Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

Page: 179 / 1518

5.19
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Functions
Over- and Underfrequency Elements
Case 2: Single-Phase PT Input, Connected to the A-Phase Input
In this case, VF01 = VA, VF02 = ZERO, and VF03 = ZERO.
Equation 5.7
Equation 5.8
Set 81UVSP to 60 percent of this value:
Equation 5.9
Equation 5.10
Case 3: Single-Phase PT Input, Connected to the B- or C-Phase Input
In this case, VF01 = ZERO, VF02 = VB, and VF03 = ZERO.
Equation 5.11
Equation 5.12
Set 81UVSP to 60 percent of this value:
Equation 5.13
Equation 5.14
Table 5.17 summarizes the results of the three cases.
Over- and Underfrequency Elements
Use the relay frequency elements for such abnormal frequency protection as
underfrequency load shedding.
Figure 5.15 shows the logic for the six levels of over- and underfrequency ele-
ments in the relay.
Valpha 2 67 V
• =
Valpha 94.75 V
=
81UVSP 0.6 94.75
•
V
=
81UVSP 56.85 V
=
Table 5.17 Table Y12. Summary of the Valpha and 81UVSP Calculations
Case PT Connections VA VB VC Valpha 0.6 • Valpha
Case 1 Three-phase 67

0° 67

–120° 67

120° 142.13 85.28
Case 2 Single-phase, VA 67

0° 0 0 94.75 56.85
Case 3 Single-phase, VB/VC 0 67

–120° 0 47.38 28.43
Valpha 2
67
2
------
V
• =
Valpha 47.37 V
=
81UVSP 0.6 47.37 V
• =
81UVSP 28.43 V
=