A Ring-Topology-Based Method and System for Power Flow Calculation in AC / DC Hybrid Distribution Networks

Abstract

This invention discloses a Ring-Topology-based method and system for calculating the power flow of a hybrid AC / DC distribution network. The method includes: constructing a steady-state power flow calculation model for the transformer, including an AC port steady-state model based on VSC (Variable Switched Controller), a DC port steady-state model based on the DC / DC converter, and a power conservation model considering the static losses of the isolation converter; dividing the distribution network into multiple AC and DC subsystems; determining the control mode and control parameters of the transformer ports; solving the DC subsystem power flow using an iterative method; and progressively solving the AC subsystem power flow. This invention significantly reduces the complexity of the solution process, improves computational efficiency, and reduces time costs, making it applicable to scenarios with high control speed requirements.

Description

Technical Field

[0001] This invention belongs to the field of power flow calculation in power systems, and involves the research on the power flow control principle of transformers and the power flow calculation method of AC / DC hybrid distribution networks. Specifically, it is a power flow calculation method and system for AC / DC hybrid distribution networks with transformers based on Ring-Topology. Background Technology

[0002] With the advancement of new power system construction, the grid integration of high-penetration renewable energy sources not only reduces the reliability of grid operation but also poses new challenges and requirements to its energy absorption capacity. Utilizing AC / DC hybrid distribution networks for AC / DC conversion is currently the most effective solution. To adapt to the development needs of AC / DC hybrid distribution networks in recent years, it is urgent to study power flow calculation methods for AC / DC distribution networks with transformers as distribution hubs.

[0003] Existing technical document 1 (CN116706921A) discloses a quantum Newton-Raphson power flow calculation method and system based on the HHL algorithm. This document adopts the quantum Newton-Raphson power flow calculation method. Although this method can theoretically improve the calculation efficiency, in practical applications, the acquisition and use of quantum computing resources are costly, the stability and reliability of quantum computing are not yet fully mature, and this scheme is mainly aimed at traditional power systems. It has low applicability and low calculation efficiency when facing complex AC and DC distribution networks.

[0004] Prior art document 2 (CN112531715A) discloses a power flow calculation method for droop control multi-terminal DC microgrids based on virtual resistance. This document uses the Newton-Raphson iterative method to solve the power flow of DC microgrids. However, it has low applicability when facing complex AC and DC distribution networks and may encounter convergence problems. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and propose a power flow calculation method for AC / DC hybrid distribution networks based on Ring-Topology. This invention studies the calculation method for power flow in AC / DC hybrid distribution networks with transformers in the scenario where the topology is ring, thereby providing a reference for power flow calculation of AC / DC hybrid distribution networks with transformers based on Ring-Topology.

[0006] This invention discloses a method for calculating the power flow of a transformer-based AC / DC hybrid distribution network based on Ring-Topology, comprising:

[0007] Construct a steady-state power flow calculation model for the transformer;

[0008] Input the system parameters of the distribution network and divide the distribution network into n1 AC subsystems and n2 DC subsystems;

[0009] Determine the control mode and control parameters for the transformer port;

[0010] The power flow of the DC subsystem is solved by iterative method to obtain the active power injected into each node in the DC subsystem;

[0011] The power flow of the AC subsystem not connected to the transformer relaxation port is solved step by step to obtain the output power of each node of the AC subsystem;

[0012] After the power flow calculations for all AC subsystems not connected to the transformer's relaxation port are completed, the active power at the transformer's relaxation port is initialized, and the initial values ​​of the control parameters are set.

[0013] The power flow of the AC subsystem connected to the transformer relaxation port is calculated step by step to obtain the injected power of the AC nodes connected to the relaxation port and the power of the transformer relaxation port.

[0014] Preferably, the AC / DC hybrid distribution network is divided into n1 AC subsystems and n2 DC subsystems, with the transformer ports as the boundaries.

[0015] Preferably, the control mode for the transformer port is determined as follows:

[0016] The control modes of transformer ports are divided into AC port control mode and DC port control mode;

[0017] The AC port of the transformer connected to the passive AC subsystem must adopt a constant AC voltage control mode;

[0018] The AC port of the transformer connected to the active AC subsystem can adopt constant power control or constant DC voltage control mode. However, in order to maintain the balance of active power inside the transformer, one side of the AC or DC port must be reserved as a relaxation port, which operates in constant DC voltage control mode.

