System and method for controlling voltage source type current converter with mixed network following and network construction proportion

Abstract

The invention relates to the technical field of converter control, and particularly discloses a network following and network construction proportion mixed voltage source type converter control system and method, and the system comprises a data obtaining module which is configured to obtain a grid connection point voltage, a GFM control reference voltage, and a GFL control reference voltage; the GFM control module is configured to calculate the grid-connected point voltage of the GFM and the current output to the power grid by the GFM, and obtain the target voltage of the GFM based on the selected control method; the GFL control module is configured to calculate the grid-connected point voltage of the GFL and the current output by the GFL to the power grid, and obtain the target voltage of the GFL based on the selected control method; and the mixing module is configured to mix the target voltage of the GFM and the target voltage of the GFL according to a selected mixing ratio in response to the switching control instruction to obtain the target voltage required by the voltage source type converter. According to the invention, constant-speed switching among different control methods can be realized; and fluctuation caused by control sudden change is avoided.

Description

Technical Field

[0001] The present invention relates to the technical field of converter control, and in particular to a voltage source converter control system and method with a mixed grid-following and grid-building ratio. 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] With the increasing popularity of renewable energy generation, voltage source converter (VSC), as an important node connecting renewable energy and power grid, has been widely used due to its flexible control and strong applicability. In order to improve the safety and stability of renewable energy generation, many VSC control strategies have been proposed, which can generally be divided into two categories: grid-connected control (GFM) and follower control (GFL).

[0004] GFL is the most commonly used control method for VSC. It tracks the phase of the voltage at the point of common coupling (PCC) through a phase-locked loop (PLL) to prevent it from forming a grid voltage, thereby achieving VSC grid connection. Due to its current source characteristics, its control method relies on a strong grid; GFM is designed to address this limitation. The synchronization method of GFM is similar to that of a synchronous generator. It adjusts the frequency of the reference voltage by exchanging power with the grid, showing voltage source characteristics. A typical GFM does not require a phase-locked loop and has the ability to automatically build voltage and frequency. Although GFM is capable of building a grid, it is more suitable for weak grids because of its relatively low stability in a strong grid.

[0005] In addition, due to their voltage source characteristics, significant short-circuit currents can be generated in the event of a short-circuit fault, which limits their application in some cases. Different types of GFMs and GFLs are suitable for different environments, each with its own advantages and limitations. However, in power system operation, VSCs may operate in different situations, requiring the use of different types of GFLs and GFMs, or operating from GFL to GFM to improve stability and resilience. Therefore, a universal VSC control architecture that combines the advantages of GFMs and GFLs is urgently needed.

[0006] To solve the above problems, the prior art proposes two solutions: The first method is to use the switching method between GFM and GFL to switch the corresponding GFM and GFL control strategies according to the operation requirements. In order to solve the disturbance stability problem caused by the switching process, some scholars have proposed adding a pre-synchronization link in the switching control. However, this requires the use of multiple switches for control switching, which increases the complexity of control.

[0007] The second method is to combine GFM control and GFL control. Hybrid control based on the parallel operation of equivalent GFM and GFL provides a new research idea. However, the ratio of GFM to GFL fluctuates with the change of circuit parameters, affecting the control performance. Starting from the basic difference between GFM and GFL, the control that combines the power synchronization loop (PSL) and the phase-locked loop (PLL) combines the characteristics of GFM and GFL. However, VSC always operates under hybrid control and does not operate as GFM or GFL alone. Since the control targets of GFM and GFL are different, the control target under hybrid control is unclear, and its operation stability needs further study.

[0008] In addition, these solutions only target a specific single control mode and are not universally applicable to a variety of GFMs and GFLs. Summary of the invention

[0009] In order to solve the above problems, the present invention proposes a voltage source converter control system and method with a mixed ratio of following the grid and building the grid. By adjusting the proportion of the target voltage of the control strategy, smooth switching between different controls is achieved, and it is also applicable to a variety of typical GFM and GFL control methods.

[0010] In some embodiments, the following technical solutions are adopted: A voltage source converter control system with a mixed grid-following and grid-building ratio, comprising: A data acquisition module, configured to acquire a grid connection point voltage, a GFM control reference voltage, and a GFL control reference voltage; The GFM control module is configured to calculate the grid connection point voltage of the GFM and the current output by the GFM to the grid, and obtain the target voltage of the GFM based on the selected control method using the voltage and current; The GFL control module is configured to calculate the grid connection point voltage of the GFL and the current output by the GFL to the grid, and obtain the target voltage of the GFL based on the selected control method using the voltage and current; The mixing module is configured to mix the target voltage of the GFM and the target voltage of the GFL according to a selected mixing ratio in response to the switching control instruction to obtain the target voltage required by the voltage source converter.

[0011] As a further solution, the switching rates in the normal operating state and the fault state are set respectively, and after receiving the switching control instruction, the target voltage required by the voltage source converter is switched at a uniform speed according to the switching rate.

[0012] As a further solution, in the GFM control module, the current output by the GFM to the grid Specifically: ; in, is the grid connection point voltage of GFM, is the GFM control reference voltage, R is the resistance between the voltage source converter and the grid connection point, L is the inductance between the voltage source converter and the grid connection point, and s is a complex variable in the frequency domain.

[0013] As a further solution, in the GFL control module, the current output by the GFL to the grid Specifically: ; in, is the grid connection point voltage of GFM, is the GFL control reference voltage, R is the resistance between the voltage source converter and the grid connection point, L is the inductance between the voltage source converter and the grid connection point, and s is a complex variable in the frequency domain.

[0014] As a further solution, in the mixing module, the target voltage of the GFM and the target voltage of the GFL are mixed according to a set ratio, specifically: ; in, is the target voltage required by the voltage source converter, is the target voltage of GFM, is the target voltage of GFL, is the proportionality coefficient, .

[0015] In other embodiments, the following technical solutions are adopted: A voltage source converter control method with a mixed grid-following and grid-building ratio, comprising: Obtain the grid connection point voltage, GFM control reference voltage and GFL control reference voltage; Calculating the grid connection point voltage of the GFM and the current output by the GFM to the grid, and using the voltage and current to obtain the target voltage of the GFM based on the selected control method; Calculate the grid connection point voltage of the GFL and the current output by the GFL to the grid, and obtain the target voltage of the GFL based on the selected control method using the voltage and current; In response to the switching control instruction, a value of the mixing ratio is selected, and the target voltage of the GFM and the target voltage of the GFL are mixed according to the mixing ratio to obtain the target voltage required by the voltage source converter.

[0016] As a further solution, the value of the mixing ratio is adjusted according to the operating state of the power grid. When k=1, it works in the GFM operating mode, and when k=0, it works in the GFL operating mode.

[0017] As a further solution, the switching rates in the normal operating state and the fault state are set respectively, and after receiving the switching control instruction, the target voltage required by the voltage source converter is switched at a uniform speed according to the switching rate.

[0018] Compared with the prior art, the present invention has the following beneficial effects: (1) The unified control system and method proposed in the present invention can realize switching between different control methods by linearly changing the proportion of the target voltage of GFM control and GFL control; the change of the proportion is obtained by linear adjustment according to the set switching rate, which can realize uniform speed switching and avoid fluctuations caused by sudden control changes. The GFM and GFL control systems always maintain a closed-loop operation state, which can ensure that no disturbance will be generated during the control switching process and achieve smooth switching.

[0019] By adjusting the proportion of the two, various control strategies such as separate GFM control, separate GFL control, uniform speed transition of GFM control and GFL control, and mixed control of different proportions can be realized according to specific working conditions to adapt to different operating conditions of VSC.

[0020] (2) The unified control system architecture of the present invention can be applied to various types of GFM and GFL control methods, and is not limited to a specific GFM and GFL control method, and has a certain degree of universality.

[0021] Other features and 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 the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the control system architecture of a voltage source converter with a mixed proportion of following the grid and building the grid in an embodiment of the present invention; Figure 2 Schematic diagram of the control architecture of GFL in an embodiment of the present invention; Figure 3 Schematic diagram of the control architecture of the GFM in an embodiment of the present invention; Figure 4 This is a block diagram of the GFL inner loop control used for stability analysis in an embodiment of the present invention; Figure 5 In the embodiment of the present invention k Root locus diagram of the system when the value changes; Figure 6 This is a topology diagram of a renewable energy 9-node system in an embodiment of the present invention; Figure 7 is an active power diagram of VSC3 under working condition 1 in an embodiment of the present invention; Figure 8 : is a system frequency diagram under working condition 1 in an embodiment of the present invention; Fig. 9 : is a voltage diagram of the grid connection point of VSC3 under working condition 1 in an embodiment of the present invention; Fig.10 is a reactive power diagram emitted by the VSC3 under working condition 1 in an embodiment of the present invention; Fig.11 is an active power diagram of VSC3 under working condition 2 in an embodiment of the present invention; Fig.12 : is a system frequency diagram under working condition 2 in an embodiment of the present invention; Fig.13 2 is a voltage diagram of the grid connection point of VSC3 under working condition 2 in an embodiment of the present invention; Fig.14 is a reactive power diagram emitted by the VSC3 under working condition 2 in an embodiment of the present invention; Fig.15 : is a diagram of VSC3 output current under working condition 3 in an embodiment of the present invention; Fig.16 3 is a voltage diagram of the grid connection point of VSC3 under working condition 3 in an embodiment of the present invention. DETAILED DESCRIPTION

[0023] It should be noted that the following detailed descriptions are illustrative and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used in the present invention have the same meanings as those commonly understood by those skilled in the art to which the present application belongs.

[0024] 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 application. 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 "comprise" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0025] Embodiment 1 In one or more embodiments, a voltage source converter control system with a mixed grid-following and grid-building ratio is disclosed, comprising: A data acquisition module, configured to acquire a grid connection point voltage, a GFM control reference voltage, and a GFL control reference voltage; The GFM control module is configured to calculate the grid connection point voltage of the GFM and the current output by the GFM to the grid, and obtain the target voltage of the GFM based on the selected control method using the voltage and current; wherein the selected control method can be any one of virtual synchronous machine control, droop control, matching control or virtual oscillation control, which has good versatility.

[0026] The GFL control module is configured to calculate the grid connection point voltage of the GFL and the current output by the GFL to the grid, and obtain the target voltage of the GFL based on the selected control method using the voltage and current; wherein the selected control method can be any one of power control, droop control or virtual synchronous machine control, which has good versatility.

[0027] The mixing module is configured to mix the target voltage of the GFM and the target voltage of the GFL according to a selected mixing ratio in response to the switching control instruction to obtain the target voltage required by the voltage source converter.

[0028] Specific, combined Figure 1 The data acquisition module obtains the three-phase voltage value of the grid connection point at the grid connection point V g,abc , GFL voltage reference value V ref , gfl,abc , GFM voltage reference value V ref,gfm,abc , and line parameters R f1 , L f , R c , L c Among them, R f1 is the resistance of the converter side of the voltage source converter and the grid connection point line, L f is the inductance of the converter side of the voltage source converter and the grid connection point line, R c is the resistance of the voltage source converter and the grid connection point on the line, L c is the inductance of the voltage source converter and the grid connection point on the line; i o,abc is the output current value.

[0029] The working process of the GFM control module is as follows: Based on the three-phase voltage value of the grid connection point V g,abc , GFM voltage reference value V ref,gfm,abc And line parameters, calculate the grid connection point voltage of GFM and the current output by GFM to the grid, where the grid connection point voltage of GFM is equal to the grid connection point voltage, that is: (1) The current output by GFM to the grid is: (2) in, is the GFM control reference voltage, which is equal to the target voltage of GFM; R is the resistance between the voltage source converter and the grid connection point, L is the inductance between the voltage source converter and the grid connection point, and s is a complex variable in the frequency domain.

[0030] In this embodiment, the subscript abc represents the corresponding value of any one of the three phases abc.

[0031] The working process of the GFL control module is as follows: Based on the grid voltage value V g,abc , GFL voltage reference value V ref,gfl,abc And line parameters, calculate the grid connection point voltage of GFL and the current output by GFL to the grid; where the grid connection point voltage of GFL is equal to the grid connection point voltage, that is: (3) The current output by GFL to the grid is: (4) in, V ref,gfm,abc It is the GFM control reference voltage, which is equal to the target voltage of GFL. In the hybrid module, the VSC controls the target voltage V ref,abc Target voltage through GFL V ref , gfl,abc The target voltage of GFM V ref,gfm,abc It is obtained by adding according to the set ratio, which is given by the following formula: (5) in, k is the percentage of GFM in VSC, ranging from [0, 1]. k By selecting different k value, it is possible to achieve separate GFM control of VSC ( k =1), single GFL control ( k =0), uniform speed switching transition between GFM control and GFL control, or mixed control with different proportions.

[0032] When a switching control instruction is received, the switching rate is selected according to the system operating conditions. g For example, the switching time is within 2 s in normal operation and within 1 ms in fault state. The corresponding switching rate can be selected according to these times. kThe value of can change uniformly at a set rate, which can ensure the stability of the VSC control mode switching process and avoid fluctuations caused by control mutations.

[0033] Embodiment 2 In one or more embodiments, a voltage source converter control method with a mixed grid-following and grid-building ratio is disclosed, specifically comprising: Obtain the grid connection point voltage, GFM control reference voltage and GFL control reference voltage; Calculating the grid connection point voltage of the GFM and the current output by the GFM to the grid, and using the voltage and current to obtain the target voltage of the GFM based on the selected control method; Calculate the grid connection point voltage of the GFL and the current output by the GFL to the grid, and obtain the target voltage of the GFL based on the selected control method using the voltage and current; In response to the switching control instruction, a value of the mixing ratio is selected, and the target voltage of the GFM and the target voltage of the GFL are mixed according to the mixing ratio to obtain the target voltage required by the voltage source converter.

[0034] The value of the mixing ratio is adjusted according to the operating status of the power grid. k =1 works in GFM operation mode. k =0 when it works in GFL operation mode. k When the value is between 0 and 1, it is the transition state of the switching between GFM and GFL control.

[0035] The switching rates in the normal operation state and the fault state are set respectively, and after receiving the switching control instruction, the target voltage required by the voltage source converter is switched at a uniform speed according to the switching rate.

[0036] The specific implementation method of the above process is the same as that in Example 1 and will not be described in detail.

[0037] In order to verify the practicability of the unified control system and control method proposed in this embodiment, the stability analysis of the proposed unified control architecture is performed. The stability analysis is performed by drawing the root locus, which reflects the scaling factor k The impact of changes on system stability.

[0038] The control architectures of GFL and GFM are as follows: Figure 2 and Figure 3 As shown in the figure, the current inner loop is the same. Considering that the outer loop has a high time constant, taking the GFL current inner loop as an example, the current target value at the input end is regarded as a constant, and other input ends are regarded as negligible disturbances. The GFL inner loop control architecture for stability analysis is shown in the figure. Figure 4 As shown; Figure 2-Figure 4The architectures shown are all prior art and therefore will not be described in detail.

[0039] Figure 5 for k The root locus diagram of the system when the value changes is given by Figure 5 It can be seen that when the percentage of GFM(k) is between 0 and 1, the root locus shows that all closed-loop poles are in the left half plane, indicating that the system is stable.

