Method and system for improving harmonic suppression of LRLC type grid-connected inverter under weak power grid

Abstract

The invention discloses a method and system for improving harmonic suppression of an LLLC type grid-connected inverter under a weak power grid, and the method comprises the steps: building a transfer function model of a current control system of the LLLC type grid-connected inverter, and enabling an active damping link of the current control system of the LLLC type grid-connected inverter to be realized through a first-order voltage feedforward controller; building a simulation model of the current control system of the LRLC type grid-connected inverter, and drawing a z-domain closed-loop zero-pole graph of the control system; and determining a parameter range of a first-order voltage feed-forward controller according to the closed-loop zero pole position of the control system containing the active damping link, and verifying the harmonic suppression capability of the LRLC type grid-connected inverter current control system containing the active damping link. On the basis of an existing sampling system of the grid-connected inverter, grid-connected low-voltage side power grid voltage feedback is adopted, an active damping feedforward controller is constructed, the harmonic suppression capacity of the LRLC type grid-connected inverter under a weak power grid is enhanced, and the current on a C branch is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of harmonic suppression of grid-connected inverters, and in particular to a method and system for suppressing harmonics of LRLC type grid-connected inverters under weak power grids. Background Art

[0002] LC filters are widely used in grid-connected inverters due to their simple structure and low cost. Since LC filters have an inherent resonant frequency, passive damping methods are often used in engineering to suppress the corresponding resonant current, such as connecting a damping resistor R in series on the C branch. In engineering applications, it is also necessary to limit the power consumption on the damping resistor R while maintaining an appropriate damping effect, so a small inductor L is generally connected in parallel to the damping resistor R to limit the current flowing through the damping resistor R. The filter with an LC series RL structure is referred to as an LRLC filter.

[0003] Under ideal grid conditions, the LRLC filter has good stability and harmonic attenuation capabilities. However, in engineering applications, the grid-connected inverter is generally isolated by a transformer and then connected to the public grid, so transformer winding leakage inductance is inevitable. As a result, for the LRLC type grid-connected inverter, the equivalent grid is no longer ideal, and the passive damping design of the LRLC filter will have a certain degree of mismatch, and the ability to suppress low-frequency harmonic currents will be weakened, and even produce an amplification effect, which will eventually lead to an increase in the low-frequency harmonic content of the output current of the grid-connected inverter, and an increase or even excess current on the C branch of the LRLC filter, threatening the safe operation of the system.

[0004] The current active damping solutions are designed for LC, LCL, LLCL and other filters. There are very few active damping design methods for LRLC filters with passive damping and their low-frequency resonance problems under weak grid conditions. And if conventional methods are used, such as capacitor branch current proportional feedback, current sensors need to be added. When multiple inverters are connected in parallel, the number of current sensors needs to be doubled, and the cost increases accordingly.

[0005] In view of this, this application is hereby filed. Summary of the invention

[0006] The purpose of the present invention is to improve the harmonic suppression capability of the LRLC type grid-connected inverter under weak power grid conditions. The design provides a method and system for improving the harmonic suppression of the LRLC type grid-connected inverter under weak power grid conditions. Based on the existing sampling system of the grid-connected inverter, the present invention adopts grid voltage feedback on the low-voltage side of the grid-connected power to construct an active damping feedforward controller (first-order voltage feedforward controller) to enhance the harmonic suppression capability of the LRLC type grid-connected inverter under weak power grid, reduce the current on the C branch, and improve the stability of system operation.

[0007] The present invention is achieved through the following technical solutions:

[0008] In a first aspect, the present invention provides a method for improving harmonic suppression of a LRLC type grid-connected inverter under a weak power grid, the method comprising:

[0009] The transfer function model of the LRLC grid-connected inverter current control system is established, and the active damping link of the LRLC grid-connected inverter current control system is realized by a first-order voltage feedforward controller;

[0010] According to the transfer function model, a simulation model of the LRLC grid-connected inverter current control system is built in Simulink;

[0011] Based on the constructed simulation model, draw the z-domain closed-loop zero-pole diagram of the control system; determine the parameter range of the first-order voltage feedforward controller based on the closed-loop zero-pole position of the control system containing the source damping link;

[0012] According to the parameter range, the harmonic suppression capability of the LRLC type grid-connected inverter current control system containing the source damping link is verified.

[0013] Furthermore, the first-order voltage feedforward controller G ff The expression of (z) is:

[0014]

[0015] Where n1 is the numerator coefficient of the first-order voltage feedforward controller; d1 is the denominator coefficient of the first-order voltage feedforward controller; z -1 For the delay link.

