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High-Current, Low-Voltage Power Net Florian Bachheibl and Dieter Gerling Institute of Electrical Drives Universitaet der Bundeswehr Muenchen, D-85577 Neubiberg, Germany e-mail: [email protected] Abstract—The ISCAD drive is the first low-voltage electrical machine applicable to automotive traction applications. Supplying the drive with high current at low voltage is both a challenge and an opportunity. This paper investigates highcurrent, low-voltage power nets for electric vehicles and presents possible realizations. The focus lies on aspects of security, feasibility and cost-effectiveness of a power net with a primary bus voltage lower than 60V. Keywords—High-Current, Low-Voltage, Automotive Power Net, Safety Concept, ISCAD, Electromagnetic Fields, Electromagnetic Compatibility I. INTRODUCTION For several reasons, car manufacturers generally rely on high-voltage power nets to supply traction motors in BEV or HEV applications, using bus voltages of 400V and above. First of all, conventional drive topologies reach their maximum efficiency at voltages between 200 and 300V for low power demand and higher voltages for high power vehicles [1]. Secondly, it is easier to integrate a high-voltage system into the stationary electric grid, since there is only a small voltage-gap to be overcome by DC-DC-converters or by transformers. Thirdly, regardless of isolation problems, the effort for operating a high-voltage power net appears to be lower than the effort for the distribution of high currents. In the light of a new machine topology with a stator-cage-winding [2], the first argument for high-voltage power-nets has lost its validity. The stator cage machine first presented during a tutorial at E|DPC 2014 requires a DC supply voltage of 24V and a peak current of 15kA and is very promising in terms of overall drive-cycle efficiency and cost-effectiveness. This paper investigates several aspects linked to supplying a machine with the given electrical characteristics. In section II, feasibility of carrying currents of that magnitude is proven realistic and in section III a general safety concept for a high-current system is investigated. EMC is investigated in section IV and general operation parameters such as losses and voltage drop across the conductors are investigated in section V. Finally, a conclusion is drawn summarizing the findings presented in the sections mentioned and showing that a low-voltage power net is a viable alternative to the high-voltage state of the art. 978-1-4799-6075-0/14/$31.00 ©2014 IEEE II. FEASIBILITY STUDY A. Current-carrying capacity A low voltage system opens up the opportunity to use the car’s ground or the battery case as conductor of the main power bus. This allows economizing at least one cable, thus cutting down costs, weight and size of the system. In Tesla’s Model S sedan, the battery pack is protected against ballistic impacts by a 6.3mm thick “ballistic shield” of solid aluminum [4] which is 1550mm in width. This yields a cross-section area of 1007.5cm² - enough to carry currents up to 50kA at a current density as low as 5A/mm². If the same current density is assumed for one discrete conductor as shown in the middle of Fig. 1 and a current of 15kA has to be transmitted, a crosssection of 30cm² and a corresponding mass of 22kg would be sufficient. However, transmitting current inside the battery is only half the challenge and linking battery and machine can be performed in a straightforward way using a structure as shown in Fig. 2. The connection between the coaxial structure and the DC-bus rings of the machine may be made by using bond wires. This allows inserting a fusible component between machine and power net by designing the bond wires such that they disconnect when overcurrent occurs. This solution already exists in a similar way in the battery pack design by Tesla Motors for the connection of each individual battery cell [10]. Single Conductor, A=30cm² Tesla “Ballistic Shield“ Fig. 1. Possible geometry of battery and conductor Fig. 2. Motor-battery link B. Evaluation of HV and LV power nets The safety requirements to automotive power nets are defined in the regulation ECE R100 [6]. According to this standard, voltages beneath 30V AC and 60V DC are considered safe and no protection is needed. For high-voltage systems, on the other hand, several layers of protection are necessary: A service switch which disconnects the HV-battery needs to be present and accessible. HV-isolation must be monitored. When inactive, both poles of the HV-battery must be disconnected from the power net. All HV-components must be located out of reach of users without special tools. In addition to these safety requirements, it