Date Code 20171021 8Instruction Manual SEL-421 Relay
8Instruction ManualInstruction Manual
SECTION 5
Protection Functions
NOTE: If the relay is using a remote
data acquisition system, such as TiDL,
the operating times will be delayed by
1.5 ms. Use caution when setting the
relay coordination times to account
for this added delay.
This section provides a detailed explanation for each of the many SEL-421 Relay
protection functions. Each subsection provides an explanation of the function,
along with a list of the corresponding settings and Relay Word bits. Logic dia-
grams and other figures are included.
Functions discussed in this section are listed below.
➤ Current and Voltage Source Selection on page 5.2
➤ Polarizing Quantity for Distance Element Calculations on page 5.14
➤ Frequency Estimation on page 5.15
➤ Undervoltage Supervision Logic on page 5.17
➤ Over- and Underfrequency Elements on page 5.19
➤ Time-Error Calculation on page 5.21
➤ Fault Location on page 5.23
➤ Open-Phase Detection Logic on page 5.24
➤ Pole-Open Logic on page 5.25
➤ Loss-of-Potential Logic on page 5.28
➤ Fault Type Identification Selection Logic on page 5.33
➤ Ground Directional Element on page 5.33
➤ Phase and Negative-Sequence Directional Elements on page 5.45
➤ Directionality on page 5.46
➤ CVT Transient Detection on page 5.47
➤ Series-Compensation Line Logic on page 5.48
➤ Load-Encroachment Logic on page 5.49
➤ Out-of-Step Logic (Conventional) on page 5.50
➤ Out-of-Step Logic (Zero Settings) on page 5.56
➤ Mho Ground-Distance Elements on page 5.72
➤ Quadrilateral Ground-Distance Elements on page 5.76
➤ Mho Phase-Distance Elements on page 5.80
➤ Quadrilateral Phase-Distance Elements on page 5.85
➤ Zone Time Delay on page 5.91
➤ Instantaneous Line Overcurrent Elements on page 5.93
➤ Inverse-Time Overcurrent Elements on page 5.99
➤ Over- and Undervoltage Elements on page 5.113
➤ Switch-Onto-Fault Logic on page 5.117
➤ Communications-Assisted Tripping Logic on page 5.120
➤ Directional Comparison Blocking Scheme on page 5.121
➤ Permissive Overreaching Transfer Tripping Scheme on page 5.124
➤ Directional Comparison Unblocking Scheme Logic on page 5.133
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