The Caprivi Link ensures reliable power transfer capability between the east and west of the Southern African Power Pool. Tim Probert takes a first-hand look at the link, which is ABB’s first HVDC Light installation built with overhead lines and is also the highest rated and longest system of its type currently in operation. This article was first published in the December 2011 issue of The Energy Industry Times.
Seven out of the world’s top ten fastest-growing economies are in sub-Saharan Africa. Much of the economic growth is being driven by mining, which needs power.
The high rate of economic growth means state utilities are constantly playing catch-up. The vast, sparsely populated nation of Namibia, where demand for power is expected to nearly double by 2014 as new mining operations come on stream, is a prime example.
NamPower is a tightly-run ship, which notched up a steady profit of $45 million in 2010 and it does not have to instigate load-shedding, as suffered in so many African nations.
Yet the 100 per cent state-owned power generation and distribution monopoly suffers a significant power generation shortfall. NamPower is only 46 per cent self-sufficient in power generation, with imports accounting for the remaining 54 per cent, of which 22 per cent is supplied by South African state utility Eskom.
Maximum demand is 511 MW, but capacity totals just 415.5 MW, including the 249 MW Ruacana hydro plant, the 120 MW Von Eck coal plant and two diesel-powered plants of a combined 66.5 MW. Furthermore, low water flow in the Kunene River at the Angolan border has impaired generation at Ruacana, while outages at Eskom’s Koeberg nuclear plant near Cape Town have left Namibia on the brink of blackouts.
NamPower is desperate to build a baseload, fossil fuel power plant, but plans for a 300 MW coal plant in Walvis Bay have been rejected, while an ambitious plan to build the integrated Kudu gas-to-power project, involving a floating gas platform 170 km offshore and an 800 MW combined cycle gas turbine plant in Oranjemund, requires some $2 billion, equivalent to 15 per cent of GDP.
So, in need of a quick fix, NamPower turned to ABB to construct the Caprivi Link, a 950 km overhead high voltage direct current (HVDC) line which runs along the narrow, tropical Caprivi Strip in extreme northeast Namibia.
While technically not an interconnector due to the converter stations being located solely in Namibia, the +300 MW, 350 kV Caprivi Link connects the Zambezi substation near Katima Mulilo, close to the Zambian border, with the Gerus substation near Otjiwarongo, around 300 km north of the Namibian capital Windhoek. The link, for which construction began in March 2007, also connects to the 220 kV HVAC transmission line from the Victoria Falls in Zambia inaugurated in 2008.
The aim of the Caprivi Link is to ensure reliable power transfer capability between the east and west of the Southern African Power Pool (SAPP). It is also the first electrical connection between the Caprivi region of Namibia and the rest of the country, and is able to supply power to the region if normal supplies from Zambia are disrupted.
Larger islanded parts of the Namibian and Zambian grids can also be supplied by the link, which maintains frequency control and thereby avoids power outages.
The Caprivi Link is somewhat of a curate’s egg. Rather than opting for cheaper HVAC or traditional HVDC, NamPower opted to utilise ABB’s HDVC Light, the Swiss firm’s brand name for HVDC with voltage source correction (VSC). HVDC Light is usually the reserve of underground or subsea links of far shorter distances, and as such it is ABB’s first installation built with overhead lines.
At 350 kV, it has the highest operating voltage for an HVDC Light system and at 950 km, is also the longest system currently in operation. Commissioned in June 2010, the Caprivi Link was jointly funded by NamPower, the European Investment Bank, the French Development Bank and the German Development Agency (KwF). Of the total N$3.2bn ($391m) cost, $180 million was booked by ABB.
The decision to use HVDC Light was made because the AC networks connecting with the HVDC Light converter stations are extremely weak at both ends, with short-circuit power levels of around 300 MVA and long AC lines connecting to remote generation stations. As a result, the AC networks are exposed to a risk of 50 Hz resonance.
Manfred Manchen, NamPower’s director of power system studies, says these factors made the design of the Caprivi Link extremely challenging. The key to achieving the desired performance under different AC network configurations, says Manchen, was robust voltage and frequency stabilising control for the connected AC network in conjunction with sufficient damping of DC resonance.
