Energy storage system for connection to AC power grid

Abstract

Disclosed herein is an energy storage system (1) for connection to an alternating current (AC) power grid (2). The energy storage system comprises a first converter (10) configured to connect an AC power 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 includes a plurality of energy storage cells (22) connected in series with 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 source or another AC source (6) as a feed from the AC grid.

Description

Technical Field

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

[0002] With the rise of renewable energy sources such as solar and wind, intermittency and volatility in the power grid have increased. Grid-connected energy storage systems address the challenges of intermittency and volatility caused by renewable energy sources. Grid-connected energy storage systems can store energy when there is excess energy in the grid and provide energy when there is a shortage of energy in the grid.

[0003] The energy storage elements in the energy storage system can be connected in series and / or in parallel to achieve the voltage and current requirements of the system. A power conversion system is often used to connect the energy storage elements to the AC grid. This allows a direct connection between the energy storage elements and the AC grid. However, this direct connection does not allow the energy storage system to balance the current distribution among the energy storage elements.

[0004] Some prior art systems allow the energy storage elements to be operated flexibly by connecting each energy storage element to a direct current / direct current (DC / DC) converter, and thereafter use a power conversion system to connect the energy storage element to the AC grid. This enables controllable voltage and current input / output to the energy storage element. However, this results in a complex and expensive system with large losses. Therefore, there is still a need for an improved grid-connected energy storage system. Summary of the invention

[0005] Therefore, an object of the present disclosure is to provide an energy storage system for connection to an alternating current (AC) grid that alleviates at least some of the disadvantages mentioned above. For example, an object of the present disclosure is to provide an energy storage system with less complexity that allows for balancing of energy storage elements and / or lower costs.

[0006] According to a first aspect of the present disclosure, there is provided an energy storage system for connecting to an AC power grid. The energy storage system includes a first converter configured to connect the AC power grid to a first terminal and a second terminal. The first converter includes an upper arm and a lower arm for each phase of the AC power grid. The upper arm is connected to the first terminal and the lower arm is connected to the second terminal, and each arm includes a plurality of converter units. The energy storage system further includes a plurality of energy storage strings, which are 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 aforementioned plurality of energy storage strings includes a plurality of energy storage units connected in series to each other. The energy storage system further includes at least one second AC / DC converter module, which is connected in series to the plurality of energy storage strings and is configured to receive AC power from an AC source or another AC source as a feed from the AC power grid.

[0007] As the amount of renewable energy in the AC power grid increases, the support function for the AC power grid plays a greater role. In the case where the aforementioned multiple energy storage strings are connected in parallel and the aforementioned multiple energy storage units are connected in series at each energy storage string, the voltage and current provided by the energy storage system can be controlled. For example, adding more energy storage strings can achieve higher currents, and adding more energy storage units can achieve higher voltages. Further, the energy storage units of the energy storage string may include several energy storage units (or elements) connected in parallel. For example, the energy storage units of the energy storage string may be supplemented by one or more additional energy storage elements connected in parallel to the energy storage units. The energy storage system can provide energy, for example, when there is a power shortage in the AC power grid.

[0008] The arrangement of the second AC / DC converter module in series to the plurality of energy storage strings, such that the second AC / DC converter module is connected to either one of the AC grid and the AC source or to both of the AC grid and the AC source, is beneficial because the second AC / DC converter module can receive AC power to recharge the energy storage units of the energy storage string, thereby avoiding replacement of the energy storage units when the stored energy has been provided to the AC grid. As mentioned above, the AC power received by the aforementioned at least one second AC / DC converter module for recharging the energy storage units of the energy storage system may be derived from the AC grid (e.g., in the event of excess power in the AC grid) or another AC source (e.g., renewable energy). Therefore, the energy storage system can be charged even if there is a power shortage in the AC grid.

[0009] As an example, the 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] The AC grid or AC source can provide energy buffering functions to the energy storage system at a low cost. With the energy buffering function, the energy rating and the ampere-hour (Ah) rating can be regarded as infinite. Therefore, energy can be buffered on the AC source instead of being stored in the energy storage unit. Using AC as a source for recharging the energy storage unit may be advantageous because it allows the use of conventional AC protection. Further, when the energy storage unit is provided with a recharging function, the complexity of the control of the energy storage system can be reduced compared to a system using a preloaded energy storage device, which requires the energy storage unit to be replaced when the energy storage unit is exhausted. The maintenance of the energy storage system is also reduced. The second AC / DC converter module allows balancing of power between multiple energy storage strings so that the charge levels between the energy storage strings are balanced. This also allows the energy storage system to contribute to improved (preferably maximized) capacity utilization because there is a balance between each energy storage string regarding the state of charge. Balancing can further ensure that each energy storage string operates at an optimal state of charge, which may result in better overall system performance. Balanced strings also allow the storage system to be scaled up or down with less risk of disrupting the performance and stability of the storage system.

[0011] By arranging the insulation of the energy storage unit accordingly, the second AC / DC converter module can be connected between the plurality of energy storage units and any one of the first terminal and the second terminal. The first terminal and the second terminal can alternatively (or interchangeably) be denoted as a first direct current (DC) pole and a second DC pole, respectively.

[0012] The energy storage system may have a unipolar DC side, wherein 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, wherein the first terminal is a first DC pole and the second terminal is connected to ground. The energy storage system may operate in the bipolar DC side with the positive polarity or with the negative polarity, or there may be one energy storage system on each side, so that both sides of the bipolar DC side are provided with a corresponding energy storage system.

[0013] The AC source may be a feed from an external AC source. In some cases, the first converter may be connected to the AC grid via a grid transformer. In this case, the AC source may be a feed from an additional winding of the grid transformer. As a further example, in a STATCOM station, there may be an auxiliary transformer that provides power to a low voltage device that may be used as an AC source.

[0014] The energy storage system may further include a control unit configured to control the first converter to follow a power reference of the AC grid. The control unit may control how much power is provided from the DC side and the energy storage system to the AC grid. The power reference may include controlling the voltage and frequency of the provided power so that it follows a reference value of the AC grid.

[0015] If the AC source is fed from the aforementioned AC power grid, the control unit may be further configured to control the first converter to compensate for the power flow between the aforementioned multiple energy storage strings and the AC source, so that the first converter follows the power reference of the AC power grid. At least one second AC / DC converter module allows power flow between the energy storage string and the AC source. The first converter can be controlled by the control unit to compensate for the power flow so that the output of the energy storage system as a whole can operate synchronously with the frequency and voltage of the AC power grid. By following the power reference of the AC power grid, the energy storage system can provide stability to the AC power grid by responding to frequency and voltage changes of the AC power grid. The energy storage system can inject or absorb power as needed to adjust these parameters, thereby preventing grid instability, voltage fluctuations and frequency deviations.

[0016] The first converter may be a modular multilevel converter (MMC). This configuration of the first converter may be suitable for providing control coordination that allows the energy storage system to follow the power reference of the AC grid while allowing power flow between the energy storage string and the AC source. Due to the modularity of the MMC, it may further provide scalability to the energy storage system, which follows the concept of balancing the energy storage strings to scale the energy storage system. As discussed earlier, power balancing between energy storage strings allows the energy storage system to be more easily scaled. The MMC can be easily scaled by adding or removing converter units. Utilizing these two features, the energy storage system can be easily scaled up or down, depending on the requirements of the AC grid to which the energy storage system is installed.

[0017] At least one second AC / DC converter module can be configured to process a portion of the power reference of the AC grid. In other words, at least one second AC / DC converter module can be a fractional converter, which is configured to change the voltage and / or frequency by a fraction. The fractional converter allows precise control of voltage and frequency.

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

[0019] There may be a DC / DC converter connected between each energy storage string and at least one second AC / DC converter module, so that there are DC / DC converters equal in number to the energy storage string. The DC / DC converter allows control of the DC power to the energy storage string so that it can be optimized for each energy storage string. Therefore, the DC power provided to each individual energy storage string can be adapted to the charging state and requirements of each energy storage string. In some configurations, the DC / DC converter can be a fractional converter, and the second AC / DC converter module can be a normal 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 second AC / DC converter connected between the common DC link and the AC source. In other words, at least one second AC / DC converter module may be a single AC / DC converter.

[0021] The common DC link is fed by the AC source through the second AC / DC converter, and the DC / DC converter adjusts the voltage and current fed to the corresponding energy storage string so that each energy storage string can be provided with optimal power. Since power flow is possible at the common DC link, it is possible to eliminate the common DC link. This allows the rating of the second AC / DC converter to be lower than the total rating of the DC / DC converter.

[0022] At least one second AC / DC converter module may include a plurality of second AC / DC converters, wherein the second AC / DC converters may be connected to corresponding energy storage strings. In this configuration, the energy storage unit of each energy storage string may be connected to an AC source first through a DC / DC converter and secondly through a second AC / DC converter, so that the AC power is first converted to DC power through the second AC / DC converter, and then the DC power is adapted to the energy storage unit of the energy storage string through the DC / DC converter.

[0023] At least one second AC / DC converter module can be an isolated AC / DC converter having multiple ports, wherein each energy storage string can be connected to a corresponding port 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 components of the energy storage system. The multiple ports of the isolated AC / DC converter allow flexibility in the design of the energy storage system, and further proportionality can be provided to the energy storage system because additional energy storage strings can be easily added to the isolated AC / DC converter.

[0024] The energy storage system may further include an isolated DC / DC converter having multiple ports, wherein each energy storage string may be connected to a corresponding port of the isolated DC / DC converter. At least one second AC / DC converter module may be an AC / DC converter connected to the isolated DC / DC converter. In other words, at least one AC / DC converter module may be a single AC / DC converter connected between the isolated DC / DC converter and the AC source or AC grid. The isolated DC / DC converter may allow the DC / DC converter to be utilized to provide the voltage required for docking purposes. Further, the multiple ports of the isolated DC / DC converter provide advantages similar to the multiple ports of the isolated AC / DC converter, namely, these 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 source.

[0026] At least one second AC / DC converter module may include a second AC / DC converter for each energy storage string, and at least one transformer may include a transformer for each second AC / DC converter, wherein each transformer may be connected between the AC source and the corresponding 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 can provide electrical isolation between the AC source and the second AC / DC converter module so that the second AC / DC converter module can have a different reference potential. Further, the transformer provides galvanic isolation between the second AC / DC converter module and the AC source, thereby providing protection from faults and voltage fluctuations from one side of the transformer that may affect the other side.

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

[0029] Therefore, it will be understood that the present disclosure is not limited to the specific component parts of the device described, but may vary.It will also be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0031] Figure 1 An energy storage system including a plurality of second AC / DC converters is illustrated.

[0032] Figure 2 The diagram shows an energy storage system having both a DC / DC converter and an AC / DC converter.

[0033] Figure 3 An energy storage system with a transformer is illustrated.

[0034] Figure 4 An energy storage system with a common DC link is illustrated.

[0035] Figure 5 An energy storage system of an isolated DC / DC converter with multiple ports is illustrated.

[0036] Figure 6 An energy storage system of an isolated AC / DC converter with multiple ports is illustrated.

[0037] Figure 7 The bipolar DC side of the first converter is illustrated, wherein the bipolar DC side has two energy storage systems. DETAILED DESCRIPTION

[0038] Now, the present disclosure will be described below with reference to the accompanying drawings, in which the currently preferred embodiments of the present disclosure are shown. However, the present disclosure can be implemented in many different forms and should not be construed as limited to the embodiments set forth herein; on the contrary, these embodiments are provided for thoroughness and completeness and to fully convey the scope of the present disclosure to the skilled person.

[0039] Figure 1 An energy storage system 1 connected to an alternating current (AC) grid 2 is illustrated. Three connecting lines 2a-2c corresponding to the three phases of the AC grid 2 are connected to the first converter 10 of the energy storage system 1 at one side of the first converter 10. The other side of the first converter 10 is connected to a first terminal 3 and a second terminal 4 at the direct current (DC) side. Each of the connecting lines 2a-2c for the three phases is connected to an upper arm 12a-12c and a lower arm 14a-14c of the converter 10. Each of the upper arms 12a-12c is connected to the first terminal 3, and each of the lower arms 14a-14c is connected to the second terminal 4. The first terminal 3 can be regarded as a first DC pole, and the second terminal 4 can be regarded as a second DC pole. Each arm 12a-12c, 14a-14c includes a plurality of converter units 16. The plurality of converter units 16 perform switching operations to convert the incoming power / voltage between AC and DC. The first converter 10 can be, for example, a modular multilevel converter. Figures 2 to 7The first converter 10 in FIG. 1 represents a converter similar to the converter described above. However, to facilitate understanding of these figures, Figures 2 to 7 In the figure, only the converter is represented by the block 10.

[0040] exist Figure 1 In the embodiment, the energy storage system 1 further includes a plurality of energy storage strings 20 , which are connected in parallel to the first converter 10 and are connected in parallel to each other between the first terminal 3 and the second terminal 4 . Figure 1 Three energy storage strings 20 are disclosed. However, as illustrated by the dashed lines, there may be Figure 1 All dashed lines in the figure indicate that more components of the same type can be added in between. Therefore, the dashed lines also illustrate that the energy storage string may include more than Figure 1 Each of the plurality of energy storage strings 20 includes a plurality of energy storage units 22 connected in series. By alternating a certain number of energy storage strings 20 connected in parallel and a certain number of energy storage units 22 connected in series, the energy storage system 1 can be designed to have different voltage ratings and current ratings.

[0041] The energy storage string 20 is connected to the second AC / DC converter module 30. The second AC / DC converter module 30 may include a plurality of second AC / DC converters or different types of AC / DC converters. In the following description, the second AC / DC converter module 30 will be described for each figure by describing different configurations of the second AC / DC converter.

[0042] exist Figure 1 In the embodiment, each of the energy storage strings 20 is connected in series to a corresponding second AC / DC converter 30. In other words, the second AC / DC converter module includes 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, an IGBT, or a MOSFET transistor. The second AC / DC converter 30 is configured to receive AC power from an AC source 6. Figure 1 In the example shown in , the AC source 6 may be a feed from an external AC source 6. In other examples, AC power may be provided to the second AC / DC converter from an AC grid.

[0043] The second AC / DC converter 30 connecting the energy storage string 20 to the AC source 6 allows the energy storage unit 22 to be recharged using energy from the AC source 6 (which energy is provided by the second AC / DC converter 30). Therefore, when the stored energy has been provided to the AC grid 2, it is not necessary to replace the energy storage unit 22. The AC source 6 provides an energy buffering function to the energy storage system 1, so that the energy rating and the ampere-hour (Ah) rating can be regarded as infinite. Therefore, energy is buffered on the AC source 6 instead of being stored in the energy storage unit.

[0044] 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 the converter unit 16. Thereby, the control unit 50 controls how much power is provided from the energy storage system 1 to the AC grid 2. The control unit 50 is configured to control the first converter 10 so that the power reference of the AC grid 2 is followed. In other words, the first converter 10 can be controlled by the control unit 50 to compensate for the power flow so that the output of the energy storage system 1 as a whole can operate synchronously with the frequency and voltage of the AC grid 2. The second AC / DC converter 30 can be configured to process a portion of the power reference of the AC grid 2. In other words, the second AC / DC converter 30 can be a fractional converter, which is configured to change the voltage and / or frequency by a portion. The fractional converter allows precise control of the voltage and frequency of the power provided to the AC grid 2.

[0045] Further, each AC / DC converter 30 may be controlled to balance the charge levels at the energy storage strings 20 so that the energy storage strings 20 are in balance with each other.

[0046] refer to Figure 2 , further embodiments of the inventive concept are described.

[0047] exist Figure 2 In FIG. 1 , the first converter 10 is connected to a grid transformer 8 , which can provide galvanic insulation between the energy storage system 1 and the AC grid 2 . Figure 2In the embodiment, the energy storage string 20 is provided with a corresponding DC / DC converter 24, which is connected between the second AC / DC converter 30 and the plurality of energy storage units 22. In other words, the DC / DC converter is connected between the first terminal 3 and the plurality of energy storage units 22 in each corresponding energy storage string 20, so that each energy storage string is provided with a corresponding DC / DC converter 24. The DC / DC converter 24 allows the DC power to be controlled to the energy storage string 20 so that it can be optimized for each energy storage string 20. Therefore, the DC power provided to each individual energy storage string 20 can be adapted to suit the charging state and requirements of each energy storage string 20. In some configurations, the DC / DC converter 24 can be a fractional converter, and the second AC / DC converter 30 can be a normal AC / DC converter.

[0048] exist Figure 2 , each of the second AC / DC converters 30 receives AC power from the AC grid 2 through an additional winding 9 of the grid transformer 8. Thus, the AC grid 2 acts as an AC source or AC provided to the second AC / DC converters 30. In this example, the AC source 6 can be considered as a feed from the AC grid 2. The second AC / DC converter transforms the AC power into DC power and transmits the DC power to the DC / DC converter 24, which adapts the DC power to the corresponding energy storage string 20 of the DC / DC converter 24. As a further example, in a STATCOM station, there may be an auxiliary transformer that provides power to a low voltage device that can be used as an AC source.

[0049] When the AC source is the AC grid 2 , the control unit 50 may be configured to control the first converter 10 such that the first converter 10 compensates for the power flow between the AC source 6 and the energy storage string 20 to follow the power reference of the AC grid 2 .

[0050] exist Figure 3 In FIG. 1 , the first converter 10 connects the DC side to the AC grid 2 via the grid transformer 8. Each of the energy storage strings 20 connected between the first terminal 3 and the second terminal 4 comprises a respective DC / DC converter 24 connected to a respective second AC / DC converter 30. Figure 3 In the embodiment of the present invention, there is a corresponding transformer 40 connected between each of the second AC / DC converters 30 and the AC source 6. The transformer 40 can provide electrical isolation between the AC source 6 and the second AC / DC converter 30, so that the second AC / DC converter 30 can have a different reference potential. Further, the transformer 40 provides galvanic isolation between the second AC / DC converter 30 and the AC source 6, thereby providing protection from faults and voltage fluctuations from one side of the transformer 40, which may affect the other side.

[0051] Steering Figure 4 , describes another embodiment of the inventive concept.

[0052] exist Figure 4 In the embodiment, 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, so 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 source 6. Since power flow is possible at the common DC link 26, it is possible to eliminate power flow in the common DC link 26, which can allow the rating of the second AC / DC converter 30 to be lower than the total rating of the DC / DC converter 24.

[0053] refer to Figure 5 , describes another embodiment of the present disclosure.

[0054] exist Figure 5 In the embodiment, the plurality of DC / DC converters 24 are replaced with an isolated DC / DC converter 25 having a plurality of ports. In other words, a multi-port DC / DC converter is used to replace the plurality of DC / DC converters 24. Each energy storage string 20 is connected to a corresponding port of the isolated DC / DC converter 25. With the isolated DC / DC converter 25, only a single second AC / DC converter 30 is required. The second AC / DC converter 30 is connected between the isolated DC / DC converter 25 and the AC source 6.

[0055] refer to Figure 6 , describes yet another embodiment of the present disclosure.

[0056] exist Figure 6 In the embodiment, the energy storage string 20 is directly connected to the corresponding port of the isolated AC / DC converter 32. In other words, Figure 6 The energy storage system of the embodiment depicted in does not include any DC / DC converter. However, in some other embodiments, a DC / DC converter may be provided for each energy storage string 20. The insulated AC / DC converter 32 may also be represented as a multi-port AC / DC converter. The use of an insulated AC / DC converter reduces the components required in the energy storage system 1.

[0057] Figures 1 to 6 The energy storage system 1 in can be regarded as a unipolar DC side, where the first terminal 3 is the first DC pole and the second terminal 4 is the second DC pole. Figure 7, a bipolar DC side is illustrated. The bipolar DC side includes a first terminal 3 (representing a positive DC pole) and a second terminal 4 (representing a negative DC pole). A third terminal 5 is provided, which represents grounding. In other words, the bipolar DC side includes a side with a positive polarity and a side with a negative polarity. The AC grid 2 is connected to two energy storage systems 1a, 1b through a transformer 8. The first of the energy storage systems 1a is connected between the first terminal 3 and the third terminal 5 with a positive polarity, and the second of the energy storage system 1b is connected between the second terminal 4 and the third terminal 5 with a negative polarity. The transformer 8 provides AC power to a first converter 10a arranged at one side of the bipolar DC side and another first converter 10b arranged at the other side of the bipolar DC side. Each energy storage system 1a, 1b includes 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 to the first converter 10a in parallel. The energy storage strings 20b of the second energy storage system 1b are connected in parallel to each other and to the first converter 10b in parallel. Each of the energy storage strings 20a, 20b includes a plurality of energy storage units 22a, 22b connected in series at each energy storage string 20a, 20b. Each energy storage string 20a, 20b is connected to a corresponding second AC / DC converter 30a, 30b, which is connected to an AC source 6. Figure 7 The first energy storage system 1a and the second energy storage system 1b are Figure 1 Thus, any energy storage system 1 described herein can be used at a bipolar DC side by connecting one at each polarity side. In some cases, only one polarity side of a bipolar DC side may be equipped with an energy storage system 1 .

[0058] It should be noted that in each of these figures, there may be components that may be added or changed. Figure 2 As shown in FIG. 1 , the use of the AC grid 2 as the AC source is also applicable to the reference Figure 1 as well as Figures 3 to 7 Further, in the embodiments described with reference to any of the accompanying drawings, a second AC / DC converter 30 and an AC source 5 may be used. Figure 3 The transformer 40 presented in FIG. 40 can be used even when the AC source 6 is the AC grid 2.

[0059] Those skilled in the art will recognize that the present disclosure is by no means limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. Additionally, from a study of the drawings, the disclosure, and the appended claims, the skilled person can understand and implement variations of the disclosed embodiments when practicing the claimed invention.

Claims

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

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

3. The energy storage system according to claim 2, wherein: If the AC source is fed from the AC grid, the control unit is further configured to control the first converter to compensate for power flow between the plurality of energy storage strings and the AC source such that the first converter follows the power reference of the AC grid.

4. An energy storage system according to any one of the preceding claims, wherein: The first converter is a modular multilevel converter (MMC).

5. The energy storage system according to any one of claims 2 to 4, 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. An energy storage system according to any one of the preceding claims, wherein: The energy storage string further comprises a DC / DC converter (24) connected between the at least one second AC / DC converter module and the plurality of series-connected energy storage cells of the energy storage string.

7. The energy storage system according to claim 6, wherein: Each DC / DC converter is connected to a common DC link (26), 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 source.

8. The energy storage system according to any one of claims 1 to 6, wherein: The at least one second AC / DC converter module includes a plurality of second AC / DC converters, wherein the second AC / DC converters are connected to corresponding energy storage strings.

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, wherein each energy storage string is connected to a corresponding port of the isolated AC / DC converter.

10. The energy storage system according to any one of claims 1 to 4, further comprising an isolated DC / DC converter (25) having a plurality of ports, wherein: Each energy storage string is connected to a corresponding port of the isolated DC / DC converter, and Wherein, the at least one second AC / DC converter module is an AC / DC converter connected to the isolated DC / DC converter.

11. An energy storage system according to any one of the preceding claims, wherein: At least one transformer (40) is connected between the at least one second AC / DC converter module and the AC source.

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, wherein each transformer is connected between the AC source and a corresponding second AC / DC converter.

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