A MMC type multi-terminal AC / DC urban power distribution system and building block modeling method

Abstract

The present invention discloses an MMC type multi-terminal AC / DC urban power distribution system and a building block modeling method, wherein the system includes: a meshed control MMC converter station, an AC distribution network, a ring / grid DC distribution network, multiple modular intelligent DC transformers, and a fault high-speed blocking DC solid-state circuit breaker. The meshed MMC converter station realizes AC / DC conversion and has the ability to support the voltage and frequency of the upper power grid; the DC transformer adopts a series-parallel hybrid topology; the MMC converter station and the DC transformer are interconnected through a DC network to realize multi-voltage level power transmission and multi-distribution area power mutual assistance. In addition, the multi-level building block modeling method divides the above-mentioned distribution system into a macro system layer and a micro functional module layer, and realizes rapid and accurate modeling of a DC distribution system with arbitrary topology through modular design and building block splicing. The present invention improves the reliability and modeling efficiency of the urban power distribution system, and provides a new solution for building a highly reliable intelligent distribution network.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics and novel power distribution system applications, and in particular relates to an MMC type multi-terminal AC / DC urban power distribution system and a building block modeling method. Background Art

[0002] With the acceleration of urbanization, the reliability and flexibility requirements of urban distribution systems are increasing. The traditional urban distribution network structure has many shortcomings when facing complex urban power loads and high reliability requirements. For example, in the actual operation of the existing single-ring network, multi-segment multi-tie power supply modes, the tie switches between feeders are usually disconnected, resulting in insufficient continuity and reliability of power supply. In addition, when facing electrical faults, traditional distribution systems usually need to remove the faults through segment switches, resulting in short-term power outages and unable to ensure the continuity of power supply. Modular multilevel converter (MMC) technology has gradually been applied in high-voltage direct current transmission systems due to its advantages such as high flexibility, high reliability and high power quality. However, there are relatively few studies on the application of MMC technology to urban flexible distribution systems, mainly focusing on multi-terminal direct current transmission systems based on MMC and direct current distribution systems based on two-level VSCs. In addition, for such a high-reliability MMC-type multi-terminal AC / DC urban distribution system, establishing an accurate and effective control simulation and systematic analysis model is another key technical problem that needs to be solved urgently. Therefore, it is of great theoretical and engineering significance to develop a high-reliability multi-terminal AC / DC urban power distribution system based on MMC and combine it with the building block modeling method to establish an accurate and efficient simulation analysis model of the system to improve the design and analysis efficiency of the system. Summary of the invention

[0003] To achieve the above-mentioned objectives, the present invention provides an MMC-type multi-terminal AC / DC urban power distribution system and a building block modeling method. First, the present invention introduces a design with high reliability, flexibility, energy efficiency and adaptability, and proposes an MMC-type multi-terminal AC / DC urban power distribution system to improve the reliability, flexibility and economy of the urban power distribution system. Secondly, for the system, a building block modeling method is proposed to ensure the efficiency, flexibility and scalability of the modeling while improving the accuracy of the modeling and the speed of the simulation, so as to solve the problems of complex modeling and difficult simulation of the AC / DC urban power distribution system based on MMC, and realize efficient simulation and optimization of the system.

[0004] Among them, an MMC type multi-terminal AC / DC urban power distribution system includes:

[0005] MMC converter station system, DC distribution network, multiple modular intelligent DC transformers, DC solid-state circuit breakers and AC circuit breakers interconnected by ports;

[0006] Wherein, the MMC converter station system and the multiple modular intelligent DC transformers are respectively connected to the DC power distribution network;

[0007] The MMC converter station system is used to realize AC / DC conversion, and is also used to perform fault self-blocking and self-healing through fault self-blocking technology and self-healing technology;

[0008] The DC power distribution network is a ring / grid type DC power distribution network with flexible interconnection of multiple voltage levels, and power transmission between different voltage levels is realized through the multiple modular intelligent DC transformers;

[0009] The multiple modular intelligent DC transformer adopts a series-parallel hybrid topology structure and has a bidirectional high-power control capability;

[0010] The DC solid-state circuit breaker is used to quickly isolate a fault area.

[0011] Preferably, the MMC converter station system adopts a grid-type control to achieve power mutual assistance and active support of AC voltage and frequency when a fault occurs at any AC end.

