Modularized high-voltage direct-current transformer and control method thereof

Abstract

The invention discloses a modular high-voltage direct-current transformer and a control method thereof. The transformer comprises a first energy storage bridge arm, a second energy storage bridge arm and a control switch, the control switch comprises a first group of thyristor valves and a second group of thyristor valves; the first group of thyristor valves comprises an upper thyristor valve and a lower thyristor valve which are respectively used for controlling the charging of the first energy storage bridge arm and the second energy storage bridge arm; the second group of thyristor valves are used for controlling the discharge of the first energy storage bridge arm and the second energy storage bridge arm; the first energy storage bridge arm, the upper thyristor valve and the first diode form a loop, and the second energy storage bridge arm, the lower thyristor valve and the second diode form another loop. According to the invention, the waveform of the bridge arm current is randomly controllable by controlling the bridge arm voltage, the control of the transmission power and the energy balance of the bridge arm are realized, and meanwhile, the active property of the bridge arm can be utilized to realize the reliable turn-off of the soft switch and the thyristor.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, and in particular to a modular high-voltage DC transformer and its control method. Background Art

[0002] For the access of new energy such as wind energy and solar energy, DC transformers provide an efficient and economical large-scale aggregation method, which helps to build an integrated DC power transmission and distribution network. DC transformers are key equipment for realizing the efficient and stable operation of power systems, and play an irreplaceable role in promoting the modernization of power systems and the wide access of new energy. Therefore, it is necessary to study new types of DC transformers to adapt to the development of new energy.

[0003] The Hybrid Modular Multilevel Direct Current Transformer (HMMDCT) combines the characteristics of the Modular Multilevel Converter (MMC) and device series connection. It can withstand high voltages, and semi-controlled power devices avoid complex series connection technologies and can also achieve soft switching. HMMDCT selects the active bridge arm based on sub-modules as the energy transfer medium and thyristors as control switches. The control of the bridge arm energy and the reliable turn-off of thyristors are major factors affecting the normal operation of the transformer. In the case of a high transformation ratio, the charging current of a single energy storage bridge arm topology is large, resulting in high device stress and high manufacturing costs.

[0004] Most existing isolated high-voltage DC transformers use large-capacity AC transformers as intermediate links, resulting in problems such as high device costs and manufacturing difficulties; non-isolated low-voltage DC transformer topologies mostly use IGBTs as switches. However, in a high-voltage background, IGBT series valves have difficulties in dynamic voltage sharing and are difficult to implement. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the defects of the above-mentioned existing technologies, and provide a modular high-voltage DC transformer based on thyristors to reduce the cost of DC transformers, and use the source nature of the bridge arm to achieve soft switching and reliable turn-off of thyristors; also provide a control method for the modular high-voltage DC transformer, which can arbitrarily control the waveform of the bridge arm current by controlling the bridge arm voltage, and achieve the control of the transmitted power and the energy balance of the bridge arm.

[0006] To this end, the present invention adopts the following technical solutions.

[0007] In a first aspect, the present invention provides a modular high-voltage DC transformer, which includes a first energy storage bridge arm, a second energy storage bridge arm and a control switch;

[0008] The control switch includes a first group of thyristor valves and a second group of thyristor valves; the first group of thyristor valves includes an upper thyristor valve T1 and a lower thyristor valve T2, which are respectively used to control the charging of the first energy storage bridge arm and the second energy storage bridge arm; the second group of thyristor valves T3 is used to control the discharging of the first energy storage bridge arm and the second energy storage bridge arm.

[0009] The first energy storage bridge arm, the upper thyristor valve T1 and the first diode D1 form a loop, and the second energy storage bridge arm, the lower thyristor valve T2 and the second diode D2 form another loop; one end of the second group of thyristor valves T3 is connected to the loop between the lower thyristor valve T2 and the second energy storage bridge arm, and the other end of the second group of thyristor valves T3 is connected to the loop between the upper thyristor valve T1 and the first energy storage bridge arm.

[0010] Further, both the first energy storage bridge arm and the second energy storage bridge arm include a plurality of series-connected half-bridge sub-modules and a bridge arm inductor.

[0011] Further, after the first energy storage bridge arm, the upper thyristor valve T1 and the first diode D1 are connected in series, they are connected in parallel with the input DC capacitor C1; after the second energy storage bridge arm, the lower thyristor valve T2 and the second diode D2 are connected in series, they are connected in parallel with the output DC capacitor C2.

