Design of an Onboard Battery Charger for an Electric Vehicle

CODEN:LUTEDX/(TEIE-5148)/1 -66/(2001 )

Simon Heckford

Department of Industrial Electrical Engineering and Automation Lund University DISCLAIMER

Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. .,, . . . . ,., +

I certify that all the material in this thesis which is not my own work has been identified and that no material is included for which a degree has previously been conferred upon me.

Signed: .&x.... w...... Simon Heckford

DESIGN OF AN ONBOARD BATTERY CHARGER FOR AN ELECTRIC VEHICLE

SIMONHECKFORD

3m YEAR INDIVIDUALPROJECT 2000 Acknowledgements

I would like to express my sincere thanks to Professor Mats Alakiila, my host supervisor for the project at the Department of Industrial Electrical Engineering and Automation, Lund Institute of Technology. Without his assistance, I would never have embarked upon the project, and he gave me invaluable support whilst undertaking the project.

I must also express my gratitude to Per Karlsson, a Ph.D. Student at the department. Without his knowledge and continual support, undertaking the project would have been a near-impossible task since I had no previous knowledge of power electronics. He was always on hand to offer guidance and during the course of the project, it became very apparent that he has a high tolerance level!!!

Other members of staff who have been of particular assistance during the project are H&an Skarrie, Bengt Simonsson, and Niklas Fridstrand,, although I must thank everyone at the department as a whole, for their constant support and goodwill. In addition, I must also thank Sabine Marksell, a student who was also working on the project and was always ready to give assistance.

In Exeter, I would of course like to thank Dr Mike Hcmvood, my home supervisor. In addition, I am very grateful to Lorna Howe and Gavin Tabor, coordinators for the Erasmus scheme. Without their organization, it would not have been possible for me to obtain the added benefit of experiencing a different culture whilst carrying out my project. Contents

f. Introduction ...... 1

1.1 Fast chargkg ...... 1 1.2 Impact on the @d ...... 3 1.3 Selected solution ...... 5 1.4 Specification ...... 6 1.5 Particular design problems ...... 7 1.6 Disposal of therepoti ...... 7

2. Modulation and Control ...... 9

3. Design of Hardware ...... 71

3.1 Semiconductors and DC-link capacitance ...... 11 3.2 AC side inductance ...... 11 3.2.1 Calculation of inductance using TFID ...... 17 3.2.2 Calculation of inductance using the IEC 1000-3-4 standard...... 19 3.3 Battery side inductance ...... 20 3.3.1 Calculation of inductance using fundamental component...... 20 3.3.2 Calculation of inductance by summing of all components ...... 24 3.4 Parasitic capacitors on the DC and battery side ...... 24

4. Analysis of Zero Sequence Current ...... 27

5. Implementation of inductances ...... 32 6. Conclusions and Recommendations ...... 33

7. References ...... 35

Appendices

Appendixl: The Electric Sniper Appendix2: AC Propulsion tzero Appendix3: Similar products Appendix 4: A Dual Purpose Battery Charger for Electric Vehicles Appendix 5: Simulink models Appendix 6: IEC 1000-3-4 standard used for determining L& Appendix 7: Data sheet giving Fourier Series Appendix 8: Data Ilom Fourier series calculations Summary

This report describes the design of an on-board battery charger for an electric sports car.

There are already various battery charger units on the market. However, these are not specifically designed for this application, and consequently do not provide an ideal solution. Because these products are not specific to one application, and instead opt to cover a variety of briefs, they are not ideal. They also tend to be heavier and more expensive than if the charger was built specifically for one purpose.

The main design considerations were that the charger should be compact and lightweight. It was also specified that the design should be able to operate using either the single- phase or three-phase AC supply.

Before the design process for the battery charger COUIC[commence, it was necessary for the author to get an appreciation of power electronics, since he had no previous experience in the subject. The author focused his attention on areas of the subject most valuable to the project, including becoming familiar with the principle behind battery chargers.

Once the required knowledge was obtained, the author could begin designing the charger. The majority of the design was actually undertaken using two soilware packages called MATLAB and Simulink, whilst also using the knowledge acquired. Regular discussions were had with the project team in order to ensure that the correct methodology was being used and a suitable design was duly developed. Possible further work was identified which could not be carried out within the time constraints of this project. 1. Introduction

With the increasing concerns over environmental ismes, alternatives to the Internal Combustion Engine (ICE) are gaining greater popularity. The electrically driven vehicle is the main environmentally friendly alternative.

The Industrial Electrical Engineering and Automation (IEA) department at the Lunds Tekniska Hogskola (or in English, Lund Institute of Technology and abbreviated to LTH) has been involved with a project designing and constructing an electric sports car - called the Electric Sniper - since autumn 1999. The car was originally built with a conventional engine fi-om Saab, but it was decided to make an electrically powered derivative.

Information regarding the Sniper electric sports car is shown in Appendix 1.

Various staff and another student from the university have been involved in the project, working on the tasks that were given to the IEA. Ths author’s role was to design the hardware for the charging unit, essentially a power converter.

The concept of an electric sports car is a fairly recent initiative and, up until recently, a contradiction in terms, since the performance of electric vehicles is impaired by the weight penalty of the batteries. However, there is an electric sports car, soon to be released on the market, whose performance exceeds that of sports cars manufacturers renowned for producing performance cars, including Porsche and Ferrari. Information on this model can be found in Appendix 2.

The design of the battery charger uses power electronics. This was a subject that was unfamiliar to the author at the start of the project. Power electronics can be defined as a process whereby the voltage and current are optimised to best suit a specific application, by controlling the flow of electric current.

The IEEE (Institute of Electrical and Electronics Engineers) provide the following formal definition for Power Electronics by stating “This technology encompasses the use of electronic components, the application of circuit theor,y and design techniques, and the development of analytical tools toward efficient electronic conversion, control, and conditioning of electric power.”

1.1 Fast charging

Charging of a battery is achieved by supplying a DC current. Through an electro- chemical process, the current provides electrical charge that is stored in the battery. Current is defined as the transport of electrical charge, per unit time. As a result, the energy delivered during the process of charging is determined by both the amount of DC

1 current supplied and the time elapsed. Electrical energy is transferred from a generator to the consumer in AC quantities. Consequently, such energy has to be converted into DC quantities. This is a primary role for the battery charger.

There is one underlying problem regarding the Electric Vehicle (EV), dictating that is not yet a very practical alternative to cars fuelled by a combustion engine. Whereas, a vehicle with an Internal Combustion Engine (ICE) can be refbeled in a couple of minutes, recharging an EV inevitably takes longer.

It is possible to charge an EV in approximately one or two hours. Charging by this method is called fast charging. In order to offer a realistic alternative to the ICE, fast charging is the only practical method of charging an EV. However, in order to provide 75 kW, a very large charger is required. This would mean that it could not be contained within the car, and hence it is mounted externally of the vehicle, and used at a “charger station”. Such a charger exists and is called the DUAL battery charger and two prototypes are in use in Sweden: one in Malmo and another in Stockholm. It is able to charge a bus using 375 V batteries, at a current of 2CIOA,corresponding to the 75 kW power.

As a result of fast charging, an owner of an EV can expect a far more substantial driving range, in comparison to the conventional charger, forth e same charge duration. This can be perhaps twice or even three times as much.

Per Karlsson and Martin Bojrup (both Ph.D. Students at the IEA) have designed a unique oflboard charger. It operates at 10 kW and a photo of this is shown in Figure 1.1. The knowledge previously acquired by the department, through undertaking this project, was of great assistance when designing the onboard charger for this project.

Figure 1.1: Offboard battery charger

2 A better alternative to the oflboard charger is to use an onboard charger. Here, the battery charger is enclosed within the vehicle chassis. This idea is beneficial since it means that the user is fhr less restricted in where he or she can charge the vehicle. By using an onboard charger, the owner has increased freedom and the process of charging the vehicle is far more convenient. The vehicle just needs to be connected to the national grid, rather than relying on charger stations, as is the case with offboard chargers. The EV can thus be charged anywhere where there is an electricity socket, for “opportunity charging”. This means that the EV could be charged overnight, with little inconvenience. Although charging will basically take longer when using an onboard charger, in comparison to the offboard charger, charging can easily be undertaken at night, usually providing adequate power for the following day. Charging the EV overnight has the added benefit that often, for instance in America, electricity is cheaper at this time.

The EVS that exist today tend to use a maximum charging current of approximately 150 A (if the batteries are liquid-cooled, the current can be increased to 200 A), and the voltage of the batteries used for European EVS is generally between 100 to 200 V (usually around 130 V).

Table 1.1 gives the typical charging power and the usual time that is required to achieve sufilcient charge to drive a distance of 100 km. The ti~blelists three charging methods for EVS (on-board, off-board and a fast charging station), in addition to a petrol station, included for comparison.

