High-voltage direct-current bus direct-hanging energy storage power grid strength supporting type equipment

Abstract

The invention provides a high-voltage direct-current bus direct-hanging energy storage power grid strength support type device, which is connected with a high-voltage alternating-current bus and comprises a plurality of cascaded direct-current sub-modules, a high-voltage direct-current bus and a converter valve, each direct current sub-module comprises an energy storage unit cluster; outgoing lines at two ends of the plurality of cascaded DC sub-modules are connected with a high-voltage DC bus, and the other side of the high-voltage DC bus is connected with a converter valve; the direct current sub-modules convert initial low-voltage direct current output by the energy storage unit cluster into low-voltage direct current needed by power grid strength support, the multiple cascaded direct current sub-modules convert the low-voltage direct current into high-voltage direct current and output the high-voltage direct current to the high-voltage direct current bus, and the high-voltage direct current output by the high-voltage direct current bus is inverted through the converter valve. Stable voltage support is provided for the high-voltage alternating-current bus; isolation of the energy storage unit cluster and the high-voltage alternating-current bus is achieved through the high-voltage direct-current bus, and the influence of reactive power fluctuation of a power grid on energy storage elements is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible AC power transmission and distribution equipment for power grids, and in particular to a power grid strength supporting equipment for directly hanging high-voltage DC busbars for energy storage. Background Art

[0002] With the construction of the "dual carbon" goal, the proportion of new energy such as wind and light in the power grid has gradually increased. Since they use current source grid-connected control, they basically do not have the grid support function, resulting in reduced grid strength and limited new energy acceptance capacity of the power system. The current method to improve the strength of the power grid is to increase voltage source support, such as installing synchronous phase regulators, but this solution has problems such as long construction period, high cost, high loss, and complex maintenance. In addition, grid-connected energy storage with voltage source control, such as grid-type controlled energy storage, can also provide voltage source support. The grid strength support equipment of this scheme combines energy storage and grid support functions, which can not only save the phase regulator of the new energy station and reduce the construction cost of the new energy station, but also has the advantages of low cost, low loss, convenient maintenance and strong controllability compared with the phase regulator.

[0003] There are currently two technical routes for grid voltage strength support equipment based on energy storage. One is to use multiple groups of energy storage converters (Power Conversion System, PCS) connected from the low-voltage side and then boosted by multi-stage transformers. Therefore, multiple groups of PCS are often required to be connected in parallel in the station to achieve the grid-connected capacity. However, the parallel connection of multiple groups of energy storage PCS with voltage source characteristics is prone to cause circulating current and multi-machine oscillation problems, resulting in decreased grid stability; the other technical route is to use high-voltage direct-mounted energy storage, which has the ability to develop large-capacity grid voltage strength support equipment. However, the traditional high-voltage direct-mounted energy storage based on the chain SVG (Static VarGenerator, static reactive generator) structure connects the DC side capacitor of the converter valve submodule in parallel with the energy storage element, resulting in direct coupling between the grid and the energy storage element, and the current fluctuation caused by reactive power flows into the energy storage element, affecting the life of the energy storage element. Summary of the invention

[0004] In order to overcome the shortcomings of the above-mentioned prior art, the present invention proposes a grid strength support type equipment for high-voltage DC busbar direct hanging energy storage, the equipment is connected to an external high-voltage AC busbar, and the equipment comprises: a plurality of cascaded DC submodules, a high-voltage DC busbar and a converter valve; each of the DC submodules comprises an energy storage unit cluster;

[0005] Lead wires at both ends of the multiple cascaded DC sub-modules are connected to the high-voltage DC bus, and the other side of the high-voltage DC bus is connected to the converter valve;

[0006] Each of the DC sub-modules converts the initial low-voltage DC power output by the energy storage unit cluster into the low-voltage DC power required for grid strength support, and the multiple cascaded DC sub-modules convert the low-voltage DC power into high-voltage DC power and output it to the high-voltage DC bus. The high-voltage DC power output by the high-voltage DC bus is inverted by the converter valve to provide stable voltage support for the high-voltage AC bus.

