GB2615813A

GB2615813A - Combined AC/DC power supply and associated methods and systems

Info

Publication number: GB2615813A
Application number: GB2202316.2A
Authority: GB
Inventors: Moller Chris
Original assignee: Open University
Current assignee: Open University
Priority date: 2022-02-21
Filing date: 2022-02-21
Publication date: 2023-08-23
Also published as: GB202202316D0

Abstract

The present disclosure provides systems for extracting, independently from each other, an AC output and a DC output from a combined AC+DC input, using an AC extraction unit and a DC extraction unit connected in parallel with each other. The AC extraction unit may comprise a supercapacitor and a voltage source to maintain a predetermined terminal voltage across the supercapacitor, and an auxiliary variable power source to maintain the terminal voltage at a value which corresponds to a DC voltage offset of the combined AC+DC input. The DC extraction unit may comprise one or more of a buck converter, boost converter or buck-boost converter.

Description

COMBINED AC/DC POWER SUPPLY AND ASSOCIATED METHODS AND SYSTEMS

TECHNICAL FIELD OF THE INVENTION

The present invention relates to methods and systems for generating combined AC and DC electrical power, and distributing it to households. In particular, the invention relates to systems and methods for independently extracting pure AC current and pure DC current from an AC+DC input, as well as new ways of supplying AC+DC power to households.

BACKGROUND TO THE INVENTION

With the advent of technology such as electric cars and heat pumps, there is a great need to increase the capacity of the final mile (i.e. the cables running from power substations to people's homes). The total load may need to double, but the current infrastructure is at capacity, and doubling the load would soon result in a damaged cable.

At present, supply of electrical to households is almost universally in the form of alternating current (AC), but these new loads, such as electrical vehicles, heat pumps, and batteries, would benefit from a supply of direct current (DC).

The present invention relates to the delivery and subsequent processing of electrical power using combined AC+DC, in a manner which does not necessitate digging up and replacement of existing cables, and which could revolutionize the supply of electricity to people's households.

SUMMARY OF THE INVENTION

Broadly speaking, the present invention relates to various methods and systems for delivering combined AC+DC power, and for processing the AC+DC power once it arrives at a given household. A first aspect of the invention relates to a system for extracting AC and DC outputs from a combined AC+DC input, using separate, independent AC and DC extraction units, so that it is possible either to extract a DC output or an AC output independently of each other, or to extract a DC output and an AC output simultaneously (but still using independent components or sets of components). Specifically, the first aspect of the invention provides a system for extracting an AC electrical output and/or a DC electrical output from a combined AC+DC supply having an AC voltage Vo and a DC offset voltage of VI, the system comprising: an AC+DC input receiving component configured to receive an AC+DC input from an external source; a DC extraction unit configured to receive the AC+DC input from the AC+DC input receiving component, and to generate a DC output; an AC extraction unit connected in parallel to the DC extraction unit, and configured to receive the AC+DC input from the AC+DC input receiving component, and to generate an AC output; wherein the DC extraction unit and the AC extraction unit and configured, respectively, to generate the DC output and the AC output independently of each other.

As will be shown later in this application, supplying power in the form of a combined AC+DC supply enables a greater power capacity of the wires which are used to convey the electrical power from e.g. an electrical substation to the households of users, the so-called "last mile" or "final mile". Systems according to the first aspect of the invention are advantageous because having separate AC extraction means and DC extraction means which are able to generate their respective outputs independently of each other means that end users can use AC-only loads or DC-only independently of each other by only using one of the extraction units at a given time. The arrangement provided is simplified relative to known arrangements for extracting AC and DC outputs from a combined AC+DC input, such as in Gai et al. (2019)1.

Herein, the term "AC+DC input" refers to an electrical power supply which has both AC and DC components. Such a signal may be regarded as an AC signal with a DC offset, or alternatively, a DC signal with added ripple. Both Gal, X.: Wang, Y.; Chen, R.; Zou, L. Research on HybrEd Microgrid Based on Simultaneous AC and DC Distribution Network and Its Power Router. Energies 20 I 9, (2, 1077. https://doi.org/10.3390/en I 204 1077 interpretations are equally valid. As discussed below, the AC component is preferably sinusoidal. In preferred cases, the voltage of the AC+DC input may be represented as: V(0 = V1 ± V0 sin(wt) This is a preferred form because AC power generated at power stations is generally sinusoidal (or can be represented well enough by a sinusoidal profile that this approximation is sufficient for any real-life calculations and applications). It should be noted that the use of a sine function is only one convenient representation which is useful for demonstrating the sinusoidal nature of the AC voltage. Other representations are equally valid, e.g. a V0cos((00, or V0exp(-jw0 or VbexpOwtY. Alternatively, any linear superpositions of these expressions are equally valid representations. Herein, the term "DC voltage" refers to V1 and the term "AC voltage" refers to Vo, i.e. the amplitude of the AC component of the total voltage V(0.

When transmitting, for example, a fixed amount of power, it is preferable to transmit at a higher voltage and a lower current, in order to stop the wires from overheating.

However, simply increasing the voltage of an AC supply is not a preferable approach, since most home appliances are designed to work at fixed (e.g. 230V) voltages. For typical home use, the value of VP is therefore preferably approximately 230 * I/2 = 325V. The value of VI is constrained at the lower limit by the value of Vo (realistically, it needs to be at least X130043) and at the upper limit by the maximum permissible voltage of the cable. In the worked examples set out later in this application, a value of 425V has been chosen.

The term "AC+DC input receiving component" is used herein to refer to any component at which or via which the AC+DC input may be received by the system. The AC+DC receiving component 2 Throughout this application, by convention j is used for the complex number defined as NC:i, in order to avoid confusion with i which is used to represent current.

may comprise, for example, a physical component such as a port in an outer housing of the system. For example, the port may be configured to receive an electrical cable, the electrical cable carrying the AC+DC input. More specifically, the port may be configured to receive a connector which is located at a distal end of such an electrical cable, the port and the connector including respective electrical contacts to enable conduction of the AC+DC input from the electrical cable to the system of the first aspect of the invention. The port may comprise a projection configured to engage with a recess in a connector at the distal end of the electrical cable, or alternatively, the port may comprise a recess configured to engage with a projection at the distal end of the electrical cable. The AC+DC input receiving component may take other forms, however. For example, in some cases, the electrical cable may connect directly to the AC extraction unit and the DC extraction unit, in which case a portion of those components may represent the AC+DC input receiving component.

We now discuss the nature of the AC extraction unit and DC extraction unit in more detail. These terms may refer to specific physical modules within the system, or may refer more generally to functional modules, i.e. arrangements of electrical components within the system which perform the required generation of AC or DC outputs.

Herein, "generating an AC output" may refer to a process in which the AC component of the AC+DC input is extracted or isolated. Alternatively, the process may viewed as suppression of the DC component of the AC+DC input. When the AC+DC input takes the form V(0 as defined earlier in this application, the AC output is preferably in the form V0sin(w0, i.e. the DC component (i.e. the DC offset) is removed. In order to remove the DC offset, the AC extraction unit may include a capacitance such as a capacitor or a supercapacitor. A capacitance is able to remove a DC offset since, when it is charged to the DC offset voltage, no additional DC current is able to flow through it. This may also be considered by considering the complex impedance arising as a result of a capacitance C which is DC current has an effective frequency of zero, which means that the complex impedance tends to infinity, preventing DC current flow.

