CN114336551A - Flexible direct current transmission system fault protection system based on MMC - Google Patents
Flexible direct current transmission system fault protection system based on MMC Download PDFInfo
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Abstract
The invention discloses a flexible direct current transmission system fault protection system based on an MMC. In the invention, a method for cutting off a filter capacitor grounding point to cut off a fault current path is designed in the fault protection module to protect system elements. Although the method can improve the insulation level of the system, the fault current path can be effectively blocked, the safe and reliable operation of the system can be maintained, meanwhile, the alternating current system single-phase earth fault protection module and the alternating current system interphase short circuit fault protection module are matched for use, and when the system has a single-pole earth short circuit fault, the operation mode of the direct current system can be changed from bipolar operation to monopolar operation. By utilizing a single-pole operation mode, the system can be effectively maintained to operate, and partial loss caused by faults is reduced, so that the protection performance of the system is improved, and the safety and stability of a power transmission system are guaranteed.
Description
Technical Field
The invention belongs to the technical field of power transmission system fault protection, and particularly relates to a flexible direct-current power transmission system fault protection system based on an MMC.
Background
As a new generation of dc transmission technology, flexible dc transmission is still composed of a converter station and a dc transmission line (usually a dc cable), similar in structure to high voltage dc transmission. The high-voltage direct-current power transmission technology based on the voltage source converter is proposed by Boon-Teck Ooi and the like of McGill university in Canada in 1990, is a novel power transmission technology based on the voltage source converter, a self-turn-off device and a Pulse Width Modulation (PWM) technology, and has the advantages of capability of supplying power to a passive network, no phase change failure, no need of communication between converter stations, easiness in forming a multi-terminal direct-current system and the like.
However, when a common system runs and a power transmission system fails, the fault information cannot be pushed in time, and a worker has a risk that the information cannot be checked in time, so that the timeliness of processing when the fault occurs is influenced.
Disclosure of Invention
The invention aims to: in order to solve the above-mentioned proposed problems, a flexible direct current transmission system fault protection system based on MMC is provided.
The technical scheme adopted by the invention is as follows: a flexible direct-current transmission system fault protection system based on MMC comprises a power switch module, a data acquisition module, a data output module, a processor module, a fault analysis module, a fault protection module, an alarm information pushing module, an information output module, a preset short message module, a network butt joint module, a sub-module fault protection module, a network side alternating-current system fault protection module, an alternating-current system single-phase ground fault protection module, an alternating-current system interphase short-circuit fault protection module, a direct-current transmission line single-pole ground fault protection module and a worker receiving terminal, wherein the output end of the power switch module is connected with the input end of the data acquisition module, the output end of the data acquisition module is connected with the input end of the data output module, the output end of the data output module is connected with the input end of the processor module, and the output end of the processor module is connected with the input end of the fault analysis module, the output end of the fault analysis module is connected with the input end of the fault protection module, and the output end of the fault protection module is connected with the input end of the alarm information pushing module;
the fault protection module is fixedly internally provided with a sub-module fault protection module, a network side alternating current system fault protection module, an alternating current system single-phase earth fault protection module, an alternating current system phase-to-phase short circuit fault protection module and a direct current transmission line single-pole earth fault protection module, and the output ends of the sub-module fault protection module, the network side alternating current system fault protection module, the alternating current system single-phase earth fault protection module, the alternating current system phase-to-phase short circuit fault protection module and the direct current transmission line single-pole earth fault protection module are connected with the input end of the fault protection module;
the alarm information pushing device comprises an alarm information pushing module, a preset short message module, a network docking module and a worker receiving terminal, wherein the alarm information pushing module is fixedly mounted inside the alarm information pushing module, and the output ends of the information output module, the preset short message module, the network docking module and the worker receiving terminal are connected with the input end of the alarm information pushing module.
In a preferred embodiment, when a bipolar short circuit occurs on the dc side of the MMC-HVDC system installed inside the fault analysis module, which is equivalent to a three-phase symmetric short circuit occurring in the ac system, the short-circuit point voltage is rapidly reduced to 0, the power transmission between the converter stations is also rapidly stopped, and the short-circuit point current is rapidly increased from a normal value to several times of a rated value; an alternating current system accessed by the converter station injects short-circuit current to a fault point through a diode D2 in each submodule of each phase bridge arm, and simultaneously before the converter station is locked, capacitors in each submodule inject capacitor discharge current to the fault point through IGBTs 1 in the submodule, so that before the converter station is locked, the short-circuit current of the direct-current side bipolar fault is formed by overlapping the two currents.
In a preferred embodiment, inside the sub-module fault protection module, when the sub-module is in the locked state: the submodule sends out a fault alarm and prohibits the fault submodule from being put into operation; (2) the submodule is in the input state: the current direction is defined as the positive current direction when the current direction is in the upper half cycle; when the fault current direction is positive, the current charges the direct current capacitor through the short circuit point; when the time specified by the time constant is reached, the voltage of the direct current capacitor reaches a rated value; when the fault current is negative, the voltage of the direct current capacitor of the submodule is gradually reduced; because the short circuit current still passes through the faulty IGBT before the bypass switch closing action is completed, this can cause secondary damage to the components; therefore, firstly, locking is carried out on the IGBT of the fault sub-module; then bypassing the faulty submodule by directly closing a bypass switch; the fault sub-module is prohibited from being introduced again by sending a fault alarm to the system; finally, inputting idle redundant sub-modules; restoring the normal operation of the system; (3) the submodule is in the cut-off state: the current direction is defined as the positive current direction when the current direction is in the upper half cycle; fault current is positive, at which time IGBT1 is shorted; the current is conducted with the IGBT2 through short circuit, and the direct current capacitor is rapidly discharged through loops from the IGBT1 to the IGBT 2; at this time, the current flowing through the IGBT1, the IGBT2, and the capacitor increases rapidly, and at this time, the short-circuit current may reach several tens of times of the rated current, so that the large short-circuit current may cause the IGBT1 and the IGBT2 to explode; however, since the IGBT is turned off by the protection control strategy after the sub-module fails, the short-circuit current flows through the IGBT1 to charge the capacitor before the bypass switch action is completed; when the fault occurs in the lower half period of which the current is less than zero, the voltage at two ends of the IGBT2 is increased to be capacitor voltage due to the short circuit of the IGBT, the IGBT2 is switched from a conducting state to a cut-off state, at the moment, the capacitor discharges through a short circuit point, and the fault range is expanded by inputting the voltage of a submodule; when the voltage drop of the capacitor is zero, the IGBT2 is cut off, the short-circuit current is continuously conducted through the freewheeling diode, and the voltage drop of the output of the submodule is zero; when the IGBT2 has a short-circuit fault, if the IGBT1 is turned on, both electrodes of the capacitor will be short-circuited, and meanwhile, a discharge path causes a large discharge current between both electrodes, and thus the fault cannot damage the device until the bypass switch is closed.
In a preferred embodiment, inside the network-side ac system fault protection module, when a network-side ac system fails, a relay protection strategy of a conventional ac system may be used to protect the system; the relay protection of the traditional alternating current system is matched with the automatic reclosing, generally instantaneous faults or misoperation and misoperation occur, a circuit breaker cuts off a circuit firstly, and then the automatic reclosing is closed; and if the fault time exceeds the automatic reclosing closing time, the breaker cuts off the fault line permanently until the fault is eliminated.
In a preferred embodiment, when a fault occurs, the filter capacitor ground and the fault ground point inside the single-phase ground fault protection module of the alternating-current system may form a closed resonant path, so that the phase voltage at the fault contains a large amount of harmonics and intermittent waves, and meanwhile, the alternating-current bias of the direct-current line may affect the stable operation of the system; when the filter capacitor has no grounding point, a closed resonant circuit is not formed when a single-phase grounding fault of an alternating current system occurs, so that alternating current bias can only occur on a direct current positive electrode and a direct current negative electrode; however, in the case of an alternating current system, the neutral point shifts, so that the non-fault phase voltage is increased to the line voltage, and the insulation performance of some equipment needs to be improved; this will increase the total construction cost of the project; the fault protection strategy can be designed to cut off the filter capacitor to be grounded when the system has single-phase earth fault of the alternating current system; in this case, for the design of grounding of the filter capacitor, whether the filter capacitor is grounded or not can be designed by referring to the design of whether the neutral point of the transformer is grounded or not of the traditional alternating current system.
