Back-to-back inverter and its start-up and normal operation control method

Abstract

The application provides a back-to-back AC / AC frequency converter and a starting and normal operation control method thereof, and the method comprises the following steps: performing capacitor charging of a sub-module of a power frequency modular multilevel converter and capacitor charging of a sub-module of a low-frequency auxiliary modular multilevel converter; controlling the DC voltage of the power frequency modular multilevel converter to be a DC voltage rated value; starting the low-frequency auxiliary modular multilevel converter, and making the low-frequency auxiliary modular multilevel converter operate in a constant AC voltage and frequency mode, controlling the frequency of the low-frequency auxiliary modular multilevel converter to be a low-frequency side rated frequency, and increasing the AC voltage of the low-frequency auxiliary modular multilevel converter from an initial value to a low-frequency AC voltage rated value; completing the starting of the back-to-back AC / AC frequency converter, and switching to a normal operation control mode. The application can provide synchronous grid-connected AC voltage for offshore wind turbines, does not need to configure large ground-occupying reactive power compensation and filtering equipment, and is relatively easy to realize black start of a wind farm.

Description

Technical Field

[0001] This invention relates to the fields of new energy power generation and flexible power transmission technology, and in particular to back-to-back AC-AC frequency converters and their start-up and normal operation control methods. Background Technology

[0002] my country possesses abundant offshore wind power resources, located near load centers in the southeast, allowing for local consumption and thus presenting broad development prospects. To access larger sea areas and more stable wind energy, the development of offshore wind power has become a key focus for countries worldwide. my country will also vigorously develop and construct large-scale offshore wind farms in its far seas. With offshore wind farms becoming increasingly farther from shore and larger in capacity, how to economically and reliably transmit offshore wind power to shore has become a critical issue. Due to cable capacitor charging current limitations, power frequency AC transmission technology is only suitable for short-distance offshore wind power transmission. High-voltage direct current (HVDC) transmission is suitable for long-distance transmission, but requires offshore converter platforms to house gigawatt-level converters, large-capacity transformers, and their auxiliary equipment, resulting in massive size and weight, significantly increasing construction difficulty and cost. Furthermore, offshore converter platforms also suffer from high operation and maintenance costs. Low-frequency AC transmission technology, by reducing the transmission frequency, can overcome the short transmission distance limitation of power frequency AC transmission technology. Compared to DC transmission technology, low-frequency AC transmission eliminates the need for offshore converter platforms, requiring only onshore AC-AC converters. This avoids the high cost and construction difficulties of offshore converter platforms and saves on maintenance costs during operation, improving the overall economic efficiency of offshore wind power transmission systems. In medium- to long-distance, large-capacity offshore wind power transmission applications, low-frequency AC transmission technology is considered a beneficial supplement to DC transmission, showing promising engineering application prospects. High-voltage, high-power AC-AC converters are used to connect and convert power between offshore low-frequency AC transmission and onshore power frequency AC grids. They are the core equipment of low-frequency AC transmission systems, and their cost and footprint are among the most critical factors determining the economic viability and feasibility of such systems. The development and practical application of offshore low-frequency AC transmission still urgently require more compact and low-cost new AC-AC converter solutions.

[0003] Based on the presence or absence of a DC bus, existing AC converter technologies can be categorized into AC-AC converters without a DC link and AC-DC-AC back-to-back converters. Currently, the mainstream AC-AC converter topology without a DC link is the Modular Multilevel Matrix Converter (M3C), which offers advantages such as high energy conversion efficiency, fewer bridge arms, and less capacitor usage. However, its main disadvantages are: firstly, M3C requires full-bridge submodules, resulting in a large number of components; secondly, due to the numerous internal circulating current channels and high coupling among its nine bridge arms, its control strategy is highly complex; thirdly, due to the absence of an intermediate DC link, M3C cannot utilize low-cost, technologically mature onshore DC power dissipation devices to achieve low-voltage ride-through of the receiving-end grid; and fourthly, M3C lacks engineering practice in high-voltage, high-capacity power transmission, and its application feasibility and technical adaptability in offshore wind power transmission technology require further verification.

[0004] Compared to M3C, back-to-back AC-DC-AC frequency converters (Btb-MMC) based on dual-ended MMCs have advantages such as mature technology and rich engineering practice. However, their disadvantages, such as a large number of sub-modules and large capacitor usage, negatively impact system lightweighting and severely affect system economy. In particular, the sub-module capacitors of the low-frequency side MMC will increase significantly due to the decrease in operating frequency. For example, when the low frequency is 50 / 3Hz, the sub-module capacitors of the low-frequency MMC should be increased to three times the original size, which will significantly increase the cost and footprint of the AC-AC frequency converter station.

[0005] Given the aforementioned shortcomings of the Btb-MMC structure, some studies have proposed using a back-to-back diode rectifier (Btb-DR) on the low-frequency side. This topology can significantly reduce device costs and energy storage requirements, enabling lightweight onshore AC-AC converters. However, due to the uncontrolled rectification characteristics of DRs, they cannot provide synchronous grid-connected AC voltage for offshore wind turbines, requiring the wind turbines to operate in a grid-based control mode where engineering technology is not yet mature. Furthermore, DRs consume significant reactive power and generate harmonics during normal operation, necessitating the installation of large-scale reactive power compensation and filtering equipment. Additionally, the Btb-DR topology makes black starting of wind farms difficult, requiring additional black-start devices, further increasing investment and land requirements. Summary of the Invention

[0006] The present invention aims to at least partially solve one of the technical problems in the related art.

[0007] Therefore, the first objective of this invention is to propose a back-to-back AC-AC converter that can provide synchronous grid-connected AC voltage for offshore wind turbines without requiring large-scale reactive power compensation and filtering equipment, and can more easily achieve black start of wind farms.

[0008] To achieve the above objectives, one embodiment of the present invention provides a back-to-back AC-AC inverter, including a low-frequency side AC switch, a low-frequency diode rectifier, a low-frequency auxiliary modular multilevel converter, a low-frequency auxiliary transformer, a power frequency side AC switch, a power frequency modular multilevel converter, a starting auxiliary diode rectifier, and a starting auxiliary transformer;

[0009] The three-phase low-frequency AC input terminal of the back-to-back AC inverter is connected to the AC input terminal of the low-frequency diode rectifier and the primary side of the low-frequency auxiliary transformer via a low-frequency side AC switch.

[0010] The secondary side of the low-frequency auxiliary transformer is connected to the AC input terminal of the low-frequency auxiliary modular multilevel converter. The positive DC terminal of the low-frequency diode rectifier is connected to the positive DC terminal of the power frequency modular multilevel converter. The negative DC terminal of the low-frequency diode rectifier is connected to the positive DC terminal of the low-frequency auxiliary modular multilevel converter and the positive DC terminal of the starting auxiliary diode rectifier.

[0011] The DC negative terminal of the low-frequency auxiliary modular multilevel converter is connected to the DC negative terminal of the power frequency modular multilevel converter and the DC negative terminal of the starting auxiliary diode rectifier. The AC output terminal of the power frequency modular multilevel converter is connected to the primary side of the starting auxiliary transformer, and then connected to the three-phase power frequency AC output terminal of the back-to-back AC-AC converter via the power frequency side AC switch. The secondary side of the starting auxiliary transformer is connected to the AC input terminal of the starting auxiliary diode rectifier.

[0012] The back-to-back AC-AC inverter of this invention can provide synchronous grid-connected AC voltage for offshore wind turbines without the need for large-scale reactive power compensation and filtering equipment, making it easier to achieve black start of wind farms.

[0013] In addition, the back-to-back AC-AC inverter according to the above embodiments of the present invention may also have the following additional technical features:

[0014] Furthermore, in one embodiment of the present invention, the low-frequency diode rectifier includes a rectifier transformer and a diode.

