Flexible direct current converter of series distributed low-voltage energy storage system and working method

Abstract

The invention discloses a flexible direct-current converter of a series distributed low-voltage energy storage system and a working method, and the method comprises the steps: enabling an energy storage type modular multi-level converter embedded in energy storage equipment to be connected in series and installed at a position, close to a grounding point, of the converter, and combining the control flexibility of flexible direct current and the energy adequacy of the energy storage system; according to the invention, a large-scale new energy overhead line power transmission channel can be constructed, the configuration of the sub-modules, the access positions, the number and the installation modes of the half-bridge sub-modules and the energy storage sub-modules are limited, and the MMC comprising the half-bridge sub-modules and the MMC comprising the energy storage sub-modules are connected in series to form the flexible direct current converter, so that the utilization efficiency of energy storage is ensured, and the energy storage efficiency is improved. According to the improved converter, under the condition that original functions are not lost, the maximum potential to ground of the energy storage MMC is lower than the voltage of the direct current bus, the insulation requirement of the energy storage valve group can be reduced, the energy storage valve group can be directly installed on the ground, the manufacturing cost of the energy storage type flexible direct current converter power station is reduced, the investment cost is reduced, and the economical efficiency of the system is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of voltage source converters, and relates to the topology and working method of a modular multilevel converter embedded with an energy storage system, and particularly relates to a flexible DC converter for a series distributed low-voltage energy storage system and a working method thereof. Background Art

[0002] Under the current background of prominent global energy security issues and severe environmental pollution problems, vigorously developing clean energy is an important measure to achieve the goals of "carbon peak and carbon neutrality". The multi-terminal DC power transmission technology based on the Modular Multilevel Converter (MMC) has the advantages of low loss, flexible power controllability, and the ability to receive power at multiple long-distance drop points, and has good application prospects in the large-scale grid connection and transmission of clean energy. However, the MMC-HVDC has a low inertia time constant and cannot effectively support the AC system. Integrating the energy storage system (ES) into the MMC not only provides the energy required for transient support, effectively improves the stability of the energy storage system, but also alleviates the problem of high uncertainty in the output of new energy and improves the power quality. Therefore, organically combining flexible DC with an energy storage system to form an energy storage type flexible DC converter has broad application prospects.

[0003] Regarding the allocation and installation of energy storage units and sub-modules, the higher the ground potential of the valve group is, the farther away from the grounding point, so the higher the insulation level is required. Since the energy storage unit is heavy and requires a low voltage, a small air clearance is required. Conventional high-voltage directly hung energy storage projects generally adopt a vertical installation method, and directly install the container composed of the energy storage unit and the converter on the ground. In the converter station of a flexible DC project, due to its high voltage level, the bridge arm valve group has high requirements for the insulation level, and the installation method is mainly a suspension installation. If the suspension installation method is used to construct an energy storage type flexible DC converter, the weight and volume of the converter valve tower will increase significantly, and the installation difficulty will increase, while the ground vertical installation scheme cannot meet the system insulation requirements. If all the converter valve group sub-modules are connected to the energy storage, the utilization efficiency of the energy storage capacity will be reduced and the cost will increase. If all energy storage sub-modules are used in the converter valve group, it will also bring the risk of too high a voltage level to the energy storage unit. These problems greatly limit the development and construction applications of energy storage type flexible DC converters.

[0004] Therefore, designing an energy storage type flexible DC converter circuit that meets the insulation level, is simpler and more convenient to install, and minimizes costs has good application value. Summary of the Invention

[0005] The main object of the present invention is to overcome the shortcomings and deficiencies of the prior art, and provide a flexible DC converter for a series distributed low-voltage energy storage system and a working method thereof.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A flexible DC converter of a series distributed low-voltage energy storage system disclosed by the present invention is hereinafter referred to as the flexible DC converter disclosed by the present invention for convenience of reference.

