Self-phase-locking network-building type inverter control method and system

Abstract

The invention belongs to the technical field of new energy grid-connected / off-grid control and micro-grid power supply conversion, and discloses a self-phase-locking grid-building type inverter control method. According to the method, a frequency-phase droop phase-locking mode is introduced on the basis of a network-forming inverter, so that an external positive feedback channel generated by power grid impedance and PLL coupling is effectively weakened, and the inverter still keeps stable operation under the condition of a weak power grid; meanwhile, through a self-phase-locking mechanism, the inverter can autonomously establish and maintain the characteristics of a voltage source without depending on an external power grid, and stable voltage building and self-synchronization operation under the off-grid condition are achieved. Furthermore, a virtual resistor is introduced on a dq axis, so that an inverter port is represented as a non-ideal voltage source, and formation of a unified modeling framework and a grid-connected analysis basis is facilitated. On the whole, the stability and robustness of the inverter under the weak network and island working conditions are remarkably improved through the self-phase-locking strategy of the network construction type inverter provided by the invention, and seamless switching of the grid-connected / off-grid operation mode is realized.

Description

Technical Field

[0001] This invention belongs to the field of new energy grid-connected / off-grid control and microgrid power conversion technology, and particularly relates to a self-locking phase grid-connected inverter control method and system. Background Technology

[0002] With the continuous increase in the penetration rate of new energy sources and the continuous decline in the short-circuit ratio of the power grid, grid-connected inverters are prone to a series of stability problems under weak power grid conditions. In weak grid environments, due to the positive feedback effect introduced by the grid impedance through the traditional dq coordinate axis uq=0 phase-locked loop, the low-frequency phase angle disturbance is amplified infinitely. The inverter grid-connected system is prone to low-frequency oscillations and PLL lockout. Based on this, a self-locked phase-locked grid-connected inverter control strategy is proposed. By introducing a frequency-phase droop phase-locked loop and a controlled source structure inside the synchronization link, a self-sustaining synchronization closed loop is constructed. This allows the inverter to autonomously generate reference values ​​of θ and ω when there is no strong external grid, maintain robust following under weak grid conditions, and achieve continuous operation using the same control framework in both grid-connected and off-grid conditions without switching. This strategy aims to suppress the positive feedback effect of the phase-locked loop, maintain synchronization stability, and ensure smooth grid connection under weak grid or even no grid conditions, thereby achieving smooth switching and stable operation between grid-connected and off-grid modes.

[0003] The closest related prior art: A method for inverter grid-forming control using virtual impedance (US202017022535A) provides a grid-forming control technology solution for inverters based on virtual impedance. This solution injects a configurable virtual impedance value through software. Regardless of the physical characteristics of the equipment, the control signal in the phase-locked loop (PLL) can be adjusted by using the voltage drop calculated based on current feedback, thereby regulating the inverter's power output and balance. This is applicable to various scenarios including wind power generation, energy storage, and STATCOM. The core technology of this patent lies in controlling the dynamic response and stability of the inverter through virtual impedance, providing strong flexible adjustment capabilities.

[0004] Although this patent enhances grid-connected stability using virtual impedance, it still relies on the traditional PLL architecture. Under weak grid conditions, it is prone to phase measurement deviations and positive feedback loops, which may lead to decreased system stability, oscillations, or loss of lock-in. In particular, it cannot effectively suppress undesirable dynamic behaviors in the phase-locked loop when the grid impedance is high. Furthermore, this scheme lacks the ability to independently and autonomously establish voltage references and synchronization in terms of off-grid self-built voltage source functionality. Once disconnected from the external grid, its synchronization mechanism is difficult to maintain, and it cannot achieve true "self-locked phase" operation. Summary of the Invention

[0005] To address the problem that traditional phase-locked loops (PLLs) are prone to positive feedback, leading to loss of lock and oscillation under weak grid conditions, and to overcome the limitation of existing grid-connected inverters lacking a reliable synchronization reference under off-grid conditions, this invention proposes a novel control method that integrates grid-connected control and a self-locking phase-locking mechanism.

