A hybrid direct current transmission system and control method thereof

Abstract

The present invention provides a hybrid direct current transmission system and a control method thereof, which belongs to the field of power electronics. The system comprises: the rectifier side circuit adopts a heterogeneous series topology structure, the high-voltage valve group adopts LCC to withstand high voltage levels, and the low-voltage valve group adopts MMC to establish and maintain the voltage stability of the grid connection point; the inverter side circuit adopts a distributed connection, wherein the high-voltage valve group adopts LCC, the low-voltage valve group adopts three parallel MMCs, and multiple inverters are distributedly connected to different AC circuits. The rectifier side is composed of LCC and MMC in series, and the inverter side is composed of three parallel MMCs and LCC in series that are distributedly connected to different AC circuits. In this way, the system can provide voltage support for the energy base, has AC and DC fault crossing function, prevents phase change failure, and improves system stability. In addition, the distributed access method can also realize flexible power distribution.

Description

Technical Field

[0001] The present invention belongs to the field of power electronics, and in particular relates to a hybrid direct current power transmission system and a control method thereof. Background Art

[0002] With the dual carbon goals proposed, my country is in a critical period of energy structure transformation, gradually shifting from a fossil energy-dominated system to a modern energy system with clean energy as the core. Due to the geographical distribution characteristics of my country's energy and power load centers, most new energy sources need to be exported over long distances and on a large scale to meet the requirements. The use of high-voltage direct current transmission technology is an inevitable choice for realizing the transmission of Shagohuang renewable energy base. At present, with the rapid development of the power system, high-voltage direct current transmission technology is also developing continuously.

[0003] In the existing technology, the DC transmission technology is the line commutated converter based high voltage direct current (LCC-HVDC) technology, which is very mature and has the advantages of low construction and operation costs, large capacity, and low loss. It has been widely used in long-distance cross-regional power transmission. However, there are also some limitations. For example, commutation failure is prone to occur on the inverter side. With the continuous improvement of the system's transmission power and voltage level, the corresponding AC system strength will be relatively reduced. However, LCC itself does not have a self-commutation function and needs to be connected to an AC system with sufficient strength to ensure normal operation. As a result, when the LCC-HVDC system is connected to a weak AC system at the inverter end, it may face system instability problems such as commutation failure. Summary of the invention

[0004] In order to solve the problem that commutation failure is prone to occur on the inverter side, the present invention provides a hybrid direct current transmission system and a control method thereof.

[0005] In order to achieve the above object, the present invention provides the following technical solutions:

[0006] A hybrid direct current power transmission system, the system comprising a rectifier side circuit and an inverter side circuit;

[0007] The rectifier side circuit comprises a grid commutation converter LCC and a modular multilevel converter MMC connected in series;

[0008] The inverter side circuit comprises an LCC and three parallel-connected MMCs, and the LCC and the three parallel-connected MMCs are connected in series;

[0009] The LCC of the rectifier-side circuit and the LCC of the inverter-side circuit are connected in series.

[0010] Optionally, the LCC includes a converter transformer and a 12-pulse converter, wherein the converter transformer is connected to the MMC, and the 12-pulse converter of the rectifier-side LCC is connected to the 12-pulse converter of the inverter-side LCC;

[0011] The 12-pulse converter includes two 6-pulse converters connected in series; the 6-pulse converter includes 6 first bridge arms of three phases abc, each first bridge arm includes a half-controlled device thyristor; the converter transformer includes two transformers with different wiring methods, one is Y△ connection and the other is Yy connection; the two 6-pulse converters are respectively connected to the two transformers with different wiring methods.

[0012] Optionally, the MMC adopts a modular multi-level structure, including 6 second bridge arms of three phases abc, each second bridge arm includes 1 inductor and multiple cascaded sub-modules; the sub-module is a half-bridge sub-module, including two IGBTs and a diode anti-parallel to the two IGBTs and a DC capacitor.

