A flexible distribution transformer topology for extracting energy from a high-voltage port and its control method

Abstract

A flexible distribution transformer topology for energy extraction from a high-voltage port and its control method. The topology includes a dual-winding main transformer, an external energy extraction transformer, a voltage-source energy extraction converter, an independent H-bridge full-bridge regulation converter, and an energy storage capacitor. The dual-winding main transformer includes a primary and a secondary winding. An independent H-bridge full-bridge regulation converter is connected in series with the primary winding, and an external energy extraction transformer is connected in parallel with the secondary winding. The external energy extraction transformer is connected in parallel with the voltage-source energy extraction converter. The energy storage capacitor is connected in parallel between the voltage-source energy extraction converter and the independent H-bridge full-bridge regulation converter. In application, energy extraction and reactive power compensation are completed by adjusting the external energy extraction transformer and the voltage-source energy extraction converter, and flexible voltage regulation is achieved by adjusting the independent H-bridge full-bridge regulation converter. Moreover, there is no need to design an internal energy extraction winding, effectively shortening the construction period, reducing the volume, and not increasing the difficulty of transformer heat dissipation. The present invention not only has a small cost but also a small volume.

Description

Technical Field

[0001] The present invention relates to a topological structure, belonging to the field of power systems, and particularly to a flexible distribution transformer topology for extracting energy from a high-voltage port and its control method. Background Art

[0002] Currently, the installed capacity of new energy is increasing year by year, and a large number of wind power, photovoltaic, and tidal power stations are connected to the grid for power transmission. As a result, the development volume of green energy and the utilization rate of new energy are increasing day by day, and a clean new power system with "more wind, light, and energy storage, less thermal power and hydropower" is being gradually built.

[0003] As the core equipment for electric energy conversion in each link of "transmission, transformation, and distribution", the electromagnetic power transformer has become the "backbone" equipment in the power system due to its high reliability and stable electric energy transmission. However, it only has a mechanical step voltage regulation function and cannot meet the requirements of rapid continuous voltage regulation and complex power flow control, nor can it meet the requirements of the new power system for comprehensive power quality management. Power electronic devices have become an effective technical means to solve the problems of comprehensive power quality management and new energy grid connection fluctuations. However, pure power electronic devices have low reliability, high construction investment, complex control circuits, and high operation and maintenance costs in high power density applications. Therefore, a flexible distribution transformer composed of an electromagnetic transformer as the main body and equipped with a DC / AC converter has emerged, which combines the advantages of high reliability of the electromagnetic transformer and high flexibility of the converter for regulation.

[0004] The patent application with the application number CN202310271987.X and the application date of March 16, 2023, discloses a configuration method and device for a flexible power transformer, which discloses a flexible power transformer composed of a three-winding power transformer with a compensation winding and a voltage source inverter. The establishment of its additional energy extraction winding requires the re-design of the topology of the electromagnetic part of the transformer, which has the defects of a long engineering development cycle and high R & D costs. At the same time, the re-design of the transformer heat dissipation and insulation indexes will also increase the volume of the main transformer. Therefore, there is an urgent need for a means to solve the above-mentioned defects existing in the prior art. Summary of the Invention

[0005] The purpose of the present invention is to overcome the above-mentioned defects and problems existing in the prior art, and provide a flexible distribution transformer topology for extracting energy from a high-voltage port and its control method with a relatively small cost and a relatively small volume.

[0006] To achieve the above purpose, the technical solution of the present invention is: A flexible distribution transformer topology for extracting energy from a high-voltage port, including a double-winding main transformer, an external energy extraction transformer, a voltage source energy extraction converter, an independent H-bridge full-bridge regulation converter, and an energy storage capacitor;

[0007] The double-winding main transformer includes a primary winding and a secondary winding. The primary winding is located on the primary side of the double-winding main transformer, and the secondary winding is located on the secondary side of the double-winding main transformer;

[0008] An independent H-bridge control converter is serially arranged at the end of the primary winding. An external energy-taking transformer is arranged in parallel at the output end of the secondary winding. The external energy-taking transformer is connected in parallel with a voltage-source energy-taking converter; The energy storage capacitor is connected in parallel between the voltage-source energy-taking converter and the independent H-bridge control converter;

[0009] The external energy-taking transformer includes an external primary winding and an external secondary winding. The external primary winding is located on the primary side of the external energy-taking transformer, and the external secondary winding is located on the secondary side of the external energy-taking transformer;

[0010] A first mechanical switch is arranged on the primary winding. A second mechanical switch is arranged on the line between the primary winding and the independent H-bridge control converter. A third mechanical switch is arranged on the line between the external secondary winding and the voltage-source energy-taking converter.

[0011] An AC system power supply is connected in parallel to the primary winding. The independent H-bridge control converter is serially connected between the end of the primary winding and the grounding end of the AC system power supply;

[0012] The external primary winding is connected to the secondary winding, and the external secondary winding is connected to the voltage-source energy-taking converter.

[0013] An L-type filter is arranged between the external secondary winding and the voltage-source energy-taking converter; An LC-type filter is arranged between the primary winding and the independent H-bridge control converter.

[0014] A load terminal is serially arranged on the secondary winding.

