Topology and control method of built-in flexible controllable converter transformer

Abstract

This invention discloses a topology and control method for a built-in flexible controllable converter transformer, belonging to the field of smart grid technology. Based on a traditional two-limb, three-winding converter transformer, it adds an independent voltage-regulating magnetic core and a matching voltage-regulating winding, as well as an energy-harvesting winding sharing a magnetic column with the grid-side winding, forming a three-limb, five-winding integrated transformer. A built-in back-to-back converter connects the energy-harvesting winding and the voltage-regulating winding, forming a power compensation channel with parallel energy harvesting and series voltage regulation. By real-time detection of grid-side voltage fluctuations, it dynamically calculates and injects a precisely controllable compensation voltage into the voltage-regulating winding, ensuring the stability of the voltage amplitude and phase on the valve side during grid-side voltage fluctuations. This invention achieves rapid, continuous, independent, and precise adjustment of voltage amplitude and phase within the transformer body, completely overcoming the technical bottlenecks of traditional mechanical tap changers—slow response, stepped adjustment, and inability to adjust phase—significantly improving the operational flexibility and voltage stability of high-voltage direct current transmission systems.

Description

Technical Field

[0001] This invention belongs to the field of smart grid technology, and more specifically, relates to a topology of a built-in flexible controllable converter transformer and its control method. Background Technology

[0002] Currently, the high proportion of renewable energy sources, represented by wind and solar power, connected to the grid poses a severe challenge to grid voltage stability. On the one hand, the strong volatility of new energy sources leads to wide and rapid fluctuations in grid voltage; on the other hand, frequent transactions in the electricity spot market require high-voltage direct current transmission (LCC-HVDC) systems based on commutation converters to have minute-level active power regulation capabilities. Both of these aspects place demands on converter transformers, as core equipment, for rapid, continuous, and precise voltage regulation.

[0003] However, the converter transformers commonly used in existing LCC-HVDC projects still rely on mechanical on-load tap changers for voltage regulation, which has the following prominent drawbacks: first, slow response speed, unable to track rapid voltage fluctuations; second, discontinuous and limited regulation range, making it difficult to achieve smooth, wide-range control; and third, only voltage amplitude can be adjusted, not phase. These problems severely restrict the DC system's rapid power regulation capability and its active support capability for the power grid.

[0004] Existing technologies struggle to simultaneously achieve rapid dynamic response, wide-range continuous regulation, and independent, precise control of voltage amplitude and phase, often facing challenges in terms of economics and engineering feasibility. This technological shortcoming makes it difficult for traditional high-voltage direct current (HVDC) transmission systems to effectively suppress wide fluctuations in grid voltage, and also limits the ability of HVDC transmission to participate in rapid power regulation and provide flexible support in the context of new power systems, becoming a key bottleneck restricting the improvement of system efficiency. Summary of the Invention

[0005] In view of the above-mentioned defects or improvement needs of the existing technology, the present invention provides a topology of a built-in flexible controllable converter transformer and its control method. The purpose is to solve the technical problem that the existing technical solutions are difficult to achieve in a balanced manner of fast dynamic response, wide-range continuous adjustment and independent and precise control of voltage amplitude and phase.

[0006] To achieve the above objectives, according to one aspect of the present invention, a topology of a built-in flexible controllable converter transformer is provided, comprising: a three-limb five-winding integrated transformer and a built-in back-to-back power converter. The three-column, five-winding integrated transformer has a middle column, a right column, and a left column. The middle column is wound with a grid-side winding W1 and an energy-harvesting winding W5. The grid-side winding W1 is used to connect to the high-voltage AC power grid and withstand the rated voltage of the system. The energy-harvesting winding W5 is wound on the same column as the energy-harvesting winding W1, and the two are tightly coupled through the magnetic circuit of the iron core. The right column is wound with a voltage-regulating winding W3, which is configured to achieve series compensation and regulation of the valve-side voltage through electromagnetic coupling. The left column is wound with valve windings W2 and W4. The built-in back-to-back power converter has its AC input side connected to the energy harvesting winding W5 and its AC output side connected to the voltage regulating winding W3.

[0007] Furthermore, the built-in back-to-back power converter includes a rectifier unit, a DC support link, and an inverter unit; the AC side of the rectifier unit is connected to the output terminal of the energy harvesting winding W5 through an input filter circuit, converting the AC power output by the energy harvesting winding W5 into DC power; the DC side of the inverter unit is connected to the DC support link, and the AC side is connected to the input terminal of the voltage regulating winding W3 through an output filter circuit, converting the DC power into an adjustable voltage with the required amplitude and phase, and injecting it into the voltage regulating winding W3.

[0008] Furthermore, the valve-side winding W2 adopts a star connection, and the valve-side winding W4 adopts a delta connection.

[0009] According to another aspect of the present invention, a control method for a built-in flexible controllable converter transformer is provided, applied to the topology of the built-in flexible controllable converter transformer, comprising: Real-time acquisition of AC voltage information from the grid side and the valve side; When amplitude or phase fluctuations of the grid-side voltage are detected: based on the amplitude or phase deviation of the grid-side voltage, the built-in back-to-back power converter is controlled to inject a corresponding compensation voltage into the regulating winding to keep the amplitude and phase of the valve-side voltage stable.

[0010] Furthermore, the control method for the built-in flexible controllable converter transformer also includes: when the grid-side voltage recovers to the rated value, controlling the built-in back-to-back power converter to reduce or exit the compensation output, so that the system returns to the transformer's natural turns ratio operating state.

[0011] Furthermore, the grid-side AC voltage information includes: grid-side voltage amplitude deviation and grid-side voltage phase deviation; the valve-side AC voltage information includes: valve-side voltage amplitude and its phase.

[0012] Furthermore, the steady-state voltage equations downstream of the valve are expressed as follows: ; ; in, The voltage vector of the valve-side winding W2. The voltage vector of valve-side winding W4. The voltage vector of the grid-side winding W1, The voltage vector of the voltage regulating winding W3. To extract the voltage vector of the energy winding W5.

[0013] Furthermore, the steady-state current equations downstream of the valve are expressed as follows:

[0014] in, The current vector of the grid-side winding W1, To obtain the current vector of the energy winding W5, The current vector of the voltage regulating winding W3, The current vector of the valve-side winding W2. This is the current vector of the valve-side winding W4.

[0015] Furthermore, in the three-phase stationary coordinate system, the amount of compensation voltage injected into the voltage regulating winding... Determined by the following formula: ;in, This is the rated value of the valve-side voltage vector. Let be the voltage vector of the grid-side winding W1.

[0016] Furthermore, decoupled control is achieved in the synchronously rotating dq coordinate system to compensate for the direct-axis component reference value of the voltage command. Reference value of cross-axis component They are represented as follows: ; in, , The direct and quadrature components of the valve-side voltage rated value vector in the dq coordinate system; , The vector of the grid-side voltage measurement value is defined by its direct and quadrature axes in the dq coordinate system.

[0017] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects: (1) This invention discloses a topology of a built-in flexible controllable converter transformer. Based on a traditional two-pillar, three-winding converter transformer, this topology adds an independent voltage-regulating core and a matching voltage-regulating winding, as well as an energy-harvesting winding sharing a magnetic column with the grid-side winding, forming a three-pillar, five-winding integrated transformer. A built-in back-to-back converter connects the energy-harvesting winding and the voltage-regulating winding, forming a power compensation channel with parallel energy harvesting and series voltage regulation. The control strategy detects grid-side voltage fluctuations in real time, dynamically calculates and injects a precisely controllable compensation voltage into the voltage-regulating winding, ensuring that the amplitude and phase of the valve-side voltage remain stable during grid-side voltage fluctuations. This invention achieves rapid, continuous, independent, and precise adjustment of voltage amplitude and phase within the transformer body, completely overcoming the technical bottlenecks of slow response, stepped adjustment, and inability to adjust phase in traditional mechanical tap changers. It significantly improves the operational flexibility and voltage stability of high-voltage direct current transmission systems in scenarios with a high proportion of new energy and frequent power market adjustments.

[0018] (2) In this scheme, the back-to-back converter is connected to the integrated transformer winding through a corresponding filter circuit. This converter structure realizes the controllable bidirectional flow of energy and electrical isolation, providing a stable power buffer for the system, enabling small-capacity converters to safely and efficiently complete power compensation for the large power grid.

[0019] (3) This invention provides a control method for a built-in flexible controllable converter transformer to perform rapid closed-loop voltage regulation control on the valve side AC voltage. This method enables the converter transformer to operate in a dynamic and continuous voltage regulation state, and its overall response time is much shorter than that of a traditional mechanical on-load tap changer. It can effectively cope with rapid fluctuations in the power grid and significantly improve the operational adaptability and voltage support capability of the LCC-HVDC system in a power grid environment with a high proportion of new energy sources.

[0020] (4) This scheme establishes the electromagnetic relationship of its specific transformer magnetic circuit structure and obtains the converted steady-state electrical equations, which show that the grid-side winding and the voltage regulating winding are connected in series, and the energy extraction winding and the grid-side winding are connected in parallel. This equivalent circuit model clearly reveals the voltage vector synthesis mechanism of "parallel energy extraction and series voltage regulation" inside the system, providing a basis for the construction of control strategies, especially the quantitative calculation of compensation voltage.

[0021] (5) The theoretical basis of the control strategy of this scheme is: in order to maintain the constant voltage on the valve side, the theoretical value of the compensation voltage that needs to be injected into the voltage regulating winding is as follows: Determined by the following formula: Based on this relationship, the controller uses a control architecture combining feedforward and feedback to generate the final compensation voltage command. This control principle is directly based on voltage vector synthesis, achieving high-performance dynamic adjustment of voltage amplitude and phase disturbances.

[0022] (6) In this scheme, the controller achieves decoupled control in a synchronously rotating dq coordinate system. The reference values ​​of the d and q axis components of the compensation voltage command are determined by the component differences between the valve-side and grid-side voltages on the d and q axes, respectively. Achieving decoupled control in the dq coordinate system transforms the complex vector calculations of AC quantities into DC control, which greatly simplifies the controller design, improves the dynamic response speed and control accuracy of the system, and makes it easy to achieve independent and precise adjustment of voltage amplitude (d axis) and phase (q axis). Attached Figure Description

[0023] Figure 1 This is a block diagram of the electrical topology of a built-in flexible controllable converter transformer provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of the single-phase magnetic circuit structure and a single-phase equivalent circuit model diagram of an integrated transformer provided in an embodiment of the present invention; Figure 3 This is a schematic diagram of the overall flow of a control method provided in an embodiment of the present invention; Figure 4 A schematic diagram of voltage vector synthesis provided in an embodiment of the present invention for illustrating the principle of amplitude and phase compensation; Figure 5 This is a block diagram of a dual closed-loop control system for an energy harvesting unit and a voltage regulation unit provided in an embodiment of the present invention; Figure 6 This is a simulation waveform diagram of grid-side voltage amplitude disturbance provided in an embodiment of the present invention; Figure 7 The simulation waveform diagram under grid-side voltage phase disturbance is provided for an embodiment of the present invention. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0025] Example 1 This embodiment provides a topology for a built-in flexible controllable converter transformer, including: a three-limb, five-winding integrated transformer and a built-in back-to-back power converter. The three-limb, five-winding integrated transformer has a middle column, a right column, and a left column. The middle column is wound with a grid-side winding W1 and a power extraction winding W5. The grid-side winding W1 is used to connect to the high-voltage AC grid and withstand the rated voltage of the system. The power extraction winding W5 is wound on the same column as the power extraction winding W1, and the two are tightly coupled through a core magnetic circuit. The right column is wound with a voltage regulating winding W3, which is configured to achieve series compensation regulation of the valve-side voltage through electromagnetic coupling. The left column is wound with valve windings W2 and W4. The built-in back-to-back power converter has its AC input side connected to the power extraction winding W5 and its AC output side connected to the voltage regulating winding W3.

[0026] Reference Figure 1 This built-in flexible controllable converter transformer comprises two core components: a three-limb, five-winding integrated transformer and a built-in back-to-back power converter. It represents a fundamental structural innovation compared to the two-limb, three-winding converter transformers used in traditional HVDC transmission converter stations. Its core employs a three-phase, three-limb structure. Taking phase A as an example (e.g.) Figure 2 (as shown in (a)).

[0027] The center column is wound with a grid-side winding W1 and an energy extraction winding W5. The grid-side winding W1 is used to connect to the high-voltage AC grid and withstand the system's rated voltage. The energy extraction winding W5 is wound on the same column as W1, and the two are tightly coupled through the iron core magnetic circuit. The design voltage of W5 is much lower than that of W1, and its core function is to provide electrically isolated, grid-synchronized control power to the subsequent power converter.

[0028] Right column: A new magnetic core column added to the traditional converter transformer, on which a voltage regulating winding W3 is wound. This winding is the key actuator of this invention, and its rated voltage and capacity determine the maximum voltage regulation capability of the system. W3 is connected to the main power transmission path in series via electromagnetic coupling.

[0029] Left column: Valve windings W2 and W4. The output voltages of the two valve-side windings have a specific relationship in amplitude and phase to meet the requirements of the converter valve forming a multi-pulse rectifier bridge. Their rated voltage is the control target that the system needs to keep constant.

[0030] In one embodiment, the built-in back-to-back power converter includes a rectifier unit, a DC support link, and an inverter unit. The AC side of the rectifier unit is connected to the output terminal of the power extraction winding W5 through an input filter circuit, converting the AC power output by the power extraction winding W5 into DC power. The DC side of the inverter unit is connected to the DC support link, and the AC side is connected to the input terminal of the voltage regulating winding W3 through an output filter circuit, converting the DC power into an adjustable voltage with the required amplitude and phase, which is then injected into the voltage regulating winding W3.

[0031] like Figure 1 As shown, the built-in back-to-back power converter is a power electronic actuator that enables the "parallel energy harvesting - series voltage regulation" function. It is directly connected between the energy harvesting winding W5 and the voltage regulating winding W3, and includes three main components: a front-end AC / DC conversion unit, a DC support component, and a back-end DC / AC inverter unit.

[0032] The front-end AC / DC converter unit: Its AC side is connected to the output terminal of the power extraction winding W5 through the input filter circuit, which converts the low-voltage AC power provided by the power extraction winding into DC power.

[0033] DC support stage: consisting of a large-capacity capacitor C dc It is designed to maintain the stability of the DC bus voltage and provide energy buffer for the two-stage converter.

[0034] The downstream DC / AC inverter unit is connected to the DC support circuit on its DC side and to the input of the voltage regulating winding W3 on its AC side through the output filter circuit. It converts the DC power into an adjustable voltage with the required amplitude and phase and injects it into the voltage regulating winding.

[0035] Example 2 This embodiment provides a control method for a built-in flexible controllable converter transformer, such as... Figure 3 As shown, the topology applied to the built-in flexible controllable converter transformer includes: real-time acquisition of grid-side AC voltage information and valve-side AC voltage information; when amplitude or phase fluctuations of the grid-side voltage are detected: based on the grid-side voltage amplitude deviation or the phase deviation, controlling the built-in back-to-back power converter to inject a corresponding compensation voltage into the regulating winding, so that the amplitude and phase of the valve-side voltage remain stable. In one embodiment, the control method of the built-in flexible controllable converter transformer further includes: when the grid-side voltage recovers to its rated value, controlling the built-in back-to-back power converter to reduce or exit the compensation output, so that the system returns to the transformer's natural turns ratio operating state. The grid-side AC voltage information includes: grid-side voltage amplitude deviation and grid-side voltage phase deviation; the valve-side AC voltage information includes: valve-side voltage amplitude and its phase.

