A VSG power angle and voltage control method and system considering fault current limiting

Abstract

The present invention provides a VSG power angle and voltage control method and system taking into account fault current limiting. The present invention uses an extended state observer to adaptively adjust the VSG output active reference to ensure the transient power angle stability of the VSG during a power grid fault, and considers the fault current constraint to provide reactive support for the grid connection point voltage, thereby achieving multiple operating goals of VSG voltage recovery and output power maximization.

Description

Technical Field

[0001] The present invention belongs to the technical field of voltage regulation, and in particular relates to a VSG power angle and voltage regulation method and system taking fault current limiting into account. 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] Virtual synchronous generator (VSG) simulates the rotor motion characteristics of synchronous generator, which enables the converter to have strong inertia damping support capability, improves the good connection of new energy to the grid and the coordinated interaction ability between the source and the grid, and has been widely studied. However, under large disturbances such as grid disconnection and short-circuit faults, the imbalance of active power input and output may drive the VSG output power angle to cross the unstable balance point, causing the converter to lose synchronization with the main grid, and may cause the current to exceed the limit, threatening the stability of the grid-connected system. According to the requirements for the grid connection of distributed generation units, during the grid fault, the distributed generation units need to provide a certain amount of reactive power to the grid to support the grid voltage, but reactive injection will further increase the output current of the VSG and increase the risk of overcurrent. Therefore, it is of great scientific and engineering significance to ensure the stable and reliable operation of VSG on the basis of current limiting and provide voltage and frequency support for the grid under fault conditions.

[0004] According to the inventors' understanding, most of the current research on VSG transient stability does not comprehensively consider the correlation between power angle stability and voltage stability under short-circuit current limiting, and cannot achieve multiple operating goals of VSG power angle stability, voltage recovery and output power maximization on the basis of ensuring that the short-circuit current does not exceed the limit. Summary of the invention

[0005] In order to solve the above problems, the present invention proposes a VSG power angle and voltage control method and system taking into account fault current limiting. The present invention uses an extended state observer (ESO) to adaptively adjust the VSG output active reference to ensure the transient power angle stability of the VSG during a power grid fault, and considers the fault current constraint to provide reactive support for the grid connection point voltage, thereby achieving multiple operating goals of VSG voltage recovery and output power maximization.

[0006] According to some embodiments, the present invention adopts the following technical solutions:

[0007] A VSG power angle and voltage control method taking into account fault current limiting comprises the following steps:

[0008] The extended state observer is used to adaptively adjust the output active power reference of the virtual synchronous generator to maintain the transient power angle stability of the virtual synchronous generator during power grid faults.

[0009] Based on the adaptively adjusted output active power reference and considering the fault current constraint, the upper limit value to which the output voltage of the virtual synchronous generator can be restored under the fault state is determined, and then the reactive power reference value is calculated. Reactive support is provided to the grid connection point voltage based on it, thereby achieving the multiple operation goals of voltage recovery and output power maximization of the virtual synchronous generator.

[0010] As an optional implementation, the extended state observer is:

[0011]

[0012] Among them, e ω Represents the observed angular frequency The deviation between the actual output angular frequency ω of the virtual synchronous generator, K represents the grid state parameter, δ represents the output power angle of the virtual synchronous generator, ω n and U n Indicates the rated angular frequency, w u It is used to simulate the disturbance of the output angular frequency caused by the change of K parameter during power grid fault. β1 and β2 represent the gain of the observer. com is the reference power compensation amount during the power grid fault process, is the active power reference of the virtual synchronous generator for adaptive regulation during power grid faults, Δω=ω-ω n ,ω andω n They represent the actual output angular frequency and rated angular frequency of the converter respectively.

[0013] As an optional implementation, the upper limit value to which the output voltage of the virtual synchronous generator can be restored under the fault state is determined based on the vector relationship between voltage and current under the fault state and the equal input and output active power in the steady state after the fault.

[0014] As a further implementation, in a fault state, the vector relationship between voltage and current is:

[0015]

[0016] Among them, I F is the current flowing through the converter under fault condition, and the grid voltage after fault is U gF ∠0°, the output voltage of the virtual synchronous generator is U pccF ∠δ F , δ F is the virtual synchronous generator power angle under fault conditions, X g is the grid impedance;

[0017] After the fault, the input and output active powers are equal in steady state, that is:

[0018]

[0019] Among them, P eF It represents the active power output of the virtual synchronous generator in steady state after a fault.

[0020] As a further embodiment, the output voltage U of the virtual synchronous generator pccF Can be restored to U oFth :

[0021]

[0022] Among them, I th is the maximum current flowing through the converter, I F =I th .

