Medium Voltage DC Technology Developments , Applications , and Trends

Electricity has become the life blood of modern society. There is no single resource commodity that we depend upon more in our daily lives. Yet, over 100 years later, from the time when AC was proven superior over DC, we still rely on electric power equipment, networks and grids that were built up primarily during the middle of the 20 Century in most developed nations, on the premise of AC technologies. At the time, selecting AC was logical with the existing equipment of the 20 century. Today, modern advances in the transportation industry have often come through the application of electronics (electric vehicles and magnetic levitation trains, for example). These innovations utilize DC power, requiring an AC to DC conversion within the current grid infrastructure. We are living in the emerging era of the micro-grid, composed of distributed generation systems that produce DC power. Finally, solutions for integrating renewable energy with storage devices are attempting to be developed at the utility scale to deliver DC power. All of these factors combine to form an opportunity for the development and further deployment of DC technology throughout the electric grid at all levels, from transmission, through distribution, to end-use. The purpose of this article is two-fold. First, the authors review select DC technologies and systems that were developed and used prior to the current discussions in the engineering community around DC. The topics include High Voltage DC (HVDC) technologies and operating experiences with DC on ships. Secondly, a new era of research and development on Medium Voltage DC (MVDC) is explained. Strictly, MVDC is a collection unit for many DC generation resources and loads and a power conversion problem being addressed by the team at the University of Pittsburgh. The MVDC collection system serves as an additional layer of infrastructure existing between transmission and distribution. The article is concluded with a discussion of several standards relating to DC applications, with the overall motivation to inspire discussions and further investigation amongst working professionals in this area of technological advancement.