A control method of a hybrid distribution transformer and related devices

Abstract

This invention discloses a control method and related apparatus for a hybrid distribution transformer, relating to the field of flexible power distribution equipment and its control technology. The control method is applied to a hybrid distribution transformer system including a balancing bridge arm. The control method includes: real-time acquisition of electrical quantity signals from the hybrid distribution transformer system; generating a first PWM drive signal based on the electrical quantity signals to control the operation of the switching transistors in the parallel converter, thereby driving the parallel converter to inject compensation current into the line; generating a second PWM drive signal based on the electrical quantity signals to control the operation of the switching transistors in the series converter, thereby driving the series converter to inject compensation voltage into the line through the series transformer; and generating a third PWM drive signal based on the electrical quantity signals to control the operation of the switching transistors in the balancing bridge arm, thereby driving the balancing bridge arm to adjust the neutral point current. This invention solves the problems of existing technologies requiring large-capacity capacitors, incurring additional losses and electromagnetic compatibility issues, and exhibiting three-phase imbalance.

Description

Technical Field

[0001] This invention relates to the field of flexible power distribution equipment and its control technology, and in particular to a control method and related device for a hybrid power distribution transformer. Background Technology

[0002] With the large-scale integration of distributed renewable energy, microgrids, and various nonlinear and unbalanced power electronic loads, power quality issues in low-voltage distribution networks, especially three-phase four-wire systems (a three-phase AC distribution system consisting of three phase lines and one neutral line, usually grounded, primarily used in low-voltage distribution networks, capable of simultaneously providing three-phase power and single-phase lighting power), are becoming increasingly prominent. Three-phase imbalance (referring to the asymmetry in amplitude and phase of three-phase voltage or current in a three-phase AC system, specifically manifested as unequal voltage / current magnitudes and phase differences deviating from 120°) can lead to additional losses in transformers and lines, excessive zero-sequence current in the neutral line, voltage deviations, and equipment overheating, severely impacting power supply reliability and power quality.

[0003] Hybrid Power Electronic Transformer (HPET), a novel device that combines the isolation characteristics of traditional transformers with the flexible control capabilities of power electronic converters, has become a research hotspot due to its ability to comprehensively compensate for power quality issues. In practical three-phase four-wire HPET systems, three-level neutral-point clamped converters have become the mainstream solution due to their advantages such as good waveform quality and moderate voltage stress. Common typical topologies include split DC bus and four-arm structures. While the split DC bus structure is simple, it requires a large-capacity capacitor to suppress neutral-point fluctuations; the four-arm structure improves neutral-point balance but introduces additional losses and electromagnetic compatibility issues; furthermore, neither the split DC bus nor the four-arm structure can effectively solve the problem of three-phase imbalance. Summary of the Invention

[0004] This invention provides a control method and related device for a hybrid distribution transformer, which solves the problems of existing technologies requiring large-capacity capacitors, having additional losses and electromagnetic compatibility issues, and having three-phase imbalance.

[0005] In view of this, the first aspect of the present invention provides a control method for a hybrid distribution transformer, the control method being applied to a hybrid distribution transformer system including a balancing bridge arm, the control method comprising:

[0006] Real-time acquisition of electrical quantity signals of the hybrid distribution transformer system, including grid-side three-phase voltage, load-side three-phase voltage and load-side three-phase current, DC-side upper and lower capacitor voltages, three-phase inductor current of series converters, three-phase inductor current of parallel converters, and DC-side midpoint capacitor current.

[0007] Based on the three-phase current on the load side and the three-phase inductor current of the parallel converter, a first PWM drive signal is generated to control the operation of the switching transistors in the parallel converter, so as to drive the parallel converter to inject compensation current into the line.

[0008] Based on the grid-side three-phase voltage, the load-side three-phase voltage, and the three-phase inductor current of the series converter, a second PWM drive signal is generated to control the operation of the switching transistors in the series converter, so as to drive the series converter to inject compensation voltage into the line through the series transformer.

[0009] Based on the voltages of the upper and lower capacitors on the DC side and the current of the midpoint capacitor on the DC side, a third PWM drive signal is generated to control the operation of the balance bridge arm switch, thereby driving the balance bridge arm to adjust the midpoint current.

[0010] Optionally, the hybrid distribution transformer system includes: an isolation transformer, a series transformer, a T-type three-level back-to-back converter, and an independent balancing bridge arm;

[0011] The isolation transformer is used to connect the high-voltage power distribution network with the low-voltage three-phase four-wire system.

[0012] The T-type three-level back-to-back converter includes a parallel converter and a series converter; the AC side of the series converter is connected in series to the line of the low-voltage three-phase four-wire system through a series transformer; the AC side of the parallel converter is connected to the load side.

[0013] The balancing arm is connected between the positive and negative DC buses, and the connection point is connected to the midpoint of the DC side of the back-to-back converter through an inductor.

[0014] Optionally, each phase arm of the series converter and the parallel converter consists of four switching transistors forming a T-type three-level structure; the independent balancing arm consists of two switching transistors and one inductor.

[0015] Optionally, the DC side of the T-type three-level back-to-back converter is composed of two supporting capacitors connected in series; one end of the inductor in the balance bridge arm is connected to the connection point of the two switching transistors, and the other end is connected to the connection point of the two supporting capacitors, forming the midpoint of the DC side;

[0016] The DC side midpoint, the low-voltage side neutral point of the isolation transformer, one end of the primary side of the series transformer, and the load neutral point are interconnected.

[0017] Optionally, the step of generating a first PWM drive signal based on the three-phase current on the load side and the three-phase inductor current of the parallel converter to control the operation of the switching transistors in the parallel converter, so as to drive the parallel converter to inject compensation current into the line, includes:

[0018] The three-phase current on the load side and the three-phase inductor current of the parallel converter are subjected to Parker transformation to obtain their load current component and first inductor current component in the synchronous rotating dq0 coordinate system.

[0019] After comparing the total DC bus voltage with the reference value, the first PI regulator processes the data and outputs the d-axis active current command.

[0020] The load current component is combined with the d-axis active current command to obtain the first current reference command of the parallel converter in the dq0 coordinate system.

[0021] The first current reference command is compared with the value of the first inductor current component in the dq0 coordinate system to obtain the current error signal, and the current error signal is sent to the corresponding proportional-integral resonant controller for processing; the output of the proportional-integral resonant controller is subjected to Park inverse transformation to obtain the three-phase voltage modulation wave of the parallel converter;

[0022] The three-phase voltage modulation wave is compared with a triangular carrier wave to generate a first PWM drive signal.

[0023] Optionally, the step of generating a second PWM drive signal based on the grid-side three-phase voltage, the load-side three-phase voltage, and the three-phase inductor current of the series converter to control the operation of the switching transistors in the series converter, so as to drive the series converter to inject compensation voltage into the line through the series transformer, includes:

[0024] The grid-side three-phase voltage, the load-side three-phase voltage, and the three-phase inductor current of the series converter are subjected to Parker transformation to obtain their grid-side voltage component, load-side voltage component, and second inductor current component in the synchronous rotating dq0 coordinate system.

[0025] A reference value for the load-side voltage in the dq0 coordinate system is set, and the reference value is compared with the grid-side voltage component to obtain the voltage error signal;

[0026] After multiplying the voltage error signal by the series transformer turns ratio, it is compared with the dq0 axis component of the actual output voltage of the series converter obtained by Parker transformation. The comparison results are sent to the corresponding proportional-integral resonant controllers, and the output of the proportional-integral resonant controllers is used as the second current reference command of the series converter.

[0027] The second current reference command is compared with the second inductor current component to obtain a current error signal. The current error signal is then sent to the corresponding proportional controller for processing. The output of the proportional controller is subjected to Parker inverse transformation to obtain the three-phase voltage modulation wave of the series converter.

[0028] The three-phase voltage modulation wave is compared with a triangular carrier wave to generate a second PWM drive signal.

