Converter, control method thereof, storage medium, and flexible direct current power transmission system

Abstract

The application provides a converter, a control method thereof, a storage medium and a flexible direct current power transmission system. The converter comprises: a multi-phase bridge arm, each phase bridge arm comprising a series connection of an upper bridge arm and a lower bridge arm, the upper bridge arm and the lower bridge arm each comprising a series connection of a switching circuit and a plurality of sub-modules, the switching circuit comprising a reactor and a plurality of switching modules, the switching module comprising a first controllable switch tube, a first diode and a first energy absorption module, the first controllable switch tube being connected in antiparallel with the first diode, the first energy absorption module being connected in parallel with the first controllable switch tube, the reactor being connected in series with the first controllable switch tube of the plurality of switching modules in turn, and neither the upper bridge arm nor the lower bridge arm comprising a lightning arrester. The application at least solves the problem of high loss of the converter in the prior art.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power electronic conversion, in particular to a converter, a control method thereof, a computer readable storage medium and a flexible direct current power transmission system. BACKGROUND

[0002] At present, the high voltage direct current based on voltage source converter (VSC-HVDC) technology has become an important technical means for high proportion of renewable energy consumption in the future because of the absence of commutation failure problem and the ability to realize active and reactive decoupling control. The existing flexible direct current converter is a modular multilevel voltage source converter (MMC), and the bridge arm is only composed of sub-modules. The upper and lower bridge arms of each phase are turned on at the same time, the AC voltage is modulated by controlling the switching of the sub-modules, and the DC voltage is supported, the number of output levels is large, the switching frequency of the device is low, and the voltage level can be flexibly expanded by increasing the number of sub-modules.

[0003] However, there is a DC circulating current in each phase bridge arm of the converter, which not only increases the loss of the system, but also may cause overheating of the switching device and the capacitor, affecting the stability of the whole system. In order to eliminate the DC circulating current, a large bridge reactor needs to be equipped on the bridge arm, resulting in high loss of the converter. SUMMARY

[0004] The main purpose of the present application is to provide a converter, a control method thereof, a computer readable storage medium and a flexible direct current power transmission system, so as to at least solve the problem of high loss of the converter in the prior art.

[0005] In order to achieve the above purpose, according to one aspect of the present application, a converter is provided, comprising: a plurality of bridge arms, each bridge arm comprising a series connection of an upper bridge arm and a lower bridge arm, the upper bridge arm and the lower bridge arm each comprising a series connection of a switching circuit and a plurality of sub-modules, the switching circuit comprising an electric reactor and a plurality of switching modules, the switching module comprising a first controllable switch, a first diode and a first energy absorption module, the first controllable switch being connected in antiparallel with the first diode, the first energy absorption module being connected in parallel with the first controllable switch, the electric reactor being connected in series with the first controllable switch of the plurality of switching modules in turn, and the upper bridge arm and the lower bridge arm each not comprising an arrester.

[0006] Optionally, the first energy absorption module comprises: a second diode, an anode of the second diode being electrically connected with a cathode of the first diode; a first energy storage device, one end of the first energy storage device being electrically connected with the cathode of the second diode, and the other end of the first energy storage device being electrically connected with the anode of the first diode; and a discharge device, the discharge device being connected in parallel with the first energy storage device.

[0007] Optionally, the first energy storage device comprises a capacitor, and the discharge device comprises a resistor.

[0008] Optionally, the first controllable switch is an IGCT (Integrated Gate-Commutated Thyristor) switch or an IGBT (Insulated Gate Bipolar Transistor) switch.

[0009] Optionally, the sub-module comprises: a second energy storage device; a second energy absorption module, the second energy absorption module comprising three terminals, a first terminal of the second energy absorption module being electrically connected with one end of the second energy storage device, and a second terminal of the second energy absorption module being electrically connected with the other end of the second energy storage device; a second controllable switch, one end of the second controllable switch being electrically connected with a third terminal of the second energy absorption module; a third diode, the third diode being connected in anti-parallel across the second controllable switch; a third controllable switch, one end of the third controllable switch being electrically connected with the other end of the second controllable switch, and the other end of the third controllable switch being electrically connected with the other end of the second energy storage device; and a fourth diode, the fourth diode being connected in anti-parallel across the third controllable switch.

[0010] Optionally, the second controllable switch and the third controllable switch are IGCT switches or IGBT switches.

[0011] Optionally, the sub-module comprises a second energy storage device, and in the bridge arm, the maximum number of the switching modules is , and the maximum number of the sub-modules is , wherein is a ceiling function, M is the maximum number of the switching modules, N is the maximum number of the sub-modules, is a DC bus voltage of the converter, is a voltage stored in the second energy storage device.

[0012] According to another aspect of the present application, a control method of a converter is provided for controlling any of the converters described above, comprising: using a power frequency square wave to alternately turn on and turn off the switching circuit of the upper bridge arm and the lower bridge arm; based on a half-wave modulation strategy and the turn-on and turn-off strategy of the switching circuit, alternately activating and locking the sub-modules of the upper bridge arm and the lower bridge arm, so that when the switching circuit is turned on, the sub-modules of the corresponding upper bridge arm or lower bridge arm are activated, and when the switching circuit is turned off, the sub-modules of the corresponding upper bridge arm or lower bridge arm are locked.

[0013] Optionally, using a power frequency square wave to alternately turn on and turn off the switching circuit of the upper bridge arm and the lower bridge arm comprises: determining the control function of the switching circuit as , according to the power frequency square wave, wherein, is the control function, is the power frequency angular frequency, and t is the time, is the power frequency phase, is a sign function, and when , the sign function is 1, and when , the sign function is 0; using the control function to control the switching circuit, so that when the control function is 1, the switching circuit of the upper bridge arm is turned on and the switching circuit of the lower bridge arm is turned off, and when the control function is 0, the switching circuit of the upper bridge arm is turned off and the switching circuit of the lower bridge arm is turned on.

[0014] Optionally, based on a half-wave modulation strategy and the turn-on and turn-off strategy of the switching circuit, alternately activating and locking the sub-modules of the upper bridge arm and the lower bridge arm comprises: generating a current reference value according to an active power reference value, a reactive power reference value, and a direct current voltage; generating a reference voltage waveform of the sub-modules of the upper bridge arm and the lower bridge arm for half-wave modulation according to the current reference value and the actual bridge arm current of the upper bridge arm and the lower bridge arm; and generating a control signal of the sub-modules of the upper bridge arm and the lower bridge arm using a nearest level approximation modulation method according to the reference voltage waveform and the turn-on and turn-off strategy, so that the sub-modules of the upper bridge arm are activated and the sub-modules of the lower bridge arm are locked, or so that the sub-modules of the lower bridge arm are activated and the sub-modules of the upper bridge arm are locked.

[0015] Optionally, generating a reference voltage waveform of the sub-modules of the upper bridge arm and the lower bridge arm for half-wave modulation according to a current reference value and an actual bridge arm current of the upper bridge arm and the lower bridge arm comprises: generating a reference voltage waveform of the sub-modules of the upper bridge arm and the lower bridge arm for half-wave modulation according to a current reference value and an actual bridge arm current of the upper bridge arm and the lower bridge arm as , wherein, the reference voltage waveform of the sub-modules of the upper bridge arm, the reference voltage waveform of the sub-modules of the lower bridge arm, a modulation ratio, a power frequency angular frequency, t is a time, a phase of an alternating current phase voltage of the jth phase.

[0016] Optionally, the control signals of the sub-modules of the upper bridge arm and the lower bridge arm are generated by using a nearest level approximation modulation method according to the reference voltage waveform and the turn-on and turn-off strategy, including: determining the number of the sub-modules of the upper bridge arm and the lower bridge arm to be turned on by using the nearest level approximation modulation method according to the reference voltage waveform and the turn-on and turn-off strategy. wherein, the number of the sub-modules of the upper bridge arm to be turned on, the number of the sub-modules of the lower bridge arm to be turned on, =0 or 1, =1 represents that the turn-on and turn-off strategy is to turn on the switching circuit of the upper bridge arm, =0 represents that the turn-on and turn-off strategy is to turn on the switching circuit of the lower bridge arm, the reference voltage waveform of the sub-modules of the upper bridge arm, the reference voltage waveform of the sub-modules of the lower bridge arm, the sub-modules including a second energy storage device, a ratio of a direct current bus voltage of the converter to a voltage stored by the second energy storage device; the control signals of the corresponding sub-modules are generated according to the number of the sub-modules of the upper bridge arm and the lower bridge arm to be turned on.

[0017] According to still another aspect of the present application, a computer readable storage medium is provided, the computer readable storage medium including a stored program, wherein the computer readable storage medium controls a device where the computer readable storage medium is located to execute any one of the methods when the program is executed.

[0018] According to still another aspect of the present application, a flexible direct current power transmission system is provided, including: any one of the converters; an electronic device including: one or more processors, a memory, and one or more programs, wherein the one or more programs are stored in the memory and configured to be executed by the one or more processors, and the one or more programs include instructions for executing any one of the methods.

[0019] According to the technical solution, in the upper bridge arm and the lower bridge arm of the converter, the switching circuit and the plurality of sub-modules are connected in series, the switching circuit comprises the reactor, a plurality of switching modules comprising the first controllable switch tube, the first diode and the first energy absorption module, the first controllable switch tube and the first diode are connected in reverse parallel, the first energy absorption module is connected in parallel across the first controllable switch tube, the reactor and the plurality of first controllable switch tubes are connected in series, and no lightning arrester is arranged in the upper bridge arm and the lower bridge arm. According to the technical solution, the switching circuit is arranged in the upper bridge arm and the lower bridge arm of the converter, the switching state of the upper bridge arm and the lower bridge arm and the switching of the sub-modules are controlled in linkage, the sub-modules on the corresponding bridge arm are put into operation when the switching circuit is turned on, and the sub-modules on the corresponding bridge arm are locked when the switching circuit is turned off, so that the upper bridge arm and the lower bridge arm are turned on alternately in different half cycles of the power frequency cycle, and then the direct current circulation cannot form a complete loop between the upper bridge arm and the lower bridge arm in a half power frequency cycle, so that the formation of the direct current circulation in the upper bridge arm and the lower bridge arm is suppressed, which means that the reactor does not need to bear too much task of suppressing the direct current circulation, and therefore the inductance requirement of the reactor in the bridge arm can be reduced. The lower inductance not only reduces the loss and response time of the whole system, but also helps to reduce the cost and physical size of the reactor. In addition, since the inductance of the inductor required by the technical solution is low, the overvoltage level caused by the low inductance is also reduced, so that the arrangement of the lightning arrester is not necessary in this case, and therefore the lightning arrester does not need to be arranged in the converter, which not only greatly reduces the overall loss of the converter, but also reduces the cost and volume of the converter. In addition, the first energy absorption module arranged in the upper bridge arm and the lower bridge arm can absorb excess energy, clamp the voltage in a safe range, and protect the controllable switch tube. BRIEF DESCRIPTION OF DRAWINGS

[0020] The drawings accompanying the specification of the present application form a part thereof, serve to provide further understanding of the application, and together with the specification explain the application. The use of the same reference symbols in different drawings indicates similar or identical components.

[0021] Figure 1 A structure diagram of a converter according to an embodiment of the present application is shown;

[0022] Figure 2 A hardware structure block diagram of a mobile terminal for executing a control method of a converter according to an embodiment of the present application is shown;

[0023] Figure 3 A flowchart of a control method of a converter according to an embodiment of the present application is shown;

[0024] Figure 4 A control block diagram of a converter according to an embodiment of the present application is shown.

[0025] In the above drawings, the following reference signs are used:

[0026] 10, switch circuit; 101, reactor; 102, first controllable switch tube; 103, first diode; 104, second diode; 105, first energy storage device; 106, discharge device; 20, sub-module; 201, second energy storage device; 202, second controllable switch tube; 203, third diode; 204, third controllable switch tube; 205, fourth diode; 206, third energy storage device; 207, fifth diode; 208, resistive element; 209, inductive element; 30, DC capacitor; 40, smoothing reactor; 502, processor; 504, memory; 506, transmission device; 508, input and output device. DETAILED DESCRIPTION

[0027] It should be noted that the embodiments and features in the present application can be combined with each other without conflict. The technical solutions in the embodiments of the present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0028] In order for those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor should be within the scope of protection of the present application.

[0029] It should be noted that the terms "first", "second" and the like in the specification and claims of the present application and the above-described drawings are used to distinguish similar objects, and do not necessarily indicate a specific order or sequence. It should be understood that the data thus used can be interchanged under appropriate circumstances, so that the embodiments of the present application described herein can be implemented. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion, for example, a process, method, system, product or device including a series of steps or units does not necessarily limit to those steps or units clearly listed, but can include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0030] As introduced in the background, the existing converter has the problem of high loss, which limits the engineering application of the converter. To solve the above technical problems, the embodiments of the present application provide a converter, a control method thereof, a computer readable storage medium and a flexible DC power transmission system.

[0031] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the accompanying drawings.

[0032] The embodiment of the present application provides a converter, Figure 1 An exemplary structure diagram of a converter according to an embodiment of the present application is shown in the figure, and the converter comprises: Figure 1

[0033] A multi-phase bridge arm, each phase bridge arm comprising a series connection of an upper bridge arm and a lower bridge arm, the upper bridge arm and the lower bridge arm each comprising a switching circuit 10 and a plurality of sub-modules 20 connected in series, the switching circuit 10 comprising an electric reactor 101 and a plurality of switching modules, the switching module comprising a first controllable switch tube 102, a first diode 103 and a first energy absorption module, the first controllable switch tube 102 and the first diode 103 being connected in anti-parallel, the first energy absorption module being connected in parallel with the first controllable switch tube 102, the electric reactor 101 and the first controllable switch tube 102 of the plurality of switching modules being connected in series, and the upper bridge arm and the lower bridge arm not comprising lightning arresters.

[0034] Specifically, the electric reactor 101 and the plurality of switching modules are connected in series. The first diode 103 is connected in anti-parallel across the first controllable switch tube 102, and the first diode 103 can play a protection role.

[0035] ​In the above-mentioned embodiments, in the upper bridge arm and the lower bridge arm of the converter, the switching circuit and the plurality of sub-modules are connected in series, the switching circuit comprises the reactor, a plurality of switching modules comprising a first controllable switch tube, a first diode and a first energy absorption module, the first controllable switch tube and the first diode are connected in antiparallel, the first energy absorption module is connected in parallel across the first controllable switch tube, and the reactor and the plurality of first controllable switch tubes are connected in series. The application sets the switching circuit in the upper and lower bridge arms of the converter, controls the switching state of the upper and lower bridge arms and the switching of the sub-modules through linkage, the sub-modules on the corresponding bridge arm are put into operation when the switching circuit is turned on, and the sub-modules on the corresponding bridge arm are locked out when the switching circuit is turned off, so that the upper and lower bridge arms are turned on alternately in different half cycles of the power frequency period, and then the direct current circulation cannot form a complete loop between the upper and lower bridge arms in a half power frequency period, thereby inhibiting the formation of the direct current circulation in the upper and lower bridge arms, which means that the reactor does not need to bear too much task of inhibiting the direct current circulation, and therefore the inductance requirement of the reactor in the bridge arm can be reduced. The lower inductance not only reduces the loss and response time of the whole system, but also helps to reduce the cost and physical size of the reactor. In addition, since the inductance of the inductor required by the application is low, the overvoltage level caused by it is also correspondingly reduced, and therefore the setting of the lightning arrester is not necessary in this case, so the lightning arrester does not need to be set in the converter of the application, which not only greatly reduces the overall loss of the converter, but also reduces the cost and volume of the converter. In addition, the first energy absorption module set in the upper and lower bridge arms can absorb excess energy, quickly absorb energy and clamp the voltage within a safe range, thereby protecting the controllable switch tube.

[0036] In actual application, when the first controllable switch tube 102 is turned off, due to the existence of inductive load in the sub-module 20, these loads will generate a back electromotive force to try to maintain the current unchanged. At this time, the first diode 103 connected in antiparallel can serve as a freewheeling path to allow the current to continue to flow instead of being suddenly interrupted, thereby preventing the generation of a high reverse voltage. This phenomenon is called "freewheeling", which can effectively avoid the damage of the switch tube caused by the inability to release energy at the moment of turning off, and therefore the first diode 103 connected in antiparallel across the first controllable switch tube 102 plays a certain protection role. In the moment of switching conversion, especially under rapidly changing conditions, voltage spikes may occur because the energy stored in the inductive load needs to find a way to release. The first diode 103 provides such a way, which can effectively clamp the voltage to prevent the first controllable switch tube 102 from being damaged. In addition, the first diode 103 connected in antiparallel can also assist in realizing soft switching technology, that is, when the first controllable switch tube 102 is turned on or turned off, it works in a zero-voltage or zero-current state, thereby greatly reducing the switching loss and switching stress, and improving the efficiency and reliability of the system.

[0037] In some embodiments, the first energy absorption module comprises: a second diode 104, an anode of the second diode 104 being electrically connected with a cathode of the first diode 103; a first energy storage device 105, one end of the first energy storage device 105 being electrically connected with a cathode of the second diode 104, and the other end of the first energy storage device 105 being electrically connected with an anode of the first diode 103; and a discharge device 106, being in parallel with the first energy storage device 105. In this embodiment, when the first controllable switch tube 102 is off, the second diode 104 is in a forward conduction state, allowing energy to flow from a high-voltage region between the first controllable switch tube 102 to the first energy storage device 105 in the first energy absorption module, and when the first controllable switch tube 102 is on, the second diode 104 is in a reverse blocking state, preventing energy from flowing reversely from the first energy storage device 105 back to the first controllable switch tube 102, so as to protect the first controllable switch tube 102 from reverse voltage and possible damage. During switching, the first energy storage device 105 can absorb and store transient energy, further playing a role in suppressing direct current circulation to prevent damage to the first controllable switch tube 102 or other circuit parts, and the first energy storage device 105 can also prevent the voltage between the first controllable switch tube 102 from appearing too high at the switching moment, protecting the first controllable switch tube 102 from over-voltage impact by limiting the voltage rate of change. When the energy in the first energy storage device 105 is no longer needed, the discharge device 106 can convert this part of energy into heat to consume or release in other ways, avoiding excessive accumulation of energy in the first energy storage device 105. After the first controllable switch tube 102 is off, quickly discharging the energy in the first energy storage device 105 through the discharge device helps to speed up the preparation for the next switching operation, reduces the waiting time during switching, and improves the response speed and efficiency of the system. In addition, by controlling the resistance or conduction time of the discharge device 106, the protection capability of the first energy absorption module can be adjusted to avoid overvoltage or overheating of the first energy storage device 105, thereby protecting the entire converter from energy impact damage.