In a preferred embodiment, the alternating-current system inter-phase short-circuit fault protection module determines whether an alternating-current system inter-phase short-circuit fault occurs according to a half of a phase voltage drop of the alternating-current system fault; when the system is judged to have the interphase short circuit fault of the alternating current system, the corresponding control protection strategy should disconnect the alternating current system and lock the converter station, and the converter station should be restarted to restore the normal operation of the system after the fault is eliminated.
In a preferred embodiment, the internal part of the dc transmission line single-pole ground fault protection module can change the operation mode of the dc system from bipolar operation to monopolar operation when the system has a single-pole ground short circuit fault; at this time, the corresponding interelectrode voltage is unchanged because the operation mode of the direct current system is changed; since one pole is grounded, the voltage drop to ground of this pole is 0; the other pole is raised to Udc; when the direct-current transmission line normally runs, the two poles of the direct-current transmission line have different effects on the rectifying station and the inverter station; setting the positive direction of the current of the electrode A as the current of the rectifying station flows to the inverting station, and setting the positive direction of the current of the electrode B as the current of the inverting station flows to the rectifying station; when the A pole has a single-pole grounding short circuit fault, the current direction of the B pole cannot meet the condition that active power flows from the rectifier station to the inverter station, and in order to enable the system to operate in a reduced power single pole, the current direction of the B pole must be changed to the opposite direction to meet the semi-active power single-pole operation condition; and when the B pole has a single-pole grounding short circuit fault, the current direction of the A pole can meet the half-power single-pole operation condition.
In a preferred embodiment, the preset short message module is configured for a sending number, a sending content, and the like that require sending a short message alarm, and the preset short message module includes: registering a user code, a receiver name, a short message receiving number, short message content and the like; each login user can set a plurality of short message receiving numbers, each receiving number can precheck corresponding short message information, the content can be sent when the power transmission system gives a fault alarm, the content can also be sent after being manually edited, and in the fault content of the power transmission system, the contact persons can be used as emergency contact persons and emergency contact phones.
In a preferred embodiment, the fault data of the power transmission system designed in the network docking module is automatically uploaded to a public security intranet, and the internet data is cleaned regularly. To realize the design, a network gate must be established, and a special channel is used for ensuring the safe transmission of data.
In a preferred embodiment, a control console, login, administrator management, role management, power transmission system area management and data statistics are arranged inside the staff receiving terminal; background management personnel can form early warning information of the power transmission system according to the statistical data or the collected information, and the early warning information is issued to a corresponding district and watched by operation staff of the power transmission system for prevention.
In summary, due to the adoption of the technical scheme, the invention has the beneficial effects that:
1. in the invention, a method for cutting off a filter capacitor grounding point to cut off a fault current path is designed in the fault protection module to protect system elements. Although the method can improve the insulation level of the system, the fault current path can be effectively blocked, the safe and reliable operation of the system can be maintained, meanwhile, the alternating current system single-phase earth fault protection module and the alternating current system interphase short circuit fault protection module are matched for use, and when the system has a single-pole earth short circuit fault, the operation mode of the direct current system can be changed from bipolar operation to monopolar operation. By utilizing a single-pole operation mode, the system can be effectively maintained to operate, and partial loss caused by faults is reduced, so that the protection performance of the system is improved, and the safety and stability of a power transmission system are guaranteed.
2. According to the alarm information pushing module, the fault information can be pushed, so that a worker can check the fault information in time through the mobile phone terminal, the problem that the fault cannot be cleared in time due to untimely information transmission is avoided, and the stability of the system is guaranteed.
Drawings
FIG. 1 is a block diagram of the system of the present invention;
FIG. 2 is a system block diagram of a fault protection module according to the present invention;
fig. 3 is a system block diagram of an alarm information pushing module according to the present invention.
