A Tariffbased Inductive Power Controller for Charging Electric Vehicles (EV s)

Udaya Madawala, Michael Neath and Duleepa Thrimawithana The Department of Electrical and Computer Engineering, The University of Auckland, New Zealand

u.madawala@auckland.ac.nz, mneaO 14@auckalanduni.ac.nz and duleepajt@gmail.com

Abstract- As EVs are becoming increasing popular, the focus

is to implement both efficient and cost effective power interfaces

for charging EVs. This paper proposes a new power controller

for charging EVs using Inductive Power Transfer (IPT)

technology. The proposed control philosophy allows for charging

multiple EVs while sharing power capability of the charging

system according to tariff and without any direct communication.

The controller utilizes two distinct jitters of frequency to convey

information with regard to tariff for charging and power

capability of the supply. Tariff for charging is varied with the

demand for power, and EVs are allowed to be charged in

accordance with their purchasing power and urgency.

Moreover, the controller facilitates operation of the IPT charging

system at tuned frequency and minimizes VAR consumption.

The validity of the tariff based power control philosophy is

demonstrated through simulated results of a 3 kW bi-directional

IPT system that is capable of charging 3 EVs. Results

convincingly indicate that proposed controller is attractive for

charging bays where multiple EVs are charged, and also

applicable for both wired and wireless charging systems as well

as vehicle-to-grid (V2G) applications.

Index Terms- wireless power transfer, contactless power

transfer, electric vehicle (EV), resonant converters, inductive

power transfer (IPT).

I. INTRODUCTION

Inductive Power Transfer (IPT) facilitates power transfer across large air-gaps and at high efficiencies, and is suitable for applications in harsh environments, being unaffected by dirt and moisture [1]-[7]. Consequently, IPT technology is widely accepted for numerous wireless power applications, ranging from low power biomedical implants to high power Electric Vehicle (EV) charging systems and material handling systems.

IPT systems have been implemented in the past for applications that require unidirectional power flow [8]-[10]. Series, parallel or a combination of series and parallel resonant networks have been used to compensate for the coil reactances and improve the overall performance of the system. Recently, IPT systems have also been developed with the capability of bidirectional power flow for applications such as Vehicle-to­Grid (V2G) systems and energy recovery systems [15].

Numerous control strategies, including DC and AC controllers, have been proposed and developed to control both unidirectional and bidirectional IPT systems with either single or multiple loads (pickups) [11 ]-[17]. Most of these systems essentially require a communication protocol between the primary and pickup sides of the system to ensure a regulated power flow in both directions. Especially, communication

978-1-4799-5115-4114/$3l.00 ©2014 IEEE

between the primary and pick-ups is vital in applications such as charging bays where multiple EVs are to be charged (or discharged) without exceeding the power handling capability of the primary supply. IPT systems with a single primary supply are advantageous for charging multiple EVs due to lower component cost, and control schemes without any direct communications have been proposed and successfully implemented for such systems [17]-[18]. However, no attempt has ever been made to implement a controller that regulates the charging rate of each EV in accordance with the demand and power handling capability of the primary supply.

This paper presents a new controller that takes into account both purchasing power of each EV and total demand to regulate and prioritise the charging processes of multiple EV s. Tariff of charging and power capability of the supply, communicated through jitters of frequency, are used to regulate and maintain the charging process within the capabilities of the power supply. The paper describes the tariff based control philosophy in detail and presents simulated results of a 3 kW bidirectionallPT system with three pick-ups to demonstrate the validity of the tariff based power controller. The proposed concept is equally applicable to both wired and uni-directional IPT chargers.

