# State Feedback Control Based Seamless Switch Control for Microgrid Inverter
## 1 Introduction  
As a popular topic in power electronics, the inverter control has confronted new challenges for microgrid (MG) applications with the massive developments and utilizations of distributed generations (DGs) [1, 2]. Microgrid inverters can be classified into two categories: voltage-controlled inverters in stand-alone (SA) mode and current-controlled inverters in grid-connected (GC) mode. The current-controlled inverters are mainly designed to deliver a specified amount of active and reactive power following the grid, while the voltage-controlled inverters are designed for autonomous operation, represented as ideal AC voltage sources, balancing the power supplies and loads [1, 2]. However, when MGs transform unintentionally from the GC mode to a SA mode, the inverter control should be designed carefully to ensure an uninterrupted power supply for critical loads such as medical facilities, data storage systems, online management systems, etc. Therefore, the new challenge for those who control inverters will show up when the tracking control structure is changed. As when current-controlled inverters shift into voltage-controlled inverters, the controlled electric quantity needs to be changed from the current to voltage. This change may arouse severe dynamic voltage distortion, which must be eliminated effectively by specific seamless switch control. The seamless switch control from GC mode to SA mode is very critical to guarantee the power supply reliability. For different scale MGs, the seamless switch control may be realized in different methods. In a large-scale MG where lots of micro power source exists, the seamless switch can be accomplished via coordinate control by multiple DGs, for example, the droop control [3, 4, 5, 6]. Nevertheless, in a small MG (less than 100 kW), this function may be achieved by one inverter that is energized by energy storages. Furthermore, in a small-scale MG, the unbalanced power in switch is not too much so that it can be compensated by a single inverter. Therefore, the control for a single inverter should be designed carefully to achieve the seamless switch in small MGs. In general, a well-designedinverter control frame can simplify the complicated switch scheduling. Conversely, if the control frame is not well designed, some complex switch scheduling is indispensable for the smooth seamless switch. At present, most of the seamless switch control algorithms for single inverter are achieved by means of switch from current control in the GC mode into voltage control in the SA mode [7; 8; 9; 10; 11; 12]. The voltage control strategies reported in the literature [7; 8; 9; 10; 11] are analogous to that adopted by regular uninterruptible power supply (UPS) and are not particularly designed for switch application. It then becomes necessary to add switch scheduling during the switchover process to suppress the dynamic impact from the unmatched power. References [7; 8] for example, adopted a traditional dual-loop control under the synchronous rotating frame (SRF). The inner loop is current loop. The outer loop shifts from power control to voltage control when the MG switches to SA mode. This shifting is completely stiff, leading to poor dynamic performance and a drastic oscillation waveform. The single-loop control under SRF was employed in [9; 10] in SA mode. However, single-loop control is hard to obtain satisfactory performance with, because the current state information is ignored. [PERSON] et al. proposed a single-loop switch control where the outputs of both the voltage loop and current loop act as feedforwards for each other to keep the control signals continuous during the mode-switch process [11]. This strategy has reduced the impact current during the period of switch. In [12], a quadrature second order generalized integrator based inner current loop is proposed to improve the transient dynamic response. Recently, some switch control algorithms were presented in [13; 14; 15; 16; 17; 18; 19; 20].