[0015] A control method for a flexible power distribution transformer topology for taking energy from a high-voltage port, the method includes:

[0016] When the topology is in the initial operation stage, the voltage-source energy-taking converter and the independent H-bridge control converter need to satisfy the following power balance equation:

[0017] U ac I1cos(θ ac +θ1)+U fe I fe cos(θ fe )=0;

[0018] Where: U ac is the ideal output voltage of the independent H-bridge control converter, I1 is the current of the primary winding, θ acTo regulate the phase of the output voltage of the independent H - type full - bridge converter, θ1 is the phase of the primary winding current, U fe is the voltage across the two ends of the external secondary winding, I fe is the ideal output current of the voltage - source energy - extraction converter, θ fe is the phase of the output current of the voltage - source energy - extraction converter;

[0019] When θ ac + θ1 > 90°, the independent H - type full - bridge converter delivers electrical energy to complete the voltage regulation task.

[0020] When the topology performs voltage regulation, the AC voltage across the two ends of the primary winding is fixed. By changing the amplitude and phase of the output voltage of the independent H - type full - bridge converter, the amplitude and phase of the actual bus voltage on the primary side of the double - winding main transformer are changed, so as to realize the control of the primary - side bus voltage;

[0021] When the actual bus voltage on the primary side of the double - winding main transformer is fixed, by changing the amplitude and phase of the output voltage of the independent H - type full - bridge converter, the amplitude of the AC voltage across the two ends of the primary winding is increased or decreased, and through the voltage transformation of the double - winding main transformer, the amplitude of the AC voltage across the two ends of the secondary winding is increased or decreased, completing the control of the secondary - side bus voltage, so as to realize the precise tracking regulation of the voltages on the primary and secondary sides of the double - winding main transformer.

[0022] When the topology performs reactive - power compensation, by changing the output current of the voltage - source energy - extraction converter and the output voltage of the independent H - type full - bridge converter, the total injected reactive power is changed. At the same time, the total reactive power must follow the power - balance equation, and its corresponding ideal maximum value is as follows:

[0023] Q in,max =U fe ·I fe +U ac ·I1;

[0024] Where: Q in，max is the ideal maximum value, U fe is the AC voltage across the two ends of the external secondary winding.

[0025] When the topology performs power - flow regulation, the topology can operate in a constant - impedance mode or a constant - power mode;

[0026] When operating in the constant - impedance mode, by regulating the output voltage of the independent H - type full - bridge converter, the output voltage of the independent H - type full - bridge converter and the output current of the voltage - source energy - extraction converter form a constant impedance, so as to realize the power - flow regulation on the primary side of the double - winding main transformer;

[0027] When operating in the constant power mode, the output voltage of the independent H-bridge control converter and the output current of the voltage-source energy extraction converter are regulated to keep the power flow values on the primary and secondary sides of the dual-winding main transformer constant.

[0028] When the topology performs voltage regulation, first, the independent H-bridge control converter subtracts the actual series output AC voltage value from the corresponding reference command value to obtain a first deviation value, and based on the first deviation value, a control reference command value is obtained. Then, the control reference command value is subtracted from the actual voltage value to obtain a second deviation value, and the second deviation value is combined with the feedforward decoupling term to obtain the output modulation voltage signal. Subsequently, based on the modulation voltage signal, a PMW trigger control signal is obtained. Finally, the PMW trigger control signal is applied to the independent H-bridge control converter to complete the voltage regulation.

[0029] When the fault current in the topology is higher than the rated current, the fault current flows through the voltage-source energy extraction converter and the independent H-bridge control converter to the primary winding of the dual-winding main transformer and the external secondary winding of the external energy extraction transformer respectively. The first mechanical switch is closed, the second mechanical switch is opened, and the third mechanical switch is opened to cut off the voltage-source energy extraction converter and the independent H-bridge control converter, thereby isolating the faulty part.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] 1. In the flexible distribution transformer topology and its control method for energy extraction from the high-voltage port of the present invention, the topology includes a dual-winding main transformer, an external energy extraction transformer, a voltage-source energy extraction converter, an independent H-bridge control converter, and an energy storage capacitor. The dual-winding main transformer includes a primary winding and a secondary winding, which are located on the primary side and the secondary side of the dual-winding main transformer respectively. The end of the primary winding is serially provided with an independent H-bridge control converter, and the output end of the secondary winding is parallely provided with an external energy extraction transformer, which is parallely connected to the voltage-source energy extraction converter. The energy storage capacitor is parallely connected between the voltage-source energy extraction converter and the independent H-bridge control converter. In the application of this design, the energy storage capacitor is charged and discharged by controlling the external energy extraction transformer and the voltage-source energy extraction converter to complete energy extraction and reactive power compensation on the secondary side. The output voltage of the independent H-bridge control converter is used to flexibly adjust the voltage on the primary side or the secondary side of the dual-winding main transformer, so that there is no need to design an internal energy extraction winding, effectively shortening the engineering construction period, reducing the volume of the main transformer, and at the same time not increasing the difficulty of transformer heat dissipation. Therefore, the present invention not only has a small cost but also a small volume.

[0032] 2. In the flexible distribution transformer topology and its control method for energy extraction from the high-voltage port of the present invention, the independent H-type full-bridge regulation converter can flexibly adjust the voltage of each winding, avoiding the disadvantages of mechanical step voltage regulation of electromagnetic transformers. At the same time, the voltage-source energy extraction converter is used to achieve comprehensive power quality control objectives such as power flow regulation, making this topology have the advantages of high reliability of electromagnetic transformers and continuous and flexible voltage regulation performance of power electronic transformers, and the grid connection capacity accounts for a relatively small proportion, which is beneficial to improving the operation stability and robustness of the clean new power system. Therefore, the present invention not only has good stability but also good robustness. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is the single-phase simplified topology diagram of the flexible distribution transformer of the present invention.

[0034] Figure 2 is the three-phase topology diagram of the flexible distribution transformer of the present invention.

[0035] Figure 3 is the schematic diagram of the switching energy extraction circuit of the present invention.

[0036] Figure 4 is the simplified equivalent circuit diagram of the flexible distribution transformer of the present invention.

[0037] Figure 5 is the electrical phasor diagram of the flexible distribution transformer of the present invention.

[0038] Figure 6 is the electrical phasor diagram of the output equivalent impedance of the regulation converter and the line equivalent impedance of the present invention.

[0039] In the figure: dual-winding main transformer 1, primary winding 11, secondary winding 12, load terminal 13, first mechanical switch 14, second mechanical switch 15, third mechanical switch 16, external energy extraction transformer 2, primary winding 21, external secondary winding 22, voltage-source energy extraction converter 3, L-type filter 31, independent H-type full-bridge regulation converter 4, LC-type filter 41, energy storage capacitor 5. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0040] The present invention will be further described in detail below with reference to the drawings and specific embodiments.

[0041] Embodiment 1:

[0042] See Figures 1-6 , a flexible distribution transformer topology for energy extraction from the high-voltage port, including a dual-winding main transformer 1, an external energy extraction transformer 2, a voltage-source energy extraction converter 3, an independent H-type full-bridge regulation converter 4, and an energy storage capacitor 5;

[0043] The double-winding main transformer 1 includes a primary winding 11 and a secondary winding 12. The primary winding 11 is located on the primary side of the double-winding main transformer 1, and the secondary winding 12 is located on the secondary side of the double-winding main transformer 1;

[0044] Preferably, the double-winding main transformer 1 adopts a Y N d11 connection, and the external energy-taking transformer 2 adopts a Δ / Y connection;

[0045] An independent H-bridge control converter 4 is serially arranged at the end of the primary winding 11. The output end of the secondary winding 12 is parallely provided with an external energy-taking transformer 2, and the external energy-taking transformer 2 is parallely connected with a voltage-source energy-taking converter 3; the energy storage capacitor 5 is parallely connected between the voltage-source energy-taking converter 3 and the independent H-bridge control converter 4.

[0046] In application, the output end of the independent H-bridge control converter 4 is serially connected to the end of the primary winding 11 on the primary side of the double-winding main transformer 1, and the Δ / Y-connected external energy-taking transformer 2 is parallely connected to the high-voltage output port of the secondary winding 12 on the secondary side of the double-winding main transformer 1. At the same time, the external energy-taking transformer 2 is parallely connected with the voltage-source energy-taking converter 3. By controlling the external energy-taking transformer 2 and the voltage-source energy-taking converter 3, the energy storage capacitor 5 is charged and discharged to complete energy taking, reactive power compensation on the secondary side, etc. By controlling and regulating the output voltage of the independent H-bridge control converter 4, the voltage on the primary side or secondary side of the double-winding main transformer 1 can be flexibly regulated;

[0047] The external energy-taking transformer 2 enables the elimination of the need to design an internal energy-taking winding, shortens the engineering construction period, reduces the initial R & D investment cost, has a low overall cost, reduces the volume of the main transformer, does not increase the heat dissipation difficulty of the transformer, and can flexibly regulate the voltage of each winding by using the independent H-bridge control converter 4, avoiding the disadvantages of mechanical sectional voltage regulation of electromagnetic transformers. The voltage-source energy-taking converter 3 is used to achieve comprehensive power quality management objectives such as power flow regulation. The topology combines the high reliability of electromagnetic transformers and the continuous and flexible voltage regulation performance of power electronic transformers, and has fast voltage regulation action and high voltage regulation accuracy. Only the converter part with a relatively small capacity ratio greatly increases the flexibility of the external output characteristics of the transformer, providing an effective technical means to solve problems such as voltage over-limitation and line power flow regulation that limit the growth of new energy grid connection capacity.

[0048] See Figure 1 As shown, the external energy-taking transformer 2 includes an external primary winding 21 and an external secondary winding 22. The external primary winding 21 is located on the primary side of the external energy-taking transformer 2, and the external secondary winding 22 is located on the secondary side of the external energy-taking transformer 2; the external primary winding 21 is connected to the secondary winding 12, and the external secondary winding 22 is connected to the voltage-source energy-taking converter 3;

[0049] In the application, the independent H-type full-bridge control converter 4 is connected to the primary winding 11 to transmit electric energy to the primary winding 11. After the voltage transformation between the primary and secondary sides, the secondary winding 12 transmits electric energy to the load side and the external primary winding 21 of the external energy-taking transformer 2. After the electric energy is transformed by the external energy-taking transformer 2, it is transmitted to the external secondary winding 22. The secondary winding 22 is connected in parallel with the voltage-source energy-taking converter 3, and the voltage-source energy-taking converter 3 is connected in parallel with the energy storage capacitor 5 to provide a path for the bidirectional flow of current between the primary side of the external energy-taking transformer 2 and the energy storage capacitor 5.

