A transient instability identification method and system for a grid-forming converter

Abstract

The present application belongs to the technical field of power system operation control, in order to solve the problem that the existing transient instability discrimination method of grid-forming converter is easy to cause the misoperation or missed judgment of control strategy, a kind of transient instability discrimination method and system of grid-forming converter is proposed, a nonlinear equivalent circuit model of grid-forming converter is established, the transient total energy and critical instability energy of the system are calculated in real time, and the control mode is adaptively switched according to the size relationship between the two; In normal operation, maintain the original virtual synchronous machine control strategy, use its damping characteristics to suppress oscillation, and maximize the voltage and frequency support capability of the power grid; When the instability risk is detected, enter the fault ride-through mode, realize the rapid and accurate identification and control of instability risk.

Description

Technical Field

[0001] This invention belongs to the technical field of power system operation and control, and particularly relates to a method and system for judging transient instability of grid-type converters. Background Technology

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

[0003] With the continuous increase in the proportion of new energy power generation, the power system exhibits significant low inertia and weak damping characteristics, posing challenges to the safe and stable operation of the power grid. Grid-type converters, due to their ability to actively construct voltage amplitude and phase, can simulate the mechanism of synchronous generators to provide the necessary voltage and frequency support for the system, and have become key equipment for improving the stability of new power systems.

[0004] Although grid-connected converters offer significant advantages in weak grid environments, they still face severe challenges in transient synchronization stability when subjected to large grid disturbances. When a short-circuit fault or phase jump occurs in the grid, the converter's active power output capacity is limited, leading to a continuous deviation between the active power reference value and the actual output value. This causes severe oscillations or monotonic divergence in the internal power angle. If the transient energy accumulated in the system during the fault exceeds the critical potential energy required to maintain synchronization, the grid-connected converter will cross the unstable equilibrium point, experiencing out-of-synchronization oscillations or even grid disconnection.

[0005] Existing transient stability assessment and enhancement strategies typically rely on threshold monitoring of single electrical quantities such as voltage amplitude or current state for triggering. However, the transient stability of grid-connected converters essentially depends on the system's energy state rather than simply the voltage level. Triggering mechanisms based on fixed thresholds often fail to accurately characterize the system's current actual stability margin, and cannot effectively distinguish whether the system is in a naturally recoverable oscillation process or about to undergo irreversible instability, easily leading to erroneous or missed detections in the control strategy. Summary of the Invention

[0006] To overcome the shortcomings of the prior art, this invention provides a transient instability identification method and system for grid-type converters. The control mode is adaptively switched based on the relationship between the real-time calculated transient total energy and the critical instability energy, thereby achieving rapid and accurate identification and adaptive control of instability risks.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a method for determining transient instability in a grid-type converter, comprising:

[0009] By introducing circuit simulation variables, the control equations of the grid converter are transformed into the corresponding circuit topology, and a nonlinear equivalent circuit model of the grid converter is established.

[0010] Based on the nonlinear equivalent circuit model of the grid converter, the transient total energy and critical instability energy of the system are calculated in real time.

[0011] If the real-time transient energy is less than the critical instability energy, the original virtual synchronous machine control strategy remains unchanged; if the real-time transient energy is not less than the critical instability energy, the fault ride-through mode is entered.

[0012] Secondly, the present invention provides a transient instability detection system for a grid-type converter, comprising:

[0013] The equivalent module is configured to: introduce circuit simulation variables to convert the control equations of the grid converter into the corresponding circuit topology and establish a nonlinear equivalent circuit model of the grid converter;

[0014] The calculation module is configured to calculate the transient total energy and critical instability energy of the system in real time based on the nonlinear equivalent circuit model of the grid-type converter.

[0015] The switching control module is configured to: if the real-time transient energy is less than the critical instability energy, maintain the original virtual synchronous machine control strategy; if the real-time transient energy is not less than the critical instability energy, enter the fault ride-through mode.

[0016] Thirdly, the present invention provides an electronic device including a memory and a processor, and computer instructions stored in the memory and running on the processor, wherein the computer instructions, when executed by the processor, perform the method described in the first aspect.

[0017] Fourthly, the present invention provides a computer-readable storage medium for storing computer instructions, which, when executed by a processor, perform the method described in the first aspect.

