Dynamic transformer energisation system and method

Abstract

The core of a power transformer of an EV charging system is pre-magnetised by energising one or more secondary windings of the power transformer so as to bring one or more attributes of one or more voltages across one or more primary windings of the power transformer into a prescribed degree of alignment with a power supply to the transformer. Pre-magnetisation can be monitored by monitoring one or more attributes of one or more voltages across the one or more primary windings of the power transformer. Alternatively, pre-magnetisation can be monitored by monitoring one or more attributes of transformer core magnetisation, either directly or inferred. Primary windings of the transformer may be connected to the power supply when the transformer core is pre- magnetised.

Description

[0001] Dynamic Transformer Energisation System and Method

[0002] FIELD

[0003] This invention relates to an EV (electric vehicle) charging system and a method of transformer energisation.

[0004] BACKGROUND

[0005] The number of EVs on the road is rapidly increasing. Presently much EV charging infrastructure is under-utilised. However, even if EVs were to completely replace internal-combustion engine vehicles and the charging infrastructure usage matched petrol station usage, the effective utilisation of the charging infrastructure will still be low, especially at night. Thereby the infrastructure is currently underutilised and projected to be underutilised in the future.

[0006] While high-power fast-charging charging station infrastructure turns itself off and uses minimal power when not charging, the larger upstream grid supply transformers remain on. While the grid transformers have minimal losses when there is no power flowing through them, they still exhibit some losses, and being energised may also result in potential lifetime reduction due to insulation degradation. The unnecessary losses and lifetime reduction is further exacerbated while EV adoption is low.

[0007] As journey EV charger powers increase to 100's of kilowatts or megawatts, when installed they are typically installed alongside a dedicated new power transformer. There are also EV fast charger topologies whereby the grid transformer is dedicated and built into the charger unit.

[0008] In applications where the EV charger has its own dedicated grid transformer, it would be possible to turn off (de-energise) the transformer to minimise losses and increase lifetime. While it is possible to turn the grid transformer on and off, turning a transformer on (energising) results in a high in-rush current from the grid, which is undesirable for several reasons:

[0009] • The inrush current results in grid voltage disturbances. On weak grids (higher grid impedance) the voltage disturbances can be sufficient to trip nearby loads. On stronger grids (lower grid impedance) the inrush currents can sustain for minutes or sometimes hours.

[0010] • The large inrush current can result in a large mechanical stress on the transformer and such damage the transformer over time.

[0011] • The large inrush currents can reduce the lifetime of upstream fuses.

[0012] All these issues typically mean that the number of transformer energisations should be minimised to typically only a few per year. As a result of this, grid transformers are typically turned on and left on, and only turned off by exception, e.g. for maintenance.

[0013] The process of putting voltage on a transformer is commonly called "magnetising" a transformer. If an "off" (de-energised) transformer is directly connected to a grid (also known as Direct On-Line or DOL), the transformer magnetic flux will saturate, which is the underlying cause of the high inrush current. If power is gradually provided to the primary windings of the transformer from the grid supply this requires large and expensive electrical components.

[0014] It is an object of the present invention to provide an improved method of providing power to a transformer at start up and a system therefor or to at least provide the public with a useful choice.

[0015] SUMMARY

[0016] According to one example embodiment there is provided a method of premagnetising the core of a power transformer of an EV charging system comprising: a. energising one or more secondary windings of the power transformer so as to achieve a desired magnetisation of the core of the transformer; b. monitoring one or more attributes of transformer core magnetisation either directly or inferred; and c. connecting one or more primary windings of the power transformer to a power supply when one or more attributes of the transformer core magnetisation have achieved a prescribed degree of alignment with the desired magnetisation of the core of the transformer.

[0017] In step a, the amplitude of a voltage applied to each secondary winding can be successively increased until one or more attributes of the transformer core magnetisation have achieved a prescribed degree of alignment with the desired magnetisation of the core of the transformer.

[0018] The amplitude of the voltage applied to each secondary winding in step a can be determined using a measured or modelled transformer flux model.

[0019] The core magnetisation can be measured directly by one or more magnetic field sensor.

[0020] Transformer core magnetisation can be inferred by monitoring the voltage across or current passing through one or more primary windings.

[0021] One or more attributes of one or more voltages across one or more primary windings of the power transformer can be monitored.

[0022] The power supply can be connected to the one or more primary windings of the power transformer when one or more attributes of one or more voltages across the one or more primary windings of the power transformer have achieved a prescribed degree of alignment with a power supply to the transformer. A bi-directional converter of an EV charging system can be used to energise one or more secondary windings of the transformer.

[0023] The bi-directional converter can be powered by the battery of an electric vehicle.

[0024] The bi-directional converter can be powered by a DC power supply.

[0025] A dedicated inverter can be used to energise one or more secondary windings of the transformer.

[0026] The dedicated inverter can be powered from a DC power supply.

[0027] The dedicated inverter can be powered by one or more battery.

[0028] The attributes can include one or more of: amplitude of the voltages across one or more primary winding of the transformer; phase of the voltages across one or more primary winding of the transformer; frequency of the voltages across one or more primary winding of the transformer.

[0029] All attributes can be monitored for one or more primary winding of the transformer.

[0030] The power supply can be an electricity supply grid.

[0031] The transformer can be a three phase transformer.

[0032] Energising one or more secondary windings of the power transformer can involve ramping the amplitude of voltage applied to each secondary winding over a plurality of iterations.

[0033] According to another example embodiment there is provided an electric vehicle charging system comprising: a. a power transformer having one or more primary windings for electrical connection to a power supply, one or more secondary windings and a core; b. an inverter electrically connected to the one or more secondary windings configured to energise the one or more secondary windings of the power transformer so as to achieve a desired magnetisation of the core of the transformer; c. a monitoring system configured to monitor one or more attributes of transformer core magnetisation either directly or inferred; and d. a switching system configured to connect the one or more primary windings of the power transformer to the power supply when one or more attributes of transformer core magnetisation have achieved a prescribed degree of alignment with the power supply to the transformer.

[0034] The monitoring system can be configured to monitor one or more attributes of one or more voltages across the one or more primary windings of the power transformer to infer one or more attributes of transformer core magnetisation and the switching system is configured to connect the one or more primary windings of the power transformer to the power supply when one or more attributes of one or more voltages across the one or more primary windings of the power transformer have achieved a prescribed degree of alignment with a power supply to the transformer.

[0035] The attributes monitored by the monitoring system can include one or more of: amplitude of the voltages across one or more primary winding of the transformer; phase of the voltages across one or more primary winding of the transformer; frequency of the voltages across one or more primary winding of the transformer.

[0036] All attributes can be monitored for one or more primary winding of the transformer. The monitoring system can include a magnetic field sensor configured to measure one or more magnetic field attribute in the core of the transformer.

[0037] The inverter can be a bidirectional converter electrically connected to the one or more secondary windings configured to: i. in a first mode energise the one or more secondary windings of the power transformer so as to bring one or more attributes of one or more voltages across one or more primary windings of the power transformer into a prescribed degree of alignment with a power supply to the transformer; and ii. in a second mode, when the power supply is connected to the one or more primary windings of the power transformer, supply power to an electric vehicle.

[0038] The bi-directional converter can be configured to be powered by a battery of an electric vehicle.

[0039] The bi-directional converter can be configured to be powered by a DC power supply.

[0040] The inverter can be an auxiliary inverter electrically connected to the one or more secondary windings configured to energise the one or more secondary windings of the power transformer so as to bring one or more attributes of one or more voltages across one or more primary windings of the power transformer into a prescribed degree of alignment with a power supply to the transformer.

[0041] The auxiliary inverter can be powered from a DC power supply.

[0042] The auxiliary inverter can be powered by one or more battery.

[0043] The one or more primary windings of the transformer can be connected to an electricity supply grid. The transformer can be a three phase transformer.