[0016] Furthermore, according to the constructed simulation model, the z-domain closed-loop zero-pole diagram of the control system is drawn; according to the closed-loop zero-pole position of the control system containing the source damping link, the parameter range of the first-order voltage feedforward controller is determined, including:

[0017] According to the built simulation model, set the numerator coefficient n1 and denominator coefficient d1 of the first-order voltage feedforward controller to 0, and draw the z-domain closed-loop zero-pole diagram of the control system; according to the z-domain closed-loop zero-pole diagram of the control system, find the low-frequency critical pole, that is, the pole with a damping coefficient less than 0.1;

[0018] Fix the numerator coefficient n1 of the first-order voltage feedforward controller to 0.5, and draw the first closed-loop zero-pole diagram of the control system when the denominator coefficient d1 has different values; determine the value range of the denominator coefficient d1 according to the variation law of the damping coefficient of the low-frequency critical pole and the stability of the control system;

[0019] The denominator coefficient d1 of the first-order voltage feedforward controller is fixed to 0.5, and the second closed-loop zero-pole diagram of the control system is plotted for different values ​​of the numerator coefficient n1. The value range of the numerator coefficient n1 is determined according to the variation law of the damping coefficient of the low-frequency critical pole and the stability of the control system.

[0020] Further, according to the parameter range, the harmonic suppression capability of the LRLC type grid-connected inverter current control system containing the source damping link is verified, including:

[0021] According to the value range of the denominator coefficient d1 and the value range of the numerator coefficient n1, a set of intermediate values ​​is selected within the value range of the denominator coefficient d1 and the value range of the numerator coefficient n1, and a closed-loop zero-pole diagram, a closed-loop frequency response curve and a frequency response curve of the C branch current of the LRLC filter are drawn;

[0022] Determine whether the three conditions are met: whether the closed-loop zero-pole is within the unit circle, whether the damping coefficient of the closed-loop zero-pole is increased by more than 2 times, and whether the low-frequency harmonic gain of the frequency response is less than 0dB; if all are met, then end; if one is not met, reselect a set of intermediate values ​​to adjust the numerator coefficient n1 and continue to judge.

[0023] Furthermore, the denominator coefficient d1 is 0.1, and the numerator coefficient n1=0.5.

[0024] In a second aspect, the present invention further provides a harmonic suppression system for an LRLC type grid-connected inverter under a weak power grid, the system comprising:

[0025] A transfer function model building unit is used to build a transfer function model of a current control system of a LRLC type grid-connected inverter, and an active damping link of the current control system of the LRLC type grid-connected inverter is realized by a first-order voltage feedforward controller;

[0026] A control system building unit, used to build a simulation model of the LRLC type grid-connected inverter current control system in Simulink according to the transfer function model;

[0027] The parameter range determination unit is used to draw a z-domain closed-loop zero-pole diagram of the control system according to the constructed simulation model; and determine the parameter range of the first-order voltage feedforward controller according to the closed-loop zero-pole position of the control system containing the source damping link;

[0028] The harmonic suppression verification unit is used to verify the harmonic suppression capability of the LRLC type grid-connected inverter current control system containing a source damping link according to the parameter range.

[0029] Furthermore, the parameter range determination unit includes:

[0030] The low-frequency critical pole finding subunit is used to set the numerator coefficient n1 and the denominator coefficient d1 of the first-order voltage feedforward controller to 0 according to the constructed simulation model, and draw the z-domain closed-loop zero-pole diagram of the control system; according to the z-domain closed-loop zero-pole diagram of the control system, find the low-frequency critical pole, that is, the pole with a damping coefficient less than 0.1;

[0031] The first parameter determination subunit is used to fix the numerator coefficient n1 of the first-order voltage feedforward controller to 0.5, and draw the first closed-loop zero-pole diagram of the control system when the denominator coefficient d1 has different values; according to the variation law of the damping coefficient of the low-frequency critical pole and the stability of the control system, the value range of the denominator coefficient d1 is determined;

[0032] The second parameter determination subunit is used to fix the denominator coefficient d1 of the first-order voltage feedforward controller to 0.5, and to draw the second closed-loop zero-pole diagram of the control system when the numerator coefficient n1 has different values; according to the variation law of the damping coefficient of the low-frequency critical pole and the stability of the control system, the value range of the numerator coefficient n1 is determined.

[0033] In a third aspect, the present invention provides an electronic device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the above-mentioned method for improving harmonic suppression of LRLC type grid-connected inverters under weak power grids when executing the computer program.

[0034] In a fourth aspect, the present invention further provides a computer-readable storage medium, wherein the computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the method for suppressing harmonics of the LRLC type grid-connected inverter under a weak power grid is implemented.

[0035] In a fifth aspect, the present invention further provides a computer program product, including a computer program / instruction, which, when executed by a processor, implements the above-mentioned method for improving harmonic suppression of LRLC type grid-connected inverters under weak power grids.

[0036] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0037] 1. The present invention improves the harmonic suppression method and system of the LRLC type grid-connected inverter under weak power grid. Based on the existing sampling system of the grid-connected inverter, the present invention adopts the grid voltage feedback on the low-voltage side of the grid-connected power to construct an active damping feedforward controller (i.e., a first-order voltage feedforward controller) to enhance the harmonic suppression capability of the LRLC type grid-connected inverter under weak power grid, reduce the current on the C branch, and improve the stability of system operation;

[0038] 2. The present invention improves the harmonic suppression method and system of the LRLC type grid-connected inverter under weak power grid. On the basis of not adding sensors, the first-order feedforward of the grid voltage is introduced to perform active damping, thereby suppressing the low-frequency harmonic current of the LRLC type grid-connected inverter under weak power grid and reducing the total current of the C branch.

[0039] 3. The present invention improves the harmonic suppression method and system of the LRLC type grid-connected inverter under weak power grid. The parameter design of the first-order feedforward link of the grid voltage is designed by graphical method based on the Z-domain closed-loop zero pole. By adjusting the coefficient of the first-order feedforward link, the damping coefficient of the critical pole of the original system is increased, thereby suppressing the forward gain of the closed-loop frequency response of the original system near the low frequency, thereby realizing the improvement of the harmonic suppression capability. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

[0041] Figure 1 The present invention is a flow chart of a method for suppressing harmonics of a LRLC type grid-connected inverter under a weak power grid;

[0042] Figure 2 This is a schematic diagram of the electrical principle of the LRLC type grid-connected inverter of the present invention;

[0043] Figure 3 It is a system transfer function block diagram of the LRLC type grid-connected inverter of the present invention;

[0044] Figure 4 A detailed flow chart of the harmonic suppression method of the LRLC type grid-connected inverter under weak power grid according to the present invention;

[0045] Figure 5 It is a schematic diagram of the closed-loop zero-pole of the system without voltage feedforward of the present invention;

[0046] Figure 6 It is the closed-loop Bode diagram of the system without voltage feedforward of the present invention;

[0047] Figure 7 The C branch frequency response curve of the LRLC filter without voltage feedforward of the present invention;

[0048] Figure 8 This is the closed-loop zero-pole diagram of the present invention with voltage feedforward (when d1 is 0.5 and n1 is different), 8(a) d1=0.5, n1=0.1; 8(b) d1=0.5, n1=0.5; 8(c) d1=0.5, n1=1.0; 8(d) d1=0.5, n1=2.0;

[0049] Fig. 9This is the closed-loop zero-pole diagram of the present invention with voltage feedforward (when n1 is 0.5 and d1 is different), 9(a) d1=0.1, n1=0.5; 9(b) d1=0.2, n1=0.5; 9(c) d1=0.8, n1=0.5; 9(d) d1=1.0, n1=0.5;

[0050] Fig.10 This is the closed-loop Bode diagram of the system with voltage feedforward in the present invention (when n1 is 0.5 and d1 is 0.1);

[0051] Fig.11 The frequency response curve of the C branch of the LRLC filter with voltage feedforward of the present invention (when n1 is 0.5 and d1 is 0.1);

[0052] Fig.12 12(a) is a schematic diagram comparing the grid-side current of the LRLC grid-connected inverter before and after adding active damping according to the present invention, wherein 12(a) is without active damping, and 12(b) is with active damping;

[0053] Fig.13 13(a) is a schematic diagram comparing the C branch current of the LRLC grid-connected inverter before and after adding active damping according to the present invention, wherein 13(a) is without active damping, and 13(b) is with active damping;

[0054] Fig.14 The present invention provides a structural block diagram of a harmonic suppression system for an LRLC type grid-connected inverter under a weak power grid. DETAILED DESCRIPTION

[0055] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary implementation modes of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0056] In order to improve the harmonic suppression capability of the LRLC type grid-connected inverter under weak power grid conditions, the present invention adopts grid voltage feedback on the low-voltage side of the grid-connected power supply on the basis of the existing sampling system of the grid-connected inverter, constructs an active damping feedforward controller (first-order voltage feedforward controller), enhances the harmonic suppression capability of the LRLC type grid-connected inverter under weak power grid conditions, reduces the current on the C branch, and improves the stability of system operation.

