GE T35 UR Series, Instruction Manual

Page: 258 / 568

5-136 T35 Transformer Protection System GE Multilin
5.6 GROUPED ELEMENTS 5 SETTINGS
5
5.6.4 PHASE CURRENT
a) INVERSE TIME OVERCURRENT CURVE CHARACTERISTICS
The inverse time overcurrent curves used by the time overcurrent elements are the IEEE, IEC, GE Type IAC, and I
2
t stan-
dard curve shapes. This allows for simplified coordination with downstream devices.
If none of these curve shapes is adequate, FlexCurves may be used to customize the inverse time curve characteristics.
The definite time curve is also an option that may be appropriate if only simple protection is required.
A time dial multiplier setting allows selection of a multiple of the base curve shape (where the time dial multiplier = 1) with
the curve shape (
CURVE
) 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 indi-
cates that the energy capacity has reached 100%, a time overcurrent element will operate. If less than 100% energy capac-
ity 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 “Timed”. The “Instan-
taneous” 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 formulae:
, (EQ 5.28)
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 5–14: OVERCURRENT CURVE TYPES
IEEE IEC GE TYPE IAC OTHER
IEEE Extremely Inverse IEC Curve A (BS142) IAC Extremely Inverse I
2
t
IEEE Very Inverse IEC Curve B (BS142) IAC Very Inverse FlexCurves A, B, C, and D
IEEE Moderately Inverse IEC Curve C (BS142) IAC Inverse Recloser Curves
IEC Short Inverse IAC Short Inverse Definite Time
Table 5–15: IEEE INVERSE TIME CURVE CONSTANTS
IEEE CURVE SHAPE A B P T
R
IEEE Extremely Inverse 28.2 0.1217 2.0000 29.1
IEEE Very Inverse 19.61 0.491 2.0000 21.6
IEEE Moderately Inverse 0.0515 0.1140 0.02000 4.85
Table 5–16: IEEE CURVE TRIP TIMES (IN SECONDS)
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
T TDM
A
I
I
pickup
----------------
 
 
p
1
–
---------------------------------- B
+
×
=
T
RESET
TDM
t
r
1
I
I
pickup
----------------
 
 
2
–
------------------------------------
×
=