Assessment of a Battery Charger for Electric Vehicles with Reactive Power Control

Transcription

1 V. Monteiro, J. G. Pinto, Bruno Exposto, Henrique Gonçalves, João C. Ferreira, Carlos Couto, João L. Afonso, Assessment of a Battery Charger for Electric Vehicles with Reactive Power Control, 38th Annual Conference of IEEE Industrial Electronics Society - IECON2012, pp , Montréal, Canada, Oct. 2012, ISBN: Assessment of a Battery Charger for Electric Vehicles with Reactive Power Control Vítor Monteiro, J. G. Pinto, Bruno Exposto, Henrique Gonçalves, João C. Ferreira, Carlos Couto, João L. Afonso Centro Algoritmi University of Minho Guimarães, Portugal s: {vitor.monteiro, gabriel.pinto, bruno.exposto, henrique.goncalves, joao.ferreira, carlos.couto, joao.l.afonso}@algoritmi.uminho.pt Abstract Batteries of Electric Vehicles (EVs) and Plug-in Hybrid Electric Vehicles (PHEVs) have a large potential not only to provide energy for locomotion of se vehicles, but also to interact, in dynamic way, with power grid. Thereby, through energy stored in batteries, se vehicles can be used to regulate active and reactive power, as local Energy Storage Systems. This way, EVs can contribute to help power grid to regulate active and reactive power flow in order to stabilize production and consumption of energy. For this propose should be defined usage profiles, controlled by a collaborative broker, taking into account requirements of power grid and conveniences of vehicle user. Besides, interface between power grid and EVs, instead of using typical power converters that only work on unidirectional mode, need to use bidirectional power converters to charge batteries (G2V - Grid-to-Vehicle mode) and to deliver part of stored energy in batteries back to power grid (V2G - Vehicle-to-Grid mode). With bidirectional power converter topology presented in this paper, consumed current is sinusoidal and it is possible to regulate power factor to control reactive power, aiming to contribute to mitigate power quality problems in power grid. To assess behavior of presented bidirectional power converter under different scenarios, are presented some computer simulations and experimental results obtained with a prototype that was developed to be integrated in an Electric Vehicle. I. INTRODUCTION Nowadays, recent and massive investments in electric mobility, mainly in Electric Vehicles (EVs) and Plug-in Hybrid Electric Vehicles (PHEVs), represents a new paradigm in transports sector, alternatively to vehicles with Internal Combustion Engines (ICE). For simplicity, from now on in this paper, it will be used terminology Electric Vehicle (EV) both for EVs and PHEVs. The proliferation of EVs will contribute do reduce strong dependence from oil and or fossil fuels, allowing an effective reduction in emissions of greenhouse gases. This way impact of transports sector in climate change will be reduced. Neverless, for electrical power grids EVs will be extra loads, which will consume energy to charge ir batteries, and in many cases at same time, and connected to same distribution transformer. With electric mobility increase, taking into account stored energy in batteries of EVs, arises a new concept in electrical power grid market denominated Vehicle-to-Grid (V2G) [1][2][3]. In this sense, besides battery charging mode, denominated as Grid-to-Vehicle (G2V), owners of se vehicles and power grid can interact, through a collaborative broker, to negotiate energy stored in batteries, respecting a schedule to charge batteries and total time required [4]. The integration of EVs in power grid distribution network, respecting technical restrictions of system, requires data analysis of electrical consumption to allow smart utilization of batteries, where charging (G2V) or discharging (V2G) processes have to be coordinated with users of vehicles [5]. Focusing integration of EVs in power grid, in order to implement V2G concept, it is necessary use of bidirectional power converters. These converters, should be designed to allow bidirectional flow of energy (with different control algorithms based on different stages of voltages and currents), and current in AC side should be sinusoidal with variable and controlled power factor. With se characteristics, it is preserved lifetime of batteries and power quality of electrical power grid. In this context, in [6][7] it is analyzed impact of battery chargers of EVs on power quality, and it is proposed a tool that can be easily applied to determine optimum charging time as function of existing available power in electrical power grid, schedule and ambient temperature. Relatively to power converters of battery chargers, which typically consist in AC-DC and DC-DC converters, re are several topologies presented in literature. In [8] are compared basic topologies for power factor correction (PFC), using DC-DC converters buck, boost, buck-boost, flyback, forward, cuk, sepic, and zeta, highlighting operation of DC-DC converters in discontinuous mode. In [9] is presented a comprehensive review of various techniques to PFC with ir control systems, advantages and disadvantages. In [10] is proposed a new low-stress buckboost converter for universal input PFC applications, and is made a comparison with boost converter. Circuit topologies for PWM boost rectifiers are presented in [11]. Due to unidirectional mode of operation, se converters are not suitable for V2G applications. For this purpose should be used topologies with bidirectional operation [12][13]. In [14] are presented basic requirements and specifications for bidirectional converters of EVs. Independently of power converter topology, battery chargers can be categorized as on-board or off-board. The onboard battery chargers are placed inside vehicles, and typically are designed to allow charging batteries in slow mode using standard plugs and sockets in connection to power grid, as in homes or public workspaces. On

2 or hand, off-board battery chargers are placed outside of vehicle, due to volume and weight, andd are designed to allow charge of batteries in a short time. In this paper is presented and analyzed t development of a bidirectional power converter to charge batteries through concept Grid-to-Vehicle grid a small amount of stored energy in (G2V), and to deliver back to power batteries, according to power grid requirements and with user agreement, through concept Vehicle-to-Grid (V2G). The bidirectional power converter was designed to preserve power quality of electrical power r grid through sinusoidal current consumption, with controlled power factor, allowing control of active a and reactive power [2]. It also was designed to allow charging batteries with different modes, based on different voltages and currents levels, aiming to preserve batteries lifetime. In order to validate different modess of operation, in a first stage bidirectional power converte topology and control algorithms were validated by computer simulations, and n were validated through experimental results with developed prototype. II. SMART GRID: ELECTRIC MOBILITY INTEGRATION With electric mobility integration, itt is predictable that in near future, in a real full scale Smartt Grid scenario, power grid should meet ncreasing demand of energyy in a reliable and efficient way, maintaining e required stability. At same time it should allow interface of renewable energy resources with ir intermittentt production. The integration of EVs in power grid is approached in several papers in literature [15][ 16][17]. In [18] is analyzed impact of EVs in power grid, but forr isolated systems. A. Energy Storage Systems With Energy Storage Systems (ESS), such as chemical batteries, flywheels or compressed air tanks, it is possible to maintain stability of power grid, mainly, to improve use of renewable energy sources (it is possible to store energy producedd when demand is low in order to bee used later when demand increases) [19]. In this scenario, with electric mobility integrationn and taking into account EVs equipped with bidirectional battery chargers, it will be possible communication of se vehicles with local utilities to ensure that batteries are charged when impact on grid is small. Also it will be possible to return part of stored energy in batteries back to power grid. This way, se vehicles can operate as local ESS controlled by a collaborative broker. Since in average most of vehicles, are a parked forr long time periods, ir batteries can be used as ESS, receiving energy during excess of production and delivering energy back to electrical grid during periods of great demand, balancing energy production and consumption. Inn this context, se vehicles are also usefull to stabilizee intermittent production from renewables, improving ir integration into power grid. In Fig. 1 is shown integration of EVs in a typical charging park operating as ESS, and integrationn of renewables in microgeneration (solar photovoltaic panels and micro wind turbines). B. Collaboration n Modes EVs can collaborate with e power grid in i several modes. The simplest process, denominated Grid-to-Vehicle (G2V), allows flow of energy from power grid to vehicles, which consists inn common procedure of charging batteries of EVs. The concept Vehicle-to-GridV d (V2G) allows bidirectional flow of energy between EVs and powerr grid. The Mid- Atlantic Grid Interactive Cars Consortium (MAGICC) defines V2G as a technology t that utilizes e stored energy in EV batteries too contribute with electricity back to grid when grid operators requestt it [20]. The operation of EVs as ESS requires use of bidirectional flow of energy. Thus, besides bidirectional controll of activee power, it is also possible to controll reactive power in order to improve operation of electrical power grid. The concept Vehicle-to-Home (V2H) allows bidirectional floww of energy between vehicle and house, minimizingg energyy transmissionn losses and cost. In this way, it is possiblee to manage and regulate profile of electricity demand in n house, controlling use of loads and stored energy available in i batteries of vehicle. The concept Vehicle-to-Building (V2B) is similar to V2G concept, and it allows use of energy stored in batteries of EVs as energy back-up system (ESS) and to compensate energy e consumption profile in commercial scale, e.g., when vehicle is parked at t work or in a shopping center. Fig. 1. Isolated systemm in Smart Grid context with power flow considering integration of EVs E (as Energy Storage Systems) and renewables.