[0044] Energising one or more secondary windings of the power transformer can involve ramping the amplitude of voltage applied to each secondary winding over a plurality of iterations.

[0045] According to a further example there is provided a method of pre-magnetising the core of a power transformer of an EV charging system comprising: a. energising one or more secondary windings of the power transformer so as to bring one or more attributes of one or more voltages across one or more primary windings of the power transformer into a prescribed degree of alignment with a power supply to the transformer; b. monitoring one or more attributes of one or more voltages across the one or more primary windings of the power transformer; and c. connecting the power transformer to the power supply when one or more attributes of one or more voltages across the one or more primary windings of the power transformer have achieved a prescribed degree of alignment with a power supply to the transformer.

[0046] According to another example embodiment there is provided a method of premagnetising the core of a power transformer of an EV charging system comprising: a. energising one or more secondary windings of the power transformer so as to achieve a desired magnetisation of the core of the transformer; b. monitoring one or more attributes of transformer core magnetisation either directly or inferred; and c. connecting the power transformer to the power supply when one or more attributes of the transformer core magnetisation have achieved a prescribed degree of alignment with the desired magnetisation of the core of the transformer.

[0047] According to another example embodiment there is provided an electric vehicle charging system comprising: a. a power transformer having one or more primary windings for electrical connection to a power supply, one or more secondary windings and a core; b. a monitoring system configured to monitor one or more attributes of one or more voltages across the one or more primary windings of the power transformer; c. a switching system configured to connect the power transformer to the power supply when one or more attributes of one or more voltages across the one or more primary windings of the power transformer have achieved a prescribed degree of alignment with a power supply to the transformer; and d. a bidirectional converter electrically connected to the one or more secondary windings configured to: i. in a first mode energise the one or more secondary windings of the power transformer so as to bring one or more attributes of one or more voltages across one or more primary windings of the power transformer into a prescribed degree of alignment with a power supply to the transformer; and ii. in a second mode, when the power supply is connected to the one or more primary windings of the power transformer, supply power to an electric vehicle. According to another example embodiment there is provided an electric vehicle charging system comprising: a. a power transformer having one or more primary windings for electrical connection to a power supply, one or more secondary windings and a core; b. a monitoring system configured to monitor one or more attributes of one or more voltages across the one or more primary windings of the power transformer; c. a switching system configured to connect the power transformer to the power supply when one or more attributes of one or more voltages across the one or more primary windings of the power transformer have achieved a prescribed degree of alignment with a power supply to the transformer; d. a converter electrically connected to the one or more secondary windings configured to supply power to an electric vehicle; and e. an auxiliary inverter electrically connected to the one or more secondary windings configured to energise the one or more secondary windings of the power transformer so as to bring one or more attributes of one or more voltages across one or more primary windings of the power transformer into a prescribed degree of alignment with a power supply to the transformer.

[0048] It is acknowledged that the terms "comprise", "comprises" and "comprising" may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, these terms are intended to have an inclusive meaning - i.e., they will be taken to mean an inclusion of the listed components which the use directly references, and possibly also of other non-specified components or elements. Reference to any document in this specification does not constitute an admission that it is prior art, validly combinable with other documents or that it forms part of the common general knowledge.

[0049] BRIEF DESCRIPTION OF THE DRAWINGS

[0050] The accompanying drawings which are incorporated in and constitute part of the specification, illustrate embodiments of the invention and, together with the general description of the invention given above, and the detailed description of embodiments given below, serve to explain the principles of the invention, in which:

[0051] Figure 1 is a block diagram of an EV charging system according to one example;

[0052] Figure 2 is a block diagram of an EV charging system according to another example;

[0053] Figure 3 is a block diagram of an EV charging system according to another example utilising transformer magnetic flux information;

[0054] Figure 4 shows a transformer with an embedded flux sensor in its core showing flux components in the transformer when energised; and

[0055] Figure 5 shows an algorithm for achieving a desired transformer flux.

