A hybrid DC transmission system with fault handling function

Abstract

The present invention discloses a hybrid DC power transmission system with a fault handling function, including a hybrid DC power transmission system and a fault handling device, wherein the fault handling device includes a blocking unit and a measuring unit; or includes a blocking unit and a bypass switch; or includes a blocking unit and a bypass switch and a knife switch assembly; or includes a blocking unit, a measuring unit and a bypass switch; or includes a blocking unit, a measuring unit and a bypass switch and a knife switch assembly. The present invention has low cost loss, can realize flexible control of the hybrid DC power transmission system through the bypass switch and the measuring unit connected across, can effectively handle the DC power transmission line fault of the hybrid DC power transmission system without locking the converter, and can realize rapid recovery of DC power after the fault, so as to better protect the safety of the AC and DC power transmission system.

Description

Technical Field

[0001] The invention belongs to the field of hybrid direct current power transmission, and in particular relates to a hybrid direct current power transmission system with a fault processing function. Background Art

[0002] High-voltage direct current transmission systems can be divided into two types: conventional direct current transmission systems (LCC-HVDC) based on thyristor technology and flexible direct current transmission systems (Flexible-HVDC) based on fully controlled power electronic device technology. Among them, conventional direct current transmission systems (LCC-HVDC) have low costs, low losses, and mature operating technology. At present, almost all direct current transmission systems in operation in the world are LCC-HVDC systems, but conventional direct current transmission systems (LCC-HVDC) have disadvantages such as easy commutation failure on the inverter side, strong dependence on the AC system, absorption of a large amount of reactive power, and large area occupied by the converter station. The new generation of flexible direct current transmission systems (Flexible-HVDC) can achieve decoupling control of active power and reactive power, can supply power to passive networks, has a compact structure and small footprint, and does not have the problem of commutation failure on the inverter side, but it has the disadvantages of high cost and large loss.

[0003] Therefore, the hybrid DC transmission system combining conventional DC transmission and flexible DC transmission will have a good prospect for engineering application. At present, the topological structure of the hybrid DC transmission system is mainly as follows: Figure 1 The symmetrical monopole hybrid two-terminal DC transmission system and Figure 2 The hybrid two-terminal DC transmission system with symmetrical bipolar connection is shown in the figure. These two systems combine the advantages of conventional DC transmission with low losses and mature operation technology, and flexible DC transmission can supply power to passive networks without commutation failure.

[0004] but Figure 1 and Figure 2In the hybrid DC transmission system, when the DC transmission line flashes due to insulation degradation or branches collide to cause ground fault, the DC system will form a short circuit for a short time, the active power cannot be output to the AC side, and a huge DC fault current is generated. However, the existing modular multi-level converter DC transmission system based on half-bridge submodules has a major defect: it cannot effectively handle the fault on the DC side and the system reliability is low. When a fault occurs on the DC side, even if all fully controlled switching devices are locked, the AC system will still feed current to the fault point through the anti-parallel freewheeling diodes of the fully controlled switching devices, which has an impact on the AC system equivalent to a three-phase short circuit. This will cause instantaneous overcurrent, so the AC circuit breaker must be tripped to cut it off, but the mechanical response time of the AC circuit breaker is at least 2 to 3 cycles, and the short-circuit overcurrent has increased to a large value during these 2 to 3 cycles; therefore, when selecting equipment, the rated parameters of the equipment have to be increased, and auxiliary measures such as high-speed bypass switches have to be configured; this greatly increases the construction cost of the converter station; therefore, the modular multilevel converter DC transmission system often needs to use cable lines with low failure rate and high cost as its DC transmission line, and cannot use overhead lines with high temporary failure rate and low cost such as flashover for power transmission, which makes it impossible to apply it to long-distance DC transmission occasions. If it is to be applied to long-distance overhead line transmission occasions, relevant measures need to be taken.

[0005] In the prior art, for DC line faults, ABB uses an additional DC circuit breaker to solve the DC overhead line fault; Siemens uses a modular multi-level converter structure based on a full-bridge submodule to solve the problem; and Alstom uses a full-bridge circuit and bridge arms connected in series with power electronic switching devices to solve the problem. The above solutions have greatly increased the construction cost of the converter station and increased the loss of the DC system. Summary of the invention

[0006] In view of the above problems, the present invention provides a hybrid DC transmission system with a fault handling function, which is applied to a hybrid DC transmission system composed of a rectifier converter station containing a thyristor converter at at least one end and an inverter converter station containing a voltage source converter at one end. The system has low cost loss, can realize flexible control of the hybrid DC transmission system, can effectively handle DC transmission line faults of the hybrid DC transmission system without locking the converter, can reliably cross the fault, realize rapid DC power recovery after the fault disappears, and better maintain the stability of the AC / DC transmission system.