[0015] Furthermore, in one embodiment of the present invention, each phase of the low-frequency auxiliary modular multilevel converter includes a first upper bridge arm and a lower bridge arm, and each bridge arm of each phase of the low-frequency auxiliary modular multilevel converter includes multiple half-bridge sub-modules and multiple full-bridge sub-modules.

[0016] Furthermore, in one embodiment of the present invention, each phase of the power frequency modular multilevel converter includes a second upper bridge arm and a lower bridge arm, and each bridge arm of each phase of the power frequency modular multilevel converter includes multiple half-bridge sub-modules.

[0017] Furthermore, in one embodiment of the present invention, the lower end of the upper arm and the upper end of the lower arm of each phase of the low-frequency auxiliary modular multilevel converter and the power frequency modular multilevel converter are connected by an inductor, with the midpoint of the inductor serving as the AC input terminal of the current phase; the upper ends of the upper arms of all phases are connected as the DC positive terminal, and the lower ends of the lower arms of all phases are connected as the DC negative terminal.

[0018] The second objective of this invention is to provide a method for controlling the start-up and normal operation of a back-to-back AC-AC inverter.

[0019] To achieve the above objectives, one embodiment of the present invention proposes a method for controlling the start-up and normal operation of a back-to-back AC-AC frequency converter, comprising:

[0020] Close the AC switch on the power frequency side to charge the sub-module capacitors of the power frequency modular multilevel converter.

[0021] By activating the auxiliary diode rectifier to output DC voltage, the submodule capacitors of the low-frequency auxiliary modular multilevel converter are charged.

[0022] Start the power frequency modular multilevel converter and put it into constant DC voltage control mode to control the DC voltage of the power frequency modular multilevel converter to the rated DC voltage value;

[0023] Close the low-frequency side AC switch, start the low-frequency auxiliary modular multilevel converter, and make the low-frequency auxiliary modular multilevel converter operate in a constant AC voltage and frequency mode. Control the AC side frequency of the low-frequency auxiliary modular multilevel converter to the rated frequency of the low-frequency side, and increase the AC side voltage of the low-frequency auxiliary modular multilevel converter from the initial value to the rated value of the low-frequency AC voltage.

[0024] After completing the startup of the back-to-back AC inverters, the system switches to normal operation control mode.

[0025] Furthermore, the normal operation control mode includes control of the low-frequency auxiliary modular multilevel converter, including:

[0026] The low-frequency AC voltage synchronization angle is obtained based on the preset low-frequency AC frequency target value;

[0027] The effective value of the fundamental wave of the low-frequency AC voltage is obtained based on the instantaneous value of the three-phase low-frequency AC voltage, and the low-frequency AC voltage harmonic components are extracted from it.

[0028] The AC voltage fundamental deviation signal is calculated based on the target value of the low-frequency AC voltage fundamental wave and the effective value of the low-frequency AC voltage fundamental wave.

[0029] The reference value of the fundamental amplitude of the AC output voltage is obtained based on the AC voltage fundamental deviation signal;

[0030] The reference value of the instantaneous value of the fundamental amplitude of the three-phase AC output voltage is calculated based on the reference value of the fundamental amplitude of the AC output voltage and the synchronization angle of the low-frequency AC voltage.

[0031] The instantaneous values ​​of the three-phase AC output voltage harmonics are obtained based on the low-frequency AC voltage harmonic components.

[0032] Based on the reference values ​​of the fundamental instantaneous value and the reference values ​​of the harmonic instantaneous value of the three-phase AC output voltage, the total reference value of the three-phase AC output voltage instantaneous value is obtained.

[0033] The target DC voltage value of the low-frequency assisted modular multilevel converter is obtained based on the submodule capacitor voltage deviation signal of the low-frequency assisted modular multilevel converter.

[0034] Based on the reference value of the instantaneous value of the total three-phase AC output voltage and the target value of the DC voltage of the low-frequency auxiliary modular multilevel converter, the reference voltages of multiple arms of the low-frequency auxiliary modular multilevel converter are calculated.

