Feasibility verification method of flexible distribution transformer based on simulation software

Abstract

A feasibility verification method for a flexible distribution transformer based on simulation software begins by constructing simulation models for the new flexible distribution transformer. These include: a dual-winding transformer simulation model, an H-bridge converter simulation model, a three-phase full-bridge converter simulation model, a photovoltaic power generation simulation model, an electric vehicle charging and discharging simulation model, a DC microgrid simulation model, an H-bridge converter control simulation model, a three-phase full-bridge converter control simulation model, and photovoltaic and electric vehicle grid-connected control simulation models. Based on this, a feasibility verification simulation model is constructed, and criteria are established to determine the feasibility of the new flexible distribution transformer, including low-voltage bus voltage deviation, voltage regulation range, and DC capacitor voltage deviation. This method aims to analyze the voltage regulation and stabilization characteristics of the new flexible distribution transformer, as well as its renewable energy absorption capacity, providing data support for the design and engineering implementation of the new flexible distribution transformer.

Description

Technical Field

[0001] The present invention relates to the field of power system electrical equipment simulation, and in particular to a feasibility verification method for a flexible distribution transformer based on simulation software. Background Art

[0002] Building a new power system dominated by renewable energy is a future development trend. However, this new power system faces several challenges, including the replacement of conventional power sources by fluctuating renewable energy, changes in the grid structure, rapid fluctuations in power and voltage due to load fluctuations, and high and low voltage thresholds. These factors limit the grid's ability to accommodate high penetration rates of renewable energy, hindering the improvement of power supply quality and the reduction of line losses. This puts tremendous pressure on the safe, stable, and economical operation of the power system, necessitating an urgent need to enhance the grid's operational flexibility and control capabilities.

[0003] Power transformers are key nodes in power transmission. To meet the demands of modern power systems for more advanced and flexible control and functionality, attention has been focused on power electronic transformers, which utilize full-power electronic components. Power electronic transformers utilize power electronics technology and components to achieve voltage conversion and energy transfer. Compared to traditional transformers, they offer greater flexibility and adjustability. However, they also face challenges and limitations. For example, the characteristics of power electronic components can result in slightly lower operating efficiency than traditional transformers. Furthermore, the high cost of power electronic components hinders the widespread application of power electronic transformers. The new flexible distribution transformer is an innovative technology that combines traditional transformers with power electronic transformers. By adding power electronic control elements to traditional transformers, it leverages the advantages of both, achieving a better economic and technical balance. The new flexible distribution transformer not only meets the voltage control requirements of modern power systems but also features DC output capabilities. By connecting to DC microgrids, it facilitates the rapid integration of renewable energy and increases the proportion of clean power generation in the power system.

[0004] In terms of simulation technology, power system simulation encompasses both electromechanical transient simulation and electromagnetic transient simulation. The electromagnetic transient simulation software PSCAD / EMTDC combines a modular user interface with an electromagnetic transient simulation engine, accurately simulating electromagnetic transient processes in power systems, including the rapid response and control characteristics of power electronic devices. Consequently, electromagnetic transient simulation has gained widespread application in engineering. For new flexible distribution transformers currently in the design phase, utilizing electromagnetic transient simulation software for development, design, and simulation verification can effectively shorten design cycles and reduce design costs. Summary of the Invention

[0005] The purpose of the present invention is to overcome the problems of complex design and difficult verification in the prior art, provide a simulation design method based on PSCAD / EMTDC, and provide a flexible distribution transformer feasibility verification method based on simulation software to determine the feasibility of a new flexible distribution transformer.

[0006] To achieve the above objectives, the technical solution of the present invention is:

[0007] The feasibility verification method of flexible distribution transformer based on simulation software includes the following steps:

[0008] S1: Construct a main circuit simulation model of a flexible distribution transformer, and respectively construct a dual-winding transformer simulation model, an H-bridge converter main circuit simulation model, and a three-phase full-bridge converter main circuit simulation model. Connect the H-bridge converter main circuit simulation model in series to the high-voltage side of the dual-winding transformer simulation model, and connect the three-phase full-bridge converter main circuit simulation model in parallel to the low-voltage side of the dual-winding transformer simulation model. The H-bridge converter main circuit simulation model and the three-phase full-bridge converter main circuit simulation model are connected via a controlled voltage source and a controlled current source to simulate a DC capacitor.

[0009] S2: Constructing a photovoltaic power generation simulation model. First, construct a photovoltaic cell simulation model, then construct a photovoltaic boost circuit simulation model. The constructed photovoltaic cell simulation model is connected to the boost circuit via a filter circuit to form a photovoltaic power generation simulation model.

[0010] S3: Constructing an electric vehicle charging and discharging simulation model. First, construct an electric vehicle battery simulation model, then construct an electric vehicle Buck-Boost circuit simulation model. The constructed electric vehicle battery simulation model is connected to the Buck-Boost circuit via a filter circuit to form an electric vehicle charging and discharging simulation model.

[0011] S4: Build a DC microgrid main circuit simulation model. First, build a DC microgrid simulation model. Then, encapsulate the DC microgrid simulation model and connect it to the DC side of the three-phase full-bridge converter to form a DC microgrid main circuit simulation model.

[0012] S5: Construct a flexible distribution transformer control circuit simulation model, and construct an H-bridge converter control simulation model and a three-phase full-bridge converter control simulation model respectively. The H-bridge converter control circuit simulation model is used to output the control signal of the H-bridge converter to control the H-bridge converter main circuit simulation model. The A, B, and C phase H-bridge converter control circuits are constructed respectively using the same method. Each phase circuit includes an H-bridge control loop module RC controller and an SPWM modulation simulation model; the three-phase full-bridge converter control circuit simulation model is used to output the control signal of the three-phase full-bridge converter to control the three-phase full-bridge converter main circuit simulation model. The three-phase full-bridge converter control circuit simulation model includes a control loop module and an SVPWM modulation module.

[0013] S6: Constructing a photovoltaic grid-connected control simulation model, wherein the photovoltaic grid-connected control simulation model includes: a photovoltaic MPPT control module and a PWM modulation circuit simulation model;

[0014] S7: constructing an electric vehicle grid-connected control simulation model, wherein the electric vehicle grid-connected control simulation model is used to generate a control signal of an electric vehicle Buck-Boost circuit simulation model;

[0015] S8: interconnecting the simulation models constructed in the above steps to form an electromagnetic transient simulation verification model based on the flexible distribution transformer;

[0016] S9: Construct a feasibility verification simulation model for flexible distribution transformers to conduct feasibility verification of flexible distribution transformers.

[0017] S10: Verify the voltage regulation, voltage stabilization and new energy absorption capabilities of flexible distribution transformers.

[0018] In S1, a dual-winding transformer simulation model is constructed. The main transformer of the flexible distribution transformer adopts a three-phase dual-winding transformer. The three-phase two-winding transformer components in PSCAD / EMTDC are selected as the main transformer of the flexible distribution transformer. The primary winding of the main transformer is set to a △ connection method; the secondary winding is set to a Y connection method.

[0019] Construct the simulation model of the main circuit of the H-bridge converter: Construct the H-bridge converters of phase A, phase B, and phase C respectively using the same method. The construction method of the H-bridge converter of phase A is as follows: Use the power electronic switch IGBT and diode in PSCAD / EMTDC to construct the main circuit of the full-bridge converter of phase A. The values ​​of the filter inductor and filter capacitor in the LC filter are obtained by calculating the system parameters and the parameters of the flexible distribution transformer; detect the input current signal IgseA, output voltage signal UseA, DC side capacitor voltage signal Udc1A and DC current signal Idc1 of the full-bridge converter of phase A. A is transmitted to S5 as the actual value input of the electrical detection of the A-phase full-bridge converter control circuit. The series control converter and the parallel energy-taking converter are connected through a controlled voltage source and a controlled current source to simulate a DC capacitor. The four IGBT trigger pulse signals of the A-phase H-bridge converter are P1seA, P2seA, P3seA and P4seA. The three H-bridge converters constitute an H-bridge converter simulation model. After the H-bridge converter simulation model is encapsulated, it is connected in series to the high-voltage side of the three-phase double-winding transformer according to its corresponding phase to form an H-bridge converter main circuit simulation model.

[0020] Construct a three-phase full-bridge converter main circuit simulation model: First, construct a three-phase bridge converter, and the construction method is as follows: use the power electronic switch IGBT and diode in PSCAD / EMTDC to construct the three-phase bridge converter main circuit, and the filter inductor and filter capacitor values ​​in the LCL filter are obtained by calculating the system parameters and flexible distribution transformer parameters; detect the three-phase bridge converter input current signal Ishg, the grid connection point voltage signal Ush and the DC side capacitor voltage signal Udc2 and transmit them to S5 as the electrical detection actual value input of the parallel energy-taking converter control circuit; the six IGBT trigger pulse signals of the three-phase bridge converter are P1sh, P2sh, P3sh, P4sh, P5sh and P6sh, and then encapsulate the three-phase bridge converter and connect it in parallel to the low-voltage side of the three-phase double-winding transformer to form a three-phase full-bridge converter main circuit simulation model connected to the AC power grid;

[0021] Construct the connection between the H-bridge converter main circuit and the three-phase full-bridge converter main circuit. The H-bridge converter main circuit simulation model and the three-phase full-bridge converter main circuit simulation model are connected through a controlled voltage source and a controlled current source to simulate a DC capacitor; the controlled voltage source external control signal U on the DC side of the H-bridge converter dc1 and the three-phase full-bridge converter DC capacitor voltage U dc2 The mathematical relationship is:

[0022] K1U dc1 =U dc2 ;

[0023] H-bridge converter DC side current I dc1A , I dc1B, I dc1C and the external control signal I of the DC side controlled current source of the three-phase full-bridge converter dc2 The mathematical relationship is:

[0024] I dc2 =K2(I dc1A +I dc1B +I dc1C );

[0025] K1 is the proportional relationship between the DC capacitor voltage and the controlled voltage source; K2 is the proportional relationship between the DC side current and the controlled current source.

[0026] In S2, a photovoltaic cell simulation model is first constructed using a photovoltaic power supply in PSCAD / EMTDC software, where Vpv is the open-circuit voltage of the photovoltaic cell, Ipv is the current output by the photovoltaic cell, G is the instantaneous value of the light intensity, and T is the instantaneous value of the battery operating temperature;

[0027] Then, a photovoltaic boost circuit simulation model is constructed using power electronic switches IGBT and diodes. The input control signal of the fully controlled power switch device IGBT is represented by g.

[0028] The photovoltaic cell simulation model is connected to the photovoltaic boost circuit simulation model through a filter circuit to form a photovoltaic power generation simulation model. In the photovoltaic power generation model, Idcl is the grid-side current, Vdcout is the grid-side voltage, Iin is the photovoltaic cell terminal current, Vin is the photovoltaic cell terminal voltage, DC1 and DC2 are DC interfaces, and the photovoltaic power generation simulation model is encapsulated in the module PV.

[0029] In the S3, firstly, an electric vehicle battery simulation model is constructed, and the single-phase voltage source model 2 in the PSCAD / EMTDC software is used to construct the electric vehicle battery simulation model. In the electric vehicle battery model, U ocv is the open circuit voltage of the electric vehicle battery, and I1 is the charging current of the electric vehicle battery;

[0030] The open circuit voltage Uocv and state of charge SOC of the battery have a fixed monotonic nonlinear functional relationship. The battery model based on the battery OCV-SOC characteristics is used to simulate the battery characteristics with SOC as a variable. The state of charge S in the electric vehicle battery model is OC The calculation formula is:

[0031]

[0032] Where S OC0 is the initial state of charge of the battery; i is the charging current; Q N is the rated capacity of the battery;

[0033] Electric vehicle battery open circuit voltage U ocv The calculation formula is:

[0034]

[0035] Among them, a, b, c, d, and e are given parameters;

[0036] Then, the electric vehicle Buck-Boost circuit simulation model is constructed using the power electronic switch IGBT and diode in PSCAD / EMTDC software. The input control signals of the two fully controlled power switch devices IGBT are represented by P1 and P2 respectively.

[0037] The constructed electric vehicle battery simulation model is connected to the Buck-Boost circuit simulation model through a filter circuit to form an electric vehicle charging and discharging simulation model. In the electric vehicle charging and discharging simulation model, U1 is the electric vehicle battery terminal voltage, I1 is the electric vehicle battery charging current, Udc is the grid-side voltage, Isingle is the grid-side current, DC1 and DC2 are DC interfaces, and the electric vehicle charging and discharging simulation model is encapsulated in the module Electric Vehicle.

[0038] In S4, a DC microgrid simulation model is first constructed. A fixed load in PSCAD / EMTDC software is used to simulate a DC load. The PV module and the Electric Vehicle module are connected in parallel to the DC bus. The DC interface DC1 is connected to the positive pole of the DC bus, and the DC interface DC2 is connected to the negative pole of the DC bus to form a DC microgrid simulation model. DCG1 and DCG2 are connection interfaces of the DC microgrid simulation model. The DC microgrid simulation model is encapsulated in the HybridMicrogrid module.

[0039] The encapsulated module HybridMicrogrid is then connected to the DC side of the three-phase full-bridge converter main circuit simulation model. The interface DCG1 in the module HybridMicrogrid is connected to the DC side interface DCP1 of the three-phase full-bridge converter main circuit simulation model. The interface DCG2 in the module HybridMicrogrid is connected to the DC side interface DCP2 of the three-phase full-bridge converter main circuit simulation model, forming a DC microgrid main circuit simulation model that is put into operation on the DC side of the transformer.

[0040] In S5, S5.1: Construct an H-bridge converter control circuit simulation model. The control principles of the three H-bridge converters are exactly the same. The same method is used to construct the A-phase, B-phase, and C-phase H-bridge converter control circuits, respectively. The construction method of the A-phase H-bridge converter control circuit is as follows:

[0041] S5.1.1: Construct a simulation model of an H-bridge control loop module, wherein the H-bridge control loop module includes a phase-locked loop module, an X-to-DQ module, a dq-axis calculation module, and a DQ-to-X module;

[0042] The phase-locked loop module uses the PLL phase-locked loop in PSCAD / EMTDC software, takes the H-bridge converter series-connected line current signal IgesA as the input of the phase-locked loop module, and uses phase vector technology to generate the ramp signal thetase;

[0043] The abc-dq coordinate transformation of the electrical signal is implemented using a self-packaged X to DQ module. The input quantities are the series connection line current signal IgseA and the voltage signal UseA across the series access point. The d-axis and q-axis components obtained are the series connection line current d-axis signal Igd, the series connection line current q-axis signal Igq, the series connection point voltage d-axis signal Used, and the series access point voltage q-axis signal Useq, respectively.

[0044] The dq-axis calculation module uses the PI controller in the PSCAD / EMTDC software. The effective value of the low-voltage line low0.4 voltage controlled by it is used as the input signal dREF of the dq-axis calculation module. The input signal reference value is subtracted from the input signal dREF and then passed through the PI controller to obtain the d-axis reference signal RT_7 of the voltage at both ends of the series connection point. The d-axis reference signal RT_7 of the voltage at both ends of the series connection point is subtracted from the d-axis signal Used of the voltage at both ends of the series connection point and then passed through the PI controller to output the d-axis reference signal Urefd. The q-axis reference signal qREF of the control loop is set to 0, and is subtracted from the q-axis signal Useq of the voltage at both ends of the series connection point and then passed through the PI controller to output the q-axis reference signal Urefq.

[0045] The dq-abc coordinate transformation of the electrical signal is implemented using the self-encapsulated DQ to X module. The input is the voltage d-axis reference signal Urefd and the voltage q-axis reference signal Urefq from the dq-axis calculation module. The output is the voltage reference signal Ref at both ends of the series connection line.

[0046] The H-bridge control loop module is encapsulated in the H-bridge control loop module RC controller. The control loop output signal Ref is the voltage reference signal Uref at both ends of the series connection line. The outer loop input dREF is the effective value of the voltage at the first end of the controlled low-voltage line Low0.4.

[0047] S5.1.2: Construct an SPWM modulation simulation model. The SPWM modulation module simulation model in the H-bridge converter control circuit uses the signal generator in the PSCAD / EMTDC software as a triangular carrier signal generator. The voltage reference signal Uref output by the control loop RC controller simulation model is input into the SPWM modulation module, namely the SPWM modulation voltage reference signal Uref. The quotient of the voltage reference signal Uref and the external control signal Udc1 of the controlled voltage source on the DC side of the H-bridge converter is used as the SPWM modulation wave. After comparing the triangular carrier with the set frequency with the modulation wave, the control signals P1seA, P2seA, P3seA, and P4seA of the IGBT in the H-bridge converter are output. The above-mentioned SPWM modulation module is encapsulated in the module SPWM single.

[0048] S5.2: Constructing a three-phase full-bridge converter control circuit simulation model: The three-phase full-bridge converter control circuit simulation model includes a control loop module and an SVPWM modulation module;

[0049] S5.2.1: Construct a simulation model of the control loop module of a three-phase full-bridge converter, wherein the control loop module includes a phase-locked loop module, an ABC to DQ module, a dq axis calculation module, and a DQ to ABC module;

[0050] The phase-locked loop module uses the PLL phase-locked loop in PSCAD / EMTDC software, takes the grid-connected point voltage signal Ush of the three-phase bridge converter as the input of the phase-locked loop module, and uses the phase vector technology to generate the ramp signal thetash;

[0051] The abc-dq coordinate transformation of the electrical signal is implemented using the self-packaged ABC to DQ module. The input quantities are the grid-connected point voltage signal Ush of the three-phase bridge converter and the grid-connected line current signal Ishg. The d-axis and q-axis components obtained are the grid-connected point voltage d-axis signal Vgd, the grid-connected point voltage q-axis signal Vgq, the grid-connected line current d-axis signal ILd, and the grid-connected line current q-axis signal ILq, respectively.

[0052] The dq-axis calculation module adopts the PI controller in the PSCAD / EMTDC software. In the constant DC voltage control loop, the DC side voltage command value of the three-phase full-bridge converter is differentially connected to the DC capacitor voltage measurement signal Udc2 and then transmitted to the PI controller. The d-axis component reference value output by the PI controller and the difference between the grid-connected line current d-axis signal ILd and ILd are used as the input of the inner-loop PI controller. The voltage signal output by the inner-loop PI controller is added with the cross-coupling term and the grid-connected point voltage d-axis signal Vgd to obtain the voltage d-axis reference Vrd. In the current q-axis signal control loop, the grid-connected line current q-axis reference value is set to 0, and the difference with the grid-connected line current q-axis signal ILq is input into the line current inner-loop PI controller. The difference between the line current d-axis signal ILd and the output of the inner-loop PI controller is used as the voltage q-axis reference Vrq.

[0053] The dq-abc coordinate transformation of the electrical signal is implemented using a self-encapsulated DQ to ABC module. The input is the voltage d-axis reference signal Vrd and the voltage q-axis reference signal Vrq from the dq-axis calculation module. The output is the three-phase full-bridge converter output voltage reference signal Vabc. The above control loop module is encapsulated in the control loop module EC controller.

[0054] S5.2.2: Construct an SVPWM modulation simulation model using interpolation sampling elements, rectangular coordinate transformation elements, and modular functions in PSCAD / EMTDC software. Input the DC capacitor voltage of the three-phase full-bridge converter into the sampling element to obtain the capacitor voltage sampling quantity Vd1 that changes in steps according to the period fs. Input the output voltage reference signal Vabc of the three-phase full-bridge converter into the self-encapsulated three-phase-to-two-phase coordinate transformation module. After passing through the sampler and rectangular coordinate transformation element, the amplitude V and phase angle theta of the output quantity under the αβ coordinates are obtained. The modulation ratio m is obtained by dividing the fundamental peak value V of the modulation wave by the fundamental peak value Vd1 of the carrier wave by the coefficient The phase angle theta is obtained by determining the sector through the custom module ChooseSector, and the positions of the phasors V1 and V2 are output. The action time of V1 and V2 in the sector is determined by T1, T2 and T0. The positions and action time of the phasors V1 and V2 are input into the custom module OutVector to output a 6-dimensional pulse sequence as the control signals P1sh, P2sh, P3sh, P4sh, P5sh, and P6sh of the three-phase full-bridge converter switching device IGBT. The above SVPWM modulation module is encapsulated in the module SVPWM.

[0055] In said S6, said photovoltaic grid-connected control simulation model includes: a photovoltaic MPPT control module and a PWM modulation circuit simulation model;

[0056] S6.1: Construct a simulation model of the photovoltaic MPPT control module using interpolation sampling elements, constant time constant differential elements, integrator elements, and range comparator elements in PSCAD / EMTDC software. First, input the photovoltaic power supply voltage and current into the sampling elements to obtain voltage and current sampling quantities that change in steps according to the period Ts. The sampling quantities are then passed through the given time constant differential element to obtain the voltage and current differential quantities dV and dI. dV × (IdV + VdI) is then fed into the range comparator for output. The range comparator output is multiplied by the given parameter signal add and then passed through the integrator to output the duty cycle D. Encapsulate the photovoltaic MPPT control simulation model in the MPPT module. The output of the MPPT module is the duty cycle D.

[0057] S6.2: Construct a PWM modulation circuit simulation model. Use the signal generator in PSCAD / EMTDC software as a triangular carrier signal generator. Compare the duty cycle D output by the photovoltaic MPPT control module with the carrier and output the power switch control signal g.

[0058] In S7, the electric vehicle grid-connected control simulation model uses the PI controller and signal generator in the PSCAD / EMTDC software as a triangular carrier signal generator. The current set value is subtracted from the measured electric vehicle battery current I1 and then passed through the PI controller to obtain the current command value Ipi. Ipi is used as the modulation wave. After comparing the triangular carrier of a certain frequency with the modulation wave, the control signals P1 and P2 of the IGBT in the Buck-Boost circuit simulation model are output to form the electric vehicle grid-connected control simulation model.

