Control method for hybrid direct current transmission system, hybrid direct current transmission system and controller

Abstract

The application provides a control method of a hybrid DC power transmission system, the hybrid DC power transmission system and a controller, relates to the field of DC power transmission control, and adopts a hybrid DC power transmission system of a sending terminal MMC+a receiving terminal HCC+uncontrolled rectifier, combines the characteristics that the MMC has network control capability and the HCC has the capability of resisting commutation failure, precisely adjusts the charging demand of the sending terminal MMC and the receiving terminal HCC by using the uncontrolled rectifier, solves the problems of starting a passive new energy isolated network and intermittent low-output DC, has the capabilities of network construction and commutation failure prevention, is suitable for large-capacity long-distance external transmission, and is more economical.

Description

Technical Field

[0001] This invention relates to the field of DC power transmission control, and in particular to a control method, a hybrid DC power transmission system, and a controller for a hybrid DC power transmission system. Background Technology

[0002] With the large-scale development of remote, isolated renewable energy sources such as desert-based renewable energy bases and deep-sea wind farms, high-capacity, long-distance DC transmission has become a core technology for transmitting renewable energy. The sending-end converter station needs to undertake the functions of constructing isolated AC voltage and balancing active and reactive power, requiring the use of MMC (Modular Multilevel Converter). The receiving end is located at the load center, where current-source converters are more economical. However, existing DC transmission technologies have the following core technical defects: 1. The pure flexible DC transmission scheme (both the sending and receiving ends are MMC) can meet the operational requirements of DC transmission of new energy from isolated grids. However, the cost of MMC at the receiving end is high, the overall project investment is large and the economic efficiency is poor, making it difficult to adapt to the low-cost construction requirements of large-scale new energy transmission.

[0003] 2. In the traditional hybrid DC scheme (sending-end MMC + receiving-end LCC), the receiving end uses the LCC (Line Commutated Converter) grid commutation converter, which has inherent commutation failure defects. When the AC grid at the receiving end fails, the fault ride-through capability is extremely poor, which cannot guarantee the stable operation of the new energy transmission system and is not suitable for new energy transmission scenarios.

[0004] 3. Existing hybrid DC solutions combining voltage source at the sending end and current source at the receiving end all assume a stable AC power grid at the sending end. These solutions are unsuitable for application scenarios such as desert areas and deep-sea regions with isolated passive renewable energy grids and no sending-end power grid, and they fail to address two key technical challenges: System startup challenge: There is no grid power at the sending end, and the MMC needs to be charged and started by the receiving end via a DC system. Existing solutions do not have a suitable startup control method. DC discontinuity risk: The output of new energy sources fluctuates randomly. When the DC current is lower than the minimum operating current of the receiving-end current source converter, DC current discontinuity is likely to occur, leading to system instability. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a control method, a hybrid DC transmission system, and a controller for a hybrid DC transmission system. This scheme adopts a hybrid architecture of a sending-end MMC + receiving-end HCC (Hybrid Commutated Converter) + uncontrolled rectifier for the hybrid DC transmission system. It combines the grid-building control capability of the MMC with the ability of the HCC to resist commutation failure. The uncontrolled rectifier is used to precisely adjust the charging demand of the sending-end MMC and the receiving-end HCC, solving the problems of DC start-up in passive renewable energy isolated grids and low-output DC intermittent problems. It has both grid-building and commutation failure prevention capabilities, is suitable for large-capacity long-distance transmission, and has better economic efficiency.

[0006] In a first aspect, embodiments of the present invention provide a control method for a hybrid DC transmission system, applied in a controller of the hybrid DC transmission system; wherein the hybrid DC transmission system is located between a sending-end islanded grid and a receiving-end grid, and includes a modular multilevel converter, an uncontrolled rectifier, and a hybrid phase-commutation converter; the method includes: After receiving power from the receiving-end grid, the uncontrolled rectifier supplies power to the modular multilevel converter and the hybrid commutator in sequence. When it is detected that both the hybrid commutator and the modular multilevel converter have reached their operating voltage, the hybrid commutator and the modular multilevel converter are sequentially controlled to be in the unlocked state. The control modular multilevel converter and hybrid commutation converter transmit the electrical energy output from the sending-end islanded grid to the receiving-end grid.

[0007] Optionally, the step of controlling the uncontrolled rectifier to sequentially supply power to the modular multilevel converter and the hybrid commutation converter after receiving power from the receiving-end grid includes: After the grid-side circuit breaker of the uncontrolled rectifier is in the closed state, the power supply from the receiving end grid is received by using the charging resistor of the uncontrolled rectifier on the AC side. When the operating voltage of the hybrid DC transmission system is detected to reach the first operating voltage, the bypass switch of the uncontrolled rectifier is closed to complete the uncontrolled power supply from the uncontrolled rectifier to the hybrid DC transmission system. The modular multilevel converter is controlled to enter a controlled charging state until the internal module voltage of the modular multilevel converter reaches the rated voltage value.

[0008] Optionally, the step of sequentially controlling the hybrid commutator and the modular multilevel converter to be in an unlocked state after detecting that both the hybrid commutator and the modular multilevel converter have reached their operating voltage includes: When the DC operating voltage in the hybrid commutator is detected to reach the second operating voltage, and the internal module voltage of the modular multilevel converter reaches the rated voltage value, the hybrid commutator is controlled to unlock, so that the hybrid commutator establishes a receiving-end power transmission channel with the receiving-end power grid through the corresponding smoothing reactor. When the receiving-end transmission channel is detected to be in the first steady state, the modular multilevel converter is unlocked, enabling the modular multilevel converter to establish a sending-end transmission channel with the sending-end isolated grid.

[0009] Optionally, the steps of controlling the modular multilevel converter and the hybrid commutation converter to transmit the electrical energy output from the sending-end islanded grid to the receiving-end grid include: When the power transmission channel at the sending end is detected to be in the second steady state, the power generation equipment in the isolated grid at the sending end is controlled to generate electricity in a predetermined batch. After the modular multilevel converter converts the electrical energy output by the power generation equipment into DC power, the DC power is transmitted to the hybrid commutator through the DC transmission line between the modular multilevel converter and the hybrid commutator. The hybrid commutator converts the received DC power into AC power and then transmits the AC power to the receiving-end power grid.

[0010] Optionally, the step of controlling the hybrid commutator to convert the received DC power into AC power and then transmitting the AC power to the receiving-end power grid includes: Using the cutoff angle reference value, AC voltage measurement value, DC voltage reference value, DC voltage measurement value, DC current reference value, and DC current measurement value as conditional parameters, an operation control strategy for the converter valve in a hybrid commutation converter is constructed. The hybrid commutation converter is controlled by an operation control strategy to convert the received DC power into AC power. The control of the hybrid commutation converter transmits AC power to the receiving-end power grid through the sending-end transmission channel.

