FIELD OF INVENTION The present invention relates to a vehicle charging system.

BACKGROUND

Electric vehicles are becoming more and more common. Because of this there is also a growing need for providing electrical vehicle charging systems.

One known vehicle charging system can be found in US 2013/0257146 where a converter converting between Alternating Current (AC) and Direct Current (DC) has an AC side on which it receives incoming power and a

DC side on which it is connected to the first side of several DC/DC converters, which DC/DC converters each have a second DC side for connection to a vehicle. Another system can be found in EP 3339084, where a charging station comprises a multi-winding transformer has one primary winding connected to an AC grid and a plurality of secondary windings electrically isolated from each other. The charging station also comprises an AC/DC converter to which some of the secondary windings are connected.

Such vehicle charging systems are typically connected to a grid. Today these grids often employ renewable energy sources. This has led to the grids being weak and often needing support. These above-mentioned charging systems generally function well.

However, it would be of interest to improve them, especially with regards to supporting the grid. SUMMARY OF THE INVENTION

The present invention is directed towards providing improvements of a vehicle charging system.

This is achieved through a vehicle charging system comprising:

at least one vehicle charging terminal,

a converter converting between Alternating Current, AC, and Direct Current, DC, and having an AC side facing an AC grid and a DC side for connection to an energy storage, and

a three-phase transformer with a single group of primary three-phase windings for connection to the power grid and at least two groups of secondary three-phase windings, where one group of the secondary three- phase windings leads to the vehicle charging terminal and is connected to the AC side of the converter via a corresponding AC link.

According to a first aspect of the vehicle charging system all groups of secondary windings lead to vehicle charging terminals.

According to a second aspect of the vehicle charging system, the vehicle charging system is configured to provide grid support functionality via the AC link. The converter converting between AC and DC may be an active converter switched at the fundamental grid frequency.

The switching of the converter at the fundamental frequency may additionally be modulated with switching at at least one higher switching frequency. The converter converting between AC and DC may thereby also be an active converter switched at the fundamental grid frequency for converting between AC and DC and comprise additional switching for performing support functionality. Thereby the switching frequency may be seen as being higher than the fundamental grid frequency.

The grid support functionality may comprise harmonic mitigation functionality implemented through the converter being controlled to perform active filtering. The grid support functionality may additionally comprise power flow control comprising the converter being configured to control the power flow towards the energy storage. The grid support functionality may also comprise voltage support functionality to the AC grid from the active converter.

The vehicle charging system may comprise a battery system. It is in one variation possible that the vehicle charging terminal is connected to the AC side of the converter and the energy storage for which the converter is provided is a battery of this battery system.

The converter may also be a passive rectifier. In this case it is possible that at least one harmonics filter is connected to the AC side of the converter. When the converter is a passive rectifier, at least parts of the grid support functionality is obtained through passive filtering using the harmonics filter.

It is additionally possible that one group of secondary three-phase windings is a first group of secondary three-phase windings, the converter is a first converter converting between AC and DC, the AC link is a first AC link and the DC link is a first DC link having a first DC voltage level and the

DC side of the first converter is connected to the charging terminal.

Thereby the previously mentioned energy storage is the battery of a vehicle when connected to the charging terminal. In this case there may

additionally be a second group of secondary three-phase windings connected to the AC side of the first converter via a second AC link. When the converter is this first converter, a first filter may be connected to the first AC link and a second filter may be connected to the second AC link. For the first converter, it is additionally possible that the voltages of the second group of secondary windings are shifted in phase by thirty degrees from the voltages of the first group of secondary windings.

When such a phase shift is used with a first converter realized as a passive rectifier, such as a diode bridge, the harmonics mitigation functionality provided by the vehicle charging system may be provided through the combination of the filters and phase shift.

It is also possible that the vehicle charging system comprises a further converter converting between the first DC level and a second DC level and having a first side connected to the DC side of the first converter and a second side for providing the vehicle charging terminal.

There may additionally be a first local power source connected to the DC side of the first converter via a corresponding first local power source converter. This first local power source may be realized as a first battery system. In this case the first local power source converter may additionally convert between the first and another DC voltage level. A power source converter is here a converter connected to a power source, for instance through interconnecting the power source with a link such as a DC link or an AC link.

