Flexible dc converter for series distributed low voltage energy storage system and working method

Abstract

The application discloses a flexible direct-current converter of a series distributed low-voltage energy storage system and a working method, wherein energy storage type modular multilevel converters embedded with energy storage devices are installed in series in the converter near grounding points; the flexible direct-current control flexibility and energy storage system energy abundance can be combined to build a large-scale new energy overhead line power transmission channel; the configuration of sub-modules, the position, quantity and installation mode of half-bridge sub-modules and energy storage sub-modules are limited; the MMC containing the half-bridge sub-modules is connected in series with the MMC containing the energy storage sub-modules to form the flexible direct-current converter, so that the utilization efficiency of the energy storage is ensured; the improved converter does not lose the original functions, the maximum ground potential of the energy storage MMC is lower than the DC bus voltage, which helps to reduce the insulation requirement of the energy storage valve group, the energy storage valve group can be directly installed on the ground, the cost of the energy storage type flexible direct-current power station is reduced, the investment cost is reduced, and the economy of the system is improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of voltage source type converters, and relates to a topology and a working method of a modular multilevel converter embedded with an energy storage system, in particular to a flexible DC converter with a series distributed low-voltage energy storage system and a working method. BACKGROUND

[0002] Under the background of the prominent global energy security problem and the serious environmental pollution problem, developing clean energy is an important measure to achieve the goal of "carbon peak and carbon neutral". The multi-terminal DC transmission technology based on the modular multilevel converter (MMC) has the advantages of small loss, flexible and controllable power, and remote multi-drop power receiving, and has good application prospects in large-scale clean energy grid connection and transmission. However, the inertia time constant of MMC-HVDC is low, which cannot effectively support the AC system. The integration of the energy storage system (ES) into the MMC not only provides the energy required for transient support and effectively improves the stability of the energy storage system, but also alleviates the problem of high uncertainty of new energy output and improves the power quality. Therefore, the combination of flexible DC and energy storage system to form an energy storage type flexible DC converter has broad application prospects.

[0003] For the distribution and installation of energy storage units and submodules, the higher the distance to the grounding point, the higher the ground potential of the valve group, so the higher the insulation level is required. Because the mass of the energy storage unit is heavy and the required voltage is low, the air clearance is small, and the conventional high-voltage direct hanging type energy storage project generally adopts a vertical installation method, and the container composed of the energy storage unit and the converter is directly installed on the ground. However, in the flexible DC project, the converter station has a high voltage level, and the insulation level of the bridge arm valve group is high, and the installation method is mainly suspension type installation. If the suspension type installation method is used to build the energy storage type flexible DC converter, the weight and volume of the converter valve tower will increase significantly, and the installation difficulty will increase, and the ground vertical installation scheme cannot meet the system insulation requirements. If all the submodules of the converter valve group are connected in the energy storage, the use efficiency of the energy storage capacity will be reduced and the cost will be increased. If all the energy storage submodules are used in the converter valve group, the voltage level of the energy storage unit will be too high. These problems greatly limit the development and construction application of the energy storage type flexible DC converter.

[0004] Therefore, it is of great application value to design an energy storage type flexible DC converter circuit that meets the insulation level, is easier to install, and reduces the cost as much as possible. SUMMARY

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

[0006] In order to achieve the above object, the application adopts the following technical solutions:

[0007] The flexible HVDC converter disclosed in the application is referred to as the flexible HVDC converter disclosed in the application.

[0008] The circuit topology of the flexible HVDC converter disclosed in the application comprises two modular multilevel converters (hereinafter referred to as MMCs) connected in series, each MMC comprising six 3-phase bridge arms, an AC output end and a DC output end, the six 3-phase bridge arms being arranged symmetrically in upper and lower positions and comprising three 3-phase upper bridge arms and three 3-phase lower bridge arms, one 3-phase upper bridge arm and one 3-phase lower bridge arm forming a bridge arm pair, and there being three bridge arm pairs, wherein the 3-phase terminals of an external AC power grid or device, the E end of a half-bridge MMC and the F end of an energy storage MMC are connected correspondingly, the D end of the 3-phase lower bridge arm of the energy storage MMC close to the grounding point is connected to the negative pole of the DC output end, the A end of the 3-phase upper bridge arm of the half-bridge MMC far from the grounding point is connected to the positive pole of the DC output end, and the C end of the 3-phase upper bridge arm of the energy storage MMC close to the grounding point is connected to the B end of the 3-phase lower bridge arm of the half-bridge MMC far from the grounding point.

