In line with the strict environmental regulations of the International Maritime Organization (IMO), various types of technologies focused on environmental protection have been developed and applied. Among these technologies, the generation system of a four-stroke dual-fuel engine using gas generated by natural vaporization during the operation of a liquefied natural gas (LNG) carrier has been developed as a major propulsion method for electric propulsion ships.

In recent years, dual fuel technology using LNG as fuel has also been applied to large two-stroke engines, and the total number of orders for electric propulsion ships has decreased. However, when special types of propulsion performance are required, such as ice breakers and LNG carriers, an electric propulsion system is preferred.

In the case of large commercial vessels with electric propulsion systems, power is supplied through a four-stroke dual-fuel generator, and a power conversion unit is used to control the speed of the propulsion motor in an alternating current (AC) power system. The power conversion unit is composed of a rectifier, direct current (DC) link terminals, and an inverter. The performance improvement of the components constituting the power conversion unit is directly related to the efficiency of the electric propulsion system.

The rectification system of the electric propulsion system for vessels is largely categorized into diode front end (DFE) rectifiers employing diode elements and active front end (AFE) rectifiers using power semiconductors. In the case of DFE rectifiers, DC output waveforms are generated in the form of 6, 12, and 18 pulses, and at this time, the input power factor decreases owing to the harmonic distortion. To resolve this problem, a high-capacity passive filter and phase-shifting transformer are installed.

In the case of AFE rectifiers, it is possible to maintain a sine wave input current by using power semiconductors capable of high-speed switching. Thus, the installation of a passive filter or phase-shifting transformer is not required compared with the case of DFE rectifiers, which is advantageous in terms of cost and space required. AFE rectifiers have not been fully applied to large-scale electric propulsion ships owing to their capacity limitations. However, recent developments in power semiconductor technology have enabled the application of AFE rectifiers in large-sized electric propulsion ships[1].

As the AFE rectifier measures and controls the phase angle of the input voltage, accurate measurements are important. Although the zero-crossing technique has been commonly used for phase-angle measurement, the corresponding accuracy in the control of the rectifier is reduced when a voltage variation of the generator occurs owing to a sudden load change[2].

In this study, we propose an AFE rectifier with the application of an improved supply voltage phase-control technique that can accurately detect the phase angle of the rectifier even with sudden load variations. In addition, the phase angles were compared according to the conditions of the supply voltage variation, and the improvements provided by the phase-angle detector method for each condition were verified.

Figure 4 shows the basic circuit diagram of the AFE three-phase rectifier. The AFE rectifier consists of three terminals and six switches, and the AC powers e_a, e_b,and e_c maintain the three-phase equilibrium. In addition, inductors L₁, L₂, and L₃ installed on the power output side are used to control the magnitude of AC currents i_a, i_b, and i_c at power conversion, and the capacitor “C”on the output side of the rectifier is installed for stable DC voltage output of the DC-link terminal even under rapid variations in the voltage[10].

Figure 4: 
Circuit diagram of AFE rectifier

The equations for the sum of the phase voltage and phase current of the three-phase AC power are expressed as follows:

where e_a, e_b, and e_c denote the supply voltages of the a, b, and c phases, respectively, and i_a, i_b, and i_c denote the respective phase currents.

The voltage equations of the AFE rectifier in each phase are as follows:

where V_a, V_b, and V_c are the input voltages of the rectifier.

As the voltage equation of the AFE rectifier is expressed as three-phase AC values with a continuous change in the values of voltage and current over time, the rectifier control becomes complicated. If the three-phase AC power can be expressed by coordinate transformation with respect to an arbitrary reference axis, the voltage equation can be converted into an equation with a constant DC value, thereby simplifying the control of the rectifier and improving the accuracy of the control. In the improved supply voltage phase-angle controller proposed in this study, the three-phase axes a, b, and c are converted into the stationary α − β axis, and the coordinate transformation is performed on the d − q axis of the synchronous rotating coordinates.

In the axis transformation process to the stationary α − β axis shown in Figure 5, the phase variable f_a for an arbitrary time can be represented as the sum of f_a cos θ, the α-axis component, f_b sin θ, and the β-axis component. Then, if phases b and c are expressed as a matrix as with the a phase, the three-phase variable is transformed into a stationary α − β component. When we set θ = 0° to match axes α and β, neglecting the zero sequence, we obtain Equation (6) as follows:

Figure 5: 
Transformation of stationary axes a, b, and c to the stationary α − β axis

After the transformation of the three-phase axes a, b, and c to the stationary α − β axis using Equation (6), the voltage equations are presented as shown in Equations (7) and (8) as follows:

In addition, the transformation of the stationary α − β axis component to the d − qaxes on the synchronous rotating coordinates can be represented as shown in Figure 6, and the values can be represented as the values of the synchronous rotating coordinates.

