GB2536653A

GB2536653A - DC power supply

Info

Publication number: GB2536653A
Application number: GB1504965.3A
Authority: GB
Inventors: Warnakulasuriya Kapila; Gurwicz David
Original assignee: CARROLL & MEYNELL TRANSFORMERS Ltd; Carroll & Meynell Transf Ltd
Current assignee: CARROLL & MEYNELL TRANSFORMERS Ltd; Carroll & Meynell Transf Ltd
Priority date: 2015-03-24
Filing date: 2015-03-24
Publication date: 2016-09-28
Also published as: GB201504965D0

Abstract

A power supply comprises a rectification stage and a plurality of DC output stages (e.g. for charging batteries). The rectification stage comprises a PFC stage including a twelve pulse transformer generating two three phase AC waveforms phase shifted with respect to each other, two bridge rectifiers, each rectifying a respective three phase AC waveform, and two interphase transformers coupling the rectified DC outputs to provide a combined DC supply, and where each DC output stage has independent current and voltage control. The two interphase transformers coupling the first and second rectified DC outputs have a lower ripple than either of the first or second rectified DC outputs. The rectification stage is arranged to attenuate harmonics transmitted to the power source up to and including the eleventh harmonic of the power line frequency and to provide a total harmonic distortion, THD, of less than 5%. The power supply circuit may be used for a charger for high power charging, such as charging the batteries of multiple electric vehicles (EVs).

Description

DC POWER SUPPLY

The present invention relates to a power supply. More particularly, the present invention relates to a power supply circuit for a charger, which can be used for high power charging, such as charging the batteries of multiple electric vehicles (EVs).

However similar power supply circuits can be used for charging a variety of batteries for energy storage.

With the increasing use of electric power for traction purposes, and the use of high power 50kW+ chargers for this application, the effect on the electrical supply network by the proliferation of these devices needs to be considered.

Electric vehicles can be charged using alternating current, AC, (via an on-board charger) or direct current, DC. However, rapid charging at high power (e.g. 50kW, 120k is normally only possible when a DC supply is used.

A typical approach to DC charging is the rectification of the incoming single or 15 three phase 50/60hz mains followed by a high frequency (15 -25 kHz) isolated DC/DC converter feeding the battery to be charged Simple bridge rectification of the incoming mains results in severe harmonic distortion of the current drawn from the supply, causing severe distortion in the supply voltage with resultant losses in the supply network and other loads fed by the same source. To avoid this, Power Factor Correction (PFC) circuits are interfaced between the supply and the rectification stage to ensure, as far as possible, a lower harmonic current draw from the mains. The complexity and size of these PFC circuits approaches that of the DC/DC conversion -isolation stage, furthermore the effect of these circuits is to increase the high-frequency noise generated, or electromagnetic interference, requiring the addition of large electromagnetic compatibility (EMC) input filters to satisfy the various standards. Whilst this is a problem when "small" chargers are fed off single-phase supplies it becomes more acute when the supply is three phase for larger chargers in excess of 6kW.

The effects of electromagnetic interference become even more significant in circuits for charging multiple batteries at such high powers. While conventional power factor correction circuits may be acceptable for individual small scale and/or low power applications, the considerable noise and interference associated with charging multiple batteries at high power cannot be reduced to acceptable levels using such circuits.

Moreover, a number of high frequency switches are typically required with complex control circuitry and sophisticated EMC filters are required to meet the exported EMC regulations. Conversion efficiency is of the order of 92%.

For this reason currently units for charging electric cars are typically designed for providing one DC and one AC charging current simultaneously.

The invention is concerned with providing a unit that is capable of simultaneously charging multiple batteries using rapid DC charging.

SUMMARY OF THE INVENTION

Aspects of the invention are set out in the independent claims and preferred features are set out in the dependent claims.

According to a first aspect there is provided a power supply system for charging a plurality of direct current, DC, batteries, the power supply system comprising: an alternating current, AC, rectification stage for providing a first DC supply, the AC rectification stage comprising: input terminals for connection to a three-phase AC power source having a power line frequency of the order of 50-60Hz; and a passive power factor control, PFC, stage comprising: an at least twelve-pulse transformer for generating a first three-phase AC waveform and a second three-phase AC waveform from the input three-phase AC power source, wherein the generated three-phase waveforms are phase-shifted with respect to each other; a first bridge rectifier arrangement connected to the at least twelve-pulse transformer for rectifying the first generated three phase AC waveform to provide a first rectified DC output; a second bridge rectifier arrangement connected to the at least twelve-pulse transformer for rectifying the second generated three-phase AC waveform to provide a second rectified DC output; and two interphase transformers coupling the first and second rectified DC outputs to provide a combined DC supply having a lower ripple than either of the first or second rectified DC outputs; wherein the rectification stage is arranged to attenuate harmonics transmitted to the power source up to and including the eleventh harmonic of the power line frequency and to provide a total harmonic distortion (THD) of less than 5%; a DC supply smoothing arrangement connected to the first DC supply, the DC supply smoothing arrangement having at least a capacitor to reduce the ripple on the first DC supply; and a plurality of DC output stages connected to the DC supply, each DC output stage for connecting to a respective DC battery, each DC output stage having an independent current and voltage output control stage for regulating DC power supply to the respective battery.

Pursuant to the invention it has been appreciated that in a high power multiple battery charger-type application it may be advantageous to provide an essentially passive rectification stage to provide an internal DC supply and multiple separate DC-DC output stages. Although this may seem counter-intuitive as the internal DC supply voltage is not actively controlled and may vary from a desired level for charging, the overall system noise and distortion can be reduced this way.

By providing a power supply circuit with an at least twelve-pulse transformer as the PFC stage, it is possible to attenuate at least the fifth and seventh harmonics of the power line frequency. The harmonic filtering arrangements may provide attenuation of other harmonics of the power line frequency, such as the eleventh harmonic. By using such components, the noise on the power source input is reduced and it has been found to result in an acceptable total harmonic distortion (THD) of <5%. Advantageously, as this PFC stage is passive it is possible to use this circuit even when delivering high power (10kW+, 50kW+ or even 100kW+) rapid charging for multiple DC batteries. By providing interphase transformers, or interphase reactors, the operation of the rectifier groups may be kept independent to help ensure correct working of the PFC stage. Providing this central PFC and rectification stage, which may ensure noise on the input power source terminals is reduced to an acceptable level, and can be more efficient than providing separate PFC and rectification for each output stage, while using independent voltage and current control for each output stage, may allow the supply to each battery to be regulated effectively in spite of the residual ripple associated with such a PFC arrangement. Thus a very efficient system for charging multiple batteries simultaneously can be provided.

Preferably, the rectification stage includes a harmonic filtering arrangement arranged to attenuate harmonics up to and including at least the thirteenth harmonic of the power line frequency and to provide a total harmonic distortion of less than 3%.

By combining this arrangement with harmonic filtering up to the thirteenth harmonic, the total harmonic distortion (THD) or electromagnetic interference on the input current can be drastically reduced (e.g. providing <3% THD), which can lower noise and ensure interference on the input power supply is reduced to acceptable levels.

Preferably, the PFC stage comprises a twelve-pulse transformer for generating the first three-phase AC waveform and the second three-phase AC waveform phase-shifted by 30degrees with respect to each other; and wherein the harmonic filtering arrangement comprises: a first harmonic trap for filtering the eleventh harmonic of the power line frequency; and a second harmonic trap for filtering the thirteenth harmonic of the power line frequency.

By using this 12-pulse configuration to generate two phase-shifted three-phase AC waveforms, 12-pulse rectification can result in attenuation of the fifth, seventh, seventeenth and nineteenth harmonics of the input power line frequency, which may be optimised at a 30degree phase-shift. It has been found that this harmonic cancellation, when combined with eleventh and thirteenth harmonic traps, may reduce the total harmonic distortion to an acceptable level even for high power charging to multiple outputs.

Alternatively there may be provided an 18-pulse configuration wherein preferably, the PFC stage transformer is arranged for generating the first three-phase AC waveform phase-shifted by +20degrees with respect to the power supply input and the second three-phase AC waveform phase-shifted by -20degrees with respect to the power supply input; wherein the power supply circuit further comprises a third bridge rectifier arrangement connected to the power source, the third bridge rectifier arrangement having a third rectified DC output; and wherein the two interphase transformers couple the first, second and third rectified DC outputs to reduce the ripple on the first DC supply.

By using three such phase-shifted three phase AC waveforms, 18-pulse rectification can result in attenuation of the fifth, seventh, eleventh and thirteenth harmonics of the input power line frequency, which may be optimised at a 20degree phase-shift. This effectively provides a harmonic attenuation arrangement without needing a separate harmonic trap.

