Microgrid overcurrent protection applicability evaluation method

Abstract

The invention discloses a microgrid overcurrent protection applicability evaluation method. In order to solve the problems that in the prior art, the influence of inverter distributed power supply access and grid-connected and off-grid switching on micro-grid current protection is not fully considered, so that new energy access capacity evaluation is inaccurate, and a flexibly-changing micro-grid operation environment is difficult to adapt, the method comprises the steps of obtaining respective short-circuit current based on grid-connected and off-grid operation topological structures; calculating a three-section current protection value of each position in the preset protection range; a high-proportion distributed photovoltaic system is connected to a micro-grid, the maximum access capacity of the distributed photovoltaic system serves as a target function, constraint conditions are established according to grid-connected and off-grid three-section current protection values and operation parameters, and an optimization model for evaluating the current protection condition is adjusted. The method has the advantages that the evaluation accuracy is improved, safe and stable operation of the micro-grid under high-proportion access of the new energy is ensured, and powerful support is provided for access of the new energy and planning operation of the micro-grid.

Description

Technical Field

[0001] The present invention relates to the field of relay protection for AC microgrids with a high proportion of new energy access, and in particular to a method for evaluating the applicability of overcurrent protection for microgrids. Background Art

[0002] The access of existing distributed power sources, new loads and energy storage to urban microgrids can make full use of local renewable energy and reduce fossil energy consumption, and is an important part of the future urban power grid. However, with the increase in the proportion of inverter-type distributed power sources, the power flow and short-circuit current distribution in the microgrid have undergone fundamental changes, affecting the reliable operation of the original protection and control devices. In addition, the flexible operation mode of the microgrid, including grid-connected and off-grid states, leads to the complexity of fault characteristics, and it is difficult for traditional current protection principles to quickly and accurately remove faults. When analyzing the applicability of current protection under the access of new energy, existing studies only consider the grid-connected operation scenario, and do not consider the boundary differences caused by grid-connected and off-grid switching. Therefore, the calculated maximum access capacity cannot reflect the new energy capacity boundary of the actual microgrid protection correct and reliable action. In response to this problem, it is urgent to study a microgrid overcurrent protection applicable boundary evaluation method taking into account grid-connected and off-grid switching to provide a calculation method for the maximum access capacity of distributed photovoltaics in microgrids in actual projects.

[0003] For example, a method for adaptively setting overcurrent protection for active distribution networks disclosed in Chinese patent literature, with announcement number CN117277207A, includes using the historical operation scenario data of the target distribution network to generate a typical operation scenario library of the target distribution network and the probability of occurrence of each typical operation scenario; constructing an inverse time overcurrent protection collaborative optimization model, generating an inverse time overcurrent protection constant value template for each typical operation scenario of the target distribution network offline for the typical operation scenario library of the target distribution network, forming an overcurrent protection operation scenario library of the target distribution network, and storing it in a central relay protection device; the central relay protection device identifies the current operation scenario of the target distribution network, finds the best matching scenario in the operation scenario library and the corresponding overcurrent protection constant value template in the storage library, and adjusts the overcurrent protection constant value of each relay protection device of the target distribution network according to the overcurrent protection constant value template. The efficiency of overcurrent protection is improved, but the consideration of the applicability of the scheme when inverter-type distributed power sources are connected and switched between grids is not comprehensive enough. Summary of the invention

[0004] The present invention mainly solves the problem that the existing technology does not fully consider the impact of inverter-type distributed power access and grid-connected and off-grid switching on microgrid current protection, resulting in inaccurate assessment of new energy access capacity and difficulty in adapting to the flexible and changing microgrid operating environment; provides a microgrid overcurrent protection applicability assessment method, constructs an optimization model with the maximum access capacity of the microgrid as the optimization target, and the current protection sensitivity and reverse fault non-false operation in the grid-connected and off-grid operating state as the constraint conditions, and obtains the new energy capacity threshold allowed for the correct and reliable operation of the current protection by solving the optimization model. When the new energy capacity connected to the system is less than the solved capacity threshold, the overcurrent protection can operate correctly and reliably.

[0005] The above technical problems of the present invention are mainly solved by the following technical solutions: The present invention includes: based on the grid-connected and off-grid operation topological structures, obtaining respective short-circuit currents, calculating the three-stage current protection values ​​at each position within the preset protection range; connecting a high proportion of distributed photovoltaics to the microgrid, taking its maximum access capacity as the objective function, establishing constraint conditions with the grid-connected and off-grid three-stage current protection values ​​and operating parameters, and adjusting the optimization model for evaluating the current protection situation. The benefit is to improve the accuracy of the evaluation, ensure the safe and stable operation of the microgrid under the high proportion of new energy access, and provide strong support for the access of new energy and the planning and operation of the microgrid.