[0008] The circuit topology of the flexible DC converter disclosed by the present invention includes 2 series-connected modular multilevel converters. Hereinafter, the modular multilevel converter is abbreviated as MMC. Each MMC includes 6 three-phase bridge arms, an AC output terminal and a DC output terminal. The 6 three-phase bridge arms are symmetrically arranged up and down, including 3 three-phase upper bridge arms and 3 three-phase lower bridge arms. One three-phase upper bridge arm and one three-phase lower bridge arm form a group of bridge arm pairs, a total of 3 groups. Among them, the three-phase terminals of the external AC power grid or equipment, the E terminal of the half-bridge MMC and the F terminal of the energy storage MMC are correspondingly connected. The D terminal of the three-phase lower bridge arm of the energy storage MMC closer to the ground connection point is connected to the negative pole of the DC output terminal. The A terminal of the three-phase upper bridge arm of the half-bridge MMC farther from the ground connection point is connected to the positive pole of the DC output terminal. The C terminal of the three-phase upper bridge arm of the energy storage MMC closer to the ground connection point is connected to the B terminal of the three-phase lower bridge arm of the half-bridge MMC farther from the ground connection point;

[0009] Among them, regardless of whether the positive or negative pole of the DC output terminal is grounded, N energy storage sub-modules and a bridge arm reactor are connected in series from the upper end C of the upper bridge arm to the lower end D of the lower bridge arm in the 6 three-phase bridge arms of the energy storage MMC closer to the ground connection point. M half-bridge sub-modules and a bridge arm reactor are connected in series from the upper end A of the upper bridge arm to the lower end B of the lower bridge arm in the 6 three-phase bridge arms of the half-bridge MMC farther from the ground connection point. To be more conducive to the installation of the equipment, the value of M is set to be greater than N. The specific topological circuit is as Figure 1 shown.

[0010] Compared with the typical energy storage type flexible DC converter circuit topology, the flexible DC converter circuit topology disclosed by the present invention adopts the series connection of a half-bridge MMC and an energy storage MMC. The MMC containing energy storage sub-modules can be directly placed on the ground and installed vertically because it is arranged close to the grounding pole line and has low insulation level requirements. The half-bridge sub-module valve bank connected in series with the MMC containing energy storage sub-modules is installed in a conventional suspension manner. This scheme of separate valve bank arrangement reduces the insulation requirements and installation difficulty of the system, effectively solves the problems of large volume, heavy weight and difficult integration of the energy storage system, and has high feasibility in actual engineering.

[0011] Further, the half-bridge sub-module, hereinafter abbreviated as HB-SM, includes 2 insulated gate bipolar transistors T 1 , T 2 , 2 power diodes D 1 , D 2and a capacitor C 0 , and the connection relationships of the components are as follows: D 1 's anode, D 2 's cathode, T 1 's emitter and T 2 's collector are connected together and serve as the positive output terminal of the half-bridge sub-module. One end of C 0 is connected to T 1 's collector, D 1 's cathode, and the three are connected together. The other end of C 0 is connected to T 2 's emitter, D 2 's anode, and the three are connected together and serve as the negative output terminal of the half-bridge sub-module.

[0012] Further, the energy storage sub-module, hereinafter referred to as the ES-SM for short, includes four insulated gate bipolar transistors T 1 , T 2 , T 3 , T 4 , 4 power diodes D 1 , D 2 , D 3 , D 4 , a capacitor C, a reactor L, and an energy storage unit ESU. And the connection relationships of the components are as follows: D 1 's anode, D 2 's cathode, T 1 's emitter and T 2 's collector are connected together and serve as the positive output terminal of the energy storage sub-module. One end of the capacitor C is connected to T 1 , T 3 's collector, D 1 , D 3 's cathode, and the five are connected together. The other end of the capacitor C is connected to T 2 , T 4 's emitter, D 2 , D 4 's anode, and the negative electrode of the ESU, and the six are connected together and serve as the negative output terminal of the full-bridge sub-module. D 3 's anode, D 4 's cathode, T 3 's emitter, T 4 's collector, and one end of L are connected together. The other end of L is connected to the positive electrode of the ESU.

