A high-voltage large-capacity DC transformer and a method thereof

Abstract

The application discloses a high-voltage and high-capacity direct-current transformer and a method thereof, which is composed of a plurality of resonant switch capacitor modules based on thyristor bridge arms. The direct-current transformer topology can realize high power transmission capacity under high transformation ratio and high voltage. Unlike the existing direct-current transformer adopting a modular multi-level structure, the direct-current transformer does not need a complex voltage balancing algorithm to ensure the balance of the module capacitor voltage, but only needs to realize self-balancing through the resonance principle. A large number of sub-modules are not needed to realize high-voltage application, and only device series connection is needed to realize high-voltage application. The application adopts semi-controlled devices instead of fully-controlled devices, and has low cost and small loss. The application can be flexibly applied to different application scenarios by changing the number of modules and the number of device series connection.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, specifically to a high-voltage, high-capacity DC transformer and its method. Background Technology

[0002] In recent years, with the gradual deterioration of the environment and the increasing scarcity of fossil fuels, renewable energy has seen significant development. Currently, large-scale onshore wind farms are difficult to find. Compared to onshore wind energy, offshore wind speeds are higher and more consistent, leading to a trend towards deeper and larger-scale offshore wind power generation. This necessitates efficient and reliable power collection and transmission systems. Offshore wind power output systems mainly include AC output and DC transmission. For deep-sea offshore wind power transmission systems, using AC transmission presents challenges such as submarine cable charging power, high terminal voltage, AC aggregation, and transmission stability. Therefore, output via a flexible DC system offers greater economic advantages and is the primary choice for transmission schemes in deep-sea wind power development and utilization.

[0003] For offshore wind power transmission systems via flexible direct current (DC) transmission, there are two main types of internal collection systems within the offshore wind farm: AC collection systems and DC collection systems. AC collection systems are currently the widely adopted solution, employing mature AC technology. However, the entire AC collection system requires multiple AC-DC conversions and large-capacity AC transformers for voltage boosting, resulting in low efficiency and high cost. In contrast, DC collection uses all-DC technology, requiring only two DC voltage boosting conversions, eliminating the need for large-capacity AC transformers and AC-DC conversion stages, making it more suitable for offshore wind power collection. The DC transformer connecting the DC wind farm and the high-voltage flexible DC transmission system is a key component for achieving all-DC collection. Especially during the secondary voltage boosting process, a high-voltage, high-capacity DC transformer is needed. Current high-voltage, high-capacity DC transformers generally employ modular multilevel technology. This approach requires complex voltage equalization control strategies to ensure stable module operation, increasing the complexity of the overall control strategy and operational reliability issues. Therefore, a simple, reliable, and low-cost DC transformer is urgently needed. Summary of the Invention

[0004] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0005] In view of the above-mentioned deficiencies of the prior art, the present invention provides a high-voltage, high-capacity DC transformer and its method. It uses semi-controlled devices to replace conventional switching devices to achieve the requirements of low cost and high capacity. It uses high-voltage semi-controlled devices and the semi-controlled devices can be easily connected in series to achieve the requirements of high voltage. It uses a modular structure to achieve the application of high turns ratio and gain adjustment design. It uses the principle of resonant switched capacitors to achieve self-balancing of capacitance between modules.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a high-voltage, high-capacity DC transformer and its method, comprising: the DC transformer being composed of multiple resonant switched capacitor modules based on thyristor bridge arms; the thyristor bridge arm being a series valve formed by multiple thyristors connected in series; the thyristor bridge arm being reliably turned off through resonant commutation; the resonant switched capacitor modules being used to construct an energy balance circuit; and controlling each module to transfer energy independently to achieve self-balancing of the DC capacitor voltage of each sub-module.

[0007] As a preferred embodiment of the high-voltage, high-capacity DC transformer and its method described in this invention, the DC transformer (100) comprises a resonant switched capacitor module (101) based on a thyristor bridge arm, an input DC capacitor (102), and an input inductor (103); the input sides (101a) of the n resonant switched modules are connected in parallel to both sides of the input DC capacitor (102) and interconnected with the medium and low voltage DC ports; the output DC capacitors of the output sides (101c) of the n resonant switched modules are connected in series and then connected in series with the input DC capacitor (102) to form the high-voltage side, thus constituting a DC transformer with a turns ratio of n+1.

