Active commutation unit and forced commutation hybrid converter topology

Abstract

The present invention discloses a hybrid converter topology structure of an active commutation unit and forced commutation, wherein the active commutation unit is arranged in the bridge arm circuit of the converter, one end of which is connected to the output end of the converter transformer, and the other end is connected to the DC bus, and the active commutation unit comprises: a main branch, on which a thyristor valve is arranged; an auxiliary branch, which is arranged in parallel with the main branch, on which a first control valve is arranged, and the first control valve has the functions of controllable shutdown of forward current and blocking of forward and reverse voltages; a second control valve, which is connected to the thyristor valve or the first control valve, and the second control valve is used to transfer current from the main branch to the auxiliary branch, and the second control valve comprises at least one power unit, and the power unit is used for controllable shutdown of forward current and controllable blocking of forward and reverse voltages. By implementing the present invention, active commutation of each bridge arm is realized, commutation failure is avoided, and stable and safe operation of the power grid is guaranteed.

Description

Technical Field

[0001] The invention relates to the technical field of commutation in power electronics, and in particular to a hybrid converter topology structure of an active commutation unit and forced commutation. Background Art

[0002] The traditional line commutated converter high voltage direct current (LCC-HVDC) transmission system has the advantages of long-distance large-capacity transmission and controllable active power, and is widely used around the world. As the core equipment of DC transmission, the converter is the core functional unit for realizing AC and DC power conversion. Its operating reliability largely determines the operating reliability of the UHV DC grid.

[0003] At present, capacitor-commutated converters or hybrid converters with turn-off devices in series with thyristors are usually used to achieve AC and DC power conversion. Among them, capacitor-commutated converters use capacitor voltage to increase the valve commutation voltage time area to ensure its reliable shutdown, but the controllable capacitor module is formed by combining power electronic switches and capacitors to achieve controllable capacitor input and voltage direction. In order to ensure reliable commutation, the single-stage capacitor value needs to be large, thereby increasing the voltage and current stress of the core component thyristor. The topological structure engineering is difficult to implement; the hybrid converter is formed by connecting the turn-off device in series with the thyristor, so that each bridge arm of the converter has the ability to be turned off, avoiding the occurrence of commutation failure, but due to the large transmission capacity of conventional DC transmission, each bridge arm of the converter is subjected to high voltage and large current, and the cost and difficulty of implementation are high. Since traditional converters mostly use semi-controlled devices thyristors as core components to form a six-pulse bridge commutation topology, each bridge arm is composed of multiple stages of thyristors and their buffer components in series. Since thyristors do not have self-shutoff capabilities, commutation failures are prone to occur in situations such as AC system failures, resulting in a surge in DC current and a rapid and large loss of DC transmission power, affecting the stable and safe operation of the power grid. Summary of the invention

[0004] In view of this, an embodiment of the present invention provides a hybrid converter topology structure of an active commutation unit and a forced commutation unit to solve the problem of commutation failure affecting the stable operation of the power grid.

[0005] According to the first aspect, an embodiment of the present invention provides an active commutation unit, which is arranged in the bridge arm circuit of the converter, one end of which is connected to the output end of the converter transformer, and the other end is connected to the DC bus, including: a main branch, on which a thyristor valve is arranged; an auxiliary branch, which is arranged in parallel with the main branch, on which a first control valve is arranged, and the first control valve has a controllable forward current shutdown function and a forward and reverse voltage blocking function; a second control valve, which is connected to the thyristor valve of the main branch or to the first control valve of the auxiliary branch, and the second control valve is used to transfer current from the main branch to the auxiliary branch, and the second control valve includes at least one power unit, and the power unit is used for controllable shutdown of forward current and controllable blocking of forward and reverse voltages.

[0006] In combination with the first aspect, in a first implementation of the first aspect, the power unit includes: a third branch, a first power device and a first diode are arranged on the first branch, and the first power device is connected in series with the first diode; a second branch, the second branch is arranged in parallel with the first branch, and the second branch includes at least one first capacitor, and the at least one first capacitor is arranged in series; a third branch, the third branch is respectively connected in parallel with the first branch and the second branch, and a second power device and a first resistor are arranged on the third branch, and the second power device is connected in series with the first resistor; the first power device and the second power device are both power electronic devices with a shutoff function.

[0007] In combination with the first aspect, in a second implementation of the first aspect, the power unit includes: a fourth branch, on which a first inductor element or a transformer is arranged; a fifth branch, which is arranged in parallel with the fourth branch; the fifth branch includes a discharge switch, a second capacitor and a first charging circuit, wherein the discharge switch is arranged in series with the second capacitor, and the charging circuit is arranged in parallel with the second capacitor.

[0008] In combination with the first aspect, in a third embodiment of the first aspect, the power unit includes: a sixth branch, including at least one third power device, and the at least one third power device is arranged in series; a seventh branch, the seventh branch has the same structure as the sixth branch, and the seventh branch is arranged in parallel with the sixth branch; the third power device is a power electronic device with a shutoff function.

[0009] In combination with the first aspect, in a fourth embodiment of the first aspect, the power unit includes: an eighth branch, including at least one first sub-branch, and the at least one first sub-branch is arranged in series; the first sub-branch includes a fourth power device and a third capacitor or a second resistor arranged in parallel; the fourth power device is a power electronic device with a shutoff function.

[0010] In combination with the first aspect, in a fifth embodiment of the first aspect, the power unit includes: a ninth branch, including at least one branch consisting of a second sub-branch, a third sub-branch and a fourth capacitor or a third resistor, and at least one of the branches is arranged in series; wherein the second sub-branch includes a fifth power device and a sixth power device arranged in series, and the first end of the fifth power device is connected to the first end of the sixth power device; the third sub-branch includes a seventh power device and an eighth power device arranged in series, and the second end of the seventh power device is connected to the second end of the eighth power device; one end of the fourth capacitor or the third resistor is connected between the fifth power device and the sixth power device, and the other end is connected between the seventh power device and the eighth power device; the fifth power device, the sixth power device, the seventh power device and the eighth power device are power electronic devices with a shutoff function.

[0011] In combination with the first aspect, in a sixth implementation of the first aspect, the power unit includes: a tenth branch, on which a ninth power device is arranged; an eleventh branch, which is arranged in parallel with the tenth branch; the eleventh branch includes a tenth power device, a second inductor, a fifth capacitor and a second charging circuit, wherein the tenth power device, the second inductor and the fifth capacitor are arranged in series, and the second charging circuit is arranged in parallel with the fifth capacitor; the ninth power device and the tenth power device are power electronic devices with a shutoff function.

[0012] In combination with the first aspect or any one of the first to sixth embodiments of the first aspect, in the seventh embodiment of the first aspect, the second control valve also includes: at least one buffer component, which is arranged in parallel in the power device; the buffer component includes: a first buffer branch composed of a sixth capacitor; or, a second buffer branch in which a fourth resistor and a seventh capacitor are connected in series; or, a third buffer branch in which the fourth resistor and the seventh capacitor are connected in parallel; or, a fourth buffer branch formed by a fifth resistor and a third diode connected in parallel and then connected in series with an eighth capacitor; or, a fifth buffer branch formed by a sixth resistor and a ninth capacitor connected in parallel and then connected in series with a fourth diode; or, a sixth buffer branch composed of a lightning arrester; or, a seventh buffer branch composed of multiple of the first buffer branch, the second buffer branch, the third buffer branch, the fourth buffer branch, the fifth buffer branch and the sixth buffer branch connected in parallel.

[0013] According to the second aspect, an embodiment of the present invention provides a hybrid converter topology with forced commutation, wherein the topology is connected to an AC power grid via a converter transformer, and the topology includes a three-phase six-bridge arm circuit, wherein each phase bridge arm includes an upper bridge arm and a lower bridge arm, and at least one upper bridge arm or a lower bridge arm is provided with the active commutation unit described in the first aspect or any embodiment of the first aspect.

[0014] The technical solution of the present invention has the following advantages:

[0015] 1. The active commutation unit provided in the embodiment of the present invention comprises a main branch and an auxiliary branch connected in parallel, and a second control valve arranged in the main branch or the auxiliary branch, wherein the main branch is provided with a thyristor valve, and the auxiliary branch is provided with a first control valve having the forward and reverse voltage blocking capability, and the second control valve is connected to the thyristor valve or to the first control valve, and comprises at least one power unit, which is used for controllable shutoff of the forward current and blocking of the forward and reverse voltages, so that the second control valve has the capability of unidirectional voltage output or unidirectional controllable shutoff, and ensures that the second control valve has a large flow capacity and carries the normal operating current, so as to transfer the current from the main branch to the auxiliary branch through the second control valve. The active commutation unit controls the forward shutoff voltage through the second control valve to extend the reverse recovery time of the thyristor valve of the main branch, thereby ensuring its reliable shutoff, and then realizing the active commutation of each bridge arm, avoiding commutation failure, and ensuring the stable and safe operation of the power grid.

[0016] 2. The forced commutation hybrid converter topology provided in the embodiment of the present invention includes a three-phase six-bridge arm circuit, each phase bridge arm includes an upper bridge arm and a lower bridge arm, and at least one upper bridge arm or a lower bridge arm is provided with an active commutation unit. The second control valve in the active commutation unit can shut off the main branch current in advance and provide a reverse voltage at the same time, thereby increasing the commutation voltage-time area of ​​the main branch thyristor valve, ensuring its reliable shutdown, avoiding the problem of commutation failure, and thus ensuring the stable and safe operation of the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0018] Figure 1 is a structural block diagram of an active commutation unit according to an embodiment of the present invention;

[0019] Figure 2 is another structural block diagram of a second control valve according to an embodiment of the present invention;

[0020] Figure 3 is another structural block diagram of a second control valve according to an embodiment of the present invention;

[0021] Figure 4 is another structural block diagram of a second control valve according to an embodiment of the present invention;

[0022] Figure 5 is another structural block diagram of a second control valve according to an embodiment of the present invention;

[0023] Figure 6 is another structural block diagram of a second control valve according to an embodiment of the present invention;

[0024] Figure 7 is another structural block diagram of a second control valve according to an embodiment of the present invention;

[0025] Figure 8 is another structural block diagram of a second control valve according to an embodiment of the present invention;

[0026] Fig. 9 is another structural block diagram of a second control valve according to an embodiment of the present invention;

[0027] Fig.10 is another structural block diagram of a second control valve according to an embodiment of the present invention;

[0028] Fig.11 is a structural block diagram of a buffer component according to an embodiment of the present invention;

[0029] Fig.12 is a structural block diagram of a thyristor valve according to an embodiment of the present invention;

[0030] Fig.13 is a structural block diagram of a first control valve according to an embodiment of the present invention;

[0031] Fig.14 is a topology diagram of a hybrid converter with forced commutation according to an embodiment of the present invention;

[0032] Fig.15 is another topological diagram of a forced-commutation hybrid converter according to an embodiment of the present invention;

[0033] Fig.16 is another topological diagram of a forced-commutation hybrid converter according to an embodiment of the present invention;

[0034] Fig.17 is a current flow path in a normal operating state of an embodiment of the present invention;

[0035] Fig.18 is the trigger control timing of the normal operating state of the embodiment of the present invention;

[0036] Fig.19is a schematic diagram of a current path for commutation from a main branch to an auxiliary branch according to an embodiment of the present invention;

[0037] Fig. 20 is a trigger control timing of a commutation failure or a short circuit fault in an embodiment of the present invention;

[0038] Fig.21 It is the control triggering timing for pre-detecting commutation failure or short circuit fault in the embodiment of the present invention. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0040] As the core equipment of DC power transmission, the converter is the core functional unit for realizing AC-DC power conversion. Its operating reliability largely determines the operating reliability of the UHV DC power grid. However, since traditional converters mostly use semi-controlled devices, thyristors, as core components to form a six-pulse bridge commutation topology, each bridge arm is composed of multiple thyristors and their buffer components in series, and since thyristors do not have self-shutoff capabilities, commutation failures are prone to occur in the event of AC system failures, resulting in a surge in DC current and a rapid and large loss of DC transmission power, affecting the stable and safe operation of the power grid.

[0041] Based on this, the technical solution of the present invention utilizes the advantages of thyristors and the first and second control valves that can be turned off, and adopts two branches in parallel. By connecting an auxiliary branch that can provide reverse voltage and self-shutoff capability in parallel on the basis of the main branch, the main branch can be reliably shut down and the entire bridge arm can be actively switched, thereby realizing the auxiliary switching function in a shorter time, avoiding the occurrence of switching failure, and ensuring the stable and safe operation of the power grid.

[0042] According to an embodiment of the present invention, an embodiment of an active commutation unit is provided, and the active commutation unit is arranged in a bridge arm circuit of a converter. One end of the active commutation unit is connected to the output end of a converter transformer, and the other end is connected to a DC bus. Figure 1 As shown in FIG. 1 , the active phase-changing unit includes: a main branch and an auxiliary branch. A thyristor valve V11 is provided on the main branch. Fig.12 As shown, the thyristor valve V11 includes at least one thyristor and a buffer component connected in parallel or in series with the thyristor, wherein at least one thyristor is arranged in series, and the buffer component is used to prevent the thyristor device from being damaged by high voltage and high current.

[0043] The auxiliary branch is connected in parallel with the main branch, and a first control valve V13 is provided on the auxiliary branch in the direction from the output end of the converter transformer to the DC bus. The first control valve V13 has a forward current controllable shutoff function and a forward and reverse voltage blocking function. Fig.13 As shown, the first control valve V13 includes a power device and a thyristor arranged in series, wherein the thyristor has a reverse blocking function, the power device has a controllable shutdown of the forward current and a blocking function of the forward and reverse voltages, and the power device is a power electronic device with a turn-off function, and the power electronic device is one or more turn-off devices such as IGBT, IGCT, IEGT, GTO or MOSFET.

[0044] The first control valve V13 is a high-pressure shut-off valve. Of course, the first control valve V13 can also be a series connection of multiple power devices and thyristors. As long as the topology can achieve the functions of controllable shutdown of forward current and forward and reverse voltage blocking capability, the present application does not limit the topology of the first control valve V13.

[0045] The second control valve V12 can be connected to the thyristor valve V11 of the main branch, or connected to the first control valve V13 on the auxiliary branch. The second control valve V12 includes: at least one power unit 11, which is used for controllable shutoff of the forward current and blocking of the forward and reverse voltages, so that the first control valve V12 has a unidirectional voltage output controllable shutoff function. The second control valve V12 can be a low-voltage shutoff valve with a unidirectional voltage controllable output capability or a unidirectional controllable shutoff function, which is used to shut off the main branch current and provide it with a reverse voltage, ensuring that the thyristor valve of the main branch has sufficient shutoff time for reliable shutoff.

[0046] The active commutation unit provided in this embodiment includes a main branch and an auxiliary branch connected in parallel, and a second control valve arranged in the main branch or the auxiliary branch, wherein the main branch is provided with a thyristor valve, and the auxiliary branch is provided with a first control valve with forward and reverse voltage blocking capability, and the second control valve is connected to the thyristor valve of the main branch or to the first control valve of the auxiliary branch, and includes at least one power unit, which is used for controllable shutoff of forward current and blocking of forward and reverse voltage, so that the second control valve has the capability of unidirectional voltage output or unidirectional controllable shutoff, and ensures that the second control valve has a large flow capacity and carries the normal operating current, so as to transfer the current from the main branch to the auxiliary branch through the second control valve. The active commutation unit controls the forward shutoff voltage through the second control valve to extend the reverse recovery time of the thyristor valve of the main branch, thereby ensuring its reliable shutoff, and then realizing active commutation of each bridge arm, avoiding commutation failure, and ensuring the stable and safe operation of the power grid.

[0047] Alternatively, if Figure 2As shown, the power unit 11 may be a power electronic unit consisting of a first branch, a second branch and a third branch.

[0048] The first branch is provided with a first power device W1 and a first diode D1, and the first power device W1 and the first diode D1 are arranged in series. Among them, the first power device W1 is a power electronic device with a turn-off function, and the power electronic device is one or more turn-off devices such as IGBT, IGCT, IEGT, GTO or MOSFET. The second branch is connected in parallel with the third branch, and at least one first capacitor C1 is arranged on the second branch, wherein at least one first capacitor C1 is arranged in series. The third branch, the third branch is respectively connected in parallel with the first branch and the second branch, and the third branch is provided with a second power device W2 and a first resistor R1, and the second power device W2 and the first resistor R1 are connected in series. Among them, the second power device W2 is a power electronic device with a turn-off function, and the power electronic device is one or more turn-off devices such as IGBT, IGCT, IEGT, GTO or MOSFET.

[0049] Alternatively, if Figure 3 As shown, the power unit 11 may be a power electronic unit consisting of a fourth branch and a fifth branch.

[0050] The fourth branch is provided with a first inductor element L1; the fifth branch is provided in parallel with the fourth branch, and the fifth branch is provided with a discharge switch K1, a second capacitor C2 and a first charging circuit P1, the discharge switch K1 and the second capacitor C2 are provided in series, and the charging circuit P1 and the second capacitor C2 are provided in parallel. Figure 3 As shown, the discharge switch K1 is composed of a thyristor and a diode, one end of the first inductor element L1 is connected to the positive electrode of the thyristor and the negative electrode of the diode, the other end of the first inductor element L1 is connected to one end of the second capacitor C2, and the other end of the second capacitor C2 is connected to the negative electrode of the thyristor and the positive electrode of the diode.

[0051] Alternatively, if Figure 4 As shown, the power unit 11 may be a power electronic unit consisting of a fourth branch and a fifth branch.

[0052] The fourth branch is provided with a transformer B1; the fifth branch is provided in parallel with the fourth branch, and the fifth branch is provided with a discharge switch K1, a second capacitor C2 and a first charging circuit P1, the discharge switch K1 and the second capacitor C2 are provided in series, and the charging circuit P1 and the second capacitor C2 are provided in parallel. Figure 4 As shown, the discharge switch K1 is composed of a thyristor and a diode, one end of the transformer B1 is connected to the positive electrode of the thyristor and the negative electrode of the diode, the other end of the transformer B1 is connected to one end of the second capacitor C2, and the other end of the second capacitor C2 is connected to the negative electrode of the thyristor and the positive electrode of the diode.

[0053] Alternatively, if Figure 5 As shown, the power unit 11 may be a power electronic unit consisting of a sixth branch and a seventh branch.

[0054] Among them, at least one third power device W3 is arranged on the sixth branch, and at least one third power device W3 is arranged in series, and the third power device W3 is a power electronic device with a turn-off function, and the power electronic device is one or more turn-off devices such as IGBT, IGCT, IEGT, GTO or MOSFET. The structure of the seventh branch is the same as that of the sixth branch, and the seventh branch is arranged in parallel with the sixth branch.

[0055] Alternatively, if Figure 6 As shown, the power unit 11 can be a power electronic unit composed of an eighth branch. The eighth branch is composed of at least one first sub-branch, and at least one first sub-branch is arranged in series, and the first sub-branch is composed of a fourth power device W4 and a third capacitor C3 arranged in parallel, wherein the fourth power device W4 is a power electronic device with a turn-off function, and the power electronic device is one or more turn-off devices such as IGBT, IGCT, IEGT, GTO or MOSFET.

[0056] Alternatively, if Figure 8 As shown, the power unit 11 can be a power electronic unit composed of an eighth branch. The eighth branch is composed of at least one first sub-branch, and at least one first sub-branch is arranged in series, and the first sub-branch is composed of a fourth power device W4 and a second resistor R2 arranged in parallel, wherein the second resistor R2 is a nonlinear resistor, and the fourth power device W4 is a power electronic device with a turn-off function, and the power electronic device is one or more turn-off devices such as IGBT, IGCT, IEGT, GTO or MOSFET.

[0057] Alternatively, if Figure 7 As shown, the power unit 11 may be a power electronic unit composed of a ninth branch, the ninth branch includes at least one branch composed of a second sub-branch, a third sub-branch and a fourth capacitor C4, and at least one branch is arranged in series.

[0058] The second sub-branch includes a fifth power device W5 and a sixth power device W6 arranged in series, each of which has a first end and a second end, and the first end of the fifth power device W5 is connected to the first end of the sixth power device W6; the third sub-branch includes a seventh power device W7 and an eighth power device W8 arranged in series, each of which has a first end and a second end, and the second end of the seventh power device W7 is connected to the second end of the eighth power device W8; one end of the fourth capacitor C4 is connected between the fifth power device W5 and the sixth power device W6, and the other end is connected between the seventh power device W7 and the eighth power device W8. The fifth power device W5, the sixth power device W6, the seventh power device W7 and the eighth power device W8 are all power electronic devices with a turn-off function, and the power electronic devices are one or more turn-off devices such as IGBT, IGCT, IEGT, GTO or MOSFET.

[0059] Alternatively, if Fig. 9 As shown, the power unit 11 may be a power electronic unit composed of a ninth branch. The ninth branch includes at least one branch composed of a second sub-branch, a third sub-branch and a third resistor R3, and at least one branch is arranged in series.

[0060] The second sub-branch includes a fifth power device W5 and a sixth power device W6 arranged in series, each of which has a first end and a second end, and the first end of the fifth power device W5 is connected to the first end of the sixth power device W6; the third sub-branch includes a seventh power device W7 and an eighth power device W8 arranged in series, each of which has a first end and a second end, and the second end of the seventh power device W7 is connected to the second end of the eighth power device W8; one end of the third resistor R3 is connected between the fifth power device W5 and the sixth power device W6, and the other end is connected between the seventh power device W7 and the eighth power device W8. The fifth power device W5, the sixth power device W6, the seventh power device W7 and the eighth power device W8 are all power electronic devices with a turn-off function, and the power electronic devices are one or more turn-off devices such as IGBT, IGCT, IEGT, GTO or MOSFET.

[0061] Alternatively, if Fig.10 As shown, the power unit 11 may be a power electronic unit consisting of a tenth branch and an eleventh branch.

[0062] Among them, the tenth branch is provided with a ninth power device W9, the eleventh branch is provided in parallel with the tenth branch, the eleventh branch includes a tenth power device W10, a second inductor L2, a fifth capacitor C5 and a second charging circuit P2, and the tenth power device W10, the second inductor L2 and the fifth capacitor C5 are provided in series, and the second charging circuit P2 is provided in parallel with the fifth capacitor C5. The ninth power device W9 and the tenth power device W10 are power electronic devices with a turn-off function, and the power electronic devices are one or more turn-off devices such as IGBT, IGCT, IEGT, GTO or MOSFET.

[0063] The above-mentioned second control valve V12 is a low-pressure shutoff valve with a unidirectional voltage-controlled output capability or a unidirectional controllable shutoff function. It is mainly used to shut off the main branch current and provide it with a reverse voltage to ensure that the thyristor valve of the main branch has sufficient shutoff time for reliable shutoff. The present application does not limit the topological form of the second control valve V12, as long as it is a topological form with a unidirectional voltage-controlled output or a unidirectional controllable shutoff function. The topological form of the power unit can also be a combination of a power electronic device without a reverse blocking function and a diode. A single-stage power electronic device without a reverse blocking function and a single-stage diode and a buffer component can be used to form a multi-stage series structure. A combination of a multi-stage power electronic device without a reverse blocking function and its buffer component and a multi-stage diode and its buffer component can be connected in series. A combination of a multi-stage power electronic device without a reverse blocking function and its buffer component and a multi-stage diode and its buffer component can also be connected in series with a multi-stage power electronic device without a reverse blocking function and a multi-stage diode alternately in series. Of course, it can also be other topological forms, which are not specifically limited here. Those skilled in the art can determine according to actual needs.

[0064] Optionally, the second control valve V12 may further include: at least one buffer component, which is arranged in parallel in the power unit 11. For those skilled in the art, they can know the parallel connection mode of the buffer component and the power unit 11, which is not specifically limited here. Fig.11 As shown, the above-mentioned buffer component is composed of one or more forms of components such as capacitors, RC circuits, diodes, inductors or lightning arresters. At least one power unit 11 can form a full-bridge circuit with at least one buffer component to ensure stable operation of the second control valve V12.

[0065] Specifically, Fig.11As shown, the buffer component can be a first buffer branch composed of a sixth capacitor; it can be a second buffer branch composed of a fourth resistor and a seventh capacitor in series; it can be a third buffer branch composed of a fourth resistor and a seventh capacitor in parallel; it can be a fourth buffer branch RCD1 composed of a fifth resistor and a third diode in parallel and an eighth capacitor in series; it can be a fifth buffer branch RCD2 composed of a sixth resistor and a ninth capacitor in parallel and a fourth diode in series; it can also be a sixth buffer branch composed of a lightning arrester; it can also be a seventh buffer branch composed of multiple of the above-mentioned first buffer branch, second buffer branch, third buffer branch, fourth buffer branch, fifth buffer branch and sixth buffer branch in parallel.

[0066] According to an embodiment of the present invention, a forced commutation hybrid converter topology is provided, and the topology is connected to an AC power grid through a converter transformer. The forced commutation hybrid converter topology includes a three-phase six-bridge arm circuit, each phase bridge arm includes an upper bridge arm and a lower bridge arm, and at least one of the upper bridge arm or the lower bridge arm is provided with the active commutation unit described in the above embodiment.

[0067] Specifically, Fig.14 The forced commutation hybrid converter topology structure includes three upper bridge arms and three lower bridge arms. Each active commutation unit acts as a commutation valve. Fig.14 The forced commutation hybrid converter topology structure includes a converter valve V1, a converter valve V2, a converter valve V3, a converter valve V4, a converter valve V5 and a converter valve V6. The main branches of the three upper bridge arms include thyristor valves V11, V31, V51 and second control valves V12, V32 and V52, respectively, and the auxiliary branches of the three upper bridge arms include first control valves V13, V33 and V53, respectively. The main branches of the three lower bridge arms include thyristor valves V21, V41, V61 and second control valves V22, V42 and V62, respectively, and the auxiliary branches of the three lower bridge arms include first control valves V23, V43 and V63, respectively. The thyristor valves, the first control valves and the second control valves are controlled to be turned off and on by controlling the trigger control system.

