A DC multiplexing weakly coupled AC converter and its control method

Abstract

The present invention belongs to the field of power electronics technology and provides a DC multiplexing weak-coupling AC converter and a control method thereof, comprising an input-side shaping circuit, an input-side conduction switch group, a constant-voltage common bridge arm group, an output-side conduction switch group, and an output-side shaping circuit; the constant-voltage common bridge arm group comprises a first bridge arm, a second bridge arm, and a third bridge arm connected in series; the first bridge arm, the second bridge arm, and the third bridge arm are each composed of a cascade of N full-bridge submodules; the input-side shaping circuit comprises a fourth bridge arm and a fifth bridge arm, and the output-side shaping circuit comprises a sixth bridge arm and a seventh bridge arm; the fourth bridge arm, the fifth bridge arm, the sixth bridge arm, and the seventh bridge arm are each composed of a cascade of N / 2 full-bridge submodules. The present invention designs a weak-coupling AC converter between direct and indirect AC converters. Under the action of the constant-voltage common bridge arm group, weak-coupling energy exchange of electrical quantities of the same or different frequencies between the power frequency side and the low frequency side is achieved, thereby realizing cross-regional power grid interconnection and energy exchange.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and in particular relates to a DC multiplexing type weakly coupled AC converter and a control method thereof. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] With the energy transformation and transition of the power industry, a new power system based on renewable energy is gradually emerging. The intermittent, random, and volatile nature of renewable energy generation can lead to power shortages during peak load periods and power surpluses during low load periods or periods of strong renewable energy generation. This poses significant challenges to balancing supply and demand in the power system. Factors such as the misalignment between energy production and load centers, the limited peak-shaving capacity of regional power grids, poor renewable energy transmission channels, and the lack of large-scale application of energy storage technology have led to constraints on the further development of the power system due to the high proportion of renewable energy being consumed.

[0004] Energy composition, load distribution, and load fluctuation patterns vary across regions. Currently, power grids generally utilize a "regional grid + key interconnection lines" structure. With the implementation of regional grid interconnection projects, the interconnection between regional grids is gradually increasing. However, the regional grid transmission structure is not perfect, and the weak coupling structure between regions has become a major factor restricting the absorption of new energy. Therefore, the research, planning, and construction of cross-regional grid interconnection technologies play a vital role in improving the interconnection structure of regional grids.

[0005] Cross-regional power grid interconnection technology has been widely used in many fields such as DC transmission, AC transmission, low-frequency transmission, flexible interconnection of distribution networks, AC / AC frequency conversion, etc. It can provide suitable solutions for multi-level and multi-terminal interconnection and control of power grids with different voltage levels, different distances, same frequency or different frequencies. At the same time, it can also realize coordinated peak-shaving operation between regional power grids, achieve power mutual assistance in emergency fault conditions, and enhance the support capacity of the power grid.

[0006] Various grid zone interconnection converters are key equipment for realizing energy exchange between grids in different regions in cross-regional grid interconnection technology, and are an important technical support in the current process of building grid zone interconnection patterns. The AC converter topology is divided into indirect AC converter and direct AC converter. Among them, the indirect AC converter contains a common DC bus, and the AC systems on both sides are independent of each other. The typical topology is the back-to-back MMC converter (back-to-back modular multilevel converter, BTB-MMC) topology; the direct AC converter does not contain a common DC bus, and the AC systems on both sides are coupled with each other. The typical topology is the modular multilevel matrix converter (M3C) topology. Among them, BTB-MMC is widely used in the field of high-voltage direct current transmission, and can realize asynchronous interconnection of long-distance, large-capacity cross-regional power grids, but the number of sub-modules required is large, the converter is large in size, and the DC circuit breaker technology is not mature; the M3C AC converter topology has obvious advantages in the field of low-frequency transmission, such as Figure 1 As shown, on the one hand, the line reactance and the charging reactive power of the submarine cable are fully reduced, and the transmission capacity is close to that of DC transmission technology; on the other hand, there is no need to build an offshore converter station, and the AC circuit breaker technology can continue to be used, which reduces project investment while improving power supply reliability.

[0007] However, the existing M3C AC switch topology has the following disadvantages:

[0008] 1) The high degree of coupling between the input and output AC systems complicates the design of the control system, and any fluctuation in the voltage of one phase will affect the three-phase voltage on the other side;

[0009] 2) The numerous internal circulation channels in the topology lead to complex circulation suppression strategies;

[0010] 3) The large number of submodules results in a large converter device and high production cost;

[0011] 4) When the input and output frequencies of the M3C topology are similar, the low-frequency ripple of the submodule capacitor voltage causes it to be in a charging and discharging state for a long time, resulting in a sharp increase in voltage amplitude and seriously exceeding the rated value of the submodule capacitor voltage. Therefore, the M3C AC converter topology is difficult to apply to the field of flexible interconnection of cross-regional power grids with the same frequency and asynchronous operation, as well as application scenarios such as AC speed regulation and power electronic transformers. Summary of the Invention

[0012] To solve the above problems, the present invention proposes a DC multiplexing weak-coupling AC converter and a control method thereof, designs a weak-coupling AC converter between direct and indirect AC converters, and uses a constant-voltage common bridge arm group shared by the input and output AC systems as a constant support for the DC voltage, thereby realizing weak coupling of different-frequency electrical quantities and same-frequency asynchronous interconnection, and further realizing cross-regional power grid interconnection and energy exchange through seven bridge arms.