[0035] The reference voltages of multiple bridge arms of the low-frequency auxiliary modular multilevel converter are pulse-width modulated to output the control pulse signal of the low-frequency auxiliary modular multilevel converter.

[0036] The back-to-back AC-AC inverter startup and normal operation control method of this invention can provide synchronous grid-connected AC voltage for offshore wind turbines, without the need for large-scale reactive power compensation and filtering equipment, and can more easily achieve black start of wind farms.

[0037] A third object of the present invention is to provide a computer device, including a processor and a memory;

[0038] The processor reads executable program code stored in the memory to run a program corresponding to the executable program code, so as to implement the start-up and normal operation control method of the back-to-back AC-AC inverter.

[0039] The fourth objective of this invention is to provide a non-transitory computer-readable storage medium storing a computer program that, when executed by a processor, implements a method for controlling the startup and normal operation of a back-to-back AC-AC inverter.

[0040] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0041] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the following description of the embodiments taken in conjunction with the accompanying drawings, wherein:

[0042] Figure 1 This is a schematic diagram of a back-to-back AC-AC inverter according to an embodiment of the present invention;

[0043] Figure 2 This is a schematic diagram of a low-frequency diode rectifier structure in a back-to-back AC-AC inverter according to an embodiment of the present invention;

[0044] Figure 3 This is a schematic diagram of a low-frequency auxiliary modular multilevel converter in a back-to-back AC-AC inverter according to an embodiment of the present invention;

[0045] Figure 4 This is a structural diagram of a half-bridge submodule in a modular multilevel converter in a back-to-back AC-AC inverter according to an embodiment of the present invention.

[0046] Figure 5 This is a structural diagram of a full-bridge submodule in a modular multilevel converter in a back-to-back AC-AC inverter according to an embodiment of the present invention;

[0047] Figure 6 This is a schematic diagram of the power frequency modular multilevel converter structure in a back-to-back AC-AC inverter according to an embodiment of the present invention;

[0048] Figure 7 This is a flowchart of a back-to-back AC-AC inverter startup and normal operation control method according to an embodiment of the present invention;

[0049] Figure 8 A flowchart of a normal operation control method for a low-frequency auxiliary modular multilevel converter in a back-to-back AC-AC inverter according to an embodiment of the present invention;

[0050] Figure 9 A computer device according to an embodiment of the present invention. Detailed Implementation

[0051] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0052] The following description, with reference to the accompanying drawings, describes a back-to-back AC inverter, a start-up and normal operation control method, an apparatus, and a storage medium according to embodiments of the present invention.

[0053] Figure 1 This is a schematic diagram of the structure of a back-to-back AC-AC inverter according to an embodiment of the present invention.

[0054] like Figure 1 As shown, the back-to-back AC-AC inverter includes: a low-frequency side AC switch 100, a low-frequency diode rectifier 200, a low-frequency auxiliary modular multilevel converter 300, a low-frequency auxiliary transformer 400, a power frequency side AC switch 500, a power frequency modular multilevel converter 600, a starting auxiliary diode rectifier 700, and a starting auxiliary transformer 800. The three-phase low-frequency AC input terminal of the back-to-back AC-AC inverter is connected via the low-frequency side AC switch 100 to the AC input terminal of the low-frequency diode rectifier 200 and the primary side of the low-frequency auxiliary transformer 400; the secondary side of the low-frequency auxiliary transformer 400 is connected to the AC input terminal of the low-frequency auxiliary modular multilevel converter 300; the positive DC terminal of the low-frequency diode rectifier 200 is connected to the positive DC terminal of the power frequency modular multilevel converter 600; the negative DC terminal of the low-frequency diode rectifier 200 is connected to the positive DC terminal of the low-frequency auxiliary modular multilevel converter 300 and the starting auxiliary diode 800. The positive DC terminal of the diode rectifier 700 is connected to the DC terminal of the low-frequency auxiliary modular multilevel converter 300, which is connected to the negative DC terminal of the power frequency modular multilevel converter 600 and the negative DC terminal of the starting auxiliary diode rectifier 700. The AC output terminal of the power frequency modular multilevel converter 600 is connected to the primary side of the starting auxiliary transformer 800, and then connected to the three-phase power frequency AC output terminal of the back-to-back AC-AC converter via the power frequency side AC switch 500. The secondary side of the starting auxiliary transformer 800 is connected to the AC input terminal of the starting auxiliary diode rectifier 700.

