A single-bipolar conversion high-voltage large-capacity DC transformer and its control method

Abstract

The present invention discloses a single-bipolar conversion high-voltage large-capacity DC transformer and a control method thereof, including: a unipolar primary side, a bipolar secondary side and an AC unit, wherein the AC unit is connected between the unipolar primary side and the bipolar secondary side. It also includes a single shift control strategy, a sub-module capacitor voltage balancing control strategy, and a DC bias suppression strategy. The AC link of the present invention reduces the cost of the entire system and can achieve bipolar operation. The voltage matching ratio can be 1 / 2, which reduces the insulation requirements of medium and high frequency isolation transformers. The introduction shift angle of the drive signal between the upper and lower bridge arms of the unipolar primary side can also be used to actively inject DC into the primary side of the AC transformer to reduce the time when the DC bias phenomenon occurs.

Description

Technical Field

[0001] The present invention relates to the fields of new energy power generation grid connection or DC grid interconnection and power electronics technology, and specifically to a single- and double-pole conversion high-voltage and large-capacity DC transformer and a control method thereof. Background Art

[0002] Compared with onshore wind power, offshore wind power can be directly connected to the load center of the southeast coast, which is easy to absorb and relieve the pressure of sending electricity from west to east. At the same time, offshore wind power has broad development space and great potential for resource development. It does not need to occupy precious land resources and has the advantages of stable wind speed and high power generation efficiency. However, as the capacity of offshore wind farm systems continues to expand and the offshore distance continues to increase, the traditional high-voltage AC transmission (HVAC) method is no longer suitable for large offshore wind farm energy transmission occasions due to problems such as large charging power, line loss, and the need for reactive power compensation devices. Instead, it is replaced by high-voltage direct current transmission (HVDC) with no charging capacitor effect and smaller line loss. DC transformers will be the core equipment for connecting DC equipment, connecting different voltage levels, connecting new energy to the grid, and interconnecting power grids in DC grids.

[0003] At present, there are two types of DC transmission wiring: unipolar wiring and bipolar wiring. The use of bipolar wiring can mainly reduce the insulation level of the DC line and improve the reliability of the system. When a line break occurs on one pole of the bipolar side, half of the energy transmission can still be guaranteed. If the output power of the DC wind farm is greater than the transmission capacity of the unipolar DC system, it is necessary to coordinate the wind farm to reduce the output power. At the same time, considering the offshore wind power transmission scenario, there is no power load on the transmission line, so a large pulsation component can be allowed on the DC. However, it should be noted that long-term unipolar operation is not conducive to the stable operation of the entire system. After a line break occurs, emergency repairs should be carried out in a short time. At the same time, the existing DC transformer is mainly used for energy transmission between unipolar wiring DC grids and unipolar wiring DC grids. The energy transmission between unipolar wiring DC grids and bipolar wiring DC grids requires two sets of DC transformers to achieve, but it also brings disadvantages such as high price, high loss, and large size. Summary of the invention

[0004] The purpose of this section is to summarize some aspects of embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the specification abstract and the invention title of this application to avoid blurring the purpose of this section, the specification abstract and the invention title, and such simplifications or omissions cannot be used to limit the scope of the present invention.

[0005] In view of the above-mentioned problems, the present invention is proposed.

[0006] Therefore, the technical problem solved by the present invention is: the existing DC transformer is mainly used for energy transmission between unipolar connected DC power grids and unipolar connected DC power grids, and the energy transmission between unipolar connected DC power grids and bipolar connected DC power grids requires two sets of DC transformers to achieve, but it also brings problems such as high price, high loss, and large size.

[0007] In order to solve the above technical problems, the present invention provides the following technical solutions: a single-bipolar conversion high-voltage large-capacity DC transformer, comprising:

[0008] A unipolar primary side, the unipolar primary side includes an A and B two-phase bridge arm circuit, a DC capacitor, a positive bus bar, and a negative bus bar;

[0009] A bipolar secondary side, the bipolar secondary side includes a C-phase bridge arm circuit, two DC capacitors, a positive bus, a negative bus and a neutral line;

[0010] An AC unit is connected between the unipolar primary side and the bipolar secondary side.