[0013] According to some embodiments, a first solution of the present invention provides a DC multiplexing weakly coupled AC converter, which adopts the following technical solution:

[0014] A DC multiplexing weak-coupling AC converter for same-frequency asynchronous interconnection, comprising an input-side shaping circuit, an input-side conduction switch group, a constant-voltage common bridge arm group, an output-side conduction switch group, and an output-side shaping circuit, wherein weak-coupling energy exchange of same-frequency or different-frequency electrical quantities between the power frequency side and the low-frequency side of the AC converter is achieved under the action of the constant-voltage common bridge arm group; wherein the constant-voltage common bridge arm group comprises a first bridge arm, a second bridge arm, and a third bridge arm connected in series; the first bridge arm, the second bridge arm, and the third bridge arm are each composed of N full-bridge sub-modules connected in cascade; the output The input-side shaping circuit includes a fourth bridge arm and a fifth bridge arm, and the output-side shaping circuit includes a sixth bridge arm and a seventh bridge arm; the fourth bridge arm, the fifth bridge arm, the sixth bridge arm and the seventh bridge arm are all composed of N / 2 full-bridge sub-modules in cascade; one phase on the industrial frequency side of the AC converter is connected to the first bridge arm and the second bridge arm respectively, and the other two phases on the industrial frequency side are connected to the input-side conduction switch group through the fourth bridge arm and the fifth bridge arm respectively, and the input-side conduction switch group, the constant voltage common bridge arm group and the output-side conduction switch group are connected in parallel.

[0015] As a further technical limitation, one phase on the low-frequency side of the AC converter is connected to the second bridge arm and the third bridge arm respectively, and the other two phases on the low-frequency side are connected to the output-side conductive switch group through the sixth bridge arm and the seventh bridge arm respectively.

[0016] As a further technical limitation, the input-side shaping circuit and the output-side shaping circuit are bidirectional equal-power transmission arrangements with a symmetrical topology structure, and the component parameters of the input-side shaping circuit and the output-side shaping circuit are consistent.

[0017] As a further technical definition, the full-bridge sub-module includes a first sub-bridge arm, a second sub-bridge arm and a DC capacitor connected in parallel; wherein, the first sub-bridge arm includes a first switch tube and a second switch tube connected in series, and the second sub-bridge arm includes a third switch tube and a fourth switch tube connected in series, the connection between the first switch tube and the second switch tube is the first output end of the full-bridge sub-module, and the connection between the third switch tube and the fourth switch tube is the second output end of the full-bridge sub-module; the first switch tube, the second switch tube, the third switch tube and the fourth switch tube are all reverse-parallel diodes.

[0018] As a further technical limitation, the input-side conduction switch group and the output-side conduction switch group both adopt a bridge circuit consisting of four conduction switches in two phases, and each of the conduction switches is composed of an IGBT cascade of several anti-parallel diodes.

[0019] Furthermore, the input-side conductive switch group includes a first conductive switch, a second conductive switch, a third conductive switch and a fourth conductive switch; the first conductive switch and the second conductive switch connected in series are connected in parallel with the third conductive switch and the fourth conductive switch connected in series; the connection point between the first conductive switch and the second conductive switch is connected to the fifth bridge arm, and the connection point between the third conductive switch and the fourth conductive switch is connected to the fourth bridge arm.

[0020] Furthermore, the output-side conductive switch group includes a fifth conductive switch, a sixth conductive switch, a seventh conductive switch and an eighth conductive switch; the fifth conductive switch and the sixth conductive switch connected in series are connected in parallel with the seventh conductive switch and the eighth conductive switch connected in series; the connection between the fifth conductive switch and the sixth conductive switch is connected to the seventh bridge arm, and the connection between the seventh conductive switch and the eighth conductive switch is connected to the sixth bridge arm.

[0021] According to some embodiments, a second solution of the present invention provides a control method for a DC multiplexing weakly coupled AC converter, which adopts the following technical solution:

[0022] A control method for a DC multiplexing weakly coupled AC converter employing the DC multiplexing weakly coupled AC converter provided by the first solution includes:

[0023] Generate a bridge arm modulation signal based on the mathematical relationship between the constant DC voltage and the AC output voltage;

[0024] At different times, the on-off state of the conduction switch is controlled while cooperating with the switching sub-module. The bridge arm modulation signal generated by the step wave is approximated with the closest level to complete the shaping of the AC voltage and realize the global energy balance control of the DC multiplexing weakly coupled AC converter.

[0025] As a further technical limitation, based on the proportional relationship between the input side AC voltage amplitude, the output side voltage amplitude and the DC constant voltage, the energy self-balancing of the second bridge arm is achieved, the energy accumulated by the first bridge arm and the third bridge arm within one cycle is complementary, and the stable operation of the AC converter is achieved through the energy exchange path established between the first bridge arm and the third bridge arm.

[0026] As a further technical limitation, when the voltage of the power frequency side phase connected to the fourth bridge arm is positive, the third conduction switch is turned on, the fourth conduction switch is turned off, and the current flows from the power frequency side to the first bridge arm through the third conduction switch; when the voltage of the power frequency side phase connected to the fourth bridge arm is negative, the third conduction switch is turned off, the fourth conduction switch is turned on, and the current flows from the power frequency side to the third bridge arm through the fourth conduction switch;

[0027] When the voltage of one phase on the power frequency side connected to the fifth bridge arm is positive, the first conduction switch is turned on, the second conduction switch is turned off, and the current flows from the power frequency side to the first bridge arm through the first conduction switch; when the voltage of one phase on the power frequency side connected to the fifth bridge arm is negative, the first conduction switch is turned off, the second conduction switch is turned on, and the current flows from the power frequency side to the third bridge arm through the second conduction switch.

