AC / DC hybrid power distribution network load flow calculation method and system based on Ring-Topology

Abstract

The invention discloses a Ring-Topology-based AC / DC hybrid power distribution network load flow calculation method and system, and the method comprises the steps: constructing a steady-state load flow calculation model of a transformer, which comprises an AC port steady-state model based on a VSC, a DC port steady-state model based on a DC / DC converter, and a power conservation model considering the static loss of an isolation converter; the power distribution network is divided into a plurality of alternating current subsystems and direct current subsystems; determining a control mode and a control parameter of the transformer port; solving the power flow of the direct-current subsystem by using an iteration method; and gradually solving the power flow of the alternating current subsystem. According to the method, the complexity of the solving process is remarkably reduced, the calculation efficiency is improved, the time cost is reduced, and the method can be suitable for scenes with high control speed requirements.

Description

Technical Field

[0001] The present invention belongs to the field of power system flow calculation, and relates to the research on the power flow control principle of transformer and the power flow calculation method of AC / DC hybrid distribution network, and specifically is a power flow calculation method and system of AC / DC hybrid distribution network containing transformer based on Ring-Topology. Background Art

[0002] With the advancement of the construction of new power systems, the high penetration rate of renewable energy grid connection not only reduces the reliability of grid operation, but also poses new challenges and requirements for its energy consumption capacity. The use of AC / DC hybrid distribution networks for AC / DC conversion is currently the most effective solution. In order to meet the development needs of AC / DC hybrid distribution networks in recent years, it is urgent to study the power flow calculation method of AC / DC distribution networks with transformers as distribution hubs.

[0003] Prior art document 1 (CN116706921A) discloses a quantum Newton-Raphson method power flow calculation method and system based on the HHL algorithm. The document adopts the quantum Newton-Raphson method power flow calculation. Although this method can theoretically improve computing efficiency, in practical applications, the cost of acquiring and using quantum computing resources is high, and the stability and reliability of quantum computing are not yet fully mature. In addition, the solution is mainly aimed at traditional power systems. Faced with complex AC and DC distribution networks, it has low applicability and low computing efficiency.

[0004] Prior art document 2 (CN112531715A) discloses a method for calculating the power flow of a multi-terminal DC microgrid with droop control based on virtual resistance. This document uses the Newton-Raphson iteration method to solve the power flow of the DC microgrid. When faced with complex AC and DC distribution networks, its applicability is low and it may be difficult to converge. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art, propose a method for calculating the power flow of an AC / DC hybrid distribution network based on Ring-Topology, and study the calculation method of the power flow of an AC / DC hybrid distribution network containing a transformer in a scenario where the topology of the AC / DC hybrid distribution network containing a transformer is a ring, so as to provide a reference for the power flow calculation of an AC / DC hybrid distribution network containing a transformer based on Ring-Topology.

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

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

[0008] Input the system parameters of the distribution network and divide the distribution network into n 1AC subsystems and n 2 A DC subsystem;

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

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

[0011] The AC subsystem power flow that is not connected to the transformer slack port is gradually solved to obtain the output power of each node of the AC subsystem;

[0012] After the power flow calculation of all AC subsystems not connected to the slack port of the transformer is completed, the active power of the slack port of the transformer is initialized and the initial value of the control parameter is set;

[0013] The AC subsystem power flow connected to the slack port of the transformer is calculated step by step, and the injected power of the AC node connected to the slack port and the power of the slack port of the transformer are obtained.

[0014] Preferably, the AC / DC hybrid distribution network is divided into n 1 AC subsystems and n 2 A DC subsystem.

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

[0016] The control mode of the transformer port is 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 the constant AC voltage control mode;

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

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

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

[0021] Preferably, the iterative method is used to solve the DC subsystem power flow as follows:

[0022] The active power injected into each node in the DC subsystem is calculated by iterative method, which can be expressed by equation (9):

[0023]

[0024] Where g is the number of DC grid nodes; P dci is the active power injected into node i in the DC distribution network; V i 、V j is the voltage amplitude of nodes i and j in the DC distribution network; Y ij is the node admittance matrix Y dc The element in row i and column j.