[0059] In the S9, an AC power grid system of equal value is connected to the primary and secondary sides of the flexible distribution transformer, wherein the AC power grid system includes a three-phase AC power supply, a transmission line, a flexible distribution transformer, and a load; a three-phase voltage source model 2 in the PSCAD / EMTDC software is used as a series simulation circuit of a three-phase AC power supply, a resistor, and an inductor, and a fixed load simulation load. The three-phase AC power supply is connected to the medium voltage side of the flexible distribution transformer through the line Mid10, and the low voltage side of the flexible distribution transformer is connected to the line Low0.4, thereby forming a feasibility verification simulation model of the flexible distribution transformer;

[0060] The key parameters that need to be set in the feasibility verification simulation model include the three-phase AC system voltage, the first section voltage of the controlled low-voltage side circuit, the effective value of the load phase voltage and the steady-state single-phase active power of the load.

[0061] S10: Verify the voltage stabilization, voltage regulation, and new energy consumption capacity of the flexible distribution transformer. If the voltage stabilization, voltage regulation, and new energy consumption capacity all meet the requirements, the simulation design scheme of the flexible distribution transformer is deemed feasible. If any one of the requirements is not met, the simulation design scheme of the flexible distribution transformer is deemed infeasible and the design scheme or parameters need to be adjusted.

[0062] Voltage stabilization simulation test: Set the simulation plan and initialize the entire simulation model first. After initialization, put the flexible distribution transformer into operation. The controlled voltage source and the controlled current source simulate the DC capacitor to start charging. After charging is completed, control the low-voltage side bus voltage of the flexible distribution transformer to 1.0pu. After the voltage boost is completed, the photovoltaic cell simulation model and the electric vehicle battery simulation model are connected to the grid in turn. After the grid connection is completed, increase the AC load by a constant power load of 0.3-0.7MW, and reduce the constant power load by 0.2-0.5MW after the set time interval. At this time, the test is completed, observe the change in the low-voltage side bus voltage of the flexible distribution transformer, and test the maximum voltage deviation of the low-voltage side bus of the distribution transformer. If the maximum voltage deviation is less than 2%, it meets the requirements.

[0063] Voltage regulation simulation test: Set the simulation plan, first initialize the entire simulation model, and after the initialization is completed, put the flexible distribution transformer into operation, and the controlled voltage source and controlled current source simulate the DC capacitor to start charging. After the charging is completed, control the low-voltage side bus voltage of the flexible distribution transformer to 1.03pu. After the voltage boost is completed, the photovoltaic cell simulation model and the electric vehicle battery simulation model are connected to the grid in turn. After the interval setting time t1, the low-voltage side bus voltage is increased by 0.06~0.08pu. After the interval setting time t2, the low-voltage side bus voltage is increased by 0.07~0.09pu. After the interval setting time t3, the low-voltage side bus voltage is reduced by 0.15~0.3pu. At this time, the test is completed; observe the change of the low-voltage side bus voltage of the flexible distribution transformer, and test the maximum voltage regulation range of the low-voltage side bus of the distribution transformer. If the maximum voltage regulation range exceeds ±10%, it meets the requirements;

[0064] Absorption capacity simulation test: Set the simulation plan, first initialize the entire simulation model, put the flexible distribution transformer into operation after initialization, and start charging the DC capacitor simulated by the controlled voltage source and the controlled current source. After charging is completed, control the low-voltage side bus voltage of the flexible distribution transformer to 1.0pu. After the boost is completed, the photovoltaic cell simulation model and the electric vehicle battery simulation model are connected to the grid in turn. After the grid connection is completed, after the set time t4, the photovoltaic light intensity of the DC microgrid is set to 1000W / m 2 After the interval setting time t5, increase the light intensity by 300~600W / m 2 After the interval setting time t6, the light intensity is increased by 300 to 600W / m 2, the test is now completed; observe the changes in the capacitor voltage on the DC side of the flexible distribution transformer, and test the maximum voltage deviation on both sides of the DC capacitor. If the maximum voltage deviation is less than 5%, it meets the requirements.

[0065] Compared with the prior art, the present invention has the following beneficial effects:

[0066] 1. The feasibility verification method of a flexible distribution transformer based on simulation software of the present invention relies on PSCAD / EMTDC simulation software. First, a main circuit simulation model of the new flexible distribution transformer system is constructed, and then its control circuit simulation model is constructed. Finally, the main circuit and control circuit models are connected to the AC power grid. After completing the simulation modeling of the new flexible distribution transformer, a new flexible distribution transformer test system model is further constructed, and corresponding evaluation indicators are proposed. The simulation results show that the present invention can complete the establishment of the electromagnetic transient simulation model of the new flexible distribution transformer based on the PSCAD / EMTDC simulation software, and evaluate the feasibility of the transformer through the simulation results, providing data support for the design and engineering implementation of the new flexible distribution transformer device.

[0067] 2. In a feasibility verification method for a flexible distribution transformer based on simulation software of the present invention, the parallel energy-taking converter side does not require a step-up or step-down transformer to transform the voltage level, but is directly connected in parallel to the low-voltage bus of the flexible distribution transformer, saving costs and reducing losses; the series control converter and the parallel energy-taking converter are connected by a controlled voltage source and a controlled current source to simulate a DC capacitor, so as to save computing resources and improve simulation speed.

[0068] 3. The feasibility verification method for a flexible distribution transformer based on simulation software in the present invention simulates and analyzes the voltage stabilization characteristics of a new flexible distribution transformer system. When the flexible distribution transformer is put into operation, the grid connection point voltage remains basically stable, providing technical support for improving the new power system's ability to absorb new energy and maintaining grid voltage stability. The voltage regulation characteristics of the new flexible distribution transformer system are simulated and analyzed. The constructed electromagnetic transient simulation model of the new flexible distribution transformer can control the grid connection point voltage to achieve rapid and smooth voltage regulation, compensating for the shortcomings of the traditional power transformer's step-type voltage regulation and providing a technical means to meet the needs of continuous voltage regulation in the new power system. The simulation analysis of the absorption capacity of the new flexible distribution transformer system shows that the system can effectively accept large-scale renewable energy and integrate it into the power system. This provides important technical support for promoting the development of renewable energy and achieving sustainable energy goals.

[0069] 4. In the feasibility verification method of a flexible distribution transformer based on simulation software of the present invention, the flexible distribution transformer does not need to add windings inside the transformer, which reduces the cost of transformer body modification, and does not require a step-down transformer to connect to the voltage source converter, thereby reducing the overall economic cost of the flexible distribution transformer. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] Figure 1 It is a schematic diagram of the main circuit model of the flexible distribution transformer in the present invention.

[0071] Figure 2 It is a schematic diagram of the simulation model of the double-winding transformer in the present invention.

[0072] Figure 3 Schematic diagram of the A-phase H-bridge converter in the present invention.

[0073] Figure 4 It is a schematic diagram of the simulation model of the main circuit of the H-bridge converter in the present invention.

[0074] Figure 5 It is a schematic diagram of the main circuit simulation model of the three-phase full-bridge converter of the present invention.

[0075] Figure 6 Schematic diagram of the photovoltaic cell simulation model of the present invention.

[0076] Figure 7 Schematic diagram of the electric vehicle battery simulation model of the present invention.

[0077] Figure 8 Schematic diagram of the DC microgrid simulation model of the present invention.

[0078] Figure 9 Schematic diagram of the H-bridge control loop module simulation model of the present invention.

[0079] Figure 10 It is a schematic diagram of the SPWM modulation simulation model of the present invention.

[0080] Figure 11 Schematic diagram of the simulation model of the H-bridge converter control circuit of the present invention.

[0081] Figure 12 It is a schematic diagram of the simulation model of the three-phase full-bridge converter control loop module of the present invention.

[0082] Figure 13 It is a schematic diagram of the SVPWM modulation simulation model of the present invention.

[0083] Figure 14 Schematic diagram of the simulation model of the three-phase full-bridge converter control circuit of the present invention.

[0084] Figure 15It is a schematic diagram of the simulation model of the photovoltaic MPPT control module of the present invention.

[0085] Figure 16 It is a schematic diagram of the photovoltaic grid-connected control simulation model of the present invention.

[0086] Figure 17 It is a schematic diagram of the electric vehicle grid-connected control simulation model of the present invention.

[0087] Figure 18 It is a schematic diagram of a feasibility verification simulation model of the flexible distribution transformer of the present invention.

[0088] Figure 19 This is a per-unit waveform diagram of the voltage of the controlled line before and after the new flexible distribution transformer of the present invention is put into operation.

[0089] Figure 20 This is a waveform diagram of the voltage regulation of the controlled line by the new flexible distribution transformer of the present invention.

[0090] Figure 21 This is a waveform diagram of the DC side capacitor voltage of the present invention. DETAILED DESCRIPTION

[0091] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0092] Example 1:

[0093] Relying on the PSCAD / EMTDC simulation platform, a feasibility verification method for a flexible distribution transformer based on simulation software is completed. The steps are as follows:

[0094] S1: Construct a main circuit simulation model of a flexible distribution transformer, and respectively construct a dual-winding transformer simulation model, an H-bridge converter main circuit simulation model, and a three-phase full-bridge converter main circuit simulation model. Connect the H-bridge converter main circuit simulation model in series to the high-voltage side of the dual-winding transformer simulation model, and connect the three-phase full-bridge converter main circuit simulation model in parallel to the low-voltage side of the dual-winding transformer simulation model. The H-bridge converter main circuit simulation model and the three-phase full-bridge converter main circuit simulation model are connected via a controlled voltage source and a controlled current source to simulate a DC capacitor.

[0095] S2: Constructing a photovoltaic power generation simulation model. First, construct a photovoltaic cell simulation model, then construct a photovoltaic boost circuit simulation model. The constructed photovoltaic cell simulation model is connected to the boost circuit via a filter circuit to form a photovoltaic power generation simulation model.

[0096] S3: Constructing an electric vehicle charging and discharging simulation model. First, construct an electric vehicle battery simulation model, then construct an electric vehicle Buck-Boost circuit simulation model. The constructed electric vehicle battery simulation model is connected to the Buck-Boost circuit via a filter circuit to form an electric vehicle charging and discharging simulation model.

[0097] S4: Build a DC microgrid main circuit simulation model. First, build a DC microgrid simulation model. Then, encapsulate the DC microgrid simulation model and connect it to the DC side of the three-phase full-bridge converter to form a DC microgrid main circuit simulation model.

[0098] S5: Construct a flexible distribution transformer control circuit simulation model, and construct an H-bridge converter control simulation model and a three-phase full-bridge converter control simulation model respectively. The H-bridge converter control circuit simulation model is used to output the control signal of the H-bridge converter to control the H-bridge converter main circuit simulation model. The A, B, and C phase H-bridge converter control circuits are constructed respectively using the same method. Each phase circuit includes an H-bridge control loop module RC controller and an SPWM modulation simulation model; the three-phase full-bridge converter control circuit simulation model is used to output the control signal of the three-phase full-bridge converter to control the three-phase full-bridge converter main circuit simulation model. The three-phase full-bridge converter control circuit simulation model includes a control loop module and an SVPWM modulation module.

[0099] S6: Constructing a photovoltaic grid-connected control simulation model, wherein the photovoltaic grid-connected control simulation model includes: a photovoltaic MPPT control module and a PWM modulation circuit simulation model;

[0100] S7: constructing an electric vehicle grid-connected control simulation model, wherein the electric vehicle grid-connected control simulation model is used to generate a control signal of an electric vehicle Buck-Boost circuit simulation model;

[0101] S8: interconnecting the simulation models constructed in the above steps to form an electromagnetic transient simulation verification model based on the flexible distribution transformer;

[0102] S9: Construct a feasibility verification simulation model for flexible distribution transformers to conduct feasibility verification of flexible distribution transformers.

[0103] S10: Verify the voltage regulation, voltage stabilization and new energy absorption capabilities of flexible distribution transformers.

[0104] Example 2:

[0105] The method for establishing the electromagnetic transient simulation model of the new flexible distribution transformer is as follows:

[0106] S1: Build a simulation model of a two-winding transformer. Figure 1A three-phase two-winding transformer (3Phase 2Winding Transformer) in PSCAD / EMTDC software was used. Its primary winding #1 is connected to the three-phase power supply line Mid10 via a series-connected H-bridge converter. Secondary winding #2 represents the low-voltage side of the two-winding transformer and is connected to the low-voltage line Low0.4 and the three-phase full-bridge converter. It supplies power to the load and energizes the DC capacitors of the three-phase full-bridge converter, enabling the H-bridge converter to control the voltage of the controlled line. Winding #1 and winding #2 are connected in a delta configuration and a Y configuration, respectively. The two-winding transformer is the main component of a new type of flexible distribution transformer. Its main structure is a multi-winding power frequency transformer. Its primary function is to transmit the majority of the power and convert the voltage levels of the primary and secondary winding buses. The main parameter settings of the dual-winding transformer simulation model are as follows: the three-phase rated capacity of the transformer is 1.6MVA; the rated line voltage effective value of winding #1 is 10kV; the rated line voltage effective value of winding #2 is 0.4kV; and the positive sequence leakage reactance between winding #1 and winding #2 is 0.04pu.

[0107] Then, the B-phase and C-phase H-bridge sub-modules are constructed in sequence. The three H-bridge sub-modules constitute the H-bridge converter simulation model. After the H-bridge converter simulation model is encapsulated, it is connected in series to the high-voltage side of the three-phase dual-winding transformer according to its corresponding phase to form the H-bridge converter main circuit simulation model.

[0108] First, a three-phase bridge converter is constructed. Then, the three-phase bridge converter is packaged and connected in parallel to the low-voltage side of a dual-winding transformer to form a simulation model of the main circuit of a three-phase full-bridge converter connected to the AC power grid.

[0109] Construct the H-bridge submodule simulation model. Figure 3 As shown in the figure, the H-bridge submodule is constructed using the Power Electronic Switch-IGBT & Diode and Single-Phase Voltage Source Model 2 in PSCAD / EMTDC software. In H-bridge submodule A, P1seA, P2seA, P3seA, and P4seA represent the control signals of the fully controlled power switch IGBTs, Udc1 represents the external control signal of the controlled voltage source, Idc1A represents the DC side current of the H-bridge submodule, Udc1A represents the DC side voltage of the H-bridge submodule, IgseA represents the current signal of the line connected in series with the H-bridge submodule, and UseA represents the voltage signal at both ends of the H-bridge submodule's serial connection point.

[0110] Construct an H-bridge converter simulation model. Figure 4As shown, the simulation models of H-bridge submodule B and H-bridge submodule C are constructed using the same method. Together, H-bridge submodules A, B, and C constitute the H-bridge converter simulation model. In the H-bridge converter model, A1, A2, B1, B2, C1, and C2 are AC interfaces. The constructed H-bridge converter simulation model is encapsulated in module RC.

[0111] Construct a simulation model of the H-bridge converter's main circuit. Connect the RC module in series to a three-phase AC system. The H-bridge submodule A interfaces A1 and A2 in the RC module are connected to phase A of the dual-winding transformer's primary winding #1 and phase A of the three-phase AC system, respectively. The H-bridge submodule B interfaces B1 and B2 in the RC module are connected to phase B of the dual-winding transformer's primary winding #1 and phase B of the three-phase AC system, respectively. The H-bridge submodule C interfaces C1 and C2 in the RC module are connected to phase C of the dual-winding transformer's primary winding #1 and phase C of the three-phase AC system, respectively. This method forms a simulation model of the H-bridge converter's main circuit connected to the dual-winding transformer and the three-phase power supply. The H-bridge converter, connected in series to the primary side of the dual-winding transformer, inverts a voltage with adjustable amplitude and phase angle, and serves as the primary control device for flexible power transformers.

[0112] The main parameters of the H-bridge converter main circuit are as follows: the on-state resistance of the anti-parallel diode and IGBT is 0.0001Ω; the off-state resistance of the anti-parallel diode and IGBT is 1×10 6 Ω; forward or reverse breakdown voltage of anti-parallel diode and IGBT 1×10 5 kV; filter inductor 2.7113mH; filter capacitor 16.6087μF.

[0113] Construct a three-phase full-bridge converter simulation model. The three-phase full-bridge converter simulation model is as follows Figure 5 As shown, a three-phase full-bridge converter is constructed using the power electronic switch (IGBT & Diode) and single-phase voltage source model 2 in PSCAD / EMTDC software. In the three-phase full-bridge converter simulation model, P1sh, P2sh, P3sh, P4sh, P5sh, and P6sh represent the input control signals for the fully controlled power switch IGBTs; Idc2 represents the external control signal for the controlled current source; Udc2 represents the DC capacitor voltage; Id1 represents the DC side current; Ush represents the grid-connection point voltage of the three-phase full-bridge converter; Ishg represents the grid-connection line current; IL represents the line current after filtering; and AC represents the AC interface. The three-phase full-bridge converter is encapsulated in module EC.

[0114] Construct a simulation model of the main circuit of a three-phase full-bridge converter. Connect the EC module in parallel to the low-voltage side of the dual-winding transformer, Low0.4, to form a simulation model of the main circuit of a three-phase full-bridge converter operating in an AC system. The three-phase full-bridge converter draws energy from the secondary side of the dual-winding transformer and primarily serves to maintain the stability of the common DC capacitor voltage. The main parameters of the three-phase full-bridge converter are: the on-state resistance of the anti-parallel diode and IGBT is 0.0001Ω; the off-state resistance of the anti-parallel diode and IGBT is 1×10 6 Ω; forward or reverse breakdown voltage of anti-parallel diode and IGBT 1×10 5 kV; filter inductor 0.505358mH; filter capacitor 497.359μF. To save computing resources and improve simulation speed, the H-bridge converter and the three-phase full-bridge converter are connected by a controlled voltage source and a controlled current source to simulate the DC capacitor.

[0115] The mathematical relationship between the external control signal Udc1 of the controlled voltage source on the DC side of the H-bridge converter and the DC capacitor voltage Udc2 of the three-phase full-bridge converter is:

[0116] K1U dc1 =U dc2

[0117] The mathematical relationship between the H-bridge converter DC-side currents Idc1A, Idc1B, and Idc1C and the external control signal Idc2 of the DC-side controlled current source of the three-phase full-bridge converter is:

[0118] I dc2 =K2(I dc1A +I dc1B +I dc1C )

[0119] K1 is the proportional relationship between the DC capacitor voltage and the controlled voltage source; K2 is the proportional relationship between the DC side current and the controlled current source, both of which are equal to 1.

[0120] S2: First, construct a photovoltaic cell simulation model, then a photovoltaic boost circuit simulation model. This model is connected to the boost circuit via a filter circuit to form a photovoltaic power generation simulation model. The photovoltaic cell simulation model is constructed using the photovoltaic source function in PSCAD / EMTDC software. In the photovoltaic cell simulation model, Vpv is the open-circuit voltage of the photovoltaic cell, Ipv is the output current of the photovoltaic cell, G is the instantaneous light intensity, and T is the instantaneous operating temperature of the cell.

[0121] Construct a photovoltaic boost circuit simulation model. This model is constructed using the power electronic switch (IGBT) and diode (Diode) in PSCAD / EMTDC software. In this model, g represents the input control signal for the fully controlled power switch (IGBT).

[0122] Construct a photovoltaic power generation simulation model. The photovoltaic power generation simulation model is as follows Figure 6 As shown, the photovoltaic cell simulation model is connected to the photovoltaic boost circuit simulation model via a filter circuit to form a photovoltaic power generation simulation model. In this photovoltaic power generation model, Idcl is the grid-side current, Vdcout is the grid-side voltage, Iin is the photovoltaic cell terminal current, Vin is the photovoltaic cell terminal voltage, and DC1 and DC2 are DC interfaces. The photovoltaic power generation simulation model is encapsulated in a PV module. The primary function of photovoltaic power generation is to provide power to DC microgrid loads and electric vehicles.

[0123] The main parameters of the photovoltaic power generation simulation model are: the on-state resistance of the anti-parallel diode and IGBT is 0.0001Ω; the off-state resistance of the anti-parallel diode and IGBT is 1×10 6 Ω; forward or reverse breakdown voltage of anti-parallel diode and IGBT 1×10 5 kV; Number of PV modules connected in series in each PV module string in the PV array: 10; Number of PV module strings connected in parallel in the PV array: 20; Number of PV cells connected in series in a PV cell string in each PV module: 60; Number of PV cell strings connected in parallel in each PV module: 6; Reference light intensity for specifying PV module parameters: 1000 W / m 2 ; The reference temperature for specifying PV module parameters is 25 degrees; filter inductor 0.0625mH; filter capacitor 2000μF; grid-side capacitor 2500μF.

[0124] S3 builds an electric vehicle battery simulation model. First, the electric vehicle battery simulation model is built, and then the electric vehicle Buck-Boost circuit simulation model is built. The constructed electric vehicle battery simulation model is connected to the Buck-Boost circuit through a filter circuit to form an electric vehicle charging and discharging simulation model.

[0125] The electric vehicle battery simulation model was constructed using the Single Phase Voltage Source Model 2 in PSCAD / EMTDC software. In the electric vehicle battery model, Uocv is the open-circuit voltage of the electric vehicle battery, and I1 is the charging current of the electric vehicle battery.

[0126] The open circuit voltage Uocv of the battery and the state of charge (SOC) have a fixed monotonic nonlinear functional relationship. This simulation uses a battery model based on the battery OCV-SOC characteristics, using SOC as a variable to simulate battery characteristics. OC The calculation formula is:

[0127]

[0128] Where S OC0 is the initial state of charge of the battery; i is the charging current; Q N is the rated capacity of the battery.

[0129] The calculation formula for the open circuit voltage Uocv of electric vehicle batteries is:

[0130]

[0131] Among them, a, b, c, d, and e are given parameters.

[0132] Construct an electric vehicle buck-boost circuit simulation model. This model uses the power electronic switch (IGBT) and diode (Diode) in PSCAD / EMTDC software. In the electric vehicle buck-boost circuit simulation model, P1 and P2 represent the input control signals for the fully controlled power switch (IGBT).

