Examination of a PHEV Bidiretional Charger System for VG Reative Power Compensation Mithat C. Kisaikoglu 1, Burak Ozpinei, and Leon M. Tolbert 1, 1 Dept. of Eletrial Engineering and Computer Siene The University of Tennessee Knoxville, TN 37996-100 Power and Energy Systems Group Oak Ridge National Laboratory Oak Ridge, TN 37831 Abstrat Plug-in hybrid eletri vehiles (PHEVs) potentially have the apability to fulfill the energy storage needs of the eletri grid by supplying anillary servies suh as reative power ompensation, voltage regulation, and peak shaving. However, in order to allow bidiretional power transfer, the PHEV battery harger should be designed to manage suh apability. While many different battery hargers have been available sine the ineption of the first eletri vehiles (EVs), on-board, ondutive hargers with bidiretional power transfer apability have reently drawn attention due to their inherent advantages in harging aessibility, ease of use, and effiieny. In this paper, a reative power ompensation ase study using just the inverter d-link apaitor is evaluated when a PHEV battery is under harging operation. Finally, the impat of providing these servies on the batteries is also explained. Keywords - PHEV; harger; VG; reative power; battery I. INTRODUCTION Today, hybrid eletri vehiles (HEVs) offer ustomers a way to inrease gasoline mileage by having batteries and eletri drive systems assist the internal ombustion engine. However, HEVs lak the availability to go for more than just short distanes at low speeds with only eletri power beause the battery is not apable of storing enough energy to power the vehile for a daily ommute. PHEVs provide eletriityonly drive option up to a speified distane, and they an help redue arbon emissions as well as other pollutants [1]. While PHEVs will provide eonomi and environmental benefits, they an also offer a potential soure of energy storage whih is valuable to the eletri power grid. The possibility of using battery-powered vehiles to support the eletri grid has been studied for more than a deade []. Reent papers inluding [3-5] have disussed several topologies and ontrol methods that an perform bidiretional power transfer using a PHEV as a distributed energy resoure. However, there has not been muh tehnial analysis about reative power ompensation using bidiretional PHEV hargers as well as the effets of suh a power support on the PHEV s battery and harger system omponents. The purpose of this study is to examine a PHEV harger system to utilize it for reative power support to the grid. The authors investigate different senarios to deliver the stored energy from vehile to grid (VG) and explain the effets of this usage on the vehile tration battery and the harger d link apaitor. In the following setion, authors disuss battery harger types briefly. Later, an analysis introdues the dynamis that govern bidiretional power flow in the system and shows how to ontrol the on-board vehile harger to provide reative power to the eletri grid. The battery of a PHEV an be used for anillary servies suh as peak power shaving and reative power support. However, in the simulation setion of this paper, it is observed that ompared to peak power shaving, reative power regulation auses no degradation at all on battery life, sine the d link apaitor is enough for supplying full reative power for level 1 harging and therefore the PHEV battery is not engaged in reative power transfer. II. ELECTRIC VEHICLE CHARGERS Battery hargers play an important role by maintaining the ondition and health of the battery while utilizing it for the best performane. A battery harger is a devie that is omposed of one or more power eletronis iruits used to onvert a eletrial energy into d with an appropriate voltage level so as to harge a battery. It has the potential to inrease harging availability of the PHEV sine it an operate as a universal onverter aepting different voltage and power levels. In addition, a battery harger should prevent overharging from happening. Espeially for lithium-ion batteries, the harger warrants a sophistiated harging ontrol algorithm to avoid overharging [6]. Also, balaning the battery ells requires speial iruitry. Consequently, the harger should protet the battery from over-urrent, overvoltage, under-voltage, and over-temperature [7]. A PHEV battery an be harged either by a separate harging iruit or via using the tration drive that serves to power the eletri motor. The first EVs used the former method. Sine this option requires an extra harging iruit, it inreases the total ost of the vehile (if the harger is on the vehile) or it requires a dediated harging station (if the harger is off the vehile). If the harger is on-board, it an be optimized to aept different harging levels as well as to math different vehile battery requirements. With an onboard harger, a vehile an be harged at any outlet that is available at home garages or workplaes with ground protetion [8]. Availability of suh harging plaes will inrease the aeptane of PHEV tehnology.
On the other hand, off-board hargers make use of fast harging and an harge a vehile in a onsiderably shorter amount of time. It is possible to harge a battery in 10 minutes to inrease its state of harge (SOC) by 50% with an off-board harger rated at 40 kw [9]. Also, aording to Nissan, its Leaf eletri ar, whih will be on the road in 010 and mass produed in 01, an be harged up to 80% SOC of its 4 kwh Li-ion battery pak in around 30 min at a quik harge station [10]. Sine on-board hargers power rating is limited due to spae and weight restritions on the vehile, it takes muh more time to fully harge a vehile battery ompared to offboard hargers. However, an integrated on-board harger utilizing the tration inverter an harge the battery at high power levels that redue the harging time [11]. Using these types of hargers whih are lassified as Level + hargers, it takes about one hour to put 80% SOC to a battery rated at 30 kwh [1]. Not only do integrated hargers onnet the vehile s battery to most available standard 10V and 40V outlets, with speial onfiguration it also ouples a PHEV to an off-board harger if faster harging is needed [11]. However, sine integrated hargers use motor indutane as inverter input indutane by onneting the neutral point of the motor to the grid, the indutane of the motor may not be the optimal value for the inverter operation. Also, this design auses the majority of the losses to be the opper losses of the motor windings [5]. An additional point a harger an offer is the apability of transferring power not only from grid to vehile but also from vehile to grid so that eah ar would operate as a distributed power soure. In summary, on-board, ondutive hargers with bidiretional power transferring apability have reently drawn attention due to their inherent advantages in ost, harging aessibility, ease of use and effiieny. The following setion will present the theoretial analysis of an onboard, ondutive harger to utilize it in bidiretional power transfer. III. THEORETICAL SYSTEM ANALYSIS OF BIDIRECTIONAL POWER TRANSFER BETWEEN A PHEV AND THE GRID A. Grid-Inverter The PHEV harger that is analyzed in this study is omposed of a full-bridge inverter/retifier and a d-d onverter. The analysis will start by investigating the interation between the grid and the inverter. In order to understand all the dynamis, the basi ideal ase is introdued with several assumptions that will make the omputation muh easier. During the analysis, the positive urrent diretion will be assumed to be from grid to the inverter as shown in Fig. 1. Therefore, positive power sign (P = ative power and Q = reative power) orresponds to the power flow from grid to the inverter. The system parameters are given as follows: v (t) instantaneous harger voltage [V], v s (t) instantaneous grid voltage [V], i (t) instantaneous harger urrent [A], oupling indutor [H], L δ phase differene between v (t) and v s (t), θ phase differene between i (t) and v s (t). Root mean square (rms) values of the instantaneous variables are given in apital ases throughout this study. The grid voltage is assumed to be purely sinusoidal, and high frequeny omponents of inverter output voltage, v (t), is negleted for analysis purposes as shown by the following equations: vs ( t) = Vs sin( wt), (1) v ( t) = V sin( wt δ ). In order to ensure power transfer from harger to the utility, a oupling indutor is used and the two voltage soures are deoupled. From Fig. 1 and applying neessary mathematial transformations, the line urrent an be written as, i ( t) = I sin( wt θ). (3) Sine the default diretion for ative and reative power transfer is from grid to harger, i (t) and v (t) are lagging the grid voltage. Also, note that the reatane is equal to X = π f L, (4) where the system frequeny, f, is 60 Hz. Table I and the P-Q plane shown in Fig. show all the different operation modes in whih the system an be working. In order to onserve the amount of energy that is drawn from the battery and to keep the battery undisturbed as muh as possible, operation in quadrants I and IV is preferred over working in quadrants II and III. In other words, PHEV battery will not provide ative power to the grid in this study. Although the utility may prefer to be able to use the PHEV as a peak shaving power soure, it may not be aepted by the vehile manufaturers and the ustomers due to safety onerns, derease in battery lifetime, and redued available battery energy. The topology that is studied here an run in all four quadrants, but for the analysis the system dynamis will be when the harger is operating in quadrants I and IV. From Fig. 1 it an also be written that Fig. 1. Representation of grid and harger. ()
