2008-06-10 - An essential HVDC (high-voltage direct current) converter station near Johannesburg is once again fully operational and delivering clean, reliable hydroelectric power from Mozambique to South African customers. ABB completed the upgrade site work in about seven months, with minimal power disruption.
Finishing upgrades like this quickly is important in a country where electricity shortages are an everyday fact of life.
Work only started on site in November 2007, and the Apollo converter station station passed all tests by June 1. "The final test was completed without any disturbances, and everything went totally according to plan," says Birger Jonsson, project manager at ABB in Ludvika. Converter stations at either end of an HVDC power line change electricity from alternating current (AC) to direct current (DC) and then back again. Electricity can be sent over vast distances with very low losses when it is transmitted as direct current. It can then be converted into useable alternating current at the other end of the line.
Apollo station is one of the biggest HVDC upgrades ABB has undertaken so far, and includes the installation of thyristor valves, AC filters and a MACH 2 digital control system, all essential parts of HVDC transmission. Thyristor valves are the components that actually switch current back and forth from AC to DC to AC. The role of AC filters is to absorb disturbances (harmonics) originating from current conversion in the thyristor valves. The ABB MACH 2 system is the world’s most commonly used control system for HVDC and FACTS (flexible AC transmission systems), and includes all functions for control, supervision and protection of the converter stations. Over 400 HVDC and FACTS systems today operate with this control system.
To avoid extended power disruptions short delivery times are critical for this type of project. Apollo convincingly demonstrates ABB's ability to complete advanced upgrades in a very short time. "Apollo station thus becomes an important reference project for ABB, " Jonsson said, adding the project team found ways to complete the work quickly and effectively .
"We saved a lot of time by reusing much of the existing valve supports in the plant when we replaced the valve system," he added.
In recent years ABB has upgraded several HVDC links, in most cases replacing old analogue control systems with ABB's digital system MACH 2. But the Apollo project was considerably more extensive because Eskom, South Africa's power utility, decided to also replace the stations thyristor valves and AC filters. This was done to minimize maintenence, improve reliability and availability, and also for environmental reasons, because the old thyristor valves contained substantional amounts of oil and the AC filters contained PCB.
The Apollo station is a critical part of the 1,400 km HVDC power link bringing badly-needed electricity into South Africa from the Cahora Bassa hydropower dam in Mozambique. The link comprises two HVDC stations – Songo in Mozambique, and Apollo station near Johannesburg. Most of the hydro-electricity produced by the Cahora Bassa dam is exported to South Africa,
Apollo was built in the seventies, and the upgrade not only significantly increases power availability for South African customers, but also prepares the way for a future upgrade of transmitted electrical capacity, from 1900 megawatts (MW) to 3900 MW.
This HVDC power link is an essential, environmentally-friendly source of electricity for South Africa, where most electricity is produced by burning coal. The link remained in operation for almost the entire upgrade, although transmission capacity had to be reduced at certain times as equipment was installed.