A direct current distribution network transient simulation method based on an average model of a converter

Abstract

This invention proposes a transient simulation method for DC distribution networks based on converter average models. It constructs average models (AVMs) of each component module of the DC distribution system, replacing switches with controllable sources to build average models of the grid-connected inverter, DC transformer, and photovoltaic array. Control schemes for each module are then designed to improve simulation efficiency. AVM-based simulations effectively reflect the steady-state and dynamic characteristics of the system, significantly improving simulation speed and providing an effective and rapid reference for the stable operation of DC distribution systems under different operating modes.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, specifically to converter modeling and simulation methods, and more particularly to a transient simulation method for DC distribution networks based on a converter average model. Background Technology

[0002] With the development of renewable energy power generation technologies and the gradual increase in DC loads such as data centers, electric vehicle charging stations, and communication equipment, the DC characteristics of the power grid are becoming increasingly prominent. Compared with AC distribution systems, DC distribution systems are easier to interface with distributed generation and energy storage devices. Furthermore, compared with traditional AC distribution systems, they offer advantages such as larger capacity, lower line losses, and higher power quality, making them a promising solution for future smart grids.

[0003] However, due to the large number of power electronic converters and switching elements in DC systems, electromagnetic transient modeling is complex, making simulations based on the detailed switching model (SM) quite complicated. Compared to the detailed switching model, the average value model (AVM) uses controlled sources to replace switches, effectively reflecting the stable operating state and dynamic characteristics of the converter. Using AVM can significantly improve simulation speed.

[0004] However, most current research on automatic motion simulation focuses on models of individual devices, and existing technologies rarely pay attention to the automatic motion simulation of the entire system with a high proportion of power electronic devices. Summary of the Invention

[0005] To address the gaps and shortcomings in existing technologies, this invention establishes a DC power distribution system simulation model based on AVM (Automatic Virtual Machine). The effectiveness of the AVM-based simulation system is verified through comparison with a switching model. Simulation results show that the AVM-based system can significantly improve simulation speed and accuracy.

[0006] This invention constructs average virtual machine (AVM) models for each component module of a DC power distribution system, replacing switches with controllable sources to build average models for grid-connected inverters, DC transformers, and photovoltaic arrays. Control schemes for each module are then designed to improve simulation efficiency. AVM-based simulations effectively reflect the steady-state and dynamic characteristics of the system, significantly increasing simulation speed and providing an effective and rapid reference for the stable operation of DC power distribution systems under different operating modes.

[0007] The present invention specifically adopts the following technical solution:

[0008] A transient simulation method for DC distribution networks based on converter average models is characterized by: constructing average models (AVMs) for each component module of the DC distribution system, and using controllable sources to replace switches to realize the construction of average models for grid-connected inverters, DC transformers, and photovoltaic arrays.

[0009] Furthermore, it includes two parts: establishing a DC distribution network model based on the converter average model and designing a control method for the DC distribution network system;

[0010] The DC distribution network model based on the converter average model consists of three parts: the grid-connected inverter average model, the DC transformer average model, and the photovoltaic array average model.

[0011] Furthermore, the establishment of the average model of the grid-connected inverter, which is the average model of the T-type three-level converter, consists of three steps:

[0012] Step A1, taking a single phase as an example, let the duty cycle of switching transistors T1 and T2 be p1, the duty cycle of switching transistors T2 and T3 be p2, and the duty cycle of switching transistors T3 and T4 be p3:

[0013]

[0014]

[0015]

[0016] Among them, i cn i is the current flowing through T4 n Let u be the current flowing through T2 and T3. pc Let i be the voltage across T1. o The output current is E, the DC voltage value is E, and the letter with a horizontal line above it indicates the average value.

[0017] Step A2, using the duty cycle d to represent the switching state of the switching transistor, the current and voltage are expressed as follows:

[0018]

[0019]

[0020]

[0021] Where, d x For the switching transistor T x The duty cycles are x = 1, 2, 3, 4;

[0022] Step A3, after extending the single-phase to three-phase system, yields the corresponding average value model:

[0023]

[0024]

[0025]

[0026]

[0027]

[0028]

[0029]

[0030]

[0031]

[0032] Where i cn y u pc y and i ny It represents the corresponding current and voltage values, d. xy It is the duty cycle of the switching transistor, i y For the corresponding phase output current, y = a, b, c, with a horizontal line above the letters indicating the average value.

[0033] Furthermore, the DC transformer average model is specifically a dual active bridge DC converter average model, which is established in three steps:

[0034] Step B1, summarizing the conduction of switches S1 and S5 and the voltage and current of the inductor in charging and discharging modes, and the time-domain expression i of the inductor current. L (t) is given by the following formula:

[0035]

[0036] In the formula, U MVDC,i and U LVDC,i These are the input and output voltages, respectively; i represents the number of dual active bridge DC-DC converters; and n is the turns ratio of the high-frequency transformer.

