Date of Award

Summer 2024

Document Type

Open Access Dissertation

Department

Computer Science and Engineering

First Advisor

Adel Nasiri

Abstract

In a grid-tied photovoltaic (PV) system connected to a medium voltage (MV) grid, an inverter is generally employed with a line frequency transformer (LFT) to connect to the grid. These LFTs are large in size and weight, exhibit high cost and losses. In recent studies, the LFTs replaced by are by (high frequency transformer) HFT embedded in DC-DC converters. These DC-DC converters are used to boost voltage levels and DC-AC inverters are employed for MV inversion. This leads to high frequency switching at MV which increases losses in inversion stage. In this study, a novel MV grid-connected solar PV inverter topology is introduced, leveraging an LLC resonant converter and a high frequency transformer. The converter stage produces a rectified sinusoidal AC output voltage, and a line frequency unfolder inverter interfaces with the MV grid. By employing a line frequency MV Unfolder, the design mitigates high-speed switching at the MV stage, thereby minimizing switching losses. A combination of phase shift and switching frequency modulation is implemented to track the reference current at the grid output. An optimal trajectory is derived through loss analysis, determining the most efficient switching frequency and phase shift angles for the LLC converter, thus minimizing overall system losses. The advantages over existing technologies include: (1) Compact transformer size leading to reduced weight, (2) Reduced switching losses in the LLC stage through optimal trajectory control, and (3) Virtually no switching losses in the unfolder inverter operating at line frequency. A straightforward design approach is proposed for resonant tank components, magnetizing inductance, and output filter components. A single-phase 1/3 MW system is designed and validated through MATLAB/Simulink simulations for line regulation, load regulation, and total harmonic distortion (THD). A MV HFT is designed, simulated, developed, and experimentally verified, achieving a peak system efficiency of approximately 98%. A 150kW rated inverter undergoes mathematical analysis, comparing proposed control methods with existing approaches, achieving a peak efficiency of 97.5% and analyzing zero voltage switching (ZVS) conditions during low voltage grid connections. The proposed topology is further validated through MATLAB/Simulink simulations with MV grid connections, and a prototype is constructed for a 4.16 kV single-phase output. Experimental results confirm zero voltage switching conditions at MV, demonstrating enhanced performance and significant size reduction compared to conventional MV PV inverters.

Rights

© 2024, Parthkumar Sureshkumar Bhuvela

Download

Share

COinS