[0011] Optionally, the steps for constructing the operation control strategy of the converter valve in a hybrid commutator, using the cut-off angle reference value, AC voltage measurement value, DC voltage reference value, DC voltage measurement value, DC current reference value, and DC current measurement value as conditional parameters, include: The first maximum firing angle allowed for the converter valve in the hybrid commutator under constant voltage control is calculated using the cut-off angle reference value and AC voltage measurement value. Using the first maximum firing angle as a constraint, the second maximum firing angle allowed for the converter valve under constant current control is calculated using the DC voltage reference value and the DC voltage measurement value. Using the second maximum firing angle as a constraint, the final firing angle of the hybrid commutator is calculated using DC current reference value and DC current measurement value, and an operation control strategy is constructed based on the first maximum firing angle, the second maximum firing angle and the final firing angle.

[0012] Optionally, the step of controlling the modular multilevel converter to convert the electrical energy output from the power generation equipment into DC power, and then transmitting the DC power to the hybrid commutation converter via the DC transmission line between the modular multilevel converter and the hybrid commutation converter, includes: The modular multilevel converter is controlled by a preset AC voltage command to establish AC voltage in the sending-end islanded grid in grid construction control mode, and the modular multilevel converter is controlled to convert the AC power corresponding to the AC voltage into DC power. DC power is transmitted to the hybrid commutator via a DC transmission line between the modular multilevel converter and the hybrid commutator.

[0013] Optionally, after the step of controlling the modular multilevel converter and the hybrid commutator to transmit the electrical energy output from the sending-end islanded grid to the receiving-end grid, the method further includes: When the output power of the DC power is detected to be lower than the preset power threshold, the power difference between the power threshold and the output power is used to enable the uncontrolled rectifier to supply power to the hybrid commutator.

[0014] Secondly, the present invention provides a hybrid DC transmission system located between a sending-end islanded grid and a receiving-end grid, comprising: a controller, a modular multilevel converter, an uncontrolled rectifier, and a hybrid phase-commutation converter; wherein the controller is connected to the modular multilevel converter, the uncontrolled rectifier, and the hybrid phase-commutation converter respectively; the modular multilevel converter and the hybrid phase-commutation converter are connected via DC transmission lines; One end of the modular multilevel converter is connected to the sending-end isolated grid, and the other end of the modular multilevel converter is connected to one end of the hybrid phase-commutation converter through a DC transmission line; the other end of the hybrid phase-commutation converter is connected to the receiving-end grid; the uncontrolled rectifier is connected in parallel with the hybrid phase-commutation converter to the receiving-end grid; In the process of controlling the transmission of DC power from the sending-end isolated grid to the receiving-end grid, the hybrid DC transmission system adopts the control method of the hybrid DC transmission system mentioned in the first aspect.

[0015] Thirdly, embodiments of the present invention also provide a controller, which includes a processor and a memory, the memory storing computer-executable instructions that can be executed by the processor, the processor executing the computer-executable instructions to implement the steps of the control method for the hybrid DC transmission system provided in the first aspect.

[0016] This invention provides a control method, a hybrid DC transmission system, and a controller for a hybrid DC transmission system. The hybrid DC transmission system is located between a sending-end islanded grid and a receiving-end grid, and includes a controller, a modular multilevel converter, an uncontrolled rectifier, and a hybrid phase-commutation converter. The modular multilevel converter and the hybrid phase-commutation converter are connected via a DC transmission line. In the process of using the controller to control the sending-end islanded grid to transmit output DC power to the receiving-end grid, firstly, the uncontrolled rectifier receives power from the receiving-end grid and then sequentially supplies power to the modular multilevel converter and the hybrid phase-commutation converter. When it is detected that both the hybrid phase-commutation converter and the modular multilevel converter have reached their operating voltage, they are sequentially controlled to be in an unlocked state. Finally, the modular multilevel converter and the hybrid phase-commutation converter are controlled to transmit the electrical energy output from the sending-end islanded grid to the receiving-end grid. This scheme adopts a hybrid DC transmission system with a hybrid architecture of sending-end MMC + receiving-end HCC + uncontrolled rectifier. It combines the grid control capability of MMC with the ability of HCC to resist commutation failure. The uncontrolled rectifier is used to precisely adjust the charging demand of sending-end MMC and receiving-end HCC, solving the problems of DC start-up of passive new energy isolated grid and low-output DC discontinuity. It has both grid construction and commutation failure prevention capabilities, is suitable for large-capacity long-distance transmission, and has better economic efficiency.

[0017] Other features and advantages of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention are realized and obtained through the structures particularly pointed out in the description and the drawings.

[0018] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0019] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0020] Figure 1 A flowchart of a control method for a hybrid DC transmission system provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of a hybrid DC transmission system provided in an embodiment of the present invention; Figure 3This is an operational characteristic diagram of a hybrid phase-commutation converter in a control method for a hybrid DC transmission system provided in an embodiment of the present invention. Figure 4 This is a schematic diagram of the operation control principle of the hybrid phase-commutation converter in a control method for a hybrid DC transmission system provided in an embodiment of the present invention. Figure 5 This is an operational characteristic diagram of a modular multilevel converter in a control method for a hybrid DC transmission system provided in an embodiment of the present invention. Figure 6 This is an operational characteristic diagram of an uncontrolled rectifier in a control method for a hybrid DC transmission system provided in an embodiment of the present invention. Figure 7 This is a schematic diagram of the stable operating point of the system during normal power transmission in a control method for a hybrid DC transmission system provided in an embodiment of the present invention; Figure 8 This is a schematic diagram of the stable operating point of the system when the power generation is insufficient in a control method for a hybrid DC transmission system provided in an embodiment of the present invention; Figure 9 This is a schematic diagram of the structure of a controller provided in an embodiment of the present invention.

