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2. A Review of Plug-in Vehicles and V2G.pdf

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A Review of Plug-in Vehicles and Vehicle-to-Grid Capability Bill Kramer, Sudipta Chakraborty, Benjamin Kroposki National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, USA Bill_Kramer, Sudipta_Chakraborty, Benjamin_Kroposki @nrel.gov Abstract- As hybrid vehicles gain popularity among the consumers, current research initiatives are focused towards developing plug-in electric and hybrid vehicles that can exploit utility power
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   A Review of Plug-in Vehicles and Vehicle-to-Grid Capability Bill Kramer, Sudipta Chakraborty, Benjamin Kroposki  National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, USA Bill_Kramer, Sudipta_Chakraborty, Benjamin_Kroposki @nrel.gov  Abstract-  As hybrid vehicles gain popularity among the consumers, current research initiatives are focused towards developing plug-in electric and hybrid vehicles that can exploit utility power to charge vehicle batteries and therefore less dependent on the gasoline usage. Power electronic systems are being developed to allow plug-in vehicles to be vehicle-to-grid (V2G) capable where the vehicles can work as distributed resources and power can be sent back to the utility. In this paper a review of different plug-in and V2G capable vehicles are given along with their power electronics topologies. The economic implication of charging the vehicle or sending power back to the utility is described in brief. Finally, all vehicles with V2G capability must meet the IEEE Standard 1547 for connecting to the utility. Brief descriptions of the requirements and testing that must be followed for V2G vehicles to conform the IEEE 1547 standards are also discussed. I.   I  NTRODUCTION  The continuous rise in gasoline prices along with the increased concerns about the pollutions produced by fossil fuel engines are forcing the current vehicle market to find new alternatives to reduce the fossil fuel usage. Along with the research on bio-fuel driven engines; different electric vehicles and hybrid electric vehicles are evolving as viable alternatives to replace, or at least reduce, the current fleet of fossil fuel driven vehicles. Although current manufactured electric/hybrid vehicles are being marketed as a way to reduce fossil fuel usage, several promising technologies are being demonstrated that can utilize power electronics to charge the battery from the utility using plug-in vehicles or act as a distributed resource to send power back to the utility with vehicle-to-grid capabilities. In this paper, different plug-in vehicle topologies are described to review the power electronics required for them. The newly evolving V2G technology is also discussed along with economics and compliance requirements to allow the vehicle to be connected to the grid. Before going into the details of power electronics required for the electric/hybrid vehicles, the common forms of these vehicles are described next to get accustomed with the terminologies.  A.    Electric Vehicles A typical electric vehicle (EV) has a battery pack connected to an electric motor and provides traction power through the use of a transmission. The batteries are charged primarily by a  battery charger that receives its power from an external source such as the electrical utility. Also during regenerative braking, the motor acts as a generator which provides power back to the  batteries and in the process slows down the vehicle. The  primary advantage of an EV is that the design is simple and has a low part count. The primary disadvantage is that the driving range of the vehicle is limited to the size of the battery and the time to re-charge the battery can be from 15 minutes to 8 hours depending on how far the vehicle was last driven, the battery type and battery charging method.  B.    Hybrid Electric Vehicles The components that make up a typical hybrid electric vehicle (HEV) include a battery pack, motor controller, motor/generator, internal combustion engine, transmission and driveline components. The batteries are charged through the use of the on-board internal combustion engine and generator. In a plug-in hybrid electric vehicle (PHEV), the batteries can also be charged through the use of a battery charger that receives its power from the utility. The best PHEV design will allow the vehicle to operate on electric power only reducing the amount of time that the engine runs. When the vehicle is not operating, the battery can be charged through the use of a  battery charger that is “plugged in” to the electrical utility or other energy sources. A PHEV normally has a larger battery  pack than a HEV. The advantage of a PHEV over an HEV is that due to external battery charging, the vehicle can run longer on electric power which in-turn reduces engine fuel consumption. C.    Fuel Cell Vehicles The prototype fuel cell vehicles (FCV) that are currently under development mostly utilize an on-board tank to store  pressurized hydrogen. Hydrogen and conditioned air are fed to a proton exchange membrane (PEM) stack to develop DC  power. The configuration is very similar to the electric vehicle configuration in which a electric motor/generator provides the mechanical power for traction. The on-board batteries allow the energy to be stored during regenerative braking and provide  peak power to the motor controller during vehicle acceleration. A plug-in fuel cell vehicle (PFCV) has a battery pack and a fuel cell that is connected to an electric motor that provides traction power to the wheels through a transmission. The  batteries can be charged by the use of a battery charger that receives its power from the utility but can also be charged by using the fuel cell.      D.   