[0007] In order to achieve the above technical objectives and the above technical effects, the present invention is implemented through the following technical solutions:

[0008] A hybrid DC power transmission system with a fault handling function, comprising a hybrid DC power transmission system, the hybrid DC power transmission system comprising a rectifier converter station for connecting to a sending-end AC power grid, an inverter station for connecting to a receiving-end AC power grid, and a DC transmission line for connecting the rectifier converter station and the inverter station, the rectifier converter station comprising at least one group of thyristor converter units, the inverter station comprising at least one group of voltage source converter units;

[0009] It is characterized in that it also includes a fault handling device, which is installed at the positive electrode outlet of the voltage source converter unit; or installed at the negative electrode outlet of the voltage source converter unit; or installed at the positive electrode outlet and the negative electrode outlet of the voltage source converter unit at the same time;

[0010] The fault handling device includes a blocking unit and a measuring unit, wherein the measuring unit is connected in parallel between one end of the blocking unit where the current flows out and the ground; or includes a blocking unit and a bypass switch, which are connected in parallel; or includes a blocking unit, a bypass switch and a knife switch assembly, wherein the blocking unit and the bypass switch are connected in parallel, and the two ends of the unit after parallel connection are respectively connected to one end connected to the knife switch, and the other end connected to the knife switch is connected in parallel to the bypass knife switch; or includes a blocking unit, a measuring unit and a bypass switch, wherein the blocking unit and the bypass switch are connected in parallel, and the measuring unit is connected in parallel between one end of the blocking unit where the current flows out and the ground; or includes a blocking unit, a measuring unit, a bypass switch and a knife switch assembly, wherein the blocking unit and the bypass switch are connected in parallel, and the two ends of the unit after parallel connection are respectively connected to one end connected to the knife switch, and the other end connected to the knife switch is connected in parallel to the bypass knife switch, and the measuring unit is connected in parallel between one end of the blocking unit where the current flows out and the ground.

[0011] In the above solution: the measuring unit is composed of a DC voltage measuring device.

[0012] In the above scheme: the DC voltage measuring device is one or more of the following: a DC electronic voltage transformer based on a voltage divider, a DC analog voltage transformer based on a voltage divider, or a DC electronic voltage transformer based on an optical voltage sensor.

[0013] In the above scheme: the blocking unit is composed of a plurality of power semiconductor devices connected in series, the power semiconductor device is a single semiconductor switching device, or a plurality of semiconductor switching devices are connected in series and / or in parallel, and the semiconductor switching device is any one or more of the following: a diode, a thyristor, an insulated gate bipolar transistor IGBT, an integrated gate-commutated thyristor IGCT, a turn-off thyristor GTO, a power field effect transistor Power MOSFET, an electron injection enhanced gate transistor IEGT, a gate-commutated thyristor GCT or a silicon carbide enhancement junction field effect transistor SiC-JFET.

[0014] In the above scheme: the blocking unit is composed of a converter unit composed of several power semiconductor devices, the power semiconductor device is a single semiconductor switching device, or a plurality of semiconductor switching devices are connected in series and / or in parallel, and the semiconductor switching device is any one or more of the following: a diode, a thyristor, an insulated gate bipolar transistor IGBT, an integrated gate-commutated thyristor IGCT, a turn-off thyristor GTO, a power field effect transistor Power MOSFET, an electron injection enhanced gate transistor IEGT, a gate-commutated thyristor GCT or a silicon carbide enhancement junction field effect transistor SiC-JFET.

[0015] In the above scheme: the converter unit is a three-phase bridge circuit or a plurality of three-phase bridge circuits connected in series and / or in parallel.

[0016] In the above solution: the bypass switch is any one or more of the following: a mechanical switch, a power electronic switch or a knife switch.

[0017] In the above scheme: the fault handling devices installed at the positive electrode outlet and / or the negative electrode outlet of the voltage source converter unit have the same or different composition structures.

[0018] In the above scheme: lightning arresters are connected in parallel at both ends of the blocking unit.

[0019] The beneficial effects of the present invention are:

[0020] 1) The present invention has low cost loss, can realize flexible control of the hybrid DC power transmission system through the jumper bypass switch and the measuring unit, can effectively handle the short circuit fault of the DC transmission line in the hybrid DC power transmission system, prevent the DC equipment from overcurrent, reliably and smoothly cross the DC line fault, and better protect the safety of the equipment.

[0021] 2) The present invention has a simple structure and is easy to operate. When a DC line fails, the converters on both sides can be kept open and reactive power support can still be provided to the connected AC system. This effectively prevents the inverter from losing both active power and reactive power during a DC line failure and maintains the voltage stability of the connected AC system.