[0029] Optionally, the step of generating a third PWM drive signal to control the operation of the balanced bridge arm switching transistor based on the DC-side upper and lower capacitor voltages and the DC-side midpoint capacitor current, so as to drive the balanced bridge arm to adjust the midpoint current, includes:

[0030] Calculate the difference between the upper capacitor voltage and the lower capacitor voltage in the DC side upper and lower capacitor voltages;

[0031] The difference is compared with the zero reference value, and the resulting error signal is processed by the second PI regulator to output a reference command for the midpoint capacitor current.

[0032] The reference command for the midpoint capacitor current is compared with the DC side midpoint capacitor current, and the resulting current error signal is sent to the proportional controller for processing. The output of the proportional controller is used as the voltage modulation signal for the balanced bridge arm.

[0033] The voltage modulation signal is compared with a triangular carrier wave to generate a third PWM drive signal.

[0034] A second aspect of the present invention provides a control device for a hybrid distribution transformer, the control device being applied to a hybrid distribution transformer system including a balancing bridge arm, the control device comprising:

[0035] The acquisition module is used to acquire electrical quantity signals of the hybrid distribution transformer system in real time. The electrical quantity signals include grid-side three-phase voltage, load-side three-phase voltage and load-side three-phase current, DC-side upper and lower capacitor voltages, three-phase inductor current of series converters, three-phase inductor current of parallel converters, and DC-side midpoint capacitor current.

[0036] The first control module is used to generate a first PWM drive signal to control the operation of the switching transistors in the parallel converter based on the three-phase current on the load side and the three-phase inductor current of the parallel converter, so as to drive the parallel converter to inject compensation current into the line side.

[0037] The second control module is used to generate a second PWM drive signal to control the operation of the switching transistors in the series converter based on the grid-side three-phase voltage, the load-side three-phase voltage and the three-phase inductor current of the series converter, so as to drive the series converter to inject compensation voltage into the line through the series transformer.

[0038] The third control module is used to generate a third PWM drive signal to control the operation of the balance bridge arm switch based on the voltage of the upper and lower capacitors on the DC side and the current of the midpoint capacitor on the DC side, so as to drive the balance bridge arm to adjust the midpoint current.

[0039] A third aspect of the present invention provides a control device for a hybrid distribution transformer, the device comprising a processor and a memory:

[0040] The memory is used to store program code and transmit the program code to the processor;

[0041] The processor is configured to execute the steps of the control method for the hybrid distribution transformer as described in the first aspect above, according to the instructions in the program code.

[0042] A fourth aspect of the present invention provides a computer-readable storage medium for storing program code for executing the control method for a hybrid distribution transformer described in the first aspect above.

[0043] As can be seen from the above technical solutions, the present invention has the following advantages:

[0044] (1) The hybrid distribution transformer topology of the present invention has an independent zero-sequence current path (the balance bridge arm is connected to the midpoint of the two capacitors on the DC side via an inductor, and at the same time, the load neutral point, the midpoint of the DC side capacitor, the neutral point of the converter filter capacitor, and the left end of the primary side of the series transformer are all connected to the neutral point of the isolation transformer T1 to form a zero-sequence current flow path), which can simultaneously solve the problem of imbalance between grid voltage and load current.

[0045] (2) The control method of the present invention effectively suppresses the fluctuation of the midpoint potential while improving the power quality by controlling the independent balanced bridge arm; it reduces the demand for DC capacitor capacity, which is conducive to the miniaturization and high efficiency of the system.

[0046] In summary, the control method for a hybrid distribution transformer provided by this invention solves the problems of existing technologies, such as the need for large-capacity capacitors, additional losses, electromagnetic compatibility issues, and three-phase imbalance. Attached Figure Description

[0047] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0048] Figure 1 A flowchart illustrating a control method for a hybrid distribution transformer provided in an embodiment of the present invention;

[0049] Figure 2A schematic diagram of a hybrid distribution transformer system with a balanced bridge arm three-level back-to-back converter provided in an embodiment of the present invention;

[0050] Figure 3(a) is a control block diagram of a parallel converter provided in an embodiment of the present invention;

[0051] Figure 3(b) is a control block diagram of a series converter provided in an embodiment of the present invention;

[0052] Figure 3(c) is a block diagram of a balanced bridge arm control provided in an embodiment of the present invention;

[0053] Figure 4 A simulation waveform diagram showing that the grid-side unbalanced voltage is compensated after adopting the control method of the present invention, provided for an embodiment of the present invention;

[0054] Figure 5 A simulation waveform diagram showing that the unbalanced current on the load side is compensated after adopting the control method of the present invention, as provided in an embodiment of the present invention;

[0055] Figure 6(a) shows the simulated DC-side voltage waveform without the addition of balanced bridge arm control according to the embodiment of the present invention;

[0056] Figure 6(b) shows the simulated DC-side voltage waveform when the balanced bridge arm control is added according to the embodiment of the present invention;

[0057] Figure 7 This is a schematic diagram of the structure of a control device for a hybrid distribution transformer provided in an embodiment of the present invention. Detailed Implementation

[0058] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0059] Note: The dq0 coordinate system is a synchronous rotating coordinate system used in electrical engineering for analyzing and controlling three-phase AC systems. Its full name is the direct axis (d-axis) - quadrature axis (q-axis) - zero axis (0-axis) coordinate system. Its core principle is to use Park's transformation to convert time-varying AC electrical quantities (such as voltage and current) in the stationary three-phase abc coordinate system into DC quantities in the synchronous rotating coordinate system, thereby simplifying system modeling and control design.

[0060] Coordinate system structure and physical meaning: d-axis (direct axis): aligned with the axis of the rotating magnetic field, mainly reflecting the active component; q-axis (intersecting axis): perpendicular to the d-axis (leading the d-axis by 90° electrical degrees), mainly reflecting the reactive component; 0-axis (zero axis): used to describe the zero-sequence component in a three-phase system, usually zero in a symmetrical three-phase system.

[0061] Example 1:

[0062] Please see Figure 1 This invention provides a control method for a hybrid distribution transformer, applicable to a hybrid distribution transformer system including a balancing bridge arm. The control method includes:

[0063] Step 101: Real-time acquisition of electrical quantity signals of the hybrid distribution transformer system. The electrical quantity signals include grid-side three-phase voltage, load-side three-phase voltage and load-side three-phase current, DC-side upper and lower capacitor voltages, three-phase inductor current of series converters, three-phase inductor current of parallel converters, and DC-side midpoint capacitor current.

[0064] It should be noted that during the operation of the hybrid distribution transformer system, the following signals are collected in real time through sensors: grid-side three-phase voltage V sa v sb v sc The three-phase voltage on the load side is v oa v ob v oc ; Load-side three-phase current i oa i ob i oc DC side upper and lower capacitor voltages v dc1 v dc2 The three-phase inductor current i of the series converter L1a i L1b i L1c The three-phase inductor current i of the parallel converter L2a i L2b i L2c DC side midpoint capacitor current i CN Furthermore, it also includes the total voltage v of the upper and lower capacitors on the DC side. dc The phase angle ɵ at the three-phase load grid connection point is described in the following corresponding embodiment.

[0065] Step 102: Based on the three-phase current on the load side and the three-phase inductor current of the parallel converter, generate the first PWM drive signal to control the operation of the switching transistors in the parallel converter, so as to drive the parallel converter to inject compensation current into the line.

[0066] It should be noted that the process of generating the first PWM drive signal based on the three-phase current on the load side and the three-phase inductor current of the parallel converter is specifically as follows: two types of current signals are acquired in real time, converted to the synchronous rotating dq0 coordinate system by Parker transformation, and the load current component and the inductor current component are obtained; then the total DC bus voltage is compared with the reference value, and the d-axis active current command is output through the PI regulator, which is combined with the load current component to form a current reference command; then the reference command is compared with the inductor current component to obtain the error, which is processed by the proportional-integral resonant controller and converted into a three-phase voltage modulation wave by Parker inverse transformation, and finally compared with the triangular carrier wave to generate a drive signal to control the operation of the switching transistors of the parallel converter. The above process is specifically described in the corresponding embodiment below. Step 102 achieves accurate monitoring and control of load current and converter current through real-time acquisition and coordinate transformation, providing a data foundation for compensation current generation; adopting a PI regulator and proportional-integral resonant controller ensures stable DC bus voltage and zero steady-state error tracking of current commands, improving compensation accuracy; the generated PWM drive signal can drive the parallel converter to inject compensation current into the line, effectively offsetting the unbalanced current on the load side, improving the current quality on the grid side, and solving the three-phase imbalance problem.

