Determination of Creepage and Clearance Distance for MVDC Shipboard Power Systems Equipment
Abstract
The high-power demand of future U.S. Navy ships requires the utilization of Medium Voltage Direct Current (MVDC) architecture for powering shipboard propulsion, sensors, and weapons. Using MVDC on shipboard power system involves the knowledge of clearance and creepage distance of electrical insulators for ungrounded-DC shipboard power systems. Also, in order to address the challenge of unprecedented levels of shipboard electrification through actively switching power electronic converters, it has been recognized that Navy???s existing standards for low-voltage creepage and clearance are outdate, and, if applied to power electronic converters the standards would lead to poor management of bus-interconnecting inductances and over-sized equipment. As there are no existing standards on clearance and creepage distance for MVDC, these distances are extrapolated from medium voltage alternating current (MVAC) standards developed for terrestrial power distribution systems. However, these extrapolations may not be accurate due to the nature of space charge being different under AC and DC conditions. To account for the differences, MVDC clearance and creepage distance would have a large factor of safety which can unnecessarily increase the size of components in the system. Furthermore, as shipboard power systems are usually ungrounded or a high-resistance ground, the voltage across the insulation can increase in the event of a single-line-to-ground fault and cause additional stress in the insulation. Therefore, calculating the clearance and creepage distances for shipboard MVDC systems is not straightforward and requires further investigation. The magnitude increase in voltage across insulation upon fault depends on the location of the insulation in the power system. The eventual breakdown of insulation upon increased voltage also depends on the amount of contamination on insulator surface and degradation of insulation prior to the fault. As a result, a more comprehensive study is needed to determine the creepage distance in MVDC systems, which takes numerous factors into consideration. The proposed research project consists of three stages, which are all necessary to determine the creepage distance of MVDC systems: (1.) determine the voltage levels across bushings, cables and buses upon fault; (2.) build an experimental setup to determine the DC tracking degradation of the insulation used and measure the space charge across the insulator surface; (3.) develop a mathematical model that calculates/estimates the creepage distance based on contamination and space charge. Accomplishment of stage (1.) requires system simulations and modeling of equipment lowest replaceable units and interconnects within the cabinet. Work on (2.) and (3.) will be informed by and, in turn, inform present work on develop scaling laws for medium voltage connected power conversion and distribution equipment. The outcomes of the research will be recommendations for creepage and clearance requirements that the Navy can apply to future ships, development of methods and models for capturing creepage and clearance impacts to equipment sizing and scaling, and explorations into new methods that will increase the power density of equipment while meeting the derived creepage and clearance standards. This project is a collaboration between three Universities: Georgia Institute of Technology, University of Wisconsin-Milwaukee and Florida State University. The effort will be led by George Institute of Technology
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jul 26, 2018
- Source ID
- N000141812622
Entities
People
- Robert Cuzner
Organizations
- Office of Naval Research
- United States Navy
- University of Wisconsin System