[0012] Further, the two groups of thyristor valves are turned on alternately according to the switching period T s Considering the reverse recovery process after the thyristor valve is turned off, when the operating frequency T w is less than the switching period T s , there are a total of four modes:

[0013] 1) Mode 1 [0 to T w / 2]. At t = 0, the upper thyristor valve T1 and the lower thyristor valve T2 are triggered to conduct, and the first energy storage bridge arm and the second energy storage bridge arm are respectively connected to the primary side to form a loop, and the energy storage bridge arm absorbs energy from the primary side; at t = 0, the voltage V arm1 of the first energy storage bridge arm is controlled to make the current i arm1 of the first energy storage bridge arm change from zero to the maximum value I arm1cha_max in the form of a trapezoidal wave; similarly, the voltage V arm2 of the second energy storage bridge arm is controlled to make the current i arm2 of the second energy storage bridge arm change from zero to the maximum value I arm2cha_max in the form of a trapezoidal wave; at t = T w / 2, the currents of both energy storage bridge arms return to zero, and this mode ends.

[0014] 2) Mode 2 [T w / 2 to T s / 2]. During this mode, the current remains zero, and the voltages V arm1 and Varm2 gradually changes to half of the secondary-side voltage V dc2 such that the sum of the voltages of the two energy storage bridge arms is equal to the secondary-side voltage V dc2 ;

[0015] 3) Mode three [T s / 2 to T w / 2 + T s / 2], when t = T s / 2, the second group of thyristor valves T3 is triggered and turned on, and the first energy storage bridge arm and the second energy storage bridge arm together form a loop with the secondary side, and the energy storage bridge arm releases energy to the secondary side; when t = T s / 2, simultaneously control the voltage V arm1 of the first energy storage bridge arm and the voltage V arm2 of the second energy storage bridge arm, so that the current i arm1 of the first energy storage bridge arm and the current i arm2 of the second energy storage bridge arm also both change from zero in reverse in the form of a trapezoidal wave to the maximum value I armrel_max ; at t = T w / 2 + T s / 2, the currents of the two energy storage bridge arms both return to zero, and this mode ends;

[0016] 4) Mode four [T w / 2 + T s / 2 to T s , during this mode, the current remains zero, and control the voltages of the two energy storage bridge arms to gradually change to the primary-side voltage V dc1 so that the voltages of the two energy storage bridge arms are equal to the primary-side voltage.

[0017] In a second aspect, the present invention provides a control method for the above-mentioned modular high-voltage DC transformer, including: the energy exchanged by the energy storage bridge arms during commutation is related to the maximum value of the current of the energy storage bridge arms. By controlling the maximum current of the inductor of the energy storage bridge arms, the energy exchanged by the energy storage bridge arms is controlled, and further the voltage of the output DC capacitor on the secondary side is controlled to achieve the control of the output voltage.

[0018] Furthermore, the control method of the modular high-voltage DC transformer includes four parts: sub-module string energy balance control, output voltage control, bridge arm inductor current control, and sub-module capacitor voltage balance control;

[0019] The sub-module string energy balance control includes: using a first proportional-integral controller to control the method of the maximum value of the absorption current of the two bridge arms to achieve sub-module string energy balance control;

[0020] The output voltage control includes: using a second proportional-integral controller to control the method of the absorption power of the output DC capacitor to achieve the stability of the output DC capacitor voltage;

[0021] The above-mentioned arm inductor current control includes: multiplying the arm absorption current and the discharge current by a waveform control signal function respectively, adding them to obtain the reference value of the arm current, and using a third proportional-integral controller for control;

[0022] The above-mentioned sub-module capacitor voltage balance control includes: adding a correction signal to adjust the capacitor voltage signal of each sub-module, and using a proportional controller for control;

[0023] Adding the sub-module string reference voltage signal and the balanced capacitor voltage difference of the sub-module to obtain the modulation signal of each sub-module, and obtaining the control signal of each sub-module through carrier phase-shifted modulation.

[0024] Furthermore, in the above-mentioned sub-module string energy balance control, the mathematical equation of the first proportional-integral controller is:

[0025]

[0026] In the formula, V c_ref is the rated capacitor voltage value of the sub-module, is the average value of the capacitor voltages of n a sub-modules in the first energy storage arm, is the average value of the capacitor voltages of n a sub-modules in the second energy storage arm; i arm1cha_ref is the reference value of the absorption current of the first energy storage arm, i arm2cha_ref is the reference value of the absorption current of the second energy storage arm; k p1cha and k p2cha are the proportional link gain coefficients of the first and second energy storage arms of the first proportional-integral controller respectively, k i1cha and k i2cha are the integral link gain coefficients of the first and second energy storage arms of the first proportional-integral controller respectively.

[0027] Furthermore, in the above-mentioned output voltage control, the mathematical equation of the second proportional-integral controller is:

[0028] i armrel_ref =k prel (V C2_ref -v C2 )+∫k irel (V C2_ref -v C2 )dt,

[0029] In the formula, V C2_ref is the reference value of the voltage of the output DC capacitor C2, V C2 is the voltage of the output DC capacitor C2, i armrel_refis the reference value of the discharge current of the first energy storage bridge arm or the second energy storage bridge arm, k prel is the proportional link gain coefficient of the second proportional-integral controller, k irel is the integral link gain coefficient of the second proportional-integral controller.

[0030] Furthermore, in the control of the arm inductor current, the mathematical equation of the third proportional-integral controller is:

[0031]

[0032] In the formula, v arm1_ref is the reference value of the capacitor voltage of the first energy storage bridge arm, v arm2_ref is the reference value of the capacitor voltage of the second energy storage bridge arm; i arm1_ref is the current reference value of the first energy storage bridge arm, i arm2_ref is the current reference value of the second energy storage bridge arm, f arm1cha and f arm2cha are the control signal function of the absorption current waveform of the first and second energy storage bridge arms respectively; f armrel is the control signal function of the discharge current waveform of the first energy storage bridge arm or the second energy storage bridge arm; k p1sum and k p2sum are the proportional link gain coefficients of the first and second energy storage bridge arms of the third proportional-integral controller respectively, k i1sum and k i2sum are the integral link gain coefficients of the first and second energy storage bridge arms of the third proportional-integral controller respectively.

[0033] Furthermore, in the control of the sub-module capacitor voltage balance, the mathematical equation of the proportional controller is:

[0034]

[0035] In the formula, v arm1_i is the capacitor voltage of the i-th sub-module in the first energy storage bridge arm, v arm2_i is the capacitor voltage of the i-th sub-module in the second energy storage bridge arm; Δv arm1_i is the balanced capacitor voltage difference of the i-th sub-module in the first energy storage bridge arm, Δv arm2_i is the balanced capacitor voltage difference of the i-th sub-module in the second energy storage bridge arm; k p1ave and k p2ave are the proportional link gain coefficients of the first and second energy storage bridge arms respectively; sgn(i arm1 ) represents the current sign function of the first energy storage bridge arm, sgn(i arm2 ) represents the current sign function of the second energy storage bridge arm.

[0036] The beneficial effects of the present invention are as follows: The DC transformer of the present invention is a low-cost modular high-voltage DC transformer based on thyristors. By utilizing the source property of the bridge arm, soft switching and reliable turn-off of the thyristors can be achieved. The control method of the present invention selects the bridge arm current as the control variable. After the waveform of the current is designed, the energy absorbed and released by the bridge arm is only related to the amplitude of the current. Therefore, by controlling the bridge arm voltage, the waveform of the bridge arm current can be arbitrarily controlled, and the control of the transmitted power and the energy balance of the bridge arm can be realized. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for the description of the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts. Among them:

[0038] Figure 1 FIG. is a topological structure diagram of a modular high-voltage DC transformer according to Embodiment 1 of the present invention;

[0039] Figure 2 FIG. is a schematic diagram of the first working mode of a modular high-voltage DC transformer according to Embodiment 1 of the present invention;

[0040] Figure 3 FIG. is a schematic diagram of the third working mode of a modular high-voltage DC transformer according to Embodiment 1 of the present invention;

[0041] Figure 4 FIG. is a waveform diagram of the current and voltage of the first energy storage bridge arm of a modular high-voltage DC transformer according to Embodiment 1 of the present invention;

[0042] Figure 5 FIG. is a waveform diagram of the current and voltage of the second energy storage bridge arm of a modular high-voltage DC transformer according to Embodiment 1 of the present invention;

[0043] Figure 6 FIG. is a schematic diagram of the control method of a modular high-voltage DC transformer according to Embodiment 2 of the present invention;

[0044] Figure 7 FIG. is a simulation curve diagram of the DC capacitor voltage on the secondary side in the simulation verification of the present invention;

[0045] Figure 8 FIG. is an overall waveform diagram of the currents of the two energy storage bridge arm branches in the simulation verification of the present invention;

[0046] Figure 9 FIG. is a detailed waveform diagram of the currents of the two energy storage bridge arm branches in the simulation verification of the present invention;

[0047] Figure 10This is the capacitance voltage fluctuation diagram of the sub-modules of two energy storage bridge arm branches in the simulation verification of the present invention. Detailed implementation manners

[0048] To make the above objects, features, and advantages of the present invention more obvious and understandable, the following will describe the detailed implementation manners of the present invention in conjunction with the accompanying drawings of the specification. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0049] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Those skilled in the art can make similar extensions without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0050] Embodiment 1

[0051] This embodiment provides a modular high-voltage DC transformer, which is composed of a first energy storage bridge arm, a second energy storage bridge arm, and a control switch, as Figure 1 shown.

