Schweitzer Engineering Laboratories SEL-351A - Page 132

4.6
SEL-351A Relay Instruction Manual Date Code 20080213
Loss-of-Potential, Load Encroachment, and Directional Element Logic
Load-Encroachment Logic (Not in SEL-351A-1)
Equation 4.2
Equation 4.3
Equation 4.4
Equation 4.5
Equation 4.6
Now, calculate the equivalent secondary impedance:
Equation 4.7
This Ω secondary value can be calculated more expediently with the following
equation:
Equation 4.8
Again, for the maximum forward load:
Equation 4.9
To provide a margin for setting ZLF, multiply by a factor of 0.9:
Equation 4.10
For the maximum reverse load:
Equation 4.11
230 kV
1
3
-------
⎝⎠
⎛⎞
• 132.8 kV line-to-neutral=
267 MVA
1
132.8 kV
----------------------
⎝⎠
⎛⎞
1000 kV
MV
--------------------
⎝⎠
⎛⎞
• • 2010 A primary=
2010 A primary
1
CT ratio
--------------------
⎝⎠
⎛⎞
•
2010 A primary
1 A secondary
400 A primary
-----------------------------------
⎝⎠
⎛⎞
• =
5.03 A secondary=
132.8 kV
1000 V
kV
-----------------
⎝⎠
⎛⎞
• 132800 V primary=
132800 V primary
1
PT ratio
-------------------
⎝⎠
⎛⎞
•
132800 V primary
1 V secondary
2000 V primary
--------------------------------------
⎝⎠
⎛⎞
• =
66.4 V secondary=
66.4 V secondary
5.03 A secondary
------------------------------------------13.2 Ω ondarysec=
line-line voltage in kV()
2
CT ratio()• []
3-phase load in MVA()PT ratio()• []
--------------------------------------------------------------------------------------------------
230()
2
400()• []
800()2000()• []
------------------------------------------13.2 Ω secondary=
ZLF 13.2 Ω secondary 0.9• =
11.90 Ω secondary=
230()
2
400()• []
500()2000()• []
------------------------------------------21.1 Ω secondary=
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