Urban power distribution network direct current interconnection site selection method and system based on mixed integer programming

Abstract

The invention provides an urban power distribution network direct current interconnection site selection method and system based on mixed integer programming, and belongs to the technical field of power distribution network planning, and the method comprises the steps: constructing an index system for evaluating the optimal site selection of voltage source converter-direct current line access; constructing a target function based on the index system; determining the number of nodes of the power distribution network, sequentially numbering buses as operable nodes, and constructing a connection matrix representing the whole power distribution network; expressing constraint conditions of the power distribution network based on the connection matrix of the whole power distribution network; and iterating the target function by using a constraint condition and an algorithm, and solving the optimal site selection of the voltage source converter-direct current line access.

Description

Technical Field

[0001] This invention belongs to the field of power distribution network planning technology, and in particular relates to a method and system for DC interconnection site selection of urban power distribution networks based on mixed integer planning. Background Technology

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

[0003] The surge in distributed renewable energy and new DC loads has spurred the development of DC-based urban power distribution networks. These new DC loads include charging stations and data centers. In traditional AC power distribution networks, AC lines are primarily used to connect DC equipment. However, the frequent switching of equipment during AC-DC power conversion has been shown to significantly increase power losses. Especially after the integration of renewable energy sources, the bidirectional flow of current further increases losses. The extensive integration of distributed power sources, energy storage, and flexible loads makes the system's control variables complex and highly coupled.

[0004] Traditional AC distribution network control strategies struggle to adapt to this complexity, leading to protection failures. Furthermore, the integration of renewable energy sources increases the complexity of fault currents, obscuring fault characteristics. Consequently, commonly used current and distance protection systems in traditional AC distribution networks may malfunction. All these issues demonstrate the significant limitations of traditional AC distribution networks in handling large-scale DC equipment integration.

[0005] Compared to AC systems, DC power distribution can handle higher power density, transmit over longer distances, and has lower overall losses in lines and conversion stages. It also maintains relatively good voltage quality without the need for large reactive power compensation devices. Furthermore, DC networks support closed-loop operation, theoretically saving 25%–30% of urban cable corridor resources. These advantages make "DCization" a significant marker of future power distribution network evolution. The traditional single-ended radial AC structure is gradually evolving into a hybrid form of "AC backbone + DC subgrid," achieving combined gains in energy efficiency, reliability, and flexibility.

[0006] However, AC / DC hybrid distribution networks differ significantly from traditional AC grids in resource allocation and operational mechanisms: rotational inertia and short-circuit capacity decrease significantly, leading to a structural shortage of resources for flexible system adjustment; distributed photovoltaic, energy storage, and V2G "source-containing loads" blur the traditional source-grid-load boundaries, resulting in multi-dimensional coupling of control objects; the randomness of renewable energy and loads transforms power balance from a deterministic problem into a high-dimensional probabilistic decision-making problem; furthermore, new stability patterns, such as DC voltage stability and broadband oscillations, are constantly emerging, weakening the overall safety and stability margin of the distribution network. The combined effect of strong source-side volatility and the trend of source-load fusion at the load end fundamentally changes the planning boundary conditions, operational constraint sets, and reliability-economic assessment methods for transmission and distribution networks.

[0007] In recent years, flexible DC technology has developed rapidly, achieving significant progress and many practical results in both power transmission and distribution network planning. In urban distribution network planning, many scholars have comprehensively described the concept of AC / DC hybrid distribution networks and verified it using various experimental data and operating conditions. Results show that AC / DC hybrid distribution networks have stronger renewable energy absorption capacity, more flexible power flow control, and higher power reliability. They represent a viable solution for addressing high losses and control difficulties under large-scale DC equipment integration conditions.

[0008] To achieve DC embedding in AC distribution networks, suitable AC-DC converters are required. Voltage source converters (VSCs) offer flexible power control capabilities. Furthermore, practical experience has shown that these devices can operate independently of the reactive power of the AC system, functioning as Static Synchronous Compensators (STATCOMs) to ensure system stability. In addition, the converter station structure for VSC-DC lines is relatively simple, requiring only the addition of VSCs at both ends of the DC line to achieve DC interconnection between AC distribution network equipment. This eliminates the need for complex harmonic filtering equipment, thus reducing construction and maintenance costs. Therefore, implementing embedded VSC-DC interconnection in urban distribution networks with distributed renewable energy and DC load access is an effective solution for reducing power loss and ensuring reliable control. Adding DC to AC distribution systems is a crucial solution for upgrading traditional distribution networks and has become a research hotspot. However, existing technologies lack research on the new characteristics of DC lines compared to AC lines, still relying on AC system standards. Moreover, existing site selection patents do not address the increased system failure risk and renewable energy absorption issues under high-proportion renewable energy integration. To promote the application of VSC-DC technology in urban power distribution networks, the first step is to solve the problem of the location of voltage source converter-DC lines.

[0009] Currently, the AC / DC transformation of distribution networks remains at the stage of directly connecting DC equipment at both ends of the line or upgrading the connection lines between two distribution networks. Most studies consider optimal economic efficiency as the optimization objective. However, due to the volatility and frequent power flow changes caused by the high proportion of new energy equipment and DC loads connected to the distribution network, line overload and loss problems arise. How to maximize the function of DC lines in urban distribution networks to improve the reliability and resilience of the distribution network is a problem that needs to be solved. Summary of the Invention

[0010] To overcome the shortcomings of the prior art, this invention provides a method and system for DC interconnection site selection in urban power distribution networks based on mixed integer programming. It takes into account the impact and improvement of DC access on transient stability, so as to select the appropriate location for installing new VSC-DC equipment or replacing existing AC lines between two areas, thereby maximizing the function of DC lines in urban power distribution networks.

[0011] To achieve the above objectives, one or more embodiments of the present invention provide the following technical solutions: Firstly, a method for DC interconnection site selection in urban distribution networks based on mixed integer programming is disclosed. include: Determine the number of nodes in the distribution network, and number the busbars as operable nodes sequentially to construct a connection matrix representing the entire distribution network; The constraints of the power distribution network are expressed based on the connection matrix of the entire power distribution network. Construct an index system for evaluating the optimal location of voltage source converter-DC line access; Construct an objective function based on an indicator system; By iterating the objective function using constraints and mixed integer programming, the optimal location for the voltage source converter-DC line connection is found.

[0012] As a further technical solution, the indicator system includes: reliability indicators and resilience indicators. The reliability indicators include active power flexibility and VSC-DC interconnect utilization rate. The resilience indicators include N-1 failure index and short-circuit current improvement number.

[0013] As a further technical solution, the active power flexibility uses the ratio of unit active power change to unit voltage change as an indicator, which can intuitively measure the stability of the distribution network under power flow changes.

[0014] As a further technical solution, the utilization rate of the VSC-DC interconnector, the value of the newly added VSC-DC tie line in guiding and utilizing the active power flow of the distribution network, if the power flow value of the newly added VSC-DC tie line is low under normal operating conditions, it indicates that its utilization rate is low and it does not participate much in the power flow distribution of the distribution network. This indicator is used to ensure that the selected location can enable the VSC-DC interconnection to play its maximum role.

[0015] As a further technical solution, the N-1 fault index is used to ensure the resilience of the power grid and to ensure that users receive continuous power supply that meets quality requirements. This index is used to measure the overall voltage drop of the remaining nodes in the event of an N-1 fault.

[0016] As a further technical solution, the short-circuit current improvement factor is improved by VSC control of the newly added VSC-DC line.

[0017] As a further technical solution, the objective function is defined as follows:

[0018]

[0019] in, w 1 and w 2 represents the weights of the reliability index system and the resilience index system in the overall objective function, respectively. α and β F represents the weight of each indicator in the two indicator systems. p For active power flexibility, F dc For VSC-DC interconnect utilization, F f F is the failure index N-1. i To improve short-circuit current.

[0020] As a further technical solution, the constraints of the entire power distribution network include: power flow constraints and power equipment constraints.

[0021] Secondly, a DC interconnection location system for urban distribution networks based on mixed integer programming is disclosed, including: The constraint construction module is configured to: determine the number of nodes in the distribution network, number the buses as operable nodes sequentially, and construct a connection matrix representing the entire distribution network. The constraints of the power distribution network are expressed based on the connection matrix of the entire power distribution network. The indicator system construction module is configured to: construct an indicator system for evaluating the optimal location of voltage source converter-DC line access; The objective function construction module is configured to: construct objective functions based on an indicator system; The solution module is configured to iterate the objective function using constraints and a mixed integer programming algorithm to find the optimal location for the voltage source converter-DC line connection.

[0022] The above one or more technical solutions have the following beneficial effects: Based on the construction of an index system for evaluating the optimal location of voltage source converter-DC line access, this method is used to assess the location selection of embedded voltage source converter-DC bus in urban power distribution networks, thus overcoming the inherent limitations of traditional AC indexes.

[0023] Compared to traditional AC line performance indicators, the proposed system for embedding DC lines in urban distribution networks considers more factors, including DC utilization and short-circuit current optimization, to quantify the performance improvement of DC lines on the distribution network. DC utilization ensures that the selected locations allow VSC-DC interconnects to fully participate in power flow distribution and maximize their effectiveness. Short-circuit current optimization addresses the challenges of large short-circuit currents and ambiguous fault characteristics in the context of numerous power electronic devices being integrated, making short-circuit fault identification difficult. Key factors such as transmission reliability and network structure flexibility are incorporated. Transmission reliability corresponds to the N-1 fault index, and network structure flexibility corresponds to the active power flexibility index, ensuring improved overall performance after selecting the appropriate VSC-DC connection locations based on voltage source converters.

