**Hyeonmin Jeon 1, Jongsu Kim <sup>1</sup> and Kyoungkuk Yoon 2,\***


Received: 1 May 2019; Accepted: 30 May 2019; Published: 1 June 2019

**Abstract:** In the case of the electric propulsion system on the vessel, Diode Front End (DFE) rectifiers have been applied for large-sized ships and Active Front End (AFE) rectifiers have been utilized for small and medium-sized ships as a part of the system. In this paper, we design a large electric propulsion ship system using AFE rectifier with the proposed phase angle detector and verify the feasibility of the system by simulation. The phase angle derived from the proposed phase angle detection method is applied to the control of the AFE rectifier instead of the zero-crossing method used to detect the phase angle in the control of the conventional AFE rectifier. We compare and analyze the speed control, Direct Current (DC)-link voltage, harmonic content and measurement data of heat loss by inverter switch obtained from the simulation of the electric propulsion system with the 24-pulse DFE rectifier, the conventional AFE rectifier, and the proposed AFE rectifier. As a result of the simulation, it was confirmed that the proposed AFE rectifier derives a satisfactory result similar to that of a 24-pulse DFE rectifier with a phase shifting transformer installed according to the speed load of the ship, and it can be designed and applied as a rectifier of a large-sized vessel.

**Keywords:** electric propulsion system; DFE rectifier; AFE rectifier; phase angle detector

#### **1. Introduction**

As environmental pollution has become a global issue, the International Maritime Organization (IMO) has been strengthening regulations on emissions of sulfur oxides, nitrogen oxides, and carbon dioxide from ships [1,2]. As a result of that various researches are being carried out in order to cope with environmental regulations that are strengthening internationally in the shipbuilding and shipping industries [3]. Moreover, the electric propulsion system of vessels with propulsion motors is also one of emerging countermeasures [4–7]. As shown in Figure 1, the order of environmentally friendly electric propulsion ship is dramatically increased on 2017 World Fleet Resister by Clarkson's Research [8].

The components of the conventional large-sized electric propulsion ship are generally composed of generator, DFE rectifier with phase shifting transformer, inverter and propulsion motor, and it is possible to design the size of the engine room with some margin [9–11]. In an electric propulsion ship, when the switching of inverters occurs, a harmonic current is generated in a power system [12]. Thus, large and small problems occur in the generator, transformer, and propulsion motor. Various methods for reducing harmonics have been studied. In the case of large electric propulsion systems, phase shifting transformer has been adopted as the most common method of installing a transformer on the output side of a generator [3,13]. There are various methods of harmonics reduction of the DFE rectifier using a phase shifting transformer, such as multi-pulse of the rectifier output [14–16], active filter installation [17], and improvement of the transformer connection method [18–20].

**Figure 1.** Annual ship newbuilding contracts.

However, when the phase shifting transformer is installed, there is a disadvantage as installation space and cost increase. Moreover, it is difficult to apply it to a small and medium-sized ship with limited space. AFE rectifiers have been mainly applied to small and medium-sized electric propulsion ships [21,22], but recently, as the technologies of power semiconductors with high capacity and high speed switching characteristics have been developed, so that it is possible to model a large-sized electric propulsion system using AFE rectifier [23]. The AFE rectifier must be designed with a control circuit that can control the semiconductor switch, and it is especially necessary to accurately detect the phase angle of the power supply voltage. Zero crossing technique that can detect the phase angle quickly is simple and has no special control method [24–27]. However, due to the fluctuation of generator output voltage in case of high load, such as propulsion motor or bow thruster. The detection of the phase angle may not be performed momentarily. Various methods have been studied to overcome the severe disadvantage of this zero-crossing technique.

In large-sized commercial vessels, it is crucial to secure the space for cargo transportation as much as possible [3,9]. However, to reduce the harmonics contained in the ship power system, the DFE rectifiers with large-sized phase shifting transformer have a disadvantage to load cargo. And the AFE rectifier using the existing zero-crossing technique also has various problems [28,29].

In this paper, an AFE rectifier using the Phase Locked Loop (PLL) method is applied to a large electric propulsion system instead of the phase angle detection method using the zero-crossing method [30–33]. We used the power analysis program, Power Simulation (PSIM), to model an AFE rectifier that uses the PLL method. Comparison simulations were performed for large-scale electric propulsion systems with the conventional DFE as well as proposed AFE rectifiers. Based on this simulation, the resulting speed of the propulsion motor, DC output of the DC link, and harmonic output characteristics of the input power supply were analyzed based on the type of rectification. In addition, the thermal loss in the switching element, which is present in the inverter when AFE rectifiers are used, as well as its stability, were evaluated. Based on these results, the characteristics of the DFE and AFE rectifiers in a large-scale electric propulsion system were compared to confirm the higher effectiveness of using the PLL-method-based AFE rectifiers in large-scale electric propulsion systems compared with the use of conventional DFE rectifiers.