[0006] Preferably, the initialization constraint condition is that the capacity value of the i-th photovoltaic access distribution network is a non-negative number and is less than the maximum photovoltaic access capacity, and i is a non-negative integer; when in the grid-connected state, a grid-connected correspondence relationship is established between the access photovoltaic in the test operation state or the normal operation state and the preset line short-circuit fault condition, and the grid-connected correspondence relationship is taken and compared with the corresponding current value of the preset line operation. The initialization constraint condition ensures the rationality and effectiveness of the photovoltaic access capacity and avoids invalid or out-of-range access; in the grid-connected state, by establishing a correspondence between the photovoltaic operation state and the line short-circuit fault, the current protection situation is accurately evaluated to improve the reliability and safety of the microgrid operation. These benefits help optimize the microgrid structure, improve the access capacity of new energy, and promote the utilization and development of renewable energy.

[0007] Preferably, the grid-connected correspondence and comparison conditions include: minimum operating mode: the value of the correspondence between the test operating state and the short-circuit fault condition at m% of the preset protection range is equal to the short-circuit current value at the time of the fault; the value of the correspondence between the test state and the short-circuit fault condition at the end of the downstream line of the preset protection range is equal to the short-circuit current value within the preset protection range at the time of the fault; maximum operating mode: the value of the correspondence between the normal operating state and the three-phase short-circuit fault condition in the first section of the preset protection range is equal to the short-circuit current value when the first end of the preset protection line fails; establish a quotient constraint range between the grid-connected correspondence value and the three-section current protection value.

[0008] The setting of the grid-connected correspondence and comparison conditions makes the current protection evaluation after photovoltaic access to the microgrid more accurate and comprehensive. The correspondence between the test operation status and the short-circuit fault condition ensures that the faults at different locations in the protection range can be accurately evaluated; the correspondence between the normal operation status and the short-circuit fault condition ensures the effectiveness of the protection when the line head fails under the maximum operation mode. By establishing the constraint range of the quotient of the grid-connected correspondence value and the three-stage current protection value, the sensitivity and selectivity of the current protection are further ensured. These benefits not only improve the safety and stability of the microgrid.

[0009] Preferably, the grid-connected quotient constraint range includes: under the minimum operating mode: when m=15, the quotient of the short-circuit current value at 15% of the preset protection range and the current value of section I is greater than 1; when a fault occurs at the end of the preset protection range, the quotient of the short-circuit current value flowing through this section of the line and the current value of section II is greater than 1.3; when a fault occurs at the end of the downstream line, the quotient of the short-circuit current value flowing through the preset protection range and the current value of section III is greater than 1.3; under the maximum operating mode: when a fault occurs at the head end of the preset protection line, the quotient of the short-circuit current value flowing through this section of the line and the current value of section III is less than 1.

[0010] The constraint range setting of the quotient during grid connection ensures the sensitivity and selectivity of current protection in different operating modes and fault conditions. By setting the quotient value range between the short-circuit current value and the current protection value, the risk of misoperation or refusal of protection is effectively avoided, and the safety and stability of the microgrid are improved. At the same time, the setting of this constraint range also optimizes the new energy access strategy, making it possible to connect a high proportion of distributed new energy to the microgrid, and promoting the widespread application and sustainable development of renewable energy. Therefore, the setting of this constraint range has important practical significance and application value.

[0011] Preferably, the initialization off-grid constraint condition is that the capacity value of the ith photovoltaic access distribution network is non-negative and less than the maximum photovoltaic access capacity at this time; establish an off-grid correspondence between the test operation state of the photovoltaic access in the off-grid state and the short-circuit fault condition of the preset line in the normal operation state, and take the value of the off-grid correspondence and compare it with the corresponding current value of the preset protection line operation. The initialization off-grid constraint condition ensures the rationality of photovoltaic access, and combines the test and operation data in the off-grid state to construct an accurate correspondence between short-circuit fault and current protection. The establishment and comparison of this correspondence not only improves the accuracy of current protection evaluation, but also strengthens the stable operation capability of the microgrid in off-grid mode.

[0012] Preferably, the comparison between the off-grid correspondence value and the corresponding current value includes: minimum operation mode: the value of the correspondence between the test operation state and the short-circuit fault condition at n% of the preset protection range is equal to the short-circuit current value at the short-circuit fault; the value of the correspondence between the test state and the short-circuit fault condition at the end of the downstream line of the preset protection range is equal to the short-circuit current value when the downstream line fails; maximum operation mode: the value of the correspondence between the normal operation state and the three-phase short-circuit fault condition in the first section of the preset protection range is equal to the short-circuit current value when the first end of the preset protection line fails; establish a quotient constraint range between the off-grid correspondence value and the three-section current protection value.

[0013] In the off-grid state, by comparing the off-grid correspondence value with the corresponding current value, the performance of the current protection when the microgrid is off-grid can be accurately evaluated. This comparison ensures that faults at different locations in the protection range can be accurately identified and handled, improving the safety and reliability of the microgrid in off-grid mode. At the same time, establishing a constraint range for the quotient of the off-grid correspondence value and the three-stage current protection value further enhances the sensitivity and selectivity of the current protection, effectively preventing the occurrence of false or refusal to operate the protection. These benefits not only help improve the fault handling capabilities of the microgrid, but also provide a strong guarantee for its stable operation under the access of a high proportion of distributed new energy.

