Active commutation unit, forced commutation hybrid converter topology and method

Abstract

The present invention discloses a hybrid converter topology structure and method with 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 converter transformer and the other end is connected to the DC bus, including: a main branch, which is provided with a thyristor valve; an auxiliary branch, which is arranged in parallel with the main branch, and a first control valve and a second control valve are arranged in sequence on the auxiliary branch, the first control valve has a unidirectional voltage output controllable shutdown function, and the second control valve has a forward current controllable shutdown function and a forward and reverse voltage blocking function. The hybrid converter topology structure with forced commutation is connected to the AC power grid through a converter transformer, and 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, and at least one upper bridge arm or a lower bridge arm is provided with an active commutation unit. By implementing the present invention, reliable shutdown of the main branch and active commutation of the entire bridge arm are achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of commutation in power electronics, and in particular to a hybrid converter topology structure and method 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] 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 and method with active commutation units and forced commutation to solve the problem that commutation failure affects the stable and safe 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 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, and on which a first control valve and a second control valve are arranged in sequence along the direction from the converter transformer to the DC bus on the auxiliary branch, the first control valve has a unidirectional voltage output controllable shutdown function, and the second control valve has a forward current controllable shutdown function and a forward and reverse voltage blocking function.

[0006] In combination with the first aspect, in a first implementation of the first aspect, the thyristor valve includes: at least one thyristor, the at least one thyristor is arranged in series; and at least one first buffer component is connected in parallel or in series with the at least one thyristor.

[0007] In combination with the first aspect, in a second implementation of the first aspect, the first control valve includes: at least one first power unit, the at least one first power unit is arranged in series; and at least one second buffer component is connected in parallel with the at least one first power unit.

[0008] In combination with the second embodiment of the first aspect, in the third embodiment of the first aspect, the first power unit includes: a first branch, on which a first power device and a diode are provided, and the first power device is a fully controlled power electronic device; a second branch, connected in parallel with the first branch, on which a first capacitor element and the first power device are provided, and the first power device and the first capacitor element are connected in series.

[0009] In combination with the second embodiment of the first aspect, in the fourth embodiment of the first aspect, the first power unit includes: a third branch, the third branch is a full-bridge circuit composed of four second power devices connected; the second power device is a fully controlled power electronic device; a fourth branch, the fourth branch is provided with a second capacitor element, and the second capacitor element is connected in parallel between the upper half bridge and the lower half bridge of the full-bridge circuit.

[0010] In combination with the first aspect, in a fifth implementation of the first aspect, the second control valve includes: at least one second power unit, the at least one second power unit is arranged in series; and at least one third buffer component is connected in parallel with the at least one second power unit.

[0011] In combination with the fifth embodiment of the first aspect, in the sixth embodiment of the first aspect, the second power unit includes: a fifth branch, on which a third power device and a first diode are arranged, and the third power device is connected in series with the first diode; or, a sixth branch, on which at least one third power device is arranged, and the at least one third power device is arranged in series; the third power device is a power electronic device that does not have a reverse blocking function; a seventh branch, connected in series with the sixth branch; and at least one second diode is arranged on the seventh branch, and the at least one second diode is arranged in series.

[0012] In combination with the fifth implementation of the first aspect, in the seventh implementation of the first aspect, the second power unit includes: an eighth branch, the eighth branch is a full-bridge circuit composed of multiple fourth power devices connected; the fourth power device is a fully controlled power electronic device.

[0013] In combination with the fifth embodiment of the first aspect, in the eighth embodiment of the first aspect, the second power unit includes: at least one ninth branch, the ninth branch includes a first sub-branch, a second sub-branch and a third sub-branch; the first sub-branch, the second sub-branch, the third sub-branch and the third buffer component constitute an H-bridge circuit; the first sub-branch is provided with a plurality of third diodes connected in series; the second sub-branch is connected in parallel between the first sub-branch and the third sub-branch, and the second sub-branch is provided with a plurality of fifth power devices connected in series, and the fifth power device is a fully controlled power electronic device; the third sub-branch is provided with a plurality of fourth diodes connected in series.

