Improved Method and System for Grid-connection Stability of VSC-HVDC under Weak Grid Based on SVG Impedance Remodeling

Abstract

The present invention discloses a method and system for improving the grid - connection stability of VSC - HVDC under a weak grid based on SVG impedance reshaping. By introducing a damping factor, this method can adjust the actual reference values of the d - axis and q - axis currents in the inner current control loop without adding an oscillation detection module and sensors, equivalently change the output impedance of the static var generator, and further improve the negative damping characteristics in the low - frequency band of the SVG, increasing the damping of the grid - connection system. The present invention can effectively suppress the grid - connection point voltage oscillation when the VSC - HVDC grid - connection system containing SVG interacts with the weak grid, enhance the stability of the grid - connection system, and improve the system stability margin.

Description

Technical Field

[0001] The present invention belongs to the field of high - voltage direct - current transmission systems, and relates to a method and system for improving the grid - connection stability of VSC - HVDC under weak grids based on SVG impedance reshaping. Background Technique

[0002] In recent years, the development of new - energy power - generation equipment such as photovoltaic and wind power grid - connected through high - voltage direct - current transmission (VSC - HVDC) has received increasing attention. To balance the reactive power of transmission lines, reactive - power compensation devices such as static var generators (SVG) are often installed on the receiving - end AC - side bus of long - distance VSC - HVDC systems. When the SVG operates in parallel with the new - energy grid - connection system, it can significantly improve the frequency stability of the system. However, research shows that there are stability problems such as voltage instability in the new - energy grid - connection system containing SVG when interacting with weak grids, especially in the low - frequency band of the coupled system.

[0003] In terms of improving stability, existing methods mainly start from three aspects: parameter optimization, device topology improvement, and control - strategy improvement. Among them, parameter optimization is difficult to fundamentally solve the stability problem, which may cause practical problems such as increased costs, and the amount of work to improve the device topology is huge in actual engineering. In contrast, the method of improving the control strategy can fundamentally achieve stability improvement and the amount of work is relatively small, which is easy to be applied in practice. However, it may add an oscillation - detection module and sensors on the basis of the original equipment, which will also increase the application cost. Therefore, it is urgent to study a more effective control strategy for improving the stability of the coupled system applied to SVG. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method and system for improving the grid - connection stability of VSC - HVDC under weak grids based on SVG impedance reshaping in view of the deficiencies of the prior art.

[0005] The technical solution adopted by the present invention is as follows:

[0006] A method for improving the grid - connection stability of VSC - HVDC under weak grids based on SVG impedance reshaping, characterized in that the method includes the following steps:

[0007] Step 1, in the outer - loop DC - voltage control link of the SVG, based on the actual value v of the DC - side voltage of the SVG dc obtain the theoretical reference value i of the d - axis current dref ;

[0008] Step 2: Multiply the obtained theoretical reference value of the d-axis current \(i\) dref by the damping factor \(\delta\) to obtain the actual reference value of the d-axis current \(i\) dr ;

[0009] Step 3: Multiply the known theoretical reference value of the q-axis current \(i\) qref by the damping factor \(\delta\) to obtain the actual reference value of the q-axis current \(i\) qr ;

[0010] Step 4: Subtract the obtained actual reference value of the d-axis current \(i\) dr from the d-axis component \(i\) d of the SVG grid-connected current, and multiply the resulting value by the transfer function \(G\) i (s) of the voltage outer-loop controller to obtain the control value of the d-axis current;

[0011] Subtract the obtained actual reference value of the q-axis current \(i\) qr from the q-axis component \(i\) q of the SVG grid-connected current, and multiply the resulting value by the transfer function \(G\) i (s) of the voltage outer-loop controller to obtain the control value of the q-axis current;

[0012] where the expression of \(G\) i (s) is:

[0013]

[0014] where \(k\) ip is the current-loop proportional coefficient, \(k\) ii is the current-loop integral coefficient, and \(s\) is the complex frequency;

[0015] Step 5: Multiply the q-axis component \(i\) q of the SVG grid-connected current, the system base angular frequency \(\omega\), and the inductance \(L\) eq of the SVG AC-side filter circuit, sum the resulting product value with the d-axis component \(v\) d of the grid-connected point voltage, subtract the control value of the d-axis current, and then divide by the given value \(v\) dcref of the SVG DC-side voltage to obtain the d-axis modulation signal \(m\) d ;

