Active damping control system for improving output impedance of current source type inverter

Abstract

The invention discloses an active damping control system for improving the output impedance of a current source type inverter, which comprises a current loop control loop, an active damping branch and an improved phase-locked loop, and is characterized in that the difference value of reference current and feedback network access current passes through a current controller to form the current loop control loop; the voltage of the grid-side inductor is fed back to the output of the current loop through the virtual resistor link to form a damping branch, and the voltage of the common coupling point enters the traditional phase-locked loop link through the low-pass filter and forms an improved phase-locked loop through phase compensation. The optimal feedback coefficient of the damping branch is designed according to the optimal damping ratio, the low-pass filter parameters are designed according to the phase at the maximum resonant frequency, the reasonable proportional gain link is designed according to the improvement of the output impedance amplitude, the resonance of the current source type inverter can be effectively inhibited, the output impedance of the inverter is increased, and the output impedance of the inverter is improved. And the quality of the network access current of the current source type inverter and the stability margin of the current source type inverter in a weak power grid are improved, and the stability of the system is improved.

Description

Technical Field

[0001] The invention relates to the technical field of grid-connected inverter control, in particular to an active damping control system for improving the output impedance of a current source inverter. Background Art

[0002] As the interface between distributed energy and the power grid, the performance of the grid-connected inverter directly determines the quality of distributed energy generation. At present, most distributed power generation uses voltage source inverters, and there is relatively little research on current source inverters. Compared with voltage source inverters, current source inverters have the following advantages: (1) The photovoltaic array exhibits current source characteristics in the stable working area, so the current source inverter is more suitable for photovoltaic grid-connected power generation; (2) The current source inverter is a boost converter, so compared with the voltage source inverter, for photovoltaic power generation, it can reduce the number of photovoltaic cells in series on the DC side, and for wind power generation, it can reduce the wind speed at which the power is cut in, that is, the current source inverter can increase the power generation of photovoltaic and wind turbines; (3) The large inductance on the DC side of the current source inverter can suppress the harmonics of the DC side current, and at the same time increase the inertia of the DC side current, which can slow down the rate of increase of the DC side current when the system is short-circuited, which is conducive to increasing the margin of the protection action time. In addition, compared with the DC filter capacitor, the DC side inductor has a longer life and higher system reliability.

[0003] However, the current source inverter uses a CL filter, which will cause a resonant peak in the system, affecting the stability of the system. The simplest and most effective method is to implement active damping on the filter capacitor voltage feedback, but since the amplitude of the filter capacitor voltage is relatively large compared to the DC side current amplitude, if active damping is directly implemented on the filter capacitor voltage feedback, it is easy to cause the inner loop to saturate when the system achieves the optimal damping ratio, thereby causing the system to become unstable. In addition, if the damping branch feedback coefficient is reduced to avoid loop saturation, the system will be insufficiently damped, which will easily cause the grid current and the AC side filter capacitor voltage to continue to oscillate when the reference current suddenly changes.

[0004] The phase-locked loop based on the synchronous coordinate system will greatly reduce the phase of the inverter output impedance within the phase-locked loop bandwidth, and may even cause the phase to be lower than -90°, thereby causing the performance of the inverter system to deteriorate under weak power grids. In the case of an ideal power grid or a relatively large short circuit, the intersection frequency of the amplitude-frequency curve of the output impedance and the grid impedance is relatively high. Since the phase-locked loop will not affect the characteristics of the output impedance in the medium and high frequency bands, the system can still maintain good stability. However, under weak power grid conditions (SCR<5), the intersection frequency of the amplitude-frequency curve of the output impedance and the grid impedance may be very low, which may cause the resonance suppression method of the system at low harmonic frequencies to fail. The phase margin of the inverter system will be greatly reduced or even fail to meet the Nyquist stability criterion, causing harmonic oscillations and even system instability. Therefore, under weak power grids, the resonance caused by the phase-locked loop needs to be considered.