[0023] As an optional implementation, the reactive power reference value of the virtual synchronous generator in the fault state is:

[0024] in, Q″ ref =3U oFth i qref , the reactive current i injected by the inverter into the grid qref The following conditions are met:

[0025]

[0026] I N is the rated current of the converter, U g is the grid voltage.

[0027] A VSG power angle and voltage flexible control system taking into account fault current limiting, comprising:

[0028] An adaptive adjustment module is configured to adaptively adjust the output active power reference of the virtual synchronous generator using an extended state observer to maintain the transient power angle stability of the virtual synchronous generator during a power grid fault;

[0029] The voltage control module is configured to determine the upper limit value to which the output voltage of the virtual synchronous generator can be restored under the fault state based on the output active power reference after adaptive adjustment and considering the fault current constraint, and then calculate the reactive power reference value, and provide reactive support to the grid connection point voltage based on it, so as to achieve the multiple operation goals of voltage recovery and output power maximization of the virtual synchronous generator.

[0030] A grid-connected converter applies the above method or includes the above system.

[0031] A computer-readable storage medium is used to store computer instructions, and when the computer instructions are executed by a processor, the steps in the above method are completed.

[0032] An electronic device comprises a memory and a processor, and computer instructions stored in the memory and executed on the processor. When the computer instructions are executed by the processor, the steps in the above method are completed.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] The present invention is different from the traditional method of improving the transient stability of VSG. It can adaptively adjust the output active and reactive power reference values ​​of VSG according to the degree of power grid fault without the need for additional adoption or hardware circuits, thereby ensuring that the grid-connected current does not exceed the limit and ensuring the safe operation of the converter equipment.

[0035] The present invention, under the condition of fault current limiting, utilizes reactive power support to restore the VSG output voltage as much as possible and improves the ability of VSG to transmit active power to the power grid, which not only ensures the stability of VSG power angle, but also restores the voltage of the grid connection point to the maximum extent, thereby improving the energy absorption capacity.

[0036] The present invention can ensure that the VSG does not go offline even if the fault is not cleared during a serious power grid fault, provide voltage and frequency support for the power grid, and avoid unplanned islanding of distributed power generation units.

[0037] 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

[0038] 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.

[0039] Figure 1 is a structural block diagram of a VSG grid-connected system of an embodiment;

[0040] Figure 2 is a block diagram of a VSG active power control structure of an embodiment;

[0041] Figure 3 is a block diagram of a VSG reactive power control structure of an embodiment;

[0042] Figure 4 is a control block diagram of an embodiment. DETAILED DESCRIPTION

[0043] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0044] 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.

[0045] 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.

[0046] In the absence of conflict, the embodiments in this application and the features in the embodiments may be combined with each other.

[0047] Embodiment 1

[0048] A VSG power angle and voltage flexible control method considering fault current limiting is proposed. This method uses an extended state observer (ESO) to adaptively adjust the VSG output active reference to ensure the transient power angle stability of the VSG during a power grid fault. Secondly, the fault current constraint is considered to provide reactive support for the grid connection point voltage, aiming to achieve multiple operating goals of VSG voltage recovery and output power maximization.

[0049] The following is a description with reference to specific application examples.

[0050] Taking grid-connected converter as an application, the following specific contents are provided.

[0051] First, in order to ensure that those skilled in the art can clearly understand the specific scheme of the present invention, the following necessary explanations are made.

[0052] This embodiment is described by taking a two-level converter as an example. First, the VSG grid-connected system structure is introduced. Figure 1 As shown in the figure, the VSG grid-connected system of the three-phase voltage source converter consists of a DC side, a converter, a filter, a grid impedance and a large grid. dc Indicates the voltage at the DC side port of the converter, L f and C f Respectively represent the inductance and capacitance of the LC filter, R g and L g Respectively represent the resistive and inductive components of the equivalent grid impedance of the transmission and distribution lines, i1 and i pcc They represent the converter port output current and the filtered grid-connected current, u pcc and U g Represent the grid connection point voltage and grid voltage respectively, ω n and U nRespectively represent the rated angular frequency and VSG output rated voltage, P e and Q e Denote the active power and reactive power of VSG output, δ, ω and U respectively. ref They represent the VSG output power angle, angular frequency and voltage amplitude reference respectively, and the VSG output electromagnetic power reference value P ref It is provided by rational coordination and flexible adjustment of new energy power generation and energy storage.

[0053] like Figure 2 As shown in Figure 2, the VSG control strategy significantly improves the inertia and damping support capabilities of the power electronics interface by simulating the electrical, mechanical and other external characteristics of the synchronous generator. The converter power angle reference δ is generated by the active power control loop.