[0014] As a preference, the off-grid quotient constraint range includes: in the minimum operation mode: when n=15, the quotient of the short-circuit current value at 15% of the preset protection range and the current value of section I is greater than 1; when the end of the preset protection range fails, the quotient of the short-circuit current value flowing through this section of the line and the current value of section II is greater than 1.3; when the end of the downstream line fails, the quotient of the short-circuit current value flowing through the preset protection range and the current value of section III is greater than 1.3; in the maximum operation mode: when the head end of the preset protection line fails, the quotient of the short-circuit current value flowing through this section of the line and the current value of section III is less than 1. The above settings make the evaluation results in the off-grid state more accurate.

[0015] Preferably, in the test state of photovoltaic access, the constraint condition for the second section of the three-section current protection value should be that the feeder and upstream photovoltaic within the preset protection range should be shut down; the constraint condition for the third section should be that the upstream photovoltaic within the preset protection range and the remaining feeder photovoltaic should be shut down. Setting specific constraints for the second and third sections of the three-section current protection value will help improve the safety and reliability of the microgrid, especially when a high proportion of distributed new energy is connected. By accurately simulating and evaluating the performance of current protection, the protection strategy can be optimized, the risk of failure can be reduced, and the overall operating efficiency of the microgrid can be improved.

[0016] As a preferred method, the capacity values ​​of all connected photovoltaics in the distribution network are compared to obtain the maximum photovoltaic capacity expression; the maximum photovoltaic capacity expression is used as the objective function, an optimization model is established according to the grid-connected and off-grid constraints, and the particle swarm algorithm is used to calculate the maximum access capacity of each location that meets the current protection requirements; when the photovoltaic capacity is less than the maximum access capacity, the current protection selectively identifies the fault; and when the connected photovoltaic capacity is greater than the maximum access capacity, the current protection will malfunction or refuse to operate. The particle swarm algorithm is used to accurately calculate the maximum access capacity of each location to ensure the effectiveness of the current protection; and it helps to optimize the photovoltaic access strategy and improve the safety and stability of the microgrid.

[0017] The beneficial effects of the present invention are: 1. A microgrid overcurrent protection applicability assessment method of the present invention can enhance the safety and stability of microgrids under a high proportion of new energy access, effectively respond to the challenges brought by a high proportion of new energy access, and ensure that the microgrid can still operate safely and stably under a high proportion of new energy access by constructing an optimization model and solving the new energy capacity threshold. This method improves the adaptability of microgrids to new energy and provides strong support for long-term stable operation.

[0018] 2. A microgrid overcurrent protection suitability assessment method of the present invention can flexibly respond to the protection needs of microgrids that switch between the grid and off-grid, and proposes a suitability assessment method that can flexibly adapt to the protection needs of different operating states by optimizing the model, ensuring that the current protection can operate correctly and reliably in various states. This method enhances the flexibility and reliability of microgrid protection.

[0019] 3. The microgrid overcurrent protection suitability assessment method of the present invention can lay the theoretical and methodological foundation for the microgrid protection suitability analysis. By solving the optimization model to obtain the new energy capacity threshold, it provides a solid theoretical basis and methodological guidance for the microgrid protection suitability analysis. This innovative method helps to optimize the microgrid protection scheme, improve the efficiency of the protection device, and provide an important reference for the planning and design of the microgrid. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a flow chart of a method for evaluating the suitability of overcurrent protection for a microgrid according to the present invention; Figure 2 A microgrid grid-connected operation topology diagram of a microgrid overcurrent protection applicability evaluation method of the present invention; Figure 3 It is a microgrid off-grid operation topology diagram of a microgrid overcurrent protection applicability evaluation method of the present invention. DETAILED DESCRIPTION

[0021] The technical solution of the present invention is further specifically described below through embodiments and in conjunction with the accompanying drawings.

[0022] Embodiment 1: A microgrid overcurrent protection applicability evaluation method of this embodiment is as follows: Figure 1 Shown include: Step 1: Calculate the current protection value.

[0023] Before starting to connect to distributed photovoltaics, it is first necessary to conduct a detailed short-circuit current analysis of the grid in two different operating modes: grid-connected and off-grid. Based on the topological structures of these two modes, the magnitude of the short-circuit current is accurately calculated, and its distribution characteristics in the grid are determined. For the preset protection range, these short-circuit current data are used to calculate the three-stage current protection values ​​required for each location. These protection values ​​are designed to ensure that when a grid fault occurs, the faulty part can be quickly and accurately cut off to prevent the fault from expanding and ensure the safe and stable operation of the grid. Among them, the grid-connected mode refers to the distributed photovoltaic system being connected to the main grid and can exchange electric energy with each other; while the off-grid mode refers to the distributed photovoltaic system operating independently and not connected to the main grid.

[0024] In the grid-connected mode, the electric energy generated by the distributed photovoltaic system can be transmitted to the main grid, and can also be obtained from the main grid to meet demand. In this mode, the current protection device needs to ensure that when a grid fault occurs, it can quickly cut off the faulty part to prevent the fault current from affecting the stable operation of the main grid. Therefore, it is necessary to accurately calculate the short-circuit current in the grid-connected mode based on factors such as the access capacity of the distributed photovoltaic system, the grid structure, and the operating parameters, and set the corresponding three-stage current protection values.

[0025] In off-grid mode, the distributed photovoltaic system needs to independently undertake the task of power supply, and its current protection strategy is different from that in grid-connected mode. The current protection device in off-grid mode mainly focuses on the fault conditions inside the distributed photovoltaic system to ensure that when a fault occurs, the faulty part can be quickly cut off to ensure the normal operation of the non-faulty part. Since there is no support from the main power grid in off-grid mode, its current protection strategy needs to be more cautious and precise.

[0026] Step 2: Establish an optimization model for photovoltaic access.

[0027] After establishing the current protection strategy, it is necessary to further consider how to achieve the maximum access capacity of distributed photovoltaics. This involves comprehensive consideration and optimization of multiple factors in grid-connected and off-grid modes. In grid-connected mode, it is first necessary to ensure that the access of distributed photovoltaic systems will not affect the stable operation of the main power grid. This includes restrictions on access capacity, optimization of the grid structure, and adjustment of operating parameters. At the same time, it is also necessary to consider how to reduce the impact of the access of distributed photovoltaic systems on the grid and improve their compatibility with the grid. This may require the use of some advanced technical means, such as intelligent control and power prediction.

[0028] In off-grid mode, more attention needs to be paid to the stability and reliability of the distributed photovoltaic system itself. Since there is no support from the main power grid in off-grid mode, once a fault occurs, the entire system may face the risk of power outage. Therefore, a series of measures need to be taken to improve the fault response capability of the distributed photovoltaic system in off-grid mode. This may include adding redundant equipment, optimizing system structure, and improving equipment performance.

[0029] Establish grid connection constraints.

[0030] In the grid-connected state, the distributed photovoltaic system needs to meet a series of constraints to ensure the safe and stable operation of the grid. These constraints mainly include the following aspects: When a distributed photovoltaic system is connected to the grid, its voltage level and power factor need to match the requirements of the grid. Mismatched voltage levels may cause equipment damage or unstable operation, while deviations in power factors may affect the power transmission efficiency of the grid. Therefore, it is necessary to set a reasonable voltage level and power factor range based on the actual situation of the grid as a constraint for the access of distributed photovoltaics.

[0031] The power flow equation is a mathematical model that describes the law of electric energy flow in the power grid. When distributed photovoltaics are connected to the power grid, it is necessary to ensure the balance of the power flow equation, that is, the power generation and power consumption in the power grid should be equal. This involves comprehensive consideration of factors such as grid structure, line impedance, and load distribution. By setting power flow equation constraints, it can be ensured that the access of distributed photovoltaics will not have too much impact on the flow of electric energy in the power grid.

[0032] The grid's acceptance capacity refers to the maximum capacity of distributed photovoltaic access that the grid can accommodate. This is affected by many factors, including the capacity, structure, and operation mode of the grid. In the grid-connected state, the grid's acceptance capacity needs to be evaluated and corresponding constraints set. This helps ensure that the access of distributed photovoltaics does not exceed the grid's carrying capacity, thereby ensuring the stable operation of the grid.

[0033] In the grid-connected mode, the access of distributed photovoltaic systems may have an impact on the short-circuit current of the power grid. Therefore, it is necessary to set corresponding constraints based on the short-circuit current limit of the power grid. This involves comprehensive consideration of the selection of distributed photovoltaic access points, access capacity restrictions, and configuration of protection devices. By setting short-circuit current constraints, we can ensure that when a fault occurs, the protection device can operate correctly, isolate the fault area, and prevent the fault from expanding.

[0034] In the grid-connected state, first ensure that the capacity value of each photovoltaic access distribution network is non-negative and does not exceed its maximum access capacity. Then establish the correspondence between the photovoltaic in the test operation state and the normal operation state and the preset line short-circuit fault condition. These correspondences compare the short-circuit current value with the three-stage current protection value to ensure that when a fault occurs, the protection device can operate correctly and isolate the fault area.

[0035] Consider the short-circuit current characteristics under different operating modes. For example, in the test operation state, pay attention to the short-circuit fault conditions at 15% of the preset protection range and the two phases at the end, and compare them with the current protection value of section I. At the same time, consider the impact of the two-phase short-circuit fault at the end of the downstream line on the preset protection range, and compare it with the current protection value of section II. In the normal operation state, focus on the three-phase short-circuit fault condition in the first section of the preset protection range, and compare it with the current protection value of section III.

[0036] Establish off-grid constraints.

[0037] In the off-grid state, the distributed photovoltaic system needs to independently undertake the power supply task, and its constraints are different from those in the grid-connected state. These constraints mainly include the following aspects: Power supply reliability refers to the ability of a distributed photovoltaic system to provide stable and continuous power supply in an off-grid state. Since there is no support from the main power grid in the off-grid mode, once a fault occurs, the entire system may face the risk of power outage. Therefore, it is necessary to set power supply reliability constraints to ensure that the distributed photovoltaic system can provide reliable power supply in both normal operation and fault conditions.