[0014] In combination with the first embodiment or the second embodiment or the fifth embodiment of the first aspect, in the ninth embodiment of the first aspect, the first buffer component, the second buffer component and the third buffer component all include: a first buffer branch composed of a capacitor; or a second buffer branch in which a resistor and the capacitor are connected in series; or a third buffer branch in which the capacitor and the resistor are connected in parallel; or a fourth buffer branch in which the resistor and a fifth diode are connected in parallel and then connected in series with the capacitor; or a fifth buffer branch in which the resistor and the capacitor are connected in parallel and then connected in series with the fifth 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.

[0015] 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.

[0016] According to the third aspect, an embodiment of the present invention provides a forced commutation control method for a hybrid converter topology with forced commutation as described in the second aspect, comprising the following steps: turning on the thyristor valve of the main branch of the i-th bridge arm of the hybrid converter topology with forced commutation; 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 with forced commutation; 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 with forced commutation; and after one control cycle, returning to turning on the thyristor valve of the main branch of the i-th bridge arm of the hybrid converter topology with forced commutation, wherein i∈[1,6].

[0017] In combination with the third aspect, in the first implementation manner of the third aspect, the method also includes: when a commutation failure or a short-circuit fault is detected in the i-th bridge arm of the hybrid converter topology structure, obtaining the duration of the commutation failure or the short-circuit fault; when the duration reaches a first preset duration, turning on the second control valve of the auxiliary branch of the i-th bridge arm, and when the duration reaches a second preset duration, turning on the first control valve of the auxiliary branch of the i-th bridge arm, to commutate the main branch to the auxiliary branch, wherein the second preset duration is greater than or equal to the first preset duration; when the current of the main branch of the i-th bridge arm of the hybrid converter topology structure is reduced to zero, and the duration reaches a third preset duration, turning off the second control valve of the auxiliary branch of the i-th bridge arm, wherein the third preset duration is greater than the second preset duration; when the thyristor valve of the main branch of the i-th bridge arm is turned on in the next control cycle, the first control valve of the auxiliary branch of the i-th bridge arm is turned off.

[0018] In combination with the first implementation manner of the third aspect, in the second implementation manner of the third aspect, the method further includes: the main branch and the auxiliary branch of the i-th bridge arm of the forced-commutation hybrid converter topology structure operate periodically and alternately.

[0019] The technical solution of the present invention has the following advantages:

[0020] 1. The active commutation unit provided in the embodiment of the present invention comprises a main branch and an auxiliary branch connected in parallel. The main branch is provided with a thyristor valve, which has a large flow capacity and carries the normal operating current; the first control valve of the auxiliary branch has a forward current controllable shutoff function, and the second control valve has a forward and reverse voltage blocking capability. The active commutation unit utilizes the advantages of the thyristor and the first control valve and the second control valve, and adopts two branches in parallel. The current transfer is realized through the first control valve in the auxiliary branch. The second control valve is used to withstand a large shutoff voltage stress in the event of a fault, and does not need to withstand current stress for a long time, thereby avoiding the increase of device loss and improving the utilization rate of the first control valve and the second control valve. By connecting an auxiliary branch that can provide reverse voltage and self-shutoff capability in parallel on the basis of the thyristor valve, the reliable shutoff of the main branch and the active commutation of the entire bridge arm are realized. When the active commutation unit operates normally, the auxiliary branch can remain in the off state and only needs to bear voltage stress; when the active commutation unit fails to commutate, the auxiliary branch is immediately turned on, the first control valve can transfer the current to the auxiliary branch and provide a reverse voltage to the thyristor valve of the main branch, and the second control valve can replace the main branch to complete the commutation, thereby realizing the auxiliary commutation function in a shorter time and avoiding the occurrence of commutation failure.

[0021] 2. The forced commutation hybrid converter topology provided in the embodiment of the present invention comprises a three-phase six-bridge arm circuit, each phase bridge arm comprises 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 first control valve of the auxiliary branch of 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.

[0022] 3. 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 of the auxiliary branch of the active commutation unit can quickly transfer the commutation current and flexibly control the commutation time. When the commutation fails, the current of the main branch is transferred to the auxiliary branch, and the commutation between the two bridge arms is completed through the second control valve, which speeds up the recovery time of the converter after the commutation failure.

