High-voltage large-capacity autotransformer and working method and control method thereof

Abstract

This invention provides a high-voltage, high-capacity autotransformer, comprising: a supporting capacitor, an energy buffer bridge arm, and a converter valve assembly. The supporting capacitor forms part of the power transmission loop; the energy buffer bridge arm realizes the main power transmission and forms another part of the power transmission loop; the transformer commutation is achieved by controlling the number of bridge arm sub-modules engaged in the energy buffer bridge arm and the trigger signal of the converter valve assembly. The invention also provides a commutation operation method and a control method for this transformer. This invention realizes energy transfer from the low-voltage side to the high-voltage side, improves transformer efficiency, and achieves energy balance in the energy buffer bridge arm; by controlling the number of energy buffer bridge arm sub-modules engaged, reliable shut-off and near-zero voltage conduction of the converter valve are achieved, while providing two closed-loop modes: DC voltage and power control. This invention realizes a high-voltage, high-capacity DC transformer, and by using a semi-controlled converter valve, reduces device costs.

Description

Technical Field

[0001] This invention relates to the fields of smart grids, power electronics, and control technology in power systems. Specifically, it relates to a high-voltage, high-capacity autotransformer and its operating and control methods, as well as a corresponding control system. Background Technology

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

[0003] Nearshore wind power development primarily employs an AC aggregation-AC transmission scheme, offering advantages such as low voltage boosting and grid connection costs. In deep-sea applications, due to issues like cable network charging power, AC aggregation-flexible DC transmission is the main solution. However, with the trend towards ultra-large capacity single offshore wind turbines, charging power issues arise in the wind farm aggregation network. Adopting a DC aggregation-transmission scheme can avoid reactive power losses or the need for reactive power compensation equipment, and effectively solve the problems of stable control of multiple turbines connected to the grid and wide-area interconnection of wind farm clusters. For deep-sea offshore wind power applications requiring all-DC transmission, high-voltage, high-capacity DC transformers are the core component and key equipment for connecting different voltage levels of DC systems in the offshore wind power aggregation and transmission network.

[0004] DC transformers can be classified into two categories based on whether electrical isolation is provided on both sides: isolated DC transformers and non-isolated DC transformers. The key characteristic of isolated DC transformers is the addition of an isolation transformer in the circuit to achieve energy transfer, such as in classic forward and flyback converters. These converters can achieve large input-output ratio conversion through a large-capacity isolation transformer, but they also suffer from issues related to the weight, size, and losses of the isolation transformer. In megawatt-level engineering applications, the manufacturing difficulty of intermediate-frequency isolation transformers is high, and mature technologies and applications are currently lacking, which to some extent limits the development of isolated DC transformers towards high-voltage, high-power transmission. Considering the stringent requirements for weight and size in offshore wind power grid-connected applications, the application scenarios for isolated DC transformers are relatively limited. In contrast, non-isolated DC transformers, by eliminating the isolation transformer, have greater advantages in system efficiency, size, and weight, making them more economical.

[0005] Compared with existing non-isolated DC transformers, it is found that although full-power DC transformers eliminate the need for AC transformers, the two-stage power conversion transmission method results in a large number of idle modules during operation, leading to a large number of power devices and high losses. Switch-coupled DC transformers do not have the problem of sub-module energy balancing and do not require the injection of AC circulating current, but their switching devices need to withstand the voltage of the grid level, which needs to be achieved by connecting the devices in series. Autotransformer DC transformers reduce the number of sub-modules by using a common bridge arm, but require the injection of AC circulating current or the addition of additional auxiliary circuits to achieve the voltage balancing of the sub-module capacitors.

[0006] In summary, there is an urgent need in this field for a solution that can realize a high-voltage, high-capacity DC transformer that meets the application requirements of low cost, high efficiency, and high reliability. Currently, no descriptions or reports of similar technologies to this invention have been found, nor have similar domestic or international materials been collected. Summary of the Invention

[0007] To address the aforementioned shortcomings in existing technologies, the present invention aims to provide a high-voltage, high-capacity autotransformer and its operating and control methods, based on the application requirements of low cost, high efficiency, and high reliability. It also provides a corresponding control system to achieve high-voltage, high-capacity DC transformers and solve problems such as reliable shut-off of thyristor converter valves and energy balance of energy buffer bridge arms.

[0008] According to one aspect of the present invention, a high-voltage, high-capacity autotransformer is provided, comprising: 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 power transmission loop for transmitting power from the low-voltage side to the high-voltage side through the support capacitor; the energy buffer bridge arm constitutes another power transmission loop for transmitting power from the low-voltage side to the high-voltage side through the energy buffer bridge arm, transmitting power from the energy buffer bridge arm to the support capacitor, and supplementing the power transmitted from the support capacitor to the high-voltage side; the transformer commutation is achieved by controlling the number of sub-modules engaged within the energy buffer bridge arm and the trigger signal of the converter valve.

[0009] in:

[0010] The supporting capacitors include supporting capacitor C1 and supporting capacitor C2, and the converter valves include a first converter valve T1 and a second converter valve D1; wherein: supporting capacitor C1 is connected in parallel with the low-voltage side, supporting capacitor C2 is connected in series with supporting capacitor C1, and the two are connected in series and then in parallel with the high-voltage side, so that the low-voltage side and the high-voltage side have a direct electrical connection, forming a part of the power transmission loop; the first converter valve T1 and the second converter valve D1 are connected in series and then 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. The negative terminal of the high-voltage side is connected, one end of the second converter valve D1 is connected to the positive terminal of the high-voltage side, and the other end of the second converter valve D1 is connected to the first converter valve T1. The converter function of the transformer is realized by the 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 supporting capacitor C1 and the supporting 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 number of sub-modules switched on and off within the bridge arm.

[0011] According to another aspect of the present invention, a method for operating a high-voltage, high-capacity autotransformer is provided, comprising:

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

[0013] In the first working mode, the first converter valve T1 is closed and the second converter valve D1 is open. At this time, the low-voltage side and the energy buffer bridge arm are connected in series to transfer power to the high-voltage side together, and the capacitor C2 is charged by the energy buffer bridge arm.

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

[0015] According to a third aspect of the present invention, a control method for a high-voltage, high-capacity autotransformer is provided, comprising:

[0016] The converter valve control section controls the current flowing through the converter valve. When a semi-controlled device is used, it is used to achieve reliable shut-off of the converter valve; when a fully controlled device is used, it is used to achieve zero-current shut-off of the converter valve and reduce shut-off losses.

[0017] The energy buffer bridge arm current control section controls the switching of the bridge arm submodules, thereby controlling the voltage of the energy buffer bridge arm and achieving closed-loop current control. Simultaneously, it generates a current reference signal i for the DC power / voltage control section and the energy balance control section of the energy buffer bridge arm. SM_Hrefand current reference signal i SM_Lref ;

[0018] The DC power / voltage control section uses the current reference signal i. SM_Href Adjustments are made to achieve closed-loop power control and stabilize the high-voltage side output voltage.

[0019] The energy balance control section of the energy buffer bridge arm controls the current reference signal i. SM_Lref Adjustments are made to maintain a constant total capacitance storage in the energy buffer bridge arm;

[0020] The grid voltage feedforward control section adjusts the voltage of the energy buffer bridge arm to achieve near-zero voltage conduction conditions for the converter valve and limit du / dt.

[0021] The submodule capacitor voltage equalization control section adjusts the voltage reference signal of each bridge arm submodule to achieve capacitor voltage equalization of the bridge arm submodules.

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

[0023] According to a fourth aspect of the present invention, a control system for a high-voltage, high-capacity autotransformer is provided, comprising:

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

[0025] The energy buffer bridge arm current control module controls the switching of the bridge arm submodules, thereby controlling the voltage of the energy buffer bridge arm and achieving closed-loop current control. It also generates a current reference signal i for both the DC power / voltage control section and the energy balance control section of the energy buffer bridge arm. SM_Href and current reference signal i SM_Lref ;

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

[0027] Energy buffer bridge arm energy balance control module, this module is used to control the current reference signal i SM_Lref Adjustments are made to maintain a constant overall capacitance storage in the energy buffer bridge arm;

[0028] The grid voltage feedforward control module is used to regulate the voltage of the energy buffer bridge arm to achieve near-zero voltage conduction condition of the converter valve and limit du / dt.

