Direct current multiplexing type weak coupling alternating current converter and control method thereof

Abstract

The invention belongs to the technical field of power electronics, and provides a direct current multiplexing type weak coupling alternating current converter and a control method thereof, and the direct current multiplexing type weak coupling alternating current converter comprises 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 which are connected in series; each of the first bridge arm, the second bridge arm and the third bridge arm is formed by cascading N full-bridge sub-modules; 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; each of the fourth bridge arm, the fifth bridge arm, the sixth bridge arm and the seventh bridge arm is formed by cascading N / 2 full-bridge sub-modules. According to the invention, a weak coupling alternating current converter between a direct type alternating current converter and an indirect type alternating current converter is designed, weak coupling energy exchange of same-frequency or different-frequency electrical quantities at a power frequency side and a low frequency side is realized under the action of a constant-voltage common bridge arm group, and cross-regional power grid interconnection and energy exchange are realized.

Description

Technical Field

[0001] The 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 with new energy as the main body is gradually emerging. New energy power generation has the characteristics of intermittent, random and volatile, which may cause the power system to face insufficient power supply during peak loads and excess power during low loads or when new energy power generation is strong, which brings great difficulties to the balance of supply and demand of the power system. The dislocation of energy production areas and load centers, the limited peak-shaving capacity of regional power grids, the poor transmission channels of new energy, the failure of large-scale application of energy storage technology and other factors have led to the further development of the power system restricted by the absorption of a high proportion of new energy.

[0004] The energy composition, load distribution and increase and decrease patterns vary from region to region. At present, the power grid generally adopts the structural mode of "regional power grid + key interconnection line". With the implementation of regional power grid interconnection projects, the degree of interconnection between power grid zones has gradually increased, but the regional power grid transmission pattern is not perfect, and the weak coupling structure between regions has become an important factor restricting the consumption of new energy. Therefore, the research, planning and construction of cross-regional power grid interconnection technology plays an extremely important role in improving the interconnection pattern of power grid zones.

[0005] Cross-regional power grid interconnection technology is 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, 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 power grid partition interconnection converters are key devices for realizing energy exchange between different regional power grids in cross-regional power grid interconnection technology, and are important technical supports in the construction process of the current power grid partition interconnection pattern. The AC converter topologies are divided into indirect AC converters and direct AC converters. Among them, the indirect AC converter contains a common DC bus, and the AC systems on both sides are independent. The typical topology is the 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 to 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 DC transmission and can realize the asynchronous interconnection of long-distance and large-capacity cross-regional power grids. However, it requires a large number of sub-modules, the converter has a large volume, and the DC circuit breaker technology is not yet mature; the M3C AC converter topology has obvious advantages in the field of low-frequency power transmission. As Figure 1 shown, on the one hand, it can fully reduce the line reactance and the charging reactive power of submarine cables, and has a transmission capacity 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 be continued to be used, reducing the project investment and improving the power supply reliability at the same time.

[0007] However, the existing M3C AC converter topology has the following disadvantages: 1) The coupling degree between the input-side and output-side AC systems is high, resulting in complex control system design, and voltage fluctuations in any one phase will affect the three-phase voltages on the other side; 2) There are many internal circulation channels in the topology, resulting in complex circulation suppression strategies; 3) The large number of sub-modules leads to a large volume of the converter device and high production costs; 4) When the input and output frequencies of the M3C topology are similar, the low-frequency ripple of the sub-module capacitor voltage causes it to be in a charging and discharging state for a long time, resulting in a sharp increase in the voltage amplitude and seriously exceeding the rated value of the sub-module capacitor voltage; therefore, the M3C AC converter topology is difficult to be applied to the fields of cross-regional power grid co-frequency asynchronous flexible interconnection and application scenarios such as AC speed regulation and power electronic transformers. Summary of the Invention

[0008] To solve the above problems, the present invention proposes a DC multiplexing type weakly coupled AC converter and its control method, designs a weakly coupled AC converter between the direct type and the indirect type AC converter, uses a constant voltage common bridge arm group shared by the input-side and output-side AC systems as the constant support of the DC voltage, realizes the weak coupling of different-frequency electrical quantities and co-frequency asynchronous interconnection, and further realizes cross-regional power grid interconnection and energy exchange through seven bridge arms.