[0022] 3) The present invention can effectively handle the DC transmission line fault of the hybrid DC transmission system without locking the converter, and can realize the rapid recovery of DC power after the fault, so as to better protect the safety of the AC / DC transmission system. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of a hybrid two-terminal DC transmission system with symmetrical monopole connection;

[0024] Figure 2 It is a schematic diagram of a hybrid two-terminal DC transmission system with symmetrical bipolar connection;

[0025] Figure 3 It is a structural schematic diagram of a first type of fault processing device of a hybrid DC fault processing device composed of a blocking unit and a measuring unit of the present invention;

[0026] Figure 4 It is a structural schematic diagram of a second type of fault processing device of a hybrid DC fault processing device composed of a blocking unit and a bypass switch of the present invention;

[0027] Figure 5 It is a structural schematic diagram of a third type of fault handling device of a hybrid DC fault handling device composed of a blocking unit, a bypass switch and a knife switch assembly of the present invention;

[0028] Figure 6 This is a structural schematic diagram of a fourth type of fault processing device of a hybrid DC fault processing device composed of a measuring unit, a blocking unit and a bypass switch of the present invention;

[0029] Figure 7 This is a structural schematic diagram of a fifth type of fault handling device of the present invention, which is a hybrid DC fault handling device consisting of a measuring unit, a blocking unit, a bypass switch and a knife switch assembly;

[0030] Figure 8 It is a schematic diagram of the structure of a blocking unit composed of a diode and a lightning arrester;

[0031] Fig. 9 It is a schematic diagram of the structure of a blocking unit composed of a thyristor and a lightning arrester;

[0032] Fig.10 The schematic diagram of the blocking unit structure is formed by a three-phase bridge circuit formed by thyristors;

[0033] Fig.11 It is a schematic diagram of a hybrid two-terminal DC power transmission system in which the fault handling device of the present invention is installed in a symmetrical monopole connection;

[0034] Fig.12 It is a schematic diagram of a fault handling device of the present invention installed on a high-voltage busbar of a voltage source converter on the inverter side of a hybrid two-terminal direct current transmission system with symmetrical bipolar connection;

[0035] Fig.13 It is a schematic diagram of a fault handling device of the present invention installed on a neutral busbar of a voltage source converter on the inverter side of a hybrid two-terminal direct current transmission system with symmetrical bipolar connection;

[0036] Fig.14 It is a schematic diagram of a fault handling device of the present invention being installed simultaneously on the high-voltage bus and the neutral bus of the voltage source converter on the inverter side of a hybrid two-terminal direct current transmission system with symmetrical bipolar connection;

[0037] Fig.15 It is a schematic diagram of a first specific fault handling device of the present invention being simultaneously installed on the inverter-side voltage source converter high-voltage busbar of a hybrid two-terminal DC power transmission system with symmetrical bipolar connection;

[0038] Fig.16 It is a schematic diagram of a second specific fault handling device of the present invention being installed simultaneously on a neutral busbar of a voltage source converter on the inverter side of a hybrid two-terminal DC power transmission system with symmetrical bipolar connection;

[0039] Fig.17 It is a schematic diagram of a fourth specific fault handling device of the present invention being simultaneously installed on the high-voltage busbar of a voltage source converter on the inverter side of a hybrid two-terminal DC power transmission system with symmetrical bipolar connection;

[0040] Fig.18 It is a schematic diagram of a first and a second specific fault handling device of the present invention being respectively installed on the high voltage bus and the neutral bus of the voltage source converter on the inverter side of a hybrid two-terminal direct current transmission system with symmetrical bipolar connection;

[0041] Fig.19 It is a schematic diagram of a fourth and a second specific fault handling device of the present invention being respectively installed on the high voltage bus and the neutral bus of the voltage source converter on the inverter side of a hybrid two-terminal DC power transmission system with symmetrical bipolar connection;

[0042] Fig. 20 It is a schematic diagram of a hybrid three-terminal DC transmission system with symmetrical unipolar connection, which contains two rectifier converter stations based on thyristor converters and one inverter converter station based on voltage source converters;

[0043] Fig.21 It is a schematic diagram of a hybrid three-terminal DC transmission system with symmetrical unipolar connection, which includes a rectifier converter station based on thyristor converter, an inverter station based on thyristor converter, and an inverter station based on voltage source converter;

[0044] Fig. 22 It is a schematic diagram of a hybrid three-terminal DC transmission system with symmetrical bipolar connection, which contains two rectifier converter stations based on thyristor converters and one inverter converter station based on voltage source converter;

[0045] Fig.23 It is a schematic diagram of a hybrid three-terminal DC transmission system with symmetrical bipolar connection, which includes a rectifier converter station based on thyristor converter, an inverter station based on thyristor converter, and an inverter station based on voltage source converter. DETAILED DESCRIPTION

[0046] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it, but the embodiments are not intended to limit the present invention.

[0047] A hybrid DC power transmission system with a fault handling function comprises a hybrid DC power transmission system and a fault handling device, wherein the hybrid DC power transmission system comprises a rectifier-converter station for connecting to a sending-end AC power grid, an inverter-converter station for connecting to a receiving-end AC power grid, and a DC transmission line for connecting the rectifier-converter station and the inverter-converter station, the inverter-converter station is connected to the receiving-end AC power grid via a transformer, the rectifier-converter station comprises at least one group of thyristor converter units, and the inverter-converter station comprises at least one group of voltage source converter units.

[0048] Generally, the rectifier-converter station adopts a converter based on thyristor devices, and the thyristor converter can be a six-pulse bridge circuit, a twelve-pulse bridge circuit or a double twelve-pulse bridge circuit; while the inverter-converter station adopts a voltage source converter based on a fully-controlled power electronic device, and the voltage source converter unit is any one or more of the following: a two-level converter, a diode-clamped multilevel converter, a modular multilevel converter MMC, a hybrid multilevel converter HMC, a two-level cascade converter CSL or a stacked two-level converter CTL, and the submodules constituting the bridge arm of the modular multilevel converter are one or more of the following: half-bridge type, full-bridge type, quasi-full-bridge type, clamped twin-module type, all of which are composed of fully-controlled power semiconductors that can be turned off, such as IGBT, IGCT, IEGT or GTO.

[0049] Figure 1 The symmetrical monopole hybrid two-terminal DC transmission system and Figure 2 The hybrid two-terminal DC power transmission system with symmetrical bipolar connection shown in the figure is a relatively common hybrid DC power transmission system. The present invention is applicable to Figure 1 and Figure 2 The hybrid DC power transmission system shown in the figure is not limited to these two power transmission systems. The present invention is applicable to all hybrid DC power transmission systems with thyristor converters on the rectifier side and voltage source converters on the inverter side. Figure 2 This will be described as a specific example.

[0050] like Figure 2As shown, the hybrid DC transmission system includes: a rectifier converter station and an inverter station, which are connected by two DC transmission lines, wherein: the rectifier converter station is used to convert the three-phase AC power of the sending-end AC power grid into DC power and then transmit it to the inverter station through the DC transmission line. The busbar of the sending-end AC power grid entering the station may be connected with a passive filter or not, which needs to be determined according to the system engineering conditions. When the sending end is composed of a thyristor converter, it is generally necessary to install a passive filter, and sometimes a reactive compensation capacitor is also required. Figure 2 The medium rectifier converter station consists of two groups of thyristor converter units connected in series, whose series nodes are connected to the grounding electrode. The positive and negative ends of the series connection are connected to the DC transmission line through smoothing reactors; at the same time, a DC filter is installed between the DC line and the ground.

[0051] The thyristor converter unit adopts a twelve-pulse bridge circuit; each bridge arm is composed of several thyristors connected in series, and the thyristor converter adopts a constant DC current control strategy. The thyristor converter is connected to the sending-end AC grid through a three-winding transformer with a connection mode of Y0 / Y / Δ, and an AC circuit breaker is installed on the primary side of the transformer. The transformer can transform the voltage level of the three-phase AC power of the sending-end AC system to adapt to the required DC voltage level. The different connection modes of the secondary side of the transformer provide three-phase AC power with a phase angle difference of 30° for the upper and lower six-pulse conversion bridges of the twelve-pulse bridge thyristor converter to reduce the harmonic current flowing into the grid.

[0052] The inverter station is used to convert DC power into three-phase AC power and then transmit it to the receiving AC power grid. It consists of two groups of voltage source converters connected in series, and the series node is connected to the grounding electrode. The voltage source converter is connected to the receiving AC power grid through a double-winding transformer with a connection method of Y0 / Δ. AC circuit breakers are installed on the primary side of the transformer. The voltage source converter is controlled by a constant DC voltage and constant reactive power control strategy.

[0053] The device also includes a fault handling device, which includes a blocking unit 1 and a measuring unit 2, wherein the measuring unit 2 is connected in parallel between a current outflow end of the blocking unit 1 and the ground. Figure 3 As shown, it is recorded as the first fault handling device, which includes a first series terminal X1 for connecting the DC positive terminal or negative terminal of the voltage source converter of the hybrid DC transmission system and one end of the device, and a second series terminal X2 for connecting the other end of the device and one end of the smoothing reactor of the hybrid DC transmission system or one end of the neutral bus; or includes a blocking unit 1 and a bypass switch 3, which are connected in parallel, as shown in FIG. Figure 4As shown, it is recorded as the second fault handling device, which includes a first series terminal X1 for connecting the DC positive terminal or negative terminal of the voltage source converter of the hybrid DC transmission system and one end of the device, and a second series terminal X2 for connecting the other end of the device and one end of the smoothing reactor of the hybrid DC transmission system or one end of the neutral bus; or includes a blocking unit 1 and a bypass switch 3 and a knife switch assembly, the blocking unit 1 is connected in parallel with the bypass switch 3, and the two ends of the parallel-connected unit are respectively connected to one end of the knife switch 4, and the other end of the knife switch 4 is connected in parallel with the bypass knife switch 5, as shown Figure 5 As shown, it is recorded as the third fault handling device, which includes a first series terminal X1 for connecting the DC positive terminal or negative terminal of the voltage source converter of the hybrid DC transmission system and one end of the device, and a second series terminal X2 for connecting the other end of the device and one end of the smoothing reactor of the hybrid DC transmission system or one end of the neutral bus; or includes a blocking unit 1, a measuring unit 2 and a bypass switch 3, wherein the blocking unit 1 is connected in parallel with the bypass switch 3, and the measuring unit 2 is connected in parallel between the end of the blocking unit 1 where the current flows out and the ground, as shown in FIG. Figure 6 As shown, it is recorded as the fourth fault handling device, which includes a first series terminal X1 for connecting the DC positive terminal or negative terminal of the voltage source converter of the hybrid DC transmission system and one end of the device, and a second series terminal X2 for connecting the other end of the device and one end of the smoothing reactor of the hybrid DC transmission system or one end of the neutral bus; or includes a blocking unit 1, a measuring unit 2 and a bypass switch 3 and a knife switch assembly, wherein the blocking unit 1 is connected in parallel with the bypass switch 3, and the two ends of the parallel-connected unit are respectively connected to one end of the connection knife switch 4, and the other end of the connection knife switch 4 is connected in parallel with the bypass knife switch 5, and the measuring unit 2 is connected in parallel between the end of the blocking unit where the current flows out and the ground, as shown Figure 7 As shown, it is recorded as the fifth fault handling device, which includes a first series terminal X1 for connecting the DC positive terminal or negative terminal of the voltage source converter of the hybrid DC transmission system and one end of the device, and a second series terminal X2 for connecting the other end of the device and one end of the smoothing reactor of the hybrid DC transmission system or one end of the neutral bus.

[0054] In the above solution: the measuring unit is composed of a DC voltage measuring device.

[0055] In the above scheme: the DC voltage measuring device is one or more of the following: a DC electronic voltage transformer based on a voltage divider, a DC analog voltage transformer based on a voltage divider, or a DC electronic voltage transformer based on an optical voltage sensor.

[0056] In the above scheme: the blocking unit is composed of a plurality of power semiconductor devices connected in series, the power semiconductor device is a single semiconductor switching device, or a plurality of semiconductor switching devices are connected in series and / or in parallel, and the semiconductor switching device is any one or more of the following: a diode, a thyristor, an insulated gate bipolar transistor IGBT, an integrated gate-commutated thyristor IGCT, a turn-off thyristor GTO, a power field effect transistor Power MOSFET, an electron injection enhanced gate transistor IEGT, a gate-commutated thyristor GCT or a silicon carbide enhancement junction field effect transistor SiC-JFET.

[0057] In the above scheme: the two ends of the blocking unit are connected in parallel with a lightning arrester, such as Figure 8 As shown, it includes a first series terminal X3 for connecting the X2 end of the fault handling device and one end of the blocking unit, and a second series terminal X6 for connecting the other end of the blocking unit and the X1 end of the fault handling device. The blocking unit is formed by a plurality of semiconductor switching devices including but not limited to a diode switch device 6 and a diode switch device 7. The cathode of the diode switch device 6 is connected to the anode of the next semiconductor switch device at X4, the cathode of the diode switch device 7 is connected to the X6 end of the blocking unit, and the two ends of the lightning arrester 8 are connected in parallel to the X3 and X6 ends of the blocking unit. Or as Fig. 9 As shown, it includes a first series terminal X3 for connecting the X2 end of the fault handling device and one end of the blocking unit, and a second series terminal X6 for connecting the other end of the blocking unit and the X1 end of the fault handling device. The blocking unit is formed by a plurality of semiconductor switching devices including but not limited to a thyristor switch device 9 and a thyristor switch device 10. The cathode of the thyristor switch device 9 is connected to the anode of the next semiconductor switch device at X4, the cathode of the thyristor switch device 10 is connected to the X6 end of the blocking unit, and the two ends of the arrester 8 are connected in parallel to the X3 and X6 ends of the blocking unit.

[0058] In the above scheme: the blocking unit is composed of a converter unit composed of several power semiconductor devices, the power semiconductor device is a single semiconductor switching device, or a plurality of semiconductor switching devices are connected in series and / or in parallel, and the semiconductor switching device is any one or more of the following: a diode, a thyristor, an insulated gate bipolar transistor IGBT, an integrated gate-commutated thyristor IGCT, a turn-off thyristor GTO, a power field effect transistor Power MOSFET, an electron injection enhanced gate transistor IEGT, a gate-commutated thyristor GCT or a silicon carbide enhancement junction field effect transistor SiC-JFET.

