Static synchronous phase modifier and submodule voltage control method and system thereof

Abstract

The invention belongs to the field of static synchronous phase modifiers, and particularly relates to a static synchronous phase modifier and a submodule voltage control method and system thereof. The method comprises the steps that when the average voltage value of a total sub-module is smaller than a first set lower limit value or larger than a first set upper limit value, direct-current additional power control is put into use, and when the average voltage value of the total sub-module recovers to be larger than a second set lower limit value or smaller than a second set upper limit value and lasts for a first set duration, direct-current additional power control is quitted, an active power instruction is set as 0; when the total sub-module voltage average value is smaller than a third set lower limit value or larger than a third set upper limit value, the network construction control is switched to the network following control, and when the total sub-module voltage average value recovers to be larger than a fourth set lower limit value or smaller than the fourth set upper limit value and lasts for a second set duration, the network following control is switched to the network construction control. Therefore, the voltage margin of the sub-module can be well considered, and modulation degree saturation or overvoltage of the sub-module and insufficient inertia supporting capability are avoided.

Description

Technical Field

[0001] This invention belongs to the field of static synchronous condensers, specifically relating to a static synchronous condenser and its sub-module voltage control method and system. Background Technology

[0002] The static synchronous condenser is based on VSG control. By expanding the DC side of the SVG and adding a supercapacitor, it achieves short-term active power-frequency support and reactive power-voltage compensation for the new energy base, meets the inertia requirements of the new energy base system, and enhances the frequency and voltage stability of the new energy base system.

[0003] Currently, the active power support function of static synchronous condensers mainly relies on inertia release or energy absorption, but this faces challenges from submodule voltage fluctuations during operation. When the submodule voltage falls below a certain threshold, modulation saturation occurs, causing control functions to fail; conversely, when the voltage exceeds the specified limit, protection mechanisms are triggered, affecting the continuous operation of the equipment. Therefore, precise voltage control of the submodules is crucial. This is a core technical aspect ensuring the safe, stable, and efficient operation of static synchronous condensers, effectively preventing control failures or protection actions caused by voltage anomalies, and providing reliable support for the dynamic stability of the power system.

[0004] Traditional grid-connected SVG submodule voltage control employs a three-layer control architecture, including intra-phase voltage equalization control, inter-phase voltage equalization control, and overall submodule voltage control. Intra-phase voltage equalization control typically uses carrier phase-shift modulation or sequential voltage equalization to adjust the charging and discharging state of individual submodules in real time, avoiding voltage deviations within a single phase caused by differences in device characteristics or fluctuations in operating conditions, thus ensuring the stability of energy distribution within each phase. Inter-phase voltage equalization control typically uses negative-sequence current control or zero-sequence voltage control to dynamically adjust the power distribution of each phase, eliminating inter-phase voltage imbalance and preventing protection actions triggered by excessively high or low voltage in any phase. Overall submodule voltage control typically uses stator voltage control, aiming to stabilize the total voltage of all submodules in the SVG. Through coordinated operation with the system's active and reactive power control, it maintains the dynamic balance of the overall energy of the submodules. For example, Chinese invention patent application publication number CN120033740A discloses a method and system for equalizing the overcapacity multi-branch of a grid-type static synchronous condenser. By generating a modified modulation wave based on the branch voltage drop data output by the proportional-integral controller, the voltage of the grid-type static synchronous condenser is controlled, and the voltage of different overcapacity branches can be dynamically adjusted to achieve voltage equalization between branches.

[0005] However, while static synchronous condensers (SSCs) add supercapacitors on the DC side to enhance energy buffering capabilities on top of the SVG topology, one of their core responsibilities as grid-connected devices is to provide inertia support through rapid energy throughput during grid frequency fluctuations. When the grid frequency drops, energy needs to be released to suppress the frequency drop, leading to capacitor discharge and voltage drop in the submodules; when the frequency rises, excess energy needs to be absorbed, causing voltage rise in the submodules. This dynamic voltage deviation is an inevitable result of achieving inertia support, while the original submodule voltage control strategy forces the voltage to stabilize near the rated value, balancing energy distribution through fine adjustment of submodule switching. Since the two fundamental objectives conflict, if the original strategy is used, the discharging or charging function of the submodules will be forcibly suppressed, hindering effective energy exchange, ultimately weakening the inertia support capability, and even causing mutual interference between control loops. Therefore, the original voltage equalization control strategy is usually unsuitable for the operational requirements of grid-connected scenarios. Summary of the Invention

[0006] The purpose of this invention is to provide a voltage control method and system for a static synchronous condenser and its sub-modules, which solves the problem that voltage control strategies in the prior art are generally unable to adapt to the operational requirements of network construction scenarios.

[0007] To achieve the above objectives, the present invention provides a submodule voltage control method for a static synchronous condenser, comprising:

[0008] When the average voltage of the total submodule is less than the first set lower limit or greater than the first set upper limit, DC additional power control is activated. When the average voltage of the total submodule recovers to a value greater than the second set lower limit or less than the second set upper limit and remains so for a first set duration, DC additional power control is deactivated and the active power command is set to 0. When the average voltage of the total submodule is less than the third set lower limit or greater than the third set upper limit, grid-based control switches to grid-following control. When the average voltage of the total submodule recovers to a value greater than the fourth set lower limit or less than the fourth set upper limit and remains so for a second set duration, grid-following control switches to grid-based control.

[0009] The first, second, third, and fourth lower limits are all less than the per-unit value, while the first, second, third, and fourth upper limits are greater than the per-unit value; the third, first, and second lower limits are successively larger, and the second lower limit is less than or equal to the fourth lower limit; the third, first, and second upper limits are successively smaller, and the second upper limit is greater than or equal to the fourth upper limit.

[0010] Furthermore, it also includes:

[0011] When the average voltage of each phase submodule is less than the fifth set lower limit or greater than the fifth set upper limit, phase-to-phase voltage control is activated; when the average voltage of each phase submodule recovers to a value greater than the sixth set lower limit or less than the sixth set upper limit, and remains so for a third set duration, phase-to-phase voltage control is deactivated.

[0012] The fifth and sixth lower limits are both less than the per-unit value, and the fifth and sixth upper limits are greater than the per-unit value; the fifth lower limit is less than the sixth lower limit, and the sixth upper limit is less than the fifth upper limit.