[0009] In the flexible HVDC converter disclosed in the application, the six 3-phase bridge arms of the energy storage MMC close to the grounding point are connected in series from the upper end C end of the upper bridge arm to the lower end D end of the lower bridge arm, and N energy storage sub-modules and one bridge arm reactor are connected in series, and the six 3-phase bridge arms of the half-bridge MMC far from the grounding point are connected in series from the upper end A end of the upper bridge arm to the lower end B end of the lower bridge arm, and M half-bridge sub-modules and one bridge arm reactor are connected in series, and the value of M is greater than the value of N, so as to facilitate the installation of the device. Figure 1 The specific topology circuit is shown in

[0010] Compared with the typical energy storage type flexible HVDC converter circuit topology, the flexible HVDC converter circuit topology disclosed in the application adopts the series connection of the half-bridge MMC and the energy storage MMC. The MMC comprising the energy storage sub-modules can be directly placed on the ground due to the low insulation level requirement caused by the proximity to the grounding pole line, and is installed vertically, and the half-bridge sub-module valve group connected in series with the MMC comprising the energy storage sub-modules is installed in a conventional suspension mode. The scheme of separate arrangement of the valve groups reduces the insulation requirement and installation difficulty of the system, effectively solves the problems of large volume, heavy weight and integration difficulty of the energy storage system, and has high feasibility in actual engineering.

[0011] Further, the half-bridge submodule, hereinafter referred to as HB-SM, comprises two insulated gate bipolar transistors T1, T2, two power diodes D1, D2 and one capacitor C0, and the connection relationship of each element 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 submodule, one end of C0 is connected with the collector of T1 and the cathode of D1, and the other end of C0 is connected with the emitter of T2 and the anode of D2 and serves as the negative output terminal of the half-bridge submodule.

[0012] Further, the energy storage submodule, hereinafter referred to as ES-SM, comprises four insulated gate bipolar transistors T1, T2, T3, T4, four power diodes D1, D2, D3, D4, one capacitor C, one reactor L and one energy storage unit ESU, and the connection relationship of each element 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 with the collector of T1, T3 and the cathode of D1, D3, the other end of the capacitor C is connected with the emitter of T2, T4, the anode of D2, 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 with the positive electrode of ESU.

[0013] For the above two steps, in order to facilitate installation, the number of half-bridge submodules is generally less than the number of energy storage submodules, and the half-bridge submodules and the energy storage submodules are located in two different MMCs respectively, the power of the upper and lower bridge arms on each MMC is equal, no fundamental frequency circulating current occurs, and the output power limit of the energy storage is reduced.

[0014] In order to achieve the above object, in a second aspect, the application further provides a control method of the flexible DC converter of the series distributed low-voltage energy storage system.

[0015] In the flexible DC converter disclosed in the application, in order to output stable DC voltage and 3-phase AC voltage by the energy storage type flexible DC converter circuit, the following operations are performed simultaneously:

[0016] S1, acquiring the d-axis phase voltage u of the MMC converter station AC bus sd , the fundamental wave amplitude U of the equivalent phase potential of the AC system sm , setting the sending end half-bridge MMC as passive control, providing voltage and frequency for the external AC power grid or equipment by the sending end half-bridge MMC, and controlling the valve side d-axis current command value i * vd u sd = U sm, by controlling the valve side q-axis current command value i * vq The MMC converter station AC bus d-axis phase voltage u sq = 0, the MMC converter station AC bus q-axis phase voltage, the control law is as follows:

[0017]

[0018] K Pd , K Id respectively, the proportional parameter and the integral parameter of the d-axis passive control PI controller; K Pq , K Iq respectively, the proportional parameter and the 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 be able to run normally.

[0019] S2, obtain the DC voltage reference value 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 DC voltage actual value 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 , 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:

[0020]

[0021] u * sd and u * sq respectively, the reference value of the d-axis and q-axis phase voltage of the MMC converter station AC bus. K P , K I respectively, the proportional parameter and the integral parameter of the DC control PI controller. Since the sending end half-bridge MMC has been controlled passively, in order to ensure that the upper and lower MMC DC voltage distribution of the sending end side is normal and the control scheme is maximally simplified, the sending end energy storage MMC selects DC voltage control. The two MMCs in series on the sending end and the receiving end are regarded as a whole, and the sending end has already supplied power to the passive network, in order to ensure that the line DC voltage is stable and the control scheme is maximally simplified, therefore the receiving end needs to control the line DC voltage.