[0012] Preferably, the system also includes a non-droop peer coordination control module, which is used to configure a non-droop peer coordination control strategy for the MMC converter station system to ensure that when a system failure occurs, the MMC converter station system can ensure continuous operation of the system without mode switching.

[0013] The present invention also provides a building block modeling method for an MMC type multi-terminal AC / DC urban power distribution system, comprising:

[0014] Adopting a multi-level decomposition strategy, the system is divided into a macro system layer and a micro functional module layer;

[0015] Based on the macro system layer, the system is divided into an MMC converter station group, a multi-port intelligent DC transformer cluster, and a multi-voltage level flexibly interconnected ring / grid DC distribution network;

[0016] Based on the micro-functional module layer, the MMC converter station system is divided into an AC electrical system, an MMC converter, a DC interface, a signal filter and a multi-mode unified controller by introducing virtual resistance; the multi-port intelligent DC transformer is divided into a power conversion unit and a unified control unit; the multi-voltage level flexible interconnected ring / grid DC distribution network is divided into branch lines and bus nodes by the circuit segmentation equivalence principle;

[0017] Each submodule of the macro system layer and the micro functional module layer is modeled separately, and through the input and output variable relationship of each module, the submodule models are spliced ​​together in a building block manner to construct a nonlinear state space mathematical model of the complete system.

[0018] Preferably, modeling the submodules of the macro system layer and the micro functional module layer respectively includes modeling the MMC converter station system;

[0019] The MMC converter station system is modeled by block decoupling and building block integration to obtain the model expression:

[0020]

[0021] In the formula, [ , , , , ; State variables [ , , , , ; Phase , , , , They are the MMC connection AC system, MMC converter station electrical system, MMC DC interface, MMC control system filter link and MMC controller state variables; Output for MMC system; input variable , , , , , ,in are the components of the AC grid voltage in the dq coordinate system, The bus voltage connected to the MMC DC side , , , Reference values ​​are given for the DC side voltage, AC side voltage, active power and reactive power of the MMC respectively.

[0022] Preferably, modeling the submodules of the macro system layer and the micro functional module layer separately includes modeling multiple modular intelligent DC transformers;

[0023] Modeling the multiple modular intelligent DC transformer includes establishing a multiple modular intelligent DC transformer electrical system model and a multiple modular intelligent DC transformer control system model;

[0024] The formula expression of the multi-modular intelligent DC transformer electrical system model includes:

[0025] and They are the high-voltage side and low-voltage side equivalent capacitances of multiple modular intelligent DC transformers; and They are the high-side and low-side voltages of multiple modular intelligent DC transformers; and They are the input current and output current of the multiple modular intelligent DC transformers respectively; and They are the equivalent controllable current source currents of the high-voltage side and the low-voltage side of the multiple modular intelligent DC transformers respectively; Equal to the DC bus node voltage connected by multiple modular intelligent DC transformers;

[0026] Then the equivalent capacitance expression is:

[0027] ,

[0028] According to the KCL equation of the low-voltage side filter capacitor node of the multi-modular intelligent DC transformer:

[0029]

[0030] Assume that the load equivalent resistance at the output end of the multiple modular intelligent DC transformer is , low voltage side output current It is expressed as:

[0031]

[0032] The KCL equation of the input capacitance of the multi-modular intelligent DC transformer is:

[0033]

[0034] The KVL equation of the input inductance of the multi-modular intelligent DC transformer is:

[0035]

[0036] Among them, multiple modular intelligent DC transformer input end H bridge current and the output H-bridge current It is expressed as:

[0037]

[0038] In the above formula, Indicates the equivalent displacement of multiple modular intelligent DC transformers. is the transformer ratio, Represents the equivalent PWM switching cycle of multiple modular intelligent DC transformers, Represents the equivalent auxiliary inductance of multiple modular intelligent DC transformers;

[0039] The formula expression of the multi-modular intelligent DC transformer control system model includes:

[0040] The input signal of the multiple modular intelligent DC transformer controller is the DC voltage on the low voltage side of the multiple modular intelligent DC transformer. , the signal is sent to the controller after filtering; the signal filter is modeled, the signal filter adopts a first-order low-pass filter, and the dynamic equation of the signal filter is expressed as:

[0041]

[0042] In the above formula, is the filter state variable, is the filter cut-off frequency;

[0043] The state equation of the phase-shift controller is:

[0044]

[0045] The process of integrating the multiple modular intelligent DC transformer electrical system model and the multiple modular intelligent DC transformer control system model includes:

[0046] According to formulas (3), (5), (6), (8), and (9), the state space model and output equation of the multi-modular intelligent DC transformer are expressed as:

[0047]

[0048] State variables ; The system output is ; Input variables , ,in Bus node voltage connected to multiple modular intelligent DC transformers It is the number of multiple modular intelligent DC transformers.

