lower conduction losses compared to silicon. However, silicon IBGTs are still the lowest cost solution for high power DC to AC inverters. There exists the need to combine the efficiency and power density benefits of fast switching power semiconductor devices while reducing the cost as a barrier to wider deployment DEFINITIONS Electrical power conversion system: Combining two or more individual converter stages. Supply connection: A supply rail, for example positive DC or zero volts. Primary DC supply: The main bulk source of DC electrical energy. Provided by, for example, a battery, a fuel cell, a supercapacitor, or the output of a rectifier when connected to an electric AC grid. The primary DC supply can provide more than two supply connections. Positive DC and negative DC: Positive is taken with reference to a reference level. The reference level might be another (lower) positive voltage potential or zero volts. In some cases, the reference level may be the negative DC. Connections to the same DC level may have different reference levels. AC to DC rectifier: A converter between AC to DC. Can be controlled or uncontrolled (using simple diodes for example). DC to DC converter (DCDC): These may use many different topologies, for example, half-bridge topology, a H-bridge topology, or a switched mode topology. Voltage level, voltage reference level: A voltage level with respect (with reference to) a voltage reference level. There will be a voltage potential seen between the voltage level and the voltage reference level. DC to AC converter (DCAC): Requires at least two DC supply connections and produces at least one AC output These may use many different topologies, for example, 2-level topology, T-type topology or half-bridge topology. Multiphase AC output demands: More than one AC output demand where the individual demands have different phase offsets from each other. An example would be a three-phase AC which is a set of sinusoidal waves each 120deg / 240deg electrically from each other. Sine PWM method: The Sine PWM (SPWM) method is a switching technique used in inverters to create an AC sine wave output from a DC input by comparing a high-frequency triangular carrier wave with a sinusoidal reference wave. The pulses generated have widths that change in proportion to the sine wave's amplitude, which is achieved by turning the inverter's switches on and off based on when the sine wave's voltage is higher or lower than the triangle wave. Space vector modulation (SVM) method: Space Vector Modulation is a digital control algorithm used to generate the pulse-width modulated (PWM) signals that control a three-phase inverter to drive electric motors. It synthesizes the required three-phase AC voltage by switching between a set of basic and zero voltage vectors, allowing for a more efficient use of the DC bus voltage compared to older methods like Sinusoidal PWM. SVM is a key component of motor control systems, especially in field-oriented control for induction and permanent magnet synchronous motors. Voltage vector magnitude and angle: A set of AC phases can be viewed on a 2D graph where the combined vectors of the phases can produce a resultant vector with a vector magnitude and angle. Modulation index: The ratio of the maximum magnitude of the output voltage (or resultant vector magnitude) to the DC supply. Region of overmodulation: The overmodulation region begins when the modulation index exceeds the linear range where a smooth sinusoidal output can be produced. This occurs when the reference voltage vector's magnitude is too large to fit within the hexagon defined by the linear SVM switching states, forcing the inverter into a non-linear mode of operation. Discontinuous PWM method: Discontinuous PWM (DPWM) is a modulation technique for power converters that improves efficiency by reducing switching losses. It works by keeping one of the converter's phase legs from switching for a portion of each cycle by clamping it to a DC voltage rail. This is achieved by injecting a zero-sequence (common offset) voltage into the reference signals, which causes one phase to be continuously switched "on" or "off while the other two are modulated. System: A system can include different function blocks each of which may be in the same or different products and / or enclosures. Multiphase electric motor: An electric motor that operates from a set of AC inputs. Flux maps: A set of tables or equations that define the optimal torque and flux currents, or voltages, given a range of operating parameters such as motor temperature and DC supply level. Torque and flux: Torque is the rotational force produced by an electric motor, which is a direct result of the interaction between the magnetic flux (the magnetic field lines) and the currentcarrying conductors in the motor's windings. Specifically, magnetic flux determines how effectively the motor can convert electrical energy into mechanical energy, and the torque is proportional to the product of both the magnetic flux and the current flowing through the conductors. Idq demands: The current demands expressed as direct (in phase) and quadrature components within a reference frame, for example the rotating field in an electric motor. Current loop and Voltage feedforward: In electric motor control, the current control loop uses feedback to ensure the actual motor current precisely matches the commanded current (which relates directly to torque / flux), while voltage feedforward enhances performance by proactively adding a voltage component to the controller output to counteract predictable disturbances, most notably the motor's back EMF. Bandwidth: In control systems, bandwidth refers to the range of frequencies over which the system can effectively respond to input changes, and it is directly related to the speed and stability of the system. A higher bandwidth results in a faster response but may lead to more noise and potential oscillations, while a lower bandwidth provides a more stable, slower, and cleaner output. Choosing the right bandwidth is a trade-off between a system's speed and its stability, which depends on the specific application's requirements. Power semiconductor: Different types such as (but not limited to) BJT, MOSFET, IGBT. And technologies, such as (but not limited to) silicon (Si), Silicon Carbide (SiC) and Gallium Nitride (GaN). While silicon (Si) has been the traditional material, Silicon Carbide (SiC) and Gallium Nitride (GaN), known as wide band-gap semiconductors, are gaining prominence. These newer materials offer superior performance, including higher operating temperatures, faster switching speeds, and significantly lower power losses, which leads to greater energy efficiency and smaller, lighter devices. Multilevel DC to AC topology: DC to AC converters / inverters require at least 2 supply connections (know as a 2-level topology). Where there are more than 2 supply connections, or where additional supply voltages (such as mid-voltage) are generated internal to the DC to AC converter / inverter topology, are often referred to as multi-level converters / inverters. Active period: The period where there is a voltage difference between at least two of the AC outputs in a multi-phase AC output. SUMMARY In a first embodiment of the disclosure there is provided an electrical power conversion system consisting of a first primary DC supply with a first positive DC input and a first negative DC input; A first DC to DC converter electrically connected to the first primary DC supply that generates a first voltage level with respect to a first voltage reference level; A first DC to AC converter electrically connected to the first voltage level and the first voltage reference level that generates a first multiphase AC output; A first set of multiphase AC output demands; Wherein the first multiphase AC output is controlled to be the first set of multiphase AC output demands; Wherein voltage between the first voltage level and the first voltage reference level is the difference between the two AC output demands that are most different from each other, within the first set of multiphase AC output demands; Wherein a phase within the first multiphase AC output generated by the first DC to AC converter is connected to the first voltage level; Wherein a phase within the first multiphase AC output generated by the first DC to AC converter is connected to the first voltage reference level; Wherein the first set of multiphase AC output demands can be expressed as a first voltage vector magnitude demand and a first angle demand. In a second embodiment of the disclosure there is provided a second DC to DC converter that generates a second voltage level and a second voltage reference level which is electrically connected to the first DC to AC converter. In a third embodiment of the disclosure the second voltage level is the AC output demand within the first set of multiphase AC output demands that is not either of the AC output demands that are the most different from each other. In a fourth embodiment of the disclosure the primary DC supply is a battery and / or a fuel cell and / or a supercapacitor and / or the output of a rectifier and / or the output of a third DC to DC converter. In a fifth embodiment of the disclosure the DC to AC converter is of a multilevel topology, for example a T-type topology. In a sixth embodiment of the disclosure the DC to AC converter is of the half-bridge topology. In a seventh embodiment of the disclosure the DC to DC converter is of the half-bridge topology and / or a H-bridge topology and / or a switched mode topology. In an eighth embodiment of the disclosure the AC output demands are based on the space vector modulation method. In a ninth embodiment of the disclosure the AC output demands include the region of overmodulation. In a tenth embodiment of the disclosure the AC output demands include periods at the first positive DC input and / or the first negative DC input. In an eleventh embodiment of the disclosure the DC to DC converter and the DC to AC converter are in different products and / or enclosures. In a twelfth embodiment of the disclosure a control loop controls which phase generated by the DC to AC converter is connected to the first voltage level and which phase generated by the DC to AC converter is connected to the first voltage reference level, in reaction to the first angle demand. In a thirteenth embodiment of the disclosure a control loop controls the first voltage level in reaction to the first voltage vector magnitude demand and the first angle demand. In a fourteenth embodiment of the disclosure a control loop controls the second voltage level in reaction to the first voltage vector magnitude demand and the first angle demand. In a fifteenth embodiment of the disclosure the AC output generated by the DC to AC converter is electrically connected to a multiphase electric motor. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described, by way of example only, and with reference to the accompanying figures: Figure 1 schematically depicts a half-bridge topology and a H-bridge topology. Figure 2 schematically depicts a switched mode topology. Figure 3 schematically depicts a T-type topology. Figure 4 schematically depicts a multiphase half-bridge topology. Figure 5 schematically depicts multiphase AC output demands and the corresponding voltage vector magnitude and angle. Figure 6 schematically depicts the difference between the two AC output demands that are most different from each other. Figure 7 schematically depicts the generation of a voltage level which is proportional to the ratio of the active period over the PWM period. Figure 8 schematically depicts the connection of phases to a variable voltage supply. Figure 9 schematically depicts the connection of phases to two variable voltage supplies. Figure 10 schematically depicts a region of overmodulation. Figure 11 schematically depicts a discontinuous PWM method. DETAILED DESCRIPTION Figure 1 schematically depicts a half-bridge topology and a H-bridge topology. Turning to image 100, which depicts a half-bridge topology. Switch SI is connected between the positive DC supply (101) and the output (103). Switch S2 is connected between the negative DC supply (102) and the output (103). The voltage level of the output is controlled by the ratio of the time that SI conducts and the time that S2 conducts. The voltage output is from zero to the range of the negative to positive DC supply. SI and S2 cannot conduct at the same time as this would cause a short circuit between the positive DC supply (101) and the negative DC supply (102). Turning to image 100, which depicts a H-bridge topology. This topology requires four switches and can be seen to resemble two half-bridge topologies and their outputs. SI and S2 provide the first half-bridge and switches S3 and S4 the second. The outputs 104 and 105 are used to supply the next stage in the overall system. The use of the two outputs enables the half-bridge topology to provide an output of minus the DC supply up to plus the DC supply, thus twice the supply range of a single half-bridge. Those skilled in the art will appreciate that many DC to AC topologies exist and that they align with the intent of the current invention. Figure 2 schematically depicts a switched mode topology. 201 represents the input DC supply and 202 the output load. The interface / driver, 207, controls the switch, 203, that periodically supplies power from the DC supply, 201, to the remainder of the circuit. Current builds up in the inductor, 204, and voltage across the output smoothing capacitor, 205. Current cannot immediately stop flowing in the inductor, 204, so when the switch, 203, is opened, current then flows through the diode, 206. The voltage at the load, 202, which is the voltage across the output smoothing capacitor, 205, can be controlled by the ratio of time that the switch, 203, is conducting and the time when it is not The period for this repeating ratio (the switching period) can be set and the size of the inductor, 204, and capacitor, 205, are determined to create the maximum level of voltage ripple required by the load, 202 given the loads power requirements and loading profile. Those skilled in the art will appreciate that many switched mode topologies exist and that they align with the intent of the current invention. Figure 3 schematically depicts a T-type topology. The mid voltage (304) is held between the positive DC (301) and negative DC (302) supply by capacitors Cl and C2. The mid-level voltage (304) may be generated and controlled internally or supplied externally. Switches S2 and S3 create a bidirectional switch as their antiparallel diodes are in opposing directions. This bidirectional switch is between the mid-level voltage (304) and the output (303). Switch SI is connected between the positive DC supply (301) and the output (303). Switch S4 is connected between the negative DC (302) supply and the output (303). Output voltages above the mid-level voltage are created by PWM between the bidirectional switch and SI. Output voltages below the mid-level voltage are created by PWM between the bidirectional switch and S4. The T-Type is one of the multi-level topologies. Those skilled in the art will appreciate that many multi-level topologies exist and that they align with the intent of the current invention. Figure 4 schematically depicts a multiphase half-bridge topology. The figure shows a three-phase power inverter for converting a DC power supply 401, 402 to an AC output 404 which may then be connected to a load (not shown). In this example, the inverter comprises three separate phases (also referred to as phases U, V, W respectively). Each phase includes two switches in series (as a half-bridge): 405, 408 in phase U; 406, 409 in phase V; and 407, 410 in phase W. Switches 405, 406 and 407 are connected to the positive rail 401 (and may be referred to as the "upper" switches) and switches 408,409 and 410 are connected to the negative rail 402 (and may be referred to as the "lower" switches). Each switch may be an IGBT (insulated gate bipolar transistor) and, for each IGBT, an associated anti-parallel diode may be used (not shown). However, switches with fast switching capability may be used, for example SiC or GaN. A control system (such as a processor) (not shown) controls the switching of the switches to control the AC output of the inverter 404. The power inverter also includes a DC bus capacitor 403, which provides a more stable DC voltage, limiting fluctuations as the inverter sporadically demands heavy current A sinusoidal output current can be created at AC output 404 by a combination of switching states of the six switches. However, the inverter must be controlled so that the two switches in the same phase are never switched on at the same time, so that the DC supply is not short circuited. Thus, if 405 is on, 408 must be off and vice versa; if 406 is on, 409 must be off and vice versa; and if 40 7 is on, 410 must be off and vice versa. The present invention can operate without the DC bus capacitor (403) or with one with very low value (for example 20uF). This enables the DC supply to change rapidly without having to charge / discharge a large capacitance. Those skilled in the art will appreciate that many multiphase DC to AC topologies exist and that they align with the intent of the current invention. Figure 5 schematically depicts multiphase AC output demands and the corresponding voltage vector magnitude and angle. Turning to image 500 which shows a set of three phase output voltages where U is 503, V is 504 and W is 505. SVM produces the "double humped” wave seen. Turning to image 501 which shows the resultant phase to phase (line to line) voltages. UV is 506, VW is 507 and WU is 508. Note thatthese are smooth sinusoids now without the "double hump”. Turning to image 502 which shows a 2D vector graph of the output phases at the time shown by the line 509 in image 500. Image 502 shows the 3 phases (U, V and W) plotted as axes 120deg apart. The corresponding U, V and W output voltages are shown as vectors on those axes. 510 is the vector for U, 511 the vector for W (not that the voltage on W is negative at line 509 and so is the direction of the corresponding vector). 512 for V. The vector sum is shown graphically, and the resultant voltage vector magnitude is 513. In this example it is aligned to the U axis, but if it was not then there would be a vector angle between the resultant voltage vector and the U phase. Those skilled in the art will appreciate that many ways to resolve AC into a vector magnitude and vector angle and that they align with the intent of the current invention. Figure 6 schematically depicts the difference between the two AC output demands that are most different from each other. The three output phases U, V and W are shown as 603, 604 and 605 respectively. The positive extent of all the phase outputs is shown as 601 and the negative extent as 602. The difference between 601 and 602 is the difference between the two AC output demands that are most different from each other. Those skilled in the art will appreciate that the actual demand AC may not be a collection of smooth waves as shown and that other wave shapes align with the intent of the current invention. Figure 7 schematically depicts the generation of a voltage level which is proportional to the ratio of the active period over the PWM period. The three output phases U, V and W are shown as 703, 704 and 705 respectively. The positive extent of all the phase outputs is shown as 701 and the negative extent as 702. The difference between 701 and 702 is the difference between the two AC output demands that are most different from each other. This difference is referenced to the 0 volts and shown as 706 which is sometimes known as the rectified voltage. In the present invention, this is voltage that will be generated by a DCDC converter supplied from a DC supply and supplying a DC AC inverter. The voltage 706 can be determined, for example, as the difference in the voltage extents of all the output voltage demands. Alternatively, when the output voltage demands are determined using the space vector modulation method, the voltage 706 is the active period multiplied by the primary DC supply level. The active period can be calculated in many ways, for example, the output voltage modulation index multiplied by the cosine of (gamma - 30degree), where gamma is the angle through the current 60degree segment and there are six 60degree segments within the whole 360degree output wave period. Those skilled in the art will appreciate that there are many ways in which the voltage 706 can be calculated that all align with the intent of the current invention. Figure 8 schematically depicts the connection of phases to a variable voltage supply. 801 is the primary power supply, the primary source of electrical energy in this example. The primary supply is the positive (802) and negative (803) supply rails. These supply the DCDC converter (804) which in turn provides a voltage level between 805 and 806 that supplies the DCAC inverter (807). The DCAC inverter (807) produces a set of AC outputs (808) that supply a load (809). Those skilled in the art will appreciate that the AC outputs could number 1 or above (3 shown) while aligning to the intent of the present invention. Those skilled in the art will appreciate that the AC load could be anything, including, but not limited to, and electric motor, a heater, an AC grid. One example load is a multiphase electric motor with controlled torque and flux. The motor controller is based on the demand motor torque and flux and produces intermediate current demands