Active power distribution network wiring mode suitable for urban and rural power grid

Abstract

The invention relates to the technical field of power distribution networks, in particular to an active power distribution network wiring mode suitable for urban and rural power grids, which comprises at least two groups of middle and low voltage looped network structures, and each middle and low voltage looped network structure comprises a middle voltage single looped network of a middle voltage layer and a low voltage single looped network of a low voltage layer; inter-ring communication is arranged between medium-voltage single ring networks of different medium-low voltage ring network structures, and a communication switch is arranged on the inter-ring communication. According to the invention, wide interconnection and multi-level mutual aid can be realized, and the power supply reliability is effectively improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power distribution network, in particular to an active power distribution network wiring mode suitable for urban and rural power distribution networks. BACKGROUND

[0002] With the development of cities, users' demand for power supply reliability is increasing. The traditional power distribution network structure, such as single radiation, single connection, etc., has limited ability to isolate and restore power supply in case of failure, and is difficult to meet the demand of high reliability power supply areas such as A+ and A class. Although the existing complex network structure such as double ring network and petal type improves the reliability, it has problems such as high investment cost, complex control logic and difficult operation and maintenance. The rural power distribution network generally adopts a radiation or simple single connection structure, which has the pain point of "one long and three less": long power supply radius, few sectional switches, few connection points and few automation terminal configurations, which easily leads to long-time power failure caused by single-point failure, voltage overrun at the end of long line, and aggravation of voltage fluctuation due to bidirectional power flow after the access of new resources (such as distributed power supply, microgrid and energy storage), random connection of distribution transformers, full feeder accompaniment during planned maintenance, low efficiency of fault location and isolation, long recovery time, etc.

[0003] Therefore, there is an urgent need for a new wiring mode and automation method with simple structure, economic feasibility, and adaptation to urban and rural power distribution networks and support for fast self-healing and access of new resources, to solve the following problems: 1. In the scenario of sparse distribution points, how to achieve N-1 transfer with the least connection switch and consider voltage quality; 2. How to design in layers and sections to compress the fault outage range and realize the fast self-healing of urban and rural power distribution feeder; 3. How to meet the access requirements and collaborative needs of new resources such as distributed power supply, microgrid and energy storage, and solve the problems of high / low voltage and reverse overload. SUMMARY

[0004] In order to solve the above technical problems, the purpose of the present application is to provide an active power distribution network wiring mode suitable for urban and rural power distribution networks, and the technical solution adopted is as follows: In the first aspect, the present application provides an active power distribution network wiring mode suitable for urban and rural power distribution networks, which comprises at least two groups of medium and low voltage ring network structures, the medium and low voltage ring network structure comprising a medium voltage single ring network of a medium voltage layer and a low voltage single ring network of a low voltage layer; an inter-ring connection is arranged between the medium voltage single ring networks of different medium and low voltage ring network structures, and a connection switch is arranged on the inter-ring connection.

[0005] In combination with the above first aspect, in some possible implementation manners, a ring network room is arranged in the medium voltage single ring network, and an inter-ring connection is arranged between the first end ring network room nodes of different medium and low voltage ring network structures.

[0006] In some possible implementation manners of the first aspect, the inter-ring connection is arranged between the low-voltage single ring networks or the transformer areas of different medium-low voltage ring network structures.

[0007] In some possible implementation manners of the first aspect, the inter-ring connection is arranged between the low-voltage single ring networks or the transformer areas of different medium-low voltage ring network structures.

[0008] In some possible implementation manners of the first aspect, the inter-ring connection is arranged between the low-voltage single ring networks or the transformer areas of different medium-low voltage ring network structures.

[0009] In some possible implementation manners of the first aspect, the inter-ring connection is arranged between the low-voltage single ring networks or the transformer areas of different medium-low voltage ring network structures.

[0010] In some possible implementation manners of the first aspect, the inter-ring connection is arranged between the low-voltage single ring networks or the transformer areas of different medium-low voltage ring network structures.

[0011] In some possible implementation manners of the first aspect, the inter-ring connection is arranged between the low-voltage single ring networks or the transformer areas of different medium-low voltage ring network structures.

[0012] In some possible implementation manners of the first aspect, the inter-ring connection is arranged between the low-voltage single ring networks or the transformer areas of different medium-low voltage ring network structures.