[0019] Each transformer DC port is considered a power point with known active power;

[0020] When the DC port of the transformer adopts the constant active power control mode, the DC node connected to the DC port is the power point;

[0021] Preferably, the iterative method for solving the power flow of the DC subsystem is as follows:

[0022] The active power injected into each node in the DC subsystem can be calculated using an iterative method, and can be expressed by equation (9), as follows:

[0023]

[0024] In the formula, g represents the number of nodes in the DC power grid; P dci V represents the active power injected into node i in a DC distribution network. i V j Y represents the voltage amplitude at nodes i and j in the DC distribution network. ij Y is the nodal admittance matrix dc The element in the i-th row and j-th column.

[0025] Preferably, the output power of node i is solved step by step. The output power of node i is equivalent to the sum of the switching power of port l and the original power of node i, which can be expressed by equation (10), as follows:

[0026]

[0027] In the formula, P aci Q aci These represent the active power and reactive power injected into node i in the AC distribution network, respectively. These are the active and reactive power setpoints exchanged between the non-relaxed AC port l and AC node i of the transformer, respectively; P aci,0 and Q aci,0 These represent the initial active and reactive power of the load at node i in the AC distribution network.

[0028] If node i of the AC grid is connected to the non-relaxed AC port l of the transformer, then the output power of node i is equivalent to the sum of the switching power of port l and the original power of node i:

[0029]

[0030] P aci Q aci These represent the active power and reactive power injected into node i in the AC distribution network, respectively. These are the active and reactive power setpoints exchanged between the non-relaxed AC port l and AC node i of the transformer, respectively; P aci,0 Q aci,0 These represent the initial active and reactive power of the load at node i in the AC distribution network.

[0031]

[0032] n represents the number of nodes in the AC power grid; U i U j G represents the voltage amplitudes at nodes i and j in the AC distribution network, respectively; ij B ij These represent the conductance and susceptance between nodes i and j, respectively; θ ij Let be the phase angle difference between nodes i and j.

[0033] Preferably, the active power at the relaxation port of the initial transformer can be expressed by equations (12) to (15), as follows:

[0034]

[0035]

[0036] In the formula, The active power absorbed by the VSC on the non-relaxed AC port l side of the transformer; This is the active power setting value for the output stage of the DC / DC converter on the DC port h side of the transformer; The active power absorbed by the transformer's relaxed AC port VSC; This refers to the active power exchanged between the transformer's relaxed AC port and the system. The node voltage amplitude of the AC power grid connected to the AC port l of the transformer;

[0037] U is the node voltage of the DC power grid connected to the DC port h of the transformer; d,ref P is the DC voltage setting value for the transformer. loss denoted as , where is the loss of the intermediate isolation DC / DC converter; g is the equivalent conductance considering VSC losses; M is the number of non-relaxed AC ports of the transformer; and N is the number of DC ports of the transformer.

[0038] and These are the active and reactive power settings for the non-relaxed AC port l side of the transformer, respectively. This is the active power setting value for the non-relaxed DC port h side of the transformer.

[0039] The initial values ​​of the control parameters can be set using equations (16) and (17), as follows:

[0040]

[0041] θ slack,0 The initial value of the angle by which the voltage phase of the AC grid node connected to the VSC on the relaxation port side of the transformer leads the voltage phase of the VSC; m slack,0 The initial value of the modulation ratio of VSC on the relaxation port side of the transformer; This is the setpoint for the reactive power exchanged between the transformer's relaxed AC port and the system. This is the set value for the node voltage amplitude of the AC power grid connected to the slack AC port of the transformer.

[0042] Preferably, the stepwise calculation of the AC subsystem power flow connected to the transformer relaxation port is specifically as follows:

[0043] Determine the initial values ​​of the control variables for the transformer relaxation port;

[0044] The power flow at the transformer relaxation port is solved step by step to obtain the power injected into the relaxation port by the AC subsystem. At each step, the derivative information of the current solution is calculated, and the solution for the next step is adjusted according to the derivative information until the calculated solution reaches the set convergence accuracy. The injected power of the AC node connected to the relaxation port is updated. If the set convergence accuracy cannot be reached, the control mode and control parameters of each port of the transformer are redefined.

