Hybrid cascade converter and control method, device, equipment and power system thereof

Abstract

The present application relates to a hybrid cascade converter and its control method, device, computer equipment, computer-readable storage medium, computer program product, and power system. The method includes: when a current abnormality of a target DC transmission line is detected, adjusting the trigger angle of a target thyristor converter so that the target thyristor converter outputs a negative voltage and keeping the working state of the modular converter unchanged. When the target thyristor converter outputs a negative voltage, if it is detected that the amplitude of the current of the target DC transmission line has not returned to normal within a preset time, the modular converter is controlled to lock. On the one hand, the target thyristor converter is used to suppress the fault current, and on the other hand, the modular converter can be used to provide AC voltage support for the AC power grid, and the AC voltage support capability is strong.

Description

Technical Field

[0001] The present application relates to the field of power transmission technology, and in particular to a hybrid cascade converter and a control method, device, computer equipment, computer readable storage medium, computer program product, and power system thereof. Background Art

[0002] With the development of electric power science and technology, the line commutated converter high voltage direct current (LCC-HVDC) technology based on line commutated converter has emerged. It has the advantages of high withstand voltage, large power capacity, and low operating loss, and is widely used in DC transmission. However, it also has poor controllability and may have commutation failure. As a result, the modular multilevel converter (MMC-HVDC) technology has emerged. It has strong controllability and no commutation failure, which makes up for the defects of LCC, but its cost and operating loss are high, and its transmission capacity is small.

[0003] In traditional technologies, in order to give full play to the advantages of LCC and MMC, a variety of new hybrid DC transmission systems have been proposed, using a hybrid cascade topology of LCC and MMC as a DC transmission system. This type of high-voltage transmission system integrates the advantages of high transmission capacity of traditional DC transmission and strong controllability of flexible DC transmission, without phase change failure, etc.

[0004] However, the LCC and MMC hybrid cascade transmission system in traditional technology requires locking the MMC when a DC short-circuit fault occurs, has poor flexibility, and cannot provide AC support. Summary of the invention

[0005] Based on this, it is necessary to provide a hybrid cascade converter and its control method, device, computer equipment, computer-readable storage medium, computer program product, and power system that can quickly limit the DC fault current in the transmission system while taking into account the AC voltage support capability in response to the above-mentioned technical problems.

[0006] In a first aspect, an embodiment of the present application provides a control method for a hybrid cascade converter, wherein the hybrid cascade converter includes a plurality of cascaded converters, wherein the plurality of converters include at least one thyristor converter and at least one modular converter, wherein one end of the first-stage converter is connected to a first DC transmission line, and one end of the last-stage converter is connected to a second DC transmission line, and the plurality of converters are respectively connected to an AC power grid through respective corresponding transformers; the method includes:

[0007] In the case of detecting a current anomaly of a target DC transmission line, adjusting a trigger angle of a target thyristor converter so that the target thyristor converter outputs a negative voltage and keeps the working state of the modular converter unchanged, wherein the target thyristor converter is at least one thyristor converter cascaded between the last-stage modular converter and the first DC transmission line, the target DC transmission line is one of the first DC transmission line and the second DC transmission line, and the negative voltage is used to suppress the current anomaly of the target DC transmission line;

[0008] In the case where the target thyristor converter outputs a negative voltage, if it is detected that the current of the target DC transmission line does not return to normal within a preset time, the modular converter is controlled to be locked.

[0009] In one of the embodiments, adjusting the trigger angle of the target thyristor converter so that the target thyristor converter outputs a negative voltage includes: adjusting the trigger angle of the target thyristor converter to a preset angle so that the target thyristor converter outputs a negative voltage, wherein the preset angle is greater than 90°; or adjusting the trigger angle of the target thyristor converter so that the DC current flowing through the target thyristor converter is limited to below a preset current value.

[0010] In one of the embodiments, the control method of the hybrid cascade converter also includes: determining a predicted commutation time of the target thyristor converter according to the parameters of the target thyristor converter and the current trigger angle; determining a current change according to the predicted commutation time and the current current value of the target DC transmission line; determining a maximum trigger angle of the target thyristor converter according to the current change, the current current value of the target DC transmission line, the parameters of the target thyristor converter, and the line voltage of the AC power grid; and limiting a range of a trigger angle of the target thyristor converter according to the maximum trigger angle and the preset angle after a set time after a current abnormality of the target DC transmission line is detected.

[0011] In one of the embodiments, the control method of the hybrid cascade converter further includes: when it is determined that the fault of the target DC transmission line is cleared, after a preset time period, controlling the target thyristor converter to operate in a normal working state, wherein, in the normal working state, the range of the trigger angle of the target thyristor converter is a preset rated range.

[0012] In one of the embodiments, the control method of the hybrid cascade converter further includes: when the target thyristor converter outputs a negative voltage and it is detected that the current of the target DC transmission line returns to normal within a preset time, maintaining the working state of the modular converter unchanged.

[0013] In a second aspect, the present application provides a hybrid cascade converter, comprising a plurality of cascaded converters, wherein the plurality of converters include at least one thyristor converter and at least one modular converter, and further comprising a plurality of transformers and controllers, wherein one end of the first-stage converter is connected to a first DC transmission line, and one end of the last-stage converter is connected to a second DC transmission line, the plurality of converters are respectively connected to an AC power grid through their respective corresponding transformers, and the controller is respectively connected to the at least one thyristor converter and the at least one modular converter;

[0014] The controller is used to adjust the trigger angle of the target thyristor converter when a current abnormality of the target DC transmission line is detected, so that the target thyristor converter outputs a negative voltage and controls the working state of the modular converter to remain unchanged; when the target thyristor converter outputs a negative voltage, if it is detected that the current of the target DC transmission line does not reach the corresponding current preset range within a preset time, the modular converter is controlled to be locked, wherein the target thyristor converter is at least one thyristor converter cascaded between the last-level modular converter and the first DC transmission line, the target DC transmission line is one of the first DC transmission line and the second DC transmission line, and the negative voltage is used to suppress the current abnormality of the target DC transmission line.

[0015] In one embodiment, the hybrid cascade converter includes a first valve group, a second valve group, and a third valve group, wherein the first end of the first valve group is connected to the first DC transmission line, the second end of the first valve group is connected to the first end of the second valve group, the second end of the second valve group is connected to the first end of the third valve group, and the second end of the third valve group is connected to the second DC transmission line, wherein the first valve group includes at least one thyristor converter, the second valve group includes at least one thyristor converter, and the third valve group includes at least one modular converter, wherein one end of at least one thyristor converter in the first valve group serves as the first end of the first valve group, and one end of at least one modular converter in the third valve group serves as the second end of the third valve group.

[0016] In one embodiment, the third valve group includes a plurality of modular converters connected in parallel, each of the modular converters being connected to an AC power grid through a corresponding transformer, wherein at least one modular converter is also used to be connected to a new energy power station; the controller is also used to control a working state of one of the modular converters to remain unchanged when an abnormal current is detected in the target DC transmission line, so as to provide AC voltage support for the AC power grid, and control at least one modular converter connected to the new energy power station to operate so as to realize power transmission between the new energy power station and the AC power grid.

[0017] In a third aspect, the present application provides a power system, comprising: the aforementioned hybrid cascade converter, an AC power grid, a first DC transmission line, and a second DC transmission line.

[0018] In a fourth aspect, the present application provides a computer device, including a memory and a processor, wherein the memory stores a computer program, and the processor implements the aforementioned control method of the hybrid cascade converter when executing the computer program.

[0019] In a fifth aspect, the present application provides a computer-readable storage medium having a computer program stored thereon, wherein the computer program, when executed by a processor, implements the aforementioned control method of the hybrid cascade converter.

[0020] In a sixth aspect, the present application provides a computer program product, which includes a computer program, and when the computer program is executed by a processor, the control method of the aforementioned hybrid cascade converter is implemented.

[0021] The above hybrid cascade converter and its control method, device, computer equipment, computer readable storage medium, computer program product, and power system. The method is applied to a hybrid cascade converter including multiple cascade converters, wherein the multiple converters include at least one thyristor converter and at least one modular converter. The method first detects whether the current of the target DC transmission line is abnormal, so as to determine whether the target DC transmission line is faulty. Then, when the current of the target DC transmission line is detected to be abnormal, the trigger angle of the target thyristor converter is adjusted so that the target thyristor converter outputs a negative voltage, and the negative voltage is used to suppress the current abnormality of the target DC transmission line, so as to suppress the abnormal rising current on the target DC transmission line, and realize the suppression and current limiting of the fault current. While the target thyristor converter outputs a negative voltage, the working state of the modular converter is kept unchanged, so that the modular converter still works normally, forms a loop with the AC power grid, and ensures the normal operation of the AC power grid, so that even when the target DC transmission line fails, it can still provide AC voltage support for the AC power grid. In the case where the target thyristor converter outputs a negative voltage, if it is detected that the current of the target DC transmission line has not returned to normal within the preset time, it means that the target thyristor converter has failed to suppress the fault current. At this time, in order to ensure the safety of the components in the circuit, it is necessary to control the modular converter to lock. After the modular converter is locked, it can prevent excessive fault current from damaging the IGBT (Insulated Gate Bipolar Transistor, Chinese name is Insulated Gate Bipolar Transistor) in the modular converter. In summary, using the method of the present application, when a fault occurs in the target DC transmission line, on the one hand, the target thyristor converter can be used to suppress the fault current, and on the other hand, the modular converter can still be used to provide AC voltage support for the AC power grid. The AC voltage support capacity is strong, which ensures the normal operation of the AC power grid and high reliability. When the target thyristor converter fails to suppress the fault current, the modular converter is locked again to prevent the IGBT in the modular converter from being damaged, which also ensures the safety of the hybrid cascade converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the conventional technology, the drawings required for use in the embodiments or the conventional technology descriptions are briefly introduced below. 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 creative work.

[0023] Figure 1 is a schematic flow chart of a control method for a hybrid cascade converter in one embodiment;

[0024] Figure 2is a schematic structural diagram of a hybrid cascade converter in one embodiment;

[0025] Figure 3 is a schematic diagram of a complete power system in one embodiment;

[0026] Figure 3a A schematic diagram of a short circuit fault in a power system in one embodiment;

[0027] Figure 3b The second schematic diagram of a short circuit fault in a power system in one embodiment;

[0028] Figure 4 A schematic diagram of the structure of an LCC in one embodiment;

[0029] Figure 5 This is a second schematic diagram of the structure of LCC in one embodiment;

[0030] Figure 6 A schematic diagram of the structure of an MMC in an embodiment;

[0031] Figure 7 Schematic diagram of the structure of the submodule of MMC in one embodiment;

[0032] Figure 8 A schematic flow chart of a method for adjusting a trigger angle in one embodiment;

[0033] Fig. 9 A schematic flow chart of a method for limiting a trigger angle in one embodiment;

[0034] Fig.10 is a schematic diagram of a fast current limiting controller in one embodiment;

[0035] Fig.11 The second flowchart of the control method of the hybrid cascade converter in one embodiment;

[0036] Fig.12 The third flowchart of a control method for a hybrid cascade converter in one embodiment;

[0037] Fig.13 The second structural schematic diagram of a hybrid cascade converter in one embodiment;

[0038] Fig.14 This is the third structural schematic diagram of a hybrid cascade converter in an embodiment.

[0039] Description of reference numerals:

[0040] 10-thyristor converter, 20-modular converter, 30-first DC transmission line, 31-second DC transmission line, 50-AC power grid, 41-first valve group, 42-second valve group, 43-third valve group, 60-bypass switch circuit, 70-AC filter device, 80-new energy power station. DETAILED DESCRIPTION

[0041] In order to facilitate understanding of the present application, the present application will be described more fully below with reference to the relevant drawings. Embodiments of the present application are provided in the drawings. However, the present application can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present application more thorough and comprehensive.

[0042] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0043] It can be understood that the terms "first", "second", etc. used in the present application can be used in this article to describe various elements, but these elements are not limited by these terms. These terms are only used to distinguish the first element from another element.

[0044] It should be noted that when an element is considered to be "connected" to another element, it can be directly connected to the other element, or connected to the other element through an intermediate element. In addition, the "connection" in the following embodiments should be understood as "electrical connection", "communication connection", etc. if there is transmission of electrical signals or data between the connected objects.

[0045] When used herein, the singular forms "a", "an", and "said / the" may also include plural forms, unless the context clearly indicates otherwise. It should also be understood that the terms "include / comprise" or "have" etc. specify the presence of stated features, wholes, steps, operations, components, parts or combinations thereof, but do not exclude the possibility of the presence or addition of one or more other features, wholes, steps, operations, components, parts or combinations thereof.

[0046] In one embodiment, Figure 1 As shown, a control method for a hybrid cascade converter is provided, such as Figure 2As shown, the hybrid cascade converter includes a plurality of cascaded converters, wherein the plurality of converters include at least one thyristor converter 10 and at least one modular converter 20, one end of the first-stage converter is connected to the first DC transmission line 30, one end of the last-stage converter is connected to the second DC transmission line 31, and the plurality of converters are connected to the AC power grid 50 through their respective corresponding transformers (TM1-TM5). Figure 2 In the example, the first stage is a thyristor converter 10 and the last stage is a modular converter 20. It can be understood that any one of the multiple converters can be a thyristor converter 10 or a modular converter 20, as long as there is at least one thyristor converter 10 and at least one modular converter 20. The method includes steps S100-110.

[0047] Step S100, when a current abnormality of a target DC transmission line is detected, adjusting a trigger angle of a target thyristor converter so that the target thyristor converter outputs a negative voltage and maintaining a working state of the modular converter unchanged.

