A direct-hanging energy storage device and a direct current transmission system

Abstract

The application provides a direct-hanging energy storage device connected between a neutral line and a direct-current transmission line of a flexible direct-current transmission system, the direct-hanging energy storage device comprising a first number of half-bridge sub-modules and a second number of full-bridge sub-modules connected in series with each other; the half-bridge sub-modules and the full-bridge sub-modules each comprise an energy storage unit, and the total number of the first number and the second number satisfies energy constraints and voltage constraints; the energy constraints are that when all the energy storage units are reduced to a lower limit of voltage, the total energy provided by the direct-hanging energy storage device is greater than or equal to a set total energy threshold, and the voltage constraints are that when all the energy storage units are reduced to the lower limit of voltage, the total voltage provided is greater than a set voltage corresponding to the direct-current transmission line. The device can realize multiple functions such as inertia support, frequency modulation support, peak clipping and valley filling, energy balance extreme scene black start and the like of the direct-current transmission system.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of direct current power transmission, and particularly relates to a direct-hanging energy storage device and a direct current power transmission system. BACKGROUND

[0002] With the development of science and technology, the power system has and will continue to undergo profound changes. The proportion of new energy and power electronic devices on the source-grid-load side is increasing, and the power system is gradually transitioning to a "double-high" stage. The composition and characteristics of the power system have undergone profound changes. In addition to traditional stability, power flow control, voltage control, and other issues, the power system faces many new challenges. On the one hand, with the large-scale access of new energy, energy storage, and other power electronic devices, the proportion of synchronous generator installed capacity in the power system is continuously decreasing, and the system is developing towards low inertia and low damping. The regulation capability of the power system is severely reduced, the anti-disturbance capability is insufficient, and the safe and stable operation is under greater pressure. On the other hand, with the increasing proportion of new energy generation and the long-distance transmission of large-scale new energy generation through high-voltage direct current power transmission, the energy balance at multiple time scales also faces new major challenges.

[0003] Direct current projects require grid-friendly energy storage devices that can provide support in inertia support, frequency modulation support, peak shaving, new energy consumption, black start in extreme scenarios, power emergency increase and decrease, and other aspects. However, there is currently a lack of related research and applications that can support the stability and reliability of direct current project power transmission. SUMMARY

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

[0005] In a first aspect, the present application provides a direct-hanging energy storage device connected between a neutral line and a direct current transmission line of a flexible direct current power transmission system. The direct-hanging energy storage device includes a first number of half-bridge sub-modules and a second number of full-bridge sub-modules connected in series with each other.

[0006] The half-bridge sub-modules and the full-bridge sub-modules each include an energy storage unit. The total number of the first number and the second number satisfies an energy constraint and a voltage constraint. The energy constraint is that when all the energy storage units are reduced to a lower limit of voltage, the total energy provided by the direct-hanging energy storage device is greater than or equal to a set total energy threshold. The voltage constraint is that when all the energy storage units are reduced to the lower limit of voltage, the total voltage provided is greater than a set voltage corresponding to the direct current transmission line.

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

[0008]

[0009] wherein, is a set total energy threshold, is a normal voltage value of the energy storage unit when fully charged, is a lower voltage limit, is a first lower limit, is a capacitance value of the energy storage unit.

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

[0011]

[0012] wherein, is a set voltage, is a second lower limit.

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

[0014] In one of the embodiments, the second number satisfies a direct current 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 are all 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 a self-clearing voltage threshold.

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

[0016]

[0017] wherein, U is the self-clearing voltage threshold, is the lower voltage limit, is the third number lower limit.

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

[0019] In one of the embodiments, a cooling unit is arranged on the energy storage unit.

[0020] In one of the embodiments, the cooling unit includes a wave-shaped heat dissipation fin.

[0021] In a second aspect, the present application provides a direct current power transmission system, comprising the direct-hanging energy storage device in any of the above embodiments.

[0022] From the above technical solutions, the embodiments of the present application have the following advantages:

[0023] In terms of energy regulation, this application's direct-connected energy storage device, through the rational configuration of a first number of half-bridge sub-modules and a second number of full-bridge sub-modules, ensures that even when the energy storage unit voltage drops to the lower limit, the total energy provided remains greater than or equal to the set total energy threshold. This guarantees stable energy output under conditions such as sudden changes in system load, maintaining system power balance and preventing operational failures due to insufficient energy. Furthermore, regarding voltage stability, even when the energy storage unit voltage is at the lower limit, the total voltage provided by the device exceeds the set voltage of the DC transmission line, effectively maintaining the stability of the DC transmission line voltage, ensuring the stability and reliability of power transmission, reducing damage to system equipment caused by voltage fluctuations, and extending equipment lifespan. Moreover, the modular design of this scheme allows for flexible adjustment of the number of half-bridge and full-bridge sub-modules according to different system requirements, improving the device's versatility and adaptability, and providing a solid guarantee for the efficient and stable operation of flexible DC transmission systems. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a schematic diagram of the structure of a direct-connected energy storage device mounted on a DC transmission system in one embodiment of this application;

[0026] Figure 2 This application provides a schematic diagram of the structure of a half-bridge submodule and a full-bridge submodule according to one embodiment. Detailed Implementation

[0027] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0028] This application provides a direct-connected energy storage device; please refer to [link / reference]. Figure 1A direct-connected energy storage device is an energy storage unit connected between the neutral line and the DC transmission line in a flexible DC transmission system. Through a specific structural composition and operating mode, it achieves energy storage and output functions, eliminating the need for a transformer on the DC side. It plays a crucial role in energy regulation and support within the flexible DC transmission system. The direct-connected energy storage device consists of a first number of half-bridge sub-modules and a second number of full-bridge sub-modules connected in series. Both half-bridge and full-bridge sub-modules contain energy storage units. The energy stored in these units can be released or absorbed when needed, and can also provide voltage support for the system. During system operation, the state of the energy storage units changes, with the most extreme state being when all energy storage units drop to the lower voltage limit. In this case, the direct-connected energy storage device must simultaneously meet both energy and voltage constraints. Energy constraint means that the total energy provided by the directly connected energy storage device must be greater than or equal to a set total energy threshold to ensure sufficient energy to support system operation under specific conditions. Voltage constraint means that the total voltage provided when all energy storage units are reduced to the lower voltage limit must be greater than the set voltage corresponding to the DC transmission line, thereby ensuring that the normal voltage level of the system can be maintained even under low voltage conditions. In this way, the directly connected energy storage device achieves effective regulation and control of energy and voltage in the flexible DC transmission system, ensuring stable system operation. In actual operation, when the load of the flexible DC transmission system changes or other operating conditions occur, the directly connected energy storage device can release or absorb energy according to system demand, maintaining the power balance and voltage stability of the system. During actual installation, the directly connected energy storage device is connected between the neutral line and the DC transmission line according to the specific parameters and requirements of the flexible DC transmission system, and corresponding electrical connections and commissioning are performed. Half-bridge submodule:

[0029] The half-bridge submodule is a component of a direct-connect energy storage system. It is connected in series with the full-bridge submodule to form the direct-connect energy storage system. Please refer to [link / reference]. Figure 2The half-bridge submodule contains an energy storage unit, which uses a specific circuit structure to store and output energy. The half-bridge submodule has four operating states: First, the active state: switch T1 is on, switch T2 is off, the half-bridge submodule outputs the voltage of the energy storage unit, and the energy storage unit discharges in this state. Second, the deactivated state: switch T1 is off, switch T2 is on, the half-bridge submodule output is nearly zero, and the energy storage unit voltage remains constant. Third, the latched state: switch T1 and switch T2 are off, and the half-bridge submodule output depends on the direction of the current flowing through the directly connected energy storage device; if the current is positive, the half-bridge submodule outputs the supercapacitor voltage, and the energy storage unit charges; if the current is negative, the half-bridge submodule output voltage is approximately zero, and the energy storage unit voltage remains constant. The fourth is the fault exit state. A bypass switch is connected in parallel to the ports of the half-bridge submodule. This bypass switch activates when a fault occurs in the half-bridge submodule, bypassing it. The bypass switch remains open in the other three states mentioned above. Switches T1 and T2 in the half-bridge submodule are fully controlled switching devices with anti-parallel diodes. Fully controlled switching devices include 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 submodule is an important component of direct-connected energy storage devices. Please refer to [link / reference]. Figure 2The full-bridge submodule contains an energy storage unit and works in conjunction with the half-bridge submodule through a specific circuit structure to achieve energy storage and output functions. The full-bridge submodule's circuit structure offers more flexible energy control and voltage output capabilities than the half-bridge submodule, and it has five operating states. The first is the positive input state, where switches T1 and T4 are on, and switches T2 and T3 are off. In this state, the full-bridge submodule outputs a positive voltage to the energy storage unit. If a positive current flows into the port, the energy storage unit charges; otherwise, it discharges. The second is the negative input state, where switches T2 and T3 are on, and switches T1 and T4 are off. In this state, the full-bridge submodule outputs a negative voltage to the energy storage unit. If a positive current flows into the port, the energy storage unit discharges; otherwise, it discharges. The third state is the off state, where switches T1 and T3 are on and switches T2 and T4 are off, or switches T2 and T4, and switches T1 and T3 are off. In this state, the output voltage of the full-bridge submodule is approximately 0, and the voltage of the energy storage unit remains unchanged. The fourth state is the latched state, where switches T1-T4 are all off. In this state, the output of the full-bridge submodule depends on the direction of the current flowing through it. If the current flowing into the port is positive, the output of the full-bridge submodule is the positive voltage of the energy storage unit, and the energy storage unit is charging; otherwise, the output of the full-bridge submodule is the negative voltage of the energy storage unit, and the energy storage unit is still charging. The fifth state is the fault exit state. A bypass switch is connected in parallel to the ports of the full-bridge submodule. When a fault occurs in the full-bridge submodule, it activates, causing the full-bridge submodule to be bypassed. The bypass switch is open in the other four states mentioned above. The switches T1 to T4 in the full-bridge submodule are all fully controlled switching devices with anti-parallel diodes, such as MOS, IGBT, GTO, GTR and other switching devices.

[0031] The most extreme operating condition for a direct-connected energy storage device is when the voltage of all energy storage units drops to the lower voltage limit. In this case, it is essential to ensure that the direct-connected energy storage device can still provide sufficient voltage and energy support for the DC transmission system. Therefore, the design of the number of full-bridge and half-bridge submodules must consider both energy and voltage constraints. The sum of the first and second quantities must simultaneously satisfy both the energy and voltage constraints of the direct-connected energy storage device.

[0032] The energy constraint is that when all energy storage units drop to their lower voltage limits, the total energy provided by the direct-connected energy storage device must be greater than or equal to a set total energy threshold. This means that when a flexible DC transmission system encounters sudden power fluctuations or load changes, even if the voltage of the energy storage units has dropped to the lower limit, the direct-connected energy storage device should be able to provide sufficient energy to stabilize the system's power balance and prevent system failures or unstable operation. The direct-connected energy storage device meets the energy constraint requirements by rationally configuring the number and capacity of energy storage units in the half-bridge and full-bridge submodules.

[0033] The voltage constraint requires that the total voltage provided by all energy storage units when their voltage drops to the lower limit must be greater than the set voltage corresponding to the DC transmission line. This voltage constraint is crucial for the stable operation of direct-connected energy storage devices in flexible DC transmission systems. As the voltage of the energy storage units gradually decreases to the lower limit, the direct-connected energy storage device must still be able to provide sufficient voltage to maintain the normal voltage level of the DC transmission line, providing voltage support for the DC transmission system. Specifically, the direct-connected energy storage device needs to meet the voltage constraint requirements by optimizing the number of half-bridge and full-bridge submodules, the lower voltage limit, and other factors. Furthermore, intelligent monitoring and diagnostic technologies can be introduced into the full-bridge submodules to monitor their operating status in real time, promptly identify potential faults, and take corresponding measures to address them, thereby improving the overall reliability and maintainability of the direct-connected energy storage device.