is technically necessary to realize a galvanic isolation of the battery management bus, introducing further complexity into the HV-system. Using Tesla’s Model S as an example, the cost of a HV-power-net is compared to the presented LV-solution in TABLE 1. It is worth noting that, although a battery disconnect switch is not required legally for a low-voltage system, it is necessary to foresee a mechanism which allows a controlled disconnection for several failures. This topic will be discussed in the following section. TABLE 1: Comparison of Costs Item Internal cabling of battery (unshielded) Cost HV 13.5€ External cabling (shielded) 3.6€ External cabling (unshielded) - Cost LV 170€ 85€ Main battery disconnect switch 112€ tbd. Isolation monitoring device 100€ - BMS optocouplers 46€ - Total 275€ 255€ III. POWER NET SAFETY CONCEPT The targeted voltage of 24V DC lies well beneath the limits set in DIN-VDE0100 (120V DC) [7] or in ECE-R100 (60V DC) [6] above which significant protection is needed. Therefore, one possible reaction to a short circuit in the power net may be the controlled depletion of all energy stored in the batteries, using the resistances occurring in a short circuit. However, when a battery chemistry is used that becomes unstable during deep discharge [8], this concept is no longer an option and circuit breakers or fuses are needed. Tesla’s solution of using bond wires to both connect and fuse the individual cells [10] is also applicable to a highly parallel battery pack. However, there are several fault scenarios which cause currents high enough to heat up parts of the car until ignition whilst not causing a battery current high enough to trigger the fuses. These scenarios require a solution to actively disconnect the battery which could be realized by a detonating fuse, e.g. an ABB Is-Limiter [12]. The operation principle of using pyrotechnical fuses is already in use in automotive applications [9]. Another way of actively disconnecting the battery pack from the power net could be to use low-voltage MOSFETswitches which can be realized in a very loss-optimized way for such low voltages [3]. The safety-concept of a low-voltage power net can be kept relatively simple when it comes to touch protection and other measures against harming humans by direct contact. The high currents involved, however, require a thorough analysis of emitted radiation which will be performed in the following section. IV. ELECTROMAGNETIC COMPATIBILITY The relevant law for EMC tolerances of vehicles in the European Union is the Commission Directive 2004/104/EC [11], which states maximum permissible amplitudes for highfrequency AC-emissions. There are no limitations concerning DC-emissions of magnetic fields in the aforementioned directive. Concerning biological safety, the International Commission on Non-Ionizing Radiation Protection (ICNIRP) issued Guidelines [13] which state an exposure limit of 400mT for the general public. However, when movement or presence of medical devices are involved, a much more restrictive limit of 50mT is suggested. A more general directive on EMC of the European Union (2004/108/EG), which is transferred to German Law in [15] defines a Flux density exposure limit of 500μT for the general public, which shall be the base for the following considerations. The setup displayed in Fig. 1 is an obvious solution as it does not require much modification of the existing Tesla battery and it allows for a simple connection between battery and machine. However, its EMCcharacteristics do not satisfy the requirements imposed by EUStandards. Fig. 3 shows the flux density distribution for the setup proposed in Fig. 1. Thereby all areas displayed in red face a flux density higher than 500μT, disqualifying the geometry as not compliant to flux density standards. An alternative solution relies completely on the protective shield of the battery by using both halves as conductors for the positive and the negative side of the DC-bus, respectively. However, this solution also radiates too high values of flux density into the environment, as shown in Fig. 4. Fig. 3. Distribution of flux density for one central conductor using the baseplate as second conductor Fig. 4. Distribution of flux density for a setup using both halves of the ballistic shield as conductors for the DC-bus The only realistic option is therefore to modify the baseplate, creating a sandwich-like structure which consists of at least three layers. In this way, one conductor is encapsulated by the other conductor, creating a shielding effect for the whole DCpower net. As Figures 5 and 6 show, a complete encapsulation of the inner conductor, including the sides is both more complicated to manufacture and less effective in shielding as there remains a high value of stray flux at the edges. Fig. 5. Distribution of flux density for a sandwich-like setup and complete encapsulation of the second conductor Fig. 6. Distribution of flux density for a sandwich-like setup without encapsulation of the front edges Fig. 7. Distribution of flux density for a sandwich-like setup with shielding of the sides In Fig. 6, the region of high flux density is very limited and can be further reduced by either increasing the number of layers, by choosing a more complex structure of the edge or by using magnetic steel to shield the edge by short circuiting the magnetic flux path, as shown in Fig. 7. However, the easier solution of Fig. 6 would already comply with standards, as the region of high flux density cannot be reached by humans during operation. At the current state of development of the system, it is not possible to calculate conducted EMI of the inverter and therefore to include it into the EMC-Concept. Yet, another source of high-frequency emission is the vehicle passing a stationary measurement point at high speeds. To model this effect, the car is assumed to pass a measurement point in a distance of 0.2m at a speed of 200km/h. A constant flux density of 0.337μT, which has been calculated by FEM, is assumed to be constant along the battery length with a decay rate of 1/r³ at both ends, yielding a Flux density waveform as displayed in Fig. 8. The chosen decay rate is a general value for magnetic fields [5],[16]. The resulting spectrum is plotted in Fig. 9 against the human exposure limits proposed by ICNIRP [14]. Although a very conservative approach has been chosen, the results show clearly that the emitted radiation is marginal. Conductive Shield Battery Pack Flux density [T] decay ~1/r³ Constant Bmax decay ~1/r³ M V=200 km/h Fig. 8. Model used to calculate high-frequency spectrum lbatt x Length of battery pack Fig. 10. Schematic drawing of battery, protective shield and electrical machine Assuming that the current passing through the cross-section of the conductive shield is: x I(x) I0 1 lbatt (1) With I0 being the DC-link current of the machine, the resulting voltage drop across the length-axis x is: U lbatt lbatt I x A dx I 0 Fig. 9. Human exposure limits and calculated spectrum V. LOSSES IN THE POWER NET The very reason for using high-voltage systems despite the dangers involved lies in the fact that power can be transmitted with lower currents, therefore causing less Joule-Loss. The purpose of this section is to show that the degrees of freedom obtained by using a safe voltage level can easily be used to reduce the Ohmic Resistance so drastically that Joule-Loss is of no more concern. As already mentioned above, the protective shielding at the bottom of the traction battery can be used as conductor for the currents inside the battery. A possible way to save material could be thinning out the profile towards the end of the battery in order to maintain the same current density all over the battery. This concept however has several disadvantages: The voltage drop remains constant over the whole length of the battery. A minimum thickness of 6.3 mm is required anyway to achieve the shielding effect. Thinking of cars propelled by more than one motor, this concept becomes obsolete. 0 l batt A 2 (2) In a similar manner, power loss in the battery pack can be calculated: P lbatt lbatt Ix 0 2 lbatt dx I02 A A 3 (3) From equation ( 3 ) it becomes obvious that the effective resistance of the conductors inside the battery is: R eff lbatt A 3 (4) The length and width of the battery pack are chosen to be equal to Tesla Model S’ battery pack. It has a width of 1556mm and a total length of 2730mm, leaving the thickness of the conductive shield plate as only variable. In order to assess the necessary values for the thickness, a drive-cycle-based analysis was performed. The drive cycles chosen for this task are the ARTEMIS Road, NEDC 2000 [17] and WLTP Class 3 [18], whose speed vs. time-characteristics can be seen in Fig. 11. Therefore, the cross-section of the baseplate remains constant along the length axis of the battery for the following considerations. The battery is assumed to be so highly parallel that the error made by distributing the current supply continuously along the length is negligible. Fig. 11. Speed vs. time for widespread driving cycles A good approximation for the calculation of required power from the drivetrain can be made by focusing on acceleration power and aerodynamic loss. According to [19], the aerodynamic drag force can be calculated by: FW c W A x Air 2 vF 2 (5) PW c W A x Air 3 vF 2 (6) The values of Table 2 are inserted into ( 3 ) in order to obtain the average losses per drive cycle for