AC extraction unit

Abstract

An AC extraction unit configured to extract an AC electrical output from a combined AC+DC input, the combined AC+DC input having an AC voltage V0 sin(wt) and a DC offset voltage V1 and to output the A

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to an AC extraction unit (which might also be called a DC offset removal unit) for extracting an AC electrical output from a combined AC+DC signal, and a system for extracting both AC and DC electrical outputs from such an input, the system including the AC extraction unit. 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. 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 IB. Fig. 1A shows a conventional, "traditional" solution for the generation of AC+DC voltage, in which the AC and DC power sources 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 used to 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. IB 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 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. To be useful, a combined AC+DC signal must be separated into AC and DC components. GB 2615813 A presented a solution to this issue, in the provision of 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 Vi. Examples of the solutions proposed by GB 2615813 A are shown in Figs. 2A and 2B of this patent application. In Fig. 2A, it may be seen that, in order to extract the AC portion of the AC+DC supply, a supercapacitor is used (a battery or battery / supercapacitor combination could also be used). The supercapacitor is charged to a terminal voltage Vi, which is equal and opposite to the DC offset voltage Vi. In this manner, the supercapacitor is able effectively to filter out the DC offset voltage, to retain only the "pure" AC signal. A more advanced arrangement is shown in Fig. 2B, in which a variable voltage supply is connected over the supercapacitor . More specifically, a means of 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 S2 and Rpc are the two components added to pre-charge the supercapacitor 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. Supercapacitors having capacitances of e.g. upwards of 10F are bulky and expensive components (a commercially available product would have weighed 205kg and required 0.2m3 of space), and it would therefore be desirable to implement the extraction of an AC output from the AC+DC supply in particular using smaller components which could be more straightforwardly integrated into a user's home. The present invention has been devised with this issue in mind. SUMMARY OF THE INVENTION The present invention provides an AC extraction unit which is configured to isolate the AC portion of an incoming AC+DC supply. The AC extraction unit of the present invention does not require the use of large, expensive supercapacitors, and may be implemented using far more compact "normal" capacitors instead. Broadly, this is achieved by connecting a DC-DC converter in series with a combined AC+DC input, the regulated side of the DC-DC converter being maintained at a voltage equal and opposite to the DC offset voltage of the combined AC+DC input. In this manner, the DC offset voltage of the combined AC+DC input is effectively cancelled out, leaving only the AC electrical output. Accordingly, a first aspect of the invention provides an AC extraction unit configured to extract an AC electrical output from a combined AC+DC input, the combined AC+DC input having an AC voltage VoSin(<Dt) and a DC offset voltage Vi and to output the AC electrical output to one or more AC loads, the AC extraction unit comprising: an AC+DC input receiving component configured to receive the combined AC+DC input from an external source; a first DC-DC converter connectable in series with the combined AC+DC input, the first DC-DC converter having a regulated side and an unregulated side, the regulated side being connected to the one or more AC loads, wherein: a feedback mechanism of the DC-DC converter is configured to maintain the regulated side of the DC-DC at a target DC voltage which is selected such that when the first DC-DC converter is connected in series with the combined AC+DC input, the DC offset voltage Vi is removed or suppressed to generate the AC electrical output for output to the AC loads. The target DC voltage is preferably equal and opposite to the DC offset voltage (or approximately equal and opposite to the DC offset voltage). 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 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(t) = 14 + y0 sin(<Dt) 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 70cos(<i)t), or Vo exp (—jojt) or Vo exp ( / lot)1. Alternatively, any linear superpositions of these expressions are equally valid representations. Herein, the term "DC voltage" or "DC offset voltage" refers to V1 and the term "AC voltage" refers to 70, i.e. the amplitude of the AC component of the total voltage 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 1 Throughout this application, by convention j is used for the complex number defined as V—1, in order to avoid confusion with i which is used to represent current. a preferable approach since most home appliances are designed to work at fixed (e.g. 230V) voltages. For typical home use, the value of Vo is therefore preferably approximately 230 * V2 = 325V. The value of is constrained at the lower limit by the value of Vo (realistically, it needs to be at least Vo X130% ) and at the upper limit by the maximum permissible voltage of the cable. 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 AC extraction unit. The AC+DC receiving component may comprise, for example, a physical component such as a port in an outer housing of the AC extraction unit. 