CN215267650U - Low-frequency power transmission system based on grounding structure - Google Patents
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Abstract
The utility model discloses a low frequency transmission system based on ground structure, include: the system comprises an alternating current power grid, an AC/AC converter, a first converter transformer, a grounding device, a second converter transformer and a low-frequency power grid. The grounding device is respectively connected with the secondary side winding of the first converter transformer and the input end of the AC/AC converter, and forms a grounding structure with the secondary side winding. The utility model discloses a different ground structure among the earthing device can regard as the neutral node of first converter transformer's secondary side winding to transmission line's too high to the ground potential ensures that low frequency transmission line can safe, reliable and stable operation.
Description
Technical Field
The utility model relates to a low frequency electric wire netting transmission of electricity technical field, concretely relates to low frequency transmission of electricity system based on ground structure.
Background
With the development of national economy and the advancement of power grid technology, the demand of vast residents or many power utilization places on electric energy is continuously increased, and the requirement on the electric energy transmission capacity of the power grid is higher and higher.
For example: in recent years, more and more island resources are developed, the tourism industry develops rapidly, the power load on the island is increased continuously, the demand of island power supply and inter-island networking is increased, the distance between islands is long, and when power frequency power transmission is used, the fluctuation of the terminal voltage of output electric energy is large, so that the offshore power supply voltage is unstable, and the requirement of using power by residents on the offshore islands cannot be met. Therefore, it is necessary to realize optimal allocation of resources by large-capacity long-distance low-frequency power transmission.
At present, in the prior art, for a low-frequency power transmission system, the output voltage of the low-frequency power transmission system is easily unstable due to the fact that the ground potential on the converter valve side of an AC/AC converter is high, and finally the low-frequency power transmission system cannot operate safely, reliably and stably.
SUMMERY OF THE UTILITY MODEL
Therefore, the to-be-solved technical problem of the present invention is to overcome the defects that the output voltage of a low frequency power transmission system is unstable and unsafe and reliable due to the high ground potential on the converter valve side of an AC/AC converter in the prior art, thereby providing a low frequency power transmission system based on a grounding structure.
According to a first aspect, an embodiment of the present invention provides a low frequency power transmission system based on a ground structure, including: an AC power grid (11) for outputting AC power; an AC/AC converter (12) for transmitting the alternating current power and converting a first frequency three-phase voltage in a power transmission line to a second frequency three-phase voltage, the first frequency being greater than the second frequency; a low frequency power grid (13) for outputting low frequency power frequency-converted by the AC/AC converter (12); a first converter transformer (14) comprising: a primary side winding located at the side end where the alternating current power grid (11) is located and a secondary side winding located at the side end where the AC/AC converter (12) is located, wherein the primary side winding is connected with the alternating current power grid (11), and the secondary side winding is connected with the AC/AC converter (12); a second converter transformer (16) comprising: a primary side winding positioned at the side end where the low-frequency power grid (13) is positioned and a secondary side winding positioned at the side end where the AC/AC converter (12) is positioned, wherein the primary side winding is connected with the low-frequency power grid (13), and the secondary side winding is connected with the output end of the AC/AC converter (12); a grounding device (15) which is respectively connected with the secondary side winding of the first converter transformer (14) and the input end of the AC/AC converter (12) and forms a grounding structure with the secondary side winding of the first converter transformer (14); or the secondary side winding of the second converter transformer (16) is connected with the output end of the AC/AC converter (12), and forms a grounding structure with the secondary side winding of the second converter transformer (16).
In one embodiment, the present invention provides a low frequency power transmission system based on a grounding structure, wherein the grounding device (15) includes: a Z-type transformer (151) including a primary winding and a secondary winding, the primary winding of which is connected to the secondary winding of the first converter transformer (14) and to an input of the AC/AC converter (12); a surge arrester (153) connected to a connection node between the primary winding and the secondary winding of the Z-type transformer (151), respectively; a first resistor (152) connected in parallel with the surge arrester (153) and also grounded at one end thereof.
In one embodiment, in the low frequency power transmission system based on the ground structure in the embodiment of the present invention, the primary winding and the secondary winding of the Z-type transformer (151) constitute a first star connection structure.