[0038] Power quality refers to the various characteristics of electric energy in the power system, such as voltage stability, frequency stability, harmonic content, etc. In the off-grid state, the distributed photovoltaic system needs to ensure that the output power quality meets the relevant standards. This involves comprehensive consideration of the performance of photovoltaic modules, the control strategy of the inverter, and the configuration of the energy storage system. By setting power quality constraints, it can be ensured that the power provided by the distributed photovoltaic system can meet the power demand of users.

[0039] Equipment life refers to the service life of each device in a distributed photovoltaic system. In an off-grid state, since the system needs to operate independently, the operating environment and conditions of the equipment may be more severe. Therefore, it is necessary to consider the constraints of equipment life to ensure that the system can operate stably for a long time. This involves comprehensive consideration of equipment selection, maintenance strategy, and replacement planning.

[0040] In off-grid mode, distributed photovoltaic systems need to achieve energy self-sufficiency. Therefore, it is necessary to ensure that the system's power generation and power consumption are balanced at any time. This involves accurate prediction and scheduling of factors such as the output power of photovoltaic modules, the capacity of the energy storage system, and load demand. By setting energy balance constraints, it can be ensured that the distributed photovoltaic system can operate stably in an off-grid state and avoid energy shortages or surpluses.

[0041] In the off-grid state, it is also necessary to ensure the rationality of the photovoltaic access capacity and the effectiveness of the current protection. Initialize the constraints and establish the correspondence between the photovoltaic in the test operation state and the normal operation state and the preset line short-circuit fault condition. These correspondences also take into account the short-circuit current characteristics under different operation modes and compare them with the corresponding three-stage current protection values.

[0042] By comparing the capacity values ​​of all connected photovoltaics in the distribution network, the maximum photovoltaic capacity is determined. Taking this maximum capacity as the objective function, combined with the constraints of grid connection and off-grid, an optimization model is established. Using optimization methods such as particle swarm optimization, this model is solved to obtain the maximum access capacity of each location that meets the current protection requirements.

[0043] Among them, the calculation of the maximum access capacity is carried out under the premise of satisfying the constraints under the above-mentioned grid-connected and off-grid states. This usually involves comprehensive modeling and simulation analysis of the power grid and distributed photovoltaic systems. First, it is necessary to establish a mathematical model of the power grid and distributed photovoltaic system, including the topological structure, line parameters, load distribution, etc. of the power grid, as well as the power generation characteristics, access location, capacity configuration, etc. of the distributed photovoltaic system. Then, the model is simulated and analyzed using simulation software to simulate the operation of the power grid and distributed photovoltaic system under different operating scenarios.

[0044] During the simulation analysis, it is necessary to consider the impact of various constraints on the access capacity of distributed photovoltaics. For voltage level and power factor constraints, the requirements can be met by adjusting the inverter control strategy of the distributed photovoltaic system; for power flow equation constraints and grid acceptance capacity constraints, it is necessary to comprehensively consider the structure and operation mode of the grid to determine the appropriate access location and capacity; for short-circuit current constraints, it is necessary to reasonably configure protection equipment to ensure correct operation in the event of a fault.

[0045] Under the premise of satisfying the constraints, the access capacity of distributed photovoltaics needs to be optimized. This can be achieved through optimization algorithms, such as particle swarm optimization, genetic algorithm, etc. These algorithms can find the optimal objective function (such as maximizing power generation, minimizing costs, etc.) under the premise of satisfying the constraints.

[0046] In practical applications, we also need to pay attention to the following points: First, when the photovoltaic capacity is less than the maximum access capacity, the current protection device can correctly identify the fault and take corresponding protection measures. However, when the connected photovoltaic capacity exceeds this maximum access capacity, the current protection may malfunction or refuse to operate. Therefore, in actual operation, it is necessary to strictly control the access capacity of photovoltaics to ensure that it does not exceed the maximum access capacity. Special attention should also be paid to the special requirements for the three-stage current protection value under the test state. In particular, in the second and third section constraints, it is necessary to consider the exit operation strategy of the feeder and upstream photovoltaic under specific circumstances to ensure the effectiveness and reliability of the current protection. With the continuous development of distributed photovoltaic technology and the continuous changes in the grid structure, the optimization model also needs to be continuously updated and improved to adapt to new application scenarios and needs.

[0047] Embodiment 2: A microgrid overcurrent protection applicability evaluation method of this embodiment is as follows: Figure 2 As shown in Figure 3, the high proportion of distributed renewable energy access and the diversification of on-grid / off-grid operation modes make it difficult for traditional protection principles and solutions to meet the requirements of safe and stable operation of microgrids. Therefore, in response to this problem, the present invention proposes an overcurrent protection applicability assessment method for AC microgrids with a high proportion of renewable energy access, taking into account the on-grid and off-grid operation states.

