Evaluate and design battery support services for the electrical grid

Brian, De Beer (2017-12)

Thesis (MEng)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: Aside from the many existing problems on the electric network; new problems have been introduced with the increased adoption of renewable energy sources. In this thesis, how battery storage systems can be applied in order to address some of the problems is investigated. These problems include: addressing the intermittent nature of photovoltaic power plants, integrating larger photovoltaic power plant into a weak network and optimising battery storage sizing for peak load shaving. The intermittent nature of photovoltaic systems becomes a problem when there is a high penetration thereof on the electric network because the fluctuating power output is reflected in the network. Fluctuation mitigation methods, incorporating battery storage systems, are investigated and the ramp-rate control strategy is chosen for further analysis. How the strategy influences the battery storage sizing, performance and cost is analysed. To analyse the impact the ramp-rate strategy has on the performance of a battery type, a battery has to be modelled. How batteries are modelled is investigated and an energythroughput model is selected to be implemented as a tool. The tool is calibrated for two battery chemistries: a lithium (LiFePO4) and lead-acid (PbSO4) chemistry. It is found that the model favours chemistries, such as the LiFePO4 chemistry, because of its linear degrading nature. The integration of larger photovoltaic installations on a weak network is investigated. Weak networks, such as high impedance radial networks, can limit power plant installations to weak connections that can restrict the power plant installation capacity. The modelling of weak networks is investigated and one such a model is implemented in DIgSILENT PowerFactory. As a solution, control systems are created where a battery storage system can work in conjunction with an on-load tap changing transformer to prevent abnormal operating conditions during a sudden power loss. Also investigated is how the battery system should be sized in order to provide this support. It is found that batteries can strengthen the network during sudden power loss conditions. It is also found that the battery systems must be sized for high power output. The last problem that is investigated is the sizing of battery storage systems for peak load shaving. Battery storage systems are usually sized for the worst case scenario but often the worst case is an anomalous case. A statistical tool called Cook’s distance is implemented to identify outlying cases in the load profiles and remove them. The original sizing strategy is optimised and implemented in Python as a tool. Finally, the original sizing strategy is compared to optimised strategy and it is found that in most of the cases the optimised strategy can improve the energy or power requirements. Finally, the costs of each of the three problems are analysed. It is found that the battery energy storage systems required for PV output fluctuation mitigation make a substantial contribution to the levelised cost of the energy of the PV installation. The same is also found with regards to the battery energy storage system used for network strengthening; however reducing the PV installation capacity can reduce the costs considerably. For the optimised battery sizing strategy, for peak load shaving; levelised costs of energy savings of up to 24% are achieved.

AFRIKAANSE OPSOMMING: Afgesien van die baie bestaande probleme op die elektriese netwerk, is nuwe probleme aan die lig gebring met die verhoogde aanname van hernubare energiebronne. In hierdie proefskrif, is daar ondersoek hoe batterystelsels toegepas kan word om sommige van hierdie probleme aan te spreek. Hierdie probleme sluit in: die aanspreek van die wisselende aard van fotovoltaïese kragsentrales, die integrasie van groter fotovoltaïese kragstasies op swak netwerke en die optimalisering van battery groottes wat gebruik is vir piek las vermindering. Die wisselende aard van fotovoltaïese stelsels is ’n probleem wanneer daar ’n hoë penetrasie daarvan op die elektriese netwerk is, omdat dit in die netwerk weerspieël word. Fluktuasie demperstrategieë wat gepaard gaan met battery stelsels, is ondersoek en ’n strategie wat omsetter-uittree beheer toepas is gekies vir verdere analise. Hoe die strategie die battery spesifikasies, prestasie en koste beïnvloed, is ontleed. Dit is nodig om die impak wat die omsetter beheer strategie het op die battery, te bestudeer. Om dit te kan doen, moet die battery gemodelleer kan word. ’n Ondersoek is gedeon op verskillende battery modelle en ’n energie-deurvoermodel is gekies om implementeer te word en te dien as instrument vir verdere ontledings. Hierdie model is gekalibreer vir twee battery chemikalieë: litium (LiFePO4) en loodsuur (PbSO4). Daar word bevind dat die model voorkeur verleen aan die chemikalie, LiFePO4, as gevolg van sy lineêre verswakkende natuur. Die integrasie van groter fotovoltaïese installasies op swak netwerke is ondersoek. Swak netwerke, soos hoë impedansie radiale netwerke, kan kragsentrales beperk tot swak verbindingspunte op die netwerk. Die swak netwerk eienskappe by die verbindingspunte kan die installasie kapasiteit van kragsentrales beperk. Die modellering van swak netwerke is ondersoek en een so ’n model is geïmplementeer in DIgSILENT PowerFactory. ’n Beheerstelsel word geskep wat die batterystelsel in staat stel om saam met die verstelbare transformator te werk. Dit is geïmplementeer as oplossing om abnormale operasionele toestande gedurende skielike kragverlies te vermy. Daar is ook ondersoek hoe groot die batterystelsels moet wees om die nodige ondersteuning te bied. Dit is gevind dat die batterystelsel wel instaat is om die network te versterk teen skielike kragverliese. Daar is ook bevind dat die batterystelsels vir hoë krag ontwerp moet word. Die laaste probleem wat ondersoek is, is die skalering van die batterystelsels vir pieklasvermiddering. Batterystelsels is gewoonlik geskaleer vir die uiterste gevalle, maar baie van die gevalle is uitskieter gevalle. ’n Statistiese instrument genaamd Cook se afstand, is geïmplementeer om afwykende gevalle te identifiseer en uit lasprofiele te verwyder. Die oorspronklike strategie waarmee batterystelsel geskaleer word, is geöptimaliseer en geïmplementeer in Python as ’n instrument. Ten slotte, is die oorspronklike strategie vergelyk met die geöptimaliseerde strategie en daar is bevind dat die geöptimaliseerde strategie die meeste van die gevalle kan verbeter ten opsigte van energie- of kragvereistes. Ten slotte is die koste van elk van die drie probleme ontleed. Daar is bevind dat die batterystelsels wat benodig is vir die fluktuasie demping van die fotovoltaïese uitset, ’n wesenlike bydrae maak tot die gelyke koste van energie van die PV kragsentrale. Dieselfde geld ook vir die batterystelsel wat gebruik is vir netwerkversterking. As die fotovoltaïese installasie kapasiteit egter verminder is, kan die koste aansienlik verminder. Vir die geöptimaliseerde battery skalering strategie vir pieklasvermindering, kan ’n kostebesparing van tot 24% behaal word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/102881
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