Dual-mode fusion control method for grid-connected inverter based on passivity

Abstract

The present invention discloses a dual-mode fusion control method for a grid-connected inverter based on passivity, and belongs to the field of electrical engineering. The present invention includes a traditional single-mode operation grid-connected inverter, which is not robust enough under weak grid conditions with high impedance and low inertia, and faces nonlinear application scenarios such as grid structure / parameter changes and inverter plug-and-play, which will cause broadband oscillations in the grid-connected voltage and current, and even cause system instability. The present invention proposes to model the grid-connected inverter based on a port-controlled Hamiltonian model, and obtain a weighted fusion control based on the passive feedback control law of the grid-connected inverter operating in the following grid and the grid-building mode. Based on the control of the dual-mode fusion control method, the grid-connected inverter can be guaranteed to operate stably under harsh grid conditions with large impedance fluctuations, and at the same time have good robustness in nonlinear working scenarios.

Description

Technical Field

[0001] The present invention relates to the field of electrical engineering, and in particular to a dual-mode fusion control method for a grid-connected inverter based on passivity. Background Art

[0002] As the penetration rate of power electronic equipment in the power grid continues to increase, the grid impedance will produce uncertainty, randomness and large-scale fluctuations, which will bring huge potential threats to the safety and stability of the power grid. The low inertia and high impedance characteristics of the current power grid have caused the grid-following (GFL) converter, which used to operate reliably, to have obvious wide-band oscillations and insufficient active support for the power grid, which seriously endangers the stable operation of the new energy power generation system. For example:

[0003] 1) Chinese patent document CN 107093909A was authorized and disclosed on August 25, 2017, "A virtual admittance method for improving the stability of grid-connected inverters under weak power grids". The present invention provides a virtual admittance method for improving the stability of grid-connected inverters under weak power grids. When the grid impedance is greater than the first grid impedance value, the grid-connected system will become unstable. The invention proposes a method of connecting a virtual admittance in the control loop, and the virtual admittance will be adaptively adjusted according to the grid impedance, so as to achieve the purpose of ensuring the stability of the grid-connected inverter. However, the accurate identification of the grid impedance is a very complex and technically difficult operation, and it is difficult to achieve real-time and accurate measurement of the grid impedance.

[0004] 2) Chinese patent document CN 114884125A authorized for publication on August 9, 2022, "A high stability control method for an LCL-type grid-connected inverter system under a weak power grid", the present invention discloses a high stability control method for an LCL-type grid-connected inverter system under a weak power grid, which is used to solve the problem that conventional inverters are prone to instability in a weak power grid environment. Aiming at the influence of increased grid impedance and small signal disturbance on the phase-locked loop and LCL filter in the grid-connected inverter system under a weak power grid, a dual-grid current loop feedback active damping strategy of second-order generalized integration is adopted, an adaptive second-order resonant integrator active damping feedback function is constructed, and a proportional resonance function is used as a current loop controller. The control system is virtual-impeded, and a voltage feedforward controller is designed; a phase-locked loop based on a dual second-order adaptive filter is designed. Although the scheme corrects the instability caused by negative impedance under weak power grid conditions through virtual impedance, it still cannot get rid of the coupling of the phase-locked loop with the grid impedance under weak power grid conditions, so there is still a risk of instability.

[0005] 3) Chinese patent document CN 106253646A, authorized and announced on December 21, 2016, "Grid-connected inverter LCL filter parameter design method for improving weak grid adaptability", the present invention discloses a grid-connected inverter LCL filter parameter design method for improving weak grid adaptability, firstly calculates the delay time of the control process, obtains the open-loop transfer function according to the current control loop, and obtains the system resonant frequency stability domain based on the generalized Nyquist stability criterion. Compared with the original parameter-heavy iterative trial-and-error method, this scheme is simple to calculate, but it is still limited to the fluctuation range of the grid impedance and cannot be applied to the current application scenarios where the grid impedance fluctuates in a larger range.