[0005] Therefore, if a new active damping method and phase-locked loop optimization method can be found, which can effectively suppress the system resonance while enabling the system to achieve the optimal damping ratio without affecting the stability of the system, it will be beneficial to improve the output impedance of the current source inverter, and can improve the phase margin and amplitude gain of the system output impedance, which is beneficial to improving the stability of the system. For this purpose, an active damping control system for improving the output impedance of the current source inverter is provided. Summary of the invention

[0006] The purpose of the present invention is to provide an active damping control system for improving the output impedance of a current source inverter in view of the defects of the prior art, so as to solve the problems raised by the above-mentioned background technology.

[0007] To achieve the above object, the present invention provides the following technical solutions: an active damping control system for improving the output impedance of a current source inverter, comprising a first subtractor, a current controller, a capacitor impedance, a grid current feedforward module, a second subtractor, a damping branch module, a third subtractor, a PWM module, a low-pass filter, a traditional delay phase-locked loop structure and a phase compensation;

[0008] The first subtractor, the current controller, the capacitor impedance, the grid current feedforward module, the second subtractor, the damping branch module, the third subtractor, and the PWM module constitute a current loop control loop and an active damping branch of a current source inverter; the current loop control loop is used to control the grid current to track the reference current, and the active damping branch is used to suppress system resonance;

[0009] The low-pass filter, the traditional delayed phase-locked loop structure and the phase compensation constitute an improved phase-locked loop control loop, and the improved phase-locked loop control loop is used to improve system stability.

[0010] As a preferred technical solution of the present invention, the input end of the first subtractor is connected to the reference current and the grid current, and the output end of the first subtractor is connected to the input end of the current controller; the first subtractor and the current controller control loop are used to control the grid current to track the reference current.

[0011] As a preferred technical solution of the present invention, the output end of the current controller is connected to the input end of the capacitive impedance, the capacitive impedance is used to control the current flowing through the capacitor, and the output end of the capacitive impedance is connected to the input end of the second subtractor to obtain the current flowing through the filter capacitor.

[0012] As a preferred technical solution of the present invention, the input end of the second subtractor is connected to the grid current feedforward module, constitutes an inverter current together with the filter capacitor current, and is connected to the input end of the third subtractor.

[0013] As a preferred technical solution of the present invention, the input end of the third subtractor is connected to the damping branch module, and together with the inverter current, forms a modulation wave.

[0014] As a preferred technical solution of the present invention, the output end of the third subtractor is connected to a PWM module, and the PWM module outputs a driving signal to control the power device to generate an inverter current.

[0015] As a preferred technical solution of the present invention, the common coupling point voltage passes through a low-pass filter, a traditional delay phase-locked loop structure and phase compensation to generate a common coupling point phase.

[0016] An active damping method for increasing the output impedance of a current source inverter is implemented by the above system, and the specific steps are as follows:

[0017] Step 1: The current controller of the current loop control loop adopts a harmonic resonance controller with phase compensation. The input is the reference grid current, the feedback is the sampled grid current, and the output is the filter capacitor voltage. The grid current is directly controlled, and the parameters of the current controller are reasonably designed to ensure that the system has a suitable bandwidth and stability margin.

[0018] Step 2: When designing the damping module, start with the passive resistor and replace the passive resistor with a virtual resistor based on the physical meaning. At the same time, find the system transfer function including the virtual resistor, and design the optimal feedback coefficient based on the optimal damping ratio. Sample the inductor voltage and feed it back to the proportional controller output through the optimal damping coefficient to form a damping branch module for suppressing system resonance.

[0019] Step 3: The output of the current loop control circuit and the output of the damping branch module form a modulation wave, which is connected to the PWM module to generate a drive signal for controlling the on and off of the power device;

[0020] Step 4: Improve the phase-locked loop control loop including the traditional delay phase-locked loop structure, low-pass filter and phase compensation; the common coupling point voltage enters the traditional delay phase-locked loop structure through the low-pass filter and undergoes phase compensation, outputs the common coupling point phase, and then outputs a reference current that is consistent with the common coupling point voltage phase.

[0021] As a preferred technical solution of the present invention, the specific steps of step 2 are as follows:

[0022] Step 2.1: Draw all passive damping methods, consider the physical meaning of passive resistance and combine it with the system control block diagram, transform the passive resistance into a virtual resistance form that can be equivalent to proportional feedback of capacitor voltage, and then continue the equivalent transformation on this basis to transform it into feedback of grid-side inductor voltage to obtain the desired system control block diagram;

[0023] Step 2.2: According to the desired system control block diagram, the system transfer function is calculated and transformed into a typical expression, and then the virtual resistance parameters are designed according to the system optimal damping ratio to suppress the system's resonant peak and ensure system stability.

[0024] As a preferred technical solution of the present invention, in step 4, the output impedance of the system is calculated according to the overall control block diagram of the system, and then the low-pass filter parameters are designed according to the phase at the maximized resonant frequency, and a reasonable proportional gain link is designed according to the increase in the output impedance amplitude, which is used to improve the phase margin and amplitude gain of the output impedance and ensure the stability of the system.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) Increasing the inverter output impedance is beneficial to suppress the impact of grid background harmonics on grid current and improve the stability margin of the current source inverter under weak grid conditions;

[0027] (2) This paper provides a method for designing the optimal damping branch feedback coefficient. The control loop is simple and easy to implement, and it can effectively improve the dynamic characteristics of the current source inverter.

[0028] (3) A phase-locked loop optimization method based on a synchronous coordinate system is provided to improve the phase margin and amplitude gain of the output impedance and enhance the stability of the system.

[0029] In summary, compared with the existing active damping method of current source inverter, the present invention feeds back the inductor voltage, which can increase the output impedance of the inverter, which is beneficial to suppress the influence of the background harmonics of the power grid on the incoming current and improve the quality of the incoming current; the feedback coefficient of the damping branch module is obtained according to the optimal damping ratio, which can effectively suppress the resonance while allowing the system to achieve the optimal damping ratio, and can effectively improve the dynamic characteristics of the current source inverter; compared with the traditional phase-locked loop structure, the improved phase-locked loop adds a low-pass filter link, a phase compensation link and a proportional gain link, which can increase the phase margin and amplitude gain of the system, improve the system stability, and improve the robustness of the inverter under weak power grids.

[0030] The present invention can improve the grid-connected current quality, stability, and dynamic characteristics of a current source inverter and improve the output impedance, thereby improving the practicability of the current source inverter. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The basic structure and control diagram of the system when the present invention is adopted;

[0032] Figure 2 It is the overall control block diagram of the system when the present invention is adopted;

[0033] Figure 3 It is the system block diagram of the inverter current to the grid current;

[0034] Figure 4 It is the Bode diagram of the transfer function from inverter current to grid current when no additional measures are taken;

[0035] Figure 5 It is a passive damping method;

[0036] Figure 6 Active damping with filter capacitor voltage feedback equivalent to passive damping;

[0037] Figure 7 Equivalent transformation of system block diagram;

[0038] Figure 8 Grid-side inductor voltage feedback active damping;

[0039] Fig. 9 The characteristics of the system after the feedback coefficient of the damping branch is designed;

[0040] Fig.10 Bode plots of active damping using proportional feedback of filter capacitor voltage and active damping using grid-side inductor voltage feedback;

[0041] Fig.11 To improve the control structure of the phase-locked loop;

[0042] Fig.12This is the equivalent model of system output impedance when the influence of the phase-locked loop is ignored;

[0043] Fig.13 This is the Bode diagram of the system output impedance when the effect of the phase-locked loop is ignored;

[0044] Fig.14 This is the equivalent model of system output impedance when considering the influence of the phase-locked loop;

[0045] Fig.15 Bode plot of the system output impedance when the phase-locked loop effect is considered;

[0046] Fig.16 To improve the phase-locked loop from u g to ref Bode plot of the transfer function;

[0047] Fig.17 Bode plot of the system output impedance when the improved phase-locked loop is used;

[0048] Fig.18 It is the overall system diagram of the present invention;

[0049] Fig.18 Among them, 1. the first subtractor; 2. the current controller; 3. the capacitor impedance; 4. the grid current feedforward module; 5. the second subtractor module; 6. the damping branch module; 7. the third subtractor; 8. the PWM module; 9. the low-pass filter; 10. the traditional delay phase-locked loop structure; 11. the phase compensation. DETAILED DESCRIPTION

[0050] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making a clearer and more definite definition of the protection scope of the present invention.