[0007] Optionally, the DC submodule further includes a fully controlled switch device and a DC capacitor;

[0008] Two ends of the fully-controlled switch device are connected to the energy storage unit cluster, and the DC capacitor is connected in parallel with the energy storage unit cluster;

[0009] By adjusting the duty cycle of the fully-controlled switch, the initial low-voltage DC power input by the energy storage unit cluster into the DC capacitor is converted into the low-voltage DC power required by the DC sub-module; the DC capacitor is charged and discharged by the energy storage unit cluster to maintain a constant voltage of the DC sub-module.

[0010] Optionally, the energy storage unit cluster includes a plurality of energy storage cells connected in series, and the energy storage cells include super capacitors and / or energy storage batteries.

[0011] Optionally, the multiple cascaded DC sub-modules constitute a high-voltage DC converter module;

[0012] The minimum required number of the DC submodules in the high-voltage DC converter module is obtained based on the rated voltage of the energy storage unit cluster, the internal resistance and rated current of the energy storage cells in the energy storage unit cluster, and the voltage of the low-voltage DC power required by the DC submodule, combined with the calculation formula for the minimum required number of the DC submodules;

[0013] The rated voltage of the energy storage unit cluster is obtained based on the number of energy storage cells in the energy storage unit cluster and the rated voltage of the energy storage cells.

[0014] Optionally, the minimum required number of DC submodules is calculated as follows:

[0015] (N-1)(U bat -mR nom I bat )≤U DC ≤N·(U bat -mR nom I bat )

[0016] Where N is the minimum required number of DC submodules; U bat represents the rated voltage of the energy storage unit cluster; m represents the number of energy storage units in the energy storage unit cluster; R nomIndicates the internal resistance of the energy storage monomer; I bat Indicates the rated current of the energy storage unit; U DC Indicates the voltage of the low-voltage DC power required by the DC submodule.

[0017] Optionally, a DC capacitor is provided on the high-voltage DC bus;

[0018] Based on the voltage change of the high-voltage DC bus, the DC capacitor is charged and discharged through the multiple cascaded DC sub-modules to maintain the voltage of the high-voltage DC bus constant.

[0019] Optionally, two sides of the converter valve are respectively connected to the high-voltage DC bus and the high-voltage AC bus.

[0020] Optionally, the converter valve includes one of the following: a two-level converter valve with a fully-controlled device series structure, a three-level converter valve with a fully-controlled device series structure, and a converter valve with a modular multi-level converter structure.

[0021] Optionally, the equipment further includes: a filter module, wherein the filter module is connected between the converter valve and the high-voltage AC bus.

[0022] Optionally, the DC submodule further includes a fuse connected in series with the energy storage unit cluster for protecting the circuit.

[0023] Compared with the closest prior art, the present invention has the following beneficial effects:

[0024] The present invention provides a grid strength support type equipment with high-voltage direct current bus directly connected to energy storage, the equipment is connected to an external high-voltage alternating current bus, and the equipment includes: a plurality of cascaded direct current sub-modules, a high-voltage direct current bus and a converter valve; each of the direct current sub-modules includes an energy storage unit cluster; the lead wires at both ends of the plurality of cascaded direct current sub-modules are connected to the high-voltage direct current bus, and the other side of the high-voltage direct current bus is connected to the converter valve; each of the direct current sub-modules converts the initial low-voltage direct current output by the energy storage unit cluster into the low-voltage direct current required for grid strength support, and the plurality of cascaded direct current sub-modules convert the low-voltage direct current into high-voltage direct current and output it to the high-voltage direct current bus, and the high-voltage direct current output by the high-voltage direct current bus is inverted by the converter valve to provide stable voltage support for the high-voltage alternating current bus. In the present invention, the connection between the converter valve and the high-voltage AC bus is achieved, so that the entire supporting equipment is directly hung on the high-voltage AC bus, avoiding the problems of circulating current and transient oscillation caused by multiple groups of low-voltage equipment after being boosted and connected to the grid, and improving the stability of the power grid; and the isolation of the energy storage unit cluster and the high-voltage AC bus is achieved through the high-voltage DC bus, reducing the high-frequency charging and discharging current of the energy storage element caused by the reactive power fluctuation of the power grid, and ensuring the life of the energy storage element. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A schematic diagram of the structure of a grid strength support type equipment for high-voltage DC busbar direct-mounted energy storage provided by the present invention;