[0133] Construct an electric vehicle charging and discharging simulation model. The electric vehicle charging and discharging simulation model is as follows: Figure 7 As shown in the figure, the EV battery simulation model is connected to the constructed Buck-Boost circuit via a filter circuit to form an EV charging and discharging simulation model. In this EV charging and discharging simulation model, U1 represents the EV battery terminal voltage, I1 represents the EV battery charging current, Udc represents the grid-side voltage, Isingle represents the grid-side current, and DC1 and DC2 represent the DC interfaces. The EV charging and discharging simulation model is encapsulated in the Electric Vehicle module. EVs charge during off-peak hours and can serve as power sources during peak hours, supplying power to the DC microgrid load.

[0134] The main parameters of the electric vehicle charging and discharging simulation model are: the on-state resistance of the anti-parallel diode and IGBT is 0.0001Ω; the off-state resistance of the anti-parallel diode and IGBT is 1×10 6 Ω; forward or reverse breakdown voltage of anti-parallel diode and IGBT 1×10 5kV; filter inductor 0.0625mH; filter capacitor 2000μF; grid-side capacitor 2500μF; electric vehicle battery internal resistance 0.508Ω.

[0135] S4: Constructing a DC microgrid main circuit simulation model. The DC microgrid main circuit model construction involves first constructing a DC microgrid simulation model, then encapsulating the DC microgrid simulation model and connecting it to the DC side of a three-phase full-bridge converter to form a DC microgrid main circuit simulation model. The specific steps for constructing the DC microgrid main circuit simulation model are as follows:

[0136] Construct a DC microgrid simulation model. Figure 8 As shown in the figure, a DC load is simulated using the Fixed Load function in PSCAD / EMTDC software. The PV module and the Electric Vehicle module are connected in parallel to the DC bus. DC interface DC1 is connected to the positive terminal of the DC bus, and DC interface DC2 is connected to the negative terminal of the DC bus, forming a DC microgrid simulation model. In the DC microgrid simulation model, DCG1 and DCG2 are the microgrid connection interfaces. The DC microgrid is encapsulated in the HybridMicrogrid module.

[0137] Construct a DC microgrid main circuit simulation model. Figure 1 As shown, the Hybrid Microgrid module is connected to the DC side of a three-phase full-bridge converter. Interface DCG1 in the Hybrid Microgrid module is connected to interface DCP1 on the DC side of the three-phase full-bridge converter, and interface DCG2 in the Hybrid Microgrid module is connected to interface DCP2 on the DC side of the three-phase full-bridge converter. This forms a simulation model of the DC microgrid's main circuit, operating on the DC side of the transformer. The connection between the DC microgrid and the DC side common capacitor of the new flexible distribution transformer allows the DC microgrid to inject excess energy into the DC side common capacitor when photovoltaic power generation is high, maintaining a balance between supply and demand in the DC microgrid.

[0138] S5: Constructing an H-bridge converter control simulation model. The H-bridge converter control circuit simulation model includes a control loop module and an SPWM modulation module.

[0139] S5.1: Construct a control loop module simulation model. The control loop simulation model in the H-bridge converter control circuit is as follows: Figure 9 As shown, the control loop module includes a phase-locked loop module, an X to DQ module, a dq axis calculation module and a DQ to X module.

[0140] S5.1.1: The phase-locked loop module uses the PLL phase-locked loop (PLL) in PSCAD / EMTDC software. The H-bridge converter is connected in series with the line current signal IgesA as the input of the phase-locked loop module, and the phase vector technology is used to generate the ramp signal thetase.

[0141] The abc-dq coordinate transformation of the electrical signal is implemented using the self-packaged X to DQ module. The input quantities are the series connection line current signal IgseA and the voltage signal UseA across the series access point. The d-axis and q-axis components obtained are the series connection line current d-axis signal Igd, the series connection line current q-axis signal Igq, the series connection point voltage d-axis signal Used, and the series access point voltage q-axis signal Useq, respectively.

[0142] The dq-axis calculation module uses the PI controller (PI Controller) in the PSCAD / EMTDC software. The effective value of the controlled low-voltage line low0.4 voltage is used as the input signal dREF of the dq-axis calculation module. The input signal reference value is subtracted from the input signal dREF and then passed through the PI controller to obtain the d-axis reference signal RT_7 of the voltage across the series connection point. The d-axis reference signal RT_7 of the voltage across the series connection point is subtracted from the d-axis signal Used of the voltage across the series connection point and then passed through the PI controller to output the d-axis reference signal Urefd. The q-axis reference signal qREF of the control loop is set to 0, and is subtracted from the q-axis signal Useq of the voltage across the series connection point and then passed through the PI controller to output the q-axis reference signal Urefq.

[0143] The dq-abc coordinate transformation of the electrical signal is implemented using the self-encapsulated DQ to X module. The input is the voltage d-axis reference signal Urefd and the voltage q-axis reference signal Urefq from the dq-axis calculation module, and the output is the voltage reference signal Ref at both ends of the series connection line.

[0144] The above modules are encapsulated in a control loop RC controller. The control loop output signal is the voltage reference signal Ref at both ends of the series connection line.

[0145] S5.1.2: Construct an SPWM modulation simulation model. The SPWM modulation module simulation model in the H-bridge converter control circuit is as follows: Figure 10As shown, the signal generator in PSCAD / EMTDC software is used as the triangular carrier signal generator. The voltage reference signal Ref output by the control loop simulation model is input into the SPWM modulation module, generating the SPWM modulation voltage reference signal Uref. The quotient of the voltage reference signal Uref and the external control signal Udc1, the DC-side controlled voltage source of the H-bridge converter, serves as the SPWM modulation wave. After comparing the triangular carrier wave of a certain frequency with the modulation wave, the control signals P1seA, P2seA, P3seA, P4seA; P1seB, P2seB, P3seB, P4seB; and P1seC, P2seC, P3seC, P4seC for the IGBTs in the H-bridge converter are output.

[0146] Construct an H-bridge converter control circuit simulation model. Figure 11 As shown, the control loop simulation model and SPWM modulation module simulation model constructed in steps S5.1.1 and S5.1.2 are encapsulated separately to form an H-bridge converter control circuit. The control loop simulation model corresponds to the controller encapsulation module, and the SPWM modulation simulation model corresponds to the SPWMsingle encapsulation module. The outer loop input dREF is the effective value of the voltage at the head end of the controlled low-voltage line Low0.4.

[0147] S5.2: Construct a three-phase full-bridge converter control circuit simulation model. The three-phase full-bridge converter control circuit simulation model includes a control loop module and an SVPWM modulation module. The specific steps for constructing the three-phase full-bridge converter control circuit simulation model are as follows:

[0148] S5.2.1: Construct a control loop module simulation model. The control loop simulation model in the three-phase full-bridge converter control circuit is as follows: Figure 12 As shown, the control loop module includes a phase-locked loop module, an ABC to DQ module, a dq axis calculation module and a DQ toABC module.

[0149] The phase-locked loop module adopts the PLL phase-locked loop (PLL) in PSCAD / EMTDC software, takes the grid-connected point voltage signal Ush of the three-phase bridge converter as the input of the phase-locked loop module, and uses phase vector technology to generate the ramp signal thetash.

[0150] The abc-dq coordinate transformation of the electrical signal is implemented using the self-packaged ABC to DQ module. The input quantities are the grid-connected point voltage signal Ush of the three-phase bridge converter and the grid-connected line current signal Ishg. The d-axis and q-axis components obtained are the grid-connected point voltage d-axis signal Vgd, the grid-connected point voltage q-axis signal Vgq, the grid-connected line current d-axis signal ILd, and the grid-connected line current q-axis signal ILq, respectively.

[0151] The dq-axis calculation module uses the PI controller in the PSCAD / EMTDC software. In the constant DC voltage control loop, the DC side voltage command value of the three-phase full-bridge converter is subtracted from the DC capacitor voltage measurement signal Udc2 and then input into the PI controller. The difference between the d-axis component reference value output by the PI controller and the grid-connected line current d-axis signal ILd is used as the input of the inner-loop PI controller. The voltage signal output by the inner-loop PI controller is added to the cross-coupling term and the grid-connected point voltage d-axis signal Vgd to obtain the voltage d-axis reference value Vrd. In the current q-axis signal control loop, the grid-connected line current q-axis reference value is set to 0, and the difference between it and the grid-connected line current q-axis signal ILq is input into the line current inner-loop PI controller. The difference between the line current d-axis signal ILd and the output of the inner-loop PI controller is used as the voltage q-axis reference value Vrq.

[0152] The dq-abc coordinate transformation of the electrical signal is implemented using the self-encapsulated DQ to ABC module. The input is the voltage d-axis reference signal Vrd and the voltage q-axis reference signal Vrq from the dq-axis calculation module, and the output is the three-phase full-bridge converter output voltage reference signal Vabc.

[0153] S5.2.2: Construct SVPWM modulation simulation model. The simulation model of SVPWM modulation module in the three-phase full-bridge converter control circuit is as follows: Figure 13 As shown, the interpolating sampling element (InterpolatingSampler), rectangular coordinate conversion element (Rectangular Coordinate Converter) and modulo function (MODULOFunction) in PSCAD / EMTDC software are used. The DC capacitor voltage of the three-phase full-bridge converter is input into the sampling element to obtain the capacitor voltage sampling quantity Vd1 that changes in steps according to the period fs. The output voltage reference signal Vabc of the three-phase full-bridge converter is input into the self-encapsulated three-phase-two-phase coordinate conversion module. After passing through the sampler and rectangular coordinate conversion element, the amplitude V and phase angle theta of the output quantity under the αβ coordinate are obtained. The modulation ratio m is obtained by dividing the fundamental peak value V of the modulation wave by the fundamental peak value Vd1 of the carrier wave by the coefficient The phase angle theta is determined by the custom module ChooseSector, which outputs the positions of the phasors V1 and V2. The action time of V1 and V2 in the sector is determined by T1, T2, and T0. The positions and action times of the phasors V1 and V2 are input into the custom module OutVector, which outputs a 6-dimensional pulse sequence as the control signals P1sh, P2sh, P3sh, P4sh, P5sh, and P6sh for the IGBTs of the three-phase full-bridge converter.

[0154] Construct a three-phase full-bridge converter control circuit simulation model, such as Figure 14 As shown, the control loop simulation model and SVPWM modulation module simulation model constructed in steps S5.2.1 and S5.2.2 are encapsulated separately to form a three-phase full-bridge converter control circuit. The control loop simulation model corresponds to the EC Controller encapsulation module, and the SVPWM modulation simulation model corresponds to the SVPWM encapsulation module. The function of the three-phase full-bridge converter control circuit is to control the common DC capacitor voltage to stabilize at the command value.

[0155] S6: Constructing a photovoltaic grid-connected control simulation model. The photovoltaic grid-connected control simulation model includes a photovoltaic MPPT (maximum power point tracking) control module and a PWM modulation circuit simulation model.