[0055] In one embodiment of the present invention, the low-frequency diode rectifier 200 is as follows: Figure 2 As shown, it includes rectifier transformer 1, rectifier transformer 2 and diodes D1 to D12.

[0056] In one embodiment of the present invention, the low-frequency auxiliary modular multilevel converter 300 is as follows: Figure 3 As shown, each phase includes an upper bridge arm and a lower bridge arm. Each bridge arm is composed of K half-bridge sub-modules and M full-bridge sub-modules cascaded together. The lower end of the upper bridge arm and the upper end of the lower bridge arm of each phase are connected together through an inductor L. The midpoint of the inductor becomes the AC input terminal of that phase. The upper ends of the upper bridge arms of all phases are connected together to form the DC positive terminal, and the lower ends of the lower bridge arms of all phases are connected together to form the DC negative terminal.

[0057] In one embodiment of the present invention, the half-bridge submodule of the low-frequency auxiliary modular multilevel converter 300 is as follows: Figure 4As shown, it includes switches S1-S2, diodes D1-D2, and DC capacitor C1. The full-bridge submodule of the low-frequency auxiliary modular multilevel converter 300 is as follows: Figure 5 As shown, it includes switches S1 to S4, diodes D1 to D4, and DC capacitor C1.

[0058] In one embodiment of the present invention, the power frequency modular multilevel converter 600 is as follows: Figure 6 As shown, each phase of the power frequency modular multilevel converter 600 includes an upper arm and a lower arm. Each arm is composed of N cascaded half-bridge submodules. The lower end of the upper arm and the upper end of the lower arm of each phase are connected together through an inductor L. The midpoint of the inductor becomes the AC input terminal of that phase. The upper ends of the upper arms of all phases are connected together to form the DC positive terminal, and the lower ends of the lower arms of all phases are connected together to form the DC negative terminal. The half-bridge submodules of the power frequency modular multilevel converter 600 are as follows: Figure 4 As shown.

[0059] The back-to-back AC-AC inverter of this invention can provide synchronous grid-connected AC voltage for offshore wind turbines, eliminating the need for bulky reactive power compensation and filtering equipment, and making black start of wind farms easier to achieve. Furthermore, this system offers advantages such as simple control, low cost, good dynamic control performance, low harmonics, high efficiency, and ease of control.

[0060] Next, with reference to the accompanying drawings, the starting and normal operation control method of the back-to-back AC inverter proposed according to an embodiment of the present invention is described.

[0061] Figure 7 This is a flowchart of a back-to-back AC-AC inverter startup and normal operation control method according to an embodiment of the present invention.

[0062] like Figure 7 As shown, the method includes the following steps:

[0063] S1, close the AC switch on the power frequency side to charge the sub-module capacitors of the power frequency modular multilevel converter;

[0064] S2, by starting the auxiliary diode rectifier to output DC voltage, charges the sub-module capacitors of the low-frequency auxiliary modular multilevel converter;

[0065] S3, start the power frequency modular multilevel converter, so that the power frequency modular multilevel converter operates in constant DC voltage control mode, and control the DC voltage of the power frequency modular multilevel converter to the rated DC voltage value;

[0066] S4, close the low-frequency side AC switch, start the low-frequency auxiliary modular multilevel converter, make the low-frequency auxiliary modular multilevel converter operate in constant AC voltage and frequency mode, control the AC side frequency of the low-frequency auxiliary modular multilevel converter to the rated frequency of the low-frequency side, and increase the AC side voltage of the low-frequency auxiliary modular multilevel converter from the initial value to the rated value of the low-frequency AC voltage.

