WO2021146995A1

WO2021146995A1 - Apparatus and method of power transmission

Info

Publication number: WO2021146995A1
Application number: PCT/CN2020/073767
Authority: WO
Inventors: Xiaobo Yang; Hailian XIE; Chunming YUAN; Mats Andersson
Original assignee: Abb Power Grids Switzerland Ag
Priority date: 2020-01-22
Filing date: 2020-01-22
Publication date: 2021-07-29
Also published as: CN114930666A

Abstract

An apparatus and method of power transmission, the apparatus of power transmission implemented at a transmitting station comprises: a first converter (210) configured to convert a first three-phase AC component of a 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 an AC transmission line (150). The apparatus of power transmission implemented at a receiving station comprises: 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 converter (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 limitation from the space charge accumulation of cable system can be eliminated, and an improvement of the power transmission capacity can be facilitated.

Description

APPARATUS AND METHOD OF POWER TRANSMISSION

FIELD

Embodiments of present disclosure generally relate to power transmission, and more specifically, to an apparatus and method of power transmission on an AC transmission line.

BACKGROUND

Nowadays, the electricity demand keeps increasing, but it is expensive to build a new AC transmission line and sometimes even difficult to find a corridor. As an alternative way, upgrading an existing AC distribution network to a DC system may increase the transmitted power with lower investment compared to building a new AC transmission line.

Currently, there are two basic converter solutions to realize AC to DC line conversion. One solution is a bipole converter solution where a first DC line consists of three conductors in parallel, which is upgraded from one of double circuit transmission lines, and a second DC line also consists of three conductors in parallel, which is upgraded from the other of double circuit transmission lines. In this solution, it fully utilizes the transmission capability of all conductors. However, it is only applicable for upgrading of double circuit transmission lines. As another solution, for single circuit transmission line, two of the three conductors will operate as DC lines and the third conductor will be reserved as a neutral line. In this case, it has lower transmission capacity as the neutral line is idle during normal operation.

Further, there is also proposed a tripole converter solution where all the three conductors in one transmission line will be fully utilized to have the maximum boosting of power transmission capacity. However, the third pole can be considered as a monopole DC system with polarity reverse capability of both voltage and current. In this solution, thermal balance of all the three conductors is realized by proper control or by modulating of the DC current at every conductor, and thus conventional topologies of tripole converters are still complex.

All of the above solutions have a common drawback when they are applied for cable system upgrading. AC cable (e.g. XLPE cable) is usually designed for 50Hz or 60Hz frequency operation and there is no specific design consideration for space charge accumulation issue. The DC operation of AC cable may result in space charge accumulation and bring the risk of cable insulation breakdown.

SUMMARY

Embodiments of the present disclosure propose an improved solution of power transmission in an AC transmission line.

In a first aspect, there is provided an apparatus of power transmission. The apparatus comprises: a first converter configured to convert a first three-phase AC component of a 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 an AC transmission line.

In a second aspect, there is provided a method of power transmission. The method comprises: converting a first three-phase AC component of a 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 causing the second three-phase AC component to be transmitted in an AC transmission line.

In a third aspect, there is provided an apparatus of power transmission. The apparatus comprises: a second controller configured to cause a second three-phase AC component of a second frequency to be received from an AC transmission line; and a second converter coupled to the controller and configured to convert 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.

In a fourth aspect, there is provided a method of power transmission. The method comprises: causing a second three-phase AC component of a second frequency to be received from an 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.

According to embodiments of the present disclosure, a solution of power transmission can be provided for an AC distribution network when it 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 from the space charge accumulation can be eliminated, and an improvement of the power transmission capacity can be facilitated.

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

DESCRIPTION OF DRAWINGS

Drawings described herein are provided to further explain the present disclosure and constitute a part of the present disclosure. The example embodiments of the disclosure and the explanation thereof are used to explain the present disclosure, rather than to limit the present disclosure improperly.

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

FIG. 2 illustrates a simplified block diagram of an apparatus of power transmission implemented at a transmitting station in accordance with embodiments of the present disclosure;

FIG. 3 illustrates a schematic diagram of an example implementation of the AC distribution network in accordance with embodiments of the present disclosure;

FIG. 4 illustrates example voltage waveforms of the pole lines with trapezoid modulation;

FIG. 5 illustrates a simplified block diagram of an apparatus of power transmission implemented at a receiving station in accordance with embodiments of the present disclosure;

FIG. 6 illustrates a flowchart of a method of power transmission implemented at the transmitting station in accordance with embodiments of the present disclosure; and

FIG. 7 illustrates a flowchart of a method of power transmission implemented at the receiving station in accordance with embodiments of the present disclosure.

Throughout the drawings, the same or similar reference numerals represent the same or similar element.

