Mmc with valve side ground fault protection capability and control method thereof

Abstract

The application discloses an MMC (Modular Multilevel Converter) with valve-side grounding fault protection capability and a control method thereof, and relates to the technical field of multilevel power electronic converters.The application comprises ABC three-phase, each phase comprising an upper bridge arm, a lower bridge arm and two bridge arm reactors; each bridge arm is connected by N sub-modules in series, wherein M full-bridge sub-modules and M-N half-bridge sub-modules are contained; the upper ends of the three-phase upper bridge arms of the converter are connected, and are connected to the DC side of a power transmission line through an isolating switch; the lower ends of the three-phase lower bridge arms are connected, and are connected to the ground through an isolating switch; the converter comprises five thyristor branches.The technical scheme can realize the same output characteristics as traditional MMC converters, and can realize rapid fault clearance by means of fewer full-bridge sub-modules for single-phase grounding faults on the valve side, and can solve problems such as sub-module capacitor overvoltage, bridge arm overcurrent and three-phase AC fault current without zero points.

Description

Technical Field

[0001] This invention relates to the field of multilevel power electronic converter technology, specifically to an MMC with valve-side ground fault protection capability and its control method. Background Technology

[0002] Modular multilevel converters (MMCs) have gained widespread attention in fields such as long-distance, high-capacity high-voltage direct current (HVDC) transmission due to their modular structure, high output power quality, high operating efficiency, and ability to achieve active and reactive power decoupling control. A symmetrical bipolar HVDC system based on MMC consists of two identical asymmetrical unipolar HVDC systems. In an asymmetrical unipolar HVDC system based on a half-bridge MMC, when an AC valve-side ground fault occurs, the unidirectional conductivity of the freewheeling diodes in the three-phase lower arm half-bridge submodule (HB-SM) leads to a lack of zero-crossing points in both the valve-side and grid-side AC fault currents. This hinders the operation of the AC circuit breaker and prevents timely clearance of the AC fault current. Prolonged AC fault current can damage transmission lines, converters, electrical loads, and valve-side transformers, and in severe cases, may damage the power electronic components in the converter.

[0003] In existing technologies, full-bridge MMCs are often used to solve the problem of no zero-crossing point in AC faults. However, when a valve-side ground fault occurs in a full-bridge MMC, it will cause severe overvoltage in the capacitors of the three-phase upper bridge arm submodules, ultimately damaging the capacitors and power electronic devices. Therefore, we propose an MMC with valve-side ground fault protection capability and its control method. Summary of the Invention

[0004] The purpose of this invention is to provide an MMC with valve-side grounding fault protection capability and its control method, which can solve the problem of single-phase grounding fault on the valve side of the MMC.

[0005] To achieve the above objectives, the present invention provides the following technical solution: an MMC with valve-side grounding fault protection capability, wherein the MMC includes three phases A, B, and C, each phase includes an upper bridge arm and a lower bridge arm, and each bridge arm includes N interconnected sub-modules, characterized in that the sub-modules in each bridge arm are a mixture of full-bridge sub-modules and half-bridge sub-modules.

[0006] The upper ends of the three-phase upper bridge arms A, B, and C are connected and connected to the DC side of the transmission line through a disconnecting switch one; the lower ends of the three-phase lower bridge arms A, B, and C are connected and grounded through a disconnecting switch two.

[0007] The MMC also includes five thyristor branches, namely thyristor branch 1 TB1, thyristor branch 2 TB2, thyristor branch 3 TB3, thyristor branch 4 TB4 and thyristor branch 5 TB5, wherein thyristor branch 1 TB1, thyristor branch 2 TB2 and thyristor branch 3 TB3 are connected in series to form a loop.

[0008] Furthermore, the MMC also includes a three-phase AC port and two bridge arm reactors, and the three-phase AC port is connected to the upper bridge arm and the lower bridge arm respectively through bridge arm reactor one and bridge arm reactor two.

[0009] Furthermore, the anode of the first thyristor branch is connected to the upper end of the lower bridge arm of phase A, and the cathode is connected to the upper end of the lower bridge arm of phase B.

[0010] The anode of the second thyristor branch is connected to the upper end of the lower bridge arm of phase B, and the cathode is connected to the upper end of the lower bridge arm of phase C.

[0011] The anode of the third thyristor branch is connected to the upper end of the lower bridge arm of phase C, and the cathode is connected to the upper end of the lower bridge arm of phase A.