[0029] The submodule capacitor voltage equalization control module is used to adjust the voltage reference signal of each bridge arm submodule to achieve voltage equalization of the bridge arm submodule capacitors.

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

[0031] By adopting 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, high-capacity autotransformer provided by this invention uses energy buffer bridge arms and supporting capacitors as a medium to realize energy transfer from the low-voltage side to the high-voltage side; at the same time, it uses thyristor converter valves to achieve zero-current turn-off, thereby improving transformer efficiency.

[0033] The present invention provides a working method and control method for a high-voltage, high-capacity autotransformer, which realizes energy balance of the energy buffer bridge arm. By controlling the number of energy buffer bridge arm sub-modules, reliable shut-off and near-zero voltage conduction of the thyristor converter valve are achieved. Furthermore, considering different application scenarios, two closed-loop modes, DC voltage control and power control, are implemented.

[0034] The high-voltage, high-capacity autotransformer and its working and control methods provided by this invention adopt an autotransformer structure and use an active energy buffer bridge arm, which can realize high-voltage, high-capacity DC transformation; at the same time, it uses a semi-controlled converter valve to reduce the cost of the device.

[0035] The high-voltage, high-capacity autotransformer and its working and control methods provided by this invention achieve high-voltage, high-capacity DC transformers through autotransformer structure and energy buffer bridge arm; and improve transformer efficiency by controlling thyristor converter valves and energy buffer bridge arm to achieve reliable thyristor converter valve turn-off and near-zero voltage conduction and turn-off.

[0036] The high-voltage, high-capacity autotransformer and its working and control methods provided by this invention can be applied to offshore wind power DC collection systems. Attached Figure Description

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

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

[0039] Figure 1 This is a basic topology diagram of a high-voltage, high-capacity autotransformer in a preferred embodiment of the present invention.

[0040] Figure 2 This is a schematic diagram of the control method for a high-voltage, high-capacity autotransformer in a preferred embodiment of the present invention.

[0041] Figure 3 This is a schematic diagram of a high-voltage, high-capacity autotransformer transmitting power from the low-voltage side to the high-voltage side in a preferred embodiment of the present invention, where the low-voltage side is connected in series with capacitor C2.

[0042] Figure 4(a) is a schematic diagram of the working mode of the high-voltage large-capacity autotransformer in a preferred embodiment of the present invention.

[0043] Figure 4(b) is a schematic diagram of the second working mode of the high-voltage, high-capacity autotransformer in a preferred embodiment of the present invention;

[0044] Figure 5 This is a schematic diagram of the current and voltage waveforms of the energy buffer bridge arm in a preferred embodiment of the present invention;

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

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

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

[0048] Figure 9 This is a control block diagram of the energy balance control section of the energy buffer bridge arm in a preferred embodiment of the present invention;

[0049] Figure 10 This is a control block diagram of the grid voltage feedforward control section in a preferred embodiment of the present invention;

[0050] Figure 11 This is a control block diagram of the carrier phase-shift modulation control section in a preferred embodiment of the present invention;

[0051] Figure 12 This is a control block diagram of the capacitor voltage equalization control section of a submodule in a preferred embodiment of the present invention. Detailed Implementation

[0052] The embodiments of the present invention are described in detail below: These embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention.

[0053] To address the lack of low-cost, high-efficiency, and high-reliability solutions for high-voltage, high-capacity DC transformers in existing technologies, one embodiment of the present invention provides a high-voltage, high-capacity autotransformer. Through an autotransformer structure and an energy buffer bridge arm, a high-voltage, high-capacity DC transformer is achieved. By controlling the thyristor converter valve and the energy buffer bridge arm, reliable turn-off of the thyristor converter valve and near-zero voltage turn-on and turn-off are achieved, thereby improving transformer efficiency.

[0054] Specifically, such as Figure 1 As shown, the high-voltage, high-capacity autotransformer provided in this embodiment may include: a supporting 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 supporting capacitor is used to form a part of the power transmission circuit, so that there is a direct electrical connection between the low-voltage side and the high-voltage side, and the two transmit power through the supporting capacitor, that is, the power transmitted by the supporting capacitor is only transmitted to the high-voltage side by the supporting capacitor C2 in series on the low-voltage side, without passing through the energy buffer bridge arm; the energy buffer bridge arm forms another part of the power transmission circuit, which is used to realize the main power transmission, and the main power transmission... The power transmission includes: the energy buffer bridge arm transmits power by connecting the low-voltage side energy buffer bridge arms in series to jointly transmit energy to the high-voltage side; simultaneously, the energy buffer bridge arm transmits energy to the supporting capacitor C2 of the supporting capacitor, supplementing the energy transferred from the supporting capacitor C2 to the high-voltage side, thus maintaining the voltage of the supporting capacitor C2 stable; the principle of the above two power transmission models is that after connecting a part of the energy storage unit in series on the low-voltage side, they jointly transmit energy to the high-voltage side, and the two modes are switched by the converter valve; the transformer commutation is realized by controlling the number of bridge arm sub-modules of the energy buffer bridge arm and the trigger signal of the converter valve;

[0055] in:

[0056] The supporting capacitors include supporting capacitor C1 and supporting capacitor C2, and the converter valves include a first converter valve T1 and a second converter valve D1. Supporting capacitor C1 is connected in parallel with the low-voltage side, and supporting capacitor C2 is connected in series with supporting capacitor C1. The two capacitors are then connected in parallel with the high-voltage side, creating a direct electrical connection between the low-voltage and high-voltage sides, forming a partial power transmission loop. The first converter valve T1 and the second converter valve D1 are connected in series and then 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 terminal of the high-voltage side. One end of the second converter valve D1 is connected to the positive terminal of the high-voltage side, and the other end is connected to the first converter valve. By alternately switching on and off the first and second converter valves, the converter function of the transformer can be realized. One end of the energy buffer bridge arm is connected to the connection point of supporting capacitors C1 and C2, and the other end is connected to the connection point of the first converter valve T1 and the second converter valve D1. By controlling the number of sub-modules switched on and off within the energy buffer bridge arm, the transformer current can be controlled.

[0057] The energy buffer bridge arm includes multiple bridge arm sub-modules SM connected in series and a buffer inductor L connected in series with the last bridge arm sub-module SM;

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

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

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

[0061] In the preferred embodiment described above, the diode can also be replaced with an IGBT; the switching device connection structure can also be replaced with other structures that have the ability to reverse the output voltage.

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

[0063] The first converter valve T1 uses a semi-controlled or fully controlled device for its unidirectional conduction.

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

[0065] exist Figure 1 Middle,U L This refers to the low-voltage side voltage of the transformer; U H This represents the high-voltage side voltage of the transformer; C1 and C2 are supporting capacitors; SM is a bridge arm submodule, which is connected in series to form an energy buffer bridge arm. The structure of the bridge arm submodule includes, but is not limited to, a full-bridge submodule, or other structures capable of reversing the output voltage. Figure 1 The middle bridge arm submodule preferably uses diodes instead of the pair of IGBTs in the full-bridge submodule; L is the buffer inductor of the energy buffer bridge arm; D1 and T1 are series-connected commutation valves, which can be composed of unidirectional conducting devices such as diodes, thyristors, IGBTs, and IGCTs connected in series, and include voltage equalization circuits for switching devices. Figure 1 T1 is preferably a thyristor converter valve, and D1 is preferably a diode converter valve.