[0057] The specific steps of the method of the present invention are: firstly, a transfer function model of an LRLC type grid-connected inverter current control system is established, and the active damping link of the LRLC type grid-connected inverter current control system is realized by a first-order voltage feedforward controller; then, a simulation model of the LRLC type grid-connected inverter current control system is built in Simulink according to the transfer function model, thereby a z-domain closed-loop zero-pole diagram of the control system can be drawn, and according to the closed-loop zero-pole position of the control system containing active damping, which is equivalent to the frequency and damping coefficient of the closed-loop zero-pole, the parameter range of the first-order voltage feedforward controller can be determined; finally, the harmonic suppression capability is verified by the closed-loop frequency response characteristics of the control system.

[0058] Example 1

[0059] like Figure 1 and Figure 4 As shown, the present invention improves the harmonic suppression method of the LRLC type grid-connected inverter under weak power grid, and the method includes:

[0060] Step 1, establishing a transfer function model of a LRLC type grid-connected inverter current control system, and the active damping link of the LRLC type grid-connected inverter current control system is realized by a first-order voltage feedforward controller;

[0061] Step 2: Building a simulation model of the LRLC grid-connected inverter current control system in Simulink according to the transfer function model;

[0062] Step 3, based on the constructed simulation model, draw the z-domain closed-loop zero-pole diagram of the control system; according to the closed-loop zero-pole position of the control system containing the source damping link, determine the parameter range of the first-order voltage feedforward controller;

[0063] Step 4: verifying the harmonic suppression capability of the LRLC type grid-connected inverter current control system including the source damping link according to the parameter range.

[0064] Figure 2 The electrical principle diagram of the LRLC grid-connected inverter mainly includes four parts: DC source, three-level converter (3L-NPC), LRLC filter and transformer. The external DC power is converted into AC power by the three-level converter, and the harmonics are filtered out by the LRLC filter. Finally, the transformer is stepped up and connected to the public grid.

[0065] The working process of the LRLC grid-connected inverter control system is as follows: First, the voltage on the low-voltage side of the transformer is detected, that is, the voltage v at the upper end of the C branch. f , according to the instantaneous power theory and the power reference P REF , Q REF , the reference i of the current loop can be calculated REF; Then detect the inverter output current, that is, the filter internal current i Li , with the reference current i REF After the difference is calculated, it is sent to the current controller to complete the current closed loop; finally, the output of the current loop is modulated by SVPWM to generate a switching sequence, thereby controlling the 3L-NPC.

[0066] In this embodiment, the transfer function model in step 1 is established as follows:

[0067] (1) The current controller adopts a proportional resonant controller:

[0068] After discretization, we can get:

[0069]

[0070] Where: G RC (s) and G RC (z): proportional resonant controller in S domain and Z domain respectively; k p , k r They are proportional gain and resonant gain respectively; ω0 is the resonant center frequency of the resonant controller; is the compensation phase of the resonant controller; Ts is the control sampling time.

[0071] (2) 3L-NPC is equivalent to a PWM wave amplifier in principle. Its mathematical model is equivalent to the PWM modulation process, that is, a 0.5-beat delay link plus a one-beat delay for sampling and control calculation, for a total of 1.5-beat delay:

[0072]

[0073] In the formula, G d (s) is the s-domain mathematical model of the delay link and PWM link of the grid-connected inverter; Ts is the control sampling time.

[0074] (3) The LRLC filter can use the KCL and KVL laws to establish a mathematical model for each component as follows:

[0075]

[0076] In the formula, v I (s) is the output voltage of the inverter; v f (s) is the output voltage of the LRLC filter; i f (s) is the current of the C branch of the LRLC filter; i Li (s) is the main inductor current of the LRLC filter; i g (s) is the output current of the LRLC filter; C f , L f , Rf are the nominal values ​​of capacitance, inductance and resistance of the C branch of the LRLC filter respectively; L i is the nominal value of the main inductance of the LRLC filter.

[0077] (4) The first-order voltage feedforward controller is G ff (z), this is to consider that the system order should not be too high, and the feedforward controller form G is set to a first-order filter, the expression is:

[0078]

[0079] Where n1 is the numerator coefficient of the first-order voltage feedforward controller; d1 is the denominator coefficient of the first-order voltage feedforward controller; z -1 For the delay link.