[0048] Among them, the current abnormality of the target DC transmission line indicates that a DC fault has occurred in the target DC transmission line, which may be a DC short circuit fault (for example, it may include a positive ground short circuit, a negative ground short circuit, or a positive and negative bipolar short circuit). The current abnormality may be an abnormal increase or decrease in the amplitude of the current or other abnormal changes that may cause damage to the components in the system. At this time, adjusting the trigger angle of the target thyristor converter can make the target thyristor converter output a negative voltage, and the negative voltage is used to suppress the current abnormality of the target DC transmission line. For example, when the current of the target DC transmission line rises abnormally, the negative voltage can be used to reduce the amplitude of the current of the target DC transmission line, thereby suppressing the abnormal increase of the current on the target DC transmission line, and achieving the suppression and current limiting of the fault current. Keeping the working state of the modular converter unchanged, the modular converter still works normally, forming a loop with the AC power grid, ensuring the normal operation of the AC power grid, and providing AC voltage support for the AC power grid, so that even if a DC fault occurs on the DC side, the AC power grid can maintain normal operation without power failure, ensuring the continuity of the AC power grid and improving efficiency.

[0049] The target DC transmission line is one of the first DC transmission line 30 and the second DC transmission line 31 .

[0050] For example, Figure 3 As shown, it is a schematic diagram of a complete power system including a positive electrode system and a negative electrode system. Figure 3 It is only used to illustrate the positive line ground short circuit, negative line ground short circuit, positive and negative bipolar short circuit. Figure 3The power system shown is explained. Figure 3 In the embodiment, the hybrid cascade converter between the positive DC line and the DC loop is the positive commutation system, and the hybrid cascade converter between the DC loop and the negative DC line is the negative commutation system. The first DC transmission line and the second DC transmission line in the present application may correspond to Figure 3 The positive DC line and DC return line in Figure 3 The DC return line and negative DC line in the Figure 3 The positive DC line and negative DC line in the circuit. The schematic diagram of the positive ground short circuit is as follows Figure 3a As shown in Figure 1, a short circuit occurs between the positive DC line and the DC return line. The schematic diagram of the negative ground short circuit is as follows: Figure 3a As shown in Figure 1, a short circuit fault occurs between the DC return line and the negative DC line. The schematic diagram of the positive and negative bipolar short circuit is shown in Figure 1. Figure 3b As shown, a short circuit fault occurs between the positive DC line and the negative DC line.

[0051] Exemplarily, the relationship between the target thyristor converter output voltage and the trigger angle is expressed as follows:

[0052]

[0053] Among them, n t is the number of commutation bridges included in the valve group of the target thyristor converter, k t is the transformer ratio of the target thyristor converter, U s is the effective value of the AC voltage on the AC grid side line, α is the trigger angle, X t is the equivalent commutation reactance of the target thyristor converter, I d is a direct current.

[0054] Among them, the thyristor converter is a line commutated converter (LCC), which has the advantages of high voltage resistance, large power capacity, and low operating loss. At the same time, it can effectively block the short-circuit current when a DC short-circuit fault occurs. For example, Figure 4As shown, the thyristor converter can be a twelve-pulse bridge structure, wherein each bridge arm is composed of a plurality of thyristors (V1-V12) connected in series, for example, including two six-pulse thyristor converters. One of the two six-pulse thyristor converters is connected to a Y / Y type transformer, and the other is connected to a Y / Δ type transformer. The phase angle difference between the two sets of transformer connection modes is 30°. The two six-pulse thyristor converters are connected together after passing through their respective transformers, and then connected to a common connection point (point of common coupling, PCC) through a circuit breaker QF1. In actual engineering applications, 6-pulse and 12-pulse converters are more common and widely used. The commutation method used by the 6-pulse converter is the three-phase bridge circuit commutation method. The 12-pulse converter is composed of two 6-pulse converters connected in series. V1-V12 are turned on in a specified order in one power frequency cycle. Therefore, the converter requires 12 sequential trigger pulses that match the same frequency as the AC system and have a pulse spacing of 30°. Figure 5As shown, the working process of the 6-pulse converter is explained. The phase is based on the sine wave. Before the AC phase line voltage Uac reaches 0°, the instantaneous value of the B phase potential is the lowest, and the instantaneous value of the C phase potential is the highest. At this time, forward voltage is applied across V5 and V6 between phases B and C, and they are in the on state. Reverse voltage is applied across the remaining four valves and they are in the off state. When the AC voltage phase is between the first angle and the second angle, the instantaneous value of the potential of phase B is the lowest, and the instantaneous value of the potential of phase A is the highest. At this time, a forward voltage is applied to both ends of V1. After passing the trigger angle, it is turned on by the trigger pulse. V6 is also in the forward voltage and can be turned on. At this time, the current passes through V1 and the load, and forms a loop through V6. At this time, a reverse voltage is applied to both ends of V5 and it is in the off state. When the AC voltage phase is between the second angle and the third angle, the instantaneous value of the potential of phase C is the lowest, and the instantaneous value of the potential of phase A is the highest. V2 is in the positive phase voltage and is turned on by the trigger pulse after the trigger angle. V6, which is in the reverse voltage, is in the off state. V1, load, and V2 form a current loop. The subsequent valve conduction rules are similar. The 12-pulse converter is composed of two 6-pulse converters connected in series. Therefore, the working principle of a 12-pulse converter is very similar to that of a 6-pulse converter. The same method is adopted, that is, the two-phase short circuit on the AC side is used for phase switching. Because the components used in the converter are thyristors, they are semi-controlled devices (semi-controlled means that they can only be controlled to be turned on but not turned off). Therefore, the converter valve must meet two conditions to be turned on: (1) The positive direction of the voltage is defined as anode-cathode, and a forward conduction voltage needs to be applied to the valve. (2) A sufficiently large trigger pulse is required to be provided to the gate. When the converter is in the rectification state, the commutation angle μ is less than 30°. If both bridges are in the non-commutation period, four valves will be in the on state at the same time, that is, two valves will be turned on in each bridge. If one bridge is in the commutation period, five valves will be in the on state at the same time, that is, two valves will be turned on in the non-commutation bridge and three valves will be turned on in the commutation bridge. Therefore, the operating condition of the system during normal operation is that 4 valves and 5 valves are turned on in turn. The thyristor converter can also adopt a controllable line commutated converter (CLCC). The CLCC structure is based on a mixed connection of a fully controlled IGBT (Insulated Gate Bipolar Transistor) and a half-controlled thyristor. Each bridge arm is composed of a main branch and an auxiliary branch in parallel. The main branch is composed of the original conventional thyristor valve and the low-voltage IGBT valve in series, and the auxiliary branch is composed of a high-voltage and low-current IGBT valve and a high-voltage and low-current thyristor valve in series.By utilizing the IGBT's turn-off capability, the main thyristor current is transferred to the auxiliary branch containing the IGBT, and its reliable recovery is achieved by increasing the thyristor turn-off time. Finally, the turn-off capability of the auxiliary branch IGBT is utilized to cut off the bridge arm current to complete the phase change between the bridge arms, thus solving the phase change failure problem in principle.

