CT SATURATION CORRECTION BASED ON THE ESTIMATED CT SATURATION TIME CONSTANT W. Rebizant*, A. Wiszniewski*, L. Schiel t *Wroclaw University of Technology, Poland, waldemar.rebizant Capwr.wroc.pl, andrzej .wiszniewski@pwr.wroc.pl t1 SIEMENS AG, PTD EA, Berlin, Germany, ludwig.schiel~siemens.com Keywords: current transformer, CT saturation correction, CT time constant, EMTP simulations. Abstract A new method for CT saturation correction is presented that is based on an on-line estimation of the CT magnetizing inductance. The approach adopted is based on numerical solution of the differential equation, which describes the saturated current transformer. The correction procedure proposed has been tested with EMTP-ATP signals proving to be an effective tool for primary current reconstruction. 1 Introduction It is obvious that protection criterion values calculated on the basis of saturated CT secondary signal may fall quite distantly from their correct values, which might have been determined if the CT primary unsaturated signal was available. Erroneous measurement may in consequence lead to false decisions (e.g. underreaching of overcurrent relays, erroneous calculation of fault loop impedance in distance relays) and protection maloperation. All those problems are well described e.g. in [7]. Thus it can be stated that CT saturation phenomenon may impair protection system reliability if appropriate algorithms for saturation detection and/or correction are not applied to eliminate the problem. A large number of papers, which dealt with the CT saturation detection and correction issue, may be grouped into four families: I- no CT current correction is performed, but the information on CT saturation is used for other purpose, e.g. for adaptation of protection settings [11I] or performing calculations during CT unsaturated periods only [1]; 2 - correction performed is based on the information extracted from the secondary current during the saturation interval, with the aim to reproduce the primary current [4, 5, 6]; 3 - correction is based on information extracted from the secondary current during the non-saturation interval, which aims at determination of the fundamental and DC components of the primary current [ 10]; 4 - CT saturation detection and/or correction is done with use of Artificial Neural Networks trained with the simulation cases of CT transients [2, 8, 12, 14]. The known in literature methods of reconstruction the current on the ground of the secondary current data during the saturated period (family 3) usually require that the CT magnetizing characteristic and its load are a'priori known, which is not always the case. In this paper a new method is presented, which estimates the magnetizing inductance of the saturated CT and the resulting time constant on-line, on the ground of secondary current and estimated primary current samples. In fact, those parameters do not appear in the final equation, which determines the next unknown sample of the corrected current. The developed CT saturation correction algorithm has been tested in simulative way with EMTP-ATP generated signals. 2 Idea of the correction method The basic idea of handling the CT saturation problem adopted here is based on splitting the task into two subtasks, namely saturation detection and correction of the distorted secondary current (Fig. 1). When the CT is not saturated (i.e. the detection block has not detected it) the correction block is not activated. Starting from the point of saturation beginning ts 5 the procedure of secondary current correction is activated. The procedure is operative until the CT goes out of saturation. Secondary CT c.rret , Figure 1: Block scheme of the proposed CT saturation correction procedure. 174