[0012] The thyristor branch four and thyristor branch five are connected in anti-parallel, wherein the anode of the thyristor branch four is grounded and the cathode is connected to the upper end of the lower bridge arm of phase C.

[0013] Furthermore, the full-bridge submodule includes first to fourth IGBTs and a first DC capacitor, each IGBT containing an anti-parallel diode, wherein the first IGBT (T F1 Emitter and second IGBT (T) F2 The collectors of the first IGBT are connected, and the connection point serves as the positive output terminal of the full-bridge submodule; F1 Collector, third IGBT (T) F3 The collector, the positive terminal of the DC capacitor, and the second IGBT (T) are connected together; F2 Emitter, fourth IGBT (T) F4 The emitter, the negative terminal of the DC capacitor, and the third IGBT (T) are connected together; F3 Emitter and fourth IGBT (T) F4 The collectors are connected, and the connection point serves as the negative output terminal of the full-bridge submodule.

[0014] Furthermore, the half-bridge submodule includes a fifth IGBT (T H1 ), the sixth IGBT (T H2 The fifth IGBT (T) contains a second DC capacitor and an anti-parallel diode. H1 Emitter and the sixth IGBT (T) H2 The collectors are connected, and the connection point serves as the positive output terminal of the half-bridge submodule; the fifth IGBT (T H1The collector of the sixth IGBT (T) is connected to the positive terminal of the second DC capacitor; H2 The emitter is connected to the negative terminal of the second DC capacitor, and the connection point serves as the negative output terminal of the half-bridge submodule.

[0015] Furthermore, the thyristor branches one to five are respectively composed of N TBi It consists of (i = 1 to 5) thyristors connected in series, where N TBi The selection method is as follows:

[0016] (1) Determine the voltage U that each thyristor can withstand based on the selected thyristor model. T ;

[0017] (2) Based on the voltage U across the thyristor branch i during rated operation of the converter TBi Through N TBi =U TBi / U T The number N of cascaded thyristors required in thyristor branch i can then be calculated. TBi .

[0018] According to one aspect of the present invention, the present invention provides a control method for an MMC with valve-side ground fault protection capability, including normal operation control, DC current clearing control and AC fault current clearing control methods.

[0019] Furthermore, the normal operation control method is as follows:

[0020] No valve-side grounding fault was detected. Thyristor branches one through five were all blocked. The MMC distributed switching signals to each submodule according to active power control, reactive power control, constant DC voltage control, constant AC voltage control, frequency control, capacitor voltage balance control, and circulating current suppression control.

[0021] In the FB-SM, the first IGBT (TF1) and the second IGBT (TF2) operate complementaryly, the third IGBT (TF3) is always on, and the fourth IGBT (TF4) is always off; in the HB-SM, the first IGBT (TH1) and the second IGBT (TH2) operate complementaryly; both disconnecting switch one (S1) and disconnecting switch two (S2) are closed.

[0022] Furthermore, the DC current clearing control method is as follows:

[0023] After detecting an AC valve-side grounding fault, the DC current clearing stage begins, and all IGBTs in the full-bridge and half-bridge submodules are turned off, while thyristor branches one through five are all turned on.

[0024] Furthermore, the AC fault current clearing control method is as follows:

[0025] After the DC current is detected to be cleared, disconnecting switch one and disconnecting switch two are disconnected to isolate the faulty converter and enter the AC fault current clearing stage. The trigger signals of thyristor branch one to thyristor branch five are removed to interrupt the AC fault current.

[0026] This invention has at least the following beneficial effects:

[0027] 1. This invention can quickly disconnect AC fault current and clear AC faults;

[0028] Traditional three-phase half-bridge MMCs will result in no zero-crossing point for the AC fault current when a valve-side ground fault occurs, hindering the timely disconnection of the fault current by the AC circuit breaker. Prolonged AC fault current can damage equipment such as transmission lines, converters, electrical loads, and valve-side transformers. For traditional three-phase full-bridge MMCs, the AC fault current has a zero-crossing point, but the fault current can only be disconnected by the AC circuit breaker. The circuit breaker's operating time is 40-100ms, resulting in a long AC fault clearing time. This invention utilizes the thyristor branch to disconnect the AC fault current, enabling the clearing of the AC fault current within 20ms.

