Control method for supporting weak power grid operation by network-forming SVG (static var generator)

Abstract

The invention discloses a control method for supporting weak power grid operation by a network construction type SVG (static var generator), relates to the technical field of power electronics, and solves the problems that in the prior art, AC voltage cannot be actively constructed, an effective dynamic coordination mechanism is lacked with DC current control of a sending-end converter station, and the reliability of the system is poor. The system voltage supporting capability is insufficient and the direct current dynamic response performance is limited under the weak power grid working condition. According to the method, cooperative control of a network-forming SVG and an LCC-HVDC sending end converter station is adopted, specifically, the stability of power transmission of the LCC-HVDC system is ensured through a direct current control mode, the network-forming SVG generates a voltage vector reference through power synchronous control and adopts voltage and current double-closed-loop control, so that the network-forming SVG has the active voltage supporting capacity, and the stability of power transmission of the LCC-HVDC system is improved. Therefore, the dynamic response speed and the overall stability of the system under the weak power grid working condition are remarkably improved, and coordinated optimization of reactive compensation and direct-current power control is achieved.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a control method for supporting weak power grid operation using a grid-type SVG. Background Technology

[0002] With the large-scale grid connection of new energy sources and the increasing demand for long-distance power transmission, the receiving-end power grid exhibits characteristics of a weak grid, including insufficient short-circuit capacity and reduced inertia. While low-voltage direct current (LCC-HVDC) technology based on grid-commutated converters offers advantages in terms of large capacity and low cost, its stable operation relies on a strong AC system, making it prone to commutation failure and voltage instability in weak grids. Static var generators (SVG), as fast reactive power compensation devices, rely on traditional grid-following control strategies that passively respond to grid voltage changes, making it difficult to actively establish a stable voltage and frequency reference and thus unable to provide effective dynamic support for LCC-HVDC.

[0003] Existing SVG and grid-type control lack the ability to actively construct voltage sources. Their output relies entirely on the accurate detection of the grid phase frequency. When LCC-HVDC power fluctuations cause severe voltage distortion in weak grids, SVG cannot quickly provide effective reactive power support and may even trigger adverse interactive oscillations. The fundamental problem is that it is difficult to form an autonomous and stable voltage reference in the near-field region of LCC-HVDC.

[0004] Therefore, a control method is needed to support the operation of weak power grids using a grid-based SVG. Summary of the Invention

[0005] To address the problem of accurate phase locking under voltage distortion in weak power grids in existing technologies, this invention provides a control method for supporting weak power grid operation using a grid-based SVG. This method switches the SVG from traditional grid-following control to grid-based control, enabling it to actively construct a stable voltage vector reference through power synchronization control without relying on grid phase detection. This fundamentally solves the problem of accurate phase locking under voltage distortion in weak power grids. The specific technical solution is as follows: A control method for supporting weak grid operation using a grid-type SVG includes the following steps: S1. Adjust the firing angle of the converter valve at the sending end of the LCC-HVDC system and maintain the stability of the DC current by adopting the DC current control mode. S2. A grid-type static var generator (SVG) is used. The AC voltage and current output of the SVG are transformed by rotating coordinates to obtain the DC components of the rotating coordinate system. The active and reactive power output of the SVG are calculated based on the DC voltage and current components in the rotating coordinate system. The outer loop of S3 and SVG uses power synchronous control to generate a voltage vector reference, while the inner loop uses voltage and current dual closed-loop control to adjust Vabc to track the reference voltage, thereby achieving stable system control.

[0006] Preferably, step S2 is as follows: S21. Convert the AC voltage and current components of the SVG output into DC voltage and current components in a rotating coordinate system through a rotational coordinate transformation.

[0007] S22. Based on the DC voltage and current components in the rotating coordinate system, the active power and reactive power output of the SVG are obtained through instantaneous power calculation.

[0008] Preferably, in step S21, the rotation coordinate transformation equation satisfies: ; in, ω 1 is the fundamental angular frequency, which can be expressed as ω 1 = 2πf1.

[0009] Preferably, in step S22, the instantaneous power calculation equation satisfies: ; Among them, V d V q For the d-axis and q-axis components of the voltage; I d I q V0 and I0 are the d-axis and q-axis components of the current; V0 and I0 are the zero-axis components of the voltage and current.

[0010] Preferably, in step S3, in the power synchronization control stage, the real-time value of active power is obtained through instantaneous power calculation. P s The active power reference value is set to 0, and virtual inertia and damping control are performed based on the active power reference value to determine the virtual phase angle of the SVG's internal potential. θ ; SVG virtual internal potential amplitude E It is controlled by a reactive power control loop.

[0011] Preferably, in step S3: in the voltage and current dual closed-loop control loop, the reference value of the inner current loop is calculated based on the virtual internal potential amplitude. I dref , I qref In the inner current loop control stage, the reference voltage in the three-phase stationary coordinate system is determined based on the inner current loop reference value and the virtual phase angle of the internal potential. V abc The pulse width modulation (PWM) generator controls the SVG main circuit based on the trigger pulse output by the reference voltage.

[0012] Preferably, step S1 specifically uses IGBT switching devices connected in parallel on the AC side of the sending-end converter station, filtered by an LC filter, and then connected to the converter bus to provide reactive power compensation to the sending-end power grid to achieve voltage support. The relevant expressions include: ; ; Where J represents the virtual moment of inertia; ꞷ The virtual angular velocity represents the internal electromotive force of the SVG output; Pref represents the active power reference value; P represents the actual active power output of the SVG AC side. ꞷ 0 represents the system's rated angular velocity; k q Represents the reactive power droop factor; U0 represents the rated voltage amplitude; Q S Q represents the actual reactive power output on the AC side of the SVG; ref represents the reactive power reference value; s represents the Laplace operator; D represents the virtual damping coefficient.

[0013] A computer-readable storage medium includes a stored program, wherein, when the program is executed, it controls the device where the computer-readable storage medium is located to perform the control method described above for supporting weak grid operation using a grid-type SVG.

[0014] A processor for running a program, wherein the program executes the control method described above for supporting weak grid operation using a grid-type SVG.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention switches the SVG from a traditional grid-following control to a grid-building control, enabling it to actively construct a stable voltage vector reference through power synchronization control without relying on grid phase detection. This fundamentally solves the problem of accurate phase locking under voltage distortion in weak grids. Simultaneously, it employs a dual closed-loop voltage and current structure to achieve fast and accurate voltage tracking. Combined with the DC current control mode at the LCC-HVDC sending end, it effectively isolates the impact of DC power fluctuations on the AC grid, allowing the SVG to independently form reliable voltage and frequency support in the LCC-HVDC near-field region. This significantly improves the carrying capacity and transient stability of weak grids for DC transmission. Attached Figure Description

[0016] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0017] Figure 1 This is a schematic diagram of a mesh-type SVG connection method; Figure 2 It is a diagram of the overall collaborative control system; Figure 3 This is a schematic diagram of dynamic voltage support. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] It should be understood that, when used in this specification and the appended claims, the terms "comprising" and "including" indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0020] It should also be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0021] It should also be further understood that the term "and / or" as used in this specification and the appended claims refers to any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0022] Example 1 This embodiment provides a control method for supporting the operation of a weak power grid using a grid-type SVG.

[0023] The aim is to overcome the disadvantages of the current grid-adaptive SVG for reactive power compensation of LCC-HVDC systems in terms of grid adaptability and support capabilities. By adopting a grid-based control method, a stable voltage and frequency can be independently established, enabling independent grid operation under grid fault or off-grid conditions, providing support for the local grid and ensuring continuous power supply to important loads.

[0024] Please refer to Figures 1-3 As shown, a control method for supporting weak grid operation using a grid-type SVG includes the following steps: Step 1: Connection method of LCC-HVDC system using grid-type SVG for reactive power compensation as follows Figure 1As shown, the rated capacity of the SVG is set to 200Mvar. It uses IGBT switching devices connected in parallel on the AC side of the sending-end converter station. After LC filtering, it is connected to the converter bus to provide reactive power compensation for the sending-end power grid to achieve voltage support.

[0025] in u g Represents the AC voltage of the power grid. L g Represents the equivalent inductive reactance of the power grid. i g Represents the alternating current on the grid side. i L This represents the AC current on the sending-end converter station side. i svg The alternating current representing SVG compensation. L F and C F These represent the filter inductor and capacitor of the SVG, respectively. U dc Represents the DC capacitor voltage. U PCC k represents the voltage at the converter bus. T This represents the turns ratio of the converter transformer. V d This represents the DC side voltage of the sending-end converter station.