Instead of using traditional feedback active power control, frequency control, and power runback systems as installed in other HVDC Light projects such as the Gotland project in Sweden and the Finland-Estonia Estlink project, a determined direct voltage and frequency control is deployed via ABB’s turn-on/turn-off insulated gate bipolar transistors (IGBT) power semiconductors.
The basic insulation levels at 350 kV DC compare quite closely with those at 400 kV AC, resulting in the assembly configuration and insulators being identical to those specified for standard 400 kV AC line designs. Without any feedback control loop, the HVDC Light system automatically changes the active power needed to keep power balance within the islanded grid so the frequency is stabilised, automatically changing the reactive power needed to keep the AC voltage at the desired level.
The VSC functionality has proved to be an effective tool in maintaining grid stability. During commissioning of the link, an AC breaker failure occurred in the Zambian grid at a time when 50 MW was being exported from Zambia to Namibia, resulting in a frequency drop in the Zambia grid of 4 Hz.
The HVDC Light system identified the critical situation and quickly changed power flow from exporting 50 MW from the Zambezi substation to importing 60 MW from Gerus.This enabled the Zambian grid frequency to recover immediately to 50 Hz, thus avoiding a blackout.
On an occasion when 80 MW was being exported from Namibia to Zambia, an overload protection tripped a 220 kV line in NamPower’s 220 kV bus-zone, which led to a sudden island condition in the Namibian grid. The sudden outage of the line led to a large frequency dip in the Namibian grid.
The Gerus substation immediately reduced the power exporting from 80 MW to almost zero and automatically switched from DC voltage control mode to voltage and frequency stabilisation control. The Zambezi substation then switched from tracking the power order to DC voltage control. About one second after the contingency, says Manchen, the Namibian grid had restored stable AC voltage in both frequency and magnitude.
At present, the Caprivi Link has proved more useful as a grid stabilisation tool than as an interconnector to boost power capacity. As Manchen said: “In the worst case of disturbances when all generators are tripped in the island grid, the link can function as a super UPS (Uninterruptible Power Supply) to feed the passive loads.”
The link has not been without problems. It has been plagued by rats, for which dozens of traps have been laid at both the Gerus and Zambezi substations. “They’ve taken a liking to the fibre-optic cable insulation,” explained Manchen. “They ate so much of it that we’ve had to use a different, less tasty type of cable. There are some things that you just can’t plan ahead for!”
Rodent issues aside, the link is not used to its full potential. Power flow is only one-way and the link significantly under-utilised. Due to the weakness of the AC grid in Zambia, only 50 MW is being transmitted, mostly hydropower from the Victoria Falls, representing just one-sixth of the link’s 300 MW capacity.
Zambian hydro plants continue to break down and cause the link to fail. However, the link’s VSC system means that generators at Zambia’s Victoria Falls hydropower plant are able to run through fault conditions for the first time, according to Reiner Jagau, chief officer of NamPower’s power system development.
The link is marketed as a key southern African interconnector, but that will only be true once Phase II is commissioned. Phase I, commissioned in 2010, is a +300 MW monopole link operated with parallel DC lines and earth return to reduce line losses. Phase II would consist of upgrading the converter stations at Zambezi and Gerus substations to a ±600 MW bipole link with zero ground current.
NamPower would also have to strengthen its AC grid, as Gerus is currently connected to the Auas substation near Windhoek only indirectly via 220 kV lines to the Omburu substation in western Nambia, from where it spurs southeast to the capital.
This would mean building a 280 km, 400 kV line from Gerus to Auas. NamPower is in negotiations with Eskom and Zambia to build Phase II, but the project has an indefinite timeline, and estimates put the commission date at 2016 at the earliest.
Another key element of Phase II is the need for a 320 kV link between Zambia’s Victoria Falls and the Hwange coal plant in Zimbabwe, for which NamPower has invested in a $40 million repowering in return for 150 MW of power capacity for five years. This line is a key part of the regional interconnection programme known as ZIZABONA (ZImbabwe, ZAmbia, BOtswana and NAmibia), but again progress is slow due to a lack of available funds in Zimbabwe.
The Caprivi Link will thus remain a useful tool to maintain grid stability and import some Zambian power but, until Phase II is commissioned, its designated function as a two-way interconnector to facilitate power trading with the rest of southern Africa will remain limited.