Segunda parte: Acertijos simples

Electric energy utilises overhead and underground means to deliver power. Overhead networks comprise relatively low-cost insulators and conductors mounted on poles made of various materials like wood, steel or concrete. Other overhead equipments are installed on some of these poles which make it more cost effective to repair and maintain. On the other hand, this direct exposure

Technical consideration and impact of converting Overhead Power Lines to Underground Power Cables

114

has a down side of being highly susceptible to malfunctioning due to environmental and man made breakdowns [29].

Undergrounding consists of maintenance holes which are commonly referred to as manholes which are tubes used to connect underground utilities to the surface. Manholes are widely used in sewer systems, electrical and communication systems. These manholes are situated at regular intervals along the utility path, to allow easy access to maintenance workers. Rubber and other insulated conductors (cables) are installed and sliced inside these underground cavities. Therefore, fault detection and repairs can be a very costly exercise [29].

Undergrounding used to be implemented in very few cases where it met strict regulatory conditions. This was primarily, due to the high cost involved in undergrounding, for example when utilities and the public dispute over private property for line easement. Over the years, various factors have played a significant role in increasing the deployment of undergrounding. Some of these core factors are: the demand for higher power consumption which has led to building more generation sites. Unfortunately, it became ultra difficult to obtain properties for such purposes. Another factor was the need for larger more economical generation units instead of local units. Finally, a more recent factor came into inception which is that health and environmental effects have been pushed quite heavily by industry lobby groups, regulators and more green governments [29].

Technical consideration and impact of converting Overhead Power Lines to Underground Power Cables

115

The aforementioned emphasis on health and environmental effects was the major driver for increased undergrounding due to the perception that underground cables will emit less electromagnetic field. Furthermore, cities began to pay extra attention to the visual appearance of their urban areas and business districts to attract more tourists and investors, therefore more power lines started to disappear in well presented paved areas. Underground cables in general operate under lower voltage levels than their overhead counterparts. Therefore, it was necessary to implement step-up and step-down transformers to maintain a long overhead line route [29].

The biggest advantage of underground installation is that it is less exposed and susceptible to external factors than overhead. However, somehow or other, this advantage can be offset by the time and effort spent in locating and repairing faults should they take place. The nature of the installation design and the complexity will be determined by the obligatory standards used for the installation [29].

Chang et al. [14], argued that although underground cables have continued, up until the present, to be more expensive than overhead lines for the same capacity, a number of European countries like the Netherlands have had underground cables installed widely since the early 70‘s. The reasons behind their ultimate decisions were based on:

Technical consideration and impact of converting Overhead Power Lines to Underground Power Cables

116

 shortage of land and its utilisation;

 recent regulation changes and opposition to overhead lines among the public.

On the other hand development and reduction in the cost of cable technology enhanced with reliable and efficient cable installation have contributed considerably in reducing the overall cost of underground cables. Meanwhile the cost ratio is less prominent at lower voltages and the economic potential of operating and maintenance costs favour underground cables and make them a dominant alternative solution to overhead lines in many cases such as:

 highly congested or populated urban areas;

 areas with historic or environmental values;

 crucial sections of the network which have low reliability records; and

 areas with a need for extra capacity where constructing a new overhead transmission line is out of question.

Adopting underground power cables has been slow in Australia in the past mainly as a consequence of the high cost of the cable technology, the low density demographic of Australian cities and limited restrictions on the use of overhead lines. Major catastrophic events have normally acted as the mechanism which drives the undergrounding of power lines. In 1974, when Cyclone Tracey hit Darwin it provided the justification for undergrounding Darwin overhead lines so subsequently by 1980, more than 50% of that city

Technical consideration and impact of converting Overhead Power Lines to Underground Power Cables

117

was serviced by underground LV and MV cables. Although there was little effort to retrofit existing areas throughout Australian cities since 1980, a steady introduction of underground cables for new residential areas has been undertaken. The bush fire risks in Australia has turned out to be more apparent due to climate change, in many cases these bushfires were results of clashing of bare conductors. Besides, the consumer demand for a more reliable and safer power supply will highlight the necessity of undergrounding the existing power network [30]. The relevant aspects of different underground cables have been elaborated.