High-voltage high-capacity self-coupling direct-current transformer and working method and control method thereof

Abstract

The invention provides a high-voltage high-capacity self-coupling direct-current transformer. The high-voltage high-capacity self-coupling direct-current transformer comprises a support capacitor, an energy buffer bridge arm and a converter valve assembly, the support capacitor is used for forming a part of power transmission loop; the energy buffer bridge arm is used for realizing main power transmission and forming the other part of power transmission loop; converting of the transformer is achieved by controlling the input number of the bridge arm sub-modules of the energy buffer bridge arm and trigger signals of the converter valve assembly. Meanwhile, the invention provides a commutation working method and a control method of the transformer. Energy transfer from a low-voltage side to a high-voltage side is realized, the efficiency of the transformer is improved, and energy balance of the energy buffer bridge arm is realized; reliable turn-off and approximate zero-voltage conduction of the converter valve are realized by controlling the input number of the energy buffer bridge arm sub-modules, and two closed-loop modes of direct-current voltage and power control are provided at the same time. High-voltage and high-capacity direct-current transformation is realized, and meanwhile, the semi-control device converter valve is used, so that the device cost is reduced.

Description

Technical Field

[0001] The present invention relates to the field of smart grid, power electronics and control technology in power systems, and in particular to a high-voltage and large-capacity autocoupled direct current transformer and a working method and a control method thereof, as well as a corresponding control system. Background Art

[0002] Wind power generation has become the most promising renewable energy generation technology in the face of energy shortages and severe climate deterioration due to its low development cost and relatively mature power generation technology.

[0003] The development of offshore wind power mainly adopts the AC convergence-AC transmission scheme, which has the advantages of low voltage boosting and networking costs. In deep sea application scenarios, due to problems such as cable network charging power, AC convergence-flexible DC transmission is the main solution. However, with the development trend of ultra-large capacity of single offshore wind power units, there are problems such as charging power in the wind farm convergence network. The use of DC convergence-transmission solutions can avoid reactive power loss or reactive power compensation equipment, and effectively solve the problems of multi-machine grid-connected stable control and wide-area interconnection networking of wind farm groups. For the application scenario of full DC transmission of deep offshore wind power, high-voltage and large-capacity DC transformers are the core link and key equipment for connecting DC systems of different voltage levels of offshore wind power convergence and transmission networks.

[0004] DC transformers can be divided into two categories: isolated DC transformers and non-isolated DC transformers according to whether the two sides are electrically isolated. The biggest feature of isolated DC transformers is that an isolation transformer is added to the circuit to achieve energy transfer, such as the classic forward converter and flyback converter. This type of converter can achieve large input and output ratio conversion through a large-capacity isolation transformer, but there are also problems such as the weight, volume and loss of the isolation transformer. In the engineering application scenario of hundreds of megawatts, the manufacturing of medium-frequency isolation transformers is difficult and there is no mature technology and application at present. This has limited the development of isolated DC transformers to high voltage and high power at the transmission level to a certain extent. Considering that the offshore wind power grid-connected application has very strict requirements on the weight and volume of the device, the application scenarios of isolated DC transformers are relatively limited. In comparison, non-isolated DC transformers have greater advantages in system efficiency, volume and weight, and are more economical because they omit the isolation transformer.

[0005] By comparing the existing non-isolated DC transformers, it is found that although the full-power DC transformer eliminates the AC transformer, the two-stage power conversion transmission method results in a large number of idle modules during the operation, resulting in a large number of power devices and large losses; the switch coupling type does not have the problem of sub-module energy balance and does not need to inject AC circulating current, but its switching devices need to withstand grid-level voltage, which needs to be achieved by connecting devices in series; the autocoupled DC transformer reduces the number of sub-modules through a common bridge arm, but needs to inject AC circulating current or add additional auxiliary circuits to achieve sub-module capacitor voltage balance.

[0006] In summary, the art is in urgent need of a solution for realizing a high-voltage and large-capacity DC transformer that can meet the application requirements of low cost, high efficiency and high reliability. Currently, no description or report of a technology similar to the present invention has been found, and similar information at home and abroad has not been collected. Summary of the invention

[0007] In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to provide a high-voltage and large-capacity autocoupled DC transformer and its working method and control method based on the application requirements of low cost, high efficiency and high reliability, and at the same time provide a corresponding control system to achieve high voltage and large capacity of the DC transformer, solve the problems of reliable shutdown of the thyristor commutator valve and energy balance of the energy buffer bridge arm, etc.

[0008] According to one aspect of the present invention, a high-voltage and large-capacity auto-coupling DC transformer is provided, comprising: a supporting capacitor, an energy buffer bridge arm and a converter valve connected between a low-voltage side and a high-voltage side of the transformer; the supporting capacitor constitutes a part of a power transmission circuit, which is used to realize power transmission from the low-voltage side to the high-voltage side through the supporting capacitor; the energy buffer bridge arm constitutes another part of a power transmission circuit, which is used to realize power transmission from the low-voltage side to the high-voltage side through the energy buffer bridge arm, power transmission from the energy buffer bridge arm to the supporting capacitor, and supplementing the power transmitted to the high-voltage side by the supporting capacitor; by controlling the number of submodules in the energy buffer bridge arm and the trigger signal of the converter valve, transformer commutation is realized;

[0009] in:

[0010] The support capacitor includes a support capacitor C1 and a support capacitor C2, and the converter valve includes a first converter valve T1 and a second converter valve D1; wherein: the support capacitor C1 is connected in parallel with the low-voltage side, the support capacitor C2 is connected in series with the support capacitor C1, and the two are connected in series with the high-voltage side, so that the low-voltage side and the high-voltage side are directly electrically connected to form a part of the power transmission circuit; the first converter valve T1 is connected in series with the second converter valve D1 and then connected in parallel with the high-voltage side, one end of the first converter valve T1 is connected to the second converter valve D1, and the other end of the first converter valve T1 is connected to the high-voltage side. One end of the energy buffer bridge arm is connected to the negative electrode of the high-voltage side, one end of the second converter valve D1 is connected to the positive electrode of the high-voltage side, and the other end of the second converter valve D1 is connected to the first converter valve T1. The commutation function of the transformer is realized by alternating conduction of the first converter valve T1 and the second converter valve D1; one end of the energy buffer bridge arm is connected to the connection between the support capacitor C1 and the support capacitor C2, and the other end of the energy buffer bridge arm is connected to the connection between the first converter valve T1 and the second converter valve D1. The control of the transformer current is realized by controlling the switching quantity of the sub-modules in the bridge arm.

[0011] According to another aspect of the present invention, there is provided a method for operating a high-voltage and large-capacity auto-coupling DC transformer, comprising:

[0012] When power flows from the low-voltage side of the transformer to the high-voltage side, there are two commutation working modes according to the switching status of the converter valve:

[0013] Working mode 1: the first converter valve T1 is turned off and the second converter valve D1 is turned on. At this time, the low-voltage side is connected in series with the energy buffer bridge arm to transfer power to the high-voltage side. The capacitor C2 is charged by the energy buffer bridge arm.

[0014] In working mode 2, the first converter valve T1 is turned on and the second converter valve D1 is turned off. At this time, the low-voltage side transfers power to the energy buffer bridge arm, and the capacitor C2 discharges to the high-voltage side.

[0015] According to a third aspect of the present invention, there is provided a control method for a high-voltage and large-capacity auto-coupling DC transformer, comprising:

[0016] The converter valve commutation control part controls the current flowing through the converter valve. When the converter valve adopts a semi-controlled device, it is used to achieve reliable shutdown of the converter valve; when the converter valve adopts a full-controlled device, it is used to achieve zero current shutdown of the converter valve and reduce shutdown loss;

[0017] The energy buffer bridge arm current control part controls the switch of the bridge arm submodule, thereby controlling the voltage of the energy buffer bridge arm to achieve closed-loop control of the current; at the same time, it generates a current reference signal i for realizing the DC power / voltage control part and the energy buffer bridge arm energy balance control part. SM_Hrefand the current reference signal i SM_Lref ;

[0018] DC power / voltage control part, the current reference signal i SM_Href Adjustment is performed to achieve power closed-loop control and high-voltage side output voltage stability;

[0019] Energy buffer bridge arm energy balance control part, the current reference signal i SM_Lref Adjustment is performed to achieve a constant overall capacitance storage in the energy buffer bridge arm;

[0020] The grid voltage feedforward control part adjusts the voltage of the energy buffer bridge arm to achieve the approximate zero voltage conduction condition of the converter valve and limit du / dt;

[0021] The submodule capacitor voltage balancing control part adjusts the voltage reference signal of each bridge arm submodule to achieve capacitor voltage balancing of the bridge arm submodule;

[0022] The carrier phase shift modulation control part adjusts the voltage reference signal of the bridge arm submodule to generate the IGBT drive signal of each bridge arm submodule.

[0023] According to a fourth aspect of the present invention, there is provided a control system for a high-voltage and large-capacity auto-coupling DC transformer, comprising:

[0024] The converter valve commutation control module is used to control the current flowing through the converter valve. When the converter valve adopts a semi-controlled device, it is used to achieve reliable shutdown of the converter valve; when the converter valve adopts a fully controlled device, it is used to achieve zero current shutdown of the converter valve and reduce shutdown losses;

[0025] The energy buffer bridge arm current control module is used to control the switch of the bridge arm submodule, thereby controlling the voltage of the energy buffer bridge arm and realizing closed-loop control of the current; at the same time, it generates a current reference signal i for realizing the DC power / voltage control part and the energy buffer bridge arm energy balance control part. SM_Href and the current reference signal i SM_Lref ;

[0026] DC power / voltage control module, which is used to control the current reference signal i SM_Href Adjust to achieve power closed-loop control and stable output voltage on the high-voltage side;

[0027] Energy buffer bridge arm energy balance control module, which is used to control the current reference signal i SM_Lref Adjustment is performed to achieve a constant overall capacitance storage in the energy buffer bridge arm;

[0028] A grid voltage feedforward control module, which is used to adjust the voltage of the energy buffer bridge arm, achieve the approximate zero voltage conduction condition of the converter valve and limit du / dt;

[0029] A submodule capacitor voltage balancing control module, which is used to adjust the voltage reference signal of each bridge arm submodule to achieve capacitor voltage balancing of the bridge arm submodule;

[0030] The carrier phase shift modulation control module is used to adjust the voltage reference signal of the bridge arm submodule and generate the IGBT drive signal of each bridge arm submodule.

[0031] Due to the adoption of the above technical solution, the present invention has at least one of the following beneficial effects compared with the prior art:

[0032] The high-voltage and large-capacity autocoupled DC transformer provided by the present invention realizes energy transfer from the low-voltage side to the high-voltage side by using the energy buffer bridge arm and the supporting capacitor as the medium; at the same time, the thyristor commutator valve is used to realize zero-current shutdown, thereby improving the efficiency of the transformer.

[0033] The working method and control method of the high-voltage and large-capacity autocoupled DC transformer provided by the present invention realize the energy balance of the energy buffer bridge arm, and realize the reliable shutdown and approximately zero-voltage conduction of the thyristor converter valve by controlling the number of energy buffer bridge arm sub-modules put into use. In addition, considering different application scenarios, two closed-loop modes of DC voltage control and power control are realized.

[0034] The high-voltage and large-capacity autocoupled DC transformer and its working method and control method provided by the present invention adopt an autocoupled structure and use an active energy buffer bridge arm to achieve high-voltage and large-capacity DC transformation; at the same time, a semi-controlled device converter valve is used to reduce the cost of the device.

[0035] The high-voltage and large-capacity autocoupled DC transformer and its working method and control method provided by the present invention realize the high voltage and large capacity of the DC transformer through the autocoupled structure and the energy buffer bridge arm; through the control of the thyristor commutator valve and the energy buffer bridge arm, the thyristor commutator valve can be reliably shut down and approximately zero-voltage conduction and zero-voltage shutdown can be achieved, thereby improving the efficiency of the transformer.

[0036] The high-voltage and large-capacity autocoupled DC transformer and its working method and control method provided by the present invention can be applied to an offshore wind power DC collection system. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0038] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0039] Figure 1 A basic topological structure diagram of a high-voltage and large-capacity auto-coupling DC transformer in a preferred embodiment of the present invention;

[0040] Figure 2 A working structure diagram of a control method for a high-voltage and large-capacity auto-coupling DC transformer in a preferred embodiment of the present invention;

[0041] Figure 3 It is a schematic diagram of a high-voltage and large-capacity auto-coupling DC transformer in a preferred embodiment of the present invention, in which the low-voltage side is connected in series with the capacitor C2 to transfer power to the high-voltage side;

[0042] FIG4( a ) is a schematic diagram of the working mode 1 of the high-voltage and large-capacity auto-coupling DC transformer in a preferred embodiment of the present invention;

[0043] FIG4( b ) is a schematic diagram of working mode 2 of a high-voltage and large-capacity auto-coupling DC transformer in a preferred embodiment of the present invention;

[0044] Figure 5 A schematic diagram of a current waveform and a voltage waveform of an energy buffer bridge arm in a preferred embodiment of the present invention;

[0045] Figure 6 A schematic diagram of a current reference value waveform introduced into a converter valve commutation control part in a preferred embodiment of the present invention;

[0046] Figure 7 A control block diagram of the current control part of the energy buffer bridge arm in a preferred embodiment of the present invention;

[0047] Figure 8 A control block diagram of a DC power / voltage control part in a preferred embodiment of the present invention;

[0048] Fig. 9 A control block diagram of an energy balance control part of an energy buffer bridge arm in a preferred embodiment of the present invention;

[0049] Fig.10 A control block diagram of a grid voltage feedforward control part in a preferred embodiment of the present invention;

[0050] Fig.11 A control block diagram of a carrier phase shift modulation control part in a preferred embodiment of the present invention;

[0051] Fig.12 This is a control block diagram of a submodule capacitor voltage balancing control part in a preferred embodiment of the present invention. DETAILED DESCRIPTION

[0052] The following is a detailed description of the embodiments of the present invention: This embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation method and a specific operation process are given. It should be pointed out that for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

[0053] In view of the problem that there is a lack of low-cost, high-efficiency and high-reliability implementation solutions for high-voltage and large-capacity DC transformers in the prior art, an embodiment of the present invention provides a high-voltage and large-capacity autocoupled DC transformer, which realizes high voltage and large capacity of the DC transformer through an autocoupled structure and an energy buffer bridge arm; through the control of the thyristor commutator valve and the energy buffer bridge arm, the thyristor commutator valve can be reliably shut down and approximately zero-voltage conduction and zero-voltage shutdown can be achieved, thereby improving the efficiency of the transformer.

[0054] Specifically, Figure 1 As shown, the high-voltage and large-capacity auto-coupling DC transformer provided in this embodiment may include: a support capacitor, an energy buffer bridge arm and a converter valve connected between the low-voltage side and the high-voltage side of the transformer; the support capacitor is used to constitute a part of the power transmission circuit, so that the low-voltage side and the high-voltage side are directly electrically connected, and the two transmit power through the support capacitor, that is, the support capacitor transmits power only by the low-voltage side connecting the support capacitor C2 in series, and jointly transmits energy to the high-voltage side without passing through the energy buffer bridge arm; the energy buffer bridge arm constitutes another part of the power transmission circuit, which is used to realize the main power transmission. Transmission, including: the energy buffer bridge arm transmits power by connecting the energy buffer bridge arm in series on the low-voltage side, and transmits energy to the high-voltage side together; at the same time, the energy buffer bridge arm transmits energy to the supporting capacitor C2 of the supporting capacitor, supplements the energy transmitted to the high-voltage side by the supporting capacitor C2, and maintains the voltage of the supporting capacitor C2 stable; the above two power transmission models are based on the principle that a part of the energy storage unit is connected in series on the low-voltage side, and then energy is transmitted to the high-voltage side together, and the two modes are switched through the converter valve; by controlling the number of bridge arm submodules of the energy buffer bridge arm and the trigger signal of the converter valve, it is used to realize transformer commutation;

[0055] in:

[0056] The support capacitor includes a support capacitor C1 and a support capacitor C2, and the converter valve includes a first converter valve T1 and a second converter valve D1; wherein: the support capacitor C1 is connected in parallel with the low-voltage side, the support capacitor C2 is connected in series with the support capacitor C1, and the two are connected in series with the high-voltage side, so that the low-voltage side and the high-voltage side are directly electrically connected, forming a part of the power transmission circuit; the first converter valve T1 is connected in series with the second converter valve D1 and then connected in parallel with the high-voltage side, one end of the first converter valve T1 is connected to the second converter valve D1, and the other end is connected to the negative electrode of the high-voltage side, one end of the second converter valve D1 is connected to the positive electrode of the high-voltage side, and the other end is connected to the first converter valve, and the converter function of the transformer can be realized by alternately conducting the first converter valve and the second converter valve; one end of the energy buffer bridge arm is connected to the connection between the support capacitors C1 and C2, and the other end is connected to the connection between the first converter valve T1 and the second converter valve D1, and the transformer current can be controlled by controlling the number of submodules switched in the energy buffer bridge arm.

