This thesis discusses a transformerless multilevel converter (MLC) applied to a domestic level renewable energy system consisting of PV panels and EV batteries in their 2nd life applications. MLCs enable the use of conventional switching devices due to reduced voltage stress. Being able to produce a multilevel output voltage waveform, MLCs require less filtering and therefore may produce better quality waveform when compared to a standard 2-level voltage source converter (VSC). In this study, various modulation techniques for MLCs are implemented and the performance of the converter analysed regarding regulations and standards.
The system is designed to have two-stage power conversion, including a DC-DC boost converter for adjusting each stage battery voltage, and maximum power point operation of the PV panels in each module. This provides a stable input voltage for the DC-AC converter stage. The cascaded H-bridge converter (CHB) is selected for the DC-AC conversion due to its isolated DC source requirement. This topology enables the separation of the total DC link voltage into different modules, increasing the accessibility of EV batteries in their 2nd life application. The base system is designed to be coupled without a transformer to the single-phase UK utility grid.
A systematic approach is adapted for examining the MLC system. The design procedure starts with system parameter definition and component selection. This is then validated using simulation analysis and hardware implementation to demonstrate the practicability of the system for the planned application. The control algorithm is implemented in a National Instruments (NI) CompactRIO FPGA that can transform graphical programming into VHDL code. To accelerate the implementation and optimisation process, a co-simulation environment is used between NI LabVIEW and NI Multisim software. This ensures the optimisation of control code before compilation and enables testing without having analogue circuitry.
Converters without galvanic isolation may exhibit ground leakage currents when coupled with grounded PV panels. This thesis analyses the common-mode and differential-mode voltages that CHB modules generate, and their effect on ground leakage current. The mathematical analysis suggests that leakage current may be supressed solely on changing the modulation method in a CHB converter. A novel leakage reduction pulse width modulation (LRPWM) technique is proposed, which successfully diminishes the ground leakage current to within the limit allowed by VDE-0126-1-1 (withdrawn, accessed in 2018) or IEC 62109-2 standard. The experimental results show that LRPWM has superior performance when compared to conventional MLC modulation technique
