Discrimination in Offshore and Marine DC Distribution Systems

In this article the design of an actual and delivered diesel electric system with DC distribution is described. The focus is on the protection philosophy in such a DC distribution system, and how it distinguishes from that in conventional offshore and marine AC distribution systems. The philosophy is based on discrimination. By discrimination it is implied that only the failure affected part is interrupted and then disconnected without interrupting the unaffected part of the system. The Description of the components directly connected to the main switchboard as well as electrical analyses for a vessel with 8 MVA installed electrical power are presented. The ultra fast bus-tie breaker called the Intelligent Load Controller (ILC, pat. pend) is considered to be the heart of the protection system. The ILC is based on IGBTs with additional isolator switches outside. In case of a short circuit failure, the IGBTs break the current before the isolator switches galvanically isolate the switchboards. The ILC is able to interrupt the bus-tie fault current within 10-20 mu s, given a low impedance fault. This time is short enough not to reduce the voltage level on the opposite side of the main switchboard capacitor banks. As a consequence, the healthy side of the main switchboard is able to operate continuously - without interruption from the failure event.