function [G,XY] = getgraphBus(nnod);
% creates the graph of "bus" topology with (nnod) nodes:

al = 0:2*pi/(nnod-1):2*pi-1e-10;
XY = [cos(al),0; sin(al),0]';
G = sparse(1:nnod-1,[2:nnod],1,nnod,nnod);
G = spones(G+G');
G = G-spdiags(diag(G),0,nnod,nnod);