直流偏磁条件下电流互感器的传变特性

Transfer Characteristics of Current Transformers With DC Bias

直流偏磁现象会改变电磁式电流互感器(current transformer,CT)的传变特性,影响继电保护可靠性和电能计量的公平性。通过建立等效电路模型,利用电磁关系方程推导剩磁和偏磁对保护用CT起始饱和时间影响的定量数值关系;利用谐波平衡原理获得了偏磁条件下计量用CT传变误差的解析表达式。分析表明,直流偏磁将加速保护用CT的饱和(达若干ms),且偏磁电流越大CT饱和越快;同时,直流偏磁可使计量用CT的角差和比差增大,导致计量结果失真。仿真算例验证了理论分析结果的正确性。该研究结果为保护用CT的选择、继电保护的整定以及计量用CT的使用提供了参考依据。

The transfer characteristics of current transformers （CT） will subject to possible changes due to DC bias, which may thereby bring about severe impacts on reliability of the relay protection and fairness of the power metering. Based on equivalent circuit models and electromagnetic equations, the integrated effects of both DC bias and remnant flux on the time-to-saturation of protective CTs were deduced, and thereby numerical expressions were quantitatively given. Mathematical formulas of phase-displace- ment and ratio error were also obtained for metering CTs with DC bias. Analysis indicates that, the DC bias current will accelerate saturation of the protective CTs up to several milliseconds, and also increase transfer errors of the metering CTs. Simulation and case study were given to verify the above analysis results. The proposed research presents referential basis for proper selection of both protective and metering CTs as well as reliable setting of corresponding relay device in power systems.