Centralized local energy storage modular multilevel converter MMC-CLES topology

Abstract

The invention relates to the technical field of flexible direct-current power transmission, in particular to a centralized local energy storage modular multilevel converter MMC-CLES topology which comprises three phase units, each phase unit comprises an upper bridge arm and a lower bridge arm, and each upper bridge arm and each lower bridge arm comprise a bridge arm reactor arranged at one end, close to a power grid, of the bridge arm; a plurality of identical sub-modules SM which are connected to a power grid through a bridge arm reactor; the upper bridge arm and the lower bridge arm form a symmetrical structure; an energy storage sub-module is arranged at the position where the three-phase unit is directly connected with the direct-current bus. According to the invention, the control system is simplified, and the hardware cost and the control complexity are reduced. Voltage stress and current stress are smaller, higher power can be borne, and the high-power capacitor is suitable for being applied to high-power occasions. And moreover, due to the symmetrical layout, the control method is relatively simple, soft switching can be realized, and the power divider can be used as a multi-port component unit.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible DC power transmission, and particularly to a centralized local energy storage modular multilevel converter MMC-CLES topology. Background Art

[0002] At present, various new energy power generation forms such as wind power generation and solar power generation are developing rapidly. However, they all have randomness and volatility, which will have a huge impact on the safe and stable operation of the power grid.

[0003] A distributed power grid based on new energy power generation technology is a small power grid dispersed around users, with good environmental compatibility. However, due to its own weak power grid attribute, there are inevitably power quality problems such as three-phase imbalance and large harmonics.

[0004] Some scholars have found that introducing energy storage technology into grid-connected converters can overcome the power quality problems brought by new energy grid connection. However, for the MMC (Modular Multilevel Converter) with a centralized energy storage structure, the flexibility of capacity selection is poor, the cost is high, and it has a great impact on the service life of energy storage components. The MMC with a distributed energy storage structure has problems such as dispersed energy storage units and inconvenient assembly and maintenance.

[0005] The information disclosed in this background art section is only intended to deepen the understanding of the overall background art of the present invention, and should not be regarded as an admission or any form of suggestion that this information constitutes the prior art known to those skilled in the art. Summary of the Invention

[0006] The present invention provides a centralized local energy storage modular multilevel converter MMC-CLES topology, thereby effectively solving the problems in the background art.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is: a centralized local energy storage modular multilevel converter MMC-CLES topology, including the following steps: Three-phase units, each phase unit includes an upper arm and a lower arm, and each of the upper arm and the lower arm includes: An arm reactor, which is arranged at one end of the arm close to the power grid; A plurality of identical sub-modules SM, and a plurality of the sub-modules SM are connected to the power grid through the arm reactor; The upper arm and the lower arm form a symmetric structure; An energy storage sub-module is arranged at the position where the three-phase unit is directly connected to the DC bus.

[0008] Further, two of the sub-modules SM of the three-phase unit are commonly used and connected to the DC bus side. Each arm has n - 1 sub-modules SM, and the three-phase unit has a total of 6(n - 1) sub-modules SM.

[0009] Further, a common energy storage sub-module ESM is shared at the position where the three-phase unit is directly connected to the DC bus, and each arm has only one shared energy storage sub-module.

[0010] Further, the sub-module SM adopts a half-bridge structure and includes: Two anti-parallel power switch tubes; A DC capacitor module connected in parallel to the series branch of the two power switch tubes; Wherein, the connection end of the common terminal of the two power switch tubes and the DC capacitor module is used as the output end of each sub-module. When one of the power switch tubes is turned on and the other is turned off, the sub-module is in the inserted or removed state. When both power switch tubes are turned off, the sub-module is in the locked state.

[0011] Further, the energy storage sub-module ESM includes: The sub-module SM structure, a dual-active bridge DAB circuit and an energy storage battery. The DAB circuit is connected in parallel with the DC capacitor module in the sub-module SM structure. After the energy storage battery is connected in series with the power switch tubes, it is then connected in parallel with the DC capacitor module.

