CN106849080A - Frequency conversion flexibility tuner, half-wavelength AC transmission system and its control method - Google Patents
Frequency conversion flexibility tuner, half-wavelength AC transmission system and its control method Download PDFInfo
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- CN106849080A CN106849080A CN201710022464.6A CN201710022464A CN106849080A CN 106849080 A CN106849080 A CN 106849080A CN 201710022464 A CN201710022464 A CN 201710022464A CN 106849080 A CN106849080 A CN 106849080A
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/01—Arrangements for reducing harmonics or ripples
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
- H02J3/1821—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/381—Dispersed generators
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
- Y02E40/30—Reactive power compensation
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
- Y02E40/40—Arrangements for reducing harmonics
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Abstract
The invention provides a kind of frequency conversion flexibility tuner, half-wavelength AC transmission system and its control method, described device includes a MMC modules and the 2nd MMC modules, and the two includes three facies units of parallel connection；Facies unit includes two bridge arms of series connection, and each bridge arm includes the reactor and power model unit of series connection；Power model unit includes the power modules of multiple series connection, and power modules are to include the half-bridge structure power modules of all-controlling power electronics device or full bridge structure power modules.Compared with prior art, a kind of frequency conversion flexibility tuner, half-wavelength AC transmission system and its control method that the present invention is provided, the electrical parameters such as output signal frequency, voltage magnitude and the phase of frequency conversion flexibility tuner can neatly be adjusted, realize grid-connected to the flexible tuning of half-wavelength AC transmission system and flexibility, adjust its systematic parameter, suppress the overvoltage and secondary arc current of half-wavelength transmission line of alternation current, and realize Fault Isolation.
Description
Technical Field
The invention relates to the technical field of half-wavelength alternating current transmission, in particular to a variable frequency flexible tuning device, a half-wavelength alternating current transmission system and a control method thereof.
Background
The development of technologies such as photovoltaic, wind power and the like promotes intensive large-scale development of resource conditions even if the resource conditions are not in the best region, so that the transmission and implementation of ultra-long-distance and high-power electric energy have wide significance and practical value.
Half-Wavelength AC Transmission (HWACT) refers to three-phase AC Transmission in which the electrical distance of Transmission is close to 1 power frequency Half-Wavelength, i.e., an ultra-long distance of 3000km (50Hz) or 2600km (60 Hz). Under the condition of no damage, the half-wavelength AC line is like an ideal transformer with the transformation ratio of-1.0, the voltage at the head end and the voltage at the tail end have the same magnitude and opposite phases. With the increasing demand for ultra-long distance and high power transmission, half-wavelength ac transmission technology, especially extra-high voltage half-wavelength ac transmission, is receiving much attention and research again. One of the advantages of the half-wavelength alternating current transmission technology is that the power factor of the half-wavelength alternating current transmission line is relatively high, and the structure of the half-wavelength alternating current transmission line is simpler than that of the existing super-long distance alternating current and direct current transmission systems under the condition that the transmission distance is equal to or slightly larger than the half wavelength; furthermore, for developing countries, the manufacture of ac transmission equipment is simpler and more economical than the introduction, operation and maintenance of converter devices.
The distance between the western energy base (such as Xinjiang coal power) and the eastern load center (such as bead triangle) in China can reach 3000km for some time, and the transmission capacity is huge. In the future, electric power energy of russia, mongolia and other countries adjacent to China can be developed and transmitted to China, Korea and other countries, and the transmission distance is over 3000 km. The ultra-long distance power transmission with special requirements enables the extra-high voltage half-wavelength alternating current transmission technology to be expected to become one of possible power transmission modes in the future in China. At present, the research on half-wavelength alternating-current transmission technology is also promoted in wide countries such as brazil, canada and the like so as to meet the increasingly urgent demand for ultra-long-distance and large-capacity transmission. Therefore, the half-wavelength alternating current transmission has very wide application prospect.
Half-wavelength ac transmission is limited by objective conditions, and the natural length of an actual line is difficult to be exactly half a wavelength. When the length of the line is insufficient or too long, the tuning circuit or the compensating circuit is needed to be used for manually compensating the electrical length of the power transmission line so as to achieve the purpose of artificial half-wavelength alternating-current power transmission line. At present, the existing half-wavelength ac transmission compensation scheme is based on passive network for transmission line tuning, such as pi-type tuning circuit/device shown in fig. 1 and T-type tuning circuit/device shown in fig. 2. The two tuning schemes influence the development and application of the half-wavelength alternating current transmission technology to a certain extent. This is because:
(1) the passive tuning circuit has relatively poor adaptability to system structure, operation mode or parameter change, and easily loses the half-wavelength characteristic;
(2) the passive tuning circuit has a single function, only solves the tuning problem and cannot adapt to the development requirements of power grid flexibility and intellectualization;
(3) when a passive tuning circuit or a half-wavelength alternating-current transmission line has a short-circuit fault, resonance is easily caused, and overvoltage with high amplitude is generated, so that the corresponding insulation requirements of the line and equipment are required to be improved, or corresponding overvoltage limiting measures are required to be installed to suppress the overvoltage of the line.
Disclosure of Invention
In order to meet the needs of the prior art, the invention provides a variable frequency flexible tuning device, a half-wavelength alternating current transmission system and a control method thereof.
In a first aspect, a technical solution of a variable frequency flexible tuning apparatus in the present invention is:
the frequency conversion flexible tuning device comprises a first MMC module and a second MMC module; the direct current side of the first MMC module is connected with the direct current side of the second MMC module, and the alternating current side is the input/output end of the variable frequency flexible tuning device;
the first MMC module and the second MMC module respectively comprise three phase units which are connected in parallel; the phase unit comprises two bridge arms connected in series, and each bridge arm comprises a reactor and a power module unit which are connected in series; the series connection point of the bridge arms in each phase unit is an alternating current terminal on the alternating current side, and the parallel connection point of each phase unit is a direct current terminal on the direct current side;
the power module unit comprises a plurality of power sub-modules connected in series, and each power sub-module is a half-bridge structure power sub-module or a full-bridge structure power sub-module comprising a full-control type power electronic device.
In a second aspect, a technical solution of a half-wavelength ac transmission system according to the present invention is:
the half-wavelength alternating current transmission system comprises a sending end alternating current system, a half-wavelength alternating current transmission line and a receiving end alternating current system which are sequentially connected, and the half-wavelength alternating current transmission system comprises two variable frequency flexible tuning devices; one frequency conversion flexible tuning device is arranged between the sending end alternating current system and the half-wavelength alternating current transmission line, and the other frequency conversion flexible tuning device is arranged between the receiving end alternating current system and the half-wavelength alternating current transmission line.
In a third aspect, a technical solution of the method for controlling a half-wavelength ac power transmission system in the present invention is:
the control method comprises the steps of carrying out flexible tuning and flexible grid connection on the half-wavelength alternating current power transmission system by adjusting the operation mode of the variable frequency flexible tuning device, and adjusting system parameters of the half-wavelength alternating current power transmission system; suppressing overvoltage and secondary arc current of a half-wavelength alternating current transmission line and realizing fault isolation; the system parameters comprise the power factor and the bus voltage of the sending-end alternating current system and the power factor and the bus voltage of the receiving-end alternating current system.
Compared with the closest prior art, the invention has the beneficial effects that:
1. the invention provides a variable-frequency flexible tuning device which comprises a first MMC module and a second MMC module, wherein the first MMC module and the second MMC module respectively comprise a plurality of power sub-modules formed by fully-controlled power electronic devices, so that input/output signals of the first MMC module and the second MMC module are changed by controlling the conduction and the locking of the fully-controlled power electronic devices, and electric quantity parameters such as frequency, voltage amplitude, phase and the like of the output signals of the variable-frequency flexible tuning device can be flexibly adjusted;
2. according to the half-wavelength alternating current transmission system, the variable-frequency flexible tuning devices are arranged at the two ends of the half-wavelength alternating current transmission line, and the half-wavelength alternating current transmission line can be tuned after the line structure, line parameters or operation mode of the half-wavelength alternating current transmission line is changed, so that the half-wavelength alternating current transmission line does not lose the half-wavelength characteristic;
3. according to the control method of the half-wavelength alternating current transmission system, the operation mode of the variable frequency flexible tuning device is changed, flexible tuning and flexible grid connection are carried out on the half-wavelength alternating current transmission system, system parameters of the half-wavelength alternating current transmission system are adjusted, overvoltage and secondary arc current of the half-wavelength alternating current transmission line are restrained, fault isolation is achieved, and the control method is flexible and simple to operate.
Drawings
FIG. 1: schematic diagram of pi-type tuning circuit/device structure;
FIG. 2: a schematic diagram of a T-type tuning circuit/device structure;
FIG. 3: the structure schematic diagram of a frequency conversion flexible tuning device in the embodiment of the invention;
FIG. 4: a schematic structural diagram of a power sub-module in a half-bridge structure;
FIG. 5: a structural schematic diagram of a power sub-module of a full-bridge structure;
FIG. 6: the invention discloses a structural schematic diagram of a half-wavelength alternating-current power transmission system.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The following describes a variable-frequency flexible tuning device provided by an embodiment of the present invention with reference to the accompanying drawings.
Fig. 3 is a schematic structural diagram of a frequency conversion flexible tuning device in an embodiment of the present invention, and as shown in the drawing, the frequency conversion flexible tuning device in this embodiment includes a first MMC module 1 and a second MMC module 2, direct current sides of the first MMC module 1 and the second MMC module 2 are connected, and an input/output end of the frequency conversion flexible tuning device is on the alternating current side.
The first MMC module 1 and the second MMC module 2 both comprise three phase units connected in parallel, each phase unit comprises two bridge arms connected in series, and each bridge arm comprises a reactor and a power module unit connected in series. The series connection point of the bridge arms in each phase unit is an alternating current terminal on the alternating current side, and the parallel connection point of each phase unit is a direct current terminal on the direct current side. Namely, the series connection point of the bridge arms in each phase unit of the first MMC module 1 is the ac terminal on the ac side thereof, and the parallel connection point of each phase unit is the dc terminal on the dc side thereof; the series connection point of the bridge arms in each phase unit of the second MMC module 2 is an ac terminal on the ac side thereof, and the parallel connection point of each phase unit is a dc terminal on the dc side thereof.
The power Module unit includes a plurality of serially connected power Sub-modules (SM), which are Half-Bridge power Sub-modules (Half-Bridge SM, HBSM) or Full-Bridge power Sub-modules (Full-Bridge SM, FBSM) including fully-controlled power electronic devices. Fig. 4 is a schematic diagram of a half-bridge power submodule structure, and fig. 5 is a schematic diagram of a full-bridge power submodule structure.
In this embodiment, the frequency conversion flexible tuning device includes a first MMC module 1 and a second MMC module 2, and the first MMC module 1 and the second MMC module 2 both include a plurality of power sub-modules formed by fully-controlled power electronic devices, so that input/output signals of the rectifier module and the inverter module are changed by controlling the conduction and locking of the fully-controlled power electronic devices, and electric parameters such as frequency, voltage amplitude, phase and the like of output signals of the frequency conversion flexible tuning device can be flexibly adjusted.
Further, the two bridge arms of the phase unit in this embodiment may be connected in the following two ways: the two bridge arms are connected through respective reactors or are connected through respective power module units. That is, the reactor may be installed at the ac terminal of the phase unit or the reactor may be installed at the dc terminal of the phase unit depending on the actual installation position of the variable frequency flexible tuning device. As shown in fig. 3, the two arms of each phase unit are connected by respective reactors in this embodiment. Meanwhile, a line is led out from each AC terminal to form an A, B, C three-phase AC line.
Further, in this embodiment, each power sub-module of the first MMC module 1 is a power sub-module with the same structure, and each reactor is a reactor with the same structure. Meanwhile, each power sub-module of the second MMC module 2 is also a power sub-module with the same structure, and each reactor is a reactor with the same structure.
Further, in this embodiment, the structures of the power sub-module of the first MMC module 1 and the power sub-module of the second MMC module 2 are the same or different, and the structures of the reactor of the first MMC module 1 and the reactor of the second MMC module 2 are the same or different. I.e. the structures of the first MMC module 1 and the second MMC module 2 may be the same or different.
Further, in this embodiment, parameters of each element included in each power sub-module in the first MMC module 1 are the same, and reactance values of each reactor are the same, so that consistency of each bridge arm of the first MMC module 1 is ensured. The parameters of each element contained in each power sub-module in the second MMC module 2 are the same, and the reactance values of each reactor are the same, so that the consistency of each bridge arm of the second MMC module 2 is ensured.
The invention also provides a half-wavelength alternating current transmission system and provides a specific embodiment.
Fig. 6 is a schematic structural diagram of a half-wavelength ac transmission system in an embodiment of the present invention, and as shown in the drawing, the half-wavelength ac transmission system in this embodiment includes a sending-end ac system, a half-wavelength ac transmission line, a receiving-end ac system, and two variable-frequency flexible tuning devices described above. One frequency conversion flexible tuning device is arranged between the sending end alternating current system and the half-wavelength alternating current transmission line, and the other frequency conversion flexible tuning device is arranged between the receiving end alternating current system and the half-wavelength alternating current transmission line. As shown in the figure, the frequency conversion flexible tuning device connected to the sending-end ac system includes a first MMC module 11 and a second MMC module 12, and the frequency conversion flexible tuning device connected to the receiving-end ac system includes a first MMC module 21 and a second MMC module 22.
In this embodiment, the variable frequency flexible tuning devices are installed at two ends of the half-wavelength ac transmission line, and can tune the half-wavelength ac transmission line after the line structure, line parameters, or operation mode of the half-wavelength ac transmission line changes, so that the half-wavelength ac transmission line does not lose the half-wavelength characteristic.
Further, in the frequency conversion flexible tuning apparatus connected to the sending end ac system in this embodiment, as shown in fig. 6, a first MMC module 11 is connected to the sending end ac system, and a second MMC module 12 is connected to the half-wavelength ac transmission line. A second MMC module 22 of the variable frequency flexible tuning device is connected with the receiving end alternating current system, and a first MMC module 21 is connected with a half-wavelength alternating current transmission line.
The invention also provides a control method of the half-wavelength alternating current transmission system, and provides a specific embodiment.
The control method of the half-wavelength alternating current transmission system in the embodiment comprises the steps of carrying out flexible tuning and flexible grid connection on the half-wavelength alternating current transmission system by adjusting the operation mode of the variable frequency flexible tuning device, adjusting system parameters of the half-wavelength alternating current transmission system, restraining overvoltage and secondary arc current of the half-wavelength alternating current transmission system, and realizing fault isolation. The system parameters comprise the power factor and the bus voltage of the sending-end alternating current system and the power factor and the bus voltage of the receiving-end alternating current system. In the embodiment, the operation mode of the variable frequency flexible tuning device is changed, flexible tuning and flexible grid connection are carried out on the half-wavelength alternating current power transmission system, system parameters of the half-wavelength alternating current power transmission system are adjusted, overvoltage and secondary arc current of the half-wavelength alternating current power transmission line are restrained, fault isolation is realized, and the operation is flexible and simple. The following specifically describes flexible tuning, flexible grid connection, system parameter adjustment, overvoltage suppression and secondary arc suppression, and fault isolation.
1. Flexible tuning
In this embodiment, the flexible tuning of the half-wavelength ac power transmission system may be performed according to the following steps:
(1) when the electrical distance of the half-wavelength alternating-current transmission line is smaller than a half wavelength corresponding to the power grid frequency, the operating frequency of the half-wavelength alternating-current transmission line is improved by controlling the output signal frequency of the variable-frequency flexible tuning device, and alternating-current transmission is carried out by adopting the increased operating frequency until the electrical distance is equal to the half wavelength corresponding to the increased operating frequency.
(2) When the electrical distance of the half-wavelength alternating-current transmission line is larger than a half wavelength corresponding to the power grid frequency, the operating frequency of the half-wavelength alternating-current transmission line is reduced by controlling the output signal frequency of the variable-frequency flexible tuning device, and alternating-current transmission is carried out by adopting the reduced operating frequency until the electrical distance is equal to the half wavelength corresponding to the reduced operating frequency.
(3) When the line structure, line parameters or operation mode of the half-wavelength alternating current transmission line changes, the operation frequency of the half-wavelength alternating current transmission line is adjusted by controlling the output signal frequency of the variable frequency flexible tuning device, and alternating current transmission is carried out by adopting the adjusted operation frequency until the electrical distance of the half-wavelength alternating current transmission line after the line structure, the line parameters or the operation mode changes is equal to one half-wavelength corresponding to the changed operation frequency.
During the flexible tuning of the half-wavelength ac transmission system, the second MMC module 12 may be equivalent to a voltage source to provide voltage to the half-wavelength ac transmission line, and the first MMC module 21 may be equivalent to a current source to be equivalent to a resistive load. Meanwhile, in this embodiment, the first MMC module 1 may work in both a rectifying state and an inverting state, and the second MMC module 2 may work in both a rectifying state and an inverting state, so that the sending-end ac system and the receiving-end ac system may be interchanged according to actual working conditions, and accordingly, the first MMC module 21 may be equivalent to a voltage source to provide a voltage to the half-wavelength ac transmission line, and the second MMC module 12 may be equivalent to a current source to be equivalent to a resistive load.