[0028] As a further technical limitation, when the voltage of the low-frequency side phase connected to the seventh bridge arm is positive, the fifth conductive switch is turned on and the sixth conductive switch is turned off, and current flows from the input-side conductive switch through the fifth conductive switch to one end of the seventh bridge arm; when the voltage of the low-frequency side phase connected to the seventh bridge arm is negative, the fifth conductive switch is turned off and the sixth conductive switch is turned on, and current flows from the connection between the input-side conductive switch group and the third bridge arm through the sixth conductive switch to one end of the seventh bridge arm;

[0029] When the voltage of one phase on the low-frequency side connected to the sixth bridge arm is positive, the seventh conduction switch is turned on and the eighth conduction switch is turned off, and the current flows from the input-side conduction switch through the seventh conduction switch to one end of the sixth bridge arm; when the voltage of one phase on the low-frequency side connected to the sixth bridge arm is negative, the seventh conduction switch is turned off and the eighth conduction switch is turned on, and the current flows from the connection between the input-side conduction switch group and the third bridge arm through the eighth conduction switch to one end of the sixth bridge arm.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] The design of the present invention contains a constant-voltage common bridge arm group shared by the input-side and output-side AC systems as a constant support for the DC voltage, realizing weak coupling of different-frequency electrical quantities and asynchronous interconnection of the same frequency, and further realizing cross-regional power grid interconnection and energy exchange through seven bridge arms; the AC converter in the present invention has no circulation path in its topological structure, so there will be no circulation, that is, there is no need to suppress or eliminate the circulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The drawings constituting a part of the specification of this embodiment are used to provide a further understanding of this embodiment. The schematic embodiments and descriptions of this embodiment are used to explain this embodiment and do not constitute an improper limitation on this embodiment.

[0033] Figure 1 Schematic diagram of the topological structure of a typical AC switch M3C in the background technology of the present invention;

[0034] Figure 2 Schematic diagram of the topology of a DC multiplexing weakly coupled AC converter in the first embodiment of the present invention;

[0035] Figure 3 Schematic diagram of the topological structure of the full-bridge submodule in the first embodiment of the present invention;

[0036] Figure 4 Schematic diagram of the topological structure of the conductive switch in the first embodiment of the present invention;

[0037] Figure 5 Schematic diagram of theoretical waveforms of voltages of three bridge arms of the constant voltage common bridge arm group in the first embodiment of the present invention;

[0038] Figure 6 Schematic diagram of the input-side shaping circuit waveform and the conduction switch drive signal in the first embodiment of the present invention;

[0039] Figure 7 Schematic diagram of theoretical waveforms of the output-side shaping circuit and the conduction switch driving signal in the first embodiment of the present invention;

[0040] Figure 8 Schematic diagram of the principle of phase b in Example 1 of the present invention;

[0041] Figure 9 Schematic diagram of the waveform generation principle of the constant voltage common bridge arm group in the first embodiment of the present invention. DETAILED DESCRIPTION

[0042] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0043] It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0044] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0045] In the present invention, terms such as "upper", "lower", "left", "right", "front", "back", "vertical", "horizontal", "side", "bottom", etc. indicating directions or positional relationships are based on the directions or positional relationships shown in the accompanying drawings. They are relational words determined only for the convenience of describing the structural relationships of the various parts or elements of the present invention, and do not specifically refer to any part or element in the present invention, and should not be understood as limiting the present invention.

[0046] In the present invention, terms such as "fixed connection," "connected," and "connection" should be interpreted broadly to mean a fixed connection, an integral connection, or a detachable connection; a direct connection or an indirect connection through an intermediary. Relevant researchers or technicians in this field may determine the specific meanings of these terms in the present invention based on specific circumstances, and they should not be construed as limitations of the present invention.

[0047] In the absence of conflict, the embodiments of the present invention and the features thereof may be combined with each other.

[0048] Example 1

[0049] The first embodiment of the present invention introduces a DC multiplexing type weakly coupled AC converter.

[0050] As the global energy system transitions toward sustainability, the vigorous development of new energy technologies is an inevitable trend. However, as the penetration rate of new energy continues to rise, isolated regional power grids are struggling to accommodate the demand. Energy composition, load distribution, and energy consumption patterns vary across regions. To scientifically address these energy distribution issues, my country's power system must vigorously develop cross-regional grid interconnection technologies to optimize the allocation of power generation resources and achieve balanced consumption on a larger scale. Grid-region interconnection converters are key components of cross-regional grid interconnection technology and are increasingly becoming a research hotspot for scholars both domestically and internationally. The typical BTB-MMC converter topology features numerous submodules, a high degree of coupling in the M3C heterogeneous AC system, and complex internal circulation, making it difficult to apply to the synchronous and asynchronous interconnection of regional power grids.

[0051] To this end, this embodiment provides a Figure 2 The DC multiplexing weakly coupled AC converter shown uses a constant-voltage bridge arm group to support the DC voltage, achieving weak coupling of heterogeneous electrical quantities within the system. This simplifies control and enables same-frequency asynchronous interconnection, overcoming the M3C same-frequency transmission problem. At the same time, at the structural level, the proposed new topology has no circulating current paths, eliminating the need for additional circulating current suppression strategies, fully realizing a compact design.

