Topology and control method of external flexible controllable converter transformer

Abstract

This invention discloses a topology and control method for an externally mounted flexible controllable converter transformer, belonging to the field of smart grid technology. The topology includes a three-phase, three-winding power frequency converter transformer and a series-connected AC / AC power electronic module. Voltage regulation is achieved by externally mounting the series-connected AC / AC power electronic module at the low-voltage side terminal of the converter transformer's grid-side winding. This allows for the upgrading and transformation of existing converter transformers in HVDC transmission projects without significant modifications or replacements to the original converter transformer itself, demonstrating good engineering feasibility and economy. This invention possesses the ability to coordinate with reactive power compensation filter switching and converter valve control, improving upon the slow response, long adjustment step size, and difficulties in coordinated control of traditional converter units. While maintaining its advantages of large capacity and high reliability, it enhances the system's adaptability to grid disturbances and power command changes.

Description

Technical Field

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

[0002] With the continuous expansion of new energy grid connection and the accelerated development of electricity spot markets and real-time trading mechanisms, DC transmission systems are gradually shifting from an operation mode primarily focused on stable power transmission to one demanding faster and more precise regulation of power and voltage. In existing LCC-HVDC (Line-Commutated Converter High-Voltage Direct Current) systems, converter units typically rely on mechanical tap switches to regulate converter transformer voltage and control system operation through reactive power compensation filters and converter valve trigger angle control. This regulation method suffers from slow response, large adjustment steps, and discrete characteristics, making it difficult to meet the demands of frequent fluctuations in new energy output and power command changes on minute-level or even shorter timescales. Furthermore, existing converter transformer voltage regulation primarily affects voltage amplitude and lacks voltage phase regulation capabilities, resulting in insufficient flexibility of converter units in weak AC grids or when grid conditions change rapidly. Frequent switching of reactive power compensation devices also easily triggers voltage fluctuations and transient impacts, increasing the difficulty of system coordination and control.

[0003] Based on the above problems, there is an urgent need for a new converter unit structure and control method that can achieve rapid, continuous, and wide-range voltage regulation and can be precisely coordinated with converter valve control and reactive power compensation devices, so as to significantly improve the system's adaptability to new energy fluctuations and power market operation demands while maintaining the advantages of large capacity and high reliability of LCC-HVDC. Summary of the Invention

[0004] In view of the above-mentioned defects or improvement needs of the existing technology, the present invention provides a topology and control method for an external flexible controllable converter transformer. Its purpose is to solve the technical problem of insufficient flexibility of the converter unit caused by the lack of voltage phase adjustment capability in the existing converter transformer voltage regulation method.

[0005] To achieve the above objectives, according to one aspect of the present invention, a topology of an external flexible controllable converter transformer is provided, comprising: a group-type three-phase three-winding power frequency converter transformer and an AC / AC power electronic module; The three-phase, three-winding power frequency converter transformer includes: two valve-side windings and one grid-side winding; both valve-side windings are connected to the high-voltage direct current transmission inverter side, the first valve-side winding adopts a star connection, and the second valve-side winding adopts a delta connection; the high-voltage terminal of the grid-side winding is connected to the receiving-end AC power grid, and the low-voltage terminal of the grid-side winding is externally connected in series with the AC / AC power electronic module; the other end of the AC / AC power electronic module is connected to the receiving-end power grid via a power extraction transformer; During operation, the AC grid voltage and power information of the receiving end are collected; when the grid voltage deviates from the preset value or the power transmission demand changes, the AC / AC power electronic module is controlled to inject an adjustable voltage with controllable amplitude and phase into the grid-side winding to regulate the voltage at both ends of the grid-side winding.

[0006] Furthermore, the amplitude and phase of the voltage across the grid-side winding are jointly determined by the receiving-end AC grid voltage and the regulating voltage injected by the AC / AC power electronic module.

[0007] Furthermore, the voltage across the grid-side winding is: ,in, For the receiving end AC grid voltage phasor, The regulated voltage phasor injected into the AC / AC power electronic module This is the voltage phasor across the grid-side winding.

[0008] Furthermore, when the voltage across the grid-side winding is controlled to a set value, the voltage of the valve-side winding is determined by the voltage clamping effect of the three-phase three-winding power frequency converter transformer, thereby realizing the controllable adjustment of the grid-side winding voltage and the valve-side winding voltage.

[0009] Furthermore, the AC / AC power electronic module includes an energy harvesting converter and a voltage regulating converter; the AC side of the voltage regulating converter is connected in series with the low-voltage end of the grid-side winding of the corresponding phase via an isolation transformer to form an AC regulating voltage with continuously adjustable amplitude and phase; the isolation transformer is used to achieve electrical isolation and match the voltage level, enabling the voltage regulating converter to operate at low voltage levels.

[0010] Furthermore, when the grid voltage deviates from the preset value or the power transmission demand changes, the voltage regulating converter is controlled to inject an amplitude- and phase-controllable regulating voltage into the grid-side winding through a series-connected AC / AC power electronic module, so as to regulate the voltage at both ends of the grid-side winding.

[0011] According to another aspect of the present invention, a method for controlling the topology of an external flexible controllable converter transformer is provided, comprising: during the operation of the external flexible controllable converter transformer, acquiring AC grid voltage and power information at the receiving end; when the grid voltage deviates from a preset value or the power transmission demand changes, controlling the AC / AC power electronic module to inject an adjustable voltage with controllable amplitude and phase into the grid-side winding to achieve adjustment of the voltage at both ends of the grid-side winding.

[0012] Furthermore, the amplitude and phase of the voltage across the grid-side winding are jointly determined by the receiving-end AC grid voltage and the regulating voltage injected by the AC / AC power electronic module.

[0013] Furthermore, the voltage across the grid-side winding is: ,in, For the receiving end AC grid voltage phasor, The regulated voltage phasor injected into the AC / AC power electronic module This is the voltage phasor across the grid-side winding.

[0014] Furthermore, when the voltage across the grid-side winding is controlled to a set value, the voltage of the valve-side winding is determined by the voltage clamping effect of the three-phase three-winding power frequency converter transformer, thereby realizing the controllable adjustment of the grid-side winding voltage and the valve-side winding voltage.

[0015] 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 proposes a topology for an external flexible controllable converter transformer, comprising a three-phase, three-winding power frequency converter transformer and a series-connected AC / AC power electronic module. Voltage regulation is achieved by externally connecting the series-connected AC / AC power electronic module to the low-voltage side terminals of the converter transformer windings. This allows for the upgrade and transformation of existing LCC-HVDC converter transformers without requiring significant modifications or replacements to the original converter transformer itself, demonstrating good engineering feasibility and economy. The external flexible controllable converter transformer provided by this invention has the ability to coordinate with the switching of reactive power compensation filters and the control of converter valves. This improves upon the slow response, long adjustment step size, and difficulty in coordinated control of traditional LCC-HVDC converter units, maintaining their advantages of large capacity and high reliability while enhancing the system's adaptability to grid disturbances and power command changes.

[0016] (2) This invention utilizes the existing power frequency converter transformer to perform the ultra-high voltage bearing and large-capacity energy transmission functions, and implements the voltage regulation function through an external series-connected AC / AC power electronic module, thus decoupling the voltage bearing and voltage regulation functions at the structural level. This reduces the voltage withstand requirements of the power electronic device, and achieves flexible and controllable voltage regulation capability while maintaining the system's voltage withstand capability and reliability.

[0017] (3) By connecting the AC / AC power electronic module in series to the low-voltage terminal of the grid-side winding, the injected regulating voltage directly participates in the formation of the equivalent voltage across the grid-side winding. Compared with indirect regulation on the AC bus side or DC side, this topology can directly regulate the voltage conditions from the input terminal of the transformer body, making the regulation action path shorter and the control link clearer, which is conducive to improving regulation accuracy and system dynamic response speed.

[0018] (4) The AC / AC power electronic module in this invention adopts a back-to-back converter structure, and the energy harvesting converter and the voltage regulating converter are connected through a DC bus. This structure allows the energy required for voltage regulation to be directly obtained from the receiving end AC grid, and the energy is buffered and distributed through the DC bus, avoiding the need for an external independent power supply, improving the overall energy utilization efficiency of the system, and supporting the bidirectional flow of regulating power, thus enhancing the system's operational flexibility.

