P.3.105
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
CT Selection Procedure
Applying Equation 3.39, we obtain the following:
Three-phase fault:
Equation 3.56
Single-phase-to-ground fault:
Equation 3.57
This result shows that the CT is suitable for both distance and current
differential applications.
ANSI Application (5 A Secondary)
Equation 3.58
On the 600:5 A tap, V
STD
= 400 V. Applying Equation 3.39, we obtain the
following:
Three-phase fault:
Equation 3.59
Single-phase-to-ground fault:
Equation 3.60
Current Differential
Three-phase fault:
Equation 3.61
Single-phase-to-ground fault:
Equation 3.62
This result shows that using the C800 CT half winding tap is not suitable for
ground faults in distance applications.
The following simulation illustrates the level of CT saturation for this case.
23136
2400
--------------- 19.303 1+15 0.25 0.1++
6
------------------------------------------------------------------------------------------
500.719=
19652
2400
--------------- 16.018 1+15 2 0.25 0.1++
6
--------------------------------------------------------------------------------------------------- 362.3=
CT Ratio: 1200/600:5 A
Accuracy Class: C800 (full winding) and C400 (half winding)
R
CT
:0.80 (full winding) and 0.40 (half winding)
R
LEAD
:0.50
Relay Burden: 0.5 VA (5 A nominal)
Z
Relay
VA
RELAY
I
NOM SEC
2
--------------------------
0.5 VA
5 A
2
------------------ 0.02 ===
400
23135
120
--------------- 19.303 1+0.5 0.02+
6
----------------------------------------------------------------------------- 339.249 V=
400
19652
120
---------------
16.01 1+20.5 0.02+
6
-----------------------------------------------------------------------------------
473.786 V=
400
23135
120
---------------
19.303 1+0.5 0.02+
7.5
----------------------------------------------------------------------------- 271.399 V=
400
19652
120
--------------- 16.01 1+20.5 0.02+
7.5
----------------------------------------------------------------------------------- 379.029 V=
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