II. BIDIRECTIONAL IPT SYSTEMS

A multi pick-up bidirectional IPT system usually consists of a single primary system, which is magnetically or inductively coupled to multiple secondary systems or pick-ups. The primary and pickup systems consist of near identical electronics, comprising a converter, an inductor-capacitor­inductor (LCL) resonant network and a controller, as shown in Fig. l. In V2G applications, the primary converter is connected to the grid through a bidirectional grid converter and the pick-up side converters are connected to EVs. As such converters on both sides of the IPT system are represented by active dc supplies. All converters are controlled independent of each other to regulate the amount and direction of power transferred to or from the pick-ups. The primary controller operates its converter, which is connected to an LCL resonant network, to produce a constant sinusoidal current, at a desired frequency, in the primary winding, represented by inductance

Lpt. The inductor Lpt is magnetically coupled to each pick-up

coil inductance Lst.n, with a mutual inductance of Mn, enabling

power to wirelessly flow from one side of the system to the other. The power flow from the primary to pick-ups is controlled via pick-up controllers. The pick-up side controllers operate respective pickup converters either in the inverting or

active rectifying mode depending on the direction of power flow. When a pick-up is operating in the rectitying mode, power is flowing from the primary to the pick-up. Conversely, when the pick-up is operating as an inverter power flows from the pick-up to the primary. The amount and direction of the power flow is controlled through the relative phase angles and magnitude of the voltages generated by the converters, as described below.

I 2nd Pick-up controller I I- isin,n

I-

I nth Pick-up controller I

Fig. I A typical N pick-up bidirectional IPT system

Operation Consider the operation of a bidirectional IPT system with N

pick-ups as shown in Fig. 1. The primary side converter produces a constant sinusoidal reference voltage VpiLO at an

angular frequency w. Assuming that the current ipt in the track

inductance Lpt is held constant, then the voltage induced in the

nth pick-up coil Lst.n, due to this current can be given by

(1) where Mn represents the mutual inductance between the

coils Lpt and Lst.n-Any pick-up may be operated as either a source or a sink by

its controller, but despite the mode of operation the voltage

vpr.n reflected on to Lpt from the current ist.n flowing in the

Lst.n winding can be given by

(2)

If the LCL circuits on both primary and pick-up sides are tuned to the frequency of w, and Lpi = Lpt, Lsi.n = Lst.n then

Under these conditions it can be shown that for N pick-ups the primary currents, ipi and ipt' can be given by

. . Vpi l = -j --pt

wL pt

Similarly, currents in the nth pick-up can be given by

. jVsr.n l· =--Sl.n wL st.n

V· . . Sl.n lst.n = -j� st.n

(4)

(5)

(6)

(7)

Using (4) to (7), it can be shown that real input power of the nth pick-up can be given by [16]

M v ·Iv. I P

_ n pl Sl.n . (e ) . - sm Sl.n L L w n

pt st.n (8)

where Vsi.nLen is the voltage produced by the nth pick-up

converter.

From (8), it is evident that the power input of any pick-up depends on both the magnitude (vsi.n) and the relative phase angle (en) between the voltages produced by primary and pick­up side converters. Thus, the power output can be regulated by controlling the voltage magnitudes and/or the phase angles with respect to the primary side voltage. Furthermore, it is evident that the maximum power transfer takes place when the phase angle is ± 900• A leading phase angle constitutes power flowing from the pick-up to primary, while a lagging phase angle constitutes power flowing from the pick-up to primary.

In a multiple pick-up bidirectional IPT system, regulation of power flow between the primary and pick-up can be achieved [16]. A power-frequency controller and a frequency jitter controller have also been reported to regulate power in multi pickup IPT systems without a communication link [17-19]. However, none of these controllers are capable of prioritising power delivery to individual pickups or provide a mechanism through which the power supplied by primary supply could be shared between pick-ups. In order overcome these drawbacks, this paper proposes a controller with two distinct frequency jitters to regulate the power flow in muti pickup IPT systems.

III. PROPOSED CONTROLLER

Controller in [18] introduces a frequency jitter around the tuned frequency of the system to regulate the power flow. In contrast, the proposed controller introduces two jitters of frequency around the tuned frequency of operation. First frequency jitter conveys information with regard to power capability of the primary supply while the second jitter of

frequency indicates the tariff for power delivery or charging in the case of EVs. The frequency jitters are confmed to small but different bandwidths, which can easily be detected and locked-in by the pick-up side controllers. Both frequency jitters are momentary and last only for a short period of time. Hence, they hardly compromise the usual operation of the IPT system at resonant or tuned frequency or the amount of power delivery. Two frequency jitters can be considered as control loops. The power frequency jitter is critical for stability and takes place at much higher rate, which can be as high as 100 times, than less critical tariff frequency jitter.