[0036] In one embodiment, the steady-state electrical equations downstream of the valve are expressed as follows: ; ; ; in, The voltage vector of the valve-side winding W2. The voltage vector of valve-side winding W4. The voltage vector of the grid-side winding W1, The voltage vector of the voltage regulating winding W3. To extract the voltage vector of the energy winding W5. The current vector of the grid-side winding W1, To obtain the current vector of the energy winding W5, The current vector of the voltage regulating winding W3, The current vector of the valve-side winding W2. This is the current vector of the valve-side winding W4.

[0037] In one embodiment, based on Figure 2 The magnetic circuit structure shown in (a) is analyzed by applying Kirchhoff's laws of magnetic circuits and Faraday's law of electromagnetic induction, and the equivalent circuit model of the system at the port is derived, as follows: Figure 2 As shown in (b) of the model, this model clearly reveals the electrical relationship of each winding after being referred to the same side (such as the valve side): from the voltage perspective, the grid-side winding and the voltage regulating winding are in series, that is... From the perspective of current and power supply, the energy extraction winding and the grid-side winding are connected in parallel, that is... Equivalent circuit relationships are the cornerstone for understanding the working principle and control strategy of the entire system.

[0038] In one embodiment, the core objective of the control is to reduce the valve-side voltage. Maintain constancy. In a three-phase stationary coordinate system, according to... Figure 2 The equivalent circuit relationship shown in (b) is theoretically based on the voltage vector composition formula: Based on this formula, the controller calculates the ideal compensation voltage vector required to counteract grid voltage disturbances. .

[0039] In one embodiment, to achieve independent and precise control of voltage amplitude and phase, the controller performs decoupled operation in a synchronously rotating dq coordinate system. After obtaining the grid synchronization angle through a phase-locked loop, the AC voltage vector is transformed into a DC quantity. The d-axis command for the compensation voltage is then used. and q-axis commands They are determined by the following formulas respectively: in, and (usually set) () is a constant valve-side voltage command. and These are the measured grid-side voltage components. The d-axis commands are mainly used to compensate for voltage amplitude fluctuations, while the q-axis commands are mainly used to compensate for voltage phase fluctuations. Figure 4 The voltage vector diagram visually illustrates this geometric relationship.

[0040] In one embodiment, the aforementioned compensation voltage command needs to be implemented via a back-to-back converter.

[0041] Front-end energy harvesting unit control (see) Figure 5 The system employs a dual closed-loop strategy consisting of an outer voltage loop and an inner current loop. The outer voltage loop stabilizes the DC bus voltage, and its output serves as a reference value for the d-axis current. The inner current loop enables rapid current tracking and controls the q-axis current to be zero, thereby ensuring that the current absorbed from the energy harvesting winding is in phase with the voltage, achieving unity power factor operation.

[0042] Post-stage voltage regulation control (see) Figure 5 Similarly, a dual closed-loop strategy of voltage outer loop and current inner loop is adopted. The voltage outer loop is generated in the example. and The command controls the voltage at the regulating winding terminals; the inner current loop ensures a rapid response of the inverter output current, thereby accurately generating the required compensation voltage waveform.

[0043] To verify its performance, a simulation study was conducted. Figure 6 and Figure 7 Simulation waveforms show that when the grid-side voltage experiences amplitude or phase step disturbances, the valve-side voltage can recover quickly and remain highly stable. The system response speed far exceeds that of traditional mechanical voltage regulators, and the system maintains unity power factor operation on the energy extraction side throughout the entire process, achieving the design goal.

[0044] The technical features of the embodiments described above can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as the combination of these technical features does not contradict each other, it should be considered within the scope of this specification. It should be noted that the terms "in one embodiment," "for example," and "again" in this invention are intended to illustrate the invention and are not intended to limit the invention.