Figure 6: 
Transformation of stationary α − β axis to rotating d − q axis

Equations (9) and (10) are voltage equations converted from those of the stationary α − β axis to the synchronous rotating d − q axis. Through the coordinate transformation process, the three-phase AC values that continuously change over time can be converted into two easy-to-control DC values, and the d − q axis value obtained by transformation to synchronous rotating coordinates is used to control the AFE rectifier.

To demonstrate the effectiveness of the improved supply voltage phase-angle detector proposed in this study, a waveform with instantaneous zero-point noise and harmonics was supplied to the supply voltage output from the generator, and the change in the phase-angle θ derived from the phase-angle detector and the resulting waveform output of the DC link voltage were analyzed using simulation. The values of the system parameters used in the simulation are listed in Table 2.

5.1 Simulation of the conventional phase-angle detector method of the AFE rectifier

Figure 10 shows a diagram of the AFE rectifier with the application of the zero-crossing phase-angle detector method, which is the conventional phase-angle detection method. In this method, the current phase-angle θ is detected by measuring the supply voltage and finding the zeroes that occur every half cycle.

Figure 10: 
Schematic diagram of AFE rectifier system with traditional AFE rectified with zero crossing

5.1.1 Simulation with zero-point noise in the supply voltage

Figure 11 shows the simulation results showing the detected phase angle and DC link voltage after the zero noise was inserted into the supply voltage at a time of 1 s. The results showed that the phase angle obtained by the zero-crossing technique was momentarily distorted in the 1-s section where the zero-point noise was generated, and the DC link voltage showed a variation in this section.

Figure 11: 
Simulation responses of AFE rectifier with zero crossing

5.1.2 Simulation responses of AFE rectifier with harmonics in the input supply voltage

Figure 12 shows the simulation results showing the detected phase angle and DC link voltage after harmonics were inserted into the supply voltage at 1 to 1.1 s. The results indicate that the phase angle obtained by the zero-crossing technique was severely distorted in the section where the harmonics were generated, and the DC link voltage showed the voltage-hunting phenomenon in this section.

Figure 12: 
Simulation responses of AFE rectifier with zero crossing

5.2 Simulation of the improved phase-angle detector method of the AFE rectifier

Figure 13 shows a schematic diagram of the AFE rectifier control system with the application of the improved phase-angle detector proposed in this study. To verify the effectiveness of the proposed method over the conventional method using the measured supply voltage, a supply voltage waveform with zero-point noise, phase change, and harmonics was supplied to the input side of the phase-angle detector. The results were analyzed using a simulation.

Figure 13: 
Schematic diagram of AFE rectifier system with proposed topology control method

5.2.1 Simulation with generation of zero-point noise in the supply voltage

Figure 14 shows the simulation results showing the detected phase angle and DC link voltage after zero noise was inserted into the supply voltage at 1 s. The results indicate that an accurate phase angle could be obtained without change even in the 1-s section, where the zero-point noise was generated using the proposed phase-angle detector method in this section. The output of the DC link voltage also improved compared with that of the conventional method.

Figure 14: 
Simulation responses of AFE rectifier with proposed control logic

5.2.2 Simulation with harmonics in the supply voltage

Figure 15 shows the simulation results, showing the detected phase angle and DC-link voltage after harmonics were inserted into the supply voltage at 1 to 1.1 s. The results indicate that an accurate phase angle could be obtained, even in the section where the harmonics were generated, with the application of the proposed phase-angle detector method, in which a stable output of the DC link voltage was obtained compared with that of the conventional method.

Figure 15: 
Simulation responses of AFE rectifier with proposed control logic

In this study, considering the voltage output characteristics of an AFE rectifier for electric propulsion systems, the voltage output obtained by applying the improved phase-angle detector method was compared to the output obtained by applying the conventional zero-crossing method.

In the conventional method, the AFE rectifier detects the phase angle from the generator supply voltage and uses the phase angle to control the rectifier. However, for the generator output voltage of a ship, there are cases of instantaneous irregular waveform output due to various causes, such as load variations and switching of the power conversion unit, which leads to errors in the phase-angle measurement and instability in the control of the rectifier.

The improved supply-voltage phase-angle controller method proposed in this study performs the coordinate transformation from the stationary α − β axis to the d − q axis of the synchronous rotating coordinates to obtain voltages e_d and e_q, and e_d is set as the active power component, whereas e_q is arbitrarily set to be the reactive power component. Accurate measurement of the phase angle is possible with the proposed method by controlling the value of e_q and maintaining the reactive power component at 0. To prove the effectiveness of the proposed phase-angle detector, a simulation of supplying an irregular waveform with zero noise and harmonics injected to the AC power of the generator supplied to the detector was simulated. As a result of the simulation, when zero noise occurred, the peak-to-peak value of the noise generated in the DC output voltage of the DC link terminal was reduced by 200 V compared with the conventional phase-angle detection method, and even when harmonics are included, the DC output voltage is more stable than that of the existing method with the desired result.