[0008] As a preferred embodiment of the high-voltage, high-capacity DC transformer and its method described in this invention, the resonant switched capacitor module (101) based on the thyristor bridge arm includes an input side (101a) composed of a set of upper and lower thyristor series arms. When the thyristor bridge arm is a series valve, it includes an auxiliary voltage equalization branch. The auxiliary voltage equalization circuit is used to realize voltage equalization of the devices and to realize reliable turn-off of the thyristor bridge arm.

[0009] In a preferred embodiment of the high-voltage, high-capacity DC transformer and its method described in this invention, the resonant switched capacitor module (101) based on the thyristor bridge arm includes a resonant branch (101b) of which serves as an energy transmission path and can employ a resonant inductor L. r and resonant capacitor C r Forming a resonant branch.

[0010] As a preferred embodiment of the high-voltage, high-capacity DC transformer and its method described in this invention, the resonant switched capacitor module (101) based on the thyristor bridge arm further includes an output side (101c) of the resonant switched capacitor module based on the thyristor bridge arm consisting of a set of upper and lower diode series arms and an output DC capacitor. The diode series arms are multiple diode series valves, and the series valves include an auxiliary voltage equalization branch. The auxiliary voltage equalization circuit is used to realize voltage equalization of the devices and to realize reliable shut-off of the series valves.

[0011] In a preferred embodiment of the high-voltage, high-capacity DC transformer and its method described in this invention, the input DC capacitor (102) of the DC transformer comprises an input DC capacitor on the input side of the DC transformer and an output DC capacitor of the resonant switching capacitor module, which together support the output voltage; the input inductance L of the DC transformer... i (103) includes the input inductance L of the DC transformer. i Used to smooth input current, L i It can also be replaced with other circuit structures that can serve a filtering function.

[0012] As a preferred embodiment of the high-voltage, high-capacity DC transformer and its method described in this invention, the working principle of the resonant switched capacitor module based on the thyristor bridge arm includes:

[0013] In mode 1, the second lower thyristor bridge arm on the input side is turned on. The voltage of the resonant capacitor is less than the voltage of the input capacitor. The second lower thyristor bridge arm of the resonant capacitor is turned on due to the forward voltage drop. The resonant branch forms a resonant circuit with the input side through the second diode bridge arm and the second thyristor bridge arm. The resonant capacitor absorbs energy and the voltage rises. The resonant current returns to 0, and the thyristor bridge arm turns off naturally.

[0014] In mode two, the voltage rises due to the energy absorbed by the resonant capacitor in the resonant branch. At this time, the voltage of the resonant capacitor is greater than the input capacitor voltage. The second lower thyristor bridge arm is in the reverse recovery phase. Since both the diode bridge arm and the thyristor bridge arm have voltage equalization branches, the resonant branch forms a reverse resonant circuit with the output side through the voltage equalization branch. The circuit parameters satisfy the overdamping condition. After the reverse resonant current passes through the voltage equalization branch of the second lower thyristor bridge arm, it will apply an approximately constant reverse voltage to the second lower thyristor bridge arm. When the time for applying the reverse voltage is greater than the turn-off time of the second lower thyristor bridge arm, the second lower thyristor bridge arm is reliably turned off. The turn-off time of the thyristor bridge arm is very short, so the duration of mode two is very short, and it is determined that the resonant capacitor voltage remains unchanged.

[0015] In mode 3, the voltage of the resonant capacitor is greater than the voltage of the input capacitor. The first upper thyristor bridge arm of the resonant capacitor is forward-biased and conducts. The resonant branch forms a resonant circuit with the input side through the first diode bridge arm and the first lower thyristor bridge arm. The resonant capacitor releases energy and the voltage drops. The resonant current returns to 0, and the thyristor bridge arm turns off naturally.

[0016] In mode four, the resonant capacitor in the resonant branch releases energy and the voltage drops. The voltage of the resonant capacitor is less than the voltage of the output capacitor. The first lower thyristor bridge arm is in the reverse recovery stage. Both the diode bridge arm and the thyristor bridge arm have voltage equalization branches. The resonant branch forms a reverse resonant circuit with the output side through the voltage equalization branch. The circuit parameters meet the overdamping condition. After the reverse resonant current passes through the voltage equalization branch of the first lower thyristor bridge arm, it will apply an approximately constant reverse voltage to the first lower thyristor bridge arm. When the time for applying the reverse voltage is greater than the turn-off time of the first lower thyristor bridge arm, the thyristor is reliably turned off.

[0017] The voltage across the resonant capacitor is equal at the beginning and end of a resonant cycle, achieving self-balancing of the capacitor voltage.

[0018] As a preferred embodiment of the high-voltage, high-capacity DC transformer and its method described in this invention, the working principle of the DC transformer includes: during the positive half-cycle, the lower thyristor bridge arm on the input side of all resonant switched capacitor modules is turned on, and the resonant capacitors of all modules form a resonant circuit with the DC capacitor through the lower thyristor bridge arm and the lower diode bridge arm on the output side, and the resonant capacitors absorb energy from the input DC capacitor; during the negative half-cycle, the upper thyristors on the input side of all resonant switched capacitor modules are turned on, and the resonant capacitors of all modules form a resonant circuit with the DC capacitor through the upper thyristor bridge arm and the upper diode bridge arm on the output side, and the resonant capacitors transfer energy to the output DC capacitor.

[0019] In a preferred embodiment of the high-voltage, high-capacity DC transformer and its method described in this invention, the parameters of the DC transformer are designed as follows:

[0020] Set the input current to I in The output current is I out =I in / (1+n), the switching period is T s The resonant period is T r ;

[0021] When operating in the first half of the cycle, for the input DC capacitor C i Its current component consists of two parts, namely, direct current I. in -I out The sum of the resonant currents of the n resonant switched capacitor modules is expressed as follows:

[0022]

[0023] During the positive half-period, [t] ai -t bi Between any given moment, the current in the input DC capacitor is less than zero, the input DC capacitor releases energy, and the capacitor voltage drops. Therefore, the maximum fluctuation value of the DC capacitor voltage can be calculated.

[0024]

[0025] in,

[0026] To satisfy the voltage fluctuation ratio being less than ε i Then the capacitance value of the input capacitor voltage needs to meet the following requirements:

[0027]

[0028] When operating in the second half of the cycle, the current expression for the nth output DC capacitor is:

[0029]

[0030] For the output DC capacitor, during the negative half-cycle, the nth DC capacitor is in [t] aon -t bon When the current between [a certain point] is greater than zero, the output DC capacitor absorbs energy, and the capacitor voltage rises. Therefore, the maximum fluctuation value of the nth DC capacitor can be calculated.

[0031]

[0032] in,

[0033] To meet the requirement that voltage fluctuation is less than ε percent n Then the capacitance value of the nth output capacitor needs to satisfy the following condition. To meet the modularity requirement, the output capacitor is set to its maximum value.

[0034]

[0035] For the input inductor, the maximum voltage drop variation is the same as that of the input DC capacitor. This is to ensure that the input current fluctuation ratio is less than ε. L Then the input inductance needs to meet the following requirements:

[0036]

[0037] For a resonant branch, the resonant frequency is determined by the resonant capacitance C. r and resonant inductance L r The decision must meet the following conditions.

[0038]

[0039] During the reverse resonance processes of modes two and four, the maximum voltage drop of the thyristor is determined by the fluctuation value of the resonant capacitor voltage, which in turn is determined by the characteristic impedance. The resonant capacitor C can be determined by giving the fluctuation value of the resonant capacitor voltage. r and resonant inductance L r Size.

[0040] As a preferred embodiment of the high-voltage, high-capacity DC transformer and its method described in this invention, the pre-charging method of the DC transformer includes: the high-voltage side of the transformer is connected to the high-voltage grid for charging through a current-limiting resistor, and the grid side is connected through a bypass switch; the lower thyristor bridge arm of all modules is turned on, the trigger pulse of the thyristor bridge arm is maintained, so that the resonant capacitor is connected in parallel to both sides of the DC capacitor for charging, charging the DC capacitor and the resonant capacitor, and after the resonant capacitor is charged to the rated voltage, the sending of the thyristor bridge arm trigger pulse is stopped, and the thyristor is naturally turned off; after the parallel resonant capacitor is disconnected, the DC capacitor continues to be charged, and after all the DC capacitors are charged to the rated value, the current-limiting resistor is bypassed and connected to the grid side; the resonant switching capacitor module based on the thyristor bridge arm can also be extended to replace the original switching tube with a thyristor of anti-parallel diode, and improved into a bidirectional resonant switching capacitor module based on the thyristor bridge arm (200), thereby forming a resonant commutator type autotransformer (300) that can realize bidirectional energy transmission, or the IGCT can be used to replace the thyristor to achieve the same function.

