Disturbance analysis method and system for node voltage and current and converter impedance mapping

Abstract

The invention discloses a disturbance analysis method and system for node voltage and current and converter impedance mapping. The method comprises the following steps: determining parameters of a network construction type converter control model and a network following type converter control model by taking stability of a system in which a network construction type converter and a network following type converter coexist as a target; establishing a frequency coupling characteristic admittance model under disturbance when the network construction type converter and the network following type converter coexist by using an alternating current port admittance model of the network construction type converter and an alternating current port admittance model of the network following type converter, which are obtained by adopting a small signal analysis method; based on the frequency coupling characteristic admittance model, constructing alternating current port admittance models of the network type converter and the network type converter, and determining equivalent output impedance of the converter; the phase angles of the equivalent output impedance and the impedance sum of the converter and the phase angle of the equivalent output impedance of the converter are obtained, system stability analysis under disturbance is carried out, and the potential instability problem is more effectively prevented and dealt with.

Description

Technical Field

[0001] This invention belongs to the field of converter and power system stability technology. Specifically, for power systems with both grid-connected and grid-connected converters, it relates to a method and system for analyzing system stability under disturbances based on the mapping of relaxed node voltage, current and converter dynamic impedance. The established mapping relationship clarifies the instability mechanism of a novel power system. Background Technology

[0002] With the high proportion of new energy sources being integrated into the grid, new power systems are trending towards a high proportion of renewable energy connected to the grid through a high proportion of power electronic devices. Consequently, the stable operation of the power system faces significant changes. Compared to traditional power systems, new power systems exhibit dynamic characteristics such as nonlinearity, time-varying nature, heterogeneity, and uncertainty, thus their stability mechanisms are no longer consistent.

[0003] Currently, grid-connected control for new energy sources is mainly based on grid-following converters. These converters achieve synchronization by following the grid voltage through phase-locked loops (PLLs) and use current vector control to manage the grid-connected current, exhibiting "current source" characteristics. However, with the large-scale grid connection of grid-following converters, the overall rigidity of the power grid weakens, and the grid strength in some areas decreases. The interaction between grid-following converters and weak grids leads to frequent broadband oscillation accidents, posing a severe challenge to the safe and stable operation of the power system. In contrast, grid-building converters, which do not rely on an external grid and can construct the voltage required by the system themselves, can operate independently or under weak grid conditions, and are beginning to be widely used in new energy grid connection. Grid-building converters achieve synchronization with the grid through power synchronization control and use voltage amplitude and frequency control methods to control AC voltage, exhibiting "voltage source" characteristics.

[0004] In existing technologies, reactive power regulation control methods for grid-connected converters achieve zero-voltage / valley-voltage turn-on of the switching transistors during reactive power regulation, effectively reducing losses caused by transistor turn-on. Control methods for energy storage converters in grid-connected / integrated distribution networks improve the transient steady-state frequency characteristics of the distribution network. Stability control methods for grid-connected converters enable stable operation under extremely weak grids with high grid impedance while also providing voltage support, thus significantly improving the operational stability of the grid-connected converters. It is evident that existing technologies primarily focus on improving the grid-connected stability of converters, or only study the grid-connected characteristics of single converters, without addressing the instability mechanisms in new power systems or the joint grid connection of multiple converters. Therefore, given that research on new power systems increasingly emphasizes system integration and coordinated control to achieve efficient grid management and optimization, a new instability criterion is needed to clarify the instability mechanisms of grid-connected and integrated systems. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a disturbance analysis method and system based on the mapping of node voltage and current with converter impedance. It analyzes system stability under disturbances based on the mapping of relaxed node voltage and current with converter dynamic impedance. The method analyzes the full-condition impedance model of a novel power system where grid-connected and grid-connected converters coexist. It analyzes the disturbance state of the system at a certain equilibrium point, obtains the changes in converter output voltage and current based on the relationship between the system's active and reactive power, and determines the mapping relationship between relaxed node voltage and current and the dynamic impedance of grid-connected and grid-connected converters based on whether the system can return to a stable state. This clarifies the instability state of the novel power system where grid-connected and grid-connected converters coexist.

[0006] The present invention adopts the following technical solution.

[0007] This invention proposes a disturbance analysis method based on the mapping of node voltage and current with converter impedance, comprising:

[0008] Obtain the operating parameters of the grid-type converter and the grid-based converter, and establish the control models of the grid-type converter and the grid-based converter; determine the parameters of the control models of the grid-type converter and the grid-based converter with the system stability of the coexisting grid-type converter as the objective;

[0009] By employing the small-signal analysis method, the control models of the grid-type converter and the root-type converter are linearly processed to obtain the AC port admittance models of the grid-type converter and the root-type converter, respectively.

[0010] Using the AC port admittance models of the grid-type converter and the root-type converter, a frequency coupling characteristic admittance model under disturbance is established when the grid-type converter and the root-type converter coexist.

[0011] Based on the frequency coupling characteristic admittance model, the AC port admittance model of the grid-type converter, and the AC port admittance model of the root-grid converter, and based on the linear mapping relationship between the voltage and current of the relaxed node under disturbance, the equivalent output impedance of the converter is determined; the phase angle of the equivalent output impedance and the impedance sum of the converter, and the phase angle of the equivalent output impedance of the converter are obtained, and the system stability analysis under disturbance is performed, where the impedance sum is the sum of the equivalent output impedance of the converter and the line impedance.

[0012] When θ is in [0, θ1+π], the power system is considered stable; when θ is in [θ1+π, π / 2], the power system is considered unstable. Here, θ is the phase angle of the equivalent output impedance of the converter and the sum of the impedances, and θ1 is the phase angle of the equivalent output impedance of the converter.