[0006] This invention is implemented as follows: a self-locked phase-locked grid inverter control strategy. Based on the grid inverter, this strategy introduces a frequency-phase droop phase-locked method, effectively weakening the external positive feedback path generated by grid impedance and PLL coupling, enabling the inverter to maintain stable operation under weak grid conditions. Simultaneously, through the self-locked phase-locked mechanism, the inverter can autonomously establish and maintain voltage source characteristics without relying on an external grid, achieving stable voltage build-up and self-synchronization operation under off-grid conditions. Furthermore, a virtual resistor is introduced on the dq axis, making the inverter port behave as a non-ideal voltage source, which is beneficial for forming a unified modeling framework and grid-connected analysis basis.

[0007] Furthermore, the aforementioned frequency-phase droop phase-locked loop method specifically includes:

[0008] Coordinate transformation and orthogonal quantity generation are used to process the grid connection point voltage and current according to phase control and then integrate them into the same control framework. The three-phase voltage and current u at the grid connection point are then used. abc i abc After dq transformation, the voltage vector and current component u in the rotating coordinate system are obtained. d ,u q i d i q The single-phase voltage U and current I are passed through an SOGI quadrature signal generator to obtain the quadrature quantity v. α ,v β， i α i β Then perform a Park transformation based on the current phase-locked angle θ to obtain u, which is equivalent to the three-phase transformation. d ,u q i d， i q This provides a controllable quantity for subsequent self-locking and damping / virtual impedance injection.

[0009] Furthermore, the self-locked phase-locked loop (PLL) mechanism, by introducing frequency-phase droop phase-locking into the traditional PLL, can achieve automatic phase correction, fundamentally suppressing the positive feedback effect formed by the traditional PLL method under weak grid conditions. Under weak grid conditions, the high grid impedance causes the inverter output current to have a significant impact on the grid voltage. Traditional PLLs, because the measured grid phase is affected by their own injected current, are prone to forming positive feedback loops that are detrimental to stability. This positive feedback weakens the system damping, causing the grid-connected inverter to oscillate or become unstable in weak grid conditions. To address this problem, this strategy adds virtual damping to the PLL structure, which is equivalent to connecting an internal damping impedance in series in the phase-locking stage, thereby blocking the positive feedback path of the PLL for the inverter under weak grid conditions. After introducing damping, the PLL's response to grid phase disturbances is additionally attenuated, significantly improving the system's small-signal damping characteristics and phase margin.

[0010] Furthermore, the introduction of a virtual resistance on the dq axis specifically includes:

[0011] A virtual resistor is introduced into the dq-axis voltage command of the inverter to adjust the output voltage reference value, constructing a non-ideal voltage source model with equivalent series impedance, and adding voltage drop compensation terms proportional to the output current to the d-axis and q-axis voltage reference values ​​respectively.

[0012] Furthermore, the method also includes: unified control process and verification for grid connection / off-grid, adopting a unified self-locking phase grid-type inverter framework, eliminating the need to switch control structures between grid connection and off-grid; verifying the effects of angle synchronization, grid connection point current, etc., under operating conditions such as "grid connection-off-grid reconnection".

[0013] Another object of the present invention is to provide a computer device, the computer device including a memory and a processor, the memory storing a computer program, the computer program being executed by the processor causing the processor to perform the steps of the self-locking grid inverter control strategy.

[0014] Another object of the present invention is to provide a computer-readable storage medium storing a computer program, which, when executed by a processor, causes the processor to perform the steps of the self-locking grid inverter control strategy.

[0015] Based on the above technical solutions and the technical problems solved, the advantages and positive effects of the technical solution to be protected by this invention are as follows:

[0016] The frequency-phase droop self-locking phase-type grid control method proposed in this invention significantly improves the dynamic stability of the system and effectively suppresses the positive feedback effect of traditional PLLs under weak grid conditions. It can maintain synchronization and stable operation even under low short-circuit ratios, frequency disturbances, or off-grid conditions. Simultaneously, the self-locking phase mechanism enables the inverter to autonomously establish phase and frequency references under off-grid conditions, exhibiting good dynamic response and ensuring smooth switching and stable operation between grid-connected and off-grid modes.