. TABLE I. CHARGER OPERATION MODES # P Q Operation Mode of the Charger 1 Zero Positive Indutive Zero Negative Capaitive 3 Positive Zero Charging a) Charging b) Disharging 4 Negative Zero Disharging 5 Positive Positive Charging and indutive 6 Positive Negative Charging and apaitive 7 Negative Positive Disharging and indutive ) Indutive operation d) Capaitive operation. 8 Negative Negative Disharging and apaitive ) Charging and indutive operation d) Charging and apaitive operation Fig. 3. Vetor diagram for different operation modes. V = V + jx I. (5) s Fig.. P-Q plane showing harger operation modes. Using (5), the system variables are shown in the phasor diagrams in Fig. 3 to illustrate the differenes between the operation modes. Only the operation modes under disussion are explained in the phasor analysis. Some onlusions drawn from the skethes will help to understand the ontrol algorithm. First, as illustrated in Fig. 3a and Fig. 3b, ative power is provided by the grid as long as v (t) lags v s (t), and it is sent to grid when v s (t) lags v (t). Sine v (t) and v s (t) are sinusoidal, i (t) is also sinusoidal as shown before. Its phase angle, θ, determines the diretion of the reative power flow. If θ is positive, reative power is sent to the grid, and if θ is negative, reative power is provided by the grid to the harger. Based on the available harging infrastruture, the system will either be harged by level 1 or level harging. Level 3 harging is not examined here. This analysis will be inluded in a future study. Therefore, the inverter urrent, i, is limited by the harging equipment to 1 A or 3 A. For all operations the ontrol algorithm should maintain that the urrent stays below either of these levels. In Table II, different harging methods in North Ameria are given for further referene. There are two ontrol methods to influene the magnitude and the diretion of P and Q. The first option is to ontrol the harger voltage, v (t), and its phase angle, δ. The seond option is to ontrol the harger urrent, i (t) and its phase angle, θ. The fundamental equations derived using these variables that govern average ative and reative power flow from grid to inverter is listed in Table III. In summary, the variables that govern the interation between the grid and the harger have been introdued in this setion. The following setion will desribe the inverter operation. B. Inverter For this study, a full-bridge PWM inverter/retifier is used as the first stage of the PHEV harger as shown in Fig. 4. Sine the PHEV harger is operating like a urrent soure, it is important that it omplies with IEEE 1547 to present the minimum urrent harmonis possible. Therefore, a hysteresisband urrent ontrol PWM is used to effetively regulate the urrent waveform. As a result, urrent and its phase angle are seleted to be the variables of the ontrol algorithm. The reason for using this topology is to have a system that is able to operate in all four quadrants of the P-Q plane. Although a half bridge inverter ould also satisfy this TABLE II. ELECTRICAL RATINGS OF DIFFERENT CHARGING METHODS IN NORTH AMERICA [13] Charging method Nominal supply voltage Maximum urrent Branh iruit breaker rating Continuous input power AC Level 1 10 V, 1-phase 1 A 15 A 1.44 kw AC Level AC Level 3 08 to 40 V, 1- phase 08 to 600 V, 3- phase 3 A 40 A 6.66 to 7.68 kw 400 A As required > 7.68 kw DC harging 600 V maximum 400 A As required <40 kw