[0019] (5) This invention provides a control method for an external ultra-high voltage flexible controllable converter transformer. The control strategy is as follows: by real-time acquisition of AC grid voltage and power information at the receiving end, when the grid voltage deviates from the preset value or the power transmission demand changes, the AC / AC power electronic module is controlled to inject an adjustable voltage with controllable amplitude and phase into the grid-side winding, thereby achieving controllable adjustment of the grid-side winding voltage and the valve-side winding voltage. Compared with the traditional converter transformer voltage regulation method that relies on mechanical tap switches, this invention is beneficial to improving the continuity and response capability of voltage regulation. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the overall electrical topology of a DC transmission system provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of an external flexible controllable converter transformer topology provided in an embodiment of the present invention; Figure 3 This is a schematic diagram of the voltage regulation principle of an external flexible controllable converter transformer according to an embodiment of the present invention; Figure 4 This is a control flowchart of an external flexible controllable converter transformer provided in an embodiment of the present invention; Figure 5 This is a control block diagram of an external flexible controllable converter transformer provided in an embodiment of the present invention; Figure 6This is a simulation verification diagram under grid voltage amplitude and phase disturbance provided in an embodiment of the present invention. Detailed Implementation

[0021] 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.

[0022] Example 1 This embodiment provides a topology for an externally mounted flexible controllable converter transformer, including: a three-phase, three-winding power frequency converter transformer and an AC / AC power electronic module. The three-phase, three-winding power frequency converter transformer includes: two valve-side windings and one grid-side winding; both valve-side windings are connected to the inverter side of a high-voltage direct current (HVDC) transmission line, the first valve-side winding is star-connected, and the second valve-side winding is delta-connected; the high-voltage terminal of the grid-side winding is connected to the receiving-end AC power grid, and the low-voltage terminal of the grid-side winding is externally connected in series with the AC / AC power electronic module; the other end of the AC / AC power electronic module is connected to the receiving-end power grid via a power strip. Specifically, the three-phase, three-winding power frequency converter transformer is a converter transformer used in existing HVDC transmission projects, and the externally mounted ultra-high voltage flexible controllable converter transformer is formed by externally connecting the AC / AC power electronic module in series with the low-voltage terminal of the grid-side winding of the converter transformer. During operation, the AC grid voltage and power information of the receiving end are collected; when the grid voltage deviates from the preset value or the power transmission demand changes, the AC / AC power electronic module is controlled to inject an adjustable voltage with controllable amplitude and phase into the grid-side winding to regulate the voltage at both ends of the grid-side winding. Figure 1 This is a schematic diagram of the overall electrical topology of the DC transmission system provided by the present invention. Figure 2 This is the topology of an externally mounted flexible controllable converter transformer.

[0023] like Figure 2As shown, the group-type three-phase three-winding power frequency converter transformer consists of three identical single-phase three-winding transformers, corresponding to phases A, B, and C of the three-phase AC system, respectively, with each phase electrically independent. Each single-phase three-winding transformer includes two valve-side windings and one grid-side winding. The two valve-side windings are connected in star and delta configurations, respectively, and are connected to the thyristor converter valves in the converter station. Their function is to provide the converter valves with a commutation voltage with a fixed phase difference to meet the phase displacement requirements of LCC-HVDC converter operation. One end of the grid-side winding is directly connected to the receiving-end AC grid as the high-voltage end, and the other end is led out as the low-voltage end for series connection with the AC / AC power electronic module, thereby making the low-voltage end a controllable potential node.

[0024] Furthermore, the amplitude and phase of the voltage across the grid-side winding are jointly determined by the receiving-end AC grid voltage and the regulating voltage injected by the AC / AC power electronic module.

[0025] Furthermore, the voltage across the grid-side winding is: ,in, For the receiving end AC grid voltage phasor, The regulated voltage phasor injected into the AC / AC power electronic module This is the voltage phasor across the grid-side winding.

