CN114930666A

CN114930666A - Apparatus and method for power transmission

Info

Publication number: CN114930666A
Application number: CN202080092877.1A
Authority: CN
Inventors: 杨晓波; 谢海莲; 苑春明; 马茨·安德森
Original assignee: Hitachi Energy Switzerland AG
Current assignee: Hitachi Energy Co ltd
Priority date: 2020-01-22
Filing date: 2020-01-22
Publication date: 2022-08-19
Also published as: WO2021146995A1

Abstract

An apparatus and method of power transmission, the apparatus of power transmission implemented in a transmitting station comprising: a first inverter (210) configured to convert the first three-phase AC component of the first frequency into 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 (220) coupled to the first converter (210) and configured to cause the second three-phase AC component to be transmitted in the AC transmission line (150). The apparatus for power transmission implemented in a receiving station comprises: a second controller (510) configured to cause a second three-phase AC component to be received from the AC transmission line (150); and a second controller (520) coupled to the second controller (510) and configured to convert the second three-phase AC component into the first three-phase AC component. In this way, the restriction of space charge accumulation of the cable system can be eliminated, and 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 on an AC transmission line.

Background

Today, power requirements are increasing, but the cost of building new AC transmission lines is high, and sometimes it is even difficult to find a channel. As an alternative, upgrading an existing AC distribution network to a DC system may increase the power transmitted with a lower investment than building a new AC transmission line.

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

In addition, a triode converter solution is proposed in which all three conductors in one transmission line are fully utilized to maximize the power transmission capacity. However, the third pole may be considered a unipolar DC system with polarity reversal capability of both voltage and current. In this solution, the thermal balance of all three conductors is achieved by suitably controlling or regulating the DC current on each conductor, and therefore the traditional topology of a triode converter is still complex.

All the above solutions have a common drawback when they are used for cable system upgrades. AC cables (e.g., XLPE cables) are typically designed for 50Hz or 60Hz frequency operation and there are no particular design considerations for space charge accumulation issues. DC operation of AC cables may lead to space charge build-up and risk of cable insulation breakdown.

Disclosure of 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 device comprises: a first inverter configured to convert the first three-phase AC component of the first frequency into 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 the AC transmission line.

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

In a third aspect, an apparatus for power transmission is provided. The device includes: a second controller configured to cause a second three-phase AC component at a second frequency to be received from the AC transmission line; and a second inverter coupled to the second controller and configured to convert the second three-phase AC component to the first three-phase AC component at a first frequency, the second frequency being less than the first frequency and greater than a predetermined frequency.

In a fourth aspect, a method of power transmission is provided. The method comprises the following steps: 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 the first three-phase AC component of the 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 distribution network may be provided in the case where the AC distribution network is upgraded or retrofitted from a Medium Voltage AC (MVAC) system to a low frequency medium voltage AC (LF-MVAC) system. With the present solution, the limitation of space charge accumulation can be eliminated, and improvement of power transmission capacity can be promoted.

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

Drawings

The drawings described herein are provided to further illustrate and form a part of the disclosure. The exemplary embodiments of the present disclosure and their description are intended to be illustrative of the disclosure, rather than to unduly limit the disclosure.

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

fig. 2 shows a simplified block diagram of an apparatus for power transmission implemented at a transmitting station in accordance with an embodiment of the present disclosure;

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

FIG. 4 shows an example voltage waveform for an epipolar line with trapezoidal modulation;

fig. 5 shows a simplified block diagram of an apparatus for power transmission implemented at a receiving station in accordance with an embodiment of the present disclosure;

fig. 6 shows a flow diagram of a method of power transmission implemented at a transmitting station in accordance with an embodiment of the present disclosure; and

fig. 7 shows a flow chart of a method of power transmission implemented at a receiving station in accordance with an embodiment of the disclosure.

Throughout the drawings, the same or similar reference numerals denote the same or similar elements.

Detailed Description

The principles of the present disclosure will now be described with reference to a few exemplary embodiments thereof as illustrated in the accompanying drawings. While the illustrative embodiments of the present disclosure have been shown in the drawings, it will be understood that they have been described herein only to assist those skilled in the art in better understanding and in carrying out the disclosure, and are not intended to limit the scope of the disclosure in any way.

