GE L90 Instruction Manual page 509

CHAPTER 5: SETTINGS
The following examples explain how the restraining signal is created for maximum sensitivity and security. These examples
clarify the operating principle and provide guidance for testing of the element.
Example 1: External single-line-to-ground fault
Given the following inputs: IA = 1 pu ∠0°, IB = 0, IC = 0, and IG = 1 pu ∠180°
The relay calculates the following values:
Igd = 0,
The restraining signal is twice the fault current. This gives extra margin if the phase or neutral CT saturates.
Example 2: External high-current SLG fault
Given the following inputs: IA = 10 pu ∠0°, IB = 0, IC = 0, and IG = 10 pu ∠–180°
The relay calculates the following values:
Igd = 0,
Example 3: External high-current three-phase symmetrical fault
Given the following inputs: IA = 10 pu ∠0°, IB = 10 pu ∠–120°, IC = 10 pu ∠120°, and IG = 0 pu
The relay calculates the following values:
Igd = 0,
Example 4: Internal low-current single-line-to-ground fault under full load
Given the following inputs: IA = 1.10 pu ∠0°, IB = 1.0 pu ∠–120°, IC = 1.0 pu ∠120°, and IG = 0.05 pu ∠0°
The relay calculates the following values:
I_0 = 0.033 pu ∠0°, I_2 = 0.033 pu ∠0°, and I_1 = 1.033 pu ∠0°
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
Figure 5-156: Restricted ground fault logic
GROUPED ELEMENTS
, and Igr = 2 pu
, and Igr = 20 pu
, and Igr = 10 pu
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