Method for suppressing harmonic voltage of grid-forming type photovoltaic inverter

Abstract

The invention provides a suppression method for harmonic voltage of a grid-forming type photovoltaic inverter, and belongs to the technical field of power electronics. Three-phase power grid voltage and three-phase power grid current are converted into a k-order harmonic synchronous rotating coordinate system, harmonic voltage and harmonic current are extracted through closed-loop filtering, and the harmonic voltage and the harmonic current respectively pass through a harmonic voltage control ring and a harmonic current control ring to generate harmonic control voltage; the harmonic control voltage is subjected to coordinate inverse transformation processing and then superposed on the fundamental frequency control voltage, and the harmonic control voltage and the fundamental frequency control voltage are modulated together to generate a trigger pulse signal and output to the network-forming converter; and the network-forming converter controls the harmonic voltage according to the network-side reference voltage information carried by the trigger pulse signal. According to the closed-loop filtering method adopted by the invention, harmonic voltage and harmonic current can be extracted more quickly and more accurately, so that harmonic waves in the voltage of the grid-connected point are effectively suppressed, the voltage quality of the grid-connected point is improved, and the stability of a power grid is effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics, and in particular to a method for suppressing harmonic voltage of a grid-connected photovoltaic inverter. Background Art

[0002] In recent years, new energy has developed rapidly, the scale of grid-connected power generation has gradually increased, and power equipment based on power electronics has gradually become an important part of the power grid. When connected to a strong power grid, power electronic equipment mainly adopts a grid-following control mode. However, in the case of a weak power grid, the grid-following control mode has problems such as low inertia and phase-locked instability. In order to ensure the stable and safe operation of the power grid under weak power grid conditions, power electronic converters need to realize or even replace the functions and roles of traditional synchronous motors. Grid-building converters have emerged and gradually become the mainstream development trend.

[0003] Grid-connected converters simulate the operating characteristics of synchronous generators to a certain extent, enabling new energy sources to have voltage construction and control capabilities. However, the access of a large number of power electronic equipment has brought harmonic pollution problems to the power grid. Especially in the case of weak power grids, the voltage quality of the grid connection point is more deeply affected by the control of power electronic equipment, resulting in reduced stability of the power grid.

[0004] In the past, harmonic suppression strategies mostly used low-pass filters to filter DC components. Low-pass filters are generally equivalent to first-order or higher-order inertia links, which will cause voltage and current phase lags. When the cutoff frequency of the low-pass filter is small, the system adjustment time will be longer and the dynamic performance will deteriorate. When the cutoff frequency is large, the filtering effect will be worse. Summary of the invention

[0005] In view of this, the present invention aims to propose a method for suppressing harmonic voltage of a grid-connected photovoltaic inverter. The closed-loop filtering method adopted in the method can extract harmonic voltage and harmonic current more quickly and accurately, so that the harmonics in the grid-connected point voltage can be effectively suppressed, the voltage quality of the grid-connected point can be improved, and the stability of the power grid can be effectively improved.

[0006] To achieve the above object, the technical solution of the present invention is implemented as follows: A method for suppressing harmonic voltage of a grid-connected photovoltaic inverter comprises the following steps:

[0007] Step 1, detecting and collecting three-phase voltage signals and three-phase current signals of the grid-connected point of the grid-connected converter;

[0008] Step 2, transforming the three-phase voltage signal and the three-phase current signal into a two-phase stationary coordinate system respectively, and then transforming the voltage signal and the current signal in the two-phase stationary coordinate system into a two-phase synchronous rotating coordinate system respectively, and obtaining the fundamental frequency voltage, fundamental frequency current, harmonic voltage, and harmonic current through a closed-loop filtering method;

[0009] Step 3, generating a base frequency control voltage by passing the base frequency voltage and the base frequency current through a base frequency control loop;

[0010] Step 4, generating a harmonic control voltage by passing the harmonic voltage and the harmonic current through a harmonic control loop;

[0011] Step 5, respectively, the base frequency control voltage and the harmonic control voltage are subjected to coordinate inverse transformation processing, so that the base frequency control voltage and the harmonic control voltage are respectively transformed into a two-phase stationary coordinate system, and then they are modulated by SVPWM together to generate a pulse signal and output it to the grid-forming converter;

[0012] Step 6: The grid-connected converter suppresses the harmonic voltage according to the reference voltage information carried by the pulse signal.

