Underground Distribution
The underground distribution system really began in 1976 using essentially the same standards and practices as the present. Until 12, 2006, underground distribution lines have been amounted up to approximately32.8% of the total circuit length that is about 96,546KM in the service area. |
| In order to meet different requirements of its service area, the underground network is configured into four topological systems, i.e. radial, normally open loop, primary selective and the secondary spot network system. Fig-1 shows schematically the configuration of a typical underground distribution system. |
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(1)Radial System The radial system using #1AWG copper cable is normally limited to single and three phases circuit with total connected load below 25 amps. These circuits are usually fed off from the main loop circuits described below and can be either overhead or nderground. (2)Normally Open Loop System The normally open loop system employs a dual source, normally open primary circuit concept. In this manner, backup service can be supplied for almost all maintenance and emergency outages. The main loop consists of two 600 amps radial circuits from two sources with the interconnection point normally open, and the branch circuits are tapped from the main loop. The branch loop is a 200 amps loop serving all low-voltage customers and most high-voltage customers. (3)Primary Selective System The primary selective system is fed by two radial feeders or double circuit looped primary feeder. Under normal operating conditions, the preferred source switch is closed while the alternative source switch is open as a standby. An automatic throw over control monitors the conditions of both power sources and initiates switching when appropriate. The control supports automatic return when the preferred source is restored. (4)Spot Network System The spot network system provides the highest level of service reliability through redundancy. A fault on a high-voltage primary feeder, or in the primary and secondary windings of the network transformer, will be automatically isolated from the unfaulted portion of the system, without causing an interruption to the load served from the spot network bus. |
(1)Cables The primary cable is single core copper cable, using XLPE insulation, extruded semi-conducting layers for both inner and outer shielding with proper size copper wire concentric neutral being applied and covered with an overall jacket of 80mil PVC. The main circuit conductor size is 500MCM copper, and branch circuit conductor size is #1AWG copper. The secondary cable is single conductor using XLPE insulation copper conductor. The main circuit conductor size is 250MCM, and branch and service circuit conductor size is 2/0 AWG. |
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(2)Switches The main circuit switches are generally limited to three sets per feeder (or 6 sets per main loop),Main circuit switches are generally 4-way type, containing two 600 amp 3 phase load break ring switches and two 200 amp 3 phase load break tap switches. The fusing provisions for branch circuits are sometimes provided in the same 4-way switch enclosure or separately housed. |
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(3)Transformers Transformer´s incorporating dead front design and proper protection features are applied on Taipower underground distribution system. To supply the low-voltage customers, single phase padmount transformers are widely used whose capacities include four types of 25,50,100 and 167KVA. For some congested districts, submersible transformers are also used. Three phase padmount transformers are generally employed in some high rise buildings to supply the three phase 220/380V low voltage customers. |
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(4)Duct and manhole All of the underground cables are adapted duct systems. Duct systems carrying primary main circuits, or exposed to heavy traffic vibrations, are always encased in concrete, Manholes are built along duct routes to accommodate cable pulling works, cable splices, major changes in circuit directions, circuit exits, and in some cases, equipment installations. All of the manholes and vaults are of precast concrete construction. |
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The recent years Taipower has actively engaged in the improvement of the underground distribution system for applicable service areas. As the cities of the island are expanding bigger and bigger with more and more population requiring ever-increasing electricity usage, the underground system has to be expanded. But in some areas the underground distribution construction is hard to carry out. Taipower will still cooperate with the government of city and to persuade some opposite standing, gradually to implement the underground distribution. To cope with congested city environment and to upgrade service quality and system operation, emphasis will be put on installing compact equipment requiring less space, completely water immersible, and above all, maintenance free. |