Calculation method of distributed photovoltaic access capacity considering distribution network protection adaptability

Abstract

The present invention proposes a distributed photovoltaic access capacity calculation method considering the adaptability of distribution network protection, including: obtaining the equivalent impedance of the outgoing line end looking at the distribution network system; calculating the maximum short-circuit current provided by the distribution network system side when the line end fails based on the equivalent impedance; calculating the current constant based on the maximum short-circuit current and considering the sensitivity of the overcurrent protection III stage; evaluating the maximum distributed photovoltaic capacity that can be accessed by the current line according to the current constant and the current limit multiple of the distributed photovoltaic connected to the distribution network. Through the implementation of the method proposed by the present invention, the maximum capacity distributed photovoltaic that can be accessed under the current working conditions is calculated, which can avoid the malfunction of the lower-level protection when a fault occurs in the upper-level network or parallel line of the distribution network.

Description

Technical Field

[0001] The present invention belongs to the technical field of distributed photovoltaic access capacity calculation, and in particular relates to a distributed photovoltaic access capacity calculation method considering the adaptability of distribution network protection. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] The new power system is mainly based on the extensive access of new energy represented by photovoltaics and wind power. After a large number of distributed photovoltaics are connected, the traditional distribution network has changed from a single-power radial network to a multi-terminal power network. At the same time, the power supply presents the characteristics of power electronics, which brings great challenges to the overcurrent protection of the traditional distribution network. There is a possibility that the traditional protection is not suitable. Under some special fault conditions, there is a risk of causing incorrect operation of the distribution network protection, which will expand the scope of power outages. Therefore, the large-scale access of distributed photovoltaics to the distribution network has brought unprecedented challenges to traditional relay protection.

[0004] According to Article 11.1 of the power grid company's enterprise standard "Q / GDW 10370-2016 Technical Guidelines for Distribution Networks", the installed capacity of distributed power sources that can be connected to a 10 kV voltage distribution network is 0.4 MW to 6 MW, and the installed capacity of distributed power sources that can be connected to a 35 kV grid distribution network is 6 MW to 20 MW. This standard defines the maximum capacity of distributed power sources that can be connected in terms of system stability, but lacks consideration of the impact on relay protection, and there is a risk that unreasonable access capacity will lead to the inadaptability of traditional protection.

[0005] In order to ensure the adaptability of distribution network protection action, a considerable number of literatures have proposed adding directional elements to traditional overcurrent protection to determine whether the fault occurs inside or outside the zone.

[0006] However, in actual power grid production, a considerable number of distribution network lines do not have the conditions to add voltage transformers (hereinafter referred to as "PT"), which results in some overcurrent protection not having the conditions to add directional protection. Therefore, when the upstream lines or parallel lines of the distribution network fail, there is a risk of causing the downstream distributed photovoltaic reverse supply short-circuit current, resulting in the risk of false operation of the downstream protection.

[0007] In addition, there is a patent application number: 201910712909.2 in the prior art, which proposes an evaluation method for the accessible capacity of distributed photovoltaic power sources, taking into account the calculation of the maximum distributed photovoltaic access capacity under extreme conditions of overcurrent I and overcurrent III, but only considering the case of providing 1.2 times the rated current during the transient short circuit of distributed photovoltaics, which lacks universality. The actual situation is that different distributed photovoltaic grid-connected controllers use different short-circuit current limiting strategies, generally 1.0 to 1.5 times the rated current is possible.

[0008] Therefore, how to calculate the distributed photovoltaic access capacity under the premise of ensuring the adaptability of distribution network protection is a technical problem to be solved by this application. Summary of the invention

[0009] In order to overcome the shortcomings of the above-mentioned prior art, the present invention provides a distributed photovoltaic access capacity calculation method considering the adaptability of distribution network protection, taking into account the traditional distribution network overcurrent protection setting calculation method, sensitivity and possible fault types and the characteristics of the distributed photovoltaic controller actually connected to the power grid. When the distribution network line does not have the conditions for installing PT, the maximum capacity distributed photovoltaic that can be connected under the current working conditions is calculated through the implementation of the method proposed in the present invention, which can avoid the false operation of the lower-level protection when a fault occurs in the upper-level network or parallel line of the distribution network.