The term "comprising" or "comprises" and variations thereof is to be understood as an open term meaning "including but not limited to". The term "or" should be understood as "and/or" unless the context clearly indicates otherwise. The term "based on" is to be understood as "based at least in part on". The term "operable to" refers to an operation that may be directed by a user or an external mechanism to achieve a function, action, motion, or state. The terms "one embodiment" and "an embodiment" should be understood as "at least one embodiment". The term "another embodiment" should be understood as "at least one other embodiment". The terms "first," "second," and the like may refer to different or the same objects. Other definitions, both explicit and implicit, may be included below. The definitions of the terms are consistent throughout the description, unless the context clearly dictates otherwise.

Unless specified or limited otherwise, the terms "mounted," "connected," "supported," and "coupled" and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, "connected" and "coupled" are not restricted to physical or mechanical connections or couplings. In the description below, like reference numerals and labels are used to describe the same, similar or corresponding parts in the drawings. Other definitions, both explicit and implicit, may be included below.

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 transmitting station for convenience) 120, a second converter station (also referred to as a receiving station for convenience) 130, and an AC transmission line 150 between the first converter station 120 and the second converter station 130. AC transmission line 150 includes line 151 of phase a (pole 1), line 152 of phase B (pole 2), and line 153 of 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 fig. 1, the AC transmission line includes a three-phase AC transmission system, but may be a multi-phase AC transmission system other than three phases.

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 to the second converter station 130 in the AC transmission line 150. Upon 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 described above, in a bipolar inverter solution, the first DC line (pole 1, e.g., line 150) includes three conductors (e.g., lines 151 and 153) in parallel, which are upgraded from one of the dual loop transmission lines, and the second DC line (pole 2, not shown, but similar to line 150) also includes three conductors in parallel, which are upgraded from the other of the dual loop transmission lines. For a single loop transmission line, two of the three wires would operate as DC lines (pole 1 and pole 2) and the third wire would remain as a neutral line.

In such AC to DC line conversion solutions, if the AC line is a cable system, DC operation of the AC transmission lines for poles 1 and 2 may result in space charge build-up and risk of line insulation breakdown.

In a tri-pole converter solution, all three conductors (e.g., lines 151 and 153) in one transmission line (e.g., line 150) will be fully utilized to maximize power transmission capacity. However, for a three-pole converter solution, the existing high voltage dc (hvdc) converter technology cannot be directly applied. A typical three-pole solution has been proposed, where the third pole (e.g., line 153) can be considered a unipolar DC system with polarity reversal capability of both voltage and current. The converter for the third pole may be a conventional Line Commutated Converter (LCC) with anti-parallel valves or anti-parallel thyristors in the same valve, or may also be a Modular Multilevel Converter (MMC) based on full-bridge submodules (FB-MMC), a Clamp Diode Submodule (CDSM) based MMC or any other MMC technology with DC voltage commutation capability. In this solution, the 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 converter system can maximize the power transmission capacity. However, the conventional topology of the three-pole converter is still complicated. For example, a third pole is usually required with an additional inverter.

To address this and potentially other issues, at least in part, embodiments of the present disclosure provide an improved three-pole solution in which upgrades are made from an MVAC system to a LF-MVAC system by using AC/AC line conversion rather than AC to DC line conversion. By low-frequency three-phase AC/AC conversion, the limitation of space charge accumulation of the cable system can be eliminated, and an increase in power transmission capacity can be promoted. This will be described in detail with reference to fig. 2 to 6.

Figure 2 shows a simplified block diagram of an apparatus 200 for power transmission implemented at a transmitting station (e.g., the first converter station 120 of figure 1) according to an embodiment of the present disclosure. In some embodiments, the arrangement 200 may be the first converter station 120 itself. In some alternative embodiments, the arrangement 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 (e.g., the AC component 110 in fig. 1) at a first frequency to a second three-phase AC component at a second frequency, wherein the second frequency is less than the first frequency. In some embodiments, the first inverter 210 may convert the AC component of each of the three phases from a first frequency to a second frequency. In some embodiments, the difference between two of the three phases may be about 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 space charge accumulation effects become insignificant. In some embodiments, the first frequency may be 50Hz or 60Hz, and the predetermined frequency may be higher than 0.01 Hz. For example, the second frequency may be set to 10 Hz. It should be noted that the above values are for illustration only, and any other suitable values may be possible.

