Power distribution network protection grading improvement method for coping with different capacity distributed photovoltaic access

Abstract

The invention discloses a power distribution network protection grading improvement method for different capacity distributed photovoltaic access. The method comprises the following steps: S1, analyzing the influence of distributed photovoltaic on current protection; s2, calculating the access capacity of the distributed photovoltaic access which does not cause instantaneous current quick-break protection maloperation; s3, according to a calculation result, performing protection improvement on the power distribution network after the distributed photovoltaic access capacity exceeds a limit value 1; and S4, improving the distribution network protection after the limit value of the distributed photovoltaic access capacity of normal operation of current protection is 2 and exceeds the limit value. The distributed photovoltaic access capacity is subjected to gradient division on the basis of the influence of distributed photovoltaic access on power distribution network protection, the method has good adaptability to the condition that the distributed photovoltaic access capacities of power distribution networks in various regions are different at present, and targeted adjustment can be performed according to the different distributed photovoltaic access capacities; and the transformation cost of protection equipment is controlled while safe and stable operation of the power distribution network is ensured.

Description

Technical Field

[0001] The present invention relates to a distribution network protection hierarchical improvement method for coping with distributed photovoltaic access in the field of relay protection, which is used to improve the protection reliability of the distribution network under the background of widespread access of distributed photovoltaics to the distribution network, and to improve the targeted protection scheme after distributed photovoltaics of different capacities are connected to the distribution network, and in particular to a distribution network protection hierarchical improvement method for coping with distributed photovoltaics of different capacities. Technical Background

[0002] With the rapid development of renewable energy, distributed photovoltaic power generation systems are increasingly used in distribution networks due to their flexibility and environmental friendliness. However, this large-scale access to distributed energy has posed new challenges to the protection system of traditional distribution networks. Traditional relay protection strategies are designed for centralized power generation and relatively fixed loads, and they usually rely on fixed parameters and preset protection logic. However, in the context of widespread access to distributed photovoltaics, these strategies may not be able to adapt to the rapid changes and dynamic characteristics of grid parameters. For example, the access of photovoltaic systems may cause the voltage of local power grids to rise, exceeding the set threshold of traditional protection devices, thereby triggering unnecessary protection actions. At the same time, the reactive power output of photovoltaic systems will also affect the reactive balance of the power grid, posing a threat to the stability of the power grid.

[0003] In summary, although distributed photovoltaic access has brought many benefits to the power grid, its impact on the distribution network protection system cannot be ignored. In order to meet these challenges, it is urgent to improve the existing relay protection methods to adapt to the current and future widespread access to distributed photovoltaics. At present, the research on the classification improvement method of distribution network protection for distributed photovoltaic access is still in its infancy, and the research on how to accurately evaluate the impact of photovoltaic access of different capacities on power grid protection and formulate corresponding protection strategies is still insufficient.

[0004] The applicant proposes a distribution network protection classification improvement method for coping with distributed photovoltaic access of different capacities, aiming to ensure that the protection system of the distribution network can respond accurately and promptly under various distributed photovoltaic access capacities through targeted protection scheme adjustments, thereby ensuring the safety and stability of the power grid. Summary of the invention

[0005] The purpose of the present invention is to provide a distribution network protection classification improvement method for distributed photovoltaic access with different capacities. Considering the distributed photovoltaic operation characteristics, the current flowing through the protection installation under different distributed photovoltaic access capacities is analyzed, and then the action of the current protection is analyzed. A distributed photovoltaic access capacity calculation method is given based on the different degrees of influence on the current protection; for different distributed photovoltaic access capacities, corresponding protection improvement plans are given to ensure that the protection system of the distribution network can respond accurately and promptly under various distributed photovoltaic access capacities, thereby ensuring the safety and stability of the power grid.

[0006] The technical solution to achieve the above purpose is: a distribution network protection classification improvement method for coping with distributed photovoltaic access of different capacities, comprising the following steps:

[0007] S1: Analyze the impact of distributed photovoltaics on current protection;

[0008] S2: Calculate the access capacity of distributed photovoltaics that will not cause false operation of instantaneous current quick-break protection;

[0009] S3: Based on the calculation results, improve the protection of the distribution network after the distributed photovoltaic access capacity exceeds the limit 1;

[0010] S4: Improvement of current protection for normal operation of distributed photovoltaic access capacity with a limit of 2 and improvement of distribution network protection after exceeding it.

