CN114930666A - Apparatus and method for power transmission - Google Patents

Abstract

An apparatus and method for power transmission, the apparatus for power transmission implemented in a sending station comprising: a first inverter (210) configured to convert a first three-phase AC component of a first frequency to a second frequency of a second three-phase AC component, the second frequency being less than the first frequency and greater than the predetermined frequency; and a first controller (220) coupled to the first inverter (210) and configured to cause the second three-phase AC component at transmission in the AC transmission line (150). The apparatus for power transmission implemented in the receiving station includes: a second controller (510) configured to cause the second three-phase AC component to be received from the AC transmission line (150); and a second controller (520) coupled to A second controller (510) is also configured to convert the second three-phase AC component to the first three-phase AC component. In this way, the limitation of space charge accumulation of the cable system can be eliminated, and the improvement of the power transmission capacity can be promoted.

Description

Apparatus and method for power transmission

technical field

Embodiments of the present disclosure relate generally to power transmission, and more particularly, to an apparatus and method of power transmission over an AC transmission line.

Background technique

Today, electricity demand is constantly increasing, but building new AC transmission lines is expensive and sometimes even difficult to find. As an alternative, upgrading an existing AC distribution network to a DC system can increase the power transmitted at a lower cost than building new AC transmission lines.

Currently, there are two basic inverter solutions for AC to DC line conversion. One solution is a bipolar inverter solution, where the first DC line consists of three conductors in parallel (the first DC line is upgraded from one of the dual loop transmission lines), and the second DC line Three conductors in parallel are also included (this second DC line is upgraded from the other line in the dual loop transmission line). In this solution, it takes full advantage of the transmission capacity of all wires. However, this solution is only applicable to the upgrade of dual loop transmission lines. As another solution, for a single loop transmission line, two of the three conductors would operate as a DC line, and the third would remain as a neutral line. In this case, since the neutral line is idle during normal operation, its transmission capacity is low.

In addition, a three-pole inverter solution is proposed, in which all three conductors in a transmission line will be fully utilized to maximize the power transmission capacity. However, the third pole can be viewed as a unipolar DC system with polarity reversal capability for both voltage and current. In this solution, the thermal balance of all three wires is achieved by appropriately controlling or regulating the DC current on each wire, and thus the traditional topology of a three-pole inverter remains complex.

All of the above solutions have a common disadvantage when they are used for cable system upgrades. AC cables (eg, XLPE cables) are typically designed for 50Hz or 60Hz frequency operation and have no special design considerations for space charge accumulation issues. DC operation of AC cables can lead to space charge accumulation and the risk of breakdown of cable insulation.

SUMMARY OF THE INVENTION

Embodiments of the present disclosure propose improved solutions for power transmission in AC transmission lines.

In a first aspect, an apparatus for power transmission is provided. The apparatus includes: a first inverter configured to convert a first three-phase AC component of a first frequency to a second three-phase AC component of a second frequency, the second frequency being less than the first frequency and greater than a predetermined frequency; and A first controller coupled to the first inverter and configured to transmit the second three-phase AC component in the AC transmission line.

In a second aspect, a method of power transmission is provided. The method includes: converting a first three-phase AC component of a first frequency to a second three-phase AC component of a second frequency, the second frequency being less than the first frequency and greater than a predetermined frequency; transmission in the transmission line.

In a third aspect, an apparatus for power transmission is provided. The apparatus includes: a second controller configured to receive a second three-phase AC component of a second frequency from the AC transmission line; and a second inverter coupled to the second controller and configured to convert the second three-phase AC component The phase AC component is converted into a first three-phase AC component of a first frequency, and the second frequency is less than the first frequency and greater than a predetermined frequency.

In a fourth aspect, a method of power transmission is provided. The method includes: receiving a second three-phase AC component of a second frequency from the AC transmission line; and converting the second three-phase AC component to a first three-phase AC component of a first frequency, the second frequency being less than the first frequency and greater than the predetermined frequency.

According to embodiments of the present disclosure, a power transmission solution for an AC power distribution network may be provided in the event that the AC power distribution network is upgraded or retrofitted from a medium voltage AC (MVAC) system to a low frequency medium voltage AC (LF-MVAC) system . With this solution, the limitation of space charge accumulation can be eliminated, and the improvement of power transmission capacity can be promoted.