[0055] Modular converters are modular multilevel converters (MMC), which have the advantages of flexible control, low harmonic content, and good voltage support capability. However, they also have the disadvantages of relatively small power capacity, high cost, and large operating losses. For example, Figure 6 As shown in the figure, the MMC in the modular converter adopts a three-phase six-bridge structure. Each bridge arm is composed of N sub-modules (SM) connected in series and then connected in series with another reactor L. The upper and lower bridge arms are connected in series to form a phase. Therefore, each converter has three phases and six bridge arms. The output voltage of the converter is obtained by connecting the working sub-module voltages in series. The MMC is connected to the AC power grid through a double-winding transformer with a connection mode of Y0 / △. The sub-module of the MMC adopts a half-bridge sub-module (HBSM). The voltage value that each sub-module can provide to the circuit when switched into the circuit is its capacitor voltage uc. The sum of the capacitor voltages of all sub-modules switched into the bridge arm determines the output voltage of the MMC. In order to prevent the sub-module from failing and causing the system to fail to operate normally, the number of sub-modules on each bridge arm of the converter must be redundant under normal circumstances. Therefore, the working state of the submodule is divided into three modes, namely, the cut-off state, the input state, and the locked state. Since the turn-off and turn-on of the fully controlled device IGBT (Insulated Gate Bipolar Transistor, Chinese name is insulated gate bipolar transistor) are determined by the applied signal, and the diode reversely connected in parallel with the IGBT has unidirectional conductivity, the on-off of the IGBT and the diode in the submodule and the flow direction of the current in the submodule determine that the submodule has two working modes in any working state. Figure 5This is a topological structure diagram of a three-phase MMC converter (modular multilevel converter). Due to the system operation stability requirements, the DC voltage output from the MMC must be stable. To achieve such a stable state, the three phase units must ensure that they have the same voltage. This depends on the number of sub-modules in the investment state being the same and constant. Even if the upper and lower bridge arms are allocated differently, the total number must be the same. For example, if you want to obtain the maximum DC voltage when all the upper bridge arm sub-modules are removed, all the lower bridge arm sub-modules must be in the investment state. In order to achieve a state that is easy to allocate, usually, the number of sub-modules invested in each phase unit is half (N) of the total number of sub-modules (2N); the output AC voltage can be adjusted by changing the allocation of the number of sub-modules invested in the upper and lower bridge arms of the three phase units. Figure 6 From the topological model, it can be seen that the bridge arm of each phase is composed of N converter units, and each converter unit has the same topological structure. Assume that the capacitor voltage of each converter unit is controlled to V c, so that the voltage output by the bridge arm will vary within the values ​​of 0, V c, 2V c, .. NV c, that is, this converter has N+1 level states. Regarding the current distribution, the three-phase topological structure is exactly the same and symmetrical, so the current will still be evenly distributed in the three phases. The DC current flowing through each phase unit accounts for 1 / 3 of the total current, and the reactor L (upper and lower bridge arms) are also equal in size. A submodule of MMC is as follows: Figure 7 As shown, it is composed of two IGBT tubes T1 and T2 and a capacitor C; wherein, the emitter of the IGBT tube T1 is connected to the collector of the IGBT tube T2 and constitutes one end of the submodule, the collector of the IGBT tube T1 is connected to one end of the capacitor C, and the emitter of the IGBT tube T2 is connected to the other end of the capacitor C and constitutes the other end of the submodule; the gates of the IGBT tubes T1 and T2 both receive switching signals provided by the outside.

[0056] The target thyristor converter is at least one thyristor converter cascaded between the last-stage modular converter and the first DC transmission line.

[0057] Exemplarily, the DC side can be connected using an asymmetric positive / negative pole connection method, an asymmetric negative pole connection method, and a true bipolar symmetrical connection method, and a bypass switch circuit (switches Q1, Q11, Q12, Q13) is connected in parallel to the DC side of the thyristor converter.

[0058] Step S110, when the target thyristor converter outputs a negative voltage, if it is detected that the current of the target DC transmission line does not return to normal within a preset time, the modular converter is controlled to be locked.

[0059] Among them, when the target thyristor converter outputs a negative voltage, if it is detected that the amplitude of the current of the target DC transmission line has not returned to normal within the preset time (for example, the amplitude of the current of the target DC transmission line rises abnormally, but when the target thyristor converter outputs a negative voltage, the current of the target DC transmission line does not reach the corresponding current preset range within the preset time), it means that the target thyristor converter has failed to suppress the fault current. At this time, in order to ensure the safety of components in the circuit (for example, the safety of the IGBT in the modular converter), it is necessary to control the modular converter to lock, that is, the IGBT in the modular converter is turned off, so as to avoid damage to the IGBT in the modular converter. When the modular converter is locked, it can prevent excessive fault current from damaging the IGBT. In this way, the fault current at the fault point will drop to zero, and the fault arc will be extinguished.

[0060] In this embodiment, the method first detects whether the current of the target DC transmission line is abnormal, so as to determine whether the target DC transmission line is faulty. Then, when the current of the target DC transmission line is detected to be abnormal, the trigger angle of the target thyristor converter is adjusted so that the target thyristor converter outputs a negative voltage, and the negative voltage is used to suppress the current abnormality of the target DC transmission line, so as to suppress the abnormal rising current on the target DC transmission line, and realize the suppression and current limiting of the fault current. While the target thyristor converter outputs a negative voltage, the working state of the modular converter is kept unchanged, so that the modular converter still works normally, forms a loop with the AC power grid, and ensures the normal operation of the AC power grid, so that even when the target DC transmission line fails, it can still provide AC voltage support for the AC power grid. In the case that the target thyristor converter outputs a negative voltage, if it is detected that the current of the target DC transmission line has not returned to normal within a preset time, it means that the target thyristor converter has failed to suppress the fault current. At this time, in order to ensure the safety of the components in the circuit, it is necessary to control the modular converter to lock. After the modular converter is locked, it can prevent the excessive fault current from damaging the IGBT. In summary, the method of the present application can, when a fault occurs in the target DC transmission line, on the one hand, use the target thyristor converter to suppress the fault current, and on the other hand, still use the modular converter to provide AC voltage support for the AC power grid. The AC voltage support capacity is strong, ensuring the normal operation of the AC power grid and high reliability. When the target thyristor converter fails to suppress the fault current, the modular converter is locked to prevent damage to the IGBT device, which also ensures the safety of the hybrid cascade converter.

[0061] In one embodiment, Figure 8 As shown, step S100, adjusting the trigger angle of the target thyristor converter to make the target thyristor converter output a negative voltage. The method includes steps S800-810.