[0011] In the above-mentioned improved method for distribution network protection classification to cope with distributed photovoltaic access of different capacities, in step S1, a combination of time-limited overcurrent protection and instantaneous current quick-break protection is used to provide protection for the urban distribution network.

[0012] In the above-mentioned distribution network protection hierarchical improvement method for distributed photovoltaic access of different capacities, in step S2, the distributed photovoltaic access capacity that will not cause the instantaneous current quick-break protection to malfunction is calculated with a limit value of 1, and the current flowing through the protection when the line is 100% faulty is equal to the instantaneous current quick-break protection setting value, so as to obtain the distributed photovoltaic short-circuit current whose protection range of the instantaneous current quick-break protection extends to the end of the line, and then consider the current limiting characteristics of the distributed photovoltaic grid-connected converter, and obtain the distributed photovoltaic access capacity that will not cause the instantaneous current quick-break protection to malfunction, and its limit value is 1.

[0013] In the above-mentioned improved distribution network protection classification method for distributed photovoltaic access with different capacities, in step S2, the distributed photovoltaic access capacity that will not cause the instantaneous current quick-break protection to malfunction is calculated. Since the power electronic devices have a weak overcurrent tolerance capability, the maximum short-circuit current that the distributed photovoltaic can provide does not exceed 2 times the rated current.PV Set to rated current I N 1.2 times of I PV and the capacity of distributed photovoltaics S PV The relationship is shown as follows:

[0014] S PV =3·I PV ·U N (10)

[0015] In the formula, S PV is the capacity of distributed photovoltaics. The short-circuit current (phase current) provided by distributed photovoltaics when a fault occurs is I PV , I N is the rated current of distributed photovoltaic, U N is the rated voltage of the distributed photovoltaic grid-connected point. According to Kirchhoff's voltage theorem, the voltage-current relationship is as follows:

[0016] E Grid =(Z1+Z Grid )·I1+Z2·I2 (11)

[0017] In the formula, E Grid is the phase voltage value of the equivalent voltage source on the grid side, I1 and I2 are the currents flowing through the first protection and

[0018] The current of the second protection, Z1 and Z2 are the line impedances, Z Grid is the equivalent impedance of the grid side. The relationship between I1 and I2 is shown in the following formula:

[0019] I1=I2-I PV (12)

[0020] Assuming that the system is in the maximum operation mode, substituting equation (3) into equation (2) yields the current I2 flowing through the second protection as shown below:

[0021]

[0022] In the formula, Z Grid-min It is the equivalent impedance of the grid side when the system is in the maximum operation mode. 2-rel Ⅰ When a fault occurs at 100% of the line under the maximum operation mode, in the absence of distributed photovoltaics, the short-circuit current is multiplied by the reliability coefficient K rel Ⅰ The setting value of the instantaneous current quick-break protection is shown as follows:

[0023]

[0024] In the formula, I 2-rel Ⅰis the instantaneous current quick-break protection setting value, K rel Ⅰ Combining equations (4) and (5), assuming that the current flowing through the second protection when the line is 100% faulty is equal to the second protection instantaneous current quick-break protection setting value, that is, I2 = I 2-set Ⅰ , the protection range of the second protection instantaneous current quick-break protection extends to the distributed photovoltaic short-circuit current at the end of the line, as shown in the following formula:

[0025]

[0026] In the above-mentioned improved distribution network protection classification method for distributed photovoltaic access with different capacities, by combining equation (6) and equation (1), it can be obtained that the protection range of the second protection instantaneous current quick-break protection extends to the distributed photovoltaic capacity at the end of the line, that is, the distributed photovoltaic access capacity S that will not cause the instantaneous current quick-break protection to malfunction when the distributed photovoltaic access is PV-1 , whose limit is 1, as shown in the following formula,

[0027]

[0028] In the formula, S PV-1 The distributed photovoltaic access capacity that will not cause the instantaneous current quick-break protection to malfunction. When the distributed photovoltaic capacity does not exceed the capacity shown in formula (7), the second protection instantaneous current quick-break protection is in normal operation.

