CN113193585A - Back-to-back system and modularized flexible interconnection system - Google Patents
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
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Abstract
The invention discloses a back-to-back system and a modular flexible interconnection system, and relates to the technical field of power systems. The back-to-back system comprises a first AC/DC converter, a second AC/DC converter and at least one first DC/DC converter; one end of the first AC/DC converter or the second AC/DC converter is connected with an alternating current network, and the other end of the first AC/DC converter or the second AC/DC converter is connected with one end of the first DC/DC converter; the first AC/DC converter and the second AC/DC converter are used for constructing a medium-voltage direct-current bus section; the first DC/DC converter is used for adjusting the voltage between the first AC/DC converter and the second AC/DC converter. The first AC/DC converter and the second AC/DC converter can construct a medium-voltage direct-current bus section for future extension interfaces, and can be connected to other medium-voltage alternating-current power distribution systems to form a multi-terminal interconnection power distribution system or connected to a low-voltage direct-current power distribution system to form a multi-stage direct-current power distribution system.
Description
Technical Field
The invention relates to the technical field of power systems, in particular to a back-to-back system and a modular flexible interconnection system.
Background
With the long-term development and wide application of new energy, new materials, information technologies and power electronic technologies, the existing ac power distribution network faces huge challenges in distributed new energy access, diversified load and power demand, complicated tidal current balance and coordination control, stability of electric energy supply, high efficiency, economy and the like. On one hand, the traditional alternating current subarea power supply cannot realize closed-loop operation and meet the requirement of a high-density load area on power supply capacity, and the traditional alternating current system is easy to cause the exceeding of short-circuit current while improving the power supply capacity. On the other hand, with the rapid development of a direct-current distributed power supply, the increase of the proportion of direct-current electric loads, the increase of the use of energy storage devices and the improvement of the power supply quality requirement of sensitive loads, the alternating-current power supply has no absolute advantages in efficiency, reliability or power quality.
Therefore, with the rapid development and mature application of the flexible direct current technology in the high voltage transmission network, the flexible direct current distribution network will gradually become an important form for future energy internet power distribution.
Disclosure of Invention
The invention aims to provide a back-to-back system and a modularized flexible interconnection system, which are combined with the existing flexible direct-current distribution network technology, realize power fusion and mutual support among different power distribution areas through flexible interconnection among medium-voltage distribution network lines, meet the friendly access requirements of direct-current loads such as distributed power supplies and electric automobiles, and improve the regulation capability of a distribution network and the reliability of regional power supply.
To achieve the above object, an embodiment of the present invention provides a back-to-back system, including a first AC/DC converter, a second AC/DC converter, and at least one first DC/DC converter;
one end of the first AC/DC converter or the second AC/DC converter is connected with an alternating current network, and the other end of the first AC/DC converter or the second AC/DC converter is connected with one end of the first DC/DC converter;
the first AC/DC converter and the second AC/DC converter are used for constructing a medium-voltage direct-current bus section;
the first DC/DC converter is used for adjusting the voltage between the first AC/DC converter and the second AC/DC converter.
Preferably, the topology of the DC/DC converter includes:
an LC series resonance structure, an LC parallel resonance structure, an LLC series parallel resonance structure, a CLLC series resonance structure or a phase shift control structure.
Preferably, the topology of the ports of the first AC/DC converter and the second AC/DC converter connected to the alternating current grid includes:
a clamp type structure, an H-bridge cascade type structure, a modular multilevel structure, or a three-phase linear cascade structure.
The embodiment of the invention also provides a modular flexible interconnection system which comprises any one of the back-to-back systems and a plurality of long-term expansion modules;
the forward-expansion module comprises one end connected between the first AC/DC converter and the first DC/DC converter or between the second AC/DC converter and the first DC/DC converter;
the other end of the long-term expansion module is connected with an alternating current power grid or a direct current power grid.
Preferably, the forward expansion module comprises a plurality of second DC/DC converters, wherein at least one of the second DC/DC converters is connected to the first AC/DC converter or the second AC/DC converter, and at least one of the second DC/DC converters is connected to a direct current power grid.
Preferably, the forward-expansion module comprises a third AC/DC converter and a plurality of third DC/DC converters; wherein the content of the first and second substances,
at least one of the third DC/DC converters is connected to the first AC/DC converter or the second AC/DC converter, and at least one of the third DC/DC converters is connected to the third AC/DC converter;
the end of the third AC/DC converter not connected to the third DC/DC converter is connected to an AC power grid.
Compared with the prior art, the invention has the following beneficial effects:
the invention discloses a back-to-back system and a modular flexible interconnection system, and relates to the technical field of power systems. The back-to-back system comprises a first AC/DC converter, a second AC/DC converter and at least one first DC/DC converter; one end of the first AC/DC converter or the second AC/DC converter is connected with an alternating current network, and the other end of the first AC/DC converter or the second AC/DC converter is connected with one end of the first DC/DC converter; the first AC/DC converter and the second AC/DC converter are used for constructing a medium-voltage direct-current bus section; the first DC/DC converter is used for adjusting the voltage between the first AC/DC converter and the second AC/DC converter. The first AC/DC converter and the second AC/DC converter can construct a medium-voltage direct-current bus section for future extension interfaces, and can be connected to other medium-voltage alternating-current power distribution systems to form a multi-terminal interconnection power distribution system or connected to a low-voltage direct-current power distribution system to form a multi-stage direct-current power distribution system.
Drawings
In order to more clearly illustrate the technical solution of the present invention, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to the drawings without creative efforts.
Fig. 1 is a schematic structural diagram of a typical interconnection system of a conventional medium voltage distribution network;
fig. 2 is a schematic structural diagram of a back-to-back system according to an embodiment of the present invention;
FIG. 3 is a schematic structural diagram of a modular flexible interconnect system according to an embodiment of the present invention;
fig. 4 is a schematic diagram of a specific arrangement structure of a back-to-back system according to an embodiment of the present invention;
fig. 5 is a schematic diagram of a fault analysis of a modular flexible interconnect system according to an embodiment of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
It should be understood that the step numbers used herein are for convenience of description only and are not intended as limitations on the order in which the steps are performed.
It is to be understood that the terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used in the specification of the present invention and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.
The terms "comprises" and "comprising" indicate the presence of the described features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
The term "and/or" refers to and includes any and all possible combinations of one or more of the associated listed items.
Typical wiring of a domestic medium-voltage distribution network at present mainly comprises typical wiring forms such as a radiation type, a single-ring network type, a double-ring network type, an N-supply-one-standby type, a multi-section N-connection type and the like. On the basis of a single-ring network (2-1 wiring), an original line tail end contact switch is cancelled, a flexible-straight-back backrest system 100 with two ends is constructed through a connection transformer and an MMC (AC/DC), loop closing operation and power transfer of alternating current lines with two ends are achieved, mutual assistance between alternating current distribution networks with two ends and mutual support under accidents are guaranteed, and therefore power supply reliability of the distribution networks is improved. Referring to fig. 1, fig. 1 is a schematic structural diagram of a typical interconnection system of a conventional medium voltage distribution network.
The typical interconnection system of the existing medium-voltage distribution network interconnects the distribution lines of two alternating-current power grids, and two ends of the typical interconnection system are respectively provided with an AC/DC converter for converting alternating current into direct current and then transmitting power. However, the AC power source needs to be transformed by a transformer before the power of the AC power grid is transmitted to the AC/DC converter.
