Broadband oscillation risk analysis method and system under distributed photovoltaic grid connection

Abstract

The invention discloses a broadband oscillation risk analysis method and system under distributed photovoltaic grid connection, and the method comprises the steps: dividing distributed photovoltaic into a plurality of clusters, and enabling the characteristics of the distributed photovoltaic in the clusters to be similar; equivalence transformation is carried out on the distributed photovoltaics with similar characteristics; constructing an equivalent network in which the equivalent units are mutually connected with the original power distribution network, and accessing the equivalent units into the original power distribution network frame through the equivalent network to realize equivalent modeling of the power distribution network; and performing frequency sweeping on the distributed photovoltaic equivalent model of the power distribution network based on a harmonic voltage injection method to obtain impedance characteristics of the distributed photovoltaic equivalent model, constructing a port equivalent model of the distributed photovoltaic cluster via the alternating current sending-out system, and further evaluating the stability of the port equivalent model of the distributed photovoltaic cluster via the alternating current sending-out system. According to the method, the accuracy and universality of broadband oscillation analysis of the power grid can be effectively improved, and the method is of great significance to construction of a safe and abundant power system.

Description

Technical Field

[0001] The present invention belongs to the technical field of grid-connected oscillation analysis, and relates to a method and system for analyzing the risk of broadband oscillation under distributed photovoltaic grid connection. Background Art

[0002] At present, in order to ensure green and sustainable development, the power system, which is mainly characterized by the large-scale development and utilization of new energy such as wind power and photovoltaics, is in a new stage of clean transformation and self-innovation, and distributed photovoltaic power generation has become an important form of new energy development. However, due to the interaction between power sources, networks and loads, there is a risk of instability oscillations in the frequency range of several hertz to several thousand hertz in the new power system, and the oscillation problem of the distribution network usually affects equipment safety, power quality and grid stability. This dynamic process in which the electrical quantity caused by the interaction between power electronic devices and between them and the power grid fluctuates periodically over time and the oscillation frequency changes in a wide range is called broadband oscillation. At the same time, there are many disadvantages in the existing methods for studying broadband oscillations. For example, the analytical calculation method is difficult to analyze the macroscopic dynamic behavior of the oscillation due to the strong nonlinearity of the complex power system; the numerical simulation method cannot obtain the intrinsic mechanism of the influence of parameters on the system oscillation; and the rationality of using the frequency scanning method to analyze the broadband oscillation characteristics of the fine equivalent model also needs to be evaluated. Summary of the invention

[0003] The purpose of the present invention is to solve the problems in the prior art that it is difficult to analyze the macroscopic dynamic behavior of broadband oscillations, it is impossible to obtain the intrinsic mechanism of the influence of parameters on system oscillations, and it is impossible to evaluate the rationality of the broadband oscillation characteristics of the fine equivalent model analyzed by the frequency scanning method, and to provide a method and system for analyzing the risk of broadband oscillations under distributed photovoltaic grid-connected conditions.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions:

[0005] A method for analyzing the risk of broadband oscillation in distributed photovoltaic grid-connected power systems, comprising:

[0006] Based on the principle of the same unit model, each distributed photovoltaic is initially grouped; and the electrical distance between each distributed photovoltaic node and the public connection point PCC is calculated;

[0007] Based on the K-means clustering algorithm and electrical distance, the distributed photovoltaics of the same type after the initial grouping are divided again to form a distributed photovoltaic cluster division result that comprehensively considers dynamic characteristics and electrical distance;

[0008] Based on the principles of "model consistency" and "power conservation", distributed photovoltaics are aggregated to build an equivalent model for distributed photovoltaic aggregation in the distribution network; based on the REI equivalent method and the WARD equivalent method, an equivalent network for the distribution network grid is built;

[0009] Based on the distributed photovoltaic cluster division results, the distribution network distributed photovoltaic aggregation equivalent model and the distribution network grid equivalent network, the distribution network distributed photovoltaic equivalent model is obtained;

[0010] Based on the harmonic voltage injection method, the distributed photovoltaic equivalent model of the distribution network is swept to obtain its impedance characteristics;

[0011] Equivalence the AC port of the distributed photovoltaic cluster and the main grid at the grid connection point of the distributed photovoltaic equivalent model of the distribution network, and construct an equivalent model of the distributed photovoltaic cluster through the AC transmission system port;

[0012] Based on the equivalent model of the system port of the distributed photovoltaic cluster via AC transmission and the impedance characteristics of the obtained distributed photovoltaic equivalent model of the distribution network, the stability of the equivalent model of the system port of the distributed photovoltaic cluster via AC transmission is evaluated.

[0013] A further improvement of the present invention is:

[0014] Furthermore, the electrical distance between each distributed photovoltaic node and the common connection point PCC is calculated, specifically: the electrical distance between each distributed photovoltaic node and the common connection point PCC is characterized based on the transfer impedance, specifically:

[0015]

[0016] Among them, I k Indicates the AC component value of the short-circuit current at point k, U i represents the voltage of the i-th distributed power source, Z ik represents the transfer impedance between the ith distributed generation node and the short-circuit point k, and n is the number of distributed photovoltaics in the cluster.

[0017] Furthermore, based on the principles of "model consistency" and "power conservation", distributed photovoltaics are aggregated to construct an equivalent model for distributed photovoltaic aggregation in the distribution network. Specifically, distributed photovoltaics of the same model are aggregated to establish an equivalent unit model, and the power at the public connection point PCC between the distribution network and the upper power grid is guaranteed to be consistent before and after the aggregation of distributed photovoltaic units, that is, the following conditions are met:

[0018]

[0019] Among them, P eq,k is the equivalent active power corresponding to cluster k; P i is the active power of the i-th distributed photovoltaic in cluster k; Q eq,k is the equivalent reactive power corresponding to cluster k; Q i is the reactive power of the i-th distributed photovoltaic unit in cluster k.