[0066] In addition to the DC transformer provided in the above embodiments of the present invention, another embodiment of the present invention provides a method for operating a high-voltage, high-capacity autotransformer. This method, when power flows from the low-voltage side to the high-voltage side of the transformer, can include the following two commutation operating modes depending on the switching status of the commutator valve:

[0067] In the first working mode, the first converter valve T1 is closed and the second converter valve D1 is open. At this time, the low-voltage side and the energy buffer bridge arm are connected in series to transfer power to the high-voltage side together, and the capacitor C2 is charged by the energy buffer bridge arm.

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

[0069] For ease of analysis, the following example uses a thyristor-based converter valve (T1) and a diode-based converter valve (D1) to further illustrate the converter operation mode provided in the above embodiments of the present invention. It should be noted that the operating principle of converter valves using other structures is the same as in this example, and will not be elaborated upon here.

[0070] The high-voltage side is equivalent to a load, the thyristor-based series converter valve is equivalent to a thyristor, and the diode-based series converter valve is equivalent to a diode. Throughout operation, when no current flows through diode D1, the low-voltage side and C2 jointly transfer power to the high-voltage side, such as... Figure 3As shown in Figures 4(a) and 4(b), when power flows from the low-voltage side to the high-voltage side, depending on the switching status of the converter valve, the high-voltage, high-capacity autotransformer provided in the above embodiments of the present invention mainly has two converter modes: Mode 1, the thyristor is turned off and the diode is turned on. At this time, the low-voltage side and the energy buffer bridge arm are connected in series to jointly transfer power to the high-voltage side, and the capacitor C2 is charged by the energy buffer bridge arm; Mode 2, the thyristor is turned on and the diode 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. Figure 5 The waveforms of the current and voltage of the energy buffer bridge arm are shown. Based on these waveforms, each circulating mode can be further divided into five different operating modes. For ease of analysis, it is assumed that capacitor C2 and the voltage across the high-voltage side remain approximately constant.

[0071] In some preferred embodiments:

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

[0073] In mode one, 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, transferring energy to the high-voltage side, and the current is I. C2_rel The energy buffer bridge arm voltage is adjusted to (U H -U L At this point, 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. Therefore, no current flows through the energy buffer bridge arm.

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

[0075] In mode two, by controlling the voltage of the energy buffer bridge arm, it is kept at its original amplitude (U H -U L Based on this, an amplitude of U was added. 0H The sine half-wave u 0H (t), at this time the voltage applied to the buffer inductor is u 0H (t), therefore, the current on the energy buffer bridge arm will rise from 0 to I. SM_H Simultaneously, as current begins to flow through the diode, the current flowing from the low-voltage side of mode one and from capacitor C2 to the high-voltage side gradually decreases to 0. Then, in reverse, the energy buffer bridge arm begins to charge capacitor C2, and the charging current increases from 0 to I as the energy buffer bridge arm current increases. C2_chaThe charging current will not flow through the low-voltage side. The purpose of this commutation waveform design method is to prevent abrupt changes in the rate of change of the energy buffer bridge arm current. Therefore, the corresponding energy buffer bridge arm output voltage waveform does not change rapidly, which reduces the difficulty of controlling the energy buffer bridge arm current. In mode two, 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 three, the energy buffer bridge arm voltage returns to and stabilizes at (U H -U L Therefore, the voltage across the buffer inductor is 0, and the current will remain at the rated value I. SM_H At this point, the power transferred from the low-voltage side and the energy buffer bridge arm to the high-voltage side will remain 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 The rated current for the energy buffer bridge arm when transferring energy to the high-voltage side;

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

[0082] In mode four, by controlling the voltage of the energy buffer bridge arm, it is kept at its original amplitude (U H -U L Based on this, a value of U was subtracted. 0H The sine half-wave u 0H (t), at this time the voltage applied to the buffer inductor is -u 0H (t), therefore, the current on the energy buffer bridge arm will be from I SM_H The current decreases to 0. Simultaneously, the charging current of the energy buffer bridge arm to capacitor C2 decreases as the energy buffer bridge arm current decreases, from I... C2_cha The power transferred from the low-voltage side and the energy buffer arm to the high-voltage side gradually decreases to 0 as the current decreases. In mode four, the power transferred from the low-voltage side and the energy buffer arm to the high-voltage side gradually decreases to 0.

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

[0084] In mode five, the current in the energy buffer bridge arm has dropped to 0. At this point, the circuit enters the dead zone, and the low-voltage side is connected in series with capacitor C2, transferring power to the high-voltage side again. The voltage regulation of the energy buffer bridge arm is (U H -U L At this point, 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. Therefore, no current flows through the energy buffer bridge arm.

[0085] The five operating modes described above all belong to commutation mode one. In this commutation mode, the main functions are to transfer energy from the low-voltage side and the energy buffer bridge arm to the high-voltage side, and to charge capacitor C2 via the energy buffer bridge arm. Specifically, when the circuit enters the dead zone, the low-voltage side and capacitor C2 jointly transfer energy to the high-voltage side. In this commutation mode, the energy buffer bridge arm's role is to transfer stored energy to the high-voltage side and capacitor C2, thus serving as a medium for power flow and ensuring the energy balance of capacitor C2.

[0086] In some preferred embodiments:

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

[0088] In mode six, 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 will continue throughout commutation mode 2, and therefore will not be elaborated upon in the subsequent modal analysis. The energy buffer arm adjusts the voltage to -U. L At this point, the low-voltage side voltage is equal to the energy buffer bridge arm voltage, and the voltage across the buffer inductor is 0. Therefore, no current flows through the energy buffer bridge arm. For the thyristor converter valve, the voltage across the converter valve is also approximately 0 at this time, creating conditions for the thyristor to conduct at near zero voltage.

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

[0090] In mode seven, by controlling the voltage of the energy buffer bridge arm, it is kept at its original amplitude -U L Based on this, an amplitude of U was added. 0L The sine half-wave u 0L (t), at this time the voltage applied to the buffer inductor is u 0L (t), therefore, the current on the energy buffer bridge arm will rise from 0 to I. SM_L 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. The commutation design method here is similar to that in mode two, which is also to limit the rate of change of the energy buffer bridge arm current from abruptly, thereby reducing the control difficulty.

[0091] (8) Modal 8 [t7,t8]

[0092] In mode eight, the energy buffer bridge arm voltage returns to and stabilizes at -U L Therefore, with the voltage across the buffer inductor at 0, the current will remain at the rated value I. SM_L At this point, the power transferred from the low-voltage side and the energy buffer bridge arm to the high-voltage side will remain 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 for the energy buffer arm when absorbing energy from the low-voltage side.

[0095] (9) Modality 9 [t8,t9]

[0096] In mode nine, by controlling the voltage of the energy buffer bridge arm, it is kept at its original amplitude -U L Based on this, a value of U was subtracted. 0L The sine half-wave u 0L (t), at this time the voltage applied to the buffer inductor is -u 0L (t), therefore, the current on the energy buffer bridge arm will be from I SM_L As the current decreases, the power transmitted by the low-voltage side energy buffer arm gradually decreases to 0.

[0097] (10) Modal + [t9,t 10 ]

[0098] In mode ten, 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 low-voltage side voltage is equal to the energy buffer bridge arm voltage, and the voltage across the buffer inductor is 0, so no current flows through the energy buffer bridge arm.

[0099] All five operating modes described above belong to commutation mode two. In this commutation mode, the main functions are to transfer energy from the low-voltage side to the energy buffer bridge arm, and to transfer energy from the low-voltage side and capacitor C2 to the high-voltage side. In this commutation mode, the energy buffer bridge arm stores the power transferred from the low-voltage side in the submodule capacitor, providing energy for the next cycle of commutation mode one.