[0054] The specific mathematical expression of the virtual rotor part is:

[0055]

[0056] Where: ω and ω n Respectively represent the actual output angular frequency and rated angular frequency of the converter, K ω is the active power-frequency droop coefficient, J and D represent the virtual moment of inertia and damping coefficient of the converter respectively. m and P e Represent the mechanical power of the system and the active power output of VSG, Δω=ω-ω n ,θ GFM Phase value of the converter output voltage.

[0057] according to Figure 1 According to the VSG grid-connected main circuit and line power transmission principle, when the inductive component of the grid impedance is much larger than the resistive component, the active power output by the VSG converter can be expressed as:

[0058]

[0059] Where U pcc Indicates the output voltage amplitude of the converter, X g represents the grid impedance, K represents the grid state parameter, where

[0060] Note that since the VSG power angle will increase during the transient process, we cannot simply assume that sinδ≈δ. In order to improve the accuracy of the state observer, sinδ is expanded by Taylor series and the first three terms are taken, that is, sinδ≈f(δ)=(δ-δ 3 / 6+δ 5 / 120).

[0061] The introduction of virtual exciter in VSG can simulate reactive power and voltage droop characteristics, including voltage regulation and reactive power-voltage droop, which interact with each other to jointly regulate the output voltage of the system.

[0062] The virtual exciter realizes reactive voltage regulation by changing the virtual excitation current. The expression is as follows:

[0063] Q me =Q ref +K q (U n -U pcc ) (3)

[0064] In order to ensure that the output reactive power tracks the reactive reference value, voltage modulation is required, and the expression is as follows:

[0065]

[0066] Where: K q is the voltage droop coefficient, K s is the integral adjustment coefficient, U pccc is the output voltage amplitude of the converter port voltage, U n is the rated output voltage of the converter, Q ref and Q e Represents reactive power reference and output value respectively. Under normal operating conditions, Q ref =0.

[0067] like Figure 4 As shown, the method of this embodiment includes two steps: active reference adaptive regulation based on an extended state observer and reactive reference adaptive regulation taking into account fault current limiting. The first step is used to reduce the active output of the VSG under a VSG fault state to ensure the power angle stability of the VSG under a power grid fault. The second step is to use reactive support under current limiting to restore the VSG output voltage as much as possible and improve the ability of the VSG to transmit active power to the power grid.

[0068] Step 1: Adaptive regulation of VSG active reference based on extended state observer

[0069] During a grid fault, the grid voltage will show different degrees of voltage loss, U g Reduced, from formula (2) we know that P e decrease, resulting in P e ≤P ref , the imbalance of active power input and output leads to an increase in the VSG output angular frequency. From formula (1), it can be seen that the increase in angular frequency will further lead to power angle instability. This design first uses the angular frequency state observer to adaptively adjust the active power reference, and combines formula (1) and formula (2) to expand the observer design as follows:

[0070]

[0071] Where e ω Represents the observed angular frequency The deviation from the actual VSG output angular frequency ω, w u It is used to simulate the disturbance of the output angular frequency caused by the change of K parameter during power grid fault. β1 and β2 represent the gain of the observer. com is the reference power compensation amount during the power grid fault process, It is the VSG active power reference for adaptive adjustment during grid faults.

[0072] Step 2: Adaptive regulation of VSG reactive power reference taking into account fault current limiting

[0073] Assume that the grid voltage after the fault is U gF ∠0°, VSG output voltage is U pccF ∠δ F , δ F is the VSG power angle in the fault state, and the current flowing through the converter in the fault state is I F From the vector relationship between voltage and current after the fault, we know that

[0074]

[0075] Then according to step 1, the output of the expanded state observer is Since the input and output active powers are equal in steady state after the fault, the following expression is obtained:

[0076]

[0077] Where P eF Indicates the active power output by the VSG in steady state after a fault.

[0078] Combining equations (6) and (7), we can obtain the fault current limiting condition, that is, let I F =I th ,VSG output voltage U pccF Can be restored to U oFth

[0079]

[0080] Where I th The maximum current flowing through the converter, which generally does not exceed the converter rated current I N 1.2 times of that.

[0081] According to formula (8), the upper limit value of the maximum recoverable output voltage of VSG under fault condition is U oFth , at this time, the reactive power reference value Q' in the VSG fault state ref As follows

[0082]

[0083] In addition, according to the national standard for distributed generation unit grid connection, when the grid voltage drops from U gF When the inverter injects reactive current i qref The following conditions must be met:

[0084]

[0085] Convert injected reactive current to injected reactive power:

[0086] Q″ ref =3U oFth i qref (11)

[0087] Combined with the fault current limiting requirements and the national standards for distributed generation grid connection, the reactive power reference value under VSG fault condition is As follows:

[0088]

[0089] During the design process, the solution of this embodiment maximizes the voltage support capability of the VSG on the basis of fault current limiting, and improves the active power transmission capability of the VSG on the basis of ensuring the stability of the power angle.

