Multi-battery Block Module Power Converter for Electric Vehicle Driven by Switched Reluctance Motors

To improve the endurance and charging flexibility of electric vehicle battery packs, this paper proposes a multi-battery block module (MBM) topology for four-phase switched reluctance motors (SRMs), which not only allows flexible electric vehicle operation, but also achieves fast demagnetization and excitation. By integrating the multi-battery block module and photovoltaic (PV)panel into an asymmetrical half-bridge (AHB)converter, the MBM topology is designed to supply a multilevel bus voltage for the SRM drive. To improve the endurance of battery packs, a PV panel is also added to the topology to charge battery packs when the system is stationary. According to the different operation requirements, multiple power supply modes and charging modes can be realized by controlling the power devices in the proposed MBM topology. The simulation results based on the MATLAB/Simulink platform and the experimental results on a four-phase 8 / 6 switched reluctance motor verify the effectiveness of the proposed design.


I. INTRODUCTION
Human survival is closely related to social progress and energy development. With the continuous progress of science and technology, people have to face new crises and challenges [1,2], especially the contradiction that the energy demand is increasing, but the existing fossil energy is increasingly short [3,4]. Developing new energy instead of fossil energy consumption to alleviate the energy crisis and reduce greenhouse gas emissions has become the main goal of global energy development in the future [5][6][7]. The development of new energy and related technologies has attracted increasing attention [8][9][10]. Making full use of renewable energy such as wind energy, solar energy and carrying out rational and effective energy management are important ways to deal with threats such as energy crises and environmental pollution [11,12]. In the field of transportation, new energy electric vehicles that rely less on traditional fossil fuels have become an emerging way to alleviate the energy crisis because their low running cost and emissions [13,14]. Simultaneously, with the development of smart grids and the popularity of charging piles, new energy electric vehicles have achieved rapid development and promotion [15,16].
The SRM is a new type of motor. Its stator and rotor are both convex pole structures. The winding is not required by the rotor and a centralized winding is employed on the stator. The SRM power converter is the core of the SRM speed control system, and the design and selection of the power converter have a direct impact on the system performance [17][18][19]. Traditional power converters cannot meet the complex power supply needs of new energy electric vehicles. Additional circuitry is needed to charge the battery packs, which increases the system complexity [20,21].
The multi-battery pack module in an electric vehicle is generally composed of multiple batteries and backup batteries, each of which has multiple single batteries. It is necessary to select different numbers of batteries that meet the operational requirements to participate in the power supply according to the operating conditions and the remaining capacity of the batteries. Therefore, it is important to design a power converter which can flexibly combine multiple battery packs to meet the complex running conditions of electric vehicles. To meet the requirements of multi-battery charging flexibility and improving the endurance of electric vehicles, Scholars have further exploited the application of batteries in electric vehicles. Cascaded converters for SRM were studied in [22][23][24]. Reference [22] integrated a battery pack into an AHB converter to realize the flexible energy conversion between the generator, battery pack and motor, which can configure the multilevel bus voltage and current capacity, so as to accelerate the excitation and demagnetization in the commutation region. However, as the DC-link voltage reaches higher levels, the switching losses increase. The integrated power converter proposed in [23] connects the Buck converter with the AHB converter in series, which can accelerate the demagnetization process. However, it is only suitable for the buck charging mode in which the charging current is low. Hence, adapting the converter to both buck and boost charging modes becomes the target of this paper. The dual-front-end circuit with photovoltaic panel (PV) feed proposed in [24] does not need to be connected to the charging station for charging. It implements six driving modes and five charging modes, but an additional converter is required during charging.
The topology of the SRM drive is optimized and improved in [25][26][27] to meet the requirements of different working conditions. Reference [25] proposed an integrated battery-driven converter for the SRM, which can charge an on-board battery. In the driving mode, the SRM drive is powered by the booster, and a well-regulated DC connection voltage is established. During each demagnetization and continuation period, the energy stored in the winding is recharged to the battery. Reference [26] proposed a fourphase hysteresis-control SRM driver for driving electric vehicles. Hysteresis control is used for low-speed operations. This simple control can be used for electric vehicles specially designed for subways. A multifunctional SRM driver for single-phase and three-phase AC charging is introduced in [27]. Power converter topologies with a large number of power devices are relatively expensive and increase the difficulty of manufacturing [28]. To overcome these inherent drawbacks of complex topologies, several modular power converters have been proposed in [28][29][30]. Reference [28] proposed an SRM modular power converter based on dual-switch modules and dual-wire windings. However, it requires two-wire windings and is not suitable for the traditional SRM. In [29], a distributed SRM driver based on a modular multilevel converter for hybrid vehicle batteries is proposed. The battery pack is connected to the half-bridge converter as a sub-module and has multiple sub-modules connected, but the bus voltage is reduced and the excitation and demagnetization processes are prolonged. The modular power converter for a four-phase SRM proposed in [30] has the advantage of overlapping current pulses, which can increase the speed of the SRM and minimize the number of power switch modules. In addition, owing to the inherent working mechanism, this modular power converter improves the motor performance throughout the variation range of the motor characteristics.
In this paper, an MBM topology is proposed to realize a flexible combination of multiple batteries and adapt to the complex operation of electrical vehicles. The two battery packs and PV panels can achieve seven power supply modes by controlling power switches, thus improving the system flexibility. Three battery static charging methods can be realized, and the charging current can be flexibly adjusted which contributes to an improved battery life.

