A phase angle limiting control method and system for a grid-connected converter

Abstract

The present invention provides a phase angle limiting control method and system for a grid-type converter, which applies limiting control to the synchronous link of the grid-type converter, limits the phase angle change during the fault of the grid-type converter, and limits the voltage of the fixed grid point to a preset range of motion during the fault. The present invention applies limiting control to the synchronous link of the GFM converter to limit the phase angle change during the fault of the GFM converter, and ensures that the GFM converter can smoothly return to a normal operating state after the fault is cleared, so as to improve the fault recovery capability of the GFM converter.

Description

Technical Field

[0001] The invention belongs to the technical field of converter control, and in particular relates to a phase angle limiting control method for a grid-type 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] The penetration rate of renewable energy in today's power system is getting higher and higher, and voltage source converters (VSC) are widely used. The new energy station obtains the phase information of the point of common coupling (PCC) through the phase-locked loop (PLL), and uses classic vector current control to adjust the angle of the VSC so that the desired power is delivered to the main grid through the VSC. This type of converter is called a grid-following (GFL) converter. Engineering practice shows that the increase of GFL converters will have an adverse effect on the stability and security of the power system, especially when they are applied to weak AC grids.

[0004] To address these challenges, it is a promising approach to change the control mode of VSC to grid-forming (GFM) converters, which can operate well as voltage sources in constant voltage control (CVC) mode, providing necessary support for the normal operation of the power grid. However, due to the voltage source characteristics of GFM converters, overcurrent protection during faults is one of the key issues that hinder the widespread application of GFM converters.

[0005] The current limiting control (CLC) strategy based on the current saturation algorithm (CSA) can effectively constrain the fault current. This strategy generates the dq axis fault current reference value through the classic vector current control. However, existing studies have pointed out that the GFM converter may be locked in the CLC mode after the fault is cleared and cannot be restored to the CVC mode. While preventing overcurrent, the GFM converter is also required to be able to return to normal operation after the fault is cleared. Summary of the invention

[0006] In order to solve the above problems, the present invention proposes a phase angle limiting control method for a grid-type converter. The present invention applies limiting control to the synchronization link of the GFM converter to limit the phase angle change during the GFM converter fault, thereby ensuring that the GFM converter can smoothly return to normal operation after the fault is cleared, thereby improving the fault recovery capability of the GFM converter.

[0007] According to some embodiments, the present invention adopts the following technical solutions:

[0008] In a first aspect, a phase angle limiting control method for a grid-connected converter is provided, comprising the following steps:

[0009] When fault recovery is unsuccessful, limiting control is applied to the synchronization link of the grid-forming converter to limit the phase angle change during the fault of the grid-forming converter and limit the voltage of the fixed grid point within a preset movement range during the fault.

[0010] To ensure that the grid-type converter can smoothly return to normal operation after the fault is cleared.

[0011] As an optional implementation, the following steps are also included:

[0012] Connecting the grid-type converter to the power grid, and controlling the grid-type converter to operate in a constant voltage control mode;

[0013] When the grid voltage drops or the grid-connected system fails, the grid-connected converter switches to the current limiting control mode;

[0014] When the grid voltage is restored and the grid-connected system fault is cleared, if the phase angle of the grid-connected converter meets the set conditions at the moment the fault is cleared, the fault is successfully recovered and the operation is switched back to the constant voltage control mode.

[0015] As a further implementation method, the specific process of controlling the grid-type converter to operate in a constant voltage control mode includes: adopting Pf droop control in the synchronization link, performing fixed grid point voltage control through double closed-loop control, and the grid-type converter operates in a constant voltage control mode.

[0016] As an optional implementation method, the specific process of switching the grid-type converter to the current limiting control mode includes: adopting a current limiting strategy for the grid-type converter, when the absolute value of the current reference value output by the voltage loop of the grid-type converter is greater than or equal to the maximum output current amplitude I cmax When the current loop input current reference value of the grid-type converter is Where φ is the angle between the output current vector and the d-axis. Otherwise, the current reference value of the current loop input of the grid-type converter is the current reference value of the voltage loop output;

[0017] In the current limiting mode, the trajectory of the grid connection point voltage is a circle with a radius equal to the grid voltage.

[0018] As an optional implementation, the setting conditions include:

[0019]

[0020] At the moment of fault clearing, when the grid connection point voltage is exactly within the circle determined above, and the system parameters also meet the following set:

[0021] Ω=[2kπ-cos -1 (m),2kπ+cos -1 (m)],k∈Z

[0022] The set satisfies:

[0023]

[0024] Among them, U p is the grid connection point voltage; U g is the grid voltage; U ref is the grid connection point voltage reference value; R g and L g are the transmission line resistance and inductance respectively; ω g is the grid angular frequency; θ is the angle between the dq coordinate system of the grid-type converter and the xy coordinate system of the grid, K pu is the voltage loop proportional coefficient, u pd and u pq are the d-axis and q-axis components of the grid-connected point voltage, I cmax is the maximum output current amplitude of the grid-type converter.

[0025] As an optional implementation, for the droop-type grid-type converter, the output power at the equilibrium point during the current limiting control period satisfies the power reference value, and the stable equilibrium point θ of the converter is solved. SEP and the unstable equilibrium point θ UEP ;

[0026] If the converter does not meet the set conditions, it is still in the current limiting control mode. If the grid voltage is at -θ UEP On the left side, the grid voltage approaches -θ in a clockwise direction. SEP , the converter will be locked at the stable equilibrium point in the current limiting control mode, and the fault recovery will fail;

[0027] If the grid voltage is at -θ UEP On the right, the grid voltage approaches -θ in a counterclockwise direction. SEP , the grid connection point voltage will meet the set conditions again, and overcurrent will no longer occur. The converter successfully switches back to the constant voltage control mode, and the fault recovery is successful.

[0028] As an optional implementation method, when the fault recovery is unsuccessful, a limiting control is applied to the synchronization link of the grid-connected converter, and the upper limit of the limiting control is θ max =θ UEP By limiting the phase angle change during the grid-connected fault, the grid-connected point voltage can meet the set conditions or fall within -θ after the fault is cleared. UEP On the right, it finally successfully switched back to constant pressure control mode.

[0029] In a second aspect, a phase angle limiting control system of a grid-type converter is provided, comprising:

[0030] The limiting control module is configured to apply limiting control to the synchronization link of the grid-forming converter when fault recovery is unsuccessful, limit the phase angle change during the grid-forming converter fault, and limit the voltage of the fixed grid point to a preset movement range during the fault.

[0031] Also includes:

[0032] A module for connecting a grid-type converter to a power grid and controlling the grid-type converter to operate in a constant voltage control mode;

[0033] A module used to switch the grid-connected converter to the current limiting control mode when the grid voltage drops or the grid-connected system fails;

[0034] It is used to restore the grid voltage and clear the grid-connected system fault. If the phase angle of the grid-connected converter meets the set conditions at the moment of fault clearing, the fault recovery is successful and the module switches back to the constant voltage control mode.

[0035] According to a third aspect, an electronic device is provided, comprising a memory and a processor, and computer instructions stored in the memory and executed on the processor, wherein the steps described in the method are completed when the computer instructions are executed by the processor.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] The present invention can achieve full process control of the droop type GFM converter by describing different operating states and switching boundaries after the GFM converter fault is cleared and providing a complete fault recovery control process of the droop type GFM converter in combination with the balance point.

[0038] The present invention can limit U p The range of motion during the fault is determined by applying a limiting control to the GFM converter synchronization link to limit the phase angle change during the GFM fault, so that U p After the fault is cleared, the conditions for successfully switching back to CVC mode are met, which improves the fault recovery capability of the GFM converter.

[0039] The present invention can be applied to the design of the phase angle limit value of the GFM converter, improve the practical value of the phase angle limit control method, and ensure that the GFM converter can resume normal operation after the fault is cleared.

[0040] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, preferred embodiments are given below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] 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.

[0042] Figure 1 is a schematic diagram of the overall method flow of this embodiment;

[0043] Figure 2 is a block diagram of a single-machine grid-connected system of a GFM converter of this embodiment;

[0044] Figure 3 Schematic diagram of the GFM converter fault recovery process of this embodiment. DETAILED DESCRIPTION

[0045] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0046] It should be noted that the following detailed descriptions are all illustrative and 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.

[0047] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit 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 "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0048] Embodiment 1

[0049] like Figure 1 As shown, the phase angle limiting control method of the grid-type converter includes the following steps:

[0050] S1: The GFM converter is connected to the grid through the transmission line. The synchronization link adopts Pf droop control and realizes the fixed grid voltage U through double closed-loop control. p Control, running in CVC mode.

[0051] S2: The grid voltage drops, the grid-connected system fails, the system operating state changes accordingly, and the GFM converter operates in CLC mode.

[0052] S3: The grid voltage is restored and the grid-connected system fault is cleared. If the phase angle of the GFM converter meets certain conditions at the moment of fault clearing, the fault is successfully restored and switched back to CVC mode.

[0053] S4: Otherwise, due to the characteristics of droop control, the GFM converter may be locked in CLC mode after the fault is cleared and the fault recovery cannot be successful.

[0054] S5: Apply limit control to the GFM converter synchronization link to limit the phase angle change during the GFM converter fault, thereby limiting U p The range of motion during the fault period ensures that the GFM converter can smoothly return to normal operation after the fault is cleared.

[0055] So far, the phase angle of the GFM converter has been successfully limited and applied to the method of improving fault recovery capability.

[0056] In step S1, the block diagram of the GFM converter single machine grid-connected system is as follows: Figure 2 As shown in the figure, the GFM converter is connected to the grid through the transmission line. The synchronization link adopts Pf droop control and the fixed grid voltage U is achieved through double closed-loop control. p Control, running in CVC mode.

[0057] The following conditions must be met for the GFM converter to operate in CVC mode without overcurrent:

[0058]

[0059] Where U p is the grid connection point voltage; U g is the grid voltage; U ref is the grid connection point voltage reference value; R g and L g are the transmission line resistance and inductance respectively; ω g is the grid angular frequency; θ is the angle between the GFM converter dq coordinate system and the grid xy coordinate system.

[0060] Square both sides of equation (1) to obtain:

[0061]

[0062] The condition shown in formula (2) does not depend on the CLC strategy and controller parameters, but only on the grid parameters.

[0063] The set of θ satisfying equation (2) is defined as Ω:

[0064] Ω=[2kπ-cos -1 (m),2kπ+cos -1 (m)],k∈Z (3)

[0065] In step S2, the grid voltage drops, the grid-connected system fails, the operating state of the system changes accordingly, and the GFM converter operates in the CLC mode.

[0066] The GFM converter adopts the following current limiting strategy:

[0067]

[0068] In the formula, The current reference value for the current loop input of the GFM converter; I is the current reference value of the voltage loop output; cmax is the maximum output current amplitude of the GFM converter; φ is the user-defined current angle, which represents the angle between the output current vector of the GFM converter and the d-axis during the CLC mode.

[0069] From the circuit equation and formula (4), we can get that U p satisfy:

[0070]

[0071]

[0072] In the formula, u pd and u pq are the d-axis and q-axis components of the grid-connected point voltage respectively; Z g is the transmission line impedance; θ Z is the transmission line impedance angle.

[0073] Further transformation of formula (5) yields:

[0074]

[0075] From formula (7), we can see that in CLC mode, U p The trajectory is a radius U g ☉E.

[0076] In step S3, the grid voltage is restored, the grid-connected system fault is cleared, and if the phase angle of the GFM converter meets certain conditions at the moment of fault clearing, the fault is successfully restored and the system switches back to the CVC mode.

[0077] Considering the GFM converter with anti-integral saturation control, the condition for switching from CLC mode back to CVC mode is:

[0078]

[0079] In the formula, K pu is the voltage loop proportional coefficient; I p is the transmission line current; C f is the filter capacitor of the LC filter.

[0080] In CLC mode, the GFM converter satisfies the following equation:

[0081]

[0082] Substituting equation (9) into equation (8) and squaring both sides, we can get:

[0083]

[0084] From formula (10), we can get that at the moment of fault clearing, when U p It is exactly within ☉F determined by equation (10), and the system parameter design is well coordinated. At the same time, equation (3) is satisfied, and no overcurrent occurs. Then the GFM converter can switch from CLC mode back to CVC mode, and the fault recovery is successful.

[0085] In step S4, for the droop type GFM converter, the output power P at the equilibrium point (EP) during the CLC period is c satisfy:

[0086] P c =I cmax U g cos(φ+θ)=P ref (11)

[0087] The stable equilibrium point (SEP) and unstable equilibrium point (UEP) of the droop type GFM converter can be obtained by formula (11):

[0088]

[0089] In the formula, k 1 and k 2 is any integer; is a set of integers.

[0090] If the fault is cleared, U p If it is not within ☉F and the conditions in step S3 are not met, and it is still in CLC mode, then due to the characteristics of droop control, the operating point of the GFM converter will always move away from UEP and toward SEP.

[0091] Figure 3The fault recovery process of the GFM converter is described. Case 1: When U p Located at -θ UEP On the left, U p will approach -θ in a clockwise direction SEP , eventually the GFM converter will be locked in the SEP of CLC mode, and the fault recovery fails; Case 2: When U p Located at -θ UEP On the right side, U p will approach -θ in a counterclockwise direction SEP In this process, U p It will move into ☉F. Assuming that overcurrent no longer occurs, the GFM converter successfully switches back to CVC mode and the fault is successfully recovered.

[0092] In step S5, in order to avoid situation 1, the core idea is to limit U p The range of motion during a fault. By applying limit control to the GFM converter synchronization link, the upper limit of the limit control θ max =θ UEP , by limiting the phase angle change during the GFM fault, let U p After the fault is cleared, it can be exactly within ☉F or fall within -θ UEP On the right, the system finally switches back to CVC mode and the fault is recovered successfully, which improves the fault recovery capability of the GFM converter.

[0093] Embodiment 2

[0094] A phase angle limiting control method for a grid-type converter comprises the following steps:

[0095] When fault recovery fails, the GFM converter synchronization link is limited to limit the phase angle change during the GFM converter fault, thereby limiting U p The range of motion during the fault period ensures that the GFM converter can smoothly return to normal operation after the fault is cleared.

[0096] Embodiment 3

[0097] A phase angle limiting control system for a grid-type converter, comprising:

[0098] The limiting control module is configured to apply limiting control to the synchronization link of the grid-forming converter when fault recovery is unsuccessful, limit the phase angle change during the grid-forming converter fault, and limit the voltage of the fixed grid point to a preset movement range during the fault.

[0099] Embodiment 4

[0100] Based on the third embodiment, some embodiments further include:

[0101] A module for connecting a grid-type converter to a power grid and controlling the grid-type converter to operate in a constant voltage control mode;

[0102] A module used to switch the grid-connected converter to the current limiting control mode when the grid voltage drops or the grid-connected system fails;

[0103] It is used to restore the grid voltage and clear the grid-connected system fault. If the phase angle of the grid-connected converter meets the set conditions at the moment of fault clearing, the fault recovery is successful and the module switches back to the constant voltage control mode.

[0104] Those skilled in the art will appreciate that embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present invention may take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0105] The present invention is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowchart and / or block diagram, as well as the combination of processes and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0106] These computer program instructions may also be stored in a computer readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer readable memory produce an article of manufacture including an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0107] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process in the computer or other programmable device. Figure 1 A process or multiple processes and / or boxes Figure 1The steps for the functions specified in one or more boxes.

[0108] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made by those skilled in the art within the spirit and principle of the present invention without creative labor shall be included in the protection scope of the present invention.

Claims

1. A phase angle limiting control method for a grid-connected converter, characterized in that: The following steps are involved: When the fault recovery is unsuccessful, a limiting control is applied to the synchronization link of the grid-forming converter to limit the phase angle change during the fault of the grid-forming converter, and the grid connection point voltage is limited to a preset movement range during the fault period; The following steps are also included: Connecting the grid-type converter to the power grid, and controlling the grid-type converter to operate in a constant voltage control mode; When the grid voltage drops or the grid-connected system fails, the grid-connected converter switches to the current limiting control mode; When the grid voltage is restored and the grid-connected system fault is cleared, if the phase angle of the grid-connected converter meets the set conditions at the moment of fault clearing, the fault recovery is successful and the operation is switched back to the constant voltage control mode; The setting conditions include: At the moment of fault clearing, when the grid connection point voltage is exactly within the circle determined by the above formula, and the system parameters also meet the following set: Ω=[2kπ-cos -1 (m),2kπ+cos -1 (m)],k∈Z The set satisfies: Among them, U p is the grid connection point voltage; U g is the grid voltage; U ref is the grid connection point voltage reference value; R g and L g are the transmission line resistance and inductance respectively; ω g is the grid angular frequency; θ is the angle between the dq coordinate system of the grid-type converter and the xy coordinate system of the grid, K pu is the voltage loop proportional coefficient, u pd and u pq are the d-axis and q-axis components of the grid-connected point voltage, I cmax is the maximum output current amplitude of the grid-type converter.