[0100] In view of the DC transformer provided in the above embodiments of the present invention, an embodiment of the present invention also provides a control method for a high-voltage, high-capacity autotransformer, such as... Figure 2As shown, the system mainly includes: a commutation control section for the commutator valve, a current control section for the energy buffer bridge arm, a DC power / voltage control section, an energy balance control section for the energy buffer bridge arm, a grid voltage feedforward control section, a submodule capacitor voltage equalization control section, and a carrier phase-shift modulation control section. The commutation control section for the commutator valve aims to achieve reliable turn-off of the thyristor commutator valve and near-zero voltage turn-on conditions; the DC power / voltage control section aims to achieve closed-loop control of the system; the energy balance control section for the energy buffer bridge arm aims to ensure stable output of the energy buffer bridge arm during circuit operation; the current control section for the energy buffer bridge arm aims to ensure good dynamic and steady-state performance of the DC power / voltage control; the grid voltage feedforward control section aims to create near-zero voltage turn-on conditions for the thyristors and limit du / dt to protect the devices from damage; the submodule capacitor voltage equalization section aims to ensure balanced voltage across the submodule capacitors, ensuring normal transformer operation; and the carrier phase-shift modulation section aims to provide corresponding IGBT control signals based on the reference values ​​of the input energy buffer bridge arm submodule capacitor voltages.

[0101] Furthermore, the control method for high-voltage, high-capacity autotransformer can further include the following operations:

[0102] The commutation control section of a converter valve controls the current flowing through the valve. When a semi-controlled device is used, it ensures reliable shut-off; when a fully controlled device is used, it achieves zero-current shut-off, reducing shut-off losses. The goal of the commutation control section is to bring the current to zero. Fully controlled devices can turn on and off regardless of whether the current is zero. However, if the device turns off when the current is not zero, it will cause shut-off losses, reduce efficiency, and lead to overheating. Controlling the current to zero before shutting off effectively reduces these losses. Semi-controlled devices, by their very nature, require the current to drop below the holding current during shut-off. This is generally achieved by controlling the current to zero; therefore, zero current in semi-controlled devices ensures reliable shut-off. Furthermore, since the shut-off current of semi-controlled devices must be approximately zero, shut-off losses can be considered negligible or minimal.

[0103] The energy buffer bridge arm current control section controls the switching of the bridge arm submodules, thereby controlling the voltage of the energy buffer bridge arm and achieving closed-loop current control. Simultaneously, it generates a current reference signal i for the DC power / voltage control section and the energy balance control section of the energy buffer bridge arm. SM_Href and current reference signal i SM_Lref ;

[0104] The DC power / voltage control section uses the current reference signal i. SM_Href Adjustments are made to achieve closed-loop power control and stabilize the high-voltage side output voltage.

[0105] The energy balance control section of the energy buffer bridge arm controls the current reference signal i. SM_Lref Adjustments are made to maintain a constant total capacitance storage in the energy buffer bridge arm;

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

[0107] The submodule capacitor voltage equalization control section adjusts the voltage reference signal of each bridge arm submodule to achieve capacitor voltage equalization of the bridge arm submodules.

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

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

[0110] Reliable turn-off of semi-controlled devices is crucial for the normal operation of a DC transformer. Failure of the thyristor commutator valve to turn off will lead to commutation failure, overcurrent, and damage to the devices. Simultaneously, the energy buffer bridge arm, as the medium for energy transfer, affects the voltage equalization of the capacitors in each submodule, thus impacting power transfer. Based on the analysis of the high-voltage, high-capacity autotransformer topology and operating modes, to achieve closed-loop control during circuit operation, it is necessary to first address the reliable turn-off of the thyristors and the control issues related to the energy buffer bridge arm and the energy balance of the submodule capacitors. DC transformer control often employs an inner current loop and an outer voltage loop. However, when the input and output voltages of the DC transformer are controlled by the preceding and following stages respectively, the outer voltage loop control increases the complexity of the control. Therefore, this embodiment proposes DC voltage / power control; when the input and output voltages are constant, a power outer loop is used to reduce control complexity.

[0111] In some preferred embodiments:

[0112] (1) Converter valve commutation control

[0113] Since the converter valve acts as the switching element of a high-voltage, high-capacity autotransformer, and the turn-off characteristic of the thyristor converter valve requires the current flowing through the thyristor to drop below the holding current, reliable turn-off of the thyristor converter valve is fundamental to stable circuit operation. On the other hand, due to the characteristics of the control method of the high-voltage, high-capacity autotransformer, the ZVS condition can be achieved in the converter valve commutation control, thereby effectively reducing switching losses and improving circuit efficiency. Figure 5 Taking the [t8, t9] period as an example, the switching control of the converter valve is achieved by introducing an additional current reference value during this period, so that the current flowing through the thyristor can stably drop below the holding current, thereby achieving reliable turn-off of the thyristor. The waveform of this control signal is generated by waveform generator 3, and is generated once within one switching cycle of the converter valve. Figure 6 A waveform diagram illustrating the current reference value introduced for the commutation control of the converter valve.

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

[0115] Based on the power regulation principle of high-voltage, high-capacity autotransformers, power transfer between the low-voltage and high-voltage sides is mediated by an energy buffer bridge arm. To ensure the transferred power meets rated requirements, the current of the energy buffer bridge arm needs to be regulated. The control method primarily employs a PI controller, introducing an inner current loop control on top of the outer loop control. Due to the characteristics of the high-voltage, high-capacity autotransformer 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 the given current reference signal is i... SM_ref By calculating its relationship with the actual current i SM The difference is then sent to the PI controller for calculation, causing its output to affect the switching drive signal of the submodule, thereby controlling the voltage of the energy buffer bridge arm and achieving closed-loop control of the current. For example... Figure 7 The block diagram for the current control of the energy buffer bridge arm is shown.

[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 jointly form a circuit. 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 It is responsible for implementing DC power / voltage control of AHVDC, while the current reference signal i SM_Lref Used to achieve energy balance in 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, high-capacity autotransformer can adopt two different control modes: DC power control or DC voltage control, such as... Figure 8 As shown. Considering practical applications, high-voltage, high-capacity autotransformers do not require additional control to maintain a constant output voltage. Instead, based on a given transmission power reference value, the current flowing through the energy buffer bridge arm and the high-voltage side needs to be calculated, and power closed-loop control is achieved by stabilizing the current. When the high-voltage and low-voltage side voltages may change, a DC voltage control mode should be adopted. This involves controlling the current and the duty cycle of the thyristor commutator valves to stabilize the high-voltage side output voltage.

[0119] Furthermore, DC power control calculates the current that should flow through the energy buffer bridge arm and the high-voltage side based on a given transmission power reference value. It then controls the voltage across the inductor of the energy buffer bridge arm by controlling the number of energy buffer bridge arm sub-modules switched on and off, thereby controlling the actual current i of the energy buffer bridge arm. SM To ensure stability and achieve closed-loop power control;

[0120] DC voltage control is activated when the voltage on the high-voltage and low-voltage sides may change, by controlling the actual current i of the energy buffer bridge arm. SM And the duty cycle of the first converter valve, to achieve stability of the high-voltage side output voltage;

[0121] in:

[0122] When the energy buffer bridge arm is connected to the high-voltage side, a reference signal i for the energy buffer bridge arm current is given. SM_Href The number of energy buffer bridge arm sub-modules switched on and off is controlled, thereby controlling the voltage across the inductor of the energy buffer bridge arm and thus the actual current i of the energy buffer bridge arm. SM Able to stably follow the current reference signal i SM_Href ,Right now Figure 5 Current reference i at time [t1,t4] SM_Href The following relationship should be satisfied:

[0123]