[0026] Figure 2 A schematic diagram of a cascade structure of multiple high-voltage DC sub-modules provided by the present invention;

[0027] Figure 3 A topological structure diagram of the DC submodule provided by the present invention;

[0028] Figure 4 A topological diagram of the energy storage battery cluster provided by the present invention;

[0029] Figure 5 A topological diagram of the supercapacitor cluster provided by the present invention;

[0030] Figure 6 A topological structure diagram of a two-level converter valve with an IGBT series structure provided by the present invention;

[0031] Figure 7 A topological structure diagram of a three-level converter valve with an IGBT series structure provided by the present invention;

[0032] Figure 8 This is a topological diagram of the MMC structure converter valve provided by the present invention. DETAILED DESCRIPTION

[0033] The specific implementation modes of the present invention are further described in detail below with reference to the accompanying drawings.

[0034] Example 1

[0035] The present invention provides a high-voltage DC busbar direct-mounted energy storage grid strength support equipment, such as Figure 1 As shown, the equipment is connected to an external high-voltage AC bus, and the equipment includes: a plurality of cascaded DC submodules, a high-voltage DC bus and a converter valve; each of the DC submodules includes an energy storage unit cluster;

[0036] Lead wires at both ends of the multiple cascaded DC sub-modules are connected to the high-voltage DC bus, and the other side of the high-voltage DC bus is connected to the converter valve;

[0037] Each of the DC sub-modules converts the initial low-voltage DC power output by the energy storage unit cluster into the low-voltage DC power required for grid strength support, and the multiple cascaded DC sub-modules convert the low-voltage DC power into high-voltage DC power and output it to the high-voltage DC bus. The high-voltage DC power output by the high-voltage DC bus is inverted by the converter valve to provide stable voltage support for the high-voltage AC bus.

[0038] like Figure 1 As shown, the two sides of the converter valve are connected to the high-voltage DC bus and the high-voltage AC bus, respectively. Through the connection between the converter valve and the high-voltage AC bus, the grid strength support equipment is directly connected to the high-voltage AC bus, that is, directly connected to the high-voltage bus of the AC grid, avoiding the problems of circulating current and transient oscillation caused by multiple groups of low-voltage equipment after boosting and grid connection; by connecting the high-voltage DC converter module including multiple energy storage unit clusters to the high-voltage DC bus, the energy storage is directly connected to the high-voltage DC bus. Compared with other devices that access energy storage through a low-voltage platform and then convert low-voltage DC power into high-voltage DC power through a converter, the steps and structure of power electronic conversion are reduced, making the structure and control of the equipment simpler.

[0039] like Figure 2 As shown, a plurality of cascaded DC sub-modules constitute a high-voltage DC converter module, the output end of each DC sub-module is cascaded, and the lead wires at both ends of the plurality of cascaded DC sub-modules are connected to the high-voltage DC bus.

