Rockwell Automation 865 Differential Protection Relay - Manual (Page 59)

Protection Functions 3-15
865-UM001A-EN-P – July 2009
The current unbalance stage protects against unbalanced phase
currents and single phasing. The protection is based on the negative
sequence current.
Both definite time and inverse time characteristics are available. The
inverse delay is based on Equation 5. Only the base frequency com-
ponents of the phase currents are used to calculate the negative
sequence value I
2
.
Inverse Delay
The inverse delay is based on the following equation.
Equation 5
2
2
2
2
1
K
I
I
K
t
N
−
⎟⎟⎠
⎞
⎜⎜⎝
⎛
= , where
t = Operation time
K
1
= Delay multiplier
I
2
= Measured and calculated negative sequence
phase current of fundamental frequency.
I
N
= Rated current
K
2
= Pick-up setting I
2
> in pu. The maximum
allowed degree of unbalance.
Example:
K
1
= 15 s
I
2
= 22.9 % = 0.229 xI
N
K
2
= 5 % = 0.05 xI
N
4 . 300
05 . 0
1
229 . 0
15
2
2
=
− ⎟
⎠
⎞
⎜
⎝
⎛
= t
The operation time in this example will be five minutes.
More Stages (Definite Time Delay only)
If more than one definite time delay stages are needed for current
unbalance protection, the freely programmable stages can be used
(Chapter 2).
Setting Groups
There are two settings groups available. Switching between setting
groups can be controlled by digital inputs, virtual inputs (mimic
display, communication, logic) and manually.
Current unbalance protection
I
2
>, I’
2
> (46)