[0124] Among them, U 0H Let ω be the angular frequency of the sinusoidal half-wave voltage applied across the buffer inductor, and P be the amplitude of the sinusoidal half-wave voltage applied across the buffer inductor. ref It is the reference value of DC power transmitted by the converter, U H For the high-voltage side voltage, I C2_cha This represents the charging current amplitude of capacitor C2 when the thyristor is turned off. If a DC voltage control mode is used, i SM_HrefThe amplitude is output by the PI controller, the purpose of which is to eliminate the high-voltage side voltage U. H Its reference value U H_ref The deviation between them. Meanwhile, in DC voltage control mode, it is important to note that the cutoff frequency of the current loop PI controller must be much higher than that of the voltage loop PI controller to achieve a dynamic structure and dynamic performance of the inner current loop. SM_Href The waveform shape is determined by waveform generator 1, which in 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 a high-voltage, high-capacity autotransformer targets the high-voltage side power or voltage. As the energy transfer medium, if the energy in the energy buffer arm cannot be balanced during operation, the controlled high-voltage side power or voltage will deviate from its rated value. Therefore, designing energy balance control for the energy buffer arm is crucial. The main purpose of this control is to maintain a constant total energy stored in the capacitors within the energy buffer arm. The energy storage element in the energy buffer arm is the capacitor in the submodule, and energy transfer is achieved through the current flowing through it, manifested externally by the capacitor voltage. Therefore, when the output voltage of the energy buffer arm is less than the rated value, more energy needs to be transferred to the submodule capacitor, i.e., the charging current needs to be increased; conversely, when the output voltage of the energy buffer arm is greater than the rated value, less energy needs to be transferred to the submodule capacitor, i.e., the charging current needs to be decreased. The control method involves adjusting the current reference i when the energy buffer arm forms a loop with the low-voltage side. SM_Lref To achieve this. SM_Lref The amplitude is generated by a PI controller, whose input is the reference value U of the submodule capacitor voltage. C_ref The average voltage U of all submodule capacitors in the measured energy buffer bridge arm C_av The deviation between them. Figure 9 This is a block diagram for the energy balance control of the energy buffer bridge arm.

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

[0128] (5) Grid voltage feedforward control

[0129] Considering that the voltage transformation of the energy buffer bridge arm is achieved by switching the submodules into their active and bypass states, and that this switching requires a certain amount of time (otherwise the device may be damaged by excessive du / dt), when the thyristor current is 0, it is necessary to adjust the voltage gradient of the energy buffer bridge arm by providing a reference signal. This ensures that the voltage across the converter valve is approximately 0 before it is turned on, thus achieving an approximately zero-voltage condition before the thyristor is turned on. Furthermore, considering the actual device performance, du / dt is limited to protect the device from damage. This value is then superimposed on the output of the energy balance control section of the energy buffer bridge arm to obtain the voltage reference signal u for the energy buffer bridge arm. ref . Figure 10 This is a block diagram of the grid voltage feedforward control.

[0130] (6) Submodule capacitor voltage equalization control and PSC-PWM (carrier phase shift modulation) control

[0131] Conventional MMC control can be achieved using PSC-PWM or a sorting algorithm. However, considering the large number of sub-modules under high-voltage conditions, using a sorting algorithm might result in a long control cycle, affecting dynamic performance. Therefore, PSC-PWM control is used for the sub-module control of high-voltage, high-capacity autotransformers. Its main function is to compare the input voltage reference signal of each sub-module with a triangular carrier wave. If the reference signal is greater than the carrier wave, the sub-module is engaged; otherwise, it is bypassed, thus generating the IGBT drive signal for each sub-module. Ideally, PSC-PWM can achieve voltage balance among the capacitors of each sub-module. However, due to differences in manufacturing processes and material properties, there are inevitably parameter differences between sub-modules, leading to voltage variations during operation. Therefore, before PSC-PWM modulation, the voltage reference of each sub-module needs to be appropriately adjusted to achieve voltage balance. Figure 11 As shown.

[0132] The specific implementation method is as follows: Figure 12 As shown, the capacitors of the sub-modules are first numbered from 1 to N, and the capacitor voltage U of the i-th sub-module (i = 1, 2, ..., N) is... C (i) The average capacitor voltage U of all submodules in the energy buffer bridge arm C_avThe deviation is fed into the proportional controller. Meanwhile, considering that the current direction of the energy buffer bridge arm does not change, there is no need to consider the impact of current direction change on the positive or negative value of the regulation, as in PSC-PWM control in MMC control. After feeding the error between the capacitor voltage and the average value into the proportional controller, the obtained proportional coefficient is directly compared with i. SM Multiplying these values ​​yields the corresponding voltage reference regulation Δu(i) for the submodule. Therefore, if the energy buffer bridge arm current charges the submodule (i... SM >0), when the capacitor voltage U of the i-th submodule C (i) lower than the average capacitor voltage U of this phase submodule C_av At this time, a positive submodule voltage reference adjustment Δu(i) will be generated, and this adjustment will be compared with the energy buffer bridge arm voltage reference u. ref Add them together to obtain the voltage reference u of the i-th submodule. ref_i This process can be represented 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] In the formula K p It is the proportional coefficient of the proportional controller.

[0135] The power absorbed by a submodule is determined by its capacitor voltage and inductor current, and the current flowing through all submodules is consistent. The magnitude of the inductor current is determined by the transmitted power, so power control can be achieved by adjusting the submodule's engagement time. When the energy buffer arm absorbs energy from the low-voltage side, the engagement time of the high-voltage module is reduced, and the engagement time of the low-voltage module is increased; when the energy buffer arm releases energy to the high-voltage side, the engagement time of the high-voltage module is increased, and the engagement time of the low-voltage module is reduced. Under this control method, submodules with higher capacitor voltages will absorb less energy (or release more energy), and vice versa, thus maintaining capacitor voltage balance among the submodules of the energy buffer arm.

[0136] Based on the same inventive concept, an embodiment of the present invention also provides a control system for a high-voltage, high-capacity autotransformer, which may include the following modules:

[0137] The converter valve switching control module is used to control the current flowing through the converter valve. When the converter valve uses a semi-controlled device, reliable shut-off of the converter valve can be achieved; when the converter valve uses a fully controlled device, zero-current shut-off of the converter valve can be achieved, reducing shut-off losses.

[0138] The energy buffer bridge arm current control module controls the switching of the bridge arm submodules, thereby controlling the voltage of the energy buffer bridge arm and achieving closed-loop current control. It also generates a current reference signal i for both the DC power / voltage control section and the energy balance control section of the energy buffer bridge arm. SM_Href and current reference signal i SM_Lref ;

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

[0140] Energy buffer bridge arm energy balance control module, this module is used to control the current reference signal i SM_Lref Adjustments are made to maintain a constant overall capacitance storage in the energy buffer bridge arm;

[0141] The grid voltage feedforward control module is used to regulate the voltage of the energy buffer bridge arm to achieve near-zero voltage conduction condition of the converter valve and limit du / dt.

[0142] The submodule capacitor voltage equalization control module is used to adjust the voltage reference signal of each bridge arm submodule to achieve voltage equalization of the bridge arm submodule capacitors.

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

[0144] It should be noted that the steps in the method provided by the present invention can be implemented 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 building the system, and will not be elaborated here.

[0145] The high-voltage, high-capacity autotransformer and its operating and control methods provided in the above embodiments of the present invention achieve energy transfer from the low-voltage side to the high-voltage side through energy buffer bridge arms and supporting capacitors as media; simultaneously, zero-current turn-off is achieved using thyristor converter valves, improving transformer efficiency. A control method for the high-voltage, high-capacity autotransformer is proposed to achieve energy balance in the energy buffer bridge arms. By controlling the number of energy buffer bridge arm sub-modules in operation, reliable turn-off and near-zero voltage conduction of the thyristor converter valves are achieved. Furthermore, considering different application scenarios, two closed-loop methods are proposed: DC voltage control and power control. Because the present invention adopts an autotransformer structure and uses active energy buffer bridge arms, high-voltage, high-capacity DC transformation can be achieved; simultaneously, the use of semi-controlled converter valves reduces device costs.

