Schweitzer Engineering Laboratories SEL-351A, Manual, Page: 132 / 666

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
ondary sec =
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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