BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for reduction of a phase imbalance in a polyphase electrical system.

It is known for a polyphase electrical system (e.g. a three-phase installation) to supply power to a plurality of types of load, including loads that are supplied by a single phase, two phases (dual-phase loads) or three phases. Depending on how these loads are distributed across these phases, a phase imbalance (defined by the vector sum of the currents on all the phases) may arise. This is particularly likely in installations where single- or dual-phase loads are installed on fixed phases. Phase imbalances can be undesirable (e.g. for a grid supplier) and in some electrical systems (e.g. within a domestic or commercial building) there can be regulatory restrictions placed on the maximum allowed magnitude of the phase imbalance.

Embodiments of the present invention seek to reduce phase imbalances.

SUMMARY OF THE INVENTION

From a first aspect, the invention provides a system comprising: a polyphase electrical system; and a control apparatus, wherein: the polyphase electrical system is supplied by a plurality of live wires each at a different respective phase; the vector sum of the current over the plurality of live wires defines a phase imbalance; the polyphase electrical system comprises a plurality of controllable loads, wherein the currents drawn by each of the respective controllable loads can be varied independently of each other; the plurality of controllable loads comprises at least a first load, powered by a first set of the plurality of live wires, and a second load powered by a second set of the plurality of live wires, wherein the second set is different from the first set; and the control apparatus is configured to receive a measurement of current through a live wire of the plurality of live wires or through a neutral wire of the polyphase electrical system, and to control a selectable one, or both, of the first load and the second load in response to the measured current so as to vary a current drawn by at least one of the first and second loads through the plurality of live wires such that the phase imbalance is reduced.

From a second aspect, the invention provides a control apparatus for controlling a plurality of controllable loads of a polyphase electrical system, wherein the polyphase electrical system is supplied by a plurality of live wires each at a different respective phase, the vector sum of the current over the plurality of live wires defining a phase imbalance, wherein the currents drawn by each of the respective controllable loads can be varied independently of each other, and the plurality of controllable loads comprises at least a first load, powered by a first set of the plurality of live wires, and a second load powered by a second set of the plurality of live wires, wherein the second set is different from the first set; wherein the control apparatus is configured to receive a measurement of current through a live wire of the plurality of live wires or through a neutral wire of the polyphase electrical system, and to control a selectable one, or both, of the first load and the second load in response to the measured current so as to vary a current drawn by at least one of the first and second loads through the plurality of live wires such that the phase imbalance is reduced.

From a third aspect, the invention provides a method of reducing phase imbalance in a polyphase electrical system, wherein the polyphase electrical system is supplied by a plurality of live wires each at a different respective phase, the vector sum of the current over the plurality of live wires defining a phase imbalance, wherein the polyphase electrical system comprises a plurality of controllable loads, wherein the currents drawn by each of the respective controllable loads can be varied independently of each other, and the plurality of controllable loads comprises at least a first load, powered by a first set of the plurality of live wires, and a second load powered by a second set of the plurality of live wires, wherein the second set is different from the first set; the method comprising: receiving a measurement of current through a live wire of the plurality of live wires or through a neutral wire of the polyphase electrical system; and controlling a selectable one, or both, of the first load and the second load in response to the measured current so as to vary a current drawn by at least one of the first and second loads through the plurality of live wires such that the phase imbalance is reduced.

The method may be carried out by a control apparatus.

Thus it will be seen that, in accordance with the invention, the control apparatus is able to reduce a phase imbalance by controlling the current drawn from the supply wires by a selectable one or both of the first and the second loads. Importantly, this is possible because the loads are powered by different (albeit potentially overlapping) sets of the live wires. Phase imbalances are typically undesirable for the grid supplier, but embodiments can advantageously reduce this. They may provide better compliance with regulatory maximum limits to the allowable phase imbalance.

Phase imbalance may also be referred to as a “neutral current”, since the phase imbalance can create a non-zero current in a neutral wire (which could be notional neutral for supplies that do not have a physical neutral wire).

The polyphase electrical system is an electrical system of a property, such as a house or a commercial premises. A property may include a domestic or commercial property, such as a residential house (i.e. a dwelling) or an office building. It may comprise a consumer unit that is supplied by the plurality of live wires (e.g. from an electricity grid outside the system).

In some embodiments, the plurality of live wires comprises at least three live wires, optionally three live wires. In some embodiments, the polyphase electrical system is a three-phase alternating-current (AC) electrical system. Thus the polyphase electrical system may comprise a first live wire, at a first phase, a second live wire, at a second phase, and a third live wire, at a third phase, where the first, second and third phases are all different. The first live wire, the second live wire and the third live wire may each be 120° out of phase with respect to each other (i.e. their currents may be 120° out of phase). The first set of the plurality of live wires, powering the first load, may comprise the first live wire and the second live wire. It may consist only of the first live wire and the second live wire. The second set of the plurality of live wires, powering the second load, may comprise the third live wire. Optionally it may comprise the first live wire, the second live wire, and the third live wire.

In some embodiments the first load may comprise a resistive heating load, or a battery (optionally together with a battery inverter), or a heat pump (optionally together with a heat pump inverter).

In some embodiments the second load comprises a polyphase power supply system (e.g. electrical vehicle supply equipment), configured to supply power to a secondary load (e.g. a battery of an electrical vehicle) that may be coupled to a power output (e.g. a charging socket) of the polyphase power supply system. Each output phase supplied at the power output of the polyphase power supply system may correspond to a phase of a respective one of the live wires of the plurality of live wires that supplies power to the polyphase power supply system. In some embodiments the correspondence between the output phases and the live wires is fixed.

The control apparatus is particularly advantageous in a system where the correspondence between the output phases and the live wires is fixed, since in such a system wiring changes cannot be made to this correspondence during operation so as to reduce the phase imbalance (e.g. to change what phases are being used to charge the electric vehicle dynamically during the charging). The control apparatus thus provides a means which allows the phase imbalance to be reduced even in such situations.

In other embodiments, the correspondence between the output phases and the live wires may be variable (i.e., after installation of the system). For example, the second load may comprise a relay matrix, arranged to connect the live wires of the plurality of live wires that supply power to the polyphase power supply system to the output phases supplied at the power output of the polyphase power supply system, such that the correspondence between the output phases and the live wires is variable (e.g. by control logic in the polyphase power supply system, which may cooperate with the control apparatus in reducing the phase imbalance of the whole polyphase electrical system). In such an arrangement power can be redirected through any of the live wires as desired. However, such an arrangement is complex and requires a significant number of relays to be provided in the second load.

In some embodiments the polyphase power supply system is arranged to selectively supply power through one, some (e.g. two) or all of the output phases, i.e. in some embodiments the polyphase power supply system is configured to selectively disconnect one or more of the live wires from the output phases. The polyphase power supply system may be configured to disconnect the live wires from the output phases all at once (i.e. only all together, such that either all are connected or none are connected). Alternatively, the polyphase power supply system may be configured to selectively disconnect a subset of the live wires from a subset of the output phases. The polyphase power supply system may be arranged to selectively supply power through certain defined subsets of the output phases. For example, where there are three live wires, they may be referred to as L1, L2, L3 (and optionally a neutral wire N), and the wires L1 and N may be selectively disconnected (and connected) from (corresponding) output phases (e.g. with a dual pole relay). The polyphase power supply system may be configured to separately disconnect and connect L2 and L3 (e.g. using an additional dual pole relay). For example, a three-phase charger could switch between just charging on a single phase (i.e., where only low power is needed for charging) and could then add in L2 and L3 if there is a higher demand or cheaper energy available. Further alternatively, the polyphase power supply system may be arranged to selectively supply power through any of the output phases individually, or in combination.

In some embodiments, the control apparatus is arranged to receive an indication of a property of the secondary load (e.g. an indication of the number of phases that a particular electric vehicle, coupled to an EVSE, should be charged with). The system may further comprise a property determination apparatus, arranged to determine the property of the secondary load. The property determination apparatus may be part of the polyphase power supply system. The polyphase power supply system may convey the property of the secondary load to the control apparatus. Alternatively, the property determination apparatus may be part of the control apparatus itself. Further alternatively the property determination apparatus may be a standalone device. The control apparatus may be configured to control the current drawn by the second load based at least in part on the property of the secondary load. The property may be the number of output phases that are used to supply power to the secondary load. It may be which output phases are used to supply power to the secondary load. The property determination device may determine the number of output phases (or which output phases) that are used to supply power to the secondary load by measuring a current on each of the output phases of the polyphase supply system.

In some embodiments, all of the wires of the plurality of live wires are connected to the second load (i.e. so that they can all supply power to the second load if they are all selected to supply power).

The system may further comprise a secondary load, connected to the second load such that the second load supplies power to the secondary load (i.e. selectively through one, some or all of the output phases). The secondary load may be an electric vehicle. In some embodiments, the second load may comprise electric vehicle supply equipment (EVSE) for supplying power to an electric vehicle. The secondary load may be an AC load.

EVSE (Electric Vehicle Supply Equipment) that is connected to all three phases of the installation may be able to charge on a single phase, dual phases or three phases. The number of phases that are supplied to a particular electric vehicle may depend on the capability of that electric vehicle’s on-board charger. Some on-board chargers are tri-phase capable or only dual phase capable, while others are only single phase capable. Three-phase capable on-board chargers commonly load all three phases equally, therefore not contributing to phase imbalance. However, in the case of single phase or dual phase charging, the electric vehicle charging can be a significant load on either one or two phases. This creates imbalance between the phases and therefore potentially significant currents may flow in the neutral conductor, if one is present. Embodiments described herein are therefore particularly advantageous in the context of a polyphase electrical system which comprises a second load that is electric vehicle supply equipment, arranged to supply power to a secondary load that is an electric vehicle, wherein the electric vehicle (when present and connected) may be arranged to charge on a single phase or on two phases. The control apparatus receives a measurement of current flow through a live wire of the plurality of live wires and/or through a neutral wire of the polyphase electrical system. Receiving a measurement through at least one live wire of the plurality of live wires is particularly advantageous since it may allow the control apparatus to preemptively determine how to reduce or remove the phase imbalance, i.e. for calculations to be made such that new currents are selected to reduce the phase imbalance. This is in contrast to a trial-and-error method in which a current (e.g. the neutral current) may be measured, and adjustments may be made, after which the current on the neutral wire may be re-measured and the changes retained only if the phase imbalance has been reduced. The control apparatus may be configured to receive a measurement of current through each live wire of the plurality of live wires (and thus to control a selectable one, or both, of the first load and the second load in response to the measured currents so as to vary a current drawn by at least one of the first and second loads through the plurality of live wires such that the phase imbalance is reduced). The control apparatus may comprise one or more current sensors, and may therefore receive a measurement directly from these one or more current sensors, or it may receive the measurement from one or more external currents sensors (e.g., by wireless transmission). Thus in some embodiments the system further comprises at least one current sensor, arranged to measure current flowing through a live wire of the plurality of live wires or through a neutral wire of the polyphase electrical system. It may comprise a plurality of current sensors, each arranged to measure current flowing in a respective one of the plurality of live wires. There may be at least one current sensor corresponding to each live wire of the plurality of wires. Additionally or alternatively, the polyphase electrical system may comprise a neutral wire and the system may comprise a current sensor arranged to measure current flowing in the neutral wire. The current sensors may be arranged in proximity to the respective wires, e.g. around the wire. Each current sensor may comprise a current transformer. Each current sensor may comprise a wireless (e.g. radio) transmitter for transmitting a current signal to the control apparatus.

The control apparatus is configured to control a selectable one, or both, of the first load and the second load. By this it should be understood that the control apparatus is able to control both of these loads (i.e. not just one load), but that at any given time it may adjust the current drawn by only one of these loads (or both of them) in order to reduce the phase imbalance. The controller itself may alter the current drawn by the load(s) (e.g. by directly reducing a voltage applied to a load), or it may output a control signal to one or more of the loads, or to a further device, such that the outputting of the control signal causes the current drawn by the respective load to vary.

The control apparatus may be in wired or wireless communication with the first load and the second load. For example, the first load and the second load may be in a network with the control apparatus. Each load may be associated with a unique identifier (e.g. a serial number). The control apparatus may be configured to transmit control signals to the first load and the second load using their unique identifiers. The control apparatus may be arranged to send a control signal transmission (e.g. a radio broadcast or an Ethernet packet) to both the first load and the second load (optionally to every load of the plurality of loads), comprising a control signal for each load. The control signal transmission may identify the control signal for each load by its respective unique identifier. The control signal may comprise a target current for each load.

The control apparatus may be separate from the polyphase electrical system, or it may be part of the polyphase electrical system. It may be configured for wired and/or wireless connection with one or more components of the polyphase electrical system.

The control apparatus may be provided by a single control unit (e.g. a central controller), or it may be distributed across a plurality of control units (e.g. a central controller and one or more edge units).

At least a part of the control apparatus may be integral with the first load or the second load, i.e. the control apparatus may be associated with or part of one of the loads of the system. It may communicate with the other load by a wired or wireless channel.

The control apparatus may implement any of the operations disclosed herein in hardware (e.g. using dedicated electrical circuitry) or in software (e.g. by executing software on one or more processors of the control apparatus) or by a combination of hardware and software.

In some embodiments, the control apparatus is configured to control a selectable one, or both, of the first load and the second load in response to the measured current so as to vary a current drawn by at least one of the first and second loads through the plurality of live wires such that the phase imbalance is reduced below a predetermined phase-imbalance limit.

