Method and system for suppressing high-frequency oscillation of grid-connected system based on cascaded SVG in weak grid state

Abstract

The application discloses a kind of weak network state based on cascade SVG's grid-connected system high-frequency oscillation suppression method and system, using SVG device capacitance voltage signal to generate preliminary modulation current component;The voltage signal of grid-connected point is passed through fundamental frequency wave trap, obtains the oscillation voltage component of network side except fundamental frequency, generates high-frequency oscillation suppression current component based on the oscillation voltage component;Based on grid-connected point voltage signal, cascade SVG device output current signal, network side current signal, generate reactive current component compensation component;Preliminary modulation current component, high-frequency oscillation suppression current component, reactive current component compensation component are added, and the reference value of cascade SVG compensation current is obtained, the difference between cascade SVG compensation current reference value and cascade SVG device output current signal is carried out, and the difference is passed through a current regulator, and the output of current regulator is passed through grid voltage feedforward control, and the cascade SVG bridge arm modulation voltage is obtained.The application realizes efficient network side reactive control while inhibiting network side high-frequency oscillation.

Description

Technical Field

[0001] This invention relates to the field of power electronic control technology, and in particular to a method and system for suppressing high-frequency oscillations in a grid-connected system based on cascaded SVG under weak grid conditions. Background Technology

[0002] With the expansion of new energy power generation, renewable energy sources, represented by wind and solar power, are being connected to the power grid on a large scale via power electronic converters. This has led to an increasingly pronounced electronic nature of the power system, significantly impacting the safe and stable operation of the grid. In recent years, numerous power accidents both domestically and internationally have been directly related to the integration of renewable energy. Renewable energy is connected to the grid through power electronic converters, which possess characteristics such as low inertia and weak damping. As a large number and variety of power electronic converters are connected to the grid, the proportion of traditional synchronous generators will continue to decline. Furthermore, renewable energy power plants are generally located at the end of the grid or in areas with relatively weak grid structures. After grid connection, renewable energy needs to be transmitted over long distances via tie lines, further highlighting the small-disturbance stability issues of renewable energy grid-connected systems.

[0003] High-frequency oscillations can damage equipment or trigger protective devices, severely restricting the efficient absorption of renewable energy and threatening the safe and stable operation of the power system, causing huge economic losses to renewable energy equipment manufacturers, power generation plants, grid companies, and important power loads. Therefore, research on high-frequency oscillation suppression in renewable energy power plants has significant engineering value for improving the safety, stability, and reliability of renewable energy grid connection and transmission systems.

[0004] The formation mechanism of high-frequency oscillations in new energy grid-connected systems can be mainly divided into two types: the inherent resonance of the LC filter and the negative damping introduced by the digital control delay in the high-frequency band, which will induce high-frequency oscillations in the system when connected to the grid. Regarding the suppression of high-frequency oscillations caused by these two factors, there has been a great deal of research both domestically and internationally on improvements to the grid-connected system's structure or control. These can be mainly summarized into three aspects: passive damping schemes, active damping schemes, and reducing the total system control delay. Based on grid-side suppression, active dampers can be added to the grid-connected side. Currently, research on active dampers is relatively limited. Existing active damper technology addresses high-frequency oscillation suppression in several ways. First, due to inaccurate high-frequency oscillation localization, suppression is primarily achieved by widening the damping bandwidth of the active damper. However, widening the damping bandwidth places high demands on the controller; excessively large bandwidth can lead to poor suppression or even control system malfunction. Second, renewable energy grid-connected systems generate or absorb reactive power under different operating conditions. This reactive power can cause abrupt changes in grid-side reactive power, leading to grid-side voltage instability. Existing active damping technology typically requires external SVG (Static Var Generator) devices for compensation, which incurs additional costs. Therefore, it is essential to simultaneously and efficiently control grid-side reactive power and suppress grid-side high-frequency oscillations. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method and system for suppressing high-frequency oscillations in a grid-connected system based on cascaded SVG under weak grid conditions, which can suppress high-frequency oscillations on the grid side while achieving efficient grid-side reactive power control.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by this invention is: a method for suppressing high-frequency oscillations in a grid-connected system based on cascaded SVG under weak grid conditions, wherein each phase of the grid-connected system includes multiple cascaded SVG devices, and the three-phase cascaded SVG devices are connected in parallel to the power grid; the method includes the following steps:

[0007] Collect the voltage signal at the grid connection point, the output current signal of the lth phase, the grid-side current signal, and the capacitor voltage signal of the SVG device;

[0008] A preliminary modulated current component is generated using the capacitor voltage signal of the SVG device.