[0045] The power flow of the AC subsystem connected to the relaxation port is solved step by step to obtain the relaxation port power. At each step, the derivative information of the current solution is calculated, and the solution of the next step is adjusted according to the derivative information until the calculated solution reaches the set convergence accuracy. If the set convergence accuracy cannot be reached, the control mode and control parameters of each port of the transformer are redefined.

[0046] Compare the power at the relaxation port with the power injected into the relaxation port by the AC subsystem. If the absolute value of the difference between the two is greater than the set convergence accuracy, it means that the convergence condition is not met. In this case, the voltage of the AC node connected to the relaxation port needs to be updated, and the initial value of the control variable of the transformer relaxation port needs to be re-determined. Otherwise, the power flow calculation result of the entire system is considered to have converged.

[0047] The second aspect of this invention discloses a power flow calculation system for a transformer-based AC / DC hybrid distribution network based on Ring-Topology, specifically comprising:

[0048] The model building module is used to build a steady-state power flow calculation model for the transformer.

[0049] The subsystem partitioning module is used to divide the distribution network into n1 AC subsystems and n2 DC subsystems;

[0050] The port setting module is used to determine the control mode and control parameters of each port of the transformer;

[0051] The DC power flow solution module is used to calculate the power flow of the DC subsystem.

[0052] The AC power flow solution module is used to calculate the power flow of the AC subsystem.

[0053] Compared with the prior art, the beneficial effects of the present invention are that it significantly reduces the complexity of the solution process, improves the computational efficiency, reduces the time cost, and can be applied to scenarios with high requirements for control speed. Attached Figure Description

[0054] Figure 1 This is a schematic diagram of a transformer-based AC / DC hybrid distribution network based on Ring-Topology.

[0055] Figure 2 This is a flowchart illustrating the power flow calculation method for a hybrid AC / DC distribution network.

[0056] Figure 3 A flowchart illustrating the power flow calculation for the AC subsystem connected to the transformer relaxation port.

[0057] Figure 4 This is a schematic diagram of the node voltage curves in a hybrid AC / DC distribution network containing transformers. Detailed Implementation

[0058] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.

[0059] The first embodiment of this invention provides a method for calculating the power flow of a hybrid AC / DC distribution network based on Ring-Topology. In a scenario where the hybrid AC / DC distribution network topology is ring-shaped, the method solves for the power flow of the hybrid AC / DC distribution network, such as... Figure 2 As shown, it includes the following steps:

[0060] Step 1: Construct a steady-state power flow calculation model for the transformer, which consists of a power electronic converter and a high-frequency transformer. The steady-state power flow calculation model includes an AC port steady-state model based on a VSC (Voltage Source Converter), a DC port steady-state model based on a DC / DC converter, and a power conservation model that takes into account the static losses of the isolation converter.

[0061] In a preferred but non-limiting embodiment of the present invention, step 1 specifically includes:

[0062] A steady-state model of the AC port based on VSC is constructed, which can be represented by equation (1), as follows:

[0063]

[0064] In the formula, These are the active and reactive power exchanged between the VSC and the connected AC grid nodes, respectively. These are the active and reactive power injected into the VSC, respectively. The voltage amplitude of the AC grid node connected to the VSC; θ represents the amplitude of the modulation voltage output from the AC side of the VSC. ac denoted as , where is the angle by which the voltage phase of the AC grid node connected to the VSC leads the voltage phase of the VSC; g and b are the equivalent conductance and equivalent susceptance, respectively, taking into account the losses of the VSC.

[0065] Among them, the amplitude of the modulation voltage output from the AC side of VSC It can be expressed by Equation (2) as follows:

[0066]

[0067] In the formula, U d is the internal DC bus voltage of the transformer, m is the modulation ratio (0 < m < 1), and μ is the DC voltage utilization rate (μ = 1).

[0068] Construct a steady-state model of the DC port based on the DC / DC converter, which can be expressed by Equations (3) to (5) as follows:

[0069] [[ID=...]]... are the currents of the input stage and output stage of the DC / DC converter respectively; r is the equivalent resistance considering the losses of the DC / DC converter; q is the ratio of the input stage to the output stage of the DC / DC converter.

[0072] Construct a power conservation model considering the static losses of the isolation converter, which can be expressed by Equation (6) as follows: <...>... is the loss of the intermediate isolation type DC / DC converter; M and N are the numbers of the AC and DC ports of the transformer respectively.

[0075] Step 2: Input the system parameters and number each port of the transformer;

[0076] Step 3: Divide the distribution network into multiple AC subsystems and DC subsystems; specifically, divide the AC-DC hybrid distribution network into multiple AC subsystems and DC subsystems with the ports of the transformer as the boundaries.

[0077] ​​​​​​​​​​​​​​​​​​​​​​As one of the most prominent and substantial features of this invention and one of the significant advancements it brings to the prior art, this invention divides the complex power distribution network into multiple subsystems, each of which can be calculated and solved independently. This significantly reduces the complexity of the solution process, improves computational efficiency, and reduces time costs, making it applicable to the control of complex AC / DC power distribution networks.

[0078] Step 4: Determine the control mode and control parameters for each port of the transformer.

[0079] In a preferred but non-limiting embodiment of the present invention, step 4 specifically includes:

[0080] Transformer control modes are classified into two main categories based on port type: AC port control mode and DC port control mode. AC port control modes can be further divided into constant AC voltage control, constant power control, and constant DC voltage control, while DC port control modes can be divided into constant active power control, constant voltage control, and droop control.

[0081] Step 4.1: Determine the control mode of the transformer's AC port.

[0082] If a hybrid AC / DC distribution network containing transformers is divided into multiple subsystems, a passive AC system may emerge. Passive AC systems are characterized by the absence of balancing nodes and a constant system load. The AC ports of the transformers connected to these systems need to assume the responsibility of balancing power; therefore, these ports must adopt a constant AC voltage control mode. For transformer AC ports connected to active AC systems, constant power control or constant DC voltage control modes can be used. However, to maintain the balance of active power within the transformer, one side of the AC or DC port must be reserved as a relaxation port, which operates in a constant DC voltage control mode.

[0083] Step 4.2: Determine the control mode of the transformer's DC port.

[0084] The DC port of a transformer is an active power exchange port based on a bidirectional DC / DC converter. The DC system has only two node types: power point and voltage point. For the transformer's internal DC bus, each DC port is considered a power point with known active power, and the node type of the DC system depends on the control mode of the DC port. When the DC port adopts a constant active power control mode, the connected DC node type is a power point; when the DC port does not adopt a constant active power control mode, the connected DC node type needs to be adjusted to a voltage point.

[0085] After determining the control mode for each port of the transformer, the control parameters for each port need to be specified. For example, for the relaxation port of the transformer, the internal DC bus voltage and the reactive power exchanged between the VSC and the AC system need to be specified.

[0086] Step 5: Solve the power flow of the DC subsystem using an iterative method.

[0087] In a preferred but non-limiting embodiment of the present invention, step 5 specifically includes:

[0088] Step 5.1, the power flow equation of the DC subsystem can be expressed by equation (7), as follows:

[0089] I dc =Y dc V (7)

[0090] Matrix I dc Inject current into the nodes of the DC subsystem; matrix Y dc is the node admittance matrix of the DC subsystem; matrix V is the node voltage of the DC distribution network;

[0091] By separating the source node of the subsystem from other nodes, the system equation can be expressed by equation (8), as follows:

[0092]

[0093] In the formula, I1 and V1 are the current and voltage vectors of the source node; I2 and V2 are the current and voltage vectors of the other nodes besides the source node; Y 11 Y is the self-admittance of the source node. 12 and Y 21 Y is the admittance between the source node and other nodes. 22 It is the admittance among the nodes other than the source node.

[0094] Step 5.2: Use the iterative method to find the node voltages V1 and V2 of the DC subsystem.

[0095] Step 5.3: Based on the node voltages of the DC subsystem calculated in Step 5.2, calculate the active power injected into each node of the DC subsystem, which can be expressed by Equation (9), as follows:

[0096]

[0097] In the formula, P dci V represents the active power injected into node i in the DC subsystem; g represents the number of nodes in the DC subsystem; V i V j Y represents the voltage amplitude at nodes i and j in the DC distribution network. ij Y is the nodal admittance matrix dc The element in the i-th row and j-th column.

[0098] Step 6: Determine the power flow calculation order of each subsystem based on whether the AC subsystem is connected to the slack port of the transformer. Specifically, prioritize the calculation of the power flow of AC subsystems not connected to the slack port of the transformer. After the power flow calculation of all AC subsystems not connected to the slack port of the transformer is completed, calculate the power flow of AC subsystems connected to the slack port of the transformer.

