Transformer Load Currents; Low-Voltage-Side Faults; Transformer Inrush Current - GE L60 Instructions Manual

DISTANCE/BACKUP SUPERVISION CHAPTER 8: APPLICATION OF SETTINGS

8.2.3 Transformer load currents

The L60 can be applied on the line with a tapped transformer. Since the tapped line can be energized from one terminal
only, 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 the
tapped transformer(s). However, this does not guarantee correct operation of the L60 during transformer energization and
LV transformer faults. Increasing the FDL and FDH settings to be immune from transformer inrush current and transformer
LV fault decreases sensitivity. As such, calculations should take into account the requirement for the pickup setting
resulting 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 faults

Use Distance Supervision to prevent maloperation of the L60 protection system during faults on the LV-side of the
transformer(s). As explained earlier, set the distance elements to overreach all line terminals, and at the same time safely
underreach the LV busbars of all the tapped transformers. This can present some challenges, particularly for long lines and
large transformers tapped close to the substations. If the L60 system retrofits distance relays, there is a good chance that
one can set the distance elements to satisfy the imposed. If more than one transformer is tapped, particularly on parallel
lines, and the LV sides are interconnected, detailed short circuit studies can be needed to determine the distance settings.

8.2.5 Transformer inrush current

The 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 line
terminal CTs contain the entire spectrum of harmonics, including 2nd, 5th, 11th and 14th. On the HV-side of the
transformer, 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 and
inductance. Therefore, it is beneficial to use total harmonic distortion (THD) for line protection. The following figure
illustrates an HV line with tapped transformer energization, depicting the difference in inrush currents to the transformer
location and line terminal CT.
Figure 8-3: Sample inrush current of the tapped line transformer energization
8
The L60 measures THD in all three phase currents and neutral current. These measurements are available for protection
purposes though FlexElements (universal comparators). The FlexElement output can be used to block sensitive neutral
instantaneous overcurrent or phase comparison on transformer energization.
8-6 L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL