Series-parallel direct-current transformer and control method thereof

Abstract

The invention provides a series-parallel direct-current transformer and a control method thereof. Wherein the series-parallel direct-current transformer comprises a first low-voltage conversion unit, a second low-voltage conversion unit, an alternating-current isolation unit and a high-voltage conversion unit, the first low-voltage conversion unit comprises a plurality of first bridge arms, and each first bridge arm comprises a current source type switching device or a plurality of current source type switching devices which are sequentially connected in series; one end of one part of the first bridge arms is connected to the positive electrode of a power grid, the other end is connected with the first end of the first voltage side of the AC isolation unit, one end of the other part of the first bridge arms is connected with the other end of one part of the first bridge arms, and the other end is connected to the negative electrode of the power grid; the AC end of the second low-voltage conversion unit is connected with the second end of the first voltage side of the AC isolation unit. The size of the series-parallel direct-current transformer can be reduced, the occupied area of the series-parallel direct-current transformer is reduced, and the economical efficiency is improved.

Description

Technical Field

[0001] The present application relates to the field of direct current power grids, and in particular to a hybrid direct current transformer and a control method thereof. Background Art

[0002] DC grid technology has shown great potential and advantages in scenarios such as large-scale distributed renewable energy access. It can effectively integrate and optimize renewable energy, reduce energy transmission losses, simplify grid-connected technical requirements, and adapt to various complex environments and application scenarios.

[0003] The construction of the DC grid is based on the development of a multi-terminal DC system, which is interconnected through multiple transmission lines between multiple converter stations to form a highly reliable network structure. This network design not only improves the redundancy of the system, but also avoids the problem of reactive power transmission, thereby ensuring the efficiency and stability of power transmission. Compared with the existing AC power grid, the control of the DC grid is more flexible, and the transmission efficiency and power density are higher. The DC transformer plays a vital role in the DC grid and is a key device for interconnecting DC lines of different voltages and types. With the continuous development of DC transformers, the role of DC transformers is not only limited to providing voltage transformation ratios, but also needs to have functions such as DC grid power flow control and fault isolation.

[0004] The DC transformers of related technologies have problems such as large size, large footprint and poor economy, and cannot meet the performance requirements of DC transformers in large-scale new energy collection and transmission and DC system interconnection scenarios. Summary of the invention

[0005] In order to solve the problems of large volume, large floor space and poor economy in the prior art, the present application provides a hybrid DC transformer and a control method thereof.

[0006] The present application provides a hybrid DC transformer, comprising: a first low-voltage conversion unit, a second low-voltage conversion unit, an AC isolation unit and a high-voltage conversion unit. The first low-voltage conversion unit comprises a plurality of first bridge arms, each of which comprises a current source type switch device or a plurality of current source type switch devices connected in series in sequence, one end of a portion of the first bridge arms is used to be connected to the positive pole of the power grid, the other end of a portion of the first bridge arms is connected to the first end of the first voltage side of the AC isolation unit, one end of another portion of the first bridge arms is connected to the other end of another portion of the first bridge arms, and the other end of another portion of the first bridge arms is used to be connected to the negative pole of the power grid, the AC end of the second low-voltage conversion unit is connected to the second end of the first voltage side of the AC isolation unit, and the second voltage end of the AC isolation unit is connected to the AC end of the high-voltage conversion unit.

[0007] Optionally, the first low-voltage conversion unit further includes a plurality of capacitors and a plurality of inductors. One end of the plurality of capacitors is connected to the other end of a portion of the first bridge arms respectively, and the other ends of the plurality of capacitors are all connected. One end of the plurality of inductors is connected to the other end of a portion of the first bridge arms respectively, and the other ends of the plurality of inductors are connected to the first end of the first voltage side of the AC isolation unit respectively.

[0008] Optionally, the hybrid DC transformer includes at least two first low-voltage conversion units, and for any two first low-voltage conversion units: the negative DC terminal of one of the first low-voltage conversion units is connected to the positive DC terminal of the other first low-voltage conversion unit; or, the positive DC terminal of one of the first low-voltage conversion units is connected to the positive DC terminal of the other first low-voltage conversion unit, and the negative DC terminal of one of the first low-voltage conversion units is connected to the negative DC terminal of the other first low-voltage conversion unit.

[0009] Optionally, the hybrid DC transformer includes at least two second low-voltage conversion units, and for any two second low-voltage conversion units: the negative DC terminal of one second low-voltage conversion unit is connected to the positive DC terminal of another second low-voltage conversion unit; or, the positive DC terminal of one second low-voltage conversion unit is connected to the positive DC terminal of another second low-voltage conversion unit, and the negative DC terminal of one second low-voltage conversion unit is connected to the negative DC terminal of another second low-voltage conversion unit.