In preferred cases, the AC extraction unit further comprises a voltage source such as a battery. The battery is preferably configured to maintain a terminal voltage of I71 across the capacitance. This ensures that the DC component is removed from the AC+DC input. In some cases, the DC offset may not remain constant (e.g. when there is a large DC power draw which might arise as a result of several households charging electric cars, or the like, which may cause a slump in the DC offset of the AC+DC input to a given system). In those cases, in order to ensure that the correct amount of DC offset voltage is "removed" from the signal, it may be necessary to vary the terminal voltage across the supercapacitor.

Accordingly, the AC extraction unit may further comprise an auxiliary variable power source configured to maintain the terminal voltage across the supercapacitor at a value which corresponds to the DC offset voltage of the AC+DC input in the event that it varies from the predetermined terminal voltage.

The variable power source may include a voltage measuring device, or equivalent component which is configured to monitor the DC component of the voltage of the AC+DC input. The variable power source may comprise a control component or controller which is configured to control the output voltage of the variable power source in response to the measured DC voltage, in order to ensure that the terminal voltage across the supercapacitor is equal or substantially equal to the DC offset voltage of the AC+DC input. The variable power source may comprise a four-quadrant isolated variable DC power supply.

The variable power supply may only be able to adjust the voltage across the supercapacitor at a finite rate. However, in some cases (including those which don't include a supercapacitor) , it is possible that the voltage may change much faster than the rate at which a variable power supply may be able to adjust the power. One example of such a case is at the start of a power cut. In such cases, it is possible that if there is a large (positive or negative) spike in the voltage and the variable power supply does not have adequate time to adjust the voltage across the supercapacitor to remove the resulting large spike in DC voltage, the AC loads will be subjected to a large spike in DC voltage, which they are not equipped to deal with. For example, the AC loads may experience a sudden 750V spike. More than likely, this would damage the AC loads, or blow the fuses in those loads. In order to combat this, in some cases, the AC extraction unit may comprise a first switch configured to disconnect the AC extraction unit from one or more connected AC loads in response to a spike in DC voltage exceeding a predetermined threshold. In preferred cases, the switch is a fast-acting switch such as a static switch, i.e. a switch which has no moving parts, such as a solid-state switch. Similarly preferably, the switch is a bidirectional switch (in order to disconnect an AC load), and may comprise two metal oxide semiconductor field-effect transistors (MOSFETs) or insulated-gate bipolar transistors (IGBTs) arranged back-to-back.

In addition to disconnecting the AC extraction unit from AC loads in the event of a spike in DC voltage, it is also desirable to ensure that the AC loads are not connected to the AC extraction unit until e.g. a supercapacitor is fully-charged, i.e. charged to the DC voltage of the AC+DC input (which may be V1 but may vary, as discussed). Again, this protects the AC loads from experiencing a large spike in DC voltage, which may be the case if the capacitance is unable to "block" the DC offset voltage from reaching the AC loads by virtue of not being fully charged. Accordingly, the AC extraction unit may further comprise a second switch configured to connect the AC extraction unit to one or more AC loads only in the event that a supercapacitor is fully-charged, i.e. is charged to a terminal voltage of the DC offset voltage of the AC+DC input. The second switch is preferably a fast-acting switch such as a static switch, and furthermore is preferably a bidirectional switch. In some cases, the first switch and the second switch may be the same switch, i.e. the function of both the switches may be performed by the same, single component.

In some cases, depending on the values of the capacitance employed in the AC extraction unit, it may take a long time for the capacitance to be fully charged by the battery and/or auxiliary variable power source. In order to avoid this delay, the AC extraction unit may further comprise an alternative current path via which the capacitance may be charged. The path may comprise a switch (e.g. a third switch), and may include a resistor.

The presence of the first switch and/or the second switch is further advantageous because it provides an important antiislanding function, ensuring that the AC loads do not receive any power when there is e.g. a power cut or other interruption in the electricity supply and any AC power sources do not feed into the supply.

Once the AC output has been generated by the AC extraction unit, the AC extraction unit may be configured to output the AC output to one or more connected AC loads or power sources.

Accordingly, the system may further comprise one or more AC output components which are configured to transmit the AC output towards one or more AC loads. The AC output components may take the same form as the AC+DC input receiving components. Specifically, the AC output component may comprise, for example, a physical component such as a port in an outer housing of the system. For example, the port may be configured to receive an electrical cable, the electrical cable configured to carry the AC output to one or more AC loads. More specifically, the port may be configured to receive a connector which is located at a distal end of such an electrical cable, the port and the connector including respective electrical contacts to enable conduction of the AC output from the electrical cable to the one or more connected loads. The port may comprise a projection configured to engage with a recess in a connector at the distal end of the electrical cable, or alternatively, the port may comprise a recess configured to engage with a projection at the distal end of the electrical cable. The AC output component may take other forms, however.

We now describe the DC extraction unit in more detail.

Analogously to the AC extraction unit, "generating a DC output" here may refer to a process in which the DC component of the AC/DC input is extracted or isolated. Alternatively, the process may involve smoothening out or flattening of the AC component of the AC+DC input. When the AC+DC input takes the form V(t) as defined earlier in this application, the DC output is preferably VI, i.e. the oscillating (AC) component is removed. In order to remove the oscillating component, the DC extraction unit may comprise an inductor, which acts to oppose any change in the current passing through it. Specifically and preferably, the DC extraction unit may comprise one or more of: a buck converter, a boost converter, or a buck-boost converter. These are components which are configured to switch a given DC voltage to a different DC voltage. The type of converter which may be used depends on the desired DC voltage for the DC load: If Vwact <V1-170, a standard buck switching regulator (i.e. a buck converter) may be used to generate the steady voltage required by the DC load.

If Vioad >VI-EV°, a standard boost regulator (i.e. a boost converter) may be used.

If -Vo < < + Vo, then a buck-boost regulator (i.e. a buck-boost converter) may be used, as described in more detail later in the application.

Once the DC output has been generated by the DC extraction unit, the DC extraction unit may be configured to output the DC output to one or more connected DC loads. Accordingly, the system may further comprise one or more DC output components which are configured to transmit the DC output towards one or more DC loads. The DC output components may take the same form as the AC+DC input receiving components. Specifically, the DC output component may comprise, for example, a physical component such as a port in an outer housing of the system. For example, the port may be configured to receive an electrical cable, the electrical cable configured to carry the DC output to one or more DC loads. More specifically, the port may be configured to receive a connector which is located at a distal end of such an electrical cable, the port and the connector including respective electrical contacts to enable conduction of the DC output from the electrical cable to the one or more connected loads. The port may comprise a projection configured to engage with a recess in a connector at the distal end of the electrical cable, or alternatively, the port may comprise a recess configured to engage with a projection at the distal end of the electrical cable. The DC output component may take other forms, however.

In the above description of the first aspect of the invention, and with particular reference to the AC extraction unit, the advantageous effects of an auxiliary variable power source, and of a fast-acting switched were explained. These features impart a technical advantage even in arrangements which do not, for example, require a DC extraction unit, and accordingly, the second and third aspects of the present invention protect AC extraction units comprising these features individually.

Specifically, a second aspect of the present invention provides an AC extraction unit comprising: an AC+DC input receiving component configured to receive an AC+DC input from an external source, the AC+DC input having an associated AC voltage with a DC offset voltage; a supercapacitor; and a variable auxiliary power source, wherein: the variable auxiliary power source is configured to maintain a terminal voltage across the supercapacitor that corresponds to the DC offset voltage of the AC+DC input; and the supercapacitor is configured to remove the DC offset voltage from the AC+DC input, thereby generating an AC output. It should be noted that the optional features set out with respect to the AC extraction unit which forms part of the system of the first aspect of the invention apply equally well to the AC extraction unit of the second aspect of the invention, and will not be repeated here, for conciseness.

A third aspect of the invention provides an AC extraction unit comprising: an AC+DC input receiving component configured to receive an AC+DO input from an external source, the AC+DC input having an associated AC voltage with a DC offset voltage; a supercapacitor at a terminal voltage corresponding to the DC offset voltage, and configured to remove the DC offset voltage from the AC+DC input, thereby generating an AC output; and a switch configured to disconnect the AC extraction unit from one or more connected AC loads in response to a spike in DC voltage exceeding a predetermined threshold. It should be noted that the optional features set out with respect to the AC extraction unit which forms part of the system of the first aspect of the invention apply equally well to the AC extraction unit of the third aspect of the invention, and will not be repeated here, for conciseness.

A fourth aspect of the invention combines the second and the third aspects of the invention, and provides an AC extraction unit comprising: an AC+DC input receiving component configured to receive an AC+DC input from an external source, the AC+DC input having an associated AC voltage with a DC offset voltage; a supercapacitor; a variable auxiliary power source; and a switch, wherein: the variable auxiliary power source is configured to maintain a terminal voltage across the supercapacitor that corresponds to the DC offset voltage of the AC+DC input; and the supercapacitor is configured to remove the DC offset voltage from the AC+DC input, thereby generating an AC output; and the switch is configured to disconnect the AC extraction unit from one or more connected AC loads in response to a spike in DC voltage exceeding a predetermined threshold. It should be noted that the optional features set out with respect to the AC extraction unit which forms part of the system of the first aspect of the invention apply equally well to the AC extraction unit of the fourth aspect of the invention, and will not be repeated here, for conciseness.

The previous aspects of the invention relate to the household-side processing of the AC+DC input in a manner which enables it to be used to power both AC and DC loads in a simple manner. We now turn to aspects of the invention which focus on the generation and transmission of the AC+DC power from power station or substation to household. As we will show later in the application, the transmission of power using combined AC+DC improves the power which can be delivered to households in the final mile safely, and without causing damage to cables. A fifth aspect of the invention relates to a system for generating and transmitting combined AC+DC electricity. Specifically, the fifth aspect of the invention provides a system for the generation and transmission of combined AC+DC power supply to households, the system comprising: an AC+DC generator configured to generate an AC+DC output, the AC+DC output having an AC component and a DC offset component; and a transmission system, the transmission system including: a first line configured to receive the AC+DC output, and to transmit it to households each comprising one or more AC and/or DC loads; and a neutral line which is maintained at zero voltage. The transmission system may be configured to receive the AC+DC output via a connector. The transmission system may include a cable which comprises the first line and the neutral line. A distal end of the cable may be configured to transmit the AC+DC output to a system according to the first aspect of the invention (where it forms the AC+DC input), e.g. the AC+DC input receiving component.

The AC+DC output may, accordingly, be generated by alternately pulling a terminal of a high-frequency inductor to one or other power rail with a frequency of tens or hundreds of kilohertz. The other terminal of the inductor is connected to the low-frequency load. The AC component of the AC+DC input may be generated by varying the mark-space ratio of the two switches. The switches may be implemented with power semiconductors, typically MOSFETs or TGETs. New semiconductor fabrication technologies (GaN, SiC) mean that both high power and high switching frequency are now possible.

The inventors have realized that further advantages may be gained by using 3-phase transmission of the AC+DC output (in contrast to conventional 3-phase transmission, which transmits only AC power). In 3-phase transmission, power is transmitted via three separate lines within a cable, the AC component within each line having a different phase from the AC components in the other lines. In implementations of the present invention which use 3-phase transmission, the transmission system may comprise: the first line which is configured to receive a first AC+DC output having an AC component in a first phase, and to transmit it towards a first household or households comprising one or more AC and/or DC loads; a second line which is configured to receive a second AC+DC output having an AC component in a second phase, and to transmit it towards a second household or households comprising one or more AC and/or DC loads; and a third line which is configured to receive a third AC+DC output, and to transmit it towards a third household or households comprising one or more AC and/or DC loads. The neutral line is preferably configured to provide a return current path for the current in each of the first line, the second line, and the third line.

The first, second, and third phases are preferably evenly spaced. For example, the second phase may be 271-radians (or 1200) spaced from the first phase, and the third phase may be 4Th 7 radians (or 240°) spaced from the first phase.

Alternatively put the third phase may be -271 radians (or - 1200) spaced from the first phase. The DC offset may also be different in different phases. For example, among the three phases, it is preferably that the DC offset voltage is positive (i.e. +1/1) for one phase, and negative (i.e. -V1) for the other two phases. It may be advantageous for the negative DC offset to predominate, since this concentrates electrolytic erosion at a sacrificial node at the substation, which will need periodic inspection.

For example, in one implementation: 171stit"(t) = 171 ± 170 sin(wt) 172nclime(t) = -171+ 170 Sin (wt -31.r) 173" Jine(t) = -171 ± 170 Sin (wt ± -231 It should be understood that other combinations of +171 and the phases are also covered by the invention, and the example voltage profiles set out above are illustrative only.

In another case, rather than using 3-phase transmission, further advantages may be achieved by using 6-phase transmission of the AC+DC output. This is analogous to 3-phase transmission, except the power is transmitted via six separate lines within a cable or alternatively two separate cables serving buildings in different directions, the AC component within each line having a different phase from the AC components in the other lines. In implementations of the present invention which use 6-phase transmission, the transmission system may comprise: the first line which is configured to receive a first AC+DC output having an AC component in a first phase, and to transmit it towards a first household or households comprising one or more AC and/or DC loads; a second line which is configured to receive a second AC+DC output having an AC component in a second phase, and to transmit it towards a second household or households comprising one or more AC and/or DC loads; a third line which is configured to receive a third AC+DC output, and to transmit it towards a third household or households comprising one or more AC and/or DC loads; the first, second and third lines may advantageously share a neutral return line in a single cable; a fourth line which is configured to receive a fourth AC+DC output having an AC component in a fourth phase, and to transmit it towards a fourth household or households comprising one or more AC and/or DC loads; a fifth line which is configured to receive a fifth AC+DC output having an AC component in a fifth phase, and to transmit it towards a fifth household or households comprising one or more AC and/or DC loads; and a sixth line which is configured to receive a sixth AC+DC output, and to transmit it towards a sixth household or households comprising one or more AC and/or DC loads; the fourth, fifth and sixth lines may also advantageously share a second neutral return line in a single cable. This second neutral line is preferably configured to provide a return current path for the current in each of the first line, the second line, and the third line.

