Method and system for suppressing high-frequency oscillation of grid-connected system based on cascaded SVG (Static Var Generator) in weak network state

Abstract

The invention discloses a cascaded SVG-based grid-connected system high-frequency oscillation suppression method and system in a weak network state. The method comprises the following steps: generating a preliminary modulation current component by using a capacitor voltage signal of an SVG device; enabling a voltage signal of a grid-connected point to pass through a fundamental frequency wave trap, obtaining an oscillation voltage component of a grid side except a fundamental frequency, and generating a high-frequency oscillation suppression current component based on the oscillation voltage component; generating a reactive current component compensation component based on the grid-connected point voltage signal, the cascade SVG device output current signal and the grid-side current signal; adding the preliminary modulation current component, the high-frequency oscillation suppression current component and the reactive current component compensation component to obtain a cascade SVG compensation current reference value, making a difference between the cascade SVG compensation current reference value and an output current signal of the cascade SVG device, enabling the difference to pass through a current regulator, and enabling the output of the current regulator to be subjected to power grid voltage feedforward control to obtain a compensation current signal of the cascade SVG device. And the cascaded SVG bridge arm modulation voltage is obtained. According to the invention, high-efficiency grid-side reactive power control is realized, and grid-side high-frequency oscillation is suppressed at the same time.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronic control, and in particular to a method and system for suppressing high-frequency oscillation of a grid-connected system based on cascaded SVG in a weak grid state. Background Art

[0002] With the expansion of the scale of renewable energy power generation, renewable energy represented by wind and solar power is connected to the power grid on a large scale through power electronic converters as interfaces. The power electronic characteristics of the power system are becoming increasingly obvious, which has a huge impact on the safe and stable operation of the power grid. In recent years, many power accidents at home and abroad are directly related to the access of renewable energy. New energy is connected to the power grid through power electronic converters. Power electronic converters have characteristics such as low inertia and weak damping. With the access of a large number of power electronic converters of various types to the power grid, the proportion of traditional synchronous generators will continue to decline. In addition, renewable energy stations are generally located at the end of the power grid or in areas with relatively weak grid structures. After renewable energy is connected to the grid, it is necessary to transmit power to the outside over long distances through interconnection lines, which makes the small disturbance stability problem of renewable energy grid-connected systems more prominent.

[0003] High-frequency oscillations can cause equipment damage or trigger the action of protective devices, which not only seriously restricts the efficient consumption of renewable energy, but also threatens the safe and stable operation of the power system, causing huge economic losses to new energy equipment manufacturers, power generation plants, power grid companies, and important power loads. Therefore, the study of high-frequency oscillation suppression in new energy stations has important engineering value for improving the safety, stability, and reliability of new energy grid connection and its transmission system.

[0004] The formation mechanism of high-frequency oscillation in the new energy grid-connected system 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 cause high-frequency oscillation in the system when connected to the grid. There are a lot of research results at home and abroad on the suppression of high-frequency oscillation caused by both. Based on the improvement of the structure or control of the grid-connected system itself, there have been a lot of research results at home and abroad, which can be summarized into three aspects: passive damping scheme, active damping scheme, and reducing the total control delay of the system. Based on the suppression on the grid side, active dampers can be added on the grid-connected side. Currently, there are relatively few studies on active dampers. The existing active damper technology suppresses the high-frequency oscillation of the system. On the one hand, due to the inaccurate positioning of high-frequency oscillation information, the high-frequency oscillation is suppressed mainly by widening the damping band of the active damper. However, widening the damping band has high requirements for the controller. If the bandwidth of the damping band is too large, it is easy to cause poor suppression effect or even control system out of control; on the other hand, because the new energy grid-connected system will generate or absorb reactive power under different working conditions, these reactive powers will cause a sudden change in reactive power on the grid side, and then cause instability in the grid side voltage. If the existing active damping technology is used, it is usually necessary to add an SVG device externally for compensation, but this will also cause additional costs. Therefore, it is very necessary to simultaneously and efficiently control the reactive power on the grid side and suppress the high-frequency oscillation on the grid side. 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 oscillation of a grid-connected system based on cascaded SVG under a weak grid state in view of the deficiencies in the prior art, so as to suppress high-frequency oscillation on the grid side while realizing efficient grid-side reactive power control.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for suppressing high-frequency oscillation of a grid-connected system based on cascaded SVG in a weak grid state, wherein each phase of the grid-connected system includes a plurality of cascaded SVG devices, and the three-phase cascaded SVG devices are connected in parallel to the grid; comprising the following steps:

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

[0008] generating a preliminary modulation current component using the capacitor voltage signal of the SVG device;

[0009] Passing the voltage signal of the grid-connected point through a fundamental frequency notch filter to obtain an oscillating voltage component other than the fundamental frequency on the grid side, and generating a high-frequency oscillation suppression current component based on the oscillating voltage component;

[0010] Generate a reactive current component compensation component based on the collected grid connection point voltage signal, the first phase output current signal, and the grid side current signal;

[0011] The preliminary modulation current component, the high-frequency oscillation suppression current component, and the reactive current component compensation component are added together to obtain a first-phase compensation current reference value, the first-phase current reference value is subtracted from the first-phase output current signal, the difference is passed through a current regulator, the output of the current regulator is subjected to grid voltage feedforward control to obtain a first-phase modulation voltage, the first-phase modulation voltage is used to obtain a drive signal to control the on and off of the switch devices of each SVG device.

[0012] The present invention does not adopt the control method of global voltage plus phase-to-phase voltage balance. In order to reduce the control complexity, the SVG submodule capacitor voltage control strategy is adopted, that is, the capacitor voltage mean value v of each submodule of the three-phase bridge arm is directly controlled. Clm (l=a, b, c), and the given reference value of the AC current is adjusted by increasing the extraction of harmonic current based on virtual resistance, so as to accurately suppress high-frequency oscillation.

[0013] The specific acquisition process of the preliminary modulation current component includes:

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

[0015] The average capacitor voltage of the SVG device is compared with the DC capacitor voltage reference command V dcref After the difference is normalized, it passes through the PI regulator to obtain the amplitude of the preliminary modulation current;

[0016] The amplitude is multiplied by sin(θ), sin(θ-2π / 3), and sin(θ+2π / 3) respectively to obtain a preliminary modulation current.

[0017] The high frequency oscillation suppresses the current component i lR The calculation formula is:

[0018]

[0019] in, v l is the three-phase voltage on the grid side, v lh is the oscillating voltage component, R v is the adaptive resistance value based on the oscillation component, ω n =2πf n , f n is the fundamental frequency, is the total harmonic component in the grid-side voltage, is the preset grid voltage harmonic content limit value, 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, k PR , k DR , k IRThey are respectively the proportional gain, differential gain and integral gain of the PID regulator in the virtual resistance adaptive control, f ch is the center frequency of the high-frequency oscillation component.

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

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

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

[0023] Real-time detection of reactive power on the grid side, and use of the reactive power as a given reactive power instruction value for the first phase;

[0024] Compare the reactive power given command value with the real-time reactive power of the first phase, input the difference into the PI regulator, and obtain the reactive power current regulation specified value;

[0025] The reactive power current adjustment designated value and the active power current adjustment designated value are subjected to coordinate transformation to obtain the three-phase reactive power current adjustment value, that is, the reactive current component compensation component; wherein the active power current adjustment designated value is set to 0.

[0026] The current regulator adopts a quasi-proportional regulator, and the transfer function of the quasi-proportional regulator is: Among them, ω c is the bandwidth of the quasi-proportional regulator, K p , K r are proportional gain and integral gain respectively, ω o =2πf o , f o Get 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 a weak grid state, comprising 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 / instruction stored thereon; the computer program / instruction implements the steps of the above method when executed by a processor.

[0029] As an inventive concept, the present invention also provides a computer program product, including a computer program / instruction; when the computer program / instruction is executed by a processor, the steps of the above method are implemented.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1. The present invention adopts an improved adaptive SOGI-FLL, that is, a second-order high-pass filter is added to the adaptive SOGI-FLL to filter out medium and low frequency oscillations, further strengthening the center frequency f of the high frequency oscillation component. ch The frequency locking accuracy effectively improves the problem of inaccurate positioning due to high-frequency oscillation information;

[0032] 2. The present invention uses a virtual resistance adaptive control algorithm based on the mean square value calculation of high-frequency harmonic components. Through a series of operations such as transformation, filtering, and comparison with preset harmonic limits, the virtual resistance is adjusted through a PID regulator, and a limit link is set to avoid system instability, so that the virtual resistance can adapt to a stable value and better suppress high-frequency oscillation.