2. A phase angle limiting control method for a grid-connected converter as claimed in claim 1, characterized in that: The specific process of controlling the grid-connected converter to operate in the constant voltage control mode includes: adopting Pf droop control in the synchronization link, controlling the grid connection point voltage through double closed-loop control, and operating the grid-connected converter in the constant voltage control mode.

3. The phase angle limiting control method of a grid-connected converter according to claim 1, characterized in that: The specific process of switching the grid-type converter to the current limiting control mode includes: adopting the current limiting strategy for the grid-type converter, when the absolute value of the current reference value output by the voltage loop of the grid-type converter is greater than or equal to the maximum output current amplitude I cmax When the current loop input current reference value of the grid-type converter is Where φ is the angle between the output current vector and the d-axis. Otherwise, the current reference value of the current loop input of the grid-type converter is the current reference value of the voltage loop output; In the current limiting mode, the trajectory of the grid connection point voltage is a circle with a radius equal to the grid voltage.

4. The phase angle limiting control method of a grid-connected converter according to claim 1, characterized in that: For the droop-type grid-connected converter, the output power at the equilibrium point during the current limiting control period satisfies the power reference value, and the stable equilibrium point θ of the converter is solved. SEP and the unstable equilibrium point θ UEP ; If the converter does not meet the set conditions, it is still in the current limiting control mode. If the grid voltage is at -θ UEP On the left side, the grid voltage approaches -θ in a clockwise direction. SEP , the converter will be locked at the stable equilibrium point in the current limiting control mode, and the fault recovery will fail; If the grid voltage is at -θ UEP On the right, the grid voltage approaches -θ in a counterclockwise direction. SEP , the grid connection point voltage will meet the set conditions again, and overcurrent will no longer occur. The converter successfully switches back to the constant voltage control mode, and the fault recovery is successful.

5. A phase angle limiting control method for a grid-connected converter as claimed in claim 4, characterized in that: When the fault recovery is unsuccessful, the limiting control is applied to the synchronization link of the grid-connected converter. The upper limit of the limiting control is θ max =θ UEP By limiting the phase angle change during the grid-connected fault, the grid-connected point voltage can meet the set conditions or fall within -θ after the fault is cleared. UEP On the right, it finally successfully switched back to constant pressure control mode.

6. A phase angle limiting control system for a grid-type converter, characterized in that it comprises: A limiting control module is configured to apply limiting control to the synchronization link of the grid-forming converter when the fault recovery is unsuccessful, so as to limit the phase angle change of the grid-forming converter during the fault period and limit the grid connection point voltage within a preset movement range during the fault period; Also includes: A module for connecting a grid-type converter to a power grid and controlling the grid-type converter to operate in a constant voltage control mode; A module used to switch the grid-connected converter to the current limiting control mode when the grid voltage drops or the grid-connected system fails; A module used to restore the grid voltage and clear the grid-connected system fault. If the phase angle of the grid-connected converter meets the set conditions at the moment of fault clearing, the fault recovery is successful and the operation is switched back to the constant voltage control mode. The setting conditions include: At the moment of fault clearing, when the grid connection point voltage is exactly within the circle determined by the above formula, and the system parameters also meet the following set: Ω=[2kπ-cos -1 (m),2kπ+cos -1 (m)],k∈Z The set satisfies: Among them, U p is the grid connection point voltage; U g is the grid voltage; U ref is the grid connection point voltage reference value; R g and L g are the transmission line resistance and inductance respectively; ω g is the grid angular frequency; θ is the angle between the dq coordinate system of the grid-type converter and the xy coordinate system of the grid, K pu is the voltage loop proportional coefficient, u pd and u pq are the d-axis and q-axis components of the grid-connected point voltage, I cmax is the maximum output current amplitude of the grid-type converter.

7. An electronic device, characterized in that: The method comprises a memory and a processor, and computer instructions stored in the memory and executed on the processor, wherein when the computer instructions are executed by the processor, the steps in the method according to any one of claims 1 to 5 are completed.

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