[0013] In some possible implementation manners of the first aspect, the inter-ring connection is arranged between the low-voltage single ring networks or the transformer areas of different medium-low voltage ring network structures.

[0014] The present application has the following advantages: by arranging the inter-ring connection between the medium-voltage single ring networks of different medium-low voltage ring network structures, and arranging the inter-ring connection switch, wide interconnection and multi-level mutual aid can be realized, and the power supply reliability is effectively improved. BRIEF DESCRIPTION OF DRAWINGS

[0015] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, and the advantages thereof, the drawings needed to be used in the embodiments or prior art description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and for those skilled in the art, other drawings can be obtained without creative labor based on these drawings.

[0016] Figure 1 It is a basic connection diagram of the city network single ring network of the embodiment of the present application. Figure 2 The figure is a schematic diagram of a "trident type" single loop network connection mode of the urban network in the embodiment of the present application. Figure 3 The figure is a schematic diagram of a single overhead contact network connection mode in the embodiment of the present application. Figure 4 The figure is a schematic diagram of a "trident type" single overhead contact network connection mode in the embodiment of the present application. Figure 5 The figure is a schematic diagram of a "trident type" single overhead contact network topology in the embodiment of the present application. Figure 6 The figure is a schematic diagram of the first network frame expansion evolution in the embodiment of the present application. Figure 7 The figure is a schematic diagram of the second network frame expansion evolution in the embodiment of the present application. DETAILED DESCRIPTION

[0017] In order to clearly illustrate the technical features of the present application, the present application will be described in detail below with reference to the specific embodiments and in conjunction with the accompanying drawings.

[0018] Embodiments of the present application will be described below in greater detail with reference to the accompanying drawings. Although some embodiments of the present application are shown in the drawings, it should be understood that the present application can be implemented in various forms, and should not be interpreted as being limited to the embodiments described herein, but rather these embodiments are provided to more thoroughly and completely understand the present application. It should be understood that the drawings and embodiments of the present application are for exemplary purposes only, and are not intended to limit the scope of protection of the present application.

[0019] It should be understood that each step described in the method embodiments of the present application can be executed in different orders, and / or in parallel. In addition, the method embodiments can include additional steps and / or omit the execution of the steps shown. The scope of the present application is not limited in this respect.

[0020] The term "comprising" and variations thereof as used in the present application are open-ended, that is, "including but not limited to". The term "based on" is "at least partially based on". The term "one embodiment" means "at least one embodiment"; the term "another embodiment" means "at least one additional embodiment"; the term "some embodiments" means "at least some embodiments". Related definitions of other terms will be given in the description below.

[0021] It should be noted that the concepts of "first", "second", etc. mentioned in the present application are only used to distinguish different devices, modules or units, and are not intended to limit the order or interdependence of the functions performed by these devices, modules or units.

[0022] Although the operations or steps are described in a particular order in the embodiments of the present application, it should not be understood that the order of the operations or steps is required to achieve desirable results, or that all illustrated operations or steps are necessary for achieving desirable results. In some embodiments of the present application, the operations or steps can be performed in series; in some embodiments of the present application, the operations or steps can be performed in parallel; and in some embodiments of the present application, some of the operations or steps can be performed.

[0023] The embodiment of the present application is based on the concept of "simplicity is reliability, reliability is practicality", and proposes a kind of active distribution network wiring mode suitable for urban and rural power grids by learning from advanced experiences such as ring distribution transformer dual access, distribution transformer dual access and transformer area interconnection. The wiring mode forms a "tripod type" single ring network wiring by improving the single ring network, and can realize extensive interconnection and multi-level mutual aid, effectively improving power supply reliability.

[0024] The active distribution network wiring mode suitable for urban and rural power grids provided by the embodiment of the present application will be described in detail below with reference to the accompanying drawings.

