A New Class of Modular Power Converters for Next-Generation Shipboard Power Systems
Abstract
The dominant solution for three-phase AC to AC conversion is the DC-link converter (Fig. 3-a), which is formed by a three-phase rectifier and a three-phase inverter, decoupled through a largeDC capacitor serving as the DC link. As mentioned in the previous section, DC-link convertershave two major shortcomings: low reliability due to the presence of large electrolytic capacitors,and low power density due to the use of bulky low frequency transformers.This project proposes a novel AC-AC converter that solves both problems associated with DClinkconverters. This class of single-stage power converters, which is shown in Fig. 4, can have anynumber of sources and loads with any form (DC or AC), any number of phases, any voltageamplitude, and any frequency. Therefore, it can be used in a wide variety of applications. Thisconverter is capable of both stepping upand stepping down the voltage.The proposed converter addressesthe ~large capacitor~ problem of DClinkconverters by placing the linkcapacitor in series with the input andoutput switch bridges, and controllingthe input and output switches such thatpower conversion occurs in only onestage instead of two stages. In DC-linkconverters, the capacitor is placed inparallel with the input and output switchbridges to decouple the inverter andrectifier. However, being single-stage,the proposed converter does not require a large decoupling capacitor, and the capacitor is only usedfor transferring the power from the input phases to the output phases. The link capacitor can bemade even smaller by operating the converter at discontinuous conduction mode or at the boundaryof continuous and discontinuous conduction modes.The proposed converter solves the ~bulky low frequency transformer~ problem of DC-linkconverters by using an integrated high-frequency transformer across the link, which is feasible dueto placing the capacitor in series.Figs. 5 and 6 show the principles of the operation of the proposed converter and itscurrent/voltage waveforms. For simplicity, the principles of the operation are shown for a nonisolatedconfiguration. Principles of the operation are explained for the time period during whichthe reference of voltage across phase pair AB at the input side is the maximum input line-to-linevoltage, and it is positive. Also it is assumed that the reference of the voltage across the outputphase pair AB has the maximum output line-to-line voltage and it is negative. In general, the phasepairs carrying the maximum line-to-line input and output voltages do not need to be the same.Moreover, the phase pairs carrying the maximum line-to-line voltages change over a cycle.In each link cycle, the link capacitor is first charged from the input phases and then dischargedinto output phases. In the three-phase AC-AC configuration, each cycle is divided into 4 modes(modes 1-4). The link capacitor will be charged from the three-phase AC source during modes 1and 2, and it will be discharged into the three-phase AC load during modes 3 and 4.Figure 4 Proposed three-phase AC-AC converter9In mode 1, the anti-parallel diodes of switches S0, S4, and S6 will be conducting. During thismode the link current (ILink) is equal to the current of one of the phases forming the maximum inputline-to-line voltage, i.e. phase A or phase B. For the case shown in Figs. 5 and 6, it is assumed thatthe current of phase A is higher than that of phase B; therefore, during mode 1 the link current isequal to the current of phase A (IA_i). The current of phase A charges the link capacitor. This isaccomplished by turning on switches S7 and S11 at the output side (these switches are selectedaccording to the output voltage references). Since the link current is positive, the link capacitor ischarged and its voltage increases. The switching frequency is much higher than the frequencies ofthe input and output currents; thus, during each charging or discharging mode, the input and outputcurrents are almost constant. Once the average of
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jan 04, 2017
- Source ID
- N000141712122
Entities
People
- Mahshid Amirabadi
Organizations
- Northeastern University
- Office of Naval Research
- United States Navy