[0020] Furthermore, based on the REI equivalent method and the WARD equivalent method, an equivalent network of the distribution network is constructed. Specifically, based on the REI equivalent method, an equivalent network between equivalent units / loads and the distributed photovoltaic access points of the original distribution network is constructed. Specifically,

[0021]

[0022] Among them, y u is the trunk admittance, y v is the branch admittance, U u is the voltage at the equivalent access point, U v is the voltage at the original access point, P eq is the equivalent power, P v is the power of the original access point; when performing load equalization, the flow direction of the load power is opposite to that of the power supply, and the positive and negative signs of the power need to be changed;

[0023] Based on the WARD equivalent method, the redundant nodes other than the equivalent units / loads and the common connection point PCC are eliminated to complete the grid equivalent and obtain the simplified node admittance matrix:

[0024]

[0025] Among them, subscript B is a boundary node, subscript I is an internal node, subscript E is an external node, matrix Y represents the node admittance matrix, matrix U represents the node voltage matrix, and matrix I represents the node current matrix.

[0026] Furthermore, based on the harmonic voltage injection method, the distributed photovoltaic equivalent model of the distribution network is swept to obtain its impedance characteristics, specifically: a harmonic voltage source is injected from the main grid side to the distribution grid side; phase voltage and current signals are collected, and positive and negative sequence separation and Fourier decomposition are performed, and for the injected harmonic voltage frequency, the voltage amplitude is divided by the current amplitude to obtain the impedance amplitude, and the voltage phase is subtracted from the current phase to obtain the impedance phase, the injected harmonic voltage frequency is changed, and the above steps are repeated until the required frequency range is swept; wherein, the amplitude of the harmonic voltage source should be greater than 1% of the rated voltage of the grid connection point and less than 5% of the rated voltage of the grid connection point.

[0027] Furthermore, the distributed photovoltaic cluster AC port of the distribution network distributed photovoltaic equivalent model and the main grid at the grid connection point are equivalent to each other, and the distributed photovoltaic cluster AC transmission system port equivalent model is constructed. Specifically, the distributed photovoltaic in the distribution network is usually in the maximum power point tracking mode MPPT or follows the dispatch instruction in the active / reactive power control mode; in this mode, the distributed photovoltaic cluster AC port presents the current source characteristics and also has output impedance; the distribution network AC port is equivalent to the ideal current source I RE Its port impedance Z REIn parallel, the main grid at the grid connection point of the equivalent model is equivalent to an ideal voltage source V g and AC grid impedance Z g Series; AC grid impedance Z g Connect an ideal current source I RE The positive pole of the ideal voltage source V g Connect an ideal current source I RE The negative electrode;

[0028] The voltage equation of the distribution network output node is:

[0029]

[0030] Among them, V out is the voltage at the distribution network output point, I out Output current for the distribution network.

[0031] Furthermore, the stability of the equivalent model of the distributed photovoltaic cluster through the AC transmission system port is evaluated, including: based on the equivalent model of the distributed photovoltaic cluster through the AC transmission system port, the Nyquist curve is obtained, and then the stability of the equivalent model of the distributed photovoltaic cluster through the AC transmission system port is determined, specifically:

[0032] According to formula (5), the distribution network output current I out for:

[0033]

[0034] The right side of the equal sign can be regarded as the multiplication of two closed-loop transfer functions, which are respectively expressed as:

[0035]

[0036] The stability of the equivalent model AC transmission system is determined by the closed-loop transfer functions F1 and F2: the system is stable if and only if the poles of F1 and F2 are both located in the left half plane of s, that is:

[0037]

[0038] When all poles of F1 are located in the left half plane of s, the system stability depends on F2, that is, the AC grid impedance Z g With the equivalent model port impedance Z RE Impedance ratio Z g / Z RE Satisfy the Nyquist criterion;

[0039] Z=PN=0 (9)

[0040] Where P is the impedance ratio Z g / Z REThe number of poles in the right half plane of s, N is the impedance ratio Z g / Z RE The number of times the trajectory circles the point (-1, j0) in the counterclockwise direction; Z is the index of the Nyquist criterion; when Z = 0, it means that the function satisfies the Nyquist criterion, and the power system is stable.

[0041] Furthermore, evaluating the stability of the equivalent model of the distributed photovoltaic cluster through the AC transmission system port also includes: obtaining the Bode diagram of the power system based on the impedance characteristics of the obtained distributed photovoltaic equivalent model of the distribution network, and then determining the stability of the equivalent model of the distributed photovoltaic cluster through the AC transmission system port, specifically:

[0042] In the Bode diagram of the power system, the equivalent model and the AC grid impedance characteristic amplitude curve intersect, that is:

[0043] Z RE (f0)|=|Z g (f0)| (10)

[0044] Among them, Z RE is the equivalent model impedance, Z g is the AC grid impedance, f0 is the frequency when the two amplitudes are equal; the stability is determined based on the sum of the two phases corresponding to the frequency;

[0045] γ=|∠Z RE (f0)+∠Z g (f0)| (11)

[0046] Determine whether the phase stability margin r is greater than the preset threshold. If it is greater, there is no oscillation risk; if it is less, a series resonant circuit is formed and there is an oscillation risk. Use the phase stability margin r to verify whether the risk assessment is correct, that is:

[0047]

[0048] In the formula, γ0 is the stability margin required for stable operation of the system. When the above conditions are met, the risk of broadband oscillation is relatively high.