[0016] Multiply the d-axis component \(i\) d of the SVG grid-connected current, the system base angular frequency \(\omega\), and the inductance \(L\) eq of the SVG AC-side filter circuit to obtain the product value \(\omega L\) eq \(i\) d , subtract the product value \(\omega L\) q from the q-axis component \(v\) eq of the grid-connected point voltage d, the obtained difference result is then subtracted by the control value of the q-axis current, and then divided by the given value v of the SVG DC-side voltage dcref , to obtain the q-axis modulation signal m q ;

[0017] Step 6, according to the d-axis modulation signal m d and the q-axis modulation signal m q generate the SVPWM wave to drive the SVG, so as to realize the impedance reshaping control for equivalently improving the damping characteristic of the SVG.

[0018] Moreover, in the SVG outer-loop DC voltage control link, based on the actual value v of the SVG DC-side voltage dc the method for obtaining the theoretical reference value i of the d-axis current is as follows: dref :

[0019] Subtract the actual value v of the SVG DC-side voltage from the given value v of the SVG DC-side voltage dcref , multiply the obtained value by the transfer function G dc of the voltage outer-loop controller v v (s), to obtain the theoretical reference value i of the d-axis current dref ,

[0020] wherein, the expression of G v (s) is:

[0021]

[0022] wherein, k vp is the voltage-loop proportionality coefficient, k vi is the voltage-loop integral coefficient, and s is the complex frequency.

[0023] Moreover, the expression of the damping factor δ is:

[0024]

[0025] wherein, τ is the filtering time constant, n is the control iteration number, v dcref is the given value of the SVG DC-side voltage, v dc is the actual value of the SVG DC-side voltage, s is the complex frequency, and i dref is the theoretical reference value of the d-axis current.

[0026] The present invention provides a VSC-HVDC grid-connection stability improvement system based on SVG impedance reshaping under a weak grid, including:

[0027] A voltage outer-loop control module, used to control the actual value of the SVG DC-side voltage to track the given value and obtain the theoretical reference value i of the d-axis current dref ;

[0028] Damping factor generation module, based on the theoretical reference value i of the d-axis current dref and the given value v of the DC side voltage of the SVG dcref and the actual value v of the DC side voltage of the SVG dc , generates a damping factor δ by setting the filter time constant and the number of control iterations;

[0029] Damping control module, used to introduce the damping factor δ into the current loop control strategy, based on the theoretical reference value i of the d-axis current dref and the theoretical reference value i of the q-axis current qref , to obtain the actual reference value i of the d-axis current dr and the actual reference value i of the q-axis current qr ;

[0030] Current control module, used to control the actual output d-axis current and q-axis current of the SVG to track the actual reference value i of the d-axis current dr and the actual reference value i of the q-axis current qr , and obtain the d-axis output voltage signal and q-axis output voltage signal for modulation;

[0031] Modulation module, based on the d-axis output voltage signal and q-axis output voltage signal for SVG modulation obtained in the current control module, obtains the d-axis modulation signal m d and the q-axis modulation signal m q , and obtains the SVPWM drive signal to drive the actual operation of the SVG.

[0032] Moreover, it also includes a damping factor generation module, based on the theoretical reference value i of the d-axis current dref , the given value v of the DC side voltage of the SVG dcref and the actual value v of the DC side voltage of the SVG dc , generates a damping factor δ by setting the filter time constant and the number of control iterations.

[0033] Moreover, the method for the voltage outer loop control module to control the actual value of the DC side voltage of the SVG to track the given value and obtain the theoretical reference value of the d-axis current is: subtract the actual value v of the DC side voltage of the SVG from the given value v of the DC side voltage of the SVG dcref and multiply the obtained value by the transfer function G dc (s) of the voltage outer loop controller to obtain i v (s), dref where, the expression of G

[0034] v v (s) is:

[0035]

[0036] where, k vp vpis the proportional coefficient of the voltage loop, k vi is the integral coefficient of the voltage loop, and s is the complex frequency.

[0037] Moreover, the expression of the damping factor δ is:

[0038]

[0039] where τ is the filtering time constant, n is the number of control iterations, v dcref is the given value of the SVG DC-side voltage, v dc is the actual value of the SVG DC-side voltage, s is the complex frequency, i dref is the theoretical reference value of the d-axis current.