[0021] icon: 100 - Controller; 200 - Modular multilevel converter; 300 - Uncontrolled rectifier; 400 - Hybrid commutated converter; 101 - Processor; 102 - Memory; 103 - Bus; 104 - Communication interface. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] To facilitate understanding of this embodiment, a control method for a hybrid DC transmission system disclosed in this invention will first be introduced. This method is applied in the controller of the hybrid DC transmission system. The controller receives system charging commands, converter unlocking commands, and renewable energy grid connection and transmission commands. These three types of commands are used to drive the constant-flexible hybrid renewable energy transmission system to complete the entire process control of startup charging, converter commissioning, and stable transmission of renewable energy power. Based on this, the method is as follows: Figure 1As shown, the hybrid DC transmission system is located between the sending-end islanded grid and the receiving-end grid, and includes a modular multilevel converter, an uncontrolled rectifier, and a hybrid phase-commutation converter. A schematic diagram of this hybrid DC transmission system is shown below. Figure 2 As shown; the modular multilevel converter acts as a rectifier, converting AC power to DC power; the hybrid commutation converter acts as an inverter, converting DC power to AC power; the uncontrolled rectifier can only perform rectification, converting AC power to DC power. Based on this, it includes: Step S101: After the uncontrolled rectifier receives power from the receiving end grid, it controls the charging uncontrolled rectifier to supply power to the modular multilevel converter and the hybrid commutation converter in sequence.

[0024] First, the charging resistor on the AC side of the uncontrolled rectifier assisted by the receiving-end power grid is activated and its grid-side circuit breaker is closed. The uncontrolled charging from the receiving-end AC power grid to the DC system is completed through the uncontrolled rectifier. After the DC system voltage stabilizes, the charging resistor of the uncontrolled rectifier is bypassed, and the modular multilevel converter (MMC) on the sending-end power grid side is controlled to enter the controllable charging state. The internal voltage is raised to the rated value through the sub-module voltage equalization control.

[0025] Step S102: When it is detected that both the hybrid commutator and the modular multilevel converter have reached their operating voltage, the hybrid commutator and the modular multilevel converter are sequentially controlled to be in the unlocked state.

[0026] Once it is detected that both the hybrid commutator (HCC) of the receiving-end grid and the modular multilevel converter (MMC) of the sending-end grid have reached their rated operating voltages, the hybrid commutator is first unlocked, and the auxiliary uncontrolled rectifier provides it with the minimum operating current to maintain low-voltage steady-state operation. After the hybrid commutator has stabilized, the modular multilevel converter is then unlocked to establish a stable AC voltage for the passive renewable energy islanded grid of the sending-end grid.

[0027] Step S103: Control the modular multilevel converter and the hybrid commutation converter to transmit the electrical energy output from the sending-end isolated grid to the receiving-end grid.

[0028] Modular multilevel converters operate with grid-type control, constructing a stable AC voltage and balancing the active and reactive power of the power generation equipment, converting the AC power generated by the power generation equipment into DC power; hybrid phase-commutation converters operate with constant DC voltage control as the main method and constant minimum DC current and constant minimum turn-off angle control as auxiliary methods, inverting DC power into AC power and sending it into the receiving-end grid; when the output of new energy is low, the uncontrolled rectifier automatically starts to supplement the current, avoiding the discontinuous DC current, and realizing the stable transmission of new energy power from the sending-end isolated grid to the receiving-end grid through the hybrid DC transmission system.

[0029] Optionally, step S101, which involves controlling the uncontrolled rectifier to sequentially supply power to the modular multilevel converter and the hybrid commutator after receiving power from the receiving-end grid, includes the following steps: Step S201: After the grid-side circuit breaker of the uncontrolled rectifier is in the closed state, the power supply from the receiving end grid is received by using the charging resistor in the AC side of the uncontrolled rectifier.

[0030] The grid-side circuit breaker of the uncontrolled rectifier is closed, connecting the uncontrolled rectifier to the receiving-end AC power grid. At this time, the AC-side charging resistor of the uncontrolled rectifier is in series connection. The charging resistor is used to limit the closing inrush current and avoid damage to the diode valve group and DC-side equipment of the uncontrolled rectifier. Subsequently, the receiving-end AC power grid supplies power to the uncontrolled rectifier through the charging resistor, starting the uncontrolled pre-charging process of the DC system and gradually raising the DC-side voltage.

[0031] Step S202: When the operating voltage of the hybrid DC transmission system is detected to reach the first operating voltage, the bypass switch of the uncontrolled rectifier is closed to complete the uncontrolled power supply from the uncontrolled rectifier to the hybrid DC transmission system.

[0032] The DC-side charging voltage of the uncontrolled rectifier is monitored in real time. When the voltage is detected to be stable and reach the first working voltage (such as the rated voltage of 0.9pu in a hybrid DC transmission system), the pre-charging is determined to be complete. The bypass switch of the uncontrolled rectifier is closed to short-circuit and disconnect the charging resistor, thereby reducing the system charging loss. The power supply mode is switched so that the uncontrolled rectifier directly supplies power to the modular multilevel converter. At the same time, the voltage equalization control strategy of the starting submodule of the modular multilevel converter is triggered to charge all submodule capacitors inside the MMC.

[0033] Step S203: Control the modular multilevel converter to enter the controllable charging state until the internal module voltage of the modular multilevel converter reaches the rated voltage value.

[0034] The modular multilevel converter is controlled to enter a controllable charging state and the capacitor voltage of each sub-module and the overall output voltage of the modular multilevel converter are continuously monitored. When the voltage is detected to meet the preset voltage equalization conditions (such as the voltage deviation of all sub-module capacitors of the modular multilevel converter ≤ ±3%, and the DC side output voltage of the modular multilevel converter stably reaches more than 95% of the rated voltage for a duration of ≥ 5s), the controllable charging of the modular multilevel converter is determined to be complete. At this time, the modular multilevel converter is controlled to supply power to the hybrid commutation converter through the DC transmission line, providing the rated pre-charge voltage for the valve group of the hybrid commutation converter before unlocking, so that the hybrid commutation converter enters the voltage preparation state for unlocking.

[0035] Optionally, step S102, which involves sequentially controlling the hybrid commutator and the modular multilevel converter to be in an unlocked state after detecting that both have reached their operating voltages, includes the following steps: Step S301: When the DC operating voltage in the hybrid commutator is detected to reach the second operating voltage, and the internal module voltage of the modular multilevel converter reaches the rated voltage value, the hybrid commutator is controlled to unlock, so that the hybrid commutator establishes a receiving-end power transmission channel with the receiving-end power grid through the corresponding smoothing reactor.

[0036] The DC operating voltage of the hybrid commutator at the receiving end is continuously monitored. When the voltage reaches the preset second operating voltage (i.e., the rated pre-charge voltage required for unlocking the hybrid commutator, close to the stable value of the rated DC voltage, ensuring that the hybrid commutator valve group has normal unlocking conditions) and the internal module voltage of the modular multilevel converter reaches the rated voltage value, the hybrid commutator is controlled to complete the unlocking operation. After unlocking, the hybrid commutator valve group enters a controllable operating state, establishing a stable receiving-end transmission channel with the receiving-end AC grid through its series smoothing reactor. At this time, the auxiliary uncontrolled rectifier continuously provides the hybrid commutator with the minimum operating current (0.1 pu) to maintain the hybrid commutator in low-voltage steady-state operation, avoiding system instability due to insufficient current after unlocking, and laying the foundation for subsequent unlocking of the modular multilevel converter.

