MMC based on thyristor ring with self-commutation capability and control method thereof

Abstract

This invention discloses a self-interrupting MMC based on a thyristor loop and its control method, relating to the field of multilevel power electronic converter technology. The MMC converter of this invention adopts a three-phase six-arm structure. The three-phase upper arms use a hybrid configuration of full-bridge and half-bridge submodules, with the full-bridge submodules blocking DC short-circuit current. The three-phase lower arms use simple half-bridge submodules. The three thyristor branches form a loop, running from the lower end of the upper arm of phase A to the lower end of the upper arm of phase B, then back to the lower end of the upper arm of phase C. When a DC-side short-circuit fault occurs, the thyristors are triggered, short-circuiting the lower ends of the three-phase upper arms, achieving AC / DC decoupling. The reverse voltage injected by the full-bridge submodules in the three-phase upper arms blocks the DC-side short-circuit current. Then, the disconnecting switch is opened to isolate the converter from the faulty line. Finally, the AC current is disconnected using the half-control characteristics of the thyristors, achieving DC-side short-circuit fault protection.

Description

Technical Field

[0001] This invention relates to the field of multilevel power electronic converter technology, specifically to an MMC based on a thyristor ring with self-interrupting capability and its control method. Background Technology

[0002] Modular multilevel converters (MMCs) are the core equipment of flexible DC transmission. Compared with traditional two-level and three-level converters, MMCs have the advantages of high efficiency, high modularity, and low output AC voltage harmonics. When a DC short-circuit fault occurs in a flexible DC system, a huge short-circuit current is generated, which can damage converters, transmission lines, and electrical loads, and may even damage the power electronic components of the converter, rendering it unable to operate. Therefore, the ability to quickly clear DC faults is directly related to the reliability of system operation.

[0003] In a traditional half-bridge MMC, the half-bridge submodule is locked out after a DC-side short-circuit fault. At this time, the output characteristics of the half-bridge submodule are equivalent to those of a diode, and the equivalent circuit of the MMC is a three-phase uncontrolled rectifier circuit. The AC side continuously injects short-circuit current into the DC fault location. Therefore, the traditional half-bridge MMC cannot achieve DC-side short-circuit fault protection by relying on submodule lockout. To address this, we propose an MMC with self-interrupting current capability based on a thyristor ring and its control method. Summary of the Invention

[0004] The purpose of this invention is to provide a thyristor ring-based MMC with self-interrupting current capability and its control method, which can realize DC side short circuit fault protection.

[0005] To achieve the above objectives, the present invention provides the following technical solution: an MMC based on a thyristor ring with self-interruption capability, wherein the MMC includes three phases A, B, and C, each phase includes an upper bridge arm and a lower bridge arm, each bridge arm includes N sub-modules, the sub-modules of the upper bridge arm are a mixture of full-bridge sub-modules and half-bridge sub-modules, the number of full-bridge sub-modules is M, and the sub-modules of the lower bridge arm are all half-bridge sub-modules;

[0006] The lower end of the upper bridge arm is short-circuited with a thyristor branch. There are three thyristor branches, namely the first branch TB1, the second branch TB2 and the third branch TB3. The three thyristor branches are connected in series to form a loop, from the lower end of the upper bridge arm of phase A to the lower end of the upper bridge arm of phase B, then from the lower end of the upper bridge arm of phase B to the lower end of the upper bridge arm of phase C, and finally from the lower end of the upper bridge arm of phase C back to the lower end of the upper bridge arm of phase A.

[0007] The MMC also includes a three-phase AC port and a DC port. The three-phase AC port is connected to the upper bridge arm and the lower bridge arm respectively through the first bridge arm inductor and the second bridge arm inductor. The DC port is connected to the DC bus through the first disconnect switch and the second disconnect switch.