It will be appreciated that, other than in the case of perfect balance between the loads, in a polyphase electrical system there will be at least a least-loaded phase and a most- loaded phase. Where more than two phases are present, there will also be one or more intermediate-loaded phases. Thus, in some embodiments, the polyphase electrical system comprises (at least at some times) a most-loaded phase, a least- loaded phase, and optionally at least one intermediate-loaded phase. Where three live wires are present there will be (at least at some times) a most-loaded phase, a least- loaded phase and one (i.e. the) intermediate-loaded phase. The current drawn on the intermediate-loaded phase will be higher than the current drawn on the least-loaded phase, and lower than the current drawn on the most-loaded phase. It will be appreciated that which of the phases of the polyphase electrical system corresponds to the most-loaded, least-loaded and intermediate-loaded may vary over time. In some embodiments the control apparatus is configured to control a selectable one, or both, of the first load and the second load to increase the current drawn on the least-loaded phase of the polyphase electrical system and to decrease the current drawn on the most-loaded phase of the polyphase electrical system, such that the current drawn on each phase approaches an intermediate value between the current drawn on the most-loaded phase and the current drawn on the least-loaded phase. In some embodiments, the plurality of live wires comprises at least three live wires, and the intermediate value is the current drawn on the third, intermediate-loaded phase. Alternatively, the intermediate value may be an average (e.g. mean) value of the current through the plurality of live wires.

In some embodiments, the control apparatus is configured to control a selectable one, or both, of the first load and the second load to drive the current drawn by the first load towards a first target current and to drive the current drawn by the second load towards a second target current. The control apparatus may transmit a first target current to the first load (e.g. in a first control signal) and a second target current to the second load (e.g. in a second control signal). Each respective load may then respond by drawing the amount of current indicated in the respective target current. A target current may be transmitted to each load at intervals, which may be regular. The control apparatus may be configured to calculate updated target currents at least once every minute, and preferably once every second. A respective target current may be calculated for (and transmitted to) each of the plurality of controllable loads.

The control apparatus may vary at least one of the first target current and the second target current in response to surplus locally-generated power available in the polyphase electrical system. For example, the control apparatus may control the controllable loads such that all surplus locally-generated power is distributed to one or more of the loads of the polyphase electrical system. Surplus locally-generated power will be understood as electrical power generated in the same property as the system, e.g. from renewable energy sources. It may for example be provided by a solar panel, or other environmental energy sources, which may in some embodiments form part of the system. It may be desirable to make use of any surplus locally-generated power by supplying it to the controllable loads in order to prevent the surplus locally-generated power from being exported to the grid (i.e. so that the system self-consumes all locally- generated power, rather than exporting it to the grid). It may be that, sometimes, only some of the loads of the plurality of controllable loads are used to receive surplus locally-generated power (e.g. only a battery or resistive heating load, and not an EVSE, where that EVSE is not currently in use to supply power to an electric vehicle).

The control apparatus may vary the first target current and the second target current (and optionally other target currents for any other loads of the plurality of controllable loads) to be above a respective minimum current threshold (e.g. a rated minimum power for that load) and/or below a respective maximum current threshold (e.g. a fuse rating or a wire rating). In addition, or alternatively, the control apparatus may vary the first target current and the second target current (and optionally other target currents for any other loads of the plurality of controllable loads) so that the sum of all of the currents is below a global maximum current threshold (e.g. a grid fuse rating). More generally, the control apparatus may be configured to control the plurality of controllable loads such that the scalar sum of the currents drawn by the plurality of live wires is below a global maximum current threshold.

The control apparatus may receive signals from one or more of the plurality of controllable loads, and may vary the respective target currents for the controllable loads based on the received signals. In some embodiments, one or more of the controllable loads may signal a minimum requested current to the control apparatus (e.g. request for a minimum current at a particular time, for example when an EVSE is charging an electric vehicle or a rated minimum power, i.e. a permanent request for that minimum power). Each controllable load may also supply a respective maximum current threshold to the control apparatus, or these may be input by another means, for example manually input during installation.

One or more of the loads of the plurality of controllable loads may be configured to transmit a boost request to the control apparatus, indicating that the load should receive maximum power (optionally up to a maximum current threshold for the load). The control apparatus may be configured to receive a boost request from one or more loads of the plurality of controllable loads, and in response to allocate the maximum possible power (e.g. taking account of other possible limitations) to that load or those loads. The boost request may be activated for a pre-set time period (e.g. according to a schedule), or may be triggered by a user request. The boost request may also be made (for some or all devices) due to an external boost request (i.e. from a device other than one of the loads, for example by a consumer unit).

The control apparatus may be configured to implement such increases or decreases immediately, but in some embodiments the control apparatus is configured to simulate the effect of increasing or decreasing the current drawn by one or more of the controllable loads on the phase imbalance before transmitting target current values to the respective loads. It may be configured to implement a succession of adjustments, e.g. as described below, by adjusting a set of assigned currents for the controllable loads in a calculation phase (i.e. by changing values in a memory of the control apparatus), so as to determine a set of target current values. Some assigned currents may be both increased and decreased by different steps, with the target current value then depending on the net effect of these changes. The control apparatus may then transmit the target current values to the respective loads in a transmission phase, after the calculation phase. It may repeat this cycle of calculation and transmission at intervals.

In some embodiments, the control apparatus is configured to control a selectable one, or both, of the first load and the second load to decrease the current drawn on the most-loaded phase of the polyphase electrical system by decreasing the current drawn by at least one load supplied by that phase (i.e. , by the most-loaded phase). It will be understood that throughout the description where a load is said to be “supplied by” a particular phase, that may mean it is supplied by that phase in addition to other phases, or may mean that it is supplied exclusively (i.e. only) by the specifically mentioned phases. The control apparatus may be configured to decrease the current drawn by a load only if this does not take the power supplied to the load below the rated minimum power of that load.

In some embodiments each of the plurality of controllable loads is assigned a priority value. The priority value may be defined by a user, or may be set based on the type of each load. Thus, it may be possible for two different loads to have the same priority value. Alternatively, the priority values may define a priority ordering (i.e. such that no two loads can have the same priority value). Thus, for example, each battery could have the same level of priority value. The current drawn by each load may be decreased (e.g. within a calculation phase) in order from the load having the lowest priority value to the load having the highest priority value. By this it will be understood that the current drawn by a load supplied only by the most-loaded phase and having the lowest priority value out of such loads will be decreased, and then the current drawn by the load having the next-lowest priority value will only be decreased if the initial decrease of the current drawn by the lowest priority load was not sufficient to bring the phase imbalance below the predetermined phase-imbalance limit.

