A method for switching a converter between on-grid and off-grid modes with the assistance of an inertial link
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- SHANDONG ELECTRIC GRP DIGITAL TECH CO LTD
- Filing Date
- 2026-02-27
- Publication Date
- 2026-06-05
Smart Images
Figure CN122159351A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of new energy power systems, specifically a method for switching between grid-connected and off-grid modes of a converter with inertial link assistance. Background Technology
[0002] Building a new power system based on new energy sources and promoting the low-carbon transformation of energy and power is a key path for my country to achieve its "dual-carbon" goals. Converters are one of the core pieces of equipment in this new power system, serving as crucial devices for AC-DC conversion and power control. However, converters face numerous challenges in actual operation, especially during the switching between grid-connected and off-grid modes. On the one hand, the sudden disappearance of grid support during the switch from grid-connected to off-grid operation leads to a momentary imbalance between power supply and demand, easily causing significant fluctuations in output voltage and frequency, threatening load safety and system stability. On the other hand, if there are deviations in amplitude, phase, or frequency between the local voltage and the grid voltage before the switch from off-grid to grid-connected operation, an inrush current will be generated at the moment of closing, leading to waveform distortion, grid connection failure, or even equipment damage.
[0003] Currently, some solutions to the grid connection / off-grid issue of converters involve analyzing virtual synchronous generator (VRG) control technology to enable energy storage systems to simulate the external characteristics of synchronous generators, achieving inertial simulation and frequency and voltage regulation functions. Others propose droop control strategies and improved pre-synchronization control to achieve seamless switching between grid connection and off-grid operation. VRG control technology simulates the rotor motion equations and electromagnetic characteristics of a synchronous generator, giving the converter inertia and damping, thus providing robust voltage and frequency support when off-grid. However, the introduced inertia also slows down the dynamic tracking speed of grid dispatch commands (such as power references) in grid-connected mode, sacrificing the speed of grid connection follow-up. Droop control and pre-synchronization technologies utilize the droop characteristics of active power-frequency and reactive power-voltage to achieve autonomous power allocation when off-grid, supplemented by pre-synchronization control to reduce grid connection impact. However, in traditional solutions, the controller's bandwidth and parameters are usually fixed or uniformly adjusted, which leads to an inherent contradiction in system characteristics between the two modes: to enhance the voltage support and disturbance rejection capability in off-grid mode (requiring lower control bandwidth and strong filtering), the fast tracking capability of power commands in grid-connected mode will be weakened (requiring higher control bandwidth and fast response); conversely, if grid-connected tracking performance is optimized, the output voltage dynamic quality and load disturbance suppression capability in off-grid mode will decrease. Summary of the Invention
[0004] To address the shortcomings of existing technologies, this invention provides an inertial link-assisted method for dual-mode switching between grid and off-grid operation of a converter. Based on a droop control strategy and a virtual synchronous machine control strategy, an inertial link is added to both the grid-connected and off-grid control algorithms of the converter before the converter current loop. By adjusting the time constant of the inertial link, the grid-connected following capability and the off-grid support capability can be adjusted respectively.
[0005] To solve the aforementioned technical problem, the present invention adopts the following technical solution: a dual-mode switching method for a converter with inertial link assistance, wherein a grid-connected inertial link is added before the current loop during grid connection, and an off-grid inertial link is added before the current loop during off-grid connection. The time constant of the off-grid inertial link is greater than that of the grid-connected inertial link, and the grid-connected following capability and the off-grid support capability are adjusted by the difference in time constant.
