Power flow control system for use in an electrical DC network

Abstract

The invention relates to a DC power flow control system (20), designed for use in an electrical DC network (10) and designed to adapt the voltage and / or the current in a DC line (12) of the DC network (10), comprising a voltage control module (30), which can be connected in series into the DC line (12) of the DC network (10), for adapting the voltage in the DC line (12); and a voltage supply unit (40), connected in parallel with the DC line (12), for supplying the voltage control module (30) with energy; wherein the DC power flow control system (20) is designed to simultaneously change voltages and potentials in the DC network (10).

Description

[0001] Rhineland-Palatinate University of Technology Kaiserslautern-Landau

[0002] Power flow control system for use in a direct current electrical network

[0003] The present invention relates to a DC power flow control system which is designed to be used in an electrical DC network and to adjust the voltage and / or current in a DC line of the network.

[0004] Electrical power grids are typically designed as AC networks or alternating current networks. This is primarily due to historical reasons. The generation, conversion, and transmission of electrical energy in three-phase networks is well established and is used almost everywhere in the world.

[0005] DC voltage grids, on the other hand, are only rarely used for high-voltage direct current transmission when energy needs to be transmitted over long distances, for example in North America and soon from northern to southern Germany, or over water, for example from offshore wind farms. However, the power electronics used today and the converters now available have eliminated one of the main reasons for using alternating current: the simple voltage conversion in AC grids using transformers. DC voltage can also now be converted quite easily. In addition, many relevant sources and loads in energy grids today, especially most electronic loads, are essentially DC devices. The emerging photovoltaics (PV) are inherently DC voltage and must first be converted to sine-wave alternating voltage before being fed into the AC grid.Motors are not directly connected to the mains supply, but rather via mains inverters to enable speed control. The motor supply frequency and voltage, which differ from the mains frequency and voltage, are usually generated by a drive inverter from a DC link. However, the DC link voltage must first be converted from the AC line voltage, which is a complex process.

[0006] Like conventional power plants, wind energy also uses generators. However, these generators are not permanently connected to the grid at a fixed speed to enable the necessary dynamic adjustment to wind conditions. Instead, they are connected via powerful electronics and a DC link, similar to motor drives.

[0007] Vehicle chargers also have a significant impact today, due to their immense power and increasing popularity. Vehicle chargers charge a DC battery. The conversion from the AC mains voltage must be provided by the charging electronics. These steps and their impact on AC and DC grids are described, for example, in the following publications:

[0008] Rivera et al. (2022) Charging Infrastructure and Grid Integration for Electromobility. Proceedings of the IEEE, 111 (4):371. doi: 10.1109 / JPROC.2022.3216362 (https: / / ieeexplore.ieee.org / abstract / document / 9940564)

[0009] Rivera et al. (2021 ) Electric Vehicle Charging Infrastructure: From Grid to Battery. IEEE Industrial Electronics Magazine, 15(2):37. doi: 10.1109 / MIE.2020.3039039 (https: / / ieeexplore.ieee.org / document / 9343759)

[0010] The consequence of these developments is that a direct current supply via a direct current grid (DC grid) can be more practical at all voltage levels, i.e. for high-voltage transmission, medium voltage, and local grids. The use of a DC supply is more direct and eliminates multiple AC-DC conversion stages. For example, energy generation with photovoltaics and subsequent intermediate storage and / or consumption by electronic devices currently requires multiple conversions. The photovoltaic system provides energy in direct current. This is converted into an AC grid. A further conversion takes place in the DC storage system and then back into the AC grid and from there, for example, into a DC vehicle converter. This means four conversions between direct current and alternating current, or vice versa, are necessary.Even with extremely high efficiencies of the individual conversions, such iterative conversion of the same energy as is currently required accumulates to immense losses and thus wasted energy. Furthermore, the conversion stages are often expensive.

[0011] At the extra-high voltage level (several 100 kV), transmission grids above certain power levels and / or for long distances are now usually implemented using direct current (HVDC). For example, most offshore wind farms are connected to onshore grids using direct current (DC), allowing subsea connections to be used.

[0012] A trend toward using DC technology even at lower voltage levels is now evident. Unipolar voltages of around 600 to 800 volts, and sometimes even up to 1500 volts, are now used in distribution and on the last mile. The voltage is usually measured against ground or bipolar with a corresponding return conductor, usually with the same voltage but negative. In the low-voltage range, some vehicle charging systems are known that operate with one or a few large AC grid converters and can feed many charging points directly from the local DC supply grid. DC supply systems are now also being used in isolated industrial applications, for example, in individual industrial plants, industrial parks, or entire industrial areas.

[0013] However, problems arise from the fact that DC networks react significantly differently than AC networks. While in AC networks, currents are limited by the AC impedances caused by inductances, inductance in DC networks only plays a transient role, not for the final value. The latter is defined purely by the resistance, which is usually dangerously low, which is desirable for lossless transmission. Therefore, extended loops are essentially unmanageable in DC networks, which is why rings are generally avoided.

[0014] Arcs that occur in AC systems are easier to extinguish than in DC systems because the current must pass zero after 10 ms at the latest. Components, devices, and the network itself in DC systems sometimes have very high capacitances, which can lead to extremely high, very short current peaks when switched on or in the event of a fault. Examples of these include inrush current peaks, which can occur in mostly inexpensive power supplies that activate the fuses when plugged in. For this reason, most DC devices use power electronics and active control, which measure the voltage and current values ​​and attempt to regulate any control error to zero.

[0015] As a result, the protection of DC networks must be designed significantly differently than for AC networks. In AC networks, the highest possible short-circuit current was desirable in the event of faults to ensure that the current reliably and relatively quickly blows the associated fuses. Impedance prevents excessive currents. In DC networks, however, a fault should be detected as immediately as possible, i.e., within milliseconds or even faster, and any fault current should be prevented, because the currents could rise virtually indefinitely.