[0067] Step 103: Based on the grid-side three-phase voltage, the load-side three-phase voltage, and the three-phase inductor current of the series converter, generate a second PWM drive signal to control the operation of the switching transistors in the series converter, so as to drive the series converter to inject compensation voltage into the line through the series transformer.

[0068] It should be noted that the process of generating the second PWM drive signal based on the grid-side three-phase voltage, the load-side three-phase voltage, and the three-phase inductor current of the series converter specifically involves real-time acquisition of three types of electrical quantity signals, performing Parker transformation on them respectively to the synchronous rotating dq0 coordinate system to obtain the grid-side voltage component, the load-side voltage component, and the second inductor current component of the series converter; then, a reference value of the load-side voltage in the dq0 coordinate system is set and compared with the grid-side voltage component to obtain a voltage error signal. This error signal is multiplied by the series transformer turns ratio and compared with the actual output voltage component of the series converter. After processing by the proportional-integral resonant controller, a second current reference command is output; then, the second current reference command is compared with the second inductor current component to obtain a current error signal, which is processed by the proportional controller and inversely transformed by Parker into a three-phase voltage modulation wave. Finally, it is compared with a triangular carrier wave to generate a drive signal to control the operation of the series converter switching transistors. The specific details of the above process are shown in the corresponding embodiments below. Step 103 achieves accurate monitoring of grid voltage and converter current through coordinate transformation and multi-signal fusion, providing data support for compensation voltage generation; adopts dual closed-loop control of proportional-integral resonant controller and proportional controller to ensure rapid tracking of voltage compensation command and zero steady-state error adjustment, improving compensation response speed; the generated PWM drive signal can drive the series converter to inject compensation voltage into the line through the series transformer, effectively eliminating grid-side voltage imbalance and sag, ensuring three-phase symmetry of load-side voltage, and improving power supply reliability and power quality.

[0069] Step 104: Based on the DC side upper and lower capacitor voltages and the DC side midpoint capacitor current, generate a third PWM drive signal to control the operation of the balanced bridge arm switch, so as to drive the balanced bridge arm to adjust the midpoint current.

[0070] It should be noted that the process of generating the third PWM drive signal based on the DC-side upper and lower capacitor voltages and the DC-side midpoint capacitor current involves: calculating the difference between the DC-side upper and lower capacitor voltages, comparing this difference with a zero reference value, processing it through a second PI regulator, and outputting a reference command for the midpoint capacitor current; then comparing this reference command with the DC-side midpoint capacitor current, and sending the resulting current error signal to a proportional controller for processing, outputting a voltage modulation signal for the balanced bridge arm; finally, comparing the voltage modulation signal with a triangular carrier wave to generate the third PWM drive signal, which controls the operation of the balanced bridge arm switching transistor. The specific details of this process are described in the corresponding embodiment below. Step 104, through the dual-loop control of the PI regulator and the proportional controller, achieves real-time monitoring and adjustment of the DC-side upper and lower capacitor voltage difference, providing precise commands for midpoint current control; the generated PWM drive signal can drive the balanced bridge arm to adjust the midpoint current, effectively suppressing DC-side midpoint potential fluctuations and maintaining a balanced voltage between the upper and lower capacitors; reducing the demand on DC capacitor capacity, which is beneficial for system miniaturization and efficiency, and improving the stability and reliability of the hybrid distribution transformer system.

[0071] Example 2:

[0072] In one embodiment, the hybrid distribution transformer system in step 101 includes: an isolation transformer, a series transformer, a T-type three-level back-to-back converter, and an independent balancing arm; the isolation transformer is used to connect the high-voltage distribution network and the low-voltage three-phase four-wire system; the T-type three-level back-to-back converter includes a parallel converter and a series converter; the AC side of the series converter is connected in series to the line of the low-voltage three-phase four-wire system through the series transformer; the AC side of the parallel converter is connected to the load side; the balancing arm is connected between the positive and negative DC busbars, wherein the connection point is connected to the midpoint of the DC side of the back-to-back converter through an inductor.

[0073] In this system, each phase arm of the series converter and the parallel converter consists of four switching transistors forming a T-type three-level structure; the independent balancing arm consists of two switching transistors and one inductor; the DC side of the T-type three-level back-to-back converter is formed by two supporting capacitors connected in series; one end of the inductor in the balancing arm is connected to the connection point of the two switching transistors, and the other end is connected to the connection point of the two supporting capacitors, forming the DC side midpoint; the DC side midpoint, the neutral point of the low-voltage side of the isolation transformer, one end of the primary side of the series transformer, and the load neutral point are interconnected.

[0074] It should be noted that the hybrid distribution transformer system with a three-level back-to-back converter and a balanced bridge arm, such as... Figure 2 As shown, the high-voltage distribution network is isolated and stepped down by the delta-star connected isolation transformer T1 to obtain a low-voltage three-phase four-wire system. Its output side (star side) is connected to one end of the secondary side of three independent series transformers T2. The other end of the secondary side of the series transformer T2 is directly connected to the left side of the three-phase load. The AC output terminal of the parallel converter (right-side converter) of the back-to-back converters is connected to the left side of the three-phase load through the filter inductor L2, and the AC output terminal of the series converter (left-side converter) is connected to both ends of the primary side of the series transformer T2 through the filter inductor L1 and the filter capacitor C1. Each phase arm of the series converter consists of four switching transistors (S... a1 –S a4 S b1 –S b4 S c1 –S c4 The parallel converter forms a T-type structure; each phase arm of the parallel converter consists of four switching transistors (Q). a1 –Q a4 Q b1 –Q b4 Q c1 –Q c4 The structure is T-shaped; the balance arm is composed of inductor L. N It consists of two switching transistors S1 and S2, and an inductor L. NOne end is connected to the connection points of two switching transistors S1 and S2, and the other end is connected to the connection points of two capacitors on the DC side. In addition, the load neutral point, the midpoint of the DC side capacitor, the neutral point of the converter filter capacitor, and the left end of the primary side of the series transformer are all connected to the neutral point of transformer T1.

[0075] Depend on Figure 2 According to Kirchhoff's laws:

[0076] (1)

[0077] (2)

[0078] The series converter branch injects a controllable compensation voltage Δv into the line through a coupling transformer. a Δv b Δv c To eliminate the three-phase voltage v on the grid side sa v sb v sc To mitigate voltage dips and imbalances, ensure the three-phase voltage v on the load side. oa v ob v oc The three-phase balance is achieved; the parallel converter injects a controllable current Δi into the line. a , Δi b , Δi c To offset the three-phase current i on the load side oa i ob i oc The unbalanced components ensure the grid-side current i sa i sb i sc The three-phase equilibrium.

[0079] Note: Kirchhoff's laws are fundamental laws in circuit theory, including Kirchhoff's Current Law (KCL) and Kirchhoff's Voltage Law (KVL). Kirchhoff's Current Law states that at any node in a circuit, the sum of the currents flowing into that node equals the sum of the currents flowing out of that node, reflecting the conservation of charge. Kirchhoff's Voltage Law states that at any closed loop in a circuit, the algebraic sum of the voltages across its components equals zero, reflecting the conservation of energy. In the analysis of the hybrid distribution transformer system of this invention, Kirchhoff's laws are used to derive the voltage-current relationship in the circuit, for example, based on the system topology (such as...). Figure 2 The three-level back-to-back converter system with a balanced bridge arm shown is used to establish the electrical equations between the series converter, the parallel converter and the balanced bridge arm through Kirchhoff's laws, providing a theoretical basis for the design of subsequent control strategies.