[0009] The voltage signal at the grid connection point is passed through a base frequency notch filter to obtain the grid-side oscillation voltage component other than the base frequency. A high-frequency oscillation suppression current component is generated based on this oscillation voltage component.

[0010] Based on the collected grid connection point voltage signal, the output current signal of the lth phase, and the grid-side current signal, a reactive current component compensation component is generated.

[0011] The initial modulation current component, the high-frequency oscillation suppression current component, and the reactive current compensation component are added together to obtain the reference value of the compensation current of the first phase. The reference value of the first phase current is subtracted from the output current signal of the first phase. The difference is passed through a current regulator. The output of the current regulator is controlled by the grid voltage feedforward to obtain the modulation voltage of the first phase. The driving signal is obtained by using the modulation voltage of the first phase to control the switching of each SVG device.

[0012] This invention does not employ a global voltage plus phase-to-phase voltage equalization control method. To reduce control complexity, it adopts an SVG submodule capacitor voltage control strategy, that is, directly controlling the average capacitor voltage v of each submodule in the three-phase bridge arm. Clm (l=a、b、c), and by increasing the extraction of harmonic current based on virtual resistance to adjust the given reference value of AC current, high-frequency oscillations are precisely suppressed.

[0013] The specific process for obtaining the initial modulation current component includes:

[0014] Calculate the average capacitor voltage of each SVG device in the three-phase bridge arm;

[0015] Compare the average capacitor voltage of the SVG device with the DC capacitor voltage reference command V. dcref The difference is calculated, and after the difference is normalized, it is passed through a PI regulator to obtain the amplitude of the initial modulation current.

[0016] Multiplying the amplitude by sin(θ), sin(θ-2π / 3), and sin(θ+2π / 3) respectively yields the initial modulation current.

[0017] The high-frequency oscillation suppression current component i lR The calculation formula is:

[0018]

[0019] in, v l The grid-side three-phase voltage, v lh R is the oscillating voltage component. v For the adaptive resistance value based on the oscillation component, ω n =2πf n f n For the fundamental frequency, This refers to the total harmonic component in the grid-side voltage. Here, s is the preset limit for grid voltage harmonic content, s is a complex variable, BW is the bandwidth of the second-order bandpass filter, ζ1 is the transfer function damping ratio coefficient of the fundamental frequency notch filter, and k is the voltage level. PR k DR k IRThese represent the proportional gain, derivative gain, and integral gain of the PID controller in virtual resistance adaptive control, respectively. ch It is the center frequency of the high-frequency oscillation component.

[0020] Total harmonic components in grid-side voltage The acquisition process includes: passing the harmonic voltage at the grid connection point through a band-stop filter to extract the harmonic components other than the fundamental frequency; then performing a Clark transform on the harmonic components other than the fundamental frequency to obtain the components V in the two-phase coordinate system. αh and V βh V αh and V βh The mean square value is sent to a low-pass filter for ripple removal to obtain the total harmonic component in the grid-side voltage.

[0021] The transfer function of the low-pass filter

[0022] The specific process for obtaining the reactive current component compensation component includes:

[0023] Real-time detection of reactive power on the grid side, and use the reactive power as the reactive power command value for the l-th phase;

[0024] The reactive power given command value is compared with the real-time reactive power of the l-th phase, and the difference is input into the PI regulator to obtain the specified value for reactive power current regulation.

[0025] By transforming the specified values ​​of reactive power current regulation and active power current regulation, the three-phase reactive power current regulation value is obtained, which is the reactive current component compensation component; wherein, the specified value of active power current regulation is set to 0.

[0026] The current regulator is a quasi-proportional regulator, and the transfer function of the quasi-proportional regulator is: Where, ω c For the bandwidth of the quasi-proportional regulator, K p K r These are the proportional gain and integral gain, respectively, ω o =2πf o f o Take the fundamental frequency.

[0027] As an inventive concept, the present invention also provides a high-frequency oscillation suppression system for a grid-connected system based on cascaded SVG under weak network conditions, including a memory, a processor, and a computer program stored in the memory; the processor executes the computer program to implement the steps of the above method.

[0028] As an inventive concept, the present invention also provides a computer-readable storage medium having a computer program / instructions stored thereon; when the computer program / instructions are executed by a processor, they implement the steps of the above-described method.