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. 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. 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, "extracting an AC electrical output" may refer to a process in which the AC component of the AC+DC input is 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 7(t) as defined earlier in this application, the AC output is preferably in the form Vosin(6)t), i.e. the DC component (i.e. the DC offset) is removed. The term "DC-DC converter" is used to refer to a component which is able to convert a source of DC from one voltage to another. According to the present invention, the first DC-DC converter has an unregulated side and a regulated side. Herein, the "regulated side" is used to refer to the side at which the voltage is fixed at a constant (or near-constant) value, i.e. the target DC voltage. Herein, the term "target DC voltage" is used to refer to a DC voltage which is maintained by the DC-DC converter in order to remove the DC offset voltage from the combined AC+DC input. The target DC voltage is preferably selectable or settable by a controller, discussed in more detail shortly. The term "target DC voltage" is used to differentiate from the "true" DC offset voltage. Accordingly, the first DC-DC converter may comprise a controller, which is configured to select or set the target DC voltage. More specifically, the feedback mechanism of the first DC-DC converter may comprise the controller which is configured to select or set the target DC voltage. In contrast, there are no restrictions on the voltage of the unregulated side, and the internal feedback mechanism acts to ensure that the unregulated voltage is converted to the regulated constant (or near-constant) voltage. In order to set the correct target DC voltage, one of two kinds of control may be executed, either a feed-back mechanism or a feed-forward mechanism. In some cases, both mechanisms may be applied in order to provide two levels of feedback. We discuss the feed-forward mechanism first. The controller may be configured to measure the DC offset voltage of the combined AC+DC input and to set the target DC voltage of the first DC-DC converter to a value which is equal and opposite to the measured DC offset voltage. In this way, it is possible to ensure that the target DC voltage is maintained at a value which is equal and opposite to the DC offset voltage at all times. The combined AC+DC input has, by definition, an alternating component as well as the DC offset voltage. Accordingly, it is necessary to measure over several cycles in order to ascertain the DC offset voltage. The controller may be configured to measure or monitor the instantaneous voltage of the combined AC+DC input over one or more cycles. Then, the controller may be configured to identify the maximum measured voltage and the minimum measured voltage and to calculate an average of the maximum measured voltage and the minimum measured voltage as the DC offset voltage. Alternatively, or additionally, the controller may be configured to integrate the voltage of the combined AC+DC input over a whole number of cycles, and to divide by the result by the time over which the integral was obtained to obtain the DC offset voltage. In order to execute this process, the controller may be configured first to measure or monitor the instantaneous voltage of the combined AC+DC input over one or more cycles. It will be appreciated that either of these modes of determining or measuring the DC offset voltage may be implemented digitally, or using electronic component such as operational amplifiers ("op-amps"). We now discuss the feed-back approach to control. At a high level, the controller may be further configured to monitor a voltage of the AC electrical output supplied to the AC loads, and if a DC offset voltage is detected in the AC electrical output supplied to the AC loads, the controller may be configured to adjust the target DC voltage of the regulated side of the first DC-DC converter to counteract the detected DC offset voltage. More specifically, an initial target DC voltage may be selected or set by the controller. Then, the controller may be configured to measure or monitor a DC offset voltage of the AC electrical output. Given that the initial target DC voltage may not be exactly equal and opposite to the DC offset voltage of the combined AC+DC input, it is possible a DC offset voltage will likely remain in the AC electrical output. This DC offset voltage of the AC electrical output may be measured by taking an average of maximum and minimum voltage values, or by integrating over one or more cycles - as outlined earlier with reference to the feed-forward control scheme. The controller may then be configured to adjust the initial target DC voltage to an adjusted target DC voltage based on the measured DC offset voltage of the AC electrical output. Specifically, the controller may be configured to select or set the adjusted target DC voltage as a voltage which is equal and opposite to the measured DC offset voltage. The adjusted target DC voltage then becomes the initial target DC voltage then repeats to ensure that the target DC voltage is always set to an appropriate value. Rather than measuring the DC offset voltage of the AC electrical output across a full cycle in order to take an average or integrate, the controller may be configured to monitor an instantaneous voltage of the AC electrical output. The controller may then be configured to compare the measured instantaneous voltage with an expected instantaneous voltage. Based on the comparison, the controller may be configured to adjust the initial target DC voltage to an adjusted target DC voltage. The expected instantaneous voltage may be based on a formula of the form x + y ■ sin(&)t) or y ■ sin(<Dt) for feed-forward and feed-back control, respectively, starting with a reasonable estimate of the values of x and y. This could be done digitally or by creating a phase-lock loop oscillator running at the AC frequency of the incoming line, for example, 50Hz. This has the potential to give an approximate answer very rapidly, and to increase the precision over time. In all cases, the measured DC offset voltage may be regarded as an error signal, in which case it may be said that the controller may be configured to set the adjusted target DC voltage based on the error signal. The error signal may have a small amplitude. The controller may therefore comprise an amplifier (e.g. an op-amp) configured to amplify the error signal. Both the feed-back and feed-forward processes may be repeated at regular intervals or continuously. We now discuss briefly how the feedback mechanism of the first DC-DC converter ensures that the regulated side of the first DC-DC converter is maintained at the target DC voltage set by the controller. The first DC-DC converter may comprise four transistors, and optionally an inductor, and the controller may be configured to control the first DC-DC converter to maintain or adjust the target DC voltage by controlling switching of the transistors. More specifically, the controller may be configured to control a respective duty cycle of each of the transistors. More specifically, bidirectional DC-DC converters employ high-frequency switching techniques to minimise energy loss. The voltage of one port is maintained at a constant value by altering the duty cycle of the switching devices in response to feedback. In order to enable highly responsive control of the duty cycle, it is preferable that a converter is implemented with fast-acting semiconductor switches such as insulated-gate bipolar transistors (IGBTs) or field-effect transistors (FETs). The transistors may be metal oxide semiconductor FETs, or MOSFETs. A high frequency inductor may also be required, as discussed above . The combined AC+DC input comprises a higher half-cycle, during which the voltage is higher than a midpoint voltage value, and a lower half-cycle, during which the voltage is lower than a midpoint voltage value. According to the invention, the voltage at the regulated side of the first converter DC-DC is maintained at a target DC voltage which may be equal and opposite to the DC offset voltage. It is therefore desirable for the AC extraction unit to comprise a component which is able to absorb electrical energy during the higher half-cycle and return this energy during the lower half-cycle, in order to maintain that voltage. In this manner, the first DC-DC converter may be referred to as a bidirectional DC-DC converter since the target DC voltage is maintained at the regulated side regardless of the direction in which the current is flowing at any given time. Accordingly, the feedback mechanism may comprise a capacitor connected to the unregulated side of the first DC-DC converter, wherein during a higher half-cycle of the combined AC+DC input, the capacitor is configured to be charged by the AC+DC input, in order to maintain the target DC voltage at the regulated side of the first DC-DC converter; and during a lower half-cycle of the combined AC+DC input, the capacitor is configured to discharge electrical energy stored therein, in order to maintain the target DC voltage at the regulated side of the first DC-DC converter. Whilst the AC extraction unit may be able to a certain extent to modify its DC offset voltage to minimize any deviation of the AC voltage from the idea sinusoidal waveform, the possibility exists of sudden deviations that are larger than the AC extraction unit's voltage feedback can compensate for. For example, events such as a power cut or a lightning strike can lead to a rapid change in the DC offset voltage. For example, in the event of a power cut, the AC loads will be subject to a voltage which is equal to the negative of the DC offset voltage. Such a large, sudden DC offset voltage can damage appliances. In order to reduce the risk of damage, in response to a detection of a spike in the DC offset voltage of the combined AC+DC input, the controller is configured to switch off all of the transistors, thereby disconnecting the combined AC+DC input from the one or more AC loads. The rapid responsiveness achievable using semiconductors between the supply and the capacitor means that the AC extraction unit according to the first aspect of the present invention is able to cope with correspondingly rapid changes in the DC offset voltage Vi of the AC+DC input by turning off all the semiconductors. Accordingly, if there is a spike in the DC offset voltage Vi, either positive or negative (such as at the onset of power cut), the switching arrangement of the present invention will be able to respond immediately to ensure that current ceases to flow through the AC loads, once small capacitor Cl has discharged. This means that AC loads connected to an output of the AC extraction unit do not experience such spikes, which could damage the AC loads (e.g. blowing fuses in household appliances). This means that AC extraction units according to the present invention do not require a switch which can disconnect AC loads in the event of spike in the DC offset voltage Vi. It has been shown that AC extraction units according to the present invention are able effectively to provide clean AC electrical outputs without the need for very high capacitance supercapacitors. The capacitor may have a capacitance of no more than 1 F, no more than 100 mF, no more than 10 mF, no more than 1 mF, no more than 100 pF, no more than 10 pF or no more than 1 pF, while still achieving good results. If the AC loads extract power asymmetrically during the higher half-cycle and the lower half-cycle, the voltage in the capacitor may increase or decrease in a runaway manner, or to an unacceptable value. In order to address this risk, the AC extraction unit may comprise a drift correction mechanism connected to the capacitor via a resistor. The drift correction mechanism may comprise a component which is capable of operating as both a current source and a current sink, the component being configured to maintain a voltage across the capacitor at a predetermined value. The predetermined value may be selected as a midpoint of an acceptable range values of a voltage across the capacitor. In some cases, the component may comprise a rechargeable battery, and in other cases, the component may comprise a second DC-DC converter having an unregulated side connected to the combined AC+DC input and a regulated side connected to the capacitor via the resistor. In those implementations having a sign of the DC offset voltage can the components which may be used: second DC-DC converter, the influence the arrangement of • When the DC offset voltage of the combined AC+DC input is a positive DC offset voltage, the second DC-DC converter may be an isolating DC-DC converter connectable between a line supplying the combined AC+DC input and a neutral line. Herein, the term "isolating DC-DC converter" is used to refer to a DC-DC converter in which the regulated side is not electrically connected to the unregulated side, for example due to the presence of a transformer. • Conversely, when the DC offset voltage of the combined AC+DC input is a negative DC offset voltage, the second DC-DC converter may be a non-isolating DC-DC converter connectable between a line supplying the combined AC+DC input and a neutral line. Here, the term "non-isolating DC-DC converter" is used to refer to a DC-DC converter in which one pole of the regulated side is electrically connected to the unregulated side. 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 the 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. The preceding disclosure is directed towards an AC extraction unit. However, it will be acknowledged that such an AC extraction unit could advantageously be included in a system for extracting both an AC electrical output and a DC electrical output from an AC+DC input. Accordingly, a second aspect of the present invention provides 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 second aspect of the invention provides a system for extracting an AC electrical output and / or a DC electrical output from a combined AC+DC input having an AC voltage VO and a DC offset voltage VI, and to output the AC electrical output to one or more AC loads and / or the DC electrical output to one or more DC loads, the system comprising: a combined AC+DC receiving component configured to receive an AC+DC input from an external source; a DC extraction unit configured to receive the combined AC+DC input from the combined AC+DC input receiving component and to generate a DC electrical output for output to the one or more DC loads; and the AC extraction unit of the first aspect of the invention, configured to receive the combined AC+DC input from the combined AC+DC input receiving component, the AC extraction unit connected in parallel with the DC extraction unit, wherein the AC extraction unit and DC extraction unit are configured, respectively, to generate the AC electrical output and the DC electrical output independently of each other. Naturally, the optional features set out previously in respect of the AC extraction unit of the first aspect of the invention apply equally well to the AC extraction unit which forms part of the system of the second aspect of the invention, except where clearly technically incompatible, or where context clearly dictates otherwise. Further optional features, related to the DC extraction unit, are set out below. 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 7(t) as defined earlier in this application, the DC output is preferably Vlr 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, the DC extraction unit may comprise a third DC-DC converter. The third DC-DC converter may comprise one or more of: a buck converter, a boost converter, a buck-boost converter. In each case, an unregulated side of the DC-DC converter may face the combined AC+DC input, and a regulated side of the DC-DC converter may face one or more DC loads. 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 Vload <— a standard buck switching regulator (i.e. a buck converter) may be used to generate the steady voltage required by the DC load. * If Vload >+ Vo, a standard boost regulator (i.e. a boost converter) may be used. * If V1-v0<vload<v1 + v0 / then a buck-boost regulator (i.e. a buck-boost converter) may be used, as described in more detail later in the application. The type of the third DC-DC converter may depend on the sign of the DC offset voltage. Specifically: « When the DC offset voltage of the combined AC+DC input is a positive DC offset voltage, the DC extraction unit may comprise a non-isolating DC-DC converter. » When the DC offset voltage of the combined AC+DC input is a negative DC offset voltage, the DC extraction unit may comprise an isolating DC-DC converter. 