[0090] The implementation of this embodiment does not require hardware equipment, such as a fault current limiter, to suppress the overcurrent of the VSG during a grid fault, thereby ensuring the stable and safe operation of the converter during a grid fault, improving the robustness and reliability of the grid-connected converter system, and ensuring that the VSG operates stably and without disconnection from the grid and has good output performance.

[0091] Embodiment 2

[0092] A VSG power angle and voltage flexible control system taking into account fault current limiting, comprising:

[0093] An adaptive adjustment module is configured to adaptively adjust the output active power reference of the virtual synchronous generator using an extended state observer to maintain the transient power angle stability of the virtual synchronous generator during a power grid fault;

[0094] The voltage control module is configured to determine the upper limit value to which the output voltage of the virtual synchronous generator can be restored under the fault state based on the output active power reference after adaptive adjustment and considering the fault current constraint, and then calculate the reactive power reference value, and provide reactive support to the grid connection point voltage based on it, so as to achieve the multiple operation goals of voltage recovery and output power maximization of the virtual synchronous generator.

[0095] Embodiment 3

[0096] A grid-connected converter applies the method provided in the first embodiment or includes the system provided in the second embodiment.

[0097] 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.

[0098] 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.

[0099] 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 comprising 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.

[0100] 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. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0101] 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 VSG power angle and voltage control method taking into account fault current limiting, characterized in that: The following steps are involved: The extended state observer is used to adaptively adjust the output active power reference value of the virtual synchronous generator to maintain the transient power angle stability of the virtual synchronous generator during power grid faults. Based on the adaptively adjusted output active power reference value, considering the fault current constraint, the upper limit value to which the output voltage of the virtual synchronous generator can be restored under the fault state is determined, and then the reactive power reference value is calculated. Based on it, reactive support is provided to the grid connection point voltage to achieve the multiple operation goals of voltage recovery and output power maximization of the virtual synchronous generator; The extended state observer is: Among them, e ω Represents the observed angular frequency The deviation between the actual output angular frequency ω of the virtual synchronous generator, K represents the grid state parameter, δ represents the output power angle of the virtual synchronous generator, ω n Indicates the rated angular frequency, w u It is used to simulate the disturbance of the output angular frequency caused by the change of K parameter during power grid fault. β1 and β2 represent the gain of the observer. com is the reference power compensation amount during the power grid fault process, is the active power reference of the virtual synchronous generator during the grid fault process, Δω=ω-ω n ,ω andω n They represent the actual output angular frequency and rated angular frequency of the converter respectively; the J parameter and D represent the virtual moment of inertia and damping coefficient of the converter respectively; According to the vector relationship between voltage and current under the fault state, combined with the equal input and output active power in the steady state after the fault, the upper limit value to which the output voltage of the virtual synchronous generator can be restored under the fault state is determined; Under fault conditions, the vector relationship between voltage and current is: Among them, I F is the current flowing through the converter under fault condition, and the grid voltage after fault is U gF ∠0°, the output voltage of the virtual synchronous generator is U pccF ∠δ F , δ F is the virtual synchronous generator power angle under fault conditions, X g is the grid impedance; After the fault, the input and output active powers are equal in steady state, that is: Among them, P eF It represents the active power output of the virtual synchronous generator in steady state after the fault; Output voltage U of virtual synchronous generator pccF Can be restored to U oFth : Among them, I th is the maximum current flowing through the converter, I F =I th ; The reactive power reference value of the virtual synchronous generator under fault condition is: in, The reactive current i injected by the inverter into the grid qref The following conditions are met: I N is the rated current of the converter, U g is the grid voltage.

2. A VSG power angle and voltage flexible control system taking into account fault current limiting, based on the method according to claim 1, characterized in that: include: An adaptive adjustment module is configured to adaptively adjust an output active power reference value of the virtual synchronous generator using an extended state observer to maintain transient power angle stability of the virtual synchronous generator during a power grid fault; The voltage control module is configured to determine the upper limit value to which the output voltage of the virtual synchronous generator can be restored under the fault state based on the output active power reference value after adaptive adjustment and taking into account the fault current constraint, and then calculate the reactive power reference value, and use it to provide reactive support to the grid connection point voltage, thereby achieving the multiple operating goals of voltage recovery and output power maximization of the virtual synchronous generator.

3. A grid-connected converter, characterized in that: The method of claim 1 is used or the system of claim 2 is included.

4. A computer-readable storage medium, characterized in that: Used to store computer instructions, which, when executed by a processor, complete the steps of the method according to claim 1.

5. 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 the steps of the method according to claim 1 are completed when the computer instructions are executed by the processor.

Images