[0013] Furthermore, the system reference angles θ and kθ and the voltage set value E of the q-axis fundamental frequency voltage are output through a meshing algorithm.

[0014] Furthermore, step 2 includes the following steps:

[0015] Step 21: convert the three-phase voltage u abc and three-phase current i abc The three-phase coordinate system is transformed into the two-phase fundamental frequency synchronous rotating coordinate system with θ as the control angle, and u dq1_0 with i dq1_0 ;u dq1_0 is the dq-axis component of the grid voltage in the fundamental frequency synchronous coordinate system; i dq1_0 is the dq-axis component of the grid current in the fundamental frequency synchronous coordinate system;

[0016] Step 22, u dq1_0 with i dq1_0 Through closed-loop filtering, the base frequency voltage u is obtained. dq1 and fundamental frequency current i dq1 ;

[0017] Step 23: convert the three-phase voltage u abc and three-phase current i abc The three-phase coordinate system is transformed into the two-phase k-time synchronous rotating coordinate system with kθ as the control angle, and the voltage component u in the k-time synchronous rotating coordinate system is obtained. dqk_0 With the current component i dqk_0 ;

[0018] Step 24: dqk_0 with i dqk_0 Through closed-loop filtering, the kth harmonic voltage u is obtained. dqk and kth harmonic current i dqk .

[0019] Furthermore, in step 21, the coordinate transformation formula is as shown in formula (1):

[0020]

[0021] Among them, T abc-dq1 is the transformation matrix from the three-phase coordinate system to the two-phase fundamental frequency synchronous rotating coordinate system, as shown in formula (2).

[0022]

[0023] Furthermore, in step 23, the coordinate transformation formula is as shown in formula (3):

[0024]

[0025] Among them, T abc-dqk is the transformation matrix from the three-phase coordinate system to the two-phase k-times synchronous rotating coordinate system, as shown in equation (4).

[0026]

[0027] Where k is the harmonic order that needs to be suppressed.

[0028] Furthermore, in step 24, a closed-loop harmonic current extraction method is used to filter the harmonic components.

[0029] Further, step 3 includes: the given value in the base frequency q-axis voltage control loop and the q-axis base frequency voltage feedback value u q1 The difference is passed through the PI controller to obtain the given value i in the base frequency q-axis current control loop. q1_ref ,i q1_ref and the q-axis fundamental frequency current feedback value i q1 The difference between the two can be obtained by the PI controller to obtain the q-axis base frequency control voltage value E q1_ref ;

[0030] The base frequency d-axis voltage control loop given value and the d-axis base frequency voltage feedback value u d1 The difference is passed through the PI controller to obtain the given value i in the base frequency d-axis current control loop. d1_ref ,i d1_ref and the d-axis fundamental frequency current feedback value i d1 The difference between the two can be passed through the PI controller to obtain the d-axis base frequency control voltage value E d1_ref .

[0031] Further, step 4 includes: the given value in the q-axis harmonic voltage control loop and the q-axis k-th harmonic voltage feedback value u qk The harmonic voltage difference is passed through the PI controller to obtain the given value i in the q-axis harmonic current control loop. qk_ref ,i qk_refand q-axis harmonic current feedback value i qk The difference between the two can be obtained by the PI controller to obtain the q-axis harmonic control voltage value E qk_ref ;

[0032] The given value in the d-axis harmonic voltage control loop and the d-axis k-th harmonic voltage feedback value u dk The harmonic voltage difference is passed through the PI controller to obtain the given value i in the d-axis harmonic current control loop. dk_ref ,i dk_ref and d-axis harmonic current feedback value i dk The difference between the two can be obtained by the PI controller to obtain the d-axis harmonic control voltage value E dk_ref .