In some embodiments, the control apparatus is configured to control a selectable one, or both, of the first load and the second load to increase the current drawn on the least-loaded phase of the polyphase electrical system by increasing the current drawn by at least one load supplied by that phase (i.e., the least-loaded phase). This control apparatus may be configured to carry out this step following the step described in the preceding paragraph (e.g. within a calculation phase). The control apparatus may be configured to increase the current drawn by a load only if this does not take the overall current drawn by that load, the polyphase electrical system and/or by any single live wire above a predetermined limit (e.g. over the global maximum current threshold, or over the respective maximum current threshold for each load).

As noted above, in some embodiments each of the plurality of controllable loads is assigned a priority value. The current drawn by each load may be increased (e.g. within a calculation phase) in order from the load having the highest priority value to the load having the lowest priority value. By this it will be understood that the current drawn by a load supplied only by the least-loaded phase and having the highest priority value out of such loads will be increased, and then the current drawn by the load having the next-highest priority value will only be increased if the initial increase of the current drawn by the highest priority load was not sufficient to bring the phase imbalance (in reality or in simulation) below the predetermined phase-imbalance limit.

In some embodiments the plurality of live wires comprises at least three live wires, and the control apparatus is configured to control a selectable one, or both, of the first load and the second load to first decrease the current drawn on the most-loaded phase of the polyphase electrical system by decreasing the current drawn by at least one load supplied by only that phase (i.e. the most-loaded phase), and then, if the phase imbalance is not reduced below the predetermined phase-imbalance limit (in reality or in simulation), to decrease the current drawn on the most-loaded phase of the polyphase electrical system by decreasing the current drawn by at least one load supplied by both that phase (i.e., the most-loaded phase) and the intermediate-loaded phase.

In some embodiments the plurality of live wires comprises at least three live wires, and the control apparatus is configured to control a selectable one, or both, of the first load and the second load to first increase the current drawn on the least-loaded phase of the polyphase electrical system by increasing the current drawn by at least one load supplied by only that phase (i.e., the least-loaded phase) and then, if the phase imbalance is not reduced below the predetermined phase-imbalance limit (in reality or in simulation), to increase the current drawn on the least-loaded phase of the polyphase electrical system by increasing the current drawn by at least one load supplied by both that phase (i.e., the least-loaded phase) and the intermediate-loaded phase.

In some embodiments, the plurality of live wires comprises at least three live wires, and the control apparatus is configured to control a selectable one, or both, of the first load and the second load to implement or calculate the effect of: a decrease of the current drawn on the most-loaded phase of the polyphase electrical system by decreasing the current drawn by at least one load supplied by that phase; then, if the phase imbalance is not reduced below a predetermined phaseimbalance limit, an increase of the current drawn on the least-loaded phase of the polyphase electrical system by increasing the current drawn by at least one load supplied by that phase; then, if the phase imbalance is not reduced below a predetermined phaseimbalance limit, a decrease of the current drawn on the most-loaded phase of the polyphase electrical system by decreasing the current drawn by at least one load supplied by both that phase and the intermediate-loaded phase; and then, if the phase imbalance is not reduced below a predetermined phaseimbalance limit, an increase of the current drawn on the least-loaded phase of the polyphase electrical system by increasing the current drawn by at least one load supplied by both that phase and the intermediate-loaded phase.

In some embodiments, the control apparatus may be arranged (e.g. after the steps laid out above) to adjust the current drawn by at least one load supplied by the intermediate-loaded phase towards a mean value, wherein the mean value is defined as the mean of the loading on the most-loaded phase and the loading on the least- loaded phase. It will be understood that by adjusting the current drawn towards the mean value it is meant that the current drawn is made as close as possible to the mean value, whilst taking account of any existing limits (e.g. rated maximum power) that may prevent the current drawn from being equal to the mean value.

In some embodiments, the control apparatus may be arranged (e.g. after the steps laid out above) to adjust the current drawn by at least one load supplied by both the highest-loaded phase and the lowest-loaded phase towards the mean value.

These adjustments (i.e. increasing or decreasing the current drawn by a load) may be an adjustment by a fixed amount (e.g. 1 A), or alternatively may be an adjustment by a fixed proportion of the presently drawn (or assigned) current (e.g. a reduction of 1% of the current presently drawn by a load). Further alternatively, the control apparatus may calculate, for a given current adjustment, a magnitude for the adjustment, e.g. it may calculate the magnitude of adjustment that would bring the phase imbalance below the predetermined phase-imbalance limit, and select this magnitude as the magnitude of the current adjustment.

In some embodiments, the control apparatus may be arranged to transmit a warning signal if it is unable to reduce the phase imbalance to below the predetermined phaseimbalance limit. The warning signal may be a display of a warning which will be visible to a user, or may be a signal transmitted to an external device which is then configured, in response to receiving the warning signal, to display a warning to a user.

Various steps are described above in which the currents drawn by various loads are varied. In some embodiments the control apparatus may carry out each variation individually (e.g. by transmitting a separate target current based on the adjusted current to be drawn), but in some preferred embodiments the control apparatus may carry out these adjustment steps as part of a series of calculation steps (i.e. adjusting a hypothetical assigned current, rather than actually adjusting the current that is presently being drawn in practice), which together determine a final current to be drawn by each of the loads. Then, once all of the necessary adjustments to the drawn currents have been carried out, the control apparatus may transmit a target current to each load of the plurality of controllable loads, indicating the current that it should draw.

Features of any aspect or embodiment described herein may, wherever appropriate, be applied to any other aspect or embodiment described herein. In particular, it will be understood that steps which the control apparatus is said to be configured to carry out may form steps of the claimed method (even where they are not carried out specifically by the control apparatus). Where reference is made to different embodiments or sets of embodiments, it should be understood that these are not necessarily distinct but may overlap.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic drawing showing a system according to an embodiment of the present invention; Figures 2a-2g together show a flow chart representing a method according to an embodiment of the present invention; and

Figures 3-8 are vector diagrams representing the current present in each of the live wires 8a, 8b, 8c on Figure 1, based on different currents assigned to the loads 12, 13, 14, 16, during the steps of the method of Figures 2a-2g.

DETAILED DESCRIPTION

Figure 1 shows a system 1 , which may be located within a domestic or commercial property, such as a residential house (i.e. a dwelling) or an office building. The system 1 includes a polyphase electrical system 2 and a control apparatus 4. The polyphase electrical system 2 is supplied with electrical power from a supply grid 6 (external to the system 1) by a plurality of live wires 8. These may be received into a consumer unit 7, also referred to as a fuse board.

In the example of Figure 1 , the plurality of live wires has three live wires 8a, 8b, 8c which are at different respective phases, each 120° out of phase with the others. There is a respective current sensor 10a, 10b, 10c, corresponding to each live wire 8a, 8b, 8c, and each current sensor 10a, 10b, 10c is arranged around its corresponding live wire 8a, 8b, 8c, so as to detect the current flowing through the wire. The vector sum of the current over the live wires 8a, 8b, 8c defines a phase imbalance.