[0068] Specifically, Fig.15 The forced commutation hybrid converter topology structure includes three upper bridge arms and three lower bridge arms. Each active commutation unit acts as a commutation valve. Fig.15The forced commutation hybrid converter topology structure includes a converter valve V1, a converter valve V2, a converter valve V3, a converter valve V4, a converter valve V5 and a converter valve V6. The main branches of the three upper bridge arms include thyristor valves V11, V31 and V51 respectively, and the auxiliary branches of the three upper bridge arms include first control valves V13, V33, V53 and second control valves V12, V32, V52 respectively. The main branches of the three lower bridge arms include thyristor valves V21, V41, V61 respectively, and the auxiliary branches of the three lower bridge arms include first control valves V23, V43, V63 and second control valves V22, V42, V62 respectively. The thyristor valve, the first control valve and the second control valve are controlled to be turned off and on by controlling the trigger control system.

[0069] Specifically, Fig.16 The hybrid converter topology shown in the figure includes three upper bridge arms and three lower bridge arms. Each active commutation unit acts as a commutation valve. Fig.16 The forced commutation hybrid converter topology structure includes a converter valve V1, a converter valve V2, a converter valve V3, a converter valve V4, a converter valve V5 and a converter valve V6. The main branches of the three upper bridge arms include thyristor valves V11, V31 and V51 respectively, and the auxiliary branches of the three upper bridge arms include first control valves V13, V33 and V53 respectively. The main branches of the three lower bridge arms include thyristor valves V21, V41 and V61 respectively, and the auxiliary branches of the three lower bridge arms include first control valves V23, V43 and V63 respectively. One end of the commutation control valve Va2, Vb2 and Vc2 is connected between the upper bridge arm and the lower bridge arm respectively, and the other end is connected to the output end of the converter transformer respectively. The commutation control valve Va2, Vb2 and Vc2 can have both the forward and reverse current shutoff function and the forward and reverse voltage output function. It can be a forward and reverse series connection of the second control valve V12 in the above embodiment, so as to be shared by the upper and lower bridge arms of each phase, reducing the number of second control valves and simplifying the topological structure of the hybrid converter. The thyristor valve, the first control valve and the second control valve are controlled by controlling the trigger control system to turn on and off, thereby realizing the reliable shutoff of the main branch and the active commutation of the entire bridge arm.

[0070] The above-mentioned forced commutation hybrid converter topology realizes reliable shutoff of the main branch and active commutation of the entire bridge arm by connecting an auxiliary branch that can provide reverse voltage and self-shutoff capability in parallel on the basis of the thyristor valve. Among them, the auxiliary branch is composed of a second control valve with bidirectional pressure-bearing capacity in series, that is, a shutoff valve is introduced for each bridge arm.

[0071] The forced commutation hybrid converter topology provided in this embodiment includes a three-phase six-bridge arm circuit, each phase bridge arm includes an upper bridge arm and a lower bridge arm, and at least one upper bridge arm or a lower bridge arm is provided with an active commutation unit. The second control valve in the active commutation unit can shut off the main branch current in advance and provide a reverse voltage at the same time, thereby increasing the commutation voltage-time area of ​​the main branch thyristor valve, ensuring its reliable shutdown, avoiding the problem of commutation failure, and thus ensuring the stable and safe operation of the power grid.

[0072] In this embodiment, a forced commutation control method is provided, which can be used for the above-mentioned forced commutation hybrid converter topology structure. Taking the second control valve V12 being set in the auxiliary branch as an example, the forced commutation control method includes the following steps:

[0073] (1) Turn on the thyristor valve of the main branch of the i-th bridge arm of the hybrid converter topology.

[0074] (2) Turning on the first control valve and the second control valve of the auxiliary branch of the i-th bridge arm of the hybrid converter topology structure.

[0075] (3) Turn off the first control valve and the second control valve of the auxiliary branch of the i-th bridge arm of the hybrid converter topology structure.

[0076] (4) After one control cycle, the thyristor valve of the main branch of the i-th bridge arm of the hybrid converter topology is turned on, where i∈[1,6].

[0077] Specifically, Fig.17 The figure shows the valve current flow path of the hybrid converter topology under normal operating conditions. The main branch is subjected to voltage and current stress periodically, and the auxiliary branch is always in the off state, and only bears voltage stress when the thyristor valve of the main branch is turned off. The first control valve and the second control valve of the auxiliary branch of the i-th bridge arm of the hybrid converter topology remain in the off state, and the thyristor valve of the main branch of the i-th bridge arm of the hybrid converter topology is turned on, thereby realizing that the hybrid converter topology with forced commutation can work in the normal commutation operation mode, that is, in the temporary commutation operation mode, the auxiliary branch is in the off state when the hybrid converter is operating normally, and only bears voltage stress, which reduces the increase in converter loss under long-term operation.