[0056] DETAILED DESCRIPTION

[0057] Figure 1 shows the general topology of an EV charging system in which power from the grid 1 can be switched in by grid breaker 2 to energise the primary windings 3 of a grid power transformer 4. It will be appreciated that the grid supply and transformer will typically be three phase and the diagram is generalised but is equally applicable to single phase or other numbers of phases. Secondary windings 5 of transformer 4 supply power to an EV charging station 6 having an outlet 7 for supplying power to an electric vehicle 10. The EV charging station 6 will typically include a grid side converter 8 receiving power from the secondary windings 5 and, in its normal operating mode, supplying power to an EV charge inverter 9 which delivers power via outlet 7 suitable for recharging an electric vehicle 10. It will be appreciated that there will typically be multiple charging stations 6 connected to common or separate secondary windings.

[0058] According to one example the grid side converter 8 of charging station 6 may be capable of bidirectional operation to operate in two modes. During normal EV charging grid breaker 2 is closed and power flows from the transformer secondary windings 5 through charging station 6 to outlet 7. However, in a second reverse mode at transformer start up when grid breaker 2 is open, power may initially flow from grid side converter 8 to energise secondary windings 5 so as to energise the core of transformer 4 prior to grid breaker 2 closing to connect the grid supply 1 to the primary windings 3.

[0059] According to one method the core of power transformer 4 may be pre-magnetised by: a) pre-charging the grid-side converter DC bus, such that it has enough DC bus headroom to magnetise the transformer core; b) whilst the grid breaker 2 is open, supplying the grid-side converter 8 from the DC bus to operate in its reverse mode and ramp up the applied voltages to one or more secondary windings 5 of the power transformer 4 (energising the one or more secondary windings) so as to bring one or more attributes of one or more voltages across one or more primary windings 3 of the power transformer 4 into a prescribed degree of alignment with the grid power supply to the transformer; c) monitoring one or more attributes of one or more voltages across the one or more primary windings of the power transformer 4; and d) connecting the primary winding(s) 3 of power transformer 4 to the power supply 1 via the grid breaker 2 when one or more attributes of one or more voltages across the one or more primary windings of the power transformer 4 have achieved a prescribed degree of alignment with the grid power supply 1 to the transformer 4. The grid-side converter 8 can be switched from its reverse mode to its normal mode at or prior to connection of the primary winding(s) 3 of power transformer 4 to the power supply 1.

[0060] The DC bus may be powered by a DC power supply 11 connected to an AC supply, a battery or other suitable source with any required conversion circuitry.

[0061] In an alternative example power from an electric vehicle may be supplied via outlet 7 through EV charge inverter 9 to grid side converter 8 to drive secondary windings 5 in its reverse mode of operation to energise the core of transformer 4. This functionality will not currently be available for all EVs and so for a universal solution this could be an optional method.

[0062] In another example shown in Figure 2 like elements have been given like numbers to those shown in Figure 1. In this case a dedicated inverter 15 is used to energise one or more secondary windings 5 of the transformer 4 instead of the grid side converter 8 of Figure 1. Controller 14 controls dedicated inverter 15 to produce the required drive waveforms for the secondary windings 5 with operation otherwise as per that of Figure 1. In this example an auxiliary transformer 13 supplies power to inverter 15 but it could be supplied from a different AC source or a DC source or battery etc. This approach may be used when the grid side converter 8 is not capable of the required bidirectional operation.

[0063] Referring again to Figure 1 an exemplary control system will be described. A controller 12 monitors voltages of one or more primary windings 3 and, optionally, one or more secondary windings 5. In this example the dashed lines are sensing and control lines but wireless or other communication techniques may be employed. Based on one or more sensed attributes of one or more voltages across the one or more primary windings 3 of the power transformer the grid side converter 8 and grid breaker 2 may be controlled. Ideally the controller will monitor the amplitude of the voltages across one or more primary windings 3 of the transformer 4; the phase of the voltages across one or more primary windings 3 of the transformer 4; and the frequency of the voltages across one or more primary windings 3 of the transformer 4. Ideally all these attributes will be monitored for all phases.

