To avoid the mal-operation of transformer differential protection when out-zone fault occurs under current transformer (CT) saturation, a new algorithm based on frequency distribution is proposed. A new current waveform is obtained by a symmetric transformation of initial part of differential current after fault occurred, and then its characteristic is extracted by frequency distribution histogram. In the case of CT’s serious saturation, medium saturation, mild saturation, the out-zone fault and the inner-zone fault can be clearly detected by the proposed algorithm. This algorithm only needs detecting the time of fault occurring, while the time of differential current increasing is no more needed, the proposed algorithm is very fast and reliable. Simulation show satisfactory results and validate its flexibility and reliability.%为有效防止变压器区外故障因 CT饱和引起的差动保护误动，提出了一种基于频数分布的检测 CT饱和的新方法。该方法截取从故障发生到差流达到第一个极值之间的部分差流波形并经适当变换，利用频数分布直方图进行电流波形处理。根据频数分布特征，能够实现 CT 严重饱和、一般饱和、轻度饱和情况下变压器区内外故障的准确检测。该方法只需定位故障发生时刻，而不需定位差流出现时刻，算法简单，快速可靠。经大量仿真实验验证了新方法的可靠性和准确性。
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