المجلة الدولية للعلوم والتقنية

Transmission lines are an important part of the power system. Transmission lines are used to transmit power and are prone to many breakdowns. Therefore, protection relays are used to protect transmission lines. The purpose of the protection system is to isolate the faulty part from the healthy part, because large currents generated by faults may cause damage to electrical equipment. One of the protection relays is the distance relay and is mainly used in the transmission line. Sometimes these relays are used for backup protection. Distance relays to determine impedance need voltage and current. Transmission lines are usually protected by a distance protection relay. Distance relays are of the high-speed class and can provide transmission lines. Protection distance relays that use impedance measurements to determine the presence and location of faults, are "fooled" by a series capacitor mounted on the line, when the presence or absence of a capacitor in the fault circuit is not known in advance. This is because the capacitor cancels or compensates for some of the line inductance, so the relay may sense a fault in its first region when the fault is actually in the second or third region of the protection. Similarly, Zone one fault can be considered reverse faults. In fact, this can cause some costly operating errors. Series compensated lines are protected from overvoltage by metal-oxide-varistors (MOVs) connected in parallel with the capacitor bank. The nonlinear characteristics of MOV devices add complexity to fault analysis and distance protection operation. During faults, the impedance of the line is modified by an equivalent impedance of the parallel MOV/capacitor circuit. Worth noting is a method that determines the L and C string values of the font and the MOV effect at the time of the error. This is done by analyzing the synchronous and sub-synchronous content of the voltage and current signals separately which provides enough information to calculate the L and C series of the line. In this paper, the transmission line impedance Z, Z(f) as a function of frequency has been calculated and plotted by the sequence characteristics and the studied MOV effect on line protection and fault location. Proper operation of the distance protection relay is also shown, and its effect on relay regions. Keywords: Distance protection relay, transmission line, zones of relay, MOV.