[0040] For example, this embodiment is simulated on a 9-node system with 100% renewable energy generation. Figure 6 As shown. The capacity of the three power sources is 200MVA, and the three power sources are connected to voltage source converters VSC1, VSC2 and VSC3 respectively; the bus voltage connected to VSC1 is 16.5kV, the output active power is 61.54MW, and the output reactive power is 5.94Mvar. The bus voltage connected to VSC2 is 18.0kV, the output active power is 125.3MW, and the output reactive power is 0.38Mvar. The bus voltage connected to VSC3 is 13.8kV, the output active power is 43.31MW, and the output reactive power is 0.35Mvar. The three load voltage levels are all 230kV, of which the 5-node load has an active power of 42.67MW and a reactive power of 16.67Mvar. The 6-node load has an active power of 1MW and a reactive power of 15Mvar. The 8-node load has an active power of 32.33MW and a reactive power of 11.67Mvar.

[0041] The control system and method proposed in this embodiment are used to control VSC3, VSC1 is controlled by the traditional GFM, and VSC2 is controlled by the traditional GFL. VSC3 initially operates in the GFL mode.

[0042] The method of this embodiment is applied to two different GFM and GFL control strategies respectively: ①GFM adopts droop control, GFL adopts PQ control system; ②GFM adopts VSG control and GFL adopts droop control.

[0043] At the same time, the existing direct control switching method and phase compensation control switching method are also applied to the above two different GFM and GFL control strategies.

[0044] The results obtained under different working conditions are compared to verify the superiority and versatility of the unified control system and method of this embodiment. Among them, the direct switching strategy is to switch directly from one control mode to another without any additional device. The phase compensation control switching strategy is to add a phase angle compensation device on the basis of the direct switching method, so that the phase angle will not change suddenly and reduce the fluctuation during the switching process.

[0045] Operating condition 1: When the power system runs for 7 seconds, VSC3 switches from GFL to GFM using the unified control method, direct control switching method and phase compensation control switching method proposed in this embodiment. The above three methods are applied to the two strategies ① and ② respectively, and six curves should be obtained. Since the direct switching method of control strategy ② will cause system collapse due to excessive interference, five curves are shown in each figure: direct switching 1, phase angle compensation switching 1, phase angle compensation switching 2, unified control architecture 1 and unified control architecture 2, which correspond to the direct switching method applied to strategy ①, the phase compensation control switching method applied to strategy ①, the phase compensation control switching method applied to strategy ②, the control method of this embodiment applied to strategy ① and the control method of this embodiment applied to strategy ②.

[0046] The obtained active power, system frequency, voltage and reactive power curves of VSC3 are as follows: Figure 7-10 It can be seen that under normal conditions, the proposed method is applicable to different GFM and GFL control strategies, and the fluctuation is significantly reduced compared with other methods.

[0047] Operating condition 2: VSC1, which operates as a GFM, disconnects after 7 seconds. At this time, only two GFLs are operating in the system. If the VSC only has GFL control and is unable to build a grid, the system will face collapse at this time. Ignoring the signal delay, VSC3 switches from GFL to GFM within 7 seconds. Each figure also shows 5 curves, and the meaning of the curves is the same as in Condition 1 and will not be described in detail.

[0048] The active power of VSC3, system frequency, voltage at PCC and reactive power are obtained as follows: Figure 11-14 As shown in the figure, it can be seen that unified control not only minimizes the interference of control switching, but also enters stable operation in the shortest time, achieving system support.

[0049] Operating condition 3: VSC3 operates in GFM mode. Figure 6 At the mark of , a single short circuit fault is set to 7s. Fig.15 and Fig.16In the figure, the comparison of the grid connection point voltage and output current of VSC3 in three different scenarios is given; each figure shows three curves: unified control architecture 1, phase angle compensation switching 1 and no switching, which correspond to the control switching of the unified control mode of this embodiment, the control switching with phase angle compensation and the grid connection point voltage or output current curve obtained by keeping GFM control applied to strategy ①. It can be seen that the unified control mode of this embodiment can minimize the short-circuit current, reduce the voltage drop and accelerate the voltage recovery.

[0050] 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 voltage source converter control system with a mixed proportion of grid-following and grid-building, characterized in that: include: A data acquisition module, configured to acquire a grid connection point voltage, a GFM control reference voltage, and a GFL control reference voltage; The GFM control module is configured to calculate the grid connection point voltage of the GFM and the current output by the GFM to the grid, and obtain the target voltage of the GFM based on the selected control method using the voltage and current; The GFL control module is configured to calculate the grid connection point voltage of the GFL and the current output by the GFL to the grid, and obtain the target voltage of the GFL based on the selected control method using the voltage and current; The mixing module is configured to mix the target voltage of the GFM and the target voltage of the GFL according to a selected mixing ratio in response to the switching control instruction to obtain the target voltage required by the voltage source converter.

2. A voltage source converter control system with a mixed grid-following and grid-building ratio as claimed in claim 1, characterized in that: The switching rates in the normal operation state and the fault state are set respectively, and after receiving the switching control instruction, the target voltage required by the voltage source converter is switched at a uniform speed according to the switching rate.

3. A voltage source converter control system with a mixed grid-following and grid-building ratio as claimed in claim 1, characterized in that: In the GFM control module, the current output by GFM to the grid Specifically: ； in, is the grid connection point voltage of GFM, is the GFM control reference voltage, R is the resistance between the voltage source converter and the grid connection point, L is the inductance between the voltage source converter and the grid connection point, and s is a complex variable in the frequency domain.

4. A voltage source converter control system with a mixed grid-following and grid-building ratio as claimed in claim 1, characterized in that: In the GFM control module, the selected control method includes: virtual synchronous machine control, droop control, matching control or virtual oscillation control.

5. A voltage source converter control system with a mixed grid-following and grid-building ratio as claimed in claim 1, characterized in that: In the GFL control module, the current output by the GFL to the grid Specifically: ； in, is the grid connection point voltage of GFM, is the GFL control reference voltage, R is the resistance between the voltage source converter and the grid connection point, L is the inductance between the voltage source converter and the grid connection point, and s is a complex variable in the frequency domain.

6. A voltage source converter control system with a mixed proportion of grid following and grid building as claimed in claim 1, characterized in that: In the GFL control module, the selected control method includes: power control, droop control or virtual synchronous machine control.

7. A voltage source converter control system with a mixed grid-following and grid-building ratio as claimed in claim 1, characterized in that: In the mixing module, the target voltage of the GFM and the target voltage of the GFL are mixed according to a set ratio, specifically: ； in, is the target voltage required by the voltage source converter, is the target voltage of GFM, is the target voltage of GFL, is the proportionality coefficient, .

8. A voltage source converter control method with a mixed grid-following and grid-building ratio, characterized in that: include: Obtain the grid connection point voltage, GFM control reference voltage and GFL control reference voltage; Calculating the grid connection point voltage of the GFM and the current output by the GFM to the grid, and using the voltage and current to obtain the target voltage of the GFM based on the selected control method; Calculate the grid connection point voltage of the GFL and the current output by the GFL to the grid, and obtain the target voltage of the GFL based on the selected control method using the voltage and current; In response to the switching control instruction, a value of the mixing ratio is selected, and the target voltage of the GFM and the target voltage of the GFL are mixed according to the mixing ratio to obtain the target voltage required by the voltage source converter.

9. A control method for a voltage source converter with a mixed proportion of following the grid and building the grid as claimed in claim 8, characterized in that: The value of the mixing ratio is adjusted according to the operating status of the power grid. When k=1, it works in the GFM operating mode, and when k=0, it works in the GFL operating mode.

10. A voltage source converter control method with a mixed proportion of following the grid and building the grid as claimed in claim 8, characterized in that: The switching rates in the normal operation state and the fault state are set respectively, and after receiving the switching control instruction, the target voltage required by the voltage source converter is switched at a uniform speed according to the switching rate.

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