5.35
Date Code 20171021 Instruction Manual SEL-421 Relay
Protection Functions
Ground Directional Element
Use caution when you set E32 = AUTO. It is not appropriate for all applications.
Systems with a strong negative-sequence source (e.g., equivalent negative-sequence
impedance of less than 2.5/I
NOM
in ohms) can use E32 = AUTO. It is best to use
E32 = AUTO2 with the settings in Table 5.34 if any of the following apply:
➤ The negative-sequence impedance of the source is greater than
2.5/I
NOM
in ohms
➤ The line impedance is unknown
➤ A non-fault condition occurs, such as a switching transformer
energization causing the negative-sequence voltage to be
approximately zero
The preferred settings in Table 5 .34 will provide equal or better protection than
E32 = AUTO for most systems.
Detailed Settings Description
If you set E32 to Y, you can change the settings listed in Table 5.33.
50FP and 50RP
Setting 50FP is the threshold for the current level detector that enables forward
decisions for both the negative- and zero-sequence voltage-polarized directional
elements. If the magnitude of 3I
2
or 3I
0
is greater than 50FP, the corresponding
directional element can process a forward decision.
Table 5.33 Ground Directional Element Settings AUTO Calculations
Setting Equation
50FP 0.12 • I
NOM
50RP 0.08 • I
NOM
Z2F 0.5 • Z1MAG
Z2R Z2F + 1/(2 • I
NOM
)
a2 0.1
k2 0.2
Z0F 0.5 • Z0MAG
Z0R Z0F + 1/(2 • I
NOM
)
a0 0.1
Table 5.34 Ground Directional Element Preferred Settings
Name 5 A nominal 1 A nominal
E32 Y Y
Z2F –0.30 –1.5
Z2R 0.30 1.5
Z0F –0.30 –1.5
Z0R 0.30 1.5
50QFP /50GFP 0. 50 A 0.10 A
50QRP /50GRP 0. 25 A 0.05 A
a2 0.10 0.10
k2 0.20 0.20
a0 0.10 0.10
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