[0025] The embodiment of the present application provides an active distribution network wiring mode suitable for urban and rural power grids, which can be divided into urban network "tripod type" single ring network wiring mode and rural network "tripod type" overhead single contact wiring mode based on different application areas. Figure 2 The urban network "tripod type" single ring network wiring mode is shown, Figure 1 The basic wiring mode of the urban network "tripod type" single ring network wiring mode is shown. Figure 2 The basic wiring mode of the urban network "tripod type" single ring network wiring mode is shown. Figure 4 The rural network "tripod type" overhead single contact wiring mode is shown, Figure 3 The basic wiring mode of the rural network "tripod type" overhead single contact wiring mode is shown. Figure 4 The basic wiring mode of the rural network "tripod type" overhead single contact wiring mode is shown. Figure 5 The "tripod type" overhead single contact topology schematic diagram is shown.

[0026] In a specific example, the application scope of the above-mentioned urban network "tripod type" single ring network wiring mode is: suitable for A+ type power supply area, A type high reliability power supply area; new area is built once, and after the stock power grid forms the target network frame, the secondary overall reconstruction and upgrading is carried out.

[0027] In a specific example, the network frame structure of the above-mentioned city network "tripod type" single ring network connection mode is: the medium voltage cable network takes the ring network room (cabinet) as the core node, adopts the power grid structure of a single ring network, forms a medium voltage single ring network, and increases the inter-ring connection between the head-end ring network room (cabinet) nodes. For the low voltage single ring network, the connection is increased at the low voltage end or between the distribution areas, and the power supply reliability is improved through the medium and low voltage collaborative power supply technology. At the same time, flexible switches are configured at the key nodes such as the intra-ring connection and the inter-ring connection of the medium and low voltage levels, and the dynamic capacity increase during normal operation of the distribution network system and the power supply under fault are realized through flexible interconnection, thereby improving the power supply reliability and the ability of distributed power supply in place / nearby.

[0028] In the above network frame structure, the city network "tripod type" single ring network connection mode takes the medium voltage ring network room (cabinet) as the backbone node, and the first and last ends of the multiple return medium voltage feeders (such as 10kV) of the same or different substations are connected to form a medium voltage single ring network. There are a plurality of ring network rooms (cabinets) in series on the ring network as core nodes, which are responsible for the distribution and transfer of power. Further, the multiple return low voltage lines (such as 0.4kV) from the same medium voltage ring network room (cabinet) or different medium voltage power points are connected through the tie switches to form a ring network, connecting the low voltage sides of multiple distribution transformers (distribution areas) to form a low voltage single ring network. On this backbone network frame, the power interconnection between the ring networks is realized by adding "inter-ring connection" between different medium voltage single ring networks, and the load transfer and energy balance between the distribution areas are realized by adding "distribution area interconnection", i.e. setting low voltage tie switches between the low voltage sides of different distribution transformers, in addition to the flexible interconnection devices, i.e. configuring flexible switches (such as SOP) at key nodes such as medium voltage inter-ring connection points and low voltage distribution area connection points, to realize flexible and accurate control of power flow, improve the ability of distributed power supply and power supply flexibility, and perform dual access of distribution transformers, i.e. important distribution transformers or users are connected to different nodes on the medium voltage single ring network through dual circuits, and backup power transfer devices (BZT) are configured to realize seamless switching in case of failure, thereby forming a "medium and low voltage collaborative interconnection system", realizing dynamic capacity increase during normal operation and cross-ring, cross-distribution area fast power supply under fault, and upgrading the single ring network to a "tripod type" target network frame with higher reliability.

[0029] In the above-mentioned city network "tripod type" single ring network connection mode, the medium voltage single ring network and the low voltage single ring network together form a stable structure similar to the ears and legs of a "tripod", and are tightly coupled through the medium and low voltage collaborative interconnection system to form a multi-level, multi-path power supply network. On this basis, the whole network is equipped with distribution automation (i.e. centralized or intelligent distributed, realizing full coverage of "three remote" functions) and flow speed break protection devices (i.e. the incoming and outgoing line switches and tie switches of the ring network cabinet adopt circuit breaker overcurrent and speed break protection), realizing millisecond-level isolation and second-level self-healing in case of failure, so that the "tripod type" network frame is stable like a tripod and flexible in power supply.

[0030] The working principle of the above-mentioned city network "tripod type" single loop network connection mode is as follows: Normal operating state: the power flow is reasonably distributed in the medium voltage and low voltage loop networks. Through the flexible interconnection device, the power flow can be actively regulated and controlled, the line load rate is optimized, and the local consumption capacity of the distributed power supply is maximized.

