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Increased Power Transmission for
Aluminum Smelter in Maputo


 

The Mozambique Transmission Company  MOTRACO S.A.R.L. owns and operates two 400 kV over head transmission lines transmitting energy generated in South African power plant Arnot and Camden to Maputo substation in Mozambique (please refer to Fig. 1).

MOTRACO is an independent transmission company owned in equal shares by three utilities: EdM from Mozambique, SEB from Swaziland and Eskom South Africa.

The bulk of the energy which is transferred via these transmission lines is for the Mozal aluminum smelter in Maputo. Due to the economical situation of the aluminum smelter, Mozal decided to increase the capacity of the smelter, connected with a increased power demand of further 425 MV.

Fig. 1: Network system overview
 

This is a typical case, where the features and benefits of the FACTS device “Fixed Series Capacitor, FSC” are of advantage, instead of building a new transmission line over hundreds of kilometers. Inserting a series capacitor in an overhead line is reducing the impedance of the line under high load conditions. The line is electrically “shorter” and more energy can be transmitted by the same line. Furthermore the stability of the net, the quality of the voltage and the load flow as well are improved by such a fixed series capacitor bank.

Based on the Motraco network configuration along with adjacent parts of the Eskom network the solution for this demand was a FSC with 535 MVAr capacitive power in the middle of the Arnot line, located in Barberton, South Africa, and in the parallel Camden transmission line a FSC at the end of the line in Maputo, Mozambique with 345 MVAr at a rated current of 2000A at both lines.

 Protection of the capacitors, which are installed in each phase of the line, is done by a chain of protection devices as arrestors, damping circuit and forced triggered spark gap on the platform and bypass breaker located in the yard (please refer to Fig. 2). The monitoring of the FSC operational situation is done by a redundant SIMADYN control and protection system. Current values of the platform equipment on high voltage level are converted by Optodyn devices into optical signals, which are transferred by fibre optic cables to the protection panels on ground level. The energy for triggering the spark gap is supplied by an optical link as well, so that no power supply is required on the platform.

Fig. 2: Main circuit components of the fixed series capacitor banks, FSC
 

  

The availability and reliability of the fixed series capacitor banks is extremely high and close to 100%. This is reached on one hand due to the design as an unattended station with redundancy in all major components of the key systems. On the other hand there are taken several measures like a comprehensive set of spare parts available on site, detailed trouble shooting manuals and intensive training of the customer’s operating and maintenance people.

 Of importance is the performance of a preventive annual maintenance as well. This maintenance is done by a crew of about three electrical/mechanical engineers/technicians familiar with HV equipment, electric/electronic equipment and analog/digital protection equipment, having participated in the FSC specific training and guided by the maintenance manual. The maintenance works are carried out within about two days.

 In May 2002, Siemens received this turnkey project on a delivery time of only 11 month. Despite the situation of starting at a green field site in South Africa (please refer to Fig. 3) and the import situation at Mozambique, Siemens succeeded to energize both plants on time middle of April 2003. Since this time both plants are in operation to MOTRACO´s satisfaction, supporting the customer fulfilling the power demands of the aluminum smelter in Maputo.

Fig. 3: Fixed Series Capacitor bank in Barberton, South Africa