Vallakati, Ranganath; Ranganathan, Prakash: 'An optimal redundancy criterion index (ORC) for optimal placement of phasor measurement units (PMU) for full observability of power grid' (Energy Engineering, 2017), 'Synchronized Phasor Measurements for Smart Grids', Chap. 2, pp. 29-58, DOI: 10.1049/PBPO097E_ch2
IET Digital Library, https://digital-library.theiet.org/;jsessionid=5cbtclmh9o2s8.x-iet-live-01content/books/10.1049/pbpo097e_ch2