[0011] As a preferred solution of the single-bipolar conversion high-voltage large-capacity DC transformer of the present invention, wherein: the A and B two-phase bridge arm circuits are composed of an upper bridge arm circuit and a lower bridge arm circuit in series, and the upper and lower bridge arm circuits of the A phase are arm au 、arm al , the upper and lower bridge arm circuits of phase B are arm bu 、arm bl , the DC capacitance is C i ; Among them, arm au and arm bu Connected to the positive bus, arm al and arm bl Connected to the negative bus, arm au 、arm al Between and arm bu 、arm bl A tap is drawn between each, and the DC capacitor C i Connected in parallel between the positive and negative busbars.

[0012] As a preferred solution of the single-pole and bipolar conversion high-voltage and large-capacity DC transformer of the present invention, wherein: the A and B two-phase bridge arm circuits include a plurality of cascaded submodules SM and a reactor L p ; Wherein, the submodule SM is composed of two IGBT switches, a diode connected in reverse parallel thereto, and a parallel DC capacitor.

[0013] As a preferred solution of the single-bipolar conversion high-voltage large-capacity DC transformer of the present invention, the C-phase bridge arm circuit is composed of an upper bridge arm circuit and a lower bridge arm circuit in series, respectively. cu and arm cl , the two DC capacitors in series are C u and C l ; Among them arm cu and C u Connected to the positive bus, arm cl and C l Connected to the negative busbar, the neutral line is connected to C u and C l Between, arm cu 、arm cl Between and C u , C l A tap is drawn between each.

[0014] As a preferred solution of the single-pole and bipolar conversion high-voltage and large-capacity DC transformer of the present invention, the C-phase bridge arm circuit is composed of a plurality of cascaded submodules SM and impedance modules SW and a reactor L s wherein each submodule SM is composed of two IGBT switches, a reverse-parallel diode therewith and a parallel DC capacitor, and the impedance module SW is composed of a switch and an impedance in parallel.

[0015] As a preferred solution of the single-polarity and bipolarity conversion high-voltage and large-capacity DC transformer described in the present invention, the AC unit (200) is a medium-high frequency isolation transformer, the primary side of the medium-high frequency isolation transformer is connected to two taps of the unipolar primary side (100), and the secondary side of the medium-high frequency isolation transformer is connected to two taps of the bipolar secondary side (300).

[0016] As a preferred solution of the single-bipolar conversion high-voltage large-capacity DC transformer and its control method described in the present invention, it includes: a single-direction control strategy, a sub-module capacitor voltage balancing control strategy, and a DC bias suppression strategy.

[0017] As a preferred solution of the single-pole and bipolar conversion high-voltage and large-capacity DC transformer and its control method described in the present invention, the single-direction control strategy includes a voltage control mode and a power control mode.

[0018] As a preferred solution of the single-pole and bipolar conversion high-voltage and large-capacity DC transformer and its control method described in the present invention, the sub-module capacitor voltage balancing control strategy includes sampling and sorting the capacitor voltages of the sub-modules SM in each bridge arm, using a driving signal with a smaller phase shift angle to control the sub-modules with lower capacitor voltages, and using a driving signal with a larger phase shift angle to control the sub-modules with higher capacitor voltages.

[0019] As a preferred solution of the single-pole and bipolar conversion high-voltage and large-capacity DC transformer and its control method described in the present invention, the DC bias suppression strategy includes introducing a phase shift angle of the drive signal between bridge arms during the modulation process of the single-pole and bipolar conversion high-voltage and large-capacity DC transformer, so that the drive signal of the upper bridge arm of phase A and the lower bridge arm of phase B is ahead of the drive signal of the lower bridge arm of phase A and the upper bridge arm of phase B, or the drive signal of the upper bridge arm of phase A and the lower bridge arm of phase B lags behind the drive signal of the lower bridge arm of phase A and the upper bridge arm of phase B.

[0020] Beneficial effects of the present invention: 1. The AC link of the present invention adopts a double-winding transformer, and only one set of converters is used on the bipolar side, which reduces the cost of the entire system and can achieve bipolar operation. When a busbar on a certain pole of the bipolar side is disconnected, the present invention can still transmit half of the rated power in a short time.

[0021] 2. The voltage matching ratio of the existing unipolar to unipolar DC transformer is 1. When there is no neutral line or the neutral point is not grounded between the two output concentrated capacitors of the present invention, the converter is converted into a unipolar to unipolar DC transformer. Since the secondary side is a modular multi-level (MMC) single-phase half-bridge structure, the voltage matching ratio can be achieved to be 1 / 2, thereby reducing the insulation requirements of the medium and high frequency isolation transformer.

