# A Decentralized Multi-Agent-Based Approach for Low Voltage Microgrid Restoration
## 2 Microgrid Control Structure for Black Start  
Figure 1 shows the structure of an inverter-based LV MG. The presence of a synchronous generator in an inverter-based LV MG is not common [8]. Generally, an LV MG includes the loads, microsources (photovoltaic (PV), wind energy conversion system (WECS), fuel cell, and microturbine), and storage devices (battery energy storage systems (BESSs) and flywheels).  
The safe, economic and stable operation of an MG in both grid connected and islanded mode depends on the existence of a proper control system [11; 32; 33; 34]. An LV MG can be controlled centrally by the MGCC installed at the LV side of MV/LV substation. The load controller (LC) and microsource controller (MC) are local controllers that control the loads and microsources, respectively, and exchange the required information (such as set-points and load/consumption situations) with the MGCC through a narrow-band communication link. LC controls the loads using the local load shedding schemes in emergency conditions while MC controls the active and reactive power of microsources [35].  
Under normal operation, the MG is connected to the MV network. However, in order to deal with the islanded mode and black start of the MG following a blackout, an emergency operation mode should be provided. If a blackout occurs, the restoration process time needs to be reduced as much as possible. The restoration plan is defined step by step. The main steps are building the LV network, connecting the microsources, connecting the controllable loads, controlling the voltage and frequency, and synchronizing the MG with the MV network, when it is available [8; 22].  
In the centralized restoration scheme, the MG black start is guided by the MGCC. The information about generation-consumption scenarios of the MG is periodically sent by the MCs and LCs to MGCC  
Figure 1: Typical low voltage (LV) microgrid (MG). MGCC: microgrid central controller; MC: microsource controller; LC: load controller; WECS: wind energy conversion system; PV: photovoltaic.  
using the communication links and they will be stored in a database. After the blackout occurrence, based on the information available in the database, the MGCC determines a sequence of restoration actions and send the proper control commands to the local controllers [8; 22].  
To implement such a centralized restoration approach, all of the MCs and LCs must have a direct communication with the MGCC. In the case of communication failure for each local controller, the centralized restoration scheme faces with some problems. That's why it is said that the centralized control schemes easily suffer from single point failure. Moreover, the centralized restoration scheme uses the information available in its database to determine the sequence of restoration actions. Since the database information is gathered during a certain period of time before the blackout, in the case of changing in the load or generation scenario or lack of preparedness for the restoration of a generation unit or load during the restoration process, the restoration scheme will be in trouble. Thus, there is a need for a decentralized restoration approach that uses the online information of the generation/consumption of the MG and determines a proper sequence of restoration actions to restore the maximum possible amount of loads in the shortest possible time.  
This paper proposes a decentralized multi-agent based approach for the MG restoration in which the online data related to the generation and consumption are used to determine a proper sequence of restoration actions. In the following sections, first, the MG components will be dynamically modeled. Then, according to the mathematical discussion on the distributed averaging problem, the proposed multi-agent based method will be presented.