The labels in the figure are: the system comprises a power switch module, a 2 data acquisition module, a 3 data output module, a 4 processor module, a 5 fault analysis module, a 6 fault protection module, a 7 alarm information pushing module, an 8 information output module, a 9 preset short message module, a 10 network docking module, a 11 sub-module fault protection module, a 12-network side alternating current system fault protection module, a 13-alternating current system single-phase grounding fault protection module, a 14-alternating current system interphase short circuit fault protection module, a 15-direct current transmission line single-pole grounding fault protection module and a 16-worker receiving terminal.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
With reference to figures 1-3 of the drawings,
a flexible direct-current transmission system fault protection system based on MMC comprises a power switch module 1, a data acquisition module 2, a data output module 3, a processor module 4, a fault analysis module 5, a fault protection module 6, an alarm information pushing module 7, an information output module 8, a preset short message module 9, a network docking module 10, a sub-module fault protection module 11, a network side alternating-current system fault protection module 12, an alternating-current system single-phase ground fault protection module 13, an alternating-current system interphase short-circuit fault protection module 14, a direct-current transmission line single-pole ground fault protection module 15 and a worker receiving terminal 16, wherein the output end of the power switch module 1 is connected with the input end of the data acquisition module 2, the output end of the data acquisition module 2 is connected with the input end of the data output module 3, and the output end of the data output module 3 is connected with the input end of the processor module 4, the output end of the processor module 4 is connected with the input end of the fault analysis module 5, the output end of the fault analysis module 5 is connected with the input end of the fault protection module 6, and the output end of the fault protection module 6 is connected with the input end of the alarm information pushing module 7;
the fault protection module 6 is internally and fixedly provided with a sub-module fault protection module 11, a network side alternating current system fault protection module 12, an alternating current system single-phase earth fault protection module 13, an alternating current system interphase short-circuit fault protection module 14 and a direct current transmission line single-pole earth fault protection module 15, and the output ends of the sub-module fault protection module 11, the network side alternating current system fault protection module 12, the alternating current system single-phase earth fault protection module 13, the alternating current system interphase short-circuit fault protection module 14 and the direct current transmission line single-pole earth fault protection module 15 are connected with the input end of the fault protection module 6;
the alarm information pushing module 7 is fixedly provided with an information output module 8, a preset short message module 9, a network docking module 10 and a worker receiving terminal 16, and the output ends of the information output module 8, the preset short message module 9, the network docking module 10 and the worker receiving terminal 16 are connected with the input end of the alarm information pushing module 7.
When a bipolar short circuit occurs at the direct current side of an MMC-HVDC system arranged in the fault analysis module 5, which is equivalent to a three-phase symmetrical short circuit of an alternating current system, the voltage of a short circuit point is rapidly reduced to 0, the power transmission between converter stations is also rapidly stopped, and the current of the short circuit point is rapidly increased to a plurality of times of a rated value from a normal value; an alternating current system accessed by the converter station injects short-circuit current to a fault point through a diode D2 in each submodule of each phase bridge arm, and simultaneously before the converter station is locked, capacitors in each submodule inject capacitor discharge current to the fault point through IGBTs 1 in the submodule, so that before the converter station is locked, the short-circuit current of the direct-current side bipolar fault is formed by overlapping the two currents.
Inside the sub-module fault protection module 11, when the sub-module is in the locked state: the submodule sends out a fault alarm and prohibits the fault submodule from being put into operation; the submodule is in the input state: the current direction is defined as the positive current direction when the current direction is in the upper half cycle; when the fault current direction is positive, the current charges the direct current capacitor through the short circuit point; when the time specified by the time constant is reached, the voltage of the direct current capacitor reaches a rated value; when the fault current is negative, the voltage of the direct current capacitor of the submodule is gradually reduced; because the short circuit current still passes through the faulty IGBT before the bypass switch closing action is completed, this can cause secondary damage to the components; therefore, firstly, locking is carried out on the IGBT of the fault sub-module; then bypassing the faulty submodule by directly closing a bypass switch; the fault sub-module is prohibited from being introduced again by sending a fault alarm to the system; finally, inputting idle redundant sub-modules; restoring the normal operation of the system; (3) the submodule is in the cut-off state: the current direction is defined as the positive current direction when the current direction is in the upper half cycle; fault current is positive, at which time IGBT1 is shorted; the current is conducted with the IGBT2 through short circuit, and the direct current capacitor is rapidly discharged through loops from the IGBT1 to the IGBT 2; at this time, the current flowing through the IGBT1, the IGBT2, and the capacitor increases rapidly, and at this time, the short-circuit current may reach several tens of times of the rated current, so that the large short-circuit current may cause the IGBT1 and the IGBT2 to explode; however, since the IGBT is turned off by the protection control strategy after the sub-module fails, the short-circuit current flows through the IGBT1 to charge the capacitor before the bypass switch action is completed; when the fault occurs in the lower half period of which the current is less than zero, the voltage at two ends of the IGBT2 is increased to be capacitor voltage due to the short circuit of the IGBT, the IGBT2 is switched from a conducting state to a cut-off state, at the moment, the capacitor discharges through a short circuit point, and the fault range is expanded by inputting the voltage of a submodule; when the voltage drop of the capacitor is zero, the IGBT2 is cut off, the short-circuit current is continuously conducted through the freewheeling diode, and the voltage drop of the output of the submodule is zero; when the IGBT2 has a short-circuit fault, if the IGBT1 is turned on, both electrodes of the capacitor will be short-circuited, and meanwhile, a discharge path causes a large discharge current between both electrodes, and thus the fault cannot damage the device until the bypass switch is closed.
