Current source converter and DC transmission system based on controllable voltage source

Abstract

This application relates to a current source converter and a DC transmission system based on a controllable voltage source. The current source converter includes a converter transformer, a converter bridge circuit, a controllable voltage source, and a filter reactor. The phase AC terminal of the converter bridge circuit is connected to the converter transformer through the filter reactor, and the controllable voltage source is connected between the phase AC terminal and the filter reactor. Thus, the filter reactor isolates the converter bridge circuit from the AC load and filters out harmonics in the output current of the converter bridge circuit, ensuring the harmonic characteristics of the grid-connected current. Furthermore, the filter reactor provides a sufficiently large equivalent impedance on the AC side, enabling the controllable voltage source to have more precise power regulation capabilities. By providing the commutation voltage required for commutation to the converter bridge circuit through the controllable voltage source, the leakage reactance of the converter transformer does not participate in commutation, completely eliminating the influence of the converter transformer leakage reactance on the commutation process and significantly reducing reactive power consumption.

Description

Technical Field

[0001] This application relates to the field of DC power transmission technology, and in particular to a current source converter and DC power transmission system based on a controllable voltage source. Background Technology

[0002] With the development of DC transmission technology, traditional synchronous power sources are gradually being replaced by large-scale new energy bases and ultra-high voltage DC systems. The trend of "hollowing out" of the sending and receiving end power grids is becoming increasingly apparent. This has led to a continuous decline in the short-circuit ratio of the sending and receiving end power grids, resulting in insufficient dynamic reactive power regulation capacity, reduced voltage stability margin, and decreased grid strength, posing a severe challenge to the safety of the large power grid. Against this backdrop, AC systems urgently need DC systems to provide necessary active support to ensure the safe and stable operation of the weak receiving end power grid.

[0003] Currently, line commutated converters (LCCs) have been widely used in ultra-high voltage direct current (UHVDC) projects. However, LCCs suffer from commutation failures when used in scenarios lacking reliable conventional power sources, which can lead to a significant shortage of active power and seriously affect the reliability of power transmission in the power grid.

[0004] As integrated gate commutated thyristors (IGCTs) continue to evolve towards higher voltage and higher current, a hybrid line commutated converter (HCC) topology based on reverse-resistance IGCT devices was proposed in 2022. This topology can perform commutation without relying on grid voltage, solving the commutation failure problem. However, because its commutation principle is the same as that of the LCC, the HCC still needs to absorb reactive power from the AC grid during operation. Its dynamic reactive power regulation capability depends on the switching of reactive power compensation devices, resulting in a slow response speed. Given the continued weakening of the power grid in the future, this topology still has room for improvement.

[0005] Since the introduction of the Modular Multilevel Converter (MMC) in 2010, it has been widely used in DC projects both domestically and internationally due to its elimination of commutation failure issues and its four-quadrant power operation capability, enabling it to provide flexible and adjustable reactive power to the grid while transmitting rated active power. However, when applied to ultra-high voltage long-distance DC transmission scenarios, it suffers from drawbacks such as difficulty in capacity expansion, low power density, high cost, and high losses. Therefore, there is an urgent need to propose a converter topology that is lower in cost and easier to upgrade in capacity.

[0006] In 2022, a multi-source composite converter (STATCOM and Line commutation converter, SLCC) based on a parallel structure of a conventional DC converter and a Static Var Compensator (STATCOM) was proposed. This effectively alleviates the problems of traditional converter technologies, such as reliance on the strength of the AC system, poor independence, and poor adaptability, while also providing reactive power support to the grid. However, because its current source section still uses thyristors, SLCC cannot avoid commutation failure when the grid voltage drops significantly, especially in scenarios with multiple DC feeds and extremely weak AC systems, where it still has limitations.

[0007] In 2022, a controllable current source converter (CSC) topology based on reverse-resistance IGCT was proposed. This topology solved the commutation failure problem by relying on IGCT devices and designed a power decoupling control strategy to achieve power decoupling in certain operating ranges, enabling the current source converter to also have four-quadrant operation capability and provide reactive power support to the grid. However, CSC operates in steady-state PWM (Pulse-Width Modulation) mode, requiring devices to have continuous active turn-off capability. Currently, the turn-off capability of reverse-resistance IGCT devices is affected by junction temperature, making it difficult to support high-frequency operation. Furthermore, increasing the switching frequency will significantly increase the difficulty of series voltage equalization of the valve arms. In addition, the parallel AC capacitor at the CSC valve outlet is composed of large-scale capacitors connected in series and parallel, resulting in a high failure rate and risk of failure.

[0008] In summary, existing converters cannot simultaneously meet the requirements of high-capacity active power transmission, strong reactive power support capability, and low cost. Summary of the Invention

[0009] Therefore, it is necessary to provide a current source converter and DC transmission system based on a controllable voltage source to address the above problems. This system has the ability to transmit large-capacity active power and strong reactive power support, and is also low in cost.

[0010] This application provides a current source converter based on a controllable voltage source, comprising: a converter transformer, a converter bridge circuit, a controllable voltage source, and a filter reactor. The DC terminal of the converter bridge circuit is used to connect to a DC load. The AC terminals of each phase of the converter bridge circuit are respectively connected to the controllable voltage source. The first terminal of each filter reactor is connected to the AC terminal of one phase and the controllable voltage source, and the second terminal of each filter reactor is respectively connected to the converter transformer. The converter transformer is used to connect to an AC load.

[0011] In one embodiment, the converter bridge circuit includes a DC reactor and converter bridge arms. Each converter bridge arm includes an upper bridge arm and a lower bridge arm. The first ends of each upper bridge arm are respectively connected to form a common terminal as a first DC terminal. The first ends of each lower bridge arm are respectively connected to form a common terminal as a second DC terminal. The second end of one upper bridge arm is correspondingly connected to the second end of one lower bridge arm, and the common terminal formed is a phase AC terminal. The first DC terminal is connected to a DC load through the DC reactor, and the second DC terminal is connected to the DC load.

[0012] In one embodiment, the converter arm includes a first current-carrying branch and a second current-carrying branch connected in reverse parallel.

[0013] In one embodiment, the first current-carrying branch includes any one of the following: a first branch formed by multiple reverse-resistance IGCT devices connected in series, a second branch formed by multiple IGBT devices connected in series, and a third branch formed by at least one asymmetric IGCT device and at least one IGBT device connected in series.

[0014] In one embodiment, the second current-carrying branch includes any one of the following: a first branch formed by multiple reverse-resistance IGCT devices connected in series, a second branch formed by multiple IGBT devices connected in series, a fourth branch formed by multiple diodes connected in series, and a fifth branch formed by multiple thyristors connected in series.

[0015] In one embodiment, the converter arm further includes a buffer branch connected in parallel with the first flow branch and the second flow branch, respectively.

[0016] In one embodiment, the buffer branch includes a first buffer branch and a second buffer branch, wherein the first buffer branch is connected in parallel with the first flow branch, and the second buffer branch is connected in parallel with the second flow branch.

[0017] In one embodiment, the converter arm further includes energy-consuming branches connected in parallel with the first flow branch and the second flow branch, respectively.

[0018] In one embodiment, the commutator bridge circuit further includes a saturation reactance, wherein the second end of the upper bridge arm is connected to the second end of the lower bridge arm through the saturation reactance, and / or, the second end of the lower bridge arm is connected to the second end of the upper bridge arm through the saturation reactance.

[0019] In one embodiment, the controllable voltage source includes multiple interconnected sub-modules, and the connection method of the sub-modules includes any one of MMC connection, delta chain connection and star chain connection.

[0020] In one embodiment, the submodule is an IGBT submodule or an IGCT submodule.

[0021] In one embodiment, the controllable voltage source establishes an AC voltage based on the angular frequency reference value obtained by the current source converter through active power control and the AC voltage amplitude reference value obtained through reactive power control to achieve grid control.

[0022] This application also provides a DC transmission system, including two current source converters as described above, one of which serves as a sending-end current source converter and the other as a receiving-end current source converter, wherein the DC terminal of the sending-end current source converter is connected to the DC terminal of the receiving-end current source converter.

[0023] In one embodiment, the sending-end current source converter adjusts the output voltage amplitude of the controllable voltage source at the sending end based on the reactive power measurement value of the sending-end grid and the DC voltage measurement value of the bridge arm of the controllable voltage source at the sending end, so as to achieve reactive power interaction with the AC grid; the receiving-end current source converter adjusts the output voltage amplitude of the controllable voltage source at the receiving end based on the reactive power measurement value of the receiving-end grid and the DC voltage measurement value of the bridge arm of the controllable voltage source at the receiving end, so as to achieve reactive power interaction with the AC grid; the sending-end current source converter performs constant DC current control by adjusting the firing angle, and the receiving-end current source converter performs constant DC voltage control by adjusting the firing angle, so as to achieve active power interaction between the sending-end current source converter and the receiving-end current source converter.

[0024] In one embodiment, the DC transmission system further includes: if a short-term fault occurs in the AC grid or DC side, the converter bridge arms of the converter bridge circuit are continuously turned off within one power frequency cycle to ensure that the AC output current is continuously controllable; if a permanent fault occurs in the receiving-end system, the upper and lower bridge arms of the same phase in the converter bridge circuit of the receiving-end current source converter are triggered to conduct, forming a bypass pair to provide a path for power transmission to the sending end.

[0025] In one embodiment, the converter bridge arm of the converter bridge circuit includes a first current-carrying branch and a second current-carrying branch connected in reverse parallel. When there is a power reverse transmission requirement, the first current-carrying branch is turned off and the second current-carrying branch is turned on. By adjusting the firing angles of the sending-end current source converter and the receiving-end current source converter, opposite potential differences are formed to achieve power reversal while the voltage polarity remains unchanged.

[0026] The aforementioned current source converter and DC transmission system include a converter transformer, a converter bridge circuit, a controllable voltage source, and a filter reactor. The phase AC terminals of the converter bridge circuit are connected to the converter transformer via the filter reactor, and a controllable voltage source is connected between the phase AC terminals and the filter reactor. The filter reactor isolates the converter bridge circuit from the AC load. This scheme has the following advantages:

[0027] 1. By constructing a second-order LC filter circuit using the filter reactance and the equivalent capacitance of the controllable voltage source, the harmonics of the output current of the converter bridge circuit can be filtered out. The harmonic characteristics of the grid current can be guaranteed without additional harmonic control strategies. At the same time, the current harmonics injected into the converter transformer are reduced, further reducing the manufacturing difficulty of the converter transformer.

[0028] 2. As an important component of the AC-side equivalent impedance, the filter reactance can be customized according to the power grid requirements. The AC-side equivalent impedance determines the reactive power regulation capability of the controllable voltage source. Appropriate impedance design can make the power regulation of the controllable voltage source more precise.

[0029] 3. The controllable voltage source provides the commutation voltage required for the commutation of the converter bridge circuit. The leakage reactance of the converter transformer does not participate in the commutation, thus completely eliminating the influence of the leakage reactance of the converter transformer on the commutation process and greatly reducing the reactive power consumption of the current source converter itself, thereby further reducing the capacity requirement of the controllable voltage source.

[0030] 4. Since the leakage reactance and filter reactance of the converter transformer do not participate in commutation, the commutation speed is accelerated, greatly reducing the risk of commutation failure.

[0031] 5. The output voltage amplitude of the controllable voltage source of the two current source converters with the above structure can be independently adjusted, which ensures the reactive power controllability of the current source converter and the AC grid. The current source converter can also adjust the firing angle according to the voltage output by the controllable voltage source, which can ensure the independent adjustment of active power. Thus, the coordinated decoupling control of active and reactive power is realized, which has both large-capacity active power transmission and strong reactive power support capabilities.

[0032] 6. No separate reactive power compensation device is required for the current source converter, nor is it necessary to connect a large-scale capacitor between the converter bridge circuit and the converter transformer, which can effectively reduce costs. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the embodiments of this application or the conventional technology, the drawings used in the description of the embodiments or the conventional technology will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 This is a schematic diagram of a current source converter structure in one embodiment of this application;

[0035] Figure 2 This is a schematic diagram of the current waveforms at different locations of a current source converter in one embodiment of this application;

[0036] Figure 3 This is a schematic diagram of the harmonic characteristics of the output current of a current source converter in one embodiment of this application;

[0037] Figure 4 This is a schematic diagram of the equivalent circuit of a current source converter connected to a single-phase AC load in one embodiment of this application;

[0038] Figure 5 This is a schematic diagram of the current source converter structure in another embodiment of this application;

[0039] Figure 6 This is a schematic diagram of the converter bridge arm structure in one embodiment of this application;

[0040] Figure 7 This is a schematic diagram of the converter bridge arm structure in another embodiment of this application;

[0041] Figure 8 This is a schematic diagram of the converter bridge arm structure in another embodiment of this application;

[0042] Figure 9 This is a schematic diagram of the converter bridge arm structure in another embodiment of this application;

[0043] Figure 10 This is a schematic diagram of the converter bridge arm structure in another embodiment of this application;

[0044] Figure 11 This is a schematic diagram of the controllable current source structure of the MMC structure in one embodiment of this application;

[0045] Figure 12 This is a schematic diagram of a controllable current source structure with an angled chain structure in one embodiment of this application;

[0046] Figure 13 This is a schematic diagram of a star-shaped chain structure controllable current source in one embodiment of this application;

[0047] Figure 14 This is a schematic diagram of the IGBT full-bridge submodule structure in one embodiment of this application;

[0048] Figure 15 This is a schematic diagram of the IGBT half-bridge submodule structure in one embodiment of this application;

[0049] Figure 16 This is a schematic diagram of the IGCT full-bridge submodule structure in one embodiment of this application;

[0050] Figure 17 This is a schematic diagram of the IGCT half-bridge submodule structure in one embodiment of this application;

[0051] Figure 18 This is a schematic diagram of forward commutation in one embodiment of this application;

[0052] Figure 19 This is a schematic diagram of the grid control logic of a current source converter in one embodiment of this application;

[0053] Figure 20 This is a schematic diagram of a DC power transmission system structure in one embodiment of this application;

[0054] Figure 21 This is a schematic diagram of the coordinated decoupling control logic of a DC transmission system in one embodiment of this application;

[0055] Figure 22 This is a schematic diagram of the power reversal control process of a DC transmission system in one embodiment of this application. Detailed Implementation

[0056] To facilitate understanding of this application, a more complete description will be provided below with reference to the accompanying drawings. Preferred embodiments of this application are shown in the drawings. However, this application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of this application.

[0057] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0058] It is understood that the terms "first," "second," etc., used herein may be used to describe various elements, but these elements are not limited by these terms. These terms are only used to distinguish one element from another. For example, without departing from the scope of this application, a first resistor may be referred to as a second resistor, and similarly, a second resistor may be referred to as a first resistor. Both the first resistor and the second resistor are resistors, but they are not the same resistor.

[0059] It is understood that the term "connection" in the following embodiments should be understood as "electrical connection," "communication connection," etc., if the connected circuits, modules, units, etc., have electrical signal or data transmission with each other.

[0060] It is understandable that "at least one" refers to one or more, and "multiple" refers to two or more. "At least a part of an element" refers to part or all of an element.

[0061] When used herein, the singular forms of “a,” “an,” and “the” may also include the plural forms unless the context clearly indicates otherwise. It should also be understood that the terms “comprising / including” or “having,” etc., specify the presence of the stated features, wholes, steps, operations, components, parts, or combinations thereof, but do not preclude the possibility of the presence or addition of one or more other features, wholes, steps, operations, components, parts, or combinations thereof. Meanwhile, the term “and / or” as used in this specification includes any and all combinations of the associated listed items.

[0062] Currently, with the development of high-voltage direct current transmission technology, the penetration rate of new energy sources in the sending and receiving end power grids is gradually increasing, and the requirements for the strength of the sending and receiving end power grids are gradually increasing. Conventional grid phase-switching converters can no longer meet the needs of high-voltage, long-distance direct current transmission.