[0049] A distributed photovoltaic grid-connected broadband oscillation risk analysis system, comprising:

[0050] The first division module, based on the principle of the same unit model, performs initial grouping of each distributed photovoltaic; and calculates the electrical distance between each distributed photovoltaic node and the common connection point PCC;

[0051] A second division module, which re-divides the distributed photovoltaics of the same type after the initial grouping based on the K-means clustering algorithm and the electrical distance to form a distributed photovoltaic cluster division result that comprehensively considers the dynamic characteristics and the electrical distance;

[0052] Aggregation module, which aggregates distributed photovoltaics based on the principles of "model consistency" and "power conservation" to construct an equivalent model for distributed photovoltaic aggregation in the distribution network; and constructs an equivalent network for the distribution network grid based on the REI equivalent method and the WARD equivalent method;

[0053] A first acquisition module, which acquires a distributed photovoltaic equivalent model of the distribution network based on the distributed photovoltaic cluster division result, the distributed photovoltaic aggregation equivalent model of the distribution network and the distribution network grid equivalent network;

[0054] A second acquisition module, wherein the second acquisition module performs a frequency sweep on a distributed photovoltaic equivalent model of the distribution network based on a harmonic voltage injection method to obtain its impedance characteristics;

[0055] A construction module, wherein the construction module performs equivalence between the AC port of the distributed photovoltaic cluster of the distribution network distributed photovoltaic equivalent model and the main network at the grid connection point, and constructs an equivalent model of the distributed photovoltaic cluster through the AC transmission system port;

[0056] An evaluation module is provided, wherein the evaluation module evaluates the stability of the equivalent model of the distributed photovoltaic cluster via an AC transmission system port based on the equivalent model of the distributed photovoltaic cluster via an AC transmission system port and based on the impedance characteristics of the obtained distributed photovoltaic equivalent model of the distribution network.

[0057] Compared with the prior art, the present invention has the following beneficial effects:

[0058] The present invention divides distributed photovoltaic into several clusters so that the distributed photovoltaic characteristics within the clusters are similar; then the distributed photovoltaic with similar characteristics is transformed into equivalent value; an equivalent network interconnected between equivalent units and the original distribution network is constructed, and the equivalent units are connected to the original distribution network grid through the equivalent network to realize the equivalent modeling of the distribution network; based on the harmonic voltage injection method, the distributed photovoltaic equivalent model of the distribution network is swept to obtain its impedance characteristics, and an equivalent model of the distributed photovoltaic cluster through the AC transmission system port is constructed, and then the stability of the equivalent model of the distributed photovoltaic cluster through the AC transmission system port is evaluated. The present invention can effectively improve the accuracy and versatility of the broadband oscillation analysis of the power grid, which is of great significance to the construction of a safe and sufficient power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0060] Figure 1 It is a schematic flow chart of a method for analyzing the risk of broadband oscillation under distributed photovoltaic grid connection of the present invention;

[0061] Figure 2 It is a schematic diagram of the equivalent model of the AC transmission system port of the distributed photovoltaic cluster;

[0062] Figure 3 It is a structural schematic diagram of a distributed photovoltaic grid-connected broadband oscillation risk analysis system of the present invention;

[0063] Figure 4 It is a structural diagram of the detailed model of IEEE33 node distribution network containing distributed photovoltaic clusters;

[0064] Figure 5 (a) Schematic diagram of PCC voltage simulation of detailed model and equivalent model;

[0065] Figure 5 (b) Schematic diagram of PCC current simulation of detailed model and equivalent model;

[0066] Figure 6 It is a schematic diagram of the Nyquist curve of the detailed model and the equivalent model when connected to the normal AC power grid;

[0067] Figure 7 It is a schematic diagram of the relationship between the amplitude of the detailed model and the equivalent model and the grid frequency when connected to the normal AC grid;

[0068] Figure 8 It is a schematic diagram of the relationship between the phase of the detailed model and the equivalent model and the grid frequency when connected to the normal AC grid;

[0069] Fig. 9 It is a schematic diagram of the Nyquist curve of the equivalent model when connected to a weak AC power grid;

[0070] Fig.10 It is a schematic diagram of the relationship between the amplitude of the equivalent model at the grid connection point and the grid frequency when connected to a weak AC grid;

[0071] Fig.11 It is a schematic diagram of the relationship between the phase and grid frequency of the equivalent model at the grid connection point when connected to a weak AC grid;

[0072] Fig.12(a) is a schematic diagram of the oscillation waveform when connected to a weak AC power grid;

[0073] Fig.12 (b) is a schematic diagram of the FFT transformation results when connected to a weak AC power grid. DETAILED DESCRIPTION

[0074] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0075] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0076] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0077] In the description of the embodiments of the present invention, it should be noted that if the terms "upper", "lower", "horizontal", "inner", etc. indicate an orientation or positional relationship based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the product of the invention is usually placed when in use, it is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. In addition, the terms "first", "second", etc. are only used to distinguish the description, and cannot be understood as indicating or implying relative importance.

[0078] In addition, if the term "horizontal" appears, it does not mean that the component must be absolutely horizontal, but can be slightly tilted. For example, "horizontal" only means that its direction is more horizontal than "vertical", which does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0079] In the description of the embodiments of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms "set", "install", "connect", and "connect" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal connection of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0080] The present invention is further described in detail below in conjunction with the accompanying drawings:

[0081] See also Figure 1 The present invention discloses a method for analyzing the risk of broadband oscillation in distributed photovoltaic grid-connected systems, comprising:

[0082] S101, based on the principle of the same unit model, the distributed photovoltaics are initially grouped; and the electrical distance between each distributed photovoltaic node and the public connection point PCC is calculated;

[0083] According to the latest power industry standard "Guidelines for Power Aggregation Equivalence and Modeling for Power System Simulation" implemented in 2024, each distributed photovoltaic is initially grouped according to the manufacturer model.