[0080] It is understood that the hybrid distribution transformer system of the present invention consists of an isolation transformer, a series transformer, a T-type three-level back-to-back converter, and an independent balancing bridge arm. The isolation transformer connects the high-voltage distribution network to the low-voltage three-phase four-wire system. The T-type three-level back-to-back converter includes a parallel converter and a series converter. The series converter is connected to the line in series via the series transformer, and the parallel converter is connected to the load side. The balancing bridge arm bridges the positive and negative DC busbars and is connected to the DC side midpoint through an inductor. Each phase bridge arm of the series and parallel converters is a four-switch T-type three-level structure. The balancing bridge arm consists of two switches and one inductor. The DC side is formed by two supporting capacitors connected in series to form the midpoint, which is connected to the low-voltage side neutral point of the isolation transformer, one end of the primary side of the series transformer, and the load neutral point. When the system is working, the series converter injects compensation voltage through the series transformer to eliminate grid-side voltage imbalance, the parallel converter injects compensation current into the line to offset load current imbalance, and the balance arm adjusts the midpoint current, together to achieve comprehensive power quality compensation.

[0081] In one embodiment, step 102 includes:

[0082] Step 1021: Perform Parker transformation on the three-phase current on the load side and the three-phase inductor current of the parallel converter to obtain the load current component and the first inductor current component in the synchronous rotating dq0 coordinate system.

[0083] It should be noted that the control process of the parallel converter is shown in Figure 3(a). Specifically, the three-phase current i on the load side is... oa i ob i oc and the three-phase inductor current i of the parallel converter L2a i L2b i L2c The inductor current component i in the synchronously rotating dq0 coordinate system is obtained through Park transformation. L2d i L2q i L20 (i.e., the first inductor current component) and the load-side current component i od i oq i o0 .

[0084] Note: Park's Transformation is a mathematical tool in electrical engineering used to convert time-varying electrical quantities such as voltage and current in a three-phase AC system into DC quantities in a synchronous rotating coordinate system (dq0 coordinate system). It was proposed by American engineer RHPark. Its core principle is to project the sinusoidal AC components in the stationary three-phase abc coordinate system onto the d-axis (direct axis), q-axis (quadrature axis), and zero axis, which rotate synchronously with the rotor, thus converting them into DC components. This simplifies the dynamic model of the motor and the design of the control system. In hybrid distribution transformer systems, Park's Transformation is used to convert the collected grid-side / load-side three-phase voltage, converter inductor current, and other signals into the dq0 coordinate system. This facilitates precise control of the compensation current and voltage through proportional-integral resonant controllers, ultimately generating PWM drive signals to regulate the converter switching action and solve power quality problems such as three-phase imbalance.

[0085] Step 1022: After comparing the total DC bus voltage with the reference value, the first PI regulator processes the data and outputs the d-axis active current command.

[0086] It should be noted that the total DC bus voltage v dc Compared with reference value v * dc The voltage error signal is obtained by subtracting the voltage from the voltage comparator. This voltage error is then fed into the first PI regulator (first proportional-integral (PI) regulator), which outputs the d-axis active current command, i.e., the active current command i required to maintain DC voltage stability. * L2d ;

[0087] Note: A PI regulator (Proportional-Integral (PI) regulator) is a closed-loop controller that combines proportional (P) and integral (I) control actions. It is primarily used to eliminate steady-state errors and improve dynamic response performance. In a hybrid distribution transformer system, the first PI regulator is specifically applied to the outer-loop voltage control of the parallel converter: it compares the total DC bus voltage with a reference value to obtain an error signal, rapidly responds to error changes through a proportional element, accumulates the error through an integral element to eliminate static deviation, and finally outputs a d-axis active current command, achieving stable control of the DC bus voltage. Its core function is to ensure the DC side voltage remains stable at the set value by dynamically adjusting the active current, providing a reliable voltage foundation for the converter's compensation function.

[0088] Step 1023: Combine the load current component with the d-axis active current command to obtain the first current reference command of the parallel converter in the dq0 coordinate system.

[0089] It should be noted that the load-side current component i od i oq i o0As a current compensation command, the first current reference command (i.e., the total reference command for the inner current loop) of the synthesized parallel converter in the dq0 coordinate system is used. * L2d +i od i oq i o0 This current reference command contains an AC component on the dq0 axis, which can be used to control the output current Δi of a parallel converter. a , Δi b , Δi c It generates an unbalanced component that has the same composition as the load current.

[0090] Step 1024: Compare the first current reference command with the value of the first inductor current component in the dq0 coordinate system to obtain the current error signal, and send the current error signal to the corresponding proportional-integral resonant controller for processing; perform Park inverse transformation on the output of the proportional-integral resonant controller to obtain the three-phase voltage modulation wave of the parallel converter;

[0091] It should be noted that, firstly, in the current comparator, the first current reference command is compared with the first inductor current component (i.e., the inductor current component i). L2d i L2q i L20 The difference between the current and inverse proportional-integral resonant (PIR) controllers is used to obtain the current error signal. These current error signals are then fed into the corresponding PIR controllers (i.e., the three PIR controllers). The PIR controllers provide high gain at the fundamental frequency and major harmonic frequencies to achieve zero steady-state error tracking of AC commands. The outputs of the PIR controllers are voltage modulation signals in the dq0 coordinate system. Next, the voltage modulation signals are transformed back to the three-phase stationary coordinate system through an inverse Parker transform, yielding the three-phase voltage modulation waves of the parallel converter.

[0092] Step 1025: Compare the three-phase voltage modulation wave with the triangular carrier wave to generate the first PWM drive signal.

[0093] It should be noted that the modulated wave is fed into the PWM modulator and compared with the triangular carrier wave to generate the Q wave. ak Q bk Q ck (That is, the first PWM drive signal controls the operation of the switching transistors of the parallel converter). The triangular carrier wave is a periodic reference signal generated by the PWM modulator based on the switching frequency of the hybrid distribution transformer system. Its frequency is consistent with the set switching frequency. The generation process is implemented through a timer or dedicated waveform generation module inside the controller, ensuring stable amplitude and frequency. It is compared with the voltage modulation signal (a three-phase modulated wave obtained through Parker inverse transformation, etc.), and the intersection of the two signals determines the turn-on / turn-off time of the converter switching transistors, thereby generating PWM drive pulses (such as Q).ak Q bk Q ck (Drive signal).

[0094] It is understandable that step 102 generates the first PWM drive signal for the parallel converter through five sub-steps: First, the three-phase current on the load side and the three-phase inductor current of the parallel converter are transformed to the synchronous rotating dq0 coordinate system through Parker transformation to obtain the load current component and the first inductor current component; then, the error after comparing the total DC bus voltage with the reference value is processed by the first PI regulator to output the d-axis active current command to maintain DC voltage stability; subsequently, the load current component and the d-axis active current command are synthesized to obtain the first current reference command containing the unbalance component; then, the reference command is compared with the first inductor current component, and the error is processed by the proportional-integral resonant controller (PIR) and obtained by inverse Parker transformation to obtain the three-phase voltage modulation wave; finally, the modulation wave is compared with the triangular carrier wave to generate the first PWM drive signal that controls the operation of the switching transistors of the parallel converter.

[0095] In one embodiment, step 103 includes:

[0096] Step 1031: Perform Parker transformation on the grid-side three-phase voltage, the load-side three-phase voltage, and the three-phase inductor current of the series converter to obtain the grid-side voltage component, the load-side voltage component, and the second inductor current component in the synchronous rotating dq0 coordinate system.

[0097] It should be noted that, as shown in Figure 3(b), the grid-side three-phase voltage v sa v sb v sc Three-phase voltage on the load side v oa v ob v oc The three-phase inductor current i of the series converter L1a i L1b i L1c The grid-side voltage component v in the synchronously rotating dq0 coordinate system is obtained by Park transformation. sd v sq v s0 Load voltage component v od v oq v o0 and the actual inductor current component i L1d i L1q i L10 (i.e., the second inductor current component).

[0098] Step 1032: Set a reference value for the load-side voltage in the dq0 coordinate system, and compare the reference value with the grid-side voltage components to obtain the voltage error signal;

[0099] It should be noted that the reference value v of the load-side voltage in the dq0 coordinate system is set. * d v * q v * 0. In the voltage comparator, the reference value is compared with the grid-side voltage component v. sd v sq The difference is calculated to obtain the voltage error signal v. * cd v * cq v * c0 .

[0100] Step 1033: Multiply the voltage error signal by the series transformer turns ratio and compare it with the actual output voltage dq0 axis component of the series converter obtained by Parker transformation. Send the comparison results to the corresponding proportional-integral resonant controllers. The output of the proportional-integral resonant controllers is used as the second current reference command of the series converter.

[0101] It should be noted that the voltage error signal v * cd v * cq v * c0 After multiplying by the series transformer turns ratio N, it is compared with the actual output voltage dq0-axis component of the series converter obtained through the Parker transformation (i.e., compared with the actual output voltage v of the series converter respectively). cd v cq v c0 The difference is calculated, and the resulting error is fed into the corresponding proportional-integral resonant controller (PIR controller). Its output serves as the current reference command i for the series converter. * L1d i * L1q i * L10 (i.e., the second current reference command).

[0102] Step 1034: Compare the second current reference command with the second inductor current component to obtain the current error signal, and send the current error signal to the corresponding proportional controller for processing. Then, perform Parker inverse transformation on the output of the proportional controller to obtain the three-phase voltage modulation wave of the series converter.

[0103] It should be noted that, firstly, in the current comparator, the current reference command i for each axis is... * L1d i * L1q i* L10 (i.e., the second current reference command) is respectively compared with the actual feedback inductor current i L1d i L1q i L10 The difference between the current components (i.e., the second inductor current component) is used to obtain the current error signal. This current error signal is then fed into the corresponding three P controllers (i.e., proportional controllers), which output the voltage modulation signal of the series converter in the dq0 coordinate system. Next, through the Parker inverse transform, the voltage modulation signal is converted back to the three-phase stationary coordinate system, yielding the three-phase voltage modulation wave of the series converter.

[0104] Step 1035: Compare the three-phase voltage modulation wave with the triangular carrier wave to generate the second PWM drive signal.

[0105] It should be noted that the three-phase voltage modulation wave is fed into the PWM modulator, compared with the triangular carrier wave, and a switching drive pulse signal S is generated. ak S bk S ck (That is, the second PWM drive signal controls the operation of the switching transistor of the series converter).

[0106] It is understandable that step 103 generates the second PWM drive signal for the series converter through five sub-steps: First, the grid-side three-phase voltage, load-side three-phase voltage, and three-phase inductor current of the series converter are converted to the synchronous rotating dq0 coordinate system via Parker transformation to obtain the grid-side voltage component, load-side voltage component, and second inductor current component; next, a reference value for the load-side voltage in the dq0 coordinate system is set and compared with the grid-side voltage component to obtain a voltage error signal; then, the voltage error signal is multiplied by the series transformer turns ratio N and compared with the actual output voltage component of the series converter, and the second current reference command is output after processing by the proportional-integral resonant controller; then, the second current reference command is compared with the second inductor current component, and the error is processed by the proportional controller and then inversely transformed by Parker to obtain the three-phase voltage modulation wave; finally, the modulation wave is compared with the triangular carrier wave to generate the second PWM drive signal that controls the operation of the switching transistors of the series converter.

[0107] In one embodiment, step 104 includes:

[0108] Step 1041: Calculate the difference between the voltage of the upper capacitor and the voltage of the lower capacitor in the DC side upper and lower capacitor voltages;

[0109] It should be noted that, as shown in Figure 3(c), the voltage of the upper capacitor and the voltage of the lower capacitor in the DC side are compared. dc1 v dc2 Take the difference and calculate the difference σ = v dc1 -v dc2 .

[0110] Step 1042: Compare the difference with the zero reference value, and process the resulting error signal through the second PI regulator to output a reference command for the midpoint capacitor current.

[0111] It should be noted that the difference between the reference value 0 and the value obtained in step 1041 is calculated and fed into the PI regulator (i.e., the second PI regulator), and its output is used as the reference value i for the midpoint capacitor current. * CN .

[0112] Step 1043: Compare the reference command of the midpoint capacitor current with the DC side midpoint capacitor current, and send the obtained current error signal to the proportional controller for processing. Use the output of the proportional controller as the voltage modulation signal of the balanced bridge arm.

[0113] It should be noted that the reference value i of the midpoint capacitor current is used in the current comparator. * CN With DC side midpoint capacitor current i CN The difference is calculated to obtain the current error signal, which is then sent to the proportional controller (inner loop) to output the voltage modulation signal of the balanced bridge arm.

[0114] Step 1044: Compare the voltage modulation signal with the triangular carrier wave to generate the third PWM drive signal S. k .

[0115] It should be noted that the voltage modulation signal is sent to the PWM modulator and compared with the triangular carrier wave to generate a third PWM drive signal, which controls the operation of the switching transistor of the balanced bridge arm, thereby adjusting the current injected into or flowing out of the midpoint so that σ tends to 0.

[0116] Understandably, step 104 generates the third PWM drive signal for the balanced bridge arm through four sub-steps: First, the difference between the upper and lower capacitor voltages on the DC side is calculated; then, this difference is compared with a zero reference value, and the error is processed by the second PI regulator to output a reference command for the midpoint capacitor current; subsequently, the reference command is compared with the actual DC side midpoint capacitor current, and the error is processed by the proportional controller to obtain the voltage modulation signal for the balanced bridge arm; finally, the voltage modulation signal is compared with a triangular carrier wave to generate the third PWM drive signal, which controls the operation of the balanced bridge arm switching transistor to adjust the current injected into or flowing out of the midpoint, so that σ tends to 0.

[0117] In summary, the control method for a hybrid distribution transformer provided by this invention first acquires the electrical quantity signals of the hybrid distribution transformer system in real time. Then, the parallel converter employs a dual closed-loop control of voltage and current based on the dq0 coordinate system. The outer voltage loop (only on the d-axis) stabilizes the DC bus voltage through a proportional-integral (PI) controller, while the inner current loop independently compensates for line imbalances and harmonic components on the dq0 axis through a proportional-integral resonant (PIR) controller. The series converter also employs a dual closed-loop control of voltage and current based on the dq0 coordinate system. The outer voltage loop generates voltage compensation commands through a PIR controller, while the inner current loop achieves rapid tracking of the compensation voltage through a proportional (P) controller to eliminate grid voltage imbalances and sags. The balancing bridge arm employs independent dual-loop control of voltage and current. The outer loop PI controller adjusts the DC-side capacitor voltage difference, while the inner loop P controller tracks the capacitor midpoint current command to suppress midpoint potential fluctuations. In summary, through the coordinated control of the parallel, series, and balancing bridge arms, coordinated control and comprehensive compensation of grid-side voltage, load-side current, and DC-side midpoint potential are achieved.

[0118] Example 3:

[0119] To test this invention, a simulation platform for a three-level inverter was built in PSIM (Power Simulation) software.

[0120] Note: PSIM (Power Simulation) is a dedicated simulation software for power electronics and motor control, developed by Powersim and currently maintained by Altair. It is primarily used for power electronic circuit analysis, control system design, motor drive system development, and modeling in the new energy field. Its core features include high-speed simulation based on ideal switching models, support for collaboration with tools such as MATLAB / Simulink, a rich built-in component library (e.g., power semiconductors, electromechanical models, photovoltaic / wind turbine modules), and professional analysis functions (e.g., harmonic spectrum calculation, switch-state frequency response analysis). It can automatically generate embedded C code and integrate with DSP hardware, and is widely used in power electronic converters, motor control, renewable energy systems, and other fields, providing an efficient simulation environment for industrial R&D and educational experiments.

[0121] The main parameters given are shown in Table 1.

[0122] Table 1. Key Parameters of the Policy

[0123]

[0124] To verify the voltage and current compensation effect of the hybrid distribution transformer, simulation tests were conducted under conditions of grid voltage imbalance and load current imbalance. Before 0.55 seconds, without the hybrid distribution transformer connected, the three-phase voltage on the grid side was unbalanced, with phase b's voltage higher than phases a and c; simultaneously, the three-phase voltage on the load side was also unbalanced, with phase b's load less than phases a and c. Under this condition, both the load-side voltage and grid-side current exhibited significant imbalances. At 0.55 seconds, after connecting the hybrid distribution transformer, the imbalance between the load-side voltage and grid-side current was eliminated, and the system returned to balanced operation. The simulation results are as follows: Figure 4 , Figure 5 As shown.

[0125] Furthermore, to verify the effectiveness of the hybrid distribution transformer in suppressing neutral point potential fluctuations, a load current imbalance condition was set up in the simulation test. Before 0.55 seconds, the system was in normal operation; after 0.55 seconds, the three phases on the load side were unbalanced, with phase b having a smaller load than phases a and c. Under this condition, the neutral point potential fluctuations were effectively suppressed after the hybrid distribution transformer was connected. The simulation results before and after connecting the hybrid distribution transformer are shown in Figures 6(a) and 6(b).

[0126] The following conclusions were drawn from the analysis of the simulation results:

[0127] (1) The proposed series converter control strategy can effectively compensate for the unbalanced voltage v on the grid side. sa v sb v sc .like Figure 4 As shown, when there is an imbalance in the grid-side voltage, the system can respond quickly by injecting compensation voltage through the series converter, so that the load-side voltage is restored to a three-phase symmetrical sine waveform, and the voltage quality is significantly improved.

[0128] (2) The proposed parallel converter control strategy can effectively suppress the unbalanced current i on the load side. oa i ob i oc .like Figure 5 As shown, when there is a severe unbalanced current on the load side, the parallel converter can generate a reverse compensation current in real time, so that the grid-side current remains three-phase symmetrical, effectively improving the grid-side current quality.

[0129] (3) The proposed balanced bridge arm control strategy can significantly suppress the DC side midpoint potential fluctuation. As shown in the comparison of Figure 6(a) and Figure 6(b), without the balanced bridge arm control, the DC side upper and lower capacitor voltages v dc1 v dc2 Significant low-frequency fluctuations exist; however, after enabling independent control of the balanced bridge arm, the midpoint potential fluctuations are effectively suppressed, the voltages of the upper and lower capacitors remain balanced, and the DC-side stability of the system is significantly enhanced.

[0130] (4) This invention enables the system to operate stably under complex conditions, providing an efficient and reliable solution for the distribution network to cope with new energy access and complex load scenarios.

[0131] Example 4:

[0132] Please see Figure 7 This invention provides a control device for a hybrid distribution transformer. The control device is applied to a hybrid distribution transformer system including a balancing bridge arm. The control device includes:

[0133] The acquisition module 201 is used to acquire electrical quantity signals of the hybrid distribution transformer system in real time. The electrical quantity signals include grid-side three-phase voltage, load-side three-phase voltage and load-side three-phase current, DC-side upper and lower capacitor voltages, three-phase inductor current of series converters, three-phase inductor current of parallel converters, and DC-side midpoint capacitor current.

[0134] It should be noted that during the operation of the hybrid distribution transformer system, the following signals are collected in real time through sensors: grid-side three-phase voltage V sa v sb v sc The three-phase voltage on the load side is v oa v ob v oc ; Load-side three-phase current i oa i ob i oc DC side upper and lower capacitor voltages v dc1 v dc2 The three-phase inductor current i of the series converter L1a i L1b i L1c The three-phase inductor current i of the parallel converter L2a i L2b i L2c DC side midpoint capacitor current i CN Furthermore, it also includes the total voltage v of the upper and lower capacitors on the DC side. dc The phase angle ɵ at the three-phase load grid connection point is described in the following corresponding embodiment.

[0135] The first control module 202 is used to generate a first PWM drive signal to control the operation of the switching transistors in the parallel converter based on the three-phase current on the load side and the three-phase inductor current of the parallel converter, so as to drive the parallel converter to inject compensation current into the line.

[0136] It should be noted that the process of generating the first PWM drive signal based on the three-phase current on the load side and the three-phase inductor current of the parallel converter is specifically as follows: two types of current signals are acquired in real time, converted to the synchronous rotating dq0 coordinate system by Parker transformation, and the load current component and the inductor current component are obtained; then the total DC bus voltage is compared with the reference value, and the d-axis active current command is output through the PI regulator, which is combined with the load current component to form a current reference command; then the reference command is compared with the inductor current component to obtain the error, which is processed by the proportional-integral resonant controller and converted into a three-phase voltage modulation wave by Parker inverse transformation, and finally compared with the triangular carrier wave to generate a drive signal to control the operation of the switching transistors of the parallel converter. The above process is specifically described in the corresponding embodiment below. The first control module 202 achieves precise monitoring and control of load current and converter current through real-time acquisition and coordinate transformation, providing a data foundation for the generation of compensation current. It adopts a PI regulator and a proportional-integral resonant controller to ensure the stability of DC bus voltage and zero steady-state error tracking of current commands, thereby improving compensation accuracy. The generated PWM drive signal can drive the parallel converter to inject compensation current into the line, effectively offsetting the unbalanced current on the load side, improving the current quality on the grid side, and solving the three-phase imbalance problem.

[0137] The second control module 203 is used to generate a second PWM drive signal to control the operation of the switching transistors in the series converter based on the grid-side three-phase voltage, the load-side three-phase voltage and the three-phase inductor current of the series converter, so as to drive the series converter to inject compensation voltage into the line through the series transformer.

[0138] It should be noted that the process of generating the second PWM drive signal based on the grid-side three-phase voltage, the load-side three-phase voltage, and the three-phase inductor current of the series converter specifically involves real-time acquisition of three types of electrical quantity signals, performing Parker transformation on them respectively to the synchronous rotating dq0 coordinate system to obtain the grid-side voltage component, the load-side voltage component, and the second inductor current component of the series converter; then, a reference value of the load-side voltage in the dq0 coordinate system is set and compared with the grid-side voltage component to obtain a voltage error signal. This error signal is multiplied by the series transformer turns ratio and compared with the actual output voltage component of the series converter. After processing by the proportional-integral resonant controller, a second current reference command is output; then, the second current reference command is compared with the second inductor current component to obtain a current error signal, which is processed by the proportional controller and inversely transformed by Parker into a three-phase voltage modulation wave. Finally, it is compared with a triangular carrier wave to generate a drive signal to control the operation of the series converter switching transistors. The specific details of the above process are shown in the corresponding embodiments below. The second control module 203 achieves accurate monitoring of grid voltage and converter current through coordinate transformation and multi-signal fusion, providing data support for compensation voltage generation. It adopts dual closed-loop control of proportional-integral resonant controller and proportional controller to ensure rapid tracking of voltage compensation command and zero steady-state error adjustment, thereby improving compensation response speed. The generated PWM drive signal can drive the series converter to inject compensation voltage into the line through the series transformer, effectively eliminating grid-side voltage imbalance and sag, ensuring three-phase symmetry of load-side voltage, and improving power supply reliability and power quality.

[0139] The third control module 204 is used to generate a third PWM drive signal to control the operation of the balance bridge arm switching transistor based on the DC side upper and lower capacitor voltages and the DC side midpoint capacitor current, so as to drive the balance bridge arm to adjust the midpoint current.

[0140] It should be noted that the process of generating the third PWM drive signal based on the DC-side upper and lower capacitor voltages and the DC-side midpoint capacitor current involves: calculating the difference between the DC-side upper and lower capacitor voltages, comparing this difference with a zero reference value, processing it through a second PI regulator, and outputting a reference command for the midpoint capacitor current; then comparing this reference command with the DC-side midpoint capacitor current, and sending the resulting current error signal to a proportional controller for processing, outputting a voltage modulation signal for the balanced bridge arm; finally, comparing the voltage modulation signal with a triangular carrier wave to generate the third PWM drive signal to control the operation of the balanced bridge arm switching transistor. The specific details of this process are described in the corresponding embodiments below. The third control module 204, through dual-loop control of the PI regulator and the proportional controller, achieves real-time monitoring and adjustment of the DC-side upper and lower capacitor voltage difference, providing precise commands for midpoint current control; the generated PWM drive signal can drive the balanced bridge arm to adjust the midpoint current, effectively suppressing DC-side midpoint potential fluctuations and maintaining a balanced voltage between the upper and lower capacitors; reducing the demand on DC capacitor capacity, which is beneficial for system miniaturization and efficiency, and improving the stability and reliability of the hybrid distribution transformer system.

[0141] In one embodiment, the hybrid distribution transformer system in the acquisition module 201 includes: an isolation transformer, a series transformer, a T-type three-level back-to-back converter, and an independent balancing bridge arm; the isolation transformer is used to connect the high-voltage distribution network and the low-voltage three-phase four-wire system; the T-type three-level back-to-back converter includes a parallel converter and a series converter; the AC side of the series converter is connected in series to the line of the low-voltage three-phase four-wire system through the series transformer; the AC side of the parallel converter is connected to the load side; the balancing bridge arm is connected between the positive and negative DC busbars, wherein the connection point is connected to the midpoint of the DC side of the back-to-back converter through an inductor.

[0142] In this system, each phase arm of the series converter and the parallel converter consists of four switching transistors forming a T-type three-level structure; the independent balancing arm consists of two switching transistors and one inductor; the DC side of the T-type three-level back-to-back converter is formed by two supporting capacitors connected in series; one end of the inductor in the balancing arm is connected to the connection point of the two switching transistors, and the other end is connected to the connection point of the two supporting capacitors, forming the DC side midpoint; the DC side midpoint, the neutral point of the low-voltage side of the isolation transformer, one end of the primary side of the series transformer, and the load neutral point are interconnected.

[0143] In one embodiment, the first control module 202 is specifically used for:

[0144] The three-phase current on the load side and the three-phase inductor current of the parallel converter are subjected to Park transformation to obtain the load current component and the first inductor current component in the synchronous rotating dq0 coordinate system.

[0145] After comparing the total DC bus voltage with the reference value, the first PI regulator processes the data and outputs the d-axis active current command.

[0146] The load current component is combined with the d-axis active current command to obtain the first current reference command of the parallel converter in the dq0 coordinate system.

[0147] The first current reference command is compared with the value of the first inductor current component in the dq0 coordinate system to obtain the current error signal, and the current error signal is sent to the corresponding proportional-integral resonant controller for processing; the output of the proportional-integral resonant controller is subjected to Park inverse transformation to obtain the three-phase voltage modulation wave of the parallel converter.

[0148] The three-phase voltage modulation wave is compared with the triangular carrier wave to generate the first PWM drive signal.

[0149] In one embodiment, the second control module 203 is specifically used for:

[0150] The grid-side three-phase voltage, the load-side three-phase voltage, and the three-phase inductor current of the series converter are subjected to Park transformation to obtain the grid-side voltage component, the load-side voltage component, and the second inductor current component in the synchronous rotating dq0 coordinate system.

[0151] Set a reference value for the load-side voltage in the dq0 coordinate system, and compare the reference value with the grid-side voltage components to obtain the voltage error signal;

[0152] After multiplying the voltage error signal by the series transformer turns ratio, it is compared with the dq0 axis component of the actual output voltage of the series converter obtained by Parker transformation. The comparison results are sent to the corresponding proportional-integral resonant controllers, and the output of the proportional-integral resonant controllers is used as the second current reference command of the series converter.

[0153] The second current reference command is compared with the second inductor current component to obtain the current error signal. The current error signal is then sent to the corresponding proportional controller for processing. The output of the proportional controller is then subjected to Parker inverse transformation to obtain the three-phase voltage modulation wave of the series converter.

[0154] The three-phase voltage modulation wave is compared with the triangular carrier wave to generate the second PWM drive signal.

[0155] In one embodiment, the third control module 204 is specifically used for:

[0156] Calculate the difference between the voltage of the upper capacitor and the voltage of the lower capacitor in the DC side upper and lower capacitor voltages;

[0157] The difference is compared with the zero reference value, and the resulting error signal is processed by the second PI regulator to output a reference command for the midpoint capacitor current.

[0158] The reference command for the midpoint capacitor current is compared with the DC side midpoint capacitor current, and the resulting current error signal is sent to the proportional controller for processing. The output of the proportional controller is used as the voltage modulation signal for the balanced bridge arm.

[0159] The voltage modulation signal is compared with the triangular carrier wave to generate the third PWM drive signal S. k .

[0160] This invention provides a control system for a hybrid distribution transformer. First, it acquires electrical signals from the hybrid distribution transformer system in real time. Then, the parallel converter employs a dual-loop voltage and current control based on the dq0 coordinate system. The outer voltage loop (only on the d-axis) stabilizes the DC bus voltage using a proportional-integral (PI) controller, while the inner current loop independently compensates for line imbalances and harmonic components on the dq0 axis using a proportional-integral resonant (PIR) controller. The series converter also employs a dual-loop voltage and current control based on the dq0 coordinate system. The outer voltage loop generates voltage compensation commands through a PIR controller, while the inner current loop achieves rapid tracking of the compensation voltage through a proportional (P) controller, eliminating grid voltage imbalances and sags. The balancing arm employs independent dual-loop voltage and current control. The outer loop PI controller adjusts the DC-side capacitor voltage difference, while the inner loop P controller tracks the capacitor midpoint current command, suppressing midpoint potential fluctuations. In summary, through the coordinated control of the parallel, series, and balancing arms, coordinated control and comprehensive compensation of grid-side voltage, load-side current, and DC-side midpoint potential are achieved.

[0161] Example 5:

[0162] This invention provides a control device for a hybrid distribution transformer, the device including a processor and a memory:

[0163] The memory is used to store program code and transfer the program code to the processor;

[0164] The processor is used to execute the steps of the control method for the hybrid distribution transformer as described in the above method embodiment, according to the instructions in the program code.

[0165] This invention provides a control device for a hybrid distribution transformer. First, it acquires electrical quantity signals from the hybrid distribution transformer system in real time. Then, the parallel converter employs a dual closed-loop control based on the dq0 coordinate system, with the outer voltage loop (only on the d-axis) stabilizing the DC bus voltage using a proportional-integral (PI) controller, and the inner current loop independently compensating for line imbalances and harmonic components on the dq0 axis using a proportional-integral resonant (PIR) controller. The series converter also employs a dual closed-loop control based on the dq0 coordinate system, with the outer voltage loop generating voltage compensation commands through a PIR controller, and the inner current loop achieving rapid tracking of the compensation voltage through a proportional (P) controller to eliminate grid voltage imbalances and sags. The balancing arm employs independent dual-loop control, with the outer loop PI controller adjusting the DC-side capacitor voltage difference and the inner loop P controller tracking the capacitor midpoint current command to suppress midpoint potential fluctuations. In summary, through the coordinated control of the parallel, series, and balancing arms, coordinated control and comprehensive compensation of grid-side voltage, load-side current, and DC-side midpoint potential are achieved.

[0166] Example 6:

[0167] This invention provides a computer-readable storage medium, characterized in that the computer-readable storage medium is used to store program code, the program code being used to execute the steps of the control method for a hybrid distribution transformer as described in the above method embodiments.

[0168] This invention provides a computer-readable storage medium that first acquires electrical quantity signals from a hybrid distribution transformer system in real time. Then, the parallel converter employs a dual-loop voltage and current control based on the dq0 coordinate system. The outer voltage loop (only on the d-axis) stabilizes the DC bus voltage using a proportional-integral (PI) controller, while the inner current loop independently compensates for line imbalances and harmonic components on the dq0 axis using a proportional-integral resonant (PIR) controller. The series converter also employs a dual-loop voltage and current control based on the dq0 coordinate system. The outer voltage loop generates voltage compensation commands through a PIR controller, while the inner current loop achieves rapid tracking of the compensation voltage through a proportional (P) controller to eliminate grid voltage imbalances and sags. The balancing bridge arm employs independent dual-loop voltage and current control. The outer loop PI controller adjusts the DC-side capacitor voltage difference, while the inner loop P controller tracks the capacitor midpoint current command to suppress midpoint potential fluctuations. In summary, through the coordinated control of the parallel, series, and balancing bridge arms, coordinated control and comprehensive compensation of grid-side voltage, load-side current, and DC-side midpoint potential are achieved.