[0041] The beneficial effects of this invention are as follows: This DC transformer topology can achieve high power transmission capability under high turns ratio and high voltage. Unlike existing DC transformers that use modular multilevel structures, it does not require complex voltage equalization algorithms to ensure the equalization of module capacitor voltages, but only needs to achieve self-balancing through the resonance principle; it does not require a large number of sub-modules to achieve high-voltage applications, but only requires large-capacity semi-controlled devices or their series valves; it uses semi-controlled devices instead of fully controlled devices, resulting in low cost and low loss; this invention can be flexibly adapted to different application scenarios by changing the number of modules and the number of devices connected in series. Attached Figure Description

[0042] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0043] Figure 1This is a schematic diagram of the topology of a high-voltage, high-capacity DC transformer and its method according to the present invention.

[0044] Figure 2 This is a schematic diagram of the resonant switch module structure based on the thyristor bridge arm in the high-voltage, high-capacity DC transformer and its method described in this invention.

[0045] Figure 3 This is a schematic diagram of the thyristor bridge arm and diode bridge arm structure of a high-voltage, high-capacity DC transformer and its method according to the present invention.

[0046] Figure 4 The present invention relates to a high-voltage, high-capacity DC transformer and its method based on the equivalent structure of a resonant switched capacitor in a thyristor bridge arm.

[0047] Figure 5 This is a schematic diagram illustrating the working principle of the resonant switched capacitor based on the thyristor bridge arm in the high-voltage, high-capacity DC transformer and its method described in this invention.

[0048] Figure 6 The typical current and voltage waveforms for the operation of the resonant switched capacitor based on the thyristor bridge arm in the high-voltage, high-capacity DC transformer and its method described in this invention are shown.

[0049] Figure 7 This is a positive half-cycle current path diagram of a high-voltage, high-capacity DC transformer and its method as described in this invention.

[0050] Figure 8 This is a current path diagram for the second half of the cycle of a high-voltage, high-capacity DC transformer and its method as described in this invention.

[0051] Figure 9 This is a schematic diagram of a derivative bidirectional resonant switched capacitor module based on the topology proposed in this invention.

[0052] Figure 10 This is a derivative of the topology proposed in this invention: a bidirectional resonant commutator type autotransformer with high voltage and large capacity.

[0053] Figure 11 This is a graph of the high-voltage side voltage in a high-voltage, high-capacity DC transformer and its method according to the first embodiment of the present invention.

[0054] Figure 12 This is a graph of the low-voltage side current in a high-voltage, high-capacity DC transformer and its method according to the first embodiment of the present invention.

[0055] Figure 13 The voltage curve of the DC capacitor is shown in the first embodiment of the present invention, which describes a high-voltage, high-capacity DC transformer and its method.

[0056] Figure 14 The resonant current i of a resonant module in the high-voltage, high-capacity DC transformer and its method described in the first embodiment of the present invention. r The curves of the resonant capacitor voltage, the voltage of thyristor bridge arm T1, and the voltage of thyristor bridge arm T2. Detailed Implementation

[0057] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the protection scope of the present invention.

[0058] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0059] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0060] This invention is described in detail with reference to the schematic diagrams. When detailing the embodiments of this invention, for ease of explanation, the cross-sectional views illustrating the device structure may be partially enlarged, not adhering to the usual scale. Furthermore, the schematic diagrams are merely examples and should not be construed as limiting the scope of protection of this invention. In actual fabrication, the three-dimensional spatial dimensions of length, width, and depth should be included.

[0061] Furthermore, in the description of this invention, it should be noted that the terms "upper," "lower," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are used solely for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. In addition, the terms "first," "second," or "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0062] Unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" in this invention should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; similarly, they can refer to mechanical connections, electrical connections, or direct connections, or indirect connections through an intermediate medium, or internal connections between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0063] The technical solution of the present invention will be further described below with reference to the accompanying drawings, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention that do not depart from the spirit and scope of the technical solution of the present invention shall be within the protection scope of the present invention.

[0064] Example 1

[0065] Reference Figures 1-10 This is the first embodiment of the present invention, providing a high-voltage, high-capacity DC transformer and its method. Figure 1 As shown, the DC transformer consists of n resonant switched capacitor modules (101) based on thyristor bridge arms. The input side of the multiple resonant switched modules is connected in parallel to the input capacitor Ci (102) as a medium voltage port. The output side and the medium voltage side of the n resonant switched modules are connected in series to form a high voltage DC port. The high voltage side is interconnected to form an autotransformer with a turns ratio of n+1.