[0022] S3, in order to make the energy storage type flexible converter circuit output stable DC voltage and 3-phase AC voltage, the following operations are performed: pulse width modulation (PWM) signals are applied to the gates of T3 and T4 of all 6N energy storage sub-modules in the converter, nearest level modulation (NLM) signals are applied to the gates of T1 and T2 of all 6N energy storage sub-modules in the converter, and NLM signals are applied to the gates of T1 and T2 of all 6M half-bridge sub-modules in the converter. The energy storage sub-module has one more control degree of freedom for energy storage output power control than the half-bridge sub-module, so PWM signals are applied to T3 and T4 of the energy storage sub-module to control the internal energy storage DC-DC converter.

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

[0024] (1) The present application improves the typical energy storage type flexible converter, and connects the half-bridge MMC and the energy storage MMC in series. The improved flexible converter combines the control flexibility of the 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 transmission channel. The configuration of the sub-modules, the position and installation method of the half-bridge sub-modules and the energy storage sub-modules are limited. The MMC containing the half-bridge sub-modules and the MMC containing the energy storage sub-modules are connected in series to form the flexible converter, which ensures the utilization efficiency of the energy storage and improves the economic benefit.

[0025] (2) The flexible converter disclosed in the present application sets the MMC close to the grounding point as the energy storage MMC and sets the MMC far from the grounding point as the half-bridge MMC. The half-bridge MMC composed of half-bridge sub-modules is suspended for installation, and the energy storage MMC composed of energy storage sub-modules with large mass and volume but low voltage is installed vertically on the ground. This can effectively reduce the insulation level of the energy storage system, reduce the installation difficulty of the converter, effectively solve the problems of large volume, heavy mass and integration difficulty of the energy storage system, and has a broad application prospect in large-scale new energy long-distance transmission scenarios. BRIEF DESCRIPTION OF DRAWINGS

[0026] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0027] Figure 1 is the flexible DC converter circuit negative ground topology of the series distributed low-voltage energy storage system disclosed in the present application;

[0028] Figure 2 is a flexible HVDC converter circuit positive pole grounding topology of the series distributed low-voltage energy storage system disclosed in the present application;

[0029] Figure 3 is a circuit topology of a double-ended power transmission system in embodiment 1 of the present application;

[0030] Figure 4 is a simulation waveform diagram of the converter AC voltage of the flexible HVDC converter disclosed in embodiment 1 of the present application;

[0031] Figure 5 is a simulation waveform diagram of the converter DC voltage of the flexible HVDC converter disclosed in embodiment 1 of the present application;

[0032] Figure 6 is a simulation waveform diagram of the converter power of the flexible HVDC converter disclosed in embodiment 1 of the present application;

[0033] Figure 7 is a circuit topology of a double-ended power transmission system in embodiment 2 of the present application;

[0034] Figure 8 is a simulation waveform diagram of the half-bridge submodule voltage and the energy storage submodule voltage of the flexible HVDC converter disclosed in embodiment 2 of the present application;

[0035] Figure 9 is a simulation waveform diagram of the energy storage MMC voltage of the flexible HVDC converter disclosed in embodiment 2 of the present application;

[0036] Figure 10 is a simulation waveform diagram of the maximum ground voltage of the energy storage MMC of the flexible HVDC converter disclosed in embodiment 2 of the present application and the maximum ground voltage of the energy storage MMC without using the flexible HVDC converter. DETAILED DESCRIPTION

[0037] In order to enable persons skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by persons skilled in the art without creative labor fall within the scope of protection of the present application.

[0038] In this application, the reference to "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this application can be combined with other embodiments.

