Method and system for connecting high-voltage alternating current generators to a high-voltage direct current transmission

Abstract

The present invention relates to a method for connecting multi-phase high-voltage (HV) alternating current generators to a high-voltage direct current (HVDC) transmission line with a HVDC nominal voltage, in which the connection is carried out by means of a plurality of serially connected feed-in units (120), in which each feed-in unit comprises a central control, a respective multi-phase HV alternating current generator (128) and, per phase, a respective modular energy storage device (121, 122, 123), which has two branches (124, 125) each with a number of submodules (109), in which each submodule comprises at least one accumulator (115), an intermediate circuit capacitor (116) and, on the input side and output side, two half-bridges (111, 112, 113, 114), in which both branches are connected at a first end via a each intermediate inductance (127) is connected to form an energy storage center tap,In which the energy storage center tap is connected to a respective phase of the HV AC generator, and in which the plurality of feed-in units is formed according to the HVDC nominal voltage divided by a partial voltage provided by the individual feed-in units, assuming a final discharge voltage of the accumulators. Furthermore, an energy transmission system on which the method is carried out is presented.

Description

The present invention relates to a method for connecting high-voltage AC generators to a line for high-voltage DC transmission by means of modular batteries. Furthermore, an energy transmission system is presented on which the method is carried out.Wind power plants provide alternators in the high-voltage range of, for example. 32 kV, which, for example, groups far in front of an coast to form so-called offshore wind farms, provide electrical powers in the up to gigawatt range. Due to their weather dependence, however, they form considerable challenges in connection with a power supply network and jeopardize its reliability. To ensure balanced electrical power over a period of time, even a portion of the wind turbines are switched over to others to counteract (wind) surge loads by braking the generators, thereby destroying usable energy and overall lowering efficiency and costability.At long distances and in cables with a high capacity covering, such as for example. Sea cable is a direct current transmission more favorable than an alternating current transmission. Therefore, a high-voltage DC transmission, abbreviated as HVDC, is used for energy transmission from wind farms far in front of the coast. For this purpose, the electrical power provided in the wind farms is transmitted to an offshore transformer substation-generally using transformers and rectifiers-for example. 320 kV is fed into an HGU line at the Festland.The document DE 10 2014 226 252 A1 discloses a system for connecting wind generators to a transmission network. Each generator has an AC / DC converter for converting the voltage. The generators are applied to a common intermediate circuit which contains a storage element. The intermediate circuit is in turn connected to a DC / DC converter.The publication DE 10 2020 000 174 A1 describes a control unit for an energy feed unit for feeding electrical energy. The energy feed unit is connected to the supply network and the energy generator. The control unit comprises a communication network which compares data and thus controls the feeding. The energy feed unit comprises a storage element for temporarily storing the energy.The document WO 2011 / 154306 A2 discloses an electrical energy storage unit having an energy store, a bidirectional DC converter with a low-voltage side and a high-voltage side. The energy is generated by wind turbines or photovoltaic installations and stored in the energy storage unit. From the energy storage unit, the energy is fed into a supply network by means of DC / AC converters.Against this background, it is an object of the present invention to present a method for connecting a plurality of alternators to a high-voltage DC transmission, in which a control power for feeding into the HVDC is ensured. The connection to the transmission network should be cost-effective. Furthermore, a system on which the method is carried out is to be presented.To achieve the object mentioned above, a method for connecting polyphase high-voltage (HV) alternators to a high-voltage direct current transmission (HVDC) line with a rated HVDC voltage is proposed, in which the connection is carried out by means of a plurality of feed units connected in series to one another. A respective feed unit comprises a central controller, a respective polyphase HV AC generator and a respective modular energy store per phase. The modular energy store has two branches, each with a number of submodules. A respective submodule comprises at least one accumulator, an intermediate circuit capacitor and, on the input side and on the output side, in each case two half bridges having a respective submodule center tap for a respective submodule connection. Both branches are connected to one another at a first end via a respective interposed inductance to form an energy storage center tap, wherein the center tap is connected to a respective phase of the HV AC generator. The central controller of the respective feed unit is configured to charge the plurality of submodules by the HV AC generator by means of suitable control of the half bridges. A DC voltage plus pole connection of the respective feed unit is formed by parallel connection of all branches of the respectively included modular energy stores to DC voltage plus potential. A DC voltage minimum pole connection of the respective feed