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Siemens 7SR21 Manual
Also see for Argus 7SR21: Installation guideManualTechnical manualManualManual
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7SR21 & 7SR22 Argus Applications GuideSection 3: CT Requirements3.1 CT Requirements for Overcurrent and Earth FaultProtection3.1.1 Overcurrent Protection CTsa) For industrial systems with relatively low fault current and no onerous grading requirements - a class10P10 with VA rating to match the load.b) For utility distribution networks with relatively high fault current and several grading stages - a class5P20, with VA rating to match the load.Note: if an accuracy limit factor is chosen which is much lower than the maximum fault current it will benecessary to consider any effect on the protection system performance and accuracy e.g. grading margins.For i.d.m.t.l. applications, because the operating time at high fault current is a definite minimum value, partialsaturation of the CT at values beyond the overcurrent factor has only a minimal effect. However, this must betaken into account in establishing the appropriate setting to ensure proper grading.c) For dtl applications utilities as for (b) above - a class 5P10 (or 20), with rated burden to suit theload.Note: Overcurrent factors do not need to be high for definite time protection because once the setting isexceeded magnitude accuracy is not important. Often, however, there is also the need to considerinstantaneous HighSet overcurrent protection as part of the same protection system and the settings wouldnormally be of the order of 10x the CT rating or higher. Where higher settings are to be used then theovercurrent factor must be raised accordingly, e.g. to P20.3.1.2 Earth Fault Protection CTsConsiderations and requirements for earth fault protection are the same as for Phase fault. Usually the relayemploys the same CT's e.g. three phase CTs star connected to derive the residual earth fault current.The accuracy class and overcurrent accuracy limit factors are therefore already determined and for both thesefactors the earth fault protection requirements are normally less onerous than for overcurrent.3.2 CT Requirements for High Impedance Restricted EarthFault ProtectionFor high impedance REF it is recommended that:Low reactance CTs to IEC 60044 Class PX are used, this allows a sensitive current setting to be applied.All CT’s should, if possible have identical turns ratios.The knee point voltage of the CTs must be greater than 2 x 64H setting voltage Vs.Where the REF function is used then this dictates that the other protection functions are also used with class PXCTs.A full explanation of how to specify CTs for use with REF protection, and set REF relays is available on ourwebsite: www.siemens.com/energy©2010 Siemens Protection Devices Limited Chapter 7 Page 28 of 39 |
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