Impacts of electric vehicle parking lots on distribution system reliability

Electrical energy used to be generally generated with large power generators in the place where the source was located and delivered to the consumer with long transmission lines. Furthermore, management of power systems (generation, transmission and distribution networks) was done by a single monopoly. Later, this structure was found to be insufficient to provide the needs of developing and growing power systems due to strict regulations that prevent effective use of energy resources. In the open-access environment with this new structure, customers are free to choose their electricity suppliers. The supply and demand of electricity is becoming more flexible with deregulated power systems. In this case, that it is needed to be done is to make improvements in the existing system, taking the needs of the consumer into consideration, or to give the consumers the opportunity to generate their own electricity and sell their excess energy. To make this change means changing the model have been used for a long time. The newly constructed structure makes distributed energy resources (DERs) with their features of low investment costs, environment-friendly and high flexibility an attractive generation and storage unit. DERs are small-scale generation and storage systems connected to the distribution systems. One of the new actors in the changing and developing power systems is electric vehicles (EVs). EVs are creating new possibilities and new challenges for distribution systems. EVs with their increasing penetration level will be a storage option as well as a load for the system that must be taken into account in the planning phase of the distribution system. It is possible to improve distribution system reliability by injecting stored energy of EV to the network when there is a supply outage and/or a fault in the system components. Besides its technical benefit, it may also give an economic opportunity that can be used for peak-shaving in the case of rising energy prices at peak hours. On the other hand, their restricted battery capacities (12-90 kWh) limit the contribution of EVs to the network. EV parking lots (PLs), because of their aggregating feature, can be used as a new peak-shaving or back-up DER. Because, EVs are parked in the PLs during most of the daytime. In this study, the impacts of stochastic storage capacity of EV PL on distribution system reliability are investigated. In metropolitans like Istanbul, there are many PLs whose car capacity is about 400-500 cars. If the change in driver behaviors in the future is considered, one PL may serve as a temporary storage unit of 1 MW along 2-3 hours during midday peak hours. In this study, to determine the impacts of the average storage capacity (ASC) of EV PL on the distribution system reliability, three basic models have been developed namely a storage model, a load model and a distribution system model. In the first stage of this study, ASC model of EV PL has been found. One of the most important parameters of this model is the arrival/departure car pattern of the PL. The probabilistic distribution of car arriving/departing times of the sample PL is calculated using the real data of Haciosman PL provided by Istanbul Car Parking Corporation (ISPARK). In order to determine ASC model of EV PL, arrival/departure patterns generated randomly according to their probability distributions, are combined with State of Charge (SoC) level of EV when arrived at the PL and charger power. Then, Monte-Carlo (MC) simulations are used to calculate ASC of EV PL for each daytime. In addition, the sensitivity analysis has been performed to determine impacts of several important parameters on the storage capacity of the PL and seasonal impacts on the storage capacity of the PL have been investigated, using a whole year data for 2013-2016 period. In the second stage, the load duration curve of the PL for G2V operations have been calculated. EVs are charged at morning hours (07.00-11.00) and if there is an outage or fault in the network, they are discharged at the period when the PL reaches the maximum storage capacity (11.00-14.00). According to this working principle, the load curve of the PL has been obtained by using the lithium-ion battery-charging characteristic and the car arrival/departure patterns which were developed in the first stage. Then, the load model of this study has been created by using load data of the distribution feeder provided by Bosphorus Electric Distribution Corporation (BEDAŞ). In the last stage, the reliability parameters of the feeder components have been determined using the data provided by BEDAŞ. The distribution system reliability indices have been calculated by combining the storage capacity model of the PL, the load models of the PL and the distribution feeder and the reliability parameters of the feeder components for four different operation scenarios. Simulation results show that the load point reliability indices can be improved up to 45% even for the considered backup operation of the EV PL. Furthermore, in order to strengthen the improvement in reliability indices of EV PL, the economic benefit from G2V-V2G operations has been calculated. The most important feature of this study is that a storage capacity model of the EV PL has been constructed using real data of a sample PL and the distribution grid has been used for quantitative assessment of the impacts of PLs on distribution grid reliability by means of the storage capacity model. In the literature, usually test systems are used and the PL, which has a very strong car capacity compared to the load capacity of distribution feeder has been analyzed; as the result of this analysis, technical improvements and economic benefits have been overestimated. These studies for the V2G operation rise expectations about the improvement of distribution system reliability in the future smart grids. Although the level of the these expectations may be a little bit overestimated; with this study, the technical and economic benefits of EV PLs in terms of power grid has become more concrete. At the same time, this study aims to inform the PL operators and distribution utilities about the benefits of aggregated V2G operation for their choices and investments. The electrical infrastructure needs of existing or newly installed PLs can be determined not only for G2V but also for V2G operations using the results of this study. Finally, on a more global scale, with the use of an existing potential, because of the fact that additional energy resource investments will be avoided, the results of the study are also important in terms of the support that the public authority provides for such initiatives. On the other hand, this study has been focused on existing or newly established PLs and distribution network information, and under these circumstances, it includes reliability evaluation for different scenarios. The siting (determining the optimal PL-network connection point) or sizing (determining the optimal PL size) problems of the PLs have not been taken into account in the study. In the future, developments in EV and battery technologies and new trends in the electricity market will further clarify the importance and significance of the results of this study.