Chris Gould

Analysis of CLL Voltage-Output ResonantConverters Using Describing Functions

A new AC equivalent circuit for the CLL voltage-output resonant converter is presented, that offers improved accuracy compared with traditional FMA-based techniques. By employing describing function techniques, the nonlinear interaction of the parallel inductor, rectifier and load is replaced by a complex impedance, thereby facilitating the use of AC equivalent circuit analysis methodologies. Moreover, both continuous and discontinuous rectifier-current operating conditions are addressed. A generic normalized analysis of the converter is also presented. To further aid the designer, error maps are used to demonstrate the boundaries for providing accurate behavioral predictions. A comparison of theoretical results with those from simulation studies and experimental measurements from a prototype converter, are also included as a means of clarifying the benefits of the proposed techniques.

EV/HEV Li-ion battery modelling and State-of-Function determination

The paper describes the application of a realtime, adaptive battery modelling methodology to Li-ion batteries. This methodology allows accurate estimation of the State-of-Function (SoF) of batteries for an Electric or Hybrid-Electric vehicle. Through use of a Kalman Estimator and online battery model parameter estimation, the voltages associated with monitoring the State of Charge (SoC) of the battery system are shown to be accurately estimated, even given erroneous initial conditions in both the model and parameters. In this way, problems such as self-discharge during non-use of the cells and SoC drift (as usually incurred by coulomb-counting methods due to over-charging or ambient temperature fluctuations) are overcome. A further benefit of the adaptive nature of the parameter estimation allows battery ageing (State of Health - SoH) to be monitored and, in the case of safety-critical systems, cell failure may be predicted in time to avoid inconvenience to passenger networks. Moreover, the ability to accurately predict the SoF and changes in battery parameters allows charging scenarios to be optimized to extend lifetime and facilitate future “Vehicle-to-Grid” (V2G) implementation.

A comparative study of on-board bidirectional chargers for electric vehicles to support vehicle-to-grid power transfer

In order to provide stabilization of the utility power supply during peak-load times, the possibility for use of an Electric Vehicles (EV) traction battery for energy buffering has been proposed, and an On-Board Bidirectional Charger is required to allow efficient battery charging regimes, whilst enabling power to be fed back to the grid pursuant to necessary standards. Here, an in-depth case-study comparison between the use of the EV motor inverter as the bidirectional charger with low frequency transformer (for galvanic isolation), and the use of a high frequency transformer with associated Dual Active Bridge topologies has been undertaken to identify the merits and issues associated with such technologies

Viability of “second-life” use of electric and hybridelectric vehicle battery packs

In order for successful second-life implementation of Electric Vehicle (EV) battery packs, the viability of the intended second-life use must be ascertained based on a cost-benefit analysis and technical appraisal of the estimated condition of the available battery packs. This paper discusses the issues in measuring State-of-Health (SoH) and other battery condition metrics of a battery pack. Measurements on real-life battery packs sent for recycling are taken that demonstrate a typical 85% SoH; slightly higher than predicted by Original Equipment Manufacturers (OEM). A model is introduced that can simulate the energy demand in a home/dwelling being met by a number of sources including mains (utility) power, photovoltaic generation (PV), and second-life battery storage. The model is applied to three scenarios using second-life battery storage, to create energy costs savings through time-shifting of energy using on-peak/off-peak electricity tariffs. For each scenario a cost-benefit analysis is produced, indicating that whilst energy costs savings can be achieved, excessive usage of the battery pack can cause the payback period of the capital investment to be longer than the predicted second-lifetime of the battery pack. However, the final scenario demonstrates that combining the battery pack with local generation, such as PV, yields cost savings that are significant at 75%, and the payback period is within the estimated lifetime of the battery pack.

