Improving Efficiency of Hybrid Electric Vehicle Using Matrix Converters

The paper deals with a novel enhanced connection of AC/AC powertrain for HEV hybrid vehicles. The substantial contribution of such a connection is the absence of 4QC auxiliary converter needed for autonomous and hybrid operational modes and its compensation by power-lesser 0×5 matrix converter. The main advantages of a simplified connection are beside smaller auxiliary converter sizing and possible better efficiency of the HEV powertrain. So, powertrain operation in autonomous traction accu-battery modes uses direct 0×5 configuration of traction 3×5 MxC matrix converter and in hybrid modes of ICE engine and accubattery, besides traction 3×5 MxC matrix converter, uses the auxiliary 0×5 matrix converter. Modeling and simulation using Matlab-Simulink environment of traction powertrain configuration in autonomous modes are presented in the paper, as well as all simulation experiment results.

central drive motor and associated transmission parts of the propulsion system of the vehicle. The main advantage of the electric motor in the wheel is adjustable traction and individual braking torque with high precision without ingestion gearbox, drive shaft, differential gear, and other complex and heavy parts of the power transmission [3]. One of such a traction system is shown in Fig. 1. A. Potential Possibilities of Improving the Efficiency of AC/AC powertrain There are following potential possibilities for direct AC/AC powertrain efficiency improving with a comparison to classical AC/DC/AC powertrain (see Fig. 1, [5], [6]) : 1. to release the traction rectifier TR and substitution of AC/DC/AC powertrain by direct AC/AC converter (matrix-or cyclo-converter type), 2. to change the classical inverse connected two IGBT switches by two antiparallel-connected reverse blocking RB_IGBT (or by two antiparallel-connected SCR thyristors), 3. to substitute classical 3-phase traction motors M1 and M2 by 5-phase motors, 4. to use independent control of two traction motors fed by single AC/AC converter, 5. to use software reconfiguration of direct AC/AC converter for all operational modes of AC/AC powertrain.
among the different number of phases traction generator and traction motor is a three-phase generator and a five-phase motor, i.e., [3×5] direct converter. Such a configuration of the AC/AC powertrain makes it possible both pure electric operating modes and/or pure engine mode, as well as hybrid mode: the vehicle is propelled by ICE engine and accubattery energy in parallel operation [1].

A. Comparison of 3-Phase VSI Powertrain and 3-Phase Matrix Powertrains
It is shown in the paper [7], [8] that when comparing matrix converter (MxC) with VSI converter with an active front-end for induction motor drive, the matrix converter's semiconductor losses are smaller at full load operation for the same silicon area in both converters (11-22 % depending on switching frequency). A one-third reduction of the device current rating of the MxC is possible, resulting in comparable thermal device stress. Therefore, if total power losses of the electrical part of powertrain (converter and motor) are, let´s say, 20 % of the transmission power, then under the converter losses are 40 % from that value. That means that the efficiency of the powertrain electrical part could be higher by 8-12 %. Regarding a number of phases, if the number of phases of the motor is three (no more), then there is not possible to connect both traction motors to one common direct traction converter [9], [10].

B. Comparison of Three-Phase and Five-Phase Matrix Converters
It is clear that the voltage is periodic and follows the desired amplitude and frequency. From [5], [11], [12], the following remarks can be deduced.
1. The five-phase application has the following advantages in comparison of the three-phases application:  The speed curve response time dynamics is faster;  The torque curve has fewer ripples rate;  The value of the output voltage of converter is higher;  Furthermore, the amplitude of the output current in five-phase is reduced compared to the three-phase (from 4.5 A to 3.5 A), which can prevent deterioration of the windings and use smaller gauge switches (reduce the cost of purchase and maintenance).
2. Both applications have nearly the same properties regarding to input voltage and output current waveforms (almost a sine waveform), and regarding to disturbance, which is applied to both machines, the curves of speed have the same behaviors of load compensations (the decrease and increase of the speed due to the variation of the load are quickly corrected by PI controllers). From the point of view of the powertrain efficiency, a simulation analysis of 3-phase and 5-phase VSI inverter is done, which can be used for indirect control operated matrix converter [11]. Modeling and simulation are described in Section III.

C. Comparison of Direct AC/AC Powertrain with 4QC Battery Converter and [0×5] MxC Battery Converter
One of the disadvantages of direct AC/AC powertrain with 4QC battery converter (Fig. 2) is that in autonomous traction mode, when powered HEV, by battery total traction power flows through two converters. Therefore, we suppose new direct AC/AC powertrain with [0×5] MxC battery converter (see Fig. 3). Using this connection shown in Fig. 3, in autonomous traction mode, when powered HEV, by battery total traction power flows just through one [0×5] MxC converter, which can be easily created from [3×5] MxC [11]. Therefore, it clear that the efficiency of such a direct AC/AC powertrain will be higher. Modeling and simulation are described in Section III.

