Functions; Power Factor (55) - GE 845 Instruction Manual

MONITORING

Functions

Power Factor (55)

4–266
It is generally desirable for a system operator to maintain the Power Factor as close to
unity as possible to minimize both costs and voltage excursions. Since the Power Factor is
variable on common non-dedicated circuits, it is advantageous to compensate for low
(lagging) Power Factor values by connecting a capacitor bank to the circuit when required.
The relay allows two stages of capacitance switching for Power Factor compensation.
The relay calculates the average Power Factor in the three phases as follows:
Average Power Factor = Total 3-Phase Real Power / Total 3-Phase Apparent Power
When the measured Power Factor becomes more lagging or leading (depending on the
user setting) than the Switch-In level, the relay operates a user-selected output contact.
This output can be used to control a switching device which connects capacitance to the
circuit, or to signal an alarm to the system operator. After entering this state, when the
Power Factor becomes less lagging or leading than the Power Factor Switch-Out level for a
time greater than the set delay, the relay resets the output contact to the non-operated
state.
For delta-connected VTs, the Power Factor feature is inhibited from operating unless all
three voltages are above a threshold and one or more currents are above 0.002 x CT.
Power Factor element delay timers are only allowed to time when the voltage threshold is
exceeded on all phases and the Power Factor remains outside of the region between the
programmed Switch-In and Switch-Out levels. In the same way, when a Power Factor
condition starts the Power Factor delay timer, if all three phase voltages fall below the
threshold before the timer has timed-out, the element resets without operating. A loss of
voltage during any state returns the Power Factor element(s) to the Reset state.
For wye-connected VTs, the power factor value is calculated from the valid phase(s) with a
voltage that is above a user selected threshold and a current that is above 0.002 x CT.
Power Factor element delay timers are only allowed to time when the supervision
conditions are met and the Power Factor remains outside of the region between the
programmed Switch-In and Switch-Out levels. In the same way, when a Power Factor
condition starts the Power Factor delay timer, if one or more valid phases no longer satisfy
the supervision conditions, the power factor will be re-calculated based on the still valid
phase(s). If the element is continuously asserted with the new power factor value, the timer
will continue timing, otherwise, the element will reset without operating.
The following figure illustrates the conventions established for use in 845 relays, where the
negative value means the lead power factor, and the positive value means the lag power
factor.
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS