Series virtual impedance method for improving grid-connected current quality of static synchronous compensator under weak grid

Abstract

The application belongs to the field of power systems and relates to a series virtual impedance method for improving grid-connected current quality of a static synchronous reactive compensator under a weak power grid. In view of the problems that the existing methods cannot simultaneously achieve stability and harmonic suppression and the THD of grid-connected current increases and the quality of grid-connected current decreases due to harmonics in a weak power grid, the application constructs a grid-connected system with a three-level diode clamped STATCOM as a core, designs a harmonic suppression module containing a multi-resonant controller and a series virtual inductor, superimposes the harmonic suppression module and an original virtual series impedance to form a composite series virtual impedance, injects the composite series virtual impedance into a shunt impedance correction feedforward path, and realizes closed-loop control of grid-connected current. The method can effectively suppress characteristic harmonics, reduce THD, ensure stable operation of the system under a weak power grid, and has good dynamic performance and is easy to implement.

Description

Technical Field

[0001] This invention belongs to the field of power systems, and specifically relates to a series virtual impedance method for improving the grid-connected current quality of static synchronous reactive power compensators in weak power grids. Background Technology

[0002] Under weak grid conditions, actual three-phase power grids contain numerous sources of harmonic pollution, including but not limited to power electronic converters and ferroresonant equipment. These sources are characterized by positive-sequence harmonics of order 6k+1 (k = 1, 2, ...) and negative-sequence harmonics of order 6k-1, while even-order and third-order harmonics are present in small amounts. These harmonics directly generate fundamental positive-sequence components, unbalanced components, and harmonic components in the grid-connected current through power connection lines; these can be collectively referred to as disturbance components of the grid-connected current. They increase the total harmonic distortion (THD) of the grid-connected current, affecting its quality, and the quality deteriorates further with increasing grid impedance.

[0003] To effectively suppress grid-connected current harmonics caused by voltage background harmonics, existing technologies typically improve the control loop using the following methods:

[0004] 1. A proportional-integral-resonant (PIR) controller is used in the rotating coordinate system, employing a repetitive controller. In the stationary coordinate system, a multi-resonant (MR) controller is used, employing grid voltage feedforward control.

[0005] When using a proportional-integral-resonant (PIR) controller in a rotating coordinate system, the stability of the control system under weak power grid conditions is worse than that when using a PR controller, which will degrade the performance of the control system. At the same time, when controlling in a rotating coordinate system, the coupling between the d-axis and q-axis needs to be considered, making the control system more complex.

[0006] 2. When using a repetitive controller, the grid-connected current loop control signal accumulates cycle by cycle, allowing the control loop to achieve a large gain at harmonic frequencies, thereby suppressing grid-connected current harmonics and unbalanced components. The structure of the repetitive controller does not become more complex as the required harmonic suppression order increases, but the dynamic performance of this method is poor, requiring combination with other instantaneous value feedback control or PI control.

[0007] 3. When using a multiresonant (MR) controller in a stationary coordinate system, by superimposing a multiresonant (MR) element on the PR compensator, the transfer function of the multiresonant controller is:

[0008]

[0009] In the formula ω n —Harmonic frequencies that need to be suppressed (n = 6k-1 or 6k+1);

[0010] K rn —The resonant coefficient corresponding to the resonant frequency;

[0011] ω i —The resonant bandwidth corresponding to the resonant frequency.

[0012] When using a multiresonator controller, the system loop gain at the harmonic frequency ω that needs to be suppressed... n The loop gain can effectively suppress the influence of background harmonics on the grid-connected current. However, when the frequency of the harmonic to be suppressed is higher than the system cutoff frequency, a negative phase shift is introduced due to the 180° phase angle jump at the harmonic frequency in the multi-resonant circuit. As the number of harmonics to be compensated gradually increases, the phase frequency response curve of the loop gain may cross -180°, causing the grid-connected system to become unstable. Therefore, an additional phase compensation circuit is required. Through design and The value of makes the frequency characteristics of the open-loop function of the control system improve the phase margin while keeping the amplitude margin unchanged; this method cannot be combined with the impedance correction method proposed in the existing public literature [1]: Author: Yang Dongsheng, Ruan Xinbo, Wu Heng, Title: Virtual impedance method to improve the adaptability of LCL grid-connected inverter to weak grid [J], Proceedings of the Chinese Society for Electrical Engineering, 2014(15):2327-2335, which will increase the complexity of the control system.

