System admittance characteristic-based stability analysis method for optical storage micro-grid in island scene

Abstract

The invention discloses a system admittance characteristic-based stability analysis method for an optical storage micro-grid in an island scene, and belongs to the technical field of distributed new energy grid-connected power generation, and the method comprises the steps: determining a main circuit filter type and a control structure of a three-phase inverter of a power generation unit; establishing a second-order admittance model of the inverter based on a harmonic linearization method; decoupling the second-order admittance model to obtain a linear admittance model under positive and negative sequences; according to the topological structure of the micro-grid, establishing a voltage and current relationship at a system common connection point based on a node voltage method, and obtaining a system admittance matrix; and drawing an amplitude-frequency curve and a phase-frequency curve of a key item in the admittance matrix of the system, determining a key characteristic frequency corresponding to a minimum value point of the amplitude-frequency curve, and determining the stability of the system according to the plus or minus of the slope of the phase-frequency curve at the key characteristic frequency. The method provided by the invention can accurately analyze the stability of the optical storage micro-grid in the island operation scene.

Description

Technical Field

[0001] The present invention relates to the technical field of distributed renewable energy grid-connected power generation, and in particular to a stability analysis method for a photovoltaic storage microgrid in an island scenario based on system admittance characteristics. Background Art

[0002] With the promotion of energy revolution, distributed power generation technology has become more and more mature, especially the penetration rate of photovoltaic power generation technology has gradually increased. However, affected by the intermittent and random nature of photovoltaics, energy storage devices are needed to provide inertia support and balance the power supply and demand in the region, and microgrid technology has emerged. However, due to the lack of support from the large power grid, the system faces stability problems such as frequency, voltage fluctuations and power balance. Therefore, it is necessary to conduct a detailed analysis of the stability of the photovoltaic storage microgrid in the isolated island operation scenario. At present, the impedance stability analysis of the photovoltaic storage microgrid in the isolated island operation scenario mainly includes the Nyquist stability criterion based on impedance ratio and the stability criterion based on frequency domain impedance. However, the existing stability analysis methods of photovoltaic storage microgrids in the isolated island operation scenario are mostly based on the Nyquist stability criterion. With the increase of heterogeneous power generation units in the microgrid, the Nyquist curve becomes more and more complicated, and the accuracy of determining the key characteristic frequency will be reduced; at the same time, the stability criterion based on frequency domain impedance does not consider the frequency coupling effect between AC and DC and the AC side, which is easy to cause misjudgment of stability. Therefore, a stability analysis method that considers multiple frequency coupling effects is urgently needed to accurately evaluate the stability of photovoltaic and energy-storage microgrids in island scenarios. Summary of the invention

[0003] The purpose of this application is to overcome the defects of the prior art and provide a stability analysis method for a photovoltaic energy storage microgrid in an island scenario based on the system admittance characteristics.

[0004] To achieve the above objectives, the present application provides a stability analysis method for a photovoltaic storage microgrid island scenario based on system admittance characteristics, comprising the following steps: Determine the main circuit filter type and control structure of the three-phase inverter of the power generation unit; Establishing a second-order admittance model of the three-phase inverter of the power generation unit based on a harmonic linearization method; Decoupling the second-order admittance model of the three-phase inverter of the power generation unit to obtain a linear admittance model under positive and negative sequences; According to the topological structure of the microgrid, the voltage-current relationship at the common connection point of the system is established based on the node voltage method to obtain the system admittance matrix; The amplitude-frequency curve and phase-frequency curve of the key items in the admittance matrix of the system are plotted to determine the key characteristic frequency corresponding to the minimum point of the amplitude-frequency curve, and the stability of the system is determined according to the positive or negative slope of the phase-frequency curve at the key characteristic frequency.

[0005] Optionally, the power generation unit includes: a photovoltaic power generation unit and an energy storage power generation unit.

[0006] Optionally, the photovoltaic power generation unit includes: a grid-following control structure and an LCL filter; the energy storage power generation unit includes: a grid-building control structure and an LC filter.

[0007] Optionally, a second-order admittance model of the three-phase inverter of the power generation unit is established based on a harmonic linearization method, comprising: The harmonic linearization method is used to consider the frequency coupling characteristics of the AC side and the frequency coupling characteristics between AC and DC to establish the first and second order admittance models. The harmonic linearization method is used to consider the frequency coupling characteristics of the AC side and establish the second-order admittance model.

[0008] Optionally, the second-order admittance model of the three-phase inverter of the power generation unit is decoupled to obtain a linear admittance model under positive and negative sequences, including: According to the physical meaning of the second-order admittance model of the three-phase inverter of the power generation unit, the response current is expressed as the product of the disturbance voltage and the self-admittance plus the product of the coupling voltage and the coupling admittance; According to the topological structure and frequency coupling relationship of the microgrid, the positive and negative sequence equivalent impedances after decoupling of the power generation unit inverter are obtained; the positive and negative sequence equivalent impedances include: a first equivalent impedance, a second equivalent impedance, a third equivalent impedance, and a fourth equivalent impedance.

[0009] Optionally, according to the topological structure of the microgrid, a voltage-current relationship at a common connection point of the system is established based on a node voltage method to obtain a system admittance matrix, including: According to the topological structure of the microgrid, an equivalent model of the microgrid in the island scenario is established; Based on the frequency coupling relationship, different power generation units in the system are aggregated separately to obtain the positive and negative sequence impedances of the inverters of different power generation units; The node voltage equation of the system is established based on the node voltage method, and combined with the conditions of the island scenario, the voltage and current relationship at the common connection point of the system is further sorted out; According to the voltage-current relationship at the common connection point of the system, a system admittance matrix is ​​obtained.