[0013] Furthermore, the methods for switching from network configuration control to network following control include:

[0014] The angle generated by the three-phase grid voltage through PLL phase-locked loop is used as the grid-following control phase angle. The control architecture adopts a voltage and current dual-loop control structure. The DC voltage of the three-phase sub-module is collected, filtered by the output filter, and the average value of the total sub-module voltage is calculated as the feedback value of the outer loop voltage loop control. The difference between the feedback value and the reference value of the outer loop voltage loop control is used to generate an active current command after passing through the PI controller, and a reactive current command is generated through virtual impedance. The output current of the static synchronous condenser is collected, filtered by the filter, and used as the feedback value of the inner loop current loop control. The difference between the reference value and the feedback value is input to the fundamental wave controller and superimposed with the grid voltage feedforward to output the fundamental wave modulation wave.

[0015] Furthermore, network control includes VSG control, excitation control, and virtual impedance control;

[0016] Methods for generating reactive current commands through virtual impedance control include:

[0017] ;

[0018] In the formula: , These are the d-axis and q-axis components of the three-phase internal potential, respectively. , These are the d-axis and q-axis components of the terminal voltage, respectively. , The d-axis and q-axis components of the stator current are respectively, and R and L are the stator resistance and stator inductance, respectively. Methods for determining the d-axis and q-axis components of the three-phase internal potential through excitation control include:

[0019] ;

[0020] In the formula, This is the unloaded internal potential; , For reactive power closed-loop PI parameters; Q represents reactive power command; Q represents VSG reactive power. This is the voltage regulation coefficient; This is the rated terminal voltage; This represents the amplitude of the terminal voltage.

[0021] Furthermore, the DC additional power control method includes: obtaining an additional active power reference value through DC droop control by means of the difference between the reference DC voltage and the DC voltage feedback value of the main submodule, which is used as the active power reference value for VSG control, and adjusting the output power of the stationary synchronous condenser in real time.

[0022] Furthermore, the methods for determining the d-axis and q-axis components of the terminal voltage include:

[0023] After the three-phase voltage of the power grid is collected and filtered by a filter, the PARK changes are performed according to the angle generated by the VSG control to generate the d and q axis components of the generator terminal voltage.

[0024] VSG-controlled models include:

[0025] ;

[0026] in ;

[0027] In the formula: The angular velocity of the VSG; ω is the rated angular velocity of the VSG; J is the moment of inertia of the VSG; and These represent the mechanical torque and electromagnetic torque of the VSG, respectively; P ref θ is the reference active power; D is the damping coefficient; θ2 is the angle generated by VSG control.

[0028] Furthermore, the methods for switching from grid-following control to grid-connected control include: using a virtual synchronous machine to generate the angle, setting the active power command to 0, using an excitation controller to generate the internal potential, generating active current and reactive current commands through virtual impedance control, and using the same current inner loop control as grid-following control.

[0029] Furthermore, the phase-to-phase voltage control employs zero-sequence voltage injection or negative-sequence current control.

[0030] The above-described technical solution of the present invention provides a novel submodule voltage control method for a static synchronous condenser. Its beneficial effects include: timely activation of DC supplementary power control when voltage control of the total submodule voltage is necessary, achieving timely voltage control; deactivation of DC supplementary power control when the voltage has returned to a relatively normal state, to avoid the voltage control strategy weakening inertia support capability; and a strategy that temporarily switches from grid-based control to grid-following control when the total submodule voltage deviates significantly from the rated voltage to prevent further voltage imbalance caused by grid-based control, and only switches from grid-following control to grid-based control to continue providing inertia support when the total submodule voltage value has basically recovered. This approach effectively balances submodule voltage margin, avoiding modulation saturation or submodule voltage overvoltage and insufficient inertia support capability.

[0031] The present invention also provides a submodule voltage control system for a static synchronous condenser, including a processor storing executable program instructions, which are executed to implement a submodule voltage control method for a static synchronous condenser, comprising:

[0032] When the average voltage of the total submodule is less than the first set lower limit or greater than the first set upper limit, DC additional power control is activated. When the average voltage of the total submodule recovers to a value greater than the second set lower limit or less than the second set upper limit and remains so for a first set duration, DC additional power control is deactivated and the active power command is set to 0. When the average voltage of the total submodule is less than the third set lower limit or greater than the third set upper limit, grid-based control switches to grid-following control. When the average voltage of the total submodule recovers to a value greater than the fourth set lower limit or less than the fourth set upper limit and remains so for a second set duration, grid-following control switches to grid-based control.

[0033] The first, second, third, and fourth lower limits are all less than the per-unit value, while the first, second, third, and fourth upper limits are greater than the per-unit value; the third, first, and second lower limits are successively larger, and the second lower limit is less than or equal to the fourth lower limit; the third, first, and second upper limits are successively smaller, and the second upper limit is greater than or equal to the fourth upper limit.

[0034] Furthermore, it also includes:

[0035] When the average voltage of each phase submodule is less than the fifth set lower limit or greater than the fifth set upper limit, phase-to-phase voltage control is activated; when the average voltage of each phase submodule recovers to a value greater than the sixth set lower limit or less than the sixth set upper limit, and remains so for a third set duration, phase-to-phase voltage control is deactivated.

[0036] The fifth and sixth lower limits are both less than the per-unit value, and the fifth and sixth upper limits are greater than the per-unit value; the fifth lower limit is less than the sixth lower limit, and the sixth upper limit is less than the fifth upper limit.

[0037] Furthermore, the methods for switching from network configuration control to network following control include:

[0038] The angle generated by the three-phase grid voltage through PLL phase-locked loop is used as the grid-following control phase angle. The control architecture adopts a voltage and current dual-loop control structure. The DC voltage of the three-phase sub-module is collected, filtered by the output filter, and the average value of the total sub-module voltage is calculated as the feedback value of the outer loop voltage loop control. The difference between the feedback value and the reference value of the outer loop voltage loop control is used to generate an active current command after passing through the PI controller, and a reactive current command is generated through virtual impedance. The output current of the static synchronous condenser is collected, filtered by the filter, and used as the feedback value of the inner loop current loop control. The difference between the reference value and the feedback value is input to the fundamental wave controller and superimposed with the grid voltage feedforward to output the fundamental wave modulation wave.

[0039] Furthermore, network control includes VSG control, excitation control, and virtual impedance control;

[0040] Methods for generating reactive current commands through virtual impedance control include:

[0041] ;

[0042] In the formula: , These are the d-axis and q-axis components of the three-phase internal potential, respectively. , These are the d-axis and q-axis components of the terminal voltage, respectively. , The d-axis and q-axis components of the stator current are respectively, and R and L are the stator resistance and stator inductance, respectively. Methods for determining the d-axis and q-axis components of the three-phase internal potential through excitation control include:

[0043] ;

[0044] In the formula, This is the unloaded internal potential; , For reactive power closed-loop PI parameters; Q represents reactive power command; Q represents VSG reactive power. This is the voltage regulation coefficient; This is the rated terminal voltage; This represents the amplitude of the terminal voltage.