The following features may apply (unless otherwise specified) to twelve-pulse, eighteen-pulse and even higher-pulse (e.g. 24-pulse) arrangements.

Preferably, each current and voltage output control stage is capable of regulating the DC output stage voltage by reducing the voltage compared to the DC supply voltage. Since there is a residual ripple on the DC bus voltage, it can be easily regulated by providing a controller/regulator which decreases the DC output stage voltage. For example, a simple buck controller could be used.

Preferably, each current and voltage output control stage is capable of regulating the DC output stage voltage by reducing or increasing the voltage compared to the DC supply voltage.

Due to variations in the power input supply, the voltage at the DC bus is sometimes not high enough to charge high voltage batteries (e.g. 500v+). Therefore, the supply may be regulated to reduce the ripple and provide a high enough charging voltage by providing a controller/regulator which can both increase and decrease the DC output stage voltage.

Preferably, each current and voltage output control stage comprises a buck-boost controller. A buck-boost controller is a simple and effective way of regulating the voltage and current at the output, whether the DC bus voltage is higher or lower than the required charging voltage.

Preferably, the DC output control stages do not provide galvanic isolation and each DC output control stage further comprises a residual current device, RCD. By providing an RCD in each individual DC output stage, the separate output charging stages can be individually protected without disconnecting, or tripping the centralised DC bus stage, or requiring isolation of each DC output stage. This is a simple way of protecting the charging circuit, which can reduces the cost and complexity of the charging circuit and may increase the efficiency compared to using an isolation approach.

In some embodiments, the output control stage comprises a high frequency inverter transformer.

A high frequency inverter transformer allows isolation between the charging outputs the source.

Preferably the PFC stage transformer is an auto-transformer.

Preferably, the plurality of batteries are batteries for electric vehicles, EVs.

Electric vehicles require high power DC charging supplies in order to be charged rapidly. Such a drive circuit significantly reduces the total harmonic distortion (THD), which allows multiple electric vehicles to be charged simultaneously by rapid DC charging.

In one embodiment, the harmonic filtering arrangement comprises at least one harmonic trap, each harmonic trap comprising: an inductor; and a capacitor; wherein the inductance and capacitance of the inductor and capacitor are selected to filter each respective harmonic. Preferably, the inductor and capacitor of each harmonic trap are connected in series.

Preferably, at least four DC output stages are connected in parallel to the DC supply. The power supply systems described herein may provide considerable improvements over previous charging systems, which can rarely provide high power rapid charging for multiple batteries from one AC source.

In some embodiments a further AC input power source, with PFC and rectification as described above, is connected to the DC outputs. E.g. there may be first and second AC input power sources, providing a first and a second DC supply, which may be connected together to provide a combined DC supply for connection to the DC outputs. This provides a convenient way to increase the power supplied by a given power supply circuit without requiring resizing the transformer arrangement and may allow large systems to be built up in a modular fashion.

Preferably, DC output stage control logic is provided for controlling the voltage and current at each DC output stage.

Preferably, there is provided a connector for electrically connecting each DC output stage to a respective DC battery and for transmitting data relating to each respective battery to the DC output stage control logic.

Preferably, the data relating to each respective battery comprises at least one of: the total energy storage capacity of the battery; the voltage capacity of the battery; and the variation in charging current required for the battery for optimised charging.

By providing such data to the control logic, it may be possible to optimise the charging current provided to the battery, for example to provide quicker charging and/or to charge with less energy wastage.

Preferably, the DC output stage control logic is configured to control the voltage and current at each DC output stage based on at least one of: a user identifier; a user input; a tariff rate for electrical power supplied at the three-phase AC power source; an indication received from the energy supplier of the three-phase AC power source; the energy storage capacity of a battery connected to the respective DC output stage; and the voltage capacity of a battery connected to the respective DC output stage.

For example, a user identifier or user input may indicate that the user has paid for a premium charging service to ensure their battery is charged at as high a power as possible for quick charging. Alternatively a user identifier/input could indicate that the user only has funds up to a certain amount and only a certain amount of charge should be provided to their battery. An energy supplier may send a signal indicative of cheap rate energy (e.g. when there is low demand for electrical energy on the network), which could result in more charging power (faster energy transfer) to the battery, or mean that consumers/users can obtain more energy for their money. It can also be beneficial to use logic to ensure that batteries are not overcharged above their voltage or energy storage capacity.