[0016] In DC networks, loops can therefore no longer be controlled beyond a certain size, as can be the case, for example, in AC local networks. Since only resistance regulates the currents between locations of different voltages, large compensating currents and even high ring currents can occur in larger loops, combined with active control of the individual connected devices. These can lead to local voltage tolerance violations, i.e., to significant voltage deviations, and thus to damage to devices and current overload on lines.

[0017] There is therefore a need for a power flow control system that can also be used with DC voltage networks with loops without causing failures or damage to equipment due to current overload or voltage fluctuations.

[0018] The present object is achieved with a DC power flow control system having the features of claim 1 and with a DC network having the features of claim 16.

[0019] A first aspect of the present invention relates to a power flow control system that is designed to be used in an electrical DC network and to adjust the voltage and / or current in a DC line of the DC network. The DC power flow control system comprises a voltage control module that can be serially connected to the DC line of the DC network in order to adjust the voltage in the DC line, and a voltage supply unit connected in parallel to the DC line in order to supply the voltage control module with energy. The voltage supply unit is preferably connected in parallel to a DC line. The power flow control system is further preferably designed to change voltages and potentials in the DC network simultaneously.

[0020] Within the scope of the invention, it was recognized that there is a need for a system that can inject a voltage in series with a DC line, in the case of a unipolar DC line into the positive branch or the negative branch, and in the case of a bipolar DC line into the positive branch, the negative branch, or both branches. In contrast to AC lines, however, there is no need for regular reversal of the polarity of the injected voltage, and this must be taken into account in the design. According to the invention, the control system can drive the current through the DC line and regulate it, even against any voltage gradient that may be present. For example, this can be used to brake current and allow it to flow more slowly than the voltage difference between two points and the line impedance would cause.

[0021] According to the invention, the DC power flow control system is designed to reduce the current or even reduce it so drastically that it is close to zero or only amounts to 1% to 30% of the rated current of the DC line, for example, if the change in the module voltage is insufficient. The DC power flow control system is preferably designed to completely regulate the current to zero. Furthermore, it can preferably allow the current to flow opposite to the direction expected based on the electrical potential conditions. The control system according to the invention has the further advantage that the (effective) line impedance can be adjusted.

[0022] In principle, a DC-DC converter could be connected to the line in a DC network. However, the converter would have to process the entire network current and be designed for the full line voltage. Consequently, the converter would have to reconvert the entire line power, which, even with extremely efficient converters, would result in very high absolute losses due to the high converted power. Such a solution is therefore impractical.

[0023] Line inverters could also, in principle, be connected to a DC line if the line inverter is not connected to ground or another line in the DC grid. However, such an inverter would require energy and power to drive current. In return, the inverter would accumulate charge and energy if the inverter essentially "brakes" a current, i.e., if it actively reduces the current level actually expected due to the potential gradient by applying a certain counter-voltage. In addition, the inverter would therefore require a power supply and a means of dissipating the accumulating energy. While the inverter could be powered via low-voltage electronics, it would not be relative to ground or another potential on the line, as this would require higher voltages. Instead, it could be powered by a supply voltage V s with an amount of V s / 2 above or below the electrical potential of the corresponding line. However, a supply with two closely spaced potential levels is complicated and unsuitable for practical use. A voltage source is also necessary. In particular, for the higher voltage level (VL + Vs / 2), the voltage is difficult to generate from the available voltages because it lies above all available voltage levels. The DC power flow control system according to the invention, on the other hand, has the advantage that it only has a very low nominal power while still controlling a high power in the DC line of the DC grid. According to the invention, the DC power flow control system is able to correct the voltage of a longer line along the line, i.e., raise or lower it, and also influence or regulate the current, i.e., drive, brake, zero, or reverse the current.

[0024] Another advantage of the DC power flow control system is that lines with different voltages can be connected via the control system despite the difference, and distortions are filtered out. Non-DC current and non-DC voltage components in the specified DC current and / or DC voltage can be filtered out.

[0025] A further advantage of the control system is that it eliminates the need for any type of power transformer, i.e. large and heavy transformers, as well as high-frequency transformers, which, despite the increased frequency, take up a relatively large amount of space and require extensive and expensive circuitry with additional passive power components and transistors.

[0026] The DC power flow control system according to the invention essentially comprises two parts: First, one or more DC power feeds, for example, one per line in bipolar networks, which are connected in series with the line. Furthermore, there is a DC voltage supply, which draws power from the available line or lines themselves and supplies the series feed (voltage control module). The DC voltage supply is connected in parallel to the DC line of the network. This supply can preferably be bidirectional and, for example, can also be provided by feeding into the line in parallel.

[0027] The DC power flow control system according to the invention is characterized by two components that are not only related in terms of energy and power, but can also each perform a separate function. The voltage control module, connected serially to the DC line, supplies voltage, i.e., it increases or decreases the line voltage across the serial section. The voltage supply unit (parallel section), connected in parallel to the DC line, not only supplies voltage but also supplies or draws current, i.e., DC current and current of any frequency can be drawn.

[0028] Both components can also be regulated to the respective other value. The serial part (voltage regulation module) can thus control or regulate the current along the node or line, since the voltage is injected between the terminals and thus along the line. In contrast, the parallel-connected voltage supply unit injects current non-specifically. It can flow out or in in both or all directions accordingly. In a control system, this would generally raise the voltage of the node relative to ground. The parallel and series parts (voltage supply unit and voltage regulation module) are formed directly via power electronics and connected to the grid. The power exchange between the voltage regulation modules and the energy supply (voltage supply unit) is solved by galvanic connection.