[0066] The resonant switched capacitor module, such as Figure 2 As shown, the resonant switched capacitor module (101) consists of an input side (101a), a resonant branch (101b), and an output side (101c). The input side consists of a set of thyristor bridge arms T1 and T2 and a DC input capacitor C. i The output side consists of a set of diode bridge arms D1 and D2 and a DC output capacitor Co. Specifically, the resonant branch uses a resonant capacitor C. r and resonant inductance L r The structure consists of diode series arms and thyristor bridge arms connected in series to achieve high voltage applications; and an energy transmission circuit is constructed through resonant branches.

[0067] A diode series arm consists of at least two diodes connected in series, each containing a voltage-equalizing branch. Similarly, a thyristor bridge arm consists of at least two thyristors connected in series, each containing a voltage-equalizing branch. See [link to relevant documentation]. Figure 3 The voltage equalization branch includes a dynamic voltage equalization branch and a static voltage equalization branch. The static voltage equalization branch is composed of resistors, and the dynamic voltage equalization branch is composed of voltage equalization resistors and voltage equalization capacitors. The voltage equalization branch is used to achieve voltage equalization of devices and ensure the reliable shut-off of the thyristor series valve.

[0068] Before analyzing the DC transformer, we will first analyze the working principle of the resonant switched capacitor module of the thyristor bridge arm.

[0069] The working principle of the resonant switched capacitor module based on the thyristor bridge arm includes the equivalent circuit structure shown in Figure 4, where a single device containing a voltage equalization branch is used to equivalently replace the series arm of the device, where (R d C d (Rt, Ct) and (Rt, Ct) are the dynamic voltage equalization parameters of the diode and thyristor, respectively. By controlling the conduction of the thyristor bridge arm, a resonant circuit is constructed on the output side and between the DC capacitor using the resonant branch, thereby achieving energy transfer. (See operating condition reference.) Figure 5 Generally, the resistance of a static voltage equalization branch is very large, and its influence can be ignored under steady state. Under this operating condition;

[0070] See current and voltage waveforms Figure 6 Based on modes, it can be divided into four stages: Mode 1 [t0-t1], Mode 2 [t1-t2], Mode 3 [t2-t3], and Mode 4 [t3-t4], which are analyzed in detail below:

[0071] Mode 1 [t0-t1]: The lower thyristor bridge arm on the input side is turned on; see current path [see details]. Figure 5 (a) Resonant capacitance C r voltage u Cr (t0) is less than the input capacitor voltage u Ci The thyristor T2 under the resonant capacitor conducts under forward voltage drop. The resonant branch forms a resonant circuit through diode bridge arm D2, thyristor bridge arm T2, and the input side. The resonant capacitor C... r As the absorbed energy increases, the resonant current returns to zero, and the thyristor bridge arm naturally turns off.

[0072] In mode two [t1-t2], the voltage of the resonant capacitor in the resonant branch absorbs energy and rises, and the voltage u of the resonant capacitor increases. cr (t1) is greater than the input capacitor voltage u co (t1) The lower thyristor bridge arm T2 is in the reverse recovery phase. However, since both the diode bridge arm and the thyristor bridge arm have voltage equalization branches, the resonant branch forms a reverse resonant circuit through the voltage equalization branch and the output side. See the current path below. Figure 5 (b) The circuit parameters satisfy the overdamping condition. After the reverse resonant current passes through the voltage equalization branch of thyristor bridge arm T2, it will apply an approximately constant reverse voltage to thyristor bridge arm T2. When the duration of the applied reverse voltage is greater than the turn-off time of the thyristor bridge arm, reliable turn-off of thyristor bridge arm T2 can be achieved. Under high voltage conditions, the number of devices connected in series is sufficient, and it is determined that the current remains constant at 0 during this stage. The turn-off time of the thyristor is very short, so the duration of mode two is very short, and the resonant capacitor voltage remains unchanged.

[0073] Mode 3 [t2-t3], resonant capacitance C rvoltage u Cr (t2) is greater than the input capacitor voltage u o The upper thyristor bridge arm T1 of the resonant capacitor conducts under forward voltage drop; see the current path below. Figure 5 (c) The resonant branch forms a resonant circuit via diode bridge arm D1, thyristor bridge arm T1, and the input side, with resonant capacitor C. r When the released energy voltage drops, the resonant current returns to 0, and the thyristor bridge arm naturally turns off.