Vd* TABLE III. FUNDAMENTAL EQUATIONS FOR ACTIVE AND REACTIVE POWER 500 PI P theta theta Control Variable P Q V s V V v (t) and δ sin(δ ) s V X 1 os( δ ) X Vs i (t) and θ V s I os(θ ) V s I sin(θ ) vd 1 Reative power Command PI P Q i* I & theta omputation P i Q vs P&Q alulation i* i PWM Gate drive with Hysteresis ontrol i vs 1 Inverter PWM Fig. 5. Control system struture of the inverter. V d nominal d link voltage [V], Δ V d rms d ripple voltage [V], Δ I ap rms d apaitor ripple urrent [A], C d d link apaitor [F]. operation, it requires two large apaitors to effetively regulate their juntion voltage. Also, using full bridge ative retifier, the d link voltage is doubled reduing the output urrent rating for the same power level. Aording to the hysteresis-band urrent ontrol PWM, the referene urrent generated by the ontroller is ompared to the atual line urrent, and the swith pairs hange their position aordingly. The inverter ontrol system shown in Fig. 5 operates with two feedbak loops, one is for reative power regulation and the other is for d voltage regulation whih indiretly failitates ative power transfer to the d-d onverter. Based on these two feedbaks, the ontroller alulates the exat I and θ values to generate i (t) * as a referene waveform to be ompared with atual line urrent. The maximum swithing frequeny that is shown using hysteresis-band urrent ontrol PWM is alulated as [14] Vd fmax =. (6) L H Fig. 4. Full bridge inverter harger. where H is the differene between upper and lower hysteresis bands and equal to 1 A. L is hosen to be 5 mh and V d to be 500 V. Therefore, the maximum swithing frequeny is alulated as 500 fmax = = 50kHz. 3 (7) 5 10 1 C. D Bus Components In this setion, the relationship between reative power transfer and d bus variables will be given. The d parameters that will be analyzed are as follows: The d link apaitor s major purpose is to regulate d voltage during battery harging. However, it an also be used for reative power regulation. First analysis shows how the reative power transfer affets V d. As it is given in [15], in a full bridge inverter, the d link voltage and urrent exhibit a ripple at double the frequeny of the line voltage with the same phase of the line urrent. For the start of the analysis, the PWM ripples are negleted and only f ripple is onsidered. Therefore, the instantaneous apaitor voltage and urrent an be defined as v i (t) = V + ΔV sin( wt θ), d d d + ( t) = CwΔV os(wt + θ) ap d. D apaitor minimum and maximum voltages our at the following time instants: π wt + θ = wt min π wt + θ = wt max π θ = and, 4 π θ =. 4 (8) (9) (10) (11) In [16], the energy onservation priniple is used assuming there is no energy loss. Similarly, it an be written that wt wt max v(t)i(t)dwt = min wt wt max min v d (t)i ap (t) dwt, (1) π VI os (δ θ) os(δ + θ) = wcvd ΔVd. (13) In addition to this, the net reative power that is sent to the harger is written as
Fig. 6. Peak-to peak voltage ripple for different reative power levels for a 500µF d apaitor. Q = V I sin( θ δ ) + wl I. (14) Using (13) and (14), the relation between reative power and the peak-to-peak d voltage ripple an be found as shown in Fig. 6. The reative power produed is not diretly related to the V d. However, the higher the d voltage, the lesser the apaitor ripple urrent. Therefore, the system will be able to supply/sink more reative power with higher V d for the same I ap levels. Similarly, the d apaitor value does not affet the reative power transfer. Rather, V d redues with inreasing apaitor rating beause the right hand side of (13) should stay onstant. Moreover, in the simulation analysis setion, the relation observed between reative power and d apaitor rms ripple urrent will be given. D. D-d Converter and Battery When harging from the grid, a bidiretional d-d onverter shown in Fig. 7 steps down the high d-link voltage and harges the battery using onstant urrent-onstant voltage (CC-CV) harging algorithm. A Li-ion battery model is implemented in Simulink using the model and parameters given in [17-19] to aount for the harging profile of a PHEV. The equivalent iruit of a Li-ion battery ell is given in Fig. 8. The nonlinear relationship between open iruit voltage, V o, and SOC is aptured using a ontrolled voltage soure. Two RC time onstants are used to mimi response to transient power. In Fig. 8, the series resistor, R Series, aounts for the instantaneous voltage drop during a step hange in the battery urrent. Also, R Transient_S, C Transient_S, R Transient_L, and C Transient_L stand for short and long time onstants that mimi the step response of the battery voltage [17]. Fig. 8. Eletri equivalent iruit of a Li-ion battery ell [17]. In PHEV appliations, the required amount of terminal voltage and apaity of the energy storage system is obtained arranging multiple battery ells in series and parallel. The ells that are in series determine the terminal