[0029] According to a second aspect, the present invention relates to a DC grid comprising such a DC power flow control system as described above. The DC grid can be single-phase (unipolar) or multi-phase (or polypolar, for example bipolar). The DC grid is preferably unipolar or bipolar. It can be controlled by the DC power flow control system such that both voltage and potential can be controlled or changed simultaneously. This makes it possible to change the currents in the DC lines of the DC grid, i.e., to increase, decrease, or slow them down.

[0030] The voltage in a line can also be changed so that the voltage across the control module is not equal to zero. Thus, the ground potential (understood as the voltage relative to ground) at the input of the voltage control module in a line is different from the ground potential at the output of the voltage control module. Thus, both the voltage and the potential can be changed simultaneously.

[0031] Preferred embodiments of the invention are described in the dependent claims. It is understood that the features mentioned above and those yet to be explained can be used not only in the respective combinations specified, but also in other combinations or alone, without departing from the scope of the present invention. In particular, the method and the computer program product can be implemented according to the embodiments described for the device in the dependent claims.

[0032] In a preferred embodiment of the DC power flow control system, the voltage supply unit is controllable such that the potential of the voltage control module follows the potential of the DC line of the DC grid in a predefined manner. Additionally or alternatively, the voltage supply unit can be controllable such that the positive potential of the voltage control module, i.e. the potential of the positive rail (positive supply rail of the module), is greater than the potential of the DC line. The voltage control module, which is connected in series to the DC line, has a positive rail and a negative rail that run parallel to each other. The potential of the positive rail of the voltage control module is therefore the positive potential (relative to ground). The potential of the negative rail of the voltage control module (negative potential) is smaller than the potential of the DC line and lies between the potential of the DC line and ground potential.

[0033] The positive and negative rails are therefore at the same potential as the DC line. They have a potential difference of only a few volts (usually less than 100 volts, preferably less than 30 V, particularly preferably less than 10 V, or, in relative terms, at most 10% of the mains voltage, preferably at most 5%, at most 4%, 3%, 2%, 1%, 0.5%). In a preferred embodiment, the voltage supply unit is configured to adjust the current flowing in the DC line. This includes both a voltage feed to increase the current in the line and an opposite voltage feed to reduce the current in the line. Not only DC currents but also any currents with any frequency or frequency components can be adjusted.

[0034] A preferred embodiment provides that the voltage supply unit is configured to drive a current into the DC line and / or suppress or compensate for distortions and / or current fluctuations. Driving a current in the DC line and drawing current from it compensate for the fluctuations.

[0035] In a preferred embodiment, the power flow control system includes an extra-low-voltage high-current transistor or a mains-voltage low-current transistor. This transistor is placed at a location that would otherwise normally be reserved for a transformer in a power flow control system in an AC network. Thus, instead of a transformer, a corresponding transistor is placed, although in principle, no transformer was originally located there and needs to be removed. Rather, such a transistor is placed during the planning stage at a location where a specialist would otherwise expect to find a transformer. Both the extra-low-voltage high-current transistors and the mains-voltage low-current transistors are low-power semiconductors that can be used almost exclusively in the DC power flow control system.Although the product of current and voltage processed by the transistors is significantly lower, they can control significantly higher power levels in the line, with the power in the small-power semiconductors preferably being lower by a factor of 5, particularly preferably by a factor of 10, very preferably by a factor of 20, and most particularly preferably by a higher factor in the range of 30 to 70.

[0036] In a preferred embodiment, the power flow control system is thus free of transformers and / or transmitters, neither at a low frequency such as 50 Hz, 60 Hz, 100 Hz, or 120 Hz, nor at a high frequency for downsizing, for example, in the kilohertz range, as is common with switching power supplies. It can therefore be constructed cost-effectively. In particular, no heavy or large transformers or transmitters are required.

[0037] In a preferred embodiment of the DC power flow control system, the voltage control module and the voltage supply unit are connected to the DC grid. They are configured and interconnected in such a way that power exchange between them occurs in a galvanic connection.

[0038] The control module is connected in such a way that a galvanically defined connection exists with the voltage supply unit, which can be controlled in such a way that the potential of the voltage control module follows the potential of the line of the DC network in a predefined manner.

[0039] A galvanically defined connection of the power supply within the meaning of this invention is to be understood as a connection of the power supply unit to the DC network or the DC line to which the one or more control modules and, via the control modules, the power supply unit are connected. A galvanically defined connection to the power supply can mean, in particular, that current can flow from a control module, e.g., the positive or negative supply rail of the control module, to the power supply unit without a compensating return current necessarily having to flow from the power supply unit to the control module.Accordingly, this current between a power supply unit and an associated control module can be a common-mode current, which contrasts with a differential-mode current that may be present and charge or discharge a capacitor connected to the positive and negative supply rails in the control module. Such common-mode currents can increase or decrease the electrical potential of the associated control module (e.g., the electrical potentials of the module's positive and negative supply rails) and are possible, for example, if a power supply unit is not galvanically isolated from a control module by a transformer or high-frequency transformer.Common-mode currents can also flow from the power supply unit to an electrical element that is either ground or has a fixed electrical potential relative to ground, such as a line in the electrical grid. This possibility can be present, for example, if a power supply unit also has a galvanically defined connection to the electrical grid.

[0040] Preferably, a galvanically defined connection in the present sense allows common-mode direct currents, i.e., constant common-mode currents as opposed to common-mode alternating currents, whose time average disappears, across the galvanically defined connection. Capacitor connections or parasitic capacitances to ground, for example, generally do not allow continuous, constant common-mode currents. Common-mode currents via the galvanically defined connection to the power supply unit preferably charge the capacitance of the respective control module to ground (common-mode capacitance), causing the electrical potential of the control module (to ground) to rise or fall, provided no other common-mode inflows or outflows are present (for example, approximately given when the power switches or transistors of the respective control module are electrically open).For example, by appropriately generating a time-varying common-mode current, the electrical potential of a control module can be adjusted with or parallel to the line voltage of a conductor of the electrical network.

[0041] Preferably, a common-mode current is actively controlled via a control loop, for example, to approximately zero or, on average, to zero, or such that the electrical potential of the associated control module runs temporally parallel to the voltage of a conductor of an electrical network, for example, such that the electrical potential of the positive rail of the associated control module is above the associated conductor voltage and the electrical potential of the negative rail of the associated control module is below the associated conductor voltage. Preferably, in addition to a galvanically defined connection of a control module to a power supply unit, there is also a galvanically defined connection of the control module to a line of an electrical network.In this case, due to the at least two parallel, galvanically defined connections to earth, to a potential with a fixed voltage condition relative to earth, or, for example, to a conductor of an electrical network, ring or circular currents are also possible. A galvanically defined connection can occur, for example, when a power supply unit is electrically configured as a voltage source (a so-called voltage-source inverter, which can also be controlled as a current source if necessary), which, in addition to a voltage between the power connections (connections to the positive or negative rail of the module), can also set an electrical potential or a voltage of the power connections to earth or another potential with a fixed reference to earth.

[0042] In a preferred embodiment, the DC power flow control system is configured to raise and / or lower the voltage across the DC line. Preferably, both the DC voltage and dynamic components of the DC line voltage are changed. Thus, voltages with different frequency components can also be changed.

[0043] Preferably, the voltage regulation module and the voltage supply unit are constructed with power electronic components. Both the voltage supply unit and the voltage regulation module therefore comprise power electronic components, such as, very preferably, transistors.

[0044] The voltage supply unit preferably has a unidirectional transmission stage. Power is preferably transmitted from the voltage supply unit to the voltage regulation module. The voltage supply unit very preferably has diodes. Such a connection can be used, for example, if there is a connection to ground, for example to limit the negative potential of the voltage regulation module (floating module). Alternatively, the diodes can be replaced by semiconductor components, such as field-effect transistors (FETs). In an alternative, but equally preferred embodiment, the voltage supply unit has a bidirectional transmission stage and comprises semiconductor components, for example FETs or MOSFETs. The semiconductor components are preferably designed such that they can be operated with the mains voltage of the DC network.Furthermore, the semiconductor components are designed such that they are preferably configured for a current of a maximum of 50% of the current of the DC line and / or the current of the voltage regulation module. The current is preferably limited to a maximum of 20%, more preferably to a maximum of 10% of the current of the DC line and / or the current of the voltage regulation module. Very preferably, the current can also be limited to smaller values, for example, to a maximum of 5%, 4%, 3%, 2%, or 1% of the current of the DC line and / or the current of the voltage regulation module.

[0045] Preferably, a portion, for example at least 40%, preferably at least 50%, of the transistors of the voltage supply unit can have a low dielectric strength and another portion of the transistors can have a higher dielectric strength. Particularly preferably, the transistors arranged on the module side of inductors (i.e. on the side of the inductors facing the voltage regulation module) can have a low dielectric strength. The dielectric strength is thus only related to the module level, i.e. the dielectric strength of the semiconductor components is slightly greater (for example + 10%) than the voltage in the voltage regulation module, or it amounts to a fraction (at most 20%, preferably at most 10%, particularly preferably at most 5%) of the dielectric strength of the other transistors of the voltage supply unit (mains-side part directly facing the DC line).Also preferably, at least half of the transistors may have a dielectric strength that corresponds to the magnitude of the DC mains voltage (deviation < ± 10%) or that is greater than the DC mains voltage.

[0046] A preferred embodiment of the DC power flow control system provides that the voltage supply unit has a ground connection for connection to ground. This can be used with unipolar DC lines of the DC grid. In a preferred embodiment, the DC power flow control system has a module DC intermediate circuit. The module DC intermediate circuit has a voltage of at most 100 volts. Preferably, the voltage is at most 60 volts, particularly preferably at most 45 volts or 30 volts. The voltage can also be lower, for example at most 25 volts, 20 volts, 15 volts, or 10 volts. Preferably, the voltage is at most 10% of the grid voltage, preferably at most 5% of the grid voltage, particularly preferably at most 2% of the grid voltage, or equally preferably at most 20%, 15%, 12%, 8%, 4%, 3%, 1%, or 0.5% of the grid voltage.

[0047] A preferred power supply unit of the DC power flow control system has a filter. The filter is preferably designed as a filter capacitor, which is particularly preferably connected in parallel with the power supply unit.

[0048] In a preferred embodiment, a DC grid with a DC power flow control system is configured such that the DC power flow control system is configured to couple the DC grid to another DC grid with a different voltage and / or potential. The DC power flow control system can preferably adjust the voltage and / or potential so that the two DC grids can be interconnected.

[0049] Preferably, the DC grid is designed to make the current flow and / or voltage differences controllable and regulatable by means of a voltage control module and / or a voltage supply unit. This is particularly preferably achieved by means of the power flow control system described above and the special design of this system.

[0050] In a preferred embodiment, the DC network comprises at least one Kirchhoff loop in which a DC power flow control system, preferably a control system according to the invention as described above, is arranged at at least one point, which is designed and configured to change the voltage between a first connection of a control module to the Kirchhoff loop and a second connection of the control module to the Kirchhoff loop or to generate a ring current component.

[0051] According to a preferred embodiment, the DC grid comprises at least one phase in which a DC power flow control system is arranged at at least one location. The DC power flow control system is designed and configured to change, increase, or reduce the voltage between a first connection of the DC power flow control system to the DC grid and a second connection of the DC power flow control system to the DC grid.