Although not shown in Figure 1 , a neutral wire may also be present, and in that case the phase imbalance results in a current flowing along the neutral wire. For this reason the phase imbalance is also referred to herein as the neutral current (but the reference to “neutral current” should not be taken to imply that a neutral wire is present in the referenced system).

The polyphase electrical system 2 comprises a plurality of controllable loads. In particular, the plurality of controllable loads in this example comprises a first load, which is a first battery 12, a second load which is electric vehicle supply equipment (EVSE) 14 (e.g. an electrical vehicle charger), a third load which is a resistive heating load 16 and a fourth load which is a second battery 13. However, other examples may have different numbers and/or types of load. The first battery 12 is supplied with power via both the second live wire 8b and the third live wire 8c. The second battery 13 is supplied with power via the third live wire 8c.

The EVSE 14 is itself a polyphase power supply system, which is arranged to supply power to a secondary load, which in this example is an electric vehicle 20. The EVSE 14 includes a property determination apparatus 22 which is arranged to determine the number of phases through which the electric vehicle 20 is to be charged. The EVSE 14 is supplied by all three live wires 8a, 8b, 8c and is able to selectively supply power from just some of these phases (or all of them) to the electric vehicle 20, depending on how many phases the electric vehicle 20 is arranged to charge on (as determined by the property determination apparatus 22). The number of phases that the electric vehicle 20 is arranged to charge on may depend on the number of phases of charging supported by the vehicle’s on-board charger. However, the wiring from the inputs of the EVSE 14 to the outputs (which connect to the electric vehicle 20) is fixed, so that each input live wire supplies a set output, and this cannot be changed. In this example the EVSE 14 has the capability to enable charging either through two live wires 8a, 8b, or through all three lives wires 8a, 8b, 8c and a neutral wire (not shown). In the example shown, the electric vehicle 20 is arranged for dual-phase charging, and therefore the EVSE 14 is loading two phases 8a, 8b. This is recognised by the property determination apparatus 22 which determines that the electric vehicle 20 is a dual-phase charging electric vehicle and may transmit this information to the control apparatus 4.

The resistive heating load 16, which may be, for example, an immersion heater of a hot water cylinder, receives power from the first live wire 8a, via a resistive load controller 18 which controls the supply of power to the resistive heating load 16.

The control apparatus 4 is arranged to communicate wirelessly with other devices in the system (as illustrated by the schematic antennae shown in the drawings). Although in this example the communication is wireless, it will be understood that the control apparatus 4 may additionally or alternatively receive signals from one or more of the other devices in the polyphase electrical system 2 via wires. Thus, the control apparatus 4 may be connected by wires to one or more of the other devices. The control apparatus 4 is shown here as a single control unit, but it could be distributed across multiple devices. Some or all of the control apparatus 4 may be part of (i.e. integral with) one of the other devices (e.g. of one of the controllable loads). For example it may be part of the EVSE — e.g. contained in a common housing with circuitry of the EVSE. The control apparatus 4 may perform steps described below in hardware (e.g. using dedicated electrical circuitry) or in software (e.g. by executing software on one or more processors) or by a combination of hardware and software.

In this example, the control apparatus 4 can communicate wirelessly with the current sensors 10a, 10b, 10c, the EVSE 14, the batteries 12, 13, and the resistive load controller 18, to receive information from those devices (e.g. about properties of the connected electric vehicle 20, and about the current flowing in the live wires), and to transmit information to those devices (e.g. to control the current they draw).

The batteries 12, 13, the EVSE 14 and the resistive load controller 18 each receive a respective target current signal from the control apparatus 4. The battery 12 receives a first target current signal, the EVSE 14 receives a second target current signal, a resistive load controller 18 receives a third target current signal, assigning a target current for the resistive heating load 16, and the second battery 13 receives a fourth target current signal.

The target currents may be set by the control apparatus 4 based on various factors.

Firstly, the control apparatus 4 may aim to distribute all surplus locally-generated power available in the polyphase electrical system 2 to the loads, so that none is exported to the grid. Surplus locally-generated energy here refers to locally-generated power — i.e. energy generation from sources in the same property as the system 2, which if not consumed at the property will lead to power being exported to the grid. It may, for example, be provided by a local solar panel 24 that is part of the polyphase power supply system 2, or other renewable energy sources within the installation.

Secondly, each controllable load may supply signals to the control apparatus 4 that affect the target current that the control apparatus 4 sets for them. For example, each device may provide the control apparatus 4 with a requested minimum current. For example, the EVSE 14 may request a minimum current (e.g. 6A per loaded phase) when the electric vehicle 20 is being charged, so that charging of the electric vehicle 20 is not interrupted even where power supplied from the solar panel 24 drops (for example due to clouds). Thus, the minimum current request may be in place only for a defined time period, or it may be permanent (i.e. a requirement for a given device).

Thirdly, each controllable load may supply signals to the control apparatus 4 including a requested (or required) maximum current. These maximum currents may arise due to fuse ratings or wire ratings for each of the loads. The control apparatus 4 may also be aware of a maximum current rating of the overall polyphase power supply system 2, i.e. a maximum grid current, and may vary the target currents of the controllable loads so that this overall limit is not exceeded.

Within the confines of these limits, the current target supplied by the control apparatus 4 may also be varied as a result of a load making a temporary “boost” request, e.g., by setting a “boost flag”. The boost request may be activated for a pre-set time period (e.g. according to a schedule), or may be triggered by a user request. For example, the resistive heating load 16 may be set to be heated during certain time periods, so that during these time periods it will make a boost request to the control apparatus.

The boost request can also be made (for some or all devices) due to an external boost request (i.e. from a device other than one of the loads). For example, the consumer unit 7 may supply a signal to the control apparatus 4, indicating that the electrical tariff is reduced (i.e. the cost is lower), and this may trigger boost requests for some or all of the loads.

These factors are not exhaustive, and there may also be other factors that the control apparatus 4 takes into account in some embodiments when determining the target currents.

As discussed below, each load is assigned a corresponding priority value. The priority values together define a priority ordering for all of the loads 12, 13, 14, 18.

The control apparatus 4 varies the target current signal supplied to each of the controllable loads 12, 13, 14, 18, and therefore varies the current drawn by each load through the plurality of live wires 8a, 8b, 8c in accordance with the above principles, but also such that the phase imbalance is reduced, in accordance with the method described below with reference to Figures 2a-2g.

The control apparatus 4 may recalculate the target currents at regular intervals, e.g. once every second, and signal any changes to the loads accordingly.

Figures 2a-2g show separate parts of a single flow diagram, with dashed lines on each page indicating where the parts of the preceding and following Figures connect to the portion of the flow diagram illustrated in that Figure.

The method starts at step 100. At this stage, the control apparatus 4 receives current measurements from the current sensors 10a, 10b, 10c, indicating the current flowing respectively in the live wires 8a, 8b, 8c (also referred to as the grid current measurements). Additionally, the control apparatus 4 also receives a report from each load (or its respective controller) indicating the current presently being drawn by each of the loads 12, 13, 14, 16 in the polyphase power system 2. Then, at stage 102, the control apparatus 4 calculates the total loading on each phase of the installation, due to both the controllable loads and any other non-controllable loads. This calculation uses both the reported currents from each load, and also the current measurements received from the current sensors 10a, 10b, 10c (i.e. the grid current measurements).

Before transmitting updated target current signals, the control apparatus 4 performs a series of steps in which it calculates the incremental effects of a succession of adjustments to the assigned currents on the phase imbalance, as described below. Once it reaches a final determination, it then transmits the current signals to effect the new current assignments. It then periodically obtains new current measurements and repeats the whole process, e.g. every second.

At stage 104, the control apparatus 4 starts by assigning devices which have made a boost request with their requested power (i.e. their highest rated power). The power is assigned to the boosted load having the highest priority value first, and then in order to each of the other boosted loads, according to their respective priority values, until all of the available power has been allocated. Thus, if there is not sufficient power to supply the boosted power amount to all boosted loads, the loads that miss out will be those assigned a lower priority value. Once the maximum power has been assigned to each of the “boosted” devices, the control apparatus 4 calculates how much surplus locally-generated power (e.g. locally- generated from renewable sources) is available on each of the three phases, at stage 106. This can be determined based on the current measured in each of the three live wires 8a, 8b, 8c, since if not all surplus locally-generated power is self-consumed by the loads in the system it will be exported to the grid as a negative current through one or more of the live wires 8a, 8b, 8c.

At stage 108, the control apparatus checks whether there is any additional surplus locally-generated power still available through the live wires, based on the calculation at stage 106.

If there is no surplus locally-generated power remaining, the control apparatus 4 proceeds straight to stage 112 (discussed below). If there is surplus locally-generated power available, the additional power is assigned to the loads in the polyphase power system 2, from highest priority to lowest priority, at stage 110, until all load requests are satisfied or until there is no remaining surplus locally-generated power. In this example, the control apparatus assigns a set “portion” of the surplus locally-generated power to each load, in priority order. Thus, if the loads with the highest priority level have been assigned all of the surplus locally-generated power that they possibly can, and there’s still remaining surplus locally-generated power, then the loads having the next highest priority level will each be assigned a portion of the surplus locally- generated power, and so on etc. until all of the available surplus locally-generated power is assigned.

At step 112 the control apparatus 4 determines the phase imbalance (referred to in the flow diagram as the neutral current) which will arise from the calculated current assignments. In this example, the phase imbalance is calculated, based on the current values assigned in the preceding steps of the method. If a neutral wire were present in the installation, the neutral current could be calculated by implementing the current assignments determined in the preceding steps, and measuring the current that was generated in the neutral wire as a result. However, this may be undesirable since the neutral current may exceed a desired threshold. So instead a predictive calculation is made before sending the target current signals. At step 114 the expected phase imbalance is compared with a “neutral limit” threshold (i.e. a predetermined phase-imbalance limit). If the phase imbalance is below this threshold, then the method can proceed to its end 116a. Upon ending, the control apparatus 4 will implement the assignments calculated in stages 100-112 by sending target current signals with the final assigned currents to each of the loads 12, 13, 14, 18.

Alternatively, if the phase imbalance exceeds the threshold, the method proceeds to step 118.

At step 118, the control apparatus 4 determines an ordering of the phases 8a, 8b, 8c from the highest-loaded phase to the lowest-loaded phase, i.e. it determines which of the three lives wires has the highest current, i.e. is the highest-loaded phase, which has the lowest current, i.e. is the lowest-loaded phase, and that the remaining wire is the intermediate-loaded phase. Then, at step 120, the control apparatus reduces the current assigned to any load which is only drawing power on a single phase, where that phase is the most highly loaded phase. At this stage, the most that the control apparatus reduces each assigned current by is to bring the current assigned to the highest-loaded phase to be equal to the current assigned to the intermediate-loaded phase. If an adjustment of the assigned current by less than this amount brings the phase imbalance below the neutral limit then the assigned current is altered by this amount. In other words, the current assigned to any load on the highest-loaded phase is reduced either by a minimum amount needed to bring the phase imbalance below the neutral limit, or at most to be the same as the current assigned to the loads on the intermediate-loaded phase, if no reduction by less than this amount brings the current below the neutral limit.

The control apparatus may compute the required adjustments to the assigned currents and make these in a single step, or it may iteratively apply reductions to the assigned currents (e.g. by steps of 1A or 1%) and calculate the effect on the phase imbalance, until a final current assignment is determined for a load which either reduces the phase imbalance below the neutral limit, or reduces the assigned current to the minimum assigned current allowed in this step, i.e. the current drawn on the intermediate-loaded phase. Initially it does this reduction only for the lowest priority load of the set of loads that only draw power on a single phase. Then, if the phase imbalance has not been sufficiently reduced by this change to below the neutral limit, the control apparatus then reduces the current assigned to the load having the next lowest priority value, and so on until the current drawn by all such loads has been reduced to the current drawn on the intermediate-loaded phase or the phase imbalance has been lowered below the threshold.

The control apparatus 4 then carries out a step referred to as a “neutral current change check” 122a. This check is carried out at multiple stages of the method, as described below. The aim of this step is to check that the preceding adjustment step did actually help to reduce the neutral current (i.e. the phase imbalance) and to revert the previous adjustment if it turns out not to have helped. Thus, the neutral current change check 122a includes a first step 124, at which the control apparatus 4 calculates the phase imbalance that will result from the currents assigned by the end of step 120, and a second step 126, at which the control apparatus 4 checks whether this is lower than the neutral current previously calculated at step 112. If the phase imbalance has not reduced then the method proceeds to step 128a at which the previous load adjustment step is reversed (i.e. the currents assigned to the singlephase loads on the highest loaded phase are increased back to their previous values).