[0052] like Figure 2 A DC multiplexing weakly coupled AC converter is shown, comprising:

[0053] An input-side shaping circuit includes a fourth bridge arm and a fifth bridge arm; each of the fourth bridge arm and the fifth bridge arm is composed of N / 2 full-bridge sub-modules cascaded;

[0054] The input side conduction switch group includes a first conduction switch S c1 , the second conduction switch S c2 , the third conduction switch S b1 and the fourth conductive switch S b2 ; The first conductive switch S connected in series c1 and the second conductive switch S c2 , and the third conduction switch S connected in series b1 and the fourth conductive switch S b2 Parallel connection; first conduction switch S c1 and the second conductive switch S c2 The connection point of is connected to the fifth bridge arm, and the third conduction switch S b1 and the fourth conductive switch S b2 The connection is connected to the fourth bridge arm;

[0055] A constant voltage common bridge arm group includes a first bridge arm m1, a second bridge arm m2, and a third bridge arm m3 connected in series; the first bridge arm m1, the second bridge arm m2, and the third bridge arm m3 are each composed of N full-bridge submodules (FBSM) cascaded;

[0056] The output-side conduction switch group includes a sixth bridge arm and a seventh bridge arm; the sixth bridge arm and the seventh bridge arm are both composed of N / 2 full-bridge sub-modules cascaded;

[0057] The output side shaping circuit includes a fifth conduction switch S v1 , the sixth conduction switch S v2 , the seventh conduction switch S w1 and the eighth conductive switch S w2 The fifth conductive switch S connected in series v1 and the sixth conductive switch S v2 , and the seventh conduction switch S connected in series w1 and the eighth conductive switch S w2 Parallel connection; fifth conduction switch S v1 and the sixth conductive switch S v2 The connection point of the seventh bridge arm is connected to the seventh conduction switch S w1 and the eighth conductive switch S w2 The connection point is connected to the sixth bridge arm.

[0058] Combine Figure 2 It can be seen that in this embodiment, one phase on the power frequency side of the AC converter is connected to the first bridge arm m1 and the second bridge arm m2 respectively, and the other two phases on the power frequency side are connected to the input side conduction switch group through the fourth bridge arm and the fifth bridge arm respectively, and the input side conduction switch group, the constant voltage common bridge arm group and the output side conduction switch group are connected in parallel; one phase on the low frequency side of the AC converter is connected to the second bridge arm m2 and the third bridge arm m3 respectively, and the other two phases on the low frequency side are connected to the output side conduction switch group through the sixth bridge arm and the seventh bridge arm respectively; under the action of the constant voltage common bridge arm group, weak coupling energy exchange of electrical quantities of the same frequency or different frequency between the power frequency side and the low frequency side is realized.

[0059] It should be noted that the input-side shaping circuit and the output-side shaping circuit in this embodiment are bidirectional equal power transmission arrangements with a symmetrical topology structure, and the component parameters of the input-side shaping circuit and the output-side shaping circuit are consistent.

[0060] It should be noted that the first conduction switch S in this embodiment c1 , the second conduction switch S c2 , the third conduction switch S b1 , the fourth conduction switch S b2 , the fifth conduction switch S v1 , the sixth conduction switch S v2 , the seventh conduction switch Sw1 and the eighth conductive switch S w2 The structures are consistent, that is, Figure 4 As shown, they are all composed of IGBT cascades with several anti-parallel diodes.

[0061] like Figure 8 As shown, this embodiment takes phase b as an example to introduce:

[0062] when u Vb Greater than 0, S b1 、S b2 The switch signals are 1 and 0 respectively, that is, S b1 conduction, S b2 Disconnect, at this time E Vb The voltage at U dc , and the b-phase shaping circuit voltage u sb Add up to get the valve side voltage on the input side u Vb , i Vb Depend on E Vb Point through S b1 Flow direction G 1 o'clock; when u Vb Less than 0, S b1 、S b2 The switch signals are 0 and 1 respectively, that is, S b1 Disconnect, S b2 Conducting, at this time E Vb The voltage at - U dc , and the b-phase shaping circuit voltage u sb Add up to get the valve side voltage on the input side u Vb , i Vb Depend on E Vb Point through S b2 Flow direction G 2 o'clock;

[0063] when u Vc Greater than 0, S c1 、S c2 The switch signals are 1 and 0 respectively, that is, S c1 conduction, S c2 Disconnect, at this time E Vc The voltage at Udc , and the c-phase shaping circuit voltage u sc Add up to get the valve side voltage on the input side u Vc , i Vc Depend on E Vc Point through S c1 Flow direction G 1 o'clock; when u Vc Less than 0, S c1 、S c2 The switch signals are 0 and 1 respectively, that is, S c1 Disconnect, S c2 Conducting, at this time E Vb The voltage at - U dc , and the c-phase shaping circuit voltage u sc Add up to get the valve side voltage on the input side u Vc , i Vc Depend on E Vc Point through S c2 Flow direction G 2 o'clock;

[0064] when u vv Greater than 0, S v1 、S v2 The switch signals are 1 and 0 respectively, that is, S v1 conduction, S v2 Disconnect, at this time E vv The voltage at U dc , and the v-phase shaping circuit voltage u sv Add up to get the valve side voltage on the input side u vv , i vv Depend on G 1 point through S v1 Flow direction E vv point; when u vv Less than 0, S v1 、S v2 The switch signals are 0 and 1 respectively, that is, S v1 Disconnect, S v2 Conducting, at this time E vv The voltage at -U dc , and the v-phase shaping circuit voltage u sv Add up to get the valve side voltage on the input side u vv , i vv Depend on G 2 points through S v2 Flow direction E vv point;

[0065] when u vw Greater than 0, S w1 、S w2 The switch signals are 1 and 0 respectively, that is, S w1 conduction, S w2 Disconnect, at this time E vw The voltage at U dc , and the w-phase shaping circuit voltage u sw Add up to get the valve side voltage on the input side u vw , i vw Depend on G 1 point through S w1 Flow direction E vw point;

[0066] when u vw Less than 0, S w1 、S w2 The switch signals are 0 and 1 respectively, that is, S w1 Disconnect, S w2 Conducting, at this time E vw The voltage at - U dc , and the w-phase shaping circuit voltage u sw Add up to get the valve side voltage on the input side u vw , i vw Depend on G 2 points through S w2 Flow direction E vw point.

[0067] It should be noted that, in this embodiment, the input-side conductive switch group and the output-side conductive switch group adopt synchronous rectification control with a driving signal phase angle difference of 180°.

[0068] In this embodiment, if Figure 2 As shown, a, b, and c are the three phases on the input side, and u, v, and w are the three phases on the output side. The input side and the output side realize direct conversion of AC power of different frequencies through the AC converter in this embodiment; wherein, "V" represents the valve side of the input side, v " indicates the valve side on the output side, O 、 O' They are the grounding points of the AC system on the input and output sides respectively; E Va It is the common connection point between the first bridge arm m1 and the second bridge arm m2 of the constant voltage common bridge arm group, E vu It is the common connection point between the second bridge arm m2 and the third bridge arm m3 of the constant voltage common bridge arm group, G 1 and G 2 are the highest points of DC voltage U dc / 2 and the lowest point - U dc / 2. E Vx ( x =b, c) is the common connection point between the input side b(c) phase shaping circuit and the conduction switch circuit, E vy ( y =v,w) is the common connection point between the output side v(w) phase shaping circuit and the conduction switch circuit; u ia 、 u ib 、 u ic is the input side AC grid voltage, u ou 、 u ov 、 u ow is the AC grid voltage at the output side. L iT 、 L oT They are the input and output side transformer leakage inductance, R iT 、 R oT They are the equivalent resistances of the short-circuit losses of the input and output side transformers respectively. u Vx , i Vx ( x =a, b, c) represents the valve side voltage and current on the input side, u vy , i vy (y =u, v, w) represents the valve side voltage and current on the output side. u sb and u sc is the output voltage of the input side b-phase and c-phase shaping circuit, u sv and u sw The output voltage of the V-phase and W-phase shaping circuits on the output side. i x1 、 i x2 Flow through S x1 、 S x2 ( x =b, c) current, i y1 、 i y2 Flow through S y1 、 S y2 ( y =v, w) current. U dc is the voltage borne by the constant voltage common bridge arm group, u m1 、 u m2 、 u m3 are the voltages of the first bridge arm m1, the second bridge arm m2, and the third bridge arm m3 of the constant voltage common bridge arm group, i m1 、 i m2 、 i m3 are the currents flowing through the first bridge arm m1, the second bridge arm m2, and the third bridge arm m3 respectively; the positive directions of the physical quantities are as follows Figure 2 shown.

[0069] This embodiment adopts Figure 3The full-bridge sub-module shown includes a first sub-bridge arm, a second sub-bridge arm, and a DC capacitor connected in parallel. The first sub-bridge arm includes a first switching transistor T1 and a second switching transistor T2 connected in series, and the second sub-bridge arm includes a third switching transistor T3 and a fourth switching transistor T4 connected in series. The connection between the first switching transistor T1 and the second switching transistor T2 forms the first output terminal of the full-bridge sub-module, and the connection between the third switching transistor T3 and the fourth switching transistor T4 forms the second output terminal of the full-bridge sub-module. The first switching transistor T1, the second switching transistor T2, the third switching transistor T3, and the fourth switching transistor T4 are connected in anti-parallel to a first diode D1, a second diode D2, a third diode D3, and a fourth diode D4, respectively.

[0070] This embodiment uses the application of AC power transmission as an example, assuming a topology of 50 Hz input and 50 / 3 Hz output. The operating principle of the AC converter proposed in this embodiment is described in detail. In actual application scenarios, when three-phase AC power passes through the AC converter and its frequency is reduced from 50 Hz to 50 / 3 Hz, the transmission capacity of AC power transmission is significantly improved.

[0071] like Figure 9 As shown in Figure 1, the constant voltage common bridge arm group is an important medium for direct energy exchange between AC currents of different frequencies on the input and output sides. It has two important functions:

[0072] 1) Support DC voltage: The cascaded submodules in the first bridge arm m1, the second bridge arm m2, and the third bridge arm m3 jointly bear the DC voltage U dc , as shown in formula (1);

[0073] 2) Shaping AC voltage: The first bridge arm m1 and the second bridge arm m2 jointly generate the input side a phase valve side voltage u Va The third bridge arm m3 and the second bridge arm m2 jointly generate the output side u-phase valve side voltage u vu , as shown in formula (2).

[0074] Combining formula (1) and formula (2), we can get the voltage of the first bridge arm m1, the second bridge arm m2 and the third bridge arm m3 of the constant voltage common bridge arm group: u m1 、 u m2 、 u m3 As shown in formula (3), the theoretical waveform of the three-bridge arm voltage is as follows: Figure 5 As shown:

[0075] (1)

[0076] (2)

[0077] (3)

[0078] In this embodiment, the input side shaping circuit, the output side shaping circuit and the first conductive switch S c1 , the second conduction switch S c2 , the third conduction switch S b1 , the fourth conduction switch S b2 , the fifth conduction switch S v1 , the sixth conduction switch S v2 , the seventh conduction switch S w1 and the eighth conductive switch S w2 Phase coordination, taking the input side b phase as an example:

[0079] like Figure 8 As shown, the first conductive switch S b1 and the second conductive switch S b2 Alternating on and off E Vb Point output contains U dc / 2 and - U dc / 2 two-level square wave, the shaping circuit formed by the cascade of sub-modules reshapes the two-level square wave voltage waveform by switching the sub-modules to obtain the b-phase valve side voltage u Vb The working principle of the shaping circuit and the conduction switch group of the input side c phase, output side v phase and w phase is the same as that of the input side b phase and output c phase valve side voltage. u Vc , v-phase valve side voltage u vv and w-phase valve side voltage u vw Input side b phase, c phase shaping circuit voltage u sb 、 u sc And the output side V phase, W phase shaping circuit voltage u sv 、 u sw The theoretical waveforms are as follows Figure 6 and Figure 7 shown.