[0049] Preferably, modeling the submodules of the macro system layer and the micro functional module layer respectively includes modeling a ring / grid type DC power distribution network with flexible interconnection of multiple voltage levels;

[0050] The process of modeling the multi-voltage level flexible interconnected ring / grid type DC distribution network includes:

[0051] The DC distribution network modeling is divided into branch lines and bus nodes, and the branch line modeling and bus node modeling are performed separately; during modeling, the circuit equivalent is merged and simplified, the variable relationship between the branch line and the bus node is spliced ​​by building blocks, all the branch lines and bus nodes are connected, and the DC distribution network model is integrated;

[0052] The formula expression of the branch line model is:

[0053]

[0054] In the above formula, and Branch lines The equivalent resistance and equivalent inductance, For the flow line The inductor current is and The maximum value is the number of branch lines and For line The voltage of the bus node connected at both ends, where the input variable is divided into two parts, the first part is the node voltage connected to the MMC , , ,in and The maximum value is the number of MMC converter stations The second part is the node voltage connecting multiple modular intelligent DC transformers , , ,in and The maximum value of is the number m of multiple modular intelligent DC transformers;

[0055] Taking the branch line current as the output, the branch line model is further organized as follows:

[0056]

[0057] State variables , input variables:

[0058] , …, …, , …, , …, ;

[0059] The formula expression of the busbar node model is:

[0060]

[0061] In the above formula, Representation Node The sum of the currents of all connected branch lines, For slave nodes The current injected into the MMC converter station or multiple modular intelligent DC transformers, For Node All branch lines connected at the node The equivalent capacitance at ;

[0062] Taking the bus node voltage as the output, the bus node model is further organized as follows:

[0063]

[0064] In the above formula, the state variable , …, , input variables:

[0065] = , …, , , , , , , , , , where is the current flowing from the DC side to the AC side of the MMC, The current flowing from the DC high voltage side to the DC low voltage side of the multiple modular intelligent DC transformers;

[0066] The process of integrating and obtaining the DC distribution network model includes:

[0067] Integrating equations (12) and (14), the state space model and output equation of the DC distribution network are expressed as:

[0068]

[0069] In the above formula, the state variable [ , = , …, , , , …, …, , , …, , …, ; The system output is ,in = , …, , = , …, , …, , = , , ; Input variables , , , where , , .

[0070] Preferably, the process of constructing a nonlinear state space mathematical model by building blocks of each submodule model through the input and output variable relationship of each module includes:

[0071] According to formulas (1), (10), and (15), the input-output relationship between the subsystems in the system is as follows:

[0072] = , …, , …, , , …, , …, , , ;

[0073] Through the input-output relationship connection system between the MMC converter station, multiple modular intelligent DC transformers and the ring / grid DC distribution network, the final MMC type multi-terminal AC / DC distribution system model is integrated.

[0074] Compared with the prior art, the present invention has the following advantages and technical effects:

[0075] (1) The reliability of urban power distribution systems has been significantly improved: By introducing advanced technologies such as MMC converter stations with self-fault blocking and self-healing capabilities, ring / grid DC distribution networks with flexible interconnection of multiple voltage levels, and multiple modular intelligent DC transformers, the high reliability and flexibility of urban power distribution systems have been achieved.

[0076] (2) Significantly improved modeling efficiency: The proposed building block modeling method significantly improves modeling efficiency while ensuring model accuracy through multi-level decomposition strategy and modular design. This method can quickly build a DC distribution system model of any topology, greatly saving simulation time, and the modeling method can also be extended to multi-terminal DC transmission systems based on MMC.