[0040] Compared with the prior art, the beneficial effects of the present invention are:

[0041] The present invention suppresses the voltage oscillation of the new energy grid-connected system by increasing the damping of the new energy grid-connected system containing SVG, that is, introducing a damping factor in the SVG control link to reshape the actual reference value of the dq-axis current, and improving the negative damping characteristic in the low-frequency band of SVG without adding an oscillation detection module and sensors.

[0042] The present invention effectively improves the damping of the new energy grid-connected system containing SVG, thereby suppressing the voltage oscillation at the system grid connection point and improving the system stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 is the structure diagram of the VSC-HVDC grid-connected system containing SVG involved in the present invention;

[0044] Figure 2 is the schematic diagram of the impedance reshaping control strategy applied to SVG involved in the present invention;

[0045] Figure 3 is the image of the SVG output impedance varying with frequency when the proposed stability improvement method is not introduced in an embodiment of the present invention; (a) is the curve of the amplitude and phase of the dd-axis component Z dd (s) of the SVG output impedance varying with frequency; (b) is the curve of the amplitude and phase of the dq-axis component Z dq (s) of the SVG output impedance varying with frequency; (c) is the curve of the amplitude and phase of the qd-axis component Z qd (s) of the SVG output impedance varying with frequency; (d) is the curve of the amplitude and phase of the qq-axis component Z dd (s) of the SVG output impedance varying with frequency;

[0046] Figure 4 The image of the SVG output impedance varying with frequency after introducing the proposed stability improvement method in an embodiment of the present invention; (a) is the magnitude and phase curves of the dd-axis component Z dd (s) of the SVG output impedance varying with frequency; (b) is the magnitude and phase curves of the dq-axis component Z dq (s) of the SVG output impedance varying with frequency; (c) is the magnitude and phase curves of the qd-axis component Z qd (s) of the SVG output impedance varying with frequency; (d) is the magnitude and phase curves of the qq-axis component Z dd (s) of the SVG output impedance varying with frequency;

[0047] Figure 5 The simulation test results of the grid-connected point voltage of the VSC-HVDC grid-connected system with SVG when the proposed impedance reshaping control strategy is not introduced in an embodiment of the present invention; (a) is the simulation curve of the grid short-circuit ratio SCR varying with time t within 0 - 6 s, the abscissa is time t, and the ordinate is the magnitude of the system short-circuit ratio SCR; (b) is the simulation curve of the effective value v of the grid-connected voltage varying with time t within 0 - 6 s, the abscissa is time, and the ordinate is the magnitude of the effective value v of the grid-connected voltage rms ; (c) is the simulation curve of the grid-connected point voltage v varying with time t within 1.9 - 2.1 s, the abscissa is time t, and the ordinate is the magnitude of the grid-connected point voltage v rms ; (d) is the simulation curve of the grid-connected point voltage v varying with time t within 3.9 - 4.1 s, the abscissa is time t, and the ordinate is the magnitude of the grid-connected point voltage v pcc ; pcc ; (d) is the simulation curve of the grid-connected point voltage v varying with time t within 3.9 - 4.1 s, the abscissa is time t, and the ordinate is the magnitude of the grid-connected point voltage v pcc ; pcc ;

[0048] Figure 6 The simulation test results of the grid-connected point voltage of the VSC-HVDC grid-connected system with SVG when the proposed impedance reshaping control strategy is introduced in an embodiment of the present invention; (a) is the simulation curve of the grid short-circuit ratio SCR varying with time t within 0 - 6 s, the abscissa is time t, and the ordinate is the magnitude of the system short-circuit ratio SCR; (b) is the simulation curve of the effective value v of the grid-connected voltage varying with time t within 0 - 6 s, the abscissa is time, and the ordinate is the magnitude of the effective value v of the grid-connected voltage rms ; (c) is the simulation curve of the grid-connected point voltage v varying with time t within 1.9 - 2.1 s, the abscissa is time t, and the ordinate is the magnitude of the grid-connected point voltage v rms ; (c) is the simulation curve of the grid-connected point voltage v varying with time t within 1.9 - 2.1 s, the abscissa is time t, and the ordinate is the magnitude of the grid-connected point voltage v pcc ; (d) is the simulation curve of the grid-connected point voltage v varying with time t within 3.9 - 4.1 s, the abscissa is time t, and the ordinate is the magnitude of the grid-connected point voltage v pcc ; (d) is the simulation curve of the grid-connected point voltage v varying with time t within 3.9 - 4.1 s, the abscissa is time t, and the ordinate is the magnitude of the grid-connected point voltage vpcc The simulation curve varying with time t, where the abscissa is time t and the ordinate is the grid connection point voltage v pcc amplitude. Specific implementation manners

[0049] The present invention will be further described in detail below through specific embodiments. The following embodiments are only descriptive and not restrictive, and the protection scope of the present invention cannot be limited thereby.