D. Comparison of IGBT and RB_IGBT
In case IGBT switches have free-wheel diodes (reverse blocking RB_IGBT), the antiparallel connection of these two RB_IGBT can be used as a bidirectional switch in matrix converter connections. Such switches with an integrated diode in the collector of IGBT are developed by such electric companies as IXYS and FUJI.
From the point of view of the powertrain efficiency, reverse blocking transistors feature by higher efficiency and lower power losses, respectively. Following Fig. 4, it shows a comparison of classical and reverse blocking IGBTs regarding all kind of losses: on-state on P , saturation sat P , switching off P , and reverse recovery rr P ones. Values of the power losses depend on the switching frequency used in the tested circuit. Figure 4 is redrawn from [13]. Rising of the frequency from 2 to 20 kHz causes the rise of the losses by 10 %. From the Fig. 4, it is clear that total losses of the reverse blocking IGBTs are lower than classical IGBTs losses. Calculation and simulation of the losses are described in [14].

E. Comparison of Three-Phase and Five-Phase IM Tractive Motors
The five-phase machines offer some inherent advantages over their three-phase counterpart as mentioned above. Major advantages of using a five-phase machine instead of the three-phase ones consist in their higher torque density, greater efficiency, and fault tolerance. Other advantage includes reduced electromagnetic torque pulsation and reduction in the required rating per inverter leg. Noise characteristics of the five-phase drives are better when compared with the three-phase ones. [10], [11], [15].
Higher phase number should yield smoother torque due to the simultaneous increase of the frequency of the torque pulsation and reduction of the torque ripple magnitude. Higher torque density in the five-phase machine is possible, although apart from the fundamental spatial field harmonics. The space field harmonics can be used to contribute to the total torque production. In a multi-phase machine with five or more phases, there are additional degrees of freedom, which can be used to enhance the torque production through the injection of higher-order current harmonics. In a fivephase induction machine, a third current harmonic injection can be used to enhance the overall torque production [10], [11], [15]. The studies on multi-phase drive system, carried out so far, are for the high-performance variable speed applications. The five-phase drive is seen as a serious contender for niche applications, such as ship propulsion, traction, lifts electric vehicles, and in safety-critical applications requiring high degree of redundancy. However, general purpose drive applications using multi-phase machines have not yet been investigated in detail.

F. Comparison of Independent Control of Two Traction Motors Fed by Single AC/AC Converter with Motors Fed by Two AC/AC Converters from the View of Efficiency
Using independent control of two traction motors fed by one single direct AC/AC converter instead of two ones (see Fig. 2), the power losses of the single converter are slightly lesser and the efficiency of the powertrain is higher, but it is not the essential increasing. However, as the authors say in [10], that although parallel multi-phase multi-motor drives and series connected multi-phase multi-motor drive systems are feasible and in principle offer good quality of dynamic performance as well, the users do not hold a real prospect for industrial applications so far.

A. Comparison of Direct AC/AC Powertrain with 4QC Battery Converter and [0×5] MxC Battery Converter of HEV Vehicle
The corresponding schemes of [3×3] and [3×5] MxC converters are presented in Fig. 6(a) and Fig. 6 The difference of () Vt where R  ... T  are logical variables for the maximal and minimal input phase voltages. The phase voltages can be calculated as in [5], [11]: .
The direct control methods of MxCs are given in [8], [16]. Similarly, in the case of [3×3] MxC converter, it can be simulated phase voltages and currents and consequently power losses and efficiencies of power electronic switches.
To modeling of VSI inverters power losses, the Matlab/Simulink programming environment is used. The simulation scheme for power losses computation in Matlab/Simulink is given in Fig. 11.
In the case of indirect controlled [3×3] MxC connection, the power losses of used IGBTs and diodes (see Appendix A) reach the following values: carried-out simulation results are shown in Fig. 12, Fig. 13, and Fig. 14.    For thermal investigation, Foster thermal model is used, which is included in the datasheet of the transistors (Fig.  15).
The average junction temperature of IGBT is approximately 65 °C, which is allowed value.
Similarly, in the case of [3×5] MxC converter, it can be simulated phase voltages and currents and consequently power losses, and efficiencies of power electronic switches. In the case of indirect controlled [3×5] MxC connection, the power losses of used IGBTs and diodes (Appendix B) reach the following values: carried-out simulation results are shown in Fig. 16, Fig. 17, and Fig. 18.   For thermal investigation, the Foster thermal model is used, which is included in the datasheet of the transistors (Fig. 19). The average junction temperature of IGBT is approximately 65°C, which is allowed value.

IV. CONCLUSIONS
Comparing calculated results given in Table I, we can conclude that the energetic efficiency of [3×3] MxC converter and [3×5] MxC converter is nearly the same under the same output phase voltages and corresponding phase currents. Actually, the calculated efficiency of the 5ph_VSI inverter is slightly better (by 0.15 %). The average junction temperature of IGBT is approximately 65 °C and it is equal in both cases. Under supposing of the same voltage and current circumstances, the similar efficiency is reached. The comparison of three-phase and five-phase VSI inverters supposed for HEV vehicles is presented in another paper (of this Conference, if accepted). To verify VSI inverters power losses and efficiencies the LT Spice circuit simulator, as well as a real test bed, in the next work are going to be used.