AC / DC multi-port flexible interconnection device, control method and its distribution network system

Abstract

This invention relates to the field of substation technology, specifically to an AC / DC multi-port flexible interconnection device, control method, and distribution network system thereof. The multi-port flexible interconnection device includes a cascaded power electronic transformer (PET), multiple cascaded H-bridges (CHBs) bridging different AC ports, and multiple sets of resonant modules. The converter topology of this invention can provide DC ports and multiple AC ports, enabling flexible interconnection of multiple AC and DC feeder systems in a distribution network. It also allows for decoupled voltage and power regulation of the DC ports and multiple AC ports, achieving flexible connection and control of the voltage and power of multiple AC and DC feeder systems in the distribution network, while simultaneously reducing the cost and size of the converter.

Description

Technical Field

[0001] This invention relates to the field of power transmission technology, specifically to AC / DC multi-port flexible interconnection devices, control methods, and their distribution network systems. Background Technology

[0002] With the increasing penetration of renewable energy sources such as wind, solar, and energy storage devices, as well as distributed power sources, into the power grid, and the large-scale integration of new loads represented by electric vehicles, the architecture and mode of distribution networks have been extensively and profoundly affected. To ensure power quality and supply reliability, and reduce system network losses, active distribution network technology based on AC / DC grid interconnection and various power electronic devices for distribution networks have received widespread attention. Among these, the key technologies of AC / DC multi-port flexible interconnection devices have gradually become a new research hotspot in the field of distribution networks.

[0003] Existing multi-port converter topologies for distribution networks are mainly divided into back-to-back cascaded H-bridge converters (BTB-CHB type) and back-to-back modular multilevel converters (BTB-MMC type). The BTB-CHB type can only provide AC ports and cannot provide DC ports. While the BTB-MMC type can provide both DC ports and multiple AC ports, it uses a larger number of switching semiconductor devices and capacitors, resulting in higher cost and larger size.

[0004] The patent application CN202111177771.4 (A multi-port converter topology for flexible distribution networks and its control method) proposes a multi-port flexible interconnection device based on CHB, which can provide one DC port and two AC ports. However, this topology cannot be further extended to three or more AC ports, the resonant capacitor is very large, and the control flexibility is limited.

[0005] The patent with application number CN201710033847.3 (Modular Multilevel Full-Bridge Resonant Power Electronic Transformer Topology) proposes a converter topology that can provide both AC and DC ports simultaneously. However, the number of AC ports in this converter topology cannot be further expanded, making it difficult to meet the needs of flexible interconnection of multiple AC feeder systems in the distribution network. Summary of the Invention

[0006] The purpose of this invention is to provide an AC / DC multi-port flexible interconnection device, a control method, and a distribution network system thereof. It can provide a DC port and multiple AC ports, which can be used for flexible interconnection of multiple AC and DC feeder systems in a distribution network. Furthermore, it can regulate the voltage and power of the DC port and multiple AC ports, realizing the connection of multiple AC and DC feeder systems in a flexible distribution network and the flexible control of the voltage and power of multiple AC and DC feeder systems, while reducing the cost and size of the converter.

[0007] The objective of this invention can be achieved through the following technical solutions:

[0008] In a first aspect, an AC / DC multi-port flexible interconnection device includes:

[0009] n AC ports, n≥2; cascaded PET, the AC terminal of the cascaded PET is connected to any AC port, and the DC terminal of the cascaded PET is connected to the DC port;

[0010] CHB, which bridges any two AC ports;

[0011] A three-phase resonant module is connected in series on the AC port. The three-phase resonant module is used for intermediate frequency resonance.

[0012] Furthermore, the sub-modules at the DC end of the cascaded PET are arranged in any of the following ways:

[0013] They are connected in series to the DC port;

[0014] They are connected in parallel to each other and then connected to the DC port;

[0015] After being mixed and connected to each other, they are connected to the DC port;

[0016] The cascaded PET sub-modules include: AC-DC sub-module, DC-DC sub-module, high-frequency transformer, and inductors and capacitors.

[0017] Furthermore, the three-phase resonant module consists of capacitors, inductors, and / or resistors with intermediate frequency resonance function.