[0037] Step S302: When the receiving end transmission channel is detected to be in the first steady state, the modular multilevel converter is controlled to unlock, so that the modular multilevel converter establishes a sending end transmission channel with the sending end isolated grid.

[0038] The system monitors the operating status of the receiving-end transmission channel in real time. When the channel is detected to be in the first steady state (i.e., the hybrid commutator is operating stably, the DC current is maintained within the minimum operating current range, the DC voltage fluctuation is ≤±2%, there are no commutation abnormalities, and the duration meets the preset threshold), it is determined that the receiving end is ready to receive power. The modular multilevel converter is then controlled to complete the unlocking operation. After the modular multilevel converter is unlocked, the grid construction control strategy is immediately activated to autonomously construct the AC voltage and frequency of the sending-end passive renewable energy islanded grid, balance the active and reactive power of the generating equipment, and establish a stable sending-end transmission channel with the sending-end islanded grid through its series smoothing reactor, preparing for the subsequent grid connection and power transmission of renewable energy units.

[0039] Optionally, step S103, which controls the modular multilevel converter and the hybrid commutation converter to transmit the electrical energy output from the sending-end islanded grid to the receiving-end grid, includes the following steps: Step S401: When the sending-end transmission channel is detected to be in the second steady state, the power generation equipment in the sending-end isolated grid is controlled to generate electricity in a predetermined batch.

[0040] When the sending-end transmission channel is detected to be in the second steady state (judgment criteria: stable operation of the modular multilevel converter after unlocking, AC side voltage and frequency reaching preset values, no voltage fluctuations, no current anomalies, meeting the conditions for new energy grid connection), the controller gradually controls the wind power, photovoltaic, and other power generation equipment in the sending-end isolated grid to start power generation according to the preset batch sequence. During the startup process, the power ramp-up rate is strictly controlled to avoid impact on the DC system due to sudden changes in power generation. At the same time, it ensures that the output power of the started power generation equipment is stable, laying the foundation for subsequent power transmission. Throughout the process, the DC current is maintained at no less than the minimum operating threshold to avoid current interruption.

[0041] Step S402: After the modular multilevel converter converts the electrical energy output by the power generation equipment into DC power, the DC power is transmitted to the hybrid commutator through the DC transmission line between the modular multilevel converter and the hybrid commutator.

[0042] The modular multilevel converter converts the AC power output from the generator started in step S401 into stable DC power output. During the conversion process, the modular multilevel converter relies on its grid control capabilities to maintain stable AC side voltage and frequency, while balancing fluctuations in renewable energy output to ensure that the output DC voltage and power meet system requirements. Subsequently, the converted DC power is smoothly transmitted to the hybrid commutation converter via a DC line. During the transmission process, a smoothing reactor is used to suppress current fluctuations and ensure the stability of power transmission.

[0043] Step S403: Control the hybrid commutator to convert the received DC power into AC power and then transmit the AC power to the receiving-end grid.

[0044] The hybrid commutator inverter converts received DC power into AC power that conforms to grid standards. During the inversion process, it strictly adheres to preset firing angle commands, combined with previously set minimum shutdown angle control, to ensure stable AC voltage and frequency after inversion. After inversion, the AC power is transmitted to the receiving-end AC grid, achieving the final absorption of renewable energy. Simultaneously, the grid's operating status is monitored in real time to ensure that power transmission matches the receiving-end grid load, avoiding problems such as voltage anomalies and power imbalances.

[0045] The control process in the above embodiment corresponds to the startup process. After startup, the modular multilevel converter, the uncontrolled rectifier, and the hybrid commutation converter adopt their respective operation control strategies to adjust the DC freewheeling current. Optionally, step S403, which controls the hybrid commutation converter to convert the received DC power into AC power and then transmits the AC power to the receiving-end power grid, includes the following steps: Step S501: Using the cut-off angle reference value, AC voltage measurement value, DC voltage reference value, DC voltage measurement value, DC current reference value, and DC current measurement value as conditional parameters, construct the operation control strategy of the converter valve in the hybrid commutation converter.

[0046] The hybrid commutation converter serves as the power balance node in a hybrid DC transmission system, employing constant DC voltage control as its primary operating mode during steady-state operation. Simultaneously, it is configured with constant minimum DC current control to limit the minimum allowable steady-state current of the converter, and constant minimum turn-off angle control to limit the minimum allowable steady-state turn-off angle of the converter. Specifically, the DC voltage setpoint U... dref =1.0pu, minimum DC current setpoint I dref =0.1pu, minimum shut-off angle setpoint γ ref At a constant angle of 10°, the operating characteristics of the receiving-end HCC are as follows: Figure 3 As shown.

[0047] Combination Figure 3 The control logic of the hybrid commutation converter uses the cut-off angle reference value, AC voltage measurement value, DC voltage reference value, DC current reference value, and DC current measurement value as core condition parameters to construct the operation control strategy of the hybrid commutation converter, focusing on determining the trigger angle adjustment rules of the converter valve. Specifically, parameters such as the cut-off angle reference value, AC voltage measurement value, DC voltage reference value, DC current reference value, and DC current measurement value are incorporated into the control model. Through a closed-loop adjustment algorithm, the trigger angle calculation logic under different operating conditions (normal output, low load) is clarified to ensure the accuracy of converter valve conduction and cut-off, laying the foundation for subsequent inverter conversion, while avoiding the risk of commutation failure. Figure 3 The control logic of the hybrid commutator remains consistent.

[0048] Step S502: Use the operation control strategy to control the hybrid commutation converter to convert the received DC power into AC power.

[0049] Using the operation control strategy constructed in step S501, the switching on and off of the converter valves of the hybrid commutation converter are controlled to invert the received DC-side power (active power from renewable energy sources) into AC output power that conforms to grid standards. During the conversion process, the AC-side voltage amplitude and frequency, as well as the DC-side voltage and current, are monitored in real time. By adjusting the trigger angle, the DC voltage is maintained stable within the rated range, while simultaneously matching the fluctuations in renewable energy output to ensure that the frequency and voltage of the converted AC power meet the grid connection requirements of the receiving end, thus avoiding problems such as commutation failure and excessive voltage fluctuations. Figure 3 The control curve (voltage-current correlation curve) of the hybrid commutation converter is accurately matched.

[0050] Step S503: Control the hybrid commutation converter to transmit AC power to the receiving-end power grid through the sending-end transmission channel.