[0013] For the above two steps, for the convenience of installation, the number of half-bridge sub-modules is generally less than that of the energy storage sub-modules, and the half-bridge sub-modules and the energy storage sub-modules are located in two different MMCs respectively. The power of the upper and lower arms of each MMC is equal, and there will be no fundamental frequency circulating current, reducing the output limit of energy storage.

[0014] To achieve the above object, in a second aspect, the present invention also provides a control method for a flexible DC converter of the above series distributed low-voltage energy storage system, adopting the following technical solutions:

[0015] In the flexible DC converter disclosed in the present invention, in order to make the energy storage type flexible DC converter circuit output a stable DC voltage and three-phase AC voltage, the following operations are carried out simultaneously:

[0016] S1. Obtain the d-axis phase voltage u sd of the AC bus of the MMC converter station, the fundamental wave amplitude U sm of the equivalent phase electromotive force of the AC system. Set the sending-end half-bridge MMC to passive control, and let the sending-end half-bridge MMC provide voltage and frequency for the external AC power grid or equipment. By controlling the d-axis current command value i * vd to make u sd =U sm ; by controlling the q-axis current command value i * vq to make the d-axis phase voltage u sq of the AC bus of the MMC converter station equal to 0. The control law of the q-axis phase voltage of the MMC converter station is as follows:

[0017]

[0018] K Pd and K Id are respectively the proportional parameter and integral parameter of the d-axis passive control PI controller; K Pq and K Iq are respectively the proportional parameter and integral parameter of the q-axis passive control PI controller. The wind farm side is equivalent to a passive network, and the MMC needs to establish a synchronous power supply for the wind farm, otherwise the wind farm will not operate normally.

[0019] S2. Obtain the DC voltage reference values of the receiving-end half-bridge MMC, the sending-end energy storage MMC, and the receiving-end half-bridge MMC, which are collectively referred to as U * dc . Obtain the actual DC voltage values of the receiving-end half-bridge MMC, the sending-end energy storage MMC, and the receiving-end half-bridge MMC, which are collectively referred to as U dc . Set the receiving-end half-bridge MMC, the sending-end energy storage MMC, and the receiving-end half-bridge MMC to DC voltage control, and the control law is as follows:

[0020]

[0021] u * sd and u * sq are respectively the reference values of the d-axis and q-axis phase voltages of the AC bus of the MMC converter station. KP and K I are the proportional parameter and integral parameter of the DC control PI controller respectively. Since the sending - end half - bridge MMC has been passively controlled, in order to ensure the normal DC voltage distribution of the upper and lower two MMCs on the sending - end side and simplify the control scheme to the greatest extent, the sending - end energy - storage MMC selects DC voltage control. Regarding the upper and lower two series - connected MMCs on the sending - end and receiving - end as a whole, the sending - end has powered the passive network. To ensure the stability of the DC line voltage and simplify the control scheme to the greatest extent, the receiving - end needs to control the DC line voltage.

[0022] S3. To make the energy - storage type flexible DC converter circuit output stable DC voltage and three - phase AC voltage, the following operations are carried out simultaneously: For T 3 and T 4 in all 6N energy - storage sub - modules in the converter, pulse - width modulation (PWM) signals are applied to their gates. For T 1 and T 2 in all 6N energy - storage sub - modules in the converter, nearest - level approximation modulation (NLM) signals are applied to their gates. For T 1 and T 2 in all 6M half - bridge sub - modules in the converter, nearest - level approximation modulation (NLM) signals are applied to their gates. The energy - storage sub - module has one more control degree of freedom for controlling the energy - storage output power compared with the half - bridge sub - module. Therefore, PWM signals are applied to T 3 and T 4 to control the internal DC - DC converter for energy - storage.