[0013] A hybrid DC power transmission control method, applied to the hybrid DC power transmission system according to claim 1, comprising:

[0014] In the rectifier side circuit, the MMC adopts a dual-loop control consisting of a power outer loop and a current inner loop to generate a trigger pulse signal for controlling each device in the MMC; the LCC adopts a constant DC current control, compares the current command value and the current measured value, performs proportional integral adjustment according to the comparison result, and controls the trigger angle of the LCC according to the adjustment result;

[0015] In the inverter side circuit, three groups of parallel MMCs adopt master-slave station switching coordinated control, one of the MMCs is set as the master station, adopting fixed DC voltage control and fixed reactive power control, and the other two MMCs are slave stations, adopting fixed active power control and fixed reactive power control; LCC adopts fixed DC voltage control, fixed DC current control and fixed shutdown angle control, and by comparing the output values ​​of the three controls of fixed DC voltage, fixed DC current and fixed shutdown angle control, the maximum value is taken as the signal for controlling the LCC trigger angle.

[0016] Optionally, the MMC adopts a dual-loop control consisting of a power outer loop and a current inner loop, and generating a trigger pulse signal for controlling each device in the MMC includes:

[0017] The output value of the outer loop control is used as the input value of the inner loop control. The inner loop control adopts PI control, and the control result is subjected to the nearest level approximation modulation and sorting method voltage balancing control to generate a trigger pulse signal for controlling each device in the MMC.

[0018] Optionally, in the inverter side circuit, the LCC also adopts a minimum arc extinction angle control strategy to prevent commutation failure caused by an arc extinction angle that is too small.

[0019] Optionally, in the event of a fault in the master control station, the first slave control station responds quickly and switches to the control mode of the master control station, switching from fixed active power control to fixed DC voltage control to maintain voltage stability of the DC system; during the first slave control response process, the control mode of the second slave control station remains unchanged.

[0020] The hybrid DC power transmission system and control method provided by the present invention have the following beneficial effects:

[0021] The transmission system proposed by the present invention adopts a structure of LCC and MMC in series on the rectifier side, which can realize the power delivery of large-scale new energy bases. The high-voltage valve group adopts LCC to withstand the high voltage level of the system. LCC has the ability of forced phase shifting and can suppress DC fault current through control strategies; the low-voltage valve group adopts MMC to establish and maintain the voltage stability of the grid connection point; the inverter side is composed of three parallel MMCs and LCCs in series that are distributedly connected to different AC circuits, and has AC and DC fault crossing functions to prevent phase switching failures, and multiple inverters are distributedly connected to different AC circuits. Such a distributed access method can also realize flexible power distribution. In addition, the present invention also implements a control strategy for the operating principle of the topological structure of the above-mentioned rectifier side and inverter side, and prevents the master-slave switching of multiple MCCs in view of the complex coupling characteristics between multiple converter stations on the inverter side, thereby avoiding the problem of phase switching failure caused by MCC failure. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the embodiment of the present invention and its design scheme, the following briefly introduces the drawings required for this embodiment. The drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0023] Figure 1 The figure is a schematic diagram of a hybrid direct current power transmission system provided according to an exemplary embodiment of the present invention.

[0024] Figure 2 The figure is a schematic diagram of the LCC structure provided according to an exemplary embodiment of the present invention.

[0025] Figure 3 The figure is a schematic diagram of the MMC structure provided according to an exemplary embodiment of the present invention.

[0026] Figure 4 The figure is a schematic diagram of the structure of an MMC submodule provided according to an exemplary embodiment of the present invention.

[0027] Figure 5 The present invention is a flow chart of a hybrid direct current transmission control method provided according to an exemplary embodiment of the present invention.

[0028] Figure 6 The figure is a schematic diagram of a rectifier-side LCC control method provided according to an exemplary embodiment of the present invention. DETAILED DESCRIPTION

[0029] In order to enable those skilled in the art to better understand the technical solution of the present invention and implement it, the present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the scope of protection of the present invention.

[0030] In the description of the present invention, it is to be understood that the terms “center”, “longitudinal”, “lateral”, “length”, “width”, “thickness”, “up”, “down”, “front”, “back”, “left”, “right”, “vertical”, “horizontal”, “top”, “bottom”, “inside”, “outside”, “axial”, “radial”, “circumferential”, etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing the technical solutions of the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0031] In addition, the terms "first", "second", etc. are used for descriptive purposes only and are not to be understood as indicating or implying relative importance. In the description of the present invention, it should be noted that, unless otherwise clearly specified or limited, the terms "connected" and "connected" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to the specific circumstances. In the description of the present invention, unless otherwise specified, "plurality" means two or more, which will not be described in detail here.

