Power Electronic Interfaces

The SoC of an energy storage element displays a direct relation with its terminal voltage. A typical ESS results in wide terminal voltage variation during charge and discharge operations. Therefore, a PE converter is employed with an ESS that accepts wide terminal voltage variations at the input and delivers a stabilized voltage at the output. Consequently, the battery pack and SC bank associated with a power

FIGURE 4.38 Passive-cascaded HESS interface.

converter to step-up their corresponding voltages and generate a constant DC link voltage. However, several interfacing topologies are available to hybridize the batteries and SCs based on their characteristics which are discussed as follows.

Passive Cascaded Configuration

The battery and SCs are connected in parallel combination with a bidirectional DC-DC PE converter associated with an SC bank as shown in Figure 4.38. The voltage rating of the battery is equal to the voltage rating of an SC bank. The battery recharges the SC bank simultaneously whenever it has been used to deliver the pulsed power. On the other hand, the bidirectional power flow enables the SC bank to discharge and recover the regenerative braking power from the load. However, this configuration requires an oversized battery that discharges onto the SCs and serving the load demand simultaneously without any control.

Active Cascaded Configuration

The aforementioned power interface is upgraded by employing a unidirectional power converter to the battery pack, which steps up the corresponding voltages as shown in Figure 4.39 [39]. This topology reduces the sizing of batteries and efficiently controls their discharge current. On the other hand, the SC bank (when detected) absorbs the transient currents from the load with the help of a bidirectional power converter.

FIGURE 4.39 Active-cascaded HESS interface.

FIGURE 4.40 Parallel-active interface.

Parallel Active Configuration

To achieve high performance and better control over battery and SC bank currents, a parallel active power interface is employed. This approach uses two bidirectional converters to the battery pack and the SC bank as shown in Figure 4.40 [40].

The HESS with this configuration facilitates lower sizing of both the battery pack and the SC banks. The power management with this topology allows the regenerative braking power to be stored in either batteries or SCs as per the requirement. However, employing two individual bidirectional DC-DC converters results in addition of the complexity of control and the overall system cost.

Multiple-Input Converter Topology

The multiple-input converter configuration uses a single DC-DC converter with dual inputs for each source. A separate switch and diode are used for each input as shown in Figure 4.41 [41]. An additional switch and diode can be included if a bidirectional operation is required. Furthermore, the converter comprises a single inductor which is shared by the battery and SC bank. As the SC bank voltage varies widely, the voltage rating of the battery is selected higher than the SC bank.

The converter is able to operate in buck, boost, and buck-boost modes as per the power flow. Moreover, the weight and volume of the converter are significantly reduced by incorporating only a single inductor for multiple inputs. However, this topology requires a complex control scheme to operate effectively.

Dual-Source Bidirectional Converters

FIGURE 4.41 Multiple-input DC-DC converter interface.

battery pack and SC bank are combined at the input stage with a single converter as shown in Figure 4.42 [42]. The total cost of this configuration is slightly lesser than the parallel-active topology. However, this circuit requires an intricate control system.

Multiple Dual-Active Bridge Converters

The hybridization of battery pack and SC bank can also be realized by employing the transformers which can be operated high frequencies. The multiple dual active bridge converter as a power interface is shown in Figure 4.43 [43].

Typically, the transformers are beneficial where they provide isolation between the input sources and the DC link. However, the transformers increase the volume and weight of the power converter. Moreover, the converter uses multiple switches, which accounts for the higher system cost.

FIGURE 4.42 Dual-source bidirectional converter interface.

FIGURE 4.43 Dual-active bridge converter interface.

Sizing of HESS for an EV

Typically, the sizing of HESS for an EV involves in determining the power and energy requirements of the propulsion system, identification of the power allocation control, and sizing of an individual ESS (battery and SCs). The steps to identify the efficient sizing parameters for HESS are discussed.

Power and Energy Ratings of EV powertrain

In practice, the EV is operated in a dynamic environment with varied driving patterns. Thus, the propulsion system of an EV is selected based on the load power requirement, which is defined from the tractive force (F,) acting on an EV for a specific driving cycle.

The overall power rating of an EV propulsion system is given by:

The total power required by the ESS (battery-SC hybrid) to drive an electric motor is obtained as:

where, V_s is the velocity of the vehicle and the total efficiency of an EV (%,,) which depends upon the transmission (rj_tr), motor (t]_mo), and the power conversion efficiencies (?7_(К) which is calculated from:

The energy storage of HESS is sized based on the total energy to be delivered by the source. This is obtained by integrating P_ESS with respect to time. However, the sizing of ESS is varied with the consideration of the regenerative braking power.

Battery and SC Sizing

The aforementioned parallel-active approach is considered for the sizing of an ESS in this study. The load power (P_load) is typically allocated between the battery pack and the SC bank. Thus, these are sized to meet the dynamic response of an EV. The following assumptions are made for the sizing: a() the battery is only used for a constant power load; (b) SC bank is used to deal with peak power demands; and (c) regenerative braking is considered which recharges only SC bank.

A power profile for a specific driving pattern is considered for sizing. The battery pack must contain the total energy that can cover a range of 100 km [44]. The recommended voltage rating of the battery is lower than the DC link voltage.

The SC bank is designed to serve only the peak power demands. As the bidirectional boost converter is associated with an SC bank, the maximum voltage rating of the bank must be lesser than the DC link voltage. Thus, the converter steps-up the corresponding voltage and matches the output voltage of the boost converter, which is connected to a battery pack.

The overall energy consideration for battery and SC bank using the parallel-active approach is given in (4.22). Therefore, the sum of energies of the battery pack (E_ball) and SC bank (£_sc) should be greater than the energy required by the load. Where £_load is the energy required by the load.

The energy of an SC bank in HESS should match with the initial energy of the bank switched SCs which is given as:

Load Power Allocation

To avoid the battery pack from serving the pulsed power demands, a load power allocation algorithm is implemented. This scheme allocates the constant power to the battery pack and peaky power demands to the SC bank. As a result, the stress on batteries is reduced and enhances their cycle life. The most widely adopted load power allocation techniques are discussed in the following section.

FIGURE 4.44 Rule-based power sharing: (a) load power profile and (b) control flow.

Rule-Based Power Allocation

The load power is differentiated using a rule-based control which splits the power profile based on the average power (P_avg). The P_avg for a specific power profile is show'n in Figure 4.44(a) and the rule-based control flow is shown in Figure 4.44(b). The flow states that if the load power is less than zero, a bidirectional DC-DC converter associated with an SC bank operates in buck mode and absorbs all the available power. On the other hand, if the P,_oad is greater than zero and less than P_avg, only battery is used to deliver the load power. Whereas, if the P_load is greater than zero and less than P_avg, then both battery pack and SC bank serve the load.

Frequency-Based Power Allocation

The power required by the load is separated and allocated using a frequency-based approach [45] as shown in Figure 4.45.

FIGURE 4.45 Frequency-based load power allocation.

It comprises a low pass filter (LPF) which eliminates the high-frequency power components and allocates it to the battery pack. On the other hand, the high- frequency power is allocated to the SC bank. However, this approach requires the tuning of the minimum frequency of an LPF.