[0057] The energy buffer bridge arm comprises a plurality of bridge arm submodules SM connected in series and a buffer inductor L connected in series with the last bridge arm submodule SM;

[0058] In some preferred embodiments, each of the bridge arm submodules SM includes: a submodule capacitor and a switch device; the submodule capacitor is used to store energy and provide voltage; the switch device is used to control the switching of the working state of the bridge arm submodule; wherein:

[0059] The switch device adopts an output voltage inversion structure, and the submodule capacitor is connected in parallel with the output voltage inversion structure of the switch device.

[0060] In some preferred embodiments, the output voltage inversion structure adopted by the switching device can be a full-bridge structure, wherein the emitter of an IGBT is connected to the cathode of a diode, and the connection point is the positive pole of the bridge arm sub-module; the collector of another IGBT is connected to the anode of another diode, and the connection point is the negative pole of the bridge arm sub-module; the two structures after the series connection are then connected in parallel, the collector of one IGBT is connected to the anode of the diode, and the emitter of the other IGBT is connected to the cathode of the diode.

[0061] In the above preferred implementation manner, the diode therein may also be replaced by an IGBT; and the switch device connection structure may also be replaced by other structures having the output voltage reverse capability.

[0062] In some preferred embodiments, the first converter valve T1 and the second converter valve D1 are respectively composed of unidirectional conductive devices connected in series; wherein:

[0063] The unidirectional conduction type device of the first converter valve T1 is a semi-controlled or fully controlled device;

[0064] The unidirectional conduction type device of the second converter valve D1 can be an uncontrolled, semi-controlled or fully controlled device.

[0065] exist Figure 1 Middle,U L is the voltage on the low voltage side of the transformer; U H is the voltage on the high-voltage side of the transformer; C1 and C2 are supporting capacitors; SM is a bridge arm submodule, and the bridge arm submodules are connected in series to form an energy buffer bridge arm, wherein the structure of the bridge arm submodule includes but is not limited to a full bridge submodule and other structures that can achieve output voltage reversal, Figure 1 The middle bridge arm submodule preferably uses a diode to replace a pair of IGBTs of the full bridge submodule; L is the buffer inductor of the energy buffer bridge arm; D1 and T1 are series-connected converter valves, which can be formed by connecting unidirectional conductive devices such as diodes, thyristors, IGBTs, IGCTs in series, including a switch device voltage equalization circuit, Figure 1 T1 preferably adopts a thyristor commutator valve, and D1 preferably adopts a diode commutator valve.

[0066] With respect to the DC transformer provided in the above embodiment of the present invention, an embodiment of the present invention further provides a working method of a high-voltage and large-capacity auto-coupling DC transformer. When power flows from the low-voltage side of the transformer to the high-voltage side, the working method may include the following two commutation working modes according to the switching conditions of the converter valve:

[0067] Working mode 1: the first converter valve T1 is turned off and the second converter valve D1 is turned on. At this time, the low-voltage side is connected in series with the energy buffer bridge arm to transfer power to the high-voltage side. The capacitor C2 is charged by the energy buffer bridge arm.

[0068] In working mode 2, the first converter valve T1 is turned on and the second converter valve D1 is turned off. At this time, the low-voltage side transfers power to the energy buffer bridge arm, and the capacitor C2 discharges to the high-voltage side.

[0069] For the convenience of analysis, the following takes the case where the first converter valve T1 is a thyristor converter valve and the second converter valve D1 is a diode converter valve as an example to further explain the commutation working mode provided by the above embodiment of the present invention. It should be noted that for converter valves with other structures, the working principle of the working mode is the same as that of this example, and will not be repeated here.

[0070] The high voltage side is equivalent to the load, the thyristor-based commutation valve is equivalent to the thyristor, and the diode-based commutation valve is equivalent to the diode. During the entire working process, when no current flows through the diode D1, the low voltage side and C2 jointly transfer power to the high voltage side, such as Figure 3As shown in the commutation modes of Figure 4 (a) and Figure 4 (b), when power flows from the low-voltage side to the high-voltage side, according to the switching conditions of the converter valve, the high-voltage and large-capacity auto-coupling DC transformer provided by the above embodiment of the present invention mainly has two commutation modes: Mode 1, the thyristor is turned off, the diode is turned on, at this time, the low-voltage side is connected in series with the energy buffer bridge arm, and power is transmitted to the high-voltage side together, and the capacitor C2 is charged by the energy buffer bridge arm; Mode 2, the thyristor is turned on, the diode is turned off, at this time, the low-voltage side transmits power to the energy buffer bridge arm, and the capacitor C2 discharges to the high-voltage side. Figure 5 are the current waveform and voltage waveform of the energy buffer bridge arm. Combining the current and voltage waveforms of the energy buffer bridge arm, each circulating current mode can be divided into 5 different working modes. For the convenience of analysis, it is assumed that the voltage across capacitor C2 and the high-voltage side is approximately unchanged.

[0071] In some preferred embodiments:

[0072] (1) Mode 1 [t0, t1]

[0073] In mode 1, the circuit is in the dead zone, and no current flows through the diode. At this time, the low-voltage side is connected in series with capacitor C2 to transfer energy to the high-voltage side. The current is I C2_rel The energy buffer bridge adjusts the voltage to (U H -U L ), at this time, the sum of the low-voltage side voltage and the energy buffer bridge arm voltage is exactly equal to the high-voltage side voltage, and the voltage across the buffer inductor is 0, so no current flows in the energy buffer bridge arm.

[0074] (2) Mode 2 [t1, t2]

[0075] In mode 2, the voltage of the energy buffer bridge arm is controlled to keep it within the original amplitude (U H -U L ), an amplitude of U is added 0H The sine half wave u 0H (t), the voltage applied to the snubber inductor is u 0H (t), therefore, the current in the energy buffer bridge arm will rise from 0 to I SM_H At the same time, as the current begins to flow through the diode, the current from the low voltage side in mode 1 and the capacitor C2 to the high voltage side gradually decreases to 0, and then reverses, and the energy buffer bridge arm begins to charge the capacitor C2. The charging current increases from 0 to I as the energy buffer bridge arm current increases. C2_cha, the charging current will not flow through the low-voltage side. The purpose of adopting this commutation waveform design method is to prevent a sudden change in the rate of change of the energy buffer bridge arm current, so that the corresponding energy buffer bridge arm output voltage waveform does not have a rapid change, which can reduce the difficulty of controlling the energy buffer bridge arm current. In mode 2, the low-voltage side and the energy buffer bridge arm begin to transfer power to the high-voltage side, and the transferred power gradually increases to the rated value as the current increases.

[0076] (3) Mode 3 [t2, t3]

[0077] In mode 3, the voltage of the energy buffer bridge arm returns to and stabilizes at (U H -U L ), therefore, the voltage across the snubber inductor is 0 and the current will remain at the rated value I SM_H At this time, the power transmitted from the low voltage side and the energy buffer bridge arm to the high voltage side will be maintained at the rated value P H , the energy transferred from the submodule to capacitor C2 is also maintained at P C2_cha .in,

[0078] P H =U H I SM_H (7)

[0079] P C2_cha =(U H -U L )·I C2_cha (8)

[0080] In the formula, I SM_H It is the rated current of the energy buffer bridge arm when transmitting energy to the high voltage side;

[0081] (4) Mode 4 [t3, t4]

[0082] In mode 4, the voltage of the energy buffer bridge arm is controlled to keep it within the original amplitude (U H -U L ), minus an amplitude of U 0H The sine half wave u 0H (t), the voltage applied to the snubber inductor is -u 0H (t), therefore, the current in the energy buffer bridge arm will be SM_H At the same time, the charging current of the energy buffer bridge arm to the capacitor C2 decreases with the energy buffer bridge arm current, from I C2_cha In mode 4, the power transmitted from the low-voltage side and the energy buffer bridge arm to the high-voltage side gradually decreases to 0 as the current decreases.

[0083] (5) Mode 5 [t4, t5]

[0084] In mode 5, the current on the energy buffer bridge arm has dropped to 0. At this time, the circuit enters the dead zone. The low-voltage side is connected in series with capacitor C2 to transfer power to the high-voltage side again. The energy buffer bridge arm voltage adjustment is (U H -U L ), at this time, the sum of the low-voltage side voltage and the energy buffer bridge arm voltage is exactly equal to the high-voltage side voltage, and the voltage across the buffer inductor is 0, so no current flows in the energy buffer bridge arm.