[0051] The control system of the hybrid commutator converts the inverted AC power into AC power through a previously established transmission channel (and...). Figure 3 The power is transmitted to the receiving-end AC grid via a DC-to-AC link. During transmission, the grid frequency and voltage stability are monitored in real time. Based on load changes in the receiving-end grid, the output amplitude of the AC power is dynamically adjusted to ensure stable power delivery. Simultaneously, the uncontrolled rectifier is activated in conjunction with the grid. If voltage or current abnormalities occur, the backup power supply is activated in a timely manner to ensure continuous and stable power delivery to the receiving-end grid, achieving the final absorption of new energy power and maintaining consistency with the overall system's power balance requirements.

[0052] Optionally, step S501, which constructs the operation control strategy for the converter valve in a hybrid commutator using the cut-off angle reference value, AC voltage measurement value, DC voltage reference value, DC voltage measurement value, DC current reference value, and DC current measurement value as conditional parameters, includes the following steps: Step S601: Calculate the first maximum firing angle allowed for the converter valve in the hybrid commutator under constant voltage control using the cut-off angle reference value and AC voltage measurement value.

[0053] To achieve the above operating characteristics, the operational control principle diagram illustrating the coordination relationship between the constant DC voltage controller, constant minimum DC current controller, and constant minimum turn-off angle controller in the hybrid commutation converter is shown below. Figure 4 As shown in the figure. The outputs of the three controllers are all converter trigger angle reference values. The angle reference value output by the minimum shut-off angle controller serves as the upper limit for the angle reference value calculated and output by the constant DC voltage controller. The angle reference value output by the constant DC voltage controller serves as the upper limit for the angle reference value calculated and output by the minimum DC current controller. The angle reference value output by the minimum DC current controller is the final trigger angle command value used to control the converter valve triggering.

[0054] Using a preset shut-off angle reference value and real-time measured AC voltage at the receiving end as input parameters, the data is fed into a minimum shut-off angle controller for closed-loop calculation. Combined with commutation characteristics, the first maximum firing angle allowed for the converter valve in the constant DC voltage control loop of the hybrid commutation converter is calculated. This first maximum firing angle serves as a top-level limit, its core function being to lock the upper limit of the firing angle at the hardware level, ensuring that the actual shut-off angle of the converter is not lower than the set minimum value, thus mitigating the risk of commutation failure under grid disturbances at the receiving end. Simultaneously, this limit is issued as an upper limit constraint for the output saturation of the subsequent constant DC voltage controller.

[0055] Step S602: Using the first maximum firing angle as a constraint, calculate the second maximum firing angle allowed for the converter valve under constant current control using the DC voltage reference value and the DC voltage measurement value.

[0056] The first maximum firing angle obtained in step S601 is used as a hard boundary constraint. Then, the reference setpoint of DC voltage and the real-time measured value of DC voltage are introduced and sent to the constant DC voltage controller for deviation closed-loop adjustment. The second maximum firing angle allowed by the converter valve in the constant minimum DC current control loop is calculated and solved.

[0057] This stage, without exceeding the turn-off angle limit of the preceding stage, aims to maintain DC voltage stability and dynamically limits the adjustment range of the lower-level controller. The output second maximum trigger angle serves as the upper limit of the saturation of the minimum DC current controller, thereby achieving amplitude limiting and isolation at the voltage control level.

[0058] Step S603: Using the second maximum firing angle as a constraint, calculate the final firing angle of the hybrid commutation converter using the DC current reference value and the DC current measurement value, and construct an operation control strategy based on the first maximum firing angle, the second maximum firing angle and the final firing angle.

[0059] Using the second maximum firing angle obtained in step S602 as the constraint boundary, a DC current reference setpoint and a real-time DC current measurement value are introduced. The current closed-loop calculation is performed by a minimum DC current controller to calculate the final firing angle command of the hybrid commutator converter valve.

[0060] Based on this, relying on Figure 4 The three-level progressive limiting architecture integrates the hierarchical constraints of the first and second maximum firing angles and the final firing angle obtained from closed-loop calculations to construct a complete converter valve operation control strategy: under normal operating conditions, the constant DC voltage controller dominates the regulation; under high-power operating conditions, the first maximum firing angle limits the regulation and the minimum turn-off angle constraint dominates the regulation; under low-output operating conditions, the second maximum firing angle limits the regulation and the constant minimum DC current controller provides a safety net. This ensures both stable DC voltage and current while taking into account the minimum turn-off angle constraint, thus avoiding commutation failure and DC current discontinuity throughout the entire process.

[0061] Optionally, step S402, which involves controlling the modular multilevel converter to convert the electrical energy output from the power generation equipment into DC power, and then transmitting the DC power to the hybrid commutation converter via the DC transmission line between the modular multilevel converter and the hybrid commutation converter, includes the following steps: Step S701: Using a preset AC voltage command, control the modular multilevel converter to establish AC voltage in the sending-end isolated grid in grid construction control mode, and control the modular multilevel converter to convert the AC power corresponding to the AC voltage into DC power.

[0062] Total output P of power generation equipment in the sending-end isolated grid dsource When the voltage is 1.0 pu, the operating characteristics of the DC side of the modular multilevel converter are as follows: Figure 5 As shown. Based on preset AC voltage amplitude and frequency commands, network control is implemented for the modular multilevel converter at the sending end. Combined with... Figure 5 The active and reactive power decoupling control logic shown adjusts the AC voltage amplitude through the reactive component and the AC system frequency through the active component, enabling the modular multilevel converter to autonomously establish a target AC voltage with stable amplitude and frequency in the passive islanded grid without the support of a large power grid. This provides reliable grid-connected voltage support for new energy power generation equipment such as wind power and photovoltaics, and balances the active and reactive power demands within the collection system.

[0063] Specifically, the controller calls preset AC voltage amplitude and frequency commands to control the modular multilevel converter at the sending end to enter the grid-connected control mode. In the isolated grid of new energy sources at the sending end without external grid support, it autonomously establishes an AC voltage with stable amplitude and frequency, providing a voltage reference for grid-connected operation of wind power, photovoltaic and other power generation equipment.

[0064] Combination Figure 5 The illustrated operating characteristics of the sending-end modular multilevel converter demonstrate that the modular multilevel converter achieves AC voltage stability through active / reactive power decoupling control: the active component regulates the AC voltage frequency, and the reactive component regulates the AC voltage amplitude. Furthermore, the modular multilevel converter converts the AC power output from the generator into DC power. The magnitude of this DC power dynamically changes with the total output of the renewable energy unit. The voltage and current on the DC side of the modular multilevel converter are not directly controlled by its own controller, but are jointly determined by the operating states of both the AC and DC sides, achieving a perfect match. Figure 5 Operating characteristics of the DC side of a modular multilevel converter.