[0010] Optionally, the hybrid DC transformer includes at least two high-voltage conversion units, and for any two high-voltage conversion units: the negative DC terminal of one of the high-voltage conversion units is connected to the positive DC terminal of the other high-voltage conversion unit; or, the positive DC terminal of one of the high-voltage conversion units is connected to the positive DC terminal of the other high-voltage conversion unit, and the negative DC terminal of one of the high-voltage conversion units is connected to the negative DC terminal of the other high-voltage conversion unit; or, one of the high-voltage conversion units has no direct connection with the other high-voltage conversion unit.

[0011] Optionally, the positive DC terminal of the first low-voltage conversion unit is connected to the positive DC terminal of the second low-voltage conversion unit, and the negative DC terminal of the first low-voltage conversion unit is connected to the negative DC terminal of the second low-voltage conversion unit; or, the first low-voltage conversion unit and the second low-voltage conversion unit are not directly connected.

[0012] The present application also provides a control method for a hybrid DC transformer, which is applied to the above-mentioned hybrid DC transformer. The high-voltage conversion unit in the hybrid DC transformer includes a capacitor, a plurality of voltage source type switching devices, and a drive controller corresponding to each voltage source type switching device. The control method includes: obtaining the active power of the DC port of the first low-voltage conversion unit and the DC voltage of the DC port of the second low-voltage conversion unit, the first low-voltage conversion unit controls the energy of the hybrid DC transformer to gradually increase, and the second low-voltage conversion unit controls the DC voltage of the hybrid DC transformer; controlling the voltage source type switching device in the high-voltage conversion unit to lock, and pre-charging the capacitor, which is used to charge the drive controller corresponding to the voltage source type switching device; unlocking each voltage source type switching device, and putting in the voltage source type switching device that does not meet the voltage requirements, and cutting off the voltage source type switching device that meets the voltage requirements.

[0013] Optionally, the control method also includes: obtaining the active power and reactive power of the AC port of the first low-voltage conversion unit; calculating the d-axis current and the q-axis current according to the active power of the DC port and the reactive power of the AC port; calculating the AC current according to the d-axis current and the q-axis current; generating a first command signal according to the AC current; and controlling the on and off of the switching device of the first low-voltage conversion unit according to the first command signal.

[0014] Optionally, the second low-voltage conversion circuit includes a switch device. The control method further includes: obtaining the reactive power of the AC port of the second low-voltage conversion unit; calculating the d-axis voltage and the q-axis voltage according to the DC voltage of the DC port and the reactive power of the AC port of the second low-voltage conversion unit; calculating the AC voltage according to the d-axis voltage and the q-axis voltage; generating a second command signal according to the AC voltage; and controlling the conduction and disconnection of the switch device of the second low-voltage conversion circuit according to the second command signal.

[0015] Optionally, the control method also includes: obtaining the AC voltage of the AC port of the high-voltage conversion unit; calculating the d-axis voltage and the q-axis voltage of the high-voltage conversion unit based on the AC voltage of the AC port; calculating the reference AC voltage based on the d-axis voltage and the q-axis voltage of the high-voltage conversion unit; generating a bridge arm voltage instruction based on the AC voltage; and controlling the conduction and shutdown of each voltage source type switching device based on the bridge arm voltage instruction.

[0016] The present application also provides a control method for a hybrid DC transformer, which is applied to the above-mentioned hybrid DC transformer. The high-voltage conversion unit in the hybrid DC transformer includes a capacitor, a plurality of voltage source type switching devices, and a drive controller corresponding to each voltage source type switching device. The control method includes: the first low-voltage conversion unit, the second low-voltage conversion unit, and each voltage source type switching device are locked, and the capacitor is pre-charged; each voltage source type switching device is unlocked, and the voltage source type switching device that does not meet the voltage requirements is put into use, and the voltage source type switching device that meets the voltage requirements is cut off; the first low-voltage conversion unit and the second low-voltage conversion unit are unlocked, and the output energy is controlled by the first low-voltage conversion unit to rise smoothly, and the DC voltage of the DC transformer is controlled by the second low-voltage conversion unit.

[0017] Optionally, the control method also includes: obtaining the active power and reactive power of the AC port of the first low-voltage conversion unit; calculating the d-axis current and the q-axis current according to the active power of the DC port and the reactive power of the AC port; calculating the AC current according to the d-axis current and the q-axis current; generating a first command signal according to the AC current; and controlling the on and off of the switching device of the first low-voltage conversion unit according to the first command signal.

[0018] Optionally, the second low-voltage conversion circuit includes a switch device. The control method further includes: obtaining the reactive power of the AC port of the second low-voltage conversion unit; calculating the d-axis voltage and the q-axis voltage according to the DC voltage of the DC port and the reactive power of the AC port of the second low-voltage conversion unit; calculating the AC voltage according to the d-axis voltage and the q-axis voltage; generating a second command signal according to the AC voltage; and controlling the conduction and disconnection of the switch device of the second low-voltage conversion circuit according to the second command signal.