[0156] S6.1: Construct a photovoltaic MPPT control simulation model, such as Figure 15 As shown, the interpolating sampling component (Interpolating Sampler), constant time constant differential component (Different Lag or Forgetting Function), integrator component (Integrator), and range comparator component (Range Comparator) in PSCAD / EMTDC software are used. First, the photovoltaic power supply voltage and current are input into the sampling component to obtain voltage and current samples with step changes according to the period Ts. The samples are then passed through the given time constant differential component to obtain the voltage and current differentials dV and dI. dV × (IdV + VdI) is then fed into the range comparator for output. The range comparator output is multiplied by the given parameter signal add and then passed through the integrator to output the duty cycle D.

[0157] S6.2: Build a PWM modulation circuit simulation model. Use the Signal Generator in PSCAD / EMTDC software as a triangular carrier signal generator to compare the duty cycle D output by the photovoltaic MPPT control module with the carrier and output the power switch off control signal g.

[0158] Construct a photovoltaic grid-connected control simulation model, such as Figure 16 The PV MPPT control simulation model is encapsulated in the MPPT module. The MPPT module's output is the duty cycle D. Steps S6.1 and S6.2 form the PV grid-connected control simulation model. PV MPPT control adjusts the PV system's operating point to maintain operation near its maximum power point, maximizing the energy output of the solar panels and providing power to DC microgrid loads and electric vehicles.

[0159] S7: Constructing a simulation model for electric vehicle grid connection control, such as Figure 17As shown, the PI controller and signal generator in PSCAD / EMTDC software are used as a triangular carrier signal generator. The current setpoint is subtracted from the measured electric vehicle battery current I1, which is then passed through the PI controller to obtain the current command value Ipi. Ipi is used as the modulation wave. After comparing the triangular carrier wave of a certain frequency with the modulation wave, the control signals P1 and P2 for the IGBT in the Buck-Boost circuit simulation model are output, forming an electric vehicle grid-connected control simulation model. The goal of electric vehicle grid-connected control is to achieve flexible energy exchange between electric vehicles and microgrids. Electric vehicles can not only draw energy from the microgrid for charging but also transmit energy stored in the battery back to the microgrid to power loads.

[0160] S8: Construct a new flexible distribution transformer simulation model. The simulation models constructed in steps S1-S7 are interconnected to form a new flexible distribution transformer electromagnetic transient simulation model based on the PSCAD / EMTDC simulation platform. The new flexible distribution transformer simulation model is as follows: Figure 1 shown.

[0161] S9: Construct a feasibility verification simulation model for a new flexible distribution transformer, such as Figure 18 As shown, an AC power grid system of equal value is connected to the primary and secondary sides of the new flexible distribution transformer, and the AC power grid system includes a three-phase AC power supply, a transmission line, a new flexible distribution transformer and a load. The three-phase voltage source model 2 (Three-Phase Voltage Source Model 2) in the PSCAD / EMTDC software is used as a series simulation line of the three-phase AC power supply, the resistance and inductance, and the fixed load (Fixed Load) simulation load. The three-phase AC power supply is connected to the medium voltage side of the new flexible distribution transformer through the line Mid10, and the low voltage side of the new flexible distribution transformer is connected to the line Low0.4, forming a feasibility verification simulation model of the new flexible distribution transformer.

[0162] The main parameters of the feasibility verification simulation model are: three-phase AC system voltage: 10kV, first section voltage of the controlled low-voltage side line: 0.4kV, effective value of load phase voltage: 0.22kV, and steady-state single-phase active power of the load: 0.2MW.

[0163] S10: In order to verify and evaluate the voltage regulation, voltage stabilization, and new energy absorption capabilities of the new flexible distribution transformer, the following simulation tests and analyses were conducted on the feasibility verification simulation model of the constructed new flexible distribution transformer based on the PSCAD / EMTDC simulation software and the aforementioned simulation parameter settings:

[0164] 1. Simulation experiment analysis of voltage stabilization characteristics of new flexible distribution transformer:

[0165] In order to verify the voltage-stabilizing capability of the new flexible distribution transformer, the simulation test process is set as follows: the test system is initialized before 0.1s, the new flexible distribution transformer is put into operation at 0.2s, starts charging the DC capacitor, and starts controlling the 0.4kV side bus voltage to 1.0pu at 0.4s. Photovoltaic and electric vehicles are connected to the grid at 0.5s and 0.7s respectively. The AC load increases by a constant power load of 0.5MW at 1.2s and reduces by a constant power load of 0.3MW at 1.5s.

[0166] Figure 19 The following diagrams show the per-unit voltage waveforms of the controlled lines before and after the commissioning of the new flexible distribution transformer. As can be seen, at 0.2s and 0.3s, the low-voltage bus voltage experiences transient fluctuations due to the operation of the three-phase full-bridge converter and the H-bridge converter. At 0.4s, the low-voltage bus voltage begins to rise and stabilizes at 1.0 pu due to the regulation of the H-bridge converter. At 0.5s and 0.7s, the low-voltage bus voltage briefly fluctuates due to the integration of photovoltaic and electric vehicles, with maximum voltage deviations of 0.42% and 0.7%, respectively. At 1.2s and 1.5s, the voltage briefly fluctuates due to the increase and decrease of AC load before returning to a stable value of 1.0 pu. The maximum voltage deviations during these periods are 0.57% and 0.45%, respectively. These voltage deviations are all within the allowable 2%, demonstrating the effective voltage stabilization of the new flexible distribution transformer.

[0167] 2. Simulation experiment analysis of the voltage regulation characteristics of the new flexible distribution transformer: In order to verify the voltage regulation capability of the hybrid power transformer, the simulation test process is set as follows: the test system is initialized before 0.1s, the new flexible distribution transformer is put into operation at 0.2s and starts to charge the DC capacitor. At 0.4s, it starts to adjust the 0.4kV side bus voltage to 1.03pu, and the grid connection point voltage command is set to 1.1pu at 0.9s, the grid connection point voltage command is increased to 1.15pu at 1.2s, and the command value is reduced to 0.85pu at 1.55s.

[0168] Figure 20The waveform diagram shows the voltage regulation of the controlled line by the new flexible distribution transformer. As can be seen from the figure, in the time period of 0.2-0.4s, when the new flexible distribution transformer is not voltage-regulated, the low-voltage side bus voltage remains at 0.989pu; during the period of 0.4-0.9s, due to the voltage regulation of the new flexible distribution transformer, the low-voltage side bus voltage rises from 0.989pu and stabilizes at 1.03pu, during which a brief voltage fluctuation occurs due to the access of new energy; at 0.9s, the command value is raised, and the low-voltage side bus voltage starts to rise from 1.03pu and stabilizes at 1.1pu; at 1.2s, the command value is raised to 1.15pu, and the bus voltage rises from 1.1pu and stabilizes at 1.131pu, indicating that the maximum positive voltage regulation range of the new flexible distribution transformer is 14.35%; at 1.55s, the command value is lowered to 0.85pu, the bus voltage drops and finally stabilizes at 0.870pu, indicating that the maximum negative voltage regulation range of the new flexible distribution transformer is -12.03%. In summary, the maximum positive voltage regulation range of the new flexible distribution transformer reaches 14.35%, and the maximum negative voltage regulation range is -12.03%, both exceeding the ±10% standard. This shows that the new flexible distribution transformer has strong voltage regulation capability in actual operation and can meet the needs of the power system.

[0169] 3. Simulation experiment analysis of the absorption capacity of the new flexible distribution transformer: In order to verify the absorption capacity of the new flexible distribution transformer, the simulation test process is set as follows: the test system is initialized before 0.1s, the new flexible distribution transformer is put into operation at 0.2s and starts charging the DC capacitor. At 1s, the photovoltaic light intensity of the DC microgrid is 1000W / m 2 , 500W / m3 is added in 1.2s 2 , 500W / m is added at 1.6s 2 .

[0170] Figure 21 This is the waveform of the DC side capacitor voltage of the new flexible distribution transformer. It can be seen from the figure that the new flexible distribution transformer starts to actively charge at 0.5s, and the DC capacitor voltage rises to 1.0pu; at 0.7s, a large fluctuation occurs due to the grid connection of electric vehicles. At 0.8s, the voltage stabilizes at 1.0pu after a brief fluctuation due to the charging and discharging of electric vehicles; at 1.0s and 1.2s, due to the increase in light intensity, the voltage fluctuates briefly and then returns to the command value of 1.0pu. The maximum voltage deviation during this period is 1.48% and 1.46%, which is within 5%, indicating that the new flexible distribution transformer has a good ability to absorb new energy.

[0171] If the voltage stabilization, voltage regulation and new energy absorption capacity all meet the requirements, the simulation design scheme of the flexible distribution transformer is determined to be feasible. If one item is not met, the simulation design scheme of the flexible distribution transformer is determined to be infeasible and the design scheme or parameters need to be adjusted.

[0172] In order to evaluate the performance of the new flexible distribution transformer, the following evaluation criteria are set, including voltage deviation, voltage regulation range and new energy consumption rate.

[0173] (1) Low-voltage bus voltage deviation ΔU. The low-voltage bus voltage deviation is used to measure the ability of the new flexible distribution transformer to maintain the stability of the low-voltage bus voltage under load changes or other interference conditions. It is shown in the following formula:

[0174]

[0175] Where, U is the actual voltage of the low-voltage busbar of the new flexible distribution transformer; U set The target voltage value set for the low-voltage side of the new flexible distribution transformer. The allowable voltage deviation is 2%.

[0176] (2) Voltage regulation range: refers to the maximum voltage regulation range that can be achieved by the new flexible distribution transformer by changing the low-voltage side bus voltage command value, including positive voltage regulation range and negative voltage regulation range, as shown in the following formula:

[0177]

[0178] Where U max It is the highest voltage that the low-voltage side bus can reach; U min It is the lowest voltage that the low-voltage side bus can reach; U initial This is the initial voltage when the voltage is not regulated. The voltage regulation range should not be less than ±10%.

[0179] (3) DC capacitor voltage deviation: The DC capacitor voltage deviation can reflect the system's ability to handle excess power. If the system can effectively absorb excess new energy, the DC capacitor voltage should be maintained within a stable range, as shown in the following formula:

[0180]

[0181] Where V dc,max is the maximum value of the DC capacitor voltage; V dc,min is the minimum value of the DC capacitor voltage; V dc,nominal is the nominal voltage of the DC capacitor. The DC capacitor voltage deviation should be within 5%.