[0023] 4. The forced commutation hybrid converter topology structure provided in the embodiment of the present invention comprises a three-phase six-bridge arm circuit, each phase bridge arm comprises 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 forced commutation hybrid converter topology structure can turn on the auxiliary branch at any time, effectively reducing the loss of the main branch, and can realize low voltage and low turn-off angle operation, thereby reducing the reactive power on the inverter side.

[0024] 5. The control method for forced commutation provided in the embodiment of the present invention is to keep 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 in the off state, and turn on the thyristor valve of the main branch of the i-th bridge arm of the hybrid converter topology structure, thereby realizing that the hybrid converter topology structure 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, thereby reducing the increase in converter loss under long-term operation. 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 one 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 normally independently, thereby ensuring that the auxiliary branch only bears the turn-off voltage stress when a fault occurs, reducing device losses and thereby extending the service life of the device.

[0025] 6. The forced commutation control method provided in the embodiment of the present invention controls the hybrid converter topology structure to start the forced commutation operation mode when a commutation failure or a short circuit fault occurs, thereby avoiding the occurrence of a commutation failure, and exits the forced commutation operation mode 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 is only subjected to the off-voltage stress when a fault occurs, reducing device losses and thereby extending the service life of the device.

[0026] 7. The forced commutation control method provided in the embodiment of the present invention 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] 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.

[0028] Figure 1 is a structural block diagram of an active commutation unit according to an embodiment of the present invention;

[0029] Figure 2 is a structural block diagram of a thyristor valve according to an embodiment of the present invention;

[0030] Figure 3 is a structural block diagram of a first control valve according to an embodiment of the present invention;

[0031] Figure 4 is another structural block diagram of a first control valve according to an embodiment of the present invention;

[0032] Figure 5 is a structural block diagram of a second control valve according to an embodiment of the present invention;

[0033] Figure 6 is a structural block diagram of a second power unit according to an embodiment of the present invention;

[0034] Figure 7 is another structural block diagram of a second control valve according to an embodiment of the present invention;

[0035] Figure 8 is another structural block diagram of a second control valve according to an embodiment of the present invention;

[0036] Fig. 9is a structural block diagram of a buffer component according to an embodiment of the present invention;

[0037] Fig.10 is a block diagram of a hybrid converter topology according to an embodiment of the present invention;

[0038] Fig.11 is a flow chart of a forced commutation control method according to an embodiment of the present invention;

[0039] Fig.12 is a current flow path of the V1 valve bridge arm in a normal operating state according to an embodiment of the present invention;

[0040] Fig.13a is a trigger control timing sequence of a normal operating state according to an embodiment of the present invention;

[0041] Fig.13b is a trigger control timing of a commutation failure or a short circuit fault according to an embodiment of the present invention;

[0042] Fig.14a is a current flow path for commutation from a main branch to an auxiliary branch according to an embodiment of the present invention;

[0043] Fig.14b is a current flow path during the auxiliary branch current flow stage according to an embodiment of the present invention;

[0044] Fig.14c is a current flow path during the auxiliary branch shutdown phase according to an embodiment of the present invention;

[0045] Fig.15 This is the periodic trigger control timing of the main branch and the auxiliary branch according to the embodiment of the present invention. DETAILED DESCRIPTION

[0046] 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.

[0047] 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.

[0048] Based on this, the technical solution of the present invention utilizes the advantages of thyristors and control valves with shutoff capabilities. By shutting down the control valve in advance to ensure that the thyristor valve has sufficient shutoff time to restore the shutoff capability, the converter can be shut down reliably, avoiding commutation failure that affects the stable and safe operation of the power grid.

[0049] 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, the active commutation unit includes: a main branch 1 and an auxiliary branch 2. The main branch 1 is provided with a thyristor valve 11; the auxiliary branch 2 is provided in parallel with the main branch 1, and a first control valve 21 and a second control valve 22 are provided in sequence on the auxiliary branch 2 along the direction from the converter transformer to the DC bus. The arrangement order of the first control valve 21 and the second control valve 22 is not specifically limited here. The first control valve 21 has a unidirectional voltage output controllable shutoff function, and the second control valve 22 has a forward current controllable shutoff function and a forward and reverse voltage blocking function.