[0034] In terms of energy regulation, this direct-connected energy storage solution utilizes a reasonable configuration of a first number of half-bridge sub-modules and a second number of full-bridge sub-modules. This ensures that even when the energy storage unit voltage drops to its lower limit, the total energy provided remains greater than or equal to the set total energy threshold. This guarantees stable energy output under conditions such as sudden changes in system load, maintaining system power balance and preventing operational failures due to insufficient energy. Furthermore, regarding voltage stability, even when the energy storage unit voltage is at its lower limit, the total voltage provided by the device exceeds the set voltage of the DC transmission line, effectively maintaining the stability of the DC transmission line voltage, ensuring the stability and reliability of power transmission, reducing damage to system equipment caused by voltage fluctuations, and extending equipment lifespan. Moreover, the modular design of this solution allows for flexible adjustment of the number of half-bridge and full-bridge sub-modules according to different system requirements, improving the device's versatility and adaptability, and providing a solid guarantee for the efficient and stable operation of flexible DC transmission systems.

[0035] In one embodiment, the first lower bound corresponding to the energy constraint is determined by the following expression:

[0036]

[0037] in, The total energy threshold is a pre-set energy value based on the operational requirements and conditions of the flexible DC transmission system. The minimum total energy required by the direct-connected energy storage device when all energy storage units in the device drop to the lower voltage limit is a key indicator of whether the device can meet the system's energy demands. This is the normal voltage value of the energy storage unit after it is fully charged. It represents the voltage level of the energy storage unit when it is in an ideal working state. This value varies for different types of energy storage units. The lower voltage limit represents the minimum voltage at which the energy storage unit can operate normally and output energy. The first lower limit is the minimum number of energy storage units required in the half-bridge and full-bridge submodules to ensure that the total energy provided by the direct-connected energy storage device is greater than or equal to the set total energy threshold when all energy storage units are reduced to the lower voltage limit. This represents the capacitance of the energy storage unit. The principle behind this expression is based on the fundamental formula for capacitor energy storage: Q = 0.5CV. 2 (Q represents the stored energy, C represents the capacitance, and V represents the voltage). In a direct-connected energy storage device, each energy storage unit can be approximated as a capacitor. When the energy storage unit is charged from its normal voltage... Drop to the lower voltage limit At that time, the energy released by each energy storage unit is the denominator of the above formula. A total energy threshold is then set. This refers to the energy that the system requires the direct-connected energy storage device to provide when the energy storage unit is at its lower voltage limit. To meet this energy requirement, through... Dividing by the energy released by each energy storage unit yields the minimum number of energy storage units required to satisfy the energy constraint, i.e., the first lower limit. This ensures that even when the energy storage unit is under low voltage, the direct-connected energy storage device can still provide sufficient energy support for the flexible DC transmission system.

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

[0039]

[0040] in, The set voltage for a DC transmission line is a pre-set voltage value based on the normal operation requirements and electrical characteristics of the DC transmission line in a flexible DC transmission system. The second lower limit is the minimum number of energy storage units required in the half-bridge and full-bridge submodules to ensure that the total voltage provided by the direct-connected energy storage device, when all energy storage units are reduced to the lower voltage limit, exceeds the set voltage corresponding to the DC transmission line. This expression is based on the voltage characteristics of a series circuit. In a direct-connected energy storage device, the energy storage units in the half-bridge and full-bridge submodules can be considered as being connected in series. When the energy storage unit is at the lower voltage limit, the voltage that each energy storage unit can provide is... And set voltage This is the minimum voltage required for the stable operation of a DC transmission line to be provided by the directly connected energy storage device. To meet the voltage constraint, the total voltage provided by the directly connected energy storage device must be greater than... By setting the voltage Divide by the voltage of a single energy storage unit at the lower voltage limit This allows us to obtain the minimum number of energy storage units required to satisfy the voltage constraint, i.e., the second lower limit. This ensures that even when the energy storage unit is in a low-voltage state, the direct-connected energy storage device can still maintain the normal voltage level of the DC transmission line.

[0041] In one embodiment, the total number is greater than or equal to the maximum of the first and second lower limits. This rule, where the total number is greater than or equal to the maximum of the first and second lower limits, is because the direct-connected energy storage device needs to simultaneously meet energy and voltage constraints. The first lower limit is the minimum number of energy storage units determined from an energy perspective, and the second lower limit is the minimum number of energy storage units determined from a voltage perspective. Taking the maximum of the two as the lower limit for the total number ensures that the direct-connected energy storage device meets the most stringent condition while also taking into account the constraints of both aspects.

