Inverter grid-connected and off-grid seamless switching control method based on virtual power control
By connecting an LC filter between the inverter and the grid and utilizing virtual power control, self-synchronization and seamless switching of the inverter in a weak grid environment are achieved, solving the instability problem caused by the phase-locked loop and improving the stability and safety of the inverter.
Patent Information
- Application Number
- CN202511875457.1
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-12-12
- Publication Date
- 2026-04-24
AI Technical Summary
In weak grid conditions, the phase-locked loop (PLL) generates coupling when detecting grid voltage, which can lead to instability in the grid-connected inverter. Furthermore, it may generate inrush currents during grid-connection and off-grid switching, damaging the inverter circuitry.
The virtual power control method is adopted, an LC filter is connected between the inverter and the grid, and the inverter can achieve seamless switching from off-grid to grid-connected by calculating and adjusting the virtual active power P and reactive power Q. Self-synchronization control is performed using virtual impedance Z.
It enables the inverter to operate automatically and switch seamlessly in weak grid environments, avoiding the use of phase-locked loops and improving the stability and security of the system.
Smart Images
Figure CN121923210A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of inverter control technology, and in particular to a method for seamless on-grid and off-grid switching control of inverters based on virtual power control. Background Technology
[0002] Typically, grid-connected inverters use phase-locked loops (PLLs) to detect the phase of the grid voltage. However, in weak grid conditions where grid impedance cannot be ignored, the PLL can couple with the current control loop through the point of common coupling (PCC), causing instability. Therefore, in weak grid conditions, DC bus voltage and reactive power control (DCVQ) grid-connected inverters with self-synchronization can be used. When a DCVQ grid-connected inverter operates in islanded mode, the amplitude, frequency, and phase of the inverter output voltage will deviate from the grid voltage. If grid connection is forced, inrush current will be generated, leading to grid connection failure or even damage to the inverter circuit. Therefore, a strategy is needed for DCVQ inverters that can achieve seamless switching between grid-connected and off-grid conditions without using a PLL. Summary of the Invention
[0003] Purpose of the invention: The purpose of this invention is to provide a seamless switching control method for grid-connected inverters based on virtual power control, so as to achieve seamless switching of grid-connected inverters from off-grid operation to grid-connected operation.
[0004] Technical Solution: To achieve the above objectives, the present invention provides a seamless switching control method for inverters between grid and off-grid operation based on virtual power control. An LC filter is connected between the inverter and the grid. When the inverter switches from grid-connected to off-grid operation, the load switch between the inverter and the grid is closed, and the grid-connected switch S... g After disconnection and off-grid operation, the power generated by the inverter flows to the load. When switching from off-grid to grid-connected operation, the voltage amplitude and phase on both the inverter side and the grid side are collected. The virtual active power P and reactive power Q flowing across the virtual impedance Z between the inverter and the grid are calculated, and P and Q are adjusted to zero. The grid-connected switch S is then closed. g This enables seamless switching of the inverter from off-grid operation to grid-connected operation.
[0005] Preferably, the active power P and reactive power Q flowing through the virtual impedance Z are expressed as:
[0006] ,
[0007] In the formula, U nm θ represents the inverter-side voltage amplitude.n U is the inverter-side voltage phase. gm θ represents the voltage amplitude on the grid side. g α represents the phase of the grid-side voltage, and α is the virtual impedance angle.
[0008] Preferably, the virtual impedance Z consists of a virtual resistance R and a virtual inductance L, and the virtual impedance angle α is specifically expressed as follows:
[0009] ,
[0010] In the formula, ω is the angular frequency.
[0011] Preferably, when the voltage phase and amplitude on the inverter side and the grid side are equal, the active power P and reactive power Q are both controlled to 0.
[0012] Preferably, the method for controlling both the active power P and reactive power Q to be 0 is as follows: Rated active power P ref and rated reactive power Q ref Set all to 0, and set P ref Q ref The difference between the virtual active power P and the virtual reactive power Q is calculated separately. After passing through a PI regulator, the frequency compensation value Δω and the voltage compensation value ΔU are obtained. Then, the obtained frequency compensation value Δω and voltage compensation value ΔU are summed with the angular frequency ω and voltage U of the virtual impedance. After gaining, the current virtual active power P and virtual reactive power Q are obtained and fed back to P. ref and Q ref Ultimately, through closed-loop regulation, both the virtual active power P and the virtual reactive power Q are adjusted to zero.