3 C. Power Quality Taking into account electric mobility integration, main problems associated with power quality [21] are: voltage and current harmonics; low power factor; noise (electromagnetic interference); and unbalances. As a way to solve se problems can be used shunt active power filters (used to solve problems related with current), series active power filters (used to solve problems related with voltage), and unified power quality conditioners (used to solve problems related with current and voltages) [22][23]. However, with topology of bidirectional converter for battery charger presented in this paper, current in AC electrical power grid side will be sinusoidal (this way it will not contribute to degrade line current and voltage), and it will be possible to control power factor. In [24] is analyzed a particular case of impact of Plug-in Hybrid Electric Vehicles (PHEVs) in electric utility system, where, basically it is approached consumption profile and how se vehicles will affect utility operation by adding additional electricity demand. III. BATTERY CHARGERS ANALYSIS In EVs battery chargers are composed by power converters, which allow transform AC power grid voltage, into DC voltage to charge batteries. The bidirectional power converter for EVs presented in this paper is composed by two power converters, one AC-DC and or DC-DC. In Fig. 2 is shown schematic of proposed converter. Due to topology of bidirectional power converter, simultaneously with charging of batteries with proper control algorithms, it is possible to consume sinusoidal current with controlled power factor. Thereby, besides controlling active power it is also possible to control reactive power. In Fig. 3 is shown flow of active and reactive power involved in interaction of this bidirectional power converter with power grid. In Tab. I are summarized different profiles of power consumption, considering inductive and power factor with different values. These different profiles of power consumption are configurable through control system. Taking into account that power grid voltage is given by (1), reference to sinusoidal current with variable power factor is given by (2), where k 2 and k 3 are amplitude of references of current, respectively. The active power is adjusted taking into account voltage and current in batteries, and reactive power is adjusted according to information provided by collaborative broker concerning current limits of bidirectional power converter. i ref v grid k 1.sin t (1) t k.cos t k 2.sin 3 (2) In a first stage to analyze behavior of bidirectional battery charger was performed several simulations with simulation tool PSIM 9.1. In Fig. 4 are shown some of obtained results, considering voltage and current in power grid with different values of in order to adjust reactive power. As shown in this figure, simulations were performed in eight different modes of operation in order to analyze behavior of power converter with unitary power factor, with inductive power factor, and with power factor. In simulation model was considered model of bidirectional power converter, an electrical model of batteries, and a digital controller (programmed in C language). v i i i Bidirectional Power Converter AC-DC DC-DC Power Grid t L filter G1T G2T C filter G3T L filter v o i o t G1B G2B G3B C filter Bank of Batteries i i v i v cc i o v o Digital Control System G1T G1B G2T G2B G3T G3B External Interface Fig. 2. Schematic of presented bidirectional power converter.

4 Fig. 3. Interface between power grid and an Electric Vehicle with possible flow of active and reactive power. p Power Factor cos( ) = 0º = 180º = 90º = -90º 0º < < 90º 90º < < 180º 180º < < 270º 270º < < 360º TABLE I DIFFERENT PROFILES OF POWER CONSUMPTION Active Power (P) Reactive Power (Q) Power Factor inductive Fig. 4 (d) Figure - Fig. 4 (a) - Fig. 4 (b) Fig. 4 (c) Fig. 4 (e) Fig. 4 (f) inductive Fig. 4 (g) inductive Fig. 4 (h) Fig. 4. Summary of some simulation results considering voltage and current in electrical power grid with different values of to adjust reactive power. IV. DEVELOPMENT OFF THE BATTERY CHARGER PROTOTYPE In order to assess operation of presented power converter under different modes of operation (considering active and reactive power control) was developed prototype presented in Fig. 5. Thus, in this section is described in detail bidirectional power converter c and digital control system developed. This battery charger was developed aiming to be integrated in an Electric Vehicle with Absorbed Glass Mat M (AGM) batteries with nominal voltage 96 V and nominal capacity 333 Ah. In accordance with manufacturer, charging algorithm should be dividedd in threee stages: Bulk; Absorption; and Float. The first stage consists in charging batteries with constant current until voltage reach 115 V. In second stage voltage is maintained at 115 V until current accepted by battery falls below than 0.1 A. Finally,, in third stage, s voltage is maintained at 108 V to compensate self-discharge. A. Bidirectional Power P Converter As referred before, presented battery chargerr is composed by two bidirectional power converters, one AC-DC power converter was used u switching technique Periodic Sampling based on o Direct Current Control (DCC). This and or DC-DC. To e AC-DC bidirectional technique does not work at fixed frequency, but it establishes a maximum frequency limit, which is 20 khzz in this case. On or hand, to DC-DC converterr was used switching technique Pulse Width Modulation (PWM), which works with a fixed frequency of 20 khz. The bidirectional flow of energy is obtained o through adequate control of IGBTs. B. Digital Control System The control system, whichh implementss algorithms during operation as G2V V and V2G, is composed by several electronic circuits with h analogue and digital signals. The control algorithms of developedd digital control system were implemented in a DSP TMS320F The internal ADCs of DSP receive voltages and currents signals that are provided by signal conditioning circuit, and in conjunction with external interface, generate control signals to command circuit. The voltage and current signals of bidirectional power converter are obtained through hall-effect LEM sensors in signal conditioning circuit. This circuitt also provides protection against overvoltages and overcurrents in both sides of power converters. The control signals to drivers of IGBTs are provided p by command circuit, which receives control signals from DSP and receives errorr signals provided by signal conditioning circuit. The operation of battery b charger should occur in accordance with orders given by collaborative broker and user interface of Electric Vehicle, which define when it works as G2V and V2G, and define value of reactive r power to be produced (without exceeding current limits of battery charger).

