Functions - GE MI-869 Instruction Manual

MONITORING

Functions

Power Factor
4–284
The 869 is applied on a synchronous machine, it is desirable not to trip or alarm on power
factor until the field has been applied. Therefore, this feature can be blocked until the
machine comes up to speed and the field is applied. From that point forward, the power
factor trip and alarm elements will be active. Once the power factor is less than either the
Lead or Lag level, for the specified delay, a trip or alarm will occur indicating a Lead or Lag
condition. The power factor alarm can be used to detect loss of excitation and out of step.
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
Two independent elements are available for monitoring Power Factor, each having Lead
and Lag trip and alarm functions.
For delta-connected VTs, the Power Factor feature is inhibited from operating unless all
three voltages are above the selected voltage threshold and one or more currents are
above the selected current threshold. Power Factor element delay timers are only allowed
to time when the voltage threshold is exceeded on all phases and current threshold is
exceeded on one phase. In the same way, when a Power Factor condition starts the Power
Factor delay timer, if all three phase voltages fall below the threshold and one phase
current threshold falls below the timer has timed-out, the element resets without
operating. A loss of voltage during any state returns both Power Factor elements to the
Reset state.
For wye-connected VTs, the power factor value is calculated from the valid phase(s) for
which voltage and current are above the user selected thresholds. Power Factor element
delay timers are only allowed to time when the supervision conditions are met. 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 is re-calculated
based on the still valid phase(s). If the element is continuously asserted with the new power
factor value, the timer would continue timing, otherwise, the element resets without
operating.
The minimum operating voltage and current are set as a threshold below which the
element is reset.
The following figure illustrates the conventions established for use in the 869 relay, where
the negative value means the lead power factor, and the positive value means the lag
power factor. For details on the convention used for measuring power, see Chapter 6:
Metering/ Power.
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS