A numerical approach to the evaluation of uncertainty in nonconventional measurements on power systems

The use of digital signal processing techniques allows measuring any quantity mathematically defined. However, the application of the rules endorsed by the ISO "Guide to the expression of uncertainty in measurement" to evaluate the measurement uncertainty can turn into a difficult task, for instance, when quantities different from those usually measured to characterize the operation of power systems must be determined. In this paper, a numerical approach is proposed to easily tackle this task. Contrary to the closed-form approach, which allows to evaluate the output uncertainty by analytically solving the uncertainty propagation law, as prescribed by the ISO Guide, the method proposed numerically finds the uncertainty by applying a recursive algorithm. First, first the validity of the numerical approach is proved in the case of the spectral analysis of a periodic signal, which is the first step for nonconventional measurement. Then, the approach is used to evaluate the uncertainty in the measurement of the harmonic losses of a power transformer supplying nonsinusoidal load currents. The results confirm that these losses can be accurately determined from the difference between two powers measured as correlated quantities at the transformer output terminals.