High-capacity single-stage AC / DC converter and control method thereof

Abstract

The invention discloses a high-capacity single-stage AC / DC converter and a control method thereof, and belongs to the field of power electronics, the converter is composed of two identical converters, and each converter is composed of a medium-voltage AC voltage regulating circuit, a single-phase intermediate-frequency transformer and a low-voltage rectifying circuit. The medium-voltage alternating-current voltage regulating circuit and the low-voltage rectifying circuit respectively use an anti-parallel thyristor valve group and a diode valve group, so that the topology efficiency and reliability are improved; a plurality of distributed high-frequency transformers are replaced by the single-phase intermediate-frequency transformer, so that the topology cost is reduced; voltage and current of a bridge arm series submodule of the medium-voltage alternating-current voltage regulation circuit have high controllability, the output end can be connected with direct-current loads of different voltage levels, and the transmission power of the power supply is remarkably improved through the structure that the two converters output in parallel. The turn-on time sequence of the periodic overturning of the thyristor valve block effectively reduces the voltage and current stress of the bridge arm of the sub-module. The converter is small in conversion stage number, large in power capacity, wide in voltage output range and suitable for a large-capacity direct-current power supply scene.

Description

Technical Field

[0001] The present invention relates to the field of power electronics, and more particularly to a large-capacity single-stage AC / DC converter and a control method thereof, which are suitable for use in fields requiring DC power supply, such as data center power supply and new energy vehicle charging stations. Background Art

[0002] With the introduction of the "dual carbon" strategy, new DC load industries such as data centers have experienced rapid development. These industries draw power from a medium-voltage alternating current (MVAC) busbar, converting it into low-voltage direct current (LVDC) to power the loads. In 2022, global data center electricity consumption reached 460 billion kWh, accounting for nearly 2% of the global total. This figure is projected to rise to over 1 trillion kWh by 2026. In 2023, my country's data center electricity consumption reached 150 billion kWh, a year-on-year increase of 15.4%, accounting for 1.63% of total electricity consumption.

[0003] Publication No. CN120033707A details a data center power supply system based on multiple multi-port solid-state transformers. However, this system suffers from numerous components, including numerous power modules and high-frequency transformers. Furthermore, the efficiency of the multi-stage rectification structure of the solid-state transformers is difficult to improve. Furthermore, the insulation design of the high-frequency transformer in the DC / DC unit within existing solid-state transformers is relatively difficult and costly, and the voltage regulation range is narrow.

[0004] As can be seen from the above, the application of CN120033707A is limited in terms of technical economy, volume, and reliability. In particular, for DC power supplies in data centers and electric vehicle charging stations, meeting the technical requirements of large capacity, wide output voltage range, controllable cost, high power density, and high reliability is a current key research issue. Summary of the Invention

[0005] The present invention aims to solve the problem that the technical requirements of DC power supply in the fields of data centers and electric vehicle charging stations in terms of large capacity, wide output voltage range, controllable cost, high power density, and high reliability cannot be met. It provides a large-capacity single-stage AC / DC converter and its control method.

[0006] The technical solution adopted in the present invention is:

[0007] A large-capacity single-stage AC / DC converter is connected to a three-phase AC grid, with phases A, B, and C.

[0008] The converter consists of two converters and an output filter capacitor C o , the two converters are converter No. 1 and converter No. 2, and the output filter capacitor C oThe two ends are the low voltage DC output terminals Udc of the converter;

[0009] Converter No. 1 and Converter No. 2 use the same circuit topology. Each converter consists of a medium-voltage AC voltage regulator circuit, a single-phase medium-frequency transformer, and a low-voltage rectifier circuit.

[0010] The medium voltage AC voltage regulation circuit in converter No. 1 includes an anti-parallel thyristor valve group T1, an anti-parallel thyristor valve group T2, a thyristor series inductor L1, a thyristor series inductor L2, a sub-module series bridge arm S1, and a sub-module series bridge arm S2;

[0011] The operating frequency of the single-phase medium frequency transformer is 300 Hz. The single-phase medium frequency transformer consists of a transformer and a transformer leakage inductance L k1 The transformer is divided into a primary side and a secondary side, wherein the primary side includes a primary side first terminal and a primary side second terminal, and the secondary side includes a secondary side first terminal and a secondary side second terminal, and the primary side first terminal and the secondary side first terminal of the transformer are the same-name terminals; the transformer ratio is n:1, and the transformer leakage inductance is L k1 ; Where n and k are both real numbers;

[0012] The low voltage rectifier circuit adopts a diode rectifier bridge, and the diode rectifier bridge is a large capacity press-fit structure, including a diode D L1 , diode D L2 , diode D L3 , diode D L4 And the output filter capacitor C o ;

[0013] One end of the thyristor series inductors L1 and L2 is connected to phase A of the three-phase AC power grid, the other end of the thyristor series inductor L1 is connected to one end of the anti-parallel thyristor valve group T1, and the other end of the anti-parallel thyristor valve group T1 is connected to one end of the submodule bridge arm S1; the other end of the thyristor series inductor L2 is connected to one end of the anti-parallel thyristor valve group T2, and the other end of the anti-parallel thyristor valve group T2 is connected to one end of the submodule bridge arm S2; the other ends of the submodule bridge arms S1 and S2 are connected to phase B of the three-phase power grid;

[0014] The thyristor series inductor L1, anti-parallel thyristor valve group T1 and submodule bridge arm S1 constitute the first bridge arm on the medium voltage side of converter No. 1, and the thyristor series inductor L2, anti-parallel thyristor valve group T2 and submodule bridge arm S2 constitute the second bridge arm on the medium voltage side of converter No. 2;

[0015] The transformer leakage inductance L of the single-phase medium frequency transformer k1 One end is connected between the anti-parallel thyristor valve group T1 in the first bridge arm and the sub-module bridge arm S1, and the transformer leakage inductance L k1The other end is connected to the first primary terminal, and the second primary terminal is connected between the anti-parallel thyristor valve group T2 in the second bridge arm and the sub-module bridge arm S2;

[0016] The diode D of the low voltage rectifier circuit L1 、D L3 connected in series and form the first arm of the diode rectifier bridge, diode D L2 、D L4 are connected in series and constitute the second bridge arm of the diode rectifier bridge; the first bridge arm of the diode rectifier bridge and the second bridge arm of the diode rectifier bridge are connected in parallel;

[0017] The first terminal of the transformer secondary side of the single-phase intermediate frequency transformer is connected to the middle node of the first bridge arm of the diode rectifier bridge, and the second terminal of the transformer secondary side of the single-phase intermediate frequency transformer is connected to the middle node of the second bridge arm of the diode rectifier bridge;

[0018] The medium voltage AC voltage regulation circuit in the second converter includes an anti-parallel thyristor valve group T3, an anti-parallel thyristor valve group T4, a thyristor series inductor L3, a thyristor series inductor L4, a sub-module series bridge arm S3, and a sub-module series bridge arm S4;

[0019] The single-phase medium frequency transformer in the second converter operates at a frequency of 300 Hz. The single-phase medium frequency transformer consists of a transformer and a transformer leakage inductance L k2 The transformer is divided into a primary side and a secondary side, wherein the primary side includes a primary side first terminal and a primary side second terminal, and the secondary side includes a secondary side first terminal and a secondary side second terminal, and the primary side first terminal and the secondary side first terminal of the transformer are the same-name terminals; the transformer ratio is n:1, and the transformer leakage inductance is L k2 ; Where n and k are both real numbers;

