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