INVESTIGATIONS OF SOLID STATE TRANSFORMERS FOR MICROGRID APPLICATIONS

Abstract

In today's fast-paced world characterized by rapid technological advancements and improved living standards, the demand for electrical energy has surged, becoming indispensable for the sustenance of the human population. However, this increased demand has taken a toll on conventional energy sources, depleting them at an alarming rate. Furthermore, the excessive consumption of these nonrenewable sources has had a detrimental impact on the environment and exacerbated the issue of global warming. In response to these challenges, there has been a growing imperative to transition from non-renewable to renewable energy sources, and solar and wind energy have emerged as the leading drivers of this global clean energy revolution. Solar energy and wind energy are spearheading the shift towards cleaner energy alternatives worldwide. Solar energy, harnessed through photovoltaic (PV) systems, utilizes the power of the sun to generate electricity. The abundance of sunlight in many regions makes solar energy an attractive and sustainable option for power generation. On the other hand, wind energy harnesses the kinetic energy of wind currents to generate electricity through wind turbines. While both solar and wind energy play vital roles in the clean energy transition, solar energy, with its consistent availability and relatively predictable output, is often favored over wind energy due to the intermittent nature of wind speed. The requirement of low maintenance and availability of the modular structure further gave an upper hand to solar PV generation systems. This favored the installation of rooftop PV arrays as small generation units and promoted the concept of microgrids. Further, the PV generation system can be broadly classified as grid-connected mode and standalone mode, but the standalone mode inherently requires an energy storage system for the instantaneous power balance matching, which results in an increment of maintenance requirement and hence, an increment in total expenditure for system operation. Hence, a grid-connected PV generation system is preferred when the grid is available. The PV generation system is required to be operated at MPPT operating point to deliver the maximum power. Hence, a DC-DC converter is required not only to implement the MPPT operation but also for the voltage boost operation to interface the low-voltage PV panels to the utility bus. So, a DCDC converter with higher power density and reliable architecture is the need of the hour. However, with the rapid introduction of power electronic converter-based load to the utility grid, various serious power quality issues such as harmonics, voltage distortion, poor power factor, etc. are also prime issues for investigation. This research work focuses on investigating a three-staged, three-wired, three-phase microgrid implementing a solid-state transformer having an active clamped L-L type current-fed front-end DCDC converter to provide the voltage boost operation and MPPT control operation. The presented SST supported the zero-voltage switching (ZVS) and zero-current switching (ZCS) operations of all primary side switches and secondary side rectifier diodes, respectively, and is used for implementation of perturb and observe (P&O) based MPPT technique. The presented SST topology was found suitable for low-power applications with lower input voltage and higher input current. The presented SST topology is then further extended for medium-power applications with a parallelinput parallel output (PIPO) connection and is being operated in an interleaved operation. Finally, for higher-power applications, the presented SST topology is connected in a series-parallel input parallel output (SPIPO) connection by utilizing four modules of the active clamped L-L type currentfed front-end DC-DC converter. The SPIPO-based SST topology also utilizes interleave operation to exploit it’s benefits and can handle higher voltage ratings than the presented SST topology. The proposed three-phase microgrid utilizes a current-controlled mode (CCM) based grid controller, which utilizes a fundamental signal extractor (FSE) for unit vector generation and grid currents harmonics mitigation. The presented work proposed four new FSEs for unit vector generation and grid currents harmonics mitigation, and their performances were compared with the help of bode plots. The presented system is simulated on MATLAB Simulink platform, and it’s performance is evaluated on various grid anomalies such as variable insolation condition, distorted grid condition, voltage sag & swell, absence of solar power, etc., and the same is validated with experimental results obtained on the OPAL-RT platform. The presented work focuses on the implementation of active clamped L-L type current-fed front-end converter topology-based SST for PV applications and provides a complete control and design approach to the proposed microgrid system.