[0008] Furthermore, the full-bridge submodule includes four IGBT switching devices (including anti-parallel diodes) and an electrolytic capacitor. The emitter of the first IGBT switch is connected to the collector of the second IGBT switch; the emitter of the third IGBT switch is connected to the collector of the fourth IGBT switch; the collectors of the first and third IGBT switches are connected to the positive terminal of the electrolytic capacitor; the emitters of the second and fourth IGBT switches are connected to the negative terminal of the electrolytic capacitor; the emitter of the first IGBT switch is the positive terminal of the full-bridge submodule, and the emitter of the third IGBT switch is the negative terminal of the full-bridge submodule.

[0009] Furthermore, the half-bridge sub-module includes two IGBT switching devices (including anti-parallel diodes) and an electrolytic capacitor, wherein the emitter of the first IGBT switch is connected to the collector of the second IGBT switch; the collector of the first IGBT switch is connected to the positive terminal of the electrolytic capacitor; the emitter of the second IGBT switch is connected to the negative terminal of the electrolytic capacitor; the emitter of the first IGBT switch is the positive terminal of the half-bridge sub-module, and the emitter of the second IGBT switch is the negative terminal of the half-bridge sub-module.

[0010] Furthermore, the specific method for calculating the number N of sub-modules is as follows:

[0011] Based on the DC bus voltage U dc The rated voltage U of the electrolytic capacitor in the sub-module cN Calculate and determine the number N of upper and lower bridge arm sub-modules:

[0012]

[0013] Furthermore, the calculation method for the number M of the upper bridge arm full-bridge submodules includes:

[0014] (1) Based on the given AC three-phase voltage U ph AC side inductor L f AC side resistance R f and bridge arm inductor L S Calculate the amplitude U of the upper arm inductor voltage during three-phase upper arm short-circuit operation. Lsm ;

[0015] (2) Based on the obtained upper bridge arm inductor voltage amplitude U Lsm The required back pressure V to be injected during the flow interruption process is obtained. neg ;

[0016]

[0017] (3), according to V neg and the rated capacitor voltage U of FB-SM cNCalculate the required number M of FB-SMs:

[0018]

[0019] Furthermore, each of the three thyristor branches contains N TB There are N series thyristors. TB The calculation method is as follows:

[0020] (1) Determine the rated voltage U of each thyristor according to its model. T ;

[0021] (2) Based on the three-phase line voltage V during rated operation on the AC side Line Through N TB =V Line / U T Calculate the number N of series-connected thyristors in each thyristor branch. TB .

[0022] According to one aspect of the present invention, the present invention provides an MMC control method based on a thyristor ring with self-interruption capability, including normal operation control, DC side short-circuit fault clearing control and AC side short-circuit fault clearing control.

[0023] Furthermore, the normal operation control method is as follows:

[0024] The half-bridge submodule blocking signal, the full-bridge submodule blocking signal, and the thyristor drive signal are all 0. The drive signals of the first IGBT switch and the second IGBT switch of all half-bridge submodules are inverted. The drive signals of the first IGBT switch and the second IGBT switch of all full-bridge submodules are inverted. The drive signal of the third IGBT switch is 0, and the drive signal of the fourth IGBT switch is 1. The first disconnect switch S1 and the second disconnect switch S2 remain closed.

[0025] Furthermore, the DC-side short-circuit fault clearing control method is as follows:

[0026] When the detected DC current increases to 1.5 times the rated value, all full-bridge and half-bridge sub-modules are locked. The first to fourth IGBT switches in the full-bridge sub-module are all turned off, and the output characteristics of the sub-module are equivalent to those of a capacitor, providing reverse voltage. The first to second IGBT switches in the half-bridge sub-module are all turned off, and the output characteristics of the sub-module are equivalent to those of a diode.

[0027] Trigger the thyristor to short-circuit the lower end of the three-phase upper bridge arm.

[0028] Furthermore, the AC side short-circuit fault clearing control method is as follows:

[0029] When the DC side short-circuit current is detected to drop to zero, the first isolation switch S1 and the second isolation switch S2 between the DC port and the DC bus are disconnected, and the thyristor loop drive signal is removed.