[0012] Further, the DAB circuit includes: Two symmetrical H-bridges HB1 and HB2; A high-frequency isolation transformer and its equivalent leakage inductance disposed between the two H-bridges; Input and output side parallel capacitors.

[0013] Further, the DAB circuit modulates the input and output DC voltages on both sides into AC square-wave voltages through phase-shift control and forms an AC / AC equivalent link in series with the equivalent leakage inductance.

[0014] Further, the HB1 is an input bridge, which is a full-bridge circuit and includes: The primary full-bridge circuit, the input side parallel capacitor C1 and the inductor L; The primary full-bridge circuit includes two sets of arms. The two sets of arms include a first arm composed of switch tubes S3 / S4 and a second arm composed of switch tubes S5 / S6. The midpoint of the second arm and the first arm is connected to the primary winding of the high-frequency isolation transformer through the inductor L. The two sets of arms are connected in parallel with the input side parallel capacitor C1; The HB2 is an output bridge, which is a full-bridge circuit and includes: The secondary full-bridge circuit is connected in parallel with the output-side parallel capacitor C2, and the secondary full-bridge circuit is in parallel with the output-side parallel capacitor C2; The midpoint of the secondary full-bridge circuit is connected to the secondary winding of the high-frequency isolation transformer. The secondary full-bridge circuit includes a third bridge arm composed of switching tubes S7 / S8 and a fourth bridge arm composed of switching tubes S9 / S 10 which is composed of

[0015] The beneficial effects of the present invention are as follows: By arranging an energy storage sub-module at the position where the three-phase unit is directly connected to the DC bus, the control system is simplified, and the hardware cost and control complexity are reduced. The voltage stress and current stress are smaller, and it can withstand a larger power, making it suitable for applications in high-power occasions. The converter with this structure has the characteristics of high power density, and due to the symmetric layout, the control method is relatively simple, and soft switching can be achieved, and it can be used as a component unit of a multi-port. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0017] Figure 1 It is a centralized local energy storage modular multilevel converter MMC-CLES topology structure; Figure 2 It is an SM sub-module topology structure; Figure 3 It is an ESM sub-module topology structure Figure 4 It is a circuit topology diagram of a DAB converter; Figure 5 It is the waveform of the DC bus voltage when the active power of the power grid increases; Figure 6 It is the waveform of the DC bus voltage when the active power of the power grid decreases. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0018] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments.

[0019] Embodiment 1: As Figure 1 shown: A centralized local energy storage modular multilevel converter MMC-CLES topology includes the following steps: Three-phase unit, each phase unit includes an upper bridge arm and a lower bridge arm, and each upper bridge arm and lower bridge arm includes: A bridge arm reactor, which is arranged at one end of the bridge arm close to the power grid; A number of identical sub-modules SM, and a number of sub-modules SM are connected to the power grid through the bridge arm reactor; The upper bridge arm and the lower bridge arm form a symmetric structure; An energy storage sub-module is arranged at the position where the three-phase unit is directly connected to the DC bus.

[0020] By arranging an energy storage sub-module at the position where the three-phase unit is directly connected to the DC bus, the control system is simplified, the hardware cost and control complexity are reduced. The voltage stress and current stress are smaller, it can withstand a larger power, and it is suitable for applications in high-power occasions. The converter with this structure has the characteristics of high power density, and because of the symmetric layout, the control method is relatively simple, soft switching can be realized, and it can be used as a component unit of a multi-port.

[0021] In this embodiment, two sub-modules SM of the three-phase unit are commonly used to connect to the DC bus side. Each bridge arm has n - 1 sub-modules SM, and the three-phase unit has a total of 6(n - 1) sub-modules SM.

[0022] A common energy storage sub-module ESM is used at the position where the three-phase unit is directly connected to the DC bus, and each bridge arm has only one common energy storage sub-module.

[0023] Among them, the sub-module SM adopts a half-bridge structure and includes: Two anti-parallel power switch tubes; A DC capacitor module connected in parallel on the series branch of the two power switch tubes; Among them, the connection end of the common terminal of the two power switch tubes and the DC capacitor module is used as the output end of each sub-module. When one of the power switch tubes is turned on and the other is turned off, the sub-module is in the inserted or removed state. When both power switch tubes are turned off, the sub-module is in the locked state.

[0024] As a preference of the above embodiment, the energy storage sub-module ESM includes: The structure of the sub-module SM, the dual active bridge DAB circuit and the energy storage battery. The DAB circuit is connected in parallel with the DC capacitor module in the sub-module SM structure. After the energy storage battery is connected in series with the power switch tube, it is then connected in parallel with the DC capacitor module.

[0025] It only contains two energy storage sub - modules, and the main components are still ordinary sub - modules. By optimizing the number of energy storage sub - modules, the control system is simplified, and the hardware cost and control complexity are reduced. The topology of the Dual Active Bridge (DAB) bidirectional DC - DC converter has symmetry, and all switching device components are active semiconductor fully - controlled types, enabling bidirectional energy transmission. The voltage stress and current stress are smaller, and it can withstand greater power, making it suitable for high - power applications. The converter with this structure has the characteristics of high power density, and due to the symmetrical layout, the control method is relatively simple, can achieve soft - switching, and can be used as a component unit of multi - ports. The resonant technology is used to solve the problem of switching loss, greatly reducing the volume and weight of the switching power supply.

[0026] In this embodiment, the DAB circuit includes: Two symmetrical H - bridges HB1 and HB2; A high - frequency isolation transformer and its equivalent leakage inductance arranged in the middle of the two H - bridges; Parallel capacitors on the input and output sides.

[0027] As a preference of the above - mentioned embodiment, the DAB circuit modulates the input and output DC voltages on both sides into AC square - wave voltages through phase - shift control, and forms an AC / AC equivalent link in series with the equivalent leakage inductance.

[0028] As a preference of the above - mentioned embodiment, HB1 is the input bridge, which is a full - bridge circuit and includes: The primary - side full - bridge circuit, the parallel capacitor C1 and the inductor L on the input side; The primary - side full - bridge circuit includes two groups of bridge arms. The two groups of bridge arms include the first bridge arm composed of switching tubes S3 / S4 and the second bridge arm composed of switching tubes S5 / S6; the mid - point of the second bridge arm and the first bridge arm is connected to the primary - side winding of the high - frequency isolation transformer through the inductor L, and the two groups of bridge arms are in parallel with the parallel capacitor C1 on the input side; HB2 is the output bridge, which is a full - bridge circuit and includes: The secondary - side full - bridge circuit and the parallel capacitor C2 on the output side, and the secondary - side full - bridge circuit is in parallel with the parallel capacitor C2 on the output side; The mid - point of the secondary - side full - bridge circuit is connected to the secondary - side winding of the high - frequency isolation transformer. The secondary - side full - bridge circuit includes the third bridge arm composed of switching tubes S7 / S8 and the fourth bridge arm composed of switching tubes S9 / S 10 ...

[0029] By using the phase difference between the driving signals of the H-bridge switching tubes on the left and right sides of the transformer, square waves with different phases are obtained on both sides of the equivalent leakage inductance, and their duty cycles remain constant. The energy transfer is achieved by adjusting the phase difference. Since the energy is transferred from the side with the leading phase to the side with the lagging phase, when the phase of HB1 is ahead of that of HB2, the converter operates in the forward mode; when the phase of HB1 lags behind that of HB2, the converter operates in the reverse mode. The transmission power of the converter is determined by the phase shift angle of the control signals of the H-bridge switching tubes on the primary and secondary sides of the transformer, and the phase shift angle is equal to the phase difference between the AC square wave voltages on both sides.