[0010] Optionally, the equivalent model of the microgrid in the island scenario includes: m Photovoltaic power generation units, n energy storage and power generation units, m Photovoltaic power generation units and n The energy storage and power generation units are connected in parallel at both ends of the load.

[0011] Optionally, the positive and negative sequence impedances of the inverters of different power generation units include: positive and negative sequence impedances of a grid-following inverter and positive and negative sequence impedances of a grid-forming inverter.

[0012] Optionally, the system admittance matrix expression is: in, m is the number of photovoltaic power generation units, n is the number of energy storage power generation units, Z c is the equivalent output impedance of the photovoltaic power generation unit, Z cl is the line impedance from each photovoltaic power generation unit to the common connection point, Z vl is the line impedance from each energy storage power generation unit to the common connection point, Z v f For the f The output impedance of an energy storage power generation unit.

[0013] Optionally, plotting the amplitude-frequency curve and the phase-frequency curve of the key items in the system admittance matrix, determining the key characteristic frequency corresponding to the minimum point of the amplitude-frequency curve, and determining the stability of the system according to the positive or negative slope of the phase-frequency curve at the key characteristic frequency, including: Plotting amplitude-frequency curves and phase-frequency curves of key items in the system admittance matrix; Selecting the minimum point of the amplitude-frequency curve as the key characteristic frequency, and observing whether the slope of the phase-frequency curve at the key characteristic frequency point is positive or negative; If the slope is positive and passes through 0° or 180°, the system is determined to be stable; if the slope is negative and passes through 0° or 180°, the system is determined to be unstable.

[0014] The present application provides a stability analysis method for a photovoltaic and energy storage microgrid in an islanded scenario based on the system admittance characteristics. By determining the inverter main circuit filter type and control structure, establishing a second-order admittance model and decoupling it, and establishing a voltage-current relationship based on the topological structure to obtain the system admittance matrix, and finally drawing a key term curve to judge the stability, the application can comprehensively consider various heterogeneous converter structures in the microgrid system, consider the frequency coupling between AC and DC and the frequency coupling effect on the AC side, thereby accurately analyzing the stability of the photovoltaic and energy storage microgrid in an islanded operation scenario, and providing strong support for system optimization and stable operation.

[0015] In order to make the above features and advantages of the invention more obvious and easy to understand, embodiments are given below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the related technologies, the drawings required for use in the embodiments or the related technical descriptions are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0017] Figure 1 This is a flowchart of a stability analysis method for a photovoltaic energy storage microgrid island scenario based on system admittance characteristics provided in one embodiment of the present application.

[0018] Figure 2 This is a main circuit diagram of a photovoltaic power generation unit in a stability analysis method for a photovoltaic microgrid island scenario based on system admittance characteristics provided in one embodiment of the present application.

[0019] Figure 3 This is a control structure diagram of the phase-locked loop SRF-PLL in a stability analysis method for a photovoltaic microgrid islanding scenario based on system admittance characteristics provided in one embodiment of the present application.

[0020] Figure 4 This is a dual closed-loop control structure diagram of a photovoltaic power generation unit and grid-following inverter in a stability analysis method for a photovoltaic energy storage microgrid island scenario based on system admittance characteristics provided in one embodiment of the present application.

[0021] Figure 5 This is a main circuit diagram of the energy storage power generation unit in a stability analysis method for a photovoltaic microgrid island scenario based on system admittance characteristics provided in one embodiment of the present application.

[0022] Figure 6 This is a diagram of the grid-type control structure of the energy storage power generation unit in the stability analysis method for a photovoltaic microgrid island scenario based on the system admittance characteristics provided in one embodiment of the present application.

[0023] Figure 7 Flow chart of step S4 in the stability analysis method for a photovoltaic microgrid island scenario based on system admittance characteristics provided in one embodiment of the present application Figure 8 This is an equivalent model diagram of a microgrid in an island scenario in a stability analysis method for a photovoltaic energy storage microgrid in an island scenario based on system admittance characteristics provided in another embodiment of the present application. DETAILED DESCRIPTION

[0024] In order to make the purpose and technical solution of the embodiment of the present application clearer, the technical solution of the embodiment of the present application will be clearly and completely described in conjunction with the drawings of the embodiment of the present application. Obviously, the described embodiment is a part of the embodiment of the present application, not all of the embodiments. Based on the described embodiment of the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0025] In one embodiment, see Figure 1 The present application provides a stability analysis method for a photovoltaic-storage microgrid in an islanded scenario based on system admittance characteristics. The stability analysis method for a photovoltaic-storage microgrid in an islanded scenario based on system admittance characteristics may include the following steps: step S1 to step S5.

[0026] Step S1: Determine the main circuit filter type and control structure of the three-phase inverter of the power generation unit.

[0027] Step S2: Establish a second-order admittance model of the three-phase inverter of the power generation unit based on the harmonic linearization method.

[0028] Step S3: Decouple the second-order admittance model of the three-phase inverter of the power generation unit to obtain a linear admittance model under positive and negative sequences.

[0029] Step S4: According to the topological structure of the microgrid, the voltage-current relationship at the system common connection point is established based on the node voltage method to obtain the system admittance matrix.

[0030] Step S5: Draw the amplitude-frequency curve and phase-frequency curve of the key items in the system admittance matrix, determine the key characteristic frequency corresponding to the minimum point of the amplitude-frequency curve, and determine the stability of the system according to the positive or negative slope of the phase-frequency curve at the key characteristic frequency.

