VSG current balance control method and system in voltage imbalance state

Abstract

The invention discloses a VSG current balance control method and system in a voltage imbalance state, and relates to the technical field of power electronics and power system application, and the method comprises the steps: taking current balance control as a core of integral control, building a VSG model, analyzing the output condition of the VSG in an imbalance power grid state, and obtaining a VSG current balance control result. VSG is controlled through active-frequency and reactive-voltage droop control, the instantaneous output power of the VSG is analyzed according to the instantaneous power theory, positive and negative sequence currents are decomposed when an unbalanced fault occurs in a power grid, and the targets of current balance and power constancy are achieved by controlling the positive and negative sequence currents and adding voltage correction. According to the invention, the current and electric energy quality control of the VSG is designed, and the output current balance, the active power constancy or the reactive power constancy are realized on the premise of not changing the characteristics of the VSG.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics and power system applications, and in particular relates to a VSG current balance 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 are expanding rapidly. In particular, wind power and solar photovoltaic technologies have become an important part of today's electricity market. However, the large-scale integration of wind power and photovoltaic power into the grid has also brought many new challenges and posed a severe test to the stability and reliability of the power system. The power generation characteristics of wind power and photovoltaic power are significantly different from those of traditional fossil fuel power generation. Traditional coal-fired, gas-fired and nuclear power plants have strong regulation capabilities and can flexibly adjust power generation output according to load demand, while the output of wind power and photovoltaic power is constrained by natural conditions and is highly intermittent and volatile. This unstable power generation characteristic makes the dispatch and balance of the power grid more complicated. Secondly, the large-scale grid connection of wind power and photovoltaic power weakens the inertia of the power grid. Traditional synchronous generators have large rotating masses. When the system is disturbed, these rotating masses can alleviate the fluctuation of the grid frequency through the inertia effect. However, wind power and photovoltaic power are mostly connected to the grid through power electronic converters, which do not provide inertial support themselves, resulting in a reduction in the overall inertia of the grid and an increase in the risk of system frequency fluctuations. In addition, after wind power and photovoltaic power are connected to the grid, the system short-circuit capacity is reduced and the voltage support capacity is weakened. The short-circuit capacity of the power system is an important indicator to measure the system's anti-interference ability. After the short-circuit capacity is reduced, the system's response ability to faults is weakened and the voltage stability decreases. This puts higher requirements on the protection and control of the power grid.

[0003] In order to meet the above challenges, Virtual Synchronous Generator (VSG) technology came into being. 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 allow 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 modulation capabilities similar to those of synchronous generators when the grid frequency fluctuates.

[0004] However, the intermittent and volatile nature of new energy sources has brought new challenges, among which the unbalanced voltage of the power grid is becoming increasingly serious, posing 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 power grid, but also realize current balance control under the condition of unbalanced voltage of the power grid. 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 fault and asymmetric grid structure. This phenomenon causes additional losses, heat and vibration in the power system equipment, seriously affecting the overall stability and operation efficiency of the power grid. In order to solve the problem of three-phase current imbalance caused by unbalanced grid voltage, it is necessary to accurately detect and separate the unbalanced current components first, suppress the negative sequence current to zero by designing a reasonable control circuit, and at the same time correct the VSG output reference voltage, and add the suppressed negative sequence voltage 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. The method suppresses the negative sequence current by adding a correction voltage to the VSG output reference voltage, and balances the 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 sequences, 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, wherein: the asymmetric fault type includes a single-phase grounding fault.

[0009] The system mathematical model established 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 a voltage imbalance state described in the present invention, the analysis of the system structure and the asymmetric fault type of the power grid includes identifying a single-phase grounding fault through an enhanced fault type identification algorithm:

[0011]

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

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

[0014]

[0015] Among them, S is the single-phase grounding fault severity index, 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 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 power 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 double closed-loop control, the main control of VSG is by the rotor mechanical equation, the expression is:

[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 the 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 value and actual value respectively.

[0031] As a preferred solution of the VSG current balance control method under voltage imbalance state described in the present invention, wherein: 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:

[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 are virtual speed and reference virtual speed respectively, a1, a2, a3, a4 are adaptive parameters respectively, which are adjusted online by fuzzy logic controller, I e is the electromagnetic current.

[0038] The improved mechanical torque and electromagnetic torque models are input into the comprehensive 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 the active power-frequency droop control through the 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 power 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 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] Incorporating 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 voltage fluctuation deviation and reactive power fluctuation deviation of the power grid, 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 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:

[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 It 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 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 It 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 obtained as follows:

[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 the 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 It 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 power grid voltage imbalance state, including eliminating voltage and current imbalance, preventing system stability from decreasing and equipment loss from increasing, and improving power quality, thereby ensuring the safe and stable operation of the power system, extending the service life of equipment, and meeting users' demand for high-quality electricity.

[0080] As a preferred solution of a 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 evaluation module identifies a single-phase grounding fault and evaluates the severity of the single-phase grounding fault 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 of the 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 having a computer program stored thereon, characterized in that 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 can identify and evaluate single-phase grounding faults in real time, optimize VSG control performance, effectively eliminate the voltage and current imbalance caused by asymmetric faults in the power grid, make the output active and reactive power constant, improve the stability of the power system, reduce equipment losses, and improve the quality of electric energy. It also has the advantages of flexible response to faults, prevention of accident expansion, strong real-time performance, easy implementation, and high 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 a fault, achieving fault crossing, and 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 use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative work, among which:

[0089] Figure 1 An overall flow chart of a VSG current balance control method under a voltage imbalance state provided by an embodiment of the present invention.

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

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

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

[0094] Figure 6 The 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 an 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 an embodiment of the present invention. DETAILED DESCRIPTION

[0097] In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the drawings of the specification. Obviously, the described embodiments are part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary persons in the art 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, but the present invention may also be implemented in other ways different from those described herein, and 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" as used herein refers to a specific feature, structure or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor is it an embodiment that is mutually exclusive with other embodiments, either individually or selectively.

[0100] The present invention is described in detail with reference to schematic diagrams. When describing the embodiments of the present invention, for the sake of convenience, the cross-sectional diagrams showing the device structure will not be partially enlarged according to the general scale, and the schematic diagrams are only examples, which should not limit the scope of protection of the present invention. In addition, in actual production, the three-dimensional dimensions of length, width and depth should be included.

[0101] At the same time, in the description of the present invention, it should be noted that the directions or positional relationships indicated by the terms "upper, lower, inner and outer" are based on the directions or positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore cannot be understood as limiting the present invention. In addition, the terms "first, second or third" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0102] In the present invention, unless otherwise clearly specified and limited, the terms "install, connect, connect" should be understood in a broad sense, for example: it can be a fixed connection, a detachable connection or an integral connection; it can also be a mechanical connection, an electrical connection or a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0103] Example 1, reference Figure 1 , which is the first embodiment of the present invention, and 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 system mathematical model 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 to ground faults through enhanced fault type identification algorithm:

[0109]

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

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

[0112]

[0113] Among them, S is the single-phase grounding fault severity index, 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 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 power 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-frequency and reactive-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 the 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 value and actual value respectively.

[0130] It should be noted that the VSG is updated to optimize the mechanical characteristics of the VSG rotor 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 are virtual speed and reference virtual speed respectively, a1, a2, a3, a4 are adaptive parameters respectively, which are adjusted online by fuzzy logic controller, I e is the electromagnetic current.

[0137] The improved mechanical torque and electromagnetic torque models are input into the comprehensive 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 the active power-frequency droop control through the 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 power 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 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] Incorporating 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 voltage fluctuation deviation and reactive power fluctuation deviation of the power grid, 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, negative sequence components will appear in the output voltage and current of the VSG. 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 It 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 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 It 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 obtained as follows:

[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 corrected voltage value is automatically generated according to the unbalanced grid voltage:

[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 It 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 the 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, at 0.5s, an unbalanced fault occurs in the power grid, the voltage of phase A of the power grid drops by 20%, and the other two phases remain unchanged.

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

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

[0184] like Figure 5 As shown, the 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 the 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, including several instructions for a computer device (which can be a personal computer, a server, or a network device, etc.) to perform all or part of the steps of the methods described in each embodiment of the present invention. The aforementioned storage medium includes: U disk, mobile hard disk, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), disk or optical disk, and other media that can store program codes.

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

[0189] More specific examples of computer-readable media (a non-exhaustive list) include the following: an electrical connection with one or more wires (electronic device), a portable computer disk case (magnetic device), a random access memory (RAM), a read-only memory (ROM), an erasable and programmable read-only memory (EPROM or flash memory), an optical fiber device, and a portable compact disk read-only memory (CDROM). In addition, the computer-readable medium may even be a 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, if necessary, processing in another suitable manner, and then stored in a computer memory.

[0190] It should be understood that the various parts of the present invention can be implemented by hardware, software, firmware or a combination thereof. In the above-mentioned embodiments, a plurality of steps or methods can be implemented by software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented by hardware, as in another embodiment, it can be implemented by any one of the following technologies known in the art or their combination: a discrete logic circuit having a logic gate circuit for implementing a logic function for a data signal, a dedicated 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 building 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 and assess the severity of single-phase grounding faults, determine the fault level, and record fault trends.

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

[0194] The VSG control module establishes the main control equation of VSG and improves it to separate the positive and negative sequence of the output current, 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 VSG plus the calculated correction voltage is subtracted from the positive sequence voltage of the power grid after dq conversion. The controller structure is designed according to the formula, and the negative sequence current is calculated to suppress the negative sequence current to zero, generate a modulation wave, and finally obtain a PWM signal.

[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 the 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 VSG mathematical model and control VSG through active-frequency and reactive-voltage control; The output current is separated into positive and negative sequence, the positive and negative sequence current reference values ​​are calculated, and the current is balanced by controlling the positive and negative sequence currents.