[0099] Step 7: Combining the steady-state model of the transformer AC port established in Step 1, progressively solve the power flow of the AC subsystem connected to the non-relaxed port of the transformer. At each step, calculate the derivative information of the current solution and adjust the solution for the next step based on the derivative information, until the set convergence accuracy of 1×10⁻⁶ is achieved. -6 Until pu.

[0100] Specifically, if node i of the AC subsystem is connected to the non-relaxed AC port l of the transformer, the output power of node i is solved step by step. The output power of node i is equivalent to the sum of the switching power of port l and the original power of node i, which can be expressed by equation (10), as follows:

[0101]

[0102] P aci Q aci These represent the active power and reactive power injected into node i in the AC distribution network, respectively. These are the active and reactive power setpoints exchanged between the non-relaxed AC port l and AC node i of the transformer, respectively; P aci,0 Q aci,0 These represent the initial active and reactive power of the load at node i in the AC distribution network.

[0103] Among them, the initial load active power P of node i in the AC distribution network aci,0 and reactive power Q aci,0 It can be represented by equation (11), as follows:

[0104]

[0105] n represents the number of nodes in the AC power grid; U i U j G represents the voltage amplitudes at nodes i and j in the AC distribution network, respectively; ij B ij These represent the conductance and susceptance between nodes i and j, respectively; θ ij Let be the phase angle difference between nodes i and j.

[0106] Step 8: Based on the steady-state power flow calculation model of the transformer established in Step 1, initialize the active power of the transformer relaxation port and set the initial values ​​of the control parameters.

[0107] In a preferred but non-limiting embodiment of the present invention, step 8 specifically includes:

[0108] Step 8.1, the active power at the relaxation port of the initial transformer can be expressed by equations (12) to (15), as follows:

[0109]

[0110] The active power absorbed by the VSC on the non-relaxed AC port l side of the transformer; This is the active power setting value for the output stage of the DC / DC converter on the non-relaxed DC port h side of the transformer. The active power absorbed by the transformer's relaxed AC port VSC; This refers to the active power exchanged between the transformer's relaxed AC port and the system. The node voltage amplitude of the AC power grid connected to the AC port l of the transformer; U is the node voltage of the DC power grid connected to the DC port h of the transformer; d,ref P is the DC voltage setting value for the transformer. loss denoted as , where is the loss of the intermediate isolation DC / DC converter; g is the equivalent conductance considering VSC losses; M is the total number of non-relaxed AC ports of the transformer; N is the total number of DC ports of the transformer. and These are the active and reactive power settings for the non-relaxed AC port l side of the transformer, respectively. This is the active power setting value for the non-relaxed DC port h side of the transformer.

[0111] Step 8.2, set the initial values ​​of the control parameters. The calculation formula is as follows:

[0112]

[0113] In the formula, θ slack,0 The initial value of the angle by which the voltage phase of the AC grid node connected to the VSC on the relaxation port side of the transformer leads the voltage phase of the VSC; m slack,0 The initial value of the modulation ratio of VSC on the relaxation port side of the transformer; This is the setpoint for the reactive power exchanged between the transformer's relaxed AC port and the system. U is the setpoint for the node voltage amplitude of the AC grid connected to the slack AC port of the transformer; b is the equivalent susceptance taking into account VSC losses; d,ref The DC voltage setting value for the transformer; The voltage amplitude of the AC grid node connected to the VSC; This refers to the active power exchanged between the transformer's relaxed AC port and the system.

[0114] Step 9, as follows Figure 3 As shown, the power flow of the AC subsystem connected to the transformer relaxation port is calculated step by step.

[0115] In a preferred but non-limiting embodiment of the present invention, step 9 specifically includes:

[0116] Step 9.1, determine the initial value θ of the transformer relaxation port control variable. slack,0 and m slack,0

[0117] Step 9.2 involves progressively solving for the power flow at the transformer's relaxed port. At each step, the derivative of the current solution is calculated, and the solution for the next step is adjusted based on this derivative information until the set convergence accuracy of 1×10⁻⁶ is achieved. -6 Once the power at the transformer's relaxation port is obtained, proceed to step 9.3; if the set convergence accuracy cannot be achieved, proceed to step 4 to redetermine the control mode and control parameters for each port of the transformer.