[0031] In the stability analysis method of the photovoltaic and energy storage microgrid in an island scenario based on the system admittance characteristics of the present application, the type of main circuit filter and control structure of the three-phase inverter of the power generation unit are determined to lay the foundation for subsequent modeling; the second-order admittance model is established and decoupled by harmonic linearization to accurately characterize the inverter characteristics; the system admittance matrix is ​​obtained based on the topological structure and node voltage method, and the overall characteristics of the system are taken into account; by plotting the amplitude-frequency curve and phase-frequency curve of key items, and judging the stability based on the slope of the phase-frequency curve at the key characteristic frequency, the stability of the system in the island scenario can be comprehensively and accurately analyzed, and considering the various characteristics of the inverter and the system topology, accurate guidance is provided for system optimization and operation, effectively improving the reliability and stability of the photovoltaic and energy storage microgrid island operation.

[0032] In step S1, see Figure 1In step S1, the main circuit filter type and control structure of the three-phase inverter of the power generation unit are determined.

[0033] As an example, the control structure and filter type of each power generation unit in the photovoltaic storage microgrid are determined, and whether the power generation unit is a mirror coupling system is determined based on the control structure.

[0034] As an example, the power generation unit may include: a photovoltaic power generation unit and an energy storage power generation unit.

[0035] As an example, the photovoltaic power generation unit may include: a grid-following control structure and an LCL filter, specifically, a phase-locked loop SRF-PLL control structure, a DC voltage outer loop, an AC current inner loop, and a capacitor current feedforward.

[0036] As an example, the energy storage power generation unit may include: a grid-type control structure and an LC filter. Specifically, it may include an active frequency loop, a reactive voltage loop, and a voltage-current dual inner loop.

[0037] As an example, see Figure 2 , Figure 2 is the main circuit diagram of the photovoltaic power generation unit. The photovoltaic current output by the photovoltaic power generation unit is i pv , the DC voltage is u dc , the DC voltage passes through the DC capacitor C dc The DC current is input into a three-phase inverter, which may include six insulated gate bipolar transistor switches IGBT, divided into upper and lower bridge arms. The three-phase inverter converts DC power into AC power and outputs a three-phase current. i a , i b , i c and three-phase voltage u a , u b , u c The AC power output by the three-phase inverter is connected to the common connection point PCC through the LCL filter, and then passes through the grid impedance Z g AC distribution network V g The LCL filter can include an inverter-side inductor L 1 , Grid-side inductance L 2 and filter capacitors C fThe LCL filter can reduce the harmonic components in the current and make the output current smoother. The entire circuit achieves stable control and efficient operation of the photovoltaic power generation unit through control strategies such as the phase-locked loop SRF-PLL, DC voltage outer loop, AC current inner loop and capacitor current feedforward.

[0038] As an example, see Figure 3 , Figure 3 The control structure diagram of the phase-locked loop SRF-PLL is shown in Figure 1. The grid voltage information obtained by the common connection point PCC is input, that is, the three phase quantities of the three-phase grid voltage u ga , u gb , u gc , through two coordinate transformation matrices and The three-phase grid voltage u ga , u gb , u gc From the three-phase stationary coordinate system abc Coordinate system conversion to two-phase rotating coordinate system dq Coordinate system, get voltage u gd and u gq ,in, is the initial reference angle, is the angle deviation; then through the phase-locked loop transfer function Perform phase-locked loop control, the phase-locked loop transfer function Implemented by a proportional-integral PI controller, including the proportional coefficient With the integral coefficient ,in is the angular frequency of the phase-locked loop output, s is the Laplace operator, and the proportional-integral PI controller can adjust the input voltage according to the input voltage. u gd and u gq to adjust the output signal; finally, the initial reference angle The phase-locked loop output angle Subtract to get the angle deviation The angle deviation is fed back to the coordinate transformation matrix to adjust the coordinate transformation, forming a closed-loop control so that the angle output by the phase-locked loop Able to track the phase of the grid voltage. The phase-locked loop control structure achieves accurate tracking of the three-phase grid voltage phase through coordinate transformation, PI control and closed-loop feedback, providing accurate phase information for the grid-following control of the photovoltaic power generation unit system, ensuring the synchronous operation of the system and the grid.

[0039] As an example, see Figure 4 , Figure 4 This is the double closed-loop control structure diagram of the grid-following inverter. When the inverter is controlled by the AC current inner loop and the DC voltage outer loop; the grid current and capacitor current form a double current inner loop, and the double current inner loop and the grid voltage form a full feedforward structure. Specifically, the DC voltage u dc With photovoltaic current i pv Input the maximum power point tracking algorithm (MPPT) to obtain the output DC voltage reference value U dcref , so that the photovoltaic power generation always works at the maximum power point to improve the power generation efficiency; then the DC voltage reference value U dcref The actual DC voltage u dc The deviation signal is input to the controller Output grid current reference value I gdref ; Three-phase grid current i ga , i gb , i gc pass abc / dq Coordinate transformation, converted to dq Current in a rotating coordinate system i gd and i gq ; Then the grid current reference value I gdref , I gqref Respectively and actually dq Current in a rotating coordinate system i gd and i gq The obtained deviation signals are input into the PI controller Capacitor current i cd and i cq After gain K ic The cross-coupling term is then fed back into the current control loop to improve the dynamic performance and stability of the system. For compensation dq The coupling effect between the axes improves the decoupling performance of the system control; after being processed by the current control loop, the output dq Voltage control signal of the axis ud and u q , used to control the inverter and other subsequent links. The dual closed-loop control structure of the grid-following inverter ensures the maximum power output of the photovoltaic cell through MPPT, realizes precise control of the grid-connected current and DC voltage through the DC voltage loop and current loop, and uses the phase-locked loop and coordinate transformation to ensure the synchronous operation of the system and the power grid.

[0040] As an example, see Figure 5 , Figure 5 This is the main circuit diagram of the energy storage power generation unit. The specific topology of the main circuit of the energy storage power generation unit can be referred to Figure 2 The specific topology of the main circuit of the photovoltaic power generation unit will not be repeated here.

[0041] As an example, see Figure 6 , Figure 6 The grid-type control structure diagram for the energy storage power generation unit includes an active frequency loop, a reactive voltage loop, and a voltage and current double inner loop. Specifically, the reference frequency The deviation from the actual frequency is proportional to the coefficient After adjustment, the active power reference value Participate in the calculation together to obtain mechanical power ,in is the expected active power value, is the set rated frequency; then the mechanical power Through the moment of inertia J Related links , and get the frequency deviation , and then with the rated frequency After adding, the integral Get the phase angle , damping coefficient D Feedback is introduced to improve the system dynamic characteristics, suppress frequency oscillation, and realize active frequency loop. u g and current i g Input into the power calculation module to get the actual active power P 0 and reactive power Q 0 ;Reactive power reference value Q dref The actual reactive power Q 0 The deviation of the proportionality coefficient k q After adjustment, the reference value of electromotive force E m Calculate the voltage reference value Edref , realize the reactive voltage loop, and control the output voltage by adjusting the reactive power. Load current i Labc and grid voltage u gabc through abc / dq Coordinate transformation, converted to dq Load current in rotating coordinate system i Ldq and grid voltage u gdq ; Voltage reference value E dref With actual d Shaft voltage u gd The deviation is transferred through the function Regulation, current reference I dref With actual d Shaft current i Ld The deviation is transferred through the function Adjustment, the two work together to obtain d Axis control signal e d ; Similarly, we get q Axis control signal e q .in and The term is used to consider the capacitance and inductance characteristics of the system and realize decoupling control. The energy storage power generation unit grid-type control structure maintains the system frequency stability through the active frequency loop, stabilizes the output voltage through the reactive voltage loop, and realizes precise regulation of the load current and output voltage through the voltage and current dual-loop control to ensure the stable operation of the microgrid.

[0042] In step S2, see Figure 1 In step S2, a second-order admittance model of the three-phase inverter of the power generation unit is established based on a harmonic linearization method.

[0043] Specifically, considering the frequency coupling effect between AC and DC and the frequency coupling effect on the AC side of the three-phase inverter of the power generation unit, the three-phase inverter of the photovoltaic power generation unit and the three-phase inverter of the energy storage power generation unit are established based on the harmonic linearization method. abc Second-order admittance model in coordinate system.

[0044] As an example, since the control loop of the photovoltaic power generation unit includes a phase-locked loop and a DC voltage outer loop, the frequency coupling characteristics of the AC side and the frequency coupling characteristics between AC and DC can be considered at the same time. Specifically, the harmonic linearization modeling method can be used to establish a sequence admittance model of the photovoltaic grid-following inverter that considers the frequency coupling effect between AC and DC and the dynamic characteristics of the maximum power point tracking MPPT on the AC side. The formula is as follows: in, It is a sequence admittance model for photovoltaic grid-following inverters that considers the frequency coupling effect between AC and DC and the dynamic characteristics of the maximum power point tracking (MPPT) on the AC side. The matrix is ​​used to describe the electrical characteristics of the inverter in the complex frequency domain and reflects the relationship between the voltage and current at the inverter port; s is the Laplace operator; It is the AC side self-admittance of the photovoltaic grid-following inverter; It is the mutual admittance of the AC side of the photovoltaic grid-following inverter; It is the mutual admittance of the AC side of the photovoltaic grid-following inverter; It is the self-admittance on the AC side of the photovoltaic grid-following inverter.

[0045] Furthermore, the element The expression is: in, is the positive and negative sequence current disturbance at the AC side grid connection point, is the positive and negative sequence voltage disturbance at the AC side grid connection point, m 1 , m 2 , m 3 , m 4 , n 1 , n 2 , n 3 , n 4 is a parameter, and the negative sign in the formula is related to the direction of the reference current.

[0046] As an example, the parameter m 1 , m 2 , m 3 , m 4 , n 1 , n 2 , n 3 , n 4 The expression is as follows: in, G PI is a PI controller, G u is the voltage controller, T PLL is the phase-locked loop period, D 0 is the initial active power, Q 0 is the initial reactive power, L 1 is the inverter side inductance, L 2 is the grid-side inductance, C is the filter capacitor, K dq is the coupling coefficient, K ic is the capacitor current feedback coefficient, I c1 is the capacitor current, I 1 is the fundamental frequency component of current, x 1 , x 2 , x 3 , x 4 is the real part of the remaining polynomial expression, y 1 , y 2 , y 3 , y 4 is the imaginary part of the remaining polynomial expression.

[0047] Furthermore, the element The expression is: in, is the positive and negative sequence current disturbance at the AC side grid connection point, It is the positive and negative sequence voltage disturbance at the AC side grid connection point.

[0048] Furthermore, the element The expression is: in, is the positive and negative sequence current disturbance at the AC side grid connection point, It is the positive and negative sequence voltage disturbance at the AC side grid connection point.

[0049] Furthermore, the element The expression is: in, is the positive and negative sequence current disturbance at the AC side grid connection point, It is the positive and negative sequence voltage disturbance at the AC side grid connection point.

