Boosting direct-current transformer with high reliability and design method thereof

Abstract

The invention discloses a boosting direct current transformer with high reliability and a design method thereof, the boosting direct current transformer comprises N power units, and each power unit comprises an input inverter circuit, a single-phase transformer and an output rectifier circuit; the input inverter circuit comprises an IGBT (Insulated Gate Bipolar Translator) device Q1 to an IGBT device Q4; the single-phase transformer comprises a primary side of the transformer, a secondary side of the transformer and a leakage inductor Lk, the primary side comprises a first terminal and a second terminal, the secondary side comprises 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 dotted terminals, and the transformation ratio of the transformer is 1: n; the output rectification circuit comprises a diode D1, a diode D2, a bridge arm reactor L1, a bridge arm reactor L2, a submodule series bridge arm S1 and a submodule series bridge arm S2. The circuit has the advantages of being small in device number, high in efficiency, low in cost and capable of achieving wide-range adjustment of input voltage.

Description

Technical Field

[0001] The invention belongs to the field of power electronics and relates to a direct current transformer, in particular to a step-up direct current transformer with high reliability and a design method thereof. Background Art

[0002] With the introduction of the carbon neutrality strategy, the proportion of new energy power generation and new power loads in the power system is increasing, which has a profound impact on the structure and transmission and distribution mode of the new power system. Among them, the DC grid has become the key solution for the current new energy transmission and distribution with its flexible power control, simple new energy access and high reliability.

[0003] Large-capacity DC transformers are core equipment in DC grid applications, requiring them to have high-reliability shutdown and commutation capabilities, wide-range voltage regulation capabilities, and higher efficiency levels. In pursuit of high efficiency, existing DC transformers use multiple resonant converter power units, which are connected in series and parallel to achieve large capacity and high voltage levels. This solution requires the use of dozens of expensive high-frequency transformers, each of which must be insulated according to the high-voltage side voltage level, resulting in expensive DC transformers and large volume and weight.

[0004] In view of the above problems, CN114123790B proposes a large-scale high-efficiency DC transformer. The isolated circuit of this scheme works at a fixed frequency, and the isolated circuit can use a structure that shares a transformer core to reduce the volume and loss of the transformer. However, this scheme still requires a huge capacitor filter on the output side, which increases the loss and volume of the DC transformer. CN113258776B proposes a high step-up ratio DC converter for large-scale offshore wind power DC transmission, which adopts a three-phase or multi-phase structure and combines an active bridge arm with a thyristor valve group and a diode valve group. The active bridge arm is used to coordinate and control the interlacing of three-phase or multi-phase current waveforms to synthesize smooth input and output currents without installing a filter on the DC side. However, the thyristor is a semi-controlled device, which is prone to phase change failure and has low reliability. In order to achieve reliable shutdown, it is necessary to reserve reverse voltage time, and the zero current time of the transformer increases, so that the active bridge arm alone supports the medium voltage side voltage for a longer time, and the current stress of the bridge arm submodule device increases, that is, the effective value of the bridge arm current and the absolute average value of the bridge arm current increase, which increases the loss of the DC transformer.

[0005] In summary, the DC transformer in the DC power grid needs to meet the characteristics of large capacity, small number of components, high efficiency, low cost, and especially high reliability. Summary of the invention

[0006] In view of the shortcomings of the prior art, the present invention provides a high-reliability step-up DC transformer and a design method thereof. The input inverter circuit of the transformer adopts a fully controlled IGBT device, which can be reliably shut down without increasing the reverse voltage time. It has high reliability and can also reduce the zero current time of the transformer, thereby improving the efficiency of the DC transformer. The output side adopts a combination of diodes and sub-module bridge arms, which has low device cost and low loss. The sub-module bridge arms adopted can control high-quality DC current waveforms, and achieve wide output voltage range regulation, and have short-circuit fault current limiting capability. The high-reliability step-up DC transformer proposed in the present invention can be designed as N units, and the phase-shifting operation between the N units can synthesize a smooth input side current, thereby reducing the current stress and device loss of the IGBT device. The present invention has the characteristics of a small number of devices, high efficiency, low cost, and wide range of input voltage regulation.

[0007] The objective of the present invention is achieved through the following technical solutions:

[0008] A step-up DC transformer with high reliability comprises N power units, wherein the input and output connections between the N power units are any combination of series and parallel connections, wherein:

[0009] The power unit comprises an input inverter circuit, a single-phase transformer, and an output rectifier circuit;

[0010] The input inverter circuit includes an IGBT device Q1, an IGBT device Q2, an IGBT device Q3, and an IGBT device Q4;

[0011] The single-phase transformer comprises a primary side of the transformer, a secondary side of the transformer and a leakage inductance L k , wherein the primary side includes a first terminal and a second terminal, 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-name terminals, and the transformation ratio of the transformer is 1:n;

[0012] The output rectifier circuit comprises a diode D1, a diode D2, a bridge arm reactor L1, a bridge arm reactor L2, a submodule series bridge arm S1 and a submodule series bridge arm S2;

[0013] The anode of the diode in the IGBT device Q1 is connected to the cathode of the diode in the IGBT device Q3 to form a first bridge arm of the input inverter circuit;

[0014] The anode of the diode in the IGBT device Q2 is connected to the cathode of the diode in the IGBT device Q4 to form a second bridge arm of the input inverter circuit;

[0015] The first terminal of the primary side of the single-phase transformer is connected to the middle node of the IGBT device Q1 and the IGBT device Q3;

[0016] The second terminal of the primary side of the single-phase transformer is connected to the middle node of the IGBT device Q2 and the IGBT device Q4;

[0017] The cathode of the diode in the IGBT device Q1 and the cathode of the diode in the IGBT device Q2 are connected together and connected to the positive electrode of the low-voltage DC input terminal of the power unit;

[0018] The anode of the diode in the IGBT device Q3 and the anode of the diode in the IGBT device Q4 are connected together and connected to the negative electrode of the low voltage DC input terminal of the power unit;

[0019] The cathode of the diode D1 is connected to the first terminal of the bridge arm reactor L1, and the anode of the diode D1 is connected to the positive electrode of the submodule series bridge arm S1, forming the first bridge arm of the output rectifier circuit;

[0020] The cathode of the diode D2 is connected to the first terminal of the bridge arm reactor L2, and the anode of the diode D2 is connected to the positive electrode of the submodule series bridge arm S2, forming the second bridge arm of the output rectifier circuit;

[0021] The first terminal of the secondary side of the single-phase transformer is connected to the middle node of the diode D1 and the submodule series bridge arm S1;

[0022] The second terminal of the secondary side of the single-phase transformer is connected to the middle node of the diode D2 and the submodule series bridge arm S2;

[0023] The second terminal of the bridge arm reactor L1 and the second terminal of the bridge arm reactor L2 are connected together and connected to one end of the medium voltage DC output port of the power unit;

[0024] The negative electrode of the submodule series bridge arm S1 and the negative electrode of the submodule series bridge arm S2 are connected together and connected to the other end of the medium voltage DC output port of the power unit.

