When an optimal power flow problem is formualted using Semi-definite programming method, the equivalent OPF problem contains one constraint
X or W = vv^H
where v is a vector of bus voltages and H is the complex conjugate transpose operator (v = [v1 v2 .... vn]^T where (1,2,...n) are buses indexes)
In various publications, authors say that this particular constraint is a rank-1 constraint and it is a non-convex?
I want to know how is this constraint a rank-1 constraint (What is actual meaning of rank-1 constraint)? Further, how is this constraint non-convex?