[0038] For example, the first energy storage device 105 comprises a capacitor, and the discharge device 106 comprises a resistor. In this embodiment, the first energy absorption module is constructed using a parallel capacitor and resistor, the capacitor absorbs energy, and the resistor discharges energy at the appropriate time, further ensuring energy absorption and discharge of the switching module when the upper and lower bridge arms are alternately turned on.

[0039] Further, the first energy storage device 105 is a capacitor, and the discharge device 106 is a resistor.

[0040] A person skilled in the art can ensure the stability and reliability of the energy absorption circuit by reasonably matching the values of the capacitor and the resistor.

[0041] In some other exemplary embodiments, the first energy storage device 105 can also be selected as a super capacitor. The energy discharge device 106 can also be selected as a transistor, thyristor, or other energy discharge device 106 or energy discharge circuit that can convert electrical energy into thermal energy, mechanical energy or chemical energy.

[0042] In yet another exemplary embodiment, the first controllable switch tube 102 is an IGCT switch or an IGBT switch. This embodiment provides a selection of two controllable switch tubes to meet the needs of different application scenarios.

[0043] In the case where the first controllable switch tube 102 is an IGCT switch, the IGCT switch is a very efficient power switching device in high-voltage, high-current power electronic applications. Its internal structure and working principle have remarkable features, especially the latch-up effect, which makes the IGCT show extremely low voltage drop in the on state, thereby greatly reducing the on-state loss. And because the on-state voltage drop of the IGCT switch is low, it means that the power loss of the IGCT switch is small in the on state, and since most of the loss occurs when the switch is on, the low on-state loss characteristic reduces the need for large-capacity buffer energy storage devices, so that μF-level capacitors are sufficient to handle the energy generated during switching.

[0044] In the case where the first controllable switch tube 102 is an IGBT switch, by introducing an IGBT-type solid-state switch in the upper and lower bridge arms of the converter, the on-off period of the upper and lower bridge arms can be accurately controlled, and the upper and lower bridge arms of each phase can be alternately turned on. The control of the sub-modules 20 on the bridge arm is not affected by the IGBT-type solid-state switch, so that the switching circuit 10 can be consistent with the control strategy of the sub-modules 20. Through the alternating on-off mode, the fundamental frequency energy fluctuation in the bridge arm sub-modules 20 can be significantly reduced, thereby reducing the need for energy storage devices in the sub-modules 20, and the device size of the energy storage devices in the sub-modules 20 can be greatly reduced.

[0045] Optionally, the reactor 101 in the present application can select an anode reactor 101 with an inductance of μH level, and the first energy storage device 105 in the present application can select a buffer capacitor with a capacitance of μF level to protect the upper and lower bridge arms, so that the overall loss of the converter is low, and the converter meets the operating requirements of existing flexible DC projects.

[0046] Exemplarily, the first diode 103 can be a fast recovery diode. The fast recovery diode is selected as the first diode 103, which can realize efficient and low-loss current rectification and freewheeling in a high-speed switching converter circuit, and maximally reduces switching noise and loss.

[0047] According to some optional schemes of the present application, the sub-module 20 comprises: a second energy storage device 201; a second energy absorption module comprising three terminals, a first terminal of the second energy absorption module being electrically connected to one end of the second energy storage device 201, a second terminal of the second energy absorption module being electrically connected to the other end of the second energy storage device 201; a second controllable switch tube 202, one end of the second controllable switch tube 202 being electrically connected to a third terminal of the second energy absorption module; a third diode 203, being connected in anti-parallel across the second controllable switch tube 202; a third controllable switch tube 204, one end of the third controllable switch tube 204 being electrically connected to the other end of the second controllable switch tube 202, the other end of the third controllable switch tube 204 being electrically connected to the other end of the second energy storage device 201; and a fourth diode 205, being connected in anti-parallel across the third controllable switch tube 204. In this embodiment, the second energy storage device 201, the second energy absorption module and the switching device are integrated in the sub-module 20, so as to realize flexible control and energy management of the bridge arm sub-module 20. When the second energy storage device 201 of each sub-module 20 is put into operation, it contributes a fixed voltage level to the output voltage of the entire converter, and the sum of the voltages of all the put-in second energy storage devices 201 constitutes the instantaneous output voltage of the bridge arm. Moreover, the fluctuations of the bridge arm power on the AC side are absorbed and released through the charging and discharging of the second energy storage device 201 in the sub-module 20, so as to avoid the penetration of these fluctuations to the DC side. The second energy absorption module performs energy buffering and absorption, so as to protect the controllable switch tube from the impact of transient energy. The switching device is used for controlling the putting-in and blocking of the sub-module 20. Through the cooperative work of the second energy storage device 201, the second energy absorption module and the switching device, the control accuracy and response speed of the sub-module 20 are improved, so that the converter can realize high-efficiency, high-quality and highly-controllable large-power electric energy conversion.

[0048] In a specific embodiment, the second energy absorption module includes: a third energy storage device 206, one end of the third energy storage device 206 being the second end of the second energy absorption module; a fifth diode 207, the cathode of the fifth diode 207 being electrically connected to the other end of the third energy storage device 206, and the anode of the fifth diode 207 being the third end of the second energy absorption module; a resistance element 208, one end of the resistance element 208 being the first end of the second energy absorption module, and the other end of the resistance element 208 being electrically connected to the cathode of the fifth diode 207; and an inductive element 209, one end of the inductive element 209 being electrically connected to one end of the resistance element 208, and the other end of the inductive element 209 being electrically connected to the anode of the fifth diode 207. In the above embodiment, the inductive element 209 can limit the current change rate of the controllable switch tube, and the third energy storage device 206 and the resistance element 208 can absorb and dissipate the energy on the inductor, thereby protecting the device from overvoltage damage.

[0049] Exemplarily, the third energy storage device 206 can be a capacitor or a super capacitor. In an optional embodiment of the present application, the third energy storage device 206 is a capacitor.

[0050] Exemplarily, the third diode 203 and the fourth diode 205 can be fast recovery diodes. By selecting fast recovery diodes as the third diode 203 and the fourth diode 205, high-efficiency and low-loss current rectification and freewheeling can be achieved in a high-speed switching inverter circuit, and switching noise and loss are minimized.

[0051] In actual application, the sub-module 20 can be a half-bridge type sub-module as shown in Figure 1 The sub-module 20 can also be a full-bridge type sub-module. Those skilled in the art can flexibly select the type of the sub-module 20 according to actual conditions.

[0052] Optionally, the second controllable switch tube 202 and the third controllable switch tube 204 are IGCT switches or IGBT switches, respectively. The present embodiment provides a selection of two kinds of controllable switch tubes to adapt to the needs of different application scenarios. IGCT has a lower on-state voltage drop and is more suitable for high power density and low loss scenarios, while IGBT can be more cost-effective under certain specific conditions.

[0053] In other embodiments, a combination of IGCT and IGBT can also be used to balance loss and cost.

[0054] As shown in Figure 1 In a specific implementation, the sub-module 20 includes a second energy storage device 201, and in the bridge arm, the maximum number of the switch modules is The maximum number of the above submodules 20 is ,in, The function is the floor function, where M is the maximum number of the aforementioned switch modules, and N is the maximum number of the aforementioned sub-modules 20. This refers to the DC bus voltage of the aforementioned converter. The voltage stored in the second energy storage device 201 mentioned above.

[0055] The above embodiment calculates the ratio between the DC bus voltage and the voltage of sub-module 20 to determine the maximum number of sub-module 20 and switch modules in order to achieve the optimal operating state. By reasonably configuring the number of sub-module 20 and switch modules, the efficiency and reliability of the converter are improved, and the stable operation and energy management of the converter are further ensured.

[0056] In some other embodiments of this application, the first energy storage device 105, the discharge device 106, and the reactor 101 can be configured as devices with adjustable parameters. At least one of the following values ​​can be adjusted in real time according to the converter's operating conditions (including load demand and grid voltage fluctuations): the capacitance of the first energy storage device 105, the resistance of the discharge device 106, and the inductance of the reactor 101. Specifically, when the load demand exceeds a first threshold or the grid voltage fluctuation is greater than a second threshold, the capacitance, resistance, and inductance are increased; when the load demand is less than or equal to the first threshold or the grid voltage fluctuation is less than or equal to the second threshold, the capacitance, resistance, and inductance are decreased. This helps to further optimize the converter's performance, improve its adaptability to complex operating environments, and reduce its losses.

[0057] The methods and embodiments provided in this application can be executed on a mobile terminal, a computer terminal, or a similar computing device. Taking running on a mobile terminal as an example, Figure 2 This is a hardware structure block diagram of a mobile terminal for a converter control method according to an embodiment of this application. Figure 2 As shown, a mobile terminal may include one or more ( Figure 2 Only one is shown in the diagram. A processor 502 (which may include, but is not limited to, a microprocessor MCU or a programmable logic device FPGA, etc.) and a memory 504 for storing data are also shown. The mobile terminal may further include a transmission device 506 for communication functions and an input / output device 508. Those skilled in the art will understand that... Figure 2 The structure shown is for illustrative purposes only and does not limit the structure of the mobile terminal described above. For example, the mobile terminal may also include components that are more... Figure 2 The more or fewer components shown, or having the same Figure 2 The different configurations shown.