[0026] The SVG main circuit structure shown in the diagram above is a voltage-mode bridge circuit. Besides this, there is also a current-mode bridge circuit. The following is a brief introduction to these two structures: The main circuit of a voltage-type bridge SVG typically adopts a three-phase full-bridge structure, with each bridge arm composed of IGBT switching devices. These switching devices generate the required AC voltage waveform through PWM modulation. The DC side of the inverter is usually connected to a large-capacity DC capacitor to ensure that the voltage does not change significantly during continuous charging and discharging or device commutation. The AC side is equipped with an LC filter (or connected to a transformer). The main circuit of the current-type bridge SVG also adopts a three-phase full-bridge inverter structure, but its DC side is an inductor instead of a capacitor, which enables the SVG to better control the output current and is suitable for applications that require fast response and high-precision current control. The AC side is usually equipped with an LCL filter to provide better harmonic suppression capability.

[0027] Since voltage-type SVG has higher operating efficiency and is more widely used in practical engineering, only voltage-type SVG will be introduced in general.

[0028] Step 2: The overall collaborative control strategy of the system, such as... Figure 2 .

[0029] As shown in the figure, the overall control system mainly consists of two parts: the LCC control element and the network-type SVG control element. (1) Sending end LCC control loop: Since the receiving end converter station is assumed to use DC voltage control mode, the sending end converter station can use DC current control mode to precisely adjust the firing angle of the converter valve and maintain the stability of DC current.

[0030] (2) Network-type SVG control loop: based on the DC voltage in the rotating coordinate system u dq Current components i dq Calculate the output power of the SVG P s , Q s The outer loop of the SVG uses power synchronous control to generate a voltage vector reference; the inner loop uses dual closed-loop control for voltage and current regulation. V abc Tracking reference voltage. The rotation coordinate transformation equation satisfies: (1) in, ω 1 is the fundamental angular frequency, which can be expressed as ω 1 = 2πf1.

[0031] The instantaneous power calculation equation satisfies: (2) Among them, V d V q For the d-axis and q-axis components of the voltage; I d I q V0 and I0 are the d-axis and q-axis components of the current; V0 and I0 are the zero-axis components of the voltage and current.

[0032] In the power synchronization control stage, the real-time value of active power is obtained through instantaneous power calculation. P s The active power reference value is set to 0. Virtual inertia and damping control are then performed based on this active power reference value to determine the virtual phase angle of the SVG's internal potential. θ The virtual internal potential amplitude of SVG; E It is controlled by a reactive power control loop.

[0033] In the voltage and current dual closed-loop control stage, the reference value of the inner current loop is calculated based on the amplitude of the virtual internal potential. I dref , I qref In the inner current loop control stage, the reference voltage in the three-phase stationary coordinate system is determined based on the inner current loop reference value and the virtual phase angle of the internal potential.V abc The pulse width modulation (PWM) generator controls the SVG main circuit based on the trigger pulse output from the reference voltage. Relevant expressions include: (3) (4) Where J represents the virtual moment of inertia; ꞷ The virtual angular velocity represents the internal electromotive force of the SVG output; Pref represents the active power reference value; P represents the actual active power output of the SVG AC side. ꞷ 0 represents the system's rated angular velocity; k q Represents the reactive power droop factor; U0 represents the rated voltage amplitude; Q S Q represents the actual reactive power output on the AC side of the SVG; ref represents the reactive power reference value; s represents the Laplace operator; D represents the virtual damping coefficient.

[0034] Step 3: To verify the feasibility of the control scheme, based on the established simulation model, the rated capacity of the SVG was set to 200Mvar. The voltage support capability and harmonic current suppression achieved by the SVG after providing reactive power compensation were verified. The relevant parameters of the LCC-HVDC system remained the same as before. A voltage drop of 20% was set at 1.0s during system operation, and the SVG was activated at 2.0s to check whether the SVG had voltage support capability.

[0035] Figure 3 The AC / DC side voltage waveforms of the converter station are used to test the voltage support capability of the SVG after applying a voltage drop condition. As can be seen from the figure, the SVG voltage support effect is good.

[0036] In summary, this invention fundamentally solves the problems of traditional grid-connected SVGs' inability to actively build voltage and lack of dynamic coordination mechanisms under weak grid conditions by coordinating the control of grid-connected SVGs with the LCC-HVDC sending-end converter station. Specifically, the DC current control mode ensures the stability of power transmission in the LCC-HVDC system, while the grid-connected SVG generates a voltage vector reference through power synchronization control and adopts dual closed-loop control of voltage and current, enabling it to have active voltage support capability. This significantly improves the system's dynamic response speed and overall stability under weak grid conditions, achieving coordinated optimization of reactive power compensation and DC power control.

