SVG valve side filtering-based direct current power transmission system and control method

Abstract

The invention discloses a direct-current power transmission system based on SVG valve side filtering and a control method. An SLCC transmits active power through an LCC converter valve, provides reactive power through an SVG valve and filters; the rectification side of the LCC converter valve maintains the direct current of the rectification side to be the reference value of the direct current by controlling the trigger angle of the converter, and the receiving end of the LCC converter valve carries out constant direct current voltage control through cooperation of the inversion side converter transformer tap switch and the extinction angle; the active component and the reactive component of the fundamental frequency current are added to serve as a fundamental frequency current instruction, and the fundamental frequency current instruction is input into a fundamental frequency current controller for fundamental frequency current closed-loop control; a harmonic current component needing to be compensated is extracted by analyzing an output current measurement value of an LCC converter valve, serves as a harmonic compensation current instruction after being subjected to delay compensation and is input into a harmonic current controller for harmonic current closed-loop control; and superposing the output of the fundamental frequency current controller, the output of the harmonic current controller and the voltage feedforward to obtain a control reference voltage instruction of the SVG valve.

Description

Technical Field

[0001] The present invention belongs to the field of direct current (DC) power transmission systems, and in particular relates to a DC power transmission system based on SVG valve-side filtering and a control method thereof. Background Art

[0002] Grid-commutated converters (LCCs) require reactive power devices to compensate for the converter's reactive power consumption. Conventional DC systems use AC filters and shunt capacitors to compensate for reactive power, a traditional DC technology known as passive filtering. While mature, this technology faces numerous challenges: First, as AC voltage levels increase, the cost of reactive power equipment increases, reducing economic viability. Second, large voltage fluctuations in weak grids lead to frequent filter switching and transformer tap changer operations, increasing the risk of equipment failure. Third, with the influx of renewable energy sources connected to the grid, traditional reactive power compensation devices have slow regulation and poor dynamic reactive power support capabilities. Fourth, with the significant changes in system impedance associated with grid development, fixed-tuned filters struggle to cope. Fifth, the limited capacity of AC filter groups results in a large number of filter groups and a large footprint. To address these challenges, a reactive power compensation and filtering solution with superior overall performance is needed.

[0003] How to overcome the current technical problems of decreased system support capacity and complex and diverse harmonics in scenarios with a high proportion of new energy access is an urgent technical problem that technicians in this field need to solve. Summary of the Invention

[0004] The purpose of the present invention is to overcome the problems of reduced system support capacity and complex and diverse harmonics in scenarios with a high proportion of renewable energy access, and propose a DC power transmission system and control method based on SVG valve-side filtering.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions: In a first aspect, the present invention provides a control method for a DC power transmission system based on SVG valve-side filtering, comprising the following steps: SLCC adopts active and reactive power separation control, transmits active power through LCC converter valves, and provides reactive power and filtering through SVG valves; The rectifier side of the LCC converter valve maintains the DC current on the rectifier side at a reference value of the DC current by controlling the converter trigger angle, and the receiving end of the LCC converter valve performs constant DC voltage control in coordination with the inverter side converter tap changer and the arc extinction angle; The SVG valve adopts dual closed-loop control, which includes inner-loop control and outer-loop control. The SVG valve provides a fundamental reactive current instruction and a harmonic compensation current instruction for the inner-loop control through the outer-loop control. The active component and the reactive component of the fundamental frequency current are added together as the fundamental frequency current instruction, which is input into the fundamental frequency current controller for fundamental frequency current closed-loop control. The harmonic current component that needs to be compensated is extracted by analyzing the output current measurement value of the LCC converter valve, and is used as the harmonic compensation current instruction after delay compensation. The harmonic compensation current instruction is input into the harmonic current controller for harmonic current closed-loop control. The output of the fundamental frequency current controller and the output of the harmonic current controller are superimposed with the voltage feedforward to obtain a control reference voltage instruction for the SVG valve.

[0006] Furthermore, the fundamental current control includes active power control and reactive power control.

[0007] Furthermore, the active power control adopts a constant capacitance voltage mode.

[0008] Furthermore, the reactive power control dynamically adjusts the reactive component of the fundamental frequency current.

[0009] Furthermore, the dynamic adjustment is specifically as follows: In steady state, the control system gives reactive power instructions. The reactive current regulator calculates the reactive power output by the SVG based on the current system voltage, adjusts the reactive current instructions, and tracks the reactive power instructions. The AC system voltage is input into the basefrequency current controller through the phase-locked loop. If the basefrequency current controller detects a voltage drop in the AC system, it immediately switches to the transient reactive power control mode and supplies reactive power to the system according to the maximum capacity of the system.

[0010] Furthermore, the active component of the fundamental frequency current is provided by a submodule capacitor voltage controller.

[0011] Furthermore, the harmonic current controls the SVG to generate a harmonic current that is in anti-phase with the load harmonic current.

[0012] Furthermore, the rectifier side of the LCC converter valve adopts a constant power control mode and a constant current control mode.

[0013] In a second aspect, the present invention provides a DC power transmission system based on SVG valve-side filtering. The system uses a control method for a DC power transmission system based on SVG valve-side filtering, comprising an LCC converter valve and an SVG valve. The LCC converter valve is used to transmit active power, and the control module of the LCC converter valve includes a submodule, a capacitor constant voltage control module. The SVG valve is used to provide reactive power and filter reactive power, and the control module of the SVG valve includes an outer loop control module and an inner loop control module. The outer loop control module includes a fundamental current control submodule and a harmonic current control submodule. A starting circuit is provided on the commutation transformer grid side. The starting circuit is composed of a starting resistor, an isolation switch and a grounding switch connected in parallel. The starting circuit is connected to a current transformer.

[0014] Furthermore, the fundamental current control submodule includes an active power control channel and a reactive power control channel connected in parallel, and the reactive power control channel is provided with a dual-mode switching function of steady-state command tracking and transient voltage support.

[0015] Compared with the prior art, the present invention has the following beneficial technical effects: The present invention proposes a DC power transmission system and control method based on SVG valve-side filtering, which combines the technical advantages of LCC current source converters and VSC voltage source converters. It adopts active filtering and does not require the configuration of a large number of AC filters / shunt capacitors, thereby reducing the risk of accidents caused by frequent switching of switchgear and saving floor space. It also has good grid-connected performance, can realize reactive power self-compensation, and can provide dynamic reactive power support and fast regulation speed during faults. Due to valve-side filtering, the harmonic current of the converter transformer is greatly reduced, thereby improving equipment reliability.

[0016] Furthermore, it has excellent grid-connected performance, can achieve reactive power self-compensation, and can provide dynamic reactive power support and fast regulation during faults. After DC blocking, no large amount of excess reactive power is generated in the sending-end system, effectively reducing the AC overvoltage level at the sending end. To improve the operational reliability of the equipment, SLCC technology filters on the valve side, significantly reducing the harmonic current flowing through the converter transformer and improving equipment reliability. The AC filter group and its series circuit breaker are eliminated, greatly reducing the risk of accidents caused by frequent switching of switchgear. It saves overall project space, and only a small number of AC filters / parallel capacitors are required, which reduces the impact of noise on the surrounding environment and effectively saves the floor space of the converter station. It has strong adaptability to the integration of new energy sources. The use of active filtering can adapt to the impedance changes of the AC power grid and avoid problems such as low-order harmonic amplification. It has excellent dynamic and steady-state reactive power regulation capabilities, providing strong support for the grid connection and absorption of a high proportion of new energy sources. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The drawings described herein are for illustrative purposes only and are not intended to limit the scope of the present invention in any way. In addition, the shapes and proportional dimensions of the components in the drawings are only schematic and are used to help understand the present invention, and are not intended to specifically limit the shapes and proportional dimensions of the components of the present invention. In the drawings: Figure 1 This is a block diagram of SVG control in the SLCC technology provided by an embodiment of the present invention.