[0051] Example: Figure 1 and Fig.18 Taking the single-phase current source grid-connected inverter shown as an example, an active damping control system for improving the output impedance of the current source inverter is based on an improved phase-locked loop, involving a control system composed of a current loop control loop, an active damping branch and a grid current feedforward branch, and involving an improved phase-locked loop control structure composed of a traditional phase-locked loop, a low-pass filter, a proportional gain link and a phase compensation link.

[0052] The above-mentioned active damping control method for improving the output impedance of the current source inverter based on the improved phase-locked loop is specifically implemented as follows: the first subtractor 1 and the current controller 2 control loop are used to control the grid current to track the reference current; the capacitor impedance 3 is used to control the current flowing through the capacitor, and it forms the inverter current through the second subtractor 5 with the grid current feedforward module 4, the grid-side inductor voltage passes through the active damping branch module 6, and the inverter current passes through the third subtractor 7 to form a modulation wave, and the modulation wave passes through the PWM module 8 to generate a drive signal. The common coupling point voltage passes through the low-pass filter 9, the traditional phase-locked loop structure 10 and the phase compensation link 11 to generate the common coupling point phase.

[0053] Combination Figure 1 , a design example of increasing the output impedance of a current source grid-connected inverter based on an improved phase-locked loop is given. Figure 3 The control block diagram of inverter current to grid current is given. Figure 3 The transfer function from inverter current to grid current can be obtained:

[0054]

[0055] From equation (1), it can be seen that the system is a second-order system, and the system does not contain a damping term, so the system is a severely underdamped system. The Bode diagram of equation (1) is plotted as follows: Figure 4 shown.

[0056] Figure 3 This proves that the system contains resonance. In the frequency band where the amplitude gain is greater than zero, the phase gain has a -180° jump. From the control point of view, the system is unstable. To increase the system damping, there are four forms of passive damping, such as Figure 5 shown.

[0057] When using methods 1 and 3, passive damping will affect the CL filter's suppression of high-order harmonics. Consider using method 2 for passive damping. This resistor can be used as the 1 / R corresponding to K1 in the current source inverter PPD controller parameters, which can solve the problem that the insulation resistance corresponding to the filter capacitor is too large, resulting in a 1 / R that is too small.

[0058] Considering that passive resistors consume power and reduce system efficiency, the resistors corresponding to method 2 are changed to virtual resistors. Figure 5 It can be seen that if the passive resistor is added, it is equivalent to adding a resistance branch on C, reducing the current flowing into C, and the magnitude of the current is u C / R, so if the resistor in mode 2 is changed to a virtual resistor, the control block diagram of inverter current to grid current can be changed to Figure 6 Feedback on the filter capacitor voltage can effectively suppress the system resonance peak. Figure 6, the output impedance can be calculated:

[0059]

[0060] However, for medium and low power current source inverters, the amplitude of the filter capacitor voltage must be greater than the DC side current amplitude. Therefore, a high-pass filter is generally required to be added to the active damping branch. The parameters of the high-pass filter will be coupled with the controller parameters, resulting in more complicated parameter design.

[0061] What is important is that the filter capacitor voltage includes the grid voltage. When the grid distortion is serious, it will undoubtedly affect the modulation wave, thereby causing grid current distortion. The following is an analysis from the perspective of inverter output impedance. Figure 6 Transform, and we get Figure 7 .

