On-load voltage regulation distribution transformer with series converter chains bearing partial power

Abstract

The invention discloses an on-load voltage regulation distribution transformer with series converter chains bearing partial power, which is characterized by comprising a group of three-phase series converter chains and a three-phase industrial frequency transformer, the three-phase series converter chain is used for three-phase voltage regulation and comprises three converter sub-chains and N4 three-phase converter sink chains, and N4 is equal to 0 or 1; the three-phase industrial frequency transformer is used for isolating and reducing voltage and providing an energy transfer port for maintaining energy conservation of the three-phase series converter chain, the three-phase industrial frequency transformer comprises 3N0 + 3N1 + 3N2 + 3N2 + 3N4 primary windings and three secondary windings, and N0 is greater than or equal to 0; the invention solves the problems of short service life of the contact and power supply interruption of the sensitive load, and realizes continuous voltage regulation with lower equipment cost.

Description

Technical Field

[0001] This invention relates to the field of power electronics and power conversion technology, specifically to an on-load tap-changing distribution transformer in which a series converter chain bears part of the power. Background Technology

[0002] With the large-scale integration of electric vehicles and distributed energy resources, the inherent randomness and intermittency of these systems combine to cause frequent voltage fluctuations in the distribution network. This not only affects the stable grid connection of distributed energy resources but may also jeopardize the safe operation of precision electrical equipment. To ensure grid voltage stability, on-load tap-changing distribution transformers have become an indispensable key piece of equipment.

[0003] Traditional on-load tap-changing distribution transformers based on mechanical tap changers achieve voltage regulation by switching taps with mechanical contacts while the transformer is energized. However, mechanical switches generate electric arcs during the switching process. Frequent operation of these switches not only leads to contact erosion and shortens equipment lifespan, but also accelerates the oxidation of the insulating medium, reducing its insulation performance and affecting the long-term reliability of the equipment.

[0004] To overcome the aforementioned shortcomings, existing research has proposed replacing mechanical switches with all-power electronic devices or hybrid mechanical-power electronic devices to achieve on-load voltage regulation. This approach avoids the load switching of mechanical switches, thus solving the problems of short contact life and low reliability of mechanical voltage regulators. However, the high-voltage power electronic devices required in these solutions are expensive and typically cannot achieve continuous voltage regulation.

[0005] Another study proposed achieving voltage regulation by combining transformer windings with switching units to change the number of turns in the windings connected to the circuit. This approach can reduce the voltage level required for power electronic devices, but it is still limited to tiered voltage regulation and cannot achieve precise and continuous voltage regulation.

[0006] To achieve continuous and smooth voltage regulation, some solutions incorporate power electronic converters. However, these converters have large capacities and high costs, and typically require additional transformers for series connection, increasing the overall system cost and severely limiting their large-scale industrial application.

[0007] Therefore, there is an urgent need to develop a new on-load tap-changing distribution transformer topology to solve the problems of short contact life and power outages for sensitive loads, while achieving continuous voltage regulation and significantly reducing system costs, so as to promote the widespread application of this technology in actual power grids. Summary of the Invention

[0008] To overcome the shortcomings of the existing technology, the present invention provides an on-load tap-changing distribution transformer in which a series converter chain bears part of the power, solving the problems of short contact life and power interruption of sensitive loads, while achieving continuous voltage regulation at a lower equipment cost.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows: An on-load tap-changing distribution transformer with a series converter chain bearing part of the power includes a three-phase series converter chain and a three-phase power frequency transformer. The three-phase series converter chain is used for three-phase voltage regulation and includes 3 converter sub-chains and N4 three-phase converter bus chains, where N4 is equal to 0 or 1; Each converter sub-chain comprises N1 two-port power frequency commutation modules with stepped voltage regulation, N2 two-port AC-AC voltage regulation modules with continuous voltage regulation, and N3 two-port single-phase AC-DC-AC voltage regulation modules with continuous voltage regulation, wherein N1, N2, and N3 are all greater than or equal to 0, and satisfy N 1+ N2+N3+N4 is greater than or equal to 1; The input and output voltages of the power frequency commutation module, the AC-AC voltage regulation module, and the single-phase AC-DC-AC voltage regulation module are denoted as u, respectively. xi v xi Where x = a, b, c represent phases A, B, and C, and i = 1, 2, ..., N1 + N2 + N3 represent the module index number in each phase subchain; The input ports of the power frequency commutation module, AC-AC voltage regulation module and single-phase AC-DC-AC voltage regulation module in the converter sub-chain are connected in series, and the two outermost terminals of the series connection are led out; the outermost terminals (X, A'), (Y, B') and (Z, C') of the three converter sub-chains constitute the input ports XA', YB' and ZC' of the converter sub-chain.

[0010] The three-phase converter bus is a three-phase two-port converter that aggregates three converter sub-chains and continuously adjusts the three-phase voltage. Its input and output port voltages are u, respectively. xt v xt The three terminals of the input port of the three-phase two-port converter are respectively connected to the external terminals (A', B', C') of the three converter sub-chains to achieve three-phase convergence. The remaining external terminals (X, Y, Z) are led out to form the input ports XYZ of the three-phase series converter chain. The output terminals of the 3N1+3N2+3N3 modules and the output terminals of the N4 three-phase converter chains in the converter sub-chains are led out to form the 3N1+3N2+3N3+N4 output ports of the three-phase series converter chain for external power supply. The input ports of the three-phase series converter chain may or may not be equipped with a centralized low-pass filter.

[0011] The three-phase power frequency transformer is used for isolation and voltage reduction, and provides an energy transfer port to maintain energy conservation in the three-phase series converter chain. It comprises four primary windings (3N0+3N1+3N2+3N3+3N) and three secondary windings, where N0 is greater than or equal to 0. The turns ratio of the primary winding to the secondary winding is... The number of turns in each winding can be set arbitrarily.

[0012] The terminals of the three secondary windings of the three-phase power frequency transformer are directly led out as output ports (aa', bb', cc'); the 3N1+3N2+3N3 primary windings of the three-phase power frequency transformer are respectively connected to the 3N1+3N2+3N3 output ports of the three-phase series converter chain; the 3N4 primary windings are connected in a star or delta structure in the three phases to form N4 three-phase interfaces, which are connected to the N4 output ports of the three-phase series converter chain.

[0013] When the number of three-phase converter chains N4 equals 0, the input ports (XA', YB', ZC') of the three converter sub-chains are connected in series with the primary windings of 3N0 transformers to obtain input ports (AA', BB', CC'). This connection method constitutes a three-phase independent basic circuit unit with input ports (AA', BB', CC') and output ports (aa', bb', cc'). The input and output ports of this three-phase independent basic circuit unit are connected in a star or delta configuration, and the terminals are led out to form an on-load tap-changing distribution transformer with external input port ABC and external output port abc.

[0014] When the number of three-phase converter chains N4 equals 1, the voltage between the input terminals (X, Y, Z) and the 3N0 ports of the three-phase series converter chain is u. pxi The primary windings of the transformer are connected in series in the three phases to obtain input terminals (A, B, C); the above connection method constitutes a three-phase coupled basic circuit unit with input ports (ABC) and output ports (aa', bb', cc'); the output ports (aa', bb', cc') of the three-phase coupled basic circuit unit are connected in a star or delta configuration, and the terminals are led out to form an on-load tap-changing distribution transformer with external input port ABC and external output port abc.

[0015] The input voltage of the on-load tap-changing distribution transformer is expressed as follows: (1) Since the input port of the three-phase series converter chain is connected in series with 3N0 primary windings, and their current values ​​are exactly the same, the power carried by the three-phase series converter chain can be expressed as: (2) In the formula px Let be the total power of the distribution transformer. Equation (2) shows that the three-phase converter chain only undertakes part of the power transmission, which greatly reduces its power loss and equipment cost.

[0016] Preferably, the three-phase two-port converter is used to act as a conduit to gather three converter sub-chains and realize the three-phase voltage continuous regulation function. It is a two-level circuit structure or a three-level circuit structure with a low-pass filter at the output port, and the input port may or may not be equipped with a low-pass filter. The switching device of the three-phase converter is a fully controllable power semiconductor device such as IGBT, MOSFET, IGCT or GaN.

[0017] When the three-phase two-port converter is a two-level structure, its circuit topology is a back-to-back three-phase two-level converter composed of two-level bridge arms. The three-phase two-level converter can be a three-phase three-wire two-level converter or a three-phase four-wire two-level converter. The midpoint of the bridge arm of the output-side three-phase converter is led out as the output port after passing through a low-pass filter, and the midpoint of the bridge arm of the input-side three-phase converter is led out directly or after passing through an optional low-pass filter as the input port. The output-side three-phase converter controls the DC bus voltage to be stabilized at V. xdc The input-side three-phase converter enables continuous voltage regulation at the input port. The mathematical relationship between the input voltage and the DC capacitor voltage is as follows: (3) In the formula, m1 is the modulation ratio of the input-side three-phase converter, which satisfies that m1 is greater than or equal to 0 and less than or equal to 1.

[0018] When the three-phase two-port converter is a three-level structure, the two-level bridge arm in the two-level structure is replaced with a three-level bridge arm, the midpoint of the three-level bridge arm is connected to the midpoint of the DC capacitor, and the remaining circuit connections remain unchanged from the two-level structure; the mathematical relationship between the output voltage and the input voltage remains the same as the formula (3) of the two-level structure. The beneficial effect of the three-level structure is that, compared with the two-level structure, the input port voltage range can be doubled when using the same withstand voltage switching devices.

[0019] The aforementioned three-phase two-port converter is used to act as a consolidation chain to gather three converter sub-chains and realize the three-phase voltage continuous regulation function. In addition, the back-to-back three-phase converter structure makes the three-phase circuit have a common DC capacitor, which causes the double frequency fluctuation power from the AC side of each phase to be canceled at the DC capacitor, greatly reducing the capacitance value.

[0020] Preferably, the power frequency commutation module is used for graded voltage regulation, and it has a two-level structure or a three-level structure; the switching device of the power frequency commutation module is a bidirectional thyristor, a reverse parallel thyristor, a reverse series IGBT, a reverse parallel IGBT bare tube, a reverse parallel gate turn-off thyristor (GTO), a reverse series integrated gate commutated thyristor (IGCT), a reverse parallel IGCT bare tube, a reverse series MOSFET, or a fully controlled or semi-controlled power semiconductor device or a combination circuit of multiple power semiconductor devices with bidirectional interruption capability.

[0021] When the power frequency commutation module has a two-level structure, it is an H-bridge circuit composed of bidirectional switches. The midpoints of the two arms of the H-bridge are led out as input ports, and the parallel terminals of the arms are led out as output ports. By controlling the on and off states of the four switches in the H-bridge, the input voltage u of the power frequency commutation module is controlled. xi Three gears -v xi ,0,v xi This enables graded voltage regulation.

[0022] When the power frequency commutation module is a three-level structure, the primary winding of the power frequency transformer connected to the power frequency commutation module needs to have a center tap, and the neutral point of the three-level structure is connected to the center tap of the primary winding. The power frequency commutation module includes a clamping three-level half-bridge circuit, a clamping three-level H-bridge circuit, a T-type three-level half-bridge circuit, or a T-type three-level H-bridge circuit. The two outer terminals of the bridge arms of the clamping three-level half-bridge circuit and the T-type three-level half-bridge circuit are led out as output ports, and the neutral point and the midpoint of the bridge arm are led out as input ports. By controlling the on and off states of the switching devices, the input voltage u of the power frequency commutation module is controlled. xi Three gears -v xi / 2, 0, v xi / 2, thereby achieving graded voltage regulation; the parallel terminals of the bridge arms of the clamping three-level H-bridge circuit and the T-type three-level H-bridge circuit are led out as output ports, and the midpoint of the two bridge arms is led out as input ports; by controlling the on and off states of the switching devices, the input voltage u of the power frequency commutation module is... xi Five gears -v xi -v xi / 2, 0, v xi / 2, v xi Compared to two-level structures and three-level half-bridge circuits, it provides more voltage levels.

[0023] The power frequency commutation module uses a less expensive semi-controlled switching device to achieve graded voltage regulation, which reduces the capacity of other modules based on fully controlled switching devices and further reduces the overall cost of the device.

[0024] Preferably, the AC-AC voltage regulation module is used for continuous voltage regulation. It has a two-level or three-level structure with a low-pass filter at the output port, and the input port may or may not have a low-pass filter. The switching device of the AC-AC voltage regulation module is a fully controllable power semiconductor device with bidirectional switching capability, such as a reverse-series IGBT, a reverse-series MOSFET, a reverse-series IGCT, or a reverse-series GaN.

[0025] When the AC-AC voltage regulation module is a two-level structure, it is an H-bridge circuit. The parallel terminals of the bridge arms of the H-bridge circuit are led out as output ports after passing through a low-pass filter. The midpoints of the two bridge arms of the H-bridge are led out directly or after passing through an optional low-pass filter as input ports. By controlling the four switching devices of the H-bridge to operate in a high-frequency chopping state, the relationship between its input voltage and output voltage is as follows: (4) Where m3 is the modulation ratio, satisfying that m3 is greater than or equal to -1 and less than or equal to 1. By controlling this modulation ratio, continuous adjustment of the input and output voltage can be achieved.

[0026] When the AC-AC voltage regulating module is a three-level structure, the primary winding of the power frequency transformer connected to the AC-AC voltage regulating module needs to have a center tap, and the neutral point of the three-level structure is connected to the center tap of the primary winding. The AC-AC voltage regulating module includes a clamping three-level half-bridge circuit, a clamping three-level H-bridge circuit, a T-type three-level half-bridge circuit, or a T-type three-level H-bridge circuit. The two outer terminals of the bridge arms of the clamping three-level half-bridge circuit and the T-type three-level half-bridge circuit are led out as output ports after passing through a low-pass filter, and the neutral point and the midpoint of the bridge arm are led out directly or after passing through a selected low-pass filter as input ports. The parallel terminals of the bridge arms of the clamping three-level H-bridge circuit and the T-type three-level H-bridge circuit are led out as output ports after passing through a low-pass filter, and the midpoints of the two bridge arms are led out directly or after passing through a selected low-pass filter as input ports. By controlling the switching devices to operate in a high-frequency chopping state, the mathematical relationships between the input voltage and output voltage of the three-level half-bridge circuit and the H-bridge circuit are respectively u xi =m3v xi / 2 and u xi =m3v xi Similarly, continuous adjustment of the input and output voltages can be achieved by controlling the modulation ratio m3.

[0027] Preferably, the single-phase AC-DC-AC voltage regulation module is used for continuous voltage regulation. It has a two-level or three-level structure with a low-pass filter at the output port, and the input port may or may not have a low-pass filter. The switching device of the single-phase AC-DC-AC voltage regulation module is one of the fully controllable power semiconductor devices such as IGBT, MOSFET, IGCT, or GaN.

[0028] When the single-phase AC-DC-AC voltage regulating module has a two-level structure, it is a back-to-back H-bridge circuit. The midpoint of the bridge arm of the output-side H-bridge is led out as the output port after passing through a low-pass filter, and the midpoint of the bridge arm of the input-side H-bridge is led out directly or after passing through an optional low-pass filter as the input port. The output-side H-bridge controls the DC bus voltage to stabilize at V. xidc The H-bridge on the input side enables continuous voltage adjustment at the input port. The relationship between the input voltage and the DC capacitor voltage is as follows: (5) In the formula, m4 is the modulation ratio of the input-side H-bridge, which satisfies that m1 is greater than or equal to 0 and less than or equal to 1.

[0029] When the single-phase AC-DC-AC voltage regulating module is a three-level structure, the single-phase AC-DC-AC voltage regulating module is a back-to-back clamped three-level half-bridge circuit, a back-to-back clamped three-level H-bridge circuit, a back-to-back T-type three-level half-bridge circuit, or a back-to-back T-type three-level H-bridge circuit; the neutral point of the above three-level structure circuit is connected to the midpoint of the DC capacitor; the output side half-bridge midpoint and neutral point of the back-to-back clamped three-level half-bridge circuit and the back-to-back T-type three-level half-bridge circuit are led out as output ports after passing through a low-pass filter, and the input side half-bridge midpoint and neutral point are led out directly or after passing through a selected low-pass filter as input ports; the output side two half-bridge midpoints of the back-to-back clamped three-level H-bridge circuit and the back-to-back T-type three-level H-bridge circuit are led out as output ports after passing through a low-pass filter, and the input side two half-bridge midpoints are led out directly or after passing through a selected low-pass filter as input ports; the relationship between the output voltage and the input voltage is the same as that of the two-level structure, which is formula (5). The advantage of the three-level structure is that, compared to the two-level structure, the input port voltage range can be doubled when using the same voltage-resistant switching devices.

[0030] Preferably, the low-pass filter can be any type of filter with low-pass filtering characteristics, such as L-type, LC-type, or LCL-type.

[0031] Preferably, in order to increase the current rating of the AC-AC voltage regulating module, the single-phase AC-DC-AC voltage regulating module, and the three-phase two-port converter, thereby expanding the capacity of the on-load tap-changing distribution transformer, the number of primary windings of the three-phase power frequency transformer in the three-phase independent basic circuit unit or the three-phase coupled basic circuit unit is expanded from 3N0+3N1+3N2+3N3+3N4 to 3N0+3N1+3iN2+3jN3+3kN4, where i, j, and k are all integers greater than or equal to 1; the 3N2 AC-AC voltage regulating modules, the 3N3 single-phase AC-DC-AC voltage regulating modules, and the three-phase two-port converter are all included. The AC / DC / AC voltage regulating modules and N4 three-phase two-port converters are expanded to 3iN2, 3jN3, and kN4, respectively. The output ports of the newly added 3(i-1)N2 AC / AC voltage regulating modules, 3(j-1)N3 single-phase AC / DC / AC voltage regulating modules, and (k-1)N4 three-phase two-port converters are connected to the primary windings of the newly added 3(i-1)N2+3(j-1)N3+3(k-1)N4 power frequency transformers. Their input ports form a structure of N2 series-i parallel, N3 series-j parallel, and N4 series-k parallel in each phase, respectively. This structure expands the current ratings of the AC / AC voltage regulating modules, single-phase AC / DC / AC voltage regulating modules, and three-phase two-port converters to times i, j, and k, respectively.

[0032] Preferably, in order to mitigate the power supply deviation of the output port under asymmetrical voltage regulation in the three-phase series converter chain, the single-phase AC-DC-AC voltage regulating modules adopt a three-phase interleaved interconnection method; the number of primary windings of the three-phase power frequency transformer in the three-phase independent basic circuit unit or three-phase coupled basic circuit unit is expanded from 3N0+3N1+3N2+3N3+3N4 to 3N0+3N1+3N2+9N3+3N4, and the N3 single-phase AC-DC-AC voltage regulating modules of the converter sub-chain are expanded to 3N3, where N3 is an integer greater than or equal to 1; the 2N3 newly added single-phase AC-DC-AC voltage regulating modules of the converter sub-chain are divided into two groups according to their quantity, and the output ports of the N3 single-phase AC-DC-AC voltage regulating modules in each group are sequentially connected to the N3 windings of the other two newly added primary windings of the power frequency transformer, so as to realize the interleaved interconnection of the output ports of the single-phase AC-DC-AC voltage regulating modules in each sub-chain, while keeping the other circuit structures and connection methods unchanged. In this connection method, if the three-phase voltage regulation is asymmetrical, the converter sub-chain with smaller voltage regulation can supply energy to the other two phases with larger voltage regulation through the interleaved interconnection structure, thereby smoothing out the energy supply deviation of the three-phase series converter chain under asymmetrical voltage regulation.

[0033] Preferably, in order to expand the capacity of the on-load tap-changing distribution transformer, m independent three-phase basic circuit units and n coupled three-phase basic circuit units are combined in series and parallel (which can be any combination before and after expansion), where m is greater than or equal to 1 and n is greater than or equal to 0. The three-phase independent basic circuit units are configured in a series-parallel configuration with input ports (A1A1', B1B1', C1C1') and output ports (a1a1', b1b1', c1c1') using a configuration of "parallel input ports, series output ports", "series input ports, parallel output ports", or a hybrid series-parallel configuration. The three-phase coupled basic circuit units are configured in a series-parallel configuration with input ports (A2B2C2) and output ports (a2a2', b2b2', c2c2'). The output ports of the three-phase independent series-parallel configuration and the three-phase coupled series-parallel configuration are connected in parallel to form a star or delta configuration, and the terminals (a1, b1, c1) are led out. The input terminals (A1', B1', C1') of the three-phase independent series-parallel configuration are... The transformer is connected to the input terminals (A2, B2, C2) of the three-phase coupled series-parallel group, and terminals (A1, B1, C1) are led out to form a large-capacity on-load tap-changing distribution transformer with external input ports A1B1C1 and external output ports a1b1c1. This connection method expands the system capacity, thereby adapting to a wide range of application scenarios.

[0034] Preferably, switches are added between each pair of the external terminals (X, Y, Z) of the three-phase series converter chain (XY, XZ, YZ) for soft starting. Before the on-load tap-changing distribution transformer starts, the three switches are closed, and the three-phase series converter chain is not connected to the circuit. The on-load tap-changing distribution transformer starts in the same way as other distribution transformers. After the on-load tap-changing distribution transformer starts and the auxiliary power supply of the control circuit is established, the three-phase series converter chain is put into operation and the output voltage is maintained at zero. Then, the three switches are disconnected, and the voltage of the three-phase series converter chain is adjusted in an orderly manner to achieve the regulation of the secondary output voltage of the distribution transformer.

[0035] The three-phase two-port converter of the on-load tap-changing distribution transformer and the single-phase AC-DC-AC voltage regulating module are subjected to any control method that can realize input port voltage control, such as DC voltage control. The AC-AC voltage regulating module is subjected to any control method that can realize input port voltage control, such as pulse width control. The power frequency commutation module is subjected to any commutation control method that can realize commutation function, such as power frequency switch control. The above-mentioned module control methods are used to realize the voltage regulation of the distribution transformer.

[0036] The beneficial effects of this invention are: (1) The on-load tap-changing distribution transformer of the present invention includes a three-phase series converter chain for three-phase voltage regulation. The three-phase series converter chain consists of a three-phase two-port converter based on fully controlled power semiconductor devices and a single-phase AC-DC-AC voltage regulation module, a power frequency commutation module based on fully controlled or semi-controlled power semiconductor devices with bidirectional breaking capability, and an AC-AC voltage regulation module based on fully controlled power semiconductor devices with bidirectional breaking capability. The mechanical contacts of the traditional mechanical distribution transformer are eliminated. With the advantage of the high switching speed of power electronic devices, the device can achieve continuous and fast voltage regulation, thereby fundamentally solving the problems of short contact life, low operational reliability and inability to continuously regulate voltage in mechanical distribution transformers.

[0037] (2) The on-load tap-changing distribution transformer of the present invention obtains the input port of the distribution transformer by connecting the N0 windings of the primary side of the power frequency transformer in series with the input port of the series converter chain, so that the series converter chain only undertakes a small portion of the transmission power. The main body of the converter chain undertaking a small portion of the power is constructed by N1 low-cost power frequency commutation modules based on semi-controlled devices, while its small remaining part is composed of higher-priced tap-changing modules based on fully controlled devices. The circuit topology and configuration of the present invention achieve continuous regulation of the output voltage of the distribution transformer at a lower cost.

[0038] (3) The on-load tap-changing distribution transformer of the present invention adopts a structure in which the primary winding of the power frequency transformer is connected in series with the isolated power supply converter chain, so that the output voltage regulation of the distribution transformer can be achieved with only one power frequency transformer, avoiding the use of an additional power frequency transformer used to connect the converter in series with the high voltage circuit in the existing scheme, and further reducing the equipment cost.

[0039] (4) The three-phase converter bus of the on-load tap-changing distribution transformer of the present invention adopts a back-to-back three-phase two-level converter or a back-to-back three-phase three-level converter. Unlike the three-phase independent structure of the existing technical solution, the three-phase converter bus of the present invention has a three-phase common DC capacitor. Therefore, the second harmonic fluctuation power of AC can be canceled at the DC capacitor of the back-to-back converter, thereby greatly reducing the DC capacitor value and further reducing the overall equipment cost. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of the structure of the on-load tap-changing distribution transformer of the present invention, in which the series converter chain bears part of the power.

[0041] Figure 2 This is a schematic diagram of the three-phase two-port converter structure of the present invention.

[0042] Figure 3 This is a schematic diagram of the power frequency commutation module structure of the present invention.

[0043] Figure 4This is a schematic diagram of the cross-current voltage regulation module structure of the present invention.

[0044] Figure 5 This is a schematic diagram of the single-phase AC / DC voltage regulating module of the present invention.

[0045] Figure 6 This is a schematic diagram of the parallel structure of the AC-AC voltage regulating module, the single-phase AC-DC-AC voltage regulating module, and the three-phase two-port converter of the present invention.

[0046] Figure 7 This is a schematic diagram of the three-phase interleaved interconnection structure of the single-phase AC / DC voltage regulating module of the present invention.

[0047] Figure 8 This is a schematic diagram of the series-parallel combination structure of the basic circuit unit of the present invention.

[0048] Figure 9 This is a schematic diagram of a structure incorporating a soft-start switch according to the present invention.

[0049] Figure 10 This is a schematic diagram of the first embodiment of the on-load tap-changing distribution transformer of the present invention.

[0050] Figure 11 This is a simulation waveform diagram of the first embodiment of the on-load tap-changing distribution transformer of the present invention.

[0051] Figure 12 This is a schematic diagram of the second embodiment of the on-load tap-changing distribution transformer of the present invention.

