Planning Strategies for EV Fast-Charging Stations combined with Battery Storage Systems in Distribution Grids

Marjan Gjelaj

    Research output: Book/ReportPh.D. thesis

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    Increasing environmental concerns are driving a change in the energy system, in which the electrification of the transport sector is considered a crucial element. The introduction of electric vehicles (EVs) can potentially reduce CO2 emissions in and around metropolitan areas. The growing number of EVs and the widespread installation of EV charging infrastructures are introducing a new high-power load to the distribution system operators (DSOs). Moreover, the replacement of internal combustion engine vehicles with EVs is expected to increase over the next ten years together with the necessary electric power generation to support the EV charging demand. More specifically, the major car manufacturers are investing resources in new models to solve the problem of a low EV range by increasing the size of the batteries. Therefore, to satisfy the EV load demand of the new EV models in urban areas, public DC fast-charging stations (DCFCSs) are essential to recharge EVs rapidly. This thesis demonstrates how an intelligent integration of DCFCSs in combination with battery energy storage (BES) can reduce the EV peak demand and the charging infrastructure costs. In particular, different aspects are addressed such as the technical and economical requirements related to EV fast-charging systems by using lithium-ion BESs within the DCFCSs. The three key topics investigated in this thesis include:
    1. Advanced charging strategies for EV load reduction considering novel design criteria of DCFCSs by using modular BESs within the fast-charging systems.
    2. Grid integration of stationary BESs within the fast-charging systems based on the prediction of the EV charging demand.
    3. Grid services provision from BES within fast-charging stations such as peak shaving and frequency regulation.
    The first topic investigates how EV charging systems in combination with BESs can mitigate the self-induced impact and dynamically reduce the EV load independently. First, the DCFCSs design criteria for enabling a bi-directional power exchange between the grid and the BESs are established. The proposed design method includes two BESs within DCFCS allowing partial decoupling between low voltage grid and the EV load. A set of monitoring and control requirements are defined in order to achieve dynamic charging coordination of the EVs through the BESs. The validation of the proposed method is performed by using a bidirectional AC/DC converter, two lithium-ion BESs and a DC/DC converter. The second topic studies EVs’ charging start times considering a stochastic planning method to predict EVs charging demand by using user behavior and the probabilistic driving patterns. According to the stochastic method, a coordinated charging demand and BES charging demand are proposed with the objective of minimizing the EV peak load and the charging infrastructure costs. The coordinated charging demand is used to control the EV charging during the EV peak hours. Instead, in the BES charging demand, an optimal BES is proposed as an alternative solution to reduce EV peak demand and DCFCSs installation costs. The proposed planning methods have demonstrated the ability to prevent the grid reinforcement costs caused by the EV demand during peak hours. The third topic examines the stochastic planning method mentioned to provide grid services from BES within the fast-charging stations. The proposed method uses the BES as a multifunctional device to provide ancillary services such as peak shaving and frequency regulation with the objective of reducing BES costs and charging infrastructure costs. The primary objective of the BES is to minimize the EV peak-load demand with peak shaving by avoiding additional grid reinforcement costs. The secondary objective is to optimize the investment costs of BES by providing primary frequency regulation to the local transmission system operator during the night when the EV charging demand can be neglected. In conclusion, this thesis proposes solutions to reduce the EVs’ charging times as well as the required power of the DCFCSs from the grid by allowing the fast-charging stations to be a cost-effective solution within the power systems. Each topic includes an economic analysis to evaluate the technical and economic aspects related to EV fast-charging infrastructures, the BES life-cycle costs as well as the financial performance of the BES costs compared to the costs of grid reinforcement.
    Original languageEnglish
    PublisherTechnical University of Denmark
    Number of pages228
    Publication statusPublished - 2019


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