Grid-connect control method and apparatus for grid-connect inverter

Abstract

Disclosed in the present invention are a grid-connect control method and apparatus for a grid-connect inverter. The method comprises: on the basis of a control structure of a grid-connect inverter in a dp domain, passing a deviation signal of a power outer loop through power outer loop controllers, so as to generate reference values of a current inner loop, and comparing the reference values of the current inner loop with an actual output current, so as to obtain error signals, wherein the control structure comprises a PQ power control outer loop and a current control inner loop, and the actual output current is obtained by means of performing phase tracking, on the basis of a synchronous reference frame phase-locked loop, on the voltage of a point of common coupling; and adjusting the error signals by means of a current inner loop controller, so as to generate modulation signals, performing pulse width modulation (PWM) on the modulation signals, so as to generate an on-off signal of an inverter bridge power semiconductor device, and controlling the state of connection and disconnection between the grid-connect inverter and a power grid on the basis of the on-off signal. In the present embodiment, the connection and disconnection between a grid-connect inverter and a power grid are controlled, so as to improve the stability of the grid-connect inverter in weak power grids.

Description

A grid-connected control method and device for a grid-connected inverter Technical Field

[0001] The present invention relates to the field of grid-connected inverter control, and in particular to a grid-connected control method and device for a grid-connected inverter. Background Art

[0002] With the rapid development of renewable energy generation technology, grid-connected inverters are increasingly used as the interface between renewable energy and the power grid. However, renewable energy sources are often distributed in remote areas, and long transmission lines and numerous transformers result in high grid impedance, a characteristic of a weak grid. Generally speaking, a grid is defined as weak when the short-circuit ratio (SCR) is ≤ 10. Due to the existence of grid impedance, the interaction between the grid-connected inverter and the grid impedance can lead to problems such as broadband oscillations in the inverter, seriously threatening the inverter's stable operation. As a critical component for achieving inverter synchronization with the grid, the phase-locked loop (PLL) (PLL) (PLL) has an even more significant impact on the stability of grid-connected inverters, especially under weak grid conditions. Traditional PLLs achieve synchronization between the inverter and the grid by controlling the q-axis voltage. This results in non-zero off-diagonal elements in the inverter's impedance matrix in the dq coordinate system, leading to frequency coupling and thus affecting the inverter's stable operation. Therefore, to eliminate the negative impact of the PLL on the stability of grid-connected inverters, existing research has primarily focused on improving the PLL itself, reducing the negative damping region of the grid-connected inverter, thereby improving system stability. Existing literature also points out that small disturbances in the phase-locked loop will introduce a positive feedback loop at the current reference value, thereby reducing the stability of the grid-connected system.

[0003] The essence of existing control methods for improving the stability of grid-connected inverters under weak power grids is to improve the amplitude-phase-frequency characteristic curve of the output impedance of the grid-connected inverter in order to adapt to a wider range of grid impedance. However, existing stability improvement methods all optimize the impedance phase or amplitude, or change the structure and parameters of the phase-locked loop, resulting in a cumbersome control process and not conducive to the promotion and transplantation of the methods. At the same time, with the continuous increase in the installed capacity of renewable energy power generation systems, the power grid will become weaker and weaker, which may make the original stability improvement methods difficult to apply. Therefore, there is an urgent need to seek more intuitive and portable stability improvement methods to provide theoretical guidance for the widespread access and safe and reliable operation of large-scale renewable energy power generation systems.

[0004] Summary of the Invention

[0005] The present invention provides a grid-connected control method and device for a grid-connected inverter, which can realize the on-off control of the grid-connected inverter and the power grid, and improve the stability of the grid-connected inverter under a weak power grid.

[0006] In order to solve the above technical problems, an embodiment of the present invention provides a grid-connected control method for a grid-connected inverter, comprising:

[0007] Based on the control structure of a grid-connected inverter in the dq domain, the deviation signal of the outer power loop is passed through the outer power loop controller to generate a reference value for the inner current loop. The reference value of the inner current loop is compared with the actual output current to obtain an error signal. The control structure includes a PQ power control outer loop and a current control inner loop. The actual output current is obtained by phase tracking the common grid connection point voltage using a phase-locked loop based on a synchronous reference coordinate system.

[0008] The error signal is adjusted through the current inner loop controller to generate a modulation signal, and the modulation signal is subjected to PWM pulse width modulation to generate a disconnection signal for the inverter bridge power semiconductor device. The on-off state of the grid-connected inverter and the power grid is controlled based on the disconnection signal.

[0009] As a preferred solution, the actual output current is obtained by phase tracking the common grid connection point voltage based on a synchronous reference coordinate system phase-locked loop, specifically:

[0010] Based on the control structure of the grid-connected inverter in the dq domain, the common grid-connected point voltage and grid-connected current are projected onto the grid dq coordinate system to obtain the active power and reactive power. The grid-connected current includes the grid d-axis current and the grid p-axis current.

