Capacitor-assisted commutation dual-converter parallel circuit, method, device, and system

Abstract

The present application provides a dual-converter parallel circuit with capacitor-assisted commutation, as well as a method, device, and system. The dual-converter parallel circuit with capacitor-assisted commutation includes a first converter, a second converter, a connecting circuit, and an auxiliary commutation capacitor circuit. The first converter includes three-phase six bridge arms, each bridge arm including a first half-controlled valve; the second converter includes three-phase six bridge arms, each bridge arm including a second valve; the connecting circuit includes four valves, the third valve connecting the positive pole of the DC bus of the first converter and the positive pole of the DC bus of the second converter, the fourth valve connecting the negative pole of the DC bus of the first converter and the negative pole of the DC bus of the second converter, the fifth valve connecting the positive pole of the DC bus of the first converter and the negative pole of the DC bus of the second converter, and the sixth valve connecting the negative pole of the DC bus of the first converter and the positive pole of the DC bus of the second converter; the first capacitor of the auxiliary commutation capacitor circuit connects the positive pole of the DC bus of the second converter and the negative pole of the DC bus of the second converter.

Description

Technical Field

[0001] The present application relates to the field of high-voltage direct current (HVDC) transmission technology, and in particular to a capacitor-assisted commutation dual-converter parallel circuit, a control method and a control device, and a HVDC transmission system. Background Art

[0002] High-voltage and ultra-high-voltage direct current (UHVDC) transmission systems have large capacity. Existing technologies use a grid-commutated converter with a twelve-pulse circuit structure. Each twelve-pulse circuit consists of two three-phase, six-arm circuits connected in series, each arm of which uses a single large-capacity thyristor in series. Because thyristors cannot control shutdown, existing converter structures are subject to commutation failure. Furthermore, large-capacity thyristors are more expensive than solutions with smaller-capacity thyristors connected in parallel.

[0003] With the increasing number of connected high-voltage and ultra-high-voltage direct current (UHVDC) transmission systems, multi-infeed DC transmission systems have been formed in multiple regional power grids. When multiple DC lines experience simultaneous commutation failures, this can pose a threat to the safe operation of the AC power grid in that region. As the proportion of renewable energy generation increases, AC voltage support capacity decreases, placing higher demands on the stable operation of DC transmission systems and their ability to suppress commutation failures. Flexible direct current (VDC) and hybrid direct current (HDC) technologies use voltage source converters (VSCs) to address commutation failure and AC voltage support issues. However, VSCs have small capacity, high losses, and the risk of oscillation. Therefore, existing HVDC, flexible direct current, and hybrid direct current technologies struggle to meet the stringent cost and performance requirements of DC transmission systems. Summary of the Invention

[0004] An embodiment of the present application provides a dual-converter parallel circuit with capacitor-assisted commutation, comprising a first converter, a second converter, a connecting circuit, and an auxiliary commutation capacitor circuit, wherein the first converter comprises six first bridge arms of three phases, each of which comprises a first half-controlled valve; the second converter comprises six second bridge arms of three phases, each of which comprises a second valve; the connecting circuit comprises a third valve, a fourth valve, a fifth valve, and a sixth valve, wherein the third valve connects the positive pole of the DC bus of the first converter and the positive pole of the DC bus of the second converter, the fourth valve connects the negative pole of the DC bus of the first converter and the negative pole of the DC bus of the second converter, the fifth valve connects the positive pole of the DC bus of the first converter and the negative pole of the DC bus of the second converter, and the sixth valve connects the negative pole of the DC bus of the first converter and the positive pole of the DC bus of the second converter; the auxiliary commutation capacitor circuit connects the positive pole of the DC bus of the second converter and the negative pole of the DC bus of the second converter, and the auxiliary commutation capacitor circuit comprises a first capacitor.

[0005] According to some embodiments, the auxiliary commutation capacitor circuit further includes a seventh valve, or a resistor and / or an inductor, and the seventh valve is connected in series with the first capacitor; the resistor and / or the inductor is connected in series with the first capacitor.

[0006] According to some embodiments, the first half-controlled valve includes a half-controlled switch, and the second valve, third valve, fourth valve, fifth valve, sixth valve, and seventh valve include at least one of a half-controlled switch, a one-way fully-controlled switch, a two-way fully-controlled switch, or an MMC single valve. Lightning arresters are respectively connected in parallel at both ends of the first half-controlled valve, the second valve, the third valve, the fourth valve, the fifth valve, the sixth valve, the seventh valve, and / or the first capacitor.

[0007] According to some embodiments, the half-controlled switch includes a thyristor and / or a diode connected in series, the fully-controlled switch includes a fully-controlled device connected in series, the fully-controlled device includes at least one of IGCT, IGBT, GTO, and MOSFET, the MMC single valve includes a half-bridge sub-module and / or a full-bridge sub-module connected in series, the half-bridge sub-module includes two of the fully-controlled devices and a capacitor, and the full-bridge sub-module includes four of the fully-controlled devices and a capacitor.

[0008] According to some embodiments, the third valve and the fourth valve include a full-control switch and a fast isolation switch connected in series.

[0009] According to some embodiments, the AC output terminals of the first converter and the second converter are connected in parallel in phase and are connected to the same converter transformer.

[0010] An embodiment of the present application further provides a high-voltage direct current (HVDC) power transmission system, comprising the aforementioned capacitor-assisted commutation dual-converter parallel circuit.

[0011] An embodiment of the present application also provides a control method for a dual-converter parallel circuit with capacitor-assisted commutation as described above, comprising: controlling the first converter to operate in an inverter state; controlling the third valve and the fourth valve to be turned on, and controlling the fifth valve and the sixth valve to be turned off, so that the first capacitor is at a positive pressure; controlling the second converter to operate in an inverter state or a locked state; when a fault occurs and may cause commutation failure of the commutation bridge arm of the first converter, comprising: locking the second converter, controlling the third valve and the fourth valve to be turned off, and transferring the current of the corresponding bridge arm of the second converter corresponding to the commutation bridge arm to the commutation bridge arm of the first converter; controlling the corresponding bridge arm of the second converter and the fifth valve or the sixth valve to be turned on, so that the first capacitor provides a negative pressure for the commutation bridge arm, the current of the commutation bridge arm is transferred to the first capacitor, and the commutation bridge arm is turned off; after the commutation bridge arm is turned off, controlling the corresponding bridge arm of the second converter and the fifth valve or the sixth valve to be turned off, thereby realizing the current transfer from the phase where the commutation bridge arm is located to another phase.

[0012] According to some embodiments, if the first capacitor is also connected in series with a seventh valve, the method further includes: controlling the seventh valve to be turned on while controlling the corresponding bridge arm of the second converter corresponding to the phase-changing bridge arm and the fifth valve or the sixth valve to be turned on; and controlling the seventh valve to be turned off while controlling the corresponding bridge arm of the second converter corresponding to the phase-changing bridge arm and the fifth valve or the sixth valve to be turned off.

[0013] The embodiment of the present application also provides a control device for a dual-converter parallel circuit with capacitor-assisted commutation as described above, comprising a detection unit and a control unit, wherein the detection unit is used to detect operating parameters and faults of the dual-converter parallel circuit with capacitor-assisted commutation; the control unit controls the first converter to operate in an inverter state based on the operating parameters of the dual-converter parallel circuit with capacitor-assisted commutation; controls the third valve and the fourth valve to be turned on, and controls the fifth valve and the sixth valve to be turned off, so that the first capacitor is at a positive pressure; controls the second converter to operate in an inverter state or a locked state; and controls the second converter to operate in an inverter state or a locked state when a fault occurs that may cause the commutation of the first converter. When the phase bridge arm commutation fails, the control unit also locks the second converter, controls the third valve and / or the fourth valve to be turned off, transfers the current of the corresponding bridge arm of the second converter corresponding to the commutation bridge arm to the commutation bridge arm of the first converter, controls the corresponding bridge arm of the second converter and the fifth valve or the sixth valve to be turned on, and the first capacitor provides negative voltage for the commutation bridge arm. The current of the commutation bridge arm is transferred to the first capacitor, and the commutation bridge arm is turned off. After the commutation bridge arm is turned off, the corresponding bridge arm of the second converter and the fifth valve or the sixth valve are controlled to be turned off, thereby realizing the current transfer from the phase where the commutation bridge arm is located to another phase.

[0014] The technical solution provided by the embodiments of this application first charges the first capacitor with a DC voltage by controlling the conduction of the third and fourth valves. Then, by controlling the conduction of the corresponding bridge arms of the second converter and the fifth or sixth valves, the reverse voltage of the first capacitor is used to assist in shutting down the commutation bridge arms of the first converter. Finally, by controlling the conduction of the corresponding bridge arms of the second converter and the fifth or sixth valves, the commutation bridge arms of the first converter are shut down, effectively preventing commutation failures. Furthermore, the parallel operation of the first and second converters enables the low-cost, high-capacity application of a dual-converter parallel circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0016] Figure 1 This is one of the schematic diagrams of a dual-converter parallel circuit with capacitor-assisted commutation provided in an embodiment of the present application.

[0017] Figure 2a-2h This is a schematic diagram of an auxiliary commutation capacitor circuit provided in an embodiment of the present application.

[0018] Figure 3a-3j This is a schematic diagram of a switch provided in an embodiment of the present application.

[0019] Figure 4 This is the second schematic diagram of a dual-converter parallel circuit with capacitor-assisted commutation provided in an embodiment of the present application.

[0020] Figure 5 This is the third schematic diagram of a dual-converter parallel circuit with capacitor-assisted commutation provided in an embodiment of the present application.

[0021] Figure 6 This is a flow chart of a control method for a capacitor-assisted commutation dual-converter parallel circuit provided in an embodiment of the present application.

[0022] Figure 7 Schematic diagram of a control device for a capacitor-assisted commutation dual-converter parallel circuit provided in an embodiment of the present application. DETAILED DESCRIPTION

[0023] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are part of the embodiments of this application, not all of them. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without making creative efforts are within the scope of protection of this application.

