10 Using of Hybrid Supply for Electric or Hybrid Vehicles

For automotive applications, the batteries are sized to ensure many constraints like startup, acceleration, braking and energy recovery. All these constraints give us a very heavy battery with very high energy compared to that required for these applications. To reduce the weight of the storage system, the battery can be associated with high power component like supercapacitors. This last one is used like power booster, and the battery is used just to ensure the energy needed for each application (hybrid or electric vehicles). The use of MatlabSimulink software allows us to make a modular simulation (Fig. 1). This software resolves the differential equations using several numerical methods (Runge-kutta, Dormand-Prince, Heun, Euler,...). This paper deals with the simulation and conception of hybrid power supply composed with battery and supercapacitors for a micro-hybrid vehicles. In this case, the battery is used as energy tank and supercapacitors as power booster. This design allows to increase the lifetime of the supply and to downsize this last one.


Introduction
For automotive applications, the batteries are sized to ensure many constraints like startup, acceleration, braking and energy recovery.All these constraints give us a very heavy battery with very high energy compared to that required for these applications.To reduce the weight of the storage system, the battery can be associated with high power component like supercapacitors.This last one is used like power booster, and the battery is used just to ensure the energy needed for each application (hybrid or electric vehicles).The use of Matlab-Simulink software allows us to make a modular simulation (Fig. 1).This software resolves the differential equations using several numerical methods (Runge-kutta, Dormand-Prince, Heun, Euler,…).This paper deals with the simulation and conception of hybrid power supply composed with battery and supercapacitors for a micro-hybrid vehicles.In this case, the battery is used as energy tank and supercapacitors as power booster.This design allows to increase the lifetime of the supply and to downsize this last one.

Topology of the power system
The hybrid storage system (battery and supercapacitors) supply the starter-alternator through an inverter.Two topologies can be used to associate the battery and the supercapacitors: in series or parallel.For this last configuration, the two components are connected to the DC link through two choppers.In the series configuration (fig.2), the supercapacitors is charged by the battery and discharged during the high power demand (startup, acceleration,…).Fig. 2. Topology of the series hybrid supply for automotive application Supercapacitors are chosen like booster because of their very high specific power, which can reach 17kw/kg.The capacitance of this component exceeds the 5000F (Maxwell technology) for a lower voltage (3V).To use this component in power applications, we must make some elements in series.

Startup torque measurement
During the starting-up, the ICE develops a load torque applied to the starter or the starteralternator.The computation of this torque allows us to validate the hybrid supply interest.
To estimate this torque we need to know the evolution of the starter torque according to its current.

Formula of the starter Torque
Because of their high starting torque, the series DC motors is used like a starter for the automobile applications.The torque developed by this motor can be written as follow: The relation between the current and the flux of this t ype of motors complica tes t he identification of this torque.For that, this motor (starter) is transformed to a separate-wound DC motor.The acquisition of the armature voltage, the armature current and the starter velocity at deferent wound current allows us to plot the evolution of the ratio K  according to the current (fig.7).
According to the results plotted on the figure 7, the ratio K  can be written as follow:

Formula of the startup torque
To identify the load torque developed by the ICE during the starting-up phase, tow startingup tests are made.The first it's without the spark plugs and the second it's with compression (with spark plug).
The first test (fig.8) allows us to estimate the inertia and the friction torque of the ICE.Using the mechanic equation we can compute the two parameters (J and f).Using the measured current and starter velocity, we can compute the parameters J, f and C0.
The second test (fig.9) allows us to estimate the rippled torque (Tcomp) due to the compression on the ICE motor.Using the parameters computed at the first test (J, f and C0), the current and the starter velocity presented in the figure 9, we can compute the torque due to the compression (Tcomp): Figure 10 presents the wave form of the compression torque Tcomp.This can be written as:

Sizing of the supercapcitors module for the startup
The supercapacitors sizing is based on the energy and power required for the starting-up of the vehicle.Figure 11 presents the evolution of the current and the voltage of the Buggy starter at the starting-up phase.At the beginning of this phase, the battery voltage deceases to reach 9V.At the same time, the current exceeds the 250A, what gives us a maximum power of 2400W.

Fig. 11. Power and energy required for the starting-up of the vehicle
In order to ensure the functioning of the boost chopper, the minimum voltage of the supercondensator should not decrease lower than 12V (battery voltage).To take a safety margin, the minimum supercapacitors voltage is chosen about 15V.For this component, more than 75% of energy is stored between the maximum voltage of supercapacitors (Vmax_SupCaps) and the half of this voltage (Vmax_SupCaps/2) : The power integral on all the starting-up duration allows us to compute the energy required for this phase.
The supercapcitors efficiency reaches 80% for 250A of RMS current.
By the neglecting of the Conv.2 losses, the starting-up energy can be written as follow: 22 max 10 10 900 3,34 33 30 The best component for this application can be chosen using the characteristics of all Maxwell cells.The element 2,5V/310F gives us the best compromise between the power and the energy of the component.

Simulation of the system
Before the development of the test bench for this application, a simulation of the operated system is carried out using Matlab-Simulink software.In this case an inverter is used to supply the starter-alternator.In the startup phase, the velocity of the starter-alternator is controlled to reach the set point value (70 rd/S).The load torque is expressed according to the velocity using the formula 7.  To calculate the parameter Maf, the starter alternator must turn at constant velocity with the variation of the inductor current.In this case, the value of the voltage between two phases can expressed as follow: The inverter is modeled using the formulas between the starter-alternator voltages (Vas,Vbs and Vcs) and the control of the semiconductors (K1,K2 and K3): The voltage of the Dc link is also one of the inputs for the inverter model.On the other hand, this model imposes the current of the hybrid supply.This last one is composed with two storage systems (battery and supercapacitors) associated with a chopper (fig.17), which is connected to the battery through an inductive filter.For the supercapacitors modeling a transmission line model is used.This approach is based on the representation of the charges propagation along the electrodes surface by RC circuits.So, the supercapacitor is considered like a capacitance distributed along the volume, with accessible areas and less accessible ones.The distribution of the capacitance depends on the electrode material and especially on the geometrical properties of the pores where ions have to access to constitute the double layer capacitance.This approach is well represented by an electrical circuit representing the transmission line and using r-c parallels branches.Then, the proposed model will only include 3 parameters (C, rs and R).