[0010] To achieve the above objectives, one or more embodiments of the present invention provide the following technical solutions:

[0011] In the first aspect, a distributed photovoltaic access capacity calculation method considering the adaptability of distribution network protection is disclosed, including:

[0012] Obtain the equivalent impedance of the outgoing line terminal looking toward the distribution network system;

[0013] Based on the equivalent impedance, the maximum short-circuit current provided by the distribution network system side when a line end fault occurs is calculated;

[0014] Calculate the current setting based on the maximum short-circuit current and considering the sensitivity of overcurrent protection stage III;

[0015] The maximum distributed photovoltaic capacity that can be connected to the current line is evaluated based on the current setting value and the current limit multiple of the distributed photovoltaic connected to the distribution network.

[0016] As a further technical solution, before the distribution network line is connected to distributed photovoltaics, when a fault occurs at the end of the line, the short-circuit current provided by the system side is calculated by the ratio of the voltage per unit value to the equivalent impedance viewed from the end of the line to the grid side.

[0017] As a further technical solution, the current constant is calculated based on the maximum short-circuit current and considering the sensitivity of the overcurrent protection stage III. The specific formula is:

[0018] I=U p.u. / X p.u.

[0019] I zd(p.u.) =I / k

[0020]

[0021] Where I represents the short-circuit current flowing through the protection device; U p.u.Indicates the voltage per unit value, usually 1; X p.u. It represents the equivalent impedance from the end of the line to the grid side; k represents the sensitivity coefficient; I zd(p.u.) Indicates the nominal value of the current flowing through the protection device; I zd It represents the nominal value of the current flowing through the protection device, in kiloamperes; S represents the reference capacity, which is generally 100 or 1000 MVA; U represents the nominal value of the calculated network voltage, in kilovolts.

[0022] As a further technical solution, when the distribution network line does not meet the conditions for installing PT, the load of the distribution network line at this level is not considered. At the same time, the short-circuit current limiting characteristics of photovoltaic grid-connected controllers of different models are considered to calculate the short-circuit current that distributed photovoltaics can provide:

[0023] I pv zd / k pv

[0024] Among them, k pv Indicates the multiple of the short-circuit current that distributed photovoltaic can provide during transient period and the rated current, I zd Indicates the nominal value of the current flowing through the protective device.

[0025] As a further technical solution, the maximum capacity of distributed photovoltaics that can be connected is calculated as follows:

[0026]

[0027] Among them, S pv Indicates the maximum accessible capacity of distributed photovoltaic power generation, in megawatts, I pv It represents the short-circuit current that distributed photovoltaic can provide, and U represents the nominal value of the calculated network voltage.

[0028] As a further technical solution, the above method is applicable to calculating the distributed photovoltaic access capacity when planning distributed photovoltaic access in a 35 kV or 10 kV distribution network.

[0029] In the second aspect, a distributed photovoltaic access capacity calculation system considering the adaptability of distribution network protection is disclosed, including:

[0030] The equivalent impedance acquisition module is configured to: obtain the equivalent impedance of the outgoing line terminal looking toward the distribution network system;

[0031] The maximum short-circuit current calculation module is configured to: calculate the maximum short-circuit current provided by the distribution network system side when a line end fault occurs based on the equivalent impedance;

[0032] The current setting value calculation module is configured to: calculate the current setting value based on the maximum short-circuit current and taking into account the sensitivity of the overcurrent protection stage III; ​

[0033] The maximum distributed photovoltaic capacity calculation module is configured to evaluate the maximum distributed photovoltaic capacity that can be connected to the current line according to the current constant value.

[0034] As a further technical solution, the maximum short-circuit current calculation module, when calculating, before the distribution network line is connected to the distributed photovoltaic, when a fault occurs at the end of the line, the short-circuit current provided by the system side is calculated by the ratio of the voltage standard value to the equivalent impedance viewed from the end of the line to the grid side.

[0035] One or more of the above technical solutions have the following beneficial effects:

[0036] When the distribution network line does not meet the conditions for PT installation, the maximum accessible capacity of distributed photovoltaics within the current protection adaptation range is calculated by considering the calculation method, sensitivity and possible fault types of the traditional distribution network overcurrent protection setting. This is to prevent the lower-level distributed photovoltaics from reversely supplying short-circuit current when the upper-level line or parallel line of the distribution network fails, causing the lower-level protection to malfunction. Distributed photovoltaics are generally connected to 35 or 10 kV distribution networks, and centralized photovoltaics are connected to voltage levels above 110. The above method is applicable to the planning of distributed photovoltaic access in 35 kV or 10 kV distribution networks.

[0037] Advantages of additional aspects of the present invention will be given in part in the following description, and in part will become obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0039] Figure 1 This is a typical distribution network topology diagram of an embodiment of the present invention. DETAILED DESCRIPTION

[0040] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0041] It should be noted that the terms used herein are for describing specific embodiments only and are not intended to be limiting of exemplary embodiments according to the present invention.