[0031] Fault or maintenance state: Medium voltage line fault: the distribution automation system quickly locates and isolates the fault section. Through the loop operation of the medium voltage single loop network itself or through the inter-loop connection, the load of the fault section is transferred to the non-fault line for power supply.

[0032] Distribution transformer or low voltage line fault: switch to another return medium voltage power supply through the backup power transfer device (BZT). At the same time, through the interconnection switch of the low voltage single loop network, part of the load of the fault area is transferred to the adjacent healthy area to realize "low voltage mutual aid". The synergistic mechanism of "medium voltage transfer" and "low voltage mutual aid" ensures the reliability of the whole process from the transformer substation to the user terminal, and realizes the goal of "all regional load transferable power".

[0033] The "tripod type" connection is based on the traditional basic network frame, adopts the medium-low voltage synergistic interconnection and interconnection mode of one medium voltage loop and one low voltage loop, realizes the target network frame through multiple flexible modes, has a certain scale and can further evolve in the mode of group, has the characteristics of evolution, is easy to build and has the implementability.

[0034] In a specific example, the transition scheme of the above-mentioned city network "tripod type" single loop network connection mode is as follows: Stock power grid transformation idea: users with access to loop network rooms access loop network rooms, build single loop networks with loop network rooms as nodes, strengthen the interconnection of key nodes between groups to form inter-loop connections, increase low voltage terminals or inter-substation connections, and important users realize double access. Users without access to loop network rooms access loop network cabinets, build single loop networks with loop network cabinets as nodes, strengthen the interconnection of key nodes between groups to form inter-loop connections, increase low voltage terminals or inter-substation connections, and important users realize double access.

[0035] Incremental power grid construction idea: first build a single loop network, strengthen the interconnection of key nodes between groups to form inter-loop connections, and increase low voltage terminals or inter-substation connections.

[0036] In a specific example, the applicable scope of the above-mentioned rural network "tripod type" overhead single connection connection mode is: suitable for B type power supply area, C / D type high reliability power supply area; new area once builds target network frame, stock power grid forms target network frame after transformation, realizes one-time and two-time unified transformation and upgrading.

[0037] In a specific example, the topology of the above-mentioned "ding-type" overhead single-contact connection mode of the rural power grid is: the medium-voltage line is an overhead insulated line, which is distributed from a 110-kilovolt substation, and a multi-section single-contact power grid structure is adopted. The large branch is preferentially connected to the nearby middle section, and the main line is preferentially connected to the nearby end. In areas with conditions, the low-voltage end or the inter-substation area can be considered to increase the contact, and the power supply reliability is improved through the medium-low voltage cooperative power supply technology.

[0038] In a specific example, for the long-term network frame: single contact is mainly used, and a clear and simple ring network structure is used to simplify the fault processing logic, and the whole line self-healing is realized according to local conditions to support the upper-level power grid. For the short-term network frame: moderate contact is mainly used, and the large branch is preferentially connected to the nearby middle section, and the main line is preferentially connected to the nearby end to realize multi-directional load / DG power supply that meets the capacity voltage constraint. Among them, for the high / low voltage problem of long line power supply, many measures (such as on-load voltage regulation / voltage regulator / line reconstruction / reactive power compensation / network type energy storage / microgrid, etc.) can be considered to solve the problem.

[0039] In a specific example, in the above-mentioned "ding-type" overhead single-contact connection mode of the rural power grid, the segmentation principle, switch configuration optimization method, distribution network automation implementation method, and protection configuration scheme are respectively: Segmentation principle: three-remote circuit breakers are used for large segmentation (such as 3 sections) at key nodes of the main line, and two-remote / three-remote load switches are used for small segmentation (such as 3-15 sections) of the main line and large branches. The goal is to compress the fault impact range to the smallest unit (1 section 1 transformer).

[0040] Switch configuration optimization method: flexible switches are configured at key nodes such as intra-ring contact and inter-ring contact at the medium-low voltage level, and dynamic capacity increase during normal operation of the distribution network system and power supply under fault are realized through flexible interconnection, which improves power supply reliability and improves the ability of distributed resources to access and consume locally / nearby.

