MMC-HVDC converter station oriented network construction type and network following type control seamless switching method and device

Abstract

The invention discloses a network construction type and network following type control seamless switching method and device oriented to an MMC-HVDC converter station, relates to the technical field of power system control, and aims at realizing seamless switching of network construction type and network following type control by constructing unified initialization setting of outer ring reference current and phase angle and voltage amplitude on the premise of not changing inner ring current control. And the continuity of the power angle and the amplitude of the output voltage in the control mode switching process is ensured. The technology can quickly respond to changes of power grid conditions, and system disturbance and instability caused by mode switching are avoided. Simulation results show that the technology effectively reduces the impact of control switching on the system, significantly improves the stability and reliability of the system, is suitable for flexible direct current transmission systems under various power grid conditions, and especially has significant advantages in weak power grid access and frequency support scenes.

Description

Technical Field

[0001] The present invention relates to the technical field of power system control, and in particular to a method and device for seamless switching between a network-building type and a network-following type control for an MMC-HVDC converter station. Background Art

[0002] Modular multilevel converter (MMC) high voltage direct current (HVDC) systems are widely used in power grids, especially in the context of rapid development of new energy, and can effectively connect large-scale renewable energy. With its flexibility and high efficiency, MMC-HVDC systems have become an ideal choice for weak grid access.

[0003] As the proportion of new energy access increases, the inertia of the power grid decreases. It is necessary to provide inertia support for the power grid through virtual synchronous machine (VSG) control technology to simulate the characteristics of synchronous generators. MMC-HVDC converter stations usually need to switch between grid-building control and grid-following control to cope with different grid conditions. Grid-building control is suitable for weak grids and provides voltage and frequency support; grid-following control is suitable for strong grids and controls system current. However, there are currently few studies on the seamless switching technology between these two control modes. Most of the existing research focuses on switching between grid-connected and off-grid modes, and research on switching between grid-building and grid-following is still insufficient. How to ensure the stability of voltage and power during the switching process is the focus of current research. Summary of the invention

[0004] In order to overcome the deficiencies of the above technologies, the present invention provides a method and device for seamless switching of grid-forming and grid-following control for MMC-HVDC converter stations, which can quickly respond to changes in grid conditions and avoid system disturbances and instability caused by mode switching.

[0005] The technical solution adopted by the present invention to overcome the technical problems is:

[0006] A method for seamless switching between a grid-building type and a grid-following type control for an MMC-HVDC converter station, comprising:

[0007] S1. Operate the MMC-HVDC system, analyze the current operating status of the MMC-HVDC system and grid conditions, and obtain raw grid data;

[0008] S2. Provide virtual inertia to the MMC-HVDC system;

[0009] S3. Construct expressions for active power and frequency;

[0010] S4. Based on the expression of active power and frequency, the active power reference value after frequency modulation with active power and frequency droop control is constructed;

[0011] S5. Obtaining the active power controller output phase angle θ based on the active power reference value;

[0012] S6. Calculate the voltage and reactive power regulation component ΔU based on the voltage and reactive power droop control q And the voltage regulation component ΔU based on voltage control v ;

[0013] S7. Voltage and reactive power regulation component ΔU based on voltage and reactive power droop control q And the voltage regulation component ΔU based on voltage control v The output voltage amplitude U of the reactive power controller is calculated m ;

[0014] S8. Based on the active controller output phase angle θ and the reactive controller output voltage amplitude U m The reference current of the grid-forming MMC-HVDC converter station and the d-axis reference current i of the grid-following MMC-HVDC converter station are obtained. d,ref and q-axis reference current i q,ref ;

[0015] S9. When the MMC-HVDC converter station switches from grid-forming control to grid-following control, the active controller output phase angle θ and reactive controller output voltage amplitude U of the current grid-forming control are used. m As the initial value of the grid-following control, when the MMC-HVDC converter station switches from the grid-following control to the grid-forming control, the current grid-following control PLL phase angle and the measured grid-side voltage amplitude are used as the initial value of the grid-forming control, and the d-axis reference current i of the grid-following MMC-HVDC converter station is converted to d,ref and q-axis reference current i q,ref Input to the inner loop current controller.

[0016] Furthermore, the virtual inertia in step S2 is a mechanical characteristic equation and an electromagnetic characteristic equation based on virtual synchronous machine control.