[0077] (3) Enhanced model flexibility and scalability: The building block modeling method makes the model flexible and scalable, making it easier to expand and optimize the system according to actual needs.

[0078] (4) Facilitate system analysis and control design: The established state-space model can be directly linearized through mathematical tools such as MATLAB to further obtain the small signal model of the system, which facilitates system stability analysis and control design. BRIEF DESCRIPTION OF THE DRAWINGS

[0079] The drawings constituting a part of this application are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an improper limitation on this application. In the drawings:

[0080] Figure 1 A schematic diagram of the basic structure of a multi-terminal DC power distribution system according to an embodiment of the present invention;

[0081] Figure 2 A schematic diagram of the overall architecture of building block modeling of a DC power distribution system according to an embodiment of the present invention;

[0082] Figure 3 A schematic diagram of an MMC represented by a controlled voltage source according to an embodiment of the present invention;

[0083] Figure 4 A DCSST circuit diagram of a multiple DAB structure according to an embodiment of the present invention;

[0084] Figure 5 is a DCSST equivalent circuit diagram of an embodiment of the present invention;

[0085] Figure 6 is an equivalent circuit diagram of a power distribution network according to an embodiment of the present invention;

[0086] Figure 7 is an equivalent circuit diagram of the MTDCDS of an embodiment of the present invention;

[0087] Figure 8 It is a block diagram of the MTDCDS model integration of an embodiment of the present invention;

[0088] Fig. 9 A voltage diagram of the DC side of a converter station according to an embodiment of the present invention;

[0089] Fig.10 This is a diagram of active power output of a converter station according to an embodiment of the present invention. DETAILED DESCRIPTION

[0090] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0091] It should be noted that the steps shown in the flowcharts of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and that, although a logical order is shown in the flowcharts, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0092] This embodiment provides an MMC type multi-terminal AC / DC urban power distribution system and a building block modeling method, including the following steps:

[0093] Step 1: Design a high-reliability MMC multi-terminal AC / DC urban power distribution system:

[0094] In response to the demand for high-reliability power supply in cities, a high-reliability MMC-type multi-terminal AC / DC urban power distribution system is designed. The system includes multiple network-controlled MMC converter station systems (including AC systems connected to MMC), ring / grid DC distribution networks, multiple modular smart DC transformers (DCSST), and DC solid-state circuit breakers. Among them, the MMC converter station and the DCSST are respectively connected to the ring / grid DC distribution network; the multiple AC / DC converter stations all adopt the MMC structure to realize AC / DC conversion, and are also used to perform fault self-blocking and self-healing through fault self-blocking technology and self-healing technology to ensure high-reliability power supply in high-load density environments in cities; the ring / grid DC distribution network is designed to be a ring / grid DC distribution network with flexible interconnection of multiple voltage levels, and power transmission between different voltage levels is realized through the DCSST; wherein the DCSST adopts a series-parallel hybrid topology structure with bidirectional high-power control capability; the DC solid-state circuit breaker is used to quickly isolate the fault area to ensure the high reliability and safety of the system.

[0095] Preferably, the coordinated control of each MMC converter station in the system adopts a non-drooping peer-to-peer coordinated control strategy, and all MMC converter stations adopt meshed control. This ensures that when a fault occurs at any AC end of the system, the MMC converter station can achieve power mutual assistance and active support of AC voltage and frequency in different distribution areas without mode switching, thereby ensuring the continuous and stable operation of the system and improving the reliability of the system.

[0096] Specifically, Figure 1As shown in the figure, a high-reliability MMC type 6-terminal AC / DC urban distribution system is designed. The system includes 3 MMC converter stations with network control, including an AC system connected to the MMC. The coordinated control of each MMC converter station adopts a non-drooping peer-to-peer coordinated control strategy; the DC distribution network adopts a ring DC distribution network with a rated DC bus voltage of ±10kV, which is suitable for urban medium-voltage distribution systems. The 3 DC SSTs adopt a series-parallel hybrid structure with a high-voltage side voltage level of ±10kV and a low-voltage side voltage level of ±375V; DC solid-state circuit breakers are installed on ±10kV and ±375V DC lines, and AC circuit breakers are installed on 10kV AC lines.

[0097] Step 2: Design the overall architecture of multi-level building block modeling of MMC multi-terminal AC / DC urban power distribution system:

[0098] like Figure 2 As shown, a multi-level decomposition strategy is adopted to divide the system into two levels: the macro system level and the micro functional module level. At the macro system level, the system is divided into MMC converter station groups, DCSST clusters, and multi-voltage level flexible interconnected ring / grid DC distribution networks; at the micro functional module level, by introducing virtual resistance, the MMC converter station is divided into an AC electrical system, an MMC converter, a DC interface, a signal filter, and a multi-mode unified controller; the DCSST is divided into a power conversion unit and a unified control unit; the multi-voltage level flexible interconnected ring / grid DC distribution network is divided into branch lines and bus nodes by the circuit segmentation equivalence principle;

[0099] After adopting the multi-level decomposition strategy, each sub-module is modeled separately, based on graph theory and circuit topology, and then through the input and output variable relationship of each module, according to Figure 2 As shown, by building block-like splicing of the sub-module models, the construction of a complete nonlinear state space mathematical model of a DC distribution system with any topology can be completed, thereby realizing fast and accurate modeling and simulation analysis of a DC distribution system with any topology.

[0100] Step 3: Detailed building block modeling of MMC multi-terminal AC / DC urban power distribution system:

[0101] Based on the above multi-level building block modeling overall architecture, detailed mathematical modeling of each key component in the system is carried out, and the accurate description and analysis of the entire system is achieved by building block splicing of the designed interface. Specifically, step 3 carries out modeling work around the MMC converter station, DCSST and ring DC network.

[0102] 3.1. Dynamic phasor model of MMC converter station;

[0103] The schematic diagram of the MMC converter station represented by the controlled voltage source is as follows Figure 3As shown in the figure, the bridge arm of the MMC is represented by an equivalent controlled voltage source series impedance. The parameters represented by each variable in the figure are shown in Table 1.

[0104] Table 1

[0105]

[0106] The detailed modeling process of each sub-module of the MMC converter station has been given in the paper (Xingfeng Xie, A Unified Modeling Scheme of Modular Multilevel Converter for Hybrid AC / DC Power Grids [J]. Frontiers in Energy Research, vol. 10, 2022.). In this embodiment, the modeling process of each sub-module of the MMC is not repeated here, and the dynamic phasor model expression of the integrated MMC converter station is directly given as:

[0107]

[0108] In the formula, [ , , , , ; State variables [ , , , , ; Phase , , , , They are the MMC connection AC system, MMC converter station electrical system, MMC DC interface, MMC control system filter link and MMC controller state variables; Output for MMC system; input variable , , , , , ,in are the components of the AC grid voltage in the dq coordinate system, The bus voltage connected to the MMC DC side , , , Reference values ​​are given for the DC side voltage, AC side voltage, active power and reactive power of the MMC respectively.

[0109] 3.2, DCSST modeling;

[0110] DCSST consists of multiple core unit dual active H-bridge converter submodules connected in series and parallel. Figure 4 The schematic diagram of the DCSST circuit is composed of modular multiple dual active full bridge circuits. The DC distribution network of this embodiment mainly focuses on the external characteristics of the DCSST. Therefore, this embodiment considers the inductance on the high-voltage side of the DCSST. The multi-modular DCSST is equivalent to a lumped parameter DAB average model. Figure 5 The DAB average equivalent circuit of DCSST is shown in Figure 2. The modeling of DCSST mainly includes the modeling of the electrical system and the control system.

[0111] 1) DCSST electrical system model;

[0112] Figure 5 middle, and They are the high-voltage side and low-voltage side equivalent capacitances of multiple modular intelligent DC transformers; and They are the high-side and low-side voltages of multiple modular intelligent DC transformers; and They are the input current and output current of the multiple modular intelligent DC transformers respectively; and They are the equivalent controllable current source currents of the high-voltage side and the low-voltage side of the multiple modular intelligent DC transformers respectively; Equal to the DC bus node voltage connected by multiple modular intelligent DC transformers;

[0113] Then the equivalent capacitance expression is:

[0114] ,

[0115] According to the DCSST low-voltage side filter capacitor node KCL equation:

[0116]

[0117] Assume that the equivalent resistance of the load at the output end of DCSST is , low voltage side output current It is expressed as:

[0118]

[0119] The KCL equation for the DCSST input capacitance is:

[0120]

[0121] The KVL equation for the inductance at the input of the DCSST is:

[0122]

[0123] Among them, the H-bridge current at the DCSST input terminal is and the output H-bridge current It is expressed as:

[0124]

[0125] In the above formula, Indicates the DCSST equivalent shift ratio, is the transformer ratio, represents the DCSST equivalent PWM switching period, Denotes the DCSST equivalent auxiliary inductance.

[0126] 2) DCSST control system model;

[0127] The input signal of the DCSST controller is the DC voltage on the low voltage side of the DCSST. , the signal is sent to the controller after filtering; the signal filter is modeled, the signal filter adopts a first-order low-pass filter, and the dynamic equation of the signal filter is expressed as:

[0128]

[0129] In the above formula, is the filter state variable, is the filter cut-off frequency;

[0130] The state equation of the phase-shift controller is:

[0131]

[0132] 3) DCSST system model integration;

[0133] According to formulas (3), (5), (6), (8), and (9), the state space model and output equation of DCSST are expressed as:

[0134]

[0135] State variables ; The system output is ; Input variables , ,in The bus node voltage connected to DCSST Wherein, m is the number of DCSSTs.

[0136] 3.3, Modeling of flexible interconnected ring DC distribution network;

[0137] The topology of the DC distribution network is complex and diverse. The existing methods are all methods for establishing the conductivity matrix of a specific network. This method is very troublesome for different networks, and the modification of the model will be very troublesome. The present invention divides the DC distribution network modeling into branch lines and bus nodes by a circuit equivalent method. When modeling, it is only necessary to connect all branch lines and bus nodes according to the variable relationship between the branch lines and the bus nodes by building blocks to obtain the model of the entire DC distribution network. Such segmented modeling can be applied to DC distribution networks of any topological structure, and can be simply and flexibly modified when the network topology changes.

[0138] 1) Branch line model

[0139] like Figure 6 As shown in the equivalent circuit of the distribution network, the DC distribution network has a total of 6 branch lines. According to KVL, the model of the branch line can be expressed as:

[0140]

[0141] In the above formula, and Branch lines The equivalent resistance and equivalent inductance, For the flow line The inductor current is and The maximum value is the number of branch lines . and For line The voltage of the bus node connected at both ends, where the input variable is divided into two parts, the first part is the node voltage connected to the MMC , , ,in and The maximum value is the number of MMC converter stations , the second part is the node voltage connected to DCSST , , ,in and The maximum value is the number m of DCSSTs.

[0142] Taking the current of the branch line as the output quantity, the model of the branch line can be organized as follows:

[0143]

[0144] State variables , input variables:

[0145] , , , , , .

[0146] 2) Busbar node model

[0147] According to KCL, the model of the busbar node can be expressed as:

[0148]

[0149] In the above formula, Representation Node The sum of the currents of all connected branch lines. For slave nodes Current injected into the MMC converter station or DCSST. For Node All branch lines connected at the node The equivalent capacitance at .

[0150] Taking the bus node voltage as the output, the bus node model can be organized as:

[0151]

[0152] In the above formula, the state variable , , , , , , input variable = , , , , , , In the formula, is the current flowing from the DC side to the AC side of the MMC, It is the current flowing from the DC high voltage side to the DC low voltage side of DCCSST.

[0153] 3) DC distribution network model integration

[0154] Integrating equations (12) and (14), the state space model and output equation of the DC distribution network are expressed as:

[0155]

[0156] In the above formula, the state variable [ , = , , , , , ;

[0157] The system output is ,in = , , = , , , = , , ; Input variables , , , where , , .

[0158] 3.4. Multi-terminal DC distribution system model integration;

[0159] Based on the model established for each part of the multi-terminal DC distribution system, Figure 1 The multi-terminal DC distribution system shown is equivalent to Figure 7 The equivalent circuit shown in Figure 1 is shown in Figure 1. Figure 7 From equations (1), (10), and (15), we can see that the input-output relationship between the subsystems in the system is as follows:

[0160] = , , , , , , , By connecting the system through the input-output relationship between the MMC converter station, DCSST and the DC distribution network, the integrated MMC multi-terminal AC / DC distribution system model can be obtained as follows: Figure 8 shown.

