GE 850 Instruction Manual page 348

CURRENT ELEMENTS
NOTE:
NOTE
6–80
In some applications, it may be necessary to apply the K-factor to compensate for any
current measurement inaccuracies from the relay. The 850 relay has the following
accuracy for RMS values in current:
± 0.25% of reading or ± 0.2% of rated (whichever is greater) from 0.1 to 2.0 x CT
± 1% of reading > 2.0 x CT
Selecting the K-factor in the range between 1 to 1.2 is sufficient.
The pickup level needs to be calculated based on the K-factor and the base current, Ib
(permissible current for the equipment to be thermally protected). The time to trip is
estimated when the square of the ratio between the actual flowing current and the
programmed pickup current is higher than 1 (i.e. 100%).
The pickup is calculated as follows:
I = K * I
pickup base
I is the pickup current (xCT) setting. K is a factor reflecting the relay current
pickup
measurement error. I
base
protected (in many cases this is the maximum continuous current for the equipment
within its thermal limits)
ALARM
Range: 70.0 to 110.0% in steps of 0.1%
Default: 80.0%
The setting sets the Alarm level for the accumulated thermal capacity above which the
element generates an alarm. The Alarm signal can be displayed by the user-
programmable LED.
HEAT TIME CONSTANT (τ
H
Range: 3.0 to 600.0 min in steps of 0.1 min
Default: 6.0 min
The time constant is used to compute the thermal capacity when the squared load/
Pickup ratio at each time step is greater than the thermal capacity computed in the
previous time step.
COOL TIME CONSTANT (τ
C
Range: 1.00 to 6.00 x τ
H
Default: 6.0 min
The time constant is used to compute the thermal capacity when the squared load/
Pickup ratio at each time step is less than the thermal capacity computed in the
previous time step.
BLOCK
Range: Off, Any FlexLogic operand
Default: Off
The element is blocked when the operand is asserted.
OUTPUT RELAY X
For details see Common
Each relay can be selected to become either energized or de-energized when operated,
and to operate as latched, self-resetting or pulsed.
EVENTS
Range: Enabled, Disabled
Default: Enabled
The selection of the Enabled setting enables the events of the function.
is the permissible current of the equipment to be thermally
)
)
in steps of 0.01 x τ
H
Setpoints.
850 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 6: PROTECTION SETPOINTS