S. Raghu Raman

Review of charge equalization schemes for Li-ion battery and super-capacitor energy storage systems

Charge equalization of series connected energy storage elements (batteries and super-capacitors) has significant ramifications on their life and also reduces their operational hazards. This paper reviews the current status and art of power electronics converter topologies employed for charge equalization of Li-ion battery and super-capacitors based energy storage systems. Charge equalization schemes are broadly categorized into passive and active cell balancing. Operating principles of various schemes are discussed and compared.

Multi-Input Switched-Capacitor Multilevel Inverter for High-Frequency AC Power Distribution

This paper proposes a switched-capacitor multilevel inverter for high-frequency ac power distribution systems. The proposed topology produces a staircase waveform with higher number of output levels employing fewer components compared to several existing switched-capacitor multilevel inverters in the literature. This topology is beneficial where asymmetric dc voltage sources are available, e.g., in case of renewable energy farms based ac microgrids and modern electric vehicles. Utilizing the available dc sources as inputs for a single inverter solves the major problem of connecting several inverters in parallel. Additionally, the need to stack voltage sources, like batteries or supercapacitors, in series which demand charge equalization algorithms, are eliminated as the voltage sources employed share a common ground. The inverter inherently solves the problem of capacitor voltage balancing as each capacitor is charged to the value equal to one of the input voltage every cycle. State analysis, losses, and the selection of capacitance are examined. Simulation and experimental results at different distribution frequencies, power levels, and output harmonic content are provided to demonstrate the feasibility of the proposed multilevel inverter topology.

Switched-capacitor multilevel inverters for high frequency AC microgrids

This paper proposes a novel topology of a switched-capacitor multilevel inverter for high frequency AC microgrids. It employs multiple DC sources which make it ideal to be employed in renewable energy sources based AC microgrids. With the proliferation in renewable energy based solar and wind farms, such multi-input topologies gain tremendous potential. Switched-capacitor based DC-DC converter in the front end produces multiple levels at the DC bus. Further, the H-bridge inverter obtains corresponding bipolar levels and a zero level at the output. The problem of capacitor voltage balancing is inherently solved as the capacitors are charged to the input voltage every cycle. Working principle and primary analysis of the topology are presented. Simulation outcomes are validated with experimental results at different distribution frequencies and power levels.

A hybrid multilevel inverter employing series-parallel switched-capacitor unit

This paper presents a new hybrid multilevel inverter (MLI) which adopts switched-capacitor (SC) to step-up the voltage and improve the output power quality. With a simple series-parallel unit, the voltage magnitude and number of output levels of the inverter are doubled. The proposed topology can be applied to arbitrary number of series-connected voltage sources. This paper also suggests the design of gate drive voltage supplies and a scheme to substantially reduce the number of active components for high power factor applications which further simplifies the design and system cost. Due to the considerable increase in number of output levels, the proposed SCMLI has satisfactory output power quality with simple staircase modulation techniques switching at fundamental frequency. The working principle of the proposed SCMLI and the associated staircase modulation technique presented here are studied and verified by simulation and experiment.

Ideas for future electric aircraft system

This design paper proposes few improvements on the existing electrical network in the aircraft system. It also suggests improvement in the starter circuitry by introducing super capacitors to reduce the number of batteries onboard to ensure improved safety and reliability. It proposes higher frequency of power distribution owing to several benefits. There are several advantages of high frequency AC power distribution over conventional DC distribution and low frequency AC power distribution. This paper explores the idea of employing switched capacitor and switched inductor converters to design multi-level inverters for high frequency AC power supplies for power distribution.

Electromagnetic scattering of high power traction transformer in high speed railway based on FEM

The paper presents the investigation of the electromagnetic interference for the traction system in a high speed railway (HSR). Tt is important to study the characteristic of the transformer, converter and power cable for HSR. Traction transformer as the one of the first equipment on board traction system for connection to pantograph, and plays a key role in the electromagnetic investigation. In this paper, the characteristic of the transformer is studied to further provide information for investigation of electromagnetic interference.

A seven level switched capacitor multilevel inverter with asymmetric input sources for microgrids

This paper presents a seven level inverter utilizing switched-capacitor technique. Proposed topology employs two asymmetric DC voltage sources as input and generates a multilevel staircase output. The structure includes a front-end switched-capacitor based DC-DC converter cascaded by a back-end H-bridge inverter. The frontend SC DC-DC converter feeds three DC voltage levels to the H-bridge inverter which produces the corresponding bipolar levels. Such inverters have the potential to be employed in AC microgrids where there are several asymmetric DC sources available. The inverter naturally solves the problem of capacitor voltage balancing as the capacitor is charged to a constant value twice every cycle. This topology also eliminates the need for series connection of individual voltage sources which require voltage balancing algorithms. Simulation and experiment results for the proposed inverter are presented.

A six-phase bi-directional interleaved converter

This paper investigates the operation of a bi-directional interleaved converter. The DC-DC converter can operate both as a buck and a boost converter as per operating requirements. Design methodology for the converter operating in either modes is presented in detail. Analysis on duty cycle limits, voltage conversion ratios and efficiency including non-idealities of the converter is presented. Extensive simulation and experiment results corroborate the theory.

Performance prediction of light electric vehicles powered by body-integrated super-capacitors

Due to higher specific power density, longer life time and safer operation, super-capacitors (ultra-capacitors or electric double layer capacitors) are being considered in applications of electric vehicles (EVs). This study deals with design of light electric vehicles or even rail vehicle powered by full or partial pure super-capacitors (SCs). The simulation model of the electric vehicles powered by super-capacitors is proposed for predicting the EV performances of acceleration and travelled distance. The design objectives of a case are presented. Moreover, the development of the super-capacitor pack consisting of body-integrated SC modules is discussed. Simulated results demonstrate that the objective of the designed light EV powered by body-integrated super-capacitor pack is achieved.

Investigation of energy distribution and power split of hybrid energy storage systems in electric vehicles

In this paper, the model of the electric vehicles (EVs) with the hybrid energy storage system (HESS) consisting of battery and super-capacitor is proposed. Using the proposed model, the operation of the EV with HESS is simulated for various energy distributions of battery and super-capacitor (SC) energy storage system. The losses of the battery and super-capacitor, the travelled distance and the ratio of the SC power to the bus power are investigated and discussed. The study in this paper reveals the relationships between the losses of the battery and the super-capacitor, the energy distribution, the power allocation, the travelled distance and the load, and shows that the optimal ratio of the SC power to the bus power can be found to minimize the losses of the battery and the SC and maximize the travelled distance of the EV. Thus, this paper offers the valuable investigation for the optimal energy management and the maximum efficiency control of HESS.