[0009] According to some embodiments, the first solution of the present invention provides a DC multiplexing type weakly coupled AC - AC converter, adopting the following technical solution: A DC multiplexing type weakly coupled AC - AC converter for synchronous - frequency asynchronous interconnection includes 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, and realizes weakly coupled energy exchange of electrical quantities with the same or different frequencies between the power - frequency side and the low - frequency side of the AC - AC converter under the action of the constant - voltage common bridge arm group; 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 each composed of N full - bridge sub - modules connected in cascade; 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 each composed of N / 2 full - bridge sub - modules connected in cascade; one phase of the power - frequency side of the AC - 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.

[0010] As a further technical limitation, one phase of the low - frequency side of the AC - AC converter is respectively connected to the second bridge arm and the third bridge arm, and the other two phases of the low - frequency side are respectively connected to the output - side conduction switch group through the sixth bridge arm and the seventh bridge arm.

[0011] As a further technical limitation, the input - side shaping circuit and the output - side shaping circuit are set for bidirectional equal - power transmission with a symmetric topological structure, and the component parameters of the input - side shaping circuit and the output - side shaping circuit are the same.

[0012] As a further technical limitation, 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, the second sub - bridge arm includes a third switching tube and a fourth switching tube connected in series, the connection point of the first switching tube and the second switching tube is the first output end of the full - bridge sub - module, and the connection point of 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 anti - parallel diodes.

[0013] As a further technical limitation, both the input - side conduction switch group and the output - side conduction switch group adopt a bridge - type circuit composed of two - phase four conduction switches, and each conduction switch is composed of several IGBTs with anti - parallel diodes connected in cascade.

[0014] Further, the input - side conduction switch group includes a first conduction switch, a second conduction switch, a third conduction switch, and a fourth conduction switch; the serially - connected first conduction switch and the second conduction switch are connected in parallel with the serially - connected third conduction switch and the fourth conduction switch; the connection point of the first conduction switch and the second conduction switch is connected to the fifth bridge arm, and the connection point of the third conduction switch and the fourth conduction switch is connected to the fourth bridge arm.

[0015] Further, the output - side conduction switch group includes a fifth conduction switch, a sixth conduction switch, a seventh conduction switch, and an eighth conduction switch; the serially - connected fifth conduction switch and the sixth conduction switch are connected in parallel with the serially - connected seventh conduction switch and the eighth conduction switch; the connection point of the fifth conduction switch and the sixth conduction switch is connected to the seventh bridge arm, and the connection point of the seventh conduction switch and the eighth conduction switch is connected to the sixth bridge arm.

[0016] According to some embodiments, the second solution of the present invention provides a control method for a DC - multiplexing type weakly - coupled AC - AC converter, adopting the following technical solutions: A control method for a DC - multiplexing type weakly - coupled AC - AC converter, which adopts the DC - multiplexing type weakly - coupled AC - AC converter provided by the first solution, includes: Generating a bridge - arm modulation signal based on the mathematical relationship between a constant DC voltage and an AC output voltage; At different moments, while controlling the on - off states of the conduction switches and cooperating with the switching sub - modules, approximating the generated bridge - arm modulation signal with the closest level through a stepped wave to complete the shaping of the AC voltage and achieve the global energy balance control of the DC - multiplexing type weakly - coupled AC - AC converter.

[0017] 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 - balance of the second bridge arm is realized, and the energies accumulated by the first bridge arm and the third bridge arm in one cycle are complementary. Through the energy exchange path established between the first bridge arm and the third bridge arm, the stable operation of the AC - AC converter is achieved.

[0018] As a further technical limitation, when the voltage of one phase on the power - frequency side 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 through the third conduction switch to the first bridge arm; when the voltage of one phase on the power - frequency side 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 through the fourth conduction switch to the third bridge arm; When the voltage of one phase on the power frequency side connected to the fifth bridge arm is positive, the first conducting switch conducts and the second conducting switch disconnects, and the current flows from the power frequency side through the first conducting switch to the first bridge arm; when the voltage of one phase on the power frequency side connected to the fifth bridge arm is negative, the first conducting switch disconnects and the second conducting switch conducts, and the current flows from the power frequency side through the second conducting switch to the third bridge arm.