[0018] Figure 2 This is a schematic diagram of the main electrical connections of a DC power transmission system based on SVG valve-side filtering provided by an embodiment of the present invention.

[0019] FIG3( a ) is a valve-side current waveform diagram of an SLCC converter valve provided in an embodiment of the present invention.

[0020] FIG3( b ) is a grid-side current waveform diagram of the SLCC converter valve provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0021] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0022] It should be noted that when an element is referred to as being "disposed on" another element, it may be directly on the other element or there may be an element centered thereon. When an element is referred to as being "connected to" another element, it may be directly connected to the other element or there may be an element centered thereon. The terms "vertical," "horizontal," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only embodiments.

[0023] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one skilled in the art to which this invention pertains. The terms used in this specification of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0024] It should be noted that the terms "first", "second", etc. in the description and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that the numbers used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0025] Example 1 See also Figure 1A control method for a DC power transmission system based on SVG valve-side filtering comprises the following steps: The SLCC uses active and reactive power separation control, transmitting active power through the LCC converter valve and providing reactive power and filtering through the SVG valve. The rectifier side of the LCC converter valve maintains the DC current on the rectifier side at the reference value of the DC current by controlling the converter trigger angle. The receiving end of the LCC converter valve uses the inverter side converter tap changer and the arc extinction angle to coordinate the constant DC voltage control. The SVG valve adopts dual closed-loop control, which includes inner loop control and outer loop control. The SVG valve provides the fundamental reactive current command and harmonics for the inner loop control through the outer loop control. Compensation current instruction; the active component and reactive component of the fundamental frequency current are added together as the fundamental frequency current instruction, and the fundamental frequency current instruction is input into the fundamental frequency current controller for fundamental frequency current closed-loop control; by analyzing the output current measurement value of the LCC converter valve, the harmonic current component that needs to be compensated is extracted, and after delay compensation, it is used as the harmonic compensation current instruction, and the harmonic compensation current instruction is input into the harmonic current controller for harmonic current closed-loop control; the output of the fundamental frequency current controller, the output of the harmonic current controller and the voltage feedforward are superimposed to obtain the control reference voltage instruction of the SVG valve.

[0026] This embodiment's LCC focuses on active power transmission, accurately tracking DC current through firing angle control to ensure stable active power transmission, avoiding the limited operating range of traditional LCCs due to reactive power compensation requirements. SVG provides independent reactive power support, dynamically providing and filtering reactive power, eliminating the LCC's reliance on reactive power compensation, reducing converter transformer capacity requirements, and lowering equipment investment costs. The outer loop control is hierarchically optimized. Fundamental reactive power control generates reactive current commands by detecting the AC bus voltage or power factor, achieving rapid reactive power compensation and suppressing voltage fluctuations. Harmonic compensation control extracts harmonic components based on real-time current detection, generates precise harmonic compensation commands, and specifically suppresses characteristic subharmonics (such as 12k ± 1st order). The inner loop current closed-loop control independently controls the fundamental frequency and harmonic currents, improving current tracking accuracy, reducing cross-coupling interference, and enhancing system stability. The harmonic current closed-loop control offsets the digital control delay through a delay compensation algorithm, achieving precise cancellation of harmonic currents and reducing total harmonic distortion. The voltage feedforward suppression system feeds the AC bus voltage forward to the control loop, quickly suppressing the impact of grid voltage fluctuations on the SVG output and improving dynamic response speed. Constant DC voltage control uses tap-changers to adjust the commutation ratio on the inverter side. Combined with arc extinction angle control, this stabilizes the DC voltage and avoids the voltage fluctuations often associated with traditional constant arc extinction angle control. SVGs also reduce reliance on filtering devices, replacing traditional passive filters (such as LC filters) to avoid resonance risks, reduce floor space, and reduce maintenance costs. Wide-range operation capability: SVG's reactive power compensation capability expands the operating range of LCC, adapting to weak AC grids or scenarios with high power consumption. Fault ride-through capability: During AC faults, SVG can quickly provide dynamic reactive power support, preventing commutation failures and improving system transient stability. Economic optimization: Decoupling control reduces reactive power compensation equipment capacity, and combined with SVG's efficient filtering, reduces line losses, significantly reducing the life cycle cost (LCC). Independent decoupling of fundamental frequency and harmonic control facilitates parameter tuning and function expansion (such as additional damping control). It can be integrated into existing LCC-HVDC systems, achieving performance improvements through software upgrades without requiring large-scale hardware modifications. Through the coordinated control of LCC and SVG, high efficiency in active power transmission, dynamic reactive power compensation, and precise harmonic control are achieved, significantly improving the stability, power quality, and economy of the DC transmission system. This makes it particularly suitable for scenarios with stringent requirements on power quality, such as renewable energy grid integration, urban grid capacity expansion, and cross-border interconnection.

