Overview of Electric Ship Propulsion and Fuel Consumption

Abstract

2-日本マリンエンジニアリング学会誌 第 00 巻 第 00 号 (2005) The system a) consists of the electric propulsion motor, PWM inverter, diode rectifier, and 12-pulse transformer (2). It is often seen that 24-pulse transformer is used for suppressing THD in large cruise liner. The system b) has higher performance to suppress THD than system a) because PWM rectifier is equipped for PFC (power factor correction) operation (3). The systems of a) and b) can use not only CPP but also FPP, because shaft speed control is available by using power electronics converters. It is required to install a break resistor in DC connection point between the rectifier and the inverter for protecting inverter from back power in system a). System b) also can have a break resistor. On the other hand, if onboard electric load is big enough in this system, it has another option to convert the back power to AC power grid. These systems can use PM (permanent magnetic) synchronous motor instead of induction motor. In generally, PM motor has good efficiency characteristics in wide range from low load to high speed torque condition compared to the induction type one. Additionally, PM motor is smaller than bulky induction type one. It utilizes permanent magnet to provide high intensity magnetic flux as field system of synchronous motor. Therefore, it is necessary to pay attention for high intensity magnetic flux when it needs to be opened for periodical maintenance. The system c) is most simple configuration, just uses a starter circuit to start the induction motors. It can drive only CPP because the speed of electric motor cannot be changed for controlling SHP. 2.2 Energy conversion loss in AC system It is difficult to compare and to discuss the energy conversion loss in the propulsion systems shown in Figure.1 because each system tends to be applied to different power scale plant. For example, system a) is often used for large scale propulsion system like LNG carrier, large cruise liner, and ice-class cargo vessel. On the other hand, system c) is seemed to be used for middle to small cargo ship and research vessel less than several 1000 ton because it's difficult to install the panel of electric converters in the limited machinery space. In this paper, however, it is assumed that SHP is 750kW for comparing them in same condition. In addition, to make the discussion simple, this paper neglects the electric demand of onboard load and side thrusters, although it is necessary to consider both of them for clarifying the electric propulsion system. To evaluate the energy conversion efficiency and their power loss from the power source to the propeller, a simulation scheme which has been developed by author's group was utilized. Figure 2 shows the efficiency comparing to the conventional diesel drive propulsion system. The power transmission efficiency from the prime mover to the propeller shaft in the conventional system is set to 100% conveniently. The system a) and b) uses FPP generally, system c) uses only CPP. Induction motor was applied to the main propulsion motor for driving propeller shaft in this simulation. PWM 70 75 80 85 90 95 100 0 150 300 450 600 750 900 SHP, kW Electric propulsion a Electric propulsion b Electric propulsion c Diesel drive Efficiency : prime mover to propeller, % Fig.2 Estimated results of total efficiency from prime mover to propeller shaft as diesel drive system's efficiency is 100% Fig.1 Electric propulsion system with AC switchboard. G D D Y AC bus Prime mover 12 pulse transformer Diode rectfier PWM inverter M propulsion motor Generator G AC bus Prime mover PWM converter PWM inverter M propulsion motor Generator DC link DC link AC filter G AC bus Prime mover M propulsion motor Generator Starter System a) System b) System c)