[0026] Furthermore, when the voltage across the grid-side winding is controlled to a set value, the voltage of the valve-side winding is determined by the voltage clamping effect of the three-phase three-winding power frequency converter transformer, thereby realizing the controllable adjustment of the grid-side winding voltage and the valve-side winding voltage.

[0027] As an optional implementation, the AC / AC power electronic module includes a power harvesting converter and a voltage regulating converter. The AC side of the voltage regulating converter is connected in series with the low-voltage end of the corresponding phase's grid-side winding via an isolation transformer to form an AC regulated voltage with continuously adjustable amplitude and phase. The isolation transformer is used to achieve electrical isolation and match the voltage level, enabling the voltage regulating converter to operate at low voltage levels. Furthermore, when a deviation of the grid voltage from a preset value or a change in power transmission demand is detected, the voltage regulating converter is controlled to inject an amplitude- and phase-controllable regulating voltage into the grid-side winding through the series-connected AC / AC power electronic module, thereby regulating the voltage across the grid-side winding.

[0028] Specifically, the AC / AC power electronic module adopts a back-to-back converter topology, including a harvester converter, a voltage regulator converter, and a DC bus connecting the two. The DC bus is composed of DC capacitors, which serve to temporarily store energy and decouple instantaneous power fluctuations between the two converters. The AC side of the harvester converter is connected to the receiving-end AC grid via a harvester winding; its function is to obtain the energy required for voltage regulation from the receiving-end AC grid and maintain the stability of the DC bus voltage. The AC side of the voltage regulator converter is connected in series with the low-voltage end of the corresponding phase's grid-side winding via an isolation transformer to form a continuously adjustable AC regulated voltage in terms of amplitude and phase. The isolation transformer provides electrical isolation and voltage matching, enabling the voltage regulator converter to operate at lower voltage levels.

[0029] Figure 3 The diagram illustrates the voltage regulation principle of the converter transformer. This analysis is based on the control objective of providing a suitable compensation voltage through a series AC / AC power electronic module before and after grid-side system voltage disturbances, thereby maintaining the grid-side and valve-side winding voltages constant. Figure 3 (a) shows the operating condition when the grid-side system voltage is in an ideal state. When the grid-side system voltage changes, such as Figure 3 The hysteresis jump in (b) or Figure 3 In step (c), the advanced switching of the voltage regulator converter, under the control of the control system, injects a compensation voltage into the low-voltage end of the grid-side winding, so that the actual voltage across the grid-side winding is determined by both the receiving-end AC grid voltage and the compensation voltage. By adjusting the amplitude and phase of the supporting voltage, continuous adjustment of the amplitude and phase of the grid-side winding voltage can be achieved. Due to the voltage transmission relationship and voltage clamping characteristics of the power frequency converter transformer, the valve-side winding voltage is adjusted synchronously with the change of the grid-side winding voltage, thereby changing the AC side voltage conditions of the converter valve.

[0030] Through the above structural design, the equivalent input voltage of the converter transformer can be controlled and adjusted without changing the main structure of the power frequency converter transformer, thus enabling the converter transformer to have flexible voltage regulation capability.

[0031] Example 2 This embodiment provides a control method for the topology of an external flexible controllable converter transformer, including: during the operation of the external flexible controllable converter transformer, collecting AC grid voltage and power information at the receiving end; when the grid voltage deviates from a preset value or the power transmission demand changes, controlling the AC / AC power electronic module to inject an adjustable voltage with controllable amplitude and phase into the grid-side winding to achieve adjustment of the voltage at both ends of the grid-side winding.

[0032] Figure 4This is a control flowchart proposed in one embodiment of the present invention. In the operation scenario of LCC-HVDC interconnection with a large AC power grid, the voltage information of the receiving-end AC power grid and the power information related to the grid operation status, including active power and reactive power information, are first collected in real time. On this basis, the real-time collected grid voltage information is compared with the preset target voltage value. When a deviation between the actual grid voltage and the expected value is detected, the voltage regulating converter is controlled to perform voltage quantitative tracking control. By injecting controllable compensation voltage into the grid-side winding, the amplitude and phase of the grid-side winding voltage and the valve-side winding voltage are corrected. At the same time, when the power flow demand of the power grid changes, the phase of the grid-side winding and the valve-side winding voltage is actively adjusted according to the voltage phase angle information corresponding to the current power flow state, so as to achieve matching control of the power transmission state.