[0033] Furthermore, in step 5, the inverse transformation formula of the fundamental frequency coordinates is as shown in formula (5):

[0034]

[0035] Among them, T dq1-αβ is the transformation matrix from the fundamental frequency synchronous rotating coordinate system to the two-phase stationary coordinate system, as shown in equation (6),

[0036]

[0037] The harmonic standard inverse transformation formula is shown in formula (7):

[0038]

[0039] Among them, T dqk-αβ is the transformation matrix from the k-times synchronous rotating coordinate system to the two-phase stationary coordinate system, as shown in equation (8),

[0040]

[0041] Compared with the prior art, the method for suppressing harmonic voltage of a grid-connected photovoltaic inverter described in the present invention has the following advantages:

[0042] (1) The method of the present invention solves the technical problem that, in the case of grid connection, especially when a large number of power electronic equipment are connected to the grid, the voltage quality at the grid connection point is affected by the control of the power electronic equipment, resulting in high harmonic content and reduced stability of the grid voltage;

[0043] (2) The present invention extracts harmonic voltage and harmonic current by closed-loop filtering, which has a small hysteresis angle and high extraction accuracy. It can speed up the adjustment speed of the system while ensuring the accuracy of the extracted harmonic components, thereby ensuring the dynamic performance of the system.

[0044] (3) The method described in the present invention can effectively suppress harmonic voltage, enhance the AC voltage control capability of the grid-type converter, and improve the voltage quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0046] Figure 1 This is the principle diagram of harmonic voltage suppression for grid-type photovoltaic inverters;

[0047] Figure 2 This is the control block diagram of the closed-loop filtering method;

[0048] Figure 3 Bode diagram of closed-loop filtering method and first-order low-pass filter;

[0049] Figure 4 It is the control block diagram of the network algorithm;

[0050] Figure 5 It is a base frequency voltage control loop and a base frequency current control loop;

[0051] Figure 6 It is the harmonic voltage control loop and the harmonic current control loop. DETAILED DESCRIPTION

[0052] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0053] The present invention will be described in detail below with reference to the accompanying drawings and in conjunction with embodiments.

[0054] The present invention is a method for suppressing harmonic voltage of a grid-connected photovoltaic inverter, comprising the following steps:

[0055] Step 1, detecting and collecting three-phase voltage signals and three-phase current signals of the grid-connected point of the grid-connected converter;

[0056] Step 2, transforming the three-phase voltage signal and the three-phase current signal into a two-phase stationary coordinate system respectively, and then transforming the voltage signal and the current signal in the two-phase stationary coordinate system into a two-phase synchronous rotating coordinate system respectively, and obtaining the fundamental frequency voltage, fundamental frequency current, harmonic voltage, and harmonic current through a closed-loop filtering method;

[0057] Step 3, generating a base frequency control voltage by passing the base frequency voltage and the base frequency current through a base frequency control loop;

[0058] Step 4, generating a harmonic control voltage by passing the harmonic voltage and the harmonic current through a harmonic control loop;

[0059] Step 5, respectively, the base frequency control voltage and the harmonic control voltage are subjected to coordinate inverse transformation processing, so that the base frequency control voltage and the harmonic control voltage are respectively transformed into a two-phase stationary coordinate system, and then they are jointly subjected to SVPWM modulation (space vector pulse width modulation) to generate a pulse signal and output it to the grid-type converter;

[0060] Step 6: The grid-connected converter suppresses the harmonic voltage according to the reference voltage information carried by the pulse signal.

[0061] The present invention includes transforming the three-phase grid voltage and the three-phase grid current into the kth harmonic synchronous rotating coordinate system, extracting the harmonic voltage and harmonic current by a closed-loop filtering method, and respectively passing the harmonic voltage and the harmonic current through the harmonic voltage control loop and the harmonic current control loop to generate a harmonic control voltage; the harmonic control voltage is subjected to coordinate inverse transformation and then superimposed on the baseband control voltage, and jointly modulated to generate a trigger pulse signal and output it to a grid-type converter; the grid-type converter controls the harmonic voltage according to the grid-side reference voltage information carried by the trigger pulse signal. The present invention effectively solves the technical problem that the poor voltage quality at the grid connection point leads to reduced grid stability. The method described in the present invention can effectively achieve harmonic voltage suppression, improve the AC voltage control capability of the grid-type converter, and improve the voltage quality. The implementation of the method described in the present invention is as follows.

[0062] like Figure 1 As shown, the three-phase voltage u abc and three-phase current i abc The three-phase coordinate system is transformed into the two-phase fundamental frequency synchronous rotating coordinate system with θ as the control angle, and u dq1_0 with i dq1_0 ;u dq1_0 is the dq-axis component of the grid voltage in the fundamental frequency synchronous coordinate system; i dq1_0 is the dq-axis component of the grid current in the fundamental frequency synchronous coordinate system; as shown in formula (1),

[0063]

[0064] Among them, T abc-dq1 is the transformation matrix from the three-phase coordinate system to the two-phase fundamental frequency synchronous rotating coordinate system, as shown in formula (2).