[0050] The active commutation unit provided in this embodiment utilizes the advantages of thyristors and the first control valve that can be turned off and the second control valve that can be turned off, adopts two branches in parallel, and realizes the current transfer through the first control valve in the auxiliary branch. The second control valve is used to withstand a large turn-off voltage stress when a fault occurs, and does not need to withstand current stress for a long time, thereby avoiding the increase of device loss and improving the utilization rate of the first control valve and the second control valve. By connecting an auxiliary branch that can provide reverse voltage and self-shutoff capability in parallel on the basis of the thyristor valve, the main branch can be reliably shut down and the active commutation of the entire bridge arm can be achieved. When the active commutation unit is operating normally, the auxiliary branch can remain in the off state and only needs to bear voltage stress; when the active commutation unit fails to commutate, the auxiliary branch is immediately turned on, and the first control valve can transfer the current to the auxiliary branch and provide a reverse voltage to the thyristor valve of the main branch, and the second control valve can replace the main branch to complete the commutation, thereby realizing the auxiliary commutation function in a relatively short time and avoiding the occurrence of commutation failure.

[0051] Optionally, the thyristor valve 11 includes at least one thyristor 111 and a first buffer component 112 connected in parallel or in series with the thyristor 111, wherein at least one thyristor is arranged in series, and the first buffer component 112 is used to prevent the thyristor device from being damaged by high voltage and high current. Figure 2 As shown, the thyristor valve 11 includes at least one thyristor 111 and first buffer components 112 respectively connected in parallel with the thyristors 111 .

[0052] Optionally, the first control valve 21 includes at least one first power unit 211 and second buffer components connected in parallel with the first power units 211 (the parallel connection method is known to those skilled in the art, but is not shown in the figure), wherein at least one first power unit is arranged in series, and the second buffer component is used to limit voltage and current stress.

[0053] Specifically, Figure 3 As shown, the first power unit 211 may be a power electronic unit consisting of a first branch and a second branch.

[0054] The first branch is provided with a first power device; the second branch is connected in parallel with the first branch, and the second branch is provided with a first capacitor element and the first power device, and the first power device and the first capacitor element are connected in series. The first power device is a fully controlled power electronic device, and the fully controlled power electronic device is one or more turn-off devices such as IGBT, IGCT, IEGT, GTO or MOSFET.

[0055] Specifically, Figure 4 As shown, the first power unit 211 may also be a power electronic unit composed of a third branch and a fourth branch.

[0056] The third branch is a full-bridge circuit composed of four second power devices connected; the fourth branch is provided with a second capacitor element, which is connected in parallel between the upper half bridge and the lower half bridge of the full-bridge circuit. The second power device is a fully controlled power electronic device, which is one or more of IGBT, IGCT, IEGT, GTO or MOSFET.

[0057] The first control valve 21 is a low-pressure shutoff valve with a unidirectional voltage-controlled output capability, and is mainly used to shut off the main branch current and provide a reverse voltage therefor, ensuring that the thyristor valve of the main branch has sufficient shutoff time for reliable shutoff, and the required number of series-connected first control valves 21 is small, resulting in a low total loss. The present application does not limit the topology of the first control valve 21, as long as it is a topology with the function of unidirectional voltage-controlled output.

[0058] Optionally, the second control valve 22 includes at least one second power unit 221 and third buffer components 222 respectively connected in parallel with the second power units 221, wherein at least one second power unit 221 is arranged in series, and the third buffer component 222 is used to limit voltage and current stress.

[0059] Specifically, Figure 5 As shown, the second power unit 221 may be a power electronic unit composed of a fifth branch.

[0060] The fifth branch is provided with a third power device and a first diode, and the third power device is arranged in series with the first diode. The third power device is a power electronic device without a reverse blocking function, and the power electronic device without a reverse blocking function is one or more of IGBT, IGCT, IEGT, GTO or MOSFET. The power electronic device without a reverse blocking function is combined in series with the first diode to form a power electronic unit with reverse blocking and forward turn-off capabilities.

[0061] Specifically, Figure 6 As shown, the second power unit 221 may also be a power electronic unit composed of a sixth branch and a seventh branch.

[0062] The sixth branch is provided with at least one third power device, and the at least one third power device is arranged in series; the seventh branch is connected in series with the sixth branch, and the seventh branch is provided with at least one second diode, and the at least one second diode is arranged in series. Among them, the third power device is a power electronic device without a reverse blocking function, and the power electronic device without a reverse blocking function is one or more of IGBT, IGCT, IEGT, GTO or MOSFET.

