Bidirectional Multilevel Converter for Grid-Tie Renewable Energy and Storage with Reduced Leakage Current

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

Suppressing leakage current for cascaded H-bridge inverters in renewable energy and storage systems

Leakage current in a transformerless cascaded H-bridge inverter is a problem that deteriorates the system performance and causes safety concerns. In this paper, a common-mode equivalent circuit is established for analyzing the occurrence of leakage current in an m-level cascaded H-bridge inverter with either asymmetrical or symmetrical inductance output filter configurations. The analysis provides a comparison between traditional phase shifted pulse width modulation, phase disposition pulse width modulation and the proposed leakage current reduction pulse width modulation. It is reported that grid leakage current cannot be suppressed in an asymmetrical inductance filter configuration solely based on modulation methods. The proposed LCRPWM pulse width modulation can effectively reduce the grid leakage current in a symmetrical filter configuration nine-level cascaded H-bridge inverter. Simulation and experimental studies for aforementioned methods are provided and their performances are evaluated

A unique pulse width modulation to reduce leakage current for cascaded H-bridge inverters in PV and battery energy storage applications

Cascaded H-bridge (CHB) inverters operate with isolated DC sources, which makes them a favorable topology for hybrid-interfaced applications. Parasitic capacitance of grounded photovoltaics (PV) results in leakage currents that may deteriorate the system performance and raise safety concerns. In order to address this problem, a common-mode equivalent circuit for a multilevel CHB is illustrated. The study reveals that common modulation techniques such as phase shifted pulse width modulation fail to suppress the leakage current. A unique modulation scheme, based on considering the switching states is proposed in order to ensure that the topology conforms to the transformerless inverter regulations and standards such as VDE-0126-1-1. Simulation study results for conventional and proposed modulation methods are presented and analyzed in a symmetrical grid inductance filter circuit

Suppressing leakage current for cascaded H-bridge converters in grid-tie renewable energy systems

Photovoltaic panels contain parasitic capacitance causing leakage currents in transformerless converters, which raises safety concerns and reduces system performance. This work presents a common-mode equivalent circuit for Cascaded H-Bridge (CHB) converters with symmetrical filter configuration. In this paper, we develop a novel modulation technique for CHB multilevel converter, which can suppress the leakage current. The simulation and experimental results for standard phase shifted pulse width modulation and novel leakage reduction pulse width modulation (LRPWM) are provided. It is found that LRPWM is more effective in suppressing leakage current and conforms VDE-0126-1-1 German leakage current standard