[0045] Furthermore, the DC additional power control method includes: obtaining an additional active power reference value through DC droop control by means of the difference between the reference DC voltage and the DC voltage feedback value of the main submodule, which is used as the active power reference value for VSG control, and adjusting the output power of the stationary synchronous condenser in real time.

[0046] Furthermore, the methods for determining the d-axis and q-axis components of the terminal voltage include:

[0047] After the three-phase voltage of the power grid is collected and filtered by a filter, the PARK changes are performed according to the angle generated by the VSG control to generate the d and q axis components of the generator terminal voltage.

[0048] VSG-controlled models include:

[0049] ;

[0050] in ;

[0051] In the formula: The angular velocity of the VSG; ω is the rated angular velocity of the VSG; J is the moment of inertia of the VSG; and These represent the mechanical torque and electromagnetic torque of the VSG, respectively; P ref θ is the reference active power; D is the damping coefficient; θ2 is the angle generated by VSG control.

[0052] Furthermore, the methods for switching from grid-following control to grid-connected control include: using a virtual synchronous machine to generate the angle, setting the active power command to 0, using an excitation controller to generate the internal potential, generating active current and reactive current commands through virtual impedance control, and using the same current inner loop control as grid-following control.

[0053] Furthermore, the phase-to-phase voltage control employs zero-sequence voltage injection or negative-sequence current control.

[0054] The technical solution of the submodule voltage control system of the static synchronous condenser described above can achieve the same beneficial effects as the submodule voltage control method of the static synchronous condenser described above.

[0055] The present invention also provides a static synchronous condenser, including a submodule voltage control system. The submodule voltage control system includes a processor storing executable program instructions, which are executed to implement a submodule voltage control method for the static synchronous condenser, comprising:

[0056] When the average voltage of the total submodule is less than the first set lower limit or greater than the first set upper limit, DC additional power control is activated. When the average voltage of the total submodule recovers to a value greater than the second set lower limit or less than the second set upper limit and remains so for a first set duration, DC additional power control is deactivated and the active power command is set to 0. When the average voltage of the total submodule is less than the third set lower limit or greater than the third set upper limit, grid-based control switches to grid-following control. When the average voltage of the total submodule recovers to a value greater than the fourth set lower limit or less than the fourth set upper limit and remains so for a second set duration, grid-following control switches to grid-based control.

[0057] The first, second, third, and fourth lower limits are all less than the per-unit value, while the first, second, third, and fourth upper limits are greater than the per-unit value; the third, first, and second lower limits are successively larger, and the second lower limit is less than or equal to the fourth lower limit; the third, first, and second upper limits are successively smaller, and the second upper limit is greater than or equal to the fourth upper limit.

[0058] Furthermore, it also includes:

[0059] When the average voltage of each phase submodule is less than the fifth set lower limit or greater than the fifth set upper limit, phase-to-phase voltage control is activated; when the average voltage of each phase submodule recovers to a value greater than the sixth set lower limit or less than the sixth set upper limit, and remains so for a third set duration, phase-to-phase voltage control is deactivated.

[0060] The fifth and sixth lower limits are both less than the per-unit value, and the fifth and sixth upper limits are greater than the per-unit value; the fifth lower limit is less than the sixth lower limit, and the sixth upper limit is less than the fifth upper limit.

[0061] Furthermore, the methods for switching from network configuration control to network following control include:

[0062] The angle generated by the three-phase grid voltage through PLL phase-locked loop is used as the grid-following control phase angle. The control architecture adopts a voltage and current dual-loop control structure. The DC voltage of the three-phase sub-module is collected, filtered by the output filter, and the average value of the total sub-module voltage is calculated as the feedback value of the outer loop voltage loop control. The difference between the feedback value and the reference value of the outer loop voltage loop control is used to generate an active current command after passing through the PI controller, and a reactive current command is generated through virtual impedance control. The output current of the static synchronous condenser is collected, filtered by the filter, and used as the feedback value of the inner loop current loop control. The difference between the reference value and the feedback value is input to the fundamental wave controller and superimposed with the grid voltage feedforward to output the fundamental wave modulation wave.

[0063] Furthermore, network control includes VSG control, excitation control, and virtual impedance control;

[0064] Methods for generating reactive current commands through virtual impedance control include:

[0065] ;

[0066] In the formula: , These are the d-axis and q-axis components of the three-phase internal potential, respectively. , These are the d-axis and q-axis components of the terminal voltage, respectively. , The d-axis and q-axis components of the stator current are respectively, and R and L are the stator resistance and stator inductance, respectively. Methods for determining the d-axis and q-axis components of the three-phase internal potential through excitation control include:

[0067] ;

[0068] In the formula, This is the unloaded internal potential; , For reactive power closed-loop PI parameters; Q represents reactive power command; Q represents VSG reactive power. This is the voltage regulation coefficient; This is the rated terminal voltage; This represents the amplitude of the terminal voltage.

[0069] Furthermore, the DC additional power control method includes: obtaining an additional active power reference value through DC droop control by means of the difference between the reference DC voltage and the DC voltage feedback value of the main submodule, which is used as the active power reference value for VSG control, and adjusting the output power of the stationary synchronous condenser in real time.

[0070] Furthermore, the methods for determining the d-axis and q-axis components of the terminal voltage include:

[0071] After the three-phase voltage of the power grid is collected and filtered by a filter, the PARK changes are performed according to the angle generated by the VSG control to generate the d and q axis components of the generator terminal voltage.

[0072] VSG-controlled models include:

[0073] ;

[0074] in ;

[0075] In the formula: The angular velocity of the VSG; ω is the rated angular velocity of the VSG; J is the moment of inertia of the VSG; and These represent the mechanical torque and electromagnetic torque of the VSG, respectively; P ref θ is the reference active power; D is the damping coefficient; θ2 is the angle generated by VSG control.

[0076] Furthermore, the methods for switching from grid-following control to grid-connected control include: using a virtual synchronous machine to generate the angle, setting the active power command to 0, using an excitation controller to generate the internal potential, generating active current and reactive current commands through virtual impedance control, and using the same current inner loop control as grid-following control.

[0077] Furthermore, the phase-to-phase voltage control employs zero-sequence voltage injection or negative-sequence current control.

[0078] The technical solution of the static synchronous condenser described above can achieve the same beneficial effects as the sub-module voltage control system of the static synchronous condenser described above. Attached Figure Description

[0079] Figure 1 This is a flowchart of the overall submodule voltage control part of the submodule voltage control method of the static synchronous condenser in the embodiment of the present invention.

[0080] Figure 2 This is a flowchart of the phase-to-phase voltage control part of the sub-module voltage control method for a static synchronous condenser in an embodiment of the present invention.