[0030] This invention has at least the following beneficial effects:

[0031] (1) Compared with the traditional half-bridge MMC, the present invention has self-interruption capability and can realize DC side short-circuit fault protection:

[0032] In a traditional half-bridge MMC, upon a DC-side short-circuit fault, all half-bridge submodules are blocked. At this point, the output characteristics of the half-bridge submodules are equivalent to diodes, and the equivalent circuit of the MMC is a three-phase uncontrolled rectifier circuit. Short-circuit current continuously injects into the DC fault location from the AC side. Therefore, traditional half-bridge MMCs cannot achieve DC-side short-circuit fault protection by blocking submodules. This invention, upon a DC-side short-circuit fault, blocks all full-bridge and half-bridge submodules, triggering the thyristor loop. The upper bridge arm full-bridge submodule suppresses the DC-side short-circuit current, and the thyristor loop forms a three-phase AC short circuit, blocking the AC-side injected short-circuit current. This is the essential difference between this invention and existing MMC topologies.

[0033] (2) Compared with traditional hybrid MMC, this invention only requires 15% of the full-bridge submodules to achieve DC-side short-circuit fault protection:

[0034] Traditional hybrid MMCs require the use of full-bridge submodules to block DC short-circuit fault currents, necessitating a full-bridge submodule ratio exceeding 50%. However, according to the present invention, the full-bridge submodule ratio in an MMC based on a thyristor ring with self-interrupting capability only needs to reach 15%, reducing the number of full-bridge submodules by at least 70%. The number of IGBT switches in a full-bridge submodule is twice that of a half-bridge submodule, and the price of IGBT switches is significantly higher than that of thyristors for the same voltage and current rating. Therefore, the present invention uses fewer full-bridge submodules, resulting in lower equipment costs compared to traditional hybrid MMCs.

[0035] Of course, any product implementing this invention does not necessarily need to achieve all of the advantages described above at the same time. Attached Figure Description

[0036] Figure 1 This is the circuit diagram of the MMC of the present invention;

[0037] Figure 2 This is a control block diagram of the MMC of the present invention.

[0038] Figure label:

[0039] 1.1 Upper bridge arm; 1.2 Lower bridge arm; 1.3 Full bridge submodule; 1.4 Half bridge submodule; 1.5 First bridge arm inductor; 1.6 Second bridge arm inductor; 1.7

[0040] 2.1 The state of the new MMC during normal operation; 2.2 The state of the new MMC during the DC side short circuit fault clearing stage; 2.3 The state of the new MMC during the AC side short circuit fault clearing stage. Detailed Implementation

[0041] The technical solutions of the embodiments of this disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this disclosure, and not all embodiments. Based on the embodiments of this disclosure, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this disclosure.

[0042] Please see Figure 1-2 The present invention provides a technical solution: an MMC based on a thyristor ring with self-interruption capability, comprising:

[0043] like Figure 1 As shown, this invention provides an MMC with self-interrupting capability based on a thyristor ring. The MMC adopts a three-phase six-arm structure, with each phase including an upper arm 1.1 and a lower arm 1.2. Each arm contains N submodules (SMs). The three-phase upper arm 1.1 adopts a mixed configuration of full-bridge submodules (FB-SM) 1.3 and half-bridge submodules (HB-SM) 1.4. The number M of full-bridge submodules 1.3 in the upper arm 1.1 is determined by short-circuit parameters. Three thyristor branches (TBs) 1.7 form a loop, namely the first branch TB1, the second branch TB2, and the third branch TB3, which are shorted to the lower end of the three-phase upper arm 1.1. Each thyristor branch contains N submodules (SMs). TB A series of thyristors, wherein the first branch TB1 connects the lower end P1 of the upper bridge arm 1.1 of phase A and the lower end P2 of the upper bridge arm 1.1 of phase B, with the cathode pointing towards P2; the second branch TB2 connects the lower end P2 of the upper bridge arm 1.1 of phase B and the lower end P3 of the upper bridge arm 1.1 of phase C, with the cathode pointing towards P3; and the third branch TB3 connects the lower end P3 of the upper bridge arm 1.1 of phase C and the lower end P1 of the upper bridge arm 1.1 of phase A, with the cathode pointing towards P1.