Inside the network side ac system fault protection module 12, when the network side ac system fails, the system can be protected by using a relay protection strategy of the conventional ac system; the relay protection of the traditional alternating current system is matched with the automatic reclosing, generally instantaneous faults or misoperation and misoperation occur, a circuit breaker cuts off a circuit firstly, and then the automatic reclosing is closed; and if the fault time exceeds the automatic reclosing closing time, the breaker cuts off the fault line permanently until the fault is eliminated.
When a fault occurs in the single-phase earth fault protection module 13 of the alternating-current system, a closed resonant path can be formed by the filter capacitor earth and the fault earth point, so that phase voltage at the fault position contains a large amount of harmonic waves and intermittent waves, and meanwhile, the stable operation of the system is influenced by alternating-current bias generated in a direct-current line; when the filter capacitor has no grounding point, a closed resonant circuit is not formed when a single-phase grounding fault of an alternating current system occurs, so that alternating current bias can only occur on a direct current positive electrode and a direct current negative electrode; however, in the case of an alternating current system, the neutral point shifts, so that the non-fault phase voltage is increased to the line voltage, and the insulation performance of some equipment needs to be improved; this will increase the total construction cost of the project; the fault protection strategy can be designed to cut off the filter capacitor to be grounded when the system has single-phase earth fault of the alternating current system; in this case, for the design of grounding of the filter capacitor, whether the filter capacitor is grounded or not can be designed by referring to the design of whether the neutral point of the transformer is grounded or not of the traditional alternating current system.
The alternating-current system interphase short-circuit fault protection module 14 judges whether an alternating-current system interphase short-circuit fault occurs according to the fact that the voltage drop of the alternating-current system fault phase is half; when the system is judged to have the interphase short circuit fault of the alternating current system, the corresponding control protection strategy should disconnect the alternating current system and lock the converter station, and the converter station should be restarted to restore the normal operation of the system after the fault is eliminated.
When the single-pole ground fault of the system occurs in the single-pole ground fault protection module 15 of the direct-current transmission line, the operation mode of the direct-current system can be changed from bipolar operation to monopolar operation; at this time, the corresponding interelectrode voltage is unchanged because the operation mode of the direct current system is changed; since one pole is grounded, the voltage drop to ground of this pole is 0; the other pole is raised to Udc; when the direct-current transmission line normally runs, the two poles of the direct-current transmission line have different effects on the rectifying station and the inverter station; setting the positive direction of the current of the electrode A as the current of the rectifying station flows to the inverting station, and setting the positive direction of the current of the electrode B as the current of the inverting station flows to the rectifying station; when the A pole has a single-pole grounding short circuit fault, the current direction of the B pole cannot meet the condition that active power flows from the rectifier station to the inverter station, and in order to enable the system to operate in a reduced power single pole, the current direction of the B pole must be changed to the opposite direction to meet the semi-active power single-pole operation condition; and when the B pole has a single-pole grounding short circuit fault, the current direction of the A pole can meet the half-power single-pole operation condition.
The preset short message module 9 sets a sending number, a sending content and the like for sending a short message alarm, and the preset short message module 9 includes: registering a user code to register a mobile phone number, a receiver name, a short message receiving number, short message content and the like; each login user can set a plurality of short message receiving numbers, each receiving number can precheck corresponding short message information, the content can be sent when the power transmission system gives a fault alarm, the content can also be sent after being manually edited, and in the fault content of the power transmission system, the contact persons can be used as emergency contact persons and emergency contact phones.