[0062] According to formula (1), the resonant frequency expression of the system can be calculated:

[0063]

[0064] Substituting the parameters into the calculation, we can see that the system resonant frequency is much greater than the fundamental frequency, so Figure 7 The negative feedback branch of the power grid voltage cannot suppress resonance. Moreover, the power grid voltage contains low-order background harmonics. Negative feedback to it will introduce low-order harmonics and affect the quality of grid-connected current. At the same time, for medium and low power current source inverters, the amplitude of the power grid voltage is much larger than the amplitude of the DC side current. Negative feedback to it can easily cause system saturation and instability. Therefore, it can be Figure 7 By simplifying and removing the negative feedback branch of the grid voltage, we can get the grid-side inductor voltage proportional feedback active strategy, such as Figure 8 shown.

[0065] according to Figure 8 The transfer function from inverter current to grid current can be obtained:

[0066]

[0067] It can be seen that the system is a second-order system. By transforming equation (4) into a typical second-order system expression, we can get:

[0068]

[0069] It can be seen that the system shown in equation (5) is a typical second-order system, and the damping ratio of the system can be expressed as:

[0070]

[0071] In order to ensure that the system has good characteristics and can effectively suppress the resonance peak of the system, let ζ equal to the optimal damping ratio, that is, let ζ = 0.707 and substitute the parameters. According to formula (6), the virtual resistance of the damping branch R = 7.07Ω can be calculated. Fig. 9 The Bode diagram of the inverter current to the grid current before and after the active damping branch is added is given. It can be seen that when the virtual resistance branch is added and the virtual resistance is designed according to the optimal damping ratio, the resonance peak is well suppressed.

[0072] When grid-side inductor voltage feedback active damping is adopted, the inverter output impedance expression is:

[0073]

[0074] Draw the Bode diagram of the inverter output impedance when using the above two active damping methods, as shown in Fig.10 Shown

[0075] Fig.10 The Bode plots of two different active damping methods are given. It can be seen from the figure that the grid-side inductor voltage feedback has a higher output impedance.

[0076] according to Figure 1 , Figure 2 As shown, we can get:

[0077]

[0078] When the phase-locked loop is ignored, (i ref Considered as a standard independent sinusoidal current source), we can get:

[0079]

[0080] According to formula (9), when the influence of the phase-locked loop is ignored, the grid-connected inverter can be equivalent to a current source and an impedance in parallel. The equivalent model of the output impedance is as follows: Fig.12 As shown, it can be concluded that the output impedance of the system when the phase-locked loop is ignored is

[0081]

[0082] Among them, G c (s) is a harmonic resonance controller with phase compensation, and its expression is:

[0083]

[0084] In the formula, w x is the controller bandwidth; k r Related to the gain at the resonant frequency; n max is the maximum harmonic number that is expected to be suppressed (n in this design max Set to 7); is the compensation phase angle at nw0.

[0085] Draw the Bode plot of the system output impedance when ignoring the phase-locked loop, such as Fig.13 As shown by Fig.13 It can be seen that the amplitude-frequency curve of the output impedance does not intersect the grid impedance (SCR=2), and the system phase is greater than -45° at this time. When the phase-locked loop is not considered, the system has strong robustness under weak grid conditions.

[0086] When considering the effect of the phase-locked loop, i ref and u g Coupling, current reference and PCC point voltage u g The relationship between can be expressed as:

[0087] i ref =I ref G PLL (s)u g (12)

[0088] Among them, G PLL (s) is the transfer function of a conventional phase-locked loop:

[0089]

[0090] will i ref Substituting into equation (9), we can get the expression of output impedance when considering the phase-locked loop:

[0091]

[0092] At this time, the equivalent model of output impedance is as follows Fig.14 As shown, by drawing the Bode diagram of formula (13), we can get the Bode diagram of the output function of the system when considering the phase-locked loop, as shown in Fig.15 shown.

[0093] Depend on Fig.15 It can be seen that when the phase-locked loop is considered, the output impedance intersects with the grid impedance at a lower frequency, and the phase near the 3rd, 5th and 7th harmonics of the output impedance is lower than -90°. At this time, the system may oscillate or even become unstable.

