Stability analysis method for islanding scenario of optical storage microgrid based on system admittance characteristics

Abstract

The application discloses a stability analysis method of a photovoltaic energy storage micro-grid in an islanding scenario based on system admittance characteristics, and belongs to the technical field of distributed new energy grid-connected power generation. The method comprises the following steps: determining the main circuit filter type and 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 linear admittance models under positive and negative sequences; establishing the voltage-current relationship at a system point of common coupling based on a node voltage method according to the topological structure of the micro-grid, and obtaining a system admittance matrix; drawing the amplitude-frequency curve and the phase-frequency curve of 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 and negative of the slope of the phase-frequency curve at the key characteristic frequency. The method can accurately analyze the stability of the photovoltaic energy storage micro-grid in the islanding operation scenario.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of distributed new energy grid-connected power generation technology, and particularly relates to a stability analysis method for an islanded scenario of a photovoltaic energy storage microgrid based on system admittance characteristics. BACKGROUND

[0002] With the promotion of energy revolution, distributed power generation technology is becoming more mature, especially the penetration rate of photovoltaic power generation technology is gradually increasing. However, due to the intermittent and random nature of photovoltaic power generation, energy storage devices are needed to provide inertia support and balance the power supply and demand in the region. Microgrid technology has emerged as the times require. However, due to the lack of support from the large power grid, the system faces stability problems such as frequency and voltage fluctuations and power balance. Therefore, it is necessary to conduct a detailed analysis of the stability of the photovoltaic energy storage microgrid in the islanded operation scenario. The current impedance stability analysis of the photovoltaic energy storage microgrid in the islanded 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 for the photovoltaic energy storage microgrid in the islanded operation scenario are mainly based on the Nyquist stability criterion. With the increasing number of heterogeneous power generation units in the microgrid, the Nyquist curve becomes increasingly complex, and the accuracy of determining the key characteristic frequency will decrease. 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 frequency coupling effect on the AC side, which may lead to misjudgment of stability. Therefore, there is an urgent need for a stability analysis method that considers multiple frequency coupling effects to accurately evaluate the stability of the photovoltaic energy storage microgrid in the islanded scenario. SUMMARY

[0003] The present application aims to overcome the defects of the prior art and provide a stability analysis method for an islanded scenario of a photovoltaic energy storage microgrid based on system admittance characteristics.

[0004] To achieve the above-mentioned purpose, the present application provides a stability analysis method for an islanded scenario of a photovoltaic energy storage microgrid based on system admittance characteristics, comprising the following steps:

[0005] determining the type of main circuit filter and the control structure of the three-phase inverter of the power generation unit;

[0006] establishing a second-order admittance model of the three-phase inverter of the power generation unit based on the harmonic linearization method;

[0007] 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;

[0008] based on the topology structure of the microgrid, establishing the voltage and current relationship at the system point of common coupling based on the node voltage method, and obtaining the system admittance matrix;

[0009] Draw the amplitude-frequency curve and the phase-frequency curve of the key item 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 and negative of the slope of the phase-frequency curve at the key characteristic frequency.

[0010] Optionally, the power generation unit comprises a photovoltaic power generation unit and an energy storage power generation unit.

[0011] Optionally, the photovoltaic power generation unit comprises a grid-following control structure and an LCL filter, and the energy storage power generation unit comprises a grid-forming control structure and an LC filter.

[0012] Optionally, the second-order admittance model of the three-phase inverter of the power generation unit is established based on a harmonic linearization method, comprising:

[0013] The first second-order admittance model is established by using the harmonic linearization method and considering the frequency coupling characteristics of the alternating current side and the frequency coupling characteristics between alternating current and direct current.

[0014] The second second-order admittance model is established by using the harmonic linearization method and considering the frequency coupling characteristics of the alternating current side.

[0015] 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, comprising:

[0016] 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.

[0017] According to the topological structure and the frequency coupling relationship of the microgrid, the positive and negative sequence equivalent impedances of the decoupled inverter of the power generation unit are obtained; the positive and negative sequence equivalent impedances comprise a first equivalent impedance, a second equivalent impedance, a third equivalent impedance, and a fourth equivalent impedance.

[0018] Optionally, according to the topological structure of the microgrid, the voltage-current relationship at the point of common coupling of the system is established based on the node voltage method to obtain the system admittance matrix, comprising:

[0019] According to the topological structure of the microgrid, an equivalent model of the microgrid in an island scenario is established.

[0020] 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 different power generation units.