[0018] Fifthly, the present invention provides a computer program product, including a computer program that, when executed by a processor, implements the method described in the first aspect.

[0019] The above one or more technical solutions have the following beneficial effects:

[0020] In this invention, a nonlinear equivalent circuit model of a grid-type converter is established to calculate the transient total energy and critical instability energy of the system in real time, and the control mode is adaptively switched according to the relationship between the two. During normal operation, the original virtual synchronous machine control strategy is maintained, and its own damping characteristics are used to suppress oscillations in order to maximize the voltage and frequency support capability of the power grid. When an instability risk is detected, the fault ride-through mode is entered to achieve rapid and accurate identification and control of the instability risk.

[0021] In this invention, in order to visually represent the change of power angle in the circuit, a unit inductor is introduced. The terminal voltage of the RC parallel circuit is applied to the two ends of the unit inductor through a controlled voltage source, and the magnitude of the inductor current directly reflects the dynamic change of the converter's power angle.

[0022] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0023] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0024] Figure 1 The nonlinear equivalent circuit diagram of a grid-type converter;

[0025] Figure 2 This is a control flowchart for a grid-type converter based on the equivalent circuit energy criterion. Detailed Implementation

[0026] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0027] It should be noted that the terminology used herein is for the purpose of describing particular implementations only and is not intended to limit the exemplary implementations of the present invention.

[0028] Where there is no conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0029] Example 1

[0030] This embodiment discloses a transient instability detection method for a grid-type converter, including:

[0031] By introducing circuit simulation variables, the control equations of the grid converter are transformed into the corresponding circuit topology, and a nonlinear equivalent circuit model of the grid converter is established.

[0032] Based on the nonlinear equivalent circuit model of the grid converter, the transient total energy and critical instability energy of the system are calculated in real time.

[0033] If the real-time transient energy is less than the critical instability energy, the original virtual synchronous machine control strategy remains unchanged; if the real-time transient energy is not less than the critical instability energy, the fault ride-through mode is entered.

[0034] This embodiment is based on the control equations of a grid-type converter, introduces circuit simulation variables, and establishes a nonlinear equivalent circuit model that maps virtual inertia, damping, and power imbalance to circuit element parameters. The transient total energy of the system is calculated in real time based on the equivalent circuit, and the critical instability energy required to maintain system synchronization is derived by combining the unstable equilibrium point under fault conditions. The transient total energy is compared with the critical instability energy in real time. If the transient total energy is less than the critical instability energy, the original virtual synchronous machine control is maintained, utilizing its own damping characteristics to suppress oscillations and maximize the voltage and frequency support capability for the power grid. If the transient total energy is greater than or equal to the critical instability energy, the fault ride-through mode is entered to prevent system loss of synchronization.

[0035] The following is combined Figure 1-Figure 2 This embodiment provides a detailed description of the transient instability detection method for a grid-type converter.

[0036] I. Establish the nonlinear equivalent circuit model of the grid-type converter.

[0037] To intuitively reveal the transient synchronization characteristics of grid-type converters under large disturbances and quantitatively characterize the system's instability boundary, this embodiment establishes a nonlinear equivalent circuit model. This nonlinear equivalent circuit model transforms the control equations of the virtual synchronous machine (VSG) into the corresponding circuit topology using circuit simulation variables. The specific modeling process is as follows:

[0038] The active-frequency control equations and reactive-voltage control equations for a grid-type converter are as follows:

[0039] (1)

[0040] (2)

[0041] in, P ref and P e These are the active power reference value and the electromagnetic power, respectively. T p This is the virtual inertial time constant; D p The damping coefficient; d′ This represents the angular velocity deviation value. d′′ The derivative of the angular velocity deviation value;Q ref and Q e The reactive power reference value and the output reactive power; V ref Reference voltage value; E e This represents the amplitude of the internal potential of the converter; T q and D q The reactive power loop time constant and droop coefficient are respectively defined.

[0042] To transform the above control equations into circuit equations, equivalent transformations are performed by introducing circuit simulation variables. First, current source variables are defined. i pref and i pe Corresponding to P ref and P e Define capacitor C p correspond T p Define conductance 1 / R p correspond D p Define node voltages U p correspond d′ .

[0043] Based on the above parameter substitutions, the active power loop control equations can be rewritten in Kirchhoff's current law form:

[0044] (3)

[0045] This equation describes a current-driven system. RC Parallel circuit. Physically, the imbalance in the input current represents the imbalance in the active power, flowing to the capacitor. C p Charging, voltage across the capacitor U p The establishment process simulates the accumulation process of rotor speed deviation; parallel resistor R p This consumes energy through a diversion effect, simulating the damping effect on oscillation suppression.

[0046] Furthermore, in order to visually represent the power angle in the circuit... d The change introduces a unit inductance with a value of 1. L p By definition, d It is angular velocity deviation d′ The integral, and in the circuit, if the current flowing through the inductor is... I Representative angle d And the voltage across the inductor is U p Then, based on the inductor's current-voltage characteristic, we can obtain... I =∫ U p dt. Therefore, by using a controlled voltage source to RC Circuit terminal voltage U p The magnitude of the inductor current applied across a unit inductor directly reflects the power angle of the converter. d The dynamic changes.

[0047] Similarly, the reactive power loop is equivalently represented. A current source is defined. i qref and i qe Corresponding reactive power reference value Q ref With reactive power output Q e Define capacitor C q Corresponding reactive time constant T q Define conductance 1 / R q Corresponding reactive power loop droop coefficient D q The reactive power equation is transformed into:

[0048] (4)

[0049] This corresponds to another RC The circuit describes the dynamic response of the internal potential amplitude under reactive power imbalance. The overall nonlinear equivalent circuit diagram is shown below. Figure 1 As shown.

[0050] II. Calculate the critical instability energy and real-time transient energy of the system.

[0051] In the equivalent circuit, the total energy of the system W The total instantaneous energy is composed of the magnetic field energy of the inductor and the electric field energy of the capacitor. For a nonlinear circuit system containing a unit inductance, an active loop equivalent capacitance, and a reactive loop equivalent capacitance, its total instantaneous energy can be expressed as:

[0052] (5)

[0053] Based on the variable correspondence established in step one, the control system state variables are substituted into the circuit parameters to obtain the real-time transient energy function of the grid-type converter:

[0054] (6)

[0055] in, This represents the amplitude of the internal potential of the converter; This refers to the angular velocity deviation. d It is angular velocity deviation d′ The integral; t represents time; T q The reactive power loop time constant; V ref This is the reference voltage value.

[0056] To determine whether a system has experienced transient loss of synchronization, it is necessary to determine the maximum energy boundary that can be tolerated to maintain synchronous operation, i.e., the critical instability energy. W th .

[0057] When a large disturbance occurs in the power grid, the power transmission characteristics of the grid-connected converter change. To determine the stable operating range of the system, it is first necessary to calculate the power angle at the unstable equilibrium point under fault conditions. d u According to the power transfer equation during a fault:

[0058] (7)

[0059] The unstable equilibrium point of the system can be obtained. d u It can be represented as:

[0060] (8)

[0061] In the formula: P ref The reference active power for grid-type converters; X The equivalent impedance of a grid-type converter to the outside; U gN The rated voltage of the external power grid; This represents the amplitude of the internal potential of the converter.

[0062] Once the converter power angle exceeds this point, the system will irreversibly lose synchronization.

[0063] In the critical instability state, the system trajectory will approach the unstable equilibrium point infinitely. d u Considering the energy dissipation characteristics of the parallel resistor in the equivalent circuit, and the discharge behavior of the capacitor when reaching the extreme point, the angular velocity deviation of the system at the critical point... d′ The decay is zero. Therefore, the critical instability energy can be calculated as:

[0064] (9)

[0065] in, P ref The reference active power for grid-type converters; X The equivalent impedance of a grid-type converter to the outside; U gN The rated voltage of the external power grid; This represents the amplitude of the internal potential of the converter; T q The reactive power loop time constant; V ref This is the reference voltage value.

[0066] III. Transient instability real-time detection and adaptive control switching mechanism.

[0067] This step is based on real-time transient energy. W(t) With critical instability energy W th Based on the comparison results, adaptive control switching is implemented for the grid-type converter.

[0068] During converter operation, transient energy is monitored in real time. W(t) With critical instability energy W th Size relationship: when W (t) < W th When the current power angle oscillation caused by the current grid disturbance is within a controllable range, the grid-type converter can suppress the power angle divergence by relying on the original virtual damping coefficient. Therefore, the original virtual synchronous machine (VSG) control strategy should be maintained to ensure the converter's ability to support the voltage and frequency of the grid.

