Kazi Saiful Alam

Converters with high boost, energy storage and bi-directional power flow in energy systems

This paper reviews the existing structures for converter stages in renewable energy and motor drive systems that offer high voltage gain at the input of an inverter. The inverter may also connect the DC supply to the utility grid. For traction motor drives in vehicle and rail transport and in high-frequency transformer connected grids, bi-directional power flow capability is also required. Cascaded high-boost non-isolated DC stage which offers multi-stage or multi-level DC inputs and output connections is the focus of attention in this paper. The high voltage DC supply to the inverter allows connections to inverters for motor drive and utility at the required voltage level. This arrangement may lead to reduced battery management issues and reduced size of storage elements, flexibility of connections with several input DC supplies at different levels, and improved reliability through series/parallel connection of converters with some redundancy. Several options for the high-boost stage which are currently available will be reviewed. This will be followed by considerations for energy storage elements and converters suitable for bi-directional power transfer capabilities. Converter topologies and control issues using a high-frequency link for bi-directional power flow between two AC grids will also be discussed.

Design of a bi-directional DC-DC converter for Solid-State Transformer(SST) application by exploiting the shoot through mode

This paper concerns the design of a Solid-State Transformer (SST) based grid integrated battery charger with an isolated bi-directional dc-dc converter to maintain the overall system competitiveness. The proposed full bridge DC-DC converter exploits the shoot through mode in the V2G mode for the boosting purpose, which is the uniqueness of the proposed topology. The inclusion of SST in the design can exhibit substantially compact size and other potential functions such as reactive power flow control, bi-directional power flow etc. The design of the high frequency transformer along with the electrical equivalent circuit obtained from FEA analysis is presented, which is one of the key contributions of this paper. As a proof of concept, simulation results obtained from Matlab-Simulink tools are presented. In summary, design considerations in building the SST based DC-fast charger in a 1-kW power level are presented which can be scaled up to the desired power level according to the requirements.

Design and comprehensive modelling of solid-state transformer(SST) based substation

The concept of Solid-State Transformer (SST) is a subject of vivid attention and extensive research in the recent decade, motivated by their potential impact extending well beyond 50 Hz transformers for power distributions. With the introduction of wide band-gap semiconductor devices such as GaN and SiC, it is starting to look more likely to see SST in field applications in the near future. This paper proposes the design of a compact 10 kVA, Solid State Transformer(SST) based substation. The design employs a dual active bridge (DAB) converter to achieve the bi-directional power flow and zero voltage switching (ZVS) in the switching devices, operated at 50 kHz switching frequency. Comprehensive modelling of the high frequency transformer is presented by considering the multiple core PCB configuration. A 2-D electromagnetic analysis is used to estimate the parameters of high frequency transformer which is implemented in a finite-element analysis software. J-A model of a hysteresis loop is also introduced to predict the magnetic hysteresis behaviour of the high frequency transformer accurately. To support the presented concept, simulation results from Matlab-Simulink tool are shown and discussed.

Multi-cell DC-DC converter based solid-state transformer (SST) design featuring medium-voltage grid-tie application

ISOV (Input Series Output Parallel) modular DAB (Dual Active Bridge) converter based topologies have been mostly considered for solid-state transformer (SST) based power substation design, involving multilevel converters at the medium-voltage side. As DAB converter is one of the core components of this topology, an important question to address is thus the freewheeling currents of DAB which increases the overall losses and reduce the effective duty cycle, in addition to the design optimization issue of the medium frequency transformer. Here, we show the design of 100-kVA SST based power substation which utilizes an isolated bi-directional dc-dc converter in ISOP modular configuration to mitigate these issues. Simulation results in Matlab-Simulink environment are presented to verify the efficacy of the proposed method.

Study on medium-frequency transformer isolated substation

In this paper study on solid-state medium-frequency transformer isolated substation is modeled in detail. The substation is formed by AC/DC rectifier, six-pulse DC/AC inverter, high-frequency transformer, DC/AC six-diode rectifier and DC/AC inverter. Both AC/DC rectifier at high AC grid-1 side and DC/AC inverter at lower grid-2 side are controlled by proportional resonant controller and are interfaced with the grids through LCL filters. LCL filters interfaced with both grids are optimized and the controller parameters are also optimized using differential evolution algorithm. It is found by adding passive filter between six-pulse inverter and medium-frequency transformer, it is possible to remove most harmonic voltage components and ensure that the voltage applied across medium- frequency transformer can be quite close to sinusoidal waveform. By doing so, the core losses could be effectively contained, thereby leading to higher frequency and efficiency operation.