Control method of LCC-full bridge MMC series connection type light HVDC converter station

Abstract

The invention relates to the technical field of high-voltage direct-current transmission, and particularly provides an LCC-full-bridge MMC series light HVDC converter station control method, which comprises the following steps: a rectification side of a power grid commutation converter adjusts direct-current power through an outer ring and an inner ring adopts linear trigger angle control to generate a trigger pulse; transformer tap adjustment is supplemented; on the basis of the established stable direct current system, an inversion side selects a minimum trigger angle as a control signal by coordinating three control rings of a turn-off angle, direct current voltage and direct current to form a control system; the full-bridge modularized multi-level converter is switched to a power grid following and network construction control mode according to power grid conditions; and when a direct current fault is detected, setting the trigger angle of the commutation converter of the power grid to be 90 degrees and switching the full-bridge modular multi-level converter to an STATCOM mode, thereby realizing direct current fault ride-through without locking the converter. According to the invention, direct-current fault rapid ride-through of the non-locking converter is realized.

Description

Technical Field

[0001] This invention relates to the field of high-voltage direct current (HVDC) transmission technology, and particularly to a control method for a lightweight HVDC converter station with LCC-full-bridge MMC series connection. Specifically, it relates to a lightweight (hybrid HVDC transmission) converter station and its control method that connects a line-commutated converter (LCC) and a full-bridge modular multilevel converter (MMC) in series, possessing DC fault ride-through capability. Background Technology

[0002] In existing technologies, long-distance, high-capacity power transmission mainly adopts thyristor-based LCC-HVDC or voltage source converter-based VSC-HVDC. LCC-HVDC has advantages such as low loss and low cost, but it has the risk of commutation failure and requires a large number of reactive power compensation devices. VSC-HVDC (such as full-bridge modular multilevel converter) can independently control active / reactive power, but due to the large number of modules, it has high volume, weight and cost, which is not conducive to space-constrained occasions such as offshore platforms. In order to balance the economy of grid commutation converters and the flexibility of voltage source converters, existing technologies have proposed a hybrid topology that connects grid commutation converters and VSC (or diode rectifiers) in parallel or series, such as the diode rectifier + MMC offshore solution proposed by Siemens (without black start capability); and parallel full-bridge modular multilevel converter active filters (requiring auxiliary power supply pre-charging), etc. However, none of the aforementioned existing technologies have solved the following technical problems: significantly reducing the size / weight of the converter station while ensuring black-start capability; and quickly suppressing the fault current without blocking the trigger pulse during DC line short-circuit faults, thus achieving DC fault ride-through. Therefore, a new topology and supporting control strategy are urgently needed to overcome these shortcomings.

[0003] Prior art 1, Chinese Patent Application No. 202411667347.1, discloses a seamless switching method for grid-connected and grid-linked control of MMC-HVDC converter stations. Without changing the inner loop current control, it ensures the continuity of the output voltage's power angle and amplitude during control mode switching by constructing a unified initialization setting for the outer loop reference current, phase angle, and voltage amplitude. While it can quickly respond to changes in grid conditions and avoid system disturbances and instabilities caused by mode switching, simulation results show that this technology effectively reduces the impact of control switching on the system, significantly improves system stability and reliability, and is suitable for flexible DC transmission systems under various grid conditions, especially in scenarios with weak grid access and frequency support. However, it mainly targets flexible DC transmission systems (MMC-HVDC) and does not address control strategies for LCC-MMC hybrid HVDC systems, nor can it effectively address commutation failure risks and DC fault ride-through problems.

[0004] Prior art two, Chinese patent application number 202411500486.5, discloses an optimized control method and system suitable for SLCC high-voltage DC systems, including: determining the topology and control strategy of the converter stations on the rectifier and inverter sides of the multi-source adaptive commutator-high-voltage DC system (SLCC-HVDC); based on the topology and control strategy, detecting the parameters of the converter transformer in the SLCC-HVDC system and obtaining the detection results; calculating the detection results to determine the reference value of the three-phase internal potential of the SVG in the LCC-HVDC system; and using the reference value of the three-phase internal potential of the SVG as the optimized control parameter to optimize the control of the SLCC-HVDC system. Although optimizing the control parameters improves the recovery speed of SLCC DC power after a fault and effectively enhances the AC fault ride-through capability of the SLCC, it fails to fully utilize the flexible adjustment capability of the MMC itself and does not involve a coordinated control mechanism between the rectifier and inverter sides, making it difficult to maintain stable system operation during DC side faults.

[0005] Prior art three, Chinese patent application number 202410760491.3, discloses a frequency stability control method, device, and medium for offshore wind farms connected to MMC-HVDC grids. Based on a control model of the second receiving-end converter station, the second grid controls the frequency of the first grid by adjusting active power. The control model is established based on a mathematical model of DC voltage reference values ​​and a frequency stability control model. The DC voltage reference value mathematical model is established by analogy and equivalence processing of the rotor motion mode and the operation mode of the first receiving-end converter station. The frequency stability control model is established based on the droop characteristics of the first grid and the grid's control mode. Although considering parameters of different dimensions to establish the control model, and utilizing the second grid to provide additional active power and inertial support to the first grid, can solve the problem of frequency stability when offshore wind farms are connected to MMC-HVDC grids, it cannot solve the DC voltage stability and commutation failure protection problems of LCC-MMC hybrid HVDC systems, and it does not address DC fault ride-through strategies.

[0006] Currently, existing technologies 1, 2, and 3 lack a clear control strategy for LCC-MMC hybrid HVDC, and the system's stability, fault tolerance, and dynamic response performance need further improvement. Therefore, this invention provides a control method for an LCC-full-bridge MMC series lightweight HVDC converter station. Summary of the Invention

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] In one aspect, the present invention provides a control method for an LCC-full-bridge MMC series-type lightweight HVDC converter station, comprising the following steps:

[0009] The rectifier side of the grid-commutated converter uses an outer loop to regulate DC power and an inner loop to use linearized trigger angle control to generate trigger pulses; supplemented by transformer tap regulation, it processes to obtain a DC system with stable power and responsiveness to the inverter side's needs;

[0010] Based on the established stable DC system, the inverter side coordinates the three control loops of turn-off angle, DC voltage and DC current, selects the minimum firing angle as the control signal, and forms a control system that is centered on preventing commutation failure, can maintain DC voltage stability and ensure system operation when the rectifier side is out of control.

[0011] The full-bridge modular multilevel converter switches between grid following and grid construction control modes according to grid conditions. When a DC fault is detected, the trigger angle of the grid-commutated converter is set to 90 degrees and the full-bridge modular multilevel converter is switched to STATCOM mode to achieve DC fault ride-through without blocking the converter.

[0012] In one optional implementation, the outer loop control of the grid-commutated converter rectifier side control detects the DC power P in real time. DC and the set value P DC ref The error signal is compared and sent to the PI controller, whose output serves as the DC current reference value I. DC ref Actual DC current I DC rec After filtering and I DC ref The error signal is then processed again by the PI controller, which outputs an initial adjustment to the firing angle. A linearization module is implemented to compensate for the nonlinear relationship between the firing angle α and the DC voltage. The phase-locked loop monitors the AC bus voltage V in real time. a V b V c Extract phase information φ PLL Based on the phase-locked loop phase and firing angle α, a firing pulse sequence is generated within a 60° phase interval to control the thyristor's conduction time; α r After filtering, the step-up / step-down logic circuit determines the result based on α. r With α r ref The deviation is detected, and a tap change command is output to adjust the transformer tap position, thereby achieving precise control of the DC voltage.