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

[0024] (1) The present invention improves the proposed typical energy - storage type flexible DC converter, connects the half - bridge MMC and the energy - storage MMC in series. The improved flexible DC converter combines the control flexibility of flexible DC and the energy abundance of the energy - storage system, improves the capacity of the converter, and can build a large - scale new - energy overhead - line power - transmission channel. Regarding the configuration of sub - modules, the access positions and installation methods of half - bridge sub - modules and energy - storage sub - modules are restricted. The MMC containing half - bridge sub - modules is connected in series with the MMC containing energy - storage sub - modules to form a flexible DC converter, which ensures the utilization efficiency of energy - storage and improves economic benefits.

[0025] (2) The flexible DC converter disclosed in the present invention sets the MMC closer to the grounding point as the energy storage MMC, and the MMC farther from the grounding point as the half-bridge MMC. The half-bridge MMC composed of half-bridge sub-modules with high insulation requirements is installed in a suspended manner, and the energy storage MMC composed of energy storage sub-modules with large mass and volume but low voltage is installed on the ground vertically. This can effectively reduce the insulation level of the energy storage system, reduce the installation difficulty of the converter, and effectively solve the problems of large volume, heavy mass, and difficult integration of the energy storage system, and has broad application prospects in the scenario of large-scale long-distance transmission of new energy. Description of the Drawings

[0026] To more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings 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.

[0027] Figure 1 It is the negative pole grounding topology diagram of the flexible DC converter circuit of the series distributed low-voltage energy storage system disclosed in the present invention;

[0028] Figure 2 It is the positive pole grounding topology diagram of the flexible DC converter circuit of the series distributed low-voltage energy storage system disclosed in the present invention;

[0029] Figure 3 It is the circuit topology diagram of the two-terminal power transmission system in Embodiment 1 of the present invention;

[0030] Figure 4 It is the schematic diagram of the simulation waveform of the converter AC voltage of the flexible DC converter disclosed in Embodiment 1 of the present invention;

[0031] Figure 5 It is the schematic diagram of the simulation waveform of the converter DC voltage of the flexible DC converter disclosed in Embodiment 1 of the present invention;

[0032] Figure 6 It is the schematic diagram of the simulation waveform of the converter power of the flexible DC converter disclosed in Embodiment 1 of the present invention;

[0033] Figure 7 It is the circuit topology diagram of the two-terminal power transmission system in Embodiment 2 of the present invention;

[0034] Figure 8 It is the schematic diagram of the simulation waveform of the half-bridge sub-module voltage and the energy storage sub-module voltage of the flexible DC converter disclosed in Embodiment 2 of the present invention;

[0035] Figure 9It is a schematic diagram of the simulation waveform of the energy storage MMC voltage of the flexible DC converter disclosed in Embodiment 2 of the present invention;

[0036] Figure 10 It is a schematic diagram of the simulation waveforms of the maximum voltage to ground of the energy storage MMC of the flexible DC converter disclosed in Embodiment 2 of the present invention and the maximum voltage to ground of the energy storage MMC without using this flexible DC converter. Detailed implementation manners

[0037] In order to enable those skilled in the art to better understand the solutions of this application, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of this application. Obviously, the described embodiments are only a part of the embodiments of this application, rather than all the embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative efforts belong to the scope of protection of this application.

[0038] The mention of "embodiment" in this application means that the specific features, structures or characteristics described in combination with the embodiment may be included in at least one embodiment of this application. The phrase appears in various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described in this application can be combined with other embodiments.