[0020] One end of the thyristor series inductors L3 and L4 is connected to phase A of the three-phase AC power grid, the other end of the thyristor series inductor L3 is connected to one end of the anti-parallel thyristor valve group T3, and the other end of the anti-parallel thyristor valve group T3 is connected to one end of the submodule bridge arm S3; the other end of the thyristor series inductor L4 is connected to one end of the anti-parallel thyristor valve group T4, and the other end of the anti-parallel thyristor valve group T4 is connected to one end of the submodule bridge arm S4; the other ends of the submodule bridge arms S3 and S4 are both connected to phase C of the three-phase power grid;

[0021] The thyristor series inductor L3, the anti-parallel thyristor valve group T3 and the submodule bridge arm S3 constitute the first bridge arm on the medium voltage side of the second converter, and the thyristor series inductor L4, the anti-parallel thyristor valve group T4 and the submodule bridge arm S4 constitute the second bridge arm on the medium voltage side of the second converter;

[0022] The transformer leakage inductance L of the single-phase medium-frequency transformer in the second converter k2 One end is connected between the anti-parallel thyristor valve group T3 in the first bridge arm and the sub-module bridge arm S3, and the transformer leakage inductance L k2 The other end is connected to the first primary terminal, and the second primary terminal is connected between the anti-parallel thyristor valve group T4 in the second bridge arm and the sub-module bridge arm S4;

[0023] Diode D in the low voltage rectifier circuit of converter No. 2 L5 、D L7 connected in series and form the first arm of the diode rectifier bridge, diode D L6 、D L8 are connected in series and constitute the second bridge arm of the diode rectifier bridge; the first bridge arm of the diode rectifier bridge and the second bridge arm of the diode rectifier bridge are connected in parallel;

[0024] The first terminal on the secondary side of the single-phase intermediate frequency transformer in the second converter is connected to the middle node of the first bridge arm of the diode rectifier bridge, and the second terminal on the secondary side of the single-phase intermediate frequency transformer is connected to the middle node of the second bridge arm of the diode rectifier bridge;

[0025] Diode D of converter No. 1 L2 The cathode and D L4 The anode of the diode D is connected to the DC output of the first converter; L6 The cathode and D L8 The anode of the two converters is connected to the DC output of the second converter; the DC outputs of the two converters are connected in parallel, and a filter capacitor C is connected in parallel to both. o , filter capacitor C o The two ends of the converter are the low-voltage DC output terminals Udc of the converter.

[0026] Furthermore, the control timing of the transformers in the No. 1 converter and the No. 2 converter differs by a quarter of the control cycle, that is, the trapezoidal wave current output by the two transformers and the double-frequency positive trapezoidal wave current output by the two converters after rectification by the low-voltage rectifier circuit have a phase difference of a quarter of the transformer operation cycle, that is, the phase difference satisfies

[0027]

[0028] Among them, T φ It is the phase difference between the trapezoidal wave currents output by the two transformers, and also the phase difference between the double-frequency positive trapezoidal wave currents output by the two converters after rectification by the low-voltage rectifier circuit. T is the transformer operating cycle, Tz is the transformer zero current time, and Tc is the transformer current adjustment time.

[0029] Furthermore, the first converter and the second converter can both operate independently.

[0030] Furthermore, the connection order of the No. 1 converter and the No. 2 converter to the A-phase terminal, B-phase terminal, and C-phase terminal of the three-phase AC power grid can also be exchanged; the input end of the medium-voltage AC voltage regulation circuit can also adopt a three-phase delta connection method.

[0031] Furthermore, the operation process of the large-capacity single-stage AC / DC converter can be divided into six stages:

[0032] Phase 1 [t0~t1]: T1 is on, T2 is off, and the voltage of the submodule bridge arm S1 is u ac , the current of T1 is controlled by the submodule bridge arm S1 to i ac , achieving unity power factor; the voltage of the submodule bridge arm S2 is u ac -nU dc , apply voltage nU on the transformer dc , control transformer current is I dc / n, by actively adjusting the voltage applied to the transformer by the first bridge arm and the second bridge arm in the No. 1 transformer, a wide DC voltage range operation is achieved;

[0033] Phase 2 [t1~t2]: The voltage of the submodule bridge arm S2 changes from u ac -nU dc Rise to u ac To prepare for thyristor commutation, the corresponding voltage change time is defined as T u At time t2, the voltage on the thyristor T2 is approximately zero, achieving zero voltage turn-on; during this process, the transformer voltage amplitude is less than nU dc , the DC side diode is in the cut-off state and the transformer current is zero; during the voltage rise of the submodule bridge arm S2, the submodule bridge arm S1 is sequentially switched on or off at short intervals to limit the du / dt generated on the thyristor T2 and prevent false turn-on; du / dt is the voltage change per unit time;

[0034] Phase 3 [t2~t3]: Thyristor commutation phase, T1 and T2 are both in the on state, by actively adjusting the voltage of the submodule bridge arms S1 and S2, a commutation voltage U with opposite amplitude is applied to the two thyristors T1 and T2 in series with the inductors L1 and L2. T , so that the current of T1 changes from i ac The current of T2 gradually decreases to zero, while the current of T2 gradually increases from zero to i ac , limit the di / dt of the current of thyristors T1 and T2, so that T1 can achieve zero current shutdown. The time used in this stage is the commutation time T c , the current flowing through the thyristor is synthesized into i ac, to ensure the continuity of medium voltage AC current, the transformer voltage amplitude at this stage is less than nU dc , the transformer current is zero; di / dt is the current change per unit time;

[0035] Phase 4 [t3~t4]: After the current in T1 decreases to zero, adjust the voltage in S1 to u ac +U R , actively apply back pressure U on T1 R , duration is T q , T1 is reliably shut down; at this time, the medium voltage AC current is only borne by T2; T q According to the thyristor turn-off time design, the turn-off time of ordinary thyristor modules is generally less than 400μs, while the turn-off time of fast switching thyristors is generally between 25μs and 80μs;

[0036] Phase 5 [t4-t5]: T1 is off, T2 remains on; the voltage of the submodule bridge arm S1 gradually drops to u ac -nU dc , du / dt is constrained during the voltage drop;

[0037] Phase 6 [t5-t6]: Submodule bridge arm S2 supports medium voltage AC voltage, and submodule bridge arm S1 voltage is u ac -nU dc The voltage difference between the bridge arms maintains the DC voltage and controls the DC current. By adjusting the voltage of the sub-module bridge arm S1, the transformer voltage amplitude is changed to achieve wide voltage range operation.