and final output voltage demands. The output voltage demands are used to control the output voltage provided by the DCAC inverter (809) and the supply voltage (805, 806) required to provide the equivalent output vector magnitude. A control loop can be used to control the supply voltage (805, 806) which is voltage 706 in figure 7, for example. The level of the supply voltage (805, 806) may be based on the voltage vector magnitude and angle of the output voltage demands. Alternatively, it may be based on just the voltage vector magnitude. A control loop controls which output phase generated by the DCAC converter is connected to the supply 805 and which output phase is connected to the supply 806, in reaction to the angle of the output voltage demands. Those skilled in the art will appreciate that the switching frequency employed in the DCDC converter could be higher than the switching frequency employed in the DCAC inverter. In systems with fixed supply rails to the DCAC inverter, the switches within the DCAC inverter see the entire DC supply range and thus the switching losses are large. The use of a DCDC to provide the required voltage, rather than the whole supply range, means that at lower speeds and motor torque there will be less switching losses within the DCAC inverter. The DCDC introduced will have its own losses, but the selection of fast switching power semiconductor technologies and efficient topologies will reduce the other all system losses. The DCDC introduced can be fitted with a small amount of output filtering (filtering of 805 and 806) if the switching frequency of the DCDC is high (>100kHz). This will reduce the switching harmonic content seen through that DCAC inverter at the load. This reduction in switching harmonic content will reduce the losses in the load. The level of filtering required in this method is much less that would have been required if the filtering were directly on the AC outputs. Figure 9 schematically depicts the connection of phases to two variable voltage supplies. Turning to image 900 which shows the three output phases U, V and W and the positive and negative extent of all the phase outputs as in figure 7. It also shows the difference between these extents is referenced to the 0 volts and shown as 901 Turning to image 920, as in figure 8, the voltage 901 is generated by the first DCDC converter 904 supplied from a DC supply (903) and supplying a DCAC inverter (909) through supply rails 908 and 906. Additionally, a second DCDC converter (905), which is again supplied from a DC supply (903), provides an additional supply rail (907) to the DCAC inverter (908). The additional supply rail (907) is at a voltage defined as in 902 in image 900. This is the voltage of the output phase that is not at the extents of the voltages and thus not directly connected by the DCAC inverter (909) to either 908 (the voltage 901) or 906 (the zero-voltage reference). The additional supply rail (907) can be determined from the voltage of the output phase that is not at the extents of all the output voltages. Those skilled in the art will appreciate that there are many ways in which the voltage 907 can be calculated that all align with the intent of the current invention. Note that in this embodiment, the lowest output supply rails from the DCDC converters are electrically connected as supply rail 906. Those skilled in the art will appreciate that the second DCDC converter (905) may not be powered by the same DC supply (903) and that the lowest output supply rails from the DCDC converters may not be electrically connected and instead the DCAC inverter (908) receives two sets of two supply rails (one set from each DCDC converter). Alternatively, the second DCDC converter (905) could be power by the output of the first DCDC converter (904). This would reduce the switching losses in the second DCDC converter (905). In an embodiment, the DCAC inverter is of the T-Type topology. In this case, the positive DC supply (as shown in figure 3) is connected to 908 (in figure 9), the negative DC supply (as shown in figure 3) is connected to 906 (in figure 9), and the mid-level voltage (as shown in figure 3) is connected to 907. In this embodiment the DCAC inverter (909) may use the angle of the output voltage demand to select which ofthe three output phases is connected to 908, which is connected to 907, and which is connected to 906. In this way the DCAC inverter (909) does not require to use PWM to ratio the voltage levels and thus can employ low-cost slow switching power semiconductor switches. This is enhanced by the supply voltages to the DCAC inverter (909) being often lower than the full DC supply voltage (903) resulting in lower switching losses independent of the power semiconductor switch technology. This results in a requirement only for low conduction losses. Those skilled in the art will appreciate that the DCAC inverter (909), in this embodiment, is only being used to direct (steer) the output voltages. The DCDC converters (904, 905) can employ high switching frequencies and power semiconductors that have lower switching losses. Thus, they can also employ only light (low value and small size) output filtering. DCAC inverters where there are more than 2 supply connections, or where additional supply voltages (such as mid-voltage) are created internal to the DC to AC converter / inverter topology, are often referred to as multi-level converters / inverters. One or both of the DCDC converters can be fitted with a small amount of output filtering before the DCAC inverter. Those skilled in the art will appreciate that the method presented in the present invention is applicable to DCAC inverters that have 2, 3 and above levels (multi-level) irrespective of the internal DCAC topology. The present invention can be extended to provide those additional supply connections by increasing the number of DCDC converters. The outputs of those additional DCDC converters would be set so that the DCAC converter is selecting which phase to connect to which supply connection as per the intent of the present invention. One embodiment of the present invention uses high bandwidth DCDC controllers (for example with switching frequency of 1MHz) to produce, after light output filtering, a smooth sinusoidally based waveform which, after the DCAC inverter / converter, results in smooth sinusoidal voltage waves at the load terminals. Presenting smooth sinusoidal voltage waves to the load reduces harmonic loss within the load and thus increases the systems overall efficiency. Those skilled in the art will appreciate that there are alternative connection methods that align with the intent of the current invention. Figure 10 schematically depicts a region of overmodulation. The figure shows the voltage 1001 generated by the first DCDC (of figure 9) and the voltage 1002 is generated by the second DCDC (of figure 9). The positive extent of all the output waves is shown as 1004 and can be seen to be saturating at the maximum (500V in this example). This is due to the modulation method entering overmodulation. The overmodulation region begins when the modulation index, a ratio of the demanded voltage vector to the maximum achievable voltage, exceeds the linear range where a smooth sinusoidal output can be produced. This occurs when the reference voltage vector's magnitude is too large to fit within the hexagon defined by the linear SVM switching states, forcing the inverter into a non-linear mode of operation. During the period shown by 1003 the voltage 1001 saturates. Those skilled in the art will appreciate that all the examples given for determining the output voltages of both the DCDC converters of figure 9 are valid with DC supply level saturation. Figure 11 schematically depicts a discontinuous PWM method. Turning to image 1100 which shows the phase voltages with the additional of a common offset voltage that clamps one phase per 60 degrees of the output waveform. Those skilled in the art will appreciate that alternative clamping and common mode voltage inject methods are possible and that the load, for example a motor, will not see the common offset voltage as its terminals are only connected to the phases. The y axis is the phase voltage level, in this example between a negative de supply of -125Vdc and a positive de supply of 125Vdc. The x axis is electrical degrees of the output waveform (0 to 360). The U phase is shown as 1102, the V phase as 1103 and the W phase as 1104. The example shown is a balanced method where each phase follows the same pattern with the V shifted 120deg electrical from the U and the W phase shifted 240 degrees electrical from the U phase. Each phase spends l / 6th of the electrical output cycle clamped to the positive DC supply and 1 / 6th of the electrical output cycle clamped to the negative DC supply. This hard clamped (no slew) method may reduce the switching loss by the theoretical 33.3% due to one phase out of three being clamped during any part of the output wave. When a phase is clamped to either the positive or negative DC bus it can stop switching. In the case of clamping to positive DC bus, the upper switch of the phase is always on (pulse extension) and the lower switch is always off (pulse dropping). The phase switches stop transitioning (stop switching), so the switching loss reduces to zero for that phase. The other phases continue to switch so the effective reduction in switching loss is ~ 1 / 3. By clamping the identified phase AC output voltage (s) to either or both of the positive or negative supply rail voltages, the switch(es) associated with the identified phase output AC voltage(s) are cooled due to the reduction in switching loss. Turning to image 1101 which shows the resulting line to line voltage as seen by the load. The y axis is the phase voltage level, and the x axis is electrical degrees of the output waveform (0 to 360). The UW phase to phase voltage is shown as 1105, the VU phase to phase voltage as 1106 and the WV phase to phase voltage as 1107. The resulting phase to phase voltage is a smooth sinusoid (as demanded) as the effect of the common offset voltage has been cancelled. Note that those skilled in the art will appreciate that adding slew control (limiting the rate of change of the common offset voltage) reduces the harmonic content in the output AC voltage(s). Those skilled in the art will appreciate that system invention here described could be implemented with circuit blocks (or circuit parts / sub-assemblies) in different products and still align with the intent of the present invention. Individual function / converter / switching blocks could be hosted on different products (a DCDC converter in a battery management system (BMS) for example) or together in one product / enclosure. 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. ASPECTS OF THE DISCLOSURE Non-limiting aspects of the disclosure are set out in the following numbered clauses. 