[0024] It should be understood that the terms "first," "second," "third," "fourth," "fifth," "sixth," "seventh," etc. in the claims, specification, and drawings of this application are used to distinguish different objects, rather than to describe a specific order. The terms "comprise" and "comprising" used in the specification and claims of this application indicate the presence of the described features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or combinations thereof.

[0025] Figure 1 This is one of the schematic diagrams of a dual-converter parallel circuit with capacitor-assisted commutation provided in an embodiment of the present application.

[0026] like Figure 1 As shown, the capacitor-assisted commutation dual-converter parallel circuit includes a first converter 1 , a second converter 2 , a connecting circuit and an auxiliary commutation capacitor circuit 3 .

[0027] The first converter 1 includes six first bridge arms of three phases, namely three first upper bridge arms and three first lower bridge arms, and the first bridge arms include but are not limited to first half-controlled valves, which include but are not limited to half-controlled switches.

[0028] like Figure 1 As shown, the first upper bridge arm of phase A of the first converter 1 is composed of the first half-controlled valve V41, the first upper bridge arm of phase B is composed of the first half-controlled valve V61, the first upper bridge arm of phase C is composed of the first half-controlled valve V21, the first lower bridge arm of phase A is composed of the first half-controlled valve V11, the first lower bridge arm of phase B is composed of the first half-controlled valve V31, and the first lower bridge arm of phase C is composed of the first half-controlled valve V51.

[0029] The second converter 2 includes six second bridge arms of three phases, namely three second upper bridge arms and three second lower bridge arms. The second bridge arms include, but are not limited to, second valves. The second valves include, but are not limited to, at least one of a half-controlled switch, a one-way fully-controlled switch, a two-way fully-controlled switch, or an MMC single valve.

[0030] like Figure 1As shown, the second upper bridge arm of phase A of the second converter 2 is composed of the second valve V43, the second upper bridge arm of phase B is composed of the second valve V63, the second upper bridge arm of phase C is composed of the second valve V23, the second lower bridge arm of phase A is composed of the second valve V13, the second lower bridge arm of phase B is composed of the second valve V33, and the second lower bridge arm of phase C is composed of the second valve V53.

[0031] The connection circuit includes, but is not limited to, a third valve V71 , a fourth valve V72 , a fifth valve V73 and a sixth valve V74 .

[0032] like Figure 1 As shown, the third valve V71 connects the positive DC bus P1 of the first converter 1 and the positive DC bus P2 of the second converter 2. The fourth valve V72 connects the negative DC bus N1 of the first converter 1 and the negative DC bus N2 of the second converter 2. The fifth valve V73 connects the positive DC bus P1 of the first converter 1 and the negative DC bus N2 of the second converter 2. The sixth valve V74 connects the negative DC bus N1 of the first converter 1 and the positive DC bus P2 of the second converter 2.

[0033] The third valve, the fourth valve, the fifth valve, and the sixth valve include but are not limited to at least one of a half-controlled switch, a one-way fully-controlled switch, a two-way fully-controlled switch, and an MMC single valve.

[0034] According to some embodiments, the third valve and the fourth valve include a full-control switch and a fast isolation switch connected in series.

[0035] The auxiliary commutation capacitor circuit 3 is connected to the DC bus positive electrode P2 of the second converter 2 and the DC bus negative electrode N2 of the second converter 2 . The auxiliary commutation capacitor circuit 3 includes but is not limited to a first capacitor.

[0036] According to some embodiments, the first capacitor includes a plurality of capacitance elements connected in series, and each of the plurality of capacitance elements is connected in parallel with a grading resistor.

[0037] According to some embodiments, the auxiliary commutation capacitor circuit 3 only includes the first capacitor C11, such as Figure 2a shown.

[0038] According to some embodiments, the auxiliary commutation capacitor circuit 3 includes a first capacitor C11 and a first resistor R11 connected in series. Figure 2b shown.

[0039] According to some embodiments, the auxiliary commutation capacitor circuit 3 includes a first capacitor C11 and a first inductor L11 connected in series. Figure 2c shown.

[0040] According to some embodiments, the auxiliary commutation capacitor circuit 3 includes a first capacitor C11, a first resistor R11 and a first inductor L11 connected in series. Figure 2d shown.

[0041] According to some embodiments, the auxiliary commutation capacitor circuit 3 includes a first capacitor C11 and a seventh valve V75 connected in series. Figure 2e shown.

[0042] According to some embodiments, the auxiliary commutation capacitor circuit 3 includes a first capacitor C11, a first resistor R11 and a seventh valve V75 connected in series. Figure 2f shown.

[0043] According to some embodiments, the auxiliary commutation capacitor circuit 3 includes a first capacitor C11, a first inductor L11 and a seventh valve V75 connected in series. Figure 2g shown.

[0044] According to some embodiments, the auxiliary commutation capacitor circuit 3 includes a first capacitor C11, a first resistor R11, a first inductor L11 and a seventh valve V75 connected in series. Figure 2h shown.

