A VSG current balance control method and system under voltage imbalance state

Abstract

The present invention discloses a method and system for controlling VSG current balance under voltage imbalance conditions. The method relates to the fields of power electronics and power system application technology. The method includes current balance control as the core of overall control, establishes a VSG model, analyzes the VSG output under unbalanced grid conditions, controls the VSG through active-frequency and reactive-voltage droop control, and then analyzes the VSG's instantaneous output power based on instantaneous power theory. When an unbalanced grid fault occurs, the positive and negative sequence currents are decomposed. By controlling the positive and negative sequence currents and incorporating voltage correction, current balance and power stability are achieved. The present invention designs current and power quality control for the VSG, achieving output current balance and maintaining active or reactive power without changing the VSG's characteristics.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics and power system applications, and particularly relates to a VSG current balancing control method and system under a voltage imbalance state. Background Art

[0002] With the global emphasis on environmental protection and sustainable development, the research and application of new energy technologies is rapidly expanding. Wind power and solar photovoltaic technologies, in particular, have become integral components of today's electricity market. However, the large-scale integration of wind and photovoltaic power into the grid has also brought many new challenges, placing severe pressure on the stability and reliability of the power system. The generation characteristics of wind and photovoltaic power differ significantly from those of traditional fossil fuel power generation. Traditional coal-fired, gas-fired, and nuclear power plants have strong regulation capabilities, allowing them to flexibly adjust their power output according to load demand. In contrast, wind and photovoltaic power output is constrained by natural conditions and is highly intermittent and volatile. This unstable generation characteristic complicates grid scheduling and balancing. Furthermore, the large-scale integration of wind and photovoltaic power reduces the inertia of the grid. Traditional synchronous generators have large rotating masses, which can mitigate grid frequency fluctuations through inertial effects when the system experiences disturbances. However, wind and photovoltaic power are mostly connected to the grid through power electronic converters, which do not inherently provide inertial support. This reduces the overall inertia of the grid and increases the risk of system frequency fluctuations. Furthermore, the integration of wind and photovoltaic power into the grid reduces the system's short-circuit capacity and voltage support capabilities. A power system's short-circuit capacity is a key indicator of its ability to resist interference. This reduction weakens the system's ability to respond to faults and reduces voltage stability. This places higher demands on grid protection and control.

[0003] To address the above challenges, Virtual Synchronous Generator (VSG) technology has emerged. VSG technology aims to simulate the inertia and damping characteristics of traditional synchronous generators through power electronic devices, thereby enhancing the stability of the power system after wind power and photovoltaic power are connected to the grid. The basic principle of VSG technology is to use advanced control algorithms to enable power electronic converters to exhibit dynamic characteristics similar to synchronous generators when connected to the grid. Specifically, VSG simulates the rotational inertia and damping characteristics of synchronous generators by adjusting the output power of the converter, thereby providing inertial response and frequency regulation capabilities similar to those of synchronous generators when the grid frequency fluctuates.

[0004] However, the intermittent and volatile nature of renewable energy sources has brought new challenges, among which the increasingly serious phenomenon of grid voltage imbalance has posed a severe test to the stability and reliability of the power system. The introduction of VSG technology can not only improve the stability of the grid, but also achieve current balance control in the case of grid voltage imbalance. Grid voltage imbalance refers to the inconsistency of the amplitude and phase of the three-phase voltage, which is mainly caused by factors such as unbalanced load, asymmetric faults and asymmetric grid structure. This phenomenon causes additional losses, heat and vibration in power system equipment, seriously affecting the overall stability and operating efficiency of the grid. To solve the problem of three-phase current imbalance caused by grid voltage imbalance, it is first necessary to accurately detect and separate the unbalanced current components. By designing a reasonable control circuit, the negative sequence current can be suppressed to zero. At the same time, the VSG output reference voltage is corrected and the suppressed negative sequence voltage is added to the reference voltage to achieve the purpose of current balance. Summary of the Invention

[0005] In view of the above existing problems, the present invention proposes a VSG current balancing control strategy under the state of grid voltage imbalance. This method suppresses negative sequence current by adding a correction voltage to the VSG output reference voltage, and balances active and reactive power at the same time.

[0006] In order to solve the above technical problems, a VSG current balance control method under voltage imbalance state is proposed, including:

[0007] Analyze the system structure and the types of asymmetric grid faults, and establish a mathematical model of the system; construct a VSG mathematical model, and control the VSG through active-frequency and reactive-voltage control; separate the output current into positive and negative sequence, calculate the positive and negative sequence current reference values, and balance the current by controlling the positive and negative sequence currents.

[0008] As a preferred solution of the VSG current balance control method under a voltage imbalance state described in the present invention, the asymmetric fault type includes a single-phase grounding fault.

[0009] The system mathematical model includes a DC power supply, an inverter, an LCL filter, a power grid and a control circuit, wherein the inverter converts DC into AC, the LCL filter can suppress high-order harmonics of the power system, and the control circuit includes VSG control and current balance control.