DETAILED DESCRIPTION OF EMBODIEMTNS

Principles of the present disclosure will now be described with reference to several example embodiments shown in the drawings. Though example embodiments of the present disclosure are illustrated in the drawings, it is to be understood that the embodiments are described only to facilitate those skilled in the art in better understanding and thereby achieving the present disclosure, rather than to limit the scope of the disclosure in any manner.

The term "comprises" or "includes" and its variants are to be read as open terms that mean "includes, but is not limited to. " The term "or" is to be read as "and/or" unless the context clearly indicates otherwise. The term "based on" is to be read as "based at least in part on. " The term "being operable to" is to mean a function, an action, a motion or a state can be achieved by an operation induced by a user or an external mechanism. The term "one embodiment" and "an embodiment" are to be read as "at least one embodiment. " The term "another embodiment" is to be read as "at least one other embodiment. " The terms "first, " "second, " and the like may refer to different or same objects. Other definitions, explicit and implicit, may be included below. A definition of a term is consistent throughout the description unless the context clearly indicates otherwise.

Unless specified or limited otherwise, the terms "mounted, " "connected, " "supported, " and "coupled" and variations thereof are used broadly and encompass direct and indirect mountings, connections, supports, and couplings. Furthermore, "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 figures. Other definitions, explicit and implicit, may be included below.

FIG. 1 illustrates an example AC distribution network 100 in which embodiments of the present disclosure may be implemented. The network 100 includes a first converter station (for convenience, also referred to as a transmitting station) 120, a second converter station (for convenience, also referred to as a receiving station) 130 and an AC transmission line 150 located between the first and

second converter stations

120 and 130. The AC transmission line 150 is composed of a line 151 of Phase A (Pole 1) , a line 152 of Phase B (Pole 2) and a line 153 of Phase C (Pole 3) . It should be noted that the number of converter stations and that of AC transmission lines in the network 100 are not limited to the above example, and the network 100 may have more converter stations and more AC transmission lines. In FIG. 1, the AC transmission lines consist of a three phase AC transmission system, but it is also possible to be a multiphase AC transmission system other than three phases.

AC component 110 from an external source is processed at the first converter station 120 in such a manner that the power transmission capacity of the network 100 is increased. The processed AC component 110 is then transmitted in the AC transmission line 150 to the second converter station 130. Upon arriving at the second converter station 130, the processed AC component 110 is processed back to the original status 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 a bipole converter solution, a first DC line (Pole 1, for example, the line 150) consists of three conductors (for example, the lines 151-153) in parallel, which is upgraded from one of double circuit transmission lines, and a second DC line (Pole 2, not shown but similar with the line 150) also consists of three conductors in parallel, which is upgraded from the other of double circuit transmission lines. For single circuit transmission line, two of the three conductors will operate as DC lines (Pole 1 and Pole 2) and the third conductor will be reserved as a neutral line.

In such AC to DC line conversion solution, if the AC line is a cable system, the DC operation of AC transmission line for Pole 1 and Pole 2 may result in space charge accumulation and bring the risk of line insulation breakdown.

In a tripole converter solution, all the three conductors (for example, the lines 151-153) in one transmission line (for example, the line 150) will be fully utilized to have the maximum boosting of power transmission capacity. However, for the tripole converter solution, the existing high voltage DC (HVDC) converter technology cannot be applied directly. A typical tripole solution has been proposed, in which the third pole (for example, the line 153) can be considered as a monopole DC system with polarity reverse capability of both voltage and current. The converter for the third pole can be a conventional line commutate converter (LCC) with anti-parallel valves or anti-parallel thyristors within same valve, or can be a full bridge sub-module based modular multilevel converter (MMC) (FB-MMC) , clamped diode sub-module (CDSM) based MMC, or any other MMC technology with DC voltage reversing capability. In this solution, thermal balance of all the three conductors is realized by proper control or by modulating of the DC current at every conductor.

It can be seen that the tripole converter system may have maximum boosting of power transmission capacity. However, conventional topologies of tripole converters are still complex. For example, it is usually required that the third pole with additional converters.

In order to at least in part solve this and potential other problems, embodiments of the present disclosure provide an improved tripole solution where a LF-MVAC system is upgraded from a MVAC system by using AC/AC line conversion, instead of the AC to DC line conversion. With the low frequency three-phase AC/AC conversion, the limitation from the space charge accumulation of cable system can be eliminated, and an improvement of the power transmission capacity can be facilitated. It will be described in detail with reference to FIGs. 2-6.

FIG. 2 illustrates a simplified block diagram of an apparatus 200 of power transmission implemented at a transmitting station (for example, the first converter station 120 in FIG. 1) in accordance with embodiments of the present disclosure. In some embodiments, the apparatus 200 may be the first converter station 120 per se. 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 comprise a first converter 210 and a first controller 220. The first converter 210 may be configured to convert a first three-phase AC component (for example, the AC component 110 in FIG. 1) of a first frequency into a second three-phase AC component of a second frequency less than the first frequency. In some embodiments, the first converter 210 may convert the AC component in each phase of the three-phase from the first frequency to the second frequency. In some embodiments, the difference between two of the three phases may be about 120 degree.