[0032] As the demand for converter performance in HVDC transmission continues to increase, a HVDC transmission technology based on modular multilevel converters (MMC-HVDC) has emerged. This technology has the ability to connect to passive or weak AC networks, has low harmonic content, does not have the risk of commutation failure, and can flexibly control active and reactive power. Compared with LCC, MMC can realize the transmission of power from large-scale new energy bases, and is widely used in large-scale onshore new energy and offshore wind farms, becoming one of the effective solutions to the problems of new energy grid connection and power consumption. However, it has large losses during operation, high initial construction and installation costs, and the MMC converter using half-bridge submodules does not have the function of self-clearing DC faults.

[0033] Based on the above-mentioned MMC-HVDC technology, in order to further improve the reliability of DC power transmission and reduce costs, the present invention proposes a hybrid DC power transmission system. The rectifier side is composed of LCC (Line-Commutated Converter) and MMC (Modular Multilevel Converter) in series, and the inverter side is composed of three parallel MMCs and LCCs in series that are distributed and connected to different AC systems. The system can provide voltage support for the Shagohuang renewable energy base, has AC and DC fault ride-through function, prevents commutation failure, and improves system stability. The distributed access method can realize flexible power distribution.

[0034] The technical solutions provided by various embodiments of the present invention are described in detail below in conjunction with the accompanying drawings.

[0035] First, the present invention provides a hybrid direct current transmission system, specifically Figure 1 As shown, including:

[0036] A rectifier side circuit and an inverter side circuit; the rectifier side circuit is connected to the transmission network of the new energy base.

[0037] The rectifier side circuit includes a grid-commutated converter LCC and a modular multilevel converter MMC connected in series; the rectifier side LCC and MMC are respectively connected to the transmission network of the new energy base; the inverter side circuit includes an LCC and three parallel MMCs, and the LCC and the three parallel MMCs are connected in series; the LCC of the rectifier side circuit and the LCC of the inverter side circuit are connected in series.

[0038] For example, the rectifier side circuit adopts a heterogeneous series topology structure, the high-pressure valve group adopts LCC to withstand high voltage levels, and the low-pressure valve group adopts MMC to establish and maintain grid-connected point voltage stability; the inverter side circuit adopts a distributed connection, in which the high-pressure valve group adopts LCC, the low-pressure valve group adopts three parallel MMCs, and multiple inverters are distributedly connected to different AC circuits.

[0039] Based on the above structure, the rectifier-side converter station adopts a heterogeneous series topology structure, which can realize the power delivery of the large-scale new energy base in Shagohuang. The high-voltage valve group adopts LCC to withstand the high voltage level of the system, and the low-voltage valve group adopts MMC to establish and maintain the voltage stability of the grid connection point. LCC has the ability of forced phase shifting and can suppress DC fault current through control strategies. In the inverter-side converter station, the high-pressure valve group and the low-pressure valve group are connected in a decentralized manner. Among them, the high-pressure valve group selects LCC, and the low-pressure valve group selects three parallel MMCs. Multiple converters on the inverter side are decentralized and connected to different AC systems to achieve flexible power distribution. At the same time, due to the existence of MMC, even if the LCC fails to change phase, the system can maintain a certain power supply capacity. This topology significantly improves the stability of the system.

[0040] In one embodiment, the LCC includes a converter transformer and a 12-pulse converter. The 12-pulse converter includes two 6-pulse converters connected in series; the 6-pulse converter includes 6 first bridge arms of three phases abc, specifically 3 common cathode converter valves and 3 common anode converter valves. These converter valves are switched on and off in a specific order to convert AC power into DC power. This configuration allows the 6-pulse converter to generate 6 pulses in a complete cycle, thereby effectively converting AC power into DC power. Each first bridge arm includes a half-controlled device thyristor; these half-controlled device thyristors are switched on and off in a specific order and control strategy to achieve the conversion of AC power to DC power. The converter transformer has the functions of adjusting the trigger angle of the converter, electrical isolation between AC and DC, and voltage conversion. It includes two transformers with different wiring methods, one is Y△ connection and the other is Yy connection, with a phase difference of 30°, which can reduce the generation of harmonics in the converter; the two 6-pulse converters are respectively connected to the two transformers with different wiring methods, specifically as Figure 2 As shown, U RL is the DC voltage of LCC on the rectifier side.