[0010] As a preferred solution of the VSG current balance control method under voltage imbalance state described in the present invention, wherein: the analysis of system structure and grid asymmetry fault type includes identifying single-phase grounding fault by using an enhanced fault type identification algorithm:

[0011]

[0012] Among them, I f To identify the current result of single-phase ground fault, V n is the nth phase voltage, is the average voltage, I n (t) is the change of the n-th phase current over time, is the average current, Γ is the sampling period, N is the number of phases, p and q are the norm exponential adjustment signal sensitivity, λ is the attenuation coefficient, and t is the time variable.

[0013] Assessing the severity of a single-phase ground fault:

[0014]

[0015] Among them, S is the severity index of single-phase grounding fault, I f To identify the change of single-phase ground fault current over time, is the average value of the identified single-phase ground fault current, is the standard deviation of the identified single-phase ground fault current, U f (t) is the change of single-phase ground fault voltage over time, is the average value of the single-phase ground fault voltage, is the standard deviation of the single-phase ground fault voltage, r and s are norm exponents, which are used to adjust the sensitivity of the voltage and current signals.

[0016] Set the imbalance threshold ΔI, when ΔI≤ΔI th1 When S is detected, it is judged that S is a minor fault, the unbalanced fault trend is recorded, the load distribution parameters of the power grid are adjusted, and the power of the controllable load is reduced.

[0017] When ΔI th1 <ΔI≤ΔI th2 When S is judged as a medium fault, VSG current balance control is performed

[0018] When ΔI>ΔI th2 When S is judged as a serious fault, VSG is updated and current balance control is performed after the voltage is stabilized.

[0019] Among them, ΔI th1 and ΔI th2 The imbalance segmentation threshold is:

[0020]

[0021] Among them, k th1 and k th2 is the base proportionality factor, and k th2 >k th1 I nomis the rated current of the grid, α and β are the adjustment coefficients affected by the load rate, I real is the real-time grid current.

[0022] As a preferred solution of the VSG current balance control method under voltage imbalance state described in the present invention, wherein: the VSG mathematical model includes, VSG control includes active-frequency control, reactive-voltage control, voltage and current dual closed-loop control, the main control of VSG is based on the rotor mechanical equation, which is expressed as:

[0023]

[0024] Among them, T m and T e are mechanical torque and electromagnetic torque respectively, P m and P e are the output mechanical power and electromagnetic power of VSG, D and J are the virtual damping coefficient and virtual inertia of VSG, ω is the actual angular frequency of VSG, ω n is the rated angular frequency, and θ is the VSG power angle.

[0025] By introducing active power-frequency droop control to simulate the effect of the speed regulator, the output of the VSG is controlled and the relationship between active power and frequency is established as follows:

[0026]

[0027] Among them, P ref is the reference power, K ω is the adjustment coefficient of the prime mover, E and U g are the inverter output voltage and grid-side voltage respectively, and X is the impedance between the inverter and the grid.

[0028] When the reactive power of the system fluctuates, the voltage changes. The reactive power-voltage droop control is designed to maintain voltage stability as follows:

[0029] U g -U ref =K q (Q ref -Q)

[0030] Among them, U ref is the reference voltage, K q is the reactive power droop coefficient, Q ref and Q are the reactive power reference and actual values ​​respectively.

[0031] As a preferred solution of the VSG current balance control method under voltage imbalance state described in the present invention, wherein: the controlling of the VSG includes updating the VSG and optimizing the mechanical characteristics of the VSG rotor by improving the mechanical torque and electromagnetic torque models:

[0032] T' m =K m ·(P m +ΔP m )·(1+γ(ω-ω n ) 2 )

[0033] T' e =K e ·(I e +ΔI e )·(1+δ(θ-θ ref ) 2 )

[0034] Among them, K m and K e is the torque gain coefficient, γ and δ are the nonlinear influence coefficients of angular frequency and power angle, θ ref is the reference power angle, ΔP m and ΔI e are the disturbance terms of mechanical power and electromagnetic current respectively, and the virtual speed is introduced:

[0035] ΔP m =a1·P m ·tanh(a2·(v s -v s,ref ))

[0036] ΔI e =a3·I e ·tanh(a4·(θ-θ ref ))

[0037] Among them, v s and v s,ref They are virtual speed and reference virtual speed respectively, a1, a2, a3, a4 are adaptive parameters respectively, which are adjusted online by fuzzy logic controller. e is the electromagnetic current.

[0038] The improved mechanical torque and electromagnetic torque models are input into the integrated torque model, where Λ is the response weight of adjusting the virtual inertia J to the change of virtual speed:

[0039] T=T′ m +T′ e +Λ·(ω-ω n )·J·(v s -vs,ref )

[0040] Update the power angle dynamic model to:

[0041]

[0042] Among them, b1 and b2 are the nonlinear influence coefficients of the torque difference on the power angle change rate.