[0017] This invention proposes a self-locked phase-locked loop (PLL) grid-type control strategy. By introducing a frequency-phase droop PLL, it achieves automatic adjustment of the error phase, blocking the positive feedback problem of the PLL in weak grid conditions. This allows the inverter to autonomously provide tracking phase even without a grid. Simultaneously, a virtual resistor is introduced on the dq axis, causing the inverter to exhibit non-ideal voltage source characteristics. Combined with the self-locking capability, the parallel and grid-connected operation of the inverter is transformed into the parallel operation of a DC power supply. This enables the inverter to autonomously provide phase reference and synchronization capabilities even without a grid, achieving true grid-type characteristics. Furthermore, it achieves stable parallel operation and smooth grid-connected / off-grid switching of the inverter without algorithm switching. Attached Figure Description

[0018] Figure 1 This is the droop characteristic curve provided in the embodiments of the present invention;

[0019] Figure 2 This refers to the ideal state of the vector relationships provided in the embodiments of the present invention;

[0020] Figure 3 These are the dynamic vector relationships provided in the embodiments of the present invention;

[0021] Figure 4 This is a block diagram of a single-phase / three-phase control system for a self-locking phase-locked loop provided in an embodiment of the present invention;

[0022] Figure 5 This is a schematic diagram of the virtual impedance in the dq coordinate system provided in an embodiment of the present invention.

[0023] Figure 6 This is a schematic diagram of the virtual impedance in the dq coordinate system provided in an embodiment of the present invention.

[0024] Figure 7 This is the equivalent port after dq-axis virtual resistance injection provided in the embodiments of the present invention;

[0025] Figure 8 This is the equivalent parallel connection model of the inverter and the power grid in the dq axis coordinate system provided in this embodiment of the invention;

[0026] Figure 9 This is a simulation of a self-locking inverter between grid and off-grid provided in an embodiment of the present invention;

[0027] Figure 10 This refers to the frequency change of a single-phase system with self-locking phase from grid connection to off-grid connection, as provided in this embodiment of the invention.

[0028] Figure 11 This embodiment of the invention describes the phase change of a self-locking inverter during the process of switching from grid connection to off-grid operation when the grid frequency is 50 Hz.

[0029] Figure 12 This refers to the phase change of the self-locking inverter provided in this embodiment of the invention during the process from grid connection to off-grid operation when the grid frequency is 51Hz. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0031] (1) Coordinate transformation and generation of orthogonal quantities

[0032] The grid-connected point voltage and current are processed separately according to phase control and then incorporated into the same control framework. The three-phase voltage and current uabc, iabc at the grid-connected point are transformed by dq to obtain the voltage vector and current components ud, uq, id, iq in the rotating coordinate system. The single-phase voltage U and current I are passed through SOGI (orthogonal signal generator) to obtain orthogonal quantities vα, vβ, iα, iβ. Then, Park transformation is performed according to the current phase-locked angle θ to obtain ud, uq, id, iq, which are equivalent to the three phases. These provide controllable quantities for subsequent self-locking and damping / virtual impedance injection.

[0033] (2) Construction of self-locking phase loop

[0034] The self-locked phase-locked loop (PLL) control strategy, by introducing frequency-phase droop phase-locking into the traditional PLL, can achieve automatic phase correction, fundamentally suppressing the positive feedback effect formed by the traditional PLL method under weak grid conditions. Under weak grid conditions, the high grid impedance causes the inverter output current to significantly affect the grid voltage. Traditional PLLs, because the measured grid phase is affected by their own injected current, are prone to forming positive feedback loops that are detrimental to stability. This positive feedback weakens system damping, causing the grid-connected inverter to oscillate or become unstable in weak grids. To address this problem, this strategy adds virtual damping to the PLL structure, equivalent to connecting an internal damping impedance in series in the phase-locking stage, thereby blocking the positive feedback path of the PLL in weak grid conditions. After introducing damping, the PLL's response to grid phase disturbances is further attenuated, significantly improving the system's small-signal damping characteristics and phase margin.

[0035] like Figure 1As shown, this invention proposes a frequency-phase droop relationship: Using the rated frequency ω0 (50Hz) as a reference, when ω=ω0, eθ=0; when ω>ω0, eθ<0, and the error phase becomes negative; when ω<ω0, eθ>0, and the error phase becomes positive. When the phase error eθ becomes negative, the system increases the angular velocity to accelerate the adjustment; when the phase error eθ becomes positive, the system decreases the angular velocity to gradually adjust the phase error, achieving automatic adjustment of the error phase. Its mathematical form can be written as:

[0036] e θ = -r(ω-ω0)

[0037] Where eθ is the error phase, ω is the actual frequency, ω0 is the reference frequency, and r is the damping droop coefficient.