[0029] In the above-mentioned distribution network protection hierarchical improvement method for coping with distributed photovoltaic access of different capacities, the current protection improvement method after the distributed photovoltaic access capacity exceeds the limit value 1 multiplies the short-circuit current when a fault occurs at the end of the line under the maximum operating mode after the distributed photovoltaic is connected by the reliability coefficient, and solves the equation to obtain the modified instantaneous current quick-break protection setting value.

[0030] In the above-mentioned improved distribution network protection classification method for distributed photovoltaic access with different capacities, when the distributed photovoltaic capacity exceeds the capacity shown in formula (7), the second protection instantaneous current quick-break protection setting value is modified to ensure that the protection range of the second protection instantaneous current quick-break protection does not extend to the lower-level line, and the short-circuit current when a fault occurs at 100% of the line under the maximum operating mode after the distributed photovoltaic access is multiplied by the reliability coefficient K rel Ⅰ The modified protection 2 instantaneous current quick-break protection setting value I can be obtained 2-set-new Ⅰ , as shown below,

[0031]

[0032] In the formula, I 2-set-new Ⅰ is the modified instantaneous current quick-break protection setting value of protection 2. After the current protection setting value is modified according to formula (8), the current protection can operate normally even if the distributed photovoltaic access capacity exceeds limit 1.

[0033] In the above-mentioned distribution network protection hierarchical improvement method for distributed photovoltaic access with different capacities, in step S4, the short-circuit current at the protection outlet under the system minimum operation mode is set equal to the modified instantaneous current quick-break protection setting value, and the protection range of the instantaneous current quick-break protection is reduced to a minimum of 0% distributed photovoltaic capacity limit, that is, the distributed photovoltaic access capacity under normal operation of the improved current protection is limited to 2.

[0034] In the above-mentioned improved distribution network protection classification method for distributed photovoltaic access with different capacities, the short-circuit current at the fault of protection 2 outlet under the minimum operation mode of the system is set to be equal to I 2-set-new Ⅰ , the protection range of the second protection instantaneous current quick-break protection is reduced to 0% distributed photovoltaic capacity limit S PV-2 , whose limit is 2, as shown in the following formula,

[0035]

[0036] In the formula, S PV-2 The protection range of the second protection instantaneous current quick-break protection is reduced to a minimum of 0% distributed photovoltaic capacity limit.

[0037] In the above-mentioned improved method for distribution network protection classification to cope with distributed photovoltaic access of different capacities, the method makes a gradient division of distributed photovoltaic access capacity based on the impact of distributed photovoltaic access on distribution network protection. It has good adaptability to the different distributed photovoltaic access capacities of distribution networks in various regions at present, and makes targeted adjustments according to different distributed photovoltaic access capacities to achieve safe and stable operation of the distribution network.

[0038] The beneficial effects of the present invention are as follows: the present invention adopts the above technical solution, and compared with the prior art, the present invention has the following advantages:

[0039] 1. The present invention divides the distributed photovoltaic access capacity into two gradients based on the impact of distributed photovoltaic access on distribution network protection. This gradient division has good adaptability to the different distributed photovoltaic access capacities of distribution networks in various regions. It can make targeted adjustments according to different distributed photovoltaic access capacities, while ensuring the safe and stable operation of the distribution network and controlling the transformation cost of the protection equipment.

[0040] 2. The present invention calculates the distributed photovoltaic access capacity (limit 1) at which distributed photovoltaic access will not cause false operation of instantaneous current quick-break protection, and provides a method for improving current protection after exceeding limit 1, thereby achieving a quantitative analysis of the impact of distributed photovoltaic access on distribution network protection, providing a theoretical basis and response method for the distributed photovoltaic access capacity that the distribution network can withstand, and improving the protection reliability of distributed photovoltaic access to the distribution network.

[0041] 3. The present invention calculates the distributed photovoltaic access capacity (limit 2) for improved normal operation of current protection, provides a theoretical basis and response method for the maximum distributed photovoltaic access capacity that the distribution network current protection can withstand, and improves the protection reliability and safe and stable operation capability of the distribution network containing a high proportion of distributed photovoltaics. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a topological diagram of the distributed photovoltaic distribution network system;

[0043] Figure 2(a)-Figure 2(b) It is the short-circuit current flowing through the second protection and the instantaneous current quick-break protection setting value of the second protection;

[0044] Figure 3(a)-Figure 3(b) Compare the short-circuit current flowing through the second protection and the modified second protection instantaneous current quick-break protection setting value;

[0045] Figure 4 It is the action status of current differential protection. DETAILED DESCRIPTION

[0046] The present invention will be further described below in conjunction with the accompanying drawings.

[0047] To achieve the above objectives, the present invention mainly includes four aspects:

[0048] S1: Analysis of the impact of distributed photovoltaics on current protection.

[0049] For urban distribution networks, current protection is the most widely used protection strategy in distribution networks, and urban distribution networks usually use a combination of time-limited overcurrent protection and instantaneous current quick-break protection to provide protection for distribution networks. For time-limited overcurrent protection, since it mainly relies on action delay to distinguish between upper and lower levels, the access of distributed photovoltaics has little effect on time-limited overcurrent protection.

[0050] For instantaneous current quick-break protection, when distributed photovoltaics are connected to the downstream of the protection, the access of distributed photovoltaics will cause external absorption of the short-circuit current flowing through the protection, causing the short-circuit current of the protection to decrease, causing the instantaneous current quick-break protection range to shorten. However, since the instantaneous current quick-break protection itself cannot protect the entire length of the line, the harm caused by the access of distributed photovoltaics to the downstream of the protection is not great; when distributed photovoltaics are connected to the upstream of the protection, the access of distributed photovoltaics will increase the short-circuit current flowing through the protection, causing the short-circuit current of the protection to increase. This will cause the protection range of the instantaneous current quick-break protection to increase. As the capacity of distributed photovoltaic access increases, when the protection range increases to 100%, there is a risk of false operation of the instantaneous current quick-break protection.

[0051] S2: Calculation of distributed photovoltaic access capacity (limit 1) that will not cause false operation of instantaneous current quick-break protection when distributed photovoltaic access is connected.

[0052] Taking the distribution network system as an example, the distributed photovoltaic access capacity that will not cause the instantaneous current quick-break protection to malfunction is calculated. Grid is the phase voltage value of the equivalent voltage source on the grid side, I1 and I2 are the currents flowing through the first protection 1 and the second protection 2, Z1 and Z2 are the line impedances,

[0053] Z Grid is the equivalent impedance of the grid side, I PV It is the short-circuit current (phase current) provided by distributed photovoltaics when a fault occurs. Since power electronic devices have a weak ability to withstand overcurrent, the maximum short-circuit current that distributed photovoltaics can provide does not exceed 2 times the rated current. Here, I PV Set to rated current I N 1.2 times of I PV and the capacity of distributed photovoltaics S PV The relationship is shown in the following formula.

[0054] S PV =3·I PV ·U N (19)

[0055] In the formula, S PV is the capacity of distributed photovoltaics. The short-circuit current (phase current) provided by distributed photovoltaics when a fault occurs is I PV , I N is the rated current of distributed photovoltaic, U N is the rated voltage of the distributed photovoltaic grid-connected point. According to Kirchhoff's voltage theorem, the voltage-current relationship is as shown below.

[0056] E Grid =(Z1+Z Grid )·I1+Z2·I2 (20)

[0057] In the formula, E Grid is the phase voltage value of the equivalent voltage source on the grid side, I1 and I2 are the currents flowing through the first protection and

[0058] The current of the second protection, Z1 and Z2 are the line impedances, Z Grid is the equivalent impedance of the power grid side. The relationship between I1 and I2 is shown in the following equation.

[0059] I1=I2-I PV (twenty one)

[0060] Assuming that the system is in the maximum operating mode, substituting (3) into (2) yields the current I2 flowing through protection 2 as shown below.

[0061]

[0062] In the formula, Z Grid-min It is the equivalent impedance of the grid side when the system is in the maximum operation mode. 2-rel Ⅰ According to the short-circuit current when a fault occurs at 100% of the line under the maximum operation mode (without distributed photovoltaics) multiplied by the reliability coefficient K rel Ⅰ Setting, protection 2 instantaneous current quick-break protection setting value is shown in the following formula.

[0063]

[0064] In the formula, I 2-rel Ⅰ is the instantaneous current quick-break protection setting value, K rel Ⅰ is the reliability coefficient of the instantaneous current quick-break protection. Combining (4) and (5), assuming that the current flowing through protection 2 when the line is 100% faulty is equal to the setting value of the instantaneous current quick-break protection of protection 2, that is, I2 = I 2-rel Ⅰ , it can be obtained that the protection range of protection 2 instantaneous current quick-break protection extends to the distributed photovoltaic short-circuit current at the end of the line, as shown in the following formula.

[0065]

[0066] Combining (6) and (1), we can obtain that the protection range of the instantaneous current quick-break protection of protection 2 extends to the distributed photovoltaic capacity at the end of the line, that is, the distributed photovoltaic access capacity S that will not cause the instantaneous current quick-break protection to malfunction. PV-1 (Limit 1), as shown in the following formula.

[0067]

[0068] In the formula, SPV-1 The distributed photovoltaic access capacity will not cause the instantaneous current quick-break protection to malfunction. When the distributed photovoltaic capacity does not exceed the capacity shown in (7), the protection 2 instantaneous current quick-break protection can operate normally.

[0069] S3: Improved method for distribution network protection after the distributed photovoltaic access capacity exceeds the limit 1.

[0070] When the distributed photovoltaic capacity exceeds the capacity shown in (7), the protection range of the protection 2 instantaneous current quick-break protection may extend to the lower-level lines, which will increase the risk of false operation of the protection 2 instantaneous current quick-break protection.

[0071] At this time, the setting value of the protection 2 instantaneous current quick-break protection can be modified to ensure that the protection range of the protection 2 instantaneous current quick-break protection does not extend to the lower-level line. The short-circuit current when a fault occurs at 100% of the line under the maximum operation mode after connecting to the distributed photovoltaic is multiplied by the reliability coefficient K. rel Ⅰ The modified protection 2 instantaneous current quick-break protection setting value I can be obtained 2-set-new Ⅰ , as shown below.

[0072]

[0073] In the formula, I 2-set-new Ⅰ This is the modified instantaneous current quick-break protection setting value of protection 2. After the current protection setting value is modified according to (8), the current protection can operate normally even if the distributed photovoltaic access capacity exceeds limit 1.

[0074] S4: Improvement method for the protection of the distributed photovoltaic access capacity (limit 2) under normal operation of current protection and the distribution network after exceeding it.

[0075] The larger the distributed photovoltaic access capacity, the larger the setting value after modification according to (8). When the distributed photovoltaic does not output current at night and the system is in the minimum operation mode, the protection range of the protection 2 instantaneous current quick-break protection may be reduced to 0%. Let the short-circuit current at the protection 2 outlet under the minimum operation mode of the system be equal to I 2-set-new Ⅰ , the protection range of protection 2 instantaneous current quick-break protection is reduced to 0% distributed photovoltaic capacity limit S PV-2 (Limit 2), as shown in the following formula.

[0076]

[0077] In the formula, S PV-2The protection range of the second protection instantaneous current quick-break protection is reduced to 0% of the distributed photovoltaic capacity limit. When the connected distributed photovoltaic exceeds the capacity shown in (9), the protection range of the night protection 2 instantaneous current quick-break protection may be reduced to 0%, and the line main protection will fail. At this time, it is recommended to use current differential protection as the line main protection.

[0078] Figure 1 The topology diagram of the distributed photovoltaic distribution network system in a certain area is given. Taking protection 2 as the analysis object, it is first necessary to calculate the distributed photovoltaic access capacity (limit 1) that will not cause the instantaneous current quick-break protection to malfunction in combination with the grid parameters, which is 1.68MW. It is known that the instantaneous current quick-break protection setting value of protection 2 is 1.672kA; the distributed photovoltaic access capacity (limit 2) for the normal operation of the improved current protection is 2.28MW.

