A photovoltaic power source equivalent method and device without reactive power support capability suitable for medium voltage distribution network fault analysis

Abstract

The present invention provides a photovoltaic power source equivalent method and device suitable for medium voltage distribution network fault analysis without reactive support capability, the method comprising: step 1: determining the overcurrent coefficient k by comparing the maximum output current of the inverter with the rated output current; step 2: determining the overall equivalent relative error D1 by using the overcurrent coefficient k; step 3: determining the equivalent relative error upper limit D2 according to actual needs; step 4: comparing the size relationship between the overall equivalent relative error D1 and the equivalent relative error upper limit D2, judging whether the equivalent method adopts overall equivalence or segmented equivalence; step 5: calculating the equivalent circuit parameters of the photovoltaic power source according to the judgment result of step 4. The present invention proposes two equivalent methods, overall and segmented equivalence, for photovoltaic power sources without reactive support capability, and adopts the best consistent approximation method to obtain the best equivalent voltage and equivalent resistance parameters, with higher simulation accuracy, without the need for complex optimization methods, and ensuring the simulation effect or expected errors.

Description

Technical Field

[0001] The present invention relates to the technical field of photovoltaic power generation, and in particular to a photovoltaic power source equivalent method and device without reactive power support capability applicable to medium voltage distribution network fault analysis. Background Art

[0002] As the density and capacity of new energy power stations such as photovoltaic power generation systems increase, the traditional distribution network that operates in a radial manner has gradually developed into an active distribution network powered by multiple power sources, and its operation mode and characteristics have changed greatly. Many experts and scholars have analyzed the output characteristics of photovoltaic power stations and established their equivalent models, especially the output characteristics and equivalent models of photovoltaic power stations after distribution network failures.

[0003] Due to the requirements of low voltage ride-through, photovoltaic equipment needs to have a certain reactive power output capability during low voltage ride-through. Therefore, the existing research on fault analysis of inverter-type power supply connected to the grid mainly focuses on photovoltaic power supply with reactive power output capability. However, in reality, there is still a certain proportion of photovoltaic grid-connected equipment that does not have reactive power output capability, and its equivalent model is still worth studying.

[0004] Working characteristics of photovoltaic power sources without reactive power support capability:

[0005] This type of photovoltaic power source will not output reactive current during the low voltage ride-through period after a fault occurs. It only protects the inverter by adjusting the output of the photovoltaic array to maintain the input and output balance of the inverter or using a crowbar circuit to limit the output of the photovoltaic array. The inverter has no special working strategy during the fault period and is mainly limited by its maximum output current. The constraints can be described as follows:

[0006]

[0007] Among them, P p is the maximum power generated by the PV array when a fault occurs, I pcc , U pcc With P pcc are the voltage, current and power at the grid connection point, I m is the maximum current that the inverter can output, I e is the rated current of the inverter output normally, k is I m with I e The relationship coefficient between .

[0008] According to the above limiting conditions, the rated value can be used as a reference to plot the per-unit value of the output current and power of the photovoltaic power source without reactive power support capability as a function of the grid connection point voltage, such as Figure 1 shown.

[0009] It can be seen that after the fault occurs, as the voltage at the grid connection point decreases, within a certain range slightly below the rated voltage, the inverter output power is limited by the maximum power that the photovoltaic array can generate, so that the output of the photovoltaic power source is reflected as a constant power source. At this time, the current is inversely proportional to the voltage, and the power remains constant. Obviously, there is a critical voltage U r , and below U r After that, as the voltage further decreases, the inverter output current reaches the upper limit of what the inverter can output, so that the photovoltaic power supply output is reflected as a constant current source. At this time, the current remains constant, and the power is proportional to the voltage.

[0010] The critical voltage U can be calculated r for:

[0011]

[0012] The following equation can be used to describe the process of current changing with voltage:

[0013]

[0014] In the early research of photovoltaic equivalent models, the inverter power supply is usually directly equivalent to a constant voltage source or a constant current source. Some scholars believe that when there are fewer inverter power supplies connected to the power system and the penetration rate is low, the inverter power supply does not contribute much to the total short-circuit current, and the short-circuit current provided by the inverter power supply can be approximately set to twice its rated current; then the nonlinear characteristics of the inverter power supply output short-circuit current and its terminal voltage are further considered, and it is equivalent to a constant voltage source model in series with a variable equivalent impedance, but the calculation method of the equivalent impedance is not given. The above studies have ignored the impact of low voltage ride-through on the output current of photovoltaic power supplies, and cannot clarify the actual simulation effect of the equivalent model.