The first, second, third, fourth, fifth, and sixth phases are preferably evenly spaced. Advantageously, the first, second, and third phases, carried in the first cable are each spaced by 271 radians from each other. The fourth, fifth, and sixth phases, carried in the second cable are also preferably spaced by 7-radians from each other. For example, the second phase may be 271 radians (or 1200) spaced from the first phase, the 4n third phase may be 7-radians (or 2400) spaced from the first phase, the fourth phase may be 7 radians (or 600) spaced from the first phase, the fifth phase may be it radians (or 180°) spaced from the first phase, and the sixth phase may be SE radians (or 300°) spaced from the first phase. This is only one way of considering the phase differences -it will be appreciated that some of the different phases may instead, and equivalently, be considered to have a negative spacing.

As with the 3-phase case, in the 6-phase case, the DC offset may be different in different phases. Specifically, it is preferred that the DC offset voltage is positive (i.e. +VI) for two phases, and negative (i.e. -V1) for the other four phases.

Again, as before, it is advantageous for the negative DC offset to predominate, since this concentrates electrolytic erosion at a sacrificial node at the substation, which will need periodic inspection.

For example, in one implementation: Vlst_fine(t) = Vo Si11(600 V2na_line (0 = -V1+ Vo sin ((ot -) V3rcuirte (t) = -171 ± 170 sin (alt -3) V4th_line = V1+ V0 sin (cot -L37) Vsth_tine(t) = -V1 + Vo sin(wt -71) V6th_line(t) = -V1 ± V0 sin (cot -1) It will be appreciated that the phase offsets could be expressed in terms of degrees, as well as radians.

The advantages of the 6-phase implementation are explained in detail later in this application, but in short it can be shown that using six phases works to even out power flows.

Additional aspects of the invention may provide methods corresponding to the aspects of the invention set out above.

For example, a sixth aspect of the invention may provide a method of extracting an AC electrical output and/or a DC electrical output from a combined AC+DC supply having an AC voltage Vo and a DC offset voltage of VI, the method comprising: receiving an AC+DC input from an external source; receiving, by a DC extraction unit, the AC+DC input, and generating a DC output; receiving, by an AC extraction unit connected in parallel to the DC extraction unit, the AC+DC input, and generating an AC output; wherein the DC extraction unit and the AC extraction unit generate the DC output and the AC output independently of each other. Optional features set out above in connection with the first aspect of the invention apply equally well to methods of the sixth aspect of the invention, except where clearly incompatible or where context dictates otherwise.

An seventh aspect of the invention provides a method of extracting an AC output from an AC+DC input, the method comprising: receiving an AC+DC input from an external source, the AC+DC input having an associated AC voltage with a DC offset voltage; maintaining, using a variable auxiliary power source, a terminal voltage across a supercapacitor that corresponds to the DC offset voltage of the AC+DC input; and removing, using the supercapacitor, the DC offset voltage from the AC+DC input, thereby generating an AC output.

A eighth aspect of the present invention provides an alternative method of extracting an AC output from an AC+DC input, the method comprising: receiving an AC+DC input from an external source, the AC+DC input having an associated AC voltage with a DC offset voltage; removing, using a supercapacitor at a terminal voltage corresponding to the DC offset voltage, the DC offset voltage from the AC+DC input, thereby generating an AC output; and in response to a spike in DC voltage exceeding a predetermined threshold, disconnecting an AC extraction unit from one or more connected AC loads.

A ninth aspect of the present invention combines the eighth and ninth aspects, and provides a method of extracting an AC output from an AC+DC input, the method comprising: receiving an AC+DC input from an external source, the AC+DC input having an associated AC voltage with a DC offset voltage; maintaining, using a variable auxiliary power source, a terminal voltage across a supercapacitor that corresponds to the DC offset voltage of the AC+DC input; removing, using the supercapacitor, the DC offset voltage from the AC+DC input, thereby generating an AC output; and in response to a spike in DC voltage exceeding a predetermined threshold, disconnecting an AC extraction unit from one or more connected AC loads.

An tenth aspect of the present invention provides a method of generating and transmitting a combined AC+DC power supply to households, the method comprising: generating an AC+DC output, the AC+DC output having an AC component and a DC offset component; and transmitting the AC+DC output to households by a transmission system including: a first line configured to receive the AC+DC output, and to transmit it to households each comprising one or more AC and/or DC loads; and a neutral line which is maintained at zero voltage.

Optional features set out earlier in respect of the second to fifth aspects of the present invention are equally applicable to the seventh to tenth aspects of the invention except where clearly incompatible or where context clearly dictates otherwise. The optional features will not be set out again, for reasons of conciseness. In other words, the invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: - Fig.1A shows a simple way for AC + DC offset to be generated.

-Fig. 1B shows a way to generate AC + DC offset using semiconductors to switch a DC supply.

- Fig.2 just shows an AC + DC signal -in this case a 230Vrms AC signal and a 425V DC signal - Fig. 3A shows how the AC+DC generated in Fig.1B may be separated at the destination into separate AC and DC power.

-Fig.3B shows a similar arrangement to Fig.3A, with additional details added.

- Fig.4 shows a bi-directional DC-DC converter.

-Fig. 5 shows how an AC+DC voltage could be applied to a 3-phase distribution network - Fig.6 shows the corresponding current waveforms. Although these are rather strange-looking waveforms, the amount of heat they generate is less than the equivalent sine wave (a surprising result).

- Fig. 7 shows the voltage, current and power delivered by the distribution cable to a single-phase AC load, with no DC load.

- Fig. 8 shows the heat dissipated in a three-phase distribution cable.

- Fig. 9 is a diagram of a standard boost converter. 5 - Fig. 10 shows a six-phase AC+DC supply.

- Fig. 11 shows the corresponding six-phase current flows.

-Fig. 12 the total power loss in the six-phase system.

DETAILED DESCRIPTION OF THE DRAWINGS

Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

Combined AC+DC power may be considered as an alternating current with a DC offset, or alternatively as a DC signal with a substantial ripple. Two ways in which AC+DC voltage may be generated are illustrated in Figs. 1A and 1B. Fig. lA shows a conventional, "traditional" solution for the generation of are traditional power sources connected in series. The solution shown is satisfactory for low power levels, but for higher powers, a zigzag transformer may advantageously be to used prevent the DC current saturating the transformer core, and the battery may be paralleled by a capacitor to reduce the amount of charging and discharging it undergoes.

Fig. 18 shows an electronic solution, in which a DC power source is interrupted at high frequency with a sinusoidally varying mark-space ratio. The voltage is averaged by the inductor to produce a sine wave.

It can be shown that transmission using a combined AC+DC voltage increases the power that may be delivered through existing cables by raising the voltage only. Increasing the voltage requires that the cables be better insulated, but this AC+DC voltage, in which the AC and DC power sources is advantageous relative to increasing the current, which would require thicker cables, to prevent excessive heating of the conductors and consequent melting of the cables/insulation. In the example used in this application, a 425V DC offset is added to a 230Vrms AC supply, using a 750V-rated cable, which is shown in Fig. 2. It will be appreciated that this is only an example, and that various AC voltages and DC offsets may be used without falling outside the scope of the invention. Increasing the power supplied using additional DC voltage is an attractive prospect, particularly when solid-state substation transformers are to be used. As we will show later, rectified 3-phase AC may also be used.

Figs. 3A and 3E show example circuit diagrams of systems in which AC+DC power is supplied by a sub-station, and then is separated and used to power AC and DC loads independently of each other in a house. In this case, the input voltage which includes AC and DC components is given as: V(t) = V1 + Vo sin(cot) We first discuss powering of the AC loads. In Fig. 3A, a battery Or supercapacitor is charged to a voltage Vu and acts effectively to filter out the DC offset, as explained previously in this application. In Fig. 3B, there are additional components. On the left-hand side, the means of driving the two MOSFETs with a 50Hz generator is shown, showing the feedback that ensures that the output of the high-speed switching of the Class D output stage does indeed produce a 50Hz sine wave. On the right-hand side, the maintaining of the equal-and-opposite DC offset voltage across the (super)capacitor by means of an auxiliary power supply is shown. Also shown is switch Si which is opened whenever the voltage that the AC load would otherwise experience is outside permissible limits. Switch 32 and Rip, are the two components added to pre-charge the (super)capacitor to the offset voltage when power is first applied. This is only necessary if the power capability of the auxiliary power supply is such that pre-charging would take unreasonably long.

We briefly discuss the requirements of the supercapacitor.

The amount of energy a supercapacitor may be required to store on the first half-cycle and return on the second is: Ebatt = Pac X T /2 For the example above and 50Ez AC: Ebatt-25000 x/ 1/100 -2501oules supercapacitor or supercapacitor-plus-battery solution may me be more attractive than a simple battery. The equivalent series resistance (FSR) of these products is also a critical parameter, as it will directly translate into energy loss and heating of the equipment.

Charge equalisation circuitry may be desirable, along with compensation circuits to keep the voltage across the supercapacitors at the right level to ensure zero DC offset for the AC loads. An electronic rapid disconnect switch may also be desirable, since a rapid fall in supply voltage would otherwise expose the AC load to a sudden large negative voltage.

A possible solution would be 14 of Eaton's HLR-51 modules in series -giving a total ESR of 70mn and a capacitance of 13F.

With a 25kW AC load, this capacitance results in a voltage change over a cycle of: pv = \12E/c = \12 250/13 =6.2V The heat dissipation with a 25kW load would be: 2P2 250002 ac C'Eutt = Resr X iric dt = Res, x -70 x 10-3 x 2 x -828W V2 3252 0 0 This is only 3.35 of the AC load (ie 96.7% efficiency), but the heat dissipation is still rather high, and it would be good if il,s, could be reduced. For this reason, the system may further comprise a temperature controlled fan, in order to reduce the temperature.

The Skeleton Technologies SkelMod171V represents another possible solution. Four of these would be required for a 700VDC system -with a total volume of 367 x 234 x 360m and weight 20.8kg.

In addition to the supercapacitor, there is a 4-quadrant variable isolated power supply, and a voltage monitor. These components act together to counteract any small changes in the DC voltage. For example, if there is a decrease in DC voltage, which may result from e.g. many users drawing power from the substation simultaneously, the voltage monitor will detect this drop. A control component (not shown) may then cause the variable power supply to increase its voltage, so that the total voltage supplied by the supercapacitor and the variable power supply is equal to the DC offset, to enable the filtering of the DC component to remain effective.

This variable power supply may have limited power, and may only be able to adjust the voltage across the supercapacitor at a finite rate. With a supercapacitance of 13F, if the power supply can source or sink 5A, dv I +5 dt 13 ±0.38 V /s Under certain circumstances, the supply voltage may change much faster than this (for example at the start of a power cut), and it will be necessary to incorporate a normally-open fast-acting bi-directional static switch Si between the supercapacitor and the AC loads, to disconnect the loads and avoid a large negative spike. When power is first applied, this switch will not turn on until the auxiliary supply has charged the supercapacitor from zero to the required voltage.

(In the example given, this would take half an hour, and an additional current path via a resistor Rpc and second switch 52 may be desirable to speed this up.) This static AC switch will also have an important role in providing an anti-islanding function for the supercapacitor. (Additional anti-islanding will be needed for the DC section if the DC installation includes power sources.) There will be a trade-off between the size of the supercapacitor and the power rating of the auxiliary power supply. In the extreme case, the supercapacitor may be eliminated completely, if the auxiliary power supply has sufficient current capability.

Returning to the power delivery in the systems shown in Figs. 3A and 3B. In this scheme, the battery or supercapacitor (or battery-supercapacitor combination) maintains a terminal potential difference equal to Vi, and is charged and discharged equally on successive half-cycles. (A small auxiliary power source may be needed to adjust the voltage to ensure that the AC loads do not experience a DC offset, but provided the AC loads do not contain a DC current component, the power dissipated in this will be small.) If Vac toad <071-VO, a standard buck switching regulator may be used to generate a steady DC voltage as required by the DC load(s). (This completely avoids commutation issues.) Alternatively, if V -aciocui> (V i+ 170, a standard boost regulator may be used. In between, a buck/boost regulator such as that shown in Fig. 4 will be needed. These are well-established, but care must be taken in the design over commutation where bucking changes to boosting. Depending on how the control electronics of the buck-boost regulator of Fig. 4, is implemented, power can flow in either direction, and the flow of power can be made independent of the voltage on either side. This form of converter may be used in preference to the buck-or boost-converter shown in Fig. 3A or 3B when the DC load on the right of the diagram also includes DC power sources.

It is assumed initially that this regulator is 100% efficient.

If the DC load(s) draw a power Pth, watts (at whatever their design voltage is), the DC current drawn from the supply will be: Pdc Pdc v (V1 + Vo sin(co0) For a load, Pa, will be positive, so the current will always be positive (DC power sources are considered below).

The AC current will be exactly as if the load were being powered by the AC alone: lac - Vbsin(co0 Rae This will be sinusoidal, with no DC offset, so will spend equal amounts of time positive and negative.

If we specify the power rather than the resistance of the AC load, Rac = 2P" sin(oit) Vo 2Pac By Kirchhoff's Law, the total current will always be: is = (lac iac) This may or may not go negative on the second half of the cycle, depending on the relative currents through the AC and DC loads.

The square of the total current drawn from the supply is: 2_c _L \2 _L 12 _L 71 s '!ac.dc) .ac.dc..ac.dc The instantaneous power loss in the cable if it has a resistance Koh will be: Pcab = X Rcab The cable losses over the full cycle are reduced, because during the second part of the cycle, 2iac lac is negative.

