## 8 Summary and Proposed Applications  
As the main findings, it should be emphasised that both applied methods--analogue and digital--can be alternatively used to implement the integration operation and estimate the magnetic flux linkage in the core of a voltage transformer. Both methods allow for determining the periodic component of the flux course. Due to the construction of the proposed analogue electrical circuit and the offset elimination in the digital algorithm, it is not possible to estimate the magnetic remanence in the core. It is also worth noting that the problem of eliminating the offset of the numerical integration operation and making it insensitive to the constant component makes the digital algorithm quite complicated. This problem was addressed in the paper [15], where, in order to avoid this problem, it was decided to perform an approximate integration operation through a digital low-pass filter. Since this filter itself also suppresses the constant component, it also allows only for estimating the periodic component of the flux course (correctly only for a limited frequency range).  
Potentially, both methods could be used to create a unique criterion for detecting ferroresonant oscillations in medium voltage networks. Phase voltages or zero sequence voltage could then be integrated. Magnetic fluxes calculated in this way could constitute an input signal for the excess element calculating the RMS value or the peak value of the periodic course. Measurements of both of these criteria quantities are, of course, associated with further problems (in particular, dependence on frequency). Nevertheless, even for a measurement window of constant width, the development of a new detection criterion using the estimated magnetic flux linkage is possible.  
Another application could also be proposed as an attempt to estimate the location of the instantaneous operating point on the magnetisation characteristic of a voltage transformer. Assuming that the shape of the magnetisation characteristic is not known, an additional measurement of the current drawn by the voltage transformer is necessary. In combination with the estimated periodic component of the magnetic flux, this would allow for the estimation of the course of the magnetic hysteresis. The measured distorted current drawn by the transformer, which is almost equal to the magnetising current, could be the basis for reproducing the magnetic remanence in the transformer core. Of course, such an application requires further research and the development of a method for measuring the very small current drawn by the voltage transformer. Despite these difficulties, magnetic flux estimation could also be useful in this case.  
Conceptualization, P.S. and A.S.; methodology, P.S.; software, P.S. and A.S.; validation, P.S.; formal analysis, P.S. and A.S.; investigation, P.S. and M.O.; resources, P.S.; data curation, P.S.; writing--original draft preparation, P.S.; writing--review and editing, P.L., A.S., S.L., M.S., A.L. and L.K.; visualisation, P.S.; supervision, P.S. and A.S.; project administration, P.S.; funding acquisition, L.N. All authors have read and agreed to the published version of the manuscript.  
This work was supported by Electrical Power Engineering Institute of Warsaw University of Technology within research subsidy, (ZPB program IE_ZPB_1_01_2022).  
Not applicable.  
Data Availability Statement: The data presented in this study are available on request from the corresponding author. The data are not publicly available due to Mr. [PERSON] will be defending his doctorate on the subject. Until the defense, he does not want to disclose this information.  
The authors declare no conflicts of interest.