[0062] Step S800: adjusting the trigger angle of the target thyristor converter to a preset angle so that the target thyristor converter outputs a negative voltage.

[0063] The preset angle is greater than 90°. The trigger angle can be designed according to the actual engineering situation, for example, it can be set to a value in the typical range of 110° to 140°, so that the target thyristor converter can quickly output a negative voltage.

[0064] Step S810, adjusting the trigger angle of the target thyristor converter so that the DC current flowing through the target thyristor converter is limited to be less than a preset current value.

[0065] Among them, the target thyristor converter can also be controlled by adopting a fixed DC current control strategy, and the DC current flowing through the target thyristor converter is set to a fixed value, such as 0A, so as to quickly suppress the fault current. Alternatively, a fixed DC voltage, fixed DC power and other control methods can be adopted, and the reference value is immediately set to a certain value that can suppress the fault current.

[0066] In this embodiment, the trigger angle of the target thyristor converter is adjusted so that the target thyristor converter outputs a negative voltage, thereby achieving fault current suppression.

[0067] In one embodiment, Fig. 9 As shown, the control method of the hybrid cascade converter also includes: steps S900-930.

[0068] Step S900, determining the predicted commutation duration of the target thyristor converter according to the parameters of the target thyristor converter and the current trigger angle.

[0069] Among them, you can use Fig.10 The fast current limiting controller shown is used to control the range of the trigger angle of the target thyristor converter, and realize the dynamic adjustment of the limit range of the trigger angle. Since the constant DC current controller or the constant DC voltage controller or the constant DC power controller will continuously increase the trigger angle to speed up the current limiting speed, the use of the fast current limiting controller can ensure that the trigger angle is within the appropriate range, thereby preventing the occurrence of commutation failure and improving the safety and reliability of the system. The fast current limiting controller specifically includes the process of steps S900-930.

[0070] Among them, the predicted duration in the fast current limiting controller is equal to the duration of the commutation angle μ in the active inverter state of the target thyristor converter during a DC short circuit fault. After obtaining the minimum allowable turn-off angle γ0 and the system angular frequency ω of the target thyristor converter, when the trigger angle α≥2*π / 3, the predicted duration calculation method is Δt=(π-γ0-α) / ω. However, when π / 2≤α<2*π / 3, the electrical angle corresponding to the actual reverse voltage bearing time is no longer equal to the turn-off angle. In order to meet the normal working requirements, it is necessary to ensure that μ≤π / 3-γ0. At this time, the predicted duration Δt=(π / 3-γ0) / ω. Corresponding Fig.10 In the example, the predicted duration is calculated based on the trigger angle α.

[0071] Step S910, determining the current variation according to the predicted commutation duration and the current current value of the target DC transmission line.

[0072] Among them, the DC current change Δi dc The DC current i dc Taylor expansion is performed and the second-order differential terms and above are ignored for approximate prediction. Therefore, the formula for predicting the change in DC current within Δt time is as follows:

[0073]

[0074] Among them, Δi dc is the current change, i dc is the current value of the target DC transmission line, and Δt is the predicted commutation time. Fig.10 According to the prediction time and DC current i dc Determine the amount of DC current change.

[0075] Step S920, determining the maximum trigger angle of the target thyristor converter according to the current variation, the current current value of the target DC transmission line, the parameters of the target thyristor converter, and the line voltage of the AC power grid.

[0076] Among them, according to the DC current change and the DC current measurement value, the maximum trigger angle allowed without commutation failure is calculated:

[0077]

[0078] Among them, α f_max is the maximum firing angle, U s is the effective value of the AC grid line voltage, k t is the transformer ratio of the target thyristor converter, X t is the transformer equivalent leakage reactance of the target thyristor converter, Δi dc is the current change, i dcis the current value of the target DC transmission line, and γ0 is the minimum allowable turn-off angle of the target thyristor converter. Fig.10 Calculate the maximum firing angle allowed without commutation failure.

[0079] Step S930: after a set time period after the current abnormality of the target DC transmission line is detected, the range of the trigger angle of the target thyristor converter is limited according to the maximum trigger angle and the preset angle.

[0080] Wherein, when the target thyristor converter is in a normal working state, the range of the trigger angle of the target thyristor converter is a preset rated range, for example, in α norm_min ~α norm_max In step S800, in order to make the target thyristor converter output a negative voltage, the trigger angle of the target thyristor converter is adjusted to a preset angle α. f That is, within the set time after the current anomaly of the target DC transmission line is detected, the trigger angle of the target thyristor converter is set to the preset angle α f After the set time, the trigger angle range of the target thyristor converter is determined according to the maximum trigger angle and the preset angle, for example, α f ~α f_max Between, where α f is the preset angle, α f_max is the maximum trigger angle, due to α f_max is the maximum trigger angle allowed without commutation failure calculated previously, so while increasing the trigger angle to speed up the current limiting speed, the trigger angle range can also be limited to α f ~α f_max The current limiting speed and reliability of the target thyristor converter are taken into account.

[0081] In this embodiment, a fast current limiting controller is used to calculate the maximum trigger angle allowed by the target thyristor converter without commutation failure, and to limit the range of the trigger angle of the target thyristor converter. This can increase the trigger angle to speed up the current limiting speed while limiting the range of the trigger angle to avoid commutation failure. This takes into account both the current limiting speed and reliability of the target thyristor converter.

[0082] In one embodiment, Fig.11 As shown, after step S100, the control method of the hybrid cascade converter further includes: steps S101-102.

[0083] Step S101, determining whether the DC current of the target DC transmission line reaches the corresponding current preset range within the preset time. If the current of the target DC transmission line does not return to normal (e.g., reach the preset range) within the preset time, then executing step S110. If the target thyristor converter outputs a negative voltage, and it is detected that the amplitude of the current of the target DC transmission line returns to normal (e.g., decreases to the preset range) within the preset time, then executing step S102.

[0084] Step S102, maintaining the working state of the modular inverter unchanged.

[0085] In this embodiment, it is determined whether the DC current of the target DC transmission line reaches the corresponding current preset range within the preset time, so as to determine whether the fault current is successfully suppressed. If it is detected that the current of the target DC transmission line does not return to normal within the preset time, it means that the target thyristor converter fails to suppress the fault current. At this time, in order to ensure the safety of components in the circuit (such as the safety of IGBT in the modular converter), it is necessary to control the modular converter to lock. If it is detected that the current of the target DC transmission line returns to normal within the preset time, it is determined that the current limiting is successful, and the working state of the modular converter remains unchanged, and the original working state is maintained, so that during the entire DC fault period, the modular converter can maintain the ability to support AC voltage and reactive power.

[0086] In one embodiment, Fig.12 As shown, the control method of the hybrid cascade converter also includes: steps S103-104.

[0087] Step S103, determining whether the fault of the target DC transmission line is cleared. If it is determined that the fault of the target DC transmission line is cleared, executing step S104. If it is determined that the fault of the target DC transmission line is not cleared, executing step S103.

[0088] The determination of whether the fault of the target DC transmission line is cleared may be based on receiving an external instruction, such as an instruction of maintenance completion, to determine whether the fault of the target DC transmission line is cleared. Alternatively, the current of the target DC transmission line may be obtained to determine whether the fault of the target DC transmission line is cleared based on the current.

[0089] Step S104: after a preset time period, control the target thyristor converter to operate in a normal working state.

[0090] Among them, under normal working conditions, the trigger angle range of the target thyristor converter is the preset rated range. The preset duration is the time to wait for the DC line to be deionized. When the DC fault is cleared, after a period of DC line deionization time, it is ensured that the arc at the fault point completely disappears and the line insulation performance returns to normal levels. The target thyristor converter resumes the control mode when it is in normal working condition, and gradually increases the system transmission power. The modular converter also operates in a normal working state, and the system returns to a normal working state.

[0091] In this embodiment, it is determined whether the fault of the target DC transmission line is cleared. If it is determined that the fault of the target DC transmission line is cleared, the target thyristor converter is controlled to operate in a normal working state after a preset time, so that the hybrid cascade converter can be restored to a normal working state as soon as possible after the DC fault is cleared.

[0092] It should be understood that although Figure 1 , 8 The steps in the flowcharts of , 9, 11, and 12 are shown in sequence as indicated by the arrows, but these steps are not necessarily executed in the order indicated by the arrows. Unless otherwise specified in this document, there is no strict order restriction for the execution of these steps, and these steps can be executed in other orders. Moreover, Figure 1 , 8 At least part of the steps in , 9, 11, and 12 may include multiple steps or multiple stages. These steps or stages are not necessarily performed at the same time, but can be performed at different times. The execution order of these steps or stages is not necessarily one by one, but can be performed in turn or alternately with other steps or at least part of the steps or stages in other steps.

[0093] In one embodiment, please continue to see Figure 2 , a hybrid cascade converter is provided, the hybrid cascade converter includes a plurality of cascaded converters, the plurality of converters include at least one thyristor converter 10 and at least one modular converter 20, and also includes a plurality of transformers and controllers (not shown in the figure), one end of the first-stage converter is connected to the first DC transmission line 30, one end of the last-stage converter is connected to the second DC transmission line 31, and the plurality of converters are respectively connected to the AC power grid 50 through their respective corresponding transformers (TM1-TM5) (in Figure 2 In the figure, the first stage is a thyristor converter 10 and the last stage is a modular converter 20 for illustration, and the controller is connected to multiple converters respectively.

[0094] The controller is used to adjust the trigger angle of the target thyristor converter 10 when the current of the target DC transmission line is detected to be abnormal, so that the target thyristor converter 10 outputs a negative voltage, and controls the working state of the modular converter 20 to remain unchanged. When the target thyristor converter 10 outputs a negative voltage, if it is detected that the current of the target DC transmission line does not reach the corresponding current preset range within a preset time, the modular converter 20 is controlled to be locked.

[0095] The target thyristor converter 10 is at least one thyristor converter 10 cascaded between the last-stage modular converter 20 and the first DC transmission line 30, the target DC transmission line is one of the first DC transmission line 30 and the second DC transmission line 31, and the negative voltage is used to suppress the current abnormality of the target DC transmission line.

[0096] In this embodiment, by setting at least one thyristor converter 10 and at least one modular converter 20 in cascade, the advantages of the thyristor converter 10, such as high withstand voltage level, large power capacity, low operating loss, and DC blocking capability, and the advantages of the modular converter 20, such as flexible control, low harmonic content, and good voltage support capability, can be combined. By setting a controller, it is first detected whether the current of the target DC transmission line is abnormal, so that it can be determined whether the target DC transmission line is faulty. Then, when the current of the target DC transmission line is detected to be abnormal, the trigger angle of the target thyristor converter 10 is adjusted so that the target thyristor converter 10 outputs a negative voltage, and the negative voltage is used to suppress the current abnormality of the target DC transmission line, so that the abnormal rising current on the target DC transmission line can be suppressed, and the fault current suppression and current limiting are achieved. While the target thyristor converter 10 outputs a negative voltage, the working state of the modular converter 20 is kept unchanged, so that the modular converter 20 still works normally, forms a loop with the AC power grid 50, and ensures the normal operation of the AC power grid 50, so that even when the target DC transmission line fails, it can still provide AC voltage support for the AC power grid 50. In the case that the target thyristor converter 10 outputs a negative voltage, if it is detected that the current of the target DC transmission line has not returned to normal within the preset time, it means that the target thyristor converter 10 has failed to suppress the fault current. At this time, in order to ensure the safety of the components in the circuit, it is necessary to control the modular converter 20 to lock. After the modular converter 20 is locked, it can prevent the excessive fault current from damaging the IGBT. In summary, the method of the present application can be used to suppress the fault current by using the target thyristor converter 10 on the one hand when the target DC transmission line fails, and on the other hand, the modular converter 20 can still be used to provide AC voltage support for the AC power grid 50. The AC voltage support capacity is strong, which ensures the normal operation of the AC power grid 50 and has high reliability. When the target thyristor converter 10 fails to suppress the fault current, the modular converter 20 is locked to prevent the IGBT device from being damaged, thereby ensuring the safety of the hybrid cascade converter.

[0097] In one embodiment, Fig.13 As shown, the hybrid cascade converter includes a first valve group 41, a second valve group 42, and a third valve group 43. The first end of the first valve group 41 is connected to the first DC transmission line, the second end of the first valve group 41 is connected to the first end of the second valve group 42, the second end of the second valve group 42 is connected to the first end of the third valve group 43, and the second end of the third valve group 43 is connected to the second DC transmission line 31, wherein:

[0098] The first valve group 41 includes at least one thyristor converter 10 , the second valve group 42 includes at least one thyristor converter 10 , and the third valve group 43 includes at least one modular converter 20 .

[0099] Among them, the first valve group 41 may include multiple thyristor converters 10, and the multiple thyristor converters 10 may be connected in series and / or in parallel. The second valve group 42 may include multiple thyristor converters 10, and the multiple thyristor converters 10 may be connected in series and / or in parallel. The third valve group 43 may include multiple modular converters 20, and the multiple modular converters 20 may be connected in series and / or in parallel. The modular converter 20 may be a half-bridge MMC, a full-bridge MMC, or a hybrid MMC. The converters in the first valve group 41, the second valve group 42, and the third valve group 43 may all be connected to the AC power grid 50, or may be partially connected to the AC power grid 50, or may be connected to an island power grid.

