Method and system for enhancing transient voltage and power angle stability of network-forming converter

Abstract

The invention discloses a method and a system for enhancing transient voltage and power angle stability of a network-forming converter, and belongs to the technical field of power electronics. The core of the method is the improved design of the reactive power controller of the network construction type converter. And in a transient working condition, switching of a reactive power control mode is realized, reactive power is switched from PI control to a voltage control mode, and the phenomenon that the voltage is continuously reduced due to transient voltage instability is avoided. And meanwhile, normal transmission of the active power can be ensured by the set voltage value, and the transient power angle instability phenomenon is avoided. According to the method provided by the invention, the transient voltage stability and the power angle stability of the converter can be enhanced at the same time, and transient instability caused by power grid faults is remarkably reduced.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, and in particular to a method and system for enhancing the transient voltage and power angle stability of a grid-connected converter. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] Renewable energy is mainly connected to the grid through power electronic converters. However, the large-scale replacement of synchronous generators by grid-connected converters has deteriorated the stability of the power grid. The reason is that mainstream grid-connected converters usually operate in a grid-following mode. Grid-following converters are equivalent to controlled current sources and have the following fundamental defects: (1) they cannot operate independently, (2) they are difficult to provide inertia, (3) they have weak frequency / voltage regulation capabilities, and (4) they are prone to instability in weak grid synchronization. To overcome the above defects, grid-connected converters have become the most suitable solution. Grid-connected converters are equivalent to controlled voltage sources and have the following advantages: (1) they can operate independently, (2) they can flexibly provide inertia, (3) they have strong frequency / voltage regulation capabilities, and (4) they have good stability in weak grids.

[0004] Compared with synchronous machines, grid-type converters have more flexible reactive power control methods. The reactive power control modes of grid-type converters include: constant reactive power model, constant power factor mode, reactive power-voltage droop mode, and constant voltage mode. The constant reactive power mode and constant power factor mode are implemented based on reactive power PI control. However, during grid faults, existing methods only focus on the transient power angle instability problem caused by active power control of grid-type converters, while ignoring the transient voltage instability problem caused by reactive power control, which makes the converter fault crossing easy to fail. Summary of the invention

[0005] In view of the deficiencies in the prior art, the purpose of the present invention is to provide a method and system for enhancing the transient voltage and power angle stability of a grid-connected converter, which can simultaneously enhance the transient voltage and power angle stability of the converter and significantly reduce transient instability caused by grid faults.

[0006] In order to achieve the above object, the present invention is implemented through the following technical solutions: A first aspect of the present invention provides a method for enhancing transient voltage and power angle stability of a grid-connected converter, comprising the following steps: Obtaining the operating state of the grid-connected converter, and building a grid-connected model of the grid-connected converter according to the operating state of the grid-connected converter; According to the grid-connected model of the grid-connected converter, a control model of the grid-connected converter is constructed based on the transient voltage instability mechanism and transient power angle instability mechanism of the grid-connected converter, wherein the control model of the grid-connected converter includes a reactive power control transient mode switching unit; The reactive power control transient mode switching unit is used to switch the mode according to the dynamic changes of the operating state of the grid-type converter, and the voltage control is used to suppress the transient voltage and power angle instability, thereby achieving enhanced stability.

[0007] Furthermore, the transient voltage instability mechanism is used to calculate the reactive power absorption limit before and after the fault under the PI control strategy, so as to identify the transient voltage instability.

[0008] Furthermore, the transient power angle instability mechanism is used to calculate the active power transfer limit under the droop control strategy, thereby identifying the transient power angle instability.

[0009] Furthermore, during normal operation, the grid-type converter adopts reactive power PI control to achieve constant reactive power control or constant power factor control; during transient operating conditions, the reactive power PI control switches to voltage control mode, sets a voltage control reference value, and adjusts the voltage control value according to the voltage control reference value, thereby avoiding transient voltage instability caused by reactive power PI control.

[0010] Furthermore, to avoid transient power angle instability caused by active power droop control, the voltage control value must meet the following requirements: , in, is the voltage control value, represents the reactance of the transmission line, is the reference value of the active power output by the grid-connected converter, Indicates the voltage amplitude of the power grid.