[0079] When the distributed photovoltaic access capacity is 1MW and 2MW respectively, when the fault occurs at 100% of the line from distribution station 4 to distribution station 3, the short-circuit current flowing through protection 2 and the instantaneous current quick-break protection setting value of protection 2 are shown in Figure 2. The values ​​in Figure 2 are shown in the following table.

[0080] Table 1 Short-circuit current and setting value flowing through protection 2 when the distributed photovoltaic access capacity is 1MW and 2MW respectively

[0081]

[0082] It can be seen that when the distributed photovoltaic access capacity is 1MW, the short-circuit current flowing through protection 2 is 1.637kA when the line end fails, which is less than the protection 2 instantaneous current quick-break protection setting value of 1.672kA, and the protection 2 instantaneous current quick-break protection is reliable and does not operate. When the distributed photovoltaic access capacity is 2MW, the short-circuit current flowing through protection 2 is 1.686kA when the line end fails, which is greater than the protection 2 instantaneous current quick-break protection setting value of 1.672kA, and the protection range of the protection 2 instantaneous current quick-break protection is extended to 100% and above, and there is a risk of false operation. It can be seen from the above table that the distributed photovoltaic access capacity (limit 1) calculated by the present invention that will not cause the instantaneous current quick-break protection to malfunction can accurately and quantitatively analyze the impact of distributed photovoltaic access on distribution network protection.

[0083] When the distributed photovoltaic access capacity is 2 and 3MW, the modified protection 2 instantaneous current quick-break protection setting values ​​are 1.770kA and 1.820kA respectively. When the distributed photovoltaic output power is zero at night and the fault occurs at the head end of the line from substation 4 to substation 3, the short-circuit current flowing through protection 2 and the modified protection 2 instantaneous current quick-break protection setting value are compared as shown in Figure 3. The values ​​in Figure 3 are shown in the following table.

[0084] Table 2 Short-circuit current and setting value of protection 2 caused by night fault when the distributed photovoltaic access capacity is 2MW and 3MW respectively

[0085]

[0086] It can be seen that when the distributed photovoltaic access capacity is 2MW, the short-circuit current flowing through protection 2 is 1.770kA, which is greater than I 2-set-new Ⅰ , protection 2 operates correctly. When the distributed photovoltaic access capacity is 3MW, the short-circuit current flowing through protection 2 is 1.820kA, which is greater than I 2-set-new Ⅰ , protection 2 refuses to operate. It can be seen that when the access capacity of distributed photovoltaic access exceeds the normal operation of improved current protection (limit 2), the current protection cannot tolerate the fluctuation of the protection range caused by the change of distributed photovoltaic operation mode. At this time, current differential protection should be configured. When the distributed photovoltaic access capacity is 3MW, the current differential protection action is as follows Figure 4 As shown, it can be seen that the current differential protection can operate correctly.

[0087] The above embodiments prove that the corresponding method is effective and reasonable.

[0088] In summary, the idea of ​​the present invention is mainly embodied in:

[0089] 1. The distributed photovoltaic access capacity (limit 1) of the present invention that will not cause the instantaneous current quick-break protection to malfunction is set to equal the instantaneous current quick-break protection setting value when the line is 100% faulty, so as to obtain the distributed photovoltaic short-circuit current whose protection range of the instantaneous current quick-break protection extends to the end of the line, and then consider the current limiting characteristics of the distributed photovoltaic grid-connected converter, and obtain the distributed photovoltaic access capacity (limit 1) that will not cause the instantaneous current quick-break protection to malfunction;

[0090] 2. The improved current protection method after exceeding the limit value 1 described in the present invention is to multiply the short-circuit current when a fault occurs at the end of the line in the maximum operating mode after connecting to the distributed photovoltaic by the reliability coefficient, and solve the equation to obtain the modified instantaneous current quick-break protection setting value;

[0091] 3. The distributed photovoltaic access capacity (limit 2) of the improved current protection for normal operation of the present invention is to make the short-circuit current at the protection outlet under the minimum operation mode of the system equal to the modified instantaneous current quick-break protection setting value, so that the protection range of the instantaneous current quick-break protection is reduced to a minimum of 0% of the distributed photovoltaic capacity limit, that is, the distributed photovoltaic access capacity (limit 2) of the improved current protection for normal operation;

[0092] 4. The invention provides a gradient division of distributed photovoltaic access capacity based on the impact of distributed photovoltaic access on distribution network protection. It has good adaptability to the different distributed photovoltaic access capacities of distribution networks in various regions. It can make targeted adjustments according to different distributed photovoltaic access capacities, while ensuring the safe and stable operation of the distribution network and controlling the transformation cost of the protection equipment.

