A power system fault feature analysis method considering multi-source heterogeneous new energy

Abstract

This application belongs to the field of power system fault analysis, specifically disclosing a method for analyzing the fault characteristics of power systems considering multi-source heterogeneous renewable energy sources. The method includes the following steps: establishing a coupled power system model containing grid-connected converters and grid-connected converters; setting faults of grounding, short-circuit, and open-circuit types near the common coupling point of the coupled power system model, and collecting time-series data of three-phase voltage and three-phase current at the common coupling point during the fault occurrence period; performing a fast Fourier transform on the time-series data to extract the amplitude and phase characteristics of the fundamental frequency component; and analyzing the impact of the coupling interaction between the grid-connected and grid-connected converters during the fault period on the dynamic characteristics of the system's electrical quantities based on the variation law of amplitude and phase characteristics with fault resistance. This application can accurately analyze the fault characteristics of power systems when grid-connected and grid-connected converters are operating in coupled mode.

Description

Technical Field

[0001] This application belongs to the field of power system fault analysis, and more specifically, relates to a method for analyzing the fault characteristics of power systems that takes into account multiple heterogeneous new energy sources. Background Technology

[0002] Driven by energy structure transformation, renewable energy sources such as wind and solar power are developing rapidly, significantly increasing the proportion of new energy installed capacity in the power system and exhibiting characteristics of a high proportion of power electronic equipment. The surge in installed capacity of new energy sources such as wind and solar power, coupled with the significant differences in topology, control logic, and dynamic characteristics among diverse power sources such as solar inverters, wind power converters, and energy storage devices, combined with the characteristics of weak end-grid networks with high impedance and long-distance distributed parameters, results in complex dynamic characteristics of the system: "low inertia, weak damping, and strong interaction," significantly exacerbating fault risks and stability challenges. Against the backdrop of high-proportion new energy penetration in the new power system, coupled operation of grid-connected and grid-connected converters has become the norm. The fundamental differences in their response mechanisms bring new challenges to system stability under fault scenarios.

[0003] Currently, most new energy equipment adopts grid-connected or grid-connected control technologies for grid connection. While these technologies offer advantages in flexible power regulation, their anti-interference capabilities and grid support performance are inferior to traditional synchronous generator units. Particularly in weak grid scenarios at the end of the line, high impedance characteristics lead to prolonged voltage dips and increased harmonic interference after faults. The phase-locked loops (PLLs) of grid-connected converters are susceptible to instability and grid disconnection due to harmonic interference, while grid-connected converters face challenges such as slow power regulation and multi-machine coupled oscillations. Furthermore, insufficient system inertia and damping levels further amplify the risks of voltage and synchronous instability. However, existing research largely focuses on fault conditions where grid-connected and grid-connected converters operate independently, lacking a comprehensive system-level analysis of fault scenarios when they operate coupled. This makes it difficult to accurately reveal the complete dynamic behavior and characteristic patterns of such hybrid systems under fault conditions.

[0004] Therefore, accurately analyzing the fault characteristics of power systems when coupled with grid-type and grid-connected converters is an urgent problem to be solved. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the purpose of this application is to provide a power system fault characteristic analysis method that takes into account multi-source heterogeneous new energy sources, which can accurately analyze the fault characteristics of power systems operating in conjunction with grid-connected and grid-connected converters.

[0006] To achieve the above objectives, in a first aspect, this application provides a method for analyzing the fault characteristics of a power system that takes into account multiple heterogeneous renewable energy sources, comprising the following steps:

[0007] S10, Establish a coupled power system model including a grid-connected converter and a grid-connected converter. The grid-connected converter adopts a current control strategy based on a phase-locked loop, and the grid-connected converter adopts a voltage control strategy based on a virtual synchronous generator. S20, near the common coupling point of the coupled power system model, faults of the types of grounding, short circuit and open circuit are set, and time series data of three-phase voltage and three-phase current at the common coupling point are collected during the fault occurrence period; S30, Perform a fast Fourier transform on the time series data to extract the amplitude and phase characteristics of the power frequency fundamental component; S40, Based on the variation law of the amplitude and phase characteristics with the fault resistance, analyze the influence of the coupling interaction between the grid-connected converter and the grid-connected converter during the fault on the dynamic characteristics of the system electrical quantities.

[0008] As a further preferred embodiment, in step S10, when establishing the model of the grid-connected converter, the phase-locked loop-based current control strategy includes a phase-locked loop control module, and the mathematical model of the phase-locked loop is:

[0009] In the formula, It is the synchronization phase angle of the phase-locked loop output; By adjusting the grid voltage Axial components The angular frequency obtained after performing proportional-integral control; It is the angular frequency of the power grid; These are the proportional parameters of the phase-locked loop; These are the integral parameters of the phase-locked loop; The d-axis component of the grid voltage; This represents the q-axis component of the grid voltage.

[0010] As a further preferred embodiment, in step S10, when establishing the model of the grid-type converter, the voltage control strategy based on the virtual synchronous generator (VSG) includes an active-frequency control loop and a reactive-voltage control loop; the mathematical model of the active-frequency control loop is:

[0011] The mathematical model of the reactive power-voltage control loop is as follows:

[0012] In the formula, Virtual rotational inertia parameters characterizing the VSG; The system's rated angular frequency parameter; This is the actual angular frequency; For virtual mechanical power input; Reflects the actual active power output of the VSG; This represents the damping coefficient of the VSG system; This is the active frequency droop factor; The power angle between the VSG output voltage and the mains voltage; This indicates the preset active power reference value; This represents the actual output reactive power of the inverter. This is a preset reactive power reference value; This is the reference amplitude of the VSG internal potential; The magnitude of the internal potential of the VSG; This is the integral gain.

[0013] As a further preferred embodiment, after establishing the coupled power system model in step S10, the method further includes a step of aggregating multiple converters with the same control mode; aggregating multiple grid-connected converters as current sources and aggregating multiple grid-connected converters as voltage sources.

[0014] As a further preferred option, the fault types set in step S20 specifically include single-phase ground fault, single-phase open-circuit fault, two-phase ground fault, two-phase open-circuit fault, two-phase short-circuit fault, three-phase open-circuit fault, and three-phase short-circuit fault.

[0015] As a further preferred embodiment, in step S20, when collecting time series data, for ground faults and short-circuit faults, the fault resistance is set to increase by an equal amount from 0 to 2Ω for multiple simulations; for open-circuit faults, the fault resistance is set to 1Ω. ~100 Multiple simulations were performed with a regular increasing pattern; each simulation collected time-series data of voltage and current at the common coupling point within 0.1 seconds after the fault occurred.

[0016] As a further preferred embodiment, step S30, specifically including the extraction of amplitude and phase characteristics of the power frequency fundamental component, includes: For the acquired discrete voltage or current time-domain signals x ( n Perform a Fast Fourier Transform to obtain the spectrum. X ( k ), where the sampling frequency is fs The number of sampling points is N Frequency index k With actual frequency The correspondence is ; Location of the frequency index closest to the 50Hz power frequency Obtain the corresponding spectral components. ; Calculate the fundamental amplitude and phase of a voltage or current signal. The fundamental amplitude is given by the spectral magnitude, and its calculation formula is as follows: .

[0017] As a further preferred embodiment, step S40, analyzing the impact of coupling interaction on the dynamic characteristics of electrical quantities, includes: Comparative analysis of the differences in the characteristics of voltage amplitude, voltage phase, current amplitude, and current phase as a function of fault resistance under grounding faults, short-circuit faults, and open-circuit faults; The fault resistance variation range for grounding faults and short-circuit faults is 0.01–2 Ω, and the fault resistance variation range for open-circuit faults is 10 Ω. - ³~10 8 Ω.

[0018] As a further preferred embodiment, step S40, the comparative analysis specifically includes: Under short-circuit faults, the voltage and current amplitudes exhibit an initial peak value followed by a decay as the fault resistance increases, while the voltage and current phases oscillate. Under grounding faults, the voltage and current amplitudes exhibit characteristics of initial impact followed by decay and then tending to steady-state values, while the voltage phase exhibits characteristics of initial oscillation followed by convergence. Under open-circuit faults, the voltage amplitude and phase exhibit continuous oscillation characteristics, while the current amplitude is lower than that of short-circuit and ground faults.

[0019] Secondly, this application provides a simulation analysis system for fault characteristics of new energy power systems, used to implement the fault characteristic analysis method for power systems considering multi-source heterogeneous new energy sources as described in any of the above-mentioned methods, including: The model building module is used to build coupled power system models that include grid-connected converters and grid-connected converters. The fault simulation and data acquisition module is used to set up multiple fault types near the common coupling point of the model, simulate the fault occurrence process, and acquire voltage and current time series data at the common coupling point. The feature extraction module is used to perform fast Fourier transform on the collected time series data to extract the amplitude and phase features of the fundamental frequency component. The coupling interaction analysis module is used to analyze the impact of the coupling interaction between the grid-connected converter and the network-connected converter on the dynamic characteristics of the system electrical quantities during a fault, based on the extracted amplitude and phase characteristics.

[0020] This application has the following advantages: By establishing a coupled power system model that simultaneously includes a grid-connected converter based on phase-locked loop current control and a grid-connected converter based on virtual synchronous generator voltage control, it can realistically reflect the actual operating structure of the system under high-proportion renewable energy access, characterized by multi-source heterogeneity and mixed control strategies. Based on this, typical faults covering grounding, short-circuit, and open-circuit types are set near the common coupling point, and three-phase electrical quantity time-series data are collected, ensuring the typicality of the fault scenarios and the completeness of the data foundation. Furthermore, a fast Fourier transform is performed on the time-series data to extract the amplitude and phase characteristics of the power frequency fundamental component. It can effectively suppress harmonic and noise interference, and extract the core characteristics of key electrical quantities that characterize the steady-state and transient behavior of the system. Finally, by analyzing the law of these amplitude and phase characteristics changing with fault resistance, it can systematically reveal how the current following characteristics of grid-connected converters and the voltage support and inertia response characteristics of grid-connected converters are coupled, superimposed and affect the overall dynamic behavior of the system under fault disturbance. This enables accurate analysis of power system fault characteristics under the complex scenario of coupled operation of grid-connected and grid-connected converters, overcoming the limitations of existing technologies that focus on the independent operation of a single type of converter. Attached Figure Description

[0021] Figure 1 This is a control structure diagram of a grid-connected converter based on current control provided in an embodiment of this application; Figure 2 This is a block diagram of a phase-locked loop control provided in an embodiment of this application; Figure 3 This is a schematic diagram of the VSG control principle provided in the embodiments of this application; Figure 4 This is a schematic diagram illustrating the aggregation of grid-connected converters and network-type converters provided in the embodiments of this application; Figure 5 This is an equivalent circuit diagram of a grid-connected converter and a network-connected converter provided in the embodiments of this application; Figure 6 This is an equivalent circuit diagram of the grid-connected and grid-coupled system when an A-phase open circuit fault occurs, as provided in the embodiments of this application. Figure 7 This is a waveform diagram of the three-phase voltage (left side of the figure) and three-phase current (right side of the figure) during normal operation of the system provided in the embodiments of this application; Figure 8 This is a waveform diagram of the three-phase voltage (left side of the figure) and three-phase current (right side of the figure) when a single-phase ground fault occurs in the system provided in this application embodiment; Figure 9 This is a waveform diagram of the three-phase voltage (left side of the figure) and three-phase current (right side of the figure) when a single-phase open circuit fault occurs in the system provided in this application embodiment; Figure 10This is a waveform diagram of the three-phase voltage (left side of the figure) and three-phase current (right side of the figure) when a two-phase short-circuit fault occurs in the system provided in this application embodiment; Figure 11 This is a diagram showing the changes in electrical quantities of phase A extracted by fast Fourier transform under a single-phase ground fault, provided in an embodiment of this application; where (a) is the amplitude of phase A voltage, (b) is the phase of phase A voltage, (c) is the amplitude of phase A current, and (d) is the phase of phase A current. Figure 12 This is a diagram of the changes in electrical quantities of phase A extracted by fast Fourier transform under a single-phase open circuit fault, provided in an embodiment of this application; wherein, (a) is the voltage amplitude of phase A, (b) is the voltage phase of phase A, (c) is the current amplitude of phase A, and (d) is the current phase of phase A. Figure 13 This is a diagram of the changes in electrical quantities of phase A extracted by fast Fourier transform under a two-phase open-circuit fault, provided in an embodiment of this application; wherein, (a) is the voltage amplitude of phase A, (b) is the voltage phase of phase A, (c) is the current amplitude of phase A, and (d) is the current phase of phase A. Figure 14 This is a diagram showing the changes in electrical quantities of phase A extracted by fast Fourier transform under a two-phase short-circuit fault, provided in an embodiment of this application; where (a) is the voltage amplitude of phase A, (b) is the voltage phase of phase A, (c) is the current amplitude of phase A, and (d) is the current phase of phase A. Figure 15 This is a diagram showing the changes in electrical quantities of phase A under a two-phase ground fault, extracted by fast Fourier transform, provided in an embodiment of this application; where (a) is the amplitude of phase A voltage, (b) is the phase of phase A voltage, (c) is the amplitude of phase A current, and (d) is the phase of phase A current. Figure 16 This is a diagram of the changes in electrical quantities of phase A extracted by fast Fourier transform under a three-phase short-circuit fault, provided in an embodiment of this application; wherein, (a) is the voltage amplitude of phase A, (b) is the voltage phase of phase A, (c) is the current amplitude of phase A, and (d) is the current phase of phase A. Figure 17 This is a diagram of the changes in electrical quantities of phase A extracted by fast Fourier transform under a three-phase open circuit fault, provided in an embodiment of this application; wherein, (a) is the voltage amplitude of phase A, (b) is the voltage phase of phase A, (c) is the current amplitude of phase A, and (d) is the current phase of phase A. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0023] To accurately analyze the fault characteristics of power systems operating in coupled mode with grid-connected and grid-connected converters, this application, under the background of large-scale grid connection of multi-source heterogeneous new energy equipment, focuses on the differences in fault response mechanisms between the two, integrates their dynamic characteristics, explores the fault characteristics of new power systems under coupled mode, and analyzes the evolution law of electrical parameters after fault.

[0024] Specifically, the power system fault characteristic analysis method considering multi-source heterogeneous renewable energy provided in this application includes the following steps: Step 1: Establish a new power system model coupled with the grid. 1. Establish a grid-connected converter model based on current control. The main control methods for grid-connected converters include phase-locked loops (PLLs), current control loops, and pulse-width modulation (PWM). Each module performs its specific function and works closely together to ensure the stable and efficient operation of the converter. Among these, the most critical control module is the PLL. The PLL estimates and tracks the voltage phase at the common coupling point, thereby achieving synchronization with the main power grid. The control structure diagram of a grid-connected converter based on current control is shown below. Figure 1 As shown.

[0025] in, It is the synchronization phase angle of the phase-locked loop output; By voltage Axial components The angular frequency obtained after proportional-integral control. In the current inner loop control, a feedforward compensation term is introduced. , achieved d shaft and q Decoupling control of the axis. Among them, and The measurements were obtained respectively dq shaft current value, and The measurements were obtained respectively dq Shaft voltage value. This is a given reference value for the DC-side voltage. This is a reference value for the AC voltage at the grid connection point. This is the actual measured value of the AC voltage at that point. Indicates the value of the filter inductance. and The corresponding AC voltages at the grid connection points are respectively d shaft and q Components on the axis. The mathematical modeling equations for the phase-locked loop are as follows:

[0026] In the formula, These are the proportional parameters of the phase-locked loop; These are the integral parameters of the phase-locked loop; For grid voltage d Axial components; For grid voltage q Axis component. Phase-locked loop control block diagram as follows: Figure 2 As shown.

[0027] 2. Establish a grid-type converter model based on virtual synchronous generator control. The control module of a Virtual Synchronous Generator (VSG) encompasses key components such as active power control loop, reactive power control loop, voltage-current dual closed-loop regulation, and Space Vector Pulse Width Modulation (SVPWM) signal generation and modulation. The core mechanism of this control strategy focuses on the active power and reactive power loops, essentially achieving dynamic synchronization with the power grid by simulating the rotor motion characteristic equations of the synchronous generator. Grid-connected converters built based on this VSG technology possess external electrical characteristics similar to traditional synchronous generators. The schematic diagrams of the active power-frequency loop and reactive power-voltage loop are shown below. Figure 3 As shown.

[0028] The mathematical model for the active-frequency loop is as follows:

[0029] In the formula, Reference power (W); This refers to the active frequency droop factor. For the angle of attack; This indicates the preset active power reference value; For virtual mechanical power input; Reflects the actual active power output of the VSG; Virtual rotational inertia parameters characterizing the VSG; This represents the damping coefficient of the VSG system; the active frequency regulation coefficient. The selection method can refer to the selection method of the power frequency regulation coefficient of synchronous generator. Too large an amount will affect the system's adjustment accuracy, while Too small a size is detrimental to system stability.

[0030] The mathematical model for the reactive power loop-voltage loop is as follows:

[0031] In the formula, This represents the actual output reactive power of the inverter. This is the reference amplitude of the VSG internal potential; The magnitude of the internal potential of the VSG; This is the integral gain.

[0032] 3. Dimensionality reduction of aggregate models In a real power grid, there may be multiple grid-connected converters and multiple network-connected converters coupled together. Therefore, it is necessary to aggregate converters with the same control method first. Aggregation follows the principle of consistent dynamic characteristics, mapping power and impedance equivalently. Since the bandwidth of the current loop of a grid-connected converter is much larger than that of the phase-locked loop, the dynamic process of the current loop can be ignored; therefore, grid-connected converters are aggregated as current sources. Conversely, the voltage and current inner loop regulation speed of a network-connected converter is much faster than that of the active and reactive power loops; the dynamic process of the voltage and current inner loop can also be ignored, so network-connected converters are aggregated as voltage sources. The control parameters and power reference values ​​of the converters change after aggregation, but the control strategy and the characteristics of the converters remain unchanged. The aggregation model is as follows: Figure 4 Equivalent model such as Figure 5 .

[0033] Figure 5 middle, This refers to the effective value of the output current of the current-controlled converter. This refers to the effective value of the output voltage of the grid-connected converter. for The phase angle difference with the power grid corresponds to the power angle of the synchronous generator, which will be referred to as the power angle below for ease of description. For voltage With current Phase angle difference; This refers to the effective value of the output voltage of the grid-connected converter. The power angle of the grid-type converter; This is the effective value of the grid voltage.

[0034] Step 2: Fault Data Acquisition 1. Set up faults This embodiment focuses on analyzing seventeen common microgrid faults, with the fault locations set near the common coupling point (CCP) of the grid converters. The impact of various faults on the voltage and current at the CCP is investigated. Fault types include single-phase ground fault, single-phase open-circuit fault, two-phase ground fault, two-phase open-circuit fault, two-phase short-circuit fault, three-phase open-circuit fault, and three-phase short-circuit fault. Single-phase ground faults are further subdivided into A-phase ground fault, B-phase ground fault, and C-phase ground fault, and the other fault types follow the same pattern, totaling seventeen fault groups. Taking a single-phase ground fault as an example, the following diagram is drawn... Figure 6 The circuit equivalent model during the fault period is shown.

[0035] 2. Fault Database Construction To clarify the changes in output electrical parameters at the common coupling point under various fault conditions, a simulation program was built in Simulink based on the aforementioned mathematical model of the coupled system. To accurately simulate actual fault conditions, 200 sets of gradient simulations were designed for each fault type. For grounding and short-circuit faults, the fault resistance was set to increase in equal increments from 0 to 2 ohms; for open-circuit faults, the fault resistance was set to 1 ohm. ~100 The voltage and current time series at the common coupling point were collected within 0.1s during the fault occurrence period to construct a fault database. The simulation parameters of the main circuit are shown in Table 1.

[0036] Table 1 Simulation parameters of the root / network coupled system

[0037] Step 3: FFT-based fault feature analysis 1. Time-domain analysis of fault characteristics Taking single-phase grounding fault, single-phase open-circuit fault, and two-phase short-circuit fault as examples, the time series of output voltage and current at the common coupling point are collected respectively. Figure 7 This figure shows the output voltage and current information under normal system operating conditions. As can be seen from the figure, the voltage remains stable under normal operating conditions, while the current reaches a steady state after 0.5 seconds. (Observation...) Figures 7-10It can be seen that the voltage and current output characteristics of the network-coupled system under different fault types show significant differences within the 1.2-1.3s fault injection interval, which is significantly different from the steady-state characteristics under normal operating conditions, where the three-phase voltage is stable at 300-400V, the three-phase current is stable at 0-40A without transient impact, and the three-phase amplitude is balanced without distortion. When a single-phase ground fault occurs, the voltage of the faulty phase shows a large transient distortion of ±400V, while the non-faulty phase only fluctuates slightly and remains at around 300V. The current of the faulty phase shows a pulse-like sudden change with a peak value exceeding 400A, while the non-faulty phase only experiences a small disturbance. Moreover, the voltage and current recover quickly after the fault ends, demonstrating the system's good transient response and current limiting capability. The transient impact of a single-phase open-circuit fault is weaker than that of a single-phase ground fault. The voltage of the faulty phase shows a spike and drop of ±450V, and the current pulse-like sudden change peak value is about 100A. The non-faulty phase has almost no obvious fluctuation, and the recovery speed of the system voltage and current is faster than that of a single-phase ground fault. Two-phase open-circuit faults cause significantly greater disturbances to the system than single-phase faults. The voltage of the faulted phase experiences a sharp drop and rebound of ±400V, while the non-faulty phases also exhibit significant fluctuations with longer transient durations. The faulty phase current surges dramatically to ±5000A, with an impact intensity far exceeding that of single-phase faults. Simultaneously, the voltage recovery speed of the grid-coupled system is significantly slower, reflecting the more severe impact of multi-phase faults on system stability and the higher requirements for current limiting and recovery mechanisms. Overall, the grid-coupled system effectively supports the stability of the non-faulty phase voltage and suppresses the continuous increase of fault current under single-phase faults, demonstrating good transient recovery capability. However, the current surge and transient duration increase significantly under two-phase open-circuit faults, and the system recovery speed slows down, exposing weaknesses in system stability control under multi-phase faults.

[0038] Overall, the core difference between the fault characteristics of the coupled network system and the traditional single network / structured network scenario lies in the coupling effect of the characteristics of the network and structured network units. This is manifested in three aspects: First, the intensity of fault disturbance. Due to the superposition of disturbances from the two types of units, the coupled system experiences a larger impact amplitude for short-circuit / grounding faults and more severe oscillations for open-circuit faults, accompanied by secondary interference. Second, the fault evolution pattern. The traditional single scenario exhibits a "monotonic change - stable convergence" characteristic after a fault, while the coupled system exhibits a "impact superposition - nonlinear decay - step convergence / continuous oscillation" characteristic due to unit interaction. Third, the recovery mechanism. The single scenario can recover quickly through its own single adjustment logic, while the coupled system requires the coordinated adaptation of the two types of units, making recovery more difficult and prone to adjustment conflicts. Specifically, short-circuit faults in traditional scenarios exhibit a single mode of "transient impact - rapid recovery," while in coupled systems they exhibit "impact superposition - nonlinear decay - step convergence." Open-circuit faults in traditional scenarios exhibit weak oscillations and a recovery trend, while in coupled systems they exhibit "continuous oscillation - no recovery trend - cross-unit diffusion." The differences between the two types of faults are more significant, which also places higher demands on fault identification and collaborative protection strategies for coupled systems.