[0063] Research has revealed that controllable current source converters, through parallel connection of high-voltage AC centralized capacitors, provide commutation voltage and reactive power to the AC grid. Their reactive power support capability depends on the capacitor value. Larger capacitance values ​​result in stronger support for the AC grid, but also increase the number of capacitor banks, leading to a larger size, increased fault risk points, and consequently higher initial construction costs. Furthermore, centralized capacitors require ground isolation to prevent the introduction of DC bias magnetization into the converter transformer, increasing the difficulty of insulation coordination.

[0064] To mitigate the aforementioned issues, a distributed design of the centralized capacitors can be considered, along with the addition of an external voltage source to provide the commutation voltage, ensuring that the leakage reactance of the converter transformer does not participate in commutation. This approach not only alleviates the drawbacks of centralized capacitors but also eliminates the impact of the converter transformer leakage reactance on commutation, enabling the current source converter to fully withstand commutation failures.

[0065] Based on the above considerations, this application provides a current source converter, including a converter transformer, a converter bridge circuit, a controllable voltage source, and a filter reactor. The phase AC terminal of the converter bridge circuit is connected to the converter transformer through the filter reactor, and a controllable voltage source is connected between the phase AC terminal and the filter reactor.

[0066] The above scheme, which distributes the capacitors in the controllable voltage source, can effectively reduce the difficulty of insulation and manufacturing. This current source converter not only has the ability to completely resist commutation failure, but also can give full play to the mutual coordination between the commutation bridge circuit and the controllable voltage source to achieve independent decoupling control of active power and independent decoupling control of reactive power.

[0067] Please see Figure 1This application provides a current source converter based on a controllable voltage source, comprising: a converter transformer 30, a converter bridge circuit 10, a controllable voltage source 20, and a filter reactor L. The DC terminal of the converter bridge circuit 10 is used to connect a DC load (not shown). Each phase AC terminal of the converter bridge circuit 10 is connected to the controllable voltage source 20. The first terminal of each filter reactor L is connected to the AC terminal of one phase and the controllable voltage source 20, and the second terminal of each filter reactor L is connected to the converter transformer 30, which is used to connect an AC load. The controllable voltage source 20 provides commutation voltage to the converter bridge circuit 10 and also determines the reactive power interaction characteristics between the current source converter and the AC power grid.

[0068] Specifically, the controllable voltage source 20 is positioned between the converter bridge circuit 10 and the converter transformer 30, connected in phase to phase. The voltage difference between the output voltage of the controllable voltage source 20 and the valve-side voltage of the converter transformer 30 determines the reactive power injected by the converter into the grid. The converter bridge circuit 10 is used to realize the AC / DC conversion function; the controllable voltage source 20 is a voltage source whose amplitude and phase of output voltage signal can be adjusted; the filter reactor L is a reactor configured between the converter bridge circuit 10 and the converter transformer 30 for filtering and isolation.

[0069] It should be noted that the type of converter bridge circuit 10 is not unique; it can be a six-pulse converter bridge circuit 10 or other types of converter bridge circuit 10, without specific limitations. The number of filter reactances L is also not unique; it varies depending on the type of converter bridge circuit 10, as long as each phase AC terminal of the converter bridge circuit 10 is connected to a corresponding filter reactance L. In one embodiment, taking a six-pulse converter bridge circuit 10 as an example, the current source converter should include three filter reactances L, respectively connected to the three phase AC terminals of the six-pulse converter bridge circuit 10.

[0070] It is understandable that the type of controllable voltage source 20 is not unique, and it will vary depending on the type of converter bridge circuit 10. Taking conventional three-phase power transmission as an example, the controllable voltage source 20 should be a three-phase controllable voltage source 20, with each phase output terminal connected to one phase AC terminal of the converter bridge circuit 10. The three-phase controllable voltage source 20 can be connected in different ways and set in parallel at the three-phase AC terminals of the converter bridge circuit 10.

[0071] In this embodiment, the filter reactance L can suppress the output current harmonics of the commutator bridge circuit 10, and also prevent DC discontinuity. Please refer to the following: Figure 2In the figure, Ica is the output current of the controllable voltage source 20, Ipa is the output current of the converter bridge circuit 10, and Iga is the grid-connected current. As can be seen from the figure, in the current source converter provided in this application, the output current Ica of the controllable voltage source 20 compensates for the output current Ipa of the converter bridge circuit 10, making the grid-connected current Iga a sinusoidal current. Through experimental analysis, as shown... Figure 3 As shown, the harmonic content of the final output current of the current source converter is about 0.3%, which shows that the current source converter has a good current harmonic suppression effect.

[0072] Furthermore, by designing the filter reactance L, the power exchange characteristics between the current source converter and the AC load can be refined, allowing the configuration capacity of the controllable voltage source 20 to be customized according to the AC load requirements.

[0073] For details, please refer to the relevant documents. Figure 4 In the single-phase equivalent circuit of a current source converter connected to an AC load, the filter reactance... 30 leakage reactance of converter transformer The series equivalent reactance is , The output voltage of the controllable voltage source 20 DC voltage For grid connection current, Apparent power, This is the mains voltage (i.e., the voltage of the AC load). Wherein, , This shows that a smaller equivalent impedance will limit the voltage regulation range of the controllable voltage source 20. In practical scenarios, the controllable voltage source 20 includes multiple sub-modules, each with the same voltage (e.g., 2.4 kV). The voltages of the connected sub-modules are superimposed to form the voltage regulation range. This means the resolution of voltage regulation ( / 2.4), will decrease as the maximum output voltage of the controllable voltage source 20 decreases.

[0074] Assumption Infinitesimals, then only and If they are not equal, then the grid-connected current will be infinite, and the reactive power injected into the grid (i.e., the AC load) will be infinite, causing the controllable voltage source 20 to lose its voltage regulation capability. Assuming... The reactive power is relatively small, but there exists a situation where it just barely meets the rated reactive power requirement. and If the voltage difference between the modules is less than the voltage of a submodule, the voltage regulation resolution of the controllable voltage source 20 is still insufficient, and it still does not have power regulation function.

[0075] In other words, only in appropriate To ensure the controllable voltage source 20 has a relatively fine power regulation capability, parameter design needs to be combined with the reactive power requirements of the system. The leakage reactance of the converter transformer is usually small; if... Including only the 30 leakage reactance of the converter transformer is usually insufficient to meet power regulation requirements.

[0076] The current source converter provided in this application operates in phase-controlled mode under steady state, requiring no active shutdown of the controllable switching devices in the converter bridge circuit 10. Commutation is achieved solely by the voltage provided by the controllable voltage source 20. In other words, the controllable degree of freedom of this current source converter under steady state is only the firing angle, resulting in a controllable degree of freedom of 1. Power decoupling is achieved by adjusting the amplitude and phase of the output voltage of the controllable voltage source 20, both of which are controllable. By adjusting the amplitude of the output voltage of the controllable voltage source 20, reactive power regulation between the current source converter and the AC load is realized. Meanwhile, the converter bridge circuit 10, by coordinating the firing angle, ensures that the DC voltage is a controllable variable under different output voltage amplitudes of the controllable voltage source 20, thereby achieving independent active power regulation between the current source converter and the AC load.

[0077] Specifically, taking two current source converters as the receiving and sending ends respectively as an example, the reactive power interaction between the sending and receiving end current source converters and the power grid is achieved by controlling the output voltage amplitude of their respective controllable voltage sources. The active power interaction between the sending and receiving end current source converters is coordinated by controlling the firing angle. Specifically, the sending end current source converter performs constant DC current control by adjusting the firing angle, and the receiving end current source converter performs constant DC voltage control by adjusting the firing angle. This coordination achieves active power control of the two end current source converters.

[0078] The output voltage amplitude of the controllable voltage source section can be independently adjusted, ensuring the reactive power controllability of the current source converter and the power grid. The current source converter, in turn, can adjust its firing angle according to the voltage output from the controllable voltage source, ensuring independent adjustment of active power under constant DC current control at the sending end and constant DC voltage control at the receiving end. Thus, coordinated decoupling control of active and reactive power between AC and DC loads is achieved.

[0079] The aforementioned current source converter includes a converter transformer 30, a converter bridge circuit 10, a controllable voltage source 20, and a filter reactor L. The phase AC terminal of the converter bridge circuit 10 is connected to the converter transformer 30 through the filter reactor L, and the controllable voltage source 20 is connected between the phase AC terminal and the filter reactor L. Thus, the filter reactor L isolates the converter bridge circuit 10 from the AC load and filters out harmonics in the output current of the converter bridge circuit 10, ensuring the harmonic characteristics of the grid-connected current. Furthermore, the filter reactor L provides a sufficiently large equivalent impedance on the AC side, enabling the controllable voltage source 20 to have more precise power regulation capabilities. The controllable voltage source 20 provides the commutation voltage required for the converter bridge circuit 10, and the leakage reactance of the converter transformer 30 does not participate in commutation, completely eliminating the influence of the converter transformer 30 leakage reactance on the commutation process and significantly reducing reactive power consumption. The above scheme not only has the ability to completely resist commutation failure, but also fully utilizes the mutual coordination between the converter bridge circuit 10 and the controllable voltage source 20 to achieve independent decoupling control of active power and independent decoupling control of reactive power.

[0080] Please see Figure 5 In one embodiment, the converter bridge circuit 10 includes a DC reactor Ldc and converter bridge arms. The converter bridge arms include upper bridge arms 11 and lower bridge arms 12. The first ends of each upper bridge arm 11 are respectively connected to form a common terminal as a first DC terminal. The first ends of each lower bridge arm 12 are respectively connected to form a common terminal as a second DC terminal. The second end of one upper bridge arm 11 is correspondingly connected to the second end of one lower bridge arm 12, and the common terminal formed is a phase AC terminal. The first DC terminal is connected to the DC load through the DC reactor Ldc, and the second DC terminal is connected to the DC load.

[0081] Specifically, the DC-side reactor is a reactor located on the DC side of the current source converter, used to connect to the DC load on the DC side. The type and number of the upper bridge arm 11 and the lower bridge arm 12 are the same, but the specific number will vary depending on the type of the converter bridge circuit 10. In one embodiment, the converter bridge circuit 10 is a six-pulse converter bridge circuit 10, which includes six bridge arms, including three upper bridge arms 11 and three lower bridge arms 12.

[0082] It is understood that, in another embodiment, in order to realize the operation control of the controllable voltage source 20, the converter bridge circuit 10, etc., the current source converter should also include a valve controller. The control terminals of the controllable switching devices of each bridge arm in the converter bridge circuit 10 and the control terminal of the controllable voltage source 20 are respectively connected to the valve controller. The valve controller can communicate with different acquisition devices in the power grid where the current source converter is located to obtain the power grid operating parameters and control the converter bridge circuit 10 and the controllable voltage source 20 in combination with the power grid operating parameters. The specific details are not elaborated here.

[0083] In the above scheme, the converter bridge circuit 10 includes a DC reactor Ldc and a converter bridge arm. The DC reactor Ldc is located between the converter bridge arm and the DC load, which can improve the operational safety of the current source converter.

[0084] It should be noted that the type of each arm in the converter bridge is not unique. In one embodiment, the upper arm 11 and the lower arm 12 can be unidirectional current-carrying arms, that is, the upper arm 11 and the lower arm 12 can be a single current-carrying branch formed by a series connection of controllable switching devices. In this embodiment, due to the single-phase conduction characteristics of the switching devices in the valve arm, the active power flow reversal (i.e., power reversal or power reverse transmission) of the current source converter is achieved by changing the polarity of the DC voltage.

[0085] To improve the efficiency of active power flow reversal in current source converters and avoid the dependence of active power flow reversal on DC voltage polarity, in one embodiment, please refer to... Figure 6 The converter bridge arm includes a first current-carrying branch 61 and a second current-carrying branch 62 connected in reverse parallel.

[0086] Specifically, when the current source converter transmits power in the first direction, the first current-carrying branch 61 is turned on and the second current-carrying branch 62 is turned off; when there is a power reversal requirement, the DC voltage at the sending end is reduced to reduce the DC current of the converter bridge circuit 10; when the DC current drops to less than or equal to a preset current threshold, the first current-carrying branch 61 is turned off and the second current-carrying branch 62 is turned on to enable the current source converter to transmit power in the second direction.

[0087] Specifically, reverse parallel connection refers to the first current-carrying branch 61 and the second current-carrying branch 62 being connected in parallel, with their current flows in opposite directions. The first direction and the second direction refer to two opposite power transmission directions, which may vary depending on the specific scenario. In this embodiment, the current source converter includes a sending-end current source converter and a receiving-end current source converter. The first direction can be power transmission from the sending-end current source converter to the receiving-end current source converter, and the second direction is power transmission from the receiving-end current source converter to the sending-end current source converter. Correspondingly, the first current-carrying branch 61 is a forward current-carrying branch, and the second current-carrying branch 62 is a reverse current-carrying branch. If the first direction is power transmission from the receiving-end current source converter to the sending-end current source converter, then the second direction is power transmission from the sending-end current source converter to the receiving-end current source converter. Accordingly, the first current-carrying branch 61 is a reverse current-carrying branch, and the second current-carrying branch 62 is a forward current-carrying branch.

[0088] Unlike the unidirectional current-carrying bridge arm in the above embodiments, this embodiment uses a bidirectional current-carrying bridge arm to build the converter bridge circuit 10. In the unidirectional current-carrying bridge arm structure, power reversal relies solely on reversing the polarity of the DC voltage. This requires first reducing the voltage of the sending and receiving end current source converters to 0 and then restarting them, changing the voltage polarity of the sending and receiving end current source converters. This entire process is slow, typically taking several seconds or even longer. However, in the bidirectional current-carrying bridge arm structure, there is no need to change the voltage polarity; the current can simply be switched to another current-carrying branch for transmission.

[0089] Specifically, when power is transmitted in the forward direction (first direction), the DC voltage at the sending end of the current source converter is higher than the DC voltage at the receiving end of the current source converter. Under the influence of line impedance, this creates conditions for power feeding from the sending end to the receiving end. When power reversal is required, the firing angle of the sending end current source converter needs to be increased, causing the DC voltage at the sending end to decrease, resulting in a gradual decrease in DC current. At this time, the first current-carrying branch 61 is activated and the second current-carrying branch 62 is deactivated. When the current drops to near zero (i.e., less than the preset current threshold), it means that the DC voltages at the sending and receiving ends are essentially equal. At this time, the first current-carrying branch 61 is deactivated and the second current-carrying branch 62 is activated. Subsequently, the firing angle of the receiving end current source converter is increased, raising its DC voltage, thus creating conditions for power feeding from the receiving end current source converter to the sending end current source converter, achieving power reversal (i.e., second direction transmission). This process eliminates the need to separately reduce the voltage of the sending and receiving end current source converters to zero and then restart them, significantly reducing the time required for active power flow reversal.

[0090] The above scheme enables bidirectional current flow through the converter bridge arm, allowing for rapid reversal of active power flow while maintaining DC voltage polarity, thus providing a solution for the power exchange flexibility between the sending and receiving grids.

[0091] For ease of understanding, when power is transmitted in the forward direction, the sending end (current source) converter is defined as the rectifier side and the receiving end (current source) converter is defined as the inverter side. When the power is reversed, the new sending end (current source) converter is defined as the original inverter side and the new receiving end (current source) converter is defined as the original rectifier side.

[0092] When power is transmitted in the forward direction, the DC voltage on the rectifier side is higher than the DC voltage on the inverter side, creating a voltage difference on the line and thus forming a forward transmission current. At this time, the firing angle on the rectifier side is approximately 17°, and the firing angle on the inverter side is 150°. Because current in a traditional LCC converter can only flow in one direction, reversing the power flow requires changing the voltage polarity of both converter sides. This involves first reducing the voltage on both sides, then restarting the converter, increasing the firing angle on the rectifier side to 150°, and decreasing the firing angle on the inverter side to 17°. This change in voltage polarity achieves the reversal process.

[0093] In this embodiment, the current is reversed by shutting off the first current-carrying branch and opening the second current-carrying branch. At this point, only a slight adjustment of the firing angles on the original rectifier and inverter sides is needed to make the DC voltage on the inverter side slightly higher than that on the rectifier side. The voltage polarity remains unchanged. This is equivalent to increasing the firing angle on the original rectifier side to 30° and the firing angle on the original inverter side to 163°. The angle change of only 13° is considered a fine adjustment, resulting in a fast response time and better facilitating power flow coordination in large power grids.