[0065] Step S702: Transmit DC power to the hybrid commutator using the DC transmission line between the modular multilevel converter and the hybrid commutator.

[0066] After completing the AC-DC conversion, the modular multilevel converter continuously outputs stable DC power. This DC power is then transmitted to the receiving-end hybrid phase-commutation converter via a DC transmission line and smoothing reactor, forming the receiving-end transmission channel. This provides continuous and stable DC power input for subsequent receiving-end inverter grid connection, ensuring continuous power transmission throughout the entire hybrid DC transmission link. During transmission, smoothing reactors connected in series at both ends of the DC line suppress DC current ripple and surges, preventing current fluctuations from damaging the converter valves of the hybrid phase-commutation converter. Simultaneously, the system's global monitoring module tracks in real-time whether the DC current is approaching the minimum operating threshold (0.1 pu). If the power output of the generator in the sending-end isolated grid decreases, causing the current to approach the threshold, a supplementary current preparation command is immediately sent to the uncontrolled rectifier, ensuring continuous current supply under low-output conditions and guaranteeing the continuity and stability of DC power transmission.

[0067] In the above process, the modular multilevel converter at the sending end adopts grid-based control to construct a stable AC voltage for the receiving end of the renewable energy source, balancing the active and reactive power of the renewable energy collection and boosting system. The active and reactive components in the control loop of the modular multilevel converter are used to control the frequency and amplitude of its AC grid-side voltage, respectively. Therefore, the voltage and current on the DC side of the modular multilevel converter are not directly controlled by its controller, but are determined jointly by the operating states of both the AC and DC sides. Without considering system losses, the DC voltage of the modular multilevel converter is stabilized at a given value by the hybrid commutation converter at the receiving end, while the DC power is always equal to the total output of the renewable energy units collected on the AC side, fluctuating randomly with changes in wind, solar, and weather conditions. Considering system losses, each of the above links is accompanied by certain active power losses. The DC voltage of the modular multilevel converter is slightly higher than the control voltage of the hybrid commutation converter, and the active power sent from the hybrid commutation converter to the AC grid is slightly lower than the total active power fed into the AC side of the modular multilevel converter.

[0068] The uncontrolled diode rectifier at the receiving end serves as an auxiliary converter, with the following functions: providing minimum operating current to the hybrid DC transmission system when the renewable energy output is too low, preventing intermittent operation of the hybrid commutation converter; and providing charging power to the modular multilevel converter during system startup. The uncontrolled rectifier consists of a three-phase diode rectifier bridge and corresponding rectifier transformers. Besides being switched on and off via circuit breakers on the grid side of the rectifier transformers, it requires no controller control. Therefore, the above functions are actually achieved through the electrical design coordination of the uncontrolled rectifier. To coordinate with the MMC-HCC hybrid DC transmission system, the design and operating characteristics of the uncontrolled rectifier are as follows: Figure 6As shown. Optionally, after controlling the modular multilevel converter and the hybrid commutator to transmit the electrical energy output from the sending-end isolated grid to the receiving-end grid, the method further includes: when the output power of the DC power is detected to be lower than a preset power threshold, using the power difference between the power threshold and the output power to enable the uncontrolled rectifier to supply power to the hybrid commutator.

[0069] Once the hybrid DC transmission system enters a stable transmission state, the DC output power transmitted from the modular multilevel converter to the hybrid commutator is continuously monitored. This power is calculated by multiplying the DC side voltage measurement value and the current measurement value. The monitoring frequency must meet the real-time requirements (typical monitoring period ≤ 50ms). Power threshold setting: The preset power threshold is determined by the product of the rated DC voltage and the minimum operating current. This threshold is the critical power value for the DC system to avoid intermittent operation. Threshold comparison and instruction generation: When the DC output power is detected to be continuously lower than the preset power threshold, it is determined that the output of the new energy source at the sending end is insufficient and there is a risk of DC current discontinuity. At this time, based on the power difference between the power threshold and the current actual output power, a power supply control instruction for the hybrid commutator converter is generated. The instruction includes: confirmation of the grid-side circuit breaker closing status of the uncontrolled rectifier, charging resistor bypass instruction, and output current target value (to meet the compensation current requirement of the power difference), ensuring that the compensation current accurately fills the gap.

[0070] Command execution preparation: After receiving the power supply control command, the uncontrolled rectifier first confirms that it is in hot standby mode. If the charging resistor is in the connected state, it immediately sends a bypass switch closing command to reduce energy loss during the current replenishment process. Precise current compensation control: The uncontrolled rectifier draws power from the receiving end AC grid, rectifies the AC power into DC power through an internal diode valve group, and supplies power to the hybrid commutator according to the output current target value set by the command, to make up for the DC power gap caused by insufficient output of new energy sources; during the current compensation process, the DC side current is monitored in real time to ensure that the current is maintained above the minimum operating current (0.1pu), completely avoiding the risk of DC interruption; Working condition linkage exit: combined with Figure 6 The closed-loop control logic automatically generates a current replenishment exit command when the DC output power is detected to rise above the preset power threshold and run stably for ≥5s. This command controls the uncontrolled rectifier to gradually reduce the output current to zero and restore the standby state, thus avoiding excessive current replenishment that could cause system power redundancy.

[0071] When the power output of the generators in the isolated grid at the sending end decreases, and the DC power fed into the sending end is insufficient to maintain the minimum DC operating current of the hybrid commutator, the power absorbed by the hybrid commutator from the hybrid DC transmission system will exceed the power fed into the sending end due to the action of the minimum DC current controller of the hybrid commutator. This causes the DC voltage to drop. When the DC voltage drops to the operating range of the uncontrolled rectifier, the uncontrolled rectifier begins to output DC current. To maintain the minimum operating current of the hybrid commutator, the rated DC current of the uncontrolled rectifier is designed to be 0.1 pu of the rated current of the hybrid DC transmission system, and the DC voltage at the rated output current is designed to be the minimum operating voltage of the hybrid DC transmission system, such as 0.8 pu.

[0072] The operating characteristics of the three converters work together to obtain a schematic diagram of the system's stable operating point when the renewable energy power generation is at any level between 0.1 pu and 1.0 pu, as shown in the figure. Figure 7 As shown in the figure; a schematic diagram of the stable operating point of the hybrid DC transmission system when the renewable energy power generation is less than 0.1 pu (taking an output of 0.05 pu as an example) is shown in the figure. Figure 8 As shown.