[0011] A second aspect of the present invention provides a grid-type converter transient voltage and power angle stability enhancement system, comprising: A model building module is configured to obtain the operating state of the grid-connected converter and build a grid-connected model of the grid-connected converter according to the operating state of the grid-connected converter; A control design module is configured to construct a grid-type converter control model based on a grid-type converter grid-connected model and a transient voltage instability mechanism and a transient power angle instability mechanism of the grid-type converter, wherein the grid-type converter control model includes a reactive power control transient mode switching unit; The mode switching module is configured to utilize the reactive power control transient mode switching unit to perform mode switching according to the dynamic changes in the operating state of the grid-type converter, and utilize voltage control to suppress transient voltage and power angle instability, thereby achieving enhanced stability.

[0012] Furthermore, the control design module includes a transient voltage instability module, which is configured to calculate the reactive power absorption limit before and after the fault under the PI control strategy using the transient voltage instability mechanism, thereby identifying transient voltage instability.

[0013] Furthermore, the control design module includes a transient power angle instability module, which is configured to calculate the active power transfer limit under the droop control strategy using the transient power angle instability mechanism, thereby identifying transient power angle instability.

[0014] Furthermore, in the mode switching module, during normal operation, the grid-type converter adopts reactive power PI control to achieve constant reactive power control or constant power factor control; during transient operating conditions, the reactive power PI control switches to the voltage control mode, sets the voltage control reference value, and adjusts the voltage control value according to the voltage control reference value, thereby avoiding transient voltage instability caused by reactive power PI control.

[0015] Furthermore, in the mode switching module, to avoid transient power angle instability caused by active power droop control, the voltage control value must meet the following requirements: , in, is the voltage control value, represents the reactance of the transmission line, is the reference value of the active power output by the grid-connected converter, Indicates the voltage amplitude of the power grid.

[0016] One or more of the above technical solutions have the following beneficial effects: The present invention discloses a method and system for enhancing the transient voltage and power angle stability of a grid-type converter. In transient working conditions, the reactive power control mode is switched, and the reactive power is switched from PI control to voltage control mode to avoid the phenomenon of continuous voltage reduction caused by transient voltage instability. At the same time, it is ensured that the set voltage value can ensure the normal transmission of active power and avoid transient power angle instability. Therefore, the method proposed in the present invention can take into account the transient voltage and power angle stability of the grid-type converter at the same time, and achieve the simultaneous enhancement of the transient voltage and power angle stability of the converter, helping the converter to successfully achieve fault ride-through, and significantly reducing the transient instability caused by power grid faults.

[0017] The method proposed by the present invention has no additional hardware cost, is simple to implement, has a rapid response, has obvious effects, and is easy to expand and apply.

[0018] Advantages of additional aspects of the present invention will be given in part in the following description, and in part will become obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0020] Figure 1 This is a flow chart of a method for enhancing transient voltage and power angle stability of a grid-type converter in Embodiment 1 of the present invention; Figure 2 Schematic diagram of the basic structure of the grid-type converter in the first embodiment of the present invention; Figure 3 Schematic diagram of reactive power PI controller of grid-type converter in Embodiment 1 of the present invention; Figure 4 Schematic diagram of active power droop controller of grid-type converter in Embodiment 1 of the present invention; Figure 5 This is a schematic diagram of a simplified grid-connected model of a grid-connected converter in Embodiment 1 of the present invention; Figure 6 It is a schematic diagram of the transient voltage instability principle of the grid-type converter in the first embodiment of the present invention; Figure 7 It is a schematic diagram of the transient power angle instability principle of the grid-type converter in the first embodiment of the present invention; Figure 8 Schematic diagram of transient mode switching of reactive power control of a grid-connected converter in Embodiment 1 of the present invention; Fig. 9 It is a simulation waveform diagram of active power, reactive power, voltage amplitude and power angle of a grid-type converter when the method for enhancing the transient voltage and power angle stability of a grid-type converter is not used in the first embodiment of the present invention; Fig.10 It is a simulation waveform diagram of active power, reactive power, voltage amplitude and power angle of a grid-type converter after using the method for enhancing transient voltage and power angle stability of a grid-type converter in the first embodiment of the present invention. DETAILED DESCRIPTION

[0021] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0022] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "include" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or their combinations; Embodiment 1: Embodiment 1 of the present invention provides a method for enhancing the transient voltage and power angle stability of a grid-connected converter, such as Figure 1 As shown, the following steps are included: Step 1: Obtain the operating status of the grid-connected converter, and build a grid-connected model of the grid-connected converter according to the operating status of the grid-connected converter.

