基于改进EMD与改进灰色相关度的防止变压器差动保护误动的新方法

为了有效防止变压器区外故障电流互感器(CT)饱和引起的差动保护误动以及区内故障CT饱和引起保护拒动，提出了一种基于改进经验模态分解(EMD)和改进灰色相关度的防止变压器差动保护误动的新方法。该方法主要利用了区内、区外故障时差流波形存在差异这一特点，只需定位故障发生时刻与第一个差流极值点出现时刻，将两时刻之间的差流波形进行关于坐标原点的对称变换，得到新的差流波形。之后将新的差流波形与正弦“小波”信号叠加得到合成波形，对合成波形进行改进EMD获得其第一个本征模态函数(IMF1)，然后求取合成波形与其IMF1的改进灰色相关度。由于该方法仅需提取故障发生时刻与第一个差流极值点出现时刻的差流波形，且区内、外故障时的改进灰色相关度数值相差甚远，因此该方法能够保证快速、准确地对变压器区内、外故障做出识别，从而保证区外故障CT达饱和时保护不误动；区内故障时CT达饱和时，保护不拒动。PSCAD及Matlab仿真实验验证了该方法的可行性和有效性。

A new method based on modified EMD and revised grey relation for avoiding mal-operation of transformer differential protection

To avoid the mal-operation of transformer differential protection when outer-zone fault occurs under current transformer (CT) saturation and the refusal-operation of the protection when inner-zone fault occurs under CT saturation, a new method based on modified Empirical Mode Decomposition (EMD) and revised grey relation is proposed in this paper. A new waveform is obtained by a symmetric transformation of the initial part of differential current after fault occurred. Then a sine signal with low amplitude is added on the new waveform. And the modified EMD is applied for the composite waveform to get its first Intrinsic Mode Function (IMF1). Then calculation of the grey relation between the composite waveform and its IMF1 is made. Due to the new method just taking the initial part of the differential current as well as the significant difference between the outer-zone and inner-zone grey relation, it is able to distinguish the two fault types quickly and accurately. Consequently, when CT goes into saturation, it can ensure that the differential protection will not operate at out-zone fault and trips correctly at inner-zone fault. Simulation results of PSCAD and Matlab validate its feasibility and availability.