A method and system for improving the harmonic transient stability of a grid-type active power filter

Abstract

This invention belongs to the field of power electronics technology and provides a method and system for improving the harmonic transient stability of a grid-type active power filter. The method involves integrating the deviation between the active power reference command and the actual measured active power value after droop control to obtain the power angle. The power angle is then added to the grid-side harmonic phase to generate a harmonic voltage phase command. The deviation between the reactive power reference command and the actual measured reactive power value is obtained through PI control to obtain the voltage deviation. This voltage deviation is then summed with the amplitude of a specified harmonic voltage on the grid side to generate a harmonic voltage amplitude command. Using the harmonic voltage amplitude command as the d-axis reference and 0 as the q-axis reference, the deviation is obtained by subtracting the d-axis and q-axis references from the harmonic voltages on the d-axis and q-axis, respectively. The dq components of the harmonic voltage reference are then obtained through PI control. This invention can improve the safety of power grid operation.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of power electronics, and particularly relates to a grid-forming active power filter harmonic transient stability improvement method and system. BACKGROUND

[0002] The statements in this section merely provide background information related to the present application and do not necessarily constitute the prior art.

[0003] With the rapid development of power electronics technology, the penetration rate of nonlinear loads and new energy devices in modern power grids has been greatly improved, resulting in increasingly prominent power quality problems. On the one hand, nonlinear loads will bring a large amount of harmonic pollution to the distribution network. Harmonic pollution will cause current / voltage waveform distortion, leading to line overheating, increased loss, and even causing protection devices to misjudge faults, threatening the safety of the power grid. Although the traditional passive filter has low cost, it has defects such as large size, limited high-frequency performance, high design complexity, and only single harmonic compensation, which cannot adapt to complex harmonic environments and limits the harmonic control scenarios. In contrast, active power filters have good dynamic performance and can be applied to rapidly changing harmonic environments, thereby improving power quality and ensuring power grid safety. On the other hand, due to the access of a large number of new energy grid-connected devices, the inertia and damping of the power system are reduced, the strength of the power grid is weakened, and instability is easily caused. Grid-forming (GFM) control can provide voltage and frequency support and enhance system stability.

[0004] There are numerous control methods for active power filters, which greatly improve the stability and harmonic compensation ability of the filter under various complex operating conditions. However, existing research has neglected the transient stability of active power filters, and transient instability problems may lead to harmonic amplification and affect the compensation effect, and grid-forming active power filters cannot meet the grid-forming needs of modern distribution networks. SUMMARY

[0005] To solve the above problems, the present application proposes a grid-forming active power filter harmonic transient stability improvement method and system. The present application focuses on the transient process of active power filter harmonic compensation and improves the stability and harmonic compensation ability of active filters by dynamically controlling the harmonic power reference to avoid power overlimiting.

[0006] According to some embodiments, the present application adopts the following technical solutions:

[0007] A grid-forming active power filter harmonic transient stability improvement method, comprising the following steps:

[0008] Based on the droop relationship between harmonic active power and frequency, the deviation between the active power reference command and the actual measured active power value is controlled by droop and then integrated to obtain the power angle. The power angle is added to the grid-side harmonic phase to generate a harmonic voltage phase command. The transmission of active power between the single-phase inverter and the grid is controlled by changing the active power reference command.

[0009] The voltage deviation is obtained by PI control of the deviation between the reactive power reference command and the actual measured reactive power value. The voltage deviation is summed with the specified harmonic voltage amplitude on the grid side to generate the harmonic voltage amplitude command. The transmission of harmonic reactive power between the single-phase inverter and the grid is controlled by changing the reactive power reference command.

[0010] Using the harmonic voltage amplitude command as the d-axis reference and 0 as the q-axis reference, the deviation is obtained by subtracting the d-axis reference and q-axis reference from the harmonic voltage on the d-axis and q-axis, respectively. The dq component of the harmonic voltage reference is obtained through PI control.

[0011] As an optional implementation, the calculation process of the harmonic active power includes: constructing an orthogonal signal of the capacitor voltage based on the filter capacitor voltage and the specified harmonic angular frequency; establishing a two-phase stationary coordinate system of voltage based on the orthogonal signal; performing dq transformation on the harmonic voltage based on the coordinate system and the harmonic phase command; converting the specified harmonic voltage into a DC quantity; converting other frequency harmonics and the fundamental voltage into AC quantities; and extracting the DC quantity to obtain the specified harmonic voltage information.

[0012] An orthogonal signal of the grid-side current is constructed based on the grid-side current and the specified harmonic angular frequency. A two-phase stationary coordinate system of the current is established based on this signal. The harmonic current is then subjected to dq transformation based on this coordinate system and the harmonic phase command. The specified harmonic current is converted into a DC quantity, while other frequency harmonics and the fundamental current are converted into AC quantities. The DC quantity is then extracted through a first-order low-pass filter to obtain the information of the specified harmonic current. The active and reactive power of the harmonics can then be calculated.

[0013] As a further defined implementation, the process of obtaining the phase of the specified subharmonic includes: constructing a quadrature signal of the grid voltage based on the grid voltage and the angular frequency of the specified subharmonic, establishing a two-phase stationary coordinate system, and extracting the grid-side specified subharmonic information. v q As a PI control input, a feedforward circuit is added, and the phase is obtained by integrating the output angular frequency.

[0014] As an optional implementation, the method further includes the following steps: calculating the active power transmission boundary under harmonic transient stability conditions based on the grid-side harmonic voltage amplitude, reactive power reference command, and line impedance between the filter capacitor and the grid side; comparing the input active power reference with the maximum transmittable active power determined by the active power transmission boundary; if the input active power reference does not exceed the limit, it is used as the active power reference command; otherwise, a new active power reference command is generated.

[0015] As an optional implementation, the method further includes the following steps: calculating the reactive power transmission boundary under harmonic transient stability conditions based on the grid-side harmonic voltage amplitude and the line impedance between the filter capacitor and the grid side; comparing the input reactive power reference with the minimum transmittable reactive power determined by the reactive power transmission boundary; if the input reactive power reference does not exceed the limit, it is used as the reactive power reference command; otherwise, a new reactive power reference command is generated.

[0016] A network-type active power filter harmonic transient stability enhancement system includes:

[0017] The active power control module is used to integrate the deviation between the active power reference command and the actual measured active power value according to the droop relationship between harmonic active power and frequency to obtain the power angle. The power angle is added to the grid-side harmonic phase to generate a harmonic voltage phase command. The active power transmission between the single-phase inverter and the grid is controlled by changing the active power reference command.

[0018] The reactive power control module is used to obtain the voltage deviation by performing PI control on the deviation between the reactive power reference command and the actual measured reactive power value. The voltage deviation is summed with the specified harmonic voltage amplitude on the grid side to generate the harmonic voltage amplitude command. The transmission of harmonic reactive power between the single-phase inverter and the grid is controlled by changing the reactive power reference command.

[0019] The voltage inner loop control module is used to take the harmonic voltage amplitude command as the d-axis reference and 0 as the q-axis reference. It calculates the deviation by subtracting the d-axis reference and q-axis reference from the harmonic voltage on the d-axis and q-axis, respectively, and obtains the dq component of the harmonic voltage reference through PI control.

[0020] As an optional implementation, it also includes a harmonic power calculation module, which is used to construct an orthogonal signal of the capacitor voltage based on the filter capacitor voltage and the specified subharmonic angular frequency, establish a two-phase stationary coordinate system of voltage based on the orthogonal signal, perform dq transformation on the harmonic phase command based on the two-phase stationary coordinate system of voltage, convert the specified subharmonic voltage into a DC quantity, convert other frequency harmonics and fundamental voltage into AC quantities, and extract the DC quantity to obtain the specified subharmonic voltage information.

[0021] An orthogonal signal of the grid-side current is constructed based on the grid-side current and the specified harmonic angular frequency. A two-phase stationary coordinate system of the current is established based on this coordinate system and the harmonic phase command output by the harmonic active power control module. The specified harmonic current is converted into a DC quantity, and other frequency harmonics and the fundamental current are converted into AC quantities. The DC quantity is then extracted through a first-order low-pass filter to obtain the information of the specified harmonic current. The active power and reactive power of the harmonics can then be calculated.

[0022] As an optional implementation, a harmonic phase detection module is also included, used to construct a quadrature signal of the grid voltage based on the grid voltage and a specified subharmonic angular frequency, establish a two-phase stationary coordinate system, and extract the specified subharmonic information from the grid side. v q As a PI control input, a feedforward circuit is added, and the phase is obtained by integrating the output angular frequency, which serves as the phase basis for the dq transform.

