9424200994 Rev N 127
Loss of Excitation - Reverse Var Based
(40Q) Protection
The loss of excitation - reverse var based (40Q) element monitors total reactive power (vars).
Element logic connections are made on the BESTlogic™Plus screen in BESTCOMSPlus® and element
operational settings are configured on the Loss of Excitation - Reverse var Based (40Q) settings screen in
BESTCOMSPlus. A summary of the logic inputs and outputs and operational settings appears at the end
of this chapter.
BESTCOMSPlus Navigation Path: Settings Explorer, Protection, Power, Loss of Excitation - Reverse
var Based (40Q)
HMI Navigation Path: Settings Explorer, Protection, Settings Group x (where x = 0 to 3), Power
Protection, Reverse var 40Q
Element Operation
When a generator loses its excitation power, it acts as a large inductor. The generator begins to absorb
large quantities of vars. The 40Q element acts on the principal that if a generator begins to absorb vars
outside of its steady-state capability curve, it has likely lost its normal excitation supply. The element is
always calibrated to the equivalent three-phase power even if the connection is single-phase. For more
information on the calibration and power calculations, refer to the Configuration chapter.
The 40Q element compares the reactive power to a map of the allowed reactive power as defined by the
Pickup setting. The 40Q remains in a pickup condition until power flow falls below the dropout ratio of
95% of the actual pickup. A time delay is recommended for tripping. For settings well outside the
generator capability curve, adding a 0.5 second time delay helps prevent transient fault conditions.
However, recovery from power system swings after a major fault may take several seconds. Therefore, if
the unit is to pick up near the steady-state capability curve of the generator, longer time delays are
recommended. See Figure 80 for details.
Figure 80. Generator Capability Curve vs. 40Q Response
BE1-11g Loss of Excitation - Reverse Var Based (40Q) Protection
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