Dynamic modelling of traction loads and renewable energy systems on shared power lines for power quality assessment

Thesis (MEng)--Stellenbosch University, 2019.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.AFRIKAANSE OPSOMMING: Eskom het onlangs begin ondersoek wat die effek is van elektriese treinlaste op hernubare energiebronne, weens die kragkwaliteitsprobleme wat verband hou met treinnetwerke. Swak kragkwaliteit wat deur middel van treinnetwerke gegenereer word, was nog altyd ‘n probleem. Die impak van die probleem het aansienlik toegeneem met die toename in die aantal hernubare kragaanlegte wat gekoppel word aan die nasionale netwerk. Studies toon ‘n toename in spanningswanbalans en harmoniek verspreiding op verskeie punte in die netwerk wat lei tot die verlies van kragproduksie in hernubare kragaanlegte. ‘n Onlangse kragkwaliteit assesseringsverslag in Eskom het aangedui dat kragkwaliteitsprobleme, veral harmoniese emissies, by hernubare energiebronne bestaan. Nie-lineêre bronne soos treinlaste dra by tot die harmoniese versteuring van spanning op die netwerk, asook harmoniese emissie van hernubare kragaanlegte. Om insig te verkry rakende die probleem, bestaan daar ‘n behoefte om treinstelsels en hernubare kragaanlegte te modelleer. DIgSILENT PowerFactory, is gekies as die sagteware simulasiepakket om generiese modelle van spanning wisselrigters in hernubare kragaanlegte en spanning gelykrigters in treine te ontwerp en te bou om dinamiese tyddomein-simulasies te doen. Die simulasie resultate is vergelyk met gemete resultate om die akkuraatheid van die modelle te bevestig. Danksy goeie korrelasie tussen die simulasie resultate en gemete resultate kan die modelle gebruik word vir toekomstige netwerkbeplanning en kragkwaliteit assessering. In hierdie studie word die kragkwaliteits probleme rakende elektriese treinlaste verder ondersoek, asook die assessering van die kragkwaliteit by die punt van konneksie van 'n plaaslike windplaas. Die assessering van die spanningswanbalans het aangedui dat elektriese treinlaste hoofsaaklik die grootste bydrae lewer tot spanningswanbalans op treinnetwerke en kan onder sekere omstandighede wisselrigter onderbrekings by hernubare kragaanlegte veroorsaak. Daar word opgemerk dat verskillende toestande soos die tipe trein, bedryfsomstandighede, beheer en drywingsaanvraag van die treinlas asook die driefasefoutvlak die spanningswanbalans wat deur treinlaste veroorsaak word, sal beïnvloed. Die impak van elektriese treine op die netwerkspanningvervorming is ondersoek en daar is vasgestel dat die generasie van klein stroomharmonieke deur elektriese treinlaste groot spanningsvervorming kan veroorsaak in die teenwoordigheid van parallelle resonansies. Die impak van impedansie en agtergrond harmonieke is ondersoek en die resultate toon dat die metodes wat in standaarde beskryf word om die impedansies te bereken vir die vasstelling van die harmoniese bydrae, nie altyd geldig sal wees veral as wisselrigters as harmoniese bronne voorkom nie. ‘n Tweepunt metingsbenadering word gevolg om die impak van stroom harmonieke in elektriese treinlaste op die assessering van stroom harmonieke in hernubare kragaanlegte te ondersoek. ‘n Metode word aangebied om die stroom harmonieke van hernubare kragaanlegte te benader sonder die impak van elektriese treinlaste op die assessering. Die resultate toon dat die harmoniese stroom van elektriese treinlaste wel die assessering van stroom harmonieke in hernubare kragaanlegte beïnvloed en dat die huidige asseseringsmetode dus nie altyd akkuraat sal wees nie