Further, in order to control the frequency conversion flexible tuning device to output a higher-frequency voltage/current signal under the same switching frequency, before controlling the output signal frequency of the frequency conversion flexible tuning device under the condition of (1), the number of power sub-modules put into operation in the first MMC module 1 and the second MMC module 2 directly connected with the half-wavelength ac power transmission line in the frequency conversion flexible tuning device may be controlled to be larger than the number of power sub-modules put into operation in the first MMC module 1 and the second MMC module 2 not directly connected with the half-wavelength ac power transmission line. Namely, the number of the power sub-modules put into operation in the second MMC module 12 is controlled to be larger than the number of the power sub-modules put into operation in the first MMC module 11, and the number of the power sub-modules put into operation in the first MMC module 21 is controlled to be larger than the number of the power sub-modules put into operation in the second MMC module 22.
2. Flexible grid connection
The present embodiment includes two grid-connected types: the method comprises the steps that firstly, a sending end alternating current system is connected with a receiving end alternating current system after being connected into a half-wavelength alternating current transmission line, and secondly, the receiving end alternating current system is connected with the sending end alternating current system after being connected into the half-wavelength alternating current transmission line. The two grid-connection type flexible grid-connection methods are specifically described below.
(1) The grid connection type is that when a sending end alternating current system is connected with a half-wavelength alternating current transmission line and then is connected with a receiving end alternating current system, the method can be implemented according to the following steps:
the method comprises the following steps: the first MMC module 11 of the frequency conversion flexible tuning device on one side of the sending end alternating current system is controlled to be in a rectification state and establish corresponding direct current voltage, the second MMC module 12 is controlled to be in an inversion state and generate alternating current voltage corresponding to the preset operation frequency of the half-wavelength alternating current transmission line, and the alternating current voltage is transmitted to the frequency conversion flexible tuning device on one side of the receiving end alternating current system through the half-wavelength alternating current transmission line.
Secondly, the step of: the first MMC module 21 of the frequency conversion flexible tuning device on one side of the receiving end alternating current system is controlled to be in a rectification state and generate corresponding direct current voltage, and the second MMC module 22 is controlled to be in an inversion state and track the voltage frequency, amplitude, phase sequence and phase position of the receiving end alternating current system.
③: when the voltage frequency, amplitude, phase sequence and phase of the output signal at the ac side are respectively the same as the voltage frequency, amplitude, phase sequence and phase of the ac system at the receiving end, that is, the grid connection condition of the power system is met, the grid connection mechanical switch between the second MMC module 22 and the ac system at the receiving end is closed to complete the grid connection. The grid-connected condition of the power system comprises the following steps: the frequency of the two systems is the same, the voltage of the two systems is the same, the phase sequence of the two systems is the same, and the phases of the two systems are the same.
(2) When the grid connection type is that the receiving end alternating current system is connected with the half-wavelength alternating current transmission line and then is connected with the sending end alternating current system, the method can be implemented according to the following steps:
the method comprises the following steps: and controlling a second MMC module 22 of the frequency conversion flexible tuning device at one side of the receiving end alternating current system to be in a rectification state and establishing corresponding direct current voltage, controlling a first MMC module 21 to be in an inversion state and generating alternating current voltage corresponding to the preset operating frequency of the half-wavelength alternating current transmission line, and transmitting the alternating current voltage to the frequency conversion flexible tuning device at one side of the sending end alternating current system through the half-wavelength alternating current transmission line.
Secondly, the step of: and controlling a second MMC module 12 of the variable frequency flexible tuning device at one side of the sending end alternating current system to be in a rectification state and generate corresponding direct current voltage, controlling a first MMC module 11 to be in an inversion state and tracking the voltage frequency, amplitude, phase sequence and phase of the sending end alternating current system.
③: when the voltage frequency, amplitude, phase sequence and phase of the output signal at the ac side are respectively the same as the voltage frequency, amplitude, phase sequence and phase of the ac system at the sending end, that is, the grid connection condition of the power system is met, the grid connection mechanical switch between the first MMC module 11 and the ac system at the sending end is closed to complete grid connection.
3. Adjusting power factor
In this embodiment, the first MMC module 11 in the variable-frequency flexible tuning device on one side of the sending-end ac system may be controlled to operate according to the whole power factor, and the first MMC module 11 is equivalent to a resistive load, thereby improving the power factor of the sending-end ac system. Namely, the power factor of the first MMC module 11 is corrected to 1, and the first MMC module 11 is controlled to operate according to the corrected power factor.