[0052] The control switch includes a first group of thyristor valves and a second group of thyristor valves; the first group of thyristor valves includes an upper thyristor valve T1 and a lower thyristor valve T2, which are respectively used to control the charging of the first energy storage bridge arm and the second energy storage bridge arm; the second group of thyristor valves T3 is used to control the discharging of the first energy storage bridge arm and the second energy storage bridge arm.

[0053] The first energy storage bridge arm, the upper thyristor valve T1, and the first diode D1 form a loop, and the second energy storage bridge arm, the lower thyristor valve T2, and the second diode D2 form another loop; one end of the second group of thyristor valves T3 is connected to the loop between the lower thyristor valve T2 and the second energy storage bridge arm, and the other end of the second group of thyristor valves T3 is connected to the loop between the upper thyristor valve T1 and the first energy storage bridge arm. The first diode D1 and the second diode D2 are used to limit the direction of the current and prevent short circuits.

[0054] Both the first energy storage bridge arm and the second energy storage bridge arm include a plurality of series-connected half-bridge sub-modules and a bridge arm inductor connected in series with the half-bridge sub-modules.

[0055] The first energy storage bridge arm, the upper thyristor valve T1, and the first diode D1 are connected in series and then connected in parallel to the input DC capacitor C1; the second energy storage bridge arm, the lower thyristor valve T2, and the second diode D2 are connected in series and then connected in parallel to the output DC capacitor C2.

[0056] The operating principle of the above DC transformer includes: two groups of thyristor valves conduct alternately according to a switching period of Ts. When considering the reverse recovery process after the thyristor valves are turned off, the operating frequency T w is less than the switching period T s , then there are a total of four modes:

[0057] 1) Mode 1 [0 to T w / 2]. At t = 0, the upper thyristor valve T1 and the lower thyristor valve T2 are triggered to conduct. The first energy storage bridge arm and the second energy storage bridge arm are respectively connected to the primary side to form a loop. The energy storage bridge arms absorb energy from the primary side, which is called the absorption current. At t = 0, control the voltage V arm1 of the first energy storage bridge arm to make the current i arm1 of the first energy storage bridge arm change from zero to the maximum value I arm1cha_max in the form of a trapezoidal wave; similarly, control the voltage V arm2 of the second energy storage bridge arm to make the current i arm2 of the second energy storage bridge arm change from zero to the maximum value I arm2cha_max in the form of a trapezoidal wave; at t = T w / 2, the currents of both energy storage bridge arms return to zero, and this mode ends.

[0058] 2) Mode 2 [T w / 2 to T s / 2]. During this mode, the current remains zero. Control the voltages V arm1 and V arm2 of the two energy storage bridge arms to gradually change to half of the secondary side voltage V dc2 , so that the sum of the voltages of the two energy storage bridge arms is equal to the secondary side voltage V dc2 , avoiding excessive stress on the switching devices in the next cycle.

[0059] 3) Mode 3 [T s / 2 to T w / 2 + T s / 2]. At t = T s / 2, the second group of thyristor valve T3 is triggered to conduct. The first energy storage bridge arm and the second energy storage bridge arm together form a loop with the secondary side. The energy storage bridge arms release energy to the secondary side, which is called the discharge current. At t = T s / 2, simultaneously control the voltage V arm1 of the first energy storage bridge arm and the voltage V arm2 of the second energy storage bridge arm to make the current i arm1 of the first energy storage bridge arm and the current i arm2 of the second energy storage bridge arm also start from zero and change in the reverse direction in the form of a trapezoidal wave to the maximum value I armrel_max ; at t = T w / 2 + T sWhen it reaches T / 2, the currents of both energy storage bridge arms return to zero, and this mode ends.

[0060] 4) Mode Four [T w / 2 + T s From T / 2 to T s , during this mode, the current remains zero, and the voltages of the two energy storage bridge arms are gradually changed to the primary side voltage V dc1 such that the voltages of the two energy storage bridge arms are equal to the primary side voltage, avoiding excessive stress on the switching devices in the next cycle.

