# The Power System and Microgrid Protection--A Review
## 1 Introduction
In Reference [35], the disadvantages of AC systems are presented in terms of DGs synchronization, power quality, and three-phase unbalance. Some of the advantages of DC systems are higher efficiency, no power factor losses, and low voltage level as well as no need for inverters and transformer. In addition, the nature of DC power facilitates exploiting renewable resources and supplying DC loads. A hybrid microgrid facilitates the direct integration of both AC- and DC-based DGs in the same distribution network. Reference [37] has proposed a fault analysis method based on a simplified model of AC/DC hybrid microgrid system. The method used a mathematical model equivalent simplification for the analysis of the characteristic system under fault conditions. Figure 3 shows a hybrid structure with both AC and DC microgrids. A smart grid provides an optimal way of distributing electricity from various sources of generation plants and DGs. The goals of employing a smart grid in the power system are as follows:  
* involving individuals as an integral part of the power system, consumers or electricity providers;
* using more renewable energy;
* decreasing the dependency on electricity generation from power plants;
* decreasing complete blackouts;  
Figure 3: A hybrid microgrid (AC and DC systems). * boosting the power system capacity to supply electricity;
* reducing the time to restore the power system after fault occurrence;
* peak shaving [38]. There have been several discussions on the protection of microgrids, each of them has focused on different issues. In Reference [36], the authors present a detailed technical overview of microgrids and smart grids in light of present developments and future trends. They described the functions of smart grid components that include smart device interface components, advanced forecasting, control of generation/storage units, data transmission/monitoring, power flow, and energy management. Reference [37] analyzed the fault current characteristics in AC and DC distribution systems, first, then it describes the protection methods for AC and DC systems, and, finally, it compares the protection methods in AC and DC networks. Reference [39] presented a review on issues and approaches for microgrid protection. This reference focused on microgrid control including centralized and decentralized controls. Reference [40] reviewed the adaptive protection methods for microgrids. It presented different methodologies of adaptive protection systems with microgrids. A review on the protection schemes and coordination techniques in microgrid systems on presented in Reference [41]. This reference concentrated on protection coordination techniques in microgrids, such as coordination using time-current discrimination using the particle swarm optimization (PSO) algorithm and a modified PSO optimization algorithm. In the present review, in addition to extending the mentioned works, the authors try to address new challenges, such as blinding zones, and future trends in protection systems. In doing so, first, the protection problems of a power system (generator, transmission line, and distribution system), especially the challenges with the advent of distributed generation networks, are discussed. As a reminder, some of these problems still exist in networks and suitable solutions need to be discussed. Then, the classification of microgrids is briefly discussed. Some problems are expressed in different ways). The last section of the paper describes future real protection systems consisting of wide-area protection and adaptive systems. Wide-area protection is based on measurements that are obtained from phasor measurement unit (PMU) and intelligence protection systems can solve many issues related to the protection systems of the future. Moreover, the wide-area protection with different data transfer processes, including remote terminal unit (RTU), supervisory control and data acquisition (SCADA), energy management systems (EMS), PMU, are reviewed. The rest of the paper is organized as follows.