The potential savings induced by the operational maintenance are most often considered secondary because the main goal of this procedure is always the qiuck restoration of desired parameters e.g.
dielectric strenght of the oil Ud (kV / 2.5mm)
or water content in the oil Qw ( ppm )
etc.
The predominant reasons for the operational maitenance are therefore the legal nature - the transformer may not be operated outside IEC norms.
On the other hand, the main goal of the preventative maintenace is dominatly the saving induced by the prolonging of the life-span of the old or aged transformer , which will be without these measures "sentenced to death" and a new one must be bought. Increased safety of the life-extending operation is evident.
Read next: 2. Estimation of the economical effect of the postponned replacement of a transformer