Mitigating the negative impact of the stray flux on the current transformers using flux equalizing windings

Abstract The accuracy of the current transformers (CTs) supplying the generator differential relay is essential in power plant stations. One important factor which decreases the CT accuracy is the stray flux originated from the adjacent phases. During external fault conditions, the currents of adjacent phases contain a considerable decaying DC component. Thus, corresponding fluxes also contain a decaying DC component which cannot be counteracted by bus bar enclosure and generator shell, completely. Hence, the CT output signal deviates from the correct waveform which may lead to undesirable operation of the differential relay supplied by the CT. In fact, there is no internal fault in the generator but the differential relay trips the breaker due to negative impact of stray flux on CT accuracy. In this paper, a real power plant bus bar is modeled using three-dimensional (3D) finite element method. Then, the effect of the stray flux on the accuracy of the CTs is studied. Finally, flux equalizing windings are considered on the CT’s secondary winding. According to finite element simulation results, the negative impact of the stray flux on the CT accuracy is decreased considerably by the flux equalizing windings.

[1]  Mehdi Vakilian,et al.  Current-Transformer Saturation Compensation for Transformer Differential Relays , 2015, IEEE Transactions on Power Delivery.

[2]  W. Rebizant,et al.  Current transformer saturation detection with genetically optimized neural networks , 2005, 2005 IEEE Russia Power Tech.

[3]  Andreas Braun.  Determination of Current Transformer Errors at Primary Currents up to 100 000 A , 1977, IEEE Transactions on Instrumentation and Measurement.

[4]  H. Khorashadi-Zadeh,et al.  Correction of saturated current transformers secondary current using ANNs , 2006, IEEE Transactions on Power Delivery.

[5]  Z. Gajic,et al.  Influence of Stray Flux on Protection Systems , 2008 .

[6]  R. A. Pfuntner.  The Accuracy of Current Transformers Adiacent to High-Current Busses , 1951, Transactions of the American Institute of Electrical Engineers.

[7]  K. Erenturk,et al.  ANFIS-Based Compensation Algorithm for Current-Transformer Saturation Effects , 2009, IEEE Transactions on Power Delivery.

[8]  Okan Ozgonenel.  Correction of saturated current from measurement current transformer , 2013 .

[9]  Carlos A. Duque,et al.  Current-Transformer Saturation Detection Using Savitzky-Golay Filter , 2016, IEEE Transactions on Power Delivery.

[10]  Mehdi Vakilian,et al.  Residual Flux Mitigation of Protective Current Transformers Used in an Autoreclosing Scheme , 2016, IEEE Transactions on Power Delivery.

[11]  M. Sanaye-Pasand,et al.  An Accurate Current Transformer Model Based on Preisach Theory for the Analysis of Electromagnetic Transients , 2008, IEEE Transactions on Power Delivery.

[13]  Paulo F. Ribeiro,et al.  A New Method of Current-Transformer Saturation Detection in the Presence of Noise , 2014, IEEE Transactions on Power Delivery.

[14]  L. Schiel,et al.  Correction of Current Transformer Transient Performance , 2008, IEEE Transactions on Power Delivery.

[15]  Jiansheng Yuan,et al.  Design of novel structure current transformer with shielding coils for overcoming the saturation of core , 2006, IEEE Transactions on Magnetics.

[16]  Bhavesh R. Bhalja,et al.  New Algorithm for current transformer saturation detection and compensation based on derivatives of secondary currents and Newton's backward difference formulae , 2014 .

[17]  Ali Hooshyar,et al.  Waveshape recognition technique to detect current transformer saturation , 2015 .

[18]  Wei Zhao,et al.  Interference mechanism of external current on heavy current transformer , 2009, 2009 IEEE Instrumentation and Measurement Technology Conference.

[19]  D. O'Kelly.  Calculation of the transient performance of protective current transformers including core hysteresis , 1992 .

[20]  Nagy I. Elkalashy,et al.  Detection of Partial Saturation and Waveform Compensation of Current Transformers , 2015, IEEE Transactions on Power Delivery.

[21]  Zarko Djekic,et al.  The Impact of Current-Transformer Saturation on Transformer Differential Protection , 2015, IEEE Transactions on Power Delivery.