Other features and advantages of embodiments of the disclosure will also be apparent from the following description of specific embodiments, when read in conjunction with the accompanying drawings, which illustrate the principles of embodiments of the disclosure.

Description of drawings

The accompanying drawings described herein are provided to further illustrate and constitute a part of this disclosure. The example embodiments of the present disclosure and their descriptions are provided to illustrate the present disclosure and not to unduly limit the present disclosure.

FIG. 1 illustrates an example AC power distribution network in which embodiments of the present disclosure may be implemented;

2 shows a simplified block diagram of an apparatus for power transmission implemented at a sending station according to an embodiment of the present disclosure;

3 shows a schematic diagram of an example implementation of an AC power distribution network according to an embodiment of the present disclosure;

FIG. 4 shows example voltage waveforms of polar lines with trapezoidal modulation;

5 shows a simplified block diagram of an apparatus for power transmission implemented at a receiving station according to an embodiment of the present disclosure;

FIG. 6 shows a flowchart of a method of power transmission implemented at a sending station according to an embodiment of the present disclosure; and

7 shows a flowchart of a method of power transfer implemented at a receiving station according to an embodiment of the present disclosure.

Throughout the drawings, the same or similar reference numbers refer to the same or similar elements.

Detailed ways

The principles of the present disclosure will now be described with reference to several example embodiments illustrated in the accompanying drawings. Although example embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that these embodiments are described only to assist those skilled in the art to better understand and thereby implement the present disclosure, and not to limit the present disclosure in any way. public scope.

The terms "including" or "comprising" and variations thereof should be understood as open-ended terms meaning "including but not limited to". The term "or" should be read as "and/or" unless the context clearly dictates otherwise. The term "based on" should be understood as "based at least in part on". The term "operable to" means that a function, action, movement, or state can be effected by manipulation directed by a user or external mechanism. The terms "one embodiment" and "an embodiment" should be understood to mean "at least one embodiment." The term "another embodiment" should be understood to mean "at least one other embodiment." The terms "first", "second", etc. may refer to different or the same objects. The following may include other explicit and implicit definitions. Definitions of terms are consistent throughout the description unless the context clearly dictates otherwise.

Unless otherwise specified or limited, the terms "mounted," "connected," "supported," and "coupled," and variations thereof, are used broadly and encompass both direct and indirect mounting, connecting, supporting, and coupling. Furthermore, "connected" and "coupled" are not limited to physical or mechanical connections or couplings. In the following description, like reference numerals and labels are used to describe the same, similar, or corresponding parts of the drawings. The following may include other explicit and implicit definitions.

FIG. 1 illustrates an example AC power distribution network 100 in which embodiments of the present disclosure may be implemented. The network 100 includes a first converter station (also referred to as a sending station for convenience) 120, a second converter station (also referred to as a receiving station for convenience) 130, and located at the first converter station 120 and the second AC transmission line 150 between converter stations 130 . The AC transmission line 150 includes a line 151 for phase A (pole 1), a line 152 for phase B (pole 2), and a line 153 for phase C (pole 3). It should be noted that the number of converter stations and the number of AC transmission lines in the network 100 are not limited to the above examples, and the network 100 may have more converter stations and more AC transmission lines. In Figure 1, the AC transmission line includes a three-phase AC transmission system, but may also be a multi-phase AC transmission system other than three-phase.

The AC component 110 from the external source is processed at the first converter station 120 in a manner that increases the power transmission capacity of the network 100 . The processed AC component 110 is then transmitted in the AC transmission line 150 to the second converter station 130 . After reaching the second converter station 130 , the processed AC component 110 is processed back to the original state at the second converter station 130 . In some embodiments, the AC component may be at least one of a voltage component or a current component.

As mentioned above, in the bipolar inverter solution, the first DC line (pole 1, eg, line 150) includes three conductors in parallel (eg, lines 151-153), the first DC line is derived from the double circuit One of the transmission lines is upgraded from the dual loop transmission line, and the second DC line (pole 2, not shown, but similar to line 150) also includes three conductors in parallel Another line was upgraded from. For a single loop transmission line, two of the three conductors will operate as DC lines (pole 1 and pole 2) and the third conductor will remain as neutral.