[0038] The vector relationships in an ideal inverter control system can be derived from... Figure 2 The following description is provided. In the diagram, the dpllqpll coordinate system is obtained by the phase-locked loop (PLL) for capacitor voltage orientation, and the angle between its dpll axis and the α axis of the αβ coordinate system is defined as θpll, which is the angle obtained by the PLL. The dsqs coordinate system is the system's synchronization coordinate system, and the angle between its ds axis and the α axis is defined as θs, which is the actual angle of the system. In steady state, the dpllqpll coordinate system coincides with the dsqs coordinate system, and θpll = θs.

[0039] The vector relationships in the dynamic inverter control system can be derived from... Figure 3 In this case, the angular velocity wpll of the phase-locked loop (PLL) will be greater than or less than the actual angular velocity w0 of the system. There will be a phase error eθ between θpll and θs, which can be adjusted using the frequency-phase droop relationship mentioned above. The projection Uq* of the AC bus voltage (terminal voltage) Ut onto qpll is calculated using eθ and used as the input to the self-locked phase control system. The formula is as follows:

[0040] Uq*=Ut sine θ

[0041] Based on the frequency-phase droop relationship, the difference between uq* and uq is used as the phase-locked error. An angular velocity ω is generated via a PI converter, and then integrated to obtain the phase angle θ, thus achieving synchronization with the power grid / reference frequency (e.g., ...). Figure 4 Simultaneously, the q-axis projected voltage is treated as a controlled power supply, and an internal resistance r and an equivalent load current model are introduced to form a self-locking characteristic. This improves the tracking stability and bandwidth under weak grid / disturbance conditions, and ensures stable operation under off-grid conditions without switching when reconnected to the grid. It is applicable to both single-phase and three-phase inverters. Its control block diagram is as follows:

[0042] (3) Virtual resistance along the dq axis

[0043] This invention introduces a virtual resistor element into the dq-axis voltage command of the inverter to adjust the output voltage reference value, constructing a non-ideal voltage source model with equivalent series impedance. Voltage drop compensation terms proportional to the output current are added to the d-axis and q-axis voltage reference values, such as... Figure 5 , 6 As shown.

[0044] Its control formula is: `

[0045]

[0046] Where Vd0 and Vq0 are the voltage references of the PI output, Rv is the set virtual resistance value, and id and iq are the actual dq-axis injected currents. Using the above formulas, the inverter is equivalent to a "voltage source + Rv series impedance" structure in the dq coordinate system, realizing the non-idealization of the voltage source. Its control principle diagram is as follows:

[0047] In the grid-connected model, the grid side can also be represented as an ideal voltage source in series with a resistor. If both are mapped to the dq rotating coordinate system, a parallel system structure of "two DC voltage sources + series resistor" is formed. In this case, the parallel circulating current is determined by the voltage difference and the series resistance, eliminating the large inrush current caused by "two ideal voltage sources in parallel." This provides effective protection for the operation of self-locking grid-connected inverters. This dq-axis series impedance modeling method not only improves system stability but also simplifies grid-connected / off-grid process modeling, facilitating stability analysis under steady-state and small disturbance conditions. Figure 8 As shown.

[0048] Unified control process and verification for both online and offline networks

[0049] The unified self-locking phase grid-type inverter framework eliminates the need to switch control structures between grid connection and off-grid operation; the effects of angle synchronization and grid connection point current are verified under operating conditions such as "grid connection-off-grid reconnection".

[0050] Figure 9 This simulation demonstrates the on-grid and off-grid operation of a self-locking inverter. It covers the entire process from grid-connected to off-grid operation and back to grid-connected operation. The PCC voltage and inductor current maintain a continuous transition without sudden jumps near the grid connection point, exhibiting good dynamic response. Furthermore, the self-locking inverter can maintain its θ and ω references independently during off-grid operation, establishing a stable voltage source on its own. This enables seamless switching and rapid reconnection to the grid within the same control framework.

[0051] Figure 10The vertical axis represents the frequency change of a single-phase system from grid connection to off-grid operation; the vertical axis represents the synchronization frequency of the self-locked phase-locked loop. Initially, it is in a grid-connected steady state with a stable frequency of 50 Hz. At 0.2 s, the grid connection point is disconnected, and the system transitions to an off-grid state. The frequency exhibits only a small instantaneous deviation before rapidly decaying and stabilizing at the grid frequency of 50 Hz. The self-locked phase-locked grid control does not require switching control modes and has the ability to maintain internal synchronization reference and quickly self-lock phase.