[0006] When faced with the new grid environment with the massive access of new energy generation and the massive use of power electronic equipment, and considering the nonlinearity of the inverter itself, especially when it is used in plug-and-play, random switching grid structure / parameter changes and nonlinear application scenarios, the adaptability of traditional single-mode grid-connected inverters is often insufficient, which may cause stability problems such as broadband oscillation. For this reason, some scholars have proposed dual-mode fusion control based on grid-following / grid-building. For example:

[0007] 1) The Chinese patent document CN 110021959A authorized and announced on August 28, 2020, "A dual-mode control method for a grid-connected inverter based on short-circuit ratio under weak power grids", discloses a dual-mode control method for a grid-connected inverter based on short-circuit ratio under weak power grids, mainly for the problem that the multi-inverter system in full current source mode under weak power grid conditions obtains the weak grid state through impedance identification, which is not intuitive and clear enough, and proposes a method to characterize the state of the weak power grid based on the short-circuit ratio to achieve the switching of the following and building modes, in order to ensure the stability of the inverter system. However, both the identification of impedance and the acquisition of the weak grid state of the power grid based on the short-circuit ratio are based on precise detection and calculation, and real-time and precise measurement and mode switching are not easy.

[0008] 2) Chinese patent document CN 114865711A, authorized and announced on August 5, 2022, "A dual-mode switching control method and system for a new energy grid-connected inverter", discloses a dual-mode switching control method and system for a new energy grid-connected inverter, and proposes that the inverter operates in a self-synchronous control mode under stable operating conditions, and switches to a constant AC voltage control mode when the grid voltage drops, providing reactive compensation for the system, in order to achieve good low-voltage ride-through capability. Automatic switching is achieved by the voltage drop depth of the grid connection point, shortening the switching time. However, this method only switches the mode for the working condition of voltage drop, and does not provide reasonable detection indicators and switching for other working conditions. Therefore, the dual-mode switching of the present invention has certain limitations.

[0009] 3) The Chinese patent document CN 115882514B authorized and announced on May 9, 2023, "A cluster fusion control method for an integrated converter cluster of a new energy power system with a grid and a network", the present invention provides a cluster aggregation control method for an integrated converter cluster of a new energy power system with a grid and a network, including: obtaining the output power response curve of the synchronous generator according to the power characteristics of the synchronous generator; equivalence of the output power of the power generation unit of the distributed energy system, so that its output power response characteristics are equivalent to a virtual synchronous generator, and obtaining its swing equation; constructing a power curve and allocating power instructions; constructing a local unified control method for grid and network construction; designing the power instruction allocation coefficient within the integrated control method for grid and network construction. The present invention uses this method to aggregate all power generation units and, based on the traditional grid construction and grid control strategies, constructs an integrated control method that combines the advantages of both. This method is a multi-mode optimization configuration scheme for a station, but the technology is relatively complex, the model structure is difficult, and the implementation is difficult.

[0010] The passivity theory is to design the control link of the controller directly based on the energy dissipation characteristics of the system. This method has an important characteristic: the new system obtained by connecting multiple passive subsystems in parallel still satisfies the passivity, and modeling based on this theory can be applied to complex high-order systems. Therefore, the controller designed based on the passivity theory provides an effective solution for the grid-connected control technology of high-penetration renewable energy power generation systems. For example:

[0011] 1) "M. Li, H. Geng and X. Zhang, Robust Passivity-Based Control for Grid-Forming Converter, CIEEC" ("Robust Control of Passivity-Based Grid-Forming Converter") published in the 2023 IEEE Sixth International Electrical and Energy Conference (CIEEC) This document proposes the problem of nonlinear control delay of passive-based controllers in practical applications, which may lead to the risk of insufficient stability margin or even instability in grid-connected converters. This paper adopts the D-division method to introduce nonlinear control delay, and provides a robust passive control method for grid-forming converters. However, the paper is aimed at the passive control law design scheme for AC output voltage control, which reflects the external characteristics of the voltage source. The present invention is related to the external characteristics of the current source characteristics. This document is aimed at the controller design and analysis of grid-forming grid-connected inverters under weak grid conditions. However, in the face of the strong volatility and randomness of today's power grids, there are still defects in dynamics and insufficient response speed.

[0012] In summary, the prior art has the following problems:

[0013] 1) Grid-connected inverters based on the traditional grid-following mode cannot operate stably under the weak grid conditions of high grid impedance and low inertia caused by large-scale access to new energy. Especially in the renewable energy power generation system, as the line impedance increases significantly due to long-distance transmission, the grid strength gradually decreases, resulting in the inverter based on the traditional grid-following mode of the phase-locked loop causing broadband oscillations in the voltage and current injected into the grid.