[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 energy storage units in the full-bridge submodule are reduced to the lower voltage limit, the total negative voltage provided by the energy storage units in the full-bridge submodule is greater than the self-clearing voltage threshold. The second quantity is the number of energy storage units in the full-bridge submodule. Half-bridge submodules do not have the ability to output negative voltage, but the direct-connected energy storage device in this embodiment also includes a portion of the full-bridge submodules, which have negative voltage output capability and can be used to clear DC fault arcs. When the full-bridge submodule is in the aforementioned negative input state, the voltage output by its energy storage units can be negative. The self-clearing voltage threshold is a pre-set voltage value based on the safe operation requirements of the direct-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 submodule is greater than this threshold, it helps to achieve fault self-clearing. Specifically, in the event of a fault, the energy storage units output negative voltage through a control circuit. When all energy storage units in the full-bridge submodule drop to the lower voltage limit, if the sum of the negative voltages provided by these energy storage units exceeds the self-clearing voltage threshold, sufficient back electromotive force can be generated to counteract the positive voltage generated by the fault current, thereby quickly cutting off the fault current and achieving fault self-clearing. This prevents the fault from escalating and ensures the safe and stable operation of the flexible DC transmission system. Therefore, under extreme operating conditions, i.e., when all energy storage units in the full-bridge submodule drop to the lower voltage limit, the sum of the negative voltages they can provide must exceed the self-clearing voltage threshold to satisfy the self-clearing constraint. Therefore, the number of full-bridge submodules must be designed appropriately.

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

[0044]

[0045] Where U is the self-clearing voltage threshold. This is the lower limit of voltage. The third lower bound refers to the minimum number of energy storage units in the full-bridge submodule required to exceed the self-clearing voltage threshold when all energy storage units in the submodule are reduced to the lower voltage limit. This expression is based on the principle of voltage superposition in series circuits. Energy storage units in the full-bridge submodule can output negative voltage during a fault. To ensure that the total negative voltage provided by the energy storage units in the full-bridge submodule exceeds the self-clearing voltage threshold at the lower voltage limit, the minimum number of energy storage units required to satisfy the self-clearing constraint—the third lower bound—is obtained by dividing the self-clearing voltage threshold by the negative voltage output by a single energy storage unit at the lower voltage limit. This ensures that, under specific fault conditions, the directly connected 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 that falls between a traditional capacitor and a battery. It features high power density, fast charging and discharging speeds, and long cycle life, enabling it to rapidly store and release large amounts of electrical energy. As an energy storage unit, the supercapacitor operates based on the electric double-layer effect. When a supercapacitor is charged, two charge layers, or electric double layers, are formed at the electrode-electrolyte interface, with positive and negative charges accumulating on opposite sides of the electrode and electrolyte, respectively, thus storing electrical energy. In a direct-connected energy storage device, when the flexible DC transmission system has excess energy, the supercapacitor can quickly absorb and store this energy; when the system needs energy, the supercapacitor can quickly release the stored energy to provide energy support to the system. Compared to traditional batteries, supercapacitors have faster charging and discharging speeds, better meeting the rapidly changing energy demands of the system. Their long cycle life also reduces maintenance and replacement costs, contributing to improved overall reliability and stability of the direct-connected energy storage device.

[0047] In one embodiment, a cooling unit is provided on the energy storage unit. Energy storage units, such as supercapacitors, generate heat during charging and discharging due to internal electrochemical reactions or resistance. If this heat cannot be dissipated in time, the temperature of the energy storage unit will rise. Excessive temperature can negatively impact the performance of the energy storage unit, such as reducing its charging and discharging efficiency, shortening its lifespan, and potentially causing safety issues. The cooling unit transfers heat to the surrounding environment through heat exchange with the energy storage unit. Common heat exchange methods include conduction, convection, and radiation. Taking an air-cooled cooling unit as an example, airflow carries away heat from the surface of the cooling unit, thereby lowering the temperature of the energy storage unit and maintaining it within a suitable operating temperature range, ensuring the stable operation of the direct-connected energy storage device and the normal performance of the energy storage unit.

[0048] In one embodiment, the cooling unit includes corrugated heat dissipation fins. Corrugated heat dissipation fins are a specific form of cooling unit, consisting of thin metal sheets with a wavy shape, which increase the heat dissipation area to accelerate heat dissipation. Energy storage units generate heat during operation; if this heat is not dissipated in time, it will affect their performance and stability. Corrugated heat dissipation fins utilize the principle of increasing the heat dissipation area to enhance heat exchange. Conventional planar heat dissipation fins have limited contact area with the air, while the corrugated design greatly increases the contact surface area between the fins and the surrounding air. When air flows over the fin surface, the larger contact area allows more heat to be transferred to the air through thermal convection, thereby accelerating heat dissipation and effectively reducing the temperature of the energy storage unit. This ensures that energy storage units, such as supercapacitors, operate in a stable temperature environment, maintaining the normal operation of direct-connected energy storage devices.

[0049] This application provides a DC transmission system including the direct-connected energy storage device in any of the above embodiments. In this DC transmission system, the direct-connected energy storage device plays a crucial regulatory role. When power fluctuations occur during system operation, such as a sudden increase or decrease in load, the energy storage units in the direct-connected energy storage device can perform rapid charging and discharging operations according to system needs. When the load increases and the system power is insufficient, the energy storage units release the stored electrical energy to supplement the DC transmission line, maintaining the system's power balance; when the load decreases and the system power is excessive, the energy storage units absorb and store the excess electrical energy. Simultaneously, by satisfying energy and voltage constraints, the direct-connected energy storage device ensures that it can still provide sufficient energy and a stable voltage to the system when the energy storage units are in a low-voltage state. Furthermore, if a fault such as a short circuit occurs in the system, the portion of the direct-connected energy storage device that meets the self-clearing constraint conditions can achieve fault self-clearing by outputting negative voltage from the energy storage units in the full-bridge submodule, ensuring the safe and stable operation of the DC transmission system.

[0050] Finally, it should be noted that in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes 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 make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A direct-connected energy storage device, characterized in that, Connected between the neutral line and the DC transmission line of the flexible DC transmission system, the direct-connected energy storage device includes a first number of half-bridge sub-modules and a second number of full-bridge modules connected in series. Both the half-bridge submodule and the full-bridge submodule include energy storage units. The total number of the first quantity and the second quantity simultaneously satisfy both energy constraints and voltage constraints. The energy constraint is that when all the energy storage units are reduced to the lower voltage limit, the total energy provided by the direct-connected energy storage device is greater than or equal to a set total energy threshold. The voltage constraint is that when all the energy storage units are reduced to the lower voltage limit, the total voltage provided is greater than the set voltage corresponding to the DC transmission line. The first lower limit corresponding to the energy constraint is determined by the following expression: ； in, To set the total energy threshold, This refers to the normal voltage value of the energy storage unit after it is fully charged. This is the lower limit of the voltage. This is the first lower limit. The capacitance value of the energy storage unit; The second lower limit corresponding to the voltage constraint is determined by the following expression: ； in, For the set voltage, This is the second lower limit; The total quantity is greater than or equal to the larger of the first lower limit and the second lower limit.

2. The direct-connected energy storage device according to claim 1, characterized in that, 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 submodule are reduced to the lower voltage limit, the total negative voltage provided by the energy storage units in the full-bridge submodule is greater than the self-clearing voltage threshold.

3. The direct-connected energy storage device according to claim 2, characterized in that, The third quantity lower limit corresponding to the self-clearing constraint is determined by the following expression: ； Wherein, U is the self-clearing voltage threshold. This is the lower limit of the voltage. This is the lower limit of the third quantity.

4. The direct-connected energy storage device according to claim 1, characterized in that, The energy storage unit includes a supercapacitor.

5. The direct-connected energy storage device according to claim 1, characterized in that, The energy storage unit is equipped with a cooling unit.

6. The direct-connected energy storage device according to claim 5, characterized in that, The cooling unit includes wave-shaped heat dissipation fins.

7. A DC transmission system, characterized in that, Includes the direct-connected energy storage device as described in any one of claims 1-6.

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