Patent ID: 12244218

In case not indicated otherwise, elements with the same reference signs or symbols denote the same or similar elements in the respective figures.

DETAILED DESCRIPTION

To support the growing power needs and to achieve physically distributed systems, the common DC bus voltage is of the Medium Voltage Direct Current (MVDC) type according to an embodiment. To minimize costs for multiple applications, which need to serve several lower power loads and sources it is beneficial, if the series connected DC/DC converters are loaded differently. Related to the state-of the art technology different loading of DC/DC converters leads to changes on the input voltage at the MVDC side. The present disclosure is able to provide, in contrast to the state-of-the-art technology having a series input and parallel output topology, individual parallel DC outputs.

FIG.5ashows a targeted functionality of the system according to the present disclosure. In particular, a MVDC input is provided which serves a plurality of DC/DC converters1in order to provide multiple Low Voltage Direct Current (LVDCx) outputs. The DC/DC converters1may form or be referred to as a converting stage. The detailed configuration of the DC/DC converter will be described below and illustrated inFIG.7. Each of the DC/DC converters may comprise at least one DC/AC converter11(shown inFIG.7, first converting stage) receiving the MVDC input and as well as at least one AC/DC converter13(shown inFIG.7, second converting stage) providing the output. According to an embodiment, the AC/DC converters13correspond to the DC/AC converters11and supply a plurality of LVDC outputs to loads. A transformer unit12may couple the DC/AC converters11to the AC/DC converters13. The DC/AC converters11are structured as a plurality of single converters connected in series, while the AC/DC converters13are connected in parallel. The proposed system thus comprises multiple isolated DC/DC converters1which provide parallel outputs. Each of the DC/DC converters1may further be equipped with a voltage control functionality. Through this, individual loads may be connected without affecting stability of the grid. In other words, all of the isolated DC/DC converters1may provide a predetermined output voltage, and the output voltages may be different. Furthermore, the grid may be connected to an energy source and redistribute the input energy to different loads. In other words, the systems according to the examples described below may be of a bi-directional type.

The system may further be enhanced with a power distribution unit as depicted inFIG.5b. Thus,FIGS.5aandbshow a MVDC/LVDC converter with series connected isolated DC/DC converters1serving multiple loads or sources. The outputs may optionally be configured to provide different power levels. This structure enhances flexibility and efficiency of power distribution and collection grids since the outputs may be configured independently or connected according to the load to provide a higher power output.

According to an embodiment, at least one of the converting stages comprises Solid State Transformers (SST). The DC/DC converters1are particularly designed as single-stage isolated DC/DC converters1comprising a voltage control configured to control a voltage of the respective DC/DC converter1.

The disclosure will be further described referring to various examples. According to a first example shown inFIG.6, a variable MVDC input is used for the system. By allowing the MVDC bus to be variable and to adapt to the needed power levels, it is possible to serve multiple differently loaded LVDC connections. The variable MVDC bus can be supplied by a grid converter supporting variable DC-Link operation, i.e., a Modular Multi-Level Converter (MMC) converter with some full-bridge cells, or by an (non-isolated) DC/DC converter3connected to the constant MVDC Link. This DC/DC conversion stage could also be distributed to the isolated DC/DC cells, allowing a more modular implementation. The DC/DC conversion stage serves as a pre-regulation and allows for a stable system. According to an example, the DC/DC conversion stage has a controlled voltage output. According to an example, the DC/DC conversion stage may be substituted by an AC/DC converter or an AC/DC conversion stage having a controlled voltage output. The AC/DC conversion stage may also act as a pre-regulation. The example according toFIG.6may also be equipped with a multi-winding transformer. Possible implementations of the power distribution unit are described below.

FIGS.7to10show further embodiments of a DC/DC converter according to the present disclosure.

In particular, according toFIG.7, a MVDC to LVDC converter with series DC/DC converters1and a multi-winding transformer12is provided. According to this example, the transformer12has a single core shared between the primary winding(s) and the secondary winding(s). The DC/DC conversion may be of the Dual Active Bridge (DAB) or the Series Resonant Converter (SRC) type. In more detail,FIG.7shows a first converting stage comprising four DC/AC converters11and a second converting stage comprising four respective AC/DC converters13. The first and second converting stages are coupled by a transformer unit12, in the present case a multi-winding transformer12having a common core. Each of the AC/DC converters13of the second converting stage provides a LVDC output to be connected to a load. The converters may be configured to provide the same or different power ratings and may also be interconnectable in order to provide higher power for higher loads such as fast charging of vehicles. This functionality may be achieved by a power distribution unit to be described below.

