# Multi-Point Time-Synchronized Waveform Recording for the Analysis of Wide-Area Harmonic Propagation
## 4 Results
### Impact of Harmonic Impedance on Voltage Distortion Levels  
In conventional power quality analysis, it is a common practice to omit influence of linear loads operating at nearly unity power factor. Such loads, however, by means of resistive nature act as a shunt impedance connected to the LV side of distribution substation. This shunt impedance in turn decreases proportionally the system harmonic impedance providing the low-impedance path for harmonics. Theoretically, this can lead to the decrease of THD levels. This effect is best described in literature as \"damping\".  
The performed experiments, however, demonstrate that in some cases this conclusion can be somewhat too optimistic. In this test sequence (B.5), we connected 24 kW of resistive loads equally divided between both distributed substations and evaluated THD at LV test days both before and after this operation has been performed. In Figure 25, the results of this experiment are shown in the representative form of box plots rendered over the whole duration of the signals. As it can be seen for both substations, the median levels (red horizontal line) of THD are actually higher after connecting the linear load banks. Here, it is important to note that, while conventional power quality studies and computer simulations (mainly because of the nature of used models of harmonic loads) rely exclusively on calculations based on magnitudes of currents and voltages, the harmonic processes in real grid scenarios are much more complex. Depending on the phase angles of harmonic current and voltage phasor (given that we did not have any control over background distortion) and new value of impedance, this interaction in some cases can lead to the rising levels of final THD values, even when expected otherwise.  
Next, a dynamically changing topology at MV levels is considered to be a known issue for harmonic analysis. The reconfiguration of the network occurring at any given time can significantly alter the voltage distortion values and if not considered properly in subsequent simulations can lead to the erroneous outcome. As per test scenario B.6, we dropped the short cable connecting Primary Switchboard to Substation G and engaged cable B with equivalent electrical distance of about 5 km (see Figure 1). After recording synchronized voltage waveforms at MV busbars of both Substation G and Primary Switchboard (Isolation Transformer), the THD values were collected in the form of  
Figure 25: Influence of linear load banks on LV-side THD levels. Test sequence B.5—public grid supply.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Harmonic Frequency, Hz** & **Emission Voltage Sub A, V** & **Emission Voltage Sub G, V** \\ \hline
250 & 0.574 & 0.043 \\
350 & 0.232 & 0.054 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Average RMS magnitude of harmonic voltage emissions per substation.  
box plot and these are presented in Figure 26. While the spread of THD values became larger, we can clearly observe the increase of median values and, in this case, increased harmonic impedance at the MV level contributed to larger values of voltage distortion.