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Enhancement of power transfer capacity in bipolar HVDC system
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Economic growth that leads to sustained increase in electricity demand resulted into extension of AC systems. This is due to the correlation between the increase in population and power consumption. It was anticipated that by now the power consumption in developing and emerging countries is expected to have increased by 220%. When such happens the power system will be experiencing problems of uncontrolled loop-flows, overloading and excess of short circuit levels, system instabilities and outages. In order to ensure the continuity of supply, power system enhancement and interconnections needs to have been in place in a form of higher voltage levels, new transmission technologies and renewable energies. As anticipated, power consumption increased to the point where power systems are constrained. The purpose of the study is to enhance the inherent power transmission capacity of the overhead lines on the overloaded existing sub-transmission and distribution networks. FACTS have been developed with the aim to better load flow and voltage control. However the devices help in increasing transport capacity, in avoiding loop power flows, in improving transient and dynamic stability etc but do not increase the inherent transmission capacity of a line. Point-to-point VSC-based HVDC transmission was used as an alternative to upgrade the existing right of way corridors. This was achieved by transformation of an existing AC line with a DC one, making maximum use of conductors and towers with up to 4 times transfer capacity increase. The studies were modelled in the software tool Digsilent Power Factory. Three scenarios were simulated under short circuit and contingency conditions where voltage was being monitored on the bursars and the capacity together with overloading were monitored on the HV lines of substations. In chapter one, the background objectives and significance of the study are presented followed by the insight on to classic HVDC transmission networks in chapter two. This matured technology was studied to trace the increased potential of HVDC applications. VSC-based converters are presented in chapter three. Amongst others, the dynamic voltage support, the system stability and the higher power transfer capacity offered by VSC based converters were the most beneficial pertaining to this thesis. Due to challenges encountered in acquiring the land for new electricity infrastructure. It has been noted that urban electrical system require easy solutions that can be attained within urban boundaries and shot lead times.as a consequence the replacement/conversion of existing AC overhead lines with DC is presented in chapter four. Among the system instability problems encountered on the network at study, Voltage instability was a key issue to be addressed and chapter five presents categories of voltage stability and mitigations thereof. In chapter six the modelling and simulation of the existing AC network to underpin the problem statement for different contingencies is presented. The results are recorded so that they can be compared to results obtained after VSC-HVDC link incorporation. Chapter seven touches on the modelling of VSC-HVDC link on Digsilent Power Factory. Following in chapter eight are the busbar voltage and the loading results after VSC-HVDC incorporation. It was evident that VSC-HVDC incorporation mitigated low voltage and overloading problems in the network. A concluding statement was reached to say the dynamic support of the AC voltage at each converter terminal improved the voltage stability while the transfer capability of the sending and receiving end AC system was improved.