TECHNICAL FIELD

The invention relates to a system for transporting electrical power from a multi-module power plant to a plurality of electrolysers, and to a power plant including such a system. The invention further relates to methods of controlling the transport of electrical power between the multi-module power plant and the plurality of electrolysers.

BACKGROUND

With a growing market share of renewable energy generation compared to fossil fuel based power plants, there is a growing need for large capacity, efficient energy storage for dealing with a fluctuating supply and demand of electric power. Various mechanical and chemical solutions have been proposed, and are used, for temporarily storing any excess of generated energy. In times of excess demand, the stored energy is released, e.g., by converting it back to electric energy. An increasingly popular technology for storing energy is the use of electrolysis to convert electric power, e.g., from wind turbines or solar panels into hydrogen. On a smaller scale, the hydrogen produced may, e.g., be used for powering fuel cells in electric vehicles. On a larger scale, the hydrogen may, e.g., be used instead of natural gas for powering large industrial facilities, such as steel, aluminium, or ammonia factories.

A typical power network architecture for powering large industrial facilities using hydrogen includes a wind turbine power plant (WPP) that is coupled to a larger national or regional electricity grid. The WPP may be located at some distance away from the industrial facility itself, while large scale electrolysers that produce the desired hydrogen may be located closer to the factories that will consume it. Between the WPP and the large scale electrolysers, multiple transformation stages are used for converting the frequency and voltage of the transmitted alternating current (AC) to reduce transmission losses and to eventually deliver the electric power in a suitable form for use by these electrolysers. The efficient transmission and conversion of the electric power generated at the WPP requires large, complex, and expensive equipment

It is against this background to which the present invention is set. SUMMARY OF THE INVENTION

According to an aspect of the invention there is provided a system for transporting electrical power from a multi-module power plant to a plurality of electrolysers. The system comprises a plurality of serially connected input converters, a medium or high voltage direct current (DC) transmission line, and a plurality of serially connected front-end converters. The input converters comprise an input converter input, connectable to a respective electrical power generating module of the multi-module power plant, and a direct current output. The medium or high voltage direct current transmission line is connected to the respective direct current outputs of a first one and a last one of the input converters. Each serially connected front-end converter comprises a front-end converter output, connectable to a respective electrolyser of the plurality of electrolysers. A first one and a last one of the front-end converters are connected to the direct current transmission line.

The series connection of the electrical power generating modules and the front end converters eliminates the need for the various converters and transformers that are otherwise needed to collect, transmit, and employ the generated electrical power. In known power transport networks for bringing electrical power to electrolysers, multiple conversions are needed between AC and DC, medium (e.g., 33 kV, 50 kV) and high (e.g., 300 kV) voltage, or different AC frequencies (e.g., 50 Hz, 60 Hz, 500 Hz, 1000 Hz). By eliminating these conversion and transformation stages, a significant cost reduction can be achieved, while providing a low maintenance network with reduce risk of technical failure. In addition thereto, the use of direct current further leads to an increased energy efficiency by a reduction of transfer losses.

It is noted that the now proposed series connection of, e.g., wind turbines or photovoltaic (PV) systems is typically advised against due to the challenges it brings with respect to controlling the power plant and the significant power drops that may occur when a single electrical power generating module fails. The inventors have, however, realised that in the specific context of powering a plurality of industrial scale electrolysers, these problems can be largely overcome by appropriately controlling the front-end converters to compensate for unexpected power fluctuations on the power plant end of the network.

In many embodiments of the power transport network according to the invention, at least one of the input converters comprises an AC/DC converter. For example, wind turbine generators typically generate electrical power in the form of an alternating current. The AC/DC converter is then used to convert the alternating current coming from one of the wind turbine generators to a medium or high voltage DC current that can be transported over the transmission line.

In some embodiments, the least one of the input converters comprises a DC/DC converter. For example, PV systems convert sunlight into direct current electricity. Normally, a PV system comprises a PV inverter for coupling a PV solar panel to the 50 Hz or 60 Hz public power grid. In power transport networks according to the current invention, this conversion to alternating current may not be necessary and a DC/DC connector can be used to just convert the lower voltage PV system power output to the medium or high voltage DC current that can be transported over the transmission line.