[0018] Furthermore, a CHB is connected in series between one end of the cascaded PET and the AC port.

[0019] Furthermore, multiple bridging CHBs are connected end to end, and multiple sets of three-phase resonant modules are merged into a single set of three-phase resonant modules.

[0020] Secondly, a control method for a multi-port flexible interconnect device, comprising the multi-port flexible interconnect device of the first aspect, further comprising:

[0021] The DC port control strategy of the cascaded PET is a single-phase shift strategy to control the DC port bus voltage.

[0022] The AC port of the cascaded PET adopts a frequency mixing modulation strategy. Its control strategy is divided into two parts: power frequency control and intermediate frequency control. The power frequency control adopts a single current loop control to control the power flow of the corresponding AC port grid; a fixed intermediate frequency control is superimposed on the power frequency modulation signal.

[0023] The control strategy of CHB is divided into two parts: power frequency control and intermediate frequency control. The power frequency control adopts a single current loop control to control the power flow of the corresponding AC port grid. The intermediate frequency control adopts a voltage and current dual closed-loop control. The voltage loop ensures that the DC capacitor voltage is constant, and the current loop controls the intermediate frequency circulating current to be in the same or opposite direction to the intermediate frequency zero-sequence voltage of the cascaded PET.

[0024] Furthermore, the resonant frequency of the resonant module is set to the intermediate frequency signal frequency of the intermediate frequency control.

[0025] Furthermore, the intermediate frequency signal for intermediate frequency control is a three-phase zero-sequence modulation signal.

[0026] Thirdly, a power distribution network system includes a converter, the converter including the multi-port flexible interconnection device of the first aspect.

[0027] The beneficial effects of this invention are:

[0028] 1. The multi-port flexible interconnection device proposed in this invention can simultaneously provide DC ports and multiple AC ports, the number of AC ports can be easily expanded, and electrical isolation and power decoupling control between AC and DC ports can be realized;

[0029] 2. Compared with existing solutions, the multi-port flexible interconnection device proposed in this invention requires fewer switching devices and passive components, thus having advantages such as lower cost, smaller size, and higher power density. Attached Figure Description

[0030] Figure 1 This is the topology of the single-phase flexible multi-port converter of the present invention;

[0031] Figure 2 This is the topology of the three-phase flexible multiport converter of the present invention;

[0032] Figure 3 This describes the connection method of the DC side of the three-phase flexible multiport converter of the present invention;

[0033] Figure 4 These are some of the optional structures for the three-phase resonant module of the present invention;

[0034] Figure 5 This invention provides a derived topology 1 for the three-phase flexible multiport converter.

[0035] Figure 6 This is the derived topology 2 of the three-phase flexible multiport converter of the present invention;

[0036] Figure 7 This is the steady-state operating vector diagram of the single-phase flexible multi-port converter of the present invention;

[0037] Figure 8DAB control strategy for cascaded PETs;

[0038] Figure 9 CHB control strategy for cascaded PET;

[0039] Figure 10 Control strategy for bridging CHB;

[0040] Figure 11 This is a waveform diagram of the AC port 1 grid voltage and current after standardization;

[0041] Figure 12 The waveforms of the AC port 2 grid voltage and current after standardization are shown.

[0042] Figure 13 The waveforms of the AC port 3 grid voltage and current after standardization;

[0043] Figure 14 This is a waveform diagram of the DC bus voltage after per-unit scaling.

[0044] Figure 15 The waveform of the DC capacitor voltage on the CHB side of AC port 1 after per-unit scaling;

[0045] Figure 16 The waveform of the DC capacitor voltage of CHB connected between AC port 1 and AC port 2 after standardization;

[0046] Figure 17 The waveform of the DC capacitor voltage of CHB connected between AC port 1 and AC port 3 after standardization;

[0047] Figure 18 This is a waveform diagram of the high-frequency current on the primary side of a high-frequency transformer.

[0048] Figure 19 The waveform of the intermediate frequency zero-sequence circulating current of the resonant LC on both sides of the AC port is shown.

[0049] Figure 20 The waveform of the intermediate frequency zero-sequence circulating current of the resonant LC on the 3rd side of the AC port is shown. Detailed Implementation

[0050] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0051] In some embodiments, this application provides an AC / DC multi-port flexible interconnection device, whose single-phase and three-phase topologies are respectively as follows: Figure 1 and Figure 2 As shown. The topology consists of a cascaded PET (cascaded power electronic transformer), multiple CHBs (cascaded H-bridges) connected between different ports, and multiple sets of resonant LCs.

[0052] The cascaded PET is connected to AC port 1 and a DC bus is led out on the DC side. Depending on the required DC bus voltage level, the various sub-modules (SMs) on the DC side of the cascaded PET can have multiple connection methods: for example, in the case of a low-voltage DC bus, the DC-side SMs of the cascaded PET can be connected in parallel to lead out the low-voltage DC bus, i.e., parallel connection between phases and parallel connection within phases, such as... Figure 3 As shown in (a); in the case of a high-voltage DC bus, the DC side SM is connected in series to the high-voltage DC bus, that is, connected in series between phases and within phases, as shown in (a). Figure 3 As shown in (b); of course, in some cases, some SMs can be connected in series and some in parallel to bring out DC buses of different voltage levels, that is, series connection between phases and parallel connection within phases or parallel connection between phases and series connection within phases, such as Figure 3 As shown in (c) and (d).

[0053] The topology described can provide multiple AC ports. A CHB is connected between AC port 1 and AC port 2 to regulate the power flow of AC port 2. A CHB is connected between AC port 1 and AC port 3 to regulate the power flow of AC port 3. Similarly, a CHB is connected between AC port 1 and AC port N to regulate the power flow of AC port N. This connection method can be continued to expand to multiple AC ports as needed.

[0054] Both the cascaded PET and the bridging CHB in the described topology employ frequency mixing modulation, superimposing a medium-frequency three-phase zero-sequence modulation signal onto the power frequency modulation signal. A corresponding resonant LC is installed at each of AC ports 2 to N, with its resonant frequency matching the frequency of the medium-frequency modulation signal in the frequency mixing modulation, providing a path for the medium-frequency zero-sequence circulating current. Since there is no zero-sequence current in a three-phase three-wire system, the frequency mixing modulation does not affect the feeder current at each AC port; the medium-frequency zero-sequence circulating current flows only through the resonant LC within the device.

[0055] Figure 4 (a)-(f) represent various optional structures for the three-phase resonant module. In addition to the structures listed, any series-parallel combination of capacitors, inductors, and resistors with intermediate frequency resonance function can be used as an optional structure for the aforementioned topological three-phase resonant module.

[0056] Figure 5 Topology 1 is derived from the aforementioned topology. Depending on the DC bus voltage level of the DC port, a portion of the cascaded PET modules can be replaced with CHBs, which can further reduce the number of switching transistors and lower equipment costs.

[0057] Figure 6 Topology 2 is derived from the aforementioned topology. Multiple bridging CHBs are connected end to end, and multiple sets of three-phase resonant LCs are merged into a single set of three-phase resonant LCs, which can further reduce the size and weight of the equipment.

[0058] Figure 7 This is a steady-state operating voltage and current vector diagram of the single-phase AC / DC multi-port flexible interconnection device. Taking an application scenario with two AC ports, AC port 1 and AC port 2, as an example, the left figure shows the power flow vector diagram of the AC grid flowing into AC port 2, and the right figure shows the power flow vector diagram of the device feeding back to the grid through AC port 2. Taking the left figure, the power flow vector diagram of the AC grid flowing into AC port 2, as an example, where θ is the voltage phase difference between AC ports 2 and 1, and U... A and U U These are the mains voltages at AC ports 1 and 2, respectively, U CHB For the voltage of CHB connected across AC ports 1 and 2, i CHB For the power frequency component of the CHB current flowing across AC ports 1 and 2, i A and i U The mains currents at AC ports 1 and 2 are respectively, i LC Let be the power frequency component of the current flowing through the resonant LC at AC port 2. According to circuit theory, the power frequency current flowing through the resonant LC is very small and can be approximated as...

[0059] I LC =ω m CU U (1)

[0060] Where ω m ω is the resonant angular frequency of the resonant LC circuit.

[0061] Ignoring the voltage drop across the AC port filter inductor, it can be assumed that...