[0017] Furthermore, the mechanical characteristic equation and electromagnetic characteristic equation based on the virtual synchronous machine control are:

[0018] Where T m is the mechanical torque, T e is the electromagnetic torque, D is the damping coefficient, ω is the angular frequency of the synchronous machine, ω0 is the rated angular frequency of the system, J is the moment of inertia, and θ is the mechanical angle of the synchronous machine.

[0019] Furthermore, the expressions of active power and frequency in step S3 are: Where P m is the synchronous machine input power, Pe is the output power of the synchronous machine.

[0020] Furthermore, through the formula P m ′=P ref +k ω (ω0-ω) calculates the active power reference value P after frequency modulation m ′, where P ref The synchronous machine input power P m In the VSG controller, it is equivalent to the reference value of active power, k ω is the frequency gain coefficient.

[0021] Further, step S5 includes the following steps:

[0022] S5-1. Set the active power reference value P m ′ is introduced into the frequency droop control, and the equation is obtained:

[0023]

[0024] S5-2. Input the equation into the PI controller to obtain the active power controller output phase angle θ.

[0025] Further, step S6 includes the following steps:

[0026] S6-1. By formula ΔU q =k q (Q ref -Q) calculates the voltage and reactive power droop control based on voltage and reactive power to obtain the voltage reactive power regulation component ΔU q , where k q is the reactive power regulation coefficient, Q ref is the MMC reactive power reference value, Q is the MMC measured value;

[0027] S6-2. By formula ΔU v =k v (Q acref -U ac ) Calculate the voltage regulation component ΔU based on voltage control v , where k v is the voltage regulation gain factor, Q acref is the change of the AC side voltage reference value, U ac is the measured value of the AC side voltage.

[0028] Further, in step S7, by formula U m =U n +ΔU q +ΔU v The output voltage amplitude U of the reactive power controller is calculated m , where Un is the reference voltage.

[0029] Further, step S8 includes the following steps:

[0030] S8-1. When the control mode of the MMC-HVDC converter station is the grid-forming type, the output phase angle θ of the active controller and the output voltage amplitude U of the reactive controller are m A reference voltage is obtained using a sine function and input into two PI controllers to generate a reference current in dq coordinates;

[0031] S8-2. When the control mode of the MMC-HVDC converter station is the grid-following type, the formula The d-axis reference current i of the grid-following MMC-HVDC converter station is calculated d,ref , where V d is the voltage dq axis component obtained by abc transformation of the actual three-phase voltage Vabc of the measured power grid, V q is the voltage dq axis component obtained by dq transformation of the actual three-phase voltage Vabc of the measured power grid; S8-3. When the control mode of the MMC-HVDC converter station is the grid-following type, the formula is used The q-axis reference current i of the grid-following MMC-HVDC converter station is calculated q,ref .

[0032] On the other hand, the present invention also relates to a seamless switching device for network-building type and network-following type control for MMC-HVDC converter station, comprising: a processor, a communication interface, a memory and a communication bus, wherein the processor, the communication interface and the memory communicate with each other through the communication bus; wherein:

[0033] The memory is used to store computer programs;

[0034] The processor is configured to execute, by running a program stored in the memory:

[0035] Run the MMC-HVDC system, analyze the current operating status of the MMC-HVDC system and grid conditions, and obtain raw grid data;

[0036] Provide virtual inertia to the MMC-HVDC system;

[0037] Construct expressions for active power and frequency;

[0038] Based on the expressions of active power and frequency, the active power reference value after frequency modulation with droop control of active power and frequency added is constructed;

[0039] Based on the active power reference value, the active power controller output phase angle θ is obtained;

[0040] Calculate the voltage and reactive power regulation component ΔU based on voltage and reactive power droop control q And the voltage regulation component ΔU based on voltage control v ;

[0041] Voltage and reactive power regulation component ΔU using droop control based on voltage and reactive power q And the voltage regulation component ΔU based on voltage control v The output voltage amplitude U of the reactive power controller is calculated m ;

[0042] Based on the active controller output phase angle θ and the reactive controller output voltage amplitude U m The reference current of the grid-forming MMC-HVDC converter station and the d-axis reference current i of the grid-following MMC-HVDC converter station are obtained. d,ref and q-axis reference current i q,ref ;

[0043] When the MMC-HVDC converter station switches from grid-forming control to grid-following control, the active controller output phase angle θ and reactive controller output voltage amplitude U of the current grid-forming control are used. m As the initial value of the grid-following control, when the MMC-HVDC converter station switches from the grid-following control to the grid-forming control, the current grid-following control PLL phase angle and the measured grid-side voltage amplitude are used as the initial value of the grid-forming control, and the d-axis reference current i of the grid-following MMC-HVDC converter station is converted to d,ref and q-axis reference current i q,ref Input to the inner loop current controller.

[0044] The beneficial effect of the present invention is that, without changing the inner loop current control, by constructing a unified outer loop reference current and the initialization settings of the phase angle and voltage amplitude, the continuity of the power angle and amplitude of the output voltage during the control mode switching process is ensured. This technology can quickly respond to changes in grid conditions and avoid system disturbances and instability caused by mode switching. Simulation results show that this technology effectively reduces the impact of control switching on the system, significantly improves the stability and reliability of the system, and is suitable for flexible DC transmission systems under various grid conditions, especially in the scenarios of weak grid access and frequency support. DETAILED DESCRIPTION

[0045] The present invention is further described below.

[0046] Example 1

[0047] A method for seamless switching between a grid-building type and a grid-following type control for an MMC-HVDC converter station, comprising:

[0048] S1. Operate the MMC-HVDC system, analyze the current operating status of the MMC-HVDC system and grid conditions, and obtain raw grid data.

[0049] S2. Provide virtual inertia to the MMC-HVDC system. Adopt virtual synchronous machine control principle, based on the mechanical characteristic equation and electromagnetic characteristic equation of synchronous machine as its control model.

[0050] S3. Construct expressions for active power and frequency.

[0051] S4. Based on the expressions of active power and frequency, a frequency-modulated active power reference value with droop control of active power and frequency is constructed.

[0052] S5. Obtain an active power controller output phase angle θ based on the active power reference value.

[0053] S6. Calculate the voltage and reactive power regulation component ΔU based on the voltage and reactive power droop control q And the voltage regulation component ΔU based on voltage control v .

[0054] S7. Voltage and reactive power regulation component ΔU based on voltage and reactive power droop control q And the voltage regulation component ΔU based on voltage control v The output voltage amplitude U of the reactive power controller is calculated m .

[0055] S8. Based on the active controller output phase angle θ and the reactive controller output voltage amplitude U m The reference current of the grid-forming MMC-HVDC converter station and the d-axis reference current i of the grid-following MMC-HVDC converter station are obtained. d,ref and q-axis reference current i q,ref .

[0056] S9. When the MMC-HVDC converter station switches from grid-forming control to grid-following control, the active controller output phase angle θ and reactive controller output voltage amplitude U of the current grid-forming control are used. m As the initial value of the grid-following control, when the MMC-HVDC converter station switches from the grid-following control to the grid-forming control, the current grid-following control PLL phase angle and the measured grid-side voltage amplitude are used as the initial value of the grid-forming control, and the d-axis reference current i of the grid-following MMC-HVDC converter station is converted to d,ref and q-axis reference current i q,ref Input to the inner loop current controller.

[0057] Implement control switching. During the control mode switching period, keep the inner loop current control unchanged, gradually apply the new reference value, and smoothly transition to the target control mode. When switching from grid-forming control to grid-following control, the phase angle generated by the grid-forming control is used as the initial value of the grid-following control. The reference value of PQ remains unchanged, and the reference current i d,ref and the reference current i q,ref When the grid-following control switches to the grid-forming control, the active power controller outputs the phase angle θ as the initial value of the grid-forming control phase angle, and the grid-side voltage amplitude measured by the grid-following control is used as the reference value of the grid-forming control voltage amplitude. d,ref and the reference current i q,ref Switch to the reference current i determined by the grid-type control d,ref and the reference current i q,ref Input into the current inner loop controller, the same current control link can be applied to two different control modes. Finally, the MMC internal controller calculates the MMC voltage modulation signal, determines the switch signal through PWM, monitors the power angle and amplitude of the output voltage during the entire switching process, ensures its continuity, and avoids mutations. After successful switching, continue to monitor the system status in real time to ensure stable operation of the system in the new mode.

[0058] In this embodiment, the virtual inertia in step S2 is a mechanical characteristic equation and an electromagnetic characteristic equation based on virtual synchronous machine control.

[0059] In this embodiment, the mechanical characteristic equation and electromagnetic characteristic equation based on virtual synchronous machine control are:

[0060] Where T m is the mechanical torque, T e is the electromagnetic torque, D is the damping coefficient, ω is the angular frequency of the synchronous machine, ω0 is the rated angular frequency of the system, J is the moment of inertia, and θ is the mechanical angle of the synchronous machine. The moment of inertia J and the damping coefficient D are adjustable control parameters. Adjusting the moment of inertia J can adjust the inertia of the system, while adjusting the damping coefficient D can adjust the damping of the system to improve robustness.

[0061] In this embodiment, the expressions of active power and frequency in step S3 are: Where P m is the synchronous machine input power, P e is the output power of the synchronous machine. VSG simulates the rotor motion equation of the synchronous machine, making VSG have virtual inertia and damping characteristics.

[0062] The expression of active power and frequency is The rotor motion equation of the synchronous machine described in FIG. 1 makes the VSG have virtual inertia and damping characteristics. By adding active power and frequency droop control, the active power reference value after frequency modulation can be obtained. Therefore, in this embodiment, the formula P m ′=P ref +k ω (ω0-ω) calculates the active power reference value P after frequency modulation m ′, where P ref The synchronous machine input power P m In the VSG controller, it is equivalent to the reference value of active power, k ω is the frequency gain coefficient. When the frequency of the AC system deviates, the active power reference value P after frequency modulation is adjusted. m ′, the active output of the system can be changed to adjust the frequency of the system, providing a stable frequency for subsequent seamless switching.

[0063] In this embodiment, step S5 includes the following steps:

[0064] S5-1. Set the active power reference value P m ′ is introduced into the frequency droop control, and the equation is obtained:

[0065]

[0066] S5-2. Input the equation into the PI controller to obtain the active power controller output phase angle θ.

[0067] In this embodiment, step S6 includes the following steps:

[0068] S6-1. In order to achieve the voltage regulation function similar to the synchronous machine, the simulated VSG simulates the excitation system of the synchronous machine by building a voltage-reactive power control loop. The voltage and reactive power of the system are controlled by controlling the output voltage of the MMC. q =k q (Q ref -Q) calculates the voltage and reactive power droop control based on voltage and reactive power to obtain the voltage reactive power regulation component ΔU q , where k q is the reactive power regulation coefficient, Q ref is the MMC reactive power reference value, and Q is the MMC measured value.

[0069] S6-2. By formula ΔU v =k v (Q acref -U ac ) Calculate the voltage regulation component ΔU based on voltage control v , where k v is the voltage regulation gain factor, Q acrefis the change of the AC side voltage reference value, U ac is the measured value of the AC side voltage.

[0070] In this embodiment, in step S7, the formula U m =U n +ΔU q +ΔU v The output voltage amplitude U of the reactive power controller is calculated m , where U n It is the reference voltage, which serves as the reference voltage for VSG control and the voltage basis for subsequent generation of seamless switching reference current.

[0071] In this embodiment, step S8 includes the following steps:

[0072] S8-1. When the control mode of the MMC-HVDC converter station is the grid-forming type, the output phase angle θ of the active controller and the output voltage amplitude U of the reactive controller are m The reference voltage is obtained using a sine function and is input into two PI controllers to generate a reference current in dq coordinates.

[0073] S8-2. When the control mode of the MMC-HVDC converter station is the grid-following type, the formula The d-axis reference current i of the grid-following MMC-HVDC converter station is calculated d,ref , where V d is the voltage dq axis component obtained by abc transformation of the actual three-phase voltage Vabc of the measured power grid, V q is the voltage dq axis component obtained by dq transformation of the actual three-phase voltage Vabc of the measured power grid. S8-3. When the control mode of the MMC-HVDC converter station is the grid-following type, the formula The q-axis reference current i of the grid-following MMC-HVDC converter station is calculated q,ref .

[0074] By designing a unified outer loop reference current for grid-building and grid-following control, the power angle and amplitude continuity of the converter station output voltage during the switching process are ensured. The current control mode (grid-building or grid-following) of the converter station and its corresponding output state (phase angle and voltage amplitude) are determined. Then the initial current reference value of the target control mode to be switched to is calculated.

[0075] Example 2

[0076] A seamless switching device for network-building type and network-following type control for MMC-HVDC converter station, comprising: a processor, a communication interface, a memory and a communication bus, wherein the processor, the communication interface and the memory communicate with each other through the communication bus; wherein:

[0077] The memory is used to store computer programs;

[0078] The processor is configured to execute, by running a program stored in the memory:

[0079] Run the MMC-HVDC system, analyze the current operating status of the MMC-HVDC system and grid conditions, and obtain the original grid data.

[0080] Provides virtual inertia to the MMC-HVDC system.

[0081] Construct expressions for active power and frequency.

[0082] The active power reference value after frequency modulation with active power and frequency droop control added is constructed based on the expressions of active power and frequency.

[0083] The active power controller output phase angle θ is obtained based on the active power reference value.

[0084] Calculate the voltage and reactive power regulation component ΔU based on voltage and reactive power droop control q And the voltage regulation component ΔU based on voltage control v .

[0085] Voltage and reactive power regulation component ΔU using droop control based on voltage and reactive power q And the voltage regulation component ΔU based on voltage control v The output voltage amplitude U of the reactive power controller is calculated m .

[0086] Based on the active controller output phase angle θ and the reactive controller output voltage amplitude U m The reference current of the grid-forming MMC-HVDC converter station and the d-axis reference current i of the grid-following MMC-HVDC converter station are obtained. d,ref and q-axis reference current i q,ref .

[0087] When the MMC-HVDC converter station switches from grid-forming control to grid-following control, the active controller output phase angle θ and reactive controller output voltage amplitude U of the current grid-forming control are used. m As the initial value of the grid-following control, when the MMC-HVDC converter station switches from the grid-following control to the grid-forming control, the current grid-following control PLL phase angle and the measured grid-side voltage amplitude are used as the initial value of the grid-forming control, and the d-axis reference current i of the grid-following MMC-HVDC converter station is converted to d,ref and q-axis reference current i q,ref Input to the inner loop current controller.

[0088] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for seamless switching between grid-building type and grid-following type control for MMC-HVDC converter stations, characterized in that: include: S1. Operate the MMC-HVDC system, analyze the current operating status of the MMC-HVDC system and grid conditions, and obtain raw grid data; S2. Provide virtual inertia to the MMC-HVDC system; S3. Construct expressions for active power and frequency; S4. Based on the expression of active power and frequency, the active power reference value after frequency modulation with active power and frequency droop control is constructed; S5. Obtaining the active power controller output phase angle θ based on the active power reference value; S6. Calculate the voltage and reactive power regulation component ΔU based on the voltage and reactive power droop control q And the voltage regulation component ΔU based on voltage control v ; S7. Voltage and reactive power regulation component ΔU based on voltage and reactive power droop control q And the voltage regulation component ΔU based on voltage control v The output voltage amplitude U of the reactive power controller is calculated m ; S8. Based on the active controller output phase angle θ and the reactive controller output voltage amplitude U m The reference current of the grid-forming MMC-HVDC converter station and the d-axis reference current i of the grid-following MMC-HVDC converter station are obtained. d,ref and q-axis reference current i q,ref ; S9. When the MMC-HVDC converter station switches from grid-forming control to grid-following control, the active controller output phase angle θ and reactive controller output voltage amplitude U of the current grid-forming control are used. m As the initial value of the grid-following control, when the MMC-HVDC converter station switches from the grid-following control to the grid-forming control, the current grid-following control PLL phase angle and the measured grid-side voltage amplitude are used as the initial value of the grid-forming control, and the d-axis reference current i of the grid-following MMC-HVDC converter station is converted to d,ref and q-axis reference current i q,ref Input to the inner loop current controller.

2. The method for seamless switching between the grid-building type and the grid-following type control for the MMC-HVDC converter station according to claim 1 is characterized in that: The virtual inertia in step S2 is a mechanical characteristic equation and an electromagnetic characteristic equation based on virtual synchronous machine control.

3. The method for seamless switching between the network-building type and the network-following type control for the MMC-HVDC converter station according to claim 2 is characterized in that: The mechanical characteristic equation and electromagnetic characteristic equation based on virtual synchronous machine control are: Where T m is the mechanical torque, T e is the electromagnetic torque, D is the damping coefficient, ω is the angular frequency of the synchronous machine, ω0 is the rated angular frequency of the system, J is the moment of inertia, and θ is the mechanical angle of the synchronous machine.

4. The method for seamless switching between the grid-building type and the grid-following type control for the MMC-HVDC converter station according to claim 3 is characterized in that: The expressions of active power and frequency in step S3 are: Where P m is the synchronous machine input power, P e is the output power of the synchronous machine.