[0022] 3. When the power transmission changes, the average value of the excitation current is not zero within a cycle. The present invention introduces a shift angle of the driving signal between the upper and lower bridge arms on the unipolar primary side to actively inject DC into the primary side of the AC transformer to reduce the time when the DC bias phenomenon occurs. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative labor. Among them:

[0024] Figure 1 A topological structure diagram of a DC transformer applied to a scenario where a unipolar DC connection and a bipolar DC connection are interconnected, provided in accordance with an embodiment of the present invention;

[0025] Figure 2 A topological structure diagram of a DC transformer applied to a scenario where a unipolar DC connection and a unipolar DC connection are interconnected, provided in accordance with an embodiment of the present invention;

[0026] Figure 3 The single-pole and bipolar conversion high-voltage and large-capacity DC transformer provided in one embodiment of the present invention is applied to a constant voltage control mode in a scenario where a new energy convergence bus is connected to a power grid transmission line (i.e., an offshore platform);

[0027] Figure 4 A single-pole and bipolar conversion high-voltage and large-capacity DC transformer provided in an embodiment of the present invention is applied to a constant power control mode in a grid bus interconnection scenario;

[0028] Figure 5 A control diagram for capacitor voltage balancing of submodules in a bridge arm in a single-pole to bipolar conversion high-voltage and large-capacity DC transformer provided by an embodiment of the present invention;

[0029] Figure 6 A DC bias suppression strategy in a single- and bipolar-conversion high-voltage and large-capacity DC transformer and a control method thereof provided in one embodiment of the present invention;

[0030] Figure 7 The output power (P) of the bipolar symmetrical operation in the single-bipolar conversion high-voltage large-capacity DC transformer and the control method thereof provided in an embodiment of the present invention is out1 =50MW, P out2 =50MW) diagram;

[0031] Figure 8 The primary and secondary voltage v of the AC side in bipolar symmetrical operation in the single-bipolar conversion high-voltage large-capacity DC transformer and its control method provided in an embodiment of the present invention is ac1 、v ac2 、The original secondary current i ac1 、i ac2 picture;

[0032] Fig. 9 The excitation current i in the single-bipolar conversion high-voltage large-capacity DC transformer and control method thereof provided in one embodiment of the present invention is m picture;

[0033] Fig.10 The arm-to-arm shift ratio d in the single-bipolar conversion high-voltage large-capacity DC transformer and control method thereof provided in one embodiment of the present invention arm The simulation experiment waveform diagram;

[0034] Fig.11 A power transmission change condition diagram of a single-bipolar conversion high-voltage large-capacity DC transformer and a control method thereof provided in one embodiment of the present invention;

[0035] Fig.12 A single-pole to bipolar conversion high voltage and large capacity DC transformer and a control method thereof are provided in one embodiment of the present invention. Fig.11 The excitation current simulation experiment waveform under the condition of power transmission variation without DC bias suppression control. The excitation current i m A DC component appears;

[0036] Fig.13 A single-pole to bipolar conversion high voltage and large capacity DC transformer and a control method thereof are provided in one embodiment of the present invention. Fig.11 The waveform of the excitation current simulation experiment under the condition of power transmission variation and DC bias suppression control. The excitation current i m A DC component appears;

[0037] Fig.14 The output power (P) after the negative busbar is disconnected in the single-pole and bipolar conversion high-voltage and large-capacity DC transformer and its control method provided in one embodiment of the present invention out1 =50MW, P out2 =0)Figure;

[0038] Fig.15 The primary and secondary voltage v of the AC side after the negative busbar is disconnected in the single-bipolar conversion high-voltage and large-capacity DC transformer and its control method provided in an embodiment of the present invention ac1 and v ac2 Waveform, current i ac1 and i ac2 Waveform graph;

[0039] Fig.16 The excitation current i after the negative busbar is disconnected in the single-pole and bipolar conversion high-voltage and large-capacity DC transformer and its control method provided in an embodiment of the present invention m Waveform graph;

[0040] Fig.17 The arm-to-arm shift ratio d after the negative busbar is disconnected in the single-pole and bipolar conversion high-voltage and large-capacity DC transformer and control method thereof provided in one embodiment of the present invention arm The simulation experiment waveform diagram;