[0013] Based on the inertia, primary frequency regulation characteristics, and primary voltage regulation characteristics of synchronous generators, active power controller models and reactive power controller models are established in a grid-type converter using virtual synchronous machine control. The control parameters for the grid-type converter and the grid-connected converter are determined with the goal of system stability in the coexistence of both grid-type and grid-connected converters, including:

[0014] A virtual impedance Z is connected in series in the current control loop. virtual (s)=R v +sL v R v For virtual resistance, L v Design R as a virtual inductor v >0.5Ω, L v <0.1mH;

[0015] The phase angle difference between the output phase angle of the phase-locked loop in the grid converter satisfies the following relationship:

[0016]

[0017] δ=θ pll -θ g

[0018] In the formula, K is the second-order linear derivative of the output phase difference δ of the SRF-PLL. p and K i For the parameters of the PI controller in the phase-locked loop, I g L represents the amplitude of the grid-connected current. g ω is the grid-side inductance value. n V is the rated angular frequency of the power grid. g θ is the effective value of the phase voltage of the power grid. g Let θ be the phase angle of the grid-connected current. pll The phase angle is output by the phase-locked loop;

[0019] Adjusting the parameter K of the PI controller in the phase-locked loop p and K i Set the phase-locked loop bandwidth of the grid converter to be less than 1 / 3 of the current loop crossover frequency.

[0020] The AC port admittance model of the grid-type converter satisfies the following relationship:

[0021]

[0022] In the formula, Y GFM For the admittance of the AC port of the grid-type converter, Λ1, Λ2, Λ θ H p (s), P v P iBoth are small-signal transfer functions of network converters, Y Cf The filter matrix is ​​used to eliminate machine-grid coupling, where I is the steady-state value of the phase current.

[0023] Λ1 and Λ2 represent the small-signal voltage when the dynamics of the active power loop are neglected. small current signal The transfer relationship, where Λ1 is related to the voltage loop transfer function H. v (s) and current loop transfer function H i The product of (s) is directly related to Λ2 and the current loop transfer function H. i (s) related;

[0024] P v P i These represent small voltage signals respectively. small current signal To active power small signal The transitive relationship;

[0025] H p (s) characterizes the small signal of active power. To the small signal of the synchronization angle The transitive relationship satisfies the following equation:

[0026]

[0027] In the formula, J is the virtual moment of inertia, and D... p This is the active damping coefficient.

[0028] The AC port admittance model of the grid converter satisfies the following relationship:

[0029]

[0030] In the formula, Y GFL To determine the admittance of the AC port of the grid converter, Λ1′, Λ′2, Λ′ θ T PLL (s), J v J i Both are small-signal transfer functions of a grid converter, Y Cf The filter matrix is ​​used to eliminate machine-grid coupling, where I is the steady-state value of the phase current.

[0031] Λ1′ and Λ′2 represent the small voltage signals when PLL dynamics are ignored. small current signal The transfer relationship, where Λ′2 is only related to the current loop transfer function H. i (s) related;

[0032] J v J iThese represent small voltage signals respectively. small current signal small signal to q-axis voltage The transitive relationship;

[0033] T PLL (s) characterizes the small signal of the q-axis voltage. To the small signal of the synchronization angle The transitive relationship satisfies the following equation:

[0034]

[0035] In the formula, H PLL (s)=(K P +K i / s) / s,K p and K i V1 represents the parameters of the PI controller in the phase-locked loop, and V2 represents the synchronization voltage signal.

[0036] The frequency coupling characteristic admittance model includes:

[0037] Current I at the disturbance frequency p1 Voltage V at the disturbance frequency p1 The admittance matrix Y between 11 Current I at the disturbance frequency p1 Voltage V at the coupling frequency p2 The admittance matrix Y between 12 Current I at the coupling frequency p2 Voltage V at the disturbance frequency p1 The admittance matrix Y between 21 and the current I at the coupling frequency p2 Voltage V at the coupling frequency p2 The admittance matrix Y between 22 .

[0038] Y sys =Y GFM +Y GFL +Y c

[0039] In the formula, Y sys For synergistic admittance, Y GFM Y is the admittance of the AC port of the grid-type converter. GFL To determine the admittance of the AC port of the grid converter, Y c Admittance for frequency coupling characteristics;

[0040] The equivalent output impedance of the converter is expressed as:

[0041]

[0042] In the formula, Z1 is the equivalent output impedance of the converter.

[0043]

[0044] In the formula, θ is the phase angle between the equivalent output impedance Z1 of the converter and the impedance Z, θ1 is the phase angle of the equivalent output impedance of the converter, X is the sum of the equivalent output reactance X1 of the converter and the line reactance X2, and R is the sum of the equivalent output resistance R1 of the converter and the line resistance R2.

[0045] This invention also proposes a disturbance analysis system that maps node voltage and current to converter impedance, comprising:

[0046] The control model establishment module is used to obtain the operating parameters of the grid-type converter and the grid-based converter, and to establish the control models of the grid-type converter and the grid-based converter. The parameters of the control models of the grid-type converter and the grid-based converter are determined with the system stability of the coexisting grid-type converter and the grid-based converter as the objective.

[0047] The admittance model is established by using small-signal analysis to linearly process the control models of the grid-type converter and the ground-type converter, respectively, to obtain the AC port admittance models of the grid-type converter and the ground-type converter. Using the AC port admittance models of the grid-type converter and the ground-type converter, a frequency coupling characteristic admittance model under disturbance is established when the grid-type converter and the ground-type converter coexist.

[0048] The disturbance analysis module is used to determine the equivalent output impedance of the converter based on the frequency coupling characteristic admittance model, the AC port admittance model of the grid-type converter, and the AC port admittance model of the root-grid converter, based on the linear mapping relationship between the voltage and current of the relaxed node under disturbance. It also obtains the phase angle of the equivalent output impedance and the impedance sum of the converter, and the phase angle of the equivalent output impedance of the converter, to perform system stability analysis under disturbance. Here, the impedance sum is the sum of the equivalent output impedance of the converter and the line impedance.

[0049] The present invention is also a terminal, including a processor and a storage medium; the storage medium is used to store instructions; the processor is used to perform operations according to the instructions to execute the steps of the method.

[0050] The present invention is also a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the method.