[0058] The memory 504 can be used to store computer programs, such as application software programs and modules, like the computer program corresponding to the converter control method in this embodiment. The processor 502 executes various functional applications and data processing by running the computer program stored in the memory 504, thus implementing the above-described method. The memory 504 may include high-speed random access memory and non-volatile memory, such as one or more magnetic storage devices, flash memory, or other non-volatile solid-state memory. In some instances, the memory 504 may further include memory remotely located relative to the processor 502, and these remote memories can be connected to the mobile terminal via a network. Examples of the aforementioned networks include, but are not limited to, the Internet, corporate intranets, local area networks, mobile communication networks, and combinations thereof. The transmission device 506 is used to receive or send data via a network. Specific examples of the aforementioned networks may include wireless networks provided by the mobile terminal's communication provider. In one example, the transmission device 506 includes a network interface controller (NIC), which can be connected to other network devices via a base station to communicate with the Internet. In one example, the transmission device 506 may be a radio frequency (RF) module, which is used to communicate with the Internet wirelessly.

[0059] This embodiment provides a control method for a converter operating on a mobile terminal, computer terminal, or similar computing device, for controlling any of the aforementioned converters. It should be noted that the steps shown in the flowcharts of the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowcharts, in some cases, the steps shown or described may be executed in a different order than that shown here.

[0060] Figure 3 This is a flowchart of a control method for a converter according to an embodiment of this application. Figure 3 As shown, the method includes the following steps:

[0061] Step S601: Use a power frequency square wave to alternately turn on and off the switching circuits of the upper and lower bridge arms;

[0062] Specifically, the aforementioned power frequency square wave refers to a square wave signal synchronized with the local power system standard frequency (also known as the grid frequency). When the upper bridge arm's switching circuit is on, the lower bridge arm's switching circuit is off; when the upper bridge arm's switching circuit is off, the lower bridge arm's switching circuit is on.

[0063] Step S602, based on the half-wave modulation strategy and the on and off strategy of the above-mentioned switch circuit, the sub-modules of the above-mentioned upper bridge arm and the above-mentioned lower bridge arm are alternately put into and locked out, so that when the above-mentioned switch circuit is turned on, the above-mentioned sub-modules of the corresponding above-mentioned upper bridge arm or the above-mentioned lower bridge arm are put into, and when the above-mentioned switch circuit is turned off, the above-mentioned sub-modules of the corresponding above-mentioned upper bridge arm or the above-mentioned lower bridge arm are locked out.

[0064] Specifically, the above-mentioned switch circuit and the above-mentioned sub-module located in the same bridge arm have a corresponding relationship. The on and off strategy of the above-mentioned switch circuit is the strategy of alternately turning on and off the switch circuit of the upper and lower bridge arms in step S601.

[0065] Through the embodiment, first, the switch circuit of the upper and lower bridge arms is alternately turned on and off by using the power frequency square wave; then, based on the half-wave modulation strategy and the on and off strategy of the switch circuit, the sub-modules of the above-mentioned upper bridge arm and the above-mentioned lower bridge arm are alternately put into and locked out, so that the switch circuit and the sub-module on the same bridge arm are synchronously turned on or synchronously turned off. By using the power frequency square wave control and the half-wave modulation strategy, the linkage control of the switching state of the upper and lower bridge arms and the switching of the sub-modules is realized, that is, when the switch circuit is turned on, the sub-modules on the corresponding bridge arm are put into, and when the switch circuit is turned off, the sub-modules on the corresponding bridge arm are locked out, so that the upper and lower bridge arms are alternately turned on in different half cycles of the power frequency cycle, and then the direct current loop cannot form a complete loop between the upper and lower bridge arms in a half power frequency cycle, thereby inhibiting the formation of the direct current loop in the upper and lower bridge arms, which means that the reactor does not need to bear too much task of inhibiting the direct current loop, so the inductance requirement of the reactor in the bridge arm can be reduced. The lower inductance not only reduces the loss and response time of the whole system, but also helps to reduce the cost and physical size of the reactor, and solves the problem of high loss of the converter caused by the direct current loop. In addition, since the inductor required by the application has a low inductance, the overvoltage level caused by it is also correspondingly reduced, so the setting of the lightning arrester is no longer necessary in this case, and therefore the lightning arrester does not need to be set in the converter of the application, which not only greatly reduces the overall loss of the converter, but also reduces the cost and volume of the converter.

[0066] In addition, in addition to the half-wave modulation strategy, multi-level modulation techniques such as space vector modulation (SVM) or carrier phase shift PWM (CPS-PWM) can also be combined to further optimize the output waveform quality, reduce THD (total harmonic distortion), and reduce the switching frequency of IGCT devices and switching loss.

[0067] In some embodiments, the switch circuit of the upper bridge arm and the lower bridge arm is alternately turned on and off by using the power frequency square wave, comprising: determining the control function of the above-mentioned switch circuit as wherein, is the control function, ω is the power frequency angular frequency, t is the time, ωt is the power frequency phase, is a symbol function, when the symbol function is 1, that is, when , the symbol function is 0, that is, when ; the control function is used to control the switch circuit, so that in the case of the control function being 1, the switch circuit of the upper bridge arm is turned on and the switch circuit of the lower bridge arm is turned off, and in the case of the control function being 0, the switch circuit of the upper bridge arm is turned off and the switch circuit of the lower bridge arm is turned on. The present embodiment determines the control function by the power frequency square wave signal, and the control function alternately turns on and turns off the switch circuits of the upper bridge arm and the lower bridge arm according to the positive and negative half cycles of the power frequency square wave, so as to realize accurate control of the bridge arm switch circuit and ensure the stability and reliability of the alternating turn-on of the bridge arm.

[0068] In other optional solutions, based on the half-wave modulation strategy and the turn-on and turn-off strategy of the switch circuit, the sub-modules of the upper bridge arm and the lower bridge arm are alternately put into operation and locked out, including: generating a current reference value according to the active power reference value, the reactive power reference value and the DC voltage; generating a reference voltage waveform of the sub-modules of the upper bridge arm and the lower bridge arm for half-wave modulation according to the current reference value and the actual bridge arm current of the upper bridge arm and the lower bridge arm; and generating control signals of the sub-modules of the upper bridge arm and the lower bridge arm by using a nearest level modulation (NLM) method according to the reference voltage waveform and the turn-on and turn-off strategy, so as to make the sub-modules of the upper bridge arm put into operation and the sub-modules of the lower bridge arm be locked out, or make the sub-modules of the lower bridge arm put into operation and the sub-modules of the upper bridge arm be locked out. In the present embodiment, the half-wave modulation strategy generates the reference voltage waveform of the sub-modules according to the turn-on state of the switch circuit of the upper and lower bridge arms, and the nearest level modulation method is used to generate the control signals of the sub-modules, so as to realize accurate control of the alternating operation and locking out of the upper and lower bridge arms and further improve the efficiency and reliability of the converter.

[0069] Specifically, the current reference value is generated according to the active power reference value, the reactive power reference value and the DC voltage, including: inputting the active power reference value, the reactive power reference value and the DC voltage into a PI controller to generate the current reference value in a two-phase rotating coordinate system (dq).

[0070] ​​​Optionally, generating the reference voltage waveform of the sub-modules of the upper bridge arm and the lower bridge arm for half-wave modulation according to the current reference value and the actual bridge arm current of the upper bridge arm and the lower bridge arm comprises: generating the reference voltage waveform of the sub-modules of the upper bridge arm and the lower bridge arm for half-wave modulation according to the current reference value and the actual bridge arm current of the upper bridge arm and the lower bridge arm is wherein, is the reference voltage waveform of the sub-modules of the upper bridge arm, is the reference voltage waveform of the sub-modules of the lower bridge arm, is the modulation ratio, is the power frequency angular frequency, and t is the time, is the phase of the j-phase alternating phase voltage.

[0071] In the above embodiment, the generated reference voltage waveform is based on the actual bridge arm current and the expected current reference value. By comparing the two and generating the corresponding voltage waveform, the deviation between the actual current and the target current can be corrected, which helps to improve the quality of the current, reduce harmonics, and make the current closer to a sine wave, meeting the standard requirements of the power system. The half-wave modulation strategy, combined with factors such as modulation ratio and power frequency angular frequency, can optimize the switching mode of the sub-modules and reduce unnecessary switching actions. This not only reduces switching loss, but also indirectly reduces the loss of energy storage devices by reducing the fundamental frequency energy pulsation of the sub-modules in the bridge arm, thereby reducing the overall operating loss of the converter and improving system energy efficiency. Compared with traditional modulation methods such as carrier layer modulation and nearest level approximation modulation, the half-wave modulation strategy is often more simple and intuitive. Based on the periodic nature of the power frequency, it can effectively suppress the direct current circulating in the direct current converter by controlling the alternating conduction of the upper bridge arm and the lower bridge arm sub-modules, thereby eliminating the use of bridge arm arresters and simplifying the design of control logic and hardware requirements. By updating and generating the reference voltage waveform in real time, the system can quickly adjust the conduction state of the bridge arm sub-modules according to the phase change of the alternating phase voltage, which is crucial for improving the dynamic response capability of the converter. Accurate reference voltage waveform and real-time monitoring of the actual bridge arm current help to maintain the stability of the direct current voltage.