[0069] when W(t) > W th When the fault disturbance is too large, the converter's power angle faces the risk of instability due to continuous divergence. The original control parameters are insufficient to maintain synchronization. At this time, the fault ride-through mode is immediately entered to prevent the grid-type converter from losing synchronization under large disturbances.

[0070] Example 2

[0071] The purpose of this embodiment is to provide a transient instability detection system for a grid-type converter, including:

[0072] The equivalent module is configured to: introduce circuit simulation variables to convert the control equations of the grid converter into the corresponding circuit topology and establish a nonlinear equivalent circuit model of the grid converter;

[0073] The calculation module is configured to: calculate the transient total energy and critical instability energy of the grid converter in real time based on the nonlinear equivalent circuit model of the grid converter;

[0074] The switching control module is configured to: if the real-time transient energy is less than the critical instability energy, maintain the original virtual synchronous machine control strategy; if the real-time transient energy is not less than the critical instability energy, control the grid-type converter to enter fault ride-through mode.

[0075] In further embodiments, the following is also provided:

[0076] An electronic device includes a memory and a processor, as well as computer instructions stored in the memory and running on the processor. When executed by the processor, the computer instructions perform the method described in Embodiment 1. For brevity, further details are omitted here.

[0077] It should be understood that in this embodiment, the processor can be a central processing unit (CPU), or it can be other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor can be a microprocessor or any conventional processor, etc.

[0078] Memory may include read-only memory and random access memory, and provides instructions and data to the processor. A portion of memory may also include non-volatile random access memory. For example, memory may also store information about the device type.

[0079] A computer-readable storage medium for storing computer instructions, which, when executed by a processor, perform the method described in Embodiment 1.

[0080] The method in Embodiment 1 can be directly implemented by a hardware processor, or implemented by a combination of hardware and software modules within the processor. The software modules can reside in readily available storage media in the art, such as random access memory, flash memory, read-only memory, programmable read-only memory, electrically erasable programmable memory, or registers. This storage medium is located in memory; the processor reads information from the memory and, in conjunction with its hardware, completes the steps of the above method. To avoid repetition, a detailed description is not provided here.

[0081] A computer program product includes a computer program that, when executed by a processor, implements the method described in Embodiment 1.

[0082] The present invention also provides at least one computer program product tangibly stored on a non-transitory computer-readable storage medium. The computer program product includes computer-executable instructions, such as instructions included in program modules, which execute in a device on a target real or virtual processor to perform the processes / methods described above. Typically, program modules include routines, programs, libraries, objects, classes, components, data structures, etc., that perform specific tasks or implement specific abstract data types. In various embodiments, the functionality of program modules can be combined or divided among program modules as needed. The machine-executable instructions for the program modules can execute within a local or distributed device. In a distributed device, the program modules can reside in both local and remote storage media.

[0083] The computer program code used to implement the methods of the present invention may be written in one or more programming languages. This computer program code may be provided to a processor of a general-purpose computer, a special-purpose computer, or other programmable data processing device, such that when executed by the computer or other programmable data processing device, the program code causes the functions / operations specified in the flowcharts and / or block diagrams to be implemented. The program code may be executed entirely on a computer, partially on a computer, as a stand-alone software package, partially on a computer and partially on a remote computer, or entirely on a remote computer or server.

[0084] In the context of this invention, computer program code or related data may be carried by any suitable carrier to enable a device, apparatus, or processor to perform the various processes and operations described above. Examples of carriers include signals, computer-readable media, and the like. Examples of signals may include electrical, optical, radio, sound, or other forms of propagation signals, such as carrier waves, infrared signals, etc.