[0011] Perform small signal linearization processing on active power and reactive power to obtain active disturbance signal and reactive disturbance signal;

[0012] Based on the synchronous reference frame phase-locked loop, active disturbance signal, and reactive disturbance signal, a small signal model between the reference value of the current inner loop and the PQ power control outer loop is constructed. The formula is as follows:

[0013] in, The projection of the disturbance signal representing the d-axis reference value of the current inner loop in the dq coordinate system of the current inner loop controller is: G represents the projection of the disturbance signal of the q-axis reference value of the current inner loop in the dq coordinate system of the current inner loop controller. P (s) represents the active power controller of the outer power loop, G Q (s) represents the power outer loop reactive controller, ΔQ represents the reactive disturbance signal, ΔQ ref Represents the reference value of the reactive disturbance signal, ΔP represents the active disturbance signal, ΔP ref Indicates the reference value of the active disturbance signal;

[0014] Based on the synchronous reference coordinate system, the phase-locked loop will perform feedback compensation on the small signal model to obtain the compensated small signal model;

[0015] Based on the compensated small signal model, the q-axis voltage component of the common grid-connected point voltage is controlled to 0 through the PI controller, and the frequency and phase of the common grid-connected point voltage are latched and tracked to obtain the actual output current.

[0016] As a preferred solution, the small signal model is feedback compensated based on the synchronous reference coordinate system phase-locked loop to obtain the compensated small signal model, specifically:

[0017] Based on the synchronous reference frame, the phase-locked loop generates a positive feedback loop at the modulation signal and the inner loop current reference disturbance value;

[0018] A compensation feedback loop is introduced into the PQ power control outer loop to eliminate the positive feedback loop, and feedback compensation is performed on the small signal model based on an overcompensation coefficient to obtain a compensated small signal model. The overcompensation coefficient is selected based on the amplitude-frequency characteristic of the phase margin.

[0019] As a preferred solution, the overcompensation coefficient is selected based on the amplitude-frequency characteristic of the phase margin, specifically:

[0020] Under different overcompensation coefficient values, the intersection frequency between different impedance matrix elements and grid impedance matrix elements is calculated;

[0021] According to different handover frequencies, different phase margins are calculated. The formula is: PM=180°-[arg(Z gqq (f c (k)))-arg(Z invqq (f c (k)))] = 90° + arg(Z invqq (f c (k)))

[0022] Among them, PM is the phase margin, Z invqq (f c (k)) is the crossover frequency of the impedance matrix elements, Z gqq (f c (k)) is the crossover frequency of the grid impedance matrix element;

[0023] According to different phase margins, the phase difference corresponding to the intersection of the amplitude-frequency characteristic curve of different impedance matrix elements and the grid impedance matrix element is determined. If the current phase difference meets the phase difference condition, the phase margin corresponding to the current phase difference is determined to obtain the current phase margin. The overcompensation coefficient is determined based on the phase compensation correspondence and the current phase margin.

[0024] In order to solve the same technical problem, an embodiment of the present invention further provides a grid-connected control device for a grid-connected inverter, comprising: a power outer loop control module and a current inner loop control module;

[0025] The power outer loop control module is used to control the inverter in the dq domain. The power outer loop deviation signal is passed through the power outer loop controller to generate a reference value for the current inner loop. The reference value of the current inner loop is compared with the actual output current to obtain an error signal. The control structure includes a PQ power control outer loop and a current control inner loop. The actual output current is obtained by phase tracking the common grid connection point voltage based on a phase-locked loop in a synchronous reference coordinate system.

[0026] The current inner loop control module is used to adjust the error signal through the current inner loop controller to generate a modulation signal, and then perform PWM pulse width modulation on the modulation signal to generate a disconnection signal for the inverter bridge power semiconductor device. Based on the disconnection signal, the on-off state of the grid-connected inverter and the power grid is controlled.

[0027] As a preferred solution, the power outer loop control module includes a phase tracking unit;

[0028] The phase tracking unit is used to project the common grid connection point voltage and grid connection current into the grid dq coordinate system based on the control structure of the grid-connected inverter in the dq domain to obtain active power and reactive power; the grid connection current includes the grid connection d-axis current and the grid connection p-axis current;

[0029] Perform small signal linearization processing on active power and reactive power to obtain active disturbance signal and reactive disturbance signal;

[0030] Based on the synchronous reference frame phase-locked loop, active disturbance signal, and reactive disturbance signal, a small signal model between the reference value of the current inner loop and the PQ power control outer loop is constructed. The formula is as follows:

[0031] in, The projection of the disturbance signal representing the d-axis reference value of the current inner loop in the dq coordinate system of the current inner loop controller is: G represents the projection of the disturbance signal of the q-axis reference value of the current inner loop in the dq coordinate system of the current inner loop controller. P (s) represents the active power controller of the outer power loop, G Q (s) represents the power outer loop reactive controller, ΔQ represents the reactive disturbance signal, ΔQ ref Represents the reference value of the reactive disturbance signal, ΔP represents the active disturbance signal, ΔP ref Indicates the reference value of the active disturbance signal;

[0032] Based on the synchronous reference coordinate system, the phase-locked loop will perform feedback compensation on the small signal model to obtain the compensated small signal model;

[0033] Based on the compensated small signal model, the q-axis voltage component of the common grid-connected point voltage is controlled to 0 through the PI controller, and the frequency and phase of the common grid-connected point voltage are latched and tracked to obtain the actual output current.

