An Active Suppression System for Submerged Current in Half-wavelength AC Transmission Lines

Abstract

This invention provides an active suppression system for residual current in half-wavelength AC transmission lines. The system includes a parallel converter, one end of which is connected to an external power source, and the other end is connected in parallel with the half-wavelength AC transmission line. The parallel converter outputs a preset current to the half-wavelength AC transmission line to suppress residual current. Compared with the prior art, the active suppression system for residual current in half-wavelength AC transmission lines provided by this invention, by controlling the parallel converter to output a preset current to the half-wavelength AC transmission line, can suppress residual current to a desired level, thereby improving the arc extinction speed at the fault point of the half-wavelength AC transmission line.

Description

Technical Field

[0001] The invention relates to the technical field of power electronics, and in particular to a system for actively suppressing a latent current of a half-wavelength AC transmission line. Background Art

[0002] The development of technologies such as photovoltaics and wind power has promoted intensive and large-scale development in areas with less than optimal resource conditions, making the transmission of ultra-long-distance, high-power electricity and its implementation of broad significance and practical value.

[0003] Half Wavelength AC Transmission (HWACT) refers to the transmission of three-phase AC power over an electrical distance close to one half wavelength of the power frequency, i.e., ultra-long-distance transmission of 3000km (50Hz) or 2600km (60Hz). A lossless half-wavelength AC line is like an ideal transformer with a transformation ratio of -1.0, where the voltage at the head and the voltage at the end are the same in magnitude and opposite in phase. With the increasing demand for ultra-long-distance and high-power transmission, half-wavelength AC transmission technology, especially ultra-high voltage half-wavelength AC transmission, has once again attracted much attention and research. One of the advantages of half-wavelength AC transmission technology is that the power factor of the half-wavelength AC transmission line is relatively high, and when the transmission distance is equal to or slightly greater than half a wavelength, its structure is simpler than the existing ultra-long-distance AC and DC transmission systems; furthermore, for developing countries, the manufacture of AC transmission equipment is simpler and more economical than the introduction, operation and maintenance of converters.

[0004] In ultra-high voltage systems, in order to improve the reliability of power supply, fast single-phase automatic reclosing is often used. However, when one phase of the system is cut off due to a single-phase grounding fault, due to the coupling of phase-to-phase mutual inductance and phase-to-phase capacitance, a certain value of grounding current, namely, latent current, still flows through the fault phase that has been cut off at the fault point. The latent current will increase the difficulty of extinguishing the arc at the fault point, resulting in failure of the automatic reclosing. Since the half-wavelength AC transmission system line is very long and the latent current is very large, it is very difficult to extinguish the arc, and certain measures need to be taken to suppress the latent current. The existing half-wavelength AC transmission latent current suppression measures are mainly passive suppression measures using passive components. In principle, a feasible solution is to install high-speed grounding switches along the line, but this suppression measure still has the following defects:

[0005] (1) The half-wavelength AC transmission system has a long line. In order to suppress the sub-current to a self-extinguishing level, a large number of high-speed grounding switches need to be installed along the line. The power supply, inspection and maintenance of high-speed grounding switches are difficult.

[0006] (2) The installation of high-speed grounding switches along the half-wavelength transmission system is likely to cause the protection device to fail to operate or malfunction, and the requirements for relay protection are relatively high;

[0007] (3) Passive suppression measures cannot flexibly and reasonably suppress the backflow current according to the fault type and fault location of the half-wavelength transmission system, and cannot adapt to the complex and changeable operating conditions of the transmission system. Summary of the invention

[0008] In order to overcome the defects of the prior art, the present invention provides a system for actively suppressing the backflow current of a half-wavelength AC transmission line, and the technical solution is:

[0009] The system includes a parallel converter, one end of which is connected to an external power supply and the other end is connected in parallel to a half-wavelength AC transmission line; the parallel converter is used to output a preset current to the half-wavelength AC transmission line to suppress the back-supply current.

[0010] Furthermore, a preferred technical solution provided by the present invention is: the system further comprises a transformer; the parallel converter is connected in parallel with the half-wavelength AC transmission line through the transformer;

[0011] Among them, the grid-side winding of the transformer is a star-connected winding, and the neutral point of the grid-side winding is grounded; the parallel converter side winding of the transformer is a delta-connected winding.

[0012] Furthermore, a preferred technical solution provided by the present invention is that the parallel converter is installed at the beginning, middle or end of the half-wavelength AC transmission line.