[0023] As an optional implementation, it also includes an active power reference adjustment module, which is used to calculate the active power transmission boundary under harmonic transient stability conditions based on the grid-side harmonic voltage amplitude, reactive power reference command, and line impedance between the filter capacitor and the grid side; compare the input active power reference with the maximum transmittable active power determined by the active power transmission boundary; if the input active power reference does not exceed the limit, it is used as the active power reference command; otherwise, a new active power reference command is generated.

[0024] As an optional implementation, a reactive power reference adjustment module is also included, which is used to calculate the reactive power transmission boundary under harmonic transient stability conditions based on the grid-side harmonic voltage amplitude and the line impedance between the filter capacitor and the grid side. The input reactive power reference is compared with the minimum transmittable reactive power determined by the reactive power transmission boundary. If the input reactive power reference does not exceed the limit, it is used as the reactive power reference command; otherwise, a new reactive power reference command is generated.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] This invention analyzes the limit values ​​of active and reactive power that the inverter and the grid can exchange under the condition of ensuring harmonic transient stability. It can extract information of each harmonic and perform independent power control. It can dynamically adjust the power reference command according to the input power reference to prevent harmonic transient instability caused by the power command exceeding the limit from threatening the safe operation of the power grid.

[0027] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0028] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0029] Figure 1 This is a block diagram of a grid-type active power filter system proposed in one embodiment;

[0030] Figure 2 This is a schematic diagram of the harmonic power calculation module structure proposed in one embodiment;

[0031] Figure 3 This is a schematic diagram of the harmonic phase detection module structure proposed in one embodiment;

[0032] Figure 4 This is a schematic diagram of the active power reference adjustment module structure proposed in one embodiment;

[0033] Figure 5 This is a schematic diagram of the reactive power reference adjustment module structure proposed in one embodiment;

[0034] Figure 6 This is a schematic diagram of the active power control module structure proposed in one embodiment;

[0035] Figure 7 This is a schematic diagram of the reactive power control module structure proposed in one embodiment;

[0036] Figure 8 This is a schematic diagram of the voltage inner loop control module structure proposed in one embodiment. Detailed Implementation

[0037] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0038] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0039] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0040] Where there is no conflict, the embodiments and features described in this application may be combined with each other.

[0041] Example 1

[0042] A transient stability enhancement control method for a grid-type active power filter is proposed. When the power grid contains harmonic components, this control method can ensure the transient stability of the filter while compensating for harmonics in the power grid.

[0043] Specifically, the following steps are included:

[0044] Based on the droop relationship between harmonic active power and frequency, the deviation between the active power reference command and the actual measured active power value is controlled by droop and then integrated to obtain the power angle. The power angle is added to the grid-side harmonic phase to generate a harmonic voltage phase command. The transmission of active power between the single-phase inverter and the grid is controlled by changing the active power reference command.

[0045] The voltage deviation is obtained by PI control of the deviation between the reactive power reference command and the actual measured reactive power value. The voltage deviation is summed with the specified harmonic voltage amplitude on the grid side to generate the harmonic voltage amplitude command. The transmission of harmonic reactive power between the single-phase inverter and the grid is controlled by changing the reactive power reference command.

[0046] Using the harmonic voltage amplitude command as the d-axis reference and 0 as the q-axis reference, the deviation is obtained by subtracting the d-axis reference and q-axis reference from the harmonic voltage on the d-axis and q-axis, respectively. The dq component of the harmonic voltage reference is obtained through PI control.

[0047] In this embodiment, the calculation process of the harmonic active power includes: constructing an orthogonal signal of the capacitor voltage based on the filter capacitor voltage and the specified harmonic angular frequency; establishing a two-phase stationary coordinate system of voltage based on the orthogonal signal; performing dq transformation on the harmonic voltage based on the coordinate system and the harmonic phase command; converting the specified harmonic voltage into a DC quantity; converting other frequency harmonics and the fundamental voltage into AC quantities; extracting the DC quantity to obtain the specified harmonic voltage information; and then calculating the harmonic active power.