Vehicle-to-Grid Capability A plug-in vehicle can also be designed to provide power for standby power applications, such as back-up power to a home, through its vehicle-to-grid (V2G) capability. For EVs and PHEVs, the amount of energy (watt-hours) that can be  provided is limited to the size of the on-board energy storage device. For PFCVs, it is limited to not only the battery pack size, but also the amount of fuel that is on-board. The unique aspect of power flow in V2G vehicles is that it is bi-directional, meaning the vehicle should be able to take power (during charging) and provide power (during discharge) from/to the grid. V2G capability is a promising technology for increasing the amount of distributed generation that can be used during  peak hours. On-going demonstrations are being conducted by utilities and customers to evaluate this new technology [1]. II.   P LUG - IN V EHICLES  According to the Electric Power Research Institute (EPRI), more than 40% of U.S. generating capacity operates overnight at a reduced load overnight, and it is during these off-peak hours that most PHEVs could be recharged. Recent studies show that if PHEVs replace one-half of all vehicles on the road  by 2050, only an 8% increase in electricity generation (4% increase in capacity) will be required [2]. Most of the electric vehicles that are of plug-in type, utilize on-board battery chargers to recharge the batteries using utility  power. The simplest form of a plug-in electric vehicle is shown in Fig. 1. This configuration consists of a battery system and a motor controller that provides power to the motor, which in-turn supplies power to the wheels for traction. Many of today’s EVs use a permanent magnet electric motor that can also act as a generator to recharge the batteries when the brakes are applied. During regenerative braking, the motor acts as a generator that provides power back to the batteries and in the  process slows down the vehicle. Friction brakes are used when the vehicle must be stopped quickly or if the batteries are at full charge. Fig. 1. Typical EV configuration The components that make up a typical HEV include a  battery pack, motor controller, motor/generator, internal combustion engine, transmission and driveline components. The primary power electronics include a DC-AC motor controller which provides three-phase power to a permanent magnet motor. The Toyota Prius HEV configuration is given in Fig. 2 (a). The Prius design uses two permanent magnet motors/generator, one of 10kW and the other of 50kW. The  battery is connected to a booster and inverter before feeding to the motor/generators. The power electronics are bidirectional and used for both charging the battery and powering the motors. The motor/generators and gasoline engine feed into a  planetary gear set. The system operates in a continuously variable transmission (CVT) mode where the gear ratio is determined by the power transfer between the battery, motor/generators and gasoline engine [3], [4]. The batteries can also be charged using regenerative braking of the large motor/generators. There is no provision to charge the batteries externally. For plug-in hybrid electric vehicles, batteries are charged when they are not being driven. This is normally accomplished through a utility connected AC-DC converter to obtain DC  power from the grid. The batteries can also be charged directly from a solar resource using a DC-DC converter or from a wind source using an AC-DC converter. Energy flow is unidirectional as power is taken from the utility to charge the  battery pack. A Toyota Prius configuration with PHEV conversion is shown in Fig. 2 (b). The battery voltage for most converted PHEVs are maintained at the same level as the srcinal design (typically 200-500 VDC) and battery modules are added in parallel to increase the energy capacity of the battery pack, thus allowing the electric motor to run more often than the srcinal HEV design. Some of the PHEV conversion companies include: CalCars, Energy CS, Hymotion, Electrovaya, and Hybrids Plus, and most of them use lithium batteries. Fig. 2. Toyota Prius configurations (a) HEV, (b) converted PHEV (a) IC Engine Fuel Tank BatteryBatteryWheelWheel10 kWPlanetaryGear Set M/GM/G 50 kWReduction Gear BoosterInverter      A typical FCV configuration is shown in Fig. 3 (a). Prototype fuel cell vehicles that are currently under development mostly utilize an on-board tank to store hydrogen  pressurized at 5,000 to 10,000 psi. Hydrogen and conditioned air are fed to a PEM stack. As the fuel flow increases, the DC output current increases. The DC output from the stack is fed into a DC-DC converter to a DC power bus. Connected to the DC bus is a battery pack and motor controller. The configuration is very similar to the electric vehicle configuration in which a motor/generator provides the mechanical power for traction. The on-board batteries allow the energy to be stored during regenerative braking and provide  peak power to the motor controller during vehicle acceleration. In field tests, more than 800,000 miles have been placed on a fleet of various fuel cell vehicles. The demonstration showed that the vehicles were performing between 52%-58% efficiency with distances ranged between 100 to 190 miles [4]. In the PFCV configuration, the batteries are primarily charged using an on-board utility-connected battery charger. This configuration typically uses a larger battery pack than a FCV to give the vehicle a longer driving range under electric  power. The configuration shown in Fig. 3(b) uses a PEM fuel cell stack that produces DC power which is then boosted to a higher voltage using a DC-DC converter. Batteries are connected to a DC bus and are used to allow the fuel cell stack to operate at more constant operating conditions. The motor controller draws it power from the DC bus and provides three- phase power to the motor/generators. Regenerative braking is also used to store power to the battery pack. An on-board  battery charger is connected to the utility to allow the batteries to be recharged when the vehicle is parked. Fig. 3. Fuel cell vehicle configuration (a) FCV, (b) PFCV In a study conducted by Parks et al. [5], the researchers concluded that the actual electricity demands associated with PHEV charging are quite modest compared to normal electricity demands. Replacing 30% of the vehicles currently in the Xcel Energy service territory with PHEVs (with a 20-mile all-electric range and deriving 39% of their miles from electricity) would increase total load by less than 3%. A very large penetration of PHEVs would place increased pressure on  peaking units if charging is completely uncontrolled. There is a natural coincidence between normal system peaks and when significant charging would occur during both the summer and winter seasons. At today’s electrical rates, the incremental cost of charging a PHEV fleet overnight will range from $90 to $140 per vehicle per year. This translates to an equivalent  production cost of gasoline of about 60 cents to 90 cents per gallon [5]. Further study is needed to determine the effects of  battery life, state of charge control, driving range, life, and any associated replacement costs. The details of current PHEV research can be found in a DOE milestone report [6]. III.   