20-08-2012, 03:20 PM
HVDC Transmission Systems-Gerus Substation
Operation of Gerus HVDC.pdf (Size: 660.21 KB / Downloads: 121)
ABSTRACT
This report presents information and data gathered during the site visit to the Gerus substation on 17th April 2012. The Gerus substation is part of the Caprivi HVDC link. It runs from the Zambezi to Otjiwarongo, for a secure long distance power transmission. The link is currently a monopole system; however the substation works both in inverter and rectifier mode.
INTRODUCTION
The advancement in the fabrication of high power electronic devices such as thyristors, Insulated Gate Bipolar Transistors (IGBTs) and other electronic switching devices has led to the development and expanded application of HVDC Schemes. Such schemes are used for bulk power transmission over long distances, inter-connections of AC systems and back-to-back arrangements for optimised power interchanges. In Namibia, the bulk generation is supplied from Ruacana hydropower station in the far north through a 520km 330kV transmission line. The Van Eck power station is also used during emergencies to supply the load centre near Windhoek. To cater for the fast growing economy and to provide a secure electric energy to the mining and mineral industry in Namibia, an HVDC link came into being as to transmit power over a long distance about 950km through overhead DC line, with the capability of transmitting 300MW with an ac voltage rating of 400Kv.
MONO-POLAR CONFIGURATION
This is a basic configuration using one conductor, usually at negative polarity and ha ground or water that acts as the current return path. But return can also be metallic in the case of high ground resistivity, or where there are objections to interference by ground or water currents.
BIPOLAR CONFIGURATION
Has two conductors, one positive and the other negative. Each may be a bundled conductor in EHV lines. Each terminal has two sets of converters of identical ratings, connected in series on the DC side. The junction between the two sets of converters is grounded at one or both ends. Normally, both poles operate at equal currents and hence there is zero ground current flowing under these conditions.
HOMO-POLAR CONFIGURATION
Has two or more conductors all having the same polarity (usually negative) and always operated with ground or metallic return. Because of the desirability of operating a DC link without ground return, bipolar links are most commonly used. Homo-polar link has the advantage of reduced insulation costs,
The Mono-Polar operation is used in the first stage of the development of a bipolar line, as the investments on converters can be deferred until the growth of load which requires bipolar operation at double the capacity of a Mono-polar link.(e.g. caprivi-link) The major components of a HVDC transmission system are converter stations where conversions from AC to DC (Rectifier station) and from DC to AC (Inverter station) are performed. A point to point transmission requires two converter stations. The role of rectifier and inverter stations can be reversed (resulting in power reversals) by suitable converter control
CONCLUSION
The HVDC system plays a big role in the Namibia power network; it is a further step in overcoming the power crisis the country is facing due to the growing mining industry and rural electrifications. NAMPOWER imports 50MW from Zambia into the Namibian network. Depending on demand the Gerus station is either in inverter or converter mode. But in most cases the Gerus station operates as an inverter. Gerus station is currently not operating according to required sequence, ABB is working on the problem. However the problem really lies in fault location and studies can still be done on fault locators to solve the problem. All in a nutshell the visit to Gerus station was a very big eye open as the student now had to see the practical side of the theory taught in class.