[0006] Furthermore, the process for switching from grid connection to off-grid is as follows: S11. Collect the three-phase voltage and three-phase current of the converter; S12. Determine whether to switch to offline mode. If yes, switch to offline mode and execute step S13. If not, continue to operate in parallel mode. S13. Calculate the required output voltage amplitude and angular frequency using a droop control algorithm or a virtual synchronous machine control algorithm; S14. Calculate the current phase angle information, and use Clark transform to obtain the voltage values in the two-phase stationary coordinate system, together with the collected three-phase voltage and three-phase current. , and current value , ,Will , and , The voltage values in the two-phase rotating coordinate system were obtained by Park transformation. , and current value , ; S15, Set voltage setpoint , ,Will As d-axis voltage loop feedback As the d-axis voltage loop reference, the d-axis voltage loop output is calculated using PI calculation. ,Will As q-axis voltage loop feedback, As the q-axis voltage loop reference, the q-axis voltage loop output is calculated using PI calculation. ; S16, Output the d-axis voltage loop. and q-axis voltage loop output As respectively Substitute the off-grid inertia components into the calculations to obtain the output. and ; S17, will As the d-axis current loop setpoint As feedback for the d-axis current loop, the d-axis current loop output is calculated using PI calculation. ,Will As the q-axis current loop setpoint. As the q-axis current loop feedback, the q-axis current loop output is calculated using PI calculation. ; S18, will and Perform inverse Park transform and inverse Clark transform to obtain , , As the PWM duty cycle for phases A, B, and C; S19. Generate the actual PWM signal based on the PWM duty cycle, and control the switching on and off of the power devices of the converter based on the PWM signal.
[0007] Furthermore, the off-grid to parallel conversion process is as follows: S21. Upon receiving the grid connection command, collect the three-phase voltage and current of the converter and the three-phase voltage of the power grid; S22. Use PLL to detect the phase angle and frequency information of the power grid, use the effective value of voltage sampling to detect the power grid voltage, take the power grid frequency as the target frequency and the power grid voltage as the target voltage, and use the droop control algorithm or virtual synchronous machine control algorithm to calculate the voltage amplitude and angular frequency to be output. S23. Calculate the current phase angle information of the converter, and use Clark transform to obtain the voltage values in the two-phase stationary coordinate system, together with the collected three-phase voltage and current of the converter. , and current value , ,Will , and , The voltage values in the rotating coordinate system of the two phase terms were obtained by Park transformation. , and current value , ; S24. Set voltage setpoint , ,Will As d-axis voltage loop feedback As the d-axis voltage loop reference, the d-axis voltage loop output is calculated using PI calculation. ,Will As q-axis voltage loop feedback, As the q-axis voltage loop reference, the q-axis voltage loop output is calculated using PI calculation. ; S25, Output the d-axis voltage loop. and q-axis voltage loop output As respectively Substitute the values into the grid-connected inertia circuitry for calculations to obtain the output. and ; S26, will As the d-axis current loop setpoint As feedback for the d-axis current loop, the d-axis current loop output is calculated using PI calculation. ,Will As the q-axis current loop setpoint. As the q-axis current loop feedback, the q-axis current loop output is calculated using PI calculation. ; S27, will and Perform inverse Park transform and inverse Clark transform to obtain , , As the PWM duty cycle for phases A, B, and C; S28. After the angular frequency, voltage amplitude and phase angle of the converter output are consistent with the power grid, close the circuit breaker to connect to the grid.
[0008] Furthermore, the transfer function for the grid-connected / off-grid inertial link is: , The difference equation for the inertial element is: , , ; Where K is the gain and T is the time constant. The sampling period is This is the output of the inertial circuit. It is the output of the previous inertial link. The time constant of the grid-connected inertial link is made smaller than that of the off-grid inertial link by setting parameters.
[0009] Furthermore, the process of calculating the voltage amplitude and angular frequency using the droop control algorithm is as follows: , In the formula and These are the active power droop coefficient and the reactive power droop coefficient, respectively. and These are the target voltage and the target frequency, respectively. and These represent the rated active power and rated reactive power output of the converter, respectively. and These are the voltage amplitude and angular frequency to be calculated, respectively. and These are the real-time active power and real-time reactive power of the converter, respectively; during grid-connected to off-grid transition... and For a given value, when switching from off-grid to on-grid, and These represent the grid voltage and grid angular frequency.