[0052] The invention is described and explained in more detail below using selected embodiments in conjunction with the accompanying drawings. They show:

[0053] Figure 1 ad Basic circuit diagrams of a DC network with control system;

[0054] Figure 2 shows a detailed sketch of a DC network with control system and unidirectional power flow with diodes and transistors;

[0055] Figure 3 shows a DC network with unidirectional power flow and bidirectional transmission stage;

[0056] Figure 4 shows a DC network with unidirectional power flow and a reduced version of a control system with only four transistors;

[0057] Figure 5 shows a bipolar DC network with a reduced variant of a control system;

[0058] Figure 6 a DC network with unidirectional power flow and diodes;

[0059] Figure 7 shows a DC network with bidirectional power flow;

[0060] Figure 8 shows a DC network with bidirectional power flow with three outgoing feeders;

[0061] Figure 9 shows the voltage conditions within the DC grid. Figures 1a to 1d each show a section of a DC grid 10 with three DC lines 12, one of which is explicitly shown. A DC power flow control system 20 is provided in this DC line 12, which includes a voltage control module 30 and a voltage supply unit 40. Each of the lines of a three-phase DC grid 10 has such a DC power flow control system 20.

[0062] Figure 1a shows a DC line 12 of the DC network 10 with a voltage regulation module 30 designed as a floating module 32, which is connected in series to the DC line 12. Optional line filters 34, which can be designed as inductors or as PI filters, can be provided at the input and output of the voltage regulation module 30.

[0063] The voltage regulation module 30 is connected to the voltage supply unit 40 and is supplied by it with the necessary voltage to balance or change voltages and potentials in the DC network 10. The voltage supply unit 40 is connected in parallel to the DC line 12 and includes a DC-DC converter. In the embodiment shown here, the input of the voltage supply unit 40 is bipolar. The voltage supply unit can, for example, be connected to an external power source.

[0064] Figure 1 b shows such an embodiment in which the voltage supply unit 40 of the DC power flow control system 20 is unipolar, so that the voltage supply unit 40 is connected between the DC line 12 and ground 42.

[0065] Figure 1c shows a DC network 10 or a DC line 12 of such a DC network with a bipolar DC line, i.e., a positive DC line 12a and a negative DC line 12b, wherein both lines have a voltage control module 30 as a floating module, which is connected in series to the DC line 12. For each of the DC lines 12 (positive DC line 12a and negative DC line 12b), a voltage supply unit 40 is provided, which is bipolarly connected to the two DC lines 12 (positive DC line 12a and negative DC line 12b). Figure 1d shows a basic arrangement of a DC network 10 with a DC power flow control system 20, wherein the voltage supply unit 40 is connected between the DC line 12 and ground 42. The voltage control module 30 is designed as a multi-node module and has several connections 36, which are also referred to as terminals.A line filter 34 is provided at each of the terminals 36 to connect the corresponding terminal 36 to a DC line 12.

[0066] Figure 2 shows an embodiment of the DC network 10 with DC power flow control system 20 for a DC line 12. The other two lines of the three-phase DC network 10 are not shown or only indicated.

[0067] The voltage control module 30 of the DC power flow control system 20 comprises a plurality of switching elements 300, which are implemented as transistors 302, preferably as low-voltage FETs (field-effect transistors). These transistors 302 are designed for high currents and low voltages. Two transistors 302 are each connected in series and in parallel to an energy storage device 304. The switching elements 300 are connected as two half-bridges.

[0068] The power supply unit 40 has an optional filter in the form of a filter capacitor 44 at each of its input terminals. The power supply unit 40 is formed from two parallel-connected half-bridges on both the input side 46 in the input section 47 and the output side 48 in the output section 49, which are connected to two parallel-connected inductors 50.

[0069] The half-bridges 52 are each formed from a transistor 54 and a diode 56, enabling a unidirectional power flow from the voltage supply unit 40 to the voltage regulation module 30. In the embodiment shown here, the voltage supply unit 40 has a connection to ground 42 so that the negative potential of the voltage regulation module 30 is also limited downwards. A module DC voltage intermediate circuit 58 formed here has only very low voltages, which are typically less than 100 volts, preferably less than 60 volts, and particularly preferably less than 30 volts or less.

[0070] An optional mains connection box 62 can be provided in a connecting line 60 between the voltage supply unit 40 and the DC line 12, which can, for example, optionally comprise an intermediate circuit pre-charge and / or a fuse and / or a current limiter and / or an emergency shutdown.

[0071] Figure 3 shows a similar configuration of a DC network 10 with a voltage control module 30, as described in Figure 2. Only the voltage supply unit 40 is formed from two half-bridges 52, each consisting only of transistors 54. No diodes 56 are provided here. This results in a fully bidirectional transmission stage 64, which enables a bidirectional power flow between the voltage control module 30 and the voltage supply unit 40.

[0072] All semiconductors or transistors 54 of this transmission stage 64, i.e., the voltage supply unit 40, are designed for the mains voltage of the DC network 10. However, they only need to carry a fraction of the current of the DC line 12 and thus only a current of the voltage regulation module 30.

[0073] In this embodiment, the voltage supply unit 40 is also connected between the DC line 12 and earth 42.

[0074] Figure 4 shows a DC network 10 with unidirectional power flow and a reduced version of a DC power flow control system 20. The DC power flow control system 20 has a transmission stage 64 with only four transistors 54 and is thus more cost-effective than the previous versions (e.g., Figure 3). The transmission stage 64 allows a bidirectional supply to the voltage control module 30. At least one of the transistors 54 can be designed as a low-voltage transistor and only needs to have the voltage level of the voltage control module 30 or be designed for it. This voltage level is approximately 10 V, approximately 24 V, or approximately 48 V (each ± ​​10%), depending on the technology or design used and the voltage.

[0075] The other transistors 54, however, must be able to block the full DC voltage of the DC network 10 against ground or be designed for such voltages. In Figure 4, these are the two lower transistors 54, which are directly connected to ground, and preferably the transistor 54 to the left of the inductor. They are designed, for example, as IGBT transistors or known alternatives such as silicon carbide transistors and are operated with the entire voltage of the network, for example, 750 V, 1500 V, or the like.

[0076] Figure 5 shows a bipolar DC network 10 with a positive DC line 12a (DC+) and a negative DC line 12b (DC-). Each of the DC lines has a DC power flow control system 20, which is implemented in a simplified version with only four transistors 54 each in the voltage supply unit 40. The transmission stages 64 allow bidirectional supply to the respective voltage control module 30.

[0077] At least the two transistors 54, which are represented on the right side by inductors and face the DC line 12, are preferably designed as low-voltage transistors, which must have or be designed for the low voltage level of the voltage regulation module 30, i.e. for less than approximately 60 V, less than approximately 30 V, or less than approximately 10 V, generally depending on the voltage level of the DC line 12.

[0078] The transistors 54, which face and are close to the optional ground connection 42, must be designed as high-voltage-resistant transistors. They are, for example, IGBTs and are designed for the entire mains voltage, i.e., approximately 750 V or 1500 V. Figures 6 to 8 each show a portion of a DC network 10 with a DC line 12 and DC power flow control system 20. The DC line 12 is designed to be unipolar, meaning the line is connected to ground.

[0079] In Figure 6, the voltage supply unit 40 is also configured with half-bridges 52, each comprising a diode 56 and a transistor 54. This results in a unidirectional power flow from the voltage supply unit 40 to the voltage regulation module 30. In contrast to the embodiment according to Figure 2, the two transformer parts are connected to each other exclusively via two inductors 50. The parallel lines, as shown in Figure 2, are omitted.

[0080] Thus, according to the embodiment of Figure 4, the charging current runs on the module side of the inductors 50 (in Figure 6 to the right of the inductors 50, output part 49) through at least two transistors 54 simultaneously, while for the charging current in the embodiments according to Figures 2 and 3 there is a quasi fixed return path to the input part 47 (in Figure 4 to the left of the inductors 50), which preferably does not run through one of the inductors 50.

[0081] Preferably, some (at least 25%) of the transistors of the voltage supply unit can have a low dielectric strength. Particularly preferably, the transistors 54 in the output section 49 (on the module side of the inductors 50) have a low dielectric strength (for example, suitable for voltages < 100 V, < 48 V, < 24 V, < 10 V). The dielectric strength is thus only related to the module level, i.e., the dielectric strength of the semiconductor components is slightly greater (+ 10%) than the voltage in the voltage regulation module 30, or it amounts to a fraction (at most 10%, preferably at most 5%) of the dielectric strength of the other transistors 54 in the input section 47 (in Figure 4, to the left of the inductors 50). The transistors 54 in the input section 47 have a dielectric strength that corresponds to the magnitude of the DC mains voltage (deviation < ± 10%) or that is greater than the DC mains voltage.

[0082] Figure 7 shows a similar network configuration of a DC network 10 with a DC power flow control system 20, wherein the voltage supply unit 40 is configured exclusively with transistors 54 as half-bridges 52 in the input section 47 and the output section 49, and a bidirectional power flow is enabled by the transistors 54. Energy can therefore flow from the voltage supply unit 40 into the voltage control module 30, but also from the voltage control module 30 into the voltage supply unit 40. Consequently, currents flowing in the DC line 12 can also be reduced.

[0083] In the embodiments according to Figures 6 and 7, the transistors 54 on the input side 46 of the voltage supply unit 40 must be designed to support the full voltage of the DC line 12. In contrast, the transistors 54 on the output side 48 of the voltage supply unit 40 can be designed as low-voltage semiconductors. They must be able to withstand the maximum voltages occurring in the voltage regulation module 30.

[0084] Figure 8 shows a DC network 10 with multiple DC lines 12, whereby here, too, only one DC power flow control system 20 of one of the DC lines 12 is shown as an example. Compared to Figure 5, this configuration differs in that the voltage control module 30 comprises a half-bridge of transistors 302 on the input side, but two half-bridges 306 connected in parallel on the output side, so that multiple connections 36, so-called terminals, are provided for additional DC lines 12.

[0085] In a particular embodiment, the voltage supply unit 40 can comprise at least one step-up DC-DC converter and one step-down DC-DC converter. The step-up DC-DC converter generates a more positive electrical potential 308 from a DC line 12 than that of the DC line 12. The step-down DC-DC converter generates a more negative electrical potential 310 from the same or a different DC line 12 than that of the DC line 12. Preferably, both convert from the same DC line 12. The embodiment can transfer these electrical potentials to a control module 30 or floating module 32, for example, an energy storage device 304 (e.g., an intermediate circuit capacitor) thereof, which supports, for example, a module DC-DC intermediate circuit 58. The step-down DC-DC converter and the step-up DC-DC converter are preferably designed for bidirectional current flow.In addition to a direct or indirect connection to a DC line 12, the step-down DC-DC converter and the step-up DC-DC converter can also have a direct or indirect connection to ground 42. This means that the step-down DC-DC converter and the step-up DC-DC converter have a galvanic connection to the DC line 12 and to ground 42. Alternatively, instead of the direct or indirect connection to ground 42, they can also have a direct or indirect connection to another DC line 12. A first DC line 12 can be a positive DC line 12a, and a second DC line 12 can be a negative DC line 12b.

