GE D90 Plus Instruction Manual page 240

GROUPED PROTECTION ELEMENTS
230
• IEEE Extremely Inverse
• IEEE Very Inverse
• IEEE Moderately Inverse
• IEC Curve A (BS142)
• IEC Curve B (BS142)
• IEC Curve C (BS142)
• IAC Extremely Inverse
• IAC Very Inverse
• 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 operates. 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 reset operation are available:
instantaneous and timed. The instantaneous method is 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.
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 curves. The
IEEE curves are derived by the operate and reset time equations.
where
• T = operate time (in seconds)
• TDM = multiplier setting
• I =input current
• I = pickup current setting
pickup
• A, B, p = constants defined in the table
• T = reset time in seconds (assuming the energy capacity is 100% and the reset
reset
mode is "Timed")
• t = characteristic constant defined in the table
r
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 7: PROTECTION
Eq. 10
Eq. 11