[0014] 2) Existing literature studies the hybrid control of following the grid / building the grid switching type. According to the grid impedance identification link, the information of the grid strength is obtained to realize the switching of the operation mode under weak grid and strong grid conditions. However, the impedance identification technology is not yet mature and there are certain technical difficulties in engineering applications.

[0015] 3) Existing hybrid control based on traditional linear architecture, such as proportional-integral controller, cannot accurately respond to nonlinear working conditions such as plug-and-play of inverters, random and uncertain fluctuations of grid line impedance, and fluctuations of line and device parameters under today's high impedance and low inertia weak grid conditions, and the dynamic response capability is insufficient. In particular, it is very easy to bring unpredictable broadband oscillations and even the risk of instability. Summary of the invention

[0016] The technical problem to be solved by the present invention is to overcome the limitation of the original dual-mode switching technical solution that requires real-time grid impedance perception. Nowadays, due to the interference of various nonlinear factors such as the weak grid characteristics of high impedance and low inertia and the plug-and-play and switching-on-at-anytime of the inverter, a control method that can widely adapt to large-scale impedance fluctuations and cope with complex nonlinear working conditions is urgently needed. Therefore, the present invention proposes a dual-mode fusion control method for grid-connected inverters based on passivity.

[0017] The technical solution of the present invention is as follows:

[0018] A dual-mode fusion control method for a grid-connected inverter based on passivity, wherein the topology involved in the grid-connected inverter includes a DC power supply, a main inverter, an LCL filter, a line equivalent inductance and a three-phase power grid connected in series in sequence, wherein the LCL filter includes a bridge arm side inductance, a bridge arm side inductance equivalent resistance, a filter capacitor and a grid side inductance; the dual-mode fusion control refers to the fusion control of the grid-connected inverter with the grid type control mode and the grid-connected inverter with the grid type control mode, and the fusion control method includes establishing a passive feedback control law of the grid-connected inverter with the grid type control mode and a passive feedback control law of the grid-connected inverter with the grid type control mode, and then introducing a weighting coefficient n to weight the two passive feedback control laws of the two control modes to obtain a dual-mode fusion passive feedback control law u, and the specific steps are as follows:

[0019] Step 1: Sample the inductor output current i on the bridge arm sidea ,i b ,i c , and the d-axis component i of the inductor output current on the bridge arm side is obtained by transforming the three-phase stationary coordinate system to the two-phase rotating coordinate system d and the q-axis component i of the inductor output current on the bridge arm side q ; Sample filter capacitor output voltage u Ca ,u Cb ,u Cc , and the d-axis component u of the filter capacitor output voltage is obtained by transforming the three-phase stationary coordinate system to the two-phase rotating coordinate system Cd And the q-axis component u of the filter capacitor output voltage Cq ; Sample three-phase grid current i ga ,i gb ,i gc , and the three-phase grid current d-axis component i is obtained by transforming the three-phase stationary coordinate system to the two-phase rotating coordinate system gd and the q-axis component i of the three-phase grid current gq ;

[0020] Step 2, obtaining the phase angle required for the grid-connected inverter to synchronize with the grid, and recording it as the synchronization phase angle θ, θ=∫ωdt, where t is the operation time of the grid-connected inverter, and ω is the reference angular frequency;

[0021] The reference angular frequency ω is obtained by active power synchronization loop control, and its expression is: ω=ω0+K P (P ref -P), where ω0 is the rated angular frequency, K p is the active power synchronization loop control proportional coefficient, P ref is the rated active power command value output by the grid-connected inverter, and P is the rated active power output by the grid-connected inverter;

[0022] Step 3: Establish the passive feedback control law of the grid-following control mode of the grid-connected inverter and record it as the grid-following passive feedback control law u1, u1 = [u d1 u q1 ], where u d1 is the d-axis modulation voltage of the grid-following passive feedback control law u1, u q1 is the q-axis modulation voltage of the grid-following passive feedback control law u1;

[0023] Step 4: Establish the passive feedback control law of the grid-connected inverter grid-connected control mode, and record it as the grid-connected passive feedback control law u2, u2 = [u d2 u q2 ], where u d2 is the d-axis modulation voltage of the network-type passive feedback control law u2, u q2is the q-axis modulation voltage of the network-type passive feedback control law u2;

[0024] Step 5, introduce the weight coefficient n to realize the weighted control of the grid-following passive feedback control law u1 and the grid-building passive feedback control law u2, and record the weighted passive feedback control law as the dual-mode fusion passive feedback control law u, which is expressed as:

[0025] u=nu1+(1-n)u2.

[0026] Preferably, the steps for solving the grid-following passive feedback control law u1 in step 3 are as follows:

[0027] In the grid-following operation mode, the d-axis component given value and the q-axis component given value of the inductor output current on the bridge arm side are recorded as The calculation formula is:

[0028]

[0029] Among them, Q ref is the rated reactive power command value output by the grid-connected inverter, and U is the effective value of the three-phase grid voltage;

[0030] Based on the interconnection and damping distribution passive control method, the grid-following passive feedback control law u1=[u d1 u q1 ] is expressed as follows:

[0031]

[0032] Among them, r f is the resistance of the inductor equivalent resistance on the bridge arm side, r L1 Inject damping gain into the grid-following mode, and L1 is the inductance of the bridge arm side.

[0033] Preferably, the steps for solving the networked passive feedback control law u2 in step 4 are as follows:

[0034] In the grid-building operation mode, the given values ​​of the d-axis component and q-axis component of the inductor output current on the bridge arm side are recorded as Its expression is:

[0035]

[0036] Where, C is the capacitance of the filter capacitor, r m1 is the first injection damping gain of the grid-forming mode, are respectively a given value of the d-axis component of the output voltage of the filter capacitor and a given value of the q-axis component of the output voltage of the filter capacitor;

[0037] Based on the interconnection and damping distribution passive control method, the networked passive feedback control law u2=[u d2 u q2 ] is expressed as follows:

[0038]

[0039] Among them, r m2 is the second injection damping gain of the lattice mode, r f is the resistance of the inductor equivalent resistor on the bridge arm side, and L1 is the inductance of the inductor on the bridge arm side.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] 1. The present invention proposes a dual-mode fusion control method for a grid-connected inverter based on passivity, which can ensure that the grid-connected inverter has strong robustness under weak grid conditions with large fluctuations in grid impedance. At the same time, from strong grids dominated by synchronous machines to weak grids with high grid impedance, this control strategy can ensure the stable operation of the grid-connected inverter.

[0042] 2. In the control method proposed in the present invention, the passive feedback control law of the grid-connected inverter following grid-type control mode and the passive feedback control law of the grid-connected inverter forming grid-type control mode are both based on the port-controlled Hamiltonian model of dissipative passivity theory, and are established using the interconnection and damping distribution passive control method. That is, the control method of the present invention performs nonlinear modeling and control of the inverter based on the port-controlled Hamiltonian model. From the energy perspective, the design of the control system of the grid-connected inverter is more intuitive. At the same time, the port-controlled Hamiltonian model is suitable for high-order nonlinear systems, so the design is more accurate.

[0043] 3. With the massive introduction of new energy power generation equipment and power electronic equipment, today's power grid presents the characteristics of high impedance and low inertia. The control method proposed in the present invention not only has higher grid-connected power quality and power regulation speed, but also has an active support role for voltage and frequency.

[0044] 4. Compared with the dual-mode fusion control of the traditional linear architecture, the control method proposed in the present invention has stronger dynamic characteristics and robustness. Therefore, it has better stability and reliability under the interference of nonlinear factors such as parameter / structural nonlinear fluctuations.

[0045] 5. The control method proposed in the present invention abandons the phase-locked loop structure commonly used in the traditional grid-following mode, and instead adopts a power synchronization link to obtain the phase of the grid voltage, avoiding the coupling of the grid impedance and the phase-locked loop under weak grid conditions, and having a stronger ability to resist grid disturbances.

[0046] 6. The method proposed in the present invention does not require other grid state perception methods such as grid impedance identification, has good robustness, and avoids the problem of control performance degradation caused by measurement errors. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 This is a schematic diagram of the control structure of the dual-mode fusion control method proposed in the present invention.