[0041] Distribution network automation implementation method: two types of centralized and on-site recloser are adopted; "three-remote" terminals should be configured for main line switches, contact switches, segmented switches, and distribution rooms with many incoming and outgoing lines; "two-remote" terminals should be configured for general nodes such as end stations without contact, and boundary switches or with remote measurement and remote signaling functions should be configured at user incoming lines.

[0042] Protection configuration scheme: circuit breaker overcurrent and instantaneous trip protection is used for contact switches, segmented switches, branch switches, and user boundary switches.

[0043] In a specific example, the transition scheme of the above-mentioned "ding-type" overhead single-contact connection mode of the rural power grid is: The stock power grid modification idea is that the overhead line with the main line end connection is considered to increase the large branch nearby middle connection, and the area with the condition can consider increasing the low-voltage end or the substation area connection; the overhead line with the main line middle connection is considered to increase the main line nearby end connection, and the area with the condition can consider increasing the low-voltage end or the substation area connection. In addition, on the basis of the main line three remote breaker large segmentation, the main line and the large branch further adopt two remote / three remote load switches for small segmentation, so as to reduce the influence of power failure range.

[0044] The incremental power grid construction idea is that for the overhead line, the large branch nearby middle connection is considered first, the main line nearby end connection is considered second, and the area with the condition can consider increasing the low-voltage end or the substation area connection; the key node of the main line adopts three remote breaker large segmentation (such as 3 segments), and the main line and the large branch adopt two remote / three remote load switch small segmentation (such as 3-15 segments), so as to reduce the influence of power failure range.

[0045] Based on the above technical scheme, the embodiment of the present application provides a kind of active distribution network connection mode suitable for city and rural power grid, which can realize feeder level fast self-healing and new resource flexible access, by using moderate connection, N-1 transfer and voltage support can be realized, by introducing flexible switching device such as flexible interconnection device, dynamic capacity increase during normal operation of distribution network system and power supply under fault can be realized, which can effectively improve power supply reliability and local / nearby access capability of flexible resources (such as distributed power supply), feeder fast fault location, isolation and self-healing are realized by configuring centralized, local type recloser type and "three remote" or "two remote" terminal, transition scheme is proposed for stock power grid modification and incremental power grid construction respectively, with replicability and popularization value.

[0046] Compared with the prior art, the above active distribution network connection mode suitable for city and rural power grid provided by the embodiment of the present application has the following advantages: High reliability: power supply capacity is reasonable and sufficient, network frame has high stability and easy reconfigurability, local fault is quickly isolated, non-fault area is quickly recovered, distributed power supply, new type energy storage, microgrid can be observed, measured and controlled.

[0047] Strong toughness: network is highly self-healing, feeder fault is self-recovered, distribution system is self-diagnosed, and the ability to resist extreme faults of power system caused by serious natural disasters is possessed.

[0048] High bearing: network is easy to plan and expand, users can plug and play, network frame is suitable for landing construction of various scenes, and is easy to build and expand, and is suitable for wide and friendly access of various new technologies, new equipment and multiple loads.

[0049] Evolution: With the characteristics of modularity, decentralization, high connectivity, extensive interconnection, flexible interconnection, cluster regulation and operation group consciousness, etc., the distribution network and microgrid are highly integrated to form a cluster-type network architecture that supports each other, and the mode is realized.

[0050] Specifically, in terms of high reliability, based on the simplified reliability evaluation model of the medium-voltage distribution network, typical reliability parameters are used to calculate the SAIDI reliability index of the typical network structure, as shown in Table 1, and the calculation results are shown in Table 2.

[0051] Table 1 Typical reliability parameter table Taking into account the failure rate, fault location and isolation time, repair time, and combining with the distribution automation coverage rate and other indicators, the power supply reliability of the "Ding-type" network structure is improved based on the traditional network structure. Among them, the power supply reliability of the cable single ring network is improved from "four nines" to "five nines", and the power supply reliability of the overhead single connection is improved from "three nines" to "four nines".