In this embodiment, the second MMC module 22 in the frequency conversion flexible tuning device on one side of the receiving-end ac system may be controlled to operate according to the whole power factor, and the second MMC module 22 is equivalent to a resistive load, thereby improving the power factor of the receiving-end ac system. That is, the power factor of the second MMC module 22 is corrected to 1, and the second MMC module 22 is controlled to operate according to the corrected power factor.
4. Regulating bus voltage
In this embodiment, the bus voltage refers to a bus voltage between the sending-end ac system and the variable-frequency flexible tuning device connected thereto, and a bus voltage between the receiving-end ac system and the variable-frequency flexible tuning device connected thereto.
(1) In this embodiment, the bus voltage of the sending-end ac system may be adjusted according to the following steps: when the bus voltage is low, controlling a first MMC module 11 of the variable frequency flexible tuning device to output capacitive reactive power to a sending end alternating current system; when the bus voltage is higher than the set value, the first MMC module 11 is controlled to output the inductive reactive power to the sending-end ac system.
(2) In this embodiment, the bus voltage of the receiving-end ac system may be adjusted according to the following steps: when the bus voltage is low, controlling a second MMC module 22 of the variable frequency flexible tuning device to output capacitive reactive power to the receiving-end alternating current system; and when the bus voltage is higher than the set value, the second MMC module 22 is controlled to output inductive reactive power to the receiving-end alternating current system.
5. Suppression of overvoltage and secondary current
In this embodiment, when the half-wavelength ac transmission system operates normally, the second MMC module 22 connected to the frequency conversion flexible tuning apparatus of the sending-end ac system may be equivalent to a voltage source to output an ac voltage to the half-wavelength ac transmission line. After the half-wavelength alternating current transmission line has a fault, the alternating current voltage output by the second MMC module 22 can be controlled to be rapidly reduced, the overvoltage and the secondary arc current can be restrained, and fault isolation can be realized.
Further, in this embodiment, in order to avoid the difficult problem of line overvoltage and secondary arc current suppression caused by the half-wavelength ac transmission line, the half-wavelength ac transmission line may be equivalent to a conventional ac line by adjusting the operation mode of the variable frequency flexible tuning device, which specifically includes: the operating frequency of the half-wavelength alternating current transmission line is reduced by controlling the output signal frequency of the variable frequency flexible tuning device until the electrical distance of the half-wavelength alternating current transmission line is far greater than the corresponding half-wavelength before the operating frequency is reduced.
Further, in order to satisfy the requirement of actual condition to the direct current transmission in this embodiment, can be through adjusting the operation mode of flexible tuning device of frequency conversion, with half wavelength alternating current transmission line equivalence as direct current circuit, specifically do: the operating frequency of the half-wavelength alternating current transmission line is reduced to 0Hz by controlling the output signal frequency of the variable frequency flexible tuning device.
It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include such modifications and variations.
Claims (17)
1. A frequency conversion flexible tuning device is characterized by comprising a first MMC module and a second MMC module; the direct current side of the first MMC module is connected with the direct current side of the second MMC module, and the alternating current side is the input/output end of the variable frequency flexible tuning device;
the first MMC module and the second MMC module respectively comprise three phase units which are connected in parallel; the phase unit comprises two bridge arms connected in series, and each bridge arm comprises a reactor and a power module unit which are connected in series; the series connection point of the bridge arms in each phase unit is an alternating current terminal on the alternating current side, and the parallel connection point of each phase unit is a direct current terminal on the direct current side;
the power module unit comprises a plurality of power sub-modules connected in series, and each power sub-module is a half-bridge structure power sub-module or a full-bridge structure power sub-module comprising a full-control type power electronic device.
2. The variable frequency flexible tuning device of claim 1,
two bridge arms in the phase unit are connected through respective reactors; or,
and two bridge arms in the phase unit are connected through respective power module units.
3. The variable frequency flexible tuning device of claim 1,
each power sub-module of the first MMC module is a power sub-module with the same structure, and each reactor is a reactor with the same structure;
and each power sub-module of the second MMC module is a power sub-module with the same structure, and each reactor is a reactor with the same structure.
4. The variable frequency flexible tuning device of claim 1,
the power sub-modules of the first MMC module and the second MMC module have the same or different structures;
and the reactor of the first MMC module and the reactor of the second MMC module have the same or different structures.