[0051] The beneficial effects of this invention are as follows, compared with the prior art, at least including: This invention proposes a study on novel power systems where grid-connected and grid-connected converters coexist; the proposed mapping relationship between relaxed node voltage and current and converter impedance provides a new approach to the stability study of systems with both grid-connected and grid-connected converters coexisting and under load disturbances; based on traditional stability criteria, it proposes a stability analysis based on the matching relationship between the converter's equivalent output impedance and the sum of the grid impedances, and the relationship between the phase angle of the converter's equivalent output impedance and the sum of the impedances, and the phase angle of the converter's equivalent output impedance. This criterion is applicable to normal operating scenarios where grid-connected or grid-connected converters exist, and is more suitable for operating scenarios where both grid-connected and grid-connected converters coexist. It can more comprehensively and accurately assess the stability of novel power systems and identify factors that may lead to instability across the entire frequency band. Moreover, the stability criterion based on two phase angles can more deeply reflect the operating state of the system and has good stability, thereby more effectively preventing and responding to potential instability problems. Attached Figure Description

[0052] Figure 1 This is a flowchart of the disturbance analysis method based on the mapping of relaxed node voltage, current and converter dynamic impedance proposed in this invention;

[0053] Figure 2 This is a specific control block diagram of a phase-locked loop (SRF-PLL) used in an embodiment of the present invention;

[0054] Figure 3 This is a block diagram of the small signal transmission of the mesh converter in an embodiment of the present invention;

[0055] Figure 4 This is a block diagram of the small signal transmission of the mesh converter in an embodiment of the present invention. Detailed Implementation

[0056] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. The embodiments described in this application are merely some embodiments of this invention, and not all embodiments. Based on the spirit of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of this invention.

[0057] This invention proposes a stability identification method based on the relaxation node voltage-current-impedance mapping in novel power systems with hybrid inverters. Before determining the mapping relationship, impedance modeling suitable for the novel power system is required. This necessitates establishing models for both types of inverters; however, for hybrid systems, the impedance model construction needs to consider more comprehensive factors. After modeling, an analysis strategy for the novel inverter system is discussed, addressing its stability process, which differs from that of traditional single-inverter grid-connected systems. Then, in the analysis of the novel power system, the method proposes to discover mapping relationships, analyzes and summarizes these relationships, and finally applies them to the stability analysis of the novel power system.

[0058] This invention proposes a disturbance analysis method based on the mapping of relaxed node voltage, current, and converter dynamic impedance, such as... Figure 1 As shown, it includes:

[0059] Step 1: Obtain the operating parameters of the grid-type converter and the follow-me-type converter, and establish the control models of the grid-type converter and the follow-me-type converter; determine the parameters of the control models of the grid-type converter and the follow-me-type converter with the system stability of the coexisting grid-type converter as the objective.

[0060] For new power systems where grid-connected and grid-connected inverters coexist, a control structure for grid-connected and grid-connected converters and impedance measurement in a hybrid power system is designed.

[0061] Specifically, step 1 includes:

[0062] Step 1.1: Based on the inertia, primary frequency regulation characteristics, and primary voltage regulation characteristics of the synchronous generator, establish the active power controller model and reactive power controller model in the grid-type converter using virtual synchronous machine control.

[0063] A grid-connected converter is a device that converts DC power to AC power. Its core technology involves the switching action of power electronic devices to achieve the conversion between DC and AC. It can serve as a voltage source for a local microgrid, acting as a voltage-source converter that generates stable voltage and frequency by simulating the power generation method of a synchronous generator. The grid-connected converter uses VSG control, primarily involving the construction of a specific control model for the VSG. The designed voltage-controlled VSG is a self-synchronizing converter, eliminating the need for a phase-locked loop (PLL). Virtual inertia and damping characteristics are introduced to simulate the dynamic behavior of a traditional synchronous generator, providing voltage and frequency support to the grid and enhancing its stability and resilience.

[0064] The active power controller of a grid-type VSG simulates the inertia and primary frequency regulation characteristics of a synchronous generator, while the reactive power controller simulates the primary voltage regulation characteristics of a synchronous generator. Therefore, the mathematical equations of the active power controller and the reactive power controller of a grid-type VSG satisfy the following relationship:

[0065] T set +(ω n -ω v )D p -T e =Jsω v

[0066]

[0067] In the formula, T set and T e These are mechanical torque and electromagnetic torque, ω v and ω n These are the VSG output angular frequency and the grid rated angular frequency, respectively. p and D q These are the active damping coefficient and reactive damping coefficient, respectively; J is the virtual moment of inertia; s is the integral operator; P... set and Q set These are the active power setpoint and reactive power setpoint, respectively, where K is the reactive inertia coefficient, and P is the reactive power setpoint. e and Q e These represent the instantaneous output active power and reactive power, respectively, where θ is the phase angle of the VSG internal potential, and V... nom V and E are the rated voltage RMS value and the output voltage RMS value, respectively. m This represents the effective value of the internal potential of the VSG;

[0068] Step 1.2: Optimize the virtual impedance of the current loop of the grid-type converter;

[0069] A virtual impedance Z is connected in series in the current control loop. virtual (s)=R v +sL v R v For virtual resistance, L v As a virtual inductor, R is designed through experiments. v >0.5Ω, L v <0.1mH, altering the damping characteristics of the grid-connected inverter at the resonant point;

[0070] A virtual impedance is connected in series in the current control loop to increase the active power loss of the system, suppress energy oscillation at resonance, and ensure stable operation when the grid-connected converter and the grid-connected converter are connected to the grid at the same time.

[0071] Step 1.3: Establish the phase-locked loop model of the grid converter;

[0072] The working principle of a grid-connected converter is based on current-source inverter technology. By connecting to the power grid, it automatically adjusts its power output to adapt to voltage and frequency fluctuations in the grid. This type of converter typically employs phase-locked loop (PLL) technology to ensure that the output current is synchronized with the phase and frequency of the grid voltage. The PLL technology precisely tracks the phase of the grid voltage, ensuring that the converter's output current is in phase with the grid voltage, achieving grid-connected operation with a power factor close to 1. Based on grid dispatch instructions or local load demand, the converter adjusts its active and reactive power output to achieve power control and maintain grid power balance.

[0073] The phase-locked loop (PLL) widely used in grid-connected converter systems is the Synchronous Reference Frame Phase-Locked Loop (SRF-PLL), and its control structure is as follows: Figure 2 As shown, under weak grid conditions, the input of the SRF-PLL is the voltage at the point of common coupling (PCC), v pq The voltage v at the PCC input of the phase-locked loop p The q-axis component obtained by dq decomposition, v pq Phase tracking is achieved through adjustment using a PI controller. The output of the SRF-PLL is the phase angle θ output by the phase-locked loop. pll With frequency f, the phase angle satisfies the following relationship:

[0074] θ pll =∫(ω n +K p v pq +K i ∫v pq )

[0075] In the formula, θ pll Let ω be the phase angle of the voltage at PCC. n K is the rated angular frequency of the power grid. p and K i For the parameters of the PI controller, v pq The voltage v at the PCC input of the phase-locked loop p The q-axis component obtained by dq decomposition;

[0076] Under weak grid conditions, the voltage at PCC is determined by two parts: the grid phase voltage and the grid-connected current flowing through the grid impedance, satisfying the following relationship:

[0077] v p =V g ∠θ g +I g Z s ∠(θ g +θ s )

[0078] In the formula, v p V is the voltage at PCC. g θ g These are the effective values ​​of the grid phase voltage and the phase angle, respectively. g Z represents the amplitude of the grid-connected current. s Let θ be the grid impedance. g Let θ be the phase angle of the grid-connected current. s The phase angle represents the grid impedance.

[0079] The q-axis component of the output is obtained by performing Park transformation on the voltage at PCC, which satisfies the following relationship:

[0080] v pq =V g sin(θ g -θ pll )+I g Z s sinθ s

[0081] Define the output phase angle difference δ of the SRF-PLL as the output phase angle θ of the phase-locked loop. pll Phase angle θ with grid current g The difference satisfies the following relationship:

[0082] δ=θ pll -θ g

[0083] Since the change in the output frequency of the phase-locked loop during the transient process will cause a change in the impedance on the grid side, the system angular frequency satisfies the following relationship:

[0084]

[0085] In the formula, ω is the system angular frequency. The first derivative of the output phase angle difference of the SRF-PLL;

[0086] The second-order linear derivative of the output phase angle difference of the phase-locked loop in the mesh converter satisfies the following relationship:

[0087]

[0088] In the formula, L is the second-order linear derivative of the output phase angle difference of the SRF-PLL. g This refers to the grid-side inductance value.

[0089] Step 1.4: Optimize the phase-locked loop bandwidth of the grid converter;

[0090] When grid-connected and grid-linked inverters are operating stably, the phase-locked loop (PLL) of the grid-linked inverter exhibits negative damping characteristics due to tracking the phase and frequency of the grid voltage. When a grid-connected inverter is connected in parallel, the negative damping characteristics of the PLL deteriorate system stability at high frequencies. To effectively suppress instability caused by the negative impedance characteristics of the PLL at high frequencies, the parameter K of the PI controller in the PLL is adjusted. p and K i Set the phase-locked loop bandwidth to be less than 1 / 3 of the current loop's crossover frequency.

[0091] This invention adapts to the synergistic effect between two types of converters by optimizing the phase-locked loop transfer function and bandwidth, and injecting virtual impedance into the current loop, so as to ensure stable operation when the grid-connected converter and the grid-connected converter are connected to the grid at the same time. The optimized control model lays the foundation for subsequent stability analysis of the system with grid-connected and grid-connected converters coexisting under disturbances.

[0092] Step 2: Using the small-signal analysis method, the control models of the grid-type converter and the root-type converter are linearly processed to obtain the AC port admittance models of the grid-type converter and the root-type converter, respectively.

[0093] Based on the control models of the grid-connected converter and the grid-connected converter after parameter determination, this invention employs small-signal analysis to perform impedance modeling for power systems with both grid-connected and grid-connected converters. Such systems exhibit complex dynamic characteristics due to the high proportion of renewable energy integration, and impedance modeling research is relatively limited. Existing technologies mostly focus on the grid-connected characteristics of a single converter. However, when facing new power systems, simply paralleling the traditional impedance models of a single converter cannot accurately and reasonably simulate the situation. In actual operation of hybrid power systems, strong frequency coupling effects exist between the converters. Injecting a positive-sequence voltage disturbance on the grid side, after passing through the self-admittance stages of both the grid-connected and grid-connected converters, will generate positive-sequence currents. Simultaneously, after passing through their respective coupling admittances, the positive-sequence voltage disturbance will generate two negative-sequence currents of different frequencies. The two positive-sequence currents are superimposed at the PCC (Power Control Center) to obtain a total positive-sequence current disturbance, which, after passing through the positive-sequence grid impedance, will generate a new positive-sequence voltage disturbance. Similarly, the two negative sequence currents are superimposed at the PCC to obtain the total negative sequence current disturbance. After passing through the negative sequence grid impedance, a new negative sequence voltage disturbance will be generated. The new positive and negative sequence voltage disturbances then generate corresponding current disturbances through the self-admittance and coupling admittance links of the grid-type converter and the network-type converter, thus forming a closed loop.

[0094] The small-signal transmission block diagram of the network converter is as follows: Figure 3 As shown, Figure 3 middle, For a frequency of f pThe positive sequence voltage small signal, The small current signal generated during the dynamic operation of the active power loop. Small current signal introduced during the dynamic operation of the active power loop The sum of, For small active power signals, For the small signal of synchronization angle, P v Positive sequence voltage small signal To active power small signal The transfer function yields the following relationship for the AC port admittance model of the grid-type converter:

[0095]

[0096] In the formula, Y GFM For the admittance of the AC port of the grid-type converter, Λ1, Λ2, Λ θ H p (s), P v P i Both are small-signal transfer functions of network converters, Y Cf The filter matrix is ​​used to eliminate machine-grid coupling, where I is the steady-state value of the phase current;

[0097] The small signal transmission block diagram of the mesh converter is as follows: Figure 4 As shown, J i Indicates small signal small signal to q-axis voltage The transfer function, the small signal of the synchronization angle small signal of q-axis voltage T is generated through PLL closed-loop control. PLL (s) represents the small signal of the q-axis voltage. To the small signal of the synchronization angle The transfer function.

[0098] The AC port admittance model of the grid converter satisfies the following relationship:

[0099]

[0100] In the formula, Y GFL To determine the admittance of the AC port of the grid converter, Λ′1, Λ′2, Λ′ θ T PLL (s), J v J i Both are small-signal transfer functions of a grid converter, Y Cf The filter matrix is ​​used to eliminate machine-grid coupling, where I is the steady-state value of the phase current;

[0101] (1)Λ1, Λ2, Λ′1, Λ′2

[0102] For a grid-connected converter (GFM), Λ1 and Λ2 characterize the small-signal voltage when the dynamics of the active loop are neglected. small current signal The transfer relationship, where Λ1 is related to the voltage loop transfer function H. v (s) and current loop transfer function H i The product of (s) is directly related to Λ2 and the current loop transfer function H. i (s) related to; Λ θ Indicates the small signal of the synchronization angle The transmission coefficient to small current signals.

[0103] For a grid-connected converter (GFL), Λ′1 and Λ′2 represent the small-signal voltages when the PLL dynamics are ignored. small current signal The transfer relationship, where Λ′2 is only related to the current loop transfer function H. i (s) related to; Λ′ θ Indicates the small signal of the synchronization angle The transmission coefficient to small current signals.

[0104] (2)P v P i J v J i

[0105] For GFM, P v P i These represent small voltage signals respectively. small current signal To active power small signal The transmission relationship between the two depends only on the steady-state operating point;

[0106] For GFL, J v J i These represent small voltage signals respectively. small current signal small signal to q-axis voltage The transitive relationship.

[0107] H p (s), T PLL (s)

[0108] For GFM, H p (s) characterizes the small signal of active power. To the small signal of the synchronization angle The transitive relationship is expressed as follows:

[0109]

[0110] In the formula, J is the virtual moment of inertia, and D...p This is the active damping coefficient;

[0111] For GFL, T PLL (s) characterizes the small signal of the q-axis voltage. To the small signal of the synchronization angle The transitive relationship is expressed as follows:

[0112]

[0113] In the formula, H PLL (s)=(K P +K i / s) / s,K p and K i V1 represents the parameters of the PI controller in the phase-locked loop, and V2 represents the synchronization voltage signal.

[0114] Step 3: Using the AC port admittance model of the grid-type converter and the AC port admittance model of the root-type converter, establish the frequency coupling characteristic admittance model under disturbance when the grid-type converter and the root-type converter coexist.

[0115] Traditional impedance models for single converters are significantly inadequate in modern power systems. For example, in traditional power grids, the main power source is synchronous generators, whose output characteristics are relatively stable, resulting in simpler impedance models. Furthermore, current research on modern power systems largely focuses on the impact of the number of grid-connected and grid-connected converters on stability. However, in the operation of modern power systems, the improved design of the control structures for grid-connected and grid-connected converters is more critical. Grid-connected converters typically rely on grid voltage for synchronization and control, while grid-connected converters can autonomously construct their own grid voltage. This difference makes existing models unable to accurately describe their characteristics when operating simultaneously. Therefore, for impedance models of both types of converters, the differences introduced by their different control structures and the strong coupling between the two inverters lead to mutual influences in power distribution, voltage regulation, and frequency control when both are connected to the grid simultaneously, due to different control strategies. For example, when grid voltage or frequency fluctuates, grid-connected inverters will follow the changes, while grid-connected inverters attempt to maintain their own set voltage and frequency. This difference can cause instability in their synergistic effect.

[0116] Based on the AC port admittance models of the grid-type converter and the root-type converter, the current I at the disturbance frequency is determined respectively. p1 Voltage V at the disturbance frequency p1 The admittance matrix Y between 11 Current I at the disturbance frequency p1 Voltage V at the coupling frequency p2 The admittance matrix Y between 12 Current I at the coupling frequencyp2 Voltage V at the disturbance frequency p1 The admittance matrix Y between 21 and the current I at the coupling frequency p2 Voltage V at the coupling frequency p2 The admittance matrix Y between 22 ;

[0117] When a positive-sequence disturbance voltage is injected into the grid-connected inverter, V is generated. p2 The reason is I p2 Equivalent impedance Z of voltage disturbance source s The coupling effect. Therefore, if the impedance of the impedance measuring device itself is extremely small compared to the impedance amplitude of the grid-connected inverter, Z can be ignored. s The influence of the resulting coupling frequency voltage component. Similarly, when injecting negative sequence disturbance voltage, the influence of Z can be ignored. s The influence of the resulting coupling frequency components only needs to consider the voltage component, current component, and coupling frequency current component in the system that have the same frequency as the disturbance signal.

[0118] The mapping relationship between voltage and current at the disturbance frequency and voltage and current at the coupling frequency is established as follows:

[0119]

[0120] Therefore, Y 11 Y 12 Y 21 and Y 22 A frequency coupling characteristic admittance model is constructed to characterize the coexistence of a grid-type converter and a parallel-grid converter under disturbances. For ease of description,

[0121] To decompose the frequency coupling impedance characteristics of a grid-connected inverter into four independent subsystems for measurement, the impedance measurement device should possess low impedance characteristics. Addressing the frequency coupling characteristics of grid-connected inverters, this invention proposes a rapid measurement method for frequency coupling impedance characteristics using a series-type impedance measurement device. Specifically, the impedance measurement device employs model predictive control (MPC) to achieve low impedance characteristics over a wide frequency band, enabling more accurate independent measurements of the grid-connected inverter.

[0122] This invention establishes a cooperative impedance model for the coexistence of grid-connected and grid-connected converters by separately performing impedance modeling on grid-connected and grid-connected converters, taking into account the frequency coupling characteristics between the two types of converters. This enables impedance modeling for a new type of power system with grid-connected and grid-connected converters coexisting, as well as impedance measurement devices.

[0123] Based on the analysis results of the impedance model, the cooperative control strategy of the grid converter and the mesh converter is optimized to improve the stability and dynamic performance of the system.

[0124] Through the above steps, an impedance model for the grid-connected converter was established, providing a theoretical basis for evaluating and optimizing its grid-connected performance. This model considers not only the converter's own dynamic characteristics but also the impact of control strategies on impedance characteristics, as well as the interaction between the grid-connected and grid-connected converters. The final power system structure diagram showing the coexistence of a current-controlled VSG grid-connected converter and a phase-locked loop-controlled grid-connected converter is shown in the figure. Figure 4 As shown.

[0125] Step 4: Based on the frequency coupling characteristic admittance model, the AC port admittance model of the grid-type converter, and the AC port admittance model of the ground-grid converter, and based on the linear mapping relationship between the voltage and current of the relaxed node under disturbance, determine the equivalent output impedance of the converter; obtain the phase angle θ of the equivalent output impedance and the impedance sum, and the phase angle θ1 of the equivalent output impedance of the converter for system stability analysis under disturbance, where the impedance sum is the sum of the equivalent output impedance of the converter and the line impedance;

[0126] Wherein, the impedance sum is the sum of the equivalent output impedance of the converter and the line impedance; when θ is in [0, θ1+π], the power system is considered stable, and when θ is in [θ1+π, π / 2], the power system is considered unstable.

[0127] Specifically, step 4 includes:

[0128] Step 4.1: Based on the frequency coupling characteristic admittance model, the AC port admittance model of the grid-type converter, and the AC port admittance model of the grid-type converter, establish a collaborative admittance model;

[0129] Synergistic Admittance Model Y sys It consists of the self-admittance and coupling admittance of two types of converters, as shown below:

[0130] Y sys =Y GFM +Y GFL +Y c

[0131] Step 4.2: Determine the equivalent output impedance of the converter using the cooperative admittance model;

[0132] Impedance measurement is used to verify the reliability of impedance modeling. Considering frequency coupling characteristics makes the impedance measurement results more accurate. Under load disturbance, the voltage and current of the relaxed node satisfy a linear mapping relationship. Therefore, the equivalent output impedance of the converter is expressed as:

[0133]

[0134] In the formula, Z1 is the equivalent output impedance of the converter;

[0135] Step 4.3: Establish the mapping relationship between the phase angle θ of the equivalent output impedance Z1 of the converter and the impedance sum Z, and the active and reactive power output of the converter to the power grid; where the impedance sum Z is the sum of the equivalent output impedance Z1 of the converter and the line impedance Z2.

[0136] At this point, assume the grid impedance is X. g R g If the phase at the grid connection point is 0°, then the converter's output power to the grid is:

[0137]

[0138] In the formula, P is the output active power, Q is the output reactive power, V0 is the converter output voltage amplitude, and V g θ g These are the effective value of the grid phase voltage and the phase angle, respectively, and δ is the output phase angle difference of the SRF-PLL;

[0139] Based on the equivalent grid-connected structure diagram of the converter, the apparent power of the converter can be expressed as:

[0140]

[0141] In the formula, U1 is the output voltage of the converter, Z is the sum of the equivalent output impedance Z1 and the line impedance Z2 of the converter, and θ is the phase angle between the equivalent output impedance Z1 and the sum of impedances Z, satisfying the following relationship:

[0142]

[0143] In the formula, X is the sum of the converter's equivalent output reactance X1 and the line reactance X2, and R is the sum of the converter's equivalent output resistance R1 and the line resistance R2.

[0144] Therefore, the active and reactive power output of the converter to the power grid can be expressed as:

[0145]

[0146] In the formula, U1 is the output voltage of the converter, and U2 is the bus voltage of the converter;

[0147] In actual operation, due to the small power angle, assuming the power angle is 0, the reactive power varies sinusoidally with θ. Since the constant power exists, the sum of the squares of the active and reactive power of the converter is constant, and the reactive power is related to the relaxed voltage and current of the converter at any given time. Therefore, based on the impedance angle stability criterion, the relationship between the relaxed node voltage / current and the converter impedance was studied.

[0148] Step 4.4: When θ is in [0, θ1+π], the power system is determined to be stable; when θ is in [θ1+π, π / 2], the power system is determined to be unstable.

[0149] Existing impedance analysis processes aim to determine the dynamic change of impedance with frequency. This invention proposes a dynamic analysis of the impedance angle and discovers its dynamic mapping relationship with relaxation nodes. This mapping relationship provides better support for the stability analysis of novel power systems. To facilitate impedance angle stability analysis, based on the mapping relationship between relaxation node voltage / current and converter impedance, power system stability analysis is performed using the phase angle θ1 of the converter's equivalent output impedance and the phase angle θ of the sum of the converter's equivalent output impedance and line impedance.

[0150] Assuming the inverter's equivalent impedance takes a fixed value in the third quadrant, and by changing the line impedance to make the combined impedance fall within different intervals, the following system stability analysis process is performed:

[0151] Taking θ in the first quadrant [0, π / 2] as an example, the rest of the analysis process is the same.

[0152] The system runs in When the reactive power balance point is reached, and the phase angle θ1 of the equivalent output impedance Z1 of the converter is in the third quadrant, the two intervals [0, θ1+π] and [θ1+π, π / 2] are obtained by dividing the equivalent output impedance of the converter into two intervals.

[0153] When θ is in the range [0, θ1+π], under load disturbance, with the apparent power constant, the system output active power increases and the system output reactive power decreases, the converter output voltage increases, and the converter output current for constant power loads decreases; based on Determine Y sysIf Z1 increases while Z2 remains constant, the combined reactive power θ increases, causing the system's reactive power balance point to shift towards a larger θ. As the reactive power balance point shifts towards a larger θ, the system's output reactive power increases, the converter output voltage decreases, and the converter output current increases. Based on the relationship between current and Z1, an increase in Z1 while Z2 remains constant leads to a decrease in θ. According to the relationship between reactive power and θ, the system's reactive power balance point will return to its original position. Based on the above analysis, regardless of the initial quadrant of θ and the quadrant of θ1, the system is stable when θ is in the range [0, θ1 + π].

[0154] When θ is in the range [θ1+π, π / 2], under load disturbance, with the apparent power remaining constant, the system output active power increases and the system output reactive power decreases, the converter output voltage increases, and the converter output current for a constant power load decreases; based on Determine Y sys If Z1 decreases while Z2 remains constant, θ will decrease, causing the reactive power balance point of the system to shift towards a smaller θ. As the reactive power balance point shifts towards a smaller θ, the reactive power output of the system decreases, eventually leading to system instability. Based on the above analysis, regardless of the initial quadrant of θ and the quadrant of θ1, the system is unstable when θ is located in [θ1+π, π / 2].

[0155] Most research on stability assessment methods focuses on voltage, frequency, and power angle. This invention innovatively proposes a stability analysis method based on impedance angle. Based on the aforementioned stability analysis relationships, the instability mechanism of a novel power system is analyzed:

[0156] Impedance analysis reveals that within the frequency band of broadband oscillations in hybrid power systems, the equivalent impedance of both converters varies with current. Furthermore, within a certain frequency band, the equivalent impedance of the converter exhibits negative resistivity-capacitance, i.e., equivalent negative impedance. The two core elements of instability in new power systems are the equivalent negative impedance of the converter and its variability with output current. Equivalent negative impedance is the root cause of system instability, while impedance variability becomes the triggering condition. When the equivalent impedance of the converter is negative, and the system's output active power is disturbed, it directly affects the system's reactive power, leading to changes in the output current. This current change causes a change in θ, which in turn affects the system's output reactive power. If the system can restore reactive power to its original equilibrium point, the system remains stable; otherwise, the system will become unstable. Since there is a functional relationship between impedance angle and frequency, impedance angle instability leads to the appearance of a corresponding oscillation frequency or band, i.e., broadband oscillation.

