Two-stage current transformer or compensated current comparator is the most commonly used high-precision standard current transformer, due to the limitation of use, it is generally only used for the traceability or transmission of the value of the power frequency current ratio. At present, China is gradually promoting the application of flexible low-frequency transmission projects in Jiangsu and Zhejiang, and low-frequency transformers are the key metering or measuring equipment for low-frequency transmission projects, and the accuracy and reliability of their values need to be calibrated by high-precision standard current transformers. There is little research on low-frequency standard current transformers, and the technical level is still in its infancy. On the basis of the traditional compensated current comparator, we have developed a low-frequency standard current transformer that uses electronic circuits to compensate the core excitation. In this paper, the basic principle structure of this low-frequency standard current transformer is expounded, the theoretical error analysis is analyzed, and the error calibration test is carried out. The test results show that the error is less than 0.0001 ppm and 3μrad at 20Hz, which can be used for high-accuracy measurement of low-frequency current.