[0013] Furthermore, a preferred technical solution provided by the present invention is: the parallel converter is a two-level converter, a multi-level converter or a modular multi-level converter; wherein the multi-level converter includes a diode clamped multi-level converter, a flying capacitor multi-level converter and a cascaded multi-level converter.

[0014] Furthermore, a preferred technical solution provided by the present invention is that the parallel converter is an AC-DC-AC converter or a DC-AC converter.

[0015] Furthermore, a preferred technical solution provided by the present invention is: the system also includes a control module for controlling the parallel converter to output a preset current to the half-wavelength AC transmission line.

[0016] Furthermore, a preferred technical solution provided by the present invention is: the control module includes a fault detection unit and a first converter driving unit;

[0017] The fault detection unit is used to detect fault information of the half-wavelength AC power transmission line;

[0018] The first converter driving unit is used to determine the current amplitude, current phase angle and injection duration injected into the half-wavelength AC transmission line according to the fault information and the operation information of the half-wavelength AC transmission line before the fault, and to output a driving pulse to the parallel converter according to the current amplitude, current phase angle and injection duration after the fault is removed for a first preset time, so as to drive the parallel converter to output a preset current;

[0019] Wherein, the first converter driving unit is further used to control the parallel converter to stop outputting the preset current to the half-wavelength AC transmission line within a second preset time before the tapping device is reclosed.

[0020] Furthermore, a preferred technical solution provided by the present invention is:

[0021] The first preset time is 1-3 cycles of the power frequency AC;

[0022] The second preset time is 1-2 cycles of the power frequency AC;

[0023] The tapping device comprises a circuit breaker.

[0024] Furthermore, a preferred technical solution provided by the present invention is: the control module also includes a second converter drive unit, which is used to control the parallel converter to output a preset reactive power to the half-wavelength AC transmission line to perform reactive power compensation for the half-wavelength AC transmission line where no faults occur.

[0025] Compared with the closest prior art, the beneficial effects of the present invention are:

[0026] The present invention provides a system for actively suppressing latent current of a half-wavelength AC transmission line. By controlling a parallel converter to output a preset current to the half-wavelength AC transmission line, the latent current can be suppressed within a desired level, thereby improving the arc extinguishing speed at the fault point of the half-wavelength AC transmission line. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 :Schematic diagram of passive suppression measures for subsurface current in point-to-grid half-wavelength AC transmission system;

[0028] Figure 2 :Schematic diagram of passive suppression measures for subsurface current in grid-to-grid half-wavelength AC transmission system;

[0029] Figure 3 : Schematic diagram of a system for actively suppressing the latent current of a half-wavelength AC transmission line in an embodiment of the present invention;

[0030] Figure 4: Schematic diagram of another system for actively suppressing the latent current of a half-wavelength AC transmission line according to an embodiment of the present invention;

[0031] Figure 5 : A schematic diagram of a two-level converter;

[0032] Figure 6 : A schematic diagram of a diode clamped three-level converter;

[0033] Figure 7 : Schematic diagram of a flying capacitor type three-level converter;

[0034] Figure 8 : A schematic diagram of a cascaded multi-level converter;

[0035] Fig. 9 : Schematic diagram of a modular multilevel converter;

[0036] Fig.10 : Schematic diagram of single-phase half-bridge structure power unit;

[0037] Fig.11 : Schematic diagram of single-phase full-bridge structure power unit;

[0038] Fig.12 : Schematic diagram of another two-level converter;

[0039] Fig.13 : Schematic diagram of another diode clamped three-level converter;

[0040] Fig.14 : Schematic diagram of another flying capacitor type three-level converter;

[0041] Fig.15 : Schematic diagram of another cascaded multilevel converter;

[0042] Fig.16 : Schematic diagram of another modular multilevel converter. DETAILED DESCRIPTION