unit is formed by parallel connection of all branches of the respectively included modular energy stores to DC voltage minimum potential. The plurality of feed units is formed in accordance with the rated HVDC voltage divided by a partial voltage provided by the individual feed units on the assumption of a discharge end voltage of the accumulators. Finally, the DC pole connection and DC pole connection located on the outside in the series of the series-connected feed units are connected to the HGU line.Due to their serial connection, the feed units are part of the entire HVDC network through the connection to the high-voltage direct current transmission (HVDC) line. A capacity of storable energy can be increased by additional parallel connection of energy stores. It is also conceivable to provide a higher number of accumulators and to interconnect them in the respective modular energy stores in order to ensure the partial voltage required for providing the rated HVDC voltage even in the case of a complete failure of an individual accumulator-a second life ("second life") of an accumulator previously used in another function and thus limited in its lifetime.In one embodiment of the method according to the invention, the accumulator is formed by a high-voltage battery or by a modular multilevel converter with a rated voltage in the high-voltage range. Such accumulators are used, for example, in electric vehicles for supplying energy to a traction system and have a rated voltage of 400 V or 800 V. By continuously alternating interconnection of the submodules with one another, it is thus possible to convert an AC voltage from the AC voltage generator to a charging current for the accumulators. This is likewise possible in the modular multilevel converter, which can already intrinsically generate a DC voltage from an AC phase for charging its energy cells. Such a modular multilevel converter is described, for example, in "Goetz, S. M.; Peterchev, A. V.; Weyh, T., "Modular Multilevel Converter With Series and Parallel Module Connectivity: Topology and Control," Power Electronics, IEEE Transactions on, vol. 30, no. 1, pp. 203-215, 2015. doi: 10.1109 / TPEL.2014.2310225.The at least one accumulator in the individual submodule is connected via half bridges of semiconductor switches, for example. MOSFETs connected to respective terminals of the submodule. Depending on the configuration, the submodule can assume different switching states, such as "serial", "parallel" or "bypass", which means a statement about the interconnection of the respective accumulator in the module string. A respective switching state then corresponds to a configuration of switching positions of the semiconductor switches in the half bridges. For each switching cycle, generally a multiple of a grid frequency, a switching command with a switching state to be assumed in each case is carried out for each battery module by the central controller,In a further embodiment of the method according to the invention, a respective output capacitor is arranged between the DC voltage connections of the respective feed unit. Since the feed units are connected to one another in series, this corresponds to a capacitive voltage divider. Thus, a rated HVDC voltage, for example, can be generated. 320 kV, to a partial voltage at the level of a rated voltage of, for example. 32 kV of a single HV AC generator.The method according to the invention feeds an electrical power provided by the HV AC generators into a power grid connected to the HVDC line, for example a power grid, in a targeted manner, and in this case uses the plurality of modular energy stores as buffers, contributing to reactive power compensation and grid stabilization. On the basis of the above functionality of the modular energy stores, their accumulators or DC voltage sources (via the half bridges connected by the central controller) can be charged with the AC current provided by the HV AC generators and thus advantageously save the use of large and heavy transformers and additional rectification.It is conceivable to form the respective HV AC generator by a respective wind turbine. Other HV AC generators for utilizing a water flow during tide stroke or in pumped storage power plants / ram power plants are also conceivable. Feeding via the modular energy stores advantageously saves a complex balancing of wind power installations by frequency stability.It is conceivable to form the semiconductor switches of the half bridges by MOSFETs. A semiconductor material with a wide band gap is selected as the semiconductor material, for example. Silicon carbide, whereby voltages are switchable and / or switched up to a 1000 V range.Furthermore, a system is claimed which is designed to connect polyphase high-voltage (HV) alternators to a high-voltage direct current transmission (HVDC) line having an HVDC rated voltage by means of a plurality of feed units connected in series with one another. A respective feed unit comprises a central controller, a respective polyphase HV AC generator and a respective modular energy store per phase. The modular energy store has two branches each having a number of submodules, wherein a respective submodule comprises at least one rechargeable battery, an intermediate circuit capacitor and, on the input side and on the output side, in each case two half bridges having a respective submodule center tap for a respective submodule connection. Both branches are connected to one another at a first end via a respective interposed inductance to form an energy storage center tap, wherein the energy storage center tap is connected to a respective phase of the HV AC generator. The central controller of the respective feed unit is configured to charge the plurality of submodules by the HV AC generator