[0030] By optimizing the number of energy storage sub-modules, the control system is simplified, and the hardware cost and control complexity are reduced. At the same time, a DAB converter is introduced to achieve electrical isolation and soft-switching technology, solve the problem of switching losses, and make the system lighter and smaller in high-voltage and high-power applications. This helps to improve the transient voltage stability.

[0031] Embodiment 2: As Figure 1 shown, the topology includes three phases, each phase unit contains two upper and lower arms, and each arm contains 1 arm reactor and n identical sub-modules, and each arm is completely symmetrical and is connected to the power grid through the arm reactor.

[0032] As Figure 2 shown, the ordinary SM sub-module adopts a half-bridge structure and is composed of two anti-parallel power switching tubes S1, S2 and a capacitor module C. The switching devices S1 and S2 are connected in series and then connected in parallel with the DC capacitor C. The common terminal of S1 and S2 and the common terminal of the capacitor C and S2 are used as the output terminals of each unit and are connected to other units. When the capacitor module operates normally, S1 and S2 conduct alternately and complementarily. When S1 conducts and S2 turns off, the sub-module is in the inserted state; when S2 conducts and S1 turns off, the sub-module is in the removed state; when S1 turns off and S2 turns off, the sub-module is in the locked state.

[0033] As Figure 3 shown, the ESM sub-module is based on the ordinary SM sub-module and a DAB circuit and an energy storage battery are connected in parallel to the capacitor C. After the energy storage battery and the switch are connected in series, they are then connected in parallel with the capacitor C.

[0034] As Figure 4As shown in the figure, the DAB circuit consists of two symmetrical H-bridges (HB1 is the input-side bridge and HB2 is the output-side bridge), a high-frequency isolation transformer in the middle, its equivalent leakage inductance, and parallel capacitors on the input and output sides. Among them, V1 is the input-side DC voltage, V2 is the output-side DC voltage, C1 is the parallel capacitor on the input side, C2 is the parallel capacitor on the output side, which plays a filtering role, L is the equivalent leakage inductance, and the turns ratio of the high-frequency transformer is set to n:1. The two sides of this dual DC converter are symmetrical with the high-frequency isolation transformer in the middle as the axis. By phase-shift control, the input and output DC voltages on both sides are modulated into AC square-wave voltages, which are connected in series with the leakage inductance to jointly form the AC / AC equivalent link of the converter. The input-side bridge HB1 consists of a full-bridge circuit, the input-side parallel capacitor C1, and the inductor L. The two bridge arms include the first bridge arm composed of the switching tubes S3 / S4 and the second bridge arm composed of the switching tubes S5 / S6. The midpoint between the second bridge arm and the first bridge arm on the primary side is connected to the primary winding of the high-frequency transformer through the inductor L, and the two bridge arms are connected in parallel with the input-side parallel capacitor C1. The output-side bridge HB2 consists of a full-bridge circuit connected in parallel with the output-side parallel capacitor C2. The midpoint of the full-bridge circuit is connected to the secondary winding of the high-frequency transformer. The secondary full-bridge circuit includes the third bridge arm composed of the switching tubes S7 / S8 and the fourth bridge arm composed of the switching tubes S9 / S 10 which is composed of

[0035] When the DC bus voltage changes unstably, using the phase difference of the driving signals of the H-bridge switching tubes on the left and right sides of the transformer, square waves with different phases are obtained on both sides of the equivalent leakage inductance, and their duty cycles remain constant. By adjusting the phase difference, energy transfer is achieved. When the DC bus voltage rises, the phase of the input-side bridge HB1 leads that of the output-side bridge HB2, and the converter is in the forward operating mode. The surplus power is absorbed by charging the battery through the DAB circuit, thereby reducing the DC bus voltage. When the DC bus voltage drops, the phase of the input-side bridge HB1 lags that of the output-side bridge HB2, and the converter is in the reverse operating mode. The battery discharges to the DC bus side through the DAB circuit, thereby causing the DC bus voltage to rise and recover.

