Application of HVDC technology in medium voltage distribution systems.

Abstract

Approximately 60% of all South Africans do not have access to electricity from the national grid and 80% of the dwellings in the rural areas are not electrified. This is due to the fact that many rural South Africans, similar to other rural markets in the developing world, live in sparsely populated, widely dispersed villages, which cannot be reached within the grid electrification program. HVDC technology provides a viable option to transmit electricity to small distant loads. The objective of the present study is to demonstrate the application of HVDC technology in a medium voltage distribution system, to provide electrical power to Kwa·Ximba, which is a small distant rural area, located in KwaZulu-Natal, South Africa. The proposed system generates electricity from a hydroelectric generation scheme namely Nagle Dam and transmits the excess power to Eskom's Caltoridge-Georgedale sub-transmission network for system enhancement purposes. Extensive technical and economical analyses of the proposed system has been conducted. An HVAC system was also considered for the same purposes in order to make technical and economical comparisons between the use of a HYDC and a HYAC system. In addition, grid extension from Eskom's Catoridge-Georgedale sub-transmission was considered to provide power to Kwa-Ximba without the use of a hydroelectric generation scheme. The proposed networks were therfeore (i) Network A:- Power supply to Kwa-Ximba. and the Catoridge-Georgedale sub-transmission network, from a hydroelectric generation scheme, using HYDC technology, (ii) Network B:- Power supply to Kwa-Ximba, and the Catoridge-Georgedale sub-transmission network, from a hydroelectric generation scheme, using HYAC technology and (iii) Network C:Power supply to Kwa-Ximba by extending Eskom's existing AC CatoridgeGeorgedale sub-transmission network with the hydroelectric generation scheme switched off. It is proposed that Nagle Dam, which is situated adjacent to Kwa-Ximba be used as a hydroelectric generation plant. In order to detennine the most efficient and cost effective use of generator sets, the flow rate, available hydraulic power and available electrical power from the year 2005 to the year 2032 were calculated. The increase in flow rate was based on an annual growth rate of l.5% in water demand. The increase in electrical power demand for Kwa-Ximba was calculated for the next 29 years based on an annual growth rate of 1.8 %. Load flow analyses was conducted on the various power line and busbars that constitute each of the networks, in order to determine the effectiveness of each network. In order to maintain flexibility in power generation, five sets of hydro electrical generators were chosen to give a combined power delivery of 20MW. The first three hydro electrical generators are rated at 5MW each, the fourth set rated at 3MW and the fifth set rated to deliver 2MW, (G I to GS operate 11 KV, 3 phase). The combination of generator sets in use (01 to 05) will vary depending on the electrical power demand in any given year. Analyses of the predicted load flow pattern revealed that in the year 2005, Kwa-Ximba will receive 10.5 MW of power while 8.64 MW of power will be used to enhance the Eskom's Catoridge-Georgedale sub-transmission network, with a 4% spinning reserve. By the year 2014 power supply to the subtransmission network will cease since Kwa-Ximba will be absorbing 12.2 MW of power with a 17.5% spinning reserve. By the year 2032, Kwa-Ximba will absorb 17MW of power with a spinning reserve of 14.63%. The converter stations required for the HYDC transmission network (Network A) will be equipped with YSC and PWM technology and have a true power rating of a 20MW. This wi ll be adequate to supply Kwa-Ximba's power demand right up until the year 2032 when the demand will be 17 MW. Converters will include 10BTs. Two 45 km long, 30 MW, 80 kV triple extruded polymetric HYDe cable will be buried 700mm below natural ground level. The Rectic Master software was used to select an appropriate overhead line for HVAC transmiss ion (Networks B and C). An aluminium, wolf conductor was selected to transmit 20MW of active power. Load flow analyses revealed that the HYDC link contributes positively to network stabil ity by absorbing more reactive power than the HYAC link. The HVDC system absorbed a combined (Kwa-Ximba, Catoridge-Georgdale sub-transmission network) reactive power of 22.04 MY AT, as opposed to the HVAC transmission system where a combined reactive power 1.89 MVAT was absorbed from the connected network. This demonstrated that the HVDC link had the ability to absorb more reactive power from the Catoridge-Georgedale sub-transmission network, therefore contributing positively to the enhancement and stabi lity of the sub-transmission network. Network A contributes more to system stability than Network B. It has also been shown that if Eskom's Catoridge-Georgedale sub-transmission network (Network C) is extended to supply electricity to Kwa-Ximba, this would result in system instability, in the long term. It is evident that Eskom would attain direct benefit from the installation of Newtork A, rather than Networks Band C. The technical and environmental differences noted in the present study, between the HVDC and HVAC systems does not, however, justify the economics to install a HVDe system in order to supply power to Kwa-Ximba. Economical analyses revealed that the implementation of Network A would cost 64% more to install and result in a 75% less annual net profit than Network B. Network B would yield the highest annual net profit for the developer. From the developer's perspective, Network B will be the most feasible network to implement. However, from Eskom's perspective, Network A will be the most bene ficial. Various recommendations have been made by the researcher that would benefit the community of Kwa-Ximba, Eskom and the developer in the long term.