[0118] Step 9.3: Update the injected power of the AC node connected to the relaxation port;

[0119] Step 9.4: Solve the power flow of the AC subsystem connected to the relaxation port step by step. Calculate the derivative information of the current solution at each step, and adjust the solution for the next step based on the derivative information until the set convergence accuracy of 1×10⁻⁶ is reached. -6 Until the power injected into the relaxation port of the AC subsystem is obtained, proceed to step 9.5; if the set convergence accuracy cannot be achieved, proceed to step 4 to redetermine the control mode and control parameters of each port of the transformer.

[0120] Step 9.5: Compare the relaxed port power calculated in Step 9.2. The power -P injected into the relaxation port of the AC subsystem obtained in step 9.4 ac,slack If the absolute value of the difference between the two is greater than the set convergence precision of 1×10, - 6 If pu indicates that the convergence condition is not met, the AC node voltage connected to the relaxation port needs to be updated before proceeding to step 9.1; otherwise, the power flow calculation results of the entire system are considered to have converged.

[0121] As one of the most prominent and substantial features of this invention and one of the significant advancements it brings to the prior art, this invention reduces the complexity of power flow calculation and the number of iterations by dividing the system into subsystems, which can achieve convergence faster and obtain accurate power flow calculation results.

[0122] A second embodiment of the present invention provides a power flow calculation system for a transformer-based AC / DC hybrid distribution network based on Ring-Topology, specifically including:

[0123] The model building module is used to build a steady-state power flow calculation model for the transformer.

[0124] The subsystem partitioning module is used to divide the distribution network into n1 AC subsystems and n2 DC subsystems;

[0125] The port setting module is used to determine the control mode and control parameters of each port of the transformer;

[0126] The DC power flow solution module is used to calculate the power flow of the DC subsystem.

[0127] The AC power flow solution module is used to calculate the power flow of the AC subsystem.

[0128] This embodiment implements the method of the present invention using specific data. The results show that the present invention can provide accurate power flow calculation results for AC / DC hybrid distribution networks with transformers based on Ring-Topology. The data in this embodiment is shown below:

[0129] This invention takes a transformer-based AC / DC hybrid distribution network based on Ring-Topology as an example, such as... Figure 1 As shown, the AC distribution network in this example is an improved IEEE 33-node system with a rated voltage of 12.66 kV. Node 3 of the original system is replaced with a four-port transformer, and a three-port transformer is connected at the end of the network, changing the radial topology to a ring topology. The DC distribution network is coupled to the AC distribution network through two transformers, with a rated voltage of 15 kV. The total active load of the system is 8799.26 kW, and the total reactive load is 4847.32 kVar. There are three distributed generation sources in the AC distribution network: nodes 6 and 28 are connected to doubly-fed asynchronous wind turbines with a rated power of 400 kW and a power factor of 0.9; node 12 is connected to a micro gas turbine (MT), which can balance the network power; both have a rated power of 500 kW and a power factor of 0.85. There are four distributed generation sources in the DC distribution network: nodes 34 and 37 are equipped with photovoltaic power generation with a rated power of 150 kW; and nodes 35 and 38 are equipped with DC wind turbines with a rated power of 200 kW. Gas turbines are controllable distributed power sources, while wind turbines and photovoltaic power sources are uncontrollable distributed power sources.

[0130] Based on the transformer port as the boundary, the AC / DC hybrid distribution network is divided into 4 regions, and the zoning results are shown in Table 1.

[0131] Table 1. Zoning of AC / DC Hybrid Distribution Network

[0132]

[0133] The capacity of both transformers is set to 5MVA. For ease of analysis and comparison, the VSC parameters of both transformers are set identically, with impedance R set to 0.0062pu and X set to 0.0196pu. Both transformers have a static active power loss of 100kW, and each AC port can provide a maximum reactive power compensation of 500kVar. The power flowing into the transformer port is considered to be in the positive direction. The transformer parameter settings in the example are shown in Table 2.