A. SWITCHED RELUCTANCE MOTOR
Ignoring the iron loss and mutual inductance between phase windings, the electromechanical energy conversion of SRM system is analyzed. The N-phase voltage is as follows where Un is the phase N supply voltage, Rn is the phase N resistance, in is the phase N current, Ln is the phase N inductance, Ψn is the phase N magnetic flux, and θn is the rotor position angle, ωr is the rotor angular velocity.

B. TOPOLOGY AND OPERATION MODE ANALYSIS
To better meet the operational requirements of battery power supply and charging in electric vehicles, a MBM power converter is designed as shown in Figure 1. The power converter is composed of a multi-battery module, chopper charging circuit and dual-bus asymmetric half-bridge circuit. The multi-battery pack module consists of two battery packs. An electric vehicle driven by an SRM, which has multiple driving and charging modes is shown in Figure 2.

Control system
Battery pack module PV Panels DC-AC    Figure 4(a) shows the normal power supply mode in which switch SK1 is on and SK2 is off. By controlling switches S3 and S5, the MBM converter can perform three different modes of power supply, namely, the battery pack BP1 participating in power supply alone, the battery pack BP2 participating in power supply alone and the battery pack BP1 and the battery pack BP2 participating in power supply together. Four methods of power supply in which PV panels participate can be implemented: the power supply with PV panels alone, the power supply with PV panels and battery pack BP1 together, power supply with PV panels and battery pack BP2 together and power supply with photovoltaic panels and battery packs BP1 and BP2 together. By controlling S4 and S6, three freewheeling modes can be achieved: freewheeling current passes through battery BP1 and battery BP2 (freewheeling mode 1), freewheeling current only passes through battery BP1(freewheeling mode 2), freewheeling current only passes through battery BP2 (freewheeling mode 3). Through different combinations, a total of 13 freewheeling charging modes can be realized under seven power supply situations. Figure 4 (a) shows the path of the phase A current in the excitation stage when battery pack BP1 supplies power alone. Under this condition, switch S3 is on and switch S5 is off, and the power supply current only flows through BP1. Figure 4(c) shows the path of the phase A current in the excitation stage when battery pack BP2 participates in the power supply separately. Under this condition, switch S5 is on, switch S3 is off, and the power supply current passes through battery pack BP2 only. Figure 4 (d) shows the circuit diagram of phase A current in the excitation stage when the battery packs BP1 and BP2 are both involved in the power supply. Under this condition, switches S3 and S5 are all on, and the power supply current passes through battery packs BP1 and BP2. By comparing the three combination forms of the battery pack, it can be found that the battery packs that participate in the power supply can be adjusted by simply controlling switches S3 and S5 to adapt to various operation states of new energy electric vehicles. Because the on-off conditions of switches S3, S4, S5, S6 are different under the seven power supply modes, the freewheeling modes that can be realized under the seven power supply modes are also different. When the battery pack BP1 participates in the power supply separately, switch S3 is on and switches S4, S5 are off, two freewheeling modes can be realized. When switch S6 is off, the SRM can realize the freewheeling mode1. The circuit diagram of the phase A current in the freewheeling stage in this mode is shown in Figure 4 (b), where the freewheeling current flows through the battery pack BP1 and the battery pack BP2. Similarly, when switch S6 is on, freewheeling mode2 can be realized. Figure 5 (a) shows that the freewheeling current of phase A only flows through the battery BP1 to charge it.
When the battery pack BP2 is independently involved in the power supply, switch S5 is on and S3 is off. Similarly, switch S6 must be turned off under the influence of switch S5. The SRM can also realize two freewheeling modes. When switch S4 is off, the freewheeling mode 2 shown in Figure 5 (a) can be realized, whereas when switch S4 is on, the freewheeling mode 2 shown in Figure 5 (b) can be realized. The freewheeling current of phase A only flows through battery pack BP2. When battery packs BP1 and BP2 participate in the power supply together, switches S3 and S5 are turned on, and switches S4 and S6 are kept shut down. Under this condition, only freewheeling mode 1, as shown in Figure 4 (b) can be realized. By controlling switches S3 and S5, three power supply modes of battery packs can be realized. Combined with the control of switches S1 and S2, a total of seven power supply modes can be implemented, including three separate power supply modes of battery packs, three hybrid power supply modes of photovoltaic panels and battery packs and the power supply of photovoltaic panels. Three types of currentcontinuum charging can be realized by controlling switches S4 and S6. However, considering that the conduction of switches S3 and S5 will affect switches S4 and S6, three freewheeling modes can be all realized only when PV panels supplies power alone. Only a part of the freewheeling mode can be realized under the other power supply modes. The combination of various power supply modes and continuous current charging forms makes the utilization of the battery pack more reasonable, and a variety of flexible control methods improve the reliability of the system.

C. OPERATION MODE ANALYSIS OF SRM
Where Ud is the voltage of phase D, Ua is the voltage of phase A, UBP1 and UBP2 is battery pack voltage, Us is the PV panel voltage, La is the phase A inductance, ia is the phase A current, θa is the rotor position angle of phase A, and ωr is the rotor angular velocity.
When phase A enters the demagnetization stage and phase B is not on yet or the excitation current ib of phase B is less than the freewheeling current ia of phase A, the freewheeling current of phase A flows back to the capacitance and the battery pack BP1. When phase A is subjected to a high reverse voltage, fast demagnetization can be achieved, where the voltage of phase A is： When phase A is off and phases B or C is in the demagnetizing stage, the phase A winding generates a small inductive current because the demagnetizing voltage is a pulse alternating voltage, not a single-level demagnetizing voltage driven by a traditional asymmetric half-bridge topology circuit. In this case, the winding voltage of phase A is： = − When the battery pack BP1 and BP2 supply power together and the battery BP2 participates in the power supply alone, the operation process of the system is similar to that of the battery pack BP1 supply power separately, but the excitation and demagnetization voltage levels of different modes are different.

D. MODEL OF PHOTOVOLTAIC PANEL
As is shown in figure 7, the paper uses a single diode model consisting of diodes and resistors for analysis. The power source coupled in parallel with a semiconductor diode shapes an ideal cell, plus the two electrical resistors RS, RP that shape the current and voltage losses [32] R P The voltage-current characteristic of a diode is expressed according to the relationship below Where ID is the current of diode, UD is the diode voltage, Is is the reverse saturation current, T is the thermodynamic temperature, K is the Boltzmann constant.
The current produced by the source Iph depends on the intensity of the solar radiation, the absorption coefficient of the wavelength of the solar radiation, and the diffusion and electron recombination characteristics of the material according to the relationship below where G is the solar radiation measured, GSTC -the solar radiation in standard conditions (1000W/m 2 ), Tc,STCtemperature at STC (298.15 K).
By applying the Kirchhoff theorem to the circuit in Fig. 8, the voltage-current characteristic of a photovoltaic cell is obtained as follows By substituting Id and Ip with their expressions we obtain: Where I is the current of PV cell, U is the volage of PV cell, ID is the current of diode, α is a constant related to the PV junction (Usually between 0 and 1)

E. STATIC CONTINUOUS CHARGING MODE
When the SRM is still, the battery is no longer involved in the power supply, switches S1, S3, andS5 are off, and the system can realize three types of charging modes. Taking the battery pack BP2 supplying power separately as an example, when the switch SK1 is off and SK2 is on, the system changes from the power supply operation mode to the charging operation mode. In order to maintain the static operation, the switch tubes on the other three-phase bridge arms of B, C, and D are kept shut off, and switches SK2, Sa1and Sa2 are controlled to enter the static continuous charging mode. As shown in Figure 8 (a), when switches Sa1, Sa2 are on and SK2 is off, the PV panels charge for phase A winding. As shown in Figure 8 (b), when switch SK2 is on and Sa1, Sa2 are off, phase A winding is charged by the battery pack BP2. The charge current can be easily regulated by controlling the PWM duty cycle and frequency of SK2, Sa1, and Sa2. The phase A winding current ia is: The battery charge current ic is: ia0 and iam are the initial phase current and the maximum phase current of phase A, respectively; D is the duty cycle of switches Sa1 and Sa2, and T is the switching period.

F. BOOST CHARGING MODE
Similarly, the battery pack BP2 supplying power separately is taken as an example for analysis. After closing switch SK1, system enters the charging operation mode of the battery pack, switch SK2 and Sa2 are controlled to enter the boost charging mode. As shown in Figure 9 (a), when switches S2, Sa1 and Sa2 are turned on and SK2 is turned off, the PV panels charge phase A winding while battery pack BP2 is charged by capacitor C1. As shown in Figure 9 (b), when switches Sa1, and SK2 are on and S2, and Sa2 are off, the PV panels and phase A winding are charged by the BP2 battery pack at the same time. The charging current can be easily regulated by controlling the PWM duty cycle and the frequencies of SK2 and Sa2.

G. BUCK CHARGING MODE
The battery pack BP2 supplying power alone is still considered as an example. Switches SK1 and Sa2 are off, and switch SK2 is on. At this time, the system enters the charging operation mode of the battery pack, and enters the buck charging mode by controlling switch Sa1. As shown in Figure  10 (a), when switch Sa1 is on, the PV panel charges the phase A winding and battery BP2. As shown in Figure 10 (b), when the Sa1 is off, the battery pack BP2 is charged by the phase A winding. The charging current can be easily regulated by controlling the PWM duty cycle and frequency of Sa1 and S2.  The perturbation observation method is employed in PV panels to guarantee a voltage of 30V at the maximum power point. Figure11 shows simulation waveforms of freewheeling mode 1 and 2 when the battery pack BP1 participates in the power supply alone, the switch S3 is on and switches S4, S5 are off, by controlling switch S6, two freewheeling modes can be realized. The simulation waveform of the freewheeling mode 1 when the switch S6 is off is shown in Figure 11 (a). The excitation voltage of phase A is 90 V which is the sum of the voltage of the two battery packs and the photovoltaic panel, and the demagnetization voltage is -36 V and -90 V. The freewheeling current flows through the battery pack BP1 and the battery pack BP2 to charge the two battery packs. When the switch S6 is on, freewheeling mode 2 can be realized. The simulation waveform of the freewheeling mode 2 is shown in figure 11 (b). It can be seen that the excitation voltage of phase A is 66 V, the demagnetization voltage is -36 V and -66 V, the freewheeling current only flows through the battery BP1 for its charging. Figure12 shows simulation waveforms of freewheeling mode 1 and 2 when the battery pack BP2 participates in the power supply alone, the switch S5 is on and switches S3, S6 are off, by controlling switch S4, two freewheeling modes can be realized. The simulation waveform of the freewheeling mode 1 when the switch S4 is off is shown in Figure 12 (a). The excitation voltage of phase A is 90 V which is the sum of the voltage of the two battery packs and the photovoltaic panel, and the demagnetization voltage is -24 V and -90 V. The freewheeling current flows through the battery pack BP1 and the battery pack BP2. When the switch S4 is on, freewheeling mode 2 can be realized. The simulation waveform of the freewheeling mode 2 is shown in figure 12 (b). It can be seen that the excitation voltage of phase A is 54 V which is the sum of the voltage of PV panel and the battery pack BP2, the demagnetization voltage is -24 V and -54 V, the freewheeling current only flows through the battery BP2 for its charging. The simulation waveform diagram of the SRM powered by batteries BP1 and BP2 together and separately by PV panels is shown in Figure 13. When the battery packs BP1 and BP2 participate in the power supply, switches S3 and S5 are on, and switches S4 and S6 are off. As shown in Figure 13 (a), there is only one freewheeling mode at this time, that is, the charging current flows through the battery pack BP1 and the battery pack BP2 to charge the two battery packs.
When powered by PV panels alone, switches S3 and S5 are off, and switches S4 and S6 are no longer affected by switches S3 and S5. As shown in Figure 13

Ⅳ. EXPERIMENT
To verify the feasibility of the proposed MBM topology, experiments were carried out on a 500 W SRM converter with the same parameters. The experimental platform is illustrated in Figure 16.    The experimental waveforms of the A-phase winding current ia, A-phase winding voltage Ua, BP1 current it1 and BP2 current it2 when the SRM is only powered by battery BP2 is shown in Figure 18. Figure 18 (a) shows the simulation waveform of freewheeling mode 1, in which switch S4 is off.
The simulation waveform diagram of freewheeling mode 2 is shown in Figure 18 (b). Similar to the operation mode in which battery BP1, supplies power alone, two freewheeling charging modes can also be realized when only powered by battery BP2, but the A-phase voltage is different from the charging target, which fully improves the flexibility of the battery charging mode. In Figure 20, the experimental waveforms under the static freewheeling charging mode are shown. Figure 20