[0093] The present invention is described in detail above in conjunction with the embodiments of the accompanying drawings. A person skilled in the art can make various variations of the present invention according to the above description. Therefore, certain details in the embodiments should not constitute a limitation of the present invention, and the scope of protection of the present invention shall be defined by the scope of the attached claims.

Claims

1. A distribution network protection hierarchical improvement method for distributed photovoltaic access with different capacities, comprising the following steps: S1: Analyze the impact of distributed photovoltaics on current protection; S2: Calculate the access capacity of distributed photovoltaics that will not cause false operation of instantaneous current quick-break protection; S3: Based on the calculation results, improve the protection of the distribution network after the distributed photovoltaic access capacity exceeds the limit 1; S4: Improvement of current protection for normal operation of distributed photovoltaic access capacity with a limit of 2 and improvement of distribution network protection after exceeding it.

2. According to claim 1, a distribution network protection hierarchical improvement method for coping with distributed photovoltaic access of different capacities is characterized by: In step S1, the urban distribution network is protected by a combination of time-limited overcurrent protection and instantaneous current quick-break protection.

3. According to claim 1, a distribution network protection hierarchical improvement method for coping with distributed photovoltaic access of different capacities is characterized by: In step S2, the distributed photovoltaic access capacity that will not cause the instantaneous current quick-break protection to malfunction is calculated with a limit value of 1, and the current flowing through the protection when the line is 100% faulty is equal to the instantaneous current quick-break protection setting value, so as to obtain the distributed photovoltaic short-circuit current whose protection range of the instantaneous current quick-break protection extends to the end of the line, and then consider the current limiting characteristics of the distributed photovoltaic grid-connected converter, and obtain the distributed photovoltaic access capacity that will not cause the instantaneous current quick-break protection to malfunction, and its limit value is 1.

4. According to claim 3, a distribution network protection hierarchical improvement method for coping with distributed photovoltaic access of different capacities is characterized by: In step S2, the distributed photovoltaic access capacity that will not cause the instantaneous current quick-break protection to malfunction is calculated. Due to the weak overcurrent tolerance of power electronic devices, the maximum short-circuit current that distributed photovoltaics can provide does not exceed 2 times the rated current. PV Set to rated current I N 1.2 times of I PV and the capacity of distributed photovoltaics S PV The relationship is shown as follows: WITH PV =3·I PV ·IN N (1) In the formula, S PV is the capacity of distributed photovoltaics. The short-circuit current provided by distributed photovoltaics when a fault occurs is I PV , I N is the rated current of distributed photovoltaic, U N is the rated voltage of the distributed photovoltaic grid-connected point. According to Kirchhoff’s voltage theorem, the voltage-current relationship is as follows: E Grid =(Z1+Z Grid )·I1+Z2·I2 (2) In the formula, E Grid is the phase voltage value of the equivalent voltage source on the grid side, I1 and I2 are the currents flowing through the first protection and the second protection, Z1 and Z2 are the line impedances, and Z Grid is the equivalent impedance of the grid side, and the relationship between I1 and I2 is shown in the following formula: I1=I2-I PV (3) Assuming that the system is in the maximum operation mode, substituting equation (3) into equation (2) yields the current I2 flowing through the second protection as shown below: In the formula, Z Grid-min is the equivalent impedance of the grid side when the system is in the maximum operation mode, and the instantaneous current quick-break protection setting value I 2-rel Ⅰ When a fault occurs at 100% of the line under the maximum operation mode, in the absence of distributed photovoltaics, the short-circuit current is multiplied by the reliability coefficient K rel Ⅰ The setting value of the instantaneous current quick-break protection is shown as follows: In the formula, I 2-rel Ⅰ is the instantaneous current quick-break protection setting value, K rel Ⅰ is the reliability coefficient of the instantaneous current quick-break protection. Combining equations (4) and (5), assuming that the current flowing through the second protection when the line is 100% faulty is equal to the instantaneous current quick-break protection setting value of the second protection, that is, I2 = I 2-set Ⅰ , the protection range of the second protection instantaneous current quick-break protection extends to the distributed photovoltaic short-circuit current at the end of the line, as shown in the following formula: The characters in the formula have the same meaning as above.