[0043] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0044] The half-wavelength AC transmission system mainly includes the point-to-grid type half-wavelength AC transmission system and the grid-to-grid type half-wavelength AC transmission system. Figure 1As shown, the point-to-grid type half-wavelength AC transmission system includes a generator, a transformer, a half-wavelength AC transmission line and a large power grid, and the generator, transformer, half-wavelength AC transmission line and the large power grid are connected in sequence. Figure 2 As shown, the grid-to-grid half-wavelength AC transmission system may include two large grids and a half-wavelength AC transmission line, and the half-wavelength AC transmission line is connected between the two large grids; the grid-to-grid half-wavelength AC transmission system may also include two large grids, a transformer and a half-wavelength AC transmission line, where one large grid is connected to one end of the half-wavelength AC transmission line through a transformer, and the other end of the half-wavelength AC transmission line is connected to another large grid. Figure 1 and 2 As shown, when passive suppression measures are adopted, in order to suppress the submerged current to a self-extinguishing level, a large number of high-speed grounding switches need to be installed along the half-wavelength AC transmission line. The power supply, inspection and maintenance of the high-speed grounding switches are difficult. At the same time, the high-speed grounding switches are prone to cause the protection device to refuse to operate or malfunction, and the requirements for relay protection are relatively high, which limits the development and application of half-wavelength AC transmission technology. In view of the defects of the passive suppression method of the submerged current of the above-mentioned half-wavelength AC transmission system, the present invention provides a submerged current active suppression system for a half-wavelength AC transmission system based on a parallel converter, which can effectively suppress the submerged current of the half-wavelength AC transmission system. At the same time, the installation location of the submerged current active suppression system is concentrated, which is convenient for technical personnel to inspect and maintain.

[0045] A system for actively suppressing latent current of a half-wavelength AC transmission line provided by an embodiment of the present invention will be described below in conjunction with the accompanying drawings.

[0046] Figure 3 This is a schematic diagram of an active suppression system for a submerged current of a half-wavelength AC transmission line in an embodiment of the present invention. As shown in the figure, the submerged current suppression system for a half-wavelength AC transmission line in this embodiment includes a parallel converter, one end of which is connected to an external power supply, and the other end is connected in parallel with the half-wavelength AC transmission line. The parallel converter is used to output a preset current to the half-wavelength AC transmission line to suppress the submerged current. The parallel converter can be installed at the head end, the middle part or the end of the half-wavelength AC transmission line. In this embodiment, the parallel converter is installed at the end of the half-wavelength AC transmission line. By centrally installing the submerged current suppression system at the head end, the middle part or the end of the half-wavelength AC transmission line, the inspection and maintenance work of the submerged current suppression system can be reduced.

[0047] In this embodiment, by controlling the parallel converter to output a preset current to the half-wavelength AC transmission line, the back-supply current can be suppressed within a desired level, thereby increasing the arc extinguishing speed at the fault point of the half-wavelength AC transmission line.

[0048] Furthermore, in this embodiment, when the external power source is an AC power source, the parallel converter may be an AC-DC-AC converter, and when the external power source is a DC power source, the parallel converter may be a DC-AC converter.

[0049] Furthermore, the parallel converter in this embodiment may adopt converters of various structures, specifically:

[0050] When the external power supply is an AC power supply, the parallel converter in this embodiment can be used Figure 5 The two-level converter, multi-level converter or Fig. 9 The modular multilevel converter shown in FIG. The multilevel converter includes Figure 6 The diode clamped multilevel converter shown in FIG. Figure 7 The flying capacitor type multilevel converter shown in FIG. Figure 8 The power modules of the modular multilevel converter in this embodiment can be Fig.10 The single-phase half-bridge structure power unit shown in the figure, or the Fig.11 The single-phase full-bridge structure power unit shown.

[0051] When the external power supply is a DC power supply, the parallel converter in this embodiment can be used Fig.12 The two-level converter, multi-level converter or Fig.16 The modular multilevel converter shown in FIG. The multilevel converter includes Fig.13 The diode clamped multilevel converter shown in FIG. Fig.14 The flying capacitor type multilevel converter shown in FIG. Fig.15 The power modules of the modular multilevel converter in this embodiment can be Fig.10 The single-phase half-bridge structure power unit shown in the figure, or the Fig.11 The single-phase full-bridge structure power unit shown.

[0052] Figure 4This is a schematic diagram of another system for suppressing the latent current of a half-wavelength AC transmission line in an embodiment of the present invention. As shown in the figure, the system for suppressing the latent current of a half-wavelength AC transmission line in this embodiment includes a parallel converter and a transformer. One end of the parallel converter is connected to an external power supply, and the other end is connected in parallel with the half-wavelength AC transmission line through a transformer. The parallel converter is used to output a preset current to the half-wavelength AC transmission line to suppress the latent current. Among them, the parallel converter can be installed at the head end, the middle part or the end of the half-wavelength AC transmission line. In this embodiment, the parallel converter is installed at the end of the half-wavelength AC transmission line. By centrally installing the active suppression system for latent current at the head end, the middle part or the end of the half-wavelength AC transmission line, the inspection and maintenance work of the active suppression system for latent current can be reduced.

