04-06-2012, 02:13 PM
Safety Aspects of GSM Systems on High-Voltage Towers: An Experimental Analysis
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Abstract
Low-voltage (LV) applications mounted on
high-voltage (HV) towers may pose a safety problem in the low
voltage network when a flashover occurs over the high-voltage
insulators. To study the effects experimentally, a 150-kV system
was made available. On a high voltage tower carrying a cellular
phone base station, one phase insulator was shorted to the tower.
A current of about 200 A was injected into that phase at a large
distance.
INTRODUCTION
THE RECENT growth of cellular phone base stations lead
to a quest for elevated locations for the base stations, particularly
in rural areas. Fig. 1 shows a common solution, a cellular
phone antenna placed on top of a high-voltage (HV) tower.
Further on we will use the European acronym GSM for “global
system for mobile communication,” originally “groupe speciale
mobile.”
MEASUREMENTS
The current distribution is given in Fig. 3, where all currents
and voltages are normalized to 100-A injection. The actual injection
current was 228 A and 1.2 kA. The current distribution
was similar at both amplitudes. The small tables represent the
measured currents (top) and the calculated values (bottom) to be
discussed in Section IV. The limited time for the measurements
did not allow to determine the current distribution at the MV/LV
transformer.
The skywires at the tower carry about 64% of the fault current.
The current toward the ground at tower 33 includes all telecom
cables. A total of 36% descends; more than half of that current
leaves the tower footing through the LV cable connected to the
base station toward the LV transformer. The measured voltage
transfers were all of the same order of magnitude. At LV1 and
LV2 we found 5.7 V and 4.7 V between the neutral and local
ground. In addition, between neutral and telecom ground at LV2
5.0 V was observed. All voltages again are scaled to 100-A injection
current.
CONCLUSION
Simulations indicated that safety could be compromised by
LV applications mounted in HV towers, in particular, cellular
phone base stations. The model has been verified by measurements
on an existing installation.
The observed rate of lightning induced flashover is about five
per year in the NUON 150- kV grid. The large number of GSM
base stations already installed and still planned, the dense population,
and the serious consequences of a flashover justify extensive
safety measures. Faults on neighboring towers are equally
dangerous as a lightning strike on the GSM tower proper.
Several solutions are proposed. An isolation transformer
combined with a suitable varistor will be imposed.