[0084] The electrical distance between each distributed photovoltaic node and the common connection point PCC is characterized based on transfer impedance, specifically:

[0085]

[0086] Among them, I k Indicates the AC component value of the short-circuit current at point k, U i represents the voltage of the i-th distributed power source, Z ik represents the transfer impedance between the ith distributed generation node and the short-circuit point k, and n is the number of distributed photovoltaics in the cluster.

[0087] S102, based on the K-means clustering algorithm and electrical distance, the distributed photovoltaics of the same type after the initial grouping are divided again to form a distributed photovoltaic cluster division result that comprehensively considers dynamic characteristics and electrical distance;

[0088] S103, based on the principles of "model consistency" and "power conservation", distributed photovoltaics are aggregated to build an equivalent model for distributed photovoltaic aggregation in the distribution network; based on the REI equivalent method and the WARD equivalent method, a distribution network grid equivalent network is built;

[0089] Based on the principles of "model consistency" and "power conservation", distributed photovoltaics are aggregated to build an equivalent model for distributed photovoltaic aggregation in the distribution network. Specifically, distributed photovoltaics of the same model are aggregated to establish an equivalent unit model, and the power at the public connection point PCC between the distribution network and the upper power grid is guaranteed to be consistent before and after the aggregation of distributed photovoltaic units, that is, the following conditions are met:

[0090]

[0091] Among them, P eq,k is the equivalent active power corresponding to cluster k; P i is the active power of the i-th distributed photovoltaic in cluster k; Q eq,k is the equivalent reactive power corresponding to cluster k; Q i is the reactive power of the i-th distributed photovoltaic unit in cluster k.

[0092] Based on the REI equivalent method and the WARD equivalent method, an equivalent network of the distribution network is constructed. Specifically, based on the REI equivalent method, an equivalent network between equivalent units / loads and the distributed photovoltaic access points of the original distribution network is constructed. Specifically,

[0093]

[0094] Among them, y u is the trunk admittance, y v is the branch admittance, U u is the voltage at the equivalent access point, U v is the voltage at the original access point, P eq is the equivalent power, P v is the power of the original access point; when performing load equalization, the flow direction of the load power is opposite to that of the power supply, and the positive and negative signs of the power need to be changed;

[0095] Based on the WARD equivalent method, the redundant nodes other than the equivalent units / loads and the common connection point PCC are eliminated to complete the grid equivalent and obtain the simplified node admittance matrix:

[0096]

[0097] Among them, subscript B is a boundary node, subscript I is an internal node, subscript E is an external node, matrix Y represents the node admittance matrix, matrix U represents the node voltage matrix, and matrix I represents the node current matrix.

[0098] S104, based on the distributed photovoltaic cluster division result, the distribution network distributed photovoltaic aggregation equivalent model and the distribution network grid equivalent network, obtain the distribution network distributed photovoltaic equivalent model;

[0099] S105, sweeping the frequency of the distributed photovoltaic equivalent model of the distribution network based on the harmonic voltage injection method to obtain its impedance characteristics;

[0100] According to the relevant requirements for broadband oscillation analysis in the latest national standard "AC Side Impedance Requirements and Test Methods for Voltage Source Converters for High Voltage Direct Current Transmission" (GB / T 43534-2023) implemented in 2024, the electrical oscillation phenomenon occurring between the AC system has an oscillation frequency ranging from several hertz to several thousand hertz; below 100Hz, there is a strong frequency coupling effect, and the influence of frequency coupling on impedance characteristics should be considered; above 100Hz, the influence of frequency coupling can usually be ignored. Because the main frequency band studied in this invention is above 100Hz, frequency coupling is ignored.

[0101] According to the above standards, the impedance-based stability analysis method is one of the effective tools for analyzing electrical oscillations, which determines the system stability by the impedance ratio of the power supply and load that constitute the system. Therefore, the present invention adopts the sweep frequency method to obtain the impedance of each model, and conducts in-depth research on broadband oscillation based on the impedance characteristics.

[0102] According to the "AC side impedance requirements and test methods for voltage source converters for high-voltage direct current transmission", the impedance test methods include: harmonic voltage injection method and harmonic current injection method. Since the voltage of the main grid is relatively stable, the harmonic voltage signal generated by the harmonic current injection method cannot be collected, so the present invention performs frequency sweeping through the harmonic voltage injection method, that is, injecting a harmonic voltage source from the main grid side to the distribution grid side; collecting phase voltage and current signals, and performing positive and negative sequence separation and Fourier decomposition processing, for the injected harmonic voltage frequency, dividing the voltage amplitude by the current amplitude to obtain the impedance amplitude, subtracting the voltage phase from the current phase to obtain the impedance phase, changing the injected harmonic voltage frequency, and repeating the above steps until the required frequency range is swept; wherein, the amplitude of the harmonic voltage source should be greater than 1% of the rated voltage of the grid connection point and less than 5% of the rated voltage of the grid connection point.

[0103] S106, equivalent the distributed photovoltaic cluster AC port of the distribution network distributed photovoltaic equivalent model and the main network at the grid connection point, and construct an equivalent model of the distributed photovoltaic cluster through the AC transmission system port;

[0104] The sweep frequency method can be used to obtain the impedance amplitude and phase of each frequency band of the equivalent model. Based on the impedance characteristics of the main grid, the Nyquist diagram and Bode diagram can be used to more intuitively analyze the system stability and further carry out research on broadband oscillation.

[0105] In order to obtain the Nyquist curve to judge the stability of the power system, it is necessary to establish an equivalent model of the main grid and the distribution grid. Distributed photovoltaics in the distribution network are usually in the maximum power point tracking mode MPPT or follow the dispatch instruction in the active / reactive power control mode; in this mode, the AC port of the distributed photovoltaic cluster presents a current source characteristic and also has output impedance; the AC port of the distribution network is equivalent to an ideal current source I RE Its port impedance Z RE In parallel, the main grid at the grid connection point of the equivalent model is equivalent to an ideal voltage source V g and AC grid impedance Z g Series; AC grid impedance Z g Connect an ideal current source I RE The positive electrode of the ideal voltage source V g Connect an ideal current source I RE negative electrode; see Figure 2 , the equivalent model of the distributed photovoltaic cluster through the AC transmission system port.