[0048] After a high proportion of distributed access to the microgrid, when the protection fails in the positive direction, the short-circuit current fed by the upstream photovoltaic and other feeder photovoltaics will increase the short-circuit current flowing through the protection, thereby increasing the range of current protection and making the protection lose selectivity. In order to ensure the selectivity of the protection, it is necessary to set the protection setting value based on the short-circuit current under photovoltaic access. However, this situation will affect the sensitivity of the protection. In addition, when the upstream line of the protection line or other feeder faults occur, the reverse short-circuit current fed by the downstream photovoltaic protection may cause the protection to malfunction.

[0049] like Figure 2Figure 3 shows the topology of the distributed photovoltaic access microgrid in grid-connected or off-grid operation. CB is the substation outlet switch, and FSW is the section switch. Comparing the topology diagrams under grid-connected and off-grid operation, it can be seen that compared with the grid-connected operation, the power flow direction of the system changes during off-grid operation, resulting in a significant change in the short-circuit current when a fault occurs in the off-grid operation state and the grid-connected state, so that the current protection setting value under the grid-connected state cannot be directly applied to the off-grid state. Therefore, in this case, it is necessary to recalculate the protection setting value through the short-circuit current calculation under off-grid operation.

[0050] To this end, a current protection applicable boundary calculation method taking into account the on-grid and off-grid states is designed, and the scheme includes the following steps: (1) According to the microgrid grid-connected operation topology and based on the short-circuit current calculation, the current I-stage protection setting value of each protection position is calculated Current II stage constant Current III stage setting

[0051] (2) According to the off-grid operation topology of the microgrid, based on the short-circuit current calculation, calculate the current I-stage protection setting value of each protection position Current II stage constant Current III stage setting

[0052] (3) For microgrids with a high proportion of distributed photovoltaic access, the maximum distributed photovoltaic access capacity is taken as the objective function, and the single photovoltaic access capacity, current protection sensitivity and reverse protection non-false operation are taken as constraints under grid-connected and off-grid operation. The applicable boundary of current protection is analyzed, and the optimization model of the maximum photovoltaic access capacity is: ①Objective function: max S sum =S pv1 +S pv2 +...+S pvn (1) In the formula, S PVn is the capacity value of the nth photovoltaic access to the distribution network.

[0053] ② Constraints on grid-connected operation: Where Pi represents line protection and i = 1, 2, 3, ..., They respectively represent the setting values ​​of stage I, stage II and stage III of the current protection Pi after the distributed photovoltaic is connected to the grid; They represent the short-circuit current flowing through this section of the line at 15% of the Pi protection range in the minimum operating mode and at the end of the fault. It represents the short-circuit current flowing through the Pi protection when a fault occurs at the end of the downstream line of the Pi protection in the minimum operation mode. Represents the short-circuit current flowing through this section of the line when the first end of the Pi protection line fails under the maximum operation mode; S represents the operating status of the connected photovoltaic. The constraint condition for the current section II should be that the feeder where the Pi protection is located and the upstream photovoltaic should be shut down; the constraint condition for the current section III should be that the Pi protection upstream photovoltaic and other feeder photovoltaics should be shut down; S N Indicates the operating status of the connected photovoltaic system. At this time, all photovoltaic systems are operating normally; They represent the 15% of Pi protection range and the two-phase short-circuit fault at the end. Indicates a two-phase short-circuit fault at the end of the downstream line of Pi protection. This is the three-phase short-circuit fault condition at the head end of Pi protection.

[0054] ③ Constraints of off-grid operation: In the formula, They represent the current protection P after the distributed photovoltaic connection in off-grid operation. i The setting values ​​of stage Ⅰ, stage Ⅱ and stage III; They represent the minimum operating mode P when off-grid. i The short-circuit current flowing through this section of the line when the end fault occurs at 15% of the protection range. Indicates the minimum operating mode P when off-grid i When the downstream line end fails, the current flows through P i Protection against short-circuit current, Represents the maximum operating mode P when off-grid i Protect the short-circuit current flowing through this section of the line when a fault occurs at the head end of the line; Respectively represent the off-grid time P i At 15% of the protection range and at the end of the two-phase short circuit fault, Indicates off-grid P i Protect the downstream line end from two-phase short circuit faults. Off-grid time P i Protect the head-end three-phase short-circuit fault.

[0055] (4) According to the optimization model with equation (1) as the objective function and equations (2) and (3) as constraints, the particle swarm algorithm is used to calculate the maximum photovoltaic capacity at each location that meets the current protection requirements. When the photovoltaic capacity is less than the calculated capacity, the current protection can sensitively and selectively identify the fault in both off-grid and off-grid conditions. However, when the connected photovoltaic capacity is greater than the maximum capacity allowed by the calculation, the current protection will malfunction or refuse to operate.

[0056] Embodiment 3: A microgrid overcurrent protection applicability evaluation method of this embodiment pays special attention to the impact of high-proportion distributed new energy access and diversified on-grid / off-grid operation modes on the safe and stable operation of the microgrid. According to the particle algorithm content in the content of embodiment 1 or 2, a current protection applicability boundary calculation method taking into account the on-grid and off-grid state is designed. The core of this method is to optimize the photovoltaic access capacity using the particle swarm algorithm to meet the needs of current protection. Using the particle swarm optimization algorithm (PSO) to solve the above optimization model includes the following contents.