[0067] S5 completes the startup of the back-to-back AC inverter and switches to normal operation control mode.

[0068] Furthermore, Figure 8 This diagram illustrates the normal operation control method for the low-frequency auxiliary modular multilevel converter in a back-to-back AC-AC inverter. Figure 8 As shown:

[0069] (1) Set the target value of the low-frequency AC frequency to ω. ref2 The target value ω of the low-frequency AC frequency ref2 The signal is fed into an integrator to obtain the low-frequency AC voltage synchronization angle θ.

[0070] (2) Acquire the instantaneous value u of the three-phase low-frequency AC voltage sa2 u sb2 u sc2 The effective value of the fundamental frequency AC voltage U was calculated. s2(1) ;

[0071] (3) Set the target value of the fundamental frequency of the low-frequency AC voltage to U. s2_ref(1) According to the effective value of the fundamental wave of low-frequency AC voltage U s2(1) Calculate the fundamental frequency deviation signal Δu of AC voltage. s(1) =U s2_ref(1) -U s2(1) ;

[0072] (4) The above AC voltage fundamental frequency deviation signal Δu s(1) After passing through the proportional-integral controller, the reference value E of the fundamental amplitude of the AC output voltage is obtained. ac2_ref(1) ;

[0073] (5) Based on the above reference value E of the fundamental amplitude of the AC output voltage ac2_ref(1) And the AC voltage synchronization angle θ, to calculate the reference value u of the fundamental instantaneous value of the three-phase AC output voltage. a2_ref(1) u b2_ref(1) u b2_ref(1) :

[0074]

[0075] (6) The instantaneous value of the three-phase low-frequency AC voltage u sa2 u sb2 u sc2Extract the low-frequency AC voltage harmonic component u. abc2(h) ;

[0076] (7) Reduce the low-frequency AC voltage harmonic components u abc2(h) After subtracting each phase from 0, the result is passed through an active filter controller to obtain the reference value u for the instantaneous harmonic values ​​of the three-phase AC output voltage. a2_ref(h) u b2_ref(h) u b2_ref(h) ;

[0077] (8) The reference value u of the fundamental instantaneous value of the three-phase AC output voltage. a2_ref(1) u b2_ref(1) u b2_ref(1) Reference value u of instantaneous harmonic values ​​of three-phase AC output voltage a2_ref(h) u b2_ref(h) u b2_ref(h) The summation operation yields the reference value u of the total instantaneous three-phase AC output voltage. a2_ref u b2_ref u c2_ref ;

[0078] (9) Set the target value of the submodule capacitor voltage of the low-frequency auxiliary modular multilevel converter to U. cap2_ref The average value u of the capacitor voltage of each submodule in the low-frequency auxiliary modular multilevel converter is calculated by collecting the capacitor voltage of all submodules. cap2 Calculate the capacitor voltage deviation signal Δu of the submodule. cap =u cap2 -U cap2_ref ;

[0079] (10) The capacitor voltage deviation signal Δu of the above sub-module cap After passing through the proportional controller, the target DC voltage value E of the low-frequency auxiliary modular multilevel converter is obtained. dc2_ref ;

[0080] (11) Based on the above reference value u of the total instantaneous three-phase AC output voltage. a2_ref u b2_ref u c2_ref and DC voltage reference value E dc2_ref The reference voltage u of the six arms of the low-frequency assisted modular multilevel converter was calculated. ap2_ref u an2_ref u bp2_ref u bn2_ref u cp2_ref u cn2_ref :

[0081]

[0082]

[0083]

[0084]

[0085]

[0086]

[0087] (12) The reference voltage u of the six arms of the above low-frequency auxiliary modular multilevel converter ap2_ref u an2_ref u bp2_ref u bn2_ref u cp2_ref u cn2_ref The signal is fed into the pulse width modulation stage to obtain the control pulse signal for the low-frequency auxiliary modular multilevel converter.