[0073] It should be noted that, Figure 3 , 5 U involved in 6, 7, and 8 dref =1.0pu、I dref =0.1pu、γ ref =10°, P dsource =1.0pu, and uncontrolled rectifier U d0 =0.9pu、U dN =0.8pu、I dN The value of 0.1pu is only an exemplary set of specific implementation methods. In actual engineering design and control command tuning, the above parameters can be set to any reasonable combination, and the resulting combination schemes should be within the protection scope of this invention.

[0074] As can be seen from the control method of the above-mentioned hybrid DC transmission system, this method adopts a hybrid DC transmission system of sending-end MMC + receiving-end HCC + uncontrolled rectifier. It combines the grid construction control capability of MMC and the ability of HCC to resist commutation failure. The uncontrolled rectifier is used to precisely adjust the charging demand of sending-end MMC and receiving-end HCC, which solves the problems of DC start-up of passive new energy isolated grid and low-output DC discontinuity. It has both grid construction and commutation failure prevention capabilities, is suitable for large-capacity long-distance transmission, and has better economic efficiency.

[0075] This invention also provides a hybrid DC transmission system, such as... Figure 2As shown, the hybrid DC transmission system is located between the sending-end islanded grid and the receiving-end grid, and includes: a controller 100, a modular multilevel converter 200, an uncontrolled rectifier 300, and a hybrid phase-commutation converter 400; wherein, the controller 100 is connected to the modular multilevel converter 200, the uncontrolled rectifier 300, and the hybrid phase-commutation converter 400 respectively; the modular multilevel converter 200 and the hybrid phase-commutation converter 400 are connected through DC transmission lines; One end of the modular multilevel converter 200 is connected to the sending-end isolated grid, and the other end of the modular multilevel converter 200 is connected to one end of the hybrid phase-commutation converter 400 through a DC transmission line; the other end of the hybrid phase-commutation converter 400 is connected to the receiving-end grid; the uncontrolled rectifier 300 is connected in parallel with the hybrid phase-commutation converter 400 to the receiving-end grid; In the process of controlling the sending-end islanded grid to transmit the output DC power to the receiving-end grid, the hybrid DC transmission system adopts the control method of the hybrid DC transmission system mentioned in the above embodiments.

[0076] As can be seen from the above hybrid DC transmission system, this system adopts a hybrid DC transmission system of sending-end MMC + receiving-end HCC + uncontrolled rectifier. It combines the grid control capability of MMC with the ability of HCC to resist commutation failure. The uncontrolled rectifier is used to precisely adjust the charging demand of sending-end MMC and receiving-end HCC, which solves the problems of DC start-up of passive new energy isolated grid and low-output DC discontinuity. It has both grid construction and commutation failure prevention capabilities, is suitable for large-capacity long-distance transmission, and has better economic efficiency.

[0077] The hybrid DC transmission system provided in this embodiment of the invention has the same implementation principle and technical effects as the control method embodiment of the aforementioned hybrid DC transmission system. For the sake of brevity, any parts not mentioned in this embodiment can be referred to the corresponding content in the control method embodiment of the aforementioned hybrid DC transmission system.

[0078] This embodiment also provides a controller, the structural schematic diagram of which is shown below. Figure 9 As shown, the device includes a processor 101 and a memory 102; wherein, the memory 102 is used to store one or more computer instructions, which are executed by the processor to implement the steps of the control method of the hybrid DC transmission system described above.

[0079] Figure 9 The controller shown also includes a bus 103 and a communication interface 104. The processor 101, the communication interface 104, and the memory 102 are connected via the bus 103.

[0080] The memory 102 may include high-speed random access memory (RAM) and may also include non-volatile memory, such as at least one disk storage device. The bus 103 may be an ISA bus, PCI bus, or EISA bus, etc. The bus can be divided into address bus, data bus, control bus, etc. For ease of representation, Figure 9 The symbol is represented by a single double-headed arrow, but this does not mean that there is only one bus or one type of bus.

[0081] The communication interface 104 is used to connect to at least one user terminal and other network units through a network interface, and to send encapsulated IPv4 packets or IPv4 packets to the user terminal through the network interface.

[0082] Processor 101 may be an integrated circuit chip with signal processing capabilities. In implementation, each step of the above method can be completed by the integrated logic circuitry in the hardware of processor 101 or by instructions in software form. The processor 101 can be a general-purpose processor, including a Central Processing Unit (CPU), a Network Processor (NP), etc.; it can also be a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field-Programmable Gate Array (FPGA), or other programmable logic devices, discrete gate or transistor logic devices, or discrete hardware components. It can implement or execute the methods, steps, and logic block diagrams disclosed in the embodiments of this disclosure. The general-purpose processor can be a microprocessor or any conventional processor. The steps of the methods disclosed in the embodiments of this disclosure can be directly manifested as execution by a hardware decoding processor, or execution by a combination of hardware and software modules in the decoding processor. The software module can reside in a mature storage medium in the art, such as random access memory, flash memory, read-only memory, programmable read-only memory, electrically erasable programmable memory, or registers. This storage medium is located in memory 102. The processor 101 reads the information in memory 102 and, in conjunction with its hardware, completes the steps of the method described in the foregoing embodiments.

[0083] This invention also provides a storage medium storing a computer program, which, when executed by a processor, performs the steps of the control method for the hybrid DC transmission system described in the foregoing embodiments.

[0084] In the several embodiments provided in this application, it should be understood that the disclosed systems, apparatuses, devices, and methods can be implemented in other ways. The system embodiments described above are merely illustrative. For example, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. Furthermore, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Additionally, the coupling or direct coupling or communication connection shown or discussed may be through some communication interfaces; the indirect coupling or communication connection between devices or units may be electrical, mechanical, or other forms.

[0085] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0086] In addition, the functional units in the various embodiments of the present invention can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit.

[0087] If the aforementioned functions are implemented as software functional units and sold or used as independent products, they can be stored in a processor-executable, non-volatile, computer-readable storage medium. Based on this understanding, the technical solution of this invention, essentially, or the part that contributes to the prior art, or a portion of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, electronic device, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of this invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0088] Finally, it should be noted that the above-described embodiments are merely specific implementations of the present invention, used to illustrate the technical solutions of the present invention, and not to limit it. The scope of protection of the present invention is not limited thereto. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that any person skilled in the art can still modify or easily conceive of changes to the technical solutions described in the foregoing embodiments within the technical scope disclosed in the present invention, or make equivalent substitutions for some of the technical features; and these modifications, changes, or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A control method for a hybrid DC transmission system, characterized in that, The method is applied in a controller for a hybrid DC transmission system; wherein the hybrid DC transmission system is located between the sending-end islanded grid and the receiving-end grid, and includes a modular multilevel converter, an uncontrolled rectifier, and a hybrid phase-commutation converter; the method includes: After the uncontrolled rectifier receives power from the receiving-end grid, it sequentially supplies power to the modular multilevel converter and the hybrid commutation converter. When it is detected that both the hybrid commutator and the modular multilevel converter have reached their operating voltage, the hybrid commutator and the modular multilevel converter are sequentially controlled to be in an unlocked state. The modular multilevel converter and the hybrid commutation converter are controlled to transmit the electrical energy output from the sending-end isolated grid to the receiving-end grid.

