Securing the electricity supply in an African country in an uncertain energy supply environment requires bold decisions. Namibia's national electric utility, NamPower, decided to adopt HVDC Light technology to take advantage of multiple energy supply sources.

For the past few decades, Namibia has imported a substantial amount of electricity from South Africa, but a recent energy crisis in this southern neighbor has forced Namibia to seek alternative sources of supply. NamPower considered various local electric generation options, including a gas-fired source from the Kudu gas field in the Atlantic Ocean and hydropower schemes on the Orange and Kunene rivers. Although these schemes are still under consideration, the country was in need of a near-term solution.

NamPower's board of directors made an investment decision in August 2007 to construct a 951-km (591-mile) ±350-kV high-voltage direct-current (HVDC) transmission line. Commenced in September 2007, this project is of strategic importance for Namibia's electricity supply. When commissioned, the line will bolster the stability of the grid and tap directly into the power pool of Namibia's northern and eastern neighbors.

Electrical Design

The Caprivi Link Interconnector will provide an asynchronous link between the Namibian, Zambian and Zimbabwean electricity networks to ensure reliable power transfer between the east and west Southern African Power Pool. The scheme is specifically designed to ensure reliable power supply to the eastern Caprivi region, which will be connected to the Namibian electricity grid. The design also allows for the Caprivi Link Interconnector to be upgraded during a second phase.

Phase one of the project comprises a 951-km ±350-kV HVDC transmission line with an earth return as well as 300-MW monopole converter stations and associated ac substation extensions at Zambezi substation (near Katima Mulilo) and Gerus substation (near Otjiwarongo). The estimated project cost for phase one is N$3.2 billion (US$0.43 billion).

Phase two consists of upgrading converter stations at Zambezi and Gerus substations to 600 MW, a 285-km (177-mile) 400-kV ac transmission line from Gerus to Auas substation (near Windhoek) and associated substation extensions at Auas and Gerus substations. This phase will be implemented if and when the need arises.

The 330-kV ac transmission infrastructure in Zambia and Zimbabwe — between Hwange and Victoria Falls — will have to be upgraded in order to transfer the additional 300 MW. This specific project is known as ZIZABONA, which is formed from the first letters of Zimbabwe, Zambia, Botswana and Namibia. It is currently under discussion and an investment decision on this project is expected soon.

To date, NamPower has awarded more than 30 contracts to local and international contractors and suppliers for the construction of the HVDC transmission line. Contracts for bush clearing, earth works, civil works and building works were all awarded to Namibian contractors, and NamPower ensured that foreign contractors made use of local labor resources as much as possible. During the last two years, 950 people were employed on the project, of which about 85% were Namibian. The project is being managed by NamPower, who is also directly involved in the erection, installation and testing work.

ABB Namibia was awarded the contract for the construction of the two converter stations at Zambezi and Gerus substations. Contrary to the classic technology used on major HVDC lines worldwide, an alternative technology known as HVDC Light was selected. With both the Namibian and Zambian transmission grids being electrically weak (with no large generation plants in the vicinity), HVDC Light was viewed as a better technology.

Used in the converter station design, this technology is based on voltage source converters employing state-of-the-art turn-on/turn-off insulated gate bipolar transistors power semiconductors. This has the capability to rapidly control both active and reactive power independently of each other to keep the voltage and frequency stable. These characteristics made HVDC Light an ideal technology application to stabilize the Zambian and Namibian networks and increase grid reliability in the Namibian electricity grid.

ABB Namibia also is responsible for system engineering, including the design, supply and installation of the two converter stations and earth electrodes. This project extends the voltage range for HVDC Light to ±350 kV and marks the first time this technology has been used on long overhead transmission lines.

Conductor selection was based on a required load transfer capacity of 600 MW and line losses at maximum power limited to 6%, excluding the 2% power loss at the inverter stations. This resulted in the use of a triple-tern (27-mm [1.06-inch] diameter) aluminum conductor steel-reinforced (ACSR) conductor bundle per pole with a design spacing of 12 m (39.4 ft).

Insulation Design

Experience has shown that the performance of HVDC lines is generally worse than high-voltage ac (HVAC) for similar voltages. One of the early contributors to this problem was applying HVAC design principles to HVDC design. While ac insulation design is largely dictated by lightning and switching overvoltages, it is pollution under normal operating conditions that is a major factor for dc insulation.

The electrostatic dc field is the additional factor and primary cause of the differences from ac:

• The partial arcs that occur under steady dc voltage, as in wet and polluted conditions, are more stable and difficult to extinguish since the currents do not pass through zero as they do with ac.

• The voltage distribution along insulators can become very non-uniform, leading to zones of high stress, thereby promoting flashovers.

• There is the flow of unidirectional ion currents in the air, causing charge accumulation on metallic floating parts, further exacerbating the non-uniform voltage distribution.

For the design philosophy employed in choosing the dc insulation levels, the basic insulation levels at ± 350 kV dc compare quite closely with those at 400 kV ac, with the result that the assembly configuration and insulators used for the project are identical to those specified for previous standard 400-kV ac line designs.

Tower Design

From a mechanical design perspective, two unique features distinguish typical dc interconnectors from ac links. First, such lines are strategically important and constitute a principle supply source, meaning that mechanical failure would constitute a loss of supply. Second, the long length of these lines increases the probability of encountering extreme climatic conditions. For these reasons, mechanical loads corresponding to a 420-year return period were selected for the structural design of the towers.

Two guyed structures were adapted from 400-kV ac designs and modified for use in the ± 350-kV dc configuration. From several support options, a standard cross-rope configuration was selected for use as the default suspension support structure, with an insulated cross-rope configuration as a support for angles up to 15 degrees. These towers have provided exceptional capital cost efficiency and operational performance in Southern Africa since 1995. The most-significant negative aspect relating to this structure type lies in the large footprint occupied by the tower stays of up to 70 m by 46 m (230 ft by 151 ft) in this case, which makes the structure type more suited to rural environments with a relatively low land cost. Such is the case in Namibia, where the average cost of land for commercial farms is N$55 per hectare. About 60% of the line also traverses tracts of state-owned land where no land-use charges were due but compensation was paid to the local community within the 80-m (262-ft) servitude (right-of-way) of the line, for relocation and damage to crops caused during construction.

The relatively low servitude cost facilitated further cost savings not usually possible with overhead lines: Originally, the servitude width was limited to 80 m, which implied a maximum span length limitation of approximately 480 m (1575 ft) on level ground under high-cross-wind conditions. By increasing the servitude width to 90 m (295 ft), the maximum span restriction was removed, permitting the use of spans of up to 550 m (1805 ft) on level ground, which was facilitated by the relatively high 35-m (115-ft) attachment height capacity of the cross-rope structures. This in turn allowed the elimination of approximately 130 structures from the line as compared to the preliminary design. Additionally, new self-supporting structures were designed around the bipolar ±350-kV dc configuration, and were developed and tested in South Africa.

Construction

The line was constructed in three concurrent sections, each in excess of 300 km (186 miles). Tenders were awarded to two international companies: Jyoti Structures Ltd. and KEC International Ltd., who in turn subcontracted construction work to three companies, two from Namibia and one from South Africa. The support structure steel and ACSR conductor were fabricated in India.

A total of seven main construction camps were established, but concrete batching plants were established at more-frequent intervals (i.e., 40 km to 60 km [25 miles to 37 miles]), which varied in accordance with ease of access along the line.

The proximity of the line route to major arterial routes, which were, on average, within 5 km (3.1 miles) of the line for about 70% of the line route, had a significant impact on the delivery of materials to the site and the rate of construction.

Geotechnical conditions along the route proved to be challenging at times, due to the large percentage of dry loose-sand conditions encountered. Approximately 70% of foundations fell into this category.

Nevertheless, a high production rate was achieved, partly due to the structural efficiency of the guyed tower configurations used. The tower erection rates achieved were impressive, with contractors completing up to 22 cross-rope towers per day. Cable-based fall-arrest systems were fitted as permanent installations on all structures during construction to increase safety during structure and hardware erection.

Kwando and Okavango River Crossings

In the wildlife-rich Caprivi Strip, the transmission line crosses the 1.5-km (0.93-mile)-wide flood plain of the Kwando River as well as the Okavango River, which is the primary tributary into the Okavango Delta. Significant concern was raised over the potential impact to bird life over these river crossings and, more specifically, the prevention of collisions with the earth wire of the line.

An ornithological study was launched to investigate and propose mitigatory methods rendering the earth wires more visible to birds. Earth wires spanning these river crossings were fitted with both spiral-type 300-mm (11.8-inch) bird diverters and solar-powered warning lights, which aid in visibility at dusk.

The spans that cross the 1.5-km-wide Kwando River were reduced to around 580 m (1903 ft) by locating two structures on small islands within the river.

Commissioning of the Line

The construction of the ±350-kV HVDC transmission line is complete. The two convertor stations are undergoing commissioning and are expected to be in commercial operation by October 2010.

Acknowledgement

The authors wish to acknowledge the contributions and support of this article given by Pierre van Niekerk, David Hechter, Viven Naidoo, Sanjay Narain, Jose Diez-Seranno and Chis van Rooyen.

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Horst Mutschler (Horst.Mutschler@nampower.com.na) holds a BSEE (heavy current) degree from the University of Stellenbosch, South Africa. He is presently employed by NamPower in the position of specialist engineer: transmission projects in the power system development BU. Mutschler has 30 years of experience in the design, specification, construction and management of projects involving overhead power lines, substations and electrification in distribution and transmission voltages ranging from 0.23 kV up to and including 400 kV). He has headed numerous large construction projects and is the overall project manager for the Caprivi Link Interconnector. Mutschler is a registered professional engineer with the Engineering Professions Association of Namibia.

Pierre Marais (pierrem@taprojects.co.za) holds a BSCE degree from the University of Natal, and a MBA degree from the University of Witwatersrand, South Africa. He is presently employed by Trans-Africa Projects as lead structural engineer and divisional manager of Trans-Africa Projects' Cape Town office. Marais has 18 years of experience in the fields of overhead line design and project management and is a registered professional engineer with the Engineering Council of South Africa.

Companies mentioned:

ABB www.abb.com

Jyoti Structures Ltd. www.jsl.in

KEC International Ltd. www.kecrpg.com