The power flow to and from MVDC and each LVDC port can be controlled separately. This requires isolated DC/DC converters1(i.e., DAB or SRC type) with voltage ratio control functionality or alternatively fixed voltage ratio supported by pre- or post-regulation stages. The number of transformer windings is flexible and can have different number on MVDC and LVDC side.

According to a preferred example shown inFIG.8, a plurality of SiC MOSFET cells each forming an DC/AC converter11is provided which are connected in series. A transformer unit12as well as an AC/DC converter13, in this case using diodes, is depicted in accordance with other embodiments. The advantage of this example is that the power rating of the multi-winding transformer is N-times higher, wherein N=number of cells with equal power. This leads to lower costs equal to (0.75{circumflex over ( )}N)/N.

FIG.9shows an alternative configuration of an MVDC/LVDC converter based on series DC/DC converters1and a multi-winding transformer12having a different number of MVDC windings with respect to the examples above. In particular, the DC/DC converter1comprises a single DC/AC converter11, a plurality of AC/DC converters13for providing an output voltage, and a transformer unit12having a primary winding for the DC/AC converter11and secondary windings for each of the AC/DC converters13. The transformer unit12also has a common core and is therefore similar to the configuration according toFIG.7. The DC/DC converter1may be of the DAB or SRC type.

InFIG.10showing a further exemplary configuration, the first converting stage, i.e., the DC/AC converter11, comprises SiC MOSFETs which are connected in series. Capacitors may optionally be provided in parallel to the respective MOSFETs. The second converting stage, i.e., the AC/DC converter13, which is connected to the first converting stage11via a transformer unit12, also comprises MOSFETs in order to convert the received AC power to a LVDC output. The functionality of the depicted example is similar to the other examples described herein.

In the example according toFIG.11, a MVDC/LVDC converter with series DC/DC converters1is shown. Furthermore, multiple multi-winding transformers12are provided. According to an embodiment, one multi-winding transformers12per DC/AC/AC/DC converter pair (1,11,12,13). An impedance Z may be needed for impedance adjustments or for controlling the energy exchange. In particular, an impedance circuit14may interconnect some or all of the transformers12. The impedance circuit14may act as a balancer. Similar results can be achieved as in the case of the examples described with reference toFIGS.7to10.

According to a further example, a pre-regulation stage for cell power exchange at the MVDC side, i.e., exchange between the respective converters of the first converting stage, may be provided. This is shown inFIG.12. The pre-regulation stage may be based on a Modular Multi-Level Converter (MMC) stage. In particular, the power redistribution grid may further comprise a plurality of second DC/DC converters upstream of and connected to each of the plurality of DC/AC converters11(which form part of the isolated DC/DC converters1). The second DC/DC converters are connected in series, wherein exemplarily an impedance is connected between two of the plurality of second DC/DC converters. Furthermore, a capacitor is connected in parallel to the plurality of second DC/DC converters. This allows a circulating current through all of the DC/DC stages and acts as a power balancer.

FIG.13shows an overview of a power grid according to the present disclosure using series DC/DC converters1which are formed as Solid State Transformers (SST). The grid comprises an energy management unit configured to control operation of the grid and a power distribution unit. According to the depicted example, a plurality of outputs 1 to L may be provided for DC loads (DC loads_1_to_L) and the outputs may be controlled or connected by the power distribution unit. Furthermore, at least one output for DC power storage DC (Storage_1_M) and at least one input for DC sources (DC sources_1_to_N) may be provided. A further connection may be provided for at least one AC load or source (AC loads or sources_1_to_P). This evidently requires a DC/AC converter as shown in the figure.

Each of the examples described with reference toFIGS.6to13may be combined with a power distribution unit2configured to connect or disconnect DC outputs to loads.

This is described with reference to an exemplary vehicle charging system using charging poles as loads22that can be connected to a vehicle to be charged. In case each charging pole22gets its own bus and each DC converter1can be connected to each of these busses, there would be nine switches (3×3) necessary for three poles and three outputs, and one hundred switches for ten outputs and ten poles. I.e., there is a quadratic growth for the number of switches related to the number of outputs. An alternative solution in order to save switches could be a ring structure, with each DC-output connectable by disconnectors or switches21to two neighboured outputs and with each charging pole22directly connectable via a disconnector21to one of the DC outputs. This is shown inFIG.14, wherein the reference sign1denotes the DC/DC converters and the respective the DC outputs and the reference sign22denotes the loads or charging poles according to the example described above. This arrangement allows to use 2×n switches21only, n being the number of outputs. A smart management of the sequence of connecting vehicles to free poles22may be employed in order to use the available power most efficiently. Target of this management would be to always provide a high probability to have at least one pole22available for fast charging with double power, which can be achieved by connecting adjacent DC outputs via the switches21, and on the other hand load all DC outputs equally.

FIG.15shows a further exemplary configuration of a power distribution and collection grid including or being connected to a power distribution unit2. In this example, the switches21also form a ring type configuration.

FIG.16depicts an alternative configuration which may be referred to as matrix configuration. In each respective output terminal of each DC/DC converter1, there are N switches21connecting with N DC loads respectively. The total number of switches are N*n, wherein N is the number of loads22, in this case charging poles, and n is the number of DC/DC converters1. The matrix type switching group can be used in an application where there is no requirement to open a short circuit current. If the switches just need to open a current of normal operation, the switching current of matrix type is just (1/N) of the current according to the ring type.

In other words and referring to the examples described above, the system may be based on a SST topology, wherein the key components in the topology are several isolated DC/DC converters1and switching groups. The isolated DC/DC converters1are connected input terminals in series. The two outermost terminals of the in-series input terminals are connected to an MV DC bus. The output terminals of each isolated DC/DC converter1connect all the electric vehicle (EV) chargers via a switching group. The switching groups control their DC/DC converters1to join into one appointed EV charger. The MV DC bus is supported by a controllable DC source.

Optionally, the grid may further comprise a control unit22and/or a bypass breaker circuit23. The control unit24may also be referred to as (coordination) controller. This controller24is proposed for smoothing operation of the proposed SST topology. The controller24generates the system operation references and switching orders, including current reference Idcreffor the DC source, DC/DC converter1voltage references UPMjref, and switching group control order SPMj_EVi.

FIG.16is thus a schematic view of a power grid and a distribution unit according to an exemplary embodiment showing the principle structure as well as optional features such as the control unit24, the bypass breaker circuit23and the matrix type configuration of the switches21.

A flow chart is depicted inFIG.17. In one control period, the controller24firstly calculates a suitable number of power modules (DC/DC converters1) for each EV charger22according to a principle of nearest mean value of all the charger powers and smallest voltage difference. The principle can be explained as the number of power modules switching to the istcharger is calculated with round-off number of the divide result of the istcharger power requirement and mean power of all chargers. The calculation may be exemplarily performed using the following formula (F1).

NPM⁢_⁢EVi=round(PEVi(PEV⁢1+PEV⁢2+…+PEVi+…+PEVm)n)(F⁢1)

Consequently, the selected power modules and their voltage references can be calculated, e.g., utilising the formulae (F2) and (F3) shown below. In particular, (F2) is used to calculate the power reference PPMjof the power module j, i.e., the respective DC/DC converter1. Using (F3), a voltage reference UPMjof the power module j, i.e., the respective DC/DC converter1, is calculated, wherein N is the number of loads and UPM_nomdenotes the nominal voltage. In addition, a switching group action can be confirmed. The MVDC bus current reference can be also calculated. The calculation may exemplarily be performed with formula (F4) shown below.