[0034] As a preferred solution, the phase tracking unit includes a feedback compensation subunit;

[0035] The feedback compensation subunit is used to generate a positive feedback loop at the modulation signal and the inner loop current reference disturbance value based on the synchronous reference coordinate system phase-locked loop;

[0036] A compensation feedback loop is introduced into the PQ power control outer loop to eliminate the positive feedback loop. Feedback compensation is performed on the small signal model based on an overcompensation coefficient to obtain a compensated small signal model. The overcompensation coefficient is selected based on the amplitude-frequency characteristic of the phase margin.

[0037] As a preferred solution, the phase tracking unit further includes an overcompensation coefficient selection subunit;

[0038] The overcompensation coefficient selection subunit is used to calculate the handover frequencies of different impedance matrix elements and grid impedance matrix elements under different overcompensation coefficient values;

[0039] According to different handover frequencies, different phase margins are calculated. The formula is: PM=180°-[arg(Z gqq (f c (k)))-arg(Z invqq (f c (k)))] = 90° + arg(Z invqq (f c (k)))

[0040] Among them, PM is the phase margin, Z invqq (f c (k)) is the crossover frequency of the impedance matrix elements, Z gqq (f c (k)) is the crossover frequency of the grid impedance matrix element;

[0041] According to different phase margins, the phase difference corresponding to the intersection of the amplitude-frequency characteristic curve of different impedance matrix elements and the grid impedance matrix element is determined. If the current phase difference meets the phase difference condition, the phase margin corresponding to the current phase difference is determined to obtain the current phase margin. The overcompensation coefficient is determined based on the phase compensation correspondence and the current phase margin.

[0042] By implementing the embodiment of the present invention, a PQ-controlled grid-connected inverter based on feedback loop over-compensation is implemented. Without changing the traditional phase-locked loop structure and parameters, the influence of the phase-locked loop is suppressed by introducing a feedback loop. Based on the instability mechanism, this method is more intuitive and simple, and is a highly portable stability improvement method. The compensation coefficient can be adjusted according to the strength of the power grid, thereby improving the stability of the grid-connected inverter and greatly enhancing the adaptability of the grid-connected inverter to the grid impedance.

[0043] Other features and advantages of the present invention will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] FIG1 is a flow chart of an embodiment of a grid-connected control method for a grid-connected inverter provided by the present invention;

[0045] FIG2 is a control structure diagram of a grid-connected inverter in the dq domain according to an embodiment of a grid-connected control method of a grid-connected inverter provided by the present invention;

[0046] FIG3 is a PLL control structure diagram of an embodiment of a grid-connected control method for a grid-connected inverter provided by the present invention;

[0047] FIG4 is a small signal control block diagram of a grid-connected inverter according to an embodiment of a grid-connected control method of the present invention;

[0048] FIG5 is a small signal control block diagram of a compensated grid-connected inverter according to an embodiment of a grid-connected control method of a grid-connected inverter provided by the present invention;

[0049] FIG6 is a phase compensation correspondence diagram of an embodiment of a grid-connected control method for a grid-connected inverter provided by the present invention;

[0050] FIG7 is a diagram showing the stability analysis results of an embodiment of a grid-connected control method for a grid-connected inverter provided by the present invention without adopting feedback loop compensation control; (a) L g =4mH when the stability analysis results and output current waveform; (b) L g =4.6mH when the stability analysis results and output current waveform;

[0051] FIG8 is a diagram showing the stability analysis results of an embodiment of a grid-connected control method for a grid-connected inverter provided by the present invention using feedback loop compensation control; (a) L g =11.5mH when the stability analysis results and output current waveform; (b) L g =12mH when the stability analysis results and output current waveform;

[0052] FIG9 is a schematic structural diagram of an embodiment of a grid-connected control device for a grid-connected inverter provided by the present invention. DETAILED DESCRIPTION

[0053] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0054] Example 1

[0055] Please refer to FIG1, which is a flow chart of a grid-connected control method for a grid-connected inverter according to an embodiment of the present invention. The grid-connected control method includes steps 101 to 102, each of which is as follows:

[0056] Step 101: Based on the control structure of the grid-connected inverter in the dq domain, the deviation signal of the power outer loop is passed through the power outer loop controller to generate a reference value of the current inner loop, and the reference value of the current inner loop is compared with the actual output current to obtain an error signal; wherein, the control structure includes a PQ power control outer loop and a current control inner loop; the actual output current is obtained by phase tracking the common grid connection point voltage based on a phase-locked loop based on a synchronous reference coordinate system.