[0045] According to some embodiments, lightning arresters are connected in parallel at both ends of the first half-controlled valve, the second valve, the third valve, the fourth valve, the fifth valve, the sixth valve or / and the first capacitor, and a second half-controlled valve is connected in parallel at both ends of the second valve, the third valve, the fourth valve, the fifth valve or the sixth valve to protect the above valves.

[0046] Half-controlled switches include, but are not limited to, thyristors connected in series. Fully controlled switches include, but are not limited to, fully controlled devices connected in series. Fully controlled devices include, but are not limited to, at least one of IGCTs (Integrated Gate Commutated Thyristors), IGBTs (Insulated Gate Bipolar Transistors), GTOs (Gate Turn-Off Thyristors), and MOSFETs (Metal Oxide Semiconductor Field Effect Transistors). MMC (Modular Multilevel Converter) single valves include, but are not limited to, half-bridge submodules and / or full-bridge submodules connected in series. A half-bridge submodule includes, but is not limited to, two fully controlled devices and a capacitor. A full-bridge submodule includes, but is not limited to, four fully controlled devices and a capacitor.

[0047] According to some embodiments, the half-controlled switch is composed of thyristors 4 connected in series, such as Figure 3aAs shown, it only controls the on-state and cannot control the off-state, and has a unidirectional current-carrying capability and a bidirectional voltage-blocking capability. Optionally, the half-controlled switch is composed of a thyristor 4 and a diode connected in series or in parallel.

[0048] According to some embodiments, the unidirectional fully controlled switch is composed of IGBT modules connected in series, such as Figure 3b As shown, it only controls the on and off in one direction, and has bidirectional current flow and unidirectional blocking voltage capabilities. The IGBT module includes an IGBT 5 and a diode 7 connected in anti-parallel therewith. Optionally, the IGBT in the figure can be a MOSFET, GTO, or IGCT.

[0049] According to some embodiments, the unidirectional fully controlled switch is composed of IGCT6 connected in series, such as Figure 3c As shown, it only controls opening and closing in one direction, and has the capability of unidirectional current flow and bidirectional blocking voltage.

[0050] According to some embodiments, the unidirectional fully controlled switch is composed of an IGBT module and a diode 7 in series, such as Figure 3d As shown, it only controls the on and off direction in one direction, and has the capability of unidirectional current flow and bidirectional blocking voltage. Optionally, the IGBT in the figure can be selected from MOSFET, GTO, or IGCT.

[0051] According to some embodiments, the unidirectional fully controlled switch is composed of an IGCT 6 and a thyristor 4 connected in anti-parallel and then in series. Figure 3e As shown, it only controls opening and closing in one direction, and has bidirectional current flow and bidirectional blocking voltage capabilities.

[0052] According to some embodiments, a bidirectional fully controlled switch is composed of forward and reverse IGBT modules connected in series, such as Figure 3f As shown, it can bidirectionally control the opening and closing, and has bidirectional current flow capability and bidirectional blocking voltage capability. Optionally, the IGBT in the figure can be selected from MOSFET, GTO, and IGCT.

[0053] According to some embodiments, the bidirectional fully controlled switch is composed of a forward IGCT 6 and a reverse IGCT 6 connected in parallel and then in series. Figure 3g As shown, it can be bidirectionally controlled to open and close, and has bidirectional current-carrying capability and bidirectional blocking voltage capability.

[0054] According to some embodiments, the MMC single valve is composed of two submodules consisting of IGBT modules and capacitor 8 connected in series, such as Figure 3h As shown, the connection point of the two IGBT modules serves as the positive terminal of the submodule, and the other end of one IGBT module serves as the negative terminal of the submodule. The submodules are connected in series and can only be turned on and off in one direction, providing bidirectional current flow capability and unidirectional blocking voltage capability. Optionally, the IGBT in the figure can be an IGCT, GTO, or MOSFET.

[0055] According to some embodiments, the MMC single valve is composed of four submodules consisting of IGBT modules and capacitor 8 connected in series, such as Figure 3i As shown, the IGBT modules are connected in series and then in parallel, and are also connected in parallel with capacitor 8. The connection points of the two IGBT modules in series serve as the positive and negative electrodes of the submodules, respectively. The submodules are connected in series, enabling bidirectional control of on and off, with bidirectional current flow capability and bidirectional blocking voltage capability. Optionally, the IGBTs in the figure can be IGCTs, GTOs, or MOSFETs.

[0056] According to some embodiments, the third valve and the fourth valve are composed of a full-control switch and a fast isolation switch 9 in series, such as Figure 3j As shown, the fast isolating switch 9 is a mechanical switch to provide sufficient withstand voltage level, reduce the conduction loss of the third valve and the fourth valve and the withstand voltage level of the full-control switch. Optionally, the IGBT in the figure can be IGCT, GTO, or MOSFET.

[0057] The thyristor 4 is equipped with a corresponding trigger circuit, the IGBT 5 is equipped with a corresponding drive circuit and a buffer circuit, and the IGCT 6 is equipped with a corresponding drive circuit and a buffer circuit. The buffer circuit is composed of at least a capacitor, or a resistor and a capacitor in series.