[0106] The voltage equation of the distribution network output node is:

[0107]

[0108] Among them, V out is the voltage at the distribution network output point, I out Output current for the distribution network.

[0109] S107, based on the distributed photovoltaic cluster AC transmission system port equivalent model and based on the acquired impedance characteristics of the distribution network distributed photovoltaic equivalent model, evaluate the stability of the distributed photovoltaic cluster AC transmission system port equivalent model.

[0110] Evaluate the stability of the equivalent model of the distributed photovoltaic cluster through the AC transmission system port, including: based on the equivalent model of the distributed photovoltaic cluster through the AC transmission system port, obtain the Nyquist curve, and then determine the stability of the equivalent model of the distributed photovoltaic cluster through the AC transmission system port, specifically:

[0111] According to formula (5), the distribution network output current I out for:

[0112]

[0113] The right side of the equal sign can be regarded as the multiplication of two closed-loop transfer functions, which are respectively expressed as:

[0114]

[0115] The stability of the equivalent model AC transmission system is determined by the closed-loop transfer functions F1 and F2: the system is stable if and only if the poles of F1 and F2 are both located in the left half plane of s, that is:

[0116]

[0117] When all poles of F1 are located in the left half plane of s, the system stability depends on F2, that is, the AC grid impedance Z g With the equivalent model port impedance Z RE Impedance ratio Z g / Z RE Satisfy the Nyquist criterion;

[0118] Z=PN=0 (9)

[0119] Where P is the impedance ratio Z g / Z RE The number of poles in the right half plane of s, N is the impedance ratio Z g / Z RE The number of times the trajectory circles the point (-1, j0) in the counterclockwise direction; Z is the index of the Nyquist criterion; when Z = 0, it means that the function satisfies the Nyquist criterion, and the power system is stable.

[0120] Evaluating the stability of the equivalent model of the distributed photovoltaic cluster through the AC transmission system port also includes: based on the impedance characteristics of the obtained distributed photovoltaic equivalent model of the distribution network, obtaining the Bode diagram of the power system, and then determining the stability of the equivalent model of the distributed photovoltaic cluster through the AC transmission system port, specifically:

[0121] In the Bode diagram of the power system, the equivalent model and the AC grid impedance characteristic amplitude curve intersect, that is:

[0122] Z RE (f0)|=|Z g (f0)| (10)

[0123] Among them, Z RE is the equivalent model impedance, Z g is the AC grid impedance, f0 is the frequency when the two amplitudes are equal; the stability is determined based on the sum of the two phases corresponding to the frequency;

[0124] γ=|∠Z RE (f0)+∠Z g (f0)| (11)

[0125] Determine whether the phase stability margin r is greater than the preset threshold. If it is greater, there is no oscillation risk; if it is less, a series resonant circuit is formed and there is an oscillation risk. Use the phase stability margin r to verify whether the risk assessment is correct, that is:

[0126]

[0127] Where γ0 is the stability margin required for stable operation of the system. When the above conditions are met, the risk of broadband oscillation is relatively high.

[0128] See also Figure 3 The present invention discloses a distributed photovoltaic grid-connected broadband oscillation risk analysis system, comprising:

[0129] The first division module, based on the principle of the same unit model, performs initial grouping of each distributed photovoltaic; and calculates the electrical distance between each distributed photovoltaic node and the common connection point PCC;

[0130] A second division module, which re-divides the distributed photovoltaics of the same type after the initial grouping based on the K-means clustering algorithm and the electrical distance to form a distributed photovoltaic cluster division result that comprehensively considers the dynamic characteristics and the electrical distance;

[0131] Aggregation module, which aggregates distributed photovoltaics based on the principles of "model consistency" and "power conservation" to construct an equivalent model for distributed photovoltaic aggregation in the distribution network; and constructs an equivalent network for the distribution network grid based on the REI equivalent method and the WARD equivalent method;

[0132] A first acquisition module, which acquires a distributed photovoltaic equivalent model of the distribution network based on the distributed photovoltaic cluster division result, the distributed photovoltaic aggregation equivalent model of the distribution network and the distribution network grid equivalent network;

[0133] A second acquisition module, wherein the second acquisition module performs a frequency sweep on a distributed photovoltaic equivalent model of the distribution network based on a harmonic voltage injection method to obtain its impedance characteristics;

[0134] A construction module, wherein the construction module performs equivalence between the AC port of the distributed photovoltaic cluster of the distribution network distributed photovoltaic equivalent model and the main network at the grid connection point, and constructs an equivalent model of the distributed photovoltaic cluster through the AC transmission system port;

[0135] An evaluation module is provided, wherein the evaluation module evaluates the stability of the equivalent model of the distributed photovoltaic cluster via an AC transmission system port based on the equivalent model of the distributed photovoltaic cluster via an AC transmission system port and based on the impedance characteristics of the obtained distributed photovoltaic equivalent model of the distribution network.

[0136] Example:

[0137] In the IEEE33 distribution network model, reliability verification of broadband oscillation based on equivalent model analysis is carried out. That is, the transient characteristics of the model before and after the equivalent model are guaranteed to be consistent. Stability analysis is performed under two different main grid conditions: normal grid and weak grid.

[0138] See also Figure 4 The present invention discloses a detailed model of IEEE33 node distribution network containing distributed photovoltaic clusters; according to the actual situation of the distribution network, distributed photovoltaic with appropriate capacity is added to some nodes, and the related parameters of distributed photovoltaic are shown in Table 1. There are 9 distributed photovoltaics and 32 loads in the model, with total capacities of 1.08MVA and (3.715+j2.3)MVA respectively.

