Rik W. De Doncker

Impedance-based non-linear dynamic battery modeling for automotive applications

This paper presents a systematic approach to employ electrochemical impedance spectroscopy for determining model structure and parameters of a simulation model for a VRLA battery. It focuses on the interpretation of the impedance data in terms of equivalent circuit models and describes the Matlab/Simulink implementation of the model as well as its time-domain verification. Furthermore, the advantages and limits of the impedance-based model as well as the possible simplifications are discussed.

Comparison of the Modular Multilevel DC Converter and the Dual-Active Bridge Converter for Power Conversion in HVDC and MVDC Grids

It is expected that in the near future the use of high-voltage dc (HVDC) transmission and medium-voltage dc (MVDC) distribution technology will expand. This development is driven by the growing share of electrical power generation by renewable energy sources that are located far from load centers and the increased use of distributed power generators in the distribution grid. Power converters that transfer the electric energy between voltage levels and control the power flow in dc grids will be key components in these systems. The recently presented modular multilevel dc converter (M2DC) and the three-phase dual-active bridge converter (DAB) are benchmarked for this task. Three scenarios are examined: a 15 MW converter for power conversion from an HVDC grid to an MVDC grid of a university campus, a gigawatt converter for feeding the energy from an MVDC collector grid of a wind farm into the HVDC grid, and a converter that acts as a power controller between two HVDC grids with the same nominal voltage level. The operation and degrees of freedom of the M2DC are investigated in detail aiming for an optimal design of this converter. The M2DC and the DAB converter are thoroughly compared for the given scenarios in terms of efficiency, amount of semiconductor devices, and expense on capacitive storage and magnetic components.

A method for measurement and interpretation of impedance spectra for industrial batteries

Abstract Impedance spectroscopy is a promising tool for the modeling and diagnosis of industrial batteries. This paper discusses methodological questions connected with the measurement and interpretation of the impedance of such batteries, especially nonlinearity, voltage drift, stability, reproducibility, half-cell measurements, model structure and parameter extraction with respect to quantities like state of charge (SOC). On the basis of this discussion, a specialized impedance spectroscope for industrial batteries has been developed, as well as modifications of the standard electrochemical impedance spectroscopy (EIS) algorithm. A mini-cycle technique is suggested that gains additional information compared to classical measurements with continuous dc current offset. Impedance spectra from lead/acid batteries for different dc currents, SOCs, and temperatures are presented and analyzed. Reference-electrode measurements allow for separation of the half-cell impedances. Emphasis is placed on the limits of experimental reproducibility due to “history” of the battery.

Dynamic and Balanced Control of Three-Phase High-Power Dual-Active Bridge DC–DC Converters in DC-Grid Applications

The three-phase dual-active bridge (DAB) is a dc–dc converter, which provides galvanic isolation, inherent soft-switching capability, and small filter size. In this study, the dynamic behavior of three-phase DAB is analyzed and a dynamic control strategy is developed. Furthermore, a compensation technique is implemented to compensate unbalanced transformer phase currents. The latter is often caused by asymmetric leakage inductances. State space averaging and first harmonic approximation models, both for the steady state and transient analysis, are developed to describe the dynamic behavior of the three-phase DAB. The accuracy of the models is compared with a detailed circuit simulation and the benefits of each model are identified. When the transferred power of the DAB changes fast, the transformer currents can become unbalanced, leading to oscillations in the output current. A unique control method is presented, which allows settling of the transformer currents within one-third of the switching period. Additionally, the transformer currents stay symmetrical and oscillations are avoided. Based on this fast current control, an outer voltage controller is designed. The comparison of the control system using the fast current control and the conventional quasi-steady-state control demonstrates the potential advantages of the new approach under dynamic conditions. In practice, it is difficult to achieve completely symmetrical short-circuit impedances in a high-power medium-voltage transformer. Asymmetric leakage inductances, however, result in unbalanced phase currents and higher dc current ripple in a three-phase DAB. The new control scheme that is developed here can be extended to compensate any unbalances in the transformer. This approach enables effectively the balancing of the three-phase currents. The new control schemes are experimentally verified.

A frequency-domain approach to dynamical modeling of electrochemical power sources

Electrochemical impedance spectroscopy can be used to obtain simulation models for the non-linear, non-stationary dynamic behavior of electrochemical power sources. Some extensions of porous-electrode theory are necessary for this purpose, which are not provided by standard EIS data evaluation software. This paper presents experimental data and consistent equivalent-circuit models for different dc conditions, regarding three electrochemical systems: supercapacitors as almost ideally blocking porous electrodes, lead/acid batteries under overcharge (water electrolysis) operation, and the same battery type under discharge/charge operation. The series inductance which masks the electrochemical behavior for frequencies as low as 100 Hz for a 100 Ah cell can be attributed mainly to the macroscopic cell geometry. The dependence of impedance parameters on direct current (non-linearity), temperature, state-of-charge, and previous discharge/charge regime is investigated. It is shown that model parameters extracted from the impedance spectra are closely linked with the charge-transfer kinetics, double-layer capacitance, transport limitation, and porous structure of the electrodes. Consequences for impedance-based determination of the batterys state-of-charge or state-of-health are discussed.

A Dual-Side Controlled Inductive Power Transfer System Optimized for Large Coupling Factor Variations and Partial Load

In this study, a 3-kW inductive power transfer system is investigated, specifically intended for contactless vehicle charging. A series-series-compensated topology with dual-side power control and a corresponding control strategy is proposed to significantly increase the overall efficiency, especially for systems with large coupling factor variations and in partial load mode. The topology, which is closely related to the dual-active bridge converter, enables the dual-side power control without adding additional dc/dc converters to the system, and thus keeping the additional hardware effort minimal. A detailed analysis of the proposed topology is provided, and the benefits of the dual-side control are demonstrated both theoretically and experimentally. A hardware prototype is built and a peak dc-to-dc efficiency of 95.8% at 100 mm air gap and a minimal efficiency of 92.1% at 170 mm air gap is measured, including the power electronic components. The partial load efficiency at 500 W output power is still as high as 90.6% at 135 mm air gap. Overall, the proposed topology provides a practical method to overcome the main drawback of most single-side controlled inductive power transfer systems, which is a significant efficiency drop outside the nominal operating point.

Advanced Electrical Drives

Electrical drives convert in a controlled manner, electrical energy into mechanical energy. Electrical drives comprise an electrical machine, i.e. an electro-mechanical energy converter, a power electronic converter, i.e. an electrical-to-electrical converter, and a controller/communication unit. Today, electrical drives are used as propulsion systems in high-speed trains, elevators, escalators, electric ships, electric forklift trucks and electric vehicles. Advanced control algorithms (mostly digitally implemented) allow torque control over a high-bandwidth. Hence, precise motion control can be achieved. Examples are drives in robots, pick-andplace machines, factory automation hardware, etc. Most drives can operate in motoring and generating mode. Wind turbines use electrical drives to convert wind energy into electrical energy. More and more, variable speed drives are used to save energy for example, in air-conditioning units, compressors, blowers, pumps and home appliances. Key to ensure stable operation of a drive in the aforementioned applications are torque control algorithms. In Advanced Electrical Drives, a unique approach is followed to derive model based torque controllers for all types of Lorentz force machines, i.e. DC, synchronous and induction machines. The rotating transformer model forms the basis for this generalized modeling approach that ultimately leads to the development of universal field-oriented control algorithms.

A dual-phase-shift control strategy for dual-active-bridge DC-DC converter in wide voltage range

This paper analyzes the dual-phase-shift (DPS) control strategy for a dual-active-bridge (DAB) converter in whole operation range. The DPS has two degree of freedom to control the transferring power, which can improve the performance of the DAB converter than single-phase-shift (SPS) control strategy. This paper derives the reactive power, the rms and the peak current flowing on the transformer and the zero-voltage-switching (ZVS) condition according to the operating conditions. Using this analysis and simulation results, a suitable control strategy for DAB converter is obtained. Experiments are performed to verify the proposed control strategy and to compare with the conventional SPS control strategy. The experimental results show that the proposed method can enhance the overall efficiency and expand the ZVS operation range.

Advantages of a variable DC-link voltage by using a DC-DC converter in hybrid-electric vehicles

In electric and hybrid-electric vehicle applications, power electronic converters are used to couple the traction machine with the battery stack. The simplest topology consists of an inverter between the battery stack and the machine. In an often discussed alternative, a DC-DC converter is used to feed the traction machine with a higher and stable voltage level. This paper deals with the differences which arise in terms of the applied control strategy and identifies operating regions of improved efficiency.

Reliability Prediction for Inverters in Hybrid Electrical Vehicles

Due to the increasing importance of power electronic components in automobiles, it becomes necessary to consider their reliability. This applies especially to hybrid electrical vehicles (HEV) where a malfunction of the power electronics may prevent the vehicle to operate. Of paramount importance for the reliability of power electronics is the component operating temperature and temperature cycling. This paper deals with the development of an advanced simulation tool which is capable of determining the component temperature of a three-phase converter over long mission profiles. In addition, the expected converter reliability is calculated. To accomplish this, losses in the semiconductors and dc-link capacitors are determined first. Next, this loss data is fed into a thermal model to compute the component temperatures, for the whole mission profile. As basis for the reliability computation, failure-rate catalogs, such as Military Handbook 217F or RDF 2000, are used. Also an approach using simple formulas for lifetime prediction is presented. According to failure-rate catalogs, temperature cycles are of particular importance for the reliability of power semiconductors. A novel algorithm, detecting all relevant temperature cycles within the computed temperature curve is developed. Finally, the applicability and significance of the presented reliability prediction methods is assessed.