[0080] In summary, the system transfer function model block diagram of the LRLC grid-connected inverter can be obtained as follows: Figure 3 shown.

[0081] Specifically, Figure 2 In the example, the LRLC grid-connected inverter system parameters are as follows:

[0082] The inverter has a rated power of 4MW, an AC rated voltage of 2.5kV, and a DC rated voltage of 4kV;

[0083] L i 450uH, L f 80uH, C f is 400uF, R f 12Ω;

[0084] The rated voltage of the transformer primary / secondary side is 10kV / 2.5kV;

[0085] The rated current of the transformer primary / secondary side is 1460.7A / 1443.4A;

[0086] Transformer short-circuit impedance: 10.17%;

[0087] The equivalent L is estimated from the transformer parameters g 560uH;

[0088] Power device: IGCT, switching frequency is 1.2kHz, dead time is 50us;

[0089] The DC bus voltage is 4kV;

[0090] The grid frequency is 50.0Hz and the grid rated voltage is 10kV;

[0091] Proportional resonant controller parameters: k p is 0.3, k ris 40, is -30, and the center frequency ω0 is 100π.

[0092] In this embodiment, according to the established system transfer function model, a corresponding control system simulation model can be established in Simulink.

[0093] In this embodiment, according to the constructed simulation model, the z-domain closed-loop zero-pole diagram of the control system is drawn; according to the closed-loop zero-pole position of the control system containing the source damping link, the parameter range of the first-order voltage feedforward controller is determined, including:

[0094] (1) According to the established simulation model, set the numerator coefficient n1 and denominator coefficient d1 of the first-order voltage feedforward controller to 0, and draw the z-domain closed-loop zero-pole diagram of the control system; according to the z-domain closed-loop zero-pole diagram of the control system, find the low-frequency critical pole, that is, the pole with a damping coefficient less than 0.1;

[0095] (2) Fix the numerator coefficient n1 of the first-order voltage feedforward controller to 0.5, and draw the first closed-loop zero-pole diagram of the control system when the denominator coefficient d1 has different values; determine the value range of the denominator coefficient d1 based on the variation law of the damping coefficient of the low-frequency critical pole and the stability of the control system;

[0096] (3) Fix the denominator coefficient d1 of the first-order voltage feedforward controller to 0.5, and draw the second closed-loop zero-pole diagram of the control system when the numerator coefficient n1 has different values; determine the value range of the numerator coefficient n1 based on the variation law of the damping coefficient of the low-frequency critical pole and the stability of the control system.

[0097] In the specific implementation, according to the built simulation model, the closed-loop zero-pole and closed-loop Bode diagram of the system without the first-order voltage feedforward controller are drawn, as shown in Figure 5 and Figure 6 As shown. Figure 5 From the closed-loop zero-pole shown, it can be seen that the system has no zero-pole outside the unit circle and is stable. However, the system has a pair of poles with a damping coefficient of 0.0439 and a frequency of 456Hz, and a pair of zeros with a damping coefficient of 0.00839 and a frequency of 316Hz. Combined with the zero-pole characteristics, it can be obtained that the system will produce a leading correction effect within 316-456Hz and introduce a large gain, resulting in the failure of harmonic suppression capability. Figure 6 The amplitude-frequency response shown in Figure 1 also confirms this phenomenon. Further drawing the frequency response curve of the C branch of the LRLC filter, as shown in Figure 2 Figure 7 As shown, it can be seen that the C branch current produces positive gain near 450Hz, and the harmonic current is amplified, which is basically consistent with the frequency response law of the main current near 450Hz.

[0098] The parameter design of the first-order voltage feedforward controller includes d1 and n1. By adjusting the parameters of the first-order voltage feedforward controller, the damping of the closed-loop critical pole of the system is increased, the low-frequency gain caused by the critical pole is reduced, and the harmonic suppression capability is improved. Therefore, according to different d1 and n1 values, the closed-loop zero-pole diagram of the new system can be drawn. Figure 8 The following is the closed-loop zero-pole diagram when d1 is 0.5 and n1 is different in value. Fig. 9 The following is the closed-loop zero-pole diagram when n1 is 0.5 and d1 is different. Figure 8 and Fig. 9 From the zero-pole trajectory shown, we can see the following rules:

[0099] The influence of n1 on the critical poles of the original system far exceeds that of d1. As n1 increases, the damping coefficient of the connecting poles of the original system increases rapidly, but at the same time, the damping of the newly added poles gradually decreases. Therefore, n1 should comprehensively consider the damping effect of the critical poles of the original system and the newly added poles, and take a value between 0.5 and 1.0.