[0052] Table 2 Reliability calculation results table (unit: strip, km / strip, segment / strip, household / strip, h / household / year, %) In terms of toughness, the "Ding-type" network structure exhibits stronger recovery ability and disaster resistance due to its multi-path characteristics. When natural disasters or other emergencies occur, the "Ding-type" network exhibits excellent load transfer capability: first, through the medium-voltage layer connection, the load of the non-fault segment is quickly transferred, ensuring that most of the area is not affected; second, for the fault segment, the load transfer of the fault segment is realized through the low-voltage layer connection, ensuring that the remaining users are not affected. This kind of load transfer capability of medium and low voltage cooperation not only guarantees the high-quality power supply demand of users, but also improves the recovery ability and disaster resistance of the system, ensures the efficient and stable operation state of the distribution network in various situations, and significantly improves the reliability of power supply.

[0053] In terms of high carrying capacity, the "Ding-type" network structure improves the reliability based on medium and low voltage double ring in the short term, and further improves the accommodation capacity of distributed power based on flexible interconnection in the long term. From bottom to top, the wide access of distributed resources is carried in stages, the low-voltage level is configured with area-level microgrid, the standardized and modular light storage socket is constructed, the network element state microgrid group is created, and efficient charging load carrying and local photovoltaic consumption are realized; The flexible switch is configured at the key nodes of the medium and low voltage level to realize capacity sharing between areas, connection groups and substations, efficient load transfer and photovoltaic bottom consumption.

[0054] In the aspect of evolution, based on the state grid standard grid, the development direction of the grid in different modes and different stages is clear, which evolves into a hierarchical and group grid pattern, has physical interfaces and functions of flexible expansion, optimized access, intelligent upgrade and wisdom integration, and can meet the development needs of self-healing technology, flexible technology and 5G communication. Figure 6 and Figure 7 The schematic diagrams of the first grid expansion evolution containing two groups of medium and low voltage ring network structures and the second grid expansion evolution containing three groups of medium and low voltage ring network structures are respectively shown.

[0055] It should be noted that the above-described embodiments are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that the technical solutions recorded in the foregoing embodiments can still be modified, or some technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application, and should be included in the protection scope of the present application.

Claims

1. An active distribution network connection mode suitable for urban and rural power grids, characterized in that, The middle-low voltage ring network structure comprises a middle pressure single ring network of a middle pressure layer and a low pressure single ring network of a low pressure layer.

2. The active distribution network connection mode suitable for urban and rural power grids according to claim 1, characterized in that, The middle pressure single ring network is provided with a ring network room, and an inter-ring connection is arranged between the first end ring network room nodes of different middle-low voltage ring network structures.

3. The active distribution network connection mode suitable for urban and rural power grids according to claim 1, characterized in that, An inter-ring connection is arranged between the middle pressure single ring networks of different middle-low voltage ring network structures in a large branch near middle section connection and a main trunk line near end connection mode.

4. The active distribution network connection mode suitable for urban and rural power grids according to any one of claims 1-3, characterized in that, An inter-ring connection is arranged between the low pressure single ring networks or the district areas of different middle-low voltage ring network structures.

5. The active distribution network connection mode suitable for urban and rural power grids according to claim 4, characterized in that, An inter-ring connection is arranged between the low pressure ends of the low pressure single ring networks of different middle-low voltage ring network structures.

6. The active distribution network connection mode suitable for urban and rural power grids according to any one of claims 1-3, characterized in that, An intra-ring connection is arranged in the middle pressure single ring network and the low pressure single ring network of the low pressure layer in the middle-low voltage ring network structure, and a connection switch is arranged on the intra-ring connection.

7. The active distribution network connection mode suitable for urban and rural power grids according to claim 6, characterized in that, The connection switch is a flexible switch.

8. The active distribution network connection mode suitable for urban and rural power grids according to claim 6, characterized in that, The connection switch is further provided with an overcurrent speed break protection device.

9. The active distribution network connection mode for urban and rural power grid according to claim 1, characterized in that, The distribution transformer or the user is connected to different nodes of the middle pressure single ring network of the middle-low voltage ring network structure through a double circuit, and a spare power automatic throw-in device is configured.

10. The active distribution network connection mode suitable for urban and rural power grids according to claim 1, characterized in that, The main trunk line key nodes in the middle pressure single ring network of the middle-low voltage ring network structure adopt a large section of three remote breakers, and the main trunk line and the large branch adopt a small section of two remote or three remote load switches.

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