[0052] Figure 13 This is a simulation waveform diagram of the second embodiment of the on-load tap-changing distribution transformer of the present invention. Detailed Implementation

[0053] The present invention will now be described in further detail with reference to the accompanying drawings.

[0054] As attached Figure 1 As shown, the present invention provides an on-load tap-changing transformer in which a series converter chain undertakes part of the power, characterized in that it includes a set of three-phase series converter chains and a three-phase power frequency transformer.

[0055] A three-phase series converter chain comprises one converter sub-chain for each phase and N4 three-phase combined converters. Each converter sub-chain includes N1 two-port power frequency commutation modules with stepped voltage regulation, N2 two-port AC-AC voltage regulation modules with continuous voltage regulation, and N3 two-port single-phase AC-DC-AC voltage regulation modules with continuous voltage regulation, wherein N1, N2, and N3 are all greater than or equal to 0, and satisfy N 1+N2+N3+N4 is greater than or equal to 1. A three-phase power frequency transformer contains 3N0+3N1+3N2+3N3+3N4 primary windings and 3 secondary windings, where N0 is greater than or equal to 0. The turns ratio of the primary windings can be set arbitrarily. By configuring the turns ratio of the secondary windings, the continuous voltage regulation range can be flexibly set, enhancing the applicability of the circuit and the degree of design freedom.

[0056] In an embodiment of the present invention, the negative terminal of the input port of the two-port commutation or voltage regulation module is sequentially connected to the positive terminal of the input port of the adjacent module to form a series converter chain. The positive terminal (X, Y, Z) of the first input port of each phase is connected to the positive terminal of the Nth input port of the adjacent module. 1+ The negative terminals (A', B', C') of the N2+N3 module input ports are led out respectively, serving as the positive and negative terminals of the three-phase converter sub-chain. The N0 windings of the primary side of each phase of the three-phase power frequency transformer are connected end to end to form the primary side sub-chain of the on-load tap-changing transformer; its first end is led out as the primary side input terminal (A, B, C) of the on-load tap-changing distribution transformer, and its end is connected to the positive terminal of each phase of the three-phase converter sub-chain. The N1+N2+N3 windings of the primary side of each phase of the power frequency transformer are connected sequentially after being aligned with the positive and negative terminals of the output ports of the two-port commutation or tap-changing modules in the converter sub-chain. Optionally, when N4 equals 0, the negative terminals (A', B', C') of each phase of the three-phase converter sub-chain are directly led out and connected in a star or delta configuration. The secondary windings (aa', bb', cc') of the power frequency transformer are also connected in a star or delta configuration to form the secondary output terminals (a, b, c). This forms an on-load tap-changing distribution transformer with external input ports ABC and external output ports abc. Optionally, when N4 equals 1, the negative terminals (A', B', C') of each phase of the three-phase converter sub-chain are connected to the input port of the three-phase converter consolidation chain. The remaining N4 windings of the primary side of the power frequency transformer are connected in a star or delta configuration and then connected to the output port of the aforementioned three-phase consolidation converter. The secondary windings (aa', bb', cc') of the power frequency transformer are also connected in a star or delta configuration to form the secondary output terminals (a, b, c). This forms an on-load tap-changing distribution transformer with external input ports ABC and external output ports abc.

[0057] Attached Figure 1 As shown, in one embodiment of the present invention, when the voltage regulating module is equipped with a low-pass filter, the converter sub-chain is not equipped with a low-pass filter; otherwise, an L-type, LC-type, or LCL-type filter with low-pass filtering characteristics should be configured between the positive and negative terminals of the converter sub-chain.

[0058] As attached Figure 2As shown in (a), as a specific implementation of the power frequency commutation module in the on-load tap-changing distribution transformer provided by the present invention, the circuit topology can be a two-level or three-level structure; the two-level structure is an H-bridge circuit composed of bidirectional switches, bidirectional switches T1 and T2 are connected in series to form the first bridge arm, bidirectional switches T3 and T4 are connected in series to form the second bridge arm, the upper and lower ends of the first and second bridge arms are connected to form the H-bridge output port, and the midpoints of the two bridge arms are respectively led out as input ports. The three-level structure is a clamping three-level half-bridge circuit, a clamping three-level H-bridge circuit, a T-type three-level half-bridge circuit, or a T-type three-level H-bridge circuit. In the clamping three-level half-bridge circuit, bidirectional switches T1, T2, T3, and T4 are connected in series to form the main bridge arm, and bidirectional switches T5 and T6 are connected in series to form the clamping bridge arm. The midpoints of switches T1 and T2 and the midpoints of switches T3 and T4 are respectively connected to the beginning and end of the clamping bridge arm. The two outer terminals of the main bridge arm are led out as output ports, the midpoint of the clamping bridge arm is led out as the neutral point, and the midpoint of the main bridge arm and the neutral point are led out as input ports. In the T-type three-level half-bridge circuit, bidirectional switches T1 and T2 are connected in series to form the main bridge arm. The beginning of bidirectional switch T3 is connected to the midpoint of the main bridge arm, and the end is connected to the neutral point. The neutral point is the two terminals on the outer side of the main bridge arm, which are led out as output ports. The midpoint of the main bridge arm and the neutral point are led out as input ports. The clamped three-level full-bridge circuit consists of two clamped half-bridges. The output ports of the first half-bridge circuit and the second half-bridge circuit are connected in parallel as the full-bridge output ports. The neutral points of the first and second half-bridge circuits are connected and led out as the full-bridge neutral point. The midpoints of the main bridge arms of the first and second half-bridge circuits are led out as full-bridge input ports. The T-type three-level full-bridge circuit consists of two T-type half-bridge circuits. The output ports of the first half-bridge circuit and the second half-bridge circuit are connected in parallel as the full-bridge output ports. The neutral points of the first and second half-bridge circuits are connected and led out as the full-bridge neutral point. The midpoints of the main bridge arms of the first and second half-bridge circuits are led out as full-bridge input ports. Preferably, when the power frequency commutation module is a three-level structure, the primary winding of the power frequency transformer connected to the power frequency commutation module needs to have a center tap. The neutral point of the three-level structure is connected to the center tap of the primary winding. With this structure, the input port voltage range can be doubled compared to the two-level structure.

[0059] As attached Figure 2 As shown in (b), the switching device of the power frequency commutation module is a fully controlled or semi-controlled power semiconductor device or a combination circuit of multiple power semiconductor devices with bidirectional interruption capability. Specifically, it can be a bidirectional thyristor, a reverse parallel thyristor, a reverse series IGBT, a reverse parallel IGBT bare tube, a reverse parallel gate turn-off thyristor (GTO), a reverse series integrated gate commutated thyristor (IGCT), a reverse parallel IGCT bare tube, a reverse series MOSFET or a reverse series GaN, or a diode bridge bidirectional switch.

[0060] As attached Figure 3As shown, as a specific implementation of the voltage regulation module, the AC-AC voltage regulation module is used for continuous voltage regulation. It has a two-level or three-level structure with a low-pass filter at the output port, and the input port may or may not have a low-pass filter.

[0061] When the AC-AC voltage regulation module has a two-level structure, it is an H-bridge circuit. The parallel terminals of the bridge arms of the H-bridge circuit are led out as output ports after passing through a low-pass filter. The midpoints of the two bridge arms are led out directly or after passing through an optional low-pass filter as input ports. The implementation method of the bidirectional switch is the same as in the power frequency commutation module, such as... Figure 3 As shown in (b).

[0062] When the AC-AC voltage regulating module is a three-level structure, the primary winding of the power frequency transformer connected to the AC-AC voltage regulating module must have a center tap, and the neutral point of the three-level structure is connected to the center tap of the primary winding. The AC-AC voltage regulating module includes a clamping three-level half-bridge circuit, a clamping three-level H-bridge circuit, a T-type three-level half-bridge circuit, or a T-type three-level H-bridge circuit. For the clamping three-level half-bridge circuit and the T-type three-level half-bridge circuit, the two outer terminals of the bridge arms are led out as output ports after passing through a low-pass filter, and the neutral point and the midpoint of the bridge arm are led out directly or after passing through an optional low-pass filter as input ports. For the clamping three-level H-bridge circuit and the T-type three-level H-bridge circuit, the parallel terminals of the bridge arms are led out as output ports after passing through a low-pass filter, and the midpoints of the two bridge arms are led out directly or after passing through an optional low-pass filter as input ports.

[0063] like Figure 4 As shown, as another specific implementation of the voltage regulation module, the single-phase AC-DC-AC module is a two-level or three-level structure with a low-pass filter at the output port, and the input port may or may not be equipped with a low-pass filter. The fully controlled switch in the implementation is an IGBT, MOSFET, GaN or IGCT.

[0064] When the single-phase AC / DC / AC module is a two-level structure, the single-phase AC / DC / AC voltage regulation module is a back-to-back H-bridge circuit. The positive terminals of the DC side of the first and second H-bridges are connected to the positive terminal of the DC capacitor, and the negative terminals of the DC side are connected to the negative terminal of the DC capacitor. The midpoint of the bridge arm of the first H-bridge is directly led out or led out after passing through a low-pass filter as an input port. The midpoint of the bridge arm of the second H-bridge is led out after passing through a low-pass filter as an input port. Optionally, when the single-phase AC / DC / AC module is a three-level structure, the single-phase AC / DC / AC voltage regulation module is a back-to-back clamped three-level half-bridge circuit, a back-to-back clamped three-level H-bridge circuit, a back-to-back T-type three-level half-bridge circuit, or a back-to-back T-type three-level H-bridge circuit. The back-to-back clamped three-level half-bridge circuit consists of two clamped three-level half-bridge circuit units. The positive terminals of the first and second clamped half-bridges are connected to the positive terminal of the split capacitor C1, and the negative terminals are connected to the negative terminal of the split capacitor C2. The neutral points of the first and second clamped half-bridges are connected to the midpoint of the split capacitors C1 and C2. The AC port of the first clamped half-bridge is directly led out or led out after passing through a low-pass filter as the input port of the single-phase AC-DC-AC converter. The AC port of the second clamped half-bridge is led out after passing through a low-pass filter as the output port of the single-phase AC-DC-AC converter. By replacing the clamped three-level half-bridge circuit units in the aforementioned back-to-back clamped three-level half-bridge circuit with clamped three-level H-bridge circuits, T-type three-level half-bridge circuits, and T-type three-level H-bridge circuits, we obtain back-to-back clamped three-level H-bridge circuits, back-to-back T-type three-level half-bridge circuits, or back-to-back T-type three-level H-bridge circuits. The connection method of the circuit units and the split capacitors, as well as the configuration method of the input and output ports, remain unchanged.

[0065] like Figure 5 As shown, the three-phase converged converter of the present invention can be a two-level three-phase back-to-back converter or a three-level three-phase back-to-back converter. The two-level three-phase three-wire structure consists of three two-level half-bridges. The midpoints of the first, second, and third half-bridges are led out as AC ports abc. The positive terminals of each phase half-bridge are connected to each other and lead out the positive terminal DC+ of the DC port. The negative terminals of each phase half-bridge are connected to each other and lead out the negative terminal DC- of the DC port. Optionally, a fourth half-bridge is introduced on the basis of the two-level three-phase three-wire structure. Its midpoint is led out as the AC neutral terminal n. The first end of the bridge arm is connected to the positive terminal DC+ of the DC port, and the last end of the bridge arm is connected to the negative terminal DC- of the DC port, forming a two-level three-phase four-wire structure. The three-phase three-wire or three-wire four-wire structure serves as the basic unit. The DC port of the first unit is connected in parallel with a DC capacitor and then connected to the DC port of the second unit. The AC port of the first unit is directly led out or led out through a filter to obtain the input port of the three-phase two-level back-to-back converter. The AC port of the second unit is led out through a low-pass filter to obtain the output port of the three-phase two-level back-to-back converter.

[0066] When a three-phase back-to-back converter is three-level, its basic unit includes four structural components. Three-level three-phase three-wire structure I consists of three clamped three-level half-bridges. The midpoints of the clamping arms of the first, second, and third half-bridges are used as AC ports abc. The positive terminals of each phase half-bridge are connected to each other and connected to the positive terminal DC+ of the DC port. The negative terminals of each phase half-bridge are connected to each other and connected to the negative terminal DC- of the DC port. The neutral points of each phase half-bridge are connected to each other and connected to the three-phase neutral point terminal N. Three-level three-phase three-wire structure II consists of three T-type three-level half-bridges. The midpoints of the main arms of the first, second, and third half-bridges are used as AC ports abc. The positive terminals of each phase half-bridge are connected to each other and connected to the positive terminal DC+ of the DC port. The negative terminals of each phase half-bridge are connected to each other and connected to the negative terminal DC- of the DC port. The neutral points of each phase half-bridge are connected to each other and connected to the three-phase neutral point terminal N. Optionally, a fourth half-bridge is introduced based on the three-level three-phase three-wire structure. The midpoint of its main bridge arm is used as the AC neutral point terminal n. The first end of the main bridge arm is connected to the positive terminal DC+ of the DC port, and the last end is connected to the negative terminal DC- of the DC port. The neutral point is connected to the neutral points of the first, second, and third half-bridges, forming a three-level three-phase four-wire structure. The three-phase three-wire or three-wire four-wire structure serves as the basic unit. The positive terminals of the DC terminals of the first and second units are connected to the positive terminal of the split capacitor C1, and the negative terminals are connected to the negative terminal of the split capacitor C2. The neutral points of the first and second units are connected to the midpoints of the split capacitors C1 and C2. The AC port of the first unit is directly led out or led out after passing through a low-pass filter as the input port of the three-phase three-level back-to-back converter. The AC port of the second unit is led out after passing through a low-pass filter as the output port of the three-phase three-level back-to-back converter.

[0067] like Figure 6As shown, the number of primary windings of the three-phase power frequency transformer in the three-phase independent basic unit or three-phase coupled basic unit is expanded from 3N0+3N1+3N2+3N3+3N4 to 3N0+3N1+3iN2+3jN3+3kN4, where i, j, and k are all integers greater than or equal to 1. The 3N2 AC-AC voltage regulating modules, 3N3 single-phase AC-DC-AC voltage regulating modules, and N4 three-phase two-port converters are expanded to 3iN2, 3jN3, and kN4, respectively. The output ports of the newly added 3(i-1)N2 AC-AC voltage regulating modules, 3(j-1)N3 single-phase AC-DC-AC voltage regulating modules, and (k-1)N4 three-phase two-port converters are connected to the primary windings of the newly added 3(i-1)N2+3(j-1)N3+3(k-1)N4 power frequency transformers. Their input ports form N2 series-i parallel, N3 series-j parallel, and N4 series-k parallel structures in each phase, respectively. Taking the 3iN2 AC-AC voltage regulating modules as an example, the input ports of the N2 AC-AC voltage regulating modules are connected in series to form an AC-AC voltage regulating module, and the input ports of the i modules are connected in parallel to form an N2 series-i parallel structure. The series and parallel connection methods of the AC-DC-AC voltage regulating modules and the single-phase AC-DC-AC voltage regulating modules are similar. The beneficial effect is that the series and parallel structure can increase the current rating of the AC-AC voltage regulating modules, the single-phase AC-DC-AC voltage regulating modules, and the three-phase two-port converters.

[0068] like Figure 7 As shown, the present invention discloses an on-load tap-changing distribution transformer with a series converter chain bearing part of the power, wherein the single-phase AC-DC-AC tap-changing modules in the converter sub-chain can be interconnected in an interleaved manner. Specifically, the number of primary windings of the three-phase power frequency transformer is expanded from 3N0+3N1+3N2+3N3+3N4 to 3N0+3N1+3N2+9N3+3N4, and the N3 single-phase AC-DC-AC tap-changing modules in the converter sub-chain are expanded to 3N3, where N3 is an integer greater than or equal to 1. The single-phase AC-DC-AC voltage regulating modules in the converter sub-chain are divided into three groups on average, and the corresponding transformer primary windings are also divided into three groups on average. The output ports of the three groups of single-phase AC-DC-AC voltage regulating modules in phase A are connected to one group of the corresponding primary windings of the transformers in phases A, B, and C, respectively. The other two phases are connected in the same way, so that the output ports of the single-phase AC-DC-AC voltage regulating modules in each sub-chain are interconnected between the three phases, while keeping other circuit structures and connection methods unchanged.

[0069] like Figure 8As shown, the on-load tap-changing distribution transformers that bear part of the power in the series converter chain can further expand their capacity through the series-parallel connection of three-phase units. First, we define the three-phase independent basic circuit unit and the three-phase coupled basic circuit unit: When N4=0, the input ports (XA', YB', ZC') of the three converter sub-chains are connected in series with the primary windings of 3N0 transformers to obtain input ports (AA', BB', CC'). This connection method constitutes a three-phase independent basic circuit unit with input ports (AA', BB', CC') and output ports (aa', bb', cc'). When N4=1, the input terminals (X, Y, Z) of the three-phase series converter chain are connected in series with the primary windings of 3N0 transformers within the three phases to obtain input terminals (A, B, C). This connection method constitutes a three-phase coupled basic circuit unit with input ports (ABC) and output ports (aa', bb', cc').