[0057] It should be noted that the single-input single-output model of the grid-connected inverter considering the influence of the modulation signal is constructed to construct the control structure of the grid-connected inverter in the dq domain, as shown in Figure 2. dc is the DC side voltage; u inv is the output voltage of the inverter bridge; the inverter side filter inductor L1, filter capacitor C f and the grid-side filter inductor L2 form an LCL filter; R d is the damping resistor for suppressing the resonance peak of the LCL filter; i1 is the output current of the bridge arm, V c is the capacitor branch voltage, i2 is the grid current, where i 2d 、i 2q is the projection of i2 in dq coordinates; V PCC is the voltage of the point of common coupling (PCC), V g is the grid voltage, Z g is the grid impedance. P / Q, P ref / Q ref are GCI active / reactive power output value and reference value respectively; I 2dref , I 2qref are the reference values ​​of the GCI output current on the d-axis and q-axis, and the reference value of the current inner loop; VMd 、V Mq is the modulation signal in the dq coordinate system; G i (s) represents the current loop controller; G P (s), G Q (s) represent the outer power loop active controller and outer power loop reactive controller, respectively; PLL stands for phase locked loop (PLL); θ is the output phase angle of the PLL. The grid-connected inverter is a grid-connected inverter, denoted simply as GCI.

[0058] In this embodiment, the control structure of the grid-connected inverter in the dq domain adopts a dual closed-loop control structure, the outer loop is the PQ power control loop (PQ power control outer loop), and the inner loop is the current control loop (current control inner loop). The deviation signal of the power outer loop is transmitted through the power outer loop controller G P (s), G Q (s) Generates the reference value I of the inner current loop 2dref , I 2qref , and then compared with the actual output current, the error signal generated by the comparison is regulated by the current inner loop controller to generate the modulation signal V Md 、V Mq ; The modulated signal is then subjected to PWM pulse width modulation to generate a switching signal for the inverter bridge power semiconductor device, thereby completing the operation of the entire circuit.

[0059] Optionally, obtaining the actual output current by performing phase tracking on the common grid connection point voltage based on a synchronous reference coordinate system phase-locked loop specifically includes steps S11 to S14, each of which is specifically as follows:

[0060] Step S11: Based on the control structure of the grid-connected inverter in the dq domain, the common grid-connected point voltage and the grid-connected current are projected onto the grid dq coordinate system to obtain active power and reactive power; wherein the grid-connected current includes the grid-connected d-axis current and the grid-connected p-axis current;

[0061] In this embodiment, according to the control structure of the grid-connected inverter in the dq domain, the active power and reactive power output by the grid-connected inverter system can be obtained as follows:

[0062] In formula (1), the superscript g represents the projection of each variable in the dq coordinate system of the power grid.

[0063] Step S12: performing small signal linearization processing on the active power and reactive power to obtain an active disturbance signal and a reactive disturbance signal;

[0064] In this embodiment, after performing small signal linearization processing on equation (1), the following equation (2) is obtained. The parameter values ​​in equation (2) are taken as equation (3), where "Δ" represents the disturbance signal.

[0065] Step S13: Based on the synchronous reference frame phase-locked loop, the active disturbance signal, and the reactive disturbance signal, a small signal model between the reference value of the current inner loop and the PQ power control outer loop is constructed. The formula is as follows:

[0066] in, The projection of the disturbance signal representing the d-axis reference value of the current inner loop in the dq coordinate system of the current inner loop controller is: G represents the projection of the disturbance signal of the q-axis reference value of the current inner loop in the dq coordinate system of the current inner loop controller. P (s) represents the active power controller of the outer power loop, G Q (s) represents the power outer loop reactive controller, ΔQ represents the reactive disturbance signal, ΔQ ref Represents the reference value of the reactive disturbance signal, ΔP represents the active disturbance signal, ΔP ref Indicates the reference value of the active disturbance signal;

[0067] In this embodiment, the small signal model between the inner loop current reference value and the power loop can be obtained according to Figure 2, as shown in Equation (4), where the superscript c represents the projection of each variable in the controller dq coordinate system, that is, the d-axis and q-axis reference values. In order to synchronize the GCI with the power grid, a synchronous reference frame phase locked loop (SRF-PLL) structure is used to achieve phase tracking. The PLL control structure is shown in Figure 3, where V pccd 、V pccq Indicates V pcc The projection in the controller dq coordinate system, H pi (s) represents the controller of PLL, ω ref Indicates the rated angular frequency, ω o The PLL output angular frequency is obtained by H pi (s)PI controller controls the PCC point voltage V pccq The voltage component is 0 to achieve PLL locking of the voltage frequency and phase of the PCC point.

[0068] Its small signal mathematical model can be expressed as:

[0069] Among them, k ppll k is the proportional coefficient of the PI controller inside the phase-locked loop,ipll is the integral coefficient of the PI controller, V m0 Indicates the voltage amplitude at the PCC point.

[0070] Step S13: performing feedback compensation on the small signal model based on the synchronous reference coordinate system phase-locked loop to obtain a compensated small signal model;

[0071] Optionally, step S13 is specifically as follows: based on the synchronous reference coordinate system phase-locked loop, a positive feedback loop is generated at the modulation signal and the inner loop current reference disturbance value; a compensation feedback loop is introduced in the PQ power control outer loop to eliminate the positive feedback loop, and the small signal model is feedback compensated based on the over-compensation coefficient to obtain a compensated small signal model; wherein the over-compensation coefficient is obtained by selecting the amplitude-frequency characteristics based on the phase margin.