Fuse / Phase Power / Type Time to achieve enough power A lme kW to drive 100 km 16 Single 3 On-board 7 hours 16 Three 10 Off-board 2 hours 120 Three 75 Fast charging 15 min

Table 1.1: Table showing the time required to achieve enough power to travel 100 km for various battery chargers for EVS (and a normal petrol station for comparison)

Information on onboard chargers that are already on the market is shown in Appendix 3.

1.2Impact on the grid

Fast chargers can result in reduced power quality in the grid. The impact that the battery charger has on the grid is most significant with offboard chargers, which typically use higher power. The problem is not really of great importance in the case of the design of this particular charger that is going to be designed as an onboard charger. Still, the

3 charger that has been designed is “grid-fi-iendly” since it draws a sinusoidal line current, which eliminates low frequency harmonics (especially the 5ti, 7“, 11* and 13* harmonics).

Neither is the effect a concern for the EV user; he or she simply connects the vehicle to charge. It is the responsibility of the power provider. ‘The current that is drawn from the grid, when using an onboard charger, can be approximated as being sinusoidal.

As a result of the non-linear characteristics of power electronics, distorted or non- sinusoidal currents on the power grid occur. Such distortion also results in voltage distortion caused by self-impedance in cables and transformers. This eventually leads to damage, and possibly malfimctions, of sensitive electrical equipment, for example, computers. The implications of this are, of course, disastrous.

Other battery chargers, based on normal diode rectifiers, increase the low order harmonic content in the grid currents. Further information concerning this area is discussed in the early paragraphs of Appendix 4.

The power grid has two stages in the process of distributing power: the transmission network and the distribution network, together forming a tree and branch-like structure. The former initially supplies power to various regions, and can provide high power transfer with low losses. On the other hand, the latter distributes power within these individual regions. The way in which this distribution occurs causes drops in the voltage and distortion can be observed. In addition, distortion can occur in the current (and consequently the voltage) when there is resonance between the conductors and capacitor banks or loads.

By matching the active power production (in the generators) and the demand (i.e. the instantaneous energy used by the consumer), accounting for losses in the two types of network, control o~-the frequency is achieved. Failure- to have such frequency control will result in an acceleration or deceleration in the generators, causing frequency variation.

There are various ways in which such distortion can be greatly reduced. Such methods include reactive power compensation. These include harmonic filtering of currents, reactive power compensation, and load balancing and peak power generation. Further analysis of such processes is discussed in Bojrup [1].

There is one important point. Charging an EV during the night can actually help to minimise the effects on the grid. The load on the power grid at night is relatively low, resulting in a high voltage. This causes power grid operators to disconnect capacitor banks, and oflen reactors are inserted in order to maintain the magnitude of the voltage. By charging EVS overnight, the operation of the power grid can be improved since the load is smoothed out over the whole day.

4 7.3 Selected solution

There are numerous possible solutions to designing a battery charger for an EV. However, the author was informed that the general specification had already been agreed.

Figure 1.2: Diagrams of a) the single-phase and b) th~ethree-phase AC supplies

The charger was designed to operate using both the single-phase and the three-phase grid supplies. Figure 1.2 shows diagrams of the single-phase and three-phase AC supplies.

Onboard chargers typically use the single-phase AC supply due to the advantage of weight and size. However, three-phase supplies have three times the charging power of an equivalent single-phase supply, and hence charging takes approximately a third of the time. This is because all of the three phases can be charged to the same level as the single-phase device charges just the one phase.

400v Me-Iii h .x,’230v rhs. rmlmd

\ \

> :

\, ; ,/ tf\ / ,’ ,’ -looViii ., ... we w, -233v h-m- nadrd

time

Figure 1.3: Diagram showing the output signal from the three phases

5 In the three-phase situation, the three different phases are called ~ S and T respectively. Their signals have exactly the same wavelength and amplitude. They differ with a certain phase shift, 2Tc/3(or 1200). This is shown in Figure 1.3.

Figure 1.4 shows a diagram of the circuit that is the basis for the design of the charger.

Figure 1.4: Diagram of circuit

On the left of the diagram is the input from the power grid. The main (central) region of the diagram shows the main components within the charger itself and finally, the far right area of the diagram shows the batteries used to power the electric vehicle.

The circuit primarily consists of a line filter, a three-phase converter, a DC-link capacitor and a smoothing inductor (which acts to reduce current ripple).

The voltage of the batteries in the vehicle, Vbati,could take a value anywhere in the region of 50-400v.

1.4Specifications

Nominal AC grid voltage v nom 400 V rms phase-phase Maximum power P llkW Nominal DC-link voltage v& either 650 V, 700 V or 750 V Nominal AC current Inom 16A Nominal DC current IdC to be decided Maximum AC ripple THD n.cm such that ~-i ~(i.)’ S 0.6% i~o~or 0.L4 nom r n=40 Maximum DC ripple Dcripple 5 A peak Switching frequency fSw either 5 kHz or 10 kHz

Table 1.2: Specification showing suggested values

6 Before any design of the battery charger could take place, it was necessa~ to ascertain various parameters. Table 1.2 shows the initial values that were agreed.

The charger has a rated power of 10kW since the fises used in Swedish households are 16A, meaning that 11 kW can be supplied ilom the household mains.

1.5 Particular design problems

Since the weight of an EV strongly influences its performance, it is important to minimise the weight as much as possible. Although the lightest design would be to only undertake the charging using single-phase AC supply, the advantage of using the three- phase grid in addition was considered to outweigh the ‘weight issue. Since inductors are heavy, one way in which it is possible to reduce weight is to attempt to reduce the size of the inductors.

The weight of the EV affects the acceleration, the hill-climbing ability, the speed and the range. In each case, the performance is reduced.

However, it is also important to ensure that there is also good stability. Therefore, the distribution of the weight is also an important consideration. This is a factor worth remembering, but is not directly associated with this project, since these constraints are mainly determined by the design of the vehicle itself.

It is also sensible to use as low a voltage as possible. For instance, although the nominal DC-link voltage has not been specified in the early stages of design, it will be preferable to use the lowest voltage possible.

1.6Disposition of the repofi

The following chapters outline the methodology used to design the overall hardware for the battery charger, and also to confm that the calculated values are suitable.

Chapter 2 discusses the modulation and control to be used in the battery charger. This includes a brief description of the fundamental kmowledge acquired during the undertaking of the project.

Chapter 3 explains the actual design of the hardware fix the charger. This includes the inductors, capacitors etc.

7 Chapter 4 describes the analysis of the zero sequence current. ‘Ibis chapter describes the investigation and experimentation into an earth protector (essentially a circuit-breaker) for the battery charger,

Chapter 5 details how the inductors will be designed and manufactured. This details the actual process by which the electrical components will be produced, once the specific values have been ascertained.

Finally, Chapter 6 sets out the conclusions that can be drawn and suggests possible recommendations on improvements to the design of the battery charger and critically assesses how the project was undertaken.

8 2, Modulation and Control

It was initially intended to describe the relevant power electronics theory in this section, and more specifically, battery current control, DC voltage control and AC rectifier control. However, unfortunately, the restriction on the length of this report dictates that it is not possible to do so, since the topic is fairly complex and hence it would take a large proportion of the allowance simply to describe this subject. Although such knowledge is a pre-requisite in the process of designing the charger, it is not really necessary to go into the matter my fbrther than the detail given in the subsequent chapters. Instead, this chapter will describe the packages used to simulate the circuitry to acquire the necessary values for the design of the battery charger.

Two software programs, called MATLAB and Sirnulink, were used in order to acquire values for the various inductors and capacitors. Appendix 5 shows various models that were used during the project.

Figure 2.1 shows part of one of the diagrams shown in Appendix 5, namely the is_x/y controller. It has been included to show an example of what MATLAB and Simulink are being used for, undertaking power electronic calculations.

. #=..”,=*) k i_sR T: I I —i (’7J I i_s I Q! b’

II IOo=pi 11. h---=-- %

Figure 2.1: One of the blocks, and the calculations that are undertaken

The block is basically calculating the following equation.

9 u= =R~i,x+#(i,x* –im)–o),L,i,y Equation 2.1

Using a systematic approach, it is possible to see how the equation is produced. The procedure to “create” the equation is shown on the annckated diagram.

These kinds of calculations are used throughout the blocks in the diagrams shown in Appendix 5. The diagrams were created by staff at the department, and not created by the author.

10 3. Design of Hardware

3.1 Semiconductors and DC-link capacitance

The semiconductors and DC-link capacitance were not an area that was the author’s responsibility. Instead, they were specified at the initial stage of design by staff at the department. For example, the value for the DC-link capacitor, shown as Cd, in Figure 1.4, was pre-determined using intelligent approximation. To calculate this capacitance value more accurately, the current spectrum would have to be analysed.

3.2AC side inductance

Initially, it was necessary to ascertain the value for the inductance that is required on the AC side of the circuit. It was decided to experiment for six cases; for switching ilequencies of both 5 kHz and 10 kHz, and for each c~fthese, using values of 650, 700 and 750 V for the DC-link voltage. The six cases are shown in Table 2.1.