[0070] Secondly, m independent three-phase basic circuit units and n coupled three-phase basic circuit units form a series-parallel combination to expand the capacity of the on-load tap-changing distribution transformer, where m is greater than or equal to 1 and n is greater than or equal to 0. The independent three-phase basic circuit units are configured in a "parallel connection of input ports and series connection of output ports", "series connection of input ports and parallel connection of output ports", or "hybrid series-parallel connection" manner to form a three-phase independent series-parallel combination with input ports (A1A1', B1B1', C1C1') and output ports (a1a1', b1b1', c1c1'); the coupled three-phase basic circuit units are configured in a "parallel connection of input ports and series connection of output ports" manner to form a three-phase coupled series-parallel combination with input ports (A2B2C2') and output ports (a2a2', b2b2', c2c2'). The three-phase independent series-parallel combination and the output port of the three-phase coupled series-parallel combination are connected in parallel to form a star or delta configuration and the terminals (a1, b1, c1) are brought out. The input terminals (A1', B1', C1') of the three-phase independent series-parallel combination are connected to the input terminals (A2, B2, C2) of the three-phase coupled series-parallel combination and the terminals (A1, B1, C1) are brought out, thus forming a large-capacity on-load tap-changing distribution transformer with external input port A1B1C1 and external output port a1b1c1.

[0071] like Figure 9As shown, the three-phase series converter chain is connected to switches for the soft-start process. Specifically, a switch is configured between terminals X and Y, and between X and Z and Y and Z respectively. The soft-start steps are as follows: Before the on-load tap-changing distribution transformer starts, the three switches are in the closed state, and the three-phase series converter chain is not connected to the circuit. After the on-load tap-changing distribution transformer starts and the auxiliary power supply of the control circuit is established, the three-phase series converter chain is put into operation and the output voltage is maintained at zero. Then, the three switches are disconnected, thereby connecting the three-phase series converter chain to the circuit. The output voltage of the distribution transformer is regulated by adjusting the output voltage of the series converter chain.

[0072] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. 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.

[0073] As attached Figure 10 To be continued Figure 11 As shown, a first specific embodiment of an on-load tap-changing distribution transformer in which a series converter chain undertakes part of the power is given.

[0074] Specifically, as shown in the attached document Figure 10 As shown, the three-phase series converter chain comprises three converter sub-chains and one three-phase converter hub. Each converter sub-chain contains two power frequency commutation modules, and the input ports of the two power frequency commutation modules are connected in series. The two outermost terminals are led out. The outermost terminals (X, A'), (Y, B'), and (Z, C') of the three converter sub-chains constitute the input ports XA', YB', and ZC' of the converter sub-chain. The three terminals of the three-phase converter hub input port are connected to the outer terminals (A', B', C') of the three converter sub-chains, respectively. The remaining outer terminals (X, Y, Z) are led out to form the input ports XYZ of the three-phase series converter chain. The output terminals of the six modules in the converter sub-chains and the output terminals of the three-phase converter hub are led out to form the seven output ports of the three-phase series converter chain. The input ports of the three-phase series converter chain are not equipped with centralized low-pass filters.

[0075] The three-phase power frequency transformer comprises 12 primary windings and 3 secondary windings. The terminals of the 3 secondary windings are directly led out as output ports (aa', bb', cc'). The 6 primary windings are connected to the output ports of the 6 power frequency commutation modules of the three-phase series converter chain. The 3 primary windings are connected in a star configuration within each of the three phases to form a three-phase interface, which is then connected to the output port of the three-phase series converter chain. The input terminals (X, Y, Z) of the three-phase series converter chain are connected in series with the remaining 3 primary windings within each of the three phases to obtain input terminals (A, B, C). The output ports (aa', bb', cc') are connected in a star configuration, and the terminals are led out to form an on-load tap-changing distribution transformer with external input ports ABC and external output ports abc.

[0076] Specifically, the turns ratio of the transformer winding is designed as follows: The three-phase two-port converter and the power frequency commutation module have a two-level circuit structure. The input port of the three-phase two-port converter is equipped with an LC-type low-pass filter, and the output port is equipped with an L-type low-pass filter.

[0077] The simulation parameters of the on-load tap-changing distribution transformer designed in this embodiment are summarized in Table 1. Based on this, the on-load tap-changing distribution transformer simulation model built using Matlab / Simulink achieved the expected design goals and realized the control function. The simulation waveforms are summarized in Appendix 1. Figure 11 middle.

[0078] Table 1 Simulation parameters of the first embodiment of the on-load tap-changing distribution transformer Input AC voltage (rms) 10kV (8-12kV) Output load voltage (rms) 380V Three-phase two-port converter 1 Number of power frequency commutation modules 6 Rated power 2MW Transformer winding turns ratio 10000：500：1500：250：380 Appendix Figure 11 These are simulation waveforms of an embodiment of the present invention. (a) is the three-phase power grid voltage waveform; (b) is the three-phase power grid current waveform; (c) is the voltage waveform of transformer winding 1; (d) is the output voltage waveform of power frequency commutation module 1; (e) is the output voltage waveform of power frequency commutation module 2; (f) is the output voltage waveform of the three-phase two-port converter; (g) is the load voltage waveform; and (h) is the load current waveform.

[0079] After the simulation model starts normally, the three-phase grid voltage drops to an effective value of 8kV. The input voltages of the two power frequency commutation modules are controlled to be -500V and -1500V respectively, and the input voltage of the three-phase two-port converter is controlled to be 0. According to the mathematical relationship that the grid voltage is equal to the series superposition of the voltages of the power frequency commutation module, the three-phase two-port converter and the transformer winding 1, the voltage of the transformer winding 1 is maintained at an effective value of 10kV, and then the output voltage is maintained at 380V through the transformer. Between 0.1s and 0.15s, the grid voltage gradually rises to 8.25kV, and the input voltage of the three-phase two-port converter continuously increases to 250V, while the input voltages of other power frequency commutation modules remain unchanged. As the grid voltage continues to rise, at 0.15s, the input voltage of power frequency commutation module 2 is controlled to 0, and the input voltage of the three-phase two-port converter is controlled to 250V. Between 0.15s and 0.2s, the input voltage of the three-phase two-port converter continuously decreases to 0V. This process continues, with the grid voltage increasing by 0.25kV every 0.05s. The input voltages of the two power frequency commutation modules and the three-phase two-port converter for each phase are adjusted to maintain the load voltage at 380V. This entire process verifies the operational feasibility and voltage regulation capability of the on-load tap-changing distribution transformer.

[0080] As attached Figure 12 To be continued Figure 13 As shown, a second specific embodiment of an on-load tap-changing distribution transformer in which a series converter chain undertakes part of the power is given.

[0081] Specifically, as shown in the attached document Figure 12 As shown, the three-phase series converter chain consists of only three converter sub-chains. Each sub-chain contains four single-phase AC / DC / AC voltage regulating modules, with their input ports connected in series. The two outermost terminals are led out. The outermost terminals (X, A'), (Y, B'), and (Z, C') of the three sub-chains constitute the input ports XA', YB', and ZC' of the sub-chain. The output terminals of the 12 modules in the sub-chains are led out to form the 12 output ports of the three-phase series converter chain.

[0082] The three-phase power frequency transformer comprises 16 primary windings and 3 secondary windings. The terminals of the 3 secondary windings are directly led out as output ports (aa', bb', cc'). The 12 primary windings of the transformer are connected to the output ports of the 12 single-phase AC / DC / AC voltage regulating modules of the three-phase series converter chain. The input terminals (X, Y, Z) of the three-phase series converter chain are connected in series with the remaining 3 primary windings of the transformer within the three phases to obtain input terminals (A, B, C). The ports (AA', BB', CC') are connected in a delta configuration, and the output ports (aa', bb', cc') are connected in a star configuration. The terminals are then led out to form an on-load tap-changing distribution transformer with external input ports ABC and external output ports abc.

[0083] Specifically, the turns ratio of the transformer winding is designed as follows: The single-phase AC-DC-AC voltage regulator module has a two-level structure, and the input port of the single-phase AC-DC-AC voltage regulator module is equipped with an LC type low-pass filter, and the output port is equipped with an L type low-pass filter.

[0084] The simulation parameters of the on-load tap-changing distribution transformer designed in this embodiment are summarized in Table 2. Based on this, the on-load tap-changing distribution transformer simulation model built using Matlab / Simulink achieved the expected design goals and realized the control function. The simulation waveforms are summarized in Appendix 2. Figure 13 middle.