[0043] Fine-tune active power-frequency droop control through frequency fine-tuning mechanism:

[0044] P m -P ref =K′ ω (ω n -ω)+ΔP adj

[0045] Among them, K' ω is the dynamically adjusted error coefficient:

[0046] K′ ω =K ω +ΔK ω

[0047] ΔP adj =ε(ff nom ) m

[0048] ΔK ω =∈(ff nom ) n

[0049] Where, f is the actual grid frequency, f nom is the rated frequency of the grid, ε and ∈ are the adjustment coefficients of the sensitivity and amplitude of the control fine-tuning, ΔK ω It is a coefficient adjusted according to the frequency deviation and system dynamic characteristics, and m and n are different system dynamic demand adjustment coefficients.

[0050] By integrating the voltage fluctuation prediction factor VFPF = η*(ΔU / U ref ), and integrated into the reference voltage to improve it to U' ref :

[0051] U′ ref =U ref *(1+VFPF)

[0052] Integrated adaptive droop coefficient ADC = ζ*(ΔQ / Q ref ), and integrated into the reactive droop coefficient to improve it to K' q :

[0053] K′ q=K q *(1+ADC)

[0054] Integrating it into reactive power-voltage droop control, the improvement is:

[0055] U′ g -U′ ref =K′ q (Q ref -Q)

[0056] Among them, ΔU and ΔQ are the grid voltage fluctuation deviation and reactive power fluctuation deviation, η is the adjustment coefficient of the quantitative voltage fluctuation prediction factor to the voltage fluctuation sensitivity, and ζ is the adjustment coefficient of the quantitative adaptive droop coefficient to the reactive power change sensitivity.

[0057] As a preferred solution of the VSG current balance control method under voltage imbalance state described in the present invention, wherein: the calculation of the positive and negative sequence current reference values ​​includes: when an unbalanced fault occurs in the power grid, the VSG output voltage and current will have a negative sequence component. According to the system mathematical model, the loop voltage equation is written using the KVL theorem as follows:

[0058]

[0059] Among them, U od and U oq is the dq component of the output voltage, and is the dq positive sequence component of the grid voltage, and is the reference value of the dq positive sequence component of the grid current, R is the equivalent resistance, and L is the equivalent inductance.

[0060] The reference value expression of the dq positive sequence component of the grid current is obtained:

[0061]

[0062] Among them, ω g is the grid angular frequency, and is the simplified dq positive sequence component of the output voltage.

[0063] Set the negative sequence current reference value to 0. When the output power still fluctuates, eliminate the active power fluctuation component to achieve current balance and set the amplitude to 0:

[0064]

[0065] in, and is the dq negative sequence component of the grid voltage and is the reference value of the dq negative sequence component of the grid current.

[0066] The reference value of the negative sequence current constant active power is:

[0067]

[0068] Let the amplitude of reactive power fluctuation component be 0 and keep reactive power constant:

[0069]

[0070] The reference value of negative sequence current constant reactive power is:

[0071]

[0072] A voltage correction is added to the reference voltage value output by the VSG to suppress the negative sequence voltage and achieve current balance.

[0073] As a preferred solution of the VSG current balance control method under voltage imbalance state described in the present invention, the voltage correction includes automatically generating an optimal correction voltage value according to the grid voltage imbalance:

[0074]

[0075] Among them, ΔU corr is the correction voltage, τ is the integral variable, i is the summation variable, I d is the reference value of the d-axis positive sequence component of the grid current, I q is the reference value of the q-axis positive sequence component of the grid current.

[0076] The obtained correction voltage ΔU corr Input to the output reference voltage of VSG:

[0077]

[0078] According to the corrected reference voltage U' od and U' oq , adjust the control strategy of VSG to suppress negative sequence voltage.

[0079] Another object of the present invention is to provide a VSG current balance control system under a voltage imbalance state. The present invention solves a series of problems under a voltage imbalance state of the power grid, including eliminating the imbalance of voltage and current, preventing the decline of system stability and increase of equipment loss, and improving the quality of electric energy, thereby ensuring the safe and stable operation of the power system, extending the service life of equipment, and meeting users' demand for high-quality electric energy.

[0080] As a preferred solution of the VSG current balance control system under a voltage imbalance state described in the present invention, it is characterized by including a fault identification and evaluation module, a system mathematical model establishment module, a VSG control module, and a voltage correction module.

[0081] The fault identification and assessment module identifies single-phase grounding faults and assesses the severity of the single-phase grounding faults through an enhanced fault type identification algorithm, determines the fault level, and records the fault trend.

[0082] The system mathematical model building module analyzes the system structure and the type of asymmetric faults in the power grid, builds a system mathematical model, and provides a theoretical basis.

[0083] The VSG control module establishes the main control equation of the VSG and improves it, separates the positive and negative sequence output current, calculates the positive and negative sequence current reference values, and realizes current balance control.

[0084] The voltage correction module calculates a correction voltage value to suppress the negative sequence voltage, inputs the correction voltage to the output reference voltage of the VSG, and adjusts the control strategy of the VSG.

[0085] A computer device includes a memory and a processor, wherein the memory stores a computer program, and is characterized in that when the processor executes the computer program, the steps of the VSG current balance control method under a voltage imbalance state are implemented.

