In the News

September 11, 2001

Lethabo power station switches to nickel-cadmium batteries for back-up power for critical systems

Nickel-cadmium technology replaces lead-acid Planté batteries for a demanding high-temperature application in South Africa.

ESKOM’s Lethabo power station in South Africa has switched to Alcad nickel-cadmium rechargeable batteries to provide a reliable, low maintenance source of standby DC power for the critical control and protection systems serving the Unit 4 generator. A total of 213 of Alcad’s M and H series cells have been installed to replace lead-acid Planté batteries in this demanding application where the ambient battery room temperature is 30°C (86°F) and often reaches 35°C (95°F).

The standby power supply for Unit 4 at Lethabo power station comprises:
- A single 220V DC system used to keep the generator’s oil and water pumps operating so as to minimise plant damage, as well as to power the emergency DC lighting which is a statutory requirement.
- Dual +24V DC and –24V DC supplies which supports the local control and instrumentation systems which interface to the overall power station control network, ensuring a safe shutdown.

Construction of the coal-fired power station started in 1980, with all six 600MW generating sets commissioned by December 1990. Planté lead-acid cells were installed as Unit back-up battery supplies for each Unit. The DC design expectation was that the cells would have a lifetime of 20 years. It has since been proved that their actual life-span has been 12 years. Rather than making a direct replacement, ESKOM decided to consider alternative battery technology.

In investigating the available alternatives ESKOM formulated its decision making criteria according to a Life Cycle Cost (LCC) analysis together with a consideration of overall battery life.

Life cycle costing
Comparative battery sizing was undertaken, comparing the existing lead-acid and Alcad’s nickel-cadmium technology. Life cycle costing models over a period of 38 years were developed. The outcome was a favourable financial return using the nickel-cadmium technology, due to longer life under high temperature conditions.

Longer life of nickel-cadmium
ESKOM’s main concern was not so much the rapid ageing of the lead-acid batteries as their unpredictable life-time. The Alcad nickel-cadmium cells have a predictable lifetime, without the risk of sudden death failure.

Tony Gordon, Project Manager with Alkaline Batteries (Pty) Ltd, who installed the replacement batteries for Lethabo power station, explains the advantages of the Alcad nickel-cadmium cells in this application:

“One of the main problems with lead-acid Planté cells is that their ageing is not linear, which makes it difficult to predict when they will need replacing. And there is always a possibility of ‘sudden death’ failure. In contrast, nickel-cadmium cells are usually considered a more reliable choice for critical system applications, especially as they are more tolerant of extreme environmental conditions such as the high temperatures in the battery room. And their ageing is linear and easily predictable – we anticipate 17 years at the battery room ambient temperature.

“Another key advantage is that the M and H type pocket plate nickel-cadmium cells we have installed at Lethabo Unit 4 require very infrequent maintenance, with electrolyte top-up intervals of 18 months. The replacement batteries for Unit 4 were installed in September 2000. If favourable results are obtained, and favourable life cycle costing models can be developed for the other five Units, there is a possibility that the power station will gradually convert all the Unit batteries to Alcad nickel-cadmium technology.”

Lethabo power station back-up power supply – technical details

For the 220V DC back-up power supply a 107 cell Planté battery was replaced with 175 Alcad MB1390 (1390Ah) nickel cadmium cells for a designed duty pattern of 300A for four hours then 350A for two minutes. The Alcad M range is specifically designed for applications where the batteries are typically required to sustain electrical loads for between 30 minutes to three hours with frequent or infrequent discharge patterns.

For the +24V DC battery system two batteries of 12 Planté cells were replaced with 19 Alcad HB640P cells to provide 850A for 30 minutes. Similarly, the –24V DC system using 12 Planté cells was replaced with two banks of 19 cells Alcad HC80P to provide 100A for 30 minutes. The Alcad H cells feature very thin plates and are designed for high discharge rate applications.

Editors Note

ESKOM is the national power utility for South Africa and produces at least 80 percent of the country’s electricity, mainly from coal fired power stations.

Press contacts:

Catharina Soderlund, Alcad. Tel: +46 491 68100 Fax: +46 491 68110
Andrew Bartlett, Roger Staton Associates. Tel: +44 (0) 1628 487222 Fax: +44 (0) 1628 487223

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