[0094] Fig.11 In order to improve the control structure of the phase-locked loop, a first-order low-pass filter is added before the PCC point voltage enters the orthogonal signal generation link, and phase compensation is performed on the improved phase-locked loop.

[0095] The transfer function of a first-order low-pass filter is

[0096]

[0097] Among them, ω cis the corner frequency of the first-order LPF, so the transfer function of the improved phase-locked loop can be expressed as:

[0098]

[0099] In the formula, It is a phase compensation link, which is used to compensate the phase angle of the phase lag at the fundamental frequency caused by the first-order LPF. The expression is

[0100]

[0101] observe Fig.15 It can be seen that due to the use of a harmonic resonant controller with phase compensation, the worst phase of the output impedance appears at 150Hz. Therefore, the design goal of the LPF is converted to maximizing the phase of the output impedance at 150Hz and increasing the amplitude of the output impedance.

[0102] Since the phase-locked loop only affects the inverter operating characteristics at lower frequencies, while the active damping and current controller affect the inverter operating characteristics at relatively high frequencies, C is ignored at 150Hz and G is considered. c The gain of (s) is very large, and the output impedance of the system at 150Hz can be written as:

[0103]

[0104] Based on the above discussion, the phase expression of the output impedance at 150Hz can be derived:

[0105] ∠Z out_PLL_150Hz =-180°-∠G PLL_150Hz +(∠G LPF_50Hz -∠G LPF_150Hz ) (19)

[0106] In this phase expression, the first two terms are related to the parameters of the inverter itself, and the last two terms are related to the parameters of the first-order LPF. Therefore, in order to maximize the phase, the last two terms need to be maximized.

[0107] In order to ensure that the added LPF does not seriously affect the gain of the output impedance at the fundamental frequency f0, and that the LPF can achieve the desired effect (i.e., there is good attenuation at 150Hz to weaken the adverse effect of the phase-locked loop on system stability), the bandwidth of the LPF should be between 50Hz and 150Hz. The phase of the LPF at 50 and 150Hz can be expressed as:

[0108]

[0109] Substituting equations (20) and (21) into equation (19), we can obtain

[0110]

[0111] Since the phase is to be maximized, equation (22) is c Take the derivative, that is:

[0112]

[0113] Let (23) be equal to 0, and we can calculate ω c =544.14rad / s is the maximum value. At this time, the improved phase-locked loop is from u g to ref The Bode diagram of the transfer function is as follows Fig.16 shown.

[0114] A proportional gain link is added to the above phase-locked loop link to increase the amplitude gain of the output impedance:

[0115]

[0116] Among them, K is the proportional gain link;

[0117] Bode diagram of the output impedance of the improved phase-locked loop and the traditional phase-locked loop Fig.17 As shown by Fig.16 , Fig.17 It can be seen that due to the use of the first-order LPF, G PLL_new (s) It has a high attenuation capability at frequencies above 100Hz, and the phase and amplitude gain of the inverter output impedance within the phase-locked loop bandwidth are greatly improved. After adopting the improved phase-locked loop, the instability problem caused by the grid impedance is solved.

[0118] The above embodiments only express the implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention.

Claims

1. An active damping control system for increasing the output impedance of a current source inverter, characterized in that: It comprises a first subtractor (1), a current controller (2), a capacitive impedance (3), a grid current feedforward module (4), a second subtractor (5), a damping branch module (6), a third subtractor (7), a PWM module (8), a low-pass filter (9), a traditional delay phase-locked loop structure (10) and a phase compensation (11); The first subtractor (1), the current controller (2), the capacitor impedance (3), the grid current feedforward module (4), the second subtractor (5), the damping branch module (6), the third subtractor (7), and the PWM module (8) constitute a current loop control loop and an active damping branch of a current source inverter; the current loop control loop is used to control the grid current to track the reference current, and the active damping branch is used to suppress system resonance; The low-pass filter (9), the traditional delayed phase-locked loop structure (10) and the phase compensation (11) constitute an improved phase-locked loop control loop, and the improved phase-locked loop control loop is used to improve system stability.

2. The active damping control system for increasing the output impedance of a current source inverter according to claim 1, characterized in that: The input end of the first subtractor (1) is connected to a reference current and a grid-connected current, and the output end of the first subtractor (1) is connected to an input end of a current controller (2); the first subtractor (1) and the current controller (2) control loop is used to control the grid-connected current to track the reference current.

3. The active damping control system for increasing the output impedance of a current source inverter according to claim 2, characterized in that: The output end of the current controller (2) is connected to the input end of the capacitive impedance (3), and the capacitive impedance (3) is used to control the current flowing through the capacitor. The output end of the capacitive impedance (3) is connected to the input end of the second subtractor (5) to obtain the current flowing through the filter capacitor.

4. The active damping control system for increasing the output impedance of a current source inverter according to claim 3, characterized in that: The input end of the second subtractor (5) is connected to the grid current feedforward module (4), and together with the filter capacitor current forms an inverter current, and is connected to the input end of the third subtractor (7).

5. The active damping control system for increasing the output impedance of a current source inverter according to claim 4, characterized in that: The input end of the third subtractor (7) is connected to the damping branch module (6), and together with the inverter current forms a modulation wave.

6. The active damping control system for increasing the output impedance of a current source inverter according to claim 5, characterized in that: The output end of the third subtractor (7) is connected to a PWM module (8), and the PWM module (8) outputs a driving signal to control the power device to generate an inverter current.

7. An active damping method for increasing the output impedance of a current source inverter, characterized in that: The method is implemented by the system according to any one of claims 1 to 6, and the specific steps are as follows: Step 1: The current controller (2) of the current loop control loop adopts a harmonic resonant controller with phase compensation, the input is the reference grid current, the feedback is the sampled grid current, and the output is the filter capacitor voltage; the grid current is directly controlled, and the parameters of the current controller (2) are reasonably designed to ensure that the system has a suitable bandwidth and stability margin; Step 2: When designing the damping module, start with the passive resistor and replace the passive resistor with a virtual resistor according to the physical meaning. At the same time, find the system transfer function including the virtual resistor, and design the optimal feedback coefficient according to the optimal damping ratio. Sample the inductor voltage and feed it back to the proportional controller output through the optimal damping coefficient to form a damping branch module (6) for suppressing system resonance. Step 3: The output of the current loop control circuit and the output of the damping branch module (6) form a modulation wave, which is connected to the PWM module (8) to generate a drive signal for controlling the on and off of the power device; Step 4: The improved phase-locked loop control circuit includes a traditional delay phase-locked loop structure (10), a low-pass filter (9) and a phase compensation (11); the common coupling point voltage enters the traditional delay phase-locked loop structure (10) through the low-pass filter (9) and undergoes phase compensation (11), outputs the common coupling point phase, and then outputs a reference current that is consistent with the common coupling point voltage phase.

8. The active damping method for increasing the output impedance of a current source inverter according to claim 7, characterized in that: The specific steps of step 2 are as follows: Step 2.1: Draw all passive damping methods, consider the physical meaning of passive resistance and combine it with the system control block diagram, transform the passive resistance into a virtual resistance form that can be equivalent to proportional feedback of capacitor voltage, and then continue the equivalent transformation on this basis to transform it into feedback of grid-side inductor voltage to obtain the desired system control block diagram; Step 2.2: According to the desired system control block diagram, the system transfer function is calculated and transformed into a typical expression, and then the virtual resistance parameters are designed according to the system optimal damping ratio to suppress the system's resonant peak and ensure system stability.

9. The active damping method for increasing the output impedance of a current source inverter according to claim 8, characterized in that: In step 4, the output impedance of the system is calculated according to the overall control block diagram of the system, and then the low-pass filter parameters are designed according to the phase at the maximized resonant frequency. A reasonable proportional gain link is designed according to the increase in the output impedance amplitude to improve the phase margin and amplitude gain of the output impedance and ensure the stability of the system.

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