[0039] Example 1

[0040] like Figure 1 As shown, this embodiment provides a flexible DC converter circuit for a series distributed low-voltage energy storage system. The characteristic feature is that one side of the converter circuit topology includes two series-connected modular multilevel converters (MMCs). Each MMC includes six three-phase arms, an AC output terminal, and a DC output terminal. The three-phase terminals of the external AC grid or equipment, the E terminal of the half-bridge MMC, and the F terminal of the energy storage MMC are connected accordingly. The D terminal of the lower three-phase arm of the energy storage MMC closer to the grounding point is connected to the negative terminal of the DC output terminal. The A terminal of the upper three-phase arm of the half-bridge MMC farther from the grounding point... The C-end of the three-phase upper bridge arm of the energy storage MMC, which is closer to the grounding point, is connected to the B-end of the three-phase lower bridge arm of the half-bridge MMC, which is farther from the grounding point. The negative terminal of the DC output is grounded. Among the six three-phase bridge arms of the energy storage MMC, which is closer to the grounding point, N energy storage sub-modules and one bridge arm reactor are connected in series from the upper C-end to the lower D-end of the bridge arm. Among the six three-phase bridge arms of the half-bridge MMC, which is farther from the grounding point, M half-bridge sub-modules and one bridge arm reactor are connected in series from the upper A-end to the lower B-end of the bridge arm. To facilitate the 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 T1 and T2, two power diodes D1 and D2, and a capacitor C0. 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 together and serve as the positive 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, serving as the negative terminal of the half-bridge sub-module.

[0042] In this embodiment, the energy storage sub-module, referred to as ES-SM in the figure, 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 element 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 sub-module, one end of the capacitor C is connected with the collectors of T1 and T3 and the cathodes of D1 and D3, the other end of the capacitor C is connected with the emitters of T2 and T4, the anodes of D2 and D4 and the negative electrode of the ESU and serves as the negative output terminal of the full-bridge sub-module, 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 with the positive electrode of the ESU

[0043] In order to show the functions and advantages of the present application, a DC side +800kV double-ended MMC-HVDC simulation model as shown in Figure 3 is built on the PSCAD / EMTDC simulation platform to simulate the normal working condition of the energy storage type flexible HVDC circuit and the control method thereof. The specific parameters of the simulation model are shown in Table 1.

[0044] Table 1. Simulation model parameter table provided by example 1

[0045] Item Example 1 value Half-bridge MMC rated capacity 1200 MW Energy storage MMC rated capacity 400 MW Grid-side AC rated voltage 500 kV Frequency 50 Hz DC rated voltage + 800 kV Half-bridge MMC DC rated voltage 600 kV Energy storage MMC DC rated voltage 200 kV Half-bridge MMC valve-side rated AC voltage 220 kV Energy storage MMC valve-side rated AC voltage 110 kV Half-bridge MMC single bridge arm sub-module number 80 Energy storage MMC single bridge arm sub-module number 20 Half-bridge MMC bridge arm reactance 0.057H Energy storage MMC bridge arm reactance 0.019H Half-bridge MMC bridge arm capacitance 3555.56 uF Energy storage MMC bridge arm reactance 2666.67 uF Energy storage element ES: super capacitor 20F Super capacitor voltage 3 kV Super capacitor rated current 840A Energy storage control carrier frequency 2 kHz

[0046] For the energy storage type flexible HVDC circuit and the control method thereof provided by this embodiment, under the normal working condition, the system is stable at about 0.75s, the energy storage is put in at the time of 2s, the energy storage power instruction is 0.5pu, and the converter port AC / DC voltage is as shown in Figure 4 , Figure 5 The power of the converter is as shown in Figure 6 The AC grid connection point grid side voltage and the DC side voltage of the MMC can be kept stable, the energy storage power effectively tracks the instruction value, which shows that the energy storage type flexible HVDC circuit proposed in the present application has the functions of voltage source converter and energy storage.

[0047] Example 2

[0048] As shown in Figure 2As shown, the embodiment provides a flexible HVDC converter circuit of a series distributed low-voltage energy storage system. The converter circuit topology on one side comprises two series-connected modular multilevel converters (hereinafter referred to as MMCs for short). Each MMC comprises six 3-phase bridge arms, an AC output end and a DC output end. The 3-phase terminals of an external AC power grid or device, the E terminals of the half-bridge MMCs and the F terminals of the energy storage MMCs are connected correspondingly. The D terminals of the 3-phase lower bridge arms of the energy storage MMCs close to the grounding point are connected to the negative pole of the DC output end. The A terminals of the 3-phase upper bridge arms of the half-bridge MMCs far from the grounding point are connected to the positive pole of the DC output end. The C terminals of the 3-phase upper bridge arms of the energy storage MMCs close to the grounding point are connected to the B terminals of the 3-phase lower bridge arms of the half-bridge MMCs far from the grounding point. The positive pole of the DC output end is grounded. N energy storage submodules and one bridge reactor are connected in series from the C terminal at the upper end of the 3-phase bridge arm to the D terminal at the lower end of the 3-phase bridge arm in the six 3-phase bridge arms of the energy storage MMCs close to the grounding point. M half-bridge submodules and one bridge reactor are connected in series from the A terminal at the upper end of the 3-phase bridge arm to the B terminal at the lower end of the 3-phase bridge arm in the six 3-phase bridge arms of the half-bridge MMCs far from the grounding point. In order to facilitate the installation of the device, the value of M is greater than that of N.