[0088] Furthermore, the power frequency modular multilevel converter is operated in a constant DC voltage control mode, and the DC voltage of the power frequency modular multilevel converter is controlled to the rated DC voltage U. dcN .

[0089] The start-up and normal operation control method for back-to-back AC-AC frequency converters proposed in this invention can provide synchronous grid-connected AC voltage for offshore wind turbines without requiring large-scale reactive power compensation and filtering equipment, making black start of wind farms easier to achieve. Furthermore, this method gives the system advantages such as simple control, low cost, good dynamic control performance, low harmonics, high efficiency, and ease of control.

[0090] To implement the methods of the above embodiments, the present invention also provides a computer device, such as... Figure 9 As shown, the computer device 600 includes a memory 601 and a processor 602; wherein, the processor 602 reads the executable program code stored in the memory 601 to run a program corresponding to the executable program code, so as to implement the various steps of the startup and normal operation control method of the back-to-back AC-AC inverter described above.

[0091] To implement the above embodiments, the present invention also proposes a non-transitory computer-readable storage medium storing a computer program thereon, which, when executed by a processor, implements the start-up and normal operation control method for a back-to-back AC-AC inverter as described in the foregoing embodiments.

[0092] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0093] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0094] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A back-to-back AC-AC frequency converter, characterized in that, include: Low-frequency side AC switch, low-frequency diode rectifier, low-frequency auxiliary modular multilevel converter, low-frequency auxiliary transformer; power frequency side AC switch, power frequency modular multilevel converter, starting auxiliary diode rectifier, starting auxiliary transformer; The three-phase low-frequency AC input terminal of the back-to-back AC inverter is connected to the AC input terminal of the low-frequency diode rectifier and the primary side of the low-frequency auxiliary transformer via a low-frequency side AC switch. The secondary side of the low-frequency auxiliary transformer is connected to the AC input terminal of the low-frequency auxiliary modular multilevel converter. The positive DC terminal of the low-frequency diode rectifier is connected to the positive DC terminal of the power frequency modular multilevel converter. The negative DC terminal of the low-frequency diode rectifier is connected to the positive DC terminal of the low-frequency auxiliary modular multilevel converter and the positive DC terminal of the starting auxiliary diode rectifier. The DC negative terminal of the low-frequency auxiliary modular multilevel converter is connected to the DC negative terminal of the power frequency modular multilevel converter and the DC negative terminal of the starting auxiliary diode rectifier. The AC output terminal of the power frequency modular multilevel converter is connected to the primary side of the starting auxiliary transformer, and then connected to the three-phase power frequency AC output terminal of the back-to-back AC-AC converter via the power frequency side AC switch. The secondary side of the starting auxiliary transformer is connected to the AC input terminal of the starting auxiliary diode rectifier.

2. The back-to-back AC-AC inverter according to claim 1, characterized in that, The low-frequency diode rectifier includes a rectifier transformer and a diode.

3. The back-to-back AC-AC inverter according to claim 1, characterized in that, Each phase of the low-frequency auxiliary modular multilevel converter includes a first upper bridge arm and a lower bridge arm, and each bridge arm of each phase of the low-frequency auxiliary modular multilevel converter includes multiple half-bridge sub-modules and multiple full-bridge sub-modules.

4. The back-to-back AC-AC inverter according to claim 3, characterized in that, Each phase of the power frequency modular multilevel converter includes a second upper bridge arm and a lower bridge arm, and each bridge arm of each phase of the power frequency modular multilevel converter includes multiple half-bridge sub-modules.

5. The back-to-back AC-AC inverter according to claim 4, characterized in that, The lower end of the upper arm and the upper end of the lower arm of each phase of the low-frequency auxiliary modular multilevel converter and the power frequency modular multilevel converter are connected by an inductor, with the midpoint of the inductor serving as the AC input terminal of the current phase; the upper ends of the upper arms of all phases are connected as the DC positive terminal, and the lower ends of the lower arms of all phases are connected as the DC negative terminal.