PPMj=PEViNPM⁢_⁢EVi(F⁢2)UPMj=PPMj∑PPMj*N*UPM⁢_⁢nom(F⁢3)Idcref=∑PPMj∑UPMj(F⁢4)

As mentioned above, the system may also comprise an optional bypass breaker circuit23. The bypass breaker circuit23is exemplarily laid out in the input terminal of each isolated DC/DC converter1as shown inFIG.16. The bypass breaker23of each isolated DC/DC converter1is used to make the converter be out of service temporarily to avoid too low input DC voltage to ensure operation with high efficiency. In some operation cases, the power difference of the isolated DC/DC converters1is very large. Thus, some converters operate in low input DC voltage and consequently have a low operational efficiency. In order to avoid this situation, some of the DC/DC converters1are stopped by closing theirs bypass breakers23. The operating DC/DC converters1will get a more uniform power requirement and input DC voltage. However, the bypass breaker23is optional in the topology. The bypass breakers23may also be controlled by the coordination controller24.

FIG.18shows a respective flow chart with bypass breaker23functionality. The respective formulae (F1) through (F4) are described above. As long as a difference of the maximum power module voltage and a minimum power module voltage max (UPMj)—min (UPMj) does not exceed a threshold voltage Utrig, a normal operation according toFIG.17is performed. However, if Utrigis exceeded, the bypass breaker circuit23is activated to bypass a respective power module, i.e., DC/DC converter, and the number of loads N is reduced.

Furthermore, the controller24, or station control, may be provided for handling assignment of loads such as charging poles22(in case of a DC charger for electric vehicles). Taking an example with four poles (cf.FIG.15, poles22), the station control24tries to keep two neighboured DC feeds (DC/DC converters1) free. I.e., after pole1was connected to DC1output, the next vehicle would either be connected to pole2or4(e.g. via signal lights with a red light above pole3indicating that pole3is occupied or not to be connected, and green lights above poles2and4indicating they are available). If done like that, after connecting the second vehicle, two neighboured DC feeds would be available for fast charging. There is no need for no-load at one DC output. In a situation when not all chargers are use, the tie switch can be connected to a neighboured DC output and the load can be shared. In the four pole system as above, when charging pole1and2are in operation, the tie switches from DC1to DC4could be closed and the tie switch between DC2and DC3. By this, all DC outputs are loaded. Even if running in a situation that poles1and3are active and a new vehicle comes in—which then could only be charged with the power of one DC output—you could share the load of the higher loaded pole of 1 or 3 with the free remaining DC output by closing the related tie switch.

The power distribution unit may also be combined with other power distribution grids and other loads or sources as presented in the examples herein.

FIG.19is a further example according to the present disclosure. Therein, a high voltage AC (HVAC) input is provided and converted into a medium voltage AC (MVAC) which is fed into an AC/DC converter. Subsequently, a plurality of DC/DC converters1which are connected in series is provided. In the example, n denotes the number of DC/DC converters1, n being an integer. Each of the plurality of DC/DC converters1comprises a DC voltage (VDC) to medium frequency AC (MFAC) converter11, a MFAC to VDC converter13and a transformer12coupling both converters11,13. The DC/DC converters may be provided as isolated single stage converters. Furthermore, any of the examples described above may be used with the example according toFIG.19and a detailed description thereof is thus omitted. The plurality of DC/DC converters1may employ a solid state transformer (SST) topology. Each of the DC output voltages of the DC/DC converters1may be provided to a plurality of switches21forming part of a low voltage DC (LVDC) bus and connector system. The LVDC bus and connector system may correspond to the power distribution unit2. The LVDC bus and connector system is connected to a plurality of loads 1 to n, in this example charging poles22. In the example ofFIG.19, the ring type switch configuration described above is shown. However, the number of switches21may correspond to the number of loads n or may be any even multiple thereof. The functionality of the example according toFIG.19is equal to the examples described above.

According to the disclosure, an improved power redistribution grid is provided capable of handling a plurality of inputs and outputs with an optimum efficiency.

The present disclosure also encompasses a corresponding method.

Other aspects, features, and advantages will be apparent from the summary above, as well as from the description that follows, including the figures and the claims.

While the disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. It will be understood that changes and modifications may be made by those of ordinary skill within the scope of the following claims. In particular, the present disclosure covers further embodiments with any combination of features from different embodiments described above and below.

Furthermore, in the claims the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single unit may fulfil the functions of several features recited in the claims. The terms “essentially”, “about”, “approximately” and the like in connection with an attribute or a value particularly also define exactly the attribute or exactly the value, respectively. Any reference signs in the claims should not be construed as limiting the scope.