## 1 Introduction  
In recent years, a distributed power generation system using a new, renewable energy source, and an eco-friendly electric vehicle has been attracted. The production and consumption of DC power have thus inevitably increased, and studies on the energy efficiency of DC power have been actively conducted.  
In order to implement a DC distribution system with the conventional AC distribution infrastructure, a high efficiency power conversion system using a power semiconductor device is required. In general, diode rectifiers have been used to convert AC power to DC power because of their relatively simple structure and operation. However, the diode rectifier is not suitable for a DC distribution system because the constant output voltage cannot be controlled when the power factor of AC input varies according to the type and size of the load. In addition, regenerative operation is not possible due to the unidirectional characteristic. Therefore, the 3-phase AC/DC pulse width modulation (PWM) converter is widely used to compensate for these drawbacks [1,2].  
The 3-phase AC/DC PWM converter used for the DC distribution, as shown in Figure 1, is composed of six insulated gate bipolar transistors (IGBTs), dc-link capacitors, and an LCL filter to the reduce harmonics on the grid side. For bidirectional power control and power factor control of this system, phase angle information of the input power source is required. In many studies [3, 4, 5], a positive sequence voltage detector using an all pass filter (APF) and synchronous reference frame phase locked loop (SRF-PLL) have been widely used; however, not only do these devices have a complicated configuration, but they are also weak in lower order harmonics [6]. In addition, problems of the grid side, such as sag, swell, and frequency fluctuation directly affect the output of the 3-phase AC/DC PWM converter. As a result, it is difficult to maintain a stable condition in the DC distribution system in which various loads are repeatedly connected [7]. In order to solve the frequency variation of the grid for the DC distribution system, improved SRF-PLL method has been researched, but the unbalanced grid voltage and voltage drop condition are not taken into account [8].  
Meanwhile, the second order generalized integrator (SOGI) algorithm was introduced by Prof. [PERSON] [9]. One of SOGI applications, SOGI-frequency locked loop (SOGI-FLL) has functions, such as the positive sequence voltage extraction and phase angle detection, as well as robust filtering characteristic. Therefore, phase synchronization can be performed using only the SOGI-FLL and it does not need any additional digital filters, such as the low pass filter (LPF) that is used in the conventional method [10,11].  
Therefore, in this paper, we propose advanced control method using the SOGI-FLL that can be applied to a 3-phase AC/DC PWM converter for the DC distribution. The proposed control method is applied to improve the stability and transient characteristic of the DC distribution system, and it is verified that the DC distribution system achieves stable and fast transient characteristics, even though unbalanced grid voltage, frequency variation, and voltage drop occur.  
This paper is organized as follows. In Section 2, we analyze the 3-phase AC/DC PWM converter, which constitutes the DC distribution system and describes the conventional method. In Section 3, we describe the phase detection and positive sequence voltage extraction while using the proposed SOGI-FLL technique. Sections 4 and 5 give the simulation and experimental results for the proposed control method based on the SOGI-FLL control, and Section 6 provides the conclusion.  
Figure 1: Configuration of 3-phase alternating current/direct current (AC/DC) pulse width modulation (PWM) converter.