6. A method for controlling the start-up and normal operation of a back-to-back AC-AC frequency converter as described in any one of claims 1-5, characterized in that, include: Close the AC switch on the power frequency side to charge the sub-module capacitors of the power frequency modular multilevel converter. By activating the auxiliary diode rectifier to output DC voltage, the submodule capacitors of the low-frequency auxiliary modular multilevel converter are charged. Start the power frequency modular multilevel converter and put it into constant DC voltage control mode to control the DC voltage of the power frequency modular multilevel converter to the rated DC voltage value; Close the low-frequency side AC switch, start the low-frequency auxiliary modular multilevel converter, and make the low-frequency auxiliary modular multilevel converter operate in a constant AC voltage and frequency mode. Control the AC side frequency of the low-frequency auxiliary modular multilevel converter to the rated frequency of the low-frequency side, and increase the AC side voltage of the low-frequency auxiliary modular multilevel converter from the initial value to the rated value of the low-frequency AC voltage. After completing the startup of the back-to-back AC inverters, the system switches to normal operation control mode.

7. The method according to claim 6, characterized in that, The normal operation control mode includes the control of the low-frequency auxiliary modular multilevel converter, including: The low-frequency AC voltage synchronization angle is obtained based on the preset low-frequency AC frequency target value; The effective value of the fundamental wave of the low-frequency AC voltage is obtained based on the instantaneous value of the three-phase low-frequency AC voltage, and the low-frequency AC voltage harmonic components are extracted from it. The AC voltage fundamental deviation signal is calculated based on the target value of the low-frequency AC voltage fundamental wave and the effective value of the low-frequency AC voltage fundamental wave. The reference value of the fundamental amplitude of the AC output voltage is obtained based on the AC voltage fundamental deviation signal; The reference value of the instantaneous value of the fundamental amplitude of the three-phase AC output voltage is calculated based on the reference value of the fundamental amplitude of the AC output voltage and the synchronization angle of the low-frequency AC voltage. The instantaneous values ​​of the three-phase AC output voltage harmonics are obtained based on the low-frequency AC voltage harmonic components. Based on the reference values ​​of the fundamental instantaneous value and the reference values ​​of the harmonic instantaneous value of the three-phase AC output voltage, the total reference value of the three-phase AC output voltage instantaneous value is obtained. The target DC voltage value of the low-frequency assisted modular multilevel converter is obtained based on the submodule capacitor voltage deviation signal of the low-frequency assisted modular multilevel converter. Based on the reference value of the instantaneous value of the total three-phase AC output voltage and the target value of the DC voltage of the low-frequency auxiliary modular multilevel converter, the reference voltages of multiple arms of the low-frequency auxiliary modular multilevel converter are calculated. The reference voltages of multiple bridge arms of the low-frequency auxiliary modular multilevel converter are pulse-width modulated to output the control pulse signal of the low-frequency auxiliary modular multilevel converter.

8. The method according to claim 6, characterized in that, The submodule capacitor voltage deviation signal of the low-frequency auxiliary modular multilevel converter includes: Obtain the target value of the submodule capacitor voltage of the low-frequency auxiliary modular multilevel converter; The average value of the sub-module capacitor voltage is obtained based on the capacitor voltage of all sub-modules of the low-frequency auxiliary modular multilevel converter. The submodule capacitor voltage deviation signal is calculated based on the target value of the submodule capacitor voltage and the average value of the submodule capacitor voltage.

9. A computer device, characterized in that, Including processor and memory; The processor reads executable program code stored in the memory to run a program corresponding to the executable program code, so as to implement the startup and normal operation control method as described in any one of claims 6-8.

10. A non-transitory computer-readable storage medium having a computer program stored thereon, characterized in that, When the program is executed by the processor, it implements the startup and normal operation control method as described in any one of claims 6-8.

Images