[0037] Those skilled in the art will recognize that the units of the various examples described in connection with the embodiments disclosed herein can be implemented in electronic hardware, computer software, or a combination of both. To clearly illustrate the interchangeability of hardware and software, the components of the various examples have been generally described in terms of functionality in the foregoing description. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementations should not be considered beyond the scope of the invention.

[0038] In the embodiments provided by the present invention, it should be understood that the division of units is only a logical functional division. In actual implementation, there may be other division methods, such as multiple units can be combined into one unit, one unit can be split into multiple units, or some features can be ignored.

[0039] 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.

[0040] 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 the present 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 to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of the present invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, read-only memory (ROM), random access memory (RAM), portable hard drives, magnetic disks, or optical disks.

[0041] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. 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 or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention.

Claims

1. A control method for supporting weak grid operation using a grid-type SVG, characterized in that, Includes the following steps: S1. Adjust the firing angle of the converter valve at the sending end of the LCC-HVDC system and maintain the stability of the DC current by adopting the DC current control mode. S2. A grid-type static var generator (SVG) is used. The AC voltage and current output of the SVG are transformed by rotating coordinates to obtain the DC components of the rotating coordinate system. The active and reactive power output of the SVG are calculated based on the DC voltage and current components in the rotating coordinate system. The outer loop of S3 and SVG uses power synchronous control to generate a voltage vector reference, while the inner loop uses voltage and current dual closed-loop control to adjust Vabc to track the reference voltage, thereby achieving stable system control.

2. The control method for supporting weak grid operation using a grid-type SVG according to claim 1, characterized in that, Step S2 is as follows: S21. Convert the AC voltage and current components of the SVG output into DC voltage and current components in a rotating coordinate system through a rotating coordinate transformation. S22. Based on the DC voltage and current components in the rotating coordinate system, the active power and reactive power output of the SVG are obtained through instantaneous power calculation.

3. The control method for supporting weak grid operation using a grid-type SVG according to claim 2, characterized in that, In step S21, the rotation coordinate transformation equation satisfies: ； in, ω 1 is the fundamental angular frequency, which can be expressed as ω 1 = 2πf1.

4. The control method for supporting weak grid operation using a grid-type SVG according to claim 2, characterized in that, In step S22, the instantaneous power calculation equation satisfies: ； Among them, V d V q For the d-axis and q-axis components of the voltage; I d I q V0 and I0 are the d-axis and q-axis components of the current; V0 and I0 are the zero-axis components of the voltage and current.

5. The control method for supporting weak grid operation using a grid-type SVG according to claim 1, characterized in that, In step S3, during the power synchronization control stage, the real-time value of active power is obtained through instantaneous power calculation. P s The active power reference value is set to 0, and virtual inertia and damping control are performed based on the active power reference value to determine the virtual phase angle of the SVG's internal potential. θ ; SVG virtual internal potential amplitude E It is controlled by a reactive power control loop.

6. The control method for supporting weak grid operation using a grid-type SVG according to claim 1, characterized in that, In step S3: In the voltage and current dual closed-loop control loop, the reference value of the inner current loop is calculated based on the virtual internal potential amplitude. I dref , I qref In the inner current loop control stage, the reference voltage in the three-phase stationary coordinate system is determined based on the inner current loop reference value and the virtual phase angle of the internal potential. V abc The pulse width modulation (PWM) generator controls the SVG main circuit based on the trigger pulse output by the reference voltage.

7. The control method for supporting weak grid operation using a grid-type SVG according to claim 1, characterized in that, In step S1, IGBT switching devices are connected in parallel on the AC side of the sending-end converter station, and after LC filtering, they are connected to the converter bus to provide reactive power compensation for the sending-end power grid to achieve voltage support.

8. A computer-readable storage medium, characterized in that, The computer-readable storage medium includes a stored program, wherein, when the program is executed, it controls the device containing the computer-readable storage medium to perform the control method for supporting weak grid operation of a grid-type SVG as described in any one of claims 1 to 7.

9. A processor, characterized in that, The processor is used to run a program, wherein the program executes the control method for supporting weak grid operation of the grid-type SVG as described in any one of claims 1 to 7.

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