[0072] In this embodiment, due to the influence of the PLL small signal disturbance, the state variables of the grid-connected inverter have the following equation relationship between the system dq coordinate system and the controller dq coordinate system:

[0073] In the above formula, the variable x can represent variables such as voltage, current, and duty cycle.

[0074] Due to the existence of small disturbances in the system, the phase-locked loop will (modulation signal) and inner loop current reference value (inner loop current reference disturbance value) generates an additional feedback loop, and the small signal control block diagram of the grid-connected inverter is shown in Figure 4. g represents the equivalent inductance on the grid side, ω1 represents the grid angular frequency, G del (s) represents the equivalent delay link in the control process, which can be expressed as G del (s)=(1-0.75sT s ) / (1+0.75sT s ), the other link parameters have been introduced above and will not be repeated here. Since the phase-locked loop introduces a positive feedback loop in the q-axis, it is not conducive to the stable operation of the grid-connected inverter system. Therefore, one of the most easily conceivable control methods to suppress the negative effects of the positive feedback loop is to compensate for it in the control loop. The small signal control block diagram of the compensated grid-connected inverter is shown in Figure 5. By introducing (K■G gc (s)G PLL (s)V g Md0 )V pccq 、(K■G gc (s)G PLL (s)I g 2d0 )V pccqTwo feedback loop terms (compensation feedback loop) to eliminate the positive feedback loop G PLL (s)V g Md0 , G PLL (s)I g 2d0 In Figure 5, K is the modulation signal and the inner loop current reference value The compensation coefficient at G is the overcompensation coefficient; gc (s) represents the conversion coefficient of the voltage in the system dq coordinate system to the control dq coordinate system. The specific expression is:

[0075] Optionally, selecting the amplitude-frequency characteristic based on the phase margin to obtain the overcompensation coefficient specifically includes S131 to S133, and each step is specifically as follows:

[0076] S131: Calculating the handover frequencies of different impedance matrix elements and grid impedance matrix elements under different overcompensation coefficient values;

[0077] In this embodiment, according to the inverter small signal model in FIG4 , the equivalent output impedance Y of the grid-connected inverter in the dq coordinate system can be derived. inv (s) is:

[0078] in,

[0079] S132: Calculate different phase margins based on different handover frequencies. The formula is:

[0080] Among them, PM is the phase margin, Z invqq (f c (k)) is the crossover frequency of the impedance matrix elements, Z gqq (f c (k)) is the crossover frequency of the grid impedance matrix element;

[0081] In this embodiment, the grid-connected inverter system is obtained by formula (8) under different compensation system K values. invqq (s) and Z gqq (s) handover frequency f c (k), and thus the phase margin PM of the grid-connected inverter is obtained.

[0082] S133: Determine the phase difference corresponding to the intersection of the amplitude-frequency characteristic curve of different impedance matrix elements and the grid impedance matrix element based on different phase margins. If the current phase difference satisfies the phase difference condition, determine the phase margin corresponding to the current phase difference to obtain the current phase margin. Determine the overcompensation coefficient based on the phase compensation correspondence and the current phase margin.

[0083] In this embodiment, the value of the overcompensation coefficient K follows the impedance matrix element Z invqq (s) and the grid impedance matrix element Z gqq Whether the phase difference corresponding to the intersection of the amplitude-frequency characteristic curve of (s) exceeds 180°, and the phase difference condition is satisfied: 20log(|Z invqq (j2πf)|)=20log(|Z gqq (j2πf)|) (11)

[0084] Based on the constraint relationship between the phase margin PM and the overcompensation coefficient K, the corresponding relationship between the phase margin of the grid-connected inverter system and the overcompensation coefficient K, i.e., the phase compensation corresponding relationship, can be obtained when the grid impedance is 11.5mH, as shown in Figure 6. As shown in Figure 6, as the overcompensation coefficient K increases, the phase margin of the grid-connected inverter will first increase and then decrease. When the overcompensation coefficient K is selected to be 3, the grid-connected system has good stability.

[0085] The overcompensation coefficient K in the compensation feedback loop (K■G gc (s)G PLL (s)V g Md0 )V pccq 、(K■G gc (s)G PLL (s)I g 2d0 )V pccq In the present invention, by analyzing and changing the value of the compensation coefficient K, the phase margin of the grid-connected inverter system can be adjusted.

[0086] Step S14: Based on the compensated small signal model, the PI controller is used to control the q-axis voltage component of the common grid-connected point voltage to 0, and the frequency and phase of the common grid-connected point voltage are latched and tracked to obtain the actual output current.

[0087] In this embodiment, in order to synchronize the GCI with the power grid, a synchronous reference frame phase locked loop (SRF-PLL) structure is used to achieve phase tracking. The PLL control structure is shown in Figure 3, where V pccd 、V pccq Indicates Vpcc The projection in the controller dq coordinate system, H pi (s) represents the controller of PLL, ω ref Indicates the rated angular frequency, ω o The PLL output angular frequency is obtained by H pi (s)PI controller controls the PCC point voltage V pccq The voltage component is 0 to achieve PLL locking of the voltage frequency and phase of the PCC point.