[0021] The node voltage equation of the system is established based on the node voltage method, and the voltage-current relationship at the point of common coupling of the system is further arranged according to the conditions of the island scenario.

[0022] According to a voltage-current relationship at a common connection point of the system, a system admittance matrix is obtained.

[0023] Optionally, the equivalent model of the micro-grid in the island scenario comprises: m a photovoltaic power generation unit, n a storage power generation unit, m a photovoltaic power generation unit and n a storage power generation unit are connected in parallel across the load.

[0024] Optionally, the positive and negative sequence impedance of the different power generation unit inverters comprises: the positive and negative sequence impedance of the grid-connected type inverter and the positive and negative sequence impedance of the grid-forming type inverter.

[0025] Optionally, the system admittance matrix expression is:

[0026]

[0027] wherein, m is the number of photovoltaic power generation units, n is the number of storage power generation units, Z c is the equivalent output impedance of the photovoltaic power generation unit, Z cl is the line impedance of each photovoltaic power generation unit to the common connection point, Z vl is the line impedance of each storage power generation unit to the common connection point, Z v f is the output impedance of the first f storage power generation unit.

[0028] Optionally, the amplitude-frequency curve and the phase-frequency curve of the key item in the system admittance matrix are drawn, the key characteristic frequency at the minimum point of the amplitude-frequency curve is determined, and the stability of the system is determined according to the positive or negative of the slope of the phase-frequency curve at the key characteristic frequency, comprising:

[0029] The amplitude-frequency curve and the phase-frequency curve of the key item in the system admittance matrix are drawn;

[0030] The minimum point of the amplitude-frequency curve is selected as the key characteristic frequency, and the positive or negative of the slope of the phase-frequency curve at the key characteristic frequency point is observed;

[0031] If the slope is positive and passes through 0° or 180°, it is determined that the system is stable; if the slope is negative and passes through 0° or 180°, it is determined that the system is unstable.

[0032] The application provides a stability analysis method for an island scene of a photovoltaic storage micro-grid based on system admittance characteristics.

[0033] In order to make the above features and advantages of the application more obvious and easy to understand, the following specific examples are described in detail below, and the accompanying drawings are described as follows. BRIEF DESCRIPTION OF DRAWINGS

[0034] In order to more clearly illustrate the technical solutions in the embodiments of the application or the related art, the following will briefly introduce the drawings needed to be used in the embodiments or the related art description. Obviously, the drawings in the following description are only some embodiments of the application, and for those skilled in the art, other drawings can be obtained from these drawings without creative labor.

[0035] Figure 1 A flow chart of the stability analysis method for an island scene of a photovoltaic storage micro-grid based on system admittance characteristics provided in an embodiment of the application.

[0036] Figure 2 A main circuit diagram of a photovoltaic power generation unit in the stability analysis method for an island scene of a photovoltaic storage micro-grid based on system admittance characteristics provided in an embodiment of the application.

[0037] Figure 3 A phase-locked loop SRF-PLL control structure diagram in the stability analysis method for an island scene of a photovoltaic storage micro-grid based on system admittance characteristics provided in an embodiment of the application.

[0038] Figure 4 A grid-connected type inverter double-loop control structure diagram of a photovoltaic power generation unit in the stability analysis method for an island scene of a photovoltaic storage micro-grid based on system admittance characteristics provided in an embodiment of the application.

[0039] Figure 5 A main circuit diagram of an energy storage power generation unit in the stability analysis method for an island scene of a photovoltaic storage micro-grid based on system admittance characteristics provided in an embodiment of the application.

[0040] Figure 6 A grid-connected type control structure diagram of an energy storage power generation unit in the stability analysis method for an island scene of a photovoltaic storage micro-grid based on system admittance characteristics provided in an embodiment of the application.

[0041] Figure 7 The flow chart of step S4 in the stability analysis method for the photovoltaic energy storage micro-grid island scenario based on the system admittance characteristics provided in an embodiment of the present application.

[0042] Figure 8 The equivalent model diagram of the micro-grid in the island scenario in the stability analysis method for the photovoltaic energy storage micro-grid island scenario based on the system admittance characteristics provided in another embodiment of the present application. DETAILED DESCRIPTION

[0043] In order to make the purpose and technical solutions of the embodiments of the present application clearer, the technical solutions of the embodiments of the present application will be described clearly and completely below with reference to the drawings of the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the described embodiments of the present application, all other embodiments obtained by those of ordinary skill in the art without any creative effort fall within the scope of protection of the present application.

