power distribution - Siemens

18TECHNICALTechnicalTypes of Power Distribution SystemsUnder normal operating conditions, theappropriate switches are closed in anattempt to divide the load equallybetween the two primary feeder circuits.Then, should a primary feeder fault occur,there is an interruption of service to onlyhalf of the load. Service can be restored toall loads by switching the deenergizedtransformers to the other primary feedercircuit. The primary selective switches areusually manually operated and outagetime for half the load is determined by thetime it takes to accomplish the necessaryswitching. An automatic throwoverswitching arrangement could be used toavoid the interruption of service to half theload. However, the additional cost of theautomatic feature may not be justified inmany applications. If a fault occurs in asecondary substation transformer, servicecan be restored to all loads except thoseserved from the faulted transformer.The higher degree of service continuityafforded by the primary selectivearrangement is realized at a costsomewhat higher than a simple radialsystem due to the extra primary cablesand switchgear.Secondary Selective SystemUnder normal conditions, the secondaryselective arrangement of Figure 5 isoperated as two separate radial systems.The secondary tie circuit breaker in eachsecondary substation is normally open.The load served from a secondaryselective substation should be dividedequally between the two bus sections. If afault occurs on a primary feeder or in atransformer, service is interrupted to allloads associated with the faulted feeder ortransformer. Service may be restored to allsecondary buses by first opening the mainsecondary switch or circuit breakerassociated with the faulted transformerand primary feeder, and then closing thetie breaker. The two transformersecondary circuit breakers in eachsubstation should be interlocked with thesecondary tie breaker in such a mannerthat all three cannot be in the closedposition simultaneously. This preventsparallel operation of the two transformersand thereby minimizes the interruptingduty imposed on the secondary switchingdevices. It also eliminates the possibility ofinterrupting service to all loads on the buswhen a fault occurs in either a primaryfeeder or a transformer.The cost of the secondary selectivesystem will depend upon the sparecapacity in the transformers and primaryfeeders. The minimum transformer andprimary feeder capacity will be determinedFigure 5. Secondary Selective SystemUsing Close-Coupled Double-EndedSubstationby essential loads that must be servedunder emergency operating conditions. Ifservice is to be provided for all loadsunder emergency conditions, then eachprimary feeder should have sufficientcapacity to carry the total load, and eachtransformer should be capable of carryingthe total load in each substation.This type of system will be moreexpensive than either the radial or primaryselective system, but it makes restorationof service to all essential loads possible inthe event of either a primary feeder ortransformer fault. The higher cost resultsfrom the duplication of transformercapacity in each secondary substation.This cost may be reduced by sheddingnonessential loads.A modification of the secondary selectivecircuit arrangement is shown in Figure 6.In this arrangement there is only onetransformer in each secondary substation,but adjacent substations areinterconnected in pairs by a normally openlow voltage tie circuit. When the primaryfeeder or transformer supplying onesecondary substation bus is out of service,the essential loads on that substation buscan be supplied over the tie circuit. Theoperating aspects of this system aresomewhat complicated if the twosubstations are separated by distance.The best arrangement is to use closecoupled,double-ended substations.Secondary Network SystemMany buildings with radial distributionsystems are served at utilization voltagefrom utility secondary network systems.The network supply system assures arelatively high degree of service reliability.The utility network may take the form of aSelectionFigure 6. Secondary Selective SystemUsing Two Single-Ended SubstationsWith Cable or Bus Tiedistributed network or a spot network. Ifthe building demand is in the order of 750kVA or higher, a spot network will often beestablished to serve the building. Inbuildings where a high degree of servicereliability is required, and where spotnetwork supply may not be available, thedistributed secondary network system isoften used. This is particularly true ofinstitutional buildings such as hospitals.The network may take the form of severalsecondary substations interconnected bylow voltage circuits. However, the mostcommon practice is to use some form ofthe spot network circuit arrangement.Figure 7. Simple Spot Network SystemA simple spot network, such as shown inFigure 7, consists of two or more identicaltransformers supplied over separateprimary feeder circuits. The transformersare connected to a common low voltage18-4Siemens Power Distribution & Control, SPEEDFAX 2007-2008 Product Catalog