L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL
8-1
L60 Line Phase Comparison System
Chapter 8: Application of settings
Application of settings
This chapter provides example(s) of use.
8.1 87PC phase comparison element
8.1.1 Overview
The L60 provides high-speed protection of transmission lines against all phase and ground faults when operated in the
“mixed-excitation” mode. The term “mixed-excitation”, when applied to phase comparison, describes a scheme that first
mixes different sequence quantities in a given proportion and phase angle, then performs a phase-comparison based on
this mix.
This section provides more information on the 87PC Scheme settings (
SETTINGS
GROUPED ELEMENTS
SETTING GROUP
1(6)
PHASE COMPARISON ELEMENTS
87PC SCHEME
) followed by an example of use.
8.1.2 Settings
87PC SIGNAL
A mixed I_2 – K × I_1 signal, mixed I_1 + I_2 / K signal, or 3I_0 signal can be chosen as the operating signal for FDH and FDL
excitation. In mixed excitation mode, the relay provides high-speed protection of transmission lines against all phase and
ground faults. However, to operate the relay only during ground faults, choose the 3I_0 mode.
87PC MIXED SIGNAL K
The K factor must be chosen for the mixed excitation operating signal. Best results are obtained using a value of 0.2 (the
default setting). The selected K value can range from 0.00 to 0.25. Setting K = 0 makes a phase comparison on the basis of
negative-sequence excitation only. In such a scheme, the relay protects against all unbalanced faults; a suitable phase-
distance relay should be used to protect against three-phase faults.
K is an important tool to set FDL and consequently FDH at the lower setting, especially in the cases when the margin
between the maximum load current and the minimum fault current is very small. Reducing K to 0.15 or 0.10 makes phase
comparison protection less sensitive to load current, which in turn allows the user to provide enough sensitivity to the fault
current. On the other hand, it makes protection less sensitive to the balanced three-phase fault, which occurs very rarely.