[0024] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0025] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0026] Figure 1 This is a schematic diagram of the connection matrix transformation representation according to an embodiment of the present invention; Figure 2 This is a topology diagram of the experimental distribution network area in an embodiment of the present invention; Figure 3 The waveforms before and after DC connection are shown in the comparison diagram. Figure 4 This is a schematic diagram of the optimal addressing topology under network and network control. Figure 5 This is a schematic diagram of the VSC-DC interconnection site selection process for urban power distribution networks based on mixed integer programming. Detailed Implementation

[0027] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0028] It should be noted that the terminology used herein is for the purpose of describing particular implementations only and is not intended to limit the exemplary implementations of the present invention.

[0029] Where there is no conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0030] Definitions: VSC is short for Voltage Source Converter. GFL is short for Grid-Following, which means "grid-following control". GFM is short for Grid-Forming, which translates to "grid control" in Chinese. STATCOM, Chinese: Static Synchronous Compensator; SCR is short for Short-Circuit Ratio. AHP is short for Analytic Hierarchy Process. PI stands for Proportional-Integral Controller. UDN is short for Urban Distribution Network. V2G is short for Vehicle-to-Grid, which translates to "car-to-grid interaction" in Chinese.

[0031] Example 1 In this implementation example, based on the operating characteristics of the VSC-DC interconnection equipment and the functional requirements of the connection area, an evaluation index system for the location selection results of the two ends of the VSC-DC interconnection equipment in the urban power distribution network is constructed, providing a quantitative reference for determining the optimal embedding location. Unlike existing location selection methods, the proposed evaluation index system additionally considers DC-related indicators such as the reliability and resilience of the urban power distribution network, in order to maximize the stability and control benefits of embedded DC lines in the urban power distribution network.

[0032] To improve the effectiveness and accuracy of DC line site selection evaluation, a stochastic optimization model for new energy equipment was considered during VSC-DC interconnection site selection. Stochastic modeling was performed for distributed photovoltaic (PV) systems and new DC loads in urban areas, using a generalized Pareto distribution model. The PV model was a Johnson bounded system. Stochastic model parameters were calculated using typical days across four seasons.

[0033] Compared to deterministic models, stochastic optimization location models better reflect the inherent complexity and uncertainty of hybrid AC / DC distribution networks. This approach has the advantage of more accurately reflecting the randomness and volatility of renewable energy and DC loads, thus contributing to the development of more practical location methods.

[0034] Furthermore, the proposed VSC-DC interconnection location method considers the control differences between grid-following control (GFL) and grid-building control (GFM). Location tests were conducted on both control methods, and based on the final location differences and score contributions, the conclusion is that grid-following control focuses on optimizing power flow and maintaining stable operation of the distribution network under various load conditions. Grid-building control focuses on the transient support capability of the distribution network. This improves the adaptability of the proposed method to location selection under different control modes. Based on the specific control difference indicators in the proposed VSC-DC interconnection point location selection evaluation index system, interconnection points can be selected according to control objectives to improve the control effect of embedded DC interconnections.

[0035] Embedded VSC-DC interconnects were added to the existing urban power distribution network, and the optimization problem was formulated as a mixed-integer programming problem. Python 3.7 was used to represent the connection matrix as integer constraints, while other constraints were non-integer constraints. Finally, the objective function derived from the indicators was solved. Specifically, the solution was achieved using the CPLEX solver. Since the goal of this study is to optimize the original power distribution network, the new VSC-DC interconnects could be included in the study without deleting or modifying the initial lines. The new VSC-DC interconnects are connected to the AC bus, and VSCs are installed at both ends of the DC lines.

[0036] The VSC (Voltage Source Converter) converter station used in this example includes a control strategy switching module. Let k be a switching parameter with a slope g as the rate of change, and let VSC's reference voltage Vref be equal to... Vref,gfm represents the voltage reference value under grid connection control of the converter, and Vref.gfl represents the voltage reference value under grid connection control of the converter. The reference voltage is smoothly switched between grid connection mode and grid connection mode by changing k from 1 to 0 or 0 to 1.

[0037] Using this function, converter stations can select the more suitable control strategy between GFM and GFL under different application conditions. The GFL control mode is only applicable to active power grids and depends on the frequency stability of the urban power grid. The outer-loop power controller can perform active and reactive power control. Through a PI regulator, the outer-loop power controller calculates and generates corresponding d-axis and q-axis current commands based on power or voltage errors. This achieves the effect of decoupling P and Q control, which improves the power quality of the selected area. The GFM control mode can simulate frequency and provide voltage regulation similar to a synchronous generator, thus independently supporting the system voltage at a selected location, enabling it to operate in islanded mode or weak grid conditions. This example will consider the optimal location of the VSC-DC under both control strategies, specifically conducting two experiments for each control mode to obtain two different site selection results.

[0038] Since the line coupling mechanism under VSC interconnection lacks a detailed and reasonable explanation in many power electronic equipment applications, addressing via mixed-integer programming is clearly the most effective, easiest to understand, and easiest to implement method. Unlike pure AC line connections, VSC interconnection can bypass the radial constraints of the line topology, meaning that all buses within the distribution area can become potential connection points. It is worth noting that due to geographical factors or other reasons, some buses may be difficult to interconnect via DC; this example will not consider such cases.

[0039] For details, please see the appendix. Figure 1 As shown, this embodiment discloses a method for DC interconnection site selection in urban distribution networks based on mixed integer programming, including: Step 1: Determine the number of nodes in the distribution network, and number the busbars as operable nodes sequentially to construct a connection matrix representing the entire distribution network; Step 2: Express the constraints of the power distribution network based on the connection matrix of the entire power distribution network, including power flow constraints and power equipment constraints; Step 3: Construct an index system for evaluating the optimal location of voltage source converter-DC line access; Step 4: Construct the objective function based on the indicator system; Step 5: Iterate the objective function using constraints and mixed integer programming algorithm to find the optimal location for the voltage source converter-DC line connection.

[0040] In one implementation example, in step one, regarding constraints and location selection methods: the number of nodes in the distribution network should also be clearly defined, and the busbars should be numbered sequentially as operable nodes. Assuming there are n nodes, then a... The square matrix L represents the connection matrix of the entire power distribution network.

[0041] in L(n i ,n j ) =0 indicates a node n i and nodes n j There is no communication line connection between them. L(n i ,n j ) =1 indicates a node n i and nodes n j There is an AC line connecting them. For example... Figure 1 As shown, select L(n i ,n j ) Elements with a value of 0 are converted to L(n i ,n j ) =2 indicates that two unconnected buses are connected through the VSC-DC interconnector.

[0042] In practical applications, not all two buses can be connected via VSC-DC interconnectors. Depending on local geographical and wiring layout conditions, some connections may need to be cancelled. Another matrix... J(n i ,n j ) Used to determine nodes n i and nodes n j Can a VSC-DC interconnect be added between them, as a condition matrix? If so... J(n i ,n j ) =1 means n i and n j There is no DC interconnection condition between them. During the selection phase, if the nodes... i and j of L(n i ,n j ) =2 and J(n i ,nj ) If the result is 1, proceed directly to the next selection.

[0043] The steady-state model of VSC is represented as follows: (1) The above line-to-line AC voltage RMS value The calculation is for DC voltage. V k dc A function of the converter modulation index M. Converter constant. K c The value depends on the type of VSC and the modulation technique. In this example, a three-phase VSC with sinusoidal pulse width modulation is used.

[0044] The relationship between the DC voltage reference and the AC voltage reference is as follows: (2) Therefore, 1 p.u DC voltage is equivalent to 1 p.u AC voltage with a unit modulation index, as shown below: (3) AC active power P vsc,ac The calculation formula is as follows: (4) in η c For VSC efficiency, I dc It is direct current.

[0045] reactive power Q vsc,ac It can be controlled by direct setpoint or based on the converter power factor angle. φ c The function is determined as shown below.

[0046] (5) Formulas (1) to (5) above are the modeling formulas for VSC. They are subsequently used as part of the overall distribution network model and are the grid equipment constraints of the objective function.

[0047] VBac is the AC voltage reference value; VBdc is the DC voltage reference value; Kc is the voltage source converter constant, the magnitude of which depends on the type of VSC and the modulation technique. This is the per-unit voltage value of DC bus i, which refers to the voltage amplitude of the AC side bus connected to VSC; The per-unit voltage value of AC bus k refers to the voltage amplitude of the DC side bus connected to VSC; M is explained above, the converter modulation index; Active power on the AC side; The voltage of DC bus k, i.e., the voltage of the VSC DC port; DC current: The current flowing through the DC side of VSC; The efficiency of VSC is calculated by dividing by the efficiency in the formula because it is derived from the DC side (source end or outlet end) to the AC side, taking into account losses. The conductance of a DC circuit is equal to the reciprocal of its DC resistance. : Voltage amplitude of AC bus i; : Voltage of adjacent DC bus j. Appears in the conductance calculation section, representing the voltage of another DC node j connected to the DC side of VSC (node ​​k), used to calculate the power flowing through the DC line.

[0048] Based on matrix L The constraints of the entire power distribution network can then be easily expressed. L(n i ,n j ) =0 indicates a node n i and nodes n j There is no communication line connection between them. L(n i ,n j ) =1 indicates a node n i and nodes n j There is an AC line connection between them. Therefore, the constraints will include... L(n i ,n j ) As a factor, it can be convenient to give the overall constraint.

[0049] Active power P ij and reactive power Q ij Define as a node i and j Power flow between V i and V j Let be the voltage at the two nodes.