[0061] Embodiment 2

[0062] This embodiment provides a control method for the modular high-voltage DC transformer described in Embodiment 1. The energy exchanged by the bridge arms during commutation is related to the maximum value of the bridge arm current. Therefore, by controlling the maximum current of the bridge arm inductance, the energy exchanged by the bridge arms can be controlled, and further the voltage of the DC capacitor V c2 can be controlled to achieve the control of the output voltage.

[0063] A control method for a modular high-voltage DC transformer of the present invention includes four parts: sub-module string energy balance control, output voltage control, bridge arm inductance current control, and sub-module capacitor voltage balance control. The control block diagram of this control method is as Figure 6 shown.

[0064] Specifically, the sub-module string energy balance control includes: the energy absorbed by the sub-modules in the bridge arm is related to the magnitude of the absorbed current. Therefore, the method of using a first proportional-integral controller to control the maximum absorbed current of the two bridge arms can be adopted to achieve the sub-module string energy balance control.

[0065] More specifically, the mathematical equation of the first proportional-integral controller is:

[0066]

[0067] In the formula, V c_ref is the rated capacitor voltage value of the sub-module, is the average value of the capacitor voltages of n a sub-modules in the first energy storage bridge arm, is the average value of the capacitor voltages of n a sub-modules in the second energy storage bridge arm; i arm1cha_ref is the reference value of the absorbed current of the first energy storage bridge arm, i arm2cha_ref is the reference value of the absorbed current of the second energy storage bridge arm; k p1cha and k p2cha are the first and second energy storage bridge arm proportional link gain coefficients of the first proportional-integral controller respectively, k i1cha and k i2chaThey are the gain coefficients of the first and second energy storage leg integral links of the first proportional-integral controller respectively.

[0068] Specifically, the output voltage control includes: when the voltage of the output DC capacitor C2 is stable, the average power absorbed by the output DC capacitor in one cycle is zero. Therefore, the method of using the second proportional-integral controller to control the power absorbed by the output DC capacitor can be adopted to achieve the stability of the output capacitor voltage.

[0069] More specifically, the mathematical equation of the second proportional-integral controller is:

[0070] i armrel_ref =k prel (V C2_ref -v C2 )+∫k irel (V C2_ref -v C2 )dt,

[0071] In the formula, V C2_ref is the reference value of the voltage of the output DC capacitor C2, V C2 is the voltage of the output DC capacitor C2, i armrel_ref is the reference value of the discharge current of the first energy storage leg or the second energy storage leg, k prel is the proportional link gain coefficient of the second proportional-integral controller, k irel is the integral link gain coefficient of the second proportional-integral controller.

[0072] Specifically, the leg inductor current control includes: multiplying the leg absorption current and the discharge current by a waveform control signal function respectively, adding them to obtain the reference value of the leg current, and using the third proportional-integral controller for control.

[0073] More specifically, the mathematical equation of the third proportional-integral controller is:

[0074]

[0075] In the formula, v arm1_ref is the reference value of the capacitor voltage of the first energy storage leg, v arm2_ref is the reference value of the capacitor voltage of the second energy storage leg; i arm1_ref is the current reference value of the first energy storage leg, i arm2_ref is the current reference value of the second energy storage leg, f arm1cha and f arm2cha are the absorption current waveform control signal functions of the first and second energy storage legs respectively; f armrel is the discharge current waveform control signal function of the first energy storage leg or the second energy storage leg; k p1sum and k p2sumare the proportional link gain coefficients of the first and second energy storage bridge arms of the third proportional integral controller, respectively, and k i1sum and k i2sum are the integral link gain coefficients of the first and second energy storage bridge arms of the third proportional integral controller, respectively.

[0076] Specifically, the capacitor voltage balance control of the sub-module includes: adding a correction signal to adjust the capacitor voltage signal of each sub-module and using a proportional controller for control;

[0077] Adding the sub-module string reference voltage signal and the balanced capacitor voltage difference of the sub-module to obtain the modulation signal of each sub-module, and the modulation signal is modulated by carrier phase shift to obtain the control signals of each sub-module.

[0078] More specifically, the mathematical equation of the proportional controller is:

[0079]

[0080] In the formula, v arm1_i is the capacitor voltage of the i-th sub-module in the first energy storage bridge arm, and v arm2_i is the capacitor voltage of the i-th sub-module in the second energy storage bridge arm; Δv arm1_i is the balanced capacitor voltage difference of the i-th sub-module in the first energy storage bridge arm, and Δv arm2_i is the balanced capacitor voltage difference of the i-th sub-module in the second energy storage bridge arm; k p1ave and k p2ave are the proportional link gain coefficients of the first and second energy storage bridge arms respectively; sgn(i arm1 ) represents the current sign function of the first energy storage bridge arm, and sgn(i arm2 ) represents the current sign function of the second energy storage bridge arm.