voltage of the battery system, and the number of parallel ells deides the urrent arrying apability of the system. The total apaity of the battery is given as C t = Ci ns np, (15) where C t is the total apaity (Ah); C i is the ell apaity (Ah); n s is the number of ells in series; and n p is the number of ells in parallel. As given in [17], the apaity of individual ells, C, i modeled is 0.85 Ah. The Li-ion battery ell model is saled up to 5 kwh to aount for the battery size as it is used in Toyota Prius Hymotion PHEV [0]. If eah ell is assumed to be operating at 3.8 V, then 53 ells in series and 9 ells in parallel onstitutes this apaity as shown below: E = Ci ns n p Vt = 0. 85 53 9 38. 5 kwh. (16) where V t is the nominal terminal voltage of eah ell (V). The implemented battery model output signal is generated in a Simulink model and then transferred to a PLECS software blok whih is embedded in Simulink for power proessing stage. E. Reative Power Support During PHEV Charging In this setion, the potential for reative power regulation during battery harging is explored using the experimental measurements of the harging power drawn by the 008 Toyota Prius PHEV [0]. For this purpose, the battery pak of the Toyota Prius has been depleted and reharged several times, and the resulting waveforms are given in Fig. 9. In Fig. 9, P1, P, and P3 stand for three different level 1 harging profiles observed when harging the PHEV. When it is first plugged in, the battery voltage level is at its minimum, and there is an exess urrent margin that an be utilized for reative power generation for about 45 minutes. During this time, the battery is harged with onstant urrent, and its terminal voltage inreases gradually; the line urrent inreases gradually too. The amount of reative power that the system an supply until the harger reahes its maximum power is alulated by the following formula: Q = V I P. s (17) Fig. 7. D-d onverter and PHEV battery.
Fig. 9. Experimental data for the harging power of Toyota Prius PHEV onverted by Hymotion. Fig. 11. Reative power demanded by the ontroller and supplied by the harger. Fig. 10. Reative power availability during onstant urrent harging of the Toyota Prius PHEV. Fig. 1. PHEV battery terminal voltage. Using (17), the available reative power is given in Fig. 10 for harge profile P3. For this data, the maximum power drawn from the grid is 1.7 kw with almost 1.0 power fator. For the reative power alulation, the apparent power is limited to be 1.7 kva to limit the peak urrent. As illustrated in Fig. 10, even during onstant harging, there is an opportunity to supply 0.45 kvar power to grid whih is still 35% of the full reative power amount that an be supplied without harging the battery. IV. SIMULATION ANALYSIS The purpose of the simulation study is to verify that the proposed system is able to work in the aforementioned operation modes. Also, the effet of different operation modes on the d variables will be given. The system will be ommanded to work in two quadrants although the topology is able to work in all four quadrants. Moreover, the system is ompatible to work with both level 1 and level harging equipment. However, sine everything exept the ratings will stay the same for the analysis, only level 1 will be evaluated. All the results have been ahieved using a 500µF d apaitor and 500 V d voltage level. Beause of the long simulation time, the system is only simulated for a few seonds. A safety limit is imposed on the line urrent not to exeed the system limitations at all operation modes. The simulation realizes the operation modes #1, #, #3, #5 and #6 as given in Table I respetfully in 11 s. In Fig. 11, the Fig. 13. PHEV battery terminal urrent. reative power ommand to the harger is given along with the harger s response as Q* and Q respetively. Also, in the same graph, the ative power sent by the grid to harge the PHEV battery is inluded. Sine it takes onsiderable amount of time to over all the harging proess of the battery, only the initial part of the onstant urrent harging is shown. Finally, towards the end of the simulation, the bidiretional harger provides reative power in response to the ontroller ommand when the PHEV is plugged in for harging operation. Note that, minus sign stands for apaitive operation and positive sign means indutive operation. Following Fig. 11, in order to show that the PHEV battery is not used to supply reative power regulation during the simulation, battery terminal voltage and urrent are given in Figs. 1 and 13, respetively. During the simulation, battery voltage and urrent have not shown any deviane from their