[0048] Figure 2 It is a topological diagram involved in the grid-connected inverter in the embodiment of the present invention.

[0049] Figure 3 It is a schematic diagram of the structure of the active power synchronization ring in the present invention.

[0050] Figure 4 It is a schematic diagram of the passive control structure of the grid-following control mode in the present invention.

[0051] Figure 5 It is a schematic diagram of the passive control structure of the networking control mode in the present invention.

[0052] Figure 6 This is the simulation waveform of the three-phase grid-connected voltage when the grid impedance fluctuates over a large range.

[0053] Figure 7 This is the simulated waveform of the three-phase grid current when the grid impedance fluctuates over a large range. DETAILED DESCRIPTION

[0054] The technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings.

[0055] Figure 2 is a topological diagram of the grid-connected inverter involved in the embodiment of the present invention. Figure 2 As shown, the topology involved in the grid-connected inverter includes a DC power supply, a main inverter, an LCL filter, a line equivalent inductor and a three-phase power grid connected in series in sequence, wherein the LCL filter includes a bridge arm side inductor, a bridge arm side inductor equivalent resistor, a filter capacitor and a grid side inductor.

[0056] exist Figure 2 In the figure, L1 is the inductance of the bridge arm side, r f is the resistance of the inductor equivalent resistance on the bridge arm side, C is the capacitance of the filter capacitor, L2 is the inductance of the grid side, L g is the inductance of the circuit equivalent inductance, V dc is the voltage at the DC power supply. In the present embodiment, L1 = 0.9 mH, r f =0.1Ω, C=30μF, L2=2mH, V dc =800V.

[0057] Figure 1This is a schematic diagram of the control structure of the dual-mode fusion control method proposed in the present invention. Figure 1 It can be seen that the dual-mode fusion control refers to the fusion control of the grid-connected inverter grid-following control mode and the grid-connected inverter grid-forming control mode. The fusion control method includes establishing a passive feedback control law of the grid-connected inverter grid-following control mode and a passive feedback control law of the grid-connected inverter grid-forming control mode, and then introducing a weighting coefficient n to weight the two passive feedback control laws of the two control modes to obtain a dual-mode fusion passive feedback control law u. The specific steps are as follows:

[0058] Step 1: Sample the inductor output current i on the bridge arm side a ,i b ,i c , and the d-axis component i of the inductor output current on the bridge arm side is obtained by transforming the three-phase stationary coordinate system to the two-phase rotating coordinate system d and the q-axis component i of the inductor output current on the bridge arm side q ; Sample filter capacitor output voltage u Ca ,u Cb ,u Cc , and the d-axis component u of the filter capacitor output voltage is obtained by transforming the three-phase stationary coordinate system to the two-phase rotating coordinate system Cd And the q-axis component u of the filter capacitor output voltage Cq ; Sample three-phase grid current i ga ,i gb ,i gc , and the three-phase grid current d-axis component i is obtained by transforming the three-phase stationary coordinate system to the two-phase rotating coordinate system gd and the q-axis component i of the three-phase grid current gq ;

[0059] Step 2, obtaining the phase angle required for the grid-connected inverter to synchronize with the grid, and recording it as the synchronization phase angle θ, θ=∫ωdt, where t is the operation time of the grid-connected inverter, and ω is the reference angular frequency;

[0060] The reference angular frequency ω is obtained by active power synchronization loop control, and its expression is: ω=ω0+K p (P ref -P), where ω0 is the rated angular frequency, K p is the active power synchronization loop control proportional coefficient, P ref is the rated active power command value output by the grid-connected inverter, and P is the rated active power output by the grid-connected inverter.

[0061] Figure 3 It is a schematic diagram of the structure of the active power synchronization ring in the present invention.

[0062] Step 3: Based on the port-controlled Hamiltonian model of dissipative passivity theory, the passive feedback control law of the grid-following control mode of the grid-connected inverter is established by adopting the interconnection and damping distribution passive control method, and recorded as the grid-following passive feedback control law u1, u1 = [u d1 u q1 ], where u d1 is the d-axis modulation voltage of the grid-following passive feedback control law u1, u q1 is the q-axis modulation voltage of the grid-following passive feedback control law u1.

