To fully utilize second-life batteries on the grid system, a hybrid battery scheme needs to be considered for several reasons: the uncertainty over using a single source supply chain for second-life batteries, the differences in evolving battery chemistry and battery configuration by different suppliers to strive for greater power levels, and the uncertainty of degradation within a second-life battery. Therefore, these hybrid battery systems could have widely different module voltage, capacity, and initial state of charge and state of health. In order to suitably integrate and control these widely different batteries, a suitable multimodular converter topology and an associated control structure are required. This paper addresses these issues proposing a modular boost-multilevel buck converter based topology to integrate these hybrid second-life batteries to a grid-tie inverter. Thereafter, a suitable module-based distributed control architecture is introduced to independently utilize each converter module according to its characteristics. The proposed converter and control architecture are found to be flexible enough to integrate widely different batteries to an inverter dc link. Modeling, analysis, and experimental validation are performed on a single-phase modular hybrid battery energy storage system prototype to understand the operation of the control strategy with different hybrid battery configurations.

This paper presents research from part of a larger project focusing on the potential development of commercial opportunities for the reuse of batteries on the electricity grid system, subsequent to their primary use in low and ultra-low carbon vehicles, and investigating the life cycle issues surrounding the batteries. The work has three main areas; examination of electric vehicle fleet data in detail to investigate usage in first life. Batteries that have passed through a battery recycler at the end of their first life have been tested within the laboratory to confirm the general assumption that remaining capacity of 80% after use in transportation is a reasonable assumption as a basis for second-life applications. The third aspect of the paper is an investigation of the equivalent usage for three different second-life applications based on connection to the electricity grid. Additionally, the paper estimates the time to cell failure of the batteries within their second-life application to estimate lifespan for use within commercial investigations.

The use of extransportation battery system (i.e., second-life electric vehicle/hybrid electric vehicle batteries) in grid applications is an emerging field of study. A hybrid battery scheme offers a more practical approach in second-life battery energy storage systems, because battery modules could be from different sources/vehicle manufacturers depending on the second-life supply chain and have different characteristics, e.g., voltage levels, maximum capacity, and also different levels of degradations. Recent research studies have suggested a dc-side modular multilevel converter topology to integrate these hybrid batteries to a grid-tie inverter. Depending on the battery module characteristics, the dc-side modular converter can adopt different modes, such as boost, buck, or boost-buck to suitably transfer the power from the battery to the grid. These modes have different switching techniques, control range, different efficiencies, which give a system designer choice on an operational mode. This paper presents an analysis and comparative study of all the modes of the converter along with their switching performances in detail to understand the relative advantages and disadvantages of each mode to help to select the suitable converter mode. Detailed study of all the converter modes and the thorough experimental results based on a multimodular converter prototype with hybrid batteries have been presented to validate the analysis.

A cascaded dc-dc boost converter is one of the ways to integrate hybrid battery types within a grid-tie inverter. Due to the presence of different battery parameters within the system such as, state-of-charge and/or capacity, a module-based distributed power sharing strategy may be used. To implement this sharing strategy, the desired control reference for each module voltage/current control loop needs to be dynamically varied according to these battery parameters. This can cause stability problem within the cascaded converters due to relative battery parameter-variations when using the conventional proportional-integral (PI) control approach. This paper proposes a new control method based on Lyapunov functions to eliminate this issue. The proposed solution provides a global asymptotic stability at a module level avoiding any instability issue due to parameter variations. A detailed analysis and design of the nonlinear control structure is presented under the distributed sharing control. At last thorough experimental investigations are shown to prove the effectiveness of the proposed control under grid-tie conditions.

There is an emerging application, which uses a mixture of batteries within an energy storage system. These hybrid battery solutions may contain different battery types. A dc-side cascaded boost converters along with a module-based distributed power sharing strategy has been proposed to cope with variations in battery parameters such as, state-of-charge (SOC) and/or capacity. This power sharing strategy distributes the total power among the different battery modules according to these battery parameters. Each module controller consists of an outer voltage-loop with an inner current-loop where the desired control reference for each control-loop needs to be dynamically varied according to battery parameters to undertake this sharing. As a result, the designed control bandwidth (BW) or stability margin of each module control-loop may vary in a wide range, which can cause a stability problem within the cascaded converter. This paper reports such a unique issue and thoroughly investigates the stability of the modular converter under the distributed sharing scheme. The paper shows that a cascaded PI control-loop approach cannot guarantee the system stability throughout the operating conditions. A detailed analysis of the stability issue and the limitations of the conventional approach are highlighted. Finally in-depth experimental results are presented to prove the stability issue using a modular hybrid battery energy storage system prototype under various operating conditions.

Recently, losses reduction gained a great deal of attention in distribution system due to low-voltage level and the high-current passing through the lines, pushing the distribution utilities to improve their profit margins on one hand by reducing the unnecessary operational cost, and improving their delivered power quality on the other hand by maintaining the system reliability, and the continuity of supply for varying load demand. Load balancing, voltage regulation, network reconfiguration and others are different techniques used to reduce the losses. This study addresses the distribution network reconfiguration to minimise the network losses. A new modified form of particle swarm optimisation (PSO) is used to identify the optimal configuration of distribution network effectively. The difference between the modified PSO (MPSO) algorithms and the typical one is the filtered random selective search space for initial position, which is proposed to accelerate the algorithm for reaching the optimum solution. The suggested MPSO is tested via 33 and 69 IEEE networks. A benchmark comparison has been conducted to prove the effectiveness of MPSO compared with previous optimisation techniques.