# The Power System and Microgrid Protection--A Review
## 3 Protection Scheme of Microgrid
### Impedance-Based Protection Method  
An impedance-based protection method protects a distribution network in the presence of DGs. Fault location is estimated by the impedance method based on the measurement of voltage and current at the relay point or at the two ends of the line [74; 75]. A distance relay is a protective device that measures the impedance of a line using the voltage and current at the relay point. Distance protection does not normally need communication among relays [74; 76]. Reference [76] described the efficiency of various types of admittance criteria during high resistance faults. Analog or digital techniques are used to realize admittance-type protections. The admittance-type protection can be used under the damaged or switched-off resistor conditions. Moreover, admittance-based protection can be widely used in resistor-grounded networks. The authors of Reference [77] studied a distance protection scheme for detecting and protecting both islanded and grid-connected microgrids. Distance relays can clear faults quickly, thereby facilitate maintaining the stability of the system during contingencies. Figure 11 shows different zones of distance relays. Distance relays usually cover three zones (zone 1, zone 2 and zone 3). Zone 1 of a distance relay commonly covers 80% of the protected line and operates without delay. The other two zones (zones 2 and 3) are overreaching zones with time delay. Zone 2 of a distance relay is usually designed to cover the protected line plus 50% of the shortest adjacent line or 120 percent of the protected line, whichever is greater. Zone 3 provides a backup protection for zones 1 and 2 of unclear faults in the adjacent sections [78]. The authors of Reference [79] studied distance relays using the wavelet transform (WT) method for detecting faults. The analysis of wavelet is widely used for signal processing that can be applied effectively to overcome difficulties of the traveling wave protection techniques. The WT method is a novel signal processing technique developed from the Fourier transform (FT) [80; 81]. A dyadic wavelet transform was used in Reference [80] for investigating transmission line protection. The discrete approximation factor of dyadic wavelet transform with Haar wavelet is used as an index for detecting the transmission line faults. The proposed algorithm technique uses a moving data window. Reference [81] presented a new wavelet transform based on fault detection. This fault detection method utilizes artificial neural networks (ANNs) to accurately approximate decomposition of phase voltages and current samples. In Reference [82], a microgrid protection scheme was proposed based on the autocorrelation of three-phase current envelopes by using the squaring and low-pass filtering techniques. The protection method included fault detection, fault direction, fault zone identification, fault classification, and tripping units. This method provides the coordination of the primary and backup protection. In Reference [83], a hybrid method was presented to detect ground fault in the blinding zones. The approach consisted of detailed coefficients of discrete wavelet transform (DWT) using a pulsation signal generator injection in an ungrounded low voltage direct current (LVDC) microgrid. The WT method is based on transformation from the time domain into the time-frequency domain. Fault location method uses two different ways based on the existing communication scheme, a single-ended method, and a double-ended method. A single end fault location scheme is also possible when the current and voltage transients are available at the relaying  
Figure 11: The zones of distance protective relays. point. In the double-ended method, the fault is recorded simultaneously at both ends of the line by two separate channels [75; 84; 85; 86]. The impedance method is the most known main-frequency component-based method. This method can calculate the fault location without requiring any special hardware/software by obtaining data [87].