Case DC-link vokage, V& / V Switching frequency, f,W/ kHz 1 650 5 2 650 10 3 700 5 4 700 10 5 750 5 6 750 10

Table 3.1: Six cases to be simulated

Figure 3.1 shows the output signal for the voltages and Figure 3.2 shows the signals for the currents for each of the six cases.

Although unfortunately, the plotted results are not very clear, there are important properties that can be seen from the plots. Firstly, the shapes of all plots are similar (both for the voltage and the current), only differing in amplitude and frequency.

The diagrams shown in Figure 3.1 actually show plots of three fimctions. The blue plot shows the triangular carrier, and the amplitude of this is the DC-link voltage. In the case of a DC-link voltage of 650 V, the maximum voltage is 325 V (half of 650 V) and the minimum value is –325 V. The yellow plot (and regrettably not really evident) shows the reference signal, a simple sinusoidal wave. Finally, the pink plot (which is of greatest importance) illustrates the same sinusoidal reference that has been symetrised, adding the

11 .— .— . “r- I . ,..

d .. + ,. t-. -– ,:, A _..~ —.

650 V, 5 kHz 650 V, 10 kHz

. —, —.r— I ——— — — —---1

L —1 % ,. ,,” .:-– A .L ,- 3.r’--

.“..”,

700 V, 5 kHz 70(1V, 10 kHz

. ,— —.

. .- . .

,.. .

,.. ..

. .I

--- . .

-?? .,, ..,. ,0- m . -?”! ,-..., *.- ,

750 V, 5 kHz 750 V, 10 kHz —

Figure 3.1: Voltage plots for the six cases

12 ~.. ___ _.. .— -r--- --—-

I *L

*A . 650 V, 5 kHz 650 V, 10 kHz

. ——.7 .—— 1“! .+——. .—— . . “

I j

-..,“’-. -.

f \. \ * _— , --- [;j~_<~ f,: . —.. — +-. ; --.’

. ——. .* _+ . . li ..L.... “’, . ..-* * . __--_-J %-, -,

700 V, 5 kHz 700 V, 10 kHz

1 ,-

“’,: \,---”~q “-~

. -

< . -..-.——-J

7“1 .-.&. ..—a . ,- ?---- u-+ 750 V, 5 kHz 750 V, 10 kHz

~igure 3.2: Current plots for the six cases

13 third order harmonic. The only difference between the outputs for the 5 and 10 kHz plots is that the wavelengths vary (by a factor of two).

At this stage it is important to look for evidence of overmodulation. Overmodulation is characterised on the voltage plots when the symetrised voltage goes outside the bounds of the triangular carrier (i.e. less than –325 V, or greater than 325 V, when a DC-link voltage of 650 V is used). It can be seen that overrnodulation does not occur in any of the six cases. If overmodulation were observed, it would instantly eradicate that particular case from firther investigation.

The diagrams in Figure 3.2 show that the amplitude of the current is in the region between –10 and 10 A. The main difference between the outputs for the two frequencies is the current ripple. This is the amount of variation of the sinusoidal wave. It is greater when using the higher frequency.

The relevant part of the diagram shown in Figure 1.4, relating to the AC side, is shown in greater detail in Figure 3.3.

@ Q.

vt- v!.

Figure 3.3: Inductors on the AC side

The diagram shows three identical sets of components in parallel, labeled a, b and c. These relate to the three phases from the grid. The voltage, u., is calculated using the following equation:

ZJa=va-vo Equation 3.1

The following relationship exists between the voltage vDand v,, vb and vC:

Va+vb+vc V. = Equation 3.2 3

14 Hence, by substituting equation 3.1 into equation 3.2,

Va+vb+vc u*=v=– Equation 3.3 3

+ 4“ ‘u=

Figure 3.4: Circuit for one phase

It is only necessary to investigate one of these sets of components since ua,ub and w are identical (simply differing in phase) would be calculated by exactly the same method. Figure 3.4 shows the components for path a.

Figure 3.5 shows the outputs for u,, ub and ~, with respect to time for case 1. The phase shill between each is clearly visible.

The equation relating the voltage drop across the inductor, assuming the resistance of the resistor, R, is zero, is given by: u~ =u~–eo Equation 3.4

It is advantageous to consider the voltage output at a given harmonic. The frequency of the grid, termed f@, is the fundamental frequency, F, which is 50 Hz in Sweden (60 Hz in the United States). A harmonic, h, is calculated as: h= ff Equation 3.5 fnomd.d = fgid

The fi-equency, f, is the frequency at a given point. Harmonics occur at multiples of F (i.e. F, 2F, 3F, 4F, 5F . . . nF).

For each harmonic, the voltage across the inductor is: v~,~= j(o~LI~ = vl,~– E~ Equation 3.6 where vl,h is the voltage at the first harmonic and Eh=E~d for h=l and Eh=O for all other values of h.

15 _— ...... --_._— ——-—..—-—. I ““”--–—————7 5ooi I I 1

$ t -500~ o 0.005 0.01 0.015 0.02 0.025 ~10.03 0.035 0.04 I time Is

I .5M: —---__/ 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 I

I 5001

-500~ I ! o 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 time Is ______~--,.- _ ..-.——. ——.-—.—.- ..—

Figure 3.5: Outputs for u,, u~ and UC,with respect to time

16 The current for each harmonic is:

1~=~ Equation 3.7 jm~L

v 1~= “h Equation 3.8 j2@1hL

The constant, fl, is the frequency of the fimdamental component, also previously termed f@&

The magnitude of Ih is:

Equation 3.9

Therefore, the modulus of Ih is proportional to VLJ/h (a term called the specific current spectrum). Figure 3.6 shows the spectrums of VLJ/h plotted against frequency, for cases 1 and 2. The results for cases 3 and 5 are practically identical to case 1, only differing in amplitude, due to the nature of the equation. Similarly, the spectrums for cases 4 and 6 are virtually the same to that of case 2.

As annotated on the spectrums, it is possible to consider the spectrums in terms of h, simply by dividing the frequency by the grid frequency, i.e. 50 Hz. Peaks are evident when h is equal to 100, 200, 300 etc. (i.e. multiples of 100). A term for these intervals is the frequency modulation ratio, mfi

3.2.1 Calculation of inductance using THD

The Total Harmonic Distortion (THD) is given by the following equation. It uses all harmonics above the H* harmonic.

~=~=~-.;”+~m Equation3.lo

I l,n I I,n Il,n

However, the value of the inductance, L, is not known: instead, it is the desired value that will be calculated. The inductance, for a given THD, Icanbe calculated. The minimum inductance value can be obtained by rearranging the equation, as follows:

17 xx .;

0.7 , , I

0.6 -

0.5-

0.4-

0.3 - ,-

0,2- -

0.1 -

0-”” ii& J,i,lm . . ...__L ., ‘. 0 2 4 6 8 G=51CM3 a) I.%=tclf=$ f A-1.(x If

“hi 0.35

0.:

0.2:

0.2

0.1

0.05

0 L — b)

Figure 3.6: Spectrums showing VL,Jh against frequency for a frequency of a) 5 kHz and b) 10 kHz

18 Equation 3.11

The THD that was suggested for use was 0.6%, complying with a standard called the IEC 1000-3-2. This indicates that the maximum AC ripple cannot exceed 0.6%. The constant, 11,~,a value of current, is 22.6 A (calculated. by 16 x 42 – since peak values have been used, but an r.m.s. value should be used instead). The most complex part of the equation to calculate is the latter component where the square root of the summation of the specific current spectrum. The aforementioned standard states that the equation should take into consideration from h=O to h=40.

The results obtained fkom calculating the minimum inductance for the six different cases are shown in Table 3.2.

Case V&/ v f,. / kHz THD LdC/ ti 1 650 10 0.0082 30.3 2 650 5 0.0075 15.2 3 700 10 0.0080 31.4 4 700 5 0.0075 15.7 5 750 10 0.0079 32.6 6 750 5 0.0079 16.3 Table 3.2: Table giving values of LdCfor the specific cases

3.2.2Calculation of inductance using the IEC 1000-3-4 standard

Having calculated the values for L& the results appeared to be a little high. The results acquired from the offboard charger described in section 1.1 were about half these values. This raised doubt as to the precision of the calculated values.

It was realised that the reason behind these excessively high values was that the THD was too restrictive. The column of Table 3.2 showing the THD indicates that the THD for eve~ case is above the 0.6°/0 (0.006) that was initially specified in Table 3.1. As a result, it was decided to “loosen” the THD as illustrated below,

Since overmodulation did not appear to occur in any of the cases, and it is advantageous to use as low a voltage as possible, a DC-link voltage of value 650 V was selected. In any case, the determining parameter affecting the value of LdCis the switching ilequency, rather than the DC-link voltage. This is shown in Table 3.2 where there is little variation between Cases 1, 3 and 5 (and similarly Cases 2, 4 and 6), where the switching frequencies are very close: just the DC-link voltage is different.

Instead of the previously-used standard, another standard, called the IEC 1000-3-4 is being introduced on 1stJanuary 2001. This standard was selected for calculating Ld..