[0074] In mode four [t3-t4], the voltage drops as the resonant capacitor in the resonant branch releases energy, and the voltage u of the resonant capacitor... Cr Less than the output capacitor voltage u o The upper thyristor bridge arm T1 is currently in the reverse recovery phase. However, since both the diode bridge arm and the thyristor bridge arm have voltage equalization branches, the resonant branch forms a reverse resonant circuit via the voltage equalization branch and the output side. See the current path diagram. Figure 5 (d) The circuit parameters satisfy the overdamping condition. After the reverse resonant current passes through the voltage equalization branch of the thyristor bridge arm T1, it will apply an approximately constant reverse voltage to the thyristor bridge arm T1. When the time for applying the reverse voltage is greater than the turn-off time of the thyristor bridge arm, the thyristor can be reliably turned off. Under high voltage conditions, the number of devices connected in series is sufficient, and the current can be considered to remain unchanged at this stage. Since the turn-off time of the thyristor bridge arm is very short, the duration of mode two is very short, and the voltage of the resonant capacitor can be considered to remain unchanged.

[0075] The working principle of a DC transformer includes: (See operating conditions for details.) Figure 6 .

[0076] In the first half of the cycle, the lower thyristor bridge arm on the input side of all switched capacitor modules is turned on, and the current path refers to... Figure 7 The resonant branch forms a resonant circuit between the thyristor bridge arm and the DC capacitor, and the resonant capacitor absorbs energy.

[0077] In the latter half of the cycle, the upper thyristor bridge arms on the input side of all switched capacitor modules are turned on, and the current path refers to... Figure 8 The resonant branch forms a resonant circuit between the thyristor bridge arm and the output DC capacitor, and the resonant capacitor releases energy to the DC output capacitor.

[0078] Figure 9 As a derivative topology of the present invention, a thyristor bridge arm with an anti-parallel diode bridge arm is used instead of a thyristor bridge arm, resulting in a bidirectional resonant switch module based on a thyristor bridge arm, which can realize bidirectional energy flow.

[0079] Figure 10This is a schematic diagram of a derivative topology of the present invention, consisting of a bidirectional resonant switch module based on a thyristor bridge arm, which enables bidirectional energy flow in a high-voltage, high-capacity DC transformer and its method.

[0080] Example 2

[0081] Reference Figures 11-14 This is the second embodiment of the present invention, which provides different solutions to illustrate the method of the present invention, so as to verify the real effect of the method.

[0082] based on Figure 1 The DC transformer structure shown is built using MATLAB / Simulink software. Figure 1 The DC transformer structure shown is used for simulation verification of this topology. The simulation parameters are shown in Table 1 below.

[0083] Table 1: Simulation Parameters of DC Transformer

[0084]

[0085]

[0086] Under the operating conditions shown in the table above, with a given input voltage of 50kV and an output voltage of 300kV, a DC transformer with a turns ratio of 6 is constructed from 5 resonant switched capacitor modules; the rated transmission power is 400MW. To ensure reliable thyristor turn-off, the switching frequency is set to 200Hz and the resonant frequency to 300Hz. The given power gradually increases from 0 to 400MW rated power between 0 and 2s. The simulation results are as follows. Figures 11-14 As shown.

[0087] Figure 11 The curve of the high-voltage side output voltage in embodiment two of the present invention shows that, under a fixed turns ratio, the voltage amplitude of the high-voltage side voltage Vdc2 fluctuates around 300kV.

[0088] Figure 12 The curve of the low-voltage side input current in embodiment two of the present invention shows that the input current gradually increases from zero to the rated value of 8000A and fluctuates around the rated value.

[0089] Figure 13 The voltage curves of the DC capacitors in Embodiment 2 of the present invention show that the voltages of all DC capacitors fluctuate around 50kV, achieving self-balancing capability.

[0090] Figure 14 The resonant current i of one of the resonant modules in embodiment two of the present invention rThe curves depict the resonant capacitor voltage, the voltage of thyristor bridge arm T1, and the voltage of thyristor T2. Specifically: When thyristor T2 is turned on, the resonant current starts resonating from zero, and the resonant capacitor voltage starts resonating and rising from its minimum value. At this point, the conduction voltage of thyristor T2 is zero. When the resonant current resonates to zero again, the thyristor bridge arm is turned off. At this time, thyristor T2 experiences a reverse voltage drop, enabling reliable thyristor turn-off.

[0091] The above examples demonstrate that the DC transformer implemented in this example, which uses a thyristor-based resonant switched capacitor module, can reliably turn off the thyristors. By utilizing the circuit structure, a stable reverse voltage is applied to the thyristors to ensure reliable turn-off. No voltage equalization control strategy is required, and self-balancing of all DC capacitor voltages is achieved.