[0037] i L (t0), i L (t1), i L (t2), i L (t3) represents the instantaneous current value at different times, and t0 is defined as the initial time and the duty cycle parameter T is introduced. s Its value is obtained from the following formula:

[0038]

[0039] Step B2, simplify T s The average power during the cycle is calculated as half of the operating cycle and is given by the following formula:

[0040]

[0041] In the formula, f s The switching frequency is T. s The reciprocal of;

[0042] Step B3, ignoring power losses in the switch, based on the power P balance from input to output, the average input current on the primary side is calculated using the following formula:

[0043]

[0044] The average output current is calculated using the following formula:

[0045]

[0046] Furthermore, the photovoltaic array averaging model adopts the pre-boost averaging model, which is determined by the following formula:

[0047] U o =dU o +U in

[0048] i o =i in -di in

[0049] In the formula, U in U o i in i o These represent the input and output voltages and currents of the BOOST converter, respectively.

[0050] Furthermore, the design of the DC distribution network system control method consists of three steps:

[0051] Step S1: The grid-connected inverter adopts DC voltage control or power control, and suppresses the circulating current through the circulating current control method.

[0052] Step S2: The dual active bridge DC-DC converter adopts voltage balance control to ensure power balance of each dual active bridge DC-DC converter module.

[0053] Step S3: The photovoltaic array uses MPPT control to track the maximum power of the photovoltaic array, and the input voltage control of the dual active bridge DC converter is used to control the voltage of the LVDC system.

[0054] Compared with the prior art, the beneficial effects of the present invention and its preferred embodiments include:

[0055] Establish an automatic dynamic simulation of the entire DC power distribution system.

[0056] The established AVM simulation can effectively reflect the steady-state and dynamic characteristics of the system, greatly improving the simulation speed.

[0057] It can provide an effective and rapid reference for the stable operation of DC power distribution systems under different working modes. Attached Figure Description

[0058] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0059] Figure 1 This is a schematic diagram of the VSC single-phase average model according to an embodiment of the present invention;

[0060] Figure 2 This is a schematic diagram of the VSC average model in an embodiment of the present invention;

[0061] Figure 3 This is a schematic diagram of the DAB averaging model according to an embodiment of the present invention;

[0062] Figure 4 This is a schematic diagram of the photovoltaic front-end BOOST average model according to an embodiment of the present invention;

[0063] Figure 5 This is a VSC control block diagram according to an embodiment of the present invention;

[0064] Figure 6 This is a DAB control block diagram according to an embodiment of the present invention;

[0065] Figure 7 This is a control block diagram of the photovoltaic array according to an embodiment of the present invention;

[0066] Figure 8 This is a diagram of the architecture of a dual-ended DC power distribution system according to an embodiment of the present invention. Detailed Implementation

[0067] In the following, specific embodiments of this application will be described in detail with reference to the accompanying drawings. Based on these detailed descriptions, those skilled in the art will be able to clearly understand and implement this application. Without departing from the principles of this application, features from various embodiments can be combined to obtain new implementations, or certain features from some embodiments can be substituted to obtain other preferred implementations.

[0068] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0069] To make the features and advantages of this patent more apparent and understandable, specific embodiments are provided below for detailed explanation:

[0070] This invention provides a transient simulation method for DC distribution networks based on a converter average model, comprising two parts: establishing a DC distribution network model based on the converter average model and designing a control method for the DC distribution network system.

[0071] Among them, the DC distribution network model based on the converter average model consists of three parts: the grid-connected inverter average model, the DC transformer average model, and the photovoltaic array average model.

[0072] The average model of the grid-connected inverter is established as the average model of the T-type three-level converter, which consists of three steps:

[0073] Step 1, taking single-phase as an example, such as Figure 1 As shown, let p1 be the duty cycle of switches T1 and T2, p2 be the duty cycle of switches T2 and T3, and p3 be the duty cycle of switches T3 and T4.

[0074]

[0075]

[0076]

[0077] Among them, i cn i is the current flowing through T4 n Let u be the current flowing through T2 and T3. pc Let i be the voltage across T1. o The output current is E, the DC voltage value is E, and the letter with a horizontal line above it indicates the average value.

[0078] Step 2, using the duty cycle d to represent the switching state of the switching transistor, the current and voltage are expressed as follows:

[0079]

[0080]

[0081]

[0082] Where, d x For the switching transistor T x The duty cycles are x = 1, 2, 3, 4.

[0083] Step 3, after extending the single-phase to three-phase system, obtain the corresponding average value model, such as... Figure 2 As shown:

[0084]

[0085]

[0086]

[0087]

[0088]

[0089]

[0090]

[0091]

[0092]

[0093] Where i cn y u pc y and i ny It represents the corresponding current and voltage values, d. xy It is the duty cycle of the switching transistor, i y For the corresponding phase output current, y = a, b, c, with a horizontal line above the letters indicating the average value.

[0094] DC transformer average model as follows Figure 3 The figure shows the average model of a dual active bridge DC-DC converter (DAB), which is established in three steps:

[0095] Step 1: Calculate the time-domain expression of the inductor current i for the conduction of switches S1 and S5 and the voltage of the inductor in charging and discharging modes. L (t) is given by the following formula:

[0096]

[0097] In the formula, U MVDC,i and U LVDC,i These are the input and output voltages, respectively. i represents the number of DABs, and n is the turns ratio of the high-frequency transformer.