[0085] The above five working modes all belong to the commutation working mode 1. In this commutation working mode, the low-voltage side and the energy buffer bridge arm transfer energy to the high-voltage side, and the energy buffer bridge arm charges the capacitor C2. Among them, when the circuit enters the dead zone, the low-voltage side and the capacitor C2 jointly transfer energy to the high-voltage side. In this commutation working mode, the role of the energy buffer bridge arm is to move the stored energy to the high-voltage side and the capacitor C2, on the one hand to realize the power transfer as a medium for power flow, and on the other hand to ensure the energy balance of the capacitor C2.

[0086] In some preferred embodiments:

[0087] (6) Mode 6 [t5, t6]

[0088] In mode 6, the low voltage side is connected in series with capacitor C2 to transfer energy to the high voltage side, and the current is I C2_rel , and this process will continue throughout the entire commutation mode 2, so it will not be repeated in the subsequent modal analysis. The energy buffer bridge arm adjusts the voltage to -U L At this time, the voltage on the low voltage side is equal to the voltage of the energy buffer bridge arm, and the voltage across the buffer inductor is 0, so no current flows through the energy buffer bridge arm. For the thyristor converter valve, the voltage across the converter valve is also approximately 0, creating conditions for the thyristor to be turned on at approximately zero voltage.

[0089] (7) Mode 7 [t6, t7]

[0090] In mode seven, the energy buffer bridge arm voltage is controlled to keep the original amplitude -U L On the basis of 0L The sine half wave u 0L (t), the voltage applied to the snubber inductor is u 0L (t), therefore, the current in the energy buffer bridge arm will rise from 0 to I SM_L The low-voltage side starts to transfer power to the energy buffer bridge arm, and the transferred power gradually increases to the rated value as the current increases. The commutation design method here is similar to that of mode 2, which is also to limit the change rate of the energy buffer bridge arm current to not change suddenly and reduce the control difficulty.

[0091] (8) Mode eight [t7, t8]

[0092] In mode eight, the voltage of the energy buffer bridge arm returns to and stabilizes at -U L Therefore, the voltage across the snubber inductor is 0 and the current will be maintained at the rated value I SM_L At this time, the power transmitted from the low voltage side and the energy buffer bridge arm to the high voltage side will be maintained at the rated value P L .in,

[0093] P L =U L I SM_L (9)

[0094] In the formula, I SM_L The rated current of the energy buffer bridge arm when absorbing energy from the low voltage side

[0095] (9) Mode nine [t8, t9]

[0096] In mode nine, the voltage of the energy buffer bridge arm is controlled to keep it within the original amplitude -U L Based on this, a value with an amplitude of U is subtracted 0L The sine half wave u 0L (t), the voltage applied to the snubber inductor is -u 0L (t), therefore, the current in the energy buffer bridge arm will be SM_L The power transmitted from the low-voltage side to the energy buffer bridge arm gradually decreases to 0 as the current decreases.

[0097] (10) Mode 10[t9,t 10 ]

[0098] In mode 10, the current on the energy buffer bridge arm has dropped to 0, and the circuit enters the dead zone. The energy buffer bridge arm voltage adjustment is -U L At this time, the voltage on the low-voltage side is equal to the voltage of the energy buffer bridge arm, and the voltage across the buffer inductor is 0, so no current flows in the energy buffer bridge arm.

[0099] The above five working modes all belong to the commutation working mode 2. In this commutation working mode, the low-voltage side transfers energy to the energy buffer bridge arm, and the low-voltage side and capacitor C2 jointly transfer energy to the high-voltage side. In this commutation working mode, the function of the energy buffer bridge arm is to store the power transferred from the low-voltage side in the submodule capacitor to provide energy for the next cycle of commutation working mode 1.

[0100] With respect to the DC transformer provided in the above embodiment of the present invention, an embodiment of the present invention further provides a control method for a high-voltage and large-capacity auto-coupling DC transformer, such as Figure 2As shown, it mainly includes: the commutation control part of the converter valve, the current control part of the energy buffer bridge arm, the DC power / voltage control part, the energy balance control part of the energy buffer bridge arm, the grid voltage feedforward control part, the submodule capacitor voltage balance control part and the carrier phase shift modulation control part. Among them, the purpose of the commutation control of the converter valve is to achieve the reliable shutdown of the thyristor converter valve and the approximate zero voltage conduction condition; the purpose of the DC power / voltage control is to achieve the closed-loop control of the system; the purpose of the energy balance control of the energy buffer bridge arm is to ensure that the output of the energy buffer bridge arm remains stable during the circuit operation process; the purpose of the energy buffer bridge arm current control is to ensure that the DC power / voltage control has good dynamic and steady-state performance; the purpose of the grid voltage feedforward control is to create the thyristor approximate zero voltage conduction condition and limit du / dt to protect the device from damage; the purpose of the submodule capacitor voltage balance is to ensure the submodule capacitor voltage balance and ensure the normal operation of the transformer; the purpose of the carrier phase shift modulation is to give the corresponding IGBT control signal according to the reference value of the input energy buffer bridge arm capacitor voltage.

[0101] Furthermore, the control method of the high-voltage and large-capacity auto-coupling DC transformer may further include the following operations:

[0102] The converter valve commutation control part controls the current flowing through the converter valve. When the converter valve adopts a semi-controlled device, it is used to achieve reliable shutdown of the converter valve; when the converter valve adopts a fully controlled device, it is used to achieve zero current shutdown of the converter valve and reduce shutdown loss. Among them, the control purpose of the converter valve commutation control part is to control the current to zero. Fully controlled switching devices can be turned on and off regardless of whether the current is 0. However, if the switching device is turned off when the current is not 0, it will cause shutdown losses, reduce efficiency, and cause problems such as heating. At this time, controlling the current to 0 and then turning it off can effectively reduce the shutdown loss. The characteristics of the semi-controlled device itself require that the current must be reduced to below the holding current when shutting down. Generally, shutdown can be achieved by controlling the current to 0. Therefore, the current of the semi-controlled device is 0 to achieve reliable shutdown. In addition, since the shutdown of the semi-controlled device requires that the current must be approximately 0, it can also be considered that there is no shutdown loss or the loss is extremely small.

[0103] The energy buffer bridge arm current control part controls the switch of the bridge arm submodule, thereby controlling the voltage of the energy buffer bridge arm to achieve closed-loop control of the current; at the same time, it generates a current reference signal i for realizing the DC power / voltage control part and the energy buffer bridge arm energy balance control part. SM_Href and the current reference signal i SM_Lref ;

[0104] DC power / voltage control part, the current reference signal i SM_Href Adjustment is performed to achieve power closed-loop control and high-voltage side output voltage stability;

[0105] Energy buffer bridge arm energy balance control part, the current reference signal i SM_Lref Adjustment is performed to achieve a constant overall capacitance storage in the energy buffer bridge arm;

[0106] The grid voltage feedforward control part adjusts the voltage of the energy buffer bridge arm to achieve the approximate zero voltage conduction condition of the converter valve and limit du / dt;

[0107] The submodule capacitor voltage balancing control part adjusts the voltage reference signal of each bridge arm submodule to achieve capacitor voltage balancing of the bridge arm submodule;

[0108] The carrier phase shift modulation control part adjusts the voltage reference signal of the bridge arm submodule to generate the IGBT drive signal of each bridge arm submodule.

[0109] For the convenience of analysis, the control method provided by the above embodiment of the present invention is further described in detail below by taking the case where the first converter valve T1 is a thyristor converter valve and the second converter valve D1 is a diode converter valve as an example. It should be noted that for converter valves with other structures, the working principle of the control method is the same as that of this example, and will not be repeated here.

[0110] The reliable shutdown of the semi-controlled device is the key to the normal operation of the DC transformer. If the thyristor commutator valve cannot be shut down, it will lead to commutation failure, overcurrent, and damage to the device. At the same time, the energy buffer bridge arm is the medium for energy transfer, and its energy balance problem affects the voltage equalization problem of each sub-module capacitor, thereby affecting power transfer. According to the topological structure and working mode of the high-voltage and large-capacity autocoupled DC transformer analyzed above, in the circuit operation process, to achieve closed-loop control, it is necessary to first solve the reliable shutdown of the thyristor, as well as the energy buffer bridge arm, sub-module capacitor energy balance and other control problems. The control of the DC transformer often adopts the current inner loop and the voltage outer loop control, but when the input voltage and output voltage of the DC transformer are controlled by the front and rear stage devices respectively, the voltage outer loop control will increase the complexity of the control. Therefore, this embodiment proposes DC voltage / power control. When the input and output voltages are constant, the power outer loop is adopted to reduce the control complexity.

[0111] In some preferred embodiments:

[0112] (1) Converter valve commutation control

[0113] Since the converter valve is the switching element of the high-voltage and large-capacity auto-coupling DC transformer, and the turn-off characteristics of the thyristor converter valve require that the current flowing through the thyristor drop below the holding current, the reliable turn-off of the thyristor converter valve is the basis for the stable operation of the circuit. On the other hand, due to the characteristics of the high-voltage and large-capacity auto-coupling DC transformer control method, the ZVS condition of the thyristor when it is turned on can be achieved in the converter valve commutation control, thereby effectively reducing the switching loss and improving the circuit efficiency. Figure 5 Taking the [t8, t9] period of the converter valve as an example, the converter valve commutation control is implemented by additionally introducing a current reference value in this period, so that the current flowing through the thyristor can stably drop below the maintenance current, thereby realizing reliable shutdown of the thyristor. The waveform of the control signal is generated by the waveform generator 3 and is generated once in a converter valve switching cycle. Figure 6 Schematic diagram of the current reference value waveform introduced for the converter valve commutation control.

[0114] (2) Energy buffer bridge arm current control

[0115] According to the power control principle of high-voltage and large-capacity auto-coupling DC transformer, the power transfer between the low-voltage side and the high-voltage side is mediated by the energy buffer bridge arm. In order to meet the rated requirements of the transferred power, the current of the energy buffer bridge arm needs to be regulated. Its control method mainly adopts the PI controller, and introduces a current inner loop control on the basis of the outer loop control. Due to the characteristics of the high-voltage and large-capacity auto-coupling DC transformer topology, the control of the energy buffer bridge arm current can be achieved by adjusting the output voltage of the energy buffer bridge arm. Assuming that the given current reference signal is i SM_ref , by calculating its difference with the actual current i SM The difference is then sent to the PI controller for calculation, so that its output affects the switch drive signal of the submodule, thereby controlling the voltage of the energy buffer bridge arm and realizing closed-loop control of the current. Figure 7 The energy buffer bridge arm current control block diagram is shown in FIG.

[0116] Among them, the energy buffer bridge arm current reference signal i SM_ref It is mainly generated by the superposition of two parts. One part is the current reference signal i when the bridge arm, high-voltage side and low-voltage side form a loop together. SM_Href The other part is the current reference signal i when the bridge arm and the low-voltage side form a loop SM_Lref The two parts are obtained through two outer loop controllers, where the current reference signal i SM_Href Responsible for realizing the DC power / voltage control of AHVDC, and the current reference signal i SM_Lref Used to achieve energy balance of the energy buffer bridge arm.

[0117] (3) DC voltage / power control

[0118] Considering the actual application scenarios of offshore wind power, the outer loop control of high-voltage and large-capacity auto-coupling DC transformers can adopt two different control modes: DC power control or DC voltage control. Figure 8 As shown. Considering the actual application scenario, the high-voltage and large-capacity auto-coupling DC transformer no longer needs to control the constant output voltage. At this time, it is necessary to calculate the current that should flow through the energy buffer bridge arm and the high-voltage side according to the given transmission power reference value, and realize power closed-loop control by controlling the stability of the current. When the voltage on the high-voltage side and the low-voltage side may change, the DC voltage control mode should be used to control the current and the duty cycle of the thyristor commutator valve to achieve the stability of the output voltage on the high-voltage side.

[0119] Furthermore, the DC power control calculates the current that should flow through the energy buffer bridge arm and the high voltage side according to the given transmission power reference value, and controls the voltage across the energy buffer bridge arm inductor by controlling the number of switching submodules of the energy buffer bridge arm, thereby controlling the actual current i of the energy buffer bridge arm. SM Stability, realizing power closed-loop control;

[0120] DC voltage control is started when the voltage on the high-voltage side and the low-voltage side may change, and the actual current i of the bridge arm is buffered by controlling the energy SM and the duty cycle of the first converter valve to achieve a stable output voltage on the high-voltage side;

[0121] in:

[0122] When the energy buffer bridge arm is connected to the high voltage side, the energy buffer bridge arm current reference signal i SM_Href , control the number of switching submodules of the energy buffer bridge arm, thereby controlling the voltage across the inductor of the energy buffer bridge arm, so that the actual current i SM Able to stably follow the current reference signal i SM_Href ,Right now Figure 5 The current reference i at time [t1, t4] SM_Href The following relationship should be satisfied:

[0123]

[0124] Among them, U 0H is the angular frequency of the half-sine wave voltage applied on both sides of the buffer inductor, ω is the angular frequency of the half-sine wave voltage applied on both sides of the buffer inductor, P ref is the DC power reference value transmitted by the converter, U H is the high voltage side voltage, I C2_cha is the charging current amplitude of capacitor C2 when the thyristor is turned off. If the DC voltage control mode is adopted, i SM_HrefThe amplitude is output by the PI controller to eliminate the high-voltage side voltage U H With its reference value U H_ref At the same time, in the DC voltage control mode, it should be noted that the cutoff frequency of the current loop PI controller must be much higher than the cutoff frequency of the voltage loop PI controller, so as to achieve the dynamic structure between the two and the dynamic performance of the current inner loop. SM_Href The waveform shape is determined by waveform generator 1, which Figure 5 A flat-top wave with an amplitude of 1 is generated during the period [t1, t4].

[0125] (4) Energy balance control of energy buffer bridge arm

[0126] The outer loop control of high-voltage and large-capacity auto-coupling DC transformer is to control the high-voltage side power or voltage. As a medium for energy transfer, if the energy of the energy buffer bridge arm cannot be balanced during operation, the controlled high-voltage side power or voltage will deviate from the rated value. Therefore, it is very important to design the energy balance control of the energy buffer bridge arm. The main purpose of this control is to achieve the constant overall energy stored in the capacitor in the energy buffer bridge arm. The energy storage element in the energy buffer bridge arm is the capacitor in the submodule, and the energy transfer of the capacitor is achieved by the current flowing through it, and the capacitor voltage is used as an external manifestation. Therefore, when the output voltage of the energy buffer bridge arm is less than the rated value, more energy needs to be transferred to the submodule capacitor, that is, the charging current is increased; conversely, when the output voltage of the energy buffer bridge arm is greater than the rated value, less energy needs to be transferred to the submodule capacitor, that is, the charging current is reduced. Its control method is to adjust the current reference i when the energy buffer bridge arm forms a loop with the low-voltage side. SM_Lref To achieve. SM_Lref The amplitude is generated by the PI controller, whose input is the submodule capacitor voltage reference value U C_ref The average voltage U of all submodule capacitors in the measured energy buffer bridge arm C_av The deviation between . Fig. 9 This is the energy balance control block diagram of the energy buffer bridge arm.

[0127] When the deviation is greater than 0, the voltage output by the energy buffer bridge arm is less than the rated value, which means that the voltage of the energy buffer bridge arm capacitor is less than the rated value, and more energy needs to be charged to achieve balance. Therefore, it is necessary to increase i through the PI controller. SM_Lref to restore the energy balance; when the deviation is less than 0, the voltage output by the energy buffer bridge arm is less than the rated value, which means that the capacitor voltage of the energy buffer bridge arm is less than the rated value, and more energy needs to be charged to achieve balance. Therefore, it is necessary to reduce i through the PI controller. SM_Lref to restore the energy balance. SM_Lref The waveform shape is determined by waveform generator 2, which Figure 5 During the period [t6, t9], a flat-top wave with an amplitude of 1 is generated.

[0128] (5) Grid voltage feedforward control

[0129] Considering that the voltage transformation of the energy buffer bridge arm is achieved by the input of the submodule and the bypass state switching, and the state switching requires a certain amount of time to achieve, otherwise the device may be damaged by excessive du / dt. Therefore, when the thyristor current is 0, it is necessary to adjust the energy buffer bridge arm voltage step-up or step-down by giving a reference signal, so that before the converter valve is turned on, the voltage at both ends is approximately 0, thereby achieving an approximate zero voltage condition before the thyristor is turned on, and considering the performance of the actual device, du / dt is limited to protect the device from damage. It is superimposed with the output of the energy balance control part of the energy buffer bridge arm to obtain the voltage reference signal u of the energy buffer bridge arm. ref . Fig.10 This is the block diagram of grid voltage feedforward control.