[0044] The MMC also includes a three-phase AC port and a DC port. The three-phase AC port is connected to the upper bridge arm 1.1 and the lower bridge arm 1.2 respectively through the first bridge arm inductor 1.5 and the second bridge arm inductor 1.6. The DC port is connected to the DC bus through the first disconnect switch S11.8 and the second disconnect switch S21.9.

[0045] It should be noted that the full-bridge submodule 1.3 includes four IGBT switching devices (including anti-parallel diodes) and one electrolytic capacitor. The emitter of the first IGBT is connected to the collector of the second IGBT; the emitter of the third IGBT is connected to the collector of the fourth IGBT; the collectors of the first and third IGBTs are connected to the positive terminal of the electrolytic capacitor; the emitters of the second and fourth IGBTs are connected to the negative terminal of the electrolytic capacitor; the emitter of the first IGBT is the positive terminal of the full-bridge submodule 1.3, and the emitter of the third IGBT is the negative terminal of the full-bridge submodule 1.3.

[0046] Furthermore, the half-bridge sub-module 1.4 includes two IGBT switching devices (including anti-parallel diodes) and an electrolytic capacitor. The emitter of the first IGBT switch is connected to the collector of the second IGBT switch; the collector of the first IGBT switch is connected to the positive terminal of the electrolytic capacitor; the emitter of the second IGBT switch is connected to the negative terminal of the electrolytic capacitor; the emitter of the first IGBT switch is the positive terminal of the half-bridge sub-module 1.4, and the emitter of the second IGBT switch is the negative terminal of the half-bridge sub-module 1.4.

[0047] The specific method for calculating the number of submodules N is as follows:

[0048] Based on the DC bus voltage U dc The rated voltage U of the electrolytic capacitor in the sub-module cN Calculate and determine the number N of upper and lower bridge arm sub-modules:

[0049]

[0050] The number M of the 1.1 full-bridge submodule 1.3FB-SM in the upper bridge arm is determined by calculation based on short-circuit parameters:

[0051] (1) Based on the given AC three-phase voltage U ph AC side inductor L f AC side resistance R f and bridge arm inductor L S Calculate the voltage amplitude U of the inductor in the upper bridge arm 1.1 during short-circuit operation of the three-phase upper bridge arm 1.1. Lsm ;

[0052] (2) Based on the obtained upper bridge arm 1.1 inductor voltage amplitude U Lsm The required back pressure V to be injected during the flow interruption process is obtained. neg ;

[0053]

[0054] (3) According to V neg and the rated capacitor voltage U of FB-SM cNCalculate the required number M of FB-SMs:

[0055]

[0056] The number N of series-connected thyristors in thyristor branch 1.7 TB The selection of thyristors and the three-phase line voltages on the AC side are determined as follows:

[0057] (1) The rated voltage U of each thyristor is determined by the selection of the thyristor. T ;

[0058] (2) Based on the line voltage V of the AC side during rated operation. Line via V Line and U T Calculate the number N of series thyristors in each thyristor branch with a configuration of 1.7. TB :

[0059]

[0060] like Figure 2 As shown, this invention provides an MMC control method based on a thyristor ring with self-interrupting current capability, including normal operation control, DC side short-circuit fault clearing control, and AC side short-circuit fault clearing control, as detailed below:

[0061] (1) 2.1 During normal operation, the blocking drive signal of the half-bridge submodule 1.4HB-SM, the blocking drive signal of the full-bridge submodule 1.3FB-SM, and the thyristor drive signal are all 0; the first IGBT switch of the half-bridge submodule 1.4HB-SM Second IGBT switching transistor Drive signal inverted; first IGBT switch of the 1.3FB-SM full-bridge submodule. Second IGBT switching transistor Drive signal inverted, third IGBT switching transistor The drive signal is 0, and the fourth IGBT switch is activated. The drive signal is 1; the first isolation switch S1 and the second isolation switch S2 remain closed; at this time, the output characteristics of the full-bridge submodule 1.3 and the half-bridge submodule 1.4 are the same, and the converter is completely equivalent to the traditional hybrid MMC.