However, in the typical interconnection system of the existing medium voltage distribution network, when the intermediate direct current bus fails, all MMCs (AC/DC) need to be locked and shut down, and the system will be out of operation. The AC distribution network systems at two ends operate independently, mutual support under the failure of the AC distribution network cannot be realized, the reliability of power supply of the system is greatly reduced, and the expansibility requirement of a subsequent system cannot be met. In the technical scheme, the connection of key elements has larger volume and weight and lower efficiency, for example, 10MW connection changes a single unit to 25 tons and two units to 50 tons; the volume of one machine is 4m multiplied by 2m multiplied by 3m, and two machines are 8m multiplied by 2m multiplied by 3 m; 98% of the efficiency of the single equipment and 96% of the two equipment cannot be realized, and the modular on-site splicing type installation of the prefabricated cabin cannot be realized, so that the modular on-site splicing type installation equipment is difficult to adapt to the application requirements of small floor area, standardized design, plug and play and the like of a medium-voltage distribution network system.
The invention aims to combine the existing flexible direct-current distribution network technology, realize power fusion and mutual support among different distribution areas through flexible interconnection among medium-voltage distribution network lines, simultaneously meet the friendly access requirements of direct-current loads of distributed power supplies, electric automobiles and the like, and greatly improve the regulation capability of the distribution network and the reliability of regional power supply. Particularly, the defects of the prior art are overcome, the system does not need to be stopped when the intermediate direct current bus fails, the single-end operation can be kept in the STATCOM mode, and the system can be unlocked and recovered to operate after the direct current bus failure is cleared. With the market scale expansion and cost reduction of distribution network flexible-straight equipment in the future, a multi-port back-to-back flexible-straight interconnection system can be used as a standard design scheme of a medium-voltage alternating-current distribution network rack N for a high-reliability power supply scene in a standby connection mode.
Referring to fig. 2, fig. 2 is a schematic structural diagram of a back-to-back system 100 (PET-BTB for short) according to an embodiment of the invention. The back-to-back system 100 provided by the present embodiment includes a first AC/DC converter 111, a second AC/DC converter 112, and at least one first DC/DC converter 113;
one end of the first AC/DC converter 111 or the second AC/DC converter 112 is connected to an AC grid, and the other end is connected to one end of a first DC/DC converter 113;
the first AC/DC converter 111 and the second AC/DC converter 112 are used to construct a medium voltage DC bus section;
the first DC/DC converter 113 is used to adjust the voltage level between the first AC/DC converter 111 and the second AC/DC converter 112.
In the embodiment of the present invention, the first AC/DC converter 111 is a flexible DC converter, and a converter based on a Modular Multilevel (MMC) topology is adopted. The MMC has the fault self-clearing capacity, a basic circuit unit of the converter is a power module, and each phase of bridge arm is formed by cascading a certain amount of power modules with the same structure and a bridge arm reactor. When the middle direct current bus breaks down, the MMC and the DC/DC at the two ends of the direct current bus are locked, the corresponding alternating current circuit breaker is disconnected, the MMC at the other end can still work in a STATCOM running mode, the system does not need to be stopped, and the direct current bus can be unlocked and restored to run after the fault is cleared.
In the embodiment of the present invention, in the back-to-back system 100, the forward ports are reserved at the ends of the first AC/DC converter 111 and the second AC/DC converter 112 that are not connected to the AC power grid to provide conditions for accessing the forward expansion module 200.
In an embodiment of the present invention, the topology of the first DC/DC converter 113 includes: an LC series resonance structure, an LC parallel resonance structure, an LLC series parallel resonance structure, a CLLC series resonance structure or a phase shift control structure.
In the embodiment of the present invention, the topology of the ports of the first AC/DC converter 111 and the second AC/DC converter 112 connected to the AC power grid includes: a clamp type structure, an H-bridge cascade type structure, a modular multilevel structure, or a three-phase linear cascade structure.
Referring to fig. 3, fig. 3 is a schematic structural diagram of a modular flexible interconnect system according to an embodiment of the present invention. The modular flexible interconnection system provided by the embodiment comprises a back-to-back system 100 and a plurality of remote expansion modules 200 according to any one of the above embodiments; the forward-expansion module 200 includes one end connected between the first AC/DC converter 111 and the first DC/DC converter 113, or between the second AC/DC converter 112 and the first DC/DC converter 113; the other end of the remote expansion module 200 is connected to an ac or dc power grid.
