GE UR series, Instruction Manual

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GE Multilin L60 Line Phase Comparison System 9-5
9 APPLICATION OF SETTINGS 9.2 DISTANCE BACKUP/SUPERVISION
9
9.2.2 LINES WITH TAPPED TRANSFORMERS
If a protected line has a tapped transformer, it is preferable to apply the L60 in a three-terminal configuration. This provides
the most secure and reliable solution. However, if current measurements or the channel between the tapped line(s) and the
two other terminals are not available, then the measures outlined in the following sections must be taken.
9.2.3 TRANSFORMER LOAD CURRENTS
The L60 can be applied on the line with a tapped transformer. Since the tapped line may be energized from one terminal
only, 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 tapped
transformer(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 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, dis-
tance supervision can be considered to prevent maloperation due to transformer load currents.
9.2.4 LV-SIDE FAULTS
Distance supervision should be used to prevent maloperation of the L60 protection system during faults on the LV side of
the transformer(s). As explained earlier, the distance elements should be set to overreach all line terminals, and at the
same time safely underreach the LV busbars of all the tapped transformers. This may present some challenges, particularly
for long lines and large transformer 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, partic-
ularly on parallel lines, and the LV sides are interconnected, detailed short circuit studies may be needed to determine the
distance settings.
9.2.5 TRANSFORMER INRUSH CURRENT
The 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 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 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 ENERGIZATION
Inrush current at XFMR location
Inrush current at line terminal
831791A1.CDR