[0081] Figure 3 This is a block diagram illustrating the principle of DC additional power control in the implementation of the submodule voltage control method for the static synchronous condenser of the present invention.

[0082] Figure 4 This is a block diagram illustrating the principle of network control in the implementation of the sub-module voltage control method for the static synchronous condenser of the present invention.

[0083] Figure 5 This is a block diagram illustrating the principle of grid control in the implementation of the voltage control method for a submodule of a static synchronous condenser of the present invention.

[0084] Figure 6 This is a block diagram illustrating the principle of the voltage control method for the submodule of the static synchronous condenser in the embodiment of the present invention. Detailed Implementation

[0085] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0086] Implementation method of voltage control method for submodule of static synchronous condenser

[0087] This embodiment provides a technical solution for a submodule voltage control method for a static synchronous condenser. By implementing phase-to-phase voltage control, DC additional power control, and grid-connected switching when voltage control is necessary, voltage control can be achieved in a timely manner. In other cases, grid-connected control is performed normally to provide inertia support, so as to take into account the voltage margin of the submodule and avoid modulation saturation or submodule voltage overvoltage and insufficient inertia support capability.

[0088] Reference Figure 1 The method includes:

[0089] When the average voltage of the total submodule is less than the first set lower limit or greater than the first set upper limit, DC additional power control is activated. When the average voltage of the total submodule recovers to a value greater than the second set lower limit or less than the second set upper limit and remains so for a first set duration, DC additional power control is deactivated and the active power command is set to 0. When the average voltage of the total submodule is less than the third set lower limit or greater than the third set upper limit, grid-based control switches to grid-following control. When the average voltage of the total submodule recovers to a value greater than the fourth set lower limit or less than the fourth set upper limit and remains so for a second set duration, grid-following control switches to grid-based control.

[0090] The first, second, third, and fourth lower limits are all less than the per-unit value, while the first, second, third, and fourth upper limits are greater than the per-unit value; the third, first, and second lower limits are successively larger, and the second lower limit is less than or equal to the fourth lower limit; the third, first, and second upper limits are successively smaller, and the second upper limit is greater than or equal to the fourth upper limit.

[0091] Therefore, this method achieves timely voltage control by promptly engaging DC additional power control when voltage control of the total submodule voltage is necessary (i.e., when the average total submodule voltage is less than the first set lower limit or greater than the first set upper limit); and disengaging DC additional power control when the voltage has returned to a relatively normal state (i.e., when the average total submodule voltage recovers to a value greater than the second set lower limit or less than the second set upper limit and remains so for a first set duration) to avoid the voltage control strategy weakening the inertia support capability. Furthermore, when the total submodule voltage deviates significantly from its rated value (i.e., when the average total submodule voltage is less than the third set lower limit or greater than the third set upper limit), the method temporarily switches from grid-based control to grid-following control to avoid modulation saturation or overvoltage; and only switches from grid-following control to grid-based control to continue providing inertia support when the total submodule voltage has basically recovered (i.e., when the average total submodule voltage recovers to a value greater than the fourth set lower limit or less than the fourth set upper limit and remains so for a second set duration). This strategy effectively balances submodule voltage margin, avoiding modulation saturation, submodule voltage overvoltage, and insufficient inertia support capability. In other words, this part mainly consists of the voltage control section of the main submodule.

[0092] Reference Figure 2 In this embodiment, the method further includes:

[0093] When the average voltage of each phase submodule is less than the fifth set lower limit or greater than the fifth set upper limit, phase-to-phase voltage control is activated; when the average voltage of each phase submodule recovers to a value greater than the sixth set lower limit or less than the sixth set upper limit, and remains so for a third set duration, phase-to-phase voltage control is deactivated.

[0094] The fifth and sixth lower limits are both less than the per-unit value, and the fifth and sixth upper limits are greater than the per-unit value; the fifth lower limit is less than the sixth lower limit, and the sixth upper limit is less than the fifth upper limit.

[0095] As can be seen, this method also includes settings for phase-to-phase voltage control: it is activated when voltage equalization is necessary (i.e., when the average voltage of each phase submodule is less than the fifth set lower limit or greater than the fifth set upper limit), and deactivated when the voltage has returned to a relatively normal state (i.e., when the average voltage of each phase submodule returns to a value greater than the sixth set lower limit or less than the sixth set upper limit, and this remains for a third set duration). This further balances the goals of timely voltage equalization, avoiding voltage imbalance, and providing sufficient inertia support. In other words, this part mainly concerns phase-to-phase voltage control.

[0096] In one specific embodiment, the average voltage of each phase submodule is detected. When the average voltage of each phase submodule is less than the set value A (i.e., the fifth set lower limit) or greater than the set value B (i.e., the fifth set upper limit), phase-to-phase voltage control is activated. When the average voltage of each phase submodule recovers and is greater than the set value C (i.e., the sixth set lower limit) or less than the set value D (i.e., the sixth set upper limit) for a set time T1, phase-to-phase voltage control is deactivated.

[0097] When the average voltage of the total submodule is less than the set value E (i.e., the first set lower limit) or greater than the set value F (i.e., the first set upper limit), DC additional power control is activated; when the average voltage of the total submodule recovers and is greater than the set value G (i.e., the second set lower limit) or less than the set value H (i.e., the second set upper limit) for a set time T2, DC additional power control is deactivated and the active power command is set to 0.

[0098] When the average voltage of the total submodule is less than the set value I (i.e., the third set lower limit) or greater than the set value J (i.e., the third set upper limit), the network control switches to network following control; when the average voltage of the total submodule recovers and is greater than the set value K (i.e., the fourth set lower limit) or less than the set value L (i.e., the fourth set upper limit) and continues for a set time T3, the network following control switches to network construction control.

[0099] Among them, the set values A and C of the phase-interleaved voltage equalization control are < 1.0 pu, the set values B and D are > 1.0 pu, and 1.0 pu represents one per-unit value; and the set value A < C, the set value D < B, T1 is the duration, which can be specifically set according to the specific situation. The set values E, G, I, and K are < 1.0 pu, the set values H, F, J, and L are > 1.0 pu, and 1.0 pu represents one per-unit value; and the set value I < E < G ≤ K, the set value L ≤ H < F < J; T2 and T3 are the durations. Parameter settings need to consider the modulation degree and the overvoltage protection of the sub-module capacitor voltage to avoid modulation degree saturation and capacitor voltage overvoltage.

[0100] In this embodiment, the method of DC additional power control includes: obtaining the additional active power reference value through DC droop control based on the difference between the reference DC voltage and the feedback value of the total sub-module DC voltage, using it as the active power reference value for VSG control, and adjusting the output power of the static synchronous compensator in real time. The specific schematic diagram is referred to Figure 3 , where U dcall is the average voltage of the total sub-modules (i.e., the feedback value of the total sub-module DC voltage), U dcref is the sub-module voltage reference value (i.e., the reference DC voltage), P e is the electromagnetic power, J is the inertia time constant, D is the damping coefficient, ω o is the rated angular frequency.

[0101] The method of switching the grid-forming control to the grid-following control includes:

[0102] Collecting the angle θ1 generated by PLL phase-locking of the three-phase grid voltage as the grid-following control phase angle, and the control architecture adopts a voltage-current double-loop control structure; after collecting the DC voltages of the three-phase sub-modules and filtering them through an output filter, calculating the average voltage of the total sub-modules as the feedback value of the outer voltage loop control, the difference between the feedback value of the outer voltage loop control and the reference value is generated into an active current command through a PI controller, and a reactive current command is generated through virtual impedance control; collecting the output current of the static synchronous compensator, filtering it through a filter as the feedback value of the inner current loop control, inputting the difference between the reference value and the feedback value into a fundamental wave controller and superimposing it with the grid voltage feedforward, and outputting a fundamental wave modulation wave.

[0103] The grid-forming control is specifically referred to Figure 4 , including VSG control, excitation control, and virtual impedance control.

[0104] The model of VSG control includes:

[0105]

[0106] Among them ;

[0107] In the formula: The angular velocity of the VSG; ω is the rated angular velocity of the VSG; J is the moment of inertia of the VSG; and These represent the mechanical torque and electromagnetic torque of the VSG, respectively; P ref The reference active power is denoted by D; the damping coefficient is θ2; and the angle generated by the VSG control is θ2. The methods for determining the d-axis and q-axis components of the three-phase internal potential include:

[0108]

[0109] In the formula, This is the unloaded internal potential; , For reactive power closed-loop PI parameters; Q represents reactive power command; Q represents VSG reactive power. This is the voltage regulation coefficient; This is the rated terminal voltage; This represents the amplitude of the terminal voltage.

[0110] The methods for determining the d-axis and q-axis components of the terminal voltage include:

[0111] Collect the three-phase voltage U of the power grid ab U bc and U ca After filtering, the PARK is adjusted according to the angle generated by VSG control, resulting in the d-axis and q-axis components of the terminal voltage I. dp I qp ;

[0112] Methods for generating reactive current commands through virtual impedance include:

[0113]

[0114] In the formula: , These are the d-axis and q-axis components of the three-phase internal potential (i.e., the d-axis and q-axis components of the internal potential generated by the excitation controller). , These are the d-axis and q-axis components of the VSG terminal voltage, respectively. , These are the d-axis and q-axis components of the stator current (i.e., reactive current command), respectively, and R and L are the stator resistance and stator inductance, respectively.

[0115] For specific details on network control, please refer to [link / reference]. Figure 5 U ab U bc U ca The three-phase voltage of the power grid. Figure 5 Middle,U dcaj * Udcbj * U dccj * These are the instantaneous voltage values ​​of the three-phase submodule, U. dca U dcb U dcc These are the average voltage values ​​of the three-phase submodules, U dcall U represents the average voltage of the total submodules. dcref This is the reference value for the total submodule voltage; I dp * I qp * These are active current command and reactive current command, respectively. dp I qp These are active current feedback and reactive current feedback, respectively, V d V q These are active voltage feedforward and reactive voltage feedforward, respectively; V om Inject compensation voltage for zero-sequence voltage or negative-sequence current; m a m b m c It is a three-phase modulated wave.

[0116] The principle of the above submodule voltage control method is as follows: Figure 6 When the detection network switching flag is 1, I dp2 * I qp2 * Switch to I dp1 * I qp1 * θ2 switches to θ1, switching from network configuration control to network following control. When the network following switching flag is detected to be 0, I... dp1 * I qp1 * Switch to I dp2 * I qp2 * θ1 is switched to θ2.

[0117] Implementation method of submodule voltage control system for static synchronous condenser

[0118] This embodiment provides a technical solution for a submodule voltage control system of a stationary synchronous condenser; the submodule voltage control system of the stationary synchronous condenser includes a processor, which stores executable program instructions, which are executed to implement the following submodule voltage control method for the stationary synchronous condenser:

[0119] When the average voltage of the total submodule is less than the first set lower limit or greater than the first set upper limit, DC additional power control is activated. When the average voltage of the total submodule recovers to a value greater than the second set lower limit or less than the second set upper limit and remains so for a first set duration, DC additional power control is deactivated and the active power command is set to 0. When the average voltage of the total submodule is less than the third set lower limit or greater than the third set upper limit, grid-based control switches to grid-following control. When the average voltage of the total submodule recovers to a value greater than the fourth set lower limit or less than the fourth set upper limit and remains so for a second set duration, grid-following control switches to grid-based control.

[0120] The first, second, third, and fourth lower limits are all less than the per-unit value, while the first, second, third, and fourth upper limits are greater than the per-unit value; the third, first, and second lower limits are successively larger, and the second lower limit is less than or equal to the fourth lower limit; the third, first, and second upper limits are successively smaller, and the second upper limit is greater than or equal to the fourth upper limit.

[0121] Therefore, this method achieves timely voltage control by promptly engaging DC additional power control when the total submodule voltage is under voltage control (i.e., when the average total submodule voltage is less than the first set lower limit or greater than the first set upper limit); and disengaging DC additional power control when the voltage has returned to a relatively normal state (i.e., when the average total submodule voltage recovers to a value greater than the second set lower limit or less than the second set upper limit and remains so for a first set duration) to avoid the voltage control strategy weakening the inertia support capability. Furthermore, it temporarily switches from grid-based control to grid-following control when the total submodule voltage deviates significantly from its rated value (i.e., when the average total submodule voltage is less than the third set lower limit or greater than the third set upper limit) to prevent further voltage imbalance caused by grid-based control; and only switches from grid-following control to grid-based control to continue providing inertia support when the total submodule voltage balance is basically restored (i.e., when the average total submodule voltage recovers to a value greater than the fourth set lower limit or less than the fourth set upper limit and remains so for a second set duration). This strategy effectively balances modulation saturation, submodule voltage overvoltage, and insufficient inertia support capability.

[0122] In this embodiment, the submodule voltage control method of the static synchronous condenser further includes:

[0123] When the average voltage of each phase submodule is less than the fifth set lower limit or greater than the fifth set upper limit, phase-to-phase voltage control is activated; when the average voltage of each phase submodule recovers to a value greater than the sixth set lower limit or less than the sixth set upper limit, and remains so for a third set duration, phase-to-phase voltage control is deactivated.

[0124] Both the fifth and sixth set lower limit values are less than the per-unit value, and the fifth and sixth set upper limit values are greater than the per-unit value; the fifth set lower limit value is less than the sixth set lower limit value, and the sixth set upper limit value is less than the fifth set upper limit value.

[0125] It can be seen that this method also sets to be put into operation when there is a need for voltage equalization in the inter-phase voltage control (that is, when the average voltage of each phase sub-module is less than the fifth set lower limit value or greater than the fifth set upper limit value), and to be withdrawn when it has returned to a relatively normal state (that is, when the average voltage of each phase sub-module has recovered to be greater than the sixth set lower limit value or less than the sixth set upper limit value and lasts for the third set duration), further taking into account the goals of timely voltage equalization, avoiding voltage imbalance, and providing sufficient inertia support.

[0126] In a specific embodiment, the average voltage of each phase sub-module is detected. When the average voltage of each phase sub-module is less than set value A or greater than set value B, the inter-phase voltage control is put into operation; when the average voltage of each phase sub-module has recovered, is greater than set value C or less than set value D and lasts for set time T1, the inter-phase voltage control is withdrawn;

[0127] When the average voltage of the total sub-modules is less than set value E or greater than set value F, the DC additional power control is put into operation; when the average voltage of the total sub-modules has recovered, is greater than set value G or less than set value H and lasts for set time T2, the DC additional power control is withdrawn, and the active power command is set to 0;

[0128] When the average voltage of the total sub-modules is less than set value I or greater than set value J, the grid-forming control is switched to the grid-following control; when the average voltage of the total sub-modules has recovered, is greater than set value K or less than set value L and lasts for set time T3, the grid-following control is switched to the grid-forming control.

[0129] Among them, for the inter-phase voltage equalization control, set values A and C < 1.0 pu, set values B and D > 1.0 pu, and 1.0 pu represents one per-unit value; and set value A < C, set value D < B, and T1 is the duration, which can be specifically set according to the specific situation. Set values E, G, I, K < 1.0 pu, set values H, F, J, L > 1.0 pu, and 1.0 pu represents one per-unit value; and set value I < E < G ≤ K, set value L ≤ H < F < J; T2 and T3 are the durations. Parameter settings need to consider the modulation degree and overvoltage of the sub-module capacitor voltage to avoid modulation degree saturation and capacitor voltage overvoltage.

[0130] In this embodiment, the method of the DC additional power control includes: obtaining the additional active power reference value through the difference between the reference DC voltage and the feedback value of the total sub-module DC voltage, and through DC droop control, as the active power reference value for VSG control, to adjust the output power of the static synchronous compensator in real time. Among them, U dcallU is the average voltage of the total submodules (i.e., the DC voltage feedback value of the total submodules). dcref P is the submodule voltage reference value (i.e., reference DC voltage). e Let J be the electromagnetic power, J be the inertial time constant, D be the damping coefficient, and ω be the electromagnetic power. o This is the rated angular frequency.

[0131] The methods for switching from network configuration control to network following control include:

[0132] The angle θ1 generated by the three-phase grid voltage through PLL phase-locked loop is used as the grid-following control phase angle. The control architecture adopts a voltage and current dual-loop control structure. The DC voltage of the three-phase sub-module is collected, filtered by the output filter, and the average value of the total sub-module voltage is calculated as the feedback value of the outer loop voltage loop control. The difference between the feedback value and the reference value of the outer loop voltage loop control is used to generate an active current command after passing through the PI controller, and a reactive current command is generated through virtual impedance control. The output current of the static synchronous condenser is collected, filtered by the filter, and used as the feedback value of the inner loop current loop control. The difference between the reference value and the feedback value is input to the fundamental wave controller and superimposed with the grid voltage feedforward to output the fundamental wave modulation wave.

[0133] The network control specifically includes VSG control, excitation control, and virtual impedance control;

[0134] VSG-controlled models include:

[0135]

[0136] in ;

[0137] In the formula: The angular velocity of the VSG; ω is the rated angular velocity of the VSG; J is the moment of inertia of the VSG; and These represent the mechanical torque and electromagnetic torque of the VSG, respectively; P ref θ is the reference active power; D is the damping coefficient; θ2 is the angle generated by VSG control.

[0138] The methods for determining the d-axis and q-axis components of the three-phase internal potential include:

[0139]

[0140] In the formula, This is the unloaded internal potential; , For reactive power closed-loop PI parameters; Q represents reactive power command; Q represents VSG reactive power. This is the voltage regulation coefficient; This is the rated terminal voltage; This represents the amplitude of the terminal voltage.

[0141] The methods for determining the d-axis and q-axis components of the terminal voltage include:

[0142] Collect the three-phase voltage U of the power grid ab U bc and U ca After filtering, the PARK is adjusted according to the angle generated by VSG control, resulting in the d-axis and q-axis components of the terminal voltage I. dp I qp ;

[0143] Methods for generating reactive current commands through virtual impedance include:

[0144]

[0145] In the formula: , These are the d-axis and q-axis components of the three-phase internal potential (i.e., the d-axis and q-axis components of the internal potential generated by the excitation controller). , These are the d-axis and q-axis components of the VSG terminal voltage, respectively. , These are the d-axis and q-axis components of the stator current (i.e., reactive current command), respectively, and R and L are the stator resistance and stator inductance, respectively.

[0146] U-band network control ab U bc U ca U is the three-phase voltage of the power grid. dcaj * U dcbj * U dccj * These are the instantaneous voltage values ​​of the three-phase submodule, U. dca U dcb U dcc These are the average voltage values ​​of the three-phase submodules, U dcall U represents the average voltage of the total submodules. dcref This is the reference value for the total submodule voltage; I dp * I qp * These are active current command and reactive current command, respectively. dp I qp These are active current feedback and reactive current feedback, respectively, V d V q These are active voltage feedforward and reactive voltage feedforward, respectively; V om Inject compensation voltage for zero-sequence voltage or negative-sequence current; ma m b m c It is a three-phase modulated wave.

[0147] Implementation of a stationary synchronous condenser

[0148] This embodiment provides a technical solution for a static synchronous condenser, which includes a sub-module voltage control system. The sub-module voltage control system includes a processor storing executable program instructions. These executable program instructions are executed to implement the following sub-module voltage control method for the static synchronous condenser:

[0149] When the average voltage of the total submodule is less than the first set lower limit or greater than the first set upper limit, DC additional power control is activated. When the average voltage of the total submodule recovers to a value greater than the second set lower limit or less than the second set upper limit and remains so for a first set duration, DC additional power control is deactivated and the active power command is set to 0. When the average voltage of the total submodule is less than the third set lower limit or greater than the third set upper limit, grid-based control switches to grid-following control. When the average voltage of the total submodule recovers to a value greater than the fourth set lower limit or less than the fourth set upper limit and remains so for a second set duration, grid-following control switches to grid-based control.

[0150] The first, second, third, and fourth lower limits are all less than the per-unit value, while the first, second, third, and fourth upper limits are greater than the per-unit value; the third, first, and second lower limits are successively larger, and the second lower limit is less than or equal to the fourth lower limit; the third, first, and second upper limits are successively smaller, and the second upper limit is greater than or equal to the fourth upper limit.

[0151] Therefore, this method achieves timely voltage control by promptly engaging DC additional power control when voltage equalization of the total submodules is necessary (i.e., when the average voltage of the total submodules is less than the first set lower limit or greater than the first set upper limit); and disengaging DC additional power control when the voltage has returned to a relatively normal state (i.e., when the average voltage of the total submodules returns to a value greater than the second set lower limit or less than the second set upper limit and remains so for a first set duration), thus preventing the voltage equalization control strategy from weakening the inertia support capability. Furthermore, this method employs a strategy of temporarily switching from grid-based control to grid-following control when the total submodule voltage deviates significantly from the rated voltage (i.e., when the average voltage of the total submodules is less than the third set lower limit or greater than the third set upper limit) to avoid modulation saturation or overvoltage, and only switching from grid-following control to grid-based control to continue providing inertia support when the voltage balance of the total submodules has basically recovered (i.e., when the average voltage of the total submodules returns to a value greater than the fourth set lower limit or less than the fourth set upper limit and remains so for a second set duration). This strategy effectively balances submodule voltage margin, avoiding modulation saturation or submodule voltage overvoltage and insufficient inertia support capability.

[0152] In this embodiment, the submodule voltage control method of the static synchronous condenser further includes:

[0153] When the average voltage of each phase submodule is less than the fifth set lower limit or greater than the fifth set upper limit, phase-to-phase voltage control is activated; when the average voltage of each phase submodule recovers to a value greater than the sixth set lower limit or less than the sixth set upper limit, and remains so for a third set duration, phase-to-phase voltage control is deactivated.

[0154] The fifth and sixth lower limits are both less than the per-unit value, and the fifth and sixth upper limits are greater than the per-unit value; the fifth lower limit is less than the sixth lower limit, and the sixth upper limit is less than the fifth upper limit.

[0155] It is evident that this method also includes settings for phase-to-phase voltage control: it is activated when voltage equalization is necessary (i.e., when the average voltage of each phase submodule is less than the fifth set lower limit or greater than the fifth set upper limit), and deactivated when the voltage has returned to a relatively normal state (i.e., when the average voltage of each phase submodule returns to a value greater than the sixth set lower limit or less than the sixth set upper limit, and this continues for the third set duration). This further balances the goals of timely voltage equalization, avoiding voltage imbalance, and providing sufficient inertia support.

[0156] In one specific embodiment, the average voltage of each phase submodule is detected. When the average voltage of each phase submodule is less than a set value A or greater than a set value B, phase-to-phase voltage control is activated. When the average voltage of each phase submodule recovers and is greater than a set value C or less than a set value D for a set time T1, phase-to-phase voltage control is deactivated.

[0157] When the average voltage of the total submodule is less than the set value E or greater than the set value F, DC additional power control is activated; when the average voltage of the total submodule recovers and is greater than the set value G or less than the set value H for a set time T2, DC additional power control is deactivated and the active power command is set to 0.

[0158] When the average voltage of the total submodule is less than the set value I or greater than the set value J, the network control switches to the network-following control; when the average voltage of the total submodule recovers and is greater than the set value K or less than the set value L for a set time T3, the network-following control switches back to the network-building control.

[0159] Among them, the set values A and C of the phase-interleaved voltage equalization control are < 1.0 pu, the set values B and D are > 1.0 pu, and 1.0 pu represents one per-unit value; and the set value A < C, the set value D < B, T1 is the duration, which can be set according to specific situations. The set values E, G, I, K are < 1.0 pu, the set values H, F, J, L are > 1.0 pu, and 1.0 pu represents one per-unit value; and the set value I < E < G ≤ K, the set value L ≤ H < F < J; T2 and T3 are the durations. Parameter settings need to consider the modulation degree to avoid modulation degree saturation.

[0160] In this embodiment, the method of DC additional power control includes: obtaining the additional active power reference value through DC droop control based on the difference between the reference DC voltage and the feedback value of the total sub-module DC voltage, and using it as the active power reference value for VSG control to adjust the output power of the static synchronous compensator in real time. Among them, U dcall is the average voltage of the total sub-module (i.e., the feedback value of the total sub-module DC voltage), U dcref is the reference voltage of the sub-module (i.e., the reference DC voltage), P e is the electromagnetic power, J is the inertia time constant, D is the damping coefficient, ω o is the rated angular frequency.

[0161] The method of switching the grid-forming control to the grid-following control includes:

[0162] Collecting the angle θ1 generated by PLL phase-locking of the three-phase grid voltage as the grid-following control phase angle, and the control architecture adopts a voltage-current double-loop control structure; after collecting the DC voltages of the three-phase sub-modules and filtering them through an output filter, calculating the average voltage of the total sub-module as the feedback value of the outer voltage loop control, the difference between the feedback value of the outer voltage loop control and the reference value generates an active current command through a PI controller, and generates a reactive current command through virtual impedance control; collecting the output current of the static synchronous compensator, filtering it through a filter as the feedback value of the inner current loop control, inputting the difference between the reference value and the feedback value into a fundamental wave controller and superimposing it with the grid voltage feedforward, and outputting a fundamental wave modulation wave.

[0163] Among them, the grid-forming control specifically includes VSG control, excitation control, and virtual impedance control;

[0164] The model of VSG control includes:

[0165]

[0166] Among them ;

[0167] In the formula: is the angular velocity of VSG; is the rated angular velocity of VSG; J is the moment of inertia of VSG; and These represent the mechanical torque and electromagnetic torque of the VSG, respectively; P ref θ is the reference active power; D is the damping coefficient; θ2 is the angle generated by VSG control.

[0168] Methods for generating reactive current commands through virtual impedance include:

[0169]

[0170] In the formula: , These are the d-axis and q-axis components of the three-phase internal potential (i.e., the d-axis and q-axis components of the internal potential generated by the excitation controller). , These are the d-axis and q-axis components of the VSG terminal voltage, respectively. , These are the d-axis and q-axis components of the stator current (i.e., reactive current command), respectively, and R and L are the stator resistance and stator inductance, respectively.