Over a full cycle, the heat dissipated in the cable will be: cab= -T1 TOT x Rcab dt As indicated earlier, where neither Vdcwaa< VO nor Vacjowl> (V1+ VO, a buck-boost converter will be required, in order to deliver a steady DC voltage. Several architectures are possible, but the four-switch converter shown in Fig. 4 has the merit of being able to cope with DC power sources as well as loads.

As discussed, the combination of AC and DC voltage may be delivered using a 3-phase system. If this scheme is to be practical for a whole street as well as individual houses, it is desirable that the existing neutral conductor is not stressed. As discussed, one phase may be generated as already discussed, and the two other phases have the DC polarity reversed, in addition to the phase shift of 1200 or 2400. It is beneficial for negative to predominate over positive in order to concentrate electrolytic corrosion at a sacrificial anode at the substation, which will need periodic inspection. An example of a 3-phase setup is discussed below, in which the following values are used: - AC voltage = 230 Vun, - DC voltage = 425 V - AC power drawn 25 kW - DC power drawn = 22 kW -Cable resistance -50 mQ The resulting line voltages are shown in Fig. 5, which, specifically, shows how the AC+DC voltage could be applied to a 3-phase distribution network in such a way (120° between phases) as to keep the (shared) neutral current no greater than any of the phase currents. Two negative offsets and one positive will concentrate electrolytic corrosion effects at the substation, but two positive and one negative offset is also possible.

The line current (-total = idc lac) is shown in Fig. 6.

The current in the neutral wire N is the sum of the currents in the other three wires. It has been observed that the shape of the current waveform over the whole cycle depends critically on the ratios of 171/ /Vo and Pacipac. Throughout the present application, it is assumed that Vi is not less than Vo, in order to avoid the voltage passing through zero twice a cycle, which would give rise to an infinite current. The current waveforms shown in Fig. 6 are somewhat unusual-looking, but the amount of heat which they generate is less than the equivalent sine wave, which is a surprising result.

In order to keep the harmonic content of the current waveform low, it is preferably that V=Vvo> L3. There is no constraint on Pacor Pa, though the current and consequently also the power flow become bidirectional when there is minimal DC load, as shown in Fig. 7. Fig. 7 shows the voltage, current and power delivered by the distribution cable to a single-phase AC load, with no DC load.

A further implication of this AC+DC load imbalance is that the peak instantaneous power may be several times the average load power (it is always twice for a pure sine wave, but in this instance it is higher for hicher DC voltages).

With 25kW of AC load, 22kW of DC load and 50mQ conductors, the power dissipated in the cable (calculated using P = (lac-Plac) 2 Rcab is: AC load (as DC load AC+DC today) only load Single-phase cable 1.18kW 1.00kW 1.04kW Three phase (4- 1.77kW 2.00kW 2.06kW core) cable In summary, for a single-phase cable, the losses are reduced by adding DC. For a 3-phase cable, they are marginally increased -but by much less than if separate conductors were used for AC and DC. For any combination of loads up to these two figures, the power dissipation is less. The waveforms associated with the cable power losses are shown in Fig. 8, which assumes a per-conductor cable resistance of 30 mQ.

It will be noted that although the heating effect is increased in the high part of the cycle, it is reduced to almost zero during part of the low half of the cycle. Naturally, cable dissipation is sensitive to phase and AC+DC load imbalance, and addition of some phase-rebalancing circuitry may be desirable to ensure that it remains low in all load imbalance situations.

Prosumers injecting power into the AC will be able to do so without modification at their end. Prosumers injecting power in the DC may need a bidirectional buck/boost converter, as shown in Fig. 4.

The details above cover the basic idea of A.C+DC delivery, but there are some second-order effects that must be considered -in particular, whether the voltage delivered to the AC loads will be an unconditionally harmonic-free and offset-free sine wave. To achieve this, additional steps will be required.

The voltage drop in the supply cable is a function of the current, so a highly non-sinusoidal current waveform may alter the shape of the voltage delivered to AC loads (voltage seen by DC loads is independent of the supply voltage), and may introduce higher harmonics. With the figures used in the first example above, the magnitude of the introduced harmonics at the end of a 50m52 cable is 12V or about 3.61 THD. However, it is unlikely this will cause problems for AC loads.

The power requirements for the substation are an important consideration. One challenging aspect of this scheme is that under conditions of no DC load, the substation may deliver excess power during the high part of the cycle, and may get the excess back in the low part of the cycle. When AC and DC loads are balanced, the power transfer is concentrated in the high part of the cycle.

A non-obvious implication of this scheme is that if there is no DC load or there are prosumers (users who both consume and generate their own electricity to return to the grid), the current between substation and AC load will reverse for the lower half of the cycle, but the voltage will not, as a result of which power is transferred from the load back to the substation. Excess power will have been taken during the high half, and it is this that is returned. It is obviously important that the substation is able to accept this power and return it to the grid or a storage capacitor, rather than to absorb it as heat. However, the inductor and two semiconductors forming the output stage of the substation act as a conventional boost DC-DC converter when current is injected into the output, see Fig. 9. This allows the returning excess energy to be returned to the source. Fig. 9 shows a boost converter with power moving from left to right. Switch S may be a MOSFET and diode D may also be a MOSFET switch. R and V may represent the electricity grid. V, may be the combined AC+DC being transmitted to the households.

From this, it will be appreciated that the circuit of Fig. 9 is effectively identical to the circuit of Fig. 3A. In other words, the circuit of Fig. 3A may be re-drawn as a conventional boost converter with no change to the power components, which is a surprising result.

The high frequency inductor may require Litz wire, and the capacitors may need to have low ESR. The semiconductors may include SiC or GaN products, in order to minimize losses A single-phase system with additional substation detail and incorporating the offset adjustment is shown in Fig. 3B.

The irregular power flows may be minimised or eliminated completely in a substation that has two (or a higher even number of) 4-conductor 3P+N branch circuits feeding the loads. In this case, a six-phase solution works well. The following values were used in this simulation: Input parameters AC RMS voltage Vrms DC offset voltage Vdc AC single-phase load Pac DC single-phase load Pdc AC load current phase angle eac AC voltage phase angle ev0 Phase conductor resistance (N=Ph.1) Rcab Ph. 1 Ph. 2 Ph. 3 Ph. 4 Ph. 5 Ph. 6 230 230 230 230 230 230 425 -425 -425 425 -425 -425 24 24 24 24 24 24 20 20 20 20 20 0 0 0 0 0 0 0 120 240 180 300 420 50 50 50 50 SO volts volts kVA kW degrees degrees The results of the simulations run for a six-phase system are shown in Figs. 10 to 12.

Fig. 10 shows a six-phase AC+DC supply. The power flow shown in Fig. 7 for the three-phase system is highly nonlinear. But for the six-phase system shown in Fig. 11, the power flow is constant, which may reduce the risk of adverse effects for the grid. Fig. 11 shows the corresponding six-phase current flows, and Fig. 12 shows the total power. The table below sets out the cable losses.

*Cable Results Phase 1 Ca max cu 2.2 Ph 0.4 0.46 base 4 Phase 5 kW

A

P1-3 Neutral rr Phase 3* Phase*6 The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

Throughout this specification, including the claims which follow, unless the context requires otherwise, the word "comprise" and "include", and variations such as "comprises", "comprising", and "including" will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

It must be noted that, as used in the specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from "about" one particular value, and/or to "about" another particular value.