19 The standard is shown in Appendix 6. It states the Admissible Harmonic Content for given values of the harmonic, expressed as a given percentage of the fimdarnental current for odd harmonics, and as either 4 divided by the himnonic number, or 0.6?40of the fimdamental of the current for the even harmonics, whichever is the higher. Table 3.3 shows the values that are obtained from the standard, the actual current for the charger, and calculated values that prove useful in comparing the two.

The calculated value for the Admissible Harmonic content can be compared to the actual current, which is found from the current spectrum. It is observed that the Admissible Harmonic Content far exceeds the actual current, for all harmonics. This indicates that the current need not be a concern at this stage.

Table 3.4 shows the results obtained for the minimum inductance for the two cases using a DC-link voltage of 650 V. These were calculated using the data previously acquired.

CkiSe Vd.iv f&/kHZ Ldc/mH 1 650 10 17 2 650 5 8.5

Table 3.4: Table giving values of LdCfor a DC-link valtage of 650 V

These newly acquired results are similar to what was exlpected.

3.3 Batiery side inductance

After earlier investigation, it was found that a DC-link voltage of 650 V would be most suitable for the battery charger. However, a decision was made to keep investigating values for both switching frequencies of 5 kHz and 10 kHz, to ascertain which is most suitable.

A few new parameters are introduced in this model.

A voltage, uba~, is calculated as being half the DC-link voltage, i.e. 325 V.

There are two methods of calculating Lb,ti. The first is to look at the fimdarnental component and the other is to look at the sum of all components.

3.3.1 Calculation of inductance using fundamental component

It was attempted to calculate the Lb,fl value by the summing method using Simulink. However, some peculiar results were obtained. On looking into the model fiu-ther and manually calculating the expected results, the error was discovered.

20 3 19 4.30 0.0125 0.0029 5 9.5 2.15 0.0022 0.0010 7 6.5 1.47 0.0016 0.0011 9 3.8 0.86 0.0012 0.0014 11 3.1 0.70 0.0010 0.0014 13 2.0 0.45 0.0008 0.0018 15 0.7 0.16 0.0008 0.0049 17 1.2 0.27 0.0006 0.0023 19 1.1 0.25 0.0006 0.0023 21 0.6 0.14 0.0005 0.0038 23 0.9 0.20 0.0005 0.0023 25 0.8 0.18 0.0004 0.0024 27 0.6 0.14 0.0004 0.0030 29 0.7 0.16 0.0004 0.0024 31 0.7 0.16 0.0004 0.0022 33 0.6 0.14 0.0003 0.0024

4 6 4/n or 0.6 0.67 or 13.6 0.67 0.0018 0.0027 8 4Jn or 0.6 0.50 or 13.6 0.:50 0.0014 0.0028 10 41n or 0.6 0.40 or 13.6 0.40 0.0011 0.0028 12 4/n or 0.6 0.33 or 13.6 0.33 0.0009 0.0027 14 41n or 0.6 0.29 or 13.6 ().:29 0.0008 0.0027 16 41n or 0.6 0.25 or 13.6 0.25 0.0007 0.0027 18 4h or 0.6 0.22 or 13.6 0.:22 0.0006 0.0027 20 4Jn or 0.6 0.20 or 13.6 (j.;~() 0.0005 0.0027 22 4/n or 0.6 0.18 or 13.6 0.18 0.0005 0.0027 24 41n or 0.6 0.17 or 13.6 0.17 0.0005 0.0027 26 4h or 0.6 0.15 or 13.6 0.15 0.0004 0.0027 28 4h or 0.6 0.14 or 13.6 0.14 0.0004 0.0027 30 41n or 0.6 0.13 or 13.6 0.13 0.0004 0.0027 32 41n or 0.6 0.13 or 13.6 0.13 0.0003 0.0027 34 4/n or 0.6 0.12 or 13.6 0.12 0.0003 0.0027

I(l_act) = 22.6 A

Table 3.3: Calculated values from IEC 1000-3-4 standard

21 The special Fourier series was found, from the data sheet given in Appendix 7, to be given by the following equation.

2h SiIl(12ZCX) a. = Equation 3.12 rm

For this general equation, h=UdC=650 V, ct=O.5 and J=: n. Jw. Hence,

f n.— Equation 3.13 f. so,

.n=*=!E!2il Equation 3.14 n—f n—f f. f.

The results iiom calculating the data using this new method are shown in Appendix 8. It was discovered that the way Simulink had calculated the equivalent to a., gave simply positive values, whereas the Fourier series method produced both positive and negative values, as shown below. This resulted in the anomalous results.

1300sin ‘z ~ ax = (/) Equation 3.15 m

Hence,

1300sinn2 =413.8 a, = (/) N.B. positive since sin(n/2) = 1 z 1300sin 2X al = (/)2 so N.B.zero since sin(x)= O 2X 1300sin3z2) as = (/= –137.9 N.B. negative since :sin(3ti2) = -1 3n

Figure 3.7 shows two plots showing a variable termed Ah.~ (used in the simulation) against h: one actually simulated by Simulink, and another created from the table of results, from manual calculation, shown in Appendix 8.

The results from calculating the inductance from the simulation model, and firther calculation, are shown in Table 3.5.

22 40 .“”’.

* 2 q ~ ......

200 ...... 2...... ,,,, ... ,. ...

100 ...... +......

0 0 h200 400 a) I

57)

m .> ? Y Iw

-X0

-Za) n ,!. , b) ., <....:

Figure 3.7: Plots of Ah.~ against h – a) simulated and b) manually calculated

23 3.3.2Calculation of inductance by summing of all components

The value for Lb.ti can be obtained in another way. For this method, the following equation is used.

. U= L$+e Equation 3.16

Hence,

~ = (U- e)At Equation 3.17 Ai

In this case, U=U& and e=U&/2, meaning the values are 650 V and 325 V respectively. The change in time, At, is equal to l/2f,W. The current, Ai, is a peak value, so is twice 5 A. So, by substituting these aforementioned values into equation 3.17, the equation is:

(650- 325) ~ 2fw _ 325 16.25 Lb~u= _— — Equation 3.18 10 2ofw = fw

This means that the corresponding values for Lb.fi,for switching frequencies of both 5kHz and 10 kHz are 3.25 mH and 1.625 rnH respectively. cue Vd. / v f&/ ~z Lb,a / ti 1stmethod 2ndmethod 1 650 10 2.6 3.3 2 650 5 1.3 1.6

Table 3.5: Values for Lb,ti, for the two methods

Table 3.5 shows the values for Lb,ti, using this method, in addition to the previous method.

The second method of calculating Lb,ti was opted for, and hereafter, these values are used.

3.4Parasitic capacitors on the DC and battery side

Figure 3.8 shows a diagram of the capacitors contained within the car. There are two on the AC side and a further two on the DC side. These are both extraneous capacitances and are “unwanted”. They occur between the circuitry and the car itself.

24 I .1 -at

6. :<:--”~A--- -r , . !1 * -, ,. \ i ~- .Jh ‘;- 1+ 443 ~. ! l=-8ke!. . ,__ —------

Figure 3.8: Diagram showing capacitors for the battery charger

To calculate the capacitance, the diagram can be simplified. This is shown in Figure 3.9.

Figure 3.9: Simplified diagram of capacitors

To calculate the capacitance, the following equation is used. c=&o$ Equation 3.19 where C is the capacitance, Cois a constant (commonly termed the dielectric constant), A is the surface area of one face of the plates and d is, the distance between the plates (contained by the dielectric).

It was estimated that the capacitance on the DC side would have a surface area of about A4 (approximated to 30 cm by 20 cm) and the distance between the plates would be around 2 cm. Similarly, on the battery side, appropriate values were thought to be 1 m2 and 10 cm respectively. The value for&o is 8.85 pFm-l (i.e. 8.85 x 10-12Fr”-l). so,

(0.30x0.20) c =8.85x10-12 X =26.55 x10-’2F &_parasitic 0.02

25 1 c =8.85x10-12 X — = 88.50 x10-12F’ butt _ parasitic 0.10

Therefore, approximate values for these values of capacitance for the DC and battery sides would be in the region of 30 pF and 90 pF respectively.

26 4. Analysis of Zero Sequence Current

The zero sequence current is a common mode current originating from a common voltage shift. This affects the DC-link bus bars when a change of switch state occurs. Such a voltage shift causes a current flow in all three phases, each with the same phase and magnitude.

It is necessary to investigate the zero sequence current, in order to ensure that it is not excessively high.

The complex equation that is used to determine the zero sequence is given below ‘0=AJk2&?JEquation 4.1 3 jdtifl+ ——1 1+ —1 2 2jco C~C Chfl [)

For example, using the values previously acquired, for a switching frequency of 5 kHz, the zero sequence current is calculated as follows: uo= U&/2 = 650/2= 325 V o.)= 2nf,w= 1000OZ l&=17mH Lb,~= 1.3 ti C&= 30 pF &~= 90 pF

Substituting the above values into Equation 4.1 gives a value of io of 2.5 mA.