[0039] Embodiment 1

[0040] As Figure 1 shown, this embodiment provides a flexible DC converter circuit for a series distributed low-voltage energy storage system, which is characterized in that the topology of one side of the converter circuit includes 2 serially connected modular multilevel converters. Hereinafter, based on the modular multilevel converter, it is abbreviated as MMC. Each MMC includes 6 three-phase bridge arms, an AC output terminal and a DC output terminal. Among them, the three-phase terminals of the external AC power grid or device, the E terminal of the half-bridge MMC and the F terminal of the energy storage MMC are correspondingly connected. The D terminal of the three-phase lower bridge arm of the energy storage MMC closer to the ground connection point is connected to the negative pole of the DC output terminal, and the A terminal of the three-phase upper bridge arm of the half-bridge MMC farther from the ground connection point is connected to the positive pole of the DC output terminal. The C terminal of the three-phase upper bridge arm of the energy storage MMC closer to the ground connection point is connected to the B terminal of the three-phase lower bridge arm of the half-bridge MMC farther from the ground connection point; among them, the negative pole of the DC output terminal is grounded. Among the 6 three-phase bridge arms of the energy storage MMC closer to the ground connection point, N energy storage sub-modules and a bridge arm reactor are serially connected from the upper end C of the bridge arm to the lower end D of the bridge arm. Among the 6 three-phase bridge arms of the half-bridge MMC farther from the ground connection point, M half-bridge sub-modules and a bridge arm reactor are serially connected from the upper end A of the bridge arm to the lower end B of the bridge arm. For more convenient installation of the equipment, the value of M is set to be greater than N.

[0041] In this embodiment, the half-bridge sub-module, referred to as HB-SM in the figure, includes two insulated gate bipolar transistors T 1 , T 2 , two power diodes D 1 , D 2 and a capacitor C 0 , and the connection relationships of the components are as follows: the anode of D 1 , the cathode of D 2 , the emitter of T 1 and the collector of T 2 are connected together and serve as the positive output terminal of the half-bridge sub-module. One end of C 0 is connected to the collector of T 1 and the cathode of D 1 . The other end of C 0 is connected to the emitter of T 2 and the anode of D 2 , and serves as the negative output terminal of the half-bridge sub-module.

[0042] In this embodiment, the energy storage sub-module, referred to as ES-SM in the figure, includes four insulated gate bipolar transistors T 1 , T 2 , T 3 , T 4 , four power diodes D 1 , D 2 , D 3 , D 4 , a capacitor C, a reactor L and an energy storage unit ESU. The connection relationships of the components are as follows: the anode of D 1 , the cathode of D 2 , the emitter of T 1 and the collector of T 2 are connected together and serve as the positive output terminal of the energy storage sub-module. One end of the capacitor C is connected to the collectors of T 1 , T 3 and the cathodes of D 1 , D 3 . The other end of the capacitor C is connected to the emitters of T 2 , T 4 , the anodes of D 2 , D 4 and the negative electrode of the ESU, and serves as the negative output terminal of the full-bridge sub-module. The anode of D 3 , the cathode of D 4 , the emitter of T 3 , the collector of T 4 and one end of L are connected together. The other end of L is connected to the positive electrode of the ESU

[0043] To demonstrate the functions and advantages of the present invention, a simulation model of a dual-terminal MMC-HVDC with a DC side voltage of +800 kV as shown in Figure 3 is built on the PSCAD / EMTDC simulation platform to simulate the normal operating conditions of the energy storage flexible DC converter circuit and its control method described in the present invention. The specific parameters of the simulation model are shown in Table 1.

[0044] Table 1. Parameter Table of the Simulation Model Provided in Embodiment 1

[0045] Item Value of Embodiment 1 Rated Capacity of Half-bridge MMC 1200MW Rated Capacity of Energy Storage MMC 400MW Rated AC Voltage on Grid Side 500kV Frequency 50Hz Rated DC Voltage +800kV Rated DC Voltage of Half-bridge MMC 600kV Rated DC Voltage of Energy Storage MMC 200kV Rated AC Voltage on Valve Side of Half-bridge MMC 220kV Rated AC Voltage on Valve Side of Energy Storage MMC 110kV Number of Sub-modules in Single Bridge Arm of Half-bridge MMC 80 Number of Sub-modules in Single Bridge Arm of Energy Storage MMC 20 Arm Reactance of Half-bridge MMC 0.057H Arm Reactance of Energy Storage MMC 0.019H Arm Capacitance of Half-bridge MMC 3555.56uF Arm Reactance of Energy Storage MMC 2666.67uF Energy Storage Element ES: Supercapacitor 20F Supercapacitor Voltage 3kV Rated Current of Supercapacitor 840A Carrier Frequency of Energy Storage Control 2kHz

