03-09-2014, 11:31 AM
Hybrid Electric Vehicle Subsystem
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1. Introduction
The near-future technologies related to hybrid electric vehicles (HEV) are the most promising alternatives to cope with the reduction of greenhouse gases in the car industry. In particular, plug-in HEV and vehicle-to-grid (VTG) concepts will have a tremendous impact not only on the reduction of greenhouse gases but also on electricity distribution systems. Above all, these new technologies will heavily depend on battery packs. It is therefore important to develop accurate battery models that can conveniently be used with simulators of power systems and on-board power electronic systems. There are basically three types of battery models reported in the literature, specifically: experimental, electrochemical and electric circuit-based. Experimental and electrochemical models are not well suited to represent cell dynamics for the purpose of state-of-charge (SOC) estimations of battery packs. However, electric circuit-based models can be useful to represent electrical characteristics of batteries. The simplest electric model consists of an ideal voltage source in series with an internal resistance.
This model, however, does not take into account the battery SOC. Another model is based on an open circuit voltage in series with resistance and parallel RC circuits with the so-called Warburg impedance. The identification of all the parameters of this model is based on a rather complicated technique called impedance spectroscopy. Shepherd developed an equation to describe the electrochemical behavior of a battery directly in terms of terminal voltage, open circuit voltage, internal resistance, discharge current and state-of-charge, and this model is applied for discharge as well as for charge. The Shepherd model is interesting but causes an algebraic loop problem in the closed-loop simulation of modular models. Battery models with only SOC as a state variable.
These models are very similar to Shepherd’s but don’t produce an algebraic loop. In this paper, a model using only SOC as a state variable is chosen in order to accurately reproduce the manufacturer’s curves for the four major types of battery chemistries. These four types are: Lead-Acid, Lithium Ion (Li-Ion), Nickel-Cadmium (NiCd) and Nickel-Metal-Hydride (NiMH). The paper is divided into three sections. In the first section, the proposed model and its parameters are described. Furthermore, a method is presented to show how to determine the model parameters from the manufacturer’s discharge curves of the battery. In the second section, discharge curves are obtained by simulation and validated with the manufacturer’s datasheets. The third section contains an example of an application where the battery model integrated to the SimPowerSystems (SPS) is used in the complete simulation of an HEV power train. The paper ends with a conclusion.
Energy Management Subsystem
The Energy Management Subsystem (EMS) determines the reference signals for the electric motor drive, the electric generator drive and the internal combustion engine in order to distribute accurately the power from these three sources. These signals are calculated using mainly the position of the accelerator, which is between -100% and 100%, and the measured HEV speed. Note that a negative accelerator position represents a positive brake position.
• The Battery management system maintains the State-Of-Charge (SOC) between 40 and 80%. Also, it prevents against voltage collapse by controlling the power required from the battery.
• The Hybrid Management System controls the reference power of the electrical motor by splitting the power demand as a function of the available power of the battery and the generator. The required generator power is achieved by controlling the generator torque and the ICE speed
4. Conclusion
The modeling of a battery is a very complex procedureand requires a thorough knowledge of electrochemistry. Thesimulation of complete systems, as with the hybrid car, doesn’trequire such a high level of precision. It is important toknow the general behavior of a battery (for example, itis important to represent the fact that the voltage availabledepends on the SOC and the current). The proposed model issimple and requires few parameters (only three points on thedischarge curve are necessary). Above all, it was shown that the model can accurately represent the discharge curves of themanufacturers. Finally, the model is inserted in a simulation model based on the Toyota Prius THS-II vehicle. The batterymodel can be used to refine the EMS in order not to exceed the limits of the battery. The results obtained show that the useof this battery model makes it possible to properly represent the transient states. It is thus possible to analyze them in orderto fine-tune the various control devices.