Single-phase or three-phase on-load voltage regulation circuit of industrial frequency transformer coupling AC-AC converter

Abstract

The invention discloses a single-phase or three-phase on-load voltage regulation circuit of an industrial frequency transformer coupled AC-AC converter. The single-phase or three-phase on-load voltage regulation circuit comprises n or 3n AC-AC converters with a stepped voltage regulation function, a single-phase or three-phase industrial frequency transformer and a special coupling mode between the single-phase or three-phase industrial frequency transformer and the AC-AC converters. Two terminals of the primary side winding are led out to form an external port, the secondary side isolation winding is connected with alternating current input ports of the n or 3n alternating current-alternating current converters, output ports of the n or 3n alternating current-alternating current converters are sequentially connected in series, and the terminals are led out to form an on-load voltage regulation port. The problems that a mechanical switch contact is short in service life and a sensitive load is powered off are fundamentally solved, and meanwhile the system cost is effectively reduced; the turn ratio of each transformer can be flexibly set according to actual requirements, so that differentiated and refined voltage regulation requirements in different application scenes can be met; the system has good expandability, and can be flexibly adapted to application scenes of different power levels through series-parallel combination.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power electronic power conversion technology, in particular to a single or three-phase on-load voltage regulating circuit of a power frequency transformer coupled AC-AC converter. BACKGROUND

[0002] Voltage fluctuation in single-phase and three-phase power systems seriously affects the power supply quality of the power grid, and even causes safety problems of equipment.

[0003] In single-phase application scenarios represented by rail transit traction power supply systems, electric locomotives as impact loads will cause severe and rapid fluctuations in the traction network voltage during startup and braking. Long-term over-limit or frequent fluctuations in voltage will directly affect the normal operation and even the safety of train operation.

[0004] In three-phase application scenarios, with a large number of high-proportion distributed new energy (such as photovoltaic and wind power) being connected, the randomness and intermittency of new energy output are superimposed on each other, which easily causes frequent over-limit of terminal voltage of three-phase power grid lines and three-phase imbalance.

[0005] In view of the above-mentioned voltage fluctuation problems in single-phase and three-phase scenarios, on-load voltage regulating circuit becomes a key technical means. The current mainstream technical routes can be divided into two categories: based on mechanical tap changer and based on unified power quality conditioner.

[0006] The on-load voltage regulator based on mechanical tap changer drives the mechanical contact to switch between different taps of the transformer winding through a servo motor, thereby realizing voltage regulation. However, this type of scheme has two problems: first, the mechanical switch switching process will produce electric arc, causing contact ablation and shortening the service life of the contact, in addition, it will also pollute the insulating medium and reduce the long-term operation reliability. Second, the response and regulation speed of the mechanical switch is slow, and when applied to sensitive load power supply, it will cause load power failure during voltage regulation; The on-load voltage regulator based on unified power quality conditioner connects the unified power quality conditioner in series between the power grid and the load, and realizes voltage regulation by controlling the output voltage. Benefiting from the significant advantage of power electronic devices in switching speed, this type of scheme can realize arcless and fast voltage regulation, solving the problems of short service life of contacts of on-load voltage regulator based on mechanical tap changer and power failure of sensitive load. However, the on-load voltage regulator based on unified power quality conditioner needs to configure two sets of power frequency transformers for parallel circuit voltage reduction and series circuit compensation, and two sets of power electronic converters for rectification and inversion respectively. Its cost is too high, which seriously hinders large-scale industrial applications.

[0007] In summary, the existing on-load voltage regulator based on mechanical tap changer has the problems of short contact life and sensitive load power-off, and the on-load voltage regulator based on unified power quality conditioner can overcome the above defects, but the circuit cost is too expensive. Therefore, it is urgent to develop a new on-load voltage regulation circuit topology, which can solve the problems of short contact life and sensitive load power-off while effectively reducing system cost. SUMMARY

[0008] In order to overcome the defects of the prior art, the present application provides a single or three-phase on-load voltage regulation circuit of a power frequency transformer coupled with an AC-AC converter, which realizes voltage regulation by combining structures such as H-bridge and three-level based on power electronic devices, and reduces the stress of power electronic devices by using the special coupling mode of power frequency transformer and AC-AC converter, thereby ensuring fast response of the system and improving the economy of the system.

[0009] In order to achieve the above purpose, the technical scheme adopted by the present application is: A single-phase on-load voltage regulation circuit of a power frequency transformer coupled with an AC-AC converter, comprising n AC-AC converters with step voltage regulation function and a single-phase power frequency transformer, which are coupled in a special way, wherein n is greater than or equal to 1; The AC-AC converter is used to generate step voltage at the on-load voltage regulation port; The single-phase power frequency transformer is used for voltage reduction to set the voltage regulation step; The single-phase power frequency transformer includes one primary winding and n mutually isolated secondary windings; two terminals (X, N) of the primary winding are led out to form an external port XN, and the corresponding terminals of the n mutually isolated secondary windings are connected with the AC input ports of the n AC-AC converters, respectively; the output ports of the n AC-AC converters are connected in series to obtain terminals (X', Y), and the terminals (X', Y) are led out to form an on-load voltage regulation port X'Y; the single-phase on-load voltage regulation circuit comprising the external port XN and the on-load voltage regulation port X'Y constitutes a basic circuit unit; The special coupling mode is that the X' terminal of the voltage regulation port is connected with the X terminal of the primary winding, so that the voltage difference between XN and YN is the voltage of the voltage regulation port, thereby forming two external ports XN and YN with special coupling relationship, and the single-phase on-load voltage regulation circuit comprising the external ports XN and YN is used for single-phase system.

[0010] The beneficial effect brought by the present application is that the voltage stress of the power electronic devices in the AC-AC converter is only the voltage of the secondary winding, which significantly reduces the voltage stress.

[0011] The number of turns of the primary winding of the single-phase power frequency transformer is , and the ratio of the number of turns of the primary winding to the number of turns of the n secondary windings is , wherein the number of turns of each winding can be set arbitrarily.

[0012] wherein, is the primary winding number of turns.

[0013] The beneficial effects brought by it are that, by reasonably configuring the turn combination of the secondary winding of the transformer, the voltage variation range of each gear can be flexibly set according to actual requirements, the differentiated and refined voltage regulation requirements in different application scenarios are met, and the applicability and design freedom of the circuit are enhanced.

[0014] Preferably, when the secondary winding of the single-phase power frequency transformer does not have a center tap, two terminals are led out from each single-phase power frequency transformer secondary winding, and the AC-AC converter with the step voltage regulation function is a two-level AC-AC H bridge. When the secondary winding of the power frequency transformer has a center tap, three terminals are led out from each single-phase power frequency transformer secondary winding, so the AC-AC converter with the step voltage regulation function is a single three-level AC-AC bridge arm or a three-level AC-AC H bridge composed of two three-level AC-AC bridge arms connected in parallel; the center tap of the secondary winding is connected with the neutral point of the three-level AC-AC bridge arm, and the basic circuit unit and the special coupling mode remain unchanged.

[0015] X'Y is the output terminal obtained by connecting the AC-AC converters in series, and the input terminal of each AC-AC converter is connected with the winding of the transformer; the input terminal has 2 or 3, depending on whether the AC-AC converter is two-level or three-level, and the output terminal has only two, which are connected in series to obtain X'Y.

[0016] Preferably, the two-level AC-AC H bridge includes 4 bidirectional switches, two of which are connected in series to form two bridge arms, the two bridge arms are connected in parallel, and the parallel terminals are led out to form an AC input port, and the midpoints of the two bridge arms are led out to form an AC output port.

[0017] Preferably, the three-level AC-AC bridge arm adopts a midpoint clamping structure or a T-shaped structure. The midpoint clamping structure includes 6 bidirectional switches, of which 4 are connected in series to form a long branch, and the other 2 are connected in series to form a short branch, and are connected across the connection points of the 1st and 2nd switches and the connection points of the 3rd and 4th switches of the long branch, and the midpoint of the short branch serves as the neutral point of the three-level AC-AC bridge arm of the midpoint clamping structure; the two outer terminals of the long branch and the terminal led out from the midpoint of the short branch constitute an AC input port. Wherein, the terminal led out from the midpoint of the short branch is connected with the center tap of the secondary winding, and the two outer terminals of the long branch are connected with the two terminals of the secondary winding.

[0018] The T-shaped structure includes three bidirectional switches, two of which are connected in series, and the third bidirectional switch is connected to the series node of the two bidirectional switches. The three bidirectional switches are interconnected in a T-shape. The outer terminal and neutral terminal of the T-shaped structure constitute an AC input port, which is respectively connected to the outer terminal and center tap of the secondary winding. The number of terminals on the secondary winding matches the number of input ports of the AC-AC converter, and the configuration of the output ports is consistent.

[0019] Preferably, when the three-level AC bridge arm with midpoint clamping structure is used alone, the midpoint of the long branch and the center tap of the secondary winding are led out to form an AC output port; when two sets of the three-level AC bridge arm with midpoint clamping structure are connected in parallel to form an H-bridge, the midpoints of the two sets of bridge arms are led out to form an AC output port; when the three-level AC bridge arm with T-shaped structure is used alone, the midpoint of the T-shaped structure three-level AC bridge arm and the center tap of the secondary winding are led out to form an AC output port; when two sets of the T-shaped structure three-level AC bridge arm are connected in parallel to form an H-bridge, the midpoints of the two sets of bridge arms are led out to form an AC output port. The beneficial effect is that the three-level AC bridge arm can expand the voltage regulation range compared to the two-level AC bridge arm.

[0020] Preferably, the bidirectional switch is any one of the following power devices or combinations of power devices with bidirectional semi-controlled / fully controlled capabilities: bidirectional thyristor, anti-parallel thyristor, bidirectional insulated gate bipolar transistor (IGBT), anti-parallel IGBT bare diode, anti-parallel gate turn-off thyristor (GTO), anti-parallel integrated gate commutated thyristor (IGCT) bare diode, anti-series IGCT, anti-series MOSFET, and diode bridge bidirectional switch. The various selections of the bidirectional switch can meet the different capacity and switching speed requirements of the voltage regulation circuit.

[0021] Preferably, the single-phase on-load tap changer circuit includes at least two basic circuit units of the above-mentioned single-phase on-load tap changer circuit. The primary windings of each basic circuit unit are connected in series, and the tap changer ports formed by the coupling of the secondary windings are connected in parallel, forming a multi-unit series-parallel combination. The combination still adopts the above-mentioned special coupling method to construct the external port of the whole machine. The beneficial effect is that the system voltage level and power capacity can be easily expanded by combining multiple basic circuit units in series and parallel. The system of the present invention can be widely used in scenarios such as rail transit traction power supply, residential areas with a high proportion of distributed photovoltaic power, and low-voltage residential households at the end of the power supply. The system supports the smooth expansion and precise matching of capacity and voltage level through the flexible series-parallel combination of standardized modules. This design can not only provide single-point treatment for specific users or line ends, but also meet the composite needs of medium and high voltage systems through integrated expansion, thereby breaking through the current capacity limitation of single devices and fundamentally avoiding the technical difficulties caused by direct parallel connection of power devices. Preferably, the two single-phase ports of the single-phase on-load tap changer circuit are configured into a single-phase on-load tap changer circuit system in either the manner of "XN connected to power supply, YN connected to load" or "XN connected to load, YN connected to power supply".

[0022] A three-phase on-load tap-changing circuit for a power frequency transformer coupled to an AC-AC converter includes one three-phase power frequency transformer and 3n AC-AC converters with stepped tap-changing function, wherein n is greater than or equal to 1; the three-phase power frequency transformer and the AC-AC converters with stepped tap-changing function are connected by a special three-phase coupling method. The primary side of the three-phase power frequency transformer includes three windings, and the terminals corresponding to the three windings are led out to form winding ports AA', BB', and CC'. The secondary side includes 3n isolation windings, which are respectively connected to the AC input ports of 3n AC-AC converters. The AC output ports of the 3n AC-AC converters are connected in series according to phases A, B, and C to form terminals (a', a), (b', b), and (c', c), which are led out to form on-load tap-changing ports a'a, b'b, and c'c. The circuit containing winding ports AA', BB', and CC' and on-load tap-changing ports a'a, b'b, and c'c constitutes a three-phase basic circuit unit. The turns ratio of each phase primary winding and n secondary windings of the three-phase power frequency transformer is: ,in, It is the number of turns in the primary winding.

[0023] The number of turns in each winding can be set arbitrarily. The beneficial effect is that by reasonably configuring the combination of turns in the secondary winding of the transformer, the voltage variation range of each level can be flexibly set according to actual needs, meeting the differentiated and refined voltage regulation requirements under different application scenarios, and enhancing the applicability and design freedom of the circuit.

[0024] The AC-AC converter is used to generate graded voltages at the on-load tap-changing port. The special three-phase coupling method involves connecting the three primary winding ports in a star or delta configuration, and leading out terminals A, B, and C to form three-phase ports. On the secondary side, the on-load tap-changing ports a', b', and c' are connected to terminals A, B, and C of the primary winding ports, respectively, and the on-load tap-changing ports a, b, and c are led out to form another three-phase port. The circuit containing ports ABC and abc is used in a three-phase system.

[0025] Preferably, when the secondary winding of the three-phase power frequency transformer does not have a center tap, the AC-AC converter with stepped voltage regulation function is a two-level AC-AC H-bridge; when the secondary winding of the three-phase power frequency transformer has a center tap, the AC-AC converter with stepped voltage regulation function is a single three-level AC-AC bridge arm or a three-level AC-AC H-bridge composed of two three-level AC-AC bridge arms connected in parallel; the center tap of the secondary winding is connected to the neutral point of the three-level AC-AC bridge arm, and the basic circuit unit and special three-phase coupling method described above remain unchanged; the three-level AC-AC bridge arm adopts a midpoint clamping structure or a T-type structure. The beneficial effect is that the three-level AC-AC bridge arm can expand the three-phase voltage regulation range compared to the two-level AC-AC bridge arm.

[0026] Preferably, the three-phase on-load tap-changing circuit includes at least two of the aforementioned three-phase basic circuit units; the primary winding ports of each three-phase basic circuit unit are connected in series according to phases A, B, and C, and the tap-changing ports formed by the coupling of the secondary windings are connected in parallel according to phases A, B, and C, forming a series-parallel combination of multiple three-phase basic circuit units; the combination still adopts the special three-phase coupling method described above to construct the three-phase ports ABC and abc of the whole machine. The beneficial effect is that the system voltage level and power capacity can be easily expanded through the series-parallel combination of multiple basic circuit units. In areas with a high proportion of photovoltaic grid connection, or at the end of rural power grids, the system can flexibly combine multiple basic units in series and parallel according to actual needs to achieve capacity expansion and voltage level matching. This can solve the single-point problem of low-voltage distribution networks and also meet the governance needs of medium- and high-voltage distribution substations through combination, covering a wide range of scenarios from substations to lines.

[0027] Preferably, the two three-phase ports of the three-phase on-load tap changer circuit are configured into a three-phase on-load tap changer circuit system in either the manner of "ABC connected to the three-phase power supply and abc connected to the three-phase load" or "abc connected to the three-phase power supply and ABC connected to the three-phase load".

[0028] Preferably, when the primary winding of the three-phase power frequency transformer is connected in a star configuration, the neutral point formed is grounded with high resistance, directly grounded, or suspended to ground according to the requirements of the power distribution network.

[0029] The beneficial effects of this invention are: (1) The single / three-phase on-load tap changer circuit of the present invention adopts an AC-AC converter based on a power electronic bidirectional switch. Its switching mechanism relies on the microsecond-level movement of semiconductor charge carriers, rather than the millisecond-level displacement of mechanical contacts. This fundamental difference enables the circuit to achieve seamless connection of output voltage at the precise zero-crossing point of the current. At the same time, replacing the mechanical switch with a power electronic device enables precise arc-free switching at the zero-crossing point, completely solving the lifespan problem caused by contact erosion in mechanical tap changers. Therefore, the present invention can solve the problems of short contact life and power failure of sensitive loads in on-load tap changers based on mechanical tap switches.

[0030] (2) In the single / three-phase on-load tap changer circuit of the present invention, the AC-AC converter and the power frequency transformer adopt a special coupling method that connects the primary winding and the tap changer port to achieve coupling between the primary and secondary sides. This method only uses one power frequency transformer and a single-stage converter. Compared with the existing on-load tap changer based on a unified power quality regulator, the capacity of the magnetic components and converter is halved, which greatly reduces the system cost.

[0031] (3) The number of turns of each winding of the power frequency transformer in the single / three-phase on-load voltage regulating circuit of the present invention can be set arbitrarily. By reasonably configuring the combination of turns of the secondary winding of the transformer, the voltage change amplitude of each level can be flexibly set according to actual needs, so as to meet the differentiated and refined voltage regulation needs under different application scenarios.

[0032] (4) The single / three-phase on-load tap changer system of the present invention has modular characteristics and can be combined in series and parallel by multiple basic circuit units to realize the expansion of system voltage and capacity, covering a wide range of application scenarios. (Overcoming the limitation of current capacity of a single device and avoiding parallel connection of power devices). Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the single / three-phase on-load tap changer circuit structure of the power frequency transformer coupled AC-AC converter of the present invention.

[0034] Figure 2 This is a schematic diagram of the topology of the single-phase on-load tap changer circuit of the present invention.

[0035] Figure 3 This is a schematic diagram of the series-parallel combination structure of the basic circuit unit of the single-phase on-load voltage regulating circuit of the present invention.

[0036] Figure 4 This is the configuration of the single-phase on-load voltage regulating circuit system of the present invention.

[0037] Figure 5 This is a schematic diagram of the topology of the three-phase on-load tap changer circuit of the present invention.

[0038] Figure 6 This is a schematic diagram of the series-parallel combination structure of the basic circuit units of the three-phase on-load tap changer circuit of the present invention.

[0039] Figure 7 This is a schematic diagram of the series-parallel combination structure of the basic circuit units of the three-phase on-load tap changer circuit of the present invention.

[0040] Figure 8 This invention relates to the grounding method of the three-phase on-load tap changer circuit.

[0041] Figure 9 This is a simulation waveform diagram of an embodiment of a single-phase on-load tap changer circuit.

[0042] Figure 10 This is a simulation waveform diagram of an embodiment of a three-phase on-load tap changer circuit. Detailed Implementation

[0043] The present invention will now be described in further detail with reference to the accompanying drawings.

[0044] As attached Figure 1 As shown in (a), a single-phase on-load tap-changing circuit of a power frequency transformer coupled AC-AC converter is composed of multiple two-level AC-AC H-bridges with tap-changing function and a single-phase power frequency transformer. Each two-level AC-AC bridge contains four pairs of anti-parallel thyristors. The anode and cathode of one thyristor are connected to the cathode and anode of another thyristor to form an anti-parallel thyristor pair. The four pairs of anti-parallel thyristors are connected in series to form two bridge arms. The two bridge arms are connected in parallel, and the parallel terminals are led out to form an AC input port. The midpoint of the two bridge arms is led out to form an AC output port. The single-phase power frequency transformer contains one primary winding and n mutually isolated secondary windings. The two terminals (X, N) of the primary winding are led out to form an external port XN. The n isolated secondary windings are respectively connected to the AC input ports of the n two-level AC-AC bridges. The output ports of the n-level AC-AC bridges are connected in series and the terminals are led out to form on-load tap-changing ports X' and Y. Finally, the X' terminal of the tap-changing port is connected to the X terminal of the primary winding to form two external ports XN and YN. The circuit containing the external port XN and the on-load tap-changing port X'Y constitutes the basic circuit unit. Thanks to the aforementioned connection structure, the voltage stress of the bidirectional switch is only the voltage amplitude of the transformer winding it is connected to, thus achieving low voltage stress in semiconductor devices.

[0045] A two-level AC-AC H-bridge, through specific logic control of its thyristors, can generate three types of voltages at its output. When thyristors 1 and 4 are simultaneously turned on, the AC output port of the two-level AC-AC H-bridge generates a voltage in phase with the input voltage; when thyristors 2 and 3 are simultaneously turned on, the AC output port generates a voltage out of phase with the input voltage; when thyristors 1 and 3 or thyristors 2 and 4 are simultaneously turned on, the AC output port generates zero voltage. By adjusting the output voltage of each two-level AC-AC H-bridge, different voltage adjustment levels can be obtained through the voltage adjustment ports X' and Y.

[0046] The turns ratio of the primary winding to the n secondary windings of a single-phase power frequency transformer is The number of turns in each winding can be set arbitrarily; the system can reasonably configure the combination of turns of the transformer secondary winding according to actual needs, thereby flexibly setting the voltage change range of each level to meet the differentiated and refined voltage regulation needs under different application scenarios.

[0047] As a typical embodiment of the present invention, the turns ratio of the n secondary windings is designed as follows: , or Any one of the above. The turns ratio of the secondary winding can be combined with the switching logic of each AC-AC converter to meet the requirements of differentiated voltage regulation levels.

[0048] As attached Figure 2 As shown in (a), the above appendix Figure 1 (a) The bidirectional thyristor in the single-phase on-load tap changer circuit shown can be replaced with the one shown in the attached diagram. Figure 2 (f) refers to any of the following power devices or combinations of power devices with bidirectional semi-controlled / fully controlled capabilities: bidirectional thyristors, anti-parallel thyristors, bidirectional insulated-gate bipolar transistors (IGBTs), anti-parallel IGBT bare diodes, anti-parallel gate turn-off thyristors (GTOs), anti-parallel integrated gate commutated thyristors (IGCTs), anti-series IGCTs, anti-series MOSFETs, and diode bridge bidirectional switches. These various implementations of bidirectional switches can meet the capacity and switching speed requirements of different application scenarios.

[0049] The above appendix Figure 2 (a) The two-level AC-AC H-bridge in the single-phase on-load tap changer circuit shown can be replaced with a three-level circuit, including a midpoint clamped three-level bridge arm, a T-type three-level bridge arm, a midpoint clamped three-level H-bridge, and a T-type three-level H-bridge. (See attached diagram) Figure 2 As shown in (b), the midpoint clamping three-level bridge arm contains six bidirectional switches. Four bidirectional switches are connected in series to form a long branch, and the other two bidirectional switches are connected in series to form a short branch that spans between the connection points of switches 1 and 2 and the connection points of switches 3 and 4 in the long branch. The midpoint of the short branch serves as the neutral point of the clamping three-level bridge arm. The two external terminals of the long branch and the terminal led out from the midpoint of the short branch constitute the AC input port. Specifically, the terminal led out from the midpoint of the short branch is connected to the center tap of the secondary winding, and the two external terminals of the long branch are connected to the two terminals of the secondary winding. When the midpoint clamping three-level bridge arm is used alone, the midpoint of the long branch and the center tap of the secondary winding are led out to form the AC output port. When two sets of midpoint clamping three-level bridge arms are connected in parallel to form an H-bridge, the midpoints of the two sets of bridge arms are led out to form the AC output port as shown in the attached figure. Figure 2 As shown in (d). Figure 2As shown in (c), the T-type three-level AC bridge arm contains three bidirectional switches interconnected in a T-shape. The external terminals and neutral terminals of the T-type structure constitute the AC input port, which is connected to the external terminals and center tap of the secondary winding, respectively. When the T-type three-level bridge arm is used alone, the midpoint of the T-type bridge arm and the center tap of the secondary winding are led out to form the AC output port. When two sets of T-type three-level bridge arms are connected in parallel to form an H-bridge, the midpoints of the two sets of bridge arms are led out to form the AC output port as shown in the attached diagram. Figure 2 As shown in (e), compared to the two-level AC bridge arm output voltage amplitude which can only be configured as the secondary winding voltage or zero, the three-level AC bridge arm output voltage amplitude can also be configured as half of the secondary winding voltage, thus expanding the voltage regulation range.

[0050] Multiple of the above-mentioned appendices Figure 2 The circuit shown can be expanded in capacity through series and parallel connections. (See attached diagram) Figure 3 As shown, the primary windings of k basic units of a single-phase on-load tap changer circuit , … A voltage regulating port is formed by the coupling of the secondary windings of k single-phase on-load tap changer circuit basic units connected in series. … and … The components are connected in parallel. Finally, the voltage regulating port X', which is connected in series, is connected to the X terminal of the primary winding to form two external ports, XN and YN. This series-parallel combination can expand the system voltage and capacity, covering a wide range of application scenarios.

[0051] The above appendix Figures 2-3 The single-phase on-load tap changer circuit can be configured with two wiring methods as shown in the attached diagram. Figure 4 As shown. Method 1 connects port XN to the power supply and port YN to the load; Method 2 connects port XN to the load and port YN to the power supply. Both connection methods allow for stepped voltage regulation.

[0052] As attached Figure 1 (b) shows a three-phase on-load tap-changing circuit for a power frequency transformer coupled AC-AC converter, which consists of multiple two-level AC-AC H-bridges with tap-changing function and a three-phase power frequency transformer; The specific structure and appendix of the two-level AC-AC H-bridge Figure 1The same applies to (a). The three-phase power frequency transformer has three windings on the primary side, with the corresponding terminals forming winding ports AA', BB', and CC'. The secondary side has 3n isolation windings, which are connected to the AC input ports of 3n AC-AC converters. The AC output ports of the 3n AC-AC converters are connected in series with n ports within each phase (A, B, C) to form terminals (a', a), (b', b), and (c', c), which are then led out to form on-load tap-changing ports a'a, b'b, and c'c. The three winding ports on the primary side are connected in a star or delta configuration, and terminals A, B, and C are led out to form three-phase ports. Finally, terminals a', b', and c' of the on-load tap-changing ports on the secondary side are connected to terminals A, B, and C of the primary winding ports, and terminals a, b, and c of the on-load tap-changing ports are led out to form another three-phase port. The circuit described above, which includes winding ports AA', BB', CC' and on-load tap changer ports a'a, b'b, c'c, constitutes a three-phase basic circuit unit. Thanks to the above connection structure, the voltage stress of the bidirectional switch is only the voltage amplitude of the transformer winding connected to it, thus achieving low voltage stress on the semiconductor device.

[0053] The turns ratio of each phase primary winding to the n secondary windings of a three-phase power frequency transformer is: The number of turns in each winding can be set arbitrarily; the system can reasonably configure the combination of turns of the transformer secondary winding according to actual needs, thereby flexibly setting the voltage change range of each level to meet the differentiated and refined voltage regulation needs under different application scenarios.

[0054] As a typical embodiment of the present invention, the turns ratio of the n windings on the secondary side of each phase can be designed as follows: , or Any one of the above. The turns ratio of the secondary winding can be combined with the switching logic of each AC-AC converter to meet the requirements of differentiated voltage regulation levels.

[0055] As attached Figure 5 As shown in (a), the above appendix Figure 1 (b) The bidirectional thyristor in the three-phase on-load tap changer circuit shown can be replaced with the one shown in the attached diagram. Figure 2 Any bidirectional switch in (f). Appendix Figure 5 (a) The two-level AC-AC H-bridge in the three-phase on-load tap changer circuit shown can be replaced with a three-level circuit, including a midpoint clamping three-level bridge arm, a T-type three-level bridge arm, a midpoint clamping three-level H-bridge, and a T-type three-level H-bridge as shown in the appendix. Figure 5 As shown in (b)-(e), the specific connection method is the same as the attached diagram. Figure 2 Similarly, I will not go into details here.

[0056] Multiple of the above-mentioned appendices Figure 5 The circuit shown can be expanded in capacity through series and parallel connections. (See attached diagram) Figure 6As shown, the primary windings of k basic units of a three-phase on-load tap changer circuit ( ), ( )...( (X represents phases A, B, and C) The voltage regulating port is formed by connecting phases A, B, and C in series. The secondary windings of k basic units of a three-phase on-load tap changer circuit are coupled together to form the voltage regulating port. … (x can be a, b, or c) and … The ports are constructed by connecting phases A, B, and C in parallel respectively. Finally, connect the series-connected voltage regulating ports. The terminal is connected to the X terminal of the primary winding to form (A, B, C) and Two three-phase external ports. The above appendix Figures 5-6 The three-phase on-load tap changer circuit can be configured with two wiring methods as shown in the attached diagram. Figure 7 As shown. Method 1 connects ports ABC to the three-phase power supply and ports abc to the three-phase load; Method 2 connects ports abc to the three-phase power supply and ports ABC to the three-phase load. Both connection methods allow for stepped voltage regulation.

[0057] The above appendix Figures 5-7 The neutral point grounding methods for a three-phase on-load tap changer circuit include three modes: high-resistance grounding, direct grounding, or floating (ungrounded). (See attached...) Figure 8 (a) illustrates a high-resistance grounding method that grounds the neutral point through a high-value resistor. This method is typically used in medium-voltage distribution networks. The resistor limits the ground fault current, reducing equipment damage and arcing risks, while also facilitating fault detection and location, and improving the system's continuous power supply capability. (See attached diagram) Figure 8 (b) The direct grounding method connects the neutral point directly to the earth, which can effectively limit overvoltage levels and reduce insulation stress, but the fault current is relatively large, requiring fast protection to clear the fault; Appendix Figure 8 (c) The floating grounding method shows that the neutral point is not connected to the earth and operates by relying on ground insulation. It can still continue to supply power for a short time in the event of a single-phase ground fault, but it may cause overvoltage problems, requiring enhanced insulation monitoring and protection. The above-mentioned neutral point grounding methods can be flexibly configured according to the technical specifications and operating requirements of the actual distribution network, so as to adapt to the electrical safety, system stability and protection coordination requirements under different application scenarios.

[0058] 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. Specific Implementation Example 1 The single-phase on-load tap changer circuit of the power frequency transformer-coupled AC-AC converter of the present invention has various circuit structures. As a preferred embodiment of the present invention, the attached diagram is provided. Figure 4 (b) shows a specific embodiment of a single-phase on-load tap-changing circuit for a power frequency transformer coupled to an AC-AC converter.

[0060] Specifically, as shown in the attached document Figure 4 As shown in (b), the AC-AC converter of the single-phase on-load voltage regulating circuit adopts a two-level H-bridge with n=2 secondary windings. The single-phase on-load voltage regulating circuit system is constructed by connecting the load with XN and the power supply with YN.

[0061] The simulation parameters of the single-phase on-load tap changer circuit designed in this embodiment are summarized in Table 1. The simulation model was built using Matlab / Simulink, and the simulation waveforms are shown in the appendix. Figure 9 middle.

[0062] Table 1 Simulation parameters of single-phase on-load tap changer At the start of the simulation, the effective value of the grid voltage was set to 8kV. The on-load tap changer circuit adjusted the load voltage to 10kV by configuring two AC-AC converters to output voltages in phase with the grid. After 0.4s, the grid voltage increased to 8.5kV. The first AC-AC converter was then configured to output zero voltage, and the second AC-AC converter to output grid voltage in phase, thus adjusting the load voltage to 10kV. This process continued, with the grid voltage increasing by 0.5kV every 0.4s. The two AC-AC converters were adjusted to flexibly output grid voltage in phase, grid voltage out of phase, or zero voltage, maintaining the load voltage at 10kV. This simulation verified the feasibility and correctness of the single-phase on-load tap changer circuit of this invention. Specific Implementation Example 2 The three-phase on-load tap changer circuit of the power frequency transformer-coupled AC-AC converter of the present invention has various circuit structures. As a preferred embodiment of the present invention, the attached diagram is shown below. Figure 7 (b) shows a specific embodiment of the three-phase on-load tap changer circuit of the power frequency transformer coupled to the AC-AC converter.

[0064] Specifically, as shown in the attached document Figure 7As shown in (b), the AC-AC converters of the three-phase on-load tap changer circuit all adopt a two-level H-bridge, with n=2 secondary windings per phase. The three-phase on-load tap changer circuit system is constructed by connecting the load with ABC and the power supply with abc.

[0065] The simulation parameters of the three-phase on-load tap changer circuit designed in this embodiment are summarized in Table 2. The simulation model was built using Matlab / Simulink, and the simulation waveforms are summarized in Appendix 2. Figure 10 middle.

[0066] Table 2 Simulation parameters of three-phase on-load tap changer At the start of the simulation, the effective value of the three-phase grid phase voltage was set to 8kV. The on-load tap changer circuit adjusted the load voltage to 10kV by configuring two AC-AC converters to output voltages in phase with the grid. After 0.4s, the grid voltage increased to 8.5kV. Then, the first AC-AC converter in each phase was configured to output zero voltage, and the second AC-AC converter in each phase was configured to output grid voltages in phase, thus adjusting the load voltage to 10kV. This process continued, with the grid voltage increasing by 0.5kV per phase every 0.4s. The two AC-AC converters in each phase were adjusted to flexibly output grid voltages in phase, grid voltages out of phase, or zero voltage, maintaining the load voltage at 10kV. This simulation verified the feasibility and correctness of the three-phase on-load tap changer circuit of this invention.

Claims

1. A single-phase on-load tap-changing circuit for a power frequency transformer-coupled AC-AC converter, characterized in that, It includes n AC-AC converters with stepped voltage regulation function and 1 single-phase power frequency transformer, which are coupled together by a special method, where n is greater than or equal to 1; The AC-AC converter is used to generate graded voltages at the on-load tap-changing port. The single-phase power frequency transformer is used to step down the voltage to set the voltage regulation level; The single-phase power frequency transformer includes one primary winding and n mutually isolated secondary windings. The two terminals (X, N) of the primary winding are led out to form an external port XN. The corresponding terminals of the n mutually isolated secondary windings are respectively connected to the AC input ports of n AC-AC converters. The output ports of the n AC-AC converters are connected in series to obtain terminals (X', Y). Terminals (X', Y) are led out to form an on-load tap-changing port X'Y. The single-phase on-load tap-changing circuit including the external port XN and the on-load tap-changing port X'Y constitutes a basic circuit unit. The special coupling method is that the X' terminal of the voltage regulating port is connected to the X terminal of the primary winding, so that the voltage difference between XN and YN is the voltage of the voltage regulating port, thereby forming two external ports XN and YN with a special coupling relationship. The single-phase on-load voltage regulating circuit containing the external ports XN and YN is used in a single-phase system.

2. The single-phase on-load tap-changing circuit of a power frequency transformer-coupled AC-AC converter according to claim 1, characterized in that, The number of turns in the primary winding of the single-phase power frequency transformer is: The turns ratio of the primary winding to the n secondary windings is The number of turns in each winding can be set arbitrarily; in, It is the number of turns in the primary winding.

3. The single-phase on-load tap-changing circuit of a power frequency transformer-coupled AC-AC converter according to claim 1, characterized in that, When the secondary winding of the single-phase power frequency transformer does not have a center tap, each secondary winding of the single-phase power frequency transformer has two terminals. The AC-AC converter with the function of step-by-step voltage regulation is a two-level AC-AC H-bridge. When the secondary winding of the power frequency transformer has a center tap, each single-phase power frequency transformer secondary winding has three terminals. Therefore, the AC-AC converter with step-by-step voltage regulation function is a single three-level AC-AC bridge arm or a three-level AC-AC H-bridge composed of two three-level AC-AC bridge arms connected in parallel. The center tap of the secondary winding is connected to the neutral point of the three-level AC-AC bridge arm, and the above basic circuit unit and special coupling method remain unchanged.

4. A single-phase on-load tap changer circuit for a power frequency transformer-coupled AC-AC converter according to claim 3, characterized in that, The two-level AC-AC bridge includes four bidirectional switches. The four bidirectional switches are connected in series in pairs to form two bridge arms. The two bridge arms are connected in parallel, and the parallel terminals are led out to form an AC input port. The midpoint of the two bridge arms is led out to form an AC output port. The three-level crossover bridge arm adopts a midpoint clamping structure or a T-shaped structure; The midpoint clamping structure includes six bidirectional switches, four of which are connected in series to form a long branch, and the other two are connected in series to form a short branch, which is connected between the connection points of the first and second switches and the connection points of the third and fourth switches in the long branch. The midpoint of the short branch serves as the neutral point of the three-level AC crossover arm of the midpoint clamping structure. The two external terminals of the long branch and the terminal led out from the midpoint of the short branch constitute an AC input port. Among them, the terminal led out from the midpoint of the short branch is connected to the center tap of the secondary winding, and the two outer terminals of the long branch are connected to the two terminals of the secondary winding. The T-shaped structure includes three bidirectional switches, two of which are connected in series, and the third bidirectional switch is connected to the series node of the two bidirectional switches. The three bidirectional switches are interconnected in a T-shape. The outer terminal and neutral terminal of the T-shaped structure constitute an AC input port, which is respectively connected to the outer terminal and center tap of the secondary winding. The number of terminals on the secondary winding matches the number of input ports of the AC-AC converter, and the configuration of the output ports is consistent.

5. A single-phase on-load tap-changing circuit for a power frequency transformer-coupled AC-AC converter according to claim 4, characterized in that, When the three-level AC crossover arm with midpoint clamping structure is used alone, the midpoint of the long branch and the center tap of the secondary winding are led out to form an AC output port; when two sets of the three-level AC crossover arm with midpoint clamping structure are connected in parallel to form an H-bridge, the midpoints of the two sets of bridge arms are led out to form an AC output port; when the three-level AC crossover arm with T-structure is used alone, the midpoint of the T-structure three-level AC crossover arm and the center tap of the secondary winding are led out to form an AC output port; when two sets of the three-level AC crossover arm with T-structure are connected in parallel to form an H-bridge, the midpoints of the two sets of bridge arms are led out to form an AC output port. The bidirectional switch is any one of the following power devices or combinations of power devices that have bidirectional semi-controlled / fully controlled capabilities: bidirectional thyristor, anti-parallel thyristor, bidirectional insulated gate bipolar transistor (IGBT), anti-parallel IGBT bare diode, anti-parallel gate turn-off thyristor (GTO), anti-parallel integrated gate commutated thyristor (IGCT bare diode), anti-series IGCT, anti-series metal-oxide-semiconductor field-effect transistor (MOSFET), and diode bridge bidirectional switch.

6. A single-phase on-load tap-changing circuit for a power frequency transformer-coupled AC-AC converter according to claim 1, characterized in that, The single-phase on-load tap changer circuit includes at least two basic circuit units of the above-mentioned single-phase on-load tap changer circuit. The primary windings of each basic circuit unit are connected in series, and the tap changer ports formed by the coupling of the secondary windings are connected in parallel to form a multi-unit series-parallel combination. The combination still adopts the above-mentioned special coupling method to construct the external port of the whole machine. The two single-phase ports of the single-phase on-load tap changer circuit are configured into a single-phase on-load tap changer circuit system in either the manner of "XN connected to power supply, YN connected to load" or "XN connected to load, YN connected to power supply". Single-phase on-load tap changer circuit systems are used in scenarios such as rail transit traction power supply, residential areas with a high proportion of distributed photovoltaic power, and low-voltage residential households at the end of the power supply.

7. A three-phase on-load tap-changing circuit for a power frequency transformer-coupled AC-AC converter, characterized in that, It includes one three-phase power frequency transformer and 3n AC-AC converters with stepped voltage regulation function as described above, where n is greater than or equal to 1; the three-phase power frequency transformer and the AC-AC converters with stepped voltage regulation function are connected by a special three-phase coupling method; The primary side of the three-phase power frequency transformer includes three windings, and the terminals corresponding to the three windings are led out to form winding ports AA', BB', and CC'. The secondary side includes 3n isolation windings, which are respectively connected to the AC input ports of 3n AC-AC converters. The AC output ports of the 3n AC-AC converters are connected in series according to phases A, B, and C to form terminals (a', a), (b', b), and (c', c), which are led out to form on-load tap-changing ports a'a, b'b, and c'c. The circuit containing winding ports AA', BB', and CC' and on-load tap-changing ports a'a, b'b, and c'c constitutes a three-phase basic circuit unit. The turns ratio of each phase primary winding and n secondary windings of the three-phase power frequency transformer is: ,in, It represents the number of turns in the primary winding; the number of turns in each winding can be set arbitrarily.

8. A three-phase on-load tap changer circuit for a power frequency transformer-coupled AC-AC converter according to claim 7, characterized in that, The AC-AC converter is used to generate graded voltages at the on-load tap-changing port. The special three-phase coupling method involves connecting the three winding ports on the primary side in a star or delta configuration, and leading out terminals A, B, and C to form a three-phase port; the on-load tap changer ports a', b', and c' on the secondary side are connected to terminals A, B, and C of the primary winding ports respectively, and the on-load tap changer ports a, b, and c are led out to form another three-phase port. The circuit containing ports ABC and abc is used in a three-phase system.

9. A three-phase on-load tap changer circuit for a power frequency transformer-coupled AC-AC converter according to claim 8, characterized in that, When the secondary winding of the three-phase power frequency transformer does not have a center tap, the AC-AC converter with stepped voltage regulation function is a two-level AC-AC H-bridge; when the secondary winding of the three-phase power frequency transformer has a center tap, the AC-AC converter with stepped voltage regulation function is a single three-level AC-AC bridge arm or a three-level AC-AC H-bridge composed of two three-level AC-AC bridge arms connected in parallel; the center tap of the secondary winding is connected to the neutral point of the three-level AC-AC bridge arm, and the basic circuit unit and special three-phase coupling method mentioned above remain unchanged; the three-level AC-AC bridge arm adopts a midpoint clamping structure or a T-type structure.

10. A three-phase on-load tap-changing circuit for a power frequency transformer-coupled AC-AC converter according to claim 9, characterized in that, The three-phase on-load tap changer circuit includes at least two of the aforementioned three-phase basic circuit units; the primary winding ports of each three-phase basic circuit unit are connected in series according to phases A, B, and C, and the tap changer ports formed by the coupling of the secondary windings are connected in parallel according to phases A, B, and C, forming a series-parallel combination of multiple three-phase basic circuit units; the combination still adopts the special three-phase coupling method described above to construct the three-phase ports ABC and abc of the whole machine; the two three-phase ports of the three-phase on-load tap changer circuit are constructed in either the manner of "ABC connected to the three-phase power supply, abc connected to the three-phase load" or "abc connected to the three-phase power supply, ABC connected to the three-phase load" to form a three-phase on-load tap changer circuit system; the neutral point formed when the primary winding of the three-phase power frequency transformer is star connected is grounded with high resistance, directly grounded, or suspended to ground according to the requirements of the distribution network.

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