[0041] Fig.18 The output power (P) after the positive busbar is disconnected in the single-pole and bipolar conversion high-voltage and large-capacity DC transformer and its control method provided in an embodiment of the present invention out1 =0,P out2 =50MW) diagram;

[0042] Fig.19The primary and secondary voltage v of the AC side after the positive busbar is disconnected in the single-bipolar conversion high-voltage and large-capacity DC transformer and its control method provided in an embodiment of the present invention ac1 and v ac2 , the original secondary current i ac1 and i ac2 Waveform graph;

[0043] Fig. 20 The excitation current i after the positive busbar is disconnected in the single-pole and bipolar conversion high-voltage and large-capacity DC transformer and its control method provided in one embodiment of the present invention m Waveform graph;

[0044] Fig.21 The arm-to-arm shift ratio d in the single-bipolar conversion high-voltage large-capacity DC transformer and control method thereof provided in one embodiment of the present invention arm The simulation experiment waveform diagram. DETAILED DESCRIPTION

[0045] In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the drawings of the specification. Obviously, the described embodiments are part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary persons in the art without creative work should fall within the scope of protection of the present invention.

[0046] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0047] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments.

[0048] The present invention is described in detail with reference to schematic diagrams. When describing the embodiments of the present invention, for the sake of convenience, the cross-sectional diagrams showing the device structure will not be partially enlarged according to the general scale, and the schematic diagrams are only examples, which should not limit the scope of protection of the present invention. In addition, in actual production, the three-dimensional dimensions of length, width and depth should be included.

[0049] At the same time, in the description of the present invention, it should be noted that the directions or positional relationships indicated by the terms "upper, lower, inner and outer" are based on the directions or positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore cannot be understood as limiting the present invention. In addition, the terms "first, second or third" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0050] In the present invention, unless otherwise clearly specified and limited, the terms "install, connect, connect" should be understood in a broad sense, for example: it can be a fixed connection, a detachable connection or an integral connection; it can also be a mechanical connection, an electrical connection or a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0051] Example 1

[0052] Reference Figures 1-2 , is an embodiment of the present invention, providing a single-bipolar conversion high voltage large capacity DC transformer, comprising:

[0053] A unipolar primary side 100 , a bipolar secondary side 300 and an AC unit 200 , wherein the AC unit 200 is connected between the unipolar primary side 100 and the bipolar secondary side 300 .

[0054] It should be noted that the medium and high frequency isolation transformer is connected at the midpoint between the upper and lower bridge arms of the original secondary side.

[0055] Furthermore, the unipolar primary side 100 includes a two-phase A and B bridge arm circuit, a DC capacitor, a positive bus and a negative bus; the two-phase A and B bridge arm circuits are composed of an upper bridge arm circuit and a lower bridge arm circuit connected in series, and the upper and lower bridge arm circuits of the A phase are respectively denoted as arm au 、arm al , the upper and lower bridge arm circuits of phase B are respectively denoted as arm bu 、arm bl , the DC capacitance is C i ; Among them, arm au and arm bu Connected to the positive bus, arm al and arm bl Connected to the negative bus, arm au 、arm al Between and arm bu 、arm bl A tap is drawn between each, and the DC capacitor C iConnected in parallel between the positive and negative busbars.

[0056] Furthermore, the A and B two-phase bridge arm circuits include a plurality of cascaded submodules SM 1 and a reactor L p ; Wherein, the submodule SM is composed of two IGBT switches, a diode connected in reverse parallel thereto, and a parallel DC capacitor.

[0057] It should be noted that the unipolar primary side 100 is a modular multi-level single-phase full-bridge structure, and the submodule SM 1 The number n is determined by the voltage level requirements of the access system.

[0058] Furthermore, the bipolar secondary side 300 includes a C-phase bridge arm circuit, two DC capacitors, a positive bus, a negative bus and a neutral line; the C-phase bridge arm circuit is composed of an upper bridge arm circuit and a lower bridge arm circuit connected in series, respectively. cu and arm cl , the two DC capacitors in series are C u and C l ; Among them arm cu and C u Connected to the positive bus, arm cl and C l Connected to the negative busbar, the neutral line is connected to C u and C l Between, arm cu 、arm cl Between and C u , C l A tap is drawn between each.

[0059] Furthermore, the C-phase bridge arm circuit is composed of several cascaded sub-modules SM 2 and impedance module SW and a reactor L s wherein each submodule SM is composed of two IGBT switches, a reverse-parallel diode therewith and a parallel DC capacitor, and the impedance module SW is composed of a switch and an impedance in parallel.

