GE 750 Instruction Manual page 171

CHAPTER 5: SETPOINTS
Note
NOTE
Note
NOTE
5.6.2 Time Overcurrent Curve Characteristics
750 FEEDER MANAGEMENT RELAY – INSTRUCTION MANUAL
or base curve values. Setting the multiplier to zero results in an instantaneous
response to all current levels above pickup.
• <Element_Name> RESET: Time overcurrent tripping time calculations are made with
an internal 'energy capacity' memory variable. When this variable indicates that the
energy capacity has reached 100%, a time overcurrent trip is generated. If less than
100% is accumulated 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
selection 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 relay must coordinate with
electromechanical units. With this setpoint, the energy capacity variable is
decremented according to the following equation.
where:T = reset time in seconds; E = energy capacity reached (per unit);
RESET
M = curve multiplier; C
FlexCurves™; 8 for IEC)
When the setpoint group changes, the setpoints for pickup, curve, multiplier and reset may
change. Accumulated 'energy capacity' continues to accumulate, using the new setpoint
group parameters. For example, assume the following conditions:
• Setpoint group 1 had curve "Definite time" set for 4 seconds.
• Setpoint group 2 had curve "Definite time" set for 8 seconds.
• Initially group 1 was active.
• Current rose above threshold at time T = 0 seconds.
• Setpoint group changed from group 1 to group 2 at time T = 1 seconds.
At this time the 'energy capacity' accumulator was at 25% = (1 second x
(100% / 4 seconds)).
• Current remained above the threshold.
• Using the new multiplier setpoint and continuing from 25% accumulated, the time
remaining after setpoint group change was 6 seconds = (100% - 25%) x 8 seconds.
• Timed element operated at time T = 7 seconds.
Graphs of standard time-current curves on 11" × 17" log-log graph paper are available
upon request. Requests may be placed with our literature department.
Definite Time Curve:
Definite Time curves trip as soon as the pickup level is exceeded for a specified period of
time. The base Definite Time curve has a delay of 0.1 seconds. The curve multiplier adjusts
this delay from 0.00 to 10.00 seconds in steps of 0.01.
T E M C × ×
=
RESET R
= characteristic constant (5 for ANSI, IAC, Definite Time, and
R
S5 PROTECTION
(EQ 5.1)
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