[0015] Therefore, for photovoltaic power sources that do not have reactive power output capability, studying the establishment method of equivalent fault analysis models with universal applicability is an important basic work in the fault research of photovoltaic power sources connected to distribution networks.

[0016] Existing equivalent methods either use ideal voltage sources and current sources directly for equivalent, which is simple, has poor simulation effects, and does not take into account the specific changing characteristics of photovoltaic power sources; or adopt a more complex equivalent form, and the resulting model can achieve a more accurate simulation effect, but the modeling or parameter calculation process is complicated. Summary of the invention

[0017] In view of the current lack of equivalent simulation of photovoltaic power sources without reactive power support capabilities, the present invention proposes a linear equivalent method, determines whether to use overall equivalence or segmented equivalence according to requirements, and gives the corresponding equivalent model and parameter calculation method, making up for the current insufficient research on photovoltaic power sources without reactive power support capabilities.

[0018] An equivalent method for photovoltaic power sources without reactive power support capabilities applicable to medium-voltage distribution network fault analysis includes the following steps

[0019] Step 1: Determine the overcurrent coefficient k by taking the ratio of the maximum output current of the inverter to the rated output current;

[0020] Step 2: Determine the overall equivalent relative error D1 using the overcurrent coefficient k;

[0021] Step 3: Determine the upper limit D2 of the equivalent relative error according to actual requirements;

[0022] Step 4: Compare the magnitude relationship between the overall equivalent relative error D1 and the upper limit D2 of the equivalent relative error, and determine whether to use overall equivalence or segmented equivalence for the equivalent method;

[0023] Step 5: Calculate the equivalent circuit parameters of the photovoltaic power source according to the judgment result of Step 4.

[0024] Further, the expression of the overcurrent coefficient k of the photovoltaic power source in Step 1 is:

[0025]

[0026] where, I m is the maximum output current of the inverter, I e is the rated current of the inverter, and k is the proportional coefficient between I m and I e , that is, the overcurrent coefficient.

[0027] Further, in Step 4, compare the magnitude relationship between the overall equivalent relative error D1 and the upper limit D2 of the equivalent relative error, and determine whether to use overall equivalence or segmented equivalence for the equivalent method. Specifically: if D1 < D2, then use overall equivalence, otherwise perform segmented equivalence.

[0028] Further, in Step 5, calculate the equivalent circuit parameters of the photovoltaic power source according to the judgment result of Step 4, specifically including:

[0029] If the judgment result in Step 4 is overall equivalence, calculate the intermediate parameters A and B as:

[0030]

[0031] where, U e is the rated voltage of the photovoltaic power source;

[0032] Calculate the equivalent resistance R eq With equivalent voltage U eq The parameters are:

[0033]

[0034] The mathematical model for determining the overall equivalent circuit of the photovoltaic power source is:

[0035]

[0036] Among them, U pcc ,I pcc They are the equivalent output voltage and current of the photovoltaic power source respectively;

[0037] If the judgment result in step 4 is segmented equivalent, calculate the critical voltage U r :

[0038]

[0039] The formula for calculating the intermediate parameter C is:

[0040]

[0041] Calculate the equivalent resistance R eq With equivalent voltage U eq The parameters are:

[0042]

[0043] The mathematical model for determining the equivalent circuit of the photovoltaic power supply segment is:

[0044]

[0045] A photovoltaic power source equivalent device without reactive power support capability suitable for medium voltage distribution network fault analysis, comprising:

[0046] An overcurrent coefficient calculation module is used to determine the overcurrent coefficient k by comparing the maximum output current of the inverter with the rated output current;

[0047] An overall equivalent relative error determination module, used to determine the overall equivalent relative error D1 using the flow coefficient k;

[0048] The overall equivalent relative error determination module is used to determine the equivalent relative error upper limit D2 according to actual needs;

[0049] The equivalent method determination module is used to compare the size relationship between the overall equivalent relative error D1 and the equivalent relative error upper limit D2, and determine whether the equivalent method adopts overall equivalence or segmented equivalence;

[0050] The equivalent circuit parameter calculation module is used to calculate the equivalent circuit parameters of the photovoltaic power source according to the judgment result of the module determined by the equivalent method.

