When I joined Tractebel Engineering in 2016, I was looking forward to the new challenges on an international level that the company could offer me after years of professional experience in my country of origin (Brazil). I was positively surprised that one of my first projects in the company would lead me to a place where my native language, Portuguese, is also spoken: Mozambique.
An iconic and impressive asset
When telling colleagues who had been working for a long time in the hydropower business in our company about this assignment, the typical reaction was: “That’s an an iconic plant!” In fact, Hidroeléctrica de Cahora Bassa (HCB) was one of the remarkable hydropower facilities erected between the 1960s and 1980s worldwide along with plants like Aswan (Egypt) and Itaipu (Brazil).
The plant and its surroundings are stunning. In a steep valley section of the Zambezi river, an impressive 171 m-high arched dam was erected in the narrowest gorge, along with a cavern-type powerhouse. Through headrace tunnels, the water flows towards five generating units rated 415 MW each, accounting for a total installed capacity of 2,075 MW. The scheme also comprises a substation with a HVDC converter system, exporting about 80% of its produced power to South Africa via 1,420-km length 533 KV DC lines to the Apollo converter station in Johannesburg.
View of the Cahora Bassa dam
How we are assisting HCB
Failures in the generators and root cause analysis
The generators in the plant were commissioned in the late 1970s. It is expected that after 40+ years in operation, electrical rotating machines are approaching the end of their lifetimes, even when subjected to the best maintenance practices. The first signs are when their electrical insulation systems start failing.
And that is how our contract started: in December 2016, generator 3 faced a stator winding phase to ground fault. After this incident, we were hired to support HCB on technical advisory and project management services. A few months later, a failure of a similar nature ocurred in generator 4 as well.
Our first task was to assist in the repair of units and investigate the failures. During the first site visits, the methodology, equipment and materials applied by the contractor were checked. At the same time, information was collected to prepare the root cause analysis (RCA) of the faults.
For those who are not familiar with how generators are built and operate: electric current flows through the stator windings when power generation occurs. In machines of the size we are handling at HCB, these windings are formed by the interconnection of insulated copper bars. These bars are suitable for a high output of power and are installed in slots within the stator core. As a consequence of ageing, a weak point in the bar insulation may progressively develop to a micro path. This happens through the action of a phenomenon called partial discharge. When a micro path is created, current suddenly flows out of the copper with a very high energy level, resulting in a short circuit.
The RCA was a crucial step to understanding the design concept and manufacturing process used at the time of the plant’s erection, as well as their influence on the ageing and residual lifetime expectancy of the windings. Some bars – including the failed one – were shipped to a specialized laboratory for dissections and other tests.
Picture: Cross section of a cut piece from a removed bar. The inner copper conductors and the insulation around them (usually referred as “groundwall insulation”) can be seen in detail. The longer the operational time, the weaker the insulation may become, due to ageing.
Diagnostics program and interventions
Based on the RCA conclusions, the client requested our assistance to develop a strategy, considering the contexts of the failure and the operation of the plant. The investigations confirmed a presumption that the windings should be fully replaced. However, such a rehabilitation project requires about 7 to 10 years from tendering to completion. The units need to be shut down one at the time for the works while the remaining ones continue to operate, meaning that each generator would still have to operate with the actual windings an additional 3 to 9 years – under an increasing failing risk level.
The objective of our project at that point became: maximizing reliability for the remaining years of operation by mitigating to the best possible extent the failure origin mechanisms.
The defined approach was through planned outages, during which each generator would be subject to a program of electrical diagnostics tests and visual inspections to further assess its integrity and identify the potential sources of contribution to the risk of faults. The scheduling of the outages is defined based on a combination of aspects, such as:
- Partial discharge measurements (machines under higher risk are prioritized)
- Annual generation plan (dry seasons, with less availability of water,
View of the powerhouse, during mobilization of the test transformer for the assessment of generator 5
For the tests, a transformer has been mobilized in order to energize the windings with AC voltage. This allows the running of a state-of-the-art test program for insulation assessment. Based on the findings, measures are specified for implementation. They have typically consisted of re-insulation of some spots, extensive cleaning to remove contamination by oil and dust, and mechanical improvements (mainly the retightening of the stator core).
This is me checking a spot with partial discharge activity in energized stator windings, using a UV camera. The view through the camera can be seen in the small photo in the lower right side.
The total outage of the unit for the tests and works normally takes around six weeks. Other Tractebel experts and I have been present on-site to coordinate, supervise and provide technical advisory. A busy but quite interesting time.
Retightening of the stator core
In different stages of the project, additional insulation failures occurred in machines that had not yet been assessed. They pretty much increased the complexity of our support to HCB. We had to quickly react by understanding the incidents, re-defining priorities and elaborating new work plans to repair and bring everything back online within the shortest possible time. Ultimately, these setbacks meant having a “second project” running in parallel.
Repair of generator 1 after a failure in 2018.
My role in the project
As the project manager, I have been in charge of leading all the coordination between Tractebel, HCB, contractor and top-level generator specialists involved in the project. The resources, risks and all other aspects have to be considered within the right balance, so that tasks can be planned and executed to effectively achieve our goal: increasing the generators’ reliability to operate the necessary additional time until they are fully rehabilitated. Since January 2017, this has been done both from our head office in Germany and on a number of missions to the site.
Cahora Bassa dam viewed from the downstream side
Source of energy in Mozambique
Cahora Bassa is one of the most important assets in Mozambique. The produced energy is essential to supply the country and many of its neighbours in the southern part of Africa. This project has shown how committed the management of HCB is on adopting a professional and proactive approach, on which effective solutions are pursued.
Given the importance of the power plant and the challenges of this assignment, I certainly consider it a major achievement in my career. The excellent collaboration with all the project members and particularly with our Mozambican friends has been a unique experience to be highlighted as well.