[0013] Preferably, when the grid-connected switch S g When disconnecting, if there is a grid connection requirement, the system first closes the enable switch S. After compensating and correcting the voltage parameters to ensure that the voltage across the enable switch S is within the allowable deviation range, the grid connection switch S is then closed. g The grid connection operation will be completed, and the enable switch S will be disconnected after grid connection.
[0014] Beneficial effects: The present invention has the following advantages: 1. The present invention can automatically track the amplitude and phase of the grid voltage to achieve self-synchronization operation; by bridging the inverter and the grid with a virtual impedance and controlling the virtual active power and virtual reactive power generated by the virtual impedance, the grid-connected inverter can achieve seamless switching from off-grid operation to grid-connected operation; 2. The present invention can achieve the switching between grid-connected and off-grid conditions without a phase-locked loop. The seamless switching control strategy between grid-connected and off-grid conditions is simple and reliable, easy to implement in a digital system, and improves the stability and safety of the inverter operation process. Attached Figure Description
[0015] Figure 1A model diagram of a grid-connected inverter with virtual impedance added;
[0016] Figure 2 A simplified model diagram of a grid-connected inverter under virtual power;
[0017] Figure 3 This is a block diagram of the active power compensation control strategy;
[0018] Figure 4 This is a block diagram of the reactive power compensation control strategy;
[0019] Figure 5 This refers to the control strategy for grid-connected inverters. Detailed Implementation
[0020] The technical solution of the present invention will be described in detail below with reference to the embodiments and accompanying drawings.
[0021] like Figure 1 The figure shows a grid-connected inverter model based on virtual power control, where U dc This is the DC bus voltage; an LC filter composed of L and C is connected between the inverter and the grid; Z g This refers to the impedance of the power grid transmission lines. The switching of grid-connected inverter operating conditions includes:
[0022] Grid-connected to off-grid switching: The load switch between the inverter and the grid is closed, and the grid-connected switch S... g After disconnection and off-grid operation, the power generated by the inverter flows to the load, avoiding the flow of power to the DC bus capacitor C. dc Accumulate knowledge and experience.
[0023] Off-grid to grid-connected operation switching: When the inverter is off-grid, the off-grid system (the system consisting of the inverter and the load) autonomously maintains the system frequency and phase. When connected to the grid, the voltage amplitude and phase on both the inverter side and the grid side are collected. The virtual active power P and reactive power Q flowing across the virtual impedance Z between the inverter and the grid are calculated, and P and Q are adjusted to zero. At this time, there is no power flow between the inverter and the grid, and the voltage on the inverter side and the grid side are consistent. The grid-connected switch S is then closed. g This enables seamless switching of the inverter from off-grid operation to grid-connected operation.
[0024] like Figure 2 As shown, U nm θ represents the inverter-side voltage amplitude. n Z represents the inverter-side voltage phase. n The inverter-side impedance; U gm θ represents the voltage amplitude on the grid side. g Z represents the phase of the grid-side voltage. gLet Sg be the grid-side impedance; ω be the grid-connected switch; ω be the angular frequency; and Z be the virtual impedance applied between the inverter and the grid, through which flows active power P and reactive power Q. The virtual impedance Z consists of virtual resistance R and virtual inductance L, and the virtual impedance angle is α. Then:
[0025] ,
[0026] The active power P and reactive power Q flowing through the virtual impedance Z are expressed as follows:
[0027] ,
[0028] As shown in the above equation, the virtual active power P is mainly affected by the voltage phase difference, while the effect of the voltage amplitude difference is relatively small. Therefore, the virtual active power P can be controlled to achieve the purpose of controlling the voltage phase on the inverter side. To simplify the calculation, the virtual impedance angle α is set as... That is, the virtual impedance only contains the virtual inductance L. Substituting the virtual impedance angle α into the above equation, we can obtain the following equation:
[0029] ,
[0030] As shown in the above equation, to achieve a smooth transition from islanded to grid-connected operation mode for a DC voltage synchronous control grid-connected inverter, the voltage phase and amplitude on both the inverter side and the grid side must be equal, i.e., U nm =U gm And θ n =θ g Therefore, both active power P and reactive power Q can be controlled to 0.