[0041] In another embodiment, if Figure 3 As shown, the MMC adopts a modular multi-level structure, including abc three-phase and a total of 6 second bridge arms, each of which includes an inductor and multiple cascaded sub-modules. The inductor can suppress the unbalanced current generated between the second bridge arms; in addition, the sub-module is a half-bridge sub-module SM 1, including two IGBTs and a diode VD connected in anti-parallel with the two IGBTs 1 And the DC capacitor C, as shown in Figure 4 As shown, VT 1 and VT 2 For IGBT.

[0042] With the above system, the rectifier side is composed of LCC and MMC in series, and the inverter side is composed of three parallel MMCs and LCC in series that are distributedly connected to different AC circuits. In this way, the system can provide voltage support for the energy base, has AC and DC fault ride-through function, prevents commutation failure, and improves system stability. In addition, the distributed access method can also achieve flexible power distribution.

[0043] Secondly, the present invention also provides a hybrid DC transmission control method, such as Figure 5 As shown, the following steps are included:

[0044] S501. In the rectifier side circuit, the MMC adopts a dual-loop control consisting of a power outer loop and a current inner loop to generate a trigger pulse signal for controlling each device in the MMC; the LCC adopts a constant DC current control, compares the current command value and the current measured value, performs proportional-integral adjustment according to the comparison result, and controls the trigger angle of the LCC according to the adjustment result.

[0045] Specifically, the outer loop control includes fixed active power control, fixed reactive power control and fixed DC voltage control. The outer loop control output value of the power outer loop is used as the inner loop control input value of the current inner loop. The inner loop control adopts PI control, and the control result is controlled by nearest level approximation modulation and sorting method to generate a trigger pulse signal for controlling each device in the MMC.

[0046] For example, the MMC converter in the hybrid DC transmission system adopts vector control. Vector control usually adopts dual-loop control, that is, it consists of a power outer loop and a current inner loop. This control is simple and has a fast response speed. The outer loop control includes fixed active power control, fixed reactive power control and fixed DC voltage control. The output value of the outer loop control is and is the input value of the inner loop control. In the inner loop control, the output value and As the input value of the nearest level approximation modulation and sorting method voltage control module, it finally generates the trigger pulse signal for controlling each device in the MMC. Among them, in order to ensure the stability of the system DC voltage, the MMC in the rectifier side circuit selects constant DC voltage and constant reactive power control.

[0047] like Figure 6 As shown, on the rectifier side, the LCC converter operates in constant DC current control. ,I dc , α R They represent the DC current command value, the DC current measured value, and the LCC trigger angle on the rectifier side. and I dc The value is used to control the trigger angle of LCC after PI adjustment.

[0048] S502. In the inverter side circuit, three groups of parallel MMCs adopt master-slave station switching coordinated control, one of the MMCs is set as the master station, adopting fixed DC voltage control and fixed reactive power control, and the other two MMCs are slave stations, adopting fixed active power control and fixed reactive power control; LCC adopts fixed DC voltage control, fixed DC current control and fixed shutdown angle control, and by comparing the output values ​​of the three controls of fixed DC voltage, fixed DC current and fixed shutdown angle control, the maximum value is taken as the signal for controlling the trigger angle of LCC.

[0049] For example, in the inverter side circuit, three groups of parallel MMCs adopt the master-slave station switching coordinated control strategy, designate MMC1 among the three groups of MMCs as the master station, select constant DC voltage and constant reactive power control, and MMC2 and MMC3 as slave stations, select constant active power and constant reactive power control. In order to facilitate analysis, the command values ​​in the constant reactive power control of the MMC are all set to 0.

[0050] In addition, the LCC in the inverter circuit adopts constant DC voltage, constant DC current, and constant turn-off angle control, and in order to prevent the arc extinction angle of the upper and lower bridges on the inverter side from being too small, resulting in commutation failure, a minimum arc extinction angle control strategy is also added. By comparing the output values ​​of the three controls of constant DC voltage, constant DC current, and constant turn-off angle control, the maximum value is taken as the signal to control the LCC trigger angle. γ I 、 , I dc , , U IL , α R They respectively represent the measured value of the arc extinction angle of the upper and lower bridges on the inverter side, the DC current command value, the measured value of the DC current, the DC voltage command value, the measured value of the DC voltage, and the LCC trigger angle on the inverter side.