[0025] Preferably, the output power of node i is gradually solved. 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 formula (10), as follows:

[0026]

[0027] Where P aci , Q aci are the active power and reactive power injected into node i in the AC distribution network respectively; are respectively the active power and reactive power setting values ​​exchanged between the non-relaxed AC port l of the transformer and the AC node i; P aci,0 and Q aci,0 are the initial load active power and reactive power of node i in the AC distribution network respectively.

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

[0029]

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

[0031]

[0032] n is the number of AC grid nodes; U i , U j are the voltage amplitudes of nodes i and j in the AC distribution network respectively; G ij , B ij are the conductance and susceptance between nodes i and j respectively; θ ij is the phase angle difference between nodes i and j.

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

[0034]

[0035]

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

[0037] is the node voltage of the DC grid connected to the DC port h of the transformer; U d,ref is the DC voltage setting value of the transformer; P loss is the loss of the intermediate isolated DC / DC converter; g is the equivalent conductance taking into account the VSC loss, 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 are respectively the active power and reactive power setting values ​​on the non-relaxed AC port l side of the transformer; It is the active power setting value on the non-relaxed DC port h side of the transformer.

[0039] Setting the initial value of the control parameter can be expressed by equation (16) and equation (17), as follows:

[0040]

[0041] θ slack,0 The initial value of the angle at which the voltage phase of the AC grid node connected to the VSC on the slack port side of the transformer leads the voltage phase of the VSC; m slack,0 is the initial value of the modulation ratio of the VSC on the slack port side of the transformer; Set the reactive power value for the transformer's slack AC port to exchange with the system; It is the set value of the node voltage amplitude of the AC power grid connected to the relaxed AC port of the transformer.

[0042] Preferably, the AC subsystem power flow connected to the transformer relaxation port is calculated step by step as follows:

[0043] Determine the initial value of the transformer relaxation port control variable;

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

[0045] The power flow of the AC subsystem connected to the relaxation port is gradually solved to obtain the relaxation port power. The derivative information of the current solution is calculated at each step, 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 achieved, the control mode and control parameters of each port of the transformer are re-determined.

[0046] Compare the power of 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, it is necessary to update the AC node voltage connected to the relaxation port and then redetermine the initial value of the control variable of the transformer relaxation port; otherwise, the power flow calculation results of the entire system are considered to have converged.

[0047] The second aspect of the present invention discloses a power flow calculation system for an AC / DC hybrid distribution network containing a transformer based on Ring-Topology, which specifically includes:

[0048] Model building module, used to build a steady-state power flow calculation model for transformers;

[0049] Subsystem division module, used to divide the distribution network into n 1 AC subsystems and n 2 A DC subsystem;

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

[0051] DC power flow solving module, used to calculate the power flow of DC subsystem;

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

[0053] Compared with the prior art, the beneficial effect of the present invention is that the present invention significantly reduces the complexity of the solution process, improves the calculation efficiency, reduces the time cost, and can be applied to scenarios with high requirements for control speed. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0055] Figure 2 The figure is a flow chart of the power flow calculation method for AC / DC hybrid distribution network.

[0056] Figure 3 Schematic diagram of the flow chart for the AC subsystem power flow calculation connected to the slack port of the transformer.

[0057] Figure 4 Schematic diagram of the node voltage curve of the AC / DC hybrid distribution network containing a transformer. DETAILED DESCRIPTION

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

[0059] The first embodiment of the present invention provides a method for calculating the power flow of an AC / DC hybrid distribution network based on Ring-Topology. In the scenario where the AC / DC hybrid distribution network topology is a ring, the power flow of the AC / DC hybrid distribution network is solved. Figure 2 As shown, the following steps are included:

[0060] Step 1, construct a steady-state power flow calculation model of the transformer, which is composed 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 VSC (Voltage Source Converter), a DC port steady-state model based on DC / DC converter, and a power conservation model taking into account the static loss of the isolation converter.

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

[0062] The steady-state model of the AC port based on VSC can be expressed by formula (1), as follows:

[0063]

[0064] In the formula, are the active and reactive powers exchanged between the VSC and the connected AC grid nodes, respectively; are the active and reactive powers injected into the VSC, respectively; is the voltage amplitude of the AC grid node to which the VSC is connected; is the modulation voltage amplitude output by the VSC AC side; θ acis 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 considering the VSC losses, respectively.