[0051] Furthermore, the overcurrent coefficient calculation module uses the overcurrent coefficient k to determine the overall equivalent relative error D1, and the calculation expression is:

[0052]

[0053] where I m is the maximum output current of the inverter, I e is the rated current of the inverter, and k is the proportional coefficient between I m and I e , that is, the overcurrent coefficient.

[0054] Furthermore, the equivalent method determination module compares the magnitude relationship between the overall equivalent relative error D1 and the equivalent relative error upper limit D2, and determines whether to adopt overall equivalence or segmented equivalence for the equivalent method. Specifically: if D1 < D2, overall equivalence is adopted; otherwise, segmented equivalence is performed.

[0055] Furthermore, the equivalent circuit parameter calculation module calculates the equivalent circuit parameters of the photovoltaic power source according to the judgment result of the equivalent method determination module, specifically including:

[0056] If the judgment result of the equivalent method determination module is overall equivalence, calculate the intermediate parameters A and B as:

[0057]

[0058] where U e is the rated voltage of the photovoltaic power source;

[0059] Calculate the equivalent resistance R eq and the equivalent voltage U eq parameters as:

[0060]

[0061] Determine the mathematical model of the overall equivalent circuit of the photovoltaic power source as:

[0062]

[0063] where U pcc , I pcc are the output voltage and current equivalent to the photovoltaic power source respectively;

[0064] If the judgment result of the equivalent method determination module is segmented equivalence, calculate the critical voltage U r :

[0065]

[0066] The formula for calculating the intermediate parameter C is:

[0067]

[0068] Calculate the equivalent resistance R eq With equivalent voltage U eq The parameters are:

[0069]

[0070] The mathematical model for determining the equivalent circuit of the photovoltaic power supply segment is:

[0071]

[0072] The present invention establishes an equivalent model of a photovoltaic power source without reactive support capability, analyzes the low voltage ride-through characteristics of the photovoltaic power source without reactive support capability during low voltage ride-through during different fault periods, and provides a calculation method for the best equivalent parameters in combination with the output characteristics of the equivalent circuit, and conducts a theoretical analysis of the equivalent relative error. The present invention proposes two equivalent methods for the photovoltaic power source without reactive support capability, namely, overall and segmented equivalence, and adopts the best consistent approximation method to obtain the best equivalent voltage and equivalent resistance parameters. The simulation accuracy is higher, and no complex optimization method is required, thereby ensuring the simulation effect or the expected error. BRIEF DESCRIPTION OF THE DRAWINGS

[0073] Figure 1 It is the power-current per unit value curve of the photovoltaic power grid connection point;

[0074] Figure 2 It is a flow chart of a photovoltaic power source equivalent method without reactive power support capability applicable to medium voltage distribution network fault analysis of the present invention;

[0075] Figure 3 is the basic characteristic of the equivalent model;

[0076] Figure 4 is a curve showing the variation of relative error of a special point of the embodiment of the present invention with k;

[0077] Figure 5 is a curve showing the variation of relative error with k at the maximum error value in the embodiment of the present invention;

[0078] Figure 6 The 400kW photovoltaic power generation system of the embodiment of the present invention is connected to a 10kV distribution network;

[0079] Figure 7 This is a waveform diagram of the output of the equivalent power supply connected to the distribution network according to an embodiment of the present invention. DETAILED DESCRIPTION

[0080] To make the objectives, 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 with reference to the accompanying drawings in the embodiments of the present invention. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0081] Please refer to Figure 2 , the embodiments of the present invention provide an equivalent method for a photovoltaic power source without reactive power support applicable to medium-voltage distribution network fault analysis. The method includes the following steps:

[0082] Step 1: Determine the overcurrent coefficient k by dividing the maximum output current of the inverter by the rated output current;

[0083] The expression of the overcurrent coefficient k of the photovoltaic power source in Step 1 is:

[0084]

[0085] Where, I m is the maximum output current of the inverter, I e is the rated current of the inverter, and k is the proportional coefficient between I m and I e , that is, the overcurrent coefficient.