[0038] Based on a control method for a large-capacity single-stage AC / DC converter, the method includes

[0039] Phase 1 [t0~t1]: T1 is on, T2 is off, and the voltage of the submodule bridge arm S1 is u ac , the current of T1 is controlled by the submodule bridge arm S1 to i ac , achieving unity power factor; the voltage of the submodule bridge arm S2 is u ac -nU dc , apply voltage nU on the transformer dc , control transformer current is I dc / n, a method for achieving wide DC voltage range operation by actively adjusting the voltage applied to the transformer by the first bridge arm and the second bridge arm in the No. 1 transformer;

[0040] Phase 2 [t1~t2]: The voltage of the submodule bridge arm S2 changes from u ac -nU dc Rise to u ac To prepare for thyristor commutation, the corresponding voltage change time is defined as T uAt time t2, the voltage on the thyristor T2 is approximately zero, achieving zero voltage turn-on; during this process, the transformer voltage amplitude is less than nU dc , the DC side diode is in the cut-off state and the transformer current is zero; during the voltage rise of the submodule bridge arm S2, the submodule bridge arm S1 is sequentially switched on or off at short intervals to limit the du / dt generated on the thyristor T2 and prevent false turn-on; du / dt is the voltage change per unit time;

[0041] Phase 3 [t2~t3]: Thyristor commutation phase, T1 and T2 are both in the on state, by actively adjusting the voltage of the submodule bridge arms S1 and S2, a commutation voltage U with opposite amplitude is applied to the two thyristors T1 and T2 in series with the inductors L1 and L2. T , so that the current of T1 changes from i ac The current of T2 gradually decreases to zero, while the current of T2 gradually increases from zero to i ac , limit the di / dt of the current of thyristors T1 and T2, so that T1 can achieve zero current shutdown. The time used in this stage is the commutation time T c , the current flowing through the thyristor is synthesized into i ac , to ensure the continuity of medium voltage AC current, the transformer voltage amplitude at this stage is less than nU dc , the method of making the transformer current zero; di / dt is the current flowing per unit time;

[0042] Phase 4 [t3~t4]: After the current in T1 decreases to zero, adjust the voltage in S1 to u ac +U R , actively apply back pressure U on T1 R , duration is T q , T1 is reliably shut down; at this time, the medium voltage AC current is only borne by T2; T q According to the thyristor turn-off time design, the turn-off time of ordinary thyristor modules is generally less than 400μs, while the turn-off time of fast switching thyristors is generally between 25μs and 80μs;

[0043] Phase 5 [t4-t5]: T1 is off, T2 remains on; the voltage of the submodule bridge arm S1 gradually drops to u ac -nU dc , a method in which du / dt is constrained during voltage drop;

[0044] Phase 6 [t5-t6]: Submodule bridge arm S2 supports medium voltage AC voltage, and submodule bridge arm S1 voltage is u ac -nU dc The voltage difference between the bridge arms maintains the DC voltage and controls the DC current. By adjusting the voltage of the sub-module bridge arm S1 and changing the transformer voltage amplitude, a method for achieving wide voltage range operation is implemented.

[0045] Thyristor conduction mode 1: When the medium-voltage AC current is in the positive half-cycle of a power frequency cycle, the anti-parallel thyristor valve groups T1 and T2 are alternately turned on, and the conduction times are the same, alternately sharing the medium-voltage AC current; when the medium-voltage AC current drops to 0, the original T2 forward conduction is naturally turned off. At this time, T2 is reversed and the sub-module bridge arm S2 in series with T2 continues to support the medium-voltage AC voltage. To ensure continuous power transmission, the S1 bridge arm voltage rises so that the voltage difference between the two bridge arms provides the transformer voltage. Thereafter, S1 and S2 alternately bear the medium-voltage AC voltage. Since the thyristor conduction timing is reversed at the zero-crossing point of the medium-voltage AC current, the maximum voltage stress of the bridge arm is the medium-voltage AC voltage, and the maximum current stress is the transformer current.

[0046] Thyristor conduction mode 2: The conduction timing of the thyristor valve group is always constant, that is, the anti-parallel thyristor valve group T1 and T2 always maintains periodic alternating conduction. When the medium voltage AC current drops to 0, the original forward conduction of T2 is naturally turned off, and at this time T1 is reversely conducted, completing the switching; at this time, the sub-module bridge arm S1 connected in series with T1 supports the medium voltage AC voltage. To ensure continuous power transmission, the voltage of the S2 bridge arm drops so that the voltage difference between the two bridge arms provides the transformer voltage. Thereafter, S1 and S2 alternately bear the medium voltage AC voltage, and the maximum voltage stress of the bridge arm is the medium voltage AC voltage superimposed on the transformer voltage; when the bridge arm supports the medium voltage AC voltage, it also provides the transformer current. Therefore, under this thyristor switching logic, the maximum current stress of the bridge arm is the transformer current superimposed on the medium voltage AC current.

[0047] Beneficial Effects: A large-capacity single-stage AC / DC converter and its control method primarily utilize an anti-parallel thyristor valve group and a submodule bridge arm to achieve a wide voltage output, requiring only a single AC / DC conversion, with a reduced number of conversion stages. The cyclically reversed conduction timing of the thyristor valve group can effectively reduce the voltage and current stress of the submodule bridge arm. In summary, the present invention has the advantages of a small number of conversion stages, a simple structure, high power capacity, low topology stress, direct medium voltage connection, and a wide voltage output range.

[0048] 1. The present invention adopts a diode and thyristor structure topology with low cost and large capacity, and the overall cost of the equipment is low, which is suitable for high-power occasions.

[0049] 2. The present invention has fewer power conversion stages and does not require a medium-voltage side rectifier bridge, thus reducing one level of AC / DC conversion and the number of devices. The anti-parallel thyristor valve group is combined with the bridge arm submodule IGBT, and the voltage regulation is flexible.

[0050] 3. Compared with the circuit topology based on solid-state transformer input series and output parallel design, the present invention uses medium-frequency transformers, with fewer transformers and lower insulation costs; fewer modules and higher overall reliability.

[0051] 4. The present invention has smooth input and output currents, low harmonic content, and high power quality.

[0052] 5. The present invention has good output voltage regulation capability and can be used in DC power supply scenarios with various voltage levels.

[0053] 6. The thyristor conduction timing logic adopted in the present invention can reduce the voltage stress and current stress of the bridge arm, effectively reduce the IGBT conduction and switching losses, and at the same time protect the device from being easily broken down.

[0054] The present invention utilizes anti-parallel thyristor valve groups and sub-module bridge arms to achieve wide voltage output, and only requires one AC / DC conversion with a small number of conversion stages, thus meeting the technical requirements of DC power supplies in data centers and electric vehicle charging stations in terms of large capacity, wide output voltage range, controllable cost, high power density, and high reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 This is a schematic diagram of the topology circuit of the large-capacity single-stage AC / DC converter proposed in the present invention;

[0056] Figure 2 This is a schematic diagram of a single-phase topology circuit of a large-capacity single-stage AC / DC converter proposed in the present invention;

[0057] Figure 3 The thyristor switching timing and main voltage and current waveforms of the large-capacity single-stage AC / DC converter proposed in the present invention;

[0058] Figure 4 This is a schematic diagram of the phase shift between the intermediate frequency transformer current and the output current of the large-capacity single-stage AC / DC converter proposed in the present invention;

[0059] Figure 5 This is a waveform diagram of the bridge arm voltage at each stage before and after thyristor commutation of the large-capacity single-stage AC / DC converter proposed by the present invention;

[0060] Figure 6 Schematic diagram of the current path of the large-capacity single-stage AC / DC converter proposed in the present invention in the first working stage;

[0061] Figure 7 Schematic diagram of the current path of the large-capacity single-stage AC / DC converter proposed in the present invention in the second working phase;

[0062] Figure 8 Schematic diagram of the current path of the large-capacity single-stage AC / DC converter proposed in the present invention in the third working stage;

[0063] Figure 9 Schematic diagram of the current path of the large-capacity single-stage AC / DC converter proposed in the present invention in the fourth working stage;

[0064] Figure 10 Schematic diagram of the current path of the large-capacity single-stage AC / DC converter proposed in the present invention in the fifth working stage;

[0065] Figure 11 Schematic diagram of the current path of the large-capacity single-stage AC / DC converter proposed in the present invention in the sixth working stage;

[0066] Figure 12 Schematic diagram of the voltage and current waveforms of the bridge arm when the thyristor conduction timing of the large-capacity single-stage AC / DC converter proposed in the present invention is constant;

[0067] Figure 13 This is a schematic diagram of the voltage and current waveforms of the bridge arm when the thyristor conduction timing cycle of the large-capacity single-stage AC / DC converter proposed in the present invention is reversed. DETAILED DESCRIPTION

[0068] The technical solution of the present invention will be further described below with reference to the accompanying drawings, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention that does not depart from the spirit and scope of the technical solution of the present invention should be included in the scope of protection of the present invention.