[0029] As an inventive concept, the present invention also provides a computer program product, including a computer program / instructions; when the computer program / instructions are executed by a processor, they implement the steps of the above-described method.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] 1. This invention employs an improved adaptive SOGI-FLL, which incorporates a second-order high-pass filter into the adaptive SOGI-FLL to filter out low- and mid-frequency oscillations, further enhancing the filtering of the center frequency f of the high-frequency oscillation component. ch The frequency locking accuracy is improved, effectively addressing the problem of inaccurate positioning due to high-frequency oscillation information;

[0032] 2. This invention employs a virtual resistance adaptive control algorithm based on the mean square value calculation of high-frequency harmonic components. Through a series of transformations, filtering, and comparisons with preset harmonic limits, the virtual resistance is adjusted via a PID controller. An amplitude limiting circuit is set to prevent system instability, enabling the virtual resistance to adaptively stabilize and better suppress high-frequency oscillations.

[0033] 3. This invention is based on a cascaded SVG device, which effectively suppresses reactive power and high-frequency oscillations on the grid side. This reduces the need for additional connected devices and lowers costs. Attached Figure Description

[0034] Figure 1 This is a flowchart of a method according to an embodiment of the present invention;

[0035] Figure 2 This is a schematic diagram of the high-frequency oscillation suppression control principle based on cascaded SVG in an embodiment of the present invention;

[0036] Figure 3 This is a schematic diagram of the capacitor voltage control principle of a submodule in an embodiment of the present invention;

[0037] Figure 4 This is a schematic diagram illustrating the extraction and suppression control principle of high-frequency oscillation components in an embodiment of the present invention.

[0038] Figure 5 This is a schematic diagram of the improved adaptive SOGI-FLL control principle according to an embodiment of the present invention;

[0039] Figure 6 The result of adaptive SOGI-FLL frequency locking should be shown in the figure;

[0040] Figure 7The result diagram shows the frequency locking performance of the improved adaptive SOGI-FLL.

[0041] Figure 8 The results are from the FFT analysis when the grid-side voltage is not suppressed.

[0042] Figure 9 This is a schematic diagram of the virtual resistance adaptive control principle.

[0043] Figure 10 The output result is a virtual resistance adaptive output diagram;

[0044] Figure 11 This is a schematic diagram of reactive power control.

[0045] Figure 12 The waveform diagram of reactive power on the grid side;

[0046] Figure 13 To suppress the voltage and current waveforms of the front and rear grid sides;

[0047] Figure 14 To suppress the front-side current FFT;

[0048] Figure 15 To suppress the back-side current FFT;

[0049] Figure 16 To suppress the front-side voltage FFT;

[0050] Figure 17 To suppress the back-side voltage FFT. Detailed Implementation

[0051] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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 embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0052] Example 1

[0053] Embodiment 1 of the present invention provides a method for suppressing high-frequency oscillations in a new energy grid-connected system based on cascaded SVG under weak grid conditions, such as... Figure 1 As shown, the main steps of this method are as follows:

[0054] Step 1: Connect the cascaded SVG device (i.e., the l-th phase bridge arm) in parallel to the new energy grid-connected system, and at the same time detect and collect the voltage signal at the grid connection point, the output current signal of the cascaded SVG device, the grid-side current signal, and the capacitor voltage signal of the SVG device in real time.

[0055] Step 2: Based on the acquired capacitor voltage signal of the SVG device, perform submodule capacitor voltage control to generate preliminary modulated current components.

[0056] Step 3: Based on the acquired grid connection point voltage signal, after processing by a baseband notch filter, the oscillation voltage components on the grid side, excluding the baseband, are obtained. Then, based on these components, high-frequency oscillation components are extracted and suppressed to generate a high-frequency oscillation suppression current component. This step includes high-frequency frequency locking adaptive control and virtual resistance adaptive control.

[0057] Step 4: Based on the collected grid connection point voltage signal, cascaded SVG device output current signal, and grid-side current signal, reactive power control based on instantaneous power theory is used to generate reactive current component compensation component.

[0058] Step 5: Add the components obtained in steps 2, 3, and 4 to form the cascaded SVG compensation current reference value. The difference between this value and the output current signal of the cascaded SVG device is then passed through a current regulator to achieve zero steady-state error tracking. After further grid voltage feedforward control, the modulation voltage of the cascaded SVG bridge arm can be obtained. CPS-PWM modulation is used to control the conduction of the switching devices in each full-bridge submodule.