[0086] Step 2: Determine the overall equivalent relative error D1 using the overcurrent coefficient k;

[0087] Step 3: Determine the upper limit D2 of the equivalent relative error according to actual requirements;

[0088] Step 4: Compare the magnitude relationship between the overall equivalent relative error D1 and the upper limit D2 of the equivalent relative error, and determine whether to use overall equivalence or segmented equivalence for the equivalent method. Specifically, if D1 < D2, overall equivalence is adopted; otherwise, segmented equivalence is performed.

[0089] Step 5: Calculate the equivalent circuit parameters of the photovoltaic power source according to the judgment result of Step 4;

[0090] Specifically, if the judgment result in Step 4 is overall equivalence, calculate the intermediate parameters A and B as:

[0091]

[0092] Where, U e is the rated voltage of the photovoltaic power source;

[0093] Calculate the equivalent resistance R eq and the equivalent voltage U eq parameters as:

[0094]

[0095] The mathematical model for determining the overall equivalent circuit of the photovoltaic power source is:

[0096]

[0097] Among them, U pcc ,I pcc They are the equivalent output voltage and current of the photovoltaic power source respectively;

[0098] If the judgment result in step 4 is segmented equivalent, calculate the critical voltage U r :

[0099]

[0100] The formula for calculating the intermediate parameter C is:

[0101]

[0102] Calculate the equivalent resistance R eq With equivalent voltage U eq The parameters are:

[0103]

[0104] The mathematical model for determining the equivalent circuit of the photovoltaic power supply segment is:

[0105]

[0106] The key technologies of the present invention mainly involve the output characteristics of the equivalent circuit, the calculation method of the optimal equivalent parameters and the theoretical analysis of the equivalent relative error, which are described in detail below.

[0107] Photovoltaic array mathematical model:

[0108] In order to facilitate the analysis of distribution network faults, it is hoped that a linear circuit can be used to equivalent the distributed power source.

[0109] The basic form of the equivalent circuit is as follows Figure 3 As shown, the relationship between the output voltage and current in this circuit can be written as:

[0110]

[0111] It can be seen from the expression that the output current I pcc With voltage U pcc There is a negative correlation between them. Its form is a straight line with a constant negative derivative in the volt-ampere characteristic, and its intersection with the current axis is (0,U eq / R eq ), and the intersection with the voltage axis is (Ueq ,0).

[0112] The basic characteristics of the equivalent model are compared with the characteristics of the above two types of photovoltaics to determine the differences, thereby ensuring the simulation effect or expected errors.

[0113] It can be seen from formula (3) that the derivative of the photovoltaic power source without reactive power support capability can always be greater than or equal to 0 as a whole, and the overall trend is similar to formula (9). Therefore, if the error is allowed, the equivalent circuit can realize the equivalent simulation of this type of photovoltaic power source; if the analysis is performed in sections, when the grid-connected point voltage is greater than the critical voltage, the equivalent circuit can be used for equivalent, and when the grid-connected point voltage is less than the rated voltage, the constant current can be used for simulation equivalent.

[0114] For photovoltaic power sources without reactive power support capability, only reactive power output is available. Figure 3 The equivalent circuit shown is equivalent to the photovoltaic power source.

[0115] a) Overall equivalence

[0116] When the photovoltaic power source of this type is equivalent as a whole, the square of the difference between the equivalent model and the photovoltaic power source without reactive power support capability can be written as:

[0117]

[0118] And integrating the above formula, we can get

[0119]

[0120] To find its minimum point, we have

[0121]

[0122] Solving equation (12), we can get

[0123]

[0124] Then, the best consistent equivalent resistance and equivalent voltage are obtained through formula (5).

[0125] It can be seen from the above results that if the rated parameters remain unchanged, the optimal parameters are only related to the flow coefficient k.