[0157] This invention also proposes a disturbance analysis system based on the mapping of node voltage and current with converter impedance, applicable to novel power systems, including:

[0158] The control model establishment module is used to obtain the operating parameters of the grid-type converter and the grid-based converter, and to establish the control models of the grid-type converter and the grid-based converter. The parameters of the control models of the grid-type converter and the grid-based converter are determined with the system stability of the coexisting grid-type converter and the grid-based converter as the objective.

[0159] The admittance model is established by using small-signal analysis to linearly process the control models of the grid-type converter and the ground-type converter, respectively, to obtain the AC port admittance models of the grid-type converter and the ground-type converter. Using the AC port admittance models of the grid-type converter and the ground-type converter, a frequency coupling characteristic admittance model under disturbance is established when the grid-type converter and the ground-type converter coexist.

[0160] The disturbance analysis module is used to determine the equivalent output impedance of the converter based on the frequency coupling characteristic admittance model, the AC port admittance model of the grid-type converter, and the AC port admittance model of the root-grid converter, based on the linear mapping relationship between the voltage and current of the relaxed node under disturbance. It also obtains the phase angle of the equivalent output impedance and the impedance sum of the converter, and the phase angle of the equivalent output impedance of the converter, to perform system stability analysis under disturbance. Here, the impedance sum is the sum of the equivalent output impedance of the converter and the line impedance.

[0161] This disclosure can be a system, method, and / or computer program product. A computer program product may include a computer-readable storage medium having computer-readable program instructions loaded thereon for causing a processor to implement various aspects of this disclosure.

[0162] Computer-readable storage media can be tangible devices capable of holding and storing instructions for use by an instruction execution device. Computer-readable storage media can be, for example—but not limited to—electrical storage devices, magnetic storage devices, optical storage devices, electromagnetic storage devices, semiconductor storage devices, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of computer-readable storage media include: portable computer disks, hard disks, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), static random access memory (SRAM), portable compact disc read-only memory (CD-ROM), digital multifunction disc (DVD), memory sticks, floppy disks, mechanical encoding devices, such as punch cards or recessed protrusions storing instructions thereon, and any suitable combination of the foregoing. The computer-readable storage media used herein are not to be construed as transient signals themselves, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through waveguides or other transmission media (e.g., light pulses through fiber optic cables), or electrical signals transmitted through wires.

[0163] The computer-readable program instructions described herein can be downloaded from computer-readable storage media to various computing / processing devices, or downloaded via a network, such as the Internet, local area network, wide area network, and / or wireless network, to an external computer or external storage device. The network may include copper transmission cables, fiber optic transmission, wireless transmission, routers, firewalls, switches, gateway computers, and / or edge servers. A network adapter card or network interface in each computing / processing device receives the computer-readable program instructions from the network and forwards them to the computer-readable storage media in the respective computing / processing device.

[0164] Computer program instructions used to perform the operations of this disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-dependent instructions, microcode, firmware instructions, status setting data, or source code or object code written in any combination of one or more programming languages, including object-oriented programming languages ​​such as Smalltalk, C++, etc., and conventional procedural programming languages ​​such as the "C" language or similar programming languages. The computer-readable program instructions may execute entirely on the user's computer, partially on the user's computer, as a standalone software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In cases involving a remote computer, the remote computer may be connected to the user's computer via any type of network—including a local area network (LAN) or a wide area network (WAN)—or may be connected to an external computer (e.g., via the Internet using an Internet service provider). In some embodiments, electronic circuitry, such as programmable logic circuitry, field-programmable gate arrays (FPGAs), or programmable logic arrays (PLAs), is personalized by utilizing the status information of the computer-readable program instructions to implement various aspects of this disclosure.

[0165] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A disturbance analysis method for node voltage-current mapping with converter impedance, characterized in that, include: Obtain the operating parameters of the grid-type converter and the grid-based converter, and establish the control models of the grid-type converter and the grid-based converter; determine the parameters of the control models of the grid-type converter and the grid-based converter with the system stability of the coexisting grid-type converter as the objective; By employing the small-signal analysis method, the control models of the grid-type converter and the root-type converter are linearly processed to obtain the AC port admittance models of the grid-type converter and the root-type converter, respectively. Using the AC port admittance models of the grid-type converter and the root-type converter, a frequency coupling characteristic admittance model under disturbance is established when the grid-type converter and the root-type converter coexist. Based on the frequency coupling characteristic admittance model, the AC port admittance model of the grid-type converter, and the AC port admittance model of the root-grid converter, and based on the linear mapping relationship between the voltage and current of the relaxed node under disturbance, the equivalent output impedance of the converter is determined; the phase angle of the equivalent output impedance and the impedance sum of the converter, and the phase angle of the equivalent output impedance of the converter are obtained, and the system stability analysis under disturbance is performed, where the impedance sum is the sum of the equivalent output impedance of the converter and the line impedance.

2. The disturbance analysis method for node voltage-current mapping with converter impedance according to claim 1, characterized in that, When θ is in [0, θ1+π], the power system is considered stable; when θ is in [θ1+π, π / 2], the power system is considered unstable. Here, θ is the phase angle of the equivalent output impedance of the converter and the sum of the impedances, and θ1 is the phase angle of the equivalent output impedance of the converter.