[0080] AC voltage on the power frequency side of the AC converter , current The expression is:

[0081] (4)

[0082] in, U m1 and I m1 is the AC voltage and current amplitude on the power frequency side, φ x For the x The initial phase angle of the phase (where φ a =0, φ b =-2π / 3, φ c =2π / 3), φ 1 is the power factor angle on the power frequency grid side.

[0083] definition and For the power frequency side x ( x =b, c) phase unit operation mode switching function, its expression is:

[0084] (5)

[0085] According to Kirchhoff's voltage theorem, the power frequency side x The expression of the phase shaping circuit voltage is:

[0086] (6)

[0087] According to Kirchhoff's current theorem, the current flowing through S x1 、 S x2 ( x =b, c) current i x1 、 i x2 for:

[0088] (7)

[0089] AC voltage on the low-frequency side of the AC converter , current The expression is:

[0090] (8)

[0091] in, U m2 and I m2 is the AC voltage and current amplitude on the low-frequency side, φ y For the y The initial phase angle of the phase (where φu =0, φ v =-2π / 3, φ w =2π / 3), φ 2 is the low-frequency grid-side power factor angle.

[0092] definition and For the power frequency side y ( y =v, w) phase unit operation mode switching function, its expression is:

[0093] (9)

[0094] According to Kirchhoff's voltage theorem, the low-frequency side y The expression of the phase shaping circuit voltage is:

[0095] (10)

[0096] According to Kirchhoff's current theorem, the current flowing through S y1 、 S y2 ( y =v,w) current i y1 、 i y2 for:

[0097] (11)

[0098] The energy balance conditions of the proposed new AC converter topology are calculated based on actual application scenarios. At this time, the power frequency and low frequency are coupled in the constant voltage common bridge arm group of the AC converter, and the periods of the power frequency system and the low frequency system are 2π and 6π, respectively. To facilitate observation of the energy flow of the AC converter, this embodiment defines the operating period of the entire AC converter as T = 6π. The submodule capacitors are the medium for power conversion between the power frequency side and the low frequency side of the AC converter. To ensure submodule capacitor voltage balance, it is necessary to ensure that the energy exchange between the shaping circuit and the outside world is zero within one cycle, that is, the energy absorbed and released should be equal.

[0099] Taking phase b as an example, the voltage of the phase b shaping circuit is u sb As shown in formula (6), the current flowing through the b-phase shaping circuit is the AC current on the power frequency side i b As shown in formula (4). u sb and i b The instantaneous power of the b-phase shaping circuit can be obtained by multiplying the expression ofP sb for:

[0100] (12)

[0101] In a period T=6π P sb The energy accumulated in one cycle of the b-phase shaping circuit can be obtained by integration:

[0102] (13)

[0103] make W sb =0, we get:

[0104] (14)

[0105] The energy balancing mechanism of phase C and phase B is the same, so the condition for energy balancing of the power frequency side shaping circuit is formula (14).

[0106] Taking the V phase as an example, the V phase shaping circuit voltage u sv As shown in formula (10), the current flowing through the V-phase shaping circuit is the low-frequency side AC current i v As shown in formula (8). u sv and i v The instantaneous power of the V-phase shaping circuit can be obtained by multiplying the expression of P sv for:

[0107] (15)

[0108] In a period T=6π P sv The energy accumulated in one cycle of the V-phase shaping circuit can be obtained by integration:

[0109] (16)

[0110] make W sv =0, we get:

[0111] (17)

[0112] The energy balancing mechanism of the w-phase and v-phase is the same, so the condition for energy balancing of the low-frequency side shaping circuit is formula (17).

[0113] The first bridge arm m1, the second bridge arm m2 and the third bridge arm m3 of the constant voltage common bridge arm group jointly support the voltageU dc , while outputting the AC voltage of phase a on the power frequency side and the AC voltage of phase u on the low frequency side. The voltages of the three bridge arms are u m1 、 u m2 、 u m3 As shown in formula (3):

[0114] According to Kirchhoff's current theorem, the currents flowing through the three bridge arms are:

[0115] (18)

[0116] Among them, the flow S x1 、 S x2 ( x =b, c) current i x1 、 i x2 As shown in formula (7), the flow S y1 、 S y2 ( y =v,w) current i y1 、 i y2 As shown in formula (11).

[0117] The voltage of the first bridge arm m1, the second bridge arm m2, and the third bridge arm m3 of the constant voltage common bridge arm group u m1 、 u m2 、 u m3 and current i m1 、 i m2 、 i m3 By multiplying them separately, we can get the instantaneous power of each of the three bridge arms. Then, by integrating the instantaneous power, we can get the energy accumulated by the three bridge arms in one cycle T=6π:

[0118] (19)

[0119] After the AC converter is put into operation, the input energy and output energy in one cycle are equal, that is,

[0120] (20)

[0121] Obviously, when formula (20) holds true, the energy of the middle bridge arm m2 of the constant voltage common bridge arm group is self-balanced.

[0122] After the energy of the bridge arms on the power frequency side and the low frequency side is balanced, that is, when formula (14) and formula (17) are satisfied at the same time, then:

[0123] (twenty one)

[0124] Combining formula (20) and formula (21), we can get:

[0125] (twenty two)

[0126] At this time, the sum of the energy accumulated in the first bridge arm m1 and the third bridge arm m3 of the constant voltage common bridge arm group in one cycle is 0, that is, W m1 + W m3 =0, the neutral points of the power frequency side and low frequency side transformers need to be connected to establish an energy exchange path between the first bridge arm m1 and the third bridge arm m3 to ensure stable operation of the topology.

[0127] At the same AC voltage level, the BTB-MMC topology uses 12N modules, the M3C topology uses 9N sub-modules, and the topology in this embodiment uses 5N sub-modules. Compared with the BTB-MMC converter topology and the M3C AC converter topology, the number of sub-modules is significantly reduced by 58.3% and 44.4%, respectively, significantly achieving a compact AC converter design.