[0040] like Figure 3 As shown, the DC submodule includes a fully controlled switch device, a DC capacitor, a DC switch and the energy storage unit cluster, and has the function of bidirectional charging and discharging of the energy storage element;

[0041] Specifically, the fully-controlled switch device forms a half-bridge structure through the switch devices and diodes in the upper tube of the bridge arm and the lower tube of the bridge arm, a bypass switch is connected in parallel to the lower tube of the bridge arm, and the two ends of the fully-controlled switch device are connected to the DC capacitor;

[0042] The DC capacitor is connected in parallel with the energy storage unit cluster and is used to charge and discharge itself through the energy storage unit cluster to maintain the voltage stability of the DC submodule;

[0043] The energy storage unit cluster is connected to the two ends of the fully controlled switch device through its own series DC switch. The high-voltage DC / DC (Direct Current) link, that is, the high-voltage DC / DC converter module, is abbreviated as the high-voltage DC converter module. In the high-voltage DC / DC link, the output DC voltage can be controlled by controlling the duty cycle of the fully controlled switch device on the bridge arm of the DC sub-module. Therefore, in the DC sub-module, by adjusting the duty cycle of the fully controlled switch, the initial low-voltage DC power input to the DC capacitor by the energy storage unit cluster is converted into the low-voltage DC power required by the DC sub-module, that is, the electric energy that the DC sub-module needs to output during the power grid failure.

[0044] The specific expression for controlling the output DC voltage and duty cycle is:

[0045] U DC =N·D·U bat

[0046] Among them, U DC represents the voltage of the low-voltage DC power required by the DC submodule; N represents the number of DC submodules, which can be obtained by the minimum required number of DC submodules; D represents the duty cycle of the upper tube of the bridge arm of the DC submodule; U bat Indicates the rated voltage of the energy storage unit cluster.

[0047] The DC submodule also includes a fuse connected in series with the energy storage element for protecting the circuit.

[0048] Each of the energy storage unit clusters includes a plurality of energy storage cells connected in series, wherein the energy storage cells include supercapacitors and / or energy storage batteries, typically lithium iron phosphate battery cells, lithium titanate battery cells, supercapacitor cells, etc. Figure 4 As shown, it is an energy storage unit cluster using energy storage batteries; Figure 5 As shown, a cluster of energy storage units using supercapacitors is shown.

[0049] The rated voltage of the energy storage unit cluster is obtained based on the number of energy storage cells in the energy storage unit cluster and the rated voltage of the energy storage cells. cell , the rated voltage of the energy storage unit cluster after m energy storage monomers are connected in series is:

[0050] U bat =mU cell

[0051] Among them, U bat represents the rated voltage of the energy storage unit cluster; m represents the number of energy storage units connected in series in the energy storage unit cluster; U cell Indicates the rated voltage of the energy storage unit.

[0052] According to the DC voltage, taking into account the loss of the internal resistance of the energy storage monomer, the minimum required number of the DC sub-modules in the high-voltage DC converter module is determined. Specifically, the minimum required number of the DC sub-modules in the high-voltage DC converter module is obtained based on the rated voltage of the energy storage unit cluster, the internal resistance and rated current of the energy storage monomer in the energy storage unit cluster, and the voltage of the low-voltage DC power required by the DC sub-module, combined with the calculation formula for the minimum required number of the DC sub-modules;

[0053] The minimum required number of DC submodules is calculated as follows:

[0054] (N-1)(U bat -mR nom I bat )≤U DC ≤N·(U bat -mR nom I bat )

[0055] Where N is the minimum required number of DC submodules; U bat represents the rated voltage of the energy storage unit cluster; m represents the number of energy storage units in the energy storage unit cluster; R nom Indicates the internal resistance of the energy storage monomer; I bat Indicates the rated current of the energy storage unit; U DC Indicates the voltage of the low-voltage DC power required by the DC submodule.

[0056] A DC capacitor is provided on the high-voltage DC bus;

[0057] Based on the voltage change of the high-voltage DC bus, the DC capacitor is charged and discharged through the multiple cascaded DC sub-modules to maintain the voltage of the high-voltage DC bus constant.

