GE T60 Instruction Manual page 452

GROUPED ELEMENTS
Having the differential and restraining signals developed, the element applies a single slope differential characteristic with
a minimum pickup as shown in the following logic diagram.
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.
5
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°
5-270
Figure 5-152: Restricted ground fault logic
T60 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS
, and Igr = 2 pu
, and Igr = 20 pu
, and Igr = 10 pu