[0052] Figure 11 The graph shows the phase change of the self-locked loop (PLL) inverter from grid connection to off-grid operation when the grid frequency is 50 Hz; blue represents the grid phase, and red represents the PLL output phase. After off-grid operation, the two phases represent the PLL output phase and the inverter output filter capacitor phase, respectively. When the system frequency remains at the reference 50 Hz, the angle setpoint stabilizes at 0, and the PLL output is consistent with the reference. This indicates that the method achieves small-deviation adaptive operation and continuous reference near the reference.

[0053] Figure 12 The graph shows the phase change of the self-locked inverter from grid connection to off-grid operation when the grid frequency is 51Hz. When the grid frequency rises to 51Hz, it can be seen from the graph that the phase tracking reference value of the phase-locked loop is not 0. The phase-locked output phase can still stably track the non-zero reference. When off-grid, the curve only shows a small transient amplitude and then recovers and remains at 51Hz. Due to the 1Hz frequency deviation, the phase error eθ does not converge to zero during the grid connection stage, but fluctuates within a bounded range near a small offset. This verifies that the control still has the synchronization capability and the characteristic of fast recovery of phase-locking under weak grid and frequency deviation conditions.

[0054] Example 1: Grid connection stability verification under weak power grid conditions

[0055] In this embodiment, the grid-connected inverter is connected to a weak grid environment with an equivalent grid impedance of 1 ohm per phase. The inverter's self-locking phase control module achieves phase locking using a frequency-phase droop method. Experiments show that, compared to the traditional phase-locked loop method, the method of this invention maintains a smooth transition of the inverter output current and does not exhibit oscillation when the grid voltage amplitude fluctuates by 5%.

[0056] Furthermore, the small-signal frequency response of the system was monitored during the experiment. The results showed that the phase margin increased from 25° under the traditional PLL structure to 47° under this method, which verified that the self-locking mechanism effectively weakens the positive feedback effect and enhances the stability under weak power grid conditions.

[0057] Example 2: Off-grid autonomous voltage building and synchronous operation

[0058] In this embodiment, the inverter is operated independently in an off-grid environment without grid support. The inverter's internal self-locking mechanism independently establishes a voltage reference and maintains the voltage amplitude at the rated 220 volts and the frequency at 50 Hz. The inverter can stably output voltage waveform without external grid signals.

[0059] When different resistive and inductive loads are connected to the inverter port, the voltage fluctuation is less than 3% of the rated value. It operates in self-synchronization mode and provides frequency fluctuation of less than 0.2 Hz for islanded power supply or independent microgrid applications, indicating that the control method can achieve stable voltage build-up and phase technology support under off-grid conditions.

[0060] Example 3: Virtual Resistance Injection Effect

[0061] In this embodiment, a virtual resistance element is introduced into the inverter's dq-axis voltage command, and the equivalent value of the virtual resistance is set to 0.2 ohms. At this time, the inverter port effectively behaves as a non-ideal voltage source, enabling the formation of reasonable impedance characteristics between voltage and current.

[0062] Comparative tests showed that when the load suddenly increased by 20%, the system without virtual resistors experienced a voltage drop of 12%, while the system with virtual resistors only experienced a voltage drop of 5%, and the recovery time was shortened by nearly 40%. This indicates that virtual resistor injection significantly improves stability during dynamic processes.

[0063] Example 4: Unified control of grid connection-off-grid connection-reconnection

[0064] In this embodiment, the inverter operates in grid-connected mode, then disconnects from the grid via the grid-connection switch, and reconnects to the grid after 3 seconds. Throughout the entire process, the control system maintains a consistent self-locking phase-type grid inverter framework without any control switching.

[0065] Test results show that at the moment of reconnection to the grid, the peak value of the inverter output current fluctuation is less than 8% of the rated value, the voltage phase angle difference is less than 3°, and the system can quickly restore synchronization. This result verifies that the method has good robustness and continuity under operating condition switching.

[0066] Example 5: Device and System Implementation

[0067] In this embodiment, an inverter control system is built based on a DSP controller. The hardware includes a signal acquisition unit, an orthogonal signal generation unit, a damping injection unit, and a virtual resistance injection module. The signal acquisition unit acquires the grid connection point voltage and current, and sends them to the control module after coordinate transformation. The controller output drives the inverter main circuit through PWM modulation.

