Designing low voltage feeders to meet quality of supply specifications for voltage variations
Master Thesis
2012
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University of Cape Town
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The provision of electricity has become a global necessity. In the developing world, residential electrification has become a tool for poverty alleviation. Unfortunately connecting residential customers to the grid, particularly in the low income communities, is more of a social task as the expected returns from the investment are unlikely to cover the costs to electrify and supply the communities. In such cases it is necessary to not over- or under-design a low voltage (LV) distribution network as this leads to unnecessary capital expenditure. The main source of uncertainty in designing LV residential distribution networks has been found to be the mode used to model the residential load. Residential electricity demand is a stochastic parameter dependant on the behaviour and occupancy patterns of household occupants. Traditionally the After Diversity Maximum Demand (ADMD), which is in essence and average value of load per household, was used to model load. However, using a singular value to describe the complex random nature of load is misleading. Probabilistic methods have been adopted to model residential load behaviour as these methods are better suited to representing the stochastic nature of the load. The Beta probability function was found to be the best representative function of residential load as its characteristics were reflective of the attributes of residential load. Studies on pre-existing LV networks in South Africa have found that these networks are operating outside of Quality of Suppy (QoS) regulation. The current QoS guideline of South Africa NRS 048-2 stipulates that 5% of measured supply voltage levels measured during a certain period are allowed to be outside the QoS compliance limits. This means that 95% QoS compliance of supply voltage levels is required for all LV networks. This QoS condition has not currently been worked into the design parameters. If a network is operating out of QoS guidelines a network upgrade is necessary. This research showed that the main source of the QoS violations of these networks was due to the risk levels used to calculate the expected voltage drops during the design stage of the networks. Typically, 10% risk is used for voltage drop calculations. This means that a best case of 90% compliance is expected which is outside the 95% compliance limit required by NRS 048- This study focused on two objectives. The first was to derive design parameters that are representative of residential load and can be used to design LV networks that comply with QoS specifications. The second was to define a means or develop a model for LV network designers to distinguish the parameters appropriate for a design, based on the customer class to be electrified. In this investigation new design parameters were derived that incorporate the 95% compliance limit of NRS 048-2 allowing LV networks built based on the new parameters, to operate within QoS limits. The parameters were derived using residential load data collected in South Africa since the early 1990's. An equation was also derived which allows countries with only ADMD data available to calculate QoS design parameters suitable for their situation.
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Kadada, H. 2012. Designing low voltage feeders to meet quality of supply specifications for voltage variations. University of Cape Town.