[0065]

[0066] After the three-phase coordinate system is transformed into the two-phase fundamental frequency synchronous rotating coordinate system with θ as the control angle, the fundamental frequency components in the three-phase voltage and three-phase current will be converted into DC components, and the harmonic components will appear as AC components. dq1_0 with u dq1_0 The base frequency voltage u can be obtained by filtering out the AC component in dq1 With the fundamental frequency current i dq1.

[0067] The three-phase voltage u abc and three-phase current i abc The three-phase coordinate system is transformed into the two-phase k-time synchronous rotating coordinate system with kθ as the control angle, and the voltage component u in the k-time synchronous rotating coordinate system is obtained. dqk_0 With the current component i dqk_0 ; As shown in formula (3),

[0068]

[0069] Among them, T abc-dqk is the transformation matrix from the three-phase coordinate system to the two-phase k-times synchronous rotating coordinate system, as shown in equation (4).

[0070]

[0071] Where k is the harmonic number that needs to be suppressed. Taking the 5th and 7th harmonics with high harmonic content in the power grid as an example, if you want to suppress the 5th and 7th harmonics, you need to reduce the three-phase voltage u abc and three-phase current i abc Transform to 5th or 7th order synchronous rotating coordinate system for subsequent control.

[0072] In the k-th synchronous rotating coordinate system, the k-th harmonic component will be converted into DC quantity, the fundamental frequency and other harmonics will be AC ​​quantity, and u dqk_0 with i dqk_0 The kth harmonic voltage u can be obtained by filtering out the AC component in dqk and kth harmonic current i dqk .

[0073] In the above links, low-pass filters are often used in previous control strategies to filter the DC components. Since low-pass filters are generally equivalent to inertial links of the first order or higher, voltage and current phases lag, the system adjustment time is long, and the dynamic performance is poor. In order to ensure the accuracy of extracting harmonic components while speeding up the adjustment speed of the system and ensuring dynamic performance, the present invention adopts a closed-loop harmonic current extraction method, namely a closed-loop filtering method, such as Figure 2 shown.

[0074] Figure 2 The input x contains both DC and AC components, and x ac contains only AC components, subtract x from x ac The DC component x can be obtained h , the order of the low-pass filter used in this method is first order.

[0075] Figure 3 K is the proportional coefficient of the PI controller in the closed-loop filtering methodp =2, integral coefficient K i = 0.1, the cut-off frequency of the low-pass filter is set to 1Hz, 5Hz, and 50Hz respectively, and the Bode diagram of the closed-loop filtering method and the first-order low-pass filter. In the figure, the curves shown in the closed-loop filtering 1 and low-pass filtering 1, the closed-loop filtering 2 and low-pass filtering 2, and the closed-loop filtering 3 and low-pass filtering 3 correspond to the situations at 1Hz, 5Hz, and 50Hz respectively.

[0076] Table 1 Figure 3 The closed-loop filtering method and the cutoff frequency and lag phase of the low-pass filter are shown in FIG. c is the frequency when the amplitude of the closed-loop filtering method and the low-pass filter drops to -3dB respectively, φ c and φ n f c and f n The lag phase of the point.

[0077] Table 1 Closed-loop filtering method and cut-off frequency and lag phase of low-pass filter

[0078]

[0079] Depend on Figure 3 From the data in Table 1, it can be seen that when the amplitude drops to -3dB, the f of the closed-loop filtering method c Smaller than the f of the low-pass filter c , while at each f n The amplitude of the point closed-loop filtering method can be reduced to -3.5dB, which can more effectively reduce the amount of AC.

[0080] From the above, it can be seen that compared with directly using a first-order low-pass filter for filtering, the closed-loop filtering method has a smaller cutoff frequency and a smaller lag phase, can more effectively reduce the AC content in the input quantity, and effectively avoids the problem of long system adjustment time caused by the low-pass filter, thereby ensuring the dynamic performance of the system.

[0081] Networking algorithms such as Figure 4 Through the active-frequency droop link and the simulated rotor motion equation, the active control loop will output the system reference angle θ, and the reactive control loop will output the voltage set value E of the q-axis fundamental frequency voltage.

