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LINE IMPEDENCELine impedence? Justmeasure it with OMICRONPrecise line parameters are required for distance protection relays & power system calculations.When performing mathematical determination of line impedances, multiple simplification stepsare taken which do not apply in practice, says Rainer Luxenburger and Wernich de Villiers.The CPC 100 + CP CU1 measuringsystem enables measurement of lineimpedances with a level ofprecision that goes far beyond anycalculations. This allows the distanceprotection and fault locators to be set upprecisely.Setting up distance protection equipmentproperly requires knowledge of thecorrect line impedance values. Line-tolineimpedances can often be calculatedwith acceptable accuracy.Line-to-ground impedances, on the otherhand, are influenced by physicalcharacteristics present such as metalpipes or neighboring cables in the groundand cannot be mathematically calculatedsufficiently accurately.This is because these calculations arebased on perfectly straight cable conduitsand homogenous ground with constantconductivity – conditions which are rarelyfound in the real world. The impedance ofa line-to-ground loop is thereforecalculated extremely inaccurately.A protective device shuts down thesystem if a short circuit occurs in thesection it is assigned to monitor. However,if a protective device cannot be set upSignificant deviations: Comparison between the imprecise, mathematicallydetermined values and the zero-sequence impedance values actually measuredwith the OMICRON system.correctly, it may also react improperly tofaults outside the protected section. Thiscan cause lines to be shut downunnecessarily, which in turn can lead topower cuts for the end customer.Imprecisely adjusted fault locators alsogive inaccurate readings of the distanceto the respective fault. In these instances,repair crews cannot be sent to preciselocations, and this increases both thetime required to carry out themaintenance work and the associatedcosts. On the other hand, if the lineimpedance is measured correctly, distanceprotection and fault locators can beaccurately adjusted. It is important tonote that the difference between themeasured values and the calculatedvalues can be quite considerable here.For example, the calculated parametersof a protective relay which disconnectedthe wrong line section from the powersystem after a fault occurred were testedby actual measurement, which can beseen in the table shown.The measurement with the OMICRONCPC 100 + CP CU1 system highlighteddeviations of 48% between the imprecise,mathematically determined values andthe zero-sequence impedance valuesactually measured.The CPC 100 from OMICRON is the idealsolution A total of seven measurementscan be performed to accurately determinethe impedances of a system.These consist of three for line-to-lineloops, three for line-to-ground loops andone for all three lines connected inparallel to ground.The measurement results can then beused to gain reliable data for adjustingprotective relays and fault locators. Withthe CPC 100 and the CP CU1, thesemeasurements provide precise values, asfrequency-selective measurement eliminatesany connected interference.The time and costs required to actuallyperform the measurements are alsoradically reduced, since portableelectronic generators can be used in placeof the devices weighing several tons.The OMICRON testing device offers asignal generator, power amplifier,measurement technology, and a PC in asingle unit.The coupling unit CP CU1 and itsgrounding box CP GB1 also secureoptimum protection throughout themeasurement process.WIPInternet linkwww.omicron.at28M ARCH 2012 • WORLDWIDE I NDEPENDENT P OWER