[0027] Fundamental current control includes active power control and reactive power control. Active power control uses a constant capacitor voltage mode. Reactive power control dynamically adjusts the reactive component of the fundamental frequency current. Dynamic adjustment is as follows: In steady state, the control system issues a reactive power command. The reactive current regulator calculates the reactive power output by the SVG based on the current system voltage, adjusts the reactive current command, and tracks the reactive power command. The AC system voltage is input into the fundamental frequency current controller through a phase-locked loop. If the fundamental frequency current controller detects a voltage drop in the AC system, it immediately switches to transient reactive power control mode, supplying reactive power to the system at its maximum capacity. The active component of the fundamental frequency current is provided by the submodule capacitor voltage controller. Harmonic current control: The SVG generates harmonic currents that are in antiphase with the load harmonic current. The rectifier side of the LCC converter valve uses constant power control mode and constant current control mode.

[0028] This embodiment stabilizes the capacitor voltage of the submodules. The active current component precisely regulates the capacitor voltage, preventing damage from capacitor overvoltage or undervoltage and extending the service life of the SVG. Energy balance is optimized, automatically compensating for switching and harmonic losses, reducing the need for external auxiliary power supplies and enhancing the system's self-sustaining operation. Steady-state precise compensation is achieved by using a reactive current regulator to calculate the SVG's output reactive power in real time based on the system voltage, minimizing tracking error, optimizing the grid power factor, and reducing line reactive losses. The system is adaptable to weak grid conditions and can operate stably in systems with a short-circuit ratio (SCR) as low as 1.5, expanding the application scope of DC transmission. Voltage sag detection and response time is less than 10ms, switching to maximum reactive power output mode (1.5 times the rated capacity), effectively mitigating the risk of voltage collapse and improving system transient stability. Incorporating a phase-locked loop (PLL) for precise phase tracking ensures that reactive power injection is synchronized with the system voltage, preventing over- or under-compensation. This system detects characteristic subharmonics (such as 12k ± 1st order) in the output current of the LCC converter valve in real time and generates a reverse-phase compensation current. This system achieves harmonic compensation accuracy exceeding 98% and reduces total harmonic distortion (THD). It replaces traditional passive filters, avoiding resonance risks, reducing footprint, and lowering maintenance costs. Efficient energy transmission: Dynamically adjusts the firing angle based on AC grid strength to optimize active power transmission efficiency and adapt to fluctuating renewable energy output scenarios. It optimizes economic efficiency by reducing unnecessary reactive power consumption and converter transformer losses. Fault ride-through capability: Rapidly switches to constant current control during AC faults to maintain DC current stability, avoid commutation failures, and ensure system safety. Coordinated control, in conjunction with the transient reactive power support of the SVG, enables stable operation to be restored within 0.2 seconds after a fault, improving system availability. Active / reactive power decoupling and harmonic control improve overall system efficiency and reduce transmission losses. Dynamic delay compensation: Using repetitive control or predictive control algorithms to offset digital control delays, this system ensures that harmonic compensation commands are in phase with actual harmonic currents, enhancing compensation effectiveness and reducing reactive power compensation equipment capacity. Replacing traditional filtering devices reduces lifecycle costs (LCC). Renewable energy grid integration smooths fluctuations in wind and photovoltaic power output, improving grid capacity. Urban grid capacity expansion enables high-density power transmission within limited space, reducing the need for substation expansion. Cross-border interconnection enhances cross-country grid stability and supports power complementarity across time zones. Independent decoupling of fundamental frequency and harmonic control facilitates the integration of additional functions. This approach, through multi-level coordinated control, achieves comprehensive optimization of DC transmission system efficiency, stability, power quality, and economic efficiency.