Sin (wt)

(a) primary

Sin (wt+us,n/2)

(b) pick-up

Fig. 2 Simplified controllers

Fig 2 shows the proposed tariff based control philosophy. The primary side controller, as shown in Fig. 2 (a), continuously monitors power input (PPin) to the primary converter in comparison to maximum power capability (P p,max) of the primary converter. The error or mismatch in power (�P) is calculated, and multiplied by a frequency jitter function (fm)' The frequency jitter function is designed ensuring that both ramping up and down of jitter frequency around the tuned frequency takes place gradually. Thus sufficient time is always available for the pick-up side controllers to be synchronised with the changing frequency of primary side controller due to frequency jitter. Furthermore, the maximum deviation of jitter frequency is limited, in this particular case, to 250 Hz. If the primary side controller detects any changes in power that is supplied by the primary converter, the varying power capability is sent through a frequency jitter at every 2 ms which will be increased to 20 ms when operating at steady state. Varying the rate of modulation ensures that the system has a fast response without affecting the V A rating of the system at rated power.

In addition to power capability, the primary controller also sends another frequency jitter, generated by function fp, to

indicate the tariff for any power intake by the pickups or EVs. The tariff is varied in accordance with demand and remaining power capability of the supply. Jitter frequency for tariff is less important and hence takes place at much lower rate in comparison to power frequency jitter. Generally, jitter frequency band for tariff is placed outside the jitter frequency for power while ensuring that no unnecessary V ARs will be drawn from the supply. In this particular case, it is set to ± 300 to ± 550 Hz.

IPT systems are designed to operate and transfer power at the tuned or resonant frequency, which is generated by the primary side controller. The pick-up controller, shown in Fig. 2 (b), constantly monitors the operating frequency of the system to which it locks in through a phase-lock-loop (PLL) to take power. Up on detection of change in the operating frequency, the pick-up controller determines from the stabilized frequency output from PLL whether the frequency jitter is associated with the tariff or the power capability. Based on the jitter frequency, the pick-up controller then regulates the power intake, taking into account its purchasing capability and power requirement, by varying the duty cycle (as,n) of the pick-up converter. Controlling the duty cycle of the pick-up converter regulates the magnitude of vSi,n and, hence according to (8), the power received from the primary [16].

Fig 3 illustrates the operating philosophy of the proposed tariff based controller. The top plots show the jitter frequencies, the middle plot shows the power capability of the primary supply and the bottom plot shows the tariff for power intake or charging. The tuned resonant frequency of operation of the system is fo while frequency jitters of power capability and tariff are set between (fo - fp,max) and (fe,min - fe,max), respectively. It should be noted that in practice jitter frequency for power occurs at much higher rate in comparison to jitter frequency for tariff. However for the purpose of illustration, it is assumed that jitter frequency for power occurs at twice the rate for tariff.

For example, consider a situation where the primary side converter of a bidirectional IPT system is idling without any pick-ups taking power or EVs are being charged. Since no power is supplied by the primary converter, the primary controller operates the system at resonant frequency fo with jitter frequencies of fp,max and fe,min indicating that the system is idling with maximum power capability (P p,max) and minimum tariff (Cmin). Approximately between t2 and t3, pick-ups (EVs) begin taking power. Noticing that power is being taken, the primary controller changes the magnitude of frequency jitters to indicate the remaining power capability and new tariff. This situation is illustrated by a smaller magnitude in power frequency jitter and higher magnitude in tariff frequency jitter after t". This process repeats as long as EV s continue to take power according to their purchasing capability and until the primary power supply reaches its capability. As the demand for charging increases, the tariff goes up with the remaining capability of the power supply. EVs that have higher purchasing capability continue to take power at higher tariff

while other EV s limit their power intake or drop off from taking further power. Eventually the system converges into a stable state at which the demand is met by the available supply on the basis of tariff.