2. The control method for a hybrid DC transmission system according to claim 1, characterized in that, The steps of controlling the uncontrolled rectifier to receive power from the receiving-end grid and then sequentially supplying power to the modular multilevel converter and the hybrid commutation converter include: After the grid-side circuit breaker of the uncontrolled rectifier is in the closed state, the power supply of the receiving-end grid is received by the charging resistor of the uncontrolled rectifier on the AC side. When the operating voltage of the hybrid DC transmission system is detected to reach the first operating voltage, the bypass switch of the uncontrolled rectifier is controlled to be in the closed state, so as to complete the uncontrolled power supply from the uncontrolled rectifier to the hybrid DC transmission system. The modular multilevel converter is controlled to enter a controllable charging state until the internal module voltage of the modular multilevel converter reaches the rated voltage value.

3. The control method for a hybrid DC transmission system according to claim 1, characterized in that, When it is detected that both the hybrid commutator and the modular multilevel converter have reached their operating voltage, the steps of sequentially controlling the hybrid commutator and the modular multilevel converter to be in an unlocked state include: When the DC operating voltage in the hybrid phase-commutation converter reaches the second operating voltage and the internal module voltage of the modular multilevel converter reaches the rated voltage value, the hybrid phase-commutation converter is unlocked, so that the hybrid phase-commutation converter establishes a receiving-end power transmission channel with the receiving-end power grid through the corresponding smoothing reactor. When the receiving-end transmission channel is detected to be in the first steady state, the modular multilevel converter is controlled to unlock, so that the modular multilevel converter establishes a sending-end transmission channel with the sending-end isolated grid.

4. The control method for a hybrid DC transmission system according to claim 3, characterized in that, The steps of controlling the modular multilevel converter and the hybrid commutation converter to transmit the electrical energy output from the sending-end isolated grid to the receiving-end grid include: When the power transmission channel at the sending end is detected to be in the second steady state, the power generation equipment in the isolated grid at the sending end is controlled to generate electricity in a preset batch. After the modular multilevel converter converts the electrical energy output by the power generation equipment into DC power, the DC power is transmitted to the hybrid phase converter through the DC transmission line between the modular multilevel converter and the hybrid phase converter. After the hybrid commutator converts the received DC power into AC power, it transmits the AC power to the receiving-end power grid.

5. The control method for a hybrid DC transmission system according to claim 4, characterized in that, The step of controlling the hybrid commutator to convert the received DC power into AC power and then transmitting the AC power to the receiving-end power grid includes: Using the cutoff angle reference value, AC voltage measurement value, DC voltage reference value, DC voltage measurement value, DC current reference value, and DC current measurement value as conditional parameters, an operation control strategy for the converter valve in the hybrid commutation converter is constructed. The operation control strategy is used to control the hybrid commutation converter to convert the received DC power into AC power; The hybrid phase converter is controlled to transmit the AC power to the receiving-end power grid through the sending-end transmission channel.

6. The control method for a hybrid DC transmission system according to claim 5, characterized in that, The steps for constructing the operation control strategy for the converter valves in the hybrid commutator, using the cutoff angle reference value, AC voltage measurement value, DC voltage reference value, DC voltage measurement value, DC current reference value, and DC current measurement value as conditional parameters, include: The first maximum firing angle allowed by the converter valve in the hybrid commutator under constant voltage control is calculated using the cut-off angle reference value and the AC voltage measurement value. Using the first maximum firing angle as a constraint, the second maximum firing angle allowed by the converter valve under constant current control is calculated using the DC voltage reference value and the DC voltage measurement value. Using the second maximum firing angle as a constraint, the final firing angle of the hybrid commutator is calculated using the DC current reference value and the DC current measurement value, and the operation control strategy is constructed based on the first maximum firing angle, the second maximum firing angle and the final firing angle.

7. The control method for a hybrid DC transmission system according to claim 4, characterized in that, The steps of controlling the modular multilevel converter to convert the electrical energy output by the power generation equipment into DC power, and then transmitting the DC power to the hybrid commutation converter via the DC transmission line between the modular multilevel converter and the hybrid commutation converter, include: The modular multilevel converter is controlled by a preset AC voltage command to establish an AC voltage in the sending-end isolated grid in a grid-building control mode, and the modular multilevel converter is controlled to convert the AC power corresponding to the AC voltage into DC power. The DC power is transmitted to the hybrid commutator via a DC transmission line between the modular multilevel converter and the hybrid commutator.

8. The control method for a hybrid DC transmission system according to claim 4, characterized in that, After controlling the modular multilevel converter and the hybrid commutation converter to transmit the electrical energy output from the sending-end isolated grid to the receiving-end grid, the method further includes: When the output power of the DC power is detected to be lower than a preset power threshold, the power difference between the power threshold and the output power is used to enable the uncontrolled rectifier to supply power to the hybrid commutator.

9. A hybrid DC transmission system, characterized in that, The hybrid DC transmission system is located between the sending-end isolated grid and the receiving-end grid, and includes: a controller, a modular multilevel converter, an uncontrolled rectifier, and a hybrid phase-commutation converter; wherein, the controller is connected to the modular multilevel converter, the uncontrolled rectifier, and the hybrid phase-commutation converter respectively; the modular multilevel converter and the hybrid phase-commutation converter are connected through a DC transmission line; One end of the modular multilevel converter is connected to the sending-end isolated grid, and the other end of the modular multilevel converter is connected to one end of the hybrid phase-commutation converter through the DC transmission line; the other end of the hybrid phase-commutation converter is connected to the receiving-end grid; the uncontrolled rectifier is connected in parallel with the hybrid phase-commutation converter to the receiving-end grid; The hybrid DC transmission system employs the control method of any one of claims 1 to 8 in the process of controlling the sending-end isolated grid to transmit the output DC power to the receiving-end grid.

10. A controller, characterized in that, The controller includes a processor and a memory, the memory storing computer-executable instructions that can be executed by the processor, the processor executing the computer-executable instructions to implement the steps of the control method for the hybrid DC transmission system according to any one of claims 1 to 8.

Images