Current Elements; Inverse Time Overcurrent Curves - GE 889 Instruction Manual

PROTECTION

Current Elements

Inverse Time
Overcurrent Curves
4–156
Figure 4-56: Current Elements Display Hierarchy
Description
The relay has six setpoint groups with phase, neutral, and ground elements per group. The
programming of the time-current characteristics of these elements is identical in all cases
and is only covered in this section. The required curve is established by programming a
Pickup Current, Curve Shape, Curve Multiplier, and Reset Time. The Curve Shape can be
either a standard shape or a user-defined shape programmed with the FlexCurve feature.
The Inverse Time Overcurrent Curves used by the Time Overcurrent elements are the IEEE,
2
IEC, GE Type IAC, ANSI, I t and I
coordination.
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 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,
4
t standard curve shapes. This allows for simplified
889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS