Current source converter based on controllable voltage source and direct current transmission system

Abstract

The invention relates to a current source converter based on a controllable voltage source and a direct current transmission system, the current source converter comprises a converter transformer, a converter bridge circuit, the controllable voltage source and a filter reactor, and the phase alternating current end of the converter bridge circuit is connected with the converter transformer through the filter reactor; and a controllable voltage source is connected between the phase alternating current end and the filter reactor. Therefore, through the arrangement of the filter reactor, the isolation between the converter bridge circuit and the alternating current load can be realized, the harmonic wave of the output current of the converter bridge circuit can be filtered through the filter reactor, and the harmonic wave characteristic of the network access current is ensured. Moreover, due to the arrangement of the filter reactance, the equivalent impedance of the alternating current side is large enough, so that the controllable voltage source has more refined power regulation capability. The commutation voltage required by commutation is provided for the converter bridge circuit through the controllable voltage source, and the leakage reactance of the converter transformer does not participate in commutation, so that the influence of the leakage reactance of the converter transformer on the commutation process is thoroughly eliminated, and the reactive power consumption is greatly reduced.

Description

Technical Field

[0001] The present application relates to the field of direct current transmission technology, and in particular to a current source converter and a direct current transmission system based on a controllable voltage source. Background Art

[0002] With the advancement of DC transmission technology, traditional synchronous generators are gradually being replaced by large-scale new energy bases and ultra-high voltage DC systems. This is leading to a trend of hollowing out of the power grids at both the transmitting and receiving ends. This has led to a continuous decline in the short-circuit ratios of both the transmitting and receiving ends, resulting in insufficient dynamic reactive power regulation capabilities, reduced voltage stability margins, and decreased grid strength. This poses a severe challenge to the security of large power grids. In this context, AC systems urgently need DC systems to provide the necessary active support to ensure the safe and stable operation of weak receiving-end power grids.

[0003] Currently, line-commutated converters (LCCs) have been widely used in ultra-high voltage direct current (UHVDC) projects. However, when LCCs are used in scenarios where there is a lack of reliable conventional power supply, they can suffer from commutation failures, leading to significant active power shortages and seriously impacting the reliability of grid power transmission.

[0004] As integrated gate-commutated thyristors (IGCTs) continue to evolve toward high-voltage and high-current capabilities, the hybrid line-commutated converter (HCC) topology based on reverse-resistance IGCTs was proposed in 2022. This topology allows for commutation independent of grid voltage, addressing commutation failure. However, because its commutation principle is the same as that of an LCC, HCC operation still requires reactive power from the AC grid. Its dynamic reactive power regulation capability depends on the switching of reactive power compensation devices, resulting in a slow response. Given the expected continued weakening of the grid in the future, this topology still has room for improvement.

[0005] Since its introduction in 2010, the modular multilevel converter (MMC) has been widely used in DC projects both domestically and internationally, thanks to its ability to operate in four quadrants while avoiding commutation failures and providing flexible and adjustable reactive power to the grid while transmitting rated active power. However, in ultra-high voltage (UHV) long-distance DC transmission scenarios, it faces limitations such as difficulty in increasing capacity, low power density, and high costs and losses. Therefore, a converter topology with lower costs and easier capacity expansion is urgently needed.

[0006] In 2022, a multi-source hybrid converter (STATCOM and line commutation converter, SLCC) based on a parallel structure of a conventional DC converter and a static VAR compensation device (STATCOM) was proposed. This effectively alleviates the problems of traditional commutation technology, such as its dependence on the strength of the AC system, its independence, and its poor adaptability, while also providing reactive power support to the grid. However, because the current source still uses thyristors, SLCC cannot avoid commutation failures when the grid voltage drops significantly, and it still has limitations in scenarios with multiple DC feeds and extremely weak power systems.

[0007] In 2022, a controllable current source converter (CSC) topology based on reverse-resistance IGCTs was proposed. This topology addresses commutation failures by leveraging IGCTs. A power decoupling control strategy was designed to achieve power decoupling in certain operating ranges, enabling the current source converter to operate in four-quadrants and provide reactive power support to the grid. However, steady-state operation of the CSC in PWM (Pulse-Width Modulation) mode requires the device to possess continuous active shutdown capability. The current shutdown capability of reverse-resistance IGCTs is affected by junction temperature and is difficult to support high-frequency operation. Increasing switching frequency will also significantly increase the difficulty of voltage balancing in series with valve arms. Furthermore, the CSC valve outlet parallel AC capacitors consist 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 large-capacity active power transmission, strong reactive power support capability and low cost. Summary of the Invention

[0009] Based on this, it is necessary to provide a current source converter and DC transmission system based on a controllable voltage source to address the above problems, which has large-capacity active power transmission and strong reactive power support capabilities, and is relatively low in cost.

[0010] The present 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, wherein the DC end of the converter bridge circuit is used to connect to a DC load; the AC end of each phase of the converter bridge circuit is respectively connected to the controllable voltage source; the first end of one of the filter reactors is connected to one of the phase AC ends and the controllable voltage source, the second end of each of the filter reactors is respectively connected to the converter transformer, and the converter transformer is used to connect to the AC load.

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

[0012] In one embodiment, the commutation bridge arm includes a first flow branch and a second flow branch connected in anti-parallel.

[0013] In one embodiment, the first current-passing branch includes any one of a first branch formed by a plurality of reverse-blocking IGCT devices connected in series, a second branch formed by a plurality of 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-passing branch includes any one of a first branch formed by a plurality of reverse-blocking IGCT devices connected in series, a second branch formed by a plurality of IGBT devices connected in series, a fourth branch formed by a plurality of diodes connected in series, and a fifth branch formed by a plurality of thyristors connected in series.

[0015] In one embodiment, the commutation bridge 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, the first buffer branch is connected in parallel with the first through-flow branch, and the second buffer branch is connected in parallel with the second through-flow branch.

[0017] In one embodiment, the commutation bridge arm further includes an energy consumption branch connected in parallel with the first flow branch and the second flow branch respectively.

[0018] In one embodiment, the commutation bridge circuit further 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.

[0019] In one embodiment, the controllable voltage source includes a plurality of connected submodules, and the connection mode of the submodules 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 to achieve network control based on an angular frequency reference value obtained by active power control of the current source converter and an AC voltage amplitude reference value obtained by reactive power control.

[0022] The present application also provides a direct current transmission system, comprising two current source converters as described above, wherein one current source converter serves as a sending-end current source converter, and the other current source converter serves as a receiving-end current source converter, and the DC end of the sending-end current source converter is connected to the DC end 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 power 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 interaction with the AC power 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 power 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 interaction with the AC power grid; the sending-end current source converter performs constant DC current control by adjusting the trigger angle, and the receiving-end current source converter performs constant DC voltage control by adjusting the trigger angle, so as to achieve active interaction between the sending-end current source converter and the receiving-end current source converter.

[0024] In one embodiment, the DC power transmission system further includes: if a short-term fault occurs in the AC grid or the DC side, the commutation bridge arm of the commutation bridge circuit is continuously shut down 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 bridge arm and the lower bridge arm of the same phase in the commutation bridge circuit of the receiving-end current source converter are triggered to be turned on to form a bypass pair to provide a path for power transmission at the sending end.

[0025] In one embodiment, the commutation bridge arm of the commutation bridge circuit includes a first current branch and a second current branch connected in reverse parallel. When there is a demand for power reverse, the first current branch is turned off and the second current branch is turned on. By adjusting the trigger angles of the sending-end current source converter and the receiving-end current source converter, an opposite potential difference is formed to achieve power reversal without changing the voltage polarity.

[0026] The above-mentioned current source converter and DC transmission system includes a converter transformer, a converter bridge circuit, a controllable voltage source, and a filter reactor. The AC end of the converter bridge circuit is connected to the converter transformer through the filter reactor, and a controllable voltage source is connected between the AC end and the filter reactor. The filter reactor can achieve isolation between the converter bridge circuit and the AC load. This solution has the following advantages:

[0027] 1. A second-order LC filter circuit is formed by combining the filter reactance and the controllable voltage source equivalent capacitor to filter out the harmonics of the output current of the converter bridge circuit. This ensures the harmonic characteristics of the grid-connected current without the need for additional harmonic control strategies. This also reduces the current harmonics injected into the converter transformer, further reducing the difficulty of converter transformer manufacturing.

[0028] 2. As an important component of the AC side equivalent impedance, the filter reactance can be customized according to the needs of the power grid. 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 refined.

[0029] 3. The controllable voltage source provides the commutation voltage required for commutation for the converter bridge circuit. The leakage reactance of the converter transformer does not participate in the commutation, which completely eliminates the influence of the leakage reactance of the converter transformer on the commutation process and greatly reduces 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 can be accelerated, greatly reducing the risk of commutation failure.

[0031] 5. With the two current source converters of the above structure, the output voltage amplitude of the controllable voltage source can be adjusted independently, ensuring the reactive power controllability of the current source converter and the AC power grid. The current source converter can adjust the trigger angle according to the voltage output by the controllable voltage source, ensuring independent regulation of active power. Thus, coordinated decoupling control of active and reactive power is achieved, with both large-capacity active power transmission and strong reactive power support capabilities.

[0032] 6. There is no need to configure an additional independent reactive compensation device for the current source converter, nor is there any need to connect a large-scale capacitor between the converter bridge circuit and the converter transformer, which can effectively reduce costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the conventional technology, the following briefly introduces the drawings required for use in the embodiments or the conventional technology descriptions. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

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

[0035] Figure 2 Schematic diagram of current waveforms at different positions of a current source converter in one embodiment of the present application;

[0036] Figure 3 Schematic diagram of output current harmonic characteristics of a current source converter in one embodiment of the present application;

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[0053] Figure 20 This is a structural diagram of a DC power transmission system in one embodiment of the present application;

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

[0055] Figure 22 FIG1 is a schematic diagram of a power reversal control process of a DC power transmission system in an embodiment of the present application. DETAILED DESCRIPTION

[0056] To facilitate understanding of the present application, the present application will be described more fully below with reference to the accompanying drawings. The accompanying drawings illustrate preferred embodiments of the present application. However, the present application may be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and comprehensive understanding of the disclosure of the present application.

[0057] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application pertains. The terms used herein in the specification of this application are for the purpose of describing specific embodiments only and are not intended to limit this application.

[0058] It will be 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 a first element from another element. For example, a first resistor may be referred to as a second resistor, and similarly, a second resistor may be referred to as a first resistor without departing from the scope of this application. The first resistor and the second resistor are both resistors, but they are not the same resistor.

[0059] It can be understood that the “connection” in the following embodiments should be understood as “electrical connection”, “communication connection”, etc. if there is transmission of electrical signals or data between the connected circuits, modules, units, etc.

[0060] It is understood that “at least one” refers to one or more, “a plurality” refers to two or more, and “at least a portion of an element” refers to a portion or all of an element.

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

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

[0063] Research has found that controlled current source converters, by connecting high-voltage AC centralized capacitors in parallel, provide both commutation voltage and reactive power to the AC grid. Their reactive power support capacity depends on the capacitor's capacitance. A higher capacitance value enhances the converter's AC grid support capability. However, this increases the number of capacitor banks, making them bulky and increasing the risk of failure, which in turn increases initial construction costs. Furthermore, centralized capacitors require ground isolation to prevent the introduction of DC bias into the commutation transformer, making insulation coordination more difficult.

[0064] To mitigate this phenomenon, a decentralized design of the centralized capacitors can be considered, with an additional voltage source providing the commutation voltage, eliminating the leakage reactance of the converter transformer from participating in commutation. This mitigates the drawbacks of centralized capacitors and eliminates the impact of the converter transformer leakage reactance on commutation, making the current source converter fully resistant to commutation failures.

[0065] Based on the above considerations, the present 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 end 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 end and the filter reactor.

[0066] The above solution disperses the capacitors in the controllable voltage source, which can effectively reduce the insulation and manufacturing difficulties. The 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 reactive power.

[0067] See also Figure 1The present 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 end of the converter bridge circuit 10 is used to connect to a DC load (not shown); each AC end of the converter bridge circuit 10 is connected to the controllable voltage source 20; the first end of each filter reactor L is connected to the AC end of a phase and the controllable voltage source 20, and the second end of each filter reactor L is connected to the converter transformer 30, which is used to connect to the AC load. The controllable voltage source 20 not only provides commutation voltage for the converter bridge circuit 10, but also determines the reactive interaction characteristics between the current source converter and the AC power grid.

[0068] Specifically, the controllable voltage source 20 is disposed between the converter bridge circuit 10 and the converter transformer 30, connected to the corresponding phases. The 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 a circuit for performing AC-DC conversion; the controllable voltage source 20 is a voltage source with adjustable amplitude and phase of the output voltage signal; and the filter reactor L is a reactor disposed between the converter bridge circuit 10 and the converter transformer 30 for filtering and isolation.

[0069] It should be noted that the type of commutation bridge circuit 10 is not unique; it can be a six-pulse commutation bridge circuit 10 or another type of commutation bridge circuit 10, without specific limitation. The number of filter reactors L is not unique and varies depending on the type of commutation bridge circuit 10. It is sufficient as long as each phase AC end of the commutation bridge circuit 10 is connected to a corresponding filter reactor L. In one embodiment, taking a six-pulse commutation bridge circuit 10 as an example, the current source converter should include three filter reactors L, each connected to the three-phase AC end of the six-pulse commutation bridge circuit 10.

[0070] It can be understood that the type of controllable voltage source 20 is not unique, and it will also vary depending on the type of the commutation 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, each phase output end of which corresponds to a phase AC end connected to the commutation bridge circuit 10. The three-phase controllable voltage source 20 can be connected in different ways and arranged in parallel at the three-phase AC end of the commutation bridge circuit 10.

[0071] In the solution of this embodiment, the filter reactor L can suppress the output current harmonics of the commutation bridge circuit 10 and prevent DC discontinuity. Figure 2In the figure, Ica is the output current of the controllable voltage source 20, Ipa is the output current of the commutation bridge circuit 10, and Iga is the grid current. As can be seen from the figure, in the current source converter provided by the present application, the output current Ica of the controllable voltage source 20 compensates the output current Ipa of the commutation bridge circuit 10, making the grid current Iga a sinusoidal current. After experimental analysis, as Figure 3 As shown in FIG, the harmonic content of the output current of the final current source converter is about 0.3%. It can be seen 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 made more refined, so that the configuration capacity of the controllable voltage source 20 can be customized according to the AC load requirements.

[0073] For details, please refer to Figure 4 In the single-phase equivalent circuit of the current source converter connected to the AC load, the filter reactance Leakage reactance of converter transformer 30 The series equivalent reactance is , is the output voltage of the controllable voltage source 20, is the DC voltage, is the grid current, is the apparent power, is the grid voltage (i.e. the voltage of the AC load). , , it can be seen that a smaller equivalent impedance will limit the voltage regulation range of the controllable voltage source 20. In actual scenarios, the controllable voltage source 20 includes multiple submodules, each of which has the same voltage (for example, 2.4 kV), and the voltages of the submodules added together form , which means the resolution of voltage regulation ( / 2.4), will decrease as the maximum output voltage of the controllable voltage source 20 decreases.

[0074] Assumptions Infinitesimal, then as long as and If they are not equal, the grid current will be infinite, and the reactive power injected into the grid (i.e., AC load) will be infinite, causing the controllable voltage source 20 to lose its voltage regulation capability. Small, there is a situation that just meets the rated reactive power, at this time and The voltage difference between the two submodules is smaller than the voltage of one submodule. In this case, the voltage regulation resolution of the controllable voltage source 20 is still insufficient and the power regulation function is still not provided.

[0075] That is, only when the appropriate Only when the controllable voltage source 20 has a relatively fine power regulation capability can it be ensured that the parameters are designed in combination with the reactive power requirements of the system. It is usually difficult to meet the power regulation requirements by only including the leakage reactance of the converter transformer 30.

[0076] The current source converter provided in this application operates in a phase-controlled mode in steady state, eliminating the need for active shutdown of the controllable switching devices in the commutation bridge circuit 10. Commutation can be accomplished by relying on the voltage provided by the controllable voltage source 20. In other words, the current source converter's only control freedom in steady state is the trigger angle, which is 1. Power decoupling relies on the amplitude and phase of the output voltage of the controllable voltage source 20, both of which are controllable. By adjusting the output voltage amplitude of the controllable voltage source 20, reactive power regulation is achieved between the current source converter and the AC load. The commutation bridge circuit 10, in part, coordinates the trigger angle, making the DC voltage 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 current source converters, respectively, as an example, the reactive power interaction between the sending and receiving current source converters and the grid is achieved by controlling the output voltage amplitude of their respective controllable voltage sources. The active power interaction between the sending and receiving current source converters is coordinated by controlling the trigger angle. The sending current source converter adjusts the trigger angle to achieve constant DC current control, while the receiving current source converter adjusts the trigger angle to achieve constant DC voltage control. This coordination achieves active power control of the current source converters at both ends.

[0078] The output voltage amplitude of the controllable voltage source can be independently adjusted, ensuring controllable reactive power for both the current source converter and the grid. The current source converter can adjust its firing angle based on the voltage output by the controllable voltage source, ensuring independent regulation of active power under constant DC current control at the sending end and constant DC voltage control at the receiving end. This achieves coordinated decoupling control of active and reactive power between the AC and DC loads.

[0079] The above-mentioned current source converter includes a converter transformer 30, a converter bridge circuit 10, a controllable voltage source 20, and a filter reactor L. The AC end of the converter bridge circuit 10 is connected to the converter transformer 30 via the filter reactor L, and the controllable voltage source 20 is connected between the AC end and the filter reactor L. Thus, the provision of the filter reactor L can achieve isolation between the converter bridge circuit 10 and the AC load. The filter reactor L can filter out harmonics in the output current of the converter bridge circuit 10, ensuring the harmonic characteristics of the grid-connected current. Furthermore, the provision of the filter reactor L makes the equivalent impedance on the AC side sufficiently large, allowing the controllable voltage source 20 to have more refined power regulation capabilities. The controllable voltage source 20 provides the converter bridge circuit 10 with the commutation voltage required for commutation. The leakage reactance of the converter transformer 30 does not participate in the commutation process, completely eliminating the impact of the leakage reactance of the converter transformer 30 on the commutation process and significantly reducing reactive power consumption. The above solution 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 10 and the controllable voltage source 20 to achieve independent decoupling control of active power and independent decoupling control of reactive power.

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

[0081] Specifically, the DC-side reactor is a reactor disposed on the DC side of the current source converter for connecting to the DC load on the DC side. The type and number of the upper bridge arms 11 and lower bridge arms 12 are consistent. The specific number may vary depending on the type of the commutation bridge circuit 10. In one embodiment, the commutation bridge circuit 10 is a six-pulse commutation bridge circuit 10, and accordingly, it includes six bridge arms, including three upper bridge arms 11 and three lower bridge arms 12.

[0082] It can be understood that in other embodiments, in order to realize the operation control of the controllable voltage source 20, the commutation bridge circuit 10, etc., the current source converter should also include a valve controller, wherein the control end of the controllable switching device of each bridge arm in the commutation bridge circuit 10 and the control end 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, obtain the power grid operation parameters, and control the commutation bridge circuit 10 and the controllable voltage source 20 in combination with the power grid operation parameters. The details are not elaborated here.

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

[0084] It should be noted that the types of the commutation arms are 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 form a single current-carrying branch by connecting controllable switching devices in series. 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 feedback) 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 of the current source converter and avoid the dependence of active power flow reversal on the polarity of the DC voltage, in one embodiment, refer to Figure 6 The commutation bridge arm includes a first flow branch 61 and a second flow branch 62 connected in reverse parallel.

[0086] Among them, when the current source converter transmits power in the first direction, the first flow branch 61 is opened and operated and the second flow branch 62 is closed; when there is a need for power reversal, 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 the preset current threshold, the first flow branch 61 is closed and the second flow branch 62 is opened and operated, so that the current source converter transmits power in the second direction.

[0087] Specifically, anti-parallel connection refers to the parallel connection of the first flow branch 61 and the second flow branch 62, with the current flowing in opposite directions. The first direction and the second direction refer to two opposite power transmission directions, which may vary depending on the actual 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. Accordingly, the first flow branch 61 is a forward flow branch, and the second flow branch 62 is a reverse flow 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 flow branch 61 is a reverse flow branch, and the second flow branch 62 is a forward flow branch.

[0088] Unlike the unidirectional current-carrying bridge arms in the above-mentioned embodiments, this embodiment uses bidirectional current-carrying bridge arms to construct the commutation bridge circuit 10. With a 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 current source converters to zero and then restarting them, changing the voltage polarity of the sending and receiving current source converters. This entire process is slow, typically taking several seconds or even longer. With a bidirectional current-carrying bridge arm structure, however, there's no need to change the voltage polarity; the current simply switches to the other current-carrying branch for transmission.

[0089] Specifically, when power is transmitted in the forward direction (first direction), the sending-end DC voltage of the sending-end current source converter is higher than the receiving-end DC voltage of the receiving-end current source converter. At this time, under the action of the line impedance, the conditions for feeding power from the sending end to the receiving end are formed. When power reversal is required, it is necessary to increase the trigger angle of the sending-end current source converter to reduce the sending-end DC voltage, so that the DC current gradually decreases. At this time, the first flow branch 61 is opened and the second flow branch 62 is closed. When the current drops to near 0 (that is, less than the preset current threshold), it means that the sending and receiving-end DC voltages are basically equal. At this time, the first flow branch 61 is closed and the second flow branch 62 is opened and operated. Subsequently, the trigger angle of the receiving-end current source converter is increased to increase its DC voltage, forming the conditions for feeding power from the receiving-end current source converter to the sending-end current source converter, and realizing power return (that is, transmission in the second direction). This process does not require the sending and receiving-end current source converters to be reduced to 0 and then restarted separately, which can significantly reduce the time for active power flow reversal.

[0090] In the above scheme, the commutation bridge arm can realize bidirectional current flow and quickly realize active power flow reversal when the DC voltage polarity remains unchanged, providing a solution for the power mutual assistance flexibility of the sending and receiving and terminal power 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] During forward power transmission, the DC voltage on the rectifier side is higher than the DC voltage on the inverter side, creating a voltage difference across the line and generating a forward current. At this point, the trigger angle on the rectifier side is approximately 17°, and the trigger angle on the inverter side is 150°. Because current in traditional LCC converters flows in only one direction, power flow reversal requires changing the voltage polarity on both sides of the converter. This involves first reducing the voltage on both sides and then restarting the converter, increasing the original trigger angle on the rectifier side to 150° and reducing the original trigger angle on the inverter side to 17°. This reversal is achieved by changing the voltage polarity.

[0093] In this embodiment, the current is reversed by shutting off the first flow branch and opening the second flow branch. In this case, only the trigger angles of the original rectifier and inverter sides need to be fine-tuned so that the DC voltage on the inverter side is slightly higher than the DC voltage on the rectifier side. The voltage polarity remains unchanged. This is equivalent to increasing the original rectifier-side trigger angle to 30° and the original inverter-side trigger angle to 163°. This angle change of only 13° is a fine adjustment, which has a fast response and is more conducive to the mutual assistance of power flows in the large power grid.

[0094] It should be pointed out that, in one embodiment, the active power flow can be reversed by the bidirectional bridge arm, but when the current source converter operates normally, the bridge arm switching devices that conduct current in the forward direction (that is, the current is transmitted from the sending end to the receiving end) need to be bidirectionally tolerant. Therefore, the first flow branch 61 and the second flow branch 62 can be configured to be constructed by controllable switching devices with bidirectional pressure-bearing capabilities, thereby further improving the operating reliability of the converter bridge circuit 10.

[0095] In one embodiment, the first flow branch 61 includes any one of a first branch formed by a plurality of reverse-blocking IGCT devices connected in series, a second branch formed by a plurality of 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, IGCT is a power integrated device that integrates a gate drive circuit with a thyristor structure; IGBT (Insulated Gate Bipolar Transistor) is a composite fully controlled voltage-driven power semiconductor device. The reverse blocking IGCT is an integrated gate commutated thyristor with reverse blocking capability. The anode of the reverse blocking IGCT device is connected to the cathode of the adjacent reverse blocking IGCT device, thereby forming a first branch in series, and the collector of the IGBT device is connected to the emitter of the adjacent IGBT device, thereby forming a second branch in series. In a similar manner, the asymmetric IGCT device and the IGBT device are connected in series to form a third branch. In actual scenarios, any one of the first branch, the second branch and the third branch can be selected as the first current-passing branch 61, and the selection can be made based on actual needs.

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

[0098] Specifically, similar to the first pass-through branch 61 in the above embodiment, in this embodiment, the second pass-through branch 62 is a reverse pass-through branch. Its structure can be the same as or different from the first pass-through branch 61, and it only needs to be connected in reverse parallel, without specific limitation. Through this solution, different combinations of first and second pass-through branches 61, 62 can be selected according to actual needs to ultimately form a commutation bridge arm, which can effectively expand the scope and scenarios of application of the current source converter.

[0099] In one embodiment, the commutation bridge arm further includes a buffer branch connected in parallel with the first through-current branch 61 and the second through-current branch 62 .

[0100] Specifically, the buffer branch is a circuit that can suppress the turn-off voltage of the switching devices of the first flow branch 61 and the second flow branch 62, thereby performing a series voltage balancing function for the switching devices. In actual scenarios, the buffer branch includes multiple buffer components. The number of switching devices connected in series in the first flow branch 61 and the second flow branch 62 is the same. For any flow branch, a buffer component is connected in parallel at both ends of each switching device. Therefore, the definition of a buffer branch connected in parallel with the flow branch in this application should be understood as the buffer components of the buffer branch being connected in parallel with the switching devices of the flow branch.

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

[0102] The above solution connects a buffer branch in parallel in the commutation bridge arm, which can realize series voltage balancing of the switching devices in the commutation bridge arm and improve the operational safety and stability of the commutation bridge arm.

[0103] It should be noted that the number of buffer branches provided in the same commutation bridge arm is not unique. In one embodiment, to reduce the volume of the current source converter and save hardware costs, a buffer branch can be configured for the first flow branch 61 and the second flow branch 62, that is, the two share a buffer branch. In this embodiment, considering that the current directions of the first flow branch 61 and the second flow branch 62 are different, in order to achieve the buffering function in both directions, the buffer component of the buffer branch should use a circuit or device without polarity restrictions. For example, it can be combined with reference to Figure 7The buffer component can be set as a circuit formed by connecting RC and MOV in parallel. In the figure, the second flow branch 62 is constructed using a diode. In other embodiments, static voltage-equalizing resistors, RC circuits, etc. can also be used, and the specific details are not limited.

[0104] In another embodiment, a buffer branch may be configured for the first flow branch 61 and the second flow branch 62 respectively, that is, the buffer branch includes a first buffer branch and a 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 pressure of the first flow branch 61 when the first flow branch 61 is opened and in operation, and the second buffer branch is used to equalize the pressure of the second flow branch 62 when the second flow branch 62 is opened and in operation. The buffer component of the first buffer branch and the buffer component of the second buffer branch can have the same structure, for example Figure 8 As shown, an RCD dynamic voltage-equalizing circuit may be used; in other embodiments, the buffer component of the first buffer branch and the buffer component of the second buffer branch may also be one or more of a static voltage-equalizing resistor, an RC circuit, and a circuit formed by connecting RC and MOV in parallel.

[0106] The structures of the first buffer branch and the second buffer branch may be different, for example Figure 9 As shown, the first buffer branch is constructed using an RCD dynamic voltage equalizing circuit, and the second buffer branch is constructed using an RC circuit. In other embodiments, the first buffer branch and the second buffer branch can also be other combinations, which are not specifically limited.

[0107] It is understood that in actual scenarios, if the first and second buffer branches employ circuits without polarity requirements, such as static voltage-equalizing resistors, RC circuits, or circuits formed by RC and MOV in parallel, they only need to be connected in parallel with the first flow branch 61 and the second flow branch 62, respectively. If the first or second buffer branch employs a circuit with polarity requirements, such as an RCD dynamic voltage-equalizing circuit, the first buffer branch needs to be connected in parallel with the first flow branch 61 in the same direction, and the second buffer branch needs to be connected in parallel with the second flow branch 62 in the same direction. In other words, the first and second buffer branches need to be connected in anti-parallel.

[0108] See also Figure 10 In one embodiment, the commutation bridge arm further includes an energy consumption branch connected in parallel with the first flow branch 61 and the second flow branch 62 respectively.

[0109] Specifically, this embodiment illustrates a scheme in which the first current-carrying branch 61 is a series-connected reverse-resistance IGCT, the second current-carrying branch 62 is a series-connected thyristor, the first buffer branch is an RCD dynamic voltage-sharing circuit, and the second buffer branch is an RC circuit. Energy-dissipating branches are also connected in parallel at both ends of the commutation bridge arm. These energy-dissipating branches not only limit device overvoltages but also provide series voltage balancing for multiple devices. It should be noted that the energy-dissipating branches are not limited to a single type; any circuit capable of achieving both of the aforementioned functions can be used.

[0110] For example, in one embodiment, the energy consumption branch includes an MOV, wherein the number of MOVs is not unique. It can be one MOV, in which case all switching devices in the entire flow branch share one MOV, or multiple switching devices in a flow branch share one MOV, or each switching device in the flow branch is configured with an MOV, without specific limitation.

[0111] Please refer to Figure 5 In one embodiment, the commutation 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 commutation bridge arm also includes a saturated reactance, which can alleviate the current intolerance phenomenon of the controllable switching devices in the commutation bridge arm, thereby improving the working reliability and operation safety of the commutation bridge arm.

[0113] It is understood that the saturated reactor can be centrally installed as in this embodiment, at both ends of the commutation bridge arm, directly connected in series with the commutation bridge arm. In other embodiments, the saturated reactors can also be distributed throughout the commutation bridge arm. Specifically, multiple controllable switches in the commutation bridge arm can be grouped together, with a saturated reactor connected in series to form a module. Subsequently, multiple similar modules can be connected in series to form the commutation bridge arm.

[0114] In one embodiment, the controllable voltage source 20 includes a plurality of connected submodules, and the connection mode of the submodules includes any one of MMC connection, delta chain connection and star chain connection.

[0115] Specifically, as shown in the above embodiment, the way in which the controllable voltage source 20 is connected to the commutation bridge circuit 10 is not unique. Under different connection modes, the connection modes of each submodule (ie, SM in the figure) in the controllable voltage source 20 will also be different. Figure 11 As shown, it includes six bridge arms, each bridge arm includes multiple cascaded sub-modules, these bridge arms are connected in pairs, and the three common ends formed are used as three-phase AC ports, that is, as shown in the figure 、 and , are connected to the three-phase AC terminals of the commutation bridge circuit 10. Figure 12 As shown, the star chain connection method is as follows Figure 13 As shown, through different connection modes of submodules, three-phase AC ports are formed respectively ( 、 and ), and are respectively connected to the three-phase AC ends of the commutation bridge circuit 10.

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

[0117] Among them, the IGBT submodule refers to a full-bridge submodule or a half-bridge submodule that uses IGBT as a controllable switch device. For details, please refer to Figure 14 and Figure 15 ; The IGCT submodule can be combined with reference to Figure 16 and Figure 17 The specific type of sub-module to be used can be selected based on the actual scenario.

[0118] In the embodiment of the present application, the commutation bridge arm uses the voltage provided by the controllable voltage source 20 to perform commutation. The leakage reactance of the converter transformer 30 does not participate in the commutation, and no external circuit is required to provide the commutation voltage. Therefore, the commutation process is fast. Figure 18 , the bridge arm S1 and the bridge arm S3, which respectively include the first flow branch 61, the first buffer branch and the energy consumption branch to perform commutation, are used as an example for description.

[0119] During normal flow, the current flows through the IGCT device of the first flow branch 61 of S1, carrying the rated current and overload current. During commutation, the first flow branch 61 of S1 actively shuts down the IGCT device according to the drive signal issued, establishes a commutation voltage, and the current of the first flow branch 61 of S1 is commutated to the first buffer branch. At this time, the diode of 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 action voltage of the energy-consuming branch (MOV), the voltage of the series module is clamped, and voltage equalization between each unit is achieved by voltage limiting. The IGCT device of the first flow branch 61 of S3 is triggered to open when subjected to positive pressure, and the DC current is commutated to S3. The current of S1 drops to 0, completing the shutdown commutation process.

[0120] Please refer to 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 network control of the entire converter.

[0121] Specifically, the core of the network control strategy is to generate the phase for generating the modulation wave without relying on the phase-locked loop based on the grid voltage. According to the droop characteristics of the synchronous generator (Formula 1), the droop control expression of the active power and reactive power of the grid-type converter can be obtained (Formula 2). According to the droop control principle, by collecting the active power P at the AC bus ac The value of ΔP is obtained by subtracting it from the rated value P0. ΔP and droop coefficient K p Multiply by the initial angular frequency By adding them together, you can get the angular frequency reference value that changes in real time according to the load change . After the integration link, the reference phase for generating the trigger signal of the IGCT valve (the commutation valve composed of the above-mentioned commutation bridge arm) can be obtained. t, at the same time, It will also be used to generate a trigger signal for the controllable voltage source 20 .

[0122] (Formula 1)

[0123] (Formula 2)

[0124] The current source converter network control strategy based on the controllable voltage source 20 needs to coordinate three control degrees of freedom, namely the trigger 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 firing angle degree of freedom It is used to coordinate the voltage balance of the full-bridge submodule in the controllable voltage source 20, through the measured value U of the DC voltage of the bridge arm of the controllable voltage source 20 dsvg With the instruction value The difference between the two values ​​can be adjusted by PI to get the Echizen trigger angle β. However, the generation of the trigger signal cannot be obtained directly from β, but needs to be obtained through the trigger angle The phase of the phase-locked loop (PLL) output generated by the receiving end grid voltage t, obtained by comparison. Combined with the formula =π-β, which can be obtained indirectly from β ,when t> At this time, the trigger signal S=1, otherwise S=0, the trigger signals of the six bridge arms of the IGCT valve are delayed by 60 degrees in sequence, that is, the trigger signal S of each bridge arm is obtained. 1-6 .

[0125] From the analysis of reactive power droop characteristics in formula 1 and formula 2, it can be seen that in the reactive power droop control link, the collected AC grid reactive power measurement value Q ac The difference between the reference value Q0 and ΔQ is obtained. ΔQ and the droop coefficient K q After multiplication, add it to the AC voltage amplitude rated value U0 to get the AC voltage amplitude reference value ,Will and AC voltage amplitude measurement value U ac The difference between the two can be obtained by closed-loop decoupling controller u cqi Among them, u cdi It is used to ensure that the controllable voltage source 20 does not exchange active power with the grid by converting the measured value of active power P svg With reference value P svg0 Make a difference and get u through closed-loop decoupling controller cdi , and then u cdi with u cqi After coordinate transformation, we can get the three-phase modulation wave u ca 、u cb 、u cc The three-phase modulated wave is modulated by the nearest level approximation, and the trigger signals of each device in the full-bridge submodule in the three-phase bridge arm of the controllable voltage source 20 are obtained, which are S a1-4 、S b1-4 、S c1-4 .

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

[0127] See also Figure 20 The present application also provides a direct current transmission system, comprising two current source converters as described above, wherein one current source converter serves as a sending-end current source converter, and the other current source converter serves as a receiving-end current source converter, and the DC end of the sending-end current source converter is connected to the DC end of the receiving-end current source converter.

[0128] Specifically, the structure of the current source converter is as shown in the above-mentioned embodiments and accompanying drawings, and will not be further described here. This current source converter completely eliminates the impact of the leakage reactance of the converter transformer 30 on the commutation process, significantly reduces reactive power consumption, and is fully resistant to commutation failures. It also provides active and reactive voltage support capabilities, making it suitable for both powering weak AC power grids at the receiving end and for transmitting renewable energy at the sending end.

[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 power 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 interaction with the AC power 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 power 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 interaction with the AC power grid; the sending-end current source converter performs constant DC current control by adjusting the trigger angle, and the receiving-end current source converter performs constant DC voltage control by adjusting the trigger angle, so as to achieve active 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 current source converters and the 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 current source converters is coordinated by controlling the trigger angle. The sending current source converter adjusts the trigger angle to achieve constant DC current control, while the receiving current source converter adjusts the trigger angle to achieve constant DC voltage control. 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 controllable reactive power for both the current source converter and the grid. The current source converter can adjust its firing angle based on the voltage output by the controllable voltage source 20. This ensures independent regulation of active power under constant DC current control at the sending end and constant DC voltage control at the receiving end. This achieves coordinated decoupling control of active and reactive power between the AC and DC loads.