3. The disturbance analysis method for node voltage-current mapping with converter impedance according to claim 1, characterized in that, Based on the inertia, primary frequency regulation characteristics, and primary voltage regulation characteristics of synchronous generators, active power controller models and reactive power controller models are established in a grid-type converter using virtual synchronous machine control. The control parameters for the grid-type converter and the grid-connected converter are determined with the goal of system stability in the coexistence of both grid-type and grid-connected converters, including: A virtual impedance Z is connected in series in the current control loop. virtual (s)=R v +sL v R v For virtual resistance, L v Design R as a virtual inductor v >0.5Ω, L v <0.1mH; The phase angle difference between the output phase angle of the phase-locked loop in the grid converter satisfies the following relationship: δ=θ pll -θ g In the formula, K is the second-order linear derivative of the output phase difference δ of the SRF-PLL. p and K i For the parameters of the PI controller in the phase-locked loop, I g L represents the amplitude of the grid-connected current. g ω is the grid-side inductance value. n V is the rated angular frequency of the power grid. g θ is the effective value of the phase voltage of the power grid. g Let θ be the phase angle of the grid-connected current. pll The phase angle is output by the phase-locked loop; Adjusting the parameter K of the PI controller in the phase-locked loop p and K i Set the phase-locked loop bandwidth of the grid converter to be less than 1 / 3 of the current loop crossover frequency.

4. The disturbance analysis method for node voltage-current mapping with converter impedance according to claim 1, characterized in that, The AC port admittance model of the grid-type converter satisfies the following relationship: In the formula, Y GFM For the admittance of the AC port of the grid-type converter, Λ1, Λ2, Λ θ H p (s), P v P i Both are small-signal transfer functions of network converters, Y Cf The filter matrix is ​​used to eliminate machine-grid coupling, where I is the steady-state value of the phase current.

5. The disturbance analysis method for node voltage-current mapping with converter impedance according to claim 3, characterized in that, Λ1 and Λ2 represent the small-signal voltage when the dynamics of the active power loop are neglected. small current signal The transfer relationship, where Λ1 is related to the voltage loop transfer function H. v (s) and current loop transfer function H i The product of (s) is directly related to Λ2 and the current loop transfer function H. i (s) related; P v P i These represent small voltage signals respectively. small current signal To active power small signal The transitive relationship; H p (s) characterizes the small signal of active power. To the small signal of the synchronization angle The transitive relationship satisfies the following equation: In the formula, J is the virtual moment of inertia, and D... p This is the active damping coefficient.

6. The disturbance analysis method for node voltage-current mapping with converter impedance according to claim 1, characterized in that, The AC port admittance model of the grid converter satisfies the following relationship: In the formula, Y GFL To determine the admittance of the AC port of the grid converter, Λ′1, Λ′2, Λ′ θ T PLL (s), J v J i Both are small-signal transfer functions of a grid converter, Y Cf The filter matrix is ​​used to eliminate machine-grid coupling, where I is the steady-state value of the phase current.

7. The disturbance analysis method for node voltage-current mapping with converter impedance according to claim 6, characterized in that, Λ′1 and Λ′2 represent the small voltage signals when PLL dynamics are ignored. small current signal The transfer relationship, where Λ′2 is only related to the current loop transfer function H. i (s) related; J v J i These represent small voltage signals respectively. small current signal small signal to q-axis voltage The transitive relationship; T PLL (s) characterizes the small signal of the q-axis voltage. To the small signal of the synchronization angle The transitive relationship satisfies the following equation: In the formula, H PLL (s)=(K P +K i / s) / s,K p and K i V1 represents the parameters of the PI controller in the phase-locked loop, and V2 represents the synchronization voltage signal.

8. The disturbance analysis method for node voltage-current mapping with converter impedance according to claim 1, characterized in that, The frequency coupling characteristic admittance model includes: Current I at the disturbance frequency p1 Voltage V at the disturbance frequency p1 The admittance matrix Y between 11 Current I at the disturbance frequency p1 Voltage V at the coupling frequency p2 The admittance matrix Y between 12 Current I at the coupling frequency p2 Voltage V at the disturbance frequency p1 The admittance matrix Y between 21 and the current I at the coupling frequency p2 Voltage V at the coupling frequency p2 The admittance matrix Y between 22 .

9. The disturbance analysis method for node voltage-current mapping with converter impedance according to claim 1, characterized in that, AND sys And GFM +And GFL +And c In the formula, Y sys For synergistic admittance, Y GFM Y is the admittance of the AC port of the grid-type converter. GFL To determine the admittance of the AC port of the grid converter, Y c Admittance for frequency coupling characteristics; The equivalent output impedance of the converter is expressed as: In the formula, Z1 is the equivalent output impedance of the converter.

10. The disturbance analysis method for node voltage-current mapping with converter impedance according to claim 9, characterized in that, In the formula, θ is the phase angle between the equivalent output impedance Z1 of the converter and the impedance Z, θ1 is the phase angle of the equivalent output impedance of the converter, X is the sum of the equivalent output reactance X1 of the converter and the line reactance X2, and R is the sum of the equivalent output resistance R1 of the converter and the line resistance R2.

11. A disturbance analysis system for mapping node voltage and current with converter impedance, utilizing the disturbance analysis method for mapping node voltage and current with converter impedance as described in any one of claims 1-10, characterized in that, The control model establishment module is used to obtain the operating parameters of the grid-type converter and the grid-based converter, and to establish the control models of the grid-type converter and the grid-based converter. The parameters of the control models of the grid-type converter and the grid-based converter are determined with the system stability of the coexisting grid-type converter and the grid-based converter as the objective. The admittance model is established by using small-signal analysis to linearly process the control models of the grid-type converter and the ground-type converter, respectively, to obtain the AC port admittance models of the grid-type converter and the ground-type converter. Using the AC port admittance models of the grid-type converter and the ground-type converter, a frequency coupling characteristic admittance model under disturbance is established when the grid-type converter and the ground-type converter coexist. The disturbance analysis module is used to determine the equivalent output impedance of the converter based on the frequency coupling characteristic admittance model, the AC port admittance model of the grid-type converter, and the AC port admittance model of the root-grid converter, based on the linear mapping relationship between the voltage and current of the relaxed node under disturbance. It also obtains the phase angle of the equivalent output impedance and the impedance sum of the converter, and the phase angle of the equivalent output impedance of the converter, to perform system stability analysis under disturbance. Here, the impedance sum is the sum of the equivalent output impedance of the converter and the line impedance.

12. A terminal, comprising a processor and a storage medium; characterized in that: The storage medium is used to store instructions; The processor is configured to operate according to the instructions to perform the steps of the method according to any one of claims 1-10.

13. A computer-readable storage medium having a computer program stored thereon, characterized in that, When executed by a processor, the program implements the steps of the method according to any one of claims 1-10.

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