However, engineers predict it will take months before the dam is running at full capacity, and it will be at least two years before the electricity it generates reaches the 1.8 million intended beneficiaries, who live in remote villages in the Helmand river valley.
Even then, electricity supplies are likely to face disruption from Taliban attacks unless the region is cleared of militants, analysts said.
The area is not densely populated, so the power lines must cover many miles of hostile land to reach the remote villages that are due to be linked up to the dam. British troops in Helmand control an area of only a few miles radius beyond the Kajaki dam, so pylons and substations will have to cross what is now a stronghold for militants operating in the region.
"The power lines coming out of Kajaki are going to be extremely vulnerable to attack," said Matthew Clements, Eurasia analyst at Jane's Defence. "The arrival of the extra turbine is a major blow to the Taliban, so they are going to be keen to make sure the project fails."
Last year, at least 700 Taliban fighters crossed from Pakistan into Afghanistan to reinforce insurgents attacking the Kajaki dam. The attacks followed operations by Nato-led troops to allow reconstruction of the dam and the power transmission lines to boost output after fighting halted refurbishment in 2006.
The Kajaki hydroelectric power plant was built with funding from US development aid in 1975. At full capacity, it can deliver 53MW of power and provide water for crop irrigation, but with only one turbine now working, it can only produce 16MW. A second turbine broke down six months ago and is awaiting spare parts. The third, a Chinese turbine, will add a further 18.9MW of power.
Fitting the new turbine will take many months, requiring heavy lifting cranes to shift the equipment. Once in place, a generator must be installed above it, all under clean conditions that prevent grit and sediments from the dam's reservoir clogging up the machinery.
The power station is connected to Helmand's provincial capital, Lashkar Gah, by a single main power line, but engineers plan to put in $77m of new transmission lines to connect up villages in the rest of the province.
"In Iraq we've seen that overhead power lines are extremely difficult to protect, and there's no point generating electricity if you can't distribute it," said Paul Smyth, head of operational studies at the Royal United Services Institute for Defence Studies.
The military convoy arrived on Tuesday bearing seven 25-tonne sections of machinery. The route passed through territory that has been held by the Taliban for two years, and was cleared by engineers who removed improvised explosives and strengthened the road.
The US-led coalition has said it killed 220 militants in Helmand province over the last week, coinciding with the turbine's movement through the region.
Brigadier Mark Carleton-Smith, commander of British forces in Afghanistan, emphasised that it would take time to connect up homes that are currently without power. "It's going to take a lot more investment and considerably more time and energy from the international community," he said. "If we're not firm in our commitment, that will resonate negatively with the Afghan civilian population."
The elaborate mission to deliver the third turbine to Kajaki suggests the military intention is to get the power plant up and running at full capacity in the hope that the security situation has improved by the time the new electric cables are ready to be put in place.
"The fact that they've been able to move this convoy through 180 miles safely already says a lot about what the military forces are able to do and what the insurgents are not able to do," said Smyth.
"This is not just about providing people with power. It's important for people there to see that life is better under the Afghan government than it is under the insurgents.
"In terms of its potential to give them electricity, to irrigate their land, and even think of having cottage industries, it could have a great effect on their lives."