[0058] According to some embodiments, the AC output terminals of the first converter 1 and the second converter 2 are connected in parallel in phase and connected to the same converter transformer, such as Figure 1 As shown, A1 and A2 are connected, B1 and B2 are connected, and C1 and C2 are connected, and are respectively connected to the three phases of the same converter transformer.

[0059] Figure 4 This is the second schematic diagram of a dual-converter parallel circuit with capacitor-assisted commutation provided in an embodiment of the present application.

[0060] exist Figure 1 Based on the embodiment, the first half-controlled valves V11, V21, V31, V41, V51, and V61 of the first converter 1 are half-controlled switches, consisting of thyristors connected in series. The second valves V13, V23, V33, V43, V53, and V63 of the second converter 2 are half-controlled switches, consisting of thyristors connected in series. The third valve V71, the fourth valve V72, the fifth valve V73, and the sixth valve V74 are unidirectional fully controlled switches, consisting of IGCTs connected in series. The auxiliary commutation capacitor is composed of a first capacitor C11 and a first resistor R11 connected in series.

[0061] The first half-controlled valves V11, V21, V31, V41, V51 and V61 of the first converter 1 are respectively connected in parallel with the first lightning arresters F11, F21, F31, F41, F51 and F61; the second valves V13, V23, V33, V43, V53 and V63 of the second converter 2 are respectively connected in parallel with the second lightning arresters F13, F23, F33, F43, F53 and F63; the third valve V71 is connected in parallel with the third lightning arrester F71; the fourth valve V72 is connected in parallel with the fourth lightning arrester F72; the fifth valve V73 is connected in parallel with the fifth lightning arrester F73; and the sixth valve V74 is connected in parallel with the sixth lightning arrester F74.

[0062] The AC output terminals of the first converter 1 and the second converter 2 are connected in parallel according to phases, that is, A1 and A2 are connected, B1 and B2 are connected, and C1 and C2 are connected, and are respectively connected to the three phases of the same converter transformer.

[0063] Figure 5 This is the third schematic diagram of a dual-converter parallel circuit with capacitor-assisted commutation provided in an embodiment of the present application.

[0064] exist Figure 1 Based on the embodiment, the first half-controlled valves V11, V21, V31, V41, V51, and V61 of the first converter 1 use half-controlled switches, consisting of thyristors connected in series. The second valves V13, V23, V33, V43, V53, and V63 use half-controlled switches, consisting of thyristors connected in series. The third valve V71, the fourth valve V72, the fifth valve V73, and the sixth valve V74 use unidirectional fully-controlled switches, consisting of IGCTs connected in series. The auxiliary commutation capacitor is composed of a first capacitor C11 and a first resistor R11 connected in series. The seventh valve uses a unidirectional fully-controlled switch, consisting of IGBT modules connected in series.

[0065] The first half-controlled valves V11, V21, V31, V41, V51 and V61 of the first converter 1 are respectively connected in parallel with the first lightning arresters F11, F21, F31, F41, F51 and F61; the second valves V13, V23, V33, V43, V53 and V63 of the second converter 2 are respectively connected in parallel with the second lightning arresters F13, F23, F33, F43, F53 and F63; the third valve V71 is connected in parallel with the third lightning arrester F71; the fourth valve V72 is connected in parallel with the fourth lightning arrester F72; the fifth valve V73 is connected in parallel with the fifth lightning arrester F73; the sixth valve V74 is connected in parallel with the sixth lightning arrester F74; and the seventh valve V75 is connected in parallel with the seventh lightning arrester F75.

[0066] The AC output terminals of the first converter 1 and the second converter 2 are connected in parallel according to phases, that is, A1 and A2 are connected, B1 and B2 are connected, and C1 and C2 are connected, and are respectively connected to the three phases of the same converter transformer.

[0067] Figure 6This is a flow chart of a control method for a capacitor-assisted commutation dual-converter parallel circuit provided in an embodiment of the present application.

[0068] In S110 , the first converter is controlled to operate in an inverter state.

[0069] like Figure 1 、 Figure 4 and Figure 5 As shown, the first converter 1 is controlled to operate in the inverter state.

[0070] In S120 , the third valve and the fourth valve are controlled to be turned on, and the fifth valve and the sixth valve are controlled to be turned off, so that the first capacitor is at a positive pressure.

[0071] like Figure 1 、 Figure 4 and Figure 5 As shown, the third valve V71 and the fourth valve V72 are controlled to be turned on, and the fifth valve V73 and the sixth valve V4 are controlled to be turned off, so that the first capacitor C11 is at a positive pressure.

[0072] The positive direction of the voltage of the first capacitor C11 is from the positive electrode P2 of the DC bus of the second converter to the negative electrode N2 of the DC bus of the second converter.

[0073] In S130, the second converter is controlled to operate in an inverter state or a blocking state.

[0074] like Figure 1 、 Figure 4 and Figure 5 As shown, the second converter 2 is controlled to operate in the inverter state or the blocking state.

[0075] In S140, when a fault occurs that may cause the commutation arm of the first converter to fail, the second converter is locked, the third valve and / or the fourth valve are controlled to be closed, and the current of the corresponding bridge arm of the second converter corresponding to the commutation bridge arm of the first converter is transferred to the commutation bridge arm of the first converter.

[0076] The above-mentioned commutation bridge arm is the bridge arm that commutates to another bridge arm during normal operation.

[0077] like Figure 1 、 Figure 4 and Figure 5 As shown, if the commutation bridge arm is the first upper bridge arm of phase A of the first converter 1, when the first upper bridge arm of phase A commutates to the first upper bridge arm of phase B, at this time, if a fault occurs that may cause the commutation of the first upper bridge arm of phase A of the first converter 1 to fail, if the second converter 2 operates in the inverter state, the second converter 2 is first locked, the third valve V71 is controlled to be closed, and the fourth valve V72 is controlled to be closed, and the current of the second upper bridge arm of phase A of the second converter 2 is transferred to the first upper bridge arm of phase A of the first converter 1.

[0078] If the commutation bridge arm is the first lower bridge arm of phase A of the first converter 1, when the first lower bridge arm of phase A commutates to the first lower bridge arm of phase B, at this time, if a fault occurs that may cause the commutation of the first lower bridge arm of phase A of the first converter 1 to fail, the third valve V71 is controlled to be closed, the fourth valve V72 is controlled to be closed, and the current of the second lower bridge arm of phase A of the second converter 2 is transferred to the first lower bridge arm of phase A of the first converter 1.

[0079] According to some embodiments, the aforementioned faults include, but are not limited to, AC system faults or DC system faults in a dual-converter parallel circuit. An AC system fault can be determined based on an increase in the zero-sequence component of the AC voltage, a sudden change in the AC voltage, a drop in the AC voltage amplitude, an increase in the AC voltage harmonics, or an increase in the DC current. A DC system fault can be determined based on, but is not limited to, a drop in the DC voltage or an increase in the DC current. The aforementioned potential commutation failure of the commutation arm of the first converter is determined based on the closing time of the first half-controlled valve of the commutation arm and the AC voltage. If the first half-controlled valve of the commutation arm has not closed at the closing time under normal AC voltage, it is determined that the commutation arm of the first converter may have failed, but is not limited to, this.

[0080] In S150, the corresponding bridge arm and the fifth valve or the sixth valve of the second converter corresponding to the commutation bridge arm are controlled to be turned on, the first capacitor provides a negative voltage for the commutation bridge arm, the current of the commutation bridge arm is transferred to the first capacitor, and the commutation bridge arm is turned off.

[0081] The corresponding bridge arm of the second converter is a bridge arm connected to the same phase and the same DC bus as the commutation bridge arm of the first converter.

[0082] like Figure 1 and Figure 4 As shown, if the commutation arm is the first upper arm of phase A of the first converter, the second upper arm of phase A of the second converter 2 and the fifth valve V73 are controlled to be conductive. The first capacitor C11 provides a reverse voltage for the first upper arm of phase A of the first converter 1. The current in the first upper arm of phase A of the first converter 1 is transferred to the first capacitor C11, and the first upper arm of phase A of the first converter 1 is turned off.

[0083] If the commutation bridge arm is the first lower bridge arm of phase A of the first converter, the second lower bridge arm of phase A of the second converter 2 and the sixth valve V74 are controlled to be turned on, the first capacitor C11 provides reverse voltage for the first lower bridge arm of phase A of the first converter 1, the current of the first lower bridge arm of phase A of the first converter 1 is transferred to the first capacitor C11, and the first lower bridge arm of phase A of the first converter 1 is turned off.

[0084] According to some embodiments, if the first capacitor is further connected in series with a seventh valve, the seventh valve is controlled to be turned on while controlling the corresponding bridge arm of the second converter and the fifth valve or the sixth valve corresponding to the commutation bridge arm to be turned on.

[0085] like Figure 5 As shown, if the commutation bridge arm is the first upper bridge arm of phase A of the first converter 1, the second upper bridge arm of phase A of the second converter 2, the fifth valve V73, and the seventh valve V75 are controlled to be conductive. The first capacitor C11 provides a reverse voltage for the first upper bridge arm of phase A of the first converter 1. The current in the first upper bridge arm of phase A of the first converter 1 is transferred to the first capacitor C11, and the first upper bridge arm of phase A of the first converter 1 is turned off.

[0086] If the commutation bridge arm is the first lower bridge arm of phase A of the first converter 1, the second lower bridge arm of phase A of the second converter 2, the sixth valve V74 and the seventh valve V75 are controlled to be turned on, the first capacitor C11 provides reverse voltage for the first lower bridge arm of phase A of the first converter 1, the current of the first lower bridge arm of phase A of the first converter 1 is transferred to the first capacitor C11, and the first lower bridge arm of phase A of the first converter 1 is turned off.

[0087] In S160, after the commutation bridge arm is turned off, the corresponding bridge arm, the fifth valve or the sixth valve of the second converter corresponding to the commutation bridge arm is controlled to be turned off, thereby realizing the current transfer from the phase where the commutation bridge arm is located to another phase.

[0088] The commutation bridge arm of the first converter is shut down when the forward current of the first half-controlled valve is less than the holding current and the forward blocking capability is restored. Specifically, restoring the forward blocking capability means restoring the forward blocking capability after the forward current is less than the holding current and the shutdown time is extended, and the shutdown time is less than 700 μs, but is not limited to this.

[0089] like Figure 1 and Figure 4 As shown, after the commutation bridge arm of the first converter is turned off, if the commutation bridge arm is the first upper bridge arm of phase A of the first converter 1, the second upper bridge arm of phase A of the second converter 2 or the fifth valve V73 is controlled to be turned off, forcing the current to switch from phase A to phase B; if the commutation bridge arm is the first lower bridge arm of phase A of the first converter 1, the second lower bridge arm of phase A of the second converter 2 or the sixth valve V74 is controlled to be turned off, forcing the current to switch from phase A to phase B.

[0090] According to some embodiments, if the first capacitor is further connected in series with a seventh valve, the seventh valve is controlled to be closed while the corresponding bridge arm, the fifth valve, or the sixth valve of the second converter corresponding to the commutation bridge arm is controlled to be closed.

[0091] like Figure 5As shown, after the commutation bridge arm of the first converter is turned off, if the commutation bridge arm is the first upper bridge arm of phase A of the first converter 1, the second upper bridge arm of phase A, the fifth valve V73 or the seventh valve V75 of the second converter 2 are controlled to be turned off, forcing the current to switch from phase A to phase B; if the commutation bridge arm is the first lower bridge arm of phase A of the first converter 1, the second lower bridge arm of phase A, the sixth valve V74 or the seventh valve V75 of the second converter 2 are controlled to be turned off, forcing the current to switch from phase A to phase B.

[0092] When the fault disappears, the third valve V71 is controlled to be turned on, the fourth valve V72 is controlled to be turned on, the second converter 2 is unlocked, and the second converter 2 is controlled to continue to operate in the inverter state.

[0093] An embodiment of the present application further provides a high-voltage direct current transmission system, comprising at least the dual-converter parallel circuit with capacitor-assisted commutation as described above.

[0094] Figure 7 3 is a schematic diagram of a control device for a capacitor-assisted commutation dual-converter parallel circuit provided in an embodiment of the present application. The control device 300 includes a detection unit 310 and a control unit 320.

[0095] The detection unit 310 is used to detect the operating parameters and faults of the dual-converter parallel circuit with capacitor-assisted commutation, including but not limited to the AC voltage, DC voltage, DC current, the operating status of the first converter 1, the second converter 2, the auxiliary commutation capacitor circuit 3, and the operating status of the third valve V71, the fourth valve V72, the fifth valve V73, the sixth valve V74, and the seventh valve V75 of the dual-converter parallel circuit with capacitor-assisted commutation.

[0096] Based on the operating parameters of the capacitor-assisted commutation dual-converter parallel circuit, the control unit 320 controls the first converter to operate in the inverter state; controls the third and fourth valves to conduct, and controls the fifth and sixth valves to close, so that the first capacitor has a positive voltage; and controls the second converter to operate in the inverter state or the locked state. When a fault occurs that may cause commutation failure in the commutation bridge arm of the first converter, the control unit 320 also locks the second converter, controls the third and / or fourth valves to close, and transfers the current from the corresponding bridge arm of the second converter corresponding to the commutation bridge arm to the commutation bridge arm of the first converter; controls the corresponding bridge arm and the fifth or sixth valve of the second converter to conduct, so that the first capacitor provides a negative voltage to the commutation bridge arm, and the current in the commutation bridge arm is transferred to the first capacitor, and the commutation bridge arm is closed; after the commutation bridge arm is closed, the corresponding bridge arm and the fifth or sixth valve of the second converter are controlled to close, thereby achieving current transfer from the phase in which the commutation bridge arm is located to another phase.

[0097] The above embodiments are only for illustrating the technical ideas of the present application and cannot be used to limit the scope of protection of the present application. Any changes made on the basis of the technical solutions in accordance with the technical ideas proposed in the present application shall fall within the scope of protection of the present application.

Claims

1. A capacitor-assisted commutation dual-converter parallel circuit, comprising: A first converter includes six first bridge arms of three phases, each of the first bridge arms including a first half-controlled valve; A second converter includes six second bridge arms of three phases, each second bridge arm including a second valve; a connection circuit, comprising a third valve, a fourth valve, a fifth valve, and a sixth valve, the third valve connecting the positive pole of the DC bus of the first converter and the positive pole of the DC bus of the second converter, the fourth valve connecting the negative pole of the DC bus of the first converter and the negative pole of the DC bus of the second converter, the fifth valve connecting the positive pole of the DC bus of the first converter and the negative pole of the DC bus of the second converter, and the sixth valve connecting the negative pole of the DC bus of the first converter and the positive pole of the DC bus of the second converter; An auxiliary commutation capacitor circuit is connected to the positive electrode of the DC bus of the second converter and the negative electrode of the DC bus of the second converter, and the auxiliary commutation capacitor circuit includes a first capacitor.

2. The dual-converter parallel circuit according to claim 1, wherein: The auxiliary commutation capacitor circuit further includes: a seventh valve connected in series with the first capacitor; or / and A resistor and / or an inductor are connected in series with the first capacitor.

3. The dual-converter parallel circuit according to claim 1, wherein: The first half-controlled valve includes a half-controlled switch, and the second valve, third valve, fourth valve, fifth valve, and sixth valve include at least one of a half-controlled switch, a one-way fully-controlled switch, a two-way fully-controlled switch, or an MMC single valve. Lightning arresters are respectively connected in parallel at both ends of the first half-controlled valve, the second valve, the third valve, the fourth valve, the fifth valve, the sixth valve, or / and the first capacitor.

4. The dual-converter parallel circuit according to claim 2, wherein: The seventh valve includes at least one of a half-controlled switch, a one-way fully-controlled switch, a two-way fully-controlled switch or an MMC single valve, and lightning arresters are connected in parallel at both ends of the seventh valve.

5. The dual-converter parallel circuit according to claim 3 or 4, wherein: The half-controlled switch includes thyristors connected in series, the fully-controlled switch includes fully-controlled devices connected in series, and the fully-controlled devices include at least one of IGCT, IGBT, GTO, and MOSFET. The MMC single valve includes a half-bridge sub-module and / or a full-bridge sub-module connected in series, and the half-bridge sub-module includes two of the fully-controlled devices and a capacitor, and the full-bridge sub-module includes four of the fully-controlled devices and a capacitor.

6. The dual-converter parallel circuit according to claim 1, wherein: The third valve and the fourth valve include a full-control switch and a fast isolation switch connected in series.

7. The dual-converter parallel circuit according to claim 1, wherein: The AC output ends of the first converter and the second converter are connected in parallel in phase and are connected to the same converter transformer.

8. A high-voltage direct current (HVDC) power transmission system, comprising the capacitor-assisted commutation dual-converter parallel circuit according to any one of claims 1 to 7.

9. A method for controlling a capacitor-assisted commutation dual-converter parallel circuit according to any one of claims 1 to 7, comprising: controlling the first converter to operate in an inverter state; controlling the third valve and the fourth valve to be turned on, and controlling the fifth valve and the sixth valve to be turned off, so that the first capacitor is at a positive pressure; controlling the second converter to operate in an inverter state or a blocking state; When a fault occurs that may cause commutation failure of the commutation bridge arm of the first converter, the method includes: Locking the second converter, controlling the third valve and / or the fourth valve to be closed, and transferring the current of the corresponding bridge arm of the second converter corresponding to the commutation bridge arm of the first converter to the commutation bridge arm of the first converter; Controlling the corresponding bridge arm of the second converter and the fifth valve or the sixth valve to be turned on, the first capacitor to provide a negative voltage for the commutation bridge arm, the current of the commutation bridge arm to be transferred to the first capacitor, and the commutation bridge arm to be turned off; When the commutation bridge arm is turned off, the corresponding bridge arm of the second converter, the fifth valve or the sixth valve is controlled to be turned off, so that the current is transferred from the phase where the commutation bridge arm is located to another phase.

10. The control method according to claim 9, wherein: If the first capacitor is further connected in series with a seventh valve, the method further includes: While controlling the corresponding bridge arm of the second converter corresponding to the commutation bridge arm and the fifth valve or the sixth valve to be turned on, controlling the seventh valve to be turned on; While controlling the corresponding bridge arm of the second converter corresponding to the commutation bridge arm, the fifth valve or the sixth valve to be turned off, the seventh valve is controlled to be turned off.

11. A control device for a capacitor-assisted commutation dual-converter parallel circuit according to any one of claims 1 to 7, comprising: a detection unit, configured to detect operating parameters and faults of the capacitor-assisted commutation dual-converter parallel circuit; The control unit controls the first converter to operate in an inverter state based on the operating parameters of the dual-converter parallel circuit with capacitor-assisted commutation; controls the third valve and the fourth valve to be turned on, and controls the fifth valve and the sixth valve to be turned off, so that the first capacitor is at a positive voltage; and controls the second converter to operate in an inverter state or a locked state. When a fault occurs that may cause commutation failure of the commutation bridge arm of the first converter, the control unit further locks the second converter, controls the third valve and / or the fourth valve to be turned off, transfers the current of the corresponding bridge arm of the second converter corresponding to the commutation bridge arm to the commutation bridge arm of the first converter, controls the corresponding bridge arm of the second converter and the fifth valve or the sixth valve to be turned on, the first capacitor provides a negative voltage for the commutation bridge arm, the current of the commutation bridge arm is transferred to the first capacitor, and the commutation bridge arm is turned off. After the commutation bridge arm is turned off, the corresponding bridge arm of the second converter and the fifth valve or the sixth valve are controlled to be turned off, thereby realizing the current transfer from the phase where the commutation bridge arm is located to another phase.

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