In this AC to DC line conversion solution, if the AC line is a cable system, the DC operation of the AC transmission line for pole 1 and pole 2 may lead to space charge accumulation and risk of line insulation breakdown .

In a three-pole inverter solution, all three conductors (eg, lines 151-153) in a transmission line (eg, line 150) will be fully utilized to maximize power transmission capacity. However, for three-pole inverter solutions, existing high-voltage DC (HVDC) inverter technology cannot be directly applied. A typical three-pole solution has been proposed, where the third pole (eg, line 153 ) can be viewed as a unipolar DC system with polarity reversal capability for both voltage and current. The converter for the third pole can be a conventional line-commutated converter (LCC) with anti-parallel valves or anti-parallel thyristors within the same valve, or it can be a modular multi-power converter based on full-bridge sub-modules. Flat-Line Converter (MMC) (FB-MMC), Clamping Diode Sub-Module (CDSM) based MMC, or any other MMC technology with DC voltage commutation capability. In this solution, thermal balance of all three wires is achieved by appropriately controlling or regulating the DC current on each wire.

It can be seen that the three-pole inverter system can maximize the power transmission capacity. However, the traditional topology of the three-pole converter is still complicated. For example, an additional inverter is often required for the third pole.

To at least partially address this and potentially other problems, embodiments of the present disclosure provide an improved three-pole solution in which an MVAC system is upgraded by using AC/AC line conversion instead of AC to DC line conversion to the LF-MVAC system. Through low frequency three-phase AC/AC conversion, the limitation of space charge accumulation in the cable system can be eliminated, and the increase of power transmission capacity can be facilitated. This will be described in detail with reference to FIGS. 2 to 6 .

FIG. 2 shows a simplified block diagram of an apparatus 200 for power transmission implemented at a sending station (eg, the first converter station 120 in FIG. 1 ) according to an embodiment of the present disclosure. In some embodiments, the apparatus 200 may be the first converter station 120 itself. In some alternative embodiments, the apparatus 200 may be a component of the first converter station 120 .

As shown in FIG. 2 , the apparatus 200 may include a first inverter 210 and a first controller 220 . The first inverter 210 may be configured to convert a first three-phase AC component of a first frequency (eg, AC component 110 in FIG. 1 ) to a second three-phase AC component of a second frequency, where the second frequency less than the first frequency. In some embodiments, the first inverter 210 may convert the AC component of each of the three phases from the first frequency to the second frequency. In some embodiments, the difference between two of the three phases may be approximately 120 degrees.

In some embodiments of the present disclosure, the second frequency may be less than the first frequency and greater than the predetermined frequency. In some embodiments, the predetermined frequency may be set such that the effect of space charge accumulation becomes insignificant. In some embodiments, the first frequency may be 50 Hz or 60 Hz, and the predetermined frequency may be higher than 0.01 Hz. For example, the second frequency may be set to 10Hz. It should be noted that the above values are for illustration only and any other suitable values may also be possible.

In this way, the voltage on all three-pole lines is a low frequency AC voltage, and thus the limitation of space charge accumulation can be removed. Compared to a 50Hz or 60Hz AC system, the transmission capacity of the low frequency AC system of this solution can be increased (due to less power loss).

Furthermore, in some embodiments of the present disclosure, the first converter 210 may include at least one modular multilevel converter (MMC). In some embodiments, for example, the first converter 210 may include at least one of a modular multilevel matrix converter (M3C) or a hexagonal modular multilevel converter (Hexverter).

The M3C topology is essentially a direct AC/AC conversion topology. Compared to the traditional indirect AC/AC solution consisting of two back-to-back (B2B) AC/DC converters, the M3C has less power electronics and therefore lower cost. Using the M3C topology, the converter topology can be simplified at a lower cost. Unless otherwise specified, MMC discussed in this context refers to M3C.

In conventional three-pole conversion, each converter station includes standard AC/DC converters and additional poles, which requires complex coordinated control. In contrast, in the proposed MMC-based conversion, the inverters can be controlled as one inverter system. Therefore, simple station-level control can be realized.

