power distribution - Siemens

TechnicalTypes of Power Distribution Systemsbus through network protectors and areoperated in parallel. A network protectoris an electrically operated power circuitbreaker controlled by network relays insuch a way that the circuit breakerautomatically opens when power flowsfrom the low voltage bus toward thetransformer. When voltages in thesystem are such that power would flowtoward the low voltage bus from thetransformer, it will close automatically.Network protectors are normallyequipped with relays which operate forfaults in the network transformer or highvoltage feeder only. The network is oftenoperated on the assumption thatnetwork failure will “burn” open.Network protectors withoutsupplementary protection do not meetthe requirements of the NEC forovercurrent, ground fault, or short circuitprotection. Protection of the network orcollector bus may be added by providingsensing devices, including ground faultdetection, with tripping of the networkprotectors. The most common use of thenetwork protector, however, has been byutilities in vaults where failure of thenetwork devices could cause damagelimited to the vault. High integrity designinvolving wide phase separation and theuse of “catastrophe” fusing minimize thedanger and extent of a network failure. Aconventional circuit breaker with timeovercurrent and instantaneous tripdevices plus network relays can meetthe NEC requirements. However, the fullreliability of the network may becompromised since selectivity betweenthese devices is difficult to obtain.Under normal operating conditions, thetotal load connected to the bus is sharedequally by the transformers. Should afault occur in a transformer or on a primaryfeeder, the network protectorassociated with the faulted transformeror feeder will open on reverse powerflow to isolate the fault from the lowvoltage bus. The remaining transformeror transformers in the substation willcontinue to carry the load and there willbe no interruption of service to the loads,except for a voltage dip during the timethat it takes for the protective equipmentto operate.If only two transformers are used in aspot network substation, eachtransformer must be capable of carryingthe total load served from the lowvoltage bus. The amount of sparetransformer capacity in the substationcan be reduced by using a primaryselective switching arrangement witheach transformer, or by using three ormore transformers. If the primaryselective switching arrangement is used,the total load can be about 160 percentof the nameplate rating of one of thetransformers. This produces an overloadon one transformer until such time asthe remaining transformer can beswitched to the other feeder in the caseof a primary feeder fault.The interrupting duty imposed on thelow voltage protective devices in a spotnetwork substation is higher than inradial, primary selective, or secondaryselective substations having the sameload capability because of the sparetransformer capacity required in the spotnetwork substation and because thetransformers are operated in parallel.Figure 8. Secondary Network SystemThe spare transformer capacity, thenetwork protectors, and the higherinterrupting duty will make the secondarynetwork arrangement much moreexpensive than the other arrangements.At the same time, these elements makethe reliability of the network systemgreater than for the other systemconfigurations.The secondary network may also takethe form shown in Figure 8. In thisarrangement there is only onetransformer in each secondarysubstation, and the substations areinterconnected by normally closed lowvoltage tie circuits. The tie circuits permitinterchange of power betweensubstations to accommodate unequalloading on the substations and to providemultiple paths of power flow to theSelectionFigure 9. Primary Selective SecondaryNetwork Systemvarious load buses. In normal operation,the substations are about equally loadedand the current flowing in the tie circuitsis relatively small. However, if a networkprotector opens to isolate a transformeron a primary feeder fault, the load on theassociated bus is then carried by theadjacent network units and is suppliedover the tie circuits. This arrangementprovides for continuous power supply toall low voltage load buses, even though aprimary feeder circuit or a transformer istaken out of service.In the network arrangement in Figure 9,if there were three incoming primaryfeeder circuits and three transformers,the combined capacity of two of thetransformers should be sufficient to carrythe entire load on the three substationson the basis that only one feeder is outof service at one time. Generally, thesetransformers would all have the sameratings. With this arrangement, as withthe spot network arrangement, areduction in spare transformer capacitycan be achieved, if a primary selectiveswitching arrangement is used at eachsubstation transformer. However, if threeor more primary feeder circuits areavailable, the reduction in transformercapacity achieved through the use of aprimary selective arrangement may besmall.Cable ties or busway ties, as shown inFigures 8 and 9, will require carefulconsideration of load distribution duringcontingencies and of the safety aspectswith regard to backfeeds. Key or othermechanical interlocking of switches orcircuit breakers may be essential.18 TECHNICALSiemens Power Distribution & Control, SPEEDFAX 2007-2008 Product Catalog 18-5