[0045] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A topology for a built-in flexible controllable converter transformer, characterized in that, include: Three-column five-winding integrated transformer and built-in back-to-back power converter; The three-column, five-winding integrated transformer has a middle column, a right column, and a left column. The middle column is wound with a grid-side winding W1 and an energy-harvesting winding W5. The grid-side winding W1 is used to connect to the high-voltage AC power grid and withstand the rated voltage of the system. The energy-harvesting winding W5 is wound on the same column as the energy-harvesting winding W1, and the two are tightly coupled through the magnetic circuit of the iron core. The right column is wound with a voltage-regulating winding W3, which is configured to achieve series compensation and regulation of the valve-side voltage through electromagnetic coupling. The left column is wound with valve windings W2 and W4. The built-in back-to-back power converter has its AC input side connected to the energy harvesting winding W5 and its AC output side connected to the voltage regulating winding W3.

2. The topology of the built-in flexible controllable converter transformer as described in claim 1, characterized in that, The built-in back-to-back power converter includes a rectifier unit, a DC support link, and an inverter unit. The AC side of the rectifier unit is connected to the output terminal of the energy harvesting winding W5 through an input filter circuit, converting the AC power output by the energy harvesting winding W5 into DC power. The DC side of the inverter unit is connected to the DC support link, and the AC side is connected to the input terminal of the voltage regulating winding W3 through an output filter circuit, converting the DC power into an adjustable voltage with the required amplitude and phase, which is then injected into the voltage regulating winding W3.

3. The topology of the built-in flexible controllable converter transformer as described in claim 1, characterized in that, Valve-side winding W2 is connected in a star configuration, while valve-side winding W4 is connected in a delta configuration.

4. A control method for a built-in flexible controllable converter transformer, characterized in that, A topology applied to the built-in flexible controllable converter transformer according to any one of claims 1-3, comprising: Real-time acquisition of AC voltage information from the grid side and the valve side; When amplitude or phase fluctuations of the grid-side voltage are detected: based on the amplitude or phase deviation of the grid-side voltage, the built-in back-to-back power converter is controlled to inject a corresponding compensation voltage into the regulating winding to keep the amplitude and phase of the valve-side voltage stable.

5. The control method for the built-in flexible controllable converter transformer as described in claim 4, characterized in that, Also includes: When the grid-side voltage returns to its rated value, the built-in back-to-back power converter is controlled to reduce or exit the compensation output, so that the system returns to the transformer's natural turns ratio operating state.

6. The control method for the built-in flexible controllable converter transformer as described in claim 4, characterized in that, The grid-side AC voltage information includes: grid-side voltage amplitude deviation and grid-side voltage phase deviation; the valve-side AC voltage information includes: valve-side voltage amplitude and phase.

7. The control method for the built-in flexible controllable converter transformer as described in claim 4, characterized in that, The steady-state voltage equations after the valve side are expressed as follows: ； ； in, The voltage vector of the valve-side winding W2. The voltage vector of valve-side winding W4. The voltage vector of the grid-side winding W1, The voltage vector of the voltage regulating winding W3. To obtain the voltage vector of the energy winding W5.

8. The control method for the built-in flexible controllable converter transformer as described in claim 7, characterized in that, The steady-state current equations after the valve side are expressed as follows: in, The current vector of the grid-side winding W1, To obtain the current vector of the energy winding W5, The current vector of the voltage regulating winding W3. The current vector of the valve-side winding W2. This is the current vector of the valve-side winding W4.

9. The control method for the built-in flexible controllable converter transformer as described in claim 8, characterized in that, In a three-phase stationary coordinate system, the amount of compensation voltage injected into the voltage regulating winding Determined by the following formula: ;in, This is the rated value of the valve-side voltage vector. Let be the voltage vector of the grid-side winding W1.

10. The control method for the built-in flexible controllable converter transformer as described in claim 8, characterized in that, Decoupling control is achieved in a synchronously rotating dq coordinate system, compensating for the direct-axis component reference value of the voltage command. Reference value of cross-axis component They are represented as follows: ; in, , The direct and quadrature components of the valve-side voltage rated value vector in the dq coordinate system; , The vector of the grid-side voltage measurement value is represented by the direct and quadrature axis components in the dq coordinate system.

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