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Abstract
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In steady-state operation, the frequency of an AC power system is constant. Because of variations in demand or generation, network operations such as line disconnections, stochastic noise, harmonics, and, on occasion, failures, the frequency in a power system is never constant and can fluctuate dramatically. In general, variations in frequency are inversely related to power system inertia, although distributed generation penetration is constantly decreasing inertia. This work proposes an estimator with frequency tracking that is based solely on phasor measurement units (PMUs) with different reporting rates. PMUs are becoming more common in power networks, primarily at the transmission level. Computed phasors are transferred at up to 100 (120 for 60Hz system) reports per second across a digital communication network to higher level applications such as the state estimator (SE). This enables the estimation to be calculated at a high refresh rate, making it possible to capture system dynamics. The estimation is done in two phases. In the first phase, frequency is calculated along with missing measurements from PMUs with low frequency reporting rates. The estimated system frequency influences the impedances of branches and shunts in the system, which is handled in the second phase. The paper presents numerical results on IEEE test systems and compares them with the results obtained by the recently published state estimators.
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