Nilanjan Mukherjee

Control of Second-Life Hybrid Battery Energy Storage System Based on Modular Boost-Multilevel Buck Converter

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.

Control of Cascaded DC–DC Converter-Based Hybrid Battery Energy Storage Systems—Part I: Stability Issue

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.

Analysis and Comparative Study of Different Converter Modes in Modular Second-Life Hybrid Battery Energy Storage Systems

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.

Adaptive control of hybrid battery energy storage systems under capacity fade

There is an increasing call for applications which use a mixture of batteries. These hybrid battery solutions may contain different battery types for example; using second life ex-transportation batteries in grid support applications or a combination of high power, low energy and low power, high energy batteries to meet multiple energy requirements or even the same battery types but under different states of health for example, being able to hot swap out a battery when it has failed in an application without changing all the batteries and ending up with batteries with different performances, capacities and impedances. These types of applications typically use multi-modular converters to allow hot swapping to take place without affecting the overall performance of the system. A key element of the control is how the different battery performance characteristics may be taken into account and the how the power is then shared among the different batteries in line with their performance. This paper proposes a control strategy which allows the power in the batteries to be effectively distributed even under capacity fade conditions using adaptive power sharing strategy. This strategy is then validated against a system of three different battery types connected to a multi-modular converter both with and without capacity fade mechanisms in place.

Control of Cascaded DC–DC Converter-Based Hybrid Battery Energy Storage Systems—Part II: Lyapunov Approach

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.

Modular multilevel converter based supercapacitor integration strategies and their comparative evaluation for railway traction drive systems

Modular multilevel converters are an emerging class of converters for traction applications. Supercapacitor energy storage integration within the sub-module provides multiple advantages compared to the traditional two-level design in terms of reliability, power quality, drive efficiency and recovery of regenerative energy. This paper explores different topologies of modular multilevel converters with integrated supercapacitor energy storage system for the typical voltages (750 V, 1.5 kV and 3 kV) of the railway traction system. The paper first derives different ways of integrating supercapacitor modules using a modular multilevel configuration. Thereafter, a detailed comparison in terms of converter cost/VA rating, reliability and suitability for traction drive systems have been presented to identify the most suitable modular converter scheme with integrated energy storage.

Modular ESS with second life batteries operating in grid independent mode

This work is part of a bigger project which aims to research the potential development of commercial opportunities for the re-use of batteries after their use in low carbon vehicles on an electricity grid or microgrid system. There are three main revenue streams (peak load lopping on the distribution Network to allow for network re-enforcement deferral, National Grid primary/ secondary/ high frequency response, customer energy management optimization). These incomes streams are dependent on the grid system being present. However, there is additional opportunity to be gained from also using these batteries to provide UPS backup when the grid is no longer present. Most UPS or ESS on the market use new batteries in conjunction with a two level converter interface. This produces a reliable backup solution in the case of loss of mains power, but may be expensive to implement. This paper introduces a modular multilevel cascade converter (MMCC) based ESS using second-life batteries for use on a grid independent industrial plant without any additional onsite generator as a potentially cheaper alternative. The number of modules has been designed for a given reliability target and these modules could be used to minimize/eliminate the output filter. An appropriate strategy to provide voltage and frequency control in a grid independent system is described and simulated under different disturbance conditions such as load switching, fault conditions or a large motor starting. A comparison of the results from the modular topology against a traditional two level converter is provided to prove similar performance criteria. The proposed ESS and control strategy is an acceptable way of providing backup power in the event of loss of grid. Additional financial benefit to the customer may be obtained by using a second life battery in this way.

A new state of charge control of modular multilevel converters with supercapacitors for traction drives

Modular multilevel converters with storage devices integrated in the submodules are emerging topologies for traction drives. This paper proposes a new state-of-charge control technique to optimally utilise supercapacitor cells that are connected to the sub-module capacitors by means of boost DC/DC converters. The proposed strategy ensures that the charging/discharging trajectory of all the supercapacitor cells during a charging or discharging cycle arrive at their final state-of-charge values at the same time, irrespective of their initial balancing. With this control, the energy stored in the super-capacitor cells is appropriately supplied/absorbed to/from the motor and, at the same time, it eliminates extra control loops for the state-of-charge balancing control. The proposed technique modulates the output voltage of the sub-module DC-DC converter according to its instantaneous state-of-charge of supercapacitor cells. The key challenge regarding the voltage based control of the modular multilevel converter is discussed in detailed. The advantages and drawbacks of this technique compared to the existing balancing technique are documented. At last, the validity of the proposed technique is presented with the help of numerical simulations in Matlab/Simulink.

A state-of-charge equalisation technique of super-capacitor energy storage systems using sub-module dc-dc converter control within modular multilevel converter (MMC) for high speed traction drive applications

This paper proposes a new technique to balance the state-of-charge of supercapacitor cells within a modular multilevel converter (MMC) for high speed traction drive applications. The proposed configuration uses an H-bridge sub-module with an integrated DC-DC converter and it controls the voltage of the sub-module capacitor according the state of charge of the storage cells. Each phase leg of the MMC is also controlled to achieve the balancing of supercapacitor cells. This paper presents in details the advantages of this voltage based control method and corresponding key challenges. At last, the validity of the proposed control technique is validated by numerical simulations using a full closed-loop model of the MMC.

Distributed control structures and their comparison in cascaded hybrid battery energy storage systems

A hybrid battery system within an energy storage system is gaining interests because it provides multiple advantages compared to the traditional single type battery system in terms of cost, volume, performance, and flexibility. Moreover, motivation on using second life ex-transportation batteries on the grid applications has given a boost on using hybrid batteries because vehicle batteries could be from different manufacturer, different sizes and capacities. A modular dc-dc converter with a battery per module is a reasonable way of integrating these batteries to a grid-tie inverter. This paper addresses the control aspect of such system and discusses two distributed control strategies which independently utilises each converter module within the system according to their module characteristics. A comparison of two control strategies in terms of operational stability and dynamic response both in charging and discharging conditions is presented to understand their suitability in this application. Detailed analysis, simulation and experimental validation of the control methods are included in grid connected mode to understand both control methods.