In this way, the voltages on all three-pole lines are low-frequency AC voltages, and therefore the limitation of space charge accumulation can be eliminated. The transmission capacity of the low frequency AC system of the present solution can be increased (due to less power loss) compared to 50Hz or 60Hz AC systems.

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

In conventional three pole conversion each converter station comprises a standard AC/DC converter and an additional pole, which requires a complex coordination control. In contrast, in the proposed MMC based conversion the converter may be controlled as a converter system. Thus, simple station-level control can be achieved.

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

An example implementation of the apparatus 200 will be described with reference to fig. 3, fig. 3 showing 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 with the first controller 220 through M3C 310. M3C 310 has 3 input terminals and 3 output terminals, which makes M3C more suitable for grid applications.

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

Due to the modular structure and application of the industrial IGBT module, M3C is less costly than the conventional B2B converter. It should be noted that M3C is only an example and that any other suitable form of MMC is feasible.

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 into 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 for an epipolar line with trapezoidal modulation. As shown in fig. 4, 410 represents the modulation waveform for pole 1 (e.g., line 151 for phase a), 420 represents the modulation waveform for pole 2 (e.g., line 152 for phase B), and 430 represents the modulation waveform for pole 3 (e.g., line 153 for phase C). The components of two of the three phases differ in phase by about 120 degrees.

By ladder 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, a first controller 220 may be coupled to the first inverter 210. The first controller 220 may be configured to cause the second three-phase AC component to be transmitted in an AC transmission line (e.g., 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 converter 210, for example in the same MMC. It should be noted that this is merely 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 (e.g., the second converter station 130 in fig. 1) according to an embodiment of the present disclosure. In some embodiments, the arrangement 500 may be the second converter station 120 itself. In some alternative embodiments, the arrangement 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 at the second frequency is received from an AC transmission line (e.g., 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 merely an example, and the second controller 510 may be implemented in any other suitable form.

A 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 at 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 about 120 degrees.

In some embodiments of the present disclosure, the second converter 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 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 by M3C 320 along with the second controller 510. M3C 320 has 3 input terminals and 3 output terminals, which makes M3C more suitable for grid applications. For the sake of brevity, further details regarding the MMC are not repeated here. It should be noted that the second controller 510 and the second converter 520 may be implemented by an MMC in any suitable manner, and details thereof are omitted herein to avoid obscuring the 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 reshaped from the trapezoidal wave into the original state, i.e., a sine wave, at the second converter station 130. 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 recovered at the second converter station 130.

Accordingly, embodiments of the present disclosure also provide methods of power transfer. This will be described below with reference to fig. 6 and 7. Fig. 6 illustrates a flow diagram of a method 600 of power transmission implemented at a transmitting station in accordance with an embodiment of the disclosure. The method 600 may be performed, for example, at the first converter station 120 in fig. 1. For purposes of discussion, the 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 the scope of the present disclosure is not limited in this respect.

As shown, 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 at block 610. 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 50Hz or 60Hz, 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 into the second three-phase AC component by 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 via 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 into a quasi-square wave.

At block 620, the first converter station 120 causes the second three-phase AC component to be transmitted in an AC transmission line (e.g., line 150 in fig. 1). In this way, all three poles are still operating at AC voltage, but with a relatively low frequency (e.g., below 50Hz or 60 Hz). Thus, 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 flow diagram of a method 700 of power transmission implemented at a receiving station in accordance with an embodiment of the disclosure. The method 700 may be performed, for example, at the second converter station 130 in fig. 1. For discussion purposes, the 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 respect.

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

At block 720, the second converter station 130 converts the second three-phase AC component to a first three-phase AC component at a 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 50Hz or 60Hz, 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 into the first three-phase AC component by 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 via 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.

Further, while operations are described in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in the order shown, or that all illustrated operations be performed, to achieve desirable results. In some cases, multitasking and parallel processing may be advantageous. Also, while several specific implementation details are included in the above discussion, 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 separate embodiments can also be implemented in combination in a single embodiment. In another aspect, 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

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.