In the event of a branch failure, the branches of the MMC can be reduced, eg, a nine-branch M3C can operate as a six-branch Hexverter. In this way, higher reliability can be obtained, and the high availability of the MMC can be further improved.

An example implementation of the apparatus 200 will be described with reference to FIG. 3 , which shows a schematic diagram 300 of an example implementation of an AC power distribution network according to an embodiment of the present disclosure.

As shown in FIG. 3 , the first inverter 210 may be implemented together with the first controller 220 through the M3C 310 . The M3C 310 has 3 input terminals and 3 output terminals, which makes the M3C more suitable for grid applications.

In some embodiments, the power electronics for the M3C may be IGBTs, IGCTs, IEGTs, or other fully controlled power electronics. In some embodiments, the sub-modules in the M3C may be full-bridge sub-modules, CDSMs, or other sub-modules where the voltage polarity on the DC side may be reversed. In some embodiments, M3C's standard controls may be employed.

Due to the modular structure and application of industrial IGBT modules, M3C is less expensive than traditional B2B converters. It should be noted that M3C is just an example and any other suitable form of MMC is possible.

In some embodiments of the present disclosure, the first inverter 210 may also be configured to shape the second three-phase AC component into a trapezoidal wave. In some embodiments, the first inverter 210 may be configured to shape the second three-phase AC component to a quasi-square wave. It should be noted that the shape of the second three-phase AC component may be any other suitable waveform.

FIG. 4 shows an example voltage waveform 400 of polar lines with trapezoidal modulation. As shown in FIG. 4, 410 represents the modulation waveform for pole 1 (eg, line 151 of phase A), 420 represents the modulation waveform for pole 2 (eg, line 152 of phase B), and 430 represents the modulation waveform for pole 3 (eg, phase C line 153) of the modulation waveform. The components of two of the three phases are about 120 degrees out of phase.

With trapezoidal modulation, a high utilization of the thermal capacity of the AC transmission line system can be achieved, and the voltage ramp rate can also be limited.

Returning to FIG. 2 , the first controller 220 may be coupled to the first inverter 210 . The first controller 220 may be configured to transmit the second three-phase AC component in an AC transmission line (eg, line 150 in FIG. 1 ). In some embodiments, the first controller 220 may include one or more switching circuits. In some embodiments, the first controller 220 may be implemented separately from the first inverter 210 . In some embodiments, the first controller 220 may be integrated with the first inverter 210, eg, in the same MMC. It should be noted that this is just an example, and the first controller 220 may be implemented in any other suitable form.

FIG. 5 shows a simplified block diagram of an apparatus 500 for power transmission implemented at a receiving station (eg, the second converter station 130 in FIG. 1 ) according to an embodiment of the present disclosure. In some embodiments, the apparatus 500 may be the second converter station 120 itself. In some alternative embodiments, the apparatus 500 may be a component of the second converter station 120 .

As shown in FIG. 5 , the apparatus 500 may include a second controller 510 and a second inverter 520 . The second controller 510 may be configured such that the second three-phase AC component of the second frequency is received from the AC transmission line (eg, line 150 in FIG. 1 ). In some embodiments, the second controller 510 may include one or more switching circuits. In some embodiments, the second controller 510 may be implemented separately from the second inverter 520 . In some embodiments, the second controller 510 may be integrated with the second inverter 520 . It should be noted that this is just an example, and the second controller 510 may be implemented in any other suitable form.

The second inverter 520 may be coupled to the second controller and may be configured to convert the second three-phase AC component to the first three-phase AC component of the first frequency. In some embodiments, the second inverter 520 may convert the AC component of each of the three phases from the second frequency to the first frequency. In some embodiments, the difference between two of the three phases may be approximately 120 degrees.

In some embodiments of the present disclosure, the second frequency may be less than the first frequency and greater than the predetermined frequency. In some embodiments, the predetermined frequency may be set such that the effect of space charge accumulation becomes insignificant. In some embodiments, the first frequency may be 50 Hz or 60 Hz, and the predetermined frequency may be higher than 0.01 Hz. For example, the second frequency may be set to 10Hz. It should be noted that the above values are for illustration only and any other suitable values may also be possible.