[0100] The impact of d1 on the system is mainly the impact on the newly added poles and zeros, and has little effect on the damping effect of the critical poles of the original system. As d1 increases, the damping effect of the newly added poles gradually decreases. When d1 exceeds 1.0, the system becomes unstable. Therefore, the value range of d1 is limited to between 0.1 and 0.2.

[0101] Combined with the above parameter optional ranges, n1 can be selected as 0.5 and d1 can be selected as 0.1.

[0102] In this embodiment, according to the parameter range, verifying the harmonic suppression capability of the LRLC type grid-connected inverter current control system containing the source damping link includes:

[0103] According to the value range of the denominator coefficient d1 and the value range of the numerator coefficient n1, a set of intermediate values ​​is selected within the value range of the denominator coefficient d1 and the value range of the numerator coefficient n1, and a closed-loop zero-pole diagram, a closed-loop frequency response curve and a frequency response curve of the C branch current of the LRLC filter are drawn;

[0104] Determine whether the three conditions are met: whether the closed-loop zero-pole is within the unit circle, whether the damping coefficient of the closed-loop zero-pole is increased by more than 2 times, and whether the low-frequency harmonic gain of the frequency response is less than 0dB; if all are met, then end; if one is not met, reselect a set of intermediate values ​​to adjust the numerator coefficient n1 and continue to judge.

[0105] In the specific implementation, according to n1, 0.5 and d1, 0.1 are selected, thereby, the closed-loop Bode diagram of the new system with source damping and the frequency response curve of the C branch of the LRLC filter can be drawn respectively, as shown in FIG. Fig.10 and Fig.11 As shown, Figure 6 and Figure 7 By comparison, it can be seen that the low-frequency gain near 450 Hz has been effectively suppressed, and the parameter design of the first-order voltage feedforward controller designed in the present invention is effective.

[0106] The present invention is based on Figure 2 The electrical schematic diagram of the LRLC grid-connected inverter and the system parameters shown can be used to build a corresponding electrical system simulation model in Simulink for simulation verification, and compare the harmonic changes of the grid-connected inverter output current and C branch current before and after adding the voltage feedforward controller.

[0107] The simulation related parameters are as follows: simulation step length: 4us; total simulation time: 6s;

[0108] Control cycle: 192us; switching cycle 768us; dead time 40us;

[0109] Simulation process: no voltage feedforward within 0s to 2.0s, voltage feedforward within 2.5s to 6.0s;

[0110] Active power setting: 4MW, reactive power setting: 0MVar.

[0111] Fig.12 The figure shows the comparison of grid-side current of the grid-connected inverter before and after adding the voltage feedforward controller. According to the FFT analysis results, it can be seen that after adding the voltage feedforward controller, the harmonic components near 400Hz are significantly reduced, and the harmonic suppression introduced by the voltage feedforward controller is real and effective. Fig.12 Here, Current represents current, Time represents time, Frequency represents frequency, and Mag represents amplitude. Fig.13 The figure shows the comparison of the C branch current of the LRLC filter before and after adding the voltage feedforward controller. According to the RMS value comparison, it can be seen that after adding the voltage feedforward controller, the C branch current of the LRLC filter has dropped by about 6A overall, which can effectively limit the overall filter current of the C branch. Fig.13 Here, Current represents current, Time represents time, and RMS represents effective value.

[0112] The advantages of the present invention are:

[0113] (1) Without adding any sensors, the first-order feedforward of the grid voltage is introduced for active damping, which suppresses the low-frequency harmonic current of the LRLC grid-connected inverter under weak grid conditions and reduces the total current of the C branch.

[0114] (2) The parameter design of the first-order feedforward link of the grid voltage is designed based on the graphical method of the Z-domain closed-loop zero poles. By adjusting the coefficients of the first-order feedforward link, the damping coefficient of the critical pole of the original system is increased, thereby suppressing the positive gain of the closed-loop frequency response of the original system near the low frequency, thereby achieving an improvement in the harmonic suppression capability.

[0115] This invention is only applicable to LRLC type grid-connected inverters.