[0050] The commutation device composed of the voltage-source energy-taking converter 3 and the independent H-type full-bridge control converter 4 is connected to the end of the primary winding 11 of the double-winding main transformer 1 and the head of the external secondary winding 22 of the external energy-taking transformer 2 at both ends. By changing the SPWM modulation voltage signal of the independent H-type full-bridge control converter 4, and then changing the PWM signal for triggering the fully controlled power device, the amplitude and phase of the series output AC voltage between the end of the primary winding 11 and the AC power supply ground point are controllable. Thus, the amplitude and phase of the voltage at both ends of the primary winding 11 of the double-winding main transformer 1 are also controllable. Due to the voltage transformation effect between the primary and secondary sides, the amplitude and phase of the voltage at both ends of the secondary winding 12 are correspondingly changed, realizing that the amplitude of the voltage of both windings on both sides of the double-winding main transformer 1 can be continuously adjusted and the phase can be flexibly changed, making this topology have strong voltage regulation performance.

[0051] Among them, the voltage regulation control logic of the independent H-type full-bridge control converter 4 is as follows:

[0052] The controller of the independent H-type full-bridge control converter 4 subtracts the actual series output AC voltage value from the corresponding reference command value to obtain the first deviation value. Under the action of the voltage outer-loop PI controller, the control reference command value required for the current inner-loop control is obtained. Then, the control reference command value is subtracted from the corresponding actual voltage value and combined with the feed-forward decoupling term to obtain the output modulation voltage signal. Then, the modulation voltage signal passes through the single-phase Park inverse transformation and the PWM trigger signal generator module to generate the PWM trigger control signal. Finally, the PWM trigger control signal acts on the IGBT of the independent H-type full-bridge control converter 4, thereby changing the series output AC voltage, so that the voltages of the primary side winding 11 and the secondary side winding 12 of the double-winding main transformer 1 can quickly and accurately track the corresponding reference command values, completing the goal of flexible voltage regulation.

[0053] A first mechanical switch 14 is provided on the primary winding 11, a second mechanical switch 15 is provided on the line between the primary winding 11 and the independent H-type full-bridge control converter 4, and a third mechanical switch 16 is provided on the line between the external secondary winding 22 and the voltage-source energy-taking converter 3;

[0054] The fault regulation logic is as follows:

[0055] When the current sensor detects a fault current that is 2 to 3 times higher than the rated current, the fault current flows through the voltage-source energy-taking converter 3 and the independent H-bridge regulation converter 4 to the primary winding 11 of the dual-winding main transformer 1 and the external secondary winding 22 of the external energy-taking transformer 2 respectively. The physical signal is transmitted to the DSP controller through the AD conversion module. Under the action of the DSP fault protection program, the first mechanical switch 14 is controlled to close, the second mechanical switch 15 is controlled to open, and the third mechanical switch 16 is controlled to open, so as to cut off the voltage-source energy-taking converter 3 and the independent H-bridge regulation converter 4, isolate the faulty converter part, and realize the basic operation of the flexible distribution transformer through the working electromagnetic transformer, ensuring the safety and stability margin of the system.

[0056] See Figure 2 , the input end of the voltage-source energy-taking converter 3 is connected to the external secondary winding 22 of the external energy-taking transformer 2, and the output end is connected in parallel with the energy storage capacitor 5. The external energy-taking transformer 2 and the voltage-source energy-taking converter 3 are used to realize bidirectional power flow, charge and discharge the energy storage capacitor 5, and use the DC voltage control circuit to ensure the stability of the DC voltage across the energy storage capacitor 5, so that the voltage of the energy storage capacitor 5 first rises and then falls during charging and first falls and then rises during discharging, and can quickly recover to the set DC voltage value, with good voltage stabilization control effect.

[0057] The input end of the independent H-bridge regulation converter 4 is connected in parallel with the energy storage capacitor 5, and the output ends of each H-bridge are respectively connected in series to the ends of the ABC-phase primary windings 11 of the dual-winding main transformer 1. The independent H-bridge regulation converter 4 is used to invert the DC voltage across the energy storage capacitor 5 into three single-phase AC voltages, making the amplitude and phase of the single-phase AC voltage controllable, and then transmitting the AC voltage to the ends of the three-phase primary windings 11 of the dual-winding main transformer 1.

[0058] The output end of the independent H-bridge regulation converter 4 is connected in series between the end of the Y-connected primary winding 11 and the ground of the AC power supply, and the potential of the output end is relatively low with respect to the ground potential. There is no need to configure a converter with a high insulation level, effectively reducing the insulation investment. At the same time, the first mechanical switch 14 and the second mechanical switch 15 are connected in series and parallel, so that the independent H-bridge regulation converter 4 can be actively cut off during a fault. This wiring method can also be applied to the medium and low voltage distribution links of the three-phase four-wire system, and the application scenarios are relatively flexible.

[0059] The energy storage capacitor 5 can provide a feeding energy output port for distributed new energy power generation units such as photovoltaic power stations, hydrogen energy power stations, wind farms, and energy storage power stations. Only a power conversion device is required to achieve energy supply output. At the same time, it can also supply power to low-capacity low-voltage loads in the adjacent area, such as the daily power consumption load in medium and high-voltage substations, reducing the number of feeders in the medium and high-voltage substations, making the topological operation mode more diverse, and facilitating the realization of diversified reception of clean electric energy.

[0060] The three-phase topological structure is the same as the simple topological principle. When the phases of the three single-phase AC voltages are in phase with the corresponding ABC three-phase voltage phases at both ends of the primary winding 11, by adjusting the same amplitude change amount of the three single-phase AC voltages, the same voltage amplitude regulation amount at both ends of the primary winding 11 and the secondary winding 12 is achieved, realizing a simple winding voltage amplitude regulation function. By separately controlling the amplitudes and phases of the three single-phase AC voltages output, the elimination of the unbalanced voltage on the primary winding 11 side can be achieved, ensuring that the phases of the three-phase voltages of the primary winding 11 are 120° out of phase with each other, and reducing the content of high-order harmonic voltages, realizing an advanced voltage quality governance function. In addition, many drawbacks of the tap step voltage regulation of electromagnetic transformers are avoided.

[0061] The voltage setting of the external secondary winding 22 of the external energy-taking transformer 2 is affected by the DC voltage given value at both ends of the energy storage capacitor 5, the capacity of the voltage-source energy-taking converter 3, and the characteristics of the fully controlled devices used. The voltage of the external secondary winding 22 is about 0.43 - 0.57 times the DC voltage given value.

[0062] When the topology performs reactive power compensation, the working state of the voltage-source energy-taking converter 3 is changed from rectification to inversion, and by changing the amplitude and phase of the inverted output current, and then through the voltage transformation of the external energy-taking transformer 2, the amplitude and phase of the current output from the external energy-taking transformer 2 to the secondary side of the double-winding main transformer 1 can be adjusted, that is, the injected reactive power is changed, realizing the compensation of reactive power on the secondary side.

[0063] The specific control logic of reactive power compensation is as follows:

[0064] The reactive power injected by the voltage-source energy-taking converter 3 is equal to the voltage on the secondary side of the double-winding main transformer 1 multiplied by the output current of the voltage-source energy-taking converter 3. When the DSP controller of the voltage-source energy-taking converter 3 receives the reactive power compensation instruction sent by the control center, through the action of the DSP reactive power compensation control program, the output current of the voltage-source energy-taking converter 3 is increased or decreased, so that the actual output reactive power tracks the received reactive power compensation instruction.

[0065] When the reactive power compensation mode is selected, synchronously performing voltage regulation will reduce the reactive power compensation ability. The maximum reactive power compensation capacity of the voltage source energy extraction converter 3 is determined by the maximum output voltage and current amplitude of the voltage source energy extraction converter 3.

[0066] When the topology performs power flow regulation, by changing the AC voltage output by the independent H-type full-bridge regulation converter 4, the output equivalent impedance is changed, thereby changing the amplitude and phase angle of the line equivalent impedance, and further changing the active power and reactive power on the line to achieve the power flow regulation goal of the line. Among them, the line equivalent impedance is equal to the sum of the power source impedance, the line impedance, and the output equivalent impedance of the independent H-type full-bridge regulation converter 4. Similarly, the power flow regulation ability is also affected by voltage regulation and reactive power compensation mode. When the amplitude of the line equivalent impedance is smaller, the power flow regulation ability of the topology line is stronger.

[0067] The specific control logic of power flow regulation is as follows:

[0068] The equivalent impedance output by the independent H-type full-bridge regulation converter 4 is equal to the AC voltage output in series by the independent H-type full-bridge regulation converter 4 divided by the current of the primary winding 11. When the DSP controller of the independent H-type full-bridge regulation converter 4 receives the power flow regulation instruction sent by the regulation center, under the action of the DSP power flow regulation program in the controller of the independent H-type full-bridge regulation converter 4, the AC voltage output in series by the independent H-type full-bridge regulation converter 4 is increased or decreased. At the same time, the equivalent impedance output by the independent H-type full-bridge regulation converter 4 increases or decreases, and then the equivalent impedance of the line is changed, so that the actual line power flow tracks the received power flow regulation instruction.

[0069] An L-type filter 31 is provided between the external secondary winding 22 and the voltage source energy extraction converter 3; an LC-type filter 41 is provided between the primary winding 11 and the independent H-type full-bridge regulation converter 4;

[0070] In application, the voltage source energy extraction converter 3 uses the L-type filter 31 for filtering, which can reduce the harmonic content in the three-phase AC current output to the external secondary winding 22. The independent H-type full-bridge regulation converter 4 uses the LC-type filter 41 for filtering, which can reduce the harmonic content of the output single-phase AC voltage.

[0071] The voltage source energy extraction converter 3 utilizes the controllability of the amplitude and phase of the output current inverted, so that the amplitude and phase of the current output by the external energy extraction transformer 2 to the secondary side of the double-winding main transformer 1 are also controllable, thereby realizing the current compensation for unbalanced loads, having the comprehensive power quality governance ability, and reducing the adverse effects of unbalanced loads on the new power system.

[0072] See Figure 3, the energy extraction effect achieved through the cooperation of the dual-winding main transformer 1 and the external energy extraction transformer 2 is the same as that of the internal energy extraction transformer, which does not affect the effect of bidirectional power flow. Moreover, the external primary winding 21 of the external energy extraction transformer 2 can be extended to extract energy from the new energy grid-connected unit, realizing flexible switching of the energy extraction method, increasing the new energy consumption, and providing a new technical means for increasing the new energy grid-connected capacity.

[0073] Embodiment 2:

[0074] Refer to Figure 4 . For the convenience of analysis, without considering the switching losses and on-off conditions of the fully controlled power devices in the converter and the leakage reactance of the electromagnetic transformer, etc., the model is simplified. The voltage-source energy extraction converter 3 is equivalent to an ideal controlled current source to stabilize the DC voltage across the energy storage capacitor 5 and achieve bidirectional power flow. The independent H-bridge full-bridge control converter 4 is equivalent to an ideal controlled voltage source to invert three single-phase AC voltages to participate in the voltage regulation of the primary and secondary sides of the dual-winding main transformer 1.

[0075] According to the electromagnetic induction law, the magnetic flux conservation law, and Kirchhoff's current law, the relationship between the electrical quantities of the dual-winding main transformer 1 and the external energy extraction transformer can be derived. The relationship between the electrical quantities is as follows:

[0076]

[0077] Where: is the voltage across both ends of the primary winding 11, is the voltage across both ends of the secondary winding 12, is the voltage across both ends of the external secondary winding 22, is the current on the primary winding 11, is the current on the secondary winding 12, I fe is the ideal output current of the voltage-source energy extraction converter 3, n fe is the voltage transformation ratio between the external primary winding 21 and the external secondary winding 22;

[0078] Refer to Figure 5 for the electrical phasor diagram. Set as the reference axis direction, and let Thus, the relationship between the electrical quantities in the topology is derived, laying a foundation for analyzing the working principle of the topology below; where: U ac is the ideal output voltage of the independent H-bridge full-bridge control converter 4, is the ideal output current of the voltage-source energy extraction converter 3.

[0079] From Figure 5The active power absorbed by the voltage-source energy-taking converter 3 and the independent H-type full-bridge control converter 4 from the AC system can be deduced as follows:

[0080] P fe =U fe I fe cos(θ fe );

[0081] P ac =U ac I1cos(θ ac +θ1);

[0082] Where: θ ac is the phase of the output voltage of the independent H-type full-bridge control converter 4, θ1 is the phase of the current in the primary winding 11, and θ fe is the phase of the output current of the voltage-source energy-taking converter 3;

[0083] When there is no external new energy power station and energy storage power station supplying power to the energy storage capacitor 5, in the initial operation stage, the inner voltage-source energy-taking converter 3 and the independent H-type full-bridge control converter 4 need to satisfy the following power balance equation:

[0084] U ac I1cos(θ ac +θ1)+U fe I fe cos(θ fe )=0;

[0085] When θ ac +θ1>90°, at this time, the independent H-type full-bridge control converter 4 delivers electric energy to the AC system to participate in voltage regulation, and realizes functions such as voltage regulation, reactive power compensation, and power flow regulation by changing the output voltage of the independent H-type full-bridge control converter 4 and the output current of the voltage-source energy-taking converter 3.

[0086] As Figure 5 shown, the actual voltage of the primary side bus of the double-winding main transformer 1 can be expressed by the following formula:

[0087]

[0088] Where: is the actual voltage, is the AC voltage at both ends of the primary winding 11, and U ac is the ideal output voltage of the independent H-type full-bridge control converter 4;

[0089] When is fixed, by changing the amplitude and phase of the output voltage of the independent H-type full-bridge control converter 4, can be changed.The amplitude and phase are used to control the primary side bus voltage, reduce the harmonic voltage and unbalanced voltage content;

[0090] When is fixed, by changing the amplitude and phase, the amplitude of can be increased or decreased. Through the voltage transformation of the dual-winding main transformer 1, the voltage amplitude at both ends of the secondary winding 12 is increased or decreased, completing the regulation of the secondary side bus voltage, and realizing the rapid and continuous tracking of the regulation command for the voltages on the primary winding 11 and secondary winding 12 sides of the dual-winding main transformer 1.

[0091] The voltage regulation ability of the said topology is as follows:

[0092] Define the actual voltage transformation ratios of the primary and secondary sides of the dual-winding main transformer 1 as and use the electrical quantity relationship formula and the above formula to replace the variables in p to obtain the transformation expression of the voltage transformation ratio n p as follows:

[0093]

[0094] Then let the output voltage of the independent H-bridge control converter 4 and the amplitude ratio of the actual primary side bus voltage be c, and substitute it into the above formula to get:

[0095]

[0096] It can be seen from the above formula that when the amplitude of is fixed, by changing the amplitude and phase of , the voltage transformation ratio n p can be changed to regulate the voltages on the primary side and secondary side of the dual-winding main transformer 1; at the same time, the upper and lower voltage regulation limits are determined by the amplitude and phase of . The larger the maximum output amplitude, the larger the voltage regulation limit range, that is, the stronger the voltage regulation performance. The larger the maximum output amplitude, the larger the voltage regulation limit range, that is, the stronger the voltage regulation performance.

[0097] The reactive power compensation ability of the said topology is as follows:

[0098] According to this topology and Figure 5 it can be known that the voltage source energy extraction converter 3 and the independent H-bridge control converter 4 can inject reactive power into the primary side and secondary side of the dual-winding main transformer 1 respectively to complete the compensation task. The total injected reactive power is shown in the following formula:

[0099]

[0100] It can be seen from the total reactive power formula that by changing the output current I of the voltage source energy extraction converter 3fe and the output voltage U of the independent H-type full-bridge control converter 4 ac the total reactive power injected into the AC system can be modified, but the injected Q in needs to satisfy the power balance equation, and the ideal maximum value is shown in the following formula:

[0101] Q in,max = U fe · I fe + U ac · I1;

[0102] The output current I of the voltage-source energy extraction converter 3 fe has the following relationship with the amplitude of the current I1 on the primary winding 11:

[0103]

[0104] Substituting the amplitude ratio c and the above formula into the ideal maximum value of the total reactive power amplitude, we can get:

[0105] Q in,max = (c + d)U1· I1;

[0106] From the above formula, it can be seen that the amplitude Q of the injected total reactive power in，max is determined by the output voltage assignment U of the independent H-type full-bridge control converter 4 ac，max and the output current assignment I of the voltage-source energy extraction converter 3 fe，max When U ac，max and I fe,max are larger, the maximum reactive power compensation capacity of the topology is larger.

[0107] In application, when the topology performs power flow control, it can operate in a constant impedance mode or a constant power mode;

[0108] When operating in the constant impedance mode, by regulating the output voltage of the independent H-type full-bridge control converter 4, the output voltage of the independent H-type full-bridge control converter 4 and the output current of the voltage-source energy extraction converter 3 form a constant impedance, so as to realize the power flow control on the primary side of the double-winding main transformer 1;

[0109] When operating in the constant power mode, by regulating the output voltage of the independent H-type full-bridge control converter 4 and the output current of the voltage-source energy extraction converter 3, the power flow power values on the primary side and the secondary side of the double-winding main transformer 1 are made constant.

[0110] When the power flow control operates in the constant impedance mode, the equivalent output impedance of the independent H-type full-bridge control converter 4 is shown in the following formula:

[0111]

[0112] and based on Figure 4 , it can be known that the current I1 on the primary winding 11 is as shown in the following formula:

[0113]

[0114] And through the above two formulas, the transformation formula of the current I1 can be deduced as shown in the following formula:

[0115]

[0116] Let the equivalent impedance and voltage difference of the line where the topology is located be Z eq and Then the following relationship exists:

[0117]

[0118] From the above formula, the phasor diagram of the equivalent impedance of the line where the topology is located can be known, as Figure 6 shown.

[0119] Through the above formula and the phasor diagram of the equivalent impedance, it can be known that by adjusting the amplitude and phase of the voltage , the amplitude and phase angle of the impedance Z ac can be changed, so that the amplitude and phase angle of Z eq change, so that the line power flow P line and P line are controllable, and the power flow control index sent by the upper level can be completed; the expressions of the P line and Q line are as follows:

[0120]

[0121] In summary, it can be known that when the amplitude of Z eq decreases, the active power P line and reactive power Q line of the line will increase, and at the same time, the change of the phase angle θ eq will also change P line and Q line . When the topology works in the constant impedance mode, by changing the amplitude and phase of the output voltage of the voltage source type energy extraction converter 3, the power flow of the primary side line of the double-winding transformer 1 can be controlled, and the flexible control and dynamic balance of the system power flow can be realized.

