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Photovoltaic based distributed generation power system protectionvan der Walt, Rhyno Lambertus Reyneke January 2017 (has links)
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
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Design considerations of South African residential distribution systems containing embedded generationKruger, Gustav Reinhold January 2017 (has links)
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
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