# Microgrid Cyber-Security: Review and Challenges toward Resilience
## 4 Perspective-Based Interventions Addressing Cyber Attacks
### Microgrid Control  
#### 4.3.1 Control Structure  
The sound operation of power management strategy (PMS) is more critical in microgrids. Reasons narrow down to the imperative adjustment of multiple interconnected DG units with significant differences in power capacities and generation system characteristics that become more and more Electronically Interfaced (EI-DG), requiring a faster response to keep dynamically changing characteristics (voltage/angle) within the appropriate margins [26]. A microgrid's control systems are a typical target for attackers. Based on the purpose of which they are implemented, microgrid's control structure can be profoundly different and the control features are limited or customized to a desired subset of functions from a larger group of options [45]. In grid-connected mode, for example, frequency and voltage values are regulated by the host grid at the point of common coupling (PCC) whereas tasks like the DER's active and reactive power accommodation, energy management and load sharing, in addition to a safe transition between connected-islanded modes are still elaborated by the microgrid's control systems. During islanded mode or in remote conditions that are completely isolated from the grid, the microgrid's local controllers take full responsibility for all stability measures which also vary depending on the microgrid type (Islanded AC, synchronized islanded AC, naturally islanded DC) [27]. Typically, having one of the DERs operating as the isochronous generator forming the microgrid voltage and frequency is quite common in islanded microgrids. In this case, the rest of the DERs could participate in supporting voltage and frequency if needed [9], whilst the complete absence of a dominant source of energy generation during the autonomous mode of operation adversely amplifies the complexity of the assigned task list. The majority of prior researches pictures the microgrid's control paradigm in a hierarchical manner, following the successfully adopted structure in the legacy grid [23]. Hierarchical control levels of microgrids are usually anticipated in three layers: primary, secondary, and tertiary. There are no definite technical boundaries between strategies of each level rather than a sort of indication based on relevant considerations, such as response rapidity interval, purpose-oriented control, and central-distributed control. However, without losing generality, we can say that primary and secondary control strategies are practically associated with operational stability and accordance between microgrid's components, while harmonization with the host grid is applied by tertiary control [27]. Primary control features the fastest response with the smallest decision time step, voltage, and frequency regulation as well as protection executed on this level, which are entirely based on local measurements and droop mechanism with no communication needed [27]. That is why operations on this level are conserved away from cyber incidents. On the contrary, secondary control operates on a slower time scale, often with a reduced communication bandwidth by using sampled measurements. It collaborates consistently with the other two levels to satisfy the requirements set by the tertiary control. The secondary control measures values across the microgrid, and accordingly, updates the desired setpoints for the primary controllers [46]. Tertiary control on the highest hierarchy collects state information of the energy system through the communication infrastructure and makes decisions to optimize the overall performances of microgrid with the longer decision time step. It may also be responsible on the economic dispatch of controllable resources and coordination with the distribution system operator with Energy Management System (EMS) ensuring power balance constraint, security constraint, and operational constraints [23]. Previous control functions can be achieved through either centralized or distributed implementation of the control architecture.