[0058] The converter valve is composed of a fully controlled device, and can be a converter valve based on an IGBT (Insulated Gate Bipolar Transistor) series structure, such as Figure 6 As shown in FIG. 1 , a two-level converter valve with an IGBT series structure is shown. Each phase bridge arm adopts an IGBT device directly connected in series structure. The DC side is connected to the high-voltage DC bus, and the AC side is connected to the A, B, and C three-phase grid (three-phase high-voltage AC bus) through the bridge arm reactance output. Figure 7 As shown in FIG. 1 , a three-level converter valve with an IGBT series structure is shown. Each phase bridge arm adopts an IGBT device directly connected in series structure. Each phase bridge arm is clamped by two groups of diodes. The DC side is connected to the high-voltage DC bus, and the AC side is connected to the A, B, and C phases of the power grid through the bridge arm reactance output. A converter valve with an MMC (Modular Multilevel Converter) structure can also be selected, such as Figure 8 As shown, each phase consists of two bridge arms, upper and lower. A single bridge arm is cascaded by submodules SM (Submodule) and connected to the grid via bridge arm reactance; the reactance is used to protect the converter valve and the grid. The submodule SM can adopt a full-bridge structure or a half-bridge structure composed of fully controlled devices and be equipped with a bypass switch. The bypass switch can be composed of a mechanical switch, a thyristor switch, or both in parallel.

[0059] Optionally, the equipment further includes a filter module, wherein the filter module is connected between the converter valve and the high-voltage AC bus, and the AC side of the converter valve is directly connected to the high-voltage bus of the AC power grid via the filter module.

[0060] In the present invention, the energy storage element (i.e., energy storage unit cluster) is integrated into the DC submodule, and the grid connection is achieved through the high-voltage DC / DC link and the DC-AC conversion link of the converter valve. The converter valve controls the output of three-phase AC current to provide support for the grid during faults. The high-voltage DC / DC link controls the charging and discharging of the energy storage unit cluster through the DC submodule to control the voltage stability of the high-voltage DC bus, ensuring that the converter valve can stably and continuously output current during grid faults, so that the equipment has the function of supporting the grid voltage in the case of voltage faults, and during frequency faults, supports the inertia of the power system and participates in the frequency modulation of the system to maintain the stable operation of the grid.

[0061] In addition to the above-mentioned traditional direct-mounted energy storage device, the DC side capacitor of the converter valve submodule is connected in parallel with the energy storage element, which will cause direct coupling between the power grid and the energy storage element. The current fluctuation caused by reactive power flows into the energy storage element, affecting the life of the energy storage element. The energy storage element is also limited by the operating range of the module DC voltage. The energy storage element has a large installed capacity but low utilization rate and high cost. In addition, due to the flammable and explosive characteristics of the energy storage element, the topological structure of the direct coupling of the converter valve and the energy storage element has problems such as low safety and difficulty in fire protection design.

[0062] In view of the large-capacity design of the above-mentioned grid strength support equipment and the requirements for improving the life and utilization rate of energy storage, the present invention provides a grid strength support equipment with direct-mounted energy storage on a high-voltage DC bus, which has the function of supporting the grid voltage in the event of a grid voltage failure, supporting the inertia of the power system during a frequency failure and participating in the frequency modulation of the system to maintain stable operation of the grid; through the connection between the converter valve and the high-voltage AC bus, the entire support equipment is directly mounted on the high-voltage AC bus, avoiding the problems of circulating current and transient oscillation caused by multiple groups of low-voltage equipment after boosting and grid connection, and realizing large-capacity design of a single group of equipment; the isolation of the energy storage unit cluster from the high-voltage AC bus is achieved through the high-voltage DC bus, reducing the high-frequency charging and discharging current of the energy storage element caused by the reactive power fluctuation of the grid, ensuring the life of the energy storage element, and facilitating the access of large-scale energy storage.

[0063] At the same time, the energy storage element and the converter valve are designed separately, which can achieve independent configuration of voltage and reactive power support capability and energy storage capacity, improve the utilization rate of the energy storage element, and can independently configure fire protection, which is highly safe.