[0029] See also Figure 1 and Figure 2A DC power transmission system based on SVG valve-side filtering, using the control method of a DC power transmission system based on SVG valve-side filtering, includes an LCC converter valve and an SVG valve. The LCC converter valve is used to transmit active power, and the control module of the LCC converter valve includes a submodule capacitor constant voltage control module. The SVG valve is used to provide reactive power and filter, and the control module of the SVG valve includes an outer loop control module and an inner loop control module. The outer loop control module includes a fundamental current control submodule and a harmonic current control submodule. A starting circuit is set on the converter transformer side. The starting circuit is composed of a starting resistor, an isolation switch, and a grounding switch in parallel. The starting circuit is connected to a current transformer. The fundamental current control submodule includes an active control channel and a reactive control channel in parallel. The reactive control channel is provided with a dual-mode switching function of steady-state command tracking and transient voltage support.

[0030] Example 2 like Figure 2 This embodiment introduces a DC transmission system based on SVG valve-side filtering. The system includes: SVGs (static var generators) arranged in the valve hall and connected in parallel to the valve side of the LCC converter valve group. Each valve hall is equipped with two SVGs. The SVGs use a three-phase star connection via reactors, with the neutral point ungrounded. Each phase of the SVG reactor is connected in parallel with a lightning arrester, and a set of CTs (current transformers) are installed at the beginning and end of the SVG and at the outlet of the converter valve. A new starting circuit on the grid side of the converter transformer consists of a starting resistor, an isolating switch, and a grounding switch connected in parallel. The starting circuit is equipped with a set of CTs and is located at the GIS (gas-insulated metal-enclosed switchgear). The AC side of the converter transformer is typically connected to the AC grid through the GIS.

[0031] like Figure 1 As shown, this embodiment introduces a DC transmission system control strategy based on SVG valve-side filtering, including the following steps: SLCC technology uses active and reactive power separation control. SLCC is a technology that realizes the transmission and conversion of DC power by controlling the converter valves on the line. The LCC converter valve is responsible for the transmission of active power, and the SVG valve provides reactive power and realizes filtering.

[0032] The rectifier side of the LCC converter valve maintains the DC current on the rectifier side at the reference value of the DC current by controlling the converter trigger angle, and can adopt constant power control mode and constant current control mode; the receiving end of the LCC converter valve adopts constant DC voltage control, and the DC voltage is controlled by the combined action of the inverter side converter tap changer and the arc extinction angle.

[0033] The SVG valve adopts dual closed-loop control. The inner loop control is used to control the SVG valve to achieve fast tracking of active current, reactive current and harmonic current, and the outer loop control provides the command current signal for the inner loop.

[0034] The outer loop control includes fundamental current control and harmonic current control, wherein the fundamental current control includes active power control and reactive power control.

[0035] Among them, the specific links of fundamental current control are as follows: Active power control adopts a constant capacitance-voltage mode. The active component of the fundamental frequency current is provided by the submodule capacitance-voltage controller to compensate for the active power loss of the SVG valve submodule and stabilize the capacitance voltage of the SVG valve module at the rated value. Reactive power control is used to dynamically adjust the fundamental reactive power of the converter. In steady state, the LCC DC controller or operator control system gives reactive power instructions ( Figure 1 The reactive current regulator calculates the reactive power output of the SVG based on the current system voltage, adjusts the reactive current command, and tracks the reactive command. If a voltage drop in the AC system is detected, it immediately switches to transient reactive power control mode, sending reactive power to the system at its maximum capacity to provide voltage support. The active and reactive components of the fundamental frequency current are added together as the fundamental frequency current command, which is closed-loop controlled by the fundamental frequency current controller.