[0013] In one optional implementation, the DC current IDC is monitored in real time in the inverter side control of the grid-commutated converter and compared with the set DC current lower limit IDC margin; the error signal is processed by the PI controller and outputs the trigger angle adjustment amount αiC; similarly, the DC voltage, AC voltage, and turn-off angle γ are detected and compared; the trigger angle adjustment amounts αiC, αiV, and αiγ output by the control loop are compared, and the smallest trigger angle is selected as the trigger angle αi; the phase-locked loop monitors the AC bus voltages Va, Vb, and Vc in real time and extracts the phase information φPLL; according to the phase of the phase-locked loop and the trigger angle αi, a trigger pulse sequence is generated within a 60° phase interval to control the conduction time of the thyristor; after αi is filtered, the step-up logic circuit outputs a tap change command according to the deviation between αi and αiref to adjust the transformer tap position and regulate the DC voltage.

[0014] In one optional implementation, the normal operation control of the full-bridge modular multilevel converter includes grid following control and grid construction control. In grid following control, the phase-locked loop extracts the phase information θGFL of the grid voltage and generates current reference values ​​Id GFM and Iq GFM based on active power and reactive power reference values. In grid construction control, the active power reference value Pref and the reactive power reference value Qref are input, and the rotor angle θGFM of the virtual synchronous generator is generated through an integrator and an oscillator. The reactive voltage regulation signal is multiplied with the sine wave sinθGFM to generate a modulation wave, and the current control signals Id GFM and IqGFM are generated through a PI controller.

[0015] In one optional implementation, the process of setting the trigger angle of the grid commutator to 90 degrees and switching the full-bridge modular multilevel converter to STATCOM mode includes the following steps:

[0016] Using the steady-state voltage value of the power-stabilized DC system as a reference, the voltage deviation of the DC bus voltage from the reference is captured in real time; at the same time, the DC current maintained under the multivariable coordinated control system is monitored, and the instantaneous change gradient of the DC current is analyzed; the voltage deviation and the instantaneous change gradient of the DC current are coupled and judged, and when the composite fault characteristic signal exceeds the preset threshold, it is determined that a DC short circuit fault has occurred, and the identification switch from the normal operation state to the fault state is completed.

[0017] For grid-commutated converters, the firing angle adjustment target is switched from power stability or prevention of commutation failure to a single current blocking target. By setting the firing angle to 90 degrees, the DC-side output voltage is brought to zero. For full-bridge modular multilevel converters, the control mode is switched from the current grid following or grid-connected state to STATCOM mode to maintain AC-side stability.

[0018] The combined effect of the grid-commutated converter output returning to zero and the DC component elimination of the full-bridge modular multilevel converter creates an environment on the DC side equivalent to a zero voltage source; the fault current begins to decay naturally, achieving fault current suppression without relying on converter blocking, and completing DC fault ride-through.

[0019] In one optional implementation, the process of coupling the voltage deviation with the instantaneous change gradient of the DC current includes the following steps:

[0020] Using the steady-state voltage value as a reference, the difference between the instantaneous DC bus voltage value and this static reference is calculated in real time to obtain the voltage deviation. At the same time, using the DC current maintained by the multivariable coordinated control system as a dynamic reference, the change of DC current within a very short control cycle is calculated in real time to obtain the instantaneous change gradient of DC current.

[0021] The voltage deviation and the instantaneous change gradient of DC current are normalized and converted into dimensionless relative intensity signals. The relative intensity signals of the processed voltage deviation and the DC current change gradient are weighted and synthesized to form a composite fault characteristic signal.

[0022] The intensity of the composite fault characteristic signal is compared with a threshold preset according to the system's tolerance capability; when the intensity of the composite signal continues to exceed the threshold, it is determined that the single parameter fluctuation has evolved into a global DC short circuit fault.

[0023] In one optional implementation, the process of forming a composite fault characteristic signal includes the following steps:

[0024] The voltage deviation is assigned a weight factor that reflects the system's steady-state tolerance; at the same time, the DC current change gradient is assigned a weight factor that reflects the system's dynamic response characteristics.

[0025] The obtained voltage deviation relative strength signal is multiplied by the weighting factor of the assigned steady-state withstand capability to obtain its weighted strength; at the same time, the DC current change gradient relative strength signal is multiplied by the weighting factor of the dynamic response characteristic to obtain its weighted strength; the two weighted strengths are superimposed, and a nonlinear operation function is introduced to enhance the superposition result to form a primary synthetic signal.

[0026] The primary synthesized signal is integrated over time, and the output value characterizes the persistence and severity of the fault features, forming a composite fault feature signal for state determination.

[0027] In one optional implementation, the process of integrating the primary synthesized signal over the time axis includes the following steps:

[0028] Based on the typical time span experienced by the actual DC short-circuit fault current from its initial growth to a significant change, a fixed integration time window is set; thus, a signal accumulation interval matching the time scale of the fault physical process is obtained.

[0029] The generated primary synthetic signal is continuously algebraically accumulated within the established signal accumulation interval to form a real-time updated signal accumulation value, which reflects the continuous intensity of the fault characteristics in the time dimension.

[0030] The obtained cumulative signal value, whose magnitude represents the sum of fault characteristic energy within a specific time window, forms a composite fault characteristic signal for state determination.

[0031] In one optional implementation, the process of forming a composite fault characteristic signal for state determination includes the following steps:

[0032] Using the established signal accumulation interval as the calculation benchmark, the output signal accumulation value is divided algebraically by the duration of the fixed time window to obtain an energy density parameter characterizing the degree of fault characteristic energy accumulation per unit time.

[0033] The obtained energy density parameters are input into a compensation function with a specific time constant. Based on the physical characteristics of energy diffusion during the fault, different weighting coefficients are assigned to the energy density contribution at a time before a specific time, forming a compensated energy density that has been weighted by the time dimension.

[0034] The compensated energy density output is normalized and mapped to a uniform numerical range by a normalization processor, forming a composite fault characteristic signal.

[0035] In one optional implementation, the process of obtaining the compensation function with a specific time constant includes the following steps:

[0036] By utilizing the established signal accumulation interval, the complete process from fault occurrence to establishment within a fixed time window is analyzed, and several specific time points where the fault energy accumulation rate changes significantly are identified; a set of key time node sequences for describing the stage characteristics of fault development are obtained.

[0037] Using the obtained key time node sequence as a benchmark, an exponential function with time as the independent variable is constructed. At the initial moment of the fault, the current energy density parameter is given the highest weight, and as time moves forward, the weight decays according to a specific exponential law, thus forming a dynamic weight allocation relationship that can quantify the importance of energy density at different times.

[0038] The established dynamic weight allocation relationship is convolved with the obtained energy density parameters over time, and the result is a compensation function with a specific time constant that can perform time-dimensional weighting of energy density, thereby mapping the energy diffusion characteristics during the fault development process.