[0046] For the energy storage flexible DC converter circuit and its control method provided in this embodiment, under normal operating conditions, the system stabilizes at about 0.75 s, energy storage is connected at 2 s, the energy storage power command is 0.5 pu, and after increasing to 1 pu at 3 s, the AC and DC voltages at the converter ports are as shown in Figure 4 、 Figure 5 ; the power of the converter is as shown in Figure 6 . The grid-side voltage and the DC-side voltage at the AC connection point of the MMC can both be kept stable, and the energy storage power effectively tracks the command value, which demonstrates that the energy storage flexible DC converter circuit proposed by the present invention has the functions of a voltage source converter and energy storage.

[0047] Embodiment 2

[0048] As shown in Figure 2 , this embodiment provides a flexible DC converter circuit for a series distributed low-voltage energy storage system. The topology of one side of the converter circuit includes 2 series-connected modular multilevel converters. Hereinafter, based on the modular multilevel converter, it is abbreviated as MMC. Each MMC includes 6 three-phase bridge arms, an AC output terminal, and a DC output terminal. Among them, the three-phase terminals of the external AC grid or device, the E terminal of the half-bridge MMC, and the F terminal of the energy storage MMC are correspondingly connected. The D terminal of the three-phase lower bridge arm of the energy storage MMC closer to the ground connection point is connected to the negative pole of the DC output terminal, the A terminal of the three-phase upper bridge arm of the half-bridge MMC farther from the ground connection point is connected to the positive pole of the DC output terminal, and the C terminal of the three-phase upper bridge arm of the energy storage MMC closer to the ground connection point is connected to the B terminal of the three-phase lower bridge arm of the half-bridge MMC farther from the ground connection point; among them, the positive pole of the DC output terminal is grounded. In the 6 three-phase bridge arms of the energy storage MMC closer to the ground connection point, N energy storage sub-modules and a bridge arm reactor are connected in series from the upper end C of the bridge arm to the lower end D of the bridge arm. In the 6 three-phase bridge arms of the half-bridge MMC farther from the ground connection point, M half-bridge sub-modules and a bridge arm reactor are connected in series from the upper end A of the bridge arm to the lower end B of the bridge arm. To be more conducive to the installation of the equipment, the value of M is set to be greater than N.

[0049] In this embodiment, the half-bridge sub-module, which is called HB-SM in the figure, includes two insulated gate bipolar transistors T1 , T 2 , two power diodes D 1 , D 2 and a capacitor C 0 , and the connection relationships of the components are as follows: the anode of D 1 , the cathode of D 2 , the emitter of T 1 and the collector of T 2 are connected together and used as the positive output terminal of the half-bridge sub-module. One end of C 0 is connected to the collector of T 1 and the cathode of D 1 . The other end of C 0 is connected to the emitter of T 2 and the anode of D 2 , and is used as the negative output terminal of the half-bridge sub-module.

[0050] In this embodiment, the energy storage sub-module, referred to as ES-SM in the figure, includes four insulated gate bipolar transistors T 1 , T 2 , T 3 , T 4 , four power diodes D 1 , D 2 , D 3 , D 4 , a capacitor C, a reactor L and an energy storage unit ESU. The connection relationships of the components are as follows: the anode of D 1 , the cathode of D 2 , the emitter of T 1 and the collector of T 2 are connected together and used as the positive output terminal of the energy storage sub-module. One end of the capacitor C is connected to T 1 , T 3 , the collector of T 1 , D 3 , D 2 , T 4 , the emitter of T 2 , D 4 , D 3 , the anode of D 4 , the cathode of D 3 , the emitter of T 4 , the collector of T

[0051] To illustrate the functions and advantages of the present invention, a simulation platform is built on PSCAD / EMTDC as shown in Figure 7The shown double-terminal MMC-HVDC simulation model with a DC side of -800 kV is used to simulate the normal operating conditions of the energy storage flexible DC converter circuit and its control method described in the present invention. The specific parameters of the simulation model are shown in Table 2.

