Mozambique is a Southern African Country with a Population of 19 Million and a land area of some 800,000 sq km (30,882 sq miles). The majority of the population lives in rural areas, surviving on subsistence agriculture, fishing and hunting. Despite making significant progress during the last 15 years, Mozambique is one of the world's poorest countries, ranked 172 of 178 countries in 2007. At present, the electrification ratio is around 20% in urban areas, but less than 2% in rural areas, with an overall energy use per capita of 89.5 kWh per annum in 2006.
The National Grid has been extended since Electricidade de Moçambique was created in 1977. EdM has an electrification master plan to provide conventional three-phase technology to supply the major cities. For rural areas, electrification poses financial and economic challenges due to the very low load density and the need to supply customers without the means to afford energy. These problems are common for utilities around the world, particularly in developing countries. Therefore, in order to provide grid-connected electricity to rural areas, it is necessary to:
Minimise investment costs.
Minimise maintenance and operational costs.
Simplify design, equipment specifications and procurement.
Establish transparent methods for system management and customer billing and account collection.
These principles form the basis of the international response and launch of Low Cost Rural Electrification programmes. One of the most frequently used forms of LCRE is a technology known as Single Wire Earth Return (SWER) that is also used in Australia, New Zealand, Canada, the United States (Alaska), South Africa, Burkina Faso, Botswana and Namibia.
EdM is currently implementing two Danish Aid-funded SWER systems to supply Mavila and Morrungula, both in the Inhambane province. This project is taking advantage of the experience of neighbouring countries, namely South Africa and Namibia, in the form of reviewing and adapting engineering standards.
The SWER distribution system uses the ground for the return path. This system is easy to construct and maintain, and is very economic overall. Often regarded as a marginal technology in Australia, SWER is a mainstream technology with some 200,000 km (124,274 miles) operating at 19.1 kV or 12.7 kV in seven states. In Mozambique, they decided to construct 19.1-kV SWER line spurs from the existing 33-kV three-phase overhead lines in rural areas as the first stage of the electrification programme.
The key components of the SWER distribution system are the poles or conductor supports, conductors and insulators. For the most economic system, it is necessary to optimise the procurement of materials and construction methods. The options available to the system designer include:
The conductor supports commonly used include wood poles, steel poles, pre-cast, pre-stressed and spun-concrete supports. In Mozambique and South Africa, there is a thriving wood pole industry using plantation timber, which results in the availability of low-cost, quality treated poles. Wood poles are very robust, can withstand transport and are the product of choice.
The available options considered include post or pin insulators for in-line and small angle sections. Since polymer insulators suffer ultraviolet damage in general, porcelain insulators are used in Africa. Standard insulators used for 12.7-kV and 19.1-kV SWER projects are rated at 22 kV and 33 kV, because they offer the opportunity for future upgrading.
In the past, No. 8 galvanised fencing wire was a common choice for SWER conductor, but more suitable conductors are now manufactured (for example, galvanised steel conductors to AS1222.1 and aluminium-clad steel conductors available from manufacturers such as Olex are used in Australia). For economic reasons, Bantam (13.1 kcmil, ¾ ACSR, 5.04 mm [0.2 inches]) or Magpie (20.87 kcmil, ¾ ACSR, 6.36 mm [0.25 inches]) conductors, which have separate outer cores of aluminium and steel inner cores, are used in Africa. As long as they are not erected in coastal areas, corrosion is not a problem.
- Vibration damage
To minimise line construction costs, SWER is designed with long spans (for example, 300 m [984 ft]) on a level terrain, based on the use of 12-m (40-ft) poles. Even longer spans may be used in undulating country. The conductors have to be strung at a higher tension, which can create damage due to conductor vibration. The use of spiral vibration dampers, retrofitted to existing line, with armour rods and preformed ties on new construction, has largely overcome this problem. An alternative solution is to use suspension insulators on the longer spans.
MAPPING THE NETWORK
Designing and routing the SWER circuits in the most cost-effective manner is a major consideration. It is almost universal that power systems develop from cities and towns with the radial distribution network and radiate out like a major artery from the main roads. The connection of the villages and farms adjacent to main roads is easy to justify financially, but the next step, the electrification of the more remote villages, is not so straightforward.
In Mozambique, detailed guidelines were established to take into account the technical, financial and socioeconomic requirements in order to prioritize the expansion of the network. It was important to package the rural electrification into definable sub-projects of between US$10 and $30 million per project, so that they could equate to the funding available from government and donor countries. The plan for the expansion of the network was represented using a geographic information system, based on an ArcView 9.2 platform from ESRI.
As is common with most of Africa and in Mozambique, there are no land registry or land ownership records. Villages and townships develop informally, without a grid pattern layout of streets and houses. This development practice makes planning the electricity network difficult, as potential customers are not marked on maps, and there are few straight routes for high-voltage and low-voltage overhead lines. Aerial photography can be used to produce a mosaic on which the network can designed, thus enabling quantities and costs to be determined. In developing countries, it is vital that where any form of imagery is used for network design, it must be regularly updated and reviewed, as houses and populations are often transitory.