[0063] Among them, the port-controlled Hamiltonian model based on dissipative passivity theory can be found in the prior art.

[0064] Step 4: Based on the port-controlled Hamiltonian model of dissipative passivity theory, the passive feedback control law of the grid-connected inverter grid-type control mode is established by using the interconnection and damping distribution passive control method, and recorded as the grid-type passive feedback control law u2, u2 = |u d2 u q2 ], where u d2 is the d-axis modulation voltage of the network-type passive feedback control law u2, u q2 is the q-axis modulation voltage of the network-type passive feedback control law u2.

[0065] Step 5, introduce the weight coefficient n to realize the weighted control of the grid-following passive feedback control law u1 and the grid-building passive feedback control law u2, and record the weighted passive feedback control law as the dual-mode fusion passive feedback control law u, which is expressed as:

[0066] u=nu1+(1-n)u2.

[0067] In this embodiment, the steps for solving the grid-following passive feedback control law u1 described in step 3 are as follows:

[0068] In the grid-following operation mode, the d-axis component given value and the q-axis component given value of the inductor output current on the bridge arm side are recorded as The calculation formula is:

[0069]

[0070] Among them, Q ref is the rated reactive power command value output by the grid-connected inverter, and U is the effective value of the three-phase grid voltage;

[0071] Based on the interconnection and damping distribution passive control method, the grid-following passive feedback control law u1=[u d1 u q1 ] is expressed as follows:

[0072]

[0073] Among them, r f is the resistance of the inductor equivalent resistance on the bridge arm side, r L1 Inject damping gain into the grid-following mode, and L1 is the inductance of the bridge arm side.

[0074] Figure 4 It is a schematic diagram of the passive control structure of the grid-following control mode in the present invention.

[0075] In this embodiment, the steps for solving the meshed passive feedback control law u2 described in step 4 are as follows:

[0076] In the grid-building operation mode, the given values ​​of the d-axis component and q-axis component of the inductor output current on the bridge arm side are recorded as Its expression is:

[0077]

[0078] Where, C is the capacitance of the filter capacitor, r m1 is the first injection damping gain of the grid-forming mode, are respectively a given value of the d-axis component of the output voltage of the filter capacitor and a given value of the q-axis component of the output voltage of the filter capacitor;

[0079] Based on the interconnection and damping distribution passive control method, the networked passive feedback control law u2=[u d2 u q2 ] is expressed as follows:

[0080]

[0081] Among them, r m2 is the second injection damping gain of the lattice mode, r f is the resistance of the inductor equivalent resistor on the bridge arm side, and L1 is the inductance of the inductor on the bridge arm side.

[0082] Figure 5 It is a schematic diagram of the passive control structure of the networking control mode in the present invention.

[0083] In the present invention, the weighting coefficient n is taken as 0.5, and then the following is adopted: Figure 1 The grid-connected inverter controlled by the passive feedback control law based on dual-mode fusion is simulated and verified. Figure 6 , Figure 7 The simulation waveforms of three-phase grid voltage and three-phase grid current are given respectively when the grid impedance fluctuates in a large range. a , U b , U c is the three-phase grid voltage, I a , I b , I cIt is the three-phase grid current.

[0084] The grid-connected inverter is set to operate under strong grid conditions in the initial state, and the grid short-circuit ratio is set to 13. Then, by changing the inductance value of the grid-side line equivalent inductance, the change of grid strength from strong to weak is simulated. At 1.2 seconds, the grid short-circuit ratio is 1.8, and the grid is in extremely weak grid conditions. The three-phase grid voltage U is measured and recorded during the whole process. a , U b , U c And the three-phase grid current I a , I b , I c The waveform of Figure 6 and Figure 7 As shown in the figure, with the large fluctuation of grid impedance, the three-phase grid voltage U a , U b , U c And the three-phase grid current I a , I b , I c The waveform still remains good, verifying the strong robustness of the control strategy.