[0019] Optionally, the control method also includes: obtaining the AC voltage of the AC port of the high-voltage conversion unit; calculating the d-axis voltage and the q-axis voltage of the high-voltage conversion unit based on the AC voltage of the AC port; calculating the reference AC voltage based on the d-axis voltage and the q-axis voltage of the high-voltage conversion unit; generating a bridge arm voltage instruction based on the AC voltage; and controlling the conduction and shutdown of each voltage source type switching device based on the bridge arm voltage instruction.

[0020] Compared with the prior art, the beneficial effects of this application are:

[0021] In the hybrid DC transformer provided in the embodiment of the present application, the first low-voltage conversion unit and the second low-voltage conversion unit can convert the DC voltage into an AC voltage. The AC isolation unit can convert the voltage level of the AC voltage. Here, the AC isolation unit can play a role of boosting or bucking. The high-voltage conversion unit can convert the AC voltage after the voltage level conversion into a DC voltage. The embodiment of the present application can utilize the large current-carrying capacity of the current source converter to increase the low-voltage side power limit of the DC transformer, thereby reducing the volume of the hybrid DC transformer, reducing the floor space of the hybrid DC transformer, and improving the economy. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the present application or the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0023] Figure 1 This is a circuit diagram of a parallel-parallel DC transformer in an embodiment of the present application;

[0024] Figure 2 This is a circuit diagram of a parallel-parallel DC transformer in an embodiment of the present application;

[0025] Figure 3 It is a circuit schematic diagram of the first bridge arm of a parallel-parallel DC transformer;

[0026] Figure 4 The circuit schematic diagram of the second low voltage conversion unit of the hybrid DC transformer;

[0027] Figure 5 is a circuit schematic diagram of the SM of the second low voltage conversion unit;

[0028] Figure 6 The circuit schematic diagram of the high voltage conversion unit of the parallel-parallel DC transformer;

[0029] Figure 7 It is a circuit schematic diagram of SM;

[0030] Figure 8 Another circuit schematic diagram of SM in the figure;

[0031] Fig. 9 This is a circuit diagram of a parallel-parallel DC transformer in an embodiment of the present application;

[0032] Fig.10 This is a circuit diagram of a parallel-parallel DC transformer in an embodiment of the present application;

[0033] Fig.11This is a circuit diagram of a parallel-parallel DC transformer in an embodiment of the present application;

[0034] Fig.12 This is a circuit diagram of a parallel-parallel DC transformer in an embodiment of the present application;

[0035] Fig.13 This is a circuit diagram of a parallel-parallel DC transformer in an embodiment of the present application;

[0036] Fig.14 This is a circuit diagram of a parallel-parallel DC transformer in an embodiment of the present application;

[0037] Fig.15 This is a circuit diagram of a parallel-parallel DC transformer in an embodiment of the present application;

[0038] Fig.16 This is a circuit diagram of a parallel-parallel DC transformer in an embodiment of the present application;

[0039] Fig.17 This is a flowchart of the startup control method in an embodiment of the present application;

[0040] Fig.18 This is a flowchart of a startup control method according to an embodiment of the present application;

[0041] Fig.19 Schematic diagram of the flow of the first low-voltage conversion unit control method in the embodiment of the present application;

[0042] Fig. 20 Schematic diagram of the flow of the second low-voltage conversion unit control method in the embodiment of the present application;

[0043] Fig.21 Schematic diagram of the flow of the high voltage conversion unit control method in the embodiment of the present application. DETAILED DESCRIPTION

[0044] The technical solution in this application will be described below in conjunction with the accompanying drawings.

[0045] The terms "first", "second", etc. in the specification embodiments, claims, and drawings of the present application are only used for the purpose of distinguishing descriptions, and cannot be understood as indicating or implying relative importance, nor can they be understood as indicating or implying order. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, including a series of steps or units. The method, system, product, or device is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products, or devices.

[0046] It should be understood that in the present application, "at least one (item)" means one or more, and "plurality" means two or more. "And / or" is used to describe the association relationship of associated objects, indicating that three relationships may exist. For example, "A and / or B" can mean: only A exists, only B exists, and A and B exist at the same time, where A and B can be singular or plural. The character " / " generally indicates that the objects associated before and after are in an "or" relationship. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single or plural items. For example, at least one of a, b or c can mean: a, b, c, "a and b", "a and c", "b and c", or "a and b and c", where a, b, c can be single or multiple.

[0047] like Figure 1 and Figure 2 As shown, the embodiment of the present application provides a hybrid DC transformer, including: a first low-voltage conversion unit 1, a second low-voltage conversion unit 2, an AC isolation unit 3 and a high-voltage conversion unit 4. The first low-voltage conversion unit 1 includes a plurality of first bridge arms 11. Figure 3 As shown, each first bridge arm 11 includes a switch device or a plurality of switch devices S connected in series, and one end of a portion of the first bridge arms 11 is used to connect to the positive pole U of the power grid. L+ , the other end is connected to the first end of the first voltage side of the AC isolation unit 3, one end of another part of the first bridge arm 11 is connected to the other end of another part of the first bridge arm 11, and the other end is used to connect to the negative pole U of the power grid L- The AC end of the second low voltage conversion unit 2 is connected to the second end of the first voltage side of the AC isolation unit 3 , and the second voltage end of the AC isolation unit 3 is connected to the AC end of the high voltage conversion unit 4 .

