Residual flux measurement in voltage transformer when power transformer is de-energized.
Magnetizing current is 90 degree leading compare to secondary voltage, hence for minimum residual magnetism, for power transformer, if switch off should done at peak voltages. Though residual magnetism depends on the core material too, different with different core materials(B-H curves). First cycle of in rush wave could give the value of residual magnetism. My paper(1) laboratory methods to generate inrush current pattern(2) CT performance for inrush could give the idea to find residual magnetism of core material. I developed one cycle wave form approached relay for power transformers, hence could refer my papers....
Thank you Bhupendra Desai sir for above update on my question.
is it accurate to measure the residual flux on the terminal side ( integrating PT seconday voltage on the de-energization of power transformer)?
If the objective is to determine when to close a circuit breaker, this is not needed. If you used a properly sized pre-insertion resistor, the inrush is limited to less than load current and the device does not need to be timed to the voltage angle. For a first estimate of inrush for the worst case and with residual flux, use this link:
https://www.southernstatesllc.com/applications/connecting-solar-to-the-grid
Frequently the residual flux is in the neighborhood of 0.7PU, 0PU and -0.7PU on three phase transformers after circuit interruption for routine load switching.