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. An important advantage of systems which are able to extract AC and DC independently from a combined AC+DC input in this manner is the ability to meter the use of AC and DC separately. Accordingly, the system may further comprise a first meter configured to measure a total amount of electrical energy consumed by the one or more DC loads, and a second meter configured to measure a total amount of electrical energy consumed by the one or more AC loads. The invention may be connected on the utility side of the existing AC meter, leaving the entire electrical installation of the building unchanged. The optional DC installation of the building is likely to be completely new, and may have its own utility meter, which is likely to be on the consumer side of the DC extraction unit (but since it is measuring total power, it could in principle be placed on either side). Additional aspects of the invention may provide methods corresponding to the aspects of the invention set out above. 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. IB shows a way to generate AC + DC offset using semiconductors to switch a DC supply. Fig. 2A shows how the AC+DC generated in Fig. IB may be separated at the destination into separate AC and DC power, according to known arrangements. Fig. 2B shows a similar arrangement to Fig. 2A, with additional details added. Fig. 3 shows a block diagram shorthand for non-isolating and isolating DC-DC converters Fig. 4 shows the revision of the right-hand part of Fig. 2A to incorporate the new DC-DC converter plus capacitor replacement for the original supercapacitor and / or battery, using the symbols defined in Fig.3. Fig. 5 shows an AC extraction unit according to the first aspect of the present invention. The AC extraction unit is connected between an AC+DC input rail and the Live AC supply to the AC loads. As discussed, the AC extraction unit of the first aspect of the invention may form part of a system for extracting both AC and DC electrical outputs from an AC+DC input. Figs. 6 and 7 are flowcharts illustrating control schemes for the DC-DC converter. Fig. 8 shows the system of Fig.4, with the rechargeable battery replaced by another (low power) DC-DC converter, using the notation of Fig. 3. Fig. 9 shows the equivalent of Fig. 6 when configured for a negative DC offset voltage. 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. Figs. 1A, IB, 2A, and 2B have been described earlier, in the background section of this patent application. Fig. 4 shows a system for extracting an AC electrical output and a DC electrical output from a combined AC+DC input, denoted: V = + Vo sin(<Dt) The combined AC+DC input is received via a live line. The system comprises DC extraction unit configured to extract a DC electrical output and deliver it to one or more DC loads, and an AC extraction unit configured to extract an AC electrical output and deliver it to one or more AC loads. The DC extraction unit comprises a non-isolating DC-DC converter, which may be in the form of buck converter, a boost converter, or a buck-boost converter. Compared with e.g. Fig. 2B, it will be acknowledged that the supercapacitor has been replaced by an arrangement comprising a non-isolating DC-DC converter (see notation in Fig. 3), whose regulated side A is connected in series with the combined AC+DC input and the one or more AC loads, and a capacitor connected to an unregulated side B of the nonisolating DC-DC converter. A target DC voltage Vdc is maintained across the regulated side A of the DC-DC converter, the target DC voltage Vdc being selected to be equal and opposite to the DC offset voltage Vi of the combined AC+DC input. By maintaining the regulated side A of the DC-DC converter at a voltage Vdc which is equal and opposite to the DC offset voltage Vi of the combined AC+DC input, the DC offset voltage Vi can effectively be suppressed or removed from the combined AC+DC input signal. The DC-DC converter is a bidirectional DC-DC converter which is able to convey current in both directions, while maintaining the regulated side at the target voltage Vdc- In order to achieve this, the AC extraction unit further comprises a capacitor (with a capacitance which may be several orders of magnitude lower than the supercapacitor of Fig. 2B) which is configured to absorb electrical energy when the current is flowing in one direction, and to discharge electrical energy when current is flowing in the other direct (i.e. during the higher half-cycle and lower half-cycle of the alternating current). By alternately storing and discharging electrical energy in this manner, the capacitor is able to ensure the bidirectional DC-DC converter is able to maintain the regulated side at the target DC voltage Vdc • Fig. 5 shows an alternative implementation, in which more detail of the DC-DC converter is shown. In the arrangement shown in Fig. 4, if there is asymmetry between the power drawn by the one or more AC loads during the higher half-cycle and the lower half-cycle, a voltage across the capacitor may increase or decrease (depending on the nature of the asymmetry), if this increase or decrease in voltage persists, operation of the AC extraction unit may eventually cease. To address this, a resistor and rechargeable battery or other voltage source that can both source and sink current is used to continually cause the capacitor voltage to tend towards the mid-point of its operation. This arrangement is referred to earlier in this patent application as a "drift correction mechanism". Referring to Fig. 5, an example of one type of DC-DC converter is shown, comprising semiconductor switches T1-T4 and inductor L. (Cl is a relatively small capacitor, which is present for system stability, but takes no other role.) During the higher half-cycle, capacitor C2 charges from its mid-point to a higher voltage, and during the lower half-cycle it returns this energy through the DC-DC converter. Any tendency for the average voltage to stray far from the mid-point is corrected