[0048] The parallel-parallel DC transformer can step up or down the voltage. Figure 1 As shown, the hybrid DC transformer of this embodiment can be used for boosting, that is, the input voltage U L Lower than the output voltage U H .like Figure 2 As shown, the hybrid DC transformer of this embodiment can be used for voltage reduction, that is, the input voltage U L Higher than the output voltage U H Moreover, in contrast Figure 1 and Figure 2 It can be seen that except for the different directions of the switch device 11 in the first voltage-down conversion unit 1 , the structures of the remaining parts are the same.

[0049] In this embodiment, the AC isolation power supply 3 may be a transformer having a high voltage end and a low voltage end. Figure 1As shown, when the hybrid DC transformer is used for boosting, the input end of the AC isolation power supply 3, that is, the side connected to the first low-voltage conversion unit 1 and the second low-voltage conversion unit 2 is the low-voltage side, and the side connected to the high-voltage conversion unit 4 is the high-voltage side. Figure 2 As shown, when the hybrid DC transformer is used for voltage reduction, the input end of the AC isolation power supply 3, that is, the side connected to the first low-voltage conversion unit 1 and the second low-voltage conversion unit 2 is the high-voltage side, and the side connected to the high-voltage conversion unit 4 is the low-voltage side.

[0050] Compared with the prior art, the beneficial effects of this application are:

[0051] In the hybrid DC transformer provided in the embodiment of the present application, the first low-voltage conversion unit 1 and the second low-voltage conversion unit 2 can convert the DC voltage into an AC voltage. The AC isolation unit 3 can convert the voltage level of the AC voltage. Here, the AC isolation unit can play a role of boosting or bucking. . The high-voltage conversion unit 4 can convert the AC voltage after the voltage level conversion into a DC voltage. The embodiment of the present application can utilize the large current-carrying capacity of the current source converter to increase the low-voltage side power limit of the DC transformer, thereby reducing the volume of the hybrid DC transformer, reducing the floor space of the hybrid DC transformer, and improving the economy.

[0052] Furthermore, if Figure 1 As shown, in this embodiment, the first low voltage conversion unit 1 may include six first bridge arms 11, wherein one end (ie, the DC end) of three first bridge arms 11 is connected to the positive DC port U of the power grid. L+ The other ends (i.e., AC ends) of the three first bridge arms 11 are respectively connected to the AC isolation unit 3. One end of the remaining three first bridge arms is respectively connected to the AC isolation unit 3, and the other ends of the three first bridge arms 11 are all connected to the negative DC port U L- Each first bridge arm 11 may be a current source type commutation component.

[0053] Optional, such as Figure 1 As shown, the first low-voltage conversion unit 1 also includes: multiple capacitors C and multiple inductors L2, one end of the multiple capacitors C is respectively connected to the other end of a portion of the first bridge arm 11, and the other ends of the multiple capacitors C are all connected; one end of the multiple inductors L2 is respectively connected to the other end of a portion of the first bridge arm 11, and the other ends of the multiple inductors L2 are respectively connected to the first end of the first voltage side of the AC isolation unit 3.

[0054] like Figure 1 As shown, the first low voltage conversion unit 1 may further include an inductor L1, one end of the inductor L1 is connected to the positive electrode U L+ The other end of the inductor L1 is connected to one end of a portion of the first bridge arm 11 .

[0055] like Figure 3 As shown, the first bridge arm 11 may include a plurality of switch modules 111 connected in series, and the structure of each switch module 111 is the same. Each switch module 111 includes a diode D1, a capacitor C1, a switch device S1, a resistor R1, a diode D2, a capacitor C2 and a resistor R2. One end of the diode D1, one end of the switch device S1, one end of the resistor R1, one end of the diode D2 and one end of the resistor R2 are connected to Figure 1 The other end of the diode D1 is connected to one end of the capacitor C1, the other end of the resistor R1 is connected to one end of the capacitor C2, the other end of the diode D2 is connected to the other end of the resistor R1, and the other end of the capacitor C1, the other end of the switch device S1, the other end of the capacitor C2 and the other end of the resistor R2 are all connected to Figure 1 The AC isolation unit 3 is shown. In this embodiment, the switch device S1 may be a current source type switch device.

[0056] Optionally, the AC isolation unit 3 includes a three-winding AC transformer. Alternatively, the AC isolation unit 3 includes a plurality of multi-winding AC transformers, and the plurality of multi-winding AC transformers are connected in series, in parallel or in cascade.

[0057] Alternatively, if Figure 4 As shown, the second low voltage conversion unit 2 may include a plurality of second bridge arms 21, each of which includes a plurality of voltage source type switching devices SM and an inductor L3 connected in series or cascade. Figure 5 As shown, each voltage source type switching device SM may include a switching device S2 and a diode D3 connected in parallel. Each second bridge arm 21 may be a voltage source type commutation component. And each voltage source type commutation component may be different. One end (i.e., the DC end) of each voltage source type commutation component is connected to the positive and negative DC ports of the power grid, and the other ends (i.e., the AC ends) of the three first bridge arms 11 are respectively connected to the AC isolation unit 3.

[0058] Alternatively, if Figure 6 As shown, the high-voltage conversion unit 4 may include a plurality of third bridge arms 41, a plurality of inductors L4 and a voltage stabilizing capacitor C3. Each third bridge arm 41 may be a voltage source type commutation component. Each third bridge arm 41 includes a plurality of voltage source type switching devices SM connected in series or in cascade. In addition, Figure 7 or Figure 8 The voltage source switching device SM shown in FIG. The AC port of the voltage source commutation component is connected to the high voltage port of the AC isolation unit 3, and the DC port of the voltage source commutation component is connected to the positive and negative DC ports of the high voltage DC grid.

[0059] Optional, such as Fig. 9As shown, the hybrid DC transformer includes at least two first low-voltage conversion units 1, and for any two first low-voltage conversion units 1: the negative DC terminal of one first low-voltage conversion unit 1 is connected to the positive DC terminal of another first low-voltage conversion unit 1. In other words, the two first low-voltage conversion units 1 are connected in series.

[0060] Or, if Fig.10 As shown, the positive DC terminal of one of the first low-voltage conversion units 1 is connected to the positive DC terminal of another first low-voltage conversion unit 1, and the negative DC terminal of one of the first low-voltage conversion units 1 is connected to the negative DC terminal of another first low-voltage conversion unit 1. In other words, the two first low-voltage conversion units 1 are connected in parallel. After parallel connection, shunt can be formed, and the current stress will be reduced, thereby increasing the commutation stress of the hybrid DC transformer, and then increasing the overall capacity.

[0061] Optional, such as Fig.11 As shown, the hybrid DC transformer includes at least two second low-voltage conversion units 2, and for any two second low-voltage conversion units 2: the negative DC end of one second low-voltage conversion unit 2 is connected to the positive DC end of another second low-voltage conversion unit 2. In other words, the two second low-voltage conversion units 2 are connected in series.

[0062] Or, if Fig.12 As shown, the positive DC terminal of one second low-voltage conversion unit 2 is connected to the positive DC terminal of another second low-voltage conversion unit 2, and the negative DC terminal of one second low-voltage conversion unit 2 is connected to the negative DC terminal of another second low-voltage conversion unit 2. In other words, the two second low-voltage conversion units 2 are connected in parallel. After parallel connection, shunt can be formed, and the current stress will be reduced, thereby increasing the commutation stress of the hybrid DC transformer, and then increasing the overall capacity.

[0063] Optional, such as Fig.13 As shown, the hybrid DC transformer includes at least two high-voltage conversion units 4. For any two high-voltage conversion units 4, the negative DC terminal of one high-voltage conversion unit 4 is connected to the positive DC terminal of another high-voltage conversion unit 4. In other words, the two high-voltage conversion units 4 are connected in series.

[0064] Or, if Fig.14 As shown, the positive DC terminal of one high-voltage conversion unit 4 is connected to the positive DC terminal of another high-voltage conversion unit 4, and the negative DC terminal of one high-voltage conversion unit 4 is connected to the negative DC terminal of another high-voltage conversion unit 4. In other words, the two high-voltage conversion units 4 are connected in parallel. After parallel connection, current shunting can be formed, and the current stress will be reduced, thereby increasing the commutation stress of the hybrid DC transformer, and then increasing the overall capacity.

[0065] Or, if Fig.15As shown, one of the high-voltage conversion units 4 is not directly connected to the other high-voltage conversion unit 4. That is, one of the high-voltage conversion units 4 and the other high-voltage conversion unit 4 are connected to different ports U H1 and U H2 , ensuring the independent power supply capabilities of different ports, each port can be connected to an independent power supply.

[0066] Optional, such as Figure 1 As shown, the positive DC terminal of the first low-voltage conversion unit 1 is connected to the positive DC terminal of the second low-voltage conversion unit 2, and the negative DC terminal of the first low-voltage conversion unit 1 is connected to the negative DC terminal of the second low-voltage conversion unit 2. In other words, Figure 1 In the embodiment, the DC end of the first low voltage conversion unit 1 is connected in parallel with the DC end of the second low voltage conversion unit 2 to form the low voltage DC end of the hybrid DC transformer. The DC end of the high voltage conversion unit 4 is the high voltage DC end of the DC transformer. Or, as Fig.15 and Fig.16 As shown, the first low voltage conversion unit 1 and the second low voltage conversion unit 2 are not directly connected. That is, the first low voltage conversion unit 1 and the second low voltage conversion unit 2 are respectively connected to different ports U L1 and U L2 , ensuring the independent power supply capabilities of different ports, each port can be connected to an independent power supply.