[0039] This invention connects a grid-commutated converter and a full-bridge modular multilevel converter in series. The grid-commutated converter handles all active power transmission, leveraging its low loss and low cost advantages. The full-bridge modular multilevel converter is only responsible for reactive power compensation, harmonic filtering, and fault ride-through assistance, significantly reducing the number of sub-modules and greatly reducing the size and weight of the converter station, thus lowering construction and maintenance costs. The introduction of the full-bridge modular multilevel converter effectively overcomes the limitation of the grid-commutated converter lacking reactive power support. Under normal operation, the full-bridge modular multilevel converter provides reactive power support and AC voltage stability to the grid in STATCOM mode. Under weak grid conditions, its grid formation control based on a virtual synchronous generator can provide virtual inertia and damping, enhancing grid stability and broadening the system's application scenarios. The DC fault ride-through strategy achieves rapid DC fault ride-through without blocking the converter by coordinating the firing angle of the grid-commutated converter and the operating mode of the full-bridge modular multilevel converter. There is no need to disconnect or restart the converter during a fault, reducing fault recovery time and operational complexity, and improving system reliability and availability. Attached Figure Description

[0040] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings:

[0041] Figure 1This is a flowchart of the control method for the LCC-full-bridge MMC series-type lightweight HVDC converter station provided in Embodiment 1 of the present invention;

[0042] Figure 2 This is a process diagram of setting the trigger angle of the grid phase-commutation converter to 90 degrees and switching the full-bridge modular multilevel converter to STATCOM mode, as provided in Embodiment 2 of the present invention.

[0043] Figure 3 This is a process diagram of coupling and judging voltage deviation with the instantaneous change gradient of DC current provided in Embodiment 3 of the present invention;

[0044] Figure 4 This is a process diagram of forming a composite fault feature signal provided in Embodiment 4 of the present invention;

[0045] Figure 5 This is a schematic diagram of the LCC-full-bridge MMC series converter station topology provided in Embodiment 8 of the present invention;

[0046] Figure 6 This is a block diagram of the LCC rectifier-side control provided in Embodiment 8 of the present invention;

[0047] Figure 7 This is the LCC inverter-side control block diagram provided in Embodiment 8 of the present invention;

[0048] Figure 8 This is a block diagram of the full-bridge MMC control provided in Embodiment 8 of the present invention;

[0049] Figure 9 This is a flowchart of the DC fault ride-through strategy provided in Embodiment 8 of the present invention;

[0050] Figure 10 This is a diagram showing the change in DC voltage waveform during a DC fault, as provided in Embodiment 8 of the present invention.

[0051] Figure 11 This is a schematic diagram of the real-time simulation system configuration provided in Embodiment 8 of the present invention;

[0052] Figures 12-17 The simulation result waveforms provided in Embodiment 8 of the present invention correspond to the key electrical quantity changes of the rectifier-side MMC station and LCC under steady-state operation, power step, DC fault and other operating conditions, respectively.

[0053] Figure 18 A block diagram of the electronic device provided by the present invention;

[0054] Figure 19 A block diagram of a computer-readable storage medium provided for this invention. Detailed Implementation

[0055] The technical solutions of the present invention will now be described with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0056] Hereinafter, the terms "first," "second," etc., are used for descriptive convenience only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0057] In this invention, unless otherwise explicitly specified and limited, the term "connection" should be interpreted broadly. For example, "connection" can be a fixed mechanical connection, a detachable mechanical connection, or an integral part; or, "connection" can be a direct connection or an indirect connection through an intermediate medium. Furthermore, unless otherwise explicitly specified and limited, the term "coupling" should be interpreted broadly. For example, "coupling" can be a direct electrical connection, such as physical contact and electrical conduction between two components; it can also be understood as an electrical connection between different components in a circuit structure through physical lines capable of transmitting electrical signals, such as copper foil or wires on a printed circuit board (PCB), to transmit electrical signals; or, "coupling" can be an indirect electrical connection between two components through an intermediate medium; or, "coupling" can be an electrical connection between two components in a non-contact manner, such as an electrical connection between two components using capacitive coupling to transmit electrical signals.

[0058] In this embodiment of the invention, directional terms such as "up," "down," "left," and "right" may be defined relative to the orientation of the components shown in the accompanying drawings. It should be understood that these directional terms can be relative concepts, used for relative description and clarification, and can change accordingly depending on the orientation of the components in the accompanying drawings.

[0059] Example 1:

[0060] like Figure 1 As shown, this embodiment of the invention provides a control method for an LCC-full-bridge MMC series-type lightweight HVDC converter station, comprising the following steps:

[0061] Step S100: The rectifier side of the grid-commutated converter generates trigger pulses by adjusting the DC power through the outer loop and using linearized trigger angle control through the inner loop; and is supplemented by transformer tap adjustment to obtain a DC system with stable power and responsive to the inverter side requirements.

[0062] Step S200: Based on the established stable DC system, the inverter side coordinates the three control loops of turn-off angle, DC voltage and DC current, selects the minimum firing angle as the control signal, and forms a control system that is centered on preventing commutation failure, can maintain DC voltage stability and ensure system operation when the rectifier side is out of control.

[0063] Step S300: The full-bridge modular multilevel converter switches between grid following and grid construction control modes according to grid conditions; when a DC fault is detected, the trigger angle of the grid-commutated converter is set to 90 degrees and the full-bridge modular multilevel converter is switched to STATCOM mode to achieve DC fault ride-through without blocking the converter.

[0064] In the above embodiments, this embodiment achieves stable power transmission through outer loop regulation of DC power on the rectifier side and inner loop linearized firing angle control, and coordinates transformer tap adjustment to adapt to the inverter side requirements. On the inverter side, by coordinating the turn-off angle, DC voltage, and current control loops, a minimum firing angle strategy is adopted to maintain DC voltage stability while preventing commutation failure, ensuring system reliability in the event of rectifier side runaway. The full-bridge modular multilevel converter switches operating modes according to grid conditions. In the event of a DC fault, non-blocking ride-through is achieved by setting the firing angle to 90 degrees and switching to STATCOM mode, ensuring continuous system operation. The entire solution achieves efficient and stable control and fault ride-through capability for lightweight HVDC systems.

[0065] This embodiment connects a grid-commutated converter and a full-bridge modular multilevel converter in series. The grid-commutated converter handles all active power transmission, leveraging its low-loss and low-cost advantages. The full-bridge modular multilevel converter is only responsible for reactive power compensation, harmonic filtering, and fault ride-through assistance, significantly reducing the number of sub-modules and greatly reducing the size and weight of the converter station, thus lowering construction and maintenance costs. The introduction of the full-bridge modular multilevel converter effectively overcomes the limitation of the grid-commutated converter lacking reactive power support. Under normal operation, the full-bridge modular multilevel converter provides reactive power support and AC voltage stability to the grid in STATCOM mode. Under weak grid conditions, its grid formation control based on a virtual synchronous generator can provide virtual inertia and damping, enhancing grid stability and broadening the system's application scenarios. The DC fault ride-through strategy achieves rapid DC fault ride-through without blocking the converter by coordinating the firing angle of the grid-commutated converter and the operating mode of the full-bridge modular multilevel converter. There is no need to disconnect or restart the converter during a fault, reducing fault recovery time and operational complexity, and improving system reliability and availability.