by the resistor R and the rechargeable battery connected across the capacitor, and which may be referred to as a drift correction mechanism. Provided that the AC loads have a symmetrical current waveform, the battery suffers no significant charging or discharging, and both battery and resistor may be of low power. A feedback mechanism of the DC-DC converter, which may be embodied in the illustrated control system ensures that the regulated side of the DC-DC converter is maintained at the target DC voltage Vdc by controlling, independently, a respective duty cycle of each of the semiconductor switches T1-T4. As explained previously, there are effectively two types of control, feed-forward, and feed-back. These are explained in turn with reference to Figs. 6 and 7 respectively. Fig. 6 is a flowchart illustrating the steps of a feed-forward control system. In a first step, S100, the control system or a component thereof measures the supply voltage and calculates or averages this voltage to determine the DC offset value. The aim of the control system is then to set the target DC voltage of the regulated voltage side of the DC-DC converter to a voltage equal and opposite to the DC offset, thereby filtering out the DC offset. A result of measuring the voltage may be a voltage measurement signal which is transmitted to the control system (or a suitable component thereof), the voltage measurement signal encoding the measured voltage, i.e. the measured DC offset voltage of the AC+DC input. In step S102, the target DC voltage of the regulated side of the DC-DC converter may then be set to the measured DC offset voltage. A suitable DC-DC converter for this purpose is the Renesas ISL8160EVAL2Z. In order to ensure that the DC offset is consistently filtered out, the control scheme of Fig. 8 may operate continuously. Another control scheme is illustrated in the flowchart of Fig. 7. This control scheme relies on feed-back method, in which the target DC voltage of the regulated side of the DC-DC converter is adjusted based on an error signal. In a first step S200, the control system may be configured to set the target DC voltage of the regulated side of the DC-DC converter to an initial DC voltage. This value may be selected as an approximate value of the DC offset voltage of the combined AC+DC input. The control system may be configured to set this voltage using control signals, as explained with reference to Fig. 8, above. If the initial DC voltage is not identical to the DC offset voltage of the combined AC+DC input, the AC electrical output will still have some DC offset voltage, albeit likely a smaller one. In step S202, the DC offset voltage of the AC electrical output is measured or calculated. This may be done by taking an average of a maximum and minimum voltage value, or by integrating across a cycle of the AC electrical output, as explained earlier in this patent application. This measured DC offset voltage represents the difference between the initial target DC voltage to which the regulated side of the DC-DC converter has been set and the "correct" DC offset voltage value which is required in order to completely suppress the DC offset of the combined AC+DC input, and hence may be referred to as an error signal. In step S204, the target DC voltage value may be adjusted to an adjusted target DC voltage value, based on the error signal. Particularly when this process is executed iteratively, the error signal may become very small as the voltage value is very close to the true DC offset voltage. Accordingly, the error signal may be amplified using the opamp. The target DC voltage of the regulated side of the DC-DC converter may be adjusted in the same manner as for the feedforward control scheme of Fig. 6. Again, to ensure that the DC offset is consistently filtered out, the control scheme of Fig. 7 may operate continuously. Figs. 8 and 9 show systems in which the drift mechanism comprising the rechargeable battery replaced by additional DC-DC converters. The correction and resistor are configuration of the drift correction mechanism depends on whether the DC offset voltage of the combined AC+DC input is a positive DC offset voltage or a negative DC offset voltage. Fig. 8 shows an arrangement in which the DC offset voltage of the combined AC+DC input is a positive DC offset voltage. In this case, the DC-DC converter is an isolating DC-DC converter which is connected between the combined AC+DC input and the neutral line. » Fig. 9 shows an arrangement in which the DC offset voltage of the combined AC+DC input is a negative DC offset voltage. In this case, the DC-DC converter is a non-isolating DC-DC converter which is connected between the combined AC+DC input and the neutral line. Although any configuration based on a positive offset voltage may in principle also be used for a negative offset voltage, simply by reversing the polarity of all the underlying components, certain polarities of semiconductors are much easier to fabricate than the opposite. As a result, semiconductors and subsystems based on a negative common rail and a positive power rail are much more easily obtained. It is therefore of value to consider how the invention may be implemented using exclusively common-negative-rail subsystems, whichever offset polarity is provided by the supply. An alternative solution would be to reverse the polarity of appliances which run on positive DC in households where the DC offset voltage of the combined AC+DC input is a negative DC offset voltage, and for appliances which run on negative DC in households where the DC offset voltage of the combined AC+DC input is a positive DC offset voltage. 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%. 10 02 25

