Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

Page: 195 / 1518

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