Paul S. Moses

Real-Time Coordination of Plug-In Electric Vehicle Charging in Smart Grids to Minimize Power Losses and Improve Voltage Profile

This paper proposes a novel load management solution for coordinating the charging of multiple plug-in electric vehicles (PEVs) in a smart grid system. Utilities are becoming concerned about the potential stresses, performance degradations and overloads that may occur in distribution systems with multiple domestic PEV charging activities. Uncontrolled and random PEV charging can cause increased power losses, overloads and voltage fluctuations, which are all detrimental to the reliability and security of newly developing smart grids. Therefore, a real-time smart load management (RT-SLM) control strategy is proposed and developed for the coordination of PEV charging based on real-time (e.g., every 5 min) minimization of total cost of generating the energy plus the associated grid energy losses. The approach reduces generation cost by incorporating time-varying market energy prices and PEV owner preferred charging time zones based on priority selection. The RT-SLM algorithm appropriately considers random plug-in of PEVs and utilizes the maximum sensitivities selection (MSS) optimization. This approach enables PEVs to begin charging as soon as possible considering priority-charging time zones while complying with network operation criteria (such as losses, generation limits, and voltage profile). Simulation results are presented to demonstrate the performance of SLM for the modified IEEE 23 kV distribution system connected to several low voltage residential networks populated with PEVs.

Power quality of smart grids with Plug-in Electric Vehicles considering battery charging profile

The impact of different battery charging rates of Plug-in Electric Vehicles (PEVs) on the power quality of smart grid distribution systems is studied in this paper. PEV battery chargers are high power nonlinear devices that can generate significant amount of current harmonics. PEVs will be an integral component to the operation of smart grids and therefore their power quality impacts must be thoroughly analyzed. Based on decoupled harmonic load flow analysis, different PEV charging scenarios (e.g., time zone scheduling, charging rate and penetration level) are tested for a typical large distribution network topology. The impacts of PEV charge rate on voltage profile, fundamental and harmonic losses, transformer loading and total harmonic distortions are demonstrated.

Impacts of battery charging rates of Plug-in Electric Vehicle on smart grid distribution systems

Plug-in Electric Vehicles (PEVs) will be an integral part of smart grids in the near future. This paper studies the impacts of different PEV battery charging profiles on the performance of smart grid distribution systems. PEVs are already growing in popularity as a low emission mode of transport versus conventional petroleum based vehicles. Utilities are becoming concerned about the potential stresses and overloads that may occur with multiple domestic PEV charging activity. Smart grids will play an important role in PEV operation because the battery chargers can be coordinated by the utility and harnessed for storing surplus grid energy and reused to support the grid during peak times. Therefore, an analysis is performed for a smart grid distribution system to demonstrate the impacts of different PEV charging scenarios. The paper compares charging rates (e.g., slow, medium and fast charging), PEV penetration levels as well as different charging periods over a 24 hour period considering existing system load profiles, and evaluates the overall performance of the distribution system. The impact on system load profile, total losses, transformer loading and voltage profile is examined.

Load management in smart grids considering harmonic distortion and transformer derating

This paper addresses the important issue of power quality management for smart grids and proposes a load management strategy based on transformer derating for minimizing harmonic distortion in distribution feeders and transformers. Ongoing development of smart grid technologies such as smart metering and smart appliances are creating new opportunities for improving distribution system performance. One area undergoing study is effective control of demand response through (semi)automated load management practices (e.g., smart appliances). Despite these developments, the impact on power quality has not been taken into consideration from a demand side management point of view. Smart grids provide an excellent opportunity to better manage power quality and reduce harmonic distortions present in power networks. In this paper, it is proposed that the impact of harmonics generated by nonlinear loads should be factored into overall load control strategies of smart appliances. This work focuses on the impact on residential distribution transformers which are adversely impacted by harmonic current distortions. A growing concern is the potentially high penetration of plug-in electric vehicles in smart grids. Load management of electric vehicles is studied for an IEEE 30-bus 23 kV distribution system to demonstrate the benefits of the proposed power quality and load management strategy. This paper proposes computing transformer K-Factor derating to control scheduling of smart appliances/loads to reduce harmonic stresses.

Voltage profile and THD distortion of residential network with high penetration of Plug-in Electrical Vehicles

This paper analyzes the potential impacts of Plug-in Electric Vehicles (PEVs) on the voltage profile, losses, power quality and daily load curve of low voltage residential network. PEVs are soon expected to grow in popularity as a low emission mode of transport compared to conventional petroleum based vehicles. Utilities are concerned about the potential detrimental impacts that multiple domestic PEV charging may have on network equipment (e.g., transformer and cable stresses). To address these issues, two charging regimes including uncoordinated (random) and coordinated (uniformly distribution) are considered. Based on harmonic analysis of a typical 19 bus low voltage (415V) residential network, different charging scenarios over a 24 hour period are compared considering voltage deviations, system losses, transformer overloading and harmonic distortions. Simulation results are used to highlight the advantages of the coordinated uniformly distributed charging of PEV in residential systems.

