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.