[0066] Example 2:

[0067] like Figure 2As shown, based on Embodiment 1, the process of setting the trigger angle of the grid phase-commutation converter to 90 degrees and switching the full-bridge modular multilevel converter to STATCOM mode in step S300 of this embodiment includes the following steps:

[0068] Step S301: Using the steady-state voltage value of the power-stabilized DC system as a reference, the voltage deviation of the DC bus voltage from the reference is captured in real time; at the same time, the DC current maintained under the multivariable coordinated control system is monitored, and the instantaneous change gradient of the DC current is analyzed; the voltage deviation and the instantaneous change gradient of the DC current are coupled and judged, and when the composite fault characteristic signal exceeds the preset threshold, it is determined that a DC short circuit fault has occurred, and the identification switch from the normal operation state to the fault state is completed.

[0069] Step S302: For grid-commutated converters, switch their firing angle adjustment target from power stability or prevention of commutation failure to a single current blocking target by setting the firing angle to 90 degrees to bring the DC-side output voltage to zero; for full-bridge modular multilevel converters, switch their control mode from the current grid following or grid-connected state to STATCOM mode to maintain AC-side stability.

[0070] Step S303: The combined effect of the grid-commutated converter output returning to zero and the DC component elimination of the full-bridge modular multilevel converter creates an environment on the DC side equivalent to a zero voltage source; the fault current begins to decay naturally, achieving fault current suppression without relying on converter blocking, and completing DC fault ride-through.

[0071] In the above embodiments, this embodiment achieves accurate fault identification by real-time monitoring of DC bus voltage deviation and current change gradient. Subsequently, the firing angle of the grid phase-commutation converter is adjusted to 90 degrees to block DC current. At the same time, the full-bridge modular multilevel converter is switched to STATCOM mode to eliminate DC component. The two work together to form a zero-voltage environment on the DC side, which promotes the natural decay of fault current. Finally, flexible DC fault ride-through without relying on blocking operation is achieved, ensuring that the system maintains AC side voltage stability and quickly restores power transmission capability during the fault.

[0072] Example 3:

[0073] like Figure 3 As shown, based on Example 2, the process of coupling and judging the voltage deviation with the instantaneous change gradient of DC current in step 301 provided in this embodiment of the invention includes the following steps:

[0074] Step S3011: Using the steady-state voltage value as a reference, calculate the difference between the instantaneous DC bus voltage value and this static reference in real time to obtain the voltage deviation; at the same time, using the DC current maintained by the multivariable coordinated control system as a dynamic reference, calculate the change of DC current in a very short control cycle in real time to obtain the instantaneous change gradient of DC current.

[0075] Step S3012: Normalize the voltage deviation and the instantaneous change gradient of DC current respectively, and convert them into dimensionless relative intensity signals; weight and synthesize the relative intensity signals of the processed voltage deviation and the DC current change gradient to form a composite fault characteristic signal.

[0076] Step S3013: Compare the intensity of the composite fault characteristic signal with a threshold preset according to the system's tolerance capability; when the intensity of the composite signal continues to exceed the threshold, it is determined that the single parameter fluctuation has evolved into a global DC short circuit fault, thereby completing the identification and switching from the normal operation state to the fault state and triggering the pass-through action.

[0077] In the above embodiments, this embodiment achieves rapid and accurate identification of DC system short-circuit faults by dynamically quantifying the degree of coordinated anomalies in voltage deviation and current change; it constructs a composite criterion using dual benchmarks of steady-state voltage and dynamic current, which not only overcomes the limitations of misjudgment by a single parameter, but also enhances the robustness of fault characteristics through normalized weighted processing; finally, it completes the fault state determination by comparing preset thresholds, providing a reliable basis for fault ride-through triggering for the system, effectively preventing fault expansion and ensuring equipment safety.

[0078] Example 4:

[0079] like Figure 4 As shown, based on Embodiment 3, the process of forming a composite fault characteristic signal in step S3012 of this embodiment of the invention includes the following steps:

[0080] Step S30121: Assign a weight factor to the voltage deviation that reflects the system's steady-state tolerance; at the same time, assign a weight factor to the DC current change gradient that reflects the system's dynamic response characteristics.

[0081] Step S30122: Multiply the obtained voltage deviation relative strength signal with the weighting factor of the assigned steady-state withstand capability to obtain its weighted strength; at the same time, multiply the DC current change gradient relative strength signal with the weighting factor of the dynamic response characteristics to obtain its weighted strength; superimpose the two weighted strengths and introduce a nonlinear operation function to enhance the superposition result to form a primary synthetic signal.

[0082] Among them, the nonlinear operation function is a mathematical relationship constructed based on the actual fault electrical quantity change law. Its core function is to transform the linear superposition result of the two weighted intensities, aiming to amplify the weak nonlinear characteristics hidden in the superposition result in the early stage of the fault, while suppressing the linear fluctuation under normal operating conditions, thereby enhancing the sensitivity and reliability of fault identification.

[0083] Step S30123: Integrate the primary synthesized signal on the time axis, and the output value characterizes the persistence and severity of the fault features; form a composite fault feature signal for state determination.

[0084] In the above embodiments, this embodiment achieves accurate extraction and enhancement of fault features through weight allocation and signal fusion. Voltage deviation and current change gradient are matched with the weights of steady-state tolerance and dynamic response characteristics, respectively, to ensure that the composite signal can reflect the static and dynamic characteristics of the system in a balanced way. The nonlinear enhancement processing after weighted superposition improves the saliency of fault features, while time integration effectively accumulates fault energy. The final composite signal has both transient sensitivity and continuous evaluation capability, providing a highly reliable quantitative basis for fault determination.

[0085] Example 5:

[0086] Based on Example 4, the process of integrating the primary synthesized signal on the time axis in step S30123 of the present invention includes the following steps:

[0087] Step S301231: Based on the typical time span experienced by the actual DC short-circuit fault current from initial growth to significant change, set a fixed integration time window; obtain a signal accumulation interval that matches the time scale of the fault physical process.

[0088] Step S301232: The generated primary synthetic signal is continuously algebraically accumulated within the established signal accumulation interval to form a real-time updated signal accumulation value, reflecting the continuous intensity of the fault characteristics in the time dimension.

[0089] Step S301233: The obtained cumulative signal value, whose magnitude represents the sum of fault characteristic energy within a specific time window, forms a composite fault characteristic signal for state determination.

[0090] In the above embodiments, this embodiment achieves a comprehensive spatiotemporal evaluation of fault characteristics by setting a window that matches the fault development time. The fixed time window design ensures that signal accumulation is synchronized with the actual physical process of the fault. Continuous algebraic accumulation operation captures the dynamic evolution law of fault characteristics. The final generated signal accumulation value quantifies the accumulation effect of fault energy in the time dimension, effectively distinguishing between instantaneous disturbances and continuous faults, and providing accurate action basis for system protection.

[0091] Example 6:

[0092] Based on Example 5, the process of forming a composite fault characteristic signal for state determination in step S301233 of this embodiment of the invention includes the following steps:

[0093] Step S3012331: Using the established signal accumulation interval as the calculation benchmark, perform algebraic division on the output signal accumulation value and the duration of the fixed time window; obtain an energy density parameter that characterizes the degree of fault characteristic energy accumulation per unit time.

[0094] Step S3012332: Input the obtained energy density parameter into a compensation function with a specific time constant. Based on the physical characteristics of energy diffusion during the fault duration, assign different weight coefficients to the energy density contribution at a time before the specific time, forming a compensated energy density after time-dimension weighting.