Battery health determination by subspace parameter estimation and sliding mode control for an all-electric Personal Rapid Transit vehicle — the ULTra

The paper describes a real-time adaptive battery modelling methodology for use in an all electric personal rapid transit (PRT) vehicle. Through use of a sliding-mode observer and online subspace parameter estimation, the voltages associated with monitoring the state of charge (SoC) of the battery system are shown to be accurately estimated, even with erroneous initial conditions in both the model and parameters. In this way, problems such as self- discharge during storage of the cells and SoC drift (as usually incurred by coulomb-counting methods due to overcharging or ambient temperature fluctuations) are overcome. Moreover, through online monitoring of the degradation of the estimated parameters, battery ageing (State of Health) can be monitored and, in the case of safety- critical systems, cell failure may be predicted in time to avoid inconvenience to passenger networks. Due to the adaptive nature of the proposed methodology, this system can be implemented over a wide range of operating environments, applications and battery topologies, by adjustment of the underlying state-space model.

Current ripple reduction in 4kW LLC resonant converter based battery charger for electric vehicles

Electric vehicles rely on efficient charging techniques to maximize battery performance, efficiency and lifetime. Most EV on-board chargers are supplied from single phase AC mains, and contain 2nd order mains frequency harmonic in the battery current. This harmonic current incurs extra loss in the battery, increases battery temperature, and hence reduces charging efficiency and battery lifetime. Conventional battery charger controllers are unable to reject this harmonic current completely. This paper describes a resonant controller employed to suppress the low-frequency current ripple in an LLC resonant converter for EV battery chargers. A small-signal model of the LLC resonant converter based battery charging system has been established from time-domain simulations by injecting a small perturbation signal into the converter switching frequency. A resonant controller is subsequently designed and the closed-loop charger system performance is analyzed. The effectiveness of the proposed resonant controller on suppressing the harmonic current is assessed by extensive simulations and experimental tests.

Prediction of device temperatures in an electric vehicle battery charger system by analysis of device thermal cross-coupling

This paper presents a practical implementation of the measurement and use of thermal cross-coupling data. Thermal cross-coupling, the thermal transfer impedance between the heat generation of one power device and the temperature of another, is characterised using pseudo-random binary sequences for a typical H-bridge arrangement. Knowledge of the cross-coupling between each pair of devices allows temperature profiles to be predicted for arbitrary input power waveforms. The technique is verified by applying arbitrary power waveforms to all devices in the experimental arrangement and measuring the temperature response. Good agreement is shown between predicted and practical device temperature waveforms, demonstrating the viability of the technique.

PWM Harmonic Signature-Based Islanding Detection for a Single-Phase Inverter With PWM Frequency Hopping

Distributed generation (DG) has gained popularity in recent years due to the increasing requirement for renewable power sources. A problem that exists with DG systems is the islanding of DG units that creates safety issues for personnel as well as the potential for damage to utility infrastructure. Therefore, islanding detection methods are utilized to mitigate the risk of islanded operation of DG units. A new passive method of islanding detection based on the signature of the PWM voltage harmonics is proposed. The viability of the algorithm is investigated with the use of an analytical and time domain model of the inverter and further validated with experimental results. Furthermore, an extension of the detection scheme is proposed for use in multi-inverter scenarios composed of adaptive frequency hopping to eliminate unwanted tripping.

PWM harmonic signature based islanding detection for a single-phase inverter with PWM frequency hopping

Distributed generation (DG) has gained popularity in recent years due to the increasing requirement for renewable power sources. A problem that exists with DG systems is the islanding of DG units that creates safety issues for personnel as well as the potential for damage to utility infrastructure. Therefore, islanding detection methods are utilized to mitigate the risk of islanded operation of DG units. A new passive method of islanding detection based on the signature of the pulse width modulation voltage harmonics is proposed. The viability of the algorithm is investigated with the use of an analytical and time-domain model of the inverter and further validated with experimental results. Furthermore, an extension of the detection scheme is proposed for use in multi-inverter scenarios composed of adaptive frequency hopping to eliminate unwanted tripping.