by means of suitable control of the half bridges. A DC voltage plus pole connection of the respective feed unit is formed by parallel connection of all branches of the respectively included modular energy stores to DC voltage plus potential. A DC voltage minimum pole connection of the respective feed unit is formed by parallel connection of all branches of the respectively included modular energy stores to DC voltage minimum potential. The plurality of feed units is formed in accordance with the rated HVDC voltage divided by a partial voltage provided by the individual feed units on the assumption of a discharge end voltage of the accumulators.In one configuration of the system according to the invention, the accumulator is formed by a high-voltage battery having a rated voltage in the high-voltage range.In another embodiment of the system according to the invention, the accumulator is formed by a modular multilevel converter with a rated voltage in the high-voltage range.In a further embodiment of the system according to the invention, a respective output capacitor is arranged between the DC voltage connections of the respective feed unit.Furthermore, a use of an accumulator in a system according to the invention is claimed, wherein a service life state of the accumulator is limited on the basis of a use with the following history: traction battery in vehicles, home storage, photovoltaic storage.In a further embodiment of the use according to the invention of the accumulator, a part of the half bridges in the respective submodule is replaced by an inverter connected to the accumulator.Furthermore, an offshore wind farm is claimed which comprises a system according to the invention and is designed to carry out a method according to the invention when accumulators are used according to the invention.Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawing.It is understood that the features mentioned above and those still to be explained below can be used not only in the respectively specified combination, but also in other combinations or alone, without departing from the scope of the present invention.The figures are described in contiguous and overlapping terms; identical components are assigned the same reference numerals. FIG. 1 shows a schematic structure of a feed unit in an embodiment of the system according to the invention. FIG. 2 shows a schematic structure of a wind turbine in the embodiment of the system according to the invention.FIG. 1 shows a schematic structure of a feed unit 120 in one embodiment of the system according to the invention. The feed unit 120 comprises a central controller, a respective polyphase HV AC generator 128 of a wind power unit 129, and a respective modular energy store 121, 122, 123 per phase of the AC generator 128. An output capacitor 126 is arranged between a DC pole terminal 107 and a DC pole terminal 108. The respective modular energy store 121, 122, 123 has an upper branch 124 and a lower branch 125 each having a number of submodules 109. The two branches are connected to one another at a first end via a respective interposed inductance 127 to an energy storage center tap which is connected to a respective phase of the HV AC generator 128. Illustration 110 shows a circuit diagram for the respective submodule 109. Two input-side half bridges 111, 112 and two output-side half bridges 113, 114 having a respective submodule center tap for a respective submodule terminal 101, 102, 103, 104 are connected in parallel to an accumulator 115 and an intermediate circuit capacitor 116. A double arrow 119 indicates the identity between the representations of submodule 109 shown.FIG. 2 shows a schematic structure 210, 220 of a wind turbine in the embodiment of the system according to the invention. In illustration 210, a plurality of feed units 211, 212, 219 are connected in series, resulting in a positive terminal 217 and a negative terminal 218 for connection to a high-voltage direct current transmission (HVDC) line. The respective output capacitor 126 can be regarded as a capacitive voltage divider in a manner analogous to known offshore installations, as shown in illustration 220 of an offshore wind farm according to the invention. A plurality of offshore feed units 221, 222, 229 are connected via their respective DC voltage plus pole connection 227 and DC voltage minus pole connection 228 to respective voltage divider capacitors 226 which are located in a transformer station 225. From there, a 320 kV high voltage DC transmission line 224 runs over a large distance 223 from the land.List of reference characters101 Submodule connection a 102 Submodule connection b 103 Submodule connection c 104 Submodule connection d 109 Submodule 107 Dc voltage plus pole connection 108 Dc voltage minus pole connection 110 Illustration Submodule 111 First half bridge 112 Second half bridge 113 Third half bridge 114 Fourth half bridge 115 Accumulator 116 Intermediate circuit capacitor 119 Identity 120 Feed unit 121 Modular energy store first phase 122 Modular energy store second phase 123 Modular energy store third phase 124 Upper branch 125 Lower branch 126 Output capacitor 127 Inductance 128 Alternator 129 Wind power unit 210 Illustration Wind turbine 211 First feed unit 212 Second feed unit 217 Positive pole connection 218 Negative pole connection 219 Nth feed unit 220 Illustration Offshore Wind farm 221 First Offshore feed unit 222 Second Offshore feed unit 223 Excess Distance from the land 224 320 kV high-voltage direct-current transmission line 225 transformer offshore 226 voltage divider capacitor 227 direct-voltage pole connection 228 direct-voltage minimum pole connection 229 Nth offshore feed unitReferences included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedDE 10 2014 226 252 A1