[0100] One end of at least one thyristor converter 10 in the first valve group 41 serves as the first end of the first valve group 41 (the thyristor converter 10 is the first-stage thyristor converter 10). One end of at least one modular converter 20 in the third valve group 43 serves as the second end of the third valve group 43 (the modular converter 20 is the last-stage modular converter 20).

[0101] The thyristor converters 10 of the first valve group 41 and the second valve group 42 may be connected in parallel with a bypass switch circuit 60, and the bypass switch circuit 60 may bypass and isolate the parallel-connected thyristor converters 10. The first valve group 41 and the target DC transmission line are also connected in series with a smoothing reactor L1. An equivalent AC inductance Lg exists between the AC grid 50 and the point of common coupling (PCC), and an AC filter device 70 is also provided between the AC grid 50 and the point of common coupling PCC.

[0102] Among them, the design of the first valve group 41, the second valve group 42, and the third valve group 43 can reduce the electrical stress that each valve group has to bear, reduce the insulation withstand voltage requirements, and share the power by three valve groups. Considering the differences in the withstand voltage levels of the core components used by LCC and MMC, the LCC in the hybrid cascade converter should transmit most of the power. In addition, in order to ensure that the LCC can provide a sufficiently large reverse voltage to suppress the discharge of the MMC capacitor during a DC short circuit fault, the LCC DC voltage should be greater than the MMC DC voltage during normal operation, so it is designed that the first valve group 41 and the second valve group 42 include LCC, and the third valve group 43 includes MMC.

[0103] Among them, when the hybrid cascade converter is operating normally, it can control the active / reactive power balance of the AC / DC system, maintain the stability of the voltage, current and frequency on the AC / DC side, and ensure the normal operation of the AC / DC system. Under normal operation, the rated DC voltages of the three valve groups, namely the first valve group 41 (LCC valve group), the second valve group 42 (LCC valve group), and the third valve group 43 (MMC valve group), each account for about one-third of the DC voltage of the entire hybrid cascade converter. The DC voltage ratio of the three can also be reasonably allocated to reduce the proportion of the DC voltage of the third valve group 43 (MMC valve group), thereby appropriately reducing the MMC capacity. The first valve group 41 and the second valve group 42 can adopt a centralized control method, and the control instructions are assigned by the upper unified controller. A separate control method can also be adopted to assign control instructions to each.

[0104] The first valve group 41 and the second valve group 42 adopt a centralized control mode, that is, the trigger pulses of the first valve group 41 and the second valve group 42 are distributed by the same controller, and the controller can adopt conventional control modes such as constant DC voltage, constant DC current, and constant DC power. In order to improve the safety of the system, a low-voltage current limiting (Voltage-Dependent Current-Order Limit, VDCOL) link is introduced before the constant DC current control, which is helpful for rapid recovery after AC or DC faults and prevents the current command from being too low under faults. It is also possible to select one of the above conventional control modes as the normal control mode, and select another one or two control modes as backup control modes. In addition, in order to avoid frequent switching between various control modes, voltage error control (VEC) and current error control (CEC) can be added between various control modes according to specific circumstances.

[0105] The first valve group 41 and the second valve group 42 adopt separate control methods, and the trigger pulses of the first valve group 41 and the second valve group 42 are independently allocated. Any one of the first valve group 41 and the second valve group 42 can select one of the control methods such as fixed minimum trigger pulse angle, fixed DC voltage, fixed DC current, and fixed DC power, and the remaining valve groups need to select another reasonable control method. The first valve group 41 and the second valve group 42 can both be equipped with a low-voltage current limiting link, or only one of them can be equipped. The first valve group 41 and the second valve group 42 can also select one of the above-mentioned conventional control methods as the normal control method, and select another one or two control methods as the backup control method. In order to avoid frequent switching between various control methods, voltage deviation control and current deviation control can be added between various control methods according to specific circumstances.

[0106] The d-axis controller of the third valve group 43 can adopt constant DC voltage control or constant active power control, and the q-axis controller can adopt constant AC voltage control or constant reactive power control. In addition, when the third valve group 43 is composed of multiple parallel connections, other control methods can also be adopted according to the nature of the MMC in the relevant third valve group 43 connected to the AC system. For example, the MMC in the third valve group 43 connected to the AC system can adopt virtual synchronization or droop control, and the MMC connected to the island power grid can adopt V / F (voltage / frequency) control. Further, the MMC can be configured with an existing AC overvoltage suppression controller to suppress the overvoltage at the sending end. The hybrid cascade converter can also be configured with existing oscillation suppression measures to suppress the oscillation generated by the access of new energy. Further, in order to facilitate the maintenance of the valve group and ensure the transmission of a small amount of power, the bypass switch circuits in the first valve group 41 and the second valve group 42 can be closed in sequence according to the actual operation requirements and the existing LCC shutdown bypass method to achieve flexible operation of the hybrid cascade converter.

[0107] In this embodiment, by setting up a hybrid cascade converter including a first valve group 41, a second valve group 42, and a third valve group 43, the operation mode of the hybrid cascade converter can be flexibly selected according to needs, and multiple valve groups can be used to share power to improve flexibility and safety.

[0108] In one embodiment, Fig.14 As shown, the third valve group 43 includes a plurality of modular converters 20 connected in parallel, each modular converter 20 is connected to the AC power grid 50 via a corresponding transformer, wherein at least one modular converter 20 is also used to connect to a new energy power station 80 .

[0109] The controller is also used to control the working state of a modular converter 20 to remain unchanged when an abnormal current is detected in the first DC transmission line 30, so as to provide AC voltage support for the AC power grid 50, and to control at least one modular converter 20 connected to the new energy power station 80 to work so as to realize the power transmission between the new energy power station 80 and the AC power grid 50.

[0110] In this embodiment, in the event of a DC fault, the modular converter of the third valve group can provide AC support for the AC power grid, power the new energy power station, and transmit clean energy to the AC power grid. The hybrid cascade converter provided in this application has a wide range of application scenarios, and a new converter station can be built in the Shagohuang area for large-scale new energy export in the "Sha, Ge, Huang" area. It is also suitable for the transformation scenario of the existing LCC or CLCC ultra-high voltage DC converter station. The transformation method is to connect the MMC valve group in series in the original valve group. It can be one or multiple parallel units, which can reduce the site selection and demonstration cost. In addition, it can be used for DC transmission projects or DC power grids of different voltage levels. When the low-voltage valve group is multiple MMCs in parallel, other DC transmission projects or DC power grids can be connected to the DC transmission project where the ultra-high voltage DC hybrid cascade converter is located through an MMC DC side.

[0111] Based on the same inventive concept, an embodiment of the present invention further provides a power system, including the hybrid cascade converter, the AC power grid, the first DC transmission line, and the second DC transmission line in any of the above embodiments. The power system also has the beneficial effects of the hybrid cascade converter in the above embodiments, and the similarities can be understood by referring to the above explanation of the hybrid cascade converter, which will not be repeated below.