[0023] like Figure 2 The basic structure of the grid-type converter is shown in Figure 1. The grid-type converter consists of a DC power supply (the DC side voltage is v gdc ) as input, through LCL Filter (by the inductor on the converter side L gi , filter capacitor C gf , grid side inductance L gg ) is connected to the grid (grid voltage is v sabc ). v gfabc and i giabc are the output voltage and current of the converter. v gabc and i ggabc The voltage and current at the point of common coupling (PCC). In addition, Figure 1 The coordinate transformation unit, control unit, power calculation unit, and PWM modulation unit of the grid-type converter are demonstrated. The coordinate transformation unit realizes the voltage and current from abc Coordinate system to dq Coordinate system transformation, the input of which is the phase of the converter voltage θ g and abc Voltage in coordinate system v gfabc (or current i ggabc ), the output is dq Voltage in coordinate system v gf_d and v gf_q (or current i gi_d and i gi_q ). The control unit includes a current controller G i ( s ), voltage controller G v ( s ), Active Power Controller G p ( s ) and reactive power controller Gq ( s ). Current controller G i ( s ) to achieve output current i gi_d and i gi_q Track its reference value i gi_ref_d and i gi_ref_q ; Voltage controller G v ( s ) to achieve output voltage v gf_d and v gf_q Track its reference value v gf_ref_d and v gf_ref_q ; Active power controller G p ( s ) and reactive power controller G q ( s ) Output phase of grid-connected converter θ g and voltage amplitude v gf_ref_d , to ensure output power p g / q g Tracks the reference value of its input p g_ref / q g_ref The power measurement unit is v gfabc and i ggabc Calculate the output power of the converter. The PWM modulation unit is responsible for controlling the output of the switching device.

[0024] In addition, the active power controller determines the phase of the grid-type converter, and the reactive power controller determines the voltage amplitude of the grid-type converter.

[0025] Step 2: According to the grid-connected model of the grid-connected converter, a control model of the grid-connected converter is constructed based on the transient voltage instability mechanism and transient power angle instability mechanism of the grid-connected converter.

[0026] Among them, the grid-connected converter control model includes a reactive power control transient mode switching unit.

[0027] Step 2.1: Construct the original control structure of the grid-type converter, such as Figure 3and Figure 4 shown. Figure 3 and Figure 4 They are the structures of reactive power PI control and active power droop control of grid-connected converters, respectively. Where ref represents the reference value, Δ represents the variation, and pu represents the per-unit value. p g and q g They are the active power and reactive power output by the grid-connected converter respectively. v gf Represents the voltage amplitude of the grid-connected converter. δ g Represents the phase difference between the grid-connected converter and the grid. S 0 is the rated power, ω 0 is the rated frequency, k pf Represents the active power droop coefficient. Figure 3 In the reactive power reference value q g_ref With output value q g The difference is passed through the PI controller ( k qp + k qi / s ,in k qp and k qi are proportional and integral coefficients respectively) to obtain the voltage change value Δ v gf . Δ v gf Voltage rating V 0 Get the reference value of the converter voltage v gf_ref_d . Figure 4 In the active power reference value p g_ref With output value p g Divide by rated power S 0 to achieve normalization, and then the difference between the two is obtained, and the droop controller ( k pf represents the droop coefficient) to obtain the per-unit frequency change Δ ω g_pu . Δ ω g_pu Rated value of multiplication frequency ω 0, and then integrate to get the phase difference between the converter voltage and the grid voltage δ g The phase difference between the converter voltage and the grid voltage is the power angle.