[0062] U CHB =U U -U A (2)

[0063] The power frequency component of the CHB current is

[0064] i CHB =i U -i LC (3)

[0065] According to the vector diagram, the angle between the power frequency voltage and current bridging CHB is θ + arctan(I LC Therefore, the charging power of the DC capacitor by the power frequency voltage and current can be calculated as / IU).

[0066]

[0067] To ensure stable voltage across the CHB DC capacitor, the discharge power of the intermediate frequency voltage and current on the capacitor must be equal to the charging power of the power frequency voltage and current on the capacitor. Therefore, the intermediate frequency zero-sequence circulating current can be obtained as follows:

[0068]

[0069] U 0 For the intermediate frequency voltage of the cascaded PET AC port, I 0 For the intermediate frequency circulating current of the resonant LC circuit, P U Let be the power flowing into the device from port 2. Equation (5) can be used to calculate the current stress of the device, providing a theoretical basis for the selection of components for the device.

[0070] In some embodiments, this application also provides a control method for a multi-port flexible interconnect device, including the multi-port flexible interconnect device described above, and further including:

[0071] The DC port control strategy of the cascaded PET is a single-phase shift strategy to control the DC port bus voltage.

[0072] The AC port of the cascaded PET adopts a frequency mixing modulation strategy. Its control strategy is divided into two parts: power frequency control and intermediate frequency control. The power frequency control adopts a single current loop control to control the power flow of the corresponding AC port grid; a fixed intermediate frequency control is superimposed on the power frequency modulation signal.

[0073] The control strategy of CHB is divided into two parts: power frequency control and intermediate frequency control. The power frequency control adopts a single current loop control to control the power flow of the corresponding AC port grid. The intermediate frequency control adopts a voltage and current dual closed-loop control. The voltage loop ensures that the DC capacitor voltage is constant, and the current loop controls the intermediate frequency circulating current to be in the same or opposite direction to the intermediate frequency zero-sequence voltage of the cascaded PET.

[0074] The resonant frequency of the resonant LC is set to the intermediate frequency signal frequency of the intermediate frequency control.

[0075] The intermediate frequency signal for intermediate frequency control is a three-phase zero-sequence modulation signal.

[0076] like:

[0077] Figure 8 For the DC port control strategy of cascaded PET, a single phase-shift strategy is used. The DC port voltage is compared with the reference value, and the control phase-shift angle of DAB is adjusted by the controller to determine the DC port bus voltage.

[0078] Figure 9This describes the control strategy for the AC port of a cascaded PET (Potentially Attached Power Source) system. The outer loop sets the DC capacitor voltage and compares the average DC capacitor voltage of the CHB (Central Attached Power Source) of the cascaded PET with its reference value, generating a d-axis current reference value via the controller. The inner loop is based on a two-phase rotating coordinate system, with the d-axis current reference value given by the outer loop and the q-axis current reference value being 0, controlling the power factor on the grid side of AC port 1. Mixer modulation is employed, superimposing a fixed intermediate frequency three-phase zero-sequence voltage onto the power frequency modulation signal.

[0079] Figure 10 The control strategy for bridging the CHB consists of two parts: power frequency control and intermediate frequency control. Power frequency control employs a single current loop, controlling the power flow of the corresponding AC port grid in a two-phase rotating coordinate system by controlling the d-axis current. Intermediate frequency control uses a dual closed-loop control system of voltage and current. The voltage loop samples the average value of the CHB DC capacitor voltage to ensure a constant DC capacitor voltage. The current loop reference value is given by the outer loop and is responsible for controlling whether the intermediate frequency circulating current and the intermediate frequency zero-sequence voltage of the cascaded PET are in the same or opposite direction.

[0080] To verify the effectiveness of this topology and its control method, simulation verification was performed on the multiport converter topology and its control method. The simulation parameters are shown in Table 1.

[0081] Table 1 Simulation parameters of AC / DC multi-port flexible interconnection device

[0082]

[0083]

[0084] Based on the parameters in Table 1, simulation verification was performed on the multi-port converter topology applied to flexible distribution networks. The simulation results are as follows: Figures 10 to 19 As shown.