1. An electrical power conversion system consisting of: A first primary DC supply providing a first positive DC input and a first negative DC input; A first DC to DC converter electrically connected to the first primary DC supply that generates a first voltage level with respect to a first voltage reference level; A first DC to AC converter electrically connected to the first voltage level and the first voltage reference level that generates a first multiphase AC output; A first set of multiphase AC output demands; Wherein the first multiphase AC output is controlled to be the first set of multiphase AC output demands; Wherein the voltage between the first voltage level and the first voltage reference level is the difference between the two AC output demands that are most different from each other, within the first set of multiphase AC output demands; Wherein a phase within the first multiphase AC output generated by the first DC to AC converter is connected to the first voltage level; Wherein a phase within the first multiphase AC output generated by the first DC to AC converter is connected to the first voltage reference level; Wherein the firstsetof multiphase AC output demands can be expressed as a first voltage vector magnitude demand and a first angle demand. Wherein the first DC to AC converter requires two or more supply connections. 2. The power conversion system of claim 1 wherein there is a second DC to DC converter that generates a second voltage level with respect to second voltage reference level which are electrically connected to the first DC to AC converter. 3. The power conversion system of claim 2 wherein the second voltage level is an AC output demand within the first set of multiphase AC output demands that is not either of the AC output demands that are the most different from each other. 4. The power conversion system of claim 2 wherein the second DC to DC converter is electrically supplied by the first voltage level and the first voltage reference level. 5. The power conversion system of claim 2 wherein the second DC to DC converter is electrically supplied by a second primary DC supply. 6. The power conversion system of claim 1 wherein the firstvoltage reference level is connected to the first negative DC input. 7. The power conversion system of claim 3 wherein the second voltage reference level is connected to the first negative DC input. 8. The power conversion system of claim 1 wherein the primary DC supply is a battery and / or a fuel cell and / or a supercapacitor and / or the output of an AC to DC rectifier and / or the output of a third DC to DC converter. 9. The power conversion system of claim 1 wherein the switching frequency employed in the first DC to DC converter is higher than the switching frequency employed in the first DC to AC converter. 10. The power conversion system of claim 3 wherein the first DC to AC converter is of the multilevel topology. 11. The power conversion system of claim 3 wherein the first DC to AC converter is of the T-type topology. 12. The power conversion system of claim 1 wherein the first DC to AC converter is of the halfbridge topology. 13. The power conversion system of claim 1 wherein the first DC to DC converter is of the halfbridge topology and / or a H-bridge topology and / or a switched mode topology. 14. The power conversion system of claim 1 wherein the first voltage level is proportional to the active period. 15. The power conversion system of claim 1 wherein the first set of multiphase AC output demands are based on the space vector modulation method. 16. The power conversion system of claim 1 wherein the first set of multiphase AC output demands include the region of overmodulation. 17. The power conversion system of claim 1 wherein the first set of multiphase AC output demands include periods at the first positive DC input and / or the first negative DC input 18. The power conversion system of claim 1 wherein the first set of multiphase AC output demands are based on a discontinuous PWM method. 19. The power conversion system of claim 1 wherein the first set of multiphase AC output demands are based on a sine PWM method. 20. The power conversion system of claim 1 wherein the first DC to DC converter and the first DC to AC converter are in different products and / or enclosures. 21. The power conversion system of claim 1 wherein a first control loop controls the first voltage level in reaction to the first voltage vector magnitude demand. 22. The power conversion system of claim 1 wherein a second control loop controls which phase generated by the first DC to AC converter is connected to the first voltage level and which phase generated by the first DC to AC converter is connected to the first voltage reference level, in reaction to the first angle demand. 23. The power conversion system of claim 1 wherein a third control loop controls the first voltage level in reaction to the first voltage vector magnitude demand and the first angle demand. 24. The power conversion system of claim 2 wherein a fourth control loop controls the second voltage level in reaction to the first voltage vector magnitude demand and the first angle demand. 25. The power conversion system of claim 1 wherein the first multiphase AC output generated by the first DC to AC converter is electrically connected to a multiphase electric motor.

