Wide-range voltage-adjustable DC transformer and current waveform design method

Abstract

The application discloses a wide-range voltage-adjustable DC transformer and a current waveform design method. The method can effectively reduce the current stress of the DC transformer and improve the power capacity and voltage grade of the DC transformer by adopting N power units in series or parallel on the input side and in parallel on the output side. The transformer current of the N power units is designed to make the transformer current between the N power units sequentially phase-shift by T / 2N, so that the smooth and continuous DC side current can be realized without a filter, thereby reducing the cost and volume of the device. The application has the characteristics of high reliability and simple structure. The output voltage and transmission power are adjusted by controlling the voltage difference of the bridge arm of the sub-module, and the application has the characteristics of wide voltage adjustment range and fault current limiting. The structure of the N power units in parallel output can significantly improve the power and waveform quality.

Description

Technical Field

[0001] This invention belongs to the field of power electronics and relates to a DC transformer with wide-range voltage adjustment and a current waveform design method. Background Technology

[0002] In desert and deep-sea renewable energy generation and new types of power loads, the proportion of renewable energy generation and new power loads in the power system is increasing, profoundly affecting the structure and transmission and distribution methods of the new power system. Among them, DC grids, with their flexible power control, simple renewable energy access, and high reliability, have become the key solution for renewable energy transmission and distribution.

[0003] High-capacity DC transformers are core equipment in DC power grid applications, requiring flexible voltage regulation capabilities to adapt to a wide range of voltage loads and provide fault current limiting capabilities. To achieve high efficiency, existing DC transformers employ multiple resonant converter power units connected in series and parallel to achieve large capacity and high voltage levels. This approach requires dozens of expensive high-frequency transformers, each with insulation designed according to the high-voltage side voltage level, resulting in high cost and large size / weight. Furthermore, resonant converters typically operate at their resonant frequency, and voltage regulation via frequency adjustment has a narrow range, lacking wide-range voltage regulation capabilities.

[0004] To address the aforementioned issues, CN117254442A proposes a flexible voltage-adjustable medium-voltage DC transformer. By connecting a switched capacitor in series with the primary winding of the transformer, the voltage gain can be flexibly adjusted, achieving a wide voltage regulation range. However, this scheme requires an additional half-bridge circuit and energy storage capacitor on the primary winding, increasing the losses and cost of the DC transformer. CN113794381A proposes an SCDAB-CLLLC composite adjustable DC transformer, employing two power units connected in series at the input and parallel at the output. When the input voltage is low, the SCDAB power unit is bypassed, and the CLLLC achieves a small-range voltage regulation. When the input voltage is high, both power units operate simultaneously to extend the voltage regulation range. This scheme suffers from inconsistent power transmission between the two power units, a large number of components, low component utilization at low input voltages, and unnecessary cost increases. The cost becomes even more significant at higher voltage levels and larger power capacities.

[0005] In summary, DC transformers in DC power grids need to meet the characteristics of large capacity, high voltage, few components, high efficiency, and low cost, and in particular, they need to have a wide voltage regulation range. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a DC transformer with wide-range voltage regulation and a current waveform design method. This method features a small number of components, high efficiency, low cost, and wide-range output voltage regulation. Specifically, the wide-range voltage-regulating DC transformer can be designed as N units; phase-shifting operation between the N units can synthesize a smooth DC-side current while reducing current stress on the transformer and sub-module bridge arms. The sub-module bridge arms can control a high-quality DC current waveform and achieve wide output voltage range regulation, possessing short-circuit fault current limiting capability.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A power unit topology for a DC transformer with wide-range voltage adjustment includes an input inverter circuit, a single-phase transformer, and an output rectifier circuit, wherein:

[0009] The input inverter circuit includes thyristor T1, thyristor T2, bridge arm reactor L1, bridge arm reactor L2, sub-module series bridge arm S1, and sub-module series bridge arm S2.

[0010] The single-phase transformer includes 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 terminals of the primary and secondary sides are of the same name. The transformer has a turns ratio of n:1 and includes a leakage inductance L. k ;

[0011] The output rectifier circuit includes IGBT device Q1, IGBT device Q2, IGBT device Q3, and IGBT device Q4;

[0012] The anode of thyristor T1 is connected to the first terminal of bridge arm reactor L1, and the cathode of thyristor T1 is connected to the positive terminal of submodule series bridge arm S1, forming the first bridge arm of the input inverter circuit; the anode of thyristor T2 is connected to the first terminal of bridge arm reactor L2, and the cathode of thyristor T2 is connected to the positive terminal of submodule series bridge arm S2, forming the second bridge arm of the input inverter circuit; one end of the medium voltage DC input port is connected to the second terminal of bridge arm reactor L1 and the second terminal of bridge arm reactor L2, and the other end of the medium voltage DC input port is connected to the negative terminal of submodule series bridge arm S1 and the negative terminal of submodule series bridge arm S2;

[0013] The first terminal on the primary side of the transformer is connected to the middle node of the series bridge arm S1 of the thyristor T1 and the submodule, and the second terminal on the primary side of the transformer is connected to the middle node of the series bridge arm S2 of the thyristor T2 and the submodule.

[0014] The diode anode in IGBT device Q1 is connected to the diode cathode in IGBT device Q3, forming the first bridge arm of the output rectifier circuit; the diode anode in IGBT device Q2 is connected to the diode cathode in IGBT device Q4, forming the second bridge arm of the output rectifier circuit.

[0015] The first terminal on the secondary side of the transformer is connected to the intermediate node of IGBT device Q1 and IGBT device Q3, and the second terminal on the secondary side of the transformer is connected to the intermediate node of IGBT device Q2 and IGBT device Q4.

[0016] The diode cathodes of IGBT device Q1 and IGBT device Q2 are connected together and connected to the positive terminal of the low-voltage DC output of the power unit; the diode anodes of IGBT device Q3 and IGBT device Q4 are connected together and connected to the negative terminal of the low-voltage DC output of the power unit.

[0017] A DC transformer with wide-range voltage adjustment and a current waveform design method, comprising the following steps:

[0018] Step 1: Power Unit Topology Design of a Wide-Range Adjustable DC Transformer

[0019] The power unit of a wide-range adjustable DC transformer includes an input inverter circuit, a single-phase transformer, and an output rectifier circuit, wherein:

[0020] The input inverter circuit includes thyristor T1, thyristor T2, bridge arm reactor L1, bridge arm reactor L2, sub-module series bridge arm S1, and sub-module series bridge arm S2.

[0021] The single-phase transformer includes 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 terminals of the primary and secondary sides are of the same name. The transformer has a turns ratio of n:1 and includes a leakage inductance L. k ;

[0022] The output rectifier circuit includes IGBT device Q1, IGBT device Q2, IGBT device Q3, and IGBT device Q4;

[0023] The anode of thyristor T1 is connected to the first terminal of bridge arm reactor L1, and the cathode of thyristor T1 is connected to the positive terminal of submodule series bridge arm S1, forming the first bridge arm of the input inverter circuit; the anode of thyristor T2 is connected to the first terminal of bridge arm reactor L2, and the cathode of thyristor T2 is connected to the positive terminal of submodule series bridge arm S2, forming the second bridge arm of the input inverter circuit; one end of the medium voltage DC input port is connected to the second terminal of bridge arm reactor L1 and the second terminal of bridge arm reactor L2, and the other end of the medium voltage DC input port is connected to the negative terminal of submodule series bridge arm S1 and the negative terminal of submodule series bridge arm S2;

[0024] The first terminal on the primary side of the transformer is connected to the middle node of the series bridge arm S1 of the thyristor T1 and the submodule, and the second terminal on the primary side of the transformer is connected to the middle node of the series bridge arm S2 of the thyristor T2 and the submodule.

[0025] The diode anode in IGBT device Q1 is connected to the diode cathode in IGBT device Q3, forming the first bridge arm of the output rectifier circuit; the diode anode in IGBT device Q2 is connected to the diode cathode in IGBT device Q4, forming the second bridge arm of the output rectifier circuit.

[0026] The first terminal on the secondary side of the transformer is connected to the intermediate node of IGBT device Q1 and IGBT device Q3, and the second terminal on the secondary side of the transformer is connected to the intermediate node of IGBT device Q2 and IGBT device Q4.

[0027] The diode cathodes of IGBT device Q1 and IGBT device Q2 are connected together and connected to the positive terminal of the low-voltage DC output of the power unit; the diode anodes of IGBT device Q3 and IGBT device Q4 are connected together and connected to the negative terminal of the low-voltage DC output of the power unit.

[0028] Step 2: Control of the power unit:

[0029] Part 1: Thyristor T1 and thyristor T2 are turned on for half a control cycle, and each thyristor maintains a fixed turn-on sequence. The working processes of submodule series bridge arm S1 and submodule series bridge arm S2 differ by half a control cycle. The output voltage and transmission power are adjusted by changing the voltage of the submodule series bridge arm.

[0030] Part Two: Let a work cycle be T, and divide it into six work stages, corresponding to seven work nodes, namely t0~t6, I M For medium-voltage DC input current, I L The output current is low-voltage DC, and the specific operating stages are as follows:

[0031] (t0~t1): During the power transmission phase, the submodule series bridge arm S1 corresponding to the conducting thyristor T1 supports U. M Transformer constant current I L / n, the voltage difference between the bridge arms maintains the transformer voltage, and most of the power is transferred to the low-voltage side via thyristors and the transformer; when the transformer current changes, the submodule corresponding to thyristor T1 is connected in series with the bridge arm S1 to support U. M The transformer current is supplied by I. L / n drops to 0, and the drop time is the transformer current change time T. c,trans The corresponding branch submodule series bridge arm S1 (transformer current flows into the bridge arm) provides the current change driving voltage;

[0032] (t1~t2): Output zero current stage, this period is the bridge arm voltage change time T. u ; Thyristor T1 is turned on, current i T1 Also for I M Thyristor T2 is scheduled to turn on, current i T2 It is still 0; the submodule series bridge arm S1 corresponding to the conducting thyristor T1 supports U. M The proposed thyristor T2 corresponding to the branch bridge arm S2 voltage is to be turned on from U. M -nU L The voltage rises to prepare for thyristor commutation. The transformer voltage is the difference between the voltage of submodule series bridge arm S1 and the voltage of submodule series bridge arm S2. As the voltage of branch submodule series bridge arm S2 rises, the transformer voltage decreases and becomes less than nU. L The transformer current remains at 0, resulting in zero output current.

