Direct-hanging energy storage device and direct-current power transmission system

Abstract

The invention provides a direct-hanging energy storage device which is connected between a neutral line and a direct-current power transmission line of a flexible direct-current power transmission system and comprises a first number of half-bridge sub-modules and a second number of full-bridge sub-modules which are connected in series. Each of the half-bridge sub-modules and the full-bridge sub-modules comprises an energy storage unit, and the total number of the first number and the second number meets the energy constraint and the voltage constraint at the same time; the energy constraint is that the total energy provided by the direct-hanging energy storage device is greater than or equal to a set total energy threshold value when all the energy storage units are reduced to a voltage lower limit, and the voltage constraint is that the total voltage provided when all the energy storage units are reduced to the voltage lower limit is greater than a set voltage corresponding to the direct-current power transmission line. The device can realize multiple functions of inertia support, frequency modulation support, peak load shifting, energy balance extreme scene black start and the like for a direct current power transmission system.

Description

Technical Field

[0001] This application relates to the technical field of DC power transmission, and particularly to a directly-connected energy storage device and a DC power transmission system. Background Art

[0002] With the development of science and technology, the power system has undergone and will continue to undergo profound changes. The proportion of new energy and power electronic devices in the source-grid-load side has increased, and the power system is gradually transitioning to the "dual-high" stage. The composition and characteristics of the power system have changed profoundly. In addition to traditional problems such as stability, power flow control, and voltage control, it also faces many new challenges: on the one hand, with the access of large-scale new energy, energy storage and other power electronic devices, the proportion of the installed capacity of synchronous generators in the power system has been continuously decreasing, and the system is developing towards low inertia and low damping. The regulation ability of the power system has seriously declined, the anti-disturbance ability is insufficient, and the safe and stable operation faces greater pressure; on the other hand, with the increase in the proportion of new energy power generation and the long-distance transmission of large-scale new energy power generation through high-voltage DC power transmission, there are also new major challenges in the energy balance on multiple time scales.

[0003] DC projects require grid-friendly energy storage devices that can provide support in aspects such as inertia support, frequency modulation support, peak shaving and valley filling, new energy consumption, black start in extreme scenarios, and rapid power increase and decrease. However, there is currently a lack of relevant research and applications that can support the stability and reliability of DC project power transmission. Summary of the Invention

[0004] The purpose of this application aims to at least solve one of the above technical defects, and particularly provides a DC energy storage device that can support the stability and reliability of DC project power transmission.

[0005] In a first aspect, this application provides a directly-connected energy storage device, which is connected between the neutral line and the DC transmission line of a flexible DC power transmission system. The directly-connected energy storage device includes a first number of half-bridge sub-modules and a second number of full-bridge sub-modules connected in series;

[0006] Both the half-bridge sub-module and the full-bridge sub-module include energy storage units. The total number of the first number and the second number simultaneously satisfies the energy constraint and the voltage constraint; the energy constraint means that when all the energy storage units are reduced to the lower limit of the voltage, the total energy provided by the directly-connected energy storage device is greater than or equal to the set total energy threshold, and the voltage constraint means that the total voltage provided when all the energy storage units are reduced to the lower limit of the voltage is greater than the set voltage corresponding to the DC transmission line.

[0007] In one embodiment, the first lower limit corresponding to the energy constraint is determined by the following expression:

[0008]

[0009] Among them, is the set total energy threshold, is the normal voltage value after the energy storage unit is fully charged, is the lower voltage limit, is the first lower limit, is the capacitance value of the energy storage unit.

[0010] In one embodiment, the second lower limit corresponding to the voltage constraint is determined by the following expression:

[0011]

[0012] Among them, is the set voltage, is the second lower limit.

[0013] In one embodiment, the total quantity is greater than or equal to the maximum value of the first lower limit and the second lower limit.

[0014] In one embodiment, the second quantity satisfies the DC overhead line fault self-clearing constraint condition, and the self-clearing constraint condition is that when the energy storage units in the full-bridge sub-module all drop to the lower voltage limit, the sum of the negative voltages provided by the energy storage units in the full-bridge sub-module is greater than the self-clearing voltage threshold.

[0015] In one embodiment, the third quantity lower limit corresponding to the self-clearing constraint condition is determined by the following expression:

[0016]

[0017] Among them, U is the self-clearing voltage threshold, is the lower voltage limit, is the third quantity lower limit.

[0018] In one embodiment, the energy storage unit is a super capacitor.

[0019] In one embodiment, a cooling unit is provided on the energy storage unit.

[0020] In one embodiment, the cooling unit includes wavy heat dissipation fins.

[0021] In a second aspect, the present application provides a DC power transmission system, including the directly hung energy storage device in any of the above embodiments.

[0022] From the above technical solutions, it can be seen that the embodiments of the present application have the following advantages:

[0023] In terms of energy regulation, the directly-connected energy storage device of the present application rationally configures the first number of half-bridge sub-modules and the second number of full-bridge sub-modules, so that even when the energy storage unit voltage drops to the lower limit, the total energy provided is still greater than or equal to the set total energy threshold, ensuring that under conditions such as sudden changes in system load, energy can be stably output, maintaining system power balance, and avoiding operation failures caused by insufficient energy. In addition, in terms of voltage stability, when the energy storage unit voltage is at the lower limit, the total voltage provided by the device is greater than the set voltage of the DC transmission line, effectively maintaining the voltage stability of the DC transmission line, ensuring the stability and reliability of power transmission, reducing the damage to system equipment caused by voltage fluctuations, and extending the service life of the equipment. Moreover, the modular design of this solution facilitates the flexible adjustment of the number of half-bridge and full-bridge sub-modules according to different system requirements, improving the versatility and adaptability of the device, and providing a solid guarantee for the efficient and stable operation of the flexible DC transmission system. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0025] Figure 1 It is a schematic structural diagram of a directly-connected energy storage device mounted on a DC transmission system in an embodiment of the present application;

[0026] Figure 2 It is a schematic structural diagram of a half-bridge sub-module and a full-bridge sub-module provided in an embodiment of the present application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] The following will clearly and completely describe the technical solutions in the embodiments of the present application with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments of the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present application.

[0028] The present application provides a directly-connected energy storage device. Please refer to Figure 1, which is an energy storage device connected between the neutral line and the DC transmission line of a flexible DC transmission system. It realizes functions such as energy storage and energy output through specific structural compositions and working methods. There is no voltage transformation link on the DC side, and it plays an important role in energy regulation and support in the flexible DC transmission system. The direct-connected energy storage device is internally composed of a first number of half-bridge sub-modules and a second number of full-bridge sub-modules connected in series. Both the half-bridge sub-modules and the full-bridge sub-modules contain energy storage units. The energy stored in these energy storage units can be released when needed, can also absorb energy when needed, and can also provide voltage support for the system. When the system is running, the state of the energy storage units will change, and its most extreme state is that all the energy storage units are reduced to the lower voltage limit. At this time, the direct-connected energy storage device needs to meet both energy constraints and voltage constraints. The energy constraint means that the total energy provided by the direct-connected energy storage device at this time needs to be greater than or equal to the set total energy threshold to ensure that sufficient energy can be provided to support the system operation under specific working conditions; the voltage constraint means that the total voltage provided when all the energy storage units are reduced to the lower voltage limit needs to be greater than the set voltage corresponding to the DC transmission line, so as to ensure that the normal voltage level of the system can be maintained under the low voltage state. In this way, the direct-connected energy storage device realizes the effective regulation and control of energy and voltage in the flexible DC transmission system, ensuring the stable operation of the system. In actual operation, when the load of the flexible DC transmission system changes or other working conditions occur, the direct-connected energy storage device can release or absorb energy according to the system requirements, maintaining the power balance and voltage stability of the system. During actual installation, according to the specific parameters and requirements of the flexible DC transmission system, the direct-connected energy storage device is connected between the neutral line and the DC transmission line, and corresponding electrical connections and commissioning are carried out. Half-bridge sub-module:

[0029] The half-bridge sub-module is one of the components of the direct-connected energy storage device. It is connected in series with the full-bridge sub-module to form the direct-connected energy storage device. Please refer to Figure 2, the half-bridge sub-module contains an energy storage unit inside, and realizes functions such as energy storage and output through a specific circuit structure. The half-bridge sub-module has four working states. The first is the input state. In this state, switch T1 is turned on and switch T2 is turned off. The output of the half-bridge sub-module is the voltage of the energy storage unit, and the energy storage unit discharges in this state. The second is the cut-off state. In this state, switch T1 is turned off and switch T2 is turned on. The output of the half-bridge sub-module is nearly 0, and the voltage of the energy storage unit remains unchanged in this state. The third is the locked state. In this state, switch T1 is turned off and switch T2 is turned off. The output of the half-bridge sub-module depends on the direction of the current flowing through the directly-connected energy storage device; if it is a positive current, the output of the half-bridge sub-module is the voltage of the super capacitor, and the energy storage unit is charged; if it is a negative current, the output voltage of the half-bridge sub-module is approximately 0, and the voltage of the energy storage unit remains unchanged. The fourth is the fault exit state. A bypass switch is connected in parallel at the port of the half-bridge sub-module and operates when the half-bridge sub-module fails, so that the half-bridge sub-module is bypassed, and the bypass switch is in the off state in the above other three states. Switches T1 and T2 in the half-bridge sub-module are fully-controlled switch devices with anti-parallel diodes. Fully-controlled switch devices include switches such as MOS (Metal-Oxide-Semiconductor Field-Effect Transistor), IGBT (Insulated Gate Bipolar Transistor), GTO (Gate Turn-Off Thyristor), and GTR (Giant Transistor).

[0030] The full-bridge sub-module is an important part of the directly-connected energy storage device. Please refer to Figure 2, which contains an energy storage unit inside and realizes collaborative work with the half-bridge sub-module through a specific circuit structure, and realizes functions such as energy storage and output through a specific circuit structure. The circuit structure of the full-bridge sub-module has more flexible energy control ability and voltage output ability than the half-bridge sub-module, and it has a total of five working states. The first is the positive input state, where switches T1 and T4 are turned on and switches T2 and T3 are turned off. At this time, the output of the full-bridge sub-module is the positive voltage of the energy storage unit. If the positive current flows in from the port at this time, the energy storage unit is charged; otherwise, the energy storage unit discharges. The second is the negative input state, where switches T2 and T3 are turned on and switches T1 and T4 are turned off. At this time, the output of the full-bridge sub-module is the negative voltage of the energy storage unit. If the positive current flows in from the port at this time, the energy storage unit discharges; otherwise, the energy storage unit discharges. The third state is the cut-off state, where switches T1 and T3 are turned on and switches T2 and T4 are turned off or switches T2 and T4 are turned on and switches T1 and T3 are turned off. At this time, the output voltage of the full-bridge sub-module is approximately 0, and the voltage of the energy storage unit remains unchanged. The fourth is the locking state, that is, switches T1 - T4 are all turned off. At this time, the output of the full-bridge sub-module depends on the direction of the flowing current. If the positive current flows in from the port at this time, the output of the full-bridge sub-module is the positive voltage of the energy storage unit, and the energy storage unit is charged; otherwise, the output of the full-bridge sub-module is the negative voltage of the energy storage unit, and the energy storage unit is still charged. The fifth state is the fault exit state. A bypass switch is connected in parallel at the port of the full-bridge sub-module and operates when the full-bridge sub-module fails, so that the full-bridge sub-module is bypassed, and the bypass switch is in the off state in the above other four states. Switches T1 to T4 in the full-bridge sub-module are all full-controlled switch devices with anti-parallel diodes, and full-controlled switch devices such as MOS, IGBT, GTO, GTR and other switch devices.

[0031] The most extreme working condition of the directly-connected energy storage device is when the voltages of all energy storage units are reduced to the lower limit of the voltage. At this time, it must be ensured that the directly-connected energy storage device can still provide sufficient voltage support and energy support for the DC power transmission system. Therefore, the number design of the full-bridge sub-module and the half-bridge sub-module must consider energy constraints and voltage constraints, and the sum of the first number and the second number must simultaneously meet the energy constraints and voltage constraints of the directly-connected energy storage device.

[0032] The energy constraint is that when all energy storage units are reduced to the lower limit of the voltage, the total energy provided by the directly-connected energy storage device is greater than or equal to the set total energy threshold. That is to say, when the flexible DC power transmission system encounters sudden power fluctuations or load changes, even if the voltage of the energy storage unit has been reduced to the lower limit, the directly-connected energy storage device should be able to provide sufficient energy to stabilize the power balance of the system and avoid system failures or unstable operations. The directly-connected energy storage device meets the requirements of energy constraints by reasonably configuring the number and capacity of energy storage units in the half-bridge sub-module and the full-bridge sub-module.

[0033] The voltage constraint is that the total voltage provided when all energy storage units are reduced to the lower voltage limit is greater than the set voltage corresponding to the DC transmission line. The voltage constraint is crucial for the stable operation of the directly-connected energy storage device in the flexible DC transmission system. When the voltage of the energy storage unit gradually decreases to the lower limit, the directly-connected energy storage device still needs to be able to provide sufficient voltage to maintain the normal voltage level of the DC transmission line and provide voltage support for the DC transmission system. Specifically, the directly-connected energy storage device needs to optimize the number of half-bridge sub-modules and full-bridge sub-modules, the lower voltage limit, etc. to meet the requirements of the voltage constraint. In addition, intelligent monitoring and diagnostic technologies can be introduced into the full-bridge sub-modules to monitor the operating status of the full-bridge sub-modules in real time, detect potential fault hazards in a timely manner, and take corresponding measures for treatment to improve the overall reliability and maintainability of the directly-connected energy storage device.

[0034] In the energy regulation of this directly-connected energy storage device solution, by reasonably configuring the first number of half-bridge sub-modules and the second number of full-bridge sub-modules, even when the voltage of the energy storage unit drops to the lower limit, the total energy provided is still greater than or equal to the set total energy threshold, ensuring that under conditions such as sudden changes in system load, it can stably output energy, maintain the system power balance, and avoid operating failures caused by insufficient energy. In addition, in terms of voltage stability, when the voltage of the energy storage unit is at the lower limit, the total voltage provided by this device is greater than the set voltage of the DC transmission line, effectively maintaining the voltage stability of the DC transmission line, ensuring the stability and reliability of power transmission, reducing the damage to system equipment caused by voltage fluctuations, and extending the service life of the equipment. Moreover, the modular design of this solution facilitates the flexible adjustment of the number of half-bridge and full-bridge sub-modules according to different system requirements, improving the versatility and adaptability of the device, and providing a solid guarantee for the efficient and stable operation of the flexible DC transmission system.

[0035] In one of the embodiments, the first lower limit corresponding to the energy constraint is determined by the following expression:

[0036]

[0037] Where, is the set total energy threshold, which is an energy value preset according to the operating requirements and conditions of the flexible DC transmission system. When all energy storage units in the directly-connected energy storage device are reduced to the lower voltage limit, the minimum total energy that the directly-connected energy storage device needs to provide is a key indicator to measure whether the directly-connected energy storage device can meet the system energy requirements. is the normal voltage value of the energy storage unit after being fully charged, which is the voltage of the energy storage unit in the fully charged state and represents the voltage level when the energy storage unit is in the ideal working state. This value varies for different types of energy storage units. is the lower voltage limit, which represents the lowest voltage value at which the energy storage unit can work normally and output energy. is the first lower limit, which is the minimum number of energy storage units in the half-bridge sub-module and full-bridge sub-module required to ensure that the total energy provided by the directly-connected energy storage device when all energy storage units drop to the lower limit of the voltage is greater than or equal to the set total energy threshold. is the capacitance value of the energy storage unit. The principle of this expression is based on the basic formula for capacitor energy storage Q = 0.5CV 2 (where Q is the stored energy, C is the capacitance, and V is the voltage). In the directly-connected energy storage device, each energy storage unit can be approximately regarded as a capacitor. When the energy storage unit drops from the normal voltage when fully charged to the lower limit of the voltage , the energy released by each energy storage unit is the denominator part of the above formula. And the set total energy threshold is the energy that the system requires the directly-connected energy storage device to provide when the energy storage unit is at the lower limit of the voltage. To meet this energy requirement, by dividing by the energy released by each energy storage unit, the minimum number of energy storage units required to meet the energy constraint can be obtained, that is, the first lower limit . This can ensure that when the energy storage unit is in a low-voltage state, the directly-connected energy storage device can still provide sufficient energy support for the flexible DC power transmission system.

[0038] In one embodiment, the second lower limit corresponding to the voltage constraint is determined by the following expression:

[0039]

[0040] where is the set voltage corresponding to the DC transmission line, which is a preset voltage value according to the normal operation requirements and electrical characteristics of the DC transmission line in the flexible DC power transmission system. is the second lower limit, which is the minimum number of energy storage units in the half-bridge sub-module and full-bridge sub-module required to ensure that the total voltage provided by the directly-connected energy storage device when all energy storage units drop to the lower limit of the voltage is greater than the set voltage corresponding to the DC transmission line. The principle of this expression is based on the voltage characteristics of a series circuit. In the directly-connected energy storage device, the energy storage units in the half-bridge sub-module and full-bridge sub-module can be regarded as connected in series. When the energy storage unit is at the lower limit of the voltage, the voltage that each energy storage unit can provide is . And the set voltage is the minimum voltage that the directly-connected energy storage device is required to provide for the stable operation of the DC transmission line. To meet the voltage constraint, that is, to make the total voltage provided by the directly-connected energy storage device greater than , by dividing the set voltage by the voltage of a single energy storage unit at the lower limit of the voltage , the minimum number of energy storage units required to meet the voltage constraint can be obtained, that is, the second lower limit This can ensure that when the energy storage unit is in a low-voltage state, the directly-connected energy storage device can still maintain the normal voltage level of the DC transmission line.

[0041] In one embodiment, the total quantity is greater than or equal to the maximum value of the first lower limit and the second lower limit. The rule that the total quantity is greater than or equal to the maximum value of the first lower limit and the second lower limit is because the directly-connected energy storage device needs to satisfy both the energy constraint and the voltage constraint simultaneously. The first lower limit is the minimum number of energy storage units determined from the energy perspective, and the second lower limit is the minimum number of energy storage units determined from the voltage perspective. Taking the maximum value of the two as the lower limit of the total quantity can ensure that the directly-connected energy storage device can take into account the two aspects of constraint requirements while meeting the most stringent conditions among them.

[0042] In one embodiment, the second quantity satisfies the self-clearing constraint condition for DC overhead line faults. The self-clearing constraint condition is that when all the energy storage units in the full-bridge sub-module are reduced to the lower voltage limit, the total negative voltage provided by the energy storage units in the full-bridge sub-module is greater than the self-clearing voltage threshold. The second quantity is the number of energy storage units in the full-bridge sub-module. The half-bridge sub-module does not have the ability to output negative voltage, but the directly-connected energy storage device in this embodiment also includes a part of full-bridge sub-modules, which have the negative voltage output ability and can be used to clear DC fault arcs. When the full-bridge sub-module is in the above negative input state, the voltage output by its energy storage unit can present a negative voltage state. The self-clearing voltage threshold is a pre-set voltage value according to the safe operation requirements of the directly-connected energy storage device and the flexible DC transmission system. When the total negative voltage provided by the energy storage units in the full-bridge sub-module is greater than this threshold, it helps to achieve self-clearing of the fault. Specifically, in the case of a fault, the control circuit is used to make the energy storage unit output negative voltage. When all the energy storage units in the full-bridge sub-module are reduced to the lower voltage limit, if the total negative voltage provided by these energy storage units is greater than the self-clearing voltage threshold, a sufficient reverse electromotive force can be generated to offset the positive voltage generated by the fault current, thereby quickly cutting off the fault current and achieving self-clearing of the fault. This can avoid the expansion of the fault and ensure the safe and stable operation of the flexible DC transmission system. Therefore, under extreme conditions, that is, when all the energy storage units of the full-bridge sub-module are reduced to the lower voltage limit, the total negative voltage that can be provided must be greater than the self-clearing voltage threshold to meet the self-clearing constraint condition. Therefore, the number of full-bridge sub-modules must be reasonably designed.

[0043] In one embodiment, the third quantity lower limit corresponding to the self-clearing constraint condition is determined by the following expression:

[0044]

[0045] where U is the self-clearing voltage threshold, is the lower voltage limit, is the third lower limit of quantity, which refers to the minimum number of energy storage units in the full-bridge submodule required when the energy storage units in the full-bridge submodule are all reduced to the voltage lower limit, and the sum of the negative pressures provided by them is greater than the self-clearing voltage threshold. This expression is based on the principle of voltage superposition in a series circuit. The energy storage units in the full-bridge submodule can output negative voltage in the event of a fault. In order to make the sum of the negative pressures provided by the energy storage units in the full-bridge submodule greater than the self-clearing voltage threshold at the voltage lower limit, the self-clearing voltage threshold is divided by the negative pressure output by a single energy storage unit at the voltage lower limit. The minimum number of energy storage units required to meet the self-clearing constraint condition can be obtained, that is, the third lower limit of quantity. This ensures that in specific fault conditions, the direct-mounted energy storage device has sufficient capacity to achieve fault self-clearing and maintain system stability.

[0046] In one embodiment, the energy storage unit is a supercapacitor. A supercapacitor is an electrochemical energy storage device between a traditional capacitor and a battery. It has the characteristics of high power density, fast charging and discharging speed, and long cycle life, and can quickly store and release a large amount of electrical energy. As an energy storage unit, the working principle of the supercapacitor is based on the double layer effect. When the supercapacitor is charged, two charge layers, namely double layers, are formed at the interface between the electrode and the electrolyte, and the positive and negative charges are respectively gathered on both sides of the electrode and the electrolyte, thereby realizing the storage of electrical energy. In the direct-hanging energy storage device, when the flexible DC transmission system has excess electrical energy, the supercapacitor can quickly absorb and store the electrical energy; and when the system needs electrical energy, the supercapacitor can quickly release the stored electrical energy to provide energy support for the system. Compared with traditional batteries, supercapacitors have faster charging and discharging speeds and can better cope with the rapidly changing energy needs in the system. At the same time, their long cycle life also reduces maintenance and replacement costs, which helps to improve the overall reliability and stability of the direct-hanging energy storage device.

[0047] In one of the embodiments, a cooling unit is provided on the energy storage unit. Energy storage units, such as supercapacitors, generate heat during the charging and discharging process due to internal electrochemical reactions or resistance and other factors. If the heat cannot be dissipated in time, the temperature of the energy storage unit will increase. Excessive temperature will have a negative impact on the performance of the energy storage unit, such as reducing its charging and discharging efficiency, shortening its service life, and may even cause safety problems. The cooling unit transfers heat to the surrounding environment by exchanging heat with the energy storage unit. Common heat exchange methods include conduction, convection and radiation. Taking an air-cooled cooling unit as an example, the air flow takes away the heat from the surface of the cooling unit, thereby reducing the temperature of the energy storage unit, keeping it in a suitable operating temperature range, and ensuring the stable operation of the direct-mounted energy storage device and the normal performance of the energy storage unit.

[0048] In one embodiment, the cooling unit includes wavy heat dissipation fins. The wavy heat dissipation fins are a specific form of the cooling unit and are metal sheets with a wavy shape. By increasing the heat dissipation area, the heat dissipation speed is accelerated. When the energy storage unit is working, heat will be generated. If the heat is not dissipated in time, its performance and stability will be affected. The wavy heat dissipation fins utilize the principle of increasing the heat dissipation area to strengthen heat exchange. The contact area between the conventional flat heat dissipation fins and the air is limited, while the wavy design greatly increases the contact surface area between the fins and the surrounding air. When the air flows over the surface of the fins, the larger contact area enables more heat to be transferred to the air through heat convection, thereby accelerating the heat dissipation speed and effectively reducing the temperature of the energy storage unit. This can ensure that the energy storage unit, such as a supercapacitor, operates in a stable temperature environment and maintains the normal operation of the directly hung energy storage device.

[0049] This application provides a DC power transmission system, including the directly hung energy storage device in any of the above embodiments. In this DC power transmission system, the directly hung energy storage device plays a key regulatory role. When there are power fluctuations during the operation of the system, such as a sudden increase or decrease in load, the energy storage unit in the directly hung energy storage device can perform rapid charge and discharge operations according to the system requirements. When the load increases and the system power is insufficient, the energy storage unit releases the stored electrical energy and supplements it into the DC power transmission line to maintain the power balance of the system; when the load decreases and the system power is excessive, the energy storage unit absorbs the excess electrical energy for storage. At the same time, the directly hung energy storage device ensures that when the energy storage unit is in a low-voltage state, it can still provide sufficient energy and stable voltage to the system by meeting the energy constraint and voltage constraint. In addition, if a fault such as a short circuit occurs in the system, the part of the directly hung energy storage device that meets the self-clearing constraint condition can output a negative voltage through the energy storage unit in the full-bridge sub-module to achieve self-clearing of the fault and ensure the safe and stable operation of the DC power transmission system.

[0050] Finally, it should also be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "including a..." does not exclude the existence of additional identical elements in the process, method, article or device including the said element.

[0051] The various embodiments in this specification are described in a progressive manner. Each embodiment focuses on the differences from other embodiments. The various embodiments can be combined as needed, and the same or similar parts can be referred to each other.

[0052] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present application. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but rather to the broadest scope consistent with the principles and novel features disclosed herein.

Claims

1. A direct-hanging energy storage device, characterized in that: Connected between the neutral line and the DC transmission line of the flexible DC power transmission system, the direct-hung energy storage device includes a first number of half-bridge submodules and a second number of full-bridge submodules connected in series; The half-bridge submodule and the full-bridge submodule both include energy storage units, and the total number of the first number and the second number satisfies both energy constraints and voltage constraints; the energy constraint is that when all the energy storage units are reduced to a voltage lower limit, the total energy provided by the direct-mounted energy storage device is greater than or equal to a set total energy threshold, and the voltage constraint is that when all the energy storage units are reduced to the voltage lower limit, the total voltage provided is greater than a set voltage corresponding to the DC transmission line.

2. The direct-hanging energy storage device according to claim 1, characterized in that: The first lower limit corresponding to the energy constraint is determined by the following expression: in, For setting the total energy threshold, is the normal voltage value of the energy storage unit after it is fully charged, is the voltage lower limit, is the first lower limit, is the capacitance value of the energy storage unit.

3. The direct-mounted energy storage device according to claim 2, characterized in that: The second lower limit corresponding to the voltage constraint is determined by the following expression: in, is the set voltage, is the second lower limit.

4. The direct-mounted energy storage device according to claim 3, characterized in that: The total number is greater than or equal to a larger value of the first lower limit and the second lower limit.

5. The direct-hanging energy storage device according to claim 1, characterized in that: The second number satisfies the self-clearing constraint condition of the DC overhead line fault, and the self-clearing constraint condition is that when the energy storage units in the full-bridge sub-module are all reduced to the voltage lower limit, the sum of the negative pressures provided by the energy storage units in the full-bridge sub-module is greater than the self-clearing voltage threshold.

6. The direct-hanging energy storage device according to claim 5, characterized in that: The third quantity lower limit corresponding to the self-clearing constraint condition is determined by the following expression: Wherein, U is the self-clearing voltage threshold, is the voltage lower limit, is the third lower limit of the quantity.

7. The direct-hanging energy storage device according to claim 1, characterized in that: The energy storage unit includes a supercapacitor.

8. The direct-hanging energy storage device according to claim 1, characterized in that: The energy storage unit is provided with a cooling unit.

9. The direct-hanging energy storage device according to claim 8, characterized in that: The cooling unit includes corrugated heat dissipation fins.

10. A direct current transmission system, characterized in that: A direct-hanging energy storage device comprising any one of claims 1-9.

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