Investigation and Control of Magnetically Coupled Impedance Source Inverters For Pv Applications

Abstract

Indian power sectors expect to use more rooftop photovoltaic (PV) inte- gration to the power grid in the near future. However, the produced power from PV is highly dependent on solar irradiation and temperature, which are irregular and hard to monitor. Therefore, power electronic converters are expected to harvest the maximum available power from PV panels and then export it to the grid based on their requirements. The commercial two-stage grid-connected PV inverters are limited to narrow range MPP voltage, which requires higher starting or wake-up voltages to start the inverters. When the PV fails to maintain the inverter’s minimum oper- ating voltage, it shuts down. A shutdown inverter must now undergo a start-up process upon the cloud clearing, at which it must monitor both grid voltage and frequency for a given period before going online. Also, these inverters cannot function in the early morning and late evening due to low string voltages. In particular, the parallel configured PV mod- ules become an ideal solution for a rooftop generation system that does not suffer from shading problems. But this design requires a power con- version with high voltage gain to match the grid voltages. Single-stage impedance source inverters (ZSI) are preferable for producing high volt- age gains over two-stage converters due to their outstanding features such as single-stage buck-boost and inversion ability, high voltage gain, and inherent short-circuit/open-circuit protections. However, the ZSIs exhibit non-minimum phase behaviour due to the right half-plane (RHP) zero in the converter transfer functions that impose a constraint on the controller design. A detailed mathematical model of the converter plays a crucial role in designing an efficient control strategy. This thesis initially develops a detailed mathematical model of non-ideal ZSIs using averaged modelling approaches. Small-signal models are used to estimate the ZSIs steady-state and dynamic performance, which are used to tune the controller’s gains for closed-loop operation. The devel- oped mathematical models are verified through simulation/experimentation in open-loop/closed-loop operations. However, the developed prototype posses low conversion efficiency due to the usage of snubber and limited iii voltage gain due to internal resistive drops of selected components. This problem could be addressed with magnetically coupled impedance source inverters (MCIS), increasing the voltage gain by increasing the turn’s ra- tio while maintaining the duty ratio minimal. However, the transformer’s magnetic flux leakage generates high switching voltage spikes leading to unwanted switch failures. Therefore, the effect of leakage inductance on converter performance must be understood before mitigation techniques are proposed. This thesis investigates different MCISs for mitigating voltage spikes and utilizes the energy stored in leakage inductance to enhance the voltage gain. Firstly, a novel active clamped Y-source impedance network and its family to limit switching voltage spikes is proposed. One additional clamping diode is added to the type-I improved Y-source network to yield the proposed active clamped Y-source converter. The proposed converter absorbs the energy stored in leakage inductance and re-utilizes the ab- sorbed energy to enhance the voltage gain for loosely coupled inductors. The thesis further investigates potential improvements in coupled inductor design and winding orientation to avoid switching voltage spikes at their origin. A family of ferrite core-based differential mode MCZSI topologies is developed by adopting the inverse coupling theory, which mitigates the switching voltage spikes without increasing the components. The ability of the proposed converters to reduce switching voltage spikes is demon- strated using simulation and experimental results. Small-signal, loss anal- ysis, and reliability studies are performed to prove the practical feasibility of designed converters. Finally, a negative embedded impedance source inverter (NEZSI) is ver- ified for low voltage harvesting in PV applications. Furthermore, an im- proved Γ-type Y-source inverter with integrated EV has been validated for PV-applications for better return of investments (ROI).