In one embodiment of the present invention, in the low frequency power transmission system based on the grounding structure described in the embodiment of the present invention, the connection node of the primary side winding and the secondary side winding of the Z-type transformer (151) is the neutral node of the first star-type connection structure.
In one embodiment, in the low frequency power transmission system based on the ground structure in the embodiment of the present invention, the primary winding and the secondary winding of the Z-type transformer (151) are three-phase windings.
In one embodiment, the low frequency power transmission system based on a grounding structure in the embodiment of the present invention, the grounding device (15) further includes: three interconnected reactors (155) and a second resistor (154), a first end of the second resistor (154) being connected to a connection node of the three interconnected reactors (155), a second end of the second resistor (154) being connected to ground.
In one embodiment, the low frequency power transmission system based on the ground structure in the embodiment of the present invention, the three interconnected reactors (155) constitute a second star connection structure.
In one embodiment, in the low frequency power transmission system based on the ground structure in the embodiment of the present invention, the connection node of the three interconnected reactors (155) is the neutral node of the second star connection structure.
In one embodiment, the low frequency power transmission system based on the grounding structure in the embodiment of the present invention, the three interconnected reactors (155) are respectively connected to the secondary side winding of the first converter transformer (14) and the input end of the AC/AC converter (12).
In an embodiment, the embodiment of the present invention provides a low frequency power transmission system based on a grounding structure, further including: a first switching device (17) connected to the primary windings of the alternating current grid (11) and the first converter transformer (14), respectively; -second switching devices (18) connected to the primary windings of the low frequency grid (13) and the second converter transformer (16), respectively.
The utility model discloses technical scheme has following advantage:
the utility model discloses a low frequency transmission system based on ground structure, include: the alternating current power grid is used for outputting alternating current electric energy; the AC/AC converter is used for transmitting alternating current electric energy and converting a first frequency three-phase voltage in the power transmission line into a second frequency three-phase voltage, and the first frequency is greater than the second frequency; the low-frequency power grid is used for outputting low-frequency electric energy subjected to frequency conversion by the AC/AC converter; a first converter transformer comprising: the primary side winding is connected with the alternating current network, and the secondary side winding is connected with the AC/AC converter; the grounding device is respectively connected with the secondary side winding of the first converter transformer and the input end of the AC/AC converter and forms a grounding structure with the secondary side winding; a second converter transformer comprising: the primary side winding is connected with the low-frequency power grid, and the secondary side winding is connected with the output end of the AC/AC converter. The utility model discloses a different ground structure among the earthing device can regard as the neutral node of first converter transformer's secondary side winding to transmission line is too high to the earth potential, ensures that low frequency transmission line can safe, reliable and stable operation.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the embodiments or the technical solutions in the prior art will be briefly described below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to these drawings without creative efforts.
Fig. 1 is a first schematic diagram of a low frequency power transmission system based on a grounding structure according to an embodiment of the present invention;
fig. 2 is a second schematic diagram of a low frequency power transmission system based on a grounding structure according to an embodiment of the present invention;
fig. 3 is a topology diagram of an AC/AC converter according to an embodiment of the present invention;
fig. 4 is a first schematic view of an embodiment of the present invention;
fig. 5 is a second schematic view of the grounding device according to the embodiment of the present invention;
fig. 6 is a third schematic diagram of a low-frequency power transmission system based on a grounding structure according to an embodiment of the present invention.
Reference numerals:
10-a bypass switching device; 11-an alternating current grid; a 12-AC/AC converter;
13-low frequency grid; 14-a first converter transformer; 15-a grounding device;
16-a second converter transformer; 17-a first switching device; 18-a second switching device;
a 151-Z transformer; 152-a first resistor;
153-a lightning arrester; 154-a second resistor; 155-reactor.
Detailed Description
The technical solution of the present invention will be described clearly and completely with reference to the accompanying drawings, and obviously, the described embodiments are some, but not all embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by a person skilled in the art without creative work belong to the protection scope of the present invention.
In the description of the present invention, it should be noted that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, and are only for convenience of description and simplification of description, but do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus, should not be construed as limiting the present invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance.
