Converter stations for integration of renewable energy

Abstract

A converter station (100) is provided. The converter station (100) comprises a modular multilevel converter (110) having an AC side and a DC side and controller (120). The modular multilevel converter MMC comprises a plurality of converter branches (111~116). A first converter branch (111) from the plurality of converter branches is coupled between an AC line at the AC side and a positive pole at the DC side, and a second converter branch (112) from the plurality of converter branches is coupled between the AC line and a negative pole at the DC side. Each of the first and second converter branches comprises a plurality of converter cells (11~10) and an inductor (71). One or more converter cells from the plurality of converter cells are configured to be coupled with renewable energy sources. The controller (120) is configured to control the one or more converter cells such that power from the renewable energy sources, which are coupled with the one or more converter cells, is at least partially transferred to at least one of the AC side and the DC side.

Description

CONVERTER STATIONS FOR INTEGRATION OF RENEWABLE ENERGYTECHNICAL FILED

[0001] The present disclosure relates to a converter station integrated with renewable energy sources and a method for operating the converter station.BACKGROUND

[0002] A large number of initiatives have been undertaken to reduce global CO2 emissions, with the aim of tackling long-term climate change. Renewable generation, which utilizes wind or solar energy, is growing rapidly as conventional generation sources are phased out. However, wind and solar energy have a high dependency on weather conditions, posing challenges to the stable operation of power systems. For example, during some periods, power generation may be in surplus, while there may also be zero generation during nights or in no-wind weather conditions. Given that energy demand and supply must always be balanced, this creates numerous challenges for power systems with high penetrations of renewable generation.

[0003] As the most common element in nature, hydrogen is considered an important energy source that contributes to the decarbonization of heavy industry, transportation, and other industrial sectors. However, the conventional process for producing hydrogen also contributes to pollution. To reduce CO2 emissions, an increasing amount of energy is generated by renewable energy sources. Green hydrogen refers specifically to hydrogen produced using electricity sourced from these renewable energy sources.

[0004] Against this background, two distinct converter systems are proposed. In the first converter system, an additional dedicated converter station is designed specifically for hydrogen production, whereas a second converter station is utilized for connection to an energy storage system, which may comprise supercapacitors, batteries, or a hybrid of both. The primary disadvantage of the first converter system lies in the necessity of employing two separate converter stations, potentially necessitating coordinated control between their respective control systems. In contrast, the second converter system features a unifieddesign, where electrolyzers and energy storage systems are interconnected to the same DC bus, either through individual DC / DC converters or directly. However, the integration of diverse energy storage systems and electrolyzers onto a common DC bus introduces complexities in terms of insulation and protection design, representing a significant challenge.SUMMARY

[0005] According to an embodiment of the present disclosure, a converter station is provided. The converter station comprises a modular multilevel converter (MMC) and a controller. The MMC has an AC side and a DC side. The MMC comprises a plurality of converter branches. A first converter branch from the plurality of converter branches is coupled between an AC line at the AC side and a positive pole at the DC side, and a second converter branch from the plurality of converter branches is coupled between the AC line and a negative pole at the DC side. Each of the first and second converter branches comprises a plurality of converter cells and an inductor. One or more converter cells from the plurality of converter cells are configured to be coupled with renewable energy sources. The controller is configured to control the one or more converter cells such that power from the renewable energy sources, which are coupled with the one or more converter cells, is at least partially transferred to at least one of the AC side and the DC side.

[0006] According to another embodiment of the present disclosure, a method for operating a converter station is provided. The method comprises: proving an MMC with a plurality of converter cells comprised in a plurality of converter branches, the MMC having an AC side and a DC side; coupling each of one or more converter cells from the plurality of converter cells with at least one renewable energy source; and controlling the one or more converter cells such that power from the at least one renewable energy source is at least partially transferred to at least one of the AC side and the DC side.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The disclosed aspects will hereinafter be described in connection with the appended drawings, which are provided to illustrate but not to limit the scope of the presentdisclosure.

[0008] Figure 1 illustrates a converter station according to an embodiment of the present disclosure.

[0009] Figure 2 illustrates an exemplary configuration for a converter cell of an MMC.

[0010] Figures 3A and 3B illustrate exemplary configurations for a BESS coupled in a converter cell.

[0011] Figures 4A to 4C illustrate examples of a DC system coupled with a DC side of the MMC.

[0012] Figure 5 illustrates an example of the converter station shown in Figure 1.

[0013] Figure 6 is a flowchart of a method for operating a converter station according to an embodiment of the present disclosure.DETAILED DESCRIPTION

[0014] The present disclosure proposes a new converter station, which is a combined or unified converter station for hydrogen production, transmission of renewable energies, as well as for grid support. The proposed converter station can be arranged in a substation or on an artificial island on the sea. The proposed converter station can be used for integrating either solar or wind energies or both into a power system, combined with hydrogen production.

[0015] In the proposed converter station, energy storage elements, photovoltaic (PV) cells, and wind turbine generators (WTG) are integrated into converter cells of a modular multilevel converter (MMC) via DC / DC and DC / AC converters. The DC side of the MMC can be connected with a load. A switch for disconnecting the load can be provided in series with the load, which can be either a mechanical switch or a power electronics-based switch. The load can be an electrolyzer for producing hydrogen (hydrogen electrolyzer) or a resistor load. One or more electrolyzers can be connected as a string of electrolyzers. Each electrolyzer in the string of electrolyzers can be connected with a bypass switch for protection. One or more strings of electrolyzers can be connected in parallel at the DC side, and each string of electrolyzers can be connected in series with a switch for disconnection. Similarly, a resistorload can comprise one or more resistors (a resistor bank or a string of resistors) and each resistor can be connected with a bypass switch for protection. One or more strings of resistors can be provided, each with an individual switch for disconnection in series.

[0016] Examples of the present disclosure have many advantages. For example, energy storage elements and loads can be separated by connecting the energy storage elements in the converter cells and connecting the loads to the DC side respectively. The DC voltage at the DC side can be controlled to a proper level that is suitable for a specific load. The same controller can be used to coordinate both the energy storage elements and the loads. The combination of the energy storage elements and the loads can contribute to peak shaving and can store surplus energy from renewable generation. The combination of the energy storage elements and the loads can also contribute to frequency support, when the converter station is used to support the frequency at the connection point that is connected to a power grid. Moreover, fault handling can be managed with the bypass switch and the control of the MMC.

[0017] According to examples of the present disclosure, a large-scale energy island at offshore level can be realized, which can reduce the footprint as well as cost for the construction of a converter station with different functionalities on the sea level.

[0018] According to examples of the present disclosure, the realization of a large-scale energy island at an offshore location is feasible, which can reduce both the footprint, and the cost associated with constructing a converter station with various functionalities on the sea surface.

[0019] Embodiments of the present disclosure will now be described with reference to the drawings.

[0020] Figure 1 shows a converter station 100 according to an embodiment of the present disclosure. The converter station 100 includes a modular multilevel converter (MMC) 110 and a controllerl20 for controlling the operation of the converter station 100.

[0021] With reference to Figure 1, the MMC 110 has an AC side coupled to an AC system 200 and a DC side coupled to a DC system 300. The DC side may have a unipolar power supply (e.g., DC+ and ground) or a bipolar power supply (e.g., DC+ and DC-).

[0022] The MMC 110 can convert an alternating current received from the AC system via the AC side into a direct current which is to be transferred to the DC system via the DC side. The MMC 110 can also convert a direct current received from the DC system via the DC side into an alternating current which is to be transferred to the AC system via the AC side.

[0023] The MMC 110 can be realized as a chain-link converter. The MMC 110 comprises a plurality of converter branches, for example, first to sixth converter branches 111~116. Each converter branch comprises a plurality of converter cells and an inductor. The inductor can be provided as spilt inductors.

[0024] With reference to Figure 1, each of the first converter branch 111 and the second converter branch 112 comprises a plurality of converter cells ll~ln and an inductor 71. The first converter branch 111 is coupled with an AC line of phase A on the AC side and with a positive pole of the DC side, while the second converter branch 112 is coupled with the same AC line of phase A but with a negative pole of the DC side. The inductor 71 in each of the first converter branch 111 and the second converter branch 112 is arranged between the AC line of phase A and the plurality of converter cells ll~ln. Similarly, each of the third converter branch 113 and the fourth converter branch 114 comprises a plurality of converter cells 21~2n and an inductor 72. The third converter branch 113 is coupled with an AC line of phase B on the AC side and with the positive pole of the DC side, while the fourth converter branch 114 is coupled with the same AC line of phase B but with the negative pole of the DC side. The inductor 72 in each of the third converter branch 113 and the fourth converter branch 114 is arranged between the AC line of phase B and the plurality of converter cells 21~2n. Each of the fifth converter branch 115 and the sixth converter branch 116 comprises a plurality of converter cells 31~3n and an inductor 73. The fifth converter branch 115 is coupled with an AC line of phase C on the AC side and with the positive pole of the DC side while the sixth converter branch 116 is coupled with the same AC line of phase B but with the negative pole of the DC side. The inductor 73 in each of the fifth converter branch 115 and sixth converter branch 116 is arranged between the AC line of phase C and the plurality of converter cells 31~3n.

[0025] In an example, each converter cell in the MMC 110 has a full-bridge topology. Forexample, a converter cell 11 with the full-bridge topology is illustrated in Figure 2. The converter cell 11 in Figure 2 comprises first to fourth semiconductor switches 11a, lib, 11c, lid, which are connected in a full-bridge configuration. The converter cell 11 further comprises an energy storage device implemented as a capacitor arrangement comprising at least one capacitor lie. The capacitor lie is configured to store electrical energy and thereby provide a voltage. The semiconductor switches 11a, lib, 11c, lid may include any selfcommutated semiconductor switches, which include at least IGBT, IGCT, IEGT, GTO, and MOSFET.

[0026] According to examples of the present disclosure, one or more converter cells in each converter branch can be coupled with renewable energy sources. For the sake of clarity, examples will be described with reference to the first converter branch 110, and other converter branches can be implemented in a similar way.

[0027] In an example, a converter cell in the first converter branch 111 is coupled with one or more renewable energy sources. The one or more renewable energy sources can include solar energy sources such as photovoltaic (PV) cells or wind energy sources such as wind turbine generators (WTGs).

[0028] In an example, as shown in Figure 1, a converter cell (e.g., the converter cell 11) in the first converter branch 111 is coupled with a first renewable energy source through a DC / DC converter, and another converter cell (e.g., the converter cell 12) in the first converter branch 111 is coupled with a second renewable energy source through a DC / AC converter. The first renewable energy source can include one or more PV cells, and the second renewable energy source can include one or more WTGs. In this example, another converter cell (e.g., the converter cell In) in the first converter branch 111 can be coupled with an energy storage system (ESS) through a DC-DC converter, and the ESS can comprise a battery energy storage system (BESS) and / or a super capacitor. In an embodiment where the ESS is implemented as a BESS, it can either receive power or provide power. For example, in the case that the BESS is charged, the control unit 120 can control the converter cell In to receive power for the BESS from one or more renewable energy sources, the AC system 200, or the DC system 300; in the case that the BESS discharges, the control unit 120 can control theconverter cell In to provide power from the BESS to the AC system 200 or the DC system 300.

[0029] Figures 3A and 3B show some other exemplary configurations for connection with a BESS, in which a converter cell is coupled to a renewable energy source that is connected in parallel with the BESS.

[0030] Figure 3A shows an exemplary configuration where the BESS is coupled with the converter cell 11 which is also coupled with one or more PV cells. As shown in Figure 3A, the BESS and the converter cell 11 are coupled to a common DC bus, and the one or more PV cells are also coupled to the common DC bus through a DC / DC converter. In such a configuration, the controller 120 can control the converter cell 11 such that both the power from the BESS and the power from the one or more PV cells are transferred to either the AC system 200 or the DC system 300. In addition, the BESS can include a battery management system (BMS) to manage charging and discharging of the BESS. For example, when the state of charge (SOC) of the BESS is too low (e.g., lower than a lower limit threshold of a predetermined SOC range), the BESS can be charged by the one or more PV cells under the control of the BMS; when the SOC of the BESS is too high (e.g., greater than a upper limit threshold of a predetermined SOC range), the BESS can discharge under the control of the BMS.

[0031] Figure 3B shows another exemplary configuration where the BESS is coupled with the converter cell 12 which is also coupled with one or more WTGs. As shown in Figure 3B, the BESS and the converter cell 12 are coupled to a common DC bus, and the one or more WTGs are also coupled with the common DC bus through an AC / DC converter. In such a configuration, the control unit 120 can control the converter cell 12 such that both the power from the BESS and the power from the one or more WTGs are transferred to either the AC system 200 or the DC system 300. In addition, the BESS can include a battery management system (BMS) to manage charging and discharging of the BESS. For example, when the state of charge (SOC) of the BESS is too low (e.g., lower than a lower limit threshold of a predetermined SOC range), the BESS can be charged by the one or more WTGs under the control of the BMS; when the SOC of the BESS is too high (e.g., greater than a upper limit threshold of a predetermined SOC range), the BESS can discharge under the control of the BMS.

[0032] Examples of the DC system 300 are described below, with reference to Figures 4A, 4B and 4C.

[0033] In an example, as shown in Figure 4A, the DC system 300 includes a load comprising a string of electrolyzers 310~31n, along with a switch for disconnection (a disconnector or a DC breaker) S301 to disconnect the entire string from the DC side. Each electrolyzer in this string is coupled with a bypass switch, specifically, electrolyzer 310 with S310, electrolyzer 311 with S311, and so on, up to electrolyzer 31n with S31n. According to the configuration in Figure 4A, the controller 120 can determine the number of electrolyzers to bypass, based on the available power, in order to efficiently produce hydrogen. For instance, when the electrolyzers rely solely on power from renewable energy sources to generate green hydrogen, the controller 120 obtains information on the amount of available power from these renewable energy sources and calculates the optimal number of electrolyzers to bypass, ensuring that the power required by the remaining electrolyzers matches the available renewable energy. Such a solution is particularly suitable for utilizing seasonal renewable energy sources to produce green hydrogen. Additionally, the controller 120 can control the MMC 110 to lower the DC voltage below the operating voltage of the electrolyzers, thereby effectively disconnecting them from the DC side, in addition to using the switch for disconnection as described. Additionally, the bypass switch can also be used to isolate the faulty unit(s) from the other parts of the system.

[0034] In another example, as shown in Figure 4B, the DC system 300 includes a load consisting of a plurality of resistors 320~32n, along with a switch for disconnection (a disconnector or a DC breaker) S302 to disconnect the entire set of resistors from the DC side. Figure 4B specifically shows these resistors connected in series, but they may also be connected in parallel or involve a combination of series and parallel connections. Each resistor is coupled with a bypass switch, specifically, resistor 320 with S320, resistor 321 with S321, and so on, up to resistor 32n with S32n. According to the configuration in Figure 4B, the controller 120 can determine the number of resistors to bypass based on the available power, in order to optimize the load on the DC side. For instance, when the DC system relies on green power from renewable energy sources, the controller 120 can obtain informationthe amount of available power and determine the number of resistors to bypass, ensuring that the power required to operate the remaining resistors matches the available renewable energy.

[0035] In yet another example, as shown in Figure 4C, the DC system 300 includes a first load consisting of a string of electrolyzers 310~31n, and a second load comprising a plurality of resistors 320~32n. This example combines the implementations of Figures 4A and 4B, and the above descriptions are also applicable here. According to the configuration of Figure 4C, both the first load and the second load, or either one of them, can be connected to or disconnected from the DC side by operating the switches for disconnection S301 and S302, respectively.

[0036] Examples of the AC system 200 are described below.

[0037] In an example, the AC system 200 includes a power grid. The controller 120 can control the MMC 110 to transfer the power generated by the renewable energy sources, which are coupled with the converter cells, to the power grid. The control unit 120 can also control the MMC 110 to transfer power from the power grid to the DC side 300.

[0038] In another example, the AC system 200 includes an AC machine which can be operated as a generator or as a motor. When the AC machine is operated as a generator, the electric power from the generator along with the electric power from the renewable energy sources, which are coupled with the one or more converter cells, is transferred to the DC system 300. When the AC machine is operated as a motor, the power from the renewable energy sources, which are coupled with the one or more converter cells, is transferred to operate the motor. This configuration is particularly suitable for pumped storage solutions. When electricity demand is low, such as at night, excess electricity generated by renewable energy sources can drive the motor to pump water into elevated reservoirs to store energy. When the peak of power demand comes, the generator can generate electricity using the potential energy of the water in the reservoir, thus improving the stability and reliability of the whole power system.

[0039] According to some examples of the present disclosure, the converter station 100 can be arranged offshore and coupled to an onshore AC network. In an example, the offshoreconverter station 100 is coupled to the onshore AC network through an AC transmission line contained in the AC system 200. In this example, power can be transferred in the form of alternating current (AC) from the converter station 100 to the onshore AC network or from the onshore AC network to the converter station 100 through the AC transmission line. In another example, the offshore converter station 100 is coupled to the onshore AC network through a DC link comprised in the DC system 300 along with an onshore DC / AC substation which is coupled to the onshore AC network. In this example, the power is first transferred in the form of direct current (DC) through a DC link (e.g., an HVDC transmission line) and then converted to alternating current (AC) in the onshore DC / AC substation for connection to the onshore AC network. Such arrangements and connections are particularly useful in scenarios such as offshore wind farms, because they allow power to be efficiently transferred from an offshore wind farm to an onshore grid network.

[0040] Figure 5 shows an exemplary implementation of the converter station 100. With reference to Figure 5, the converter cells in the MMC 110 are coupled exclusively with renewable energy sources and energy storage systems (ESS), while the DC side of the MMC 110 is solely connected to DC loads. Furthermore, the AC side of the MMC 110 is connected to a power grid or an AC machine. This configuration physically segregates the energy sources (renewable energy sources and ESS) from the loads within the system design, with the energy sources being coupled in the converter cells of the MMC 110 and the loads connected to the DC side of the MMC 110. With this setup, the MMC 110 facilitates the independent connection of diverse energy sources and loads, fostering a non-interfering environment. Additionally, it enhances system flexibility and reliability by enabling seamless expansion and maintenance operations. This architecture promotes the seamless integration and optimal utilization of renewable energy, improves energy transmission efficiency, and reinforces the overall flexibility and reliability of the system.

[0041] Examples of operation of the converter station 100 are described below.

[0042] In an example, in the case where the power available from the renewable energy sources can just meet the requirements of either the AC system 200 or the DC system 300, the controller 120 can control the converter cells of the MMC 100 such that power from therenewable energy sources is fully transferred to the AC system 200 or the DC system 300.

[0043] In another example, in the case where the power available from the renewable energy sources exceeds the required power for the DC system 300, the controller 120 can control the converter cells of the MMC 100 such that a portion of the power is transferred to the DC system 300 to meet its required power, with the remaining power being stored in the ESS.

[0044] In yet another example, in the case where the power available from the renewable energy sources exceeds the required power for the AC system 200, the controller 120 can control the converter cells of the MMC 100 such that a portion of the power is transferred to the AC system 200 to meet its required power, with the remaining power being stored in the ESS.

[0045] In yet another example, in the case where the power available from the renewable energy sources is less than the required power for the DC system 300, the controller 120 can control the converter cells of the MMC 100 such that both the power from the renewable energy sources and the power from the power grid coupled at the AC side are simultaneously transferred to the DC system 300.

[0046] In yet another example, in the case where the power available from the renewable energy sources exceeds the combined required power of both the AC system 200 and the DC system 300, the controller 120 prioritizes the allocation of power based on predefined criteria, such as system priority, energy efficiency, or economic factors. For example, if the loads in the DC system 300 are designated as the highest priority, the controller 120 first controls the MMC 110 to transfer power from the renewable energy sources to the loads in the DC system 300 to satisfy the requirements of the DC system 300, and then controls the MMC 110 to transfer the remaining power, after satisfying the DC system's requirements, to the AC system 200. In addition, any remaining power not required by either the AC system 200 or the DC system 300 can then be stored in the ESS.

[0047] In some other examples, to further optimize energy usage and reduce costs, an energy storage management strategy can be incorporated for peak shaving. During periodsof low power demand and high renewable energy generation, the controller 120 can control the MMC 110 to transfer excess power from the renewable energy sources to the ESS for storage. When power demand increases or renewable energy generation decreases, the controller 120 can communicate with the management system of the ESS and control the MMC 110 such that the ESS discharges power to supplement the power supply to the AC system 200 or the DC system 300, thereby reducing the need for additional power generation or grid imports during peak hours.

[0048] In some other examples, to guarantee the reliability and resilience of the entire power system, a fault tolerance and redundancy strategy can be implemented. If a fault is detected in any component of the MMC 110, the AC system 200, the DC system 300, or the renewable energy sources, the controller 120 can promptly isolate the malfunctioning component and redistribute power flow to sustain the operation of the entire power system. Furthermore, a backup power supply, such as a diesel generator or a battery bank, can be integrated to provide an alternative source of power in the event of component failure.

[0049] In some other examples, the converter station 100 can provide a grid service. For example, the controller 120 receives information regarding a change in power frequency and / or voltage of the power grid, which is coupled at the AC side of the MMC 110. Based on this change, the controller 120 determines the active and / or reactive power that should be injected into or absorbed from the power grid. The controller 120 then controls the MMC 110 to transfer the determined active and / or reactive power from or to the MMC 110, in order to provide a grid service. Specifically, depending on the amount of frequency change and / or voltage change, the controller 120 utilizes model calculation or a lookup table to determine the amount of active power and / or the amount of reactive power that should be injected into the grid or absorbed from the grid.

[0050] Figure 6 is a flowchart of a method 600 for operating a converter station according to an embodiment of the present disclosure.

[0051] With reference to Figure 6, at block 602, a modular multilevel converter (MMC) comprising a plurality of converter cells in a plurality of converter branches is provided. The MMC has an AC side and a DC side.

[0052] At block 604, each of one or more converter cells from the plurality of converter cells is coupled with at least one renewable energy source.

[0053] At block 606, the one or more converter cells are controlled such that power from the at least one renewable energy source is at least partially transferred to both the AC side and the DC side, or to one of them.

[0054] In an example, an amount of power to be transferred from the one or more converter cells is controlled such that power from the renewable energy sources is fully transferred to the AC side or fully to the DC side.

[0055] In an example, a converter cell from the plurality of converter cells is coupled with an ESS that comprises batteries and / or super capacitors.

[0056] In an example, a converter cell from the plurality of converter cells is coupled with a renewable energy source that is connected in parallel with an ESS. The converter cell is operated to transfer both power from the renewable energy source and power from the ESS to either the AC side or the DC side.

[0057] While the present invention has been illustrated in the appended drawings and the foregoing description, such illustration is to be considered illustrative or exemplifying and not restrictive; the present invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the appended claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

Claims:

1. A converter station (100), comprising: a modular multilevel converter, MMC (110) having an AC side and a DC side, the MMC (110) comprising a plurality of converter branches (111~116), a first converter branch (111) from the plurality of converter branches being coupled between an AC line at the AC side and a positive pole at the DC side, a second converter branch (112) from the plurality of converter branches being coupled between the AC line and a negative pole at the DC side, each of the first and second converter branches comprising a plurality of converter cells (ll~10) and an inductor (71), wherein one or more converter cells from the plurality of converter cells are configured to be coupled with renewable energy sources; and a controller (120) configured to control the one or more converter cells such that power from the renewable energy sources, which are coupled with the one or more converter cells, is at least partially transferred to at least one of the AC side and the DC side.

2. The converter station (100) of claim 1, wherein a converter cell from the plurality of converter cells is configured to be coupled to one or more photovoltaic (PV) cells via a DC-DC converter.

3. The converter station (100) of claim 1, wherein a converter cell from the plurality of converter cells is configured to be coupled to one or more wind turbine generators (WTGs) via a DC-AC converter.

4. The converter station (100) of claim 1, wherein the converter station is an integrated converter station configured to couple two or more renewable energy sources by coupling one or more photovoltaic cells via a DC-DC converter with a first converter cell from the plurality of converter cells; and by coupling one or more wind turbine generators via a AC-DC converter with a second converter cell from the plurality of converter cells.

5. The converter station (100) of claim 1, wherein a converter cell from the plurality of converter cells is configured to be coupled to an energy storage system (ESS), and the ESS comprises batteries and / or super capacitors.

6. The converter station (100) of claim 5, wherein the ESS is a battery energy storage system (BESS), and the controller is configured to control the converter cell, which is coupled with the BESS, such that the BESS either receives power from at least one of the renewable energy sources, energy sources coupled at the AC side, and energy sources coupled at the DC side, or provides power to at least one of AC loads coupled at the AC side and DC loads coupled at the DC side.

7. The converter station (100) of claim 1, wherein a converter cell from the plurality of converter cells is configured to be coupled to a renewable energy source that is connected in parallel with a BESS; and wherein the converter cell is configured to transfer both power from the renewable energy source and power from the BESS to either the AC side or the DC side, and wherein the BESS is operated to either be charged by the renewable energy source or to discharge power to supplement the power supplied by the renewable energy source.

8. The converter station (100) of claim 1, wherein the converter station is configured to individually couple a variety of renewable energy sources with converter cells, and to supply power to DC loads connected to the DC side or to supply power to AC loads connected to the AC side.

9. The converter station (100) of claim 1, wherein the plurality of converter cells are coupled exclusively with energy sources comprising renewable energy sources and an ESS, and the plurality of converter cells comprise at least one of: a converter cell which is coupled with one or more PV cells; a converter cell which is coupled with one or more WTGs;a converter cell which is coupled with one or more fuel cells; and a converter cell which is coupled with the ESS, wherein the DC side is solely coupled with DC loads comprising at least one of an electrolyzer and a resistor.

10. The converter station (100) of claim 1, wherein the AC side is coupled to a power grid and the controller is configured to control the MMC to provide a grid service by controlling active power and / or reactive power transfer from and / or to the MMC in response to a change in power frequency or voltage of the power grid.

11. The converter station (100) of claim 1, wherein the AC side is coupled to an AC machine which can be operated as a generator or as a motor, coupled with the MMC at the AC side; and when operated as a generator, the generator transfers electric power, along with the electric power from the renewable sources coupled with the one or more converter cells, to the DC side, and when operated as a motor, the power from the renewable sources couple with the one or more converter cells is transferred to operate the motor.

12. The converter station (100) of claim 1, wherein the power from the renewable energy sources coupled with the one or more converter cells is at least partially transferred to at least one of the AC side and the DC side, and the controller is configured to control an amount of power to be transferred from the one or more converter cells such that power from the renewable energy sources is fully transferred to the AC side or fully to the DC side.

13. The converter station (100) of claim 1, wherein the controller (120) is configured to control the one or more converter cells such that both power from the renewable energy sources and power from the AC power network are transferred to the DC side.

14. The converter station (100) of claim 1, wherein the DC side is configured to be coupled with a string of electrolyzers, each of which is coupled with a by-pass switch, and the power transferred to the DC side is variable according to the available power from the renewable energy sources, and the number of electrolyzers to be bypassed is determined based on the available power to efficiently produce hydrogen.

15. The converter station (100) of claim 1, wherein the DC side is configured to be coupled with a resistor, which is connected in series with a switch for disconnection to disconnect the resistor from the DC side and is connected in parallel with a bypass switch to bypass the resistor.

16. The converter station (100) of claim 1, wherein the converter station is arranged offshore and coupled to an onshore AC network through an AC transmission line coupled to the AC side, or through a DC link coupled to the DC side and subsequently interfaces with an onshore DC-AC substation, which is coupled to the onshore AC network.

17. A method for operating a converter station, comprising: providing a modular multilevel converter (MMC) comprising a plurality of converter cells in a plurality of converter branches, the MMC having an AC side and a DC side; coupling each of one or more converter cells from the plurality of converter cells with at least one renewable energy source; and controlling the one or more converter cells such that power from the at least one renewable energy source is at least partially transferred to at least one of the AC side and the DC side.

18. The method of claim 17, wherein controlling the one or more converter cells comprises:controlling an amount of power to be transferred from the one or more converter cells such that power from the renewable energy sources is fully transferred to the AC side or fully to the DC side.

19. The method of claim 17, wherein providing the MMC comprises: coupling a converter cell from the plurality of converter cells with an ESS that comprises batteries and / or super capacitors.

20. The method of claim 17, wherein the method comprises: coupling a converter cell from the plurality of converter cells to a renewable energy source that is connected in parallel with an ESS; and operating the converter cell to transfer both power from the renewable energy source and power from the ESS to either the AC side or the DC side.

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