[0049] In the embodiment, the half-bridge submodule, referred to as HB-SM in the figure, comprises two insulated gate bipolar transistors T1 and T2, two power diodes D1 and D2 and one capacitor C0, and the connection relationship of the elements 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 pole outgoing terminal of the half-bridge submodule; one end of C0 is connected to the collector of T1 and the cathode of D1; the other end of C0 is connected to the emitter of T2 and the anode of D2 and serves as the negative pole outgoing terminal of the half-bridge submodule.

[0050] In the embodiment, the energy storage submodule, referred to as ES-SM in the figure, comprises four insulated gate bipolar transistors T1, T2, T3 and T4, four power diodes D1, D2, D3 and D4, one capacitor C, one reactor L and one energy storage unit ESU, and the connection relationship of the elements 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 pole outgoing 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 pole of ESU and serves as the negative pole outgoing 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; the other end of L is connected to the positive pole of ESU.

[0051] In order to show the functions and advantages of the present application, a simulation platform is built on PSCAD / EMTDC as shown inFigure 7 The DC side-800kV double-ended MMC-HVDC simulation model shown is used to simulate the normal working condition of the energy storage type flexible HVDC circuit and the control method thereof. The specific parameters of the simulation model are shown in Table 2.

[0052] Table 2. Simulation model parameter table provided by Example 2

[0053] Item Example 2 value Half-bridge MMC rated capacity 1200 MW Energy storage MMC rated capacity 400 MW Grid-side AC rated voltage 500 kV Frequency 50 Hz DC rated voltage - 800 kV Half-bridge MMC DC rated voltage - 600 kV Energy storage MMC DC rated voltage - 200 kV Half-bridge MMC valve-side rated AC voltage 220 kV Energy storage MMC valve-side rated AC voltage 110 kV Half-bridge MMC single bridge arm sub-module number 80 Energy storage MMC single bridge arm sub-module number 20 Half-bridge MMC bridge arm reactance 0.057H Energy storage MMC bridge arm reactance 0.019H Half-bridge MMC bridge arm capacitance 3555.56 uF Energy storage MMC bridge arm reactance 2666.67 uF Energy storage element ES: super capacitor 20F Super capacitor voltage 3 kV Super capacitor rated current 840A Energy storage control carrier frequency 2 kHz

[0054] For the energy storage type flexible HVDC circuit and the control method thereof provided by the present embodiment, in the normal working state, the system is stable at about 0.75s, the energy storage is put in at the time of 2s, the energy storage power instruction is 1pu, the energy storage power instruction value is reduced to 0.5pu at 3s, and the response of the system is shown in Figure 8 , Figure 9 The voltage of the half-bridge sub-module of the energy storage type flexible HVDC and the voltage of the energy storage sub-module can be kept stable, and the maximum ground potential of the energy storage MMC is lower than the DC bus voltage. The comparison of the maximum ground voltage of the energy storage MMC obtained by the present flexible HVDC and the full energy storage MMC using only one sub-module with a number of 100 is shown in Figure 10 It can be concluded that the flexible HVDC can effectively reduce the maximum ground voltage of the energy storage MMC.

[0055] It is shown that the energy storage type flexible HVDC circuit proposed in the present embodiment has the ability of stable operation and the ground potential of the energy storage MMC is low, which can effectively reduce the insulation level of the energy storage system and reduce the difficulty of construction of the HVDC.

[0056] The technical features of the above embodiments can be combined arbitrarily. In order to make the description simple, all possible combinations of the technical features in the above embodiments are not described, however, as long as the combination of the technical features does not exist contradictory, it should be considered as the scope of the present application.