[0064] The controller 12 can also monitor voltages of the grid power supply 1. Controller 12 can be configured to monitor the amplitude of the voltages across one or more phases of grid power supply 1; the phase of the voltages of grid power supply 1; and the frequency of the of grid power supply 1. It can monitor all phases, if required.

[0065] Whilst it is preferred to monitor all attributes set out in the preceding two paragraphs, fewer attributes could be monitored in a simplified control method.

[0066] The controller 12 can optionally also monitor voltages of one or more secondary windings 5. As controller 14 controls the drive signals for grid side converter 8 it is not essential for the secondary windings to be monitored but this may provide enhanced monitoring and control. Controller 12 can be configured to monitor the amplitude of the voltages across one or more secondary windings 5 of the transformer 4; the phase of the voltages across one or more secondary windings 5 of the transformer 4; and the frequency of the voltages across one or more secondary windings 5 of the transformer 4. It can monitor all phases, if required.

[0067] The attributes necessary to be monitored by controller 12 will depend upon all sources of information available to the controller 12. For example grid supply attributes may be monitored elsewhere and this information provided to controller 12. Further, a combination of information sources may be utilised such as voltages and transformer core flux etc.

[0068] Controller 12 controls the grid side converter 8 in step b above to ramp up the voltage amplitudes applied to the secondary windings 5 over a number of iterations so as to drive the attributes of the induced voltages on the primary windings 3 to match those of the grid power supply 1. Ideally the voltage amplitudes, phases and frequencies of at least one phase of the primary windings 3 and corresponding phase of the grid power supply 1 will be brought into a required degree of alignment. This may be done across all phases on the primary windings of the transformer 4 and grid power supply 1 too. It will be appreciated that exact alignment of primary winding and grid power supply attributes may not be required and the degree of alignment may be selected based on the application.

[0069] Once the attributes of the voltages of one or more primary windings 3 have brought into a required alignment with the attributes of one or more phases of the grid power supply controller 12 can switch in grid breaker 2 and stop grid side converter 8 from driving any secondary winding 5 and adopt normal EV charging operation (i.e. supply power from secondary windings 5 to EV charge inverter 9 to supply to outlet 7).

[0070] By ramping the transformer voltage from zero to the grid voltage (step b in the process above) the transformer does not saturate and does not exhibit the detrimental effects noted above.

[0071] The controller 12 may be integrated into a charging station 6 or be integrated with the magnetising inverter 15 in Figure 2 or be a separate control module. It will be appreciated that controller 14 shown in Figure 2 operates in a similar manner but drives magnetising inverter 15 instead of grid side inverter 8. Referring now to Figure 3 an EV charging system according to another example is shown which utilises transformer magnetic flux information. Like integers to the previous examples have been given like numbers. In this example instead of sensing transformer winding voltages (inferred transformer core magnetisation), the magnetic flux of the transformer core is monitored directly. A magnetic flux sensor 16 provides flux field strength and direction information to controller 17. The flux sensor may be a Hall effect sensor or other suitable sensor.

[0072] Where an E core transformer is employed, such as shown in Figure 4, a single magnetic flux sensor 16 can be sufficient. Additional sensors may be employed for different transformer topologies or to provide redundancy or additional data. Figure 4 shows the positioning of an embedded flux sensor 16 in the core 18 of a transformer 4. It will be appreciated that the required flux can be modelled based on the flux generated by windings in the three cores taking into account flux leakage and zero-sequence fluxes. Thus controller 17 can determine a required flux amplitude, phase and frequency to be sensed by flux sensor 16 that would correspond to the magnetic flux generated when the grid power supply is applied to the primary windings.