In some cases, the combined power capacity of the plurality of DC output stages is greater than the total power capacity of the AC rectification stage(s); and wherein the DC output stage control logic comprises power regulation logic for regulating the power consumed at each DC output stage such that the total power consumed at the plurality of DC output stages does not exceed the total power capacity of the AC rectification stage(s).

By providing DC output stages with a total combined maximum power output greater than that of the AC supply, it may be possible to provide faster charging (i.e. more power) when one, or only a few, of the output stages are in use, but still provide charging (albeit at lower power) for a larger number of output stages when all the charging stages are in use.

Preferably, the power regulation logic is configured to regulate the power consumed at each DC output stage independently based on at least one control parameter other than current drawn by default to the load. More preferably, the power regulation logic is configured to regulate the power consumed at each DC output stage by prioritising the power supplied to each DC output stage based on at least one of: a user identifier; a user input; a tariff rate for electrical power supplied at the three-phase AC power source; an indication received from the energy supplier of the three-phase AC power source; and the energy storage capacity of a battery connected to the respective DC output stage. For example, a user may have paid a premium rate to ensure their battery is charged faster than others connected to the AC supply.

Preferably, the DC output stage control logic further comprises metering logic for measuring the amount of energy provided to a battery connected to a DC output stage.

Preferably, the power regulation logic further comprises safety control logic for preventing unsafe power supply to a DC output stage. For example, to prevent power supply at a DC output when the battery is not properly connected.

There is also described herein a power supply unit for the power supply system according to any preceding claim, the unit comprising: a weatherproof housing enclosing the power supply system of any preceding claim, the housing having a plurality of connection points for connecting a battery to one of the plurality of DC output stages.

Preferably, the weatherproof housing comprises: a central AC connection module housing enclosing the AC rectification stage(s) and the DC bus; and a plurality of charging modules, each charging module having a housing enclosing a DC output stage and having a connection point for connecting a battery to the enclosed DC output stage.

By providing such a modular system for housing the power supply circuit, the system may have improved safety, be rapidly deployable, easily scalable and replaceable. Each of the plurality of connections may be flexible for improving ease of installation and use.

Preferably, the central AC connection module comprises a plurality of apertures comprising a connection point for attaching a central connector, wherein each aperture has a removable panel for covering the aperture when a central connector is not attached.

There is also described herein an electric vehicle charging station comprising a power supply system or unit as previously described; and a plurality of vehicle connection points. There is also described herein a filling station or motorway service station comprising such an electric vehicle charging station. There is also described herein a section of electric power transmission network comprising a power supply system or station as previously described.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way of example only and with reference to the accompanying drawings, in which: Figure 1 illustrates a circuit diagram of a power factor correction stage comprising an auto-transformer according to one embodiment, Figure 2 illustrates the vector diagram of the basic auto-transformer according to the embodiment of Figure 1; Figure 3 illustrates a circuit diagram of a buck regulator; Figure 4 illustrates a circuit diagram of a boost regulator; Figure 5 illustrates a circuit diagram of a combined buck-boost controller according to one embodiment; Figure 6 illustrates a general configuration of an output current control stage comprising a high frequency inverter/transformer stage for situations in which isolation is required; Figure 7 illustrates the vector diagram of a Zig-Zag auto-transformer connection; and Figure 8 illustrates a circuit diagram for a power supply circuit according to one embodiment, including a central rectification stage, harmonic cancellation and individual output chargers controlled by buck-boost regulators.

DETAILED DESCRIPTION OF THE INVENTION

Our proposal is to replace the active PFC stage with a passive one resulting in substantially reduced complexity and EMC generation. Efficiency should be of the order of 98% -a substantial improvement. The basic passive approach is based on an auto-transformer and auxiliary windings producing two output three-phase waveforms spaced by 30degrees, these result in cancellation of the 51h and 71h current harmonics taken from the AC power source.

A circuit diagram of an example embodiment of the PFC stage 300 is shown in Figure 1. The three-phase voltage input 310 is connected to a 12-pulse transformer 320. The output of the first waveform from the 12-pulse transformer 320 is connected to a first diode bridge rectifier, BR1, and the output of the second waveform is connected to a second diode bridge rectifier, BR2. The 12-pulse transformer 320 produces two output three phase waveforms, for example by using a delta-type and/or star-type (also known as a "Y"-or Wye-type) autotransformer connection. In a preferred embodiment, the phase shift angle is 30degrees, and the windings on each limb of the auto-transformer are adjusted accordingly.