[0134] Table 2 Control Modes and Parameter Settings for Transformer Ports

[0135]

[0136] According to the transformer control modes in Table 1, the relaxation control ports for the two transformers are D1 and D7, respectively. The balancing nodes are set as node 1 of the AC subnet, node 22 of the AC subnet 2, node 12 of the AC grid 3, and node 33 of the DC subnet. The system's base capacity is set to 10 MVA, the allowable deviation range of the system node voltage amplitude is 0.95–1.05 pu, and the convergence accuracy is 1 × 10⁻⁶. -6 The allowable range of the transformer's internal DC bus voltage is set to 1.8–2.2 pu. The doubly-fed asynchronous wind turbines connected to nodes 6 and 28 each have an active power output of 200 kW and a reactive power output of 96 kW. The photovoltaic power outputs installed at nodes 34 and 37 are each 80 kW, and the DC wind turbines installed at nodes 35 and 38 each have an output of 100 kW. An iterative method is used to solve the example. The power flow calculation results for the two transformers are shown in Table 3. All port variables of the transformers are within the allowable range, proving that the transformer control mode and the power flow calculation method for AC / DC hybrid distribution networks containing transformers adopted in this paper are effective and feasible.

[0137] Table 3 Power flow calculation results for transformers

[0138]

[0139]

[0140] Depend on Figure 4 It can be seen that in the AC distribution network section, the node voltage distribution of each branch generally shows a gradual decreasing trend towards the end of the distribution network. In the DC ring network section, the voltage of each node is approximately the same. The above voltage distribution characteristics are consistent with the voltage distribution law of AC / DC hybrid distribution networks. In addition, the voltage amplitude of all nodes in the system is above 0.95 pu, indicating the accuracy of the proposed power flow calculation method for AC / DC hybrid distribution networks with transformers based on Ring-Topology.

[0141] The above examples illustrate the effects of implementing the present invention under specific circumstances. However, the implementation of the present invention is not limited to the above examples. Any modifications, alterations, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for calculating power flow in a transformer-based AC / DC hybrid distribution network based on Ring-Topology, characterized in that, The method includes: Construct a steady-state power flow calculation model for the transformer; Input the system parameters of the distribution network and divide the distribution network into n1 AC subsystems and n2 DC subsystems; Determine the control mode and control parameters for the transformer port; The power flow of the DC subsystem is solved by iterative method to obtain the active power injected into each node in the DC subsystem; The power flow of the AC subsystem connected to the non-relaxed port of the transformer is solved step by step to obtain the output power of each node of the AC subsystem; After the power flow calculations for all AC subsystems not connected to the transformer's relaxation port are completed, the active power at the transformer's relaxation port is initialized, and the initial values ​​of the control parameters are set. The initial active power at the transformer's relaxation port is expressed by the following formula: In the formula, The active power absorbed by the VSC on the non-relaxed AC port l side of the transformer; This is the active power setting value for the output stage of the DC / DC converter on the non-relaxed DC port h side of the transformer. The active power absorbed by the transformer's relaxed AC port VSC; This refers to the active power exchanged between the transformer's relaxed AC port and the system. The node voltage amplitude of the AC power grid connected to the non-relaxed AC port l of the transformer; U is the node voltage of the DC grid connected to the non-relaxed DC port h of the transformer; d,ref P is the DC voltage setting value for the transformer. loss denoted as , where is the loss of the intermediate-isolation DC / DC converter; g is the equivalent conductance considering VSC losses; M is the number of non-relaxed AC ports of the transformer; N is the number of non-relaxed DC ports of the transformer. and These are the active and reactive power settings for the non-relaxed AC port l side of the transformer, respectively. This is the active power setting value for the non-relaxed DC port h side of the transformer; The initial values ​​of the control parameters are set using the following formula: In the formula, θ slack,0 The initial value of the angle by which the voltage phase of the AC grid node connected to the VSC on the relaxation port side of the transformer leads the voltage phase of the VSC; m slack,0 The initial value of the modulation ratio of VSC on the relaxation port side of the transformer; This is the setpoint for the reactive power exchanged between the transformer's relaxed AC port and the system. U is the setpoint for the node voltage amplitude of the AC grid connected to the slack AC port of the transformer; b is the equivalent susceptance taking into account VSC losses; d,ref The DC voltage setting value for the transformer; The voltage amplitude of the AC grid node connected to the VSC; This refers to the active power exchanged between the transformer's relaxed AC port and the system. The power flow of the AC subsystem connected to the transformer relaxation port is calculated step by step to obtain the injected power of the AC nodes connected to the relaxation port and the power of the transformer relaxation port.

2. The method for calculating power flow in a transformer-based AC / DC hybrid distribution network based on Ring-Topology as described in claim 1, characterized in that: The power distribution network is divided into n1 AC subsystems and n2 DC subsystems, with the transformer ports as the boundaries.