The fault data of the power transmission system designed in the network docking module 10 is automatically uploaded to a public security intranet, and the internet data is regularly cleaned. To realize the design, a network gate must be established, and a special channel is used for ensuring the safe transmission of data.
A control console, login, administrator management, role management, power transmission system area management and data statistics are arranged in the staff receiving terminal 16; background management personnel can form early warning information of the power transmission system according to the statistical data or the collected information, and the early warning information is issued to a corresponding district and watched by operation staff of the power transmission system for prevention.
The fault protection module 6 is internally provided with a method for cutting off a filter capacitor grounding point to cut off a fault current path so as to protect system elements. Although the method can improve the insulation level of the system, the fault current path can be effectively blocked, the safe and reliable operation of the system can be maintained, meanwhile, the alternating-current system single-phase earth fault protection module 13 and the alternating-current system interphase short circuit fault protection module 14 are matched for use, and when the system has a single-pole earth short circuit fault, the operation mode of the direct-current system can be changed from bipolar operation to monopolar operation. By utilizing a single-pole operation mode, the system can be effectively maintained to operate, and partial loss caused by faults is reduced, so that the protection performance of the system is improved, and the safety and stability of a power transmission system are guaranteed.
Alarm information push module 7 can carry out the propelling movement to fault information to make the work people say and can in time look over fault information through the cell-phone end, avoided the trouble that information transmission leads to untimely can not in time clear up, provide the guarantee for the stability of this system.
The above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (10)
1. The utility model provides a flexible direct current transmission system fault protection system based on MMC, including switch module (1), data acquisition module (2), data output module (3), processor module (4), fault analysis module (5), fault protection module (6), alarm information push module (7), information output module (8), predetermine SMS module (9), network butt joint module (10), submodule piece fault protection module (11), net side alternating current system fault protection module (12), alternating current system single-phase earth fault protection module (13), alternating current system interphase short-circuit fault protection module (14), direct current monopole transmission line earth fault protection module (15), staff receiving terminal (16), its characterized in that: the output end of the power switch module (1) is connected with the input end of the data acquisition module (2), the output end of the data acquisition module (2) is connected with the input end of the data output module (3), the output end of the data output module (3) is connected with the input end of the processor module (4), the output end of the processor module (4) is connected with the input end of the fault analysis module (5), the output end of the fault analysis module (5) is connected with the input end of the fault protection module (6), and the output end of the fault protection module (6) is connected with the input end of the alarm information pushing module (7);
the fault protection system comprises a fault protection module (6), a sub-module fault protection module (11), a network side alternating current system fault protection module (12), an alternating current system single-phase ground fault protection module (13), an alternating current system interphase short-circuit fault protection module (14) and a direct current transmission line single-pole ground fault protection module (15) are fixedly installed inside the fault protection module (6), and the output ends of the sub-module fault protection module (11), the network side alternating current system fault protection module (12), the alternating current system single-phase ground fault protection module (13), the alternating current system interphase short-circuit fault protection module (14) and the direct current transmission line single-pole ground fault protection module (15) are connected with the input end of the fault protection module (6);
the alarm information pushing device is characterized in that an information output module (8), a preset short message module (9), a network docking module (10) and a worker receiving terminal (16) are fixedly mounted inside the alarm information pushing module (7), and the output ends of the information output module (8), the preset short message module (9), the network docking module (10) and the worker receiving terminal (16) are connected with the input end of the alarm information pushing module (7).
2. The MMC-based, flexible direct current power transmission system fault protection system of claim 1, wherein: when a bipolar short circuit occurs at the direct current side of an MMC-HVDC system arranged in the fault analysis module (5), which is equivalent to a three-phase symmetrical short circuit of an alternating current system, the voltage of a short circuit point is rapidly reduced to 0, power transmission between converter stations is also rapidly stopped, and the current of the short circuit point is rapidly increased to a plurality of times of a rated value from a normal value; an alternating current system accessed by the converter station injects short-circuit current to a fault point through a diode D2 in each submodule of each phase bridge arm, and simultaneously, before the converter station is locked, capacitors in each submodule inject capacitor discharge current to the fault point through IGBTs 1 in the submodule.