[0029] 2. This invention avoids overvoltage of submodule capacitors and overcurrent of bridge arms;

[0030] Traditional three-phase full-bridge MMCs can solve the problem of no zero-crossing point for three-phase fault current, but they can cause severe overvoltage in the three-phase upper bridge arm submodule capacitors, damaging capacitor components and power electronic switching devices. In addition, traditional MMCs can cause large bridge arm overcurrents when a valve-side ground fault occurs, which can damage the power electronic switching devices in the converter. This invention clamps the voltage of the three-phase lower bridge arm in the converter by conducting the thyristor branch, avoiding overvoltage of the submodule capacitors. At the same time, the three-phase fault current mainly flows through the thyristor branch, avoiding the generation of bridge arm overcurrent.

[0031] 3. This invention has relatively low converter losses during normal operation;

[0032] Traditional three-phase full-bridge MMC can solve the problem of AC fault current without zero crossing point, but its operating loss is large. The MMC with valve side grounding fault protection capability proposed in this invention only requires 7% of FB-SM. Under normal operation, the converter proposed in this invention reduces converter loss by 35.7% compared with traditional three-phase full-bridge MMC, thereby significantly improving the operating efficiency of the converter.

[0033] 4. The basic construction cost of this invention is low;

[0034] Traditional three-phase full-bridge MMCs can solve the problem of AC fault currents without zero crossing points, but they have many power electronic devices and high converter infrastructure costs. The MMC with valve-side ground fault protection capability proposed in this invention only requires 7% of the FB-SM. Although it adds 5 thyristor branches, the infrastructure cost of this solution is reduced by 42.2% compared to traditional three-phase full-bridge MMCs because the price of thyristors is much lower than that of IGBT switching devices at the same voltage and current level.

[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 a schematic diagram of the MMC circuit structure according to an embodiment of the present invention;

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

[0038] Figure label:

[0039] 1. Upper bridge arm; 2. Lower bridge arm; 3. Full bridge submodule; 4. Half bridge submodule; 5. Bridge arm reactor one; 6. Bridge arm reactor two; 7. Thyristor branch; 8. Disconnecting switch one; 9. Disconnecting switch two. Detailed Implementation

[0040] 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.

[0041] Please see Figures 1-2 The present invention provides a technical solution: an MMC with valve side grounding fault protection capability. The MMC includes three phases A, B and C. Each phase includes an upper bridge arm 1 and a lower bridge arm 2. Each bridge arm includes N interconnected sub-modules. The sub-modules in each bridge arm are a mixture of full-bridge sub-modules 3 and half-bridge modules 4.

[0042] The upper ends of the three-phase upper bridge arm 1 (A, B, and C) are connected and connected to the DC side of the transmission line through disconnector switch 8; the lower ends of the three-phase lower bridge arm 2 (A, B, and C) are connected and grounded through disconnector switch 9.

[0043] The MMC also includes five thyristor branches 7, namely thyristor branch 1 TB1, thyristor branch 2 TB2, thyristor branch 3 TB3, thyristor branch 4 TB4 and thyristor branch 5 TB5.

[0044] The specific implementation steps are as follows:

[0045] like Figure 1 As shown, the MMC has valve-side ground fault protection capability. The MMC consists of three phases, A, B, and C. Each phase includes an upper bridge arm 1, a lower bridge arm 2, and two bridge arm reactors L. s Each bridge arm consists of N sub-modules connected in series, including M full-bridge sub-modules 3 and MN half-bridge modules 4; the upper ends of the MMC three-phase upper bridge arms are connected and connected to the DC side of the transmission line through disconnector switch one S18; the lower ends of the three-phase lower bridge arms are connected and grounded through disconnector switch two S29.

[0046] Full-bridge submodule 3 includes first to fourth IGBTs and a first DC capacitor. Each IGBT includes an anti-parallel diode, wherein the first IGBT (T F1 Emitter and second IGBT (T) F2 The collectors of the first IGBT are connected, and the connection point serves as the positive output terminal of the full-bridge submodule; F1 Collector, third IGBT (T) F3 The collector, the positive terminal of the DC capacitor, and the second IGBT (T) are connected together; F2 Emitter, fourth IGBT (T) F4 The emitter, the negative terminal of the DC capacitor, and the third IGBT (T) are connected together; F3 Emitter and fourth IGBT (T) F4 The collectors are connected, and the connection point serves as the negative output terminal of the full-bridge submodule.

[0047] Half-bridge submodule 4 includes the fifth IGBT (T H1 ), the sixth IGBT (T H2 The fifth IGBT (T) contains a second DC capacitor and an anti-parallel diode. H1 Emitter and the sixth IGBT (T) H2 The collectors are connected, and the connection point serves as the positive output terminal of the half-bridge submodule; the fifth IGBT (T H1 The collector of the sixth IGBT (T) is connected to the positive terminal of the second DC capacitor; H2 The emitter is connected to the negative terminal of the second DC capacitor, and the connection point serves as the negative output terminal of the half-bridge submodule.

