Fault Resistance of the Fault Arc

Source: Jianping Wang, ABB, 2013-06-14 KTH

Arc which developed at the transmission line during fault occurrence can be due to an object very near to the line or due to lightning which make the fault current to jump to the transmission tower (earth). This arc representing an equivalent value of fault resistance, Rf. This Rf value is depends on the arc length and fault current. The longer the arc length, the higher the Rf while the higher the fault current the lower the fault resistance as shown by the equation in the figure above. 

The value of Rf can affects the accuracy of fault locating device and also the protection relay which uses the impedance based method (such as distance relay). For impedance based method relay, Rf can affects the accuracy of the relay in determining the tripping signal decision especially for fault near to the setting of zone reach. 

Charging Current Compensation of Differential Relay

For underground cables and long transmission lines, there will be significant effects of charging current on the accuracy of the protection relay. This charging current is the result of the distributed capacitance along the transmission line or underground cables as shown by figure below. The effects of the charging current for short transmission line is insignificant and can be neglected.

This charging current can introduces error in fault detection by installed protection relays. Some protection relays adapted with charging current compensation to eliminate the error in their algorithms.

One of the relay is line differential relay GRL100 by Toshiba. The charging current is also depending on value of the line voltage. So, the charging current is continuously calculated by the relay depending on the value of current line voltage and compensates it in the differential current calculation.

The procedure is the capacitance of the line be divided into two. Then, each terminal of the relay calculates the value of charging current at each terminal using the equation below.

Importants of Protective Apparatus in Power System Protection

Sweet Memories - Working as Protection Engineer at Tenaga Nasional Berhad

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