[0116] Example 2

[0117] like Fig.14 As shown, the difference between this embodiment and embodiment 1 is that this embodiment provides a harmonic suppression system for improving the LRLC type grid-connected inverter under weak power grid, and the system corresponds to the harmonic suppression method for improving the LRLC type grid-connected inverter under weak power grid in embodiment 1 in a one-to-one manner; the system includes:

[0118] A transfer function model building unit is used to build a transfer function model of a current control system of a LRLC type grid-connected inverter, and an active damping link of the current control system of the LRLC type grid-connected inverter is realized by a first-order voltage feedforward controller;

[0119] A control system building unit, used to build a simulation model of the LRLC type grid-connected inverter current control system in Simulink according to the transfer function model;

[0120] The parameter range determination unit is used to draw a z-domain closed-loop zero-pole diagram of the control system according to the constructed simulation model; and determine the parameter range of the first-order voltage feedforward controller according to the closed-loop zero-pole position of the control system containing the source damping link;

[0121] The harmonic suppression verification unit is used to verify the harmonic suppression capability of the LRLC type grid-connected inverter current control system containing a source damping link according to the parameter range.

[0122] As a further implementation, the parameter range determination unit includes:

[0123] The low-frequency critical pole finding subunit is used to set the numerator coefficient n1 and the denominator coefficient d1 of the first-order voltage feedforward controller to 0 according to the constructed simulation model, and draw the z-domain closed-loop zero-pole diagram of the control system; according to the z-domain closed-loop zero-pole diagram of the control system, find the low-frequency critical pole, that is, the pole with a damping coefficient less than 0.1;

[0124] The first parameter determination subunit is used to fix the numerator coefficient n1 of the first-order voltage feedforward controller to 0.5, and draw the first closed-loop zero-pole diagram of the control system when the denominator coefficient d1 has different values; according to the variation law of the damping coefficient of the low-frequency critical pole and the stability of the control system, the value range of the denominator coefficient d1 is determined;

[0125] The second parameter determination subunit is used to fix the denominator coefficient d1 of the first-order voltage feedforward controller to 0.5, and to draw the second closed-loop zero-pole diagram of the control system when the numerator coefficient n1 has different values; according to the variation law of the damping coefficient of the low-frequency critical pole and the stability of the control system, the value range of the numerator coefficient n1 is determined.

[0126] Among them, the execution process of each unit can be executed according to the process steps of the method for improving harmonic suppression of LRLC type grid-connected inverter under weak power grid in Example 1, and will not be repeated one by one in this embodiment.

[0127] At the same time, the present invention also provides an electronic device, including a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, the above-mentioned method for suppressing harmonics of the LRLC type grid-connected inverter under a weak power grid is implemented.

[0128] At the same time, the present invention also provides a computer-readable storage medium, which stores a computer program. When the computer program is executed by a processor, it implements the above-mentioned method for improving harmonic suppression of LRLC type grid-connected inverter under weak power grid.

[0129] At the same time, the present invention also provides a computer program product, including a computer program / instruction, which, when executed by a processor, implements the above-mentioned method for suppressing harmonics of a LRLC type grid-connected inverter under a weak power grid.

[0130] Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems, or computer program products. Therefore, the present application may adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the present application may adopt the form of a computer program product implemented in one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code.

[0131] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1A device that provides the functions specified in a block or multiple blocks.

[0132] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0133] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0134] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. Improve the harmonic suppression method of LRLC type grid-connected inverter under weak power grid, characterized in that: The method includes: The transfer function model of the LRLC grid-connected inverter current control system is established, and the active damping link of the LRLC grid-connected inverter current control system is realized by a first-order voltage feedforward controller; Based on the transfer function model, a simulation model of the LRLC type grid-connected inverter current control system is built in Simulink; Based on the constructed simulation model, draw the z-domain closed-loop zero-pole diagram of the control system; determine the parameter range of the first-order voltage feedforward controller based on the closed-loop zero-pole position of the control system containing the source damping link; According to the parameter range, the harmonic suppression capability of the LRLC type grid-connected inverter current control system containing the source damping link is verified.

2. The method for suppressing harmonics of LRLC type grid-connected inverter under weak power grid according to claim 1, characterized in that: The first-order voltage feedforward controller G ff The expression of (z) is: Where n1 is the numerator coefficient of the first-order voltage feedforward controller; d1 is the denominator coefficient of the first-order voltage feedforward controller; z -1 For the delay link.