fe,max fe.min ." . ' . ' fp,max . ' . ' . ' · . · . · . f� fo · . · . Pp,max

0 �r-�---+4--r---r--�--+_--r-----�

Cs.max Cmin +---_____ J

C �--�--_+--_+--�--_r--�--+_--�� t to

Fig. 3 Controller operation

IV. RESULTS

In order to verify the applicability of the proposed control technique a bi-directional IPT systems with a 3 kW primary and three pick-ups rated for 2 kW, representing 3 EVs, were simulated using PLECS, a MA TLAB Simulink based software package. Parameters of the simulated IPT system are given in table I. The primary converter, supplied from a 400 V dc

supply, was controlled to produce a constant rms current ipt of

60 A in the primary winding. The primary winding was coupled to three pick-up converters to either provide or extract power from the primary converter. The pick-up converters were operated from 200 V dc supplies with a 90° phase delay with respect to the primary, ensuring that the system was operated at unity power factor. The pick-up controllers vary the

phase shift as,n between the two legs of their full-bridge

converters to control the amount of power being supplied or extracted. Using the proposed technique the amount of power and price of the power each pick-up takes from the primary is controlled.

Fig. 4 shows a situation where the proposed controller regulates the charging process of 3 EVs. Fig. 4(a) shows the power handling capability of the primary supply and EVs while Fig. 4(b) shows the price or tariff for charging. The broken lines of both plots indicate the maximum power handling capability and purchasing power or the maximum price that each EV is willing to pay for charging. Initially, the system is idling and hence the tariff is lowest at I 0 Cents/kWh. Approximately at the 30th hour, two EVs with maximum purchasing capability of 20 Cents/k W begin charging or taking power at 10 Cents/kWh. Primary controllers detects that power is being taken and sends frequency jitters, indicating the level of remaining power and tariff. As the level of remaining power decreases, the magnitude of frequency jitter for power decreases, too. Controllers on EVs monitors the decreasing

magnitude of the frequency jitter for power, and continue to take power as long the tariff is within their purchasing limit. As soon as the primary supply reaches its capability of 3 k W, frequency jitter for tariff increases while that for power is zero. EV controllers notice that no additional power is available to be taken and limit the power intake to 1.5 kW at maximum purchasing rate of 20 Cent/kWh. As evident, even though each EV is capable of taking 2 kW of power, the intake is limited to 1.5 kW at the maximum purchasing capability of 20 Cents/kWh. Noticing that there is no additional demand for power, primary controller maintains its tariff just over 20 Cents/kWh.

�o2 : [� 1 : � 3

�� 1 . 5 1 0.. 0 � 3

�� 1.5 1 0.. 0

� 3

1 C'J 1.5 '

0.. 0 0

1-- :-1-- :-

: 60

. . . . . : . -;� -;� l

120

time (hours) (a)

: l t � l t � l : l

180 240

�o :f Z,.,;-: --Ar--ww----il �.:t�z'7'""'":--: --------il �� : t � 2""""""--: �:�: ------ll

30 r------,-------T-------r------.

�'J �� �I �r�" ...... ·· .. � .... : .. · ... _·· .... ··1 o 60 120

time (hours) (b)

180 240

Fig. 4 System operating with two different price levels (a) power and (b) price of power

A 3rd EV, which is willing to pay 30 Cent/kWh for charging, arrives at the 120th hour and begins to take power. Detecting the increase in demand, the primary controller increases tariff above 20 Cents/kWh through jitter frequency for tariff. The 3rd EV continues to take power but the other two EV s decrease the power intake, not willing to pay more than 20 Cents/kWh. This process continues until the 3rd EV reaches its

power requirement of 2 kW at approximately 21 Cents/kWh while the other EV s reduce intake to 1 k W at 20 Cents/kWh. The system reaches a stable state at which the supply capability of 3 kW meets the demand of three EV s, based on the tariff for charging.

�o2 :�F �:""""""""""�" i "�""'�i �l � 3 r------�------�------�------�

��1 ·�L..--1 �. _:�·�p-------ll � 3r------�------�------�----�

�� ' : ,----I -----l...--2'_.�. " " , ,�: \ ------1' � l �� ' : L----I �! _, -�-: ---------"---� -----'- l

o 60 120

time (hours) (a)

180 240

30r------�-------.------�----�

����L..--I �./_. :�. �(_l �·::=I =: ==,. =(=1

30r------�-------r_------r_----�

�� ��'----t� -----L..-.-- /_-, -:�,- ,- ��( ------11 30 r------,-------,-----�r------.

