Schober, F., A. Krieg, A. Küchler, M. Liebschner, and F. Berger. “Parameter Influences on HVDC Transformer Insulation and Its Link to Conduction Processes”. International Journal of Electrical and Computer Engineering Systems, vol. 6, no. 2, Dec. 2015, pp. 57-62, https://ijeces.ferit.hr/index.php/ijeces/article/view/825.