power distribution - Siemens

TechnicalTypes of Power Distribution SystemsThere are several basic considerationswhich must be included by the systemdesign engineer to select and design thebest power distribution system which willsupply power to both present and futureloads most economically. Among theseare:▪ Safety▪ Reliability▪ Maintenance▪ Flexibility▪ Voltage Regulation▪ Initial Investment▪ Simplicity of OperationThe characteristics of electrical serviceavailable at the building site, the types ofloads, the quality of service required, andthe size and configuration of building arealso important factors that will influencesystem design and circuit arrangement.Four basic circuit arrangements are usedfor the distribution of electric power. Theyare the radial, primary selective, secondaryselective, and secondary network circuitarrangements. The following discussion ofthese circuit arrangements covers boththe high-voltage and low-voltage circuits.The reader should recognize that the highvoltagecircuits and substations may beowned by either the utility company or thebuilding owner, depending upon theelectric rates, the practice, andrequirements of the particular electricutility serving the specific building site.Radial SystemIf power is brought into a building atutilization voltage, the simplest and thelowest cost means of distributing thepower is to use a radial circuitarrangement. The radial system is thesimplest that can be used, and has thelowest system investment. It is suitable forsmaller installations where continuity ofservice is not critical.The low voltage service entrance circuitcomes into the building through serviceentrance equipment and terminates at amain switchgear assembly, switchboard orpanelboard. Feeder circuits are provided tothe loads or to other subswitchboards,distribution cabinets, or panelboards.Figure 1 shows the two forms of radialcircuit arrangements most frequentlyused. Under normal operating conditions,the entire load is served through thesingle incoming supply circuit, and in thecase of high voltage service, through thetransformer. A fault in the supply circuit,the transformer, or the main bus willcause an interruption of service to allloads. A fault on one of the feeder orbranch circuits should be isolated from therest of the system by the circuit protectivedevice on that circuit. Under thiscondition, continuity of service ismaintained for all loads except thoseserved from the faulted circuit.The need for continuity of service oftenrequires multiple paths of power supply asopposed to the single path of powersupply in the radial system.Figure 2. Expanded Radial System—SinglePrimary FeederA fault in a primary feeder in thearrangement shown in Figure 2 will causethe main protective device to operate andinterrupt service to all loads. If the faultwere in a transformer, service could berestored to all loads except those servedfrom that transformer. If the fault were ina primary feeder, service could not berestored to any loads until the source oftrouble had been eliminated. Since it is tobe expected that more faults will occur onthe feeders than in the transformers, itbecomes logical to consider providingindividual circuit protection on the primaryfeeders as shown in Figure 3. Thisarrangement has the advantage ofSelectionFigure 3. Expanded Radial Systemsindividual Primary Feeder Protectionmaking it possible to limit outages due toa feeder or transformer fault to the loadsassociated with the faulted equipment. Ifcircuit breakers are used for primaryfeeder protection, the cost of this systemwill be high. Even if fused switches areused, the cost of the arrangement ofFigure 3 will exceed the cost of thearrangement of Figure 2.Primary Selective SystemThe circuit arrangement of Figure 4provides means of reducing both theextent and duration of an outage causedby a primary feeder fault. This operatingfeature is provided through the use ofduplicate primary feeder circuits and loadinterrupter switches that permitconnection of each secondary substationtransformer to either of the two primaryfeeder circuits. Each primary feeder circuitmust have sufficient capacity to carry thetotal load in the building.Figure 4. Primary Selective Systems18 TECHNICALFigure 1. Radial SystemsSiemens Power Distribution & Control, SPEEDFAX 2007-2008 Product Catalog 18-3