In some embodiments of the present disclosure, the second frequency may be less than the first frequency and greater than a predetermined frequency. In some embodiments, the predetermined frequency may be set so as to cause the space charge accumulation effort to become insignificant. In some embodiments, the first frequency may be 50Hz or 60Hz, and the predetermined frequency may be higher than 0.01Hz. For example, the second frequency may be set to 10Hz. It should be noted that, the above value is merely for illustration, and any other suitable values can also be feasible.

In this way, the voltages at all the three pole lines are of low frequency AC voltage, and thus the limitation from space charge accumulation can be eliminated. Compared with 50Hz or 60Hz AC system, transmission capacity of low frequency AC system of the present solution can be increased due to less power losses.

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

M3C topology is essentially a direct AC/AC conversion topology. Compared with conventional indirect AC/AC solution which consists of two back to back (B2B) AC/DC converters, the M3C has less power electronics devices and thus has a lower cost. With the M3C topology, the converter topology can be simplified with a lower cost. Unless otherwise specified, the MMC discussed in the context refers to M3C.

In conventional tripole conversion, each converter station consists of a standard AC/DC converter and an additional pole, which requires complex coordinated control. In contrast, in the proposed MMC based conversion, the converter can be controlled as one converter system. Thus, a simple station level control can be achieved.

In case that a branch failure occurs, the branches of the MMC can be reduced, for example, the nine-branch M3C may be operated as a six-branch Hexverter. In this way, higher reliability can be attained, and a further increase of the MMC high availability also can be reached.

An example implementation of the apparatus 200 will be described with reference to FIG. 3 which illustrates a schematic diagram 300 of an example implementation of an AC distribution network in accordance with embodiments of the present disclosure.

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

In some embodiments, the power electronics devices used for the M3C can be IGBT, IGCT, IEGT or other full controlled power electronics device. In some embodiments, the sub-module in the M3C can be full bridge sub-module, CDSM or other submodule which voltage polarity on DC side can be reversed. In some embodiments, standard control of M3C may be employed.

Because of the modular structure and application of industrial IGBT modules, M3C has lower cost than conventional B2B converter.. It should be noted that M3C is merely an example, and any other suitable forms of MMC are also feasible.

In some embodiments of the present disclosure, the first converter 210 may be further configured to shape the second three-phase AC component into a trapezoid wave. In some embodiments, the first converter 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 in any other suitable waveforms.

FIG. 4 illustrates example voltage waveforms 400 of the pole lines with trapezoid modulation. As shown in FIG. 4, 410 represents a modulation waveform of Pole 1 (for example, the line 151 of Phase A) , 420 represents a modulation waveform of Pole 2 (for example, the line 152 of Phase B) , and 430 represents a modulation waveform of Pole 3 (for example, the line 153 of Phase C) . Components in two of the three phases differ about 120 degree in phases.

With the trapezoid modulation, high utilization of the thermal capability of the AC transmission line system can be achieved, and the voltage ramping rate can also be limited.

Returning to FIG. 2, the first controller 220 may be coupled to the first converter 210. The first controller 220 may be configured to cause the second three-phase AC component to be transmitted in an AC transmission line (for example, the line 150 in FIG. 1) . In some embodiments, the first controller 220 may comprises one or more switch circuits. In some embodiments, the first controller 220 may be implemented separately from the first converter 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 can be achieved in any other suitable forms.

FIG. 5 illustrates a simplified block diagram of an apparatus 500 of power transmission implemented at a receiving station (for example, the second converter station 130 in FIG. 1) in accordance with embodiments of the present disclosure. In some embodiments, the apparatus 500 may be the second converter station 120 per se. 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 comprise a second controller 510 and a second converter 520. The second controller 510 may be configured to cause a second three-phase AC component of a second frequency to be received from an AC transmission line (for example, the line 150 in FIG. 1) . In some embodiments, the second controller 510 may comprises one or more switch circuits. In some embodiments, the second controller 510 may be implemented separately from the second converter 520. In some embodiments, the second controller 510 may be integrated with the second converter 520. It should be noted that this is merely an example, and the second controller 510 can be achieved in any other suitable forms.

The second converter 520 may be coupled to the second controller and may be configured to convert the second three-phase AC component into a first three-phase AC component of a first frequency. In some embodiments, the second converter 520 may convert the AC component in each phase of the three-phase from the second frequency to the first frequency. In some embodiments, the difference between two of the three phases may be about 120 degree.

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

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

With reference to FIG. 3 again, the second converter 520 may be implemented together with the second controller 510 by a M3C 320. The M3C 320 has 3 input terminals and 3 output terminals, which makes M3C more suitable for grid application. For concise, other details about MMC are not repeated here. It should be noted that the second controller 510 and the second converter 520 may be implemented by a MMC in any suitable ways, and its details are omitted here so as to avoid confusing the present invention.

In some embodiments where the received second three-phase AC component is shaped into a trapezoid wave at the first converter station 120, the received second three-phase AC component may be reshaped at the second converter station 130 from the trapezoid wave to the original status, i.e., a sine wave. In these embodiments, the second converter 520 may be further 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 transmission. This will be described below with reference to FIGs. 6 and 7. FIG. 6 illustrates a flowchart of a method 600 of power transmission implemented at the transmitting station in accordance with embodiments of the present disclosure. For example, the method 600 may be performed at the first converter station 120 in FIG. 1. For the purpose of discussion, the method 600 will be described with reference to FIG. 1. It is to be understood that method 600 may further include additional blocks not shown and/or omit some shown blocks, and the scope of the present disclosure is not limited in this regard.

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

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 into the second three-phase AC component by at least one of a M3C or a Hexverter.

In some embodiments, the first converter station 120 may further shape the second three-phase AC component into a trapezoid 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 (for example, the line 150 in FIG. 1) . In this way, all of the three poles are still operated under AC voltage but with a relative low frequency (for example, lower than 50Hz or 60Hz) . Thus, the limitation from the space charge accumulation is eliminated. The method 600 corresponds to the above operations described about the apparatus 200, and thus other details are not repeated here.

FIG. 7 illustrates a flowchart of a method 700 of power transmission implemented at the receiving station in accordance with embodiments of the present disclosure. For example, the method 700 may be performed at the second converter station 130 in FIG. 1. For the purpose of discussion, the method 700 will be described with reference to FIG. 1. It is to be understood that method 700 may further include additional blocks not shown and/or omit some shown blocks, and 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 a second three-phase AC component of a second frequency to be received from an AC transmission line (for example, the line 150 in FIG. 1) .

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

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 into the first three-phase AC component by at least one of a M3C or a Hexverter.

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

Further, while operations are depicted in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Likewise, while several specific implementation details are contained in the above discussions, these should not be construed as limitations on the scope of the present 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 may 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 may also be implemented in multiple embodiments separately or in any suitable sub-combination.

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

An apparatus of power transmission comprising:

a first converter configured to convert a first three-phase AC component of a 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 an AC transmission line.
The apparatus of Claim 1, wherein the first converter is further configured to shape the second three-phase AC component into a trapezoid wave.
The apparatus of Claim 1, wherein the first frequency is 50Hz or 60Hz, and the predetermined frequency is higher than 0.01Hz.
The apparatus of Claim 1, wherein the first converter comprises at least one modular multilevel converter.
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.
A method of power transmission comprising:

converting a first three-phase AC component of a 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

causing the second three-phase AC component to be transmitted in an AC transmission line.
The method of Claim 6, wherein converting the first three-phase AC component into the second three-phase AC component further comprises:

shaping the second three-phase AC component into a trapezoid wave.
The method of Claim 6, wherein the first frequency is 50Hz or 60Hz, and the predetermined frequency is higher than 0.01Hz.
The method of Claim 6, wherein converting the first three-phase AC component into the second three-phase AC component comprises:

converting, by at least one modular multilevel converter, the first three-phase AC component into the second three-phase AC component.
The method of Claim 9, wherein converting the first three-phase AC component into the second three-phase AC component comprises:

converting, by at least one of a modular multilevel matrix converter or a hexagonal modular multilevel converter, the first three-phase AC component into the second three-phase AC component.
An apparatus of power transmission comprising:

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

a second converter coupled to the second controller and configured to convert 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.
The apparatus of Claim 11, wherein the second converter is further configured to shape the second three-phase AC component into a sine wave.
The apparatus of Claim 11, wherein the first frequency is 50Hz or 60Hz, and the predetermined frequency is higher than 0.01Hz.
The apparatus of Claim 11, wherein the second converter comprises at least one modular multilevel converter.
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.
A method of power transmission comprising:

causing a second three-phase AC component of a second frequency to be received from an 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.
The method of Claim 16, wherein converting the second three-phase AC component into the first three-phase AC component further comprises:

shaping the first three-phase AC component from a trapezoid wave into a sine wave.
The method of Claim 16, wherein the first frequency is 50Hz or 60Hz, and the predetermined frequency is higher than 0.01Hz.
The method of Claim 16, wherein converting the second three-phase AC component into the first three-phase AC component comprises:

converting, by at least one modular multilevel converter, the second three-phase AC component into the first three-phase AC component.
The method of Claim 19, wherein converting the second three-phase AC component into the first three-phase AC component comprises:

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