[0161] Step 4: Model simulation verification:

[0162] In order to verify the modeling accuracy of the state space model proposed in this paper, Figure 1 The 6-terminal DC power distribution test system based on MMC is established on the MATLAB / Simulink platform. Figure 8 The MMC type multi-terminal DC distribution system model (Math) and the average value electromagnetic transient simulation model (AVE-EMT) established as shown are compared, where the parameters of the multi-terminal DC distribution system are shown in Table 2.

[0163] Voltage on the DC side of the converter station under disturbance The waveform of (per unit value) is as follows Fig. 9 As shown in the figure, the waveform of active power (per unit value) output by each converter station is as follows Fig.10 As shown. Fig. 9 and Fig.10 It can be seen that the step responses of the state-space model and the AVE-EMT simulation model are highly consistent, which verifies the accuracy and effectiveness of the proposed building block modeling method. In addition, Table 3 shows the comparison results of the actual simulation time of the state-space model proposed in this paper and AVE-EMT. Under the same simulation conditions, the simulation efficiency of the system mathematical model established by the proposed method is at least 14 times higher than that of AVE-EMT. Therefore, the proposed model greatly speeds up the simulation while achieving higher accuracy.

[0164] Table 2

[0165]

[0166] Table 3

[0167]

[0168] The above are only preferred specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A building block modeling method for an MMC type multi-terminal AC / DC urban power distribution system, characterized in that: include: Adopting a multi-level decomposition strategy, the system is divided into a macro system layer and a micro functional module layer; Based on the macro system layer, the system is divided into an MMC converter station group, a multi-port intelligent DC transformer cluster, and a multi-voltage level flexibly interconnected ring / grid DC distribution network; Based on the micro-functional module layer, the MMC converter station system is divided into an AC electrical system, an MMC converter, a DC interface, a signal filter and a multi-mode unified controller by introducing virtual resistance; the multi-port intelligent DC transformer is divided into a power conversion unit and a unified control unit; the multi-voltage level flexible interconnected ring / grid DC distribution network is divided into branch lines and bus nodes by the circuit segmentation equivalence principle; Each submodule of the macro system layer and the micro functional module layer is modeled separately, and through the input and output variable relationship of each module, the submodule models are spliced ​​together in a building block manner to construct a nonlinear state space mathematical model of the complete system.

2. The modular modeling method of the MMC multi-terminal AC / DC urban power distribution system according to claim 1 is characterized in that: Modeling the submodules of the macro system layer and the micro functional module layer respectively includes modeling the MMC converter station system; The MMC converter station system is modeled by block decoupling and building block integration to obtain the model expression: (1) In the formula, [ , , , , ; State variables [ , , , , ; Phase , , , , They are the MMC connection AC system, MMC converter station electrical system, MMC DC interface, MMC control system filter link and MMC controller state variables; Output for MMC system; input variable , , , , , ,in The AC grid voltage is dq The components in the coordinate system, The bus voltage connected to the MMC DC side , , , Reference values ​​are given for the DC side voltage, AC side voltage, active power and reactive power of the MMC respectively.

