High power clean DC bus generation using AC-link AC to DC power voltage conversion, DC regulation, and galvanic isolation

Abstract

The proliferation of large non linear loads such as main propulsion and thruster variable frequency drives (VFDs) on both warships and commercial vessels, high voltage power supplies for lasers and other advanced weapons on warships require both AC voltage transformation and power conversion from AC to DC. For VFDs, rectification from AC to DC and inversion from DC back to AC is necessary; a process also applicable for some types of naval electric weapons and defensive systems. AC power transformation and rectification requires large conventional, low frequency transformers (i.e. 50 Hz or 60 Hz operation) in order to achieve voltage transformation and then rectification to provide the required level of DC voltage. The rectification process results in the production of harmonic currents and subsequent voltage distortion which, as is widely acknowledged, can adversely affect the operational integrity of the vessel or installation. The purpose of this paper is to propose a marine power system distribution system based on 'clean DC power' for some classes of naval vessels, commercial shipping and drilling/offshore applications. The paper introduces AC Linktrade technology, a 'soft switching' (i.e. it has no switching losses), high frequency technology which can transform voltages from any AC source within the converter (i.e. no conventional transformers are required) to either low voltage or medium-high voltage as part of the AC to DC conversion process. The AC Linktrade converter draws virtually harmonic free AC current (THD<1%) at unity displacement power factor and can provide single or multiple, regulated and galvanically isolated DC outputs (or AC outputs if required). The DC power can be supplied without any harmonic ripple, based on various 'active transformer' derived configurations: a) as discrete front end rectifiers for dedicated systems; b) as the DC supply for common DC bus systems and c) as the input converter stage to low voltage or medium to high voltage DC ring main systems. Voltage transformation (AC or DC) is via internal, high frequency transformers (20 kHz) which are a fraction of the size and weight of conventional transformers. This paper also outlines important weaknesses in present harmonic mitigations technologies with respect to cost, real world performance and physical size. The proposes an exciting, advanced technology which is poised to take marine, offshore, and industrial electrical power system engineering into the 21st Century.