GE D90 Plus Instruction Manual page 232

GROUPED PROTECTION ELEMENTS
IEEE curves
222
• IAC Inverse.
• IAC Short Inverse.
2
• I t.
• FlexCurves™ A, B, C, and D.
• Recloser curves.
• Definite Time.
A multiplier (time dial) setting allows 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 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 methods of this resetting operation are available:
instantaneous and timed. The instantaneous method 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 method can be used where the
relay must coordinate with electromechanical relays.
The IEEE time overcurrent curve shapes conform to industry standards and the IEEE
C37.112-1996 curve classifications for extremely, very, and moderately inverse curves. The
IEEE curves are derived by the operate and reset time equations.
The terms in the above equations are defined as follows.
• T represents the operate time in seconds.
operate
• TDM represents the multiplier setting.
• I represents the input current.
• I represents the pickup current setting value.
pickup
• A, B, and p are constants and are defined in the table below.
• T represents reset time in seconds, assuming the energy capacity is 100% and the
reset
reset mode is timed.
• t represents the characteristic constant and is defined in the table below.
r
Table 13: IEEE inverse time curve constants
IEEE curve shape
Extremely inverse
Very inverse
Moderately inverse
A B
28.2 0.1217
19.61 0.491
0.0515 0.1140
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 7: PROTECTION
Eq. 8
Eq. 9
p t
r
2.0000 29.1
2.0000 21.6
0.020000 4.85