[0112] In one embodiment, a computer device is provided, including a memory and a processor, wherein a computer program is stored in the memory, and the processor implements the steps in the above-mentioned method embodiments when executing the computer program.

[0113] In one embodiment, a computer-readable storage medium is provided, on which a computer program is stored. When the computer program is executed by a processor, the steps in the above-mentioned method embodiments are implemented.

[0114] In one embodiment, a computer program product is provided, including a computer program, which implements the steps in the above method embodiments when executed by a processor.

[0115] Those of ordinary skill in the art can understand that all or part of the processes in the above-mentioned embodiment methods can be completed by instructing the relevant hardware through a computer program, and the computer program can be stored in a non-volatile computer-readable storage medium. When the computer program is executed, it can include the processes of the embodiments of the above-mentioned methods. Among them, any reference to memory, storage, database or other media used in the embodiments provided in this application can include at least one of non-volatile and volatile memory. Non-volatile memory can include read-only memory (ROM), magnetic tape, floppy disk, flash memory or optical memory, etc. Volatile memory can include random access memory (RAM) or external cache memory. As an illustration and not limitation, RAM can be in various forms, such as static random access memory (SRAM) or dynamic random access memory (DRAM).

[0116] In the description of this specification, the description with reference to the terms "some embodiments", "other embodiments", "ideal embodiments", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic descriptions of the above terms do not necessarily refer to the same embodiment or example.

[0117] The technical features of the above embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0118] The above-mentioned embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the attached claims.

Claims

1. A control method for a hybrid cascade converter, characterized in that: The hybrid cascade converter comprises a plurality of cascaded converters, wherein the plurality of converters comprises at least one thyristor converter and at least one modular converter, one end of the first-stage converter is connected to a first DC transmission line, one end of the last-stage converter is connected to a second DC transmission line, and the plurality of converters are respectively connected to an AC power grid through respective corresponding transformers; the method comprises: In the case of detecting a current anomaly of a target DC transmission line, adjusting a trigger angle of a target thyristor converter so that the target thyristor converter outputs a negative voltage and keeps the working state of the modular converter unchanged, wherein the target thyristor converter is at least one thyristor converter cascaded between the last-stage modular converter and the first DC transmission line, the target DC transmission line is one of the first DC transmission line and the second DC transmission line, and the negative voltage is used to suppress the current anomaly of the target DC transmission line; In the case where the target thyristor converter outputs a negative voltage, if it is detected that the current of the target DC transmission line does not return to normal within a preset time, the modular converter is controlled to be locked.

2. The control method of the hybrid cascade converter according to claim 1, characterized in that: The step of adjusting the trigger angle of the target thyristor converter so that the target thyristor converter outputs a negative voltage includes: adjusting the trigger angle of the target thyristor converter to a preset angle so that the target thyristor converter outputs a negative voltage, wherein the preset angle is greater than 90°; or, The trigger angle of the target thyristor converter is adjusted so that the direct current flowing through the target thyristor converter is limited to be below a preset current value.

3. The control method of the hybrid cascade converter according to claim 2, characterized in that: The method further comprises: Determining a predicted commutation duration of the target thyristor converter according to the parameters of the target thyristor converter and the current trigger angle; Determining a current change amount according to the predicted commutation time and a current current value of the target DC transmission line; Determining a maximum trigger angle of the target thyristor converter according to the current variation, the current current value of the target DC transmission line, the parameters of the target thyristor converter, and the line voltage of the AC power grid; After a set time period after the current abnormality of the target DC transmission line is detected, the range of the trigger angle of the target thyristor converter is limited according to the maximum trigger angle and the preset angle.

4. The control method of the hybrid cascade converter according to claim 1, characterized in that: The method further comprises: When it is determined that the fault of the target DC transmission line is cleared, after a preset time period, the target thyristor converter is controlled to operate in a normal working state, wherein in the normal working state, the range of the trigger angle of the target thyristor converter is a preset range.

5. The control method of the hybrid cascade converter according to claim 1, characterized in that: The method further comprises: When the target thyristor converter outputs a negative voltage and it is detected that the current of the target DC transmission line returns to normal within a preset time, the working state of the modular converter is kept unchanged.

6. A hybrid cascade converter, characterized in that: The invention comprises a plurality of cascaded converters, wherein the plurality of converters include at least one thyristor converter and at least one modular converter, and also includes a plurality of transformers and controllers. One end of the first-stage converter is connected to a first DC transmission line, and one end of the last-stage converter is connected to a second DC transmission line. The plurality of converters are respectively connected to an AC power grid through their respective corresponding transformers, and the controller is respectively connected to the at least one thyristor converter and the at least one modular converter; wherein, The controller is used to adjust the trigger angle of the target thyristor converter when a current abnormality of a target DC transmission line is detected, so that the target thyristor converter outputs a negative voltage, and controls the working state of the modular converter to remain unchanged; when the target thyristor converter outputs a negative voltage, if it is detected that the current of the target DC transmission line does not reach the corresponding current preset range within a preset time, the modular converter is controlled to be locked, wherein the target thyristor converter is at least one thyristor converter cascaded between the last-stage modular converter and the first DC transmission line, the target DC transmission line is one of the first DC transmission line and the second DC transmission line, and the negative voltage is used to suppress the current abnormality of the target DC transmission line.

7. The hybrid cascade converter according to claim 6, characterized in that: The invention comprises a first valve group, a second valve group and a third valve group, wherein the first end of the first valve group is connected to the first DC transmission line, the second end of the first valve group is connected to the first end of the second valve group, the second end of the second valve group is connected to the first end of the third valve group, and the second end of the third valve group is connected to the second DC transmission line, wherein: The first valve group includes at least one thyristor converter, the second valve group includes at least one thyristor converter, and the third valve group includes at least one modular converter, wherein one end of at least one thyristor converter in the first valve group serves as the first end of the first valve group, and one end of at least one modular converter in the third valve group serves as the second end of the third valve group.

8. The hybrid cascade converter according to claim 7, characterized in that: The third valve group includes a plurality of modular converters connected in parallel, each of the modular converters is connected to an AC power grid through a corresponding transformer, wherein at least one modular converter is also used to connect to a new energy power station; The controller is also used to control the working state of one of the modular converters to remain unchanged to provide AC voltage support for the AC power grid when an abnormal current of the target DC transmission line is detected, and to control at least one modular converter connected to the new energy power station to work so as to realize power transmission between the new energy power station and the AC power grid.

9. A power system, characterized in that: include: The hybrid cascade converter, the AC power grid, the first DC transmission line, and the second DC transmission line as described in any one of claims 6 to 8.

10. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that: When the processor executes the computer program, the steps of the method according to any one of claims 1 to 5 are implemented.

11. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the method according to any one of claims 1 to 5 are implemented.

12. A computer program product, comprising a computer program, characterized in that When the computer program is executed by a processor, the steps of the method according to any one of claims 1 to 5 are implemented.

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