No standardised well-defined voltage boundaries for medium voltage direct current (MVDC) and high voltage direct current (HVDC) are defined. For the purpose of this disclosure, we assume MVDC to range from roughly 2 kV (±1 kV) to about 200 kV (±100 kV). HVDC thus covers anything above about 200 kV (±100 kV). In preferred embodiments, the MVDC or HVDC transmission line is configured to transmit power at a voltage of at least ±50kV to ensure efficient power transmission. In further embodiments, the transmission line may be a HVDC transmission line configured to transmit power at a voltage of at least ±300kV.

According to an aspect of the invention there is provided a power plant comprising a plurality of electrical power generating modules, a system for transporting electrical power as described above, and a plurality of electrolysers, wherein the electrical power generating modules are connected to the input converter input of respective input converters and the electrolysers to the front-end converter output of respective front-end converters of the system for transporting electrical power. The electrical power generating modules may, e.g., comprise wind turbines, photovoltaic systems, or other systems for generating electricity, preferably from renewable energy sources.

In preferred embodiments, the power plant is off-grid and the electrical power generating modules are exclusively used for powering the electrolysers. The electrolysers may, e.g., be used for providing hydrogen to large industrial factories for producing steel, aluminium, or ammonia. By not being connected to a larger public power grid, According to a further aspect of the invention there is provided a method for controlling a power plant or a system for transporting electrical power as described above. The method comprises steps of monitoring a power and/or voltage output of the multi-module power plant, observing a change in the power and/or voltage output of the multi-module power plant, and controlling at least one of the front-end converters to connect or disconnect at least one electrolyser of the plurality of electrolysers in dependence of the observed change in the power and/or voltage output of the multi-module power plant. With this control method, it is possible to balance the supply of and demand for electrical power, thereby ensuring that the voltage on the transmission line is kept stable. When a sudden drop in available electrical power is observed, for example because of a change in wind speed, clouds moving between the sun and a photovoltaic system, or a technical failure of one of the electrical power generating modules, one or more of the electrolysers can be temporarily disconnected to compensate for the reduced power generation. Conversely, a sudden increase in energy production can be reacted to by reconnecting one or more of the previously disconnected electrolysers.

A similar control method comprises the steps of detecting a malfunction in an electrical power generating module of the multi-module power plant, disconnecting the at least one electrical power generating module wherein the malfunction is detected, selecting one or more electrolysers of the plurality of electrolysers, such that a total power consumption of the selected electrolysers is of a similar magnitude as a rated power output of the electrical power generating module wherein the malfunction is detected, and controlling the respective front-end converters to disconnect the selected electrolysers. For example, when detecting the malfunction of a 10 MW wind turbine, this wind turbine can be disconnected from the network for further inspection and repair. When simultaneously disconnecting a 10 MW electrolyser or two 5 MW electrolysers, power supply and demand are kept in-balance and the voltage on the transmission line is kept stable.

Preferably, the one or more front-end converters are selected for disconnection in such a way that the total power consumption of the respective electrolysers connected to the selected front-end converters is at least 50%, preferably at least 70%, even more preferably at least 90%, of the rated power output of the electrical power generating module wherein the malfunction is detected. Especially when the power plant is a self- contained, off-grid power network, it is important for the amount of generated power to be of similar magnitude as the total amount of consumed power. BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 schematically illustrates a wind power plant using a power transport system according to an embodiment of the invention.

Figure 2 shows some additional detail of part of the wind power plant of Figure 1 .

Figure 3 shows a flow chart of a control method according to an embodiment of the invention.

Figure 4 shows a flow chart of a further control method according to an embodiment of the invention.

DETAILED DESCRIPTION

Figures 1 and 2 schematically illustrate a wind power plant 110 using a power transport system 100 according to an embodiment of the invention. The wind power plant 110 setup shown here is an example of a Power-to-X (also P2X) plant, such as is nowadays used to convert electric power to various other forms of energy storage. Power-to-X is typically used for temporary energy storage during periods where fluctuating renewable energy generation exceeds load. Power-to-X conversion technologies allow for the decoupling of power from the electricity sector for use in other sectors (such as transport or chemicals). The X in Power-to-X can, for example, refer to ammonia, gas, or methane. The power transport systems 100 described herein are primarily adapted for use with large scale industrial electrolysers 120 for the production of hydrogen. It is, however, to be noted that the front-end converters 150 of this power transport system 100 may alternatively be configured to be connected to equipment used to physically store electric energy in other forms than hydrogen.

The wind power plant 110 of Figures 1 and 2 comprises a plurality of wind turbines 112 which are each connected to an AC input terminal 132 of a respective input converter 130. A wind turbine 112 typically delivers electric power in the form of an alternating current. The input converters 130 are configured to convert the incoming alternating current into a direct current at a prescribed voltage or inside a prescribed voltage range. For this purpose, the input converter 130 comprises at least one AC/DC converter 136. In this exemplary embodiment, the initial AC/DC conversion is directly followed by a DC/DC conversion that adapts the voltage level of the produced direct current. The DC/DC converter 138 may, for example, comprise a DC/AC converter for converting the direct current to an alternating current of a desired frequency (e.g. 1000 Hz), a transformer for bringing the alternating current to the desired voltage level, and a DC/AC converter to obtain the desired direct current output voltage of the input converter 130. Preferably, the DC/DC converters are galvanically separated from the transmission line 140, e.g., through medium frequency power transformers.

The DC output terminals 134 of all input converters 130 are connected in series. The resulting voltage between the DC output terminals 134 of a first one and a last one of the serially connected input converters 130 equals the sum of the output voltages of all connected individual input converters 130. If one of the wind turbines 112 is out of service for maintenance, repair or other reasons, it is functionally disconnected from its respective input converter 130, such that the input converter 130 only serves to provide a direct link between the output terminals 134 of the preceding and following input converters 130.

The wind power plant 110 is designed such that the DC output voltage of the wind power plant as a whole is either in a medium voltage direct current (MVDC) or a high voltage direct current (HVDC) range. No standardised well-defined voltage boundaries for MVDC and HVDC are defined. Therefore, for the purpose of this disclosure, we assume MVDC to range from roughly 2 kV (±1 kV) to about 200 kV (±100 kV), such that HVDC covers anything above about 200 kV (±100 kV). In preferred embodiments, the wind power plant 110 is configured to provide an output voltage of at least ±50kV to ensure efficient power transmission towards the electrolysers 120 that may be located at a considerable distance away from the wind power plant 110. In further embodiments, the wind power plant output voltage may be at least ±300kV.

The direct current outputs 134 of the first and last ones of the serially connected input converters 130 of the wind power plant 110 are each connected to one line of a medium or high voltage direct current transmission line 140. At its other end, the two lines of the medium or high voltage direct current transmission line 140 are connected to a first one and a last one of a plurality of serially connected front-end converters 150. In practice, as shown in the drawings, both lines of the medium or high voltage direct current transmission line 140 will likely be bundled in a single cable 140. Alternatively, each line may run its own separate way towards the serially connected front-end converters 150.

The front-end converters 150 (also shown in Figure 2) each have their front-end converter output 154 connected to a respective electrolyser 120. Electrolysers 120 use a direct current to split water into hydrogen and oxygen. The front-end converters 150, provide a DC/DC conversion of the input current from the direct current transmission line 140 to the input current for the electrolyser 120. In this exemplary embodiment, the DC/DC conversion is very similar to the DC/DC conversion described earlier for the input converters 130 of the wind power plant 110. Each front-end converter 150 comprises a DC/AC converter for converting the incoming direct current to an alternating current of a desired frequency (e.g. 1000 Hz), a transformer for bringing the alternating current to the desired voltage level, and a DC/AC converter to obtain the desired direct input voltage for the electrolyser 120. Preferably, the DC/DC converters are galvanically separated from the transmission line 140, e.g., through medium frequency power transformers. Each frontend converter 150 may, for example, be located in a separate 20 or 40 feet container arrangement, or the front-end converters 150 may all be located together inside a building. The control of the front-end converters 150 is preferably coordinated with the control of the input converters 130 that are coupled to the wind turbines 112. Exemplary control methods are described in more detail below with reference to the flow charts of Figures 3 and 4.

Compared to prior art Power-to-Hydrogen power plants, wherein both the wind turbines 112 and the electrolysers 120 are connected in parallel, the series connection of the electrical power generating modules 112 and the electrolysers 120 leads to a simplified system, requiring less capital intensive electrical equipment. No switch gears are involved at either the power collection side or at the power delivery side of this Power-to-Hydrogen power plant. The now provided series connection, e.g., eliminates the need for the various converters and transformers that are otherwise needed to collect, transmit, and employ the generated electrical power. In known power transport networks for bringing electrical power to electrolysers, multiple conversions are needed between AC and DC, medium (e.g., 33 kV, 50 kV) and high (e.g., 300 kV) voltage, or different AC frequencies (e.g., 50 Hz, 60 Hz, 500 Hz, 1000 Hz). By eliminating these conversion and transformation stages, a significant cost reduction can be achieved, while providing a low maintenance network with reduce risk of technical failure. In addition to the reduced need for copper and iron on magnetics and transformers, the power transfer system now presented, also reduces the geographical footprint of the power plant, which makes the now proposed solution especially beneficial for application on energy islands or substations of limited size. In addition thereto, the use of direct current further leads to an increased energy efficiency by a reduction of transfer losses.

Each electrolysers 120 is connected to a front-end converter output 154 of a respective front-end converter. If one of the electrolysers 120 is out of service for maintenance, repair or other reasons, it is functionally disconnected from the transmission line 140, such that the front-end converter 150 only serves to provide a direct link between the preceding and following front-end converters 150. The hydrogen produced by the electrolysers 120 may be collected in a central hydrogen pipeline 160 through which the hydrogen can be supplied to, e.g., a steel, aluminium, or ammonia factory 170, or distributed to transport vehicles that bring the hydrogen to other possible users.

In alternative embodiments, some or all of the electrical power generating modules 112 may be photovoltaic (PV) systems that convert solar energy to electricity. PV systems generate electricity in the form of DC currents. Consequently, the input converters 130 coupling the PV modules to other electrical power generating modules 112 and the transmission line 140 will generally comprise a DC/DC converter for bringing the produced electrical current to the desired voltage level.

Figure 3 shows a flow chart of a control method 30 according to an embodiment of the invention. This control method 30 comprises a first step 301 of monitoring a power and/or voltage output of the multi-module power plant 110, a second step 302 of observing a change in the power and/or voltage output of the multi-module power plant 110, and a third step of controlling at least one of the front-end converters 150 to connect or disconnect at least one electrolyser 120 of the plurality of electrolysers 120 in dependence of the observed change in the power and/or voltage output of the multi-module power plant 110. With this control method 30, it is possible to balance the supply of and demand for electrical power, thereby ensuring that the voltage on the transmission line 140 is kept stable. When a sudden drop in available electrical power is observed, for example because of a change in wind speed, clouds moving between the sun and a photovoltaic system, or a technical failure of one of the electrical power generating modules 112, one or more of the electrolysers 120 can be temporarily disconnected to compensate for the reduced power generation. Conversely, a sudden increase in energy production can be reacted to by reconnecting one or more of the previously disconnected electrolysers 120. Figure 4 shows a flow chart of a further control method 40 according to an embodiment of the invention. This further control method 40 may be implemented alongside the control method described above with reference to Figure 3 and aims to further adapt the power supply by the wind turbines 112 or other electrical power generating modules to the power demand of the operating electrolysers 120. For that purpose, this further control method 40 comprises a first step 401 of detecting a malfunction in an electrical power generating module 112 of the multi-module power plant 110, a second step 402 of disconnecting the at least one electrical power generating module 112 wherein the malfunction is detected, a third step 403 of selecting one or more electrolysers 120 of the plurality of electrolysers 120, such that a total power consumption of the selected electrolysers 120 is of a similar magnitude as a rated power output of the electrical power generating module 112 wherein the malfunction is detected, and controlling the respective front-end converters 150 to disconnect the selected electrolysers 120. For example, when detecting the malfunction of a 10 MW wind turbine 112, this wind turbine 112 can be disconnected from the network for further inspection and repair. When simultaneously disconnecting a 10 MW electrolyser 120 or two 5 MW electrolysers 120, power supply and demand are kept in-balance and the voltage on the transmission line is kept stable.