There may also be a second local power source connected to the DC side of the first converter via a corresponding second local power source converter. The second local power source may be a photo voltaic system and the second local power source converter may be a converter converting between the first and a further DC voltage level. There may additionally be a third local power source connected to the DC side of the first converter via a corresponding third local power source converter. The third local power source may be a first wind farm system and the third local power source converter may convert between AC and DC.

Another possible variation is that an internal power source is connected to another of the groups of secondary windings via a corresponding converter and a corresponding AC link. It is thus possible that the internal power source is connected to another group of secondary windings than the first and second groups of secondary windings. It is additionally possible that this converter is a converter converting between AC and DC and that the internal power source is a second battery system connected to the DC side of this converter, while the AC link is connected to the AC side of the converter. It is additionally possible that an alternating current vehicle charging terminal is connected to the AC link between this other of the groups of secondary windings and the corresponding converter. Another possible internal power source is a second wind farm system connected to another of the group of secondary windings. It is thus possible that the internal power source is connected to another group of secondary windings than the first and second groups of secondary windings. Furthermore, the connection may be made via a further power source converter, for instance a wind farm converter, converting between two AC levels.

A further possible internal power source is a second photo voltaic system connected to yet another of the group of secondary windings. It is thus possible that the internal power source is connected to another group of secondary windings than the first and second groups of secondary windings. Furthermore, the connection may be made via another power source converter, for instance a photo voltaic converter, converting between AC and DC.

The vehicle charging system may also comprise a control unit configured to perform power flow control through controlling a local power source converter connected between the DC link and a corresponding local power source to supply power to the vehicle charging terminal, to control the converter connected between another group of secondary windings, i.e. another group than the first and second groups of secondary windings, and the other power source to supply further power to the three-phase transformer for the charging if the power of the local power source is insufficient and to control the first converter to receive the further power via the first and second groups of secondary windings and to forward this power to the vehicle charging terminal. Therefore, the first converter may in this case be an active converter.

The control unit may additionally balance the load on the different groups of secondary windings. Balancing may be achieved through controlling the power output by the converters connected to the different groups of windings in a direction away from the three-phase transformer.

The control may additionally comprise controlling the first converter to receive power from the AC grid in case the power from local and internal power sources is insufficient.

The invention has a number of advantages. It provides a flexible vehicle charging system capable of supporting the AC grid. It also allows an efficient winding use in the transformer with regard to charging. All windings can be used for charging vehicles. The invention also limits the size of the elements used. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will in the following be described with reference being made to the accompanying drawings, where fig. l schematically shows a first realization of a first exemplifying vehicle charging system comprising a transformer with a first group of primary windings and eight groups of secondary windings, each connected to a corresponding DC link,

fig. 2 schematically shows a second realization of the vehicle charging system,

fig. 3 schematically shows a first realization of the connection of a first and second group of secondary windings to a first AC/DC converter via a first and a second AC link for use with a DC charging terminal and where the AC links are connected to first and second filters,

fig. 4 schematically shows a second realization of the connection shown in fig. 3 but without the filters,

fig. 5 schematically shows the connection of a seventh and eighth group of secondary windings connected to a fourth and a fifth AC/DC converter, where the AC sides of the converters have AC charging terminals, and fig. 6 shows the charging made via first charging terminal in the first exemplifying vehicle charging system with power obtained from within the energy charging system. DETAILED DESCRIPTION OF THE INVENTION

In the following, a detailed description of preferred embodiments of the invention will be given. Fig. l shows one example of a vehicle charging system toA that comprises at least one vehicle charging terminal. The vehicle charging system toA is connected to a medium voltage (MV) alternating current (AC) grid ACG via a single group of primary windings PWG of a three-phase transformer TR 12, which group is a three-phase group and thus comprises three windings. There are also at least two groups of secondary three-phase windings, where each group is a three-phase group and thus comprises three windings. As an example, there is a first group of secondary windings SiG, a second group of secondary windings S2G, a third group of secondary windings S3G, a fourth group of secondary windings S4G, a fifth group of secondary windings S5G, a sixth group of secondary windings S6G, a seventh group of secondary windings S7G and an eighth group of secondary windings S8G, where all groups of secondary windings are magnetically coupled to the group of primary windings PWG. Moreover, the voltages of the second group of secondary windings S2G are shifted in phase by thirty degrees from the voltages of the first group of secondary windings SiG, the voltages of the fourth group of secondary windings S4G are shifted in phase by thirty degrees from the voltages of the third group of secondary windings S3G and the voltages of the sixth group of secondary windings S6G are shifted in phase by thirty degrees from the voltages of the fifth group of secondary windings S5G.