[0033] (t2~t3): Zero current output stage; this period is the thyristor commutation time T. q ; Thyristor T1 current i T1 From I M Reduced to 0; thyristor T2 current i T2 From 0 to I M The thyristor undergoes commutation, and the voltage of the series bridge arm S1 of the submodule is U. M +U T The voltage of the submodule series bridge arm S2 is U M -U T The two sub-modules connected in series provide the commutation voltage, and the sub-modules connected in series with the bridge arm together support U. M The transformer voltage is less than nU. L The transformer current remains at 0, resulting in zero output current.

[0034] (t3~t4): Zero current output stage; this period is the thyristor reverse voltage time T. c,Thy When thyristor T1 is turned off, the current i T1 The current is 0; thyristor T2 is turned on, and the current iT2 For I M ; Submodule series bridge arm S1 voltage holding U M +U T This provides reverse voltage to thyristor T1, which can reliably turn off under reverse voltage. The voltage of submodule series bridge arm S1 is U. M The transformer voltage is less than nU L The transformer current remains at 0, resulting in zero output current.

[0035] (t4~t5): Output zero current stage, this period is the time T of bridge arm voltage change. u When thyristor T1 is turned off, the current i T1 The current is 0; thyristor T2 is turned on, and the current i T2 For I M The submodule corresponding to the conducting thyristor T2, series bridge arm S2, supports U. M The voltage of the series bridge arm S1 corresponding to the turned-off thyristor T1 is from U M Descending to U M -U T To prepare for power transmission;

[0036] Output zero current time T z It can be obtained from the following formula:

[0037] T z =T c,Thy +2T u +T q (1)

[0038] (t5~t6): Power transmission phase, entering the second half of the cycle, normal power transmission; the submodule series bridge arm S2 corresponding to the conducting thyristor T2 supports U. M Transformer constant current - I L / n, the voltage difference between the bridge arms maintains the transformer voltage; most of the power is transferred to the low-voltage side transformer via thyristors and the transformer; when the current changes, the submodule corresponding to thyristor T2 is connected in series with the bridge arm S2 to support U. M The transformer current is controlled by -I. L / n rises to 0, and the rise time is the transformer current change time T. c,trans The corresponding branch submodule series bridge arm S2 (transformer current flows into the bridge arm) provides the current change driving voltage.

[0039] Step 3: Topology Design of N Power Units

[0040] Wide-range adjustable DC transformers use N identical power units to improve power transmission. For the input side of the topology, when operating in a boost configuration, the large input current can be evenly distributed using parallel connections; when operating in a buck configuration, the large input voltage can be evenly distributed using series connections. For the output side of the topology, parallel connections are always used, and multiple power units operate with phase shifting in timing to maintain continuous output current.

[0041] Step 4: Design of current waveforms for N power units

[0042] This invention addresses the proposed wide-range adjustable DC transformer by designing a current waveform method when using N identical power units. Each power unit must complete the bridge arm voltage rise and fall process, thyristor commutation, and thyristor reverse-voltage turn-off process within the zero-current output time. By using N power units connected in parallel on the output side, when the output current of any one power unit is zero, the other N-1 power units will evenly distribute the output current instead of taking turns bearing it. Therefore, the output current of each power unit is I. L / (n-1), instead of I L Therefore, the current stress of the DC transformer with wide-range voltage adjustment is reduced. After N power units are connected in parallel on the output side, N converters must operate with phase shifting to ensure that when the output current of any one power unit is zero, the other N-1 power units will evenly distribute the output current. The phase shift angle is T / 2N, and the following conditions must be met:

[0043]

[0044] Step 5: Wide-range voltage regulation design of DC transformers

[0045] The DC transformer with wide-range voltage regulation proposed in this invention has a wide-range voltage regulation capability. The voltage of the sub-module bridge arm can be flexibly adjusted, and the voltage difference between the two sub-modules connected in series bridge arms is the transformer voltage amplitude nU. L The transformer voltage amplitude nU L With output voltage U L Since the sizes are matched, the voltage regulation function of the DC transformer can be achieved by adjusting the voltage difference between the bridge arms. This utilizes the flexible voltage regulation characteristics of the submodules connected in series to achieve a wide range of voltage regulation. For N power units, the specific voltage regulation process is as follows:

[0046] During the t0 to t1 phase, thyristor T1 is turned on, and the submodule series bridge arm S1 supports the input voltage U. M The voltage of the submodule series bridge arm S2 is U M -nU L Therefore, the voltage difference between the bridge arms is nU. LBy adjusting the voltage U of the series bridge arm S2 of the submodule M -nU L The size of nU can change L The size of U is thus changed. L The size of the voltage is adjusted to achieve the voltage regulation function;

[0047] During the t1 to t5 stage, power unit one is in the zero-current output stage, and the voltage difference between submodule series bridge arm S1 and submodule series bridge arm S2 is less than nU. L Power unit one cannot adjust the voltage; the output voltage adjustment is achieved by the remaining N-1 power units. Since the N power units are phase-shifted, the remaining N-1 power units are in the same working state as power unit one in the t0~t1 stage, thus realizing the voltage regulation function of the DC transformer.

[0048] During the t5 to t6 phase, thyristor T2 is turned on, and the submodule series bridge arm S2 supports the input voltage U. M The voltage of the series bridge arm S1 of the submodule is U. M -nU L Therefore, the voltage difference between the bridge arms is -nU. L After passing through the output rectifier circuit, the output voltage is U. L By adjusting the voltage U of the series bridge arm S1 of the submodule M -nU L The size of U can be changed. L The size of the voltage is adjusted to achieve the voltage regulation function.

[0049] In this invention, the thyristors T1 and T2 in the input inverter circuit can be composed of several thyristors connected in series. T1 and T2 can also be composed of one or more of the following devices in series or a combination of series and parallel: thyristors, insulated gate bipolar transistors (IGBTs), MOSFETs, injection enhancement gate transistors (IEGTs), integrated gate commutation thyristors (IGCTs), anti-parallel thyristors, and reverse-conducting thyristors.

[0050] In this invention, the sub-module series bridge arm S1 and sub-module series bridge arm S2 in the input inverter circuit can be composed of several half-bridge sub-modules connected in series, and the sub-module series bridge arm S1 and sub-module series bridge arm S2 can also be composed of full-bridge sub-modules connected in series, half-bridge and full-bridge hybrid sub-modules connected in series, or other feasible sub-module structures connected in series.

[0051] In this invention, the IGBT devices Q1, Q2, Q3, and Q4 in the output rectifier circuit can also be composed of one or more of the following devices in series or a combination of series and parallel: diodes, thyristors, MOSFETs, injection enhancement gate transistors (IEGT), integrated gate commutation thyristors (IGCT), anti-parallel thyristors, and reverse-conducting thyristors.

[0052] Compared with the prior art, the present invention has the following advantages:

[0053] 1. Compared with the existing series-parallel structure of resonant converters, the present invention uses sub-module bridge arms to adjust the output voltage, which has the advantage of a wide voltage adjustment range.

[0054] 2. Compared with the existing series-parallel structure of resonant converters, this invention eliminates a large number of expensive high-frequency transformers and uses a few large-capacity intermediate frequency transformers, which has the advantage of high reliability.

[0055] 3. Compared with existing DC transformers with voltage regulation capabilities, this invention does not require an additional voltage regulation stage, and has fewer power units, saving a large number of semiconductor devices. It has the advantages of high cost, low device loss, and simple structure.

[0056] 4. By employing N power units connected in series or parallel on the input side and in parallel on the output side, this invention can effectively reduce the current stress of the DC transformer and improve the power capacity and voltage level of the DC transformer.

[0057] 5. This invention uses the transformer current of N power units for design, so that the transformer current of the N power units is shifted by a phase of T / 2N in sequence, which can realize a smooth and continuous DC side current, eliminating the need for a filter and reducing the cost and size of the device.

[0058] 6. In this invention, the input inverter circuit and output rectifier circuit use thyristors and IGBTs respectively, which can be replaced with semiconductor devices of any load power level, and have the characteristics of high reliability and simple structure; the output voltage and transmission power are adjusted by controlling the voltage difference of the sub-module bridge arm, which has the characteristics of wide voltage regulation range and fault current limiting; the structure of N power unit outputs in parallel can significantly improve power and waveform quality. Attached Figure Description

[0059] Figure 1 This is a schematic diagram of the DC transformer power unit topology circuit with wide-range voltage adjustment proposed in this invention;

[0060] Figure 2 The thyristor drive timing and main voltage and current waveforms of the DC transformer power unit with wide voltage adjustment range proposed in this invention;

[0061] Figure 3 This is a schematic diagram of the current path of the DC transformer power unit with wide voltage adjustment range proposed in this invention during the first stage of operation.

[0062] Figure 4 This is a schematic diagram of the current path of the DC transformer power unit with wide voltage adjustment range proposed in this invention during the second stage of operation.

[0063] Figure 5 This is a schematic diagram of the current path of the DC transformer power unit with wide voltage adjustment range proposed in this invention during the third stage of operation.

[0064] Figure 6 This is a schematic diagram of the current path of the DC transformer power unit with wide voltage adjustment range proposed in this invention during the fourth stage of operation.

[0065] Figure 7 This is a schematic diagram of the current path in the fifth stage of operation of the DC transformer power unit with wide voltage adjustment range proposed in this invention.

[0066] Figure 8 This is a schematic diagram of the current path of the DC transformer power unit with wide voltage adjustment range proposed in this invention during the sixth stage of operation.

[0067] Figure 9 The circuit diagram shows the topology of the DC transformer with wide voltage adjustment proposed in this invention, in which the inputs are connected in series and the outputs are connected in parallel.

[0068] Figure 10 The waveforms of the transformer current and output current of the DC transformer with wide voltage adjustment proposed in this invention are shown in the figure. The inputs of the two power units are connected in series and the outputs are connected in parallel.

[0069] Figure 11 The circuit diagram shows the topology of the DC transformer with wide voltage adjustment proposed in this invention, in which the inputs are connected in series and the outputs are connected in parallel.

[0070] Figure 12 The waveforms of the transformer current and output current of the DC transformer with wide voltage adjustment proposed in this invention are shown in the figure. The inputs of the three power units are connected in series and the outputs are connected in parallel.

[0071] Figure 13 The circuit diagram shows the topology of the DC transformer with wide voltage adjustment proposed in this invention, in which the four power units are connected in series at the input and in parallel at the output.

[0072] Figure 14 The waveforms of the transformer current and output current of the DC transformer with wide voltage adjustment proposed in this invention are shown in the figure. The four power units are connected in series at the input and in parallel at the output.

[0073] Figure 15The circuit diagram shows the topology of the DC transformer with wide voltage adjustment proposed in this invention, in which N power units are connected in series at the input and in parallel at the output. Detailed Implementation

[0074] The technical solution of the present invention will be further described below with reference to the accompanying drawings, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention that do not depart from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention.

[0075] Note: When multiple power units are used, they are referred to as Power Unit 1, Power Unit 2, and so on.

[0076] Example 1: Two power units are connected in series at the input and in parallel at the output.

[0077] The power unit of the DC transformer with wide voltage adjustment proposed in this invention is as follows: Figure 9 As shown, the DC transformer with wide-range voltage adjustment includes two power units. 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.

[0078] Taking power unit one as an example, the input inverter circuit includes thyristor T1, thyristor T2, bridge arm reactor L1, bridge arm reactor L2, submodule series bridge arm S1, and submodule series bridge arm S2; the single-phase transformer includes a primary side and a secondary side, wherein the primary side of the transformer includes a first terminal and a second terminal, and the secondary side of the transformer includes a first terminal and a second terminal, and the first terminal of the primary side and the first terminal of the secondary side of the transformer are of the same name, the transformer turns ratio is n:1, and it includes leakage inductance L. k The output rectifier circuit includes IGBT device Q1, IGBT device Q2, IGBT device Q3, and IGBT device Q4.

[0079] The anode of thyristor T1 is connected to the first terminal of bridge arm reactor L1, and the cathode of thyristor T1 is connected to the positive terminal of sub-module series bridge arm S1, forming the first bridge arm of the input inverter circuit; the anode of thyristor T2 is connected to the first terminal of bridge arm reactor L2, and the cathode of thyristor T2 is connected to the positive terminal of sub-module series bridge arm S2, forming the second bridge arm of the input inverter circuit.

[0080] One end of the medium-voltage DC input port of the power unit is connected to the second terminal of bridge arm reactor L1 and the second terminal of bridge arm reactor L2, and the other end is connected to the negative terminal of submodule series bridge arm S1 and the negative terminal of submodule series bridge arm S2.

[0081] The first terminal on the primary side of the single-phase transformer is connected to the intermediate node of the thyristor T1 and the sub-module series bridge arm S1; the second terminal on the primary side of the single-phase transformer is connected to the intermediate node of the thyristor T2 and the sub-module series bridge arm S2.

[0082] The diode anode in IGBT device Q1 is connected to the diode cathode in IGBT device Q3, forming the first bridge arm of the output rectifier circuit; the diode anode in IGBT device Q2 is connected to the diode cathode in IGBT device Q4, forming the second bridge arm of the output rectifier circuit.

[0083] The first terminal on the secondary side of the single-phase transformer is connected to the intermediate node between IGBT devices Q1 and Q3; the second terminal on the secondary side of the single-phase transformer is connected to the intermediate node between IGBT devices Q2 and Q4.

[0084] The diode cathodes of IGBT device Q1 and IGBT device Q2 are connected together and connected to the positive terminal of the low-voltage DC output of the power unit; the diode anodes of IGBT device Q3 and IGBT device Q4 are connected together and connected to the negative terminal of the low-voltage DC output of the power unit.

[0085] In this embodiment, the circuits and components of the power unit two are the same as those of the power unit one, and the internal circuit connection methods of the power unit two and the power unit one are the same. That is, the connection methods of the input inverter circuit, single-phase transformer and output rectifier circuit in the power unit two are consistent with those in the power unit one.

[0086] In this embodiment, the input terminal of the DC transformer with wide voltage adjustment range has four terminals, namely the first terminal N. i1 Second terminal N i2 Third terminal N i3 and the fourth terminal N i4 The low-voltage DC output terminal also contains 4 terminals, namely the first terminal N. o1 Second terminal N o2 Third terminal N o3 and the fourth terminal N o4 .

[0087] In this embodiment, the input side of the DC transformer with wide voltage adjustment range is connected in series, that is, the second terminal N i2 With the third terminal N i3 Connected, first terminal N i1 As the positive terminal of the DC medium voltage input, the fourth terminal N i4 As the negative terminal of the DC medium-voltage input, each unit shares the higher voltage on the medium-voltage side; the output side adopts parallel connection, that is, the first terminal N o1 With the third terminal No3 Connected to the positive terminal of the low-voltage DC output, the second terminal N o2 With the fourth terminal N o4 Connected to serve as the negative terminal of the low-voltage DC output.

[0088] The thyristor drive timing and main voltage and current waveforms of the wide-range adjustable DC transformer unit proposed in this embodiment are as follows: Figure 2 As shown, taking power unit one as an example, thyristors T1 and T2 are turned on for half a control cycle, and each thyristor maintains a fixed turn-on sequence. The operation of submodule series bridge arm S1 and submodule series bridge arm S2 differs by half a control cycle. The output voltage and transmitted power are adjusted by changing the voltage of the submodule series bridge arm. Power unit two also uses the same control method. Among them, I M i represents the current after the medium-voltage AC is rectified by the input rectifier circuit of the converter. T1 and i T2 The currents of thyristor valve group T1 and thyristor valve group T2 in converter one are respectively, i t1 The transformer current, u, is the current in power unit one. S1 and u S2 These are the voltages of submodule series bridge arm S1 and submodule series bridge arm S2 in converter one, respectively. O1 and i O2 These are the output currents of power unit one and power unit two, respectively. Assuming one operating cycle is T, with a time period from t0 to t6, taking converter one as an example, the specific operating stages are as follows:

[0089] Work phase one: (t0~t1), such as Figure 3 As shown, in this stage, the submodule series bridge arm S1 corresponding to the conducting thyristor T1 supports U. M Transformer constant current I L / n, the voltage difference between the bridge arms maintains the transformer voltage, and most of the power is transferred to the low-voltage side via thyristors and the transformer. When the transformer current changes, the submodule corresponding to thyristor T1 is turned on, and the series bridge arm S1 supports U. M The transformer current is supplied by I. L / n drops to 0, and the drop time is the transformer current change time T. c,trans The corresponding branch submodule series bridge arm S1 (transformer current flows into the bridge arm) provides the current change driving voltage.

[0090] Phase 2 of work: (t1~t2), such as Figure 4 As shown, this time period is the time T of the bridge arm voltage change. u When thyristor T1 is turned on, the current i T1 Also for I M Thyristor T2 is scheduled to turn on, current i T2It is still 0. The submodule corresponding to the conducting thyristor T1, in series, supports U via bridge arm S1. M The proposed activation of the voltage across the series bridge arm S2 of the submodule corresponding to thyristor T2 is based on the voltage from U. M -nU L The voltage rises to prepare for thyristor commutation. The transformer voltage is the difference between the voltage of submodule series bridge arm S1 and the voltage of submodule series bridge arm S2. As the voltage of branch submodule series bridge arm S2 rises, the transformer voltage decreases and becomes less than nU. L The transformer current remains at 0, resulting in zero output current.

[0091] Work phase three: (t2~t3), such as Figure 5 As shown, this period is the thyristor commutation time T. q Thyristor T1 current i T1 From I M Reduced to 0; thyristor T2 current i T2 From 0 to I M The thyristor undergoes commutation, and the voltage of the series bridge arm S1 of the submodule is U. M +U T The voltage of the submodule series bridge arm S2 is U M -U T The two bridge arms provide the commutation voltage and together support U. M The transformer voltage is less than nU. L The transformer current remains at 0, resulting in zero output current.

[0092] Working phase four: (t3~t4), such as Figure 6 As shown, this period is the thyristor reverse voltage time T. c,Thy When thyristor T1 is turned off, the current i T1 The current is 0; thyristor T2 is turned on, and the current i T2 For I M The voltage of the submodule series bridge arm S1 is maintained by U. M +U T This provides reverse voltage to thyristor T1, which can reliably turn off under reverse voltage. The voltage of submodule series bridge arm S1 is U. M The transformer voltage is less than nU L The transformer current remains at 0, resulting in zero output current.

[0093] Work phase five: (t4~t5), such as Figure 7 As shown, this time period is the time T of the bridge arm voltage change. u When thyristor T1 is turned off, the current i T1 The current is 0; thyristor T2 is turned on, and the current i T2 For I M The submodule corresponding to the conducting thyristor T2 is connected in series with the bridge arm S2, which supports U. MThe voltage of the series bridge arm S1 corresponding to the turned-off thyristor T1 is from U M Descending to U M -U T This is to prepare for power transmission.

[0094] Output zero current time T z It can be obtained from the following formula

[0095] T z =T c,Thy +2T u +T q (3)

[0096] Work phase six: (t5~t6), such as Figure 8 As shown, this stage enters the second half of the cycle, with normal power transmission. The submodule corresponding to the conducting thyristor T2, in series with the bridge arm S2, supports U. M Transformer constant current - I L / n, the voltage difference between the bridge arms maintains the transformer voltage, and most of the power is transferred to the low-voltage side transformer via thyristors and the transformer. When the current changes, the submodule corresponding to thyristor T2 is connected in series with the bridge arm S2 to support U. M The transformer current is controlled by -I. L / n rises to 0, and the rise time is the transformer current change time T. c,trans The corresponding branch submodule series bridge arm S2 (transformer current flows into the bridge arm) provides the current change driving voltage.

[0097] When power unit one and power unit two have their inputs connected in series and their outputs connected in parallel, the topology is as follows: Figure 9 As shown, the corresponding transformer current and output current waveforms are as follows: Figure 10 As shown. Power unit one and power unit two operate alternately. During the time when one transformer has zero current and zero output current, the other converter outputs current. The trapezoidal waves of the output current alternate for T / 4, and the combined amplitude is I. L It provides smooth DC current without the need for a filter.

[0098] Example 2: Three power units are connected in series at the input and in parallel at the output.

[0099] When power unit 1, power unit 2, and power unit 3 have their inputs connected in series and their outputs connected in parallel, the topology is as follows: Figure 11 As shown, the corresponding transformer current and output current waveforms are as follows: Figure 12 As shown. The three power units operate alternately. During the zero-current period of one transformer, the other two converters evenly distribute the output current, resulting in staggered trapezoidal waves of output current with a combined amplitude of I. L The smooth DC current of / 2 eliminates the need for filters and reduces current stress on bridge arms and transformers.

[0100] When N power units are connected in series at their inputs and in parallel at their outputs, the topology is as follows: Figure 15 As shown. N converters operate alternately. During the zero-current period of one transformer, the output current is evenly distributed among the other (N-1) converters. The trapezoidal waves of the output current are interleaved with T / (2N), and the combined amplitude is I. L The smooth DC current of / (N-1) eliminates the need for filters and reduces current stress on bridge arms and transformers.

Claims

1. A method for designing the current waveform of a DC transformer with wide-range voltage adjustment, characterized in that... The method includes the following steps: Step 1: Power Unit Topology Design of a Wide-Range Adjustable DC Transformer The power unit of a wide-range adjustable DC transformer includes an input inverter circuit, a single-phase transformer, and an output rectifier circuit, wherein: The input inverter circuit includes thyristor T1, thyristor T2, bridge arm reactor L1, bridge arm reactor L2, sub-module series bridge arm S1, and sub-module series bridge arm S2. The single-phase transformer includes 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 terminals of the primary and secondary sides are of the same name. The transformer has a turns ratio of n:1 and includes a leakage inductance L. k ; The output rectifier circuit includes IGBT device Q1, IGBT device Q2, IGBT device Q3, and IGBT device Q4; The anode of thyristor T1 is connected to the first terminal of bridge arm reactor L1, and the cathode of thyristor T1 is connected to the positive terminal of submodule series bridge arm S1, forming the first bridge arm of the input inverter circuit; the anode of thyristor T2 is connected to the first terminal of bridge arm reactor L2, and the cathode of thyristor T2 is connected to the positive terminal of submodule series bridge arm S2, forming the second bridge arm of the input inverter circuit; one end of the medium voltage DC input port is connected to the second terminal of bridge arm reactor L1 and the second terminal of bridge arm reactor L2, and the other end of the medium voltage DC input port is connected to the negative terminal of submodule series bridge arm S1 and the negative terminal of submodule series bridge arm S2; The first terminal on the primary side of the transformer is connected to the middle node of the series bridge arm S1 of the thyristor T1 and the submodule, and the second terminal on the primary side of the transformer is connected to the middle node of the series bridge arm S2 of the thyristor T2 and the submodule. The diode anode in IGBT device Q1 is connected to the diode cathode in IGBT device Q3, forming the first bridge arm of the output rectifier circuit; the diode anode in IGBT device Q2 is connected to the diode cathode in IGBT device Q4, forming the second bridge arm of the output rectifier circuit. The first terminal on the secondary side of the transformer is connected to the intermediate node of IGBT device Q1 and IGBT device Q3, and the second terminal on the secondary side of the transformer is connected to the intermediate node of IGBT device Q2 and IGBT device Q4. The diode cathodes of IGBT device Q1 and IGBT device Q2 are connected together and connected to the positive terminal of the low-voltage DC output of the power unit; the diode anodes of IGBT device Q3 and IGBT device Q4 are connected together and connected to the negative terminal of the low-voltage DC output of the power unit. Step 2: Control of the power unit: Part 1: Thyristor T1 and thyristor T2 are turned on for half a control cycle, and each thyristor maintains a fixed turn-on sequence. The working processes of submodule series bridge arm S1 and submodule series bridge arm S2 differ by half a control cycle. The output voltage and transmission power are adjusted by changing the voltage of the submodule series bridge arm. Part Two: Let a work cycle be T, and divide it into six work stages, corresponding to seven work nodes, t0~t6, I. M For medium-voltage DC input current, I L The output current is low-voltage DC, and the specific operating stages are as follows: t0~t1: During the power transmission phase, the submodule series bridge arm S1 corresponding to the conducting thyristor T1 supports U M Transformer constant current I L / n, the voltage difference between the bridge arms maintains the transformer voltage, and most of the power is transferred to the low-voltage side via thyristors and the transformer; when the transformer current changes, the submodule corresponding to thyristor T1 is connected in series with the bridge arm S1 to support U. M The transformer current is supplied by I. L / n drops to 0, and the drop time is the transformer current change time T. c,trans The corresponding branch submodule series bridge arm S1 provides the current change drive voltage; t1~t2: Output zero current stage, this period is the time T of bridge arm voltage change. u ; Thyristor T1 is turned on, current i T1 Also for I M Thyristor T2 is to be turned on, current i T2 It is still 0; the submodule series bridge arm S1 corresponding to the conducting thyristor T1 supports U. M The proposed thyristor T2 corresponding to the branch bridge arm S2 voltage is to be turned on from U. M -nU L The voltage rises to prepare for thyristor commutation. The transformer voltage is the difference between the voltage of submodule series bridge arm S1 and the voltage of submodule series bridge arm S2. As the voltage of branch submodule series bridge arm S2 rises, the transformer voltage decreases and becomes less than nU. L The transformer current remains at 0, resulting in zero output current. t2~t3: Zero current output stage, this period is the thyristor commutation time T. q ; Thyristor T1 current i T1 From I M Reduced to 0; thyristor T2 current i T2 From 0 to I M The thyristor undergoes commutation, and the voltage of the series bridge arm S1 of the submodule is U. M +U T The voltage of the submodule series bridge arm S2 is U M -U T The two sub-modules connected in series provide the commutation voltage, and the sub-modules connected in series with the bridge arm together support U. M The transformer voltage is less than nU. L The transformer current remains at 0, resulting in zero output current. t3~t4: Zero current output stage, this period is the thyristor reverse voltage time T c,Thy When thyristor T1 is turned off, the current i T1 The current is 0; thyristor T2 is turned on, and the current i T2 For I M ; Submodule series bridge arm S1 voltage holding U M +U T This provides reverse voltage to thyristor T1, which can reliably turn off under reverse voltage. The voltage of submodule series bridge arm S1 is U. M The transformer voltage is less than nU L The transformer current remains at 0, resulting in zero output current. t4~t5: Output zero current stage, this period is the time T of bridge arm voltage change. u When thyristor T1 is turned off, the current i T1 The current is 0; thyristor T2 is turned on, and the current i T2 For I M The submodule corresponding to the conducting thyristor T2, series bridge arm S2, supports U. M The voltage of the series bridge arm S1 corresponding to the turned-off thyristor T1 is from U M Descending to U M -U T To prepare for power transmission; t5~t6: Power transmission phase, entering the second half of the cycle, normal power transmission; the submodule series bridge arm S2 corresponding to the conducting thyristor T2 supports U. M Transformer constant current - I L / n, the voltage difference between the bridge arms maintains the transformer voltage; most of the power is transferred to the low-voltage side transformer via thyristors and the transformer; when the current changes, the submodule corresponding to thyristor T2 is connected in series with the bridge arm S2 to support U. M The transformer current is controlled by -I. L / n rises to 0, and the rise time is the transformer current change time T. c,trans The corresponding branch submodule series bridge arm S2 provides the current change driving voltage; Step 3: Topology Design of N Power Units Wide-range adjustable DC transformers use N identical power units to improve transmission power. For the input side of the topology, when the topology operates in a boost mode, the large current on the input side is evenly distributed by parallel connection of the inputs. When the topology operates in a buck mode, the large voltage on the input side is evenly distributed by series connection of the inputs. For the output side of the topology, parallel connection is always used, and multiple power units operate in phase shifting order to maintain continuous output current. Step 4: Design of current waveforms for N power units When N power units are connected in parallel on the output side, if the output current of any one power unit is zero, the other N-1 power units will distribute the output current equally, and the output current of each power unit will be I. L / (n-1), the phase shift angle is T / 2N, and it must satisfy: Step 5: Wide-range voltage regulation design of DC transformers For N power units, the specific voltage regulation process is as follows: During the t0~t1 phase, thyristor T1 is turned on, and the submodule series bridge arm S1 supports the input voltage U. M The voltage of the submodule series bridge arm S2 is U M -nU L Therefore, the voltage difference between the bridge arms is nU. L By adjusting the voltage U of the series bridge arm S2 of the submodule M -nU L The size of nU can change L The size of U is thus changed. L The size of the voltage is adjusted to achieve the voltage regulation function; During the t1~t5 stage, power unit one is in the zero-current output stage, and the voltage difference between submodule series bridge arm S1 and submodule series bridge arm S2 is less than nU. L Power unit one cannot adjust the voltage; the output voltage adjustment is achieved by the remaining N-1 power units. Since the N power units are phase-shifted, the remaining N-1 power units are in the same working state as power unit one in the t0~t1 stage, thus realizing the voltage regulation function of the DC transformer. During the t5~t6 phase, thyristor T2 is turned on, and the submodule series bridge arm S2 supports the input voltage U. M The voltage of the series bridge arm S1 of the submodule is U. M -nU L Therefore, the voltage difference between the bridge arms is -nU. L After passing through the output rectifier circuit, the output voltage is U. L By adjusting the voltage U of the series bridge arm S1 of the submodule M -nU L The size of U can be changed. L The size of the voltage is adjusted to achieve the voltage regulation function.

2. The current waveform design method for a wide-range adjustable DC transformer according to claim 1, characterized in that... The thyristors T1 and T2 are composed of several thyristors connected in series.

3. The current waveform design method for a wide-range adjustable DC transformer according to claim 1, characterized in that... The thyristors T1 and T2 are composed of one or more of the following: thyristors, insulated gate bipolar transistors (IGBTs), MOSFETs, injection enhancement gate transistors (IEGTs), integrated gate commutation thyristors (IGCTs), anti-parallel thyristors, and reverse-conducting thyristors, in series or a combination of series and parallel configurations.

4. The current waveform design method for a wide-range adjustable DC transformer according to claim 1, characterized in that... The sub-module series bridge arm S1 and sub-module series bridge arm S2 in the input inverter circuit are composed of several half-bridge sub-modules connected in series, full-bridge sub-modules connected in series, or a mixture of half-bridge and full-bridge sub-modules connected in series.

5. The current waveform design method for a wide-range adjustable DC transformer according to claim 1, characterized in that... The IGBT devices Q1, Q2, Q3, and Q4 are composed of one or more of the following: diodes, thyristors, MOSFETs, injection enhancement gate transistors (IEGT), integrated gate commutation thyristors (IGCT), anti-parallel thyristors, and reverse-conducting thyristors, in series or a combination of series and parallel configurations.

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