[0095] Step S3012333: The output compensated energy density is mapped to a uniform numerical range by a normalization processor to form a composite fault characteristic signal that can reflect both instantaneous intensity and continuous effect.

[0096] In the above embodiments, this embodiment achieves refined characterization of fault features through energy density calculation and time-weighted compensation. The energy density parameter per unit time quantifies the instantaneous concentration of fault intensity, and the compensation function assigns dynamic weights to historical data based on energy diffusion characteristics. After normalization processing, a standardized composite signal is formed, which not only preserves the transient response characteristics of fault features but also highlights the cumulative effect of persistent faults through time-dimensional weighting, providing the protection system with a judgment basis that is both timely and accurate.

[0097] Example 7:

[0098] Based on Example 6, the process of obtaining the compensation function with a specific time constant in step S3012332 provided in this embodiment of the invention includes the following steps:

[0099] Step S30123321: Using the established signal accumulation interval, analyze the complete process from fault occurrence to establishment within a fixed time window, identify several specific time points where the fault energy accumulation rate changes significantly, and obtain a set of key time node sequences to describe the stage characteristics of fault development.

[0100] Step S30123322: Using the obtained key time node sequence as a benchmark, construct an exponential function with time as the independent variable. At the initial moment of the fault, assign the current energy density parameter the highest weight, and as time moves forward, the weight decays according to a specific exponential law; thus forming a dynamic weight allocation relationship that can quantify the importance of energy density at different times.

[0101] Step S30123323: The established dynamic weight allocation relationship is convolved with the obtained energy density parameters on the time series. The result is a compensation function with a specific time constant that can perform time-dimensional weighted processing of energy density, thereby achieving an accurate mapping of energy diffusion characteristics during fault development.

[0102] In the above embodiments,

[0103] Example 8:

[0104] Based on Examples 1-7, the control method for an LCC-full-bridge MMC series-type lightweight HVDC converter station provided in this invention specifically includes the following steps:

[0105] Step S1: Rectifier side control of grid-commutated converter: Constant DC current control is adopted. The DC current is adjusted through the outer loop, and the linearized firing angle control is adopted in the inner loop to maintain the DC power stability and meet the power demand of the inverter side.

[0106] The DC voltage on the rectifier side of the grid-commutated converter can be expressed as:

[0107]

[0108] The specific execution method of step S1 is as follows: Figure 6 The outer loop control detects the DC power P in real time. DC and the set value P DC ref The error signal is compared and sent to the PI controller, whose output serves as the DC current reference value I. DC ref Actual DC current I DC rec After filtering and I DC ref The error signal is then processed again by the PI controller, which outputs an initial adjustment to the firing angle. Subsequently, to compensate for the nonlinear relationship between the firing angle α and the DC voltage, a linearization module is implemented to ensure that the firing angle can quickly and accurately track system requirements. A phase-locked loop (PLL) monitors the AC bus voltage V in real time. a V b V c Extract phase information φ PLL Based on the PLL phase and firing angle α, a firing pulse sequence is generated within a 60° phase interval to control the thyristor's on-time. α rAfter filtering, the step-up / step-down logic circuit determines the result based on α. r With α r ref The system detects deviations and outputs tap change commands to adjust the transformer tap positions, achieving precise control of the DC voltage. Through this control logic, the rectifier side of the grid-commutated converter can maintain stable DC power while rapidly responding to changes in power demand on the inverter side.

[0109] Step S2: Inverter side control of grid commutation converter: The inverter side adopts a multi-variable coordinated control strategy with the main goal of preventing commutation failure. It combines turn-off angle control, DC voltage control and DC current control to ensure the stability of the inverter DC voltage and maintain system operation when the rectifier side is out of control.

[0110] The DC voltage on the inverter side of the grid-commutated converter is expressed as:

[0111]

[0112] The specific execution method of step S2 is as follows: Figure 7 First, there is DC current control: real-time detection of DC current I. DC and the set DC current lower limit I DC margin Comparison. The error signal is processed by a PI controller, and the output is the firing angle adjustment α. iC Similarly, the DC voltage, AC voltage, and turn-off angle γ are detected and compared. The firing angle adjustment α output by the above control loop is then used. iC α iV α iγ The smallest firing angle is selected as the final firing angle α through comparison. i This ensures commutation margin and prevents commutation failure. The PLL monitors the AC bus voltage V in real time. a V b V c Extract phase information φ PLL Based on the PLL phase and firing angle αi, a firing pulse sequence is generated within a 60° phase interval to control the thyristor's on-time. α i After filtering, the step-up / step-down logic circuit determines the result based on α. i With α i ref The deviation is used to output a tap change command to adjust the DC voltage by adjusting the position of the transformer taps.

[0113] Step S3: Normal operation control of the full-bridge modular multilevel converter. The control strategies include: reactive power compensation and AC voltage support, submodule capacitor voltage balancing and suppression of internal circulating current. Under weak grid conditions, the full-bridge modular multilevel converter switches to the virtual synchronous generator (VSG) control mode to provide virtual inertia and voltage frequency support.

[0114] The dynamics of a full-bridge modular multilevel converter can be described as follows:

[0115]

[0116] The specific implementation method is as follows: Figure 8 This includes grid following control and grid construction control. The former uses a phase-locked loop to extract the phase information θ of the grid voltage. GFL Based on the active and reactive power reference values, a current reference value I is generated. d GFM I q GFM The latter inputs the active power reference value P. ref and reactive power reference value Q ref The rotor angle θ of the virtual synchronous generator is generated through an integrator and an oscillator. GFM The reactive power regulation signal is compared with a sine wave sinθ. GFM The multiplication generates a modulated wave, which is then used by a PI controller to generate a current control signal I. d GFM I q GFM This enables precise regulation of the grid current, providing stable voltage and frequency support for the grid.

[0117] Under weak grid conditions, the full-bridge modular multilevel converter switches to VSG control mode. The VSG control model adopts the mechanical and electromagnetic equations of a synchronous generator.

[0118]

[0119] Instead, active power:

[0120]

[0121] Where Pm is the input power of the synchronous machine, which is equivalent to the active power reference power in the VSG controller, and Pe is the output power of the synchronous machine. The control structure for VSG voltage amplitude and reactive power is as follows: Figure 6 As shown.

[0122] DC fault ride-through strategy:

[0123] Step S4: Fault Detection: Monitor the DC bus voltage and fault current rise rate in real time, and quickly determine the occurrence of faults by setting thresholds (such as voltage drop and current surge). Once a DC short circuit fault is detected, the fault ride-through procedure is started immediately.

[0124] Step S5: Action Execution: Quickly set the trigger angle of the grid-commutated converter to 90 degrees, causing the DC output voltage of the grid-commutated converter to drop to zero; simultaneously, switch the full-bridge modular multilevel converter to STATCOM mode, ensuring that the DC component of its arm voltage is also zero, thereby rapidly suppressing the fault current and achieving DC fault ride-through without blocking the converter. The specific execution method is as follows: Figure 9, Figure 10 As shown, the description is as follows: Upon the occurrence of a fault, the firing angle of the grid-commutated converter is immediately adjusted to 90 degrees, putting it into a current-blocking state; simultaneously, the full-bridge modular multilevel converter operates in STATCOM mode, and the DC component of the arm voltage is also reduced to zero. Therefore, the topology can withstand DC faults without blocking the converter.