When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent "about," it will be understood that the particular value forms another embodiment. The term "about" in relation to a numerical value is optional and means

for example +/-10.

Claims

CLAIMS1. A system for extracting an AC electrical output and/or a DC electrical output from a combined AC+DC supply having an AC voltage Vo and a DC offset voltage of VI, the system comprising: an AC+DC input receiving component configured to receive an AC+DC input from an external source; a DC extraction unit configured to receive the AC+DC input from the AC+DC input receiving component, and to generate a DC output; an AC extraction unit connected in parallel to the DC extraction unit, and configured to receive the AC+DC input from the AC+DC input receiving component, and to generate an AC output; wherein the DC extraction unit and the AC extraction unit and configured, respectively, to generate the DC output and the AC output independently of each other.
2. The system of claim 1, wherein: the AC extraction using comprises a supercapacitor.
3. The system of claim 2, wherein: the AC extraction unit comprises a voltage source configured to maintain a predetermined terminal voltage across the supercapacitor.
4. The system of claim 3, wherein: the AC extraction unit further comprises an auxiliary variable power source configured to maintain the terminal voltage across the supercapacitor at a value which corresponds to a DC voltage offset of the AC+DC input, in the event that it varies from the predetermined terminal voltage.
5. The system of claim 4, wherein: the variable power source comprises: a voltage measuring device configured to monitor the DC component of the voltage of the AC+DC input; and a controller configured to control the output voltage of the variable power source in response to the measured DC voltage, to ensure that the terminal voltage across the supercapacitor is equal to the DC offset voltage of the AC+DC input.
6. The system of claim 5, wherein: the variable power source comprises a four-quadrant isolated variable DC power supply.
7. The system of any one of claims 1 to 6, wherein: the AC extraction unit further comprises a fast-acting solid state bidirectional switch configured to disconnect the AC extraction unit from one or more connected AC loads in response to a spike in DC voltage exceeding a predetermined threshold.
8. The system of claim 7, wherein: the switch is configured to connect the AC extraction unit to the one or more AC loads only in the event that the supercapacitor is charged to a terminal voltage equal to the DC offset voltage of the AC+DC input.
9. The system of any one of claims 2 to 8, wherein: the AC extraction unit further comprises an alternative current path via which the supercapacitor may be charged.
10. The system of claim 9, wherein: the alternative current path comprises a switch and a resistor.
11. The system of any one of claims 1 to 10, wherein: the DC extraction unit comprises one or more of: a buck converter; a boost converter; and a buck-boost converter.
12. An AC extraction unit comprising: an AC+DC input receiving component configured to receive an AC+DC input from an external source, the AC+DC input having an associated AC voltage with a DC offset voltage; a supercapacitor; and a variable auxiliary power source, wherein: the variable auxiliary power source is configured to maintain a terminal voltage across the supercapacitor that corresponds to the DC offset voltage of the AC+DC input; and the supercapacitor is configured to remove the DC offset voltage from the AC+DC input, thereby generating an AC output.
13. An AC extraction unit comprising: an AC+DC input receiving component configured to receive an AC+DC input from an external source, the AC+DC input having an associated AC voltage with a DC offset voltage; a supercapacitor at a terminal voltage corresponding to the DC offset voltage, and configured to remove the DC offset voltage from the AC+DC input, thereby generating an AC output; and a switch configured to disconnect the AC extraction unit from one or more connected AC loads in response to a spike in DC voltage exceeding a predetermined threshold.
14. An AC extraction unit according to claim 13, wherein: the switch is a fast-acting solid-state bidirectional switch.
15. A system for the generation and transmission of combined AC+DC power supply to households, the system comprising: an AC+DC generator configured to generate an AC+DC output, the AC+DC output having an AC component and a DC offset component; and a transmission system, the transmission system including: a first line configured to receive the AC+DC output, and to transmit it to households each comprising one or more AC and/or DC loads; and a neutral line which is maintained at zero voltage.
16. The system of claim 15, wherein: the transmission system comprises: the first line which is configured to receive a first AC+DC output having an AC component in a first phase, and to transmit it towards a first household or households comprising one or more AC and/or DC loads; a second line which is configured to receive a second AC+DC output having an AC component in a second phase, and to transmit it towards a second household or households comprising one or more AC and/or DC loads; and a third line which is configured to receive a third AC+DC output, and to transmit it towards a third household or households comprising one or more AC and/or DC loads; and the neutral line is configured to provide a return current path for the current in each of the first line, the second line, and the third line.
17. The system of claim 16, wherein: 27r the second phase is -T radians (or 1200) spaced from the first phase; and the third phase is radians (or 2400) spaced from the first phase.
18. The system of claim 17, wherein: the DC offset voltage is positive for one phase and negative for the other two phases.
19. The system of claim 14, wherein: the transmission system comprises: the first line which is configured to receive a first AC+DC output having an AC component in a first phase, and to transmit it towards a first household or households comprising one or more AC and/or DC loads; a second line which is configured to receive a second AC+DC output having an AC component in a second phase, and to transmit it towards a second household or households comprising one or more AC and/or DC loads; a third line which is configured to receive a third AC+DC output, and to transmit it towards a third household or households comprising one or more AC and/or DC loads; a fourth line which is configured to receive a fourth AC+DC output having an AC component in a fourth phase, and to transmit it towards a fourth household or households comprising one or more AC and/or DC loads; a fifth line which is configured to receive a fifth AC+DC output having an AC component in a fifth phase, and to transmit it towards a fifth household or households comprising one or more AC and/or DC loads; and a sixth line which is configured to receive a sixth AC+DC output, and to transmit it towards a sixth household or households comprising one or more AC and/or DC loads.
20. The system of claim 19, wherein: the first line, the second line and the third line share the neutral line in a first cable; and the fourth line, the fifth line, and the sixth line share a second neutral line in a second cable.
21. The system of claim 19 or claim 20, wherein: the second phase is 27/ radians (or 120°) spaced from the first phase; the third phase is 171 radians (or 240°) spaced from the first phase; the fourth phase is 'I radians (or 60°) spaced from the first phase; the fifth phase is Jr radians (or 180°) spaced from the first phase; and the sixth phase is radians (or 300°) spaced from the first phase.
22. The system of claim 21, wherein: the DC offset voltage is positive for two phases, and negative for the other four phases.
23. A method of extracting an AC electrical output and/or a DC electrical output from a combined AC+DC supply having an AC voltage Vo and a DC offset voltage of VI, the method comprising: receiving an AC+DC input from an external source; receiving, by a DC extraction unit, the AC+DC input, and generating a DC output; receiving, by an AC extraction unit connected in parallel to the DC extraction unit, the AC+DC input, and generating an AC output; wherein the DC extraction unit and the AC extraction unit generate the DC output and the AC output independently of each other.