Claims

1. A feasibility verification method for flexible distribution transformers based on simulation software, characterized by: S1: Construct a main circuit simulation model of a flexible distribution transformer, and respectively construct a dual-winding transformer simulation model, an H-bridge converter main circuit simulation model, and a three-phase full-bridge converter main circuit simulation model. Connect the H-bridge converter main circuit simulation model in series to the high-voltage side of the dual-winding transformer simulation model, and connect the three-phase full-bridge converter main circuit simulation model in parallel to the low-voltage side of the dual-winding transformer simulation model. The H-bridge converter main circuit simulation model and the three-phase full-bridge converter main circuit simulation model are connected via a controlled voltage source and a controlled current source to simulate a DC capacitor. S2: Constructing a photovoltaic power generation simulation model. First, construct a photovoltaic cell simulation model, then construct a photovoltaic boost circuit simulation model. The constructed photovoltaic cell simulation model is connected to the boost circuit via a filter circuit to form a photovoltaic power generation simulation model. S3: Constructing an electric vehicle charging and discharging simulation model. First, construct an electric vehicle battery simulation model, then construct an electric vehicle Buck-Boost circuit simulation model. The constructed electric vehicle battery simulation model is connected to the Buck-Boost circuit via a filter circuit to form an electric vehicle charging and discharging simulation model. S4: Build a DC microgrid main circuit simulation model. First, build a DC microgrid simulation model. Then, encapsulate the DC microgrid simulation model and connect it to the DC side of the three-phase full-bridge converter to form a DC microgrid main circuit simulation model. S5: Construct a flexible distribution transformer control circuit simulation model, and construct an H-bridge converter control simulation model and a three-phase full-bridge converter control simulation model respectively. The H-bridge converter control circuit simulation model is used to output the control signal of the H-bridge converter to control the H-bridge converter main circuit simulation model. The A, B, and C phase H-bridge converter control circuits are constructed respectively using the same method. Each phase circuit includes an H-bridge control loop module RC controller and an SPWM modulation simulation model; the three-phase full-bridge converter control circuit simulation model is used to output the control signal of the three-phase full-bridge converter to control the three-phase full-bridge converter main circuit simulation model. The three-phase full-bridge converter control circuit simulation model includes a control loop module and an SVPWM modulation module. S6: Constructing a photovoltaic grid-connected control simulation model, wherein the photovoltaic grid-connected control simulation model includes: a photovoltaic MPPT control module and a PWM modulation circuit simulation model; S7: constructing an electric vehicle grid-connected control simulation model, wherein the electric vehicle grid-connected control simulation model is used to generate a control signal for an electric vehicle Buck-Boost circuit simulation model; S8: interconnecting the simulation models constructed in the above steps to form an electromagnetic transient simulation verification model based on the flexible distribution transformer; S9: Construct a feasibility verification simulation model for flexible distribution transformers to verify the feasibility of flexible distribution transformers; S10: Verify the voltage regulation, voltage stabilization and new energy absorption capabilities of flexible distribution transformers.

2. The feasibility verification method for a flexible distribution transformer based on simulation software according to claim 1 is characterized in that: In S1, a dual-winding transformer simulation model is constructed. The main transformer of the flexible distribution transformer adopts a three-phase dual-winding transformer. The three-phase two-winding transformer components in PSCAD / EMTDC are selected as the main transformer of the flexible distribution transformer. The primary winding of the main transformer is set to a △ connection method; the secondary winding is set to a Y connection method. Construct the simulation model of the main circuit of the H-bridge converter: Construct the H-bridge converters of phase A, phase B, and phase C respectively using the same method. The construction method of the H-bridge converter of phase A is as follows: Use the power electronic switch IGBT and diode in PSCAD / EMTDC to construct the main circuit of the full-bridge converter of phase A. The values ​​of the filter inductor and filter capacitor in the LC filter are obtained by calculating the system parameters and the parameters of the flexible distribution transformer; detect the input current signal IgseA, output voltage signal UseA, DC side capacitor voltage signal Udc1A and DC current signal Idc1 of the full-bridge converter of phase A. A is transmitted to S5 as the actual value input of the electrical detection of the A-phase full-bridge converter control circuit. The series control converter and the parallel energy-taking converter are connected through a controlled voltage source and a controlled current source to simulate a DC capacitor. The four IGBT trigger pulse signals of the A-phase H-bridge converter are P1seA, P2seA, P3seA and P4seA. The three H-bridge converters constitute an H-bridge converter simulation model. After the H-bridge converter simulation model is encapsulated, it is connected in series to the high-voltage side of the three-phase double-winding transformer according to its corresponding phase to form an H-bridge converter main circuit simulation model. Construct a three-phase full-bridge converter main circuit simulation model: First, construct a three-phase bridge converter, and the construction method is as follows: use the power electronic switch IGBT and diode in PSCAD / EMTDC to construct the three-phase bridge converter main circuit, and the filter inductor and filter capacitor values ​​in the LCL filter are obtained by calculating the system parameters and flexible distribution transformer parameters; detect the three-phase bridge converter input current signal Ishg, the grid connection point voltage signal Ush and the DC side capacitor voltage signal Udc2 and transmit them to S5 as the electrical detection actual value input of the parallel energy-taking converter control circuit; the six IGBT trigger pulse signals of the three-phase bridge converter are P1sh, P2sh, P3sh, P4sh, P5sh and P6sh, and then encapsulate the three-phase bridge converter and connect it in parallel to the low-voltage side of the three-phase double-winding transformer to form a three-phase full-bridge converter main circuit simulation model connected to the AC power grid; Construct the connection between the H-bridge converter main circuit and the three-phase full-bridge converter main circuit. The H-bridge converter main circuit simulation model and the three-phase full-bridge converter main circuit simulation model are connected through a controlled voltage source and a controlled current source to simulate a DC capacitor; the controlled voltage source external control signal U on the DC side of the H-bridge converter dc1 and the three-phase full-bridge converter DC capacitor voltage U dc2 The mathematical relationship is: K1U dc1 =U dc2 ; H-bridge converter DC side current I dc1A , I dc1B , I dc1C and the external control signal I of the DC side controlled current source of the three-phase full-bridge converter dc2 The mathematical relationship is: I dc2 =K2(I dc1A +I dc1B +I dc1C ); K1 is the proportional relationship between the DC capacitor voltage and the controlled voltage source; K2 is the proportional relationship between the DC side current and the controlled current source.

3. The feasibility verification method for a flexible distribution transformer based on simulation software according to claim 1 is characterized in that: In S2, a photovoltaic cell simulation model is first constructed using a photovoltaic power supply in PSCAD / EMTDC software, where Vpv is the open-circuit voltage of the photovoltaic cell, Ipv is the current output by the photovoltaic cell, G is the instantaneous value of the light intensity, and T is the instantaneous value of the battery operating temperature; Then, a photovoltaic boost circuit simulation model is constructed using power electronic switches IGBT and diodes. The input control signal of the fully controlled power switch device IGBT is represented by g. The photovoltaic cell simulation model is connected to the photovoltaic boost circuit simulation model through a filter circuit to form a photovoltaic power generation simulation model. In the photovoltaic power generation model, Idcl is the grid-side current, Vdcout is the grid-side voltage, Iin is the photovoltaic cell terminal current, Vin is the photovoltaic cell terminal voltage, DC1 and DC2 are DC interfaces, and the photovoltaic power generation simulation model is encapsulated in the module PV.

4. The feasibility verification method for a flexible distribution transformer based on simulation software according to claim 1 is characterized in that: In S3, first, an electric vehicle battery simulation model is constructed, and the electric vehicle battery simulation model is constructed using the single-phase voltage source model 2 in the PSCAD / EMTDC software. In the electric vehicle battery model, Uocv is the open circuit voltage of the electric vehicle battery, and I1 is the charging current of the electric vehicle battery; The open circuit voltage Uocv and state of charge SOC of the battery have a fixed monotonic nonlinear functional relationship. The battery model based on the battery OCV-SOC characteristics is used to simulate the battery characteristics with SOC as a variable. The state of charge S in the electric vehicle battery model is OC The calculation formula is: Where S OC0 is the initial state of charge of the battery; i is the charging current; Q N is the rated capacity of the battery; The calculation formula for the open circuit voltage Uocv of electric vehicle batteries is: Among them, a, b, c, d, and e are given parameters; Then, the electric vehicle Buck-Boost circuit simulation model is constructed using the power electronic switch IGBT and diode in PSCAD / EMTDC software. The input control signals of the two fully controlled power switch devices IGBT are represented by P1 and P2 respectively. The constructed electric vehicle battery simulation model is connected to the Buck-Boost circuit simulation model through a filter circuit to form an electric vehicle charging and discharging simulation model. In the electric vehicle charging and discharging simulation model, U1 is the electric vehicle battery terminal voltage, I1 is the electric vehicle battery charging current, Udc is the grid-side voltage, Isingle is the grid-side current, DC1 and DC2 are DC interfaces, and the electric vehicle charging and discharging simulation model is encapsulated in the module Electric Vehicle.

5. The feasibility verification method for a flexible distribution transformer based on simulation software according to claim 1 is characterized in that: In S4, a DC microgrid simulation model is first constructed. A fixed load in PSCAD / EMTDC software is used to simulate a DC load. The PV module and the Electric Vehicle module are connected in parallel to the DC bus. The DC interface DC1 is connected to the positive pole of the DC bus, and the DC interface DC2 is connected to the negative pole of the DC bus to form a DC microgrid simulation model. DCG1 and DCG2 are connection interfaces of the DC microgrid simulation model. The DC microgrid simulation model is encapsulated in the HybridMicrogrid module. The encapsulated module HybridMicrogrid is then connected to the DC side of the three-phase full-bridge converter main circuit simulation model. The interface DCG1 in the module HybridMicrogrid is connected to the DC side interface DCP1 of the three-phase full-bridge converter main circuit simulation model. The interface DCG2 in the module HybridMicrogrid is connected to the DC side interface DCP2 of the three-phase full-bridge converter main circuit simulation model, forming a DC microgrid main circuit simulation model that is put into operation on the DC side of the transformer.

6. The feasibility verification method for a flexible distribution transformer based on simulation software according to claim 2 is characterized in that: In S5, S5.1: Construct an H-bridge converter control circuit simulation model. The control principles of the three H-bridge converters are exactly the same. The same method is used to construct the A-phase, B-phase, and C-phase H-bridge converter control circuits, respectively. The construction method of the A-phase H-bridge converter control circuit is as follows: S5.1.1: Construct a simulation model of an H-bridge control loop module, wherein the H-bridge control loop module includes a phase-locked loop module, an X to DQ module, a dq axis calculation module, and a DQ to X module; The phase-locked loop module uses the PLL phase-locked loop in PSCAD / EMTDC software, takes the H-bridge converter series-connected line current signal IgesA as the input of the phase-locked loop module, and uses phase vector technology to generate the ramp signal thetase; The abc-dq coordinate transformation of the electrical signal is implemented using a self-packaged X to DQ module. The input quantities are the series connection line current signal IgseA and the voltage signal UseA across the series access point. The d-axis and q-axis components obtained are the series connection line current d-axis signal Igd, the series connection line current q-axis signal Igq, the series connection point voltage d-axis signal Used, and the series access point voltage q-axis signal Useq, respectively. The dq-axis calculation module uses the PI controller in the PSCAD / EMTDC software. The effective value of the low-voltage line low0.4 voltage controlled by it is used as the input signal dREF of the dq-axis calculation module. The input signal reference value is subtracted from the input signal dREF and then passed through the PI controller to obtain the d-axis reference signal RT_7 of the voltage at both ends of the series connection point. The d-axis reference signal RT_7 of the voltage at both ends of the series connection point is subtracted from the d-axis signal Used of the voltage at both ends of the series connection point and then passed through the PI controller to output the d-axis reference signal Urefd. The q-axis reference signal qREF of the control loop is set to 0, and is subtracted from the q-axis signal Useq of the voltage at both ends of the series connection point and then passed through the PI controller to output the q-axis reference signal Urefq. The dq-abc coordinate transformation of the electrical signal is implemented using the self-encapsulated DQ to X module. The input is the voltage d-axis reference signal Urefd and the voltage q-axis reference signal Urefq from the dq-axis calculation module. The output is the voltage reference signal Ref at both ends of the series connection line. The H-bridge control loop module is encapsulated in the H-bridge control loop module RC controller. The control loop output signal Ref is the voltage reference signal Uref at both ends of the series connection line. The outer loop input dREF is the effective value of the voltage at the first end of the controlled low-voltage line Low0.