[0125] Topology such as Figure 5 As shown, the grid-commutated converter is responsible for the transmission of all active power, while the full-bridge modular multilevel converter only undertakes reactive power compensation, harmonic filtering, and fault ride-through auxiliary functions, thereby significantly reducing the system size, weight, and cost. At the same time, in the event of a DC short-circuit fault, DC fault ride-through of the converter is achieved without blocking the converter by coordinating the firing angle of the grid-commutated converter and the operating mode of the full-bridge modular multilevel converter.

[0126] The following simulation verification, along with the accompanying drawings, provides a more detailed explanation of this embodiment.

[0127] The performance of the hybrid topology was verified through simulation. This platform was built on a real-time simulation target with a CPU and FPGA. The system performance under steady-state and DC fault conditions was studied. The parameters of the simulation system are shown in Table 1, and the simulation system description is as follows: Figure 10 As shown, the simulation system consists of a power grid phase-commutation converter and a full-bridge modular multilevel converter.

[0128] Table 1 Experimental parameters

[0129] Parameter name Symbol Value Rated power of grid phase converter <![CDATA[P 电网换相换流器 ]]> 5000 MW Full-bridge modular multilevel converter rated power <![CDATA[P 全桥模块化多电平变换器 ]]> 1000 MW Number of sub-modules per bridge arm N 50 AC line voltage (RMS) <![CDATA[V LL ]]> 500 kV DC bus voltage <![CDATA[V dc ]]> 500 kV AC system frequency f 50 Hz Full-bridge modular multilevel converter bridge arm inductor <![CDATA[L arm ]]> 2.5 mH Submodule capacitor C 10 mF

[0130] Step 1: Simulation System Configuration: Build a real-time simulation platform, such as... Figure 11 As shown. The converter station includes a grid-commutated phase converter and a full-bridge modular multilevel converter. The grid-commutated phase converter has a rated power of 5000MW, and the full-bridge modular multilevel converter has a rated power of 1000MW. The DC bus voltage is 500kV, the effective value of the AC line voltage is 500kV, and the system frequency is 50Hz. Each arm of the full-bridge modular multilevel converter contains 50 full-bridge sub-modules, with a sub-module capacitance of 10mF and an arm inductance of 2.5mH.

[0131] Step 2: Following the control strategies described in steps S1 to S3, precisely control the grid-commutated converter and the full-bridge modular multilevel converter. The grid-commutated converter employs constant DC current control on the rectifier side and coordinated control of the turn-off angle, DC voltage, and DC current on the inverter side. The full-bridge modular multilevel converter uses STATCOM mode for reactive power compensation and harmonic filtering during normal operation, switching to VSG grid control during weak grid conditions or black start situations to provide stable support for the grid.

[0132] Step 3: Implementation of DC fault ride-through strategy: Following the process described in steps S4 and S5, when a DC short-circuit fault is detected, the firing angle of the grid-commutated converter is quickly adjusted to 90 degrees, and the full-bridge modular multilevel converter is switched to STATCOM mode. Through the synergistic effect of the two, the fault current is quickly suppressed, and DC fault ride-through without blocking the converter is achieved.

[0133] A. Steady-state performance

[0134] The steady-state results of the rectifier-side full-bridge modular multilevel converter are as follows: Figure 12 As shown. In Figure 12 In (a), the DC current flowing through the full-bridge modular multilevel converter is the same as the DC line current. For example... Figure 12 As shown in (b), the DC voltage is close to zero, and the AC side voltage, current, and power are as follows: Figure 12 As shown in (c)-(e), the active power is zero and the reactive power is 0.2 pu, indicating that the full-bridge modular multilevel converter is operating in STATCOM mode. The arm current and voltage are as follows... Figure 12 As shown in (f) and (g). Because the DC-side current flows through the arms of the full-bridge modular multilevel converter, the arm current has a DC shift, while the arm voltage does not have a DC shift because it is modulated as a pure sine wave. The average voltage across the capacitor is as follows: Figure 12 As shown in (h), the capacitor voltage of the full-bridge modular multilevel converter remains balanced.

[0135] The results on the rectifier side of the power grid commutator converter are as follows: Figure 13 As shown. In Figure 13 In (c) and (d), the DC side voltage and current of the grid-commutated converter are given, kept near 1 p.u., with the reference power set at 1 p.u. Figure 13 In (e), the firing angle of the grid-commutated converter is around 17 degrees.

[0136] B. Power step performance

[0137] The power step size results of the rectifier-side full-bridge modular multilevel converter station are as follows: Figure 14 As shown, the active power step from 1 p.u. to 0.8 p.u. is applied to the grid-commutated converter within 3 seconds.

[0138] exist Figure 14 In (a), the DC current through the full-bridge modular multilevel converter decreases as the active power of the grid commutator decreases. For example... Figure 14 As shown in (b), the DC side voltage remains close to zero, while the AC side voltage, current, and power are as follows: Figure 14 As shown in (c)-(e). Active power is zero, reactive power remains at 0.2 pu. Arm current and voltage are as follows... Figure 14 As shown in (f) and (g), the DC displacement of the arm current decreases due to the reduction in the DC-side current flowing through the full-bridge modular multilevel converter arm. The average voltage of the capacitor is as follows. Figure 14 As shown in (h), this demonstrates that the full-bridge modular multilevel converter maintains good energy balance during power steps and that its ripple is reduced due to the decrease in current.

[0139] The results on the rectifier side of the grid commutator are as follows: Figure 15 As shown. In Figure 15 In (b), due to the decrease in active power, the AC current of the grid commutator decreases. For example... Figure 15 As shown in (d), since the reference power is set to 0.8 pu, the DC-side current of the grid-commutated converter decreases to 0.8 pu. Figure 15 In (e), the firing angle of the grid-commutated converter increases to around 23 degrees.

[0140] C. DC fault performance

[0141] DC fault results of the rectifier-side full-bridge modular multilevel converter station are as follows: Figure 16 As shown. A DC fault is applied for 5 seconds.

[0142] exist Figure 16 In (a), due to the lack of power transmission during a DC fault, the DC current through the full-bridge modular multilevel converter is reduced to zero. For example... Figure 16 As shown in (b), the DC voltage oscillates and then drops to zero. The AC side voltage, current, and power are as follows: Figure 16 As shown in (c)-(e), the active power is zero, while the reactive power remains at 0.2 pu. Figure 16 (f) and (g) describe the arm current and voltage. Since the DC-side current flowing through the arm of the full-bridge modular multilevel converter is zero, the DC displacement of the arm current is zero. The average voltage across the capacitor is as follows: Figure 16 As shown in (h), this indicates that the submodule capacitor voltages of the full-bridge modular multilevel converter remain balanced during DC faults.

[0143] Simulation results of the rectifier side of the power grid commutator are as follows: Figure 17 As shown. In Figure 17 In (d), the DC fault current of the grid-commutated converter drops to 0. Figure 17 In (e), the firing angle of the grid-commutated converter is set to 90 degrees, which can generate zero DC voltage to overcome DC faults.