[0085] Table 2 Simulation parameters of the second embodiment of the on-load tap-changing distribution transformer Input AC voltage (rms) 10kV (8-12kV) Output load voltage (rms) 380V Number of single-phase AC / DC-AC voltage regulating modules 12 Rated power 2MW Transformer winding turns ratio 10000：500：500：500：500：220 Appendix Figure 13 These are simulation waveforms of an embodiment of the present invention. (a) is the three-phase power grid voltage waveform; (b) is the three-phase power grid current waveform; (c) is the voltage waveform of transformer winding 1; (d) is the output voltage waveform of single-phase AC-DC-AC voltage regulating module 1; (e) is the output voltage waveform of single-phase AC-DC-AC voltage regulating module 2; (f) is the output voltage waveform of single-phase AC-DC-AC voltage regulating module 3; (g) is the output voltage waveform of single-phase AC-DC-AC voltage regulating module 4; (h) is the load voltage waveform; and (i) is the load current waveform.

[0086] When the simulation model starts normally, the effective value of the three-phase grid voltage drops to 8kV. The input voltage of the four single-phase AC-DC-AC voltage regulating modules is -500V. According to the mathematical relationship that the grid voltage is equal to the series superposition of the input voltage of the four single-phase AC-DC-AC voltage regulating modules and the voltage of transformer winding 1, the effective value of the voltage of transformer winding 1 is maintained at 10kV, and then the output voltage is maintained at 380V through the transformer. Between 0.1s and 0.2s, the grid voltage gradually rises to 8.5kV. The input voltage of the single-phase AC / DC / AC module 4 is continuously increased from -500V to 0V, while the input voltages of the other single-phase AC / DC / AC modules remain unchanged, maintaining the output load voltage at 380V. Between 0.2s and 0.3s, the grid voltage continues to rise to 9kV. The input voltage of the single-phase AC / DC / AC module 3 is continuously increased from -500V to 0V, while the input voltages of the other single-phase AC / DC / AC modules remain unchanged. This process continues, with the grid voltage increasing by 0.5kV every 0.1s, adjusting the input voltages of the four single-phase AC / DC / AC modules to maintain the load voltage at 380V. This entire process further verifies the operational feasibility and voltage regulation capability of the on-load tap-changing distribution transformer.

Claims

1. An on-load tap-changing distribution transformer with a series converter chain bearing part of the power, characterized in that, It includes one set of three-phase series converter chains and one three-phase power frequency transformer; The three-phase series converter chain is used for three-phase voltage regulation. It includes 3 converter sub-chains and N4 three-phase converter bus chains, where N4 is equal to 0 or 1. Each converter sub-chain comprises N1 two-port power frequency commutation modules with stepped voltage regulation, N2 two-port AC-AC voltage regulation modules with continuous voltage regulation, and N3 two-port single-phase AC-DC-AC voltage regulation modules with continuous voltage regulation, wherein N1, N2, and N3 are all greater than or equal to 0, and satisfy N 1+ N2+N3+N4 is greater than or equal to 1; The input ports of the power frequency commutation module, AC-AC voltage regulation module and AC-DC-AC voltage regulation module in the converter sub-chain are connected in series, and the two outermost terminals of the series connection are led out. The outermost terminals (X, A'), (Y, B'), and (Z, C') of the three converter sub-chains constitute the input ports XA', YB', and ZC' of the converter sub-chains; the three-phase converter chain is a three-phase two-port converter with the function of connecting three converter sub-chains and continuously adjusting the three-phase voltage. The three terminals of its input port are respectively connected to the outer terminals (A', B', C') of the three converter sub-chains to realize the three-phase connection, and the remaining outer terminals (X, Y, Z) are led out to form the input port XYZ of the three-phase series converter chain; The output terminals of the 3N1+3N2+3N3 modules and the output terminals of the N4 three-phase converter junctions in the converter sub-chain are respectively led out to form 3N1+3N2+3N3+N4 output ports of the three-phase series converter chain for external power supply; the input ports of the three-phase series converter chain may or may not be equipped with centralized low-pass filters. The three-phase power frequency transformer is used for isolation and voltage reduction and provides an energy transfer port to maintain the energy conservation of the three-phase series converter chain. It includes four primary windings (3N0+3N1+3N2+3N3+3N) and three secondary windings, where N0 is greater than or equal to 0. The terminals of the three secondary windings of the three-phase power frequency transformer are directly led out as output ports (aa', bb', cc'). The 3N1+3N2+3N3 primary windings of the three-phase power frequency transformer are respectively connected to the 3N1+3N2+3N3 output ports of the three-phase series converter chain. The 3N4 primary windings are connected in a star or delta structure in the three phases to form N4 three-phase interfaces, which are connected to the N4 output ports of the three-phase series converter chain.

2. The on-load tap-changing distribution transformer with a series converter chain bearing part of the power according to claim 1, characterized in that, When the number of three-phase converter chains N4 is equal to 0, the input ports (XA', YB', ZC') of the three converter sub-chains are connected in series with the primary windings of 3N0 transformers to obtain input ports (AA', BB', CC'). The above connection method constitutes a three-phase independent basic circuit unit with input ports (AA', BB', CC') and output ports (aa', bb', cc'). The input ports and output ports of this three-phase independent basic circuit unit are connected in a star or delta configuration, and the terminals are led out to form an on-load tap-changing distribution transformer with external input port ABC and external output port abc. When the number of three-phase converter chains N4 equals 1, the input terminals (X, Y, Z) of the three-phase series converter chain are connected in series with the primary windings of 3N0 transformers in the three phases to obtain input terminals (A, B, C); the above connection method constitutes a three-phase coupled basic circuit unit with input ports (ABC) and output ports (aa', bb', cc'); the output ports (aa', bb', cc') of the three-phase coupled basic circuit unit are connected in a star or delta configuration, and the terminals are led out to form an on-load tap-changing distribution transformer with external input port ABC and external output port abc.

3. The on-load tap-changing distribution transformer with a series converter chain bearing part of the power according to claim 2, characterized in that, The three-phase two-port converter is used to act as a consolidation chain to gather three converter sub-chains and realize the three-phase voltage continuous regulation function. The output port is equipped with a two-level circuit structure or a three-level circuit structure with a low-pass filter, and the input port may or may not be equipped with a low-pass filter. The switching device of the three-phase converter is a fully controllable power semiconductor device. When the three-phase two-port converter is a two-level structure, its circuit topology is a back-to-back three-phase two-level converter composed of two-level bridge arms. The three-phase two-level converters are either three-phase three-wire two-level converters or three-phase four-wire two-level converters. The midpoint of the bridge arm of the output-side three-phase converter is led out as the output port after passing through a low-pass filter, and the midpoint of the bridge arm of the input-side three-phase converter is led out directly or after passing through an optional low-pass filter as the input port. When the three-phase two-port converter is a three-level structure, the two-level bridge arm in the two-level structure is replaced with a three-level bridge arm. The midpoint of the three-level bridge arm is connected to the midpoint of the DC capacitor. The midpoint of the bridge arm of the output-side three-phase converter is led out as the output port after passing through a low-pass filter. The midpoint of the bridge arm of the input-side three-phase converter is led out directly or after passing through an optional low-pass filter as the input port. The fully controllable power semiconductor device is an IGBT, MOSFET, IGCT, or GaN.

4. The on-load tap-changing distribution transformer with a series converter chain bearing part of the power according to claim 2, characterized in that, The power frequency commutation module is used for graded voltage regulation and has a two-level or three-level structure; the switching device of the power frequency commutation module is a fully controlled or semi-controlled power semiconductor device or a combination circuit of multiple power semiconductor devices with bidirectional switching capability. When the power frequency commutation module is a two-level structure, the power frequency commutation module is an H-bridge circuit composed of bidirectional switches. The midpoint of the two arms of the H-bridge is led out as the input port, and the parallel terminals of the arms of the H-bridge are led out as the output port. When the power frequency commutation module is a three-level structure, the primary winding of the power frequency transformer connected to the power frequency commutation module must have a center tap, and the neutral point of the three-level bridge arm is connected to the center tap of the primary winding; the power frequency commutation module includes a clamping three-level half-bridge circuit, a clamping three-level H-bridge circuit, a T-type three-level half-bridge circuit, or a T-type three-level H-bridge circuit. The clamping three-level half-bridge circuit and the T-type three-level half-bridge circuit have two outer terminals on the bridge arm as output ports, and the neutral point and the midpoint of the bridge arm as input ports; the clamping three-level H-bridge circuit and the T-type three-level H-bridge circuit have parallel terminals on the bridge arm as output ports, and the midpoints of the two bridge arms as input ports. The fully controlled or semi-controlled power semiconductor device with bidirectional switching capability is a bidirectional thyristor, a reverse parallel thyristor, a reverse series IGBT, a reverse parallel IGBT bare diode, a reverse parallel gate turn-off thyristor (GTO), a reverse series integrated gate commutated thyristor (IGCT), a reverse parallel IGCT bare diode, a reverse series MOSFET, or a reverse series GaN.