[0039] 2. Frequency domain analysis of voltage and current amplitude extracted by FFT In the study of power electronic grid-connected systems and faults, the fundamental frequency components of voltage and current signals contain crucial information about the system's operating state and fault characteristics. Since the actual acquired time-domain signals typically contain harmonic components and noise interference, direct time-domain analysis is insufficient to accurately characterize their steady-state properties. Therefore, it is necessary to use frequency-domain analysis methods to process the signals. The Fast Fourier Transform (FFT), as a highly efficient spectrum analysis tool, is widely used in power system signal processing to extract the fundamental amplitude and phase information of voltage and current signals at the power frequency. Let the sampled discrete voltage or current time-domain signal be... Its sampling frequency is The number of sampling points is N The FFT can be used to transform a time-domain signal into the frequency domain, and its spectral expression is as follows:

[0040] Among them, frequency index k With actual frequency The correspondence is In power systems, the power frequency is typically 50 Hz, therefore, the frequency index closest to 50 Hz can be used to determine the frequency. Obtain the spectral components of the signal at the power frequency. Based on this spectral component, the fundamental amplitude and phase of the voltage or current signal can be further calculated. The fundamental amplitude is given by the spectral magnitude, and its calculation formula is:

[0041] By performing FFT analysis on the three-phase voltage and current signals, the amplitude and phase characteristics of each phase under the power frequency fundamental frequency can be obtained. Under different fault conditions, such as ground faults, short-circuit faults, and open-circuit faults, these fundamental frequency characteristics show significant differences with the change of fault impedance. For example, ground faults and short-circuit faults usually lead to a significant increase in the fundamental current amplitude and a decrease in the voltage amplitude, while open-circuit faults show a sharp decrease in the current amplitude and a significant change in phase characteristics. Therefore, using the FFT method to extract the fundamental amplitude and phase of voltage and current signals under the power frequency can not only effectively suppress harmonic and noise interference, but also obtain key characteristics reflecting the system's operating state with low computational complexity, laying a reliable data foundation for power electronic system fault analysis. Due to the large amount of data, this embodiment focuses on analyzing the seven most representative fault types: single-phase ground fault, two-phase ground fault, two-phase short circuit, three-phase short circuit, single-phase open circuit, two-phase open circuit, and three-phase open circuit. One set of each fault type is selected, and the fundamental amplitude and phase characteristics of its A-phase voltage and current under the power frequency are analyzed. These seven fault groups are: phase A grounding, phase AB short circuit, phase ABC three-phase short circuit, phase AC two-phase grounding, phase A open circuit, phase AB two-phase open circuit, and phase ABC three-phase open circuit. The extracted features are as follows: Figures 11-17 As shown.

[0042] Based on the 50Hz fundamental component extracted by FFT, the variation characteristics of phase A voltage, current amplitude, and phase with fault resistance for seven types of faults in a coupled system (i.e., a multi-source heterogeneous scenario integrating both the grid-connecting unit and the grid-connecting unit) show significant differences compared to traditional single grid-connecting scenarios and single grid-connecting scenarios (containing only the grid-connecting converter VSG). Due to the coupling between the constant power control characteristics of the grid-connecting unit and the inertia support and voltage regulation characteristics of the grid-connecting unit, the fault propagation path, disturbance superposition effect, and recovery mechanism in the coupled system exhibit unique characteristics. The specific core differences from traditional single scenarios are as follows: Short-circuit faults are the most severe type of fault in multi-source heterogeneous systems, with fault resistance varying linearly within the range of 0.01–2Ω. In the initial stage of the fault, phase A voltage amplitude typically exhibits a significant peak, which rapidly decays as the fault resistance increases; the voltage phase is accompanied by ±10... 4 The system exhibits severe oscillations on the order of deg; the current amplitude initially surges dramatically before rapidly decaying to near zero, with the current phase also displaying high-frequency, large-amplitude oscillations. Compared to traditional single-grid systems, the grid-connected power supply in a multi-source heterogeneous system maintains bus voltage rigidity, allowing it to continuously inject current into the fault point for a short period, thus significantly amplifying the current surge amplitude. Internal comparisons show that the voltage and current surge amplitudes of a three-phase short circuit (exceeding 100 V / 800 A) are significantly higher than those of a two-phase short circuit (approximately 50 V / 500 A), reflecting that under grid-supported conditions, the more faulty phases there are, the more severe the system power and electromagnetic stress surges.