[0088] Step 102: The error signal is regulated by a current inner loop controller to generate a modulation signal, and the modulation signal is subjected to PWM pulse width modulation to generate a switching signal for the inverter bridge power semiconductor device. The on / off state of the grid-connected inverter and the grid is controlled based on the switching signal.

[0089] In this embodiment, to verify the effectiveness of the aforementioned method for improving grid-connected inverter stability based on feedback loop overcompensation, stability analysis results and output current waveforms before and after feedback loop compensation control are performed on the grid-connected inverter are shown in Figures 7 and 8. Comparing the theoretical analysis and simulation waveforms shows that feedback loop compensation control can improve the grid impedance adaptability of the grid-connected inverter from 4mH to 11.5mH.

[0090] By implementing the embodiments of the present invention, the positive feedback link introduced by the phase-locked loop in the equivalent control block diagram of the PQ-controlled grid-connected converter is eliminated, the stability of the grid-connected converter is improved, and the overcompensation coefficient introduced in the control block diagram of the PQ-controlled grid-connected converter is rationally designed. Based on the instability mechanism of the PQ-controlled grid-connected converter, a corresponding feedback link is specifically introduced to suppress the negative impact of the phase-locked loop. Compared with the traditional stability improvement method based on impedance amplitude correction, the stability improvement method proposed in this article starts from the inherent mechanism of converter instability, is intuitive, and has strong portability. This control method greatly improves the adaptability of the grid-connected converter to weak power grids while reducing the difficulty of analysis and design.

[0091] Example 2

[0092] Accordingly, referring to Figure 9, Figure 9 is a schematic diagram of the structure of a second embodiment of a grid-connected control device for a grid-connected inverter provided by the present invention. As shown in Figure 9, the grid-connected control device for a grid-connected inverter includes a power outer loop control module 901 and a current inner loop control module 902;

[0093] The power outer loop control module 901 is used to generate a reference value for the current inner loop based on the control structure of the grid-connected inverter in the dq domain, pass the deviation signal of the power outer loop through the power outer loop controller, compare the reference value of the current inner loop with the actual output current, and obtain an error signal; wherein the control structure includes a PQ power control outer loop and a current control inner loop; the actual output current is obtained by phase tracking the common grid connection point voltage based on a synchronous reference coordinate system phase-locked loop;

[0094] The power outer loop control module 901 includes a phase tracking unit;

[0095] The phase tracking unit is used to project the common grid connection point voltage and grid connection current into the grid dq coordinate system based on the control structure of the grid-connected inverter in the dq domain to obtain active power and reactive power; the grid connection current includes the grid connection d-axis current and the grid connection p-axis current;

[0096] Perform small signal linearization processing on active power and reactive power to obtain active disturbance signal and reactive disturbance signal;

[0097] Based on the synchronous reference frame phase-locked loop, active disturbance signal, and reactive disturbance signal, a small signal model between the reference value of the current inner loop and the PQ power control outer loop is constructed. The formula is as follows:

[0098] in, The projection of the disturbance signal representing the d-axis reference value of the current inner loop in the dq coordinate system of the current inner loop controller is: G represents the projection of the disturbance signal of the q-axis reference value of the current inner loop in the dq coordinate system of the current inner loop controller. P (s) represents the active power controller of the outer power loop, G Q (s) represents the power outer loop reactive controller, ΔQ represents the reactive disturbance signal, ΔQ ref Represents the reference value of the reactive disturbance signal, ΔP represents the active disturbance signal, ΔP ref Indicates the reference value of the active disturbance signal;

[0099] Based on the synchronous reference coordinate system, the phase-locked loop will perform feedback compensation on the small signal model to obtain the compensated small signal model;

[0100] Based on the compensated small signal model, the q-axis voltage component of the common grid-connected point voltage is controlled to 0 through the PI controller, and the frequency and phase of the common grid-connected point voltage are latched and tracked to obtain the actual output current.

[0101] The phase tracking unit includes a feedback compensation subunit;

[0102] The feedback compensation subunit is used to generate a positive feedback loop at the modulation signal and the inner loop current reference disturbance value based on the synchronous reference coordinate system phase-locked loop;

[0103] A compensation feedback loop is introduced into the PQ power control outer loop to eliminate the positive feedback loop. Feedback compensation is performed on the small signal model based on an overcompensation coefficient to obtain a compensated small signal model. The overcompensation coefficient is selected based on the amplitude-frequency characteristic of the phase margin.

[0104] The phase tracking unit also includes an overcompensation coefficient selection subunit;

[0105] The overcompensation coefficient selection subunit is used to calculate the handover frequencies of different impedance matrix elements and grid impedance matrix elements under different overcompensation coefficient values;

[0106] According to different handover frequencies, different phase margins are calculated. The formula is: PM=180°-[arg(Z gqq (f c (k)))-arg(Z invqq (f c (k)))] = 90° + arg(Z invqq (f c (k)))

[0107] Among them, PM is the phase margin, Z invqq (f c (k)) is the crossover frequency of the impedance matrix elements, Z gqq (f c (k)) is the crossover frequency of the grid impedance matrix element;

[0108] According to different phase margins, the phase difference corresponding to the intersection of the amplitude-frequency characteristic curve of different impedance matrix elements and the grid impedance matrix element is determined. If the current phase difference meets the phase difference condition, the phase margin corresponding to the current phase difference is determined to obtain the current phase margin. The overcompensation coefficient is determined based on the phase compensation correspondence and the current phase margin.

[0109] The current inner loop control module 902 is used to adjust the error signal through the current inner loop controller to generate a modulation signal, and perform PWM pulse width modulation on the modulation signal to generate a switching signal for the inverter bridge power semiconductor device, and control the on and off status of the grid-connected inverter and the power grid based on the switching signal.

[0110] By implementing the embodiments of the present invention, a targeted feedback loop is introduced to address the instability mechanism of a PQ-controlled grid-connected converter, suppressing the negative effects of the phase-locked loop (PLL). Compared to traditional stability enhancement methods based on impedance amplitude correction, the proposed stability enhancement method addresses the inherent mechanism of converter instability, is intuitive, and highly portable. This control method significantly improves the grid-connected converter's adaptability to weak grid conditions while reducing analysis and design complexity.

[0111] The aforementioned grid-connected inverter grid-connected control device can implement the grid-connected inverter grid-connected control method of the aforementioned method embodiment. The optional options in the aforementioned method embodiment also apply to this embodiment and are not described in detail here. The remaining contents of the embodiments of this application can refer to the contents of the aforementioned method embodiment and are not further described in this embodiment.

[0112] The above specific embodiments further illustrate the objectives, technical solutions, and beneficial effects of the present invention. It should be understood that the above are merely specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. In particular, it should be noted that 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 for those skilled in the art.

Claims

1. A grid-connected control method for a grid-connected inverter, characterized in that: include: Based on the control structure of the grid-connected inverter in the dq domain, the deviation signal of the power outer loop is passed through the power outer loop controller to generate a reference value of the current inner loop, and the reference value of the current inner loop is compared with the actual output current to obtain an error signal; wherein the control structure includes a PQ power control outer loop and a current control inner loop; the actual output current is obtained by phase tracking the common grid-connected point voltage based on a synchronous reference coordinate system phase-locked loop; The error signal is adjusted through a current inner loop controller to generate a modulation signal, and the modulation signal is subjected to PWM pulse width modulation to generate a disconnection signal of the inverter bridge power semiconductor device, and the on-off state of the grid-connected inverter and the power grid is controlled based on the disconnection signal.

2. The grid-connected control method of the grid-connected inverter according to claim 1, characterized in that: The actual output current is obtained by performing phase tracking on the common grid connection point voltage based on a synchronous reference coordinate system phase-locked loop, specifically: Based on the control structure of the grid-connected inverter in the dq domain, the common grid-connected point voltage and the grid-connected current are projected on the grid dq coordinate system to obtain active power and reactive power; wherein the grid-connected current includes a grid-connected d-axis current and a grid-connected p-axis current; Performing small signal linearization processing on the active power and the reactive power to obtain an active disturbance signal and a reactive disturbance signal; According to the synchronous reference coordinate system phase-locked loop, the active disturbance signal and the reactive disturbance signal, a small signal model between the reference value of the current inner loop and the PQ power control outer loop is constructed, and the formula is as follows: in, The disturbance signal representing the d-axis reference value of the current inner loop is controlled by the current inner loop. The projection of the device in the dq coordinate system, The projection of the disturbance signal representing the q-axis reference value of the current inner loop in the dq coordinate system of the current inner loop controller, G P (s) represents the active power controller of the outer power loop, G Q (s) represents the power outer loop reactive controller, ΔQ represents the reactive disturbance signal, ΔQ ref represents the reference value of the reactive disturbance signal, ΔP represents the active disturbance signal, ΔP ref represents a reference value of the active disturbance signal; Based on the synchronous reference coordinate system phase-locked loop, feedback compensation is performed on the small signal model to obtain a compensated small signal model; Based on the compensated small signal model, the q-axis voltage component of the common grid-connected point voltage is controlled to be 0 through a PI controller, and the frequency and phase of the common grid-connected point voltage are latched and tracked to obtain the actual output current.

3. The grid-connected control method of the grid-connected inverter according to claim 2, characterized in that: The phase-locked loop based on the synchronous reference coordinate system performs feedback compensation on the small signal model to obtain a compensated small signal model, which is specifically: Based on the synchronous reference coordinate system phase-locked loop, a positive feedback loop is generated at the modulation signal and the inner loop current reference disturbance value; A compensation feedback loop is introduced into the PQ power control outer loop to eliminate the positive feedback loop, and feedback compensation is performed on the small signal model based on an overcompensation coefficient to obtain a compensated small signal model; wherein the overcompensation coefficient is obtained by selecting the amplitude-frequency characteristic based on the phase margin.