[0052] Table 2. Parameter table of the simulation model provided in Embodiment 2

[0053] Item Value of Embodiment 2 Rated Capacity of Half-bridge MMC 1200MW Rated Capacity of Energy Storage MMC 400MW Rated AC Voltage on Grid Side 500kV Frequency 50Hz Rated DC Voltage -800kV Rated DC Voltage of Half-bridge MMC -600kV Rated DC Voltage of Energy Storage MMC -200kV Rated AC Voltage on Valve Side of Half-bridge MMC 220kV Rated AC Voltage on Valve Side of Energy Storage MMC 110kV Number of Sub-modules in Single Bridge Arm of Half-bridge MMC 80 Number of Sub-modules in Single Bridge Arm of Energy Storage MMC 20 Arm Reactance of Half-bridge MMC 0.057H Arm Reactance of Energy Storage MMC 0.019H Arm Capacitance of Half-bridge MMC 3555.56uF Arm Reactance of Energy Storage MMC 2666.67uF Energy Storage Element ES: Supercapacitor 20F Supercapacitor Voltage 3kV Rated Current of Supercapacitor 840A Carrier Frequency of Energy Storage Control 2kHz

[0054] For the energy storage flexible DC converter circuit and its control method provided in this embodiment, in the normal working state, the system stabilizes at about 0.75 s, energy storage is input at 2 s, the energy storage power command is 1 pu, and the energy storage power command value is reduced to 0.5 pu at 3 s. The response of the system is as Figure 8 、 Figure 9 shown. The voltages of the half-bridge sub-modules and the energy storage sub-modules of the energy storage flexible DC converter can both be kept stable, and the maximum ground potential of the energy storage MMC is lower than the DC bus voltage. Comparing the maximum ground voltage of the energy storage MMC obtained by using this flexible DC converter with that of a full energy storage MMC with only one sub-module number of 100, as Figure 10 shown, it can be concluded that the flexible DC converter can effectively reduce the maximum ground voltage of the energy storage MMC.

[0055] This indicates that the energy storage flexible DC converter circuit proposed in the present invention has the ability to operate stably, and the ground potential of the energy storage MMC is relatively low, which can effectively reduce the insulation level of the energy storage system and reduce the construction difficulty of the converter.

[0056] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as the combinations of these technical features do not conflict, they should be considered as the scope described in this specification.

[0057] The above embodiments are the preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. A flexible DC converter for a series-connected distributed low-voltage energy storage system, characterized in that: The circuit topology of the converter includes two modular multilevel converters connected in series, hereinafter referred to as MMC, each MMC includes six three-phase bridge arms, an AC output terminal and a DC output terminal, the six three-phase bridge arms are arranged symmetrically up and down, including three three-phase upper bridge arms and three three-phase lower bridge arms, one three-phase upper bridge arm and one three-phase lower bridge arm form a group of bridge arm pairs, a total of three groups, wherein the three-phase terminals of the external AC power grid or equipment, the E end of the half-bridge MMC and the F end of the energy storage MMC are connected correspondingly, the D end of the three-phase lower bridge arm of the energy storage MMC closer to the grounding point is connected to the negative pole of the DC output terminal, the A end of the three-phase upper bridge arm of the half-bridge MMC farther from the grounding point is connected to the positive pole of the DC output terminal, and the C end of the three-phase upper bridge arm of the energy storage MMC closer to the grounding point is connected to the B end of the three-phase lower bridge arm of the half-bridge MMC farther from the grounding point; Among them, no matter whether the positive or negative pole of the DC output terminal is grounded, N energy storage sub-modules and a bridge arm inductor are connected in series from the upper end C of the upper bridge arm to the lower end D of the lower bridge arm in the six three-phase bridge arms of the energy storage MMC closer to the grounding point, and M half-bridge sub-modules and a bridge arm inductor are connected in series from the upper end A of the upper bridge arm to the lower end B of the lower bridge arm in the six three-phase bridge arms of the half-bridge MMC farther from the grounding point, and the value of M is set greater than N.