[0013] 4. When using the grid voltage feedforward control method, the grid voltage is sampled and then fed forward through the feedforward function G. ff (s) Modulated wave v superimposed on the Static Synchronous Var Compensator (STATCOM) x The above, which cancels out the influence of grid voltage on grid-connected current, can suppress harmonic components and unbalanced components, and improve the quality of grid-connected current. For STATCOM connected to LCL type filter, the control method of proportional feedforward, positive feedback of filter capacitor voltage and current or full feedforward of grid voltage is generally adopted. Among them, the full feedforward method of grid voltage can obtain grid-connected current with better quality. The full feedforward control method of grid voltage is equivalent to the parallel impedance correction method in reference [1], and the feedforward function G ff The expressions for (s) are all formulas (2):

[0014]

[0015] This method can effectively suppress grid-connected current harmonics and unbalanced components caused by grid voltage background harmonics. However, this method is proposed under strong grid conditions. When the grid impedance increases, the grid-connected system may not be able to operate stably, thus leading to the development of a grid voltage-weighted feedforward strategy. This strategy is based on the feedforward function G... ff Introducing weight K in (s)f The new feedforward function G′ is obtained. ff (s)=K f G ff (s). Simultaneously, because the grid impedance Z... g (s) and output impedance Z o (s) will not intersect across the entire frequency band; therefore, to maintain better harmonic suppression capability, different weights K can be introduced for different harmonic components. fn (K fn ≤1). For harmonic frequencies not near the intersection frequency, we can let K be... fn =1, while other harmonic frequencies that may appear near the intersection frequency need to be adjusted according to the phase margin requirements of the control system. fn The design was carried out. This method weakens the original feedforward function G to some extent. ff The cancellation term introduced in (s) is a control strategy that compromises between the grid voltage weighted feedforward method and the grid voltage full feedforward method. Essentially, it represents a trade-off between the stability of the grid-connected system and the ability to suppress grid current harmonics, and it still has certain limitations. This method may fail when the short-circuit ratio (SCR) < 10. Summary of the Invention

[0016] To address the issues of high harmonic distortion and poor current quality in the grid-connected current of Static Synchronous Var Compensators (STATCOMs) under weak grid conditions due to background harmonics in grid voltage and increased grid impedance, existing control methods suffer from drawbacks such as difficulty in balancing stability and harmonic suppression capabilities, insufficient dynamic performance, or complex control systems. This invention provides a series virtual impedance method to improve the grid-connected current quality of STATCOMs under weak grid conditions. While ensuring system stability, this method effectively suppresses characteristic harmonics in the grid-connected current, thereby improving the grid-connected current quality.

[0017] To achieve the above objectives, the technical solution adopted by this invention is as follows: A series virtual impedance method for improving the grid-connected current quality of a static synchronous reactive power compensator in a weak power grid, comprising the following steps:

[0018] S1. Construct the STATCOM grid-connected system topology: Using a three-level diode clamped STATCOM as the core, its output is connected to the weak grid through an LCL filter to obtain the grid connection point voltage, grid-side current and STATCOM operating status parameters;

[0019] S2. Design of the harmonic suppression module: The harmonic suppression module consists of a multi-resonance controller and a series virtual inductor at the corresponding resonant frequency, and its transfer function is: In the formula,

[0020] ω n—The harmonic frequencies that need to be suppressed are n = 6k-1 or 6k+1;

[0021] K n —Resonance coefficient, used to adjust the harmonic extraction effect, preferably K5=K7=0.5;

[0022] ω ih —The resonant bandwidth of the harmonic suppression module ensures normal operation of the controller when the power grid frequency fluctuates; ω is preferred. ih =10πrad / s;

[0023] L sn —The series inductor corresponding to the resonant frequency dampens the harmonic components using high-frequency impedance characteristics, and its value is set to 20 times the basic series virtual inductance L. s .

[0024] S3. Integration of Harmonic Suppression Module and Impedance Correction Channel: Integrating the harmonic suppression module with the virtual series impedance L in the original STATCOM impedance correction method. s The superposition forms a composite series virtual impedance, which is specifically achieved as follows:

[0025] S31. Use a multi-resonant controller to extract the 5th and 7th characteristic harmonic components from the grid-side current;

[0026] S32. The extracted harmonic components are compared with the corresponding series virtual inductance L. sn Multiplication achieves harmonic damping;

[0027] S33. Connect the processed harmonic suppression signal to the foundation in series with the virtual impedance L. s The voltage signal is superimposed and injected into the feedforward path of the parallel impedance correction channel;

[0028] S4. Realize grid-connected current closed-loop control: Combined with the original control strategy of STATCOM, the output signal of the above composite series virtual impedance is used as a feedforward compensation quantity and superimposed on the modulation wave of STATCOM to realize real-time adjustment of grid-connected current.

[0029] Furthermore, in step S31, the characteristic harmonic components include the 5th negative sequence harmonic and the 7th positive sequence harmonic.

[0030] Furthermore, in step S33, the basic series virtual impedance L s The voltage signal is sL s The product of the grid-side current and the grid-side current.

[0031] Furthermore, in step S4, the original control strategy of STATCOM includes voltage outer loop control and current inner loop control.

[0032] Furthermore, the LCL-type filter includes an inductor L1 on the converter side and a filter capacitor C. f And the grid-side inductor L2.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] 1. Significant harmonic suppression effect: By accurately extracting the 5th and 7th characteristic harmonics through a multi-resonant controller and combining the high-frequency damping characteristics of the series virtual inductor, the 6k±1st characteristic harmonics in weak power grids can be specifically suppressed, reducing the grid-connected current THD (Total Harmonic Distortion) to below 5%, meeting grid connection standards.

[0035] 2. Good system stability: The harmonic suppression module works synergistically with the series virtual impedance in the original impedance correction method to suppress harmonics without reducing the system phase margin. Even under weak grid conditions with a short-circuit ratio SCR < 3, the STATCOM grid-connected system can still be guaranteed to operate stably.

[0036] 3. Excellent dynamic performance: It does not rely on slow dynamic components such as repetitive controllers. By combining instantaneous value feedback with harmonic suppression modules, it improves the system's response speed to sudden changes in grid impedance and harmonic disturbances.

[0037] 4. Simple to implement: The harmonic suppression module has a simple parameter design and is compatible with existing STATCOM control loops. It does not require major changes to the hardware structure and is easy to implement in engineering. Attached Figure Description

[0038] Figure 1 This is a diagram of the topology of a three-level diode-clamped STATCOM.

[0039] Figure 2 The equivalent control block diagram is shown for the voltage feedforward strategy based on a multi-resonant controller.

[0040] Figure 3 For the output impedance Z o3 (s), Z o2 (s) and Z o (s) and grid impedance Z g Frequency response diagram of (s);

[0041] Figure 4 The diagrams show a comparison of virtual series and parallel integrated impedance correction methods, including (a) the circuit diagram of virtual series and parallel integrated impedance correction, and (b) the equivalent transformation diagram of the control block diagram.

[0042] Figure 5 This is a block diagram illustrating the technical implementation of the series virtual impedance method for improving the grid-connected current quality of static synchronous reactive power compensators in weak power grids according to the present invention.

[0043] Figure 6The 5th and 7th harmonic components extracted for the multiresonant controller;

[0044] Figure 7 The figures show the voltage and current waveforms under different grid impedances when using the series and parallel combined impedance correction method. Among them, (a) L g =2mH, (b)L g =4.66mH;

[0045] Figure 8 The figures show the voltage and current waveforms under different grid impedances when using series virtual impedance technology and impedance correction method, where (a) L g =4.66mH, (b)L g =7mH, (c)a phase current THD = 3.27% (L g =4.66mH), (d)a phase current THD = 4.27% (L g =7mH). Detailed Implementation

[0046] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0047] Example 1

[0048] like Figure 1 As shown, a three-level diode-clamped STATCOM consists of a DC-side capacitor, a power switch, and a clamping diode. Its output is connected to the grid connection point PCC via an LCL filter. Wherein, v pcc V is the voltage at the common coupling point between the STATCOM and the power grid. g For grid-side voltage, i g This represents the grid-side current.

[0049] In the series and parallel combined impedance correction method, the virtual impedance Z of the parallel circuit is... p (s)=-Z o (s), at this time the equivalent output impedance Infinity can eliminate quantities related to grid voltage in the grid-side current expression, thereby eliminating the influence of grid voltage background harmonics on the grid-connected current.

[0050] However, in practical applications, due to the equivalent delay function G d The effect of (s), the practically achievable equivalent output impedance It is not infinite, and in practical applications, it cannot completely eliminate the influence of grid-side voltage on grid-connected current. Furthermore, the series virtual impedance Z in the output impedance correction method... sThe purpose of this method is to improve the phase of the output impedance in the high-frequency range and enhance the stability of the STATCOM grid-connected system. However, the influence of grid voltage background harmonics was not considered when designing the series virtual impedance. Therefore, the proposed impedance correction method suffers from the equivalent delay function G d Due to the influence of (s) and other external conditions, the harmonic suppression capability of the grid-connected current may not be ideal in practical applications, especially when the weak grid conditions deteriorate further (SCR<3), which may lead to a decline in the quality of the grid-connected current and failure to meet the grid connection standards.

[0051] To effectively suppress grid-connected current harmonics, most current methods improve upon the full voltage feedforward strategy, i.e., the parallel impedance correction method. To improve the grid voltage-weighted feedforward strategy, a grid voltage-weighted feedforward strategy based on a multi-resonant controller is introduced. This involves introducing a series of multi-resonant components into the full grid voltage feedforward path to replace the weights K in the grid voltage-weighted feedforward strategy. f To suppress harmonic components in the grid-connected current, a weighted feedforward control strategy based on a multi-resonant controller is employed. This strategy involves feeding forward the background harmonics of the grid voltage and, through careful design of the multi-resonant components and their weights at different harmonic frequencies. The formula for the feedforward function in this strategy is: Among them, R n (s) is the multi-resonance control component, and its expression is:

[0052]

[0053] In the formula, ω rn —Harmonic characteristic angular frequency;

[0054] K rn —The resonant coefficient corresponding to the resonant frequency;

[0055] ω in —The resonant bandwidth corresponding to the resonant frequency.

[0056] When the grid impedance changes, there exists a boundary frequency f. d That is, only at frequencies higher than f d At that time, Z g (s) and Z o Only when the amplitude-frequency curves of (s) intersect will f d It is determined by the main circuit and control parameters of the power grid. Since the background harmonics in the power grid voltage are distributed only as multiples of the fundamental frequency, different resonance coefficients K can be introduced for different power grid voltage harmonics. rn , below f d At the frequency, the resonant coefficient K rn Set it to slightly greater than 1 to ensure stronger harmonic suppression of the grid-connected current; for values ​​higher than f d The frequency, resonance coefficient K rnIt should be less than 1 to ensure the phase margin of the grid-connected system. At the same time, the coupling problem between the various multiresonant components must be considered; therefore, the multiresonant gain R at adjacent harmonics can be set to less than 1. n (s) not exceeding 5% of the gain at the target harmonic. Taking all the above requirements into account, the resonant bandwidth ω can be obtained. in Harmonic characteristic angular frequency ω rn and the resonant coefficient K of the multi-resonator rn The calculation formulas for ω and ω are respectively formulas (4), (5), and (6). To simplify the calculation, it can be assumed that the resonant bandwidth at all harmonic frequencies is equal, i.e., ω in It is a constant independent of n.

[0057]

[0058] R n (s) is added to the parallel impedance correction channel, and the impedance correction method is considered comprehensively to obtain the equivalent control block diagram of the voltage feedforward strategy based on the multi-resonant controller, as shown below. Figure 2 As shown. Voltage v at the grid connection point. pcc After passing through a multi-resonant controller and merging with a virtual series impedance branch, it is then combined with an impedance correction method via a parallel impedance correction channel. This section considers the case where the voltage background harmonics contain 5th negative-sequence and 7th positive-sequence harmonics, and obtains R. n The expression for (s) is (7). After performing an equivalent transformation on the control block diagram, the new output impedance Z can be obtained. o3 The expression for (s) is (8).

[0059]

[0060] After designing the parameters based on the above analysis process, the output impedance Z of the STATCOM grid-connected system using a voltage feedforward strategy based on a multi-resonant controller can be plotted. o3 (s) Using series and parallel output impedance correction method Z o2 (s) and the original output impedance Z o (s) and grid impedance Z g The frequency response curve of (s) is as follows: Figure 3 As shown in the figure, the grid impedances are 1.4mH and 4.66mH, respectively, and the corresponding SCRs are 10 and 3.

[0061] According to the Bode plot, at the harmonic frequencies of 250Hz and 350Hz where suppression is required, Z... o3 The amplitude of (s) is greater than Z. o2 (s) is greater than Z o(s) indicates that when using a voltage feedforward strategy based on a multi-resonant controller, the harmonic suppression capability of the grid-connected current is enhanced at a specified subharmonic frequency. However, at other frequencies, Z... o3 The amplitude of (s) is less than Z. o2 (s) will result in a certain loss of harmonic suppression capability. Simultaneously, with the increase of grid impedance, Z o3 (s) and Z g The intersection frequency of (s) continuously decreases, as the grid impedance L g When Z = 4.66mH, o3 (s) and Z g The intersection frequency of (s) appears near the harmonic frequency that needs to be suppressed, which greatly reduces the phase margin of the grid-connected system and affects its stability. Therefore, the grid voltage feedforward strategy based on the multi-resonant controller is essentially a control method that balances the stability of the grid-connected system and the ability to suppress grid current harmonics. It has certain limitations and may not be applicable when the short-circuit ratio (SCR) is small.

[0062] Therefore, it is possible to consider using the duality principle to improve the series virtual impedance channel based on the concept of a multi-resonant controller. In the series and parallel impedance correction method, a series virtual impedance Z is introduced. s =sL s The aim is to improve the phase of the output impedance in the high-frequency range. The basic equivalent circuit of the series and parallel combined impedance correction method is as follows: Figure 4 As shown. According to Figure 4 (a) and Figure 4 (b) It can be seen that, considering only the grid voltage, when looking from the grid connection point PCC towards the converter side, the current direction is from PCC to the converter, which is consistent with the virtual inductance L. s The current reference direction is opposite. Therefore, the virtual series inductance L s At the grid connection point PCC, it is equivalent to a negative inductor. The virtual series and parallel impedance is implemented by using L... s The voltage obtained by multiplying the voltage by the grid-side current is directly proportional to the grid connection point voltage V. pcc The voltages are then superimposed, and the resulting voltage is applied to the parallel impedance correction stage. Therefore, physically speaking, the series virtual inductance L... s Its essential working mechanism is that it can interact with the grid impedance L, which is positive on the grid side. g This offsets the impact of grid impedance on the STATCOM grid-connected system, thereby improving the stability of the STATCOM grid-connected system.

[0063] Based on the above analysis, a harmonic suppression module can be added to the series impedance correction channel of the output impedance correction method. This module utilizes the combination of a multi-resonant controller and series virtual impedance to suppress harmonic components in the grid-connected current, further improving the quality of the grid-connected current. Harmonic suppression module G fn (s) is controlled by a multi-resonant controller G n (s) and virtual series impedance Z sn The composition is expressed as formula (9).

[0064]

[0065] In the formula ω n —Harmonic frequencies that need to be suppressed (n = 6k-1 or 6k+1);

[0066] K n —The resonant coefficient of the harmonic suppression module;

[0067] ω ih —The resonant bandwidth of the harmonic suppression module;

[0068] L sn —The series inductance corresponding to the resonant frequency.

[0069] The function of the multiresonant controller is to extract the harmonic components at the harmonic frequencies that need to be suppressed in the grid-connected current, and connect them in series with the virtual inductor L. sn The role of virtual inductance L in impedance correction methods s Different, L sn Its function is to utilize the characteristic that inductors have a large impedance to high-frequency signals to dampen the harmonic components extracted by the multi-resonant controller. To further improve the grid-connected current quality, L... sn Reference direction and L s To maintain consistency and achieve better damping effect, L sn Set the size to 20 times the size of L s Left and right. Resonant bandwidth ω ih Its function is to ensure that the harmonic controller can still operate normally when the power grid frequency fluctuates. To simplify the calculation, the resonant bandwidth ω at all harmonic frequencies can be set as follows: ih Equal, i.e., ω in It is a constant independent of n, and the resonant bandwidth ω is chosen. ih =2πΔf=10π. K n The value of G and the harmonic suppression module n(s) The effect of extracting a specified harmonic is related to the case where k=1, i.e., the 5th negative sequence and 7th positive sequence harmonic components in the voltage background harmonics are taken into account. To achieve a better harmonic suppression effect, the following parameters are set for the multi-resonant controller: resonant coefficient K5=K7=0.5, harmonic angular frequency ω5=500πrad / s, ω7=700πrad / s, and resonant bandwidth ω ih = 10πrad / s (corresponding to frequency fluctuation ±5Hz), series virtual inductance parameter: L s =0.1=0.1mH, then L s5 =L s7 =2mH(20 times L) S ).

[0070] In summary, the main idea of ​​the proposed series virtual impedance technique is: after extracting the main harmonic components that need to be suppressed in the grid-connected current using a multi-resonant controller, and then connecting them with the series inductor L at the corresponding resonant frequency. sn Harmonic damping is achieved through multiplication. Then, the harmonic suppression module is connected to the virtual series impedance L in the impedance correction method. s The superposition, multiplied by the grid-side current, is then applied to the feedforward path of the parallel impedance correction method. The implementation block diagram of the proposed series virtual impedance technique for improving grid-connected current quality is shown in Figure 5, and the control implementation process is as follows:

[0071] 1. Collect grid-side current i g After being transformed to a stationary coordinate system by Clark transformation, the fundamental and harmonic components are separated.

[0072] 2. The multi-resonant controller extracts the 5th and 7th harmonic components, and multiplies them by Ls5 and Ls7 respectively to generate harmonic damping voltages;

[0073] 3. The voltage signal (sLs·i) of the harmonic damping voltage and the virtual impedance Ls in series with the foundation. g By superimposing these values, a composite series virtual impedance voltage is obtained.

[0074] 4. Inject the composite series virtual impedance voltage into the feedforward path of the parallel impedance correction channel, and connect it to the grid connection point voltage v. pcc After superposition, the signal is sent to the modulation module of STATCOM to generate the switching transistor drive signal.

[0075] Example 2

[0076] Simulation Verification: To verify the proposed series virtual impedance method for improving the grid-connected current quality of static synchronous reactive power compensators (SNC-RCCs) in weak power grids, a simulation model was built using the Matlab / Simulink simulation platform. The proposed series virtual impedance method was analyzed, and the simulation parameters and harmonic suppression module parameter settings are given in Tables 1 and 2, respectively. To simulate worse weak power grid conditions and further increase the amplitude of grid voltage background harmonics, the grid-side power supply was superimposed with 5th negative-sequence and 7th positive-sequence harmonic sources on top of the ideal power supply. The injected harmonic orders and their amplitudes and phases compared to the fundamental wave are shown in Table 3.

[0077] Table 1 Simulation parameters of the STATCOM grid-connected system

[0078]

[0079] Table 2 Simulation parameters of the harmonic suppression module

[0080]

[0081] Table 3. Number and amplitude of injected harmonics

[0082]

[0083] The 5th and 7th harmonic components extracted from the grid-connected current after passing through the multi-resonant controller are as follows: Figure 6 As shown, the frequencies of the two waveforms are 250Hz and 350Hz respectively, proving the effectiveness of the multi-resonant controller G. n The correctness of (s).

[0084] like Figure 7 As shown, the waveforms of phase a grid-side voltage and current under different grid impedances are obtained when using a series and parallel combined correction method. As the amplitude of the grid voltage background harmonics increases, the THD of the grid-connected current will exceed the grid connection standard, and the quality of the grid-connected current will deteriorate. With further increases in grid impedance, the waveform of the grid-connected current will further deteriorate.

[0085] like Figure 8 As shown, when the proposed series virtual impedance technology and series-parallel integrated impedance correction method are used, the waveforms of phase a grid side voltage and current under different grid impedances and their corresponding THD diagrams are displayed. The corresponding short-circuit ratios (SCRs) are 3 and 2, respectively. Figure 6 and Figure 7Comparisons show that under weak grid conditions, without using series virtual impedance technology to suppress harmonic components in the grid-connected current, the THD of the grid-connected current is large and the waveform quality is poor. However, with series virtual impedance technology, the THD of the grid-connected current is small, and the waveform quality is further improved based on the adaptive impedance correction method, verifying the effectiveness of the proposed series virtual impedance technology. Furthermore, the harmonic suppression module introduced by the series virtual impedance technology reduces the harmonic components of the grid-connected current without degrading the stability of the STATCOM grid-connected system.

Claims

1. A method for improving the grid-connected current quality of a static synchronous reactive power compensator (SNC-RCC) in a weak power grid, characterized in that, Includes the following steps: S1. Construct the STATCOM grid-connected system topology: Using a three-level diode clamped STATCOM as the core, its output is connected to the weak grid via an LCL filter to obtain the grid connection point voltage, grid-side current, and STATCOM operating status parameters; S2. Design of the harmonic suppression module: The harmonic suppression module consists of a multi-resonance controller and a series virtual inductor corresponding to the resonant frequency, and its transfer function is: , In the formula, ω n —The harmonic frequencies that need to be suppressed are n=6k-1 or 6k+1; K n —Resonance coefficient; ω ih —The resonant bandwidth of the harmonic suppression module; L sn —The series inductance corresponding to the resonant frequency; S3. Integration of Harmonic Suppression Module and Impedance Correction Channel: This integrates the harmonic suppression module with the virtual series impedance L in the original STATCOM impedance correction method. s The superposition forms a composite series virtual impedance, which is specifically achieved as follows: S31. Use a multi-resonant controller to extract the 5th and 7th characteristic harmonic components from the grid-side current; S32. The extracted harmonic components are compared with the corresponding series virtual inductance L. sn Multiplying these values ​​yields the harmonic damping voltage, thus achieving harmonic damping. The harmonic damping voltage is a harmonic suppression signal. S33. Connect the processed harmonic suppression signal to the foundation in series with the virtual impedance L. s The voltage signal is superimposed and injected into the feedforward path of the parallel impedance correction channel; S4. Achieve closed-loop control of grid-connected current: Combining the original control strategy of STATCOM, the output signal of the above composite series virtual impedance is used as a feedforward compensation quantity and superimposed on the modulation wave of STATCOM to achieve real-time adjustment of grid-connected current.

2. The series virtual impedance method for improving the grid-connected current quality of a static synchronous reactive power compensator in a weak power grid, as described in claim 1, is characterized in that... In step S2, the resonance coefficient K n The value of is 0.5, and the resonance coefficients for the 5th and 7th harmonics satisfy K5=K7.

3. The series virtual impedance method for improving the grid-connected current quality of a static synchronous var compensator in a weak power grid, as described in claim 1, is characterized in that... In step S2, the resonant bandwidth ω ih =10πrad / s.

4. The series virtual impedance method for improving the grid-connected current quality of a static synchronous reactive power compensator in a weak power grid, as described in claim 1, is characterized in that... In step S2, the series inductor L sn The value of L is based on the series virtual inductance. s 20 times, that is, L sn =20L s .

5. The series virtual impedance method for improving the grid-connected current quality of a static synchronous reactive power compensator in a weak power grid, as described in claim 1, is characterized in that... In step S31, the characteristic harmonic components include the 5th negative sequence harmonic and the 7th positive sequence harmonic.

6. The series virtual impedance method for improving the grid-connected current quality of a static synchronous reactive power compensator in a weak power grid, as described in claim 1, is characterized in that... In step S33, the basic series virtual impedance L s The voltage signal is sL s The product of the grid-side current and the grid-side current.

7. The series virtual impedance method for improving the grid-connected current quality of a static synchronous reactive power compensator in a weak power grid, as described in claim 1, is characterized in that... In step S4, the original control strategy of STATCOM includes voltage outer loop control and current inner loop control.

8. A series virtual impedance method for improving the grid-connected current quality of a static synchronous reactive power compensator in a weak power grid, as described in claim 1, is characterized in that... The LCL-type filter includes an inductor L1 on the converter side and a filter capacitor C. f And the grid-side inductor L2.

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