[0092] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A high-voltage, high-capacity DC transformer, characterized in that: include, The DC transformer is composed of multiple resonant switched capacitor modules based on thyristor bridge arms; The thyristor bridge arm is a series valve formed by multiple thyristors connected in series. The thyristor bridge arm is reliably turned off through resonant commutation. The resonant switched capacitor module is used to construct an energy balance circuit; Each module is controlled to transfer energy independently, achieving self-balancing of the DC capacitor voltage in each sub-module; The DC transformer (100) comprises a resonant switched capacitor module (101) based on a thyristor bridge arm, an input DC capacitor (102), and an input inductor (103). The input side (101a) of the resonant switched capacitor module is connected in parallel to both sides of the input DC capacitor (102) and interconnected with the medium and low voltage DC port. The output DC capacitor of the output side (101c) of the resonant switched capacitor module is connected in series and then connected in series with the input DC capacitor (102) to form the high voltage side, thus forming a DC transformer with a turns ratio of n+1. The resonant switched capacitor module (101) based on thyristor bridge arms includes an input side (101a) consisting of a set of upper and lower thyristor series arms, the thyristor bridge arms being series valves, and including an auxiliary voltage equalization branch. The auxiliary voltage equalization circuit is used to achieve voltage equalization of the devices and to achieve reliable turn-off of the thyristor bridge arms. The resonant switched capacitor module (101) based on the thyristor bridge arm includes a resonant branch (101b) of the resonant switched capacitor module based on the thyristor bridge arm as an energy transmission path, which is composed of a resonant inductor Lr and a resonant capacitor Cr. The resonant switched capacitor module (101) based on the thyristor bridge arm further includes an output side (101c) of the resonant switched capacitor module based on the thyristor bridge arm consisting of a set of upper and lower diode series arms and an output DC capacitor. The diode series arms are multiple diode series valves, and the series valves include an auxiliary voltage equalization branch. The auxiliary voltage equalization circuit is used to realize voltage equalization of the devices and to realize reliable shut-off of the series valves.

2. The high-voltage, high-capacity DC transformer as described in claim 1, characterized in that: The input DC capacitor (102) of the DC transformer includes the input DC capacitor on the input side of the DC transformer and the output DC capacitor of the resonant switching capacitor module, which together support the output voltage. The input inductance Li (103) of the DC transformer is used to smooth the input current.

3. A control method for a high-voltage, high-capacity DC transformer as described in claim 2, characterized in that: The working principle of the resonant switched capacitor module based on the thyristor bridge arm includes: In mode 1, the second lower thyristor bridge arm on the input side is turned on. The voltage of the resonant capacitor is less than the voltage of the input capacitor. The second lower thyristor bridge arm of the resonant capacitor is turned on due to the forward voltage drop. The resonant branch forms a resonant circuit with the input side through the second diode bridge arm and the second thyristor bridge arm. The resonant capacitor absorbs energy and the voltage rises. The resonant current returns to 0, and the thyristor bridge arm turns off naturally. In mode two, due to the energy absorbed by the resonant capacitor voltage in the resonant branch, the voltage rises. At this time, the voltage of the resonant capacitor is greater than the input capacitor voltage. The second lower thyristor bridge arm is in the reverse recovery phase. Since both the diode bridge arm and the thyristor bridge arm have voltage equalization branches, the resonant branch forms a reverse resonant circuit with the output side through the voltage equalization branch. The circuit parameters satisfy the overdamping condition. After the reverse resonant current passes through the voltage equalization branch of the second lower thyristor bridge arm, it will apply an approximately constant reverse voltage to the second lower thyristor bridge arm. When the time for applying the reverse voltage is greater than the turn-off time of the second lower thyristor bridge arm... Then, the reliable turn-off of the second lower thyristor bridge arm is achieved; the turn-off time of the thyristor bridge arm is very short, so the duration of mode two is very short, and it is determined that the resonant capacitor voltage remains unchanged. In mode 3, the voltage of the resonant capacitor is greater than the voltage of the input capacitor. The first upper thyristor bridge arm of the resonant capacitor is forward-biased and conducts. The resonant branch forms a resonant circuit with the input side through the first diode bridge arm and the first lower thyristor bridge arm. The resonant capacitor releases energy and the voltage drops. The resonant current returns to 0, and the thyristor bridge arm turns off naturally. In mode four, the resonant capacitor in the resonant branch releases energy and the voltage drops. The voltage of the resonant capacitor is less than the voltage of the output capacitor. The first lower thyristor bridge arm is in the reverse recovery stage. Both the diode bridge arm and the thyristor bridge arm have voltage equalization branches. The resonant branch forms a reverse resonant circuit with the output side through the voltage equalization branch. The circuit parameters meet the overdamping condition. After the reverse resonant current passes through the voltage equalization branch of the first lower thyristor bridge arm, it will apply an approximately constant reverse voltage to the first lower thyristor bridge arm. When the time for applying the reverse voltage is greater than the turn-off time of the first lower thyristor bridge arm, the thyristor is reliably turned off. The voltage across the resonant capacitor is equal at the beginning and end of a resonant cycle, achieving self-balancing of the capacitor voltage.