usual profile satisfying safe operation regulations. In other words, the battery is always operated suh that the urrent and voltage ripple presented are the same as it is during a normal harging operation. After onfirming that the designed system is able to operate at the planned operation modes without putting adverse effets on the battery, the effets of the different operation modes on the d bus variables should be presented to investigate if the d dynamis of the system pose a danger on the d link apaitor. First, the d link apaitor voltage has shown a profile with a frequeny that is double the line frequeny as expeted. Fig. 14 shows the regulation of d voltage during the simulation. When the PHEV is plugged in to be harged at 4.5 s, the d link voltage suddenly drops and then regulates itself. Fig. 15 shows the hanges in the d apaitor peak to peak ripple voltage when the system operates at different modes. For reative power only operation, the d link apaitor is exposed to ~13 V peak-to-peak voltage ripple when absorbing 1.7 kvar from grid (mode #1) and ~18 V when supplying 1.7 kvar to grid (mode #). Beause of the oupling indutor, it requires less voltage ripple to absorb reative power. These results onfirm well with the initial analysis equations, (8) - (13) and Fig. 6. If the PHEV is used to sink reative power from the grid during harging (mode #5), it only requires ~1 V more peak-to-peak ripple voltage and around ~V more for apaitive operation (mode #6). Fig. 16 also illustrates how the apaitor ripple urrent hanges with different operation modes. The net hange in the rms ripple urrent is small when the harger swithed between different operation modes keeping the d link apaitor in its safe operating limits. The results onfirm that, with level 1 harging, supplying anillary servies suh as reative power ompensation an be ahieved with an on-board, ondutive, and bidiretional harger without using the PHEV battery and keeping the d link apaitor in its operating limits. V. CONCLUSION The basis of this paper is to introdue the tehnial understanding of the VG reative power ompensation. Therefore, VG operation is shown by simulating different modes of operation out of whih reative power supply/sink with/without PHEV battery harging being the most important ones. The simulation study showed that with level 1 harging, it is possible to fulfill reative power ompensation without any power demand from the battery. Moreover, the d link apaitor of the bidiretional harger is used to supply reative power. The results show that reative power ompensation an be aomplished with/without battery harging and it does not put stress on the d link apaitor. The peak to peak ripple of the d voltage and d apaitor rms ripple urrent are observed for safety of the d link apaitor. Future study will be on engaging the PHEV battery for the reative power support at higher power levels and showing if there are adverse effets of this operation on the battery. Fig. 14. D link voltage when harging PHEV battery. Fig. 15. Peak-to-peak ripple voltage seen on the d link apaitor for different operation modes (#1,,3,5, and 6 in Table I). Fig. 16. Rms d link apaitor urrent for different operation modes (#1,,3,5, and 6 in Table I). REFERENCES [1] Eletriity advisory ommittee, Bottling eletriity: storage as a strategi tool for managing variability and apaity onerns in the modern grid, Deember 008. [] W. Kempton, and A. E. Letendre, Eletri vehiles as a new soure for eletri utilities, Transport. Res. Part D Transport. Envir., vol., no 3, pp. 157-175, September 1997. [3] X. Zhou, et. al., Design and ontrol of grid-onneted onverter in bidiretional battery harger for plug-in hybrid eletri vehile appliation, Vehile Power and Propulsion Conferene (VPPC 09), Dearborn, MI, USA, 7-10 September 009. [4] I. Cvetkovi, et. al. Future home uninterruptable renewable energy system with vehile-to-grid tehnology, Energy Conversion Congress & Exposition (ECCE'09), San Jose, CA, USA, September 0-4, 009.
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