[0042] In the absence of conflict, the embodiments of the present invention and the features of the embodiments may be combined with each other.

[0043] Embodiment 1

[0044] This embodiment discloses a method for calculating the access capacity of distributed photovoltaic power generation taking into account the adaptability of distribution network protection, including:

[0045] Obtain the equivalent impedance of the outgoing line terminal looking toward the distribution network system;

[0046] Based on the equivalent impedance, the maximum short-circuit current provided by the distribution network system side when a line end fault occurs is calculated;

[0047] Calculate the current setting based on the maximum short-circuit current and considering the sensitivity of overcurrent protection stage III;

[0048] The maximum distributed photovoltaic capacity that can be connected to the current line is evaluated based on the current setting value and the current limit multiple of the distributed photovoltaic connected to the distribution network.

[0049] At present, the protection adopted by the distribution network is basically three-stage overcurrent protection, among which overcurrent stage I is the instantaneous stage with the largest current setting value, which protects against metallic faults on the outlet side of the outgoing circuit breaker; overcurrent stage II is time-limited quick-break, which generally takes a time difference of 0.4 seconds with overcurrent stage I, and the current setting value is coordinated with branch or boundary protection; overcurrent stage III is timed quick-break, which generally takes a time difference of 0.2 seconds with overcurrent stage II, and the current setting value is the smallest in the three-stage protection, which protects against metallic faults at the end of the line.

[0050] In order to conservatively consider the maximum distributed photovoltaic capacity that can be connected, the smallest current setting value among the three-stage overcurrent protection is generally taken as the basis. The calculation method for the overcurrent protection setting value of the traditional distribution network protection is:

[0051] (1) Short-circuit current calculation

[0052] Before the distribution network line is connected to the distributed photovoltaic, when a fault occurs at the end of the line, the short-circuit current provided by the system side is

[0053] I=U p.u. / X p.u. (1)

[0054] Where I represents the short-circuit current flowing through the protection device; U p.u. Indicates the voltage per unit value, usually 1; X p.u. It represents the equivalent impedance viewed from the end of the line to the grid side.

[0055] (2) Calculation of overcurrent protection setting

[0056] In order to ensure that the circuit breaker outlet does not have a metallic short circuit fault, the protection should have a certain sensitivity, and the current setting value is

[0057] I zd(p.u.) =I / k (2)

[0058]

[0059] Among them, I zd Indicates the nominal value of the current flowing through the protection device, in kiloamperes; I zd(p.u.) It represents the per unit value of the current flowing through the protection device; S represents the reference capacity, which is 100 MVA or 1000 MVA according to the overall consideration of the setting calculation; U represents the nominal value of the calculated network voltage, in kilovolts; k represents the sensitivity coefficient, which is generally 1.3 to 1.5.

[0060] (3) Short-circuit current that can be provided by downstream distributed photovoltaics without considering directional elements

[0061] When the distribution network line does not have the conditions for installing PT, the maximum short-circuit current that the distributed photovoltaic can provide should not be greater than the overcurrent protection setting value of this level. For conservative calculation, the load of the distribution network line at this level is not considered. At the same time, the short-circuit current limiting characteristics of photovoltaic grid-connected controllers of different models are considered. The short-circuit current that the distributed photovoltaic can provide is calculated as

[0062] I pv zd / k pv (4)

[0063] Among them, k pv It indicates the multiple of the short-circuit current that distributed photovoltaic can provide during transient period and the rated current, and is generally taken as 1.0 to 1.5.

[0064] (4) Calculate the maximum capacity of distributed photovoltaics that can be connected

[0065]

[0066] Among them, S pv It indicates the maximum accessible capacity of distributed photovoltaic power generation, in megawatts.

[0067] Figure 1 In the figure, QF1~QF16 represent the circuit breakers of the relevant feeders and branches, all of which are equipped with overcurrent protection function, and DG1~DG3 represent three 10 kV distributed photovoltaic stations.

[0068] like Figure 1 The distribution network shown in Figure 1. In the early stage of distribution network planning, there is no distributed photovoltaic station DG3, and the overcurrent protection setting value configured for the QF5 circuit breaker only considers the metallic short-circuit fault at the circuit breaker outlet. The overcurrent protection setting value is calculated by equations (1) to (3). Since the protection range of QF5 is between QF5 and QF6, after the distributed photovoltaic DG3 is connected, if a fault occurs between QF5 and QF6, the short-circuit current provided by DG3 will not flow through QF5, so the setting value of QF5 is generally not adjusted after the distributed photovoltaic is connected.