[0130] (6) Submodule capacitor voltage balancing control and PSC-PWM (Phase Shift Modulation) control

[0131] Conventional MMC control can be controlled by PSC-PWM or sorting algorithm. Considering that there are a large number of sub-modules under high voltage conditions, if the sorting algorithm is used for control, it may lead to a long control cycle and affect the dynamic performance of the control. Therefore, the sub-module control of the high-voltage and large-capacity autocoupled DC transformer adopts PSC-PWM control. Its main function is to compare the voltage reference signal of each sub-module with the triangular carrier according to the input. When it is greater than the triangular carrier, the sub-module is put into operation. Otherwise, the sub-module is bypassed to generate the IGBT drive signal of each sub-module. Under ideal conditions, PSC-PWM can achieve the balance of the capacitor voltage of each sub-module. However, due to the differences in actual manufacturing processes, material properties, etc. between each sub-module, there must be certain parameter differences, which will cause differences in capacitor voltage during operation. Therefore, before PSC-PWM modulation, the voltage reference of each sub-module needs to be appropriately adjusted to achieve sub-module capacitor voltage balance, such as Fig.11 shown.

[0132] Specific implementation methods such as Fig.12 As shown, first, the submodule capacitors are numbered from 1 to N, and the capacitor voltage U of the i-th submodule (i = 1, 2, ..., N) is C (i) and the average voltage U of all submodule capacitors in the energy buffer bridge arm C_avThe deviation is sent to the proportional controller. At the same time, considering that the current direction of the energy buffer bridge arm will not change, there is no need to consider the impact of the change in current direction on the positive and negative adjustment amount as in the PSC-PWM control in the MMC control. After the error between the capacitor voltage and the average value is sent to the proportional controller, the obtained proportional coefficient is directly compared with i SM The corresponding submodule voltage reference adjustment value Δu(i) can be obtained by multiplying them. Therefore, if the energy buffer bridge arm current charges the submodule (i SM >0), when the capacitor voltage U C (i) Lower than the average capacitor voltage U of the phase submodule C_av When the positive submodule voltage reference adjustment value Δu(i) is generated, the adjustment value is combined with the energy buffer bridge arm voltage reference u ref Add together to get the voltage reference u of the i-th submodule ref_i This process can be expressed by equation (6).

[0133] u ref_i =u ref +Δu(i)=u ref +K p [U C_av -U C (i)]·i SM (12)

[0134] Where K p is the proportional coefficient of the proportional controller.

[0135] The power absorbed by the submodule is determined by the capacitor voltage and inductor current of the module, and the current flowing through all submodules is consistent. The size of the inductor current is determined by the transmission power, so the power control can be achieved by the length of time the submodule is put into operation. When the energy buffer bridge arm absorbs energy from the low-voltage side, the time of the module with high voltage is reduced and the time of the module with low voltage is increased; when the energy buffer bridge arm releases energy to the high-voltage side, the time of the module with high voltage is increased and the time of the module with low voltage is reduced. Under this control mode, the submodule with high capacitor voltage will absorb less energy (or release more energy), and vice versa, thereby maintaining the capacitor voltage balance between the submodules of the energy buffer bridge arm.

[0136] Based on the same inventive concept, an embodiment of the present invention further provides a control system for a high-voltage and large-capacity auto-coupling DC transformer, which may include the following modules:

[0137] The converter valve control module is used to control the current flowing through the converter valve. When the converter valve adopts a semi-controlled device, the converter valve can be reliably shut down; when the converter valve adopts a fully controlled device, the converter valve can be shut down at zero current to reduce the shutdown loss;

[0138] The energy buffer bridge arm current control module is used to control the switch of the bridge arm submodule, thereby controlling the voltage of the energy buffer bridge arm and realizing closed-loop control of the current; at the same time, it generates a current reference signal i for realizing the DC power / voltage control part and the energy buffer bridge arm energy balance control part. SM_Href and the current reference signal i SM_Lref ;

[0139] DC power / voltage control module, which is used to control the current reference signal i SM_Href Adjust to achieve power closed-loop control and stable output voltage on the high-voltage side;

[0140] Energy buffer bridge arm energy balance control module, which is used to control the current reference signal i SM_Lref Adjustment is performed to achieve a constant overall capacitance storage in the energy buffer bridge arm;

[0141] A grid voltage feedforward control module, which is used to adjust the voltage of the energy buffer bridge arm, achieve the approximate zero voltage conduction condition of the converter valve and limit du / dt;

[0142] A submodule capacitor voltage balancing control module, which is used to adjust the voltage reference signal of each bridge arm submodule to achieve capacitor voltage balancing of the bridge arm submodule;

[0143] The carrier phase shift modulation control module is used to adjust the voltage reference signal of the bridge arm submodule and generate the IGBT drive signal of each bridge arm submodule.

[0144] It should be noted that the steps in the method provided by the present invention can be implemented by using corresponding modules, devices, units, etc. in the system. Those skilled in the art can refer to the technical solution of the method to realize the composition of the system, that is, the embodiments in the method can be understood as preferred examples for constructing the system, which will not be elaborated here.

[0145] The high-voltage and large-capacity auto-coupling DC transformer and its working method and control method provided by the above-mentioned embodiment of the present invention realize energy transfer from the low-voltage side to the high-voltage side through the energy buffer bridge arm and the supporting capacitor as the medium; at the same time, the thyristor converter valve is used to realize zero-current shutdown and improve the efficiency of the transformer. A control method for a high-voltage and large-capacity auto-coupling DC transformer is proposed to realize the energy balance of the energy buffer bridge arm, and the reliable shutdown and approximate zero-voltage conduction of the thyristor converter valve are realized by controlling the number of energy buffer bridge arm sub-modules, and considering the different application scenarios, two closed-loop methods of DC voltage control and power control are proposed. Since the present invention adopts an auto-coupling structure and uses an active energy buffer bridge arm, high-voltage and large-capacity DC transformation can be realized; at the same time, a semi-controlled device converter valve is used to reduce the cost of the device.

[0146] All matters not covered in the above embodiments of the present invention are well known in the art.

[0147] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A high voltage and large capacity auto-coupling DC transformer, characterized in that: include: A support capacitor, an energy buffer bridge arm and a converter valve connected between the low-voltage side and the high-voltage side of the transformer; the support capacitor constitutes a part of the power transmission circuit, which is used to realize the transmission of power from the low-voltage side to the high-voltage side through the support capacitor; the energy buffer bridge arm constitutes another part of the power transmission circuit, which is used to realize the transmission of power from the low-voltage side to the high-voltage side through the energy buffer bridge arm, the transmission of power from the energy buffer bridge arm to the support capacitor, and the supplement of the power transmitted to the high-voltage side by the support capacitor; by controlling the number of bridge arm submodules of the energy buffer bridge arm and the trigger signal of the converter valve, the transformer commutation is realized; in: The support capacitor includes a support capacitor C1 and a support capacitor C2, and the converter valve includes a first converter valve T1 and a second converter valve D1; wherein: the support capacitor C1 is connected in parallel with the low-voltage side, the support capacitor C2 is connected in series with the support capacitor C1, and the two are connected in series with the high-voltage side, so that there is a direct electrical connection between the low-voltage side and the high-voltage side, forming a part of the power transmission circuit; the first converter valve T1 is connected in series with the second converter valve D1 and then connected in parallel with the high-voltage side, one end of the first converter valve T1 is connected to the second converter valve D1, and the other end of the first converter valve T1 is connected to the high-voltage side. One end of the energy buffer bridge arm is connected to the connection between the support capacitor C1 and the support capacitor C2, and the other end of the energy buffer bridge arm is connected to the connection between the first converter valve T1 and the second converter valve D1. The transformer current is controlled by controlling the switching quantity of the sub-modules in the energy buffer bridge arm.

2. The high-voltage and large-capacity auto-coupling DC transformer according to claim 1 is characterized in that: The energy buffer bridge arm comprises a plurality of bridge arm submodules SM connected in series and a buffer inductor L connected in series with the last bridge arm submodule SM; Each of the bridge arm submodules SM includes: a submodule capacitor and a switch device; the submodule capacitor is used to store energy and provide voltage; the switch device is used to control the switching of the working state of the bridge arm submodule; wherein: The switch device adopts an output voltage inversion structure, and the submodule capacitor is connected in parallel with the output voltage inversion structure of the switch device.