5. The method for seamless switching between the network-building type and the network-following type control for the MMC-HVDC converter station according to claim 4 is characterized in that: By formula P m ′=P ref +k ω (ω0-ω) calculates the active power reference value P after frequency modulation m ′, where P ref The synchronous machine input power P m In the VSG controller, it is equivalent to the reference value of active power, k ω is the frequency gain coefficient.

6. The method for seamless switching between grid-building type and grid-following type control for MMC-HVDC converter station according to claim 5 is characterized in that: Step S5 includes the following steps: S5-1. Set the active power reference value P m ′ is introduced into the frequency droop control, and the equation is obtained: S5-2. Input the equation into the PI controller to obtain the active power controller output phase angle θ.

7. The method for seamless switching between grid-building type and grid-following type control for MMC-HVDC converter station according to claim 1, characterized in that: Step S6 includes the following steps: S6-1. By formula ΔU q =k q (Q ref -Q) calculates the voltage and reactive power droop control based on voltage and reactive power to obtain the voltage reactive power regulation component ΔU q , where k q is the reactive power regulation coefficient, Q ref is the MMC reactive power reference value, Q is the MMC measured value; S6-2. By formula ΔU v =k v (Q acref -U ac ) Calculate the voltage regulation component ΔU based on voltage control v , where k v is the voltage regulation gain factor, Q acref is the change of the AC side voltage reference value, U ac is the measured value of the AC side voltage.

8. The method for seamless switching between the grid-building type and the grid-following type control for the MMC-HVDC converter station according to claim 7 is characterized in that: In step S7, the formula U m =U n +ΔU q +ΔU v The output voltage amplitude U of the reactive power controller is calculated m , where U n is the reference voltage.

9. The method for seamless switching between grid-building type and grid-following type control for MMC-HVDC converter station according to claim 7, characterized in that: Step S8 includes the following steps: S8-1. When the control mode of the MMC-HVDC converter station is the grid-forming type, the output phase angle θ of the active controller and the output voltage amplitude U of the reactive controller are m A reference voltage is obtained using a sine function and input into two PI controllers to generate a reference current in dq coordinates; S8-2. When the control mode of the MMC-HVDC converter station is the grid-following type, the formula The d-axis reference current i of the grid-following MMC-HVDC converter station is calculated d,ref , where V d is the voltage dq axis component obtained by abc transformation of the actual three-phase voltage Vabc of the measured power grid, V q is the voltage dq axis component obtained by dq transformation of the actual three-phase voltage Vabc of the measured power grid; S8-3. When the control mode of the MMC-HVDC converter station is the grid-following type, the formula is used The q-axis reference current i of the grid-following MMC-HVDC converter station is calculated q,ref .

10. A seamless switching device for network construction type and network following type control for MMC-HVDC converter station, characterized in that: include: A processor, a communication interface, a memory and a communication bus, wherein the processor, the communication interface and the memory communicate with each other via the communication bus; wherein: The memory is used to store computer programs; The processor is configured to execute, by running a program stored in the memory: Run the MMC-HVDC system, analyze the current operating status of the MMC-HVDC system and grid conditions, and obtain raw grid data; Provide virtual inertia to the MMC-HVDC system; Construct expressions for active power and frequency; Based on the expressions of active power and frequency, the active power reference value after frequency modulation with droop control of active power and frequency added is constructed; Based on the active power reference value, the active power controller output phase angle θ is obtained; Calculate the voltage and reactive power regulation component ΔU based on voltage and reactive power droop control q And the voltage regulation component ΔU based on voltage control v ; Voltage and reactive power regulation component ΔU using droop control based on voltage and reactive power q And the voltage regulation component ΔU based on voltage control v The output voltage amplitude U of the reactive power controller is calculated m ; Based on the active controller output phase angle θ and the reactive controller output voltage amplitude U m The reference current of the grid-forming MMC-HVDC converter station and the d-axis reference current i of the grid-following MMC-HVDC converter station are obtained. d,ref and q-axis reference current i q,ref ; When the MMC-HVDC converter station switches from grid-forming control to grid-following control, the active controller output phase angle θ and reactive controller output voltage amplitude U of the current grid-forming control are used. m As the initial value of the grid-following control, when the MMC-HVDC converter station switches from the grid-following control to the grid-forming control, the current grid-following control PLL phase angle and the measured grid-side voltage amplitude are used as the initial value of the grid-forming control, and the d-axis reference current i of the grid-following MMC-HVDC converter station is converted to d,ref and q-axis reference current i q,ref Input to the inner loop current controller.

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