[0050] Figure 1 It is the structure diagram of the VSC-HVDC grid-connected system with SVG involved in the present invention, including a weak grid, a VSC-HVDC AC side filter inductor, a VSC-HVDC inverter station converter, an SVG AC side filter, an SVG converter, and an SVG DC side capacitor.

[0051] Figure 2 It is the schematic diagram of the impedance reshaping control strategy applied to SVG involved in the present invention. The specific implementation steps include:

[0052] Step 1, in the SVG outer loop DC voltage control link, based on the actual value v dc of the SVG DC side voltage, obtain the theoretical reference value i dref of the d-axis current. Subtract the actual value v dcref of the SVG DC side voltage from the given value v dc of the SVG DC side voltage, and multiply the obtained value by the transfer function G v (s) of the voltage outer loop controller to obtain the theoretical reference value i dref of the d-axis current. Among them, the expression of G v (s) is:

[0053]

[0054] Among them, k vp is the voltage loop proportional coefficient, k vi is the voltage loop integral coefficient, and s is the complex frequency;

[0055] Step 2, multiply the theoretical reference value i dref of the d-axis current by the damping factor δ to obtain the actual reference value i dr of the d-axis current. Among them, the expression of the damping factor δ is:

[0056]

[0057] Among them, τ is the filtering time constant, n is the control iteration number, v dcref is the given value of the SVG DC side voltage, v dc is the actual value of the SVG DC side voltage, and s is the complex frequency;

[0058] Step 3, multiply the q-axis current theoretical reference value \(i\) qref by the damping factor \(\delta\) to obtain the q-axis current actual reference value \(i\) qr ;

[0059] Step 4, subtract the d-axis component \(i\) dr of the SVG grid-connected current from the d-axis current actual reference value \(i\) d , multiply the obtained value by the transfer function \(G\) i (s) of the voltage outer-loop controller to obtain the control value of the d-axis current; subtract the q-axis component \(i\) qr of the SVG grid-connected current from the q-axis current actual reference value \(i\) q , multiply the obtained value by the transfer function \(G\) i (s) of the voltage outer-loop controller to obtain the control value of the q-axis current; where the expression of \(G\) i (s) is:

[0060]

[0061] where \(k\) ip is the current-loop proportional coefficient, \(k\) ii is the current-loop integral coefficient, and \(s\) is the complex frequency;

[0062] Step 5, add the product value of the q-axis component \(i\) q of the SVG grid-connected current and \(\omega L\) eq to the d-axis component \(v\) d of the grid connection point voltage, subtract the control value of the d-axis current, and then divide by the SVG DC-side voltage given value \(v\) dcref to obtain the d-axis modulation signal \(m\) d ; subtract the product value of the d-axis component \(i\) q of the SVG grid-connected current and \(\omega L\) d from the q-axis component \(v\) eq of the grid connection point voltage, subtract the control value of the q-axis current, and then divide by the SVG DC-side voltage given value \(v\) dcref to obtain the q-axis modulation signal \(m\) q ;

[0063] where \(\omega\) is the system reference angular frequency, and \(L\) eq is the inductance of the SVG AC-side filter circuit;

[0064] Step 6, generate an SVPWM wave to drive the SVG based on the d-axis modulation signal \(m\) d and the q-axis modulation signal \(m\) q to achieve impedance reshaping control for equivalently improving the SVG damping characteristic.

[0065] Figure 3 and Figure 4 、 Figure 5 and Figure 6There are two sets of comparative examples, where

[0066] Figure 3 is the image of the SVG output impedance varying with frequency when the proposed stability improvement method is not introduced in an embodiment of the present invention. The dd-axis component Z dd (s) and the qq-axis component Z qq (s) are in the negative damping range with a phase curve less than -90° in the low-frequency range of 0 - 200 Hz. At this time, it is easy to cause voltage oscillation at the grid connection point of the coupled system composed of the SVG, the grid-side converter of the VSC-HVDC system, and the weak grid.

[0067] Figure 4 is the image of the SVG output impedance varying with frequency after introducing the proposed stability improvement method. Compared with Figure 3 the phase curves of the dd-axis component Z dd (s) and the qq-axis component Z qq (s) of the SVG output impedance varying with frequency in, after introducing the proposed stability improvement method, the phase curves varying with frequency are not in the negative damping range at this time, indicating that the proposed stability improvement method can improve the negative damping characteristics of the SVG output impedance and help enhance the system stability margin.

[0068] Figure 5 is the simulation test result of the grid connection point voltage v pcc and the effective value v rms of the grid connection voltage of the VSC-HVDC grid-connected system containing SVG varying with the grid short-circuit ratio SCR when the proposed stability improvement method is not introduced in an embodiment of the present invention. At 2 s, when the grid short-circuit ratio SCR decreases from 3 to 2, the waveform of the grid connection point voltage v pcc only changes slightly and then returns to stability, and the effective value v rms of the grid connection voltage does not change significantly. However, at 4 s, when the grid short-circuit ratio SCR decreases from 2 to 1, the waveforms of the grid connection point voltage v pcc and the effective value v rms of the grid connection voltage change significantly, and the effective value v rms of the grid connection voltage gradually rises to nearly 5000 kV, indicating that the grid-connected system shows obvious oscillation until it completely loses stability at this time.

[0069] Figure 6 is the simulation test result of the grid connection point voltage v pcc and the effective value v rms of the grid connection voltage of the VSC-HVDC grid-connected system containing SVG varying with the grid short-circuit ratio SCR after introducing the proposed impedance reshaping control strategy in an embodiment of the present invention. At 2 s, when the grid short-circuit ratio SCR decreases from 3 to 2, the grid connection point voltage v pcc and the effective value v rmsThe waveform does not change significantly and can return to the state before the change of the short-circuit ratio SCR of the power grid. At 4 s, the short-circuit ratio SCR of the power grid decreases from 2 to 1, and the grid-connected voltage v pcc and the effective value of the grid-connected voltage v rms can return to the state before the change of the short-circuit ratio SCR of the power grid after a short change. The results show that the proposed control strategy can enhance the stability of the system, improve the stability margin of the system, and ensure the safe and stable operation of the VSC-HVDC grid-connected system with SVG under weak power grids.

[0070] In summary, the advantages of the present invention are as follows:

[0071] 1. There is no need to additionally introduce sensors and detection devices, thus achieving the purpose of reducing costs;

[0072] 2. By determining the theoretical reference value of the d-axis current and controlling different forms of the introduced damping factor, the output impedance characteristics of SVG can be flexibly controlled, which has the characteristics of flexibility and strong applicability;

[0073] 3. The introduction of the damping factor does not change the size of the equivalent filter inductance on the AC side of SVG, so it can be applied to different grid strengths, making its applicability stronger.

[0074] Although the embodiments and drawings of the present invention are disclosed for illustrative purposes, those skilled in the art can understand that: without departing from the spirit and scope of the present invention and the appended claims, various substitutions, changes, and modifications are possible. Therefore, the scope of the present invention is not limited to the content disclosed in the embodiments and drawings.

Claims

1. An improved method for the grid connection stability of VSC-HVDC under weak power grids based on SVG impedance reshaping, characterized in that: The method includes the following steps: Step 1, in the SVG outer-loop DC voltage control link, based on the actual value v of the SVG DC-side voltage dc obtain the theoretical reference value i of the d-axis current dref ; Step 2, multiply the obtained theoretical d-axis current reference value \(i\) dref by the damping factor \(\delta\) to obtain the actual d-axis current reference value \(i\) dr ; Step 3, multiply the known theoretical reference value of the q-axis current i qref by the damping factor δ to obtain the actual reference value of the q-axis current i qr ; The expression of the damping factor δ is: Among them, τ is the filtering time constant, n is the number of control iterations, v dcref is the given value of the SVG DC-side voltage, v dc is the actual value of the SVG DC-side voltage, s is the complex frequency, i dref is the theoretical reference value of the d-axis current; Step 4: Subtract the obtained actual reference value of the d-axis current \(i_{d}^{ref}\) dr from the d-axis component \(i_{d}\) of the SVG grid-connected current d and multiply the resulting value by the transfer function \(G_{v}(s)\) of the voltage outer-loop controller i to obtain the control value of the d-axis current; The obtained actual reference value of the q-axis current i qr is subtracted from the q-axis component i q of the SVG grid-connected current. The resulting value is multiplied by the transfer function G i (s) of the voltage outer-loop controller to obtain the control value of the q-axis current; Among them, G i (s) is expressed as: where k ip is the proportional coefficient of the current loop, and k ii is the integral coefficient of the current loop, and s is the complex frequency; Step 5, multiply the q-axis component i of the SVG grid-connected current, q the system reference angular frequency ω, and the inductance L of the SVG AC-side filter circuit eq together, add the obtained product value to the d-axis component v of the grid connection point voltage d , subtract the control value of the d-axis current, and then divide by the given value v of the SVG DC-side voltage dcref to obtain the d-axis modulation signal m d ; Multiply the d-axis component i of the SVG grid-connected current, the system base angular frequency ω, and the inductance L of the AC-side filter circuit of the SVG d to obtain the product value ωLi eq ; subtract the product value ωLi eq from the q-axis component v of the grid connection point voltage, subtract the control value of the q-axis current from the resulting difference, and then divide by the given value v of the SVG DC-side voltage d to obtain the q-axis modulation signal m q ; eq d dcref q ​​​​ Step 6, generate an SVPWM wave to drive the SVG based on the d-axis modulation signal m d and the q-axis modulation signal m q to achieve impedance reshaping control for equivalently improving the damping characteristics of the SVG.