5. The variable frequency flexible tuning device of claim 1,
parameters of all elements contained in each power sub-module in the first MMC module are the same, and reactance values of all reactors are the same;
and parameters of all elements contained in each power sub-module in the second MMC module are the same, and reactance values of all reactors are the same.
6. A half-wavelength ac transmission system comprising a sending end ac system, a half-wavelength ac transmission line and a receiving end ac system connected in sequence, characterized in that the half-wavelength ac transmission system comprises two variable frequency flexible tuning devices according to any one of claims 1-5; one frequency conversion flexible tuning device is arranged between the sending end alternating current system and the half-wavelength alternating current transmission line, and the other frequency conversion flexible tuning device is arranged between the receiving end alternating current system and the half-wavelength alternating current transmission line.
7. A half-wavelength AC transmission system according to claim 6,
in the variable-frequency flexible tuning device, a first MMC module is connected with a sending end alternating current system, and a second MMC module is connected with a half-wavelength alternating current transmission line; and a second MMC module in the other variable frequency flexible tuning device is connected with a receiving end alternating current system, and a first MMC module is connected with a half-wavelength alternating current transmission line.
8. The control method of the half-wavelength alternating-current transmission system according to claim 6 or 7, characterized by comprising the steps of performing flexible tuning and flexible grid connection on the half-wavelength alternating-current transmission system by adjusting the operation mode of the variable-frequency flexible tuning device, adjusting system parameters of the half-wavelength alternating-current transmission system, suppressing overvoltage and secondary current of the half-wavelength alternating-current transmission line and realizing fault isolation; the system parameters comprise the power factor and the bus voltage of the sending-end alternating current system and the power factor and the bus voltage of the receiving-end alternating current system.
9. A method of controlling a half-wavelength ac transmission system according to claim 8, wherein said flexibly tuning the half-wavelength ac transmission system comprises:
when the electrical distance of the half-wavelength alternating-current transmission line is smaller than a half wavelength corresponding to the power grid frequency, the operating frequency of the half-wavelength alternating-current transmission line is increased by controlling the output signal frequency of the variable-frequency flexible tuning device until the electrical distance is equal to a half wavelength corresponding to the increased operating frequency;
when the electrical distance of the half-wavelength alternating-current transmission line is greater than a half wavelength corresponding to the power grid frequency, reducing the operating frequency of the half-wavelength alternating-current transmission line by controlling the output signal frequency of the variable-frequency flexible tuning device until the electrical distance is equal to a half wavelength corresponding to the reduced operating frequency;
when the line structure, line parameters or operation mode of the half-wavelength alternating current transmission line changes, the operation frequency of the half-wavelength alternating current transmission line is adjusted by controlling the output signal frequency of the frequency conversion flexible tuning device until the electrical distance of the half-wavelength alternating current transmission line after the line structure, line parameters or operation mode changes is equal to a half-wavelength corresponding to the changed operation frequency.
10. The method of claim 9, wherein controlling the frequency of the output signal of the variable frequency flexible tuning device at a half wavelength for which the electrical distance is less than the grid frequency comprises:
and controlling the number of the power sub-modules which are put into operation in the first MMC module and the second MMC module which are directly connected with the half-wavelength alternating-current transmission line in the frequency conversion flexible tuning device to be larger than the number of the power sub-modules which are put into operation in the first MMC module and the second MMC module which are not directly connected with the half-wavelength alternating-current transmission line.
11. A method of controlling a half-wavelength AC transmission system according to claim 8,
the grid connection type of the flexible grid connection comprises that the sending end alternating current system is connected with the receiving end alternating current system after being connected into the half-wavelength alternating current transmission line, and the receiving end alternating current system is connected with the sending end alternating current system after being connected into the half-wavelength alternating current transmission line.
12. The method according to claim 11, wherein the flexible grid connection of the half-wavelength ac transmission system is performed when the sending-end ac transmission system is connected to the half-wavelength ac transmission line and then is connected to the receiving-end ac transmission system after the sending-end ac transmission system is connected to the half-wavelength ac transmission line, and the flexible grid connection of the half-wavelength ac transmission system includes:
controlling a first MMC module of a variable-frequency flexible tuning device at one side of the sending end alternating current system to be in a rectification state and a second MMC module to be in an inversion state, and controlling an alternating current side of the second MMC module to generate alternating current voltage corresponding to the preset operating frequency of the half-wavelength alternating current transmission line;
controlling a first MMC module of a variable-frequency flexible tuning device at one side of the receiving end alternating current system to be in a rectification state and a second MMC module to be in an inversion state, and controlling an alternating current side output signal of the second MMC module to track the voltage frequency, the amplitude, the phase sequence and the phase of the receiving end alternating current system; and when the voltage frequency, the amplitude, the phase sequence and the phase of the output signal of the alternating current side are respectively the same as the voltage frequency, the amplitude, the phase sequence and the phase of the receiving end alternating current system, closing a grid-connected mechanical switch between the second MMC module and the receiving end alternating current system to complete grid connection.