[0078] When a commutation failure or an AC short circuit fault occurs, the first control valve and the second control valve of the auxiliary branch of the i-th bridge arm of the hybrid converter topology structure are turned on; the current of the main branch is forced to be transferred to the auxiliary branch, and when the current transfer is completed, the first control valve and the second control valve of the auxiliary branch of the i-th bridge arm of the hybrid converter topology structure are turned off to realize the forced commutation of the hybrid converter. After a control cycle, the step of turning on the thyristor valve of the main branch of the i-th bridge arm of the hybrid converter topology structure is returned, and the main branch continues to operate independently and normally, thereby ensuring that the auxiliary branch only bears the turn-off voltage stress when a fault occurs, reducing device loss, and thus extending the service life of the device.

[0079] Fig.18 The trigger control timing in normal operation mode is given, and t0 in the figure represents the initial trigger time.

[0080] Fig.19 When the main branch is commutating to the auxiliary branch, the V1 valve is closed and the auxiliary branch begins to bear voltage stress. This process is divided into three stages: Fig.19 (Left) The commutation stage from the main branch to the auxiliary branch. In this stage, the auxiliary branch receives a trigger signal and turns on. Then, the auxiliary branch valves V12 and V13 receive a turn-on signal, transferring the current of the main branch to the auxiliary branch and applying a reverse voltage to the main branch. Fig.19 (Middle) is the auxiliary branch current flow stage, in which the main branch has been completely shut down and the main branch current has been completely transferred to the auxiliary branch; Fig.19 (Right) is the auxiliary branch shut-off stage. In this stage, when the shut-off signal is received, the auxiliary branch V13 valve is shut off first. At this time, the V1 valve is in the shut-off state to withstand the positive voltage. Then, before or at the same time as the V11 valve of the main branch is opened in the next control cycle, the V12 valve is shut off. The above operation process can be put into operation when a commutation fault occurs or a commutation fault is predicted.

[0081] Fig. 20 The invention discloses a trigger control timing of a hybrid converter topology structure with forced commutation when a commutation failure or an AC short circuit occurs. Fig. 20 In t f After the failure of the commutation from the V1 valve to the V3 valve is detected, the auxiliary branch V13 valve is turned on after the first preset time Δt1, and the auxiliary branch V12 valve is turned on after the second preset time Δt2, and the commutation process from the main branch to the auxiliary branch is performed, and Δt2≥Δt1≥0. The main branch current I11 gradually decreases to zero, and the auxiliary branch current I12 gradually increases. After the third preset time Δt3, the auxiliary branch V13 valve is turned off, and the time from the main branch current passing through zero to the V13 valve being turned off is the thyristor valve turn-off time t off , where t offIt is greater than the minimum off time of the thyristor valve to ensure that the thyristor valve V11 has enough time to shut down. After the auxiliary branch V13 valve is shut down, the auxiliary branch current will be commutated to the V3 valve until it reaches the DC current Id, thus completing the commutation from the V1 valve to the V3 valve, successfully resisting the commutation failure fault, and then shutting down the auxiliary branch V12 valve before the V11 valve is opened in the next control cycle. Starting this operation mode when it is predicted that a commutation failure will occur or when a commutation failure is detected can successfully avoid the commutation failure. When the converter commutation process returns to normal, the operation mode is exited, the auxiliary branch remains in the off state, and the main branch operates independently and normally.

[0082] In the event of commutation failure or short circuit fault, the hybrid converter topology is controlled to start the forced commutation operation mode to avoid the occurrence of commutation failure, and the forced commutation operation mode is exited when the commutation process of the hybrid converter returns to normal. The auxiliary branch continues to remain in the off state, and the main branch operates independently and normally, thereby ensuring that the auxiliary branch only bears the off-voltage stress in the event of a fault, reducing device losses and thereby extending the service life of the device.

[0083] Fig.21 The figure shows the control triggering sequence of the forced commutation hybrid converter topology structure when the commutation failure or short circuit fault is detected in advance, and the control triggering sequence of each valve when the main branch and the auxiliary branch of the V1 valve are operated periodically and alternately. The specific operation process is as follows Fig.19 As shown. At the beginning of the phase change of the V1 valve and the V3 valve, that is, the V1 valve trigger pulse Sg1 is delayed by 120°, or the auxiliary branch V13 valve is triggered near this moment, and the auxiliary branch V12 valve is opened after a short time (such as 1s, 5s, etc.), so as to realize the commutation from the main branch to the auxiliary branch. After the main branch current passes through zero, the main branch V11 valve is closed and bears the reverse voltage, and the time from the main branch current passing through zero to the auxiliary branch V13 valve being closed is the thyristor valve closing time t off , and t off The minimum off time of the thyristor valve is greater than that of the thyristor valve to ensure its reliable off. At this point, the current of the V1 valve is completely transferred to the auxiliary branch. After Δt, the auxiliary branch V13 valve begins to shut down, and the V1 valve begins to bear the positive voltage. Then, before or at the same time as the V11 valve opens in the next working cycle, the auxiliary branch V12 valve is shut down. In this operating mode, the main branch and the auxiliary branch in the bridge arm of the hybrid converter topology structure with forced commutation operate alternately periodically. On the basis of having the ability to resist commutation failure, there is no need to predict commutation failure. At the same time, the hybrid converter can be put into a small off-angle operation mode to reduce the reactive power consumption of the hybrid converter.