[0060] It should be noted that the bipolar secondary side 300 is a modular multi-level single-phase half-bridge structure.

[0061] Furthermore, the AC unit 200 is a medium-high frequency isolation transformer, the primary side of the medium-high frequency isolation transformer is connected to the two taps of the unipolar primary side 100, and the secondary side of the medium-high frequency isolation transformer is connected to the two taps of the bipolar secondary side 300.

[0062] It should be noted that the frequency of the medium and high frequency isolation transformer and the IGBT inside the sub-module is 1000 Hz, which can reduce the volume and weight of the equipment, make the equipment lightweight, reduce the manufacturing cost of the equipment, facilitate the transportation and assembly of the equipment, and reduce the footprint.

[0063] During normal operation, the A phase upper bridge arm circuit arm au And B phase lower bridge arm circuit arm bl The submodule SM is put into operation or removed at the same time, and the lower bridge arm circuit arm of phase A al And B phase upper bridge arm circuit arm bu The sub-modules SM of the upper bridge arm circuit and the lower bridge arm circuit of the same phase are put into operation or removed at the same time, and the sub-modules SM of the upper bridge arm circuit and the lower bridge arm circuit of the same phase are put into operation complementary to each other; the sub-modules SM of the upper and lower bridge arm circuits of the C phase are put into operation complementary to each other, and the switch of the impedance module SW is closed.

[0064] When the positive busbar on the bipolar side is disconnected, the C phase upper arm circuit arm cu The switch of the impedance module SW is disconnected; that is, when the output power of the positive bus is zero, the negative bus can still output power; when the negative bus on the bipolar side is disconnected, the C phase lower bridge arm circuit arm cl The switch of the impedance module SW is disconnected. When the output power of the negative bus is zero, the positive bus can still output power. This mode is called unipolar operation mode, which can temporarily prevent power transmission interruption. The existing solution will interrupt power transmission once a disconnection fault occurs.

[0065] Therefore, it can be seen that the advantage also lies in that the bipolar side of the existing single-stage and double-stage conversion DC transformer is generally cascaded by two sets of independent converters to realize the bipolar wiring mode, in which the AC link generally adopts a three-winding transformer or directly adopts two double-winding transformers in parallel. The AC link of this device adopts a double-winding transformer, and only one set of converters is used on the bipolar side, which reduces the cost of the entire system. At the same time, the voltage matching ratio of the existing unipolar to unipolar DC transformer is 1. When there is no neutral line or the neutral point is not grounded between the two output concentrated capacitors of the present invention, the converter is converted into a unipolar to unipolar DC transformer. Since the secondary side is a modular multi-level (MMC) single-phase half-bridge structure, the voltage matching ratio can be achieved to be 1 / 2, which reduces the insulation requirements of the medium and high frequency isolation transformer.

[0066] Example 2

[0067] Reference Figures 3 to 6 , which is an embodiment of the present invention, provides a single-bipolar conversion high-voltage large-capacity DC transformer and a control method thereof, including: a single-direction control strategy, a sub-module capacitor voltage balancing control strategy, and a DC bias suppression strategy.

[0068] Furthermore, the single-direction control strategy includes a voltage control mode and a power control mode.

[0069] It should be noted that the single-direction control strategy is used to control the magnitude of power transmission.

[0070] Specifically, the voltage control mode is mainly used in the interconnection between the renewable energy power generation side and the transmission system. The control takes the unipolar side voltage as the control object. The system collects the unipolar side DC voltage and compares it with the preset reference value. The difference is the power shift angle of the primary and secondary side voltage of the system after passing through the PI controller. Power shift angle It should be kept within the range of [-π / 2, π / 2], and after limiting the shift angle, the pulse signal required by the switching device inside each sub-module SM is generated through pulse width modulation.

[0071] Specifically, the power control mode is mainly used in the interconnection between power grids. The control takes the transmission power of the converter as the control object. The system collects the input power of the unipolar side and compares it with the preset reference value. The difference is the power shift angle of the primary and secondary voltages of the system after passing through the PI controller. Power shift angle It should be kept within the range of [-π / 2, π / 2], and after limiting the shift angle, the pulse signal required by the internal switching device of each sub-module SM is generated through pulse width modulation.