[0086] The step-down DC-DC converter preferably comprises at least one half-bridge comprising at least one transistor and one diode and / or one transistor. Preferably, the electrical path between the common node of the transistor and the diode and / or the transistor of the bridge and a control module 30 or floating module 32 comprises an inductor. A second terminal of the bridge can be electrically connected directly or indirectly to a DC line 12. A third terminal of the bridge can be electrically connected directly or indirectly to ground or another DC line 12 or the positive point of another bridge.

[0087] The step-up DC-DC converter preferably comprises at least one half-bridge, each comprising at least one transistor and one diode and / or one transistor. Preferably, the electrical path between the common node of the transistor and the diode and / or the transistor of the bridge and a DC line 12 comprises an inductor. A second terminal of the bridge can be electrically connected directly or indirectly to a control module 30 or floating module 32. A third terminal of the bridge can be electrically connected directly or indirectly to ground or another DC line 12 or the positive point of another bridge. Figure 9 shows the voltage conditions in the DC network 10 and in relation to the DC lines 12 and the voltage control module 30. The DC network 10 is designed as a bipolar network. It has a positive DC line 12a and a negative DC line 12b, as well as an optional ground line 41 with a connection to ground 42.The DC grid 10 is asymmetrical in that the potentials of the positive DC line 12a relative to ground 42 and the negative DC line 12b relative to ground 42 are different. The DC grid voltage 14 is applied between the positive DC line 12a and the negative DC line 12b.

[0088] The voltages within the voltage regulation module 30 between a positive rail 308 and a negative rail 310 are defined as the module voltage 312. The voltage of the positive rail 308 relative to the positive DC line 12a is a potential reserve 314 that must be applied; it is positive here. The potential of the positive rail 308 is thus greater than the potential of the positive DC line 12a. This potential difference (voltage) must be generated and provided by the voltage supply unit 40.

[0089] A voltage regulation module 30 connected in series with the negative DC line 12b can equalize or compensate for a voltage drop in the line. The same applies to a negative voltage across the voltage regulation module 30, i.e., when the potential between the module's input and output is negative. The module voltage 312 must be adjusted accordingly. Furthermore, the potential of the negative line 12b relative to ground 42 can also be changed in a predefined or desired manner. For this purpose, the negative potential reserve 314 must be applied or consumed.

[0090] The different configurations of the DC power flow control system 20 described here allow voltages to be injected into the DC line 12, i.e., along the line. For example, the voltage on a long line can be increased again if many loads are connected to the line. It is also possible to reduce the voltage when many sources are present, which is referred to, for example, as a voltage restorer. Furthermore, the DC power flow control system 20, as described with reference to the figures, is designed to connect DC networks 10 with different voltages.

[0091] In summary, it can be said that the DC power flow control system 20 according to the invention is designed to fulfill two tasks:

[0092] 1 . In a line, for example in a star-shaped network, the DC power flow control system 20 allows the voltage of a DC line 12 to be raised or lowered by building up a voltage difference across the series-connected voltage control module 30. This can create a voltage difference upstream and downstream of the voltage control module 30, i.e. such that the DC line 12 upstream of the voltage control module 30 (to the left of the module 30 in Figures 2 to 6) has a different voltage than this DC line downstream of the voltage control module 30 (to the right of the module 30 in Figures 2 to 6). This is the case when the DC power flow control system 20 is connected in series to the DC line 12 or is switch-modulated between a series mode and a bypass mode or, alternatively, two series modes with inverse polarity.For example, a so-called voltage restorer can be enabled on a long DC line 12 to raise the voltage for multiple loads or lower it for multiple connected sources. The DC power flow control system 20 offers the possibility of connecting two or more DC grids 10, even with only slightly different voltages. The same applies to connecting multiple DC grids 10 with powerful sources.

[0093] 2. In a ring or mesh of a meshed network, the DC power flow control system 20 allows current control and / or voltage control. Current control is influenced by voltage control, and vice versa.

[0094] The DC power flow control system 20 enables grid control via power flow regulators owned and accessible by the grid operators using voltage injections and virtual power impedances, instead of distributed control across multiple sources that are often inaccessible to the grid operator. While the known sources receive simple power overvoltage parameterization (droop), the inventive DC power flow control system 20 can dynamically increase or decrease the voltage in the DC line 12 of the DC grid 10 to distribute the load between sources or to regulate it.

[0095] The DC power flow control system 20 according to the invention makes it possible to implement meshed DC networks 10 without uncontrolled compensating currents or ring currents. This enables the operation of a DC network 10 with lines of different strengths (DC lines 12 with different loads), where coordination of the load of the individual lines is necessary. At the same time, the DC power flow control system 20 makes it possible to slow short-circuit currents and isolate network faults, for example, by reducing the current flowing into a damaged line element (partial line). Damage to the line element can be caused, for example, by a short circuit or the like.

[0096] Furthermore, the power flow control system 20 enables filtering of voltage or current fluctuations or preventing flow.

[0097] In the context of the invention, a voltage is understood to be a potential difference that occurs between two lines or along a line, for example, via a control module connected in series in a line, i.e., across the two terminals of the control module. A potential is measured against ground. It is therefore the voltage relative to ground. The voltage between the positive and negative rails of the control module is referred to as the module voltage.

[0098] The invention has been comprehensively described and explained with reference to the drawings and the description. The description and explanation are to be understood as exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other embodiments or variations will become apparent to those skilled in the art upon use of the present invention and upon careful analysis of the drawings, the disclosure, and the following claims.

[0099] In the claims, the words "comprising" and "having" do not exclude the presence of further elements or steps. The undefined article "a" or "an" does not exclude the presence of a plurality. A single element or unit can perform the functions of several of the units recited in the claims. An element, unit, device, and system can be partially or completely implemented in hardware and / or software. The mere mention of some measures in several different dependent claims should not be understood to mean that a combination of these measures cannot also be used advantageously. Reference signs in the claims are not to be understood as limiting.