[0073] In this example the core of power transformer 4 can be pre-magnetised by: a. when grid breaker 2 is open, the controller 17 controlling the grid side converter 8 to energise one or more secondary windings 5 of the power transformer 4 so as to achieve a desired magnetisation of the core of the transformer; b. Controller 17 monitoring via sensor 16 one or more attributes (esp. magnitude, phase and frequency) of transformer core magnetisation either directly or inferred; and c. Controller 17 controlling the grid breaker 2 to close, connecting the power transformer 4 to the grid power supply 1, when one or more attributes of the transformer core magnetisation have achieved a prescribed degree of alignment with the desired magnetisation of the core of the transformer. Grid side converter 8 would then revert to its normal EV charging mode. It will be appreciated that this approach may be similarly applied to an implementation using an auxiliary inverter as per the example shown in Figure 2.

[0074] As in previous examples the amplitude of the voltage applied to each secondary winding in step a is successively increased until one or more attributes of the transformer core magnetisation have achieved a prescribed degree of alignment with the desired magnetisation of the core of the transformer.

[0075] The amplitude of the voltage applied to the secondary windings can be iteratively increased until a required transformer flux (measured or inferred) is achieved. In this way the transformer is energised with no inrush current.

[0076] An alternative to ramping is to use measured, modelled or inferred transformer flux and implement an algorithm which aligns the flux as required. An example algorithm is shown in Figure 5.

[0077] Whilst the examples have been described in relation to three phase embodiments it will be appreciated that they may be implemented in single phase embodiments too. It will also be appreciated that not all secondary windings need to be driven to generate a pre-magnetising flux. In some cases partial pre-magnetisation by driving only one secondary winding may be sufficient or in some applications driving only two secondary windings may be sufficient.

[0078] It will also be appreciated that common aspects of the charging systems in each example are interchangeable and the description in one example may be applied to another example where appropriate. While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of the Applicant's general inventive concept.

Claims

CLAIMS:

1. A method of pre-magnetising the core of a power transformer of an EV charging system comprising: a. energising one or more secondary windings of the power transformer so as to achieve a desired magnetisation of the core of the transformer; b. monitoring one or more attributes of transformer core magnetisation either directly or inferred; and c. connecting one or more primary windings of the power transformer to a power supply when one or more attributes of the transformer core magnetisation have achieved a prescribed degree of alignment with the desired magnetisation of the core of the transformer.

2. A method as claimed in claim 1 wherein, in step a, the amplitude of a voltage applied to each secondary winding is successively increased until one or more attributes of the transformer core magnetisation have achieved a prescribed degree of alignment with the desired magnetisation of the core of the transformer.

3. A method as claimed in claim 2 wherein the amplitude of the voltage applied to each secondary winding in step a is determined using a measured or modelled transformer flux model.

4. A method as claimed in any one of claims 1 to 3 wherein the core magnetisation is measured directly by one or more magnetic field sensor.

5. A method as claimed in claim 1 wherein transformer core magnetisation is inferred by monitoring the voltage across or current passing through one or more primary windings.

6. A method as claimed in claim 5 wherein one or more attributes of one or more voltages across one or more primary windings of the power transformer are monitored.

7. A method as claimed in claim 6 wherein the power supply is connected to the one or more primary windings of the power transformer when one or more attributes of one or more voltages across the one or more primary windings of the power transformer have achieved a prescribed degree of alignment with a power supply to the transformer.

8. A method as claimed in any one of the preceding claims wherein a bidirectional converter of an EV charging system is used to energise one or more secondary windings of the transformer.

9. A method as claimed in claim 8 wherein the bi-directional converter is powered by the battery of an electric vehicle.

10. A method as claimed in claim 8 wherein the bi-directional converter is powered by a DC power supply.

11. A method as claimed in claim any one of claims 1 to 7 wherein a dedicated inverter is used to energise one or more secondary windings of the transformer.

12. A method as claimed in claim 11 wherein the dedicated inverter is powered from a DC power supply.

13. A method as claimed in claim 11 wherein the dedicated inverter is powered by one or more battery.

14. A method as claimed in claim 6 or claim 7 wherein the attributes include one or more of: amplitude of the voltages across one or more primarywinding of the transformer; phase of the voltages across one or more primary winding of the transformer; frequency of the voltages across one or more primary winding of the transformer.