[0057] The above embodiments are the preferred embodiments of the present application, but the embodiments of the present application are not limited by the above embodiments, and any changes, modifications, substitutions, combinations and simplifications made without departing from the spirit and principles of the present application shall be equivalent replacement methods and shall be included in the protection scope of the present application.

Claims

1. A flexible DC converter for a series 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 3-phase bridge arms, an AC output terminal, and a DC output terminal. The six 3-phase bridge arms are arranged symmetrically, including three 3-phase upper bridge arms and three 3-phase lower bridge arms. One 3-phase upper bridge arm and one 3-phase lower bridge arm form a bridge arm pair, for a total of three pairs. 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 connected accordingly. The D terminal of the 3-phase lower bridge arm of the energy storage MMC that is closer to the grounding point is connected to the negative terminal of the DC output terminal. The A terminal of the 3-phase upper bridge arm of the half-bridge MMC that is farther from the grounding point is connected to the positive terminal of the DC output terminal. The C terminal of the 3-phase upper bridge arm of the energy storage MMC that is closer to the grounding point is connected to the B terminal of the 3-phase lower bridge arm of the half-bridge MMC that is farther from the grounding point. In this case, regardless of whether the positive or negative terminal of the DC output is grounded, in the six three-phase bridge arms of the energy storage MMC that are closer to the grounding point, N energy storage sub-modules and one 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 six three-phase bridge arms of the half-bridge MMC that are farther from the grounding point, M half-bridge sub-modules and one 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. The value of M is set to be greater than N. The half-bridge submodule, hereinafter referred to as HB-SM, includes two insulated-gate bipolar transistors T1 and T2, two power diodes D1 and D2, and a capacitor C0. 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 together and serve as the positive terminal of the half-bridge submodule. 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 terminal of the half-bridge submodule. The energy storage submodule, hereinafter referred to as ES-SM, comprises four insulated-gate bipolar transistors (IGBTs) T1, T2, T3, and T4, four power diodes D1, D2, D3, and D4, a capacitor C, a reactor L, and an energy storage unit (ESU). The connections of each component are as follows: the anode of D1, the cathode of D2, the emitter of T1, and the collector of T2 are connected together and serve as the positive terminal of the energy storage submodule. One end of capacitor C is connected to the collectors of T1 and T3 and the cathodes of D1 and D3. The other end of capacitor C is connected to the emitters of T2 and T4, the anodes of D2 and D4, and the negative terminal of ESU and serves as the negative 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 together. The other end of L is connected to the positive terminal of ESU.

2. A method for operating a flexible DC converter in a series distributed low-voltage energy storage system, based on the flexible DC converter in a series distributed low-voltage energy storage system as described in claim 1, characterized in that, The working method includes the following steps: S1. Obtain the d-axis phase voltage u of the MMC converter station AC bus. sd The amplitude of the fundamental wave of the equivalent phase potential of the AC system, U sm The sending-end half-bridge MMC is configured as a passive control, providing voltage and frequency to the external AC power grid or equipment via the sending-end half-bridge MMC, and controlling the d-axis current command value i on the control valve side. * vd make u sd =U sm By controlling the q-axis current command value i on the valve side * vq Make the q-axis phase voltage u of the MMC converter station AC bus sq =0, the q-axis phase voltage of the MMC converter station AC bus, the control law is as follows: u * sd with u * sq These are the reference values ​​for the d-axis and q-axis phase voltages of the AC bus at the MMC converter station, respectively, K. Pd K Id These are the proportional and integral parameters of the d-axis passive PI controller; K Pq K Iq These are the proportional and integral parameters of the q-axis passive 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 DC voltage 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 The receiving-end half-bridge MMC, the sending-end energy storage MMC, and the receiving-end half-bridge MMC are all set to DC voltage control, and the control law is as follows: K P K I These are the proportional and integral parameters of the DC control PI controller; S3. In order to make the flexible DC converter circuit output a stable DC voltage and a 3-phase AC voltage, the following operations are performed simultaneously: apply pulse width modulation (PWM) signals to the gates of T3 and T4 in all 6N energy storage submodules in the converter; apply nearest level approximation modulation (NLM) signals to the gates of T1 and T2 in all 6N energy storage submodules in the converter; and apply nearest level approximation modulation (NLM) signals to the gates of T1 and T2 in all 6M half-bridge submodules in the converter.

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