Technical Challenges of Plug-In Hybrid Electric Vehicles and Impacts to the US Power System: Distribution System Analysis

Abstract

Replacing a significant portion of the nation's light vehicle fleet with plug-in hybrid electric vehicles (PHEVs) offers the potential of reducing our dependence on petroleum fuels together with important economic and environmental benefits. The electric power grid is built to support peak loads and, as a consequence, suffers from low asset utilization rates in off-peak periods. In principle, this under-utilized capacity could effectively power a national fleet of PHEVs with little need to increase the energy delivery capacity of the existing grid infrastructure. In practice, this ideal opportunity may be compromised by several factors including the size and distribution of the PHEV fleet, and the timing of vehicle charging activity. This report documents work conducted by Pacific Northwest National Laboratory (PNNL) for the Department of Energy (DOE) to address three basic questions concerning how typical existing electrical distribution systems would be impacted by the addition of PHEVs to residential loads. These questions are:  How many vehicles could the existing power delivery system support in the near future?  What time of day would PHEVs be charged?  Where would the vehicles be charged? The present study complements other research being performed for the DOE by the University of Michigan (UM) in collaboration with PNNL on various issues relating PHEV adoption and marketability. The primary focus of this study was to estimate impacts of PHEV charging on the components of residential feeders. The first part of the analysis was designed to study PHEV charging impacts on the distribution system as a whole. A load flow study was performed for various PHEV penetrations to observe whether any principal components of main feeders exceed their capacity because of PHEV charging. The second part of the analysis focused on the potential impact of the additional load on the distribution transformers serving each individual residence. In both parts of the study computerized modeling of feeder-level load flow compared the existing loading profile and component-specific loads with those created by the superimposition of a hypothetical PHEV load distribution. Three electric utilities: Franklin PUD, Snohomish PUD and Puget Sound Energy (PSE) collaborated with this study by providing information that allowed system-specific feeder modeling. A principal conclusion drawn from this study is that the addition of PHEV load at an individual residence may or may not have noticeable impacts on the existing distribution system depending on the overall penetration level and charging strategy used. The distribution utility's system planning philosophy used to size distribution transformers and the backbone feeder system also plays a large part in determining whether or not, or when in the future, additional PHEV load would impact the system. Acronyms EV electric vehicle PHEV plug-in hybrid electric vehicle LDV light-duty vehicles

Local versus centralized charging strategies for electric vehicles in low voltage distribution systems

“…While each of the charging units has the ability to vary their output in a continuous manner, the charging rate limits are defined in (2), where P EVmax is the rated output of each charging unit.…”

Safety control of PHEVs in distribution networks using finite state machines with variables

“…The research reported in [1] and [9] considered the management of PHEVs in traditional distribution networks As the smart grid technologies advance, it is necessary to study PHEV's interface to future smart distribution networks. We are facing great challenges in how to utilize the smart distribution network technologies (e.g., smart meters) to provide service to more PHEVs and how to reduce cost while developing a new paradigm of distribution networks with more reliable and flexible control strategies.…”

“…term problems (i.e., undesired power quality and compromised system stability and reliability) and long term issues such as reduced lifetime of distribution transformers if they are overloaded [1], [5].…”

“…They first extend the scope of the traditional DES control to include both event disablement and event enforcement. Then they propose an offline safety control synthesis procedure that takes the advantage of both event disablement and enforcement in order to prevent the controlled system from venturing into the 1 In this paper, a distribution network is modeled by an FSMwV. We consider both conventional uncontrollable loads and controllable loads (such as plug-in hybrid electric vehicles) by using appropriate variables to avoid possible state explosion.…”

Residential electrical vehicle charging strategies: the good, the bad and the ugly

“…However, as EV penetration increases, the extra demand due to EV charging will have a considerable impact on the design and operation of electrical power systems [4]. In [5][6][7][8][9][10], it has been shown that the widespread adoption of EVs could negatively impact the distribution network if charging is not properly coordinated. Grid impacts of uncoordinated charging include, but are not limited to, increased voltage imbalance [7], increased grid losses [8], overloading [6], fluctuation of grid frequencies [11], and increased harmonic distortion [12].…”