3. The method for calculating power flow in a transformer-based AC / DC hybrid distribution network based on Ring-Topology as described in claim 1, characterized in that: The specific control mode for the transformer port is determined as follows: The control modes of transformer ports are divided into AC port control mode and DC port control mode; The AC port of the transformer connected to the passive AC subsystem must adopt a constant AC voltage control mode; The AC port of the transformer connected to the active AC subsystem adopts constant power control or constant DC voltage control mode. However, in order to maintain the balance of active power inside the transformer, one side of the AC or DC port must be reserved as a relaxation port, which operates in constant DC voltage control mode.

4. The method for calculating power flow in a transformer-based AC / DC hybrid distribution network based on Ring-Topology according to claim 3, characterized in that: Each transformer DC port is considered a power point with known active power; When the DC port of the transformer adopts the constant active power control mode, the DC node connected to the DC port is the power point; When the DC port does not use the constant active power control mode, the DC node connected to the DC port is adjusted to a voltage point.

5. The method for calculating power flow in a transformer-based AC / DC hybrid distribution network based on Ring-Topology according to claim 1, characterized in that: The iterative method for solving the power flow of the DC subsystem is as follows: The node voltages of the DC subsystem are determined using an iterative method. Based on the calculated node voltages, the active power injected into each node in the DC subsystem is calculated and expressed by the following formula: In the formula, P dci V represents the active power injected into node i in the DC subsystem; g' represents the number of nodes in the DC subsystem; V i V j Y represents the voltage amplitude at nodes i and j in the DC distribution network. ij The element in the i-th row and j-th column of the node admittance matrix represents the admittance of the node.

6. The method for calculating power flow in a transformer-based AC / DC hybrid distribution network based on Ring-Topology according to claim 1, characterized in that: The stepwise solution to the power flow of the AC subsystem connected to the non-relaxed port of the transformer is as follows: If the nodes of the AC subsystem are connected to the non-relaxed AC ports of the transformer, the output power of the AC subsystem nodes is solved step by step. The output power of the AC subsystem nodes is expressed by the following formula: In the formula, P aci Q aci These represent the active power and reactive power injected into node i in the AC distribution network, respectively. These are the active and reactive power setpoints exchanged between the non-relaxed AC port l and AC node i of the transformer, respectively; P aci,0 and Q aci,0 These represent the initial active and reactive power of the load at node i in the AC distribution network.

7. The method for calculating power flow in a transformer-based AC / DC hybrid distribution network based on Ring-Topology according to claim 1, characterized in that: The power flow of the AC subsystem connected to the transformer relaxation port is calculated step by step as follows: Determine the initial values ​​of the transformer relaxation port control parameters; The power flow at the relaxed port of the transformer is solved step by step to obtain the power at the relaxed port. At each step, the derivative information of the current solution is calculated, and the solution for the next step is adjusted according to the derivative information until the calculated solution reaches the set convergence accuracy. If the set convergence accuracy cannot be reached, the control mode and control parameters of each port of the transformer are redefined. The power flow of the AC subsystem connected to the relaxation port is solved step by step to obtain the power injected into the relaxation port by the AC subsystem. At each step, the derivative information of the current solution is calculated, and the solution of the next step is adjusted according to the derivative information until the calculated solution reaches the set convergence accuracy. The injected power of the AC node connected to the relaxation port is updated. If the set convergence accuracy cannot be reached, the control mode and control parameters of each port of the transformer are redefined. Compare the power at the relaxation port with the power injected into the relaxation port by the AC subsystem. If the absolute value of the difference between the two is greater than the set convergence accuracy, it means that the convergence condition is not met. In this case, the voltage of the AC node connected to the relaxation port needs to be updated, and the initial values ​​of the transformer relaxation port control parameters need to be re-determined. Otherwise, the power flow calculation results of the entire system are considered to have converged.

8. A power flow calculation system for a transformer-based AC / DC hybrid distribution network using Ring-Topology, comprising the power flow calculation method for a transformer-based AC / DC hybrid distribution network according to any one of claims 1 to 7, characterized in that: The model building module is used to build a steady-state power flow calculation model for the transformer. The subsystem partitioning module is used to divide the distribution network into n1 AC subsystems and n2 DC subsystems; The port setting module is used to determine the control mode and control parameters of each port of the transformer; The DC power flow solution module is used to calculate the power flow of the DC subsystem. The AC power flow solution module is used to calculate the power flow of the AC subsystem.

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