3. The MMC-based, flexible direct current power transmission system fault protection system of claim 1, wherein: inside the sub-module fault protection module (11), when the sub-module is in a locked state: the submodule sends out a fault alarm and prohibits the fault submodule from being put into operation; the submodule is in the input state: the current direction is defined as the positive current direction when the current direction is in the upper half cycle; when the fault current direction is positive, the current charges the direct current capacitor through the short circuit point; when the time specified by the time constant is reached, the voltage of the direct current capacitor reaches a rated value; when the fault current is negative, the voltage of the direct current capacitor of the submodule is gradually reduced; this can cause secondary damage to the components because the short circuit current still passes through the faulty IGBT before the bypass switch closing action is completed.
4. The MMC-based, flexible direct current power transmission system fault protection system of claim 1, wherein: when the network side alternating current system fails, a relay protection strategy of a traditional alternating current system is utilized to protect the system inside the network side alternating current system fault protection module (12); then automatically reclosing and closing; and if the fault time exceeds the automatic reclosing closing time, the breaker cuts off the fault line permanently until the fault is eliminated.
5. The MMC-based, flexible direct current power transmission system fault protection system of claim 1, wherein: the filter capacitor grounding and the fault grounding point can form a closed resonant path when a fault occurs in the interior of the single-phase ground fault protection module (13) of the alternating-current system.
6. The MMC-based, flexible direct current power transmission system fault protection system of claim 1, wherein: the alternating-current system interphase short-circuit fault protection module (14) judges whether an alternating-current system interphase short-circuit fault occurs according to the fact that the voltage drop of a fault phase of the alternating-current system is half; when the system is judged to have the interphase short circuit fault of the alternating current system, the corresponding control protection strategy should disconnect the alternating current system and lock the converter station, and the converter station should be restarted to restore the normal operation of the system after the fault is eliminated.
7. The MMC-based, flexible direct current power transmission system fault protection system of claim 1, wherein: when the single-pole grounding short circuit fault occurs in the system, the operation mode of the direct current system can be changed from bipolar operation to monopolar operation inside the direct current transmission line single-pole grounding fault protection module (15); at this time, the corresponding interelectrode voltage is unchanged because the operation mode of the direct current system is changed; since one pole is grounded, the voltage drop to ground of this pole is 0; the other pole is raised to Udc; when the direct-current transmission line normally runs, the two poles of the direct-current transmission line have different effects on the rectifying station and the inverter station; setting the positive direction of the current of the electrode A as the current of the rectifying station flows to the inverting station, and setting the positive direction of the current of the electrode B as the current of the inverting station flows to the rectifying station; when the A pole has a single-pole grounding short circuit fault, the current direction of the B pole cannot meet the condition that active power flows from the rectifier station to the inverter station, and in order to enable the system to operate in a reduced power single pole, the current direction of the B pole must be changed to the opposite direction to meet the semi-active power single-pole operation condition; and when the B pole has a single-pole grounding short circuit fault, the current direction of the A pole can meet the half-power single-pole operation condition.
8. The MMC-based, flexible direct current power transmission system fault protection system of claim 1, wherein: the preset short message module (9) is set for the sending number, the sending content and the like of the short message alarm needing to be sent, and the preset short message module (9) comprises: registering a mobile phone number, a receiver name, a short message receiving number, short message content and the like; each login user can set a plurality of short message receiving numbers, each receiving number can precheck corresponding short message information, the content can be sent when the power transmission system gives a fault alarm, the content can also be sent after being manually edited, and in the fault content of the power transmission system, the contact persons can be used as emergency contact persons and emergency contact phones.
9. The MMC-based, flexible direct current power transmission system fault protection system of claim 1, wherein: and fault data of a power transmission system designed in the network docking module (10) are automatically uploaded to a public security intranet, and the internet data are regularly cleaned.
10. The MMC-based, flexible direct current power transmission system fault protection system of claim 1, wherein: a control console, login, administrator management, role management, power transmission system area management and data statistics are arranged in the staff receiving terminal (16); background management personnel can form early warning information of the power transmission system according to the statistical data or the collected information, and the early warning information is issued to a corresponding district and watched by operation staff of the power transmission system for prevention.
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