Modulation of a Bidirectional DC-AC Converter for an AC Grid Interface in Bipolar DC Microgrids

Abstract

As renewable energy sources and DC energy storage continue to increase in modern power systems, the power electronic converters required to interface them with existing power grids are also rapidly increasing. This is resulting in increasing amounts of the total power exchanged on the grids being converted between DC and AC. Additionally, the majority of today's grid loads ultimately convert their input power to some form of DC whether they are connected to an AC or DC grid. Therefore, the concept of DC grids is becoming more attractive for applications such as DC distribution systems and electric vehicle charging stations. However, the existing AC grids will continue to play a fundamental role in the power infrastructure. Therefore, it is, important to consider how LVDC distribution grids will be connected to, and exhange power with, the AC grid. A transformerless interface between LVDC distribution grids and the LV AC grid via a power electronic converter is an attractive option in terms of size and cost reduction. However, the challenge of ground leakage current is a major challenge for such an interface.

This thesis assesses the different possible three-phase DC-AC converter topologies for a transformerless interface between a bipolar DC microgrid and the LV AC grid. The DC-AC converter modulation is explored to understand the challenges of ground leakage current and determine the feasibility of a transformerless interface. A detailed analysis of the common-mode voltage, common-mode impedance and resulting circulating currents in a grid-tied DC-AC converter is performed. The effect of this circulating current on AC grid protection equipment is considered.

A survey of converter modulation methods for common-mode reduction and DC bus voltage balancing is conducted. Simulations are used to verify the performance of the different modulation methods in terms of common-mode reduction and DC balancing capabilities in the bipolar DC grid. A new modulation method is proposed for simultaneously mitigating ground leakage current and providing DC voltage balancing. The limitations of this method are analyzed and simulations are performed to evaluate its effectiveness. The developed modulation method is also shown to eliminate the low-frequency voltage ripple in the DC bus caused by the conventional modulation methods. An experimental hardware prototype DC-AC converter is designed and built for testing the different modulation methods. Finally, recommendations for future work are made.