[0082] The fundamental frequency voltage and current loops are shown in Figure 5. Among them, the given value in the q-axis fundamental frequency voltage control loop is the output E of the reactive power control loop in the network-building algorithm, and E is related to the q-axis fundamental frequency voltage feedback value u q1 The difference is passed through the PI controller to obtain the given value i in the q-axis base frequency current control loop. q1_ref ,i q1_ref and the q-axis fundamental frequency current feedback value i q1The difference between the two can be obtained by the PI controller to obtain the q-axis base frequency control voltage value E q1_ref ;

[0083] The given value of the d-axis base frequency voltage control loop is set to 0, which is consistent with the d-axis base frequency voltage feedback value u d1 The difference is passed through the PI controller to obtain the given value i in the d-axis base frequency current control loop. d1_ref ,i d1_ref and the d-axis fundamental frequency current feedback value i d1 The difference between the two can be obtained by the PI controller to obtain the d-axis base frequency control voltage value E d1_ref .

[0084] Harmonic voltage and current loops Figure 6 As shown in the figure, the given values ​​of the d-axis and q-axis in the harmonic voltage control loop are both 0.

[0085] The feedback value is the kth harmonic voltage u qk , the given value in the q-axis harmonic voltage control loop and the q-axis kth harmonic voltage feedback value u qk The harmonic voltage difference is passed through the PI controller to obtain the given value i in the q-axis harmonic current control loop. qk_ref ,i qk_ref and q-axis harmonic current feedback value i qk The difference between the two can be obtained by the PI controller to obtain the q-axis harmonic control voltage value E qk_ref ;

[0086] The feedback value is the kth harmonic voltage u dk , the given value in the d-axis harmonic voltage control loop and the d-axis k-th harmonic voltage feedback value u dk The harmonic voltage difference is passed through the PI controller to obtain the given value i in the d-axis harmonic current control loop. dk_ref ,i dk_ref and d-axis harmonic current feedback value i dk The difference between the two can be obtained by the PI controller to obtain the d-axis harmonic control voltage value E dk_ref .

[0087] E dq1_ref 、E dqk_ref The coordinates are inversely transformed into a two-phase stationary coordinate system, and then a pulse signal is generated through SVPWM modulation to control the grid-type converter.

[0088] The inverse transformation formula of the fundamental frequency coordinates is shown in formula (5):

[0089]

[0090] Among them, T dq1-αβ is the transformation matrix from the fundamental frequency synchronous rotating coordinate system to the two-phase stationary coordinate system, as shown in equation (6),

[0091]

[0092] The harmonic standard inverse transformation formula is shown in formula (7):

[0093]

[0094] Among them, T dqk-αβ is the transformation matrix from the k-times synchronous rotating coordinate system to the two-phase stationary coordinate system, as shown in equation (8),

[0095]

[0096] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for suppressing harmonic voltage of a grid-connected photovoltaic inverter, characterized in that: The following steps are involved: Step 1, detecting and collecting three-phase voltage signals and three-phase current signals of the grid-connected point of the grid-connected converter; Step 2, transforming the three-phase voltage signal and the three-phase current signal into a two-phase stationary coordinate system respectively, and then transforming the voltage signal and the current signal in the two-phase stationary coordinate system into a two-phase synchronous rotating coordinate system respectively, and obtaining the fundamental frequency voltage, fundamental frequency current, harmonic voltage, and harmonic current through a closed-loop filtering method; Step 3, generating a base frequency control voltage by passing the base frequency voltage and the base frequency current through a base frequency control loop; Step 4, generating a harmonic control voltage by passing the harmonic voltage and the harmonic current through a harmonic control loop; Step 5, respectively, the base frequency control voltage and the harmonic control voltage are subjected to coordinate inverse transformation processing, so that the base frequency control voltage and the harmonic control voltage are respectively transformed into a two-phase stationary coordinate system, and then they are modulated by SVPWM together to generate a pulse signal and output it to the grid-forming converter; Step 6: The grid-connected converter suppresses the harmonic voltage according to the reference voltage information carried by the pulse signal.

2. The method for suppressing harmonic voltage of a grid-connected photovoltaic inverter according to claim 1, characterized in that: The system reference angles θ and kθ and the voltage reference value E of the q-axis fundamental frequency voltage are output through a meshing algorithm.

