**2. Conventional Methods for Marine Electrical Prolusion Systems**

### *2.1. Background*

The DFE rectifier with a phase shifting transformer is mostly used for high-power drives, such as the motors, fans, and compressors installed in the large plant in the industrial field [34]. Thanks to its long history of operation with know-how and track records accumulated in the industrial field, it was proved that stable operation would be possible. Therefore, the same high-power drive system of the existing industrial field has been applied in the early large electric propulsion system [6].

As mentioned in the previous section, thus far, large-scale electric propulsion systems have primarily consisted of a generator, phase shifting transformer, DFE rectifier, inverter, and propulsion motor [35]. The generators that supply power to the large-scale electric propulsion systems are typically brushless synchronous generators that can generate high voltages, such as 3300 V or 6600 V. To reduce the detrimental effects of the aforementioned harmonics produced in these power systems, including on the voltage and current of the generator, and to improve the output waveform of the rectified DC current, a phase shifting transformer is installed before the DFE rectifier [36,37]. Furthermore, to control the speed of the propulsion motor, an inverter which can control voltage and frequency is installed. Induction motors are often used as propulsion motors because it is easy to control the torque and speed of such motors. In addition, their maintenance is simple [38,39]. Figure 2 shows the schematic diagram of a large-scale electric propulsion system.

**Figure 2.** Schematic diagram of a large-scale electric propulsion system.

Although contributing to decreasing the total harmonics distortion, the DFE rectifier with the transformer is subject to the increase of volume and weight for the system as well as the design complexity for the phase shifting transformer to obtain a linear DC waveform by increasing the number of pulses. For example, Figure 3 shows the electric drawing of the electric propulsion system of 'A' company with a 24-pulse rectifier. Table 1 shows the comparison when an AFE rectifier is installed instead of a 24-pulse rectifier [40]. The total volume and weight of the system will be increased inevitably. Figure 4 shows the actual application of the transformer installed on the ship.


**Table 1.** Comparison of AFE rectifier vs. 24-pulse rectifier.

**Figure 3.** Typical configuration for a twin skeg electric propulsion ship.

**Figure 4.** Phase shifting transformer installed on large-scale electric propulsion system.

## *2.2. DFE Rectification Method in Large-Scale Electric Propulsion Systems*

Electric propulsion systems require the AC current generated by the generator to be converted into DC current. The conventional method involves the use of a DFE rectifier that employs a diode element to generate 6-pulses. However, as shown in Table 2 below, 6-pulse rectifiers lead to high harmonic distortion so that it cannot suit ship application. In order to reduce the harmonic distortion, as shown in Figure 3, a typical electric propulsion system maker chooses the phase shifting transformer to reduce the level of harmonics distortion by making 12-pulse, 24-pulse DC output [14,15]. Therefore, taking into account the harmonics distortion in the existing electric propulsion system, complex structures of phase shifting transformer have been applied with a number of DFE rectifiers. As a result, not only does the initial installation cost increase but also the volume and weight of the system [35].

Consequently, the overall efficiency of the system is reduced because of this decrease in the input power factor, as well as the severely distorted waveforms, owing to the DC output in a pulse form [41]. To resolve these problems, a high-capacity passive filter and phase shifting transformer must be installed, which, in turn, have their own drawbacks in that they considerably increase the overall system size and installation costs associated with the system [9].


**Table 2.** Total harmonic distortion for type of rectifier

#### 2.2.1. 12-Pulse Rectifier

One rectification method for improving the harmonic characteristics of the output of the power supply is to install a phase shifting transformer before the DFE rectifier to produce DC waveforms with 12-pulses in each cycle [15]. Figure 5 shows the block diagram of a DFE-style 12-pulse rectifier that uses a phase shifting transformer. The connections on the transformer's secondary side consist of Y-Y and Y-D connections. The phase shift angle for creating 12-pulses per cycle is 30◦ between each phase, as indicated by Equation (1).

$$
\Delta = \angle \mathbf{e\_{ab}} - \angle \mathbf{e\_{AB}} = \mathbf{30}^{\circ} \tag{1}
$$

where, Δ is the phase shift angle, ∠*eab* is the line voltage of the primary side of the rectifier, and ∠*eAB* is the line voltage of the secondary side of the rectifier.

**Figure 5.** Block diagram and Waveforms of a DFE-style 12-pulse rectifier.

Thus, in this case, for the 12-pulse DC waveforms that occur during one cycle, there is a 30◦ difference in the phases of the Y and D connections on the secondary side of the phase shifting transformer. In addition, there is a 30◦ difference in the phases of the 6-pulse DC waveforms generated by each unit, which produce a 12-pulse-per-cycle DC waveform in the DC link unit. In terms of the total harmonic distortion in the DC output waveform of the 12-pulse rectifier, the fifth order and seventh order harmonics are entirely eliminated, and only the harmonics that are 11th order and above remain. Thus, the rectifier effectively reduces the harmonic characteristics more effectively compared with a 6-pulse rectifier.