[0048] In this embodiment, the calculation process of the harmonic reactive power includes: constructing an orthogonal signal of the grid-side current based on the grid-side current and the specified harmonic angular frequency, establishing a two-phase stationary coordinate system for the current, performing dq transformation on the harmonic current based on the coordinate system and the harmonic phase command, converting the specified harmonic current into a DC quantity, converting other frequency harmonics and the fundamental current into AC quantities, extracting the DC quantity, obtaining the specified harmonic current information, and then calculating the harmonic reactive power.

[0049] In this embodiment, the calculation process for obtaining the specified harmonic phase includes: constructing a grid voltage quadrature signal based on the grid voltage and the specified harmonic angular frequency, establishing a two-phase stationary coordinate system, and extracting the specified harmonic information from the grid side. v qAs a PI control input, a feedforward circuit is added, and the harmonic phase is obtained by integrating the output angular frequency.

[0050] In this embodiment, the following steps are also included: calculating the active power transmission boundary under harmonic transient stability conditions based on the grid-side harmonic voltage amplitude, reactive power reference command, and line impedance between the filter capacitor and the grid side; comparing the input active power reference with the maximum transmittable active power determined by the active power transmission boundary; if the input active power reference does not exceed the limit, it is used as the active power reference command; otherwise, a new active power reference command is generated.

[0051] In this embodiment, the following steps are also included: calculating the reactive power transmission boundary under harmonic transient stability conditions based on the grid-side harmonic voltage amplitude and the line impedance between the filter capacitor and the grid side; comparing the input reactive power reference with the minimum transmittable reactive power determined by the reactive power transmission boundary; if the input reactive power reference does not exceed the limit, it is used as the reactive power reference command; otherwise, a new reactive power reference command is generated.

[0052] Example 2

[0053] A network-type active power filter transient stability improvement control system, such as Figure 1 As shown, it comprises a harmonic power calculation module, a harmonic phase detection module, an active power reference adjustment module, a reactive power reference adjustment module, an active power control module, a reactive power control module, and a voltage inner loop control module. The harmonic power calculation module is capable of calculating... n The first harmonic active / reactive power is used for independent power control of multiple harmonics. The harmonic phase detection module can acquire... n The phase information of the second harmonic is used for active power control. Multiple harmonic reference voltages are generated through power control and voltage control. v ref_n ( n =1, 2, 3, ...).

[0054] Figure 1 This is a block diagram of the proposed grid-type active power filter system. The circuit consists of a DC power supply, a single-phase half-bridge inverter, and... LCL Filters, power grids, and their line impedances constitute the system. LCL The filter consists of an inverter-side inductor. L gi Filter capacitor C gf and grid-side inductor L gg Composition. A power grid with harmonic interference is simulated by ideal voltage sources in series, wherein... v s_1 Indicates the fundamental frequency. vs_n express n Second harmonic, simulating grid voltage. v s .

[0055] like Figure 2 As shown, the harmonic power calculation module uses the filter capacitor voltage... v gf Grid-side current i s as well as n Subharmonic angular frequency ω n As input, orthogonal signals are generated through a second-order generalized integrator (SOGI) to construct an α-β coordinate system. The components of capacitor voltage and line current in the α-β coordinate system are then plotted. v αβ0 and i αβ0 According to the harmonic phase command output by the active power control module θ ref_n Perform dq transformation, n The second harmonic component is converted into a DC quantity, and other frequency components are converted into AC quantities. By filtering out the AC quantities using a low-pass filter, the harmonic voltage and current information can be extracted. Further calculations can then be performed to obtain... n Active and reactive power of subharmonics.

[0056] like Figure 3 As shown, the harmonic phase detection module utilizes the grid voltage v s and n Subharmonic angular frequency ω n Generating grid voltage via SOGI v s The orthogonal signals are used to construct an α-β coordinate system. Then, based on the network-side signals output by this module... n Phase information of subharmonics θ n By performing a dq transform and using a low-pass filter to remove the AC components, the result can be obtained from... n DC information from subharmonic conversion. Setting the q-axis voltage component. v q The reference value is 0, and the frequency deviation is obtained through PI control, where... n Subharmonic angular frequency ω n As a feedforward element, it can improve transient performance; the phase information can be obtained by integrating the output frequency. θ n .when vq When it is stable at 0 θ n That is n The real-time phase of the second harmonic. The proportional gain of the PI control in the diagram is 16, and the integral gain is 10. The closed-loop transfer function of the SOGI system is:

[0057] (1)

[0058] (2)

[0059] in, k The smaller the value of SOGI, the stronger its frequency selection capability. k The value is 0.5.