[0019] As a further technical limitation, when the voltage of one phase on the low-frequency side connected to the seventh bridge arm is positive, the fifth conducting switch conducts and the sixth conducting switch disconnects, and the current flows from the input-side conducting switch through the fifth conducting switch to one end of the seventh bridge arm; when the voltage of one phase on the low-frequency side connected to the seventh bridge arm is negative, the fifth conducting switch disconnects and the sixth conducting switch conducts, and the current flows from the connection point between the input-side conducting switch group and the third bridge arm through the sixth conducting 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 conducting switch conducts and the eighth conducting switch disconnects, and the current flows from the input-side conducting switch through the seventh conducting 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 conducting switch disconnects and the eighth conducting switch conducts, and the current flows from the connection point between the input-side conducting switch group and the third bridge arm through the eighth conducting switch to one end of the sixth bridge arm.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention is designed with 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 synchronous asynchronous interconnection, and further realizing cross-regional power grid interconnection and energy exchange through seven bridge arms; the AC-DC converter in the present invention has no circulating current path in the topological structure, so there will be no circulating current, that is, there is no need to suppress and eliminate the circulating current. Description of the Drawings

[0021] The specification drawings forming a part of this embodiment are used to provide a further understanding of this embodiment. The schematic embodiments and descriptions thereof are used to explain this embodiment and do not constitute an improper limitation to this embodiment.

[0022] Figure 1 It is a schematic diagram of the typical AC-DC converter M3C topological structure in the background technology of the present invention; Figure 2 It is a schematic diagram of the topological structure of a DC multiplexing type weak-coupling AC-DC converter in the first embodiment of the present invention; Figure 3 It is a schematic diagram of the topological structure of the full-bridge sub-module in the first embodiment of the present invention; Figure 4 It is a schematic diagram of the topological structure of the conducting switch in the first embodiment of the present invention; Figure 5 Schematic diagram of the theoretical waveforms of the three-arm voltages of the constant-voltage common bridge arm group in Embodiment 1 of the present invention; Figure 6 Schematic diagram of the waveforms of the input-side shaping circuit and the driving signals of the conducting switches in Embodiment 1 of the present invention; Figure 7 Schematic diagram of the theoretical waveforms of the output-side shaping circuit and the driving signals of the conducting switches in Embodiment 1 of the present invention; Figure 8 Schematic diagram of the principle of phase b in Embodiment 1 of the present invention; Figure 9 Schematic diagram of the principle of waveform generation of the constant-voltage common bridge arm group in Embodiment 1 of the present invention. Detailed implementation manners

[0023] The present invention will be further described below in conjunction with the drawings and embodiments.

[0024] It should be noted that the following detailed descriptions are all exemplary and are intended to provide further explanations of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0025] It should be noted that the terms used herein are only for describing specific implementation manners 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 forms are also intended to include the plural forms. 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.

[0026] In the present invention, terms such as "upper", "lower", "left", "right", "front", "rear", "vertical", "horizontal", "side", "bottom", etc. indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only relationship terms determined for the convenience of describing the structural relationships of the various components or elements of the present invention, rather than specifically referring to any component or element of the present invention, and should not be construed as a limitation to the present invention.

[0027] In the present invention, terms such as "fixedly connected", "connected", "connected" should be understood in a broad sense, indicating that it can be a fixed connection, an integral connection or a detachable connection; it can be directly connected or indirectly connected through an intermediate medium. For those related scientific research or technical personnel in the field, the specific meanings of the above terms in the present invention can be determined according to specific circumstances, and should not be construed as a limitation to the present invention.

[0028] Without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other.

[0029] Embodiment 1 Embodiment 1 of the present invention introduces a DC multiplexing type weakly coupled AC converter.

[0030] In the macro process of the global energy system's transformation towards sustainability, vigorously developing new energy technologies is an inevitable trend. However, as the penetration rate of new energy continues to increase, self - contained regional power grids are difficult to shoulder the heavy responsibility of new energy consumption. The energy composition, load distribution, and increase - decrease laws vary among different regions. To scientifically solve the above - mentioned energy distribution problems, it is required that China's power system vigorously develop cross - regional power grid interconnection technologies, so as to optimize the allocation of power generation resources and achieve consumption balance on a larger scale. Grid - partitioned interconnection converters are key devices for cross - regional power grid interconnection technologies and have gradually become a research hotspot among domestic and foreign scholars. The typical converter topology BTB - MMC has a large number of sub - modules, a high degree of coupling in the M3C heterogeneous AC system, complex internal circulating currents, and is difficult to be applied to the field of synchronous - asynchronous interconnection of regional power grids with the same frequency.

[0031] Therefore, this embodiment provides a DC multiplexing type weakly coupled AC converter as shown in Figure 2 . It uses a constant - voltage bridge arm group to support the DC voltage, realizes weak coupling of different - frequency electrical quantities inside the system, has simple control, realizes synchronous - asynchronous interconnection, and overcomes the problem of M3C synchronous transmission; at the same time, at the structural level, the proposed new topology has no circulating current path and does not require additional circulating current suppression strategies, fully realizing a compact design.