[0028] Step 2.2: Simplify the original control structure of the grid-connected converter to obtain Figure 5 The simplified model of the grid-connected grid-connected converter is shown in Figure 1. Since the AC voltage control speed is fast and the accuracy is high, the grid-connected converter can be regarded as a controllable AC voltage source. The converter and grid voltage are represented as voltage vectors v gf ∠ δ g and v s ∠0, v gf and v s They represent the voltage amplitude of the grid-connected converter to the grid, and the phase of the grid is taken as zero radians as a reference. In addition, X gs and R gs represent the reactance and resistance of the transmission line respectively, and the dynamics of the transmission line are neglected. In addition, due to X gs Often much greater than R gs Therefore, the influence of line resistance is ignored. Further, the reactive and active power transmitted from the converter to the grid can be expressed as: (1) , (2).

[0029] Step 2.3: Use the transient voltage instability mechanism to calculate the reactive power absorption limit before and after the fault under the PI control strategy, so as to identify transient voltage instability.

[0030] In a specific implementation, the transient voltage instability mechanism of the grid-connected converter is as follows: Figure 6 As shown. Figure 6 , and ignore the influence of the inner loop control (i.e. v gf = v gf_ref_d ), the reactive power PI control can be expressed as: (3).

[0031] in and They represent the proportional gain and integral gain of the reactive power controller respectively. Indicates the voltage reference value of the grid-connected converter. Substituting (1) into (3) yields: (4).

[0032] According to formula (1), Figure 6 Shows the before and after of the failure q g – v gf curve. q g_min represents the limit of reactive power absorption, and it can be deduced that: (5).

[0033] Obviously, after the failure q g_min If it appears q g_min > q g_ref , the equilibrium point, i.e. Figure 6 In v gf1 and v gf2 will be missing. According to formula (4), d v gf / d t Always less than 0 ，v gf It keeps decreasing, resulting in transient voltage instability.

[0034] Step 2.4: Calculate the active power transfer limit under the droop control strategy using the transient power angle instability mechanism, thereby identifying transient power angle instability.

[0035] In a specific implementation, the transient power angle instability mechanism of the grid-connected converter is as follows: Figure 7 As shown. Figure 7 , the active power droop control can be expressed as: (6).

[0036] Substituting formula (2) into formula (6), we can get the expression of active power control: (7).

[0037] According to formula (2), Figure 7 Shows the before and after of the failure p g – δ g Curve. Among them, p g_max represents the limit of active power transfer and can be derived as: (8).

[0038] After a fault such as a grid voltage drop, p g_max If it occurs pg_ref > p g_max , the equilibrium point, that is Figure 7 In δ gf1 and δ gf2 will be missing, d δ g / d t Always greater than 0 , δ g Divergence and transient power angle instability occur.

[0039] Step 2.5: Design a new grid-connected converter reactive power controller, such as Figure 8 shown.

[0040] Step 3: Use the reactive power control transient mode switching unit to switch the mode according to the dynamic changes in the operating state of the grid-connected converter, and use voltage control to suppress transient voltage and power angle instability, thereby enhancing stability.

[0041] In a specific implementation, the reactive power controller of the grid-type converter includes a reactive power control transient mode switching unit to achieve switching between reactive power PI control and voltage control modes.

[0042] Specifically, Figure 8 As shown, During normal operation, the mode switching unit switches S q = 0, the grid-type converter adopts reactive power PI control to achieve constant reactive power control or constant power factor control, which can ensure the economic operation of the converter. The structure of the reactive PI controller is: reactive power reference value q g_ref With output value q g The difference is passed through the PI controller ( k qp + k qi / s ) to obtain the voltage change value Δ v gf , Δ v gf Voltage rating V 0 Get the reference value of the converter voltage v gf_pi .

[0043] In transient conditions, the switch S q= 1, reactive power PI control switches to voltage control mode. Reactive power PI control is removed and switched to voltage control mode, directly setting the reference value of voltage control v gf_vo In the voltage control mode, the voltage control reference value is set, and the voltage control value is adjusted according to the voltage control reference value, thereby avoiding the transient voltage instability caused by reactive power PI control.