[0100] Reference symbol

[0101] DC grid 304 Energy storage DC line 306 Half bridgea Positive DC line 308 Positive railb Negative DC line 310 Negative rail Grid voltage 312 Module voltage DC power flow 314 Potential reserve Control system Voltage control module Floating module Line filter Connections Power supply unit Earth line Earth

[0102] Filter capacity Input side Input part Output side Output part Inductance Half bridge Transistor Diode Module DC voltage intermediate circuit Connection cable Mains connection box Transmission stage 0 Switching element 2 Transistor

Claims

Patent claims 1. A DC power flow control system (20) configured for use in an electrical DC network (10) and configured to adjust the voltage and / or current in a DC line (12) of the DC network (10), comprising a voltage control module (30) that can be serially connected to the DC line (12) of the DC network (10) for adjusting the voltage in the DC line (12); and a voltage supply unit (40) for supplying the voltage control module (30) with energy.

2. DC power flow control system (20) according to claim 1, characterized in that the DC power flow control system (20) is set up to change voltages and potentials in the DC network (10) simultaneously and that the voltage supply unit (40) is controllable such that the potential of the voltage control module (30) follows the potential of the DC line (12) of the DC network (10) in a predefined manner and / or that the positive potential of the voltage control module (30) is greater than the potential of the DC line (12).

3. DC power flow control system (20) according to claim 1 or 2, characterized in that the voltage supply unit (40) is connected in parallel to a DC line (12) and that the voltage supply unit (40) is configured to effect an adjustment of the current flowing in the DC line (12).

4. DC power flow control system (20) according to the preceding claim, characterized in that the voltage supply unit (40) is configured to inject a current into the DC line (12) and / or to absorb or compensate for distortions and / or current fluctuations.

5. DC power flow control system (20) according to one of the preceding claims, characterized in that the power flow control system (20) comprises a low-voltage high-current transistor or a mains voltage low-current transistor at a location provided for a transformer or transmitter.

6. DC power flow control system (20) according to one of the preceding claims, characterized in that the DC power flow control system (20) is free of transformers or transmitters.

7. DC power flow control system (20) according to one of the preceding claims, characterized in that the voltage control module (30) and the voltage supply unit (40) are connected to the DC network (10) and are designed and connected in such a way that a power exchange between them takes place in a galvanically connected manner.

8. DC power flow control system (20) according to one of the preceding claims, characterized in that the DC power flow control system (20) is configured to raise and / or lower the voltage across the DC line (12), wherein preferably both the DC voltage and dynamic components of the voltage of the DC line (12) are variable.

9. DC power flow control system (20) according to one of the preceding claims, characterized in that the voltage control module (30) and the voltage supply unit (40) comprise power electronic components, preferably transistors (54).

10. DC power flow control system (20) according to one of the preceding claims, characterized in that the voltage supply unit (40) has a unidirectional transmission stage (64), wherein preferably a power transmission takes place from the voltage supply unit (40) to the voltage control module (30) and the voltage supply unit (40) has diodes (56).

11. DC power flow control system (20) according to one of claims 1 to 9, characterized in that the voltage supply unit (40) has a bidirectional transmission stage (64) and comprises semiconductor components, wherein the semiconductor components are preferably designed such that they can be operated with mains voltage of the DC network (10) and that they are preferably designed for a current of a maximum of 50% of the current of the DC line (12) and / or the voltage control module (30), particularly preferably for a maximum of 20%, very preferably for a maximum of 10% of the current of the DC line (12) and / or the voltage control module (30).

12. DC power flow control system (20) according to one of the preceding claims, characterized in that the voltage supply unit (40) has an earth terminal for connection to earth (42).

13. DC power flow control system (20) according to one of the preceding claims, characterized in that the DC power flow control system (20) has a module DC voltage intermediate circuit (58) which has a voltage of less than 100 volts, preferably less than 60 volts, particularly preferably less than 30 volts or a voltage of less than 10% of the mains voltage, preferably less than 5% of the mains voltage, particularly preferably less than 2% of the mains voltage.

14. DC power flow control system (20) according to one of the preceding claims, characterized in that the voltage supply unit (40) has a filter, preferably a filter capacitor (44), which is particularly preferably connected in parallel.

15. DC power flow control system according to one of claims 1 to 14, characterized in that the voltage supply unit (40) comprises a boost converter which generates an electrical potential which is higher than the potential of the DC line (12) and a buck converter which generates an electrical potential which is lower than the potential of the DC line (12).

16. DC network (10) with a DC power flow control system (20) according to one of the preceding claims.

17. DC network (10) according to claim 16, characterized in that the DC power flow control system (20) is designed to couple the DC network (10) to another DC network (10) with a different voltage and / or different potential and to adjust the voltage and / or the potential.

18. DC network according to claim 16 or 17, characterized in that the current flow and / or voltage differences can be controlled and regulated by means of a voltage control module (30) and / or a voltage supply unit (40), preferably by means of the DC power flow control system (20).

19. DC network according to claim 16 or 17, characterized in that the DC network (10) comprises at least one Kirchhoff loop in which a DC power flow control system (20) is arranged at least at one point, which is designed and configured to change the voltage between a first connection to the Kirchhoff loop and a second connection to the Kirchhoff loop or to generate a ring current component.

20. DC network according to claim 16 or 17, characterized in that the DC network (10) comprises at least one strand in which a DC power flow control system (20) is arranged at least at one point, wherein the DC power flow control system (20) is designed and configured to change the voltage between a first connection of the DC power flow control system (20) to the DC network (10) and a second connection of the DC power flow control system (20) to the DC network (10).

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