After this, or directly after step 126 if the phase imbalance has reduced, the control apparatus 4 checks whether the phase imbalance is still higher than the threshold, at step 130a. If the phase imbalance is now below the threshold, then the method proceeds to its end 116b. Otherwise, the method continues to step 132. At step 132, the control apparatus 4 increases the current assigned to any load on the least loaded phase. This stage either increases the assigned current until the phase imbalance no longer exceeds the neutral limit, or at most it increases the assigned current until it reaches the current assigned to loads on the intermediate-loaded phase. The control apparatus may compute the required adjustments to the assigned currents and make these in a single step, or it may iteratively apply increases to the assigned currents (e.g. by steps of 1A or 1%) and calculate the effect on the phase imbalance, until a final current assignment is determined for a load which either reduces the phase imbalance below the neutral limit, or increases the assigned current to the maximum assigned current allowed in this step, i.e. the current drawn on the intermediate-loaded phase. It starts by increasing the current assigned to the highest priority load which is supplied only by the least loaded phase, and only if this does not reduce the phase imbalance to below the neutral limit it then proceeds through the priority ordering for any other loads, checking whether each reduces the phase imbalance below the neutral limit.

Next, the control apparatus 4 again carries out a Neutral Current Change Check 122b, and if the phase imbalance has not reduced then the method proceeds to step 128b at which the previous load adjustment step is reversed (i.e. the currents assigned to the single-phase loads on the lowest loaded phase are decreased back to their previous values). After this, or directly after step 122b if the phase imbalance has reduced, the control apparatus 4 checks whether the phase imbalance is still higher than the threshold, at step 130b. If the phase imbalance is now below the threshold, then the method proceeds to its end 116c. Otherwise, the method continues to step 134.

At step 134, the control apparatus 4 reduces the current assigned to any load that is loaded on two phases, where those phases are the highest two loaded phases out of the three phases. The maximum reduction for this stage is to bring the current drawn on the highest two loaded phases down until the current drawn on the lowest out of these two phases is equal to the current drawn on the lowest-loaded phase. If a smaller adjustment brings the phase imbalance below the target current then this will be used instead. Again, the adjustment to the assigned currents may be computed by the control apparatus in a single stage, or it may be determined by incrementally adjusting the assigned current (e.g. by amounts of 1A or 1%) and determining the effect of these changes on the phase imbalance. It starts by decreasing the current assigned to the lowest priority load which is supplied by the two most loaded phases, and then proceeds through the priority ordering for any other loads from lowest up to highest priority value (again checking each time whether the phase imbalance has reduced below the threshold and only moving on to the next load in the priority ordering if it has not).

Next, the control apparatus 4 again carries out a Neutral Current Change Check 122c, and if the phase imbalance has not reduced then the method proceeds to step 128c at which the previous load adjustment step is reversed (i.e. the current assigned to the dual phase loads on the highest two loaded phases is increased). After this, or directly after step 122c if the phase imbalance has reduced, the control apparatus 4 checks whether the phase imbalance is still higher than the threshold, at step 130c. If the phase imbalance is now below the threshold, then the method proceeds to its end 116d. Otherwise, the method continues to step 136.

At step 136, the control apparatus 4 increases the current assigned to any load that is loaded on two phases, where those phases are the two lowest loaded phases out of the three phases. The assigned currents are increased until either the phase imbalance is reduced below the neutral limit, or until the maximum increase has been made. The maximum increase for this stage is to increase the current drawn on the lowest two loaded phases until the current drawn on the highest out of these two phases is equal to the current drawn on the highest-loaded phase. Again, the adjustment to the assigned currents may be computed by the control apparatus in a single stage, or it may be determined by incrementally adjusting the assigned current (e.g. by amounts of 1A or 1%) and determining the effect of these changes on the phase imbalance. It starts by increasing the current assigned to the highest priority load which is supplied by the two least loaded phases, and then proceeds through the priority ordering for any other loads from highest down to lowest priority value (again checking each time whether the phase imbalance has reduced below the threshold and only moving on to the next load in the priority ordering if it has not).

Next, the control apparatus 4 again carries out a Neutral Current Change Check 122d, and if the phase imbalance has not reduced then the method proceeds to step 128d at which the previous load adjustment step is reversed (i.e. the current assigned to the dual phase loads on the lowest two loaded phases is decreased). After this, or directly after step 122d if the phase imbalance has reduced, the control apparatus 4 checks whether the phase imbalance is still higher than the threshold, at step 130d. If the phase imbalance is now below the threshold, then the method proceeds to its end 116e. Otherwise, the method continues to step 138.

At step 138, the control apparatus 4 adjusts the current assigned to any load that is loaded only on a single phase, where that phase is the intermediate-loaded phase. It adjusts the assigned current towards a value that is the mean value between the current on the highest-loaded phase and the current on the lowest-loaded phase. The assigned current may be adjusted in increments or decrements (e.g. 0.1A, 1A or 1% steps) or directly (or as close to as possible) to the mean value (i.e. if no other limits or restrictions prevent this). Optionally this step can take into account the priority value assigned to the loads. If it is taken into account, then the control apparatus will incrementally increase the assigned current in the order of highest to lowest priority but will incrementally decrease (i.e. decrement) the assigned currents in the order of lowest to highest priority.

Next, the control apparatus 4 again carries out a Neutral Current Change Check 122e, and if the phase imbalance has not reduced then the method proceeds to step 128e at which the previous load adjustment step is reversed (i.e. the current assigned to single-phase loads that are supplied by the intermediate-loaded phase are reverted to their previous values). After this, or directly after step 122e if the phase imbalance has reduced, the control apparatus 4 checks whether the phase imbalance is still higher than the threshold, at step 130e. If the phase imbalance is now below the threshold, then the method proceeds to its end 116f. Otherwise, the method continues to step 140.

At step 140 the control apparatus checks if there are any loads that are supplied by both the highest phase and the intermediate phase. If there are, the device then checks at step 142 if there are loads supplied by both the highest and the lowest phases. If there are, then the method proceeds to step 144. At step 144 the control apparatus reduces the current assigned to both of these sets of loads, i.e. both to loads supplied by the highest and intermediate-loaded phase and to loads supplied by the highest and lowest phases. Again, this adjustment may be made incrementally or computed by the control apparatus. Where the reduction is iterative, the control apparatus will calculate for the next increment whether the phase imbalance is reduced, if it is then it will move to the next increment for calculation, if it is not then the preceding current assignment value (i.e. the previous iteration) will be used to provide the limit for the assigned current (i.e. the process will stop once a further incremental decrease is determined to not further decrease the phase imbalance). This step is similar to step 134, but the intervening assignment steps may have the effect that which phase is the highest-loaded phase may have changed as a result of the changes made, compared to which were the highest and intermediate loaded phases when step 134 was carried out. Alternatively, if at step 140 the control apparatus determines that there are no loads that are loaded on the highest and intermediate phases, then the method proceeds to step 146 at which the device then checks if there are loads supplied by both the highest and the lowest phases. If there are, then the method proceeds to step 148. At step 148 the control apparatus reduces the current assigned to these loads, to move it towards the value of the load on the intermediate-loaded phase, or towards a mean value between the highest and the lowest loaded phase. Again, this may be done iteratively or through calculation by the control apparatus.