5 (a) Fig. 5. Battery charger prototype developed to operatee as G2V and V2G, and to t control reactive power, to be implemented inn an Electric Vehicle. V.. EXPERIMENTAL RESULTS In this section are presented experimental results obtained with developed battery charger with bidirectional power converter considering t different modes of operation presented in Tab. II. The presented results were obtained with AGM batteries with nominal voltage 96 V and nominal capacity 33 Ah, and respecting three stages of charging (Bulk, Absorption, Float), and controlling energy that is delivered back to powerr grid (limiting Depth-of-Discharge). The experimental results, presented in Fig. 6, were obtained in steady state operation with a digital oscilloscope YOKOGAWA DL708E. As shown in this figure, f due to or non-linear loads existing in electrical installation, waveform of power grid voltage is distorted. Neverless, in all modes of operation e current consumed by bidirectional power converte is sinusoidal, contributing to preserve power quality. (b) (c) TABLE II MODES M OF OPERATION OF THE BIDIRECTIONAL POWER CONVERTERR TO OBTAIN THE EXPERIMENTAL RESULTS Power Factor cos( ) Mode of Operation Figure = 0º - G2V Fig. 6 (a)) = 180º - V2G Fig. 6 (b)) = 45º G2V and V2G Fig. 6 (c)) = -45º inductive G2V and V2G Fig. 6 (d)) (d) Fig. 6. Experimental results r of bidirectional power converter c controlling active and reactivee power: (a) Operation with = 0º º; (b) Operation with = 180º; (c) Operation with = 45º º; and (d) Operation with = -45º.

6 VI. CONCLUSIONS In this paper was presented a battery charger for Electric Vehicles (EVs) aiming ir integration in Smart Grids. This battery charger allows interaction with power grid to charge batteries (Grid-to-Vehicle - G2V mode) and to deliver part of stored energy in batteries back to power grid (Vehicle-to-Grid - V2G mode). Through operation of this bidirectional battery charger EVs can be used to regulate reactive power and to act as local Energy Storage Systems (ESS). This way, EVs will be able to help power grid to regulate active and reactive power flow, stabilizing production and consumption of energy. Taking into account bidirectional power converter topology that was presented in this paper, consumed current is sinusoidal and it is possible to regulate power factor (to control reactive power), aiming to contribute to mitigate power quality problems in power grid. The reactive power control is performed considering user interface of Electric Vehicle, requirements of power grid defined by a collaborative broker, and concerning current limits of bidirectional power converter. In a first stage, behavior of bidirectional power converter was evaluated under different scenarios through computer simulations. Then behavior of bidirectional power converter was evaluated with a prototype, which was developed aiming to reduce its volume and weight, in order to be integrated in an Electric Vehicle with Absorbed Glass Mat (AGM) batteries. In this paper were presented simulations and experimental results obtained. ACKNOWLEDGMENT This work is financed by FEDER Funds, through Operational Programme for Competitiveness Factors COMPETE, and by National Funds through FCT Foundation for Science and Technology of Portugal, under projects: FCOMP FEDER , PTDC/EEA- EEL/104569/2008 and MIT-PT/EDAM-SMS/0030/2008. REFERENCES [1] B. Kramer, S. Chakraborty, B. 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