[0057] The particle swarm optimization algorithm is an optimization algorithm based on swarm intelligence. It simulates the foraging behavior of bird flocks and finds the optimal solution through information sharing and collaboration between individuals in the group. In the current protection applicability assessment, we use the access capacity of each photovoltaic as the position of the particle, and the speed of the particle represents the adjustment direction of the access capacity. By continuously updating the position and speed of the particle, we find the maximum photovoltaic access capacity that meets the constraints.

[0058] The following are the specific steps of the particle swarm algorithm: Initialize the particle swarm: Set the size of the particle swarm (i.e., the number of particles), each particle represents a possible solution, i.e., the access capacity of each photovoltaic. Initialize the position and speed of the particles, which can be generated randomly or based on some heuristic strategy.

[0059] Calculate the fitness function value: For each particle (i.e., each combination of photovoltaic access capacity), calculate its fitness function value according to the objective function (Formula 1) and the constraints (Formula 2 and Formula 3). The fitness function value can be defined based on the sensitivity and selectivity of the current protection. For example, the probability of false operation and refusal of the current protection can be calculated and used as a component of the fitness function.

[0060] Update individual optimal solution and global optimal solution: For each particle, compare its current fitness function value with the fitness function value of the individual historical optimal solution. If the current value is better, update the individual optimal solution. At the same time, compare the individual optimal solutions of all particles to find the global optimal solution, that is, the optimal solution found in the entire particle group.

[0061] Update particle speed and position: Update the speed and position of each particle based on the individual optimal solution and the global optimal solution. Usually, the particle speed update takes into account the current speed, the position of the individual optimal solution, and the position of the global optimal solution, and performs a weighted sum with a certain weight. Then, adjust the particle position based on the updated speed.

[0062] Iteration and termination condition: Repeat steps 2 to 4 until the termination condition is met. The termination condition can be that the preset maximum number of iterations is reached, or the fitness function value reaches a preset threshold, or the position and speed changes of the particle swarm are less than a preset threshold, etc.

[0063] Output results: When the algorithm terminates, the global optimal solution is output, that is, the maximum photovoltaic capacity at each location that meets the current protection requirements.

[0064] Through the application of particle swarm optimization, the problem of current protection applicability evaluation when a high proportion of distributed photovoltaics is connected to the microgrid can be effectively solved, and the maximum photovoltaic access capacity that meets the current protection requirements can be found, providing a strong guarantee for the safe and stable operation of the microgrid.

[0065] It should be noted that the performance of the particle swarm algorithm is affected by many factors, such as particle swarm size, number of iterations, weight parameters, etc. In practical applications, it is necessary to adjust and optimize according to specific problems to obtain better optimization results. In addition, since the topology and operation mode of the microgrid may change at any time, in practical applications, the applicability of the current protection needs to be regularly evaluated and adjusted to ensure the safe and stable operation of the microgrid.

[0066] The particle swarm algorithm is an efficient global optimization algorithm that can quickly search for optimal solutions that meet the constraints by simulating the information sharing and collaboration mechanism in the foraging behavior of bird flocks. In the evaluation of the suitability of microgrid current protection, the particle swarm algorithm can quickly find the maximum photovoltaic access capacity that meets the current protection requirements, greatly improving the efficiency and accuracy of the evaluation.

[0067] It has strong robustness and adaptability. The topology and operation mode of the microgrid may change with the access of new energy and changes in load, and the particle swarm algorithm can adaptively adjust the search strategy and parameters according to the changed conditions to maintain high optimization performance. Therefore, even if the operating environment of the microgrid changes, the particle swarm algorithm can still effectively evaluate the applicability of current protection.

[0068] The maximum photovoltaic access capacity obtained through particle swarm algorithm optimization can provide an important basis for the planning and operation of microgrids. The planning of microgrids needs to consider the access capacity of new energy and the requirements of current protection, and the particle swarm algorithm can help planners find a solution to maximize the access capacity of new energy while meeting the current protection requirements. At the same time, during the operation process, according to the optimization results obtained by the particle swarm algorithm, the photovoltaic access capacity and the current protection constant can be adjusted to ensure the safe and stable operation of the microgrid.

[0069] Using particle swarm algorithm to evaluate the suitability of current protection can also reduce the operation risk of microgrids. By optimizing the calculated photovoltaic access capacity and current protection setting, the possibility of current protection misoperation or refusal to operate can be reduced, and the fault identification and self-protection capabilities of microgrids can be improved. This helps to reduce the failure rate and maintenance cost of microgrids and improve the reliability and economy of the system.

[0070] It should be understood that the embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope limited by the appended claims of the application.

Claims

1. A method for evaluating the suitability of overcurrent protection of a microgrid, comprising obtaining the microgrid on-grid and off-grid operation topologies respectively, characterized in that: Also includes: Based on the grid-connected and off-grid operation topologies, the respective short-circuit currents are obtained, and the three-stage current protection values ​​at each location within the preset protection range are calculated; A high proportion of distributed photovoltaics is connected to the microgrid, and its maximum access capacity is used as the objective function. Constraints are established using the three-stage current protection values ​​and operating parameters of grid-connected and off-grid, and the optimization model for evaluating the current protection situation is adjusted.