In the embodiment of the present invention, the first AC/DC converter 111 and the second AC/DC converter 112 of the back-to-back system 100 can each construct a medium voltage DC bus segment for future expansion interface. A DC/DC and/or DC/AC converter can be additionally arranged at the medium-voltage direct-current bus so as to be connected to other medium-voltage alternating-current power distribution systems to form a multi-terminal interconnection power distribution system; or the DC/DC converter is additionally arranged to be connected to a low-voltage direct-current power distribution system to form a multi-stage direct-current power distribution system, and meanwhile, the multi-stage direct-current power distribution system can be used as an access point of a distributed power supply such as photovoltaic power generation.
In an embodiment of the present invention, the forward expansion module 200 comprises a plurality of second DC/DC converters 210, wherein at least one second DC/DC converter 210 is connected to the first AC/DC converter 111 or the second AC/DC converter 112, and at least one second DC/DC converter 210 is connected to the DC power grid.
In an embodiment of the present invention, the forward expansion module 200 includes a third AC/DC converter 220 and a plurality of third DC/DC converters 221; wherein, at least one third DC/DC converter 221 is connected to the first AC/DC converter 111 or the second AC/DC converter 112, and at least one third DC/DC converter 221 is connected to the third AC/DC converter 220; the third AC/DC converter 220 is connected to the AC grid at the end not connected to the third DC/DC converter 221.
Referring to fig. 4, fig. 4 is a schematic diagram illustrating an exemplary layout structure of a back-to-back system according to an embodiment of the present invention. Due to the fact that the selected voltage class is low (+ -10 kV), the number of required converter power modules is small, convenience in operation and maintenance, good operation environment and compactness of equipment arrangement are considered, and the back-to-back system provided by the embodiment adopts a modular prefabricated cabin compact arrangement mode.
A first port: in fig. 4, a first prefabricated cabin at the lower part is a basic module i, namely an AC/DC module, and a starting cabinet, a bridge arm reactor, an MMC valve bank, a direct current disconnecting link cabinet and a water cooling unit are sequentially arranged in the cabin; the second prefabricated cabin is a secondary and communication module;
and a second port: the first prefabricated cabin at the lower part in FIG. 4 is a basic module I, namely an AC/DC module; the second prefabricated cabin is a basic module II, namely a DC/DC module, and a DCT valve bank, a water cooling unit and a DCT control cabinet are arranged in the second prefabricated cabin.
If the system needs to be expanded into a three-port or four-port system in the future, namely, corresponding AC/DC modules and DC/DC modules are added according to the application requirements of the ports, the layout is clear in level, the modularization degree is high, the wiring is simple and convenient, and the system is suitable for popularization.
1) In the scheme of the invention, the two-port back-to-back system has three operation modes: back-to-back mode, UPS power mode, and STATCOM mode.
(1) Back-to-back mode
The two-end alternating current system and the PET-BTB work normally, the MMC (AC/DC) at two ends in the PET-BTB controls the voltage of a direct current end, the DC/DC controls the power exchange of the two-end alternating current transformer substations according to a system instruction, the load can be guaranteed to be equally divided, and the power supply efficiency and the equipment utilization rate are improved.
When the AC power distribution branch circuit has a fault (the PET-BTB access branch circuit is not included), the AC system cuts off the fault area, and the system still works in a back-to-back mode.
(2) UPS supply mode
When the bus area of the alternating current system on one side has a fault, the alternating current system cuts off the alternating current bus area, and the alternating current distribution network line connected with the side is supplied with voltage by the alternating current system on the other side through the PET-BTB system to maintain power to supply power in an emergency support mode. Under the mode, the AC/DC of the normal operation end works in a direct current voltage control mode, the DC/DC also works in a direct current voltage mode to control the direct current voltage of the normal operation end, and the AC/DC of the fault end works in an alternating current voltage control mode to supply power to a load on a feeder line of the fault side.