The results acquired for the zero sequence current, botlh using a DC link voltage of 650 V, and for both the single and three-phase cases, are shcwn in Table 4.1.

Switching frequency, f,~ / kHz Zero sequence current, io/ rnA 5 2.5 10 4.9

Table 4.1: Zero sequence current results for both the single-phase and three-phase scenarios

By plotting the zero sequence current as a function c)f the frequency it is possible to confirm that these values are correct. Figure 4.1 shows such plots.

27 ..

...... /

......

. . ...

1.4 a) .-

Figure 4.1: Plots of the zero sequence current as a function of frequency - for switching frequencies of a) 5 kHz and b) 10 kHz

28 The peaks that are evident in the plots are due to there being no damping in the system. The relationship between the frequency and the zero sequence current can be considered as a continuous line.

The overall load is capacitive. This means that the current magnitude is most significantly determined by the capacitance, and the capacitive current leads the voltage by 90°.

The earth protector that will be incorporated into the battery charger will break the circuit, and consequently the charger will cease operating, when there is a current of 30 mA. However, there was uncertainty as to what frequencies this applies. As a result, it was suggested that experimentation be undertaken in order to distinguish this. Figure 4.2 shows the layout of the apparatus for the experiment.

Figure 4.2: Diagram showing equipment used for experiment

Figure 4.3 shows photos of the equipment used to unde@ce this experiment.

29 a)

Figure 4.3: Photo of apparatus used in experiment – a) whole circuit including oscilloscope, b) just the electronic circuitry and c) the earth protector

Unfortunately, the voltage required in order to operate the earth protector is f= greater than that supplied by the mains supply. As a result, the voltage needed to be increased using a transformer. This was beyond the scope of the project and is therefore an area for fbrther investigation. The process that would have been used is detailed below.

For various frequencies, the voltage at which the earth protector switches would be taken from the oscilloscope. Using Ohm’s Law, the current can simply be calculated, as follows:

V=IR

30 where V is the voltage, I is the current and R is the resistance.

Hence,

Since the resistance being used is 100 Q, for this case, IJ- 100

The department will undertake firther analysis, if it is considered necessary, at a later date. However, it was thought that the zero sequence current would not actually present a problem.

31 5. Implementation of Inductances

In order to build the charger itsel~ firther time would be required. However, this task is more time-consuming, rather than challenging. As a result there is little to be gained by carrying out the construction of the inductors.

The process of acquiring the values required gave the author an understanding of power electronics and although it is unfortunate that construction could not be undertaken, this would only serve as practical experience. The department has technicians whose responsibility is to undertake such tasks.

Hi3kan Skarrie is a Ph.D. Student at the IEA. He is engaged in a project making inductors and transformers with iron powder or fibre cores. He required some values of inductors to test his research and it was decided that the results from this project would be ideal for this purpose. Therefore, he was given the aforementicmed values and the responsibility of implementation of the inductances was assigned to him, so that the theory fkom his project could be applied to a real application.

32 6. Conclusions and Recommendations

The onboard battery charger that has been designed for the Electric Sniper is such that it not only fblfils the brief, but it is also unique in its design, having been designed with this application solely in mind. This means that the design was carefully considered and all criteria have been optimised for the charger. For example, the type of inductors used, which strongly affect the overall weight, has been vigilantly considered.

This onboard charger also uses the three-phase, in addition to the single-phase, AC supply. This makes it fairly exclusive, in that most on’board battery chargers use simply the single-phase AC supply. Using the three-phase supply and fhst-charging methodology means that it is superior to the older, conventional method, since, as clearly indicated by it’s name, charging occurs at a greater s,peed, and range is therefore also increased (for the same length of charge).

Also, onboard chargers, when used at night, can actually help by reducing the impact on the grid. Also, charging at this time may cost less.

Although primarily designed for use with the Electric Sniper, it is very feasible to use this design for charging the batteries of other EVS. The unique features that are characteristic in this charger mean that it can be considered as a strong competitor in the market for battery chargers for use with EVS.

There is fi,uther work that would ideally have been camied out, if more time had been available. Such improvements would generally be a case of investigating in greater detail the processes involved in designing the battery charger. These would include experimenting fhrther with the earth protector, described in Chapter 4.

It is unfortunate that the time restrictions dictated that building of the components, and the charger on the whole, could not be undertaken by the author. It was recognised that it would be impossible to undertake all areas of the charger design and construction, particularly since the author had to acquire a large degree of understanding of power electronics - which took a large proportion of the project duration - before any design could take place. Even with the necessary knowledge and experience in power electronics it would have been difilcult to complete all the tasks in the given time. Although not critical to the finished product, but more a matter of increased practical input for the author, it would have been nice to have built the electrical components, and battery charger itself.

Non-availability of the necessary transformer at the time of experimentation meant that a full analysis of the zero sequence current could not be undertaken. This is of course an area that could be investigated if more time was available. If considered beneficial, such experimentation will be carried out by members of slaff at the department. However

33 such analysis is likely to prove insignificant, since the results seem to suggest that the zero sequence current will not be an area of concern in the design of the charger.

Overall, the author found the project stimulating, rewarding and educational and hopes that his work proved usefbl to the project team.

34 7. References

1. M. Bojrup, Advanced Control of Active Filters in a Battery Charger Application, Licentiate’s Thesis, Department of Industrial Electrical Engineering and Automation, Lund Institute of Technology, Nov, 1999, P. 7-18

2. P. Karlsson, Quasi Resonant DC Link Converters, Analysis and Design for a Battery Charger Application, Licentiate’s Thesis, Department of Industrial Electrical Engineering and Automation, Lund Institute of Technology, 1999

3. Department of Electrical Engineering and Automation, Lund Institute of Technology, Elmaskinsystem, 2000

4. M. Bojrup, P. Karlsson, B. Simonsson and M. Alakula, A Dual Purpose Battery Charger for Electric Vehicles, PESC 98 Conference Proceedings, Fukona, Japan, Vol. 1, May 1998, pp. 565-570

5. N. Mohan, T. M. Underland and W. P. Robbins, Power Electronics; Converters, Applications and Design, 2ndEdition, Wiley, New York USA 1995

6. D.I. Crecraft, Gorharn and Sparkes, Electronics, Chapman and Hall, 1993 Appendix 1

The Electric Sniper

●“ ProjektEl Page 1of 2

The Electric Sniper

Pa sv enska. tac k!

Since autumn 1999 ~ is involved in a project concerning the construction of an electric sports car. The car is a kit car externally designed by Juha Huhtilainen from Finland and it is called.“the Sniper”. It is manufactured in Eslov, Scania, Sweden. Originally this car is supposed to hold an engine from a regular Saab 99900. We however, supply it with an electric drive system developed by ABBespeciallyforelectricalvehicles.Hereis somepreliminarytechnical informationabout “theSniper”:

Table: Preliminary data

Motor permanent magnet synchronous motor (PMSM) t On board charger k~ Batteries =~ Length E~ Wdth =~ llHeight 11735mm II 1124kg excluding driver and passenger 1!Weight IL II Acceleration O.. 100 Icrn/b w~ Top Speed .w~

Important System Components

file: //C:’\WINDOWSWesktopl,Simon’Projekt E1.htm 13/09100 ProjektEl Page 2 of 2

Component ‘=~ Chassisand body =~ DC/DC-converter NIR4 ComponentsAB IntegralDriveSystem ABBCorporateResearch GenesisSealedLeadAcidBatteries =~ Onboardcharger,systemdevelopmen~batterymanagement system,etc =~ ToyoProxes@low-rollmg-resistanceties c~

The creationof the caris a cooperationbetween

. Erik Holmqvist,Eslov, Sweden . ABB Corporate Research . NIRA ComponentsAB ● IEA, LundUniversity

Thereare also studentsfromIEA involved in this project.

Our purpose is currently NOT to produce a series built car, but to:

. Create an attractive experimental platform for student and master thesis projects . Make PR within and outside Lund Institute of Technology . Participate in races, for example “&esundsrallyt 2000”, August the 25th-27th in the year 2000.