[0072] According to some embodiments of the present application, generating the control signal of the sub-modules of the upper bridge arm and the lower bridge arm according to the reference voltage waveform and the on and off strategy by using the nearest level approximation modulation method comprises: determining the number of sub-modules of the upper bridge arm and the lower bridge arm to be put into according to the reference voltage waveform and the on and off strategy by using the nearest level approximation modulation method is wherein, is the number of sub-modules of the upper bridge arm to be put into, the number of the sub-modules of the upper bridge arm, =0 or 1, =1 represents that the on and off strategy is to turn on the switch circuit of the upper bridge arm, =0 represents that the on and off strategy is to turn on the switch circuit of the lower bridge arm, the reference voltage waveform of the sub-modules of the upper bridge arm, the reference voltage waveform of the sub-modules of the lower bridge arm, the sub-modules comprising a second energy storage device, is the ratio of the DC bus voltage of the converter to the voltage stored in the second energy storage device; according to the number of the sub-modules of the upper bridge arm and the lower bridge arm, the control signal of the corresponding sub-module is generated.

[0073] In the above embodiment, the NLM method is used to make the number of the sub-modules close to the reference voltage waveform, so as to realize accurate control of the output voltage of the converter, which can generate an output voltage closest to the reference voltage waveform, reduce voltage deviation, improve voltage quality, and ensure the stability of the voltage in the power system; the NLM method also simplifies the control logic and avoids complex multi-level PWM modulation strategies; according to the on and off strategy, the switch circuit of the upper bridge arm or the lower bridge arm is selectively turned on, which can avoid unnecessary sub-module switching actions, thereby significantly reducing the switching loss; by using the reference voltage waveform to guide the sub-module, the energy pulsation of the second energy storage device in the upper and lower bridge arms can be effectively suppressed, and under the strategy of alternating turn-on, the charging and discharging processes of the second energy storage devices of the upper bridge arm and the lower bridge arm are evenly distributed, which reduces the energy fluctuation in the second energy storage device and prolongs the service life of the second energy storage device.

[0074] According to still some embodiments of the present application, a multi-objective optimization control strategy can also be introduced to control the converter, which includes: determining the control objectives of the converter, which include at least part of power factor correction (PFC), harmonic suppression, voltage stability and energy recovery efficiency; based on the dynamic model of the converter, converting each control objective into a mathematical expression as a target function to obtain a set of target functions, for example, the target function can include minimizing the deviation between the instantaneous power factor and the ideal value, which is expressed as minimizing the deviation of the d-axis component in the dq coordinate system; for another example, the target function can include minimizing the harmonic content of the output current or voltage, which quantifies the harmonic distortion by establishing a target function based on Fourier technology decomposition; for another example, the target function can include voltage stability, which is expressed as minimizing the deviation between the voltage and the set reference value; defining the constraint conditions that meet the safety and physical limitations of the converter, including but not limited to the amplitude limiting of current and voltage, the limitation of IGCT switching on time, etc.; determining the search space, i.e. the range of control parameters such as modulation ratio, carrier frequency, threshold value of switching action, etc.; selecting an optimization algorithm, such as particle swarm optimization, genetic algorithm, differential evolution or multi-objective optimization variants such as NSGA-II, MOEA / D algorithm, and executing the optimization algorithm to iteratively find the optimal solution set that meets the set of target functions, and control the operation of the converter according to the optimal solution set.

[0075] In order to enable those skilled in the art to more clearly understand the technical solutions of the present application, the implementation process of the control method of the converter of the present application will be described in detail below in conjunction with specific embodiments.

[0076] The present embodiment relates to a specific control method of a converter, and the specific structure of the converter is as shown in Figure 1 as shown in Figure 1As shown, the above converter is a three-phase six-bridge-arm topology, that is, the converter comprises six bridge arms, each of which is provided with a series-connected switching circuit 10 and N sub-modules 20, the switching circuit 10 comprises a series-connected reactor 101 and M switching modules. Each of the above switching modules comprises a first controllable switch tube 102, a first diode 103, a second diode 104, a first energy storage device 105, and a bleeding device 106, the first controllable switch tube 102 is connected in antiparallel with the first diode 103, the anode of the second diode 104 is electrically connected to the cathode of the first diode 103, the cathode of the second diode 104 is electrically connected to one end of the first energy storage device 105, the other end of the first energy storage device 105 is electrically connected to the anode of the first diode 103, and the bleeding device 106 is connected in parallel across the first energy storage device 105. Each sub-module 20 comprises a second energy storage device 201, a third energy storage device 206, a second controllable switch tube 202, a third diode 203, a third controllable switch tube 204, a fourth diode 205, a fifth diode 207, a resistor element 208, and an inductor element 209, the second controllable switch tube 202 is connected in series with the third controllable switch tube 204, the third diode 203 is connected in antiparallel across the second controllable switch tube 202, the fourth diode 205 is connected in antiparallel across the third controllable switch tube 204, the anode of the fifth diode 207 is electrically connected to the cathode of the third diode 203, the cathode of the fifth diode 207 is electrically connected to one end of the third energy storage device 206, the other end of the third energy storage device 206 is electrically connected to the anode of the fourth diode 205, one end of the inductor element 209 is electrically connected to the anode of the fifth diode 207, the other end of the inductor element 209 is electrically connected to one end of the second energy storage device 201, the other end of the second energy storage device 201 is electrically connected to the anode of the fourth diode 205, one end of the resistor element 208 is electrically connected to the cathode of the fifth diode 207 and one end of the third energy storage device 206 respectively, and the other end of the resistor element 208 is electrically connected to the other end of the inductor element 209 and one end of the second energy storage device 201 respectively. The DC end of the above converter comprises a DC capacitor 30 and a smoothing reactor 40. Among them, the first controllable switch tube 102, the second controllable switch tube 202, and the third controllable switch tube 204 are all IGCTs, the first diode 103, the third diode 203, and the fourth diode 205 are all fast recovery diodes, the second energy storage device 201 is a DC capacitor 30, the inductor element 209 and the reactor 101 are both anode reactors, the first energy storage device 105 and the third energy storage device 206 are both absorption capacitors, and the bleeding device 106 is an absorption resistor.

[0077] Among them, M and N satisfy the following formula:

[0078] , .

[0079] The control block diagram of the above-mentioned converter is shown in Figure 4 The embodiment provides a converter control method based on a half-wave modulation strategy and alternating conduction of an upper bridge arm and a lower bridge arm of an IGCT type. The IGCT type switch circuit 10 of the upper bridge arm and the lower bridge arm in the above-mentioned converter adopts power frequency square wave control, and the control function of the IGCT type switch circuit 10 of the jth (j=a, b, c) phase unit bridge arm is When , the first controllable switch tube 102 of the upper bridge arm is turned on, and the first controllable switch tube 102 of the lower bridge arm is turned off; when , the first controllable switch tube 102 of the lower bridge arm is turned on, and the first controllable switch tube 102 of the upper bridge arm is turned off.

[0080] The control function of the IGCT type switch circuit 10 of the jth phase unit bridge arm is The control function satisfies:

[0081] Wherein, is a symbol function, the symbol function is 1 when , and the symbol function is 0 when .

[0082] The IGCT type sub-module 20 of the bridge arm of the above-mentioned converter adopts a control method based on a half-wave modulation strategy, as shown in Figure 4 According to the active power reference value , the reactive power , and the reference value of the DC bus voltage , an outer ring power P / Q (or voltage ) control is adopted to generate the dq axis current reference value , , and then according to the current reference value , and each phase reference wave , an inner ring current control is adopted to generate the reference wave and of the upper and lower bridge arm IGCT type sub-modules 20 of each phase unit, and the reference wave satisfies the following formula:

[0083] ;

[0084] If the control function of the phase unit IGCT type switch circuit 10 is , the IGCT type sub-module 20 of the upper bridge arm of the phase unit generates a control signal according to a nearest level modulation method, and the IGCT type sub-module 20 of the lower bridge arm is locked; otherwise, when , the IGCT type sub-module 20 of the lower bridge arm of the phase unit generates a control signal according to an NLM modulation method, and the IGCT type sub-module 20 of the upper bridge arm is locked.

[0085] The number of IGCT type sub-modules 20 put into each phase unit upper and lower bridge arm and Satisfy the formula:

[0086] Wherein N0=U dc / U SM , U dc is the DC bus voltage of the above-mentioned converter, U SM is the voltage stored by the second energy storage device 201, and is also the average voltage of the second energy storage device 201 in each sub-module 20.

[0087] The control function of the IGCT type switching circuit 10 of each phase unit is a square wave of the power frequency, and the IGCT type sub-modules 20 of each bridge arm are only allowed to be put into during the half cycle in which the IGCT type switching circuit 10 of the bridge arm is turned on, and are blocked during the half cycle in which the IGCT type switching circuit 10 of the bridge arm is turned off, so that the DC circulating current path between the phase units and between the phase unit and the DC side capacitor is blocked, thereby suppressing the DC circulating current, and the bridge arm reactor 101 and the arrester on the bridge arm can be cancelled. The topology structure and control method of the above-mentioned converter in which the IGCT type bridge arm sub-modules 20 are alternately turned on greatly reduce the volume of the converter, improve the power density of the valve body, reduce the maximum fundamental frequency energy pulsation of the bridge arm sub-modules 20 by about 2 / 3, and solve the problem of high loss of the existing bridge arm alternately turned on converter, thereby meeting the operation requirements of the flexible DC project. In addition, the controllable switching tubes in the sub-modules 20 and the switching circuits 10 of the above-mentioned converter all use IGCT devices, and the conduction loss is smaller; since the bridge arm reactor 101 is cancelled, only μF level buffer capacitor and μH level anode reactance need to be provided in the switching circuit 10, and the loss in the switching process is smaller, so that the overall loss of the converter is lower than that of the existing MMC, thereby meeting the operation requirements of the flexible DC project.

[0088] The embodiment of the present application further provides a control device of a converter. It should be noted that the control device of the converter of the embodiment of the present application can be used to execute the control method for the converter provided by the embodiment of the present application. The device is used to realize the above-mentioned embodiment and preferred embodiment, and will not be described here. As used below, the term "module" can be a combination of software and / or hardware that realizes a predetermined function. Although the device described in the following embodiment is preferably realized in software, the realization of hardware, or a combination of software and hardware, is also possible and conceived.

[0089] The control device of the converter includes a processor and a memory, and units for implementing the control method of the converter are stored in the memory as program units. The processor executes the program units stored in the memory to implement the corresponding functions. The modules are located in the same processor, or the modules are located in different processors in any combination.

[0090] The processor includes a core, and the core retrieves the corresponding program units from the memory. The core can be one or more, and the core parameters are adjusted to at least solve the problem of high loss of the converter in the prior art.

[0091] The memory can include a non-persistent memory in a computer readable medium, a random access memory (RAM), and / or a non-volatile memory such as a read-only memory (ROM) or a flash memory (flash RAM), and the memory includes at least one memory chip.

[0092] The embodiments of the present application also provide a flexible direct current power transmission system, which includes:

[0093] The converter described above;

[0094] An electronic device includes one or more processors, a memory, and one or more programs, wherein the one or more programs are stored in the memory and configured to be executed by the one or more processors, and the one or more programs include a program for executing any of the methods described above.

[0095] Through the above embodiments, in the flexible direct current power transmission system, the switching circuit is arranged in the upper and lower bridge arms of the converter, and the control method of the converter is executed by the electronic device. The method realizes the linkage control of the switching state of the upper and lower bridge arms and the switching of the sub-modules through the power frequency square wave control and the half-wave modulation strategy, that is, the sub-modules on the corresponding bridge arm are put into operation when the switching circuit is turned on, and the sub-modules on the corresponding bridge arm are locked out when the switching circuit is turned off, so that the upper and lower bridge arms are alternately turned on in different half cycles of the power frequency cycle. Then, the direct current loop cannot form a complete loop between the upper and lower bridge arms in a half power frequency cycle, thereby inhibiting the formation of the direct current loop in the upper and lower bridge arms, which means that the reactor does not need to bear too much task of inhibiting the direct current loop, and therefore the inductance requirement of the reactor in the bridge arm can be reduced. The lower inductance not only reduces the loss and response time of the entire system, but also helps to reduce the cost and physical size of the reactor, and solves the problem of high loss of the converter caused by the direct current loop. In addition, since the inductor inductance required by the present application is low, the overvoltage level caused by the low inductor inductance is also reduced, and therefore the setting of the lightning arrester is no longer necessary in this case. Therefore, the lightning arrester does not need to be arranged in the converter of the present application, which not only greatly reduces the overall loss of the converter, but also reduces the cost and volume of the converter.

[0096] The embodiment of the present application provides a computer readable storage medium, which comprises a stored program, wherein the program controls a device where the computer readable storage medium is located to perform the control method of the converter when the program is running.

[0097] Specifically, the control method of the converter comprises:

[0098] In step S601, the switch circuit of the upper bridge arm and the lower bridge arm is alternately turned on and turned off by using a power frequency square wave.

[0099] Specifically, the power frequency square wave refers to a square wave signal synchronized with a local power system standard frequency (also called grid frequency). In the case that the switch circuit of the upper bridge arm is turned on, the switch circuit of the lower bridge arm is turned off; in the case that the switch circuit of the upper bridge arm is turned off, the switch circuit of the lower bridge arm is turned on.

[0100] In step S602, based on a half-wave modulation strategy and the turn-on and turn-off strategy of the switch circuit, the sub-modules of the upper bridge arm and the lower bridge arm are alternately put into operation and locked out, so that the sub-modules of the corresponding upper bridge arm or lower bridge arm are put into operation when the switch circuit is turned on, and the sub-modules of the corresponding upper bridge arm or lower bridge arm are locked out when the switch circuit is turned off.

[0101] Specifically, the switch circuit and the sub-module located in the same bridge arm have a corresponding relationship. The turn-on and turn-off strategy of the switch circuit is the strategy of alternately turning on and turning off the switch circuit of the upper and lower bridge arms in step S601.

[0102] Optionally, alternately turning on and turning off the switch circuit of the upper bridge arm and the lower bridge arm by using a power frequency square wave comprises: determining a control function of the switch circuit as f (t) = sin (ωt + φ) according to the power frequency square wave, wherein ω is a power frequency angle frequency, t is a time, and φ is a power frequency phase. The control function is f (t) = sin (ωt + φ). ω is a power frequency angle frequency, t is a time, and φ is a power frequency phase. The power frequency phase is φ. The sign function is a sign function, which is 1 when t < 0 and 0 when t > 0. The sign function is 1 when t < 0 and 0 when t > 0. The sign function is 0 when t > 0; and the switch circuit is controlled by using the control function, so that the switch circuit of the upper bridge arm is turned on and the switch circuit of the lower bridge arm is turned off when the control function is 1, and the switch circuit of the upper bridge arm is turned off and the switch circuit of the lower bridge arm is turned on when the control function is 0.

[0103] ​Optionally, based on the half-wave modulation strategy and the turn-on and turn-off strategy of the switch circuit, the alternating investment and locking of the sub-modules of the upper bridge arm and the lower bridge arm comprises: generating a current reference value according to the active power reference value, the reactive power reference value and the DC voltage; generating a reference voltage waveform of the sub-modules of the upper bridge arm and the lower bridge arm for half-wave modulation according to the current reference value and the actual bridge arm current of the upper bridge arm and the lower bridge arm; and generating control signals of the sub-modules of the upper bridge arm and the lower bridge arm by using the nearest level approximation modulation method according to the reference voltage waveform and the turn-on and turn-off strategy, so that the sub-modules of the upper bridge arm are invested and the sub-modules of the lower bridge arm are locked, or the sub-modules of the lower bridge arm are invested and the sub-modules of the upper bridge arm are locked.

[0104] Optionally, the generating of the reference voltage waveform of the sub-modules of the upper bridge arm and the lower bridge arm for half-wave modulation according to the current reference value and the actual bridge arm current of the upper bridge arm and the lower bridge arm comprises: generating the reference voltage waveform of the sub-modules of the upper bridge arm and the lower bridge arm for half-wave modulation according to the current reference value and the actual bridge arm current of the upper bridge arm and the lower bridge arm is wherein, is the reference voltage waveform of the sub-modules of the upper bridge arm, is the reference voltage waveform of the sub-modules of the lower bridge arm, is a modulation ratio, is a power frequency angular frequency, and t is a time, is a phase of an alternating current phase voltage of the j phase.

[0105] Optionally, the generating of the control signals of the sub-modules of the upper bridge arm and the lower bridge arm by using the nearest level approximation modulation method according to the reference voltage waveform and the turn-on and turn-off strategy comprises: determining the number of the sub-modules of the upper bridge arm and the lower bridge arm to be invested by using the nearest level approximation modulation method according to the reference voltage waveform and the turn-on and turn-off strategy is wherein, is the number of the sub-modules of the upper bridge arm to be invested, is the number of the sub-modules of the lower bridge arm to be invested, =0 or 1, =1 represents that the turn-on and turn-off strategy is to turn on the switch circuit of the upper bridge arm, =0 represents that the turn-on and turn-off strategy is to turn on the switch circuit of the lower bridge arm, is the reference voltage waveform of the sub-modules of the upper bridge arm, is the reference voltage waveform of the sub-modules of the lower bridge arm, and the sub-modules comprise a second energy storage device. The ratio of the DC bus voltage of the converter to the voltage stored in the second energy storage device is used; the control signal for the corresponding sub-module is generated based on the number of sub-modules deployed in the upper and lower bridge arms.

[0106] This application provides a processor for running a program, wherein the program executes the control method of the converter.

[0107] This application provides a device, which includes a processor, a memory, and a program stored in the memory and executable on the processor. When the processor executes the program, it performs at least the following steps:

[0108] Step S601: Use a power frequency square wave to alternately turn on and off the switching circuits of the upper and lower bridge arms;

[0109] Specifically, the aforementioned power frequency square wave refers to a square wave signal synchronized with the local power system standard frequency (also known as the grid frequency). When the upper bridge arm's switching circuit is on, the lower bridge arm's switching circuit is off; when the upper bridge arm's switching circuit is off, the lower bridge arm's switching circuit is on.

[0110] Step S602: Based on the half-wave modulation strategy and the on and off strategies of the switching circuit, the sub-modules of the upper bridge arm and the lower bridge arm are alternately engaged and disengaged, such that when the switching circuit is engaged, the corresponding sub-module of the upper bridge arm or the lower bridge arm is engaged, and when the switching circuit is disengaged, the corresponding sub-module of the upper bridge arm or the lower bridge arm is disengaged.

[0111] Specifically, the aforementioned switching circuits and sub-modules located in the same bridge arm have a corresponding relationship. The switching circuit's on and off strategy is the same as the strategy of alternately turning on and off the switching circuits of the upper and lower bridge arms in step S601.

[0112] The devices mentioned in this article can be servers, PCs, tablets, mobile phones, etc.