[0081] In order to verify the beneficial effects of the present invention, scientific demonstration is carried out through comparative experiments.

[0082] Based on Figure 1 the DC transformer structure shown, a DC transformer structure is built using MATLAB / Simulink software, and simulation verification is carried out for this topology. The simulation parameters are shown in Table 1 below.

[0083] Table 1: DC transformer simulation parameters

[0084] Parameter Value Parameter Value Input voltage 20 kV Output voltage 50 kV Switching frequency 200 Hz Operating frequency 300 Hz Transmission power 10 MW Transformation ratio 2：5

[0085] Under the working conditions shown in the above table, the given input voltage is 20 kV, the output voltage is 50 kV, and the rated transmission power is 10 MW. To ensure reliable turn-off of the thyristor, the switching frequency is set to 150 Hz, and the simulation results are as Figure 7 - 10 shown. Figure 8 - 10In this case, the upper energy storage bridge arm P is the first energy storage bridge arm, and the lower energy storage bridge arm N is the second energy storage bridge arm.

[0086] Figure 7 This is the waveform diagram of the transmission power and the secondary side voltage in the simulation verification of the present invention. As the transmission power increases, the fluctuation of the secondary side voltage also increases accordingly. During the whole process, the voltage always remains stable at about 50 kV.

[0087] Figure 8 This is the overall waveform diagram of the currents of the two energy storage bridge arm branches in the simulation verification of the present invention. It can be seen that as the transmission power increases, the amplitude of the current also increases accordingly. Figure 9 This is the detailed waveform diagram of the currents of the two energy storage bridge arm branches. It can be seen that the current waveforms of the two energy storage bridge arm branches change in the form of trapezoidal waves, which is consistent with the theoretical analysis.

[0088] Figure 10 This is the voltage fluctuation diagram of the sub-modules of the two energy storage bridge arm branches in the simulation verification of the present invention. It can be seen that as the transmission power increases, the voltage fluctuation of the sub-module capacitors also increases accordingly. During the whole process, the voltage always remains stable at about 2.5 kV.

[0089] Through the above simulation verification, the effectiveness of the control method of the present invention is proved.

[0090] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered by the scope of the claims of the present invention.

[0091] Those skilled in the art should understand that the embodiments of the present invention can be provided as a method, a system, or a computer program product. Therefore, the present invention can be implemented in the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present invention can be implemented in the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk memories, CD-ROMs, optical memories, etc.) containing computer-usable program codes. The solutions in the embodiments of the present invention can be implemented in various computer languages, for example, object-oriented programming languages such as Java and interpreted scripting languages such as JavaScript.

[0092] The present invention is described with reference to flowchart illustrations and / or block diagram illustrations of methods, apparatus (systems), and computer program products according to embodiments of the invention. It should be understood that each flow and / or block in the flowchart illustrations and / or block diagram illustrations, and combinations of flows and / or blocks in the flowchart illustrations and / or block diagram illustrations, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions executed by the processor of the computer or other programmable data processing apparatus create means for implementing the functions specified in the flowchart flow or flows and / or block or blocks. Figure 1 one flow or flows and / or one block or blocks Figure 1 for implementing the functions specified in one flow or flows and / or one block or blocks.

[0093] These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means for implementing the functions specified in one flow or flows and / or one block or blocks. Figure 1 one flow or flows and / or one block or blocks Figure 1 for implementing the functions specified in one flow or flows and / or one block or blocks.

[0094] These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process, such that the instructions executed on the computer or other programmable apparatus provide steps for implementing the functions specified in one flow or flows and / or one block or blocks. Figure 1 one flow or flows and / or one block or blocks Figure 1 for implementing the functions specified in one flow or flows and / or one block or blocks.

[0095] Although the preferred embodiments of the present invention have been described, additional changes and modifications can be made by those skilled in the art once they learn of the basic inventive concept. Therefore, the appended claims are intended to be construed to include the preferred embodiments and all changes and modifications falling within the scope of the present invention.

[0096] It is apparent that those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention is also intended to include these modifications and variations.