[0053] In this embodiment, by controlling the parallel converter to output a preset current to the half-wavelength AC transmission line, the back-supply current can be suppressed within a desired level, thereby increasing the arc extinguishing speed at the fault point of the half-wavelength AC transmission line.

[0054] Furthermore, in this embodiment, the grid-side winding of the transformer is a star-connected winding, and the neutral point of the grid-side winding is grounded. This connection method can reduce the phase voltage, thereby reducing losses and insulation costs. The parallel converter side winding of the transformer is a delta-connected winding. This connection method can prevent the harmonics generated by the parallel converter from entering the half-wavelength AC transmission line, and can also prevent the zero-sequence current generated by the parallel converter due to a fault from adversely affecting the protection device of the half-wavelength AC transmission system.

[0055] Furthermore, in this embodiment, when the external power source is an AC power source, the parallel converter may be an AC-DC-AC converter, and when the external power source is a DC power source, the parallel converter may be a DC-AC converter.

[0056] Furthermore, the parallel converter in this embodiment may adopt converters of various structures, specifically:

[0057] When the external power supply is an AC power supply, the parallel converter in this embodiment can be used Figure 5 The two-level converter, multi-level converter or Fig. 9 The modular multilevel converter shown in FIG. The multilevel converter includes Figure 6 The diode clamped multilevel converter shown in FIG. Figure 7 The flying capacitor type multilevel converter shown in FIG. Figure 8 The power modules of the modular multilevel converter in this embodiment can be Fig.10 The single-phase half-bridge structure power unit shown in the figure, or the Fig.11 The single-phase full-bridge structure power unit shown.

[0058] When the external power supply is a DC power supply, the parallel converter in this embodiment can be used Fig.12 The two-level converter, multi-level converter or Fig.16 The modular multilevel converter shown in FIG. The multilevel converter includes Fig.13 The diode clamped multilevel converter shown in FIG. Fig.14 The flying capacitor type multilevel converter shown in FIG. Fig.15 The power modules of the modular multilevel converter in this embodiment can be Fig.10 The single-phase half-bridge structure power unit shown in the figure, or the Fig.11 The single-phase full-bridge structure power unit shown.

[0059] In a preferred embodiment provided by the present invention, the system for actively suppressing the backflow current further includes a control module for controlling the parallel converter to output a preset current to the half-wavelength AC transmission line. In this embodiment, the control module includes a fault detection unit and a first converter drive unit, specifically:

[0060] The fault detection unit is used to detect the fault information of the half-wavelength AC transmission line. The fault information may include the fault phase, fault type and fault location where the fault occurs, and the fault type mainly includes a short circuit fault.

[0061] The first converter drive unit is used to determine the current amplitude, current phase angle and injection duration injected into the half-wavelength AC transmission line according to the fault information and the operation information of the half-wavelength AC transmission line before the fault, and to output a drive pulse to the parallel converter according to the current amplitude, current phase angle and injection duration after the fault is removed for a first preset time, so as to drive the parallel converter to output a preset current. The first preset time may be 1-3 cycles of the power frequency AC. In this embodiment, the first converter drive unit may also be used to control the parallel converter to stop outputting the preset current to the half-wavelength AC transmission line within a second preset time before the tapping device is reclosed. The second preset time is 1-2 cycles of the power frequency AC, and the tapping device mainly includes a circuit breaker.

[0062] Furthermore, the control module in this embodiment may also include a second converter drive unit for controlling the parallel converter to output a preset reactive power to the half-wavelength AC transmission line, thereby playing the role of a static synchronous compensator (STATCOM) to achieve reactive compensation of the half-wavelength AC transmission line.

[0063] Furthermore, in this embodiment, the control module controls the parallel converter to output a preset current to the half-wavelength AC transmission line in the following steps:

[0064] 1. When the half-wavelength AC transmission system generates a back-supply current due to a fault, the fault detection unit detects the fault information of the half-wavelength AC transmission line.

[0065] 2. The first converter drive unit determines the current amplitude, current phase angle and injection duration injected into the half-wavelength AC transmission line based on the fault information and the operation information of the half-wavelength AC transmission line before the fault and by using a table lookup method.

[0066] 3. After the fault is cleared for a first preset time, the first converter drive unit outputs a drive pulse to the parallel converter according to the current amplitude, current phase angle and injection duration, as well as the type of the parallel converter, to drive the parallel converter to output a preset current and suppress the back-supply current.

[0067] 4. The first converter drive unit controls the parallel converter to stop outputting a preset current to the half-wavelength AC transmission line within a second preset time before the tapping device is reclosed.

[0068] 5. When the half-wavelength AC transmission line fails again, the control module repeats steps 1 to 4 until the half-wavelength AC transmission system resumes normal operation.

[0069] It should be noted that the above embodiments illustrate the present invention rather than limit it, and that those skilled in the art may devise alternative embodiments without departing from the scope of the appended claims. In the claims, any reference symbol between brackets shall not be construed as a limitation on the claims. The word "comprising" does not exclude the presence of elements or steps not listed in the claims. The word "one" or "an" preceding an element does not exclude the presence of a plurality of such elements. The present invention may be implemented with the aid of hardware comprising a number of different elements and with the aid of a suitably programmed PC. In a unit claim enumerating a number of devices, several of these devices may be embodied by the same hardware item. The use of the words first, second, and third, etc., does not indicate any order. These words may be interpreted as names.

[0070] Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems, or computer program products. Therefore, the present application may adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the present application may adopt the form of a computer program product implemented in one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that include computer-usable program code.

[0071] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0072] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0073] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0074] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these 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 these modifications and variations.

Claims

1. A system for actively suppressing the backflow current of a half-wavelength AC transmission line, characterized in that: The system includes a parallel converter, one end of which is connected to an external power source, and the other end of which is connected in parallel to a half-wavelength AC transmission line; the parallel converter is used to output a preset current to the half-wavelength AC transmission line to suppress the backflow current; The system further comprises a control module for controlling the parallel converter to output a preset current to the half-wavelength AC transmission line; The control module includes a fault detection unit and a first converter driving unit; The fault detection unit is used to detect fault information of the half-wavelength AC power transmission line; The first converter driving unit is used to determine the current amplitude, current phase angle and injection duration injected into the half-wavelength AC transmission line according to the fault information and the operation information of the half-wavelength AC transmission line before the fault, and by using a table lookup method; And, after the fault removal reaches a first preset time, outputting a driving pulse to the parallel converter according to the current amplitude, current phase angle and injection duration, so as to drive the parallel converter to output a preset current; Wherein, the first converter driving unit is further used to control the parallel converter to stop outputting the preset current to the half-wavelength AC transmission line within a second preset time before the tapping device is reclosed.

2. The system for actively suppressing the backflow current of a half-wavelength AC transmission line according to claim 1, characterized in that: The system further comprises a transformer; the parallel converter is connected in parallel with the half-wavelength AC transmission line through the transformer; Among them, the grid-side winding of the transformer is a star-connected winding, and the neutral point of the grid-side winding is grounded; the parallel converter side winding of the transformer is a delta-connected winding.

3. A system for actively suppressing the backflow current of a half-wavelength AC transmission line according to claim 1 or 2, characterized in that: The parallel converter is installed at the line head end, the line middle part or the line end of the half-wavelength AC transmission line.

4. A system for actively suppressing the backflow current of a half-wavelength AC transmission line according to claim 1 or 2, characterized in that: The parallel converter is a two-level converter, a multi-level converter or a modular multi-level converter; wherein the multi-level converter includes a diode clamped multi-level converter, a flying capacitor multi-level converter and a cascaded multi-level converter.

5. A system for actively suppressing the backflow current of a half-wavelength AC transmission line according to claim 1 or 2, characterized in that: The parallel converter is an AC-DC-AC converter or a DC-AC converter.

6. The system for actively suppressing the backflow current of a half-wavelength AC transmission line according to claim 1, characterized in that: The first preset time is 1-3 cycles of the power frequency AC; The second preset time is 1-2 cycles of the power frequency AC; The tapping device comprises a circuit breaker.

7. The system for actively suppressing the backflow current of a half-wavelength AC transmission line according to claim 1, characterized in that: The control module also includes a second converter driving unit for controlling the parallel converter to output a preset reactive power to the half-wavelength AC transmission line, so as to perform reactive power compensation on the half-wavelength AC transmission line where no fault occurs.

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