Derating of Asymmetric Three-Phase Transformers Serving Unbalanced Nonlinear Loads

A new analysis into the steady-state operation and derating of three-phase transformers under nonsinusoidal and asymmetric operating conditions is proposed. The combined effects of transformer core and load asymmetry, nonlinearity, and harmonics, as well as nonsinusoidal input excitation are examined. A time-domain nonlinear model for three-phase three-leg transformers is implemented. Transformer derating is estimated by modeling additional power losses due to harmonics generated by the iron core, nonsinusoidal excitation, and nonlinear (rectifier and electric drive) loading. Laboratory tests are performed to verify simulated waveforms. The contribution of this paper is a nonlinear transformer modeling technique for steady-state operation under unbalanced, asymmetric, and nonsinusoidal operation, capable of computing derating factors.

Dynamic Modeling of Three-Phase Asymmetric Power Transformers With Magnetic Hysteresis: No-Load and Inrush Conditions

A new dynamic model of a three-phase three-leg transformer for steady-state and transient operating conditions is proposed in this paper. With very few exceptions, the existing models oversimplify the magnetic interactions in multileg core topologies and employ single-value nonlinear functions for modeling core nonlinearities. Unfortunately, this does not provide sufficient accuracy for a wide range of dynamic disturbances such as dc bias, ferroresonance, and inrush. For this purpose, a new time-domain model based on magnetic-circuit theory is developed to include dynamic-hysteresis behavior (major and minor loops) in asymmetric three-leg core topologies. Furthermore, a new analysis is performed to study the impacts of hysteresis on no-load and transient inrush current behavior in three-phase transformers. Simulation results have been successfully compared with measurements to verify the accuracy of the proposed model.

Impacts of Hysteresis and Magnetic Couplings on the Stability Domain of Ferroresonance in Asymmetric Three-Phase Three-Leg Transformers

This paper investigates the stability domain of ferroresonance in asymmetric three-phase three-leg transformers considering magnetic couplings and hysteresis effects of the core. A newly developed and accurate time-domain transformer model capable of simulating dynamic and transient operating conditions is implemented in this study. The model is based on electromagnetic circuit theory and considers dynamic hysteresis effects (major and minor loops) as well as core topology, asymmetry, and magnetic flux cross-coupling interactions of the core legs. Unbalanced switching with series and shunt capacitances, which is known to increase the risk of ferroresonance, is studied with the developed model. The validity of the model under ferroresonant conditions is confirmed by comparisons with extensive experimental data. The main contribution is a new analysis of (a)symmetric three-phase transformer ferroresonance behavior with an accurate core model capable of predicting ferroresonance modes.

Distribution transformer stress in smart grid with coordinated charging of Plug-In Electric Vehicles

Coordinated charging of Plug-In Electric Vehicles (PEVs) in residential distribution systems is a new concept currently being explored in the wake of smart grids. Utilities are exploring these options as there are concerns about potential stresses and network congestions that may occur with random and uncoordinated multiple domestic PEV charging activities. Such operations may lead to degraded power quality, poor voltage profiles, overloads in transformer and cables, increased power losses and overall a reduction in the reliability and economy of smart grids. Future smart grids communication network will play an important role in PEV operation because the battery chargers can be remotely coordinated by the utility and harnessed for storing surplus grid energy and reused to support the grid during peak times. Based on a recently proposed PEV charging algorithm, this paper focuses on the impact of coordinated charging on distribution transformer loading and performance. Simulation results are presented to explore the ability of the PEV coordination algorithm in reducing the stress on distribution transformers at different PEV penetration levels. The performance of various distribution transformers within the simulated smart grid is examined for a modified IEEE 23 kV distribution system connected to several low voltage residential networks populated with PEVs.

Overloading of distribution transformers in smart grid due to uncoordinated charging of plug-In electric vehicles

Random uncoordinated charging of multiple plug-In electric vehicles (PEVs) in residential distribution systems could become a reality in the very near future. The potential for stresses and network congestion is significant since PEV charging represents a sizeable and possibly random operation in distribution systems. Coordinated and smart charging regimes are currently being investigated as possible long-term solutions. However, it could take several years before smart grid infrastructure is ready to support smart coordinated charging. Therefore, until smart charging becomes available, it must be determined how present-day distribution systems will cope with uncoordinated random PEV charging activities. In particular, the burden on local distribution circuits such as transformers and cables, which are the critical links in distribution systems, must be investigated under PEV loads. This paper carries out an analysis into the impacts of random uncoordinated PEV charging on the performance of distribution transformers. Such operation may lead to an overall reduction in the reliability and economy of future smart grids. Simulation results are presented which demonstrate various random uncoordinated PEV charger activities for a modified IEEE 23 kV distribution system connected to several low voltage residential networks populated with PEVs. The performance of various distribution transformers within the simulated smart grid is examined.