In the description of the present invention, it is to be noted that, unless otherwise explicitly specified or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, and may be, for example, fixedly connected, detachably connected, or integrally connected; can be mechanically or electrically connected; the two elements may be directly connected or indirectly connected through an intermediate medium, or may be communicated with each other inside the two elements, or may be wirelessly connected or wired connected. The specific meaning of the above terms in the present invention can be understood in specific cases to those skilled in the art.
Furthermore, the technical features mentioned in the different embodiments of the invention described below can be combined with each other as long as they do not conflict with each other.
In a long-distance low-frequency power transmission system, although low-frequency power transmission can solve the problem that the fluctuation of the voltage at the tail end of output electric energy is large, the output voltage of the low-frequency power transmission system is still unstable due to the fact that the distance between power transmission places is long and the ground potential on the converter valve side of an AC/AC converter is high, and finally the low-frequency power transmission system cannot run safely and stably. Therefore, the utility model discloses a low frequency transmission system based on ground structure.
The utility model discloses a low frequency transmission system based on ground structure, as shown in FIG. 1, include respectively: an alternating current grid 11, an AC/AC converter 12, a low frequency grid 13, a first converter transformer 14, a grounding device 15 and a second converter transformer 16.
In fig. 1, an ac grid 11 is shown for outputting ac power. The ac grid 11 here is a grid on the power frequency side for outputting electric power of an ac signal, and the power frequency side is a grid side generally used for outputting ac power having a frequency of 50 Hz. For example: the ac power grid 11 can be a land-based substation and/or power station and/or distribution substation, which supplies electrical energy primarily to land-based consumers. Another example is: the ac grid 11 may also be a new energy generation base that is currently popular.
In fig. 1, an AC/AC converter 12 is used for transmitting AC power and converting a first frequency three-phase voltage in a power transmission line into a second frequency three-phase voltage, wherein the first frequency is greater than the second frequency. Since the low-frequency power transmission system is often applied to large-capacity long-distance power transmission, in order to improve transmission efficiency, enhance transmission capacity, and further reduce impedance and transmission distance of a power transmission line, the AC power transmitted by the AC power grid 11 is subjected to frequency conversion by the AC/AC inverter 12, wherein a range of a first frequency and a second frequency is determined according to practical application, the low-frequency power transmission system based on the grounding structure is applied to large-capacity long-distance power transmission, the range of the first frequency and the second frequency is determined according to practical application, the preferred range of the first frequency is 75Hz or less, and the second frequency is only required to be less than the first frequency, for example, the first frequency is 60Hz, and the second frequency may be any frequency less than 60 Hz. In the embodiment, because the power frequency of the power grid in China is 50Hz, the first frequency is set to be 50 Hz; the second frequency was set to 50/3 Hz; with the reduction of the transmission frequency, the current of the cable is increased, the insulation performance is reduced, the 50/3Hz low-frequency transmission frequency is adopted after the influence of various factors such as insulation, cost and the like is comprehensively considered, and the purposes of increasing the transmission capacity by 3 times, reducing the line impedance and increasing the transmission distance can be achieved.
In one embodiment, as shown in fig. 2, the AC/AC converter 12 is connected in a structure, in fig. 1, the AC grid 11 is a phase a, a phase B, and a phase C, and the output voltages of the AC grid 11 are VA, VB, and VC. As shown in fig. 3, AC/AC converter 12 includes three commutation units 121, and commutation unit 121 includes three commutation bridge arms, each commutation bridge arm includes an inductor 1211 and an H-bridge 1212, a first end of inductor 1211 is connected to a first end of H-bridge 1112, a second end of inductor 1211 serves as an input end of the commutation bridge arm, and a second end of H-bridge 1212 serves as an output end of the commutation bridge arm; the AC/AC converter 12 has 9 legs consisting of cascaded H-bridges.