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

[0069]

[0070]

[0071] In the formula, is the active power exchanged between the DC / DC converter and the DC grid node it is connected to, that is, the active power of the input stage; is the active power injected by the DC / DC converter into the internal DC bus of the transformer, that is, the active power of the output stage; are the voltages of the input stage and output stage of the DC / DC converter, respectively; 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 turns ratio from 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), specifically as follows:

[0073]

[0074] P k,VSC 、P k,DC / DC are the active powers injected into the internal DC bus by the AC and DC ports of the transformer, respectively; P loss is the loss of the intermediate isolation type DC / DC converter; M and N are the numbers of 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, dividing the distribution network into a plurality of AC subsystems and DC subsystems; specifically, dividing the AC / DC hybrid distribution network into a plurality of AC subsystems and DC subsystems with the ports of the transformer as the boundary.

[0077] As a prominent substantive feature of the present invention and one of the significant advances it brings to the prior art, the present invention divides a complex distribution network into multiple subsystems, each of which can be calculated and solved independently, which significantly reduces the complexity of the solution process, improves calculation efficiency, reduces time cost, and can be applied to the control of complex AC and DC distribution networks.

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

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

[0080] The control mode of the transformer is divided into two categories according to the port type: AC port control mode and DC port control mode. The AC port control mode can be divided into constant AC voltage control, constant power control and constant DC voltage control, and the DC port control mode 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 AC port.

[0082] If the AC / DC hybrid distribution network with transformers is divided into multiple subsystems, a passive AC system may appear. The characteristics of a passive AC system are that there is no balancing node and the system load is constant. The AC port of the transformer connected to it needs to assume the responsibility of balancing power, so the port must adopt a constant AC voltage control mode. For the AC port of the transformer connected to the active AC system, a constant power control or constant DC voltage control mode can be adopted, but in order to maintain the active power balance inside the transformer, one side of the AC or DC port must be left as a slack port, which works in a constant DC voltage control mode.

[0083] Step 4.2, determine the control mode of the transformer DC port.

[0084] The DC port of the 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 internal DC bus of the transformer, each DC port is regarded as 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 the fixed active power control mode, the connected DC node type is a power point. When the DC port does not adopt the fixed active power control mode, the connected DC node type needs to be adjusted to a voltage point.

[0085] After determining the control mode of each port of the transformer, it is necessary to give the control parameters of each port. For example, for the slack port of the transformer, it is necessary to give the internal DC bus voltage and the reactive power exchanged between the VSC and the AC system.

[0086] Step 5: Use the iterative method to solve the DC subsystem power flow.

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

[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; the matrix V is the node voltage of the DC distribution network;

[0091] If the source node of the subsystem is separated from other nodes, the system equation can be expressed as formula (8), as follows:

[0092]

[0093] In the formula, I 1 、V 1 is the current and voltage vector of the source node; I 2 、V 2 is the current and voltage vector of the nodes except the source node, Y 11 is the self-admittance of the source node, Y 12 and Y 21 is the admittance between the source node and other nodes, Y 22 is the admittance between the remaining nodes except the source node.

[0094] Step 5.2, calculate the node voltage V of the DC subsystem by iterative method 1 , V 2 .

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

[0096]

[0097] Where P dci is the active power injected into node i in the DC subsystem; g is the number of nodes in the DC subsystem; V i、V j is the voltage amplitude of nodes i and j in the DC distribution network; Y ij is the node admittance matrix Y dc The element in row i and column j.

[0098] Step 6, according to whether the AC subsystem is connected to the slack port of the transformer, the order of power flow calculation of each subsystem is determined; specifically, the power flow of the AC subsystem not connected to the slack port of the transformer is calculated first, and after the power flow calculation of all AC subsystems not connected to the slack port of the transformer is completed, the power flow of the AC subsystem connected to the slack port of the transformer is calculated.

[0099] Step 7: Combined with the steady-state model of the transformer AC port established in step 1, the AC subsystem power flow connected to the non-relaxed port of the transformer is gradually solved. The derivative information of the current solution is calculated at each step, and the solution of the next step is adjusted according to the derivative information until the set convergence accuracy of 1×10 is reached. -6 pu until now.

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

[0101]

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

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

[0104]

[0105] n is the number of AC grid nodes; U i , U j are the voltage amplitudes of nodes i and j in the AC distribution network respectively; G ij , B ij are the conductance and susceptance between nodes i and j respectively; θ ijis the phase angle difference between nodes i and j.

[0106] Step 8, combining 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 value of the control parameter.

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

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

[0109]

[0110] is the active power absorbed by the VSC on the non-relaxed AC port l side of the transformer; is the active power setting value of the DC / DC converter output stage on the non-relaxed DC port h side of the transformer; The active power absorbed by the slack AC port VSC of the transformer; is the active power exchanged between the slack AC port of the transformer and the system; is the node voltage amplitude of the AC grid connected to the AC port l of the transformer; is the node voltage of the DC grid connected to the DC port h of the transformer; U d,ref is the DC voltage setting value of the transformer; P loss is the loss of the intermediate isolated DC / DC converter; g is the equivalent conductance taking into account the VSC loss; 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 are respectively the active power and reactive power setting values ​​on the non-relaxed AC port l side of the transformer; It is the active power setting value on the non-relaxed DC port h side of the transformer.

[0111] Step 8.2, set the initial value of the control parameter, the calculation formula is as follows:

[0112]

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

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

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

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

[0117] Step 9.2, gradually solve the power flow of the transformer relaxation port, calculate the derivative information of the current solution at each step, and adjust the next solution according to the derivative information until the set convergence accuracy of 1×10 is reached. -6 pu, the power of the transformer relaxation port is obtained, and then go to step 9.3; if the set convergence accuracy cannot be achieved, go to step 4 and redefine the control mode and control parameters of each port of the transformer;

[0118] Step 9.3, updating the injected power of the AC node connected to the slack port;

[0119] Step 9.4, gradually solve the power flow of the AC subsystem connected to the relaxation port, calculate the derivative information of the current solution at each step, and adjust the solution of the next step according to the derivative information until the set convergence accuracy of 1×10 is reached. -6 pu, the power injected into the relaxation port by the AC subsystem is obtained, and then go to step 9.5; if the set convergence accuracy cannot be achieved, go to step 4 and redefine 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 injected into the relaxation port by the AC subsystem calculated in step 9.4 is -P ac,slack If the absolute value of the difference between the two is greater than the set convergence accuracy 1×10 - 6 pu, indicating that the convergence condition is not met, it is necessary to update the AC node voltage connected to the relaxation port and then go to step 9.1; otherwise, the power flow calculation results of the entire system are considered to have converged.

[0121] As a prominent substantive feature of the present invention and one of the significant advances brought to the prior art, the present invention reduces the complexity of power flow calculation and the number of iterations by dividing the subsystem, and can reach convergence faster to obtain accurate power flow calculation results.

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

[0123] Model building module, used to build a steady-state power flow calculation model for transformers;

[0124] Subsystem division module, used to divide the distribution network into n 1 AC subsystems and n 2 A DC subsystem;

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

[0126] DC power flow solving module, used to calculate the power flow of DC subsystem;

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

[0128] This embodiment implements the method of the present invention through specific data, and the results show that the present invention can provide accurate power flow calculation results for the AC / DC hybrid distribution network containing transformers based on Ring-Topology. The data of this embodiment are as follows:

[0129] The present invention takes the AC / DC hybrid distribution network containing transformer based on Ring-Topology as an example. Figure 1As shown in the figure, the AC distribution network of this example is an improved IEEE33 node system with a rated voltage of 12.66kV. Node 3 of the original system is changed into a four-port transformer, and a three-port transformer is connected at the end of the network to change the radial topology into a ring topology. The DC distribution network is coupled with the AC distribution network through two transformers, and its rated voltage is 15kV. The total active load of the system is 8799.26kW, and the total reactive load is 4847.32kVar. There are 3 distributed power sources in the AC distribution network. Nodes 6 and 28 are connected to double-fed asynchronous wind turbines with a rated power of 400kW and a power factor of 0.9. Node 12 is connected to a micro gas turbine. The micro gas turbine MT can balance the network power. The rated power is 500kW and the power factor is 0.85. There are 4 distributed power sources in the DC distribution network. Nodes 34 and 37 are equipped with photovoltaic power generation with a rated power of 150kW, and nodes 35 and 38 are equipped with DC wind turbines with a rated power of 200kW. Gas turbines are controllable distributed power sources, while wind turbines and photovoltaic power sources are uncontrollable distributed power sources.