[0033] like Figure 5 As shown, both the harvested converter and the voltage-regulating converter employ a dual closed-loop control structure combining an outer voltage loop and an inner current loop. The outer voltage loop of the harvested converter controls the DC bus voltage, while the outer voltage loop of the voltage-regulating converter controls the filtered AC voltage. Both generate current commands via a voltage regulator by comparing the sampled voltage value with a corresponding reference value. The inner current loop controls the three-phase AC current flowing through the filter inductor, enabling rapid tracking control of the current commands.

[0034] In one embodiment, the external ultra-high voltage flexible controllable converter transformer control method involves both the energy harvesting converter and the voltage regulating converter using phase-locked loops (PLLs) based on the receiving-end AC grid voltage to obtain the phase angle and angular frequency of a synchronous rotating coordinate system. The phase angle is then used to transform the sampled three-phase voltage and current signals from the stationary coordinate system to the synchronous rotating coordinate system. In the synchronous rotating coordinate system, a current inner loop decoupling control is used to generate voltage commands, which are then used by a pulse width modulation module to generate drive signals. The synchronous rotating coordinate system uses the receiving-end AC grid voltage phasor as its orientation axis, and the combined controllable adjustment of the grid-side winding voltage amplitude and phase is achieved by adjusting the d-axis and q-axis control quantities, respectively. The d-axis control quantity is used to adjust the voltage amplitude, and the q-axis control quantity is used to adjust the voltage phase. Furthermore, in the external ultra-high voltage flexible controllable converter transformer control method, the voltage outer loop reference values ​​of the energy harvesting converter and the voltage regulating converter are set or adjusted according to the grid operating status, disturbance type, or system operating requirements.

[0035] During the control process, both types of converters use a phase-locked loop (PLL) based on the receiving-end AC grid voltage to acquire the phase angle and angular frequency information of the synchronous rotating coordinate system. The phase angle is then used to transform the sampled three-phase voltage and current signals from the stationary coordinate system to the synchronous rotating coordinate system. In the synchronous rotating coordinate system, a cross-coupling compensation term based on the filter inductor parameters is introduced into the inner current loop to reduce the coupling effect between the d-axis and q-axis currents. The voltage command signal generated after current regulation and decoupling compensation is sent to the pulse width modulation module to generate the corresponding converter drive signal, thereby achieving stable control of the DC bus voltage and precise output of the regulated voltage.

[0036] Figure 6 A simulation verification diagram is presented for a receiving-end AC grid voltage amplitude and phase under combined disturbances. All analyses below are based on per-unit (pu) and consider the symmetry of the three-phase system. Voltage phase analysis focuses only on phase A; the other two phases are essentially the same as phase A. Assume the initial receiving-end grid phase A voltage is 1∠0°, and neglect the leakage withstand voltage drop of the converter transformer. Within a short period, frequent combined disturbances to the receiving-end AC grid voltage amplitude and phase are applied. Based on the aforementioned goal of maintaining constant voltage on both the grid side and valve side windings, a control strategy is applied. The accuracy of the topology and its control strategy in achieving precise control of the grid side winding voltage is determined by observing whether the compensation voltage injection value matches the expectation and whether the grid-side phase A winding voltage is consistently controlled to 1∠0°. Figure 6 As can be seen in (a), after each disturbance, the voltage of the grid-side A-phase winding is quickly controlled to an operating state with an amplitude of 1, and the amplitude of the injected compensation voltage is also consistent with the expectation. The expected value is the theoretical value required for the compensation voltage assuming that the grid-side A-phase winding voltage is 1∠0° after control. Figure 6 As can be seen in (b), even with frequent disturbances in the initial phase of the A-phase voltage of the receiving-end AC grid, the initial phase of the A-phase winding voltage on the grid side can always be guaranteed to be 0°, and the amplitude and phase of the grid-side winding voltage are precisely controlled.

[0037] In one embodiment, the outer-loop voltage reference values ​​of the energy harvesting converter and the voltage regulating converter can be flexibly set and switched according to the grid operating status and system operating objectives. The aforementioned analysis is based on the purpose of maintaining the grid-side and valve-side winding voltages constant, which provides the control strategy and outer-loop voltage reference values. A new control strategy, namely the voltage compensation maximization strategy, does not consider phase compensation after each disturbance in the receiving-end grid voltage, but instead focuses on rapidly supporting the amplitude of the winding voltage. In this case, the compensation voltage should be in phase or out of phase with the disturbed receiving-end AC grid voltage to maximize the ability to maintain the stability of the winding voltage amplitude. Therefore, through the above analysis, it can be seen that the outer-loop voltage reference values ​​of the energy harvesting converter and the voltage regulating converter can be flexibly set and switched according to different grid operating states, disturbance types, and system operating objectives.

[0038] In the aforementioned voltage regulation process, by introducing an external ultra-high voltage flexible controllable converter transformer, a clear division of labor is established among the various regulation objects within the converter unit in terms of time scale and regulation responsibilities. Specifically, rapid and continuous voltage regulation no longer primarily relies on the switching operation of reactive power compensation filter devices, but is achieved by the external ultra-high voltage flexible controllable converter transformer through the AC / AC power electronics module. This converter transformer can inject adjustable voltage with controllable amplitude and phase into the grid-side winding within a short time scale, thereby rapidly correcting the AC side voltage conditions of the converter valve and enabling timely response to fluctuations in renewable energy output or changes in power market power commands.

[0039] Based on this, the functional positioning of reactive power compensation filter devices has shifted from the traditional "rapid voltage regulation means" to "steady-state reactive power support and harmonic suppression device." Its switching process is mainly adjusted based on the system's steady-state operating state or slowly changing reactive power demand, avoiding frequent operations within a short timescale. By reducing the high-frequency switching of reactive power compensation devices, not only is the probability of voltage steps and transient impacts reduced, but the electrical and mechanical stresses on the reactive power compensation equipment are also decreased, which helps improve the operational reliability of the device.

[0040] Meanwhile, the converter valve control no longer needs to undertake the main voltage regulation task by significantly adjusting the firing angle. Instead, based on the pre-adjustment of the AC side voltage conditions by the external ultra-high voltage flexible controllable converter transformer, it completes active power regulation within a smaller range. As a result, the operating point of the converter valve changes more smoothly, the firing angle adjustment range is reduced, which helps maintain good commutation margin and reduce the risk of commutation failure.

[0041] Through the above methods, a multi-level collaborative control system is formed within the converter unit, consisting of a fast voltage regulation layer centered on an externally mounted ultra-high voltage flexible controllable converter transformer, a steady-state support layer based on a reactive power compensation filter device, and a power execution layer based on converter valve control. This collaborative system, without altering the basic structure of the converter valves and reactive power compensation filters in traditional grid-based phase-commutation converter-based high-voltage direct current transmission (LCC-HVDC), achieves precise and rapid coordination between converter transformer voltage regulation, reactive power compensation switching, and valve control. This enables the converter unit to maintain the speed, wide range, and continuous smoothness of voltage regulation under conditions of new energy fluctuations and electricity market operation.

[0042] Furthermore, the amplitude and phase of the voltage across the grid-side winding are jointly determined by the receiving-end AC grid voltage and the regulating voltage injected by the AC / AC power electronic module.

[0043] Furthermore, the voltage across the grid-side winding is: ,in, For the receiving end AC grid voltage phasor, The regulated voltage phasor injected into the AC / AC power electronic module This is the voltage phasor across the grid-side winding.

[0044] Furthermore, when the voltage across the grid-side winding is controlled to a set value, the voltage of the valve-side winding is determined by the voltage clamping effect of the three-phase three-winding power frequency converter transformer, thereby realizing the controllable adjustment of the grid-side winding voltage and the valve-side winding voltage.

[0045] The topology of the external flexible controllable converter transformer provided by this invention, combined with its control method, can achieve fast, wide-range, and smooth voltage regulation capabilities. Therefore, it is considered to achieve precise and rapid coordinated control between the transformer and the reactive power compensation filter switching and valve control to meet the requirements of the new energy and power market.