[0059] Medium and high voltage SVG commonly adopts a cascaded H-bridge topology (single-star MMC). Single-star MMCs lack arm circulating current and corresponding circulating current control strategies. Addressing the shortcomings of existing technologies, this suppression strategy is based on improvements to the typical control structure of cascaded SVGs. It not only achieves reactive power control but also effectively suppresses high-frequency oscillations caused by grid-connected renewable energy systems such as direct-drive wind turbines and photovoltaic inverters, thereby enhancing the stability of renewable energy grid-connected systems. For example... Figure 2 As shown, unlike the typical control structure of a cascaded SVG, this suppression strategy does not employ a global voltage plus phase-to-phase voltage equalization control method for the DC-side capacitor voltage of the SVG. To reduce control complexity, it adopts an SVG sub-module capacitor voltage control strategy, that is, directly controlling the average capacitor voltage v of each sub-module in the three-phase bridge arm. Clm (l=a、b、c), and by increasing the extraction of harmonic current based on virtual resistance to adjust the given reference value of AC current, high-frequency oscillations are precisely suppressed.

[0060] Depend on Figure 2 As can be seen, the control structure is mainly divided into two layers: upper-layer control and lower-layer control. The upper-layer control mainly includes submodule capacitor voltage control, system reactive power control, high-frequency oscillation component extraction and suppression control, SVG compensation current negative feedback, quasi-proportional resonant control, and grid voltage feedforward, etc. (Figure L) arm For the bridge arm inductance, R arm V is the equivalent resistance of the bridge arm inductance. lm(l = a, b, c) represents the total voltage of each phase arm.

[0061] First, there's the upper-level control; the submodule capacitor voltage control generates the initial modulation current i. clref (l = a, b, c); i clref (l=a、b、c), high-frequency oscillation suppression current component i based on virtual resistance extraction lR (l=a、b、c), reactive current component compensation value i generated by system reactive power control ql The sum of (l = a, b, c) forms the cascaded SVG compensation current reference value i. lref (l = a, b, c); then after SVG compensation current negative feedback regulation, which adopts quasi-proportional resonance control; finally, after grid voltage feedforward regulation and normalization, the modulation voltage m of the three-phase bridge arm is obtained. lm (l = a, b, c). The lower-level control mainly utilizes the three-phase bridge arm modulation voltage m generated by the upper-level control. lm (l = a, b, c) is used to generate the drive signals for each switching device in the SVG submodule, which mainly includes two parts: CPS-PWM and submodule capacitor voltage sorting and balancing.

[0062] In step two, for the submodule capacitor voltage control, equation (1) is used to calculate the average capacitor voltage v of each submodule in the three-phase bridge arm. Clm (l = a, b, c), where N is the number of cascaded SVG full-bridge submodules connected in series. For example... Figure 3 As shown in the control structure block diagram, v Clm (l = a, b, c) and DC capacitor voltage reference command V dcref After differential calculation and per-unit scaling, the signal passes through a PI controller to obtain the amplitude I of the initial modulation current. cl (l=a, b, c), then multiply by sin(θ), sin(θ-2π / 3), and sin(θ+2π / 3) respectively to obtain the initial modulation current i. clref (l=a、b、c), θ is obtained from the PLL phase-locked loop of the grid-side voltage.

[0063]

[0064] In step three, the extraction and suppression control structure of the high-frequency oscillation component in this suppression method is shown in the diagram below. Figure 4 As shown. The grid-side voltage v l (l=a、b、c) passes through a fundamental frequency notch filter to obtain the grid-side oscillation voltage component v other than the fundamental frequency. lh (l=a、b、c), the cutoff frequency ω of the transfer function of the fundamental frequency notch filter in equation (2) n =2πf n fn The fundamental frequency is set to 50Hz, and the damping ratio coefficient ξ1 is set to 0.04. Based on v lh (l=a、b、c) respectively perform virtual resistance adaptive control and high frequency locking adaptive control.

[0065]

[0066] High-frequency frequency locking adaptive control is used to determine the center frequency f of the high-frequency oscillation band. ch The purpose of virtual resistance adaptive control is to obtain an adaptive resistance value R based on the oscillation component. v Inverting and reciprocating it is to convert the harmonic voltage into a reverse harmonic suppression current i based on virtual resistance. lh (l = a, b, c); then, using a second-order bandpass filter with appropriately set bandwidth, extract the f-based... ch High-frequency oscillation suppression current component i lR (l = a, b, c). For example... Figure 2 As shown, i lR (l = a, b, c), i is the output of the submodule capacitor voltage control. clref (l=a、b、c) and the reactive power control output i ql (l = a, b, c) together constitute the SVG compensation current reference value i lref (l = a, b, c).

[0067] For high-frequency oscillation suppression, it is crucial to quickly and accurately locate the high-frequency oscillation frequency. Furthermore, since this suppression method extracts the high-frequency oscillation current component using a second-order bandpass filter, and the center frequency of this bandpass filter is determined by the frequency-locking stage, the accuracy of the frequency-locking stage significantly affects the extraction of the high-frequency oscillation current, thus impacting the oscillation suppression effect.

[0068] This suppression method employs an improved adaptive SOGI-FLL, based on the grid voltage harmonic component v. lh (l = a, b, c) is obtained through an improved adaptive SOGI-FLL frequency locking circuit. For example... Figure 5As shown, taking phase a voltage as an example, the traditional SOGI-FLL aims to overcome the drawback of a large deviation in the output result due to the presence of a DC component in the input signal. While the adaptive SOGI-FLL effectively solves this problem, its purpose in this suppression method is to detect the high-frequency oscillation component of the voltage; therefore, simply eliminating the DC component is insufficient. The adaptive SOGI-FLL directly introduces kE(s) into qv', but this causes it to contain all harmonic frequency components except the fundamental frequency. To reduce the influence of other frequency bands on the frequency locking of the high-frequency components, an improved adaptive SOGI-FLL is used. This involves adding a second-order high-pass filter to the adaptive SOGI-FLL to filter out mid- and low-frequency oscillations, aiming to further enhance the suppression of the high-frequency oscillation component's center frequency f. ch Frequency locking accuracy.

[0069] The transfer function of the second-order high-pass filter selected for the improved adaptive SOGI-FLL is shown in equation (3). Where the cutoff frequency ω of the second-order high-pass filter is... n1 =2πf n1 Regarding the wideband oscillation frequency division of new energy grid-connected systems, when the oscillation frequency is greater than 420Hz, it belongs to high-frequency oscillation, therefore f n1 The frequency is set to 420Hz, and the damping ratio coefficient ξ is set to 0.707.

[0070]

[0071] like Figure 5 As shown, the working principle of the improved adaptive SOGI-FLL is as follows: First, v' and qv' are obtained through the improved SOGI circuit. v' and qv" are mutually orthogonal signals, and qv' is the target signal after removing the influence of low- and mid-frequency signals from qv". The transfer function of the SOGI part in the improved adaptive SOGI-FLL is shown in equations (4) and (6). The coefficient k is taken according to the optimal industrial coefficient of a typical second-order system. ω ch =2πf ch .

[0072]

[0073] Substituting equation (3) into equation (5) and simplifying, we get:

[0074]

[0075] Ideally, v' and v ah They are in phase and amplitude, but when the frequency ω of the FLL stage outputs... chWhen the frequency is inconsistent with the center frequency of the high-frequency voltage oscillation, the frequency deviation will cause the error E(s) to be too large. The FLL element of the improved adaptive SOGI-FLL uses E(s) to adjust the frequency ω in reverse. ch E(s) is not 0, and the FLL element is sensitive to frequency ω. ch The adjustment will not stop until E(s) is 0, at which point the frequency ω of the FLL circuit outputs will be... ch The frequency is equal to the center frequency of the high-frequency voltage oscillation, thus achieving the purpose of high-frequency frequency locking adaptive control. The parameter Γ is taken as an empirical value of 46. The qv' signal in the improved adaptive SOGI-FLL filters out the influence of low- and mid-frequency oscillations, further improving the frequency locking accuracy of the FLL stage for the center frequency of the high-frequency oscillation band.

[0076] Further Simulink simulation verification was conducted by connecting a high-frequency oscillation source to the power grid. Figure 8 The FFT analysis results of the grid-side voltage at the time without suppression clearly show that the center frequency of the high-frequency oscillation is at 1600Hz.

[0077] Comparing the two frequency locking effects, the improved adaptive SOGI-FLL frequency locking result ultimately stabilizes at 1599Hz, almost identical to the high-frequency oscillation center frequency. While the adaptive SOGI-FLL frequency locking result also achieves stable frequency locking, the final result stabilizes at 1549Hz, significantly different from the oscillation center frequency of 1600Hz. Although theoretically, increasing the bandwidth of the bandpass filter can somewhat compensate for the inaccurate frequency locking, if the center frequency difference is too large, an excessively large bandwidth of the bandpass filter can easily lead to uncontrolled extraction and suppression of the high-frequency oscillation current component. This is why this control method uses a bandpass filter instead of a high-pass filter when extracting the high-frequency oscillation component. Therefore, using the improved adaptive SOGI-FLL for high-frequency oscillation component frequency locking is essential.

[0078] Since this suppression method employs high-frequency oscillation suppression based on virtual resistance, determining the virtual resistance value is another crucial factor affecting the high-frequency oscillation suppression effect. Typically, the virtual resistance value required for simulation can be obtained through trial and error, i.e., a virtual resistance value is directly assigned, but it is not an adaptive value. If the oscillation frequency and the virtual resistance value are not set appropriately, it will result in a very large command current, easily causing system instability. Therefore, it is not suitable for engineering implementation.

[0079] This suppression method employs a virtual resistance adaptive control algorithm based on the mean square value calculation of oscillation components. The harmonic voltage at the grid connection point is passed through a band-stop filter to extract harmonic components other than the fundamental frequency. These components are then subjected to Clark transform to obtain the components in the two-phase coordinate system. This is achieved by using V... αh and V βhThe mean square value is sent to the low-pass filter G. LPF (s) Remove ripples to obtain Used to represent the total harmonic component in the grid-side voltage. G LPF (s) can be set as a second-order low-pass filter with a cutoff frequency of ω. n =2πf n f n Using a fundamental frequency of 50Hz and a damping ratio coefficient ξ of 0.707, the second-order low-pass filter exhibits a fast roll-off speed, rapidly attenuating high-frequency signals with a narrow transition band. This results in a more precise transition between the passband and stopband, leading to a purer filtered signal. This achieves a balance between ripple suppression and dynamic response of the extracted harmonic voltage.

[0080]

[0081] Then, With preset harmonic limits Compare and and The difference is transmitted to a virtual resistor regulator, which can be implemented using a PID controller. PID controllers offer high control precision, accurately tracking the setpoint, good stability, excellent dynamic performance, fast response, and strong anti-interference capabilities. Because in The value of the virtual resistance can increase significantly during dynamic and transient changes. Therefore, the virtual resistance value will surge, potentially causing irreversible system instability. Adding a limiting circuit at the end of the output effectively avoids this. v Excessively large.

[0082] To limit the harmonic content of the grid voltage, according to GB / T14549, the harmonic requirement for 35kV grid-connected voltage must meet 4% V. g In the following simulation, V is taken as... lim =2%V g V g This refers to the standard value of the phase voltage amplitude of a 35kV power grid, for example:

[0083]

[0084] based on Figure 9 The virtual resistor adaptive control structure block diagram shown is built and Simulink simulation is run. The total simulation time is 4s. Suppression is performed at 2s. It is obvious that the virtual resistor can eventually adapt to a stable value.

[0085] In the simulation, the reciprocal of the virtual resistance adaptive result is taken, i.e., 1 / R. v Then, by taking the negative of this value and multiplying it by the oscillation voltage value, the reference value of the reverse total oscillation current i can be obtained. lh(l = a, b, c). Used for the next step of extracting the SVG high-frequency oscillation suppression current component.

[0086]

[0087] As shown in equation (13), the reverse total oscillation current reference value i is then used. lh (l=a、b、c) can be passed through a second-order bandpass filter to obtain the high-frequency oscillation suppression current component i. lR (l = a, b, c), the center frequency of this second-order bandpass filter is taken from the frequency-locked result ω of the improved adaptive SOGI-FLL. ch =2πf ch The bandwidth BW can be appropriately increased based on the high-frequency oscillation situation.

[0088] Damping coefficient ξ of a second-order bandpass filter ch The relationship between the center frequency and the calculation is shown in equation (14).

[0089]

[0090] In step four, because the renewable energy grid-connected system generates or absorbs reactive power under different operating conditions, it causes sudden changes in reactive power on the grid side. For example... Figure 11 As shown in the block diagram of the reactive power control structure, the reactive power control element of this suppression method adopts a control method based on instantaneous reactive power theory. By detecting the reactive power on the grid side in real time, it is used as the reactive power command value Q of the cascaded SVG. ref The reactive power Q of the cascaded SVG is compared with the reactive power current Q, and after passing through a PI regulator, a specified value i for reactive power current regulation is generated. qref There is a specified value i for power current adjustment. dref The value is set to 0, and then the three-phase reactive power current regulation value i is generated through coordinate transformation. ql This enables reactive power regulation using the suppression method described herein.

[0091]

[0092]

[0093] In equation (19), k PQ k IQ The control parameters of the PI regulator are given by equation (20), where the coordinate transformation angle θ is obtained from the PLL phase-locked loop of the grid-side voltage.

[0094] Based on the reactive power control structure block diagram, a Simulink simulation was built and run. The total simulation time was 4 seconds, with suppression implemented at 2 seconds. The grid-side reactive power was as follows: Figure 12As shown, 2 seconds ago, the new energy grid-connected system generated a large amount of reactive power, which flowed to the grid side, causing an increase in grid-side reactive power. After 2 seconds, the reactive power suppression strategy was introduced, and the cascaded SVG absorbed the reactive power generated by the new energy grid-connected system, resulting in a decrease in grid-side reactive power.

[0095] In step five, the initial modulation current i clref (l=a、b、c), high-frequency oscillation suppression current component i lR (l=a、b、c), reactive current component compensation value i ql The sum of (l = a, b, c) forms the reference value i for the three-phase alternating current. lref (l = a, b, c).

[0096] i lref =i ql +i lR +i clref (twenty one)

[0097] The current regulator in this suppression method uses a quasi-proportional regulator. This is beneficial for eliminating the dynamic error (transient error) of the system. The transfer function of the quasi-proportional regulator is shown in equation (22), where K p K r These are the proportional gain and integral gain, respectively; resonant frequency ω. o =2πf o f o The base frequency is set to 50Hz; as for the bandwidth ω c The value of ω is typically taken considering the grid voltage ±0.5Hz. c =π.

[0098]

[0099] The proportional regulator, after being processed by grid voltage feedforward control and normalization, yields the modulation voltage m of the three-phase bridge arm. lm (l = a, b, c). Employing grid voltage feedforward can increase the grid-side voltage response speed while improving the grid-side power quality.

[0100] Finally, the CPS-PWM modulation method and the submodule capacitor voltage sorting and equalization are used to determine the conduction of each full-bridge submodule switching device on each phase arm.

[0101] This high-frequency oscillation source is generated by the coupling between the LC filter of the direct-drive wind turbine system and the grid-side impedance. Under weak grid conditions, even a low level of high-frequency current oscillation can cause significant grid voltage oscillation. Figure 13 The graphs show the grid-side voltage and current waveforms before and after the implementation of this suppression method. It is clearly evident that the high-frequency oscillations in the grid voltage and current are significantly improved after the implementation of this suppression method.

[0102] Perform FFT analysis on the grid voltage and current before and after suppression, as follows: Figures 14 to 17 As shown, it is clear that the amplitude at the center frequency of the high-frequency oscillation band has been effectively suppressed. The center frequency of the grid-side current oscillation has decreased from 4% to 0.57%, and the center frequency of the grid-side voltage oscillation has decreased from 60% to 2.75%. At the same time, the cascaded SVG absorbs the reactive power on the grid side, so that the amplitude of the grid voltage fundamental wave remains stable, which further illustrates the feasibility of this suppression method.

[0103] Example 2

[0104] Embodiment 2 of the present invention provides a suppression system corresponding to Embodiment 1 above, which includes a memory, a processor and a computer program stored in the memory; the processor executes the computer program in the memory to implement the steps of the method of Embodiment 1 above.

[0105] In some implementations, the memory may be high-speed random access memory (RAM), and may also include non-volatile memory, such as at least one disk storage device.

[0106] In other implementations, the processor can be any type of general-purpose processor, such as a central processing unit (CPU) or a digital signal processor (DSP), and there is no limitation here.

[0107] Example 3

[0108] Embodiment 3 of the present invention provides a computer-readable storage medium corresponding to Embodiment 1 above, on which a computer program / instructions are stored. When the computer program / instructions are executed by a processor, they implement the steps of the method of Embodiment 1 above.

[0109] A computer-readable storage medium can be a tangible device that holds and stores instructions for use by an instruction execution device. A computer-readable storage medium can be, for example, but not limited to, an electrical storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any combination thereof.

[0110] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code. The solutions in the embodiments of this application can be implemented in various computer languages, such as the object-oriented programming language Java and the interpreted scripting language JavaScript.

[0111] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. 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... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0112] 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.

[0113] Although preferred embodiments of this application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this application.

[0114] Obviously, those skilled in the art can make various modifications and variations to this application without departing from the spirit and scope of this application. Therefore, if such modifications and variations fall within the scope of the claims of this application and their equivalents, this application also intends to include such modifications and variations.

Claims

1. A method for suppressing high-frequency oscillations in a grid-connected system based on cascaded SVG under weak grid conditions, wherein each phase of the grid-connected system includes multiple cascaded SVG devices, and the three-phase cascaded SVG devices are connected in parallel to the power grid; characterized in that, Includes the following steps: Collect the voltage signal at the grid connection point, the output current signal of the lth phase of the SVG device, the grid-side current signal, and the capacitor voltage signal of the SVG device; A preliminary modulated current component is generated using the capacitor voltage signal of the SVG device. The voltage signal at the grid connection point is passed through a base frequency notch filter to obtain the oscillating voltage component on the grid side other than the base frequency. A high-frequency oscillation suppression current component is generated based on this oscillating voltage component. Based on the collected grid connection point voltage signal, the output current signal of the l-th phase, and the grid-side current signal, a reactive current component compensation component is generated. ; The initial modulation current component, the high-frequency oscillation suppression current component, and the reactive current compensation component are added together to obtain the compensation current reference value of the l-th phase. The difference between the cascaded SVG compensation current reference value and the output current signal of the l-th phase is calculated. The difference is passed through a current regulator. The output of the current regulator is passed through the grid voltage feedforward control to obtain the modulation voltage of the l-th phase. The modulation voltage of the l-th phase is used to obtain the drive signal to control the on / off state of the switching devices of each SVG device. The high-frequency oscillation suppresses the current component The calculation formula is: ； ； Among them, the damping coefficient of the second-order bandpass filter ; ; ; For grid-side three-phase voltage, It is the oscillating voltage component. The cutoff frequency of the transfer function of the fundamental frequency notch filter is based on the adaptive resistance value of the oscillation component. , For the fundamental frequency, This refers to the total harmonic component in the grid-side voltage. Here, s is the preset limit for grid voltage harmonic content, s is a complex variable, and BW is the bandwidth of the second-order bandpass filter. The transfer function damping ratio coefficient of the fundamental frequency notch filter. , , These represent the proportional gain, derivative gain, and integral gain of the PID controller in virtual resistance adaptive control. The center frequency of the high-frequency oscillation component, and the center frequency of the second-order bandpass filter. Total harmonic components in grid-side voltage The acquisition process includes: passing the harmonic voltage at the grid connection point through a band-stop filter to extract the harmonic components other than the fundamental frequency; then performing Clark transform on the harmonic components other than the fundamental frequency to obtain the components in the two-phase coordinate system. and ,Will and The mean square value is sent to a low-pass filter for ripple removal to obtain the total harmonic component in the grid-side voltage. .

2. The method for suppressing high-frequency oscillations in a grid-connected system based on cascaded SVG under weak network conditions as described in claim 1, characterized in that, The specific process for obtaining the initial modulation current component includes: Calculate the average capacitor voltage of each SVG device in the three-phase bridge arm; Compare the average capacitor voltage of the SVG device with the DC capacitor voltage reference command. The difference is calculated, and after the difference is normalized, it is passed through a PI regulator to obtain the amplitude of the initial modulation current. Multiply the amplitude by respectively , , This means obtaining the initial modulation current.

3. The method for suppressing high-frequency oscillations in a grid-connected system based on cascaded SVG under weak network conditions as described in claim 1, characterized in that, The transfer function of the low-pass filter .

4. The method for suppressing high-frequency oscillations in a grid-connected system based on cascaded SVG under weak network conditions as described in claim 1, characterized in that, The specific process for obtaining the reactive current component compensation component includes: Real-time detection of reactive power on the grid side, and use the reactive power as the reactive power command value for the l-th phase; The reactive power given command value is compared with the real-time reactive power of the l-th phase, and the difference is input into the PI regulator to obtain the specified value for reactive power current regulation. By transforming the specified values ​​of reactive power current regulation and active power current regulation, the three-phase reactive power current regulation value is obtained, which is the reactive current component compensation component; wherein, the specified value of active power current regulation is set to 0.

5. The method for suppressing high-frequency oscillations in a grid-connected system based on cascaded SVG under weak network conditions as described in claim 1, characterized in that, The current regulator is a quasi-proportional regulator, and the transfer function of the quasi-proportional regulator is: ;in, The bandwidth of the quasi-proportional regulator. , These represent the proportional gain and integral gain, respectively, and the resonant frequency of the quasi-proportional regulator. , =50Hz.

6. A high-frequency oscillation suppression system for a grid-connected system based on cascaded SVG under weak network conditions, comprising a memory, a processor, and a computer program stored in the memory; characterized in that, The processor executes the computer program to implement the steps of the method according to any one of claims 1 to 5.

7. A computer-readable storage medium having a computer program / instructions stored thereon; characterized in that, When the computer program / instructions are executed by the processor, they implement the steps of the method described in any one of claims 1 to 5.

8. A computer program product comprising a computer program / instructions; characterized in that, When the computer program / instruction is executed by the processor, it implements the steps of the method described in any one of claims 1 to 5.

Images