An auto-transformer can be smaller, cheaper and more efficient than a conventional isolation transformer. The windings on the limbs of the transformer can be adjusted to give a 30degree between the two generated waveforms. For an 18-pulse embodiment, adjustment of the windings to produce a 40degree phase shift would be preferred.

In a twelve-pulse system, a 30 degree phase-shift between the two three-phase waveforms optimises the attenuation of the fifth, seventh, seventeenth and nineteenth harmonics. In an 18-pulse system, a 20 degree shift between each of the three three-phase waveforms optimises the attenuation of the fifth, seventh, eleventh and thirteenth harmonics.

The vector diagram of the output of a basic auto-transformer, such as the one shown in Figure 1, is shown in Figure 2. The original input three-phase vector (Va, Vb, Vb) is shown, along with the two three-phase vectors (Va., Vu, Vu) and (Va.., Vu., Vc).

Two inter-phase transformers are provided in the positions indicated in Figure 1, between the rectifier bridges BR1, BR2, and the output towards a buck/boost circuit. In order to meet the limits for harmonic current emission set out in BS EN61000 harmonic traps tuned to the eleventh and thirteenth harmonic are added, shown in Figure 1. These harmonic filters comprise inductors (L1-L6) and capacitors (C1-C6).This arrangement, where at least the fifth, seventh, eleventh and thirteenth harmonics are cancelled/attenuated has been found not to be problematic for such applications.

Being totally passive, the PFC stage can be easily scaled to achieve high power outputs. Advantageously, the input current total harmonic distortion (THD) of such arrangements can be reduced to <3%.

In order to reduce fluctuations in the charging current and voltage, which can occur particularly when charging multiple batteries, a current and voltage control stage follows the resulting DC supply, or DC bus. If complete isolation is required a high frequency inverter/transformer may be provided. However, in a preferred embodiment a non-isolated output control stage is used, protected by a residual current device (ROD), since the vehicle battery systems are generally well isolated from ground. This would substantially reduce cost and complexity and increase efficiency.

To satisfy EV charging requirements it would appear that a universal charging unit has to be capable of feeding a battery of nominally 500volts. If the DC supply, or bus, always exceeded 500volts by an ample margin, a simple buck regulator 500 would suffice, as shown in Figure 3. The buck regulator 500 comprises a voltage input Vin. A switch 510 and a diode 520 are connected in series with the voltage input Vin. An inductor 530 and connections to a voltage output Vout are connected across the diode 520. This results in a lower voltage output Vout than voltage input Vin.

In the case of the cross-connected auto-transformer described above the relationship between the line-line supply voltage and the DC supply is approximately 1.37:1 and for a nominal 400v AC input, the output is 548v DC. However, if supply tolerances are considered and with one unit satisfying the European and US markets, the line-line supply could be as low as 340v and the resulting DC bus would be 466v, i.e. too low to allow power transfer between the DC bus and the battery. Due to the variation in supply, such a circuit would not normally be considered suitable for charging electric vehicles at high voltages. However, a boost controller can be used, which allows a controlled transfer of current from a source at a low potential to a load at a higher potential. Figure 4 shows a boost controller or regulator 600, comprising a voltage input Vin. An inductor 630 and a switch 610 are connected in series with the voltage input Vin.

A diode 620 and connections to a voltage output Vout are connected across the switch 610. This results in a higher voltage output Vout than Voltage input Vin. The two circuits can be combined into a single buck-boost controller or regulator 700, as shown in Figure 5. The buck-booster regulator 700 comprises a Voltage input Vin, connected to a first switch 710 and a first diode 720. Across the first diode 720, an inductor 730 and second switch 715 are connected. A second diode 725 and connections to a voltage output, Vout, are connected across the second switch 715. Thus the buck-boost controller 700 produces a controlled output voltage Vout which is either greater than or less than the voltage input Vin, depending on the value of the voltage input Vin.

