Applications

A microgrid is a decentralized group of electricity sources and loads that normally operates connected to and synchronous with the traditional wide area synchronous grid, but can also disconnect to "island mode" — and function autonomously as physical or economic conditions dictate. In this way, a microgrid can effectively integrate various sources of distributed generation (DG), especially Renewable Energy Sources (RES) - renewable electricity, and can supply emergency power, changing between island and connected modes.

There are many types of microgrids. Based on applications and sizes, they can be classified as Campus Environment/Institutional Microgrids, Community Microgrids, Remote Off-grid Microgrids, Military Base Microgrids and Commercial and Industrial (C&I) Microgrids. In terms of electrical structures, they include AC microgrids, DC microgrids and hybrid AC/DC microgrids.

A microgrid is capable of operating in grid-connected and stand-alone modes and of handling the transition between the two. Microgrids offer an option to balancing the need to reduce carbon emissions while continuing to provide reliable electric energy in periods of time that renewable sources of power are not available. Microgrids also offer the power security and shorten power outage time in the events of severe weather and natural disasters.

Microgrids, and the integration of distributed energy resource (DER) units in general, introduce a number of operational challenges that need to be addressed. Bidirectional power flows and stability issues are top two of them. Interactions between distributed energy generator units may create local oscillations, requiring a thorough small-disturbance stability analysis. Moreover, transition activities between the grid-connected and islanding (stand-alone) modes of operation in a microgrid can create transient instability. Recent studies have shown that direct-current (DC) microgrid interface can result in a significantly simpler control structure, more energy efficient distribution and higher current carrying capacity for the same line ratings.

A typical hybrid microgrid structure[1]

A typical hybrid microgrid has the structure as shown above. The core components of the microgrid is bidirectional AC/DC and DC/DC converters. For safety and reliability reasons, the converters need to be isolated, so any load or energy source failure would not propagate the problem to the power bus/grid.

Bidirectional Dual Active Full Bridge Converter

PV to DC Grid Converter

2-Level Bidirectional AC/DC Converter

Most grid-connected AC/DC and DC/DC converters need to operate in bidirectional energy flow, which requires a switching device to serve as an active switch in one energy flowing direction, but to function as a diode or synchronous MOSFET in the other energy flowing direction. SiC MOSFETs, with near zero reverse recovery body diode, are an ideal option in the applications, especially for hard switching topologies. For the bidirectional three AC/DC converters, Vienna topology is no more valid. The 2-level three-phase AC/DC topology becomes a preferred choice due to its simplicity. SiC MOSFETs not just enable many bidirectional topologies in this application area, their superior switching characters make the solutions more efficient, compact and even less expensive with further SiC price reducing.

[1] Chendan Li, Sanjay Kumar Chaudhary, Josep M. Guerrero “Power flow analysis for droop controlled LV hybrid AC-DC microgrids with virtual impedance,” 2014 IEEE PES General Meeting | Conference & Exposition