4.7
Date Code 20060320
Instruction Manual
SEL-311C Relay
Loss-of-Potential, CCVT Transient Detection, Load-Encroachment, and Directional Element Logic
Load-Encroachment Logic
f.
132800 V primary • (1/PT ratio)
= 132800 V primary • (1 V secondary/2000 V primary)
= 66.4 V secondary
Step 2. Now, calculate the equivalent secondary impedance:
a. 66.4 V secondary/5.03 A secondary = 13.2
Ω
secondary
This
Ω
secondary value can be calculated more
expediently with the following equation:
b. [(line-line voltage in kV)
2
• (CT ratio)]/[(3-phase load
in MVA) • (PT ratio)]
Again, for the maximum forward load:
[(230)
2
• (400)]/[(800) • (2000)] = 13.2
Ω
secondary
Step 3. To provide a margin for setting ZLF, multiply by a factor of 0.9:
a. ZLF = 13.2
Ω
secondary • 0.9 = 11.90
Ω
secondary
Step 4. For the maximum reverse load:
a. [(230)
2
• (400)]/[(500) • (2000)] = 21.1
Ω
secondary
Again, to provide a margin for setting ZLR:
b. ZLR = 21.1
Ω
secondary • 0.9 = 19.00
Ω
secondary
Convert Power Factors to Equivalent Load Angles
The power factor (forward load) can vary from 0.90 lag to 0.95 lead.
Setting PLAF = cos
–1
(0.90) = 26°
Setting NLAF = cos
–1
(0.95) = –18°
The power factor (reverse load) can vary from 0.80 lag to 0.95 lead.
Setting PLAR = 180° – cos
–1
(0.95) = 180° – 18° = 162°
Setting NLAR = 180° + cos
–1
(0.80) = 180° + 37° = 217°
Apply Load-Encroachment Logic to a Phase Time-Overcurrent
Again, from
ZLOAD =
ZLOUT + ZLIN
Summary of Contents for SEL-311C
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