[0060] like Figure 4 As shown, the active power reference adjustment module adjusts according to the grid side. n Second harmonic voltage amplitude v s_n , n Subharmonic reactive power reference command q ref_n It can calculate the target n Maximum active power that a subharmonic inverter can transmit p max_n . p max_n The calculation formula is as follows:

[0061] (3)

[0062] (4)

[0063] in, L s This represents the line inductance between the inverter and the power grid. L gg express LCL The inductor on the grid side, X s_n Represented as n The line inductive reactance between the inverter and the power grid is a subharmonic.

[0064] When input n Reference value of subharmonic active power p ref Greater than p max_n This can sometimes lead to transient power angle instability. To avoid this, the adjustment module will... n Reference value of subharmonic active power p ref and p max_n Compare them. Whenp ref p max_n season p ref_n = p ref ;when p ref > p max_n season p ref_n = p max_n .therefore, p ref_n It can always meet the transient power angle stability requirements.

[0065] like Figure 5 As shown, the reactive power reference adjustment module adjusts according to the grid side. n Second harmonic voltage amplitude v s_n It can calculate the target n Minimum reactive power that a subharmonic inverter can transmit q min_n . q min_n The calculation formula is as follows:

[0066] (5)

[0067] When input n Reference value of secondary harmonic reactive power q ref Less than q min_n This can sometimes lead to transient voltage instability. To avoid this, the regulation module will input... q ref and q min_n Compare them. When q ref q min_n season q ref_n = q ref ;when q ref < q min_n season q ref_n = q min_n .therefore, q ref_n It can always meet the requirements for transient voltage stability.

[0068] like Figure 6 As shown, the active power control module uses n Active power of subharmonics p g_n and its reference instructions p ref_n As input, the deviation between the two is normalized and then droop control is applied. Based on the droop relationship between active power and frequency, the droop control is applied by changing... p ref_n It can change the output frequency, among which f n for n The power angle can be obtained by integrating the de-standardized angular frequency output. δ n , functional angle δ n Phase with grid harmonic voltage θ n The summation can generate a harmonic voltage phase reference command. θ ref_n The droop control factor for droop control P in the figure is 0.01.

[0069] like Figure 7 As shown, the reactive power control module uses n Reactive power of subharmonics q g_n and its reference instructions q ref_n As input, the deviation between the two is normalized and then PI control is applied. PI control can achieve zero steady-state error tracking, that is, under stable conditions... q g_n Able to achieve q ref_n Δ is obtained by inverse permutation after passing through the controller. v n , and the net side n Second harmonic voltage amplitude v s_n The summation yields the harmonic voltage amplitude reference command. v ref_n_d ,in v s_n can be Figure 2 middle n Subharmonic DC flow v d Obtain the value. In the diagram, the proportional coefficient for PI control is 0.5, and the integral coefficient is 1.5.

[0070] like Figure 8 As shown, the voltage inner loop control module uses n Reference command for subharmonic voltage amplitude v ref_n_d0 and 0 are used as voltage reference commands for the d-axis and q-axis, respectively, and are used in the harmonic power calculation module. n The deviation is obtained by subtracting the real-time voltage components of the second harmonic on the dq axis. The deviation between the d and q axes can be controlled by a PI controller to achieve zero steady-state error tracking. The feedforward quantity on the d-axis... v s_n This can avoid inrush current. v dc The DC voltage amplitude, with a gain of 1 / 2 due to the single-phase half-bridge structure, is the final output. n Reference voltage command for subharmonics in the dq coordinate system v ref_n_dq0 Based on the phase command generated by the active power control module, an inverse dq transformation is performed to generate... n Reference voltage command for subharmonics v ref_n The two PI controls in the diagram have the same parameters: a proportional coefficient of 30 and an integral coefficient of 20.

[0071] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of one or more computer-usable storage media (including, but not limited to, disk storage, etc.) containing computer-usable program code. CD - ROM It takes the form of a computer program product implemented on (such as optical memory, etc.).

[0072] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0073] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1The function specified in one or more boxes.