[0059] In the above scheme: the converter unit is a three-phase bridge circuit or multiple three-phase bridge circuits connected in series and / or in parallel, such as Fig.10As shown, it includes a first series terminal P for connecting the X2 end of the fault handling device and one end of the blocking unit, a second series terminal N for connecting the other end of the blocking unit and the X1 end of the fault handling device, and three AC terminals A, B, and C are respectively used to connect the ABC three-phase power supply of the AC system, and the power semiconductor device of each bridge arm in the three-phase bridge circuit is composed of multiple semiconductor switching devices connected in series.

[0060] In the above solution: the bypass switch is any one or more of the following: a mechanical switch, a power electronic switch or a knife switch.

[0061] In the above scheme: the fault handling device is installed at the positive electrode outlet of the voltage source converter unit, or installed at the negative electrode outlet of the voltage source converter unit, such as Fig.12 , Fig.13 As shown, or installed at the positive and negative outlets of the voltage source converter unit at the same time, such as Fig.11 , Fig.14 shown.

[0062] In the above scheme: the fault handling devices installed at the positive electrode outlet and / or the negative electrode outlet of the voltage source converter unit have the same or different structures, such as Fig.15 , Fig.16 , Fig.17 , Fig.18 , Fig.19 As shown. Fig.15 The figure shows the installation of the first fault handling device at the high voltage bus terminal of the voltage source converter of the inverter station. Fig.16 The second fault handling device is installed at the low voltage bus terminal of the voltage source converter of the inverter station. Fig.17 The fourth fault handling device is installed at the high voltage bus terminal of the voltage source converter of the inverter station. Fig.18 The figure shows that the first fault handling device is installed at the high voltage bus end of the voltage source converter of the inverter station, and the second fault handling device is installed at the low voltage bus end. Fig.19 The figure shows that the fourth fault handling device is installed at the high-voltage bus end of the voltage source converter of the inverter station, and the second fault handling device is installed at the low-voltage bus end.

[0063] The following is only Fig.19 A specific embodiment of the hybrid DC transmission system shown is used to describe the working process.

[0064] like Fig.19A hybrid two-terminal DC transmission system with a DC voltage of 500 kV is shown, wherein the thyristor converter is in the rectifier station, and the voltage source converter is in the inverter station, wherein the voltage source converter adopts a modular multilevel converter based on a half-bridge submodule, and a fault handling device is installed at the high-voltage outlet of the modular multilevel converter, including a blocking unit, a measuring unit and a bypass switch, wherein the blocking unit is connected in parallel with the bypass switch, and the measuring unit is connected in parallel between one end of the current outflow of the blocking unit and the ground, and the blocking unit is composed of a plurality of diodes connected in series and in parallel, and lightning arresters are connected in parallel at both ends, wherein the cathode end of the blocking unit diode is connected in series with the high-voltage outlet of the modular multilevel converter, and the anode end is connected in series with one end of the smoothing reactor, and the measuring unit adopts a DC electronic voltage transformer based on a voltage divider, one end of which is connected between the cathode end of the blocking unit diode and the high-voltage outlet of the modular multilevel converter, and the other end is connected to the ground. The bypass switch adopts a mechanical switch commonly used in DC transmission systems. A fault handling device is also installed at the low-voltage outlet end of the modular multilevel converter, including a blocking unit and a bypass switch, which are connected in parallel. The blocking unit is composed of multiple diodes connected in series and in parallel, and lightning arresters are connected in parallel at both ends. The anode end of the blocking unit diode is connected in series with the low-voltage outlet end of the modular multilevel converter, and the cathode end is connected in series with one end of the neutral bus. The bypass switch adopts a mechanical switch commonly used in DC transmission systems.

[0065] During the startup and operation of the hybrid DC transmission system, the modular multilevel converter requires a grounding point. Originally, the hybrid DC transmission system is connected to a grounding electrode, which can provide a grounding point. However, due to the serial connection of the fault handling device, the connection between the modular multilevel converter and the grounding electrode is blocked. Therefore, during startup, the bypass switch NBK2 in the fault handling device at the low-voltage outlet end can be closed to bypass the blocking unit, thereby providing a reliable connection between the modular multilevel converter and the grounding electrode, thereby realizing the pre-charging process of the modular multilevel converter. At the same time, the bypass switch NBK2 can be opened after the DC current is established and the blocking unit is turned on, without affecting the blocking function of the fault handling device. After the pre-charging is completed, the modular multilevel converter normally unlocks the DC voltage of the control system. Since the high-voltage outlet end of the modular multilevel converter is connected in series with a fault handling device, which realizes the isolation between the high-voltage outlet end of the modular multilevel converter and the DC line, the DC voltage measured by the normally configured DC line voltage measuring device UDL cannot correctly reflect the DC voltage at both ends of the modular multilevel converter at this time, which will bring problems to the flexible control of the entire hybrid DC transmission system. This problem can be solved in three ways: a. Avoid using the inverter-side modular multilevel converter to control the DC voltage, and instead use the rectifier station thyristor converter to control the DC voltage, and the inverter-side modular multilevel converter to control the power. Although this solution can solve the above problems, it solidifies the control mode of the converter stations at both ends, and the operation of the entire system is not flexible enough. At the same time, it will cause problems for the coordination of the controllers of the converters of the two stations, which is very likely to cause DC overvoltage. b. The DC voltage UDP measured by the measuring unit of the fault handling device is used to realize the DC voltage control of the system. Since the DC voltage UDP measured by the measuring unit truly reflects the DC voltage at both ends of the modular multilevel converter, the above-mentioned problem can be solved and the flexible control of the DC voltage can be realized. c. The bypass switch NBK1 of the fault handling device is used to close the bypass switch NBK1 and bypass the blocking unit to provide a reliable connection between the modular multilevel converter and the DC line, thereby realizing the reliable measurement of the DC voltage at both ends of the modular multilevel converter and realizing the control of the DC voltage. At the same time, the bypass switch NBK1 can be opened after the DC current is established and the blocking unit is turned on, without affecting the blocking function of the fault handling device. In addition, when the blocking unit of the fault handling device fails, the bypass switch NBK1 and the bypass switch NBK2 can be bypassed, thereby not affecting the normal and stable operation of the entire hybrid DC transmission system.

[0066] When the inverter station collects and detects that the rate of change of the DC voltage UDL is less than -580kv / ms and the duration exceeds 1ms, the inverter station determines that the DC transmission line has a fault at this time, and sends the fault signal to the rectifier station through the communication between the two stations. After receiving the fault signal, the rectifier station immediately increases the trigger angle quickly to make the thyristor converter at the sending end switch to inverter operation, extract the DC side fault energy, and the DC side current quickly drops to zero; at the same time, the inverter station uses the unidirectional conductive characteristics of the blocking unit in the fault handling device to block the AC system through the anti-parallel freewheeling diode of the fully controlled switch device to feed current to the fault point. Cut off the fault current, at this time, the modular multi-level converter of the inverter station does not need to be locked, and can continue to operate in the STATCOM mode to provide reactive voltage support to the connected AC system.

[0067] When the arc-off time of 250ms is reached, the thyristor converter of the rectifier station immediately cancels the intervention on the trigger angle and starts to reduce the trigger angle to try to establish a DC voltage. During this process, if the DC voltage is successfully established, the DC transmission line fault disappears, and the thyristor converter of the rectifier station restores the transmitted power to the power level before the fault. The modular multilevel converter of the inverter station restores the generated DC voltage and output AC current to the level before the fault, and the hybrid DC transmission system returns to normal.

[0068] If during this process, the rectifier station has not detected that the DC voltage of the hybrid DC transmission system is greater than 0.5 times the rated DC voltage of 250kV, it is determined that the DC transmission line fault still exists at this time, and the sending-end thyristor converter will immediately switch to the inverter operation state to extract the DC side fault energy; at the same time, the inverter station still maintains the STATCOM mode unchanged. Wait for 250ms again to extinguish the arc. When the waiting arc extinguishing time arrives, the thyristor converter of the rectifier station immediately cancels the intervention on the trigger angle and starts to reduce the trigger angle to try to establish the DC voltage. During this process, if the DC voltage is successfully established, the DC transmission line fault disappears, the thyristor converter of the rectifier station will restore the transmitted power to the power level before the fault, and the modular multilevel converter of the inverter station will restore the generated DC voltage and output AC current to the level before the fault, and the hybrid DC transmission system will return to normal.

[0069] It should be pointed out that Fig. 20 , Fig.21 The symmetrical monopole hybrid three-terminal DC transmission system shown in FIG. Fig. 22 , Fig.23 The hybrid three-terminal DC power transmission system with symmetrical bipolar connection shown is also applicable to the present invention. The present invention is applicable to all hybrid DC power transmission systems with thyristor converters on the rectifier side and voltage source converters on the inverter side.

[0070] 1) The present invention has low cost loss, can realize flexible control of the hybrid DC power transmission system through the jumper bypass switch and the measuring unit, can effectively handle the short circuit fault of the DC transmission line in the hybrid DC power transmission system, prevent the DC equipment from overcurrent, reliably and smoothly cross the DC line fault, and better protect the safety of the equipment.