[0067] In addition, the embodiment of the present application can also use the voltage source converter of the second low voltage conversion unit 2 to realize the power return and no-load operation of the hybrid DC transformer. The sub-winding AC isolation unit is used to realize the boost or buck function of the DC transformer. The voltage source converter in the high voltage conversion unit 4 is used to realize the high voltage and large capacity transmission power requirements of the hybrid DC transformer. As a result, while being able to reduce the volume, it is also possible to improve the economy of the hybrid DC transformer.

[0068] Based on the same inventive concept, the present application also provides a control method for a parallel-parallel DC transformer, which is applied to the parallel-parallel DC transformer. Figure 6 As shown, the high voltage conversion unit 4 in the hybrid DC transformer includes a capacitor, a plurality of voltage source type switching devices SM, and a drive controller corresponding to each voltage source type switching device SM ( Figure 6 not shown).

[0069] An embodiment of the present application provides a control method for a parallel-parallel DC transformer, including a startup control method and a steady-state control method.

[0070] In one example, if Fig.17 As shown, the startup control method includes:

[0071] S171, obtaining the active power of the DC port of the first low-voltage conversion unit and the DC voltage of the DC port of the second low-voltage conversion unit.

[0072] The energy of the hybrid DC transformer is gradually increased by controlling the first low-voltage conversion unit, and the DC voltage of the hybrid DC transformer is controlled by the second low-voltage conversion unit.

[0073] S172, calculating and obtaining the d-axis current and the q-axis current of the first low-voltage conversion unit, and the d-axis voltage and the q-axis voltage of the second low-voltage conversion unit.

[0074] S173, generating a first command signal for controlling the first low-voltage conversion unit and a second command signal for controlling the second low-voltage conversion unit.

[0075] S174, controlling the voltage source type switch device in the high voltage conversion unit to lock, and pre-charging the capacitor, the capacitor is used to charge the drive controller corresponding to the voltage source type switch device.

[0076] S175, determining whether the driver of the voltage source type switch device in the high voltage conversion unit can obtain energy from the capacitor.

[0077] If so, execute S176 to unlock each voltage source type switching device, and put the voltage source type switching device that does not meet the voltage requirement into operation, and cut off the voltage source type switching device that meets the voltage requirement.

[0078] If not, return to step S174.

[0079] S177, determining whether the voltage of the capacitor reaches the rated voltage or meets the steady-state operation requirement.

[0080] If yes, the hybrid DC transformer enters the steady-state operation mode. If no, the process returns to step S176.

[0081] In another example, if Fig.18 As shown, the startup control method includes:

[0082] S181, each voltage source type switch device in the first low voltage conversion unit, the second low voltage conversion unit and the high voltage conversion unit is locked, and the capacitor is pre-charged.

[0083] S182, determining whether the driver of the voltage source type switching device in the high voltage conversion unit can obtain energy from the capacitor.

[0084] If yes, execute S183 to unlock the voltage source type switching device of the high voltage conversion unit, and the high voltage conversion unit controls the switch of the voltage source type switching device.

[0085] The voltage source type switching devices that do not meet the voltage requirements in the high-voltage conversion unit are put into use, and the voltage source type switching devices that meet the voltage requirements are cut off.

[0086] If not, return to step S181.

[0087] S184, determining whether the capacitor voltage is charged to the rated voltage or meets the voltage requirement.

[0088] If so, execute step S185 to unlock the first low-voltage conversion unit and the second low-voltage conversion unit, and control the output energy to rise smoothly by the first low-voltage conversion unit, and control the DC voltage of the DC transformer by the second low-voltage conversion unit.

[0089] S185, calculating and obtaining the d-axis current and q-axis current of the first low-voltage conversion unit and the d-axis voltage and q-axis voltage of the second low-voltage conversion unit.

[0090] S186, generating a first command signal for controlling the first low-voltage conversion unit and a second command signal for controlling the second low-voltage conversion unit.

[0091] The hybrid DC transformer enters the steady-state operation mode.

[0092] Optional, such as Fig.19 As shown, the steady-state control method may include a first low-voltage conversion unit control method, a second low-voltage conversion unit control method and a high-voltage conversion unit control method. The first low-voltage conversion unit control method includes:

[0093] S191, obtaining active power and reactive power of an AC port of a first low-voltage conversion unit.

[0094] S192, calculating the d-axis current and the q-axis current according to the active power and reactive power of the AC port.

[0095] S193, calculating the AC current based on the d-axis current and the q-axis current.

[0096] S194, generating a first command signal according to the alternating current.

[0097] S195, controlling the on and off of the switch device of the first low-voltage conversion unit according to the first command signal.

[0098] The first command signal is connected to the driver of the current source type switch device to control the on and off of the current source type switch device, thereby realizing the control of the active power and reactive power of the AC port of the first low-voltage conversion unit.