[0113] Optionally, using a power frequency square wave, the switching circuits of the upper and lower bridge arms are alternately turned on and off, including: determining the control function of the switching circuits based on the aforementioned power frequency square wave. ,in, For the above control function, Where t is the power frequency angular frequency, and t is the time. For power frequency phase, For the sign function, in When the above symbolic function is 1, in The above control function is 0; the above switch circuit is controlled by the above control function, so that the above switch circuit of the upper bridge arm is turned on and the above switch circuit of the lower bridge arm is turned off when the above control function is 1, and the above switch circuit of the upper bridge arm is turned off and the above switch circuit of the lower bridge arm is turned on when the above control function is 0.

[0114] Alternatively, based on the half-wave modulation strategy and the turn-on and turn-off strategy of the above switch circuit, the sub-modules of the upper bridge arm and the lower bridge arm are alternately turned on and locked out, including: generating a current reference value according to the active power reference value, the reactive power reference value and the direct current voltage; generating a reference voltage waveform of the sub-modules of the upper bridge arm and the lower bridge arm for half-wave modulation according to the current reference value and the actual bridge arm current of the upper bridge arm and the lower bridge arm; and generating control signals of the sub-modules of the upper bridge arm and the lower bridge arm by using the nearest level approximation modulation method according to the reference voltage waveform and the turn-on and turn-off strategy, so that the sub-modules of the upper bridge arm are turned on and the sub-modules of the lower bridge arm are locked out, or the sub-modules of the lower bridge arm are turned on and the sub-modules of the upper bridge arm are locked out.

[0115] Alternatively, generating a reference voltage waveform of the sub-modules of the upper bridge arm and the lower bridge arm for half-wave modulation according to the current reference value and the actual bridge arm current of the upper bridge arm and the lower bridge arm includes: generating a reference voltage waveform of the sub-modules of the upper bridge arm and the lower bridge arm for half-wave modulation according to the current reference value and the actual bridge arm current of the upper bridge arm and the lower bridge arm is wherein, is the reference voltage waveform of the sub-modules of the upper bridge arm, is the reference voltage waveform of the sub-modules of the lower bridge arm, is a modulation ratio, is a power frequency angular frequency, and t is a time, is a phase of the j-phase alternating current phase voltage.

[0116] Alternatively, generating control signals of the sub-modules of the upper bridge arm and the lower bridge arm by using the nearest level approximation modulation method according to the reference voltage waveform and the turn-on and turn-off strategy includes: determining the number of turned-on sub-modules of the upper bridge arm and the lower bridge arm by using the nearest level approximation modulation method according to the reference voltage waveform and the turn-on and turn-off strategy is wherein, is the number of turned-on sub-modules of the upper bridge arm, is the number of turned-on sub-modules of the lower bridge arm, =0 or 1, = 1 represents that the on and off strategy of the switch circuit is to turn on the upper bridge arm, = 0 represents that the on and off strategy of the switch circuit is to turn on the lower bridge arm, is the reference voltage waveform of the sub-module of the upper bridge arm, is the reference voltage waveform of the sub-module of the lower bridge arm, and the sub-module comprises a second energy storage device, is the ratio of the DC bus voltage of the converter to the voltage stored in the second energy storage device; according to the number of the sub-modules of the upper bridge arm and the lower bridge arm, the control signal of the corresponding sub-module is generated.

[0117] The application also provides a computer program product adapted to execute the program of at least the following method steps when executed on a data processing device:

[0118] Step S601, alternatingly turning on and off the switch circuits of the upper bridge arm and the lower bridge arm by using a power frequency square wave;

[0119] Specifically, the power frequency square wave refers to a square wave signal synchronized with the local power system standard frequency (also known as grid frequency). In the case that the switch circuit of the upper bridge arm is turned on, the switch circuit of the lower bridge arm is turned off; in the case that the switch circuit of the upper bridge arm is turned off, the switch circuit of the lower bridge arm is turned on.

[0120] Step S602, based on the half-wave modulation strategy and the on and off strategy of the switch circuit, alternatingly inputting and locking the sub-modules of the upper bridge arm and the lower bridge arm, so that when the switch circuit is turned on, the sub-modules of the corresponding upper bridge arm or lower bridge arm are inputted, and when the switch circuit is turned off, the sub-modules of the corresponding upper bridge arm or lower bridge arm are locked.

[0121] Specifically, the switch circuit and the sub-module located in the same bridge arm have a corresponding relationship. The on and off strategy of the switch circuit is the strategy of alternatingly turning on and off the switch circuits of the upper and lower bridge arms in step S601.

[0122] Optionally, alternatingly turning on and off the switch circuits of the upper bridge arm and the lower bridge arm by using a power frequency square wave comprises: determining the control function of the switch circuit as wherein, is the control function, is the power frequency angular frequency, and t is the time, is the power frequency phase, is a sign function, and when the sign function is 1, and when the control function is 1, the switch circuit of the upper bridge arm is turned on and the switch circuit of the lower bridge arm is turned off, and when the control function is 0, the switch circuit of the upper bridge arm is turned off and the switch circuit of the lower bridge arm is turned on.

[0123] Alternatively, based on the half-wave modulation strategy and the turn-on and turn-off strategy of the switch circuit, the alternating investment and locking of the sub-modules of the upper bridge arm and the lower bridge arm comprises: generating a current reference value according to the active power reference value, the reactive power reference value and the direct current voltage; generating a reference voltage waveform of the sub-modules of the upper bridge arm and the lower bridge arm for half-wave modulation according to the current reference value and the actual bridge arm current of the upper bridge arm and the lower bridge arm; and generating control signals of the sub-modules of the upper bridge arm and the lower bridge arm by using the nearest level approximation modulation method according to the reference voltage waveform and the turn-on and turn-off strategy, so that the sub-modules of the upper bridge arm are invested and the sub-modules of the lower bridge arm are locked, or the sub-modules of the lower bridge arm are invested and the sub-modules of the upper bridge arm are locked.

[0124] Alternatively, generating a reference voltage waveform of the sub-modules of the upper bridge arm and the lower bridge arm for half-wave modulation according to the current reference value and the actual bridge arm current of the upper bridge arm and the lower bridge arm comprises: generating a reference voltage waveform of the sub-modules of the upper bridge arm and the lower bridge arm for half-wave modulation according to the current reference value and the actual bridge arm current of the upper bridge arm and the lower bridge arm is wherein, is the reference voltage waveform of the sub-modules of the upper bridge arm, is the reference voltage waveform of the sub-modules of the lower bridge arm, is a modulation ratio, is a power frequency angular frequency, and t is a time, is a phase of the j-phase alternating current phase voltage.

[0125] Alternatively, generating control signals of the sub-modules of the upper bridge arm and the lower bridge arm by using the nearest level approximation modulation method according to the reference voltage waveform and the turn-on and turn-off strategy comprises: determining the number of sub-modules of the upper bridge arm and the lower bridge arm to be invested by using the nearest level approximation modulation method according to the reference voltage waveform and the turn-on and turn-off strategy is wherein, is the number of sub-modules of the upper bridge arm to be invested, is the number of sub-modules of the lower bridge arm to be invested, = 0 or 1, = 1 represents that the on and off strategy is to turn on the upper bridge arm of the switch circuit, = 0 represents that the on and off strategy is to turn on the lower bridge arm of the switch circuit, is the reference voltage waveform of the sub-module of the upper bridge arm, is the reference voltage waveform of the sub-module of the lower bridge arm, and the sub-module comprises a second energy storage device, is the ratio of the DC bus voltage of the converter to the voltage stored in the second energy storage device; according to the number of the sub-modules of the upper bridge arm and the lower bridge arm, the control signal of the corresponding sub-module is generated.

[0126] Obviously, those skilled in the art should understand that the modules or steps of the present application can be realized by a general computing device, which can be concentrated on a single computing device or distributed on a network composed of multiple computing devices, and can be realized by program codes executable by the computing device, so that they can be stored in a storage device and executed by the computing device, and in some cases, the steps shown or described can be executed in different order, or they can be manufactured into individual integrated circuit modules, or multiple modules or steps can be manufactured into a single integrated circuit module. Thus, the present application is not limited to any specific hardware and software combination.

[0127] Those skilled in the art should understand that the embodiments of the present application can be provided as a method, a system, or a computer program product. Therefore, the present application can take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can take the form of a computer program product implemented on one or more computer usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer usable program codes.

[0128] The present application is described with reference to flowcharts and / or block diagrams according to the methods, devices (systems), and computer program products of the embodiments of the present application. It should be understood that each flow and / or block in the flowcharts and / or block diagrams, and the combination of the flows and / or blocks in the flowcharts and / or block diagrams can be realized by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing devices produce a device that implements the functions specified in the flowcharts and / or block diagrams. Figure 1 one flow or multiple flows and / or blocks Figure 1 one block or multiple blocks.

[0129] These computer program instructions can also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instructions which implement the flow Figure 1 one or more flows and / or blocks Figure 1 one or more blocks or multiple blocks.

[0130] These computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flow Figure 1 one or more flows and / or blocks Figure 1 one or more blocks or multiple blocks.

[0131] In a typical configuration, a computing device includes one or more processors (CPUs), input / output interfaces, network interfaces, and memory.

[0132] The memory can include non-persistent memory and / or volatile memory, such as random access memory (RAM) about which the computer stores information such as computer program instructions. Memory is an example of computer readable media.

[0133] Computer readable media includes permanent and non-permanent, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. Examples of computer storage media include, but are not limited to, phase change memory (PRAM), static random access memory (SRAM), dynamic random access memory (DRAM), other types of random access memory (RAM), read only memory (ROM), electrically erasable programmable read only memory (EEPROM), flash memory or other memory technology, compact disc read only memory (CD-ROM), digital versatile discs (DVD) or other optical storage, magnetic cassettes, magnetic disks storage or other magnetic storage devices, or any other non-transmission medium that can be used to store information accessible to computing devices. According to the definition herein, computer readable media does not include transitory media, such as modulated data signals and carrier waves.