In fig. 3, the fully-controlled H-bridge 1212 includes two sets of power electronic device bridge arms connected in parallel, each power electronic device bridge arm including two power electronic devices connected in series, and a dc capacitor connected in parallel with the power electronic device bridge arms; the power electronic device includes an Insulated Gate Bipolar Transistor (IGBT) and an anti-parallel diode connected in parallel with the IGBT. Of course, in other embodiments, the power electronic device may also be a metal-oxide-semiconductor (MOS) field effect Transistor (MOS) or a Bipolar Junction Transistor (BJT), and the like, and may be reasonably arranged as needed. In fig. 3, since the voltage class that can be borne by one fully-controlled H-bridge 1212 is limited, a plurality of fully-controlled H-bridges 1212 may be set for cascading according to the requirement of the power transmission line, and in other embodiments, the number of cascades of the fully-controlled H-bridges 1212 may be set reasonably according to the requirement.
In fig. 1, among others, a first converter transformer 14 includes: a primary winding at the side of the AC grid 11 and a secondary winding at the side of the AC/AC converter 12, wherein the primary winding is connected to the AC grid 11 and the secondary winding is connected to the input of the AC/AC converter 12. In fig. 1, it can be seen that the first converter transformer 14 is disposed between the AC power grid 11 and the AC/AC converter 12, the primary winding at the side of the AC power grid 11 is in a star connection structure, and the primary winding is connected to the ground, the secondary winding at the side of the AC/AC converter 12 is in a delta connection structure, and the delta connection structure does not have a ground point, so that a to-be-grounded point needs to be disposed at the secondary winding to prevent the power transmission line from having a high ground potential.
A second converter transformer 16 comprising: a primary winding at the side of the low frequency grid 13 and a secondary winding at the side of the AC/AC converter 12, wherein the primary winding is connected to the low frequency grid 13 and the secondary winding is connected to the output of the AC/AC converter 12. Of these, a primary side winding thereof is connected to an AC grid 11, and a secondary side winding thereof is connected to an AC/AC converter 12. In fig. 1, it can be seen that the second converter transformer 16 is arranged between the low frequency grid 13 and the AC/AC converter 12, and the primary side winding at the side end of the low frequency grid 13 is in a star connection configuration, and the primary side winding thereof is connected to ground. The second converter transformer 16 and the first converter transformer 14 have the same structure, and the main functions of the second converter transformer 16 and the first converter transformer 14 are to adjust the transmission voltages at the low-frequency grid 13 side end and the power-frequency grid side end and the output voltages of the sub-modules included in the AC/AC converter 12, so that the stable and reliable operation of the flexibly controlled low-frequency transmission system can be improved.
In fig. 1, the grounding device 15 is connected to the secondary winding of the first converter transformer 14 and the input terminal of the AC/AC converter 12, and forms a grounding structure with the secondary winding of the first converter transformer 14. In fig. 1, it can be seen that the grounding arrangement 15 is arranged between the first converter transformer 14 and the AC/AC converter 12. As another alternative, the grounding device 15 may be connected between the second converter transformer 16 and the AC/AC converter 12, to the secondary winding of the second converter transformer 16 and the output terminal of the AC/AC converter 12, and form a grounding structure with the secondary winding of the second converter transformer (16). The grounding device 15 may have a Z-type transformer 151 grounding structure as a grounding node in which the secondary winding of the first converter transformer 14 has a delta structure, and the grounding device 15 may have three reactors 155 connected to each other as a grounding node in a grounding structure. Therefore, the grounding device 15 is intended to ground the secondary winding of the first converter transformer 14 in order to form a grounding node, so as to prevent the potential of the transmission line to the ground potential node from being too high, thereby ensuring the safe and stable operation of the transmission line.
In a preferred embodiment, as shown in fig. 4, the grounding device 15 in the embodiment of the present invention includes: a Z-type transformer 151, a surge arrester 153, and a first resistor 152. The lightning arrester 153 is connected with a connection node of a primary winding and a secondary winding of the Z-type transformer 151; and a first resistor 152 connected in parallel with the surge arrester 153 and also grounded at one end thereof.
In fig. 4, a Z-type transformer 151, which includes a primary side winding and a secondary side winding, has its primary side winding connected to the secondary side winding of the first converter transformer 14 and the input terminal of the AC/AC converter 12. The primary winding and the secondary winding of the Z-type transformer 151 are three-phase windings, the primary winding and the secondary winding of the Z-type transformer 151 form a first star connection structure, and a connection node of the primary winding and the secondary winding of the Z-type transformer 151 is a neutral node of the first star connection structure. In fig. 4, three-phase windings of the primary winding are a1, B1, and C1, three-phase windings of the secondary winding are a2, B2, and C2, a winding a1 on the primary side is connected to a winding B2 on the secondary side, a winding B1 on the primary side is connected to a winding C2 on the secondary side, and a winding C1 on the primary side is connected to a winding a2 on the secondary side. Therefore, the primary windings C1, a1, and B1 of the Z-type transformer 151 are connected end to end with the secondary windings a2, B2, and C2, respectively, to form a first star connection structure, where a2, B2, and C2 are neutral nodes of the first star connection structure, and the neutral nodes are connected to the surge arrester 153 and the first resistor 152, respectively. In fig. 4, one end of the surge arrester 153 connected in parallel with the first resistor 152 is connected to the neutral nodes a2, B2, C2, and the other end of the surge arrester 153 connected in parallel with the first resistor 152 is grounded.
Although the Z-transformer 151 is a common power electronic device in an ac/dc transmission system, the zero-sequence reactance is small and no choke effect is generated on the zero-sequence current because the zero-sequence current directions in the two half windings on the same column of the Z-transformer 151 are opposite. When the neutral point of the Z-transformer 151 is connected to the arc suppression coil, the arc suppression coil compensation current can be freely passed, and the Z-transformer 151 is widely used as a grounding transformer. Therefore, the Z-type transformer 151 is used as a grounding structure for the secondary winding of the first converter transformer 14. In fig. 1, it can be seen that the secondary winding of the first converter transformer 14 is a triangular connection structure, and the triangular connection structure has no neutral node, so that direct grounding cannot be performed, which results in a higher ground potential of the transmission line, and finally affects reliable and stable operation of low-frequency transmission. In this embodiment, the Z-type transformer 151 may be configured to form a grounding structure, so as to ensure that the secondary winding of the first converter transformer 14 can be grounded safely even if it is in a delta connection structure, thereby ensuring more stable and reliable low-frequency power transmission.
In another alternative embodiment, as shown in fig. 5, in the low-frequency power transmission system based on the grounding structure in the embodiment of the present invention, the grounding device 15 further includes: three reactors 155 and a second resistor 154 connected to each other, a first end of the second resistor 154 is connected to a connection node of the three reactors 155 connected to each other, and a second end of the second resistor 154 is grounded. The three interconnected reactors 155 constitute a second star connection. The connection node of the three interconnected reactors 155 is the neutral node of the second star connection. Three reactors 155 connected to each other are connected to the secondary winding of the first converter transformer 14 and the input terminal of the AC/AC converter 12, respectively. In fig. 5, the neutral node of three interconnected reactors 155 is one end of a second resistor 154, and the other end of the second resistor 154 is grounded. In fig. 5, the three interconnected reactors 155 and the second resistor 154 also form a grounding structure as a neutral node of the secondary winding of the first converter transformer 14, so as to better enable the first converter transformer 14 to be reliably grounded to prevent it from being higher to the ground potential in the power transmission line, and finally ensure that the low-frequency power transmission line can be operated safely and stably.
As shown in fig. 6, a grounding structure formed by three reactors 155 and second resistors 154 connected to each other is applied to a low-frequency power transmission line.
In an embodiment, the low frequency power transmission system based on the grounding structure in the embodiment of the present invention further includes, in fig. 1 or fig. 6: a first switching device 17, a second switching device 18.
Therein, a first switching device 17 is connected to the ac grid 11 and the primary winding of the first converter transformer 14, respectively. And a second switching device 18 connected to the primary windings of the low frequency grid 13 and the second converter transformer 16, respectively. The main function of the first switching device 17 and the second switching device 18 is to turn on or off the grid transmission line. In fig. 6, a bypass switching device 10 is further provided on the primary winding of the first converter transformer 14 and a bypass switching device 10 is further provided on the primary winding of the second converter transformer 16.
