Electrifying the desert

Other external factors, such as sand, salt and humidity, can also influence the performance of an electricity transmission and distribution network and therefore need to be taken into account when designing technology and selecting materials.

Heat exhaustion

The main challenge facing cable producers in the Middle East is to supply conductors that will not overheat. Jon Vail general manager, technical, for Dubai-based cable firm Ducab explains: “When a current passes through a cable it generates heat. If the ambient temperature is also high, less current can go through the cable, so you need bigger sized conductors.

High-voltage substation cables have to be specially designed taking the local environment and operating conditions into consideration. “Here, instead of using aluminium cables, which are less conductive, copper cables are preferred as they can carry more current,” says Vail. “To design a cable to cope with heat you need to use more metal, more copper.” Larger cables are required regardless of whether the lines are to be laid above or below ground, as arid soil retains heat.

But conductor size adjustments are not the only modifications needed. Changes are also made to the properties of the plastic that houses them.

Manufacturers add ultra-violet (UV) radiation stabilisers to the polymer mix for cables that will be permanently exposed to direct sunlight, as overtime, strong UV rays fatigue the external protective casing. Likewise, in areas of high rainfall, or where the water table is high, water-resistant cables are used.

It is worth mentioning as well that, in the Middle East, fitters have to avoid installing cables in extreme heat as the outer plastic can soften, making the conductors more susceptible to damage. The opposite happens in cold climates, where the protective layer can become brittle in severe wintry conditions.

Aggressive soil

Cable makers also have to contend with the region’s soil conditions. Alkaline soil is aggressive and can damage aluminium sheaths on underground cables. To counter this problem, Ducab uses lead-cased cables, which are more corrosion resistant.

The presence of sulphur underground is another potential problem. Sulphur can attack the colour-coded plastic used to differentiate cable voltages, turning red polyvinyl chloride (PVC) black. So, lead-free PVC, which is more stable, is often used for underground cabling.

Sand blast

Sand has much less of an impact on the performance of cables than heat, thanks to their multiple protective layers. Contamination is only really likely if jointing work is carried out in windy conditions.

Insulators on power lines and in substations, however, are very vulnerable to contamination by sand and other airborne particles.

The Oman Electricity Transmission Company (OETC) recently presented its research on the impact of the environment on overhead power lines in its transmission system, at the GCC Power conference organised by Cigre International.

The OETC operates the main interconnected transmission grid in the northern part of the Oman, which sees a variety of weather patterns and environmental influences, such as sand, sea-salt, wind, rain and dew, as well as air pollution from industrial plants.

In 2006, the OETC recorded 154 incidents of power lines tripping or partial disturbances to supply on its network. Of these, 38 were categorised as faults of a critical nature. Analysis showed that the majority of significant incidents occurred in August and September, when the country’s humidity levels peak.

Line tripping was also reported more frequently in coastal areas and in other humidity prone zones. So, the OETC was able to establish a direct correlation between humidity and system losses.

Furthermore, it noted that around 50% of the outages happened during inclement weather, including sand storms, heavy wind and rain. The OETC, therefore, assessed that bad weather was having an impact on Oman’s overhead transmission and distribution lines as well.

Additional studies showed insulator contamination was a third key causative element in line tripping. For example, faults were more frequently reported in places where a high proportion of roads were un-surfaced, while, in areas with little environmental pollution, few problems were seen.

The OETC concluded that the accumulation of airborne contaminants on the insulators of overhead power lines, combined with moisture in the atmosphere, was responsible for the high number of tripping incidents on its transmission grid. The operator said: “Under adverse environmental conditions, pollution is deposited on the insulator surface, which enhances the chances of flashover. Under dry conditions the contaminated surfaces do not conduct, and thus contamination is of little importance in dry periods.

“In cases when there is light rain, fog or dew, the contamination on the surface dissolves. This promotes a conducting layer on the surface of the insulator and the line voltage initiates the leakage current.” It added that flashover voltages became more likely as the severity of contamination increased.

The OETC criticised the original design of the system for having selected inappropriate materials for the local environmental conditions.