Furthermore, the first and second groups of secondary windings SiG and S2G are both connected to an AC side of a first converter 14 via

corresponding first and second AC links ACLi and ACL2. The first converter 14 converts between AC and Direct Current (DC) and is therefore also a first AC/DC converter 14. It can also be seen that the first AC/DC converter 14 has an AC side facing the AC grid and a DC side, which as will be described later, is provided for connection to an energy storage. The third and fourth groups of secondary windings S3G and S4G are both connected to an AC side of a second AC/DC converter 16 via corresponding third and fourth AC links ACL3 and ACL4, and the fifth and sixth groups of secondary windings S5G and S6G are both connected to an AC side of a third AC/DC converter 18 via corresponding fifth and sixth AC links ACL5 and ACL6. The seventh group of windings S7G is connected to the AC side of a fourth AC/DC converter 20 via a seventh AC link ACL7 and the eighth group of windings S8G is connected to the AC side of a fifth AC/DC converter 22 via an eighth AC link ACL8.

The first AC/DC converter 14 has a DC side connected to a first end of a first low voltage DC link LVDCi having a first DC voltage level VDC _I . The first DC link LVDCi is in this case made up of a first and a second pole Pi and P2. A first DC vehicle charging terminal EVi is provided at a second end of this DC link LVDCi. The energy storage for which the first AC/DC converter 14 is primarily provided is thereby the battery of a vehicle connected to the charging terminal EVi when this battery is to be charged. To the DC link LVDCi there is also connected a first side of a first DC/DC converter 24, which first DC/DC converter 24 has a second side connected to a first battery system BSi. This first battery system BSi is also a first local power source connected to the DC side of the first converter via a corresponding first local power source converter, where the local power source converter is the first DC/DC converter 24 converting between the first and another DC voltage level. The connection of the first DC/DC converter 24 to the first DC link LVDCi is made between the two previously mentioned ends.

The second and third AC /DC converters 16 and 18 are also connected to corresponding DC links LVDC2 and LVDC3 providing DC vehicle charging terminals (not shown). It is also possible that battery systems are connected to these DC links via corresponding DC/DC converters in the same way as was described above.

Thereby one group, and in the present case two groups, of the secondary three-phase windings leads to a vehicle charging terminal and is connected to the AC side of a corresponding converter via a corresponding AC link.

The system also comprises a second and a third electric vehicle charging terminal EV2 and EV3, which are both AC charging terminals. The second charging terminal EV2 is connected to the seventh AC link ACL7 between the seventh group of secondary windings S7G and an AC side of the fourth AC/DC converter 20. The fourth AC/DC converter 20 also has a DC side and this DC side is connected to a second battery system BS2. The third charging terminal EV3 is connected to the eighth AC link ACL8 between the eighth group of secondary windings S8G and an AC side of the fifth AC/DC converter 22. The fifth AC/DC converter 22 also has a DC side and this DC side is connected to a third battery system BS3. It can thereby also be seen that the fourth and fifth AC/DC converters 20 and 22 each has an AC side facing the AC grid and a DC side provided for connection to an energy storage. Moreover, one group of the secondary three-phase windings thereby leads to a corresponding AC vehicle charging terminal and is connected to the AC side of an AC/DC converter associated with this group via a corresponding AC link. It can also be seen that the second and third vehicle charging terminals are each connected to the AC side of the corresponding AC/DC converter and the energy storage for which the converter is provided is in this case a battery of the battery system on the DC side of the converter. The second and third battery systems BS2 and BS3 with the fourth and fifth AC/DC converters may in some instances each be considered to be internal power sources connected to another of the groups of secondary windings than the first and second groups of secondary windings via corresponding converters and a corresponding AC links.