[0032] As shown in Figure 2 , a DC multiplexing type weakly coupled AC converter includes: An input - side shaping circuit, including a fourth bridge arm and a fifth bridge arm; both the fourth bridge arm and the fifth bridge arm are composed of N / 2 full - bridge sub - modules connected in cascade; An input - side conduction switch group, including a first conduction switch S c1 , a second conduction switch S c2 , a third conduction switch S b1 , and a fourth conduction switch S b2 ; the first conduction switch S c1 and the second conduction switch S c2 connected in series are connected in parallel with the third conduction switch S b1 and the fourth conduction switch S b2 connected in series; the connection point of the first conduction switch S c1 and the second conduction switch S c2 is connected to the fifth bridge arm, and the connection point of the third conduction switch S b1 and the fourth conduction switch S b2 is connected to the fourth bridge arm; The 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 sub-modules (FBSM) connected in cascade. 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 each composed of N / 2 full-bridge sub-modules connected in cascade. The output-side shaping circuit includes a fifth conduction switch S v1 、a sixth conduction switch S v2 、a seventh conduction switch S w1 and an eighth conduction switch S w2 ; the fifth conduction switch S v1 and the sixth conduction switch S v2 connected in series are connected in parallel with the seventh conduction switch S w1 and the eighth conduction switch S w2 connected in series; the connection point of the fifth conduction switch S v1 and the sixth conduction switch S v2 is connected to the seventh bridge arm, and the connection point of the seventh conduction switch S w1 and the eighth conduction switch S w2 is connected to the sixth bridge arm.

[0033] Combined Figure 2 it can be seen that in this embodiment, one phase of the power frequency side of the AC converter is respectively connected to the first bridge arm m1 and the second bridge arm m2, 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. 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 of the low-frequency side of the AC converter is respectively connected to the second bridge arm m2 and the third bridge arm m3, and the other two phases of the low-frequency side are respectively connected to the output-side conduction switch group through the sixth bridge arm and the seventh bridge arm; under the action of the constant-voltage common bridge arm group, weak-coupling energy exchange of the same-frequency or different-frequency electrical quantities between the power frequency side and the low-frequency side is realized.

[0034] It should be noted that the input-side shaping circuit and the output-side shaping circuit in this embodiment are set for bidirectional equal-power transmission with a symmetrical topological structure, and the component parameters of the input-side shaping circuit and the output-side shaping circuit are the same.

[0035] It should be noted that the first conduction 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 conduction switch Sw2 The structures are all the same, that is, as Figure 4 shown, they are all composed of cascaded IGBTs with several antiparallel diodes.

[0036] As Figure 8 shown, in this embodiment, taking phase b as an example, the introduction is as follows: When u Vb is greater than 0, the switching signals of S b1 and S b2 are 1 and 0 respectively, that is, S b1 conducts and S b2 disconnects. At this time E Vb the voltage at U dc is u sb Adding it to the voltage of the phase b shaping circuit u Vb to obtain the valve-side voltage i Vb of the input side. E Vb flows from point b1 through S G to point u Vb When b1 is less than 0, the switching signals of S b2 and S b1 are 0 and 1 respectively, that is, S b2 disconnects and S E Vb the voltage at U dc is - u sb Adding it to the voltage of the phase b shaping circuit u Vb to obtain the valve-side voltage i Vb of the input side. E Vb flows from point b2 through S G to point When u Vc is greater than 0, the switching signals of S c1 and S c2 are 1 and 0 respectively, that is, S c1 conducts and S c2 disconnects. At this time E Vc the voltage at U dc is u sc Adding it to the voltage of the phase c shaping circuitAdd them to obtain the valve-side voltage on the input side u Vc , i Vc From E Vc point through S c1 flow to G point 1; When u Vc is less than 0, the switch signals of S c1 and S c2 are 0 and 1 respectively, that is, S c1 is off and S c2 is on. At this time E Vb the voltage at the place is - U dc , and add it to the voltage of the c-phase shaping circuit u sc to obtain the valve-side voltage on the input side u Vc , i Vc From E Vc point through S c2 flow to G point 2; When u vv is greater than 0, the switch signals of S v1 and S v2 are 1 and 0 respectively, that is, S v1 is on and S v2 is off. At this time E vv the voltage at the place is U dc , and add it to the voltage of the v-phase shaping circuit u sv to obtain the valve-side voltage on the input side u vv , i vv From G point 1 through S v1 flow to E vv point; When u vv is less than 0, the switch signals of S v1 and S v2 are 0 and 1 respectively, that is, S v1 is off and S v2 is on. At this time E vv the voltage at the place is - U dc , and add it to the voltage of the v-phase shaping circuit u svAdded together to obtain the valve-side voltage on the input side u vv , i vv From G 2 points pass through S v2 Flowing to E vv Point; When u vw Is greater than 0, the switch signals of S w1 、S w2 Are 1 and 0 respectively, that is, S w1 Conducts, S w2 Disconnects. At this time E vw The voltage at is U dc , added to the voltage of the w-phase shaping circuit u sw Added together to obtain the valve-side voltage on the input side u vw , i vw From G 1 point passes through S w1 Flowing to E vw Point; When u vw Is less than 0, the switch signals of S w1 、S w2 Are 0 and 1 respectively, that is, S w1 Disconnects, S w2 Conducts. At this time E vw The voltage at is - U dc , added to the voltage of the w-phase shaping circuit u sw Added together to obtain the valve-side voltage on the input side u vw , i vw From G 2 points pass through S w2 Flowing to E vw Point.