[0139] Table 1 Distributed photovoltaic parameters

[0140]

[0141] According to the method disclosed in the present invention, the above 9 distributed photovoltaics are equivalent, and the equivalent results include 5 equivalent power sources and 1 equivalent load, as well as the equivalent grid between the access points of the two and the PCC.

[0142] In order to verify the reliability of the research based on the equivalent model, a three-phase fault module of Simulink is set outside the distribution network system under study. A three-phase short circuit fault occurs somewhere on the line at 0.5s, and the protection action clears the fault at 0.6s. The simulation dynamic change process of the electrical quantity of the detailed model and the equivalent model at the common connection point PCC with the upper power grid under fault disturbance is shown in the following figure. Figure 5 (a) and Figure 5 (b) as shown.

[0143] Figure 5 (a) and Figure 5 (b) shows the transient voltage and current response curves of the detailed model and the equivalent model respectively. When the short-circuit fault does not occur, the distribution network is in a steady-state operation process. The PCC voltage and current of the equivalent model are highly consistent with the detailed model, and can accurately reflect the steady-state operation of the voltage and current at the PCC of the distribution network and the upper power grid. When a permanent fault occurs outside the distribution network, the distribution network is disturbed by the fault, and the PCC shows voltage drop and current oscillation. The equivalent model comprehensively considers the electrical distance between the distributed photovoltaic and PCC and the dynamic characteristics of the unit, and has a high simulation accuracy under fault conditions. In summary, the proposed equivalent model can accurately reflect the response of the distribution network containing large-scale distributed photovoltaics under normal and fault conditions, verifying the reliability of the equivalent model.

[0144] The reliability verification of the equivalent model was completed, and the feasibility of conducting a study on broadband oscillation of distributed photovoltaic power distribution networks based on the equivalent model was clarified, which effectively improved the research efficiency of broadband oscillation. The same distribution network model was used to analyze the stability of broadband oscillation.

[0145] In order to comprehensively analyze the impact of different strength AC main grid systems on the broadband oscillation of the distribution network, the concept of system short-circuit ratio is introduced, that is, the short-circuit ratio SCR is the short-circuit capacity S of the main grid. ac With the distribution network rated power S N Ratio:

[0146]

[0147] The strength of different AC main grid systems is distinguished according to the size of SCR, and stability analysis is performed under two different main grid conditions: access to a normal grid and a weak grid.

[0148] (1) Connect to normal AC grid (SCR is larger)

[0149] The normal AC grid SCR is set to 3.7, and the AC grid impedance per unit value is j0.773. At the grid connection point in the area where the distributed photovoltaic cluster is located, a controllable voltage harmonic source with an amplitude of 15V is set. The frequency of the voltage source is set in a logarithmic interval from 1 to 10000Hz, and the current signal of the model response is measured. The impedance characteristics of the equivalent model and the detailed model at the common connection point PCC are calculated by the two signals, as shown in Figure 6 , Figure 7 and Figure 8 Shown

[0150] Firstly, the frequency sweep curves of each model under normal AC power grid connection are analyzed; Figure 6 , Figure 7 and Figure 8 The system Nyquist curve of each model and the relationship between the amplitude, phase and grid frequency of each model are shown respectively.

[0151] The system Nyquist curve of the equivalent model is obtained by the frequency sweep method. Compared with the system Nyquist curve of the detailed model, Figure 6 It can be seen that the proposed equivalent model can fit the detailed model curve well, verifying the reliability of the study of broadband oscillation based on the equivalent model and the accuracy of the power system stability judgment. When the IEEE33 distribution network model is connected to the normal power grid, the system Nyquist curve does not pass through the (-1, j0) point, which shows that there is no broadband oscillation risk in the system.

[0152] At the same time, the impedance characteristics of the equivalent model and the detailed model at the grid connection point are compared. The comparison curves are shown in Figure 7 and Figure 8The equivalent model can well reflect the impedance characteristics of the detailed model, and also verifies the reliability of studying broadband oscillation based on the equivalent model and the accuracy of oscillation frequency positioning.

[0153] (2) Access to weak AC grid (small SCR)

[0154] Analogous to the case of access to a normal AC power grid, frequency sweep experiments of each model were also carried out simultaneously when connected to a weak AC power grid. The SCR of the weak AC power grid during a fault is set to 0.6, and the per-unit value of the AC power grid impedance is j4.765. At the grid connection point in the area where the distributed photovoltaic cluster is located, a controllable voltage harmonic source with an amplitude of 15V is set. The frequency of the voltage source is set in a logarithmic interval from 1 to 10000Hz, and the current signal of the model response is measured. The impedance characteristics of the equivalent model and the detailed model at the common connection point PCC are calculated through the two signals. Combined with the experimental result curve, it can be seen that the proposed equivalent method has high accuracy in different scenarios.

[0155] When connected to a weak AC grid, a wide-band oscillation analysis is performed based on the sweep frequency curve of the equivalent model. Fig. 9 , Fig.10 and Fig.11 The Nyquist curve of the equivalent model and the relationship between the amplitude, phase and grid frequency of the equivalent model at the grid connection point are shown respectively. The system Nyquist curve of the equivalent model is obtained by the sweep frequency method. Fig. 9 It can be seen that the curve passes through the point (-1, j0), which shows that the system has the risk of broadband oscillation; the impedance characteristic curve of the equivalent model at the grid connection point is shown in Fig.10 and Fig.11 As shown in the figure, in some frequency bands between 100 Hz and 1000 Hz, the impedance of the distribution network is capacitive, and the impedance of the distribution network intersects with the impedance of the AC grid at an amplitude of 705 Hz. At this time, the phase stability margin is 8.72°. For the IEEE33 system, the stability margin is insufficient, and there is an oscillation risk, and 705 Hz oscillation will occur.