(3) STATCOM mode
When the STATCOM mode is operated, MMC (AC/DC) at two ends of a back-to-back system independently operates in the STATCOM mode, and reactive compensation at an alternating current side is achieved. It should be noted that although in other modes, the back-to-back system can also regulate reactive power, the reactive capacity is not large, mainly for active control.
In addition, when an alternating current access point at one end of the PET-BTB fails, the alternating current system cuts off an alternating current branch fault area, and other areas still keep normal power supply. The fault end side MMC (AC/DC) of the PET-BTB is locked, and the normal operation end side MMC (AC/DC) can still work in the STATCOM operation mode.
When the PET-BTB direct current bus has a fault, the MMC and the DC/DC at two ends of the direct current bus are locked, the corresponding alternating current circuit breaker is disconnected, and the MMC at the other end can still work in the STATCOM operation mode.
2) When the two-port scheme is extended to three ports in a long term, in addition to the above operation modes, the control modes of the direct-current bus are different along with the difference of fault points under the condition of the fault of the direct-current bus. Referring to fig. 5, fig. 5 is a schematic diagram illustrating a fault analysis of a modular flexible interconnect system according to an embodiment of the present invention.
If a direct current fault occurs between the AC/DC module and the high-frequency isolation DC/DC module of one port (as shown in the situation 2 or 3 in FIG. 5), the AC/DC module and the high-frequency isolation DC/DC module of the port are locked, and due to the fault isolation effect of the DC/DC module, the operation of the other two ports cannot be influenced, and the power fusion of the remaining 2-loop line can still be realized; if a direct current side fault occurs in a single MMC (AC/DC) module (as in case 1 in FIG. 5), the high frequency isolation DC/DC connected in the MMC (AC/DC) of the current port and the other two ports can block and isolate the fault, and the MMCs (AC/DC) of the other two ports can independently run in the STATCOM mode.
The invention meets the requirement of high-reliability horizontal power supply of the medium-voltage distribution network and improves the utilization rate of the distribution network lines. Through the power output or the compensating current of control VSC transverter, provide nimble tide circulation confession and reactive compensation ability, improve the power supply reliability of the interchange generating line who is connected with it, exchange distribution network system reliability index and have a great promotion than the conventionality. In addition, the maximum average utilization rate of the two-supply one-standby connection of the conventional alternating current distribution network is only 60%, the two-port system can realize loop closing operation, the average utilization rate of the line can reach 66.7%, and the line utilization rate of the three-port system can be increased from 65% to 75% for the three-supply one-standby connection.
The invention solves the problem of unbalanced load of the power distribution network and reduces the line loss. Through the flexible interconnection system, the power supply capacity support of more than 50% can be provided for the target feeder line in two directions, and the load balance distribution among lines is realized through the supply transfer channel, so that on one hand, conditions and guarantee are provided for the heavy load operation of the target feeder line, the power supply operation pressure of the lines is relieved, meanwhile, the overload risk of the lines at the peak load period is eliminated, and the hidden danger of line equipment is reduced; on the other hand, the power exchange between the nodes of the alternating current feeder line and between the branch lines is reduced through the real-time flexible power flow regulation and control of the flexible straight ring sections, and the line loss is reduced.
The invention improves the power quality of the medium-voltage alternating-current distribution network connected with the medium-voltage alternating-current distribution network, solves the problem of voltage sag caused by inductive load switching, and improves the voltage quality of sensitive loads. The alternating current feeder line is regulated and controlled in real time through the flexible interconnection system, voltage flicker caused by impact load can be greatly reduced, voltage drop is isolated, harmonic waves are controlled, and power supply quality is guaranteed. A direct-current distribution network is constructed through a DC/DC port of the system, a special power supply channel is provided for sensitive loads to improve the quality of electric energy, and the problems of frequency fluctuation and current distortion caused by asymmetric loads are thoroughly solved.
The invention meets the access and coordinated operation of distributed energy sources, an energy storage system and an electric vehicle charging pile. The multi-port flexible interconnection system can provide a grid-connected access point for a distributed power supply, an energy storage system and an electric automobile charging facility, reduces voltage transformation links, and reduces equipment investment and operation loss; meanwhile, the regulation and control technology based on the flexible-direct interconnection link can further reduce the influence caused by uncertain factors and ensure the source-storage-charging coordinated operation.
The invention adopts a flexible interconnection system based on the power electronic transformer, saves two ends of the transformer, reduces the weight of equipment and the occupied area of the system, adopts a modular prefabricated cabin installation mode, has clear layout level, high modularization degree, simple and convenient field wiring, has strong engineering feasibility and reproducibility, and is suitable for popularization and application in the future.
While the foregoing is directed to the preferred embodiment of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention.
Claims (6)
1. A back-to-back system comprising a first AC/DC converter, a second AC/DC converter and at least one first DC/DC converter;
one end of the first AC/DC converter or the second AC/DC converter is connected with an alternating current network, and the other end of the first AC/DC converter or the second AC/DC converter is connected with one end of the first DC/DC converter;
the first AC/DC converter and the second AC/DC converter are used for constructing a medium-voltage direct-current bus section;
the first DC/DC converter is used for adjusting the voltage between the first AC/DC converter and the second AC/DC converter.
2. The back-to-back system of claim 1, wherein the topology of the DC/DC converter comprises:
an LC series resonance structure, an LC parallel resonance structure, an LLC series parallel resonance structure, a CLLC series resonance structure or a phase shift control structure.
3. The back-to-back system of claim 1, wherein the topology of the ports at which the first and second AC/DC converters are connected to the AC power grid comprises:
a clamp type structure, an H-bridge cascade type structure, a modular multilevel structure, or a three-phase linear cascade structure.
4. A modular flexible interconnect system comprising a back-to-back system according to any of claims 1-3 and a plurality of remote expansion modules;
the forward-expansion module comprises one end connected between the first AC/DC converter and the first DC/DC converter or between the second AC/DC converter and the first DC/DC converter;
the other end of the long-term expansion module is connected with an alternating current power grid or a direct current power grid.
5. The modular flexible interconnect system of claim 4, wherein the forward expansion module comprises a plurality of second DC/DC converters, wherein at least one of the second DC/DC converters is connected to the first AC/DC converter or the second AC/DC converter, and at least one of the second DC/DC converters is connected to a DC power grid.
6. The modular flexible interconnect system of claim 4, wherein the forward expansion module comprises a third AC/DC converter and a number of third DC/DC converters; wherein the content of the first and second substances,
at least one of the third DC/DC converters is connected to the first AC/DC converter or the second AC/DC converter, and at least one of the third DC/DC converters is connected to the third AC/DC converter;
the end of the third AC/DC converter not connected to the third DC/DC converter is connected to an AC power grid.
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Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN113972680A (en) * | 2021-10-20 | 2022-01-25 | 广东电网有限责任公司 | High-frequency isolated medium-voltage distribution network flexible interconnection system |
| CN114362182A (en) * | 2021-12-28 | 2022-04-15 | 国网北京市电力公司 | Four-end flexible direct-current interconnection double-loop network power distribution system |
| CN114865634A (en) * | 2022-01-17 | 2022-08-05 | 全球能源互联网研究院有限公司 | Flexible interconnection wiring structure of alternating current power grid and control method |
| CN116632924A (en) * | 2023-07-26 | 2023-08-22 | 广东电网有限责任公司佛山供电局 | Grid-connected starting system and method for multi-port flexible interconnection device |
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| CN114362182B (en) * | 2021-12-28 | 2024-04-26 | 国网北京市电力公司 | Four-terminal flexible direct-current interconnection dicyclo network distribution system |
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