Rockwell Automation 865 Differential Protection Relay - Manual (Page 82)

3-38 Protection
Functions
865-UM001A-EN-P – July 2009
Figure 3.18 – IEC Very Inverse Delay Figure 3.19 – IEC Long Time Inverse Delay
IEEE/ANSI Inverse Time Operation
There are three different delay types according IEEE Std C37.112-
1996 (MI, VI, EI) and many de facto versions according Table 3.24.
The IEEE standard defines inverse delay for both trip and release
operations. However, in the Allen-Bradley relay only the trip time is
inverse according the standard but the release time is constant.
The operation delay depends on the measured value and other
parameters according to Equation 7. Actually this equation can only
be used to draw graphs or when the measured value I is constant
during the fault. A modified version is implemented in the relay for
real time usage.
Equation 7:
⎥
⎥
⎥
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎢
⎢
⎢
⎣
⎡
+
−
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
= B
I
I
A
k t
C
pickup
1
t = Operation delay in seconds
k = User’s multiplier
I = Measured value
I
pickup
= User’s pick up setting
A,B,C = Constant parameter according to Table 3.24