Claims

1. A modular high-voltage DC transformer, characterized in that: It includes a first energy storage bridge arm, a second energy storage bridge arm and a control switch; The control switch includes a first group of thyristor valves and a second group of thyristor valves; the first group of thyristor valves includes an upper thyristor valve T1 and a lower thyristor valve T2, which are respectively used to control the charging of the first energy storage bridge arm and the second energy storage bridge arm; the second group of thyristor valves T3 is used to control the discharge of the first energy storage bridge arm and the second energy storage bridge arm; The first energy storage bridge arm, the upper thyristor valve T1 and the first diode D1 form a circuit, and the second energy storage bridge arm, the lower thyristor valve T2 and the second diode D2 form another circuit; one end of the second group of thyristor valves T3 is connected to the circuit between the lower thyristor valve T2 and the second energy storage bridge arm, and the other end of the second group of thyristor valves T3 is connected to the circuit between the upper thyristor valve T1 and the first energy storage bridge arm.

2. The modular high-voltage DC transformer according to claim 1, characterized in that: The first energy storage bridge arm and the second energy storage bridge arm each include a plurality of half-bridge sub-modules connected in series and a bridge arm inductor.

3. The modular high-voltage DC transformer according to claim 1, characterized in that: The first energy storage bridge arm, the upper thyristor valve T1 and the first diode D1 are connected in series and then connected in parallel to the input DC capacitor C1; the second energy storage bridge arm, the lower thyristor valve T2 and the second diode D2 are connected in series and then connected in parallel to the output DC capacitor C2.

4. The modular high-voltage DC transformer according to any one of claims 1 to 3, characterized in that: Two groups of thyristor valves are switched according to the switching cycle T s When the reverse recovery process of the thyristor valve is taken into account, the operating frequency T w Less than the switching period T s , there are four modes in total: 1) Mode 1 [0 to T w / 2], when t=0, the upper thyristor valve T1 and the lower thyristor valve T2 are triggered to conduct, and the first energy storage bridge arm and the second energy storage bridge arm respectively form a loop with the primary side, and the energy storage bridge arm absorbs energy from the primary side; when t=0, the voltage V arm1 Make the current i of the first energy storage bridge arm arm1 It changes from zero to the maximum value I in the form of a trapezoidal wave arm1cha_max ; Similarly control the voltage V of the second energy storage bridge arm arm2 The current i of the second energy storage bridge arm arm2 It changes from zero to the maximum value I in the form of a trapezoidal wave arm2cha_max ; at t = T w / 2, the currents of the two energy storage bridge arms return to zero, and this mode ends; 2) Mode 2 [T w / 2 to T s / 2], during this mode, the current remains zero, controlling the voltage V of the two energy storage bridge arms arm1 and V arm2 Gradually changes to the secondary side voltage V dc2 half of the secondary voltage V dc2 ; 3) Mode Three [T s / 2 to T w / 2+T s / 2], t = T s / 2, the second group of thyristor valves T3 is triggered to conduct, the first energy storage bridge arm and the second energy storage bridge arm together with the secondary side form a loop, and the energy storage bridge arm releases energy to the secondary side; t = T s / 2, the voltage V of the first energy storage bridge arm is controlled at the same time arm1 and the voltage V of the second energy storage bridge arm arm2 , so that the current i of the first energy storage bridge arm arm1 and the current i of the second energy storage bridge arm arm2 Similarly, they all start from zero and change in the reverse direction in the form of a trapezoidal wave to the maximum value; at t = T w / 2+T s / 2, the currents of the two energy storage bridge arms return to zero, and this mode ends; 4) Mode Four [T w / 2+T s / 2 to T s ], during this mode, the current remains zero, and the voltage of the two energy storage bridge arms is controlled to gradually change to the primary side voltage V dc1 , making the voltage of the two energy storage bridge arms equal to the primary measured voltage.

5. The control method of the modular high-voltage DC transformer according to any one of claims 1 to 4, characterized in that: include: The energy interacting between the energy storage bridge arms during commutation is related to the maximum value of the energy storage bridge arm current. The energy interacting between the energy storage bridge arms is controlled by controlling the maximum current of the energy storage bridge arm inductance, thereby controlling the voltage of the output DC capacitor on the secondary side to achieve output voltage control.

6. The control method of the modular high-voltage DC transformer according to claim 5, characterized in that: It includes four parts: submodule string energy balance control, output voltage control, bridge arm inductor current control and submodule capacitor voltage balance control; The submodule string energy balance control includes: using a first proportional integral controller to control the maximum absorption current of two bridge arms to achieve submodule string energy balance control; The output voltage control includes: using a second proportional-integral controller to control the output DC capacitor to absorb power to achieve the stability of the output DC capacitor voltage; The bridge arm inductor current control includes: the bridge arm absorption current and discharge current are respectively multiplied by a waveform control signal function, and a reference value of the bridge arm current is obtained after adding them, and the third proportional integral controller is used for control; The submodule capacitor voltage balance control includes: adding a correction signal to adjust the capacitor voltage signal of each submodule, and controlling it using a proportional controller; The submodule string reference voltage signal and the submodule equalizing capacitor voltage difference are added to obtain the modulation signal of each submodule, and the modulation signal is modulated by carrier phase shift to obtain the control signal of each submodule.

7. The control method of the modular high-voltage DC transformer according to claim 6, characterized in that: In the submodule string energy balance control, the mathematical equation of the first proportional integral controller is: Where V c_ref is the rated capacitance voltage value of the submodule, is the first energy storage bridge arm n a The average value of the capacitor voltage of each submodule, is the second energy storage bridge arm n a The average value of the capacitor voltage of each submodule; arm1cha_ref is the reference value of the first energy storage bridge arm absorbing current, i arm2cha_ref k is the reference value of the second energy storage bridge arm absorbing current; p1cha and k p2cha are the proportional link gain coefficients of the first and second energy storage bridge arms of the first proportional-integral controller, k i1cha and k i2cha They are respectively the integral link gain coefficients of the first and second energy storage bridge arms of the first proportional-integral controller.

8. The control method of the modular high-voltage DC transformer according to claim 6, characterized in that: In the output voltage control, the mathematical equation of the second proportional-integral controller is: and armrel_ref =k prel (V C2_ref -v C2 )+∫k irel (V C2_ref -v C2 )dt, Where V C2_ref is the reference value of the output DC capacitor C2 voltage, V C2 is the output DC capacitor C2 voltage, i armrel_ref is the reference value of the discharge current of the first energy storage bridge arm or the second energy storage bridge arm, k prel is the proportional link gain coefficient of the second proportional-integral controller, k irel is the integral link gain coefficient of the second proportional-integral controller.

9. The control method of the modular high-voltage DC transformer according to claim 6, characterized in that: In the bridge arm inductor current control, the mathematical equation of the third proportional integral controller is: In the formula, v arm1_ref is the reference value of the capacitor voltage of the first energy storage bridge arm, v arm2_ref is the reference value of the capacitor voltage of the second energy storage bridge arm; i arm1_ref is the current reference value of the first energy storage bridge arm, i arm2_ref is the current reference value of the second energy storage bridge arm, f arm1cha and f arm2cha are the first and second energy storage bridge arm absorption current waveform control signal functions respectively; f arm1rel and f arm2rel are the discharge current waveform control signal functions of the first and second energy storage bridge arms respectively; k p1sum and k p2sum are the proportional link gain coefficients of the first and second energy storage bridge arms of the third proportional-integral controller, k i1sum and k i2sum are the integral link gain coefficients of the first and second energy storage bridge arms of the third proportional integral controller respectively; i arm1 Represents the current of the first energy storage bridge arm; i arm2 Represents the current of the second energy storage bridge arm; i arm1cha_ref is the reference value of the first energy storage bridge arm absorbing current, i arm2cha_ref is the reference value of the current absorbed by the second energy storage bridge arm; i armrel_ref is a reference value of the discharge current of the first energy storage bridge arm or the second energy storage bridge arm.

10. The control method of the modular high-voltage DC transformer according to claim 6, characterized in that: In the submodule capacitor voltage balance control, the mathematical equation of the proportional controller is: In the formula, v arm1_i is the capacitor voltage of the ith submodule in the first energy storage bridge arm, v arm2_i is the capacitor voltage of the i-th submodule in the second energy storage bridge arm; Δv arm1_i is the voltage difference of the balancing capacitor of the i-th submodule in the first energy storage bridge arm, Δv arm2_i is the voltage difference of the balancing capacitor of the i-th submodule in the second energy storage bridge arm; k p1ave and k p2ave are the proportional gain coefficients of the first and second energy storage bridge arms respectively; sgn(i arm1 ) represents the current sign function of the first energy storage bridge arm, sgn(i arm2 ) represents the current sign function of the second energy storage bridge arm; n a is the number of submodules in the first or second energy storage bridge arm; i arm1 Represents the current of the first energy storage bridge arm; i arm2 Represents the current of the second energy storage bridge arm; is the first energy storage bridge arm n a The average value of the capacitor voltage of each submodule, is the second energy storage bridge arm n a The average value of the capacitor voltage of each submodule.

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