Are you yourself interested in participating in the E-sniper-project, or do you want more information? Please contact us!: mats.al~la@ieaml~.se or [email protected]~ose.

file://C:\WINDOWSU)esktop\SimonWrojektE1.htm 13/09/00 Appendi:x2

AC Propulsion tzero tzerohome Page 1of 1

Clneof the world’s fastest- accelerating cars is also one of the world’s most efficient cars

by AC Propulsion

O to 60 mph 4.1 seconds 1/4 mile 13.2 seconds Ra[

~rs- 200 horsepower 20 kW on-board charger 7 Powerful anti-slip regeneration

Performance Features FAQs tzero GalIesy How to get the tzero

Hk.m.eI what’s New IM-we horn presidentIQmz?.awHistw IpressReleases Productsand TechnologyItzero[In theNewsIWhitelx IEV In&structure EmploymentODportunitiesIContactUs ILinks

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Appendi:x3

Similar Prodlucts a) K & W Engineering BC-20 b) Brusa NLG41x

, K&WEngineering,Inc. Page 1of 1

—— —— BC-20

Specifications:

● Battery pack voltage (user selectable): 72 to 108 volts DC; 114 to 144 volts with optional LB-20. ● Charging current: Adjustable 0-20 amperes. O-25ampere front pa]rnelmeter. ● Float voltage: Adjustable 2.0 to 2.5 volts/cell. ● Input voltage: 110 to 125 VAC, 50/60 Hz ● Brownout/blackout protection: Protecte~ automatic resumption. ● Input/output connections: Screw terminals on barrier-style termin:alblock. ● Safety protection: 30 ampere AC circuit breaker. AC Ground Fault Interrupter. 30 Ampere DC output fuse. Overhe off, automatic resumption after unit cools down). ✎ Radio Frequency Interference (RFI) suppression: AC RFl faltering included. ● Siie: Height: 6 inches (15.2 cm) Width: 10.5 inches (26.7 cm) Depth: 4.25 inches (10.8 cm) ● Weighh 10 lbs (4.5 kg) ● Cooling Internal ball-bearing fan. ● Mounting Threaded inserts (1/4-20) back and bottom (four each surface). ● Environmental restrictions: Splash-resistant

..— Pages:lHmg/[BC-21V[;C-2501 DownIoad:[BC-250Batterv Charrer Instinction A4anuaIl~nstructionManual for BC-2o Batter-vCharger and LB-20 Line BoostezJ Dism”butors:[him:hww.innevat”on.s.comkhar~ers.htm~&@. .hww,kta-ev.co~ E-Mail: [kwencineerin_@varldn;t.att.n@l

13/09/00 :file://C:\WINDOWSU)esktop\SimonWC2O.HTM I Battery Charger NLG41 x -~

BruSa Elektmik Battery Ladegerat— 3.4 kW CH -9473 Gains Tel 0317603530 Fax 081750353$

NLG41X

Data / D*I Value 1V&i Cunditix / 6edingungw

Nan bakry vdage / Battedenennqx mung Ug,.>. 60 c 372 b see ordering &@?Wofr$ I Nom pmw / Nennleistung I1% 3.6 w I A4ax,mehs cwrwt/ max.Netzstror I!!(J?M Cwrenl phase angle/ Strwophasenwinkel Dstorfkm factor/ Klirrfaktor 403 1,1= 16/

0.92 AUG472 I 7,3kg I Afr-wokd wrehw /LMtqekUhl@ Version ~wlalicm dandert /Mu&art C@emfhr fernp. cmge / Betriebtemratur Rxfwed oufpuii Leistungsreduktbr :~ Watewu&edmWon 1UUasse~ekWta Vwsim In.wbtion sfancfm lSchutzart I //=64 Operdtin Imp, mge I BetrieWxnperatw T. -25”,, +60 “( Reduced cdputl Leistungsreduktbr T@ above W ‘C coolant&imp. [retake) I /

Appendi:x4

A Dual Purpose Battery Charger for Electric Vehicles A Dual Purpose Battery Charger for Electric Vehicles

M.Bojrup, P. Karlsson, M. Alaki,ila, B. Simonsson

Department of Industrial Electrical Engineering and Automation Lund Institute of Technology P.O. BOX 118 22100 Lund Sweden

ABSTRACT

A dual purpose, high power, off board battery ch~ger This paper introduces a new charger concept which for electric vehicles is presented. For higher viability addresseathe problems indicated. Central to our design and increasedusage of the charger, grid conditioning is a dual functionality which implies that the unit has capabilities are included in the concept. Conventional an efficient charging capability as well as a power line power electronicconverter topologies are used in a new conditioning role, refer to Figure 1. The latter is benefi- application, which combines fast EV battery charging cial to the grid as power quaMy is improved. The mar- with active filtering. Furthermore hi-directional power gin between the rated power and the actual charging flow capabilitiesare provided to accommodate grid peak power can be used for power line conditioning, thus power requirements. Both simulation and experimental operating the line converter at ratedpower and mlucing results are presented in this paper which demonstrate the the amount of grid conditioning dependenton the charg- capabilitiesof the new charger. ing power. The approach suggested here gives better utilisation of both the power grid and the battery 1. INTRODUCTION charger, which is beneficial to power distributors ad EV users. Electric vehicles will become an attractive alternative to internal combustion engine vehicles in the event that Given the above it is apparent that the dual pwpose their range can be extended.One way to achieve this in battery charger should have the following capabilities: the short term is to provide a fast charger infrastructure.

Such a structure would provide greater mobility for the ● Fast charging of EV batteries

EV user, since during short stops (<1 hour) the EV ● Grid conditioning capabilities batteriescould be charged from typically 20 to 80 % of ● Harmonic filtering of currents nominal charge. This would significantly extend the EV ● Reactive power compensation range. ● Load balancing

● Peak power generation There are mainly two reasons, why a fast charger infrastructure has not been built. Firstly, the cost of charg- An added controvemialcapabilityincludedhere relatesto ers and corresponding controllers is high. Secondly, peak power generation which implies that the charger chargers could adversely affect the grid power quality. should be albleto act as an “electronic gasturbine”, i.e. The reason for this is that such an increase of power defiverpower to the utility grid during periods of high electronic loads (mainly dicde rectifiem and thyristor power demi~d or at emergency situations. This can be bridge converters) in the distribution network, could done since EV batteries attachedto chargers representa result in voltage distortion and current harmonics. Poor large potential energy reserve, during a short period of power quality is aheady a problem in many countries time energy could be borrowed from the EV batteries in [1]. order to support the grid. This is similar to [2], where a superconductingmagnetic energy storage device is used to balance the fluctuating load from a large saw mill.

2. HARDWARE CONFIGURATION

+ The general specifications of the dual purpose battery Voad charger [3] are listed in Table 1. In [3], further back- ground infcmnation about the concept is also given. Figure 1. Grid and battery charger Table 1. General specifications of the dual purpose for frequenciesabove the resonancefrequencyconqxued battery charger to the 20 dB/decadefor the L-filter. Second, the total Ratedpower Sn 75 kVA inductance(ofthe LCL-filter is smaller than for the L Grid voltage v“ 400V filter, this can reduce the physical size of the filter ad Gridfrequency fn 50 Hz simplifies the design and manufactureof the inductors. EMC, cable IEC 1000-2-4 Third, since the total reactance of the LCL-filter is less at lower frequencies, the DC link voltage can be de Battery voltage ‘bat, 96 to 600 V cnased since the reactive voltage drop in the passive Chargingcurrent 1~,,, %200A UP to75 kW filter is smaller for the same current and frequency for

Current ripple AIb,,, Illm. lo~o Oflb,U the LCL-fil.ter as the voltage drop is for the L-filter. These benefits can be concluckl from the brxieplot of Grid conditioning S,~ Limited by the margin the transfer function from converter output voltage to between ratedpower and line current, see Figure 3. actual charging power

Though this is a prototype pre-commercird fast charger, the Swedish and European EV market and ongoing .> .,... . standardisationhas been considered. This means that the ...... >. . .-%.... chargeris designedfor conductive charging with the 11- ...... pin Marechal connector system. This system is deemed ...... to be standard on electric vehicles from French car :...... manufacturers...... 100 [r-b] 1000 lm The dual puqmse battery charger is designed with conventional power electronic converter topologies. The Figure 3. Transfer function from converter output desiredgrid conditioning capabilities in terms of peak vol[age to line current (LCLtilter solid, powergenerationand harmonic filtering calls for a self Lfilter dashed) commutated converter topology with bi-dhectional power flow, This leads to a block schematic representa- The major disadvantages with a LCLfilter are the high tion of the charger as indicated in Figure 2. current ripple in LI (the inductors nearest to the converters) causedby the PWM and the increased number of transducersneeded for controlpurposes. Furthermore, the reactivepower requirementsof the capacitos present in the passive line filter has to be accommodated by the convetter as to prohibit reactive grid current when the unit is operating under idle conditions.

