Online Estimation of Cable Harmonic Impedance in Low-Voltage Distribution Systems

Incorrect values of cable harmonic impedance can
lead to wrong conclusions when assessing an impact of current
distortion on the power network. A theoretical derivation
of resistance and reactance for harmonic frequencies is not
sufficient as it neglects operating conditions of cables as well
as their configuration and actual installation pattern. In this
paper, a noninvasive procedure for estimating parameters of the
low-voltage cable is proposed. On contrary to invasive methods,
the proposed technique does not require any external sources
of harmonic excitation, and consequently, it does not introduce
additional disturbances to the system. A disconnection of the
cable is not required as the procedure meant to be performed
online. The methodology employs natural variations of harmonic
currents and voltages; however, the impedance angle is preserved
due to the utilization of synchronized measurement samples. The
developed signal processing algorithm provides noise reduction
capabilities and smoothing of the output in presence of fluctuating
harmonics. In addition, an attention is given to the metrological
characterization of the measurement system and evaluation of
uncertainty of impedance values. The results of the laboratory
experiment indicate that for selective harmonics the variations
of impedance estimates are no more than 10 mOhm, which is in the
range of calculated uncertainty values. The practical applicability
of the method is discussed together with its limitations.