[0010] Furthermore, the process of calculating the voltage amplitude and angular frequency using the virtual synchronous machine control algorithm is as follows: , , in It is the moment of inertia. It is the angular frequency that the converter needs to output. For the target angular frequency, For the set active power, Where is the current active power, and D is the virtual damping coefficient. It is the output voltage. It is the target voltage. It is the reactive power setpoint. This is the current reactive power; when switching from grid connection to off-grid, and For a given value, when switching from off-grid to on-grid, and These represent the grid voltage and grid angular frequency.
[0011] Furthermore, the transfer functions for the current loop and voltage loop are: , in It is proportional gain. It is the integral gain.
[0012] Furthermore, the voltage setpoint , , This refers to the voltage amplitude that the converter should output.
[0013] Furthermore, step S14 calculates the current phase angle information using the formula: Current phase angle = Original phase angle + angular frequency In step S23, the current phase angle information is calculated by gradually reducing the phase difference between the converter output and the power grid through closed-loop regulation, and integrating the angular frequency calculated in step S22 to obtain the current phase angle.
[0014] Furthermore, in step S12, if the island detection algorithm detects that the network has been disconnected or a disconnection command has been issued, it switches to disconnection mode.
[0015] The beneficial effects of this invention are as follows: Based on a droop control algorithm or a virtual synchronous machine control algorithm, this invention adds a grid-connected inertial link before the current loop during grid-connected operation and an off-grid inertial link before the current loop during off-grid operation. The time constants of the grid-connected and off-grid inertial links are different; the time constant of the inertial link is smaller during grid-connected operation, resulting in better tracking performance, while the time constant is larger during off-grid operation, indicating stronger voltage support capability. Attached Figure Description
[0016] Figure 1 A flowchart for the grid-connected to off-grid transition; Figure 2 This is a flowchart for the off-grid to on-grid conversion process. Detailed Implementation
[0017] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.
[0018] Example 1 This embodiment discloses a converter dual-mode switching method for grid-connected and off-grid operation assisted by an inertial link, introducing an inertial link based on a pseudo-synchronous machine and droop control. A grid-connected inertial link is added before the current loop during grid-connected operation, and an off-grid inertial link is added before the current loop during off-grid operation. The time constants of the grid-connected and off-grid inertial links are different (the time constant for off-grid operation is greater than that for grid-connected operation). The smaller time constant during grid-connected operation results in better tracking performance, while the larger time constant during off-grid operation provides stronger voltage support. Furthermore, the time constants are settable, allowing for more flexible adjustment between grid-connected and off-grid operation.
[0019] like Figure 1 As shown, the grid-connected to off-grid transition process is as follows: S11. The three-phase voltage and three-phase current of the converter are acquired by using the ADC peripheral of the DSP. S12. Determine whether to switch to offline mode. Use the island detection algorithm to detect whether the network is currently offline or whether an external command to disconnect has been issued. If yes, switch to offline mode and execute step S13. If not, continue to operate in network mode. S13. Calculate the required output voltage amplitude and angular frequency using a droop control algorithm or a virtual synchronous machine control algorithm; S14. Calculate the current phase angle information, and use Clark transform to obtain the voltage values in the two-phase stationary coordinate system, together with the collected three-phase voltage and three-phase current. , and current value , ,Will , and , The voltage values in the two-phase rotating coordinate system were obtained by Park transformation. , and current value , ;in , This is the actual voltage feedback value. , This is the actual current feedback value; S15, Set voltage setpoint 、 ,Will As d-axis voltage loop feedback As the d-axis voltage loop reference, the d-axis voltage loop output is calculated using PI calculation. ,Will As q-axis voltage loop feedback, As the q-axis voltage loop reference, the q-axis voltage loop output is calculated using PI calculation. ; S16, Output the d-axis voltage loop. and q-axis voltage loop output As respectively Substitute the off-grid inertia components into the calculations to obtain the output. and ; S17, will As the d-axis current loop setpoint As feedback for the d-axis current loop, the d-axis current loop output is calculated using PI calculation. ,Will As the q-axis current loop setpoint. As the q-axis current loop feedback, the q-axis current loop output is calculated using PI calculation. ; S18, will and Perform inverse Park transform and inverse Clark transform to obtain , , As the PWM duty cycle for phases A, B, and C; S19. Generate the actual PWM signal based on the PWM duty cycle, and control the switching on and off of the power devices of the converter based on the PWM signal.
[0020] like Figure 2As shown, the off-grid to parallel conversion process is as follows: S21. Upon receiving the grid connection command, the converter's three-phase voltage and current, and the grid's three-phase voltage are acquired using the DSP's ADC peripheral. S22. Use PLL to detect the phase angle and frequency information of the power grid, use the effective value of voltage sampling to detect the power grid voltage, take the power grid frequency as the target frequency and the power grid voltage as the target voltage, and use the droop control algorithm or virtual synchronous machine control algorithm to calculate the voltage amplitude and angular frequency to be output. S23. Calculate the current phase angle information of the converter, and use Clark transform to obtain the voltage values in the two-phase stationary coordinate system, together with the collected three-phase voltage and current of the converter. , and current value , ,Will , and , The voltage values in the rotating coordinate system of the two phase terms were obtained by Park transformation. , and current value , ; S24. Set voltage setpoint 、 ,Will As d-axis voltage loop feedback As the d-axis voltage loop reference, the d-axis voltage loop output is calculated using PI calculation. ,Will As q-axis voltage loop feedback, As the q-axis voltage loop reference, the q-axis voltage loop output is calculated using PI calculation. ; S25, Output the d-axis voltage loop. and q-axis voltage loop output As respectively Substitute the values into the grid-connected inertia circuitry for calculations to obtain the output. and ; S26, will As the d-axis current loop setpoint As feedback for the d-axis current loop, the d-axis current loop output is calculated using PI calculation. ,Will As the q-axis current loop setpoint. As the q-axis current loop feedback, the q-axis current loop output is calculated using PI calculation. ; S27, will and Perform inverse Park transform and inverse Clark transform to obtain , , As the PWM duty cycle for phases A, B, and C; S28. After the angular frequency, voltage amplitude and phase angle of the converter output are consistent with the power grid, close the circuit breaker to connect to the grid. S29. After successful grid connection, issue the given active power... ( ) and no work ( The output voltage and frequency are calculated by the droop control algorithm (virtual synchronous machine algorithm), and then the active and reactive power control of the converter when it is connected to the grid is realized through steps S23, S24, S25, S26, S27, and S28.
[0021] In this embodiment, the transfer function of the grid-connected / off-grid inertial link is: , The difference equation for the inertial element is: , , ; Where K is the gain and T is the time constant. The sampling period is This is the output of the inertial circuit. This is the output of the previous inertial circuit. The time constant of the grid-connected inertial circuit is made smaller than that of the off-grid inertial circuit by setting parameters.
[0022] The droop control algorithm calculates the required output voltage of the converter when it is off-grid based on a given voltage amplitude and a given angular frequency. And angular frequency, the calculation process is as follows: , In the formula and These are the active power droop coefficient and the reactive power droop coefficient, respectively. and These are the target voltage and the target frequency, respectively. and These represent the rated active power and rated reactive power output of the converter, respectively. and These are the voltage amplitude and angular frequency to be calculated, respectively. and These are the real-time active power and real-time reactive power of the converter, respectively; during grid-connected to off-grid transition... and For a given value, when switching from off-grid to on-grid, and These represent the grid voltage and grid angular frequency.
