The measurement error of such a large rectified output is caused by the measurement method. First of all, the waveform after SCR rectification is not pure DC. If the DC current measurement method is used, the error will definitely be larger. Secondly, in order to measure such a large current, it is necessary to measure the harmonic interference caused by transformer and thyristor, which leads to the superposition of harmonic components in the signal measured by transformer, and there must be great errors. The so-called formula can't solve this problem.

You need to analyze the waveforms, compare the deviations, and then correct them.