[0134] The technical features of the above embodiments can be combined in any manner. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described, however, it is understood that any combination of the technical features is within the scope of the present specification.

[0135] It should also be noted that the terms "comprising", "comprises" or any other variation thereof are intended to cover a non-exclusive inclusion, such that a process, method, article or apparatus that comprises a list of elements does not include only those elements but can also include other elements not expressly listed or inherent to such process, method, article or apparatus. Without further limitation, an element defined by the statement "comprising a" does not exclude the presence of additional identical elements in the process, method, article or apparatus that includes the element.

[0136] From the above description, it can be seen that the above-mentioned embodiments of the present application achieve the following technical effects:

[0137] In the upper bridge arm and the lower bridge arm of the converter of the present application, the switching circuit and the plurality of sub-modules are connected in series, the switching circuit includes the reactor, a plurality of switching modules including the first controllable switch tube, the first diode and the first energy absorption module, the first controllable switch tube and the first diode are connected in reverse parallel, the first energy absorption module is connected in parallel across the first controllable switch tube, the reactor and the plurality of first controllable switch tubes are connected in series, and neither the upper bridge arm nor the lower bridge arm is provided with a lightning arrester. The present application sets the switching circuit in the upper and lower bridge arms of the converter, controls the switching state of the upper and lower bridge arms and the switching of the sub-modules through linkage, the sub-modules on the corresponding bridge arm are put into operation when the switching circuit is turned on, and the sub-modules on the corresponding bridge arm are locked out when the switching circuit is turned off, so that the upper and lower bridge arms are turned on alternately in different half cycles of the power frequency cycle, and then the direct current circulating current cannot form a complete loop between the upper and lower bridge arms in a half power frequency cycle, thereby inhibiting the formation of the direct current circulating current in the upper and lower bridge arms, which means that the reactor does not need to bear too much task of inhibiting the direct current circulating current, so that the inductance requirement of the reactor in the bridge arm can be reduced, and the lower inductance not only reduces the loss and response time of the entire system, but also helps to reduce the cost and physical size of the reactor. In addition, since the inductor required by the present application has a lower inductance, the overvoltage level caused by it is also correspondingly reduced, so that the setting of the lightning arrester is no longer necessary in this case, and therefore the converter of the present application does not need to be provided with a lightning arrester, which not only greatly reduces the overall loss of the converter, but also reduces the cost and volume of the converter. In addition, the first energy absorption module is provided in the upper and lower bridge arms to absorb excess energy, which can quickly absorb energy and clamp the voltage within a safe range, thereby protecting the controllable switch tube.

[0138] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A flexible direct current power transmission system, characterized by The converter comprises a plurality of phase arms, each phase arm comprising a series connection of an upper bridge arm and a lower bridge arm, each of the upper bridge arm and the lower bridge arm comprising a series connection of a switching circuit and a plurality of sub-modules, the switching circuit comprising a reactor and a plurality of switching modules, each switching module comprising a first controllable switch, a first diode and a first energy absorption module, the first controllable switch being connected in antiparallel with the first diode, the first energy absorption module being connected in parallel with the first controllable switch, the reactor being connected in series with the first controllable switch of each of the plurality of switching modules, and each of the upper bridge arm and the lower bridge arm not comprising an arrester, wherein the first energy absorption module comprises a second diode, an anode of the second diode being electrically connected to a cathode of the first diode, a first energy storage device, one end of the first energy storage device being electrically connected to a cathode of the second diode, the other end of the first energy storage device being electrically connected to an anode of the first diode, and a bleeder device connected in parallel with the first energy storage device. The electronic device comprises one or more processors, a memory, and one or more programs, wherein the one or more programs are stored in the memory and configured to be executed by the one or more processors, and the one or more programs comprise a control method for the converter, the method comprising: alternately turning on and off the first controllable switch in the switching circuit of the upper bridge arm and the lower bridge arm using a power frequency square wave; alternately turning on and off the sub-modules of the upper bridge arm and the lower bridge arm based on a half-wave modulation strategy and a turn-on and turn-off strategy of the first controllable switch, so that the sub-modules of the corresponding upper bridge arm or lower bridge arm are turned on when the first controllable switch is turned on, and the sub-modules of the corresponding upper bridge arm or lower bridge arm are turned off when the first controllable switch is turned off. The first energy storage device comprises a capacitor, and the bleeder device comprises a resistor.

2. The flexible HVDC power transmission system of claim 1, wherein, The first controllable switch is an IGCT switch or an IGBT switch.

3. The flexible HVDC power transmission system of claim 1, wherein, The sub-module comprises:

4. The flexible HVDC power transmission system of claim 1, wherein, a second energy storage device; a second energy absorption module comprising three terminals, a first terminal of the second energy absorption module being electrically connected to one end of the second energy storage device, a second terminal of the second energy absorption module being electrically connected to the other end of the second energy storage device; a second controllable switch, one end of the second controllable switch being electrically connected to a third terminal of the second energy absorption module; a third diode connected in antiparallel across the second controllable switch; a third controllable switch, one end of the third controllable switch being electrically connected to the other end of the second controllable switch, the other end of the third controllable switch being electrically connected to the other end of the second energy storage device; a fourth diode connected in antiparallel across the third controllable switch. The second controllable switch and the third controllable switch are IGCT switches or IGBT switches.

5. The flexible HVDC power transmission system of claim 4, wherein, Alternately turning on and off the first controllable switch in the switching circuit of the upper bridge arm and the lower bridge arm using a power frequency square wave comprises:

6. The flexible HVDC power transmission system of claim 1, wherein, The sub-modules comprise second energy storage devices, in the bridge arm, the maximum number of the switching modules is , the maximum number of the sub-modules is , wherein, is a ceiling function, M is the maximum number of the switching modules, N is the maximum number of the sub-modules, is a DC bus voltage of the converter, is a voltage stored by the second energy storage device.

7. The flexible HVDC power transmission system of claim 1, wherein, ​ According to the power frequency square wave, a control function of the first controllable switch tube is determined as wherein, is the control function, is a power frequency angular frequency, and t is a time, is a power frequency phase, is a sign function, and when the sign function is 1, and when the sign function is 0. The first controllable switch tube is controlled by using the control function, so that when the control function is 1, the first controllable switch tube of the upper bridge arm is turned on and the first controllable switch tube of the lower bridge arm is turned off, and when the control function is 0, the first controllable switch tube of the upper bridge arm is turned off and the first controllable switch tube of the lower bridge arm is turned on.

8. The flexible HVDC power transmission system of claim 1, wherein, Based on the half-wave modulation strategy and the turn-on and turn-off strategy of the first controllable switch tube, the sub-modules of the upper bridge arm and the lower bridge arm are alternately put into operation and locked out, comprising: According to the active power reference value, the reactive power reference value and the direct current voltage, a current reference value is generated; According to the current reference value and the actual bridge arm current of the upper bridge arm and the lower bridge arm, a reference voltage waveform of the sub-modules of the upper bridge arm and the lower bridge arm for half-wave modulation is generated; According to the reference voltage waveform and the turn-on and turn-off strategy, the control signals of the sub-modules of the upper bridge arm and the lower bridge arm are generated by using the nearest level approximation modulation method, so that the sub-modules of the upper bridge arm are put into operation and the sub-modules of the lower bridge arm are locked out, or the sub-modules of the lower bridge arm are put into operation and the sub-modules of the upper bridge arm are locked out.

9. The flexible HVDC power transmission system of claim 8, wherein, According to the current reference value and the actual bridge arm current of the upper bridge arm and the lower bridge arm, a reference voltage waveform of the sub-modules of the upper bridge arm and the lower bridge arm for half-wave modulation is generated, comprising: The reference voltage waveform of the sub-modules of the upper bridge arm and the lower bridge arm for half-wave modulation is generated according to the current reference value, actual bridge arm currents of the upper bridge arm and the lower bridge arm, and is wherein, is the reference voltage waveform of the sub-modules of the upper bridge arm, is the reference voltage waveform of the sub-modules of the lower bridge arm, is a modulation ratio, is a power frequency angular frequency, t is a time, is a phase of an alternating current phase voltage of the j phase.

10. The flexible HVDC power transmission system of claim 8, wherein, According to the reference voltage waveform and the turn-on and turn-off strategy, the control signals of the sub-modules of the upper bridge arm and the lower bridge arm are generated by using the nearest level approximation modulation method, comprising: According to the reference voltage waveform and the turn-on and turn-off strategy, the number of the sub-modules of the upper bridge arm and the lower bridge arm is determined by using a nearest level approximation modulation method wherein, is the number of the sub-modules of the upper bridge arm, is the number of the sub-modules of the lower bridge arm, =0 or 1, =1 represents that the turn-on and turn-off strategy is to turn on the first controllable switch tube of the upper bridge arm, =0 represents that the turn-on and turn-off strategy is to turn on the first controllable switch tube of the lower bridge arm, is the reference voltage waveform of the sub-modules of the upper bridge arm, is the reference voltage waveform of the sub-modules of the lower bridge arm, the sub-modules comprising a second energy storage device, is the ratio of the DC bus voltage of the converter to the voltage stored in the second energy storage device; According to the number of sub-modules of the upper bridge arm and the lower bridge arm, the control signals of the corresponding sub-modules are generated.

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