If these changes are made, in either step 144 or 148, the control apparatus 4 then carries out a Neutral Current Change Check 122f, and if the phase imbalance has not reduced then the method proceeds to step 128f at which the previous load adjustment step (either at 144 or 148) is reversed. After this, or directly after step 142 or 146 if there are no suitable dual-phase loads for which the assigned current can be reduced, the control apparatus checks whether the phase imbalance is still higher than the threshold, at step 130f. If the phase imbalance is now below the threshold, then the method proceeds directly to its end 116g. If all of these phase imbalance reduction steps have still not successfully reduced the phase imbalance to below the threshold then a warning is displayed to the user, at stage 150. After this the method ends at step 116g.

It will be understood that once the method ends the control apparatus 4 then implements the currents assigned during the method described above (i.e. the control apparatus need not actually make any adjustments to the devices as the currents are assigned, until a final determination is made as to the target currents, at which point these finalised current assignments are transmitted to the controllable loads).

An example of the application of this method of reducing a phase imbalance will now be described with reference to the vector diagrams of Figures 3-8.

Each of Figures 3 to 8 shows vectors 300, 302, 304 representing the current in each of the three live wires 8a, 8b, 8c. The length of each vector represents the magnitude of the current and the angle of each vector represents the relative phase of each live wire 8a, 8b, 8c. In this example system 1, the current in each of the three live wires is 120° out of phase with respect to the other two. The solid vector 300 represents the current in the first live wire 8a. The dashed vector 302 represents the current in the second live wire 8b. The dotted and dashed vector 304 represents the current in the third live wire 8c. The dotted vector 306 represents the phase imbalance (also referred to as the neutral current) which arises as a result of the three currents 300, 302, 304.

Figure 3 shows the effect on the supply of an initial default assignment of currents to the loads in a simulated example system 1, while Figures 4 to 8 simulate the progressive reduction in the magnitude of the phase-imbalance vector 306 that can be achieved by successively performing control steps as disclosed above.

In accordance with the method described above with reference to Figures 2a-2g, the current assigned to single phase loads on the highest loaded phase is reduced first, as in step 120. In a starting assignment represented in Figure 3, the highest loaded phase is the first live wire 8a. Referring to Figure 1 it can be seen that the resistive heating load 16 is supplied only by the first live wire 8a. Thus, the current assigned to the resistive heating load 16 (i.e., sent to the resistive load controller 18) is reduced first. In this example, the current assigned to the resistive heating load 16 is reduced as much as possible until the magnitude of the current 300 flow through the first-phase live wire 8a will equal the current 302 flow through the second-highest loaded live-wire phase 8b. Thus at most the current 300 flow through the first-phase live wire 8a will be reduced until it equals the current 302 flow through the second-highest loaded live- wire 8b. If a smaller reduction brings the phase imbalance to below the neutral limit, then the current 300 flow through the first-phase live wire 8a will be reduced but will remain higher than the current 302 flow through the second-highest loaded live-wire 8b and the control apparatus will move on to the next step. Depending on the nature of the loads, it may not always be possible to achieve parity; however, in this example it is possible to do so and the effect of this reduction is represented in Figure 4, in which the magnitude of the current 300 in the first live wire 8a is reduced to equal that of the second live wire 8b, and therefore the phase imbalance 306 is reduced.

Next, as explained above in step 132 of the method, the current 304 assigned to devices that are supplied by the lowest loaded phase, in this case the third live wire 8c, is increased. As explained above, the assigned current will not be increased above any limit indicated by the relevant device, e.g. the maximum fuse rating for that device. In this example, as seen in Figure 1 , the second battery 13 is the only single-phase load that is supplied by the third live wire 8c. Thus, the current assigned to the second battery 13 is increased (e.g. either until the phase imbalance no longer exceeds the neutral limit, or up to the current assigned to loads on the intermediate-loaded phase). The effect of this is represented in Figure 5, in which the current 304 in the third live wire 8c has increased, and as a result the phase imbalance 306 has reduced further.

The control apparatus 4 then reduces the current assigned to loads that are powered by both of the highest two loaded phases, in accordance with step 134. In this example the highest loaded phases are the first live wire 8a and the second live wire 8b, and therefore the only such load is the EVSE 14, which is selectively supplying power to the electric vehicle 20 only through the first and second live wires 8a, 8b). The current assigned to the EVSE 14 is therefore reduced, either by the amount required to bring the phase imbalance below the neutral limit, or until the current drawn on the lowest out of these two phases is equal to the current drawn on the lowest- loaded phase. The assigned current will not be reduced below the minimum current value if there is either a pre-set minimum (i.e. a rated minimum) or it is a period of time in which the EVSE 14 has requested the supply of a minimum current. This results in the currents 300, 302 in both the first and second lives wires 8a, 8b, being reduced by equal amounts, and therefore the phase imbalance 306 is reduced still further, as shown in Figure 6.

Next, in accordance with step 136, the control apparatus 4 increases the current assigned to loads that are supplied by the two least loaded phases. In this example the least loaded phases are now the second live wire 8b and the third live wire 8c. The first battery 12 is supplied by both the second and third live wires 8b, 8c. Therefore, at this stage, the current assigned to the first battery 12 is increased, either until the phase imbalance is reduced below the neutral limit, or if that is not achieved, until the maximum increase has been made. The maximum increase for this stage is to increase the current drawn on the lowest two loaded phases until the current drawn on the highest out of these two phases is equal to the current drawn on the highest- loaded phase. The result of this is that the currents 302, 304 are both increased, further reducing the phase imbalance 306, as shown in Figure 7.

As laid out in step 138, the current supplied to any loads that are single-phase loads supplied by the intermediate-loaded phase (which by this point in the assignment process is the first live wire 8a) are then adjusted towards the mean value between the highest phase 8b and the lowest phase 8c. In this case the resistive heating load 16 is supplied only by the first live wire 8a. Thus, according to step 138, the current assigned to the resistive heating load 16 (i.e. sent to the resistive load controller 18) is adjusted so that its magnitude is equal to the mean magnitude value of the current on the second phase 302 and the current on the third phase 304. This results in the current assigned to the resistive heating load 16 being decreased, resulting in a decrease in the current on the first live wire 300. The resulting phase imbalance 306 is represented in Figure 8.

In this particular example the magnitude of the final resulting phase imbalance is smaller than the pre-set threshold, and therefore the process ends with the loading illustrated in Figure 8, and the control apparatus 4 then transmits target current signal to each load 12, 13, 14, 18 that implement the assigned currents determined in the method above, which resulted in the loading represented in Figure 8.

It will be appreciated by those skilled in the art that the invention has been illustrated by describing one or more specific embodiments thereof, but is not limited to these embodiments; many variations and modifications are possible, within the scope of the accompanying claims.