[0069] The high-capacity, single-stage AC / DC converter proposed in this invention comprises two identical converters. Converter 1 and Converter 2 employ the same circuit topology and are star-connected to the medium-voltage AC grid. Both converters include a medium-voltage AC voltage regulation circuit consisting of an anti-parallel thyristor valve assembly and submodule bridge arms, a single-phase medium-frequency transformer, and a low-voltage rectifier circuit formed by a crimped diode.

[0070] Introducing components

[0071] Taking converter 1 as an example, the circuits of each part are as follows:

[0072] The medium voltage AC voltage regulation circuit in this topology includes: anti-parallel thyristor valve group T1, anti-parallel thyristor valve group T2, thyristor branch series inductor L1, thyristor branch series inductor L2, submodule bridge arm S1 and submodule bridge arm S2. The anti-parallel thyristor valve group is composed of several anti-parallel thyristors in series, and the submodule bridge arm is composed of several submodules in series. The operating frequency of the single-phase medium frequency transformer is usually 300 Hz. The structure is divided into primary and secondary sides, wherein the primary side includes a first terminal and a second terminal, and the secondary side includes a first terminal and a second terminal, and the first terminal of the primary side of the transformer and the first terminal of the secondary side of the transformer are the same terminal. The transformer ratio is n:1, and the transformer leakage inductance is L k1 The low voltage rectifier circuit diode adopts a large capacity press-fit structure, including diode D L1 , diode D L2 , diode D L3 , diode DL4 And the output filter capacitor C o ;

[0073] The circuits and components of the second converter are exactly the same as those of the first converter.

[0074] Introducing the connection method

[0075] The connection method of the large-capacity single-stage AC / DC converter is as follows: the upper terminals of the thyristor series inductors L1 and L2 are connected to phase A of the three-phase AC grid, and the lower terminals of the thyristor series inductors L1 and L2 are connected to the anti-parallel thyristor valve groups T1 and T2 respectively. The lower terminals of the submodule bridge arms S1 and S2 are connected to phase B of the three-phase grid. The thyristor series inductor L1, anti-parallel thyristor valve group T1, and submodule bridge arm S1 constitute the first bridge arm on the medium voltage side of the converter, and the thyristor series inductor L2, anti-parallel thyristor valve group T2, and submodule bridge arm S2 constitute the second bridge arm on the medium voltage side of the converter. The leakage inductance L of the medium frequency transformer k1 Returned to the primary side, leakage inductance L k1 The left terminal is connected between the first bridge arm anti-parallel thyristor valve group T1 and the sub-module bridge arm S1, and the leakage inductance L k1 The right terminal of is connected to the first terminal of the primary side, and the second terminal of the primary side is connected between the second bridge arm anti-parallel thyristor valve group T2 and the submodule bridge arm S2. L1 、D L3 The first arm of the diode rectifier bridge is formed by diode D L2 、D L4 It constitutes the second bridge arm of the diode rectifier bridge. The first terminal of the secondary side of the medium frequency transformer is connected to the middle node of the first bridge arm of the diode rectifier bridge, and the second terminal of the secondary side of the medium frequency transformer is connected to the middle node of the second bridge arm of the diode rectifier bridge. The first terminals of the primary and secondary sides of the medium frequency transformer constitute the same-name terminals. The upper terminals of the thyristor series inductors L3 and L4 are connected to phase A of the three-phase AC power grid, and the lower terminals of the thyristor series inductors L3 and L4 are connected to the anti-parallel thyristor valve groups T3 and T4 respectively. The lower terminals of the submodule bridge arms S3 and S4 are connected to phase C of the three-phase power grid. The thyristor series inductor L3, the anti-parallel thyristor valve group T3, and the submodule bridge arm S3 constitute the first bridge arm on the medium voltage side of the converter two, and the thyristor series inductor L4, the anti-parallel thyristor valve group T4, and the submodule bridge arm S4 constitute the second bridge arm on the medium voltage side of the converter two. The leakage inductance L of the medium frequency transformer k2 Returned to the primary side, leakage inductance L k2 The left terminal is connected between the first bridge arm anti-parallel thyristor valve group T3 and the sub-module bridge arm S3, and the leakage inductance L k2The right terminal of is connected to the first terminal of the primary side, and the second terminal of the primary side is connected between the second bridge arm anti-parallel thyristor valve group T4 and the submodule bridge arm S4. L5 、D L7 The first arm of the diode rectifier bridge is formed by diode D L6 、D L8 The first terminal of the secondary side of the intermediate frequency transformer is connected to the middle node of the first bridge arm of the diode rectifier bridge, and the second terminal of the secondary side of the intermediate frequency transformer is connected to the middle node of the second bridge arm of the diode rectifier bridge. The first terminals of the primary and secondary sides of the intermediate frequency transformer form the same-name terminals. The DC output ports of the two converters are connected in parallel, and a filter capacitor C is also connected in parallel. o .

[0076] Working principle:

[0077] The control method and working principle of large-capacity single-stage AC / DC converter include the following parts:

[0078] Part 1: Taking a single converter as an example, within a power frequency cycle, the two anti-parallel thyristor valve groups in the two bridge arms alternately conduct to ensure the continuity of the input medium-voltage AC current and cooperate with the submodule bridge arms to generate a trapezoidal AC voltage across the transformer. Before the thyristors turn on, the voltage in the corresponding bridge arm rises to the medium-voltage AC voltage, ensuring that the thyristors turn on with minimal voltage stress, thus reducing thyristor turn-on stress. When the thyristors in one bridge arm turn on, the submodule in that same bridge arm supports the input AC voltage and simultaneously controls the medium-voltage AC current. The thyristors in the other bridge arm turn off, and the voltage across the corresponding bridge arm is the difference between the medium-voltage AC voltage and the transformer primary voltage. After the thyristors turn off, the other bridge arm can also actively apply reverse voltage. The thyristor commutation process between the two bridge arms has a certain commutation time to limit the di / dt. By adjusting the transformer voltage amplitude, a wide range of DC output voltages can be achieved. The transformer current is controlled to a trapezoidal waveform, and the rectified DC current becomes a trapezoidal waveform with a frequency twice the transformer frequency. The two converters work simultaneously, and the output trapezoidal waves are superimposed to obtain a stable DC output.

[0079] Part 2: In the large-capacity single-stage AC / DC converter proposed by the present invention, the control timing of the transformers of converter 1 (converter No. 1) and converter 2 (converter No. 2) differs by a quarter of the control cycle, that is, the trapezoidal wave current output by the two transformers and the double-frequency positive trapezoidal wave current output by the two converters after rectification by the low-voltage rectifier circuit have a phase difference of a quarter of the transformer operation cycle, that is, the phase difference satisfies Among them, T φIt is the phase difference between the trapezoidal wave currents output by the two transformers, and also the phase difference between the double-frequency positive trapezoidal wave currents output by the two converters after rectification by the low-voltage rectifier circuit. T is the transformer operating cycle, Tz is the transformer zero current time, and Tc is the transformer current adjustment time.

[0080] Part 3: Specifically, taking a single converter as an example, the operation process of this large-capacity single-stage AC / DC converter can be divided into six stages:

[0081] Phase 1 [t0~t1]: T1 is on, T2 is off, and the voltage of the submodule bridge arm S1 is u ac , the current of T1 is controlled by the submodule bridge arm S1 to i ac , achieving unity power factor; the voltage of the submodule bridge arm S2 is u ac -nU dc , apply voltage nU on the transformer dc , control transformer current is I dc / n, a method for achieving wide DC voltage range operation by actively adjusting the voltage applied to the transformer by the first bridge arm and the second bridge arm in the No. 1 transformer;

[0082] Phase 2 [t1~t2]: The voltage of the submodule bridge arm S2 changes from u ac -nU dc Rise to u ac To prepare for thyristor commutation, the corresponding voltage change time is defined as T u At time t2, the voltage on the thyristor T2 is approximately zero, achieving zero voltage turn-on; during this process, the transformer voltage amplitude is less than nU dc , the DC side diode is in the cut-off state and the transformer current is zero; during the voltage rise of the submodule bridge arm S2, the submodule bridge arm S1 is sequentially switched on or off at short intervals to limit the du / dt generated on the thyristor T2 and prevent false turn-on; du / dt is the voltage change per unit time;

[0083] Phase 3 [t2~t3]: Thyristor commutation phase, T1 and T2 are both in the on state, by actively adjusting the voltage of the submodule bridge arms S1 and S2, a commutation voltage U with opposite amplitude is applied to the two thyristors T1 and T2 in series with the inductors L1 and L2. T , so that the current of T1 changes from i ac The current of T2 gradually decreases to zero, while the current of T2 gradually increases from zero to i ac , limit the di / dt of the current of thyristors T1 and T2, so that T1 can achieve zero current shutdown. The time used in this stage is the commutation time T c , the current flowing through the thyristor is synthesized into i ac , to ensure the continuity of medium voltage AC current, the transformer voltage amplitude at this stage is less than nUdc , the method of making the transformer current zero; di / dt is the current flowing per unit time;

[0084] Phase 4 [t3~t4]: After the current in T1 decreases to zero, adjust the voltage in S1 to u ac +U R , actively apply back pressure U on T1 R , duration is T q , T1 is reliably shut down; at this time, the medium voltage AC current is only borne by T2; T q According to the thyristor turn-off time design, the turn-off time of ordinary thyristor modules is generally less than 400μs, while the turn-off time of fast switching thyristors is generally between 25μs and 80μs;

[0085] Phase 5 [t4-t5]: T1 is off, T2 remains on; the voltage of the submodule bridge arm S1 gradually drops to u ac -nU dc , a method in which du / dt is constrained during voltage drop;

[0086] Phase 6 [t5-t6]: Submodule bridge arm S2 supports medium voltage AC voltage, and submodule bridge arm S1 voltage is u ac -nU dc The voltage difference between the bridge arms maintains the DC voltage and controls the DC current. By adjusting the voltage of the sub-module bridge arm S1 and changing the transformer voltage amplitude, a method for achieving wide voltage range operation is implemented.

[0087] Part 4: The conduction timing of the anti-parallel thyristor valve group of the large-capacity single-stage AC / DC converter is divided into conduction mode 1 and conduction mode 2. The voltage and current stresses borne by the bridge arm in different conduction modes are different, as follows.

[0088] Thyristor conduction mode 1, please refer to Figure 12 . When the medium-voltage AC current is in the positive half-cycle of an industrial frequency cycle, the anti-parallel thyristor valve groups T1 and T2 are turned on alternately, and the number of times they are turned on is the same, alternatingly sharing the medium-voltage AC current. When the medium-voltage AC current drops to 0, the original forward-conducting T2 is naturally turned off. At this time, T2 is reverse-conducted, and the sub-module bridge arm S2 in series with T2 continues to support the medium-voltage AC voltage. In order to ensure continuous power transmission, the voltage of the S1 bridge arm rises so that the voltage difference between the two bridge arms provides the transformer voltage. After that, S1 and S2 alternately bear the medium-voltage AC voltage. Since the thyristor conduction timing is reversed at the zero-crossing point of the medium-voltage AC current, the maximum voltage stress of the bridge arm is the medium-voltage AC voltage, and the maximum current stress is the transformer current.

[0089] Thyristor conduction mode 2, please refer to Figure 13. The conduction timing of the thyristor valve group is always constant, that is, the anti-parallel thyristor valve group T1 and T2 always maintains periodic alternating conduction. When the medium-voltage AC current drops to 0, the original forward conduction of T2 is naturally turned off, and at this time, T1 is reversely conducted to complete the switching. At this time, the sub-module bridge arm S1 connected in series with T1 supports the medium-voltage AC voltage. In order to ensure continuous power transmission, the voltage of the S2 bridge arm drops so that the voltage difference between the two bridge arms provides the transformer voltage. After that, S1 and S2 alternately bear the medium-voltage AC voltage, and the maximum voltage stress of the bridge arm is the medium-voltage AC voltage superimposed on the transformer voltage. When the bridge arm supports the medium-voltage AC voltage, it also provides transformer current. Therefore, under this thyristor switching logic, the maximum current stress of the bridge arm is the transformer current superimposed on the medium-voltage AC current.

[0090] Example 1:

[0091] The high-capacity, single-stage AC / DC converter proposed in this invention comprises two identical converters. Converter 1 and Converter 2 employ the same circuit topology and are star-connected to the medium-voltage AC grid. Both converters include a medium-voltage AC voltage regulation circuit consisting of an anti-parallel thyristor valve assembly and submodule bridge arms, a single-phase medium-frequency transformer, and a low-voltage rectifier circuit formed by a crimped diode.

[0092] Take a single converter as an example, Figure 2 As shown, the circuits of each part are:

[0093] The medium voltage AC voltage regulation circuit in this topology includes: anti-parallel thyristor valve group T1, anti-parallel thyristor valve group T2, thyristor series inductor L1, thyristor series inductor L2, submodule bridge arm S1 and submodule bridge arm S2. The anti-parallel thyristor valve group is composed of several anti-parallel thyristors connected in series, and the submodule bridge arm is composed of several submodules connected in series. The operating frequency of the single-phase medium frequency transformer is usually 300 Hz, and the structure is divided into a primary side and a secondary side. The primary side includes a first terminal and a second terminal, and the secondary side includes a first terminal and a second terminal. The first terminal of the primary side of the transformer and the first terminal of the secondary side of the transformer are the same terminal. The transformer ratio is n:1, and the transformer leakage inductance is L k1 The low voltage rectifier circuit diode adopts a large capacity press-fit structure, including diode D L1 , diode D L2 , diode D L3 , diode D L4 . Diode D L1 With diode D L3 The first arm of the diode rectifier bridge is formed by diode D L2 With diode D L4 The second bridge arm of the diode rectifier bridge is formed, and the output filter capacitor C o Connected in parallel at both ends of the rectifier bridge arm.

[0094] The devices and components of the second converter are exactly the same as those of converter one.

[0095] The connection method of the large-capacity single-stage AC / DC converter is as follows: the upper terminals of the thyristor series inductors L1 and L2 are connected to phase A of the three-phase AC grid, and the lower terminals of the thyristor series inductors L1 and L2 are connected to the anti-parallel thyristor valve groups T1 and T2 respectively. The lower terminals of the submodule bridge arms S1 and S2 are connected to phase B of the three-phase grid. The thyristor series inductor L1, anti-parallel thyristor valve group T1, and submodule bridge arm S1 constitute the first bridge arm on the medium voltage side of the converter, and the thyristor series inductor L2, anti-parallel thyristor valve group T2, and submodule bridge arm S2 constitute the second bridge arm on the medium voltage side of the converter. The leakage inductance L of the medium frequency transformer k1 Returned to the primary side, leakage inductance L k1 The left terminal is connected between the first bridge arm anti-parallel thyristor valve group T1 and the sub-module bridge arm S1, and the leakage inductance L k1 The right terminal of is connected to the first terminal of the primary side, and the second terminal of the primary side is connected between the second bridge arm anti-parallel thyristor valve group T2 and the submodule bridge arm S2. L1 、D L3 The first arm of the diode rectifier bridge is formed by diode D L2 、D L4 It constitutes the second bridge arm of the diode rectifier bridge. The first terminal of the secondary side of the medium frequency transformer is connected to the middle node of the first bridge arm of the diode rectifier bridge, and the second terminal of the secondary side of the medium frequency transformer is connected to the middle node of the second bridge arm of the diode rectifier bridge. The first terminals of the primary and secondary sides of the medium frequency transformer constitute the same-name terminals. The upper terminals of the thyristor series inductors L3 and L4 are connected to phase A of the three-phase AC power grid, and the lower terminals of the thyristor series inductors L3 and L4 are connected to the anti-parallel thyristor valve groups T3 and T4 respectively. The lower terminals of the submodule bridge arms S3 and S4 are connected to phase C of the three-phase power grid. The thyristor series inductor L3, the anti-parallel thyristor valve group T3, and the submodule bridge arm S3 constitute the first bridge arm on the medium voltage side of the converter two, and the thyristor series inductor L4, the anti-parallel thyristor valve group T4, and the submodule bridge arm S4 constitute the second bridge arm on the medium voltage side of the converter two. The leakage inductance L of the medium frequency transformer k2 Returned to the primary side, leakage inductance L k2 The left terminal is connected between the first bridge arm anti-parallel thyristor valve group T3 and the sub-module bridge arm S3, and the leakage inductance L k2 The right terminal of is connected to the first terminal of the primary side, and the second terminal of the primary side is connected between the second bridge arm anti-parallel thyristor valve group T4 and the submodule bridge arm S4. L5 、D L7 The first arm of the diode rectifier bridge is formed by diode D L6 、D L8The first terminal of the secondary side of the intermediate frequency transformer is connected to the middle node of the first bridge arm of the diode rectifier bridge, and the second terminal of the secondary side of the intermediate frequency transformer is connected to the middle node of the second bridge arm of the diode rectifier bridge. The first terminals of the primary and secondary sides of the intermediate frequency transformer form the same-name terminals. The DC output ports of the two converters are connected in parallel, and a filter capacitor C is used at the same time. o Replace the filter capacitors in converters 1 and 2.

[0096] The switching sequence of the thyristor valve group of the large-capacity single-stage AC / DC converter proposed by the present invention and the main voltage and current waveforms are as follows: Figure 3 As shown, the phase shift design of the two medium frequency transformers is as follows Figure 4 As shown, the bridge arm voltage waveform is as follows Figure 5 As shown in the figure. Taking the converter 1 as an example, the operation process is divided into six stages, as shown in the figure. Figure 6 ~ Figure 11 The voltage and current waveforms of the bridge arm in the following thyristor conduction mode are shown as follows: Figure 12 As shown, the bridge arm voltage and current waveforms under the thyristor conduction mode 2 are as follows Figure 13 The working process of each stage has been mentioned in the working principle section and will not be repeated here.

[0097] The first converter and the second converter in the large-capacity single-stage AC / DC converter of the present invention can both operate independently;

[0098] The connection order of the input terminals of the large-capacity single-stage AC / DC converter described in the present invention and the A-phase terminal, B-phase terminal, and C-phase terminal of the three-phase AC power grid can also be interchanged; the medium voltage input terminal can also use the three-phase delta connection method.

[0099] The large-capacity single-stage AC / DC converter described in the present invention can be expanded to use multiple medium-voltage AC voltage regulation circuits in parallel, series, or a combination of series and parallel, and respectively connected to multiple medium-frequency transformers; the secondary sides of multiple transformers can be connected in parallel, series, or a combination of series and parallel, and multiple low-voltage rectifier circuits can also be connected in parallel, series, or a combination of series and parallel.

[0100] The present invention discloses a large-capacity single-stage AC / DC converter, which is composed of two identical converters, converter one and converter two, and is directly connected to a medium-voltage AC power grid. Each converter is composed of an anti-parallel thyristor valve group, a sub-module bridge arm, a single-phase medium-frequency transformer, and a diode rectifier bridge. The medium-voltage AC voltage regulation circuit and the low-voltage rectifier circuit respectively use an anti-parallel thyristor valve group and a diode valve group with low equipment cost. The thyristors and diodes have low conduction losses and mature series technology, which can improve topology efficiency and reliability; two medium-frequency transformers replace multiple distributed high-frequency transformers, further reducing topology costs; the voltage and current of the bridge arm series sub-module are highly controllable, and the output end can be connected to DC loads of different voltage levels. The structure of the two converter outputs in parallel can significantly improve the transmission power of the power supply. The conduction timing of the thyristor valve group's periodic reversal can effectively reduce the voltage and current stress of the sub-module bridge arm. Overall, this large-capacity single-stage AC / DC converter has the advantages of fewer conversion stages, simple structure, large power capacity, low topology stress, direct medium voltage connection, and a wide voltage output range. It is very suitable for large-capacity DC power supply scenarios.

[0101] The present invention is described by way of several specific embodiments. It should be understood by those skilled in the art that various modifications and equivalent substitutions may be made to the present invention without departing from the scope of the present invention. In addition, various modifications may be made to the present invention for specific situations or circumstances without departing from the scope of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed, but is intended to include all embodiments falling within the scope of the claims.

Claims

1. A large-capacity single-stage AC / DC converter connected to a three-phase AC grid, with phases A, B, and C; characterized in that: The converter consists of two converters and an output filter capacitor C o , the two converters are converter No. 1 and converter No. 2, and the output filter capacitor C o The two ends are the low voltage DC output terminals Udc of the converter; Converter No. 1 and Converter No. 2 use the same circuit topology. Each converter consists of a medium-voltage AC voltage regulator circuit, a single-phase medium-frequency transformer, and a low-voltage rectifier circuit. The medium voltage AC voltage regulation circuit in converter No. 1 includes an anti-parallel thyristor valve group T1, an anti-parallel thyristor valve group T2, a thyristor series inductor L1, a thyristor series inductor L2, a sub-module series bridge arm S1, and a sub-module series bridge arm S2; The operating frequency of the single-phase medium frequency transformer is 300 Hz. The single-phase medium frequency transformer consists of a transformer and a transformer leakage inductance L k1 The transformer is divided into a primary side and a secondary side, wherein the primary side includes a primary side first terminal and a primary side second terminal, and the secondary side includes a secondary side first terminal and a secondary side second terminal, and the primary side first terminal and the secondary side first terminal of the transformer are the same-name terminals; the transformer ratio is n:1, and the transformer leakage inductance is L k1 ; Where n and k are both real numbers; The low voltage rectifier circuit adopts a diode rectifier bridge, and the diode rectifier bridge is a large capacity press-fit structure, including a diode D L1 , diode D L2 , diode D L3 , diode D L4 And the output filter capacitor C o ; One end of the thyristor series inductors L1 and L2 is connected to phase A of the three-phase AC power grid, the other end of the thyristor series inductor L1 is connected to one end of the anti-parallel thyristor valve group T1, and the other end of the anti-parallel thyristor valve group T1 is connected to one end of the submodule bridge arm S1; the other end of the thyristor series inductor L2 is connected to one end of the anti-parallel thyristor valve group T2, and the other end of the anti-parallel thyristor valve group T2 is connected to one end of the submodule bridge arm S2; the other ends of the submodule bridge arms S1 and S2 are connected to phase B of the three-phase power grid; The thyristor series inductor L1, anti-parallel thyristor valve group T1 and submodule bridge arm S1 constitute the first bridge arm on the medium voltage side of converter No. 1, and the thyristor series inductor L2, anti-parallel thyristor valve group T2 and submodule bridge arm S2 constitute the second bridge arm on the medium voltage side of converter No. 2; The transformer leakage inductance L of the single-phase medium frequency transformer k1 One end is connected between the anti-parallel thyristor valve group T1 in the first bridge arm and the sub-module bridge arm S1, and the transformer leakage inductance L k1 The other end is connected to the first primary terminal, and the second primary terminal is connected between the anti-parallel thyristor valve group T2 in the second bridge arm and the sub-module bridge arm S2; The diode D of the low voltage rectifier circuit L1 、D L3 connected in series and form the first arm of the diode rectifier bridge, diode D L2 、D L4 are connected in series and constitute the second bridge arm of the diode rectifier bridge; the first bridge arm of the diode rectifier bridge and the second bridge arm of the diode rectifier bridge are connected in parallel; The first terminal of the transformer secondary side of the single-phase intermediate frequency transformer is connected to the middle node of the first bridge arm of the diode rectifier bridge, and the second terminal of the transformer secondary side of the single-phase intermediate frequency transformer is connected to the middle node of the second bridge arm of the diode rectifier bridge; The medium voltage AC voltage regulation circuit in the second converter includes an anti-parallel thyristor valve group T3, an anti-parallel thyristor valve group T4, a thyristor series inductor L3, a thyristor series inductor L4, a sub-module series bridge arm S3, and a sub-module series bridge arm S4; The single-phase medium frequency transformer in the second converter operates at a frequency of 300 Hz. The single-phase medium frequency transformer consists of a transformer and a transformer leakage inductance L k2 The transformer is divided into a primary side and a secondary side, wherein the primary side includes a primary side first terminal and a primary side second terminal, and the secondary side includes a secondary side first terminal and a secondary side second terminal, and the primary side first terminal and the secondary side first terminal of the transformer are the same-name terminals; the transformer ratio is n:1, and the transformer leakage inductance is L k2 ; Where n and k are both real numbers; One end of the thyristor series inductors L3 and L4 is connected to phase A of the three-phase AC power grid, the other end of the thyristor series inductor L3 is connected to one end of the anti-parallel thyristor valve group T3, and the other end of the anti-parallel thyristor valve group T3 is connected to one end of the submodule bridge arm S3; the other end of the thyristor series inductor L4 is connected to one end of the anti-parallel thyristor valve group T4, and the other end of the anti-parallel thyristor valve group T4 is connected to one end of the submodule bridge arm S4; the other ends of the submodule bridge arms S3 and S4 are both connected to phase C of the three-phase power grid; The thyristor series inductor L3, the anti-parallel thyristor valve group T3 and the submodule bridge arm S3 constitute the first bridge arm on the medium voltage side of the second converter, and the thyristor series inductor L4, the anti-parallel thyristor valve group T4 and the submodule bridge arm S4 constitute the second bridge arm on the medium voltage side of the second converter; The transformer leakage inductance L of the single-phase medium-frequency transformer in the second converter k2 One end is connected between the anti-parallel thyristor valve group T3 in the first bridge arm and the sub-module bridge arm S3, and the transformer leakage inductance L k2 The other end is connected to the first primary terminal, and the second primary terminal is connected between the anti-parallel thyristor valve group T4 in the second bridge arm and the sub-module bridge arm S4; Diode D in the low voltage rectifier circuit of converter No. 2 L5 、D L7 connected in series and form the first arm of the diode rectifier bridge, diode D L6 、D L8 are connected in series and constitute the second bridge arm of the diode rectifier bridge; the first bridge arm of the diode rectifier bridge and the second bridge arm of the diode rectifier bridge are connected in parallel; The first terminal on the secondary side of the single-phase intermediate frequency transformer in the second converter is connected to the middle node of the first bridge arm of the diode rectifier bridge, and the second terminal on the secondary side of the single-phase intermediate frequency transformer is connected to the middle node of the second bridge arm of the diode rectifier bridge; Diode D of converter No. 1 L2 The cathode and D L4 The anode of the diode D is connected to the DC output of the first converter; L6 The cathode and D L8 The anode of the two converters is connected to the DC output of the second converter; the DC outputs of the two converters are connected in parallel, and a filter capacitor C is connected in parallel to both. o , filter capacitor C o The two ends of the converter are the low-voltage DC output terminals Udc of the converter.

2. A large-capacity single-stage AC / DC converter according to claim 1, characterized in that: The control timing of the transformers in the No. 1 converter and the No. 2 converter differs by a quarter of the control cycle, that is, the trapezoidal wave current output by the two transformers and the double-frequency positive trapezoidal wave current output by the two converters after rectification by the low-voltage rectifier circuit have a phase difference of a quarter of the transformer operation cycle, that is, the phase difference satisfies in, It is the phase difference between the trapezoidal wave currents output by the two transformers, and also the phase difference between the double-frequency positive trapezoidal wave currents output by the two converters after rectification by the low-voltage rectifier circuit. T is the transformer operating cycle, Tz is the transformer zero current time, and Tc is the transformer current adjustment time.

3. A large-capacity single-stage AC / DC converter according to claim 1, characterized in that: The first converter and the second converter can both operate independently.

4. A large-capacity single-stage AC / DC converter according to claim 1, characterized in that: The connection order of the No. 1 converter and the No. 2 converter to the A-phase terminal, B-phase terminal, and C-phase terminal of the three-phase AC power grid can also be exchanged; the input end of the medium-voltage AC voltage regulation circuit can also adopt a three-phase delta connection method.

5. A large-capacity single-stage AC / DC converter according to any one of claims 1, 2, 3 or 4, characterized in that: The operation process can be divided into six stages: Phase 1 [t0~t1]: T1 is on, T2 is off, and the voltage of the submodule bridge arm S1 is u ac , the current of T1 is controlled by the submodule bridge arm S1 to i ac , achieving unity power factor; the voltage of the submodule bridge arm S2 is u ac -nU dc , apply voltage nU on the transformer dc , control transformer current is I dc / n, by actively adjusting the voltage applied to the transformer by the first bridge arm and the second bridge arm in the No. 1 transformer, a wide DC voltage range operation is achieved; Phase 2 [t1~t2]: The voltage of the submodule bridge arm S2 changes from u ac -nU dc Rise to u ac To prepare for thyristor commutation, the corresponding voltage change time is defined as T u At time t2, the voltage on the thyristor T2 is approximately zero, achieving zero voltage turn-on; during this process, the transformer voltage amplitude is less than nU dc , the DC side diode is in the cut-off state and the transformer current is zero; during the voltage rise of the submodule bridge arm S2, the submodule bridge arm S1 is sequentially switched on or off at short intervals to limit the du / dt generated on the thyristor T2 and prevent false turn-on; du / dt is the voltage change per unit time; Phase 3 [t2~t3]: Thyristor commutation phase, T1 and T2 are both in the on state, by actively adjusting the voltage of the submodule bridge arms S1 and S2, a commutation voltage U with opposite amplitude is applied to the two thyristors T1 and T2 in series with the inductors L1 and L2. T , so that the current of T1 changes from i ac The current of T2 gradually decreases to zero, while the current of T2 gradually increases from zero to i ac , limit the di / dt of the current of thyristors T1 and T2, so that T1 can achieve zero current shutdown. The time used in this stage is the commutation time T c , the current flowing through the thyristor is synthesized into i ac , to ensure the continuity of medium voltage AC current, the transformer voltage amplitude at this stage is less than nU dc , the transformer current is zero; di / dt is the current change per unit time; Phase 4 [t3~t4]: After the current in T1 decreases to zero, adjust the voltage in S1 to u ac +U R , actively apply back pressure U on T1 R , duration is T q , T1 is reliably shut down; at this time, the medium voltage AC current is only borne by T2; T q According to the thyristor turn-off time design, the turn-off time of ordinary thyristor modules is generally less than 400μs, while the turn-off time of fast switching thyristors is generally between 25μs and 80μs; Phase 5 [t4-t5]: T1 is off, T2 remains on; the voltage of the submodule bridge arm S1 gradually drops to u ac -nU dc , du / dt is constrained during the voltage drop; Phase 6 [t5-t6]: Submodule bridge arm S2 supports medium voltage AC voltage, and submodule bridge arm S1 voltage is u ac -nU dc The voltage difference between the bridge arms maintains the DC voltage and controls the DC current. By adjusting the voltage of the sub-module bridge arm S1, the transformer voltage amplitude is changed to achieve wide voltage range operation.

6. The control method of a large-capacity single-stage AC / DC converter according to claim 5, characterized in that: The method involves running the process in six stages: Phase 1 [t0~t1]: T1 is on, T2 is off, and the voltage of the submodule bridge arm S1 is u ac , the current of T1 is controlled by the submodule bridge arm S1 to i ac , achieving unity power factor; the voltage of the submodule bridge arm S2 is u ac -nU dc , apply voltage nU on the transformer dc , control transformer current is I dc / n, a method for achieving wide DC voltage range operation by actively adjusting the voltage applied to the transformer by the first bridge arm and the second bridge arm in the No. 1 transformer; Phase 2 [t1~t2]: The voltage of the submodule bridge arm S2 changes from u ac -nU dc Rise to u ac To prepare for thyristor commutation, the corresponding voltage change time is defined as T u At time t2, the voltage on the thyristor T2 is approximately zero, achieving zero voltage turn-on; during this process, the transformer voltage amplitude is less than nU dc , the DC side diode is in the cut-off state and the transformer current is zero; during the voltage rise of the sub-module bridge arm S2, the sub-module bridge arm S1 is sequentially switched on or off at short intervals to limit the du / dt generated on the thyristor T2 and prevent false turn-on; du / dt is the voltage change per unit time; Phase 3 [t2~t3]: Thyristor commutation phase, T1 and T2 are both in the on state, by actively adjusting the voltage of the submodule bridge arms S1 and S2, a commutation voltage U with opposite amplitude is applied to the two thyristors T1 and T2 in series with the inductors L1 and L2. T , so that the current of T1 changes from i ac The current of T2 gradually decreases to zero, while the current of T2 gradually increases from zero to i ac , limit the di / dt of the current of thyristors T1 and T2, so that T1 can achieve zero current shutdown. The time used in this stage is the commutation time T c , the current flowing through the thyristor is synthesized into i ac , to ensure the continuity of medium voltage AC current, the transformer voltage amplitude at this stage is less than nU dc , the method of making the transformer current zero; di / dt is the current flowing per unit time; Phase 4 [t3~t4]: After the current in T1 decreases to zero, adjust the voltage in S1 to u ac +U R , actively apply back pressure U on T1 R , duration is T q , T1 is reliably shut down; at this time, the medium voltage AC current is only borne by T2; T q According to the thyristor turn-off time design, the turn-off time of ordinary thyristor modules is generally less than 400μs, while the turn-off time of fast switching thyristors is generally between 25μs and 80μs; Phase 5 [t4-t5]: T1 is off, T2 remains on; the voltage of the submodule bridge arm S1 gradually drops to u ac -nU dc , a method in which du / dt is constrained during voltage drop; Phase 6 [t5-t6]: Submodule bridge arm S2 supports medium voltage AC voltage, and submodule bridge arm S1 voltage is u ac -nU dc The voltage difference between the bridge arms maintains the DC voltage and controls the DC current. By adjusting the voltage of the sub-module bridge arm S1 and changing the transformer voltage amplitude, a method for achieving wide voltage range operation is implemented.

7. The control method of a large-capacity single-stage AC / DC converter according to claim 5, characterized in that: Thyristor conduction mode 1: When the medium-voltage AC current is in the positive half-cycle of a power frequency cycle, the anti-parallel thyristor valve groups T1 and T2 are alternately turned on, and the conduction times are the same, alternately sharing the medium-voltage AC current; when the medium-voltage AC current drops to 0, the original T2 forward conduction is naturally turned off. At this time, T2 is reversed and the sub-module bridge arm S2 in series with T2 continues to support the medium-voltage AC voltage. To ensure continuous power transmission, the S1 bridge arm voltage rises so that the voltage difference between the two bridge arms provides the transformer voltage. Thereafter, S1 and S2 alternately bear the medium-voltage AC voltage. Since the thyristor conduction timing is reversed at the zero-crossing point of the medium-voltage AC current, the maximum voltage stress of the bridge arm is the medium-voltage AC voltage, and the maximum current stress is the transformer current.

8. The control method of a large-capacity single-stage AC / DC converter according to claim 5, characterized in that: Thyristor conduction mode 2: The conduction timing of the thyristor valve group is always constant, that is, the anti-parallel thyristor valve group T1 and T2 always maintains periodic alternating conduction. When the medium voltage AC current drops to 0, the original forward conduction of T2 is naturally turned off, and at this time T1 is reversely conducted, completing the switching; at this time, the sub-module bridge arm S1 connected in series with T1 supports the medium voltage AC voltage. To ensure continuous power transmission, the voltage of the S2 bridge arm drops so that the voltage difference between the two bridge arms provides the transformer voltage. Thereafter, S1 and S2 alternately bear the medium voltage AC voltage, and the maximum voltage stress of the bridge arm is the medium voltage AC voltage superimposed on the transformer voltage; when the bridge arm supports the medium voltage AC voltage, it also provides the transformer current. Therefore, under this thyristor switching logic, the maximum current stress of the bridge arm is the transformer current superimposed on the medium voltage AC current.

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