Influence Of DC Magnetic Bias On Transformer Differential Protection And Suppression

DC magnetic bias means that after the DC current into the transformer, the transformer magnetic flux saturation, the excitation current waveform is no longer symmetric, distorted, and produced a large amount of harmonic and a series of electromagnetic effect. DC biasing can have a serious impact on the transformer, such as the transformer noise increases rapidly, vibration intensified, release a lot of heat, especially affect the differential protection, therefore, to study on influence of DC magnetic bias differential protection is a problem worthy of study.There are verities of reasons that may cause DC magnetic bias.This paper studies the effect on transferring characteristics of current transformers(CT) casued by GIC and propounds new means of preventing maloperation of transformer differential protection.In order to suppress DC bias, so that the safe operation of power grid.This paper analysis the direct inhibition of several measures of DC bias and compares the advantages and disadvantages. Referring to GIC-type DC magnetic bias, the functions of magnetizing current are derived in condition that the primary current consists of components with different frequencies, and the variation of magnetizing inductance is dicussed based on physical significant. Furthermore, we bring in the concept of time-scale to analyze the effects on the transient transferring characteristics of CT and the transfer error of power frequency current during different stages of GIC. Based on typical waveform feature of GIC an equivalent source model of GIC is constructed, and the saturation characteristic of CT under the affect of GIC with different magnitude and frequency is simulated, and the resulting affect on transformer differential protection is researched.Besides, considering that GIC possesses the feature of high-amplitude, low-frequency, time-variance in regional power grid, it may lead to serious saturation of CT, so the unbalanced current calculated by transformer differential protection is enlarged and the content of the second order harmonic in the differential current will drop below the restraint threshold, and finally the maloperation of transformer differential protection occurs. To prevent such a maloperation, the measure based on differential current and the variation tendency of the content of second order harmonic in it can be adopted.