The inductcmare fabricated in tape wound C-cores with Figure 2. The dual purpose battery charger iron tape thickness of 0.1 mm and the windings are separatedalongthe airgap to reduce airgap losses in the It is evident from Figure 2, the dual purpose battery windingsdue to fringing flux (especially important for charger consists of Pulse Width Modulated (PWM) the imer inductomdue to the high switching fkquency Voltage Source Convefiers (WCS) based on IGBTs for current ripple). Polypropylene capacitom of low ESR high performanceand passive filters for proper attenua- type mainly intended for DC applicationsare used. tion of current harmonics caused by the PWM of the converters. The three phase PWM VSC is well suited The control algorithms are implemented on a Multi to its role as line side converter, since the controllabil- Input Multi Output DSP cad, based on the ity of the currents is very high. On the battery side, a TMS320C30 floating point processor. The DSP catd two quadrantDC-DC (Buck-Boost) converter is used contains a serial communication interface (RS422). which fulfils the requirements of bidirectional charg- This interface is used for control of the dual purpose ing. battery charger by a supervisory controller unit. ‘Ihe supervisory controller unit contains a payment system, Third order passive filters (LCL-filters) are chosen to serial communication with the EV Battery Management attenuate undesired current harmonics caused by the System (BMS) for battery current reference and the converters [4]. There are three major benefits with a possibility for a power distributor operator to remotely LCL-filter compared to the first oder filter (L-filter). (via a modem) set the conditioning priorities or getthe First, higher attenuation at high frequencies since the operating data of the dual purposebattery charger. LCL-filter increases the attenuation with 60 dB/ckcade The power electronics of the dual purpose battery ance or harmonic currents. The stationary currents, ir~~, chargerconsists IGBT modules. The DC link voltage, in the d- and q-axis provides information on the reactive U~C,is chosen to 750 V. A 5 kHz triangular carder and active current components of the load. Negative wave modulatesthe converters which leads to a switch- sequencecurrents, i~, appear at twice the fundamental ing frequency, f,W,of 5 kHz, The controllers are sampled grid frequency in the dq-frame,since the negative se- with a time interval, T,, of 100 ps. quencevector rotates backwards with fundamental grid frequencyin the stationary c$-frame. The current har- The charger construction is in the format of an ahunin- monics, i~, are visible at even higher multiples of the ium cabinet in which the internal sections (line filter, fundamentalflequencyin the dq-frame. By appropriate power electronics, battery filter and control electronics) filtering the reactive, negative sequence and harmonic are totally shielded from each other. This constructional currents can.be identified from the load current and thus approach minimises EMC problems and promotes a be fomvardedto the input of the Ioad current compensa- modular implementation of the major component tors given in Figure 5. groups, which in turn simplifies maintenance. ~ 3. CONTROL Lowpass fund r—-----!I filter ] Activefi~er +. In this section the controllers are presented. The overall Ifoad Ban(jpaSqIn Lead-Lag: laf + ~.current control structure contains a DC link voltage controller, filter 7“C21OOHZ limitation a battery current controller and a line current controller $ +“ in the synchronous dq-fmme. For grid conditioning Highpass Ih ‘Mfi regis’te~ & fiHer 3 sample purposes, a load current identifier is neededto extract %: prediction “L “ the current components to be conditioned, Figure 4 shows the block diagram of the overall control system. Figure 5, Load current identifierand compensator

. i? Uc i,l Ubatt The response time from line current dq-fmm references to line currents is approximately three samples, calcr.da- tion time in the DSP takes one sample and the delay through the LCL-filter is approximately two samples for frequencies which are of interest for conditioning. ‘:~ This results in a phase differencebetween the injected +* Load current Iaf currents compmd to the desiredcurrent references. To identifier and +-++= avoid this cffeet a phase compensation scheme of the compensator 12 Uc 11 ! I grid conditioning current referencesis introduced. TIE integral pantof the d-current controller takes care of the Figure 4. Block diagram of the overall control system reactive cunent, since it is representedas a non-varying valuein the dq-frame. For the negativesequence cunen~ The DC link voltage controller indicated in Figure 4 is a simple first order lead lag filter acting on the ~ference an anti-windup PI-controller with f&edfor-wad of the is sufilcient, since only one iiequency is of interest. battery current as to achieve a fast line cument response. The phase compensator for the current harmonics util- The fked fomard term decxeasesthe DC link voltage ises the fact that the harmonics are periodic, i.e. what variation encountenxi during changes in the charging happens during a given pericd will approximately hap- current.The bandwidth of the DC link voltage control- pen in the following period. By shifting the harmonic ler is limited by a lowpass filter which in effect decou- current referenceinto a shift register with a length ox- ples the latter for the active filter control function. me responding to the lowest harmonic frequencyof interest grid conditioning current references are derived from a and then reading a value from the previous pericd thee load cumentidentifier, which extracts the reactive, har- samples ahead an adequatethree sample prediction of monic and imbalance (negative sequence) components the harmonic currents is achieved as may be obserwxi from the load current. These conditioning components, from Figure 5. Also shown in this figure is the active i,~, are addd to the DC link voltage controller output, filter current limitation. This module ensures that the i2~,and acts as the line current EfeEn@ vector, iz, as line side converter is not overloaded.The latter is real- may be observed from Figure 4. ised by en:wing that the final current refemces ate limited to fit inside the rated current vector length. The The grid conditioning current references are identified current limitation is determined by the conditioning fmm the load current in the synchronous dq-tlame. An priorities, the output ratios set by an operator and the advantageof using the synchronous dq-frame in the amount of capacity that is left over for conditioning identifieris that everything that is not stationary non- purposes. varyingcan be attributed to either load changes, imbal- The line current controllers are based on the vector The block diagram representation of the line current equationsin the synchronous reference dq-frame,as this controllers in the dq-frarne, including the Smith com- simplifies the derivation and the implementation of the pensator, is shown in Figure 6. controller stnwture. The differential equations of the passive line filter in the synchronous dq-flarnemay be written as

C.-$iic =-jCOC.iic+7~-~ (1)

L2. ~%=–(R2+jcoL2)” &+iiC-Z (

Cascadedline current controllers with anti-windup of the integral parts and feed forwtud of the grid voltage, are derivedfrom the discretizedform of eq. (1) as indi- Figure 6. Block diagram of the line current controllers cated in eq. (2). Usually, deadbeat controllers are pre- fened for high system bandwidth, but in the case of a The battery current controller is based on the differential third order filter some careful considerations with ~gatd equationsof the third order passivefilter to the controller gains has to be done in order to avoid an oscillatory or unstable system. In any event, the L1. ZZLd ‘ =–R1. il–uc+ul bandwidth is more or less limited by the resonance frequencyof the third order filter. These considemtions C .-$uc=il–iz (4) dictateathe values scale factors, k, from the dead beat L2. $2 = –R2 .iz + Uc – uba~ gain for each controller in the cascade structure. The discrete line current controller equations are given below In a similar way as for the line current controllers, a cawadedbarte~ current controller with anti-windup of the integral part in the inner loop and feed forward of the battery voltageis derived by discretizationof eq. (4). (2) Again deadbeat controllers for high system bandwidth ate desimt. The gain in the proportional parts of the controllerincludes scale factors, k, of the dead beat gain for a less oscillatory system. The discrete battery current controllerequations are representedas

L u~=k. ~.(i~–i2)+R2.12+ubotf.* RI 12 T’ f=q i; =kuc &.(u:_ucT )+; (-3 kil =0.5 (3) s km =0.2 u~=kil~.‘1 (i~–il)+~.~(i~–il) +4 ( ) ki2 =1 s where To compensate for the dead time introduced by the DSP, we have chosen to use Smith compensators for RI systems with delays as suggested in [5]. The Smith ‘“q compensator estimates what has happened during the kil =0.5 last sample with the previous control signal set. The (6) predictedresult is then forwarded to the measured signal kuC=O for a gcod estimation of the actual value, which the ki2 =1 controller acts upon. The Smith compensator has the advantageof coping with overmodulation of the converter compacedwith the dead time compensation used Sufficient performanceof the system is achieved within [4]. This stabilises the system so that higher gains out a P-controller on the capacitor voltage (k@) ad can be chosen in the current controllers. thus it is left out in the controller, thus reducing tie number of measured signals with one. Only feed forward of the capacitor voltage refemce is implemented as may be observed from eq (5). The battery cmrent The charging current error is due to the large time con- controller in block diagram representation, including the stant, z, of the integral part in the battery curnmt con- Smith compensator, is given in Figure 7. troller, The ringing of the battery current directly at% the load is applied is due to the resonance of the third T5 I LI order filter, the amplitude can be deaeased by limiting z-(l+R1*Ts / LI ) the slope of the step change in the battery current.

i2* Figure 9 shclwsthe load current spectrum together with , kil(:+~) the resulting line current spectrum for these two modes + R2 of operation. In both cases the harmonic content of the T~ ]J ‘Tx line current issignificantly reduced, i2 -@4 Iqz 9 TS

160 .: ...:.~....~.- {Ioad .1. . Figure 7. The cascadedbattery current controller T t lline~(l) ~ ...... - {MS (2) .~.. . . . 4. SIMULATION i: . . I.

Simulations are used to verify the function and the capabilities of the dual purpose battery charger. Figure 8 and 9 shows the results of a simulation, The load is ,. . . , I . . modelled as an ideal diode rectifier of 100 kW in parallel “ 1 57 11 13 17 19 with an inductive load of 37.5 kVAr, The charger is Harmonic order operated in two different modes. Firstly, the charger is IFigure9. Phase current spectrum set to idle in which case all the capacity of the unit is used for grid conditioning (case 1), After 50 ms, charg- 5. EXPERIMENTAL RESULTS ing of a 190 V battery with 200 A is started (case 2) and only harmonic compensation of the grid is per- Measurementson the dual purpose battery charger have formed as not to overload the line convener. been carried out at two different locations and dudng diffenmt operating conditions. At the fimt location, tests were made on a battery package used for a hybrid vehicle systtm, were charging/dischargingof the battery package at rated power and current has verified the charging characteristics. As expected the current harmonics due to PWM rwe properly attenuated by the

I I passive filters, both on the line and on the battery side. I ...... 1 The idle productionof reactivecurrent (=4 kVAr) in the capacitors of the passive filter is consumed by the converter, i.e. no reactive current of the unit end up in the grid.