2. The flexible DC converter of the series-connected distributed low-voltage energy storage system disclosed in claim 1 is characterized in that: The half-bridge sub-module, hereinafter referred to as the half-bridge sub-module HB-SM, includes two insulated gate bipolar transistors T1 and T2, two power diodes D1 and D2 and a capacitor C0, and the connection relationship of each component is as follows: the anode of D1, the cathode of D2, the emitter of T1 and the collector of T2 are connected and serve as the positive output terminal of the half-bridge sub-module, one end of C0 is connected to the collector of T1 and the cathode of D1, and the other end of C0 is connected to the emitter of T2 and the anode of D2, and serves as the negative output terminal of the half-bridge sub-module.

3. The flexible DC converter of the series-connected distributed low-voltage energy storage system disclosed in claim 1 is characterized in that: The energy storage submodule, hereinafter referred to as the energy storage submodule ES-SM, comprises four insulated gate bipolar transistors T1, T2, T3, T4, four power diodes D1, D2, D3, D4, a capacitor C, a reactor L and an energy storage unit ESU, and the connection relationship of each component is as follows: the anode of D1, the cathode of D2, the emitter of T1 and the collector of T2 are connected and serve as the positive output terminal of the energy storage submodule, one end of the capacitor C is connected to the collectors of T1 and T3 and the cathodes of D1 and D3, the other end of the capacitor C is connected to the emitters of T2 and T4, the anodes of D2 and D4 and the negative electrode of ESU and serves as the negative output terminal of the full-bridge submodule, the anode of D3, the cathode of D4, the emitter of T3, the collector of T4 and one end of L are connected, and the other end of L is connected to the positive electrode of ESU.

4. A working method of a flexible DC converter of a series-connected distributed low-voltage energy storage system, based on any one of the flexible DC converters of a series-connected distributed low-voltage energy storage system disclosed in claims 1 to 3, characterized in that: The working method comprises the following steps: S1. Obtain the d-axis phase voltage u of the AC busbar of the MMC converter station sd , AC system equivalent phase potential fundamental amplitude U sm , the sending-end half-bridge MMC is set to passive control, and the sending-end half-bridge MMC provides voltage and frequency for the external AC power grid or equipment, and controls the d-axis current command value i on the valve side * vd make u sd =U sm , through the control valve side q axis current command value i * vq Make the q-axis phase voltage u of the AC busbar of the MMC converter station sq =0, the q-axis phase voltage of the AC busbar of the MMC converter station, the control law is as follows: u * sd with u * sq are the reference values ​​of the d-axis and q-axis phase voltages of the AC busbar of the MMC converter station, respectively. Pd , K Id are the proportional parameter and integral parameter of the d-axis passive control PI controller; K Pq , K Iq They are the proportional and integral parameters of the q-axis passive control PI controller; S2, obtain the DC voltage reference values ​​of the receiving end half-bridge MMC, the sending end energy storage MMC, and the receiving end half-bridge MMC, collectively referred to as U * dc , obtain the actual values ​​of the DC voltages of the receiving end half-bridge MMC, the sending end energy storage MMC, and the receiving end half-bridge MMC, which will be collectively referred to as U dc , the receiving end half-bridge MMC, the sending end energy storage MMC, and the receiving end half-bridge MMC are set to DC voltage control, and the control law is as follows: K P , K I They are the proportional and integral parameters of the DC control PI controller respectively; S3. In order to make the energy storage type flexible DC converter circuit output stable DC voltage and three-phase AC voltage, the following operations are performed simultaneously: a pulse width modulation PWM signal is applied to the gates of T3 and T4 in all 6N energy storage submodules in the converter, a nearest level approximation modulation NLM signal is applied to the gates of T1 and T2 in all 6N energy storage submodules in the converter, and a nearest level approximation modulation NLM signal is applied to the gates of T1 and T2 in all 6M half-bridge submodules in the converter.

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