[0094] It should be noted that, in one embodiment, the bidirectional current-carrying bridge arm can achieve active power flow reversal, but when the current source converter is operating normally, the bridge arm switching device needs to withstand bidirectional forward current (i.e., current is transmitted from the sending end to the receiving end). Therefore, the first current-carrying branch 61 and the second current-carrying branch 62 can both be configured to be built with controllable switching devices with bidirectional pressure bearing capacity, thereby further improving the operational reliability of the converter bridge circuit 10.

[0095] In one embodiment, the first current-carrying branch 61 includes any one of the following: a first branch formed by multiple reverse-resistance IGCT devices connected in series, a second branch formed by multiple IGBT devices connected in series, and a third branch formed by at least one asymmetric IGCT device and at least one IGBT device connected in series.

[0096] Specifically, an IGCT is a power integrated device that integrates the gate drive circuit and the thyristor structure; an IGBT (Insulated Gate Bipolar Transistor) is a composite fully controllable voltage-driven power semiconductor device. A reverse-resistance IGCT is an integrated gate-commutated thyristor with reverse blocking capability. The anode of a reverse-resistance IGCT is connected to the cathode of an adjacent reverse-resistance IGCT, forming the first branch. The collector of an IGBT is connected to the emitter of an adjacent IGBT, forming the second branch. Similarly, asymmetric IGCT and IGBT devices are connected in series to form the third branch. In practical applications, any one of the first, second, or third branches can be selected as the first current-carrying branch 61, depending on the specific requirements.

[0097] In one embodiment, the second current-carrying branch 62 includes any one of the following: a first branch formed by multiple reverse-resistance IGCT devices, a second branch formed by multiple IGBT devices connected in series, a fourth branch formed by multiple diodes connected in series, and a fifth branch formed by multiple thyristors connected in series.

[0098] Specifically, similar to the first current-carrying branch 61 in the above embodiment, in this embodiment, the second current-carrying branch 62 is a reverse current-carrying branch. Its structure can be the same as or different from the first current-carrying branch 61, as long as they are connected in reverse parallel. No specific limitation is made. Through this scheme, different combinations of the first current-carrying branch 61 and the second current-carrying branch 62 can be selected according to actual needs to ultimately obtain the converter bridge arm, which can effectively improve the applicability and scenarios of the current source converter.

[0099] In one embodiment, the converter arm further includes buffer branches connected in parallel with the first flow branch 61 and the second flow branch 62, respectively.

[0100] Specifically, the buffer branch is a circuit that suppresses the turn-off voltage of the switching devices in the first current-carrying branch 61 and the second current-carrying branch 62, thereby achieving the effect of series voltage equalization of the switching devices. In practical scenarios, the buffer branch includes multiple buffer components. The number of switching devices connected in series in the first current-carrying branch 61 and the second current-carrying branch 62 is the same. For any current-carrying branch, a buffer component is connected in parallel across each switching device. Therefore, the parallel connection of the buffer branch and the current-carrying branch as defined in this application should be understood as the buffer components of the buffer branch being connected in parallel with the switching devices of the current-carrying branch, respectively.

[0101] It is understood that the type of buffer component is not unique. Any circuit or device with voltage equalization function can be used. For example, in one embodiment, the buffer component can be one or more of the following: a static voltage equalization resistor, an RCD (resistor, capacitor and diode) dynamic voltage equalization circuit, an RC (resistor and capacitor in series) circuit, or a circuit formed by RC and MOV (Metal Oxide Varistor) in parallel. There is no specific limitation. The selection can be made according to the actual needs.

[0102] The above scheme, by connecting a buffer branch in parallel in the converter bridge arm, can achieve series voltage equalization of the switching devices in the converter bridge arm, thereby improving the operational safety and stability of the converter bridge arm.

[0103] It should be noted that the number of buffer branches in the same converter bridge arm is not unique. In one embodiment, to reduce the size of the current source converter and save hardware costs, the first current-carrying branch 61 and the second current-carrying branch 62 can share a single buffer branch, that is, they share one buffer branch. In this embodiment, considering that the current directions of the first current-carrying branch 61 and the second current-carrying branch 62 are different, in order to achieve buffering function in both directions, the buffering components of the buffer branch should use circuits or devices without polarity restrictions. For example, refer to [reference needed]. Figure 7The buffer component can be configured as a circuit formed by RC and MOV in parallel. In the figure, the second current-carrying branch 62 is built with diodes. In other embodiments, static voltage equalizing resistors, RC circuits, etc. can also be used, and there is no specific limitation.

[0104] In another embodiment, buffer branches may be configured for the first flow branch 61 and the second flow branch 62 respectively. That is, the buffer branches include the first buffer branch and the second buffer branch. The first buffer branch is connected in parallel with the first flow branch 61, and the second buffer branch is connected in parallel with the second flow branch 62.

[0105] Specifically, the first buffer branch is used to equalize the voltage of the first current-carrying branch 61 when it is turned on, and the second buffer branch is used to equalize the voltage of the second current-carrying branch 62 when it is turned on. The buffer components of the first buffer branch and the buffer components of the second buffer branch can have the same structure, for example... Figure 8 As shown, both can adopt an RCD dynamic voltage equalization circuit; in another embodiment, the buffer component of the first buffer branch and the buffer component of the second buffer branch can also be one or more of the following: a static voltage equalization resistor, an RC circuit, or a circuit formed by RC and MOV in parallel.

[0106] The structures of the first buffer branch and the second buffer branch can be different, for example... Figure 9 As shown, the first buffer branch is constructed using an RCD dynamic voltage equalization circuit, and the second buffer branch is constructed using an RC circuit. In another embodiment, the first and second buffer branches can also be other combinations, without limitation.

[0107] It is understandable that, in practical scenarios, if the first and second buffer branches use circuits without polarity requirements, such as static equalizing resistors, RC circuits, or circuits formed by parallel RC and MOV circuits, they can simply be connected in parallel with the first current-carrying branch 61 and the second current-carrying branch 62, respectively. If the first or second buffer branch uses circuits with polarity requirements, such as RCD dynamic equalizing circuits, the first buffer branch must be connected in parallel with the first current-carrying branch 61 in the same direction, and the second buffer branch must be connected in parallel with the second current-carrying branch 62 in the same direction; that is, the first and second buffer branches must be connected in parallel in opposite directions.

[0108] Please see Figure 10 In one embodiment, the converter arm further includes energy-consuming branches connected in parallel with the first current-carrying branch 61 and the second current-carrying branch 62, respectively.

[0109] Specifically, in this embodiment, the scheme is illustrated by using a first current-carrying branch 61 constructed with a series-connected reverse-resistance IGCT, a second current-carrying branch 62 constructed with a series-connected thyristor, a first buffer branch as an RCD dynamic voltage equalization circuit, and a second buffer branch as an RC circuit. Energy-dissipating branches are also connected in parallel at both ends of the converter bridge arm. The energy-dissipating branches serve two purposes: limiting overvoltage of the devices and equalizing voltage across multiple devices in series. It should be noted that the type of energy-dissipating branch is not unique; any circuit capable of achieving the above two functions is acceptable.

[0110] For example, in one embodiment, the power dissipation branch includes an MOV. The number of MOVs is not unique. It can be a single MOV in which all switching devices of the entire current-carrying branch share one MOV, or multiple switching devices of a current-carrying branch can share one MOV, or each switching device of the current-carrying branch can be configured with one MOV. The specific configuration is not limited.

[0111] Please refer to the following: Figure 5 In one embodiment, the commutator bridge circuit 10 further includes a saturated reactance, the second end of the upper bridge arm 11 is connected to the second end of the lower bridge arm 12 through the saturated reactance, and / or the second end of the lower bridge arm 12 is connected to the second end of the upper bridge arm 11 through the saturated reactance.

[0112] Specifically, the converter arm also includes saturated reactance, which can alleviate the current intolerance of the controllable switching devices in the converter arm and improve the working reliability and operational safety of the converter arm.

[0113] It is understood that the saturated reactor can be centrally located in this embodiment, placed at both ends of the converter bridge arm and directly connected in series with the converter bridge arm. In another embodiment, the saturated reactor can be distributed among the converter bridge arms. Specifically, multiple controllable switches in the converter bridge arm can be grouped together and connected in series with a saturated reactor to form a module. Then, multiple similar modules can be connected in series to form the converter bridge arm.

[0114] In one embodiment, the controllable voltage source 20 includes multiple interconnected sub-modules, and the sub-modules are connected in any one of the following ways: MMC connection, delta chain connection, and star chain connection.

[0115] Specifically, as shown in the above embodiment, the way the controllable voltage source 20 is connected to the converter bridge circuit 10 is not unique. Under different connection methods, the connection methods of each sub-module (i.e., the SM shown in the figure) in the controllable voltage source 20 will also be different. MMC connection method as follows: Figure 11 As shown, it includes six bridge arms, each of which includes multiple cascaded sub-modules. These bridge arms are connected in pairs, and the three common terminals formed serve as three-phase AC ports, as illustrated in the diagram. , and These are connected to the three-phase AC terminals of the converter bridge circuit 10, respectively. The delta-chain connection is as follows: Figure 12 As shown, the star-chain connection method is as follows: Figure 13 As shown, three-phase AC ports are formed through different connection methods of the submodules. , and ), and connected to the three-phase AC terminals of the converter bridge circuit 10 respectively.

[0116] It should be noted that the specific type of submodule is not unique; it can be a full-bridge submodule, a half-bridge submodule, etc., without any specific limitation. Furthermore, depending on the type of controllable switching device used in the submodule, it can also be divided into IGBT submodules or IGCT submodules.

[0117] The IGBT submodule refers to a full-bridge or half-bridge submodule that uses IGBTs as controllable switching devices. For details, please refer to [reference needed]. Figure 14 and Figure 15 The IGCT submodule can be referenced in conjunction with other relevant documents. Figure 16 and Figure 17 The specific type of submodule to use can be selected based on the actual scenario.

[0118] In this embodiment of the application, the converter bridge arm uses the voltage provided by the controllable voltage source 20 for commutation. The leakage reactance of the converter transformer 30 does not participate in commutation, and no external circuit is required to provide commutation voltage. Therefore, the commutation process is fast. For details, please refer to the relevant documentation. Figure 18 The following explanation will be based on the example of commutation performed by bridge arms S1 and S3, which respectively include the first flow branch 61, the first buffer branch and the energy dissipation branch.

[0119] During normal operation, current flows through the IGCT device in the first current-carrying branch 61 of S1, carrying the rated current and overload current. During commutation, the IGCT device in the first current-carrying branch 61 of S1 actively turns off according to the issued drive signal, establishing a commutation voltage. The current in the first current-carrying branch 61 of S1 commutates to the first buffer branch. At this time, the diode in the first buffer branch bypasses the resistor to improve the voltage equalization effect of the capacitor. When the voltage of the first buffer branch is charged to the operating voltage of the energy dissipation branch (MOV), the voltage of the series module is clamped, and voltage equalization between units is achieved by limiting the voltage. The IGCT device in the first current-carrying branch 61 of S3 is triggered to turn on when subjected to positive voltage, and the DC current commutates to S3. The current in S1 drops to 0, completing the turn-off commutation process.

[0120] Please refer to the following: Figure 19In one embodiment, the controllable voltage source 20 can establish an AC voltage based on the angular frequency reference value obtained by the active power droop controller and the AC voltage amplitude reference value obtained by the reactive power control, thereby realizing the grid-connected control of the entire converter.

[0121] Specifically, the core of the grid control strategy lies in the ability to generate the phase for producing the modulation wave without relying on a phase-locked loop based on grid voltage. Based on the droop characteristics of synchronous generators (Formula 1), the droop control expressions for active and reactive power of grid-type converters can be obtained (Formula 2). According to the droop control principle, the active power P at the AC bus is collected... ac The value of is calculated, and the difference between it and the nominal value P0 is taken to obtain ΔP. ΔP is then compared with the droop coefficient K. p Multiply and then add to the initial angular frequency By adding them together, a reference value of the angular frequency that changes in real time according to the load can be obtained. . After integration, the reference phase for generating the trigger signal of the IGCT valve (the converter valve composed of the aforementioned converter bridge arms) can be obtained. t, at the same time, It will also be used to generate the trigger signal for the controllable voltage source 20.

[0122] (Formula 1)

[0123] (Formula 2)

[0124] The grid control strategy for a current source converter based on a controllable voltage source 20 requires coordinating three control degrees of freedom, namely the firing angle of the IGCT valve. And the dq-axis voltage component u of the controllable voltage source 20 cdi with u cqi Among them, the trigger angle degree of freedom Used to coordinate the voltage balance of the full-bridge submodules in the controllable voltage source 20, the measured value U of the DC voltage of the bridge arm of the controllable voltage source 20 is used. dsvg With instruction value The difference between the two values ​​can be used to adjust the PI control to obtain the leading trigger angle β. However, the trigger signal cannot be directly obtained from β; it requires the trigger angle to be used. The phase output of the phase-locked loop (PLL) generated by the receiving-end grid voltage. t is obtained through comparison. Combined with the formula... =π-β, which can be indirectly obtained from β. ,when t> When the trigger signal S=1, it is always true; otherwise, it is true. The trigger signals of the six bridge arms of the IGCT valve are delayed by 60° sequentially, thus obtaining the trigger signal S of each bridge arm. 1-6 .

[0125] Analysis of the reactive power droop characteristics in Formulas 1 and 2 shows that, in the reactive power droop control loop, the measured reactive power value Q of the AC power grid is collected. ac The difference between this value and the reference value Q0 is ΔQ. ΔQ is then compared with the droop coefficient K. q After multiplying, add the result to the rated AC voltage amplitude U0 to obtain the AC voltage amplitude reference value. ,Will With AC voltage amplitude measurement value U ac The difference can be obtained by passing through a closed-loop decoupling controller. cqi Among them, u cdi This is used to ensure that the controllable voltage source 20 does not exchange active power with the power grid, by measuring the active power value P. svg Compared with reference value P svg0 By performing the difference operation and passing it through a closed-loop decoupling controller, u can be obtained. cdi Then u cdi with u cqi After coordinate transformation, the three-phase modulated wave u can be obtained. ca u cb u cc By applying the closest-level approximation modulation to the three-phase modulated wave, the trigger signals of each device in the full-bridge submodule of the three-phase bridge arm of the controllable voltage source 20 can be obtained, namely S. a1-4 S b1-4 S c1-4 .

[0126] Through the above-mentioned grid control strategy, the current source converter can supply power to weak grids and even isolated scenarios, and can also be used in scenarios with a high proportion of new energy access, with strong applicability and technical and economic efficiency.

[0127] Please see Figure 20 This application also provides a DC transmission system, including two current source converters as described above, one of which serves as the sending-end current source converter and the other as the receiving-end current source converter, with the DC terminal of the sending-end current source converter connected to the DC terminal of the receiving-end current source converter.

[0128] Specifically, the structure of the current source converter is as shown in the above embodiments and accompanying drawings, and will not be repeated here. Through the above-described current source converter, the influence of the 30 leakage reactance of the converter transformer on the commutation process can be completely eliminated, reactive power consumption can be significantly reduced, it has the ability to completely resist commutation failure, and it also has active and reactive voltage support capabilities. It is suitable for both supplying power to the receiving end weak AC grid and for sending-end renewable energy transmission scenarios.

[0129] In one embodiment, the sending-end current source converter adjusts the output voltage amplitude of the controllable voltage source at the sending end based on the reactive power measurement value of the sending-end grid and the DC voltage measurement value of the bridge arm of the controllable voltage source at the sending end, so as to achieve reactive power interaction with the AC grid; the receiving-end current source converter adjusts the output voltage amplitude of the controllable voltage source at the receiving end based on the reactive power measurement value of the receiving-end grid and the DC voltage measurement value of the bridge arm of the controllable voltage source at the receiving end, so as to achieve reactive power interaction with the AC grid; the sending-end current source converter performs constant DC current control by adjusting the firing angle, and the receiving-end current source converter performs constant DC voltage control by adjusting the firing angle, so as to achieve active power interaction between the sending-end current source converter and the receiving-end current source converter.