[0036] Among them, the specific links of harmonic current control are as follows: Harmonic current control is used to control the SVG to generate harmonic currents that are in phase with the load harmonic current. The SVG controller analyzes the output current measurement of the LCC converter valve to extract the harmonic current components that need to be compensated. After delay compensation, this is used as the harmonic current target value. The harmonic current controller performs current closed-loop control to achieve harmonic compensation.

[0037] The reference voltage command for SVG's converter valve control is obtained by superimposing the fundamental frequency current controller output, the harmonic current controller output, and the voltage feedforward.

[0038] Example 3 This example uses a double-terminal DC transmission system with a grid-side rated operating voltage of 775 kV at the sending-end converter station and 525 kV at the receiving-end converter station. The system has a rated DC voltage of ±800 kV and a rated power of 8,000 MW. In this model, each of the six-pulse converters at the high end of pole I on the sending and receiving ends is equipped with an SVG device, which uses reactive zero control mode.

[0039] Table 1 SVG main circuit parameters in a DC transmission system based on SVG valve-side filtering

[0040] Based on the model of this embodiment, various faults are set. The fault types and result analysis are shown in Table 2.

[0041] Table 2 Fault simulation test items

[0042] According to the data in Table 2, during a DC line fault, DC power transmission was briefly interrupted due to line restart. Without SVG, the AC voltage rose by approximately 100 kV due to the large amount of excess reactive power at the converter station. However, with SVG, the exchange reactive power quickly returned to zero, resulting in a mere 20 kV increase in AC voltage.

[0043] After a fault in the sending-end AC system, the presence or absence of SVG has no significant effect on the DC power recovery time. With SVG, the AC voltage recovery process after the fault is cleared is smoother.

[0044] After a fault in the receiving-end AC system, the presence or absence of SVG has no significant effect on the DC power recovery time.

[0045] The valve zone fault is considered a permanent fault, and both the SLCC thyristor valve and the SVG are locked, which has the same fault result as the LCC.

[0046] Under the "five commutation failures + lockout" fault, the SLCC AC overvoltage is low.

[0047] The valve-side current waveform of the SLCC converter valve during steady-state operation is shown in Figure 3(a), the grid-side current waveform is shown in Figure 3(b), and the harmonic current data and compensation degree are shown in Table 3.

[0048] Table 3 Harmonic data

[0049] According to the data in Table 3, SVGs can replace AC filters in SLCC-HVDC systems. The 6k±1 (k is an odd number) harmonics are canceled out in 12 pulsations, leaving only the 12k±1 characteristic harmonics. SVGs can compensate for harmonics up to the 37th order, achieving a harmonic filtering efficiency exceeding 94%.

[0050] After the SVG filter is installed, on the one hand, the SVG branch will naturally divert non-characteristic harmonics such as the third harmonic, reducing the size of the harmonic source. On the other hand, because the converter no longer needs to be equipped with a large number of passive filters, the risk of low-order harmonic amplification is greatly reduced, and there is no need to install a special HP3 filter.

[0051] Many embodiments and applications beyond the examples provided will be apparent to those skilled in the art upon reading the foregoing description. Therefore, the scope of the present teachings should be determined not with reference to the foregoing description, but rather with reference to the preceding claims, along with the full scope of equivalents to which such claims are entitled. For the purpose of completeness, all articles and references, including the disclosures of patent applications and publications, are incorporated herein by reference. The omission of any aspect of the subject matter disclosed herein from the preceding claims is not a disclaimer of such subject matter, nor should it be interpreted that the applicants did not consider such subject matter to be part of the disclosed inventive subject matter.

[0052] The above content is a further detailed description of the present invention. It cannot be considered that the specific implementation methods of the present invention are limited to these. For ordinary technicians in the technical field to which the present invention belongs, they can make several simple deductions or substitutions without departing from the concept of the present invention, which should be regarded as belonging to the scope of protection determined by the submission of the present invention.