[0050] As an example, m 1 , m 2 , m 3 , m 4 , n 1 , n 2 , n 3 , n 4 Satisfies the following expression: in, , is the positive and negative sequence current disturbance at the AC side grid connection point, , It is the positive and negative sequence voltage disturbance at the AC side grid connection point.

[0051] As an example, for the energy storage power generation unit, due to the existence of the power calculation link, only the frequency coupling effect on the AC side is considered. Specifically, the equivalent output admittance model of the virtual synchronous generator VSG energy storage power generation unit considering the frequency coupling effect on the AC side can be established: The formula is as follows: in, It is an equivalent output admittance model of VSG energy storage generation unit considering the frequency coupling effect on the AC side. The matrix is ​​used to describe the electrical characteristics of the inverter in the complex frequency domain and reflects the relationship between the voltage and current at the inverter port; s is the Laplace operator; The AC side self-admittance of the grid-type inverter for energy storage; Mutual admittance of AC side of grid-connected inverter for energy storage; Mutual admittance of AC side of grid-connected inverter for energy storage; The AC side self-admittance of the energy storage grid-type inverter.

[0052] Furthermore, the element , , , It can be expressed as follows: in, M v1 for, M v2 for, M i1 for, M i2 for, N v1 for, N v2 for, N i1 for, N i2 As a parameter, the specific expression is as follows: in, G i is the current converter, G u is a voltage converter, G f is the frequency converter, E 0 is the initial electrical energy, W p is the gain factor.

[0053] Specifically, M v1 , M v2 , M i1 , M i2 , N v1 , N v2 , N i1 , N i2 Satisfies the following expression: in, , is the positive and negative sequence current disturbance at the AC side grid connection point, 、 It is the positive and negative sequence voltage disturbance at the AC side grid connection point.

[0054] In step S3, see Figure 1 In step S3, the second-order admittance model of the three-phase inverter of the power generation unit is decoupled to obtain a linear admittance model under positive and negative sequences.

[0055] As an example, according to the physical meaning of the two-dimensional admittance matrix, the response current is expressed as the product of the disturbance voltage and the self-admittance plus the product of the coupling voltage and the coupling admittance, and the coupled part is expressed as a controlled current source. The corresponding positive and negative sequence controlled currents are 、 The expression is: in, s is the Laplace operator, is the angular frequency corresponding to the fundamental frequency, is the positive sequence disturbance voltage applied at the grid connection point, is the negative sequence disturbance voltage at the AC side grid connection point, , is the mutual admittance on the AC side, The effect of the negative sequence voltage coupled with the positive sequence voltage disturbance on the positive sequence response current is shown in Figure 2. This is the effect of the positive-sequence voltage disturbance on the negative-sequence coupling current.

[0056] Furthermore, according to the topological structure and frequency coupling relationship of the microgrid, the positive and negative sequence equivalent impedances after the decoupling of the power generation unit inverter can be obtained. The positive and negative sequence equivalent impedances include: positive sequence forward equivalent impedance Z ppsl , Negative sequence forward equivalent impedance Z nnsl , Positive sequence reverse equivalent impedance Z ppsm , Negative sequence reverse equivalent impedance Z nnsm , the expression is: in, Z ppsl is the positive sequence forward equivalent impedance, which reflects the equivalent impedance characteristics of the inverter during forward transmission under the action of the positive sequence voltage component; Z nnsl It is the negative sequence forward equivalent impedance, which reflects the equivalent impedance characteristics of the inverter during forward transmission under the action of the negative sequence voltage component; Z ppsm is the positive sequence reverse equivalent impedance, which reflects the equivalent impedance characteristics of the inverter during reverse transmission under the action of the positive sequence voltage component; Z nnsm It is the negative sequence reverse equivalent impedance, which reflects the equivalent impedance characteristics of the inverter during reverse transmission under the action of the negative sequence voltage component;s is the Laplace operator; is the angular frequency corresponding to the fundamental frequency; It is the influence of the negative sequence voltage coupled after applying the positive sequence disturbance voltage disturbance on the positive sequence corresponding current; is the negative sequence current after applying the negative sequence voltage disturbance; is the positive sequence current that responds after applying the positive sequence voltage disturbance; is the positive sequence disturbance voltage applied at the grid connection point; is the negative sequence disturbance voltage applied at the grid connection point; is the grid admittance; is the self-admittance on the AC side; is the mutual admittance on the AC side; is the mutual admittance on the AC side; is the self-admittance on the AC side.

[0057] It should be noted that the sequence impedance models of the photovoltaic power generation unit and the energy storage power generation unit are different, so the four admittance elements are different, and only the same representation symbols are used here.

[0058] In step S4, see Figure 1 In step S4, according to the topological structure of the microgrid, a voltage-current relationship at the system common connection point is established based on a node voltage method to obtain a system admittance matrix.

[0059] As an example, see Figure 7 , step S4 may include the following steps: S41~S44.

[0060] Step S41: Establish an equivalent model of the microgrid in an island scenario according to the topological structure of the microgrid.

[0061] Step S42: Based on the frequency coupling relationship, different power generation units in the system are aggregated respectively to obtain the positive and negative sequence impedances of the inverters of the different power generation units.

[0062] Step S43: establishing a node voltage equation of the system based on the node voltage method, and further sorting out the voltage-current relationship at the common connection point of the system in combination with the conditions of the island scenario.

[0063] Step S44: Obtain the system admittance matrix according to the voltage-current relationship at the system common connection point.