[0068] Experiments conducted on this system platform showed that the inverter did not exhibit oscillations when operating under a weak grid, maintained stable voltage source characteristics in off-grid mode, and successfully completed voltage and phase synchronization during grid-to-off-grid switching. These verifications demonstrate that both the device and the system effectively support the technical solution of this invention.

[0069] Evidence related to the technical effects obtained by the embodiments of the present invention:

[0070] 1. Main circuit and model configuration

[0071] DC side: Constant voltage source U dc →DC bus capacitor.

[0072] Inverter bridge: three-phase two-level IGBT six-switch, each phase bridge arm is triggered by carrier comparison / PWM.

[0073] Filtering / Grid Connection / Off-Grid: The inverter uses an LC filter and is connected to the grid equivalent module; grid connection and off-grid are achieved through commands to connect the circuit breaker at the PCC terminal, with the grid side on the right.

[0074] Sampling and Transformation: Voltage / Current Measurement Block V abc I abc It is connected in parallel to the abc / dq and dq / abc conversion modules; the signals from each measurement point are sent to the control via the bus.

[0075] 2. Control framework for self-locking grid-type inverters

[0076] Self-locked phase-locked grid control is implemented using S-Function: This involves controlling the grid connection point voltage and current u... abc i abc With parameter bus (self-locked phase loop proportional gain k) p Integral gain k i The module takes the damping coefficient r, the given frequency f0, etc., as inputs, performs the Park transformation within the module, and outputs U. q Synchronization error is constructed, and ω and θ are generated through a discrete PI converter and integrator. A damping term r is introduced into the synchronization loop to achieve self-locking when disconnected from the grid. Externally, a voltage / current dual loop is used, with the outer voltage loop (PI) outputting u in the dq coordinate system. d0 ,u q0 And obtain v by superimposing virtual impedance terms. d * and v q * After passing through the current loop, it is finally converted by dq / abc and modulated to generate PWM to drive the bridge arm; and before grid connection, the external logic can only allow closing and grid connection according to the criteria of |Δθ|<5°, |Δf|<0.1Hz, and |ΔV|<5%.

[0077] 3. Simulation conditions and processes

[0078] (1) Operating condition 1 is a simulation of the self-locking inverter's grid connection and disconnection. The grid connection / disconnection is achieved by a circuit breaker that is connected in series between the PCC and the grid equivalent: the circuit breaker is enabled by external control, and the control terminal com is driven by the synchronization logic; during the grid connection phase in the simulation, the circuit breaker com=1 is set, and at t1, the disconnection command is pressed to set com=0 to enter the island, and the self-locking module continuously outputs θ,ω to keep the voltage source running; when the synchronization criterion is met, com=1 is set at t2 to complete the grid connection, and the grid connection voltage and current transition smoothly, and the control architecture does not need to be switched.

[0079] (2) Operating condition 2 is a simulation of the phase change of the self-locking inverter from grid connection to off-grid when the grid frequency is 50Hz. The grid frequency is 50Hz. The grid connection starts in steady state. The self-locking control has matched the device phase θ with the grid phase θ. g Alignment; the above image also plots θ. g (Blue) and the device output phase θ (red), the following figure uses sum to calculate the phase error e. θ =θ g -θ. At time t1, the device transitions to an off-grid state via the circuit breaker (COM is changed from 1 to 0). Subsequently, the red phase trajectory extends continuously without a step, maintaining a reference 50Hz slope. The phase error is generated at the moment of switching, causing a brief, small offset, which then rapidly decays to near zero and remains stable, with only a small pulsation appearing near the switching point. Under the 50Hz reference, the device maintains a constant synchronization reference during the grid-to-off-grid transition, and the phase is quickly recovered via self-locking.

[0080] (3) Operating condition 3 is a simulation of the phase change of the self-locking inverter from grid connection to off-grid when the grid frequency is 51Hz. In the main model, the grid frequency is set to 51Hz. A circuit breaker is connected in series between the PCC and the grid and external control is selected. The circuit breaker is initially closed with com=1. The given frequency f0 of 50Hz is connected to the self-locking S-Function. At time t1, Step is used to change com:1→0 to make it switch to the off-grid state. The total simulation time is set to about 0.6s to record the entire process. This process highlights the difference from the 50Hz operating condition and realizes the phase change during the grid connection period. θ Offset frequency tracking ≠ 0.