[0031] Based on the above analysis, the block diagram of the inverter's seamless grid-to-off-grid switching control strategy based on virtual power control can be obtained, as follows: Figure 3 , Figure 4 As shown. Wherein, the rated active power P ref and rated reactive power Q ref Set all to 0, and set P ref Q ref The difference between the virtual active power P and the virtual reactive power Q is calculated separately. After passing through a PI regulator, the frequency compensation value Δω and the voltage compensation value ΔU are obtained. Then, the obtained frequency compensation value Δω and voltage compensation value ΔU are summed with the angular frequency ω and voltage U of the virtual impedance. After gaining, the current virtual active power P and virtual reactive power Q are obtained and fed back to P. ref and Q ref Ultimately, by adjusting both the virtual active power P and the virtual reactive power Q to zero through closed-loop regulation, the grid connection requirements can be met, thereby achieving a seamless switch from off-grid to grid-connected operation.
[0032] like Figure 5 As shown, when Sg When disconnecting, if grid connection is required, the system must first compensate and correct the voltage parameters to ensure that the voltage across enable switch S is within the allowable deviation range before closing S. g For grid connection operations, S needs to be disconnected after grid connection.
Claims
1. A method for seamless on-grid / off-grid switching control of inverters based on virtual power control, characterized in that, An LC filter is connected between the inverter and the grid. When the inverter switches from grid-connected to off-grid operation, the load switch between the inverter and the grid is closed, and the grid-connected switch S... g After disconnection and off-grid operation, the power generated by the inverter flows to the load. When switching from off-grid to grid-connected operation, the voltage amplitude and phase on both the inverter side and the grid side are collected. The virtual active power P and reactive power Q flowing across the virtual impedance Z between the inverter and the grid are calculated, and P and Q are adjusted to zero. The grid-connected switch S is then closed. g This enables seamless switching of the inverter from off-grid operation to grid-connected operation.
2. The inverter-to-grid seamless switching control method based on virtual power control according to claim 1, characterized in that, The active power P and reactive power Q flowing through the virtual impedance Z are expressed as: ， In the formula, U nm θ represents the voltage amplitude on the inverter side. n U is the inverter-side voltage phase. gm Let θ be the voltage amplitude on the grid side. g α represents the phase of the grid-side voltage, and α is the virtual impedance angle.
3. The inverter-to-grid seamless switching control method based on virtual power control according to claim 1, characterized in that, The virtual impedance Z consists of a virtual resistance R and a virtual inductance L, and the virtual impedance angle α is specifically expressed as follows: ， In the formula, ω is the angular frequency.
4. The inverter-to-grid seamless switching control method based on virtual power control according to claim 1, characterized in that, When the voltage phase and amplitude on the inverter side and the grid side are equal, the active power P and reactive power Q are both controlled to 0.
5. The inverter-to-grid seamless switching control method based on virtual power control according to claim 4, characterized in that, The method to control both the active power P and reactive power Q to 0 is as follows: Rated active power P ref and rated reactive power Q ref Set all values to 0, and set P to 0. ref Q ref The difference between the virtual active power P and the virtual reactive power Q is calculated separately. After passing through a PI regulator, the frequency compensation value Δω and the voltage compensation value ΔU are obtained. Then, the obtained frequency compensation value Δω and voltage compensation value ΔU are summed with the angular frequency ω and voltage U of the virtual impedance. After gaining, the current virtual active power P and virtual reactive power Q are obtained and fed back to P. ref and Q ref Ultimately, through closed-loop regulation, both the virtual active power P and the virtual reactive power Q are adjusted to zero.
6. The inverter on-grid / off-grid seamless switching control method based on virtual power control according to claim 4, characterized in that, When the grid-connected switch S g When disconnecting, if there is a grid connection requirement, the system first closes the enable switch S. After compensating and correcting the voltage parameters to ensure that the voltage across the enable switch S is within the allowable deviation range, the grid connection switch S is then closed. g The grid connection operation will be completed, and the enable switch S will be disconnected after grid connection.