It should be understood that the above examples are only for clarity of illustration and are not intended to limit the embodiments. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. And obvious variations or modifications can be made without departing from the scope of the invention.
Claims (10)
1. A low frequency power transmission system based on a grounded structure, comprising:
an AC power grid (11) for outputting AC power;
an AC/AC converter (12) for transmitting the alternating current power and converting a first frequency three-phase voltage in a power transmission line to a second frequency three-phase voltage, the first frequency being greater than the second frequency;
a low frequency power grid (13) for outputting low frequency power frequency-converted by the AC/AC converter (12);
a first converter transformer (14) comprising: a primary side winding located at the side end where the alternating current power grid (11) is located and a secondary side winding located at the side end where the AC/AC converter (12) is located, wherein the primary side winding is connected with the alternating current power grid (11), and the secondary side winding is connected with the AC/AC converter (12);
a second converter transformer (16) comprising: a primary side winding positioned at the side end where the low-frequency power grid (13) is positioned and a secondary side winding positioned at the side end where the AC/AC converter (12) is positioned, wherein the primary side winding is connected with the low-frequency power grid (13), and the secondary side winding is connected with the output end of the AC/AC converter (12);
a grounding device (15) which is respectively connected with the secondary side winding of the first converter transformer (14) and the input end of the AC/AC converter (12) and forms a grounding structure with the secondary side winding of the first converter transformer (14); or the secondary side winding of the second converter transformer (16) is connected with the output end of the AC/AC converter (12), and forms a grounding structure with the secondary side winding of the second converter transformer (16).
2. A low frequency power transmission system based on a grounding structure according to claim 1, characterized in that said grounding means (15) comprise:
a Z-type transformer (151) including a primary winding and a secondary winding, the primary winding of which is connected to the secondary winding of the first converter transformer (14) and to an input of the AC/AC converter (12);
a surge arrester (153) connected to a connection node between the primary winding and the secondary winding of the Z-type transformer (151), respectively;
a first resistor (152) connected in parallel with the surge arrester (153) and also grounded at one end thereof.
3. The ground structure based low frequency power transmission system according to claim 2, characterized in that the primary and secondary windings of the Z-transformer (151) constitute a first star connection structure.
4. The ground based low frequency power transmission system of claim 3, characterized in that the connection node of the primary and secondary windings of the Z-type transformer (151) is the neutral node of the first star connection.
5. The low frequency power transmission system based on the grounding structure according to any one of claims 2 to 4, characterized in that the primary winding and the secondary winding of the Z-type transformer (151) are both three-phase windings.
6. A low frequency power transmission system based on a grounding structure according to claim 1, characterized in that said grounding device (15) further comprises: three interconnected reactors (155) and a second resistor (154), a first end of the second resistor (154) being connected to a connection node of the three interconnected reactors (155), a second end of the second resistor (154) being connected to ground.
7. A low frequency power transmission system based on a grounding structure according to claim 6 characterized in that said three interconnected reactors (155) constitute a second star connection.
8. A ground structure based low frequency power transmission system according to claim 7, characterized in that the connection node of said three interconnected reactors (155) is the neutral node of said second star connection.
9. The ground based low frequency transmission system according to any of claims 6 to 8, characterized in that said three interconnected reactors (155) are connected to the secondary winding of said first converter transformer (14) and to the input of said AC/AC converter (12), respectively.
10. The ground structure based low frequency power transmission system of claim 1, further comprising:
a first switching device (17) connected to the primary windings of the alternating current grid (11) and the first converter transformer (14), respectively;
-second switching devices (18) connected to the primary windings of the low frequency grid (13) and the second converter transformer (16), respectively.
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| CN202121592222.9U CN215267650U (en) | 2021-07-13 | 2021-07-13 | Low-frequency power transmission system based on grounding structure |
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Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2024098381A1 (en) * | 2022-11-11 | 2024-05-16 | 国网智能电网研究院有限公司 | Alternating current power transmission system |
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Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2024098381A1 (en) * | 2022-11-11 | 2024-05-16 | 国网智能电网研究院有限公司 | Alternating current power transmission system |
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