[0048] The MMC contains five thyristor branches 7, among which thyristor branch 1 TB1, thyristor branch 2 TB2 and thyristor branch 3 TB3 are connected in series to form a loop.

[0049] The anode of thyristor branch 1 TB1 is connected to the upper end P1 of the lower bridge arm of phase A, and the cathode is connected to the upper end P2 of the lower bridge arm of phase B; the anode of thyristor branch 2 TB2 is connected to the upper end P2 of the lower bridge arm of phase B, and the cathode is connected to the upper end P3 of the lower bridge arm of phase C; the anode of thyristor branch 3 TB3 is connected to the upper end P3 of the lower bridge arm of phase C, and the cathode is connected to the upper end P1 of the lower bridge arm of phase A; thyristor branch 4 TB4 and thyristor branch 5 TB5 are connected in anti-parallel, wherein the anode of thyristor branch 4 TB4 is grounded, and the cathode is connected to the upper end P3 of the lower bridge arm of phase C.

[0050] It should be noted that thyristor branches one through five are respectively controlled by N TBi i = 1 to 5 cascaded thyristors, where N TBi The selection method is as follows:

[0051] (1) Determine the voltage U that each thyristor can withstand based on the selected thyristor model. T ;

[0052] (2) Based on the voltage U across the thyristor branch i during rated operation of the converter TBi Through N TBi =U TBi / U T The number N of cascaded thyristors required in thyristor branch i7 can then be calculated. TBi .

[0053] Furthermore, the MMC also includes a three-phase AC port, which is connected to the upper bridge arm 1 and the lower bridge arm 2 respectively through the bridge arm reactor 1 and the bridge arm reactor 2.

[0054] like Figure 2 As shown, this invention provides a control method for an MMC with valve-side ground fault protection capability, including control during normal operation, DC current clearing control, and AC fault current clearing control. The specific steps are as follows:

[0055] S1. No valve-side grounding fault was detected. That is, during normal MMC operation, the MMC distributes switching signals to each submodule based on active power control, reactive power control, constant DC voltage control, constant AC voltage control, frequency control, capacitor voltage balance control, and circulating current suppression control; among which, the first IGBT in FB-SM... F1 Second IGBTT F2 Complementary operation, third IGBTT F3 Always on, fourth IGBT F4 Always off; First IGBTT in HB-SM H1 Second IGBTT H2 Complementary operation; all five thyristor branches 7 are locked; disconnector switch S18 and disconnector switch S29 are both closed.

[0056] S2. After detecting a valve-side grounding fault, the DC current clearing phase begins, and all submodules within the MMC are locked, i.e., T in FB-SM. F1 T F2 T F3 and T F4 All are off, T in HB-SM H1 and T H2 All are off; all five thyristor branches 7 are on.

[0057] S3. After detecting the DC current clearance, disconnect disconnector switch one S18 and disconnector switch two S29, enter the AC fault current clearance stage, remove the trigger signals of the five thyristor branches 7, and interrupt the AC fault current.

[0058] In summary, the technical solution of this invention can achieve the same output characteristics as traditional MMC converters. At the same time, for valve-side single-phase grounding faults, it can achieve rapid fault clearing with fewer full-bridge sub-modules, solving problems such as sub-module capacitor overvoltage, bridge arm overcurrent, and no zero-crossing point of three-phase AC fault current.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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 multiphase control module (MMC) with valve-side ground fault protection capability, wherein the MMC comprises three phases A, B, and C, each phase includes an upper bridge arm and a lower bridge arm, and each bridge arm includes N interconnected sub-modules, characterized in that... Each of the bridge arm submodules is a hybrid of full-bridge and half-bridge submodules; The upper ends of the three-phase upper bridge arms A, B, and C are connected and connected to the DC side of the transmission line through a disconnecting switch one; the lower ends of the three-phase lower bridge arms A, B, and C are connected and grounded through a disconnecting switch two. The MMC also includes five thyristor branches, each with a thyristor branch of one... TB 1. Thyristor Branch Two TB 2. Thyristor Branch Three TB 3. Thyristor Branch Four TB 4 and thyristor branch five TB 5, of which the thyristor branch is one TB 1. Thyristor Branch Two TB 2 and thyristor branch three TB 3 connected in series to form a loop; The MMC also includes a three-phase AC port and two bridge arm reactors, and the three-phase AC port is connected to the upper bridge arm and the lower bridge arm respectively through bridge arm reactor one and bridge arm reactor two. The anode of the first thyristor branch is connected to the upper end of the lower bridge arm of phase A, and the cathode is connected to the upper end of the lower bridge arm of phase B. The anode of the second thyristor branch is connected to the upper end of the lower bridge arm of phase B, and the cathode is connected to the upper end of the lower bridge arm of phase C. The anode of the third thyristor branch is connected to the upper end of the lower bridge arm of phase C, and the cathode is connected to the upper end of the lower bridge arm of phase A. The thyristor branch four and thyristor branch five are connected in anti-parallel, wherein the anode of the thyristor branch four is grounded and the cathode is connected to the upper end of the lower bridge arm of phase C.

