Stability enhancement of the motor drive DC input voltage of an electric vehicle using on-board hybrid energy storage systems

Highlights

• A stability enhancement of EV motor drive DC input voltage is proposed.

• The weaknesses and strengths of hybrid-ESS topologies are addressed.

• A reduced-scale power level hardware-in-the-loop system is specified and used.

• A comparison of different on-board hybrid-ESS topologies is made.

• Plus 80% of power transferred by improved EMS, keeping good voltage stability.

Abstract

There are several advantages in keeping motor drive DC input voltage stable around its nominal value especially when it comes to minimize losses. This paper deals with the stability enhancement of the motor drive DC input voltage of an electric vehicle with on-board hybrid energy storage system. On one hand, the natural voltage variation at the output battery pack can be avoided by using a DC/DC converter connected between the battery and the motor drive. On the other hand, the incorporation of supercapacitors in small urban electric vehicles enables handling power peaks thus reducing the battery current root-mean-square value, which in turn increases the battery lifetime. Several topologies can be considered for the coupling of supercapacitors with the vehicle energy system. In this paper, three topologies are studied: battery-only, direct hybrid coupling, and active parallel hybrid coupling of batteries and supercapacitors. A reduced-scale power level hardware-in-the-loop test-bench has been built to analyze the performance of the hybrid topologies under the ARTEMIS driving cycle. Experimental results show the effectiveness of the active parallel configuration controlled by an improved energy management strategy that dynamically regulates the supercapacitors state-of-charge. The analysis performed demonstrates that the use of this configuration coupled with an improved management strategy can increase the power transferred by 80% compared with a battery-only configuration, and by 40% compared with direct hybrid coupling or active parallel coupling configuration with a traditional management strategy, keeping the voltage stability of the DC Link.

Introduction

In the last few years, all major world leader automotive companies have developed serial-produced Electric Vehicles (EVs). EVs are certainly the most efficient vehicles from the environmental and energy consumption points of view [1], [2], [3]. However, there is still a need for the development of on-board energy storage or power generation systems with higher specific energy performance. Hence, only full EVs with relatively low driving range are being produced and available at reasonable price tags. For these reasons, Plug-In Hybrid EVs can only really compete with conventional Internal Combustion Engine (ICE) cars in today’s automotive market [1], [4], [5]. Plug-In Hybrid EVs also require an effective on-board power system with high levels of energy and power density for enhanced emission reduction and fuel consumption. Since no on-board energy sources or Energy Storage Systems (ESS) exist that have simultaneously high specific energy and high specific power, the hybridization (e.g. batteries and supercapacitors) of multiple on-board ESS has been proposed to ensure a satisfactory driving range and EV dynamics [1], [2], [4], [5]. The main aim underlying the hybridization concept is the combination of multiple ESS with complementary properties [6]. Recent research proposes the use of the on-board battery as an energy source coupled with another ESS power source to obtain a global supply system with improved characteristics [2], [5], [6]. Other authors propose the substitution of large capacity batteries by fuel cells as primary source of energy, and powerful batteries with lower energy capacity, SuperCapacitors (SCs), and also flywheels as a source of power [1], [7], [8], [11].

Extensive research is being carried out with significant progress in the development of efficient hybrid ESS for EV application [2], [8], [9]. In general, the specifications of the systems determine the technical and global operational performance, with respect to specific energy, specific power, lifecycle, security, cost and the different degrees of potential contribution of each ESS type to the overall supply system. Some of these studies are extending to other fields in which stand-alone power systems are used, like telecommunications and isolated systems supplied by renewable sources [12], [13].

In the design process of on-board hybrid ESS, two main aspects should be considered: technical challenges and economic evaluation of circuit topology efficiency, considering its control and strategy systems [10]. Despite the significant amount of research and technology surveys [5], [7], [11], [14], a deeper analysis of hybrid ESS topologies and their impact on the powertrain performance remains relevant [15]. A rule-based-optimization strategy considering simultaneously the power demand and the State-of-Charge (SoC) of the battery and SCs systems is proposed in [5]. An adaptive optimal power splitting strategy for fuel cell and battery hybrid system is proposed in [7]. An optimization-based strategy (λ-control) and a rule-based strategy (filtering) are compared in [11]. An integrated optimization framework for battery sizing, charging, and on-road power management in plug-in HEVs is developed in [14]. In [15], a generic control scheme for hybrid ESS is proposed by inversion of the model developed using the Energetic Macroscopic Representation (EMR) formalism to enable different energy management strategies, using a distribution input to share the energy between two different ESS. Also, in [16] EMR is used to develop an improved Energy Management Strategy (EMS) to a dual ESS for a three-wheel EV. An off-line dynamic programing approach is used to optimize the hybrid ESS in [17]. In [18], a power distribution combined with a three-mode rule-based strategy is proposed to minimize the total energy in battery/SCs EVs. A model-predictive control was tested to regulate the power allocation of a hybrid ESS system in a plug-in hybrid EV in [19].

Thereby, the analysis of recent literature reveals that the most common hybrid system in the automotive field is the coupling of batteries and SCs [6], [20]. Although batteries and SCs are electrochemical-based devices, their characteristics differ significantly and complement one another [5], [6], [11], [20]. Table 1 presents a comparison of the main characteristics of different battery types and SCs cells of commercial products. In general, batteries have a high specific energy and short lifecycle, while SCs have high specific power and large lifecycle. A comparison of three different electrochemical ESSs for a hybrid bus powertrain is proposed in [21]. In [22], the potential of different technology combinations is analyzed using standardized duty cycles enhanced with gradient profiles related to suburban, regional and shunting operations. In [23], a procedure is proposed for developing an optimal management strategy, which considers battery durability and longevity. A hybrid ESS is optimally sized and managed with a dynamic battery heath model presented in [24].