[0086] A computer-readable storage medium stores a computer program thereon, wherein when the computer program is executed by a processor, the steps of the VSG current balance control method under a voltage imbalance state are implemented.

[0087] The beneficial effects of the present invention are as follows: the present invention optimizes the VSG control performance by real-time identification and evaluation of single-phase grounding faults, effectively eliminates the voltage and current imbalance caused by asymmetric faults in the power grid, makes the output active and reactive power constant, improves the stability of the power system, reduces equipment losses, improves the quality of electric energy, and has the advantages of flexible response to faults, prevention of accident expansion, strong real-time performance, easy implementation and high degree of intelligence, thereby ensuring the safe and stable operation of the power system, extending the service life of equipment, meeting users' demand for high-quality electric energy, ensuring that the converter can operate normally during the fault, achieving fault ride-through, and also providing a new solution for virtual synchronous machines to deal with faults. BRIEF DESCRIPTION OF THE DRAWINGS

[0088] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive work, among which:

[0089] Figure 1 The present invention provides an overall flow chart of a method for controlling VSG current balance under voltage imbalance conditions according to an embodiment of the present invention.

[0090] Figure 2 A grid voltage diagram of a VSG current balancing control method under voltage imbalance state provided by one embodiment of the present invention.

[0091] Figure 3 A grid current diagram of a VSG current balancing control method under voltage imbalance state provided by one embodiment of the present invention.

[0092] Figure 4 A positive and negative zero-sequence diagram of the grid current of a VSG current balancing control method under a voltage imbalance state provided by one embodiment of the present invention.

[0093] Figure 5 An active power and reactive power diagram of a VSG current balance control method under a voltage imbalance state provided by one embodiment of the present invention.

[0094] Figure 6 This is a system overall structure diagram of a VSG current balance control system under a voltage imbalance state provided by one embodiment of the present invention.

[0095] Figure 7 A circuit control diagram of a VSG current balance control system under a voltage imbalance state provided by one embodiment of the present invention.

[0096] Figure 8 A current balance control diagram of a VSG current balance control system under a voltage imbalance state is provided by one embodiment of the present invention. DETAILED DESCRIPTION

[0097] To make the above-mentioned objects, features, and advantages of the present invention more clearly understood, the following detailed description of the specific embodiments of the present invention is given in conjunction with the accompanying drawings. It is obvious that the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary persons in this field without creative work should fall within the scope of protection of the present invention.

[0098] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0099] Secondly, the term "one embodiment" or "embodiment" herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in various places throughout this specification does not necessarily refer to the same embodiment, nor does it individually or selectively refer to an embodiment that is mutually exclusive of other embodiments.

[0100] The present invention is described in detail with reference to schematic diagrams. For ease of illustration, cross-sectional views of device structures may be partially enlarged and not to scale when describing embodiments of the present invention. Furthermore, the schematic diagrams are merely illustrative and should not limit the scope of the present invention. Furthermore, in actual production, the three-dimensional dimensions of length, width, and depth should be included.

[0101] In the description of the present invention, it should be noted that the terms "upper, lower, inner, and outer" and other references to orientations or positional relationships are based on the orientations or positional relationships shown in the accompanying drawings and are intended solely to facilitate and simplify the description of the present invention. They are not intended to indicate or imply that the devices or components referred to must have, be constructed, or operate in a specific orientation, and therefore should not be construed as limitations on the present invention. Furthermore, the terms "first, second, or third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0102] In this disclosure, unless otherwise specified or limited, the terms "mounted," "connected," and "connected" should be interpreted broadly. For example, they may refer to fixed, removable, or integral connections. They may also refer to mechanical, electrical, or direct connections, indirect connections through an intermediary, or internal communication between two components. Those skilled in the art will understand the specific meanings of these terms in this disclosure.

[0103] Example 1, with reference to Figure 1 , which is the first embodiment of the present invention, provides a VSG current balance control method under a voltage imbalance state, comprising:

[0104] S1: Analyze the system structure and the types of asymmetric faults in the power grid, and establish a mathematical model of the system.

[0105] Furthermore, a mathematical model of the system is established including the DC power supply, inverter, LCL filter, power grid and control circuit.

[0106] Among them, the inverter converts DC into AC, the LCL filter can suppress the high-order harmonics of the power system, and the control circuit includes VSG control and current balance control.

[0107] It should be noted that the present invention only studies the asymmetric fault type of single-phase grounding fault.

[0108] Identify single-phase grounding faults using an enhanced fault type identification algorithm:

[0109]

[0110] Among them, I f To identify the current result of single-phase ground fault, V n is the nth phase voltage, is the average voltage, I n (t) is the change of the n-th phase current over time, is the average current, Γ is the sampling period, N is the number of phases, p and q are the norm exponential adjustment signal sensitivity, λ is the attenuation coefficient, and t is the time variable.