2. The MMC with valve-side grounding fault protection capability according to claim 1, characterized in that: The full-bridge submodule includes first to fourth IGBTs and a first DC capacitor. Each IGBT contains an anti-parallel diode, wherein the first IGBT (T) F1 Emitter and second IGBT (T) F2 The collectors of the first IGBT (T) are connected, and the connection point serves as the positive output terminal of the full-bridge submodule; F1 Collector, third IGBT (T) F3 The collector, the positive terminal of the DC capacitor, and the second IGBT (T) are connected together; F2 Emitter, fourth IGBT (T) F4 The emitter, the negative terminal of the DC capacitor, and the third IGBT (T) are connected together; F3 Emitter and fourth IGBT (T) F4 The collectors are connected, and the connection point serves as the negative output terminal of the full-bridge submodule.

3. The MMC with valve-side grounding fault protection capability according to claim 2, characterized in that: The half-bridge submodule includes a fifth IGBT (T H1 ), the sixth IGBT (T H2 The fifth IGBT (T) contains a second DC capacitor and an anti-parallel diode. H1 Emitter and the sixth IGBT (T) H2 The collectors are connected, and the connection point serves as the positive output terminal of the half-bridge submodule; the fifth IGBT (T H1 The collector of the sixth IGBT (T) is connected to the positive terminal of the second DC capacitor; H2 The emitter is connected to the negative terminal of the second DC capacitor, and the connection point serves as the negative output terminal of the half-bridge submodule.

4. The MMC with valve-side grounding fault protection capability according to claim 1, characterized in that: The thyristor branches one through five are respectively composed of N TBi ( i =1~5) thyristors connected in series, among which N TBi The selection method is as follows: (1) Determine the voltage that each thyristor can withstand based on the selected thyristor model. U T ; (2) Based on the thyristor branch during rated operation of the converter i Voltage borne by both ends U TBi ,pass N TBi = U TBi / U T The thyristor branch can then be calculated. i The number of cascaded thyristors required N TBi .

5. A control method for an MMC with valve-side ground fault protection capability, using an MMC with valve-side ground fault protection capability as described in any one of claims 1 to 4, characterized in that: This includes normal operation control, DC current clearing control, and AC fault current clearing control methods.

6. The control method for MMC with valve-side ground fault protection capability according to claim 5, characterized in that, The normal operation control method is as follows: No valve-side grounding fault was detected. Thyristor branches one through five were all blocked. The MMC distributed switching signals to each submodule according to active power control, reactive power control, constant DC voltage control, constant AC voltage control, frequency control, capacitor voltage balance control, and circulating current suppression control. In the FB-SM, the first IGBT (TF1) and the second IGBT (TF2) operate complementaryly, the third IGBT (TF3) is always on, and the fourth IGBT (TF4) is always off; in the HB-SM, the first IGBT (TH1) and the second IGBT (TH2) operate complementaryly; both disconnecting switch one (S1) and disconnecting switch two (S2) are closed.

7. The control method for MMC with valve-side ground fault protection capability according to claim 6, characterized in that, The DC current clearing control method is as follows: After detecting an AC valve-side grounding fault, the DC current clearing stage begins, and all IGBTs in the full-bridge and half-bridge submodules are turned off, while thyristor branches one through five are all turned on.

8. The control method for MMC with valve-side ground fault protection capability according to claim 6, characterized in that: The AC fault current clearing control method is as follows: After the DC current is detected to be cleared, disconnecting switch one and disconnecting switch two are disconnected to isolate the faulty converter and enter the AC fault current clearing stage. The trigger signals of thyristor branch one to thyristor branch five are removed to interrupt the AC fault current.

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