Finally, there is an optional control unit (CU) 25 that is used for power flow control. This control unit 25 is shown as sending control signals to the first, second, third, fourth and fifth AC/DC converters 14, 16, 18, 20 and 22 as well as to the first DC/DC converter 24, where these control signals are indicated via dashed arrows.

Another observation that can be made in this first example is that all groups of secondary windings lead to vehicle charging terminals. The charging system shown in fig. l is merely an example and may be varied in a number of ways. One such variation is shown in fig. 2. The second example of the vehicle charging system 10B comprises a single group of primary windings PWG, seven groups of secondary windings SiG, S2G, S3G, S4G, S5G, S6G and S7G as well as three AC/DC converters 14, 16 and 18 connected to the first, second, third, fourth, fifth and sixth groups of secondary windings SiG, S2G, S3G, S4G, S5G, S6G via first, second, third, fourth, fifth and sixth AC links ACLi, ACL2, ACL3, ACL4, ACL5, ACL6, where each of these AC/DC converters 14, 16 and 18 is connected to a corresponding DC link LVDCi, LVDC2 LVDC3. The above- mentioned elements are connected in the same way as in fig. 1. However, in this case, as opposed to in fig. 1, the seventh group of secondary windings S7G is connected to a first AC/AC converter 26, which in turn is connected to a second wind farm system WFS2. The second wind farm system with the first AC/AC converter 26 may in some instances be considered to be an internal power source connected to another of the group of secondary windings than the first and second group of secondary windings via a further power source converter, where the first AC/AC converter 26 is this further power source converter.

There are also no AC charging terminals. It can also be seen that the first electrical vehicle charging terminal EVi is connected to the first DC link LVDCi via a second DC/DC converter 28, which thus converts between the first DC voltage level VDC _I of the DC link LVDCi and a second DC voltage level V _DC2 of the first electrical charger terminal EVi. The second DC/DC converter 28 is thereby a further converter converting between the first DC level and a second DC level and having a first side connected to the DC side of the first AC/DC converter 14 and a second side for providing the first vehicle charging terminal EVi. The previously described first DC/DC converter 24 and first battery system BSi are also connected to the first DC link LVDCi, thereby providing a first local power source and first local power source converter. There is also a photo voltaic system PVi connected to the first DC link LVDCi via a third DC/DC converter 30. The photo voltaic system PVi is a second local power source, which is connected to the DC side of the first AC/DC converter 14 via a corresponding second local power source converter in the form of the third DC/DC converter 30. Finally, there is a first windfarm system WFSi connected to the first DC link LVDCi via a further AC/DC converter 32. The first windfarm system WFSi is an example of a third local power source connected to the DC side of the first AC/DC converter 14 via a corresponding third local power source converter, which further AC/DC converter 32 in this case is the third local power source converter. The first, second and third DC/DC converters 24, 28 and 30 as well as the further AC/DC converter 32 are thereby connected to the first DC link LVDCi in parallel with each other. The second and third DC links LVDC2 and LVDC3 may here have the same or similar realizations with connected battery systems, photo voltaic systems and/or wind farm systems.

Fig. 3 schematically shows one realization of the group of primary windings PWG and the first and second groups of secondary windings SiG and S2G together with the first AC/DC converter 14. The windings of the first group SiG are here delta connected, while the windings of the second group S2G are wye connected. Each winding of the first group SiG is connected to a midpoint of a corresponding phase leg in a first converter stage of the first AC/DC converter 14, while each winding of the second group S2G is connected to the midpoint of a corresponding phase leg of a second converter stage of the first AC/DC converter 14, which converter stages are connected in cascade between the first and second poles Pi and

P2 of the first DC link LVDCi. The phase legs of both the first and the second converter stage each comprise an upper and a lower valve, where the lower valves in the first converter stage are interconnected and connected to all the, also interconnected, upper valves of the second converter stage. The junction between these interconnected lower valves of the upper converter stage and upper valves of the lower converter stage may additionally be connected to the midpoint of a capacitor string connected between the first and second DC pole Pi and P2. Additionally, the upper valves of the first converter stage are connected to the first pole Pi, while the lower valves of the second converter stage are connected to the second pole P2. The converter stages may be two-level voltage source converter stages. However also other converter stage realizations are possible, such as neutral-point clamped and Modular Multilevel Converter (MMC) stages. The valve may in turn be realized through a transistor with or without antiparallel diode, where one type of transistor that may be used is an Insulated Gate Bipolar Transistor (IGBT). However also other types of transistors are contemplated, such as metal oxide semiconductor field effect transistor (MOSFET) or junction field effect transistor (JFET).