In connection to the tests of the charging characteristics, power conditioning of a six-pulse thyristor bridge converter were performed. The results are shown in Figures 10 and 11.

35 . : ...... {...... ;....;.. ,, . . q) . “..,.....:...,;...... ;...,:,. . . ..loa~ ...... 11 1 ...... — {line:. .I .4 ... .,.. .:...... : . .-...... JCOrnP...;...... ’. . .:, ...... ;, ,., ......

o 10 20 30 40 50 60 70 80 W . . . . Time [ins] lid K1 !4 J Figure 8. Simulated currents in the time-domain, 1 57 11 13 17 19 case 1) 0..50 ms and case 2) 50..100 ms Hmmonic order Figure 10.Phase current spectrum - ...... ,, ...... 325 - . . - Ilead : ...... - {line...... -...... Icornp...... :...... , ...... ;. ...:...... : ...... r . . . ,...... :. .,,...... , .,., ...... /...... [~~ ~--: . . 1 57 11 13 17 19 . . . . . HarmoNc order I I . ,. . . . Figure 13. Phase current spectrum

6. CONCLUSIONS

A new dual purpose battery chargerprototype has been designd implemented and tested. Simulation results and field measurements confirm that fast charging ad 0 5 10 15 20 25 30 35 grid conditioning capabilities can be successfully Time [ins] merged which leads to a viable and high performance Figure 11. Phase currents in the time-domain unit that is beneficial to power distributors and EV users. Third. order passive fiIters are successfully im- The second test was made at a district heating plant, plemented to attenuate current harmonics due to the whe~ the current harmonics from a large water pump PWM of the converters instead of the more common drive were filtered. The test should give a further verifi- first order filter. cation of the power conditioning capabilities of the dual purpose battery charger, since the harmonics generated 7. REFERENCES by the load w higher than the rating of the unit. Fig- ures 12 and 13 shows the results of grid conditioning, [1] H. Akagi, “New Trends in Active Filters for Power when all the capacity of the dual purpose battery charger Conditioning”, IEEE Trans. on Industry Applica- was used for harmonic compensation. tions, vol. 32, No 6, Dec. 1996, pp 1312-1322. [2] 0, Simon, H. Spaeth, K.P. Juengst, P. Komarek, “Expen.mentalSetup of a Shunt Active Filter Using a SuperconductingMagneticEnergy Storage Device”, Proceedings EPE, Trondheim 1997, vol. 1, pp 1447-1452. [3] P. Btickstrom,“The DUAL concept. A way of making EV fast charging more viable.”, Proceed- ings EVS14. [4] M. Lindgren, “Filtering and Control of a Grid- connectedVoltage Source Converted’,Technical Repmt No. 208 L, Chahners University of Tech- nology, Goteborg, Sweden, 1995. [5] J.W, Lee, “An Intelligent Current Controller Using Delay Compensation for PWM Converters”, Pro- ceedings EPE, Trondheim 1997,vol. 1, pp 1342- 1346.

Figure 12. Phase currents in the time-domain Appendix 5

Simulink Mc)dels a) Three-phase case b) Three-phase case, with battery side c) Single-phase case, with battery side I I + Mux ~

~w’kspa” _: Scopel

(.s...-.d,--,.. , II,, ,,, -.. —. uStep T=J--Lm r .“”.. . + .

+ ~ + J?#i?l 111- ++--j/l>: c Stepl z:; e+j 1 c Cz To Workspace is_xly - controller 3 phase 2 level modulator abc 9> vector vector E)> abc Symmetrize

[

, +“’~1. I I -----+- MATLAB ps~ Function e-j r “--p MATLAB Fcn 3-phase grid n

m Rs=O.I ; > Ls=O.01 ; Clock2 )) Ts=O.0001 ; Scope3 b ~ Psim=400/1 00/pi; Scope2 To Workspace I Cl=kl

AI 11+1=1----l II L!?h--5- l--w To Workipace2 u. k- li7’vsaL’l.& “1’L==r “. t L.—.—J i___ Modulator Invarterl

‘ h u’ 1’

I c1n

1 I I # I-8X” Usx” ➤ F?q ~ l_sx Ua, Sa - 1, p Utr sb - * l-s~ k :~; + E UC. Sc usy- r, :1 + I-sy F Udc Tri - y ;dc w 1 ; e+j 1 I 1 Vc --l C10Ck3 is_xty - wntrder 3 phase 2 level modulator atw vector vector D> atx 1 I 3 phase 2 level inverter Symmetrize To Workspace3

— — psl u

.s-, I 1 s_bstai ,.. T, .0 c... —,, h 1 ,.,,-.,L7.”,W, --T==’= I e_alfa1 3-phase grid o 0=--E!l n b Effektl Udc3 43Udc2 c10 Effek12 Effekt4

P Effekt3 I r’I

I “U* I w Effekt5 I ++

a > T !-l n h x ●I*

19 Y

+11 a.=‘0 +- .-‘1

-7al D d?-

3I i-

—

--Q Appendix 6

IEC 1000-3-4 standard used for determining L~C -+,’ ..-

. ~,+:.-...,. > ., .-

,,.

./ 7,., ,,. , . .

●

.,. . . ,, -. “. . . . , -, *-. . ,.. ; . .,,, .’ “

. Appendi:~7

Data sheet giving Fourier Series

Appendi:~8

Data from Fourier series calculations h U(dc) 650 alpha 0.5 f(switch) 5000 f n a(n) o 0 #DIV/O! 500 0.1 647.3303 1000 0.2 639.3606 1500 0.3 626.2085 2000 0.4 608.068 2500 0.5 585,2056 3000 0.6 557.9559 3500 0.7 526.7158 4000 0.8 491.9374 4500 0.9 454.1203 5000 1 413.8029 5500 1.1 371.553 6000 1.2 327.9582 6500 1.3 283.6162 7000 1.4 239.124 7500 1.5 195.0685 8000 1.6 152.017 8500 1.7 110.5074 9000 1.8 71.04006 9500 1.9 34.07001 10000 2 2.53E-14 10500 2.1 -30.8253 11000 2.2 -58.1237 11500 2.3 -81.6794 12000 2.4 -101.345 !2500 2.5 -117.041 13000 2.6 -128.759 d3500 2.7 -136.556 14000 2.8 -140.554 14500 2.9 -140.934 15000 3 -137.934 15500 3.1 -131.841 16000 3.2 -122.984 16500 3.3 -111.728 17000 3.4 -98.4628 17500 3.5 -83.6008 18000 3.6 -67.5631 18500 3.7 -50.7737 19000 3.8 -33.6506 19500 3.9 -16.5982 20000 4 -2.5E-14 20500 4.1 15.78854 21000 4.2 30.44574 21500 4.3 43.68897 22000 4.4 55.27891 22500 4.5 65.02285 23000 4.6 72.77686 23500 4.7 78.44703 h U(dc) 650 alpha 0.5 f(switch) 5000 fl 50 mf 100

Ahat ExpectedAhat (from from fractionof f n h a(n) sqrt[a(n)A2] MatLab) initial amplitude 5000 1 100 413.802852 413.802852 413.5444 10000 2 200 2.53485E-14 2.53485E-14 15000 3 300 -137.934284 137.934284 137.7083 1/3 137.8481333 20000 4 400 -2.53485E-14 2.53485E-!4 25000 5 500 82.76057041 82.76057041 82.4647 1/5 82.70888 30000 6 600 2.53485E-14 2.53485E-14 35000 7 700 -59.11469315 59.11469315 58.7311 1/7 59.07777143 40000 8 800 -2.53485E-14 2.53485E-14 45000 9 900 45.97809467 45.97809467 45.5 1/9 45.94937778 50000 10 1000 2.53485E-14 2.53485E-14 55000 11 1100 -37.61844109 37.61844109 37.0444 1/11 37.59494545 fmnnn. . . . . q~ Iznn -2=53485E-14 2.53485E-14 65000 13 1300 31.83098862 31.83098862 31.16 1/13 31.81110769 70000 14 1400 2.53485E-14 2.53485E-14 75000 15 1500-27.5868568 27.5868568 26.8182 1/15 27.56962667 80000 16 1600 -2,53485E-14 2.53485E-14 85000 17 170024.34134424 24.34134424 23.475 1/17 24.32614118 90000 18 1800 2.53485E-14 2.53485E-14 95000 19 1900-21.77909748 21.77909748 20.815 1/19 21.76549474 100000 20 2000 -2.53485E-14 2.53485E-14 .,..,,