[0111] Assessing the severity of a single-phase ground fault:

[0112]

[0113] Among them, S is the severity index of single-phase grounding fault, I f To identify the change of single-phase ground fault current over time, is the average value of the identified single-phase ground fault current, is the standard deviation of the identified single-phase ground fault current, U f (t) is the change of single-phase ground fault voltage over time, is the average value of the single-phase ground fault voltage, is the standard deviation of the single-phase ground fault voltage, r and s are norm exponents, which are used to adjust the sensitivity of the voltage and current signals.

[0114] Set the imbalance threshold ΔI, when ΔI≤ΔI th1 When S is detected, it is judged that S is a minor fault, the unbalanced fault trend is recorded, the load distribution parameters of the power grid are adjusted, and the power of the controllable load is reduced.

[0115] When ΔI th1 <ΔI≤ΔI th2 When S is judged as a medium fault, VSG current balance control is performed

[0116] When ΔI>ΔI th2 When S is judged as a serious fault, VSG is updated and current balance control is performed after the voltage is stabilized.

[0117] Among them, ΔI th1 and ΔI th2 The imbalance segmentation threshold is:

[0118]

[0119] Among them, k th1 and k th2 is the base proportionality factor, and k th2 >k th1 I nom is the rated current of the grid, α and β are the adjustment coefficients affected by the load rate, I real is the real-time grid current.

[0120] S2: Construct a VSG mathematical model and control the VSG through active power-frequency and reactive power-voltage control.

[0121] Furthermore, VSG control includes active-frequency control, reactive-voltage control, and voltage-current dual closed-loop control. The main control of VSG is based on the rotor mechanical equation, which is expressed as:

[0122]

[0123] Among them, T m and T e are mechanical torque and electromagnetic torque respectively, P m and P e are the output mechanical power and electromagnetic power of VSG, D and J are the virtual damping coefficient and virtual inertia of VSG, ω is the actual angular frequency of VSG, ω n is the rated angular frequency, and θ is the VSG power angle.

[0124] By introducing active power-frequency droop control to simulate the effect of the speed regulator, the output of the VSG is controlled and the relationship between active power and frequency is established as follows:

[0125]

[0126] Among them, P ref is the reference power, K ω is the adjustment coefficient of the prime mover, E and U g are the inverter output voltage and grid-side voltage respectively, and X is the impedance between the inverter and the grid.

[0127] When the reactive power of the system fluctuates, the voltage changes. The reactive power-voltage droop control is designed to maintain voltage stability as follows:

[0128] U g -U ref =K q (Q ref -Q)

[0129] Among them, U ref is the reference voltage, K q is the reactive power droop coefficient, Q ref and Q are the reactive power reference and actual values ​​respectively.

[0130] It should be noted that the VSG is updated and the mechanical characteristics of the VSG rotor are optimized by improving the mechanical torque and electromagnetic torque models:

[0131] T' m =K m ·(P m +ΔP m )·(1+γ(ω-ω n ) 2

[0132] T' e =K e ·(I e +ΔI e )·(1+δ(θ-θ ref ) 2 )

[0133] Among them, K m and K e is the torque gain coefficient, γ and δ are the nonlinear influence coefficients of angular frequency and power angle, θ ref is the reference power angle, ΔP m and ΔI e are the disturbance terms of mechanical power and electromagnetic current respectively, and the virtual speed is introduced:

[0134] ΔP m =a1·P m ·tanh(a2·(v s -v s,ref ))

[0135] ΔI e =a3·I e ·tanh(a4·(θ-θ ref ))

[0136] Among them, v s and v s,ref They are virtual speed and reference virtual speed respectively, a1, a2, a3, a4 are adaptive parameters respectively, which are adjusted online by fuzzy logic controller. e is the electromagnetic current.

[0137] The improved mechanical torque and electromagnetic torque models are input into the integrated torque model, where Λ is the response weight of adjusting the virtual inertia J to the change of virtual speed:

[0138] T=T′m +T′ e +Λ·(ω-ω n )·J·(v s -v s,ref )

[0139] Update the power angle dynamic model to:

[0140]

[0141] Among them, b1 and b2 are the nonlinear influence coefficients of the torque difference on the power angle change rate.

[0142] Fine-tune active power-frequency droop control through frequency fine-tuning mechanism:

[0143] P m -P ref =K′ ω (ω n -ω)+ΔP adj

[0144] Among them, K' ω is the dynamically adjusted error coefficient:

[0145] K′ ω =K ω +ΔK ω

[0146] ΔP adj =ε(ff nom ) m

[0147] ΔK ω =∈(ff nom ) n

[0148] Where, f is the actual grid frequency, f nom is the rated frequency of the grid, ε and ∈ are the adjustment coefficients of the sensitivity and amplitude of the control fine-tuning, ΔK ω It is a coefficient adjusted according to the frequency deviation and system dynamic characteristics, and m and n are different system dynamic demand adjustment coefficients.