[0126] Obviously, the maximum error between the equivalent circuit and the volt-ampere characteristic of the photovoltaic power source will appear in U pcc The value is 0, U r and U e The relative error formula of each point can be written as follows:

[0127]

[0128]

[0129]

[0130] The overcurrent capacity of photovoltaic power supply is very limited, and the maximum overcurrent coefficient k is generally not more than 2. The relative error of each point can be plotted as k changes, such as Figure 4 As shown:

[0131] For the common grid-following photovoltaic inverters in the market, under the premise of meeting the relevant standards and ensuring low costs, the k value is generally only 1.1, such as Figure 4 As shown in , the maximum absolute value of the relative error among the three curves is U ppc =U e The maximum relative error is 8.2% at U, but when k exceeds 1.13, the maximum relative error will also exceed 10%. The absolute values ​​of the maximum relative errors of the three curves are always lower than 20%, and when k exceeds 1.85, the maximum relative error absolute value is at U ppc =U r Place.

[0132] It can be seen from the above results that if the accuracy required for the analyzed problem is low, that is, when the absolute value of the relative error is required to be greater than 20%, the value of the overcurrent coefficient k can be ignored under general conditions; for the k value used by common photovoltaic power sources, that is, k = 1.1, or even power sources with lower overcurrent capacity, the overall equivalent method can generally be adopted.

[0133] For situations where the overcurrent coefficient is large and certain accuracy is required, it is necessary to perform equivalent on the volt-ampere characteristic curve in sections.

[0134] b) Segmental equivalence

[0135] For the section below the critical voltage, a constant current source can be used for equivalent, so only the rated voltage to critical voltage section needs to be analyzed. Further, considering that at the critical voltage, the equivalent curves at both ends need to remain continuous, the volt-ampere curve of the equivalent circuit needs to pass through (U r ,kI e ), then the expression of the equivalent model can be written as:

[0136] I pcc =C(U pcc -U r )+kI e U r <U pcc <U e (9)

[0137] It can be written that within this range, the square of the difference between the equivalent model and the photovoltaic power source without reactive support capability is:

[0138]

[0139] And integrate the above formula and express the result as follows:

[0140]

[0141] Furthermore, the parameter C with the minimum error is obtained by solving the following equation:

[0142]

[0143] Can get

[0144]

[0145] And can be obtained, in U pcc When the following values ​​are taken, the relative error will reach the maximum point:

[0146]

[0147] Using formulas (21) and (22), we can draw a curve showing the relative error at the maximum error value as a function of k: Figure 5 shown.

[0148] Depend on Figure 5 It can be seen that under normal conditions, the error caused by segmented equivalence will not be greater than 8%, and for photovoltaic power sources with generally low overcurrent coefficients (k less than 1.3), the error will be less than 1%.

[0149] The following is a detailed description using a specific example:

[0150] Take the example of a 400kW photovoltaic power generation system connected to a 10kV ungrounded system. Figure 6 As shown:

[0151] The power grid mainly includes the system power supply voltage U s , system voltage impedance Zs, Z M , Z1, Z2, Z N1 , Z N2 , Z N3 , Z N4The impedance of each section of the line, etc. The system has a total of 5 cable and overhead line hybrid lines, namely L1-L5, with line lengths of 13km, 4km, 4.5km, 4km, and 5km respectively. The load capacity of each line is 2MVA, of which line L1 is the fault line. The overhead line parameters use standard parameters: positive / negative sequence parameters: R1=0.12Ω / km, L1=1.02mH / km, C1=0.12μF / km; zero sequence parameters: R0=0.35Ω / km, L0=4.78mH / km, C0=0.004μF / km. Cable line parameters: positive / negative sequence parameters: R1=0.08Ω / km, L1=0.27mH / km, C1=0.47μF / km; zero sequence parameters: R0=0.8Ω / km, L0=0.96mH / km, C0=0.47μF / km. It is assumed that a three-phase short circuit fault occurs at the fault point F of line L1, which is 6 km away from the busbar, in the time period of 0.4-0.6s. The transition resistance of the grounding point is 10kΩ. A 400kW photovoltaic power source is connected to a 10kV ungrounded system. The rated voltage of the photovoltaic power source is U e =10.5kV, rated output current I e =21.99A, maximum output current I m =24.19A. Taking this as an example, the above system is used to verify the effectiveness of the method of the present invention. The specific implementation steps are as follows:

[0152] Step 1: Use the maximum output current I of the inverter m =24.19A and rated output current I e =21.99A as a comparison, determine the current coefficient k = 1.1;

[0153] Step 2: Determine the relative error of the equivalent power supply based on the overcurrent coefficient k, Figure 4 The maximum relative error is D1 = 8.2%;

[0154] Step 3: Determine the upper limit of the relative error of the equivalent power supply, assuming that the upper limit of the error is D2 = 15%;

[0155] Step 4: According to step 2 and step 3, D1 < D2, the relative error of the equivalent power supply is lower than the upper limit of the relative error, which meets the requirements, and the overall equivalent method is adopted for the photovoltaic power supply;

[0156] Step 5: Equivalent the photovoltaic power source and use formula (5) to obtain the intermediate parameters A and B:

[0157]

[0158] Substituting into formula (6), the optimal equivalent parameter U of the equivalent power supply is eq =440.8kV, R eq =10437.76Ω.

[0159] Model and simulate the connection of photovoltaic power sources and equivalent power sources without reactive power support capabilities to the 10kV distribution network, and analyze and record the voltage and output current at the grid connection point. Compare and analyze the output waveforms of photovoltaic power sources and equivalent power sources without reactive power support capabilities when they are connected to the distribution network and a fault occurs, such as Figure 7 shown.

[0160] Depend on Figure 7 It can be seen that:

[0161] For the grid connection point voltage, the effective value of the grid connection point voltage of the photovoltaic power source after the fault occurs is U pcc =270.89V, the voltage of the equivalent power supply grid connection point is U pcc =271.21V. Therefore, the grid voltage error is ΔU pcc =0.12%. For the output current, the effective value of the photovoltaic power supply output current is I pcc =25.81A, the effective value of the equivalent power supply output current is I pcc =24.52A, so the output current error of the photovoltaic power supply is ΔI pcc =4.99%. Both errors are within the error requirements and meet the equivalence of photovoltaic power sources.

[0162] The present invention has the following advantages:

[0163] (1) Linear equivalence: The present invention adopts linear analysis for the equivalence of photovoltaic power sources. The volt-ampere characteristic curve can be used to clearly and intuitively observe the relationship between the power and current at the photovoltaic power grid connection point, and the complex relationship can be simplified for analysis.

[0164] (2) Simple calculation: The introduction of intermediate parameters A, B, and C makes the parameters of equivalent resistance Req and equivalent voltage Ueq simpler and more efficient; and a mathematical model of the segmented equivalent circuit on both sides of the critical voltage Ur of the photovoltaic power source is given, which has strong applicability.

[0165] (3) Consider different accuracy requirements: If the accuracy required for the problem being analyzed is low, the overall equivalent method can generally be used; if the accuracy requirements are even higher, it is necessary to perform segmented equivalent on the photovoltaic power source. Therefore, the analysis of the applicable accuracy range of the equivalent method for the photovoltaic power source is more comprehensive.

[0166] (4) High reliability: Not only are the equivalent formulas of the overall equivalent model and the segmented equivalent model of the photovoltaic power source given, but the curves of the relative error of each special point changing with the overcurrent coefficient k are also drawn; the segmented analysis of the equivalent model and the analysis and calculation of the critical points also make the model more reliable.

[0167] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A photovoltaic power source equivalent method without reactive power support capability suitable for medium voltage distribution network fault analysis, characterized in that: Including the following steps Step 1: Determine the overcurrent coefficient k by taking the ratio of the maximum output current to the rated output current of the inverter; Step 2: Determine the overall equivalent relative error D1 using the overcurrent coefficient k; Step 3: Determine the upper limit D2 of the equivalent relative error according to the actual requirements; Step 4: Compare the magnitude relationship between the overall equivalent relative error D1 and the upper limit D2 of the equivalent relative error, and determine whether to adopt overall equivalence or segmented equivalence for the equivalent method; Step 5: Calculate the equivalent circuit parameters of the photovoltaic power source according to the judgment result of Step 4; Step 4 compares the magnitude relationship between the overall equivalent relative error D1 and the upper limit D2 of the equivalent relative error, and determines whether to adopt overall equivalence or segmented equivalence for the equivalent method. Specifically: if D1 < D2, overall equivalence is adopted; otherwise, segmented equivalence is performed; Step 5 calculates the equivalent circuit parameters of the photovoltaic power source according to the judgment result of Step 4, specifically including: If the judgment result in Step 4 is overall equivalence, calculate the intermediate parameters A and B as: Among them, U e is the rated voltage of the photovoltaic power source; Calculate the equivalent resistance R eq With equivalent voltage U eq The parameters are: Determine the mathematical model of the overall equivalent circuit of the photovoltaic power source as: Among them, U pcc ,I pcc They are the equivalent output voltage and current of the photovoltaic power source respectively; If the judgment result in step 4 is segmented equivalent, calculate the critical voltage U r : The formula for calculating the intermediate parameter C is: Calculate the equivalent resistance R eq With equivalent voltage U eq The parameters are: Determine the mathematical model of the segmented equivalent circuit of the photovoltaic power source as:

2. The photovoltaic power source equivalent method without reactive power support capability applicable to medium voltage distribution network fault analysis as claimed in claim 1, characterized in that: The expression of the overcurrent coefficient k of the photovoltaic power source in Step 1 is: Among them, I m is the maximum output current of the inverter, I e is the rated current of the inverter, k is I m with I e The proportional coefficient between them is the overcurrent coefficient.

3. A photovoltaic power source equivalent device without reactive power support capability suitable for medium voltage distribution network fault analysis, characterized in that: Including: An overcurrent coefficient calculation module for determining the overcurrent coefficient k by taking the ratio of the maximum output current to the rated output current of the inverter; An overall equivalent relative error determination module for determining the overall equivalent relative error D1 using the overcurrent coefficient k; An overall equivalent relative error determination module for determining the upper limit D2 of the equivalent relative error according to the actual requirements; An equivalent method determination module for comparing the magnitude relationship between the overall equivalent relative error D1 and the upper limit D2 of the equivalent relative error, and determining whether to adopt overall equivalence or segmented equivalence for the equivalent method; An equivalent circuit parameter calculation module for calculating the equivalent circuit parameters of the photovoltaic power source according to the judgment result of the equivalent method determination module; The equivalent method determination module compares the magnitude relationship between the overall equivalent relative error D1 and the upper limit D2 of the equivalent relative error, and determines whether to adopt overall equivalence or segmented equivalence for the equivalent method. Specifically: if D1 < D2, overall equivalence is adopted; otherwise, segmented equivalence is performed; The equivalent circuit parameter calculation module calculates the equivalent circuit parameters of the photovoltaic power source according to the judgment result of the equivalent method determination module, specifically including; If the judgment result of the equivalent method determination module is overall equivalence, calculate the intermediate parameters A and B as: Among them, U e is the rated voltage of the photovoltaic power source; Calculate the equivalent resistance R eq With equivalent voltage U eq The parameters are: Determine the mathematical model of the overall equivalent circuit of the photovoltaic power source as: Among them, U pcc ,I pcc They are the equivalent output voltage and current of the photovoltaic power source respectively; If the equivalent method determines that the module is segmented equivalent, calculate the critical voltage U r : The formula for calculating the intermediate parameter C is: Calculate the equivalent resistance R eq With equivalent voltage U eq The parameters are: Determine the mathematical model of the segmented equivalent circuit of the photovoltaic power source as:

4. The photovoltaic power source equivalent device without reactive power support capability suitable for medium voltage distribution network fault analysis as claimed in claim 3, characterized in that: The overcurrent coefficient calculation module determines the overall equivalent relative error D1 using the overcurrent coefficient k, and the calculation expression is: Among them, I m is the maximum output current of the inverter, I e is the rated current of the inverter, k is I m with I e The proportional coefficient between them is the overcurrent coefficient.

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