In some embodiments of the present disclosure, the second inverter 520 may include at least one MMC. In some embodiments, for example, the second inverter 520 may include at least one of M3C or Hexverter.

As discussed with respect to the first converter 210, with the MMC topology, the converter topology can be simplified at a lower cost, and simple station-level control can be achieved with higher reliability and higher MMC high availability.

Referring again to FIG. 3 , the second inverter 520 may be implemented together with the second controller 510 through the M3C 320 . The M3C320 has 3 input terminals and 3 output terminals, which makes the M3C more suitable for grid applications. For the sake of brevity, other details about MMC are not repeated here. It should be noted that the second controller 510 and the second inverter 520 may be implemented in any suitable manner by MMC, and details thereof are omitted here to avoid obscuring the present disclosure.

In some embodiments, where the received second three-phase AC component is shaped into a trapezoidal wave at the first converter station 120 , the received second three-phase AC component may be at the second converter station 130 Reshape from a trapezoidal wave to its original state, a sine wave. In these embodiments, the second inverter 520 may also be configured to shape the second three-phase AC component into a sine wave. In this way, the AC component 110 is restored at the second converter station 130 .

Accordingly, embodiments of the present disclosure also provide methods of power transmission. This will be described below with reference to FIGS. 6 and 7 . FIG. 6 shows a flowchart of a method 600 of power transmission implemented at a sending station according to an embodiment of the present disclosure. For example, the method 600 may be performed at the first converter station 120 in FIG. 1 . For discussion purposes, method 600 will be described with reference to FIG. 1 . It should be understood that method 600 may also include additional blocks not shown and/or omit some of the blocks shown, and that the scope of the present disclosure is not limited in this regard.

As shown, at block 610, the first converter station 120 converts the first three-phase AC component at the first frequency to the second three-phase AC component at the second frequency. In some embodiments, the second frequency is less than the first frequency and greater than the predetermined frequency. In some embodiments, the first frequency may be 50 Hz or 60 Hz, and the predetermined frequency may be higher than 0.01 Hz.

In some embodiments, the first converter station 120 may convert the first three-phase AC component to the second three-phase AC component through at least one MMC. In some embodiments, the first converter station 120 may convert the first three-phase AC component to the second three-phase AC component through at least one of M3C or Hexverter.

In some embodiments, the first converter station 120 may also shape the second three-phase AC component into a trapezoidal wave. In some embodiments, the first converter station 120 may shape the second three-phase AC component to a quasi-square wave.

At block 620, the first converter station 120 transmits the second three-phase AC component in an AC transmission line (eg, line 150 in FIG. 1). In this way, all three poles still operate at AC voltage, but at a relatively low frequency (eg, below 50 Hz or 60 Hz). Therefore, the limitation of space charge accumulation is eliminated. The method 600 corresponds to the operations described above with respect to the apparatus 200, and thus other details are not repeated here.

FIG. 7 shows a flowchart of a method 700 of power transmission implemented at a receiving station according to an embodiment of the present disclosure. For example, the method 700 may be performed at the second converter station 130 in FIG. 1 . For discussion purposes, method 700 will be described with reference to FIG. 1 . It should be understood that method 700 may also include additional blocks not shown and/or omit some of the blocks shown, and that the scope of the present disclosure is not limited in this regard.

As shown in FIG. 7, at block 710, the second converter station 130 causes the second three-phase AC component of the second frequency to be received from the AC transmission line (eg, line 150 in FIG. 1).

At block 720, the second converter station 130 converts the second three-phase AC component to the first three-phase AC component at the first frequency. In some embodiments, the second frequency may be less than the first frequency and greater than the predetermined frequency. In some embodiments, the first frequency may be 50 Hz or 60 Hz, and the predetermined frequency may be higher than 0.01 Hz.

In some embodiments, the second converter station 130 may convert the second three-phase AC component to the first three-phase AC component through at least one MMC. In some embodiments, the second converter station 130 may convert the second three-phase AC component to the first three-phase AC component through at least one of M3C or Hexverter.

In some embodiments, the second converter station 130 may also shape the first three-phase AC component from a trapezoidal wave to a sine wave. In this way, the AC component 110 is recovered at the second converter station 130 . The method 700 corresponds to the operations described above with respect to the apparatus 500, and thus other details are not repeated here.