If isolation is required the output current control stage could alternatively be a high frequency inverter/transformer stage, as shown in Figure 6, instead of the Buck or Buck-boost regulator described above. Figure 6 shows a high frequency inverter/transformer circuit 200. A first switch, 01, and a second switch, 02 are connected in series with an input voltage, yin. An inductor, Lr, and a first capacitor, Cr, are connected in series with the primary winding, np, on a transformer. The inductor, Lr, first capacitor, Cr, and primary winding, np, are connected at point Va, such that they are in parallel with the second switch, 02. The transformer also comprises two secondary windings, ns. Each secondary winding, ns, is in series with a diode, D1, 02, and connected across the voltage output, Vo. A second capacitor, Cf, is also connected across the voltage output, Vo.

An alternative to the cross-connected auto-transformer is the Zig-Zag auto-transformer connection, vector diagram shown in Figure 7. Whilst the transformer itself is larger than the cross-connected version the ratio of DC supply, or bus, to line-line supply voltage is now 1.67:1 as opposed to the 1.37:1 referred to above and a DC bus of approximately 550v can be realised even at the minimum supply voltage of 340v line-line and a simple buck regulator can be used as the control device.

As the input PFC stage is passive in the circuits described herein there should be no EMC or CE (European Community) implications and the only development required would be the output control to suit various batteries.

A similar approach could be employed for the proposed multiple output charging car parks at motorway service stations where a large amount (e.g. 4, 6 or 10 or more) of high power rated (50kVV) individual chargers would be required. What is proposed is a central rectification stage including harmonic cancellation, with the individual chargers being buck-boost regulators, as shown in Figure 8. The power supply charging circuit of Figure 8 shows a 400v 3-phase AC input, connected to a harmonic trap (for the 11th and le harmonics of the AC input power line frequency) and a 12-pulse auto-transformer, followed by a 12-pulse rectifier and a DC bus. Individual buck-boost regulators are provided for the charging output for each EV battery. In Figure 8 the circuit is shown with outputs for charging only three EV batteries, however it is anticipated that such circuits would be used for charging more batteries (e.g. 4+ or even 10+).

Such power supply circuits may be used with a conventional AC power source, preferably greater than 200v, more preferably around 400v. The chargers may need to provide an output stage voltage of at least 450v, preferably at least 500v for charging batteries. Normally the voltage would be less than lkv.

Preferably, such power supply circuits would be capable of providing a charging power to each output stage of more than 6kW, preferably more than 10kW, preferably 10 more than 40kW, or more preferably more than 50kW. Normally the power provided would be less than 500kW.

Preferably, each DC output stage may be capable of providing a current of more than 50 amperes, preferably more than 80 amperes, or preferably more than 100 amperes. Each DC output stage would normally provide a current of less than 500 amperes, less than 350 amperes, or less than 200 amperes.

Central passive harmonic cancellation and rectification with multiple controlled charger outputs can result in a good power factor reduced EMC and increased efficiency compared to the multiple single point chargers previously envisaged. In particular, these charging circuits can provide <3%, or even <2% THD. Advantageously, conversion efficiency is around 98%, compared to previous power factor correction circuits, which have efficiencies of only 92%.

Any system feature as described herein may also be provided as a method feature, and vice versa. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to system aspects, and vice versa. Furthermore, any, some and/or all features in one aspect can be applied to any, some and/or all features in any other aspect, in any appropriate combination.

It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and/or supplied and/or used independently.