[0130] Taking the transformer port as the boundary, the AC / DC hybrid distribution network is divided into four areas. The zoning results are shown in Table 1.

[0131] Table 1 AC / DC hybrid distribution network partition

[0132]

[0133] The capacity of the two transformers is set to 5MVA. For the convenience of analysis and comparison, the parameters of the VSC in the two transformers are set the same, the impedance parameter R is set to 0.0062pu, and X is set to 0.0196pu. The 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 regarded as the positive direction of power. The parameter setting values ​​of the transformer in the example are shown in Table 2.

[0134] Table 2 Control mode and parameter settings of transformer ports

[0135]

[0136] According to the control mode of the transformer in Table 1, the relaxation control ports of 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 base capacity is set to 10MVA, the allowable offset range of the system node voltage amplitude is 0.95~1.05pu, and the convergence accuracy is 1×10 -6pu. The allowable range of the internal DC bus voltage of the transformer is set to 1.8~2.2pu. The active output of the double-fed asynchronous wind turbines connected to nodes 6 and 28 is 200kW, and the reactive output is 96kW. The photovoltaic output installed at nodes 34 and 37 is 80kW, and the DC wind turbine output installed at nodes 35 and 38 is 100kW. The iterative method is used to solve the example. The power flow calculation results of the two transformers are shown in Table 3. The variables of each port of the transformer are within the allowable range, which proves that the transformer control mode and the power flow calculation method of the AC / DC hybrid distribution network containing transformers adopted in this paper are effective and feasible.

[0137] Table 3 Transformer power flow calculation results

[0138]

[0139]

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

[0141] The above examples are the implementation effects of the present invention under special circumstances, but the implementation of the present invention is not limited to the above examples. Any other modifications, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and should be included in the protection scope of the present invention.

Claims

1. A method for calculating power flow of AC / DC hybrid distribution network containing transformer based on Ring-Topology, characterized in that: The method comprises: Construct a steady-state power flow calculation model for transformers; 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 of the transformer port; The DC subsystem power flow is solved by iterative method to obtain the active power injected into each node in the DC subsystem; The AC subsystem power flow connected to the non-relaxed port of the transformer is gradually solved to obtain the output power of each node of the AC subsystem; After the power flow calculation of all AC subsystems not connected to the slack port of the transformer is completed, the active power of the slack port of the transformer is initialized and the initial value of the control parameter is set; The AC subsystem power flow connected to the slack port of the transformer is calculated step by step, and the injected power of the AC node connected to the slack port and the power of the slack port of the transformer are obtained.

2. The method for calculating power flow of a transformer-containing AC / DC hybrid distribution network based on Ring-Topology according to claim 1, characterized in that: The 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 of a transformer-containing AC / DC hybrid distribution network based on Ring-Topology according to claim 1, characterized in that: Determine the control mode of the transformer port as follows: The control mode of the transformer port is 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 the constant AC voltage control mode; The AC port of the transformer connected to the active AC subsystem can adopt a constant power control or a constant DC voltage control mode, but in order to maintain the active power balance inside the transformer, one side of the AC or DC port must be reserved as a slack port, which works in a constant DC voltage control mode.

4. The method for calculating power flow of a transformer-containing AC / DC hybrid distribution network based on Ring-Topology according to claim 3 is characterized in that: Each transformer DC port is regarded as a power point with known active power; When the DC port of the transformer adopts the fixed active power control mode, the DC node connected to the DC port is the power point; When the DC port does not adopt the fixed 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 of AC / DC hybrid distribution network containing transformer based on Ring-Topology according to claim 1, characterized in that: The specific solution of the DC subsystem power flow using the iteration method is: The active power injected into each node in the DC subsystem is calculated by iterative method, which can be expressed by equation (9): Where P dci is the active power injected into node i in the DC subsystem; g is the number of nodes in the DC subsystem; V i 、V j is the voltage amplitude of nodes i and j in the DC distribution network; Y ij is the element in the i-th row and j-th column of the node admittance matrix, representing the admittance of the node.