[0072]

[0073] in,

[0074] Where: P is the transmission power, N is the transformer ratio, V dc1 is the primary input voltage on the unipolar side, V dc2 is the bipolar side secondary output voltage, is the power transfer angle, D is the power transfer ratio, T h is half a switching cycle, d n is the maximum shift ratio between the submodules in each bridge arm circuit of phase A and phase B, d m is the maximum shift ratio between the submodules in each bridge arm circuit of phase C, L eq is the equivalent inductance of the device, L p is the primary bridge arm inductance, L s is the secondary bridge arm inductance, L t For externally connected inductor.

[0075] The above formula shows that by adjusting the power shift angle (Power shift ratio D) controls the size of power P, and this strategy formula is more general and universal.

[0076] Furthermore, the sub-module capacitor voltage balancing control strategy includes sampling and sorting the capacitor voltage of the sub-module SM in each bridge arm, using a driving signal with a smaller phase shift angle to control a sub-module with a lower capacitor voltage, and using a driving signal with a larger phase shift angle to control a sub-module with a higher capacitor voltage.

[0077] It should be noted that the submodule capacitor voltage balancing control ensures the stability of the capacitor voltage inside the submodule SM in the bridge arm during normal operation.

[0078] Specifically, energy balance of the submodule SM is a necessary condition for normal operation of the device. The energy of the submodule SM is reflected in the capacitor voltage of the submodule SM. Each submodule in the bridge arm is driven by a corresponding driving signal.

[0079]

[0080] Where: ΔQ is S k+1 The submodules of the driver and S k The difference in the energy of the driven submodule capacitance, d k is the shift ratio between the kth submodules, d k+1 is the shift ratio between the k+1th submodules, d n is the maximum shift ratio between submodules, and D is the power shift ratio.

[0081] where d k+1 >d k , d n <<D, so the right side of the inequality is less than zero. The above formula shows that using a larger shift ratio to drive the submodule makes the corresponding submodule have less energy in one switching cycle.

[0082] At the same time, the capacitor voltage of the submodule SM in each bridge arm is sampled and sorted, and a driving signal with a smaller phase shift angle is used to control the submodule with a lower capacitor voltage, and a driving signal with a larger phase shift angle is used to control the submodule with a higher capacitor voltage. This control method does not require measuring the direction of the bridge arm current.

[0083] Furthermore, DC bias suppression strategies include:

[0084] S1: introducing a driving signal shift angle between the upper and lower bridge arm circuits of the A and B phases of the single- and bipolar conversion high-voltage large-capacity DC transformer;

[0085] It should be noted that during the modulation process of the single-bipolar conversion high-voltage and large-capacity DC transformer, a phase shift angle of the driving signal between the bridge arms is introduced, that is, the driving signal of the upper bridge arm of phase A and the lower bridge arm of phase B is ahead of the driving signal of the lower bridge arm of phase A and the upper bridge arm of phase B, or the driving signal of the upper bridge arm of phase A and the lower bridge arm of phase B is lagged behind the driving signal of the lower bridge arm of phase A and the upper bridge arm of phase B; the advance or lag depends on the direction of the required injected DC component.

[0086] Specifically,

[0087]

[0088] Wherein, γ is the phase shift angle of the driving signal, d arm is the displacement ratio between bridge arms.

[0089] S2: Relying on bridge arm reactor L p Active power is transferred between the upper and lower bridge arm circuits, thereby changing the voltage of the upper and lower bridge arms;

[0090] It should be noted that due to the phase shift angle of the driving signal, an AC voltage will be generated on the bridge arm inductor, and the AC voltage acts on the bridge arm reactor L p An active AC current is generated, which is equivalent to opening up a power transmission channel between the upper and lower bridge arm circuits.

[0091] At the same time, due to the effect of the power transmission channel, the energy of the bridge arm changes, and the energy of the bridge arm is reflected by the capacitor voltage of the bridge arm submodule, thereby changing the voltage of the upper and lower bridge arms;

[0092] As a feasible method, the absolute value of the voltage change of the upper and lower bridge arms can be expressed as:

[0093]

[0094] Among them, d arm is the shift ratio between bridge arms, L p The bridge arm inductance is, f is the switching frequency, V dc1 is the DC voltage on the unipolar side, R eq is the equivalent resistance of the bridge arm resistance of the equipment and the leakage resistance of the AC transformer, ΔV is the absolute value of the change in the bridge arm voltage, and P is the transmission power.