[0037] It should be noted that in this embodiment, the input-side conduction switch group and the output-side conduction switch group adopt synchronous rectification control with a phase angle difference of 180° between the driving signals.

[0038] In this embodiment, as Figure 2As 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 are directly converted into alternating current with different frequencies through the AC converter in this embodiment. Among them, "V" represents the valve side of the input side, " v " represents the valve side of the output side, O 、 O' are the grounding points of the AC systems on the input and output sides respectively; E Va is the common connection point between the first arm m1 and the second arm m2 of the constant-voltage common arm group, E vu is the common connection point between the second arm m2 and the third arm m3 of the constant-voltage common arm group, G 1 and G 2 are respectively the highest point U dc / 2 and the lowest point - U dc / 2 of the DC voltage. E Vx ( x = b, c) is the common connection point between the shaping circuit and the conduction switch circuit of the b (c) phase on the input side, E vy ( y = v, w) is the common connection point between the shaping circuit and the conduction switch circuit of the v (w) phase on the output side; u ia 、 u ib 、 u ic are the AC grid voltages on the input side, u ou 、 u ov 、 u ow are the AC grid voltages on the output side. L iT 、 L oT are respectively the leakage inductances of the transformers on the input and output sides, R iT 、 R oT are respectively the equivalent resistances of the short-circuit losses of the transformers on the input and output sides. 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 are the output voltages of the shaping circuits for phases b and c on the input side, u sv and u sw are the output voltages of the shaping circuits for phases v and w on the output side. i x1 , i x2 are respectively the currents flowing through S x1 , S x2 ( x = b, c), i y1 , i y2 are respectively the currents flowing through S y1 , S y2 ( y = v, w). U dc is the voltage borne by the constant-voltage common bridge arm group, u m1 , u m2 , u m3 are respectively 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 respectively the currents flowing through the first bridge arm m1, the second bridge arm m2, and the third bridge arm m3; the positive directions of each physical quantity are as Figure 2 shown.

[0039] This embodiment adopts a full-bridge sub-module as shown in Figure 3 , which includes a first sub-bridge arm, a second sub-bridge arm, and a DC capacitor connected in parallel; among them, the first sub-bridge arm includes a first switching tube T1 and a second switching tube T2 connected in series, the second sub-bridge arm includes a third switching tube T3 and a fourth switching tube T4 connected in series, the connection point of the first switching tube T1 and the second switching tube T2 is the first output end of the full-bridge sub-module, and the connection point of the third switching tube T3 and the fourth switching tube T4 is the second output end of the full-bridge sub-module; the first switching tube T1, the second switching tube T2, the third switching tube T3, and the fourth switching tube T4 are respectively anti-parallelly connected with a first diode D1, a second diode D2, a third diode D3, and a fourth diode D4.

[0040] This embodiment takes the application in AC power transmission as an example, sets the topology to industrial frequency 50 Hz input and low frequency 50 / 3 Hz output, and launches a detailed introduction to the working principle of the AC converter proposed in this embodiment: In actual application scenarios, when the three-phase AC power passes through the AC converter and its frequency is reduced from industrial frequency 50 Hz to low frequency 50 / 3 Hz, the transmission capacity of AC power transmission is significantly improved.

[0041] like Figure 9 As shown in the figure, the constant voltage common bridge arm group is an important medium for direct energy exchange between the input side and the output side of the alternating current of different frequencies, and has two important functions: 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); 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).

[0042] 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 Figure 5 As shown: (1) (2) (3) In this embodiment, the input side shaping circuit, the output side shaping circuit and the first conduction 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 conduction switch S w2 Phase coordination, taking the input side b phase as an example: like Figure 8As shown, the first conduction switch S b1 and the second conduction switch S b2 alternately turn on and off at E Vb The output at the point contains U dc / 2 and - U dc / 2, a square wave with two levels. The shaping circuit formed by cascading sub-modules reshapes the two-level square wave voltage waveform by switching the sub-modules to obtain the valve-side voltage of phase b u Vb . The working principle of the shaping circuits on the input side of phase c, output sides of phases v and w and the conduction switch group is the same as that on the input side of phase b, and the valve-side voltages of output phase c u Vc , valve-side voltage of phase v u vv and valve-side voltage of phase w u vw are obtained. The theoretical waveforms of the voltages of the shaping circuits on the input sides of phases b and c u sb , u sc and the voltages of the shaping circuits on the output sides of phases v and w u sv , u sw are respectively as shown in Figure 6 and Figure 7 .

[0043] The expressions for the AC voltage and current on the power frequency side of the AC-DC converter are: (4) where U m1 and I m1 are the amplitudes of the AC voltage and current on the power frequency side, φ x is the initial phase angle of the x th 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.

[0044] Define and as the power frequency side x ( x= b, c) is the operating mode switching function of the phase unit, and its expression is: (5) According to Kirchhoff's voltage law, the expression of the voltage of the phase shaping circuit on the power frequency side is: x (6) (6) According to Kirchhoff's current law, the current flowing through S x1 、 S x2 ( x = b, c) is i x1 、 i x2 : (7) The AC voltage and current on the low-frequency side of the AC converter are 、 The expression is: (8) Among them, U m2 And I m2 Are the amplitudes of the AC voltage and current on the low-frequency side, φ y Is the initial phase angle of the y Phase (where φ u = 0, φ v = -2π / 3, φ w = 2π / 3), φ 2 is the power factor angle on the low-frequency grid side.

[0045] Define And As the operating mode switching functions of the y ( y = v, w) phase units of the power frequency side, and their expressions are: (9) According to Kirchhoff's voltage law, the expression of the voltage of the phase shaping circuit on the low-frequency side is: y (10) (10) According to Kirchhoff's current law, the current flowing through S y1 、 S y2 ( y = v, w) is iy1 , i y2 is: (11) Based on the actual application situation, calculate the energy balance condition of the proposed novel AC - AC converter topology. At this time, the power frequency and low - frequency are coupled in the constant - voltage common bridge arm group of the AC - AC converter, and the periods of the power - frequency system and the low - frequency system are 2π and 6π respectively. To facilitate observing the energy flow of the AC - AC converter, the operation period of the entire AC - AC converter is defined as T = 6π in this embodiment. The sub - module capacitor is the medium for power conversion on the power - frequency side and low - frequency side of the AC - AC converter. To ensure the balance of the sub - module capacitor voltage, it is necessary to ensure that the energy exchange between the shaping circuit and the outside world within one period is zero, that is, the absorbed energy and the released energy should be equal.

[0046] Taking the b - phase as an example, the voltage of the b - phase shaping circuit u sb As shown in formula (6), the current flowing through the b - phase shaping circuit is the power - frequency side alternating - current i b As shown in formula (4). u sb Multiplying with i b the expression of can obtain the instantaneous power of the b - phase shaping circuit P sb is: (12) Integrating within one period T = 6π for P sb can obtain the energy accumulated by the b - phase shaping circuit within one period as: (13) Let W sb = 0, we can get: (14) The energy - balance mechanism of the c - phase and the b - phase is the same, so the energy - balance condition of the power - frequency side shaping circuit is formula (14).

[0047] Taking the v - phase as an example, the voltage of the v - phase shaping circuit u sv As shown in formula (10), the current flowing through the v - phase shaping circuit is the low - frequency side alternating - current i v As shown in formula (8). u sv Multiplying with i v the expression of can obtain the instantaneous power of the v - phase shaping circuit P sv is: (15) Over one period \(T = 6\pi\), integrating P sv yields the energy accumulated by the v-phase shaping circuit over one period as: (16) Let W sv = 0, then we get: (17) The energy equalization mechanism for the w-phase and v-phase is the same. Therefore, the condition for energy equalization in the low-frequency side shaping circuit is formula (17).