6. The method for calculating power flow of AC / DC hybrid distribution network containing transformer based on Ring-Topology according to claim 1, characterized in that: The AC subsystem power flow connected to the non-relaxed port of the transformer is gradually solved 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 nodes of the AC subsystem is gradually solved. The output power of the nodes of the AC subsystem can be expressed by formula (10), as follows: Where P aci , Q aci are the active power and reactive power injected into node i in the AC distribution network respectively; are respectively the active power and reactive power setting values ​​exchanged between the non-relaxed AC port l of the transformer and the AC node i; P aci,0 and Q aci,0 are the initial load active power and reactive power of node i in the AC distribution network respectively.

7. The method for calculating power flow of AC / DC hybrid distribution network containing transformer based on Ring-Topology according to claim 1, characterized in that: The active power of the initial transformer relaxation port can be expressed by equations (12) to (15), as follows: In the formula, is the active power absorbed by the VSC on the non-relaxed AC port l side of the transformer; is the active power setting value of the DC / DC converter output stage on the non-relaxed DC port h side of the transformer; The active power absorbed by the slack AC port VSC of the transformer; is the active power exchanged between the slack AC port of the transformer and the system; is the node voltage amplitude of the AC grid connected to the AC port l of the transformer; is the node voltage of the DC grid connected to the DC port h of the transformer; U d,ref is the DC voltage setting value of the transformer; P loss is the loss of the intermediate isolated DC / DC converter; g is the equivalent conductance taking into account the VSC loss; M is the number of non-relaxed AC ports of the transformer; N is the number of DC ports of the transformer; and are respectively the active power and reactive power setting values ​​on the non-relaxed AC port l side of the transformer; It is the active power setting value on the non-relaxed DC port h side of the transformer.

8. The method for calculating power flow of AC / DC hybrid distribution network containing transformer based on Ring-Topology according to claim 6 is characterized by: Setting the initial value of the control parameter can be expressed by equation (16) and equation (17), as follows: In the formula, θ slack,0 The initial value of the angle at which the voltage phase of the AC grid node connected to the VSC on the slack port side of the transformer leads the voltage phase of the VSC; m slack,0 is the initial value of the modulation ratio of the VSC on the slack port side of the transformer; Set the reactive power value for the transformer's slack AC port to exchange with the system; is the set value of the node voltage amplitude of the AC grid connected to the relaxed AC port of the transformer; b is the equivalent susceptance taking into account the VSC loss; U d,ref is the DC voltage setting value of the transformer; is the voltage amplitude of the AC grid node to which the VSC is connected; It is the active power exchanged between the slack AC port of the transformer and the system.

9. The method for calculating power flow of AC / DC hybrid distribution network containing transformer based on Ring-Topology according to claim 1, characterized in that: The AC subsystem power flow connected to the transformer slack port is calculated step by step as follows: Determine the initial value of the transformer relaxation port control variable; The power flow of the transformer relaxation port is solved step by step to obtain the power injected into the relaxation port by the AC subsystem. The derivative information of the current solution is calculated at each step, and the solution of the next step is adjusted according to the derivative information until the calculated solution reaches the set convergence accuracy. The injection power of the AC node connected to the relaxation port is updated; if the set convergence accuracy cannot be achieved, the control mode and control parameters of each port of the transformer are re-determined; The power flow of the AC subsystem connected to the relaxation port is gradually solved to obtain the relaxation port power. The derivative information of the current solution is calculated at each step, 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 achieved, the control mode and control parameters of each port of the transformer are re-determined. Compare the power of 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, it is necessary to update the AC node voltage connected to the relaxation port and then redetermine the initial value of the control variable of the transformer relaxation port; otherwise, the power flow calculation results of the entire system are considered to have converged.

10. A power flow calculation system for an AC / DC hybrid distribution network containing a transformer based on Ring-Topology, running a power flow calculation method for an AC / DC hybrid distribution network containing a transformer based on Ring-Topology according to any one of claims 1 to 9, characterized in that: Model building module, used to build a steady-state power flow calculation model for transformers; A subsystem division module is used to divide the distribution network into n1 AC subsystems and n2 DC subsystems; A port setting module is used to determine the control mode and control parameters of each port of the transformer; DC power flow solving module, used to calculate the power flow of DC subsystem; The AC power flow solving module is used to calculate the power flow of the AC subsystem.

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