Electric vehicle integration - grid stability concerns and countermeasures

Abstract

The electrification of transportation has become a sustainable solution to the global economic and environmental challenges associated with the fossil fuel-dependent transportation. Substantial electric vehicle (EV) integration has resulted in the recent past owing to the distinct advantages and various incentives provided by governments. It can be expected that EV penetration will further accelerate along with technological advancements. This strategic shift of primary source of transportation energy from oil pipelines to power grids will inevitably bring numerous challenges to power grids around the world. The potential impacts of EV integration on power grids are yet to be discovered. This research concentrated on modelling the EV charging load, evaluating its impact on power system stability and identifying remedies. Further, two computationally efficient indexes were developed to identify voltage stability and oscillatory stability prudent charging solutions. A comprehensive EV charging infrastructure planning strategy was also developed. This research investigated the probable grid impacts associated with EV charging by comprehensively reviewing the available literature. Even though there were number of system studies on wide variety of grid impacts, scant attention has been paid to impact of EV charging on system stability. Electrification of transportation brings significant load integration to the grid. Hence, it is important to understand the EV charging impact on power system stability. The extant literature is largely based on conventional load characteristics rather than actual EV load behaviours, due to the unavailability of proper load models. Hence, this study develops static and dynamic EV load models following analytical and numerical methodologies, as an essential basis for accurate system stability studies. It is identified that the static load model of a power electronically controlled EV load can be best described by a combination of constant power and negative exponential load models. The factors affecting the load model parameters are also evaluated. Subsequently, a dynamic load model is derived, considering the dynamics of the EV charger and the battery. The EV charging load dynamics could be described by an eleventh order dynamic model. The developed static and dynamic load models are then utilised to evaluate the impacts of EV charging on power system voltage stability and oscillatory stability. Power system static voltage stability and oscillatory stability studies with EV charging loads are carried out on different test systems, under several practical scenarios. The results show that the EV is an onerous load, and the existing studies which have modelled EVs with conventional load characteristics have resulted in conservative outcomes. Further investigations are performed iv | P a g e to identify the factors affecting the voltage stability and oscillatory stability of the power grid in the presence of the EV charging load. Remedies which can be implemented to mitigate impact of EV charging load on static voltage stability are investigated. Effectiveness of load bus voltage control to mitigate the voltage stability impact of EV charging load, is tested and verified. In addition, importance of giving priority to implement public charging stations to ease the grid impacts caused by distributed home based chargers in the distribution system, is highlighted in this research. It is found that proper planning can be done to minimise the grid impacts, proactively. Two computationally efficient indexes are derived to identify the relative static voltage stability and oscillatory stability status of the power system in different planning cases. The developed indexes having good physical interpretations are proven to be simple and easy to incorporate in distribution system planning exercises to obtain a stability preserved planning solution. A metaheuristic technique has been incorporated to facilitate EV charging infrastructure planning. Several planning cases are discussed to represent different planning requirements. A comprehensive EV charging infrastructure planning framework is developed by incorporating consumer, investor and power grid requirements within the planning objectives and constraints. This could identify solutions which cause less impact on the grid in terms of losses, voltage regulation, asset overloading, grid voltage stability and oscillatory stability, while optimally satisfying the EV customer and investor requirements. The optimal capacity and placement of a fixed capacitor to minimise grid impacts through effective reactive power compensation have also been identified. Further, the possibility of enhancing EV charging facilities while minimising the grid impacts is proven with optimum utilisation of renewable energy sources. Overall, the contribution made by this thesis promotes greener transportation with fewer associated grid impacts.