[0050] (6) (7) (8) (9) in, N b This refers to the set of nodes in the distribution network. P ac,ij Representative node i and nodes j Active power transmission in AC lines. P ac,ij,max represent i , j The maximum rated value of active power that the AC line between them can transmit. P dc,ij Represents a node i and nodes j Active power transmission of the newly added DC lines between them P dc,ij,max represent i , j The newly added DC line can transmit the maximum rated value of active power. V max,i Represents a node i The maximum voltage it can withstand. T ij It is a derived function that distinguishes between DC line interconnections between nodes and other cases.

[0051] Formulas (10)-(13) are used to calculate the busbars respectively. i Active power injection P i inj ， Calculated active power P i cal Reactive power injection Q i inj Calculated values ​​of reactive power Q i cal .

[0052] Only after performing active and reactive power flow calculations can the voltage at each point, line power flow, and power of VSC-DC interconnected lines be obtained. All parameters in the indicators must be obtained through these calculations.

[0053] (10) (11) (12) (13) in, ηI,i VSC is on the node i The above represents the inverter's efficiency, expressed as a percentage. Similarly, η R,i It refers to the efficiency of the rectifier. G ij and G ij dc Indicates busbar i and j The conductivity of AC and DC lines between them. Q gen,i ac and Q load,i ac These are the reactive power of the AC generator and the bus power, respectively. i The reactive power of the AC load. Q genc,i dc and Q loadc,i dc They are nodes i The reactive power injected and absorbed by the VSC at the DC interconnect. Q c,ij This refers to the reactive power on the AC side of the VSC.

[0054] P gen,i and Q load,i They are nodes i The active power output of the generator and the node i Active load, θ ij It is a bus stop i and j The voltage phase angle difference between them.

[0055] sign( x (Using the symbolic function to compare bus nodes) i and j The magnitude of the voltage between them determines the direction of the power flow. G ij and G ij dc Let represent the conductance of the AC and DC lines between bus nodes i and j, respectively. B ij and B ij dc Representing bus stops i and j The susceptance of the AC and DC lines between them.

[0056] (14) (15) The integer variable constraints are expressed as (16) and (17).

[0057] (16) (17) Where, sign( x (Using the symbolic function to compare bus nodes) i and j The magnitude of the voltage between them determines the direction of the power flow. x The defined parameters are used to pass into the symbolic function. M nm With M mn This is the converter modulation index.

[0058] Specifically, in step three, in this implementation example, an index system is constructed to find the optimal location for VSC-DC access, and the above indexes are used as the objective function. Accordingly, a distribution network model for location selection is introduced. Constraints and a mixed-integer programming algorithm are used to iterate the location selection. The following is a detailed explanation.

[0059] The distribution network model for site selection incorporates line constraints from VSC modeling formulas 1-5 and 14-23. A mixed-integer programming algorithm is used, programmed in Python and solved using the CPLEX solver.

[0060] First, in order to maximize the reliability and resilience of the urban power distribution network, the following indicator system is proposed: The resilience of a distribution network refers to its ability to return to operational status after being disrupted. Currently, there is no clearly unified definition for this, either domestically or internationally. Because the distribution network structure is relatively weaker than the main grid, improving its reliability and resilience is of great significance.

[0061] Traditional urban power distribution networks are susceptible to failure under the influence of natural disasters. Furthermore, data anomalies in extreme situations can significantly reduce power supply reliability. Power can only be restored to a limited extent through relay protection devices, which can easily lead to prolonged power outages and the expansion of outage areas. Therefore, it is necessary to study the resilience of power distribution networks against natural disasters. Research on the disaster resistance of power distribution networks must first consider their ability to withstand damage from natural disasters. Therefore, the resilience index quantitatively describes the resilience of power distribution networks by measuring the power supply stability of critical loads and the disaster resistance of the network structure.

[0062] All the following indicators assume that the number of busbar nodes in the distribution network is 1.N b .

[0063] Reliability index system Active power flexibility: (18) in k i for (19) in, N b Let i be the number of busbar nodes in the distribution network, and i be any busbar node, with values ​​ranging from 1 to... N b Δu represents the unit voltage change at the bus node after adding the VSC-DC interconnect. Δp represents the power change at the bus node under the corresponding unit voltage change. k i' This represents the ratio of the change in unit active power to the change in unit voltage on bus i after the addition of VSC-DC interconnection.

[0064] This indicator is inspired by the definition of Short-Circuit Ratio (SCR), but differs from the SCR indicator because the impedance of the SCR indicator is calculated based on the equivalent circuit of the AC system and cannot accurately reflect the situation of the distribution network system with VSC-DC interconnection. Therefore, its definition has been modified for greater intuitiveness. VSC-DC tie lines have the advantages of increasing transmission capacity and flexibility in active / reactive power regulation.