3. The modular modeling method of the MMC multi-terminal AC / DC urban power distribution system according to claim 1 is characterized in that: Modeling the submodules of the macro system layer and the micro functional module layer respectively includes modeling multiple modular intelligent DC transformers; Modeling the multiple modular intelligent DC transformer includes establishing a multiple modular intelligent DC transformer electrical system model and a multiple modular intelligent DC transformer control system model; The formula expression of the multi-modular intelligent DC transformer electrical system model includes: and They are the high-voltage side and low-voltage side equivalent capacitances of multiple modular intelligent DC transformers; and They are the high-voltage side and low-voltage side voltages of multiple modular intelligent DC transformers respectively; and They are the input current and output current of the multiple modular intelligent DC transformers respectively; and They are the equivalent controllable current source currents of the high-voltage side and the low-voltage side of the multiple modular intelligent DC transformers respectively; Equal to the voltage of the DC bus node connected by multiple modular intelligent DC transformers; Then the equivalent capacitance expression is: ， (2) According to the KCL equation of the low-voltage side filter capacitor node of the multi-modular intelligent DC transformer: (3) Assume that the load equivalent resistance at the output end of the multiple modular intelligent DC transformer is , low voltage side output current It is expressed as: (4) The KCL equation of the input capacitance of the multi-modular intelligent DC transformer is: (5) The KVL equation of the input inductance of the multi-modular intelligent DC transformer is: (6) Among them, multiple modular intelligent DC transformer input end H bridge current and the output H-bridge current It is expressed as: (7) In the above formula, Indicates the equivalent displacement of multiple modular intelligent DC transformers. is the transformer ratio, Represents the equivalent PWM switching cycle of multiple modular intelligent DC transformers, Represents the equivalent auxiliary inductance of multiple modular intelligent DC transformers; The formula expression of the multi-modular intelligent DC transformer control system model includes: The input signal of the multiple modular intelligent DC transformer controller is the DC voltage on the low voltage side of the multiple modular intelligent DC transformer. , the signal is sent to the controller after filtering; the signal filter is modeled, the signal filter adopts a first-order low-pass filter, and the dynamic equation of the signal filter is expressed as: (8) In the above formula, is the filter state variable, is the filter cut-off frequency; The state equation of the phase-shift controller is: (9) The process of integrating the multiple modular intelligent DC transformer electrical system model and the multiple modular intelligent DC transformer control system model includes: According to formulas (3), (5), (6), (8), and (9), the state space model and output equation of the multi-modular intelligent DC transformer are expressed as: (10) State variables ; The system output is ; Input variables , ,in Bus node voltage connected to multiple modular intelligent DC transformers It is the number of multiple modular intelligent DC transformers.

4. The building block modeling method of the MMC type multi-terminal AC / DC urban power distribution system according to claim 1 is characterized in that: Modeling the submodules of the macro system layer and the micro functional module layer respectively includes modeling a ring / grid type DC distribution network with flexible interconnection of multiple voltage levels; The process of modeling the multi-voltage level flexible interconnected ring / grid type DC distribution network includes: The DC distribution network modeling is divided into branch lines and bus nodes, and the branch line modeling and bus node modeling are performed separately; during modeling, the circuit equivalent is merged and simplified, the variable relationship between the branch line and the bus node is spliced ​​by building blocks, all the branch lines and bus nodes are connected, and the DC distribution network model is integrated; The formula expression of the branch line model is: (11) In the above formula, and Branch lines The equivalent resistance and equivalent inductance, For the flow line The inductor current is and The maximum value is the number of branch lines and For Line The voltage of the bus node connected at both ends, where the input variable is divided into two parts, the first part is the node voltage connected to the MMC , , ,in and The maximum value is the number of MMC converter stations The second part is the node voltage connecting multiple modular intelligent DC transformers , , ,in and The maximum value of is the number m of multiple modular intelligent DC transformers; Taking the branch line current as the output, the branch line model is further organized as follows: (12) State variables , input variables: ，…， …， ， …， ，…， ； The formula expression of the busbar node model is: (13) In the above formula, Representation Node The sum of the currents of all connected branch lines, For slave nodes The current injected into the MMC converter station or multiple modular intelligent DC transformers, For Node All branch lines connected at the node The equivalent capacitance at ; Taking the bus node voltage as the output, the bus node model is further organized as follows: (14) In the above formula, the state variable , …, , input variables: = , …, , , , , , , , , , where is the current flowing from the DC side to the AC side of the MMC, The current flowing from the DC high voltage side to the DC low voltage side of the multiple modular intelligent DC transformers; The process of integrating and obtaining the DC distribution network model includes: Integrating equations (12) and (14), the state space model and output equation of the DC distribution network are expressed as: (15) In the above formula, the state variable [ , = , …, , , , …, …, , , …, , …, ; The system output is ,in = , …, , = , …, , …, , = , , ; Input variables , , , where , , .

5. The building block modeling method of the MMC type multi-terminal AC / DC urban power distribution system according to claim 1 is characterized in that: Through the input and output variable relationship of each module, the sub-module models are connected in a building block manner to construct a nonlinear state space mathematical model, which includes: According to formulas (1), (10), and (15), the input-output relationship between the subsystems in the system is as follows: = ，…， ，…， ， ，…， ，…， ， ， ； Through the input-output relationship connection system between the MMC converter station, multiple modular intelligent DC transformers and the ring / grid DC distribution network, the final MMC type multi-terminal AC / DC distribution system model is integrated.

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