[0051] In one possible implementation, when a fault occurs at the master control station, the first slave control station responds quickly and switches to the control mode of the master control station, switching from fixed active power control to fixed DC voltage control to maintain voltage stability of the DC system; during the first slave control response process, the control mode of the second slave control station remains unchanged.

[0052] For example, in a hybrid DC power transmission system, a permanent AC fault occurs at the outlet of the inverter side MMC1, which will threaten the stability of the entire DC power transmission system, and the power balance of the system will also be affected. The present invention proposes a master-slave station switching coordination control strategy suitable for parallel MMC converter stations in a hybrid DC power transmission system. When a fault occurs, the master control station MMC1 exits operation due to the fault and loses the ability to control the system DC voltage. In order to ensure the stability of the system DC voltage, the slave control station MMC2 responds quickly and switches to the control mode of the master control station, switching from fixed active power control to fixed DC voltage control to maintain the voltage stability of the DC system and avoid system instability or failure caused by voltage fluctuations. During the whole process, the MMC3 control mode remains unchanged. At the same time, the output power originally borne by the inverter side MMC1 is shared by MMC2 and MMC3.

[0053] In order to further illustrate the present invention, a mathematical model can be established according to the characteristics of the topological structure of the hybrid DC transmission system, and the operating principle and control strategy are deeply studied based on the mathematical model. At the same time, further interaction mechanism analysis is carried out for the complex coupling characteristics between multiple converter stations on the inverter side.

[0054] Specifically, before controlling the hybrid DC power transmission system, it is necessary to construct a transmission parameter model of the hybrid DC power transmission system, including:

[0055] The DC voltage of the LCC on the rectifier side is U RL , the DC voltage of the MMC on the rectifier side is U RM , the DC voltage of the inverter-side LCC is U IL , the DC voltage of the inverter side MMC is U IM , the system DC current is I dc , assuming the DC line resistance is R , then the voltage equation model of the entire system is:

[0056] U RL +U RM =I dc R+U IL +U IM .

[0057] The active power output of LCC is P RL , the reactive power isQ RL , the transmission parameter equation model of the rectifier side LCC is as follows:

[0058] .

[0059] P RL = U RL I dc .

[0060] .

[0061] .

[0062] in, E RL is the effective value of the no-load line voltage on the valve side, X RL is the commutation reactance, is the trigger angle, is the commutation angle, is the power factor angle.

[0063] In the above Figure 3 middle, U RM is the DC voltage value output by the MMC on the rectifier side, , as well as are the input voltages of phase a, phase b, and phase c respectively. Since the MMC three-phase structure is highly symmetrical, phase a is taken as the research object for analysis, and the mathematical expression is as follows:

[0064] i a =-i a1 +i a2 .

[0065] in, i a is the current of phase a on the AC side of the MMC, i a1 is the current of the upper bridge arm of phase a on the AC side of the MMC, i a2 is the current of the lower bridge arm of phase a on the AC side of the MMC.

[0066] .

[0067] .

[0068] in, u ap andu an are the voltages of the upper bridge arm and the lower bridge arm respectively, u a1 and u a2 They are the output voltages of the upper bridge arm and lower bridge arm submodules respectively. L is the bridge arm inductance.

[0069] In order to analyze the mathematical model of MMC, the bridge arm inductance is ignored, and the following is obtained:

[0070] .

[0071] .

[0072] .

[0073] From the above formula, it can be concluded that the voltage of the upper bridge arm and the lower bridge arm together constitute the DC side voltage.

[0074] The above control method is applied to the hybrid DC transmission system proposed in the present invention. The rectifier side adopts the structure of LCC and MMC in series, which can realize the power delivery of large-scale new energy bases. The high-voltage valve group adopts LCC to withstand the high voltage level of the system. LCC has the ability of forced phase shifting and can suppress DC fault current through control strategy; the low-voltage valve group adopts MMC to establish and maintain the voltage stability of the grid connection point; the inverter side is composed of three parallel MMCs and LCCs in series that are distributedly connected to different AC circuits, and has AC and DC fault crossing functions to prevent phase switching failures, and multiple inverters are distributedly connected to different AC circuits. Such a distributed access method can also realize flexible power distribution. In addition, the present invention also implements a control strategy for the operating principle of the topological structure of the above-mentioned rectifier side and inverter side, and prevents the master-slave switching of multiple MCCs for the complex coupling characteristics between multiple converter stations on the inverter side. The problem of phase switching failure caused by MCC failure is avoided.