Fig. 19. Transmission line model
The modelling of the chopper is made making the formulas between the inputs and outputs of this converter (fig.20).

Simulation results
Two simulations of the system functioning are made using the developed model.The first model it's to validate the startup of the vehicle using the hybrid supply.In this case, the supercapacitor discharge and the voltage of the DC link decreases.The second test is to validate the electric power generation and the charge of the battery.

Startup of the vehicle using the hybrid supply
To turn-on the vehicle, the velocity of the starter-alternator is controlled to reach the set point value (32rd/s).During this phase, the supercapacitor discharge.Figure 22 presents the waveform of the starter-alternat or velocity (real velocity) and that estimated using the hall sensors.This figure shows also that the starter-alternator velocity reaches the set-point value after 50 ms.
Figure 23 presents the waveform of the electric angle and that estimated using the hall signals.This result shows the deference between the real value of the electric angle and that estimated for the low velocity.After that the estimation gives almost the real value.During the startup phase, the supercapacitors current reaches -90A at the beginning and stabilizes after that at -38A.In this case, the supercapacitors discharge and the voltage of this component deceases.During this phase, the battery charges the supercapacitors at low current (20A) and the SOC of this component decreases from 100% to 99.8%.

Electric power generation
When the engine is turned-on, the starter alternator charges the battery and the supercapacitors.Firstly, the supercapacitors is charged with high current to reach 30V (fig.27).In the same time, the chopper insures the charge of the battery.In this case, the voltage of the battery is controlled to reach the set point value (13V).During this time, the state of charge increases (fig.28).
The battery current is limited between -20A and +20A.At the beginning, the battery participate to charge the supercapacitor module and after that the battery current reaches 20A, in the same time battery SOC increases during this phase.
The ICE drive the starter-alternator and this last one provide the power needed to charge the battery (fig.28 and 29).The current in this case is very low compared to that of the startup phase (280A for the sturtup and 4A to charge the battery).In the power generation phase, the inverter is used like a synchronous rectifier and the signal period depends to the engine velocity.

Conclusion
The batteries used for the micro-hybrid vehicles have very high specific energy.
Unfortunately, the power of this technology is very low compared to other technologies (Ni-Mh, Li-Ion,…).The association of battery for this application (micro-hybrid) with powerful component like supercapacitors can decrease the weight of the battery and/or ameliorate the lifetime of the supply.In this paper, hybrid supply for micro-hybrid vehicle has been presented.In this case, two storage components are used: supercapacitors like power supply and battery like energy supply.The power density of supercapacitors makes them very interesting for the applications which need high power during short time.
Two architectures are possible to implement the association of supercapacitors and battery: a parallel architecture by connecting the two sources to DC bus through two choppers.The drawback of this architecture is the very large size of the induction coil in series with the supercapacitor module.In the case of series architecture, the size of the induction coil used to smooth the battery current is less important because of the weakness of the current charge/discharge.
After the choose of the architecture of the power supply and the identification of the load torque, the parameters of the starter-alternator are identified.In the next part, the sizing of the two storage system (battery and supercapacitors) is presented.
Matlab-simulink software is used to simulate the supplying of the starter-alternator by the hybrid supply through an inverter.The association of battery and supercapacitors is made using a boost chopper.For the startup phase, the velocity of the starter-alternator is controlled to follow the set point value (32rd/s).

Fig. 3 .
Fig. 3. Using the hybrid supply with starter or starter-alternator

Figure 6
Figure 6 presents the transformation of the series DC motor to a separate-wound DC motor.This transformation allows us to identify their torque.

Fig. 7 .
Fig. 7. Evolution of the ratio K  according to the wound current

Fig. 8 .
Fig. 8. Evolution of the current ant the starter velocity for the test without spark plugs

Fig. 12 .
Fig. 12. Simulink model of the global system

Fig. 13 .
Fig. 13.Test bench developed on the mechatronic laboratoryTo avoid the angular representation, the starter-alternator is modeled with two phases inputs/outputs (d,q).The model is transformed into three phases inputs/outputs using the Park's transformations.The rotoric current and the load torque generates by the ICE are considered also like inputs.The starter alternator equations can write as follow:

Fig. 15 .
Fig. 15.Identification of the starter-alternator inductances The first position (fig.16.a) gives us the value of the direct inductance (Lsd), and The second position of the rotor (fig.16.b) gives us the value of the quadratic inductance (Lsq).The same formula is used to calculate the two inductances:

Fig. 17 .
Fig. 17.Simulink model of the hybrid supply Two models are used to represent the battery behaviour.The first one is the simplified R-C model which uses just a resistance and capacitance to modelling the battery.The second representation is based on the frequency behaviour of the battery.This method allows the representation of the component using the Shepherd model (fig.18)

Fig
Fig. 20.Chopper inputs/outputs Fig. 22. Velocity of the starter-alternator during the start-up phase

Fig. 25 .Fig
Fig. 25.Current and voltage of the supercapacitor module during the start-up phase

Fig. 29 .
Fig. 29.Waveform of the phase current during the power generation phase (during 2 seconds)

Fig. 30 .
Fig. 30.Waveform of the current during the power generation phase (during 0.2 seconds )