4. S5.1.2: Construct an SPWM modulation simulation model. The SPWM modulation module simulation model in the H-bridge converter control circuit uses the signal generator in the PSCAD / EMTDC software as a triangular carrier signal generator. The voltage reference signal Uref output by the control loop RC controller simulation model is input into the SPWM modulation module, namely the SPWM modulation voltage reference signal Uref. The quotient of the voltage reference signal Uref and the external control signal Udc1 of the controlled voltage source on the DC side of the H-bridge converter is used as the SPWM modulation wave. After comparing the triangular carrier with the set frequency with the modulation wave, the control signals P1seA, P2seA, P3seA, and P4seA of the IGBT in the H-bridge converter are output. The above-mentioned SPWM modulation module is encapsulated in the module SPWM single. S5.2: Constructing a three-phase full-bridge converter control circuit simulation model: The three-phase full-bridge converter control circuit simulation model includes a control loop module and an SVPWM modulation module; S5.2.1: Construct a simulation model of the control loop module of a three-phase full-bridge converter, wherein the control loop module includes a phase-locked loop module, an ABC to DQ module, a dq axis calculation module, and a DQ to ABC module; The phase-locked loop module uses the PLL phase-locked loop in PSCAD / EMTDC software, takes the grid-connected point voltage signal Ush of the three-phase bridge converter as the input of the phase-locked loop module, and uses the phase vector technology to generate the ramp signal thetash; The abc-dq coordinate transformation of the electrical signal is implemented using the self-packaged ABC to DQ module. The input quantities are the grid-connected point voltage signal Ush of the three-phase bridge converter and the grid-connected line current signal Ishg. The d-axis and q-axis components obtained are the grid-connected point voltage d-axis signal Vgd, the grid-connected point voltage q-axis signal Vgq, the grid-connected line current d-axis signal ILd, and the grid-connected line current q-axis signal ILq, respectively. The dq-axis calculation module uses the PI controller in PSCAD / EMTDC software. In the constant DC voltage control loop, the DC side voltage command value of the three-phase full-bridge converter is subtracted from the DC capacitor voltage measurement signal Udc2 and then transmitted to the PI controller. The difference between the d-axis component reference value output by the PI controller and the grid-connected line current d-axis signal ILd is used as the input of the inner-loop PI controller. The voltage signal output by the inner-loop PI controller is added to the cross-coupling term and the grid-connected point voltage d-axis signal Vgd to obtain the voltage d-axis reference value Vrd. In the current q-axis signal control loop, the grid-connected line current q-axis reference value is set to 0, and the difference between the grid-connected line current q-axis signal ILq is input into the line current inner-loop PI controller. The difference between the line current d-axis signal ILd and the output of the inner-loop PI controller is used as the voltage q-axis reference value Vrq. The dq-abc coordinate transformation of the electrical signal is implemented using a self-encapsulated DQ to ABC module. The input is the voltage d-axis reference signal Vrd and the voltage q-axis reference signal Vrq from the dq-axis calculation module. The output is the three-phase full-bridge converter output voltage reference signal Vabc. The above control loop module is encapsulated in the control loop module EC controller. S5.2.2: Construct an SVPWM modulation simulation model using interpolation sampling elements, rectangular coordinate transformation elements, and modular functions in PSCAD / EMTDC software. Input the DC capacitor voltage of the three-phase full-bridge converter into the sampling element to obtain the capacitor voltage sampling quantity Vd1 that changes in steps according to the period fs. Input the output voltage reference signal Vabc of the three-phase full-bridge converter into the self-encapsulated three-phase-to-two-phase coordinate transformation module. After passing through the sampler and rectangular coordinate transformation element, the amplitude V and phase angle theta of the output quantity under the αβ coordinates are obtained. The modulation ratio m is obtained by dividing the fundamental peak value V of the modulation wave by the fundamental peak value Vd1 of the carrier wave by the coefficient The phase angle theta is obtained by determining the sector through the custom module ChooseSector, and the positions of the phasors V1 and V2 are output. The action time of V1 and V2 in the sector is determined by T1, T2 and T0. The positions and action time of the phasors V1 and V2 are input into the custom module OutVector to output a 6-dimensional pulse sequence as the control signals P1sh, P2sh, P3sh, P4sh, P5sh, and P6sh of the three-phase full-bridge converter switching device IGBT. The above SVPWM modulation module is encapsulated in the module SVPWM.

7. The feasibility verification method for a flexible distribution transformer based on simulation software according to claim 3 is characterized in that: In said S6, said photovoltaic grid-connected control simulation model includes: a photovoltaic MPPT control module and a PWM modulation circuit simulation model; S6.1: Construct a simulation model of the photovoltaic MPPT control module using interpolation sampling elements, constant time constant differential elements, integrator elements, and range comparator elements in PSCAD / EMTDC software. First, input the photovoltaic power supply voltage and current into the sampling elements to obtain voltage and current sampling quantities that change in steps according to the period Ts. The sampling quantities are then passed through the given time constant differential element to obtain the voltage and current differential quantities dV and dI. dV × (IdV + VdI) is then fed into the range comparator for output. The range comparator output is multiplied by the given parameter signal add and then passed through the integrator to output the duty cycle D. Encapsulate the photovoltaic MPPT control simulation model in the MPPT module. The output of the MPPT module is the duty cycle D. S6.2: Construct a PWM modulation circuit simulation model. Use the signal generator in PSCAD / EMTDC software as a triangular carrier signal generator. Compare the duty cycle D output by the photovoltaic MPPT control module with the carrier and output the power switch control signal g.

8. The feasibility verification method for a flexible distribution transformer based on simulation software according to claim 4 is characterized in that: In S7, the electric vehicle grid-connected control simulation model uses the PI controller and signal generator in the PSCAD / EMTDC software as a triangular carrier signal generator. The current set value is subtracted from the measured electric vehicle battery current I1 and then passed through the PI controller to obtain the current command value Ipi. Ipi is used as the modulation wave. After comparing the triangular carrier of a certain frequency with the modulation wave, the control signals P1 and P2 of the IGBT in the Buck-Boost circuit simulation model are output to form the electric vehicle grid-connected control simulation model.

9. The feasibility verification method for a flexible distribution transformer based on simulation software according to claim 1 is characterized in that: In the S9, an AC power grid system of equal value is connected to the primary and secondary sides of the flexible distribution transformer, wherein the AC power grid system includes a three-phase AC power supply, a transmission line, a flexible distribution transformer, and a load; a three-phase voltage source model 2 in the PSCAD / EMTDC software is used as a series simulation circuit of a three-phase AC power supply, a resistor, and an inductor, and a fixed load simulation load. The three-phase AC power supply is connected to the medium voltage side of the flexible distribution transformer through the line Mid10, and the low voltage side of the flexible distribution transformer is connected to the line Low0.4, thereby forming a feasibility verification simulation model of the flexible distribution transformer; The key parameters that need to be set in the feasibility verification simulation model include the three-phase AC system voltage, the first section voltage of the controlled low-voltage side circuit, the effective value of the load phase voltage and the steady-state single-phase active power of the load.

10. The feasibility verification method for a flexible distribution transformer based on simulation software according to claim 1 is characterized in that: S10: Verify the voltage stabilization, voltage regulation, and new energy consumption capacity of the flexible distribution transformer. If the voltage stabilization, voltage regulation, and new energy consumption capacity all meet the requirements, the simulation design scheme of the flexible distribution transformer is deemed feasible. If any one of the requirements is not met, the simulation design scheme of the flexible distribution transformer is deemed infeasible and the design scheme or parameters need to be adjusted. Voltage stabilization simulation test: Set the simulation plan and initialize the entire simulation model first. After initialization, put the flexible distribution transformer into operation. The controlled voltage source and the controlled current source simulate the DC capacitor to start charging. After charging is completed, control the low-voltage side bus voltage of the flexible distribution transformer to 1.0pu. After the voltage boost is completed, the photovoltaic cell simulation model and the electric vehicle battery simulation model are connected to the grid in turn. After the grid connection is completed, increase the AC load by a constant power load of 0.3-0.7MW, and reduce the constant power load by 0.2-0.5MW after the set time interval. At this time, the test is completed, observe the change in the low-voltage side bus voltage of the flexible distribution transformer, and test the maximum voltage deviation of the low-voltage side bus of the distribution transformer. If the maximum voltage deviation is less than 2%, it meets the requirements. Voltage regulation simulation test: Set the simulation plan, first initialize the entire simulation model, and after the initialization is completed, put the flexible distribution transformer into operation, and the controlled voltage source and controlled current source simulate the DC capacitor to start charging. After the charging is completed, control the low-voltage side bus voltage of the flexible distribution transformer to 1.03pu. After the voltage boost is completed, the photovoltaic cell simulation model and the electric vehicle battery simulation model are connected to the grid in turn. After the interval setting time t1, the low-voltage side bus voltage is increased by 0.06~0.08pu. After the interval setting time t2, the low-voltage side bus voltage is increased by 0.07~0.09pu. After the interval setting time t3, the low-voltage side bus voltage is reduced by 0.15~0.3pu. At this time, the test is completed; observe the change of the low-voltage side bus voltage of the flexible distribution transformer, and test the maximum voltage regulation range of the low-voltage side bus of the distribution transformer. If the maximum voltage regulation range exceeds ±10%, it meets the requirements; Absorption capacity simulation test: Set the simulation plan, first initialize the entire simulation model, put the flexible distribution transformer into operation after initialization, and start charging the DC capacitor simulated by the controlled voltage source and the controlled current source. After charging is completed, control the low-voltage side bus voltage of the flexible distribution transformer to 1.0pu. After the boost is completed, the photovoltaic cell simulation model and the electric vehicle battery simulation model are connected to the grid in turn. After the grid connection is completed, after the set time t4, the photovoltaic light intensity of the DC microgrid is set to 1000W / m 2 After the interval setting time t5, increase the light intensity by 300~600W / m 2 After the interval setting time t6, the light intensity is increased by 300 to 600W / m 2 , the test is now completed; observe the changes in the capacitor voltage on the DC side of the flexible distribution transformer, and test the maximum voltage deviation on both sides of the DC capacitor. If the maximum voltage deviation is less than 5%, it meets the requirements.

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