4. The method as described in claim 3, characterized in that: The working principle of the DC transformer includes: During the positive half-cycle, the lower thyristor bridge arm on the input side of all resonant switched capacitor modules is turned on. The resonant capacitors of all modules form a resonant circuit with the DC capacitor through the lower thyristor bridge arm and the lower diode bridge arm on the output side. The resonant capacitor absorbs energy from the input DC capacitor. During the negative half-cycle, the upper thyristors on the input side of all resonant switched capacitor modules are turned on. The resonant capacitors of all modules form a resonant circuit with the DC capacitor through the upper thyristor bridge arm and the upper diode bridge arm on the output side. The resonant capacitors supply energy to the output DC capacitor.

5. The method as described in claim 4, characterized in that: The parameter design of the DC transformer includes, The input current is set to I in, the output current is Iout = I in / (1+n), the switching period is Ts, and the resonant period is Tr; When operating in the first half of the cycle, for the input DC capacitor Ci, its current component consists of two parts: DC current I. in -I out The sum of the resonant currents of the n resonant switched capacitor modules is expressed as follows: During the positive half-period, [t] ai _t bi Between moments, the input DC capacitor C i When the current is less than zero, the input DC capacitor releases energy, causing the capacitor voltage to drop. Therefore, the maximum fluctuation value of the DC capacitor voltage can be calculated. in, To ensure that the voltage fluctuation ratio is less than εi, the capacitance value of the input capacitor must meet the following requirements: When operating in the second half of the cycle, the current expression for the nth output DC capacitor is: For the output DC capacitor, during the negative half-cycle, the nth DC capacitor is in [[t an _t bn When the current between [a certain point] is greater than zero, the output DC capacitor absorbs energy, and the capacitor voltage rises. The maximum fluctuation value of the nth DC capacitor voltage can then be determined. ： in, ; To meet the requirement that voltage fluctuation is less than ε percent n Then the capacitance value Cn of the nth output capacitor needs to satisfy the following condition. To meet the modularity requirement, the output capacitor takes the maximum value of . ； For the input inductor, the maximum voltage drop variation is the same as that of the input DC capacitor. This is to ensure that the input current fluctuation ratio is less than ε. L Then the input inductance needs to meet the following requirements: For a resonant branch, the resonant frequency is determined by the resonant capacitance Cr and the resonant inductance Lr, and must satisfy the following conditions: ； During the reverse resonance processes of modes two and four, the maximum voltage drop of the thyristor is determined by the fluctuation value of the resonant capacitor voltage, which in turn is determined by the characteristic impedance. The magnitudes of the resonant capacitor Cr and the resonant inductor Lr can be determined by considering the fluctuation value of the resonant capacitor voltage.

6. The method as described in claim 5, characterized in that: The pre-charging method of the DC transformer includes: the high-voltage side of the transformer is connected to the high-voltage grid for charging through a current-limiting resistor, and the grid side is connected through a bypass switch; the lower thyristor bridge arm of all modules is turned on, the trigger pulse of the thyristor bridge arm is maintained, so that the resonant capacitor is connected in parallel to both sides of the DC capacitor for charging, charging the DC capacitor and the resonant capacitor; after the resonant capacitor is charged to the rated voltage, the sending of the thyristor bridge arm trigger pulse is stopped, and the thyristor is turned off naturally. After disconnecting the parallel resonant capacitor, continue charging the DC capacitors. Once all DC capacitors are charged to their rated values, bypass the current-limiting resistor and connect to the grid side.

7. The method as described in claim 6, characterized in that: The resonant switched capacitor module based on the thyristor bridge arm replaces the original switching transistor with a thyristor with an anti-parallel diode, and is improved into a bidirectional resonant switched capacitor module based on the thyristor bridge arm (200), which in turn forms a resonant commutator type autotransformer (300) that can realize bidirectional energy transmission.

8. The method as described in claim 7, characterized in that: The same function can also be achieved by replacing the thyristor with an IGCT.

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