Parallel Operation of Permanent Magnet Generators and Diode Rectifiers in Maritime DC Power System Considering Fuel Economy

Abstract

The dc grid system is attracting attention as a new electric power distribution method for several vessel kinds due to its high fuel efficiency compared with the ac grid. The combination of a wound rotor generator (WRG) and a diode rectifier [diode front end (DFE)] has been typically applied as a generation part of the dc grid. Still, the combination of a permanent magnet generator (PMG) and a DFE has been recently studied to take advantage of their simplicity, cost-effectiveness, robustness, high reliability, and high efficiency. However, although a single-operation mode of the “PMG + DFE” system was analyzed in the literature, research for the parallel operation of the “PMG + DFE” system has not yet been conducted. The “PMG + DFE” system should be operated along the optimal operating line (OOL) in the power–voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$P$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V$ </tex-math></inline-formula> ) domain to minimize fuel consumption. This OOL in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$P$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V$ </tex-math></inline-formula> domain includes negative, zero, and positive slopes under light, medium, and heavy loads, respectively. Therefore, if the conventional droop control is applied to this system, it cannot be operated along the OOL. This article introduces modified droop control for operating paralleled “PMG + DFE” system along the OOL in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$P$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V$ </tex-math></inline-formula> domain. Furthermore, a mode change strategy from the single-operation mode to the parallel-operation mode is proposed. The validity of the proposed modified droop control is verified by computer simulation and a small-scaled experimental test.