3. The high-voltage and large-capacity auto-coupling DC transformer according to claim 1, characterized in that: The first converter valve T1 and the second converter valve D1 are respectively composed of unidirectional conductive devices connected in series; wherein: The unidirectional conduction type device of the first converter valve T1 is a semi-controlled or fully controlled device; The unidirectional conduction type device of the second converter valve D1 can be an uncontrolled, semi-controlled or fully controlled device.

4. A method for operating a high-voltage and large-capacity auto-coupling DC transformer, characterized in that: include: When power flows from the low-voltage side of the transformer to the high-voltage side, there are two commutation working modes according to the switching status of the converter valve: Working mode 1: the first converter valve T1 is turned off and the second converter valve D1 is turned on. At this time, the low-voltage side is connected in series with the energy buffer bridge arm to transfer power to the high-voltage side. The capacitor C2 is charged by the energy buffer bridge arm. In working mode 2, the first converter valve T1 is turned on and the second converter valve D1 is turned off. At this time, the low-voltage side transfers power to the energy buffer bridge arm, and the capacitor C2 discharges to the high-voltage side.

5. The working method of the high-voltage and large-capacity auto-coupling DC transformer according to claim 4 is characterized in that: Combined with the current and voltage waveforms of the energy buffer bridge arm, assuming that the voltage across capacitor C2 and the high-voltage side of the transformer remains approximately unchanged, the working mode 1 includes the following modes: Mode 1 [t0, t1], in mode 2, the circuit is in the dead zone, no current flows through the second converter valve D1, at this time the low voltage side is connected in series with capacitor C2 to transfer energy to the high voltage side, the current is I C2_rel ; The energy buffer bridge arm adjusts the voltage to (U H -U L ), where U H is the voltage on the high voltage side of the transformer, U L is the voltage on the low-voltage side of the transformer. At this time, the sum of the low-voltage side voltage and the energy buffer bridge arm voltage is equal to the high-voltage side voltage. The voltage across the buffer inductor L of the energy buffer bridge arm is 0, and no current flows through the energy buffer bridge arm. Mode 2 [t1, t2], in mode 2, by controlling the voltage of the energy buffer bridge arm, the voltage is kept at the original amplitude (U H -U L ), plus an amplitude of U 0H The sine half wave u 0H (t), at this time the voltage applied to the snubber inductor L is u 0H (t), the current on the energy buffer bridge arm rises from 0 to I SM_H At the same time, as the current starts to flow through the second converter valve D1, the current from the low-voltage side of mode 1 and the capacitor C2 to the high-voltage side gradually decreases to 0, and then reverses, and the energy buffer bridge arm starts to charge the capacitor C2. The charging current increases from 0 to I as the energy buffer bridge arm current increases. C2_cha ; Mode 3 [t2, t3], in mode 3, the voltage of the energy buffer bridge arm returns to and stabilizes at (U H -U L ), the voltage across the snubber inductor L is 0, and the current is maintained at the rated value I SM_H At this time, the power transmitted from the low voltage side and the energy buffer bridge arm to the high voltage side is maintained at the rated value P H The energy transferred from the bridge arm submodule to capacitor C2 is maintained at P C2_cha ;in: P H =U H I SM_H (1)P C2_cha =(U H -U L )·I C2_cha (2) In the formula, I SM_H It is the rated current of the energy buffer bridge arm when transmitting energy to the high voltage side; Mode 4 [t3, t4], in mode 4, by controlling the voltage of the energy buffer bridge arm, the voltage is kept within the original amplitude (U H -U L ), minus an amplitude of U 0H The sine half wave u 0H (t), at this time the voltage applied to the snubber inductor L is -u 0H (t), the current on the energy buffer bridge arm changes from I SM_H At the same time, the charging current of the energy buffer bridge arm to the capacitor C2 decreases with the energy buffer bridge arm current, from I C2_cha Down to 0; Mode 5 [t4, t5], in mode 5, the current on the energy buffer bridge arm has dropped to 0, and the circuit enters the dead zone; the low-voltage side is connected in series with capacitor C2 to transfer power to the high-voltage side again; the energy buffer bridge arm adjusts the voltage to (U H -U L ), at this time, the sum of the low-voltage side voltage and the energy buffer bridge arm voltage is equal to the high-voltage side voltage, the voltage across the buffer inductor L is 0, and no current flows in the energy buffer bridge arm.

6. The working method of the high-voltage and large-capacity auto-coupling DC transformer according to claim 4, characterized in that: Combined with the current and voltage waveforms of the energy buffer bridge arm, assuming that the voltage across capacitor C2 and the high-voltage side of the transformer remains approximately unchanged, the second working mode includes the following modes: Mode 6 [t5, t6], in mode 6, the low voltage side is connected in series with capacitor C2 to transfer energy to the high voltage side, and the current is I C2_rel This process lasts throughout the entire working mode 2; the energy buffer bridge arm adjusts the voltage to -U L , at this time, the voltage on the low voltage side is equal to the voltage of the energy buffer bridge arm, the voltage across the buffer inductor L is 0, and no current flows through the energy buffer bridge arm; at this time, the voltage across the first converter valve T1 is approximately 0, which establishes the conduction condition for the first converter valve T1 to be approximately zero voltage turned on; Mode 7 [t6, t7], in mode 7, by controlling the voltage of the energy buffer bridge arm, the voltage is kept at the original amplitude -U L On the basis of 0L The sine half wave u 0L (t), at this time the voltage applied to the snubber inductor L is u 0L (t), therefore, the current in the energy buffer bridge arm will rise from 0 to I SM_L ; At the same time, the low-voltage side begins to transfer power to the energy buffer bridge arm, and the transferred power gradually increases to the rated value as the current increases; Mode eight [t7, t8], in mode eight, the voltage of the energy buffer bridge arm returns to and stabilizes at -U L , the voltage across the snubber inductor L is 0, and the current is maintained at the rated value I SM_L At this time, the power transmitted by the low-voltage side energy buffer bridge arm is maintained at the rated value P L ;in: P L =U L I SM_L (3) In the formula, I SM_L It is the rated current of the energy buffer bridge arm when absorbing energy from the low voltage side; Mode nine [t8, t9], in mode nine, by controlling the voltage of the energy buffer bridge arm, the voltage is kept at the original amplitude -U L Based on this, subtract an amplitude of U 0L The sine half wave u 0L (t), at this time the voltage applied to the snubber inductor L is -u 0L (t), the current on the energy buffer bridge arm changes from I SM_L The power transmitted from the low-voltage side to the energy buffer bridge arm gradually decreases to 0 as the current decreases; Mode 10[t9,t 10 ], in mode 10, the current on the energy buffer bridge arm has dropped to 0, and the circuit enters the dead zone; the energy buffer bridge arm adjusts the voltage to -U L At this time, the voltage on the low-voltage side is equal to the voltage of the energy buffer bridge arm, the voltage across the buffer inductor L is 0, and no current flows through the energy buffer bridge arm.

7. A control method for a high-voltage and large-capacity auto-coupling DC transformer, characterized in that: include: The converter valve commutation control part controls the current flowing through the converter valve. When the converter valve adopts a semi-controlled device, it is used to achieve reliable shutoff of the converter valve; When the converter valve adopts a fully controlled device, it is used to achieve zero current shutdown of the converter valve and reduce shutdown loss; The energy buffer bridge arm current control part controls the switch of the bridge arm submodule, thereby controlling the voltage of the energy buffer bridge arm to achieve closed-loop control of the current; at the same time, it generates a current reference signal i for realizing the DC power / voltage control part and the energy buffer bridge arm energy balance control part. SM_Href and the current reference signal i SM_Lref ; DC power / voltage control part, the current reference signal i SM_Href Adjustment is performed to achieve power closed-loop control and high-voltage side output voltage stability; Energy buffer bridge arm energy balance control part, the current reference signal i SM_Lref Adjustment is performed to achieve a constant overall capacitance storage in the energy buffer bridge arm; The grid voltage feedforward control part adjusts the voltage of the energy buffer bridge arm to achieve the approximate zero voltage conduction condition of the converter valve and limit du / dt; The submodule capacitor voltage balancing control part adjusts the voltage reference signal of each bridge arm submodule to achieve capacitor voltage balancing of the bridge arm submodule; The carrier phase shift modulation control part adjusts the voltage reference signal of the bridge arm submodule to generate the IGBT drive signal of each bridge arm submodule.

8. The control method of the high-voltage and large-capacity auto-coupling DC transformer according to claim 7, characterized in that: The converter valve commutation control part introduces an additional current reference value. When the first converter valve T1 adopts a half-controlled device, the current flowing through the first converter valve T1 is stably reduced to below the maintenance current, thereby achieving reliable shutdown of the first converter valve T1; when the first converter valve T1 adopts a full-controlled device, zero-current shutdown of the first converter valve T1 is achieved.