NETWORK DESIGN CONSIDERATIONS
Special measures are recommended in areas with a high keraunic level to protect the SWER network from outages and component damage due to lightning strikes. For example, for 19.1-kV networks with pin insulators, the earth wire is fixed to the pole with a 20-mm (0.8-inch)-wide aluminum strap. The wire is bent to form a U with approximately 10 mm (0.40 inches) of the U sticking out above the aluminum strap. This bend is created to prevent the buildup of a dense electrical field and the possibility of flashovers. The earth lead acts as a "rod gap," and lighting current flows through the earth wire rather than the pole. These ground wires are normally installed on the pole prior to site erection. Distribution and isolating transformers are particularly vulnerable to lightning strikes, and earth wires are installed from ground level to the line insulator on the final three poles before the transformer installation, together with the high-voltage and low-voltage surge arresters used to attenuate the effects of lightning strikes.
As SWER uses the earth as the ground return path, the effectiveness of the earthing assemblies are crucial for network performance. The conventional way of ensuring satisfactory earthing is to measure the ground resistance prior to when the proposed route is surveyed using specialist test equipment. A series of earth rods are then driven deep if the measured earth resistance is too high. With time and an increase in load, earth connections can deteriorate, so regular earthing testing is essential. The SWER primary earth is designed to ensure the resistance is not so low that the current caused by lightning exceeds the capacity of the surge arrester and not high enough to endanger staff, the population and livestock due to the magnitude of the step and touch potential. As a guideline, the SWER earth resistance should limit the rise in potential to about 30 V above ground level.
The design and height of the conductor supports used on SWER lines in Africa must take into account the hazard to giraffes caused by inadequate ground clearance. Records confirm the tallest-ever giraffe was measured at 5.88 m (19.3 ft), although the average height is 5.3 m (17.4 ft), and reports confirm that species have been electrocuted where the ground clearance was less than 5.2 m (17 ft). In and adjacent to game reserves, where visual amenity is also an issue, underground cables are installed.
The inline pole is the key to low-cost electrification using SWER technology, and it is essential to use maximum span lengths, as there is no conductor clashing to consider, thereby reducing the pole count. The isolating transformer keeps the SWER earth return current from the network, ensuring that the feeder ground fault protection at the source substation operates correctly. This transformer not only adds cost to the SWER system, it also makes it uneconomical to extend the SWER line less than 6 to 10 km (3.7 to 6.2 miles) from the main feeder. Therefore, it is important that the SWER line construction savings offset the cost of the isolating transformer.
The distribution transformer is a standard component that requires good earthing. The ground terminal on the primary winding can be specified as fully insulated at an extra cost, which allows the distribution transformer to be upgraded to a three-phase system when required.
With SWER, there is always a residual current flowing through the earth between the distribution transformer and isolating transformer. Sensitive earth-fault protection is difficult to apply downstream of the isolating transformer to sectionalise the feeder using auto-reclosers. However, Schneider Electric's NuLec Industries has developed the W-series single-pole recloser, which includes sensitive earth-fault protection that can be used to sectionalise faulted segments of the SWER feeders, thus improving system reliability. However, the detection of low-level earth faults on SWER systems remains a problem.
In Mozambique, the majority of materials and equipment is imported, because the local manufacturing industry needs to be rebuilt following years of conflict and the low rate of rural electrification. Therefore, materials and equipment generally are sourced from South Africa, due to its close proximity, but some supplies come from European and North American companies.
The renders for the construction work associated with the Mavila and Morrungula projects were circulated in the summer of 2007, and site work is in progress. In order to determine the cost of the LCRE programme in Mozambique, the unit rates shown in the table (at left) were used to determine budget costs.
A long-term plan for Mozambique's rural electrification programme has been determined. This nationwide project has established achievement targets as documented in a report prepared by Norconsult/Swedpower in 2004 for the period 2009 to 2020. The overall long-term goal is to move the percent electrification in Mozambique from the current 5% overall to almost 15% by 2020.
|400 kV to 330 kV||125 km (78 miles)|
|275 kV||109 km (68 miles)|
|220 kV||1465 km (911 miles)|
|110 kV||2188 km (1360 miles)|
|66/60 kV||324 km (201 miles)|
|33-kV Three-Phase Distribution|
|33-kV, three-phase auto recloser||each||$31,000|
|33-kV LCRE distribution line||circuit km / circuit mile||$9800 / $15,770|
|33/19.1-kV isolation substation||each||$51,000|
|19.1-kV SWER LCRE distribution line||circuit km / circuit mile||$4500 / $7240|
|House connection density type 1||each||$340|
|House connection density type 2||each||$459|
|House connection density type 3||each||$582|
|District of PA office||each||$2000|
|230-V Household Service|
|Service connection, 30 m, airdac 6-mm2 cable with ready board, flat-rate prepayment meter and kicker pole||each||$510|
Conrad Holland ([email protected]) is a power systems consulting engineer in the International Transmission and Distribution Group of Maunsell Ltd. (Auckland, New Zealand). Holland was awarded a BE degree (electrical) from Auckland University. Specializing in power systems engineering, he has worked internationally in rural electrification and utility rehabilitation projects since 1992. Holland is registered as a chartered professional engineer in New Zealand.
Henk Meyer ([email protected]) is the principle of Meyer Consulting Engineers (Windhoek, Namibia, South West Africa) and director of Power Systems Dynamics (Pty) Ltd. Meyer was awarded his BE degree (electrical and electronics) from Rand Afrikaans University and a Master of Business Leadership degree from the University of South Africa. Meyer has specialist expertise in electrical distribution and visual amenity projects, and has extensive experience in the application of low-cost solutions, particularly SWER technologies. Meyer is a member of the Engineering Council of Namibia and the Association of Consulting Engineers, Namibia.