13. The method according to claim 11, wherein the flexible grid connection of the half-wavelength ac transmission system is performed when the receiving-side ac transmission system is connected to the half-wavelength ac transmission line and then connected to the sending-side ac transmission system after being connected to the half-wavelength ac transmission line, and the method includes:
controlling a second MMC module of the frequency conversion flexible tuning device at one side of the receiving end alternating current system to be in a rectification state and a first MMC module to be in an inversion state, and controlling the alternating current side of the first MMC module to generate alternating current voltage corresponding to the preset operating frequency of the half-wavelength alternating current transmission line;
controlling a second MMC module of the variable-frequency flexible tuning device at one side of the sending end alternating current system to be in a rectification state and a first MMC module to be in an inversion state, and controlling an alternating current side output signal of the first MMC module to track the voltage frequency, the amplitude, the phase sequence and the phase of the sending end alternating current system; and when the voltage frequency, the amplitude, the phase sequence and the phase of the output signal of the alternating current side are respectively the same as the voltage frequency, the amplitude, the phase sequence and the phase of the sending end alternating current system, closing a grid-connected mechanical switch between the first MMC module and the sending end alternating current system to complete grid connection.
14. A method of controlling a half-wavelength AC transmission system according to claim 8,
the adjusting of the power factor of the sending-end alternating current system comprises: controlling a first MMC module of a variable-frequency flexible tuning device at one side of the sending end alternating current system to operate according to a whole power factor;
the adjusting the power factor of the receiving end alternating current system comprises the following steps: and controlling a second MMC module of the variable-frequency flexible tuning device at one side of the receiving end alternating current system to operate according to the integral power factor.
15. A method of controlling a half-wavelength AC transmission system according to claim 8,
the bus voltage of the sending end alternating current system is adjusted by the following steps: when the bus voltage between the sending end alternating current system and the variable frequency flexible tuning device connected with the sending end alternating current system is low, controlling a first MMC module of the variable frequency flexible tuning device to output capacitive reactive power to the sending end alternating current system; when the bus voltage is higher than a set value, controlling the first MMC module to output inductive reactive power to a sending end alternating current system;
the adjusting of the bus voltage of the receiving end alternating current system comprises: when the bus voltage between the receiving-end alternating current system and the variable-frequency flexible tuning device connected with the receiving-end alternating current system is low, controlling a second MMC module of the variable-frequency flexible tuning device to output capacitive reactive power to the receiving-end alternating current system; and when the bus voltage is higher than a set value, the second MMC module is controlled to output inductive reactive power to the receiving end alternating current system.
16. A method of controlling a half-wavelength ac transmission system according to claim 8, wherein said suppressing overvoltage and secondary current of the half-wavelength ac transmission line comprises: and when the half-wavelength alternating current transmission line has a fault, controlling the alternating voltage output by the second MMC module in the frequency conversion flexible tuning device at one side of the sending end alternating current system to be quickly reduced so as to inhibit the overvoltage and the secondary arc current and realize fault isolation.
17. A method of controlling a half-wavelength AC transmission system according to claim 8,
the control method further comprises the step of enabling the half-wavelength alternating current transmission line to be equivalent to a conventional alternating current line by adjusting the operation mode of the variable frequency flexible tuning device, and specifically comprises the following steps: reducing the operating frequency of a half-wavelength alternating current transmission line by controlling the output signal frequency of the variable frequency flexible tuning device until the electrical distance of the half-wavelength alternating current transmission line is far greater than a corresponding half-wavelength before the operating frequency is reduced;
the control method further comprises the step of enabling the half-wavelength alternating-current transmission line to be equivalent to a direct-current line by adjusting the operation mode of the variable-frequency flexible tuning device, and specifically comprises the following steps: and reducing the operating frequency of the half-wavelength alternating current transmission line to 0Hz by controlling the frequency of an output signal of the variable-frequency flexible tuning device.
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