Claims

CLAIMS: 1. A power supply system for charging a plurality of direct current, DC, batteries, the power supply system comprising: an alternating current, AC, rectification stage for providing a first DC supply, the AC rectification stage comprising: input terminals for connection to a three-phase AC power source having a power line frequency of the order of 50-60Hz; and a passive power factor control, PFC, stage comprising: an at least twelve-pulse transformer for generating a first three-phase AC waveform and a second three-phase AC waveform from the input three-phase AC power source, wherein the generated three-phase waveforms are phase-shifted with respect to each other; a first bridge rectifier arrangement connected to the at least twelve-pulse transformer for rectifying the first generated three-phase AC waveform to provide a first rectified DC output; a second bridge rectifier arrangement connected to the at least twelve-pulse transformer for rectifying the second generated three-phase AC waveform to provide a second rectified DC output; and two interphase transformers coupling the first and second rectified DC outputs to provide a combined DC supply having a lower ripple than either of the first or second rectified DC outputs; wherein the rectification stage is arranged to attenuate harmonics transmitted to the power source up to and including the eleventh harmonic of the power line frequency and to provide a total harmonic distortion, THD, of less than 5%; a DC supply smoothing arrangement connected to the first DC supply, the DC supply smoothing arrangement having at least a capacitor to reduce the ripple on the first DC supply; and a plurality of DC output stages connected to the first DC supply, each DC output stage for connecting to a respective DC battery, each DC output stage having an independent current and voltage output control stage for regulating DC power supply to the respective battery.
2. A power supply system according to claim 1, wherein the rectification stage includes a harmonic filtering arrangement arranged to attenuate harmonics up to and including at least the thirteenth harmonic of the power line frequency and to provide a total harmonic distortion of less than 3%.
3. A power supply system according to claim 2, wherein the power factor control stage comprises a twelve-pulse transformer for generating the first three-phase AC waveform and the second three-phase AC waveform phase-shifted by 30degrees with respect to each other; and wherein the harmonic filtering arrangement comprises: a first harmonic trap for filtering the eleventh harmonic of the power line frequency; and a second harmonic trap for filtering the thirteenth harmonic of the power line frequency.
4. A power supply system according to claim 1 or 2, wherein the PFC stage transformer is arranged for generating the first three-phase AC waveform phase-shifted by +20degrees with respect to the power supply input and the second three-phase AC waveform phase-shifted by -20degrees with respect to the power supply input; wherein the power supply circuit further comprises a third bridge rectifier arrangement connected to the power source, the third bridge rectifier arrangement having a third rectified DC output; and wherein the two interphase transformers couple the first, second and third rectified DC outputs to reduce the ripple on the first DC supply.
5. A power supply system according to any preceding claim, wherein each current and voltage output control stage is capable of regulating the DC output stage voltage by reducing the voltage compared to the DC supply voltage.
6. A power supply system according to claim 5, wherein each current and voltage output control stage is capable of regulating the DC output stage voltage by reducing or increasing the voltage compared to the DC supply voltage.
7. A power supply system according to claim 6, wherein each current and voltage output control stage comprises a buck-boost controller.
8. A power supply system according to any preceding claim, wherein the DC output control stages do not provide galvanic isolation and wherein each DC output control stage further comprises an individual residual current device, ROD.
9. A power supply system according to any of claims 1 to 6, wherein the output control stage comprises a high frequency inverter transformer.
10. A power supply system according to any preceding claim, wherein the power factor control stage transformer is an auto-transformer.
11. A power supply system according to any preceding claim, wherein the plurality of batteries are batteries for electric vehicles, EVs.
12. A power supply system according to claim 2, or any of claims 3 to 11 when dependent on claim 2, wherein the harmonic filtering arrangement comprises at least one harmonic trap, each harmonic trap comprising: an inductor; and a capacitor; wherein the inductance and capacitance of the inductor and capacitor are selected to filter each respective harmonic.
13. A power supply system according to claim 12, wherein the inductor and capacitor of each harmonic trap are connected in series.
14. A power supply system according to any preceding claim, wherein each DC output stage is arranged to provide a power of at least 6kW and the power supply circuit is capable of providing a power across the plurality of DC output power stages of at least 50kW.
15. A power supply system according to any preceding claim, wherein at least four DC output stages are connected in parallel to the DC supply.
16. A power supply system according to any preceding claim, having a plurality of said AC rectification stages and wherein the DC supplies of the AC rectification stages are each connected to a respective subset of the plurality of DC output stages.
17. A power supply system according to any of claims 1 to 15, having a plurality of said AC rectification stages and wherein the DC supplies of the plurality of AC rectification stages are connected in parallel.
18. A power supply system according to any preceding claim, further comprising DC output stage control logic for controlling the voltage and current at each DC output stage.
19. A power supply system according to claim 18, further comprising: a connector for electrically connecting each DC output stage to a respective DC battery and for transmitting data relating to each respective battery to the DC output stage control logic.