different values of thickness, as shown in Fig. 13. The vertical line drawn at a thickness of 6.3mm marks the already available thickness of Tesla’s ballistic shield. Notably, a thickness up to this point does not have any effect on the battery cost if compared to the actually existing vehicle. and therefore: The instantaneous power of acceleration at a given speed can be obtained by Pa m a v (7) The aerodynamic parameters of the Model S are known to be 0.24 for cw and 2.34m² for the projected surface [20]. The curb weight amounts to 2108kg [21], allowing to calculate the required power to follow each of the driving cycles and the current needed to supply that power at a voltage of 24V, as shown in Fig. 12. Fig. 13. Loss in conductive plate for various driving conditions In addition to the losses, it is very important to guarantee a very small voltage drop across the conductor since it is not only used to transfer current to the machine but also to connect the cells in parallel. Equation ( 2 ) is used to calculate the values displayed in Fig. 14. Fig. 12. Cycle-based current consumption The current waveforms of Fig. 12 are simplified by calculating their effective values according to: 1T Ieff I 2dt T 0 (8) which yields the values of TABLE 2 for Ieff. TABLE 2: Effective currents for different driving cycles Driving Cycle Effective Current at U=24V ARTEMIS Road 724.6A NEDC 2000 389.3A WLTP 548.0A Peak Power (360 kW) 15.0kA Nominal Power (70 kW) 2.91kA Fig. 14. Voltage drop in bottom plate VI. CONCLUSION A power net concept for a low-voltage, high-current drive system has been proposed. Feasibility of such a system was analyzed and validated considering cost, material expense, safety and EMC. It is worth noting that the cost projection of Table 1 would be much more in favor of a low voltage system if a sandwich-structure for the conductors had been used and the conductor inside the battery had not been necessary. Further analyses need to be done with respect to a solution for actively disconnecting the battery in an emergency case. VII. REFERENCES [1] R. De Doncker and D. U. Sauer, “Annual Report 2011”, Institut für Stromrichtertechnik und Elektrische Antriebe (ISEA), Aachen, 2011. [2] G. Dajaku and D. Gerling, “Low Costs and High Efficiency Asynchronous Machine with Stator Cage Winding”, IEEE International Electric Vehicle Conference, Florence, 2014, to be published. [3] A. Patzak and D. Gerling, “Design of a Multi-Phase Inverter for Low Voltage High Power Electric Vehicles”, IEEE International Electric Vehicle Conference, Florence, 2014, to be published. [4] P.D. Rawlinson, “Vehicle Battery Pack Ballistic Shield”, U.S. Patent 020120312615A1, August 13, 2012. [5] S. Brandt and H.D. Dahmen, Elektrodynamik, 4th ed. Heidelberg: Springer, 2005. [6] United Nations, “Agreement – Concerning the Adoption of Uniform Technical Prescriptions for Wheeled Vehicles”, Addendum 99, Regulation No. 100, 1995. [7] K-H Krefter and H. Schmolke, DIN VDE 0100, 3rd ed. Berlin: VDE-Verlag, 2012. [8] S. Levy and P. Bro, Battery Hazards and Accident Prevention, New York: Plenum Press, 2004. [9] W. Hentschel, “Fuse for a Motor Vehicle Power Line”, U.S. Patent 20130009745A1, January 10, 2013. [10] J. Straubel et. al., “System and method for fusibly linking batteries”, U.S. Patent 20070188147A1, August 16, 2007. [11] Commission Directive 2004/104/EC, Official Journal of the European Union Series L, 2004. [12] ABB Product Sheet. (2013, April 11). Is-limiter – The world fastest limiting and switching device [Online]. Available : http://www05.abb.com/global/scot/scot235.nsf/veritydisplay/1e8df815dc e50e6bc1257b4a0048e083/$file/2493%20Is-Limiter%20GB.pdf [13] “ICNIRP Guidelines on limits of exposure to static magnetic fields”, Health Physics, 96(4):504-514, 2009. [14] “ICNIRP Guidelines for limiting exposure to time-varying electric and magnetic fields (1Hz-100kHz)”, Health Physics 99(6):818-836, 2010. [15] Bundesamt für Strahlenschutz, “26. Verordnung zur Durchführung des Bundes-Immissionsschutzgesetzes (Verordnung über elektromagnetische Felder – 26. BlmSchV)”, 2013. [16] K. Küpfmüller et al., Theoretische Elektrotechnik. Berlin Heidelberg: Springer, 2013. [17] T.J. Barlow et al., “A reference book of driving cycles for use in the measurement of road vehicle emissions”, TRL Limited, 2009. [18] M. Tutuianu et. al., “Development of a World-wide Worldwide harmonized Light duty driving Test Cycle (WLTC)”, UNECE Informal document GRPE-68-03, 2014 [19] W-H. Hucho, Aerodynamik des Automobils, Wiesbaden:Springer, 2013. [20] D. Sherman, ”Drag Queens: Aerodynamics Compared“, Car and Driver, Ann Arbor, 2014. [21] Tesla Motors Website, Specificactions of Tesla Model S [Online]. Available: http://www.teslamotors.com/models/specs