# Optimal Design and Real Time Implementation of Autonomous Microgrid Including Active Load
## 1 Introduction  
Nowadays, different distributed generation (DG) resources--such as wind and photovoltaic arrays (PVs)--are being rapidly connected in the utility grid to overcome the environmental problems and global warming gas emissions [1]. To operate a microgrid at the distribution level, constant impedance loads (CILs) and constant power loads (CPLs) are connected with DGs [2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14]. For inverter-based microgrids, stability analysis is one of the most important concerns [4; 5; 6; 7; 8]. In an autonomous microgrid, achieving accurate power-sharing while regulating the magnitude and the frequency of the microgrid is the main DGs control objective [4]. For enhancing and improving microgrid dynamic performance, researchers have used different centralized and decentralized control schemes [8; 9; 10; 11; 12]. The schemes advantages and disadvantages have been shortened in [8]. Different centralized control schemes to coordinate between parallel multiple inverters and to maximize the DGs productions and power exchanges of a microgrid with the main grid [8; 9]. Centralized control in remote areas with long distance between inverters is impractical and costly to maintain a communication link [9]. Therefore, decentralized schemes are used to avoid the communication link cost [10; 11]. Theseschemes are mostly based on the droop concept. Through droop control, emulating synchronous generators, sharing powers between inverter-based DGs was firstly proposed [6]. Droop control schemes with many advantages such as flexibility, redundancy, and expandability can be categorized for four main groups: (1) conventional and variants of the droop control [6; 13]; (2) virtual framework structure-based method [14; 15]; (3) \"construct and compensate\" based methods [16; 17]; and (4) the hybrid droop/signal injection method [18; 19]. The details and characters of various control schemes were illustrated in [5]. Another updated revision for the power-sharing schemes was presented in [20]. It was stated that load demand and the controller power-sharing coefficients dominantly distress the low frequency modes; while filter parameters, load dynamics, and the controller parameters of the inner voltage and outer current controllers distress the medium and high frequency damped modes [5]. Moreover, constant load perturbation and load dynamics significantly distress microgrid stability [3; 21]. Considering CPLs and/or CILs, it is essential to understand microgrid dynamic behavior [22]. With similar frequency range, control loop dynamic of the inverter and negative incremental resistance are the main important characteristics of the CPLs [12]. These properties reduce the system damping especially when CPL consumes a significant portion of the power [23]. Behind that, active load needs to be synchronized with the microgrid. Phase locked loop (_PLL_) is used for synchronization [24; 25]. Difficult tuning of the _PLL_ parameters and its negative effect on the controller performance are almost the main _PLL_ drawbacks [24]. Moreover, the coupling effect and interactions between the _PLL_ itself and the system impedance lead to a potential instability issue [25]. Therefore, studying microgrid stability considering CPLs is very important [26; 27; 28; 29; 30; 31]. Large-signal study is used to explore the microgrid stability with CPL [29; 30; 31]. A traditional Proportional Integral (PI) controller has been extensively used to control the DG inverters [29]. Many studies show that fixed-gain PI controllers cannot easily acclimate to load changes and disturbances, especially in large microgrids [30]. Many trial-and-error-based studies have significant drawbacks such as optimal settings difficulties and being time-consuming [12; 32; 33]. In [12], the analysis revealed that when the active load DC-voltage controller is designed with large gains, the voltage controller of the inverter becomes unstable. In addition, the controller design problem cannot be solved systematically.