9. The control method of the high-voltage and large-capacity auto-coupling DC transformer according to claim 7, characterized in that: The energy buffer bridge arm current control part adopts a PI controller, and introduces a current inner loop control on the basis of the outer loop control; assuming that the given energy buffer bridge arm current reference signal is i SM_ref , by calculating the current reference signal and the actual current i SM The difference is then sent to the PI controller for calculation, so that the output result of the PI controller affects the switch drive signal of the bridge arm submodule, thereby controlling the voltage of the energy buffer bridge arm and realizing closed-loop control of the current.

10. The control method of the high-voltage and large-capacity auto-coupling DC transformer according to claim 9, characterized in that: The energy buffer bridge arm current reference signal i SM_ref It is mainly generated by the superposition of two parts, one of which is the current reference signal i when the bridge arm, high-voltage side and low-voltage side form a loop together SM_Href The other part is the current reference signal i when the bridge arm and the low-voltage side form a loop SM_Lref ; These two parts are obtained through two outer loop controllers, where the current reference signal i SM_Href Used to implement the DC power / voltage control part, the current reference signal i SM_Lref Used to realize the energy balance control part of the energy buffer bridge arm.

11. The control method of the high-voltage and large-capacity auto-coupling DC transformer according to claim 7, characterized in that: The DC voltage / power control part includes: DC power control and DC voltage control; wherein: The DC power control calculates the current that should flow through the energy buffer bridge arm and the high voltage side according to the given transmission power reference value, and controls the voltage across the energy buffer bridge arm inductor by controlling the number of switching submodules of the energy buffer bridge arm, thereby controlling the actual current i of the energy buffer bridge arm. SM Stability, realizing power closed-loop control; The DC voltage control is started when the voltages on the high voltage side and the low voltage side may change, and the actual current i of the energy buffer bridge arm is controlled. SM and the duty cycle of the first converter valve to achieve a stable output voltage on the high-voltage side; in: When the energy buffer bridge arm is connected to the high voltage side, the energy buffer bridge arm current reference signal i SM_Href , control the number of switching submodules of the energy buffer bridge arm, thereby controlling the voltage across the inductor of the energy buffer bridge arm, so that the actual current i SM Able to stably follow the current reference signal i SM_Href ; Current reference signal i SM_Href The following relations are satisfied: Where U 0H is the angular frequency of the half-sine wave voltage applied on both sides of the buffer inductor, ω is the angular frequency of the half-sine wave voltage applied on both sides of the buffer inductor, P ref is the DC power reference value transmitted by the converter, U H is the high voltage side voltage, I C2_cha is the charging current amplitude of capacitor C2 when the thyristor is turned off; When DC voltage control is used, the current reference signal i SM_Href The amplitude of the high voltage side voltage U H With its reference value U H_ref At the same time, the cut-off frequency of the current loop PI controller is much higher than the cut-off frequency of the voltage loop PI controller, thus achieving the dynamic structure between the two and the dynamic performance of the current inner loop.

12. The control method of the high-voltage and large-capacity auto-coupling DC transformer according to claim 7, characterized in that: The energy buffer bridge arm energy balance control part adjusts the current reference signal i when the energy buffer bridge arm and the low voltage side form a loop. SM_Lref To achieve the overall constant of the capacitance storage in the energy buffer bridge arm, when the output voltage of the energy buffer bridge arm is less than the rated value, increase the current reference signal i SM_Lref ; On the contrary, when the output voltage of the energy buffer bridge arm is greater than the rated value, the current reference signal i is reduced. SM_Lref ;in: The current reference signal i SM_Lref Generated by the PI controller, whose input is the bridge arm submodule capacitor voltage reference value U C_ref The average voltage U of all bridge arm submodule capacitors in the measured energy buffer bridge arm C_av When the deviation is greater than 0, the voltage output by the energy buffer bridge arm is less than the rated value, and the current reference signal i is increased by adjusting the PI controller. SM_Lref amplitude to restore energy balance; When the deviation is less than 0, the voltage output by the energy buffer bridge arm is less than the rated value, and the current reference signal i is reduced by adjusting the PI controller. SM_Lref amplitude to restore the energy balance.

13. The control method of the high voltage and large capacity auto-coupling DC transformer according to claim 7, characterized in that: The grid voltage feedforward control part, when the current of the first converter valve is 0, controls the grid voltage feedforward by giving a reference signal U g_ref , adjusting the voltage of the energy buffer bridge arm to rise or fall in steps, so that before the first converter valve is turned on, the voltage across both ends is approximately 0, thereby achieving an approximately zero voltage condition before the first converter valve is turned on, and limiting du / dt; The grid voltage feedforward control reference signal U g_ref The voltage reference signal u of the energy buffer bridge arm is obtained by superimposing it with the output of the energy balance control part of the energy buffer bridge arm. ref .

14. The control method of the high voltage and large capacity auto-coupling DC transformer according to claim 7, characterized in that: The submodule capacitor voltage balancing control part adjusts the voltage reference signal of each bridge arm submodule, so as to achieve capacitor voltage balancing of the bridge arm submodule; the carrier phase shift modulation control part compares the input adjusted voltage reference signal of each bridge arm submodule with the triangular carrier, and when it is greater than the triangular carrier, the bridge arm submodule is put into operation, otherwise, the bridge arm submodule is bypassed, so as to generate the IGBT drive signal of each bridge arm submodule; wherein: First, the bridge arm submodule capacitors are numbered from 1 to N, and the capacitor voltage U of the i-th bridge arm submodule (i = 1, 2, ..., N) is C (i) and the average voltage U of all bridge arm submodule capacitors in the energy buffer bridge arm C_av The deviation is sent to the proportional controller, and the obtained proportional coefficient is directly compared with the actual current i of the energy buffer bridge arm. SM Multiply them to get the voltage reference adjustment value Δu(i) of the corresponding bridge arm submodule; when the energy buffer bridge arm current charges the bridge arm submodule, i SM >0, when the capacitor voltage U C (i) Lower than the average capacitor voltage U of the bridge arm submodule of the phase C_av When the voltage reference adjustment value Δu(i) of the positive bridge arm submodule is generated, the adjustment value is combined with the energy buffer bridge arm voltage reference signal u ref Add together to get the voltage reference signal u of the i-th bridge arm submodule ref_i , then: u ref_i =u ref +Δu(i)=u ref +K p [U C_av -U C (i)]·i SM (6) In the formula, K p is the proportional coefficient of the proportional controller; When the energy buffer bridge arm absorbs energy from the low-voltage side, the input time of the high-voltage bridge arm sub-module is reduced, and the input time of the low-voltage bridge arm sub-module is increased; when the energy buffer bridge arm releases energy to the high-voltage side, the input time of the high-voltage bridge arm sub-module is increased, and the input time of the low-voltage bridge arm sub-module is reduced, thereby maintaining the capacitor voltage balance between the bridge arm sub-modules of the energy buffer bridge arm.

15. A control system for a high-voltage and large-capacity auto-coupling DC transformer, characterized in that: include: The converter valve commutation control module is used to control the current flowing through the converter valve. When the converter valve adopts a semi-controlled device, it is used to achieve reliable shutdown of the converter valve; when the converter valve adopts a fully controlled device, it is used to achieve zero current shutdown of the converter valve and reduce shutdown losses; The energy buffer bridge arm current control module is used to control the switch of the bridge arm submodule, thereby controlling the voltage of the energy buffer bridge arm and realizing closed-loop control of the current; at the same time, it generates a current reference signal i for realizing the DC power / voltage control part and the energy buffer bridge arm energy balance control part. SM_Href and the current reference signal i SM_Lref ; DC power / voltage control module, which is used to control the current reference signal i SM_Href Adjust to achieve power closed-loop control and stable output voltage on the high-voltage side; Energy buffer bridge arm energy balance control module, which is used to control the current reference signal i SM_Lref Adjustment is performed to achieve a constant overall capacitance storage in the energy buffer bridge arm; A grid voltage feedforward control module, which is used to adjust the voltage of the energy buffer bridge arm, achieve the approximate zero voltage conduction condition of the converter valve and limit du / dt; A submodule capacitor voltage balancing control module, which is used to adjust the voltage reference signal of each bridge arm submodule to achieve capacitor voltage balancing of the bridge arm submodule; The carrier phase shift modulation control module is used to adjust the voltage reference signal of the bridge arm submodule and generate the IGBT drive signal of each bridge arm submodule.

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