4. The grid-connected control method of the grid-connected inverter according to claim 3, characterized in that: The overcompensation coefficient is obtained by selecting the amplitude-frequency characteristic based on the phase margin, specifically: Under different over-compensation coefficient values, the handover frequencies of different impedance matrix elements and grid impedance matrix elements are calculated; According to the different handover frequencies, the different phase margins are calculated, and the formula is: PM=180°-[arg(Z gqq (f c (k)))-arg(Z invqq (f c (k)))] =90°+arg(Z invqq (f c (k))) Among them, PM is the phase margin, Z invqq (f c (k)) is the crossover frequency of the impedance matrix element, Z gqq (f c (k)) is the handover frequency of the grid impedance matrix element; According to the different phase angle margins, the phase differences corresponding to the intersection points of the amplitude-frequency characteristic curves of the different impedance matrix elements and the grid impedance matrix elements are determined. If the current phase difference meets the phase difference condition, the phase angle margin corresponding to the current phase difference is determined to obtain the current phase angle margin, and the over-compensation coefficient is determined according to the phase compensation correspondence and the current phase angle margin.

5. A grid-connected control device for a grid-connected inverter, characterized in that: include: Power outer loop control module and current inner loop control module; The power outer loop control module is used to generate a reference value of the current inner loop based on the control structure of the grid-connected inverter in the dq domain, and the deviation signal of the power outer loop is passed through the power outer loop controller to compare the reference value of the current inner loop with the actual output current to obtain an error signal; wherein the control structure includes a PQ power control outer loop and a current control inner loop; the actual output current is obtained by phase tracking the common grid-connected point voltage based on a synchronous reference coordinate system phase-locked loop; The current inner loop control module is used to adjust the error signal through the current inner loop controller to generate a modulation signal, and perform PWM pulse width modulation on the modulation signal to generate a switching signal of the inverter bridge power semiconductor device, and control the on and off state of the grid-connected inverter and the power grid based on the switching signal.

6. The grid-connected control device of the grid-connected inverter according to claim 5, characterized in that: The power outer loop control module includes a phase tracking unit; The phase tracking unit is used to project the common grid-connected point voltage and grid-connected current into the grid dq coordinate system based on the control structure of the grid-connected inverter in the dq domain to obtain active power and reactive power; wherein the grid-connected current includes a grid-connected d-axis current and a grid-connected p-axis current; Performing small signal linearization processing on the active power and the reactive power to obtain an active disturbance signal and a reactive disturbance signal; According to the synchronous reference coordinate system phase-locked loop, the active disturbance signal and the reactive disturbance signal, a small signal model between the reference value of the current inner loop and the PQ power control outer loop is constructed, and the formula is as follows: in, The projection of the disturbance signal representing the d-axis reference value of the current inner loop in the dq coordinate system of the current inner loop controller, The projection of the disturbance signal representing the q-axis reference value of the current inner loop in the dq coordinate system of the current inner loop controller, G P (s) represents the active power controller of the outer power loop, G Q (s) represents the power outer loop reactive controller, ΔQ represents the reactive disturbance signal, ΔQ ref represents the reference value of the reactive disturbance signal, ΔP represents the active disturbance signal, ΔP ref represents a reference value of the active disturbance signal; Based on the synchronous reference coordinate system phase-locked loop, feedback compensation is performed on the small signal model to obtain a compensated small signal model; Based on the compensated small signal model, the q-axis voltage component of the common grid-connected point voltage is controlled to be 0 through a PI controller, and the frequency and phase of the common grid-connected point voltage are latched and tracked to obtain the actual output current.

7. The grid-connected control device of the grid-connected inverter according to claim 6, characterized in that: The phase tracking unit includes a feedback compensation subunit; Wherein, the feedback compensation subunit is used to generate a positive feedback loop at the modulation signal and the inner loop current reference disturbance value based on the synchronous reference coordinate system phase-locked loop; A compensation feedback loop is introduced into the PQ power control outer loop to eliminate the positive feedback loop, and feedback compensation is performed on the small signal model based on an overcompensation coefficient to obtain a compensated small signal model, wherein the overcompensation coefficient is selected based on the amplitude-frequency characteristic of the phase margin.

8. The grid-connected control device of the grid-connected inverter according to claim 7, characterized in that: The phase tracking unit also includes an overcompensation coefficient selection subunit; Wherein, the overcompensation coefficient selection subunit is used to calculate the handover frequency of different impedance matrix elements and grid impedance matrix elements under different overcompensation coefficient values; According to the different handover frequencies, the different phase margins are calculated, and the formula is: PM=180°-[arg(Z gqq (f c (k)))-arg(Z invqq (f c (k)))] =90°+arg(Z invqq (f c (k))) Among them, PM is the phase margin, Z invqq (f c (k)) is the crossover frequency of the impedance matrix element, Z gqq (f c (k)) is the handover frequency of the grid impedance matrix element; According to the different phase angle margins, the phase differences corresponding to the intersection points of the amplitude-frequency characteristic curves of the different impedance matrix elements and the grid impedance matrix elements are determined. If the current phase difference meets the phase difference condition, the phase angle margin corresponding to the current phase difference is determined to obtain the current phase angle margin, and the over-compensation coefficient is determined according to the phase compensation correspondence and the current phase angle margin.

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