[0085] Those skilled in the art will recognize that the units and algorithm steps described in conjunction with the embodiments herein can be implemented in electronic hardware or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0086] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this 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 without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for determining transient instability in a grid-type converter, characterized in that, include: By introducing circuit simulation variables, the control equations of the grid-type converter are transformed into the corresponding circuit topology, and a nonlinear equivalent circuit model of the grid-type converter is established; specifically: Define the active power reference value and electromagnetic power corresponding to the current source variable, define the capacitor corresponding to the reactive power loop time constant, define the conductance corresponding to the reactive power loop droop coefficient, and define the node voltage corresponding to the angular velocity deviation value. The active power loop control equation of the grid converter is described as an RC parallel circuit driven by the current source. Define the reactive power reference value and output reactive power corresponding to the current source, define the reactive power time constant corresponding to the capacitor, define the reactive power loop droop coefficient corresponding to the conductance, and describe the RC circuit form of the reactive power loop control equation of the grid converter to describe the dynamic response of the internal potential amplitude under reactive power imbalance drive. Based on the nonlinear equivalent circuit model of the grid converter, the transient total energy and critical instability energy of the grid converter are calculated in real time. If the real-time transient energy is less than the critical instability energy, the original virtual synchronous machine control strategy remains unchanged. If the real-time transient energy is not less than the critical instability energy, the grid-type converter will enter the fault ride-through mode. The critical instability energy The calculation is as follows: ； in, P ref The reference active power for grid-type converters; X The equivalent impedance of a grid-type converter to the outside; U gN The rated voltage of the external power grid; This represents the amplitude of the internal potential of the converter; T q The reactive power loop time constant; V ref This is the reference voltage value.

2. The transient instability detection method for a grid-type converter as described in claim 1, characterized in that, By introducing a unit inductor, the terminal voltage of the RC parallel circuit is applied to both ends of the unit inductor through a controlled voltage source, and the dynamic change of the converter's power angle is reflected by the magnitude of the inductor current.

3. The transient instability detection method for a grid-type converter as described in claim 1, characterized in that, The determination of the critical instability energy is specifically as follows: Based on the power transfer equation during the fault, the power angle of the unstable equilibrium point under fault conditions is calculated. Under the critical instability state, the system trajectory will infinitely approach the unstable equilibrium point. Considering the energy dissipation characteristics of the parallel resistor in the equivalent circuit and the discharge behavior of the capacitor when it reaches the extreme point, the angular velocity deviation at the critical point decays to zero, and the critical instability energy is determined.

4. The transient instability detection method for a grid-type converter as described in claim 1, characterized in that, In the equivalent circuit, the total energy of the system is composed of the magnetic field energy of the inductor and the electric field energy of the capacitor; based on the established variable correspondence in the equivalent circuit, the real-time transient energy of the grid-type converter is determined.

5. A transient instability detection system for a grid-type converter, characterized in that, include: The equivalent module is configured to: introduce circuit simulation variables to transform the control equations of the grid-type converter into the corresponding circuit topology, and establish a nonlinear equivalent circuit model of the grid-type converter; specifically: Define the active power reference value and electromagnetic power corresponding to the current source variable, define the capacitor corresponding to the reactive power loop time constant, define the conductance corresponding to the reactive power loop droop coefficient, and define the node voltage corresponding to the angular velocity deviation value. The active power loop control equation of the grid converter is described as an RC parallel circuit driven by the current source. Define the reactive power reference value and output reactive power corresponding to the current source, define the reactive power time constant corresponding to the capacitor, define the reactive power loop droop coefficient corresponding to the conductance, and describe the RC circuit form of the reactive power loop control equation of the grid converter to describe the dynamic response of the internal potential amplitude under reactive power imbalance drive. The calculation module is configured to: calculate the transient total energy and critical instability energy of the grid-type converter in real time based on the nonlinear equivalent circuit model of the grid-type converter; the critical instability energy The calculation is as follows: ； in, P ref The reference active power for grid-type converters; X The equivalent impedance of a grid-type converter to the outside; U gN The rated voltage of the external power grid; This represents the amplitude of the internal potential of the converter; T q The reactive power loop time constant; V ref Reference voltage value; The switching control module is configured to: if the real-time transient energy is less than the critical instability energy, maintain the original virtual synchronous machine control strategy; if the real-time transient energy is not less than the critical instability energy, control the grid-type converter to enter fault ride-through mode.

6. An electronic device, characterized in that, It includes a memory and a processor, as well as computer instructions stored in the memory and running on the processor, which, when executed by the processor, perform the method according to any one of claims 1-4.

7. A computer-readable storage medium, characterized in that, Used to store computer instructions, which, when executed by a processor, perform the method described in any one of claims 1-4.

8. A computer program product, characterized in that, Includes a computer program, which, when executed by a processor, implements the method described in any one of claims 1-4.

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