�� �� ,----t - ----L...--- S/_:�: ------11 o 60 120

time (hours) (b)

180 240

Fig. YY System operating with three different price levels (a) power and (b) price of power

Fig. 5 shows a situation for which three tariff levels are used to establish stability in power sharing among 3 EVs, rated for 2 kW but with different purchasing capabilities. Initially EV 3, which is capable of paying 20 Cents/kWh, takes its rated power of 2 kW at minimum tariff of 10 Cents/kWh. As the primary supply is operating below its 3 kW power rating, the tariff is set to 10 Cents/kWh and frequency jitters are sent indicating that power is still available to be taken at 10 Cents/kWh. The second EV, which can pay up to 25 Cents/kWh, begins taking power at the 60th hour at 10 Cents/kWh. Primary controller detects the increase in demand above its rated power level of 3 k W and increases tariff which exceeds 20 Cents/kWh at approximately 90lh hour. EV 2 continues to take power at 21 Cents/kWh but EV 3 reduces its intake at 20 Cents/kWh. The system reaches a stable point of

operation at the 90th hour, as the 3 kW demand meets the rated supply. EV2 takes 2 kW of power while EV3 takes only 1 kW of power but at a reduced tariff. At around the 180lh hour EV I , which is willing to pay 30 Cents/kWh, begins taking power and forcing the demand to exceed the capability of primary supply. The magnitude of jitter frequency is increased to indicate that tariff has now been raised above 21 Cents/kWh to meet the increased demand. Not willing to pay the increased tariff, EV3 cuts off its power intake while both EVI and EV2 continue to take power at the increasing tariff. Approximately at the 200th hour, the tariff reaches 25 Cents/kWh, exceeding the maximum tariff that EV2 is prepared to pay. Consequently, EV2 reduces its intake while EVI continues to take power at higher tariff. Finally, the system reaches a stable state of operation at which EV3 takes no power, EV2 takes 1 kW at 25 Cents/kWh and EVI takes 2 kW at 26 Cents/kWh.

Table I : Parameters of the simulated system

Parameter Value Vpin 400 V

Vsin.v VSin.2 and VSin.3 200 V Lpt and Lpi 46.2 flH

Cpt 1.4 flF

Lst.v LSt.2 and LSt•3 26.1 flH Lsi.V Lsi.2 and Lsi.3 26.1 flH Cst.v CSt.2 and CSt.3 2.5 flF

MVM2 and M3 4 flH Pp,max 3kW ps.max 2kW

fa 20 kHz fe.max 20.55 kHz fe.min 20.3 kHz fp.max 20.25 kHz tltmin 2ms tltmax 20ms

tj 500 flS

V. CONCLUSIONS

A new controller that is suitable for charging multiple EVs using IPT has been proposed. A tariff based technique has been utilized to establish power sharing among EVs for any given capability of the power supply. Two distinct jitters of frequency have been employed to inform EVs of the remaining power capability of the IPT supply and tariff for power intake. The validity of the controller has been demonstrated by charging 3 EVs using a bi-directional IPT system. The proposed controller is applicable to uni-directional as well as wired charging systems.

REFERENCES

[1] S. Y. R. Hui, and W. W. C. Ho, "A New Generation of Universal Contactless Battery Charging Platform for Portable

Consumer Electronic Equipment," IEEE Trans. Power

Electron, vol. 20, no. 3, pp. 620-627, May 2005.

[2] P. Si, A. P. Hu, J. W. Hsu, M. Chiang, Y. Wang, S. Malpas, and D. Budgett, "Wireless Power Supply for Implantable Biomedical Device Based on Primary Input Voltage Regulation," in IEEE Conference on Industrial Electronics and Applications, 2007, pp. 235 -239.

[3] J. Yungtaek and M. M. Jovanovic, "A contactless electrical energy transmission system for portable-telephone battery chargers," IEEE Transactions on Industrial Electronics, vol. 50, no. 3, pp. 520- 527, Jun. 2003.