[0132] For more details, please refer to Figure 21 The coordinated control of the sending and receiving ends 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 of the sending and receiving ends.

[0133] Among them, the decoupling control level between the single-ended current source and the controllable voltage source 20 is consistent in the control mode of the sending and receiving ends. The coordination mode of the receiving end voltage source and the current source is as follows: Assuming that the sending end DC current I dc Constant, then the active power control between the receiving end current source converter and the grid depends on the DC voltage U dcThe control is carried out by measuring the DC voltage U dc With reference value The difference between the two values ​​can be obtained by introducing the PI link to obtain the Echizen trigger angle β. Since the generation of the trigger signal cannot be obtained directly from β, it is necessary to use the trigger angle The phase of the phase-locked loop (PLL) output generated by the receiving end grid voltage t is obtained by comparison, combined with the formula =π-β, which can be obtained indirectly from β ,when t> When the trigger signal S=1, otherwise S=0, the trigger signals of the six bridge arms of the IGCT valve are delayed by 60° in sequence to obtain the trigger signal S of each bridge arm. 1-6 .

[0134] The reactive power of the converter is achieved by controlling the voltage amplitude output by the controllable voltage source 20, and the reactive power measurement value Q of the receiving grid is converted into aci and reactive power reference value The difference between the two, the closed-loop decoupling controller is introduced to obtain the q-axis component of the voltage modulation wave , and then combine it with the d-axis component of the modulation wave generated by the submodule voltage control , and perform coordinate transformation together. The three-phase voltage modulation wave of the controllable voltage source 20 can be obtained. By performing the nearest level approximation modulation on the above modulation wave, the trigger signals of each device in the full bridge submodule in the three-phase bridge arm of the controllable voltage source 20 can be obtained, which are S a1-4 、S b1-4 、S c1-4 Among them, the d-axis component of the modulation wave output by the submodule voltage control , is obtained by converting the measured value U of the DC voltage of the bridge arm of the controllable voltage source 20 dsvg With the instruction value The difference between the trigger angle and the current is adjusted by PI and then the closed-loop decoupling controller is used to obtain the active power of the converter. Control, reactive power is controlled by With the adjustment of reactive power, 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 will be further coordinated. , thus realizing the decoupling control of active and reactive power of the receiving-end current source converter.

[0135] The coordination method of the sending end voltage source and current source is as follows, assuming that the receiving end DC current U dc constant, then the active power transmitted by the sending-end current source converter depends on the DC current I dc The control is carried out by measuring the DC current value Idc With reference value The difference between the two values ​​can be obtained by introducing the PI link to obtain the Echizen trigger angle β. Since the generation of the trigger signal cannot be obtained directly from β, it is necessary to use the trigger angle α r The phase of the phase-locked loop (PLL) output generated by the receiving end grid voltage t is obtained by comparison, combined with formula α r =π-β, α can be obtained indirectly from β r .when t>α r When the trigger signal S=1, otherwise S=0, the trigger signals of the six bridge arms of the IGCT valve are delayed by 60° in sequence to obtain the trigger signal S of each bridge arm. 1-6 Among them, for the sending-end current source converter, in order to ensure the reliable triggering of the power device under the minimum triggering angle condition, it is necessary to set the minimum triggering angle limit, α r The engineering experience value is 5°, so the upper limit value of β is β max Take 175°.

[0136] The reactive power of the converter is achieved by controlling the voltage amplitude output by the controllable voltage source 20, and the reactive power measurement value Q of the sending-end grid is converted into acr and reactive power reference value By introducing the closed-loop decoupling controller, we can get the q-axis component of the voltage modulation wave. , and then combine it with the d-axis component of the modulation wave generated by the submodule voltage control By performing coordinate transformation together, the three-phase voltage modulation wave of the controllable voltage source 20 can be obtained. By performing the nearest level approximation modulation on the above modulation wave, the trigger signals of each device in the full bridge submodule in the three-phase bridge arm of the controllable voltage source 20 can be obtained, which are S a1-4 、S b1-4 、S c1-4 Among them, the d-axis component of the modulation wave output by the submodule voltage control , is obtained by converting the measured value U of the DC voltage of the bridge arm of the controllable voltage source 20 dsvg With the instruction value The difference between the two is PI-adjusted and then obtained by the closed-loop decoupling controller. r Control, reactive power is controlled by With the adjustment of reactive power, 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 , thus realizing the decoupling control of active and reactive power of the sending-end current source converter.

[0137] In the coordinated control of active power at the sending and receiving ends, the sending end adopts constant (DC) current control, and the receiving end adopts constant (DC) voltage control. dc (Sending end) and U dc Independent control of the (receiving end) is achieved, thereby realizing coordinated control of the active power of the DC system.

[0138] In one embodiment, if a short-term fault occurs in the AC grid or the DC side, the commutation bridge arms of the commutation bridge circuit are continuously shut down 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 bridge arm and the lower bridge arm of the same phase in the commutation bridge circuit of the receiving-end current source converter are triggered to conduct to form a bypass pair, thereby providing a path for power transmission at the sending end.

[0139] Specifically, a short-term AC system fault typically refers to a brief circuit interruption or anomaly in the AC power grid caused by various reasons (such as line aging, poor contact, or equipment defects). This fault may manifest as abnormal current, voltage fluctuations, or equipment tripping. A short-term DC side fault typically occurs on the DC side of the converter valve and may be caused by a DC power supply failure, poor line contact, or an internal equipment short circuit. This fault may result in abnormal DC voltage and current fluctuations. The power frequency cycle refers to the time it takes for AC power to complete a positive-negative alternation within one cycle, typically 50 Hz (expressed in time as 0.02 seconds). When a short-term AC system or DC side fault occurs in a DC transmission system, the AC output current at the receiving end is continuously controlled by controlling the commutation bridge arm to continuously shut down within one power frequency cycle. This effectively suppresses the magnitude of the fault current, compared to a thyristor valve that fails to actively commutate, thereby ensuring reliable valve operation.