[0144] Simulation results:

[0145] Steady-state operating conditions: Figure 12The steady-state operation results of the full-bridge modular multilevel converter station on the rectifier side are presented. The DC current is stable, the DC voltage is close to zero, and the AC side voltage, current, and harmonic characteristics are good. Active power is zero, and reactive power is 0.2 pu, indicating that the full-bridge modular multilevel converter successfully operates in STATCOM mode, providing stable reactive power support to the grid. Simultaneously, the capacitor voltage remains balanced, and the arm current and voltage waveforms are normal, verifying the effectiveness of the control strategy.

[0146] Power step operation: Figure 15 The response of the full-bridge modular multilevel converter on the rectifier side under power step is presented. When the active power of the grid commutator drops from 1 p.u. to 0.8 p.u., the DC current of the full-bridge modular multilevel converter decreases accordingly, but it can still maintain stable reactive power output and reduce capacitor voltage ripple, demonstrating the system's good adaptability and energy balance capability under power changes.

[0147] DC fault conditions: Figure 16 The performance of the rectifier-side full-bridge modular multilevel converter station and the grid-commutated converter during a DC fault was demonstrated. After the fault occurred, the DC current rapidly decreased to zero, the DC voltage oscillated before dropping to zero, while the AC side voltage, current, and power remained stable, reactive power continued to be output, and the capacitor voltage remained balanced. This verifies the speed and effectiveness of the DC fault ride-through strategy; the system can successfully ride through the fault without blocking the converter, ensuring the continuity and reliability of power transmission.

[0148] This embodiment proposes a compact HVDC topology that enhances DC fault ride-through capability. Based on the series integration of a grid-commutated converter and a full-bridge modular multilevel converter (MMC), the grid-commutated converter handles most of the active power transmission, while the MMC is dedicated to reactive power compensation and active harmonic filtering. The control algorithm and coordinated fault ride-through strategy fully utilize the complementary characteristics of the grid-commutated converter and the MMC. By integrating the grid-commutated converter and the MMC in series, significant reductions in system cost, size, and weight are achieved, while effectively improving system flexibility and reliability. When a DC fault occurs, by coordinating the operating modes of the grid-commutated converter and the MMC, the fault can be quickly overcome without blocking the converter, greatly improving system availability and security. Real-time simulation results confirm the proposed topology's ability to block DC faults and maintain system stability, fully verifying the effectiveness of the proposed topology and control strategy, and demonstrating its significant advantages and broad application prospects in long-distance, high-capacity power transmission, especially in space-constrained applications such as offshore platforms.

[0149] Figure 18A block diagram of an exemplary electronic device suitable for implementing embodiments of the present invention is shown.

[0150] Electronic devices may include a central processing unit / microprocessor / main control chip, etc.; a storage medium coupled to the central processing unit / microprocessor / main control chip, etc., and storing computer-executable instructions therein for performing the steps of various methods of embodiments of the present invention when executed by a processor.

[0151] The central processing unit / microprocessor / main control chip, etc., may include, but are not limited to, one or more processors or microprocessors.

[0152] Storage media may include, but are not limited to, random access memory (RAM), read-only memory (ROM), flash memory, EPROM memory, EEPROM memory, registers, and computer storage media (such as hard disks, floppy disks, solid-state drives, removable disks, CD-ROMs, DVD-ROMs, Blu-ray discs, etc.).

[0153] In addition, the electronic device may include (but is not limited to) a data bus, an input / output bus / external bus / device bus, a display, and input / output devices (e.g., keyboard, mouse, speaker, etc.).

[0154] The central processing unit / microprocessor / main control chip, etc., can communicate with external devices via the I / O bus through a wired or wireless network (not shown).

[0155] The storage medium may also store at least one computer-executable instruction for performing the steps of various functions and / or methods in the embodiments described herein when run by a central processing unit / microprocessor / main control chip, etc.

[0156] In one embodiment, the at least one computer-executable instruction may also be compiled into or comprise a software product, wherein one or more computer-executable instructions are executed by a processor to perform the steps of the various functions and / or methods in the embodiments described herein.

[0157] Figure 19 A schematic diagram of a computer-readable storage medium according to an embodiment of the present invention is shown.

[0158] like Figure 19As shown, instructions, such as computer-readable instructions, are stored on a non-transitory computer-readable storage medium. When the computer-readable instructions are executed by a processor, the various methods described above can be performed. The non-transitory computer-readable storage medium includes, but is not limited to, volatile memory and / or non-volatile memory. Volatile memory may include, for example, random access memory (RAM) and / or cache memory. Non-transitory non-volatile memory may include, for example, read-only memory (ROM), hard disk, flash memory, etc. For example, the non-transitory computer-readable storage medium can be connected to a computing device such as a computer, and then, when the computing device executes the computer-readable instructions stored on the computer-readable storage medium, the various methods described above can be performed.

[0159] In the several embodiments provided by this invention, it should be understood that the disclosed apparatus and methods can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative; for instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be through some interfaces; the indirect coupling or communication connection between apparatuses or units may be electrical, mechanical, or other forms.

[0160] 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.

[0161] Furthermore, 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. The integrated unit can be implemented in hardware or as a software functional unit.

[0162] If the integrated unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of this invention, in essence, or the part that contributes to the prior art, or all or part 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 for executing all or part of the steps of the methods of the various embodiments of this invention through a computer device (which may be a personal computer, server, or network device, etc.). The aforementioned storage medium includes: USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, optical disks, and other media capable of storing program code.

[0163] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications 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.

Claims

1. A control method for an LCC-full-bridge MMC series-type lightweight HVDC converter station, characterized in that, Includes the following steps: The rectifier side of the grid-commutated converter uses an outer loop to regulate DC power and an inner loop to use linearized trigger angle control to generate trigger pulses; supplemented by transformer tap regulation, it processes to obtain a DC system with stable power and responsiveness to the inverter side's needs; Based on the established stable DC system, the inverter side coordinates the three control loops of turn-off angle, DC voltage and DC current, selects the minimum firing angle as the control signal, and forms a control system that is centered on preventing commutation failure, can maintain DC voltage stability and ensure system operation when the rectifier side is out of control. The full-bridge modular multilevel converter switches between grid following and grid construction control modes according to grid conditions. When a DC fault is detected, the trigger angle of the grid-commutated converter is set to 90 degrees and the full-bridge modular multilevel converter is switched to STATCOM mode to achieve DC fault ride-through without blocking the converter.

2. The control method for an LCC-full-bridge MMC series-type lightweight HVDC converter station as described in claim 1, characterized in that, In the rectifier-side control of the power grid commutator, the outer loop control monitors the DC power P in real time. DC and the set value P DCref The error signal is compared and sent to the PI controller, whose output serves as the DC current reference value I. DCref ;actual DC current I DCrec After filtering and I DCref The error signal is then processed again by the PI controller, which outputs an initial adjustment to the firing angle. A linearization module is implemented to compensate for the nonlinear relationship between the firing angle α and the DC voltage. The phase-locked loop monitors the AC bus voltage V in real time. a V b V c Extract phase information Based on the phase-locked loop phase and firing angle α, a firing pulse sequence is generated within a 60° phase interval to control the thyristor's conduction time; α r After filtering, the step-up / step-down logic circuit determines the result based on α. r With α rref The deviation is detected, and a tap change command is output to adjust the transformer tap position, thereby achieving precise control of the DC voltage.