[0156] In order to verify the accuracy of the above analysis, a time domain simulation of the oscillation risk in this scenario was carried out. Harmonics of various frequency bands were added to the distribution network side. When the harmonic frequency was 705Hz, the oscillation was more obvious. The oscillation waveform and FFT results are shown in the figure below. Fig.12 As shown. Fig.12 (a) It can be seen that the grid-connected current waveform oscillates significantly. The FFT results are as follows Fig.12 As shown in (b), it can be seen that 705Hz oscillation will occur when connected to a weak AC power grid. The simulation results are consistent with the frequency domain analysis, which verifies the correctness of the broadband oscillation risk analysis based on the equivalent model.

[0157] In summary, the proposed equivalent model can accurately reflect the impedance characteristics of the distribution network with large-scale distributed photovoltaic access under two different main grid conditions. The broadband oscillation analysis based on the equivalent model is also consistent with the simulation results of the detailed model.

[0158] The above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the same. Although the present invention has been described in detail with reference to the above embodiments, a person skilled in the art should understand that the specific implementation modes of the present invention can still be modified or replaced by equivalents, and any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be included in the scope of protection of the claims of the present invention.

Claims

1. A method for analyzing the risk of broadband oscillation in distributed photovoltaic grid-connected power system, characterized in that: include: Based on the principle of the same unit model, each distributed photovoltaic is initially grouped; And calculate the electrical distance between each distributed photovoltaic node and the common connection point PCC; Based on the K-means clustering algorithm and electrical distance, the distributed photovoltaics of the same type after the initial grouping are divided again to form a distributed photovoltaic cluster division result that comprehensively considers dynamic characteristics and electrical distance; Based on the principles of "model consistency" and "power conservation", distributed photovoltaics are aggregated to build an equivalent model for distributed photovoltaic aggregation in the distribution network; based on the REI equivalent method and the WARD equivalent method, an equivalent network for the distribution network grid is built; Based on the distributed photovoltaic cluster division results, the distribution network distributed photovoltaic aggregation equivalent model and the distribution network grid equivalent network, the distribution network distributed photovoltaic equivalent model is obtained; Based on the harmonic voltage injection method, the distributed photovoltaic equivalent model of the distribution network is swept to obtain its impedance characteristics; Equivalence the AC port of the distributed photovoltaic cluster and the main grid at the grid connection point of the distributed photovoltaic equivalent model of the distribution network, and construct an equivalent model of the distributed photovoltaic cluster through the AC transmission system port; Based on the equivalent model of the system port of the distributed photovoltaic cluster via AC transmission and the impedance characteristics of the obtained distributed photovoltaic equivalent model of the distribution network, the stability of the equivalent model of the system port of the distributed photovoltaic cluster via AC transmission is evaluated.

2. The method for analyzing the risk of broadband oscillation in distributed photovoltaic grid-connected power system according to claim 1, characterized in that: The electrical distance between each distributed photovoltaic node and the common connection point PCC is calculated as follows: the electrical distance between each distributed photovoltaic node and the common connection point PCC is characterized based on the transfer impedance, specifically: Among them, I k Indicates the AC component value of the short-circuit current at point k, U i represents the voltage of the i-th distributed power source, Z ik represents the transfer impedance between the ith distributed generation node and the short-circuit point k, and n is the number of distributed photovoltaics in the cluster.

3. The method for analyzing the risk of broadband oscillation in distributed photovoltaic grid-connected power system according to claim 2, characterized in that: The distributed photovoltaics are aggregated based on the principles of "model consistency" and "power conservation" to construct an equivalent model for the distributed photovoltaic aggregation of the distribution network. Specifically, the distributed photovoltaics of the same model are aggregated to establish an equivalent unit model, and the power at the public connection point PCC of the distribution network and the upper power grid is ensured to be consistent before and after the aggregation of the distributed photovoltaic units, that is, the following conditions are met: Among them, P eq,k is the equivalent active power corresponding to cluster k; P i is the active power of the i-th distributed photovoltaic in cluster k; Q eq,k is the equivalent reactive power corresponding to cluster k; Q i is the reactive power of the i-th distributed photovoltaic unit in cluster k.

4. The method for analyzing the risk of broadband oscillation in distributed photovoltaic grid-connected power system according to claim 3, characterized in that: The construction of the distribution network grid equivalent network based on the REI equivalent method and the WARD equivalent method is specifically as follows: the construction of the equivalent network between the equivalent units / loads and the original distribution network distributed photovoltaic access points based on the REI equivalent method is specifically as follows: Among them, y u is the trunk admittance, y v is the branch admittance, U u is the voltage at the equivalent access point, U v is the voltage at the original access point, P eq is the equivalent power, P v is the power of the original access point; when performing load equalization, the flow direction of the load power is opposite to that of the power supply, and the positive and negative signs of the power need to be changed; Based on the WARD equivalent method, the redundant nodes other than the equivalent units / loads and the common connection point PCC are eliminated to complete the grid equivalent and obtain the simplified node admittance matrix: Among them, subscript B is a boundary node, subscript I is an internal node, subscript E is an external node, matrix Y represents the node admittance matrix, matrix U represents the node voltage matrix, and matrix I represents the node current matrix.

5. The method for analyzing the risk of broadband oscillation in distributed photovoltaic grid-connected power system according to claim 4, characterized in that: The method based on harmonic voltage injection is used to sweep the frequency of the distributed photovoltaic equivalent model of the distribution network to obtain its impedance characteristics, specifically: injecting a harmonic voltage source from the main grid side to the distribution grid side; collecting phase voltage and current signals, and performing positive and negative sequence separation and Fourier decomposition processing, for the injected harmonic voltage frequency, dividing the voltage amplitude by the current amplitude to obtain the impedance amplitude, subtracting the current phase from the voltage phase to obtain the impedance phase, changing the injected harmonic voltage frequency, and repeating the above steps until the required frequency range is swept; wherein, the amplitude of the harmonic voltage source should be greater than 1% of the rated voltage of the grid connection point and less than 5% of the rated voltage of the grid connection point.