[0095] S3: Changing the DC component of the primary voltage of the AC transformer to achieve active DC injection;

[0096] Specifically, the DC component of the primary voltage of the AC transformer is

[0097]

[0098] In the above formula, V al For the bridge arm al Voltage, V au For the bridge arm au Voltage, V bl For the bridge arm bl Voltage, V bu For the bridge arm bu The above formula shows that the inconsistency of the upper and lower bridge arm voltages (Val ≠V au or V bl ≠V bu ) will superimpose a DC voltage on the primary side of the AC transformer, thereby achieving active DC injection.

[0099] S4: Adjust the injected DC component I by changing the shift ratio between the bridge arms dc , thereby suppressing the DC bias phenomenon of the AC transformer.

[0100] It should be noted that the DC bias suppression strategy is mainly used to prevent magnetic saturation in medium and high frequency AC transformers.

[0101] The shift ratio between bridge arms and the DC component I dc The relationship is

[0102]

[0103] In the above formula, d arm is the shift ratio between bridge arms, L p The bridge arm inductance is, f is the switching frequency, V dc1 is the DC voltage on the unipolar side, I dc is the magnitude of the injected DC component, R eq is the equivalent resistance of the bridge arm resistance of the equipment and the leakage resistance of the AC transformer, and P is the transmission power. The above formula shows that by changing the shift ratio d between the bridge arms arm The injected DC component I can be adjusted dc .

[0104] In the modulation process of the DC transformer, a phase shift angle of the driving signal between the bridge arms is introduced, so that the driving signal of the upper bridge arm of phase A and the lower bridge arm of phase B is ahead of the driving signal of the lower bridge arm of phase A and the upper bridge arm of phase B, or the driving signal of the upper bridge arm of phase A and the lower bridge arm of phase B lags behind the driving signal of the lower bridge arm of phase A and the upper bridge arm of phase B, thereby changing the instantaneous voltage of the upper and lower bridge arms, and actively injecting a DC current into the primary side of the transformer to suppress the DC bias magnetic phenomenon that may occur in the AC transformer. This DC bias magnetic suppression strategy is not limited to the topological structure of the present invention, and can also be applied to a common unipolar to unipolar modular multi-level DC transformer (MMDC).

[0105] And its advantage also lies in that, when the power transmission changes, the average value of the excitation current is not zero within a cycle, that is, a DC bias current is generated. Under normal circumstances, the DC bias current is slowly consumed through the parasitic resistance of the component itself. The present invention introduces a shift angle of the drive signal between the upper and lower bridge arms on the unipolar primary side to actively inject DC into the primary side of the AC transformer to reduce the time when the DC bias phenomenon occurs.

[0106] Example 3

[0107] Reference Figures 7 to 21 In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail and its effects are verified in combination with the accompanying drawings and specific embodiments. Figure 1 The single-pole and bipolar conversion DC transformer shown in the figure is modeled on MATLAB / Simulink software for verification, and the simulation parameters are shown in Table 1.

[0108] Table 1

[0109]

[0110]

[0111] Figure 7 to Figure 10 is the output power of bipolar symmetrical operation (P out1 =50MW, P out2 =50MW), AC side primary and secondary voltage v ac1 、v ac2 , the original secondary current i ac1 、i ac2 , excitation current i m , the displacement ratio between bridge arms d arm The simulation experiment waveform diagram. It can be seen from the simulation waveform diagram that the voltage and current of the primary and secondary sides remain stable, and the excitation current of the isolation transformer has no DC component, indicating that the equipment can maintain a good bipolar symmetrical operation state.

[0112] Fig.12 For power transfer changes (at Fig.11 Under the condition of power transmission change), the excitation current simulation experiment waveform without DC bias suppression control, the excitation current i m A DC component appears; Fig.13 For power transfer changes (at Fig.11 Under the condition of power transmission change), the excitation current simulation experiment waveform under DC bias suppression control is shown in Figure 1. The excitation current i m A DC component appears. Fig.12 and Fig.13 It can be seen from the comparison of the simulation experiment waveforms that in the case of a sudden change in transmission power, through the design of our DC transformer and the application of DC bias suppression strategy, the time interval of the DC bias phenomenon is shortened from 3 seconds (1s-4s) to 0.25 seconds (1s-1.25s). Fig.12 and Fig.13 The comparison illustrates the effectiveness of DC bias suppression control, which enables the transformer to recover the optimal operating state as quickly as possible under changes in power transmission.