24. A method of extracting an AC output from an AC+DC input, the method comprising: receiving an AC+DC input from an external source, the AC+DC input having an associated AC voltage with a DC offset voltage; maintaining, using a variable auxiliary power source, a terminal voltage across a supercapacitor that corresponds to the DC offset voltage of the AC+DC input; removing, using the supercapacitor, the DC offset voltage from the AC+DC input, thereby generating an AC output; and in response to a spike in DC voltage exceeding a predetermined threshold, disconnecting an AC extraction unit from one or more connected AC loads.
25. A method according to claim 24, wherein: the step of disconnecting is performed by a fast-acting solid-state bidirectional switch.CLAIMS1. A system for extracting an AC electrical output and a DC electrical output from a combined AC+DC supply having an AC voltage Vn and a DC offset voltage of Vi, the system comprising: an AC+DC input receiving component configured to receive an AC+DC input from an external source; a DC extraction unit comprising one or more of a buck converter, a boost converter and a buck-boost converter, the DC extraction unit configured to receive the AC+DC input from the AC+DC input receiving component, and to generate a DC output; an AC extraction unit connected in parallel tc the DC extraction unit, the AC extraction unit comprising a supercapacitor and a voltage source configured to maintain a COpredetermined terminal voltage across the supercapacitor, and configured to receive the AC+DC input from the AC+DC input receiving component, and to cenerate an AC output; CDwherein the DC extraction unit and the AC extraction unit and configured, respectively, to generate the DC output and the AC output independently of each other.2. The system of claim 1, wherein: the AC extraction unit further comprises an auxiliary variable power source configured to maintain the terminal voltage across the supercapacitor at a value which corresponds to a DC voltage offset of the AC+DC input, in the event that it varies from the predetermined terminal voltage.3. The system of claim 2, wherein: the variable power source comprises: a voltage measuring device configured to monitor the DC component of the voltage of the AC+DC input; and a controller configured to control the output voltage of the variable power source in response to the measured DC voltage, to ensure that the terminal voltage across the supercapacitor is equal to the DC offset voltage of the AC+DC input.4. The system of claim 3, wherein: the variable power source comprises a four-quadrant isolated variable DC power supply.5. The system of any one of claims 1 to 4, wherein: the AC extraction unit further comprises a solid state bidirectional switch configured to disconnect the AC extraction unit from one or more connected AC loads in response to a spike in DC voltage exceeding a predetermined threshold.6. The system of claim 5, wherein: the switch is configured to connect the AC extraction unit to the one or more AC loads only in the event that the supercapacitor is charged to a terminal voltage equal to the DC offset voltage of the AC+DC input.C\J 7. The system of any one of claims 1 to 6, wherein: the AC extraction unit further comprises an alternative CDcurrent path via which the supercapacitor may be charged.8. The system of claim 7, wherein: the alternative current path comprises a switch and a resistor.9. An AC extraction unit comprising: an AC+DC input receiving component configured to receive an AC+DC input from an external source, the AC+DC input having an associated AC voltage with a DC offset voltage; a supercapacitor; and a variable auxiliary power source, wherein: the variable auxiliary power source is configured to maintain a terminal voltage across the supercapacitor that corresponds to the DC offset voltage of the AC+DC input; and the supercapacitor is configured to remove the DC offset voltage from the AC+DC input, thereby generating an AC output.10. An AC extraction unit comprising: an AC+DC input receiving component configured to receive an AC+DC input from an external source, the AC+DC input having an associated AC voltage with a DC offset voltage; a supercapacitor at a terminal voltage corresponding to the DC offset voltage, and configured to remove the DC offset voltage from the AC+DC input, thereby generating an AC output; and a solid-state bidirectional switch configured to disconnect the AC extraction unit from one or more connected AC loads in response to a spike in DC voltage exceeding a predetermined threshold.11. A system for the generation and transmission of combined AC+DC power supply to households, the system comprising: an AC+DC generator configured to generate an AC+DC output, the AC+DC output having an AC component and a DC offset component; and a transmission system, the transmission system including: a first line which is configured to receive a first AC+DC output having an AC component in a first phase, and to transmit it towards a first household or households comprising one or more AC and/or DC loads; a second line which is configured to receive a second AC+DC output having an AC component in a second phase, and to transmit it towards a second household or households comprising one or more AC and/or DC loads; and a third line which is configured to receive a third AC+DC output, and to transmit it towards a third household or households comprising one or more AC and/or DC loads; and a neutral line which is maintained at zero voltage, the neutral line being configured to provide a return current path for the current in each of the first line, the second line, and the third line.12. The system of claim 11, wherein: 2n.the second phase is --radians (or 1200) spaced from the first phase; and 4rz the third phase is --radians (or 240°) spaced from the first phase.13. The system of claim 12, wherein: the DC offset voltage is positive for one phase and negative for the other two phases.14. The system of claim 11, wherein: the transmission system further comprises: a fourth line which is configured to receive a fourth AC+DC output having an AC component in a fourth phase, and to transmit it towards a fourth household or households comprising one or more AC and/or DC loads; a fifth line which is configured to receive a fifth AC+DC output having an AC component in a fifth phase, and to transmit it towards a fifth household or households comprising one or more AC and/or DC loads; and a sixth line which is configured to receive a sixth AC+DC output, and to transmit it towards a sixth household or households comprising one or more AC and/or DC loads.13. The system of claim 14, wherein: the first line, the second line and the third line share the neutral line in a first cable; and the fourth line, the fifth line, and the sixth line share a second neutral line in a second cable.16. The system of claim 14 or claim 15, wherein: the second phase is 2n radians (or 1200) spaced from the first phase; 4n the third phase iS -- radians (or 240°) spaced from the first phase; 3 the fourth phase is radians (or 60°) spaced from the first phase; 3 first the fifth phase is Jr radians (or 1800) spaced from the phase; and the sixth phase is (i71 radians (or 300°) spaced from the first phase. 3 17. The system of claim 16, wherein: the DC offset voltage is positive for two phases, and negative for the other four phases.18. A method of extracting an AC electrical output and a DC electrical output from a combined AC+DC supply having an AC voltage Vo and a DC offset voltage of Vi, the method comprising: receiving an AC+DC input from an external source; receiving, by a DC extraction unit comprising one or more of a buck converter, a boost converter and a buck-boost converter, the AC+DC input, and generating a DC output using the DC extraction unit; receiving, by an AC extraction unit connected in parallel to the DC extraction unit, the AC extraction unit comprising a supercapacitor and a voltage source configured to maintain a predetermined terminal voltace across the supercapacitor, the AC+DC input, and generating an AC output using the AC extraction unit; wherein the DC extraction unit and the AC extraction unit C\J generate the DC output and the AC output independently of each other.CD19. A method of extracting an AC output from an AC+DC input, the method comprising: receiving an AC+DC input from an external source, the AC+DC input having an associated AC voltage with a DC offset voltage; maintaining, using a variable auxiliary power source, a terminal voltage across a supercapacitor that corresponds to the DC offset voltage of the AC+DC input; removing, using the supercapacitor, the DC offset voltage from the AC+DC input, thereby generating an AC output; and in response to a spike in DC voltage exceeding a predetermined threshold, disconnecting an AC extraction unit from one or more connected AC loads using a solid-state bidirectional switch.