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

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

Claims

1. A method for operating a high-voltage, high-capacity autotransformer, characterized in that, The high-voltage, high-capacity autotransformer includes: A supporting capacitor, an energy buffer bridge arm, and a converter valve are connected between the low-voltage and high-voltage sides of the transformer. The supporting capacitor forms part of the power transmission loop, used to transmit power from the low-voltage side to the high-voltage side through the supporting capacitor. The energy buffer bridge arm forms another part of the power transmission loop, used to transmit power from the low-voltage side to the high-voltage side through the energy buffer bridge arm, to the supporting capacitor through the energy buffer bridge arm, and to supplement the power transmitted from the supporting capacitor to the high-voltage side. Transformer commutation is achieved by controlling the number of bridge arm sub-modules engaged in the energy buffer bridge arm and the trigger signal of the converter valve. The supporting capacitor includes a supporting capacitor... C 1 and supporting capacitor C 2. The switching valve includes a first switching valve. T 1 and second converter valves D 1; wherein: the supporting capacitor C 1. Connected in parallel with the low-voltage side, the supporting capacitor C 2 and the supporting capacitor C 1. Series connection; the two are connected in series and then in parallel with the high-voltage side, creating a direct electrical connection between the low-voltage and high-voltage sides, forming part of the power transmission circuit; the first converter valve T 1 and the second switching valve D 1. Connected in series and then in parallel with the high-voltage side, the first converter valve T One end of 1 is connected to the second converter valve D 1 connected, the first switching valve T The other end of 1 is connected to the negative terminal of the high-voltage side, and the second converter valve D One end of 1 is connected to the positive terminal of the high-voltage side, and the second converter valve D The other end of 1 is connected to the first converter valve. T 1 is connected, through the first switching valve T 1 and the second switching valve D The alternating conduction of 1 enables the transformer to perform its commutation function; one end of the energy buffer bridge arm is connected to the supporting capacitor. C 1 and the supporting capacitor C At connection point 2, the other end of the energy buffer bridge arm is connected to the first converter valve. T 1 and the second switching valve D At the connection point of 1, the transformer current is controlled by controlling the number of sub-modules switched on and off within the energy buffer bridge arm; The working method includes: When power flows from the low-voltage side of the transformer to the high-voltage side, depending on the switching status of the converter valve, there are two converter operating modes: Operating mode 1, first converter valve T 1. Shut down the second converter valve. D 1. When the circuit is turned on, the low-voltage side and the energy buffer bridge arm are connected in series to jointly transfer power to the high-voltage side. (The capacitor...) C 2. Charging is performed by the energy buffer bridge arm; Operating mode two, first converter valve T 1 is turned on, second converter valve D 1. When the circuit is turned off, the low-voltage side energy buffer bridge arm transfers power, and the capacitor... C 2. Discharge to the high-voltage side; It also includes any one or more of the following: The first step involves considering the current and voltage waveforms of the energy buffer bridge arm and setting the capacitor... C 2. If the voltage across the high-voltage side of the transformer remains approximately constant, then the first operating mode includes the following modes: Mode 1 [ t 0, t [1] In the first mode, the circuit is in a dead zone, and the second commutator valve D 1. No current flows through the capacitor at this time. C Two units are connected in series to transfer energy to the high-voltage side, with a current of... I C2_rel ; Adjust the voltage of the energy buffer bridge arm to ( U H - U L ),in, U H This refers to the voltage on the high-voltage side of the transformer. U L This is the low-voltage side voltage of the transformer. At this time, the sum of the low-voltage side voltage and the energy buffer bridge arm voltage equals the high-voltage side voltage. The buffer inductance of the energy buffer bridge arm... L The voltage across the two ends is 0, and no current flows through the energy buffer bridge arm; Mode 2 [ t 1, t [2] In mode two, by controlling the voltage of the energy buffer bridge arm, the voltage is kept at its original amplitude ( U H - U L Based on this, add an amplitude value. U 0H half-sine wave u 0H ( t At this time, the inductor applied to the buffer inductor L The voltage on is u 0H ( t The current on the energy buffer bridge arm rises from 0 to... I SM_H Meanwhile, due to the second converter valve D Current begins to flow through 1, in mode one, the low-voltage side and the capacitor C 2. The current flowing to the high-voltage side gradually decreases to 0, and then reverses, starting from the energy buffer bridge arm supplying the capacitor. C 2. Charging begins, and the charging current increases from 0 to [a higher value] as the energy buffer bridge arm current increases. I C2_cha ; Modal 3 [ t 2, t [3], under mode three, the energy buffer bridge arm voltage returns to and stabilizes at ( U H - U L ), buffer inductor L The voltage across the terminals is 0, and the current is maintained at the rated value. I SM_H At this time, the power transferred from the low-voltage side and the energy buffer bridge arm to the high-voltage side remains at the rated value. P H Bridge arm submodule to capacitor C 2. The energy transferred is maintained at P C2_cha ;in: (1) (2) In the formula, I SM_H The rated current for the energy buffer bridge arm when transferring energy to the high-voltage side; Mode 4 t 3, t [4] In mode four, by controlling the voltage of the energy buffer bridge arm, the voltage is kept at its original amplitude ( U H - U L Based on ), subtract an amplitude value. U 0H half-sine wave u 0H ( t At this time, the inductor applied to the buffer inductor L The voltage on is - u 0H ( t The current on the energy buffer bridge arm flows from... I SM_H The energy drops to 0; simultaneously, the energy buffer bridge arm supplies energy to the capacitor. C The charging current of 2 decreases as the energy buffer bridge arm current decreases, from I C2_cha Drop to 0; Modal 5 [ t 4, t [5] In mode five, the current on the energy buffer bridge arm has dropped to 0, at which point the circuit enters the dead zone; the low-voltage side and the capacitor C 2 are connected in series, transferring power to the high-voltage side again; the energy buffer bridge arm voltage is adjusted as follows: U H - U L At this point, the sum of the low-voltage side voltage and the energy buffer bridge arm voltage equals the high-voltage side voltage, and the buffer inductor... L The voltage across the two ends is 0, and no current flows through the energy buffer bridge arm; The second step involves considering the current and voltage waveforms of the energy buffer bridge arm and setting the capacitor... C 2. If the voltage across the high-voltage side of the transformer remains approximately constant, then the second operating mode includes the following modes: Modal 6 [ t 5, t [6], under mode six, the low-voltage side and the capacitor C Two units are connected in series to transfer energy to the high-voltage side, with a current of... I C2_rel This process continues throughout operating mode two; the energy buffer bridge arm adjusts the voltage to - U L At this time, the low-voltage side voltage is equal to the energy buffer bridge arm voltage, and the buffer inductor... L The voltage across the terminals is 0, and no current flows through the energy buffer bridge arm; at this time, the first converter valve T The voltage across terminals 1 is approximately 0, which is the voltage across the first converter valve. T 1. Near-zero voltage conduction establishes the conduction condition; Modal 7 [ t 6, t 7], in mode seven, by controlling the energy buffer bridge arm voltage, the voltage is kept within its original amplitude. U L On top of that, add an amplitude value U 0L half-sine wave u 0L ( t At this time, the inductor applied to the buffer inductor L The voltage on is u 0L ( t Therefore, the current on 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. Modal 8 [ t 7, t [8], under mode eight, the energy buffer bridge arm voltage returns to and stabilizes at - U L Buffer inductor L The voltage across the terminals is 0, and the current is maintained at the rated value. I SM_L At this time, the power transmitted by the low-voltage lateral energy buffer arm remains at the rated value. P L ;in: (3) In the formula, I SM_L The rated current for the energy buffer arm when it absorbs energy from the low-voltage side; Modal Nine [ t 8, t [9], in mode nine, by controlling the voltage of the energy buffer bridge arm, the voltage is kept within its original amplitude. U L Based on this, subtract an amplitude value U 0L half-sine wave u 0L ( t At this time, the inductor applied to the buffer inductor L The voltage on is - u 0L ( t The current on the energy buffer bridge arm flows from... I SM_L As the current decreases, the power transmitted by the low-voltage side energy buffer bridge arm gradually decreases to 0. Modal 10 [ t 9, t 10 In mode 10, the current on the energy buffer bridge arm has dropped to 0, and the circuit has entered the dead zone; the energy buffer bridge arm adjustment voltage is - U L At this time, the low-voltage side voltage is equal to the energy buffer bridge arm voltage, and the buffer inductor... L The voltage across the two ends is 0, and no current flows through the energy buffer bridge arm.

