Energy storage system for connecting to the AC grid

Abstract

This specification discloses an energy storage system (1) for connection to an alternating current (AC) grid (2). The energy storage system comprises a first converter (10) configured to connect the AC grid to a first terminal (3) and a second terminal (4). The energy storage system further comprises a plurality of energy storage strings (20) connected in parallel to the first converter and connected in parallel to each other between the first terminal and the second terminal. Each of the plurality of energy storage strings comprises a plurality of energy storage units (22) connected in series to each other. The energy storage system further comprises at least one second AC / DC converter module (30) connected in series to the plurality of energy storage strings and configured to receive AC power from an AC power source, which is an input supply from the AC grid or another AC power source (6).

Description

Technical Field

[0001] Technical Field The present disclosure generally relates to an energy storage system for connection to an alternating current (AC) grid.

Background Art

[0002] Background With the increase of renewable energy sources such as photovoltaic power generation and wind power, intermittency and fluctuation of power grids have been increasing. Grid-connected energy storage systems address the challenges of intermittency and fluctuation caused by renewable energy sources. A grid-connected energy storage system can store energy when there is excess energy in the power grid, and supply energy when energy in the power grid is insufficient.

[0003] Energy storage elements in an energy storage system may be connected in series and / or in parallel to meet the voltage and current requirements of the system. Power conversion systems are often used to connect energy storage elements to an alternating current (AC) grid, which enables direct connection between the energy storage elements and the AC grid. However, such a direct connection does not allow the energy storage system to balance the current distribution among the energy storage elements.

[0004] Some conventional systems allow for flexible operation of energy storage elements by connecting each element to a DC / DC (direct current / direct current) converter, and then a power conversion system is used to connect the energy storage elements to the AC grid. This achieves the ability to control the voltage and current input / output to and from the energy storage elements. However, this results in a complex and expensive system with high losses. Therefore, improved grid-connected energy storage systems are still needed. [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] overview Therefore, an object of this disclosure is to provide an energy storage system for connection to an alternating current (AC) grid that mitigates at least some of the aforementioned drawbacks. For example, an object of this disclosure is to provide a less complex energy storage system that allows for the equilibriumization of energy storage elements and / or is less expensive. [Means for solving the problem]

[0006] According to a first aspect of the present disclosure, an energy storage system for connection to an AC grid is provided. The energy storage system comprises a first converter configured to connect an AC grid to a first terminal and a second terminal. The first converter comprises an upper arm and a lower arm for each phase of the AC grid. The upper arm is connected to the first terminal, and the lower arm is connected to the second terminal, and each arm comprises a plurality of converter cells. The energy storage system further comprises a plurality of energy storage strings connected in parallel to the first converter and connected in parallel to each other between the first terminal and the second terminal. Each of the plurality of energy storage strings comprises a plurality of energy storage units connected in series to each other. The energy storage system further comprises at least one second AC / DC converter module connected in series to the plurality of energy storage strings and configured to receive AC power from an AC power source, which is an input supply from an AC grid or another AC power source.

[0007] As the amount of renewable energy sources in the AC grid increases, the support function for the AC grid plays a greater role. By connecting multiple energy storage strings in parallel and multiple energy storage units in each energy storage string in series, the voltage and current supplied by the energy storage system can be controlled. For example, higher currents can be achieved by adding more energy storage strings, and higher voltages can be achieved by adding more energy storage units. Furthermore, the energy storage units of an energy storage string may include several energy storage units (or elements) connected in parallel. For example, the energy storage units of an energy storage string may be complemented by one or more additional energy storage elements connected in parallel to the energy storage units. The energy storage system can supply energy, for example, when there is a power shortage in the AC grid.

[0008] Placing the second AC / DC converter module in series with multiple energy storage strings, such that the second AC / DC converter module is connected to either the AC grid or the AC power supply, or both, is beneficial in that the second AC / DC converter module can receive AC power to recharge the energy storage units of the energy storage strings, thereby avoiding the replacement of energy storage units when stored energy is supplied to the AC grid. As described above, the AC power received by at least one second AC / DC converter module for recharging the energy storage units of the energy storage system may originate from the AC grid, for example, in the case of a power surplus in the AC grid, or from another AC power source (e.g., a renewable energy source). Therefore, the energy storage system can be charged even when there is a power shortage in the AC grid.

[0009] As an example, at least one second AC / DC converter module may include one or more AC / DC converters, which may include diodes, thyristors, insulated-gate bipolar transistors (IGBTs), or MOSFET transistors.

[0010] An AC grid or AC power supply can provide low-cost energy buffering functionality to an energy storage system. Energy buffering allows the energy rating and ampere-hour (Ah) rating to be considered infinite. Therefore, energy can be buffered on the AC power supply instead of being stored in the energy storage unit. Using AC as a source for recharging the energy storage unit can be advantageous in that it allows the use of conventional AC protection. Furthermore, if the energy storage unit has a recharging function, the complexity of controlling the energy storage system can be reduced compared to a system using pre-charged energy storage, where the energy storage unit needs to be replaced when depleted. Maintenance of the energy storage system is also reduced. A second AC / DC converter module allows for power balancing between multiple energy storage strings, thereby balancing the charge levels between the energy storage strings. Because the charge states are balanced between each energy storage string, this also allows the energy storage system to contribute to an improved (preferably maximized) capacity utilization rate. Balancing can further ensure that each energy storage string operates at an optimal charge state, which can result in better overall system performance. Equilibrium energy storage strings also allow energy storage systems to scale up or down with less risk of compromising their performance and stability.

[0011] The second AC / DC converter module may be connected between multiple energy storage units and either the first terminal or the second terminal by arranging and configuring the isolation of the energy storage units. The first terminal and the second terminal may be alternatively (or interchangeably) designated as the first direct current (DC) pole and the second DC pole, respectively.

[0012] The energy storage system may have a unipolar DC side in which the first terminal is a first direct current (DC) pole and the second terminal is a second DC pole. However, the energy storage system may also be part of a bipolar DC side in which the first terminal is a first DC pole and the second terminal is connected to ground. The energy storage system may operate on either the positive polarity bipolar DC side or the negative polarity side, or there may be one energy storage system on each side of the bipolar DC side, with each side having its own energy storage system.

[0013] The AC power supply may be supplied from an external AC power source. In some cases, the first converter may be connected to the AC grid through a grid transformer. In such cases, the AC power supply may be supplied from an extra winding of the grid transformer. As a further example, there may be an auxiliary transformer in the STATCOM station that supplies power to low-voltage equipment that can be used as an AC power source.

[0014] The energy storage system may further include a control unit configured to control a first converter to conform to a power standard of the AC grid. The control unit can control the amount of power supplied from the DC side and the energy storage system to the AC grid. The power standard may include controlling the voltage and frequency of the supplied power to conform to a standard value of the AC grid.

[0015] If the AC power source is an input from the AC grid, the control unit may be further configured to control the first converter to compensate for the power flow between the multiple energy storage strings and the AC power source so that the first converter conforms to the power reference of the AC grid. At least one second AC / DC converter module enables the power flow between the energy storage strings and the AC power source. The first converter may be controlled by the control unit to compensate for the power flow so that the output of the entire energy storage system can operate in sync with the frequency and voltage of the AC grid. By conforming to the power reference of the AC grid, the energy storage system can provide stability to the AC grid by responding to changes in the frequency and voltage of the AC grid. The energy storage system can inject or absorb power as needed to adjust these parameters, preventing grid instability, voltage fluctuations, and frequency deviations.

[0016] The first converter may be a modular multi-level converter (MMC). Such a configuration of the first converter may be suitable for providing control coordination that enables the energy storage system to conform to the power standards of the AC grid while allowing power flow between the energy storage string and the AC power source. Due to the modularity of the MMC, it may further provide the energy storage system with scalability that follows the idea of ​​balancing the energy storage string for scaling the energy storage system. As mentioned above, power balancing between energy storage strings enables easier scaling of the energy storage system. The MMC can be easily scaled by adding or removing converter cells. These two features allow the energy storage system to be easily scaled up or down depending on the requirements of the AC grid in which the energy storage system is installed.

[0017] At least one second AC / DC converter module may be configured to handle fractions of the power reference of the AC grid. In other words, at least one second AC / DC converter module may be a fractional converter configured to vary the voltage and / or frequency by a fraction. The fractional converter allows for precise control of voltage and frequency.

[0018] The energy storage string may further include a DC / DC converter connected between at least one second AC / DC converter module and a plurality of series-connected energy storage units of the energy storage string.

[0019] A DC / DC converter may be connected between each energy storage string and at least one second AC / DC converter module, so that there are as many DC / DC converters as there are energy storage strings. The DC / DC converters enable control of the DC power to the energy storage strings so that it can be optimized for each energy storage string. Thus, the DC power supplied to each distinct energy storage string can be adapted to match the charge state and requirements of each energy storage string. In some configurations, the DC / DC converters may be fractional converters, and the second AC / DC converter module may be a standard AC / DC converter.

[0020] Each DC / DC converter may be connected to a common DC link. At least one second AC / DC converter module may be a single second AC / DC converter connected between the common DC link and the AC power supply. In other words, the above at least one second AC / DC converter module may be a single AC / DC converter.

[0021] The common DC link is powered by an AC power source through a second AC / DC converter, which adjusts the voltage and current supplied to each energy storage string, thereby ensuring that each energy storage string receives optimal power. Because power flow is possible in the common DC link, it can be deactivated. This may allow the second AC / DC converter to have a lower rating than the total rating of the DC / DC converter.

[0022] At least one second AC / DC converter module may include multiple second AC / DC converters, each of which may be connected to a respective energy storage string. In such a configuration, the energy storage units of each energy storage string may be connected to an AC power source first through a DC / DC converter and second through a second AC / DC converter, thereby converting the AC power to DC power by the second AC / DC converter, and then the DC power to the energy storage units of the energy storage string by the DC / DC converter.

[0023] At least one second AC / DC converter module may be an isolated AC / DC converter with multiple ports, and each energy storage string may be connected to one of the ports of the isolated AC / DC converter. This allows each energy storage string to be connected to a single second AC / DC converter, thereby reducing the number of components in the energy storage system. Since additional energy storage strings can be easily added to the isolated AC / DC converter, the multiple ports of the isolated AC / DC converter allow for design flexibility in the energy storage system and can provide further scalability to the energy storage system.

[0024] The energy storage system may further comprise an isolated DC / DC converter having a plurality of ports, and each energy storage string may be connected to a respective one port of the isolated DC / DC converter. The at least one second AC / DC converter module may be an AC / DC converter connected to the isolated DC / DC converter. In other words, the at least one AC / DC converter module may be a single AC / DC converter connected between the isolated DC / DC converter and an AC power source or an AC grid. The isolated DC / DC converter can enable the DC / DC converter to be utilized to provide a voltage required for interface purposes. Furthermore, the plurality of ports of the isolated DC / DC converter provide similar advantages to those of the plurality of ports of an isolated AC / DC converter, that is, the advantage that the ports provide scalability to the energy storage system.

[0025] At least one transformer may be connected between the at least one second AC / DC converter module and the AC power source.

[0026] The at least one second AC / DC converter module may comprise one second AC / DC converter for each energy storage string, the at least one transformer may comprise one transformer for each second AC / DC converter, and each transformer may be connected between the AC power source and the respective second AC / DC converter. In other words, there may be a single second AC / DC converter for each energy storage string, and there may be a single transformer for each second AC / DC converter.

[0027] The transformer may provide electrical insulation between an AC power source and the second AC / DC converter module, so that the second AC / DC converter modules can have different reference potentials. Further, the transformer provides galvanic insulation between the second AC / DC converter module and the AC power source, which provides protection against faults and voltage fluctuations from one side of the transformer that could otherwise affect the other side.

[0028] Further scope of applicability of the present disclosure will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the disclosure, are given by way of illustration only, since various changes and modifications within the scope of the disclosure will become apparent to those skilled in the art from this detailed description.

[0029] Therefore, it should be understood that the present disclosure is not limited to the specific components of the described device which may vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting.

[0030] Brief Description of the Drawings The present disclosure will be described in more detail by way of example with reference to the accompanying drawings, which show exemplary embodiments of the present disclosure. Brief Description of the Drawings

[0031] [Figure 1] It is a diagram showing an energy storage system comprising a plurality of second AC / DC converters. [Figure 2] It is a diagram showing an energy storage system having both a DC / DC converter and an AC / DC converter. [Figure 3] It is a diagram showing an energy storage system having a transformer. [Figure 4] It is a diagram showing an energy storage system having a common DC link. [Figure 5]This figure shows an energy storage system having an isolated DC / DC converter with multiple ports. [Figure 6] This figure shows an energy storage system having an isolated AC / DC converter with multiple ports. [Figure 7] This diagram shows the bipolar DC side of the first converter, which has two energy storage systems. [Modes for carrying out the invention]

[0032] Detailed explanation Hereafter, this disclosure is described in its entirety with reference to the accompanying drawings illustrating currently preferred embodiments of this disclosure. However, this disclosure can be embodied in many different forms and should not be construed as being limited to the embodiments described herein. Rather, these embodiments are provided for thoroughness and completeness and fully convey the scope of this disclosure to those skilled in the art.

[0033] Figure 1 shows an energy storage system 1 connected to an alternating current (AC) grid 2. Three connection lines 2a to 2c, corresponding to the three phases of the AC grid 2, are connected to the first converter 10 of the energy storage system 1 on one side of the first converter 10. The other side of the first converter 10 is connected to the direct current (DC) side to the first terminal 3 and the second terminal 4. Each of the connection lines 2a to 2c for the three phases is connected to the upper arms 12a to 12c and the lower arms 14a to 14c of the converter 10. Each of the upper arms 12a to 12c is connected to the first terminal 3, and each of the lower arms 14a to 14c is connected to the second terminal 4. The first terminal 3 may be considered a first DC pole, and the second terminal 4 may be considered a second DC pole. Each arm 12a to 12c, 14a to 14c comprises a plurality of converter cells 16. Multiple converter cells 16 perform switching operations to convert incoming power / voltage between AC and DC. The first converter 10 may be, for example, a modular multilevel converter. The first converter 10 in Figures 2 to 7 represents a converter similar to the one described above. However, for ease of understanding the drawings, the converter is represented only by box 10 in Figures 2 to 7.

[0034] In Figure 1, the energy storage system 1 further comprises a plurality of energy storage strings 20 connected in parallel to the first converter 10 and connected in parallel to each other between the first terminal 3 and the second terminal 4. Figure 1 discloses three energy storage strings 20. However, as indicated by the dotted lines, there may be more energy storage strings not shown in Figure 1. All dotted lines in the figure indicate that more components of the same type may be added between them. Thus, the dotted lines also indicate that an energy storage string may contain more energy storage units than those shown in Figure 1. Each of the plurality of energy storage strings 20 comprises a plurality of energy storage units 22 connected in series. By alternating the number of parallel-connected energy storage strings 20 and the number of series-connected energy storage units 22, the energy storage system 1 can be designed with different voltage and current ratings.

[0035] The energy storage string 20 is connected to a second AC / DC converter module 30. The second AC / DC converter module 30 may comprise multiple second AC / DC converters or AC / DC converters of different types. In the following description, the second AC / DC converter module 30 will be described with each drawing by describing different configurations of the second AC / DC converters.

[0036] In Figure 1, each of the energy storage strings 20 is connected in series with its respective second AC / DC converter 30. In other words, the second AC / DC converter module comprises one second AC / DC converter 30 for each energy storage string 20. The second AC / DC converter 30 may include, for example, a diode, a thyristor, an insulated-gate bipolar transistor (IGBT), or a MOSFET transistor. The second AC / DC converter 30 is configured to receive AC power from an AC power supply 6. In the example shown in Figure 1, the AC power supply 6 may be an input supplied from an external AC power supply 6. In other examples, the AC power may be supplied to the second AC / DC converter from an AC grid.

[0037] A second AC / DC converter 30, which connects the energy storage string 20 to the AC power supply 6, allows the energy storage unit 22 to be recharged using energy from the AC power supply 6 (supplied through the second AC / DC converter 30). Therefore, there is no need to replace the energy storage unit 22 when the stored energy is supplied to the AC grid 2. The AC power supply 6 provides an energy buffering function to the energy storage system 1, such that its energy rating and ampere-hour (Ah) rating can be considered infinite. Thus, energy is buffered on the AC power supply 6 instead of being stored in the energy storage unit.

[0038] The energy storage system may include a control unit 50 configured to control the power flow of the first converter 10, more specifically by controlling a converter cell 16. This allows the control unit 50 to control the amount of power supplied from the energy storage system 1 to the AC grid 2. The control unit 50 is configured to control the first converter 10 to conform to the power reference of the AC grid 2. In other words, the first converter 10 may be controlled by the control unit 50 to compensate for the power flow so that the output of the entire energy storage system 1 can operate in sync with the frequency and voltage of the AC grid 2. A second AC / DC converter 30 may be configured to handle fractions of the power reference of the AC grid 2. In other words, the second AC / DC converter 30 may be a fractional converter configured to vary the voltage and / or frequency by a fraction. The fractional converter allows for precise control of the voltage and frequency of the power supplied to the AC grid 2.

[0039] Furthermore, each AC / DC converter 30 may be controlled to balance the charge levels in the energy storage strings 20 so that the energy storage strings 20 are in equilibrium with each other.

[0040] Further embodiments of the concept of the present invention will be described with reference to Figure 2. In Figure 2, the first converter 10 is connected to a grid transformer 8 which can provide galvanic isolation between the energy storage system 1 and the AC grid 2. In Figure 2, each energy storage string 20 is provided with a DC / DC converter 24 connected between the second AC / DC converter 30 and the multiple energy storage units 22. In other words, a DC / DC converter is connected between the first terminal 3 in each energy storage string 20 and the multiple energy storage units 22, thereby providing each energy storage string with one DC / DC converter 24. The DC / DC converter 24 enables control of the DC power to the energy storage string 20 so that it can be optimized for each energy storage string 20. Thus, the DC power supplied to each separate energy storage string 20 can be adapted to match the charge state and requirements of each energy storage string 20. In some configurations, the DC / DC converter 24 may be a fractional converter, and the second AC / DC converter 30 may be a standard AC / DC converter.

[0041] In Figure 2, each of the second AC / DC converters 30 receives AC power from the AC grid 2 through the extra windings 9 of the grid transformer 8. Thus, the AC grid 2 acts as the AC power supply or AC supplied to the second AC / DC converters 30. In this example, the AC power supply 6 can be considered as the input supply from the AC grid 2. The second AC / DC converters convert the AC power to DC power and transmit the DC power to the DC / DC converter 24, which then adapts the DC power to the respective energy storage strings 20 of the DC / DC converter 24. In a further example, there may be an auxiliary transformer in the STATCOM station that supplies power to low-voltage equipment that can be used as an AC power supply.

[0042] If the AC power source is AC grid 2, the control unit 50 may be configured to control the first converter 10 so as to compensate for the power flow between the AC power source 6 and the energy storage string 20 so that the first converter 10 conforms to the power standard of AC grid 2.

[0043] In Figure 3, the first converter 10 connects its DC side to the AC grid 2 through a grid transformer 8. Each of the energy storage strings 20 connected between the first terminal 3 and the second terminal 4 comprises a DC / DC converter 24 connected to a second AC / DC converter 30. However, in Figure 3, a transformer 40 is connected between each of the second AC / DC converters 30 and the AC power supply 6. The transformer 40 can provide electrical isolation between the AC power supply 6 and the second AC / DC converters 30 so that the second AC / DC converters 30 may have different reference potentials. Furthermore, the transformer 40 provides galvanic isolation between the second AC / DC converters 30 and the AC power supply 6, which provides protection against faults and voltage fluctuations from one side of the transformer 40 that could affect the other side.

[0044] Another embodiment of the concept of the present invention will be described with reference to Figure 4. In Figure 4, the energy storage system 1 is provided with a common DC link 26. Each DC / DC converter 24 is connected to the common DC link 26 such that only a single second AC / DC converter 30 is required. The second AC / DC converter 30 is connected between the common DC link 26 and the AC power supply 6. Because power flow is possible in the common DC link 26, power flow cancellation is possible in the common DC link 26, which may allow the second AC / DC converter 30 to have a lower rating than the total rating of the DC / DC converters 24.

[0045] Another embodiment of this disclosure will be described with reference to Figure 5. In Figure 5, the multiple DC / DC converters 24 are replaced by an isolated DC / DC converter 25 having multiple ports. That is, a multi-port DC / DC converter is used instead of the multiple DC / DC converters 24. Each energy storage string 20 is connected to one port of the isolated DC / DC converter 25. The isolated DC / DC converter 25 requires only a single second AC / DC converter 30. The second AC / DC converter 30 is connected between the isolated DC / DC converter 25 and the AC power supply 6.

[0046] Further embodiments of this disclosure will be described with reference to Figure 6. In Figure 6, the energy storage strings 20 are directly connected to each port of the isolated AC / DC converter 32. In other words, the energy storage system of the embodiment shown in Figure 6 does not include any DC / DC converters. However, in some other embodiments, a DC / DC converter may be provided for each energy storage string 20. The isolated AC / DC converter 32 may also be referred to as a multi-port AC / DC converter. By using an isolated AC / DC converter, the number of components required for the energy storage system 1 is reduced.

[0047] The energy storage systems in Figures 1 to 6 can be considered as unipolar DC sides, where the first terminal 3 is the first direct current (DC) pole and the second terminal 4 is the second DC pole. However, Figure 7 shows a bipolar DC side. The bipolar DC side includes a first terminal 3 representing the positive DC pole and a second terminal 4 representing the negative DC pole. A third terminal 5 is provided to represent ground. In other words, the bipolar DC side includes a side with positive polarity and a side with negative polarity. The AC grid 2 is connected to two energy storage systems 1a and 1b through a transformer 8. The first energy storage system 1a is connected with positive polarity between the first terminal 3 and the third terminal 5, and the second energy storage system 1b is connected with negative polarity between the second terminal 4 and the third terminal 5. Transformer 8 supplies AC power to a first converter 10a located on one side of the bipolar DC side, and another first converter 10b located on the other side of the bipolar DC side. Each energy storage system 1a, 1b comprises a plurality of energy storage strings 20a, 20b. The energy storage strings 20a of the first energy storage system 1a are connected in parallel to each other and in parallel to the first converter 10a. The energy storage strings 20b of the second energy storage system 1b are connected in parallel to each other and in parallel to the first converter 10b. Each of the energy storage strings 20a, 20b includes a plurality of energy storage units 22a, 22b connected in series in each energy storage string 20a, 20b. Each energy storage string 20a, 20b is connected to a second AC / DC converter 30a, 30b, each connected to an AC power source 6. The first energy storage system 1a and the second energy storage system 1b in Figure 7 are two copies of the system in Figure 1. Therefore, any of the energy storage systems 1 described herein can be used on the bipolar DC side by connecting one to each polarity side. In some situations, only one of the polarities on the bipolar DC side may be equipped with the energy storage system 1.

[0048] Note that each figure may contain components that can be added or modified. Using the AC grid 2 as an AC power source, as shown in Figure 2, is equally applicable to the embodiments described with reference to Figures 1 and 3-7. Furthermore, the transformer 40 shown in Figure 3 may be used between the second AC / DC converter 30 and the AC power source 5 in the embodiments described with reference to any of the figures. The transformer 40 may also be used when the AC power source 6 is the AC grid 2.

[0049] Those skilled in the art will recognize that this disclosure is by no means limited to the preferred embodiments described above. Conversely, many modifications and variations are possible within the scope of the appended claims. In addition, changes to the disclosed embodiments can be understood and implemented by those skilled in the art in practice of the claimed invention, based on a study of the drawings, disclosure, and the appended claims.

Claims

1. An energy storage system (1) for connecting to an alternating current (AC) grid (2), wherein the energy storage system is A first converter (10) is configured to connect the AC grid to a first terminal (3) and a second terminal (4), wherein the first converter comprises upper arms (12a-c) and lower arms (14a-c) for each phase of the AC grid, the upper arms are connected to the first terminal, the lower arms are connected to the second terminal, and each arm comprises a plurality of converter cells (16), and the first converter (10) A plurality of energy storage strings (20) are connected in parallel to the first converter and in parallel with each other between the first terminal and the second terminal, and each of the plurality of energy storage strings comprises a plurality of energy storage units (22) connected in series with each other, At least one second AC / DC converter module (30) connected in series to the plurality of energy storage strings and configured to receive AC power from an AC power source, which is an input from the AC grid or another AC power source (6) and An energy storage system (1) comprising:

2. The energy storage system according to claim 1, further comprising a control unit (50) configured to control the first converter in accordance with the power standards of the AC grid.

3. The energy storage system according to claim 2, wherein, if the AC power source is an input supply from the AC grid, the control unit is further configured to control the first converter to compensate for the power flow between the plurality of energy storage strings and the AC power source so that the first converter conforms to the power reference of the AC grid.

4. The energy storage system according to claim 1, wherein the first converter is a modular multi-level converter (MMC).

5. The energy storage system according to claim 2, wherein the at least one second AC / DC converter module is configured to process a portion of the power reference of the AC grid.

6. The energy storage system according to claim 1, further comprising a DC / DC converter (24) connected between the at least one second AC / DC converter module and a plurality of series-connected energy storage units of the energy storage string.

7. Each DC / DC converter is connected to a common DC link (26). The energy storage system according to claim 6, wherein the at least one second AC / DC converter module is a second AC / DC converter connected between the common DC link and the AC power supply.

8. The energy storage system according to any one of claims 1 to 6, wherein the at least one second AC / DC converter module comprises a plurality of second AC / DC converters, each of which is connected to an energy storage string.

9. The energy storage system according to any one of claims 1 to 6, wherein the at least one second AC / DC converter module is an isolated AC / DC converter (32) having a plurality of ports, and each energy storage string is connected to each of the ports of the isolated AC / DC converter.

10. The system further comprises an isolated DC / DC converter (25) having multiple ports, each energy storage string being connected to one of the ports of the isolated DC / DC converter. The energy storage system according to any one of claims 1 to 4, wherein the at least one second AC / DC converter module is an AC / DC converter connected to the isolated DC / DC converter.

11. The energy storage system according to any one of claims 1 to 7, wherein at least one transformer (40) is connected between the at least one second AC / DC converter module and the AC power supply.

12. The energy storage system according to claim 11, wherein the at least one second AC / DC converter module includes one second AC / DC converter for each energy storage string, and the at least one transformer includes one transformer for each second AC / DC converter, and each transformer is connected between the AC power supply and each second AC / DC converter.

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