[0033] 3. The present invention is based on cascaded SVG devices, and can effectively suppress the reactive power on the grid side and the high-frequency oscillation on the grid side, thereby reducing the number of additional connected devices and the cost. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

[0037] Figure 4 The figure is a schematic diagram of the extraction and suppression control of high-frequency oscillation components according to an embodiment of the present invention;

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

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

[0040] Figure 7This is the frequency locking result diagram of the improved adaptive SOGI-FLL;

[0041] Figure 8 This is the FFT analysis result when the grid-side voltage is not suppressed;

[0042] Fig. 9 This is the principle diagram of virtual resistance adaptive control;

[0043] Fig.10 This is the result diagram of the virtual resistance adaptive output;

[0044] Fig.11 It is the principle diagram of reactive power control;

[0045] Fig.12 It is the grid-side reactive power waveform diagram;

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

[0047] Fig.14 To suppress the front grid side current FFT;

[0048] Fig.15 To suppress the FFT of the rear grid-side current;

[0049] Fig.16 To suppress the front grid side voltage FFT;

[0050] Fig.17 To suppress the FFT of the rear grid side voltage. DETAILED DESCRIPTION

[0051] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work 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 oscillation of a new energy grid-connected system based on cascaded SVG in a weak grid state. Figure 1 As shown, the main steps of this method are as follows:

[0054] Step 1: Connect the cascaded SVG device (i.e., the first phase bridge arm) in parallel to the new energy grid-connected system, and simultaneously detect and collect the voltage signal of the grid-connected 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 collected capacitor voltage signal of the SVG device, the sub-module capacitor voltage is controlled to generate a preliminary modulated current component.

[0056] Step 3: Based on the collected grid connection point voltage signal, after being processed by a baseband notch filter, the grid-side oscillation voltage component other than the baseband is obtained, and then based on this component, the high-frequency oscillation component is 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, the output current signal of the cascaded SVG device, and the grid-side current signal, reactive power control based on instantaneous power theory is adopted to generate reactive current component compensation components.

[0058] Step 5: Add the components obtained in steps 2, 3, and 4 to form a cascaded SVG compensation current reference value, which is used as a difference between the output current signal of the cascaded SVG device and passed through a current regulator to achieve zero-static tracking. After grid voltage feedforward control, the cascaded SVG bridge arm modulation voltage can be obtained. The CPS-PWM modulation method is used to control the conduction of the switch devices of each full-bridge sub-module.

[0059] Medium and high voltage SVG generally adopts cascade H-bridge topology (single star MMC), and single star MMC does not have bridge arm circulation and corresponding circulation control strategy. In view of the shortcomings of the existing technology, this suppression strategy is based on the improvement of the typical control structure of cascaded SVG, which can not only realize reactive power control, but also effectively suppress the high-frequency oscillation caused by new energy grid-connected systems such as direct-drive wind turbines and photovoltaic inverters, thereby enhancing the stability of new energy grid-connected systems. Figure 2 As shown in the figure, different from the typical control structure of cascaded SVG, for the SVG DC side capacitor voltage, this suppression strategy does not adopt the control method of global voltage plus phase-to-phase voltage sharing. In order to reduce the control complexity, it adopts the SVG sub-module capacitor voltage control strategy, that is, directly controlling the capacitor voltage mean value v of each sub-module of 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 oscillation is accurately suppressed.

[0060] Depend on Figure 2 It can be seen that the control structure is mainly divided into two layers, namely upper control and lower control. The upper 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 resonance control, grid voltage feedforward, etc. arm is the bridge arm inductance, R arm is the equivalent resistance of the bridge arm inductance, v lm(l=a, b, c) is the total voltage of each phase bridge arm.

[0061] First, the upper layer controls the submodule capacitor voltage control to generate the initial modulation current i clref (l=a、b、c); i clref (l = a, b, c), high-frequency oscillation suppression current component i extracted based on virtual resistance lR (l = a, b, c), the system reactive power control generates reactive current component compensation value i ql (l=a、b、c) The three are added to form the cascade SVG compensation current reference value i lref (l=a, b, c); then it is adjusted by SVG compensation current negative feedback, which adopts quasi-proportional resonance control; finally, it is adjusted by grid voltage feedforward and normalized to obtain the modulation voltage m of the three-phase bridge arm. lm (l=a, b, c). The lower control mainly uses the three-phase bridge arm modulation voltage m generated by the upper control lm (l=a, b, c) is used to generate driving signals for each switch device of the SVG submodule, which mainly includes two parts: CPS-PWM and submodule capacitor voltage sorting and balancing.

