4–154 845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
PROTECTION CHAPTER 4: SETPOINTS
A time dial multiplier setting allows the selection of a multiple of the base curve shape
(where the time dial multiplier = 1) with the curve shape setting. Unlike the
electromechanical time dial equivalent, operate times are directly proportional to the time
multiplier (TD MULTIPLIER) setting value. For example, all 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.
Time Overcurrent 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 element will operate. If less than 100% energy capacity 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 types of this resetting operation are available:
“Instantaneous” and “Timed”. The “Instantaneous” selection is intended for applications
with other relays, such as most static relays, which set the energy capacity directly to zero
when the current falls below the reset threshold. The “Timed” selection can be used where
the relay must coordinate with electromechanical relays.
IEEE CURVES
The IEEE Time Overcurrent curve shapes conform to industry standards and the IEEE
C37.112-1996 curve classifications for extremely, very, and moderately inverse. The IEEE
curves are derived from the formula:
Where:
T = operate time (in seconds)
TDM = Multiplier setting
I = input current
I
pickup
= Pickup Current setting
A, B, p = constants
T
RESET
= reset time in seconds (assuming energy capacity is 100% and RESET is
“Timed”)
t
r
= characteristic constant
Table 4-24: IEEE INVERSE TIME CURVE CONSTANTS
Table 4-25: IEEE CURVE TRIP TIMES (IN SECONDS)
IEEE CURVE SHAPE A B P t
r
IEEE Extremely Inverse 28.2 0.1217 2.000 29.1
IEEE Very Inverse 19.61 0.491 2.000 21.6
IEEE Moderately Inverse 0.0515 0.1140 0.02000 4.85
MULTIPLIER
(TDM)
CURRENT (I/I
pickup
)
1.5 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
IEEE EXTREMELY INVERSE
0.5 11.341 4.761 1.823 1.001 0.648 0.464 0.355 0.285 0.237 0.203
1.0 22.682 9.522 3.647 2.002 1.297 0.927 0.709 0.569 0.474 0.407
2.0 45.363 19.043 7.293 4.003 2.593 1.855 1.418 1.139 0.948 0.813
4.0 90.727 38.087 14.587 8.007 5.187 3.710 2.837 2.277 1.897 1.626
6.0 136.090 57.130 21.880 12.010 7.780 5.564 4.255 3.416 2.845 2.439
8.0 181.454 76.174 29.174 16.014 10.374 7.419 5.674 4.555 3.794 3.252
10.0 226.817 95.217 36.467 20.017 12.967 9.274 7.092 5.693 4.742 4.065
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