[0064] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit its protection scope. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that after reading the present invention, those skilled in the art can still make various changes, modifications or equivalent substitutions to the specific implementation methods of the application, but these changes, modifications or equivalent substitutions are all within the protection scope of the claims to be approved.

Claims

1. A grid strength support equipment for high-voltage DC busbar direct hanging energy storage, characterized in that: The equipment is connected to an external high-voltage AC bus, and comprises: a plurality of cascaded DC submodules, a high-voltage DC bus and a converter valve; each of the DC submodules comprises an energy storage unit cluster; Lead wires at both ends of the multiple cascaded DC sub-modules are connected to the high-voltage DC bus, and the other side of the high-voltage DC bus is connected to the converter valve; Each of the DC sub-modules converts the initial low-voltage DC power output by the energy storage unit cluster into the low-voltage DC power required for grid strength support, and the multiple cascaded DC sub-modules convert the low-voltage DC power into high-voltage DC power and output it to the high-voltage DC bus. The high-voltage DC power output by the high-voltage DC bus is inverted by the converter valve to provide stable voltage support for the high-voltage AC bus.

2. The device according to claim 1, characterized in that The DC submodule also includes a fully controlled switch device and a DC capacitor; Two ends of the fully-controlled switch device are connected to the energy storage unit cluster, and the DC capacitor is connected in parallel with the energy storage unit cluster; By adjusting the duty cycle of the fully-controlled switch, the initial low-voltage DC power input by the energy storage unit cluster into the DC capacitor is converted into the low-voltage DC power required by the DC sub-module; the DC capacitor is charged and discharged by the energy storage unit cluster to maintain a constant voltage of the DC sub-module.

3. The device according to claim 1, characterized in that: The energy storage unit cluster includes a plurality of energy storage cells connected in series, and the energy storage cells include super capacitors and / or energy storage batteries.

4. The device according to claim 3, characterized in that The multiple cascaded DC sub-modules constitute a high-voltage DC converter module; The minimum required number of the DC submodules in the high-voltage DC converter module is obtained based on the rated voltage of the energy storage unit cluster, the internal resistance and rated current of the energy storage cells in the energy storage unit cluster, and the voltage of the low-voltage DC power required by the DC submodule, combined with the calculation formula for the minimum required number of the DC submodules; The rated voltage of the energy storage unit cluster is obtained based on the number of energy storage cells in the energy storage unit cluster and the rated voltage of the energy storage cells.

5. The device according to claim 4, characterized in that The minimum required number of DC submodules is calculated as follows: (N-1)(U bat -mR nom I bat )≤U Dc ≤N·(U bat -mR nom I bat ) Where N is the minimum required number of DC submodules; U bat represents the rated voltage of the energy storage unit cluster; m represents the number of energy storage units in the energy storage unit cluster; R nom Indicates the internal resistance of the energy storage monomer; I bat Indicates the rated current of the energy storage unit; U DC Indicates the voltage of the low-voltage DC power required by the DC submodule.

6. The apparatus according to claim 1, characterized in that A DC capacitor is provided on the high-voltage DC bus; Based on the voltage change of the high-voltage DC bus, the DC capacitor is charged and discharged through the multiple cascaded DC sub-modules to maintain the voltage of the high-voltage DC bus constant.

7. The apparatus according to claim 1, characterized in that Both sides of the converter valve are connected to the high-voltage DC bus and the high-voltage AC bus respectively.

8. The apparatus according to claim 7, characterized in that The converter valve includes one of the following: a two-level converter valve with a fully-controlled device series structure, a three-level converter valve with a fully-controlled device series structure, and a converter valve with a modular multi-level converter structure.

9. The apparatus according to claim 7, characterized in that Also includes: A filter module is connected between the converter valve and the high-voltage AC bus.

10. The apparatus according to claim 2, characterized in that The DC submodule also includes a fuse connected in series with the energy storage unit cluster for protecting the circuit.

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