[0036] The specific steps are as follows: Step 1: Build a three-phase 23-level MMC-CLES system in the Matlab / Simulink environment. The selected DC voltage of the system is 11 kV, and the grid AC voltage is 6.6 kV; Step 2: Determine the battery model according to the established power system. The selected lithium battery has a rated voltage of 450 V and a rated capacity of 200 Ah; Step 3: Build a DAB converter. The output-side capacitor C2 is 2 mF, and the frequency is 10 kHz; Verification experiment When configuring the topology of the centralized local energy storage modular multilevel converter MMC-CLES, a fault is simulated at 0.5 s. The active power of the power grid increases from 2 MW to 3 MW, and the DC bus voltage rises. Energy storage is put into operation at 1 s, and the DC bus voltage of the system is as Figure 5 shown. The surplus power is absorbed by charging the battery through the forward working mode of the DAB converter, and the DC bus voltage drops accordingly.

[0037] The active power of the power grid decreases from 2 MW to 1 MW, and the DC bus voltage drops accordingly. Energy storage is put into operation at 1 s, and the DC bus voltage of the system is as Figure 6 shown. The DAB converter is in the reverse working mode, and the battery discharges to the DC bus side through the DAB circuit, so that the DC bus voltage rises and recovers.

[0038] It is verified that the proposed topology has good stability after the battery is connected, can effectively suppress the power fluctuation, and realizes the stable operation of the system.

[0039] In the description of the present invention, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features. The meaning of "plurality" is two or more unless otherwise specifically defined.

[0040] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed", etc. shall be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or integrated; it may be a mechanical connection or an electrical connection; it may be directly connected or indirectly connected through an intermediate medium, and it may be the internal communication of two components or the interaction relationship between two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

[0041] In the description of this specification, the description with reference to terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0042] Any process or method description represented in a flowchart or otherwise described herein can be understood to represent a module, segment, or portion of code including one or more executable instructions for implementing a specific logical function or process. The scope of the preferred embodiments of the present invention includes additional implementations where functions may be executed not in the order shown or discussed, including in a substantially simultaneous manner according to the functions involved or in a reverse order, which should be understood by those skilled in the art to which the embodiments of the present invention pertain.

[0043] The logic and / or steps represented in a flowchart or otherwise described herein, for example, can be considered a sequenced list of executable instructions for implementing a logical function and can be embodied in any computer-readable medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, a system including a processor, or other systems that can fetch and execute instructions from the instruction execution system, apparatus, or device. For the purposes of this specification, a "computer-readable medium" can be any device that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. More specific examples (a non-exhaustive list) of the computer-readable medium include the following: an electrical connection having one or more wires (electronic device), a portable computer diskette (magnetic device), a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber device, and a portable compact disc read-only memory (CDROM). Additionally, the computer-readable medium can even be paper or other suitable medium on which the program can be printed, as the program can be obtained electronically, for example, by optically scanning the paper or other medium, followed by editing, interpretation, or other suitable processing as necessary, and then stored in a computer memory.

[0044] It should be understood that various parts of the present invention can be implemented in hardware, software, firmware, or a combination thereof. In the above-described embodiments, multiple steps or methods can be implemented in software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented in hardware, as in another embodiment, any one of the following techniques known in the art or a combination thereof can be used: discrete logic circuits having logic gate circuits for implementing logical functions on data signals, application specific integrated circuits having appropriate combinational logic gate circuits, programmable gate arrays (PGA), field programmable gate arrays (FPGA), etc.

[0045] Those of ordinary skill in the art can understand that all or part of the steps carried out in implementing the above-described embodiment methods can be completed by a program instructing relevant hardware. The program can be stored in a computer-readable storage medium. When the program is executed, it includes one or a combination of the steps of the method embodiment.