[0044] In one embodiment, referring to Figure 1 The present application provides a stability analysis method for a photovoltaic energy storage micro-grid island scenario based on system admittance characteristics, which can include the following steps: steps S1-S5.

[0045] Step S1: determining the main circuit filter type and control structure of the three-phase inverter of the power generation unit.

[0046] Step S2: establishing a second-order admittance model of the three-phase inverter of the power generation unit based on the harmonic linearization method.

[0047] Step S3: 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.

[0048] Step S4: establishing the voltage and current relationship at the system point of common coupling based on the node voltage method according to the topological structure of the micro-grid, and obtaining the system admittance matrix.

[0049] Step S5: drawing the amplitude-frequency curve and 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 and negative of the slope of the phase-frequency curve at the key characteristic frequency.

[0050] In the stability analysis method of the photovoltaic and energy storage micro-grid island scene based on the system admittance characteristics, the type and control structure of the three-phase inverter main circuit filter of the power generation unit are determined, laying a foundation for subsequent modeling; the second-order admittance model is established and decoupled by using harmonic linearization, accurately depicting the inverter characteristics; the system admittance matrix is obtained based on the topological structure and node voltage method, considering the overall characteristics of the system; by drawing the amplitude-frequency curve and phase-frequency curve of the key items, and judging the stability according to the slope of the phase-frequency curve at the key characteristic frequency, the stability of the system in the island scene can be comprehensively and accurately analyzed, considering the various characteristics of the inverter and the system topological structure, providing accurate guidance for system optimization and operation, and effectively improving the reliability and stability of the photovoltaic and energy storage micro-grid island operation.

[0051] In step S1, please refer to Figure 1 , the type and control structure of the three-phase inverter main circuit filter of the power generation unit are determined.

[0052] As an example, the control structure and filter type of each power generation unit in the photovoltaic and energy storage micro-grid are determined, and whether the power generation unit is a mirror coupled system is determined according to the control structure.

[0053] As an example, the power generation unit can include a photovoltaic power generation unit and an energy storage power generation unit.

[0054] As an example, the photovoltaic power generation unit can include a grid-connected control structure and an LCL type filter. Specifically, it can include a phase-locked loop SRF-PLL control structure, a direct current voltage outer ring, an alternating current current inner ring, and a capacitor current feedforward.

[0055] As an example, the energy storage power generation unit can include a grid-connected control structure and an LC type filter. Specifically, it can include an active frequency ring, a reactive voltage ring, and a voltage and current double inner ring.

[0056] As an example, please refer to Figure 2 , Figure 2 , which 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 direct current voltage is u dc , and the direct current voltage is filtered by a direct current capacitor C dc . The direct current is input into a three-phase inverter, which can include six insulated gate bipolar transistor switches IGBTs, divided into upper and lower bridge arms. The three-phase inverter converts direct current into alternating current and outputs three-phase current i a , i b , i cand 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 the inverter side inductor L 1 , grid-side inductance L 2 and filter capacitors C f The LCL filter reduces harmonics in the current, making the output current smoother. The entire circuit achieves stable control and efficient operation of the photovoltaic power generation unit through a control strategy that includes a phase-locked loop (SRF-PLL), a DC voltage outer loop, an AC current inner loop, and capacitor current feedforward.

[0057] 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 from 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 converted 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 and the integral coefficient ,in The angular frequency of the phase-locked loop output, s is the Laplace operator, the proportional-integral PI controller can adjust the output signal according to the input voltage u gd With u gq Adjust the output signal; finally, the initial reference angle Subtract the phase-locked loop output angle Get the angle deviation , the angle deviation is fed back to the coordinate transformation matrix for adjusting the coordinate transformation, forming a closed-loop control, so that the phase-locked loop output angle Can track the phase of the grid voltage. The phase-locked loop control structure realizes accurate tracking of the phase of the three-phase grid voltage through coordinate transformation, PI control and closed-loop feedback, etc. It provides accurate phase information for the grid-connected control of the photovoltaic power generation unit system and ensures the synchronous operation of the system and the grid.