[0062] (2)2.2 When DC current i is detected dc When increased to 1.5 times the rated value, the full-bridge submodule 1.3FB-SM and the half-bridge submodule 1.4HB-SM are locked, and the first to fourth IGBT switching transistors in the full-bridge submodule 1.3FB-SM are locked. and With all modules off, the output characteristic of the full-bridge submodule 1.3FB-SM is equivalent to a capacitor, providing reverse voltage; the first IGBT switch in the half-bridge submodule 1.4HB-SM... Second IGBT switch T h2 When all circuits are turned off, the output characteristics of the half-bridge submodule 1.4HB-SM are equivalent to those of a diode. The thyristor is triggered, shorting the three-phase upper bridge arm 1.1, decoupling the AC and DC circuits. The energy stored in the DC bus inductor is absorbed by the full-bridge submodule 1.3, and the DC side short-circuit current gradually decreases until it is eliminated.

[0063] (3)2.3 When the DC side short-circuit current is detected to drop to zero, disconnect the first isolation switch S1 and the second isolation switch S2 between the DC port of the converter and the DC bus, and then remove the thyristor loop drive signal. Due to the semi-controlled characteristics of the thyristor, when the AC side short-circuit current crosses zero, the thyristor turns off and the AC short-circuit current is eliminated.

[0064] In summary, the technical solution proposed in this invention has self-interruption capability compared to traditional half-bridge MMC, enabling DC-side fault protection; compared to traditional hybrid MMC, it uses fewer full-bridge sub-modules 1.3 to achieve DC-side short-circuit fault protection, showing significant engineering application prospects in the field of flexible DC transmission, and is currently being gradually promoted and applied in fields such as power quality management, high-voltage DC power conversion, and electric drive.

[0065] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0066] For those skilled in the art, the specific meaning of the above terms in this invention can be understood according to the specific circumstances. When an element is referred to as being "assembled on," "mounted on," "fixed to," or "set on" another element, it may be directly on the other element or there may be an intermediate element present. When an element is considered to be "connected to" another element, it may be directly connected to the other element or there may be an intermediate element present. The terms "vertical," "horizontal," "upper," "lower," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only possible embodiments.

[0067] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

[0068] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this disclosure. 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.

Claims

1. A thyristor ring-based MMC with self-interrupting capability, the MMC comprising three phases A, B, and C, each phase including an upper bridge arm and a lower bridge arm, each bridge arm including N sub-modules, characterized in that, The sub-modules of the upper bridge arm are a mixture of full-bridge sub-modules and half-bridge sub-modules, with the number of full-bridge sub-modules being M, and the sub-modules of the lower bridge arm are all half-bridge modules. The lower end of the upper bridge arm is short-circuited with a thyristor branch. There are three thyristor branches, namely the first branch TB1, the second branch TB2 and the third branch TB3. The three thyristor branches are connected in series to form a loop, from the lower end of the upper bridge arm of phase A to the lower end of the upper bridge arm of phase B, then from the lower end of the upper bridge arm of phase B to the lower end of the upper bridge arm of phase C, and finally from the lower end of the upper bridge arm of phase C back to the lower end of the upper bridge arm of phase A. The MMC also includes a three-phase AC port and a DC port. The three-phase AC port is connected to the upper bridge arm and the lower bridge arm respectively through the first bridge arm inductor and the second bridge arm inductor. The DC port is connected to the DC bus through the first disconnect switch and the second disconnect switch. The calculation method for the number M of the full-bridge sub-modules in the upper arm includes: (1) Based on the given three-phase phase voltage U on the AC side ph AC side inductor L f AC side resistance R f and bridge arm inductor L S Calculate the amplitude U of the upper arm inductor voltage during three-phase upper arm short-circuit operation. Lsm ; (2) Based on the obtained upper bridge arm inductor voltage amplitude U Lsm The required back pressure V to be injected during the flow interruption process is obtained. neg ; (3) According to V neg and the rated capacitor voltage U of FB-SM cN Calculate the required number M of FB-SMs: 。 2. The MMC based on a thyristor ring with self-interrupting current capability according to claim 1, characterized in that: The full-bridge submodule includes four IGBT switching devices and one electrolytic capacitor. The emitter of the first IGBT is connected to the collector of the second IGBT; the emitter of the third IGBT is connected to the collector of the fourth IGBT; the collectors of the first and third IGBTs are connected to the positive terminal of the electrolytic capacitor; the emitters of the second and fourth IGBTs are connected to the negative terminal of the electrolytic capacitor; the emitter of the first IGBT is the positive terminal of the full-bridge submodule, and the emitter of the third IGBT is the negative terminal of the full-bridge submodule.