[0023] The virtual synchronous machine algorithm calculates the required output voltage based on the current active and reactive power inputs and their current values. And angular frequency, the calculation formula is: , , in It is the moment of inertia. It is the angular frequency that the converter needs to output. For the target angular frequency, For the set active power, Where is the current active power, and D is the virtual damping coefficient. It is the output voltage. It is the target voltage. It is the reactive power setpoint. This is the current reactive power; when switching from grid connection to off-grid, and For a given value, when switching from off-grid to on-grid, and These represent the grid voltage and grid angular frequency.
[0024] In this embodiment, the transfer functions of the current loop and the voltage loop are: , in It is proportional gain. It is the integral gain.
[0025] In this embodiment, the formula for calculating the current phase angle information in step S14 is: Current phase angle = Original phase angle + angular frequency In step S23, the current phase angle information is calculated by gradually reducing the phase difference between the converter output and the power grid through closed-loop regulation, and integrating the angular frequency calculated in step S22 to obtain the current phase angle.
[0026] The Clark and Park transformations used in this embodiment are conventional techniques in the field and will not be described in detail here.
[0027] The above description is merely the basic principle and preferred embodiment of the present invention. Improvements and substitutions made by those skilled in the art based on the present invention are within the scope of protection of the present invention.
Claims
1. A method for inertial link-assisted converter switching between grid and off-grid modes, characterized in that: When connected to the grid, a grid-connected inertial link is added before the current loop; when disconnected from the grid, an off-grid inertial link is added before the current loop. The time constant of the off-grid inertial link is greater than that of the grid-connected inertial link. The different time constants are used to adjust the grid-connected following ability and the off-grid support ability.
2. The inertial link-assisted converter on-grid / off-grid dual-mode switching method according to claim 1, characterized in that: The process for switching from grid connection to off-grid is as follows: S11. Collect the three-phase voltage and three-phase current of the converter; S12. Determine whether to switch to offline mode. If yes, switch to offline mode and execute step S13. If not, continue to operate in parallel mode. S13. Calculate the required output voltage amplitude and angular frequency using a droop control algorithm or a virtual synchronous machine control algorithm; S14. Calculate the current phase angle information, and use Clark transform to obtain the voltage values in the two-phase stationary coordinate system, together with the collected three-phase voltage and three-phase current. , and current value , ,Will , and , The voltage values in the two-phase rotating coordinate system were obtained by Park transformation. , and current value , ; S15, Set voltage setpoint , ,Will As d-axis voltage loop feedback As the d-axis voltage loop reference, the d-axis voltage loop output is calculated using PI calculation. ,Will As q-axis voltage loop feedback, As the q-axis voltage loop reference, the q-axis voltage loop output is calculated using PI calculation. ; S16, Output the d-axis voltage loop. and q-axis voltage loop output As respectively Substitute the off-grid inertia components into the calculations to obtain the output. and ; S17, will As the d-axis current loop setpoint As feedback for the d-axis current loop, the d-axis current loop output is calculated using PI calculation. ,Will As the q-axis current loop setpoint. As the q-axis current loop feedback, the q-axis current loop output is calculated using PI calculation. ; S18, will and Perform inverse Park transform and inverse Clark transform to obtain , , As the PWM duty cycle for phases A, B, and C; S19. Generate the actual PWM signal based on the PWM duty cycle, and control the switching on and off of the power devices of the converter based on the PWM signal.
