power distribution - Siemens

18TECHNICALTechnicalSystem AnalysisGeneralProper system design requires that thesystem be coordinated so the interruptingcapacity and / or short circuit withstandcapabilities of the various components inthe system are not exceeded for anyoperating situation. Good practice alsorequires that the system be selective, thatis, that the minimum portion of thesystem be interrupted on occurrence of afault. The need for selectivity must alwaysbe balanced against the requirements ofeconomics and coordination with theoverall process needs.At the conceptual phase of a project,several distribution system alternativesshould be considered, and examined bothtechnically and economically. This studyshould include sufficient detail for athorough understanding of the systemalternatives. The conceptual study shoulddetermine the optimal distribution systemconfiguration for the project, on whichdefinitive design can proceed.At all stages of design, the principalobjectives of personnel safety, equipmentprotection, process continuity, faultclearing, and service continuity should beconsidered.In designing a new or modifieddistribution system, the following types ofsystem studies may be needed:1. Short Circuit Studies: three phase,line-to-line, and line-to-ground faultscan be calculated for both close-andlatchand interrupting conditions,necessary for checking interruptingdevice and related equipment ratings,and setting protective devices.2. Circuit Breaker Application Studies:consider the AC and DC decrementsin the fault current, and the speed ofthe various medium voltage circuitbreakers, to determine close-and-latchand interrupting duties.3. Protective Device CoordinationStudies: determine characteristicsand settings of protective devices,e.g., relays, trip devices, fuses, etc.The coordination study should providea balance between protection ofsystem equipment and continuityof service.4. Load Flow Studies: calculate voltages,phase angles, real and reactivepower, line and transformer loadingsunder simulated conditions to aid indetermining the performance of anew or revamped system during theplanning stage.5. Motor Starting Studies: determineseverity of voltage dips and adequacyof load accelerating torque when startinglarge motors on a weak system.Today, most studies are performed usingcomputers. Some specialized studiesrequire large computing resources, butmany studies can now be performed onpersonal computers. A wide variety ofsoftware packages are available. Inaddition, many specialty firms exist whichprovide engineering service to performsuch studies.Short Circuit CalculationsThe single-line diagram serves as thestarting point for the system study andselection of equipment ratings. Thesingle-line must be modified to show allpower sources and capacities, andsystem impedances. Sources of shortcircuit current include utility connections,local generation, and all rotating machinesconnected to the system at the instantthe fault occurs. The system study shouldconsider various fault types (line-to-lineand line-to-ground) and fault locations.The value of normal load current in acircuit depends on the load connected,and is essentially independent of thecapacity of the power system. On theother hand, the short circuit currentdepends almost entirely on the capacityof the power system, not the size of theload.The total fault current consists of asymmetrical AC component,superimposed on a DC (offset)Selectioncomponent. Hence, the total fault currentis asymmetric with respect to the currentaxis. The value of the DC componentdepends on the point of the voltage waveat which the fault was initiated. Forsystem studies, it is assumed that thefault is initiated at the worst point, toproduce a “fully offset” fault current. Thisis illustrated in Figure 25.Short circuit currents are determined bythe system impedance, including bothreactance and resistance. The effect ofthe reactance is to cause the initial faultcurrent to be high, with the fault currentdeclining as time proceeds. This isrepresented as the summation of a DCcomponent which decays relativelyrapidly over time, and an AC component,which decays at a slower rate. The rate ofdecay of the components depends on thesystem X / R ratio.Since the reactance of rotating machinesvaries with the time from fault initiation,the short circuit calculations must use theappropriate machine reactance values.Subtransient reactance (X” d ) governscurrent flow for approximately the first 6cycles of a fault. Then, transientreactance (X’ d ) determines current flowup to around 30-120 cycles, depending onthe machine. After this, synchronousreactance (X d ) applies, but studiesseldom use this value as faults are notusually allowed to persist for this lengthof time.For transformers, the actual tested valueof the transformer impedance is used. Ifthis is not available, use designimpedance adjusted to the minimumvalue allowed by manufacturing toleranceof + – 7.5%. For example, a 5.75% designunit has a tolerance range of 5.32-6.18%,and 5.32% would be used in a systemstudy prior to manufacture.Figure 25. Structure of Asymmetrical Current Wave (Fully Offset)18-14Siemens Power Distribution & Control, SPEEDFAX 2007-2008 Product Catalog