- Wide band-gap devices (GaN, SiC) based converters and active gate driving
- Design optimization of the power converter
- High efficiency high power density power converter for electric mobility
- Renewable energy integration
- Flexible AC/DC smart energy systems and micro-grids
- Medium voltage DC-DC converter for future DC grid applications
- Medium frequency transformers, magnetic material characterization
Gate Driver for Medium Voltage (10 kV) SiC MOSFET
Medium Voltage (MV) Silicon Carbide (SiC) devices have opened up new areas of applications which were previously dominated by silicon-based IGBTs and can achieve high efficiency and power density in MV power conversion applications. However, when SiC devices are used in these applications, they are exposed to high peak stress (5 kV to 10 kV) and a very high dv/dt (10 kV/us to 100 kV/us). Using these devices calls for a gate driver with dc-dc isolation stage which has ultra-low coupling capacitance, in addition to be able to withstand the high isolation voltage. MV gate driver design to address these issues while maintaining a minimal footprint for the gate driver. A medium voltage isolation transformer is designed with a low inter-winding capacitance while maintaining the clearance, creepage, as well as insulation standards. The key features include low input common-mode current, and a short circuit protection scheme specifically designed for 10 kV SiC MOSFETs.
Research focus includes:
- Intelligent gate driver
- Gade driver power supply with low coupling capacitance
- Junction temperature estimation
1500V SiC MOSFETs based Photovoltaic Inverter
Use of 1500 Vdc PV system and cost consideration in design optimization enables capital and installation cost (combiner boxes, cabling, trenching, and labor cost) reduction. The innovative Silicon Carbide (SiC) based soft-switched PV inverter concept along with the novel control scheme result in >33\% lower power conversion losses, thereby improving the power production and generated revenue. The DFR approach, where the reliability of the PV inverter is included at the design stage, along with the control schemes to reduce thermo-mechanical stresses in semiconductor devices and dc-link capacitors ensures highly reliable PV inverter leading to reduced downtime and improved revenue generation. Employ comprehensive mission profile based multi-objective design optimization approach to select design parameters, components, and magnetic material to reduce energy losses, volume, and cost.
Research focus includes:
- SiC Inverter
- Soft-switched PV inverter
- Control and modulation scheme to reduce thermo-mechanical stresses in the semiconductor
- Multi-objective design optimization
Four Quadrant GaN Switch: Application and Characterization
Conventional approaches of ac-dc conversion (ac source feeding the dc load), ac-ac conversion (motor drives), and dc-ac conversion (renewable energy integration) involve multiple power stages. These multiple stages are often decoupled from each other using intermediate passive elements, which occupy significant volume and compromise the power density and reliability. The conventional approach of isolated dc-ac power conversion uses three cascade stages. Efficiency is compromised due to multiple stages of power conversion. DC-link capacitors are used to decouple the dynamics of ac-dc rectifier stage and dc-ac inverter stage, which reduces reliability and power density. Same functionality can be obtained using high frequency link inverter. A cyclo-converter is used on the secondary side of the medium frequency transformer, which combines the functionality of rectifier and inverter stages. A basic building block of the cyclo-converter is the four quadrant switch. The four quadrant switch also finds its application in matrix converters, power supplies, solid-state transformers, Vienna rectifiers, battery chargers, etc.
We explore the use of four quadrant single device GaN switch for cyclo-converter and matrix converter applications. Research focus includes:
- Four quadrant GaN switch characterization
- High frequency link inverter design using four quadrant GaN switch
- Gate driver design and converter packaging
- Modulation and control of cyclo-converter and matrix converter
Parallel Interleaved Multi-level Grid Interactive Inverters
Voltage Source Converter (VSC) are commonly used in many dc/ac power electronics applications and often connected in parallel to increase the current handling capability. The typical semiconductor devices used in high power applications suffer from excessive losses if the switching frequency is increased beyond a few kHz. As a result, large passive filter components are generally employed to comply with the stringent power quality requirements, which lead to the increased cost, size and losses. For a given switching frequency, one of the ways to reduce the filtering requirement is to employ a multi-level VSC. For the parallel connected VSCs, multi-level voltage waveforms can be achieved by the interleaved operation of the parallel VSCs. However, the issue of the circulating current between the parallel VSCs should be addressed to realize the full potential of the interleaved operation. Moreover, there is a scope for improving the harmonic performance of the multi-level voltage waveforms by employing modified multi-level modulation techniques.
