The aim of the paper is to present a new method for estimation of the CT (Current Transformer) rated primary current and / or load of the secondary winding derating levels for transformation of distorted current in relation to the rated values fixed for transformation of sinusoidal current. This ensures the values of the CT composite errors/transformer correction factors as resulting from its IEC 60044-1/IEEE C57.13 accuracy class in condition of distorted currents transformation.
 A. Cataliotti, D. Di Cara, A.E. Emanuel, and S. Nuccio, “A novel approach to current transformer characterization in the presence of harmonic distortion”, IEEE Trans. Instrum. and Meas. 58 (5), 1446-1453 (2009).
 A. Wiszniewski, W. Rebizant, and L. Schiel, “Correction of current transformer transient performance”, IEEE Trans. Power Delivery 23 (2), 624-632 (2008).
 M. Kaczmarek and R. Nowicz, “Application of instrument transformers in power quality assessment”, Proc. Modern Electric Power Systems Conf. IEEE Xplore 1, CD-ROM (2010).
 M. Kaczmarek and R. Nowicz, “Simulation of the influence of conductive disturbances on accuracy of the voltage transformers during measurements of the power quality,” Proc. Electric Power Quality and Utilization Conf., IEEE Xplore 1, CD-ROM (2009).
 M. Kaczmarek, ”The influence of the sinusoidal conductive interferences on metrological properties of voltage transformers under the change of their operation conditions”, Przeglad Elektrotechniczny 86 (3), 34-35 (2010).
 M. Kaczmarek, “Method of current transformer metrological properties estimation for transformation of distorted currents”, Proc. IEEE Int. Power Modulator and High Voltage Conf. IEEE Xplore 1, CD-ROM (2012).
 A. Kasprzak, M. Orlikowski, and D. Brodecki, “Operation of voltage transformer in grids with distorted signals”, Bull. Pol. Ac.: Tech. 59 (4), 551-554 (2011).
 IEEE Std. C57.13-2008, IEEE Standard Requirements for Instrument Transformers.
 IEC 61869-2, Instrument Transformers - Part 2: Additional Requirements for Current Transformers, IEC, Geneva, 2012.
 G. Benysek, M.P. Kazmierkowski, J. Popczyk, and R. Strzelecki, “Power electronic systems as a crucial part of Smart Grid infrastructure - a survey”, Bull. Pol. Ac.: Tech.59 (4), 455-473 (2011).
 M.B.B. Sharifian, J. Faiz, S.A. Fakheri, and A. Zraatparvar, “Derating of distribution transformers for non-sinusoidal load currents using finite element method”, Proc. 10th IEEE Int. Conf. Electronics, Circuits and Systems 2, 754-757 (2003).
 E.F. Fuchs, Lin Dingsheng, and J. Martynaitis, “Measurement of three-phase transformer derating and reactive power demand under nonlinear loading conditions”, IEEE Trans. on Power Deliv. 21 (2), 665-672 (2006).
 A.W. Kelley, S.W. Edwards, J.P. Rhode, and M.E. Baran, “Transformer derating for harmonic currents: a wide-band measurement approach for energized transformers”, IEEE Trans. on Ind. App. 35 (6), 1450-1457 (1999).
 IEEE Std, C57.110-2008, IEEE Recommended Practice for Establishing Transformer Capability When Supplying Nonsinusoidal Load Currents.