[0044] The voltage control mode of the grid-type converter can realize constant voltage operation of the grid-type converter and ensure that the active power transmission limit is not lower than the active power reference value.

[0045] Specifically, according to formula (8), the transmission limit of active power is p g_max The voltage amplitude of the grid-connected converter v gf About. p g_max > p g_ref The transient power angle phenomenon caused by active power exceeding the limit can be avoided. Combined with formula (8), it can be obtained that in order to avoid the transient power angle instability phenomenon caused by active power droop control, the voltage control value needs to satisfy: (9).

[0046] in, is the voltage control value, represents the reactance of the transmission line, is the reference value of the active power output by the grid-connected converter, Indicates the voltage amplitude of the power grid.

[0047] Therefore, during the transient period, the reference value of the voltage control is v gf_vo Greater than .

[0048] It should be noted that the reactive power controller of the grid-type converter including the reactive power control transient mode switching unit can be applied to the reactive power PI control of the grid-type converter, and other control strategies such as reactive power droop control and reactive power droop control with inertia can also be used.

[0049] Fig. 9 It shows that when the method of the present invention is not used, the voltage amplitude of the grid-type converter is v g , Active Power p g , reactive power q g , power angle δ g and currenti gg As shown in the figure, after the fault (grid voltage drop) occurs, the converter voltage amplitude v g Continue to decline or even collapse, power angle δ g The divergence situation means that the converter has transient voltage and power angle instability at the same time.

[0050] Fig.10 It shows that after using the method of the present invention, the voltage amplitude of the grid-type converter is reduced. v g , Active Power p g , reactive power q g , power angle δ g and current i gg As shown in the figure, after the fault occurs, the grid-connected converter can operate stably, indicating that this method can simultaneously enhance the transient voltage and power angle stability of the grid-connected converter.

[0051] Embodiment 2: Embodiment 2 of the present invention provides a grid-connected converter transient voltage and power angle stability enhancement system, including: A model building module is configured to obtain the operating state of the grid-connected converter and build a grid-connected model of the grid-connected converter according to the operating state of the grid-connected converter; A control design module is configured to construct a grid-type converter control model based on a grid-type converter grid-connected model and a transient voltage instability mechanism and a transient power angle instability mechanism of the grid-type converter, wherein the grid-type converter control model includes a reactive power control transient mode switching unit; The control design module includes a transient voltage instability module and a transient power angle instability module, wherein the transient voltage instability module is configured to calculate the reactive power absorption limit before and after the fault under the PI control strategy using the transient voltage instability mechanism, thereby identifying transient voltage instability. The transient power angle instability module is configured to calculate the active power transmission limit under the droop control strategy using the transient power angle instability mechanism, thereby identifying transient power angle instability.

[0052] The mode switching module is configured to utilize the reactive power control transient mode switching unit to perform mode switching according to the dynamic changes in the operating state of the grid-type converter, and utilize voltage control to suppress transient voltage and power angle instability, thereby achieving enhanced stability.

[0053] In the mode switching module, during normal operation, the grid-type converter adopts reactive power PI control to achieve constant reactive power control or constant power factor control; during transient conditions, the reactive power PI control switches to voltage control mode, sets the voltage control reference value, and adjusts the voltage control value according to the voltage control reference value, thereby avoiding transient voltage instability caused by reactive power PI control.

[0054] In the mode switching module, to avoid transient power angle instability caused by active power droop control, the voltage control value must meet the following requirements: .

[0055] in, is the voltage control value, represents the reactance of the transmission line, is the reference value of the active power output by the grid-connected converter, Indicates the voltage amplitude of the power grid.

[0056] The steps involved in the above embodiment 2 correspond to those in the method embodiment 1. For the specific implementation method, please refer to the relevant description part of the embodiment 1.

[0057] Those skilled in the art should understand that the modules or steps of the present invention described above can be implemented by a general-purpose computer device, or alternatively, they can be implemented by a program code executable by a computing device, so that they can be stored in a storage device and executed by the computing device, or they can be made into individual integrated circuit modules, or multiple modules or steps therein can be made into a single integrated circuit module for implementation. The present invention is not limited to any specific combination of hardware and software.

[0058] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. A method for enhancing transient voltage and power angle stability of a grid-connected converter, characterized in that: The following steps are involved: Obtaining the operating state of the grid-connected converter, and building a grid-connected model of the grid-connected converter according to the operating state of the grid-connected converter; According to the grid-connected model of the grid-connected converter, a control model of the grid-connected converter is constructed based on the transient voltage instability mechanism and transient power angle instability mechanism of the grid-connected converter, wherein the control model of the grid-connected converter includes a reactive power control transient mode switching unit; The reactive power control transient mode switching unit is used to switch the mode according to the dynamic changes of the operating state of the grid-type converter, and the voltage control is used to suppress the transient voltage and power angle instability, thereby achieving enhanced stability.

2. The method for enhancing transient voltage and power angle stability of a grid-connected converter according to claim 1, characterized in that: The transient voltage instability mechanism is used to calculate the reactive power absorption limit before and after the fault under the PI control strategy, so as to identify the transient voltage instability.

3. The method for enhancing transient voltage and power angle stability of a grid-connected converter according to claim 1, characterized in that: The transient power angle instability mechanism is used to calculate the active power transfer limit under the droop control strategy, thereby identifying the transient power angle instability.

4. The method for enhancing transient voltage and power angle stability of a grid-connected converter according to claim 1, characterized in that: During normal operation, the grid-type converter adopts reactive power PI control to achieve constant reactive power control or constant power factor control; during transient operating conditions, the reactive power PI control switches to voltage control mode, sets the voltage control reference value, and adjusts the voltage control value according to the voltage control reference value, thereby avoiding transient voltage instability caused by reactive power PI control.

5. The method for enhancing transient voltage and power angle stability of a grid-connected converter according to claim 4, characterized in that: To avoid transient power angle instability caused by active power droop control, the voltage control value must meet the following requirements: ， in, is the voltage control value, represents the reactance of the transmission line, is the reference value of the active power output by the grid-connected converter, Indicates the voltage amplitude of the power grid.

6. A grid-type converter transient voltage and power angle stability enhancement system, characterized in that: include: A model building module is configured to obtain the operating state of the grid-connected converter and build a grid-connected model of the grid-connected converter according to the operating state of the grid-connected converter; A control design module is configured to construct a grid-type converter control model based on a grid-type converter grid-connected model and a transient voltage instability mechanism and a transient power angle instability mechanism of the grid-type converter, wherein the grid-type converter control model includes a reactive power control transient mode switching unit; The mode switching module is configured to utilize the reactive power control transient mode switching unit to perform mode switching according to the dynamic changes in the operating state of the grid-type converter, and utilize voltage control to suppress transient voltage and power angle instability, thereby achieving enhanced stability.

7. The grid-type converter transient voltage and power angle stability enhancement system according to claim 6, characterized in that: The control design module includes a transient voltage instability module, which is configured to calculate the reactive power absorption limit before and after the fault under the PI control strategy using the transient voltage instability mechanism, so as to identify the transient voltage instability.

8. The grid-connected converter transient voltage and power angle stability enhancement system according to claim 6, characterized in that: The control design module includes a transient power angle instability module, which is configured to calculate the active power transfer limit under the droop control strategy using the transient power angle instability mechanism, thereby identifying the transient power angle instability.

9. The grid-connected converter transient voltage and power angle stability enhancement system according to claim 6, characterized in that: In the mode switching module, during normal operation, the grid-type converter adopts reactive power PI control to achieve constant reactive power control or constant power factor control; during transient conditions, the reactive power PI control switches to voltage control mode, sets the voltage control reference value, and adjusts the voltage control value according to the voltage control reference value, thereby avoiding transient voltage instability caused by reactive power PI control.

10. The grid-connected converter transient voltage and power angle stability enhancement system according to claim 9, characterized in that: In the mode switching module, to avoid transient power angle instability caused by active power droop control, the voltage control value must meet the following requirements: ， in, is the voltage control value, represents the reactance of the transmission line, is the reference value of the active power output by the grid-connected converter, Indicates the voltage amplitude of the power grid.

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