3. The MMC based on a thyristor ring with self-interrupting current capability according to claim 1, characterized in that: The half-bridge submodule includes two IGBT switching devices and an electrolytic capacitor. The emitter of the first IGBT switch is connected to the collector of the second IGBT switch. The collector of the first IGBT switch is connected to the positive terminal of the electrolytic capacitor. The emitter of the second IGBT switch is connected to the negative terminal of the electrolytic capacitor. The emitter of the first IGBT switch is the positive terminal of the half-bridge submodule, and the emitter of the second IGBT switch is the negative terminal of the half-bridge submodule.

4. The MMC based on a thyristor ring with self-interrupting current capability according to claim 1, characterized in that: The specific method for calculating the number N of sub-modules is as follows: Based on the DC bus voltage U dc The rated voltage U of the electrolytic capacitor in the sub-module cN Calculate and determine the number N of upper and lower bridge arm sub-modules: 。 5. The MMC based on a thyristor ring with self-interrupting current capability according to claim 1, characterized in that: Each of the three thyristor branches contains N TB There are N series thyristors. TB The calculation method is as follows: (1) Determine the rated voltage U of each thyristor according to its model. T ; (2) Based on the three-phase line voltage V during rated operation on the AC side Line Through N TB =V Line / U T Calculate the number N of series-connected thyristors in each thyristor branch. TB .

6. A method for controlling an MMC based on a thyristor ring with self-interrupting current capability, employing the MMC based on a thyristor ring with self-interrupting current capability as described in any one of claims 1 to 5, characterized in that, This includes normal operation control, DC side short-circuit fault clearing control, and AC side short-circuit fault clearing control.

7. The MMC control method based on a thyristor ring with self-interruption capability according to claim 6, characterized in that: The normal operation control method is as follows: The half-bridge submodule blocking signal, the full-bridge submodule blocking signal, and the thyristor drive signal are all 0. The drive signals of the first IGBT switch and the second IGBT switch of all half-bridge submodules are inverted. The drive signals of the first IGBT switch and the second IGBT switch of all full-bridge submodules are inverted. The drive signal of the third IGBT switch is 0, and the drive signal of the fourth IGBT switch is 1. The first disconnect switch S1 and the second disconnect switch S2 remain closed.

8. The MMC control method based on a thyristor ring with self-interruption capability according to claim 7, characterized in that: The DC-side short-circuit fault clearing control method is as follows: When the detected DC current increases to 1.5 times the rated value, all full-bridge and half-bridge sub-modules are locked. The first to fourth IGBT switches in the full-bridge sub-module are all turned off, and the output characteristics of the sub-module are equivalent to those of a capacitor, providing reverse voltage. The first to second IGBT switches in the half-bridge sub-module are all turned off, and the output characteristics of the sub-module are equivalent to those of a diode. Trigger the thyristor to short-circuit the lower end of the three-phase upper bridge arm.

9. The MMC control method based on a thyristor ring with self-interruption capability according to claim 7, characterized in that: The AC side short-circuit fault clearing control method is as follows: When the DC side short-circuit current is detected to drop to zero, the first isolation switch S1 and the second isolation switch S2 between the DC port and the DC bus are disconnected, and the thyristor loop drive signal is removed.

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