# AC Microgrids Protection: A Digital Coordinated Adaptive Scheme
The coordination of the over-current relay (OCR) is being studied by different researchers to minimize the operating time of the primary and backup relay [23; 24]. A communication-assisted protection scheme for MGs is proposed for both the grid-connected and islanded modes of operation in [25]. The dual setting of DOCR by considering multiple fault locations is investigated and the protection coordination problem is solved by using optimization algorithms with voltage constraints [26; 27]. It should be noted here that when the fault path impedance is high, the current direction may not change, which makes the directional protection ineffective. To address this concern, recently, the superimposed component-based DOCR setting methods have gained attention from researchers. In [28], a superimposed current-based DOCR protection is proposed while the superimposed component of transient energy has not been utilized for DOCR protection. The DOCR protection scheme proposed in [29] is based on the modified squared poverty gap (MSPG) index calculated using the superimposed component of current. Moreover, a fault current limiter (FCL) based approach is proposed for coordination of DOCR in [18]. In [30; 31], a protection system by using DOCR and an FCL is proposed for both of the connection modes of operation. In [32], by employing artificial neural networks (ANNs) and a transient monitoring function (TMF), fault is detected discriminated that the output current magnitudes of DER units are limited, and after fault clearance, the microgrid is restored to its normal operating conditions. An adaptive OCR protection based on micro-phasor measurement units (\(\mu\)PMUs) for MGs system is proposed in [33]. The \(\mu\)PMUs are installed at PCC and other parts of the MGs system to detect any change in the system and outage of line. The monitored data by \(\mu\)PMUs are sent to the phasor data concentrators and then OCRs coordination is updated accordingly. The line outage, DG disconnection, and network configuration changes are monitored by Thevenin impedance estimation at PCC. An adaptive zonal protection algorithm is proposed for ring MGs that updates the relay settings according to the line current and DG status [34]. Moreover, an adaptive protection scheme for MGs is proposed by using a signal processing-based fault detection method to detect the changes in the system network and optimally identify the pickup current. This method is implemented by using a fast recursive discrete Fourier transform (FRDFT) algorithm and fuzzy logic interface for a fast and effective fault detection [35]. Lack of a comprehensive protection scheme is sensed in most of the MG protection researches. For example, ref. [36] proposes a voltage-based protection scheme for an inverter-based MGs; however, it only considers the islanded mode of operation. In [37; 38], sequence components of current are used for fault detection of islanded MGs. In a similar manner, authors in [39] have proposed a protection scheme for LV MGs, which is applicable for autonomous operation of an MG. In addition, single-phase tripping is supported. However, all of the downstream elements of a feeder should be tripped of a fault in the feeder's zone, which is a drawback and undesirable for sensitive loads. In [40], a comprehensive protection scheme is proposed, although its parameters are not set properly based on the system's feature. The communication system failure and its impact on adaptive protection of MGs are investigated in [41], where MGs fault detection and recent development in protection systems along with adaptive protection have been briefly reviewed. Authors have shown studies carried out to identify the optimal pickup current (Ip) setting and minimized approaches of coordination time interval (CTI) of the primary and backup relays. It has been revealed that a number of the reviewed methods are depended to the operation mode, and in some others, because of employing one directional OCRs, all of the downstream DGUs are tripped.