GE Multilin L60 Line Phase Comparison System 9-59 APPLICATION OF SETTINGS 9.2 DISTANCE BACKUP/SUPERVISION99.2.2 LINES WITH TAPPED TRANSFORMERSIf a protected line has a tapped transformer, it is preferable to apply the L60 in a three-terminal configuration. This providesthe most secure and reliable solution. However, if current measurements or the channel between the tapped line(s) and thetwo other terminals are not available, then the measures outlined in the following sections must be taken.9.2.3 TRANSFORMER LOAD CURRENTSThe L60 can be applied on the line with a tapped transformer. Since the tapped line may be energized from one terminalonly, or there may 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 the tappedtransformer(s). However, this does not guarantee correct operation of L60 during transformer energization and LV trans-former faults. Increasing the FDL and FDH settings to be immune from transformer inrush current and transformer LV faultdecreases sensitivity – as such, calculations should take into account the requirement for the pickup setting resulting fromline charging currents as well. Certainly, a security factor must be applied to the above stability conditions. Alternatively, dis-tance supervision can be considered to prevent maloperation due to transformer load currents.9.2.4 LV-SIDE FAULTSDistance supervision should be used to prevent maloperation of the L60 protection system during faults on the LV side ofthe transformer(s). As explained earlier, the distance elements should be set to overreach all line terminals, and at thesame time safely underreach the LV busbars of all the tapped transformers. This may present some challenges, particularlyfor long lines and large transformer tapped close to the substations. If the L60 system retrofits distance relays, there is agood chance that one can set the distance elements to satisfy the imposed. If more than one transformer is tapped, partic-ularly on parallel lines, and the LV sides are interconnected, detailed short circuit studies may be needed to determine thedistance settings.9.2.5 TRANSFORMER INRUSH CURRENTThe L60 has the capability to detect harmonics caused by transformer inrush current or other phenomena like sub-synchro-nous oscillations caused by active power system components. During transformer energization, current at the line terminalCTs contain the entire spectrum of harmonics, including 2nd, 5th, 11th and 14th. On the HV-side of the transformer, the 2ndharmonic 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 and inductance. Therefore, it isbeneficial to use THD for line protection. The figure below illustrates an HV line with tapped transformer energization,depicting the difference in inrush currents to the transformer location and line terminal CT.Figure 9–3: SAMPLE INRUSH CURRENT OF THE TAPPED LINE TRANSFORMER ENERGIZATIONInrush current at XFMR locationInrush current at line terminal831791A1.CDR