V EHICLE - TO -G RID C APABILITY  A plug-in vehicle can only charge its batteries using AC  power typically provided by utility grid. EV’s, PHEV’s and PFCV’s can also be designed so that power can be sent back to the grid. A vehicle with this type of technology is defined as  being V2G capable. All of the topologies, as discussed in the  previous section, utilize a battery pack to store DC power that must be converted to AC power to connect to the utility for V2G applications. The individual battery cells are generally connected in different series and/or parallel configurations to achieve the required voltage and current outputs. The power conditioning systems include inverters and motor controllers. The unique aspect of power electronics for V2G vehicles is that they must be bi-directional, that is capable of both taking  power (during charging) and providing power (during discharge) from/to the grid. The V2G vehicles for distributed energy applications can  provide voltage and frequency regulation, spinning reserves, and electrical demand side management. If used in large numbers, V2G vehicles have the potential to absorb excess electricity produced by renewable sources, such as wind  power, when the grid is operated at low load conditions. Studies show that V2G vehicles could be a significant enabling factor for increased penetration of wind energy [7]. Controls can be developed that would allow an operator to dispatch these renewable resources through the use of the vehicle’s  battery when they are needed by the utility. During periods of low demand, excess generation can be used to charge the on- board batteries which can then be used by the driver to run an electric motor and offset fuel consumption. A set of fleet vehicles that are parked at a company’s facility could  potentially be used to provide electricity during periods of high demand to offset the facility’s electrical demand charges. Motor Controller WheelWheelBatteryBatteryBattery M/G Fuel Cell StackPressurizedHydrogen Fuel TankDC-DCConverter  (a)(b) Motor Controller WheelWheel M/G Fuel Cell StackPressurizedHydrogen Fuel TankDC-DCConverter BatteryBatteryBatteryBatteryBatteryBatteryBattery Charger Utility       Fig. 4. Schematic of an ISO controlled V2G topology  A proposed wireless configuration for independent system operator (ISO) to control charging and discharge of a V2G  battery is shown in Fig. 4. Each V2G capable vehicle must have three required elements: a power connection to the grid for electrical energy flow, control or logical connection necessary for communication with grid operators, and on-board  precision metering [8]. The configuration depicted in Fig. 4 shows that the power electronics being controlled using a wireless cell connection to communicate with the V2G capable vehicles. While V2G capable cars could provide peak power demand-response resource, their economic values do not generally justify the expense.   These services are needed for  just a few hours each year, thus the potential revenue from  providing these services is limited. It is suggested by the researchers that the most promising markets for V2G are for those services that the electric industry refers to as ancillary services such as voltage and frequency regulation [9], [10]. A V2G vehicle can be designed to provide frequency regulation services by absorbing or providing power back to the utility to match generation with the load. A grid operator could provide commands to the V2G capable vehicle to allow the vehicle to absorb or produce power in order to keep the utilities Area Control Error (ACE) low. ACE is a measure that indicates the deviation of the generation in a power system area from the load. The ACE is generally controlled to by controlling individual generators within that control area so that it complies with the National Electric Reliability Council (NERC) and the areas electricity governing council prescribed acceptable limits [9], [10]. At today’s U.S. gas prices, a vehicle that can provide regulation services is expected to provide the highest return to the V2G owner [8], [11]. An annual revenue estimate for a 10kW V2G capable vehicle could yearly provide between $920 to $1,117 for spinning reserves and $2,497 to $3,285 for regulation for the PJM and ERCOT territories [11]. State-of-charge regulation, battery life, power capacity, energy capacity, and available power connection will be critical factors in the design of these vehicles. The number of battery discharges, charges and state-of-charge control directly effects  battery life. It is expected that with today’s battery technology, designing a vehicle that can provide spinning reserve capability will be easier than a vehicle that provides regulation. The number of charges and discharges, energy capacity, and range of state-of-charge control will be less for spinning reserve over regulation that will require deeper discharges and more frequent operations. V2G vehicles typically use a high power, high energy  battery pack and a bi-directional inverter and controller. An electronic control module controls the power electronics to operate in charge, discharge, or standby modes. Typical V2G vehicles utilize either a Nickel Metal Hydride or Lithium Ion  battery pack. Fig. 5. EV power electronics configuration with V2G   
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