5. The on-load tap-changing distribution transformer with a series converter chain bearing part of the power according to claim 2, characterized in that, The AC-AC voltage regulation module is used for continuous voltage regulation. It has a two-level or three-level structure with a low-pass filter at the output port, and the input port may or may not have a low-pass filter. The switching device of the AC-AC voltage regulation module is a fully controllable power semiconductor device with bidirectional switching capability. When the AC-AC voltage regulation module is a two-level structure, the AC-AC voltage regulation module is an H-bridge circuit. The parallel terminals of the bridge arms of the H-bridge circuit are led out as output ports after passing through a low-pass filter. The midpoints of the two bridge arms of the H-bridge are led out directly or after passing through an optional low-pass filter as input ports. When the AC-AC voltage regulating module is a three-level structure, the primary winding of the power frequency transformer connected to the AC-AC voltage regulating module must have a center tap, and the neutral point of the three-level structure is connected to the center tap of the primary winding; the AC-AC voltage regulating module is a clamping three-level half-bridge circuit, a clamping three-level H-bridge circuit, a T-type three-level half-bridge circuit, or a T-type three-level H-bridge circuit. The two outer terminals of the clamping three-level half-bridge circuit and the T-type three-level half-bridge circuit are led out as output ports after passing through a low-pass filter, and the neutral point and the midpoint of the bridge arm are led out directly or after passing through an optional low-pass filter as input ports; the parallel terminals of the bridge arms of the clamping three-level H-bridge circuit and the T-type three-level H-bridge circuit are led out as output ports after passing through a low-pass filter, and the midpoints of the two bridge arms are led out directly or after passing through an optional low-pass filter as input ports. The fully controllable power semiconductor device with bidirectional switching capability is a reverse-series IGBT, a reverse-series MOSFET, a reverse-series IGCT, or a reverse-series GaN. The single-phase AC-DC-AC voltage regulation module is used for continuous voltage regulation. It has a two-level or three-level structure with a low-pass filter at the output port, and the input port may or may not have a low-pass filter. The switching device of the single-phase AC-DC-AC voltage regulation module is a fully controllable power semiconductor device. The fully controllable power semiconductor device is an IGBT, MOSFET, IGCT, or GaN; When the single-phase AC-DC-AC voltage regulating module is a two-level structure, the single-phase AC-DC-AC voltage regulating module is a back-to-back H-bridge circuit. The midpoint of the bridge arm of the output-side H-bridge is led out as the output port after passing through a low-pass filter, and the midpoint of the bridge arm of the input-side H-bridge is led out directly or after passing through an optional low-pass filter as the input port. When the single-phase AC-DC-AC voltage regulating module is a three-level structure, it can be a back-to-back clamped three-level half-bridge circuit, a back-to-back clamped three-level H-bridge circuit, a back-to-back T-type three-level half-bridge circuit, or a back-to-back T-type three-level H-bridge circuit. The neutral point of each of these three-level circuits is connected to the midpoint of a DC capacitor. The output half-bridge midpoints and neutral points of the back-to-back clamped three-level half-bridge circuit and the back-to-back T-type three-level half-bridge circuit are led out as output ports after passing through a low-pass filter. The input half-bridge midpoints and neutral points are led out directly or after passing through an optional low-pass filter as input ports. The output two half-bridge midpoints of the back-to-back clamped three-level H-bridge circuit and the back-to-back T-type three-level H-bridge circuit are led out as output ports after passing through a low-pass filter. The input two half-bridge midpoints are led out directly or after passing through an optional low-pass filter as input ports.

6. A series converter chain-driven on-load tap-changing distribution transformer according to any one of claims 1-5, characterized in that, The low-pass filter is any type of filter with low-pass filtering characteristics, such as L-type, LC-type, or LCL-type.

7. A series converter chain-driven on-load tap-changing distribution transformer according to any one of claims 1-6, characterized in that, The number of primary windings of the three-phase power frequency transformer in the three-phase independent basic circuit unit or three-phase coupled basic circuit unit is expanded from 3N0+3N1+3N2+3N3+3N4 to 3N0+3N1+3iN2+3jN3+3kN4 to increase the capacity of the distribution transformer, where i, j, and k are all integers greater than or equal to 1. The 3N2 AC-AC voltage regulating modules, 3N3 single-phase AC-DC-AC voltage regulating modules, and N4 three-phase two-port converters are expanded to 3iN2, 3jN3, and kN4, respectively. The output ports of the newly added 3(i-1)N2 AC-AC voltage regulating modules, 3(j-1)N3 single-phase AC-DC-AC voltage regulating modules, and (k-1)N4 three-phase two-port converters are connected to the primary windings of the newly added 3(i-1)N2+3(j-1)N3+3(k-1)N4 power frequency transformers. Their input ports form N2 series-i parallel, N3 series-j parallel, and N4 series-k parallel structures in each phase, respectively, to increase the current rating of the AC-AC voltage regulating modules, single-phase AC-DC-AC voltage regulating modules, and three-phase two-port converters.

8. A series converter chain-driven on-load tap-changing distribution transformer according to any one of claims 1-6, characterized in that, The number of primary windings of the three-phase power frequency transformer in the three-phase independent basic circuit unit or three-phase coupled basic circuit unit is expanded from 3N0+3N1+3N2+3N3+3N4 to 3N0+3N1+3N2+9N3+3N4. The N3 single-phase AC-DC-AC voltage regulating modules of the converter sub-chain are expanded to 3N3 for three-phase interleaved interconnection, where N3 is an integer greater than or equal to 1. The 2N3 newly added single-phase AC-DC-AC voltage regulating modules of the converter sub-chain are divided into two groups according to their quantity. The output ports of the N3 single-phase AC-DC-AC voltage regulating modules in each group are sequentially connected to the N3 windings of the other two newly added primary windings of the power frequency transformer, so as to realize the interleaved interconnection of the output ports of the single-phase AC-DC-AC voltage regulating modules in each sub-chain. The three-phase interleaved interconnection of the single-phase AC-DC-AC voltage regulating modules is used to smooth the power supply deviation of the output port of the three-phase series converter chain under asymmetrical voltage regulation.

9. A series converter chain-driven on-load tap-changing distribution transformer according to any one of claims 1-8, characterized in that, The system comprises m independent three-phase basic circuit units and n coupled three-phase basic circuit units, forming a series-parallel combination to expand the capacity of the on-load tap-changing distribution transformer, where m is greater than or equal to 1 and n is greater than or equal to 0. The independent three-phase basic circuit units are configured in a series-parallel combination with input ports (A1A1', B1B1', C1C1') and output ports (a1a1', b1b1', c1c1') using a parallel input port, series output port, series output port, or hybrid series-parallel configuration. The coupled three-phase basic circuit units are configured in a series-parallel combination with input ports (A2B2C2', b2b2', and c1c1') using a parallel input port, series output port configuration. The three-phase coupled series-parallel combination (c2c2') is formed by connecting the output ports of the three-phase independent series-parallel combination and the three-phase coupled series-parallel combination in parallel to form a star or delta configuration and leading out the terminals (a1, b1, c1). The input terminals (A1', B1', C1') of the three-phase independent series-parallel combination are connected to the input terminals (A2, B2, C2) of the three-phase coupled series-parallel combination and leading out the terminals (A1, B1, C1), thus forming a large-capacity on-load tap-changing distribution transformer with external input ports A1B1C1 and external output ports a1b1c1.

10. A series converter chain-driven on-load tap-changing distribution transformer according to any one of claims 1-9, characterized in that, Switches are connected between each pair of the external terminals (X, Y, Z) of the three-phase series converter chain (XY, XZ, YZ) for soft starting. Before the on-load tap-changing distribution transformer starts, the three switches are closed and the three-phase series converter chain is not connected to the circuit. After the on-load tap-changing distribution transformer starts and the auxiliary power supply of the control circuit is established, the three-phase series converter chain is put into operation and the three switches are disconnected, thereby connecting the three-phase series converter chain into the circuit to complete the soft start. The three-phase two-port converter and single-phase AC-DC-AC voltage regulating module of the on-load tap-changing distribution transformer are subjected to any control algorithm that can realize input port voltage control and any control method that can realize DC capacitor voltage regulation and stabilization. The AC-AC voltage regulating module is subjected to any control method that can realize input port voltage control, and the power frequency commutation module is subjected to any commutation control method that can realize commutation function. The above-mentioned module control methods are used to realize the start-up and output voltage regulation of the distribution transformer.

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