Through the converter being a voltage source converter it is also an active converter. As an alternative the converter may be a passive converter for instance realized as two cascaded diode bridges. The first AC link ACLi interconnecting the first group of secondary windings SiG with the upper converter stage may be connected to a first filter Ft, while the second AC link ACL2 interconnecting the second group of secondary windings with the second converter stage may be connected to a second filter F2. The first filter Ft may thereby be connected to the first AC link ACLi and the second filter F2 to the second AC link ACL2.

Fig. 4 schematically shows a variation of the elements used in fig. 3. As can be seen in fig. 4 it is possible to omit the filters Ft and F2, which is possible if the converter 14 is actively controlled. Fig. 5 schematically shows one realization of the group of primary windings PWG with the seventh and eighth groups of secondary windings S7G and S8G together with the fourth and the fifth AC/DC converters 20 and 22. The seventh group of secondary windings S7G are here wye connected and connected to the AC side of the fourth AC/DC converter 20 via the seventh AC link ACL7, while the DC side of the fourth AC/DC converter 20 is connected to the second battery system BS2. The seventh AC link ACL7 is additionally connected to a third filter F3. The second AC charging terminal EV2 is also connected to each of the AC links of the seventh AC link ACL7 in order to provide three-phase charging of a connected vehicle.

In a similar manner the eighth group of secondary windings S8G are wye connected and connected to the AC side of the fifth AC/DC converter 22 via the eighth AC link ACL8, while the DC side of the fifth AC/DC converter 22 is connected to the third battery system BS3. The eighth AC link ACL8 is in turn connected to a fourth filter F4. A third AC charging terminal EV3 is also in this case connected to each of the AC links of the eighth AC link ACL8 in order to provide three-phase charging of a connected vehicle. The fourth and fifth AC/DC converters 20 and 22 are with advantage active converters allowing bidirectional power transfer. Additionally, they typically provide galvanic isolation between the charging terminals and the battery systems. This can be achieved through the converters employing transformers in the conversion. Furthermore, the use of a wye connection of the seventh and eight groups of secondary windings S7G and S8G is no requirement. A delta connection can also be used.

The filters used in fig. 3 and 5 may be filters set to filter output harmonics of the AC grid frequency.

The vehicle charging system is provided for charging batteries of electric or hybrid vehicles. The first charging terminal EVi may for instance be used for such charging. This charging is according to aspects of the invention combined with supporting the AC grid ACG.

In the system there is therefore provided grid support functionality via one or more active voltage source converters or passive rectifiers connected to the grid via one or two groups of secondary windings. The grid support is also provided via the AC link or links of the converter.

At least some of the converters used in the support may thereby be directly connected to a group of secondary windings. Examples of converters involved in such support is the first AC/DC converter 14 and the fourth AC/DC converter 20. The support functionality may comprise power flow control, AC voltage support and/or harmonics mitigation. In the case of an active converter, such as the first AC/DC converter 14, the converter may operate at the fundamental frequency of the AC grid in order to converter between AC and DC. It may thus switch at the fundamental grid frequency. In the case of an active converter performing harmonics mitigation, the converter may also perform additional switching. The active converter may additionally be controlled to limit harmonics. It may thus perform active filtering. When this is combined with a thirty-degree phase shift between two groups of secondary windings connected to the converter, the harmonics of the AC voltage on the AC side of the converter may be limited enough so that any filters on this AC side are unnecessary. The additional filters connected to the AC link may thus be removed. This can be seen in fig. 4, where the system lacks filters on the AC side of the first AC/DC converter.

Harmonics mitigation in the first AC/DC converter 14 may also be achieved through the use of the filters Ft and F2 connected to the AC side of the first AC/DC converter 14 possibly only together with the above- mentioned phase shift. In case there is no need for any power delivery out from the AC side of the first AC/DC converter 20, this converter 14 may furthermore be a rectifier that may additionally be a passive rectifier realized as a diode bridge. The converter may thus be a passive rectifier. In this case at least parts of the grid support functionality is obtained through passive filtering using the harmonics filter with the rest of the support obtained through the phase shift. The harmonics mitigation functionality may thereby be provided through the combination of the filters and phase shift.

Harmonics mitigation may also be carried out using a converter connected to the grid via a single group of secondary windings, such as the fourth AC/DC converter 20. In this case there is naturally no use of the phase shift by thirty degrees. The harmonics mitigation may in this case be achieved using the filter F3 together with fundamental frequency switching modulated with additional switching that performs harmonics mitigation control of the fourth AC/DC converter 20, which is thus an active voltage source converter.

An active converter, such as the first AC/DC converter 14 or the fourth AC/DC converter 20, may additionally or instead provide reactive or active power support of the AC grid as well as voltage support of the grid, where voltage support may involve keeping the AC link voltage at a desired voltage level.

The control functionality of an active converter may be implemented through Model Predictive Control (MPC).

In addition to or instead of the above-mentioned support, one or more of the converters may be involved in power flow control. It may for instance be desirable to support the grid through limiting the load on the AC grid as much as possible when a vehicle battery is to be charged. This type of support is made through performing power flow control in the vehicle charging system. An active converter may be involved in power flow control through controlling the power flow towards the battery of a vehicle being charged.

The switching of the converter at the fundamental frequency may thus be modulated with switching at at least one higher switching frequency for performing grid support. The converter converting between AC and DC may thereby also be an active converter switched at the fundamental grid frequency and comprise additional switching performing support functionality, such as power flow control and harmonics mitigation.

Thereby the switching frequency may additionally be seen as being higher than the fundamental grid frequency.

Before power flow control is described some more, the vehicle charging system will be discussed in some more detail.

An AC/DC converter leading to a DC link may form a subsystem together with this DC link and the various elements connected to it.

Through the use of the transformer 12 it can be seen that each subsystem is galvanically isolated from each other. Further galvanic isolation within a subsystem is also possible which is indicated through the use of the first DC/DC converter 24 in fig. 1. Many DC/DC converters are centred on the use of a transformer. Thereby galvanic isolation is inherently provided. Each subsystem formed by the converter and DC link connected to a group or two groups of secondary windings are thereby galvanically isolated from the other subsystems, where a subsystem may comprise one or more charging terminals with or without one or more of a Photo Voltaic system, battery system or a windfarm system.

In fig. 1 there is thus provided a first subsystem connected to the first and second groups of secondary windings, a second subsystem connected to the third and fourth groups of secondary windings, a third subsystem connected to the fifth and sixth groups of secondary windings, a fourth subsystem connected to the seventh group of secondary windings and a fifth subsystem connected to the eighth group of secondary windings, while in fig. 2 there are only four subsystems.

It is possible that it is desirable to use the internal power sources of the vehicle charging system as much as possible. It may additionally be desirable to use the power sources as locally as possible in the vehicle charging system, meaning that if there are power sources in the subsystem comprising a charging terminal to which a vehicle is connected the battery of which is to be charged, then these power sources should be used first and thereafter internal power sources in other subsystems may be used.

For this reason, power flow control may be used in the vehicle charging system. The power flow control may be performed by the control unit 25.

Fig. 6 shows one example of how the control may be carried out for the system in fig. 1.

If for instance the battery of a vehicle connected to the first charging terminal EVi in the first subsystem is to be charged, it is possible that the first local power source in the form of the first battery system BSi is to have priority followed by an internal power source of another subsystem if the power deliverable from the first battery system BSi is insufficient. It may as an example be possible to receive power from the second and/or third battery system BS2 or BS3 or from another of the subsystems. In the example of fig. 6, the second battery system BS2 is used.