[0046] 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.

[0047] 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 an externally mounted flexible controllable converter transformer, characterized in that, include: Grouped three-phase three-winding power frequency converter transformer and AC / AC power electronic module; The three-phase, three-winding power frequency converter transformer includes: two valve-side windings and one grid-side winding; both valve-side windings are connected to the high-voltage direct current transmission inverter side, the first valve-side winding adopts a star connection, and the second valve-side winding adopts a delta connection; the high-voltage terminal of the grid-side winding is connected to the receiving-end AC power grid, and the low-voltage terminal of the grid-side winding is externally connected in series with the AC / AC power electronic module; the other end of the AC / AC power electronic module is connected to the receiving-end power grid via a power extraction transformer; During operation, the AC grid voltage and power information of the receiving end are collected; when the grid voltage deviates from the preset value or the power transmission demand changes, the AC / AC power electronic module is controlled to inject an adjustable voltage with controllable amplitude and phase into the grid-side winding to regulate the voltage at both ends of the grid-side winding.

2. The topology of the external flexible controllable converter transformer as described in claim 1, characterized in that, The amplitude and phase of the voltage across the grid-side winding are jointly determined by the receiving-end AC grid voltage and the regulating voltage injected by the AC / AC power electronic module.

3. The topology of the external flexible controllable converter transformer as described in claim 2, characterized in that, The voltage across the grid-side winding is: ,in, For the receiving end AC grid voltage phasor, The regulated voltage phasor injected into the AC / AC power electronic module This is the voltage phasor across the grid-side winding.

4. The topology of the external flexible controllable converter transformer as described in claim 2, characterized in that, When the voltage across the grid-side winding is controlled to a set value, the voltage of the valve-side winding is determined by the voltage clamping effect of the three-phase three-winding power frequency converter transformer, thereby realizing the controllable adjustment of the grid-side winding voltage and the valve-side winding voltage.

5. The topology of the external flexible controllable converter transformer as described in claim 1, characterized in that, AC / AC power electronic modules include energy harvesting converters and voltage regulating converters; The AC side of the voltage regulating converter is connected in series with the low-voltage end of the grid-side winding of the corresponding phase via an isolation transformer to form an AC regulating voltage with continuously adjustable amplitude and phase. The isolation transformer is used to achieve electrical isolation and match the voltage level, enabling the voltage regulating converter to operate at low voltage levels.

6. The topology of the external flexible controllable converter transformer as described in claim 5, characterized in that, When the grid voltage deviates from the preset value or the power transmission demand changes, the voltage regulating converter is controlled to inject an adjustable voltage with controllable amplitude and phase into the grid-side winding through the AC / AC power electronic module set in series, so as to regulate the voltage at both ends of the grid-side winding.

7. A control method for the topology of an externally mounted flexible controllable converter transformer, characterized in that, include: During the operation of the external flexible controllable converter transformer as described in any one of claims 1-6, the voltage and power information of the receiving-end AC power grid are collected; When the grid voltage deviates from the preset value or the power transmission demand changes, the AC / AC power electronic module is controlled to inject an adjustable voltage with controllable amplitude and phase into the grid-side winding to regulate the voltage at both ends of the grid-side winding.

8. The control method for the topology of the external flexible controllable converter transformer as described in claim 7, characterized in that, The amplitude and phase of the voltage across the grid-side winding are jointly determined by the receiving-end AC grid voltage and the regulating voltage injected by the AC / AC power electronic module.

9. The control method for the topology of the external flexible controllable converter transformer as described in claim 8, characterized in that, The voltage across the grid-side winding is: ,in, For the receiving end AC grid voltage phasor, The regulated voltage phasor injected into the AC / AC power electronic module This is the voltage phasor across the grid-side winding.

10. The control method for the topology of the external flexible controllable converter transformer as described in claim 8, characterized in that, When the voltage across the grid-side winding is controlled to a set value, the voltage of the valve-side winding is determined by the voltage clamping effect of the three-phase three-winding power frequency converter transformer, thereby realizing the controllable adjustment of the grid-side winding voltage and the valve-side winding voltage.

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