[0063] The topological form of the above-mentioned second power unit is a power electronic device without reverse blocking function in combination with a first diode. It can be a multi-stage series structure formed by a single-stage power electronic device without reverse blocking function, a single-stage diode and a buffer component. It can be a combination of a multi-stage power electronic device without reverse blocking function and its buffer component and a combination of a multi-stage diode and its buffer component in series. It can also be a combination of a multi-stage power electronic device without reverse blocking function and a multi-stage diode alternately connected in series. Of course, it can also be other topological forms, which are not specifically limited here. Technical personnel in the field can determine according to actual needs.

[0064] Specifically, Figure 7As shown, the second power unit 221 can also be a power electronic unit composed of an eighth branch. The eighth branch is a full-bridge circuit composed of a plurality of fourth power devices connected, wherein the fourth power device is a fully controlled power electronic device, and the fully controlled power electronic device is one or more of IGBT, IGCT, IEGT, GTO or MOSFET.

[0065] The full-bridge circuits are connected in series in sequence to achieve forward and reverse current control, complete the transfer of the main branch current to the auxiliary branch at any time, and can withstand forward and reverse voltages. At the same time, each bridge arm in the full bridge is a single-stage structure or a multi-stage series structure composed of fully controlled power electronic devices and diodes. Of course, it can also be other topological forms, which are not specifically limited here, and technical personnel in this field can determine according to actual needs.

[0066] Specifically, Figure 8 As shown, the second power unit 221 can also be a power electronic unit composed of a ninth branch, the ninth branch including a first sub-branch, a second sub-branch and a third sub-branch. The first sub-branch, the second sub-branch, the third sub-branch and the third buffer component form an H-bridge circuit.

[0067] Among them, a plurality of third diodes connected in series are arranged on the first sub-branch; the second sub-branch is connected in parallel between the first sub-branch and the third sub-branch, and a plurality of fifth power devices connected in series are arranged on the second sub-branch, wherein the fifth power device is a fully controlled power electronic device, and the fully controlled power electronic device is one or more of IGBT, IGCT, IEGT, GTO or MOSFET; a plurality of fourth diodes connected in series are arranged on the third sub-branch. The fully controlled power electronic devices and diodes in the H-bridge circuit can be a single-stage structure or a multi-stage series structure, and the H-bridge circuits can be connected in series in sequence to realize the bidirectional flow and bidirectional shutdown functions.

[0068] The above-mentioned second control valve 22 is a high-pressure shut-off valve with the ability of controllable shutoff of forward current and blocking of forward and reverse voltages. The present application does not limit the topological form of the second control valve 22, as long as it is a topological form with the function of controllable shutoff of forward current and blocking of forward and reverse voltages.

[0069] Optionally, the auxiliary branch may be formed by connecting the first control valve 21 and the second control valve 22 in series, or may be formed by connecting units of the first control valve 21 and the second control valve 22 alternately in series.

[0070] Optionally, the first buffer component 112, the second buffer component and the third buffer component 222 are all composed of one or more forms of components such as capacitors, resistor-capacitor circuits, diodes, inductors or lightning arresters.

[0071] Specifically, Fig. 9As shown, the first buffer component 112, the second buffer component and the third buffer component 222 can be a first buffer branch composed of a capacitor; can be a second buffer branch composed of a resistor and a capacitor in series; can be a third buffer branch composed of a capacitor and a resistor in parallel; can be a fourth buffer branch RCD1 composed of a resistor and a fifth diode in parallel and a capacitor in series; can be a fifth buffer branch RCD2 composed of a resistor and a capacitor in parallel and a fifth diode in series; can also be a sixth buffer branch composed of a lightning arrester; 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.

[0072] According to an embodiment of the present invention, a forced commutation hybrid converter topology is provided, which is connected to an AC power grid via a converter transformer. Fig.10 As shown, the forced commutation hybrid converter topology structure 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.