[0128] This embodiment uses a constant-voltage common bridge arm group shared by the input-side and output-side AC systems as a constant support for the DC voltage, achieving weak coupling of different-frequency electrical quantities and asynchronous interconnection of the same frequency. At the same time, when a single-side fault occurs, the weak-coupling topological characteristics can achieve fault isolation to a certain extent, with less impact on the AC system on the fault-free side. Cross-regional power grid interconnection and energy exchange are further achieved through the seven bridge arms.

[0129] Example 2

[0130] The second embodiment of the present invention introduces a control method for a DC multiplexing weakly coupled AC converter.

[0131] A DC multiplexing weakly coupled AC converter, comprising:

[0132] Generate a bridge arm modulation signal based on the mathematical relationship between the constant DC voltage and the AC output voltage;

[0133] At different times, the on-off state of the conduction switch is controlled while cooperating with the switching sub-module. The bridge arm modulation signal generated by the step wave is approximated with the closest level to complete the shaping of the AC voltage and realize the global energy balance control of the DC multiplexing weakly coupled AC converter.

[0134] As one or more embodiments, when u Vb Greater than 0, S b1 、S b2 The switch signals are 1 and 0 respectively, that is, S b1 conduction, S b2 Disconnect, at this time E Vb The voltage at U dc , and the b-phase shaping circuit voltage u sb Add up to get the valve side voltage on the input side u Vb , i Vb Depend on E Vb Point through S b1 Flow direction G 1 o'clock; when u Vb Less than 0, S b1 、S b2 The switch signals are 0 and 1 respectively, that is, S b1 Disconnect, S b2 Conducting, at this time E Vb The voltage at - U dc , and the b-phase shaping circuit voltage u sb Add up to get the valve side voltage on the input side u Vb , i Vb Depend on E Vb Point through S b2 Flow direction G 2 o'clock;

[0135] when u Vc Greater than 0, S c1 、S c2 The switch signals are 1 and 0 respectively, that is, S c1 conduction, S c2 Disconnect, at this time E Vc The voltage at U dc , and the c-phase shaping circuit voltage usc Add up to get the valve side voltage on the input side u Vc , i Vc Depend on E Vc Point through S c1 Flow direction G 1 o'clock; when u Vc Less than 0, S c1 、S c2 The switch signals are 0 and 1 respectively, that is, S c1 Disconnect, S c2 Conducting, at this time E Vb The voltage at - U dc , and the c-phase shaping circuit voltage u sc Add up to get the valve side voltage on the input side u Vc , i Vc Depend on E Vc Point through S c2 Flow direction G 2 o'clock;

[0136] As one or more embodiments, when u vv Greater than 0, S v1 、S v2 The switch signals are 1 and 0 respectively, that is, S v1 conduction, S v2 Disconnect, at this time E vv The voltage at U dc , and the v-phase shaping circuit voltage u sv Add up to get the valve side voltage on the input side u vv , i vv Depend on G 1 point through S v1 Flow direction E vv point; when u vv Less than 0, S v1 、S v2 The switch signals are 0 and 1 respectively, that is, S v1 Disconnect, S v2 Conducting, at this time E vv The voltage at - U dc, and the v-phase shaping circuit voltage u sv Add up to get the valve side voltage on the input side u vv , i vv Depend on G 2 points through S v2 Flow direction E vv point;

[0137] when u vw Greater than 0, S w1 、S w2 The switch signals are 1 and 0 respectively, that is, S w1 conduction, S w2 Disconnect, at this time E vw The voltage at U dc , and the w-phase shaping circuit voltage u sw Add up to get the valve side voltage on the input side u vw , i vw Depend on G 1 point through S w1 Flow direction E vw point;

[0138] when u vw Less than 0, S w1 、S w2 The switch signals are 0 and 1 respectively, that is, S w1 Disconnect, S w2 Conducting, at this time E vw The voltage at - U dc , and the w-phase shaping circuit voltage u sw Add up to get the valve side voltage on the input side u vw , i vw Depend on G 2 points through S w2 Flow direction E vw point.

[0139] The detailed steps are the same as the working principle of the DC multiplexing weakly coupled AC converter provided in Example 1, and will not be repeated here.

[0140] The above description is merely a preferred embodiment of this embodiment and is not intended to limit this embodiment. Those skilled in the art will readily appreciate that this embodiment may be modified and varied in various ways. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this embodiment shall be within the scope of protection of this embodiment.

Claims

1. A DC multiplexing weakly coupled AC converter, characterized in that: It is used for same-frequency asynchronous interconnection, including an input-side shaping circuit, an input-side conduction switch group, a constant-voltage common bridge arm group, an output-side conduction switch group and an output-side shaping circuit. Under the action of the constant-voltage common bridge arm group, weak-coupling energy exchange of same-frequency or different-frequency electrical quantities on the power frequency side and low-frequency side of the AC converter is realized; wherein, the constant-voltage common bridge arm group includes a first bridge arm, a second bridge arm and a third bridge arm connected in series; the first bridge arm, the second bridge arm and the third bridge arm are all composed of N full-bridge sub-modules cascaded; the input-side shaping circuit includes The fourth bridge arm and the fifth bridge arm, the output-side shaping circuit includes a sixth bridge arm and a seventh bridge arm; the fourth bridge arm, the fifth bridge arm, the sixth bridge arm, and the seventh bridge arm are each composed of N / 2 full-bridge sub-modules in cascade; one phase of the power frequency side of the AC converter is respectively connected to the first bridge arm and the second bridge arm, and the other two phases of the power frequency side are respectively connected to the input-side conduction switch group through the fourth bridge arm and the fifth bridge arm, and the input-side conduction switch group, the constant voltage common bridge arm group, and the output-side conduction switch group are connected in parallel; One phase on the low-frequency side of the AC converter is connected to the second bridge arm and the third bridge arm respectively, and the other two phases on the low-frequency side are connected to the output-side conductive switch group through the sixth bridge arm and the seventh bridge arm respectively.