20. A power supply system according to claim 19, wherein the data relating to each respective battery comprises at least one of: the total energy storage capacity of the battery; the voltage capacity of the battery; and the variation in charging current required for the battery for optimised charging.
21. A power supply system according to any of claims 18 to 20, wherein the DC output stage control logic is configured to control the voltage and current at each DC output stage based on at least one of: a user identifier; a user input; a tariff rate for electrical power supplied at the three-phase AC power source; an indication received from the energy supplier of the three-phase AC power source; the energy storage capacity of a battery connected to the respective DC output stage; and the voltage capacity of a battery connected to the respective DC output stage.
22. A power supply system according to any of claims 18 to 21, wherein the combined power capacity of the plurality of DC output stages is greater than the total power capacity of the AC rectification stage(s); and wherein the DC output stage control logic comprises power regulation logic for regulating the power consumed at each DC output stage such that the total power consumed at the plurality of DC output stages does not exceed the total power capacity of the AC rectification stage(s).
23. A power supply system according to claim 22, wherein the power regulation logic is configured to regulate the power consumed at each DC output stage independently based on at least one control parameter other than current drawn by default to the load.
24. A power supply system according to claim 23, wherein the power regulation logic is configured to regulate the power consumed at each DC output stage by prioritising the power supplied to each DC output stage based on at least one of: a user identifier; a user input; a tariff rate for electrical power supplied at the three-phase AC power source; an indication received from the energy supplier of the three-phase AC power source; and the energy storage capacity of a battery connected to the respective DC output stage.
25. A power supply system according to any of claims 18 to 24, wherein the DC output stage control logic further comprises metering logic for measuring the amount of energy provided to a battery connected to a DC output stage.
26. A power supply system according to any of claims 18 to 25, wherein the power regulation logic further comprises safety control logic for preventing unsafe power supply to a DC output stage.
27. A power supply system for charging a plurality of direct current, DC, batteries, the power supply system comprising: an alternating current, AC, rectification stage for providing a first DC supply, the AC rectification stage comprising: input terminals for connection to a three-phase AC power source having a power line frequency of the order of 50-60Hz; a passive power factor control, PFC, stage comprising: a twelve-pulse transformer for generating a first three-phase AC waveform and a second three-phase AC waveform from the input three-phase AC power source, wherein the generated three-phase waveforms are phase-shifted by 30 degrees with respect to each other; a first harmonic trap for filtering the eleventh harmonic of the power line frequency; and a second harmonic trap for filtering the thirteenth harmonic of the power line frequency; a first bridge rectifier arrangement connected to the at least twelve-pulse transformer for rectifying the first generated three-phase AC waveform to provide a first rectified DC output; a second bridge rectifier arrangement connected to the at least twelve-pulse transformer for rectifying the second generated three-phase AC waveform to provide a second rectified DC output; and two interphase transformers coupling the first and second rectified DC outputs to provide a combined DC supply having a lower ripple than either of the first or second rectified DC outputs; wherein the rectifier arrangements and harmonic filters are arranged to attenuate harmonics up to at least the nineteenth harmonic of the power line frequency and to provide a total harmonic distortion of less than 3%; a DC supply smoothing arrangement connected to the DC supply, the DC supply smoothing arrangement having at least a capacitor to reduce the ripple on the DC supply; a plurality of DC output stages connected to the DC supply, each DC output stage for connecting to a respective DC battery, each DC output stage having an independent current and voltage output control stage for regulating DC power supply to the respective battery; wherein each current and voltage output control stage comprises a buck-boost controller capable of regulating the DC output stage voltage by reducing or increasing the voltage compared to the DC supply voltage; wherein the DC output control stages do not provide galvanic isolation and wherein each DC output control stage further comprises an individual residual current device, ROD.
28. A power supply unit for the power supply system according to any preceding claim, the unit comprising: a weatherproof housing enclosing the power supply system of any preceding claim, the housing having a plurality of connection points for connecting a battery to one of the plurality of DC output stages.
29. A power supply unit according to claim 28, wherein the weatherproof housing comprises: a central AC connection module housing enclosing the AC rectification stage(s) and the DC supply; and a plurality of charging modules, each charging module having a housing enclosing a DC output stage and having a connection point for connecting a battery to the enclosed DC output stage.
30. A power supply unit according to claim 29, wherein the central AC connection module comprises a plurality of apertures comprising a connection point for attaching a central connector, wherein each aperture has a removable panel for covering the aperture when a central connector is not attached.
31. An electric vehicle charging station comprising: a power supply system or unit according to any preceding claim-and a plurality of vehicle connection points.
32. A filling station or motorway service station comprising an electric vehicle charging station according to claim 31.
33. A section of electric power transmission network comprising a power supply system or station according to any preceding claim.
34. An apparatus substantially as hereinbefore described in relation to the Figures.
35. A method substantially as hereinbefore described in relation to the Figures.