[4] C. G. Kim, D. H. Seo, J. S. You, J. H. Park, and B. H. Cho, "Design of a Contactless Battery Charger for Cellular Phone," IEEE Transactions on Industrial Electronics, vol. 48, no. 6, pp. 1238-1247, Dec. 2001.

[5] J. Sallan, J. L. Villa, A. Llombart, and J. F. Sanz, "Optimal Design of ICPT Systems Applied to Electric Vehicle Battery Charge," IEEE Transactions on Industrial Electronics, vol. 56, no. 6, pp. 2140 -2149, Jun. 2009.

[6] X. Liu and S. Y. Hui, "Optimal Design of a Hybrid Winding Structure for Planar Contactless Battery Charging Platform," IEEE Transactions on Power Electronics, vol. 23, no. 1, pp. 455 --463. Jan. 2008.

[7] U. K. Madawala and D. J. Thrimawithana, "A new technique for Inductive Power Transfer using a single controller, " lET

Trans. on Power Electronics, vol.5, no.2, pp.248-256, Feb. 2012.

[8] B. Choi, J. Nho, H. Cha, T. Ahn, and S. Choi, "Design and Implementation of Low-Profile Contactless Battery Charger Using Planar Printed Circuit Board Windings as Energy Transfer Device," IEEE Trans on Industrial Electronics, vol. 51, no. 1, pp. 140-147, Feb. 2004.

[9] N. A. Keeling, G. A. Covic, and J. T. Boys, "A Unity-Power­Factor IPT Pickup for High-Power Applications," IEEE

Transactions on Industrial Electronics, vol. 57, no. 2, pp. 744-751, Feb. 2010.

[10] D. J. Thrimawithana and U. K. Madawala, "A generalized mathematical model for Bi-directionaliPT systems, " in IEEE

Trans. on Power Electronics, vol. 28, no. 10, pp. 4681-4689, Oct. 2013.

[11] J. T. Boys, G. A. Covic, and A. W. Green, "Stability and control of inductively coupled power transfer systems," lEE

Proceedings - Electric Power Applications, vol. 147, no. 1, pp. 37--43,Jan. 2000.

[12] L. Chen, S. Liu, Y. C. Zhou, and T. J. Cui, "An Optimizable Circuit Structure for High-Efficiency Wireless Power Transfer," IEEE Trans. Industrial Electronics, vol. 60, no. 1, pp. 339-349,Jan. 2013.

[13] H. Matsumoto, Y. Neba, K. Ishizaka, and R. Ttoh, "Model for a Three-Phase Contactless Power Transfer System," IEEE

Trans. on Power Electron., vol. 26, no. 9, pp. 2676-2687, Sep. 2011.

[14] A. J. Moradewicz and M. P. Kazmierkowski, "Contactless Energy Transfer System With FPGA-Controlled Resonant Converter," IEEE Transactions on Industrial Electronics, vol. 57, no. 9, pp. 3181 -3190, Sep. 201 O.

[15] R. M. Miskiewicz, A. J. Moradewicz, and M. P. Kazmierkowski. "Contactless Battery Charger with Bi­Directional Energy Transfer for Plug-In Vehicles With Vehicle-to-Grid Capability," in IEEE International Symposium on Industrial Electronics, 2011, pp. 1969-1973.

[16] U. K. Madawala and D. J. Thrimawithana, "A Bidirectional Inductive Power Interface for Electric Vehicles in V2G Systems," IEEE Transactions on Industrial Electronics, vol. 58, no. 10, pp. 4789--4796, Oct. 2011.

[17] U. K. Madawala. M. Neath. and D. J. Thrimawithana. "A Power-Frequency Controller for Bidirectional Inductive Power Transfer Systems," IEEE Transactions on Industrial

Electronics, vol. 60, no. 1, pp. 310 -317, Jan. 2013. doi: 10.1109/TIE.2011.2174537.

[18] M. J. Neath, U. K. Madawala and D. J. Thrimawithana, "Frequency jitter control of a multiple pick-up Bidirectional Inductive Power Transfer system," in IEEE International

Conference on Industrial Technology (ICIT), 2013, pp. 521 -526.