[0122] Although the embodiments of the present invention have been shown and described above, it should be understood that the above embodiments are exemplary and cannot be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A control method for a flexible distribution transformer topology that draws energy from a high voltage port, characterized in that: The flexible distribution transformer topology for taking energy from a high-voltage port comprises a dual-winding main transformer (1), an external energy-taking transformer (2), a voltage source energy-taking converter (3), an independent H-type full-bridge control converter (4) and an energy storage capacitor (5); The dual-winding main transformer (1) comprises a primary winding (11) and a secondary winding (12), wherein the primary winding (11) is located on the primary side of the dual-winding main transformer (1), and the secondary winding (12) is located on the secondary side of the dual-winding main transformer (1); An independent H-type full-bridge control converter (4) is arranged in series at the end of the primary winding (11); an external energy-taking transformer (2) is arranged in parallel at the output end of the secondary winding (12); the external energy-taking transformer (2) is connected in parallel with the voltage source type energy-taking converter (3); the energy storage capacitor (5) is connected in parallel between the voltage source type energy-taking converter (3) and the independent H-type full-bridge control converter (4); The external energy transformer (2) comprises an external primary winding (21) and an external secondary winding (22), wherein the external primary winding (21) is located on the primary side of the external energy transformer (2), and the external secondary winding (22) is located on the secondary side of the external energy transformer (2); A first mechanical switch (14) is provided on the primary winding (11), a second mechanical switch (15) is provided on the line between the primary winding (11) and the independent H-type full-bridge control converter (4), and a third mechanical switch (16) is provided on the line between the external secondary winding (22) and the voltage source type energy-taking converter (3); The control method of the flexible distribution transformer topology for taking energy from a high-voltage port comprises: When the topology is in the initial operation stage, the voltage source type energy-taking converter (3) and the independent H-type full-bridge control converter (4) need to satisfy the following power balance equation: U ac I1cos(θ ac +θ1)+U fe I fe cos(θ fe )=0; Among them: U ac is the ideal output voltage of the independent H-type full-bridge control converter (4), I1 is the current of the primary winding (11), θ ac is the phase of the output voltage of the independent H-type full-bridge control converter (4), θ1 is the phase of the current of the primary winding (11), U fe is the voltage across the external secondary winding (22), I fe is the ideal output current of the voltage source energy converter (3), θ fe The phase of the output current of the voltage source type energy-taking converter (3); When θ ac When +θ1>90°, the independent H-type full-bridge control converter (4) transmits electric energy to complete the voltage regulation task; When the topology performs voltage regulation, the AC voltage at both ends of the primary winding (11) is fixed, and the amplitude and phase of the output voltage of the independent H-type full-bridge control converter (4) are changed to change the amplitude and phase of the actual bus voltage on the primary side of the double-winding main transformer (1), thereby achieving control of the primary side bus voltage; When the actual bus voltage on the primary side of the dual-winding main transformer (1) is fixed, the amplitude and phase of the output voltage of the independent H-type full-bridge control converter (4) are changed to achieve the increase or decrease of the amplitude of the AC voltage at both ends of the primary winding (11), and after the voltage conversion of the dual-winding main transformer (1), the amplitude of the AC voltage at both ends of the secondary winding (12) is increased or decreased to complete the control of the secondary side bus voltage, thereby achieving accurate tracking and regulation of the primary and secondary side voltages of the dual-winding main transformer (1).

2. The control method of a flexible distribution transformer topology for taking energy from a high voltage port according to claim 1 is characterized in that: The primary winding (11) is connected in parallel with an AC system power supply (17), and the independent H-type full-bridge control converter (4) is connected in series between the end of the primary winding (11) and the grounding terminal of the AC system power supply (17); The external primary winding (21) is connected to the secondary winding (12), and the external secondary winding (22) is connected to the voltage source type energy-taking converter (3).

3. The control method of a flexible distribution transformer topology for taking energy from a high voltage port according to claim 1 is characterized in that: An L-type filter (31) is provided between the external secondary winding (22) and the voltage source type energy-taking converter (3); and an LC-type filter (41) is provided between the primary winding (11) and the independent H-type full-bridge control converter (4).

4. The control method of a flexible distribution transformer topology for taking energy from a high voltage port according to claim 1 is characterized in that: The secondary winding (12) is connected in series with a load terminal (13).

5. The control method of a flexible distribution transformer topology for taking energy from a high-voltage port according to claim 1 is characterized in that: When the topology performs reactive power compensation, the total reactive power injected is changed by changing the output current of the voltage source energy-taking converter (3) and the output voltage of the independent H-type full-bridge control converter (4). At the same time, the total reactive power must follow the power balance equation, and its corresponding ideal maximum value is as follows: Q in，max =U fe ·I fe +U ac ·I1; Where: Q in，maX is the ideal maximum value, U fe is the AC voltage across the external secondary winding (22).

6. The control method of a flexible distribution transformer topology for taking energy from a high voltage port according to claim 1, characterized in that: When the topology performs power flow control, the topology may operate in a constant impedance mode or a constant power mode; When operating in a constant impedance mode, the output voltage of the independent H-type full-bridge control converter (4) is regulated so that the output voltage of the independent H-type full-bridge control converter (4) and the output current of the voltage source type energy-taking converter (3) form a constant impedance, thereby realizing power flow regulation on the primary side of the dual-winding main transformer (1); When operating in a constant power mode, the output voltage of the independent H-type full-bridge control converter (4) and the output current of the voltage source type energy-taking converter (3) are regulated, thereby achieving constant power flow values ​​on the primary side and the secondary side of the dual-winding main transformer (1).

7. The control method of a flexible distribution transformer topology for taking energy from a high voltage port according to claim 1, characterized in that: When the topology performs voltage regulation, the independent H-type full-bridge control converter (4) firstly makes a difference between the AC voltage value of the actual series output and the corresponding reference command value to obtain a first deviation value, and obtains a control reference command value based on the first deviation value, and then makes a difference between the control reference command value and the actual voltage value to obtain a second deviation value, and combines the second deviation value with the feedforward decoupling term to obtain an output modulation voltage signal, and then obtains a PMW trigger control signal based on the modulation voltage signal, and finally applies the PMW trigger control signal to the independent H-type full-bridge control converter (4) to complete voltage regulation.

8. The control method of a flexible distribution transformer topology for taking energy from a high voltage port according to claim 1, characterized in that: When a fault current higher than the rated current occurs in the topology, the fault current flows through the voltage source type energy-taking converter (3) and the independent H-type full-bridge control converter (4) to the primary winding (11) of the double-winding main transformer (1) and the external secondary winding (22) of the external energy-taking transformer (2), respectively. The first mechanical switch (14) is closed, the second mechanical switch (15) is opened, and the third mechanical switch (16) is opened, so that the voltage source type energy-taking converter (3) and the independent H-type full-bridge control converter (4) are cut off, thereby isolating the faulty part.

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