[0130] The reactive power interaction between the sending and receiving end current source converters and the power grid is achieved by controlling the output voltage amplitude of their respective controllable voltage sources 20. The active power interaction between the sending and receiving end current source converters is coordinated by controlling the firing angle. Specifically, the sending end current source converter performs constant DC current control by adjusting the firing angle, and the receiving end current source converter performs constant DC voltage control by adjusting the firing angle. This coordination achieves active power control of the current source converters at both ends.

[0131] The voltage amplitude output by the controllable voltage source 20 can be independently adjusted, ensuring the reactive power controllability of the current source converter and the power grid. The current source converter, in turn, can adjust its firing angle according to the voltage output by the controllable voltage source 20, ensuring independent adjustment of active power under constant DC current control at the sending end and constant DC voltage control at the receiving end. Thus, coordinated decoupling control of active and reactive power between AC and DC loads is achieved.

[0132] Specifically, please refer to the following: Figure 21 The coordinated control of the current source converter based on the controllable voltage source 20 can be divided into two aspects: decoupling control between the single-ended current source and the controllable voltage source 20, and coordinated control of the active power at the sending and receiving ends.

[0133] The decoupling control layer between the single-ended current source and the controllable voltage source (20) uses the same control method at both the sending and receiving ends. The coordination method between the receiving-end voltage source and the current source is as follows, assuming the sending-end DC current I... dc If the current source converter at the receiving end is constant, then the active power control between the converter and the grid depends on the DC voltage U. dcControlled by measuring the DC voltage U at the receiving end. dc Compared with reference value The difference can be used to obtain the leading trigger angle β by introducing a PI circuit. Since the trigger signal cannot be directly obtained from β, but requires the trigger angle... The phase output of the phase-locked loop (PLL) generated by the receiving-end grid voltage. t is obtained by comparison, combined with the formula =π-β, which can be indirectly obtained from β. ,when t> When the trigger signal is active, the trigger signal S=1; otherwise, S=0. The trigger signals of the six bridge arms of the IGCT valve are sequentially delayed by 60° to obtain the trigger signal S of each bridge arm. 1-6 .

[0134] The reactive power of the converter is controlled by adjusting the voltage amplitude output of the controllable voltage source 20, which is achieved by measuring the reactive power Q of the receiving-end grid. aci Reactive power reference value The difference can be used to obtain the q-axis component of the voltage modulation wave by introducing a closed-loop decoupling controller. Then, it is combined with the d-axis component of the modulation wave generated by the voltage control of the submodule. They then perform a coordinate transformation together. This yields the three-phase voltage modulation wave of the controllable voltage source 20. By performing near-level approximation modulation on the above modulation wave, the trigger signals of each device in the full-bridge submodule of the three-phase bridge arm of the controllable voltage source 20 can be obtained, namely S. a1-4 S b1-4 S c1-4 Among them, the d-axis component of the modulated wave output by the submodule voltage control. This is achieved by measuring the DC voltage U of the bridge arm of the controllable voltage source 20. dsvg With instruction value The difference is obtained by PI regulation and then through a closed-loop decoupling controller. Thus, the active power of the converter is determined by the firing angle. Controlled, reactive power is controlled by Control is implemented, and as the reactive power is adjusted, the output voltage amplitude of the controllable voltage source 20 will be adjusted accordingly. At this time, in order to ensure constant DC power, the firing angle will be further coordinated. This achieves decoupled control of active and reactive power in the receiving-end current source converter.

[0135] The coordination method between the sending-end voltage source and the current source is as follows, assuming the receiving-end DC current U dc If the current source converter at the sending end is constant, then the active power transmitted by the converter depends on the DC current I. dc Controlled by measuring the DC current at the sending end, Idc Compared with reference value The difference between the two can be used to obtain the leading trigger angle β by introducing a PI circuit. Since the trigger signal cannot be directly obtained from β, but requires the trigger angle α... r The phase output of the phase-locked loop (PLL) generated by the receiving-end grid voltage. The result is obtained by comparing t and combining it with the formula α. r =π-β, from which α can be indirectly obtained. r .when t>α r When the trigger signal is active, the trigger signal S=1; otherwise, S=0. The trigger signals of the six bridge arms of the IGCT valve are sequentially delayed by 60° to obtain the trigger signal S of each bridge arm. 1-6 For the sending-end current source converter, in order to ensure reliable triggering of power devices under the minimum firing angle condition, a minimum firing angle limit needs to be set, α. r The empirical value for β is 5°, therefore the upper limit of β is β. max Take 175°.

[0136] The reactive power of the converter is controlled by adjusting the voltage amplitude output from the controllable voltage source 20, which is achieved by measuring the reactive power Q of the sending-end grid. acr Reactive power reference value By taking the difference and introducing a closed-loop decoupling controller, the q-axis component of the voltage modulation wave can be obtained. Then, it is combined with the d-axis component of the modulation wave generated by the voltage control of the submodule. By performing a coordinate transformation together, the three-phase voltage modulation wave of the controllable voltage source 20 can be obtained. By performing near-level approximation modulation on the above modulation wave, the trigger signals of each device in the full-bridge submodule of the three-phase bridge arm of the controllable voltage source 20 can be obtained, namely S. a1-4 S b1-4 S c1-4 Among them, the d-axis component of the modulated wave output by the submodule voltage control. This is achieved by measuring the DC voltage U of the bridge arm of the controllable voltage source 20. dsvg With instruction value The difference is obtained by PI regulation and then through a closed-loop decoupling controller. Thus, the active power of the converter is determined by the firing angle α. r Controlled, reactive power is controlled by Control is implemented, and as the reactive power is adjusted, the output voltage amplitude of the controllable voltage source 20 will be adjusted. At this time, in order to ensure the constant DC power, the trigger angle α will be further coordinated. r This achieves decoupling control of active and reactive power in the sending-end current source converter.

[0137] In the coordinated control of active power at both the sending and receiving ends, the sending end adopts constant (DC) current control, and the receiving end adopts constant (DC) voltage control. This is achieved by controlling the IV of the converters at both ends. dc (Sending end) and U dc Independent control of the receiving end enables coordinated control of the active power of the DC system.

[0138] In one embodiment, if a short-term fault occurs in the AC power grid or DC side, the converter bridge arms of the converter bridge circuit are continuously turned off within one power frequency cycle to ensure that the AC output current is continuously controllable; if a permanent fault occurs in the receiving-end system, the upper and lower bridge arms of the same phase in the converter bridge circuit of the receiving-end current source converter are triggered to conduct, forming a bypass pair to provide a path for the transmission of power to the sending end.

[0139] Specifically, short-term faults in AC systems typically refer to temporary circuit interruptions or abnormalities in AC power grids caused by various reasons (such as line aging, poor contact, equipment defects, etc.). These faults may manifest as abnormal current, voltage fluctuations, or equipment tripping. Short-term faults on the DC side usually occur on the DC side of the converter valve and may be caused by DC power supply failures, poor line contact, or internal equipment short circuits. These faults may lead to abnormal DC voltage and current fluctuations. The power frequency cycle refers to the time it takes for AC current to complete one positive-to-negative alternation within one cycle, typically 50Hz (0.02 seconds in time terms). When short-term faults occur in the AC system or DC side of a DC transmission system, the converter bridge arm is continuously turned off within one power frequency cycle, ensuring continuous control of the AC output current at the receiving end. Thus, compared to the situation where thyristor valves cannot actively commutate, leading to commutation failure, the IGCT valve used in this application can effectively suppress the amplitude of the fault current, which is beneficial for ensuring the reliable operation of the valve.

[0140] A permanent fault in the receiving-end system refers to a fault occurring in the receiving end of a DC transmission system that cannot be automatically recovered or requires a long recovery time. These faults typically involve critical components such as DC lines and converter station equipment. In such cases, all upper and lower arms of the controllable converter valve (the converter bridge arm in the converter bridge circuit) are triggered, forming a bypass pair. The three-phase bypass pairs can be used alternately as required. This provides a path for power transfer to the sending end, preventing the risk of overvoltage due to sudden load shedding or damage to the sending-end power supply.