[0046] The above-mentioned storage medium can be a read-only memory, a magnetic disk, an optical disc, etc. Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A centralized local energy storage modular multilevel converter MMC-CLES topology, characterized in that Comprising: Three-phase unit, each phase unit includes an upper arm and a lower arm, and each of the upper arm and the lower arm includes: Arm reactor, which is arranged at one end of the arm close to the power grid; A number of identical sub-modules SM, and a number of the sub-modules SM are connected to the power grid through the arm reactor; The upper arm and the lower arm form a symmetric structure; An energy storage sub-module is arranged at the position where the three-phase unit is directly connected to the DC bus.

2. The centralized local energy storage modular multilevel converter MMC-CLES topology according to claim 1, characterized in that, Two of the sub-modules SM of the three-phase unit are commonly connected to the DC bus side, each arm has n - 1 of the sub-modules SM, and the three-phase unit has a total of 6(n - 1) of the sub-modules SM.

3. The centralized local energy storage modular multilevel converter MMC-CLES topology according to claim 2, characterized in that, A common energy storage sub-module ESM is used at the position where the three-phase unit is directly connected to the DC bus, and each arm has only one common energy storage sub-module.

4. The centralized local energy storage modular multilevel converter MMC-CLES topology according to claim 1, wherein The sub-module SM adopts a half-bridge structure and includes: Two anti-parallel power switch tubes; A DC capacitor module connected in parallel to the series branch of the two power switch tubes; Wherein, the connection end of the common end of the two power switch tubes and the DC capacitor module is used as the output end of each sub-module, and when one of the power switch tubes is turned on and the other is turned off, the sub-module is in the input or cut-off state, and when both power switch tubes are turned off, the sub-module is in the locked state.

5. The centralized local energy storage modular multilevel converter MMC-CLES topology according to claim 4, characterized in that, The energy storage sub-module ESM includes: Sub-module SM structure, dual-active-bridge DAB circuit and energy storage battery, the DAB circuit is connected in parallel with the DC capacitor module in the sub-module SM structure, and the energy storage battery is connected in series with the power switch tubes and then connected in parallel with the DC capacitor module.

6. The centralized local energy storage modular multilevel converter MMC-CLES topology according to claim 5, characterized in that, The DAB circuit includes: Two symmetric H-bridges HB1 and HB2; A high-frequency isolation transformer and its equivalent leakage inductance arranged between the two H-bridges; Input and output side parallel capacitors.

7. The centralized local energy storage modular multilevel converter MMC-CLES topology according to claim 6, characterized in that, The DAB circuit modulates the input and output DC voltages on both sides into AC square wave voltages through phase-shift control and forms an AC / AC equivalent link in series with the equivalent leakage inductance.

8. The centralized local energy storage modular multilevel converter MMC-CLES topology according to claim 6, characterized in that, The HB1 is an input bridge, which is a full-bridge circuit and includes: Primary full-bridge circuit, input side parallel capacitor C1 and inductor L; The primary full-bridge circuit includes two groups of arms, and the two groups of arms include a first arm composed of switch tubes S3 / S4 and a second arm composed of switch tubes S5 / S6; the midpoint of the second arm and the first arm is connected to the primary winding of the high-frequency isolation transformer through the inductor L, and the two groups of arms are connected in parallel with the input side parallel capacitor C1; The HB2 is an output bridge, which is a full-bridge circuit and includes: Secondary full-bridge circuit and output side parallel capacitor C2, and the secondary full-bridge circuit is connected in parallel with the output side parallel capacitor C2; The midpoint of the secondary full-bridge circuit is connected to the secondary winding of the high-frequency isolation transformer. The secondary full-bridge circuit includes a third bridge arm formed by switching transistors S7 / S8 and a fourth bridge arm formed by switching transistors S9 / S 10 10.

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