[0169] Those skilled in the art will understand that, for the sake of convenience and brevity, the specific working process of the system and modules described above can be referred to the corresponding process in the foregoing method embodiments, and will not be repeated here.

[0170] In the embodiments provided by this invention, it should be understood that the disclosed systems, apparatuses, and methods can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative; for instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be through some interfaces, indirect coupling or communication connection between apparatuses or units, and may be electrical, mechanical, or other forms.

[0171] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0172] Furthermore, the functional units in the various embodiments of the present invention can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0173] If the integrated unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art, or all or part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods of the various embodiments of the present invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0174] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A control method for a hybrid distribution transformer, characterized in that, The control method is applied to a hybrid distribution transformer system including a balancing bridge arm, and the control method includes: Real-time acquisition of electrical quantity signals of the hybrid distribution transformer system, including grid-side three-phase voltage, load-side three-phase voltage and load-side three-phase current, DC-side upper and lower capacitor voltages, three-phase inductor current of series converters, three-phase inductor current of parallel converters, and DC-side midpoint capacitor current. Based on the three-phase current on the load side and the three-phase inductor current of the parallel converter, a first PWM drive signal is generated to control the operation of the switching transistors in the parallel converter, so as to drive the parallel converter to inject compensation current into the line. Based on the grid-side three-phase voltage, the load-side three-phase voltage, and the three-phase inductor current of the series converter, a second PWM drive signal is generated to control the operation of the switching transistors in the series converter, so as to drive the series converter to inject compensation voltage into the line through the series transformer. Based on the voltages of the upper and lower capacitors on the DC side and the current of the midpoint capacitor on the DC side, a third PWM drive signal is generated to control the operation of the balance bridge arm switch, thereby driving the balance bridge arm to adjust the midpoint current.

2. The control method for a hybrid distribution transformer according to claim 1, characterized in that, The hybrid power distribution transformer system includes: an isolation transformer, a series transformer, a T-type three-level back-to-back converter, and an independent balancing bridge arm; The isolation transformer is used to connect the high-voltage power distribution network with the low-voltage three-phase four-wire system. The T-type three-level back-to-back converter includes a parallel converter and a series converter; the AC side of the series converter is connected in series to the line of the low-voltage three-phase four-wire system through a series transformer; the AC side of the parallel converter is connected to the load side. The balancing arm is connected between the positive and negative DC buses, and the connection point is connected to the midpoint of the DC side of the back-to-back converter through an inductor.

3. The control method for a hybrid distribution transformer according to claim 2, characterized in that, Each phase arm of the series converter and the parallel converter consists of four switching transistors forming a T-type three-level structure; the independent balanced arm consists of two switching transistors and one inductor.

4. The control method for a hybrid distribution transformer according to claim 3, characterized in that, The DC side of the T-type three-level back-to-back converter is composed of two supporting capacitors connected in series; one end of the inductor in the balance bridge arm is connected to the connection point of the two switching transistors, and the other end is connected to the connection point of the two supporting capacitors, forming the midpoint of the DC side. The DC side midpoint, the low-voltage side neutral point of the isolation transformer, one end of the primary side of the series transformer, and the load neutral point are interconnected.

5. The control method for a hybrid distribution transformer according to claim 4, characterized in that, The step of generating a first PWM drive signal based on the three-phase current on the load side and the three-phase inductor current of the parallel converter to control the operation of the switching transistors in the parallel converter, thereby driving the parallel converter to inject compensation current into the line, includes: The three-phase current on the load side and the three-phase inductor current of the parallel converter are subjected to Parker transformation to obtain their load current component and first inductor current component in the synchronous rotating dq0 coordinate system. After comparing the total DC bus voltage with the reference value, the first PI regulator processes the data and outputs the d-axis active current command. The load current component is combined with the d-axis active current command to obtain the first current reference command of the parallel converter in the dq0 coordinate system. The first current reference command is compared with the value of the first inductor current component in the dq0 coordinate system to obtain the current error signal, and the current error signal is sent to the corresponding proportional-integral resonant controller for processing; the output of the proportional-integral resonant controller is subjected to Park inverse transformation to obtain the three-phase voltage modulation wave of the parallel converter; The three-phase voltage modulation wave is compared with a triangular carrier wave to generate a first PWM drive signal.

6. The control method for a hybrid distribution transformer according to claim 4, characterized in that, The step of generating a second PWM drive signal based on the grid-side three-phase voltage, the load-side three-phase voltage, and the three-phase inductor current of the series converter to control the operation of the switching transistors in the series converter, thereby driving the series converter to inject compensation voltage into the line through the series transformer, includes: The grid-side three-phase voltage, the load-side three-phase voltage, and the three-phase inductor current of the series converter are subjected to Parker transformation to obtain their grid-side voltage component, load-side voltage component, and second inductor current component in the synchronous rotating dq0 coordinate system. A reference value for the load-side voltage in the dq0 coordinate system is set, and the reference value is compared with the grid-side voltage component to obtain the voltage error signal; After multiplying the voltage error signal by the series transformer turns ratio, it is compared with the dq0 axis component of the actual output voltage of the series converter obtained by Parker transformation. The comparison results are sent to the corresponding proportional-integral resonant controllers, and the output of the proportional-integral resonant controllers is used as the second current reference command of the series converter. The second current reference command is compared with the second inductor current component to obtain a current error signal. The current error signal is then sent to the corresponding proportional controller for processing. The output of the proportional controller is subjected to Parker inverse transformation to obtain the three-phase voltage modulation wave of the series converter. The three-phase voltage modulation wave is compared with a triangular carrier wave to generate a second PWM drive signal.

7. The control method for a hybrid distribution transformer according to claim 4, characterized in that, The step of generating a third PWM drive signal based on the DC-side upper and lower capacitor voltages and the DC-side midpoint capacitor current to control the operation of the balanced bridge arm switching transistor, thereby driving the balanced bridge arm to adjust the midpoint current, includes: Calculate the difference between the upper capacitor voltage and the lower capacitor voltage in the DC side upper and lower capacitor voltages; The difference is compared with the zero reference value, and the resulting error signal is processed by the second PI regulator to output a reference command for the midpoint capacitor current. The reference command for the midpoint capacitor current is compared with the DC side midpoint capacitor current, and the resulting current error signal is sent to the proportional controller for processing. The output of the proportional controller is used as the voltage modulation signal for the balanced bridge arm. The voltage modulation signal is compared with a triangular carrier wave to generate a third PWM drive signal.

8. A control device for a hybrid distribution transformer, characterized in that, The control device is applied to a hybrid distribution transformer system including a balancing bridge arm, and the control device includes: The acquisition module is used to acquire electrical quantity signals of the hybrid distribution transformer system in real time. The electrical quantity signals include grid-side three-phase voltage, load-side three-phase voltage and load-side three-phase current, DC-side upper and lower capacitor voltages, three-phase inductor current of series converters, three-phase inductor current of parallel converters, and DC-side midpoint capacitor current. The first control module is used to generate a first PWM drive signal to control the operation of the switching transistors in the parallel converter based on the three-phase current on the load side and the three-phase inductor current of the parallel converter, so as to drive the parallel converter to inject compensation current into the line. The second control module is used to generate a second PWM drive signal to control the operation of the switching transistors in the series converter based on the grid-side three-phase voltage, the load-side three-phase voltage and the three-phase inductor current of the series converter, so as to drive the series converter to inject compensation voltage into the line through the series transformer. The third control module is used to generate a third PWM drive signal to control the operation of the balance bridge arm switch based on the voltage of the upper and lower capacitors on the DC side and the current of the midpoint capacitor on the DC side, so as to drive the balance bridge arm to adjust the midpoint current.

9. A control device for a hybrid distribution transformer, characterized in that, The device includes a processor and a memory: The memory is used to store program code and transmit the program code to the processor; The processor is used to execute the control method of the hybrid distribution transformer according to any one of claims 1-7 according to the instructions in the program code.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium is used to store program code for executing the control method of the hybrid distribution transformer according to any one of claims 1-7.

Images