2. The operating method of the high-voltage, high-capacity autotransformer according to claim 1, characterized in that, The energy buffer bridge arm includes multiple bridge arm sub-modules SM connected in series and a buffer inductor connected in series with the last bridge arm module SM. L ; Each of the bridge arm submodules SM includes: a submodule capacitor and a switching device; the submodule capacitor is used to store energy and provide voltage; the switching device is used to control the switching of the bridge arm submodule's operating state; wherein: The switching device adopts an output voltage inverse structure, and the submodule capacitor is connected in parallel with the output voltage inverse structure of the switching device.

3. The operating method of the high-voltage, high-capacity autotransformer according to claim 1, characterized in that, First converter valve T 1 and the second switching valve D 1 is composed of unidirectional conductive devices connected in series; wherein: First converter valve T 1. Unidirectional conduction devices use semi-controlled or fully controlled devices; Second converter valve D 1. Unidirectional conduction devices use uncontrolled, semi-controlled, or fully controlled devices.

4. A control method for a high-voltage, high-capacity autotransformer, characterized in that, The high-voltage, high-capacity autotransformer includes: A supporting capacitor, an energy buffer bridge arm, and a converter valve are connected between the low-voltage and high-voltage sides of the transformer. The supporting capacitor forms part of the power transmission loop, used to transmit power from the low-voltage side to the high-voltage side through the supporting capacitor. The energy buffer bridge arm forms another part of the power transmission loop, used to transmit power from the low-voltage side to the high-voltage side through the energy buffer bridge arm, to the supporting capacitor through the energy buffer bridge arm, and to supplement the power transmitted from the supporting capacitor to the high-voltage side. Transformer commutation is achieved by controlling the number of bridge arm sub-modules engaged in the energy buffer bridge arm and the trigger signal of the converter valve. The supporting capacitor includes a supporting capacitor... C 1 and supporting capacitor C 2. The switching valve includes a first switching valve. T 1 and second converter valves D 1; wherein: the supporting capacitor C 1. Connected in parallel with the low-voltage side, the supporting capacitor C 2 and the supporting capacitor C 1. Series connection; the two are connected in series and then in parallel with the high-voltage side, creating a direct electrical connection between the low-voltage and high-voltage sides, forming part of the power transmission circuit; the first converter valve T 1 and the second switching valve D 1. Connected in series and then in parallel with the high-voltage side, the first converter valve T One end of 1 is connected to the second converter valve D 1 connected, the first switching valve T The other end of 1 is connected to the negative terminal of the high-voltage side, and the second converter valve D One end of 1 is connected to the positive terminal of the high-voltage side, and the second converter valve D The other end of 1 is connected to the first converter valve. T 1 is connected, through the first switching valve T 1 and the second switching valve D The alternating conduction of 1 enables the transformer to perform its commutation function; one end of the energy buffer bridge arm is connected to the supporting capacitor. C 1 and the supporting capacitor C At connection point 2, the other end of the energy buffer bridge arm is connected to the first converter valve. T 1 and the second switching valve D At the connection point of 1, the transformer current is controlled by controlling the number of sub-modules switched on and off within the energy buffer bridge arm; The control method includes: The converter valve control section controls the current flowing through the converter valve. When a semi-controlled device is used, it is used to achieve reliable shut-off of the converter valve; when a fully controlled device is used, it is used to achieve zero-current shut-off of the converter valve and reduce shut-off losses. The energy buffer bridge arm current control section controls the switching of the bridge arm submodules, thereby controlling the voltage of the energy buffer bridge arm and achieving closed-loop current control. Simultaneously, it generates current reference signals for the DC power / voltage control section and the energy balance control section of the energy buffer bridge arm. i SM_Href and current reference signal i SM_Lref ; The DC power / voltage control section uses the current reference signal. i SM_Href Adjustments are made to achieve closed-loop power control and stabilize the high-voltage side output voltage. The energy balance control section of the energy buffer bridge arm controls the current reference signal. i SM_Lref Adjustments are made to maintain a constant total capacitance storage in the energy buffer bridge arm; The grid voltage feedforward control section regulates the voltage of the energy buffer bridge arm to achieve near-zero voltage conduction conditions for the converter valve and limit d. u / d t ; The submodule capacitor voltage equalization control section adjusts the voltage reference signal of each bridge arm submodule to achieve capacitor voltage equalization of the bridge arm submodules. The carrier phase-shift modulation control section adjusts the voltage reference signal of the bridge arm submodule to generate the IGBT drive signal for each bridge arm submodule.

5. The control method for a high-voltage, high-capacity autotransformer according to claim 4, characterized in that, The switching valve switching control section introduces an additional current reference value when the first switching valve... T 1. When using semi-controlled devices, the flow through the first converter valve is... T The current of valve 1 steadily decreases below the holding current, thereby realizing the first switching valve. T 1. Reliable shutdown; when the first switching valve T 1. When using fully controlled devices, realize the first converter valve T 1. Zero current turn-off.

6. The control method for a high-voltage, high-capacity autotransformer according to claim 4, characterized in that, The energy buffer bridge arm current control section employs a PI controller, introducing an inner current control loop on top of the outer loop control; assuming 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 of the PI controller affects the switching 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.

7. The control method for a high-voltage, high-capacity autotransformer according to claim 6, characterized in that, The energy buffer bridge arm current reference signal i SM_ref It is generated by superimposing two parts, one of which is the current reference signal when the bridge arm, high-voltage side, and low-voltage side jointly form a circuit. i SM_Href The other part is the current reference signal when the bridge arm and the low-voltage side form a circuit. i SM_Lref These two parts are obtained through two outer-loop controllers, where the current reference signal... i SM_Href Current reference signal used to implement DC power / voltage control section i SM_Lref Used to implement the energy balance control section of the energy buffer bridge arm.

8. The control method for a high-voltage, high-capacity autotransformer according to claim 4, characterized in that, The DC power / voltage control section 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 based on a given transmission power reference value. It then controls the voltage across the inductor of the energy buffer bridge arm by controlling the number of energy buffer bridge arm sub-modules switched on and off, thereby controlling the actual current flowing through the energy buffer bridge arm. i SM To ensure stability and achieve closed-loop power control; The DC voltage control is activated when the voltage on the high-voltage and low-voltage sides may change, by controlling the actual current of the energy buffer bridge arm. i SM And the duty cycle of the first converter valve, to achieve stability of the high-voltage side output voltage; in: When the energy buffer bridge arm is connected to the high-voltage side, a reference signal for the energy buffer bridge arm current is given. i SM_Href The number of energy buffer bridge arm sub-modules switched on and off is controlled, thereby controlling the voltage across the inductor of the energy buffer bridge arm and thus the actual current of the energy buffer bridge arm. i SM Able to stably follow the current reference signal i SM_Href Current reference signal i SM_Href The following relationship must be satisfied: (4) (5) In the formula, U 0H The angular frequency is the amplitude of the sinusoidal half-wave voltage applied across the buffer inductor. ω The angular frequency of the sinusoidal half-wave voltage applied across the buffer inductor. P ref The reference value for DC power transmitted by the converter. U H This is the high-voltage side voltage. I C2_cha For capacitor C 2. The amplitude of the charging current when the thyristor is turned off; When DC voltage control is used, the current reference signal i SM_Href The amplitude is controlled by a PI controller to eliminate the high-voltage side voltage. U H Its reference value U H_ref The deviation between them; at the same time, the cutoff frequency of the current loop PI controller is much higher than that of the voltage loop PI controller, thereby achieving the dynamic structure between the two and the dynamic performance of the inner current loop.