[0140] A permanent fault in the receiving system refers to a fault that occurs in the receiving portion of a DC transmission system and cannot be automatically recovered, or that requires a long time to recover. This type of fault typically involves critical components such as the DC line and converter station equipment. In this case, the upper and lower arms of the converter valve (the commutation bridge arm of the converter bridge circuit) can be controlled to trigger, forming a bypass pair. These three-phase bypass pairs can be used alternately as required. This provides a path for power transmission to the sending end, preventing the risk of overvoltage caused by sudden load shedding or damage to the sending-end power supply.

[0141] In one embodiment, the commutation bridge arm includes a first current branch 61 and a second current branch 62 connected in reverse parallel. When there is a need for power reverse, the first current branch 61 is turned off and the second current branch 62 is turned on. By adjusting the trigger angles of the sending-end current source converter and the receiving-end current source converter, an opposite potential difference is formed to achieve power reversal without changing the voltage polarity.

[0142] For details, please refer to 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] During forward power transmission, the DC voltage on the rectifier side is higher than the DC voltage on the inverter side, creating a voltage difference across the line and generating a forward current. At this point, the trigger angle on the rectifier side is approximately 17°, and the trigger angle on the inverter side is 150°. Because current in traditional LCC converters flows in only one direction, power flow reversal requires changing the voltage polarity on both sides of the converter. This involves first reducing the voltage on both sides and then restarting the converter, increasing the original trigger angle on the rectifier side to 150° and reducing the original trigger angle on the inverter side to 17°. This reversal is achieved by changing the voltage polarity.

[0144] In this embodiment, upon receiving the power reversal command Fz, the first flow-through branch 61 is shut off and the second flow-through branch 62 is opened, thereby reversely transmitting the current. The primary sending end (i.e., the primary sending end current source converter) still uses constant DC current control, while the primary receiving end (i.e., the primary receiving end current source converter) still uses voltage control. In other words, the DC current command remains unchanged, reducing the DC current at the primary sending end and achieving current reverse transmission.

[0145] This process only requires fine-tuning the original trigger angles of the rectifier side and the inverter side so that the DC voltage on the inverter side is slightly higher than the DC voltage on the rectifier side. The voltage polarity does not change, which is equivalent to increasing the original trigger angle on the rectifier side to 30° and the original trigger angle on the inverter side to 163°. The angle change of only 13° is a fine-tuning with a fast response speed, which is more conducive to the power flow mutual assistance of the large power grid.

[0146] This solution can achieve rapid power flow reversal while maintaining the DC voltage polarity unchanged. On the one hand, it can be applied to scenarios with high requirements for power flow reversal speed, such as power grid interconnection in areas with a high penetration of volatile renewable energy. On the other hand, it also solves the problem of power flow reversal difficulties in multi-terminal DC networking.

[0147] In the description of this specification, reference to the terms "some embodiments" or "other embodiments" means that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic descriptions of the above terms do not necessarily refer to the same embodiment or example.

[0148] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned 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 above-described embodiments merely represent several implementation methods of the present application. 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 a person of ordinary skill in the art may make various modifications and improvements without departing from the spirit of the present application, and these modifications and improvements fall within the scope of protection of the present application. Therefore, the scope of protection of the present patent application shall 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 commutation bridge circuit, wherein the DC end of the commutation bridge circuit is used to connect a DC load; A controllable voltage source, to which the AC ends of each phase of the commutation bridge circuit are directly connected; A filter reactor, wherein a first end of the filter reactor is connected to one of the phase AC ends and the controllable voltage source, and a second end of each of the filter reactors is respectively connected to the converter transformer, and the converter transformer is used to connect an AC load.

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

3. The current source converter according to claim 2, characterized in that: The commutation bridge arm comprises a first flow branch and a second flow branch which are connected in reverse parallel.

4. The current source converter according to claim 3, characterized in that: The first current-passing branch includes any one of a first branch formed by a plurality of reverse-blocking IGCT devices connected in series, a second branch formed by a plurality of 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-passing branch includes any one of a first branch formed by a plurality of reverse-blocking IGCT devices connected in series, a second branch formed by a plurality of IGBT devices connected in series, a fourth branch formed by a plurality of diodes connected in series, and a fifth branch formed by a plurality of thyristors connected in series.

6. The current source converter according to any one of claims 3 to 5, characterized in that: The commutation bridge arm further includes a buffer branch 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, the first buffer branch is connected in parallel with the first through-flow branch, and the second buffer branch is connected in parallel with the second through-flow branch.

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

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

10. The current source converter according to any one of claims 1 to 5, characterized in that: The controllable voltage source includes a plurality of connected submodules, and the connection mode of the submodules 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 submodule of the controllable voltage source is an IGBT submodule or an IGCT submodule.

12. The current source converter according to any one of claims 1 to 5, characterized in that: Also includes: The controllable voltage source establishes an AC voltage to achieve network control according to an angular frequency reference value obtained by the current source converter through active power control and an AC voltage amplitude reference value obtained through reactive power control.

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

14. The direct current power transmission system according to claim 13, characterized in that: 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 power grid and the bridge arm DC voltage measurement value of the controllable voltage source at the sending end, so as to achieve reactive interaction with the AC power 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 power grid and the bridge arm DC voltage measurement value of the controllable voltage source at the receiving end, so as to achieve reactive interaction with the AC power grid; The sending-end current source converter performs constant DC current control by adjusting the trigger angle, and the receiving-end current source converter performs constant DC voltage control by adjusting the trigger angle, so as to realize active interaction between the sending-end current source converter and the receiving-end current source converter.

15. The direct current power transmission system according to claim 13, characterized in that: Also includes: If a short-term fault occurs in the AC grid or the DC side, the commutation bridge arms of the commutation bridge circuit are continuously shut down within one power frequency cycle, so that the AC output current can be continuously controlled; If a permanent fault occurs in the receiving end system, the upper bridge arm and the lower bridge arm of the same phase in the commutation bridge circuit of the receiving end current source converter are triggered to conduct to form a bypass pair to provide a path for power transmission at the sending end.

16. The direct current power transmission system according to claim 13, characterized in that: The commutation bridge arm of the commutation bridge circuit includes a first current branch and a second current branch connected in reverse parallel. When there is a demand for power reverse transmission, the first current branch is turned off and the second current branch is turned on. By adjusting the trigger angles of the sending-end current source converter and the receiving-end current source converter, an opposite potential difference is formed to achieve power reversal without changing the voltage polarity.

Images