[0062] In step 2, for submodule capacitor voltage control, equation (1) is used to calculate the capacitor voltage mean value v of each submodule in the three-phase bridge arm: Clm (l=a、b、c), N is the number of full bridge submodules in cascaded SVG series. Figure 3 As shown in the control structure diagram, v Clm (l=a, b, c) and DC capacitor voltage reference command V dcref After the difference is made and the current is normalized, it passes through a PI regulator to obtain the initial modulation current amplitude I cl (l=a, b, c), and then multiply by sin(θ), sin(θ-2π / 3), sin(θ+2π / 3) respectively, to obtain the preliminary modulation current i clref (l=a, b, c), θ is obtained by the PLL phase-locked loop of the grid-side voltage.

[0063]

[0064] In step 3, the extraction of high-frequency oscillation components and the control structure diagram of the suppression method are shown in the following figure: Figure 4 As shown. l (l=a, b, c) After a fundamental frequency notch filter, the oscillating voltage component v other than the fundamental frequency is obtained on the grid side lh (l = a, b, c), the transfer function cutoff frequency ω of the baseband notch filter in formula (2) n =2πf n , fn The fundamental frequency is 50Hz and the damping ratio coefficient ξ1 is 0.04. lh (l=a, b, c) respectively perform virtual resistance adaptive control and high frequency locking adaptive control.

[0065]

[0066] High frequency locking adaptive control is to determine the center frequency f of the high frequency oscillation band ch The purpose of the virtual resistance adaptive control is to obtain an adaptive resistance value R based on the oscillation component. v The purpose of negating and reciprocating it is to convert the harmonic voltage into a reverse harmonic suppression current i based on a virtual resistor. lh (l=a, b, c); then pass it through a second-order bandpass filter, set its bandwidth reasonably, and extract the f-based ch The high-frequency oscillation suppression current component i lR (l=a、b、c). Figure 2 As shown, lR (l=a, b, c), the submodule capacitor voltage control output i clref (l=a, b, c) and 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, how to quickly and accurately locate the high-frequency oscillation frequency is extremely important. At the same time, since this suppression method extracts the high-frequency oscillation current component through a second-order bandpass filter when suppressing the high-frequency oscillation current, and the center frequency of this bandpass filter is determined by the frequency locking link, the accuracy of the frequency locking link will greatly affect the extraction of the high-frequency oscillation current, and thus affect the oscillation suppression effect.

[0068] This suppression method adopts the improved adaptive SOGI-FLL, which is composed of the grid voltage harmonic component v lh (l = a, b, c) is obtained through the improved adaptive SOGI-FLL frequency locking link. Figure 5As shown, taking the a-phase voltage as an example, the traditional SOGI-FLL will cause the output result to have a large deviation in order to overcome the DC component of the input signal. Although the adaptive SOGI-FLL can effectively solve this problem, since its purpose in this suppression method is to detect the high-frequency oscillation component of the voltage, it is not enough to simply eliminate the influence of the DC component. The adaptive SOGI-FLL directly introduces kE(s) into qv', but it will also contain all harmonic frequency components except the fundamental frequency. In order to reduce the influence of other frequency bands on the frequency locking of the high-frequency band components, an improved adaptive SOGI-FLL is used, that is, a second-order high-pass filter is added to the adaptive SOGI-FLL to filter out medium and low-frequency oscillations, in order to further enhance the center frequency f of the high-frequency oscillation component. ch The frequency locking accuracy.

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

[0070]

[0071] like Figure 5 As shown in Figure 1, the working principle of the improved adaptive SOGI-FLL is as follows: first, v' and qv' are obtained through the improved SOGI link. v' and qv' are mutually orthogonal signals, and qv' is the target signal after qv' removes the influence of the medium and low frequency signals. 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 the typical second-order system. ω ch =2πf ch .

[0072]

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

[0074]

[0075] Ideally, v' and v ah are of the same amplitude and phase, but when the frequency ω output by the FLL link chWhen the frequency is inconsistent with the center frequency of the voltage high-frequency oscillation, the frequency deviation will cause the error E(s) to be too large. The FLL link of the improved adaptive SOGI-FLL uses E(s) to adjust the frequency ω in turn. ch , E(s) is not 0, the FLL link has a frequency of ω ch The regulation will not stop until E(s) is 0 and the frequency ω output by the FLL link is ch The qv' signal in the improved adaptive SOGI-FLL filters out the influence of low- and medium-frequency oscillations, further improving the frequency locking accuracy of the FLL link to the center frequency of the high-frequency oscillation band.

[0076] Further simulink simulation verification is carried out to connect the high-frequency oscillation source to the power grid. Figure 8 This is the FFT analysis result of the grid-side voltage when no suppression is performed. It can be clearly seen 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 finally stabilizes at 1599Hz, which is the same as the center frequency of the high-frequency oscillation. Although the adaptive SOGI-FLL frequency locking result can also stably lock the frequency, the final result of the frequency locking is stabilized at 1549Hz, which is quite different from the 1600Hz center frequency of the oscillation. Although in theory the influence of inaccurate frequency locking can be compensated to a certain extent by increasing the bandwidth of the bandpass filter, if the center frequency difference is too large and the bandwidth of the bandpass filter is too large, it is easy to cause the entire high-frequency oscillation current component extraction and suppression to be out of control. This is also the reason why this control method uses a bandpass filter instead of a high-pass filter when extracting high-frequency oscillation components. Therefore, it is necessary to use the improved adaptive SOGI-FLL to lock the high-frequency oscillation component.

[0078] Since this suppression method uses high-frequency oscillation suppression based on virtual resistance, the determination of the virtual resistance value is another important factor affecting the high-frequency oscillation suppression effect. Usually, the virtual resistance value required in simulation can be obtained through trial and error, that is, a virtual resistance value is directly given, but it is a non-adaptive value. If a certain oscillation frequency and virtual resistance value are not set appropriately, the command current will be very large, which can easily cause system instability, so it is not applicable in engineering implementation.

[0079] This suppression method adopts a virtual resistor adaptive control algorithm based on the mean square value calculation of the oscillation component. The harmonic voltage of the grid connection point is extracted through a band-stop filter except the fundamental frequency, and then the harmonic components are obtained by Clark transformation in the two-phase coordinate system. αh and V βhThe mean square value is sent to the low-pass filter G LPF (s) removes the ripples and obtains Used to indicate the total harmonic component in the grid-side voltage. LPF (s) can be set as a second-order low-pass filter with a cutoff frequency of ω n =2πf n , f n The base frequency is 50Hz, and the damping ratio coefficient ξ is 0.707. The second-order low-pass filter has a fast roll-off speed, can quickly attenuate high-frequency signals, and has a narrow transition band. The transition between the passband and the stopband is more accurate, and the filtered signal is purer. This achieves a balance between ripple suppression and dynamic response of the extracted harmonic voltage.

[0080]

[0081] Then, With preset harmonic limits to compare and and The difference is transmitted to the virtual resistance regulator, which can be realized by PID regulator. PID regulator has high control accuracy, can accurately track the set value, has good stability, excellent dynamic performance, fast response speed and strong anti-interference ability. The dynamic transient change process may increase greatly. Therefore, the virtual resistance value will increase suddenly, which will cause irreversible instability of the system. Therefore, adding a limiting link at the end of the output can effectively avoid R v Too big a situation.

[0082] To set the grid voltage harmonic content limit, according to GB / T14549, the 35KV grid voltage harmonic requirement must meet 4%V g In the following, V lim =2%V g , V g It is the standard value of phase voltage amplitude of 35KV power grid, for example:

[0083]

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

[0085] In the simulation, the virtual resistance adaptive result is taken as the inverse, that is, 1 / R v , and then multiply the negative value by the oscillation voltage value to obtain the reverse total oscillation current reference value i lh(l=a, b, c) is used to extract the SVG high-frequency oscillation suppression current component in the next step.

[0086]

[0087] As shown in formula (13), the total oscillating current reference value i in the reverse direction is lh (l = a, b, c) through a second-order bandpass filter, the high-frequency oscillation suppression current component i can be obtained lR (l = a, b, c), the center frequency of the second-order bandpass filter is taken from the improved adaptive SOGI-FLL frequency locking result ω ch =2πf ch , the bandwidth BW can be appropriately increased according to the high-frequency oscillation situation.

[0088] Second-order bandpass filter damping coefficient ξ ch The calculation relationship with the center frequency is shown in formula (14).

[0089]

[0090] In step 4, the new energy grid-connected system will generate or absorb reactive power under different working conditions, resulting in a sudden change in reactive power on the grid side. Fig.11 As shown in the reactive power control structure diagram, the reactive power link of this suppression method adopts a control method based on instantaneous reactive power theory, which detects the reactive power on the grid side in real time and uses it as the reactive power given instruction value Q of the cascaded SVG. ref , compared with the real-time reactive power Q of the cascaded SVG, and through a PI regulator, a reactive power current regulation specified value i is generated qref , with power current regulation specified value i dref Take it as 0, and then generate the three-phase reactive power current adjustment value i after coordinate change ql , thereby realizing the reactive power regulation of this suppression method.

[0091]

[0092]

[0093] In formula (19), k PQ , k IQ is the control parameter of the PI regulator, and the coordinate transformation angle θ in equation (20) is obtained by the PLL phase-locked loop of the grid-side voltage.

[0094] Based on the reactive power control structure diagram, a simulink simulation is built and run. The total simulation time is 4s. Suppression is performed at 2s. The reactive power on the grid side is Fig.12As shown in the figure, 2s ago, the renewable energy grid-connected system generated a large amount of reactive power, which flowed to the grid side, causing the reactive power on the grid side to increase. 2s later, with the addition of the reactive power suppression strategy, the cascaded SVG absorbed the reactive power generated by the renewable energy grid-connected system, causing the reactive power on the grid side to decrease.

[0095] In step 5, 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 (l = a, b, c) The three are added to form the three-phase AC current reference value i lref (l=a, b, c).

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

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

[0098]

[0099] The quasi-proportional regulator is subjected to grid voltage feed-forward control and normalization to obtain the modulation voltage m of the three-phase bridge arm. lm (l = a, b, c). Using grid voltage feedforward can increase the grid voltage response speed and improve the power quality on the grid side.

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

[0101] The 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, relatively low-content high-frequency current oscillations can cause large grid voltage oscillations. Fig.13 The following are the voltage and current waveforms of the grid before and after the suppression method is added. It can be seen intuitively that the high-frequency oscillation of the grid voltage and current has been significantly improved after the suppression method is added.

[0102] Perform FFT analysis on the grid voltage and current before and after suppression, such as Figure 14 to Figure 17 As shown, it is obvious that the amplitude at the center frequency of the high-frequency oscillation band is effectively suppressed, the center frequency of the grid-side current oscillation is reduced from 4% to 0.57%, and the center frequency of the grid-side voltage oscillation is reduced from 60% to 2.75%. At the same time, the cascaded SVG absorbs the grid-side reactive power, 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 the above-mentioned embodiment 1, 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 the above-mentioned embodiment 1.

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

[0106] In some other implementations, the processor may be a central processing unit (CPU), a digital signal processor (DSP), or other general-purpose processors of various types, which are not limited herein.

[0107] Example 3

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

[0109] Computer readable storage media can be tangible devices that hold and store instructions used by instruction execution devices. Computer readable storage media can be, for example, but not limited to, electronic storage devices, magnetic storage devices, optical storage devices, electromagnetic storage devices, semiconductor storage devices, or any combination thereof.

[0110] Those skilled in the art will appreciate that the embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the present application can adopt the form of complete hardware embodiments, complete software embodiments, or embodiments in combination with software and hardware. Moreover, the present application can adopt 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.) that contain computer-usable program code. The scheme in the embodiments of the present application can be implemented in various computer languages, for example, object-oriented programming language Java and literal scripting language JavaScript, etc.

[0111] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0112] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 A step that specifies a function in one or more boxes.

[0113] Although the preferred embodiments of the present application have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications falling within the scope of the present application.

[0114] Obviously, those skilled in the art can make various changes and modifications to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalents, the present application is also intended to include these modifications and variations.

Claims

1. A method for suppressing high-frequency oscillation of a grid-connected system based on cascaded SVG in a weak grid state, wherein each phase of the grid-connected system includes a plurality of cascaded SVG devices, and the three-phase cascaded SVG devices are connected in parallel to the grid; characterized in that: The following steps are involved: Collect the voltage signal of the grid connection point, the output current signal of the first phase, the grid-side current signal, and the capacitor voltage signal of the SVG device; generating a preliminary modulation current component using the capacitor voltage signal of the SVG device; Passing the voltage signal of the grid-connected point through a fundamental frequency notch filter to obtain an oscillating voltage component other than the fundamental frequency on the grid side, and generating a high-frequency oscillation suppression current component based on the oscillating voltage component; 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; l = a, b, c; The preliminary modulation current component, the high-frequency oscillation suppression current component, and the reactive current component compensation component are added together to obtain the compensation current reference value of the first phase, the cascaded SVG compensation current reference value 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 subjected to grid voltage feedforward control to obtain the modulation voltage of the first phase, the modulation voltage of the first phase is used to obtain a drive signal to control the on and off of the switch devices of each SVG device.

2. The method for suppressing high-frequency oscillation of a grid-connected system based on cascaded SVG under a weak grid state according to claim 1 is characterized in that: The specific acquisition process of the preliminary modulation current component includes: Calculate the average capacitor voltage of each SVG device in the three-phase bridge arm; The average capacitor voltage of the SVG device is compared with the DC capacitor voltage reference command V dcref After the difference is normalized, it passes through the PI regulator to obtain the amplitude of the preliminary modulation current; The amplitude is multiplied by sin(θ), sin(θ-2π / 3), and sin(θ+2π / 3) respectively to obtain a preliminary modulation current.

3. The method for suppressing high-frequency oscillation of a grid-connected system based on cascaded SVG under a weak grid state according to claim 1, characterized in that: The high frequency oscillation suppresses the current component i lR The calculation formula is: in, v l is the three-phase voltage on the grid side, v lh is the oscillating voltage component, R v is the adaptive resistance value based on the oscillation component, ω n =2πf n , f n is the fundamental frequency, is the total harmonic component in the grid-side voltage, is the preset grid voltage harmonic content limit value, s is a complex variable, BW is the bandwidth of the second-order bandpass filter, ξ1 is the transfer function damping ratio coefficient of the fundamental frequency notch filter, k PR , k DR , k IR They are respectively the proportional gain, differential gain and integral gain of the PID regulator in the virtual resistance adaptive control, f ch is the center frequency of the high-frequency oscillation component.

4. The method for suppressing high-frequency oscillation of a grid-connected system based on cascaded SVG under a weak grid state according to claim 3 is characterized in that: Total harmonic components in the grid voltage The acquisition process includes: passing the harmonic voltage of the grid connection point through a band-stop filter to extract the harmonic components except the fundamental frequency, and then performing Clark transformation on the harmonic components except the fundamental frequency to obtain the component V in the two-phase coordinate system. αh and V βh , V αh and V βh The mean square value is sent to the low-pass filter to remove the ripple and obtain the total harmonic component in the grid-side voltage 5. The method for suppressing high-frequency oscillation of a grid-connected system based on cascaded SVG under a weak grid state according to claim 4 is characterized in that: The transfer function of the low-pass filter is 6. The method for suppressing high-frequency oscillation of a grid-connected system based on cascaded SVG under a weak grid state according to claim 1, characterized in that: The specific process of obtaining the reactive current component compensation component includes: Real-time detection of reactive power on the grid side, and use of the reactive power as a given reactive power instruction value for the first phase; Compare the reactive power given command value with the real-time reactive power of the first phase, input the difference into the PI regulator, and obtain the reactive power current regulation specified value; The reactive power current adjustment designated value and the active power current adjustment designated value are subjected to coordinate transformation to obtain the three-phase reactive power current adjustment value, that is, the reactive current component compensation component; wherein the active power current adjustment designated value is set to 0.

7. The method for suppressing high-frequency oscillation of a grid-connected system based on cascaded SVG under a weak grid state according to claim 1, characterized in that: The current regulator adopts a quasi-proportional regulator, and the transfer function of the quasi-proportional regulator is: Among them, ω c is the bandwidth of the quasi-proportional regulator, K p , K r are proportional gain and integral gain respectively, ω o =2πf o , f o Get the fundamental frequency.

8. A high-frequency oscillation suppression system for a grid-connected system based on cascaded SVG in a weak grid state, 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 7.

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

10. A computer program product comprising a computer program / instructions; characterized in that When the computer program / instructions are executed by a processor, the steps of the method described in any one of claims 1 to 7 are implemented.

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