3. The method for suppressing harmonic voltage of a grid-connected photovoltaic inverter according to claim 2, characterized in that: Step 2 includes the following steps: Step 21: convert the three-phase voltage u abc and three-phase current i abc The three-phase coordinate system is transformed into the two-phase fundamental frequency synchronous rotating coordinate system with θ as the control angle, and u dq1_0 with i dq1_0 ,u dq1_0 is the dq-axis component of the grid voltage in the fundamental frequency synchronous coordinate system; i dq1_0 is the dq-axis component of the grid current in the fundamental frequency synchronous coordinate system; Step 22, u dq1_0 with i dq1_0 Through closed-loop filtering, the base frequency voltage u is obtained. dq1 and fundamental frequency current i dq1 ; Step 23: convert the three-phase voltage u abc and three-phase current i abc The three-phase coordinate system is transformed into the two-phase k-time synchronous rotating coordinate system with kθ as the control angle, and the voltage component u in the k-time synchronous rotating coordinate system is obtained. dqk_0 With the current component i dqk_0 ; Step 24: dqk_0 with i dqk_0 Through closed-loop filtering, the kth harmonic voltage u is obtained. dqk and kth harmonic current i dqk .

4. The method for suppressing harmonic voltage of a grid-connected photovoltaic inverter according to claim 3, characterized in that: In step 21, the coordinate transformation formula is as shown in formula (1): Among them, T abc-dq1 is the transformation matrix from the three-phase coordinate system to the two-phase fundamental frequency synchronous rotating coordinate system, as shown in formula (2).

5. The method for suppressing harmonic voltage of a grid-connected photovoltaic inverter according to claim 3, characterized in that: In step 23, the coordinate transformation formula is shown in formula (3): Among them, T abc-dqk is the transformation matrix from the three-phase coordinate system to the two-phase k-times synchronous rotating coordinate system, as shown in equation (4). Where k is the harmonic number that needs to be suppressed.

6. The method for suppressing harmonic voltage of a grid-connected photovoltaic inverter according to claim 3, characterized in that: In step 24, a closed-loop harmonic current extraction method is used to filter the harmonic components.

7. The method for suppressing harmonic voltage of a grid-connected photovoltaic inverter according to claim 2, characterized in that: Step 3 includes: the given value in the base frequency q-axis voltage control loop and the q-axis base frequency voltage feedback value u q1 The difference is passed through the PI controller to obtain the given value i in the base frequency q-axis current control loop. q1_ref ,i q1_ref and the q-axis fundamental frequency current feedback value i q1 The difference between the two can be obtained by the PI controller to obtain the q-axis base frequency control voltage value E q1_ref ; The base frequency d-axis voltage control loop given value and the d-axis base frequency voltage feedback value u d1 The difference is passed through the PI controller to obtain the given value i in the base frequency d-axis current control loop. d1_ref ,i d1_ref and the d-axis fundamental frequency current feedback value i d1 The difference between the two can be passed through the PI controller to obtain the d-axis base frequency control voltage value E d1_ref .

8. The method for suppressing harmonic voltage of a grid-connected photovoltaic inverter according to claim 2, characterized in that: Step 4 includes: the given value in the q-axis harmonic voltage control loop is compared with the q-axis k-th harmonic voltage feedback value u qk The harmonic voltage difference is passed through the PI controller to obtain the given value i in the q-axis harmonic current control loop. qk_ref ,i qk_ref and q-axis harmonic current feedback value i qk The difference between the two can be obtained by the PI controller to obtain the q-axis harmonic control voltage value E qk_ref ; The given value in the d-axis harmonic voltage control loop and the d-axis k-th harmonic voltage feedback value u dk The harmonic voltage difference is passed through the PI controller to obtain the given value i in the d-axis harmonic current control loop. dk_ref ,i dk_ref and d-axis harmonic current feedback value i dk The difference between the two can be obtained by the PI controller to obtain the d-axis harmonic control voltage value E dk_ref .

9. The method for suppressing harmonic voltage of a grid-connected photovoltaic inverter according to claim 1, characterized in that: In step 5, the inverse transformation formula of the fundamental frequency coordinates is shown in formula (5): Among them, T dq1-αβ is the transformation matrix from the fundamental frequency synchronous rotating coordinate system to the two-phase stationary coordinate system, as shown in equation (6), The harmonic standard inverse transformation formula is shown in formula (7): Among them, T dqk-αβ is the transformation matrix from the k-times synchronous rotating coordinate system to the two-phase stationary coordinate system, as shown in equation (8),

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