[0099] Optionally, the second low voltage conversion circuit includes a switching device, such as Fig. 20 As shown, the second low voltage conversion unit control method includes:

[0100] S201, obtaining a DC voltage of a DC port and reactive power of an AC port of a second low voltage conversion unit.

[0101] S202, calculating a d-axis voltage and a q-axis voltage according to the DC voltage of the DC port and the reactive power of the AC port of the second low-voltage conversion unit.

[0102] S203, calculating the AC voltage according to the d-axis voltage and the q-axis voltage.

[0103] S204, generating a second command signal according to the AC voltage.

[0104] The second command signal is used to control the on and off of the voltage source type switching device in the second low voltage conversion unit.

[0105] S205, controlling the on and off of the switch device of the second low voltage conversion circuit according to the second command signal.

[0106] The second command signal is connected to the drive of the voltage source type switching device to control the on and off of the voltage source type switching device in the second low-voltage conversion circuit, thereby realizing the control of the DC voltage of the DC port and the reactive power of the AC port of the second low-voltage conversion unit.

[0107] Optional, such as Fig.21 As shown, the high voltage conversion unit control method includes:

[0108] S211, obtaining an AC voltage of an AC port of a high-voltage conversion unit;

[0109] S212, calculating the d-axis voltage and the q-axis voltage of the high-voltage conversion unit according to the AC voltage of the AC port;

[0110] S213, calculating a reference AC voltage according to the d-axis voltage and the q-axis voltage of the high-voltage conversion unit;

[0111] S214, generating a bridge arm voltage instruction according to the AC voltage;

[0112] S215, controlling the on and off of each voltage source type switch device of the high voltage conversion unit according to the bridge arm voltage instruction.

[0113] The above are merely embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are included in the scope of the claims of the present invention to be approved.

Claims

1. A hybrid DC transformer, characterized in that: include: A first low-voltage conversion unit, a second low-voltage conversion unit, an AC isolation unit and a high-voltage conversion unit, wherein the first low-voltage conversion unit includes a plurality of first bridge arms, each of the first bridge arms includes a current source type switching device or a plurality of current source type switching devices connected in series in sequence, one end of a portion of the first bridge arms is used to be connected to the positive pole of the power grid, the other end of a portion of the first bridge arms is connected to the first end of the first voltage side of the AC isolation unit, one end of another portion of the first bridge arms is connected to the other end of a portion of the first bridge arms, and the other end of another portion of the first bridge arms is used to be connected to the negative pole of the power grid, the AC end of the second low-voltage conversion unit is connected to the second end of the first voltage side of the AC isolation unit, and the second voltage end of the AC isolation unit is connected to the AC end of the high-voltage conversion unit.

2. The hybrid DC transformer according to claim 1, characterized in that: The first low-voltage conversion unit further includes a plurality of capacitors and a plurality of inductors, one end of the plurality of capacitors is connected to the other end of a portion of the first bridge arms respectively, and the other ends of the plurality of capacitors are all connected; One ends of the multiple inductors are respectively connected to the other ends of a portion of the first bridge arms, and the other ends of the multiple inductors are respectively connected to the first end of the first voltage side of the AC isolation unit.

3. The hybrid DC transformer according to claim 1, characterized in that: The hybrid DC transformer includes at least two of the first low-voltage conversion units. For any two of the first low-voltage conversion units: The negative DC terminal of one of the first low-voltage conversion units is connected to the positive DC terminal of another of the first low-voltage conversion units; Or, the positive DC terminal of one of the first low-voltage conversion units is connected to the positive DC terminal of another of the first low-voltage conversion units, and the negative DC terminal of one of the first low-voltage conversion units is connected to the negative DC terminal of another of the first low-voltage conversion units.

4. The hybrid DC transformer according to claim 1, characterized in that: The hybrid DC transformer includes at least two of the second low-voltage conversion units; For any two of the second low-voltage conversion units: A negative DC terminal of one of the second low-voltage conversion units is connected to a positive DC terminal of another second low-voltage conversion unit; Or, the positive DC terminal of one of the second low-voltage conversion units is connected to the positive DC terminal of another of the second low-voltage conversion units, and the negative DC terminal of one of the second low-voltage conversion units is connected to the negative DC terminal of another of the second low-voltage conversion units.

5. The hybrid DC transformer according to claim 1, characterized in that: The hybrid DC transformer includes at least two high-voltage conversion units; For any two of the high-voltage conversion units: The negative DC terminal of one of the high-voltage conversion units is connected to the positive DC terminal of another of the high-voltage conversion units; Or, the positive DC terminal of one of the high-voltage conversion units is connected to the positive DC terminal of another of the high-voltage conversion units, and the negative DC terminal of one of the high-voltage conversion units is connected to the negative DC terminal of another of the high-voltage conversion units; Alternatively, one of the high-voltage conversion units is not directly connected to another of the high-voltage conversion units.

6. The hybrid DC transformer according to claim 1, characterized in that: The positive DC terminal of the first low-voltage conversion unit is connected to the positive DC terminal of the second low-voltage conversion unit, and the negative DC terminal of the first low-voltage conversion unit is connected to the negative DC terminal of the second low-voltage conversion unit; Alternatively, the first low-voltage conversion unit and the second low-voltage conversion unit are not directly connected.

7. A control method for a hybrid DC transformer, characterized in that: The high-voltage conversion unit in the hybrid DC transformer includes a capacitor, a plurality of voltage source type switching devices, and a drive controller corresponding to each of the voltage source type switching devices; the control method includes: Acquire the active power of the DC port of the first low-voltage conversion unit and the DC voltage of the DC port of the second low-voltage conversion unit, control the energy of the hybrid DC transformer to gradually increase by the first low-voltage conversion unit, and control the DC voltage of the hybrid DC transformer by the second low-voltage conversion unit; Controlling the voltage source type switch device in the high voltage conversion unit to lock, and pre-charging the capacitor, wherein the capacitor is used to charge the drive controller corresponding to the voltage source type switch device; Unlock each of the voltage source type switching devices, put the voltage source type switching devices that do not meet the voltage requirements into operation, and cut off the voltage source type switching devices that meet the voltage requirements.

8. The control method according to claim 7, characterized in that: The control method further comprises: Acquiring active power and reactive power of the AC port of the first low-voltage conversion unit; Calculating the d-axis current and the q-axis current according to the active power of the DC port and the reactive power of the AC port; Calculating an alternating current based on the d-axis current and the q-axis current; generating a first command signal according to the alternating current; The on and off of the switch device of the first low-voltage conversion unit is controlled according to the first instruction signal.

9. The control method according to claim 7, characterized in that: The second low voltage conversion circuit includes a switch device, and the control method further includes: Acquiring reactive power of an AC port of a second low voltage conversion unit; Calculating a d-axis voltage and a q-axis voltage according to the DC voltage of the DC port and the reactive power of the AC port of the second low-voltage conversion unit; Calculating an AC voltage based on the d-axis voltage and the q-axis voltage; generating a second command signal according to the AC voltage; The on and off of the switching device of the second low-voltage conversion circuit is controlled according to the second instruction signal.

10. The control method according to claim 7, characterized in that: The control method further comprises: Acquiring an AC voltage of an AC port of the high-voltage conversion unit; Calculating the d-axis voltage and the q-axis voltage of the high-voltage conversion unit according to the AC voltage of the AC port; Calculating a reference AC voltage according to the d-axis voltage and the q-axis voltage of the high-voltage conversion unit; generating a bridge arm voltage instruction according to the AC voltage; According to the bridge arm voltage instruction, the on and off of each of the voltage source type switching devices is controlled.

11. A control method for a hybrid DC transformer, characterized in that: The high-voltage conversion unit in the hybrid DC transformer includes a capacitor, a plurality of voltage source type switching devices, and a drive controller corresponding to each of the voltage source type switching devices; the control method includes: The first low voltage conversion unit, the second low voltage conversion unit and each of the voltage source type switching devices in the hybrid DC transformer are all locked, and the capacitor is pre-charged; Unlocking each of the voltage source type switching devices, and putting into operation the voltage source type switching devices that do not meet the voltage requirements, and removing the voltage source type switching devices that meet the voltage requirements; The first low-voltage conversion unit and the second low-voltage conversion unit are unlocked, and the first low-voltage conversion unit controls the output energy to rise smoothly, and the second low-voltage conversion unit controls the DC voltage of the DC transformer.

12. The control method according to claim 11, characterized in that: The control method further comprises: Acquiring active power and reactive power of the AC port of the first low-voltage conversion unit; Calculating the d-axis current and the q-axis current according to the active power of the DC port and the reactive power of the AC port; Calculating an alternating current based on the d-axis current and the q-axis current; generating a first command signal according to the alternating current; The on and off of the switch device of the first low-voltage conversion unit is controlled according to the first instruction signal.

13. The control method according to claim 11, characterized in that: The second low voltage conversion circuit includes a switch device, and the control method further includes: Acquiring reactive power of an AC port of a second low voltage conversion unit; Calculating a d-axis voltage and a q-axis voltage according to the DC voltage of the DC port and the reactive power of the AC port of the second low-voltage conversion unit; Calculating an AC voltage based on the d-axis voltage and the q-axis voltage; generating a second command signal according to the AC voltage; The on and off of the switching device of the second low-voltage conversion circuit is controlled according to the second instruction signal.

14. The control method according to claim 11, characterized in that: The control method further comprises: Acquiring an AC voltage of an AC port of the high-voltage conversion unit; Calculating the d-axis voltage and the q-axis voltage of the high-voltage conversion unit according to the AC voltage of the AC port; Calculating a reference AC voltage according to the d-axis voltage and the q-axis voltage of the high-voltage conversion unit; generating a bridge arm voltage instruction according to the AC voltage; According to the bridge arm voltage instruction, the on and off of each of the voltage source type switching devices is controlled.

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