Johns, A.T.; El-Alaily, A.A.: 'Variable-characteristic generalised techniques for distance protection: Double-circuit-application studies', Proceedings of the Institution of Electrical Engineers, 1974, 121, (12), p. 1582-1584, DOI: 10.1049/piee.1974.0327
IET Digital Library, https://digital-library.theiet.org/;jsessionid=1ub5dwt5o7t8t.x-iet-live-01content/journals/10.1049/piee.1974.0327