Claims

1. A dual-mode fusion control method for a grid-connected inverter based on passivity, wherein the topology involved in the grid-connected inverter includes a DC power supply, a main inverter, an LCL filter, a line equivalent inductance and a three-phase power grid connected in series in sequence, wherein the LCL filter includes a bridge arm side inductance, a bridge arm side inductance equivalent resistance, a filter capacitor and a grid side inductance; characterized in that: The dual-mode fusion control refers to the fusion control of the grid-connected inverter grid-following control mode and the grid-connected inverter grid-forming control mode. The fusion control method includes establishing a passive feedback control law of the grid-connected inverter grid-following control mode and a passive feedback control law of the grid-connected inverter grid-forming control mode, and then introducing a weighting coefficient n to weight the two passive feedback control laws of the two control modes to obtain a dual-mode fusion passive feedback control law u. The specific steps are as follows: Step 1: Sample the inductor output current i on the bridge arm side a ,i b ,i c , and the d-axis component i of the inductor output current on the bridge arm side is obtained by transforming the three-phase stationary coordinate system to the two-phase rotating coordinate system d and the q-axis component i of the inductor output current on the bridge arm side q ; Sample filter capacitor output voltage u Ca ,u Cb ,u Cc , and the d-axis component u of the filter capacitor output voltage is obtained by transforming the three-phase stationary coordinate system to the two-phase rotating coordinate system Cd And the q-axis component u of the filter capacitor output voltage Cq ; Sample three-phase grid current i ga ,i gb ,i gc , and the three-phase grid current d-axis component i is obtained by transforming the three-phase stationary coordinate system to the two-phase rotating coordinate system gd and the q-axis component i of the three-phase grid current gq ; Step 2, obtaining the phase angle required for the grid-connected inverter to synchronize with the grid, and recording it as the synchronization phase angle θ, θ=∫ωdt, where t is the operation time of the grid-connected inverter, and ω is the reference angular frequency; The reference angular frequency ω is obtained by active power synchronization loop control, and its expression is: ω=ω0+K p (P ref -P), where ω0 is the rated angular frequency, K p is the active power synchronization loop control proportional coefficient, P ref is the rated active power command value output by the grid-connected inverter, and P is the rated active power output by the grid-connected inverter; Step 3: Establish the passive feedback control law of the grid-following control mode of the grid-connected inverter and record it as the grid-following passive feedback control law u1, u1 = [u d1 u q1 ], where u d1 is the d-axis modulation voltage of the grid-following passive feedback control law u1, u q1 is the q-axis modulation voltage of the grid-following passive feedback control law u1; The steps for solving the grid-following passive feedback control law u1 are as follows: In the grid-following operation mode, the d-axis component given value and the q-axis component given value of the inductor output current on the bridge arm side are recorded as The calculation formula is: Among them, Q ref is the rated reactive power command value output by the grid-connected inverter, and U is the effective value of the three-phase grid voltage; Based on the interconnection and damping distribution passive control method, the grid-following passive feedback control law u1=[u d1 u q1 ] is expressed as follows: Among them, r f is the resistance of the inductor equivalent resistance on the bridge arm side, r L1 Inject damping gain for the grid-following mode, L1 is the inductance of the bridge arm side; Step 4: Establish the passive feedback control law of the grid-connected inverter grid-connected control mode, and record it as the grid-connected passive feedback control law u2, u2 = [u d2 u q2 ], where u d2 is the d-axis modulation voltage of the network-type passive feedback control law u2, u q2 is the q-axis modulation voltage of the network-type passive feedback control law u2; The steps for solving the networked passive feedback control law u2 are as follows: In the grid-building operation mode, the given values ​​of the d-axis component and q-axis component of the inductor output current on the bridge arm side are recorded as Its expression is: Where, C is the capacitance of the filter capacitor, r m1 is the first injection damping gain of the grid-forming mode, are respectively a given value of the d-axis component of the output voltage of the filter capacitor and a given value of the q-axis component of the output voltage of the filter capacitor; Based on the interconnection and damping distribution passive control method, the networked passive feedback control law u2=[u d2 u q2 ] is expressed as follows: Among them, r m2 is the second injection damping gain of the lattice mode, r f is the resistance value of the inductor equivalent resistor on the bridge arm side, and L1 is the inductance value of the inductor on the bridge arm side; Step 5, introduce the weight coefficient n to realize the weighted control of the grid-following passive feedback control law u1 and the grid-building passive feedback control law u2, and record the weighted passive feedback control law as the dual-mode fusion passive feedback control law u, which is expressed as: u=nu1+(1-n)u2.

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