[0065] VSC-DC interconnect utilization: (20) in, P dc The active power on the newly built VSC-DC interconnection line. P in,i Indicates the injection node i active power, P in,j Indicates the injection node j active power, P in,i ' , P in,j ' Representing nodes respectively i and nodes j The active power injected after adding VSC-DC access, F dc VSC-DC interconnect utilization.

[0066] VSC-DC interconnect utilization reflects the value of newly added VSC-DC tie lines in guiding and utilizing the active power flow of the distribution network. If the power flow of the newly added VSC-DC tie line is small under normal operating conditions, it indicates that its utilization rate is low and it does not participate much in the power flow distribution of the distribution network. This indicator is used to ensure that the selected location allows the VSC-DC tie line to achieve maximum effectiveness.

[0067] If the newly built VSC-DC interconnection line has a low power flow value under normal operating conditions, it indicates that the newly built VSC-DC interconnection line does not participate much in the power flow distribution of the distribution network, that is, the line utilization rate is low. This indicator reflects the role of the newly built line in guiding and distributing the active power flow of the distribution network.

[0068] Elasticity index: The N-1 fault index is an important standard for evaluating the safety and stability of power systems. It has two meanings: first, ensuring the stability of the power grid; and second, ensuring that users receive a continuous power supply that meets quality requirements. Specifically, the N-1 criterion is used for static security analysis of power systems under fault-free disconnection conditions, or for dynamic security analysis of power system stability after a fault disconnection.

[0069] (twenty one) (twenty two) in, j Indicates the bus node where a short-circuit fault occurred. N rest Indicates except j The remaining nodes, η j This indicates the degree to which the voltage of the remaining nodes is maintained in the event of a fault at node j, where V i This represents the voltage at which node i fails. V b Represents the bus reference voltage. F f The failure index is N-1. η j This indicator can be used to measure the overall voltage drop of the remaining nodes under N-1 fault conditions, to ensure that the power supply capacity of the remaining buses in the distribution network is not significantly affected in the event of any fault.

[0070] Improved short-circuit current: (twenty three) In the formula, where I i,sc and I ' i,sc These are the front and rear nodes of the VSC-DC interconnection access.i The three-phase steady-state short-circuit current, and I i,op This is the average normal operating current of node i in the original UDN. It can be seen that... F i The improvement effect of UDN on short-circuit current after VSC-DC interconnection embedding can be measured. I ' i,sc The closer I i,op , Fi The larger the value, the better the mitigation effect on short-circuit current.

[0071] Large short-circuit currents in urban power distribution networks are a pressing issue that AC systems struggle to handle. Short-circuit currents can be mitigated by implementing VSC control on newly connected VSC-DC converters.

[0072] Specifically, in step four, the active power flexibility mentioned above is inspired by the short-circuit ratio index of the power grid, but the definition of the short-circuit ratio (SCR) index is modified to avoid the problem that the traditional SCR index, based on the equivalent circuit calculation of the AC system, cannot accurately reflect the situation of the distribution network with VSC-DC tie line. Using the ratio of unit active power change to unit voltage change as an index, the stability of the distribution network under the influence of power flow changes can be directly measured.

[0073] VSC-DC interconnect utilization rate indicates the value of the newly added VSC-DC tie line in guiding and utilizing the active power flow of the distribution network. If the power flow value of the newly added VSC-DC tie line is low under normal operating conditions, it indicates that its utilization rate is low and it does not participate much in the power flow distribution of the distribution network. This indicator is used to ensure that the selected location allows the VSC-DC interconnect to achieve maximum effectiveness.

[0074] The N-1 Fault Index serves as an indicator for assessing the safety and resilience of a power system. It encompasses two aspects: 1. Ensuring the resilience of the power grid. 2. Ensuring users receive a continuous power supply that meets quality requirements. This index measures the overall voltage drop at remaining nodes in the event of an N-1 fault (a single component failure). This ensures that the power supply capacity of the remaining buses in the distribution network is not significantly affected when any fault occurs.

[0075] The short-circuit current improvement formula was proposed to address the problem of excessive short-circuit current caused by the integration of numerous power electronic devices in new distribution networks, a problem that AC systems struggle to handle. It improves short-circuit current through VSC control of the newly added VSC-DC lines.

[0076] Combining the indicators proposed above and considering them, the objective function is defined as follows: [Evaluation based on weighted average to ensure the reliability and resilience of the distribution network] (26) (27) in, w 1 and w 2 represents the weights of the reliability index system and the resilience index system in the overall objective function, respectively. α and β These represent the weights of each indicator within the two indicator systems. F p For active power flexibility, F dc For VSC-DC interconnect utilization, F f F is the failure index N-1. i To improve short-circuit current.

[0077] The objective function can be calculated by simultaneously calculating node and line parameters using the above formula and the iterative location analysis of the connection matrix. This achieves accurate and optimal location for VSC-DC interconnection. Finally, the optimal location is determined by ranking the highest-scoring location scheme obtained through iterative connection matrix analysis to solve the objective function.