[0149] By integrating the voltage fluctuation prediction factor VFPF = η*(ΔU / U ref ), and integrated into the reference voltage to improve it to U' ref :

[0150] U′ ref =U ref *(1+VFPF)

[0151] Integrated adaptive droop coefficient ADC = ζ*(ΔQ / Q ref), and integrated into the reactive droop coefficient to improve it to K' q :

[0152] K′ q =K q *(1+ADC)

[0153] Integrating it into reactive power-voltage droop control, the improvement is:

[0154] U′ g -U′ ref =K′ q (Q ref -Q)

[0155] Among them, ΔU and ΔQ are the grid voltage fluctuation deviation and reactive power fluctuation deviation, η is the adjustment coefficient of the quantitative voltage fluctuation prediction factor to the voltage fluctuation sensitivity, and ζ is the adjustment coefficient of the quantitative adaptive droop coefficient to the reactive power change sensitivity.

[0156] S3: Separate the output current into positive and negative sequence, calculate the positive and negative sequence current reference values, and balance the current by controlling the positive and negative sequence currents.

[0157] Furthermore, when an unbalanced fault occurs in the power grid, the VSG output voltage and current will have a negative sequence component. According to the system mathematical model, the loop voltage equation can be written using the KVL theorem as follows:

[0158]

[0159] Among them, U od and U oq is the dq component of the output voltage, and is the dq positive sequence component of the grid voltage, and is the reference value of the dq positive sequence component of the grid current, R is the equivalent resistance, and L is the equivalent inductance.

[0160] The reference value expression of the dq positive sequence component of the grid current is obtained:

[0161]

[0162] Among them, ω g is the grid angular frequency, and is the simplified dq positive sequence component of the output voltage.

[0163] Set the negative sequence current reference value to 0. When the output power still fluctuates, eliminate the active power fluctuation component to achieve current balance and set the amplitude to 0:

[0164]

[0165] in, and is the dq negative sequence component of the grid voltage and is the reference value of the dq negative sequence component of the grid current.

[0166] The reference value of the negative sequence current constant active power is:

[0167]

[0168] Let the amplitude of reactive power fluctuation component be 0 and keep reactive power constant:

[0169]

[0170] The reference value of negative sequence current constant reactive power is:

[0171]

[0172] A voltage correction is added to the reference voltage value output by the VSG to suppress the negative sequence voltage and achieve current balance.

[0173] It should also be noted that the optimal correction voltage value is automatically generated according to the grid voltage imbalance:

[0174]

[0175] Among them, ΔU corr is the correction voltage, τ is the integral variable, i is the summation variable, I d is the reference value of the d-axis positive sequence component of the grid current, I q is the reference value of the q-axis positive sequence component of the grid current.

[0176] The obtained correction voltage ΔU corr Input to the output reference voltage of VSG:

[0177]

[0178] According to the corrected reference voltage U' od and U' oq , adjust the control strategy of VSG to suppress negative sequence voltage.

[0179] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

[0180] Example 2, reference Figure 2-Figure 5 An embodiment of the present invention provides a VSG current balance control method under a voltage imbalance state. In order to verify the beneficial effects of the present invention, scientific demonstration is carried out through experiments.

[0181] like Figure 2 As shown in Figure 1, at 0.5s, an unbalanced fault occurs in the power grid, the voltage of phase A drops by 20%, and the other two phases remain unchanged.

[0182] like Figure 3 As shown in the figure, the current fluctuates at 0.5s and stabilizes after 0.6s, achieving the purpose of current balance.

[0183] like Figure 4 As shown in Figure 1, the negative sequence current fluctuates at 0.5s and is suppressed to 0 at 0.6s.

[0184] like Figure 5 As shown in Figure 1, active power and reactive power fluctuate at 0.5s and recover after 0.6s.

[0185] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

[0186] Embodiment 3, the third embodiment of the present invention, is different from the first two embodiments in that:

[0187] If the functions are implemented in the form of software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, or the part of the technical solution, can be embodied in the form of a software product. The computer software product is stored in a storage medium and includes several instructions for enabling a computer device (which can be a personal computer, server, or network device, etc.) to execute all or part of the steps of the method described in each embodiment of the present invention. The aforementioned storage medium includes various media that can store program codes, such as a USB flash drive, a mobile hard disk, a read-only memory (ROM), a random access memory (RAM), a magnetic disk, or an optical disk.

[0188] The logic and / or steps represented in the flowcharts or otherwise described herein, for example, can be considered as an ordered list of executable instructions for implementing the logical functions, and can be embodied in any computer-readable medium for use by, or in conjunction with, an instruction execution system, apparatus, or device (e.g., a computer-based system, a system including a processor, or other system that can fetch and execute instructions from an instruction execution system, apparatus, or device). For purposes of this specification, a "computer-readable medium" can be any device that can contain, store, communicate, propagate, or transport a program for use by, or in conjunction with, an instruction execution system, apparatus, or device.

[0189] More specific examples (a non-exhaustive list) of computer-readable media include the following: an electrical connection with one or more wires (electronic devices), a portable computer disk cartridge (magnetic devices), a random access memory (RAM), a read-only memory (ROM), an erasable and programmable read-only memory (EPROM or flash memory), a fiber optic device, and a portable compact disc read-only memory (CDROM). In addition, the computer-readable medium may even be paper or other suitable medium on which the program is printed, since the program may be obtained electronically, for example, by optically scanning the paper or other medium, followed by editing, deciphering, or processing in another suitable manner as necessary, and then stored in a computer memory.