[0171] The methods for determining the d-axis and q-axis components of the terminal voltage include:

[0172] Collect the three-phase voltage U of the power grid ab U bc and U ca After filtering, the PARK is adjusted according to the angle generated by VSG control, resulting in the d-axis and q-axis components of the terminal voltage I. dp I qp ;

[0173] The methods for determining the d-axis and q-axis components of the three-phase internal potential include:

[0174]

[0175] In the formula, This is the unloaded internal potential; , For reactive power closed-loop PI parameters; Q represents reactive power command; Q represents VSG reactive power. This is the voltage regulation coefficient; This is the rated terminal voltage; This represents the amplitude of the terminal voltage.

[0176] U-band network control ab U bc U ca U is the three-phase voltage of the power grid. dcaj * U dcbj * U dccj * These are the instantaneous voltage values ​​of the three-phase submodule, U.dca U dcb U dcc These are the average voltage values ​​of the three-phase submodules, U dcall U represents the average voltage of the total submodules. dcref This is the reference value for the total submodule voltage; I dp * I qp * These are active current command and reactive current command, respectively. dp I qp These are active current feedback and reactive current feedback, respectively, V d V q These are active voltage feedforward and reactive voltage feedforward, respectively; V om Inject compensation voltage for zero-sequence voltage or negative-sequence current; m a m b m c It is a three-phase modulated wave.

[0177] It should be understood that the above-described specific embodiments of the present invention are merely illustrative or explanatory of the principles of the present invention, and do not constitute a limitation thereof.

Claims

1. A method of sub-module voltage control for a static synchronous compensator, characterized by, The method comprises the following steps: When the average value of the total submodule voltage is less than a first lower limit value or greater than a first upper limit value, the direct current additional power control is started; when the average value of the total submodule voltage is restored to be greater than a second lower limit value or less than a second upper limit value and the state is maintained for a first set time, the direct current additional power control is exited, and the active power instruction is set to 0; when the average value of the total submodule voltage is less than a third lower limit value or greater than a third upper limit value, the grid-forming control is switched to the grid-following control; when the average value of the total submodule voltage is restored to be greater than a fourth lower limit value or less than a fourth upper limit value and the state is maintained for a second set time, the grid-following control is switched to the grid-forming control. The first, second, third and fourth lower limit values are all less than the per unit value, and the first, second, third and fourth upper limit values are all greater than the per unit value; the third, first and second lower limit values are greater in turn, and the second lower limit value is less than or equal to the fourth lower limit value; the third, first and second upper limit values are smaller in turn, and the second upper limit value is greater than or equal to the fourth upper limit value.

2. The method of claim 1, wherein, The method further comprises the following steps: When the average value of each phase submodule voltage is less than a fifth lower limit value or greater than a fifth upper limit value, the inter-phase voltage control is started; when the average value of each phase submodule voltage is restored to be greater than a sixth lower limit value or less than a sixth upper limit value and the state is maintained for a third set time, the inter-phase voltage control is exited. The fifth and sixth lower limit values are both less than the per unit value, and the fifth and sixth upper limit values are both greater than the per unit value; the fifth lower limit value is less than the sixth lower limit value, and the sixth upper limit value is less than the fifth upper limit value.

3. The method of voltage control of a sub-module of a static synchronous compensator according to claim 1 or 2, characterized in that, The grid-forming control switching to the grid-following control comprises the following steps: The angle generated by the three-phase grid voltage through PLL phase locking is collected as the phase angle of the grid-following control, and a voltage-current double-loop control structure is adopted; the direct current voltage of the three-phase submodule is collected after being filtered by an output filter, and the average value of the total submodule voltage is calculated as the feedback value of the outer loop voltage loop control; the difference between the feedback value and the reference value of the outer loop voltage loop control is input into a PI controller to generate an active current instruction, and a virtual impedance is used to generate a reactive current instruction; the output current of the static synchronous compensator is collected after being filtered by a filter, and the feedback value of the inner loop current loop control is obtained; the difference between the reference value and the feedback value is input into a fundamental wave controller and superimposed with the grid voltage feedforward to output a fundamental wave modulation wave.

4. The method of claim 1 or 2, wherein The grid-forming control comprises VSG control, excitation control and virtual impedance control. The method for generating the reactive current instruction through the virtual impedance control comprises the following steps: ； wherein: , are the d, q-axis components of the three-phase internal potential, respectively, , are the d, q-axis components of the machine terminal voltage, respectively, , are the d, q-axis components of the stator current, respectively, R, L are the stator resistance and stator inductance, respectively; The method for determining the d, q axis components of the three-phase internal electromotive force through the excitation control comprises the following steps: ； wherein, V0 is the no-load internal potential; , KQ is the reactive power closed-loop PI parameter; Q is the VSG reactive power; K is the voltage regulation coefficient; Vn is the rated terminal voltage; V is the terminal voltage amplitude.

5. The method of claim 4, wherein, The method for the direct current additional power control comprises the following steps: the difference between the reference direct current voltage and the feedback value of the total submodule direct current voltage is input into a direct current droop control to obtain an additional active power reference value, which is used as the active power reference value of the VSG control, and the output power of the static synchronous compensator is adjusted in real time.

6. The method of claim 4, wherein, The method for determining the d, q axis components of the machine terminal voltage comprises the following steps: The three-phase grid voltage is collected after being filtered by a filter, and the d, q axis components of the machine terminal voltage are generated by PARK transformation according to the angle generated by the VSG control; The model of the VSG control comprises the following steps: ； wherein ; wherein: is the angular velocity of the VSG; is the rated angular velocity of the VSG; J is the moment of inertia of the VSG; and are the mechanical and electromagnetic torques of the VSG, respectively; P ref is the reference active power; D is the damping coefficient; θ2is the angle generated by the VSG control.

7. The method of claim 1 or 2, wherein The mode of switching from grid-following control to grid-forming control includes: generating an angle by a virtual synchronous machine, giving an active power instruction as 0, generating an internal potential by an excitation controller, generating an active current instruction and a reactive current instruction by a virtual impedance control, and performing current inner loop control in the same way as grid-following control.

8. The method of claim 1 or 2, wherein, The inter-phase voltage control adopts zero sequence voltage injection or negative sequence current control.

9. A sub-module voltage control system of a static synchronous compensator comprising a processor having executable program instructions stored therein, the processor being configured to: The executable program instructions are used to be executed to implement the sub-module voltage control method of the static synchronous compensator in any one of claims 1-8.

10. A static synchronous compensator comprising a sub-module voltage control system, characterized in that, The sub-module voltage control system is the sub-module voltage control system of the static synchronous compensator in claim 9.

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