Energy storage system for a direct current transmission system and method for exchanging energy with a direct current transmission system

Abstract

Disclosed herein is an energy storage system (1) for a direct current, DC, power transmission system, the energy storage system being configured to be connected to a DC link (2). The energy storage system comprises a first system terminal (10), a second system terminal (20), a first converter (30) connected to the first system terminal, and a second converter (40) connected to the first converter and the second system terminal. The energy storage system further comprises an alternating current, AC, loop device (50) providing an AC path, and a plurality of energy storage devices (60) connected in parallel with the second converter, the energy storage devices comprising a battery cell (62) with a power electronic switch (63) and an energy storage element (64) connected to the battery cell, wherein the battery cell is individually switchable. The disclosure further relates to a method (200) for providing energy storage to a DC power transmission system.

Description

Technical Field

[0001] This disclosure relates to an energy storage system for a DC transmission system and a method for storing energy in a DC transmission system. Background Technology

[0002] The increase in renewable energy generation in the power grid system has led to an increase in energy sources such as wind and solar power. The replacement of conventional power generation with renewable energy sources results in a reduction in the inertia of the power system. This reduction in inertia leads to decreased grid stability and an inability to cope with sudden changes in electricity consumption or production. Such changes could be, for example, power outages by large consumers or producers.

[0003] To support the inertia of power systems, inertial simulations have been performed using power electronic devices. One example of such simulation is through power electronic converters (such as AC-DC converters, which behave similarly to synchronous machines), where the DC-DC converter is placed on the DC side to interface with energy storage devices. By providing energy storage capabilities to the power electronic converters, they can extract or inject power into the power system to mimic inertia.

[0004] However, existing DC-DC converters with energy storage capabilities are insufficient in terms of design flexibility. For the power grid, a DC-DC converter with higher availability and reliability is needed. Furthermore, a DC-DC converter with higher transmission capacity and lower converter stress is required. Summary of the Invention

[0005] In view of the above, the purpose of this disclosure is to provide an improved energy storage system for DC transmission systems.

[0006] Another objective of this disclosure is to provide an energy storage system that is more flexible in design and has a higher power transmission capacity.

[0007] Another objective is to provide an energy storage system with higher reliability and availability.

[0008] The purpose of this disclosure is also to provide a more cost-effective energy storage system.

[0009] To achieve at least one of the foregoing objectives and others as will become apparent from the following description, an energy storage system as defined in claim 1 is provided according to this disclosure. Preferred variations of the energy storage system will become apparent from the dependent claims.

[0010] More specifically, according to a first aspect of this disclosure, an energy storage system (ESS) for a direct current (DC) transmission system is provided, the energy storage system being configured to connect to a DC link of the DC transmission system. The energy storage system includes a first system terminal and a second system terminal, a first converter connected to the first system terminal, a second converter connected to the first converter and the second system terminal, an AC loop device (providing an AC path) connected to the first system terminal and the second system terminal and in parallel with the first and second converters, and a plurality of energy storage devices connected in parallel with the second converter and in series with the first converter. Each energy storage device includes a battery cell with a power electronic switch and an energy storage element connected to the battery cell, wherein the battery cell can be switched individually.

[0011] By connecting multiple energy storage devices in parallel with a second converter, power exchange with the DC transmission system can be flexibly controlled. For example, the DC voltage across the multiple energy storage devices can be controlled to be significantly lower than the full DC voltage of the DC transmission system, and the number of energy storage devices can be kept at a reasonable level. Furthermore, due to the individual switching of the battery cells in the energy storage devices, the voltage delivered by the energy storage system is less dependent on the minimum permissible state of charge of the energy storage devices. Therefore, a certain degree of freedom is introduced into the energy storage system.

[0012] Energy storage systems can be used in medium-voltage direct current (MVDC) transmission systems or high-voltage direct current (HVDC) transmission systems. By connecting the energy storage system to the DC link, energy support functions can be provided, such as absorbing energy through charging when there is excess power in the DC transmission system, or discharging and supplying power to the DC transmission system when there is a power shortage.

[0013] The first converter and the second converter can be a full-bridge arrangement, a half-bridge arrangement, or a combination thereof.

