Sonel PQM analyzers case study #13: Interference in medium-voltage lines

Medium-voltage overhead lines often operate close to trees or shrubs. During strong winds, vegetation can cause transients and threats to the continuity of the power supply in the form of sags and blackouts. It is thus important for energy services to take proper care of the transmission infrastructure, e.g. by cutting down trees and shrubs that threaten the lines.

Description of the identified problem

During the normal operation of the distribution network, there was a period of very strong winds, following which the threat of the distance to ground obstacles in the form of tree branches being reduced increased to a considerable extent. What was the interference pattern and impact on the deterioration of power supply performance?

Measurement tools used

• A Sonel PQM-703 analyser connected behind the voltage transformers (see successor Sonel PQM-711)
• A set of C-6A clamps (In=10 A) for measurements behind the current transformers
• Sonel Analysis software

 

Figure 1 Graph of voltages and flicker index Plt in 15 kV line during gusty winds with marked voltage sags (markers no. 1, 2 and 3)

 

Figure 2 RMS ½ graph of voltages and currents during a voltage sag (marker no. 4) in a 15 kV medium voltage line

 

Figure 3 Oscillogram of instantaneous values of voltages and currents during a voltage sag (marker no. 5) in MV 15 kV line

 

Figure 4 Oscillogram of the transient (tick no. 6) in the 5 MHz band in a 15 kV line.

 

FINAL CONCLUSIONS:

1. Gusty winds resulting in a reduction in the distance between the overhead line conductors and obstacles and between each other can lead to a puncture of the aerial insulation.
2. The wind, combined with rainfall, has caused unstable network operating conditions, manifested by the occurrence of power supply sags (Figs. 1, 2, 3).
3. Equally adverse phenomena are the transients that occur (Fig. 4).
4. From a power quality point of view, voltage sags deteriorate the statistics of the flicker index (Fig. 1), but have little effect on the physical aspect of flicker. Keep this in mind when generating the EN 50160 compliance report and consider the exclusion of flagged flicker samples.

RECOMMENDATIONS:

1. The MV lines should be reviewed to detect obstructions that may cause problems during gusty winds.

 

Authors: Krzysztof Lorek, Marcin Szkudniewski