In recent years, the world has seen a significant growth in energy requirements. To meet this requirement and also driven by environmental issues with conventional power plants, engineers and consumers have started a growing trend in the deployment of distributed renewable power plants such as photovoltaic (PV) power plants and wind turbines. The introduction of distributed generation pose some serious issues for power system protection and control engineers. One of the major challenges are power system protection. Conventional distribution power systems take on a radial topology, with current flowing from the substation to the loads, yielded unidirectional power flow. With the addition of distributed generation, power flow and fault current are becoming bi-directional. This causes loss of coordination between conventional overcurrent protection devices. Adding power sources downstream of protection devices might also cause the upstream protection device to be blinded from faults. Conventional overcurrent protection is mainly based on the fault levels at specific points along the network. By adding renewable sources, the fault levels increase and become dynamic, based on weather conditions. In this dissertation, power system faults are modelled with sequence components and simulated with Digsilent PowerFactory power system software. The modeling of several faults under varying power system parameters are combined with different photovoltaic penetration levels to establish a framework under which protection challenges can be better defined and understood. Understanding the effects of distributed generation on three phase power systems are simplified by modeling power systems with sequence networks. The models for asymmetrical faults shows the limited affect which distributed generation has on power system protection. The ability of inverter based distributed generators to provide active control of phase current, irrespective of unbalanced voltage occurring in the network limits their influence during asymmetrical faults. Based on this unique ability of inverter based distributed generators (of which PV energy sources are the main type), solutions are proposed to mitigate or prevent the occurrence of loss of protection under increasing penetration levels of distributed generation. The solutions include using zero and negative sequence overcurrent protection, and adapting the undervoltage disconnection time of distributed generators based on the unique network parameters where it is used. Repeating the simulations after integrating the proposed solutions show improved results and better protection coordination under high penetration levels of PV based distributed generation. / Dissertation (MEng)--University of Pretoria, 2017. / Electrical, Electronic and Computer Engineering / MEng / Unrestricted

The electricity generation composition in the South African national grid has changed in recent years from mostly thermal generation to a combination of thermal generation plants and a variety of plants owned and operated by Renewable Energy Independent Power Producers (REIPPs). The need arises to determine whether the existing planning and design guidelines of distribution networks in South Africa are sufficient in terms of equipment specifications and general sizing and rating principles, used during the network planning process, under increasing penetration levels of embedded generation. The correlation between increases in embedded generation penetration levels and voltage variation, unbalance and harmonic emissions are determined by simulating various operating scenarios of varying load and short circuit level for penetration levels of 10%, 25% and 40%. The existing distribution grid planning standard NRS 097 allows for a 25% penetration level where several consumers share one feeder or distribution transformer. Some of the limits contained in the South African power quality standards NRS 048 and the distribution grid planning guidelines NRS 097 are exceeded when penetration levels of grid connected Photovoltaic (PV) generation exceeds certain levels. - Switching embedded generation in or out of service does not cause voltage variations that exceed the planning limit of 3% at the shared feeder. - Voltage unbalance due to embedded generation connected to the same phase does not cause the compatibility limit of 3% to be exceeded. - Current unbalance should be monitored as it is very likely that equipment ratings may be exceeded when the integration of embedded generation is not coordinated. - Voltage harmonic limits of the odd harmonic which are multiples of 3 are exceeded. - Current harmonic planning limits of several harmonics are exceeded for penetration levels of 25%. The criteria and limits contained in the standards and guidelines relating to current unbalance and harmonic currents should be reviewed to ensure that future grids with high penetration levels of embedded generation can withstand the inherent power quality challenges without having an adverse effect on distribution equipment. Distribution transformers can age faster when they are subjected to harmonic currents and voltages exceeding their design parameters [12]. The distribution transformer isolates the Medium Voltage (MV) distribution grid from the 400 V residential grid. The voltage harmonics and voltage unbalance on the Low Voltage (LV) grid therefore do not permeate to the MV grid. Proposed future work includes translating the qualitative suggestions made in this dissertation into quantitative terms that can be included in revisions of the distribution equipment standards and grid planning guidelines. / Dissertation (MEng)--University of Pretoria, 2017. / Electrical, Electronic and Computer Engineering / MEng / Unrestricted