[0014] The energy storage system may further include an energy storage device controller. The first and second converters may be configured to control a first DC voltage between the first and second system terminals, a second DC voltage between the interconnection point of the first and second converters and the second system terminal, and first and second AC voltages across the first and second converters (30, 40), respectively. The energy storage device controller may be configured to control the insertion or bypass of each battery cell into an energy storage element, thereby controlling power exchange with multiple energy storage devices and adapting the energy storage voltage, which is the total output voltage of the multiple energy storage devices, to the second DC voltage and the second AC voltage.

[0015] Energy storage device controllers improve the control of energy storage devices. They can control these devices based on information such as the state of charge of each device and / or the power balance in the DC transmission system. When needed, the controller can communicate with other components in the DC transmission system. An example of such a component could be a chopper resistor controller. Using information from other components to achieve efficient control of the energy storage system can be advantageous.

[0016] At least one of the first converter and the second converter can generate AC current, which can be allowed to circulate through the first converter, the second converter, and the AC loop device.

[0017] AC current may be advantageous because it provides power balance between the first and second converters.

[0018] Each of the first and second converters can be a modular multilevel converter.

[0019] Modular multilevel converters are examples of advanced voltage source converters suitable for operation in energy storage systems. Modular multilevel converters offer advantages due to their high output performance quality and lower requirements on the voltage and current ratings of the power switches.

[0020] AC circuit equipment may include at least one of the arrangements of passive components and arrangements of active components.

[0021] A filter is provided by combining components of an AC loop device. The filter can be tuned at the AC frequency of the circulating AC current to provide a path for the circulating AC current by offering a low-impedance path. Examples of passive components are inductors and capacitors, while examples of passive components are individual battery cells.

[0022] Each energy storage element can be a supercapacitor and / or a battery.

[0023] Supercapacitors and batteries are well-suited for use in energy storage systems within DC transmission systems. Supercapacitors are appropriate for DC transmission systems where rapid power generation from energy storage devices is required. Their rapid charging and discharging times allow them to provide power during DC transmission system power shortages. Supercapacitors are also suitable for energy storage systems operating at varying temperatures, and can function over a wide temperature range.

[0024] Batteries may be suitable for DC transmission systems where energy storage systems need to provide power over extended periods. Batteries can store relatively large amounts of energy and exhibit low energy loss over time.

[0025] The energy storage system may further include a braking device and a switching structure. The braking device includes a braking resistor arranged in parallel with multiple energy storage devices, and the switching structure includes a first switch connected in series with multiple energy storage devices and a second switch connected in parallel with the braking resistor.

[0026] When the state of charge of the energy storage device has reached 100% and there is excess energy in the DC transmission system, it is advantageous for the energy storage system to dissipate power from the DC transmission system. A resistor and switch structure achieves this braking operation of the DC transmission system by using a switch to guide current through the resistor. Thus, even when the state of charge of the energy storage device is 100%, the energy storage system can still dissipate power.

[0027] The energy storage system may further include a braking device and a switching structure. The braking device includes a braking resistor arranged in parallel with multiple energy storage devices. The switching structure includes a first switch connected in series with multiple energy storage devices and a power electronic switch, wherein the power electronic switch is connected in series with the braking resistor.

[0028] Multiple energy storage devices may include a branch consisting of several energy storage devices connected in series.

[0029] By connecting several energy storage devices in series, the voltage provided by the energy storage system can be altered to adapt to the voltage of a DC transmission system. Furthermore, by connecting several energy storage devices in series, the energy storage system can be designed in various ways to suit DC transmission systems. As an example, the energy storage system can be designed to achieve the required voltage rating by connecting the desired number of energy storage devices in series in a single branch. To adapt the energy storage system to the energy storage requirements of a DC transmission system, several branches can exist, each containing energy storage devices connected in series, and these branches can be connected in parallel.

[0030] The energy storage may further include an AC suppression circuit, which is connected in series with multiple energy storage devices between the energy storage devices and the first and second converters.

[0031] AC suppression circuits can be used to prevent AC current from entering energy storage devices. AC suppression circuits can be advantageous because they provide protection for energy storage devices and also eliminate the need for the DC ESS voltage to follow the AC voltage to limit AC current entering the energy storage device. AC suppression circuits can be designed as notch filters, incorporating an inductor and a capacitor connected in parallel.

[0032] The DC link may have a first pole and a second pole. The energy storage system may further include a third converter, a fourth converter, and a plurality of other energy storage devices interconnected in a manner similar to the first converter, the second converter, and the plurality of energy storage devices, and include a third system terminal and a fourth system terminal that are respectively similar to the first system terminal and the second system terminal, wherein the first system terminal is connected to the first pole, the second system terminal is connected to ground, the third system terminal is connected to the second pole, and the fourth system terminal is connected to ground.

[0033] The third and fourth converters, along with several additional energy storage devices, allow the energy storage system to operate not only in unipolar DC links but also in bipolar and same-pole DC links. This introduces additional flexibility to the use cases of the energy storage system. When the energy storage system is grounded, it also allows each pole to operate independently when attached to a bipolar DC link.

[0034] According to a second aspect of this disclosure, a method for exchanging energy with a DC transmission system via an energy storage system is provided, the energy storage system being configured to connect to a DC link of the DC transmission system. The energy storage system includes: a first system terminal and a second system terminal, a first converter connected to the first system terminal, a second converter connected to the first converter and the second system terminal, an AC loop device (providing an AC path) connected to and in parallel with the first and second system terminals, and a plurality of energy storage devices connected in parallel with the second converter and in series with the first converter. Each energy storage device includes a battery cell having a power electronic switch and an energy storage element connected to the battery cell, wherein the battery cell is individually switchable. The method includes: absorbing energy from the DC transmission system by controlling at least one of the plurality of energy storage devices to absorb energy from the energy storage element; and releasing electrical energy from the energy storage system to the DC transmission system by controlling at least one of the energy storage devices to release energy from the energy storage element.

[0035] The effects and features of the second aspect can be substantially similar to those described above in conjunction with the first aspect. The embodiments mentioned in the first aspect can be at least substantially compatible with the second aspect. It should further be noted that, unless explicitly stated otherwise, this disclosure relates to all possible combinations of features.

[0036] The method may further include: controlling a first DC voltage between a first system terminal and a second system terminal and a second DC voltage between the interconnection point of the first and second converters and the second system terminal via a first converter and a second converter; controlling a first AC voltage across the first converter and a second AC voltage across the second converter; and controlling each battery cell to be inserted into or bypassed by an energy storage element, thereby controlling power exchange with multiple energy storage devices and adapting the total output voltage of the multiple energy storage devices to the second DC voltage and the second AC voltage.

[0037] The method may further include generating AC current and circulating the AC current through a first converter, a second converter, and an AC loop device.

[0038] The method may further include controlling the total voltage across the energy storage device to follow the amplitude variation of the modulated AC voltage across the second converter.

[0039] By following the AC voltage across the second converter, AC current is prevented from entering the energy storage device. This step is advantageous because it allows the energy storage system to be designed with limited or no AC limiting circuitry, or to use a DC choke instead of the AC limiting circuitry.

[0040] The further scope of this disclosure will become apparent from the detailed description given below. However, it should be understood that while the detailed description and specific examples indicate preferred embodiments of this disclosure, they are given by way of illustration only, as various variations and modifications within the scope of this disclosure will become apparent to those skilled in the art from this detailed description.

[0041] Therefore, it should be understood that this disclosure is not limited to the specific components of the described apparatus or the steps of the described method, as such apparatus and methods can vary. It should also be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. Attached Figure Description

[0042] The present disclosure will be described in more detail by way of example and with reference to the accompanying drawings, which illustrate exemplary embodiments of the present disclosure.

[0043] Figure 1 The diagram shows an enlarged view of the energy storage system and energy storage device in a unipolar DC link.

[0044] Figure 2 The diagram illustrates some of the voltages and currents that occur in an energy storage system.

[0045] Figure 3 The diagram illustrates the energy storage system in a bipolar DC link.

[0046] Figure 4 The diagram illustrates an energy storage system with a combination of alternative components.

[0047] Figure 5 The illustration shows a method for providing energy storage to a DC transmission system. Detailed Implementation

[0048] This disclosure will now be described more fully with reference to the accompanying drawings, in which presently preferred embodiments of the disclosure are shown. However, this disclosure may be implemented in many different forms and should not be construed as limiting itself to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness and to fully convey the scope of this disclosure to those skilled in the art.

[0049] Figure 1 An energy storage system ESS1 for a DC transmission system is illustrated, configured to be connected to a DC link 2 of the DC transmission system. The DC link can be a unipolar DC link with a positive pole 4. Other types of DC links are also feasible, such as different types of unipolar and bipolar DC links. The energy storage system 1 can be used in a medium-voltage DC MVDC transmission system or a high-voltage DC HVDC transmission system. By connecting the energy storage system 1 to the DC link, energy support functions can be provided, such as absorbing energy through charging when there is excess power in the DC transmission system, or discharging and supplying power to the DC transmission system when there is a power shortage. Therefore, the energy storage system 1 can act as a balancer for the DC transmission system when the power balance changes abruptly, for example, due to an inertial simulation provided by an AC / DC converter to the AC system.

[0050] Energy storage system 1 includes a first system terminal 10 and a second system terminal 20. The first system terminal 10 is configured to connect to the positive terminal 4 of DC link 2. The second system terminal 20 is configured to connect to ground 4. A first converter 30 is connected to the first system terminal 10. A second converter 40 is connected to the first converter 30 and the second system terminal 20. The first converter 30 and the second converter 40 are interconnected at an interconnection point 35. In this embodiment, the first converter 30 and the second converter 40 are modular multilevel converters, wherein the first converter 30 has a full-bridge arrangement and the second converter 40 has a half-bridge arrangement. Modular multilevel converters are suitable because they provide high output performance quality and have lower requirements for the voltage and current ratings of the power switches. However, in other embodiments, other types of converters can be used, and both converters can be either full-bridge or half-bridge arrangements. Those skilled in the art can adapt the converters to the requirements of the DC link and the DC transmission system.

[0051] AC loop device 50 is connected to the first system terminal 10 and the second system terminal 20, and is connected in parallel with the first converter 30 and the second converter 40. AC loop device 50 provides an AC path. Preferably, AC loop device 50 has low impedance to a specific AC current frequency. The AC path provides an AC loop in which AC current can flow to balance the first converter 30 and the second converter 40, which will combine... Figure 2 Further description is required.

[0052] Multiple energy storage devices 60 are connected in parallel with the second converter 40 and in series with the first converter 30. Therefore, the multiple energy storage devices 60 are connected directly or via additional components to the interconnection point 35 (as illustrated below) and directly or via additional components to the second system terminal 20. Each energy storage device 60 includes a battery cell 62 with a power electronic switch 63 and an energy storage element 64 connected to the battery cell 62, wherein the battery cell 62 can be switched individually. The multiple energy storage devices 60 include a branch formed by connecting several energy storage devices 60 in series. The multiple energy storage devices 60 may also include multiple branches in which the energy storage devices 60 are connected in series, and these branches are then connected in parallel with each other. With different configurations of the multiple energy storage devices 60, the energy storage system 1 can also be designed in different ways suitable for DC transmission systems. As an example, the energy storage system 1 can be designed to achieve the required voltage rating by connecting the required number of energy storage devices 60 in series into a single branch. In order to adapt the energy storage system 1 to the energy storage requirements of the DC transmission system, there may be several branches, each with energy storage devices 60 connected in series, and these branches are connected in parallel.

[0053] In this embodiment, the power electronic switches 63 of each battery cell 62 are arranged in a full-bridge configuration because this distributes stress across four power electronic switches instead of two. However, in other embodiments, these power electronic switches may be arranged in a half-bridge configuration. The power switches 63 provide controllability over the energy storage element 64, allowing the energy storage element to be switched on and off individually. The energy storage elements 64 are adapted to the DC transmission system in which they operate. As an example, supercapacitors have rapid charging and discharging times, which allows them to generate power when there is a power shortage in the DC transmission system. Therefore, supercapacitors can be suitable for DC transmission systems where a rapid generation of power from the energy storage device 60 is required. Supercapacitors are also suitable for energy storage systems 1 operating at varying temperatures because they can operate over a wide temperature range. As another example, batteries may be more suitable in DC transmission systems where the energy storage system 1 needs to provide power over longer periods. Batteries are capable of storing relatively large amounts of energy and have low energy loss over time.

[0054] The operability of the energy storage devices 60 can be individually switched, thus allowing for a distributed supply of energy storage devices 60. Distributed energy storage devices 60 can operate only a few, and these devices can be switched sequentially between rest and operation. This is advantageous because it allows the energy storage system 1 to adjust the voltage supplied by multiple energy storage devices 60. For example, the DC voltage across multiple energy storage devices 60 can be controlled to be significantly lower than the full DC voltage of the DC transmission system, and the number of energy storage devices 60 can be kept at a reasonable level. Furthermore, multiple energy storage devices 60 can compensate for voltage drops caused by the loss of state of charge of already operated energy storage devices 60, because another energy storage device 60 can be switched to begin operation, allowing the energy storage devices 60 to more fully deplete their state of charge, thereby achieving more efficient use of the energy storage devices 60. In other words, the voltage supplied by the energy storage system 1 will be less dependent on the minimum permissible state of charge of each energy storage device 60. Therefore, a certain degree of freedom is introduced into the energy storage system 1. This degree of freedom allows for a reduction in the number of energy storage devices 60 in the energy storage system 1 while still providing the same amount of energy support to the DC transmission system. By reducing the number of energy storage devices 60, a more cost-effective energy storage system 1 can be provided. Greater flexibility is also achieved in the design of the energy storage system. The individually switchable energy storage devices 60 also allow the energy storage system 1 to reach higher voltage values, thereby enabling the energy storage system 1 to have a higher power capacity.

[0055] Go to Figure 2 This will describe some of the operating voltages and currents of energy storage system 1. Figure 2 In this embodiment, the energy storage system 1 further includes an energy storage device controller 70. The energy storage device controller 70 can be used to control multiple energy storage devices 60, such that it controls the insertion or bypass of each battery cell 62 into the energy storage element 64, thereby efficiently controlling the energy exchange between the DC transmission system and the multiple energy storage devices 60. The energy storage device controller 70 can control the energy storage devices 60 based on information such as, for example, the state of charge of each energy storage device 60 and / or the power balance in the DC transmission system. When needed, the energy storage device controller 70 can be able to communicate with other components in the DC transmission system. An example of such a component could be a chopper resistor controller. Using information from other components to achieve efficient control of the energy storage system 1 may be advantageous.

[0056] The first converter 30 and the second converter 40 are configured to control a first DC voltage V1 between the first system terminal 10 and the second system terminal 20, and a second DC voltage V2 between the interconnection point of the first and second converters and the second system terminal 20. The first DC voltage V1 may be the terminal voltage of the DC link and will be the voltage supplied across the first converter 30 and the second converter 40. The second DC voltage V2 is the voltage across the second converter 40 and also the voltage across the multiple energy storage devices 60. When the energy storage devices 60 are activated and, for example, supply power to a DC transmission system, the output DC current Ia from the multiple energy storage devices 60 will be shunt between the first converter 30 and the second converter 40 into a first DC current Ib and a second DC current Ic. Controlling the output DC current Ia controls the discharge of the multiple energy storage devices 60, thereby generating a modulated DC voltage in each converter 30, 40. However, the shunt output DC current Ia causes the power flow of the two converters to be different. Therefore, since the current flows through the converters 30, 40 in opposite directions, a power imbalance problem will exist between the first converter 30 and the second converter 40. When energy is drawn from the first converter 30, the energy is inserted into the second converter 40. Therefore, an AC path is provided by the AC loop device 50. At least one of the first converter 30 and the second converter 40 generates an AC current Iac, which is allowed to circulate between the first converter 30 and the second converter 40 and through the AC loop device 50 to achieve the opposite effect, i.e., drawing energy from the second converter 40 and inserting the energy into the first converter 30. This allows the AC voltage, shifted by 180° between the first converter 30 and the second converter 40, to also be modulated by the converters 30 and 40. The AC current Iac allows power to be transferred between the first converter 30 and the second converter 40, thus enabling the converters to achieve balance when the power balance between the converters becomes unbalanced due to the different directions and levels of the first DC current Ib and the second DC current Ic.

[0057] The output voltage of multiple energy storage devices 60 can be controlled to follow the second DC voltage V2 and the AC voltage, thereby preventing AC current from entering the energy storage device 60.

[0058] For those skilled in the art, other discharge and charge states certainly exist. One example is the opposite of the situation described above, where multiple energy storage devices 60 are typically charging, while the power balance of the first converter 30 and the second converter 40 is reversed, causing the circulating AC current Iac to result in the transfer of power from the first converter 30 to the second converter 40.

[0059] It should also be noted that the individual battery cells 62 of the energy storage device 60 can be individually controlled to simultaneously charge some of the individual energy storage elements 64, discharge some of the energy storage elements, and bypass some of the energy storage elements, or any other combination of these states.

[0060] Go to Figure 3 The diagram illustrates an energy storage system using a bipolar DC link. In other embodiments, the bipolar DC link can be other types of DC links with two poles and a ground. Figure 3 In this configuration, the energy storage system 1 is connected to a DC link 2 having a positive first pole 4 and a negative second pole 6. To achieve this, the energy storage system 1 includes a first converter 30, a second converter 40, an AC loop device 50, and multiple energy storage devices 60. These components are designed to work in conjunction with… Figure 1 The energy storage system 1 under discussion is connected in a similar manner. A first converter 30 is connected to a first system terminal 10, which is configured to connect to the positive first pole 4 of the DC link 2. A second converter 40 is connected to a second system terminal 20, which is configured to connect to ground 5. Alternative connections for the second system terminal 5 may be, for example, solid ground, high-impedance ground, or floating. The energy storage system 1 further includes a third converter 130, a fourth converter 140, additional AC loop devices 150, and additional energy storage devices 160 interconnected in a similar manner to the first converter 30, the second converter 40, the AC loop device 50, and multiple energy storage devices 60. The energy storage system 1 further includes a third system terminal 110 and a fourth system terminal 120, similar to the first system terminal 10 and the second system terminal 20. The third system terminal 110 is connected to the negative second pole 6, and the fourth system terminal 120 is connected to ground 5. This configuration of the energy storage system 1 allows the energy storage system 1 to have a second set of components with additional pole-to-ground connections. The third converter 130, the fourth converter 140, and several other energy storage devices 160 allow the energy storage system 1 to operate not only in unipolar DC links, but also in bipolar DC links and same-pole DC links. This introduces additional flexibility to the use cases of the energy storage system 1. When the energy storage system 1 is grounded, it also allows each pole to operate independently when attached to a bipolar DC link.

[0061] Go to Figure 4 , Figure 1 The energy storage system 1 is disclosed as having additional components, which may be used in some embodiments to further improve the energy storage system 1. It should be noted that... Figure 4The additional components illustrated herein can be added individually or in combination, but for efficiency and understanding of the application, all of these additional components are shown in one figure. Components described in the previous figures will no longer be described.

[0062] Figure 4 The energy storage system may further include a braking device 80 and a switching structure 84. The braking device includes a braking resistor 82 arranged in parallel with the plurality of energy storage devices 60 and in series with the second converter 40. The switching structure is configured to disconnect the plurality of energy storage devices 60 and redirect current to flow through the resistor 82. The switching structure 84 includes a first switch 86 connected in series with the plurality of energy storage devices 60 and a second switch 88 connected in parallel with the braking resistor 82. When the state of charge of the plurality of energy storage devices has reached 100% and there is excess energy in the DC transmission system, the braking device 80 can redirect the excess power to the braking resistor 82. The second switch 88 is closed during normal operation, thereby bypassing the braking resistor 82. Advantageously, the energy storage system 1 can also dissipate power from the DC transmission system when the plurality of energy storage devices 60 are fully charged. However, the braking resistor 82 can also be used when the excess power in the DC transmission system exceeds the processing capacity of the plurality of energy storage devices 60. In this configuration, braking device 80 can be used to connect energy storage device 60 and braking resistor 82, thereby dissipating additional power. Braking device 80 can be controlled by energy storage device controller 70 to achieve efficient control of energy storage system 1.

[0063] As an alternative, such as Figure 4 As shown by the dashed lines, the braking device 100 may include a braking resistor 102 arranged in parallel with a plurality of energy storage devices 60, and a switching structure 84 including a first switch 86 and a power electronic switch 104. The power electronic switch 104 is connected in series with the braking resistor 102, thereby forming a so-called link. Therefore, the link is connected in parallel with the series connection of the first switch 86 and the plurality of energy storage devices 60.

[0064] AC loop device 50 and other loop devices 150 may include an inductor 52 and a capacitor 54 connected in series. A filter is provided through the inductor 52 and capacitor 54 in AC loop device 50. The filter can be tuned to a desired AC frequency corresponding to the frequency of the circulating AC current. A tuned filter is advantageous because it can improve harmonic performance by reducing harmonic voltage and current distortion. Therefore, unwanted frequencies can be reduced or substantially eliminated. Loop device 50 and other loop devices 150 may include at least one arrangement of passive components and an arrangement of active components. For example, each of loop device 50 and other loop devices 150 may individually include an inductor and a capacitor connected in series, an inductor and several battery cells connected in series, or an inductor, a capacitor, and several battery cells connected in series. The battery cells can be used to tune the frequency at which they provide low impedance to the circulating AC current. Similarly, combinations of inductors and capacitors form AC frequency-tunable filters.

[0065] In all configurations, the energy storage system 1 may further include an AC suppression circuit 90, which is connected in series with the energy storage device 60 between the energy storage device 60 and the first converter 30 and the second converter 40. The AC suppression circuit 90 can be used to prevent circulating AC current from entering the energy storage device 60. The AC suppression circuit 90 may be advantageous because it provides protection for the energy storage device 60 and also eliminates the need for the output voltage of the energy storage device 60 to follow the AC voltage to limit AC current from entering the energy storage device 60. The AC suppression circuit 90 may be designed as a notch filter with an inductor 94 and a capacitor 92 connected in parallel, or with additional and alternative components (such as individual battery cells).

[0066] Go to Figure 5 A method 200 is provided for exchanging energy with a DC transmission system via an energy storage system 1, wherein the energy storage system is configured to be connected to a DC link 2 of the DC transmission system. The energy storage system 1 used in the method may be... Figures 1 to 4 Any one or another combination of the embodiments described herein.

[0067] Method 200 includes absorbing energy 210 from a DC transmission system by controlling at least one of a plurality of energy storage devices 60 to absorb energy from an energy storage element 64.

[0068] Method 200 further includes releasing 220 energy from energy storage system 1 to DC transmission system by controlling at least one of energy storage devices 60 to release energy from energy storage element 64.

[0069] The method may further include controlling a first DC voltage V1 between the first system terminal 10 and the second system terminal 20 and a second DC voltage V2 between the interconnection point 35 of the first converter 30 and the second converter 40 and the second system terminal 20 via the first converter 30 and the second converter 40.

[0070] The method may further include controlling a first AC voltage across the first converter and a second AC voltage across the second converter.

[0071] The method may further include controlling each battery cell 62 to insert into or bypass the energy storage element 64, thereby controlling the power exchange with the plurality of energy storage devices 60, and making the total output voltage of the plurality of energy storage devices 60 compatible with the second DC voltage V2 and the second AC voltage.

[0072] The method may further include generating a 260 AC current and circulating the AC current 270 through a first converter 30, a second converter 40, and an AC loop device 50.

[0073] The method may further include controlling the total voltage across the 280 energy storage device 60 to follow the amplitude variation of the modulated AC voltage across the second converter 40.

[0074] The first AC voltage and the second AC voltage across the first converter 30 and the second converter 40 are modulated such that the second AC voltage is phase-shifted by 180 degrees relative to the first AC voltage. Thus, the first and second AC voltages interact with the circulating AC current Iac to dissipate energy from the capacitors inherent in the submodules or battery cells of the second converter 40 and inject the same amount of energy into the capacitors of the first converter 30.

[0075] Those skilled in the art will recognize that this disclosure is by no means limited to the preferred embodiments described above. Rather, various modifications and variations are possible within the scope of the appended claims. Furthermore, through a study of the drawings, this disclosure, and the appended claims, those skilled in the art can understand and implement variations of the disclosed embodiments when practicing the claimed invention.

Claims

1. An energy storage system (1) for a direct current (DC) transmission system, the energy storage system (1) being configured to be connected to a DC link of the DC transmission system, the energy storage system (1) comprising: First system terminal (10) and second system terminal (20). The first converter (30) is connected to the first system terminal (10). The second converter (40) is connected to the first converter (30) and the second system terminal (20). An AC loop device (50) is provided, which is connected to the first system terminal (10) and the second system terminal (20) and is connected in parallel with the first converter (30) and the second converter (40). The AC loop device (50) provides an AC path. Multiple energy storage devices (60) are connected in parallel with the second converter (40) and in series with the first converter (30). Each energy storage device (60) includes a battery cell (62) with a power electronic switch (63) and an energy storage element (64) connected to the battery cell (62), wherein the battery cell can be switched on and off individually.

2. The energy storage system (1) according to claim 1, comprising an energy storage device controller (70). in, The first converter (30) and the second converter (40) are configured to control a first DC voltage (V1) between the first system terminal (10) and the second system terminal (20) and a second DC voltage (V2) between the interconnection point (35) of the first converter and the second converter and the second system terminal (20), and to control the first AC voltage and the second AC voltage across the first and second converters (30, 40) respectively. The energy storage device controller (70) is configured to control each battery cell (62) to be inserted into or bypass the energy storage element (64), thereby controlling the power exchange with the plurality of energy storage devices (60) and making the energy storage voltage, which is the total output voltage of the plurality of energy storage devices, compatible with the second DC voltage (V2) and the second AC voltage.

3. The energy storage system (1) according to claim 1 or 2, wherein, At least one of the first converter (30) and the second converter (40) generates an AC current that is allowed to circulate through the first and second converters (30, 40) and the AC loop device (50).

4. The energy storage system (1) according to claim 1 or 2, wherein, Each of the first converter (30) and the second converter (40) is a modular multilevel converter.

5. The energy storage system (1) according to claim 1 or 2, wherein, The AC circuit device (50) includes at least one of an arrangement of passive components and an arrangement of active components.

6. The energy storage system (1) according to claim 1 or 2, wherein, Each energy storage element (64) is a supercapacitor and / or a battery.

7. The energy storage system (1) according to claim 1 or 2, further comprising a braking device (80) and a switching structure (84), the braking device comprising a braking resistor (82) arranged in parallel with the plurality of energy storage devices (60) and in series with the second converter (40), wherein, The switching structure includes a first switch (86) connected in series with the plurality of energy storage devices (60) and a second switch (88) connected in parallel with the braking resistor (82). The switching structure (84) is configured to disconnect the plurality of energy storage devices (60) and redirect current to flow through the braking resistor (82).

8. The energy storage system (1) according to claim 1 or 2, further comprising a braking device (100) and a switching structure (84), the braking device comprising a braking resistor (102) arranged in parallel with the plurality of energy storage devices (60), and the switching structure comprising a first switch (86) connected in series with the plurality of energy storage devices (60) and a power electronic switch (104), wherein, The power electronic switch (104) is connected in series with the braking resistor (102).

9. The energy storage system (1) according to claim 1 or 2, wherein, The plurality of energy storage devices (60) includes a branch consisting of several energy storage devices (60) connected in series.

10. The energy storage system (1) according to claim 1 or 2, wherein, The DC link has a first pole and a second pole. The energy storage system (1) further includes a third converter (130), a fourth converter (140), and a plurality of other energy storage devices (160) interconnected in the same manner as the first converter (30), the second converter (40), and the plurality of energy storage devices (60). It also includes a third system terminal (110) and a fourth system terminal (120), wherein the first system terminal (10) is connected to the first pole, the second system terminal (20) is connected to ground, the third system terminal (110) is connected to the second pole, and the fourth system terminal (120) is connected to ground.

11. A method (200) for exchanging energy with a DC transmission system via an energy storage system (1), said energy storage system being configured to be connected to a DC link of said DC transmission system, wherein, The energy storage system (1) includes: First system terminal (10) and second system terminal (20). The first converter (30) is connected to the first system terminal (10). The second converter (40) is connected to the first converter (30) and the second system terminal (20). AC loop device (50), the AC loop device being connected to the first system terminal (10) and the second system terminal (20) and connected in parallel with the first converter (30) and the second converter (40), the AC loop device (50) providing an AC path, and Multiple energy storage devices (60) are connected in parallel with the second converter (40) and in series with the first converter (30). Each energy storage device (60) includes a battery cell (62) having a power electronic switch (63) and an energy storage element (64) connected to the battery cell (62), wherein the battery cell (62) can be switched on and off individually, and the method includes: Energy is absorbed (210) from the DC transmission system by controlling at least one of the plurality of energy storage devices (60) to absorb energy from the energy storage element (64), and Energy is released (220) from the energy storage system (1) to the DC transmission system by controlling at least one of the energy storage devices (60) to release energy from the energy storage element (64).

12. The method (200) according to claim 11, comprising: The first DC voltage between the first system terminal (10) and the second system terminal (20) and the second DC voltage (V2) between the interconnection point of the first converter (30) and the second converter (40) and the second system terminal (20) are controlled by the first converter and the second converter (230). Control (240) the first AC voltage across the first converter and the second AC voltage across the second converter, and Control (250) each battery cell (62) to insert into or bypass the energy storage element (64), thereby controlling the power exchange with the plurality of energy storage devices (60) and making the total output voltage of the plurality of energy storage devices (60) compatible with the second DC voltage (V2) and the second AC voltage.

13. The method (200) according to claim 11 or 12, comprising: Generate (260) AC current; as well as The AC current is circulated (270) through the first converter (30) and the second converter (40) and the AC loop device (50).

14. The method (200) according to claim 13, comprising: Control (280) the total voltage across the energy storage device (60) to follow the amplitude variation of the modulated AC voltage across the second converter (40).

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