[0085] Figure 11 , 12 Figures 1 and 3 show the voltage and current waveforms of the grid at AC port 1 after per-unit scaling. At AC ports 1 and 3, current flows into the grid, so the voltage and current are in phase. At AC port 2, current flows into the equipment, so the voltage and current are in opposite phases.

[0086] Figure 14 This is the per-unit DC bus voltage. Its voltage stabilizes at the rated value within 0.1s with almost no overshoot.

[0087] Figure 15 , 16 17 represents the per-unit voltage of the cascaded PET, the CHB connected across ports 1 and 2, and the CHB connected across ports 1 and 3, respectively. All voltages are stable at their rated values, and the voltage equalization effect is good.

[0088] Figure 18 This refers to the high-frequency three-phase current flowing through the high-frequency transformer in the cascaded PET circuit.

[0089] Figure 19 and 20 This represents the current of the resonant LC circuit corresponding to AC port 2 and AC port 3. Since the resonant frequency is 500Hz, its power frequency current component is very small, and the main current component is the 500Hz intermediate frequency zero-sequence current component.

[0090] According to the simulation waveforms of the AC / DC multi-port flexible interconnection device, the power flow of each AC port and DC port is controlled to the set value. The DC port and AC port have achieved independent voltage control and decoupled power control. The voltage of each DC capacitor is stable at the rated value, achieving the control purpose and expected effect, and verifying the effectiveness and feasibility of the AC / DC multi-port flexible interconnection device.

[0091] The technical means disclosed in this invention are not limited to those disclosed in the above embodiments, but also include technical solutions composed of any combination of the above technical features. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications are also considered within the scope of protection of this invention.

[0092] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0093] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. An AC / DC multi-port flexible interconnection device, comprising: It has n AC ports, n≥2; its characteristic is that... Cascaded PET: The AC terminal of a cascaded PET is connected to any AC port, and the DC terminal of a cascaded PET is connected to a DC port. CHB, which bridges any two AC ports; A three-phase resonant module is connected in series on the AC port. The three-phase resonant module is used for intermediate frequency resonance. A CHB is connected in series between one end of the cascaded PET and the AC port; Multiple CHBs are connected end to end, and multiple sets of three-phase resonant modules are merged into a single set of three-phase resonant modules.

2. The multi-port flexible interconnection device according to claim 1, characterized in that, The DC-side sub-modules of a cascaded PET are arranged in any of the following ways: 1) Connect them in series to the DC port; 2) Connect them in parallel to each other and then connect them to the DC port; 3) After being mixed and connected, they are connected to the DC port; The cascaded PET sub-modules include: AC-DC sub-module, DC-DC sub-module, high-frequency transformer, and inductors and capacitors.

3. The multi-port flexible interconnection device according to claim 1, characterized in that, A three-phase resonant module consists of capacitors, inductors, and / or resistors with intermediate frequency resonance functionality.

4. A control method for a multi-port flexible interconnection device, characterized in that, The multi-port flexible interconnect device according to any one of claims 1-3 further includes: The DC port control strategy of the cascaded PET is a single-phase shift strategy to control the DC port bus voltage. The AC port of the cascaded PET adopts a frequency mixing modulation strategy. Its control strategy is divided into two parts: power frequency control and intermediate frequency control. The power frequency control adopts a single current loop control to control the power flow of the corresponding AC port grid; a fixed intermediate frequency control is superimposed on the power frequency modulation signal. The control strategy of CHB is divided into two parts: power frequency control and intermediate frequency control. The power frequency control adopts a single current loop control to control the power flow of the corresponding AC port grid. The intermediate frequency control adopts a voltage and current dual closed-loop control. The voltage loop ensures that the DC capacitor voltage is constant, and the current loop controls the intermediate frequency circulating current to be in the same or opposite direction to the intermediate frequency zero-sequence voltage of the cascaded PET.

5. The control method according to claim 4, characterized in that, The resonant frequency of the resonant module is set to the intermediate frequency signal frequency of the intermediate frequency control.

6. The control method according to claim 5, characterized in that, The intermediate frequency signal for intermediate frequency control is a three-phase zero-sequence modulation signal.

7. A power distribution network system, characterized in that, It includes a converter, which includes a multi-port flexible interconnect device according to any one of claims 1-3.

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