[0048] The first arm m1, the second arm m2, and the third arm m3 of the constant-voltage common bridge arm group jointly support the voltage U dc , and at the same time output the industrial-frequency side a-phase AC voltage and the low-frequency side u-phase AC voltage. The voltages of the three arms u m1 、 u m2 、 u m3 are as shown in formula (3): According to Kirchhoff's current law, the currents flowing through the three arms are respectively: (18) Among them, the currents flowing through S x1 、 S x2 ( x = b, c) i x1 、 i x2 are as shown in formula (7), and the currents flowing through S y1 、 S y2 ( y = v, w) i y1 、 i y2 are as shown in formula (11).

[0049] The voltages u m1 、 u m2 、 u m3 and the currents i m1 、i m2 and i m3 Multiplying them respectively can obtain the instantaneous power of each of the three bridge arms, and integrating the instantaneous power can obtain that the energies accumulated by the three bridge arms within a period T = 6π are respectively: (19) After the AC - AC converter is put into operation, the input energy and output energy within one period are equal, that is (20) Obviously, when formula (20) holds, the energy of the middle bridge arm m2 of the constant - voltage common bridge arm group is self - balanced.

[0050] After the energies of the bridge arms on the power - frequency side and the low - frequency side are balanced, that is, when formulas (14) and (17) are satisfied simultaneously, then there is: (21) Combining formula (20) and formula (21), we can get: (22) At this time, the sum of the energies accumulated by the first bridge arm m1 and the third bridge arm m3 of the constant - voltage common bridge arm group within one period is 0, that is W m1 + W m3 = 0. It is necessary to connect the neutral points of the power - frequency side and the low - frequency side transformers to establish an energy exchange path between the first bridge arm m1 and the third bridge arm m3 to ensure the stable operation of the topology.

[0051] Under the same AC voltage level, BTB - MMC uses 12N modules, 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 - AC converter topology, the number of sub - modules is reduced by 58.3% and 44.4% respectively, significantly realizing the compact design of the AC - AC converter.

[0052] This embodiment uses a constant - voltage common bridge arm group shared by the input - side and output - side AC systems as the constant support of the DC voltage, realizing weak coupling of different - frequency electrical quantities and synchronous - frequency asynchronous interconnection; at the same time, when a single - side fault occurs, the weak - coupling topological characteristics can, to a certain extent, achieve fault isolation and have little impact on the AC system on the non - fault side; cross - regional power grid interconnection and energy exchange are further realized through seven bridge arms.

[0053] Embodiment Two Embodiment Two of the present invention introduces a control method for a DC - multiplexing - type weak - coupling AC - AC converter.

[0054] A DC multiplexing type weakly coupled AC converter, comprising: Generating a leg modulation signal based on the mathematical relationship between the constant DC voltage and the AC output voltage; At different times, while controlling the on-off state of the conducting switch and cooperating with the switching sub-module, approximating the generated leg modulation signal with the closest level through a stepped wave to complete the shaping of the AC voltage and achieve the global energy balance control of the DC multiplexing type weakly coupled AC converter.

[0055] As one or more embodiments, when u Vb is greater than 0, the switching signals of S b1 and S b2 are 1 and 0 respectively, that is, S b1 is conducting and S b2 is off. At this time, E Vb the voltage at U dc is u sb which is added to the voltage of the b-phase shaping circuit u Vb to obtain the valve-side voltage i Vb of the input side. E Vb flows from the b1 point through S G to the u Vb When b1 is less than 0, the switching signals of S b2 and S b1 are 0 and 1 respectively, that is, S b2 is off and S E Vb the voltage at U dc is - u sb which is added to the voltage of the b-phase shaping circuit u Vb to obtain the valve-side voltage i Vb of the input side. E Vb flows from the b2 point through S G to the When u Vc is greater than 0, the switching signals of S c1 and S c2 are 1 and 0 respectively, that is, S c1 is conducting and S c2 is off. At this time, EVc The voltage at U dc , is added to the voltage of the c-phase shaping circuit u sc to obtain the valve-side voltage on the input side u Vc , i Vc From E Vc point, it flows through S c1 to G point 1; when u Vc is less than 0, the switching signals of S c1 and S c2 are 0 and 1 respectively, that is, S c1 is disconnected and S c2 is conducting. At this time E Vb the voltage at U dc is - u sc , which is added to the voltage of the c-phase shaping circuit u Vc to obtain the valve-side voltage on the input side i Vc From E Vc point, it flows through S c2 to G point 2; As one or more embodiments, when u vv is greater than 0, the switching signals of S v1 and S v2 are 1 and 0 respectively, that is, S v1 is conducting and S v2 is disconnected. At this time E vv the voltage at U dc is u sv , which is added to the voltage of the v-phase shaping circuit u vv to obtain the valve-side voltage on the input side i vv From G point 1, it flows through S v1 to E vv point; when u vv is less than 0, the switching signals of S v1 and S v2 are 0 and 1 respectively, that is, S v1 is disconnected and S v2 is conducting. At this timeE vv The voltage at U dc is - u sv added to the voltage of the v-phase shaping circuit to obtain the valve-side voltage on the input side u vv , i vv From G point 2 through S v2 flows to E vv point; When u vw is greater than 0, the switching signals of S w1 and S w2 are 1 and 0 respectively, that is, S w1 conducts and S w2 disconnects. At this time E vw the voltage at U dc is u sw added to the voltage of the w-phase shaping circuit to obtain the valve-side voltage on the input side u vw , i vw From G point 1 through S w1 flows to E vw point; When u vw is less than 0, the switching signals of S w1 and S w2 are 0 and 1 respectively, that is, S w1 disconnects and S w2 conducts. At this time E vw the voltage at U dc is - u sw added to the voltage of the w-phase shaping circuit to obtain the valve-side voltage on the input side u vw , i vw From G point 2 through S w2 flows to E vw point.