[0074] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0075] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made by those skilled in the art without creative effort within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A network-configuration type active power filter harmonic transient stability improvement method, characterized in that, The method comprises the following steps: Based on the droop relationship between harmonic active power and frequency, the deviation between the active power reference command and the actual measured active power value is controlled by droop and then integrated to obtain the power angle. This power angle is added to the grid-side harmonic phase to generate a harmonic voltage phase command. The transmission of active power between the single-phase inverter and the grid is controlled by changing the active power reference command. The active power control module uses the active power p of the nth harmonic... g_n and its reference instruction p ref_n As input, the deviation between the two is normalized and then droop control is applied; based on the droop relationship between active power and frequency, the droop control is applied by changing p. ref_n It can change the output frequency, where f n The frequency is the nth harmonic; by integrating the inversely standardized, permuted angular frequency, the power angle δ can be obtained. n δ n Phase θ with grid harmonic voltage n The summation generates a harmonic voltage phase reference command θ. ref_n The droop control coefficient for droop control P is 0.

01. The deviation amount of the reactive power reference instruction and the actually measured reactive power value is subjected to PI control to obtain a voltage deviation amount, the voltage deviation amount is summed with the grid-side specified harmonic voltage amplitude to generate a harmonic voltage amplitude instruction, and the transmission of harmonic reactive power between the single-phase inverter and the grid is controlled by changing the reactive power reference instruction; the reactive power control module takes the n-th harmonic reactive power q g_n and its reference instruction q ref_n as input, and the deviation amounts of the two are subjected to normalization and then PI control; the PI control can realize zero-static-error tracking, i.e., under the premise of stability, q g_n can reach q ref_n ; after the controller, the post-normalization deviation amount Δv n is obtained, which is added to the grid-side n-th harmonic voltage amplitude v s_n to obtain a harmonic voltage amplitude reference instruction v ref_n_d , wherein v s_n is obtained from the n-th harmonic direct current v d ; the proportional coefficient of the PI control is 0.5, and the integral coefficient is 1.5; The harmonic voltage reference dq component is obtained by taking the harmonic voltage amplitude instruction as the d-axis reference, taking 0 as the q-axis reference, taking the difference between the d-axis reference and the q-axis reference and the harmonic voltage on the d-axis and the q-axis respectively, and performing PI control.

2. The network-configuration active power filter harmonic transient stability improvement method of claim 1, characterized in that, The calculation process of the harmonic active power comprises the following steps: constructing a quadrature signal of the filter capacitor voltage according to the filter capacitor voltage and a specified harmonic angular frequency, establishing a voltage two-phase static coordinate system based on the quadrature signal, performing dq transformation on the harmonic voltage based on the voltage two-phase static coordinate system and a harmonic phase instruction, converting the specified harmonic voltage into a direct current, converting other frequency harmonics and fundamental wave voltage into alternating current, and extracting the direct current to obtain the specified harmonic voltage information; The current two-phase static coordinate system is established according to the grid-side current and the specified harmonic angular frequency, the dq transformation is performed on the harmonic current based on the coordinate system and the harmonic phase instruction, the specified harmonic current is converted into a direct current, other frequency harmonics and fundamental wave current are converted into alternating current, and then the direct current is extracted through a first-order low-pass filter to obtain the specified harmonic current information, and then the active power and the reactive power of the harmonic are calculated.

3. The networked active power filter harmonic transient stability improvement method of claim 2, wherein, The phase in the process of dq transformation depends on the calculation process, which comprises: constructing grid voltage quadrature signals according to grid voltage and specified harmonic angular frequency, establishing a two-phase stationary coordinate system, extracting grid-side specified harmonic information, and calculating the phase according to the grid voltage quadrature signals and the specified harmonic information v q As the PI control input, the feedforward link is added, and the phase is obtained by integrating the output angular frequency q is the q-axis voltage component.