[0004] DE 10 2020 000 174 A1

[0005] WO 2011 / 154306 A2

[0006] Cited Non-Patent LiteratureGoetz, S. M.; Peterchev, A. V.; Weyh, T., "Modular Multilevel Converter With Series and Parallel Module Connectivity: Topology and Control," Power Electronics, IEEE Transactions on, vol. 30, no. 1, pp. 203-215, 2015. doi: 10.1109 / TPEL.2014.2310225

[0010]

Claims

Method for connecting polyphase high-voltage (HV) alternators to a high-voltage direct current transmission (HVDC) line with a rated HVDC voltage, in which the connection is carried out by means of a plurality of feed units (120, 211, 212, 219, 221, 222, 229) connected in series to one another, in which a respective feed unit (120, 211, 212, 219, 221, 222, 229) comprises a central controller, a respective polyphase HV AC alternator (128) and a respective modular energy store (121, 122, 123) per phase, in which the modular energy store (121, 122, 123) has two branches (124, 125) each having a number of submodules (109), in which a respective submodule (109) comprises at least one accumulator (115), an intermediate circuit capacitor (116) and, on the input side and on the output side, in each case two half bridges (111, 112, 113, 114) having a respective submodule center tap for a respective submodule terminal (101, 102, 103, 104), in which the two branches (124, 125) are connected to one another at a first end via a respective interposed inductance (127) to form an energy store center tap, in which the energy store center tap is connected to a respective phase of the HV AC generator (128), in which the central controller of the respective feed unit (120, 211, 212, 219, 221, 222, 229) is configured to charge the plurality of submodules (109) by the HV AC generator (128) by means of suitable control of the half bridges (111, 112, 113, 114), in which a DC pole terminal (107) of the respective feed unit (120, 211, 212, 219, 221, 222, 229) is formed by parallel connection of all branches (124, 125) of the respectively included modular energy stores (121, 122, 123) with DC voltage plus potential, in which a DC voltage minus pole connection (108) of the respective feed unit (120, 211, 212, 219, 221, 222, 229) is formed by parallel connection of all branches (124, 125) of the respectively included modular energy stores (121, 122, 123) with DC voltage minus potential, and in which the plurality of feed units (120, 211, 212, 219, 221, 222, 229) is formed in accordance with the HGU rated voltage divided by a partial voltage provided by the individual feed units (120, 211, 212, 219, 221, 222, 229) assuming a discharge end voltage of the accumulators (115).Method according to Claim 1, in which the accumulator (115) is formed by a high-voltage battery or by a modular multilevel converter with a rated voltage in the high-voltage range.Method according to one of the preceding claims, in which a respective output capacitor (126) is arranged between the DC voltage connections (107, 108) of the respective feed unit (120, 211, 212, 219, 221, 222, 229).A system which is configured to connect polyphase high-voltage (HV) alternators (128) to a high-voltage direct current transmission (HVDC) line (224) having a rated HVDC voltage by means of a plurality of feed units (120, 211, 212, 219, 221, 222, 229) connected in series to one another, wherein a respective feed unit (120, 211, 212, 219, 221, 222, 229) comprises a central controller, a respective polyphase HV AC alternator (128) and a respective modular energy store (121, 122, 123) per phase, wherein the modular energy store (121, 122, 123) has two branches (124, 125) each having a number of submodules (109), wherein a respective submodule (109) comprises at least one accumulator (115), an intermediate circuit capacitor (116) and, on the input side and on the output side, in each case two half bridges (111, 112, 113, 114) having a respective submodule center tap for a respective submodule connection (101, 102, 103, 104), wherein the two branches (124, 125) are connected to one another at a first end via a respective interposed inductance (127) to form an energy store center tap, wherein the energy store center tap is connected to a respective phase of the HV AC generator (128), wherein the central controller of the respective feed unit (120, 211, 212, 219, 221, 222, 229) is configured to charge the plurality of submodules (109) by the HV AC generator (128) by means of suitable control of the half bridges (111, 112, 113, 114), wherein a DC pole connection (107) of the respective feed unit (120, 211, 212, 219, 221, 222, 229) is formed by parallel connection of all branches (124, 125) of the respectively included modular energy stores (121, 122, 123) having a DC voltage plus potential, wherein a DC voltage minus pole connection (108) of the respective feed unit (120, 211, 212, 219, 221, 222, 229) is formed by parallel connection of all branches (124, 125) of the respectively included modular energy stores (121, 122, 123) having a DC voltage minus potential, and wherein the plurality of feed units (120, 211, 212, 219, 221, 222, 229) is formed in accordance with the HVDC rated voltage divided by a partial voltage provided by the individual feed units (120, 211, 212, 219, 221, 222, 229) assuming a discharge end voltage of the accumulators (115).The system according to claim 4, wherein the accumulator (115) is formed by a high-voltage battery having a nominal voltage in the high-voltage range.The system of claim 4, wherein the accumulator (115) is formed by a modular multilevel converter having a nominal voltage in the high voltage range.The system according to any one of claims 4 to 6, wherein a respective output capacitor (126) is arranged between the DC voltage terminals (107, 108) of the respective feed unit (120, 211, 212, 219, 221, 222, 229).Use of an accumulator in a system according to one of Claims 4 to 7, wherein a service life state of the accumulator (115) is restricted on the basis of use with the following history: traction battery in vehicles, domestic storage, photovoltaic storage.Use of an accumulator according to Claim 8, wherein a part of the half bridges (111, 112, 113, 114) in the respective submodule (109) is replaced by an inverter connected to the accumulator (115).Offshore wind farm comprising a system according to one of claims 4 to 7 and configured to carry out a method according to one of claims 1 to 3 and thereby use accumulators (115) according to one of claims 8 or 9.

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