# The Power System and Microgrid Protection--A Review
## 3 Protection Scheme of Microgrid
### Adaptive Protection  
The protection system cannot protect the power system against widespread changes. An adaptive protection scheme can change the relay setting/protection requirement according to the new prevailing conditions such as operational or topological changes [111; 112; 113; 114]. The power system protection is improved, and system security is enhanced by following adaptive protection philosophy. The adaptive protection schemes are more effective for the protection of such a power system [46]. Adaptive protection is \"an online activity that modifies the preferred protective response to a change in system conditions or requirements in a timely manner using externally generated signals or control action\". Adaptive protection has been developed since the 1980s [61]. An adaptive protection scheme solves the problem of both grid-connected and islanded modes of operation [115]. In an adaptive protection system, there is an automatic readjustment of relay settings, when microgrid mode changes from grid-connected to islanded and from islanded to grid-connected. The authors of References [116; 117] investigated an adaptive OC-based microgrid protection scheme that only considers synchronous generators and lacks HIF detection. The authors of Reference [118] used an adaptive protection only for a power system with synchronous generators that have a significant fault current contribution. The proposed method is unable to detect a single-phase fault and every fault type. References [119; 43; 120] have proposed a method using communication-assisted adaptive protection schemes. This method is used for looped/meshed distribution systems with DG penetration. The method is based on each DOC relays of the setting group in advance using optimization algorithms. Reference [118] also used adaptive schemes that include two sets of relay settings, one for the grid-connected and the other for islanded mode. This adaptive scheme determines the relay communication protocol/communication infrastructure for interacting among the relays and master controller. The relays update the settings with the master controller that senses the topology of the microgrid and communicates [46; 118]. Reference [111] proposed an adaptive multi-stage OC relay protection scheme in the presence of DGs in the distribution network. The adaptive DOC relay coordination was performed using the ant colony optimization (ACO) and genetic algorithm (GA) [112]. In Reference [113], an adaptive protection coordination scheme has been proposed for numerical DOC relays by using the commercial AMPL (a mathematical programming Language)-based IPOPT (interior point optimization) solver. An adaptive protection system was studied in References [67; 114] using the optimal coordination of OC relays. The method is divided into two separate parts: an adaptive protection device and an adaptive protection system. The device, in general, uses actual data and can adapt to the changes in the processing mode for certain zones. The adaptive protection scheme based on GPS in the distribution network with high penetration of DGs is presented in References [29; 41; 121]. The distribution network was divided into several zone breakers according to a reasonable balance of DGs and local loads. Tables 1-3 list the classification of different items of the review. Nowadays, different protection schemes are increasingly used in the real world. Table 1 presents the items in the literature for the protection of microgrids including the type of DGs, protection scheme, protective relay, the simulation situation (including simulation of a real network or simulation of a test case), and the year of publication. Some protection schemes in the table have been implemented in the real world, and their simulation and experimental results are given in the paper. A comparison between simulation and experimental results confirms the research quality. The empirical results are according to real-time conformity.