In order to realize full potential of the interleaved operation, circulating current between the parallel VSCs should be suppressed to some acceptable limits. One of the ways to address this issue is to introduce sufficient impedance in the circulating current path. We focus on the magnetic component design (coupled inductor, common-mode inductor, integrated inductor) and modulation technique to reduce/suppress the circulating current.
Harmonic performance improvement
The contradictory requirements of the switching loss reduction and the power density improvement can be achieved by the sequential switching of the parallel VSCs, which leads to the multi-level voltage waveforms. Further improvement can be achieved by optimizing the harmonic performance of multi-level output voltage. The sequential switching can be achieved by using the phase-shifted carrier signals. This scheme is widely reported in the literature, as it offers natural balancing of the flux in the inductor employed for the circulating current suppression and it is easy to implement. However, the harmonic performance is not optimal. We investigate modulation techniques to improve the harmonic performance of the multi-level voltage waveforms without compromizing the flux balancing.
High power medium voltage converter using the series and parallel connection of two-level VSCs
The concept of parallel interleaved VSCs can be further expanded to dual converter fed open-end transformer topology for the medium-voltage converter system. In a dual converter fed open-end transformer topology, the open-end transformer winding is fed from both the ends using the converters. Since these two converters are series connected, they have to process the rated current. In many high power applications, single two-level VSC may not be able to supply the rated current. To overcome this problem, parallel connection of the two-level VSCs in each of the converter groups of the open-end transformer topology can be used. In this way, both the voltage and the current handling capability of the converter can be increased. Moreover, the harmonic quality can be improved by the interleaved operation. We study the insulation coordination issues and control of such converters.
Mission-profile Based Multi-objective Design Optimization
The power converter optimization is often performed with an objective to minimize losses, volume or cost. However, the volume optimized design may have higher losses. Therefore it is important to investigate the tradeoff between losses and volume, which can be achieved by performing multi-objective optimization. Multi-objective optimization of power converter is often carried out for a specific loading condition (typically for rated load conditions). However the obtained solution may be sub-optimal if the load varies in a large range, which is the case in most of the power electronics applications. We conduct research on mission-profile based multi-objective optimization approach for designing power converter, where the objective is to minimize the energy loss for a given load profile as well as the volume of the converter.
The procedure has been demonstrated by carrying out multi-objective optimization of 2 MW, 690 V Wind Energy Conversion System (WECS) connected to 30 kV Medium Voltage (MV) grid using the step-up transformer. The volume and energy loss (and not the power loss) have been minimized for a given wind profile. Variation and frequency of the recorded wind speed is obtained using the Weibull distribution function. Using Weibull distribution and exponential power curve expression, different loading conditions and associated time duration is obtained. For these loading conditions, multi-objective optimization has been performed. Accurate loss and volume models of the semiconductor and passive components have been derived.
Flexible Smart Energy Systems
In last century, the power grids were generally used to carry power over long distances from a few centralized generating stations to a large number of loads. The power flow was unidirectional and the system was inefficient, with around 8-20% energy lost during the transmission and distribution, while almost 20-25% of the generation capacity existed to meet the peak power demand. Driven by the high fuel prices and environmental concerns, attention was primarily on optimizing power production and transmission by the end of the 20th century. However the distribution and consumption was completely ignored. In last decade, the use of information and commutation technology is explored to make the complex electrical grid system more efficient and reliable. This can be done by providing monitoring and communication capabilities to the energy producing/consuming devices and other essential equipments of the electrical network. With the communication capabilities, advance metering infrastructure can be created to enable Demand Response (DR). If implemented properly, DR can reduce the requirement of installed peak power plants and can facilitate reduction in energy price. However, better control over the load is required to implement DR effectively. Moreover, with the increased penetration of the distributed generators (majority of them are intermittent sources, such as PV and wind), more control over this sources will be required to reduce its negative impact on the system stability. The power electronics can provide these control capabilities. Therefore, the communication along with the power electronics enabled control is required to achieve common objectives such as energy efficiency, cost saving, and reliable operation.
Our resaerch focus is on power electronics enabled flexible grid concept with objectives to:
- Promote the use of hybrid AC/DC system to increase energy efficiency.
- Decouple prosumer (Producer + Consumer) from low-voltage DC distribution system.
- Enable flexible grid operation to enhance stability.
- Foster autonomy of prosumer by local generation and storage and enable power balance at lowest possible level.
- Enable highly decentralized operation.
- Achieve grid resiliency.