[0058] As an example, please refer to Figure 4 , Figure 4 The double-closed-loop control structure diagram of the grid-connected inverter, when the inverter is controlled by the AC current inner loop and the DC voltage outer loop; the grid current and the 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 And the photovoltaic current i pv Input the maximum power point tracking algorithm (MPPT) to get the output DC voltage reference value U dcref Make the photovoltaic always work at the maximum power point to improve the power generation efficiency; then compare the DC voltage reference value U dcref With the actual DC voltage u dc The deviation signal is input to the controller , and the output grid current reference value I gdref ; the three-phase grid current i ga , i gb , i gc Through abc / dq Coordinate transformation, convert to dq Current in the rotating coordinate system i gd And i gq ; then compare the grid current reference value I gdref , I gqref With the actual dqCurrent in the rotating coordinate system i gd With i gq The deviation signals obtained are input into PI controllers Capacitance current i cd With i cq After gain K ic feedback into the current control loop, used to improve the dynamic performance and stability of the system; cross-coupling terms used to compensate dq the coupling effect between the axes, improve the decoupling performance of the system control; after processing by the current control loop, the voltage control signals of the axes are output dq u d With u q , used to control the subsequent links such as inverters. The double closed-loop control structure of the grid-connected inverter ensures the maximum power output of the photovoltaic cell through MPPT, realizes accurate control of the grid-connected current and the direct-current voltage through the direct-current voltage loop and the current loop, and ensures the synchronous operation of the system and the power grid through the phase-locked loop and the coordinate transformation.

[0059] As an example, refer to Figure 5 , Figure 5 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 refer to the specific topology of the main circuit of the photovoltaic power generation unit in Figure 2 , which will not be repeated here.

[0060] As an example, refer to Figure 6 , Figure 6 is the grid-connected type control structure diagram of the energy storage power generation unit, including the active frequency loop, the reactive voltage loop, and the voltage and current double inner loop. Specifically, the reference frequency and the actual frequency deviation are adjusted by a proportional coefficient , and are calculated together with the active power reference value to obtain the mechanical power , wherein is the expected active power value, is the set rated frequency; then the mechanical power is processed by the rotational inertia J related link to obtain the frequency deviation , which is added to the rated frequency and then processed by the integral link to obtain the phase angle , and the 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 and actual reactive power Q 0 The deviation of the proportional coefficient k q After adjustment, the reference value of electromotive force E m Calculate the voltage reference value E dref , 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 and reality d Shaft voltage u gd The deviation of the transfer function Regulation, current reference I dref and reality d Shaft current i Ld The deviation of the transfer 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 take into account the capacitance and inductance characteristics of the system and achieve decoupling control. The energy storage and generation unit grid-based control structure maintains system frequency stability through the active frequency loop, stabilizes output voltage through the reactive voltage loop, and achieves precise regulation of load current and output voltage through dual voltage and current loop control, ensuring stable operation of the microgrid.

[0061] In step S2, please refer to Figure 1 In step S2, please refer to

[0062] Specifically, considering the AC-DC inter-frequency coupling effect and the AC side frequency coupling effect, the second-order admittance model of the photovoltaic power generation unit three-phase inverter and the energy storage power generation unit three-phase inverter in the abc coordinate system is established based on the harmonic linearization method.

[0063] As an example, since the control loop of the photovoltaic power generation unit contains a phase-locked loop and a DC voltage outer loop, the AC side frequency coupling characteristics and the AC-DC inter-frequency coupling characteristics can be considered simultaneously. Specifically, the harmonic linearization modeling method can be used to establish the sequence admittance model of the photovoltaic grid-connected inverter considering the AC-DC inter-frequency coupling effect and the AC side maximum power point tracking (MPPT) dynamic characteristics The formula is as follows:

[0064]

[0065] wherein, is the sequence admittance model of the photovoltaic grid-connected inverter considering the AC-DC inter-frequency coupling effect and the AC side maximum power point tracking (MPPT) dynamic characteristics, which is a matrix used to describe the electrical characteristics of the inverter in the complex frequency domain, reflecting the relationship between the port voltage and the current of the inverter; s is the Laplace operator; is the AC side self-admittance of the photovoltaic grid-connected inverter; is the AC side mutual admittance of the photovoltaic grid-connected inverter; is the AC side mutual admittance of the photovoltaic grid-connected inverter; is the AC side self-admittance of the photovoltaic grid-connected inverter.

[0066] Further, the element is expressed as:

[0067] wherein, is the AC side grid point positive sequence current disturbance, is the AC side grid point positive sequence voltage disturbance, m 1, m 2, m 3, m 4, n 1, n 2, n 3, n 4 are parameters, and the minus sign in the formula is related to the direction selection of the reference current.

[0068] As an example, the parameterm 1、 m 2、 m 3、 m 4、 n 1、 n 2、 n 3、 n 4The expression is as follows:

[0069]

[0070]

[0071] wherein, G PI is a PI controller, G u is a voltage controller, T PLL is a phase-locked loop period, D 0 is an initial active power, Q 0 is an initial reactive power, L 1 is an inverter-side inductance, L 2 is a grid-side inductance, C is a filter capacitance, K dq is a coupling coefficient, K ic is a capacitance current feedback coefficient, I c1 is a capacitance current, I 1 is a current fundamental component, x 1 , x 2 , x 3 , x 4 is a real part of the remaining polynomial expression, y 1 , y 2 , y 3 , y 4 is an imaginary part of the remaining polynomial expression.

[0072] Further, the element is expressed as:

[0073]

[0074] wherein, For positive sequence current disturbance of AC side grid-connection point, For negative sequence voltage disturbance of AC side grid-connection point.

[0075] Further, the element The expression is:

[0076]

[0077] Wherein, For negative sequence current disturbance of AC side grid-connection point, For positive sequence voltage disturbance of AC side grid-connection point.

[0078] Further, the element The expression is:

[0079]

[0080] Wherein, For negative sequence current disturbance of AC side grid-connection point, For negative sequence voltage disturbance of AC side grid-connection point.

[0081] As an example, m 1, m 2, m 3, m 4, n 1, n 2, n 3, n 4 satisfies the following expression:

[0082]

[0083] Wherein, , For positive and negative sequence current disturbance of AC side grid-connection point, , For positive and negative sequence voltage disturbance of AC side grid-connection point.

[0084] As an example, for the energy storage power generation unit, since there is a power calculation link, only the AC side frequency coupling effect is considered. Specifically, the equivalent output admittance model of the virtual synchronous generator (VSG) energy storage power generation unit considering the AC side frequency coupling effect can be established The formula is as follows:

[0085]

[0086] Wherein, The equivalent output admittance model of the VSG energy storage power generation unit considering the AC side frequency coupling effect is a Matrix, which is used to describe the electrical characteristics of the inverter in the complex frequency domain, and reflects the relationship between the inverter port voltage and current.s is the Laplace operator; is the self-admittance of the energy storage grid forming inverter AC side; is the mutual admittance of the energy storage grid forming inverter AC side; is the mutual admittance of the energy storage grid forming inverter AC side; is the self-admittance of the energy storage grid forming inverter AC side.

[0087] Further, the element , , , can be expressed by the following formula:

[0088]

[0089] wherein, M v1 is, M v2 is, M i1 is, M i2 is, N v1 is, N v2 is, N i1 is, N i2 is a parameter, and the specific expression is as follows:

[0090]

[0091]

[0092] wherein, G i is a current transformer, G u is a voltage transformer, G f is a frequency transformer, E 0 is an initial electric energy, W p is a gain coefficient.

[0093] Specifically, M v1 , M v2 , M i1 , M i2 , N v1 , N v2 ,N i1 、 N i2 satisfies the following expression:

[0094]

[0095] wherein, 、 is the positive and negative sequence current disturbance of the AC side grid-connection point, 、 is the positive and negative sequence voltage disturbance of the AC side grid-connection point.

[0096] In step S3, please refer to step S3 in Figure 1 , 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.

[0097] 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 part with coupling is expressed as a controlled current source, then the corresponding positive and negative sequence controlled currents 、 The expression is:

[0098]

[0099] wherein, 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 of the AC side grid-connection point, 、 is the mutual admittance of the AC side, is the influence of the negative sequence voltage coupled out after the positive sequence disturbance voltage disturbance on the positive sequence response current, is the influence of the negative sequence coupling current after the positive sequence disturbance voltage disturbance.

[0100] Further, according to the topological structure of the microgrid and the frequency coupling relationship, the positive and negative sequence equivalent impedances of the inverter of the power generation unit after decoupling 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:

[0101]

[0102]

[0103]

[0104]

[0105] wherein, Z ppsl is the positive sequence forward equivalent impedance, reflecting the equivalent impedance characteristics of the inverter when transmitting forward under the action of the positive sequence voltage component; Z nnsl is the negative sequence forward equivalent impedance, reflecting the equivalent impedance characteristics of the inverter when transmitting forward under the action of the negative sequence voltage component; Z ppsm is the positive sequence reverse equivalent impedance, reflecting the equivalent impedance characteristics of the inverter when transmitting reverse under the action of the positive sequence voltage component; Z nnsm is the negative sequence reverse equivalent impedance, reflecting the equivalent impedance characteristics of the inverter when transmitting reverse under the action of the negative sequence voltage component; s is the Laplace operator; is the angular frequency corresponding to the fundamental frequency; is the influence of the coupled negative sequence voltage on the positive sequence corresponding current after the positive sequence disturbance voltage disturbance is applied; is the negative sequence current after the response of the applied negative sequence disturbance voltage disturbance; is the positive sequence current after the response of the applied positive sequence disturbance 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 of the AC side; is the mutual admittance of the AC side; is the mutual admittance of the AC side; is the self-admittance of the AC side.

[0106] 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 here the same symbols are used.

[0107] In step S4, please refer to step S4 in Figure 1 , according to the topological structure of the microgrid, the voltage and current relationship at the system public connection point is established based on the node voltage method, and the system admittance matrix is obtained.

[0108] As an example, please refer to Figure 7 , step S4 can include the following steps: S41-S44.

[0109] Step S41: According to the topological structure of the microgrid, an equivalent model of the microgrid in the island scenario is established.

[0110] Step S42: Based on the frequency coupling relationship, different power generation units in the system are aggregated respectively to obtain positive and negative sequence impedances of different power generation unit inverters.

[0111] Step S43: The node voltage equation of the system is established based on the node voltage method, and the voltage and current relationship at the system public connection point is further arranged based on the conditions of the island scenario.

[0112] Step S44: According to the voltage and current relationship at the system public connection point, the admittance matrix of the system is obtained.

[0113] As an example, in step S41, based on the topological structure of the microgrid, the overall equivalent model of the microgrid in the island scenario is established. Figure 8 For the overall equivalent model of the microgrid in the island scenario, as shown in Figure 8 , the system can include m a photovoltaic power generation unit, n a storage power generation unit, the m a photovoltaic power generation unit and n a storage power generation unit are connected in parallel across the load. Specifically, I c,f is the equivalent current source of the f th grid-following inverter, f =1,2,… m ; Z c,f is the output impedance of the f th grid-following inverter, f =1,2,… m ; U v,f is the equivalent voltage source of the f th grid-forming inverter, f =1,2,… n ; Z v,f is the output impedance of the f th grid-forming inverter, f =1,2,… n ; Z cl,f is the line impedance from the f th grid-following inverter to the grid connection point, f =1,2,… m ; Z vl,f is the line impedance from the f th grid-forming inverter to the grid connection point,f =1,2,… n ; Z g is the grid impedance; Z load is the load impedance; U PCC is the grid-connected voltage at the point of common coupling (PCC).

[0114] As an example, in step S42, based on the frequency coupling relationship, each power generation unit in the system is aggregated respectively to obtain positive and negative sequence impedances of different power generation unit inverters. The positive and negative sequence impedances of the different power generation unit inverters include: positive and negative sequence impedances of grid-connected type inverters Z sm and positive and negative sequence impedances of grid-forming type inverters Z sn .

[0115] As an example, the positive and negative sequence impedances of the grid-connected type inverters Z sm are expressed as:

[0116]

[0117] wherein, Z ppsm is the positive sequence reverse equivalent impedance of the grid-connected type inverters; Z nnsm is the negative sequence reverse equivalent impedance of the grid-connected type inverters; Z load is the load impedance; Z vl is the line impedance of 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 and negative sequences; Z cl is the line impedance of each photovoltaic power generation unit to the PCC point; Y den1 is the equivalent admittance seen from the port of each photovoltaic power generation unit, i.e., the equivalent admittance seen from the port of each grid-connected type inverter; s is the Laplace operator; is the angular frequency corresponding to the fundamental frequency; is the self-admittance on the alternating current side; is the mutual admittance on the alternating current side; is the mutual admittance on the alternating current side; is the self-admittance on the alternating current side; m is the number of photovoltaic power generation units; n is the number of energy storage power generation units.

[0118] As an example, the positive and negative sequence impedance of the grid-forming inverter Z sn The expression is:

[0119]

[0120] wherein, Z ppsn is the positive sequence backward equivalent impedance of the grid-forming inverter; Z nnsn is the negative sequence backward equivalent impedance of the grid-forming inverter; Z load is the load impedance; Z vl is the line impedance from each energy storage generation unit to the PCC point; Z c is the equivalent output impedance of the photovoltaic generation unit; Z cl is the line impedance from each photovoltaic generation unit to the PCC point; Y den2 is the equivalent admittance seen from each energy storage generation unit port, i.e. the equivalent admittance seen from each grid-forming inverter port; 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 generation units; n is the number of energy storage generation units.

