Application experience with line current differential relays on three terminal series compensated lines

In order to increase the power transfer capability of transmission lines a series capacitance is added to the line. Addition of this series capacitance reduces the net reactance of the line thereby increasing the power transfer. Series capacitors introduce a number of problems in protecting transmission lines due to the fact that the state of the series capacitor (whether it is inserted or bypassed) can change dynamically by gaps flashing or metal oxide varistor operation during high current conditions. The problem is further compounded by the amount of series compensation applied to the line, and the location of the series capacitors on the line. Add to this that the line is a three terminal line and one realizes the complex issue Bonneville Power Administration had to resolve to protection several transmission lines. This paper will describe several three terminal lines series compensation that BPA protection engineers were charged with protecting. Some were heavily loaded. One of the lines also has four reactors to extinguish the secondary arc during single pole open and allow for successful reclose. Some transmission lines had near 100% compensation.