Rockwell Automation 865 Differential Protection Relay - Manual (Page 77)

Protection Functions 3-33
865-UM001A-EN-P – July 2009
Inverse Time Operation
The inverse time operation – i.e. inverse delay minimum time (IDMT)
type of operation – is available for several protection functions. The
common principle, formula and graphic representations of the
available inverse delay types are described in this chapter.
Inverse delay means that the operation time depends on the measured
real time process values during a fault. When utilizing an overcurrent
stage using an inverse delay, a large fault current results in faster
operation. The alternative to inverse delay is definite delay. With
definite delay a preset time is used and the operation time does not
depend on the size of a fault.
Stage Specific Inverse Delay
Some protection functions have their own specific type of inverse
delay. Details of these dedicated inverse delays are described with
the appropriate protection function.
Operation Modes
There are three operation modes to use the inverse time
characteristics:
Standard delays
Using standard delay characteristics by selecting a curve family
(IEC, IEEE, IEEE2, RI) and a delay type (Normal inverse, Very
inverse etc). See “Standard Inverse Delays IEC, IEEE, IEEE2,
RI”.
Standard delay formula with parameters
Selecting a curve family (IEC, IEEE, IEEE2) and defining one's
own parameters for the selected delay formula. This mode is
activated by setting delay type to ‘Parameters’, and then editing
the delay function parameters A ... E. See “Parameter setting
using IEC, IEEE and IEEE2 Equations”.
Fully programmable inverse delay characteristics
Building the characteristics by setting 16 [current, time] points.
The relay interpolates the values between given points with 2nd
degree polynomials. This mode is activated by setting curve
family to ‘PrgN’'. There are maximum three different
programmable curves available at the same time. Each
programmed curve can be used by any number of protection
stages. See “Programmable Inverse Time Curves”.
Local Panel Graph
The relay will show a graph of the currently used inverse delay on
the local panel display. Up and down keys can be used for zooming.
Also the delays at 20xI
SET
, 4xI
SET
and 2xI
SET
are shown.