2. An improved method for the grid connection stability of VSC-HVDC under a weak grid based on SVG impedance reshaping according to claim 1, characterized in that: In the DC voltage control link of the SVG outer loop, based on the actual value v of the DC side voltage of the SVG dc to obtain the theoretical reference value i of the d-axis current dref The method is as follows: Subtract the actual value v of the SVG DC-side voltage from the given value v of the SVG DC-side voltage dcref Multiply the resulting value by the transfer function G dc (s) of the voltage outer-loop controller to obtain the theoretical reference value i v of the d-axis current dref , Among them, G v (s) is expressed as: Among them, k vp is the proportional coefficient of the voltage loop, and k vi is the integral coefficient of the voltage loop, and s is the complex frequency.

3. A VSC-HVDC grid-connected stability improvement system based on SVG impedance reshaping under weak power grids, characterized in that: including: The voltage outer loop control module is used to control the actual value of the SVG DC side voltage to track the given value and obtain the theoretical reference value i of the d-axis current dref ; Damping factor generation module, based on the theoretical reference value i of the d-axis current dref , the given value v of the DC side voltage of the SVG dcref and the actual value v of the DC side voltage of the SVG dc , generates a damping factor by setting a filtering time constant and the number of control iterations; A damping control module, which is used to introduce a damping factor δ into the current loop control strategy, and based on the d-axis current theoretical reference value i dref , the q-axis current theoretical reference value i qref , to obtain the actual d-axis current reference value i dr , the actual q-axis current reference value i qr ; The current control module is used to control the d-axis current and q-axis current actually output by the SVG to respectively track the actual reference value i of the d-axis current dr and the actual reference value i of the q-axis current qr , and obtain the d-axis output voltage signal and q-axis output voltage signal for modulation; The modulation module obtains the d-axis modulation signal m and the q-axis modulation signal m based on the d-axis output voltage signal and the q-axis output voltage signal for SVG modulation obtained in the current control module, and obtains the SVPWM drive signal for driving the actual operation of the SVG. d and the q-axis modulation signal m q and obtains the SVPWM drive signal for driving the actual operation of the SVG.

4. An improved system for the grid connection stability of VSC-HVDC under a weak grid based on SVG impedance reshaping according to claim 3, characterized in that: The voltage outer loop control module is used to control the actual value of the SVG DC-side voltage to track the given value, and the method for obtaining the theoretical reference value of the d-axis current is as follows: Subtract the actual value v dcref of the SVG DC-side voltage from the given value v dc of the SVG DC-side voltage, multiply the obtained value by the transfer function G v (s) of the voltage outer loop controller to obtain i dref , Among them, G v (s) is expressed as: where k vp is the voltage loop proportionality coefficient, and k vi is the voltage loop integral coefficient, and s is the complex frequency.

5. An improved system for the grid connection stability of VSC-HVDC under a weak grid based on SVG impedance reshaping according to claim 3, characterized in that: The expression of the damping factor δ is: Among them, τ is the filtering time constant, n is the control iteration number, v dcref is the given value of the SVG DC-side voltage, v dc is the actual value of the SVG DC-side voltage, s is the complex frequency, i dref is the theoretical reference value of the d-axis current.

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