Claims

1. An electrical power conversion system comprising:a first primary DC supply providing a first positive DC input and a first negative DC input; a first DC to DC converter electrically connected to the first primary DC supply and configured to generate a first voltage level with respect to a first voltage reference level, the first voltage level and the first voltage reference level being maintained on a respective first supply rail and second supply rail;a first DC to AC converter electrically connected to the first supply rail and the second supply rail and configured to generate a first multiphase AC output;a first set of multiphase AC output demands, expressible as a first voltage vector magnitude demand and a first angle demand;wherein the first multiphase AC output is controlled to correspond to the first set of multiphase AC output demands;wherein the voltage between the first voltage level and the first voltage reference level is the difference between the instantaneous maximum and the instantaneous minimum of the phase-voltage demands within the first set of multiphase AC output demands;wherein, at any instant, one phase within the first multiphase AC output generated by the first DC to AC converter is connected to the first supply rail and a different phase within the first multiphase AC output is connected to the second supply rail;wherein a control loop controls which phase generated by the first DC to AC converter is connected to the first supply rail and which phase is connected to the second supply rail, in reaction to the first angle demand; andwherein the first DC to AC converter requires two or more supply connections.

2. The power conversion system of claim 1 wherein there is a second DC to DC converter that generates a second voltage level with respect to a second voltage reference level, the second voltage level and the second voltage reference level being maintained on a respective third supply rail and fourth supply rail electrically connected to the first DC to AC converter.

3. The power conversion system of claim 2 wherein the second voltage level corresponds to the phase-voltage demand within the first set of multiphase AC output demands that is neither the instantaneous maximum nor the instantaneous minimum of the phase-voltage demands.

4. The power conversion system of claim 2 wherein the second DC to DC converter is electrically supplied from the first supply rail and the second supply rail.

5. The power conversion system of claim 2 wherein the second DC to DC converter is electrically supplied by a second primary DC supply.

6. The power conversion system of claim 1 wherein the second supply rail is connected to the first negative DC input.

7. The power conversion system of claim 3 wherein the fourth supply rail is connected to the first negative DC input.

8. The power conversion system of claim 1 wherein the primary DC supply is a battery and / or a fuel cell and / or a supercapacitor and / or the output of an AC to DC rectifier and / or the output of a further DC to DC converter.

9. The power conversion system of claim 1 wherein the switching frequency employed in the first DC to DC converter is higher than the switching frequency employed in the first DC to AC converter.

10. The power conversion system of claim 3 wherein the first DC to AC converter is of the multilevel topology.

11. The power conversion system of claim 3 wherein the first DC to AC converter is of the T-type topology.

12. The power conversion system of claim 1 wherein the first DC to AC converter is of the halfbridge topology.

13. The power conversion system of claim 1 wherein the first DC to DC converter is of the halfbridge topology and / or a H-bridge topology and / or a switched mode topology.

14. The power conversion system of claim 1 wherein the first voltage level is proportional to a ratio of an active period to a pulse-width-modulation period, the active period being a period during which there is a voltage difference between at least two of the phases of the first multiphase AC output15. The power conversion system of claim 1 wherein the first set of multiphase AC output demands are based on the space vector modulation method.

16. The power conversion system of claim 15 wherein the first set of multiphase AC output demands include the region of overmodulation.

17. The power conversion system of claim 16 wherein the first set of multiphase AC output demands include periods at which a phase-voltage demand is limited to the first voltage level and / or the first voltage reference level.

18. The power conversion system of claim 1 wherein the first set of multiphase AC output demands are based on a discontinuous PWM method.

19. The power conversion system of claim 1 wherein the first set of multiphase AC output demands are based on a sine PWM method.

20. The power conversion system of claim 1 wherein the first DC to DC converter and the first DC to AC converter are in different products and / or enclosures.

21. The power conversion system of claim 1 wherein a control loop controls the difference between the first voltage level and the first voltage reference level in reaction to the first voltage vector magnitude demand.

22. The power conversion system of claim 1 wherein a further control loop controls the difference between the first voltage level and the first voltage reference level in reaction to the first voltage vector magnitude demand and the first angle demand.

23. The power conversion system of claim 2 wherein a control loop controls the second voltage level in reaction to the first voltage vector magnitude demand and the first angle demand.

24. The power conversion system of claim 1 wherein the first multiphase AC output generated by the first DC to AC converter is electrically connected to a multiphase electric motor.T +44(0)30 0300 2000A

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