3-10
Line Current Differential, Distance, Out-of-Step, Overcurrent,
Date Code 20011112
Voltage, Synchronism Check, and Frequency Elements
SEL-311L Instruction Manual
nominal load current to 45 ohms worst case at nominal load current, as shown in Figure 3.8. This
sensitivity is in secondary ohms referenced to the highest CTR.
R
F
(
Ω
sec.)
Load Current (Per Unit of Nominal)
1/3
2/3
1
45
Ω
132.8
Ω
M311L092
Figure 3.8: Ground Fault Sensitivity of 87L2 and 87LG Elements With 87L2P = 0.5 or
87LGP = 0.5
Other Unbalanced Fault Types
Identical considerations for other internal unbalanced fault types (phase-to-phase, and phase-to-
phase-to-ground), give identical settings for the negative-sequence 87L elements.
Factory Settings Give Excellent Security During External Faults
In summary, the 87L element settings derived above give excellent speed, sensitivity, and
dependability for internal faults of all types. Recall the Alpha plane ratio ideally lies at 1
∠
180°
for external faults. The restraint regions defined by the settings derived above surround 1
∠
180°
and must include possible sources of error. To check these settings for security during external
faults, consider all possible sources of ratio and angle error on the Alpha plane during an external
fault.
First, consider negative-sequence element 87L2. During an external three-phase fault, the CTs at
one terminal could saturate, while the CTs at the other terminal do not. This can produce false
negative-sequence current at one terminal, and essentially zero negative-sequence current at the
other terminal. Since all of the measured negative-sequence current is errant, the angle and ratio
on the negative-sequence Alpha plane are not easy to predict. To prevent the 87L2 element from
misoperating during an external three-phase fault which produces CT saturation at one line
terminal, the 87L2 element is blocked if all three-phase currents from any line terminal exceed
300% of nominal current (15 A for a 5 A relay).
A similar argument holds for the zero-sequence or ground differential element. To prevent the
87LG element from misoperating for an external three-phase or phase-to-phase fault that causes
CT saturation at one line terminal, the ground differential element is blocked if two or three of the
phase currents from any line terminal exceed 300% of nominal current (15 A for a 5 A relay). If
the negative-sequence element is not used, set the phase elements to detect internal phase faults
AND internal phase-to-phase-to-ground faults, and set the ground element to detect internal
phase-to-ground faults.
Summary of Contents for SEL-311L
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