3. The method for suppressing harmonics of LRLC type grid-connected inverter under weak power grid according to claim 1, characterized in that: Based on the established simulation model, draw the z-domain closed-loop zero-pole diagram of the control system; based on the closed-loop zero-pole positions of the control system containing the source damping link, determine the parameter range of the first-order voltage feedforward controller, including: Based on the established simulation model, set the numerator coefficient n1 and denominator coefficient d1 of the first-order voltage feedforward controller to 0, and draw the z-domain closed-loop zero-pole diagram of the control system; based on the z-domain closed-loop zero-pole diagram of the control system, find the low-frequency critical pole, that is, the pole with a damping coefficient less than 0.1; Fix the numerator coefficient n1 of the first-order voltage feedforward controller to 0.5 and plot the first closed-loop zero-pole diagram of the control system for different values ​​of the denominator coefficient d1. Determine the value range of the denominator coefficient d1 based on the variation pattern of the damping coefficient of the low-frequency critical pole and the stability of the control system. The denominator coefficient d1 of the first-order voltage feedforward controller is fixed at 0.5, and the second closed-loop zero-pole diagram of the control system is plotted for different values ​​of the numerator coefficient n1. The value range of the numerator coefficient n1 is determined based on the variation law of the damping coefficient of the low-frequency critical pole and the stability of the control system.

4. The method for suppressing harmonics of LRLC type grid-connected inverter under weak power grid according to claim 3 is characterized in that: Based on the parameter range, the harmonic suppression capability of the LRLC type grid-connected inverter current control system with source damping link is verified, including: According to the value range of the denominator coefficient d1 and the value range of the numerator coefficient n1, a set of intermediate values ​​is selected within the value range of the denominator coefficient d1 and the value range of the numerator coefficient n1, and a closed-loop zero-pole diagram, a closed-loop frequency response curve, and a frequency response curve of the LRLC filter C branch current are drawn; Determine whether the three conditions are met: whether the closed-loop zero-pole is within the unit circle, whether the damping coefficient of the closed-loop zero-pole is increased by more than 2 times, and whether the low-frequency harmonic gain of the frequency response is less than 0dB; if all are met, end; if one is not met, reselect a set of intermediate values ​​to adjust the numerator coefficient n1 and continue the judgment.

5. The method for suppressing harmonics of LRLC type grid-connected inverter under weak power grid according to claim 4 is characterized in that: The denominator coefficient d1 is 0.1, and the numerator coefficient n1 is 0.

5.

6. Improve the harmonic suppression system of LRLC type grid-connected inverter under weak power grid, which is characterized by: The system includes: A transfer function model building unit is used to build a transfer function model of a current control system of an LRLC type grid-connected inverter, wherein an active damping link of the current control system of the LRLC type grid-connected inverter is realized by a first-order voltage feedforward controller; A control system building unit, used to build a simulation model of the LRLC type grid-connected inverter current control system in Simulink according to the transfer function model; The parameter range determination unit is used to draw the z-domain closed-loop zero-pole diagram of the control system based on the constructed simulation model; and determine the parameter range of the first-order voltage feedforward controller based on the closed-loop zero-pole position of the control system containing the source damping link; The harmonic suppression verification unit is used to verify the harmonic suppression capability of the LRLC type grid-connected inverter current control system containing the source damping link according to the parameter range.

7. The harmonic suppression system for LRLC type grid-connected inverter under weak power grid according to claim 6 is characterized in that: The parameter range determining unit includes: The low-frequency critical pole search subunit is used to set the numerator coefficient n1 and denominator coefficient d1 of the first-order voltage feedforward controller to 0 based on the established simulation model, and draw the z-domain closed-loop zero-pole diagram of the control system; based on the z-domain closed-loop zero-pole diagram of the control system, find the low-frequency critical pole, that is, the pole with a damping coefficient less than 0.1; The first parameter determination subunit is used to fix the numerator coefficient n1 of the first-order voltage feedforward controller to 0.5 and draw the first closed-loop zero-pole diagram of the control system for different values ​​of the denominator coefficient d1; based on the variation law of the damping coefficient of the low-frequency critical pole and the stability of the control system, the value range of the denominator coefficient d1 is determined; The second parameter determination subunit is used to fix the denominator coefficient d1 of the first-order voltage feedforward controller to 0.5, and draw the second closed-loop zero-pole diagram of the control system when the numerator coefficient n1 has different values; according to the variation law of the damping coefficient of the low-frequency critical pole and the stability of the control system, the value range of the numerator coefficient n1 is determined.

8. An electronic device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein: When the processor executes the computer program, the method for suppressing harmonics of an LRLC type grid-connected inverter under a weak power grid is implemented as described in any one of claims 1 to 5.

9. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by a processor, the method for suppressing harmonics of an LRLC type grid-connected inverter under a weak power grid as described in any one of claims 1 to 5 is implemented.

10. A computer program product comprising a computer program / instructions, characterized in that When the computer program / instruction is executed by a processor, the method for suppressing harmonics of an LRLC type grid-connected inverter under a weak power grid as described in any one of claims 1 to 5 is implemented.

Images