2. A DC multiplexing weakly coupled AC converter as claimed in claim 1, characterized in that: The input-side shaping circuit and the output-side shaping circuit are arranged in a bidirectional equal-power transmission manner in a symmetrical topological structure, and the component parameters of the input-side shaping circuit and the output-side shaping circuit are consistent.

3. A DC multiplexing weakly coupled AC converter as claimed in claim 1, characterized in that: The full-bridge sub-module includes a first sub-bridge arm, a second sub-bridge arm, and a DC capacitor connected in parallel; wherein, the first sub-bridge arm includes a first switching tube and a second switching tube connected in series, and the second sub-bridge arm includes a third switching tube and a fourth switching tube connected in series, the connection between the first switching tube and the second switching tube is the first output end of the full-bridge sub-module, and the connection between the third switching tube and the fourth switching tube is the second output end of the full-bridge sub-module; the first switching tube, the second switching tube, the third switching tube, and the fourth switching tube are all connected in anti-parallel with diodes.

4. A DC multiplexing weakly coupled AC converter as claimed in claim 1, characterized in that: The input-side conduction switch group and the output-side conduction switch group both adopt a bridge circuit consisting of four conduction switches in two phases, and each of the conduction switches is composed of an IGBT cascade of several anti-parallel diodes.

5. A DC multiplexing weakly coupled AC converter as claimed in claim 4, characterized in that: The input-side conductive switch group includes a first conductive switch, a second conductive switch, a third conductive switch, and a fourth conductive switch; the first conductive switch and the second conductive switch are connected in series, and are connected in parallel with the third conductive switch and the fourth conductive switch, which are connected in series; the connection point between the first conductive switch and the second conductive switch is connected to the fifth bridge arm, and the connection point between the third conductive switch and the fourth conductive switch is connected to the fourth bridge arm; The output-side conductive switch group includes a fifth conductive switch, a sixth conductive switch, a seventh conductive switch, and an eighth conductive switch; the fifth conductive switch and the sixth conductive switch connected in series are connected in parallel with the seventh conductive switch and the eighth conductive switch connected in series; the connection point between the fifth conductive switch and the sixth conductive switch is connected to the seventh bridge arm, and the connection point between the seventh conductive switch and the eighth conductive switch is connected to the sixth bridge arm.

6. A control method for a DC multiplexing weakly coupled AC converter, using the DC multiplexing weakly coupled AC converter according to any one of claims 1 to 5, characterized in that: include: Generate a bridge arm modulation signal based on the mathematical relationship between the constant DC voltage and the AC output voltage; At different times, the on-off state of the conduction switch is controlled while cooperating with the switching sub-module to complete the shaping of the AC voltage through the bridge arm modulation signal generated by the step wave approximation, thereby realizing the global energy balance control of the DC multiplexing weakly coupled AC converter.

7. A control method for a DC multiplexing weakly coupled AC converter as claimed in claim 6, characterized in that: Based on the proportional relationship between the input side AC voltage amplitude, the output side voltage amplitude and the DC constant voltage, the energy of the second bridge arm is self-balanced, and the energy accumulated by the first bridge arm and the third bridge arm in one cycle is complementary. Through the energy exchange path established between the first bridge arm and the third bridge arm, the stable operation of the AC converter is achieved.

8. A control method for a DC multiplexing weakly coupled AC converter as claimed in claim 6, characterized in that: When the voltage of the power frequency side phase connected to the fourth bridge arm is positive, the third conduction switch is turned on and the fourth conduction switch is turned off, and the current flows from the power frequency side to the first bridge arm through the third conduction switch; when the voltage of the power frequency side phase connected to the fourth bridge arm is negative, the third conduction switch is turned off and the fourth conduction switch is turned on, and the current flows from the power frequency side to the third bridge arm through the fourth conduction switch; When the voltage of one phase on the power frequency side connected to the fifth bridge arm is positive, the first conduction switch is turned on, the second conduction switch is turned off, and the current flows from the power frequency side to the first bridge arm through the first conduction switch; when the voltage of one phase on the power frequency side connected to the fifth bridge arm is negative, the first conduction switch is turned off, the second conduction switch is turned on, and the current flows from the power frequency side to the third bridge arm through the second conduction switch.

9. A control method for a DC multiplexing weakly coupled AC converter as claimed in claim 6, characterized in that: When the voltage of the low-frequency side phase connected to the seventh bridge arm is positive, the fifth conductive switch is turned on and the sixth conductive switch is turned off, and the current flows from the input-side conductive switch through the fifth conductive switch to one end of the seventh bridge arm; when the voltage of the low-frequency side phase connected to the seventh bridge arm is negative, the fifth conductive switch is turned off and the sixth conductive switch is turned on, and the current flows from the connection between the input-side conductive switch group and the third bridge arm through the sixth conductive switch to one end of the seventh bridge arm; When the voltage of one phase on the low-frequency side connected to the sixth bridge arm is positive, the seventh conduction switch is turned on and the eighth conduction switch is turned off, and the current flows from the input-side conduction switch through the seventh conduction switch to one end of the sixth bridge arm; when the voltage of one phase on the low-frequency side connected to the sixth bridge arm is negative, the seventh conduction switch is turned off and the eighth conduction switch is turned on, and the current flows from the connection between the input-side conduction switch group and the third bridge arm through the eighth conduction switch to one end of the sixth bridge arm.

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