[0078] Specifically, in step five, to verify the effectiveness of the proposed method, a simulation experiment was conducted using a new distribution network area model located in Shandong Province, China. This model includes 29 bus nodes. See the attached diagram for the experimental distribution network topology. Figure 2 As shown.

[0079] In this scenario, the photovoltaic active power is modeled using historical data from the city's power distribution network as stochastic parameters. Therefore, the embedded VSC-DC interconnection location selection problem is a multi-objective optimization problem. To mitigate the limitations of single-method allocation of various indicator weights, this paper uses the Analytic Hierarchy Process (AHP) to determine the subjective indicator weights and the entropy weight method to calculate the objective indicator weights.

[0080] As can be seen from Table 1, for the GFL control strategy, adding a flexible VSC-DC connection between node 7 and node 9 can maximize the overall benefits of the urban power distribution network and is the optimal location for the urban power distribution network area.

[0081] Table 1. Optimal Location for GFL Control Strategy Position Selection Ranking Overall Score <![CDATA[Two end nodes of the line ( n x - n y )]]> 1 1 7-9 2 0.98909 2-9 3 0.94791 21-29 4 0.94151 11-15 5 0.88716 1-21 6 0.86591 3-23 7 0.86074 6-18 8 0.85824 3-24 9 0.85716 3-5 10 0.80955 7-17 As shown in Table 2, when using the GFM strategy, adding a VSC-DC connection between nodes 21 and 27 is the optimal choice. Compared to the VSC grid-based control strategy, this location is clearly farther from the root node of the urban distribution network. This is reasonable because in GFM operation mode, the VSC has reactive power compensation capabilities, which can support nodes with weaker voltage. It is worth noting that in this experiment, nodes 4 and 10 were considered unsuitable for installing VSC and VSC-DC interconnection equipment, and therefore these two cases were omitted.

[0082] Table 2 Optimal Location for GFM Control Strategy Position Selection Ranking Overall Score <![CDATA[Nodes at both ends of the line ( n x - n y )]]> 1 1 21-27 2 0.96644 16-19 3 0.95814 3-18 4 0.9145 19-29 5 0.91447 24-29 6 0.91225 17-27 7 0.87025 15-29 8 0.85121 7-13 9 0.84673 7-28 10 0.84544 6-18 To verify whether the performance of the distribution network improved after the VSC-DC interconnection device was connected, Figure 5 In the urban power distribution network, a short-circuit fault point is selected, and then various power indicators of the distribution network before and after connecting the VSC-DC interconnection device are analyzed. The simulation model is built and run on PscadV5, and the location selection algorithm and distribution network constraints are implemented using Python 3.12.

[0083] Comparison before and after DC input, for example Figure 3 As shown, fault selection occurs in 1 second and lasts for 0.2 seconds. (a) shows a comparison of voltage drops at adjacent nodes caused by VSC short-circuit faults under the GFM control strategy. It can be clearly seen that the minimum voltage under fault conditions was 0.258 before the VSC-DC interconnection device was connected. After connecting the VSC-DC interconnection device, this value rose to 0.342, an increase of 32.5%, indicating that the new line has voltage support capability. (b) shows that under the GFL control strategy, VSC can significantly reduce the short-circuit current amplitude at the same fault point compared to other location selection strategies. (c) shows that the VSC-DC interconnection line can significantly improve the power capacity of the distribution network from 2.5MW to approximately 2.8MW.

[0084] The indicator system in this example is used to evaluate the location selection of embedded voltage source converter-DC bus in urban power distribution networks, overcoming the inherent limitations of traditional AC indicators. The proposed framework incorporates key factors such as transmission reliability and network structure flexibility, ensuring improved overall performance after incorporating VSC-DC access location selection based on voltage source converters.

[0085] To verify the effectiveness of this indicator system, a mixed integer programming model for optimizing VSC-DC access location selection was developed and tested using a real urban power distribution network. Simulation results are shown in the appendix. Figure 4As shown, under the proposed index system, performance was significantly improved: the voltage of adjacent nodes increased by 32.5% during short-circuit faults, while the average short-circuit current amplitude decreased by 23.9% under optimized site selection. Furthermore, the power transmission capacity of the urban distribution network also increased by 12%, indicating that the proposed site selection index and method are highly effective in guiding the selection of DC interconnection nodes.

[0086] Example 2 The purpose of this embodiment is to provide a computer device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor executes the program to implement the steps of the above-described method.

[0087] Example 3 The purpose of this embodiment is to provide a computer-readable storage medium.

[0088] A computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, performs the steps of the above method.

