The cascaded H-bridge converter for large-scale PV grid-integration with isolated DC/DC converters

Abstract

As a well-established multilevel converter topology, the cascaded H-bridge (CHB) is suitable for large-scale PV grid-integration due to its high-voltage and high-power capability, high power quality, modular structure, and multiple DC-links which are able to incorporate a large number of PV generators. Since PV system grounding is required by most countries grid codes and the modulation process of the CHB introduces high potentials upon the DC-side of the converter, galvanic isolation between the PV and the grid is essential to avoid the potential induced degradation (PID) of the PV modules, and to prevent the leakage current, as a result of the PV panel-to-ground parasitic capacitance, from flowing through the converter and ground, posing serious safety concerns. PV isolation can be achieved by the implementation of isolated DC/DC converters. The DC/DC converters connect the PV sources to the CHB converter through DC-links, performing maximum power point tracking (MPPT) and necessary DC voltage amplification, while the imbedded high-frequency transformers isolate the PV from the AC-side.

The thesis presents the system validation and documents the performance evaluation of the CHB converter with isolated DC/DC converters for large-scale PV grid-integration, featuring different power and voltage level configurations, tailored with two control approaches, and through specific case studies. Two isolated DC/DC converter topologies, namely the boost-half-bridge (BHB) and the flyback are implemented.

The CHB converter provides good quality voltage and current waveforms and is capable of medium-voltage grid-connection of the PV system. The system can be well regulated with voltage-oriented control (VOC) schemes both in the synchronous frame using Proportional-Integral (PI) controllers, and in the stationary frame using Proportional-Resonant (PR) controllers. Both DC/DC converters are able to perform MPPT under Standard Test Conditions (STC) and track the varying input voltage reference signal in a wide range fast and accurately. Compared with the flyback, the BHB converter presents lower ripples on the control variables and offers better MPPT efficiency at a higher component count. As the building block of the multiphase DC/DC converters, which feature high-voltage and high-current capability with modular and interleaved structures, the BHB makes a competitive candidate for this application.