Claims

1. An AC extraction unit configured to extract an AC electrical output from a combined AC+DC input, the combined AC+DC input having an AC voltage Vo and a DC offset voltage Vi and to output the AC electrical output to one or more AC loads, the AC extraction unit comprising:an AC+DC input receiving component configured to receive the combined AC+DC input from an external source; anda first DC-DC converter connectable in series with the combined AC+DC input, the first DC-DC converter having a regulated side and an unregulated side, the regulated side being connected to the one or more AC loads, wherein:a feedback mechanism of the DC-DC converter is configured to maintain the AC load side of the DC-DC converter at a target DC voltage which is selected such that when the first DC-DC converter is connected in series with the combined AC+DC input, the DC offset voltage Vi is removed or suppressed to generate the generate the AC electrical output for output to the AC loads;the feedback mechanism comprises a capacitor connected to the unregulated side of the first DC-DC converter;during a higher half-cycle of the combined AC+DC input, the capacitor is configured to be charged by the AC+DC input, in order to maintain the target DC voltage at the regulated side of the first DC-DC converter; andduring a lower half-cycle of the combined AC+DC input, the capacitor is configured to discharge electrical energy stored therein, in order to maintain the target DC voltage at the regulated side of the first DC-DC converter.

2. The AC extraction unit of claim 1, wherein:the target DC voltage is equal and opposite to the DC offset voltage.

3. The AC extraction unit of claim 1, wherein:the capacitor has a capacitance of no more than lOOmF.10 02 254. The AC extraction unit of claim 1 or claim 3, further comprising :a drift correction mechanism connected to the capacitor via a resistor, the drift correction mechanism capable of acting as both a current source and a current sink; andthe drift correction mechanism is configured to maintain a voltage across the capacitor at a predetermined value.

5. The AC extraction unit of claim 4, wherein:the predetermined value is selected as a midpoint of an acceptable range of values of a voltage across the capacitor.

6. The AC extraction unit of claim 4 or claim 5, wherein: the drift correction mechanism comprises a rechargeable battery.