The main purpose of the hybridization of ESS is to extend the battery lifetime, by reducing its Root-Mean-Square (RMS) current value and the associated degradation phenomena which entail the life cycle cost of the EVs supply system [8]. Recently, several studies concerned with the aging parameters of batteries have been carried out [3], [11], [15]. Globally, stress factors such as high variations of the SoC, high current rates, low and high temperatures can be pointed out. The battery RMS current value provides a fair correlation with the battery aging phenomenon [11], [15], [25]. Indeed, by reducing the battery RMS current value, the occurrences of high magnitude battery current are minimized, leading to lower SoC variations and lower battery heating at the same time.

These well-known characteristics underlie the relevance of the hybridization concept, that is, using the batteries for average EV power demand while the SCs deal with higher power demand and regenerative energy, as in acceleration, braking or driving uphill of EV operation. Since both sources are designed to supply a DC on-board voltage network, several hybrid ESS combinations are possible [19]. Some of these combinations require bidirectional DC/DC converters, some not. Power devices used for the global supply system (ESSs and converters) are qualified by mass, volume and cost as a function of their power. In the past, the combinations with bidirectional DC/DC converters had concerns regarding the system cost. Recently, development in power electronic and optimization in system integration design fields have again launched the debate on the implementation of different topologies.

The analysis in this paper is based on this requirement (stable motor drive DC input voltage), which is considered in all topologies. One of the simplest topology possible, with battery-only, is a battery linked to the DC link through a DC/DC converter, as presented in Fig. 1a (T1). Most commercially available HEVs use this topology, as for example Toyota Prius, Honda Insight, and Ford Escape [5]. The addition of SCs to improve this topology is then evaluated. Two approaches could be followed leading to different costs and efficiency results. The first approach is the direct coupling of the batteries and the SCs as presented in Fig. 1b (T2). This topology is closely linked to the batteries and SCs characteristics with commensurate voltage but its performance and lifetime should be correctly assessed to maximize its utilization [5]. The second approach is based on an active parallel coupling topology (T3), where the use of one DC/DC converter per ESS is requested, as presented in Fig. 1c. In topology T3, the DC/DC converter for the batteries has a significantly smaller power than the ones used in topologies T1 and T2 [5]. Also, the use of the active parallel topology allows different ESS voltage combinations and does not limit the usage of the batteries to the lower voltage limit of the SCs.

Topology T3 is the most promising one to extend the typical 200 km driving range of EVs and increase the efficiency of the hybrid EVs powertrain. So far, the automotive industry shows this topology as a high cost option, but the trends of extending the driving range and longer life-time batteries further raise the challenges to develop new hybrid ESS, increase its performance and reduce DC/DC converter mass, volume and cost [26], [27].

On the methodological side of the research, Hardware-In-the-Loop (HIL) simulation is increasingly used in the automotive industry to test the control loops interaction with power electronics and/or the traction system. Usually, full-scale HIL simulations are mandatory due to production requirements, but reduced-scale HIL simulations could be used with advantage as intermediary steps in some situations. For instance, the development of a range-extended EV, where the powertrain was integrated and optimized through full-scale HIL prior to vehicle installation, is developed in [28]. In [29], a laboratory full-scale test-bench is presented to perform the analysis of a Zebra battery plus SCs-based propulsion systems. This full-scale test-bench was also used to perform several experimental studies on the performance of a Zebra battery based propulsion system in urban commercial vehicles [30]. A reduced-scale HIL simulation for initial validation of an innovative subway train on a reduced-power experimental test-bench is successfully used in [31]. A reduced-scale HIL platform is implemented to study several hybridization schemes of EVs in [32]. Regarding the capabilities of this approach to reproduce several experimental validations with the same conditions, it was selected to evaluate the more suitable topology regarding the stability enhancement of the motor drive DC input voltage of EVs using multiple ESSs.

The aim of this paper is to perform a comparative and accurate analysis of three configurations of hybrid ESSs with batteries (high specific energy) and SCs (high specific power), with respect to the stability of the motor drive DC input voltage. There are three major contributions. First, a global model of the urban EV under study using the EMR formalism is performed, to obtain a unified organization of the various subsystem models, respecting the causality principle. Second, the application of the EMR formalism leads to a systematic development of a common control scheme architecture that facilitates the articulation with any generic EMS. Finally, an effective reduced-scale power level HIL test-bench is specified and used for performance analysis of the alternative hybrid ESSs with different system control approaches. To the authors’ knowledge, this is the first study of stability enhancement of motor drive DC input using hybrid ESSs regarding motor drive losses minimization.

The proposed study can be extended to other type of EVs, since the model is developed with the EMR formalism and can be easily parameterized for any kind of EV, power supply topology and ESS. A 1 kW reduced-scale power-level HIL test-bench has been used, which allows the extrapolation of the results to real EVs. This extrapolation will be instrumental in the design of a suitable hybrid ESS to improve the performance of the EV prototype under study. We anticipate this is a relevant issue in future work for the on-board implementation of hybrid ESSs for the new generation of EVs.

This paper is organized as follows. The motivation, literature survey, contributions, and organization have been addressed in this introduction. In Section 2, the EV prototype is described and its generic model using EMR including the global maximum control structure is presented. Comprehensive models using the EMR approach and the control layer of different configurations of the hybrid ESSs are developed in Section 3, considering battery-only (T1), direct coupling (T2) and active parallel coupling of batteries and SCs packs (T3). The reduced-scale prototype to analyze the performance of the hybrid ESSs and the process emulation approach are presented in Section 4. The comparative experimental results are fully discussed in Section 5. Conclusions are drawn and future work is outlined in Section 6.