2. A microgrid overcurrent protection applicability evaluation method according to claim 1, characterized in that: The initialization constraint condition is that the capacity value of the i-th photovoltaic access to the distribution network is non-negative and less than the maximum photovoltaic access capacity, and i is a non-negative integer; When in the grid-connected state, a grid-connected correspondence relationship between the connected photovoltaic in the test operation state or the normal operation state and the preset line short-circuit fault condition is established, and a value of the grid-connected correspondence relationship is taken and compared with the corresponding current value of the preset line operation.

3. A microgrid overcurrent protection applicability evaluation method according to claim 2, characterized in that: The grid-connected correspondence and comparison situations include: Minimum operation mode: the value of the corresponding relationship between the test operation state and the short-circuit fault condition at m% of the preset protection range is equal to the short-circuit current value at the time of the fault; the value of the corresponding relationship between the test state and the short-circuit fault condition at the end of the downstream line of the preset protection range is equal to the short-circuit current value within the preset protection range at the time of the fault; Maximum operation mode: The corresponding relationship between the normal operation state and the three-phase short-circuit fault condition in the first section of the preset protection range is equal to the short-circuit current value when the first end of the preset protection line fails; Establish the quotient constraint range of the grid-connected corresponding relationship value and the three-stage current protection value.

4. A microgrid overcurrent protection applicability assessment method according to claim 3, characterized in that: The scope of grid connection constraints includes: In the minimum operation mode: the quotient of the short-circuit current value at 15% of the preset protection range and the current value of section I is greater than 1; the quotient of the short-circuit current value flowing through this section of the line and the current value of section II when a fault occurs at the end of the preset protection range is greater than 1.3; the quotient of the short-circuit current value flowing through the preset protection range and the current value of section III when a fault occurs at the end of the downstream line is greater than 1.3; Under the maximum operation mode: when the first end of the preset protection line fails, the quotient of the short-circuit current value flowing through this section of the line and the current value of section III is less than 1.

5. A microgrid overcurrent protection applicability assessment method according to claim 1, characterized in that: The initial off-grid constraint condition is that the capacity value of the ith photovoltaic access distribution network is non-negative and less than the maximum photovoltaic access capacity at this time; The off-grid correspondence between the test operation state of photovoltaic access and the normal operation state and the preset line short-circuit fault condition is established respectively, and the off-grid correspondence is taken and compared with the corresponding current value of the preset protection line operation.

6. A microgrid overcurrent protection applicability assessment method according to claim 5, characterized in that: The comparison between the off-grid corresponding relationship value and the corresponding current value includes: Minimum operation mode: the value of the corresponding relationship between the test operation state and the short-circuit fault condition at n% of the preset protection range is equal to the short-circuit current value at the short-circuit fault; the value of the corresponding relationship between the test state and the short-circuit fault condition at the end of the downstream line of the preset protection range is equal to the short-circuit current value when the downstream line fails; Maximum operation mode: The corresponding relationship between the normal operation state and the three-phase short-circuit fault condition in the first section of the preset protection range is equal to the short-circuit current value when the first end of the preset protection line fails; Establish a quotient constraint range between the off-grid corresponding relationship value and the three-stage current protection value.

7. A microgrid overcurrent protection applicability assessment method according to claim 6, characterized in that: Off-grid business constraints include: In the minimum operation mode: the quotient of the short-circuit current value at 15% of the preset protection range and the current value of section I is greater than 1; the quotient of the short-circuit current value flowing through this section of the line and the current value of section II when a fault occurs at the end of the preset protection range is greater than 1.3; the quotient of the short-circuit current value flowing through the preset protection range and the current value of section III when a fault occurs at the end of the downstream line is greater than 1.3; Under the maximum operation mode: when the first end of the preset protection line fails, the quotient of the short-circuit current value flowing through this section of the line and the current value of section III is less than 1.

8. A microgrid overcurrent protection applicability assessment method according to claim 3 or 6, characterized in that: In the test state of photovoltaic access, the constraint condition for the second section of the three-section current protection value is that the feeder and upstream photovoltaic within the preset protection range are out of operation; the constraint condition for the third section is that the upstream photovoltaic within the preset protection range and the remaining feeder photovoltaics are out of operation; and m or n indicates that the range is within the preset protection range.

9. A method for evaluating the suitability of overcurrent protection for a microgrid according to claim 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8, characterized in that: The capacity values ​​of all connected photovoltaics in the distribution network are compared to obtain the maximum photovoltaic capacity value; the maximum photovoltaic capacity value is used as the objective function, and an optimization model is established according to the grid-connected and off-grid constraints. The particle swarm algorithm is used to calculate the maximum access capacity of each location that meets the current protection requirements; When the photovoltaic capacity is less than the maximum accessible capacity, the current protection selectively identifies the fault; when the connected photovoltaic capacity is greater than the maximum accessible capacity, the current protection is activated and malfunctions or refuses to operate.

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