[0064] As an example, in step S41, based on the topological structure of the microgrid, an overall equivalent model of the microgrid in an island scenario is established. Figure 8 This is the overall equivalent model diagram of the microgrid in the island scenario, such as Figure 8 As shown, the system may include m Photovoltaic power generation units, nenergy storage and power generation units, m Photovoltaic power generation units and n The energy storage and power generation units are connected in parallel at both ends of the load. I c,f For the f The equivalent current source of a grid-following inverter is: f =1,2,… m ; Z c,f For the f The output impedance of a grid-following inverter is f =1,2,… m ; U v,f For the f The equivalent voltage source of a grid-connected inverter is: f =1,2,… n ; Z v,f For the f The output impedance of a grid-connected inverter is f =1,2,… n ; Z cl,f For the f The line impedance from the grid-connected inverter to the grid connection point is f =1,2,… m ; Z vl,f For the f The line impedance from the grid-connected inverter to the grid connection point is f =1,2,… n ; Z g is the grid impedance; Z load is the load impedance; U PCC is the grid voltage at the public connection point.

[0065] As an example, in step S42, based on the frequency coupling relationship, each power generation unit in the system is aggregated to obtain the positive and negative sequence impedances of the inverters of different power generation units. The positive and negative sequence impedances of the inverters of different power generation units include: the positive and negative sequence impedances of the grid-following inverter Z sm Positive and negative sequence impedance of grid-connected inverter Z sn .

[0066] As an example, the positive and negative sequence impedance of the grid-following inverter Z sm The expression is: in,Z ppsm is the positive sequence reverse equivalent impedance of the grid-following inverter; Z nnsm is the negative sequence reverse equivalent impedance of the grid-following inverter; Z load is the load impedance; Z vl is the line impedance from each energy storage power generation unit to the PCC point; Z v is the equivalent output impedance of the energy storage power generation unit, which can represent both positive sequence and negative sequence; Z cl is the line impedance from each photovoltaic power generation unit to the PCC point; Y den1 is the equivalent admittance seen from the ports of each photovoltaic power generation unit, that is, the equivalent admittance seen from the ports of each grid-connected inverter; s is the Laplace operator; is the angular frequency corresponding to the fundamental frequency; is the self-admittance on the AC side; is the mutual admittance on the AC side; is the mutual admittance on the AC side; is the self-admittance on the AC side; m is the number of photovoltaic power generation units; n is the number of energy storage power generation units.

[0067] As an example, the positive and negative sequence impedance of the grid-connected inverter is Z sn The expression is: in, Z ppsn is the positive sequence reverse equivalent impedance of the grid-connected inverter; Z nnsn is the negative sequence reverse equivalent impedance of the grid-connected inverter; Z load is the load impedance; Z vl is the line impedance from each energy storage power generation unit to the PCC point; Z c is the equivalent output impedance of the photovoltaic power generation unit; Z cl is the line impedance from each photovoltaic power generation unit to the PCC point; Y den2 is the equivalent admittance seen from the ports of each energy storage power generation unit, that is, the equivalent admittance seen from the ports of each grid-type inverter; s is the Laplace operator; is the angular frequency corresponding to the fundamental frequency; is the self-admittance on the AC side; is the mutual admittance on the AC side; is the mutual admittance on the AC side; is the self-admittance on the AC side; m is the number of photovoltaic power generation units; n is the number of energy storage power generation units.

[0068] As an example, in step S43, according to the circuit structure of the overall equivalent model and based on the node voltage method, the node voltage equation of the system is established as follows: in, m is the number of photovoltaic power generation units, n is the number of energy storage power generation units, Z load is the load impedance, Z c is the equivalent output impedance of the photovoltaic power generation unit, Z cl is the line impedance from each photovoltaic power generation unit to the PCC point, Z vl is the line impedance from each energy storage power generation unit to the PCC point, Z v is the equivalent output impedance of the energy storage power generation unit, I c is the ideal current source equivalent to each photovoltaic power generation unit, U v is the ideal voltage source equivalent to each energy storage and power generation unit, U pcc is the grid voltage at the public connection point.

[0069] Furthermore, in an island scenario, the load current provided by the system to the load is I gl Grid voltage to the public connection point U pcc Satisfies the following expression: in, Z load is the load impedance, I gl is the load current, U PCC is the grid voltage at the public connection point.

[0070] Furthermore, the grid-connected voltage equation of the common connection point of the system is sorted out, and the voltage-current relationship at the common connection point of the system is obtained as follows: in, mis the number of photovoltaic power generation units, n is the number of energy storage power generation units, I gl is the load current, I c is the ideal current source equivalent to each photovoltaic power generation unit, U v is the ideal voltage source equivalent to each energy storage and power generation unit, Z c is the equivalent output impedance of the photovoltaic power generation unit, Z cl is the line impedance from each photovoltaic power generation unit to the PCC point, Z vl is the line impedance from each energy storage power generation unit to the PCC point, Z v f For the f The output impedance of a grid-connected inverter.

[0071] As an example, in step S44, the system admittance matrix is ​​obtained according to the voltage and current relationship at the system common connection point: Z s for: in, m is the number of photovoltaic power generation units, n is the number of energy storage power generation units, Z c is the equivalent output impedance of the photovoltaic power generation unit, Z cl is the line impedance from each photovoltaic power generation unit to the PCC point, Z vl is the line impedance from each energy storage power generation unit to the PCC point, Z v f For the f The output impedance of a grid-connected inverter.

[0072] In step S5, refer to Figure 1 In step S5, the amplitude-frequency curve and the phase-frequency curve of the key items in the system admittance matrix are plotted, the key characteristic frequency corresponding to the minimum point of the amplitude-frequency curve is determined, and the stability of the system is determined according to the positive or negative slope of the phase-frequency curve at the key characteristic frequency.

[0073] As an example, observe that the system admittance matrix Z sFrom the structure of the formula, it can be found that under reasonable parameter settings, each grid-connected power generation unit can operate independently and stably, and the load can operate stably under the action of an ideal current source or voltage source. The stability of the power generation unit microgrid system in the island scenario mainly depends on the following two items: in, m is the number of photovoltaic power generation units, n is the number of energy storage power generation units, N 1 is the first main item, N 2 The second main item is Z c is the equivalent output impedance of the photovoltaic power generation unit, Z cl is the line impedance from each photovoltaic power generation unit to the PCC point, Z vl is the line impedance from each energy storage power generation unit to the PCC point, Z v f For the f The output impedance of a grid-connected inverter.

[0074] Furthermore, it can be verified by both theory and simulation N 2 There is always a positive stability margin, so the stability of the microgrid in the island scenario mainly depends on N 1 , according to N 1 The zero point of is used to determine the stability of the system. Specifically, if N 1 There exists a pair of conjugate zeros λ 0, 1 = σ 0 ± jω 0 , a certain oscillation mode of the reaction system; N ( jω )= x + jy Represents the polynomial expression of the remaining terms, then N 1 Expands to: in, σ 0 is the damping of the corresponding oscillation mode, ω 0 is the angular frequency of the corresponding oscillation mode, x is the real part of the remaining polynomial expression, yis the imaginary part of the remaining polynomial expression.

[0075] Furthermore, by setting the imaginary part to zero, we can obtain the horizontal coordinate of the zero-crossing point of the imaginary part-frequency curve: ω n The corresponding values ​​are: in, ω n is the horizontal coordinate of the zero-crossing point of the imaginary part-frequency curve, σ 0 is the damping of the corresponding oscillation mode, ω 0 is the angular frequency of the corresponding oscillation mode, x is the real part of the remaining polynomial expression, y is the imaginary part of the remaining polynomial expression, m is the number of photovoltaic power generation units.

[0076] As an example, in the weakly damped oscillation mode, At this time, the horizontal coordinate of the imaginary part-frequency curve at the zero point ω n Approximately the horizontal coordinate of the oscillation mode, that is Substituting the approximate conclusion back into N 1 The expression of N 1 The expression for the real part is: in, σ 0 is the damping of the corresponding oscillation mode, ω 0 is the angular frequency of the corresponding oscillation mode, x is the real part of the remaining polynomial expression, y is the imaginary part of the remaining polynomial expression.

[0077] Further, N 1 The conjugate zero of λ 0，1 exist ω 0 The slope at k n for: in, m is the number of photovoltaic power generation units, n is the number of energy storage power generation units, ω is the angular frequency, σ 0 is the damping of the corresponding oscillation mode.

[0078] Furthermore, through the above two equations, we can find that: k n When it is less than 0, N 1 The real part of the real part-frequency curve at the zero point has the same sign as the real part of the conjugate zero point; when k n When greater than 0, N 1 The real part of the real part-frequency curve at the zero point has the opposite sign to the real part at the conjugate zero point. N 1 The minimum point of the real part-frequency curve is taken as the key characteristic frequency, and the slope of the imaginary part-frequency curve at the key characteristic frequency point is observed. If the slope is positive and passes through 0° or 180°, the system is considered stable; if the slope is negative and passes through 0° or 180°, the system is considered unstable.

[0079] In the stability analysis method of the photovoltaic storage microgrid in the island scenario based on the system admittance characteristics of the present application, by determining the main circuit filter type and control structure of the three-phase inverter of the power generation unit, it can provide a basic basis for subsequent precise modeling and clarify the basic framework of the system; the second-order admittance model is established and decoupled by the harmonic linearization method, which can accurately characterize the characteristics of the inverter at different frequencies and fully consider the frequency coupling effect between AC and DC and the AC side; based on the microgrid topology and node voltage method to obtain the system admittance matrix, it can comprehensively consider the overall characteristics of the system and reasonably aggregate different power generation units; by drawing the amplitude-frequency curve and phase-frequency curve of the key items of the system admittance matrix, the stability is judged according to the slope of the phase-frequency curve at the key characteristic frequency, which can comprehensively and accurately evaluate the stability of the system in the island scenario, and timely discover potential unstable factors. The physical meaning is clear, and accurate conclusions of stability analysis can be drawn, and the accuracy of extracting key characteristic frequencies is guaranteed, providing scientific and accurate guidance for the optimization and operation of the photovoltaic storage microgrid, and effectively improving the reliability and stability of its island operation.

[0080] It should be understood that, although the steps in the flowchart of the accompanying drawings are displayed in sequence as indicated by the arrows, these steps are not necessarily executed in sequence in the order indicated by the arrows. Unless otherwise specified herein, there is no strict order restriction on the execution of these steps, and these steps can be executed in other orders. Moreover, at least a portion of the steps in the accompanying drawings may include multiple sub-steps or multiple stages, and these sub-steps or stages are not necessarily executed at the same time, but can be executed at different times, and the execution order of these sub-steps or stages is not necessarily sequential, but can be executed in turn or alternately with other steps or at least a portion of the sub-steps or stages of other steps.

[0081] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0082] Although the present application has been disclosed as above with the embodiments, it is not intended to limit the present application. Any person with ordinary knowledge in the technical field can make some changes and modifications without departing from the spirit and scope of the present application. Therefore, the scope of protection of the present application shall be determined by the scope of the attached patent application.

Claims

1. A stability analysis method for a photovoltaic microgrid with energy storage in an island scenario based on system admittance characteristics, characterized in that: The following steps are involved: Determine the main circuit filter type and control structure of the three-phase inverter of the power generation unit; Establishing a second-order admittance model of the three-phase inverter of the power generation unit based on a harmonic linearization method; Decoupling the second-order admittance model of the three-phase inverter of the power generation unit to obtain a linear admittance model under positive and negative sequences; According to the topological structure of the microgrid, the voltage-current relationship at the common connection point of the system is established based on the node voltage method to obtain the system admittance matrix; The amplitude-frequency curve and phase-frequency curve of the key items in the system admittance matrix are plotted to determine the key characteristic frequency corresponding to the minimum point of the amplitude-frequency curve. The stability of the system is determined based on the positive or negative slope of the phase-frequency curve at the key characteristic frequency.

2. The stability analysis method of the photovoltaic energy storage microgrid in an island scenario based on system admittance characteristics according to claim 1 is characterized in that: The power generation unit includes: a photovoltaic power generation unit and an energy storage power generation unit.

3. The stability analysis method of the photovoltaic storage microgrid in an island scenario based on system admittance characteristics according to claim 2 is characterized in that: The photovoltaic power generation unit includes: a grid-following control structure and an LCL filter; the energy storage power generation unit includes: a grid-building control structure and an LC filter.

4. The stability analysis method of the photovoltaic energy storage microgrid in an island scenario based on system admittance characteristics according to claim 1 is characterized in that: A second-order admittance model of the three-phase inverter of the power generation unit is established based on the harmonic linearization method, including: The harmonic linearization method is used to consider the frequency coupling characteristics of the AC side and the frequency coupling characteristics between AC and DC to establish the first and second order admittance models. The harmonic linearization method is used to consider the frequency coupling characteristics of the AC side and establish the second-order admittance model.

5. The stability analysis method of the photovoltaic energy storage microgrid in an island scenario based on system admittance characteristics according to claim 1 is characterized in that: The second-order admittance model of the three-phase inverter of the power generation unit is decoupled to obtain a linear admittance model under positive and negative sequences, including: According to the physical meaning of the second-order admittance model of the three-phase inverter of the power generation unit, the response current is expressed as the product of the disturbance voltage and the self-admittance plus the product of the coupling voltage and the coupling admittance; According to the topological structure and frequency coupling relationship of the microgrid, the positive and negative sequence equivalent impedances after decoupling of the power generation unit inverter are obtained; the positive and negative sequence equivalent impedances include: a first equivalent impedance, a second equivalent impedance, a third equivalent impedance, and a fourth equivalent impedance.

6. The stability analysis method of the photovoltaic energy storage microgrid in an island scenario based on system admittance characteristics according to claim 1 is characterized in that: According to the topological structure of the microgrid, the voltage-current relationship at the common connection point of the system is established based on the node voltage method to obtain the system admittance matrix, including: According to the topological structure of the microgrid, an equivalent model of the microgrid in the island scenario is established; Based on the frequency coupling relationship, different power generation units in the system are aggregated separately to obtain the positive and negative sequence impedances of the inverters of different power generation units; The node voltage equation of the system is established based on the node voltage method, and combined with the conditions of the island scenario, the voltage and current relationship at the common connection point of the system is further sorted out; According to the voltage-current relationship at the common connection point of the system, a system admittance matrix is ​​obtained.

7. The stability analysis method of the photovoltaic energy storage microgrid in an island scenario based on system admittance characteristics according to claim 6 is characterized in that: The equivalent model of the microgrid in the island scenario includes: m Photovoltaic power generation units, n energy storage and power generation units, m Photovoltaic power generation units and n The energy storage and power generation units are connected in parallel at both ends of the load.

8. The stability analysis method of the photovoltaic energy storage microgrid in an island scenario based on system admittance characteristics according to claim 6 is characterized in that: The positive and negative sequence impedances of the inverters of different power generation units include: the positive and negative sequence impedances of the grid-following inverter and the positive and negative sequence impedances of the grid-building inverter.

9. The stability analysis method of the photovoltaic energy storage microgrid in an island scenario based on system admittance characteristics according to claim 6 is characterized in that: The system admittance matrix expression is: in, m is the number of photovoltaic power generation units, n is the number of energy storage power generation units, Z c is the equivalent output impedance of the photovoltaic power generation unit, Z cl is the line impedance from each photovoltaic power generation unit to the common connection point, Z vl is the line impedance from each energy storage power generation unit to the common connection point, Z v f For the f The output impedance of an energy storage power generation unit.

10. The stability analysis method of the photovoltaic energy storage microgrid in an island scenario based on system admittance characteristics according to claim 1 is characterized in that: Plotting the amplitude-frequency curve and phase-frequency curve of the key items in the admittance matrix of the system, determining the key characteristic frequency corresponding to the minimum point of the amplitude-frequency curve, and determining the stability of the system according to the positive or negative slope of the phase-frequency curve at the key characteristic frequency, including: Plotting amplitude-frequency curves and phase-frequency curves of key items in the system admittance matrix; Selecting the minimum point of the amplitude-frequency curve as the key characteristic frequency, and observing whether the slope of the phase-frequency curve at the key characteristic frequency point is positive or negative; If the slope is positive and passes through 0° or 180°, the system is determined to be stable; if the slope is negative and passes through 0° or 180°, the system is determined to be unstable.

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