8-6 L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUALDISTANCE/BACKUP SUPERVISION CHAPTER 8: APPLICATION OF SETTINGS88.2.3 Transformer load currentsThe L60 can be applied on the line with a tapped transformer. Since the tapped line can be energized from one terminalonly, or there can be a low current flowing through the line, the phase-comparison element must set to provide stability.Accordingly, the FDH pickup setting must be high enough to prevent maloperation from the total load current of thetapped transformer(s). However, this does not guarantee correct operation of the L60 during transformer energization andLV transformer faults. Increasing the FDL and FDH settings to be immune from transformer inrush current and transformerLV fault decreases sensitivity. As such, calculations should take into account the requirement for the pickup settingresulting from line charging currents as well. Certainly, a security factor must be applied to the above stability conditions.Alternatively, distance supervision can be considered to prevent maloperation due to transformer load currents.8.2.4 Low-voltage-side faultsUse Distance Supervision to prevent maloperation of the L60 protection system during faults on the LV-side of thetransformer(s). As explained earlier, set the distance elements to overreach all line terminals, and at the same time safelyunderreach the LV busbars of all the tapped transformers. This can present some challenges, particularly for long lines andlarge transformers tapped close to the substations. If the L60 system retrofits distance relays, there is a good chance thatone can set the distance elements to satisfy the imposed. If more than one transformer is tapped, particularly on parallellines, and the LV sides are interconnected, detailed short circuit studies can be needed to determine the distance settings.8.2.5 Transformer inrush currentThe L60 has the capability to detect harmonics caused by transformer inrush current or other phenomena like sub-synchronous oscillations caused by active power system components. During transformer energization, current at the lineterminal CTs contain the entire spectrum of harmonics, including 2nd, 5th, 11th and 14th. On the HV-side of thetransformer, the 2nd harmonic prevails and is used to detect transfer inrush for transformer differential inhibit. However,on the line terminal CT, the shunt reactor harmonic spectrum contains additional harmonics due to line capacitance andinductance. Therefore, it is beneficial to use total harmonic distortion (THD) for line protection. The following figureillustrates an HV line with tapped transformer energization, depicting the difference in inrush currents to the transformerlocation and line terminal CT.Figure 8-3: Sample inrush current of the tapped line transformer energizationThe L60 measures THD in all three phase currents and neutral current. These measurements are available for protectionpurposes though FlexElements (universal comparators). The FlexElement output can be used to block sensitive neutralinstantaneous overcurrent or phase comparison on transformer energization.