[0071] 2) The present invention has a simple structure and is easy to operate. When a DC line fails, the converters on both sides can be kept open and reactive power support can still be provided to the connected AC system. This effectively prevents the inverter from losing both active power and reactive power during a DC line failure and maintains the voltage stability of the connected AC system.

[0072] 3) The present invention can effectively handle the DC transmission line fault of the hybrid DC transmission system without locking the converter, and can realize the rapid recovery of DC power after the fault, so as to better protect the safety of the AC / DC transmission system.

[0073] The above are only preferred embodiments of the present invention, and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A hybrid DC power transmission system with a fault handling function, comprising a hybrid DC power transmission system, wherein the hybrid DC power transmission system comprises a rectifier-converter station for connecting to a sending-end AC power grid, an inverter-converter station for connecting to a receiving-end AC power grid, and a DC transmission line for connecting the rectifier-converter station and the inverter-converter station, wherein the rectifier-converter station comprises at least one group of thyristor converter units, and the inverter-converter station comprises at least one group of voltage source converter units; It is characterized in that It also includes a fault handling device, which is installed at the positive electrode outlet of the voltage source converter unit; or installed at the negative electrode outlet of the voltage source converter unit; or installed at both the positive electrode outlet and the negative electrode outlet of the voltage source converter unit; The fault handling device comprises a blocking unit and a measuring unit, wherein the measuring unit is connected in parallel between one end of the blocking unit where the current flows out and the earth; or comprises a blocking unit and a bypass switch, which are connected in parallel; or comprises a blocking unit, a bypass switch and a knife switch assembly, wherein the blocking unit and the bypass switch are connected in parallel, and the two ends of the unit after the parallel connection are respectively connected to one end connected to the knife switch, and the other end connected to the knife switch is connected in parallel to the bypass knife switch; or comprises a blocking unit, a measuring unit and a bypass switch, wherein the blocking unit and the bypass switch are connected in parallel, and the measuring unit is connected in parallel between one end of the blocking unit where the current flows out and the earth; or comprises a blocking unit, a measuring unit, a bypass switch and a knife switch assembly, wherein the blocking unit and the bypass switch are connected in parallel, and the two ends of the unit after the parallel connection are respectively connected to one end connected to the knife switch, and the other end connected to the knife switch is connected in parallel to the bypass knife switch, and the measuring unit is connected in parallel between one end of the blocking unit where the current flows out and the earth; The blocking unit is composed of a plurality of power semiconductor devices connected in series, wherein the power semiconductor device is a single semiconductor switch device, or a plurality of semiconductor switch devices connected in series and / or in parallel; or, The blocking unit is composed of a converter unit composed of a plurality of power semiconductor devices, wherein the power semiconductor device is a single semiconductor switch device, or a plurality of semiconductor switch devices are connected in series and / or in parallel; When a fault occurs in a DC transmission line, the inverter station uses the unidirectional conductive characteristics of the blocking unit in the fault handling device to block the path for the AC system to feed current to the fault point through the anti-parallel freewheeling diode of the fully controlled switching device.

2. A hybrid DC power transmission system fault handling device with fault handling function according to claim 1, characterized in that: The measuring unit is composed of a DC voltage measuring device.

3. A hybrid DC power transmission system fault handling device with fault handling function according to claim 2, characterized in that: The DC voltage measuring device is one or more of the following: a DC electronic voltage transformer based on a voltage divider, a DC analog voltage transformer based on a voltage divider, or a DC electronic voltage transformer based on an optical voltage sensor.

4. A hybrid DC power transmission system fault handling device with fault handling function according to claim 3, characterized in that: The semiconductor switching device is any one or more of the following: a diode, a thyristor, an insulated gate bipolar transistor IGBT, an integrated gate-commutated thyristor IGCT, a turn-off thyristor GTO, a power field effect transistor Power MOSFET, an electron injection enhanced gate transistor IEGT, a gate-commutated thyristor GCT or a silicon carbide enhancement junction field effect transistor SiC-JFET.

5. A hybrid DC power transmission system fault handling device with fault handling function according to claim 1, characterized in that: The converter unit is a three-phase bridge circuit or a plurality of three-phase bridge circuits connected in series and / or in parallel.

6. A hybrid DC power transmission system fault handling device with fault handling function according to claim 1, characterized in that: The bypass switch is any one or more of the following: a mechanical switch or a power electronic switch.

7. A hybrid DC power transmission system fault handling device with fault handling function according to claim 1, characterized in that: The fault handling devices installed at the positive electrode outlet and / or the negative electrode outlet of the voltage source converter unit may have the same or different components and structures.

8. A hybrid DC power transmission system fault handling device with fault handling function according to any one of claims 1 to 7, characterized in that: Both ends of the blocking unit are connected in parallel with lightning arresters.

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