[0073] Specifically, Fig.10 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.10 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; the auxiliary branches of the three upper bridge arms include first control valves V13, V33 and V53 respectively; the auxiliary branches of the three upper bridge arms include second control valves V12, V32 and V52 respectively; the main branches of the three lower bridge arms include thyristor valves V21, V41 and V61 respectively; the auxiliary branches of the three lower bridge arms include first control valves V23, V43 and V63 respectively; the auxiliary branches of the three lower bridge arms include second control valves V22, V42 and V62 respectively, and 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.

[0074] 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 has self-shutoff capability in parallel on the basis of the thyristor valve. Among them, the auxiliary branch is composed of a first control valve with reverse voltage capability and a second control valve with bidirectional pressure bearing capability in series, that is, a shutoff valve is introduced for each bridge arm.

[0075] The forced commutation hybrid converter topology structure 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 first control valve of the auxiliary branch of the active commutation unit can shut off the main branch current in advance and provide a reverse voltage at the same time to realize active commutation of the entire bridge arm. The forced commutation hybrid converter topology structure increases the commutation voltage-time area of ​​the main branch thyristor valve to ensure its reliable shutdown and avoid the problem of commutation failure, thereby ensuring the stable and safe operation of the power grid.

[0076] According to an embodiment of the present invention, an embodiment of a forced commutation control method is provided. It should be noted that the steps shown in the flowchart of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and although a logical order is shown in the flowchart, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0077] In this embodiment, a forced commutation control method is provided, which can be used for the above-mentioned forced commutation hybrid converter topology structure. Fig.11 FIG. 4 is a flow chart of a forced commutation control method according to an embodiment of the present invention. Fig.11 As shown, the process includes the following steps:

[0078] S21, turning on the thyristor valve of the main branch of the i-th bridge arm of the hybrid converter topology structure.

[0079] S22, 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.

[0080] S23, shutting 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.

[0081] S24, after one control cycle, turns on the thyristor valve of the main branch of the i-th bridge arm of the hybrid converter topology structure, where i∈[1,6].

[0082] Specifically, Fig.12 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 is only subjected to voltage stress when the thyristor valve of the main branch is turned off.

[0083] The control method for forced commutation provided in this embodiment is that 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 remain in the closed state, and the thyristor valve of the main branch of the i-th bridge arm of the hybrid converter topology structure is turned on, thereby realizing that the hybrid converter topology structure with forced commutation can operate in the normal commutation operation mode, that is, in the temporary commutation operation mode, the auxiliary branch is in the closed state when the hybrid converter is operating normally, and only bears voltage stress, thereby reducing the increase in converter loss under long-term operation.

[0084] 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.

[0085] Fig.13a The trigger control timing in normal operation mode is given, and t0 in the figure represents the initial trigger time.

[0086] Fig.14a , Fig.14b and Fig.14c 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.14a In the phase of commutation from the main branch to the auxiliary branch, the auxiliary branch receives a trigger signal and turns on, and then the auxiliary branch V12 valve and V13 valve 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.14b This is the auxiliary branch current flow stage, during which the main branch has been completely shut down and the main branch current has been fully transferred to the auxiliary branch; Fig.14c This is the auxiliary branch shut-off stage. When the shut-off signal is received in this stage, 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.

[0087] Fig.13b 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.13b In t fAfter 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 off It 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.

[0088] The forced commutation control method provided in the present embodiment controls the hybrid converter topology structure to start the forced commutation operation mode when a commutation failure or a short circuit fault occurs, thereby avoiding the occurrence of a commutation failure, and exits the forced commutation operation mode 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 is subjected to the off voltage stress only when a fault occurs, reducing device losses and thereby extending the device service life.

[0089] Fig.15 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.14a , Fig.14b and Fig.14c 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 offThe 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.

[0090] 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.