[0089] Example 4 The purpose of this embodiment is to provide a DC interconnection site selection system for urban distribution networks based on mixed integer programming, including: The indicator system construction module is configured to: construct an indicator system for evaluating the optimal location of voltage source converter-DC line access; The objective function construction module is configured to: construct objective functions based on an indicator system; The constraint construction module is configured to: determine the number of nodes in the distribution network, number the buses as operable nodes sequentially, and construct a connection matrix representing the entire distribution network. The constraints of the power distribution network are expressed based on the connection matrix of the entire power distribution network. The solution module is configured to iterate the objective function using constraints and algorithms to find the optimal location for the voltage source converter-DC line connection.

[0090] Example 5 The purpose of this embodiment is to provide a computer program product containing instructions that, when run on a computer, cause the computer to perform the methods and functions involved in any of the above embodiments.

[0091] The steps and methods involved in the apparatus of the above embodiments correspond to those in Embodiment 1. For specific implementation details, please refer to the relevant description section of Embodiment 1. The term "computer-readable storage medium" should be understood as a single medium or multiple media including one or more instruction sets; it should also be understood as including any medium capable of storing, encoding, or carrying an instruction set for execution by a processor and enabling the processor to perform any of the methods in this invention.

[0092] Those skilled in the art will understand that the modules or steps of the present invention described above can be implemented using general-purpose computer devices. Optionally, they can be implemented using computer-executable program code, thereby allowing them to be stored in a storage device for execution by a computer device, or they can be fabricated as separate integrated circuit modules, or multiple modules or steps can be fabricated as a single integrated circuit module. The present invention is not limited to any particular combination of hardware and software.

[0093] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A site selection method for DC interconnection of urban distribution networks based on mixed-integer programming, characterized by: include: Determine the number of nodes in the distribution network, and number the busbars as operable nodes sequentially to construct a connection matrix representing the entire distribution network; The constraints of the power distribution network are expressed based on the connection matrix of the entire power distribution network. Construct an index system for evaluating the optimal location of voltage source converter-DC line access; Construct an objective function based on an indicator system; By iterating the objective function using constraints and mixed integer programming, the optimal location for the voltage source converter-DC line connection is found.

2. The urban distribution network DC interconnection site selection method based on mixed integer programming as described in claim 1, characterized in that, The indicator system includes: reliability indicators and resilience indicators. The reliability indicators include active power flexibility and VSC-DC interconnect utilization rate. The resilience indicators include N-1 failure index and short-circuit current improvement number.

3. The urban distribution network DC interconnection site selection method based on mixed integer programming as described in claim 1, characterized in that, The active power flexibility is measured by the ratio of unit active power change to unit voltage change, which can directly measure the stability of the distribution network under the influence of power flow changes.

4. The urban distribution network DC interconnection site selection method based on mixed integer programming as described in claim 1, characterized in that, The utilization rate of the VSC-DC interconnect refers to the value of the newly added VSC-DC tie line in guiding and utilizing the active power flow of the distribution network. If the power flow value of the newly added VSC-DC tie line is low under normal operating conditions, it indicates that its utilization rate is low and it does not participate much in the power flow distribution of the distribution network. This indicator is used to ensure that the selected location can maximize the effectiveness of the VSC-DC interconnect.

5. The urban distribution network DC interconnection site selection method based on mixed integer programming as described in claim 1, characterized in that, The N-1 fault index is used to ensure the resilience of the power grid and to ensure that users receive continuous power supply that meets quality requirements. This index is used to measure the overall voltage drop of the remaining nodes in the event of an N-1 fault. The short-circuit current improvement is achieved by improving the short-circuit current through VSC control of the newly added VSC-DC line.

6. The urban distribution network DC interconnection site selection method based on mixed integer programming as described in claim 1, characterized in that, The objective function is defined as follows: in, w 1 and w 2 represents the weights of the reliability index system and the resilience index system in the overall objective function, respectively. α and β F represents the weight of each indicator in the two indicator systems. p For active power flexibility, F dc For VSC-DC interconnect utilization, F f F is the failure index N-1. i To improve short-circuit current; The constraints of the entire power distribution network include power flow constraints and power equipment constraints.

7. A DC interconnection location system for urban distribution networks based on mixed-integer programming, characterized in that, include: The constraint construction module is configured to: determine the number of nodes in the distribution network, number the buses as operable nodes sequentially, and construct a connection matrix representing the entire distribution network. The constraints of the power distribution network are expressed based on the connection matrix of the entire power distribution network. The indicator system construction module is configured to: construct an indicator system for evaluating the optimal location of voltage source converter-DC line access; The objective function construction module is configured to: construct objective functions based on an indicator system; The solution module is configured to iterate the objective function using constraints and a mixed integer programming algorithm to find the optimal location for the voltage source converter-DC line connection.

8. A computer program product, comprising a computer program, characterized in that, When the computer program is executed by a processor, it implements the method of any one of claims 1 to 6.

9. A computer device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the processor executes the program, it implements the steps of the method described in any one of claims 1-6.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the program is executed by the processor, it performs the steps of the method described in any one of claims 1-6 above.

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