15. A method as claimed in claim 14 wherein all attributes are monitored for one or more primary winding of the transformer.

16. A method as claimed in any one of the preceding claims wherein the power supply is an electricity supply grid.

17. A method as claimed in claim 16 wherein the transformer is a three phase transformer.

18. A method as claimed in any one of the preceding claims wherein energising one or more secondary windings of the power transformer involves ramping the amplitude of voltage applied to each secondary winding over a plurality of iterations.

19. An electric vehicle charging system comprising: a. a power transformer having one or more primary windings for electrical connection to a power supply, one or more secondary windings and a core; b. an inverter electrically connected to the one or more secondary windings configured to energise the one or more secondary windings of the power transformer so as to achieve a desired magnetisation of the core of the transformer; c. a monitoring system configured to monitor one or more attributes of transformer core magnetisation either directly or inferred; andd. a switching system configured to connect the one or more primary windings of the power transformer to the power supply when one or more attributes of transformer core magnetisation have achieved a prescribed degree of alignment with the power supply to the transformer.

20. An electric vehicle charging system as claimed in claim 19 wherein the monitoring system is configured to monitor one or more attributes of one or more voltages across the one or more primary windings of the power transformer to infer one or more attributes of transformer core magnetisation and the switching system is configured to connect the one or more primary windings of the power transformer to the power supply when one or more attributes of one or more voltages across the one or more primary windings of the power transformer have achieved a prescribed degree of alignment with a power supply to the transformer.

21. An electric vehicle charging system as claimed in claim 20 wherein the attributes monitored by the monitoring system include one or more of: amplitude of the voltages across one or more primary winding of the transformer; phase of the voltages across one or more primary winding of the transformer; frequency of the voltages across one or more primary winding of the transformer.

22. An electric vehicle charging system as claimed in claim 21 wherein all attributes are monitored for one or more primary winding of the transformer.

23. An electric vehicle charging system as claimed in claim 20 wherein the monitoring system includes a magnetic field sensor configured to measure one or more magnetic field attribute in the core of the transformer.

24. An electric vehicle charging system as claimed in any one of claims 19 to 23 wherein the inverter is a bidirectional converter electrically connected to the one or more secondary windings configured to: i. in a first mode energise the one or more secondary windings of the power transformer so as to bring one or more attributes of one or more voltages across one or more primary windings of the power transformer into a prescribed degree of alignment with a power supply to the transformer; and ii. in a second mode, when the power supply is connected to the one or more primary windings of the power transformer, supply power to an electric vehicle.

25. An electric vehicle charging system as claimed in claim 24 wherein the bi-directional converter is configured to be powered by a battery of an electric vehicle.

26. An electric vehicle charging system as claimed in claim 24 wherein the bi-directional converter is configured to be powered by a DC power supply.

27. An electric vehicle charging system as claimed in any one of claims 19 to 23 wherein the inverter is an auxiliary inverter electrically connected to the one or more secondary windings configured to energise the one or more secondary windings of the power transformer so as to bring one or more attributes of one or more voltages across one or more primary windings of the power transformer into a prescribed degree of alignment with a power supply to the transformer.

28. An electric vehicle charging system as claimed in claim 27 wherein the auxiliary inverter is powered from a DC power supply.

29. An electric vehicle charging system as claimed in claim 27 wherein the auxiliary inverter is powered by one or more battery.

30. An electric vehicle charging system as claimed in any one of claims 21 to29 wherein the one or more primary windings of the transformer are connected to an electricity supply grid.

31. An electric vehicle charging system as claimed in any one of claims 21 to30 wherein the transformer is a three phase transformer.

32. An electric vehicle charging system as claimed in any one of claims 21 to31 wherein energising one or more secondary windings of the power transformer involves ramping the amplitude of voltage applied to each secondary winding over a plurality of iterations.

Images