Browsing by Author "Naude, Hein"

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    Dynamic modelling of traction loads and renewable energy systems on shared power lines for power quality assessment
    (Stellenbosch : Stellenbosch University, 2019-04) Naude, Hein; Beukes, Johan; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: Eskom has recently started investigating the effect of traction on renewable energy sources due to the power quality problems associated with traction networks. Poor power quality generated by means of traction networks have always been of concern. The impact of the traction load power quality issues has greatly increased due to the increasing number of renewable power producers (RPPs) being connected to the national grid. Studies has shown an increase in voltage unbalance and harmonic distribution at various points of concern in the network which leads to the loss of power production from the RPPs. Recent power quality assessment reports from Eskom has indicated that power quality problems, particular harmonic emissions, exist at RPPs. Harmonic sources such as non-linear (traction loads) are contributors to voltage harmonic distortion on the network in addition to harmonic emissions of RPPs. To gain insight into the problem the need exist to model and simulate traction drive systems and renewable power plants. DIgSILENT PowerFactory, was chosen as the software simulation package to design and build generic models of renewable system inverters and traction load rectifiers to conduct dynamic time domain simulations. To validate the accuracy of the models, the simulation results were compared to measured results. Due to good correlation, the models can be used for future network planning and power quality assessment. The aim of this thesis is further to investigate the power quality issues related to traction loads and to perform a power quality assessment at the POC of a local wind farm. The assessment of voltage unbalance indicated that traction loads is generally the largest contributor to voltage unbalance on a traction network and can cause inverter trips at RPPs at certain conditions. It is observed that various conditions such as the traction load type, operating conditions and control of the traction load, power demand of the traction loads and three-phase fault level will impact the voltage unbalance caused by traction loads. The impact of traction loads on the network voltage distortion is investigated and it is determined that small current harmonics emissions of traction loads can generate large voltage distortion at the presence of a parallel resonance. The impact of impedance and background harmonics is investigated and the results show that the methods often described in standards for calculating impedances to establish harmonic contribution will not always be valid, especially when having inverters as harmonic sources. A two-point measurement approach is followed for investigating the impact of traction load current emissions on the assessment of RPP current emissions based on international guidelines. A method is presented to approximate current emissions of the RPP without the impact of the traction load current emissions on the assessment. The results show that traction loads do impact the harmonic assessment of RPPs and therefore the current assessment method will not always be accurate.