[0091] 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 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 and a second control valve are sequentially arranged on the auxiliary branch in a direction from the converter transformer to the DC bus, wherein the first control valve has a unidirectional voltage output controllable shutoff function, and the second control valve has a forward current controllable shutoff function and a forward and reverse voltage blocking function; Wherein, the first control valve comprises: at least one first power unit, the at least one first power unit is arranged in series; at least one second buffer component is connected in parallel with the at least one first power unit; the first power unit comprises: a first branch, the first branch is provided with a first power device, the first power device is a fully controlled power electronic device; a second branch is connected in parallel with the first branch, the second branch is provided with a first capacitor element and the first power device, the first power device and the first capacitor element are connected in series; Among them, the second control valve includes: at least one second power unit, which is arranged in series; at least one third buffer component, which is connected in parallel with the at least one second power unit; the second power unit includes: a ninth branch, which includes a first sub-branch, a second sub-branch and a third sub-branch; the first sub-branch, the second sub-branch, the third sub-branch and the third buffer component constitute an H-bridge circuit; the first sub-branch is provided with a plurality of third diodes connected in series; the second sub-branch is connected in parallel between the first sub-branch and the third sub-branch, and the second sub-branch is provided with a plurality of fifth power devices connected in series, and the fifth power device is a fully controlled power electronic device; the third sub-branch is provided with a plurality of fourth diodes connected in series.

2. The active commutation unit according to claim 1, characterized in that: The thyristor valve comprises: at least one thyristor, the at least one thyristor being arranged in series; At least one first buffer component is connected in parallel or in series with the at least one thyristor.

3. The active commutation unit according to claim 1, characterized in that: The first power unit comprises: A third branch, wherein the third branch is a full-bridge circuit formed by connecting four second power devices; the second power devices are fully controlled power electronic devices; A fourth branch is provided with a second capacitance element, and the second capacitance element is connected in parallel between the upper half bridge and the lower half bridge of the full-bridge circuit.

4. The active commutation unit according to claim 1, characterized in that: The second power unit comprises: A fifth branch, wherein a third power device and a first diode are arranged on the fifth branch, wherein the third power device is connected in series with the first diode; and the third power device is a power electronic device without a reverse blocking function; or, A sixth branch, wherein at least one of the third power devices is disposed on the sixth branch, and the at least one third power device is disposed in series; The seventh branch is connected in series with the sixth branch; the seventh branch is provided with at least one second diode, and the at least one second diode is arranged in series.

5. The active commutation unit according to claim 1, characterized in that: The second power unit comprises: The eighth branch is a full-bridge circuit composed of a plurality of fourth power devices connected together; the fourth power devices are fully controlled power electronic devices.

6. The active commutation unit according to claim 1 or 2, characterized in that: The first buffer component, the second buffer component and the third buffer component each include: A first buffer branch composed of a capacitor; or, a second buffer branch in which a resistor and the capacitor are connected in series; or, a third buffer branch in which the capacitor and the resistor are connected in parallel; Or, the resistor and the fifth diode are connected in parallel, and then connected in series with the capacitor to form a fourth buffer branch; Or, the resistor and the capacitor are connected in parallel and then connected in series with the fifth 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.

7. 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 6 is arranged on at least one upper bridge arm or a lower bridge arm.

8. A forced commutation control method for the forced commutation hybrid converter topology structure according to claim 7, characterized in that: The steps include: Turning on the thyristor valve of the main branch of the i-th bridge arm of the forced commutation 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 the forced-commutation 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 forced commutation hybrid converter topology structure; After one control cycle, the thyristor valve of the main branch of the i-th bridge arm of the forced-commutation hybrid converter topology structure is turned back on, where i∈[1,6].

9. The method according to claim 8, characterized in that Also includes: When a commutation failure or a short circuit fault occurs in the i-th bridge arm of the hybrid converter topology structure, the duration of the commutation failure or the short circuit fault is obtained; When the duration reaches a first preset duration, the second control valve of the auxiliary branch of the i-th bridge arm is turned on, and when the duration reaches a second preset duration, the first control valve of the auxiliary branch of the i-th bridge arm is turned on to commutate from the main branch to the auxiliary branch, wherein the second preset duration is greater than or equal to the first preset duration; When the current of the main branch of the i-th bridge arm of the hybrid converter topology structure decreases to zero and the duration reaches a third preset duration, the second control valve of the auxiliary branch of the i-th bridge arm is turned off, wherein the third preset duration is greater than the second preset duration; When the thyristor valve of the main branch of the i-th bridge arm is turned on in the next control cycle, the first control valve of the auxiliary branch of the i-th bridge arm is turned off.

10. The method according to claim 9, characterized in that Also includes: The main branch and the auxiliary branch of the i-th bridge arm of the forced-commutation hybrid converter topology structure operate alternately and periodically.

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