[0075] It should be noted that the above specific implementation method can enable those skilled in the art to understand the invention more comprehensively, but does not limit the invention in any way. Therefore, although the present invention has been described in detail in this specification, those skilled in the art should understand that the invention can still be modified or replaced by equivalents; and all technical solutions and improvements that do not deviate from the spirit and scope of the invention are included in the protection scope of the patent for the invention. Any figure mark in the claims should not be regarded as limiting the claims involved.

Claims

1. A hybrid direct current transmission control method, characterized in that: The method is implemented by using the following system, which includes a rectifier side circuit and an inverter side circuit; The rectifier side circuit comprises a grid commutation converter LCC and a modular multilevel converter MMC connected in series; The inverter side circuit comprises an LCC and three parallel-connected MMCs, and the LCC and the three parallel-connected MMCs are connected in series; The LCC of the rectifier side circuit and the LCC of the inverter side circuit are connected in series; The method comprises: In the rectifier side circuit, the MMC adopts a dual-loop control consisting of a power outer loop and a current inner loop to generate a trigger pulse signal for controlling each device in the MMC; the LCC adopts a constant DC current control, compares the current command value and the current measured value, performs proportional integral adjustment according to the comparison result, and controls the trigger angle of the LCC according to the adjustment result; In the inverter side circuit, three groups of parallel MMCs adopt master-slave station switching coordinated control, one of the MMCs is set as the master station, adopting fixed DC voltage control and fixed reactive power control, and the other two MMCs are slave stations, adopting fixed active power control and fixed reactive power control; LCC adopts fixed DC voltage control, fixed DC current control and fixed shutdown angle control, and by comparing the output values ​​of the three controls of fixed DC voltage, fixed DC current and fixed shutdown angle control, the maximum value is taken as the signal for controlling the LCC trigger angle.

2. The hybrid DC power transmission control method according to claim 1, characterized in that: The LCC includes a converter transformer and a 12-pulse converter, wherein the converter transformer is connected to the MMC, and the 12-pulse converter of the rectifier-side LCC is connected to the 12-pulse converter of the inverter-side LCC; The 12-pulse converter includes two 6-pulse converters connected in series; the 6-pulse converter includes 6 first bridge arms of three phases abc, each first bridge arm includes a half-controlled device thyristor; the converter transformer includes two transformers with different wiring methods, one is Y△ connection and the other is Yy connection; the two 6-pulse converters are respectively connected to the two transformers with different wiring methods.

3. The hybrid DC power transmission control method according to claim 1, characterized in that: The MMC adopts a modular multi-level structure, including abc three-phases with a total of 6 second bridge arms, each of which includes an inductor and multiple cascaded sub-modules; the sub-module is a half-bridge sub-module, including two IGBTs and a diode anti-parallel to the two IGBTs, and a DC capacitor.

4. The hybrid DC power transmission control method according to claim 1, characterized in that: The MMC adopts a dual-loop control consisting of a power outer loop and a current inner loop, and generates trigger pulse signals for controlling various devices in the MMC, including: The outer loop control output value of the power outer loop is used as the inner loop control input value of the current inner loop. The inner loop control adopts PI control, and the control result is controlled by nearest level approximation modulation and sorting method to generate a trigger pulse signal for controlling each device in the MMC.

5. The hybrid DC power transmission control method according to claim 1, characterized in that: In the inverter side circuit, the LCC also adopts a minimum arc extinction angle control strategy to prevent commutation failure caused by an excessively small arc extinction angle.

6. The hybrid DC power transmission control method according to claim 1, characterized in that: In the event of a fault at the master control station, the first slave control station responds quickly and switches to the control mode of the master control station, switching from fixed active power control to fixed DC voltage control to maintain voltage stability of the DC system; during the first slave control response process, the control mode of the second slave control station remains unchanged.

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