4. The networked active power filter harmonic transient stability improvement method of claim 1, wherein, The method further comprises the following steps: The active power transmission boundary under the harmonic transient stability condition is calculated according to the grid-side harmonic voltage amplitude, the reactive power reference instruction and the line impedance between the filter capacitor and the grid side; the input active power reference is compared with the maximum transmissible active power determined by the active power transmission boundary, if the input active power reference is not out of limit, the input active power reference is taken as the active power reference instruction, otherwise a new active power reference instruction is generated; The active reference regulating module regulates the grid-side n Sub-harmonic voltage amplitude v s_n , n Sub-harmonic reactive power reference instruction q ref_n The maximum active power that the sub-harmonic inverter can transmit can be calculated n Sub-harmonic inverter can transmit p max_n ; p max_n The calculation formula is as follows: wherein, L s Lgrid represents the line inductance between the inverter and the grid, L gg Lgrid represents the line inductance between the inverter and the grid, LCL Lgrid represents the line inductance between the inverter and the grid, X s_n Lgrid represents the line inductance between the inverter and the grid, n Lgrid represents the line inductance between the inverter and the grid, ω n Lgrid represents the line inductance between the inverter and the grid, n Lgrid represents the line inductance between the inverter and the grid, When the input n sub-harmonic active power reference value p ref greater than p max_n , transient power angle instability will be triggered; to avoid this situation, the adjustment module will n sub-harmonic active power reference value p ref compared with p max_n ; when p ref p max_n , let p ref_n = p ref ; when p ref > p max_n , let p ref_n = p max_n ; therefore, p ref_n the transient power angle stability requirement can always be met.

5. The networked active power filter harmonic transient stability improvement method of claim 4, wherein, The method further comprises the following steps: The reactive power transmission boundary under the harmonic transient stability condition is calculated according to the grid-side harmonic voltage amplitude and the line impedance between the filter capacitor and the grid side, the input reactive power reference is compared with the minimum transmissible reactive power determined by the reactive power transmission boundary, if the input reactive power reference is not out of limit, the input reactive power reference is taken as the reactive power reference instruction, otherwise a new reactive power reference instruction is generated. The reactive reference regulating module regulates the grid-side n Sub-harmonic voltage amplitude v s_n The minimum reactive power that the sub-harmonic inverter can transmit can be calculated for n Sub-harmonic inverter can transmit q min_n ; q min_n The calculation formula is as follows: When the input n sub-harmonic reactive power reference value q ref less than q min_n will cause transient voltage instability; to avoid this situation, the adjustment module will input q ref and q min_n compare; when q ref q min_n , let q ref_n = q ref ; when q ref < q min_n , let q ref_n = q min_n ; therefore, q ref_n can always meet the transient voltage stability requirements.

6. A network configuration type active power filter harmonic transient stability improvement system, characterized in that, The method comprises: The active power control module is used for obtaining a power angle by integral of deviation between an active power reference instruction and an actually measured active power value according to a droop relationship between the harmonic active power and the frequency, adding the power angle and a grid-side harmonic phase to generate a harmonic voltage phase instruction, and controlling transmission of the active power between the single-phase inverter and the grid by changing the active power reference instruction; the active power control module takes the active power p g_n of the n-th harmonic and the reference instruction p ref_n as inputs, applies droop control to a deviation between the two after normalization, changes p ref_n based on the droop relationship between the active power and the frequency, and changes the output frequency, wherein f n is the n-th harmonic frequency; integral of an angular frequency output after denormalization can obtain a power angle δ n , the power angle δ n is added to a grid harmonic voltage phase θ n to generate a harmonic voltage phase reference instruction θ ref_n ; a droop control coefficient of the droop control P is 0.

01. The reactive power control module is used for PI control of a deviation amount between a reactive power reference instruction and an actually measured reactive power value to obtain a voltage deviation amount, the voltage deviation amount is summed with a grid side specified harmonic voltage amplitude to generate a harmonic voltage amplitude instruction, and transmission of harmonic reactive power between the single-phase inverter and the grid is controlled by changing the reactive power reference instruction; the reactive power control module takes the n-th harmonic reactive power q g_n and a reference instruction q ref_n of the n-th harmonic reactive power as input, PI control is applied to a deviation amount between the two after normalization; the PI control can realize zero static error tracking, that is, under the premise of stability q g_n can reach q ref_n ; after the controller, a reverse normalization is performed to obtain Δv n , which is added with a grid side n-th harmonic voltage amplitude v s_n to obtain a harmonic voltage amplitude reference instruction v ref_n_d , wherein v s_n is obtained from an n-th harmonic direct current v d ; a proportional coefficient of the PI control is 0.5, and an integral coefficient is 1.5; The voltage inner loop control module is used for taking the harmonic voltage amplitude instruction as the d-axis reference, taking 0 as the q-axis reference, taking the difference between the d-axis reference and the q-axis reference and the harmonic voltage on the d-axis and the q-axis respectively, and obtaining the dq component of the harmonic voltage reference through PI control.