3. The control method for an LCC-full-bridge MMC series-type lightweight HVDC converter station as described in claim 1, characterized in that, In the inverter-side control of the grid-connected converter, the DC current IDC is monitored in real time and compared with the set lower limit IDC margin. The error signal is processed by the PI controller, which outputs the firing angle adjustment αiC. Similarly, the DC voltage, AC voltage, and turn-off angle γ are detected and compared. The firing angle adjustment αiC, αiV, and αiγ output by the control loop are compared, and the smallest firing angle is selected as the firing angle αi. The phase-locked loop monitors the AC bus voltages Va, Vb, and Vc in real time and extracts phase information. Based on the phase-locked loop phase and trigger angle αi, a trigger pulse sequence is generated within a 60° phase interval to control the conduction time of the thyristor; after αi is filtered, the step-up logic circuit outputs a tap change command based on the deviation between αi and αi ref to adjust the transformer tap position and regulate the DC voltage.

4. The control method for an LCC-full-bridge MMC series-type lightweight HVDC converter station as described in claim 1, characterized in that, The normal operation control of the full-bridge modular multilevel converter includes grid following control and grid construction control. In grid following control, the phase-locked loop extracts the phase information θGFL of the grid voltage and generates current reference values ​​IdGFM and IqGFM based on the active power and reactive power reference values. In grid construction control, the active power reference value Pref and the reactive power reference value Qref are input, and the rotor angle θGFM of the virtual synchronous generator is generated through an integrator and an oscillator. The reactive voltage regulation signal is multiplied with the sine wave sinθGFM to generate a modulation wave, and the current control signals IdGFM and IqGFM are generated through the PI controller.

5. The control method for an LCC-full-bridge MMC series-type lightweight HVDC converter station as described in claim 1, characterized in that, The process of setting the trigger angle of the grid-commutated converter to 90 degrees and switching the full-bridge modular multilevel converter to STATCOM mode includes the following steps: Using the steady-state voltage value of the power-stabilized DC system as a reference, the voltage deviation of the DC bus voltage from the reference is captured in real time; at the same time, the DC current maintained under the multivariable coordinated control system is monitored, and the instantaneous change gradient of the DC current is analyzed; the voltage deviation and the instantaneous change gradient of the DC current are coupled and judged, and when the composite fault characteristic signal exceeds the preset threshold, it is determined that a DC short circuit fault has occurred, and the identification switch from the normal operation state to the fault state is completed. For grid-commutated converters, the firing angle adjustment target is switched from power stability or prevention of commutation failure to a single current blocking target. By setting the firing angle to 90 degrees, the DC-side output voltage is brought to zero. For full-bridge modular multilevel converters, the control mode is switched from the current grid following or grid-connected state to STATCOM mode to maintain AC-side stability. The combined effect of the grid-commutated converter output returning to zero and the DC component elimination of the full-bridge modular multilevel converter creates an environment on the DC side equivalent to a zero voltage source; the fault current begins to decay naturally, achieving fault current suppression without relying on converter blocking, and completing DC fault ride-through.

6. The control method for an LCC-full-bridge MMC series-type lightweight HVDC converter station as described in claim 5, characterized in that, The process of coupling voltage deviation with the instantaneous gradient of DC current includes the following steps: Using the steady-state voltage value as a reference, the difference between the instantaneous DC bus voltage value and this static reference is calculated in real time to obtain the voltage deviation. At the same time, using the DC current maintained by the multivariable coordinated control system as a dynamic reference, the change of DC current within a very short control cycle is calculated in real time to obtain the instantaneous change gradient of DC current. The voltage deviation and the instantaneous change gradient of DC current are normalized and converted into dimensionless relative intensity signals. The relative intensity signals of the processed voltage deviation and the DC current change gradient are weighted and synthesized to form a composite fault characteristic signal. The intensity of the composite fault characteristic signal is compared with a threshold preset according to the system's tolerance capability; when the intensity of the composite signal continues to exceed the threshold, it is determined that the single parameter fluctuation has evolved into a global DC short circuit fault.

7. The control method for an LCC-full-bridge MMC series-type lightweight HVDC converter station as described in claim 6, characterized in that, The process of forming a composite fault characteristic signal includes the following steps: The voltage deviation is assigned a weight factor that reflects the system's steady-state tolerance; at the same time, the DC current change gradient is assigned a weight factor that reflects the system's dynamic response characteristics. The obtained voltage deviation relative strength signal is multiplied by the weighting factor of the assigned steady-state withstand capability to obtain its weighted strength; at the same time, the DC current change gradient relative strength signal is multiplied by the weighting factor of the dynamic response characteristic to obtain its weighted strength; the two weighted strengths are superimposed, and a nonlinear operation function is introduced to enhance the superposition result to form a primary synthetic signal. Integrating the primary synthesized signal over time, the output value characterizes the persistence and severity of the fault features; A composite fault characteristic signal is formed for state determination.

8. The control method for an LCC-full-bridge MMC series-type lightweight HVDC converter station as described in claim 7, characterized in that, The process of integrating the primary synthesized signal over time includes the following steps: Based on the typical time span experienced by the actual DC short-circuit fault current from its initial growth to a significant change, a fixed integration time window is set; thus, a signal accumulation interval matching the time scale of the fault physical process is obtained. The generated primary synthetic signal is continuously algebraically accumulated within the established signal accumulation interval to form a real-time updated signal accumulation value, which reflects the continuous intensity of the fault characteristics in the time dimension. The obtained cumulative signal value, whose magnitude represents the sum of fault characteristic energy within a specific time window, forms a composite fault characteristic signal for state determination.

9. The control method for an LCC-full-bridge MMC series-type lightweight HVDC converter station as described in claim 8, characterized in that, The process of forming a composite fault characteristic signal for state determination includes the following steps: Using the established signal accumulation interval as the calculation benchmark, the output signal accumulation value is divided algebraically by the duration of the fixed time window; We obtain an energy density parameter that characterizes the degree of energy accumulation of fault features per unit time. The obtained energy density parameters are input into a compensation function with a specific time constant. Based on the physical characteristics of energy diffusion during the fault, different weighting coefficients are assigned to the energy density contribution at a time before a specific time, forming a compensated energy density that has been weighted by the time dimension. The compensated energy density output is normalized and mapped to a uniform numerical range by a normalization processor, forming a composite fault characteristic signal.

10. The control method for an LCC-full-bridge MMC series-type lightweight HVDC converter station as described in claim 9, characterized in that, The process of obtaining a compensation function with a specific time constant includes the following steps: By utilizing the established signal accumulation interval, the complete process from fault occurrence to establishment within a fixed time window is analyzed, and several specific time points where the fault energy accumulation rate changes significantly are identified; a set of key time node sequences for describing the stage characteristics of fault development are obtained. Using the obtained key time node sequence as a benchmark, an exponential function with time as the independent variable is constructed. At the initial moment of the fault, the current energy density parameter is given the highest weight, and as time moves forward, the weight decays according to a specific exponential law, thus forming a dynamic weight allocation relationship that can quantify the importance of energy density at different times. The established dynamic weight allocation relationship is convolved with the obtained energy density parameters over time, and the result is a compensation function with a specific time constant that can perform time-dimensional weighting of energy density, thereby mapping the energy diffusion characteristics during the fault development process.

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