2. A VSG current balance control method under voltage imbalance state as claimed in claim 1, characterized in that: The asymmetric fault types include single-phase grounding fault; The system mathematical model established 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.

3. A VSG current balance control method under voltage imbalance state as claimed in claim 2, characterized in that: The analysis of the system structure and the asymmetric fault type of the power grid includes identifying a single-phase grounding fault by using an enhanced fault type identification algorithm: Among them, I f To identify the current result of a single-phase ground fault, V n is the nth phase voltage, is the average voltage, I n (t) is the variation of the nth phase current with time, is the average current, Γ is the sampling period, N is the number of phases, p and q are the sensitivity of the norm exponential adjustment signal, λ is the attenuation coefficient, and t is the time variable; Assessing the severity of a single-phase to ground fault: Among them, S is the single-phase grounding fault severity index, 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, σ If 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, σ Uf 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; Set the imbalance threshold ΔI, when ΔI≤ΔI th1 When S is judged as 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 ΔI th1 <ΔI≤ΔI th2 When S is judged as a medium fault, VSG current balance control is performed 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; Among them, ΔI th1 and ΔI th2 The imbalance segmentation threshold is: 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 power grid, α and β are the adjustment coefficients affected by the load rate, I real is the real-time grid current.

4. A VSG current balance control method under voltage imbalance state as claimed in claim 3, characterized in that: The VSG mathematical model includes active-frequency control, reactive-voltage control, and voltage-current double closed-loop control in VSG control. The main control of VSG is based on the rotor mechanical equation, which is expressed as follows: 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, 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: 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; When the reactive power of the system fluctuates, the voltage changes. The reactive power-voltage droop control is designed to maintain the voltage stability as follows: U g -U ref =K q (Q ref -Q) 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 value and actual value respectively.

5. A VSG current balance control method under voltage imbalance state as claimed in claim 4, 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: T' m =K m ·(P m +ΔP m )·(1+γ(ω-ω n ) 2 ) T' e =K e ·(I e +ΔI e )·(1+δ)θ-θ ref ) 2 ) 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: ΔP m =a1·P m ·tanh(a2·(v s -v s,ref )) I e =a3·I e ·tanh(a4·(θ-θ ref )) Among them, v s and v s,ref are virtual speed and reference virtual speed respectively, a1, a2, a3, a4 are adaptive parameters respectively, which are adjusted online by fuzzy logic controller, I e is the electromagnetic current; The improved mechanical torque and electromagnetic torque models are input into the comprehensive torque model, where Λ is the response weight of adjusting the virtual inertia J to the change of virtual speed: T=T' m +T' e +Λ·(ω-ω n )·J·(v s -v s,ref ) Update the power angle dynamic model to: Among them, b1 and b2 are the nonlinear influence coefficients of the torque difference on the power angle change rate; Fine-tune the active power-frequency droop control through the frequency fine-tuning mechanism: P m -P ref =K' ω (oh n -ω)+ΔP adj Among them, K' ω is the dynamically adjusted error coefficient: K' ω =K ω +ΔK ω ΔP adj =ε(ff nom ) m ΔK ω =∈(ff nom ) n Where, f is the actual grid frequency, f nom is the rated frequency of the power grid, ε and ∈ are the adjustment coefficients of the sensitivity and amplitude of the control fine-tuning, ΔK ω It is the 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 VFPF = η*(ΔU / U ref ), and integrated into the reference voltage to improve it to U' ref : IN' ref =U ref *(1+VFPF) Integrated adaptive droop coefficient ADC = ζ*(ΔQ / Q ref ), and integrated into the reactive droop coefficient to improve it to K' q : K' q =K q *(1+ADC) Incorporating it into reactive power-voltage droop control, the improvement is: U' g -U' ref =K' q (Q ref -Q) Among them, ΔU and ΔQ are the voltage fluctuation deviation and reactive power fluctuation deviation of the power grid, 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.

6. A VSG current balance control method under voltage imbalance state as claimed in claim 5, 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: 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; The reference value expression of the dq positive sequence component of the grid current is obtained: Among them, ω g 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 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 obtained as follows: 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 obtained as follows: A voltage correction is added to the reference voltage value output by the VSG to suppress the negative sequence voltage and achieve current balance.

7. A VSG current balance control method under voltage imbalance state as claimed in claim 6, characterized in that: The voltage correction includes automatically generating an optimal correction voltage value according to the unbalanced voltage of the power grid: 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; The obtained correction voltage ΔU corr Input to the output reference voltage of VSG: According to the corrected reference voltage U' od and U' oq , adjust the control strategy of VSG to suppress negative sequence voltage.

8. A system using a VSG current balance control method under voltage imbalance state as claimed in any one of claims 1 to 7, 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 the main control equation of the VSG and makes improvements to separate the positive and negative sequence of the 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.

9. 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 a VSG current balance control method under a voltage imbalance state as described in any one of claims 1 to 7 are implemented.

10. 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 as described in any one of claims 1 to 7 are implemented.

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