In particular, it said the decision to use porcelain insulators had been wrong. “Ceramic insulators are brittle and have a high value of surface free energy, which results in a great adhesion to water. This property of ceramic insulators makes it readily wettable and consequently has a negative impact on contamination based flashover performance,” the operator stated.

But by gradually replacing glass and porcelain insulators with composite ones, such as hydrophobic rubber silicon insulators, the OETC was later able to mitigate the effects of the pollution and humidity and cut the number of line failures.

Gis Substations

To avoid having to battle the difficult environmental conditions in the Middle East, an increasing number of the region’s power firms are choosing to invest in gas-insulated switchgear (GIS) substations.

In these types of substations, all the live equipment is housed in metal enclosures, often made from aluminium alloy, which is corrosion resistant. Consequently, these substations are less sensitive to air pollution and to interference from salt and sand particles than conventional air-insulated switchgear (AIS) substations.

In a GIS substation, the electrical conductors are insulated by sulfur hexafluoride (SF6) gas. SF6 has a higher dielectric strength than air, so it can prevent unwanted voltage flashovers more effectively. As a result, GIS substations are less prone to faults.

Using SF6 gas as an insulating medium also reduces the clearance distance between active and non-active parts of a switchgear, meaning less space is required to house the substation. Manufacturers say these systems occupy just 5-20% of the space used by the AIS design. For this reason, GIS substations are also becoming the technology of choice in congested cities, where they can even be located underground.

ABB has been building turnkey GIS substations for more than 35 years, delivering in excess of 10 000 high-voltage switchgear bays worldwide. Juliane Lenzner, corporate communications ABB, believes the system is well suited for use in the Middle East: “The very compact GIS equipment is usually installed inside a building, where the climate conditions can be regulated. In this way we have a climatic reliable operation.

“Each substation is designed and built to address different market requirements, such as capacity, availability, location and so on. The modular design of the ABB GIS high-voltage substations provides a high versatility to meet challenging requirements. GIS substations are less sensitive to heat, cold and pollution, as well as salt, sand or even large amounts of snow.

Lenzner says the region’s harsh climate is not the only reason why GIS substations are growing in popularity in the Middle East: “The [extreme environment] is one reason. The completely encapsulated GIS design is also the newest technology and its high reliability and efficiency helps increase the availability of the power, which is a key requirement here.

Growing pains

The urgent need to increase power generation capacity in order to support the Middle East’s rapid industrial and population growth means electricity transmission and distribution networks are constantly being upgraded and extended.

But installing additional power cables near to where existing lines already run can throw up more challenges for cable manufacturers. The original cable configuration often has to be recalculated and new conductor sizes adjusted accordingly in order to reach the optimum load.

“If you design a cable to work in isolation then subsequently locate more cables alongside as demand increases when cities and power loads grow, then the original cable might not be able to work at its optimum rate because cables located next to each other heat each other up. If the original cable cannot take the power it was designed to take, then there is a risk of overload,” Vail explains.

Some power firms are now choosing to use a hybrid version of the GIS system when expanding networks, whereby gas-insulated modules are incorporated into air-insulated substations. In this way, the extended section is unaffected by external influences.

Easier times ahead

Designing and equipping a substation to operate in desert conditions is a significant challenge. And there is, certainly, some additional expense involved – mostly due to the need for larger cables, particularly as prices for copper and other commodity metals have shot up during the past few years.

But the development of GIS substations over the last 25 years has, without a doubt, made the task of generating electricity in harsh climates far easier.

While the Middle East’s hot, humid and dusty environment does pose specific challenges to equipment suppliers, these challenges are not dramatically worse, nor much more costly to address, than those in other regions, where flooding, snow or dense pollution are the main environmental issues.

Furthermore, with the current pace of technological advancement and greater automation, systems will continue to improve and become more sophisticated. So no problem is likely to remain insurmountable for long.

As a result, Middle East energy firms are better equipped than ever to continue the electrification of desert areas and to satisfy the region’s growing hunger for power.