In this case the control unit 25 may be configured to perform power flow control through controlling a local power source converter connected between the first DC link LVDCi and a corresponding local power source to supply power to the first vehicle charging terminal EVi and to control the converter connected between another group of secondary windings, i.e. another group than the first and second groups of secondary windings, and another internal power source to supply further power to the three-phase transformer for the charging if the power of the local power source is insufficient and to control the first AC/DC converter 14 to receive the further power via the first and second groups of secondary windings and to forward this power to the vehicle charging terminal EVi. This control may be done through the control unit 25 controlling the first DC/DC converter 24 to supply power to the first charging terminal EVi within the first subsystem and if this is not enough to control the fourth AC/DC converter 20 to supply additional power from the second battery system BS2 and to control the first AC/DC converter 14 to receive this additional power from the second battery system BS2 and supply it to the first charging terminal EVi. As an alternative or in addition it is possible to supply additionally power from another subsystem. It is for instance possible that the battery of a vehicle connected to a charging terminal of the subsystem formed by the third AC/DC converter 18 or that a further battery system of this subsystem may be used.

The first AC/DC converter 14 of the first subsystem thus receives power internally in the vehicle charging system from another subsystem, here exemplified by the fourth subsystem and the second battery system BS2, without drawing power from the grid ACG and then supplies this power to the first charging terminal EVi at the same time as the first DC/DC converter 14 supplies power from the power source in the first subsystem, which in this case is the first battery system BSi. It can also be seen that in this case both sources are galvanically isolated from the first charging terminal EVi through the use of the transformer 12 and first DC/DC converter 22.

If the first charging terminal EVi was to be charged in the second variation of the vehicle charging system 10B shown in fig. 2, then a combination of the first battery system BSi, the first photo voltaic systems PVi and/or the first wind farm system WFSi could be used possibly together with the second wind farm system WFS2 and/or one or more batteries connected to the DC links LVDCi, LVDC2 and LVDC3.

If the local energy is not sufficient, then it is of course also possible to use power from the grid. The control may therefore additionally comprise controlling the first AC/DC converter 14 to receive power from the AC grid ACG in case the power from local and internal power sources is

insufficient. The vehicle may thereby be charged from several different sources.

The control unit may additionally balance the load on the different groups of secondary windings. Balancing may be achieved through controlling the power output in a direction away from the three-phase transformer by the converters connected to the different groups of windings. If these converters are AC/DC converters, the power may be the power output from the DC sides of these AC/DC converters. The converters connected between the different groups of secondary windings and the different subsystems may thus be controlled so that the power input into the subsystems is balanced.

In the example given above, the charging terminal was the first charging terminal EVi connected to the first DC link, which is thus a DC charging terminal. It is in a similar way possible to charge a vehicle connected to an AC charging terminal, such as the second charging terminal EV2 shown in fig. 1 using power from the own subsystem, exemplified by the second battery system BS2 and/or from other subsystems, such as from the first subsystem with the first battery system BSi and the first charging terminal EVi as well as from the grid. It can be seen that also here there is galvanic isolation between the power sources and the charging terminal because of the use of the transformer 12 and the specific realization of the AC/DC converter 20. Examples given of internal power sources were battery systems and windfarm systems. Naturally also a photo voltaic system can be an internal power source. A further possible internal power source is thereby a second photo voltaic system connected to another of the group of secondary windings than the first and second groups, which connection may be made via another power source converter, for instance a photo voltaic converter, converting between AC and DC.

It can be seen that the invention provides a flexible vehicle charging system capable of supporting the AC grid. It may also provide a more efficient winding use in the transformer with regard to charging. All windings can thus be used for charging vehicles. The invention also limits the size of the elements used. The power rating of the transformer can be made lower than the peak output vehicle charging power. The power rating of each converter can be made lower than the peak output vehicle charging power.

The control unit 25 may be implemented through a computer or a processor with associated program memory or dedicated circuit such as Field-Programmable Gate Arrays (FPGAs) or Application Specific

Integrated Circuits (ASICs).

The control unit may thus be realized in the form of discrete components, such as FPGAs or ASICs. However, it may also be implemented in the form of a processor with accompanying program memory comprising computer program code that performs the desired power flow control functionality when being run on the processor. A computer program product carrying this code can be provided as a data carrier such as a memory carrying the computer program code, which performs the above-described power flow control functionality when being loaded into a computer.

From the foregoing discussion it is evident that the present invention can be varied in a multitude of ways. It shall consequently be realized that the present invention is only to be limited by the following claims.