Additionally, although operations are described in a particular order, this should not be construed as requiring that the operations be performed in the particular order or sequence shown, or that all operations shown be performed to achieve desirable results. In some cases, multitasking and parallel processing may be advantageous. Likewise, although the above discussion contains several specific implementation details, these should not be construed as limitations on the scope of the disclosure, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in the context of different embodiments can also be implemented in combination in a single embodiment. On the other hand, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination.

Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.

Claims (20)

1. An apparatus for power transmission, the apparatus comprising:

a first inverter configured to convert a first three-phase AC component of a first frequency to a second three-phase AC component of a second frequency, the second frequency being less than the first frequency and greater than a predetermined frequency; and

a first controller coupled to the first converter and configured to cause the second three-phase AC component to be transmitted in an AC transmission line.

2. The apparatus of claim 1, wherein the first inverter is further configured to shape the second three-phase AC component into a trapezoidal wave.

3. The apparatus of claim 1, wherein the first frequency is 50Hz or 60Hz, and the predetermined frequency is higher than 0.01 Hz.

4. The apparatus of claim 1, wherein the first converter comprises at least one modular multilevel converter.

5. The apparatus of claim 4, wherein the first converter comprises at least one of a modular multilevel matrix converter or a hexagonal modular multilevel converter.

6. A method of power transmission, the method comprising:

converting a first three-phase AC component of a first frequency to a second three-phase AC component of a second frequency, the second frequency being less than the first frequency and greater than a predetermined frequency; and

transmitting the second three-phase AC component in an AC transmission line.

7. The method of claim 6, wherein converting the first three-phase AC component to the second three-phase AC component further comprises:

shaping the second three-phase AC component into a trapezoidal wave.

8. The method according to claim 6, wherein the first frequency is 50Hz or 60Hz and the predetermined frequency is higher than 0.01 Hz.

9. The method of claim 6, wherein converting the first three-phase AC component to the second three-phase AC component comprises:

converting the first three-phase AC component to the second three-phase AC component by at least one modular multilevel converter.

10. The method of claim 9, wherein converting the first three-phase AC component to the second three-phase AC component comprises:

converting the first three-phase AC component to the second three-phase AC component by at least one of a modular multilevel matrix converter or a hexagonal modular multilevel converter.

11. An apparatus for power transmission, the apparatus comprising:

a second controller configured to cause a second three-phase AC component at a second frequency to be received from an AC transmission line; and

a second inverter coupled to the second controller and configured to convert the second three-phase AC component to a first three-phase AC component at a first frequency, the second frequency being less than the first frequency and greater than a predetermined frequency.

12. The apparatus of claim 11, wherein the second inverter is further configured to shape the second three-phase AC component into a sine wave.

13. The apparatus of claim 11, wherein the first frequency is 50Hz or 60Hz, and the predetermined frequency is higher than 0.01 Hz.

14. The apparatus of claim 11, wherein the second converter comprises at least one modular multilevel converter.

15. The apparatus of claim 14, wherein the second converter comprises at least one of a modular multilevel matrix converter and a hexagonal modular multilevel converter.

16. A method of power transmission, the method comprising:

causing a second three-phase AC component at a second frequency to be received from the AC transmission line; and

converting the second three-phase AC component into a first three-phase AC component of a first frequency, the second frequency being less than the first frequency and greater than a predetermined frequency.

17. The method of claim 16, wherein converting the second three-phase AC component to the first three-phase AC component further comprises:

shaping the first three-phase AC component from a trapezoidal wave to a sine wave.

18. The method of claim 16, wherein the first frequency is 50Hz or 60Hz, and the predetermined frequency is higher than 0.01 Hz.

19. The method of claim 16, wherein converting the second three-phase AC component to the first three-phase AC component comprises:

converting the second three-phase AC component to the first three-phase AC component by at least one modular multilevel converter.

20. The method of claim 19, wherein converting the second three-phase AC component to the first three-phase AC component comprises:

converting the second three-phase AC component to the first three-phase AC component by at least one of a modular multilevel matrix converter and a hexagonal modular multilevel converter.

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