7. The AC extraction unit of claim 4 or claim 5, wherein: the drift correction mechanism comprises a second DC-DC converter having an unregulated side connected to the combined AC+DC input and a regulated side connected to the capacitor.

8. The AC extraction unit of claim 7, wherein:the DC offset voltage of the combined AC+DC input is a positive DC offset voltage; andthe second DC-DC converter is an isolating DC-DC converter connectable between a line supplying the combined AC+DC input and a neutral line.

9. The AC extraction unit of claim 7, wherein:the DC offset voltage of the combined AC+DC input is a negative DC offset voltage; andthe second DC-DC converter is a non-isolating DC-DC converter connectable between a line supplying the combined AC+DC input and a neutral line.

10. The AC extraction unit of any one of claims 1 to 9, further comprising :a controller configured to select or set the target DC voltage .

11. The AC extraction unit of claim 10, wherein:the controller is configured to measure the DC offset10 02 25voltage of the combined AC+DC input and to set the target DC voltage to a voltage equal and opposite to the DC offset voltage of the combined AC+DC input.

12. The AC extraction unit of claim 10 or claim 11, wherein: the controller is configured to monitor the voltage of the AC electrical output supplied to the AC loads, and if a DC offset voltage is detected in the AC electrical output supplied to the AC loads, the controller is configured to adjust the target DC voltage of the regulated side of the DC-DC converter to counteract the detected DC offset voltage.

13. The AC extraction unit of claim 12, wherein: the controller is configured to select or set an initial target DC voltage;the controller is configured to measure or monitor a DC offset voltage of the AC electrical output; andthe controller is configured to adjust the initial target DC voltage to an adjusted target DC voltage based on the measured DC offset voltage of the AC electrical output.

14. The AC extraction unit of claim 13, wherein: the controller is configured to select or set the adjusted target DC voltage as a voltage value which is equal and opposite to the measured DC offset voltage.

15. The AC extraction unit of any one of claims 10 to 14, wherein: the first DC-DC converter unit comprises four transistors, and the controller is configured to control the first DC-DC converter to maintain or adjust the target DC voltage by controlling switching of the transistors.

16. The AC extraction unit of claim 15, wherein: controlling switching of the transistors comprises controlling a respective duty cycle of each of the transistors .

17. The AC extraction unit of any one of claims 10 to 16, wherein: in response to a detection of a spike in the DC offset voltage of the combined AC+DC input, the controller is configured to switch off all of the transistors, thereby disconnecting the combined AC+DC input from the AC loads.18 .10 02 2519.20.21.22 .23.A system for extracting an AC electrical output and / or a DC electrical output from a combined AC+DC input having an AC voltage Vo and a DC offset voltage Vi, and to output the AC electrical output to one or more AC loads and / or the DC electrical output to one or more DC loads, the system comprising :a combined AC+DC receiving component configured to receive an AC+DC input from an external source;a DC extraction unit configured to receive the combined AC+DC input from the combined AC+DC input receiving component and to generate a DC electrical output for output to the one or more DC loads; andthe AC extraction unit of any one of claims 1 to 12 configured to receive the combined AC+DC input from the combined AC+DC input receiving component, the AC extraction unit connected in parallel with the DC extraction unit, wherein the AC extraction unit and DC extraction unit are configured, respectively, to generate the AC electrical output and the DC electrical output independently of each other.The system of claim 18, wherein:the DC extraction unit comprises a third DC-DC converter.The system of claim 19, wherein:the DC offset voltage of the combined AC+DC input is a positive DC offset voltage; andthe third DC-DC converter is a non-isolating third DC-DC converter.The system of claim 19, wherein:the DC offset voltage of the combined AC+DC input is a negative DC offset voltage; andthe third DC-DC converter is an isolating third DC-DC converter.The system of claim 19 or claim 20, wherein:the third DC-DC converter is a buck converter, a boost converter, or a buck-boost converter.The system of any one of claims 18 to 22, further comprising: a first meter configured to measure a total amount of electrical energy consumed by the one or more DC loads; anda second meter configured to measure a total amount of electrical energy consumed by the one or more AC loads.LO CXICXI

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