To maintain the reliable operation of the iThemba LABS sub-atomic particle accelerator in the Western Cape, the facility was equipped with a high-power uninterruptable power supply (UPS) to ensure a stable and steady supply of electricity. This article describes the recent replacement of the UPS batteries undertaken by a local battery company, Silicon Engineering.
A South African nuclear facility, iThemba Laboratory for Accelerator Based Science (LABS), provides scientists who use a number of sub-atomic particle accelerators in their work with the necessary laboratory and facilities to enable them to undertake physical, medical and biological scientific developments. The oganisation operates on two sites, one in Gauteng and another in the Western Cape.
The Western Cape site houses two injector cyclotrons, one providing intense beams of light ions, and the other, beams of polarised light ions or heavy ions, and a 6 MV Van de Graaff electrostatic accelerator.
This highly specialised equipment is used for the treatment of cancers and the production of unique radioisotopes which are used in radiation medicine and nuclear science and technologies to advance the knowledge and health of the people of Africa. The facility requires a continuous, steady and reliable source of electricity to operate its accelerators and other sensitive scientific equipment.
A UPS was installed to maintain the correct voltage to the sensitive equipment in the event of the electrical supply from Eskom suffering a power dip (voltage fluctuation). This UPS stores electrical energy in rechargeable batteries which can provide 4,4 MVA of clean power to the laboratories for up to 15 minutes in the event of a complete loss of power from Eskom (Fig. 3).
Electricity dips at the research facility are very costly, both in scientists’ time and materials. For this reason, the UPS must be totally reliable and is therefore heavily dependent upon its batteries. The UPS comprises four separate UPS units, each with a capacity of 1,1 MVA.
Silicon Engineering, a Johannesburg-based specialist battery company, recently installed over one thousand 2 V, 2000 Ah, vented lead-acid cells at iThemba LABS’ Western Cape site. These cells, known as OGi cells, replaced the UPS’s existing valve-regulated lead-acid cells.
The company installed four banks of 264 cells each to create four batteries: one for each UPS unit. The cells are connected in series and supply 500 V DC, 2000 A, to each UPS unit (Fig. 3). The UPSs, in turn, supply the facility with electricity, the standard AC voltage, from either Eskom’s supply or the UPS’s storage batteries.
The installation required a set of four purpose-built battery stands to be designed and erected in the battery room on site (Fig. 4). The floor of the battery room slopes toward a centrally-located drain which carries any liquid which may be present out of the room. This, however, complicated the installation of the battery stands; shims had be fitted under the stands to ensure that the cells were perfectly level.
The cells are housed in blocks of three, and connected in series by means of a rigid insulated copper connector. Keeping the cells level with one another ensures that no mechanical strain is applied to the cell’s terminals.
There are four racks, each being 60 m long, with a total length of 240 m, each supporting two rows – one beneath the other – of 88 blocks, 44 (i.e. 132 cells) on the upper tier and 44 on the lower tier. These racks have to be able to carry a mass of over 19 800 kg, as each block weighs about 150 kg.
To ensure that these cells provide 20 years of service, the manufacturer uses a round-grid flat-plate design for its OGi cells. That, combined with a high lead-mass and grid plate design, promises a long operational life and a very good high-current performance. The straight-walled containers and bridge supported plates provide a high power-density in a compact foot-print of 210 x 440 x 675 mm. The transparent container allows visibility and control for easier maintenance and service.
The company also developed a special method of isolating the lead pole from the rubber seal. Called a “panzerpol”, the post is provided with a plastic coating where it meets the rubber seal under the top of the cell. This prevents the electrolyte from seeping past the lead-to-rubber seal where the cell’s terminals pass through the plastic
case (Fig. 5).
The electrolyte, sulfuric acid, is highly corrosive, particularly to metal, and will cause irreparable harm to exposed metals such as battery stands, cables and connectors. Preventing the electrolyte from leaking not only extends the life of the cells but also protects other equipment in the battery room from premature failure due to corrosion and decreases maintenance and cleaning.
Erecting the stands and installing the cells took two months to complete. Over 6000 terminals had to bolted to a predetermined torque setting, cables had to be connected and laid to feed the four UPS units. In preparation for this, the walls, ceiling and floor of the battery room had been specially prepared with an acid-resistant paint. Special hydrogen sensors were installed to monitor the battery’s gassing – which is vital as hydrogen is an explosive gas; and a ventilation system was installed to prevent the room from exceeding the ideal operating temperature of the cells.
Silicon Engineering remains responsible for the maintenance of the UPS’s battery. Servicing is done every three months and takes a week. This includes reading the specific gravity of the wet electrolyte, which falls as the battery discharges, and the voltage of each cell – all 1056 of them.