  Phasor Measurement Units (PMUs) are essential measuring devices for
monitoring, control and protection of power systems. The objective of the
optimal PMU placement (OPP) problem is to minimize the number of PMUs and
select the bus locations to make a power system completely observable. In this
paper, the OPP problem is formulated as a nonlinear programming (NLP) problem
and a sequential quadratic programming (SQP) method is used for its solution.
Simulations are carried out on IEEE standard test systems, using MATLAB. The
numerical results are compared to those obtained by a binary integer
programming (BIP) model, also implemented in MATLAB. The comparative study
shows that the proposed formulation yields the same number of PMUs as the BIP
model. The fundamental contribution of this paper lies in investigating the
feasibility of using NLP for the solution of the OPP problem and the ability of
the proposed methodology to provide multiple solutions in contrast to the
binary integer programming model. The System Observability Redundancy Index is
adopted to further rank the multiple solutions.