[0056] The detailed steps are the same as the working principle of a DC multiplexing type weak-coupling AC-DC converter provided in Embodiment 1 and will not be elaborated here.

[0057] The above are only the preferred embodiments of this embodiment and are not intended to limit this embodiment. For those skilled in the art, various modifications and changes can be made to this embodiment. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of this embodiment shall be included within the protection scope of this embodiment.

Claims

1. A DC multiplexing weakly coupled AC converter, characterized in that: Used for same-frequency asynchronous interconnection, including input-side shaping circuit, input-side conduction switch group, constant-voltage common bridge arm group, output-side conduction switch group and output-side shaping circuit, under the action of the constant-voltage common bridge arm group, the same-frequency or different-frequency electrical quantity weak coupling energy exchange between the power frequency side and the 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 all composed of N / 2 full-bridge sub-modules cascaded; one phase of the industrial 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 industrial 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.

2. A DC multiplexing weakly coupled AC converter as claimed in claim 1, characterized in that: 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.

3. 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 bidirectional equal-power transmission arrangements in a symmetrical topological structure, and the component parameters of the input-side shaping circuit and the output-side shaping circuit are consistent.

4. 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 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.

5. 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 two-phase four conduction switches, and each of the conduction switches is composed of an IGBT cascade of several anti-parallel diodes.

6. A DC multiplexing weakly coupled AC converter as claimed in claim 5, characterized in that: The input-side conduction switch group includes a first conduction switch, a second conduction switch, a third conduction switch and a fourth conduction switch; the first conduction switch and the second conduction switch connected in series are connected in parallel with the third conduction switch and the fourth conduction switch connected in series; the connection point of the first conduction switch and the second conduction switch is connected to the fifth bridge arm, and the connection point of the third conduction switch and the fourth conduction 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 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.

7. A control method for a DC multiplexing weakly coupled AC converter, using the DC multiplexing weakly coupled AC converter as claimed in any one of claims 1 to 6, characterized in that: include: generating a bridge arm modulation signal based on a mathematical relationship between a constant DC voltage and an AC output voltage; At different times, the on-off state of the conduction switch is controlled while cooperating with the switching sub-module to approximate the generated bridge arm modulation signal with the closest level through the step wave to complete the shaping of the AC voltage and realize the global balanced control of the energy of the DC multiplexing weakly coupled AC converter.

8. A control method for a DC multiplexing weakly coupled AC converter as claimed in claim 7, 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 self-balancing of the second bridge arm is achieved, 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.

9. A control method for a DC multiplexing weakly coupled AC converter as claimed in claim 7, characterized in that: When the voltage of one phase on the power frequency side 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 one phase on the power frequency side 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; 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.

10. A control method for a DC multiplexing weakly coupled AC converter as claimed in claim 7, characterized in that: When the voltage of one phase on the low-frequency side connected to the seventh bridge arm is positive, the fifth conduction switch is turned on, the sixth conduction switch is turned off, and the current flows from the input-side conduction switch to one end of the seventh bridge arm through the fifth conduction switch; when the voltage of one phase on the low-frequency side connected to the seventh bridge arm is negative, the fifth conduction switch is turned off, the sixth conduction switch is turned on, and the current flows from the connection between the input-side conduction switch group and the third bridge arm to one end of the seventh bridge arm through the sixth conduction switch; 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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