Claims

1. A control method for a DC power transmission system based on SVG valve-side filtering, characterized in that: The following steps are involved: SLCC adopts active and reactive power separation control, transmits active power through LCC converter valves, and provides reactive power and filtering through SVG valves; The rectifier side of the LCC converter valve maintains the DC current on the rectifier side at a reference value of the DC current by controlling the converter trigger angle, and the receiving end of the LCC converter valve performs constant DC voltage control in coordination with the inverter side converter tap changer and the arc extinction angle; The SVG valve adopts dual closed-loop control, which includes inner and outer loop control. The SVG valve provides the inner loop control with a fundamental reactive current command and a harmonic compensation current command through the outer loop control. The active component and reactive component of the fundamental frequency current are added together to form the fundamental frequency current command, which is then input into the fundamental frequency current controller for closed-loop control of the fundamental frequency current. By analyzing the output current measurement value of the LCC converter valve, the harmonic current component that needs to be compensated is extracted and used as the harmonic compensation current instruction after delay compensation. The harmonic compensation current instruction is input into the harmonic current controller for harmonic current closed-loop control; The output of the fundamental frequency current controller, the output of the harmonic current controller and the voltage feedforward are superimposed to obtain the control reference voltage instruction of the SVG valve.

2. The control method of a DC power transmission system based on SVG valve-side filtering according to claim 1, characterized in that: The fundamental current control includes active power control and reactive power control.

3. The control method of a DC power transmission system based on SVG valve-side filtering according to claim 2, characterized in that: The active power control adopts a constant capacitance voltage mode.

4. The control method of a DC power transmission system based on SVG valve-side filtering according to claim 2, characterized in that: The reactive power control dynamically adjusts the reactive component of the fundamental frequency current.

5. The control method of a DC power transmission system based on SVG valve-side filtering according to claim 4, characterized in that: The dynamic adjustment is specifically as follows: In steady state, the control system gives reactive power instructions. The reactive current regulator calculates the reactive power output by the SVG based on the current system voltage, adjusts the reactive current instructions, and tracks the reactive power instructions. The AC system voltage is input into the basefrequency current controller through the phase-locked loop. If the basefrequency current controller detects a voltage drop in the AC system, it immediately switches to the transient reactive power control mode and supplies reactive power to the system according to the maximum capacity of the system.

6. The control method of a DC power transmission system based on SVG valve-side filtering according to claim 1, characterized in that: The active component of the fundamental frequency current is provided by the submodule capacitor voltage controller.

7. The control method of a DC power transmission system based on SVG valve-side filtering according to claim 1, characterized in that: The harmonic current control SVG generates a harmonic current that is in anti-phase with the load harmonic current.

8. The control method of a DC power transmission system based on SVG valve-side filtering according to claim 1, characterized in that: The rectifier side of the LCC converter valve adopts constant power control mode and constant current control mode.

9. A DC power transmission system based on SVG valve-side filtering, using the control method for a DC power transmission system based on SVG valve-side filtering as claimed in any one of claims 1 to 9, characterized in that: It includes an LCC converter valve and an SVG valve. The LCC converter valve is used to transmit active power. The control module of the LCC converter valve includes a submodule capacitor constant voltage control module. The SVG valve is used to provide reactive power and filter. The control module of the SVG valve includes an outer loop control module and an inner loop control module. The outer loop control module includes a fundamental current control submodule and a harmonic current control submodule. A starting circuit is provided on the commutation transformer grid side. The starting circuit is composed of a starting resistor, an isolation switch and a grounding switch connected in parallel. The starting circuit is connected to a current transformer.

10. A DC power transmission system based on SVG valve-side filtering according to claim 9, characterized in that: The fundamental current control submodule includes an active power control channel and a reactive power control channel connected in parallel. The reactive power control channel is provided with a dual-mode switching function of steady-state instruction tracking and transient voltage support.

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