6. The method for analyzing the risk of broadband oscillation in distributed photovoltaic grid-connected power system according to claim 5, characterized in that: The distributed photovoltaic cluster AC port of the distribution network distributed photovoltaic equivalent model and the main grid at the grid connection point are equivalent to construct an equivalent model of the distributed photovoltaic cluster through AC transmission system port, specifically: the distributed photovoltaic in the distribution network is usually in the maximum power point tracking mode MPPT or follows the dispatch instruction in the active / reactive power control mode; in this mode, the distributed photovoltaic cluster AC port presents a current source characteristic and also has an output impedance; the distribution network AC port is equivalent to an ideal current source I RE Its port impedance Z RE In parallel, the main grid at the grid connection point of the equivalent model is equivalent to an ideal voltage source V g and AC grid impedance Z g Series; AC grid impedance Z g Connect an ideal current source I RE The positive pole of the ideal voltage source V g Connect an ideal current source I RE The negative electrode; The voltage equation of the distribution network output node is: Among them, V out is the voltage at the distribution network output point, I out Output current for the distribution network.

7. The method for analyzing the risk of broadband oscillation in distributed photovoltaic grid-connected power system according to claim 6, characterized in that: The evaluation of the stability of the equivalent model of the distributed photovoltaic cluster through the AC transmission system port includes: based on the equivalent model of the distributed photovoltaic cluster through the AC transmission system port, obtaining the Nyquist curve, and then determining the stability of the equivalent model of the distributed photovoltaic cluster through the AC transmission system port, specifically: According to formula (5), the distribution network output current I out for: The right side of the equal sign can be regarded as the multiplication of two closed-loop transfer functions, which are respectively expressed as: The stability of the equivalent model AC transmission system is determined by the closed-loop transfer functions F1 and F2: the system is stable if and only if the poles of F1 and F2 are both located in the left half plane of s, that is: When all poles of F1 are located in the left half plane of s, the system stability depends on F2, that is, the AC grid impedance Z g With the equivalent model port impedance Z RE Impedance ratio Z g / Z RE Satisfy the Nyquist criterion; Z=PN=0 (9) Where P is the impedance ratio Z g / Z RE The number of poles in the right half plane of s, N is the impedance ratio Z g / Z RE The number of times the trajectory circles the point (-1, j0) in the counterclockwise direction; Z is the index of the Nyquist criterion; when Z = 0, it means that the function satisfies the Nyquist criterion, and the power system is stable.

8. The method for analyzing the risk of broadband oscillation in distributed photovoltaic grid-connected power system according to claim 7, characterized in that: The evaluation of the stability of the equivalent model of the distributed photovoltaic cluster through the AC transmission system port also includes: obtaining the Bode diagram of the power system based on the impedance characteristics of the distributed photovoltaic equivalent model of the distribution network obtained, and then determining the stability of the equivalent model of the distributed photovoltaic cluster through the AC transmission system port, specifically: In the Bode diagram of the power system, the equivalent model and the AC grid impedance characteristic amplitude curve intersect, that is: |Z RE (f0)|=|Z g (f0)| (10) Among them, Z RE is the equivalent model impedance, Z g is the AC grid impedance, f0 is the frequency when the two amplitudes are equal; the stability is determined based on the sum of the two phases corresponding to the frequency; γ=|∠Z RE (f0)+∠Z g (f0)| (11) Determine whether the phase stability margin r is greater than the preset threshold. If it is greater, there is no oscillation risk; if it is less, a series resonant circuit is formed and there is an oscillation risk. Use the phase stability margin r to verify whether the risk assessment is correct, that is: Where γ0 is the stability margin required for stable operation of the system. When the above conditions are met, the risk of broadband oscillation is relatively high.

9. A distributed photovoltaic grid-connected broadband oscillation risk analysis system, characterized in that: include: A first grouping module, wherein the first grouping module performs initial grouping of distributed photovoltaics based on the principle of the same unit model; And calculate the electrical distance between each distributed photovoltaic node and the common connection point PCC; A second division module, which re-divides the distributed photovoltaics of the same type after the initial grouping based on the K-means clustering algorithm and the electrical distance to form a distributed photovoltaic cluster division result that comprehensively considers the dynamic characteristics and the electrical distance; Aggregation module, which aggregates distributed photovoltaics based on the principles of "model consistency" and "power conservation" to build an equivalent model for distributed photovoltaic aggregation in the distribution network; and builds an equivalent network for the distribution network grid based on the REI equivalent method and the WARD equivalent method; A first acquisition module, which acquires a distributed photovoltaic equivalent model of the distribution network based on the distributed photovoltaic cluster division result, the distributed photovoltaic aggregation equivalent model of the distribution network and the distribution network grid equivalent network; A second acquisition module, wherein the second acquisition module performs a frequency sweep on a distributed photovoltaic equivalent model of the distribution network based on a harmonic voltage injection method to obtain its impedance characteristics; A construction module, wherein the construction module performs equivalence between the AC port of the distributed photovoltaic cluster of the distribution network distributed photovoltaic equivalent model and the main network at the grid connection point, and constructs an equivalent model of the distributed photovoltaic cluster through the AC transmission system port; An evaluation module is provided, wherein the evaluation module evaluates the stability of the equivalent model of the distributed photovoltaic cluster via an AC transmission system port based on the equivalent model of the distributed photovoltaic cluster via an AC transmission system port and based on the impedance characteristics of the obtained distributed photovoltaic equivalent model of the distribution network.

Images