7. The networked active power filter harmonic transient stability improvement system of claim 6, wherein, The harmonic power calculation module is used for constructing a quadrature signal of the filter capacitor voltage according to the filter capacitor voltage and a specified harmonic angular frequency, establishing a voltage two-phase static coordinate system based on the quadrature signal, performing dq transformation on the harmonic voltage based on the voltage two-phase static coordinate system and a harmonic phase instruction, converting the specified harmonic voltage into a direct current, converting other frequency harmonics and fundamental wave voltage into alternating current, and extracting the direct current to obtain the specified harmonic voltage information; According to the grid-side current and the specified harmonic angle frequency, the orthogonal signal of the grid-side current is constructed, and the two-phase stationary coordinate system of the current is established. Based on the coordinate system and the harmonic phase instruction output by the harmonic active power control module, dq transformation is carried out, the specified harmonic current is converted into direct current, other frequency harmonics and fundamental wave current are converted into alternating current, and then the direct current is extracted through a first-order low-pass filter to obtain the specified harmonic current information, and then the active power and the reactive power of the harmonic are calculated.

8. The networked active power filter harmonic transient stability improvement system of claim 6, wherein, Also include harmonic phase detection module, for according to the grid voltage and the specified harmonic angle frequency constructs grid voltage quadrature signal, establishes two-phase stationary coordinate system, extracts the grid side specified harmonic information, to v q As the PI control input, add the feedforward link, integrate the output angular frequency to obtain the phase, as the phase basis of dq transformation; q For q-axis voltage component.

9. The networked active power filter harmonic transient stability improvement system of claim 6, wherein, Further comprising an active reference regulation module, configured to calculate the active power transmission boundary under the harmonic transient stability condition according to the grid-side harmonic voltage amplitude, the reactive power reference instruction and the line impedance between the filter capacitor and the grid side; compare the input active power reference with the maximum transmissible active power determined by the active power transmission boundary, if the input active power reference is not over limited, the input active power reference is taken as the active power reference instruction, otherwise a new active power reference instruction is generated; The active reference regulating module regulates the grid-side n Sub-harmonic voltage amplitude v s_n , n Sub-harmonic reactive power reference instruction q ref_n The maximum active power that the sub-harmonic inverter can transmit can be calculated n Sub-harmonic inverter can transmit p max_n ; p max_n The calculation formula is as follows: wherein, L s represents the line inductance between the inverter and the grid, L gg represents LCL the inductance on the grid side, X s_n represents n the line reactance of the line between the inverter and the grid for the second harmonic, ω n represents n the second harmonic angular frequency; When the input n sub-harmonic active power reference value p ref greater than p max_n will cause transient power angle instability; to avoid this situation, the adjustment module will n sub-harmonic active power reference value p ref compared with p max_n ; when p ref p max_n , let p ref_n = p ref ; when p ref > p max_n , let p ref_n = p max_n ; therefore, p ref_n can always meet the requirements of transient power angle stability.

10. The networked active power filter harmonic transient stability improvement system of claim 6, wherein, Further comprising a reactive reference regulation module, configured to calculate the reactive power transmission boundary under the harmonic transient stability condition according to the grid-side harmonic voltage amplitude and the line impedance between the filter capacitor and the grid side; compare the input reactive power reference with the minimum transmissible reactive power determined by the reactive power transmission boundary, if the input reactive power reference is not over limited, the input reactive power reference is taken as the reactive power reference instruction, otherwise a new reactive power reference instruction is generated; The reactive reference regulating module regulates the grid-side n Sub-harmonic voltage amplitude v s_n The minimum reactive power that the sub-harmonic inverter can transmit can be calculated for n Sub-harmonic inverter can transmit q min_n ; q min_n The calculation formula is as follows: When the input n sub-harmonic reactive power reference value q ref less than q min_n transient voltage instability; to avoid this situation, the adjustment module will input q ref with q min_n comparison; when q ref q min_n , q ref_n = q ref ; when q ref < q min_n , q ref_n = q min_n ; therefore, q ref_n always be able to meet the transient voltage stability requirements.

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