5. According to claim 4, a distribution network protection hierarchical improvement method for coping with distributed photovoltaic access of different capacities is characterized by: Combining equation (6) and equation (1), we can get that the protection range of the second protection instantaneous current quick-break protection extends to the distributed photovoltaic capacity at the end of the line, that is, the distributed photovoltaic access capacity S that will not cause the instantaneous current quick-break protection to malfunction PV-1 , whose limit is 1, as shown in the following formula, In the formula, S PV-1 The distributed photovoltaic access capacity will not cause the instantaneous current quick-break protection to malfunction when the distributed photovoltaic access is connected. When the distributed photovoltaic capacity does not exceed the capacity shown in formula (7), the second protection instantaneous current quick-break protection is in normal operation.

6. A distribution network protection hierarchical improvement method for coping with distributed photovoltaic access of different capacities according to claim 5, characterized in that: The improved method for current protection after the distributed photovoltaic access capacity exceeds the limit value 1 is to multiply the short-circuit current when a fault occurs at the end of the line under the maximum operating mode after the distributed photovoltaic access by the reliability coefficient, and solve the equation to obtain the modified instantaneous current quick-break protection setting value.

7. A distribution network protection hierarchical improvement method for coping with distributed photovoltaic access of different capacities according to claim 6, characterized in that: When the distributed photovoltaic capacity exceeds the capacity shown in formula (7), the second protection instantaneous current quick-break protection setting value is modified to ensure that the protection range of the second protection instantaneous current quick-break protection does not extend to the lower-level line. The short-circuit current when a fault occurs at 100% of the line under the maximum operation mode after connecting to the distributed photovoltaic is multiplied by the reliability coefficient K rel Ⅰ The modified protection 2 instantaneous current quick-break protection setting value I can be obtained 2-set-new Ⅰ , as shown below, In the formula, I 2-set-new Ⅰ is the modified protection 2 instantaneous current quick-break protection setting value. After the current protection setting value is modified according to formula (8), the current protection can operate normally even if the distributed photovoltaic access capacity exceeds limit 1.

8. A distribution network protection hierarchical improvement method for coping with distributed photovoltaic access of different capacities according to claim 7, characterized in that: In step S4, the short-circuit current at the protection outlet when the fault occurs under the minimum operating mode of the system is set equal to the modified instantaneous current quick-break protection setting value, and the protection range of the instantaneous current quick-break protection is reduced to a minimum of 0% distributed photovoltaic capacity limit, that is, the distributed photovoltaic access capacity under normal operation of the improved current protection is limited to 2.

9. A distribution network protection hierarchical improvement method for coping with distributed photovoltaic access of different capacities according to claim 8, characterized in that: Let the short-circuit current when the protection 2 outlet fails under the system minimum operation mode be equal to I 2-set-new Ⅰ , the protection range of the second protection instantaneous current quick-break protection is reduced to 0% distributed photovoltaic capacity limit S PV-2 , whose limit is 2, as shown in the following formula, In the formula, S PV-2 The protection range of the second protection instantaneous current quick-break protection is reduced to a minimum of 0% distributed photovoltaic capacity limit.

10. The method for improving distribution network protection classification for distributed photovoltaic access with different capacities according to claim 1, characterized in that: The method is to divide the distributed photovoltaic access capacity into gradients based on the impact of distributed photovoltaic access on distribution network protection. It has good adaptability to the different distributed photovoltaic access capacities of distribution networks in various regions, and makes targeted adjustments according to different distributed photovoltaic access capacities to achieve safe and stable operation of the distribution network.

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