[0141] In one embodiment, the converter bridge arm includes a first current-carrying branch 61 and a second current-carrying branch 62 connected in reverse parallel. When there is a power reverse transmission requirement, the first current-carrying branch 61 is turned off and the second current-carrying branch 62 is turned on. By adjusting the firing angles of the sending-end current source converter and the receiving-end current source converter, opposite potential differences are formed to achieve power reversal while the voltage polarity remains unchanged.

[0142] Specifically, please refer to the following: Figure 22 When power is transmitted in the forward direction, the original sending-end current source converter is defined as the rectifier side and the original receiving-end current source converter is defined as the inverter side. When the power is reversed, the new sending-end current source converter is defined as the original inverter side and the new receiving-end current source converter is defined as the original rectifier side.

[0143] When power is transmitted in the forward direction, the DC voltage on the rectifier side is higher than the DC voltage on the inverter side, creating a voltage difference on the line and thus forming a forward transmission current. At this time, the firing angle on the rectifier side is approximately 17°, and the firing angle on the inverter side is 150°. Because current in a traditional LCC converter can only flow in one direction, reversing the power flow requires changing the voltage polarity of both converter sides. This involves first reducing the voltage on both sides, then restarting the converter, increasing the firing angle on the rectifier side to 150°, and decreasing the firing angle on the inverter side to 17°. This change in voltage polarity achieves the reversal process.

[0144] In this embodiment, after receiving the power reversal command Fz, the current is reversed by turning off the first current-carrying branch 61 and turning on the second current-carrying branch 62. The original sending end (i.e., the original sending end current source converter) still uses constant DC current control, and the original receiving end (i.e., the original receiving end current source converter) still uses voltage control. That is, the DC current command remains unchanged, which reduces the DC current at the original sending end, thereby achieving the reverse current transmission.

[0145] This process only requires fine-tuning the firing angles of the original rectifier side and the inverter side so that the DC voltage on the inverter side is slightly higher than that on the rectifier side. The voltage polarity remains unchanged. This is equivalent to increasing the firing angle of the original rectifier side to 30° and the firing angle of the original inverter side to 163°. The angle change is only 13°, which is a fine-tuning. The response speed is fast and it is more conducive to the power flow mutual assistance of the large power grid.

[0146] This scheme can achieve rapid power flow reversal while maintaining the DC voltage polarity. On the one hand, it can be applied to scenarios with high requirements for power flow reversal speed, such as grid interconnection in areas with a high proportion of fluctuating new energy sources. On the other hand, it also solves the problem of difficult power flow reversal under multi-terminal DC grids.

[0147] In the description of this specification, references to terms such as "some embodiments," "other embodiments," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative descriptions of the above terms do not necessarily refer to the same embodiments or examples.

[0148] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0149] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A current source converter based on a controllable voltage source, characterized in that, include: Converter transformer; A converter bridge circuit, wherein the DC terminal of the converter bridge circuit is used to connect a DC load; A controllable voltage source is provided, wherein each phase AC terminal of the converter bridge circuit is directly connected to the controllable voltage source; the controllable voltage source is a voltage source whose amplitude and phase of the output voltage signal can be adjusted. The filter reactor has a first end connected to a phase AC terminal and a controllable voltage source, and a second end of each filter reactor is connected to the converter transformer, which is used to connect to the AC load. The controllable voltage source is used to establish an AC voltage based on the angular frequency reference value obtained by the current source converter through active power control and the AC voltage amplitude reference value obtained by reactive power control, so as to realize grid control. In this circuit, the only degree of control freedom of the current source converter in steady state is the firing angle. By adjusting the output voltage amplitude of the controllable voltage source, the reactive power regulation between the current source converter and the AC load is realized. The active power regulation between the current source converter and the AC load is realized by adjusting the firing angle according to the voltage signal output by the controllable voltage source.

2. The current source converter according to claim 1, characterized in that, The converter bridge circuit includes a DC reactor and converter bridge arms. Each converter bridge arm includes an upper bridge arm and a lower bridge arm. The first ends of each upper bridge arm are connected to form a common terminal as a first DC terminal. The first ends of each lower bridge arm are connected to form a common terminal as a second DC terminal. The second end of one upper bridge arm is connected to the second end of one lower bridge arm, and the common terminal is a phase AC terminal. The first DC terminal is connected to a DC load through the DC reactor, and the second DC terminal is connected to the DC load.

3. The current source converter according to claim 2, characterized in that, The converter bridge arm includes a first current-carrying branch and a second current-carrying branch connected in opposite parallel.

4. The current source converter according to claim 3, characterized in that, The first current-carrying branch includes any one of the following: a first branch formed by multiple reverse-resistance IGCT devices connected in series, a second branch formed by multiple IGBT devices connected in series, and a third branch formed by at least one asymmetric IGCT device and at least one IGBT device connected in series.

5. The current source converter according to claim 3, characterized in that, The second current-carrying branch includes any one of the following: a first branch formed by multiple reverse-resistance IGCT devices connected in series, a second branch formed by multiple IGBT devices connected in series, a fourth branch formed by multiple diodes connected in series, and a fifth branch formed by multiple thyristors connected in series.

6. The current source converter according to any one of claims 3-5, characterized in that, The converter bridge arm also includes buffer branches connected in parallel with the first flow branch and the second flow branch, respectively.

7. The current source converter according to claim 6, characterized in that, The buffer branch includes a first buffer branch and a second buffer branch, wherein the first buffer branch is connected in parallel with the first flow branch, and the second buffer branch is connected in parallel with the second flow branch.

8. The current source converter according to any one of claims 3-5, characterized in that, The converter bridge arm also includes energy-consuming branches connected in parallel with the first flow branch and the second flow branch, respectively.

9. The current source converter according to any one of claims 2-5, characterized in that, The converter bridge circuit also includes a saturated reactance, and the second end of the upper bridge arm is connected to the second end of the lower bridge arm through the saturated reactance, and / or the second end of the lower bridge arm is connected to the second end of the upper bridge arm through the saturated reactance.

10. The current source converter according to any one of claims 1-5, characterized in that, The controllable voltage source includes multiple interconnected sub-modules, and the connection method of the sub-modules includes any one of MMC connection, delta chain connection and star chain connection.

11. The current source converter according to claim 10, characterized in that, The controllable voltage source submodule is an IGBT submodule or an IGCT submodule.

12. A DC transmission system, characterized in that, It includes two current source converters as described in any one of claims 1-11, wherein one current source converter serves as the sending-end current source converter and the other current source converter serves as the receiving-end current source converter, and the DC terminal of the sending-end current source converter is connected to the DC terminal of the receiving-end current source converter.

13. The DC transmission system according to claim 12, characterized in that, The sending-end current source converter adjusts the output voltage amplitude of the sending-end controllable voltage source based on the reactive power measurement value of the sending-end power grid and the DC voltage measurement value of the bridge arm of the sending-end controllable voltage source, so as to achieve reactive power interaction with the AC power grid. The receiving-end current source converter adjusts the output voltage amplitude of the receiving-end controllable voltage source based on the reactive power measurement value of the receiving-end power grid and the DC voltage measurement value of the bridge arm of the receiving-end controllable voltage source, so as to realize reactive power interaction with the AC power grid. The sending-end current source converter performs constant DC current control by adjusting the firing angle, and the receiving-end current source converter performs constant DC voltage control by adjusting the firing angle, so as to realize the active power interaction between the sending-end current source converter and the receiving-end current source converter.

14. The DC transmission system according to claim 12, characterized in that, Also includes: If a short-term fault occurs on the AC power grid or DC side, the converter bridge arm of the converter bridge circuit will be continuously shut off within one power frequency cycle to ensure that the AC output current is continuously controllable. If a permanent fault occurs in the receiving-end system, the upper and lower bridge arms of the same phase in the converter bridge circuit of the receiving-end current source converter will be triggered to conduct, forming a bypass pair to provide a path for power transfer to the sending end.

15. The DC transmission system according to claim 12, characterized in that, The converter bridge arm of the converter bridge circuit includes a first current-carrying branch and a second current-carrying branch connected in reverse parallel. When there is a power reverse transmission requirement, the first current-carrying branch is turned off and the second current-carrying branch is turned on. By adjusting the firing angles of the sending end current source converter and the receiving end current source converter, opposite potential differences are formed to achieve power reversal while the voltage polarity remains unchanged.

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