9. The control method for a high-voltage, high-capacity autotransformer according to claim 4, characterized in that, The energy balance control section of the energy buffer bridge arm adjusts the current reference signal when the energy buffer bridge arm and the low-voltage side form a circuit. i SM_Lref To maintain a constant overall capacitance storage in the energy buffer bridge arm, when the output voltage of the energy buffer bridge arm is less than the rated value, the current reference signal is increased. i SM_Lref ; Conversely, when the output voltage of the energy buffer bridge arm is greater than the rated value, the current reference signal is reduced. i SM_Lref ;in: The current reference signal i SM_Lref Generated by a PI controller, its input is the reference value of the bridge arm submodule capacitor voltage. U C_ref The average voltage of the capacitors of all bridge arm submodules in the measured energy buffer bridge arm. U C_av The deviation between; when the deviation is greater than 0, the voltage output of the energy buffer bridge arm is less than the rated value, and the current reference signal is increased by adjusting the PI controller. i SM_Lref The amplitude is adjusted to restore energy balance; when the deviation is less than 0, the voltage output of the energy buffer bridge arm is less than the rated value, and the current reference signal is reduced by adjusting the PI controller. i SM_Lref The amplitude is adjusted to restore energy balance.

10. The control method for a high-voltage, high-capacity autotransformer according to claim 4, characterized in that, The grid voltage feedforward control section, when the first converter valve current is 0, provides a grid voltage feedforward control reference signal. U g_ref The voltage step of the energy buffer bridge arm is adjusted to rise or fall, so that the voltage across the first converter valve is approximately zero before it is turned on, thus achieving an approximately zero voltage condition before the first converter valve is turned on, and for d u / d t To impose restrictions; feedforward control reference signal of grid voltage U g_ref The voltage reference signal of the energy buffer bridge arm is obtained by superimposing it with the output of the energy balance control section of the energy buffer bridge arm. u ref .

11. The control method for a high-voltage, high-capacity autotransformer according to claim 4, characterized in that, The submodule capacitor voltage equalization control section adjusts the voltage reference signals of each bridge arm submodule to achieve capacitor voltage equalization. The carrier phase shift modulation control section compares the adjusted voltage reference signals of each bridge arm submodule with a triangular carrier wave. If the signal is greater than the triangular carrier wave, the bridge arm submodule is engaged; otherwise, it is bypassed, thereby generating IGBT drive signals for each bridge arm submodule. First, increase the capacitor of the bridge arm submodule from 1 to... N Number them, and put the first i Each bridge arm submodule ( i =1, 2, … , N capacitor voltage Average voltage of capacitors in all bridge arm submodules of the energy buffer bridge arm U C_av The deviation is fed into the proportional controller, and the obtained proportional coefficient is directly compared with the actual current of the energy buffer bridge arm. i SM Multiplying these values ​​yields the voltage reference regulation for the corresponding bridge arm submodule. When the energy buffer bridge arm current charges the bridge arm submodule, i SM >0, when the first i Capacitor voltage of each bridge arm submodule Lower than the average capacitor voltage of the phase bridge arm submodule U C_av At this time, a positive voltage reference regulation will be generated for the bridge arm submodule. The adjustment amount is compared with the energy buffer bridge arm voltage reference signal. u ref Add them together to get the first one. i Voltage reference signal of each bridge arm submodule u ref_i Then we have: (6) In the formula, K p The proportional coefficient of the proportional controller; When the energy buffer bridge arm absorbs energy from the low-voltage side, the activation time of the high-voltage bridge arm submodule is reduced, and the activation time of the low-voltage bridge arm submodule is increased; when the energy buffer bridge arm releases energy to the high-voltage side, the activation time of the high-voltage bridge arm submodule is increased, and the activation time of the low-voltage bridge arm submodule is reduced, thereby maintaining the capacitor voltage balance among the bridge arm submodules of the energy buffer bridge arm.

12. The control method for a high-voltage, high-capacity autotransformer according to claim 4, characterized in that, The energy buffer bridge arm includes multiple bridge arm sub-modules SM connected in series and a buffer inductor connected in series with the last bridge arm module SM. L ; Each of the bridge arm submodules SM includes: a submodule capacitor and a switching device; the submodule capacitor is used to store energy and provide voltage; the switching device is used to control the switching of the bridge arm submodule's operating state; wherein: The switching device adopts an output voltage inverse structure, and the submodule capacitor is connected in parallel with the output voltage inverse structure of the switching device.

13. The control method for a high-voltage, high-capacity autotransformer according to claim 4, characterized in that, First converter valve T 1 and the second switching valve D 1 is composed of unidirectional conductive devices connected in series; wherein: First converter valve T 1. Unidirectional conduction devices use semi-controlled or fully controlled devices; Second converter valve D 1. Unidirectional conduction devices use uncontrolled, semi-controlled, or fully controlled devices.

14. A control system for a high-voltage, high-capacity autotransformer, characterized in that, The high-voltage, high-capacity autotransformer includes: A supporting capacitor, an energy buffer bridge arm, and a converter valve are connected between the low-voltage and high-voltage sides of the transformer. The supporting capacitor forms part of the power transmission loop, used to transmit power from the low-voltage side to the high-voltage side through the supporting capacitor. The energy buffer bridge arm forms another part of the power transmission loop, used to transmit power from the low-voltage side to the high-voltage side through the energy buffer bridge arm, to the supporting capacitor through the energy buffer bridge arm, and to supplement the power transmitted from the supporting capacitor to the high-voltage side. Transformer commutation is achieved by controlling the number of bridge arm sub-modules engaged in the energy buffer bridge arm and the trigger signal of the converter valve. The supporting capacitor includes a supporting capacitor... C 1 and supporting capacitor C 2. The switching valve includes a first switching valve. T 1 and second converter valves D 1; wherein: the supporting capacitor C 1. Connected in parallel with the low-voltage side, the supporting capacitor C 2 and the supporting capacitor C 1. Series connection; the two are connected in series and then in parallel with the high-voltage side, creating a direct electrical connection between the low-voltage and high-voltage sides, forming part of the power transmission circuit; the first converter valve T 1 and the second switching valve D 1. Connected in series and then in parallel with the high-voltage side, the first converter valve T One end of 1 is connected to the second converter valve D 1 connected, the first switching valve T The other end of 1 is connected to the negative terminal of the high-voltage side, and the second converter valve D One end of 1 is connected to the positive terminal of the high-voltage side, and the second converter valve D The other end of 1 is connected to the first converter valve. T 1 is connected, through the first switching valve T 1 and the second switching valve D The alternating conduction of 1 enables the transformer to perform its commutation function; one end of the energy buffer bridge arm is connected to the supporting capacitor. C 1 and the supporting capacitor C At connection point 2, the other end of the energy buffer bridge arm is connected to the first converter valve. T 1 and the second switching valve D At the connection point of 1, the transformer current is controlled by controlling the number of sub-modules switched on and off within the energy buffer bridge arm; The control system includes: The converter valve switching control module is used to control the current flowing through the converter valve. When the converter valve uses a semi-controlled device, it is used to achieve reliable shut-off of the converter valve; when the converter valve uses a fully controlled device, it is used to achieve zero-current shut-off of the converter valve and reduce shut-off losses. The energy buffer bridge arm current control module controls the switching of the bridge arm submodules, thereby controlling the voltage of the energy buffer bridge arm and achieving closed-loop current control. It also generates current reference signals for the DC power / voltage control section and the energy balance control section of the energy buffer bridge arm. i SM_Href and current reference signal i SM_Lref ; DC power / voltage control module, which is used for current reference signal i SM_Href Adjustments are made to achieve closed-loop power control and stable high-voltage side output voltage; Energy buffer bridge arm energy balance control module, this module is used for current reference signal i SM_Lref Adjustments are made to maintain a constant overall capacitance storage in the energy buffer bridge arm; The grid voltage feedforward control module is used to regulate the voltage of the energy buffer bridge arm, achieving near-zero voltage conduction of the converter valve and limiting d. u / d t ; The submodule capacitor voltage equalization control module is used to adjust the voltage reference signal of each bridge arm submodule to achieve voltage equalization of the bridge arm submodule capacitors. The carrier phase-shift modulation control module is used to adjust the voltage reference signal of the bridge arm sub-module and generate the IGBT drive signal for each bridge arm sub-module.

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