[0113] Figure 14 to Figure 17 is the output power after the negative busbar is disconnected (Pout1 =50MW, P out2 =0), AC side primary and secondary voltage v ac1 and v ac2 , the original secondary current i ac1 and i ac2 , excitation current i m , the displacement ratio between bridge arms d arm The simulation waveform diagram shows that after the negative busbar is disconnected, the positive busbar can still maintain power transmission; the excitation current i m There is no DC component, which means that the isolation transformer can still maintain operation.

[0114] Figure 18 to Figure 21 is the output power after the positive busbar is disconnected in the implementation scheme (P out1 =0,P out2 =50MW), AC side primary and secondary voltage v ac1 and v ac2 , the original secondary current i ac1 and i ac2 , excitation current i m , the displacement ratio between bridge arms d arm The simulation waveform diagram shows that after the negative busbar is disconnected, the positive busbar can still maintain power transmission; the excitation current i m There is no DC component, which means that the isolation transformer can still maintain operation.

[0115] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A single-pole and bipolar conversion high-voltage and large-capacity DC transformer, characterized in that: include: A unipolar primary side (100), the unipolar primary side (100) comprising two-phase A and B bridge arm circuits, a DC capacitor, a positive bus bar, and a negative bus bar; A bipolar secondary side (300), the bipolar secondary side (300) comprising a C-phase bridge arm circuit, two DC capacitors, a positive bus bar, a negative bus bar and a neutral line; An alternating current unit (200), the alternating current unit (200) being connected between the unipolar primary side (100) and the bipolar secondary side (300); The A and B two-phase bridge arm circuits are composed of an upper bridge arm circuit and a lower bridge arm circuit in series, and the upper and lower bridge arm circuits of phase A are arm au 、arm al , the upper and lower bridge arm circuits of phase B are arm bu 、arm bl , the DC capacitance is C i ; Among them, arm au and arm bu Connected to the positive bus, arm al and arm bl Connected to the negative bus, arm au 、arm al Between and arm bu 、arm bl A tap is drawn between each, and the DC capacitor C i Connect in parallel between the positive and negative busbars; The A and B two-phase bridge arm circuits include a plurality of cascaded submodules SM and a reactor L p ; Wherein, the submodule SM is composed of two IGBT switches, a diode connected in reverse parallel thereto, and a parallel DC capacitor; The C-phase bridge arm circuit is composed of an upper bridge arm circuit and a lower bridge arm circuit in series. cu and arm cl , the two DC capacitors in series are C u and C l ; Among them arm cu and C u Connected to the positive bus, arm cl and C l Connected to the negative busbar, the neutral line is connected to C u and C l Between, arm cu 、arm cl Between and C u , C l A tap is drawn between each; The C-phase bridge arm circuit is composed of several cascaded sub-modules SM, impedance modules SW and a reactor L s Each submodule SM is composed of two IGBT switches, a diode connected in reverse parallel thereto, and a DC capacitor connected in parallel, and the impedance module SW is composed of a switch and an impedance connected in parallel; The AC unit (200) is a medium-high frequency isolation transformer, the primary side of the medium-high frequency isolation transformer is connected to two taps of the unipolar primary side (100), and the secondary side of the medium-high frequency isolation transformer is connected to two taps of the bipolar secondary side (300); Introducing a drive signal phase shift angle between the upper and lower bridge arm circuits of the A and B phases of the single- and bipolar conversion high-voltage large-capacity DC transformer; Relying on the bridge arm reactor L p Active power is transferred between the upper and lower bridge arm circuits, thereby changing the instantaneous voltage of the upper and lower bridge arms; Active DC injection is achieved by changing the DC component of the primary voltage of the AC transformer; By changing the bridge arm shift ratio, the injected DC component I dc , thereby suppressing the DC bias phenomenon of the AC transformer.

2. The single-pole and bipolar conversion high-voltage and large-capacity DC transformer according to claim 1, characterized in that: The driving signal phase shift angle is obtained by outputting the Pi controller.

3. The single-pole and bipolar conversion high-voltage and large-capacity DC transformer according to claim 2, characterized in that: The DC component of the primary voltage includes: Among them, V al For the bridge arm al Voltage, V au For the bridge arm au Voltage, V bl For the bridge arm bl Voltage, V bu For the bridge arm bu voltage.

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