Where 200mA is the current drawn by the IED circuit and 50mA is the current drawn
by each CT just at pickup. The magnetizing current is taken from the magnetizing
curve of the current transformer cores, which should be available. The current value at
T
ripPickup is taken.
7.2.4.5 Restricted earth fault protection (87N)
M16850-156 v10
For solidly grounded systems a restricted earth fault protection REFPDIF (87N) is
often provided as a complement to the normal transformer differential function. The
advantage with the restricted ground fault functions is the high sensitivity for internal
earth faults in the transformer winding. Sensitivities of 2-8% can be achieved whereas
the normal differential function will have sensitivities of 20-40%. The sensitivity for
high impedance restricted ground fault function is mostly dependent on the current
transformers magnetizing currents.
The connection of a restricted earth fault function is shown in Figure
64. It is
connected across each directly or low impedance grounded transformer winding.
ANSI05000177 V1 EN-US
Figure 64: Application of HZPDIF (87) function as a restricted earth fault
protection for a star connected winding of an YNd transformer
Section 7 1MRK 502 071-UUS A
Differential protection
174 Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062
Application manual
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