power distribution - Siemens

Low Voltage SwitchgearType WL Circuit BreakersRogowski Coil CT TechnologyA Rogowski coil is an ‘air-core’ toroidalcoil placed round the conductor. Thealternating magnetic field produced bythe current induces a voltage in the coilwhich is proportional to the rate ofchange of current.The direct output from the coil is givenby Vout=M dI/dt Where M is the mutualinductance of the coil and dI/dt is therate of change of current. To completethe transducer the voltage is integratedelectronically so that the output from theintegrator is a voltage that accuratelyreproduces the current waveform.Coil and Integrator:The combination of a coil and anintegrator provides an exceptionallyversatile current-measuring systemwhich can be designed to accommodatea vast range of frequencies, currentlevels and conductor sizes. The output isindependent of frequency, has anaccurate phase response and canmeasure complex current waveformsand transients.Linearity:One of the most important properties ofa Rogowski coil measuring system isthat it is inherently linear. The coilcontains no saturable components andthe output increases linearly inproportion to current right up to theoperating limit determined by voltagebreakdown. The integrator is alsoinherently linear up to the point wherethe electronics saturates. Linearitymakes Rogowski coils easy to calibratebecause a transducer can be calibratedat any convenient current level and thecalibration will be accurate for allcurrents including very large ones. Also,because of their linearity, thetransducers have a very wide dynamicrange and an excellent transientresponse.Coil WindingWith a Rogowski coil it is important toensure that the winding is as uniform aspossible. A non-uniform winding makesthe coil susceptible to magnetic pickupfrom adjacent conductors or othersources of magnetic fields. We havedeveloped special machines for makingaccurate windings. Coils come in a rangeof styles including rigid and flexible coilsbut we have developed several othervariations to meet specific needs.Output IndicationThe output from the integrator can beused with any form of electronicindicating device that has an inputimpedance greater than about 10kohmsuch as a voltmeter, oscilloscope,transient recorder or protection system.Split CoilsSome designs of coil can be fitted onthe conductor without the need todisconnect the conductor. Most flexiblecoils can be fitted this way and it is alsopossible to build split rigid coils. Splitiron-cored devices such as currenttransformers are subject to appreciableamplitude and phase errors if the halvesare misaligned by even a small amount.Rogowski coils do not have thisproblem. Misalignment of the joiningfaces of a split Rogowski coil has only asmall effect on the amplitude and noeffect on the phase.Electronic Trip UnitSelection Criteria for WL CircuitBreakersThe basic criteria for selecting circuitbreakers is:Maximum Available Short Circuit at theinstallation point. This value determinesthe short circuit current interruptingGeneralrating or short circuit current withstandrating of the circuit breaker.Rated Current In which is to flowthrough the respective circuit breakercontinuously. This value may not begreater than the maximum rated currentof the circuit breaker. The rated currentfor the WL is determined by the ratingplug, up to the maximum frame rating.Ambient Temperature of the circuitbreaker. This is usually the temperatureinside the cubicle.Design of the circuit breaker.Protective Functions of the circuitbreaker.These are determined by theselection of the appropriate trip unit.Dynamic Arc-Flash Sentry (PatentPending) A unique feature of the WL tripunit allows the system designer toachieve lower levels of arc flash energyand delayed tripping for selective tripcoordination purposes.Rating PlugThe Rating Plug is a replaceable modulethat enables users to reduce the rateddevice current for optimum adaptation tothe system; e.g., during startup of a plantsection. The Rating Plug should beselected so that it corresponds to therated current of the system.Switch-selectable I2t or I4t CharacteristicCurve Improved Overload Protection.The best possible protection is assuredwhen all protective devices in the systemare optimally coordinated. To achieveoptimum selectivity and coordination, thelong-time characteristic can be switchedbetween l2t and l4t, to improvecoordination with fuses or inverse relays.Switchable Parameter SetsTo allow the protection to adapt tochanges in system needs such asswitching between utility and generatorfeeds, WL Circuit Breakers support ETUswith two independent parameter sets.Switching between the parameter setsoccurs in less than 100 ms and can bedone remotely or via a contact input to anoptional CubicleBUS module.13 SWITCHGEARFor more information, please visit http://automation.usa.siemens.com/consultant/ or contact your local sales office.Siemens Power Distribution & Control, SPEEDFAX 2007-2008 Product Catalog 13-49