PMU Placement using Graph Theoretic Approach and State Estimation W. A. Gavhane* and R.N.Awale Department of Electrical Engineering, Veermata Jijabai Technological institute, Matunga, Mumbai India *e-mail : walmikgavhane@yahoo.co.in Abstract: The Phasor Measurement Unit (PMU) is considered to be one of the most important measuring devices in the future of power systems. The distinction comes from its unique ability to provide synchronized phasor measurements of voltages and currents from widely dispersed locations in an electric power grid. Global Positioning Satellite (GPS) with accuracy of timing pulses in the order of 1 microsecond made possible the commercial production of PMU. Simulations and field experiences suggest that PMUs can revolutionize the way power systems are monitored and controlled. However, it is perceived that costs and communication links will affect the number of PMUs to be installed in any power system. A novel method of PMU placement based on incomplete observability using graph theoretic approach is proposed. The objective is to reduce the required number of PMUs by intentionally creating widely dispersed pockets of unobserved buses in the network. Observable buses enveloped such pockets of unobserved regions thus enabling the interpolation of the unknown voltages. The concept of depth of unobservability is introduced. It is a general measure of the physical distance of unobserved buses from those known. Keywords: Global Positioning Satellite (GPS); Phasor Measurement Unit (PMU); Wide Area Network (WAN) 1. Introduction The Phasor Measurement Unit (PMU) is a power system device capable of measuring the synchronized voltage and current phasor in a power system. Synchronicity among PMUs is achieved by same-time sampling of voltage and current waveforms using a common synchronizing signal from the global positioning satellite (GPS). The ability to calculate synchronized phasors makes the PMU one of the most important measuring devices in the future of power system monitoring and control. The technology behind PMUs traced back to the field of computer relaying.[1] The phasor are calculated via Discrete Fourier Transform (DFT) applied on a moving data window whose width can vary from fraction of a cycle to multiple of a cycle. Equation (1) shows how the fundamental frequency component X of the DFT is calculated from the collection of Xk waveform samples.[4] = √  ∑     ∈ / (1) Synchronization of sampling was achieved using a common timing signal available locally at the substation. Timing signal accuracy in the order of msec suffices for this relaying application. It became clear that the same approach of calculating phasors for computer relaying could be extended to the field of power system monitoring. However the phasor calculations demand greater than the 1-millisecond accuracy. It is only with the opening for commercial use of GPS that PMU was finally developed. GPS is capable of providing timing signal of the order of 1 μsec at any locations around the world. It basically solved the