[0043] Grounding faults can be considered a branch of short-circuit faults, and their fault resistance also varies linearly within the range of 0.01–2 Ω. Under this type of fault, the voltage amplitude of phase A experiences a peak in the initial stage of the fault, and then rapidly decays to a stable low value as the resistance increases; the voltage phase initially exhibits significant oscillations, but generally shows a smooth convergence trend; the current amplitude experiences a significant initial impact, followed by rapid decay, and the current phase oscillation amplitude is relatively limited. Compared to traditional single-grid systems, multi-source heterogeneous systems are highly sensitive to unbalanced voltages, making the phase and current transient characteristics more pronounced during grounding faults. Internal differences show that the voltage and current impact amplitudes of two-phase grounding (approximately 45 V / 480 A) are significantly greater than those of single-phase grounding (approximately 15 V / 200 A), and the phase oscillation amplitude also significantly increases with the number of grounded phases, indicating that multi-phase grounding causes stronger unbalanced disturbances to multi-source heterogeneous systems.

[0044] The fault resistance of open circuit faults is 10 - ³~10 8 The logarithmic increase of Ω indicates that the smaller the resistance, the more severe the circuit rupture. Under this type of fault, the voltage amplitude and phase of phase A exhibit continuous high-frequency oscillations throughout the fault process, with almost no obvious convergence trend; the current amplitude is only in the ampere range, significantly lower than that of short-circuit and ground faults, but its phase also shows continuous oscillation characteristics. Unlike traditional single-network systems where circuit ruptures mainly cause power redistribution, in multi-source heterogeneous systems, the different power supply control characteristics and equivalent impedance differences make it difficult for the system to quickly form a unified and stable phase reference, thus causing phase oscillations to persist for a long time. Internal comparisons show that the voltage amplitude and phase oscillations are most severe in two-phase circuit ruptures (peak values ​​exceeding 150 V); single-phase circuit ruptures have the largest current amplitude (3.42–3.56 A) because the remaining phase maintains the current path; while three-phase circuit ruptures have the smallest amplitude (1.5–1.65 A) because the current loop is completely cut off and the current is maintained only by parasitic channels.

[0045] Overall, the analysis results of the 50 Hz fundamental component extracted by FFT show that in multi-source heterogeneous grid-connected / grid-connected systems, the voltage, current amplitude, and phase of phase A under seven types of faults exhibit significantly different dynamic characteristics compared to traditional single grid-connected or single grid-connected systems. The fundamental reason is that the grid-connected power supply provides the voltage and phase reference, while the grid-connected power supply synchronizes and injects current through a PLL. Both participate in regulation during the fault, resulting in the superposition of "voltage-supported" and "current-following" control behaviors in the system response. Compared to single-control-mode systems, multi-source heterogeneous systems exhibit stronger transient impacts, more complex phase evolution processes, and more pronounced multi-timescale coupling characteristics in the early stages of a fault, making the influence of fault type and phase number combination on electrical quantity characteristics more sensitive. A comparison table of output characteristics under various fault types is shown below.

[0046] Table 2 Comparison of Fundamental Electrical Quantity Characteristics under Three Types of Faults

[0047] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for analyzing fault characteristics of power systems considering multi-source heterogeneous renewable energy sources, characterized in that, Includes the following steps: S10, Establish a coupled power system model including a grid-connected converter and a grid-connected converter. The grid-connected converter adopts a current control strategy based on a phase-locked loop, and the grid-connected converter adopts a voltage control strategy based on a virtual synchronous generator. S20, near the common coupling point of the coupled power system model, faults of the types of grounding, short circuit and open circuit are set, and time series data of three-phase voltage and three-phase current at the common coupling point are collected during the fault occurrence period; S30, Perform a fast Fourier transform on the time series data to extract the amplitude and phase characteristics of the power frequency fundamental component; S40, Based on the variation law of the amplitude and phase characteristics with the fault resistance, analyze the influence of the coupling interaction between the grid-connected converter and the grid-connected converter during the fault on the dynamic characteristics of the system electrical quantities.

2. The power system fault characteristic analysis method considering multi-source heterogeneous new energy sources as described in claim 1, characterized in that, In step S10, when establishing the model of the grid-connected converter, the phase-locked loop-based current control strategy includes a phase-locked loop control module, and the mathematical model of the phase-locked loop is as follows: In the formula, It is the synchronization phase angle of the phase-locked loop output; By adjusting the grid voltage Axial components The angular frequency obtained after performing proportional-integral control; It is the angular frequency of the power grid; These are the proportional parameters of the phase-locked loop; These are the integral parameters of the phase-locked loop; The d-axis component of the grid voltage; This represents the q-axis component of the grid voltage.

3. The power system fault characteristic analysis method considering multi-source heterogeneous new energy sources as described in claim 1, characterized in that, In step S10, when establishing the model of the grid-type converter, the voltage control strategy based on the virtual synchronous generator (VSG) includes an active-frequency control loop and a reactive-voltage control loop; the mathematical model of the active-frequency control loop is: The mathematical model of the reactive power-voltage control loop is as follows: In the formula, Virtual rotational inertia parameters characterizing the VSG; The system's rated angular frequency parameter; This is the actual angular frequency; For virtual mechanical power input; Reflects the actual active power output of the VSG; This represents the damping coefficient of the VSG system; This is the active frequency droop factor; The power angle between the VSG output voltage and the mains voltage; This indicates the preset active power reference value; This represents the actual output reactive power of the inverter. This is a preset reactive power reference value; This is the reference amplitude of the VSG internal potential; The magnitude of the internal potential of the VSG; This is the integral gain.

4. The power system fault characteristic analysis method considering multi-source heterogeneous new energy sources as described in claim 1, characterized in that, In step S10, after establishing the coupled power system model, the method further includes a step of aggregating multiple converters with the same control mode; aggregating multiple grid-connected converters as current sources and aggregating multiple grid-connected converters as voltage sources.

5. The power system fault characteristic analysis method considering multi-source heterogeneous new energy sources as described in claim 1, characterized in that, In step S20, the specific fault types set include single-phase ground fault, single-phase open circuit fault, two-phase ground fault, two-phase open circuit fault, two-phase short circuit fault, three-phase open circuit fault, and three-phase short circuit fault.

6. The power system fault characteristic analysis method considering multi-source heterogeneous new energy sources as described in claim 1 or 5, characterized in that, In step S20, when collecting time series data, for ground faults and short circuit faults, the fault resistance is set to increase by an equal amount from 0 to 2Ω for multiple simulations. For open circuit faults, set the fault resistance to 1. ~100 Multiple simulations were performed with a regular increasing pattern; in each simulation, the voltage and current time series data at the common coupling point were collected within 0.1 seconds after the fault occurred.

7. The power system fault characteristic analysis method considering multi-source heterogeneous new energy sources as described in claim 1, characterized in that, In step S30, the extraction of the amplitude and phase characteristics of the power frequency fundamental component specifically includes: For the acquired discrete voltage or current time-domain signals x ( n Perform a Fast Fourier Transform to obtain the spectrum. X ( k ), where the sampling frequency is fs The number of sampling points is N Frequency index k With actual frequency The correspondence is ; Location of the frequency index closest to the 50Hz power frequency Obtain the corresponding spectral components ; Calculate the fundamental amplitude and phase of a voltage or current signal. The fundamental amplitude is given by the spectral magnitude, and its calculation formula is as follows: .

8. The power system fault characteristic analysis method considering multi-source heterogeneous new energy sources as described in claim 1, characterized in that, In step S40, the analysis of the impact of coupling interaction on the dynamic characteristics of electrical quantities includes: Comparative analysis of the differences in the characteristics of voltage amplitude, voltage phase, current amplitude, and current phase as a function of fault resistance under grounding faults, short-circuit faults, and open-circuit faults; The fault resistance variation range for grounding faults and short-circuit faults is 0.01–2 Ω, and the fault resistance variation range for open-circuit faults is 10 Ω. - ³~10 8 Ω.

9. The power system fault characteristic analysis method considering multi-source heterogeneous new energy sources as described in claim 8, characterized in that, In step S40, the comparative analysis specifically includes: Under short-circuit faults, the voltage and current amplitudes exhibit an initial peak value followed by a decay as the fault resistance increases, while the voltage and current phases oscillate. Under grounding faults, the voltage and current amplitudes exhibit characteristics of initial impact followed by decay and then tending to steady-state values, while the voltage phase exhibits characteristics of initial oscillation followed by convergence. Under open-circuit faults, the voltage amplitude and phase exhibit continuous oscillation characteristics, while the current amplitude is lower than that of short-circuit and ground faults.

10. A simulation analysis system for fault characteristics of a new energy power system, characterized in that, The method for analyzing the fault characteristics of a power system considering multiple heterogeneous new energy sources as described in any one of claims 1 to 9 includes: The model building module is used to build coupled power system models that include grid-connected converters and grid-connected converters. The fault simulation and data acquisition module is used to set up multiple fault types near the common coupling point of the model, simulate the fault occurrence process, and acquire voltage and current time series data at the common coupling point. The feature extraction module is used to perform fast Fourier transform on the collected time series data to extract the amplitude and phase features of the fundamental frequency component. The coupling interaction analysis module is used to analyze the impact of the coupling interaction between the grid-connected converter and the network-connected converter on the dynamic characteristics of the system electrical quantities during a fault, based on the extracted amplitude and phase characteristics.

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