power distribution - Siemens

TechnicalOvercurrent Protection and CoordinationCoordination of a power distributionsystem requires that circuit protectivedevices be selected and set so thatelectrical disturbances, such as over-loadsor short circuits, will be cleared promptlyby isolating the faulted equipment withminimum service disruption of thedistribution system. Time / CurrentCharacteristic Curves are available forcircuit protective devices, such as circuitbreakers and fuses, which show howquickly they will operate at various valuesof overload and short circuit current.Coordination can be obtained bycomparing these curves for each device inseries in the system.In developing the system, it will be notedthat many compromises must be madebetween the various objectives:1. System reliability.2. Continuity of service.3. Equipment and system protection.4. Coordination of protective devices.5. System cost.Preliminary steps in Coordination study:A) One-line diagram: used as a base onwhich to record pertinent data andinformation regarding relays, circuitbreakers, fuses, current transformers, andoperating equipment while at the sametime, providing a convenientrepresentation of the relationship of circuitprotective devices with one another.B) Short-circuit study: record all applicableimpedances and ratings; using thesevalues, a short-circuit study is made todetermine currents available at anyparticular point in the system.C) Determine maximum load currentswhich will exist under normal operatingconditions in each of the power-systemcircuits, the transformer magnetizinginrush currents, and times, and thestarting currents, and accelerating times oflarge motors. These values will determinethe maximum currents which circuitprotective devices must carry withoutoperating. The upper boundary of currentsensitivity will be determined by thesmallest values resulting from thefollowing considerations:1) Maximum available short-circuitcurrent obtained by calculation.2) Requirements of applicable codesand standards for the protection ofequipment such as cable, motors,and transformers.3) Thermal and mechanical limitationsof equipment.D) Time / current characteristic curves ofall the protective devices to be coordinatedmust be obtained. These should beplotted on standard log-log coordinationpaper to facilitate the coordination study.Mechanics Of Achieving Coordination:The process of achieving coordinationamong protective devices in series isessentially one of selecting individual unitsto match particular circuit or equipmentprotection requirements, and of plottingthe time/current characteristic curves ofthese devices on a single overlay sheet oflog-log coordination paper.The achievement of coordination is a trialand-errorroutine in which the varioustime / current characteristic curves of theseries array of devices are matched oneagainst another on the graph plot.When selecting protective devices onemust recognize ANSI and NECrequirements and adhere to the limitingfactors of coordination such as loadcurrent, short-circuit current, and motorstarting. The protective devices selectedmust operate within these boundarieswhile providing selective coordinationFigure 24. Coordination of Example SystemSelectionwhere possible. Selective coordination isusually obtained in low voltage systemswhen the log-log plot of time / currentcharacteristics displays a clear spacebetween the characteristics of theprotective devices operating in series,that is, no overlap should exist betweenany two time/current characteristics iffull selective coordination is to beobtained. Allowance must be made forrelay overtravel and for relay and fusecurve accuracy. Quite often thecoordination study will stop at a pointshort of complete selective coordinationbecause a compromise must be madebetween the competing objectives ofmaximum protection and maximumservice continuity.Computer Aided Coordination:The philosophy discussed above appliesto the “classical” practice of performingcoordination studies manually. Today,however, there are numerous personalcomputer software programs availablefor performing coordination studies.18 TECHNICALSiemens Power Distribution & Control, SPEEDFAX 2007-2008 Product Catalog 18-13