[0098] i L (t0), i L (t1), i L (t2), i L (t3) represents the instantaneous current value at different times, and t0 is defined as the initial time and the duty cycle parameter T is introduced. s Its value can be obtained from the following formula:

[0099]

[0100] Step 2, Simplify T sThe average power during the cycle is calculated as half of the operating cycle and is given by the following formula:

[0101]

[0102] In the formula, f s The switching frequency is T. s The reciprocal of.

[0103] Step 3, ignoring power losses in the switch, based on the power P balance from input to output, the average input current on the primary side is calculated using the following formula:

[0104]

[0105] The average output current is calculated using the following formula:

[0106]

[0107] The photovoltaic array averaging model adopts the pre-boost averaging model, such as... Figure 4 As shown, it is determined by the following formula:

[0108]

[0109] In the formula, U in U o i in i o These represent the input and output voltages and currents of the BOOST converter, respectively.

[0110] The design of control methods for DC distribution networks consists of three steps:

[0111] Step 1, the grid-connected inverter uses DC voltage control, such as... Figure 5 As shown;

[0112] Step 2: The DAB uses voltage balance control to ensure power balance among the DAB modules, such as... Figure 6 As shown;

[0113] Step 3, Photovoltaic Array: MPPT control is used to track the maximum power of the photovoltaic array, and input voltage control of the DAB converter is used to control the voltage of the LVDC system, such as... Figure 7 As shown.

[0114] Finally, a typical two-terminal DC power supply system is established, such as Figure 8 As shown above, the preferred embodiments of the present invention have been described in detail. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various equivalent transformations can be made to the technical solutions of the present invention, and these equivalent transformations all fall within the protection scope of the present invention.

[0115] This patent is not limited to the above-described preferred embodiment. Anyone can derive other forms of transient simulation methods for DC distribution networks based on the converter average model under the guidance of this patent. All equivalent changes and modifications made within the scope of this patent application shall fall within the scope of this patent.

Claims

1.A DC distribution network transient simulation method based on an average model of a converter, characterized in that: A mean virtual machine (AVM) model is constructed for each component module of the DC power distribution system. A controllable source is used to replace the switch in order to build the mean virtual machine model for the grid-connected inverter, DC transformer and photovoltaic array. It includes two parts: establishing a DC distribution network model based on the converter average model and designing a control method for the DC distribution network system; The DC distribution network model based on the converter average model consists of three parts: the grid-connected inverter average model, the DC transformer average model, and the photovoltaic array average model. The average model of the grid-connected inverter is established as the average model of the T-type three-level converter, which consists of three steps: Step A1, taking a single phase as an example, let the duty cycle of switching transistors T1 and T2 be p1, the duty cycle of switching transistors T2 and T3 be p2, and the duty cycle of switching transistors T3 and T4 be p3: Where, i cn i is the current flowing through T4 n Let u be the current flowing through T2 and T3. pc Let i be the voltage across T1. o The output current is E, the DC voltage value is E, and the letter with a horizontal line above it indicates the average value. Step A2, using the duty cycle d to represent the switching state of the switching transistor, the current and voltage are expressed as follows: where d x is the duty cycle of the switch T x x = 1, 2, 3, 4. Step A3, after extending the single-phase to three-phase system, yields the corresponding average value model: Where i cn y u pc y and i ny It represents the corresponding current and voltage values, d. xy It is the duty cycle of the switching transistor, i y For the corresponding phase output current, The letter with a horizontal line above it indicates the average value. The DC transformer average model is specifically a dual active bridge DC converter average model, which is established in three steps: Step B1: The time-domain expressions for the conduction of switches S1 and S5, the voltage of the inductor in charging and discharging modes, and the inductor current are calculated. It is given by the following formula: wherein and are the input and output voltages, respectively, i denotes the number of dual active bridge DC converters, and n is the turns ratio of the high frequency transformer. , , , denotes the current instantaneous value at different time instants, t0is defined as the initial time instant and the duty cycle parameter T is introduced s whose value is obtained from the following equation: Step B2, Simplify T s The calculation of the average power in T is half the duty cycle and is given by: where f is the switching frequency, and T is the period of the switching frequency. s s the inverse of T.​ Step B3, ignoring power losses in the switch, based on the power P balance from input to output, the average input current on the primary side is calculated using the following formula: The average output current is calculated using the following formula: The photovoltaic array average model adopts the pre-boost average model, which is determined by the following formula: wherein , , , BOOST converter input and output voltages, input and output currents, respectively; The design of the DC distribution network system control method consists of three steps: Step S1: The grid-connected inverter adopts DC voltage control or power control, and suppresses the circulating current through the circulating current control method. Step S2: The dual active bridge DC-DC converter adopts voltage balance control to ensure power balance of each dual active bridge DC-DC converter module. Step S3: The photovoltaic array uses MPPT control to track the maximum power of the photovoltaic array, and the input voltage control of the dual active bridge DC converter is used to control the voltage of the LVDC system.

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