3. The inertial link-assisted converter on-grid / off-grid dual-mode switching method according to claim 1, characterized in that: The process for switching from off-grid to parallel operation is as follows: S21. Upon receiving the grid connection command, collect the three-phase voltage and current of the converter and the three-phase voltage of the power grid; S22. Use PLL to detect the phase angle and frequency information of the power grid, use the effective value of voltage sampling to detect the power grid voltage, take the power grid frequency as the target frequency and the power grid voltage as the target voltage, and use the droop control algorithm or virtual synchronous machine control algorithm to calculate the voltage amplitude and angular frequency to be output. S23. Calculate the current phase angle information of the converter, and use Clark transform to obtain the voltage values in the two-phase stationary coordinate system, together with the collected three-phase voltage and current of the converter. , and current value , ,Will , and , The voltage values in the rotating coordinate system of the two phase terms were obtained by Park transformation. , and current value , ; S24. Set voltage setpoint , ,Will As d-axis voltage loop feedback As the d-axis voltage loop reference, the d-axis voltage loop output is calculated using PI calculation. ,Will As q-axis voltage loop feedback, As the q-axis voltage loop reference, the q-axis voltage loop output is calculated using PI calculation. ; S25, Output the d-axis voltage loop. and q-axis voltage loop output As respectively Substitute the values into the grid-connected inertia circuitry for calculations to obtain the output. and ; S26, will As the d-axis current loop setpoint As feedback for the d-axis current loop, the d-axis current loop output is calculated using PI calculation. ,Will As the q-axis current loop setpoint. As the q-axis current loop feedback, the q-axis current loop output is calculated using PI calculation. ; S27, will and Perform inverse Park transform and inverse Clark transform to obtain , , As the PWM duty cycle for phases A, B, and C; S28. After the angular frequency, voltage amplitude and phase angle of the converter output are consistent with the power grid, close the circuit breaker to connect to the grid.
4. The inertial link-assisted converter on-grid / off-grid dual-mode switching method according to claim 2 or 3, characterized in that: The transfer function of the grid-connected / off-grid inertial link is: ， The difference equation for the inertial element is: , , ; Where K is the gain and T is the time constant. The sampling period is This is the output of the inertial circuit. It is the output of the previous inertial link. The time constant of the grid-connected inertial link is made smaller than that of the off-grid inertial link by setting parameters.
5. The inertial link-assisted converter on-grid / off-grid dual-mode switching method according to claim 2 or 3, characterized in that: The process of calculating voltage amplitude and angular frequency using the droop control algorithm is as follows: ， In the formula and These are the active power droop coefficient and the reactive power droop coefficient, respectively. and These are the target voltage and the target frequency, respectively. and These represent the rated active power and rated reactive power output of the converter, respectively. and These are the voltage amplitude and angular frequency to be calculated, respectively. and These are the real-time active power and real-time reactive power of the converter, respectively; during grid-connected to off-grid transition... and For a given value, when switching from off-grid to on-grid, and These represent the grid voltage and grid angular frequency.
6. The inertial link-assisted converter on-grid / off-grid dual-mode switching method according to claim 2 or 3, characterized in that: The process of calculating voltage amplitude and angular frequency using a virtual synchronous machine control algorithm is as follows: ， ， in It is the moment of inertia. It is the angular frequency that the converter needs to output. For the target angular frequency, For the set active power, Where is the current active power, and D is the virtual damping coefficient. It is the output voltage. It is the target voltage. It is the reactive power setpoint. This is the current reactive power; when switching from grid connection to off-grid, and For a given value, when switching from off-grid to on-grid, and These represent the grid voltage and grid angular frequency.
7. The inertial link-assisted converter on-grid / off-grid dual-mode switching method according to claim 2 or 3, characterized in that: The transfer functions for the current loop and voltage loop are: ， in It is proportional gain. It is the integral gain.
8. The inertial link-assisted converter on-grid / off-grid dual-mode switching method according to claim 2 or 3, characterized in that: Voltage setpoint , , This refers to the voltage amplitude that the converter should output.
9. The inertial link-assisted converter on-grid / off-grid dual-mode switching method according to claim 2 or 3, characterized in that: Step S14 calculates the current phase angle using the formula: Current phase angle = Original phase angle + angular frequency In step S23, the current phase angle information is calculated by gradually reducing the phase difference between the converter output and the power grid through closed-loop regulation, and integrating the angular frequency calculated in step S22 to obtain the current phase angle.
10. The inertial link-assisted converter on-grid / off-grid dual-mode switching method according to claim 2, characterized in that: In step S12, if the island detection algorithm detects that the network has been disconnected or a disconnection command has been issued, it switches to disconnection mode.