GE 489 - Manual (Page 71)

GE Multilin 489 Generator Management Relay 4-21
4 SETPOINTS 4.6 S5 CURRENT ELEMENTS
4
4.6S5 CURRENT ELEMENTS 4.6.1 INVERSE TIME OVERCURRENT CURVE CHARACTERISTICS
a) DESCRIPTION
The 489 inverse time overcurrent curves may be either ANSI, IEC, or GE Type IAC standard curve shapes. This allows for
simplified coordination with downstream devices. If however, none of these curve shapes is adequate, the FlexCurve™
may be used to customize the inverse time curve characteristics. Definite time is also an option that may be appropriate if
only simple protection is required.
A multiplier setpoint allows selection of a multiple of the base curve shape that is selected with the curve shape setpoint.
Unlike the electromechanical time dial equivalent, trip times are directly proportional to the time multiplier setting value. For
example, all trip times for a multiplier of 10 are 10 times the multiplier 1 or base curve values. Setting the multiplier to zero
results in an instantaneous response to all current levels above pickup.
Regardless of the trip time that results from the curve multiplier setpoint, the 489 cannot trip any quicker
than one to two cycles plus the operate time of the output relay.
Time overcurrent tripping time calculations are made with an internal “energy capacity” memory variable. When this vari-
able indicates that the energy capacity has reached 100%, a time overcurrent trip is generated. If less than 100% is accu-
mulated in this variable and the current falls below the dropout threshold of 97 to 98% of the pickup value, the variable must
be reduced. Two methods of this resetting operation are available, “Instantaneous” and “Linear”. The Instantaneous selec-
tion is intended for applications with other relays, such as most static units, which set the energy capacity directly to zero
when the current falls below the reset threshold. The Linear selection can be used where the 489 must coordinate with
electromechanical units. With this setting, the energy capacity variable is decremented according to the following equation.
(EQ 4.3)
where: T
RESET
= reset time in seconds
E = energy capacity reached (per unit)
M = curve multiplier
C
R
= characteristic constant (5 for ANSI, IAC, Definite Time and FlexCurves™, 8 for IEC curves)
b) ANSI CURVES
The ANSI time overcurrent curve shapes conform to industry standard curves and fit into the ANSI C37.90 curve classifica-
tions for extremely, very, normally, and moderately inverse. The 489 ANSI curves are derived from the formula:
(EQ 4.4)
where: T = Trip Time in seconds; M = Multiplier setpoint
I = Input Current; I
pickup
= Pickup Current setpoint
A, B, C, D, E = Constants
Table 4–1: 489 OVERCURRENT CURVE TYPES
ANSI IEC GE TYPE IAC OTHER
ANSI Extremely Inverse IEC Curve A (BS142) IAC Extremely Inverse FlexCurve™
ANSI Very Inverse IEC Curve B (BS142) IAC Very Inverse Definite Time
ANSI Normally Inverse IEC Curve C (BS142) IAC Inverse
ANSI Moderately Inverse IEC Short Inverse IAC Short Inverse
Table 4–2: ANSI INVERSE TIME CURVE CONSTANTS
ANSI CURVE SHAPE CONSTANTS
A B C D E
EXTREMELY INVERSE 0.0399 0.2294 0.5000 3.0094 0.7222
VERY INVERSE 0.0615 0.7989 0.3400 –0.2840 4.0505
NORMALLY INVERSE 0.0274 2.2614 0.3000 –4.1899 9.1272
MODERATELY INVERSE 0.1735 0.6791 0.8000 –0.0800 0.1271
NOTE
T
RESET
E M C
R
Ч Ч
100
----------------------------- =
T M A
B
I I
pickup
⁄ ( ) C –
-------------------------------------
D
I I
pickup
⁄ ( ) C
–
( ) 2
---------------------------------------------
E
I I
pickup
⁄ ( ) C
–
( ) 3
--------------------------------------------- + + +
⎝ ⎠
⎜ ⎟
⎛ ⎞
Ч =