[0069] ​If QF5 is connected to DG3 and the site does not have the conditions for installing PT, the overcurrent protection configured by QF5 will not be able to determine the direction. At this time, if a short circuit occurs at F1, QF5 will flow the reverse short-circuit current provided by DG3. If the capacity of DG3 is too large, and the current setting value of QF5 is less than the reverse short-circuit current provided by DF3 during the setting calculation, it may cause the QF5 overcurrent protection to malfunction, expanding the scope of the accident.

[0070] In view of the above reasons, the current QF5 protection adaptability needs to be considered when performing DG3 distributed photovoltaic network access planning calculations.

[0071] The first step is to calculate the QF5 overcurrent protection setting according to equations (1) to (3).

[0072] The second step is to reversely deduce the maximum short-circuit current that DG3 can provide under the condition that QF5 overcurrent protection is adapted according to formula (4).

[0073] The third step is to calculate the maximum capacity that DG3 can access based on the maximum short-circuit current that DG3 can provide and considering the limiting characteristics of the renewable energy grid-connected controller.

[0074] At this point, the calculation of the maximum accessible capacity of distributed photovoltaics is completed on the basis of adapting to traditional overcurrent protection.

[0075] Different distributed photovoltaic grid-connected controllers have different performances. When evaluating the maximum accessible capacity, it is necessary to determine it based on the rated current of the inverter currently used by the user and the performance of the controller. Therefore, the present invention has a wider range of application and is more practical.

[0076] Embodiment 2

[0077] The purpose of this embodiment is to provide a computer device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the steps of the above method when executing the program.

[0078] Embodiment 3

[0079] The purpose of this embodiment is to provide a computer-readable storage medium.

[0080] A computer-readable storage medium stores a computer program, which executes the steps of the above method when executed by a processor.

[0081] Embodiment 4

[0082] The purpose of this embodiment is to provide a distributed photovoltaic access capacity calculation system that takes into account the adaptability of distribution network protection, including:

[0083] The equivalent impedance acquisition module is configured to: obtain the equivalent impedance of the outgoing line terminal looking toward the distribution network system;

[0084] The maximum short-circuit current calculation module is configured to: calculate the maximum short-circuit current provided by the distribution network system side when a line end fault occurs based on the equivalent impedance;

[0085] The current setting value calculation module is configured to: calculate the current setting value based on the maximum short-circuit current and taking into account the sensitivity of the overcurrent protection stage III;

[0086] The maximum distributed photovoltaic capacity calculation module is configured to evaluate the maximum distributed photovoltaic capacity that can be connected to the current line according to the current constant value.

[0087] The above-mentioned maximum short-circuit current calculation module, when calculating, before the distribution network line is connected to the distributed photovoltaic, when a fault occurs at the end of the line, the short-circuit current provided by the system side is calculated by the ratio of the voltage per unit value to the equivalent impedance viewed from the end of the line to the grid side.

[0088] The steps involved in the apparatuses of the above embodiments 2, 3 and 4 correspond to the method embodiment 1, and the specific implementation methods can refer to the relevant description part of embodiment 1. The term "computer-readable storage medium" should be understood as a single medium or multiple media including one or more instruction sets; it should also be understood to include any medium that can store, encode or carry an instruction set for execution by a processor and enable the processor to execute any method in the present invention.

[0089] Those skilled in the art should understand that the modules or steps of the present invention described above can be implemented by a general-purpose computer device, or alternatively, they can be implemented by a program code executable by a computing device, so that they can be stored in a storage device and executed by the computing device, or they can be made into individual integrated circuit modules, or multiple modules or steps therein can be made into a single integrated circuit module for implementation. The present invention is not limited to any specific combination of hardware and software.

[0090] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. A distributed photovoltaic access capacity calculation method considering the adaptability of distribution network protection is characterized by: include: Obtain the equivalent impedance of the outgoing line terminal looking toward the distribution network system; Based on the equivalent impedance, the maximum short-circuit current provided by the distribution network system side when a line end fault occurs is calculated; Calculate the current setting based on the maximum short-circuit current and considering the sensitivity of overcurrent protection stage III; According to the current setting value and the current limit multiple of the distributed photovoltaic connected to the distribution network, the maximum distributed photovoltaic capacity that can be connected to the current line is evaluated; Before the distribution network line is connected to the distributed photovoltaic, when a fault occurs at the end of the line, the short-circuit current provided by the system side is calculated by the ratio of the voltage per unit value to the equivalent impedance viewed from the end of the line to the grid side, specifically: I=U p.u. / X p.u. Where I represents the short-circuit current flowing through the protection device; U p.u. Indicates the voltage per unit value; X p.u. It represents the equivalent impedance viewed from the end of the line to the grid side; The current setting is calculated based on the maximum short-circuit current and taking into account the sensitivity of the overcurrent protection stage III. The specific formula is: I zd(p.u.) =I / k Among them, I zd Indicates the nominal value of the current flowing through the protection device, in kiloamperes; I zd(p.u.) It indicates the nominal value of the current flowing through the protection device; S indicates the reference capacity; U indicates the nominal value of the calculated network voltage, in kilovolts; k indicates the sensitivity coefficient; When the distribution network line does not meet the conditions for installing PT, the load of the distribution network line at this level is not considered. At the same time, the short-circuit current limiting characteristics of photovoltaic grid-connected controllers of different models are considered to calculate the short-circuit current that distributed photovoltaics can provide: I pv <I zd / k pv Among them, k pv It indicates the multiple of the short-circuit current and rated current that distributed photovoltaic can provide during transient period, I zd Indicates the nominal value of the current flowing through the protection device; Calculate the maximum capacity of distributed photovoltaics that can be connected, specifically: Among them, S pv Indicates the maximum accessible capacity of distributed photovoltaic power generation, in megawatts, I pv It represents the short-circuit current that distributed photovoltaic can provide, and U represents the nominal value of the calculated network voltage.

2. The distributed photovoltaic access capacity calculation method considering the adaptability of distribution network protection as claimed in claim 1 is characterized in that: The method is applicable to calculating the distributed photovoltaic access capacity when planning distributed photovoltaic access in a 35 kV or 10 kV distribution network.

3. A distributed photovoltaic access capacity calculation system considering the adaptability of distribution network protection is characterized by: include: The equivalent impedance acquisition module is configured to: obtain the equivalent impedance of the outgoing line terminal looking toward the distribution network system; The maximum short-circuit current calculation module is configured to: calculate the maximum short-circuit current provided by the distribution network system side when a line end fault occurs based on the equivalent impedance; The current setting value calculation module is configured to: calculate the current setting value based on the maximum short-circuit current and taking into account the sensitivity of the overcurrent protection stage III; The maximum distributed photovoltaic capacity calculation module is configured to: evaluate the maximum distributed photovoltaic capacity that can be connected to the current line according to the current constant value; Before the distribution network line is connected to the distributed photovoltaic, when a fault occurs at the end of the line, the short-circuit current provided by the system side is calculated by the ratio of the voltage per unit value to the equivalent impedance viewed from the end of the line to the grid side, specifically: I=U p.u. / X p.u. Where I represents the short-circuit current flowing through the protection device; U p.u. Indicates the voltage per unit value; X p.u. It represents the equivalent impedance viewed from the end of the line to the grid side; The current setting is calculated based on the maximum short-circuit current and taking into account the sensitivity of the overcurrent protection stage III. The specific formula is: I zd(p.u.) =I / k Among them, I zd Indicates the nominal value of the current flowing through the protection device, in kiloamperes; I zd(p.u.) It indicates the nominal value of the current flowing through the protection device; S indicates the reference capacity; U indicates the nominal value of the calculated network voltage, in kilovolts; k indicates the sensitivity coefficient; When the distribution network line does not meet the conditions for installing PT, the load of the distribution network line at this level is not considered. At the same time, the short-circuit current limiting characteristics of photovoltaic grid-connected controllers of different models are considered to calculate the short-circuit current that distributed photovoltaics can provide: I pv <I zd / k pv Among them, k pv It indicates the multiple of the short-circuit current and rated current that distributed photovoltaic can provide during transient period, I zd Indicates the nominal value of the current flowing through the protection device; Calculate the maximum capacity of distributed photovoltaics that can be connected, specifically: Among them, S pv Indicates the maximum accessible capacity of distributed photovoltaic power generation, in megawatts, I pv It represents the short-circuit current that distributed photovoltaic can provide, and U represents the nominal value of the calculated network voltage.

4. The distributed photovoltaic access capacity calculation system considering the adaptability of distribution network protection as claimed in claim 3 is characterized in that: The maximum short-circuit current calculation module calculates the short-circuit current provided by the system side by the ratio of the voltage per unit value to the equivalent impedance viewed from the end of the line to the grid side when a fault occurs at the end of the line before the distribution network line is connected to the distributed photovoltaic.

5. A computer device comprising a memory, a processor and a computer program stored in the memory and executable on the processor, wherein: When the processor executes the program, the steps of the method described in any one of claims 1 to 2 are implemented.

6. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the steps of the method described in any one of claims 1 to 2 are performed.

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