[0190] It should be understood that various parts of the present invention can be implemented using hardware, software, firmware, or a combination thereof. In the above-described embodiments, multiple steps or methods can be implemented using software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented using hardware, as in another embodiment, any one of the following technologies known in the art or a combination thereof can be used: a discrete logic circuit having a logic gate circuit for implementing a logic function on a data signal, an application-specific integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc.

[0191] Example 4, reference Figure 6-Figure 8 , which is the fourth embodiment of the present invention, provides a VSG current balance control system under voltage imbalance state, including a fault identification and evaluation module, a system mathematical model establishment module, a VSG control module, and a voltage correction module.

[0192] The fault identification and assessment module uses an enhanced fault type identification algorithm to identify single-phase grounding faults, assess the severity of single-phase grounding faults, determine the fault level, and record fault trends.

[0193] The system mathematical model establishment module analyzes the system structure and the types of asymmetric faults in the power grid, constructs a system mathematical model, and provides a theoretical basis.

[0194] The VSG control module establishes the main control equation of the VSG and improves it to separate the positive and negative sequence output currents, calculate the positive and negative sequence current reference values, and achieve current balance control.

[0195] The voltage correction module calculates the correction voltage value to suppress the negative sequence voltage, inputs the correction voltage to the output reference voltage of the VSG, and adjusts the control strategy of the VSG.

[0196] Figure 7 The main circuit is composed of a DC source, an inverter, an LCL filter, and a power grid. When an asymmetric fault occurs in the power grid, the grid-side current and voltage signals are transmitted to the control circuit, and after processing, PWM signals are generated to control the operation of the inverter.

[0197] Figure 8 The reference voltage signal generated by the VSG is added to the calculated correction voltage, which is then subtracted from the positive sequence voltage of the grid after dq conversion. The controller structure is designed according to the formula, and after the negative sequence current is calculated, the negative sequence current is suppressed to zero, the modulation wave is generated, and finally the PWM signal is obtained.

[0198] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A VSG current balance control method under voltage imbalance state, characterized by: include, Analyze the system structure and the types of asymmetric faults in the power grid and establish a mathematical model of the system; Construct a VSG mathematical model and control the VSG through active power-frequency and reactive power-voltage control; Separate the output current into positive and negative sequence, calculate the positive and negative sequence current reference values, and balance the current by controlling the positive and negative sequence currents; Wherein, the asymmetric fault type includes single-phase grounding fault; The system mathematical model includes a DC power supply, an inverter, an LCL filter, a power grid, and a control circuit. The inverter converts DC into AC, the LCL filter can suppress high-order harmonics in the power system, and the control circuit includes VSG control and current balance control. The analysis of the system structure and the type of asymmetric fault in the power grid includes identifying single-phase grounding faults using an enhanced fault type identification algorithm: in, To identify the current results of a single-phase ground fault, is the nth phase voltage, is the average voltage, is the variation of the n-th phase current with time, is the average current, is the sampling period, is the phase number, p and q are the norm exponents that adjust the sensitivity of the signal, is the attenuation coefficient, t is the time variable; Assessing the severity of a single-phase ground fault: Among them, S is the severity index of single-phase grounding fault, To identify the change of single-phase ground fault current over time, is the average value of the identified single-phase ground fault current, is the standard deviation of the identified single-phase ground fault current, is the change of single-phase ground fault voltage over time, is the average value of the single-phase ground fault voltage, is the standard deviation of the single-phase ground fault voltage, r and s are norm exponents used to adjust the sensitivity of the voltage and current signals; Setting the Imbalance Threshold ,when When S is detected, it is judged that S is a minor fault, the unbalanced fault trend is recorded, the load distribution parameters of the power grid are adjusted, and the power of the controllable load is reduced; when When S is judged as a medium fault, VSG current balance control is performed when When S is judged as a serious fault, VSG is updated and current balance control is performed after the voltage is stabilized; in, and The imbalance segmentation threshold is: in, and is the base scale factor, and ; is the rated current of the grid, and is the adjustment coefficient affected by the load factor, is the real-time grid current; The VSG mathematical model includes active-frequency control, reactive-voltage control, and voltage-current dual closed-loop control. The main control of the VSG is based on the rotor mechanical equation, which is expressed as follows: in, and are mechanical torque and electromagnetic torque respectively, and are the output mechanical power and electromagnetic power of VSG, D and J are the virtual damping coefficient and virtual inertia of VSG, ω is the actual angular frequency of VSG, is the rated angular frequency, θ is the VSG power angle; By introducing active power-frequency droop control to simulate the effect of the speed regulator, the output of the VSG is controlled and the relationship between active power and frequency is established as follows: in, is the reference power, is the adjustment coefficient of the prime mover, E and are the inverter output voltage and grid-side voltage respectively, and X is the impedance between the inverter and the grid; When the reactive power of the system fluctuates, the voltage changes. The reactive power-voltage droop control is designed to maintain voltage stability as follows: in, is the reference voltage, is the reactive power droop coefficient, and Q are the reactive power reference and actual values ​​respectively.