[0084] The forced commutation control method provided in this embodiment can not only resist commutation failure, but also eliminate the need to predict commutation failure through the periodic alternating operation of the main branch and the auxiliary branch. At the same time, it ensures that the hybrid converter operates in a small shut-off angle operation mode, reducing the reactive power consumption of the hybrid converter.

[0085] Although the embodiments of the present invention have been described in conjunction with the accompanying drawings, those skilled in the art may make various modifications and variations without departing from the spirit and scope of the present invention, and such modifications and variations are all within the scope defined by the appended claims.

Claims

1. An active commutation unit, arranged in the bridge arm circuit of the converter, one end of which is connected to the output end of the converter transformer and the other end is connected to the DC bus, characterized in that: include: A main branch, wherein a thyristor valve is arranged on the main branch; An auxiliary branch is arranged in parallel with the main branch, and a first control valve is arranged on the auxiliary branch, and the first control valve has a forward current controllable shutoff function and a forward and reverse voltage blocking function; a second control valve connected to the thyristor valve of the main branch or to the first control valve of the auxiliary branch, the second control valve being used to transfer current from the main branch to the auxiliary branch, the second control valve comprising at least one power unit, the power unit being used for controllable shutoff of forward current and controllable blocking of forward and reverse voltages; Wherein, the power unit comprises: A first branch, wherein a first power device and a first diode are arranged on the first branch, and the first power device is connected in series with the first diode; A second branch, the second branch is arranged in parallel with the first branch, the second branch comprises at least one first capacitor, and the at least one first capacitor is arranged in series; a third branch, the third branch being connected in parallel with the first branch and the second branch respectively, the third branch being provided with a second power device and a first resistor, the second power device being connected in series with the first resistor; the first power device and the second power device being both power electronic devices with a turn-off function; The control method for the hybrid converter topology structure formed by the active commutation unit includes: Turning on the thyristor valve of the main branch of the i-th bridge arm of the hybrid converter topology; Turning on a first control valve and a second control valve of an auxiliary branch of an i-th bridge arm of a hybrid converter topology structure; Turning off the first control valve and the second control valve of the auxiliary branch of the i-th bridge arm of the hybrid converter topology structure; After one control cycle, the thyristor valve of the main branch of the i-th bridge arm of the hybrid converter topology is turned on, where: .

2. An active commutation unit and control method, wherein the active commutation unit is arranged in the bridge arm circuit of the converter, one end of which is connected to the output end of the converter transformer and the other end is connected to the DC bus, characterized in that: include: A main branch, wherein a thyristor valve is arranged on the main branch; An auxiliary branch is arranged in parallel with the main branch, and a first control valve is arranged on the auxiliary branch, and the first control valve has a forward current controllable shutoff function and a forward and reverse voltage blocking function; a second control valve connected to the thyristor valve of the main branch or to the first control valve of the auxiliary branch, the second control valve being used to transfer current from the main branch to the auxiliary branch, the second control valve comprising at least one power unit, the power unit being used for controllable shutoff of forward current and controllable blocking of forward and reverse voltages; Wherein, the power unit comprises: a tenth branch, wherein a ninth power device is disposed on the tenth branch; an eleventh branch, the eleventh branch being arranged in parallel with the tenth branch; the eleventh branch comprising a tenth power device, a second inductor, a fifth capacitor and a second charging circuit, wherein the tenth power device, the second inductor and the fifth capacitor are arranged in series, and the second charging circuit is arranged in parallel with the fifth capacitor; The ninth power device and the tenth power device are power electronic devices with a shutoff function; The control method for the hybrid converter topology structure formed by the active commutation unit includes: Turning on the thyristor valve of the main branch of the i-th bridge arm of the hybrid converter topology; Turning on a first control valve and a second control valve of an auxiliary branch of an i-th bridge arm of a hybrid converter topology structure; Turning off the first control valve and the second control valve of the auxiliary branch of the i-th bridge arm of the hybrid converter topology structure; After one control cycle, the thyristor valve of the main branch of the i-th bridge arm of the hybrid converter topology is turned on, where: .

3. The active commutation unit according to claim 1 or 2, characterized in that: The second control valve further comprises: at least one buffer component, the buffer component being arranged in parallel in the power device; The buffer component comprises: a first buffer branch composed of a sixth capacitor; or, a second buffer branch connected in series with a fourth resistor and a seventh capacitor; or, a third buffer branch in which the fourth resistor and the seventh capacitor are connected in parallel; Or, a fourth buffer branch is formed by connecting a fifth resistor and a third diode in parallel and then connecting an eighth capacitor in series; Or, a sixth resistor and a ninth capacitor are connected in parallel, and then connected in series with the fourth diode to form a fifth buffer branch; or, a sixth buffer branch consisting of a lightning arrester; Or, a seventh buffer branch formed by connecting in parallel a plurality of the first buffer branch, the second buffer branch, the third buffer branch, the fourth buffer branch, the fifth buffer branch and the sixth buffer branch.

4. A forced commutation hybrid converter topology structure, the topology structure is connected to the AC power grid through a converter transformer, the topology structure includes a three-phase six-bridge arm circuit, each phase bridge arm includes an upper bridge arm and a lower bridge arm, characterized in that: An active commutation unit according to any one of claims 1 to 3 is arranged on at least one upper bridge arm or a lower bridge arm.

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