[0025] A design method of the above-mentioned step-up DC transformer with high reliability comprises the following steps:

[0026] Step 1: Basic power unit topology

[0027] The power unit includes an input inverter circuit, a single-phase transformer, and an output rectifier circuit, wherein:

[0028] The input inverter circuit includes an IGBT device Q1, an IGBT device Q2, an IGBT device Q3, and an IGBT device Q4;

[0029] The single-phase transformer comprises a primary side of the transformer, a secondary side of the transformer and a leakage inductance L k, wherein the primary side includes a first terminal and a second terminal, 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-name terminals, and the transformation ratio of the transformer is 1:n;

[0030] The output rectifier circuit comprises a diode D1, a diode D2, a bridge arm reactor L1, a bridge arm reactor L2, a submodule series bridge arm S1 and a submodule series bridge arm S2;

[0031] The anode of the diode in the IGBT device Q1 is connected to the cathode of the diode in the IGBT device Q3 to form a first bridge arm of the input inverter circuit;

[0032] The anode of the diode in the IGBT device Q2 is connected to the cathode of the diode in the IGBT device Q4 to form a second bridge arm of the input inverter circuit;

[0033] The first terminal of the primary side of the single-phase transformer is connected to the middle node of the IGBT device Q1 and the IGBT device Q3;

[0034] The second terminal of the primary side of the single-phase transformer is connected to the middle node of the IGBT device Q2 and the IGBT device Q4;

[0035] The cathode of the diode in the IGBT device Q1 and the cathode of the diode in the IGBT device Q2 are connected together and connected to the positive electrode of the low-voltage DC input terminal of the power unit;

[0036] The anode of the diode in the IGBT device Q3 and the anode of the diode in the IGBT device Q4 are connected together and connected to the negative electrode of the low voltage DC input terminal of the power unit;

[0037] The cathode of the diode D1 is connected to the first terminal of the bridge arm reactor L1, and the anode of the diode D1 is connected to the positive electrode of the submodule series bridge arm S1, forming the first bridge arm of the output rectifier circuit;

[0038] The cathode of the diode D2 is connected to the first terminal of the bridge arm reactor L2, and the anode of the diode D2 is connected to the positive electrode of the submodule series bridge arm S2, forming the second bridge arm of the output rectifier circuit;

[0039] The first terminal of the secondary side of the single-phase transformer is connected to the middle node of the diode D1 and the submodule series bridge arm S1;

[0040] The second terminal of the secondary side of the single-phase transformer is connected to the middle node of the diode D2 and the submodule series bridge arm S2;

[0041] The second terminal of the bridge arm reactor L1 and the second terminal of the bridge arm reactor L2 are connected together and connected to one end of the medium voltage DC output port of the power unit;

[0042] The negative electrode of the submodule series bridge arm S1 and the negative electrode of the submodule series bridge arm S2 are connected together and connected to the other end of the medium voltage DC output port of the power unit.

[0043] Step 2: Control method of power unit:

[0044] Step 21: IGBT device Q1, IGBT device Q4 and IGBT device Q2, IGBT device Q3 are turned on alternately at intervals of half a cycle, and all maintain a fixed turn-on timing. The turn-on time is less than half a cycle, and the size is the difference between half a cycle and a commutation time, a bridge arm voltage rise time, and a bridge arm voltage fall time; diode D1 and diode D2 are turned on for half a control cycle respectively, and the working process of sub-module series bridge arm S1 and sub-module series bridge arm S2 differs by half a control cycle, and the output voltage and transmission power are adjusted by changing the voltage of the sub-module series bridge arm;

[0045] Step 22: Assume a working cycle as T, and divide it into five working stages, corresponding to eight working nodes, namely t0~t7, I L is the low voltage DC input current, I M For medium voltage DC output current, the specific working stages are:

[0046] Working stage 1, power transmission stage t0~t1: The submodule corresponding to the conducting diode D1 is connected in series with the bridge arm S1 to support U M ; Transformer constant current I L / n, the bridge arm voltage difference maintains the transformer voltage, and most of the power is transmitted to the medium voltage side through the IGBT and transformer; when the transformer current changes, the sub-module corresponding to the diode D1 is connected in series with the bridge arm S1 to support U M ; Transformer current is I L / n drops to 0, and the drop time is the transformer current change time T c,trans , the submodule is connected in series with the bridge arm S2 to provide a current change driving voltage;

[0047] Working stage 2, output zero current stage t1~t2: This period of time is the bridge arm voltage change time T u , IGBT device Q1 and IGBT device Q4 are turned off, diode D1 is turned on, and the output current i D1 Also for I M ; Diode D2 is to be turned on, current i D2 The submodule series bridge arm S1 corresponding to the conducting diode D1 supports U M ; The voltage of the submodule series bridge arm S2 corresponding to the diode D2 to be turned on is from U M -nU LThe transformer voltage rises to prepare for diode commutation. The transformer voltage is the difference between the voltage of the bridge arm S1 and the voltage of the bridge arm S2 connected in series. As the voltage of the bridge arm S2 connected in series increases, the transformer voltage decreases and is less than nU L , the transformer current is maintained at 0, and the output current is zero;

[0048] Working stage three, output zero current stage t2~t3: This period of time is the diode commutation time T q , diode D1 current i D1 From I M Reduced to 0; diode D2 current i D2 From 0 to I M ; The diode undergoes current commutation, and the voltage of the submodule series bridge arm S1 is U M -U T , the voltage of the bridge arm S2 of the submodule series connection is U M +U T , the two bridge arms provide commutation voltage, and the bridge arms jointly support U M ; Transformer voltage is less than nU L , the transformer current is maintained at 0, and the output current is zero;

[0049] Working stage 4, output zero current stage t3~t4: This period of time is the bridge arm voltage change time T u , diode D1 is turned off, current i D1 is 0; diode D2 is turned on, current i D2 For I M ; The submodule corresponding to the conducting diode D2 is connected in series with the bridge arm S2 to support U M , the voltage of the submodule series bridge arm S1 corresponding to the turn-off diode D1 changes from U M Descend to U M +U T , prepare for transmission power;

[0050] Working stage 5, power transmission stage t4~t7: Entering the second half cycle, normal power transmission, among which t4~t5 is the transformer current change stage, that is, the IGBT device Q2, IGBT device Q3 conduction current rising stage, the sub-module corresponding to the conduction diode D2 is connected in series with the bridge arm S2 to support U M , the submodule series bridge arm S1 is U M -nU L +U T , providing commutation voltage for current change; in the t5-t6 stage, the submodule series bridge arm S2 corresponding to the conducting diode D2 supports U M , transformer constant current - I L / n, the bridge arm voltage difference maintains the transformer voltage, and most of the power is transmitted to the low-voltage side transformer through the thyristor and transformer; t6~t7 is the transformer current change stage, that is, the IGBT device Q2, IGBT device Q3 conduction current decrease stage, the sub-module corresponding to the conduction diode D2 is connected in series with the bridge arm S2 to support U M , the transformer current is -I L / n drops to 0, and the drop time is the transformer current change time T c,trans , the submodule is connected in series with the bridge arm S1 to provide a current change driving voltage;

[0051] Step 3: N power unit topology:

[0052] For the input side of the topology, a parallel connection is adopted, and multiple power units are operated with phase shift in timing to maintain the continuity of the input current, or a series connection is adopted to evenly distribute the input side voltage; for the output side of the topology, an output parallel connection is adopted to evenly distribute the output side current, or an output series connection is adopted to evenly distribute the high voltage on the output side.

[0053] Step 4: Design method of current waveform of N power units:

[0054] The input current of a single power unit is intermittent and the output current is continuous. When the input current is zero, that is, during the zero current time of the transformer, the single power unit completes the bridge arm voltage rise and fall process and the diode commutation process. After N power units are connected in series on the input side, the N power units evenly distribute the input voltage, and the voltage stress of the IGBT device is reduced. After N power units are connected in parallel on the input side, when the output current of any power unit is zero, the other N-1 power units will evenly distribute the output current, and the output current of each power unit is I M / (n-1); After N power units are connected in parallel on the input side, the N converters are operated in phase shift to ensure that when the transformer current of any power unit is zero, the other N-1 power units will evenly distribute the transformer current, and the phase shift angle is T / 2N, and must meet the following requirements:

[0055]

[0056] Where, T z is the output zero current time.

[0057] Compared with the prior art, the present invention has the following advantages:

[0058] 1. The input side of the present invention adopts a fully controlled IGBT device, which has higher reliability than using a half-controlled thyristor device.

[0059] 2. The present invention adopts fully controlled IGBT devices on the input side, which eliminates the reverse voltage time required for reliable shutdown, reduces the zero current time of the transformer, reduces the time that the bridge arm supports the medium voltage side alone, reduces the effective value of the bridge arm current and the absolute average value of the bridge arm current, reduces the conduction loss and switching loss of the IGBT device, and improves the efficiency of the DC transformer.

[0060] 3. Compared with the existing series-parallel structure of resonant converters, the present invention saves a large number of expensive high-frequency transformers and adopts a small number of large-capacity medium-frequency transformers, which has the advantages of small size and weight.

[0061] 4. Compared with the existing step-up DC transformer, the present invention does not require an additional voltage regulation stage, and has a small number of power units, eliminating a large number of semiconductor devices, and has the advantages of high cost, low device loss and simple structure.

[0062] 5. The output current of the present invention is an interlaced trapezoidal wave, and the output current is smooth and continuous, without the need for a filter, thereby reducing the cost and volume of the device.

[0063] 6. The present invention adopts N power units connected in parallel on the input side and in series or in parallel on the output side, so that the transformer currents between the N power units are shifted in phase by T / 2N in sequence, and the input current is continuous, which can effectively reduce the current stress of the IGBT device on the input side of the DC transformer and improve the power capacity and voltage level of the DC transformer.

[0064] 7. In the present invention, IGBT devices and diodes are used in the input inverter circuit and the output rectifier circuit respectively, which have the characteristics of high reliability and simple structure; the output current of a single power unit is an interlaced trapezoidal wave with high waveform quality and no filter is required; the structure of N power units input in parallel can significantly reduce the current stress of the input side device, reduce its conduction loss, and improve the efficiency of the DC transformer. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 The schematic diagram of the topology circuit of the boost DC transformer power unit with high reliability;

[0066] Figure 2 The thyristor drive timing and main voltage and current waveforms of the boost DC transformer power unit with high reliability;

[0067] Figure 3 A schematic diagram of a current path of a high-reliability boost DC transformer power unit in working phase 1;

[0068] Figure 4 A schematic diagram of a current path of a high-reliability boost DC transformer power unit in working phase 2;

[0069] Figure 5It is a schematic diagram of the current path of the power unit of the high-reliability step-up DC transformer in the third working stage;

[0070] Figure 6 It is a schematic diagram of the current path of the boost DC transformer rate unit with high reliability in the fourth working stage;

[0071] Figure 7 It is a schematic diagram of the current path of the power unit of the step-up DC transformer with high reliability in the fifth working stage;

[0072] Figure 8 A topological circuit diagram of a step-up DC transformer with high reliability, wherein two power units are connected in parallel and outputs are connected in series;

[0073] Fig. 9 The two power units of the step-up DC transformer with high reliability are input in parallel, and the transformer current and output current waveforms are output in series;

[0074] Fig.10 A topological circuit diagram of a high-reliability step-up DC transformer with three power units in parallel input and series output;

[0075] Fig.11 The three power units of the step-up DC transformer with high reliability are input in parallel, and the transformer current and output current waveforms are output in series;

[0076] Fig.12 A topological circuit diagram of a high-reliability step-up DC transformer with four power units in parallel input and series output;

[0077] Fig.13 The four power units of the step-up DC transformer with high reliability are input in parallel, and the transformer current and output current waveforms are output in series;

[0078] Fig.14 The topological circuit diagram of a high-reliability step-up DC transformer with N power units having parallel input and series output. DETAILED DESCRIPTION

[0079] The technical solution of the present invention is further described below in conjunction with the accompanying drawings, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be included in the protection scope of the present invention.

[0080] A step-up DC transformer with high reliability includes N power units, the input and output connections between the N power units can be connected in series and parallel in any combination, and the power units use the same circuit topology and control method, such as Figure 1As shown, they all include input inverter circuit, single-phase transformer, and output rectifier circuit, among which:

[0081] The input inverter circuit includes an IGBT device Q1, an IGBT device Q2, an IGBT device Q3, and an IGBT device Q4;

[0082] The single-phase transformer comprises a primary side of the transformer, a secondary side of the transformer and a leakage inductance L k , wherein the primary side includes a first terminal and a second terminal, 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-name terminals, and the transformation ratio of the transformer is 1:n;

[0083] The output rectifier circuit comprises a diode D1, a diode D2, a bridge arm reactor L1, a bridge arm reactor L2, a submodule series bridge arm S1 and a submodule series bridge arm S2;

[0084] The anode of the diode in the IGBT device Q1 is connected to the cathode of the diode in the IGBT device Q3 to form a first bridge arm of the input inverter circuit;

[0085] The anode of the diode in the IGBT device Q2 is connected to the cathode of the diode in the IGBT device Q4 to form a second bridge arm of the input inverter circuit;

[0086] The first terminal of the primary side of the single-phase transformer is connected to the middle node of the IGBT device Q1 and the IGBT device Q3;

[0087] The second terminal of the primary side of the single-phase transformer is connected to the middle node of the IGBT device Q2 and the IGBT device Q4;

[0088] The cathode of the diode in the IGBT device Q1 and the cathode of the diode in the IGBT device Q2 are connected together and connected to the positive electrode of the low-voltage DC input terminal of the power unit;

[0089] The anode of the diode in the IGBT device Q3 and the anode of the diode in the IGBT device Q4 are connected together and connected to the negative electrode of the low voltage DC input terminal of the power unit;

[0090] The cathode of the diode D1 is connected to the first terminal of the bridge arm reactor L1, and the anode of the diode D1 is connected to the positive electrode of the submodule series bridge arm S1, forming the first bridge arm of the output rectifier circuit;

[0091] The cathode of the diode D2 is connected to the first terminal of the bridge arm reactor L2, and the anode of the diode D2 is connected to the positive electrode of the submodule series bridge arm S2, forming the second bridge arm of the output rectifier circuit;

[0092] The first terminal of the secondary side of the single-phase transformer is connected to the middle node of the diode D1 and the submodule series bridge arm S1;

[0093] The second terminal of the secondary side of the single-phase transformer is connected to the middle node of the diode D2 and the submodule series bridge arm S2;

[0094] The second terminal of the bridge arm reactor L1 and the second terminal of the bridge arm reactor L2 are connected together and connected to one end of the medium voltage DC output port of the power unit;

[0095] The negative electrode of the submodule series bridge arm S1 and the negative electrode of the submodule series bridge arm S2 are connected together and connected to the other end of the medium voltage DC output port of the power unit.

[0096] A design method of the above-mentioned step-up DC transformer with high reliability comprises the following steps:

[0097] Step 1: Basic power unit topology

[0098] The power unit includes an input inverter circuit, a single-phase transformer, and an output rectifier circuit, wherein:

[0099] The input inverter circuit includes an IGBT device Q1, an IGBT device Q2, an IGBT device Q3, and an IGBT device Q4;

[0100] The single-phase transformer comprises a primary side of the transformer, a secondary side of the transformer and a leakage inductance L k , wherein the primary side includes a first terminal and a second terminal, 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-name terminals, and the transformation ratio of the transformer is 1:n;

[0101] The output rectifier circuit comprises a diode D1, a diode D2, a bridge arm reactor L1, a bridge arm reactor L2, a submodule series bridge arm S1 and a submodule series bridge arm S2;

[0102] The anode of the diode in the IGBT device Q1 is connected to the cathode of the diode in the IGBT device Q3 to form a first bridge arm of the input inverter circuit;

[0103] The anode of the diode in the IGBT device Q2 is connected to the cathode of the diode in the IGBT device Q4 to form a second bridge arm of the input inverter circuit;

[0104] The first terminal of the primary side of the single-phase transformer is connected to the middle node of the IGBT device Q1 and the IGBT device Q3;

[0105] The second terminal of the primary side of the single-phase transformer is connected to the middle node of the IGBT device Q2 and the IGBT device Q4;

[0106] The cathode of the diode in the IGBT device Q1 and the cathode of the diode in the IGBT device Q2 are connected together and connected to the positive electrode of the low-voltage DC input terminal of the power unit;

[0107] The anode of the diode in the IGBT device Q3 and the anode of the diode in the IGBT device Q4 are connected together and connected to the negative electrode of the low voltage DC input terminal of the power unit;

[0108] The cathode of the diode D1 is connected to the first terminal of the bridge arm reactor L1, and the anode of the diode D1 is connected to the positive electrode of the submodule series bridge arm S1, forming the first bridge arm of the output rectifier circuit;

[0109] The cathode of the diode D2 is connected to the first terminal of the bridge arm reactor L2, and the anode of the diode D2 is connected to the positive electrode of the submodule series bridge arm S2, forming the second bridge arm of the output rectifier circuit;

[0110] The first terminal of the secondary side of the single-phase transformer is connected to the middle node of the diode D1 and the submodule series bridge arm S1;

[0111] The second terminal of the secondary side of the single-phase transformer is connected to the middle node of the diode D2 and the submodule series bridge arm S2;

[0112] The second terminal of the bridge arm reactor L1 and the second terminal of the bridge arm reactor L2 are connected together and connected to one end of the medium voltage DC output port of the power unit;

[0113] The negative electrode of the submodule series bridge arm S1 and the negative electrode of the submodule series bridge arm S2 are connected together and connected to the other end of the medium voltage DC output port of the power unit.

[0114] Step 2: Control method of power unit:

[0115] Step 21: IGBT device Q1, IGBT device Q4 and IGBT device Q2, IGBT device Q3 are turned on alternately at intervals of half a cycle, and all maintain a fixed turn-on timing. The turn-on time is less than half a cycle. The turn-on time is determined by the bridge arm voltage change time and the diode commutation time, and its size is the difference between half a cycle and one commutation time, one bridge arm voltage rise time and one bridge arm voltage fall time; diode D1 and diode D2 are turned on for half a control cycle respectively, and the working process of sub-module series bridge arm S1 and sub-module series bridge arm S2 differs by half a control cycle. The output voltage and transmission power are adjusted by changing the voltage of the sub-module series bridge arm.

[0116] Step 22: Assume a working cycle as T, and divide it into five working stages, corresponding to eight working nodes, namely t0~t7, I L is the low voltage, DC input current, I M For medium voltage DC output current, the specific working stages are:

[0117] Working stage 1, transmission power stage (t0~t1): Figure 3 As shown, the submodule series bridge arm S1 corresponding to the conducting diode D1 supports U M ; Transformer constant current I L / n, the bridge arm voltage difference maintains the transformer voltage, and most of the power is transmitted to the medium voltage side through the IGBT and transformer; when the transformer current changes, the sub-module corresponding to the diode D1 is connected in series with the bridge arm S1 to support U M ; Transformer current is I L / n drops to 0, and the drop time is the transformer current change time T c,trans , the sub-module series bridge arm S2 (transformer current flows into the bridge arm) provides a current change driving voltage.

[0118] Working stage 2, output zero current stage (t1~t2): This period of time is the bridge arm voltage change time T u ,like Figure 4 As shown, IGBT devices Q1 and Q4 are turned off, diode D1 is turned on, and the output current i D1 Also for I M ; Diode D2 is to be turned on, current i D2 The submodule series bridge arm S1 corresponding to the conducting diode D1 supports U M ; The voltage of the submodule series bridge arm S2 corresponding to the diode D2 to be turned on is from U M -nU L The transformer voltage rises to prepare for diode commutation. The transformer voltage is the difference between the voltage of the bridge arm S1 and the voltage of the bridge arm S2 connected in series. As the voltage of the bridge arm S2 connected in series increases, the transformer voltage decreases and is less than nU L , the transformer current remains at 0 and the output current is zero.

[0119] Working stage 3, output zero current stage (t2~t3): This period of time is the diode commutation time T q ,like Figure 5 As shown, the diode D1 current i D1 From I M Reduced to 0; diode D2 current i D2 From 0 to I M ; The diode undergoes current commutation, and the voltage of the submodule series bridge arm S1 is U M -U T , the voltage of the bridge arm S2 of the submodule series connection is UM +U T , the two bridge arms provide commutation voltage, and the bridge arms jointly support U M ; Transformer voltage is less than nU L , the transformer current remains at 0 and the output current is zero.

[0120] Working stage 4, output zero current stage (t3~t4): This period of time is the bridge arm voltage change time T u ,like Figure 6 As shown, diode D1 is turned off and the current i D1 is 0; diode D2 is turned on, current i D2 For I M ; The submodule corresponding to the conducting diode D2 is connected in series with the bridge arm S2 to support U M , the voltage of the submodule series bridge arm S1 corresponding to the turn-off diode D1 changes from U M Descend to U M +U T , in preparation for transmission power.

[0121] Output zero current time T z It can be obtained according to the following formula:

[0122] T z =2T u +T q (1)

[0123] Working stage 5: Transmission power stage (t4~t7): Figure 7 As shown, it enters the second half cycle and transmits power normally, wherein (t4-t5) is the transformer current change stage, i.e., the IGBT device Q2 and IGBT device Q3 are conducting current rising stage, and the submodule series bridge arm S2 corresponding to the conducting diode D2 supports U M , the submodule series bridge arm S1 is U M -nU L +U T , providing commutation voltage for current change; (t5~t6) stage, the submodule series bridge arm S2 corresponding to the conducting diode D2 supports U M , transformer constant current - I L / n, the bridge arm voltage difference maintains the transformer voltage, and most of the power is transmitted to the low-voltage side transformer through the thyristor and transformer; (t6~t7) is the transformer current change stage, that is, the IGBT device Q2, IGBT device Q3 conduction current decrease stage, the sub-module corresponding to the conduction diode D2 is connected in series with the bridge arm S2 to support U M , the transformer current is -I L / n drops to 0, and the drop time is the transformer current change time T c,trans, the sub-module series bridge arm S1 (transformer current flows into the bridge arm) provides current change driving voltage.

[0124] Step 3: N power unit topology:

[0125] A high-reliability step-up DC transformer can use N power units with the same structure to increase the transmission power. For the topology input side, a parallel connection can be used, and multiple power units can be operated in phase shift in timing to maintain the continuity of the input current; a series connection can also be used to evenly distribute the input side voltage. For the topology output side, the output parallel connection can be used to evenly distribute the output side current, or the output series connection can be used to evenly distribute the high output side voltage.

[0126] Step 4: Design method of current waveform of N power units:

[0127] The present invention designs a current waveform method for the proposed step-up DC transformer with high reliability when using N power units with the same structure. A single power unit has intermittent input current and continuous output current. When the input current is zero, that is, within the zero current time of the transformer, a single power unit needs to complete the bridge arm voltage rise and fall process and the diode commutation process. After N power units are connected in series on the input side, the N power units evenly distribute the input voltage, and the voltage stress of the IGBT device is reduced. After N power units are connected in parallel on the input side, when the output current of any one power unit is zero, the other N-1 power units will evenly distribute the output current, and the output current of each power unit is I M / (n-1), so the current stress of the diode is reduced. After N power units are connected in parallel on the input side, the N converters are operated in phase shift to ensure that when the transformer current of any power unit is zero, the other N-1 power units will evenly distribute the transformer current, and the phase shift angle is T / 2N, and must meet the following requirements:

[0128]

[0129] In the present invention, fully controlled IGBT devices and diodes are used in the input inverter circuit and the output rectifier circuit to bear the current and transmit power, which has the characteristics of high reliability, high efficiency and low cost; the input voltage and transmission power are adjusted by controlling the voltage at both ends of the bridge arm of the submodule in series, which has the characteristics of wide voltage regulation range and fault current limiting. The present invention proposes a current waveform design method, N power units are connected in series and parallel, and phase-shifted operation can be performed, so that the input current can be evenly distributed by the other (N-1) power units in the zero current stage of the transformer of one power unit, synthesizing a smooth DC current and reducing the current stress of the IGBT device.

[0130] On the whole, the high-reliability step-up DC transformer proposed in the present invention has the advantages of small number of components, high efficiency, low cost, small current stress of IGBT devices on the input side, smooth current waveform, high reliability and fault current limiting capability. It is very suitable for new DC loads and energy storage systems, new energy DC collection and transmission systems and other fields.

[0131] Embodiment 1: Two power units are connected in parallel for input and in series for output:

[0132] The power unit of the step-up DC transformer with high reliability proposed in this embodiment is as follows: Figure 8 As shown, the high-reliability step-up DC transformer includes two power units: power unit one and power unit two. The two power units adopt the same circuit topology and control method. Each power unit includes an input inverter circuit, a single-phase transformer, and an output rectifier circuit.

[0133] Taking power unit 1 as an example, the input inverter circuit includes IGBT devices Q1, IGBT devices Q2, IGBT devices Q3, and IGBT devices Q4; the single-phase transformer includes the primary side of the transformer, the secondary side of the transformer, and the leakage inductance L k , wherein the primary side includes a first terminal and a second terminal, 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, and the transformation ratio of the transformer is 1:n; the output rectifier circuit includes a diode D1, a diode D2, a bridge arm reactor L1, a bridge arm reactor L2, a submodule series bridge arm S1 and a submodule series bridge arm S2, wherein:

[0134] The anode of the diode in the IGBT device Q1 is connected to the cathode of the diode in the IGBT device Q3 to form a first bridge arm of the input inverter circuit;

[0135] The anode of the diode in the IGBT device Q2 is connected to the cathode of the diode in the IGBT device Q4 to form a second bridge arm of the input inverter circuit;

[0136] The first terminal of the primary side of the single-phase transformer is connected to the middle node of the IGBT device Q1 and the IGBT device Q3;

[0137] The second terminal of the primary side of the single-phase transformer is connected to the middle node of the IGBT device Q2 and the IGBT device Q4;

[0138] The cathode of the diode in the IGBT device Q1 and the cathode of the diode in the IGBT device Q2 are connected together and connected to the positive electrode of the low-voltage DC input terminal of the power unit;

[0139] The anode of the diode in the IGBT device Q3 and the anode of the diode in the IGBT device Q4 are connected together and connected to the negative electrode of the low voltage DC input terminal of the power unit;

[0140] The cathode of the diode D1 is connected to the first terminal of the bridge arm reactor L1, and the anode of the diode D1 is connected to the positive electrode of the submodule series bridge arm S1, forming the first bridge arm of the output rectifier circuit;

[0141] The cathode of the diode D2 is connected to the first terminal of the bridge arm reactor L2, and the anode of the diode D2 is connected to the positive electrode of the submodule series bridge arm S2, forming the second bridge arm of the output rectifier circuit;

[0142] The first terminal of the secondary side of the single-phase transformer is connected to the middle node of the diode D1 and the submodule series bridge arm S1;

[0143] The second terminal of the secondary side of the single-phase transformer is connected to the middle node of the diode D2 and the submodule series bridge arm S2;

[0144] The second terminal of the bridge arm reactor L1 and the second terminal of the bridge arm reactor L2 are connected together and connected to one end of the medium voltage DC output port of the power unit;

[0145] The negative electrode of the submodule series bridge arm S1 and the negative electrode of the submodule series bridge arm S2 are connected together and connected to the other end of the medium voltage DC output port of the power unit;

[0146] The circuits and components of the power unit 2 are consistent with those of the power unit 1, and the internal circuit connection method of the power unit 2 is the same as that of the power unit 1, that is, the input inverter circuit, single-phase transformer, and output rectifier circuit in the power unit 2 are connected in the same way as those in the power unit 1.

[0147] In this embodiment, the input end of the step-up DC transformer with high reliability includes four terminals, namely, a first terminal N i1 、Second terminal N i2 、The third terminal N i3 and the fourth terminal N i4 The low voltage DC output terminal also has 4 terminals, namely the first terminal N o1 、Second terminal N o2 、The third terminal N o3 and the fourth terminal N o4 The input side of the high-reliability step-up DC transformer is connected in parallel, that is, the first terminal N i1 With the third terminal N i3 connected, as the low voltage DC input positive pole, the second terminal N i2 With the fourth terminal N i4connected as the negative pole of low voltage DC input; the output side adopts series connection, that is, the second terminal N o2 With the third terminal N o3 Connected, the first terminal N o1 As the positive pole of the medium voltage DC output terminal, the fourth terminal N o4 As the negative pole of the medium voltage DC output terminal, each unit shares the higher voltage on the medium voltage side.

[0148] The thyristor driving timing and main voltage and current waveforms of the high reliability step-up DC transformer unit proposed by the present invention are as follows: Figure 2 As shown, taking power unit 1 as an example, IGBT devices Q1, IGBT devices Q4 and IGBT devices Q2, IGBT devices Q3 are turned on alternately at intervals of half a cycle, and all maintain a fixed turn-on timing. The turn-on time is less than half a cycle. The turn-on time is determined by the bridge arm voltage change time and the diode commutation time. The size is the difference between half a cycle and a commutation time, a bridge arm voltage rise time and a bridge arm voltage fall time. Diode D1 and diode D2 are turned on for half a control cycle respectively. The working process of sub-module series bridge arm S1 and sub-module series bridge arm S2 differs by half a control cycle. The output voltage and transmission power are adjusted by changing the voltage of the sub-module series bridge arm; power unit 2 also adopts the same control method. Among them, I L is the current after low voltage AC is rectified by the converter input rectifier circuit, i q1 and i q2 are the currents of IGBT device Q1, IGBT device Q4, IGBT device Q2, and IGBT device Q3 in converter 1, respectively. t1 is the transformer current of power unit 1, u S1 and u S2 are the voltages of the submodule series bridge arm S1 and the submodule series bridge arm S2 in converter 1, respectively. O1 and i O2 are the output currents of power unit 1 and power unit 2 respectively. Assuming a working cycle is T, the time is t0~t6, taking converter 1 as an example, the specific working stages are:

[0149] Working stage 1: (t0~t1), such as Figure 3 As shown, in this stage, the submodule series bridge arm S1 corresponding to the diode D1 supports U M ; Transformer constant current I L / n, the bridge arm voltage difference maintains the transformer voltage, and most of the power is transmitted to the medium voltage side through the IGBT and transformer. When the transformer current changes, the submodule corresponding to the diode D1 is connected in series with the bridge arm S1 to support U M ; Transformer current is I L / n drops to 0, and the drop time is the transformer current change time Tc,trans , the sub-module series bridge arm S2 (transformer current flows into the bridge arm) provides a current change driving voltage.

[0150] Working stage 2: (t1~t2), such as Figure 4 As shown, this period of time is the bridge arm voltage change time T u IGBT device Q1 and IGBT device Q4 are turned off, diode D1 is turned on, and the output current i D1 Also for I M ; Diode D2 is to be turned on, current i D2 The submodule series bridge arm S1 corresponding to the conducting diode D1 supports U M ; The voltage of the bridge arm S2 of the submodule corresponding to the diode D2 to be turned on is from U M -nU L The transformer voltage rises to prepare for diode commutation. The transformer voltage is the difference between the voltage of the submodule series bridge arm S1 and the voltage of the submodule series bridge arm S2. As the voltage of the submodule series bridge arm S2 rises, the transformer voltage decreases and is less than nU L , the transformer current remains at 0 and the output current is zero.

[0151] Working stage three: (t2~t3), such as Figure 5 As shown, this period of time is the diode commutation time T q . Diode D1 current i D1 From I M Reduced to 0; diode D2 current i D2 From 0 to I M The diode commutates, and the voltage of the submodule series bridge arm S1 is U M -U T , the voltage of the bridge arm S2 of the submodule series connection is U M +U T , the two bridge arms provide commutation voltage, and the bridge arms jointly support U M ; Transformer voltage is less than nU L , the transformer current remains at 0 and the output current is zero.

[0152] Working stage 4: (t3~t4), such as Figure 6 As shown, this period of time is the bridge arm voltage change time T u . Diode D1 is turned off, and the current i D1 is 0; diode D2 is turned on, current i D2 For I M The submodule series bridge arm S2 corresponding to the conducting diode D2 supports U M , the voltage of the submodule series bridge arm S1 corresponding to the turned-off diode D1 changes from U M Descend to U M +U T, in preparation for transmission power.

[0153] Output zero current time T z It can be obtained according to the following formula:

[0154] T z =2T u +T q (1)

[0155] Working stage five: (t4~t6), such as Figure 7 As shown, this stage enters the second half cycle and transmits power normally. Among them, (t4~t5) is the transformer current change stage, that is, the IGBT device Q2, IGBT device Q3 conduction current rising stage, the sub-module series bridge arm S2 corresponding to the conduction diode D2 supports U M , the submodule series bridge arm S1 is U M -nU L +U T , providing commutation voltage for current change. In the (t5-t6) stage, the submodule corresponding to the conducting diode D2 is connected in series with the bridge arm S2 to support U M ; Transformer constant current - I L / n, the bridge arm voltage difference maintains the transformer voltage, and most of the power is transmitted to the low-voltage side transformer through the thyristor and transformer. (t6~t7) is the transformer current change stage, that is, the IGBT device Q2, IGBT device Q3 conduction current decrease stage, the sub-module corresponding to the conduction diode D2 is connected in series with the bridge arm S2 to support U M ; Transformer current is -I L / n drops to 0, and the drop time is the transformer current change time T c,trans , the sub-module series bridge arm S1 (transformer current flows into the bridge arm) provides current change driving voltage.

[0156] When the input of power unit 1 and power unit 2 are connected in series and the output is connected in parallel, the topological circuit diagram is as follows Figure 8 As shown, the corresponding transformer current and output current waveforms are as follows Fig. 9 As shown. Power unit 1 and power unit 2 operate alternately. During the zero current and output zero current time of one transformer, the other converter outputs current. The output current trapezoidal wave is staggered by T / 4, and the composite amplitude is I L Smooth DC current without filters.

[0157] Embodiment 2: Three power units are connected in series for input and in parallel for output:

[0158] When the inputs of power unit 1, power unit 2 and power unit 3 are connected in parallel and the outputs are connected in series, the topological circuit diagram is as follows: Fig.10 As shown, the corresponding transformer current and output current waveforms are as follows Fig.11The three power units operate in an interleaved manner. During the zero current time of one transformer, the other two converters evenly distribute the input current. The input current trapezoidal wave is staggered by T / 6, and the composite amplitude is 2I L The smooth DC current can reduce the current stress of IGBT devices.

[0159] Embodiment 3: N power units are connected in series for input and in parallel for output:

[0160] When N power units are connected in parallel and in series, the topology circuit diagram is as follows: Fig.14 As shown. N converters are staggered, and during the zero current time of one transformer, the other (N-1) converters evenly distribute the input current, and the output current trapezoidal wave is staggered by T / (2N), and the composite amplitude is (N-1)I L The smooth DC input current can reduce the current stress of IGBT devices.

Claims

1. A step-up DC transformer with high reliability, characterized in that The step-up DC transformer comprises N power units, wherein: The power unit comprises an input inverter circuit, a single-phase transformer, and an output rectifier circuit; The input inverter circuit includes an IGBT device Q1, an IGBT device Q2, an IGBT device Q3, and an IGBT device Q4; The single-phase transformer comprises a primary side of the transformer, a secondary side of the transformer and a leakage inductance L k , wherein the primary side includes a first terminal and a second terminal, 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-name terminals, and the transformation ratio of the transformer is 1:n; The output rectifier circuit comprises a diode D1, a diode D2, a bridge arm reactor L1, a bridge arm reactor L2, a submodule series bridge arm S1 and a submodule series bridge arm S2; The anode of the diode in the IGBT device Q1 is connected to the cathode of the diode in the IGBT device Q3 to form a first bridge arm of the input inverter circuit; The anode of the diode in the IGBT device Q2 is connected to the cathode of the diode in the IGBT device Q4 to form a second bridge arm of the input inverter circuit; The first terminal of the primary side of the single-phase transformer is connected to the middle node of the IGBT device Q1 and the IGBT device Q3; The second terminal of the primary side of the single-phase transformer is connected to the middle node of the IGBT device Q2 and the IGBT device Q4; The cathode of the diode in the IGBT device Q1 and the cathode of the diode in the IGBT device Q2 are connected together and connected to the positive electrode of the low-voltage DC input terminal of the power unit; The anode of the diode in the IGBT device Q3 and the anode of the diode in the IGBT device Q4 are connected together and connected to the negative electrode of the low voltage DC input terminal of the power unit; The cathode of the diode D1 is connected to the first terminal of the bridge arm reactor L1, and the anode of the diode D1 is connected to the positive electrode of the submodule series bridge arm S1, forming the first bridge arm of the output rectifier circuit; The cathode of the diode D2 is connected to the first terminal of the bridge arm reactor L2, and the anode of the diode D2 is connected to the positive electrode of the submodule series bridge arm S2, forming the second bridge arm of the output rectifier circuit; The first terminal of the secondary side of the single-phase transformer is connected to the middle node of the diode D1 and the submodule series bridge arm S1; The second terminal of the secondary side of the single-phase transformer is connected to the middle node of the diode D2 and the submodule series bridge arm S2; The second terminal of the bridge arm reactor L1 and the second terminal of the bridge arm reactor L2 are connected together and connected to one end of the medium voltage DC output port of the power unit; The negative electrode of the submodule series bridge arm S1 and the negative electrode of the submodule series bridge arm S2 are connected together and connected to the other end of the medium voltage DC output port of the power unit.

2. The step-up DC transformer with high reliability according to claim 1 is characterized in that The input and output connections between the N power units are any combination of series and parallel.

3. A design method for a high-reliability step-up DC transformer, characterized in that The method comprises the following steps: Step 1: Basic power unit topology The power unit includes an input inverter circuit, a single-phase transformer, and an output rectifier circuit, wherein: The input inverter circuit includes an IGBT device Q1, an IGBT device Q2, an IGBT device Q3, and an IGBT device Q4; The single-phase transformer comprises a primary side of the transformer, a secondary side of the transformer and a leakage inductance L k , wherein the primary side includes a first terminal and a second terminal, 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-name terminals, and the transformation ratio of the transformer is 1:n; The output rectifier circuit comprises a diode D1, a diode D2, a bridge arm reactor L1, a bridge arm reactor L2, a submodule series bridge arm S1 and a submodule series bridge arm S2; The anode of the diode in the IGBT device Q1 is connected to the cathode of the diode in the IGBT device Q3 to form a first bridge arm of the input inverter circuit; The anode of the diode in the IGBT device Q2 is connected to the cathode of the diode in the IGBT device Q4 to form a second bridge arm of the input inverter circuit; The first terminal of the primary side of the single-phase transformer is connected to the middle node of the IGBT device Q1 and the IGBT device Q3; The second terminal of the primary side of the single-phase transformer is connected to the middle node of the IGBT device Q2 and the IGBT device Q4; The cathode of the diode in the IGBT device Q1 and the cathode of the diode in the IGBT device Q2 are connected together and connected to the positive electrode of the low-voltage DC input terminal of the power unit; The anode of the diode in the IGBT device Q3 and the anode of the diode in the IGBT device Q4 are connected together and connected to the negative electrode of the low voltage DC input terminal of the power unit; The cathode of the diode D1 is connected to the first terminal of the bridge arm reactor L1, and the anode of the diode D1 is connected to the positive electrode of the submodule series bridge arm S1, forming the first bridge arm of the output rectifier circuit; The cathode of the diode D2 is connected to the first terminal of the bridge arm reactor L2, and the anode of the diode D2 is connected to the positive electrode of the submodule series bridge arm S2, forming the second bridge arm of the output rectifier circuit; The first terminal of the secondary side of the single-phase transformer is connected to the middle node of the diode D1 and the submodule series bridge arm S1; The second terminal of the secondary side of the single-phase transformer is connected to the middle node of the diode D2 and the submodule series bridge arm S2; The second terminal of the bridge arm reactor L1 and the second terminal of the bridge arm reactor L2 are connected together and connected to one end of the medium voltage DC output port of the power unit; The negative electrode of the submodule series bridge arm S1 and the negative electrode of the submodule series bridge arm S2 are connected together and connected to the other end of the medium voltage DC output port of the power unit. Step 2: Control method of power unit: Step 21: IGBT device Q1, IGBT device Q4 and IGBT device Q2, IGBT device Q3 are turned on alternately at intervals of half a cycle, and all maintain a fixed turn-on timing. The turn-on time is less than half a cycle, and the size is the difference between half a cycle and a commutation time, a bridge arm voltage rise time, and a bridge arm voltage fall time; diode D1 and diode D2 are turned on for half a control cycle respectively, and the working process of sub-module series bridge arm S1 and sub-module series bridge arm S2 differs by half a control cycle, and the output voltage and transmission power are adjusted by changing the voltage of the sub-module series bridge arm; Step 22: Assume a working cycle as T, and divide it into five working stages, corresponding to eight working nodes, namely t0~t7, I L is the low voltage DC input current, I M For medium voltage DC output current, the specific working stages are: Working stage 1, power transmission stage t0~t1: The submodule corresponding to the conducting diode D1 is connected in series with the bridge arm S1 to support U M ; Transformer constant current I L / n, the bridge arm voltage difference maintains the transformer voltage, and most of the power is transmitted to the medium voltage side through the IGBT and transformer; when the transformer current changes, the sub-module corresponding to the diode D1 is connected in series with the bridge arm S1 to support U M ; Transformer current is I L / n drops to 0, and the drop time is the transformer current change time T c,trans , the submodule is connected in series with the bridge arm S2 to provide a current change driving voltage; Working stage 2, output zero current stage t1~t2: This period of time is the bridge arm voltage change time T u , IGBT device Q1 and IGBT device Q4 are turned off, diode D1 is turned on, and the output current i D1 Also for I M ; Diode D2 is to be turned on, current i D2 The submodule series bridge arm S1 corresponding to the conducting diode D1 supports U M ; The voltage of the submodule series bridge arm S2 corresponding to the diode D2 to be turned on is from U M -nU L The transformer voltage rises to prepare for diode commutation. The transformer voltage is the difference between the voltage of the bridge arm S1 and the voltage of the bridge arm S2 connected in series. As the voltage of the bridge arm S2 connected in series increases, the transformer voltage decreases and is less than nU L , the transformer current is maintained at 0, and the output current is zero; Working stage three, output zero current stage t2~t3: This period of time is the diode commutation time T q , diode D1 current i D1 From I M Reduced to 0; diode D2 current i D2 From 0 to I M ; The diode undergoes current commutation, and the voltage of the submodule series bridge arm S1 is U M -U T , the voltage of the bridge arm S2 of the submodule series connection is U M +U T , the two bridge arms provide commutation voltage, and the bridge arms jointly support U M ; Transformer voltage is less than nU L , the transformer current is maintained at 0, and the output current is zero; Working stage 4, output zero current stage t3~t4: This period of time is the bridge arm voltage change time T u , diode D1 is turned off, current i D1 is 0; diode D2 is turned on, current i D2 For I M ; The submodule corresponding to the conducting diode D2 is connected in series with the bridge arm S2 to support U M , the voltage of the submodule series bridge arm S1 corresponding to the turn-off diode D1 changes from U M Descend to U M +U T , prepare for transmission power; Working stage 5, power transmission stage t4~t7: Entering the second half cycle, normal power transmission, among which t4~t5 is the transformer current change stage, that is, the IGBT device Q2, IGBT device Q3 conduction current rising stage, the sub-module corresponding to the conduction diode D2 is connected in series with the bridge arm S2 to support U M , the submodule series bridge arm S1 is U M -nU L +U T , providing commutation voltage for current change; in the t5-t6 stage, the submodule series bridge arm S2 corresponding to the conducting diode D2 supports U M , transformer constant current - I L / n, the bridge arm voltage difference maintains the transformer voltage, and most of the power is transmitted to the low-voltage side transformer through the thyristor and transformer; t6~t7 is the transformer current change stage, that is, the IGBT device Q2, IGBT device Q3 conduction current decrease stage, the sub-module corresponding to the conduction diode D2 is connected in series with the bridge arm S2 to support U M , the transformer current is -I L / n drops to 0, and the drop time is the transformer current change time T c,trans , the submodule is connected in series with the bridge arm S1 to provide a current change driving voltage; Step 3: N power unit topology: For the input side of the topology, a parallel connection is adopted, and multiple power units are operated with phase shift in timing to maintain the continuity of the input current, or a series connection is adopted to evenly distribute the input side voltage; for the output side of the topology, an output parallel connection is adopted to evenly distribute the output side current, or an output series connection is adopted to evenly distribute the high voltage on the output side. Step 4: Design method of current waveform of N power units: The input current of a single power unit is intermittent and the output current is continuous. When the input current is zero, that is, during the zero current time of the transformer, the single power unit completes the bridge arm voltage rise and fall process and the diode commutation process. After N power units are connected in series on the input side, the N power units evenly distribute the input voltage, and the voltage stress of the IGBT device is reduced. After N power units are connected in parallel on the input side, when the output current of any power unit is zero, the other N-1 power units will evenly distribute the output current, and the output current of each power unit is I M / (n-1); After N power units are connected in parallel on the input side, the N converters are operated in phase shifting mode to ensure that when the transformer current of any power unit is zero, the other N-1 power units will evenly distribute the transformer current, and the phase shift angle is T / 2N.

4. The design method of a step-up DC transformer with high reliability according to claim 3 is characterized in that The phase shift angle satisfies: Where, T z is the output zero current time.

5. The design method of a step-up DC transformer with high reliability according to claim 3, characterized in that The output zero current time T z According to the following formula: T z =2T u +T q 。

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