2. The VSG current balance control method under voltage imbalance state according to claim 1, characterized in that: The control of the VSG includes updating the VSG and optimizing the mechanical characteristics of the VSG rotor by improving the mechanical torque and electromagnetic torque models: in, and is the torque gain coefficient, and is the nonlinear influence coefficient of angular frequency and power angle, is the reference power angle, and are the disturbance terms of mechanical power and electromagnetic current respectively, and the virtual speed is introduced: in, and are virtual speed and reference virtual speed respectively, 、 、 、 They are adaptive parameters, which are adjusted online by fuzzy logic controller, is the electromagnetic current; The improved mechanical torque and electromagnetic torque models are input into the integrated torque model, where To adjust the response weight of the virtual inertia J to the change of virtual speed: Update the power angle dynamic model to: in, and is the nonlinear influence coefficient of the torque difference on the power angle change rate; Fine-tune active power-frequency droop control through frequency fine-tuning mechanism: in, is the dynamically adjusted error coefficient: Where, f is the actual grid frequency, is the rated frequency of the grid, and is the adjustment coefficient that controls the sensitivity and amplitude of fine-tuning, It is a coefficient adjusted according to the frequency deviation and system dynamic characteristics, and m and n are different system dynamic demand adjustment coefficients; By integrating the voltage fluctuation prediction factor , and integrated into the reference voltage to improve : Integrated adaptive droop coefficient , and integrated into the reactive droop coefficient to improve it to : Integrating it into reactive power-voltage droop control, the improvement is: in, and is the grid voltage fluctuation deviation and reactive power fluctuation deviation, is the adjustment coefficient of the quantitative voltage fluctuation prediction factor to the voltage fluctuation sensitivity, It is an adjustment factor that quantifies the sensitivity of the adaptive droop coefficient to reactive power changes.

3. The VSG current balance control method under voltage imbalance state according to claim 2, characterized in that: The calculation of the positive and negative sequence current reference values ​​includes that when an unbalanced fault occurs in the power grid, the VSG output voltage and current will have a negative sequence component. According to the system mathematical model, the loop voltage equation is written using the KVL theorem as follows: in, and is the dq component of the output voltage, and is the dq positive sequence component of the grid voltage, and is the reference value of the dq positive sequence component of the grid current, R is the equivalent resistance, and L is the equivalent inductance; The reference value expression of the dq positive sequence component of the grid current is obtained: in, is the grid angular frequency, and is the simplified dq positive sequence component of the output voltage; Set the negative sequence current reference value to 0. When the output power still fluctuates, eliminate the active power fluctuation component to achieve current balance and set the amplitude to 0: in, and is the dq negative sequence component of the grid voltage and is the reference value of the dq negative sequence component of the grid current; The reference value of the negative sequence current constant active power is: Let the amplitude of reactive power fluctuation component be 0 and keep reactive power constant: The reference value of negative sequence current constant reactive power is: A voltage correction is added to the reference voltage value output by the VSG to suppress the negative sequence voltage and achieve current balance.

4. The VSG current balance control method under voltage imbalance state according to claim 3, characterized in that: The voltage correction includes automatically generating an optimal correction voltage value according to the grid voltage imbalance: in, To correct the voltage, is the integration variable, i is the summation variable, is the reference value of the d-axis positive sequence component of the grid current, is the reference value of the q-axis positive sequence component of the grid current; The corrected voltage Input to the output reference voltage of VSG: According to the corrected reference voltage and , adjust the control strategy of VSG to suppress negative sequence voltage.

5. A system using the VSG current balance control method under voltage imbalance state according to any one of claims 1 to 4, characterized in that: It includes fault identification and evaluation module, system mathematical model building module, VSG control module, and voltage correction module; The fault identification and assessment module identifies single-phase grounding faults and assesses the severity of single-phase grounding faults through an enhanced fault type identification algorithm, determines the fault level, and records the fault trend; The system mathematical model building module analyzes the system structure and the type of asymmetric faults in the power grid, builds a system mathematical model, and provides a theoretical basis; The VSG control module establishes and improves the main control equation of the VSG, separates the positive and negative sequence output current, calculates the positive and negative sequence current reference values, and realizes current balance control; The voltage correction module calculates a correction voltage value to suppress the negative sequence voltage, inputs the correction voltage to the output reference voltage of the VSG, and adjusts the control strategy of the VSG.

6. A computer device comprising a memory and a processor, wherein the memory stores a computer program, wherein: When the processor executes the computer program, the steps of the VSG current balance control method under a voltage imbalance state according to any one of claims 1 to 4 are implemented.

7. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of a VSG current balance control method under a voltage imbalance state according to any one of claims 1 to 4 are implemented.

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