[0081] It should be noted that embodiments of the present invention can be implemented in hardware, software, or a combination of both. The hardware portion can be implemented using dedicated logic; the software portion can be stored in memory and executed by a suitable instruction execution system, such as a microprocessor or dedicated-design hardware. Those skilled in the art will understand that the above-described devices and methods can be implemented using computer-executable instructions and / or included in processor control code, for example, such code provided on a carrier medium such as a disk, CD, or DVD-ROM, a programmable memory such as read-only memory (firmware), or a data carrier such as an optical or electronic signal carrier. The devices and modules of the present invention can be implemented by hardware circuitry such as very large-scale integrated circuits or gate arrays, semiconductors such as logic chips, transistors, or programmable hardware devices such as field-programmable gate arrays, programmable logic devices, etc., or by software executed by various types of processors, or by a combination of the above-described hardware circuitry and software, such as firmware.

[0082] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions, and improvements made by those skilled in the art within the scope of the technology disclosed in the present invention, and within the spirit and principles of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A control method for a self-locking phase-locked grid inverter, characterized in that, The method includes: Introducing a phase-locked loop (PLL) method based on frequency and phase droop into the control framework of a grid-connected inverter weakens the positive feedback path generated by the coupling between grid impedance and the PLL, enabling the inverter to maintain stable operation under weak grid conditions. The inverter establishes voltage source characteristics and maintains phase self-synchronization operation in the off-grid state by using a self-locking phase mechanism; Virtual resistance elements are introduced into the d-axis and q-axis voltage commands of the rotating coordinate system, making the inverter port behave as a non-ideal voltage source with equivalent series impedance.

2. The method according to claim 1, characterized in that, The phase-locked loop methods with frequency and phase droop include: The voltage and current components at the grid connection point are obtained by performing a coordinate transformation from three-phase to two-phase. The orthogonal quantities of voltage and current are obtained by an orthogonal signal generator, and then Park transformation is performed. The obtained d-axis and q-axis components are used as inputs to the self-locking phase circuit and the virtual damping injection.

3. The method according to claim 1, characterized in that, The self-locking phase mechanism achieves automatic phase correction by introducing frequency and phase droop control into the traditional phase-locked loop, thereby suppressing the positive feedback effect caused by the inverter output current affecting the grid voltage.

4. The method according to claim 3, characterized in that, A damping element is connected in series in the phase detection and tracking stage of the phase-locked loop to effectively increase the internal damping impedance, reduce the excessive response of the phase-locked loop to power grid phase disturbances, and thus improve the small-signal damping characteristics and phase margin.

5. The method according to claim 1, characterized in that, The virtual resistance element includes: Introduce voltage drop compensation proportional to the d-axis output current in the d-axis voltage command; Introduce voltage drop compensation proportional to the q-axis output current in the q-axis voltage command; The above-mentioned compensation structure possesses the characteristics of a non-ideal voltage source with equivalent series impedance.

6. The method according to claim 1, characterized in that, The method further includes: adopting a unified self-locking phase-type grid inverter control framework, which eliminates the need to switch control structures during grid connection, off-grid and re-grid processes, maintaining angle synchronization and smooth current transition.

7. A self-locking phase-locked grid inverter device, characterized in that, include: The main body of a grid-type inverter is used to realize power conversion; The self-locked phase control module is used to achieve self-synchronization control in grid-connected and off-grid conditions based on the phase-locking method of frequency and phase droop. The virtual resistance injection module is used to introduce current-related compensation in the d-axis and q-axis voltage commands to form an equivalent series impedance.

8. The apparatus according to claim 7, characterized in that, The self-locking phase control module includes: The signal acquisition unit is used to acquire the voltage and current at the grid connection point. The coordinate transformation and orthogonal signal generation unit is used to generate voltage and current components in the rotating coordinate system; Damping injection unit is used to add a damping element to the phase-locked loop structure to suppress positive feedback channels.

9. The apparatus according to claim 7, characterized in that, The virtual resistance injection module generates a voltage drop compensation signal proportional to the d-axis and q-axis output current through a proportional arithmetic unit, and superimposes it onto the corresponding voltage command to realize the non-ideal voltage source characteristics of the inverter's equivalent electrical model.

10. A self-locking phase-locked grid inverter control system, characterized in that, The system includes: The grid-type inverter device as described in claim 7; Grid connection interface unit, used to connect the device to the power grid; The controller is used to uniformly control and verify the operating status of the device during grid connection, off-grid, and re-grid processes, and to maintain voltage phase synchronization and output current stability.

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