[0121] As an example, in step S43, the node voltage equations of the system are established according to the circuit structure of the overall equivalent model and based on the node voltage method as follows:

[0122]

[0123] wherein, m is the number of photovoltaic generation units, n is the number of energy storage generation units, Z load is the load impedance, Z c is the equivalent output impedance of the photovoltaic generation unit, Z cl is the line impedance from each photovoltaic generation unit to the PCC point, Z vl is the line impedance from each energy storage generation unit to the PCC point, Z v is the equivalent output impedance of the energy storage 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 point of common connection.

[0124] Furthermore, in an island scenario, the load current provided by the system to the load I gl Grid voltage at the point of public connection U pcc Satisfies the following expression:

[0125]

[0126] in, Z load is the load impedance, I gl is the load current, U PCC is the grid voltage at the point of common connection.

[0127] Furthermore, the grid-connected voltage equation of the system's common connection point is sorted out, and the voltage-current relationship at the system's common connection point is obtained as follows:

[0128]

[0129] in, m is 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-following inverter.

[0130] As an example, in step S44, the system admittance matrix is ​​obtained based on the voltage and current relationship at the system common connection point: Z sFor:

[0131]

[0132] wherein, m is the number of photovoltaic generation units, n is the number of energy storage generation units, Z c is the equivalent output impedance of the photovoltaic generation units, Z cl is the line impedance from each photovoltaic generation unit to the PCC point, Z vl is the line impedance from each energy storage generation unit to the PCC point, Z v f is the output impedance of the nth grid-connected inverter. f In step S5, referring to step S5 in

[0133] Figure 1 draw the amplitude-frequency curve and the phase-frequency curve of the key items in the system admittance matrix, determine the key characteristic frequency at the minimum point of the amplitude-frequency curve, and determine the stability of the system according to the positive or negative of the slope of the phase-frequency curve at the key characteristic frequency.

[0134] As an example, observe the system admittance matrix Z s The structure of the formula can be found that, under the condition that the parameters are reasonably set, each grid-connected generation unit can be independently and stably operated, and the load can be stably operated under the action of an ideal current source or a voltage source. Then, the stability of the generation unit microgrid system in the islanding scenario mainly depends on the following two items:

[0135]

[0136] wherein, m is the number of photovoltaic generation units, n is the number of energy storage generation units, N 1 is the first main item, N 2 is the second main item, Z c is the equivalent output impedance of the photovoltaic generation units, Z cl is the line impedance from each photovoltaic generation unit to the PCC point, Z vl is the line impedance from each energy storage generation unit to the PCC point, Z v f is the output impedance of the nth grid-connected inverter. f ​​

[0137] Further, it can be verified by theory and simulation that N 2always has positive stability margin, so the stability of the microgrid in islanding scenario mainly depends on N 1. Specifically, if N 1has a pair of conjugate zeros N 1has a pair of conjugate zeros lambda 0, 1 = sigma 0± j omega 0, it means that there is a certain oscillation mode in the system; N ( j omega )= x + j y , where N 1can be expanded as:

[0138]

[0139] where sigma 0is the damping of the corresponding oscillation mode, omega 0is the angular frequency of the corresponding oscillation mode, x is the real part of the rest of the polynomial expression, y is the imaginary part of the rest of the polynomial expression.

[0140] Further, by setting the imaginary part to zero, the abscissa omega n of the zero-crossing point of the imaginary part-frequency curve can be obtained, and the corresponding value is:

[0141]

[0142] where omega n is the abscissa of the zero-crossing point of the imaginary part-frequency curve, sigma 0is the damping of the corresponding oscillation mode, omega 0is the angular frequency of the corresponding oscillation mode, x is the real part of the rest of the polynomial expression, y is the imaginary part of the rest of the polynomial expression, m is the number of photovoltaic power generation units.

[0143] As an example, in the weakly damped oscillation mode, it satisfies . At this time, the abscissa omega n of the zero-crossing point of the imaginary part-frequency curve is approximately equal to the abscissa of the oscillation mode, i.e. . By substituting the expression of N 1back into the expression, the expression of the real part of N 1is obtained as:

[0144]

[0145] in, sigma 0 is the damping of the corresponding oscillation mode, omega 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.

[0146] Furthermore, N Conjugate zero of 1 lambda 0，1 exist omega Slope at 0 k n for:

[0147]

[0148] in, m is the number of photovoltaic power generation units, n is the number of energy storage power generation units, omega is the angular frequency, sigma 0 corresponds to the damping of the oscillation mode.

[0149] Furthermore, through the above two formulas, we can find that: k n When it is less than 0, N 1The real part of the real part of the 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 1The real part of the frequency curve at the zero point has the opposite sign to the real part at the conjugate zero point. Therefore, select N The minimum point of the real part-frequency curve is taken as the key characteristic frequency. 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.

[0150] In the stability analysis method of the photovoltaic storage micro-grid island scene based on the system admittance characteristics, the type and control structure of the main circuit filter of the three-phase inverter of the power generation unit are determined, which can provide a basis for subsequent accurate modeling and clarify the basic framework of the system; the second-order admittance model is established and decoupled by using the harmonic linearization method, which can accurately describe the characteristics of the inverter at different frequencies and fully consider the frequency coupling effect between AC and DC and the frequency coupling effect on the AC side; the system admittance matrix is obtained based on the micro-grid topology structure and the node voltage method, which can comprehensively consider the overall characteristics of the system and reasonably aggregate different power generation units; by drawing the amplitude-frequency curve and the phase-frequency curve of the key items of the system admittance matrix, and judging the stability according to the slope of the phase-frequency curve at the key characteristic frequency, the stability of the system in the island scene can be comprehensively and accurately evaluated, potential instability factors can be found in time, the physical meaning is clear, accurate conclusions of the stability analysis can be drawn, and the accuracy of extracting the key characteristic frequency is ensured, which provides scientific and accurate guidance for the optimization and operation of the photovoltaic storage micro-grid and effectively improves the reliability and stability of the island operation.

[0151] It should be understood that, although each step in the flowchart of the accompanying drawings is shown in sequence according to the direction of the arrow, these steps are not necessarily executed in sequence according to the direction of the arrow. Unless otherwise specified herein, the execution of these steps is not strictly limited in sequence, and these steps can be executed in other sequences. Moreover, at least part of the steps in the accompanying drawings can include multiple sub-steps or multiple stages, which are not necessarily executed at the same time, but can be executed at different times, and the execution sequence of these sub-steps or stages is not necessarily sequential, but can be executed alternately or alternately with at least part of other steps or sub-steps or stages of other steps.

[0152] The technical features of the above embodiments can be combined in any manner. In order to make the description concise, not all possible combinations of the technical features in the above embodiments are described, but as long as the combinations of the technical features do not exist contradictory, it should be considered as the scope of the present application.

[0153] Although the present application has been disclosed as above with examples, it is not intended to limit the present application, and anyone with ordinary knowledge in the art can make some changes and modifications without departing from the spirit and scope of the present application, therefore the protection scope of the present application shall be subject to the appended patent claim scope.

Claims

1. A stability analysis method for a photovoltaic microgrid with energy storage in an islanded 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; Based on the harmonic linearization method, the second-order admittance model of the three-phase inverter of the power generation unit is established by considering the frequency coupling effect between AC and DC and the frequency coupling effect on the AC side; Decoupling a 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, including expressing a response current as a product of a disturbance voltage and a self-admittance plus a product of a coupling voltage and a coupling admittance according to the physical meaning of the second-order admittance model of the three-phase inverter of the power generation unit; According to the topological structure and frequency coupling relationship of the microgrid, the positive and negative sequence equivalent impedance of the power generation unit inverter after decoupling is 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; According to the topology of the microgrid, a voltage-current relationship at the system common connection point is established based on the node voltage method to obtain the system admittance matrix, including: establishing an equivalent model of the microgrid in an island scenario according to the topology of the microgrid; aggregating different power generation units in the system based on the frequency coupling relationship to obtain the positive and negative sequence impedances of the inverters of different power generation units; establishing the node voltage equation of the system based on the node voltage method, and combining the conditions of the island scenario to further organize the voltage-current relationship at the system common connection point; and obtaining the system admittance matrix based on the voltage-current relationship at the system common connection point; Drawing 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: drawing the amplitude-frequency curve and the phase-frequency curve of the key items in the system admittance matrix; selecting the minimum point of the amplitude-frequency curve as the key characteristic frequency, observing the positive or negative slope of the phase-frequency curve at the key characteristic frequency point; if the slope is positive and passes through 0° or 180°, then it is determined that the system is stable; if the slope is negative and passes through 0° or 180°, then it is determined that the system is unstable; 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 each energy storage power generation unit.

2. The stability analysis method for a photovoltaic microgrid in an islanded 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 for a photovoltaic microgrid in an islanded 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 for a photovoltaic microgrid in an islanded scenario based on system admittance characteristics according to claim 1 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.

5. The stability analysis method for a photovoltaic microgrid in an islanded scenario based on system admittance characteristics according to claim 1 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-forming inverter.

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